Metabolic reprogramming of the heart through stearoyl-CoA desaturase.

Progress in Lipid Research 57 (2015) 1–12

Contents lists available at ScienceDirect

Progress in Lipid Research journal homepage: www.elsevier.com/locate/plipres

Review

Metabolic reprogramming of the heart through stearoyl-CoA desaturase Pawel Dobrzyn a,⇑, Tomasz Bednarski a, Agnieszka Dobrzyn b,⇑ a b

Laboratory of Molecular and Medical Biochemistry, Nencki Institute of Experimental Biology, Warsaw, Poland Laboratory of Cell Signaling and Metabolic Disorders, Nencki Institute of Experimental Biology, Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 22 July 2014 Received in revised form 25 November 2014 Accepted 25 November 2014 Available online 5 December 2014 Keywords: Heart function Fatty acids Metabolism Stearoyl-CoA desaturase (SCD) Hypertrophy Atherosclerosis

a b s t r a c t Stearoyl-CoA desaturase (SCD), a central enzyme in lipid metabolism that synthesizes monounsaturated fatty acids, has been linked to tissue metabolism and body adiposity regulation. Recent studies showed that SCD has the ability to reprogram cardiac metabolism, thereby regulating heart function. In the heart, the lack of SCD1 enhances glucose transport and metabolism at the expense of fatty acid (FA) uptake and oxidation. The metabolic changes associated with SCD1 deficiency protect cardiac myocytes against both necrotic and apoptotic cell death and improve heart function. Furthermore, SCD4, a heart-specific isoform of SCD, is specifically repressed by leptin and the lack of SCD1 function in leptin-deficient ob/ob mice results in a decrease in the accumulation of neutral lipids and ceramide and improves the systolic and diastolic function of a failing heart. Large-population human studies showed that the plasma SCD desaturation index is positively associated with heart rate, and cardiometabolic risk factors are modulated by genetic variations in SCD1. The current findings indicate that SCD may be used to reprogram myocardial metabolism to improve cardiac function. Here, we review recent advances in understanding the role of SCD in the control of heart metabolism and its involvement in the pathogenesis of lipotoxic cardiomyopathies. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stearoyl-CoA desaturase (SCD): what does it do? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isoforms of SCD expressed in the heart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of SCD in cardiac metabolism and function: what we have learned from SCD1 knockout mice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Fatty acid and glucose oxidation in the heart in SCD1/ mice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Role of AMPK and PPARa pathways in SCD-dependent regulation of cardiac substrate utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Echocardiographic analysis of SCD1-deficient heart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SCD1 deletion improves heart function in obese mouse models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SCD1 and cardiomyocyte apoptosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physiological and pathological left ventricular hypertrophy: role of SCD and lipid metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Is SCD involved in postnatal dietary fat-induced cardiac hypertrophy? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regulation of cardiac metabolism and function by oleic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plasma desaturation index as a predictor of cardiac health in human . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SCD and atherosclerosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and future direction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 3 4 4 5 5 5 6 7 8 8 8 9 9

Abbreviations: AMPK, AMP-activated protein kinase; CD36, fatty acid translocase CD36; CPT1, carnitine palmitoyltransferase; CVD, cardiovascular disease; DAG, diacylglycerol; DAGL, diacylglycerol lipase; FA, fatty acid; FAS, fatty acid synthase; FATP, fatty acid transport protein; GLUT4, glucose transporter 4; GPAT, glycerol3-phosphate acyltransferase; IL, interleukin; iNOS, inducible nitric oxide synthase; IR, insulin receptor; IRS, insulin receptor substrate; LV, left ventricle; MUFA, monounsaturated fatty acids; NO, nitric oxide; PI3K, phosphatidylinositol 3-kinase; PLs, phospholipids; PPAR, peroxisome proliferator-activated receptors; PUFA, polyunsaturated fatty acids; SCD, stearoyl-CoA desaturase; SFA, saturated fatty acids; SREBP, sterol regulatory element-binding protein; TG, triacylglycerol; TO, trioleate; TS, tristearate. ⇑ Corresponding authors at: Laboratory of Molecular and Medical Biochemistry, Nencki Institute of Experimental Biology, 3 Pasteur Street, 02-093 Warsaw, Poland (P. Dobrzyn). Laboratory of Cell Signaling and Metabolic Disorders, Nencki Institute of Experimental Biology, 3 Pasteur Street, 02-093 Warsaw, Poland (A. Dobrzyn). E-mail addresses: [email protected] (P. Dobrzyn), [email protected] (A. Dobrzyn). http://dx.doi.org/10.1016/j.plipres.2014.11.003 0163-7827/Ó 2014 Elsevier Ltd. All rights reserved.

2

P. Dobrzyn et al. / Progress in Lipid Research 57 (2015) 1–12

Conflict of interest. . . . . . Transparency Document . Acknowledgements . . . . . References . . . . . . . . . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

1. Introduction Myocardial metabolism plays an important role in maintaining proper heart function and therefore is strictly regulated. Under aerobic conditions, the heart derives 60–90% of the energy necessary for contractile function from fatty acid (FA) oxidation, whereas the remainder is obtained mainly from carbohydrates (glucose and lactate) [1,2]. Evidence suggests that impaired cardiomyocyte metabolism contributes to contractile dysfunction and the progressive left-ventricular remodeling that is characteristic of heart failure. In disease states, such as ischemia-reperfusion, diabetes, and obesity, cardiac substrate utilization shifts to the excessive use of FAs in place of glucose [1–3]. This shift in metabolism has been suggested to play a role in the development of cardiomyopathy, leading to both impaired contractile function and ischemic injury [2,4]. In contrast, readjusting cardiomyocyte metabolic pathways to favor glucose oxidation leads to ischemia resistance in the heart [5] and protects against lipotoxic heart disease [6]. Patients with congenital lipodystrophy, a rare disorder in which the absence of adipocytes results in the accumulation of lipid in non-adipose tissues, or with inherited mitochondrial fatty acid oxidation defects develop premature cardiomyopathy [7]. In animal models of obesity and diabetes, such as leptin-deficient ob/ob mice, leptin receptor-deficient db/db mice and Zucker Diabetic Fatty rats, lipid accumulation within cardiomyocytes and dysregulation in cardiac metabolism are associated with impaired contractile function [7]. To date, metabolic alterations in the failing heart have been considered a part of the phenotype (i.e., a consequence of the development of cardiac dysfunction). However, some observations suggest the intriguing possibility that the disruption of normal glucose or FA metabolism may indeed be a primary factor responsible for the development of heart failure. Thus, understanding the regulatory mechanisms that are responsible for reprogramming cardiomyocyte metabolism is imperative to discover new treatments to improve cardiac function. Many studies underscore the important role of lipogenic enzymes in the regulation of cardiac metabolism and function and suggest that the role of lipogenic genes in cardiomyocytes may be distinct from other tissues. Cardiac sterol regulatory element-binding protein 1 (SREBP1), a key lipogenic transcription factor, was shown to activate G-protein-coupled inwardly reflecting K+ channels, leading to enhanced acetylcholine-sensitive K+ currents and reduced arrhythmias postmyocardial infraction [8]. The transgenic overexpression of fatty acid transport protein 1 (FATP1) in the heart has also been shown to cause lipotoxic cardiomyopathy [9]. The heart-specific knockdown of peroxisome proliferatoractivated receptor c (PPARc) [10] and acyl-CoA synthase 1 (ACS-1) [11] induces cardiac hypertrophy. Fatty acid synthase (FAS), the enzyme that catalyzes de novo FA synthesis, is involved in the regulation of the ability of the heart to respond to stress through the activation of Ca2+/calmodulin-dependent protein kinase II [12]. Diacylglycerol acyltransferase 1 overexpression improves heart function in long-chain acyl-CoA synthetase-expressing mice, which develop lipotoxic cardiomyopathy, by reducing the levels of cardiac ceramide and diacylglycerol (DAG), decreasing cardiomyocyte apoptosis, but increasing FA oxidation [13]. Recent studies showed that stearoyl-CoA desaturase (SCD), an enzyme involved in the biosynthesis of monounsaturated fatty acids (MUFAs), induces the reprogramming of cardiomyocyte

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

10 10 10 10

metabolism, thereby playing an important role in the regulation of cardiac function [6,14,15]. The lack of SCD1 expression decreases FA uptake and oxidation and increases glucose transport and oxidation in the heart [14]. Disruption of the SCD1 gene improves cardiac function in obese leptin-deficient ob/ob mice by correcting systolic and diastolic dysfunction [6]. The improvement is associated with a reduction of the expression of genes involved in FA transport and lipid synthesis within the heart, together with decreases in cardiac free fatty acid (FFA), DAG, triacylglycerol (TG), and ceramide levels and reduced cardiomyocyte apoptosis [6]. Additionally, recent studies showed that physiological hypertrophy induced by endurance training is accompanied by the increased expression of SCD1 and SCD2 [15]. Here, we review recent advances in understanding the role of SCD in the control of heart metabolism and its involvement in the pathogenesis of lipotoxic cardiomyopathy. 2. Stearoyl-CoA desaturase (SCD): what does it do? SCD is the rate-limiting enzyme that catalyzes the synthesis of MUFAs, mainly oleate and palmitoleate (Fig. 1), which are used as substrates for the synthesis of TG, wax esters, cholesteryl esters, and phospholipids (PLs) [16]. The degree of the unsaturation of cellular lipids can also play a role in membrane fluidity and cell signaling. Therefore, SCD is highly conserved, with multiple isoforms that provide overlapping but distinct tissue and substrate specificity. Four isoforms of SCD have been identified in the mouse (SCD1-4) [17–20], and two isoforms (SCD1 and 5) have been identified in the human genome [21,22]. Human SCD1 shows 85% homology with murine SCD1 [21]. In the adult mouse, SCD1 is expressed in lipogenic tissues, including the liver and adipose tissue. SCD2 is ubiquitously expressed in most tissues except the liver, where it is only expressed at early stages of life (embryonic and neonatal); at weaning, it is replaced by SCD1 [23]. SCD3 expression is restricted to sebocytes in the skin, harderian gland, and preputial gland [24]. SCD4 is expressed exclusively in the heart [20]. Human SCD1 is expressed in adult adipose tissue, the liver, the lungs, the brain, the heart, the pancreas, and skeletal muscles [25]. Human SCD5 is expressed predominantly in the brain and pancreas, with some limited expression in the heart, kidneys, lungs, and placenta [22,25,26]. The physiological role of each SCD isoform and the reason for having multiple SCD gene isoforms that share considerable sequence homology and catalyze the same biochemical reactions are currently under investigation. Although MUFA products of SCD are abundant in the diet, SCD1 is highly regulated, indicating a critical role for endogenously synthesized MUFAs. The regulation of SCD1 has been reviewed elsewhere [27]. Briefly, the SCD1 gene is positively regulated by insulin, transcription factor SREBP1c, the liver X receptor, and numerous dietary and cellular factors, including glucose, fructose, and saturated FA (SFA). Negative regulation of the SCD1 gene is affected by the actions of polyunsaturated fatty acids (PUFA) and leptin [28]. SCD1 may also be regulated at the protein level and subjected to degradation by proteases and through the proteasomal pathway [29,30]. Research over the past decade has identified SCD as an important regulator of body adiposity and lipid partitioning. High SCD activity favors fat storage, whereas the suppression of the enzyme activates metabolic pathways that promote the burning of fat and

3


O

9

CoA-S NAD(P)H + H +

NAD(P)

10

Cytochrome b5 reductase (FADH 2)

cytochrom b5 Fe 2+

Cytochrome b5 reductase (FAD)

cytochrom b5 Fe3+ O

Stearoyl-CoA O2

SCD H2 O

9 10

CoA-S Oleoyl-CoA

Fig. 1. Pathway of electron transfer in the desaturation of fatty acids by stearoyl-CoA desaturase (SCD).

decreases lipid synthesis in white adipose tissue and the liver [31]. SCD1 deficiency upregulates insulin-signaling components and affects glycogen metabolism in insulin-sensitive tissues [32,33]. Much evidence indicates that the direct anti-steatotic effect of SCD1 deficiency stems from increased FA oxidation in the liver and skeletal muscle, reduced lipid synthesis, and increased thermogenesis [32,34–36]. The molecular mechanisms of these effects are not completely understood. However, our study established that one likely mechanism is increased activation of the AMPactivated protein kinase (AMPK) pathway [32,37]. The antisteatotic impact of SCD1 deficiency also involves transcriptional effects. The loss of SCD1 function downregulates SREBP-1c, thereby reducing the expression of lipogenic enzymes, such as FAS, acetyl-CoA carboxylase, and glycerol-3-phosphate acyltransferase (GPAT) in

the liver [35,38]. SCD1 deficiency also upregulates the expression of genes that are involved in FA b-oxidation [39]. The mechanisms by which SCD1 deficiency affects the expression of genes are still unknown. Several very comprehensive reviews on the role of SCD in the liver, skeletal muscle, and skin have recently been published [27,40]. Therefore, we focus on cardiac SCD in the present review.

