Differential hormone-dependent phosphorylation of progesterone receptor A and B forms revealed by a phosphoserine site-specific monoclonal antibody

Triple SILAC identified progestin-independent and dependent PRA and PRB interacting partners in breast cancer

Progesterone receptor (PR) isoforms, PRA and PRB, act in a progesterone-independent and dependent manner to differentially modulate the biology of breast cancer cells. Here we show that the differences in PRA and PRB structure facilitate the binding of common and distinct protein interacting partners affecting the downstream signaling events of each PR-isoform. Tet-inducible HA-tagged PRA or HA-tagged PRB constructs were expressed in T47DC42 (PR/ER negative) breast cancer cells. Affinity purification coupled with stable isotope labeling of amino acids in cell culture (SILAC) mass spectrometry technique was performed to comprehensively study PRA and PRB interacting partners in both unliganded and liganded conditions. To validate our findings, we applied both forward and reverse SILAC conditions to effectively minimize experimental errors. These datasets will be useful in investigating PRA- and PRB-specific molecular mechanisms and as a database for subsequent experiments to identify novel PRA and PRB interacting proteins that differentially mediated different biological functions in breast cancer.

Systemic distribution of progesterone receptor subtypes in human tissues

Progesterone receptor (PR) is expressed in a wide variety of human tissues, including both reproductive and non-reproductive tissues. Upon binding to the PR, progesterone can display several non-reproductive functions, including neurosteroid activity in the central nervous system, inhibition of smooth muscle contractile activity in the gastrointestinal tract, and regulating the development and maturation of the lung. PR exists as two major isoforms, PRA and PRB. Differential expression of these PR isoforms reportedly contributes to different biological activities of the hormone. However, the distribution of the PR isoforms in human tissues has remained virtually unexplored. In this study, we immunolocalized PR expression in various human tissues using PR (1294) specific antibody, which is capable of detecting both PRA and PRB, and PRB (250H11) specific antibody. Tissues from the uterus, ovary, breast, placenta, prostate, testis, cerebrum, cerebellum, pituitary, spinal cord, esophagus, stomach, small intestine, colon, pancreas, liver, kidney, urinary bladder, lung, heart, aorta, thymus, adrenal gland, thyroid, spleen, skin, and bone were examined in four different age groups (fetal, pediatric, young, and old) in male and female subjects. PR and PRB were detected in the nuclei of cells in the female reproductive system, in both the nuclei and cytoplasm of pituitary gland and pancreatic acinar cells, and only in the cytoplasm of cells in the testis, stomach, small intestine, colon, liver, kidney, urinary bladder, lung, adrenal gland, and skin. Of particular interest, total PRB expression overlapped with that of total PR expression in most tissues but was negative in the female fetal reproductive system. The findings indicate that progesterone could affect diverse human organs differently than from reproductive organs. These findings provide new insights into the novel biological roles of progesterone in non-reproductive organs.

Differential quantitative proteomics reveals key proteins related to phenotypic changes of breast cancer cells expressing progesterone receptor A

Progesterone receptor isoforms A and B exert different biological effects in breast cancer cells. Alteration of PRA/PRB ratio is often observed during breast cancer progression. High PRA/PRB ratios in breast cancer patients are associated with resistance to chemotherapy and poor prognosis. While it is well accepted that PRA and PRB regulate different sets of genes, how the expression of PRA and PRB alters breast cancer proteomes has not been fully investigated. To directly investigate the effects of PR isoform expression on the breast cancer proteome, both in the presence and absence of progestin, PRA and PRB were independently stably expressed in T47DC42 PR-null breast cancer cells using a doxycycline (Dox)-regulated promoter. Dox induction dose-dependently increased PRA and PRB expression. Dox-induced PRA and PRB showed normal receptor localization and were transcriptionally active. Differential quantitative proteomic analysis by stable isotope dimethyl labeling was performed to quantitatively examine how PR isoforms altered global breast cancer proteomes. Cells expressing PRA in the absence of progestin were enriched in proteins involved in the TCA cycle and enriched in proteins involved in glycolysis in the presence of progestin, whilst cells expressing PRB in the absence and presence progestin were significantly enriched in proteins involved in the cell cycle and cell apoptosis pathways. This proteomic data revealed a link between PR isoform expression and alteration in cell metabolism, cell proliferation, and apoptosis. The enrichment of proteins involved in the glycolytic pathway in breast cancer cells expressing PRA is consistent with stem cell-like properties, previously reported in PRA-rich breast cancer cells. Moreover, compared to liganded PRB, liganded PRA differentially upregulated proteins involved in chromatin remodeling, such as linker histone H1.2. Silencing H1.2 gene expression suppressed PRA-mediated cell proliferation and promoted G2/M and S phase entry of the cell cycle. Additionally, liganded PRA upregulated the expression of cathepsin D (CTSD) protease, whose expression is associated with poor prognosis in breast cancer patients. Together, our data demonstrated that the expression of PRA or PRB dramatically and differentially altered breast cancer cell proteomes. These isoform-specific changes in the breast cancer proteome will help to explain the distinct phenotypic properties of breast cancer cells expressing different levels of PRA and PRB.

Molecular mechanisms underlying progesterone receptor action in breast cancer: Insights into cell proliferation and stem cell regulation

The ovarian steroid hormone progesterone and its nuclear receptor, the Progesterone Receptor (PR), play an essential role in the regulation of cell proliferation and differentiation in the mammary gland. In addition, experimental and clinical evidence demonstrate their critical role in controlling mammary gland tumorigenesis and breast cancer development. When bound to its ligand, the main action of PR is as a transcription factor, which regulates the expression of target genes networks. PR also activates signal transduction pathways through a rapid or non-genomic mechanism in breast cancer cells, an event that is fully integrated with its genomic effects. This review summarizes the molecular mechanisms of the ligand-activated PR actions that drive epithelial cell proliferation and the regulation of the stem cell population in the normal breast and in breast cancer.

Progesterone receptor isoform B expression in pulmonary neuroendocrine cells decreases cell proliferation

The progesterone receptor (PR) has been reported to play important roles in lung development and function, such as alveolarization, alveolar fluid clearance (AFC) and upper airway dilator muscle activity. In the lung, pulmonary neuroendocrine cells (PNECs) are important in the etiology and progression of lung neuroendocrine tumors (NETs). Women with lung NETs had significantly better survival rates than men, suggesting that sex steroids and their receptors, such as the PR, could be involved in the progression of lung NETs. The PR exists as two major isoforms, PRA and PRB. How the expression of different PR isoforms affects proliferation and the development of lung NETs is not well understood. To determine the role of the PR isoforms in PNECs, we constructed H727 lung NET cell models expressing PRB, PRA, Green Fluorescence Protein (GFP) (control). The expression of PRB significantly inhibited H727 cell proliferation better than that of PRA in the absence of progestin. The expression of the unrelated protein, GFP, had little to no effect on H727 cell proliferation. To better understand the role of the PR isoform in PNECs, we examined PR isoform expression in PNECs in lung tissues. A monoclonal antibody specific to the N-terminus of PRB (250H11 mAb) was developed to specifically recognize PRB, while a monoclonal antibody specific to a common N-terminus epitope present in both PRA and PRB (1294 mAb) was used to detect both PRA and PRB. Using these PR and PRB-specific antibodies, we demonstrated that PR (PRA&PRB) and PRB were expressed in the PNECs of the normal fetal and adult lung, with significantly higher PR expression in the fetal lung. Interestingly, PRB expression in the normal lung was associated with lower cell proliferation than PR expression, suggesting a distinct role of PRB in the PNECs. A better understanding of the molecular mechanism of PR and PR isoform signaling in lung NET cells may help in developing novel therapeutic strategies that will benefit lung NET patients in the future.

Phosphorylated Progesterone Receptor Isoforms Mediate Opposing Stem Cell and Proliferative Breast Cancer Cell Fates

Progesterone receptors (PRs) are key modifiers of estrogen receptor (ER) target genes and drivers of luminal breast cancer progression. Total PR expression, rather than isoform-specific PR expression, is measured in breast tumors as an indicator of functional ER. We identified phenotypic differences between PR-A and PR-B in luminal breast cancer models with a focus on tumorsphere biology. Our findings indicated that PR-A is a dominant driver of cancer stem cell (CSC) expansion in T47D models, and PR-B is a potent driver of anchorage-independent proliferation. PR-A+ tumorspheres were enriched for aldehyde dehydrogenase (ALDH) activity, CD44+/CD24-, and CD49f+/CD24- cell populations relative to PR-B+ tumorspheres. Progestin promoted heightened expression of known CSC-associated target genes in PR-A+ but not PR-B+ cells cultured as tumorspheres. We report robust phosphorylation of PR-A relative to PR-B Ser294 and found that this residue is required for PR-A-induced expression of CSC-associated genes and CSC behavior. Cells expressing PR-A S294A exhibited impaired CSC phenotypes but heightened anchorage-independent cell proliferation. The PR target gene and coactivator, FOXO1, promoted PR phosphorylation and tumorsphere formation. The FOXO1 inhibitor (AS1842856) alone or combined with onapristone (PR antagonist), blunted phosphorylated PR, and tumorsphere formation in PR-A+ and PR-B+ T47D, MCF7, and BT474 models. Our data revealed unique isoform-specific functions of phosphorylated PRs as modulators of distinct and opposing pathways relevant to mechanisms of late recurrence. A clear understanding of PR isoforms, phosphorylation events, and the role of cofactors could lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target CSCs in luminal breast cancer.

