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(Hypertension. 2006;48:677.)
© 2006 American Heart Association, Inc.
Original Articles |
From the Johannes Gutenberg University Hospital (M.O., A.D., P.W., U.H., E.S., H.M., A.L.K., A.M., T.M.), Division of Cardiology, Mainz, Germany; J.W. Goethe-University (R.P.B.), Institut für Kardiovaskuläre Physiologie, Frankfurt am Main, Germany; Institut für Pharmakologie (M.H., H.L., U.F.), Johannes Gutenberg-Universität Mainz, Mainz, Germany; and University Hospital Eppendorf (A.v.S.), Division of Cardiology, Hamburg, Germany.
Correspondence to Thomas Münzel, Klinikum der Universität Mainz, II Med Klinik u Poliklinik, Langenbeckstr 1, 55101 Mainz, Germany. E-mail tmuenzel{at}uni-mainz.de
| Abstract |
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Key Words: angiotensin II nitric oxide synthase endothelium oxidative stress vasodilation
| Introduction |
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Adrenergic ß-receptor antagonists are standard drugs for the treatment of hypertension and coronary artery disease. Some of these ß-blockers, such as carvedilol and nebivolol, have been shown recently to exhibit antioxidative properties.10,11 In vitro stimulation of the ß2-receptors on endothelial cells by nebivolol metabolites increased endothelial [Ca2+] levels and, accordingly, NOS III activity.12 We have shown recently that chronic treatment with nebivolol could normalize vascular superoxide formation, as well as endothelial dysfunction in Watanabe heritable hyperlipidemic rabbits, which served as a model of hyperlipidemia and early stage atherosclerosis.11 Nebivolol also exerted this antioxidative effect in vitro when whole blood, as well as isolated neutrophils or macrophages, were stimulated by a phorbol ester derivative. This indicates that the protective mechanism was mainly based on suppression of the phagocytic NADPH-dependent superoxide formation by either direct inhibition of the enzyme activity or inhibition of its activation by PKC.9,11 The antioxidative effect was not stereoselective, because it was mediated by D- as well as L-nebivolol excluding an involvement of the ß receptor and pointing toward a free-radical scavenging effect of the molecule itself for these protective effects.13
Chronic treatment with nebivolol has also been shown to improve endothelial function in patients with essential hypertension.14 This protective effect was explained by a nebivolol-mediated decrease in oxidative stress as shown by decreased levels of the isoprostane 8-iso-prostaglandin F2
in patients15 and decreased expression of endothelin-1 in endothelial and smooth muscle cells.16 It was shown recently that Nox1 overexpression in transgenic mice potentiates Ang IIinduced hypertension, which likewise improved in Nox1-deficient mice.17,18 With the present study, we tested whether the antioxidative effects of the ß-blocker nebivolol also comprises inhibitory effects on the nonphagocytic, vascular NADPH oxidase in the setting of Ang II infusion. We also tested whether this may lead to a preservation of the NO signaling pathway and endothelial function. In vitro experiments using cardiac membrane preparations from Ang IIinfused animals and HEK cells overexpressing NADPH oxidases were performed to address whether nebivolol may interfere with the process of NADPH oxidase assembly.
We show here for the first time that nebivolol improves endothelial function in Ang IIinduced hypertension by preventing increases in NADPH oxidase activity and expression and by preventing NOS III uncoupling.
