Identification of novel LRH-1 target genes in breast cancer cells

The orphan nuclear receptor liver receptor homolog-1 (LRH-1) plays important roles in embryonic development, lipid homeostasis and steroidogenesis, and has been implicated in driving several cancers. In breast cancer, LRH-1 is expressed in tumour epithelial cells of invasive ductal carcinomas. We hypothesized that LRH-1 regulates epithelial cell proliferation and invasiveness to drive breast tumour progression. The overall goal of this study was to identify molecular mechanisms regulated by LRH-1 related to cell proliferation and epithelial to mesenchymal transition (EMT). Previously, expression-profiling experiments identified the heterogeneous nuclear ribonucleoprotein (hnRNP) gene family as potential LRH-1 target genes. The specific aim of this project was to define the effects of LRH-l on hnRNPA1 and hnRNPA2 gene expression, cell proliferation and EMT using normal and transformed human mammary epithelial cell lines as models. The siRNA-mediated knockdown of LRH-1 significantly decreased expression of hnRNPs at the mRNA and protein levels. This correlated with reduced cell proliferation, an effect that was potentiated when hnRNPA1 were knocked down in parallel. Evidence was also obtained to suggest that alternative splicing of the hnRNPs-regulated enzyme pyruvate kinase resulted in a switch in energy metabolism (from aerobic glycolysis to oxidative phosphorylation) that contributes to increased cell proliferation. Using a well-established model of EMT (the hMLE epithelial cell line), we demonstrated that LRH-1 expression increases during TGFβ-induced EMT, and that knockdown of LRH-l partially reverses the EMT phenotype. hnRNPs expression decreased during EMT, however, and knockdown experiments did not support a role for LRH-1 induced hnRNPs expression in EMT. In conclusion, this study has identified hnRNPs family as LRH-1 target genes. The data support a role for LRH-1 in mediating breast cancer proliferation via multiple mechanisms, including metabolic regulation. We also demonstrate an important role for LRH-1 in EMT, although LRH-1-induced hnRNPs expression does not appear to be involved in this process.