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Signalling pathways activated by hydrogen peroxide in vascular smooth muscle

Sobey, Christopher G; Miller, Alyson A

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doi: 10.1097/01.hjh.0000184409.96672.ab
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Reactive oxygen species (ROS), generated in the walls of blood vessels, are now recognized as important second-messenger molecules [1]. 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 [2]. 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) [5]. 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 [6] and is a powerful cerebral vasodilator in vivo[7]. 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 [10]. 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 [13]. Indeed, H2O2 has been implicated as a mediator in the growth-promoting effects of angiotensin II [14] and platelet-derived growth factor [15], 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 [3]. Over the past few years, many redox-sensitive proteins have been identified, including receptor tyrosine kinases, non-receptor tyrosine kinases and MAP kinases [1]. 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 [1]. 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. [18], 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. [18] 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. [18] 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. [18] provide a strong rationale for evaluating inhibitors of tyrosine kinases for their future therapeutic use in redox-related vascular diseases.

References

1 Griendling KK, Sorescu D, Lassegue B, Ushio-Fukai M. Modulation of protein kinase activity and gene expression by reactive oxygen species and their role in vascular physiology and pathophysiology. Arterioscler Thromb Vasc Biol 2000; 20:2175–2183.
2 Cai H, Harrison DG. Endothelial dysfunction in cardiovascular diseases: the role of oxidant stress. Circ Res 2000; 87:840–844.
3 Cai H. Hydrogen peroxide regulation of endothelial function: origins, mechanisms, and consequences. Cardiovasc Res 2005; in press.
4 Cai H. NAD(P)H oxidase-dependent self-propagation of hydrogen peroxide and vascular disease. Circ Res 2005; 96:818–822.
5 Faraci FM, Didion SP. Vascular protection: superoxide dismutase isoforms in the vessel wall. Arterioscler Thromb Vasc Biol 2004; 24:1367–1373.
6 Zembowicz A, Hatchett RJ, Jakubowski AM, Gryglewski RJ. Involvement of nitric oxide in the endothelium-dependent relaxation induced by hydrogen peroxide in the rabbit aorta. Br J Pharmacol 1993; 110:151–158.
7 Sobey CG, Heistad DD, Faraci FM. Mechanisms of bradykinin-induced cerebral vasodilatation in rats. Evidence that reactive oxygen species activate K+ channels. Stroke 1997; 28:2290–2295.
8 Cai H, Li Z, Davis ME, Kanner W, Harrison DG, Dudley SC Jr. Akt-dependent phosphorylation of serine 1179 and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 cooperatively mediate activation of the endothelial nitric-oxide synthase by hydrogen peroxide. Mol Pharmacol 2003; 63:325–331.
9 Thomas SR, Chen K, Keaney JF Jr. Hydrogen peroxide activates endothelial nitric-oxide synthase through coordinated phosphorylation and dephosphorylation via a phosphoinositide 3-kinase-dependent signaling pathway. J Biol Chem 2002; 277:6017–6024.
10 Drummond GR, Cai H, Davis ME, Ramasamy S, Harrison DG. Transcriptional and posttranscriptional regulation of endothelial nitric oxide synthase expression by hydrogen peroxide. Circ Res 2000; 86:347–354.
11 Yada T, Shimokawa H, Hiramatsu O, Kajita T, Shigeto F, Goto M, et al. Hydrogen peroxide, an endogenous endothelium-derived hyperpolarizing factor, plays an important role in coronary autoregulation in vivo. Circulation 2003; 107:1040–1045.
12 Iida Y, Katusic ZS. Mechanisms of cerebral arterial relaxations to hydrogen peroxide. Stroke 2000; 31:2224–2230.
13 Griendling KK, Ushio-Fukai M. Redox control of vascular smooth muscle proliferation. J Lab Clin Med 1998; 132:9–15.
14 Zafari AM, Ushio-Fukai M, Akers M, Yin Q, Shah A, Harrison DG, et al. Role of NADH/NADPH oxidase-derived H2O2 in angiotensin II-induced vascular hypertrophy. Hypertension 1998; 32:488–495.
15 Sundaresan M, Yu ZX, Ferrans VJ, Irani K, Finkel T. Requirement for generation of H2O2 for platelet-derived growth factor signal transduction. Science 1995; 270:296–299.
16 Xu Q, Liu Y, Gorospe M, Udelsman R, Holbrook NJ. Acute hypertension activates mitogen-activated protein kinases in arterial wall. J Clin Invest 1996; 97:508–514.
17 Touyz RM, Deschepper C, Park JB, He G, Chen X, Neves MF, et al. Inhibition of mitogen-activated protein/extracellular signal-regulated kinase improves endothelial function and attenuates Ang II-induced contractility of mesenteric resistance arteries from spontaneously hypertensive rats. J Hypertens 2002; 20:1127–1134.
18 Tabet F, Schiffrin EL, Touyz RM. MAP Kinase activation by H2O2 is mediated through tyrosine kinsase-dependent, PKC-independent pathways in vascular smooth muscle cells-upregulation in SHR. J Hypertens 2005; 23:2005–2012.
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