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| Results |
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Effects of Nebivolol Treatment on Vascular Superoxide Production and NOS III Uncoupling Assessed by Dihydroethidine-Derived Fluorescence and Plasma Nitrite/HemoglobinNO Levels
Dihydroethidine (DHE) staining revealed vascular superoxide throughout the vessel wall in vessels from Ang IItreated animals (Figure 2). Although vascular superoxide in control rats was not modified by nebivolol treatment, a marked reduction was observed in vessels from hypertensive rats treated with nebivolol (Figure 2A). To assess the contribution of NOS III to superoxide formation because of NOS III uncoupling, rat aortic tissue was incubated with a NOS III inhibitor, NG-nitro-L-arginine (L-NNA). L-NNA increased DHE-derived fluorescence within the endothelial monolayer from control rats (marked with E in Figure 2A). This finding indicates that in normotensive rats, NOS IIIderived NO quenches basal levels of superoxide by concomitant formation of, for example, peroxynitrite, which is not detected by DHE. In contrast, incubation of vessels from Ang IItreated rats with L-NNA decreased DHE-derived fluorescence exclusively within the endothelium, thereby identifying NOS III as a significant superoxide source. This effect of L-NNA was not observed in vessels from Ang IIand nebivolol-treated rats, indicating that nebivolol treatment was able to prevent NOS III uncoupling (Figure 2A).
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Ang II treatment also markedly decreased NO bioavailability (measured as plasma nitrite and whole blood hemoglobin [Hb]NO), which was substantially corrected even above control levels by in vivo nebivolol treatment (Figure 2B and 2C and Table II). The nitrate/nitrite ratio was increased in response to Ang II and normalized by nebivolol cotreatment (Figure II and serum parameters, available online).
Effects of Nebivolol Treatment on Vascular Superoxide Assessed by Lucigenin- and L-012Enhanced Chemiluminescence
Ang II treatment increased vascular superoxide 4-fold as assessed by lucigenin-derived chemiluminescence ([LDCL] 5 µmol/L). Treatment with nebivolol decreased LDCL by 50%, whereas superoxide production of control vessels was not modified at all by nebivolol (Figure 3A). Likewise, Ang II infusion significantly increased NADPH oxidase activity in heart membranes, which was significantly inhibited by treatment with nebivolol but not with metoprolol (Figure 4A).
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In heart membranes isolated from Ang II treated rats, L-012enhanced chemiluminescence was markedly increased, and nebivolol cotreatment did not modify the L-012 signal in control subjects but markedly decreased the signal in the Ang II infusion group (Table III).
Effects of Nebivolol Treatment on Vascular Expression and Distribution of the NADPH Oxidase Subunits p67phox and Rac1
Ang II treatment upregulated the expression of p67phox and Rac1 in aortic membrane fractions and in heart membranes, all of which was normalized by in vivo nebivolol treatment (Figures 3B and 4
B). In addition, we found significant increases in the expression of Nox1 in heart membranes that were decreased by treatment with nebivolol. By RT-PCR measurements we established significant increases in Nox2 (gp91phox), p47phox, and p22phox mRNA in aortic and heart tissue ranging from 2- to 4-fold, all of which was almost normalized by nebivolol but not metoprolol cotreatment in vivo (Figure I).
Effects of In Vitro Nebivolol on Superoxide Production and Membrane Association of NADPH Oxidase Subunits p67phox and Rac1 in Heart Membrane From Ang IITreated Animals
Superoxide production (NADPH oxidase activity) in heart membranes of Ang IItreated animals was concentration-dependently inhibited by nebivolol (Figure 5A). With respect to other ß-blockers, carvedilol but not atenolol and metoprolol were able to inhibit superoxide production to a similar extent (Figure 5A). Likewise, membrane association of NADPH- oxidase subunits p67phox and Rac1 was dose-dependently inhibited by 1 to 100 µmol/L of nebivolol (Figure 5B) resulting in significant inhibition at 100 µmol/L. Although carvedilol showed a similar inhibitory effect on LDCL, the inhibition of membrane association was less pronounced. Atenolol and metoprolol clearly had no effect at all on this parameter (Figure 5B).
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Effects of In Vitro Nebivolol on P-VASP Levels in Vessels Exposed to Phorbol Ester
Incubation of rat aortic vessels with a phorbol ester derivative (10 µmol/L) dramatically decreased NO/cGMP signaling as assessed by the P-VASP at serine239, all of which was concentration-dependently prevented by preincubation with nebivolol and normalized at 100 µmol/L of nebivolol (Figure 5C).