3. Isoforms of SCD expressed in the heart Three SCD isoforms (SCD1, SCD2, and SCD4) are expressed in the mouse heart. The expression of SCD1 is several times higher in the murine heart compared with SCD4 [6,20]. However, SCD4

SCD1 SCD2 SCD3 SCD4

1 1 1 1

MPAHMLQ MPAH I LQ M PG H L L Q M T A H L PQ

E -

E I SSSYTTTTTITAPPSGNE - - - REK E I SG A Y SA T T T I T A P P S GGQ Q NG G E K EMTPS YTTTTTITAPPSGS L QNGREK E I S S RC S - T TNIMEPH S R R Q QD G E E K

SCD1 SCD2 SCD3 SCD4

55 58 59 53

YQDEEGP YQDDEGP YQDEEGP YQDEEGP

P P P P

PK PK PK PK

LE LE LE LE

SCD1 SCD2 SCD3 SCD4

115 118 119 113

A A A A

HRLWS HRLWS HRLWS HRLWS

H H H H

R R R R

TY TY TY TY

KARL KARL KARL KARL

SCD1 SCD2 SCD3 SCD4

175 178 179 173

F F F F

S S S S

H V GW H V GW H V GW H V GW

L L L L

L L L L

VR VR VR VR

SCD1 SCD2 SCD3 SCD4

235 238 239 233

GE GE GE GE

T T T T

FV FV FL FQ

N N N H

S S S S

L L F L

F V CV YV CV

SCD1 SCD2 SCD3 SCD4

295 298 299 292

NY NY NY NY

HHT HHA HHA HHT

F F F F

P P P P

F D Y S A S E Y R WH I N F Y D Y S A S E Y R WH I N F Y D Y S A S E Y R WH I N F Y D Y S V S E Y RWH I NF

Y V WRN Y V WRN Y V WRN Y V WRN

I I I I

I LM V L I LM A L I LM A L I FM A L

T T T T

ALFG CLFA CLFA WL L G

I FY YL Y FVY VFY

Y MT S A L G Y V I SALG Y V I S I EG N VV AG L G

F QN D V Y DWARDHRA F QN D V Y EWARDHRA F QN D V Y EWARDHRA F QN D V Y EWARDHRA

H H H H

H KF S H KF S H KF S H KF S

E E E E

TH TH TH TH

A A A A

D PHN SR R GF F D PHN SR R GF F D PHN SR R GF F D PHN SR R GF F

K H P A V K E K GG K L D M S D L K H P A V K E K GG K L D M S D L K H P A V K E K GG K L D M S D L K H P A V K E KG K N L D M S D L

K K K K

AEK AEK AEK AEK

P P P L

G LL LMCF DLL L MCF G I L L MCF AVTLMF I

I V I I

L L L L

S S A S

S S S S

A AH L Y G Y RP YDKN I AAH L Y G YR PYDKN I AAH L Y G YR PYDKN I A AH L Y G YR PYDRG I

QSREN SSRE N D PRQN GAREN

I L V S L G AVG E G F H I L V S MGAV G E R F H A L V S L GSMG EG F H P F V S M AS L G EG F H

C C C C

MA A L G L MA L L G L MA A L G L MA A L G L

KATVL R AAVL K ATVL K AVVL

AR AR AR AR

I I I I

H H H H

L GGL YG L GALY G V GALY G V GALY G

IANTM IANTM IANTM MA N T M

A A A A

T F L R YT LVLN AT WL V N T F L R Y A VV L N A T W L V N T L L R Y A VV L N A T W L V N N F L R YAVLLN FT WL V N TT F TT F TT F TT F

F F F F

I I I I

D D D D

I I I I

L L L L

I T T T

RPEMKED I HDPTYQ R P E L K DD L Y D P TY Q RPEMKEDI YDPTYQ R P E I K DD L Y D PSY Q

L V P SC K L Y L V P SC K L Y L V P SC K L Y L V P SC KV Y

P L R I FL P L R L FL P L R I FL P L R I FL

L L L L

VKTVPLH L EED I F E KSSHH W G AD V VKTVPLY L EED I - - - MPL - Q AED I

V M F Q RR Y Y K V M FQ RRYY K V M FQ RRYY K V M FQ RRYY K

A Y DRK K V S A Y DRK R V S A Y DRK R V S A Y DRK K V S

I I I I

I I I I

P T L V PW P T L V PW P T L V PW P T L V PW

KR KR KR KR

TAG TAG GAG TAG

YCW YCW YCW Y LW

TGD G SH KS S TGD G SC KSG TGD G SH KSG TGD GS H KS S

Fig. 2. Amino acid sequence alignment of mouse SCD1, SCD2, SCD3 and SCD4. The nonconserved amino acids are shaded in gray. The three HXXHH motifs that are 100% conserved are shaded in black. The transmembrane domain amino acids are underlined.

4


expression is significantly increased after a high-fat diet in obese leptin-deficient mice [6,20] and in states of SCD1 deficiency [14,20]. Cardiac SCD4 cDNA encodes a 353-amino-acid residue protein with four transmembrane regions that is more than 80% homologous to the other three mouse SCD genes [20]. SCD4 also contains three conserved histidine-rich motifs that are present in the other SCD isoforms and essential for D9-desaturase function [17–20]. This enzyme desaturates both stearate and palmitate to the corresponding MUFA [20]. SCD4 maps to mouse chromosome 19 D2, where three other SCD genes are located within a 200-kb region. SCD4 lies between the SCD1 and SCD2 genes, indicating that this region is composed of a cluster of D9-desaturases. The 3.1-kb transcript is the predominant species; thus, SCD4 mRNA is smaller than the 4.9 kb of the other SCD isoforms because of a shorter 30 -untranslated region [20]. Amino acid sequence alignment of SCD4 with SCD1, SCD2, and SCD3 indicates sequence identity of greater than 77% and includes 100% conservation of the three histidine motifs HRLWSH, HRAHH, and HNYHH that exist in other desaturases (Fig. 2). There were found two minor transcripts (2.8 and 1.5 kb) of SCD4 rising from the use of multiple GAAA repeats present within exon 6 as cleavage sites for the addition of polyadenylate in the heart [20]. The mouse SCD1, SCD2, and SCD3 have no additional polyadenylation sites, but a second polyadenylation site has been found in the human SCD [21] and postulated to be involved in regulating its expression. Whether polyadenylation is a mode of regulation of SCD4 in the heart will require further studies. In the liver, SCD1 is a major target gene of leptin [41]. In the heart, leptin represses the expression of SCD4 but not SCD1 or SCD2 [20], suggesting differential physiological role of SCD1 between liver and heart. It also suggests that leptin signaling in the heart is different from that in the liver. As shown by Miyazaki et al. [20], both SCD1 and SCD4 gene expression is induced by high carbohydrate diet, it was proposed that both of these SCD isoforms are targets of SREBP-1c in the heart. However, analysis of the SCD4 promoter sequence did not reveal an SREBP binding site that is well conserved in the SCD1 and SCD2 promoters [20,42]. Also, while SCD1, and to a smaller extent SCD2, are repressed by PUFA in the heart, SCD4 gene is not sensitive to PUFA repression [20],

FA

IR

CD36 FATP

FA uptake

and its promoter sequence lacks the PUFA response element that is present in the SCD1 and SCD2 promoters [20]. SCD1, SCD2, and SCD4 are induced by the liver X receptor a agonists, suggesting that the three isoforms can be regulated by cholesterol in the heart [20]. All these findings indicate that there is a tissue-specific regulation of the SCD isoforms in response to particular regulatory factors. 4. Role of SCD in cardiac metabolism and function: what we have learned from SCD1 knockout mice Studies on mouse strains that have a mutation in the SCD1 gene have provided evidence that SCD1 is an important control point in lipid metabolism and body weight regulation [24,27,31]. Mice with targeted disruption of the SCD1 gene exhibited increased energy expenditure, reduced body adiposity, and increased insulin sensitivity and are resistant to diet-induced obesity [16,40,33]. SCD1 was found to be specifically repressed during leptin-mediated weight loss, and leptin-deficient ob/ob mice that lack SCD1 exhibited markedly reduced adiposity, despite higher food intake [41]. Additionally, SCD1 deficiency completely corrected the hypometabolic phenotype and hepatic steatosis in ob/ob mice [41] and low-density lipoprotein receptor-deficient mice [43]. The lack of SCD1 function also attenuates fasting-induced liver steatosis in PPARa deficient mice [44]. SCD1 knockout was recently shown to significantly affect myocardial substrate utilization and protect the heart from obesity-induced steatosis [6,13]. 4.1. Fatty acid and glucose oxidation in the heart in SCD1/ mice Myocardial substrate selection is not simply dictated by its relative abundance at a given time but rather subjected to regulation according to the developmental, hormonal, and pathophysiological status of the organism. SCD1 is among the factors that modify cardiac substrate utilization [14]. One of the key control points for FA b-oxidation is the rate of FA transfer into mitochondria through carnitine palmitoyltransferase (CPT1) [45]. CPT1 activity and the rate of FA b-oxidation are increased in the liver, skeletal muscle, and brown adipose tissue

insulin

P

glucose

P

GLUT4

IRS2 IRS1 P

FA CoA

PI3K

P

glucose uptake

FA-CoA X TG ceramide 1

SCD1

CPT

lipogenesis

glucose oxidation

β-oxidation mitochondria

PPARα β -oxidation gene expression nucleus

Fig. 3. Effect of SCD1 ablation on fatty acid and glucose oxidation pathways in the heart. In the absence of SCD1, the uptake of FA and lipogenesis are significantly reduced which lead to a decrease in the intracellular accumulation of FA, FA-CoA, TG and ceramide. Also the expression of fatty acid oxidation genes and the rate of b-oxidation are significantly decreased in SCD1-deficient heart. The reduction of the lipid content IRSs and Akt kinase and consequently activates PI3K leading to increased GLUT4 membrane translocation and enhanced glucose uptake and oxidation. Abbreviations: CD36, fatty acid translocase CD36; CPT1, carnitine palmitoyltransferase; FA, fatty acid; FA-CoA, fatty acyl-CoA; FATP, fatty acid transport protein; GLUT4, glucose transporter 4; IR, insulin receptor; IRS, insulin receptor substrate; PI3K, phosphatidylinositol 3-kinase; PPAR, peroxisome proliferator-activated receptors; SCD, stearoyl-CoA desaturase; TG, triacylglycerol.