The Flavonoid Apigenin Is a Progesterone Receptor Modulator with In Vivo Activity in the Uterus

Apigenin is a flavonoid with well-documented anti-cancer properties; however, its mechanisms of action are still unclear. We previously identified apigenin as a potential phytoprogestin, a natural product with a chemical scaffold that interacts with the progesterone receptor (PR). Our objective was to characterize the ability of apigenin to interact with PR through molecular docking studies, in vitro activity assays, and the ability of apigenin to elicit progestin-like effects in vivo. Molecular docking confirmed that apigenin could interact with PR, though with lower affinity than progesterone due to fewer van der Waals interactions. In Ishikawa cells stably expressing PR-B, apigenin significantly increased progesterone response element/luciferase (PRE/Luc) activity at 5 and 10 μM, but not in the parental Ishikawa cells that lack PR expression. In the presence of 100 nM of progesterone, 10 μM apigenin reduced PRE/Luc activity, indicative of mixed agonist activity. Apigenin also triggered degradation of PR in Ishikawa PR-B cells as measured by western blot. Apigenin reduced proliferation of Ishikawa cells, but through a PR-independent mechanism. In contrast, apigenin and progesterone both stimulated proliferation of T47D cells, an effect blocked by RU486. Apigenin activated other nuclear receptors evidenced by increased luciferase activity in MDA-MB-231 cells, which are PR negative. In vivo, apigenin blocked the genistein-stimulated increase in uterine epithelial cell height; stimulated endometrial expression of Hand2, a transcription factor stimulated by PR, and significantly reduced genistein-induced proliferation. In summary, apigenin is a phytoprogestin, with mixed agonist activity that demonstrates activity in vivo by hindering estrogen receptor-mediated uterine proliferation.

Quantification of Breast Cancer Protein Biomarkers at Different Expression Levels in Human Tumors

Liquid chromatography-selected reaction monitoring (LC-SRM) mass spectrometry has developed into a versatile tool for quantification of proteins with a wide range of applications in basic science, translational research, and clinical patient assessment. This strategy uniquely complements traditional pathology approaches, like hematoxylin and eosin (H&E) staining and immunohistochemistry (IHC). The multiplexing capabilities offered by mass spectrometry are currently unmatched by other techniques. However, quantification of biomarkers in tissue specimens without the other data obtained from H&E-stained slides or IHC, including tumor cellularity or percentage of positively stained cells inter alia, may not provide as much information that is needed to fully understand tumor biology or properly assess the patient. Therefore, additional characterization of the tissue proteome is needed, which in turn requires the ability to assess protein markers across a wide range of expression levels from a single sample. This protocol provides an example of multiplexed analysis in breast tumor tissue quantifying specific biomarkers, specifically estrogen receptor, progesterone receptor, and the HER2 receptor tyrosine kinase, in combination with other proteins that can report on tissue content and other aspects of tumor biology.

Inhibiting Nuclear Phospho-Progesterone Receptor Enhances Antitumor Activity of Onapristone in Uterine Cancer

Although progesterone receptor (PR)-targeted therapies are modestly active in patients with uterine cancer, their underlying molecular mechanisms are not well understood. The clinical use of such therapies is limited because of the lack of biomarkers that predict response to PR agonists (progestins) or PR antagonists (onapristone). Thus, understanding the underlying molecular mechanisms of action will provide an advance in developing novel combination therapies for cancer patients. Nuclear translocation of PR has been reported to be ligand-dependent or -independent. Here, we identified that onapristone, a PR antagonist, inhibited nuclear translocation of ligand-dependent or -independent (EGF) phospho-PR (S294), whereas trametinib inhibited nuclear translocation of EGF-induced phospho-PR (S294). Using orthotopic mouse models of uterine cancer, we demonstrated that the combination of onapristone and trametinib results in superior antitumor effects in uterine cancer models compared with either monotherapy. These synergistic effects are, in part, mediated through inhibiting the nuclear translocation of EGF-induced PR phosphorylation in uterine cancer cells. Targeting MAPK-dependent PR activation with onapristone and trametinib significantly inhibited tumor growth in preclinical uterine cancer models and is worthy of further clinical investigation. Mol Cancer Ther; 17(2); 464-73. ©2017 AACR.

Quantitative proteomics of breast tumors: Tissue quality assessment to clinical biomarkers

Liquid chromatography-selected reaction monitoring mass spectrometry (LC-SRM) is not only a proven tool for clinical chemistry, but also a versatile method to enhance the capability to quantify biomarkers for tumor biology research. As the treatment of cancer continues to evolve, the ability to assess multiple biomarkers to assign cancer phenotypes based on the genetic background and the signaling of the individual tumor becomes paramount to our ability to treat the patient. In breast cancer, the American Society of Clinical Oncology has defined biomarkers for patient assessment to guide selection of therapy: estrogen receptor, progesterone receptor, and the HER2/Neu receptor tyrosine kinase; therefore, these proteins were selected for LC-SRM assay development. Detailed molecular characterization of these proteins is necessary for patient treatment, so expression and phosphorylation assays have been developed and applied. In addition, other LC-SRM assays were developed to further evaluate tumor biology (e.g. Ki-67 for proliferation and vimentin for tumor aggressiveness related to the epithelial-to-mesenchymal transition). These measurements combined with biomarkers for tissue quality and histological content are implemented in a three-tier multiplexed assay platform, which is translated from cell line models into frozen tumor tissues banked from breast cancer patients.

The Progestin Receptor Interactome in the Female Mouse Hypothalamus: Interactions with Synaptic Proteins Are Isoform Specific and Ligand Dependent

Progestins bind to the progestin receptor (PR) isoforms, PR-A and PR-B, in brain to influence development, female reproduction, anxiety, and stress. Hormone-activated PRs associate with multiple proteins to form functional complexes. In the present study, proteins from female mouse hypothalamus that associate with PR were isolated using affinity pull-down assays with glutathione S-transferase–tagged mouse PR-A and PR-B. Using complementary proteomics approaches, reverse phase protein array (RPPA) and mass spectrometry, we identified hypothalamic proteins that interact with PR in a ligand-dependent and isoform-specific manner and were confirmed by Western blot. Synaptic proteins, including synapsin-I and synapsin-II, interacted with agonist-bound PR isoforms, suggesting that both isoforms function in synaptic plasticity. In further support, synaptogyrin-III and synapsin-III associated with PR-A and PR-B, respectively. PR also interacted with kinases, including c-Src, mTOR, and MAPK1, confirming phosphorylation as an integral process in rapid effects of PR in the brain. Consistent with a role in transcriptional regulation, PR associated with transcription factors and coactivators in a ligand-specific and isoform-dependent manner. Interestingly, both PR isoforms associated with a key regulator of energy homeostasis, FoxO1, suggesting a novel role for PR in energy metabolism. Because many identified proteins in this PR interactome are synaptic proteins, we tested the hypothesis that progestins function in synaptic plasticity. Indeed, progesterone enhanced synaptic density, by increasing synapsin-I–positive synapses, in rat primary cortical neuronal cultures. This novel combination of RPPA and mass spectrometry allowed identification of PR action in synaptic remodeling and energy homeostasis and reveals unique roles for progestins in brain function and disease.

Posttranslationally modified progesterone receptors direct ligand-specific expression of breast cancer stem cell-associated gene programs

Background

Estrogen and progesterone are potent breast mitogens. In addition to steroid hormones, multiple signaling pathways input to estrogen receptor (ER) and progesterone receptor (PR) actions via posttranslational events. Protein kinases commonly activated in breast cancers phosphorylate steroid hormone receptors (SRs) and profoundly impact their activities.

Methods

To better understand the role of modified PRs in breast cancer, we measured total and phospho-Ser294 PRs in 209 human breast tumors represented on 2754 individual tissue spots within a tissue microarray and assayed the regulation of this site in human tumor explants cultured ex vivo. To complement this analysis, we assayed PR target gene regulation in T47D luminal breast cancer models following treatment with progestin (promegestone; R5020) and antiprogestins (mifepristone, onapristone, or aglepristone) in conditions under which the receptor is regulated by Lys388 SUMOylation (K388 intact) or is SUMO-deficient (via K388R mutation to mimic persistent Ser294 phosphorylation). Selected phospho-PR-driven target genes were validated by qRT-PCR and following RUNX2 shRNA knockdown in breast cancer cell lines. Primary and secondary mammosphere assays were performed to implicate phospho-Ser294 PRs, epidermal growth factor signaling, and RUNX2 in breast cancer stem cell biology.

Results

Phospho-Ser294 PR species were abundant in a majority (54%) of luminal breast tumors, and PR promoter selectivity was exquisitely sensitive to posttranslational modifications. Phospho-PR expression and target gene programs were significantly associated with invasive lobular carcinoma (ILC). Consistent with our finding that activated phospho-PRs undergo rapid ligand-dependent turnover, unique phospho-PR gene signatures were most prevalent in breast tumors clinically designated as PR-low to PR-null (luminal B) and included gene sets associated with cancer stem cell biology (HER2, PAX2, AHR, AR, RUNX). Validation studies demonstrated a requirement for RUNX2 in the regulation of selected phospho-PR target genes (SLC37A2). In vitro mammosphere formation assays support a role for phospho-Ser294-PRs via growth factor (EGF) signaling as well as RUNX2 as potent drivers of breast cancer stem cell fate.

