Differential cellular localization of estrogen receptor α 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 cell surface. In this work, we present immunological data supporting extra-nuclear ER alpha localization in uterine (SHM) and mammary (MCF-7) cell lines. Immunocytological studies performed on SHM cells revealed that native ERs mainly localize as a perinuclear cytoplasmic ring. The receptors were rapidly translocated to the nucleus by 17 beta -estradiol. In addition to nuclear ERs, a peripheral reservoir of ER alpha immuno reactivity, most probably associated with the plasma membrane, was detected in MCF-7 cells. These results were confirmed by the detection of membrane estrogen binding sites using fluorescent estrogen-BSA derivatives and ligand binding assays to intact cells, where [H-3]-estradiol could be partly displaced by impeded estrogen conjugates. Partial inhibition of radioligand binding by an antibody against the steroid binding domain of the ER alpha suggests that the isoform faces the extracellular media in MCF-7 cells. Moreover, ER alpha -like proteins (similar to 67 kDa) were found to be associated in isolated membrane subfractions from the cells. However, immunocytology of COS-1 (ER-negative) and SHM cells transfected with the complete cDNA coding for the cloned ERa and beta isoforms showed exclusive nuclear localization of the overexpressed ERs, The non-classical distribution of native ER alpha -like proteins in each cell line, suggests an alternative mode of ER alpha. cellular localization/function. Cell type-dependent processing may account for the differential localization shown by native and expressed receptors in the systems considered. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.