Surgery for Obesity and Related Diseases ] (2014) 00–00
Original article
Sleeve gastrectomy reduces xanthine oxidase and uric acid in a rat model of morbid obesity Andreas Oberbach, M.D., Ph.D., M.P.H.a,b, Jochen Neuhaus, Ph.D.c, Nadine Schlichting, Ph.D.d, Joachim Kugler, M.D., Ph.D.b, Sven Baumann, Ph.D.e, Holger Till, M.D., Ph.D.f,* a Department of Cardiac Surgery, University of Leipzig, Heart Center, Leipzig, Germany Department of Health Sciences / Public Health, University of Dresden, Dresden, Germany c Department of Urology, University of Leipzig, Leipzig, Germany d Integrated Research and Treatment Center (IFB) Adiposity Diseases, University of Leipzig, Leipzig, Germany e Department of Metabolomics, Helmholtz Centre for Environmental Research, Leipzig, Germany f Department of Pediatrics and Adolescent Surgery, Medical University of Graz, Graz, Austria b
Abstract
Background: Serum uric acid (sUA) plays a major role in the development of morbidities associated with obesity, especially cardiovascular diseases. Within the purine pathway, xanthine oxidase (XOD) represents the key enzyme. The aim of this study was to investigate the dynamics of sUA and XOD following sleeve gastrectomy (SG) in a rat model of high-fat-diet (HFD) induced obesity. Patients: Over a period of 11 weeks, 30 rats received a HFD, and 10 rats received a low fat diet (LFD). Thereafter, 10 randomly selected HFD rats and 10 LFD rats were sacrificed. The remaining 20 HFD rats were randomly assigned to either SG or sham operation (SH) and studied 14 days postoperatively. Methods: The white adipose tissues (WAT) from visceral (intestinal and retroperitoneal) and inguinal (subcutaneous) depots were collected. sUA and urine UA (uUA) were measured by high performance liquid chromatography-mass spectrometry (HPLC-MS/MS). Abundance and activity of XOD was investigated in the liver, colon, adipose tissue, and skeletal muscle by enzyme-linked immunosorbent assay (ELISA). Results: HFD led to significant weight gain, elevated sUA levels, increased WAT and increase of XOD activity. Fourteen days postoperatively, SG rats showed a significant decrease of weight and adipose tissue, improved glucose metabolism, and changes of gut hormones. The sUA and uUA levels were significantly decreased following SG. Furthermore, XOD activity was significantly down-regulated in WAT. Conclusion: HFD induces elevated sUA levels by gain of WAT and increase of XOD activity. Following SG, the reduction of WAT as the major source of XOD and the lowering of XOD activity are the basis for the decrease of sUA. (Surg Obes Relat Dis 2014;]:00–00.) r 2014 American Society for Metabolic and Bariatric Surgery. All rights reserved.
Keywords:
Rat; Obesity; High fat diet; Sleeve gastrectomy; Uric acid; Xanthine oxidase
This work was supported by the Federal Ministry of Education and Research (BMBF), Germany (Integrated Research and Treatment Center IFB “Adiposity Diseases”). * Correspondence: Holger Till, M.D., Ph.D, Department of Pediatric and Adolescent Surgery, Medical University of Graz, Auenbruggerplatz 34, A-8036 Graz, Austria. E-mail: [email protected]
In patients with morbid obesity, elevated serum uric acid (sUA) levels are associated with an increased mortality [1]. Hyperuricemia plays a major role in the development of hypertension, insulin resistance (IR), and renal and cardiovascular diseases (CVD) [2]. Additionally, high fat diet (HFD) induces abdominal fat accumulation associated with hyperuricemia [3] while weight loss reverses both [2]. Within the purine pathway xanthine oxidase (XOD)
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A. Oberbach et al. / Surgery for Obesity and Related Diseases ] (2014) 00–00
represents the key enzyme leading to uric acid (UA) production [4]. To date it remains unclear, whether XOD activity and sUA production are affected by bariatric surgery such as sleeve gastrectomy (SG). The present study in rats tested the hypotheses that (1) HFD has an effect on sUA, XOD, glucose homeostasis, and gut hormones and (2) that SG influences these parameters in HFD-treated rats compared to sham operation (SH). Methods Experimental setting This study was approved by the local Council of Animal Research (Landesdirektion Leipzig, TVV 32/11 and TVV 32/12). Forty male Sprague-Dawley rats (Medical Experimental Center, University of Leipzig) were subjected to different dietary regimens for 11 weeks. Thirty animals received HFD (fat content 45% of energy, based on Atwater-calculation, ssniff-Spezialdiaeten GmbH, Soest, Germany). Ten animals received a low-fat diet (LFD, fat content 18% of energy, based on Atwater-calculation; ssniff-Spezialdiaeten GmbH, Soest, Germany). All animals were kept on a 12:12-h light–darkness cycle. Weight gain and food intake were monitored once a week, and metabolic cages were used to collect individual urine samples for 24 h. After 11 weeks of dietary treatment, 10 randomly selected HFD rats and 10 LFD rats were sacrificed to be studied preoperatively. The remaining 20 HFD rats were randomized to receive either SG or sham operation (SH). Surgical interventions Before SG and SH, all animals were fasted for 24 hours with free access to water. SG in rats was performed according to Chambers et al. [5]. SH consisted of an equivalent laparotomy only. Postoperatively, SG and SH animals were kept on a HFD for 14 days. In the SG group 2 animals died following SG before reaching the primary endpoint at 14 days postoperatively. Then, all animals were sacrificed for tissue and blood collection. Tissue and blood sample collection Arterial blood was drawn from the heart into ethylenediaminetetraacetic acid-coated vials. After centrifugation (10 min 2500 xg at 41C) plasma was stored at –801C until analysis. Tissue samples from skeletal muscle (M. quadriceps femoris), liver, colon, adipose tissue (visceral and subcutaneous), and feces were collected, snap-frozen in liquid nitrogen, and stored at −801C until use. Adipose tissues from abdominal and inguinal regions (subcutaneous fat) and visceral fat from the intestine and from the retroperitoneal region were collected (visceral fat) as described elsewhere [6]. Plasma measurements. After 11 weeks on HFD, fasting glucose was analyzed during oral glucose tolerance test (oGTT). This test was performed after an overnight fast
