Inhibition of estradiol receptor/Src association and cell growth by an estradiol receptor α tyrosine-phosphorylated peptide

This report offers direct evidence that association of the estradiol receptor (ER) with Src triggered by steroid agonists or growth factors controls breast and prostate cancer cell growth. This association is abolished in whole cells and in vitro by a six-amino-acid peptide that mimics the sequence around the phosphotyrosine residue in position 537 of the human ER alpha. The phosphorylated peptide, at nanomolar concentrations, is taken up by MCF-7 and LNCaP cells derived from human mammary and prostate cancers, respectively. In addition, to block the ER/Src interaction, the phosphopeptide inhibits Src/Erk pathway, cyclin D1 expression, and DNA synthesis induced by estradiol or androgen or triggered by epidermal growth factor. In contrast, no inhibition of the Src-mediated epidermal growth factor action on DNA synthesis is detectable in human mammary cancer cells that do not express ER (MDA-MB231), indicating that the peptide specifically targets the ER-associated Src. Remarkably, the peptide, in contrast with classic steroid antagonists, does not interfere in ER- or androgen receptor-dependent transcriptional activity. Nevertheless, it markedly inhibits the growth of MCF-7 cell xenografts induced in immunodepressed and estradiol-treated mice. The present report suggests that inhibition of association of steroid receptors with Src or other signaling effectors may have therapeutic applications for patients with ER-positive tumors.