Abstract P2-05-27: The ratio of progesterone receptor isoform A to B determines the effect of antiprogestins on preclinical models of breast cancer metastasis
Authors: Lanari C , Alvarez M , Giulianelli S , Marina R , Sequeira G , May M , Novaro V , Sahores A , Lamb C 
Published: May 2016

In previous studies using several experimental models expressing different progesterone receptor (PR) isoform ratios, we have shown that only those with high levels of isoform A (PRA) are inhibited by antiprogestins whereas those with high levels of isoform B (PRB) are resistant to antiprogestin therapy. Moreover, results obtained using tissue cultures of breast cancer patients confirmed the data observed in these experimental models (May and Rojas et al., ASCO meeting 2015).

The aim of this study was to evaluate the role of progestins and antiprogestins on the outgrowth of spontaneous metastatic foci of mammary carcinomas with different PR isoform ratios.

We used metastatic tumors from the murine medroxyprogesterone acetate (MPA)-induced breast cancer model: C7-2-HI (PRA&gt;PRB) and C7-HI (PRA&lt;PRB), and human MDA-MB-231 cells genetically modified to overexpress PRB. Tumors were orthotopically inoculated in the fourth mammary gland and Mifepristone (MFP; 6 mg), Telapristone (TLP; 6 mg), MPA (20 mg) or vehicle silastic pellets were sc implanted when the tumors reached a size of 25 mm2. Tumors were measured twice a week and animals were followed to detect the presence of axillary lymph node metastasis. Animals were euthanized before tumor size exceeded 2 cm at the largest diameter or showed a deterioration of the physical conditions.

Both antiprogestins had similar effects, MFP induced almost complete regression of the C7-2-HI tumor as already described, and TLP inhibited significantly its growth as compared with control tumors, while no changes in tumor size were observed in MPA-treated mice. Lung or lymph node studies confirmed the lack of metastatic foci in MFP-treated mice while a significant decrease (p&lt;0.05) and increase (p&lt;0.05) was observed in TLP- and MPA-treated animals respectively, as compared to control mice. In C7-HI tumors, with higher levels of PRB than PRA, MFP clearly stimulated the number of lung foci as compared with control animals (p&lt;0.05). The effects of TLP and MPA on tumor metastasis, although not inhibitory, need to be confirmed. Studies performed using MDA-MB-231-PRB xenografts growing in NSG mice showed that MFP stimulated the growth of lung foci, while MPA was inhibitory. No significant changes were observed in mice inoculated with empty vector transfected cells. To investigate the mechanisms underlying the metastatic process, we evaluated the expression of a metastatic suppressor gene (Nm23-H1) and a prometastatic enzyme (MMP-2). We found a lower expression by immunohistochemistry of the former and an increase in the latter by qPCR in MFP-treated MDA-MB-231-PRB xenografts compared with untreated tumors.

In conclusion, we have demonstrated that MFP impedes the metastatic process in tumors with higher levels of PRA than PRB while it might promote metastasis in those with the opposite ratio. We suggest that MFP, through PRB receptors, downregulate the expression of Nm23-H1 and increase the expression of MMP-2. The conclusive effects of TLP and MPA need further investigation. These data underscore the importance of isoform ratio determination before treatment of breast cancer patients with endocrine therapies that target PR.

Citation Format: Lanari C, Alvarez M, Giulianelli S, Marina R, Sequeira G, May M, Novaro V, Sahores A, Lamb C. The ratio of progesterone receptor isoform A to B determines the effect of antiprogestins on preclinical models of breast cancer metastasis. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-05-27.