Role of S-adenosylhomocysteine in cardiovascular disease and its potential epigenetic mechanism

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Role of S-adenosylhomocysteine in cardiovascular disease and its potential epigenetic mechanism夽 Yunjun Xiao a,∗ , Xuefen Su b , Wei Huang a , Jinzhou Zhang a , Chaoqiong Peng a , Haixiong Huang a , Xiaomin Wu a , Haiyan Huang a , Min Xia c , Wenhua Ling c,∗∗ a Department of Nutrition and Food Hygiene, Key Laboratory of Modern Toxicology of Shenzhen, Shenzhen Center for Disease Control and Prevention, Shenzhen, China b The Jockey Club School of Public Health and Primary Care, School of Public Health, The Chinese University of Hong Kong, Hong Kong, China c Guangdong Provincial Key Laboratory of Food, Nutrition and Health, Department of Nutrition, School of Public Health, Sun Yat-sen University, Guangzhou, China

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Article history: Received 2 March 2015 Received in revised form 8 June 2015 Accepted 16 June 2015 Available online xxx Keywords: Homocysteine S-adenosylhomocysteine Cardiovascular disease Atherosclerosis Epigenetic

a b s t r a c t Transmethylation reactions utilize S-adenosylmethionine (SAM) as a methyl donor and are central to the regulation of many biological processes: more than fifty SAM-dependent methyltransferases methylate a broad spectrum of cellular compounds including DNA, histones, phospholipids and other small molecules. Common to all SAM-dependent transmethylation reactions is the release of the potent inhibitor S-adenosylhomocysteine (SAH) as a by-product. SAH is reversibly hydrolyzed to adenosine and homocysteine by SAH hydrolase. Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. However, a major unanswered question is if homocysteine is causally involved in disease pathogenesis or simply a passive and indirect indicator of a more complex mechanism. A chronic elevation in homocysteine levels results in a parallel increase in intracellular or plasma SAH, which is a more sensitive biomarker of cardiovascular disease than homocysteine and suggests that SAH is a critical pathological factor in homocysteine-associated disorders. Previous reports indicate that supplementation with folate and B vitamins efficiently lowers homocysteine levels but not plasma SAH levels, which possibly explains the failure of homocysteine-lowering vitamins to reduce vascular events in several recent clinical intervention studies. Furthermore, more studies are focusing on the role and mechanisms of SAH in different chronic diseases related to hyperhomocysteinemia, such as cardiovascular disease, kidney disease, diabetes, and obesity. This review summarizes the current role of SAH in cardiovascular disease and its effect on several related risk factors. It also explores possible the mechanisms, such as epigenetics and oxidative stress, of SAH. This article is part of a Directed Issue entitled: Epigenetic dynamics in development and disease. © 2015 Published by Elsevier Ltd.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Metabolism of SAH in the methionine cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Role of SAH in diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. SAH and cardiovascular disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. SAH and atherosclerosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3. SAH and endothelial dysfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.4. SAH and renal disease, diabetes, and obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

夽 This article is part of a Directed Issue entitled: Epigenetic dynamics in development and disease. ∗ Corresponding author at: Shenzhen Center for Disease Control and Prevention, 8 Longyuan Road, 518055 Shenzhen, China. Tel.: +86 0755 25617321; fax: +86 0755 25500660. ∗∗ Corresponding author at: Sun Yat-sen University, 74 Zhongshan Road 2, 510080 Guangzhou, China. Tel.: +86 20 87331597; fax: +86 20 87330446. E-mail addresses: [email protected] (Y. Xiao), [email protected] (W. Ling). http://dx.doi.org/10.1016/j.biocel.2015.06.015 1357-2725/© 2015 Published by Elsevier Ltd.

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The underlying mechanisms of SAH in CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.1. Epigenetic mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4.2. Other potential mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction More than 40 years ago, McCully (1969) first found a link between homocysteine and atherothrombotic vascular disease. Since then, many epidemiological studies have confirmed that elevated plasma levels of total homocysteine (tHcy) are associated with an increased risk of cardiovascular and cerebrovascular diseases (Brattstrom and Wilcken, 2000; Clarke et al., 1991; Fryer et al., 1993; Kang et al., 1992). Of these studies, nested case–control studies showed that a 5 ␮mol/L increment in tHcy results in a 20–30% increase in cardiovascular risk, which is substantially lower than the 60–90% risk enhancement shown in retrospective case–control studies. Furthermore, a meta-analysis of eight cohort studies showed that a 5 ␮mol/L increase in serum homocysteine results in a 35% increase in dementia risk (Wald et al., 2011). During the past decades, animal models of hyperhomocysteinemia (HHcy) were developed using methods including the use of diets high in methionine and/or low in folate (Zhou et al., 2001) and genetic approaches, such as the generation of knockout mice with a targeted disruption of the cystathionine ␤-synthase (Cbs) (Watanabe et al., 1995), methylene tetrahydrofolate reductase (Mthfr) (Chen et al., 2001), or methionine synthase (Mtr) genes (Swanson et al., 2001). Severe HHcy animals (>100 ␮mol/L) develop endothelial dysfunction and atherosclerosis (Wang et al., 2003) that was not observed in mild HHcy animals (≈15 ␮mol/L) (Troen et al., 2003), which have levels approaching the physiological levels in the general population. The mechanisms by which homocysteine induce atherosclerosis include oxidative stress, endoplasmic reticulum (ER) stress, and proinflammatory responses (Austin et al., 2004). However, there is no generally accepted mechanism for the pathophysiology of elevated plasma homocysteine as a cause of vascular disease. Furthermore, several recent large-scale intervention studies using B vitamins and folate to lower plasma homocysteine levels have not shown overall clinical benefits (Albert et al., 2008; Bonaa et al., 2006; Clarke et al., 2010; Jamison et al., 2007; Lonn et al., 2006). Thus, a causal relationship between homocysteine and vascular disease remains controversial (Brattstrom and Wilcken, 2000; Ueland et al., 2000). One potential explanation for these conflicting results is that HHcy may not be the causative agent in vascular disease, but instead may be a marker for another risk factor (Lu et al., 2010). Recent studies suggest an indirect mechanism for homocysteine toxicity that is secondary to S-adenosylhomocysteine (SAH) accumulation. SAH is the precursor of homocysteine and is reversibly hydrolyzed to homocysteine and adenosine by SAH hydrolase (SAHH) (Stipanuk, 2004). SAH is produced as a product of methylation reactions involving S-adenosylmethionine (SAM) as the methyl donor (Grillo and Colombatto, 2008). Several previous cross-sectional and case–control studies have indicated that plasma SAH may be a better indicator of cardiovascular disease than homocysteine (Kerins et al., 2001; Valli et al., 2008; Wagner and Koury, 2007). A recent prospective cohort study also found that higher plasma SAH levels were significantly associated with an increased risk of cardiovascular events and were a better predictor of cardiovascular risk than homocysteine in coronary angiographic patients (Xiao et al., 2013). Furthermore, Liu et al. (2008) showed that plasma SAH levels were a more sensitive biomarker of atherosclerosis than homocysteine and were associated with DNA

