AP-1 transcription factor is involved in breast cancer cell proliferation mediated by progestins

MARÍA C. DÍAZ FLAQUÉ, WENDY BEGUELIN, CINTHIA ROSEMBLIT, CECILIA PROIETTI, MARTIN A. RIVAS, MERCEDES TKACH, EDUARDO H. CHARREAU, ROXANA SCHILLACI, PATRICIA ELIZALDE

Washington DC, USA

Congreso; 101st Annual Meeting American Association for Cancer Research; 2010

American Association for Cancer Research

Accumulating evidence has shown the involvement of the progesterone receptor (PR) in breast cancer development. We and others have also shown that progestin are able to induce cyclin D1 expression, a key regulatory molecule that does not contain a progesterone response element (PRE) in the promoter region. In the present study, we propose a novel mechanism in which progestin controls breast cancer growth through the integration of rapid PR signaling and a transcriptional mechanism (tethering), that involves progesterone-bound PR interaction with AP-1 transcription factor (composed of Jun and Fos family members), at specific AP-1 binding sites (TRE) in the cyclin D1 promoter. MPA treatment of breast cancer cells induced an increase in the levels of cyclin D1 protein. We have also shown that MPA treatment of breast cancer cells induced an increase in the levels of c-jun and c-fos phosphorylation. To study the effect of MPA on AP-1-mediated transcriptional activity, cells were transiently transfected with a luciferase reporter plasmid containing three copies of TRE. We found that MPA enhanced AP-1 transcriptional activity and this effect was abolished by the antiprogestin RU486. We assessed the specific association of AP-1 and PR to the TRE region of the cyclin D1 gene in the context of living cells, by performing Chromatin Immunoprecipitation Assays. We found that MPA treatment induced c-jun, c-fos and PR recruitment to the cyclin D1 promoter. These data identify, for the first time, the interaction between AP-1 and PR regulating cyclin D1 transcription by tethering to DNA-bound at TRE site. Furthermore, we found that the inhibition of c-fos and c-jun activation by the use of dominant negative forms of these proteins (A-Fos and TAM-67 respectively) completely blocked progestin-induced cyclin D1 expression and in vitro breast cancer cell growth. Finally, we addressed the effect of targeting AP-1 in in vivo MPA-dependent growth of C4HD progestin-dependent murine mammary tumor. Transfection of C4HD cells with TAM-67 or A-Fos DN expression vectors significantly inhibited these cells’ ability to form tumors in syngeneic mice. Histological examination determined a striking decrease in histological grade in both groups in comparison to control group.