EDITORIAL COMMENT URRENT C OPINION
Atherosclerosis: cell biology and lipoproteins Harry Bjo¨rkbacka
Although low-circulating HDL-C levels have been shown over and over to be associated with increased cardiovascular risk, a detailed understanding of which molecular functions associated with the HDL particle that explain the risk association is lacking [1 ,2 ]. Several recent reports shed light on the anti-inflammatory functions of HDL and the role of dysfunctional HDL in cardiovascular disease risk prediction. As low-circulating HDL-C levels are associated with increased cardiovascular risk, a goal has been to pharmacologically increase HDL-C levels. Such efforts have, however, not been fruitful so far, even if many drugs that increase HDL-C, including fibrates, niacin or inhibitors of cholesteryl ester transfer protein, have been tested [2 ]. The raising of HDL-C levels as a pharmacological target has also been put in question by Mendelian randomization studies indicating that genetic variants that raise HDL-C levels do not lower the risk of myocardial infarction [3,4]. Instead, there has been an increased attention to HDL function, fueled by studies showing that cholesterol efflux function is a better predictor of risk than HDL levels alone [5]. Functions associated with the HDL particle, in addition to mediating the removal of cholesterol from foam cell macrophages in atherosclerotic lesions, include antioxidant, antithrombotic, and anti-inflammatory properties, which may all contribute to the reduced cardiovascular risk associated with HDL. The anti-inflammatory properties of HDL from a molecular standpoint have been poorly understood. Recently, however, De Nardo et al. [6 ] show that HDL can suppress Toll-like receptor-stimulated expression of several proinflammatory cytokines on the transcriptional level. The authors identify the transcription regulator ATF3 (activating transcription factor 3; cyclic AMP-dependent transcription factor) as a HDL-inducible target that directly regulates several key proinflammatory genes. The ability of HDL to modulate Toll-like receptormediated responses raises the possibility that HDL could be useful in treating inflammatory disorders not directly linked to cardiovascular disease. The anti-inflammatory properties of HDL can be impaired during inflammatory conditions, such as during an acute phase response, and the HDL may &&
&&
&&
&&
become dysfunctional or even gain proinflammatory properties [7–9]. Recently, an impaired antiinflammatory capacity of HDL, determined as the ability to suppress tumor necrosis factor-a-induced vascular cell adhesion molecule-1 (VCAM-1) mRNA expression in endothelial cells in vitro, was shown to be predictive for future secondary major adverse cardiovascular events [10 ]. Both circulating HDL and apoA1 recovered from human atherosclerotic lesions show an association between oxidative modifications of apoA1 and an impaired cholesterol efflux function, as well as acquisition of proinflammatory activity [8,11]. The oxidative modifications of apoA1 leading to dysfunctional HDL are in part formed by myeloperoxidase (MPO)-derived oxidants [8,11]. Consistent with this, a recent study also indicates that the MPO to paraoxonase 1 (PON1) ratio in serum could be a useful indicator of the functional properties of HDL [12 ]. Another aspect to consider when comparing measurements of HDL-C versus the evaluation of functional properties of HDL is the fact that apoA1 protein found in atherosclerotic lesion or normal aortic tissue has been found to be lipid-poor, highly cross-linked, and displaying impaired cholesterol efflux activity to a much greater extent than apoA1 found in plasma [13]. Recently, however, two studies showed that circulating apoA1 modified by the MPO–hydrogen peroxide–chloride system could still be useful for monitoring cardiovascular disease status. Huang et al. [14 ] show that only a fraction (0.007%) of apoA1 in plasma harbored an oxidized tryptophan 72 moiety that was relatively common (20%) in apoA1 isolated from atherosclerotic arteries. Still, the presence of this specific apoA1 modification was found to be higher in individuals that had clinical evidence of cardiovascular &
