Membrane association of estrogen receptor alpha mediates estrogen effect on MAPK activation

Estrogen rapidly activates MAPK in many cell types but the mechanisms have not been fully understood. We previously demonstrated that 17-beta-estradiol (estradiol) rapidly induced membrane translocation of estrogen receptor alpha (ERalpha) and activated MAPK in MCF-7 breast cancer cells. This study further determines the cause and effect relationship between the presence of membrane ERalpha and MAPK activation. ERalpha with a membrane localization signal (HE241G-mem) was expressed and compared with the ones in nucleus (HEGO) or cytosol (HE241G) localization. Confocal microscopy showed that HE241G-mem was expressed in the cell membrane as well as in the cytosol in COS-1 cells. HE241G localized in the cytosol and HEGO in the nucleus. Functional studies showed that only membrane ERalpha, not cytosol and nuclear ones, responded to estradiol by inducing MAPK phosphorylation. HE241G-mem neither increased basal nor estradiol-induced ERE promoter activation, indicating no transcriptional action involved. Our data support the view that membrane-associated ERalpha is critical in estrogen-initiated MAPK activation.