Membrane estrogen receptors identified by multiple antibody labeling and impeded-ligand binding

GH₃/B6 rat pituitary tumor cells exhibit rapid prolactin release (within 5 min) when treated with nanomolar amounts of estrogen. However, the putative protein mediator of this nongenomic action has not been described. Using antibodies directed against a peptide representing the hinge region of the intracellular estrogen receptor (iER), we have demonstrated that these cells contain a membrane ER (mER). We now report that confocal scanning laser microscopy of cells labeled live with the anti-peptide antibody further supports a membrane localization of ER. The monoclonal antibodies H226 and H222 and a polyclonal antibody, ER21, each recognizing a unique epitope on iER (NH₂ terminal to the DNA-binding region, within the steroid binding region, and the NH₂-terminal end, respectively), also immunohistochemically label membrane proteins of immuno-selected GH₃/B6 cells. These cells also specifically bind a fluorescent estrogen-BSA conjugate. Coincubation of cells with anti-ER antibody and the fluorescent estrogen-BSA conjugate reveals that these labels colocalize on cells. These results suggest that mER may be structurally similar to iER.