hypomethylation in HHcy mice. Moreover, Luo et al. (2012) used an SAHH inhibitor and shRNA interference to induce elevated SAH levels, which promoted early atherosclerosis by inducing the proliferation and migration of smooth muscle cells through the oxidative stress-extracellular signal-regulated protein kinases 1 and 2 pathways. Although oxidative stress has been proposed to explain the atherogenic effects of SAH, recent in vivo and in vitro studies demonstrated that SAH accelerated atherosclerosis and was associated with epigenetic regulation of ER stress (Xiao et al., 2015). This review summarizes the current role of SAH in cardiovascular disease and several related risk factors. It also explores possible pathological mechanisms of SAH, such as epigenetics and oxidative stress. 2. Metabolism of SAH in the methionine cycle The methionine cycle is formed by the synthesis of SAM by methionine adenosyltranserase. SAM is the methyl donor of numerous specific transmethylation reactions including DNA, RNA, histone, phospholipids, and other proteins (Grillo and Colombatto, 2008). A methyl group is removed by different methyltransferases to form SAH, which is further hydrolyzed to homocysteine and adenosine by SAHH. The reaction is reversible with the equilibrium favoring the formation of SAH unless the products adenosine and homocysteine are effectively removed in vivo. Adenosine is converted to inosine by the enzyme adenosine deaminase. Homocysteine is at the intersection of the remethylation and transsulfuration pathways in the methionine cycle (Stipanuk, 2004). In remethylation, homocysteine can acquire a methyl group from N-5-methyltetrahydrofolate by the vitamin B-12 dependent enzyme system methionine synthase or can accept a methyl group from betaine to form methionine by the vitamin B-12independent enzyme betaine homocysteine methyltransferase. In the transsulfuration pathway, homocysteine condenses with serine to generate cystathionine in an irreversible reaction catalyzed by the pyridoxal-5 -phosphate (PLP)-containing enzyme Cbs. Cystathionine is further degraded to form cysteine by a second PLP-containing enzyme, ␥-cystathionase. Excess cysteine is oxidized to taurine or inorganic sulfates or is renally excreted. Thus, in addition to the synthesis of cysteine, this transsulfuration pathway effectively catabolizes excess homocysteine, which is not required for methyl transfer. It is important to note that genetic or nutritional perturbations that hinder efficient removal of homocysteine or adenosine will induce a reversal of the SAHH reaction, leading to an intracellular accumulation of SAH. Elevated cellular concentrations of this metabolite are likely to precede and accompany all forms of HHcy. 3. Role of SAH in diseases A possible alternative cause of the pathophysiology associated with HHcy is SAH. However, relatively few studies have directly compared plasma homocysteine and SAH concentrations in diseases, probably because of the complex methods involved measuring of plasma SAH in large studies (Gellekink et al., 2005; Struys et al., 2000). However, growing evidence has shown that SAH has an important role in different chronic diseases.

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3.1. SAH and cardiovascular disease Supporting a role for SAH in cardiovascular disease (CVD) risk, Wagner et al. (Kerins et al., 2001) conducted a case–control study in CVD patients and found that plasma SAH concentrations were significantly higher in the patients than controls. However, there was no significant difference in homocysteine concentrations between the patients and controls. The authors concluded that an increase in plasma SAH is a much more sensitive indicator of CVD than an increase in plasma homocysteine. Although the inability to discriminate between patients with CVD and controls using homocysteine concentrations was likely due to the small sample of patients (n = 30) and controls (n = 29) in the study, this insensitivity illustrates one of the major problems with using plasma homocysteine as an indicator of CVD risk. There was a large overlap in plasma homocysteine concentrations between the patients and controls, which makes it impossible to predict if a person with moderately elevated plasma homocysteine is at a greater risk than another person with the same plasma homocysteine concentration (Wagner and Koury, 2007). However, because plasma SAH concentrations are ≈1–500th that of plasma homocysteine, a small change in homocysteine concentrations may result in a relatively large increase in plasma SAH concentrations. Plasma SAH values in both patients and control subjects overlapped much less than those for homocysteine. Therefore, SAH might be a better indicator of vascular disease than homocysteine. Furthermore, because a high concentration of homocysteine can directly promote SAH synthesis in vivo through a reversible reaction catalyzed by SAHH, plasma SAH concentrations are often highly correlated with parallel increases in plasma homocysteine concentrations. For example, Yi et al. (2000) reported that a chronic elevation in plasma homocysteine levels in healthy young subjects was associated with a concomitant increase in plasma or intracellular SAH levels. Moreover, in HHcy patients with occlusive vascular disease, Loehrer et al. (2001) found elevated plasma SAH level in both plasma and erythrocytes. Castro et al. (2003) also showed that patients with vascular disease had significantly higher plasma SAH and homocysteine concentrations than age- and sex-matched controls. Therefore, this study could not exclude the effect of SAH on vascular disease related to HHcy. Plasma SAH concentrations were not measured in the majority of previous studies on plasma homocysteine and CVD. However, although plasma SAH and homocysteine concentrations would be expected to change in the same way, this is not always the case. For example, Green et al. (2010) showed that B vitamin supplementation in older people lowered plasma homocysteine concentrations but had no effect on plasma SAH concentrations. Becker et al. (2003b) showed that, in contrast to plasma homocysteine concentrations, plasma SAH concentrations were not associated with folate concentrations in healthy subjects. Thus, if SAH is a toxic agent, rather than homocysteine, then the use of folic acid supplementation to reduce plasma homocysteine concentrations will not reduce the risk of vascular disease. This is noteworthy in view of recent large clinical trials showing that folate supplementation did not reduce the risk of vascular disease (Loscalzo, 2006), and selectively lowered plasma homocysteine concentrations but not plasma SAH. Despite the aforementioned small-sample size cross-sectional and case–control studies reporting the relationship between plasma SAH and CVD, large-scale prospective cohort studies on the relationship between plasma levels of SAH and vascular disease risk are lacking. To the best of our knowledge, only one recent cohort study prospectively evaluated the association between plasma SAH levels and CVD risk in patients undergoing coronary angiography (Xiao et al., 2013). The study found that higher plasma SAH concentrations were significantly associated with an increased risk of cardiovascular events, including fatal CVD, nonfatal myocardial

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infarction, and stroke. Because of the biological interplay between SAH and homocysteine and the association between high SAH and homocysteine concentrations and increased CVD risk, the authors further analyzed the effect of homocysteine on-cardiovascular events by adjusting for SAH levels and vice versa. There was a significant positive association between SAH and the risk of cardiovascular events even after adjusting for homocysteine. However, the association between homocysteine and CVD risk was no longer significant after adjusting for SAH. Interestingly, combination analyses of the effect of SAH and homocysteine on the risk of CVD showed that higher plasma homocysteine concentrations strengthened the positive association between SAH and risk of cardiovascular events. These findings suggest that plasma SAH concentrations are independently associated with an increased risk of cardiovascular events and might be a better predictor of cardiovascular risks than plasma homocysteine. Notably, the association between homocysteine and cardiovascular risk might depend on the effect of SAH, but high homocysteine could synergistically increase the effect of SAH on cardiovascular risk.

3.2. SAH and atherosclerosis Atherosclerosis, a disease of the large arteries, is the primary cause of heart disease and stroke (Lusis, 2000). Recent evidence reported a relationship between plasma SAH and atherosclerosis in human and animal studies and supported the hypothesis that certain detrimental effects hitherto attributed to homocysteine may, in fact, be caused by SAH. Zawada et al. (2014) showed that plasma SAH, not homocysteine, was strongly associated with traditional cardiovascular risk factors and subclinical atherosclerosis by measuring common carotid intima-media-thickness in 402 subjects with low cardiovascular risk, suggesting that SAH may serve as a sensitive marker of early atherosclerotic disease. Moreover, Xiao et al. (2013) recently found that plasma SAH was significantly associated with severity and extent of coronary artery disease in 1003 patients undergoing coronary angiography. Pathophysiological effects of SAH on atherosclerosis have also been demonstrated in experimental studies. Liu et al. (2008) increased plasma SAH and homocysteine concentrations by feeding a high methionine diet deficient in folate and B vitamins to apolipoprotein E-deficient (apoE−/−) mice. Atherosclerotic lesion areas and plasma SAH and homocysteine concentrations were significantly increased in these mice. However, when this diet was supplemented with B vitamins, plasma homocysteine concentrations returned to normal levels, but the plasma SAH concentrations remained significantly elevated and the atherosclerotic lesion areas remained increased. Plasma SAH concentrations and aortic sinus lesion areas were positively correlated. These results suggest that plasma SAH is a better biomarker of atherosclerosis than homocysteine and may accelerate the development of atherosclerotic lesions in apoE−/− mice that have been fed a high methionine diet. To further explore the role of SAH in atherosclerosis and prove the speculation that detrimental effects formerly ascribed to homocysteine are indeed mediated via SAH, Luo et al. (2012) established an animal model of chronically increased plasma SAH concentrations in apoE−/− mice by feeding the mice a diet supplemented with the SAHH inhibitor adenosine-2,3-dialdehyde. They found that plasma SAH accumulation accelerated the development of early atherosclerotic lesion areas. To exclude the effect of the inhibitor itself, the authors constructed a shRNA interference plasmid for SAHH and intravenously injected apoE−/− mice with a retrovirus that expressed SAHH shRNA semi-weekly for 8 weeks. They found early atherosclerotic lesion areas were also increased in these mice. Next, Xiao et al. (2015) treated apoE−/− mice at 8 weeks with SAHH inhibitor and shRNA for an additional 16 weeks and found increased plasma SAH