5


in SCD1/ mice compared with SCD1+/+ mice [32,36,37]. Interestingly, in the heart, SCD1 deficiency decreases CPT1 mRNA and protein levels and activity. As a result of lower FA availability and uptake and a reduced rate of FA transport into mitochondria by CPT1, the rate of mitochondrial FA oxidation is significantly decreased in the heart in SCD1/ animals (Fig. 3). Remarkably, no significant difference was found in SCD enzyme activity in either group of mice [14]. Furthermore, no significant differences were observed in the levels of palmitoleic acid, a product of de novo desaturation by SCD, in cardiac lipids between SCD1+/+ and SCD1/ mice. These results, together with increased SCD4 mRNA levels in SCD1/ mice, indicate that SCD4 compensates for the lack of SCD1 in the heart [14,20]. This relative lack of a change in cardiac D9-desaturase activity may account for the tissue-specific differences in fat oxidation observed in the heart, as opposed to other SCD1-deficient tissues, such as skeletal muscles, the liver, and brown adipose tissue. However, the potential role of SCD4 in the regulation of FA oxidation requires further studies. The utilization of FA and glucose is tightly coupled in the myocardium [1,2]. When FAs are unavailable as a source of ATP, the heart extends its use of carbohydrates as an energy supply. This effect (i.e., increased glucose oxidation) was observed in the heart in SCD1/ mice to compensate for ATP supply. This was accompanied by an increase in the tyrosine phosphorylation of the insulin receptor (IR) and IR substrate (IRS)-1 and a greater IRS association with the ap85 subunit of phosphatidylinositol 3-kinase in the heart, despite lower levels of plasma insulin [14]. Increases in insulin signaling are responsible for enhanced glucose uptake in the heart in SCD1/ mice (Fig. 3). A marked increase in insulinstimulated glucose flux in the heart in SCD1/ mice was also observed by Flowers et al. [46] in studies that utilized an hyperinsulinemic-euglycemic clamp. The decrease in fat oxidation coupled with increased insulin sensitivity led to a shift in substrate utilization from FAs to glucose in the SCD1-deficient heart [14]. The important role of SCD1 in regulating cardiac myocyte substrate utilization was also shown in studies of neonatal cardiomyocytes, in which SCD1 overexpression attenuated palmitic acid oxidation and restored the palmitate-induced suppression of glucose oxidation in the heart [47]. 4.2. Role of AMPK and PPARa pathways in SCD-dependent regulation of cardiac substrate utilization AMPK is an important factor that regulates metabolic pathways, such as FA oxidation, glucose transporter (GLUT4) translocation, glucose uptake, and glycolysis [48]. SCD1 deficiency is known to activate AMPK in the liver and skeletal muscle [32,37]. However, in the heart, AMPK phosphorylation and protein levels were unaffected by SCD1 deficiency, indicating that AMPK is unlikely to play a role in the shift in substrate oxidation in the myocardium in SCD1/ mice [14]. The transcription factor PPARa is highly expressed in tissues with a high capacity for FA oxidation, including hepatocytes, cardiomyocytes, the renal cortex, and skeletal muscles. Its activation promotes FA oxidation, ketone body synthesis, and glucose sparing [49]. The cardiac-specific overexpression of PPARa has been shown to cause insulin resistance and increased FA oxidation in the heart [50], whereas the ablation of fatty acid translocase CD36 (CD36) in the context of PPARa cardiac overexpression largely reversed these effects and promoted glucose uptake and oxidation in the heart [51]. These studies underscore the role of PPARa in regulating cardiac substrate utilization. The expression of PPARa is significantly decreased in the myocardium in SCD1/ mice. Therefore, the decreased expression of genes that encode proteins involved in FA oxidation, such as CPT1 and acyl-CoA oxidase, may be mediated by decreased PPARa activity in the heart

Table 1 Echocardiograpy results of heart function and structure of SCD1+/+ and SCD1/ mice.

HR (bpm) AWd (mm) PWd (mm) LVDd (mm) LV mass/BW (mg/g) % Fractional shortening IVRT (s) MPI Ea/Aa E/Ea

SCD1+/+

SCD1/

445.00 ± 31.1 0.80 ± 0.2 0.81 ± 0.1 3.30 ± 0.4 3.63 ± 0.5 51.33 ± 6.3 0.012 ± 0.004 0.44 ± 0.1 1.87 ± 0.6 30.83 ± 3.0

476.13 ± 26.0 0.84 ± 0.1 0.86 ± 0.1 3.79 ± 0.3* 4.18 ± 0.3* 54.05 ± 3.7 0.017 ± 0.002 0.48 ± 0.2 1.63 ± 0.2 27.62 ± 4.1

HR, heart rate in beats per minute; AWd, anterior wall in diastole; PWd, posterior wall in diastole; LVDd, left ventricular diameter in diastole; LV mass/BW, left ventricular mass in milligrams/body weight in grams; Fractional shortening, (LVDdLVDs)/LVDd; IVRT, isovolumic relaxation time in seconds (the time between the closure of the aortic valve and the opening of the mitral valve); MPI, myocardial performance index = the ratio of isovolumic contraction and relaxation to ejection time (MPI = (a b)/b where a = the time of mitral value closure and b = aortic ejection time); Ea, early diastolic maximal velocity from tissue Doppler; Aa, late diastolic maximal velocity from tissue Doppler; E, transmitral early filling velocity. * p < 0.05 vs. SCD1+/+ mice.

in SCD1/ mice (Fig. 3) [14]. Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1a and nonesterified PUFAs are two main regulators of PPARa activity [28,52]. Although cardiac PGC1a expression is unaffected by SCD1 deficiency, intracellular PUFA content is reduced by 30% in the heart in SCD1/ mice, suggesting a viable mechanism for the decreased PPARa activity observed in SCD1-deficient hearts [14]. 4.3. Echocardiographic analysis of SCD1-deficient heart Because of its detrimental effect on contractile function, the shift in cardiac substrate utilization might lead to cardiac structural and/or functional abnormalities [2]. Transthoracic echocardiography with Doppler flow analysis was used to assess the structure and function of an SCD1/ heart [14]. Left ventricle (LV) weights that were normalized to body weight (measured by both wet weights obtained at necropsy and echocardiography) were 15% higher in SCD1/ mice compared with SCD1+/+ mice. The LV diameter in the heart in SCD1/ mice was significantly larger, but the LV posterior and anterior wall thicknesses were similar in SCD1-deficient and wildtype hearts (Table 1). The structural changes in the heart in SCD1/ mice were not accompanied by functional abnormalities. Heart rate (beats/min), measured during echocardiography, was similar in SCD1/ and SCD1+/+ mice. Left ventricle fractional shortening, which is used as a measure of systolic function, and the isovolumetric relaxation time, which is used as a measure of diastolic function, were not significantly affected by SCD1 deficiency. The myocardial performance index, a Doppler-based measure of left ventricular function, and the velocity of blood flow across the mitral valve in early diastole were unchanged in SCD1/ mice compared with SCD1+/+ mice [14]. These results indicate that changes in substrate utilization caused by SCD1 deficiency do not disturb heart function. 5. SCD1 deletion improves heart function in obese mouse models The ectopic deposition of lipids in the myocardium may lead to functional impairments, as observed in obese leptin-resistant Zucker diabetic rats (ZDF) [53], db/db mice, and leptin-deficient ob/ob mice [54]. Ob/ob mice develop pathologic LV hypertrophy, together with elevated TG content and increased cardiomyocyte apoptosis [20,54]. Numerous studies have suggested an important role for SCD in the pathogenesis of lipid-induced heart disease.

6


Table 2 Echocardiography results of heart function and structure of ob/ob and ob/ob;SCD1/ mice.

HR (bpm) AWd (mm) PWd (mm) LVDd (mm) LV mass/BW (mg/g) % Fractional shortening IVRT (s) MPI Ea/Aa E/Ea

ob/ob

ob/ob;SCD1/

515.13 ± 36.0 0.89 ± 0.1 0.92 ± 0.1 3.79 ± 0.3 2.01 ± 0.2 43.1 ± 3.7 0.020 ± 0.002 0.57 ± 0.2 1.33 ± 0.2 17.6 ± 5.1

481.33 ± 67.1 0.85 ± 0.1 0.87 ± 0.1 3.34 ± 0.2# 2.69 ± 0.4# 55.3 ± 7.8# 0.020 ± 0.003 0.50 ± 0.1 1.07 ± 0.2# 22.3 ± 2.4

HR, heart rate in beats per minute; AWd, anterior wall in diastole; PWd, posterior wall in diastole; LVDd, left ventricular diameter in diastole; LV mass/BW, left ventricular mass in milligrams/body weight in grams; Fractional shortening, (LVDdLVDs)/LVDd; IVRT, isovolumic relaxation time in seconds (the time between the closure of the aortic valve and the opening of the mitral valve); MPI, myocardial performance index = the ratio of isovolumic contraction and relaxation to ejection time (MPI = (a b)/b where a = the time of mitral value closure and b = aortic ejection time); Ea, early diastolic maximal velocity from tissue Doppler; Aa, late diastolic maximal velocity from tissue Doppler; E, transmitral early filling velocity. # p < 0.05 vs ob/ob mice.

changes in relative wall thickness. Consequently, these rats exhibited impaired systolic function, reflected by decreased fractional shortening, increased wall stress, increased diastolic stiffness, decreased developed pressure, and decreased LV contractility. Additionally, diastolic, systolic, and stroke volumes and cardiac output were all elevated in rats fed a high-carbohydrate, high-fat diet, with no changes in heart rate [57]. These negative changes in cardiac function were accompanied by increased SCD activity. Numerous studies have shown that increased amounts of a-linoleic acid, docosahexaenoic acid, and eicosapentaenoic acid suppress SCD1 expression [58,59]. a-Linoleic acid, docosahexaenoic acid, and eicosapentaenoic acid supplementation in high-carbohydrate, high-fat diet-induced metabolic syndrome in rats caused lipid redistribution away from the abdominal area and favorably improved glucose tolerance, insulin sensitivity, dyslipidemia, hypertension, and left ventricular dimensions, contractility, volumes, and stiffness. These effects were associated with the complete suppression of SCD activity [60], further emphasizing the importance of SCD for heart function regulation.

