Differential Effects of Iron Overload and Iron Repletion on Bone and Mineral Metabolism in Growing Mice

Background: Altered iron status is associated with bone and mineral metabolism disorders, including bone loss, increased risk of fracture and altered production and cleavage of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23). In mice and humans, iron deficiency is associated with increased risk of osteoporosis and stimulates FGF23 production and cleavage, leading to excess circulating carboxy-terminal (Cter)-FGF23 fragments, high total FGF23 (cFGF23) but only slightly elevated intact (i) FGF23. However, the effects of iron repletion on bone and mineral metabolism remain to be determined. Methods: We fed 3-week-old male mice a control (Ctr), an iron deficient (ID) or a high iron diet (HI) for 3 weeks until 6 weeks of age. In addition, we fed 3 week-old male mice an ID diet for one week followed by HI for 2 weeks to study the effects of iron repletion (IR) on the skeleton. In all mice, we analyzed biochemical parameters of mineral metabolism, hematological parameters, and 3D bone microarchitecture. Results: Compared to Ctr mice, ID mice developed anemia, marked by decreased serum iron and hemoglobin (Hb) content, and showed elevated serum iFGF23 (1.6-fold) and cFGF23 (3-fold) leading to a 7-fold decrease in intact to total (i/c)FGF23 ratio (p<0.05). As expected, IR increased serum iron and IR mice were no longer anemic, as shown by corrected Hb levels. IR also decreased iFGF23 (30%) with no impact on cFGF23 compared to Ctr. Interestingly, iron overload also led to anemia in HI mice, despite higher serum iron (2.5-fold) and TSAT (2-fold) levels, and to a 2-fold increase in cFGF23, but reduced iFGF23 and i/cFGF23 by 30 and 70%, respectively (p<0.05). ID and IR showed a modest impact on the skeleton. In sharp contrast, and in absence of changes in PTH and phosphate levels, iron overload led to a severe bone loss with a dramatic reduction in BV/TV (-38%), trabecular number (-32%), cortical thickness (-31%) and higher cortical porosity (+36%). Conclusion: In aggregate, our findings show that dietary iron intake in iron depleted mice corrects anomalies associated with iron deficiency. However, excessive iron consumption leads to extensive skeletal alterations, demonstrating that dietary iron intake affects bone development and FGF23 regulation differently, depending on iron status. Funding: NIDDK Support