Progesterone receptor isoforms, agonists and antagonists differentially reprogram estrogen signaling

Abstract

Major roadblocks to developing effective progesterone receptor (PR)-targeted therapies in breast cancer include the lack of highly-specific PR modulators, a poor understanding of the pro- or anti-tumorigenic networks for PR isoforms and ligands, and an incomplete understanding of the cross talk between PR and estrogen receptor (ER) signaling. Through genomic analyses of xenografts treated with various clinically-relevant ER and PR-targeting drugs, we describe how the activation or inhibition of PR differentially reprograms estrogen signaling, resulting in the segregation of transcriptomes into separate PR agonist and antagonist-mediated groups. These findings address an ongoing controversy regarding the clinical utility of PR agonists and antagonists, alone or in combination with tamoxifen, for breast cancer management. Additionally, the two PR isoforms PRA and PRB, bind distinct but overlapping genomic sites and interact with different sets of co-regulators to differentially modulate estrogen signaling to be either pro- or anti-tumorigenic. Of the two isoforms, PRA inhibited gene expression and ER chromatin binding significantly more than PRB. Differential gene expression was observed in PRA and PRB-rich patient tumors and PRA-rich gene signatures had poorer survival outcomes. In support of antiprogestin responsiveness of PRA-rich tumors, gene signatures associated with PR antagonists, but not PR agonists, predicted better survival outcomes. The better patient survival associated with PR antagonists versus PR agonists treatments was further reflected in the higher in vivo anti-tumor activity of therapies that combine tamoxifen with PR antagonists and modulators. This study suggests that distinguishing common effects observed due to concomitant interaction of another receptor with its ligand (agonist or antagonist), from unique isoform and ligand-specific effects will inform the development of biomarkers for patient selection and translation of PR-targeted therapies to the clinic.

Progesterone and Breast Cancer: an NCI Workshop Report

“…PR isoforms have different binding sites in chromatin, and the coregulators associated with PR isoforms are different (1); more recent studies have expanded the repertoire of isoformspecific PR binding partners as measured by Rapid Immunoprecipitation Mass Spectrometry of Endogenous proteins (RIME) (2). Distinct but overlapping gene sets are regulated by two major PR isoforms which can form either homodimers or heterodimers at hormone-responsive elements in chromatin.…”

“…PR-B (but not PR-A) primarily cross-talks with ER at estrogen-regulated genes via direct association as measured by co-IP and ChIP assays performed in human breast cancer cell lines and breast tumors (3), as well as in intact cells by proximity ligand assays (TH Truong and CA Lange unpublished data). The ER/PR interaction with c-Src kinase and hormone-dependent activation of downstream MAPKs was first defined by seminal studies in 1998 (4) and a role for ER/PR transcriptional complexes in the regulation of global gene expression was later demonstrated by gene profiling and RIME studies performed in breast cancer models (2,3,5).…”

“…R5020 is a more stable PR ligand relative to P4 since P4 is actively metabolized within the cell and R5020 is not. Deep sequencing studies have established that the effect of P4 is identical to that of R5020 (2). For the most part, P4 and progestins have similar biological effects except for medroxyprogesterone 17-acetate (MPA) which can also activate either GR (26) or AR (27) in biochemical assays conducted in vitro (as do many synthetic antiprogestins).…”

Biological and clinical impact of imbalanced progesterone receptor isoform ratios in breast cancer

“…More recently, Singhal et al reported that in T47D xenografts, the antiprogestin telapristone (TLP) induced transient inhibition of tumor growth, while the combined administration together with tamoxifen induced an almost complete tumor regression (Singhal et al 2016). A similar effect was obtained with two other antiprogestins: CDB 4453 and EC313 (Singhal et al 2018). These results are in line with the inhibitory effect previously observed in T47D xenografts treated with MFP (Wargon et al 2015).…”

“…Tumors were (Knutson et al 2012) and with our preclinical studies suggesting that PRB-H tumors may be endocrineresistant tumors (Lanari et al 2012, Wargon et al 2015. Singhal et al (2018) using the raw RNA-seq data from our study (Rojas et al 2017) compared the list of differential genes expressed in T47D-YA and T47D-YB xenografts and those differentially regulated in PRA-H or PRB-H patients and concluded that a high PRA/PRB ratio might correlate with worse prognosis. Although the total number of samples included in both studies coincides, for the PAM50 analysis, we excluded the HER2+ cases, a lobular tumor and a metastatic tumor from a premenopausal patient, to compare a homogeneous cohort of patients.…”