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concentrations and accelerated the development of atherosclerotic lesion areas in both groups of mice. Comparing the effects of homocysteine and SAH on atherosclerosis, plasma homocysteine concentrations in the apoE−/− mice fed the high-methionine diet increased more than 3-fold relative to the control mice with normal homocysteine levels (≈5 ␮mol/L). No acceleration of atherosclerotic lesion formation was observed in mice with mild HHcy (≈15 ␮mol/L). In contrast, in an animal model of chronically elevated plasma SAH levels, a 2-fold increase in plasma SAH levels accelerated the development of atherosclerosis, whereas a 10-fold increase in plasma homocysteine levels (≈100 ␮mol/L) was required to cause severe HHcy and accelerated atherosclerotic progression. This evidence indicates that a mild elevation in plasma SAH levels is more likely to promote the development of atherosclerosis than a mild increase in homocysteine levels. It also supports plasma SAH is a more sensitive marker of atherosclerosis and CVD than homocysteine. In addition, it may be concluded that the effect of plasma SAH on atherosclerosis is independent of homocysteine, which is consistent with findings in human studies that indicates that plasma SAH concentrations are associated with a cardiovascular risk independent of plasma homocysteine levels. 3.3. SAH and endothelial dysfunction Endothelial dysfunction is commonly caused by an impairment of an endothelium-dependent relaxation of blood vessels, and is predictive of adverse cardiovascular outcomes (Schachinger et al., 2000). Many studies have demonstrated that HHcy induces endothelial dysfunction in animal models and human subjects (Eberhardt et al., 2000; Kanani et al., 1999; Wilson et al., 2007). However, only a few studies have reported on a relationship between SAH and endothelial dysfunction. Dayal et al. (2001) found that increased tissue levels of SAH were associated with endothelial dysfunction in heterozygous Cbs-deficient mice with HHcy, which suggests that SAH may contribute to vascular dysfunction in HHcy. However, the ability of SAH to induce vascular dysfunction independently of homocysteine is unknown. De Vriese et al. (2004) investigated the effect of acute and chronic HHcy on a renal vasodilatory response mediated by endothelium-derived hyperpolarizing factor (EDHF), which is a major regulator of vascular function in small vessels. The authors observed that acute HHcy induced by an intravenous infusion with methionine, but not homocysteine, impaired the EDHF-mediated vasodilatory response. Further, when a methionine infusion was preceded by adenosine periodate oxidized, an inhibitor of SAHH, to prevent the cleavage of SAH to homocysteine and adenosine, a similar impairment of EDHF was observed, but with normal homocysteine levels. The results indicate that SAH but not homocysteine itself might be a mediator of vascular dysfunction. However, in a population-based study of 608 elderly people, Spijkerman et al. (2005) found that endothelium-dependent, flow-mediated vasodilation was significantly associated with high SAM and low 5-methyltetrahydrofolate but not SAH or homocysteine. Additional studies are needed to clarify these conflicting results. 3.4. SAH and renal disease, diabetes, and obesity Although it has not been fully explained, there also appears to be a relationship between kidney function and plasma SAH. Loehrer et al. (1998) observed a 45-fold increase in plasma SAH compared with a four-fold increase in plasma homocysteine in patients with end-stage renal failure. However, the authors drew no conclusions about which measurement was a more sensitive indicator of renal function. Subsequently, Wagner et al. (2004) measured plasma SAH and total homocysteine in 36 patients with renal insufficiency and

17 control subjects. They found less overlap between the values of plasma SAH between the patients and control subjects compared to plasma homocysteine. Thus, the authors suggested that plasma SAH might also be a more sensitive indicator than homocysteine for renal disease. Because patients with kidney disease generally are older and have other diseases such as diabetes and hypertension, which are associated with vascular disease and could reduce renal function, it is difficult to determine if renal insufficiency alone was the cause of the elevated homocysteine and SAH. To rule out complicating factors as the cause of elevated SAH in renal disease, Jabs et al. (2006) measured SAH and homocysteine in 23 patients between the ages of 1 and 18 years old with a wide range of renal functions without complicating factors. They did not observe a statistical correlation between glomerular filtration rate and plasma homocysteine. However, there was a strong correlation between glomerular filtration rate and plasma SAH, which suggests that plasma SAH, rather than homocysteine, is the metabolite primarily affected in renal disease. Similarly, a recent case–control study investigated the serum concentrations of SAM, SAH and total homocysteine in 124 stage 5 chronic kidney disease patients and 47 control subjects (Valli et al., 2008). Of the investigated sulfur amino acids, only SAH was independently associated with the presence of clinical signs of CVD. The results suggest that homocysteine might be influenced by a number of confounding factors and SAH levels may better reflect an increased cardiovascular risk. Furthermore, it is interesting to note that Stam et al. (2004) observed that patients with end-stage renal disease had impaired homocysteine clearance by transsulfuration and remethylation, which were inversely associated with elevated whole blood SAH levels. These findings suggest that the metabolism dysfunction of homocysteine may be mediated by increased SAH due to renal dysfunction. Garibotto et al. (2009) explored why plasma levels of SAH are increased in patients with chronic kidney diseases and found that kidneys extracted 40% of SAH. A SAH arterio-venous difference across the kidney was directly and significantly related to SAH arterial levels, which further confirmed the pivotal role for kidneys in SAH metabolism. Renal function in Type 2 diabetes appears to change with the progression of diabetes: hyperfiltration in the early stages and progressive deterioration as the disease progresses (Wijekoon et al., 2007). Therefore, diabetes provides an interesting model in which changes in kidney function are superimposed on previously existing changes in the metabolic milieu. Herrmann et al. (2005) measured serum or plasma concentrations of methionine cycle intermediates SAH, SAM, and homocysteine in 93 patients with renal failure and type 2 diabetes. They found that increased plasma concentrations of SAH, SAM, and total homocysteine were related to the degree of renal insufficiency in patients with type 2 diabetes. Furthermore, persons with type 2 diabetes are more susceptible to the harmful effects of HHcy than nondiabetic individuals. Erythrocytes or plasma SAH concentrations are also significantly increased in patients with type 2 diabetes compared to nondiabetic patients (Becker et al., 2003a). However, no study has shown that diabetic patients with increased plasma SAH have a higher cardiovascular risk than normal SAH levels patients. Further, prospective studies are needed to investigate harmful effects of increased plasma SAH in patients with type 2 diabetes. Obesity is a risk factor for many chronic diseases such as CVD, diabetes, and fatty liver disease. Many studies have shown that aberrant metabolism of methionine and its metabolites are associated with fat mass and adiposity. For example, high plasma cysteine is associated with an increased body mass index (BMI) and fat mass in children and adults (Elshorbagy et al., 2012b). Feeding cysteine to mice induces lipogenic enzymes, lowers metabolic rate, and increases visceral adiposity (Elshorbagy et al., 2012a). Apart from cysteine, total homocysteine was reported to be associated with obesity (Elshorbagy et al., 2008). Plasma SAH and SAM are

