Effects of estrogen on stress-induced premature senescence of vascular smooth muscle cells: a novel mechanism for the "time window theory" of menopausal hormone therapy.

To investigate the effects of estrogen on stress-induced premature senescence of vascular smooth muscle cells (VSMCs) and the underlying mechanisms. VSMCs of passage 2-3 cultured from young (2 months) and old (18 months) female SD rats were induced into premature senescence by exposure to 150 μmol/L H(2)O(2) in the presence or absence of different concentrations of 17β-estradiol (E(2)). The expression or activation of senescence-associated beta-galactosidase (SA-β-Gal), DcR2, oncogene Ras, p38, PRAK, p53, p21, p16 and Rb was detected by flow cytometry, pull-down assay or Western blot. Flow cytometry analysis showed that in the VSMCs from young rats pre-administration of E(2) significantly suppressed the H(2)O(2)-induced premature senescence (reducing both percentage of SA-β-Gal positive cells and cellular expression of DcR2) in a dose-dependent manner; these senescent-inhibiting effects of E(2) could be blocked by an estrogen receptor antagonist ICI 182,780 (10(-5)mol/L). Pull-down assay or Western blot analysis revealed that pre-administration of 10(-8)mol/L E(2) significantly reduced the H(2)O(2)-induced activation of oncogene Ras, as well as activity of p16 and p38 MAPK, and expression of PRAK, p53, p21 and p-Rb. Unexpectedly, in the VSMCs from old rats the senescent-inhibiting effect of E(2) disappeared and switched to a senescent-promoting action at 10(-8)mol/L. This senescent-promoting effect could be enhanced by ICI 182,780 and eliminated by a cytochrome P450s inhibitor ABT. Estrogen inhibits stress-induced premature senescence of VSMCs from young female through its receptor-mediated suppression of both Ras-p38-PRAK-p53-p21-Rb and Ras-p16-Rb pathways, but this effect disappeared and even more switched to a senescent-promoting action in the cells from old body probably due to a side effect of estrogen metabolites.