Fibroblast growth factor 23 (FGF23) is produced by bone cells and regulates renal phosphate and vitamin D metabolism, as well as causing left ventricular hypertrophy. FGF23 deficiency results in rapid aging, whereas high plasma FGF23 levels are found in several disorders, including kidney or cardiovascular diseases. Regulators of FGF23 production include parathyroid hormone (PTH), calcitriol, dietary phosphate, and inflammation. We report that insulin and insulin-like growth factor 1 (IGF1) are negative regulators of FGF23 production. In UMR106 osteoblast-like cells, insulin and IGF1 down-regulated FGF23 production by inhibiting the transcription factor forkhead box protein O1 (FOXO1) through phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB)/Akt signaling. Insulin deficiency caused a surge in the serum FGF23 concentration in mice, which was reversed by administration of insulin. In women, a highly significant negative correlation between FGF23 plasma concentration and increase in plasma insulin level following an oral glucose load was found. Our results provide strong evidence that insulin/IGF1-dependent PI3K/PKB/Akt/FOXO1 signaling is a powerful suppressor of FGF23 production in vitro as well as in mice and in humans.

Fibroblast growth factor 23 (FGF23) is mainly produced in the bone and, upon secretion, forms a complex with a FGF receptor and coreceptor αKlotho. FGF23 can exert several endocrine functions, such as inhibiting renal phosphate reabsorption and 1,25‐dihydroxyvitamin D3 production. Moreover, it has paracrine activities on several cell types, including neutrophils and hepatocytes. Klotho and Fgf23 deficiencies result in pathologies otherwise encountered in age‐associated diseases, mainly as a result of hyperphosphataemia‐dependent calcification. FGF23 levels are also perturbed in the plasma of patients with several disorders, including kidney or cardiovascular diseases. Here, we review mechanisms controlling FGF23 production and discuss how FGF23 regulation is perturbed in disease.

Sources of AGEs induce the transcription of Fgf23 in UMR cells. At least in part, the effect is mediated through up-regulation of NFκB and subsequent SOCE. AGE-induced FGF23 production may contribute to increased FGF23 serum levels observed in chronic disease.

Serine/threonine protein kinase C (PKC) is activated by diacylglycerol that is released from membrane lipids by phospholipase C in response to activation of G protein-coupled receptors or receptor tyrosine kinases. PKC isoforms are particularly relevant for proliferation and differentiation of cells including osteoblasts. Osteoblasts/osteocytes produce fibroblast growth factor 23 (FGF23), a hormone regulating renal phosphate and vitamin D handling. PKC activates NFκB, a transcription factor complex controlling FGF23 expression. Here, we analyzed the impact of PKC on FGF23 synthesis. Fgf23 expression was analyzed by qRT-PCR in UMR106 osteoblast-like cells and in IDG-SW3 osteocytes, and FGF23 protein was measured by ELISA. Phorbol ester 12-O-tetradecanoylphorbol-13-acetate (PMA), a PKC activator, up-regulated FGF23 production. In contrast, PKC inhibitors calphostin C, Gö6976, sotrastaurin and ruboxistaurin suppressed FGF23 formation. NFκB inhibitor withaferin A abolished the stimulatory effect of PMA on Fgf23 . PKC is a powerful regulator of FGF23 synthesis, an effect which is at least partly mediated by NFκB.

Fibroblast growth factor 23 (FGF23) inhibits renal phosphate reabsorption and calcitriol formation, effects depending on Klotho as a co-receptor for FGF23. In addition, FGF23/Klotho strongly influences aging and the onset of age-associated diseases. The synthesis of FGF23 by bone cells is induced by store-operated Ca entry (SOCE) through Orai1 in UMR106 osteoblast-like cells. Ca entry activates the phosphatase calcineurin in many cell types which dephosphorylates nuclear factor of activated T cells (NFAT) thereby stimulating its transcriptional activity. Here, we explored whether calcineurin-NFAT signaling impacts on FGF23 production. Fgf23 transcripts were determined by qRT-PCR and FGF23 protein by ELISA. Calcineurin as well as NFAT expression were quantified by RT-PCR in UMR106 cells. UMR106 cells expressed calcineurin subunits Ppp3r1, Ppp3ca, Ppp3cb, and Ppp3cc as well as NFATc1, NFATc3, and NFATc4. Calcineurin inhibitors ciclosporin A (CsA) and tacrolimus (FK-506) decreased Fgf23 gene expression and FGF23 protein production. Moreover, calcineurin-NFAT interaction inhibitor INCA-6 reduced the abundance of Fgf23 transcripts as well as FGF23 protein. Calcineurin-NFAT signaling is a potent regulator of FGF23 formation.

Serine/threonine protein kinase C (PKC) is activated by diacylglycerol that is released from membrane lipids by phospholipase C in response to activation of G protein-coupled receptors or receptor tyrosine kinases. PKC isoforms are particularly relevant for proliferation and differentiation of cells including osteoblasts. Osteoblasts/osteocytes produce fibroblast growth factor 23 (FGF23), a hormone regulating renal phosphate and vitamin D handling. PKC activates NFκB, a transcription factor complex controlling FGF23 expression. Here, we analyzed the impact of PKC on FGF23 synthesis. Fgf23 expression was analyzed by qRT-PCR in UMR106 osteoblastlike cells and in IDG-SW3 osteocytes. Phorbol ester 12-O-tetradecanoylphorbol-13-acetate (PMA), a PKC activator, up-regulated Fgf23 expression. In contrast, PKC inhibitors calphostin C, Gö6976, sotrastaurin and ruboxistaurin supressed Fgf23 gene expression. NFκB inhibitor withaferin A abolished the stimulatory effect of PMA on Fgf23. PKC is a powerful regulator of FGF23 synthesis, an effect which is at least partly mediated by NFκB.

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