Differences in bone microarchitecture between genetic and secondary iron-overload mouse models suggest a role for hepcidin deficiency in iron-related osteoporosis

Iron overload is one of the secondary osteoporosis etiologies. Cellular and molecular mechanisms involved in iron-related osteoporosis are not fully understood.Aim: The aim of the study was to investigate the respective roles of iron excess and hepcidin, the systemic iron regulator, in the development of iron-related osteoporosis.Material and Methods: We used mice models with genetic iron overload (GIO) related to hepcidin deficiency (Hfe(-/-) and Bmp6(-/-)) and secondary iron overload (SIO) exhibiting a hepcidin increase secondary to iron excess. Iron concentration and transferrin saturation levels were evaluated in serum and hepatic, spleen, and bone iron concentrations were assessed by ICP-MS and Perl's staining. Gene expression was evaluated by quantitative RT-PCR. Bone micro-architecture was evaluated by micro-CT. The osteoblastic MC3T3 murine cells that are able to mineralize were exposed to iron and/or hepcidin.Results: Despite an increase of bone iron concentration in all overloaded mice models, bone volume/total volume (BV/TV) and trabecular thickness (Tb.Th) only decreased significantly in GIO, at 12 months for Hfe(-/-) and from 6 months for Bmp6(-/-). Alterations in bone microarchitecture in the Bmp6(-/-) model were positively correlated with hepcidin levels (BV/TV (rho = +.481, p < .05) and Tb.Th (rho = +.690, p < .05). Iron deposits were detected in the bone trabeculae of Hfe(-/-) and Bmp6(-/-) mice, while iron deposits were mainly visible in bone marrow macrophages in secondary iron overload. In cell cultures, ferric ammonium citrate exposure abolished the mineralization process for concentrations above 5 mu M, with a parallel decrease in osteocalcin, collagen 1, and alkaline phosphatase mRNA levels. Hepcidin supplementation of cells had a rescue effect on the collagen 1 and alkaline phosphatase expression level decrease.Conclusion: Together, these data suggest that iron in excess alone is not sufficient to induce osteoporosis and that low hepcidin levels also contribute to the development of osteoporosis.