The role of STAT, AP-1, E-box and TIEG motifs in the regulation of hepcidin by IL-6 and BMP-9: lessons from human HAMP and murine Hamp1 and Hamp2 gene promoters.

Hepcidin, the principal regulator of the iron metabolism, is up-regulated in response to inflammatory stimuli, bone morphogenic proteins (BMPs) and iron excess. There are two murine hepcidin genes: hepcidin-1 (Hamp1) and hepcidin-2 (Hamp2). Hamp1 gene responds to both IL-6 and BMPs while Hamp2 responds to neither. We replaced the putative functional regulatory motifs of the Hamp1 promoter with the corresponding putative "non-functional" Hamp2 motifs and vice versa in reporter constructs. Conversion of the Hamp1 STAT site into the Hamp2 site reduced the basal level of reporter expression but did not affect IL-6 and BMP responsiveness; replacing Hamp2 site with the Hamp1 site only resulted in partial responsiveness. These data are in contrast to the role of the STAT site in the human hepcidin promoter which is important in both basal level and IL-6 inducible promoter activity. The murine AP1, E-box and TIEG motifs were found to neither influence the basal level of expression of Hamp1 and HAMP promoters nor play a critical role in the IL-6 and BMP-9 induced response. Our data suggest that the STAT site (nt -148 to -130) is important for the regulation of basal level expression of Hamp1 but there are additional regions that are responsible for the IL-6 and BMP-9 responsiveness within the Hamp1 promoter.