The apparent cardioprotective effects of endogenous estrogens in pre-menopausal women is due, at least in part, to direct estrogenic modification of vascular cell function. In vitro, estrogen has been shown to promote proliferation and migration of vascular endothelial cells (EC) and to inhibit these processes in vascular smooth muscle cells (VSMC). The molecular mechanisms accounting for these differences are not well understood. Previously, our laboratory demonstrated that estradiol (E2) induces activity of EC mitogen activated protein kinases (MAPKs), an important event in signal transduction. To further elucidate the mechanism of estrogen actions on vascular cells, we evaluated the effect of E2 on activation of the MAPKs, extracellular signal-regulated kinases 1 and 2 (ERK 1/2), in VSMC.

Actively replicating human coronary artery and rat aortic VSMC were made quiescent by incubating in 0.5% bovine calf serum (BCS) media for 24h. Quiescent cells were treated with E2 (3nM) for 10 min, 1h, 4h, or 20h and subsequently stimulated with either PDGF-BB (10ng/ml) or serum (10% BCS). ERK 1/2 activation was evaluated using an in-vitro kinase assay. PDGF and serum stimulated ERK activation 2-13 fold in both human and rat VSMC with maximal stimulation after 30 min. In human and rat VSMC, E2 treatment inhibited PDGF- and serum-stimulated ERK activation at early but not late timepoints. The most significant inhibition was seen at 10 min and 1h of E2 treatment and ranged from 20-50% of maximal activation. By 20h, ERK activity returned to untreated levels.

In summary, whereas E2 activates MAPK in EC, E2 treatment of cultured VSMC inhibits PDGF- and serum-stimulated activation of ERK 1/2. Because activation of MAPK in VSMC occurs through multiple pathways, partial inhibition by E2 of MAPK activation suggests that estrogen inhibits one or more mechanisms by which PDGF and serum activate MAPK. Inhibition at early but not later timepoints suggests that estrogen has rapid, direct effects on at least some of these mechanisms. Estrogen effects on cell signaling through MAPK and related pathways could contribute to the differential response to stimuli by endothelial and vascular smooth muscle cells.