Breast cancer cells can recognize and respond to different levels of progestins to achieve different phenotypic outputs

The steroid hormone progesterone, acting through its nuclear progesterone receptor (PR), has complex physiologic activities with different levels of hormones manifesting distinct and sometimes opposing phenotypic responses in target tissues. However, most of what is currently known about the transcriptional activity of PR comes from studies performed using progestins at levels that are in the high physiologic range ([≥]10nM), relevant only in the luteal phase of the reproductive cycle and pregnancy in humans. These studies do not consider the non-linearity of responses to progestins that exist in physiology and are not informative as to the mechanisms by which low levels of progestins, as occurs during menopause, exert their biological activities. Thus, we undertook to define the mechanisms which enable cells to recognize and respond to different levels of progestins. Using a PR expressing cell model of luminal breast cancer (T47D cells) we demonstrated that low concentration progestins (0.1-0.3nM) drive proliferation while high dose progestins ([≥]10nM) inhibit proliferation. Using both unbiased and targeted approaches, we found that low dose progestins facilitate cell cycle entry by enhanced expression of CCND1 and SGK1, which are both required to initiate a signaling cascade that leads to increased phospho-Rb and E2F1 transcriptional activity. CCND1 cooperates with CDK4/6 to phosphorylate Rb, while SGK1 phosphorylates p21, thereby excluding it from the nucleus and inhibiting its anti-proliferative function. Expression of CCND1 and SGK1 mRNAs are primary responses to low dose progestin treatment. However, these responses occur at very low levels of receptor occupancy and in the absence of receptor phosphorylation events that have been shown to be required for nuclear translocation and transcriptional activity. These findings challenge the assumption of linearity in response to progestin dose. Further, they suggest that concentrations of progestins found in post-menopausal women (0.1-0.3nM) have the potential to exert proliferative responses in PR expressing cancers.