(16 hr) by oral application of 2 g/kg weight 20% glucose solution between 7:00 a.m. and 9:00 a.m. Blood glucose levels were measured from tail tip capillary region at 0 (baseline), 15, 30, 45, 60, and 120 minutes after injection using a hand-held glucometer (GlucoCheckExcellent, aktivmed GmbH, Augsburg, Germany). Unless stated otherwise, all reagents were purchased from Sigma or Merck at the highest purity grade available. Fasting plasma insulin was determined using immunoassay kit (Rat Insulin ELISA Kit, Alpco Diagnostics, Salem, NH). Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated by multiplying fasting plasma glucose (mg/dL) by fasting plasma insulin (mU/ mL) divided by 2.430 according to the method described by Cacho et al. [7]. Plasma levels of total cholesterol, HDLcholesterol, triglycerides, interleukine-6 and C-reactive protein were measured using commercial enzymatic assay kits (Total Cholesterol Assay Kit, Cell Biolabs Inc., San Diego, CA; HDL Cholesterol Assay Kit, Cell Biolabs Inc.; Serum Triglyceride Quantification Kit, Cell Biolabs Inc., San Diego, CA; Rat IL-6 Quantikine ELISA Kit, R&D Systems, Minneapolis, MN; AssayMax Rat C-reactive protein ELISA Kit, Assaypro, St. Charles, MO). LDL cholesterol was calculated by Friedewald's formula: LDL ¼ TC HDL TG/2.2 (mmol/L) [8]. Creatinine clearance (mL/min) was calculated using the standard formula U*V/P, while U is the urinary creatinine concentration (mg/dL), V is the 24 h urine volume (mL/ min) and P represents the serum creatinine concentration (mg/dL). Plasma and urinary creatinine were determined by Creatinine (serum) Assay Kit and Creatinine (urinary) Assay Kit (Cayman Chemicals, Ann Arbor, MI). Gut hormones. Levels of total protein of the orexigenic gut hormones Ghrelin, glucagon-like peptide 1 (GLP-1), peptide tyrosine (PYY), and acylGhrelin and active GLP-1 were measured by ELISA at 0, 30, 60, 120, and 180 minutes. Quantification of Serum UA. UA sample preparation was based on a modified method by Kim et al. [8,9]. Briefly, 10 ml serum was diluted 10-fold with water including [1,3-15 N2]-UA (Euriso-top, Saarbrücken, Germany) (final concentration 50 mM) and treated with 20 ml 10% trichloroacetic acid TCA (w/v). The samples were vortexed for 1 minute and centrifuged at 15,000 xg for 2 minutes. Supernatants were loaded into autosampler vials and analyzed by LC-MS/MS. All analyses were carried out on an Agilent 1100 series binary high performance liquid chromatography (HPLC) system (Agilent Technologies, Waldbronn, Germany) coupled to a 4,000 QTrap mass spectrometer (AB Sciex, Concord, Canada) equipped with TurboIon spray source. Liquid chromatography was performed in gradient elution mode using Chromolith Flash RP-18 e (25 4.6 mm, Merck, Darmstadt, Germany) at 401C with eluent A (0.1% formic acid in water) and eluent B (0.1% formic acid in acetonitrile) at a flow rate of 400 ml/min. The injection volume of each sample was 5 ml. For sUA quantification MS was operated in negative ion and multiple reaction
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
monitoring mode. Data were acquired and analyzed using Analyst software version 1.4.2 (AB Sciex). Tissue measurements. Xanthine oxidase protein expression and enzymatic activity were analyzed in plasma and tissue homogenates (M. quadriceps femoris, liver, colon, visceral fat, subcutaneous fat) using a rat XOD ELISA Kit (Cusabio Biotech, Wuhan, China) and XOD activity assay Kit (Cayman Chemicals). Total protein amount of plasma and tissue homogenates was determined by BCA Protein Assay Kit (Thermo Fisher Scientific, IL) using BSA as standard. Total free fatty acids (FFA) in feces and serum were quantified by a commercial Free Fatty Acid Quantification Kit (BioVision, CA) according to the manufacturer’s instructions. 20 mg feces were homogenized in assay buffer and spun 5 minutes at top speed in a microcentrifuge. For quantification, 10 ml serum and 20 ml of feces supernatant were used. Statistical analysis. Data analysis was performed using Prism v5.0 (GraphPad Software, La Jolla, CA). Statistical differences were calculated by independent t test statistics. For correlation analysis Pearson r and linear regression model was calculated. A P value r 0.05 was considered statistically significant. All data are presented as means ⫾ SD.
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(Fig. S1 A-B) compared to LFD. The authors found significant changes in fasting glucose metabolism and lipid metabolism, except for HDL- and LDL-cholesterol, which was elevated in HFD, but did not reach a significanct level (Table 1). Additionally, protein levels of inflammatory markers like IL-6 and C-reactive protein were significantly higher in HFD (Table 1). Gut-related hormones were not influenced by HFD when measured under fasting conditions (Table 1). Furthermore, HFD animals showed increased abundance of sUA, while uUA was unchanged (Fig. 1A-B). XOD abundance and activity were both significantly elevated following HFD (Fig. 1C-D). Tissue-based analysis of XOD revealed higher levels in subcutaneous adipose tissue and in skeletal muscle (SkM) after HFD, while lower XOD levels were measured in visceral fat. Liver and colon XOD levels were not significantly altered (Fig. 1E). In parallel, HFD increased XOD activity in subcutaneous fat and SkM while visceral fat, liver, and colon showed no significant alterations (Fig. 1F). Effects of SG versus SH on HFD rats
Results Effects of HFD versus LFD (before surgery) Eleven weeks of HFD resulted in significant increases of weight (Table 1) and visceral and subcutaneous fat mass
Fourteen days postoperatively, SG rats showed lower weight (Table 1) while still being fed HFD. Moreover, a significant decrease of visceral and subcutaneous adipose tissue compared to SH (Fig. S1) was documented. Both
Table 1 Clinical characteristics of the rats phenotype LFD (n ¼ 10) Age [weeks] Time of follow up [days] Weight [g] Gastric hormones Ghrelin (total) [ng/mL] GLP-1 (total) [pmol/L] PYY [ng/mL] Glucose metabolism (fasting) Glucose [mmol/L] Insulin [ng/mL] HOMA-IR Glucose 2 hoGTT [AUC] Lipid metabolism Triglyceride [mmol/L] Total-cholesterol [mmol/L] HDL-cholesterol [mmol/L] LDL-cholesterol [mmol/L] Inflammatory markers and renal function Interleukin-6 [pg/mL] C-reactive protein [mg/mL] Urin volume [mL/d] Creatinine clearance [mL/min]
