Our lab has previously reported the identification of a novel endogenous 19-nor steroid, estradienolone (ED), in pregnant women that strongly bound to sex hormone binding globulin. Estrogen-receptor related receptors (ERRs), which have no known natural ligands, are a family of orphan receptors consisting of 3 isoforms: ERRalpha, ERRbeta and ERRgamma. The ERRs have been shown to actively modulate estrogenic responses, to play an essential role in pregnancy, and are implicated in breast cancer prognosis. My results show that ED acts as an antagonist of the ERRalpha confirming preliminary results obtained by our group. Studies of cellular responses demonstrate that ED has strong anti-mitogenic properties. ED inhibited the growth of both estrogen receptor (ER)-positive (MCF-7) and ER-negative (MDA-MB-231) breast cancer cells in a dose-dependent manner but did not have any effects on the proliferation of the non-cancerous immortalized epithelial breast MCF-10A cells. The finding that ED inhibits proliferation of both ER negative and ER positive breast cancer cells, and regulate ERR transcriptional activity may have important ramifications in breast cancer therapy.

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