&
&&
Department of Clinical Sciences, Ska˚ne University Hospital, Lund University, Malmo¨, Sweden Correspondence to Harry Bjo¨rkbacka, Experimental Cardiovascular Research, CRC Lund University, Building 91 : 12, Jan Waldenstro¨ms gata 35, Malmo¨ University Hospital, SE-205 02 Malmo¨, Sweden. Tel: +46 40 391205; fax: +46 40 391212; e-mail: harry.bjorkbacka@ med.lu.se Curr Opin Lipidol 2014, 25:319–320 DOI:10.1097/MOL.0000000000000101
0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Bimonthly update Atherosclerosis: cell biology and lipoproteins &&
disease. In line with these results, Shao et al. [15 ] recently reported that the plasma levels of apoA1 with chlorinated tyrosine 192 and oxidized methionine 148 measured by mass spectrometry were higher in individuals with stable coronary artery disease or acute coronary syndrome compared to control individuals. Furthermore, the specific apoA1 modifications described in both studies were associated with an impaired ABCA1-mediated cholesterol efflux capacity, independently of HDL-C levels. Thus, although the usefulness of measuring specific modifications of apoA1 as a marker of dysfunctional HDL and its value in risk prediction will have to be further evaluated, the findings are promising in that circulating levels of dysfunctional apoA1 seem to reflect the pathophysiological process within the vascular wall. In conclusion, although HDL-C may still be useful in risk prediction, detailed understanding of the functional properties of HDL now shows great promise to provide even better cardiovascular disease prediction and stratification. It remains to be seen if these insights into HDL function can be exploited as therapeutic targets. Acknowledgements None. Conflicts of interest There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Feig JE, Hewing B, Smith JD, et al. High-density lipoprotein and atherosclerosis regression: evidence from preclinical and clinical studies. Circ Res 2014; 114:205–213. Authoritative review that discusses the importance of measuring HDL function in addition to plasma levels of HDL cholesterol. It emphasizes the evidence that support atheroprotection by HDL if the number of functional particles is increased. 2. Lu¨scher TF, Landmesser U, von Eckardstein A, et al. High-density lipoprotein: && vascular protective effects, dysfunction, and potential as therapeutic target. Circ Res 2014; 114:171–182. Authoritative review that discusses HDL as a therapeutic target, taking into account recent finding that HDL may become dysfunctional in patients with cardiovascular disease due to oxidative modifications and subsequent alterations in the proteome composition of HDL. 3. Voight BF, Peloso GM, Orho-Melander M, et al. Plasma HDL cholesterol and risk of myocardial infarction: a mendelian randomisation study. Lancet 2012; 380:572–580. &&
320
www.co-lipidology.com
4. Haase CL, Tybjærg-Hansen A, Qayyum AA, et al. LCAT, HDL cholesterol and ischemic cardiovascular disease: a Mendelian randomization study of HDL cholesterol in 54,500 individuals. J Clin Endocrinol Metab 2012; 97:E248– E256. 5. Khera AV, Cuchel M, de la Llera-Moya M, et al. Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis. N Engl J Med 2011; 364:127–135. 6. De Nardo D, Labzin LI, Kono H, et al. High-density lipoprotein mediates anti&& inflammatory reprogramming of macrophages via the transcriptional regulator ATF3. Nat Immunol 2014; 15:152–160. Shows that HDL suppresses the ability of Toll-like receptors to induce the expression of pro-inflammatory cytokines on the transcriptional level following stimulation. Identify ATF3 as an HDL-inducible negative regulator of several key pro-inflammatory genes that are directly targeted by ATF3 after its induction by HDL. The findings raise the possibility to use HDL-based therapeutics in treating inflammatory disorders not directly linked to hypercholesterolemia. 