In
breast cancer,
prolactin-induced activation of the
transcription factor STAT5a results from the phosphorylation of STAT5a
tyrosine residue 694. However, its role in mammary
oncogenesis remains an unsettled debate as STAT5a exhibits functional dichotomy with both pro-differentiative and pro-proliferative target genes. Phosphorylation of STAT5a
serine residues, S726 and S780, may regulate STAT5a in such a way to underlie this duality. Given hematopoiesis studies showing phospho-
serine STAT5a as necessary for transformation, we hypothesized that
serine phosphorylation regulates STAT5a activity to contribute to its role in mammary
oncogenesis, specifically in
luminal breast cancer. Here, phosphorylation of S726-, S780-, and Y694-STAT5a in response to
prolactin in MCF7
luminal breast cancer cells was investigated with STAT5a knockdown and rescue with Y694F-, S726A-, or S780A-STAT5a, where the phospho-sites were mutated.
RNA-sequencing and subsequent Ingenuity Pathway Analysis predicted that loss of each phospho-site differentially affected both
prolactin-induced gene expression as well as functional pathways of
breast cancer (e.g. cell survival, proliferation, and colony formation). In vitro studies of anchorage-independent growth and proliferation confirmed distinct phenotypes: whereas S780A-STAT5a decreased clonogenicity, S726A-STAT5a decreased proliferation in response to
prolactin compared to wild type STAT5a. Collectively, these studies provide novel insights into STAT5a activation in
breast cancer pathogenesis.