Estrogen activates mitogen-activated protein kinase in native, nontransfected CHO-K1, COS-7, and RAT2 fibroblast cell lines

CHO-K1, COS-7, and Rat2 fibroblast cell lines are generally believed to be devoid of estrogen receptors (ERs) and have been widely used to study the functions of ER-alpha and ER-beta after transfection of their cDNAs. Numerous studies have demonstrated that transfected ER-alpha or ER-beta mediates estradiol-induced activation of multiple signaling pathways, including the MAPK/ERK pathways. We report here for the first time that both 17alpha-estradiol and 17beta-estradiol elicit activation of MAPK/ERK in native, nontransfected CHO-K1, COS-7, and Rat2 fibroblast cell lines. We further report that, contrary to the generally held belief, these cell lines are not unresponsive to estradiol in their native, nontransfected state, and that this estrogen responsiveness is associated with estrogen binding. Using multiple ER antibodies, we failed to find ER-alpha or ER-beta isoforms or even ER-X. In view of these findings, researchers, using such cells as models to investigate mechanisms of estrogen action, must always take into account the existence of endogenous estrogen binding proteins other than ER-alpha, ER-beta, or ER-X.