Conclusions

We conclude that PR Ser294 phosphorylation is a common event in breast cancer progression that is required to maintain breast cancer stem cell fate, in part via cooperation with growth factor-initiated signaling pathways and key phospho-PR target genes including SLC37A2 and RUNX2. Clinical measurement of phosphorylated PRs should be considered a useful marker of breast tumor stem cell potential. Alternatively, unique phospho-PR target gene sets may provide useful tools with which to identify patients likely to respond to selective PR modulators that block PR Ser294 phosphorylation as part of rational combination (i.e., with antiestrogens) endocrine therapies designed to durably block breast cancer recurrence.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0462-7) contains supplementary material, which is available to authorized users.

The Germ Cell Gene TDRD1 as an ERG Target Gene and a Novel Prostate Cancer Biomarker

BACKGROUND

TMPRSS2-ERG fusion occurs in about half of prostate cancers and results in over-expression of the oncogenic ERG protein in the prostate. The mechanism by which ERG contributes to prostate cancer initiation and progression remains largely unknown. Because ERG is a transcriptional activator, we reasoned that the target genes regulated by ERG could contribute to prostate cancer development.

METHODS

In a search for ERG target genes, we took advantage of published datasets from the MSKCC Prostate Oncogene Project, in which a comprehensive analysis was applied to define transcriptomes in 150 prostate tumors. We retrieved the mRNA expression dataset, split them based on ERG expression, and identified genes whose expression levels are associated with ERG mRNA levels.

RESULTS

mRNA expression levels of 21 genes were found to be significantly increased, while for one gene it was decreased in ERG-positive prostate tumors. Among them, the expression of TDRD1 was the most significantly increased in ERG-positive tumors. Among 131 primary prostate tumors which were primarily from European American patients, TDRD1 is over-expressed in 68% of samples, while ERG is overexpressed in 48% of samples, suggesting an additional ERG-independent mechanism of TDRD1 overexpression. In African American prostate tumors, TDRD1 mRNA is expressed in 44%, while ERG is expressed in 24% of samples. In normal tissues, TDRD1 mRNA is exclusively expressed in germ cells and its protein is also known as cancer/testis antigen 41.1 (CT41.1). We generated a mouse monoclonal antibody that recognizes human TDRD1 protein with high specificity and sensitivity. By Western blot analysis and immunohistochemistry (IHC) staining, we demonstrate that TDRD1 protein is expressed in the majority of human prostate tumors, but not in normal prostate tissue. Finally, TDRD1 is not induced in the prostate of ERG overexpression transgenic mice, suggesting that such model does not fully recapitulate the TMPRSS2/ERG fusion-dependent human prostate cancer development.

CONCLUSIONS

Our results suggest TDRD1 as a novel prostate cancer biomarker. As an ERG target gene, TDRD1 might play an important role in human prostate cancer development, and as a cancer/testis antigen, TDRD1 might have long-term potential to be a therapeutic target for prostate cancer immunotherapy. Prostate 76:1271-1284, 2016. © 2016 Wiley Periodicals, Inc.

Modifications to glucocorticoid and progesterone receptors alter cell fate in breast cancer

Steroid hormone receptors (SRs) are heavily posttranslationally modified by the reversible addition of a variety of molecular moieties, including phosphorylation, acetylation, methylation, SUMOylation, and ubiquitination. These rapid and dynamic modifications may be combinatorial and interact (i.e. may be sequential, complement, or oppose each other), creating a vast array of uniquely modified receptor subspecies that allow for diverse receptor behaviors that enable highly sensitive and context-dependent hormone action. For example, in response to hormone or growth factor membrane-initiated signaling events, posttranslational modifications (PTMs) to SRs alter protein-protein interactions that govern the complex process of promoter or gene-set selection coupled to transcriptional repression or activation. Unique phosphorylation events allow SRs to associate or disassociate with specific cofactors that may include pioneer factors and other tethering partners, which specify the resulting transcriptome and ultimately change cell fate. The impact of PTMs on SR action is particularly profound in the context of breast tumorigenesis, in which frequent alterations in growth factor-initiated signaling pathways occur early and act as drivers of breast cancer progression toward endocrine resistance. In this article, with primary focus on breast cancer relevance, we review the mechanisms by which PTMs, including reversible phosphorylation events, regulate the closely related SRs, glucocorticoid receptor and progesterone receptor, allowing for precise biological responses to ever-changing hormonal stimuli.

Progesterone receptor (PR) polyproline domain (PPD) mediates inhibition of epidermal growth factor receptor (EGFR) signaling in non-small cell lung cancer cells

Recent evidence has suggested a possible role for progesterone receptor (PR) in the progression of non-small cell lung cancer (NSCLC). However, little is known concerning roles of PR in NSCLC. PR contains a polyproline domain (PPD), which directly binds to the SH3 domain of signaling molecules. Because PPD-SH3 interactions are essential for EGFR signaling, we hypothesized that the presence of PR-PPD interfered with EGFR-mediated signaling and cell proliferation. We examined the role of PR-PPD in cell proliferation and signaling by stably expressing PR-B, or PR-B with disrupting mutations in the PPD (PR-BΔSH3), from a tetracycline-regulated promoter in A549 NSCLC cells. PR-B dose-dependently inhibited cell growth in the absence of ligand, and progestin (R5020) treatment further suppressed the growth. Treatment with RU486 abolished PR-B- and R5020-mediated inhibition of cell proliferation. Expression of PR-BΔSH3 and treatment with R5020 or RU486 had no effect on cell proliferation. Furthermore, PR-B expression but not PR-BΔSH3 expression reduced EGF-induced A549 proliferation and activation of ERK1/2, in the absence of ligand. Taken together, our data demonstrated the significance of PR extranuclear signaling through PPD interactions in EGFR-mediated proliferation and signaling in NSCLC.

Active FOXO1 Is a Key Determinant of Isoform-Specific Progesterone Receptor Transactivation and Senescence Programming

Progesterone promotes differentiation coupled to proliferation and prosurvival in the breast, but inhibits estrogen-driven growth in the reproductive tract and ovaries. Herein, it is demonstrated, using progesterone receptor (PR) isoform-specific ovarian cancer model systems, that PR-A and PR-B promote distinct gene expression profiles that differ from PR-driven genes in breast cancer cells. In ovarian cancer models, PR-A primarily regulates genes independently of progestin, while PR-B is the dominant ligand-dependent isoform. Notably, FOXO1 and the PR/FOXO1 target gene p21 (CDKN1A) are repressed by PR-A, but induced by PR-B. In the presence of progestin, PR-B, but not PR-A, robustly induced cellular senescence via FOXO1-dependent induction of p21 and p15 (CDKN2B). Chromatin immunoprecipitation (ChIP) assays performed on PR isoform-specific cells demonstrated that while each isoform is recruited to the same PRE-containing region of the p21 promoter in response to progestin, only PR-B elicits active chromatin marks. Overexpression of constitutively active FOXO1 in PR-A-expressing cells conferred robust ligand-dependent upregulation of the PR-B target genes GZMA, IGFBP1, and p21, and induced cellular senescence. In the presence of endogenous active FOXO1, PR-A was phosphorylated on Ser294 and transactivated PR-B at PR-B target genes; these events were blocked by the FOXO1 inhibitor (AS1842856). PR isoform-specific regulation of the FOXO1/p21 axis recapitulated in human primary ovarian tumor explants treated with progestin; loss of progestin sensitivity correlated with high AKT activity.

IMPLICATIONS

This study indicates FOXO1 as a critical component for progesterone signaling to promote cellular senescence and reveals a novel mechanism for transcription factor control of hormone sensitivity.

A role for site-specific phosphorylation of mouse progesterone receptor at serine 191 in vivo

Progesterone receptors (PRs) are phosphorylated on multiple sites, and a variety of roles for phosphorylation have been suggested by cell-based studies. Previous studies using PR-null mice have shown that PR plays an important role in female fertility, regulation of uterine growth, the uterine decidualization response, and proliferation as well as ductal side-branching and alveologenesis in the mammary gland. To study the role of PR phosphorylation in vivo, a mouse was engineered with homozygous replacement of PR with a PR serine-to-alanine mutation at amino acid 191. No overt phenotypes were observed in the mammary glands or uteri of PR S191A treated with progesterone (P4). In contrast, although PR S191A mice were fertile, litters were 19% smaller than wild type and the estrous cycle was lengthened slightly. Moreover, P4-dependent gene regulation in primary mammary epithelial cells (MECs) was altered in a gene-selective manner. MECs derived from wild type and PR S191A mice were grown in a three-dimensional culture. Both formed acinar structures that were morphologically similar, and proliferation was stimulated equally by P4. However, P4 induction of receptor activator of nuclear factor-κB ligand and calcitonin was selectively reduced in S191A cultures. These differences were confirmed in freshly isolated MECs. Chromatin immunoprecipitation analysis showed that the binding of S191A PR to some of the receptor activator of nuclear factor-κB ligand enhancers and a calcitonin enhancer was substantially reduced. Thus, the elimination of a single phosphorylation site is sufficient to modulate PR activity in vivo. PR contains many phosphorylation sites, and the coordinate regulation of multiple sites is a potential mechanism for selective modulation of PR function.

A single-dose PK study of onapristone including the effect of food on absorption

PURPOSE

Onapristone is an antiprogestin with activity in breast cancer and is under investigation for use in endometrial, ovarian and prostate cancers. Megestrol acetate and abiraterone generally show variability in absorption and, depending on the formulation, food effect. This study was conducted to determine the effect of food on 10 mg oral immediate-release (IR) onapristone and to help identify a formulation to minimize variability.