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also strongly correlated with BMI in women (Inoue-Choi et al., 2012; van Driel et al., 2009). However, recently Elshorbagy et al. (2013) reported that plasma SAM, but not SAH, is independently associated with fat mass and trunk adiposity in older adults. In an in vitro study, Ngo et al. (2014) found that expansion and differentiation of murine 3T3-L1 preadipocytes in the presence of SAH impaired both basal and induced glucose uptake and lipolysis compared with untreated controls. However, elevated intracellular SAH did not alter preadipocyte factor 1 and peroxisome proliferator activated receptor ␥2 . These findings suggested that SAH did not affect adipogenesis per se but altered adipocyte functionality, such that they exhibited altered glucose disposal and lipolysis. Future studies are required to explore possible causes of the contradictory results between in vitro and in vivo studies. 4. The underlying mechanisms of SAH in CVD Because SAH, which is released as a by-product of SAMdependent methyltransferase reactions, is a potent product inhibitor of most SAM-dependent methyltransferases, such as DNA, histone, and phospholipid methyltransferases, it has been considered a sensitive marker for the cellular methylation status reflected by a SAM/SAH ratio. It can be used to predict methylation deficiency, e.g., DNA hypomethylation. However, the molecular mechanisms triggered by elevated SAH levels in the pathogenesis of diseases are entirely not understood. 4.1. Epigenetic mechanism Recently, it has been shown that epigenetic regulation of gene expression plays an important role in the development of vascular disease and has attracted considerable attention (Matouk and Marsden, 2008). Epigenetic alterations encompass hereditary traits mediated by changes, other than nucleotide sequences, in DNA (Bernstein et al., 2007). Methylation is a postsynthetic DNA modification that contributes to epigenetic modulation of gene expression. DNA methylation is a leading epigenetic mechanism involved in the pathological consequences of SAH accumulation. DNA methylation is the covalent addition of a methyl group at the 5 carbon of a cytosine ring and results in 5-methylcytosine (Bird, 2002). The methylation of a DNA sequence is catalyzed by at least five independent DNA methyltransferases, including DNMT1, DNMT2, DNMT3A, DNMT3B, and DNMT3L. Maintenance of DNA methylation is mediated by DNMT1, while de novo methylation is carried out by DNMT3A and DNMT3B (Fatemi et al., 2002). Because SAH is a potent inhibitor of DNA methyltransferases, DNA methylation can be regulated at the SAH level in vivo and in vitro. SAH reduces the expression and activity of these DNA methyltransferases. For example, an elevation in SAH levels has been associated with decreased mRNA expression of DNMT3A and DNMT3B and O6-methylguanine DNA methyltransferase (Hermes et al., 2008; Tao et al., 2008). Human studies have shown that elevated plasma and lymphocyte SAH levels were associated with increased DNA hypomethylation in CVD patients and healthy adults (Castro et al., 2003; Yi et al., 2000). Furthermore, the effect of SAH on DNA methylation has also been demonstrated in animal studies. Cbs−/− mice that have diminished activity of the Cbs-dependent transsulfuration pathway can develop HHcy and concomitant increases in SAH (Dayal et al., 2001). Severe HHcy in Cbs−/− mice can induce endothelial dysfunction and promote atherosclerotic formation (Dayal et al., 2001; Wang et al., 2003), which may be mediated by SAH-induced DNA hypomethylation. Caudill et al. (2001) fed heterozygous Cbs mice a control or methyl-deficient diet for 24 weeks and found that a decrease in SAM alone was not sufficient

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to affect DNA methylation, whereas an increase in SAH, either alone or associated with a decrease in SAM, was most consistently associated with DNA hypomethylation. These results suggest increased plasma SAH concentrations may provide a more sensitive biomarker for cellular DNA methylation status than a decrease in the SAM/SAH ratio, which might be not sufficient to affect DNA methylation if SAM alone is depleted. Another animal study showed that SAH concentrations were significantly higher in all tissues of homozygous mutant Cbs mice compared with wild-type mice, whereas the genomic DNA methylation status in Cbs−/− mice was lower in the liver and kidney but did not differ in the brain compared with wild-type mice (Choumenkovitch et al., 2002). The finding that a deficiency in Cbs had differential effects on SAH concentrations and DNA methylation in different tissues suggests that regulation of biological methylation by SAH is a complex tissuespecific phenomenon. Additionally, Jiang et al. (2007) proposed that in rat with mild and moderate HHcy, homocysteine might primarily influence the epigenetic regulation of gene expression through SAH-mediated DNA hypomethylation. In accordance with this, Liu et al. (2008) observed a negative correlation between plasma SAH concentrations and both the global DNA methylation status and DNA methyltransferase activity in the aortic tissue of mice with atherosclerotic lesions. Moreover, Castro et al. (2005) used adenosine-2,3-dialdehyde, an inhibitor of SAHH, to increase intracellular SAH concentrations in endothelium cells and substantiated the effect of intracellular SAH accumulation on DNA methylation patterns. The authors observed that supplementation of the inhibitor had no cytotoxic effect and increased the intracellular SAH concentrations in a dose-dependent manner, which was associated with decreased genomic DNA methylation status. The findings strongly point to the importance of SAH as a pivotal biomarker of genomic DNA methylation status. In vivo and in vitro evidence consistently suggest that SAH-mediated DNA hypomethylation and associated alterations in gene expression be a new epigenetic mechanism for pathogenesis of atherosclerotic CVD related to HHcy (James et al., 2002). Another epigenetic modification is histone methylation, which may be involved in a pathological mechanism of SAH. Histones can be methylated on either lysine (K) or arginine (R) residues. Lysine residues can be monomethylated, dimethylated, or trimethylated in vivo (Bernstein et al., 2007). It has been demonstrated that trimethylated histone H3 K4 (3meH3K4) is strongly associated with transcriptional activation, whereas 3meH3K9 is predominantly correlated with transcriptional repression (Bhaumik et al., 2007). For example, dysregulation of these histone methylation markers resulted in activation or repression of proinflammatory genes and endothelial or inducible nitric oxide synthase genes expression in diabetes mellitus or vascular disease (Chan et al., 2005; Matouk and Marsden, 2008; Reddy et al., 2008; Villeneuve et al., 2008). However, the involvement of SAH in the regulation of gene expression via histone methylation remains poorly understood. Recently, Ara et al. (2008) showed that SAM and its metabolites could inhibit lipopolysaccharide (LPS)-induced expression of tumor necrosis factor ␣ and inducible nitric oxide synthase genes in murine macrophage cells and LPS-treated mice. This might be mediated by SAH-reduced occupancy of 3meH3K4 at the promoters of these genes. In contrast, Xiao et al. (2015) observed no effect of plasma SAH accumulation on 3meH3K4 in an animal model of atherosclerosis. Furthermore, Esfandiari et al. (2010) reported that elevated hepatic SAH levels were associated with decreased occupancy of 3meH3K9 at the promoters of ER stress-related genes in mice with alcoholic liver injury. Consistent with this finding, increased plasma SAH concentrations were also significantly correlated with reduced expression of 3meH3K9 and decreased occupancy of 3meH3K9 at the promoters of ER stress-related genes in the aortic tissue of mice with atherosclerotic lesions. Further, in vivo and in vitro studies

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have shown that SAH-inhibited expression of 3meH3K9 might be mediated by an inhibition of total H3K9 methyltransferase activity and a reduction in the mRNA expression of Suv39h1 and G9a because of inhibited promoter activity (Xiao et al., 2015). Supporting this, a study reported that DZNep, an inhibitor of SAHH, increased intracellular SAH concentration and regulated both the expression of the 3meH3K27 histone methyltransferase EZH2 and expression of the H3K9 histone methyltransferase SETDB1 at the transcriptional level (Lee and Kim, 2013). Thus, these findings suggest that SAH is not only a potent inhibitor of the activity of several histone methyltransferases but also regulates the expression of histone methyltransferases at the transcriptional level. In particular, the histone modification 3meH3K27 has been widely linked to the repression of transcription elongation leading to an inhibition of gene expression (Guenther et al., 2007). In adipocytes undergoing differentiation in the presence of elevated SAH, Ngo et al. (2014) found elevated intracellular SAH concentrations significantly reduced the expression of the CAAT enhancer binding proteins (Cebp)˛, Cebpˇ and retinoid x receptor (Rxr)˛ and increased Rxr˛, but not Cebpˇ, methylation. Further, SAH significantly enhanced occupancy of 3meH3K27 on the promoters of Cebp˛ and Rxr˛. These findings suggest that the SAH-induced repression of these adipocyte functionality genes occurred through an epigenetic mechanism. 4.2. Other potential mechanisms In addition to epigenetic mechanisms, several other potential pathological mechanisms of SAH have been proposed. First, homocysteine contains a highly reactive thiol group and is readily oxidized to form reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and peroxynitrite, suggesting that homocysteine induces cell injury through a mechanism involving oxidative damage (Starkebaum and Harlan, 1986; Weiss et al., 2002). In addition to auto-oxidative ROS production, homocysteine exerts direct biological damage on vascular cells and tissue though an oxidative mechanism that damages lipids, nucleic acids, and proteins. Furthermore, the injurious oxidative stress exerted by homocysteine is thought to have an indirect effect on vascular redox reactions by decreasing the expression and activity of the antioxidant enzymes superoxide dismutase, heme oxygenase-1, and glutathione peroxidase (Steed and Tyagi, 2011). Endotheliumdependent factors such as nitric oxide (NO) and NO-containing intermediate products that appear to be key components for the production of the free radicals most likely play pivotal roles in endothelial dysfunction, which is thought to be the initial and significant event in the development of vascular disease. Direct toxic endothelium damage mediated by homocysteine has been shown to be initiated by the generation of potent ROS. Oxidative radicals induced by homocysteine are responsible for the diminished production and bioavailability of NO. Selley (2003) found a highly significant, negative association between plasma homocysteine concentrations and NO (as nitrate and nitrite) in patients with Alzheimer’s disease. Furthermore, it has also been proposed that homocysteine can increase NO production by increasing the activity of neural NOS and inducible NOS. The increase of NO in a highly oxidative environment, such as in HHcy, can lead to additional peroxynitrite formation. It has been already demonstrated that energy deficiency and mitochondrial DNA overproliferation and/or deletion appear to be primary target in HHcy-dependent vascular diseases. Tyagi et al. (2006) reported that homocysteinemediated ROS production promotes endothelial cell death in part by disturbing mitochondrial membrane potential accompanied by the release of cytochrome-c. Concerning the sites of action in the mitochondrial respiratory chain, homocysteine increased mitochondrial ROS generation at both complex I and complex III.

