E2f1 and RB are common mediators of the inhibitory effects prompted by the combination of Mifepristone and Palbociclib on breast cancer cells expressing progesterone receptor Isoform A

Abstract

Palbociclib (PALBO), a CDK 4/6 inhibitor, is currently used in combination with endocrine therapy targeting estrogen receptors to treat advanced luminal breast cancer. However, with time tumors become resistant to these treatments highlighting the need to develop other therapeutic strategies. Our laboratory focuses on the use of therapies targeting progesterone receptors (PR). We have previously shown that PALBO inhibits luminal breast cancer cell proliferation regardless of the prevailing PR isoform expressed and that mifepristone (MFP), an antiprogestin, potentiates this effect only in cells expressing PR isoform A (PRA). The aim of this study was to evaluate the role of two key cell cycle proteins, RB and E2F1 as mediators of this effect. T47D-YA or T47D-YB cells, expressing respectively PRA or PRB were treated with MFP, PALBO, or MFP+PALBO. The expression of E2F1 and pRB was evaluated by western blots. In agreement with data obtained in cell proliferation studies, a significant decrease of both protein levels (p<0.05) was observed only in T47D-YA cells treated with MFP+PALBO, whereas slight decreases were noted with single treatments. Contrarily, in T47D-YB cells, the effects of combined drugs were similar to those induced by PALBO. The in vivo growth of T47D cells expressing equimolar levels of PRA and PRB was also inhibited by the combined therapies and the strongest inhibition of pRB was registered by immunohistochemistry in tumors treated with both agents. Our results suggest that E2F1 and RB are key players mediating the inhibition of cell proliferation induced by PALBO and MFP combination exclusively in PRA-expressing cells. Mechanistic studies are underway to explore the direct involvement of PRA on the E2F1 promoter.