Sanvito, Paola (2008) Effetti vascolari degli estrogeni: importanza della selettivitĂ Â recettoriale e della durata della deprivazione ormonale. [Ph.D. thesis]
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17?-estradiol exerts beneficial effects on the cardiovascular system, as shown by several epidemiological studies, which have demonstrated a several-fold higher prevalence of coronary heart disease in men compared with women before the age of 65. Experimental results showed that estradiol acts through two estrogen receptor isoforms, ER? and ER?, which are both espressed in the vascular wall, where estrogen displays anti-inflammatory and anti-proliferative activity in smooth muscle cells and causes vasodilation through both genomic and nongenomic activation of endothelial nitric oxide synthase (eNOS).
Experimental evidence and observational studies also suggest that postmenopausal hormone therapy protects against cardiovascular disease, whereas randomized clinical trials are not consistent with these findings, as regards primary and secondary prevention of cardiovascular diseases in post-menopausal women treated with hormone replacement therapy. So, it has been hypothesized that the receptor selectivity of estrogen compounds and the time since menopause are both crucial for cardiovascular protection of estrogen treatment. It is known that ER? mediates the majority of vascular protective effects of estrogen, such as the re-endothelialization process, NO and prostacyclin production and the anti-inflammatory action in the brain. A recent retrospective study also concluded that the time of therapy initiation is likely to have an impact on cardiovascular risk. However, experimental and/or clinical evidence demonstrating this hypothesis is missing.
The involvement of each ER isoform in the short-term responses to estradiol in isolated rat aortas was first evaluated. In order to assess the relative contribution of ER? and ER? to vascular responses, subtype-selective agonists and antagonist were used, because estradiol binds with the same affinity to the two isoforms. The influence of circulating hormone levels was evaluated using both intact and ovariectomized (OVX) rats. Finally, the role of endothelium and NO in these short?term responses was also studied. Acute treatment with the ER?-agonist PPT dose-dependently relaxed aortic rings from intact females; the response induced by 17?-estradiol fully overlapped with that elicited by PPT. Conversely, the ER??agonist had no effects on aortic vasomotion. The vasorelaxation elicited by the selective ER?-agonist as well as by 17?-estradiol was NO- and endothelium-dependent and was mediated by interaction with ER-, as shown using a selective ER?-antagonist, although the expression of both ER proteins was detected in isolated rat aortic endothelial cells. Levels of circulating hormones influenced the extent of estrogen vascular responses. In aortic rings from estrogen-depleted rats, neither ER?-agonist nor 17?-estradiol induced acute vascular relaxation. The short-term effect of both compounds was restored in tissues from estrogen-replaced ovariectomized animals, whereas DPN remained ineffective. These findings may have implications for selective targeting of ER subtypes in treating vascular disease.
Next, the influence of prolonged ovarian hormone deprivation on estrogen-induced vascular responses was studied. Experiments were performed in aortic rings from ovariectomized rats at 1, 4 or 8 months since surgery, in order to mimic differential time since hormone deprivation. eNOS and phosphorylated eNOS (peNOS) protein expression, as well as ER? mRNA levels, were also evaluated in the aortic endothelium
obtained from each experimental group. The results show that prolonged estrogen deprivation abolished vascular relaxation elicited by ER? and significantly reduced acetylcholine-induced endothelium-dependent vasodilation. Histological analysis provided no evidence for visible injury or gross vascular changes irrespective of ovariectomy duration and estrogen- replacement, suggesting the occurrence of functional rather than morphological impairment of the endothelium after long?term hypoestrogenity. Total eNOS and peNOS protein levels in the aortic endothelium of ovariectomized rats were increased by estrogen treatment after 1 or 4 months from surgery, whereas the expression of both protein was not improved by estrogen treatment after prolonged hormone deprivation. This was associated with a 3-fold decline in ER? mRNA levels in the aortic endothelium from 8-month vs 1-month ovariectomized rats. Immunohistochemical analysis for ER? and peNOS in aortic tissues from 1-month as compared with 8-months OVX E2-treated animals revealed marked negative regulation of the former proteins. The present findings document that the functional impairment of the ER?/eNOS signaling network after an extended period of hypoestrogenicity was insensitive to estrogen replacement, thus providing experimental support to early initiation of estrogen replacement with preferential ER? drugs to improve cardiovascular outcomes.
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