Protective Effect Of 17β-estradiol Research Articles

Stimulation of Tetrahydrobiopterin Synthesis by 17β-Estradiol in Brain Microvascular Endothelial Cells

Abstract The purpose of this study was to examine whether 17β-estradiol stimulates the synthesis of tetrahydrobiopterin : BH4), which is one of the cofactors of nitric oxide (NO) synthase, in mouse brain microvascular endothelial cells. Addition of 17()-estradiol to endothelial cells time- and concentration-dependently increased intracellular BH4 level. 17β-Estradiol also stimulated the mRNA level of GTP-cyclohydrolase I (GTPCH), which is a rate-limiting enzyme of the de novo BH4 synthetic pathway. In addition, the 17β-estradiol-induced expression of GTPCH mRNA was strongly attenuated by treatment with an inhibitor of 17β-estradiol receptor 4-hydroxy-tamoxlfen. These results suggest that 17β-estradiol stimulates BH4 synthesis through the induction of GTPCH by tamoxifensensitive receptor in vascular endothelial cells. The 17β-estradiol-induced increase in BH4 level might be implicated in not only NO production, but also protective effects of 17β-estradiol against ischemic brain damage and atherosclerosis, since BH4 is an intracellular antioxidant.

Involvement of potassium channels in the protective effect of 17β-estradiol on hypercholesterolemic rabbit carotid artery

The involvement of endothelium-derived hyperpolarizing factor (EDHF) in the protective effect of 17β-estradiol was investigated on the phenylephrine-precontracted carotid artery from cholesterol fed rabbits. Animals were fed for 8 weeks as follows: control group, standard chow; (control+estradiol) group, standard chow+17β-estradiol; standard chow+1% cholesterol, cholesterol group; or (cholesterol+estradiol) group, 1% cholesterol chow+17β-estradiol. Relaxations to acetylcholine (ACh) (3 nM–30 μM) were performed with N ω nitro- l-arginine methyl ester (300 μM) and indomethacin (10 μM). Charybdotoxin (50 nM)+apamin (50 nM), glibenclamide (10 μM) or 4-aminopyridine (1 mM) were used to block, respectively, calcium-activated-K +, adenosine triphosphate (ATP)-sensitive-K + and voltage-dependent K + channels. In the control group, ACh induced a residual concentration-dependent relaxation. This response was impaired by hypercholesterolemia and restored by 17β-estradiol. In control and cholesterol groups, 4-aminopyridine or glibenclamide did not affect this relaxation, but in (control+estradiol) and (cholesterol+estradiol) groups, glibenclamide suppressed it. In all groups, this persisting relaxation was completely abolished by charybdotoxin alone or with apamin, by hemoglobin (10 μM), a nitric oxide scavenger, or by LY83183 (10 μM), a guanylate cyclase inhibitor. Thus, in the rabbit carotid artery, the protective effect of 17β-estradiol against hypercholesterolemia is probably mediated by a nitric oxide/cyclic GMP pathway which activates calcium-targeted and ATP-dependent K + channels.

Involvement of Cyclooxygenase 2 in the Protective Effect of 17β-Estradiol on Hypercholesterolemic Rabbit Aorta

The involvement of cyclooxygenase (COX) in the effects of 17β-estradiol was investigated on hypercholesterolemic rabbits aorta. Acetylsalycilic acid, nimesulide, or SQ22536 was used as respective antagonist of COX-1, COX-2, or adenylate cyclase using aortic rings precontracted with phenylephrine and exposed to cumulative concentrations of acetylcholine (ACh). The relaxation effect of ACh was impaired by hypercholesterolemia and restored by an 8-week 17β-estradiol treatment. In the control group treated with estrogen, nimesulide, acetylsalycilic acid, or SQ22536 slightly reduced the response to ACh. In hypercholesterolemic rabbits treated with estrogen, nimesulide significantly reduced the maximal relaxation and shifted to the right the relaxation curve of ACh, whereas acetylsalycilic acid did not modify the maximal response to ACh but displaced slightly the concentration–response curve. SQ22536 reduced the relaxant effect of ACh down to the level obtained in the presence of nimesulide. These results suggest that the protective effect of 17β-estradiol against hypercholesterolemia involved COX-2/adenylate cyclase pathway.

The Protective Effect of 17β-Estradiol on Vasomotor Responses of Aorta from Cholesterol-Fed Rabbit Is Reduced by Inhibitors of Superoxide Dismutase and Catalase

The involvement of endogenous antioxidant enzymes in the protective effect of 17β-estradiol against hypercholesterolemia was evaluated on aorta from cholesterol-fed rabbits treated with estradiol. 17β-Estradiol, more potent than α-tocopherol, restored the acetylcholine-induced relaxation, which was impaired by cholesterol chow, and enhanced the vasorelaxation induced by sodium nitroprusside. Diethyldithiocarbamate (5 mM), a superoxide dismutase (SOD) inhibitor, reduced relaxation to acetylcholine down to the level obtained in cholesterol-fed rabbits not treated with estrogen. This inhibition was dose-dependently reversed by addition of exogenous SOD. Aminotriazole (5 mM), a catalase inhibitor, reduced slightly this response, which was not reversed by exogenous catalase. A similar concentration of diethyldithiocarbamate prevented the potentiation of response to sodium nitroprusside induced by estrogen, whereas aminotriazole had no effect. These results suggest that, on aorta from cholesterol-fed rabbit, superoxide dismutase and catalase are involved in the protective effect of estrogen on the vasomotor response to acetylcholine, but only superoxide dismutase participates in response to sodium nitroprusside.