Prolactin Inhibits BCL6 Expression in Breast Cancer through a Stat5a-Dependent Mechanism

Abstract

BCL6 is a transcriptional repressor that recognizes DNA target sequences similar to those recognized by signal transducer and activator of transcriptions 5 (Stat5). BCL6 disrupts differentiation of breast epithelia, is downregulated during lactation, and is upregulated in poorly differentiated breast cancer. In contrast, Stat5a mediates prolactin-induced differentiation of mammary epithelia, and loss of Stat5 signaling in human breast cancer is associated with undifferentiated histology and poor prognosis. Here, we identify the mammary cell growth factor prolactin as a potent suppressor of BCL6 protein expression in human breast cancer through a mechanism that requires Stat5a, but not prolactin-activated Stat5b, MEK-ERK, or PI3K-AKT pathways. Prolactin rapidly suppressed BCL6 mRNA in T47D, MCF7, ZR75.1, and SKBr3 breast cancer cell lines, followed by prolonged reduction of BCL6 protein levels within 3 hours. Prolactin suppression of BCL6 was enhanced by overexpression of Stat5a but not Stat5b, was mimicked by constitutively active Stat5a, but did not require the transactivation domain of Stat5a. Stat5 chromatin immunoprecipitation demonstrated physical interaction with a BCL6 gene regulatory region, and BCL6 transcript repression required histone deacetylase activity based on sensitivity to trichostatin A. Functionally, BCL6 overexpression disrupted prolactin induction of Stat5 reporter genes. Prolactin suppression of BCL6 was extended to xenotransplant tumors in nude mice in vivo and to freshly isolated human breast cancer explants ex vivo. Quantitative immunohistochemistry revealed elevated BCL6 in high-grade and metastatic breast cancer compared with ductal carcinoma in situ and nonmalignant breast, and cellular BCL6 protein levels correlated negatively with nuclear Stat5a (r = −0.52; P < 0.001) but not with Stat5b. Loss of prolactin-Stat5a signaling and concomitant upregulation of BCL6 may represent a regulatory switch facilitating undifferentiated histology and poor prognosis of breast cancer.

STAT5 Outcompetes STAT3 To Regulate the Expression of the Oncogenic Transcriptional Modulator BCL6

“…It was recently demonstrated independently that STAT5 bound to region B in breast cancer cells (20), which is consistent with our data in both hematopoietic and breast cells. In addition, we have found that these STATs compete for binding to this site, and STAT5 can outcompete STAT3, thereby preventing BCL6 gene expression.…”

“…BCL6 and STATs also compete to regulate some of the same genes (20,21,26). In liver cells, there are sex-biased genes regulated by STAT5 that affect many cellular processes (22).…”

Determination of Molecular Markers for BRCA1 and BRCA2 Heterozygosity Using Gene Expression Profiling

“…For instance, basal activation of STAT5 in healthy human breast epithelia is frequently lost in invasive and metastatic human breast cancer. Indeed, loss of active STAT5 in breast cancer is correlated with poorly differentiated histology and poor prognosis (26). In addition, most human mammary tumors displayed decreased expression of the transcription-related gene ELF1, which was also found downregulated in our study (27).…”

STAT5 in Cancer and Immunity

“…Peck et al showed that a loss of pSTAT5 from the nucleus predicts for a poor clinical outcome along with the possibility of failure to respond to endocrine therapy in ER+ve breast cancer patients [151]. Interestingly, the expression of Bcl6 is inhibited by prolactin mediated activation of STAT5 in breast cancer [152]. Again, it has been shown that the STAT3 and STAT5 signaling pathway is integrally involved in endocrine resistance and more so in the growth factor-stimulated cases [153].…”