Estrogen Enhances Proliferative Capacity of Cardiac Fibroblasts by Estrogen Receptor- and Mitogen-Activated Protein Kinase-Dependent Pathways

The role of female hormones in the prevalence of cardiac diseases are recognized but not fully explored. Proliferation of cardiac fibroblasts, the cellular origin of the extracellular matrix proteins, growth factors and cytokines in the heart, is an important underlying mechanism in the pathophysiological remodeling of the myocardium. In this study, we have investigated the effect of estrogen (17 beta-estradiol) on proliferative capacity of cardiac fibroblasts obtained from adult female rat heart. DNA synthesis, as determined by incorporation of 3H-thymidine into DNA, increased in estrogen-treated cells. In the presence of tamoxifen, an anti-estrogen with high affinity for estrogen receptor. 17 beta-estradiol-induced stimulation of DNA synthesis was abolished. Alpha-estradiol, a stereo-isomer which does not bind the estrogen receptor, did not change DNA synthesis. In the presence of a synthetic inhibitor of MAP kinase pathway. PD98059, estrogen failed to stimulate DNA synthesis. In-gel kinase assays showed rapid and transient increased phosphorylation of MAP kinase substrate, myelin basic protein (MBP), at 42 and 44 kDa by 17 beta-estradiol, which was not observed in the presence of PD98059 and tamoxifen, not induced by alpha-estradiol and persisted in the absence of protein kinase C. In vitro kinase assay confirmed 17 beta-estradiol-induced activation of ERK1 and ERK2, with predominant effect on ERK2 in cardiac fibroblasts. The results of immunofluorescent light microscopy using anti-type alpha and beta estrogen receptor antibodies showed the expression of estrogen receptor types alpha and beta in control untreated cells, and indicated that type beta receptor is the predominant type with both cytoplasmic and nuclear localization. 17 beta-estradiol treatment of cardiac fibroblasts induced the translocation of receptor protein to the nuclei. Together, these data provide evidence that cardiac fibroblasts are cellular targets for direct effects of estrogen, and that this hormone enhances proliferative capacity of cardiac fibroblasts via estrogen receptor- and MAP kinase-dependent mechanisms. These data further suggest that estrogen, by its growth-enhancing effects in cardiac fibroblasts, can regulate the remodeling of the extracellular matrix and alter the microenvironment of cardiac cells, and hence exert an impact on the integrity of myocardial function.