METHODS

This is an open-label, randomized, crossover study to determine the pharmacokinetic profile of onapristone and its main metabolite, N-mono-desmethyl onapristone. Twelve healthy female subjects received 10 mg of oral IR onapristone after an overnight fast, or within 30 min of a high-fat, high-calorie meal with a 2-week washout between dosing periods.

RESULTS

Onapristone plasma t1/2 (mean ± SD) was 4.36 ± 0.81 h for the fasted state and 3.76 ± 0.36 h for the fed state. Following food, onapristone tmax was delayed from 1 to 4 h. Food intake was also associated with a small increase in AUC0-∞ of approximately 13 % and a statistically significant decrease in Cmax of approximately 18 %. One subject experienced a 23-day delay in menses after one 10 mg onapristone dose, while another subject experienced transient grade 2 NCI-CTCAE liver enzyme elevation 3 weeks post dose.

CONCLUSION

The results are consistent with previous observations, indicating that there is a small increase in onapristone exposure and a significant decrease in Cmax when taken with food. These changes are within acceptable limits set out by the FDA. Thus, our findings indicate that onapristone could be administered without regard to food.

Progesterone receptor signaling in the initiation of pregnancy and preservation of a healthy uterus

Infertility and reproductive-associated disease are global problems in the world today affecting millions of women. A successful pregnancy requires a healthy uterus ready to receive and support an implanting embryo. As an endocrine organ, the uterus is dependent on the secretions of the ovarian hormones estrogen and progesterone which signal via their cognate receptors, the estrogen and progesterone receptors. The progesterone receptor not only functions using classical nuclear receptor signaling, but also participates in non-genomic signaling at the cellular membrane. The complexity of progesterone signaling is further enhanced by the existence of multiple isoforms and post-translational regulation via kinases and transcription coregulators. This dynamic means of regulation of the progesterone receptor is evidenced in its necessary role in a successful pregnancy. Within early pregnancy, the progesterone receptor elicits activation of its target genes in a spatiotemporal manner in order to allow for successful embryo attachment and uterine decidualization. Additionally, appropriate progesterone signaling is important for the prevention of uterine disease such as endometrial cancer, endometriosis, and leiomyoma. The utilization of progesterone receptor modulators in the treatment of these devastating uterine diseases is promising. This review presents a general overview of progesterone receptor structure, function, and regulation and highlights its important role in the establishment of pregnancy and as a therapeutic target in uterine disease.

Molecular determinants of context-dependent progesterone receptor action in breast cancer

The ovarian steroid hormone, progesterone, and its nuclear receptor, the progesterone receptor, are implicated in the progression of breast cancer. Clinical trial data on the effects of hormone replacement therapy underscore the importance of understanding how progestins influence breast cancer growth. The progesterone receptor regulation of distinct target genes is mediated by complex interactions between the progesterone receptor and other regulatory factors that determine the context-dependent transcriptional action of the progesterone receptor. These interactions often lead to post-translational modifications to the progesterone receptor that can dramatically alter receptor function, both in the normal mammary gland and in breast cancer. This review highlights the molecular components that regulate progesterone receptor transcriptional action and describes how a better understanding of the complex interactions between the progesterone receptor and other regulatory factors may be critical to enhancing the clinical efficacy of anti-progestins for use in the treatment of breast cancer.

Lysine methylation of progesterone receptor at activation function 1 regulates both ligand-independent activity and ligand sensitivity of the receptor

Progesterone receptor (PR) exists in two isoforms, PRA and PRB, and both contain activation functions AF-1 and AF-2. It is believed that AF-1 is primarily responsible for the ligand-independent activity, whereas AF-2 mediates ligand-dependent PR activation. Although more than a dozen post-translational modifications of PR have been reported, no post-translational modification on AF-1 or AF-2 has been reported. Using LC-MS/MS-based proteomic analysis, this study revealed AF-1 monomethylation at Lys-464. Mutational analysis revealed the remarkable importance of Lys-464 in regulating PR activity. Single point mutation K464Q or K464A led to ligand-independent PR gel upshift similar to the ligand-induced gel upshift. This upshift was associated with increases in both ligand-dependent and ligand-independent PR phosphorylation and PR activity due to the hyperactivation of AF-1. In contrast, mutation of Lys-464 to the bulkier phenylalanine to mimic the effect of methylation caused a drastic decrease in PR activity. Importantly, PR-K464Q also showed heightened ligand sensitivity, and this was associated with increases in its functional interaction with transcription co-regulators NCoR1 and SRC-1. These results suggest that monomethylation of PR at Lys-464 probably has a repressive effect on AF-1 activity and ligand sensitivity.

Phosphorylation: a fundamental regulator of steroid receptor action

Steroid hormone receptors (SHRs) are hormone-activated transcription factors involved in numerous cellular functions and in health and disease. Their activities depend on the cellular level of the receptor, the presence of coregulator proteins, and the cell signaling pathways that are active in the cell. SHRs and their coregulators are phosphorylated on multiple sites by a wide variety of kinases. Each site may contribute to multiple functions and the net effect of an individual phosphorylation depends on the activating kinase. Here we discuss functions of known SHR phosphorylation sites, kinase regulation, evidence of translational relevance, and crosstalk between SHRs and cell signaling pathways. Understanding how cell signaling pathways regulate SHRs might yield novel therapeutic targets for multiple human diseases.

Regulation of the structurally dynamic N-terminal domain of progesterone receptor by protein-induced folding

The N-terminal domain (NTD) of steroid receptors harbors a transcriptional activation function (AF1) that is composed of an intrinsically disordered polypeptide. We examined the interaction of the TATA-binding protein (TBP) with the NTD of the progesterone receptor (PR) and its ability to regulate AF1 activity through coupled folding and binding. As assessed by solution phase biophysical methods, the isolated NTD of PR contains a large content of random coil, and it is capable of adopting secondary α-helical structure and more stable tertiary folding either in the presence of the natural osmolyte trimethylamine-N-oxide or through a direct interaction with TBP. Hydrogen-deuterium exchange coupled with mass spectrometry confirmed the highly dynamic intrinsically disordered property of the NTD within the context of full-length PR. Deletion mapping and point mutagenesis defined a region of the NTD (amino acids 350-428) required for structural folding in response to TBP interaction. Overexpression of TBP in cells enhanced transcriptional activity mediated by the PR NTD, and deletion mutations showed that a region (amino acids 327-428), similar to that required for TBP-induced folding, was required for functional response. TBP also increased steroid receptor co-activator 1 (SRC-1) interaction with the PR NTD and cooperated with SRC-1 to stimulate NTD-dependent transcriptional activity. These data suggest that TBP can mediate structural reorganization of the NTD to facilitate the binding of co-activators required for maximal transcriptional activation.

The requirement for p42/p44 MAPK activity in progesterone receptor-mediated gene regulation is target gene-specific

Recent studies have suggested that progestins play a role in the etiology of breast cancer; however, the mechanisms by which progestins promote tumor formation/progression have not been defined. Progestin action, in target tissues such as the breast, is mediated by the progesterone receptor (PR). PR signaling is complex and PR regulates transcription of target genes through a variety of mechanisms. Many cell signaling pathways are activated inappropriately in breast cancer cells and these pathways can regulate PR activity. For example, the p42/p44 MAPK pathway can regulate PR function by altering phosphorylation of PR, as well as its coregulators. We found that inhibition of the p42/p44 MAPK signaling pathway with a MEK inhibitor (U0126) impairs PR-mediated gene induction, but not gene repression. In addition, the effects of U0126 on PR-mediated gene transcription are much greater with long-term versus short-term inhibition and are gene-specific. Finally, treatment with U0126 delays phosphorylation of Ser294, but does not block phosphorylation completely, suggesting that p42/p44 MAPK kinase is not the dominant kinase responsible for phosphorylating this site. Collectively, these studies suggest that in addition to the p42/p44 MAPK pathway, other signaling pathways are also important for PR transcriptional activity in breast cancer cells. The integration of PR transcriptional effects and cell signaling pathways has implications for the initiation or progression of breast cancer. Understanding how these pathways interact may aid in the development of prevention and/or treatment strategies for the disease.

Differential phosphorylation of perilipin 1A at the initiation of lipolysis revealed by novel monoclonal antibodies and high content analysis

Lipolysis in adipocytes is regulated by phosphorylation of lipid droplet-associated proteins, including perilipin 1A and hormone-sensitive lipase (HSL). Perilipin 1A is potentially phosphorylated by cAMP(adenosine 3′,5′-cyclic monophosphate)-dependent protein kinase (PKA) on several sites, including conserved C-terminal residues, serine 497 (PKA-site 5) and serine 522 (PKA-site 6). To characterize perilipin 1A phosphorylation, novel monoclonal antibodies were developed, which selectively recognize perilipin 1A phosphorylation at PKA-site 5 and PKA-site 6. Utilizing these novel antibodies, as well as antibodies selectively recognizing HSL phosphorylation at serine 563 or serine 660, we used high content analysis to examine the phosphorylation of perilipin 1A and HSL in adipocytes exposed to lipolytic agents. We found that perilipin PKA-site 5 and HSL-serine 660 were phosphorylated to a similar extent in response to forskolin (FSK) and L-γ-melanocyte stimulating hormone (L-γ-MSH). In contrast, perilipin PKA-site 6 and HSL-serine 563 were phosphorylated more slowly and L-γ-MSH was a stronger agonist for these sites compared to FSK. When a panel of lipolytic agents was tested, including multiple concentrations of isoproterenol, FSK, and L-γ-MSH, the pattern of results was virtually identical for perilipin PKA-site 5 and HSL-serine 660, whereas a distinct pattern was observed for perilipin PKA-site 6 and HSL-serine 563. Notably, perilipin PKA-site 5 and HSL-serine 660 feature two arginine residues upstream from the phospho-acceptor site, which confers high affinity for PKA, whereas perilipin PKA-site 6 and HSL-serine 563 feature only a single arginine. Thus, we suggest perilipin 1A and HSL are differentially phosphorylated in a similar manner at the initiation of lipolysis and arginine residues near the target serines may influence this process.