Regarding the effect of SAH on oxidative stress, evidence from the literature has shown that SAH is the only precursor of homocysteine and that they have similar structures and characteristics. Oxidative damage of homocysteine in endothelial cells is also SAHdependent (Mansoor et al., 1995). To demonstrate this, Sipkens et al. (2012) found that intracellular SAH accumulation, independent of homocysteine, significantly induced apoptosis and increased the expression of NADPH oxidase (NOX) and production of ROS in endothelial cells. Inhibition of NOX-mediated ROS by a flavoenzyme inhibitor significantly decreased SAH-induced apoptosis and ROS production. Further, in vivo and in vitro studies showed that increased plasma or intracellular SAH levels promoted vascular smooth muscle cell proliferation and migration related to the early formation of atherosclerosis through an oxidative stressdependent activation of the ERK1/2 pathway. These effects were significantly attenuated after a preincubation with superoxide dismutase (Luo et al., 2012). A more recent study showed that an accumulation of intracellular SAH concentration decreased tRNAsec methylation, which reduced the expression of selenoprotein glutathione peroxidase-1 and increased the oxidative stress-induced inflammatory activation of endothelial cells (Barroso et al., 2014). However, it is still uncertain if SAH accumulation could directly induce inflammation in vivo and in vitro, or if inflammation mediated the effect of SAH in vascular disease. The exact mechanism is unknown. Second, SAH accumulation can directly and indirectly induce DNA damage and inhibit DNA repair. Yang et al. (2003) observed that incubation with different doses of SAH dosedependently enhanced hydrogen peroxide-induced DNA damage and inhibited DNA repair in mouse endothelial cells and human intestinal cells. These effects were related to increased uracil misincorporation. Homocysteine had much weaker effects. Further, Lin et al. (2007) found synergistic effects of SAH and homocysteine on DNA damage, which was mediated by the induction of ROS and inhibited by the addition of superoxide dismutase. DNA damage induced by SAH also involves DNA hypomethylation, which enhances the sensitivity of DNA to SAH toxicity. Moreover, SAH is a risk factor for neurodegenerative diseases such as Alzheimer’s disease (Kennedy et al., 2004; Selley, 2007), for which ␤-Amyloid formation is a major risk factor (Hensley et al., 1994). A recent study found SAH increased DNA damage in mouse microglial BV2 cells possibly by increasing ␤-Amyloid formation and leading to increased formation of ROS (Lin et al., 2011). Furthermore, DNA damage was enhanced by SAH by the inhibition of DNMT1 activity and hypomethylation of 8-oxo-deoxyguanosine DNA glycosylases I. Third, it is recognized that prolonged activation of the ER stress pathway, known as the unfolded protein response (UPR), occurs during all stages of atherogenesis (Hotamisligil, 2010a,b; Zhou et al., 2004). A prior study showed that an increase of plasma SAH-accelerated atherosclerotic lesions was linked with the activation of ER stress in apoE−/− mice (Xiao et al., 2015). Although the author suggested that SAH induced ER stress might be through an epigenetic regulation mechanism, the underlying mechanism is still unknown. One proposed explanation is that a deficiency in phospholipid methylation due to SAH accumulation leads to an increase in saturated phosphatidylcholine molecular species in ER membranes followed by ER stress, protein misfolding, induction of UPR and activation of lipid metabolism (Ariyama et al., 2010; Innis et al., 2003). Finally, another important methylation reaction that can promote atherogenesis, independently of changes in gene expression is the methylation of Larginine to asymmetric dimethylarginine and SAH as a by-product (Doshi et al., 2005). Asymmetric dimethylarginine can inhibit the activity of eNOS and is associated with an increased risk of vascular disease in HHcy (Boger et al., 2000). Therefore, the effect of SAH on vascular disease may be mediated by its regulation of asymmetric dimethylarginine production through methylation.

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5. Conclusions and future directions Although an elevated homocysteine level is an independent risk factor for CVD and several cellular mechanisms have been proposed to explain the effects of homocysteine on endothelial dysfunction and atherosclerosis, a causal relationship between homocysteine and CVD remains controversial. Most importantly, more evidence suggests that SAH rather than homocysteine might be the culprit for several HHcy-associated pathologies. The role of SAH in CVD has been recognized. SAH is considered a more sensitive biomarker of CVD than homocysteine. However, to demonstrate the relationship between SAH and CVD, more large prospective studies are needed. Furthermore, the role of SAH in several CVD risk factors such as renal disease, diabetes, obesity, hypertension and its effects on cholesterol and lipid metabolisms need further studies. Although evidence has suggested that SAH accumulation induces endothelial dysfunction and accelerates atherosclerosis, the mechanisms responsible for the pathological consequences associated with SAH accumulation are still elusive. As a product inhibitor of a number of SAM-dependent methyltransferases, SAH can trigger multiple pathological mechanisms. Alterations in DNA or histone methylation and gene expression secondary to SAH accumulation and how SAH accumulation is related to the pathogenesis of chronic diseases associated with HHcy should provide a fertile area for future research. Moreover, to demonstrate a causal relationship between SAH and CVD or atherosclerosis, new intervention approaches to decrease the elevated SAH levels should be explored, with the exception of folate and B vitamins, which cannot lower SAH levels. Additionally, further human or animal studies are needed to determine if lowering SAH levels could reduce the risk of CVD or atherosclerotic lesions. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (81402672 and 81130052), the Guangdong Provincial Traditional Chinese Medicine Research Fund (20131037), the Shenzhen Municipal Science and Technology Project (201302139), and the Guangdong Provincial Medical Research Fund (B2014356). References Albert, C.M., Cook, N.R., Gaziano, J.M., Zaharris, E., MacFadyen, J., Danielson, E., et al., 2008. Effect of folic acid and B vitamins on risk of cardiovascular events and total mortality among women at high risk for cardiovascular disease: a randomized trial. JAMA 299, 2027–2036. Ara, A.I., Xia, M., Ramani, K., Mato, J.M., Lu, S.C., 2008. S-adenosylmethionine inhibits lipopolysaccharide-induced gene expression via modulation of histone methylation. Hepatology 47, 1655–1666. Ariyama, H., Kono, N., Matsuda, S., Inoue, T., Arai, H., 2010. Decrease in membrane phospholipid unsaturation induces unfolded protein response. J. Biol. Chem. 285, 22027–22035. Austin, R.C., Lentz, S.R., Werstuck, G.H., 2004. Role of hyperhomocysteinemia in endothelial dysfunction and atherothrombotic disease. Cell Death Differ. 11 (Suppl. 1), S56–S64. Barroso, M., Florindo, C., Kalwa, H., Silva, Z., Turanov, A.A., Carlson, B.A., et al., 2014. Inhibition of cellular methyltransferases promotes endothelial cell activation by suppressing glutathione peroxidase 1 protein expression. J. Biol. Chem. 289, 15350–15362. Becker, A., Henry, R.M., Kostense, P.J., Jakobs, C., Teerlink, T., Zweegman, S., et al., 2003a. Plasma homocysteine and S-adenosylmethionine in erythrocytes as determinants of carotid intima-media thickness: different effects in diabetic and non-diabetic individuals. The Hoorn Study. Atherosclerosis 169, 323–330. Becker, A., Smulders, Y.M., Teerlink, T., Struys, E.A., de Meer, K., Kostense, P.J., et al., 2003b. S-adenosylhomocysteine and the ratio of S-adenosylmethionine to S-adenosylhomocysteine are not related to folate, cobalamin and vitamin B6 concentrations. Eur. J. Clin. Invest. 33, 17–25. Bernstein, B.E., Meissner, A., Lander, E.S., 2007. The mammalian epigenome. Cell 128, 669–681. Bhaumik, S.R., Smith, E., Shilatifard, A., 2007. Covalent modifications of histones during development and disease pathogenesis. Nat. Struct. Mol. Biol. 14, 1008–1016.

