Progesterone receptors (PR) are critical mediators of mammary gland development and contribute to breast cancer progression. Progestin-induced rapid activation of cytoplasmic protein kinases leads to selective regulation of growth-promoting genes by phospho-PR species. Herein, we show that phosphorylation of PR Ser81 is ck2 dependent and progestin regulated in intact cells but also occurs in the absence of PR ligands when cells enter the G 1 /S phase of the cell cycle. T47D breast cancer cells stably expressing a PR-B mutant receptor that cannot be phosphorylated at Ser79/81 (S79/81A) formed fewer soft agar colonies. Regulation of selected genes by PR-B, but not PR-A, also required Ser79/81 phosphorylation for basal and/or progestinregulated (BIRC3, HSD11␤2, and HbEGF) expression. Additionally, wild-type (wt) PR-B, but not S79/81A mutant PR, was robustly recruited to a progesterone response element (PRE)-containing transcriptional enhancer region of BIRC3; abundant ck2 also associated with this region in cells expressing wt but not S79/81A PR. We conclude that phospho-Ser81 PR provides a platform for ck2 recruitment and regulation of selected PR-B target genes. Understanding how ligand-independent PRs function in the context of high levels of kinase activities characteristic of breast cancer is critical to understanding the basis of tumor-specific changes in gene expression and will speed the development of highly selective treatments.The ovarian steroid hormone progesterone acts by binding to and activating progesterone receptor (PR) A, B, and C isoforms expressed in target tissues. In the normal breast, PR-A and PR-B are typically expressed in a minority population (7 to 10%) of luminal epithelial cells. PR-B is required for mammary gland development during puberty and pregnancy and acts by contributing to lobulo-alveolar proliferation and ductal side branching (8, 46). Studies from PR-knockout mice show that these mice have significant defects in mammary gland morphology (primarily PR-B dependent) and reproductive abnormalities (primarily PR-A driven) (46, 54). Additionally, the presence of PR was shown to be required for the formation of mammary tumors in a carcinogen-induced mouse model of breast cancer (47). Finally, recent clinical data have shown that women taking hormone replacement therapy (HRT) whose regimens included both estrogen and a progestin, but not estrogen alone, experienced increased breast tumor numbers and sizes (1,5,12). Interestingly, the effect of combined HRT on breast cancer risk was reversible (5, 13), suggestive of epigenetic events.In the absence of progesterone, PR molecules rapidly shuttle between the cytoplasm and the nucleus; cytoplasmic PRs contain membrane-associated species capable of direct binding and signaling to mitogenic protein kinases (c-Src, MAPK, PI3K) (3,7,25,50). Following ligand binding, PRs dissociate from heat shock protein-containing chaperone complexes, undergo dimerization, and are largely retained in the nucleus. Nuclear receptors activate transcription of ...

The recent discovery of a novel, membrane localized progestin receptor (mPR) unrelated to the classical progesterone receptor (PR) in fishes and its subsequent identification in mammals suggests a potential mediator of non-traditional progestin actions, particularly in tissues where PR is absent. While early studies on mPR focused on final oocyte maturation in fishes, more current studies have examined mPRs in multiple mammalian systems in both reproductive and nonreproductive tissues as well as in diseased tissues. Here we review the current data on mPR in mammalian systems including male and female reproductive tracts, liver, neuroendocrine tissues, the immune system and breast and ovarian cancer. We also provide new data demonstrating mPR expression in the RAW 264.7 immune cell line and bone marrow-derived macrophages as well as mPR expression and downstream gene regulation in ovarian cancer cells.

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Membrane progesterone receptors (mPRs) have been detected in breast cancer cells and tissues, but their roles in cancer progression remain unclear. Here, we demonstrate the localization, signaling, and antiapoptotic actions of mPRs in two nuclear progesterone receptor (PR)-negative breast cancer cell lines, SKBR3 and MDA-MB-468 (MB468), and mPR expression in human breast tumor biopsies. mPRα, mPRβ, and mPRγ subtypes were detected in both cell lines as well as in breast tumor tissues from 13 individuals irrespective of nuclear steroid receptor expression. Competitive receptor binding studies with a selective PR ligand, R5020, and an mPR agonist, Org OD 02-0 confirmed the presence of functional mPRs on both cancer cell lines. Progesterone treatment of either cell line caused rapid activation of an inhibitory G protein, as well as activation of p42/44 MAP kinase. Treatment with progesterone or Org OD 02-0 significantly decreased cell death and apoptosis in response to serum starvation, whereas testosterone, 17β-estradiol, dexamethasone, and R5020 and RU486 were ineffective. Progesterone treatment of MB468 cells also increased mitochondrial membrane potential and Akt activity, but no decrease in caspase 3 activity was observed. Knockdown of mPRα expression in MB468 cells by siRNA transfection blocked the inhibitory effects of progesterone on cell death. The results indicate that progesterone can act through mPRs to inhibit apoptosis in breast cancer cells. The involvement of mPRs in the development or progression of breast tumor growth through inhibition of cell death is an intriguing possibility and requires further investigation.

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