Abstract 700: Prolactin negates the ability of BRCA1 to increase p21 expression and the ability of already synthesized p21 to inhibit the cell cycle

Abstract

Abstract The literature supports a role for prolactin (PRL) in the development of some cancers through PRL's promotion of the cell cycle and/or inhibition of apoptosis. The selective PRL receptor modulator, S179D PRL, on the other hand, inhibits cancer cell proliferation and promotes differentiation and/or apoptosis. BRCA1 is an important tumor suppressor, shown to play a role in breast (B), ovarian (O), and prostate (P) cancer. The purpose of the current study was to determine the effect of the functionally distinct ligands for the PRL receptor on aspects of BRCA1 biology. Contrary to expectation, both PRL and S179D PRL (at 500 ng/ml) elevated levels of BRCA1 protein (∼3 fold in 24h and up to 10 fold in 48h) in all cancer types examined (B, O, &P). The promoter for the cell cycle inhibitory molecule, p21, has a BRCA1 binding site, but only S179D PRL increased levels of p21 protein as a result of BRCA1 elevation. Using deletion analysis of a p21 promoter-luciferase construct, we confirmed that the BRCA1 binding site (−84 to −129) was crucial to p21 expression in response to S179D PRL. An important question therefore was how elevated BRCA1 in response to each ligand had a different end result in terms of p21 expression. Examination of nuclear translocation of BRCA1 in response to cell stimulation with each ligand showed that both promoted movement into the nucleus. Moreover, both ligands similarly increased BRCA1 interaction with the important region (−84 to −129) of the endogenous promoter of the p21 gene (ChIP assay). A major difference in the signaling resulting from receptor engagement by PRL versus S179D PRL is that only PRL activates Stat5. With PRL, but not S179D PRL, BRCA1 formed a complex with phospho-Stat5 and this complex was present in the nucleus. Since both ligands increase BRCA1 interaction with the p21 promoter, but only PRL results in a BRCA1-phospho-Stat5 complex, it appears that binding of phospho-Stat5 to BRCA1, while not inhibiting its ability to bind to the promoter, negates the ability of BRCA1 to transactivate the p21 promoter. For already synthesized p21, additional studies demonstrated that stimulation of cells with PRL, but not S179D PRL, resulted in phosphorylation of p21, a posttranslational modification that inhibits nuclear translocation and therefore the ability of p21 to inhibit the cell cycle. This study has uncovered two ways in which p21 function is hampered by PRL and hence additional ways in which PRL promotes cell proliferation. At the same time, since S179D PRL inhibits Stat5 activation in response to PRL in addition to itself signaling to p21 elevation, this work also furthers our understanding of the anti-cancer activities of this selective PRL receptor modulator. Supported by California Breast Cancer Research program # 10PB-0127 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 700.