Thioredoxin in the cardiovascular system
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- World, C.J., Yamawaki, H. & Berk, B.C. J Mol Med (2006) 84: 997. doi:10.1007/s00109-006-0109-6
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The thioredoxin (TRX) system (TRX, TRX reductase, and NADPH) is a ubiquitous thiol oxidoreductase system that regulates cellular reduction/oxidation (redox) status. The impairment of cell redox state alters multiple cell pathways, which may contribute to the pathogenesis of cardiovascular disorders including hypertension, atherosclerosis, and heart failure. In this manuscript, we review the essential roles that TRX plays by limiting oxidative stress directly via antioxidant effects and indirectly by protein–protein interactions with key signaling molecules such as thioredoxin interacting protein (TXNIP). TRX and its endogenous regulators may represent important future targets to develop clinical therapies for diseases associated with oxidative stress.
KeywordsAntioxidantsCardiovascular diseasesEndotheliumSmooth muscleSignal transduction
Apoptosis signal-regulating kinase 1
Jun N-terminal kinase
Platelet-derived growth factor
Reactive oxygen species
Vascular cell adhesion molecule
Vascular smooth muscle cell
Introduction: general function of thioredoxin system
TRX function in vascular endothelial and smooth muscle cells
TRX is ubiquitously expressed in endothelial cells (EC)  and protects EC from H2O2-induced cytotoxicity . Because treatment with H2O2 increased TRX expression in EC, it appears that TRX is a ROS-inducible protein. Interestingly, a low concentration of H2O2 (10–50 μM) protects EC from apoptosis by increasing TRX expression . TRX is also ubiquitously expressed in vascular smooth muscle cells (VSMC) of normal arteries . Schulze et al.  showed, in human aortic VSMC, that adenoviral gene transfer of TRX increased DNA synthesis, suggesting a role for TRX in VSMC proliferation. Importantly, the expression of TRX in VSMC is not regulated by ROS, since no change in TRX expression was observed after treatment with H2O2 or platelet-derived growth factor (PDGF).
Given that EC and SMC respond differently to a pathological insult, it would be interesting to address the apparently opposing observations that TRX promotes EC survival and intimal SMC proliferation. It seems possible that there are distinct cell-specific regulatory mechanisms that may include different binding partners and changes in sub-cellular localization. These issues represent a novel avenue of research for the investigation of how TRX activity is regulated.
TRX function in atherosclerosis, hypertension, and vascular injury
In human coronary atherosclerotic specimens, TRX expression is enhanced throughout the vessel wall . The greatest increases were observed in EC and infiltrating macrophages within the neointimal plaques [10, 14]. Furman et al.  recently found that TRX reductase, a ubiquitous 55 kDa selenoprotein, was upregulated in human atherosclerotic plaques. They showed that oxidized low-density lipoproteins increased TRX reductase expression in human monocyte-derived macrophages. The results suggest that TRX and TRX reductase cooperate to work for antioxidant defense mechanisms in atherosclerosis.
Since oxidative stress plays a crucial role in the development and pathogenesis of hypertension, it is likely that TRX–TRX reductase function may be altered in various hypertensive models. Tanito et al.  recently showed increased oxidative stress in tissues (aorta, heart, and kidney) of spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive (SHRSP) rats compared to Wistar Kyoto (WKY) rats. The expression of TRX was markedly decreased in SHR and SHRSP tissues. Furthermore, the induction of TRX was impaired after angiotension II treatment of peripheral blood mononuclear cells isolated from SHR and SHRSP compared to WKY. Yamagata et al.  reported similar data for cultured cortical neurons isolated from SHRSP after ischemia reperfusion. These studies suggest that decreased TRX in tissues of the SHR and SHRSP genetically hypertensive rats may contribute to hypertension and its sequelae.
In balloon-injured rat carotid arteries, TRX expression increased in regenerating EC. Takagi et al.  suggested that NO, produced by inducible nitric oxide synthase (iNOS), plays a crucial role in the induction of TRX, since the localization of iNOS strongly correlated with TRX. Because excess NO production by iNOS may be cytotoxic by forming peroxynitrite , the findings suggest that induction of TRX represents a protective mechanism against nitrosative and oxidative stress. This was confirmed by the demonstration that TRX-transfected cells were more resistant to peroxynitrite-induced cytotoxicity than control-transfected cells .
Thus, it appears that TRX acts to reduce cellular oxidative stress, and therefore, plays a crucial role in attenuating the pathology associated with vascular disease. It is now important to identify the mechanism by which TRX acts. As described in this review, the biochemical actions of TRX are diverse and include the ability to regulate signal transduction and gene expression. Thus, the ability of TRX to limit vascular and cardiac disease may include the regulation of cell processes via or independent of its action as an antioxidant.
