Review ArticleCysteine/cystine redox signaling in cardiovascular disease
Introduction
Oxidative stress has been implicated in the progression of many diseases including cardiovascular disease (CVD), cancer, and neurodegenerative disease. Numerous studies supporting an important role for free radicals in disease provided a basis to use antioxidants such as vitamin C and E in prevention and treatment trials for CVD; however, these trials showed that these antioxidant vitamins have limited benefits [1], [2], [3]. Moreover, a systematic review on the effects of antioxidant supplements indicated that treatment with β-carotene, vitamin A, and vitamin E may increase mortality rather than improve health in randomized primary and secondary prevention trials [4]. Therefore, development of therapeutics for preventing CVD using different approaches is critical.
Proteins present in extracellular fluids and on the surface of cells are susceptible to oxidation through reactive thiols in cysteine (Cys) residues. Reactive thiols often form transient catalytic intermediates in the reaction cycle of many enzymes or serve as a site of covalent modification to regulate biological activity. Alterations in protein activity by modifying the redox state of functionally essential thiols affect cellular signaling mechanisms, which couples protein redox state directly to functional activity. Oxidation and modification of extracellular thiols have a significant effect on lymphocyte proliferation and function, illustrating the importance of maintaining extracellular thiols in cell signaling [5], [6], [7]. These changes in lymphocyte function have been implicated in CVD [8], [9]. In addition to CVD, the progression of numerous diseases involves oxidation and modification of thiols. For instance, oxidation of thiols sensitized cells to radiation-induced death, indicating that thiol plays a critical role in protecting cells from a pathologic event [10]. Available data indicate that control of protein redox state via thiol/disulfide switching is critical for normal cellular activities and for maintaining physiological functions. Consequently, thiol oxidation offers an alternative mechanism by which oxidative stress could contribute to disease with little or no dependence upon free radicals.
Earlier studies suggested that high plasma homocysteine is associated with other risk factors for CVD [11], [12]. More recent evidence has suggested that these risk factors are specifically associated with the redox state of thiol/disulfide systems [11], [13], [14], [15], [16]. This fact was supported by multiple studies suggesting that increased plasma homocysteine levels of patients with peripheral vascular disease and decreased plasma albumin levels were associated with oxidation of plasma redox state [14], [15]. High methionine (Met) levels in association with hyperhomocystenemia lead to atherosclerosis in the coronary artery, which is exacerbated when combined with high dietary cholesterol [17], [18]. Met intake is positively associated with coronary artery disease and death, whereas protein intake is negatively associated and has no relation to homocysteine levels [17]. Earlier studies also showed that increased total Cys was associated with pathologic conditions such as CVD [19], [20], [21], [22]. However, this interpretation was not correct because of a lack of separate quantification of the reduced form, Cys; the oxidized form, CySS; and the mixed protein Cys disulfide. Later, the increased level of total Cys was redefined as an increase in the oxidized form of Cys and the protein-bound disulfide form. Because Cys/CySS is the most abundant low-molecular-weight thiol/disulfide couple in human plasma [14], [23], the value of the plasma redox state is largely determined by the redox state of Cys/CySS.
In addition to the critical function of Cys in proteins, the redox state of free Cys and CySS in human plasma has attracted attention as a means to measure oxidative stress in the clinical setting. This review summarizes relevant literature showing that the balance of Cys and CySS modulates cellular events relevant to CVD, including early proinflammatory signaling-controlled cell adhesion [24], [25], [26], cell proliferation [27], and resistance to apoptosis [28]. The clinical research showing that Cys becomes more oxidized in association with age [23], [29], smoking [30], and age-related diseases [31] is summarized. This includes evidence that increased oral intake of zinc and sulfur amino acid supplements modulates the Cys and CySS concentrations in plasma [32], thereby providing possible approaches to decrease the risk of CVD. The advances in the understanding of Cys/CySS redox signaling and control suggest that free radical scavenging trials may have failed because these antioxidants do not correct oxidative stress-associated disruption of thiol/disulfide systems in vascular diseases.
Section snippets
Effects of oxidized extracellular EhCySS on proinflammatory signaling
The innermost cell layer of blood vessels is the endothelium. These cells play a vital role in maintaining vascular health by responding to physical and biochemical changes in the blood. The normal endothelium promotes vasodilatation, is anti-inflammatory, and inhibits thrombosis. When the endothelium becomes dysfunctional, many of these properties are altered. Endothelial injury and dysfunction, highly associated with increased oxidative stress and inflammation, stimulate atherosclerotic
Oxidation of plasma redox in association with risk factors for CVD
The concept that EhCySS modulation can affect cell signaling provides a clear alternative to free radical mechanisms for oxidative stress in CVD. Oxidized EhCySS is sensed by cell-surface thiols in endothelial cells and monocytes. In endothelial cells, oxidants activate NF-κB and increase expression of cell adhesion molecules. In monocytes, oxidation activates proinflammatory cytokine IL-1β and TNF-α production. Together, these processes enhance the critical early event of vascular monocyte
Cys/CySS redox interactions with sulfur amino acid metabolism
Previous studies have extensively addressed the roles of homocysteine and GSH in CVD [19], [75]; these two molecules are connected with Cys metabolism in multiple ways and the quantitative relationships have not been fully explored (Fig. 4). Cys and CySS metabolism involves both synthesis and degradation of the carbon skeleton and also reversible oxidation–reduction of the thiol. Studies of Cys/CySS turnover using stable isotopic tracer methods in humans have addressed only the carbon skeleton
Current understanding of the regulation of plasma EhCySS
The quantitatively important mechanisms involved in regulation of plasma and extracellular EhCySS are not known, but several human, rodent, and cellular studies have provided possible mechanisms of regulation. Studies with cellular systems indicate that the balance of “reduction” and “oxidation” is largely determined by transport. Release of both GSH and Cys from cells probably contributes to supplying thiol, whereas thiol loss occurs by Cys uptake and by oxidation to form disulfide. Cystine
Global effects on cell signaling mechanisms and functions controlled by extracellular EhCySS
In addition to the proinflammatory signaling in monocytes and endothelial cells described above, a number of other redox signaling pathways have been described. These are summarized in Fig. 6 and briefly discussed because of possible relevance to the integrated effects of diet, lifestyle, and metabolism in CVD. Global effects of Eh on signaling in other cell types and in organ systems could have important indirect effects on vascular function and thereby an impact on CVD.
The earlier findings on
Conclusions
Accumulating data on Cys/CySS signaling provide a mechanism for oxidative stress in CVD. Both endothelial and monocytic cells in culture respond to oxidized Cys/CySS with proinflammatory signaling and increased cell adhesion. The signaling responses occur over an oxidized range of EhCySS that occurs in humans in association with age, cigarette smoking, obesity, and alcohol abuse, known risk factors for CVD. Although free radical mechanisms have long been considered central to CVD, these
Acknowledgments
This work was supported by NIH Grants ES011195 and ES009047. We gratefully acknowledge Drs. David G. Harrison, Young Sup Yoon (Cardiology, Emory University), and Siobhan E. Craige (Cardiology, University of Massachusetts) for their reading of and critical comments on the manuscript.
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