Targeted disruption of the p160 coactivator interface of androgen receptor (AR) selectively inhibits AR activity in both androgen-dependent and castration-resistant AR-expressing prostate cancer cells

The evidence that androgen blockade-resistant prostate cancer, termed castration resistant, remains androgen receptor (AR) dependent is compelling. AR is re-activated through multiple mechanisms including expression of constitutively active splice variants that lack hormone binding domains (HBDs). This highlights the need to develop therapies that target regions other than the HBD. Because the p160 coactivators interact most strongly with the amino-terminus of AR, we examined the consequences of disrupting this interaction. We identified two overlapping SRC-1 peptides that interact with AR, but not with progesterone receptor. These peptides reduce AR and AR variant AR-V7 dependent induction of an AR responsive reporter. Using mammalian two hybrid assays, we found that the peptides interrupt the AR/SRC-1, AR/SRC-2 and AR N/C interactions, but not SRC-1/CARM-1 interactions. Consistent with the SRC-1 dependence of induced, but not repressed genes, in LNCaP cells, the peptides inhibited hormone dependent induction of endogenous target genes including PSA and TMPRSS2, but did not block AR dependent repression of UGT2B17 or inhibit vitamin D receptor activity. Simultaneous detection of SRC-1 peptides and PSA by double immunofluorescence in transfected LNCaP cells clearly demonstrated a strong reduction in PSA levels in cells expressing the peptides. The peptides also inhibited the AR dependent expression of PSA in castration resistant C4-2 cells. Moreover they inhibited androgen dependent proliferation of LNCaP cells and proliferation of C4-2 cells in androgen depleted medium without affecting AR negative PC-3 cells. Thus, the p160 coactivator binding site is a novel potential therapeutic target to inhibit AR activity.

Role of phosphorylation in progesterone receptor signaling and specificity

Progesterone receptors (PR), in concert with peptide growth factor-initiated signaling pathways, initiate massive expansion of the epithelial cell compartment associated with the process of alveologenesis in the developing mammary gland. PR-dependent signaling events also contribute to inappropriate proliferation observed in breast cancer. Notably, PR-B isoform-specific cross talk with growth factor-driven pathways is required for the proliferative actions of progesterone. Indeed, PRs act as heavily phosphorylated transcription factor "sensors" for mitogenic protein kinases that are often elevated and/or constitutively activated in invasive breast cancers. In addition, phospho-PR-target genes frequently include the components of mitogenic signaling pathways, revealing a mechanism for feed-forward signaling that confers increased responsiveness of, PR +mammary epithelial cells to these same mitogenic stimuli. Understanding the mechanisms and isoform selectivity of PR/kinase interactions may yield further insight into targeting altered signaling networks in breast and other hormonally responsive cancers (i.e. lung, uterine and ovarian) in the clinic. This review focuses on PR phosphorylation by mitogenic protein kinases and mechanisms of PR-target gene selection that lead to increased cell proliferation.

Phosphorylated and sumoylation-deficient progesterone receptors drive proliferative gene signatures during breast cancer progression

INTRODUCTION

Progesterone receptors (PR) are emerging as important breast cancer drivers. Phosphorylation events common to breast cancer cells impact PR transcriptional activity, in part by direct phosphorylation. PR-B but not PR-A isoforms are phosphorylated on Ser294 by mitogen activated protein kinase (MAPK) and cyclin dependent kinase 2 (CDK2). Phospho-Ser294 PRs are resistant to ligand-dependent Lys388 SUMOylation (that is, a repressive modification). Antagonism of PR small ubiquitin-like modifier (SUMO)ylation by mitogenic protein kinases suggests a mechanism for derepression (that is, transcriptional activation) of target genes. As a broad range of PR protein expression is observed clinically, a PR gene signature would provide a valuable marker of PR contribution to early breast cancer progression.

METHODS

Global gene expression patterns were measured in T47D and MCF-7 breast cancer cells expressing either wild-type (SUMOylation-capable) or K388R (SUMOylation-deficient) PRs and subjected to pathway analysis. Gene sets were validated by RT-qPCR. Recruitment of coregulators and histone methylation levels were determined by chromatin immunoprecipitation. Changes in cell proliferation and survival were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and western blotting. Finally, human breast tumor cohort datasets were probed to identify PR-associated gene signatures; metagene analysis was employed to define survival rates in patients whose tumors express a PR gene signature.

RESULTS

'SUMO-sensitive' PR target genes primarily include genes required for proliferative and pro-survival signaling. DeSUMOylated K388R receptors are preferentially recruited to enhancer regions of derepressed genes (that is, MSX2, RGS2, MAP1A, and PDK4) with the steroid receptor coactivator, CREB-(cAMP-response element-binding protein)-binding protein (CBP), and mixed lineage leukemia 2 (MLL2), a histone methyltransferase mediator of nucleosome remodeling. PR SUMOylation blocks these events, suggesting that SUMO modification of PR prevents interactions with mediators of early chromatin remodeling at 'closed' enhancer regions. SUMO-deficient (phospho-Ser294) PR gene signatures are significantly associated with human epidermal growth factor 2 (ERBB2)-positive luminal breast tumors and predictive of early metastasis and shortened survival. Treatment with antiprogestin or MEK inhibitor abrogated expression of SUMO-sensitive PR target-genes and inhibited proliferation in BT-474 (estrogen receptor (ER)+/PR+/ERBB2+) breast cancer cells.

CONCLUSIONS

We conclude that reversible PR SUMOylation/deSUMOylation profoundly alters target gene selection in breast cancer cells. Phosphorylation-induced PR deSUMOylation favors a permissive chromatin environment via recruitment of CBP and MLL2. Patients whose ER+/PR+ tumors are driven by hyperactive (that is, derepressed) phospho-PRs may benefit from endocrine (antiestrogen) therapies that contain an antiprogestin.

Structural and functional analysis of domains of the progesterone receptor

Steroid hormone receptors are multi-domain proteins composed of conserved well-structured regions, such as ligand (LBD) and DNA binding domains (DBD), plus other naturally unstructured regions including the amino-terminal domain (NTD) and the hinge region between the LBD and DBD. The hinge is more than just a flexible region between the DBD and LBD and is capable of binding co-regulatory proteins and the minor groove of DNA flanking hormone response elements. Because the hinge can directly participate in DNA binding it has also been termed the carboxyl terminal extension (CTE) of the DNA binding domain. The CTE and NTD are dynamic regions of the receptor that can adopt multiple conformations depending on the environment of interacting proteins and DNA. Both regions have important regulatory roles for multiple receptor functions that are related to the ability of the CTE and NTD to form multiple active conformations. This review focuses on studies of the CTE and NTD of progesterone receptor (PR), as well as related work with other steroid/nuclear receptors.

Evaluation of human first trimester decidual and telomerase-transformed endometrial stromal cells as model systems of in vitro decidualization

BACKGROUND

Decidualization, the differentiation process of maternal uterine stromal cells into secretory decidual cells, is a prerequisite for successful implantation and progression of pregnancy. For in vitro differentiation mostly primary human endometrial stromal cells (HESC) isolated from uterine samples after hysterectomy for benign gynaecological diseases are utilised. However, a continuous supply of endometrial tissue is often lacking. Hence, we analysed whether cultivated human decidual stromal cells (HDSC) prepared from first trimester pregnancy terminations may represent an alternative model system for in vitro decidualization. Moreover, based on the expression of critical marker genes these cells were compared to a previously established endometrial stromal cell line during in vitro differentiation.

METHODS

HDSC isolated from decidual tissue attached to first trimester placentae, and telomerase-transformed human endometrial stromal cells (THESC) were characterised by immunofluorescence and differentiated in vitro using either cyclic adenosine monophosphate (cAMP) and/or estrogen (E2)/progesterone (P4). Proliferation was measured by analyzing cumulative cell numbers. Expression of mRNAs encoding progesterone receptor (PR), prolactin (PRL), insulin-like growth factor binding protein-1 (IGFBP1), and Dickkopf-1 (DKK1) was evaluated using quantitative PCR after 3, 6, 9 and 12 days of in vitro differentiation. PRL and IGFBP-1 protein expression was investigated by enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively. Furthermore, forkhead box O1A (FOXO1A), a critical transcription factor in decidualization, was analysed by immunofluorescence and Western blotting at two different time points of differentiation.

RESULTS

Treatment with cAMP provoked morphological changes and growth arrest of THESC and HDSC, the latter showing loss of cells after 6 days of treatment. E2P4 stimulation did neither affect cell morphology nor proliferation of THESC and HDSC. Upon cAMP stimulation PR mRNA was suppressed in HDSC but not in THESC, whereas E2P4 did not alter transcript levels in both cell types. Protein expression of PR-A and PR-B was detectable in HDSC and diminished under cAMP, whereas THESC failed to produce the nuclear receptors. Supplementation of cAMP induced mRNA and protein expression of PRL and IGFBP-1 in both cell types at day 3, 6, 9, and 12 of treatment. In HDSC stimulation with E2P4 increased PRL and IGFBP-1 mRNA and protein production, whereas hormone treatment did not induce the two factors in THESC. E2P4 increased DKK1 mRNA at all time points in HDSC and cAMP provoked induction at day 9 and 12 of differentiation. In contrast, cAMP suppressed DKK1 mRNA in THESC, whereas E2P4 was ineffective. In both cell types combined treatments with cAMP and E2P4 provoked higher expression levels of PRL and IGFBP1 mRNA and protein as compared to cAMP stimulation alone. FOXO1A protein and its nuclear abundance were increased by cAMP in both cell types. However, reduction of its nuclear localisation upon E2P4 treatment could only be observed in HDSC.

