Raloxifene induces nucleolar translocation of the estrogen receptor

Raloxifene (RLX), a selective estrogen receptor modulator (SERM), binds to the estrogen receptor alpha (ERα) and acts as an agonist in some tissues, and as an antagonist in others. To clarify the molecular mechanism underlying the tissue specificity of SERMs, we examined the intracellular localization of ERα using a green fluorescent protein (GFP)-tagged protein in culture cells from various tissues. Although ERα formed intranuclear foci in the presence of estradiol (E2), RLX translocated ERα into the nucleoli in breast cancer cell lines. This phenomenon was not observed in cells from other tissues. Immunofluorescence staining revealed that endogenous ERα was also translocated into the nucleoli in the presence of RLX. Mutation analyses demonstrate that helix 12 of ERα is essential to the nucleolar translocation of ERα. These results suggest that translocation of ERα into the nucleoli is RLX-specific and is a key event for RLX-induced growth repression of mammary gland cells.