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

Abstract Objectives To investigate the effects of estrogen on stress-induced premature senescence of vascular smooth muscle cells (VSMCs) and the underlying mechanisms. Methods 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. Results 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, p21and 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. Conclusion Estrogen inhibits stress-induced premature senescence of VSMCs from young female through its receptor-mediated surpression 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.