Hemochromatosis Proteins Hemojuvelin and Homeostatic Iron Regulator in Bone Morphogenetic Protein‐Mediated Hepcidin Regulation and Iron Homeostasis

The bone morphogenetic protein (BMP)-SMAD signaling pathway is central to regulating hepcidin, the master regulator of systemic iron homeostasis. We have previously demonstrated that BMP6, BMP2, and, to a lesser extent, BMP5 are the major ligands contributing to hepcidin and iron homeostasis regulation in vivo. Hemojuvelin (HJV) and homeostatic iron regulator (HFE) are hepcidin modulators that are mutated in hereditary hemochromatosis. Although both HJV and HFE regulate hepcidin, at least partly, by functionally interacting with the BMP-SMAD pathway, the mechanisms are incompletely understood. Notably, both HJV and HFE can regulate hepcidin in a BMP6-independent manner. To understand whether HJV and HFE influence hepcidin regulation by BMP2 and/or BMP5, we investigated the iron phenotype of mice with combined mutations in endothelial Bmp2/Hjv and Bmp5/Hfe. We found that endothelial Bmp2/Hjv double knockout (KO) mice exhibit more severe hepcidin deficiency and iron overload than single endothelial Bmp2 or Hjv KO mice, similar to previous findings in mice with double endothelial Bmp2/Hfe KO and Bmp6/Hjv KO, or a functional loss of both Bmp6 and Hfe. Moreover, we found that iron completely fails to induce hepcidin in both endothelial Bmp2/Hjv and Bmp2/Hfe double KO mice. In contrast, a functional loss of BMP5 does not worsen hemochromatosis in Hfe KO mice. Together with other published data, these findings suggest a model whereby BMP2 and BMP6 can signal to hepcidin induction independently of HJV and HFE and vice versa. In contrast, BMP5, HJV, and HFE are all required for iron-mediated hepcidin regulation in the absence of BMP2 and BMP6.