TRX function in cardiac disease
Turoczi et al.  found in ex vivo working rat heart that reperfusion of ischemic myocardium down-regulated TRX expression. They also showed that TRX-over-expressing mouse hearts had improved post-ischemic ventricular recovery and reduced myocardial infarct size compared to wild-type hearts. Conversely transgenic mice with cardiac-specific over-expression of a dominant negative TRX mutant (Cys-32/35-Ser) exhibited increased cardiac oxidative stress and hypertrophy both under basal conditions and in response to pressure overload . It was recently demonstrated in adult rat cardiomyocyte that TRX over-expression prevented α-adrenergic receptor-stimulated hypertrophy by inhibiting Ras activation . These results implicate a protective role for endogenous TRX in failing heart. Other investigators found important roles for exogenous TRX in decreasing reperfusion-induced arrhythmias , adriamycin-induced cytotoxic injury , and in autoimmune myocarditis . When TRX was partially S-nitrosated at Cys-69, its cardioprotective effect was markedly enhanced , a result in line with the demonstration that nitrosylation of Cys-69 accounts in part for the anti-apoptotic activity of TRX . Though these results suggest that both endogenous and exogenous TRX provide protection against ROS-mediated cardiotoxicity, the mechanism by which this occurs remains unknown. Given a role of TRX for regulating NFκB to regulate the transcription of anti-oxidant and pro-survival genes, it would be worthwhile investigating changes in gene expression associated with TRX-mediated cardiac protection via cDNA array analysis.
Plasma TRX level: a marker for oxidative stress and inflammation-related disease?
Plasma TRX levels are elevated in conditions associated with oxidative stress and inflammation such as HIV  and rheumatoid arthritis . Kishimoto et al.  reported that plasma TRX levels were significantly elevated in patients with acute coronary syndromes and dilated cardiomyopathy compared with the control subjects. In addition, plasma TRX levels correlated positively with the severity of New York Heart Association functional class and negatively with left ventricular ejection fraction. These results suggest a possible association between TRX secretion and the severity of heart failure. Plasma TRX is also increased in patients with chronic heart failure , acute myocardial infarction , and angina [32, 33]. These results support the positive association between plasma TRX levels and heart failure.
Another key mechanism by which TRX mediates cell protection is via binding to signaling molecules and modulating their function. Below, we discuss several examples with important cardiovascular effects.
Apoptosis signal-regulating kinase 1
Apoptosis signal-regulating kinase 1 (ASK1), a mitogen-activated protein (MAP) kinase kinase kinase, plays an essential role in stress-induced apoptosis . ASK1 is activated by many stress- and cytokine-related stimuli and activates JNK and p38 MAP kinases. Through genetic screening for ASK1-binding proteins, Saitoh et al.  found that TRX bound directly to the N-terminus of ASK1 and inhibited ASK1 kinase activity and ASK1-dependent apoptosis. The interaction between TRX and ASK1 was regulated by TRX redox status, since the interaction was observed only under reducing conditions.
TRX interacting protein
Thioredoxin interacting protein (TXNIP; also termed VDUP1 for vitamin D3-upregulated protein) was originally identified in HL-60 leukemia cells treated with 1,25-dihydroxyvitamin D3 . Thereafter, Nishiyama et al.  isolated TXNIP as a TRX-binding protein using a yeast two-hybrid system. Biochemical analysis showed that TXNIP inhibits TRX activity by interacting with the catalytic site of TRX, suggesting that TXNIP is an endogenous inhibitor of TRX [37, 38].
There is accumulating evidence that TXNIP plays a pivotal role in cardiovascular disorders, functioning as a sensor for biomechanical and oxidative stress. Schulze et al.  recently reported in VSMC that hyperglycemia increased oxidative stress by inducing TXNIP and inhibiting the antioxidant function of TRX. They also showed that diabetic animals exhibited increased vascular expression of TXNIP. Yoshioka et al.  reported that TXNIP expression was decreased in pressure-overload cardiac hypertrophy followed by TRX-induced stimulation of cardiac cell growth. These results support the emerging concept that TXNIP is a critical regulator of biomechanical signaling in cardiovascular disorders.
It was also demonstrated in VSMC that PDGF suppressed TXNIP expression with increases in TRX activity and DNA synthesis . Conversely, over-expression of TXNIP abolished PDGF-induced TRX activity and DNA synthesis. These results suggest that TXNIP has pro-apoptotic effects in VSMC through the suppression of TRX activity. We recently found in the endothelium of intact rabbit aorta that exposure to physiologic fluid shear stress (12 dyn/cm2, 24 h) decreased TXNIP expression and increased TRX activity . Physiologic flow inhibited TNF stimulation of JNK, p38, and vascular cell adhesion molecule 1 (VCAM1) expression in aortic EC . In cultured EC, decreasing TXNIP by RNA interference increased TRX binding to ASK1 and inhibited TNF stimulation of JNK, p38, and VCAM1 expression . These data demonstrate a novel mechanism for the anti-inflammatory effects of fluid shear stress via decreased TXNIP, increased TRX activity, and decreased activity of JNK, p38, and VCAM1. Since inflammation and apoptosis are key mechanisms in atherosclerosis, we propose that TXNIP is a novel biomechanical effector of atherosclerosis.
