Differential cellular localization of estrogen receptor alpha in uterine and mammary cells

The classical a isoform of the estrogen receptor (ER) has been reported to localize almost exclusively in the nucleus. However, studies on non-genomic steroid effects have also suggested the existence of ERs residing at the c ell surface. In this work, we present immunological data supporting extra-n uclear ER alpha localization in uterine (SHM) and mammary (MCF-7) cell line s. Immunocytological studies performed on SHM cells revealed that native ER s mainly localize as a perinuclear cytoplasmic ring. The receptors were rap idly translocated to the nucleus by 17 beta -estradiol. In addition to nucl ear ERs, a peripheral reservoir of ER alpha immuno reactivity, most probabl y associated with the plasma membrane, was detected in MCF-7 cells. These r esults were confirmed by the detection of membrane estrogen binding sites u sing fluorescent estrogen-BSA derivatives and ligand binding assays to inta ct cells, where [H-3]-estradiol could be partly displaced by impeded estrog en conjugates. Partial inhibition of radioligand binding by an antibody aga inst the steroid binding domain of the ER alpha suggests that the isoform f aces the extracellular media in MCF-7 cells. Moreover, ER alpha -like prote ins (similar to 67 kDa) were found to be associated in isolated membrane su bfractions from the cells. However, immunocytology of COS-1 (ER-negative) a nd SHM cells transfected with the complete cDNA coding for the cloned ERa a nd beta isoforms showed exclusive nuclear localization of the overexpressed ERs, The non-classical distribution of native ER alpha -like proteins in e ach cell line, suggests an alternative mode of ER alpha. cellular localizat ion/function. Cell type-dependent processing may account for the differenti al localization shown by native and expressed receptors in the systems cons idered. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.