استرس اکسیداتیو و آسیب کاردیوواسکولار : مکانیزم های عمومی و نظارت بر ROS در داخل بدن

ABSTRACT Oxidative Stress and Cardiovascular Injury - Part I: Basic Mechanisms and In Vivo Monitoring of ROS

There are many ROS that play central roles in vascular physiology (Figure 1) and pathophysiology, the most important of which are nitric oxide (NO·), superoxide (O₂^·−), hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^·−). NO· is normally produced by endothelial nitric oxide synthase (eNOS) in the vasculature, but in inflammatory states, inducible NOS can be expressed in macrophages and smooth muscle cells and contributes to NO·production. NO·is a crucial mediator of endothelium-dependent vasodilation and may also play a role in platelet aggregation and in maintaining the balance between smooth muscle cell growth and differentiation. Superoxide results from one electron reduction of oxygen by a variety of oxidases (Figure 2). When O₂^·− is produced in concert with NO·, they rapidly react to form the highly reactive molecule ONOO^·−. ONOO^·− is an important mediator of lipid peroxidation and protein nitration, including oxidation of LDL, which has dramatic proatherogenic effects. In the absence of immediately accessible NO·, O₂^·− is rapidly dismutated to the more stable ROS, H₂O₂, by superoxide dismutase, which is then converted to H₂O by either catalase or glutathione peroxidase. The effects of O₂^·− and H₂O₂on vascular function depend critically on the amounts produced. When formed in low amounts intracellularly, they can act as intracellular second messengers, modulating the function of biochemical pathways mediating such responses as growth of vascular smooth muscle cells (VSMCs) and fibroblasts. Higher amounts of ROS can cause DNA damage, significant toxicity, or even apoptosis, as demonstrated in endothelial cells¹ and smooth muscle cells.

Cellular and Enzymatic Sources of ROS in the Vessel Wall

Each of these ROS derives from specific enzymatic or chemical reactions. NO· is produced in endothelial cells by activation of eNOS during the normal functioning of the vessel wall. Vasodilator hormones raise intracellular Ca²⁺, leading to an increase in eNOS activity and NO· release. Physical forces, such as fluid shear stress, activate eNOS via protein kinase A- or Akt-dependent phosphorylation. Pathophysiological expression of inducible NOS in both macrophages and VSMCs elevated cytokine levels, resulting in localized inflammation. This, in turn, results in production of NO· in the absence of further stimuli. Moreover, under some circumstances, eNOS becomes uncoupled and O₂^·− is made rather than NO·.³ The NOS enzymes are thus potentially important sources of both NO· and O₂^·−, depending on the surrounding environment.

Virtually all types of vascular cells produce O₂^·− and H₂O₂.⁴ In addition to mitochondrial sources of ROS, O₂^·− and/or H₂O₂ can be made by many enzymes (Figure 2). Two of the most important sources in the normal vessel are thought to be cytochrome P450 and the membrane-associated NAD(P)H oxidase(s).^5,6 A cytochrome P450 isozyme homologous to CYP 2C9 has been identified in coronary arteries and has been shown to produce O₂^·− in response to bradykinin.⁷ NAD(P)H oxidases that are similar in structure to the neutrophil respiratory burst NADPH oxidase, but produce less O₂^·− for a longer time, have been identified in vascular cells. The endothelial, VSMC, and fibroblast enzymes are not identical but have unique subunit structures and mechanisms of regulation.⁴ One important aspect of ROS production by at least the VSMC NAD(P)H oxidase is that it occurs largely intracellularly, making it ideally suited to modify signaling pathways and gene expression.