6. SCD1 and cardiomyocyte apoptosis SCD1 expression and activity are significantly increased in the myocardium in ob/ob and db/db mice [55]. The activity of SCD is also significantly induced in the hearts of obese diabetic rats that are fed a high-sucrose diet, and SCD1 overexpression markedly induces lipid droplet accumulation in isolated neonatal rat cardiac myocytes that are incubated with palmitate [47]. Furthermore, cardiac SCD1 and SCD4 are overexpressed in ACS transgenic mice, which develop severe lipotoxic cardiomyopathy [56]. The expression of SCD1 appears to be more abundant in the hearts of diabetic patients than in the hearts of lean subjects and non-diabetics [47]. SCD1 repression has been proposed to be a viable approach for decreasing FA uptake and oxidation in the heart, thereby aiding in the prevention and treatment of lipotoxic cardiomyopathy observed in diabetic and obese states. To test this hypothesis, an ob/ob;SCD1 double-knockout mouse model was generated [6]. Transthoracic echocardiography analysis showed that ob/ob; SCD1/ mice had a smaller LV diameter and significantly reduced LV mass compared with ob/ob mice, although the LV wall thickness was unaffected by SCD1 deficiency [6]. Because the body mass of the ob/ob mice was much larger compared with either wild type or ob/ob;SCD1/ counterparts, the LV mass/body weight ratio was significantly increased in ob/ob;SCD1/ mice (Table 2). Ob/ob mice appeared to have a significant reduction of systolic function, reflected by an impairment in fractional shortening [6]. SCD1 deficiency increased the percentage of fractional shortening by 28% in the heart in ob/ob;SCD1/ mice compared with ob/ob mice. The percentage of fractional shortening in ob/ob;SCD1/ mice was not different from wildtype animals [6]. The myocardial performance index, a Doppler-based measure of LV function, was marginally different between ob/ob and ob/ob;SCD1/ mice. The Doppler flow analysis demonstrated that the E/Ea ratio was reduced by 57% in ob/ob mice compared with wildtype mice, indicating diastolic dysfunction in ob/ob mice [6]. Although the E/Ea ratio in the heart in ob/ob;SCD1/ mice was still significantly lower than in wildtype mice, SCD1 deficiency increased the E/Ea ratio by 27% in the heart in these mice compared with ob/ob mice. These results showed that disruption of the SCD1 gene improved cardiac function in obese leptin-deficient ob/ob mice (Table 2). The reduction of SCD1 activity was also shown to exert a positive effect on heart function in animal models of diet-induced obesity. Rats fed a high-carbohydrate, high-fat diet developed eccentric hypertrophy, a characteristic of increased preload, defined by increased LV internal diameter in diastole without

Excessive deposition of intramyocardial TG enlarges the intracellular pool of fatty acyl-CoA, thereby providing a substrate for nonoxidative metabolic pathways, such as ceramide synthesis [61]. Increased levels of ceramide cause apoptosis of cardiac myocytes, which can result in LV chamber expansion, contractile dysfunction, and impaired diastolic filling, contributing to the cardiomyopathy observed in states of obesity and diabetes [53]. This phenomenon, broadly referred to as ‘‘cardiac lipotoxicity,’’ has been observed in several genetic models in mice and rats. Obese ZDF rats develop cardiac dilatation and reduced contractility at 14 weeks of age, effects that are associated with elevated intramyocardial TG and ceramide levels and increased apoptosis [53]. Troglitazone therapy in ZDF rats decreased cardiac TG and ceramide levels, which were associated with the complete prevention of apoptosis and restoration of cardiac function. The accumulation of intramyocardial ceramide and TG, LV chamber enlargement, and impaired contractile function were also observed in obese leptin-deficient ob/ob mice [6]. Interestingly, knockout of the SCD1 gene resulted in decreased TG and ceramide content in the heart in ob/ob;SCD1/ mice compared with control ob/ob mice. Notably, ceramide levels in the heart in ob/ob;SCD1/ mice were comparable to values in the myocardium in wildtype counterparts [6]. Ceramide may be formed by the hydrolysis of sphingomyelin, de novo synthesis via the condensation of palmitoyl-CoA and serine, glycosphingolipid breakdown, or the conversion of other sphingolipids [62]. The decrease in ceramide content in SCD1-deficient hearts appears to be attributable to a decrease in de novo synthesis, reflected by decreased serine palmitoyltransferase activity and gene expression and reduced incorporation of [14C] palmitate into ceramide [6]. The reduced intracellular palmitate level in the heart in ob/ob;SCD1/ mice that resulted from decreased lipogenesis and reduced FA uptake may also be a rate-limiting factor in de novo ceramide synthesis. Similar effects, including reduced SCD activity and decreased intramuscular palmitoyl-CoA content, concomitant with the downregulation of serine palmitoyltransferase activity and reduction of ceramide synthesis, were observed in oxidative skeletal muscles in SCD1/ and SCD1-deficient ob/ob mice [32]. The ceramide pathway is the most important of the lipoapoptotic routes in cardiomyocytes [63], and decreased ceramide content caused by SCD1 deficiency resulted in a reduced rate of apoptosis in the heart in ob/ob mice (Fig. 4). Two key markers of ceramide-induced apoptosis, nitric oxide production (reflected by


SCD1 CPT1

FFA DAG TG ceramide

β-oxidation Bcl-2

ROS production

NO

iNOS

apoptosis

IMPROVED CARDIAC FUNCTION IN LEPTIN-DEFICIENT HEART Fig. 4. Proposed model for the effect of SCD1 gene deletion on heart lipid metabolism and left ventricle function in leptin deficiency. The reduction of myocardial lipid accumulation and inhibition of lipid-induced apoptosis appear to be the main mechanisms responsible for improved cardiac function in leptindeficient ob/ob mice caused by a lack of SCD1 function. Abbreviations: Bcl-2, an antiapoptotic factor; CPT1, carnitine palmitoyltransferase 1; DAG, diacylglycerol; FFA, free fatty acid; iNOS, inducible nitric oxide synthase; NO, nitric oxide; SCD, stearoyl-CoA desaturase; TG, triacylglycerol.

inducible nitric oxide synthase [iNOS] activity) and caspase-3 activity, were significantly reduced in the hearts in ob/ob;SCD1/ double-mutant mice [6]. The decreased activity of iNOS and caspase-3 might result from the inhibition of de novo ceramide synthesis caused by SCD1 deficiency (Fig. 4). Bielawska and coworkers [64] proposed that ceramide upregulates iNOS expression and increases NO production, causing an increase in peroxynitrite and apoptosis. Additionally, the downregulation of caspase-3 was often linked to ceramide action [65,66]. Ravid et al. [65] showed that ceramide-mediated apoptosis is blocked by the general caspase inhibitor Boc-D-fluoromethylketone. Ruvolo et al. [67] reported that exogenous ceramide can downregulate antiapoptotic factor Bcl-2 expression and phosphorylation, thereby activating caspase-3 and apoptosis. The antiapoptotic effect of Bcl-2 has also been suggested to occur via the modulation of ceramide production and prevention of ceramide-mediated caspase activation [67,68]. Increased Bcl-2 mRNA levels were observed in the heart in ob/ob;SCD1/ mice compared with ob/ob controls. Thus, the increased gene expression of Bcl-2 could be another factor that contributes to the downregulation of caspase-3 activity and reduction of the rate of apoptosis in the heart in ob/ob;SCD1/ doubleknockout mice (Fig. 4). Furthermore, the increased oxidation of palmitate through CPT1 has been suggested to be involved in induction of oxidative stress and apoptosis in cardiomyocytes. Previous studies established that palmitate-induced cell death was enhanced by carnitine, a cofactor that is needed for palmitate transport into mitochondria via CPT1 [69]. CPT1 mRNA levels were decreased in the heart in ob/ ob;SCD1/ mice compared with ob/ob mice [6]. Therefore, decreased mitochondrial FA oxidation may account for decreased ROS production and thus reduced apoptosis in the heart in SCD1deficient ob/ob mice (Fig. 4). As described above, knockout of the SCD1 gene significantly improved diastolic and systolic LV function in ob/ob mice (Table 2). Reduced apoptosis in cardiomyocytes was shown to improve cardiac function [54], and the inhibition of

7

lipid-induced apoptosis caused by SCD1 deficiency might be directly responsible for the improvement in heart function in ob/ob mice (Fig. 4). Studies on isolated cardiomyocytes showed that SCD1 had protective effects against SFA-induced apoptosis [47]. Palmitic acid and stearic acid increased the activity of caspases 3 and 7 and the number of TUNEL-positive stained cells in neonatal rat cardiomyocytes. In contrast, SCD1 overexpression significantly attenuated SFA-induced apoptotic changes. These results suggest that SCD1 suppresses SFA-induced apoptosis in cardiac myocytes, as first shown by Listenberger et al. [70] in Chinese hamster ovary cells, but this effect was observed in studies performed only in vitro. Interestingly, all studies carried out in vivo show positive correlation between high SCD1 expression and increased apoptosis [6,32,54,71]. Thus results obtained in cellular models indicating protective role of SCD1 overexpression against SFA-induced apoptosis outcome should be more carefully evaluated. There is a strong probability that in vitro models do not fully reflect the complexity of SCD1-dependent regulatory mechanisms.

7. Physiological and pathological left ventricular hypertrophy: role of SCD and lipid metabolism Cardiac hypertrophy is associated with extensive remodeling, which eventually will affect cardiac function and ultimately contribute to the transition from compensatory hypertrophy to cardiac failure [72]. Both physiological and pathological cardiac hypertrophy causes changes in lipid metabolism in the heart, although little is known about the underlying molecular changes. Previous studies have primarily focused on the FA oxidation pathway and reported the downregulation of genes involved in FA transport and oxidation in pathological hypertrophy, whereas the upregulation of several of these genes was observed in adaptive hypertrophy [73]. The decreased expression of genes that are involved in FA oxidation that is observed in pathological hypertrophy is believed to be attributable to a decline in the activity of PPARa [72,74,75]. Moreover, the expression of PPARa and its downstream targets (i.e., medium-chain acyl-CoA dehydrogenase and CPT1) was upregulated after endurance training [75]. Therefore, one proposal is that the higher rates of FA oxidation observed in trained hearts may partially prevent the accumulation of FAs and FA-CoAs to limit their toxic effects in the cytosol and mitochondria [76]. The available data show that SCD1 upregulation plays an important role in the adaptation of skeletal muscle to endurance training [77,78]. Our recent results demonstrate that physiological hypertrophy induced by endurance training is accompanied by the increased expression of SCD1, SCD2, and other lipogenic genes and activation of SREBP-1c and Akt signaling. None of the analyzed lipogenic genes or SREBP-1c was upregulated in the heart with pathological hypertrophy induced by abdominal aortic banding [15]. These data show that although the myocardium has a low capacity for de novo lipogenesis, lipogenic genes play significant roles in the control of cardiac metabolism and remodeling. The effects of physiological and pathological hypertrophy on cardiac lipid metabolism are more complex. Although the expression of lipogenic genes and the levels of FA transport proteins (CD36 and FATP1) were unchanged or reduced, pathological hypertrophy led to cardiac TG and DAG accumulation compared with the sham group. A possible explanation for this phenomenon is a decrease in lipolysis, reflected by the increased protein content of G0/G1 switch gene 2 (G0S2), a small basic protein that functions as endogenous inhibitor of adipose triglyceride lipase, a key enzyme in intracellular lipolysis, increased phosphorylation of hormone-sensitive lipase at Ser565, and decreased protein levels of DAG lipase a (DAGLa) and DAGLb, which attenuate TG and DAG content [15]. The