Effects of Nebivolol and Other ß-Blockers on Reactive Oxygen Species Production and Rac1 Translocation in Transfected HEK293 Cells
HEK293 cells were transfected with Nox1, Noxa1, and Noxo1 (the analogs of p47phox and p67phox), and basal reactive oxygen species (ROS) production was measured using a chemiluminescence-based luminol-peroxidase assay. Nebivolol, as well as carvedilol, significantly decreased the chemiluminescence signal when used at 100 µmol/L, whereas atenolol and metoprolol had no effect (Figure 6). Nebivolol (100 µmol/L) inhibited the phorbol myristate (PMA)induced translocation of Rac1 in the transfected cells (Figure 6B).
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| Discussion |
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We have shown recently that in vivo treatment with the ß-blocker nebivolol markedly improved endothelial dysfunction and NO signaling in vessels from hypercholesterolemic Watanabe heritable hyperlipidemic rabbits, which was at least in part secondary to a normalization of vascular superoxide production.11 Importantly, in this particular study, nebivolol was able to inhibit superoxide production by macrophages and neutrophils activated in vitro by Ang II and by phorbol ester. Both compounds induce the oxidative burst in these inflammatory cells mainly by activating the NADPH oxidase. Interestingly, other ß-blockers, such as atenolol and metoprolol, were not able to inhibit superoxide production,11 which may explain why nebivolol but not metoprolol is able to reduce the progression of atherosclerosis in the setting of hypercholesterolemia.11,19
Up to now, there are no data available on whether ß-blockers are able to inhibit superoxide production by the vascular NADPH oxidase, enzymes that have been identified to represent some of the major superoxide-producing enzymes in the setting of atherosclerosis,20 diabetes mellitus,21 heart failure,22 and arterial hypertension.23 In animals infused with Ang II, we have demonstrated recently that there is a marked degree of endothelial dysfunction, increased oxidative stress because of upregulation of the activity and expression of the NADPH oxidase and by NOS III uncoupling, and that there is a strong inhibition of NO signaling as evidenced by the marked reduction of the vascular levels of the P-VASP, a surrogate marker for the activity of the cGK-I.4,24
To address the possibility that nebivolol treatment is able to prevent NOS III uncoupling, vessels from Ang IIinfused animals with and without nebivolol were incubated with the NOS inhibitor L-NNA. In vessels from Ang IIinfused animals, the superoxide signal within endothelial cells was almost eliminated, identifying NOS III as a significant superoxide source. In vessel from Ang IIinfused animals treated with nebivolol, L-NNA increased the superoxide signal like in control animals, providing evidence, as shown before,21,22,24 that under these conditions, NOS III is recoupled and that NO produced by NOS III metabolizes superoxide produced by the endothelium under baseline conditions. This hypothesis is further strengthened by the observed decrease in plasma nitrite levels (indirect measure of NO) and HbNO in response to Ang II, which is compatible with NOS III uncoupling, and the correction of these parameters by in vivo treatment with nebivolol.
Importantly, the increase in superoxide production induced by Ang II was not only restricted to the endothelium but also involved the media, as well as the adventitia, as shown by the DHE experiments. Treatment with nebivolol was able to drastically reduce superoxide production throughout the vessel wall. One of the candidate enzymes responsible for the increased superoxide production throughout the vessel wall is the vascular NADPH oxidase.25 Importantly, in vivo nebivolol treatment resulted in a normalization of the NADPH oxidase activity in membrane fractions from the heart, whereas metoprolol was virtually ineffective (Figure 4A). In addition, nebivolol therapy prevented the upregulation of the NADPH oxidase subunit expression, such as p22phox, gp91phox (Nox2), Nox1, p47phox, and p67phox, as quantified by quantitative RT-PCR and Western blotting technique (in the heart and aorta). In contrast, in vivo treatment with metoprolol failed to demonstrate inhibitory effects on NADPH oxidase expression (please see the online data supplement).