7. Van Lenten BJ, Hama SY, de Beer FC, et al. Anti-inflammatory HDL becomes pro-inflammatory during the acute phase response. Loss of protective effect of HDL against LDL oxidation in aortic wall cell cocultures. J Clin Invest 1995; 96:2758–2767. 8. Undurti A, Huang Y, Lupica Ja, et al. Modification of high density lipoprotein by myeloperoxidase generates a pro-inflammatory particle. J Biol Chem 2009; 284:30825–30835. 9. Besler C, Heinrich K, Rohrer L, et al. Mechanisms underlying adverse effects of HDL on eNOS-activating pathways in patients with coronary artery disease. J Clin Invest 2011; 121:2693–2708. 10. Dullaart RPF, Annema W, Tio Ra, et al. The HDL anti-inflammatory function is & impaired in myocardial infarction and may predict new cardiac events independent of HDL cholesterol. Clin Chim Acta 2014; 433C:34–38. The first study that investigates the predictive value of measuring HDL function for the future development of new major adverse cardiovascular events. 11. Zheng L, Nukuna B, Brennan M, et al. Apolipoprotein A-I is a selective target for myeloperoxidase-catalyzed oxidation and functional impairment in subjects with cardiovascular disease. J Clin Invest 2004; 114:529–541. 12. Haraguchi Y, Toh R, Hasokawa M, et al. Serum myeloperoxidase/paraoxonase 1 & ratio as potential indicator of dysfunctional high-density lipoprotein and risk stratification in coronary artery disease. Atherosclerosis 2014; 234:288–294. Shows that HDL isolated from patients with a high MPO/PON1 ratio in serum inhibits VCAM-1 expression less than HDL isolated from patients with a low MPO/ PON1 ratio. The cholesterol efflux capacity of apolipoprotein B-depleted serum from patients with high MPO/PON1 ratio was also significantly decreased compared to serum from patients with low MPO/PON1 ratio. Indicates that the MPO/ PON1 ratio could be a useful indicator of the functional properties of HDL. 13. DiDonato Ja, Huang Y, Aulak KS, et al. Function and distribution of apolipoprotein A1 in the artery wall are markedly distinct from those in plasma. Circulation 2013; 128:1644–1655. 14. Huang Y, Didonato Ja, Levison BS, et al. An abundant dysfunctional apoli&& poprotein A1 in human atheroma. Nat Med 2014; 20:193–203. Identify an oxindolyl alanine moiety formed at tryptophan 72 of apoA1 by the MPOhydrogen peroxide-chloride system. The tryptophan 72 of apoA1 is important for the cholesterol efflux activity of apoA1 and oxidation of this moiety bestows apoA1 pro-inflammatory activity on endothelial cells. ApoA1 containing oxidized tryptophan 72 was found in low abundance in plasma but was common in apoA1 isolated form atherosclerotic arteries. The presence of oxidized tryptophan 72 in apoA1 was higher in plasma from subjects that had clinical evidence of cardiovascular disease. Although there was no significant gain in the receiver operating characteristic curve, the net reclassification index was significantly increased by 5.9% upon inclusion of oxidized tryptophan 72 apoA1 in a fully adjusted model, indicating that this apoA1 modification could be used to monitor the atherogenic process in the artery. 15. Shao B, Tang C, Sinha A, et al. Humans with atherosclerosis have impaired && ABCA1 cholesterol efflux and enhanced HDL oxidation by myeloperoxidase. Circ Res 2014; 114:1733–1742. doi: 10.1161/CIRCRESAHA.114.303454. Quantify the presence of chlorinated tyrosine 192 and oxidized methionine 148 modifications in apoA1 with mass spectrometry in control subjects and subjects with stable coronary artery disease or acute coronary syndrome. Subjects with cardiovascular disease displayed higher levels of both modified amino acids in plasma apoA1 than the control subjects and the levels were inversely associated with cholesterol efflux capacity. The association with atherosclerotic disease status remained significant after adjusting for HDL-cholesterol levels, indicating that the levels of chlorinated tyrosine 192 and oxidized methionine 148 in plasma HDL could be useful indicators of the risk of cardiovascular disease.