8


increase in TG and DAG accumulation in abdominal aortic bandinginduced hypertrophy might have lipotoxic effects, thereby predisposing individuals to future cardiomyopathy and heart failure. 7.1. Is SCD involved in postnatal dietary fat-induced cardiac hypertrophy? Maternal overnutrition during pregnancy predisposes offspring to a higher prevalence of metabolic disorders and cardiovascular diseases [79,80]. Additionally, maternal fat exposure can be more deleterious if offspring are exposed to dietary fat later in life [81,82]. Diet-induced obesity in postnatal life is accompanied by insulin resistance, cardiac hypertrophy, myocardial dysfunction, and downregulated mitochondrial oxidative phosphorylation [83,84]. Recent studies by Turdi and co-workers [71] showed that SCD1 protein levels are elevated in male offspring of low-fat diet-fed dams that are postnatally fed a high-fat diet and in male offspring of high-fat diet-fed dams that are postnatally fed a high-fat or low-fat diet. In maternally low-fat and postnatally high-fat diet-fed offspring and maternally high-fat and postnatally high-fat diet-fed offspring, increased SCD1 protein expression was accompanied by an elevated amount of lipids as demonstrated by Oil Red O staining. These two groups of animals were also characterized by decreased insulin-stimulated glucose uptake and increased plasma leptin and TG levels. Postnatal high-fat feeding increased LV mass, LV end-systolic diameter, and interstitial fibrosis and resulted in an enlarged cardiomyocyte cross-sectional area. Peak shortening (PS), maximal velocity of shortening/relengthening (±dL/dt), and prolonged time-to-PS and time-to-90% relengthening (TR90) were significantly reduced in postnatally high-fat diet-fed animals. Additionally, postnatal (but not maternal) fat exposure elicited overt apoptosis and reactive oxygen species accumulation [71]. These changes in cardiac metabolism are at least partially similar to those caused by SCD1 downregulation in the heart and other tissues. However, further studies are needed to establish whether SCD1 directly affects heart structure and function in postnatally high-fat diet-fed rats. 8. Regulation of cardiac metabolism and function by oleic acid Dietary stearic acid intake ranks second among the SFA consumed in the United States, accounting for 25.8% of SFA intake and 2.9% of total kcal [85]. Stearate is a precursor for the endogenous synthesis of oleate by SCD [28]. Oleic acid is the most relevant intermediate between SFA and PUFA, and it is best suited for storage or incorporation into glycerolipids and modulation of the basic features of biomembranes [86]. Because of its unique features, oleate plays an important metabolic role in the cell (e.g., it directly affects signal transduction by activating the isoenzymes of protein kinase C), regulating the phosphorylation of proteins and modulating the expression of genes [87,88]. In the heart, oleate plays an important role in the regulation of substrate metabolism and appears to be involved in the pathogenesis of lipotoxic cardiomyopathy [89–91]. The majority of FA that undergoes b-oxidation in the heart is not SFA but rather MUFA, and oleic acid is the preferred substrate [4]. An increased rate of FA b-oxidation was found in the heart in rats fed trioleate- (TO) or tristearate- (TS) supplemented diets, suggesting that both dietary and endogenously synthesized oleate are able to activate oxidative pathways [91]. Very little is known about the potential mechanisms involved in the regulation of cardiac FA oxidation by oleate. One of the possible mechanisms that link oleate and FA oxidation may involve the activation of PPARa. Transgenic mice with an overexpression of cardiac PPARa exhibit an increase in myocardial FA uptake, cardiac TG and ceramide accumulation, cardiac hypertrophy, and LV dilation [74,92]

and clear adverse effects on cardiac function and structure [9,74]. PPARa activation increases FA oxidation and protects the heart from substrate-induced contractile dysfunction when there is an oversupply of FAs and TG [93]. Gilde et al. [94] showed that the administration of long-chain SFA and MUFA results in moderate increases in the mRNA expression of several PPARa-regulated genes, but this effect was only seen in isolated cardiomyocytes [94]. Recent studies demonstrated that dietary or de novo-synthesized oleate increased the expression and activity of PPARa [91]. Furthermore, feeding with TS- and TO-supplemented diets resulted in the increased phosphorylation and activity of AMPK in the heart, another important factor for regulating cardiac FA oxidation and GLUT4 membrane translocation [4,48]. Analyses performed on the perfused heart showed that oleic acid activated AMPK in the myocardium [95]. Thus, the activation of both the AMPK and PPARa pathways by oleate may be a viable mechanism for increasing mitochondrial FA oxidation in response to increased oleate availability. As mentioned above, oleate can be easily incorporated into glycerolipids for storage or into PLs, leading to changes in plasma membrane properties. As shown using TS- and TO-fed rats, the augmentation of cardiac oleate is accompanied by increased protein levels of CD36 [91], the major protein responsible for membrane FA transport [96]. This change could be especially valuable for mitochondrial FA oxidation because the overexpression of CD36 in skeletal muscle leads to an enhanced level of mitochondrial FA oxidation [97]. An increase in oleate availability also results in increased TG accumulation in the myocardium [91]. The increase in TG levels in the heart in TO- and TS-fed rats is likely related to the fact that oleate is the preferred substrate for TG esterification. However, both TO- and TS-supplemented diets resulted in the upregulation of cardiac lipogenic genes (i.e., FAS, acyl-CoA synthetase, and GPAT) in the heart. The parallel upregulation of SCD1/oleate availability, FAS, and GPAT under conditions of increased cardiac FA oxidation suggests that these proteins may cooperate in maintaining intracellular oleate levels within a narrow range and thus may play a role in the regulation of cardiac energy metabolism. Increased levels of dietary or endogenously synthesized oleate may be responsible for increasing the rate of FA oxidation and reducing glucose uptake in the heart [91]. Recent studies showed that long-chain FAs affect GLUT4 expression, membrane translocation, and Akt phosphorylation in cardiac tissue. Reporter studies in H9C2 cardiomyotubes have shown that in vitro hyperlipidemia, which is induced by high levels of stearic and oleic acids, repressed transcription of the GLUT4 promoter [98]. Hommelberg et al. [99] observed a reduced rate of GLUT4 translocation and deoxyglucose uptake in L6 skeletal muscle cells incubated with palmitate and stearate. Oleate was shown to reduce Akt phosphorylation in perfused rat livers [100] and primary hepatocytes [101]. Furthermore, the prolonged exposure of hepatocytes to oleate decreased the insulin-induced tyrosine phosphorylation of IRS1/2 while slightly increasing the serine phosphorylation of IRS [101]. Therefore, dietary or de novo-synthesized oleate from stearic acid may decrease the rate of glucose uptake in the heart by reducing Akt phosphorylation and GLUT4 membrane translocation, as was found in TSand TO-fed rats [91]. These data are consistent with results obtained in SCD1/ mice, in which increased cardiac glucose transport and oxidation were accompanied by decreased oleic acid content [14]. 9. Plasma desaturation index as a predictor of cardiac health in human The role of SCD1 in human heart function and heart health has not been investigated directly, rather the MUFA to SFA ratios in


plasma and the risk of cardiovascular disease (CVD) has been explored. Warensjo et al. [102] showed that the plasma SCD desaturation index is elevated by a diet that is high in saturated fat compared with a diet that is rich in unsaturated fat. The FA composition of plasma cholesteryl esters was evaluated in relation to CVD and total mortality in over 2000 individuals in the Uppsala Longitudinal Study of Adult Man (ULSAM) [103]. An increased risk of CVD mortality and total mortality was associated with an increased plasma cholesteryl ester palmitoleate/palmitate ratio. The authors suggested that endogenous FA desaturation, which is partially independent of diet, is associated with and might even contribute to mortality. Thus, desaturase indices may be used to predict cardiovascular mortality risk [103]. These results were confirmed by Djousse et al. [104], who showed that plasma stearoylCoA desaturase activity (palmitoleate/palmitate ratio) was positively associated with the risk of heart failure, whereas oleic acid and cis-vaccenic acid concentrations were unrelated to heart failure risk. Moreover, the plasma SCD desaturation index was positively associated with heart rate [105]. Additionally, a study that included 93 healthy volunteers showed that the SCD desaturation index was closely associated with the features of cardiometabolic risks (i.e., increased body mass index and TG content as well as decreased HDL-cholesterol level) in Koreans [106]. Additionally, assessment of the FA composition of plasma cholesteryl esters, PLs, and TG in a large population study (approximately 2400 men and women) revealed a strong positive association between palmitoleate in plasma lipid fractions and age, body mass index, and plasma total cholesterol and high-density lipoprotein cholesterol [107]. Altogether, these results suggest that the level of FA desaturation in blood plasma is related to heart rate and function and possibly related to arrhythmia and sudden death, which would partially explain the observed association between cardiovascular mortality risk and SCD activity. The study of the association between palmitoleate in plasma PLs and CVD risk in 3500 individuals from the Cardiovascular Health Study cohort showed that men and women who previously had ischemic heart disease had a significantly lower abundance of the plasma PL palmitoleate compared with individuals who had never had ischemic heart disease [108]. Interpreting these data is difficult because the usefulness of the product-to-precursor ratio from the plasma PL pool is potentially limited. Plasma PLs are not recommended as a biomarker of SCD activity because FA desaturation ratios are very low in plasma PLs and tightly regulated and reflect a number of different lipid pools. Notably, most of the human studies used the FA ratio as indices of SCD1 activity without validation against specific measurements of enzyme activity. The use of the palmitoleate/palmitate ratio has only been validated for plasma total VLDL and VLDL-TG for hepatic SCD1 expression in humans [109].

10. SCD and atherosclerosis Atherosclerosis is a multifactorial complex disease, which is responsible for approximately 50% of deaths in Western world, mainly due to CVD, including heart disease and stroke [110]. Risk factors for the development of atherosclerotic diseases include diabetes, hypertension, hyperlipidemia, and a lack of physical activity [111]. Interestingly, although SCD1 deficiency reduces plasma TG and provides protection from obesity and insulin resistance [16,27,40], which would be predicted to be associated with reduced susceptibility to atherosclerosis, SCD1-deficient mice developed greater atherosclerotic lesions [112]. In the LDL receptor knockout (LDLR/) mouse model of atherosclerosis, Asebia mice with naturally occurring global SCD1 deletion exhibited an increase in atherosclerosis in aorta, despite improvements in

9

metabolic parameters, such as decreased adiposity, reduced liver steatosis, and increased insulin sensitivity [43,112]. This acceleration in atherosclerosis was likely to result from chronic inflammation, primarily of the skin, which then leads to changes in markers of inflammation (i.e., increased interleukin-6 [IL-6], IL-1b, IL12p70, and sICAM-1 levels) in plasma and proinflammatory changes in high-density lipoprotein [112]. Studies in LDLR/ mice that overexpressed human ApoB100 also identified severe atherosclerosis in aorta after SCD1 was knocked down by antisense oligonucleotides against SCD1 [113]. SCD1 inhibition promoted the accumulation of SFA in plasma and tissues and reduced plasma TG in LDLR/ mice that overexpressed ApoB100. The accumulation of SFA in macrophage membranes enhanced the toll-like receptor 4-driven tyrosine phosphorylation of STAT1 and ultimately enhanced inflammatory cytokine secretion. This proinflammatory phenotype thereby promoted atherosclerosis in a hyperlipidemic setting, which could not be reversed by dietary oleate [113]. In contrast, in LDLR/ mice that overexpressed ApoB100 and were fed fish oil, SCD1 inhibition did not augment the macrophage inflammatory response or severe atherosclerosis. It was caused by the prevention of SFA-driven toll-like receptor 4 hypersensitivity and accelerated atherosclerosis seen with SCD1 inhibition by fish oil-derived omega-3 PUFA [113]. Contrary to the results presented above, another study [114] reported that chronic intermittent hypoxia, which is associated with atherosclerosis, was accompanied in human subjects by an increase in SCD1 levels in liver. In contrast, SCD1 antisense oligonucleotide inhibition in C57BL/6 J mice that were exposed to chronic intermittent hypoxia for 10 weeks exhibited significant reductions of both dyslipidemia and atherosclerosis [114]. Dietary a-linolenic acid, an omega-3 PUFA, repressed SCD1 expression in macrophage-derived foam cells, favorably increasing cholesterol efflux and decreasing cholesterol accumulation in foam cells [115]. This may be one mechanism by which dietary omega-3 PUFA promotes atherosclerosis regression [115]. Because of some controversy regarding the role of SCD1 in the pathogenesis of atherosclerosis, further studies are needed to establish either a pro- or anti-atherosclerotic role for the desaturase.