CONCLUSION

Both HDSC and THESC may represent suitable model systems for cAMP-dependent in vitro decidualization. Since cAMP decreases cell viability of HDSC after 6 days of incubation, this substance should be preferentially used in short-term experiments. Progesterone treatment of THESC might not be applicable since these cells lack progesterone response and PR protein. In contrast, stimulation of PR-expressing HDSC with E2P4 or cAMP/E2P4 may represent an appropriate protocol for human in vitro decidualization inducing and maintaining expression of critical marker genes in a time-dependent manner.

Discovery of a novel mechanism of steroid receptor antagonism: WAY-255348 modulates progesterone receptor cellular localization and promoter interactions

WAY-255348 is a potent nonsteroidal progesterone receptor (PR) antagonist previously characterized in rodents and nonhuman primates. This report describes the novel mechanism by which WAY-255348 inhibits the activity of progesterone. Most PR antagonists bind to and block PR action by inducing a unique "antagonist" conformation of the PR. However, WAY-255348 lacks the bulky side chains or chemical groups that have been associated with the conformation changes of helix 12 that lead to functional antagonism. We show that WAY-255348 achieves antagonist activity by binding to and subsequently preventing progesterone-induced nuclear accumulation, phosphorylation and promoter interactions of the PR. This effect was concentration dependent, as high concentrations of WAY-255348 alone are able to induce nuclear translocation, phosphorylation and subsequent promoter interactions resulting in partial agonist activity at these concentrations. However, at lower concentrations where nuclear accumulation and phosphorylation are prevented, the progesterone-induced DNA binding is blocked along with PR-dependent gene expression. Analysis of the PR conformation induced by WAY-255348 using a limited protease digestion assay, suggested that the WAY-255348 bound PR conformation was similar to that of a progesterone agonist-bound PR and distinct from steroidal antagonist-bound PR conformations. Furthermore, the recruitment and binding of peptides derived from nuclear receptor co-activators is consistent with WAY-255348 inducing an agonist-like conformation. Taken together, these data suggest that WAY-255348 inhibits PR action through a novel molecular mechanism that is distinct from previously studied PR modulators and may be a useful tool to further understanding of PR signaling pathways. Development of therapeutic molecules with this 'passive' antagonism mechanism may provide distinct advantages for patients with reproductive disorders or PR positive breast cancers.

BRCA1 counteracts progesterone action by ubiquitination leading to progesterone receptor degradation and epigenetic silencing of target promoters

Germ-line mutations in the BRCA1 gene increase the risk of breast cancer in women, but the precise mechanistic basis for this connection remains uncertain. One popular hypothesis to explain breast tissue specificity postulates a link between BRCA1 and the action of the ovarian hormones estrogen and progesterone. Given the relevance of progesterone for normal mammary development and breast cancer formation, we searched for a functional relationship between BRCA1 and progesterone receptor (PR) in the PR-positive breast cancer cell line T47D. Here, we report that BRCA1 inhibits the transcriptional activity of PR by at least 2 mechanisms involving the E3 ubiquitin ligase activity of BRCA1. First, BRCA1 has a direct effect on the cellular level of PR and, hence, on the extent of PR recruitment to target promoters through the promotion of its ligand-independent and -dependent degradation. Through in vitro and in vivo assays, we found that BRCA1/BARD1 may be the main E3 ubiquitin ligase responsible for ubiquitination and degradation of PR in the absence of hormone. Second, after hormone treatment of cells, the BRCA1/BARD1 complex is recruited via interaction with PR to the hormone-responsive regions of PR target genes, affecting local levels of monoubiquitinated histone H2A and contributing to epigenetic silencing of these promoters. The connections between BRCA1/BARD1 and PR activity suggested by our findings may help explain why host mutations in BRCA1 exert a tissue specificity in preferentially elevating the risk of breast cancer.

p38 and p42/44 MAPKs differentially regulate progesterone receptor A and B isoform stabilization

Progesterone receptor (PR) isoforms (PRA and PRB) are implicated in the progression of breast cancers frequently associated with imbalanced PRA/PRB expression ratio. Antiprogestins represent potential antitumorigenic agents for such hormone-dependent cancers. To investigate the mechanism(s) controlling PR isoforms degradation/stability in the context of agonist and antagonist ligands, we used endometrial and mammary cancer cells stably expressing PRA and/or PRB. We found that the antiprogestin RU486 inhibited the agonist-induced turnover of PR isoforms through active mechanism(s) involving distinct MAPK-dependent phosphorylations. p42/44 MAPK activity inhibited proteasome-mediated degradation of RU486-bound PRB but not PRA in both cell lines. Ligand-induced PRB turnover required neosynthesis of a mandatory down-regulating partner whose interaction/function is negatively controlled by p42/44 MAPK. Such regulation strongly influenced expression of various endogenous PRB target genes in a selective manner, supporting functional relevance of the mechanism. Interestingly, in contrast to PRB, PRA stability was specifically increased by MAPK kinase kinase 1-induced p38 MAPK activation. Selective inhibition of p42/p44 or p38 activity resulted in opposite variations of the PRA/PRB expression ratio. Moreover, MAPK-dependent PR isoforms stability was independent of PR serine-294 phosphorylation previously proposed as a major sensor of PR down-regulation. In sum, we demonstrate that MAPK-mediated cell signaling differentially controls PRA/PRB expression ratio at posttranslational level through ligand-sensitive processes. Imbalance in PRA/PRB ratio frequently associated with carcinogenesis might be a direct consequence of disorders in MAPK signaling that might switch cellular responses to hormonal stimuli and contribute towards pathogenesis.

Progesterone receptor directly inhibits β-casein gene transcription in mammary epithelial cells through promoting promoter and enhancer repressive chromatin modifications

Differentiated HC-11 cells ectopically expressing progesterone receptor (PR) were used to explore the molecular mechanisms by which progesterone suppresses β-casein gene transcription induced by prolactin (PRL) and glucocorticoids in the mammary gland. As detected by chromatin immunoprecipitation assays, treatment of cells with the progestin agonist R5020 induced a rapid recruitment (5 min) of PR to the proximal promoter (-235 bp) and distal enhancer (-6 kb upstream of transcription start site) of β-casein. PR remained bound for 4 h and was dissociated by 24 h after treatment. Despite efficient binding, the hormone agonist-occupied PR did not stimulate transcription of the β-casein gene. Recruitment of signal transducer and activator of transcription 5a, glucocorticoid receptor, and the CCAAT enhancer binding protein β to the enhancer and proximal promoter of β-casein induced by PRL and glucocorticoids was blocked by progestin cotreatment, whereas PR binding was induced under these conditions. PRL/glucocorticoid-induced histone acetylation and the recruitment of the coactivator p300 and RNA polymerase II required for gene activation were also inhibited by progestin. In addition, progestin prevented dissociation of the corepressors Yin and Yang 1 and histone deacetylase 3 from the promoter, and demethylation of lysine 9 of histone 3 induced by PRL and glucocorticoids. These studies are consistent with the conclusion that progesterone interferes with PRL/glucocorticoid induction of β-casein transcription by a physical interaction of PR with the promoter and enhancer that blocks assembly of a transcriptional activation complex and dissociation of corepressors and promotes repressive chromatin modifications. These studies define a novel mechanism of steroid receptor-mediated transcriptional repression of a physiologically important gene in mammary gland development and differentiation.

ck2-dependent phosphorylation of progesterone receptors (PR) on Ser81 regulates PR-B isoform-specific target gene expression in breast cancer cells

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 progestin-regulated (BIRC3, HSD11β2, 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.

Interaction between FGFR-2, STAT5, and progesterone receptors in breast cancer

Fibroblast growth factor (FGF) receptor 2 (FGFR-2) polymorphisms have been associated with an increase in estrogen receptor and progesterone receptor (PR)-positive breast cancer risk; however, a clear mechanistic association between FGFR-2 and steroid hormone receptors remains elusive. In previous works, we have shown a cross talk between FGF2 and progestins in mouse mammary carcinomas. To investigate the mechanisms underlying these interactions and to validate our findings in a human setting, we have used T47D human breast cancer cells and human cancer tissue samples. We showed that medroxyprogesterone acetate (MPA) and FGF2 induced cell proliferation and activation of ERK, AKT, and STAT5 in T47D and in murine C4-HI cells. Nuclear interaction between PR, FGFR-2, and STAT5 after MPA and FGF2 treatment was also showed by confocal microscopy and immunoprecipitation. This effect was associated with increased transcription of PRE and/or GAS reporter genes, and of PR/STAT5-regulated genes and proteins. Two antiprogestins and the FGFR inhibitor PD173074, specifically blocked the effects induced by FGF2 or MPA respectively. The presence of PR/FGFR-2/STAT5 complexes bound to the PRE probe was corroborated by using NoShift transcription and chromatin immunoprecipitation of the MYC promoter. Additionally, we showed that T47D cells stably transfected with constitutively active FGFR-2 gave rise to invasive carcinomas when transplanted into NOD/SCID mice. Nuclear colocalization between PR and FGFR-2/STAT5 was also observed in human breast cancer tissues. This study represents the first demonstration of a nuclear interaction between FGFR-2 and STAT5, as PR coactivators at the DNA progesterone responsive elements, suggesting that FGFRs are valid therapeutic targets for human breast cancer treatment.