Volume 25 Number 4 August 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Atherosclerosis: cell biology and lipoproteins Harry Bjo¨rkbacka
Although low-circulating HDL-C levels have been shown over and over to be associated with increased cardiovascular risk, a detailed understanding of which molecular functions associated with the HDL particle that explain the risk association is lacking [1 ,2 ]. Several recent reports shed light on the anti-inflammatory functions of HDL and the role of dysfunctional HDL in cardiovascular disease risk prediction. As low-circulating HDL-C levels are associated with increased cardiovascular risk, a goal has been to pharmacologically increase HDL-C levels. Such efforts have, however, not been fruitful so far, even if many drugs that increase HDL-C, including fibrates, niacin or inhibitors of cholesteryl ester transfer protein, have been tested [2 ]. The raising of HDL-C levels as a pharmacological target has also been put in question by Mendelian randomization studies indicating that genetic variants that raise HDL-C levels do not lower the risk of myocardial infarction [3,4]. Instead, there has been an increased attention to HDL function, fueled by studies showing that cholesterol efflux function is a better predictor of risk than HDL levels alone [5]. Functions associated with the HDL particle, in addition to mediating the removal of cholesterol from foam cell macrophages in atherosclerotic lesions, include antioxidant, antithrombotic, and anti-inflammatory properties, which may all contribute to the reduced cardiovascular risk associated with HDL. The anti-inflammatory properties of HDL from a molecular standpoint have been poorly understood. Recently, however, De Nardo et al. [6 ] show that HDL can suppress Toll-like receptor-stimulated expression of several proinflammatory cytokines on the transcriptional level. The authors identify the transcription regulator ATF3 (activating transcription factor 3; cyclic AMP-dependent transcription factor) as a HDL-inducible target that directly regulates several key proinflammatory genes. The ability of HDL to modulate Toll-like receptormediated responses raises the possibility that HDL could be useful in treating inflammatory disorders not directly linked to cardiovascular disease. The anti-inflammatory properties of HDL can be impaired during inflammatory conditions, such as during an acute phase response, and the HDL may &&
&&
&&
&&
become dysfunctional or even gain proinflammatory properties [7–9]. Recently, an impaired antiinflammatory capacity of HDL, determined as the ability to suppress tumor necrosis factor-a-induced vascular cell adhesion molecule-1 (VCAM-1) mRNA expression in endothelial cells in vitro, was shown to be predictive for future secondary major adverse cardiovascular events [10 ]. Both circulating HDL and apoA1 recovered from human atherosclerotic lesions show an association between oxidative modifications of apoA1 and an impaired cholesterol efflux function, as well as acquisition of proinflammatory activity [8,11]. The oxidative modifications of apoA1 leading to dysfunctional HDL are in part formed by myeloperoxidase (MPO)-derived oxidants [8,11]. Consistent with this, a recent study also indicates that the MPO to paraoxonase 1 (PON1) ratio in serum could be a useful indicator of the functional properties of HDL [12 ]. Another aspect to consider when comparing measurements of HDL-C versus the evaluation of functional properties of HDL is the fact that apoA1 protein found in atherosclerotic lesion or normal aortic tissue has been found to be lipid-poor, highly cross-linked, and displaying impaired cholesterol efflux activity to a much greater extent than apoA1 found in plasma [13]. Recently, however, two studies showed that circulating apoA1 modified by the MPO–hydrogen peroxide–chloride system could still be useful for monitoring cardiovascular disease status. Huang et al. [14 ] show that only a fraction (0.007%) of apoA1 in plasma harbored an oxidized tryptophan 72 moiety that was relatively common (20%) in apoA1 isolated from atherosclerotic arteries. Still, the presence of this specific apoA1 modification was found to be higher in individuals that had clinical evidence of cardiovascular &
