doi: 10.1161/01.RES.0000137726.23347.11
© 2004 American Heart Association, Inc.
Editorials |
And What About the Endothelium?
On the Predominance of Cerebral Superoxide Formation for Angiotensin II–Induced Systemic Hypertension
From the Institut für Kardiovaskuläre Physiologie Klinikum der J.W. Goethe-Universität, Theodor-Stern-Kai 7 D-60596, Frankfurt am Main, Germany.
Correspondence to Ralf P. Brandes, Institut für Kardiovaskuläre Physiologie Klinikum der J.W. Goethe-Universität Theodor-Stern-Kai 7 D-60596, Frankfurt am Main, Germany. E-mail r.brandes{at}em.uni-frankfurt.de
See related article, pages 210–216
Key Words: superoxide dismutase • oxidative stress • angiotensin II • hypertension • NADPH oxidase
The hypertensive action of angiotensin II (Ang II) is a consequence of its effects on the kidney, the nervous system, and the vasculature resulting in direct vasoconstriction, increase in sympathetic outflow and sodium and water retention. In addition to these effects, Ang II enhances the generation of superoxide anions (O2–) within the vasculature by inducing and activating NADPH oxidases.1 O2– rapidly reacts with nitric oxide (NO) and thereby limits the bioavailability of this endogenous vasodilator. The reaction product of O2– and NO, peroxynitrite, is a highly aggressive compound that oxidizes proteins, lipids, and enzyme cofactors.1 Therefore, one important consequence of increased plasma levels of Ang II is endothelial dysfunction, usually identified as an attenuated endothelium-dependent, NO-mediated vasodilatation. Under normal conditions, the endothelium continuously releases NO in response to fluid shear stress and endothelium-derived NO continuously and significantly lowers vascular resistance. The loss of this "basal" NO production in endothelial NO synthase knockout mice (eNOS–/–) mice is thought to underlie the hypertensive phenotype in these animals. The logical consequence of this is that the scavenging of NO by O2– during Ang II treatment may increase peripheral resistance and thus contribute to hypertension.1
The study by Zimmermann et al2 in this issue of Circulation Research challenges the physiological importance of this paradigm and reminds us of the extraordinary role of the nervous system in the control of blood pressure.2 In this article, which is part of a series of articles from the same group,3–6 the authors provide another piece of information to substantiate their proposal that Ang II–induced hypertension is predominantly the consequence of cerebral redox-mediated regulatory processes. They demonstrate that Ang II–dependent O2– production is not restricted to the cardiovascular system but also occurs in neurons where it has an important impact on function. More specifically, the authors observed that a continuous systemic infusion of a low dose of Ang II in mice increases the formation of O2– in the subfornical organ. In vivo gene therapy with Cu-Zn-superoxide dismutase (CuZnSOD) in this region of the brain basically abrogated the hypertensive effects of Ang II. The Ang II–induced cardiac hypertrophy, in contrast, was not affected by cerebral CuZnSOD overexpression, indicating that the cardiac effects are a direct consequence of the growth-promoting actions of Ang II and that the action of CuZnSOD is specific and restricted to the brain.
The subfornical organ expresses high concentrations of AT1 receptors and belongs to the circumventricular organs of the brain that are not protected by the blood–brain barrier. It is thought that the subfornical organ is a neuronal sensor for Ang II, acting as an the interface to the neurons of the paraventricular nucleus, the ventrolateral medulla, the nucleus tractus solitarius, and the nucleus ambiguous, which are involved in the control of the sympathetic outflow.7,8
The observations by Zimmermann et al2 demonstrate that a systemic increase in blood pressure in response to Ang II is primarily a consequence of an alteration in cerebral regulatory processes, involving oxygen-derived free radicals. Moreover, they imply that endothelial dysfunction and impaired NO bioavailability play only a minor role in the regulation of blood pressure.