HFD (n ¼ 10)
SH (n ¼ 10)
SG (n ¼ 8)
15 0 430 ⫾ 25.6
533 ⫾ 23.9*
17 14 490 ⫾ 70
431 ⫾ 33*
3.53 ⫾ .69 17.78 ⫾ 6.55 0.47 ⫾ .05
3.1 ⫾ 1.23 18.23 ⫾ 4.4 0.43 ⫾ .07
3.88 ⫾ .51 20.59 ⫾ 1.55 0.42 ⫾ .08
2.15 ⫾ .23* 30.74 ⫾ 6.66* 1.12 ⫾ .25*
5.48 ⫾ .63 1.25 ⫾ .14 1.47 ⫾ .3 63.8 ⫾ 4.54
6.30 ⫾ .91* 2.08 ⫾ .78* 2.88 ⫾ 1.4* 71.6 ⫾ 5.64*
6,4 ⫾ 0,66 0,85 ⫾ 0,16 1,17 ⫾ 0,26 72.6 ⫾ 2.67
6,2 ⫾ 0,8 0,8 ⫾ 0,19 1,1 ⫾ 0,39 71.6 ⫾ 2.75
0.69 ⫾ .49 2.79 ⫾ 1.03 0.97 ⫾ .25 1.41 ⫾ .83
1.3 ⫾ .75* 3.88 ⫾ .86* 1.29 ⫾ .44 2 ⫾ .51
0,47 ⫾ 0,24 4.81 ⫾ .37 0.79 ⫾ .18 1.73 ⫾ .26
0,36 ⫾ 0,18 3.33 ⫾ .48* 1.32 ⫾ .24* 0.71 ⫾ .21*
223 ⫾ 40.2 378 ⫾ 63 10.5 ⫾ 3.51 0.86 ⫾ .17
269 ⫾ 32.3* 522.3 ⫾ 30* 6.37 ⫾ 1.77* 0.8 ⫾ .27
288.2 ⫾ 52.2 379 ⫾ 23 7.15 ⫾ 2.71 0.51 ⫾ .15
225.1 ⫾ 46.9 366 ⫾ 42 7.3 ⫾ 3.24 0.54 ⫾ .38
Clinical characteristics of the study population phenotype. Clinical characteristics of the animal groups after 15 weeks and after intervention. Values are means ⫾ SD of different groups. A P value o 0.05 was taken to indicate statistical significance. GLP-1 ¼ glucagon-like peptide 1; HFD ¼ high fat diet; HOMA-IR ¼ homeostatic model assessment of insulin resistance; LFD ¼ low fat diet; PYY ¼ peptide tyrosine; SG ¼ sleeve gastrectomy; SH ¼ sham operation.
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Fig. 1. Levels of uric acid and xanthine oxidase (XOD) in different tissues of high fat diet (HFD) rats. HFD rats showed significantly increased levels of uric acid in serum (sUA; A) but not in urine (uUA; B) compared to low fat diet (LFD). Analysis of serum xanthine oxidase showed increased XOD abundance (C) and XOD activity (D) in HFD. (E) XOD abundance was unchanged in liver and colonic samples but increased in subcutaneous fat and skeletal muscle (SkM) in HFD while XOD was decreased in visceral fat compared to LFD. (F) XOD activity in these tissues was increased in subcutaneous adipose tissue and in SkM but unchanged in liver, colon, and visceral fat comparing HFD to LFD.
locations of adipose tissues, visceral and retroperitoneal were affected (Fig. S1 A;C). Quantification of FFA revealed significant diminished FFA levels in serum and increased FFA excretion via feces (Fig. S1E,F).
significant level (Fig. 2A). The postoperative food intake in SG rats was significantly lower compared to SH (Fig. 2B) and in parallel, SG rats showed a reduction of weight in 14 days follow-up observation (Fig. 2C).
Changes in gut hormones and long term metabolic afferent signals
Regulation of UA and XOD following SG
While fasting serum ghrelin was significantly decreased in SG, levels of fasting GLP-1 and PYY were significantly elevated (Table 1). Interestingly, parameters of glucose metabolism were unaffected, and parameters of lipid metabolism were significantly altered in SG rats, except for triglycerides. Renal function and inflammatory markers were not different between SG and SH after 14 days (Table 1). To investigate meal-related changes of gastric hormones, a 2-hour oGTT was performed, and the acylGhrelin, active GLP-1 and PYY were measured. At SG, rats revealed an altered expression of gastric hormones compared to SH (Fig. 2). Following glucose stimulation, SG rats showed lower acylGhrelin levels while PYY level was increased. The observed decrease of active GLP-1 did not reach a
sUA and uUA concentrations were significantly decreased following SG compared to SH rats (Fig. 3A-B). In serum, the abundance of XOD was increased in SG rats (Fig. 3 C), while XOD activity was unchanged (Fig. 3D). The measurement of XOD abundance revealed a significant up-regulation in colonic samples compared to adipose tissue, liver, and skeletal muscle (P o 0.01) (Fig. 3E). XOD activity was significantly increased in colonic samples while both subcutaneous and visceral adipose tissue showed decreased XOD activity in SG rats (Fig. 3F). Furthermore, the authors found a significant positive correlation between sUA and subcutaneous (P o 0.001; r2 ¼ 0.39) and visceral (P o 0.001; r2 ¼ 0.44) adipose tissue and between subcutaneous adipose tissue and XOD activity (P o 0.001; r2 ¼ 0.28; Fig. S2A-B;D).
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
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Fig. 2. Weight, food intake, and gut hormones after sleeve gastrectomy (SG) versus sham operation (SH) (A). Dynamics of the serum levels of gastric hormones acylGhrelin, active glucagon-like peptide 1 (GLP-1) and peptide tyrosine (PYY) after oral glucose tolerance test (oGTT) in SG rats (white circles) compared to SH (black circles). Values are represented as means ⫾ SD. (B) Increase in daily food intake of SG operated rats compared to SH group. (C) Trend in weight of SG and SH. Body weight was monitored between 11 and 14 weeks of age only, because oGTT performed at 15 weeks of age required 24 hours fasting which interfered with normal weight gain.
Discussion HFD-induced adipose tissue as a major source of UA metabolism UA represents the end product of the purine nucleotide metabolism and has been associated with obesity and related co-morbidities [10]. Previous studies in humans showed that fat mass correlated positively with serum UA concentrations [11]. The authors’ present study in rats confirmed that adipose tissue represents the location with the most XOD activity compared to liver, colon, or skeletal muscle (Fig. 1E and 1F). HFD increased adipose tissue, which showed a significant increase of XOD activity. Interestingly, LFD revealed higher XOD abundance in visceral adipose tissue than HFD (Fig. 1E). It is well known that following HFD, adipocytes can react with hypertrophy and hyperplasia, which might influence the relative distribution of a protein of interest [12]. XOD
activity and visceral and subcutaneous fat XOD activity were significantly increased (Fig. 1F). As XOD is known to be the key enzyme in UA metabolism, increased XOD activity leads to increased sUA levels and this is in line with a wide range of clinical studies correlating fat mass with sUA [13].
