Differential Regulation of Breast Cancer-Associated Genes by Progesterone Receptor Isoforms PRA and PRB in a New Bi-Inducible Breast Cancer Cell Line

Abstract

Progesterone receptor isoforms (PRA and PRB) are expressed at equal levels in normal mammary cells. However, alteration in PRA/PRB expression is often observed in aggressive breast cancer suggesting differential contribution of PR isoforms in carcinogenesis. The mechanisms underlying such processes remain to be established mainly due to paucity of appropriate cellular models. To investigate the role of PR isoforms and the impact of imbalanced PRA/PRB ratio in transcriptional regulation, we have generated an original human breast cancer cell line conditionally expressing PRA and/or PRB in dose-dependence of non-steroid inducers. We first focused on PR-dependent transcriptional regulation of the paracrine growth factor gene amphiregulin (AREG) playing important role in cancer. Interestingly, unliganded PRA increases AREG expression, independently of estrogen receptor, yet inhibitable by antiprogestins. We show that functional outcome of epidermal growth factor (EGF) on such regulation is highly dependent on PRA/PRB ratio. Using this valuable model, genome-wide transcriptomic studies allowed us to determine the differential effects of PRA and PRB as a function of hormonal status. We identified a large number of novel PR-regulated genes notably implicated in breast cancer and metastasis and demonstrated that imbalanced PRA/PRB ratio strongly impact their expression predicting poor outcome in breast cancer. In sum, our unique cell-based system strongly suggests that PRA/PRB ratio is a critical determinant of PR target gene selectivity and responses to hormonal/growth factor stimuli. These findings provide molecular support for the aggressive phenotype of breast cancers with impaired expression of PRA or PRB.

Marginal activity of progesterone receptor B (PR-B) in dogs but high incidence of mammary cancer

“…The progesterone-activated cPR-B stimulated the expression of FKBP5, SGK, and MT1 although to a much lower extent than hPR-B and hBUS did. With the exception of FKBP5 all other genes are regulated in a modified T47D human breast cancer cells also when only the PR-A isoform is expressed [49] in line with the idea that there may be some crosstalk of the overexpressed cPR-B with PR-A regulated gene expression.…”

New Insight into Progesterone-dependent Signalization

“…The nPRB/nPRA ratio determines the response of the tissue to P4 agonists or antagonists [22]. Disturbed isoform balance or overexpression of one of them could contribute to the development of carcinomas in mammary glands, ovary and endometrium, or correlate with tumor progression into more aggressive forms [24].…”

Progesterone Receptor Isoforms A and B in Pancreatic Neuroendocrine Tumor

“…PNET cases with markedly increased PR expression were reported to be significantly associated with better disease-free survival [9] , suggesting PR to be a potential prognostic marker in PNET. However, the status of PR isoform expression has not been investigated in PNET, and therefore it is not known which PR isoform plays the functional role of progesterone in NET mentioned above, since a higher ratio of PRA to PRB was reported to influence the clinical outcome in breast [12] and endometrial cancer patients [14] . The results of our in vitro experiment indicated that PRB was the only isoform expressed in the NET cell lines examined in this study.…”

“…In humans, PR exists in two isoforms, PRA and PRB [12,13] , but the exact difference between these two isoforms has not been fully clarified in NET. The results of the present study revealed that PRA was more frequent than PRB in PNETs and that PRA immunoreactivity plays an inhibitory role in cell proliferation.…”

“…Aside from the pancreas, NET has also been reported to express PR in the breast in a great majority of cases, and treatment with hormonal therapy of these NET patients was associated with a better overall survival as compared with that of untreated patients [11] . However, to the best of our knowledge, the significance of PR as a new therapeutic target has not been studied at all in PNET. PR has been demonstrated to exist in two isoforms, PRA and PRB, generated from a single gene by alternative transcriptional promoters [12,13] . Microarray analyses of human breast cancer cells demonstrated that despite the high sequence similarity between these two isoforms, PRA and PRB have a distinctive set of target genes [5] and display differential physiological responses to progesterone administration in target tissues through individual isoforms [12] which exhibit a distinctive response to progestin antagonists [13] .…”