Cyclin D1 and
insulin-like growth factor 1 receptor (IGF-1R) are key regulators of cell proliferation that are overexpressed in most breast
cancers. The purpose of the present study was to investigate the molecular mechanism by which
hemin exerts its inhibitory effects on aggressive
breast cancer cells. We found that
hemin regulates
cyclin D1 and IGF-1R
proteins and
insulin-like growth factor-1 gene expression through STAT5b in
breast cancer cells. We confirmed that STAT5b,
cyclin D1, and IGF-1R is up-regulated by
hypoxia, and the increased STAT5b binds strongly to the STAT5-binding sites contained within the distal 5'-flanking region of
IGF-1 gene in
breast cancer cells. EMSA studies showed that STAT5 binding activity to the
IGF-1 and
cyclin D1 promoter was distinctly decreased by
hemin in STAT5b-transfected COS-7 or MDA-MB 231 cells.
IGF-1 gene expression was also decreased by
hemin in mammary epithelial cells. STAT5b expression was inhibited in
siRNA experiments and by
hemin, leading to decreased levels of
IGF-1. These results provide a basis for molecular targets in
cancer treatment via the STAT5b/IGF-1 or /
cyclin D1 pathway in solid
tumor cells. These data indicate that
hemin inhibits the
cyclin D1 and
IGF-1 expression via STAT5b under
hypoxia in
ERalpha-negative
breast cancer cells. These findings are valuable toward understanding the role of
hemin-induced inhibition of
cyclin D1 and
IGF-1 expression under
hypoxia in invasive and metastatic
breast cancer.