When Is Progesterone Anti-Cancer? When Is It Pro-Cancer?

Majid Ali, M.D.

Natural Progesterone Is Anti-Cancer. Synthetic Progesterone Is Pro-Cancer 

Below is text from an important paper by Mohammed et al. published in journal Nature on July 16, 2015.

Progesterone receptor modulates ERα action in breast Cancer – Abstract

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.

There is compelling evidence that inclusion of a progestogen as part of hormone replacement therapy increases the risk of breast cancer, implying that PR signalling can contribute towards tumour formation1. However, the increased risk of breast cancer associated with progestogen-containing hormone replacement therapy is mainly attributed to specific synthetic progestins, in particular medroxyprogesterone acetate (MPA), which is known to also have androgenic properties2. The relative risk is not significant when native progesterone is used3. In ERα+ breast cancers, PR is often used as a positive prognostic marker of disease outcome4, but the functional role of PR signalling remains unclear. While activation of PR may promote breast cancer in some women and in some model systems, progesterone treatment has been shown to be antiproliferative in ERα+ PR+ breast cancer cell lines567, and progestogens have been shown to oppose oestrogen-stimulated growth of an ERα+ PR+ patient-derived xenograft8. In addition, exogenous expression of PR in ERα+ breast cancer cells blocks oestrogen-mediated proliferation and ERα transcriptional activity9. Furthermore, in ERα+ breast cancer patients, PR is an independent predictor of response to adjuvant tamoxifen10, high levels of PR correlate with decreased metastatic events in early stage disease11, and administration of a progesterone injection before surgery can provide improved clinical benefit12. These observations imply that PR activation in the context of oestrogen-driven, ERα+ breast cancer, can have an anti-tumorigenic effect. In support of this, PR agonists can exert clinical benefit in ERα+ breast cancer patients that have relapsed on ERα antagonists13.

Breast cancers are typically assessed for ERα, PR and HER2 expression to define histological subtype and guide treatment options. PR is an ERα-induced gene14 and ERα+ PR+ HER2tumours tend to have the best clinical outcome because PR positivity is thought to reflect a tumour that is driven by an active ERα complex and therefore likely to respond to endocrine agents such as tamoxifen or aromatase inhibitors1015. While ERα+ PR+ tumours have a better clinical outcome than ERα+ PR tumours4, clinical response to ERα antagonists can vary, even among tumours with similar ERα and PR status1516, and recent evidence suggests that PR may be prognostic, but not predictive17. Some ERα+ PR tumours that are resistant to one class of ERα antagonists gain clinical benefit from another class, suggesting that in a subset of ERα+ PR breast cancers, the lack of PR expression does not reflect a non-functional ERα complex. It has been proposed that the non-functional ERα complex theory cannot completely explain PR negativity18. An alternative hypothesis is that other factors contribute to the loss of PR expression, which consequently influences breast tumour responses to ERα target therapies.

PR is recruited to the ERα complex

Given the controversial and complex interplay between the ERα and PR pathways in breast cancer, we explored the possible functional crosstalk between these two transcription factors and the implications for clinical prognosis in ERα+ disease. Ligand-activated ERα and PR protein complexes were purified to ascertain interplay between these two transcription factors. Asynchronous ERα+PR+ MCF-7 and T-47D breast cancer cells were grown in stable isotope labelling by amino acids in cell culture (SILAC) growth media, which contains sufficient oestrogen to elicit maximal ERα binding to chromatin19. Oestrogen treatment is required to induce detectable levels of PR in MCF-7 cells, but not in T-47D cells20. The two cell lines were subsequently treated with vehicle or one of two progestogens: native progesterone or the synthetic progestin R5020. Cells were cross-linked following hormone treatment and endogenous PR was immunopurified followed by mass spectrometry, using a technique we recently developed called RIME (rapid immunoprecipitation mass spectrometry of endogenous proteins)21. Under oestrogenic conditions, progesterone treatment induced an interaction between PR and ERα in the MCF-7 and T-47D cell lines, in support of previous findings showing a physical interaction between these two nuclear receptors22. In addition to ERα, progesterone treatment induced interactions between PR and known ERα-associated cofactors, including NRIP1, GATA3 and TLE3 (ref. 21) in both cell lines (Fig. 1a). As expected, treatment with natural ligand decreased interaction between PR and chaperone/co-chaperone proteins such as HSP90 and FKBP4/FKBP5 (Fig. 1a). The same findings were observed when R5020 was used as a synthetic ligand (Extended Data Fig. 1). Interestingly, when ERα was purified under the same treatment conditions, PR was the only differentially recruited protein in both cell lines (Fig. 1b). Moreover, the interaction between ERα and known ERα-associated cofactors was not differentially affected by progesterone treatment. A list of all interacting proteins under all experimental conditions is provided in Supplementary Table 1. The progesterone-induced ERα–PR interaction was confirmed by standard co-immunoprecipitation experiments in both MCF-7 and T-47D cells (Fig. 1c). We conclude that activation of PR results in a robust association between PR and the ERα complex. However, it remains unclear what effect this may have on ERα/PR DNA binding or what the primary DNA tethering mechanism may be (Fig. 1d).

Nature 2015;523:313-321.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s