11. Conclusion and future direction SCD, a central enzyme in lipid metabolism that synthesizes MUFAs, could be considered a house-keeping enzyme because its main product, oleate, is abundant in many dietary sources and tissue lipids. However, research over the past decade has identified SCD as an important regulator of tissue metabolism and provided evidence that SCD, in fact, is a key regulatory enzyme of body adiposity and insulin signaling. Furthermore, recent studies showed that SCD has the ability to reprogram cardiac metabolism and thereby regulate heart function. In the heart, the lack of SCD1 enhanced glucose transport and metabolism at the expense of FA uptake and oxidation. This shift in cardiac substrate utilization from FAs to glucose was caused by the upregulation of insulin signaling, decreased FA availability, and the reduced expression of FA oxidation genes. The metabolic changes associated with SCD1 deficiency lead to the protection of cardiac myocytes against both necrotic and apoptotic cell death and improvements in heart function. The concept that SCD plays an important role in cardiac physiology is additionally supported by the fact that one of the SCD isoforms, SCD4, is expressed exclusively in the heart and specifically regulated by the hormone leptin. This raises the intriguing possibility SCD repression may mediate the metabolic effects of leptin in the heart. Indeed, the lack of SCD1 function in leptin-deficient ob/ob mice results in a decrease in the accumulation of neutral

10


lipids and ceramide, thereby inhibiting apoptotic pathways in the LV and improving systolic and diastolic function of the failing heart. Furthermore, a recent study showed that MUFAs have signaling properties that regulate myocardial metabolism. Both exogenous and de novo-synthesized oleate affects substrate utilization in the heart and leads to LV hypertrophy. Large-population human studies showed that the level of FA desaturation in blood plasma is related to heart rate and function and possibly related to arrhythmia and sudden death, suggesting that desaturase indices may be used to predict cardiovascular mortality risk. The limitation of these studies was that they were not supported by SCD activity measurements, but the high correlation between the palmitoleate/palmitate ratio in plasma CE and TG and CVD risk, regardless of dietary MUFA intake, strongly supports the involvement of SCD. Further studies that involve human cardiomyocyte-specific genetic gain- and loss-of-function models would provide much needed information about the contribution of SCD in heart physiology and disease. Altogether, although the role of human SCD in the heart is still debatable, the studies conducted to date in in vitro model systems and animal models suggest that SCD could be used to reprogram myocardial metabolism to improve cardiac function. Therefore, a tempting speculation is that the pharmacological manipulation of desaturase activity might be a very promising target for the treatment of heart dysfunction. However, the potential use of an SCD inhibitor as a therapeutic agent awaits a more complete understanding of the mechanisms that underlie the effects of SCD deficiency and an indication that inhibition of this enzyme is both safe and effective.

Conflict of interest The authors declare that there are no conflicts of interest.

Transparency Document The Transparency document associated with this article can be found in the online version. Acknowledgements P. Dobrzyn is supported by Grants from by the National Science Centre (UMO-2011/01/D/NZ3/04777) and National Centre for Research and Development (LIDER/19/2/L-2/10/NCBiR/2011). A. Dobrzyn is supported by a Grant from the Foundation For Polish Science (TEAM/2010-5/2). References [1] Sambandam N, Lopaschuk GD. AMP-activated protein kinase (AMPK) control of fatty acid and glucose metabolism in the ischemic heart. Prog Lipid Res 2003;42:238–56. [2] Stanley WC, Recchia FA, Lopaschuk GD. Myocardial substrate metabolism in the normal and failing heart. Physiol Rev 2005;85:1093–129. [3] Chaitman BR, Pepine CJ, Parker JO, Skopal J, Chumakova G, Kuch J, et al. Effects of ranolazine with atenolol, amlodipine, or diltiazem on exercise tolerance and angina frequency in patients with severe chronic angina: a randomized controlled trial. JAMA 2004;291:309–16. [4] Lopaschuk GD, Ussher JR, Folmes CD, Jaswal JS, Stanley WC. Myocardial fatty acid metabolism in health and disease. Physiol Rev 2010;90:207–58. [5] Kivelä R, Bry M, Robciuc MR, Räsänen M, Taavitsainen M, Silvola JM, et al. VEGF-B-induced vascular growth leads to metabolic reprogramming and ischemia resistance in the heart. EMBO Mol Med 2014;6:307–21. [6] Dobrzyn P, Dobrzyn A, Miyazaki M, Ntambi JM. Loss of stearoyl-CoA desaturase 1 rescues cardiac function in obese leptin-deficient mice. J Lipid Res 2010;51:2202–10. [7] Unger RH. The physiology of cellular liporegulation. Annu Rev Physiol 2003;65:333–47.

[8] Park HJ, Georgescu SP, Du C, Madias C, Aronovitz MJ, Welzig CM, et al. Parasympathetic response in chick myocytes and mouse heart is controlled by SREBP. J Clin Invest 2008;118:259–71. [9] Chiu HC, Kovacs A, Blanton RM, Han X, Courtois M, Weinheimer CJ, et al. Transgenic expression of fatty acid transport protein 1 in the heart causes lipotoxic cardiomyopathy. Circ Res 2005;96:225–33. [10] Duan SZ, Ivashchenko CY, Russell MW, Milstone DS, Mortensen RM. Cardiomyocyte-specific knockout and agonist of peroxisome proliferatoractivated receptor-gamma both induce cardiac hypertrophy in mice. Circ Res 2005;97:372–9. [11] Ellis JM, Mentock SM, Depetrillo MA, Koves TR, Sen S, Watkins SM, et al. Mouse cardiac acyl coenzyme a synthetase 1 deficiency impairs fatty acid oxidation and induces cardiac hypertrophy. Mol Cell Biol 2011;31:1252–62. [12] Razani B, Zhang H, Schulze PC, Schilling JD, Verbsky J, Lodhi IJ, et al. Fatty acid synthase modulates homeostatic responses to myocardial stress. J Biol Chem 2011;286:30949–61. [13] Liu L, Shi X, Bharadwaj KG, Ikeda S, Yamashita H, Yagyu H, et al. DGAT1 expression increases heart triglyceride content but ameliorates lipotoxicity. J Biol Chem 2009;284:36312–23. [14] Dobrzyn P, Sampath H, Dobrzyn A, Miyazaki M, Ntambi JM. Loss of stearoylCoA desaturase 1 inhibits fatty acid oxidation and increases glucose utilization in the heart. Am J Physiol Endocrinol Metab 2008;294:E357–64. [15] Dobrzyn P, Pyrkowska A, Duda MK, Bednarski T, Maczewski M, Langfort J, et al. Expression of lipogenic genes is upregulated in the heart with exercise training-induced but not pressure overload-induced left ventricular hypertrophy. Am J Physiol Endocrinol Metab 2013;304:E1348–58. [16] Dobrzyn P, Jazurek M, Dobrzyn A. Stearoyl-CoA desaturase and insulin signaling -what is the molecular switch? Biochim Biophys Acta 2010;1797: 1189–94. [17] Ntambi JM, Buhrow SA, Kaestner KH, Christy RJ, Sibley E, Kelly TJ, et al. Differentiation-induced gene expression in 3T3-L1 preadipocytes. Characterization of a differentially expressed gene encoding stearoyl-CoA desaturase. J Biol Chem 1988;263:17291–300. [18] Kaestner KH, Ntambi JM, Kelly TJ, Lane MD. Differentiation-induced gene expression in 3T3-L1 preadipocytes. A second differentially expressed gene encoding stearoyl-CoA desaturase. J Biol Chem 1989;264:14755–61. [19] Zheng Y, Prouty SM, Harmon A, Sundberg JP, Stenn KS, Parimoo S. Scd3: a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin. Genomics 2001;71:182–91. [20] Miyazaki M, Jacobson MJ, Man WC, Cohen P, Asilmaz E, Friedman JM, et al. Identification and characterization of murine SCD4, a novel heart-specific stearoyl-CoA desaturase isoform regulated by leptin and dietary factors. J Biol Chem 2003;278:33904–11. [21] Zhang L, Ge L, Parimoo S, Stenn K, Prouty SM. Human stearoyl-CoA desaturase: alternative transcripts generated from a single gene by usage of tandem polyadenylation sites. Biochem J 1999;340:255–64. [22] Beiraghi S, Zhou M, Talmadge CB, Went-Sumegi N, Davis JR, Huang D, et al. Identification and characterization of a novel gene disrupted by a pericentric inversion inv(4)(p13.1q21.1) in a family with cleft lip. Gene 2003;309:11–21. [23] Miyazaki M, Dobrzyn A, Elias PM, Ntambi JM. Stearoyl-CoA desaturase-2 gene expression is required for lipid synthesis during early skin and liver development. Proc Natl Acad Sci USA 2005;102:12501–6. [24] Ntambi JM, Miyazaki M. Regulation of stearoyl-CoA desaturases and role in metabolism. Prog Lipid Res 2004;43:91–104. [25] Bene H, Lasky D, Ntambi JM. Cloning and characterization of the human stearoyl-CoA desaturase gene promoter: transcriptional activation by sterol regulatory element binding protein and repression by polyunsaturated fatty acids and cholesterol. Biochem Biophys Res Commun 2001;284:1194–8. [26] Wang J, Yu L, Schmidt RE, Su C, Huang X, Gould K, et al. Characterization of HSCD5, a novel human stearoyl-CoA desaturase unique to primates. Biochem Biophys Res Commun 2005;332:735–42. [27] Hodson L, Fielding BA. Stearoyl-CoA desaturase: rogue or innocent bystander? Prog Lipid Res 2013;52:15–42. [28] Sampath H, Ntambi JM. Polyunsaturated fatty acid regulation of genes of lipid metabolism. Annu Rev Nutr 2005;25:317–40. [29] Heinemann FS, Korza G, Ozols J. A plasminogen-like protein selectively degrades stearoyl-CoA desaturase in liver microsomes. J Biol Chem 2003;278: 42966–75. [30] Kato H, Sakaki K, Mihara K. Ubiquitin-proteasome-dependent degradation of mammalian ER stearoyl-CoA desaturase. J Cell Sci 2006;119:2342–53. [31] Dobrzyn P, Ntambi JM, Dobrzyn A. Stearoyl-CoA desaturase: a novel control pint of lipid metabolism and insulin sensitivity. Eur J Lipid Sci Technol 2008;110:93–100. [32] Dobrzyn A, Dobrzyn P, Lee SH, Miyazaki M, Cohen P, Asilmaz E, et al. StearoylCoA desaturase-1 deficiency reduces ceramide synthesis by downregulating serine palmitoyltransferase and increasing beta-oxidation in skeletal muscle. Am J Physiol Endocrinol Metab 2005;288:E599–607. [33] Rahman SM, Dobrzyn A, Dobrzyn P, Lee SH, Miyazaki M, Ntambi JM. StearoylCoA desaturase 1 deficiency elevates insulin-signaling components and down-regulates protein-tyrosine phosphatase 1B in muscle. Proc Natl Acad Sci USA 2003;100:11110–5. [34] Dobrzyn A, Ntambi JM. The role of stearoyl-CoA desaturase in body weight regulation. Trends Cardiovasc Med 2004;14:77–81. [35] Miyazaki M, Dobrzyn A, Man WC, Chu K, Sampath H, Kim HJ, et al. StearoylCoA desaturase 1 gene expression is necessary for fructose-mediated induction of lipogenic gene expression by sterol regulatory element-


