Low and High Dose Progestins Elicit Distinct Breast Cancer Biology

An incomplete understanding of dose-dependent progesterone receptor (PR) biology has led to the suboptimal exploitation of PR as a therapeutic target. While hormone replacement therapies (HRT) containing low-doses of synthetic progestogens (progestins) have been associated with an increased risk of breast cancer, high-dose progestin therapies have beneficial therapeutic effects in some patients with breast cancer. Herein we report that PR subcellular localization, post-trans3lational modifications, stability, chromatin binding, responsive gene expression and downstream biology differ considerably as a function of progestin concentration. Dose-dependent transcriptional profiling revealed that different concentrations of progestin elicit classic sigmoidal and/or biphasic gene induction at distinct target gene sets. Notably, when compared to high doses of progestin, low-dose treatments lead to minimal nuclear translocation of PR while supporting robust induction of a subset of target genes, indicating that PR occupancy has a complex relationship to target gene induction. Interestingly, high but not low-dose progestin treatments lead to increased phosphorylation at Ser294 and is associated with higher receptor turnover. Furthermore, while low-dose treatments stimulate cell proliferation, high-dose treatments do not result in significant cell cycle progression to S phase. Similar biphasic and sigmoidal patterns of gene induction were observed in human breast tumor explants treated with various concentrations of progestin, underscoring the need to fine tune the dosage of PR-targeting therapies. Given the wide range of medical applications of PR target therapy, our discovery of distinct dose-dependent PR biology may have important clinical implications. Overall design: Cells were grown in DMEM supplemented with 8% fetal bovine serum (FBS) and 1% penicillin streptomycin. Prior to experiments, cells were cultured for 48 hours in phenol red free media supplemented with 0.1% charcoal-stripped FBS and 1% penicillin streptomycin (steroid-deprived media). R5020 (PerkinElmer #NLP004005MG) dissolved in ethanol (vehicle) was used at a final concentration as mentioned in the manuscript.