17β-Estradiol attenuates PDGF signaling in vascular smooth muscle cells at the postreceptor level

Estrogens are known to display significant vasoprotective effects in premenopausal women. PDGF is an important mediator of vascular smooth muscle cell (VSMC) migration and proliferation, and thus atherogenesis. We analyzed the effects of 17β-estradiol (E2) on β-PDGF receptor (β-PDGFR) expression/activation and PDGF-dependent VSMC proliferation, migration, and downstream signaling events. Pretreatment of VSMCs with E2 (0.3 μM–0.1 mM) for 24 h concentration-dependently inhibited PDGF-induced proliferation and migration up to 85.5 ± 15.8% and 79.4 ± 9.8%, respectively (both P < 0.05). These effects were prevented by coincubation with the ER antagonist ICI-182780. E2 did not alter β-PDGFR expression, nor did it impair the ligand-induced tyrosine phosphorylation of the β-PDGFR and consecutive binding of the receptor-associated signaling molecules Src homology region 2-containing phosphatase-2, PLC-γ, phosphatidylinositol 3-kinase, and RasGAP. Thus estrogens inhibited PDGF-induced cellular responses at the postreceptor level. Although stimulation of VSMCs with PDGF-BB led to a transient increase of rac-1 activity, pretreatment with E2 for 24 h concentration-dependently inhibited PDGF-induced rac-1 activation. Furthermore, inhibition of rac-1 by Clostridium sordellii lethal toxin or overexpression of dominant-negative rac-1 (rac-N17) significantly inhibited PDGF-induced VSMC migration, indicating that rac-1 activity is essential for PDGF-dependent cellular responses. E2 did not further reduce PDGF-induced migration in rac-N17-overexpressing cells, suggesting that it diminishes VSMC migration by altering rac-1 activity. We conclude that E2 attenuates PDGF-dependent cellular functions of VSMCs downstream of the β-PDGFR via inhibition of rac-1. These observations offer a molecular explanation for the vasoprotective effects of estrogens.