Effects of SG on UA metabolism Metabolic surgery such as SG is a potent procedure to reduce fat mass [14]. To investigate the surgical effects on UA metabolism, the authors performed SG on HFD rats and continued HFD postoperatively. They observed a significant decrease in fat mass at 14 days postoperatively (Fig. S1 CþD). In parallel, XOD activity was significantly decreased in subcutaneous and visceral adipose tissues (Fig. 3F). Those results suggest that nucleotide metabolism is down-
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Fig. 3. Levels of uric acid (UA) and XOD in various tissues after SG. SG rats showed significantly decreased levels of uric acid in serum (sUA; A) and in urine (uUA; B) compared to SH rats. Analysis of XOD showed increased XOD abundance (C) in SG rats while XOD activity (D) was unchanged. (E) XOD abundance in liver, adipose tissue, skeletal muscle on protein level revealed no alteration between SG and SH, while SG rats showed higher XOD abundance in colonic samples. The adipose tissue showed the highest XOD abundance in both groups (F). XOD activity was decreased in adipose tissue, increased in colon but unchanged in liver and muscle comparing SG to SH.
regulated as a consequence of undernourishment of adipose cells. However, the question remains as to which factors caused a decrease of sUA following SG. Recent evidence suggests that SG leads to a decreased food intake not only by mechanical restriction of the stomach but also by reducing a subject’s appetite [15]. SG is supposed to modulate meal-related hormones of the gut-brain axis such as Ghrelin (signal ¼ hunger) or PYY (slows gastric emptying) or GLP-1 (decreases food intake) [16]. In the present study, the authors found that rats following SG had a decreased food intake and, in parallel, a reduction of weight (Fig. 2BþC). At 14 days postoperatively, acyl Ghrelin levels were lower in SG (Fig. 2A) while PYY was increased. So, in this experimental setting, SG modulated food intake and specific gut hormones. Of course, diminished food intake leading to a loss of fat mass intervenes with UA metabolism [17]. At 14 days postoperatively, UA levels were decreased (Fig. 3AþB). One explanation might be that SG-mediated food restriction provoked catabolism leading to down-regulation of weight and reduction of fat mass. Katabolic metabolism following weight loss is correlated to ketosis [18]. Ketosis impairs the
ability of the kidney to reabsorb UA, which would lead to enhanced UA secretion [17]. Fig. 4 summarizes the authors’ findings and effects of SG on food intake, gut-brain hormones, adipose tissues, and uric acids. Another factor influencing UA metabolism could be the association of gut manipulation and XOD activity. Recently, Garcia described that local hyperoxia during colonic surgery is associated with a reduction of XOD activity and oxidative stress in the colonic mucosa [19]. However, in the present study, the colon of HFD rats had not been manipulated during SG. In contrast to Garcia, the authors found a local up-regulation of XOD abundance and activity in colonic samples of SG animals (Fig. 3E), which did not translate into a systemic increase of sUA and UA levels in SG rats. Finally, other major metabolic pathways such as glucose homeostasis could interfere with the purine pathway and the authors’ postoperative findings. Especially hyperglycemia can induce hyperuricemia as a consequence of elevated serum insulin levels, which have been shown to stimulate renal reabsorption of UA [20]. Consequently, the authors investigated the glucose homeostasis following SG in their HFD rats. As a result neither fasting nor oGGT-related glucose and insulin levels were altered at 14 days after SG.
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
Fig. 4. Hypothetical mechanism of SG: SG modulates hormones of the gutbrain axis responsible for appetite regulation, which in turn leads to diminished food intake. Besides the effects of weight and especially adipose tissue on serum uric acid, diminished food intake directly affects serum uric acid. White open arrows indicate HFD regulation; black arrows indicate regulation of HFD rats 14 days after SG.
Thus, glucose impairment did not influence the significant down-regulation of sUA and uUA levels the authors found after SG. In conclusion, the authors report for the first time that fat mass reduction following SG in a HFD model resulted primarily in UA reduction via down-regulation of XOD activity. Therefore, therapies such as diets, exercise, or weight loss surgery, focusing on reduction of adipose tissue, may have beneficial effects on systemic UA metabolism. Moreover, the authors outlined that SG is a potent therapy to reduce cardiovascular risk factors such as adipose tissuerelated hyperuricemia. Disclosures The authors declare no conflicts of interests. Appendix Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.soard.2013.12.010. References [1] Fang J, Alderman MH. Serum uric acid and cardiovascular mortality the NHANES I epidemiologic follow-up study, 1971-1992. National Health and Nutrition Examination Survey. JAMA 2000;283:2404–10.
