Activation of estrogen receptor-α reduces aortic smooth muscle differentiation

Women are at high risk of dying from unrecognized cardiovascular disease. Many differences in cardiovascular disease between men and women appear to be mediated by vascular smooth muscle cells (SMC). Because estrogen reduces the proliferation of SMC, we hypothesized that activation of estrogen receptor-alpha (ER alpha) by agonists or by growth factors altered SMC function. To determine the effect of growth factors, estrogen, and ER alpha expression on SMC differentiation, human aortic SMC were cultured in serum-free conditions for 10 days. SMC from men had lower spontaneous expression of ER alpha and higher levels of the differentiation markers calponin and smooth muscle alpha-actin than SMC from women. When SMC containing low expression of ER alpha were transduced with a lentivirus containing ER alpha, activation of the receptor by ligands or growth factors reduced differentiation markers. Conversely, inhibiting ER alpha expression by small interfering RNA ( siRNA) in cells expressing high levels of ER alpha enhanced the expression of differentiation markers. ER alpha expression and activation reduced the phosphorylation of Smad2, a signaling molecule important in differentiation of SMC and initiated cell death through cleavage of caspase-3. We conclude that ER alpha activation switched SMC to a dedifferentiated phenotype and may contribute to plaque instability.