Membrane-associated binding sites for estrogen contribute to growth regulation of human breast cancer cells

Membrane-associated binding sites for estrogen may mediate rapid effects of estradiol-17 beta that contribute to proliferation of human breast cancers . After controlled homogenization and fractionation of MCF-7 breast cancer cells, the bulk of specific estradiol binding is found in nuclear fractions . However, a significant portion of specific, high-affinity estradiol-17 be ta binding-sites are also enriched in plasma membranes. These estradiol bin ding-sites co-purify with 5'-nucleotidase, a plasma membrane-marker enzyme, and are free from major contamination by cytosol or nuclei. Electrophoresi s of membrane fractions allowed detection of a primary 67-kDa protein and a secondary 46-kDa protein recognized by estradiol-17 beta and by a monoclon al antibody directed to the ligand-binding domain of the nuclear form of es trogen receptor. Estrogen-induced growth of MCF-7 breast cancer cells in vi tro was blocked by treatment with the antibody to estrogen receptor and cor related closely with acute hormonal activation of mitogen-activated protein kinase and Akt kinase signaling. Estrogen-promoted growth of human breast cancer xenografts in nude mice was also significantly reduced by treatment in vivo with the estrogen receptor antibody. Thus, membrane-associated form s of estrogen receptor may play a role in promoting intracellular signaling for hormone-mediated proliferation and survival of breast cancers and offe r a new target for antitumor therapy.