Improvement of Nitric Oxide–Dependent Vasodilatation by HMG-CoA Reductase Inhibitors Through Attenuation of Endothelial Superoxide Anion Formation

Abstract

—Three 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (HCRIs), atorvastatin, pravastatin, and cerivastatin, inhibited phorbol ester–stimulated superoxide anion (O₂⁻) formation in endothelium-intact segments of the rat aorta in a time- and concentration-dependent manner (maximum inhibition of 70% after 18 hours at 1 to 10 μmol/L). The HMG-CoA reductase product mevalonic acid (400 μmol/L) reversed the inhibitory effect of the HCRIs, which, conversely, was mimicked by inactivation of p21 Rac with Clostridium sordellii lethal toxin but not by inactivation of p21 Rho with Clostridium botulinum exoenzyme (C3). A mevalonate-sensitive inhibition of phorbol ester–stimulated O₂⁻ formation by atorvastatin was also observed in porcine cultured endothelial cells and in a murine macrophage cell line. In the rat aorta, no effect of the HCRIs on protein kinase C, NADPH oxidase, or superoxide dismutase (SOD) activity and expression was detected, whereas that of endothelial nitric oxide (NO) synthase was enhanced ≈2-fold. Moreover, exposure of the segments to atorvastatin resulted in a significant improvement of endothelium-dependent NO-mediated relaxation, and this effect was abolished in the presence of SOD. Taken together, these findings suggest that in addition to augmenting endothelial NO synthesis, HCRIs inhibit endothelial O₂⁻ formation by preventing the isoprenylation of p21 Rac, which is critical for the assembly of NADPH oxidase after activation of protein kinase C. The resulting shift in the balance between NO and O₂⁻ in the endothelium improves endothelial function even in healthy blood vessels and therefore may provide a reasonable explanation for the beneficial effects of HCRIs in patients with coronary heart disease in addition to or as an alternative to the reduction in serum LDL cholesterol.