Calmodulin enhances the stability of the estrogen receptor

The estrogen receptor mediates breast cell proliferation and is the princip al target for chemotherapy of breast carcinoma. Previous studies have demon strated that the estrogen receptor binds to calmodulin-Sepharose in vitro, However, the association of endogenous calmodulin with endogenous estrogen receptors in intact cells has not been reported, and the function of the in teraction is obscure. Here we demonstrate by co-immunoprecipitation from MC F-7 human breast epithelial cells that endogenous estrogen receptors bind t o endogenous calmodulin, Estradiol treatment of the cells had no significan t effect on the interaction, However, incubation of the cells with tamoxife n enhanced by 5-10-fold the association of calmodulin with the estrogen rec eptor and increased the total cellular content of estrogen receptors by 1.5 -2-fold. In contrast, the structurally distinct calmodulin antagonists trif luoperazine and CGS9343B attenuated the interaction between calmodulin and the estrogen receptor and dramatically reduced the number of estrogen recep tors in the cell. Neither of these agents altered the amount of estrogen re ceptor mRNA, suggesting that calmodulin stabilizes the protein. This hypoth esis is supported by the observation that, in the presence of Ca2+, calmodu lin protected estrogen receptors from in vitro proteolysis by trypsin. Furt hermore, overexpression of wild type calmodulin, but not a mutant calmoduli n incapable of binding Ca2+, increased the concentration of estrogen recept ors in MCF-7 cells, whereas transient expression of a calmodulin inhibitor peptide reduced the estrogen receptor concentration. These data demonstrate that calmodulin binds to the estrogen receptor in intact cells in a Ca2+-d ependent, but estradiol-independent, manner, thereby modulating the stabili ty and the steady state level of estrogen receptors.