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[2] Li C, Hsieh MC, Chang SJ. Metabolic syndrome, diabetes, and hyperuricemia. Curr Opin Rheumatol 2013;25:210–6. [3] Lin ZJ, Zhang B, Liu XQ, Yang HL. Abdominal fat accumulation with hyperuricemia and hypercholesterolemia quail model induced by high fat diet. Chin Med Sci J 2009;24:191–4. [4] Hare JM, Johnson RJ. Uric acid predicts clinical outcomes in heart failure: insights regarding the role of xanthine oxidase and uric acid in disease pathophysiology. Circulation 2003;107:1951–3. [5] Chambers AP, Stefater MA, Wilson-Perez HE, et al. Similar effects of roux-en-Y gastric bypass and vertical sleeve gastrectomy on glucose regulation in rats. Physiol Behav 2011;105:120–3. [6] Ribot J, Rodriguez AM, Rodriguez E, Palou A. Adiponectin and resistin response in the onset of obesity in male and female rats. Obesity (Silver Spring) 2008;16:723–30. [7] Cacho J, Sevillano J, de Castro J, Herrera E, Ramos MP. Validation of simple indexes to assess insulin sensitivity during pregnancy in Wistar and Sprague-Dawley rats. Am J Physiol Endocrinol Metab 2008;295:E1269–76. [8] Kaplan SA, Te AE, Blaivas JG. Urodynamic findings in patients with diabetic cystopathy. J Urol 1995;153:342–4. [9] Oberbach A, Neuhaus J, Inge TH, et al. Bariatric surgery in severly obese adolescents improves major comorbidities including hyperuricemia. Metabolism 2013, http://dx.doi.org/10.1016/j.metabol. 2013.11.012. [10] Ford ES, Li C, Cook S, Choi HK. Serum concentrations of uric acid and the metabolic syndrome among US children and adolescents. Circulation 2007;115:2526–32. [11] Takahashi S, Yamamoto T, Tsutsumi Z, Moriwaki Y, Yamakita J, Higashino K. Close correlation between visceral fat accumulation and uric acid metabolism in healthy men. Metabolism 1997;46:1162–5. [12] Sun K, Kusminski CM, Scherer PE. Adipose tissue remodeling and obesity. J Clin Invest 2011;121:2094–101. [13] Chuang HH, Li WC, Sheu BF, et al. Correlation between body composition and risk factors for cardiovascular disease and metabolic syndrome. Biofactors 2012;38:284–91. [14] Stefater MA, Perez-Tilve D, Chambers AP, et al. Sleeve gastrectomy induces loss of weight and fat mass in obese rats, but does not affect leptin sensitivity. Gastroenterology 2010;138:2426–36, (2436.e1-3). [15] Dimitriadis E, Daskalakis M, Kampa M, Peppe A, Papadakis JA, Melissas J. Alterations in gut hormones after laparoscopic sleeve gastrectomy: a prospective clinical and laboratory investigational study. Ann Surg 2013;257:647–54. [16] Basso N, Capoccia D, Rizzello M, et al. First-phase insulin secretion, insulin sensitivity, ghrelin, GLP-1, and PYY changes 72 h after sleeve gastrectomy in obese diabetic patients: the gastric hypothesis. Surg Endosc 2011;25:3540–50. [17] Forster H. [Possibilities for weight reduction by means of diet]. Fortschr Med 1979;97:1339–44. [18] Sumithran P, Proietto J. Safe year-long use of a very-low-calorie diet for the treatment of severe obesity. Med J Aust 2008;188:366–8. [19] Garcia-de-la-Asuncion J, Barber G, Rus D, et al. Hyperoxia during colon surgery is associated with a reduction of xanthine oxidase activity and oxidative stress in colonic mucosa. Redox Rep 2011;16: 121–8. [20] Facchini F, Chen YD, Hollenbeck CB, Reaven GM. Relationship between resistance to insulin-mediated glucose uptake, urinary uric acid clearance, and plasma uric acid concentration. JAMA 1991;266: 3008–11.
Original article
Sleeve gastrectomy reduces xanthine oxidase and uric acid in a rat model of morbid obesity Andreas Oberbach, M.D., Ph.D., M.P.H.a,b, Jochen Neuhaus, Ph.D.c, Nadine Schlichting, Ph.D.d, Joachim Kugler, M.D., Ph.D.b, Sven Baumann, Ph.D.e, Holger Till, M.D., Ph.D.f,* a Department of Cardiac Surgery, University of Leipzig, Heart Center, Leipzig, Germany Department of Health Sciences / Public Health, University of Dresden, Dresden, Germany c Department of Urology, University of Leipzig, Leipzig, Germany d Integrated Research and Treatment Center (IFB) Adiposity Diseases, University of Leipzig, Leipzig, Germany e Department of Metabolomics, Helmholtz Centre for Environmental Research, Leipzig, Germany f Department of Pediatrics and Adolescent Surgery, Medical University of Graz, Graz, Austria b
Abstract
Background: Serum uric acid (sUA) plays a major role in the development of morbidities associated with obesity, especially cardiovascular diseases. Within the purine pathway, xanthine oxidase (XOD) represents the key enzyme. The aim of this study was to investigate the dynamics of sUA and XOD following sleeve gastrectomy (SG) in a rat model of high-fat-diet (HFD) induced obesity. Patients: Over a period of 11 weeks, 30 rats received a HFD, and 10 rats received a low fat diet (LFD). Thereafter, 10 randomly selected HFD rats and 10 LFD rats were sacrificed. The remaining 20 HFD rats were randomly assigned to either SG or sham operation (SH) and studied 14 days postoperatively. Methods: The white adipose tissues (WAT) from visceral (intestinal and retroperitoneal) and inguinal (subcutaneous) depots were collected. sUA and urine UA (uUA) were measured by high performance liquid chromatography-mass spectrometry (HPLC-MS/MS). Abundance and activity of XOD was investigated in the liver, colon, adipose tissue, and skeletal muscle by enzyme-linked immunosorbent assay (ELISA). Results: HFD led to significant weight gain, elevated sUA levels, increased WAT and increase of XOD activity. Fourteen days postoperatively, SG rats showed a significant decrease of weight and adipose tissue, improved glucose metabolism, and changes of gut hormones. The sUA and uUA levels were significantly decreased following SG. Furthermore, XOD activity was significantly down-regulated in WAT. Conclusion: HFD induces elevated sUA levels by gain of WAT and increase of XOD activity. Following SG, the reduction of WAT as the major source of XOD and the lowering of XOD activity are the basis for the decrease of sUA. (Surg Obes Relat Dis 2014;]:00–00.) r 2014 American Society for Metabolic and Bariatric Surgery. All rights reserved.
