Hypoxia, a common consequence of solid
tumor growth in
breast cancer or other
cancers, serves to propagate a cascade of molecular pathways which include angiogenesis, glycolysis, and various cell-cycle control
proteins. As we have shown previously,
hypoxia activates STAT5 (
signal transducer and activator of transcription 5) and increases its binding activity to the GAS
element in mammary epithelial cells. In this study we attempted to elucidate the mechanism by which
cyclin D1 is regulated by the
STAT5 protein under hypoxic conditions. Our data demonstrate that
hypoxia (2% O(2)) or
desferrioxamine (DFO) induces
tyrosine and
serine phosphorylation of STAT5 in human
breast cancer cells (MCF-7) and mammary epithelial cells (HC11). Imunoprecipitation and subsequent Western analysis showed that Jak2 leads to the
tyrosine phosphorylation and activation of STAT5a or STAT5b under hypoxic conditions. Using a transfected COS-7 cell model system, we demonstrate that the activity of a
cyclin D1 promoter-
luciferase construct increased under hypoxic conditions or DFO treatment. The activity of the STAT5b/
cyclin D1 promoter increased significantly by 12 h of
hypoxia, whereas the activity of the STAT5a/
cyclin D1 promoter was unaffected under hypoxic conditions. These increases in promoter activity are predominantly mediated by the Jak2/STAT5b signaling pathway. We have shown by EMSA that
hypoxia induces STAT5 to bind to the
cyclin D1 promoter (GAS-1) in MCF-7 and HC11 cells. These data suggest that STAT5b may mediate the transcriptional activation of
cyclin D1 after hypoxic stimulation.