However, before such conclusions can be drawn, it is essential to consider some of the experimental conditions used in this study. First of all, the authors restricted their analysis of the effects of Ang II and SOD therapy to the measurement of blood pressure and organ weight; this however makes it impossible to assess the true nature of the hypertension and the antihypertensive effects observed. Secondly, a relatively low dose of Ang II was used, which does not result in an immediate hypertensive response but which elicits a slowly developing increase in blood pressure over several days. Such a response can clearly involve indirect mechanisms and may be a consequence of a complex interplay of different regulatory processes. It is likely that this slow onset hypertension to some extent represents the human situation, where hypertension is rarely a consequence of excessively elevated Ang II level, but it can also be assumed that higher doses of Ang II would result in different effects.
It remains to be determined whether the observations made by Zimmerman et al2 are restricted to Ang II or whether other hypertensive stimuli also enhance cerebral radical formation as a signaling pathway. In a previous study from the same group, the hypertensive effect of acute intraventricular application of carbachol was found not to be associated with increased radical formation.4 On the other hand, it was recently reported that the stimulation of 
-adrenergic receptors on vascular smooth muscle cells increases the vascular O2– formation by activating the NADPH oxidase.9 This latter observation contrasts with previous vascular studies in which adrenergic stimulation did not increase vascular radical generation,10 and this discrepancy was attributed to the different experimental conditions used.
In order to attenuate superoxide levels, the authors overexpressed CuZnSOD in the subfornical region of the brain and convincingly demonstrated using the dihydroethium technique that this maneuver suppressed O2–. The clear advantage of CuZnSOD over an unspecific antioxidative therapy with radical scavengers is the extremely high efficacy of this enzyme compared with the radical scavengers that have to be given at high concentrations. Moreover, the possibility of targeting the antioxidative action to certain tissues, as in the study by Zimmerman et al, and its specificity for O2– makes CuZnSOD gene therapy an unique tool studying O2–-mediated effects. SOD, however, will not only reduce the concentration of O2– but also that of peroxynitrite, and would be expected to result in the accumulation of NO and H2O2.
It is possible that such side effects might be of mechanistic relevance for the effects observed in this study. NO can activate the soluble guanylyl cyclase and leads to the accumulation of cGMP. NO and cGMP are both capable of opening potassium channels, at least in the vasculature, and thus to elicit hyperpolarization (Figure). The role of H2O2 in this system is even less clear. H2O2 is thought to be the effector of O2– with respect to signaling and gene expression. However, whether overexpression of SOD really increases cellular H2O2 level is controversial.11
|
Although it is unknown whether NO is an important modulator in the subfornical organ, NOS expression in this region has been detected and certainly further studies using isolated neurons from this region of the brain should be conducted to address the functional effects of the interventions performed in this study and of the different radicals generated after Ang II stimulation. Little is known about the mechanisms underlying alterations of neuronal function in response to low, noncytotoxic, nonapoptotic oxidative stress, and differential effects of the various radical species have not been carefully addressed.
What are the enzymatic sources of O2– generation in the present model? It is tempting to speculate that as in other organs, isoforms of the leukocyte NADPH oxidase are expressed and generate radicals in response to Ang II. Subunits of the NADPH oxidase have been detected in mouse neurons12 but the experiments performed in mice lacking NADPH oxidase subunits have been somewhat inconclusive regarding the role of this enzyme in the development of hypertension. The gp91phox subunit seems to play only a minor role, if any, in Ang II-induced hypertension.13 P47phox–/– mice exhibited a more pronounced attenuation of the vasopressor effect of Ang II,14 but still the effects were less marked than those observed in the study by Zimmerman et al.2 Whether these differences can be attributed to the experimental model (Ang II concentration, method of blood pressure measurements), to a different subunit composition of the neuronal NADPH oxidase, or even to the activation of other enzymatic sources of O2– in the brain will require additional studies.
One important observation made by Zimmerman et al was that CuZnSOD, but not extracellular SOD (ecSOD) was able to prevent the development of hypertension. Indeed, at least in the murine vasculature it has been observed that the expression of ecSOD is so high that a further increase in enzyme levels—even in situations of oxidative stress—has no effect on blood pressure and vascular function.15 Although a complete lack of ecSOD leads to exacerbation of hypertension,15 it remains to be determined whether under certain circumstances a shortage of ecSOD activity in the brain or the blood vessels occurs and whether ecSOD is a suitable therapeutic agent for oxidative stress in man.
