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Inflammation leads to functional iron deficiency, by increasing the expression of the hepatic iron regulatory peptide, hepcidin. Inflammation also stimulates fibroblast growth factor 23 (FGF23) production by increasing both Fgf23 transcription and FGF23 cleavage, which paradoxically leads to excess in C-terminal FGF23 peptides (Cter-FGF23), rather than intact hormone (iFGF23). We determined that the major source of Cter-FGF23 are the osteocytes and investigated whether Cter-FGF23 peptides play a direct role in the regulation of hepcidin and iron metabolism in response to acute inflammation. Mice harboring an osteocyte-specific deletion of Fgf23 showed a ~90% reduction in Cter-FGF23 levels during acute inflammation. Reduction in Cter-FGF23 led to a further decrease in circulating iron in inflamed mice due to excessive hepcidin production. We observed similar results in mice showing impaired FGF23 cleavage due to osteocyte-specific deletion of Furin. We next showed that Cter-FGF23 peptides bind members of the bone morphogenic protein (BMP) family, BMP2 and BMP9, which are established inducers of hepcidin. Co-administration of Cter-FGF23 and BMP2 or BMP9 prevented the increase in Hamp mRNA and circulating hepcidin levels induced by BMP2/9, resulting in normal serum iron levels. Finally, injection of Cter-FGF23 in inflamed Fgf23KO mice and genetic overexpression of Cter-Fgf23 in WT mice also resulted in lower hepcidin and higher circulating iron levels. In conclusion, during inflammation, bone is the major source of Cter-FGF23 secretion and independently of iFGF23, Cter-FGF23 reduces BMP-induced hepcidin secretion in the liver.

Excess fibroblast growth factor (FGF) 23 signaling in patients with chronic kidney disease induces left ventricular hypertrophy. In this issue, Yanucil et al. investigated the interaction of soluble klotho and heparin with FGF23 and FGF receptor isoforms. They concluded that heparin promotes the FGF23-FGF receptor isoform 4 interaction and FGF23 pathogenic effects, supporting an important role of heparin in the pathogenesis of FGF23-mediated left ventricular hypertrophy in chronic kidney disease.