As mentioned before, nebivolol has been demonstrated to suppress the NADPH oxidasemediated oxidative burst in inflammatory cells in vitro.11 It remains to be established whether it may have similar inhibitory effects on superoxide production by the vascular NADPH oxidase in vitro. In the present studies, we have chosen several methods to approach this point. First we incubated isolated rings with phorbol ester (Figure 5C), a PKC-dependent activator of the vascular NADPH oxidase, and subsequently quantified the activity of the cGK-I by measuring P-VASP levels in vascular tissue. The phorbol estertriggered activation of NADPH oxidasedependent superoxide formation impaired NO signaling by 70%, all of which was prevented when aortic rings were coincubated with nebivolol. These data indicate that nebivolol may be able to inhibit the activation of the NADPH oxidase. These observations are further strengthened by experiments performed in HEK293 overexpressing Nox1-containing NADPH oxidase, facilitating a direct analysis of the effects of ß-blockers on NADPH oxidase function. Also, in these experiments nebivolol strongly inhibited oxidase activity, and this effect was associated with a dissociation of Rac1 from the activated enzyme complex.
Based on these observations, it might be valid to speculate regarding the underlying mechanism. The interaction of Rac1 and the cytosolic phox subunits with the membranebound main oxidase complex depends on the association of the PX domain of p47phox with the plasma membrane and the Rac anchoring in the membrane via its geranyl-geranylated tail (for review see Reference 26). It, therefore, seems probable that nebivolol at the concentrations reached in these studies may have direct effects on membrane properties (such as influencing the fluidity) attenuating the anchoring of cytosolic proteins in the process of cellular activation. This concept could also nicely account for the beneficial effects of nebivolol on endothelial NO synthase activity.11
Using HEK293 cells, we could demonstrate that nebivolol, but not metoprolol or atenolol, decreased the basal ROS formation and prevented translocation of Rac1 to the membrane. In addition, we were also able to show that when the NADPH oxidase is already assembled, such as in hearts from Ang IIinfused animals, nebivolol, but not metoprolol or atenolol, was able to dissociate the NADPH oxidase subunits Rac1 and p67phox from the membrane and simultaneously reduced NADPH oxidase activity (Figure 5A and 5B).
Perspectives
The present study indicates that the selective ß1-receptor blocker nebivolol is able to prevent the adverse effects of Ang II hypertension with respect to NO/cGMP/cGK-I signaling and endothelial function by inhibiting increases in oxidative stress. Nebivolol treatment prevented increases in the expression of the NADPH-oxidase subunits Nox1, Nox2, p22phox, p47phox, p67phox, and Rac1 and recoupled an uncoupled NOS III. Importantly, the acute challenge of heart membranes from Ang IIinfused animals with nebivolol normalized superoxide production by dissociating the NADPH-oxidase subunits Rac1 and p67phox from the membranes. In cultured cells transfected with NADPH oxidase subunits, nebivolol but not metoprolol or atenolol was able to prevent PMA-induced Rac1 translocation and simultaneously inhibited basal superoxide production. These results point to powerful, pleiotropic, "antioxidative" effects of nebivolol, which will ultimately lead to an increase in vascular NO bioavailability, all of which may explain at least in part the positive effects on prognosis in patients with congestive heart failure as demonstrated in the Study of the Effects of Nebivolol Intervention on Outcomes and Rehospitalization in Seniors With Heart Failure (SENIORS),27 as well as the beneficial effects of nebivolol as compared with atenolol on endothelial function in patients with essential hypertension.14,28
| Acknowledgments |
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Sources of Funding
This study was supported by a vascular biology grant from Berlin Chemie (Berlin, Germany), grants from the Deutsche Forschungsgemeinschaft (BR1839/2-3 to R.P.B., SFB 553A1 to H.L. and U.F., and SFB 553C17 to A.D. and T.M.), and by the European Vascular Genomic Network, a Network of Excellence supported by the European Communitys sixth Framework Program (Contract LSHM-CT-2003-503254).
Disclosures
None.
| Footnotes |
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Received April 6, 2006; first decision April 26, 2006; accepted July 24, 2006.
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