[36]

[37]

[38]

[39]

[40] [41]

[42]

[43]

[44]

[45] [46]

[47]

[48] [49]

[50]

[51]

[52] [53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

binding protein-1c-dependent and -independent mechanisms. J Biol Chem 2004;279:25164–71. Lee SH, Dobrzyn A, Dobrzyn P, Rahman SM, Miyazaki M, Ntambi JM. Lack of stearoyl-CoA desaturase 1 upregulates basal thermogenesis but causes hypothermia in a cold environment. J Lipid Res 2004;45:1674–82. Dobrzyn P, Dobrzyn A, Miyazaki M, Cohen P, Asilmaz E, Hardie DG, et al. Stearoyl-CoA desaturase 1 deficiency increases fatty acid oxidation by activating AMP-activated protein kinase in liver. Proc Natl Acad Sci USA 2004;101:6409–14. Dobrzyn A, Dobrzyn P, Miyazaki M, Sampath H, Chu K, Ntambi JM. StearoylCoA desaturase 1 deficiency increases CTP:choline cytidylyltransferase translocation into the membrane and enhances phosphatidylcholine synthesis in liver. J Biol Chem 2005;280:23356–62. Ntambi JM, Miyazaki M, Stoehr JP, Lan H, Kendziorski CM, Yandell BS, et al. Loss of stearoyl-CoA desaturase-1 function protects mice against adiposity. Proc Natl Acad Sci USA 2002;99:11482–6. Sampath H, Ntambi JM. Role of stearoyl-CoA desaturase-1 in skin integrity and whole body energy balance. J Biol Chem 2014;289:2482–8. Cohen P, Miyazaki M, Socci ND, Hagge-Greenberg A, Liedtke W, Soukas AA, et al. Role for stearoyl-CoA desaturase-1 in leptin-mediated weight loss. Science 2002;297:240–3. Tabor DE, Kim JB, Spiegelman BM, Edwards PA. Identification of conserved cis-elements and transcription factors required for sterol-regulated transcription of stearoyl-CoA desaturase 1 and 2. J Biol Chem 1999;274: 20603–10. MacDonald ML, Singaraja RR, Bissada N, Ruddle P, Watts R, Karasinska JM, et al. Absence of stearoyl-CoA desaturase-1 ameliorates features of the metabolic syndrome in LDLR-deficient mice. J Lipid Res 2008;49:217–29. Miyazaki M, Dobrzyn A, Sampath H, Lee SH, Man WC, Chu K, et al. Reduced adiposity and liver steatosis by stearoyl-CoA desaturase deficiency are independent of peroxisome proliferator-activated receptor-alpha. J Biol Chem 2004;279:35017–24. Awan MM, Saggerson ED. Malonyl-CoA metabolism in cardiac myocytes and its relevance to the control of fatty acid oxidation. Biochem J 1993;295:61–6. Flowers JB, Rabaglia ME, Schueler KL, Flowers MT, Lan H, Keller MP, et al. Loss of stearoyl-CoA desaturase-1 improves insulin sensitivity in lean mice but worsens diabetes in leptin-deficient obese mice. Diabetes 2007;56:1228–39. Matsui H, Yokoyama T, Sekiguchi K, Iijima D, Sunaga H, Maniwa M, et al. Stearoyl-CoA desaturase-1 (SCD1) augments saturated fatty acid-induced lipid accumulation and inhibits apoptosis in cardiac myocytes. PLoS ONE 2012;7:e33283. Towler MC, Hardie DG. AMP-activated protein kinase in metabolic control and insulin signaling. Circ Res 2007;100:328–41. Ferre P. The biology of peroxisome proliferator-activated receptors: relationship with lipid metabolism and insulin sensitivity. Diabetes 2004;53:S43–50. Park SY, Cho YR, Finck BN, Kim HJ, Higashimori T, Hong EG, et al. Cardiacspecific overexpression of peroxisome proliferator-activated receptor-alpha causes insulin resistance in heart and liver. Diabetes 2005;54:2514–24. Yang J, Sambandam N, Han X, Gross RW, Courtois M, Kovacs A, et al. CD36 deficiency rescues lipotoxic cardiomyopathy. Circ Res 2007;100: 1208–17. Finck BN, Kelly DP. PGC-1 coactivators: inducible regulators of energy metabolism in health and disease. J Clin Invest 2006;116:615–22. Zhou YT, Grayburn P, Karim A, Shimabukuro M, Higa M, Baetens D, et al. Lipotoxic heart disease in obese rats: implications for human obesity. Proc Natl Acad Sci USA 2000;97:1784–9. Barouch LA, Gao D, Chen L, Miller KL, Xu W, Phan AC, et al. Cardiac myocyte apoptosis is associated with increased DNA damage and decreased survival in murine models of obesity. Circ Res 2006;98:119–24. Ge F, Hu C, Hyodo E, Arai K, Zhou S, Lobdell H, et al. Cardiomyocyte triglyceride accumulation and reduced ventricular function in mice with obesity reflect increased long chain fatty acid uptake and de novo fatty acid synthesis. J Obes 2012;1:205648. Lee Y, Naseem RH, Duplomb L, Park BH, Garry DJ, Richardson JA, et al. Hyperleptinemia prevents lipotoxic cardiomyopathy in acyl CoA synthase transgenic mice. Proc Natl Acad Sci USA 2004;101:13624–9. Poudyal H, Panchal SK, Waanders J, Ward L, Brown L. Lipid redistribution by a-linolenic acid-rich chia seed inhibits stearoyl-CoA desaturase-1 and induces cardiac and hepatic protection in diet-induced obese rats. J Nutr Biochem 2012;23:153–62. Hofacer R, Magrisso IJ, Jandacek R, Rider T, Tso P, Benoit SC, et al. Omega-3 fatty acid deficiency increases stearoyl-CoA desaturase expression and activity indices in rat liver: positive association with non-fasting plasma triglyceride levels. Prostaglandins Leukot Essent Fatty Acids 2012;86:71–7. Poudyal P, Brown L. Stearoyl-CoA desaturase: a vital checkpoint in the development and progression of obesity. Endocr Metab Immune Disord Drug Targets 2011;11:217–31. Poudyal H, Panchal SK, Ward LC, Brown L. Effects of ALA, EPA and DHA in high-carbohydrate, high-fat diet-induced metabolic syndrome in rats. J Nutr Biochem 2013;24:1041–52. Mengi SA, Dhalla NS. Carnitine palmitoyltransferase-I, a new target for the treatment of heart failure: perspectives on a shift in myocardial metabolism as a therapeutic intervention. Am J Cardiovasc Drugs 2004;4:201–9. Hanada K. Serine palmitoyltransferase, a key enzyme of sphingolipid metabolism. Biochim Biophys Acta 2003;1632:16–30.

11

[63] Shimabukuro M, Higa M, Zhou YT, Wang MY, Newgard CB, Unger RH. Lipoapoptosis in beta-cells of obese prediabetic fa/fa rats. Role of serine palmitoyltransferase overexpression. J Biol Chem 1998;273:32487–90. [64] Bielawska AE, Shapiro JP, Jiang L, Melkonyan HS, Piot C, Wolfe CL, et al. Ceramide is involved in triggering of cardiomyocyte apoptosis induced by ischemia and reperfusion. Am J Pathol 1997;151:1257–63. [65] Ravid T, Tsaba A, Gee P, Rasooly R, Medina EA, Goldkorn T. Ceramide accumulation precedes caspase-3 activation during apoptosis of A549 human lung adenocarcinoma cells. Am J Physiol Lung Cell Mol Physiol 2003;284: L1082–92. [66] Castillo SS, Levy M, Wang C, Thaikoottathil JV, Khan E, Goldkorn T. Nitric oxide-enhanced caspase-3 and acidic sphingomyelinase interaction: a novel mechanism by which airway epithelial cells escape ceramide-induced apoptosis. Exp Cell Res 2007;313:816–23. [67] Ruvolo PP, Clark W, Mumby M, Gao F, May WS. A functional role for the B56 alpha-subunit of protein phosphatase 2A in ceramide-mediated regulation of Bcl2 phosphorylation status and function. J Biol Chem 2002; 277:22847–52. [68] Sawada M, Nakashima S, Banno Y, Yamakawa H, Takenaka K, Shinoda J, et al. Influence of Bax or Bcl-2 overexpression on the ceramide-dependent apoptotic pathway in glioma cells. Oncogene 2000;19:3508–20. [69] Kong JY, Rabkin SW. Palmitate-induced cardiac apoptosis is mediated through CPT-1 but not influenced by glucose and insulin. Am J Physiol Heart Circ Physiol 2002;282:H717–25. [70] Listenberger LL, Han X, Lewis SE, Cases S, Farese Jr RV, Ory DS, et al. Triglyceride accumulation protects against fatty acid-induced lipotoxicity. Proc Natl Acad Sci USA 2003;100:3077–82. [71] Turdi S, Ge W, Hu N, Bradley KM, Wang X, Ren J. Interaction between maternal and postnatal high fat diet leads to a greater risk of myocardial dysfunction in offspring via enhanced lipotoxicity, IRS-1 serine phosphorylation and mitochondrial defects. J Mol Cell Cardiol 2013;55: 117–29. [72] Van Bilsen M, van Nieuwenhoven FA, van der Vusse GJ. Metabolic remodelling of the failing heart: beneficial or detrimental? Cardiovasc Res 2009;81:420–8. [73] Strom CC, Aplin M, Ploug T, Christoffersen TE, Langfort J, Viese M, et al. Expression profiling reveals differences in metabolic gene expression between exercise-induced cardiac effects and maladaptive cardiac hypertrophy. FEBS J 2005;272:2684–95. [74] Finck BN, Lehman JJ, Leone TC, Welch MJ, Bennett MJ, Kovacs A, et al. The cardiac phenotype induced by PPARa overexpression mimics that caused by diabetes mellitus. J Clin Invest 2002;109:121–30. [75] Rimbaud S, Sanchez H, Garnier A, Fortin D, Bigard X, Veksler V, et al. Stimulus specific changes of energy metabolism in hypertrophied heart. J Mol Cell Cardiol 2009;46:952–9. [76] Burelle Y, Wambolt RB, Grist M, Parsons HL, Chow JC, Antler C, et al. Regular exercise is associated with a protective metabolic phenotype in the rat heart. Am J Physiol Heart Circ Physiol 2004;287:H1055–63. [77] Dobrzyn P, Pyrkowska A, Jazurek M, Szymanski K, Langfort J, Dobrzyn A. Endurance training-induced accumulation of muscle triglycerides is coupled to upregulation of stearoyl-CoA desaturase 1. J Appl Physiol 2010;109: 1653–61. [78] Rogowski MP, Flowers MT, Stamatikos AD, Ntambi JM, Paton CM. SCD1 activity in muscle increases triglyceride PUFA content, exercise capacity, and PPARd expression in mice. J Lipid Res 2013;54:2636–46. [79] Handschin C, Spiegelman BM. Peroxisome proliferator-activated receptor gamma coactivator 1 coactivators, energy homeostasis, and metabolism. Endocr Rev 2006;27:728–35. [80] Rankin J, Tennant PW, Stothard KJ, Bythell M, Summerbell CD, Bell R. Maternal body mass index and congenital anomaly risk: a cohort study. Int J Obes (Lond) 2010;34:1371–80. [81] Bentham J, Michell AC, Lockstone H, Andrew D, Schneider Jr E, Brown NA, et al. Maternal high-fat diet interacts with embryonic Cited2 genotype to reduce Pitx2c expression and enhance penetrance of left-right patterning defects. Hum Mol Genet 2010;19:3394–401. [82] Rajia S, Chen H, Morris MJ. Maternal overnutrition impacts offspring adiposity and brain appetite markers-modulation by postweaning diet. J Neuroendocrinol 2010;22:905–14. [83] Dong F, Li Q, Sreejayan N, Nunn JM, Ren J. Metallothionein prevents high-fat diet induced cardiac contractile dysfunction: role of peroxisome proliferator activated receptor gamma coactivator 1alpha and mitochondrial biogenesis. Diabetes 2007;56:2201–12. [84] Zhang J, Kris-Etherton PM, Thompson JT, Hannon DB, Gillies PJ, Heuvel JP. Alpha-linolenic acid increases cholesterol efflux in macrophage-derived foam cells by decreasing stearoyl CoA desaturase 1 expression: evidence for a farnesoid-X-receptor mechanism of action. J Nutr Biochem 2012;23: 400–9. [85] Kris-Etherton PM, Griel AE, Psota TL, Gebauer SK, Zhang J, Etherton TD. Dietary stearic acid and risk of cardiovascular disease: intake, sources, digestion, and absorption. Lipids 2005;40:1193–200. [86] Kamp F, Guo W, Souto R, Pilch PF, Corkey CE, Hamilton JA. Rapid flip-flop of oleic acid across the plasma membrane of adipocytes. J Biol Chem 2003;278: 7988–95. [87] Kennedy A, Martinez K, Chuang CC, LaPoint K, McIntosh M. Saturated fatty acid-mediated inflammation and insulin resistance in adipose tissue: mechanisms of action and implications. J Nutr 2009;139:1–4.