&
&&
Department of Clinical Sciences, Ska˚ne University Hospital, Lund University, Malmo¨, Sweden Correspondence to Harry Bjo¨rkbacka, Experimental Cardiovascular Research, CRC Lund University, Building 91 : 12, Jan Waldenstro¨ms gata 35, Malmo¨ University Hospital, SE-205 02 Malmo¨, Sweden. Tel: +46 40 391205; fax: +46 40 391212; e-mail: harry.bjorkbacka@ med.lu.se Curr Opin Lipidol 2014, 25:319–320 DOI:10.1097/MOL.0000000000000101
0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Bimonthly update Atherosclerosis: cell biology and lipoproteins &&
disease. In line with these results, Shao et al. [15 ] recently reported that the plasma levels of apoA1 with chlorinated tyrosine 192 and oxidized methionine 148 measured by mass spectrometry were higher in individuals with stable coronary artery disease or acute coronary syndrome compared to control individuals. Furthermore, the specific apoA1 modifications described in both studies were associated with an impaired ABCA1-mediated cholesterol efflux capacity, independently of HDL-C levels. Thus, although the usefulness of measuring specific modifications of apoA1 as a marker of dysfunctional HDL and its value in risk prediction will have to be further evaluated, the findings are promising in that circulating levels of dysfunctional apoA1 seem to reflect the pathophysiological process within the vascular wall. In conclusion, although HDL-C may still be useful in risk prediction, detailed understanding of the functional properties of HDL now shows great promise to provide even better cardiovascular disease prediction and stratification. It remains to be seen if these insights into HDL function can be exploited as therapeutic targets. Acknowledgements None. Conflicts of interest There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Feig JE, Hewing B, Smith JD, et al. High-density lipoprotein and atherosclerosis regression: evidence from preclinical and clinical studies. Circ Res 2014; 114:205–213. Authoritative review that discusses the importance of measuring HDL function in addition to plasma levels of HDL cholesterol. It emphasizes the evidence that support atheroprotection by HDL if the number of functional particles is increased. 2. Lu¨scher TF, Landmesser U, von Eckardstein A, et al. High-density lipoprotein: && vascular protective effects, dysfunction, and potential as therapeutic target. Circ Res 2014; 114:171–182. Authoritative review that discusses HDL as a therapeutic target, taking into account recent finding that HDL may become dysfunctional in patients with cardiovascular disease due to oxidative modifications and subsequent alterations in the proteome composition of HDL. 3. Voight BF, Peloso GM, Orho-Melander M, et al. Plasma HDL cholesterol and risk of myocardial infarction: a mendelian randomisation study. Lancet 2012; 380:572–580. &&
320
www.co-lipidology.com
4. Haase CL, Tybjærg-Hansen A, Qayyum AA, et al. LCAT, HDL cholesterol and ischemic cardiovascular disease: a Mendelian randomization study of HDL cholesterol in 54,500 individuals. J Clin Endocrinol Metab 2012; 97:E248– E256. 5. Khera AV, Cuchel M, de la Llera-Moya M, et al. Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis. N Engl J Med 2011; 364:127–135. 6. De Nardo D, Labzin LI, Kono H, et al. High-density lipoprotein mediates anti&& inflammatory reprogramming of macrophages via the transcriptional regulator ATF3. Nat Immunol 2014; 15:152–160. Shows that HDL suppresses the ability of Toll-like receptors to induce the expression of pro-inflammatory cytokines on the transcriptional level following stimulation. Identify ATF3 as an HDL-inducible negative regulator of several key pro-inflammatory genes that are directly targeted by ATF3 after its induction by HDL. The findings raise the possibility to use HDL-based therapeutics in treating inflammatory disorders not directly linked to hypercholesterolemia. 