Reactive oxygen species (ROS), generated in the walls of blood vessels, are now recognized as important second-messenger molecules . Moreover, compelling evidence implicates chronically elevated levels of vascular ROS (often referred to as ‘oxidative stress’) as a common factor in the pathogenesis of many cardiovascular diseases, including hypertension . Over the last decade, it has been reported that ROS exert a wide range of potentially deleterious effects in the vascular wall. These include inactivation of endothelium-derived nitric oxide (NO), oxidation of lipoproteins, promotion of endothelial cell apoptosis, upregulation of pro-inflammatory adhesion molecules, and promotion of vascular smooth muscle (VSM) cell and endothelial cell growth and migration [1,3,4].
The ROS parent molecule, superoxide, is generated by a range of vascular oxidases via the one-electron reduction of molecular oxygen. This highly reactive and toxic molecule is well known for reacting with NO, resulting in the formation of another harmful species, peroxynitrite (ONOO−). Alternatively, superoxide can be dismutated, either spontaneously or catalysed by superoxide dismutase, to form the more stable and cell-permeable ROS, hydrogen peroxide (H2O2) . Emerging evidence suggests that H2O2 might be the most important cell signalling ROS molecule in vascular cells. It has been reported that H2O2 can elicit endothelium-dependent vasorelaxation  and is a powerful cerebral vasodilator in vivo. Indeed, H2O2 can acutely stimulate endothelial NO synthase (eNOS) production of NO via PI3-kinase/AKT and members of the mitogen-activated protein (MAP) family of kinases [8,9]. In addition, H2O2 has been reported to potently increase the expression of eNOS . Furthermore, in some vascular beds, there is evidence that this ROS molecule may actually serve as an endothelium-derived hyperpolarizing factor [11,12].
In addition to its effects on vascular tone, recent studies suggest that H2O2 plays an important role in regulating endothelial cell and VSM cell growth, hypertrophy, differentiation and migration . Indeed, H2O2 has been implicated as a mediator in the growth-promoting effects of angiotensin II  and platelet-derived growth factor , both of which are associated with vascular disease. In addition, there is evidence that this ROS molecule can also directly promote endothelial and VSM cell growth . Over the past few years, many redox-sensitive proteins have been identified, including receptor tyrosine kinases, non-receptor tyrosine kinases and MAP kinases . Evidence suggests that the growth-promoting effects of H2O2 may ultimately occur through activation of members of MAP kinases, including extracellular signal-regulated kinases (ERK1/2), p38MAP kinases, c-Jun N-terminal kinases (JNK) and ERK5 . Importantly, enhanced activation of vascular MAP kinases has been reported in experimental hypertension, and is implicated in hypertensive vascular remodelling and target organ damage [16,17].
The upstream signalling pathways involved in H2O2-mediated activation of MAP kinases in vascular cells have not previously been clarified. Because tyrosine kinases (receptor and non-receptor) and protein kinase C (PKC) are signalling molecules upstream of MAP kinases, the study by Tabet et al. , as reported in this issue of the journal, was designed to test whether tyrosine kinases and PKC participate in the activation of MAP kinases (ERK1/2 and p38MAP kinase) by H2O2 in cultured VSM cells. A second goal of the study was to determine whether the growth-promoting effects of H2O2 on MAP kinases are differentially regulated in VSM cells derived from chronically hypertensive animals. Thus, the authors used both pharmacological and molecular approaches to study mesenteric artery VSM-cultured cells from normotensive (Wistar–Kyoto, WKY) rats and spontaneously hypertensive rats (SHR).
In an elegant series of experiments, Tabet et al.  present clear evidence that exposure of VSM cells to 100 μmol/l exogenous H2O2 increases ERK1/2 and p38MAP kinase activation (i.e. phosphorylation) through mechanisms dependent on tyrosine kinase, but not PKC activity. Specifically, ERK1/2 was found to be regulated by both receptor and non-receptor tyrosine kinases, whereas p38MAP kinase was regulated only by non-receptor tyrosine kinases. Moreover, H2O2-induced phosphorylation of both ERK1/2 and p38MAP kinase was significantly greater in cells from SHR versus WKY rats, which is consistent with a contribution by these H2O2-induced processes in enhanced redox-dependent MAP kinase vascular signalling and growth during hypertension.
The significant strengths of this study include the use of H2O2, a defined and endogenously relevant ROS stimulus of signalling in vascular smooth muscle, the efficacious and selective effects of the pharmacological inhibitors used, and a comparison between data from normotensive and hypertensive tissues. Hence, the study by Tabet et al.  has provided new insight regarding precisely how H2O2 brings about its vascular growth-promoting effects, including the augmented effects in hypertensive arteries. The limitations of the present work include the use of exogenously applied H2O2 in a relatively high (i.e. supraphysiological) concentration, rather than being endogenously generated in response to a pathophysiological stimulus. Thus, such treatment with exogenous H2O2 may not precisely simulate its normal levels and spatial distribution within the arterial wall, despite its relative diffusability. However, the authors do acknowledge this limitation, and argue that levels of vascular H2O2 may increase to μmol/l concentrations in some disease states. It is also possible that the endothelial and adventitial cell layers might normally modulate these signalling events in intact arteries, especially in the presence of blood flow and pulsatile pressure. Moreover, the greater effect of H2O2 in cells derived from SHR versus WKY rat arteries need not have necessarily been due to pressure-related strain differences because the causes of hypertension in the SHR, and the precise differences between the two genotypes are complex and undefined. Future studies will need to clarify whether the signalling elicited by H2O2 is due, for example, to poorer antioxidant (e.g. glutathione peroxidase and catalase) activity in cells from SHR resulting in a higher effective concentration of H2O2, or if there is a greater amplification at some downstream point(s) in the signal transduction pathway in SHR. Furthermore, if the observed differences are indeed pressure-related, it remains to be determined whether hypertension is the cause or the effect. Finally, the findings of Tabet et al.  provide a strong rationale for evaluating inhibitors of tyrosine kinases for their future therapeutic use in redox-related vascular diseases.
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