7

Bird, A., 2002. DNA methylation patterns and epigenetic memory. Genes Dev. 16, 6–21. Boger, R.H., Bode-Boger, S.M., Sydow, K., Heistad, D.D., Lentz, S.R., 2000. Plasma concentration of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase, is elevated in monkeys with hyperhomocyst(e)inemia or hypercholesterolemia. Arterioscler. Thromb. Vasc. Biol. 20, 1557–1564. Bonaa, K.H., Njolstad, I., Ueland, P.M., Schirmer, H., Tverdal, A., Steigen, T., et al., 2006. Homocysteine lowering and cardiovascular events after acute myocardial infarction. N. Engl. J. Med. 354, 1578–1588. Brattstrom, L., Wilcken, D.E., 2000. Homocysteine and cardiovascular disease: cause or effect? Am. J. Clin. Nutr. 72, 315–323. Castro, R., Rivera, I., Martins, C., Struys, E.A., Jansen, E.E., Clode, N., et al., 2005. Intracellular S-adenosylhomocysteine increased levels are associated with DNA hypomethylation in HUVEC. J. Mol. Med. 83, 831–836. Castro, R., Rivera, I., Struys, E.A., Jansen, E.E., Ravasco, P., Camilo, M.E., et al., 2003. Increased homocysteine and S-adenosylhomocysteine concentrations and DNA hypomethylation in vascular disease. Clin. Chem. 49, 1292–1296. Caudill, M.A., Wang, J.C., Melnyk, S., Pogribny, I.P., Jernigan, S., Collins, M.D., et al., 2001. Intracellular S-adenosylhomocysteine concentrations predict global DNA hypomethylation in tissues of methyl-deficient cystathionine beta-synthase heterozygous mice. J. Nutr. 131, 2811–2818. Chan, G.C., Fish, J.E., Mawji, I.A., Leung, D.D., Rachlis, A.C., Marsden, P.A., 2005. Epigenetic basis for the transcriptional hyporesponsiveness of the human inducible nitric oxide synthase gene in vascular endothelial cells. J. Immunol. 175, 3846–3861. Chen, Z., Karaplis, A.C., Ackerman, S.L., Pogribny, I.P., Melnyk, S., Lussier-Cacan, S., et al., 2001. Mice deficient in methylenetetrahydrofolate reductase exhibit hyperhomocysteinemia and decreased methylation capacity, with neuropathology and aortic lipid deposition. Hum. Mol. Genet. 10, 433–443. Choumenkovitch, S.F., Selhub, J., Bagley, P.J., Maeda, N., Nadeau, M.R., Smith, D.E., et al., 2002. In the cystathionine beta-synthase knockout mouse, elevations in total plasma homocysteine increase tissue S-adenosylhomocysteine, but responses of S-adenosylmethionine and DNA methylation are tissue specific. J. Nutr. 132, 2157–2160. Clarke, R., Daly, L., Robinson, K., Naughten, E., Cahalane, S., Fowler, B., et al., 1991. Hyperhomocysteinemia: an independent risk factor for vascular disease. N. Engl. J. Med. 324, 1149–1155. Clarke, R., Halsey, J., Lewington, S., Lonn, E., Armitage, J., Manson, J.E., et al., 2010. Effects of lowering homocysteine levels with B vitamins on cardiovascular disease, cancer, and cause-specific mortality: meta-analysis of 8 randomized trials involving 37 485 individuals. Arch. Intern. Med. 170, 1622–1631. Dayal, S., Bottiglieri, T., Arning, E., Maeda, N., Malinow, M.R., Sigmund, C.D., et al., 2001. Endothelial dysfunction and elevation of S-adenosylhomocysteine in cystathionine beta-synthase-deficient mice. Circ. Res. 88, 1203–1209. De Vriese, A.S., Blom, H.J., Heil, S.G., Mortier, S., Kluijtmans, L.A., Van de Voorde, J., et al., 2004. Endothelium-derived hyperpolarizing factor-mediated renal vasodilatory response is impaired during acute and chronic hyperhomocysteinemia. Circulation 109, 2331–2336. Doshi, S., McDowell, I., Goodfellow, J., Stabler, S., Boger, R., Allen, R., et al., 2005. Relationship between S-adenosylmethionine, S-adenosylhomocysteine, asymmetric dimethylarginine, and endothelial function in healthy human subjects during experimental hyper- and hypohomocysteinemia. Metab. Clin. Exp. 54, 351–360. Eberhardt, R.T., Forgione, M.A., Cap, A., Leopold, J.A., Rudd, M.A., Trolliet, M., et al., 2000. Endothelial dysfunction in a murine model of mild hyperhomocyst(e)inemia. J. Clin. Invest. 106, 483–491. Elshorbagy, A.K., Church, C., Valdivia-Garcia, M., Smith, A.D., Refsum, H., Cox, R., 2012a. Dietary cystine level affects metabolic rate and glycaemic control in adult mice. J. Nutr. Biochem. 23, 332–340. Elshorbagy, A.K., Nijpels, G., Valdivia-Garcia, M., Stehouwer, C.D., Ocke, M., Refsum, H., et al., 2013. S-adenosylmethionine is associated with fat mass and truncal adiposity in older adults. J. Nutr. 143, 1982–1988. Elshorbagy, A.K., Nurk, E., Gjesdal, C.G., Tell, G.S., Ueland, P.M., Nygard, O., et al., 2008. Homocysteine, cysteine, and body composition in the Hordaland Homocysteine Study: does cysteine link amino acid and lipid metabolism? Am. J. Clin. Nutr. 88, 738–746. Elshorbagy, A.K., Valdivia-Garcia, M., Graham, I.M., Palma Reis, R., Sales Luis, A., Smith, A.D., et al., 2012b. The association of fasting plasma sulfur-containing compounds with BMI, serum lipids and apolipoproteins. Nutr. Metab. Cardiovasc. Dis. 22, 1031–1038. Esfandiari, F., Medici, V., Wong, D.H., Jose, S., Dolatshahi, M., Quinlivan, E., et al., 2010. Epigenetic regulation of hepatic endoplasmic reticulum stress pathways in the ethanol-fed cystathionine beta synthase-deficient mouse. Hepatology 51, 932–941. Fatemi, M., Hermann, A., Gowher, H., Jeltsch, A., 2002. Dnmt3a and Dnmt1 functionally cooperate during de novo methylation of DNA. Eur. J. Biochem. 269, 4981–4984. Fryer, R.H., Wilson, B.D., Gubler, D.B., Fitzgerald, L.A., Rodgers, G.M., 1993. Homocysteine, a risk factor for premature vascular disease and thrombosis, induces tissue factor activity in endothelial cells. Arterioscler. Thromb. 13, 1327–1333. Garibotto, G., Valli, A., Anderstam, B., Eriksson, M., Suliman, M.E., Balbi, M., et al., 2009. The kidney is the major site of S-adenosylhomocysteine disposal in humans. Kidney Int. 76, 293–296. Gellekink, H., van Oppenraaij-Emmerzaal, D., van Rooij, A., Struys, E.A., den Heijer, M., Blom, H.J., 2005. Stable-isotope dilution liquid

