P2-17-05: Antiprogestin Proellex Suppresses Proliferation of Aromatase Overexpressing and Letrozole Resistant T47D Breast Cancer Cells.
Authors: Gupta A , Mehta R , Alimirah F , Murillo G , Peng X , Wiehle R , Mehta R 
Published: June 2012

Background: Aromatase is a cytochrome P450 CYP19A1 enzyme responsible for the conversion of C19 androgens to C18 estrogen. Aromatase overexpression leads to increase in local estrogen concentration in post menopausal women diagnosed for breast cancer. Aromatase inhibitors (AI) are used as one of the first line therapies for the treatment of ER+PR+ breast cancers. However, many patients acquire resistance to AI treatment. Therefore alternative approaches are being sought for patients with AI resistance. Previous reports suggest that upregulation of progesterone receptor can lead to enhanced expression of EGFR/ERK/MAPK involved in acquiring resistance. In this study, we evaluated the effect of a selective progesterone receptor modulator, CDB4124 (Proellex) with low glucocorticoid activity on aromatase overexpressing and Letrozole resistant T47D cells.

Methods: Aromatase overexpressing T47D (T47Darom) and respective control (T47Dcon) breast cancer cell lines were generated by stable transfection with plasmid containing CYP19A1 coding region, or empty vector respectively. Letrozole resistant cell line (T47DaromLR) was generated by incubating T47Darom for 75 weeks in the presence of 10μM Letrozole. Cell proliferation was determined by MTT or Crystal violet assays. Gene expressions were quantitated by qRT-PCR whereas proteins were identified by western blot analyses, flow cytometry and immunofluorescence staining. Aromatase activity was determined by estradiol ELISA. The effects of Proellex on anchorage independent growth were measured by soft agar colony formation. Statistical differences between the various groups were determined by Student's ‘T’ test or ANOVA followed by Bonferroni's post hoc test.

Results: T47Darom and T47DaromLR cell lines had significantly higher aromatase expression (mRNA and protein) and as a result exhibited increased conversion of testosterone to estradiol as compared to T47Dcon. Both these cell lines showed enhanced growth in the presence of Testosterone. In T47Darom cells increased PR-B and EGFR expression as compared to T47Dcon cells was observed. Proellex, Letrozole and other known AI (Anastrozole, Exemestane) inhibited testosterone induced cell proliferation and anchorage independent growth of T47Darom cells. The inhibition of cell proliferation was significantly greater when cells were treated with Proellex in combination to other AIs. Proellex inhibited mRNA and protein levels of PR-B, reduced PRB/p300 complex formation in the nuclei and significantly reduced EGFR expression in T47Darom cells. Our results in the present study indicate that antiproliferative effect of Proellex may be due to PR-B/EGFR modulation in ER+PR+, aromatase overexpressing cells.

Conclusion: Overall these results suggest that antiprogestin, Proellex could be developed as a possible treatment strategy for aromatase overexpressing ER+/PR+ breast cancer patients as well as for AI resistant breast cancer patients.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-17-05.