It is essential to stress that the observations made in mice cannot be directly extrapolated to humans. Indeed, the cerebral effects of Ang II differ substantially even between different rodents. Nevertheless, the exciting report by Zimmermann et al, leads the way for many future studies and will probably enforce a reassessment of the role of vascular NO and O2– in the control of blood pressure.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
1. Cai H, Harrison DG. Endothelial dysfunction in cardiovascular diseases: the role of oxidant stress. Circ Res. 2000; 87: 840–844.
2. Zimmerman MC, Lazartigues E, Sharma RV, Davisson RL. Hypertension caused by angiotensin II infusion involves increased superoxide production in the central nervous system. Circ Res. 2004; 95: 210–216.
3. Davisson RL, Walton TM, Johnson AK, Lewis SJ. Cardiovascular effects produced by systemic injections of nitro blue tetrazolium in the rat. Eur J Pharmacol. 1993; 241: 135–137.[CrossRef][Medline] [Order article via Infotrieve]
4. Zimmerman MC, Lazartigues E, Lang JA, Sinnayah P, Ahmad IM, Spitz DR, Davisson RL. Superoxide mediates the actions of angiotensin II in the central nervous system. Circ Res. 2002; 91: 1038–1045.
5. Zimmerman MC, Davisson RL. Redox signaling in central neural regulation of cardiovascular function. Prog Biophys Mol Biol. 2004; 84: 125–149.[CrossRef][Medline] [Order article via Infotrieve]
6. Lindley TE, Doobay MF, Sharma RV, Davisson RL. Superoxide is involved in the central nervous system activation and sympathoexcitation of myocardial infarction-induced heart failure. Circ Res. 2004; 94: 402–409.
7. Ganong WF. Circumventricular organs: definition and role in the regulation of endocrine and autonomic function. Clin Exp Pharmacol Physiol. 2000; 27: 422–427.[CrossRef][Medline] [Order article via Infotrieve]
8. Johnson AK, Gross PM. Sensory circumventricular organs and brain homeostatic pathways. FASEB J. 1993; 7: 678–686.[Abstract]
9. Bleeke T, Zhang H, Madamanchi N, Patterson C, Faber JE. Catecholamine-induced vascular wall growth is dependent on generation of reactive oxygen species. Circ Res. 2004; 94: 37–45.
10. Laursen JB, Rajagopalan S, Galis Z, Tarpey M, Freeman BA, Harrison DG. Role of superoxide in angiotensin II–induced but not catecholamine-induced hypertension. Circulation. 1997; 95: 588–593.
11. Teixeira HD, Schumacher RI, Meneghini R. Lower intracellular hydrogen peroxide levels in cells overexpressing CuZn-superoxide dismutase. Proc Natl Acad Sci U|S|A. 1998; 95: 7872–7875.
12. Serrano F, Kolluri NS, Wientjes FB, Card JP, Klann E. NADPH oxidase immunoreactivity in the mouse brain. Brain Res. 2003; 988: 193–198.[CrossRef][Medline] [Order article via Infotrieve]
13. Jung O, Schreiber JG, Geiger H, Pedrazzini T, Busse R, Brandes RP. gp91phox-containing NADPH oxidase mediates endothelial dysfunction in renovascular hypertension. Circulation. 2004; 109: 1795–1801.
14. Landmesser U, Cai H, Dikalov S, McCann L, Hwang J, Jo H, Holland SM, Harrison DG. Role of p47(phox) in vascular oxidative stress and hypertension caused by angiotensin II. Hypertension. 2002; 40: 511–515.
15. Jung O, Marklund SL, Geiger H, Pedrazzini T, Busse R, Brandes RP. Extracellular superoxide dismutase is a major determinant of nitric oxide bioavailability: in vivo and ex vivo evidence from ecSOD-deficient mice. Circ Res. 2003; 93: 622–629.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||