Estrogen and raloxifene induce apoptosis by activating p38 mitogen-activated protein kinase cascade in synthetic vascular smooth muscle cells

Abstract

Proliferation of vascular smooth muscle cells (VSMC) plays a major role as an initiating event of atherosclerosis. Although estrogen directly inhibits the proliferation of VSMC, the mechanism has not been firmly established. In addition, the effect of raloxifene on VSMC remains unknown. 17β-Estradiol (E2) and raloxifene significantly inhibited the growth of VSMC under growth-stimulated conditions. Since mitogen-activated protein (MAP) kinases have been implicated in VSMC proliferation, the role of MAP kinases in both the E2- and raloxifene-induced growth inhibition of VSMC was studied. Both E2 and raloxifene caused rapid, transient phosphorylation and activation of p38 that was not affected by actinomycin D and was blocked by ICI 182,780. In contrast with p38 phosphorylation, extracellular signal-regulated protein kinase (ERK) phosphorylation was significantly inhibited and c-Jun N-terminal kinase (JNK) phosphorylation was not changed by E2. Because VSMC expressed both estrogen receptor (ER) α and ERβ, it is not known which of them mediates the E2-induced phosphorylation of p38. Although E2 did not affect the p38 phosphorylation in A10 smooth muscle cells, which express ERβ but not ERα, transfection of ERα expression vector into A10 cells rendered them susceptible to induction of p38 phosphorylation by E2. We then examined whether E2 and raloxifene induce apoptosis through a p38 cascade. Both E2 and raloxifene induced apoptosis under growth-stimulated conditions. The p38 inhibitor SB 203580 completely blocked the E2-induced apoptosis. Our findings suggest that both E2- and raloxifene-induced inhibition of VSMC growth is due to induction of apoptosis through a p38 cascade whose activation is mediated by ERα via a nongenomic mechanism.