12


[88] Dziewulska A, Dobrzyn P, Jazurek M, Pyrkowska A, Ntambi JM, Dobrzyn A. Monounsaturated fatty acids are required for membrane translocation of protein kinase C-theta induced by lipid overload in skeletal muscle. Mol Membr Biol 2012;29:309–20. [89] Posner BM, Cobb JL, Belanger AJ, Cupples LA, D’Agostino RB, Stokes J. Dietary lipid predictors of coronary heart disease. Men Arch Intern Med 1991;151: 1181–7. [90] Xu J, Eilat-Adar S, Loria C, Goldbourt U, Howard BV, Fabsitz RR, et al. Dietary fat intake and risk of coronary heart disease: the Strong Heart Study. Am J Clin Nutr 2006;84:894–902. [91] Dobrzyn P, Pyrkowska A, Jazurek M, Dobrzyn A. Increased availability of endogenous and dietary oleic acid contributes to the upregulation of cardiac fatty acid oxidation. Mitochondrion 2012;12:132–7. [92] Finck BN, Han X, Courtois M, Aimond F, Nerbonne JM, Kovacs A, et al. A critical role for PPAR-mediated lipotoxicity in the pathogenesis of diabetic cardiomyopathy: modulation by dietary fat content. Proc Natl Acad Sci USA 2003;100:1226–31. [93] Morgan EE, Dennison JH, Young ME, McElfresh TA, Kung TA, Tserng KY, et al. Effects of chronic activation of peroxisome proliferator-activated receptoralpha or high-fat feeding in a rat infarct model of heart failure. Am J Physiol Heart Circ Physiol 2006;290:H1899–904. [94] Gilde AJ, Van Der Lee KA, Willemsen PH, Chinetti G, Van Der Leij FR, van der Vusse GJ, et al. Peroxisome proliferator-activated receptor (PPAR)-a and PPARb/c, but not PPARc, modulate the expression of genes involved in cardiac lipid metabolism. Circ Res 2003;92:518–24. [95] Clark H, Carling D, Saggerson D. Covalent activation of heart AMP-activated protein kinase in response to physiological concentrations of long-chain fatty acids. Eur J Biochem 2004;271:2215–24. [96] Carley AN, Severson DL. Fatty acid metabolism is enhanced in type 2 diabetic hearts. Biochim Biophys Acta 2005;1734:112–26. [97] Nickerson JG, Alkhateeb H, Benton CR, Lally J, Nickerson J, Han XX, et al. Greater transport efficiencies of the membrane fatty acid transporters FAT/ CD36 and FATP4 compared with FABPpm and FATP1 and differential effects on fatty acid esterification and oxidation in rat skeletal muscle. J Biol Chem 2009;284:16522–30. [98] Armoni M, Harel C, Bar-Yoseph F, Milo S, Karnieli E. Free fatty acids repress the GLUT4 gene expression in cardiac muscle via novel response elements. J Biol Chem 2005;280:34786–95. [99] Hommelberg PP, Plat J, Langen RC, Schols AM, Mensink RP. Fatty acid-induced NF-kappaB activation and insulin resistance in skeletal muscle are chain length dependent. Am J Physiol Endocrinol Metab 2009;296:E114–20. [100] Anderwald C, Brunmair B, Stadlbauer K, Krebs M, Furnsinn C, Roden M. Effects of free fatty acids on carbohydrate metabolism and insulin signalling in perfused rat liver. Eur J Clin Invest 2007;37:774–82. [101] Liu HY, Collins QF, Xiong Y, Moukdar F, Lupo EG, Liu Z, et al. Prolonged treatment of primary hepatocytes with oleate induces insulin resistance through p38 mitogen-activated protein kinase. J Biol Chem 2007;282: 14205–12.

[102] Warensjo E, Risérus U, Gustafsson IB, Mohsen R, Cederholm T, Vessby B. Effects of saturated and unsaturated fatty acids on estimated desaturase activities during a controlled dietary intervention. Nutr Metab Cardiovasc Dis 2008;18:683–90. [103] Warensjo E, Sundström J, Vessby B, Cederholm T, Risérus U. Markers of dietary fat quality and fatty acid desaturation as predictors of total and cardiovascular mortality: a population-based prospective study. Am J Clin Nutr 2008;88:203–9. [104] Djoussé L, Weir NL, Hanson NQ, Tsai MY, Gaziano JM. Plasma phospholipid concentration of cis palmitoleic acid and risk of heart failure. Circ Heart Fail 2012;5:703–9. [105] Ebbesson SO, Lopez-Alvarenga JC, Okin PM, Devereux RB, Tejero ME, Harris WS, et al. Heart rate is associated with markers of fatty acid desaturation: the GOCADAN study. Int J Circumpolar Health 2012;71:17343. [106] Do HJ, Chung HK, Moon J, Shin MJ. Relationship between the estimates of desaturase activities and cardiometabolic phenotypes in Koreans. J Clin Biochem Nutr 2011;49:131–5. [107] Crowe FL, Skeaff CM, Green TJ, Gray AR. Serum fatty acids as biomarkers of fat intake predict serum cholesterol concentrations in a population-based survey of New Zealand adolescents and adults. Am J Clin Nutr 2006;83:887–94. [108] Mozaffarian D, Cao H, King IB, Lemaitre RN, Song X, Siscovick DS, et al. Circulating palmitoleic acid and risk of metabolic abnormalities and newonset diabetes. Am J Clin Nutr 2010;92:1350–8. [109] Peter A, Cegan A, Wagner S, Lehmann R, Stefan N, Konigsrainer A, et al. Hepatic lipid composition and stearoyl-coenzyme A desaturase 1 mRNA expression can be estimated from plasma VLDL fatty acid ratios. Clin Chem 2009;55:2113–20. [110] Libby P, Ridker PM, Hansson GK. Progress and challenges in translating the biology of atherosclerosis. Nature 2011;473:317–25. [111] Palmefors H, Dutta Roy S, Rundqvist B, Börjesson M. The effect of physical activity or exercise on key biomarkers in atherosclerosis – A systematic review. Atherosclerosis 2014;235:150–61. [112] MacDonald ML, van Eck M, Hildebrand RB, Wong BW, Bissada N, Ruddle P, et al. Despite antiatherogenic metabolic characteristics, SCD1-deficient mice have increased inflammation and atherosclerosis. Arterioscler Thromb Vasc Biol 2009;29:341–7. [113] Brown JM, Chung S, Sawyer JK, Degirolamo C, Alger HM, Nguyen TM, et al. Combined therapy of dietary fish oil and stearoyl-CoA desaturase 1 inhibition prevents the metabolic syndrome and atherosclerosis. Arterioscler Thromb Vasc Biol 2010;30:24–30. [114] Savransky V, Jun J, Li J, Nanayakkara A, Fonti S, Moser AB, et al. Dyslipidemia and atherosclerosis induced by chronic intermittent hypoxia are attenuated by deficiency of stearoyl coenzyme A desaturase. Circ Res 2008;103: 1173–80. [115] Zhang Y, Yuan M, Bradley KM, Dong F, Anversa P, Ren J. Insulin-like growth factor 1 alleviates high-fat diet-induced myocardial contractile dysfunction: role of insulin signaling and mitochondrial function. Hypertension 2012;59: 680–93.

Roles of StearoylCoA Desaturase-1 in the Regulation of Cancer Cell Growth, Survival and Tumorigenesis.

Editorial: Exploring Cancer Metabolic Reprogramming through Molecular Imaging.

Metabolic Reprogramming in Glioma.

VEGF-B-induced vascular growth leads to metabolic reprogramming and ischemia resistance in the heart.

Energy metabolic reprogramming in the hypertrophied and early stage failing heart: a multisystems approach.

Metabolic reprogramming during neuronal differentiation.

EZH2 promotes metabolic reprogramming in glioblastomas through epigenetic repression of EAF2-HIF1α signaling.

Metabolic reprogramming of stromal fibroblasts through p62-mTORC1 signaling promotes inflammation and tumorigenesis.

Metabolic reprogramming of myeloid-derived suppressive cells.

Metabolic reprogramming in triple-negative breast cancer through Myc suppression of TXNIP.

Sqstm1 promotes malignancy of HCV-positive hepatocellular carcinoma through Nrf2-dependent metabolic reprogramming.

Metabolic Reprogramming of Stem Cell Epigenetics.

Metabolic reprogramming through fatty acid transport protein 1 (FATP1) regulates macrophage inflammatory potential and adipose inflammation.

Similarities in the Metabolic Reprogramming of Immune System and Endothelium.

mTORC2 in the center of cancer metabolic reprogramming.

TORquing metabolic reprogramming in cancer cells.

Metastasis: metabolic reprogramming in disseminated cells.

HIF1α and metabolic reprogramming in inflammation.

Targeting Lipid Metabolic Reprogramming as Anticancer Therapeutics.

T cell metabolic reprogramming and plasticity.

Metabolic reprogramming in mutant IDH1 glioma cells.

Heart regeneration: a tale of cell reprogramming.

Lipid metabolic reprogramming in cancer cells.

Metabolic reprogramming: the emerging concept and associated therapeutic strategies.