Progesterone receptor modulates ERα action in breast cancer.

Progesterone receptor (PR) expression is used as a biomarker of oestrogen receptor-α (ERα) function and breast cancer prognosis. Here we show that PR is not merely an ERα-induced gene target, but is also an ERα-associated protein that modulates its behaviour. In the presence of agonist ligands, PR associates with ERα to direct ERα chromatin binding events within breast cancer cells, resulting in a unique gene expression programme that is associated with good clinical outcome. Progesterone inhibited oestrogen-mediated growth of ERα(+) cell line xenografts and primary ERα(+) breast tumour explants, and had increased anti-proliferative effects when coupled with an ERα antagonist. Copy number loss of PGR, the gene coding for PR, is a common feature in ERα(+) breast cancers, explaining lower PR levels in a subset of cases. Our findings indicate that PR functions as a molecular rheostat to control ERα chromatin binding and transcriptional activity, which has important implications for prognosis and therapeutic interventions.

Androgen receptor driven transcription in molecular apocrine breast cancer is mediated by FoxA1.

Breast cancer is a heterogeneous disease and several distinct subtypes exist based on differential gene expression patterns. Molecular apocrine tumours were recently identified as an additional subgroup, characterised as oestrogen receptor negative and androgen receptor positive (ER- AR+), but with an expression profile resembling ER+ luminal breast cancer. One possible explanation for the apparent incongruity is that ER gene expression programmes could be recapitulated by AR. Using a cell line model of ER- AR+ molecular apocrine tumours (termed MDA-MB-453 cells), we map global AR binding events and find a binding profile that is similar to ER binding in breast cancer cells. We find that AR binding is a near-perfect subset of FoxA1 binding regions, a level of concordance never previously seen with a nuclear receptor. AR functionality is dependent on FoxA1, since silencing of FoxA1 inhibits AR binding, expression of the majority of the molecular apocrine gene signature and growth cell growth. These findings show that AR binds and regulates ER cis-regulatory elements in molecular apocrine tumours, resulting in a transcriptional programme reminiscent of ER-mediated transcription in luminal breast cancers.

Androgen and Estrogen Receptors in Breast Cancer Coregulate Human UDP-Glucuronosyltransferases 2B15 and 2B17.

Glucuronidation is an enzymatic process that terminally inactivates steroid hormones, including estrogens and androgens, thereby influencing carcinogenesis in hormone-dependent cancers. While estrogens drive breast carcinogenesis via the estrogen receptor alpha (ERα), androgens play a critical role as prohormones for estrogen biosynthesis and ligands for the androgen receptor (AR). In this study, the expression and regulation of two androgen-inactivating enzymes, the UDP-glucuronosyltransferases UGT2B15 and UGT2B17, was assessed in breast cancer. In large clinical cohorts, high UGT2B15 and UGT2B17 levels positively influenced disease-specific survival in distinct molecular subgroups. Expression of these genes was highest in cases positive for ERα. In cell line models, ERα, AR, and the transcription factor FOXA1 cooperated to increase transcription via tandem binding events at their proximal promoters. ERα activity was dependent on FOXA1, facilitated by AR activation, and potently stimulated by estradiol as well as estrogenic metabolites of 5α-dihydrotestosterone. AR activity was mediated via binding to an estrogen receptor half-site 3' to the FOXA1 and ERα-binding sites. Although AR and FOXA1 bound the UGT promoters in AR-positive/ERα-negative breast cancer cell lines, androgen treatment did not influence basal transcription levels. Ex vivo culture of human breast tissue and ERα+ tumors provided evidence for upregulation of UGT2B15 and UGT2B17 by estrogen or androgen treatment. ERα binding was evident at the promoters of these genes in a small cohort of primary tumors and distant metastases. Collectively, these data provide insight into sex steroid receptor-mediated regulation of androgen-inactivating enzymes in ERα+ breast cancer, which may have subtype-specific consequences for disease progression and outcomes. Cancer Res; 76(19); 5881-93. ©2016 AACR.

FOXA1 mutations in hormone-dependent cancers.

The forkhead protein, FOXA1, is a critical interacting partner of the nuclear hormone receptors, oestrogen receptor-α (ER) and androgen receptor (AR), which are major drivers of the two most common cancers, namely breast and prostate cancer. Over the past few years, progress has been made in our understanding of how FOXA1 influences nuclear receptor function, with both common and distinct roles in the regulation of ER or AR. Recently, another level of regulation has been described, with the discovery that FOXA1 is mutated in 1.8% of breast and 3-5% prostate cancers. In addition, a subset of both cancer types exhibit amplification of the genomic region encompassing the FOXA1 gene. Furthermore, there is evidence of somatic changes that influence the DNA sequence under FOXA1 binding regions, which may indirectly influence FOXA1-mediated regulation of ER and AR activity. These recent observations provide insight into the heterogeneity observed in ER and AR driven cancers.

Endogenous purification reveals GREB1 as a key estrogen receptor regulatory factor.

Estrogen receptor-α (ER) is the driving transcription factor in most breast cancers, and its associated proteins can influence drug response, but direct methods for identifying interacting proteins have been limited. We purified endogenous ER using an approach termed RIME (rapid immunoprecipitation mass spectrometry of endogenous proteins) and discovered the interactome under agonist- and antagonist-liganded conditions in breast cancer cells, revealing transcriptional networks in breast cancer. The most estrogen-enriched ER interactor is GREB1, a potential clinical biomarker with no known function. GREB1 is shown to be a chromatin-bound ER coactivator and is essential for ER-mediated transcription, because it stabilizes interactions between ER and additional cofactors. We show a GREB1-ER interaction in three xenograft tumors, and using a directed protein-protein approach, we find GREB1-ER interactions in half of ER(+) primary breast cancers. This finding is supported by histological expression of GREB1, which shows that GREB1 is expressed in half of ER(+) cancers, and predicts good clinical outcome. These findings reveal an unexpected role for GREB1 as an estrogen-specific ER cofactor that is expressed in drug-sensitive contexts.

FoxA1 is a key mediator of hormonal response in breast and prostate cancer.

Hormonally regulated breast and prostate cancers are the most common cause of cancer in females and males respectively. FoxA1 acts as a pioneer factor for both androgen receptor (AR) and estrogen receptor-α (ER), dictating the binding location, and therefore function of these transcription factors. It is an essential protein for the transcriptional activity of both ER and AR, yet it has distinct roles with the two different nuclear receptors. In both malignancies, FoxA1 plays a pivotal role from early stage cancer through to drug resistant and metastatic disease. Due to this key role in mediating ER and AR function, FoxA1 is not only an attractive therapeutic target but could potentially function as a novel biomarker.