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chromatography–electrospray injection tandem mass spectrometry method for fast, selective measurement of S-adenosylmethionine and S-adenosylhomocysteine in plasma. Clin. Chem. 51, 1487–1492. Green, T.J., Skeaff, C.M., McMahon, J.A., Venn, B.J., Williams, S.M., Devlin, A.M., et al., 2010. Homocysteine-lowering vitamins do not lower plasma S-adenosylhomocysteine in older people with elevated homocysteine concentrations. Br. J. Nutr. 103, 1629–1634. Grillo, M.A., Colombatto, S., 2008. S-adenosylmethionine and its products. Amino Acids 34, 187–193. Guenther, M.G., Levine, S.S., Boyer, L.A., Jaenisch, R., Young, R.A., 2007. A chromatin landmark and transcription initiation at most promoters in human cells. Cell 130, 77–88. Hensley, K., Carney, J.M., Mattson, M.P., Aksenova, M., Harris, M., Wu, J.F., et al., 1994. A model for beta-amyloid aggregation and neurotoxicity based on free radical generation by the peptide: relevance to Alzheimer disease. Proc. Natl. Acad. Sci. U. S. A. 91, 3270–3274. Hermes, M., Geisler, H., Osswald, H., Riehle, R., Kloor, D., 2008. Alterations in S-adenosylhomocysteine metabolism decrease O6-methylguanine DNA methyltransferase gene expression without affecting promoter methylation. Biochem. Pharmacol. 75, 2100–2111. Herrmann, W., Schorr, H., Obeid, R., Makowski, J., Fowler, B., Kuhlmann, M.K., 2005. Disturbed homocysteine and methionine cycle intermediates S-adenosylhomocysteine and S-adenosylmethionine are related to degree of renal insufficiency in type 2 diabetes. Clin. Chem. 51, 891–897. Hotamisligil, G.S., 2010a. Endoplasmic reticulum stress and the inflammatory basis of metabolic disease. Cell 140, 900–917. Hotamisligil, G.S., 2010b. Endoplasmic reticulum stress and atherosclerosis. Nat. Med. 16, 396–399. Innis, S.M., Davidson, A.G., Chen, A., Dyer, R., Melnyk, S., James, S.J., 2003. Increased plasma homocysteine and S-adenosylhomocysteine and decreased methionine is associated with altered phosphatidylcholine and phosphatidylethanolamine in cystic fibrosis. J. Pediatr. 143, 351–356. Inoue-Choi, M., Nelson, H.H., Robien, K., Arning, E., Bottiglieri, T., Koh, W.P., et al., 2012. One-carbon metabolism nutrient status and plasma S-adenosylmethionine concentrations in middle-aged and older Chinese in Singapore. Int. J. Mol. Epidemiol. Genet. 3, 160–173. Jabs, K., Koury, M.J., Dupont, W.D., Wagner, C., 2006. Relationship between plasma S-adenosylhomocysteine concentration and glomerular filtration rate in children. Metab. Clin. Exp. 55, 252–257. James, S.J., Melnyk, S., Pogribna, M., Pogribny, I.P., Caudill, M.A., 2002. Elevation in S-adenosylhomocysteine and DNA hypomethylation: potential epigenetic mechanism for homocysteine-related pathology. J. Nutr. 132, 2361S–2366S. Jamison, R.L., Hartigan, P., Kaufman, J.S., Goldfarb, D.S., Warren, S.R., Guarino, P.D., et al., 2007. Effect of homocysteine lowering on mortality and vascular disease in advanced chronic kidney disease and end-stage renal disease: a randomized controlled trial. JAMA 298, 1163–1170. Jiang, Y., Sun, T., Xiong, J., Cao, J., Li, G., Wang, S., 2007. Hyperhomocysteinemia-mediated DNA hypomethylation and its potential epigenetic role in rats. Acta Biochim. Biophys. Sin. 39, 657–667. Kanani, P.M., Sinkey, C.A., Browning, R.L., Allaman, M., Knapp, H.R., Haynes, W.G., 1999. Role of oxidant stress in endothelial dysfunction produced by experimental hyperhomocyst(e)inemia in humans. Circulation 100, 1161–1168. Kang, S.S., Wong, P.W.K., Malinow, M.R., 1992. Hyperhomocyst(e)inemia as a risk factor for occlusive vascular disease. Annu. Rev. Nutr. 12, 279–298. Kennedy, B.P., Bottiglieri, T., Arning, E., Ziegler, M.G., Hansen, L.A., Masliah, E., 2004. Elevated S-adenosylhomocysteine in Alzheimer brain: influence on methyltransferases and cognitive function. J. Neural Transm. 111, 547–567. Kerins, D.M., Koury, M.J., Capdevila, A., Rana, S., Wagner, C., 2001. Plasma S-adenosylhomocysteine is a more sensitive indicator of cardiovascular disease than plasma homocysteine. Am. J. Clin. Nutr. 74, 723–729. Lee, J.K., Kim, K.C., 2013. DZNep, inhibitor of S-adenosylhomocysteine hydrolase, down-regulates expression of SETDB1 H3K9me3 HMTase in human lung cancer cells. Biochem. Biophys. Res. Commun. 438, 647–652. Lin, H.C., Song, T.Y., Hu, M.L., 2011. S-Adenosylhomocysteine enhances DNA damage through increased beta-amyloid formation and inhibition of the DNA-repair enzyme OGG1b in microglial BV-2 cells. Toxicology 290, 342–349. Lin, P.Y., Yang, T.H., Lin, H.G., Hu, M.L., 2007. Synergistic effects of S-adenosylhomocysteine and homocysteine on DNA damage in a murine microglial cell line. Clin. Chim. Acta 379, 139–144. Liu, C., Wang, Q., Guo, H., Xia, M., Yuan, Q., Hu, Y., et al., 2008. Plasma S-adenosylhomocysteine is a better biomarker of atherosclerosis than homocysteine in apolipoprotein E-deficient mice fed high dietary methionine. J. Nutr. 138, 311–315. Loehrer, F.M., Angst, C.P., Brunner, F.P., Haefeli, W.E., Fowler, B., 1998. Evidence for disturbed S-adenosylmethionine: S-adenosylhomocysteine ratio in patients with end-stage renal failure: a cause for disturbed methylation reactions? Nephrol. Dial. Transplant. 13, 656–661. Loehrer, F.M., Tschopl, M., Angst, C.P., Litynski, P., Jager, K., Fowler, B., et al., 2001. Disturbed ratio of erythrocyte and plasma S-adenosylmethionine/S-adenosylhomocysteine in peripheral arterial occlusive disease. Atherosclerosis 154, 147–154. Lonn, E., Yusuf, S., Arnold, M.J., Sheridan, P., Pogue, J., Micks, M., et al., 2006. Homocysteine lowering with folic acid and B vitamins in vascular disease. N. Engl. J. Med. 354, 1567–1577.