Physiological actions of TXNIP
With the knowledge that TXNIP is an endogenous inhibitor of TRX activity and function , there is, therefore, much to deduce about TRX actions from the studies of TXNIP expression and physiological function. Our own unpublished data demonstrate that TXNIP is an early growth response gene with a half-life of approximately 15 min, thus, changes in its expression are primarily regulated at the level of transcription. In general, it has been shown that increases in TXNIP mRNA expression are mirrored by a concomitant decrease in TRX expression [43, 44]. This is most evident in cells undergoing apoptosis in a variety of cells including pancreatic beta cells [45, 46], cerebellar granule neurons , and cardiomyocytes , suggesting a role for TXNIP as a pro-apoptotic factor. In line with this role as a regulator of cell death is the observation that TXNIP over-expression in insulinoma beta cells acts as a transcriptional repressor suppressing the expression of pro-survival genes such as BcL2-like 1 and voltage-dependent anion channel 1 . Furthermore, it has been demonstrated that adenovirus-mediated over-expression of TXNIP suppresses TRX activity, induces cardiomyocyte death, and sensitizes cells to oxidative stress-mediated apoptosis . In an animal model of myocardial ischemia, it was further shown that TXNIP knockdown by DNA enzymes promoted cardiomyocyte survival, reduced cardiac expression of pro-collagen type I α2, and left ventricular scar formation coupled with a significant improvement in cardiac function . Recently it was shown that TXNIP is localized to the mitochondria and is capable of interacting with the apoptasome complex cofactor cytochrome C . However, the nature of this interaction and the nature of its contribution to apoptosis and mitochondrial oxidative stress remain unknown. It is now well established that TXNIP contributes to cell death likely due to its ability to antagonize the pro-survival actions of TRX. Thus, due to the extent of apoptosis evident within the vessel wall of vascular lesions, it is anticipated that the ability to regulate TXNIP expression in vascular cells will represent a strategy by which to treat human diseases such as atherosclerosis and inflammation following angioplasty.
The ability of TXNIP to act as a transcriptional repressor (as described above) was initially demonstrated in response to growth inhibitors such as transforming growth factor-beta and vitamin D3 . Furthermore, it was shown that the transfection of TXNIP in tumor cells reduced cell growth by acting as a tumor-suppressor inducing cell cycle arrest at G0/G1 phase . In addition, it was demonstrated that TXNIP promoted the stability and nuclear localization of the cyclin-dependent kinase inhibitor p27kip1 , thereby inhibiting fibroblast growth and retarding T-cell growth by reducing retinoblastoma phosphorylation and p16 expression . The growth-suppressive activity of TXNIP is paralleled by the demonstration that it is primarily localized in the nucleus [51, 55]. In line with its ability to act as a suppressor of cell growth is the demonstration that TXNIP expression is decreased in several human tumors [54, 56, 57]. Moreover, a potent anti-oncogenic molecule, suberoylanilide hydroxamic acid was shown to enhance the expression of TXNIP accompanied by a significant reduction in TRX . Significantly, it has been shown that TXNIP over-expression inhibits PDGF-mediated increases in TRX activity and DNA synthesis , thereby demonstrating a role for TXNIP as a critical molecule in the transduction of pro-oxidant mitogenic signals.
To date, the mechanism by which TRX and TXNIP translocate and enter the nucleus is yet to be elucidated. Furthermore, given TRX has numerous binding partners found in distinct subcellular locales, it remains likely that changes in TRX localization represent a novel mechanism to regulate TRX activity. Thus, an area of further research is investigating the hypothesis that regulating TRX localization is a more efficient clinical strategy than simply enhancing TRX expression.
In conjunction with the ability of high glucose to increase TXNIP expression [45, 46, 58], it has also been demonstrated that hyperglycemia-induced oxidative stress in the vasculature inhibits TRX activity and increases TXNIP expression , evidently via the action of a glucocorticoid response element found within the TXNIP promoter . Furthermore, in HcB-19 mice where there is a nonsense mutation that eliminates TXNIP expression, there is a three-fold increase in insulin secretion under fasting conditions . It is therefore implicated that TXNIP, as a regulator of insulin secretion and inhibitor of the anti-oxidative actions of TRX, contributes as an important mechanism for vascular oxidative stress and the pathology of diabetes mellitus.
This study was supported by the NIH grants (HL-62826 and HL-64839) to BCB.