Keywords:
Rat; Obesity; High fat diet; Sleeve gastrectomy; Uric acid; Xanthine oxidase
This work was supported by the Federal Ministry of Education and Research (BMBF), Germany (Integrated Research and Treatment Center IFB “Adiposity Diseases”). * Correspondence: Holger Till, M.D., Ph.D, Department of Pediatric and Adolescent Surgery, Medical University of Graz, Auenbruggerplatz 34, A-8036 Graz, Austria. E-mail: [email protected]
In patients with morbid obesity, elevated serum uric acid (sUA) levels are associated with an increased mortality [1]. Hyperuricemia plays a major role in the development of hypertension, insulin resistance (IR), and renal and cardiovascular diseases (CVD) [2]. Additionally, high fat diet (HFD) induces abdominal fat accumulation associated with hyperuricemia [3] while weight loss reverses both [2]. Within the purine pathway xanthine oxidase (XOD)
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represents the key enzyme leading to uric acid (UA) production [4]. To date it remains unclear, whether XOD activity and sUA production are affected by bariatric surgery such as sleeve gastrectomy (SG). The present study in rats tested the hypotheses that (1) HFD has an effect on sUA, XOD, glucose homeostasis, and gut hormones and (2) that SG influences these parameters in HFD-treated rats compared to sham operation (SH). Methods Experimental setting This study was approved by the local Council of Animal Research (Landesdirektion Leipzig, TVV 32/11 and TVV 32/12). Forty male Sprague-Dawley rats (Medical Experimental Center, University of Leipzig) were subjected to different dietary regimens for 11 weeks. Thirty animals received HFD (fat content 45% of energy, based on Atwater-calculation, ssniff-Spezialdiaeten GmbH, Soest, Germany). Ten animals received a low-fat diet (LFD, fat content 18% of energy, based on Atwater-calculation; ssniff-Spezialdiaeten GmbH, Soest, Germany). All animals were kept on a 12:12-h light–darkness cycle. Weight gain and food intake were monitored once a week, and metabolic cages were used to collect individual urine samples for 24 h. After 11 weeks of dietary treatment, 10 randomly selected HFD rats and 10 LFD rats were sacrificed to be studied preoperatively. The remaining 20 HFD rats were randomized to receive either SG or sham operation (SH). Surgical interventions Before SG and SH, all animals were fasted for 24 hours with free access to water. SG in rats was performed according to Chambers et al. [5]. SH consisted of an equivalent laparotomy only. Postoperatively, SG and SH animals were kept on a HFD for 14 days. In the SG group 2 animals died following SG before reaching the primary endpoint at 14 days postoperatively. Then, all animals were sacrificed for tissue and blood collection. Tissue and blood sample collection Arterial blood was drawn from the heart into ethylenediaminetetraacetic acid-coated vials. After centrifugation (10 min 2500 xg at 41C) plasma was stored at –801C until analysis. Tissue samples from skeletal muscle (M. quadriceps femoris), liver, colon, adipose tissue (visceral and subcutaneous), and feces were collected, snap-frozen in liquid nitrogen, and stored at −801C until use. Adipose tissues from abdominal and inguinal regions (subcutaneous fat) and visceral fat from the intestine and from the retroperitoneal region were collected (visceral fat) as described elsewhere [6]. Plasma measurements. After 11 weeks on HFD, fasting glucose was analyzed during oral glucose tolerance test (oGTT). This test was performed after an overnight fast
(16 hr) by oral application of 2 g/kg weight 20% glucose solution between 7:00 a.m. and 9:00 a.m. Blood glucose levels were measured from tail tip capillary region at 0 (baseline), 15, 30, 45, 60, and 120 minutes after injection using a hand-held glucometer (GlucoCheckExcellent, aktivmed GmbH, Augsburg, Germany). Unless stated otherwise, all reagents were purchased from Sigma or Merck at the highest purity grade available. Fasting plasma insulin was determined using immunoassay kit (Rat Insulin ELISA Kit, Alpco Diagnostics, Salem, NH). Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated by multiplying fasting plasma glucose (mg/dL) by fasting plasma insulin (mU/ mL) divided by 2.430 according to the method described by Cacho et al. [7]. Plasma levels of total cholesterol, HDLcholesterol, triglycerides, interleukine-6 and C-reactive protein were measured using commercial enzymatic assay kits (Total Cholesterol Assay Kit, Cell Biolabs Inc., San Diego, CA; HDL Cholesterol Assay Kit, Cell Biolabs Inc.; Serum Triglyceride Quantification Kit, Cell Biolabs Inc., San Diego, CA; Rat IL-6 Quantikine ELISA Kit, R&D Systems, Minneapolis, MN; AssayMax Rat C-reactive protein ELISA Kit, Assaypro, St. Charles, MO). LDL cholesterol was calculated by Friedewald's formula: LDL ¼ TC HDL TG/2.2 (mmol/L) [8]. Creatinine clearance (mL/min) was calculated using the standard formula U*V/P, while U is the urinary creatinine concentration (mg/dL), V is the 24 h urine volume (mL/ min) and P represents the serum creatinine concentration (mg/dL). Plasma and urinary creatinine were determined by Creatinine (serum) Assay Kit and Creatinine (urinary) Assay Kit (Cayman Chemicals, Ann Arbor, MI). Gut hormones. Levels of total protein of the orexigenic gut hormones Ghrelin, glucagon-like peptide 1 (GLP-1), peptide tyrosine (PYY), and acylGhrelin and active GLP-1 were measured by ELISA at 0, 30, 60, 120, and 180 minutes. Quantification of Serum UA. UA sample preparation was based on a modified method by Kim et al. [8,9]. Briefly, 10 ml serum was diluted 10-fold with water including [1,3-15 N2]-UA (Euriso-top, Saarbrücken, Germany) (final concentration 50 mM) and treated with 20 ml 10% trichloroacetic acid TCA (w/v). The samples were vortexed for 1 minute and centrifuged at 15,000 xg for 2 minutes. Supernatants were loaded into autosampler vials and analyzed by LC-MS/MS. All analyses were carried out on an Agilent 1100 series binary high performance liquid chromatography (HPLC) system (Agilent Technologies, Waldbronn, Germany) coupled to a 4,000 QTrap mass spectrometer (AB Sciex, Concord, Canada) equipped with TurboIon spray source. Liquid chromatography was performed in gradient elution mode using Chromolith Flash RP-18 e (25 4.6 mm, Merck, Darmstadt, Germany) at 401C with eluent A (0.1% formic acid in water) and eluent B (0.1% formic acid in acetonitrile) at a flow rate of 400 ml/min. The injection volume of each sample was 5 ml. For sUA quantification MS was operated in negative ion and multiple reaction
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
monitoring mode. Data were acquired and analyzed using Analyst software version 1.4.2 (AB Sciex). Tissue measurements. Xanthine oxidase protein expression and enzymatic activity were analyzed in plasma and tissue homogenates (M. quadriceps femoris, liver, colon, visceral fat, subcutaneous fat) using a rat XOD ELISA Kit (Cusabio Biotech, Wuhan, China) and XOD activity assay Kit (Cayman Chemicals). Total protein amount of plasma and tissue homogenates was determined by BCA Protein Assay Kit (Thermo Fisher Scientific, IL) using BSA as standard. Total free fatty acids (FFA) in feces and serum were quantified by a commercial Free Fatty Acid Quantification Kit (BioVision, CA) according to the manufacturer’s instructions. 20 mg feces were homogenized in assay buffer and spun 5 minutes at top speed in a microcentrifuge. For quantification, 10 ml serum and 20 ml of feces supernatant were used. Statistical analysis. Data analysis was performed using Prism v5.0 (GraphPad Software, La Jolla, CA). Statistical differences were calculated by independent t test statistics. For correlation analysis Pearson r and linear regression model was calculated. A P value r 0.05 was considered statistically significant. All data are presented as means ⫾ SD.