Affinity of estrogens for human progesterone receptor A and B monomers and risk of breast cancer: a comparative molecular modeling study

Background

The human progesterone receptor (hPR) belongs to the steroid receptor family. It may be found as monomers (A and B) and or as a dimer (AB). hPR is regarded as the prognostic biomarker for breast cancer. In a cellular dimer system, AB is the dominant species in most cases. However, when a cell coexpresses all three isoforms of hPR, the complexity of the action of this receptor increases. For example, hPR A suppresses the activity of hPR B, and the ratio of hPR A to hPR B may determine the physiology of a breast tumor. Also, persistent exposure of hPRs to nonendogenous ligands is a common risk factor for breast cancer. Hence we aimed to study progesterone and some nonendogenous ligand interactions with hPRs and their molecular docking.

Methods and results

A pool of steroid derivatives, namely, progesterone, cholesterol, testosterone, testolectone, estradiol, estrone, norethindrone, exemestane, and norgestrel, was used for this in silico study. Dockings were performed on AutoDock 4.2. We found that estrogens, including estradiol and estrone, had a higher affinity for hPR A and B monomers in comparison with the dimer, hPR AB, and that of the endogenous progesterone ligand. hPR A had a higher affinity to all the docked ligands than hPR B.

Conclusion

This study suggests that the exposure of estrogens to hPR A as well as hPR B, and more particularly to hPR A alone, is a risk factor for breast cancer.

Mechanisms of progesterone receptor inhibition of inflammatory responses in cellular models of breast cancer

Both pro- and antimitogenic activities have been ascribed to progesterone receptor (PR) agonists and antagonists in breast cancer cells; however, the transcriptional responses that underlie these paradoxical functions are not apparent. Using nontransformed, normal human mammary epithelial cells engineered to express PR and standard microarray technology, we defined 2370 genes that were significantly regulated by the PR agonist R5020. Gene ontology (GO) analysis revealed that GO terms involved in inflammation and nuclear factor-κB (NF-κB) signaling were among the most significantly regulated. Interestingly, on those NF-κB responsive genes that were inhibited by agonist-activated PR, antagonists either 1) mimicked the actions of agonists or 2) reversed the inhibitory actions of agonists. This difference in pharmacological response could be attributed to the fact that although agonist- and antagonist-activated PR is recruited to NF-κB-responsive promoters, the physical presence of PR tethered to the promoter of some genes is sufficient for transcriptional inhibition, whereas on others, an agonist-activated PR conformation is required for inhibition of NF-κB signaling. Importantly, the actions of PR on the latter class of genes were reversed by an activation function-2-inhibiting, LXXLL-containing peptide. Consideration of the relative activities of these distinct antiinflammatory pathways in breast cancer may be instructive with respect to the likely therapeutic activity of PR agonists or antagonists in the treatment of breast cancer.

Peroxisome proliferator-activated receptor gamma activation inhibits progesterone-stimulated human MUC1 expression

Mucin 1 (MUC1) is a type I transmembrane glycoprotein abundantly expressed on nearly all epithelial tissues and overexpressed by many cancer cells. Previous studies from our lab showed that progesterone receptor (PR)B is a strong stimulator of MUC1 gene expression. It is reported that liganded peroxisome proliferator-activated receptor gamma (PPARgamma) stimulates Muc1 expression in murine trophoblast. Here, we demonstrate that although the PPARgamma ligand, rosiglitazone, stimulates the murine Muc1 promoter in HEC1A, a human uterine epithelial cell line, rosiglitazone alone, has no significant effect on basal human MUC1 promoter activity. In fact, rosiglitazone treatment antagonizes progesterone-stimulated human MUC1 promoter activity and protein expression in two human uterine epithelial cell lines and T47D human breast cancer cells. This response is antagonized by the PPARgamma antagonist, GW9662, as well as a dominant-negative form of PPARgamma, demonstrating the response is mediated by PPARgamma. Additional studies indicate that PPARgamma activation does not change PR binding to the MUC1 promoter but generally antagonizes progesterone activity by stimulating PRB degradation and inhibiting progesterone-induced PRB phosphorylation. Collectively, these studies indicate that PPARgamma activation inhibits PRB activity through both acute (phosphorylation) and long-term (PRB degradation) pathways.

The emerging role of Krüppel-like factors in endocrine-responsive cancers of female reproductive tissues

Krüppel-like factors (KLFs), of which there are currently 17 known protein members, belong to the specificity protein (Sp) family of transcription factors and are characterized by the presence of Cys(2)/His(2) zinc finger motifs in their carboxy-terminal domains that confer preferential binding to GC/GT-rich sequences in gene promoter and enhancer regions. While previously regarded to simply function as silencers of Sp1 transactivity, many KLFs are now shown to be relevant to human cancers by their newly identified abilities to mediate crosstalk with signaling pathways involved in the control of cell proliferation, apoptosis, migration, and differentiation. Several KLFs act as tumor suppressors and/or oncogenes under distinct cellular contexts, underscoring their prognostic potential for cancer survival and outcome. Recent studies suggest that a number of KLFs can influence steroid hormone signaling through transcriptional networks involving steroid hormone receptors and members of the nuclear receptor family of transcription factors. Since inappropriate sensitivity or resistance to steroid hormone actions underlies endocrine-related malignancies, we consider the intriguing possibility that dysregulation of expression and/or activity of KLF members is linked to the pathogenesis of endometrial and breast cancers. In this review, we focus on recently described mechanisms of actions of several KLFs (KLF4, KLF5, KLF6, and KLF9) in cancers of the mammary gland and uterus. We suggest that understanding the mode of actions of KLFs and their functional networks may lead to the development of novel therapeutics to improve current prospects for cancer prevention and cure.

Partial agonist activity of the progesterone receptor antagonist RU486 mediated by an amino-terminal domain coactivator and phosphorylation of serine400

Jun dimerization protein-2 (JDP-2) is a progesterone receptor (PR) coregulatory protein that acts by inducing structure and transcriptional activity in the disordered amino-terminal domain (NTD) of PR. JDP-2 can also potentiate the partial agonist activity of the PR antagonist RU486 by mechanisms that have not been defined. Functional mutagenesis experiments revealed that a subregion of the NTD (amino acids 323-427) was required for the partial agonist activity of RU486 induced by PR interaction with JDP-2. However, this subregion was not required for JDP-2 enhancement of the activity of progestin agonists. Mutation of phosphorylation sites within this region of the NTD showed that phosphorylation of serine 400 was required for the partial agonist activity of RU486 stimulated by JDP-2, but was not required for activity of hormone agonist, either in the presence or absence of JDP-2. Cyclin-dependent kinase 2 (Cdk2)/cyclin A is a novel PR coregulator that binds the NTD and acts by phosphorylating steroid receptor coactivator-1 and modulating steroid receptor coactivator-1 interaction with PR. Cdk2/cyclin A also potentiated the partial agonist activity of RU486; however, phosphorylation of serine 400 was not required, indicating that JDP-2 and Cdk2/cyclin A act by distinct mechanisms. We conclude that PR bound to RU486 and associated with JDP-2 adopts an active conformation in a subregion of the NTD requiring phosphorylation of serine 400 that is distinct from that promoted by progestin agonists. These data underscore the structural flexibility of the NTD of PR, and the ability of steroid ligands together with interacting proteins to affect the conformation and activity of the NTD.

Steroid receptor phosphorylation: Assigning function to site-specific phosphorylation

Steroid receptors (SRs) are hormone-activated transcription factors important for a wide variety of cellular functions. Post-translational modifications of SRs, including phosphorylation, ubiquitination, acetylation, and sumoylation regulate their expression and function. The remarkable number of phosphorylation sites in these receptors and the wide variety of kinases shown to modulate phosphorylation influence the integration between cell-signaling pathways and SR action. These phosphorylation sites have been identified in all of the functional domains with the majority being located within the amino-terminal portions of the receptors. The regulation of function is receptor specific, site specific, and often dependent on the cellular context. Numerous roles for site-specific phosphorylation have been elucidated including sensitivity of hormone response, DNA binding, expression, stability, subcellular localization, dimerization, and protein-protein interactions that can determine the regulation of specific target genes. This review summarizes the current knowledge regarding receptor site-specific phosphorylation and regulation of function. As functional assays become more sophisticated, it is likely that additional roles for phosphorylation in receptor function will be identified.

