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

The classical α 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 ERα 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 17β-estradiol. In addition to nuclear ERs, a peripheral reservoir of ERα immunoreactivity, 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 [³H]-estradiol could be partly displaced by impeded estrogen conjugates. Partial inhibition of radioligand binding by an antibody against the steroid binding domain of the ERα suggests that the isoform faces the extracellular media in MCF-7 cells. Moreover, ERα-like proteins (∼ 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 ERα and β isoforms showed exclusive nuclear localization of the overexpressed ERs. The non-classical distribution of native ERα-like proteins in each cell line, suggests an alternative mode of ERα cellular localization/function. Cell type-dependent processing may account for the differential localization shown by native and expressed receptors in the systems considered.