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(Fig. S1 A-B) compared to LFD. The authors found significant changes in fasting glucose metabolism and lipid metabolism, except for HDL- and LDL-cholesterol, which was elevated in HFD, but did not reach a significanct level (Table 1). Additionally, protein levels of inflammatory markers like IL-6 and C-reactive protein were significantly higher in HFD (Table 1). Gut-related hormones were not influenced by HFD when measured under fasting conditions (Table 1). Furthermore, HFD animals showed increased abundance of sUA, while uUA was unchanged (Fig. 1A-B). XOD abundance and activity were both significantly elevated following HFD (Fig. 1C-D). Tissue-based analysis of XOD revealed higher levels in subcutaneous adipose tissue and in skeletal muscle (SkM) after HFD, while lower XOD levels were measured in visceral fat. Liver and colon XOD levels were not significantly altered (Fig. 1E). In parallel, HFD increased XOD activity in subcutaneous fat and SkM while visceral fat, liver, and colon showed no significant alterations (Fig. 1F). Effects of SG versus SH on HFD rats
Results Effects of HFD versus LFD (before surgery) Eleven weeks of HFD resulted in significant increases of weight (Table 1) and visceral and subcutaneous fat mass
Fourteen days postoperatively, SG rats showed lower weight (Table 1) while still being fed HFD. Moreover, a significant decrease of visceral and subcutaneous adipose tissue compared to SH (Fig. S1) was documented. Both
Table 1 Clinical characteristics of the rats phenotype LFD (n ¼ 10) Age [weeks] Time of follow up [days] Weight [g] Gastric hormones Ghrelin (total) [ng/mL] GLP-1 (total) [pmol/L] PYY [ng/mL] Glucose metabolism (fasting) Glucose [mmol/L] Insulin [ng/mL] HOMA-IR Glucose 2 hoGTT [AUC] Lipid metabolism Triglyceride [mmol/L] Total-cholesterol [mmol/L] HDL-cholesterol [mmol/L] LDL-cholesterol [mmol/L] Inflammatory markers and renal function Interleukin-6 [pg/mL] C-reactive protein [mg/mL] Urin volume [mL/d] Creatinine clearance [mL/min]
HFD (n ¼ 10)
SH (n ¼ 10)
SG (n ¼ 8)
15 0 430 ⫾ 25.6
533 ⫾ 23.9*
17 14 490 ⫾ 70
431 ⫾ 33*
3.53 ⫾ .69 17.78 ⫾ 6.55 0.47 ⫾ .05
3.1 ⫾ 1.23 18.23 ⫾ 4.4 0.43 ⫾ .07
3.88 ⫾ .51 20.59 ⫾ 1.55 0.42 ⫾ .08
2.15 ⫾ .23* 30.74 ⫾ 6.66* 1.12 ⫾ .25*
5.48 ⫾ .63 1.25 ⫾ .14 1.47 ⫾ .3 63.8 ⫾ 4.54
6.30 ⫾ .91* 2.08 ⫾ .78* 2.88 ⫾ 1.4* 71.6 ⫾ 5.64*
6,4 ⫾ 0,66 0,85 ⫾ 0,16 1,17 ⫾ 0,26 72.6 ⫾ 2.67
6,2 ⫾ 0,8 0,8 ⫾ 0,19 1,1 ⫾ 0,39 71.6 ⫾ 2.75
0.69 ⫾ .49 2.79 ⫾ 1.03 0.97 ⫾ .25 1.41 ⫾ .83
1.3 ⫾ .75* 3.88 ⫾ .86* 1.29 ⫾ .44 2 ⫾ .51
0,47 ⫾ 0,24 4.81 ⫾ .37 0.79 ⫾ .18 1.73 ⫾ .26
0,36 ⫾ 0,18 3.33 ⫾ .48* 1.32 ⫾ .24* 0.71 ⫾ .21*
223 ⫾ 40.2 378 ⫾ 63 10.5 ⫾ 3.51 0.86 ⫾ .17
269 ⫾ 32.3* 522.3 ⫾ 30* 6.37 ⫾ 1.77* 0.8 ⫾ .27
288.2 ⫾ 52.2 379 ⫾ 23 7.15 ⫾ 2.71 0.51 ⫾ .15
225.1 ⫾ 46.9 366 ⫾ 42 7.3 ⫾ 3.24 0.54 ⫾ .38
Clinical characteristics of the study population phenotype. Clinical characteristics of the animal groups after 15 weeks and after intervention. Values are means ⫾ SD of different groups. A P value o 0.05 was taken to indicate statistical significance. GLP-1 ¼ glucagon-like peptide 1; HFD ¼ high fat diet; HOMA-IR ¼ homeostatic model assessment of insulin resistance; LFD ¼ low fat diet; PYY ¼ peptide tyrosine; SG ¼ sleeve gastrectomy; SH ¼ sham operation.
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Fig. 1. Levels of uric acid and xanthine oxidase (XOD) in different tissues of high fat diet (HFD) rats. HFD rats showed significantly increased levels of uric acid in serum (sUA; A) but not in urine (uUA; B) compared to low fat diet (LFD). Analysis of serum xanthine oxidase showed increased XOD abundance (C) and XOD activity (D) in HFD. (E) XOD abundance was unchanged in liver and colonic samples but increased in subcutaneous fat and skeletal muscle (SkM) in HFD while XOD was decreased in visceral fat compared to LFD. (F) XOD activity in these tissues was increased in subcutaneous adipose tissue and in SkM but unchanged in liver, colon, and visceral fat comparing HFD to LFD.
locations of adipose tissues, visceral and retroperitoneal were affected (Fig. S1 A;C). Quantification of FFA revealed significant diminished FFA levels in serum and increased FFA excretion via feces (Fig. S1E,F).
significant level (Fig. 2A). The postoperative food intake in SG rats was significantly lower compared to SH (Fig. 2B) and in parallel, SG rats showed a reduction of weight in 14 days follow-up observation (Fig. 2C).
Changes in gut hormones and long term metabolic afferent signals
Regulation of UA and XOD following SG
While fasting serum ghrelin was significantly decreased in SG, levels of fasting GLP-1 and PYY were significantly elevated (Table 1). Interestingly, parameters of glucose metabolism were unaffected, and parameters of lipid metabolism were significantly altered in SG rats, except for triglycerides. Renal function and inflammatory markers were not different between SG and SH after 14 days (Table 1). To investigate meal-related changes of gastric hormones, a 2-hour oGTT was performed, and the acylGhrelin, active GLP-1 and PYY were measured. At SG, rats revealed an altered expression of gastric hormones compared to SH (Fig. 2). Following glucose stimulation, SG rats showed lower acylGhrelin levels while PYY level was increased. The observed decrease of active GLP-1 did not reach a
sUA and uUA concentrations were significantly decreased following SG compared to SH rats (Fig. 3A-B). In serum, the abundance of XOD was increased in SG rats (Fig. 3 C), while XOD activity was unchanged (Fig. 3D). The measurement of XOD abundance revealed a significant up-regulation in colonic samples compared to adipose tissue, liver, and skeletal muscle (P o 0.01) (Fig. 3E). XOD activity was significantly increased in colonic samples while both subcutaneous and visceral adipose tissue showed decreased XOD activity in SG rats (Fig. 3F). Furthermore, the authors found a significant positive correlation between sUA and subcutaneous (P o 0.001; r2 ¼ 0.39) and visceral (P o 0.001; r2 ¼ 0.44) adipose tissue and between subcutaneous adipose tissue and XOD activity (P o 0.001; r2 ¼ 0.28; Fig. S2A-B;D).
