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Even though mutations in the tumor suppressor, BRCA1, markedly increase the risk of breast and ovarian cancer, most breast and ovarian cancers express wild type BRCA1. An important question is therefore how the tumor-suppressive function of normal BRCA1 is overcome during development of most cancers. Because prolactin promotes these and other cancers, we investigated the hypothesis that prolactin interferes with the ability of BRCA1 to inhibit the cell cycle. Examining six different cancer cell lines with wild type BRCA1, and making use of both prolactin and the growth-inhibiting selective prolactin receptor modulator, S179D PRL, we demonstrate that prolactin activation of Stat5 results in the formation of a complex between phospho-Stat5 and BRCA1. Formation of this complex does not interfere with nuclear translocation or binding of BRCA1 to the p21 promoter, but does interfere with the ability of BRCA1 to transactivate the p21 promoter. Overexpression of a dominant-negative Stat5 in prolactin-stimulated cells resulted in increased p21 expression. We conclude that prolactin inhibits a major tumor-suppressive function of BRCA1 by interfering with BRCA1's upregulation of expression of the cell cycle inhibitor, p21.

Signal transducer and activator of transcription STAT5 and its upstream activating kinase JAK2 are essential mediators of cytokine signaling. Their activity is normally tightly regulated and transient. However, constitutive activation of STAT5 is found in numerous cancers and a driving force for malignant transformation. We describe here the identification of the synthetic chalcone alpha-Br-2',3,4,4'-tetramethoxychalcone (alpha-Br-TMC) as a novel JAK/STAT inhibitor. Using the non-transformed IL-3-dependent B cell line Ba/F3 and its oncogenic derivative Ba/F3-1*6 expressing constitutively activated STAT5, we show that alpha-Br-TMC targets the JAK/STAT pathway at multiple levels, inhibiting both JAK2 and STAT5 phosphorylation. Moreover, alpha-Br-TMC alters the mobility of STAT5A/B proteins in SDS-PAGE, indicating a change in their post-translational modification state. These alterations correlate with a decreased association of STAT5 and RNA polymerase II with STAT5 target genes in chromatin immunoprecipitation assays. Interestingly, expression of STAT5 target genes such as Cis and c-Myc was differentially regulated by alpha-Br-TMC in normal and cancer cells. While both genes were inhibited in IL-3-stimulated Ba/F3 cells, expression of the oncogene c-Myc was down-regulated and that of the tumor suppressor gene Cis was up-regulated in transformed Ba/F3-1* 6 cells. The synthetic chalcone alpha-Br-TMC might therefore represent a promising novel anticancer agent for therapeutic intervention in STAT5-associated malignancies.