Signaling inputs to progesterone receptor gene regulation and promoter selectivity

Progesterone receptors (PR) select and control genetic programs in the breast during normal mammary gland development, and progestin-driven processes contribute to the initiation and/or progression of breast cancer [Beral, V., 2003. Breast cancer and hormone-replacement therapy in the Million Women Study. Lancet 362, 419-427; Chlebowski, R.T., Hendrix, S.L., Langer, R.D., Stefanick, M.L., Gass, M., Lane, D., Rodabough, R.J., Gilligan, M.A., Cyr, M.G., Thomson, C.A., et al., 2003. Influence of estrogen plus progestin on breast cancer and mammography in healthy postmenopausal women: the Women's Health Initiative Randomized Trial. JAMA 289, 3243-3253]. Throughout the mammalian life span, progesterone exerts varying biological consequences on the mammary epithelial compartment, from brief proliferative spurts that occur with each luteal phase of the menstrual cycle to the massive expansion of the pregnant gland in preparation for lactation [Brisken, C., Park, S., Vass, T., Lydon, J.P., O'Malley, B.W., Weinberg, R.A., 1998. A paracrine role for the epithelial progesterone receptor in mammary gland development. Proc. Natl. Acad. Sci. U.S.A. 95, 5076-5081; Ismail, P.M., Amato, P., Soyal, S.M., DeMayo, F.J., Conneely, O.M., O'Malley, B.W., Lydon, J.P., 2003. Progesterone involvement in breast development and tumorigenesis-as revealed by progesterone receptor "knockout" and "knockin" mouse models. Steroids 68, 779-787]. These processes, while important developmentally, can become deregulated in breast cancer, thereby contributing to unchecked proliferation, increased survival, and invasive behaviors. Recently, our lab has focused on the molecular mechanisms, including phosphorylation events, by which PRs select specific target genes in response to progestins and other mitogenic hormonal signals (i.e. EGF, heregulin). Herein, we discuss the actions of cytoplasmic signaling molecules such as c-Src and mitogen-activated protein kinases as key mediators of PR promoter selectivity.

The cytoplasmic 60 kDa progesterone receptor isoform predominates in the human amniochorion and placenta at term

BACKGROUND

The mechanism that initiates human parturition has been proposed to be 'functional progesterone withdrawal' whereby the 116 kDa B-isoform of the progesterone receptor (PR-B) switches in favour of the 94 kDa A-isoform (PR-A) in reproductive tissues. Recently, other PR isoforms, PR-S, PR-C and PR-M generated from the same gene have been identified and partially characterised.

METHODS AND RESULTS

Using immunohistochemical, western blotting and RT-PCR techniques, evidence is provided that indicates the major PR isoform present in human term fetal membranes (amnion and chorion) and syncytiotrophoblast of the placenta is neither of the classical nuclear PR-B or PR-A isoforms but is the N-terminally truncated 60 kDa PR-C isoform. Evidence is also provided that this 60 kDa isoform resides in the cytoplasm of the expressing cell types. Data are also presented to show that PR-B, PR-A and PR-S isoforms are essentially absent from the amnion and chorion, whereas PR isoforms A, B, C and S are all present in the decidua, with PR-A being the major isoform. The syncytiotrophoblast of the placenta contains the cytoplasmic 60 kDa isoform, but not isoforms PR-A, PR-B or PR-S.

CONCLUSION

The major PR isoform in the amnion, chorion and placenta is a 60 kDa protein that could be PR-C, suggesting that the cytoplasmic isoform has a specific role in extra-embryonic tissues and may be involved in the regulation of human parturition.

Transcriptional response of the murine mammary gland to acute progesterone exposure

Our mechanistic understanding of progesterone's involvement in murine mammary morphogenesis and tumorigenesis is dependent on defining effector pathways responsible for transducing the progesterone signal into a morphogenetic response. Toward this goal, microarray methods were applied to the murine mammary gland to identify novel downstream gene targets of progesterone. Consistent with a tissue undergoing epithelial expansion, mining of the progesterone-responsive transcriptome revealed the up-regulation of functional gene classes involved in epithelial proliferation and survival. Reassuringly, signaling pathways previously reported to be responsive to progesterone were also identified. Mining this informational resource for rapidly induced genes, we identified "inhibitor of differentiation 4" (Id4) as a new molecular target acutely induced by progesterone exposure. Mammary Id4 is transiently induced during early pregnancy and colocalizes with progesterone receptor (PR) expression, suggesting that Id4 mediates the early events of PR-dependent mammary morphogenesis. Chromatin immunoprecipitation assay detecting direct recruitment of ligand occupied PR to the Id4 promoter supports this proposal. Given that Id4 is a member of the Id family of transcriptional regulators that have been linked to the maintenance of proliferative status and tumorigenesis, the establishment of a mechanistic link between PR signaling and Id4 promises to furnish a wider conceptual framework with which to advance our understanding of normal and abnormal mammary epithelial responses to progestins. In sum, the progesterone-responsive transcriptome described herein not only reinforces the importance of progesterone in mammary epithelial expansion but also represents an invaluable information resource with which to identify novel signaling paradigms for mammary PR action.

Carcinoma-associated fibroblasts activate progesterone receptors and induce hormone independent mammary tumor growth: A role for the FGF-2/FGFR-2 axis

The mechanisms by which mammary carcinomas acquire hormone independence are still unknown. To study the role of cancer-associated fibroblasts (CAF) in the acquisition of hormone-independence we used a hormone-dependent (HD) mouse mammary tumor and its hormone-independent (HI) variant, which grows in vivo without hormone supply. HI tumors express higher levels of FGFR-2 than HD tumors. In spite of their in vivo differences, both tumors have the same hormone requirement in primary cultures. We demonstrated that CAF from HI tumors (CAF-HI) growing in vitro, express higher levels of FGF-2 than HD counterparts (CAF-HD). FGF-2 activated the progesterone receptors (PR) in the tumor cells, thus increasing cell proliferation in both HI and HD tumors. CAF-HI induced a higher proliferative rate on the tumor cells and in PR activation than CAF-HD. The blockage of FGF-2 in the co-cultures or the genetic or pharmacological inhibition of FGFR-2 inhibited PR activation and tumor cell proliferation. Moreover, in vivo, the FGFR inhibitor decreased C4-HI tumor growth, whereas FGF-2 was able to stimulate C4-HD tumor growth as MPA. T47D human breast cancer cells were also stimulated by progestins, FGF-2 or CAF-HI, and this stimulation was abrogated by antiprogestins, suggesting that the murine C4-HI cells respond as the human T47D cells. In summary, this is the first study reporting differences between CAF from HD and HI tumors suggesting that CAF-HI actively participate in driving HI tumor growth.

Integration of progesterone receptor action with rapid signaling events in breast cancer models

Recent discoveries suggest that several protein kinases are rapidly activated in response to ligand binding to cytoplasmic steroid hormone receptors (SRs), including progesterone receptors (PRs). Thus, PRs act as ligand-activated transcription factor "sensors" for growth factor-initiated signaling pathways in hormonally regulated tissues, such as the breast. Induction of rapid signaling upon progestin binding to PR-B provides a means to ensure that receptors and co-regulators are appropriately phosphorylated as part of optimal transcription complexes. Alternatively, PR-B activated kinase cascades provide additional avenues for progestin-regulated gene expression independent of PR nuclear action. Herein, an overview of progesterone/PR and signaling cross-talk in breast cancer models is provided. Kinases are emerging as key mediators of PR action. Cross-talk between SR and membrane-initiated signaling events suggests a mechanism for coordinate regulation of gene subsets by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to contribute to cancer biology.

Progesterone receptor action: translating studies in breast cancer models to clinical insights

Progesterone receptors (PR) are useful prognostic indicators of breast cancers likely to respond to anti-estrogen receptor (ER) therapies. However, the role of progesterone, therapeutic progestins, or unliganded or liganded PRin breast cancer development or progression remains controversial. PR are ligand-activated transcription factors that act in concert with intracellular signaling pathways as "sensors" of multiple growth factor inputs to hormonally regulated tissues, such as the breast. The recently defined induction of rapid signaling events upon progestin-binding to PR-B provides a means to ensure that receptors and coregulators are appropriately phosphorylated as part of optimal transcription complexes. PR-activated kinase cascades may provide additional avenues for progestin-regulated gene expression independent of PR nuclear action. Herein, we present an overview ofprogesterone/PR and signaling cross-talk in breast cancer models and discuss the potential significance ofprogestin/PR action in breast cancer biology using examples from both in vitro and in vivo models, as well as limited clinical data. Kinases are emerging as key mediators of PR action. Cross-talk between PR and membrane-initiated signaling events suggests a mechanism for coordinated regulation ofgene subsets by mitogenic stimuli in hormonally responsive normal tissues. Dysregulation of this cross-talk mechanism may contribute to breast cancer biology; further studies are needed to address the potential for targeting PR in addition to ER and selected protein kinases as part of more effective breast cancer therapies.

Identification of a novel mitotic phosphorylation motif associated with protein localization to the mitotic apparatus

The chromosomal passenger complex (CPC) is a crucial regulator of chromosome, cytoskeleton and membrane dynamics during mitosis. Here, using liquid chromatography coupled to mass spectrometry (LC-MS), we identified phosphopeptides and phosphoprotein complexes recognized by a phosphorylation-specific antibody that labels the CPC. A mitotic phosphorylation motif {PX[G/T/S][L/M]S(P) P or WGLS(P) P} was identified by MS in 11 proteins, including FZR1 (Cdh1) and RIC8A-two proteins with potential links to the CPC. Phosphoprotein complexes contained the known CPC components INCENP, Aurora-B (Aurkb) and TD-60 (Rcc2, RCC1-like), as well as SMAD2, 14-3-3 proteins, PP2A and Cdk1 (Cdc2a), a probable kinase for this motif. Protein sequence analysis identified phosphorylation motifs in additional proteins, including SMAD2, PLK3 and INCENP. Mitotic SMAD2 and PLK3 phosphorylation was confirmed using phosphorylation-specific antibodies, and, in the case of Plk3, phosphorylation correlated with its localization to the mitotic apparatus and the midbody. A mutagenesis approach was used to show that INCENP phosphorylation is required for its localization to the midbody. These results provide evidence for a shared phosphorylation event that regulates localization of crucial proteins during mitosis.