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
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Fig. 2. Weight, food intake, and gut hormones after sleeve gastrectomy (SG) versus sham operation (SH) (A). Dynamics of the serum levels of gastric hormones acylGhrelin, active glucagon-like peptide 1 (GLP-1) and peptide tyrosine (PYY) after oral glucose tolerance test (oGTT) in SG rats (white circles) compared to SH (black circles). Values are represented as means ⫾ SD. (B) Increase in daily food intake of SG operated rats compared to SH group. (C) Trend in weight of SG and SH. Body weight was monitored between 11 and 14 weeks of age only, because oGTT performed at 15 weeks of age required 24 hours fasting which interfered with normal weight gain.
Discussion HFD-induced adipose tissue as a major source of UA metabolism UA represents the end product of the purine nucleotide metabolism and has been associated with obesity and related co-morbidities [10]. Previous studies in humans showed that fat mass correlated positively with serum UA concentrations [11]. The authors’ present study in rats confirmed that adipose tissue represents the location with the most XOD activity compared to liver, colon, or skeletal muscle (Fig. 1E and 1F). HFD increased adipose tissue, which showed a significant increase of XOD activity. Interestingly, LFD revealed higher XOD abundance in visceral adipose tissue than HFD (Fig. 1E). It is well known that following HFD, adipocytes can react with hypertrophy and hyperplasia, which might influence the relative distribution of a protein of interest [12]. XOD
activity and visceral and subcutaneous fat XOD activity were significantly increased (Fig. 1F). As XOD is known to be the key enzyme in UA metabolism, increased XOD activity leads to increased sUA levels and this is in line with a wide range of clinical studies correlating fat mass with sUA [13].
Effects of SG on UA metabolism Metabolic surgery such as SG is a potent procedure to reduce fat mass [14]. To investigate the surgical effects on UA metabolism, the authors performed SG on HFD rats and continued HFD postoperatively. They observed a significant decrease in fat mass at 14 days postoperatively (Fig. S1 CþD). In parallel, XOD activity was significantly decreased in subcutaneous and visceral adipose tissues (Fig. 3F). Those results suggest that nucleotide metabolism is down-
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Fig. 3. Levels of uric acid (UA) and XOD in various tissues after SG. SG rats showed significantly decreased levels of uric acid in serum (sUA; A) and in urine (uUA; B) compared to SH rats. Analysis of XOD showed increased XOD abundance (C) in SG rats while XOD activity (D) was unchanged. (E) XOD abundance in liver, adipose tissue, skeletal muscle on protein level revealed no alteration between SG and SH, while SG rats showed higher XOD abundance in colonic samples. The adipose tissue showed the highest XOD abundance in both groups (F). XOD activity was decreased in adipose tissue, increased in colon but unchanged in liver and muscle comparing SG to SH.
regulated as a consequence of undernourishment of adipose cells. However, the question remains as to which factors caused a decrease of sUA following SG. Recent evidence suggests that SG leads to a decreased food intake not only by mechanical restriction of the stomach but also by reducing a subject’s appetite [15]. SG is supposed to modulate meal-related hormones of the gut-brain axis such as Ghrelin (signal ¼ hunger) or PYY (slows gastric emptying) or GLP-1 (decreases food intake) [16]. In the present study, the authors found that rats following SG had a decreased food intake and, in parallel, a reduction of weight (Fig. 2BþC). At 14 days postoperatively, acyl Ghrelin levels were lower in SG (Fig. 2A) while PYY was increased. So, in this experimental setting, SG modulated food intake and specific gut hormones. Of course, diminished food intake leading to a loss of fat mass intervenes with UA metabolism [17]. At 14 days postoperatively, UA levels were decreased (Fig. 3AþB). One explanation might be that SG-mediated food restriction provoked catabolism leading to down-regulation of weight and reduction of fat mass. Katabolic metabolism following weight loss is correlated to ketosis [18]. Ketosis impairs the
ability of the kidney to reabsorb UA, which would lead to enhanced UA secretion [17]. Fig. 4 summarizes the authors’ findings and effects of SG on food intake, gut-brain hormones, adipose tissues, and uric acids. Another factor influencing UA metabolism could be the association of gut manipulation and XOD activity. Recently, Garcia described that local hyperoxia during colonic surgery is associated with a reduction of XOD activity and oxidative stress in the colonic mucosa [19]. However, in the present study, the colon of HFD rats had not been manipulated during SG. In contrast to Garcia, the authors found a local up-regulation of XOD abundance and activity in colonic samples of SG animals (Fig. 3E), which did not translate into a systemic increase of sUA and UA levels in SG rats. Finally, other major metabolic pathways such as glucose homeostasis could interfere with the purine pathway and the authors’ postoperative findings. Especially hyperglycemia can induce hyperuricemia as a consequence of elevated serum insulin levels, which have been shown to stimulate renal reabsorption of UA [20]. Consequently, the authors investigated the glucose homeostasis following SG in their HFD rats. As a result neither fasting nor oGGT-related glucose and insulin levels were altered at 14 days after SG.
SG Reduces Uric Acid in Obese Rats / Surgery for Obesity and Related Diseases ] (2014) 00–00
Fig. 4. Hypothetical mechanism of SG: SG modulates hormones of the gutbrain axis responsible for appetite regulation, which in turn leads to diminished food intake. Besides the effects of weight and especially adipose tissue on serum uric acid, diminished food intake directly affects serum uric acid. White open arrows indicate HFD regulation; black arrows indicate regulation of HFD rats 14 days after SG.
Thus, glucose impairment did not influence the significant down-regulation of sUA and uUA levels the authors found after SG. In conclusion, the authors report for the first time that fat mass reduction following SG in a HFD model resulted primarily in UA reduction via down-regulation of XOD activity. Therefore, therapies such as diets, exercise, or weight loss surgery, focusing on reduction of adipose tissue, may have beneficial effects on systemic UA metabolism. Moreover, the authors outlined that SG is a potent therapy to reduce cardiovascular risk factors such as adipose tissuerelated hyperuricemia. Disclosures The authors declare no conflicts of interests. Appendix Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.soard.2013.12.010. References [1] Fang J, Alderman MH. Serum uric acid and cardiovascular mortality the NHANES I epidemiologic follow-up study, 1971-1992. National Health and Nutrition Examination Survey. JAMA 2000;283:2404–10.
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