Endothelial Bmp2 knockout exacerbates hemochromatosis in Hfe knockout mice but not Bmp6 knockout mice

: Bone morphogenetic proteins BMP2 and BMP6 play key roles in systemic iron homeostasis by regulating production of the iron hormone hepcidin. HFE also regulates hepcidin through a mechanism that intersects with the BMP-SMAD1/5/8 pathway. However, the relative roles of BMP2 compared with BMP6 and whether HFE regulates hepcidin through a BMP2-dependent mechanism remain uncertain. We therefore examined the iron phenotype of mice deficient for both Bmp2 and Bmp6 or both Bmp2 and Hfe compared with single knockout (KO) mice and littermate controls. Eight-week-old double endothelial Bmp6/Bmp2 KO mice exhibited a similar degree of hepcidin deficiency, serum iron overload, and tissue iron overload compared with single KO mice. Notably, dietary iron loading still induced liver SMAD5 phosphorylation and hepcidin in double Bmp6/endothelial Bmp2 KO mice, although no other BMP ligand mRNAs were increased in the livers of double KO mice, and only Bmp6 and Bmp2 mRNA were induced by dietary iron loading in wildtype mice. In contrast, double Hfe/endothelial Bmp2 KO mice exhibited reduced hepcidin and increased extrahepatic iron loading compared to single Hfe or endothelial Bmp2 KO mice. Liver phosphorylated SMAD5 and the SMAD1/5/8 target Id1 mRNA were also reduced in double Hfe/endothelial Bmp2 KO compared with single endothelial Bmp2 KO females. Finally, hepcidin and Id1 mRNA induction by homodimeric BMP2, homodimeric BMP6, and heterodimeric BMP2/6 were blunted in Hfe KO primary hepatocytes. CONCLUSION: These data suggest that BMP2 and BMP6 work collaboratively to regulate hepcidin expression, that BMP2- and BMP6-independent SMAD1/5/8 signaling contributes a non-redundant role to hepcidin regulation by iron, and that HFE regulates hepcidin at least in part through a BMP2-independent, but SMAD1/5/8-dependent, mechanism.