ETHINYLESTRADIOL DOES NOT ENHANCE THE EXPRESSION OF NITRIC-OXIDE SYNTHASE IN BOVINE ENDOTHELIAL-CELLS BUT INCREASES THE RELEASE OF BIOACTIVE NITRIC-OXIDE BY INHIBITING SUPEROXIDE ANION PRODUCTION

Estradiol is known to exert a protective effect against the developmen t of atherosclerosis, but the mechanism by which this protection is me diated is unclear. Since animal studies strongly suggest that producti on of endothelium-derived relaxing factor is enhanced by estradiol, we have examined the effect of estrogens on nitric oxide (NO) synthase ( NOS) activity, protein, and mRNA in cultured bovine aortic endothelial cells. in reporter cells rich in guanylate cyclase, it has been obser ved that long-term treatment (greater than or equal to 24 hr) with eth inylestradiol (EE(2)) dose-dependently increased guanylate cyclase-act ivating factor activity in the conditioned medium of endothelial cells . However, conversion of L-[C-14]arginine to L-[C-14]citrulline by end othelial cell homogenate or quantification of nitrite and nitrate rele ased by intact cells in the conditioned medium did not reveal any chan ge in NOS activity induced by EE(2) treatment. Similarly, Western and Northern blot analyses did not reveal any change in the endothelial NO S protein and mRNA content in response to EE(2). However, EE(2) dose- and time-dependently decreased superoxide anion production in the cond itioned medium of endothelial cells with an EC(50) value (0.1 nM) clos e to that which increased guanylate cyclase-activating, factor activit y (0.5 nM). Both of these effects were completely prevented by the ant iestrogens tamoxifen and RU54876. Thus, endothelium exposure to estrog ens appears to induce a receptor-mediated antioxidant effect that enha nces the biological activity of endothelium-derived NO. These effects could account at least in part for the vascular protective properties of these hormones.