Heme Activates Macrophage Hepcidin Expression via Toll like Receptor 4 and Extracellular Signal-Regulated Kinases Signaling Pathway

Tight regulation of systemic and cellular iron levels is required for good health. This control is ensured by hepcidin, a small peptide hormone produced predominantly by the liver. Lack of hepcidin expression or mutations affecting regulators of hepcidin expression, cause common genetic iron disorders. Hepcidin is also expressed in myeloid cells and its expression is increased after infections and in response to lipopolysaccharide. Our study uncovers that macrophages rapidly increase hepcidin expression in response to excess of heme. Moreover, we demonstrate that the underlying mechanism by which heme triggers hepcidin activation in macrophages depends on the Toll Like Receptor (TLR)-4 and the contribution of Extracellular Signal-Regulated Kinases (ERK) pathway. Our data propose the contribution of hepcidin, locally produced by macrophages, to the pathology of disorders characterized by excess of free heme, such as certain bacterial infections and hemolytic disorders. Finally, using macrophages from Hfe-deficient mice, we demonstrate that the lack of Hfe is not critical for the hepcidin induction by heme but is required to maintain basal hepcidin expression in macrophages. The findings that the levels of hepcidin in macrophages are directly controlled by the actions of Hfe in these cells expand our view on Hfe beyond the liver and as mere regulator of iron levels.