The oxidative paradox: another piece in the puzzle.

Because atherosclerosis is a disease of many risk factors, a complex pathology, and diverse clinical manifestations, the as yet unattained Holy Grail of atherosclerosis research is a unifying pathway to explain its many aspects and provide a single point at which to measure risk or intervene in its clinical course. If vascular biology possesses such a relic, it would have many of the characteristics attributed to oxidative stress. Risk factors for atherosclerosis, such as hypertension1 and hyperlipidemia,2 are potent stimuli for the generation of reactive oxygen species (ROS) in experimental systems, and it is likely that cigarette smoking and diabetes mellitus share oxidative heritages. At the molecular level, signaling in response to proatherogenic agents, such as a-thrombin,3 platelet-derived growth factor,4 and angiotensin II,5,6 requires the generation of ROS via oxidase systems. In fact, many of the hallmarks of atherosclerosis, including endothelial dysfunction, smooth muscle cell (SMC) proliferation, and inflammatory recruitment, occur in response to ROS. In this issue of Circulation Research, Schieffer et al7 contribute to our understanding of these events by demonstrating that angiotensin II–induced cytokine activation via the Janus kinase/signal transducer and activator of transcription (STAT) pathway requires oxidant generation by the NAD(P)H oxidase in SMCs. Their data are consistent with the notion that ROS are central mediators of adverse events in atherosclerosis. The oxidative paradox, then, is the inability of clinical investigators to definitively demonstrate that modulation of ROS by the use of antioxidant therapies has any effect on atherogenesis. Angiotensin II is the best-characterized stimulus for ROS generation in SMCs.5,6 The study by Schieffer et al7 describes the role of ROS as stimuli for inflammatory events that may contribute to plaque instability. Although many of the issues raised in this study have been addressed previously,3,5 the nature of the NAD(P)H oxidase that likely generates ROS in SMCs merits attention. The classical NAD(P)H oxidase described in neutrophils consists of several components, p22, p67, p47, gp91, and a rac GTPase, that are recruited to create the oxidative burst. The study by Schieffer et al7 is the second to implicate a particular component of this oxidase, p47, in the oxidase of SMCs.3 Using electroporated antibodies to target p47, they find that this protein (and presumably the oxidase in which it participates) is necessary for ROS generation, STAT activation, and interleukin-6 synthesis elicited by angiotensin II. It should be noted that their data, although compelling, are not conclusive in this regard. Decreased p47 protein in cells electroporated with an anti-p47 antibody may indicate decreased de novo p47 protein synthesis. However, neutralizing-antibody experiments are notoriously difficult to interpret, and alternative explanations include the possibility that p47antibody complexes are insoluble or that antibody binding to p47 elicits its degradation. p47 is not the only component of the SMC oxidase shared with the neutrophil oxidase. There is also good evidence that p22 is present in both.8 However, equally compelling evidence points to critical differences in the phagocytic and nonphagocytic oxidases. First, these oxidases have important differences in substrate use. The neutrophil oxidase consumes NADPH preferentially, whereas the SMC oxidase favors NADH.3,6 Second, the amounts of ROS generated by these oxidases differ by several orders of magnitude and also differ in the rate by which ROS is generated. Third, evidence is mounting that the components of these two oxidases differ structurally. A member of the gp91 family, NOX1, probably participates in the SMC oxidase in place of gp919; and p67 has been difficult to detect in SMCs,3 indicating that it may either be dispensable or replaced by another component. The novel structure of the SMC oxidase (and perhaps other nonphagocytic oxidases) may explain how these oxidases mediate nonmicrobicidal functions, particularly by generation of ROS at slower rates and lower intracellular concentrations. Perhaps more importantly, it is possible that the function of the SMC oxidase can be regulated independently of oxidase systems in other cell types, a significant point if oxidases in nonvascular cells are essential for other purposes. The present study provides confirmatory evidence that p47 is a component of the SMC oxidase. Identification and reconstitution of the SMC oxidase, which we anticipate would include additional novel components in comparison with the neutrophil oxidase, will be crucial to our understanding of the atherogenic role of ROS. It should also be emphasized that a component of the SMC oxidase, the Rac1 GTPase, may also regulate signaling independently of ROS generation by directly interacting with and activating STAT proteins.10 The relevance of such interactions, as demonThe opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the University of North Carolina at Chapel Hill, Program in Molecular Cardiology, Division of Cardiology and Department of Medicine (C.P., N.R.M., M.S.R.) and Lineberger Comprehensive Cancer Center (C.P.), Chapel Hill, NC. Correspondence to Marschall S. Runge, MD, PhD, Chairman, Department of Medicine, University of North Carolina at Chapel Hill, 3033 Old Clinic Building, Campus Box 7005, Chapel Hill, NC 27599-7005. E-mail [email protected] (Circ Res. 2000;87:1074-1076.) © 2000 American Heart Association, Inc.