Background: The vasoprotective effects of estrogen are mediated by estrogen receptors (ERs). ERs are transcription factors that require coactivators to exert transcriptional activity. The steroid receptor coactivator-3 (SRC-3, also known as pCIP, AIB1, ACTR, and TRAM-1) interacts with estrogen-bound ERs and strongly coactivates the transcription of target genes in cultured cells. This study has characterized the expression of SRC-3 in cardiovascular tissue and the role of SRC-3 in estrogen-dependent vasoprotection from vascular injury.
Methods and results: Phenotypically normal SRC-3(+/-) mice with a knock-in LacZ reporter were used to characterize SRC-3 expression by X-gal staining within the cardiovascular system. Staining signals were specifically detected in vascular smooth muscle cells and endothelial cells but not in myocardial cells. The role of SRC-3 during vascular remodeling was analyzed using a unilateral carotid ligation model. The extent of neointima formation in SRC-3(-/-) mice was significantly higher than in wild-type mice, and this difference was diminished after depletion of estrogen by ovariectomy. After ovariectomy, neointimal growth in wild-type mice was almost completely inhibited by estrogen treatment but only partially inhibited in SRC-3(-/-) mice. Furthermore, estrogen treatment resulted in reduced inhibition of intimal cell proliferation in SRC-3(-/-) mice.
Conclusions: SRC-3 is highly expressed in vascular smooth muscle cells and endothelial cells. The loss of SRC-3 function causes a decrease in sensitivity of estrogen-mediated inhibition of neointimal growth, which may be attributable to an insufficient suppression of vascular cell proliferation. These results indicate that SRC-3 largely facilitates ER-dependent vasoprotective effects under conditions of vascular trauma.