7. Van Lenten BJ, Hama SY, de Beer FC, et al. Anti-inflammatory HDL becomes pro-inflammatory during the acute phase response. Loss of protective effect of HDL against LDL oxidation in aortic wall cell cocultures. J Clin Invest 1995; 96:2758–2767. 8. Undurti A, Huang Y, Lupica Ja, et al. Modification of high density lipoprotein by myeloperoxidase generates a pro-inflammatory particle. J Biol Chem 2009; 284:30825–30835. 9. Besler C, Heinrich K, Rohrer L, et al. Mechanisms underlying adverse effects of HDL on eNOS-activating pathways in patients with coronary artery disease. J Clin Invest 2011; 121:2693–2708. 10. Dullaart RPF, Annema W, Tio Ra, et al. The HDL anti-inflammatory function is & impaired in myocardial infarction and may predict new cardiac events independent of HDL cholesterol. Clin Chim Acta 2014; 433C:34–38. The first study that investigates the predictive value of measuring HDL function for the future development of new major adverse cardiovascular events. 11. Zheng L, Nukuna B, Brennan M, et al. Apolipoprotein A-I is a selective target for myeloperoxidase-catalyzed oxidation and functional impairment in subjects with cardiovascular disease. J Clin Invest 2004; 114:529–541. 12. Haraguchi Y, Toh R, Hasokawa M, et al. Serum myeloperoxidase/paraoxonase 1 & ratio as potential indicator of dysfunctional high-density lipoprotein and risk stratification in coronary artery disease. Atherosclerosis 2014; 234:288–294. Shows that HDL isolated from patients with a high MPO/PON1 ratio in serum inhibits VCAM-1 expression less than HDL isolated from patients with a low MPO/ PON1 ratio. The cholesterol efflux capacity of apolipoprotein B-depleted serum from patients with high MPO/PON1 ratio was also significantly decreased compared to serum from patients with low MPO/PON1 ratio. Indicates that the MPO/ PON1 ratio could be a useful indicator of the functional properties of HDL. 13. DiDonato Ja, Huang Y, Aulak KS, et al. Function and distribution of apolipoprotein A1 in the artery wall are markedly distinct from those in plasma. Circulation 2013; 128:1644–1655. 14. Huang Y, Didonato Ja, Levison BS, et al. An abundant dysfunctional apoli&& poprotein A1 in human atheroma. Nat Med 2014; 20:193–203. Identify an oxindolyl alanine moiety formed at tryptophan 72 of apoA1 by the MPOhydrogen peroxide-chloride system. The tryptophan 72 of apoA1 is important for the cholesterol efflux activity of apoA1 and oxidation of this moiety bestows apoA1 pro-inflammatory activity on endothelial cells. ApoA1 containing oxidized tryptophan 72 was found in low abundance in plasma but was common in apoA1 isolated form atherosclerotic arteries. The presence of oxidized tryptophan 72 in apoA1 was higher in plasma from subjects that had clinical evidence of cardiovascular disease. Although there was no significant gain in the receiver operating characteristic curve, the net reclassification index was significantly increased by 5.9% upon inclusion of oxidized tryptophan 72 apoA1 in a fully adjusted model, indicating that this apoA1 modification could be used to monitor the atherogenic process in the artery. 15. Shao B, Tang C, Sinha A, et al. Humans with atherosclerosis have impaired && ABCA1 cholesterol efflux and enhanced HDL oxidation by myeloperoxidase. Circ Res 2014; 114:1733–1742. doi: 10.1161/CIRCRESAHA.114.303454. Quantify the presence of chlorinated tyrosine 192 and oxidized methionine 148 modifications in apoA1 with mass spectrometry in control subjects and subjects with stable coronary artery disease or acute coronary syndrome. Subjects with cardiovascular disease displayed higher levels of both modified amino acids in plasma apoA1 than the control subjects and the levels were inversely associated with cholesterol efflux capacity. The association with atherosclerotic disease status remained significant after adjusting for HDL-cholesterol levels, indicating that the levels of chlorinated tyrosine 192 and oxidized methionine 148 in plasma HDL could be useful indicators of the risk of cardiovascular disease.
Volume 25 Number 4 August 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