Loscalzo, J., 2006. Homocysteine trials – clear outcomes for complex reasons. N. Engl. J. Med. 354, 1629–1632. Lu, S.C., Chen, Y.J., Chang, P.Y., 2010. Letter to the editor: is homocysteine the culprit molecule in vascular diseases or just a bystander? Am. J. Physiol. Cell Physiol. 299, C873, author reply C4. Luo, X., Xiao, Y., Song, F., Yang, Y., Xia, M., Ling, W., 2012. Increased plasma S-adenosyl-homocysteine levels induce the proliferation and migration of VSMCs through an oxidative stress-ERK1/2 pathway in apoE(−/−) mice. Cardiovasc. Res. 95, 241–250. Lusis, A.J., 2000. Atherosclerosis. Nature 407, 233–241. Mansoor, M.A., Bergmark, C., Svardal, A.M., Lonning, P.E., Ueland, P.M., 1995. Redox status and protein binding of plasma homocysteine and other aminothiols in patients with early-onset peripheral vascular disease. Homocysteine and peripheral vascular disease. Arterioscler. Thromb. Vasc. Biol. 15, 232–240. Matouk, C.C., Marsden, P.A., 2008. Epigenetic regulation of vascular endothelial gene expression. Circ. Res. 102, 873–887. McCully, K.S., 1969. Vascular pathology of homocysteinemia: implications for the pathogenesis of arteriosclerosis. Am. J. Pathol. 56, 111–128. Ngo, S., Li, X., O’Neill, R., Bhoothpur, C., Gluckman, P., Sheppard, A., 2014. Elevated S-adenosylhomocysteine alters adipocyte functionality with corresponding changes in gene expression and associated epigenetic marks. Diabetes 63, 2273–2283. Reddy, M.A., Villeneuve, L.M., Wang, M., Lanting, L., Natarajan, R., 2008. Role of the lysine-specific demethylase 1 in the proinflammatory phenotype of vascular smooth muscle cells of diabetic mice. Circ. Res. 103, 615–623. Schachinger, V., Britten, M.B., Zeiher, A.M., 2000. Prognostic impact of coronary vasodilator dysfunction on adverse long-term outcome of coronary heart disease. Circulation 101, 1899–1906. Selley, M.L., 2003. Increased concentrations of homocysteine and asymmetric dimethylarginine and decreased concentrations of nitric oxide in the plasma of patients with Alzheimer’s disease. Neurobiol. Aging 24, 903–907. Selley, M.L., 2007. A metabolic link between S-adenosylhomocysteine and polyunsaturated fatty acid metabolism in Alzheimer’s disease. Neurobiol. Aging 28, 1834–1839. Sipkens, J.A., Hahn, N.E., Blom, H.J., Lougheed, S.M., Stehouwer, C.D., Rauwerda, J.A., et al., 2012. S-Adenosylhomocysteine induces apoptosis and phosphatidylserine exposure in endothelial cells independent of homocysteine. Atherosclerosis 221, 48–54. Spijkerman, A.M., Smulders, Y.M., Kostense, P.J., Henry, R.M., Becker, A., Teerlink, T., et al., 2005. S-adenosylmethionine and 5-methyltetrahydrofolate are associated with endothelial function after controlling for confounding by homocysteine: the Hoorn Study. Arterioscler. Thromb. Vasc. Biol. 25, 778–784. Stam, F., van Guldener, C., ter Wee, P.M., Kulik, W., Smith, D.E., Jakobs, C., et al., 2004. Homocysteine clearance and methylation flux rates in health and end-stage renal disease: association with S-adenosylhomocysteine. Am. J. Physiol. Renal Physiol. 287, F215–F223. Starkebaum, G., Harlan, J.M., 1986. Endothelial cell injury due to copper-catalyzed hydrogen peroxide generation from homocysteine. J. Clin. Invest. 77, 1370–1376. Steed, M.M., Tyagi, S.C., 2011. Mechanisms of cardiovascular remodeling in hyperhomocysteinemia. Antioxid. Redox Signal. 15, 1927–1943. Stipanuk, M.H., 2004. Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine. Annu. Rev. Nutr. 24, 539–577. Struys, E.A., Jansen, E.E., de Meer, K., Jakobs, C., 2000. Determination of S-adenosylmethionine and S-adenosylhomocysteine in plasma and cerebrospinal fluid by stable-isotope dilution tandem mass spectrometry. Clin. Chem. 46, 1650–1656. Swanson, D.A., Liu, M.L., Baker, P.J., Garrett, L., Stitzel, M., Wu, J., et al., 2001. Targeted disruption of the methionine synthase gene in mice. Mol. Cell. Biol. 21, 1058–1065. Tao, J., Yang, M., Chen, Z., Huang, Y., Zhao, Q., Xu, J., et al., 2008. Decreased DNA methyltransferase 3A and 3B mRNA expression in peripheral blood mononuclear cells and increased plasma SAH concentration in adult patients with idiopathic thrombocytopenic purpura. J. Clin. Immunol. 28, 432–439. Troen, A.M., Lutgens, E., Smith, D.E., Rosenberg, I.H., Selhub, J., 2003. The atherogenic effect of excess methionine intake. Proc. Natl. Acad. Sci. U. S. A. 100, 15089–15094. Tyagi, N., Ovechkin, A.V., Lominadze, D., Moshal, K.S., Tyagi, S.C., 2006. Mitochondrial mechanism of microvascular endothelial cells apoptosis in hyperhomocysteinemia. J. Cell. Biochem. 98, 1150–1162. Ueland, P.M., Refsum, H., Beresford, S.A., Vollset, S.E., 2000. The controversy over homocysteine and cardiovascular risk. Am. J. Clin. Nutr. 72, 324–332. Valli, A., Carrero, J.J., Qureshi, A.R., Garibotto, G., Barany, P., Axelsson, J., et al., 2008. Elevated serum levels of S-adenosylhomocysteine, but not homocysteine, are associated with cardiovascular disease in stage 5 chronic kidney disease patients. Clin. Chim. Acta 395, 106–110. van Driel, L.M., Eijkemans, M.J., de Jonge, R., de Vries, J.H., van Meurs, J.B., Steegers, E.A., et al., 2009. Body mass index is an important determinant of methylation biomarkers in women of reproductive ages. J. Nutr. 139, 2315–2321. Villeneuve, L.M., Reddy, M.A., Lanting, L.L., Wang, M., Meng, L., Natarajan, R., 2008. Epigenetic histone H3 lysine 9 methylation in metabolic memory and inflammatory phenotype of vascular smooth muscle cells in diabetes. Proc. Natl. Acad. Sci. U. S. A. 105, 9047–9052. Wagner, C., Koury, M.J., 2007. S-Adenosylhomocysteine: a better indicator of vascular disease than homocysteine? Am. J. Clin. Nutr. 86, 1581–1585.

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Wagner, C., Stone, W.J., Koury, M.J., Dupont, W.D., Kerins, D.M., 2004. S-adenosylhomocysteine is a more sensitive indicator of renal insufficiency than homocysteine. Nutr. Res. 24, 487–494. Wald, D.S., Kasturiratne, A., Simmonds, M., 2011. Serum homocysteine and dementia: meta-analysis of eight cohort studies including 8669 participants. Alzheimers Dement. 7, 412–417. Wang, H., Jiang, X., Yang, F., Gaubatz, J.W., Ma, L., Magera, M.J., et al., 2003. Hyperhomocysteinemia accelerates atherosclerosis in cystathionine beta-synthase and apolipoprotein E double knock-out mice with and without dietary perturbation. Blood 101, 3901–3907. Watanabe, M., Osada, J., Aratani, Y., Kluckman, K., Reddick, R., Malinow, M.R., et al., 1995. Mice deficient in cystathionine beta-synthase: animal models for mild and severe homocyst(e)inemia. Proc. Natl. Acad. Sci. U. S. A. 92, 1585–1589. Weiss, N., Heydrick, S., Zhang, Y.Y., Bierl, C., Cap, A., Loscalzo, J., 2002. Cellular redox state and endothelial dysfunction in mildly hyperhomocysteinemic cystathionine beta-synthase-deficient mice. Arterioscler. Thromb. Vasc. Biol. 22, 34–41. Wijekoon, E.P., Brosnan, M.E., Brosnan, J.T., 2007. Homocysteine metabolism in diabetes. Biochem. Soc. Trans. 35, 1175–1179. Wilson, K.M., McCaw, R.B., Leo, L., Arning, E., Lhotak, S., Bottiglieri, T., et al., 2007. Prothrombotic effects of hyperhomocysteinemia and hypercholesterolemia in ApoE-deficient mice. Arterioscler. Thromb. Vasc. Biol. 27, 233–240. Xiao, Y., Huang, W., Zhang, J., Peng, C., Xia, M., Ling, W., 2015. Increased plasma S-adenosylhomocysteine-accelerated atherosclerosis is associated with

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epigenetic regulation of endoplasmic reticulum stress in apoE−/− mice. Arterioscler. Thromb. Vasc. Biol. 35, 60–70. Xiao, Y., Zhang, Y., Wang, M., Li, X., Su, D., Qiu, J., et al., 2013. Plasma S-adenosylhomocysteine is associated with the risk of cardiovascular events in patients undergoing coronary angiography: a cohort study. Am. J. Clin. Nutr. 98, 1162–1169. Yang, T.H., Yang, N.C., Hu, M.L., 2003. S-adenosylhomocysteine enhances hydrogen peroxide-induced DNA damage by inhibition of DNA repair in two cell lines. Nutr. Cancer 47, 70–75. Yi, P., Melnyk, S., Pogribna, M., Pogribny, I.P., Hine, R.J., James, S.J., 2000. Increase in plasma homocysteine associated with parallel increases in plasma S-adenosylhomocysteine and lymphocyte DNA hypomethylation. J. Biol. Chem. 275, 29318–29323. Zawada, A.M., Rogacev, K.S., Hummel, B., Berg, J.T., Friedrich, A., Roth, H.J., et al., 2014. S-adenosylhomocysteine is associated with subclinical atherosclerosis and renal function in a cardiovascular low-risk population. Atherosclerosis 234, 17–22. Zhou, J., Moller, J., Danielsen, C.C., Bentzon, J., Ravn, H.B., Austin, R.C., et al., 2001. Dietary supplementation with methionine and homocysteine promotes early atherosclerosis but not plaque rupture in ApoE-deficient mice. Arterioscler. Thromb. Vasc. Biol. 21, 1470–1476. Zhou, J., Werstuck, G.H., Lhotak, S., de Koning, A.B., Sood, S.K., Hossain, G.S., et al., 2004. Association of multiple cellular stress pathways with accelerated atherosclerosis in hyperhomocysteinemic apolipoprotein E-deficient mice. Circulation 110, 207–213.

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