Significance: Fibroblast growth factor 23 (FGF23) regulates phosphate and vitamin D metabolism and induces left heart hypertrophy. The importance of FGF23 is supported by the consequences of FGF23 deficiency: FGF23-deficient mice have a significantly reduced life span and recapitulate human age-associated diseases. Moreover, FGF23 has gained attention as a potential disease biomarker due to its positive correlation with disease activity, progression, and outcome in chronic kidney disease or cardiovascular disorders. It is, however, not entirely clear whether FGF23 merely indicates disease or actively contributes to disease progression. Therefore, it is important to explore the regulation of FGF23 production, which is similarly incompletely understood. Our paper identifies fundamental insulin/IGF1-dependent PI3K/Akt/FOXO1 signaling as a key suppressor of FGF23 formation.

Significance: The PI3 kinase-dependent inactivation of glycogen synthase kinase (GSK) 3 is an important aspect of normal insulin signaling. Surprisingly, transgenic mice expressing PI3 kinase-resistant GSK3 (gsk3^KI) have been found not to be insulin-resistant. We show that gsk3^KImice are even protected from the development of metabolic syndrome characterized by insulin resistance, obesity, and dyslipidemia, which all could readily be induced by a high-fat diet in gsk3^WTmice. The metabolic syndrome affects millions of patients and is associated with a high prevalence of cardiovascular disease and a significant reduction of life span. Exploring the underlying mechanism, we found that the production of the hormone adiponectin, which provides protection from metabolic syndrome, is controlled by a GSK3-dependent pathway.

Background/objectives: Bone-derived fibroblast growth factor 23 (FGF23) is a hormone that suppresses renal phosphate reabsorption and calcitriol (i.e., 1,25(OH)₂D₃) formation together with its co-receptor Klotho. FGF23- or Klotho-deficient mice suffer from rapid aging with multiple age-associated diseases, at least in part due to massive calcification. FGF23 is considered as a disease biomarker since elevated plasma levels are observed early in patients with acute and chronic disorders including renal, cardiovascular, inflammatory, and metabolic diseases. An energy-dense diet, which induces sequelae of the metabolic syndrome in humans and mice at least in part by enhancing pro-inflammatory TNFα formation, has recently been demonstrated to stimulate FGF23 production. Methods: We investigated the relevance of TNFα for high-fat diet (HFD)-induced FGF23 formation in wild-type (tnf^+/+) and TNFα-deficient (tnf^−/−) mice. Results: Within 3 weeks, HFD feeding resulted in a strong increase in the serum FGF23 level in tnf^+/+ mice. Moreover, it caused low-grade inflammation as evident from a surge in hepatic Tnfα transcript levels. TNFα stimulated Fgf23 transcription in UMR106 osteoblast-like cells. Serum FGF23 was significantly lower in tnf^−/− mice compared to tnf^+/+ mice following HFD. Serum phosphate and calcitriol were not significantly affected by genotype or diet. Conclusions: We show that HFD feeding is a powerful stimulator of murine FGF23 production through TNFα formation.

Transforming growth factor-β (TGF-β) is a cytokine produced by many cell types and implicated in cell growth, differentiation, apoptosis, and inflammation. It stimulates store-operated calcium entry (SOCE) through the calcium release-activated calcium (CRAC) channel Orai1/Stim1 in endometrial Ishikawa cells. Bone cells generate fibroblast growth factor (FGF) 23, which inhibits renal phosphate reabsorption and 1,25(OH)₂D₃ formation in concert with its co-receptor Klotho. Moreover, Klotho and FGF23 counteract aging and age-related clinical conditions. FGF23 production is dependent on Orai1-mediated SOCE and inflammation. Here, we explored a putative role of TGF-β2 in FGF23 synthesis. To this end, UMR106 osteoblast-like cells were cultured, Fgf23 transcript levels determined by qRT-PCR, FGF23 protein measured by ELISA, and SOCE analyzed by fluorescence optics. UMR106 cells expressed TGF-β receptors 1 and 2. TGF-β2 enhanced SOCE and potently stimulated the production of FGF23, an effect significantly attenuated by SB431542, an inhibitor of the transforming growth factor-β (TGF-β) type I receptor activin receptor-like kinases ALK5, ALK4, and ALK7. Furthermore, the TGF-β2 effect on FGF23 production was blunted by SOCE inhibitor 2-APB. We conclude that TGF-β2 induces FGF23 production, an effect involving up-regulation of SOCE.

Glycogen synthase kinase (GSK)‐3 is a ubiquitously expressed kinase inhibited by insulin‐dependent Akt/PKB/SGK. Mice expressing Akt/PKB/SGK‐resistant GSK3α/GSK3β ( gsk3 ^KI ) exhibit enhanced sympathetic nervous activity and phosphaturia with decreased bone density. Hormones participating in phosphate homeostasis include fibroblast growth factor (FGF)‐23, a bone‐derived hormone that inhibits 1,25‐dihydroxyvitamin D ₃ (1,25(OH) ₂ D ₃ ; calcitriol) formation and phosphate reabsorption in the kidney and counteracts vascular calcification and aging. FGF23 secretion is stimulated by the sympathetic nervous system. We studied the role of GSK3‐controlled sympathetic activity in FGF23 production and phosphate metabolism. Serum FGF23, 1,25(OH) ₂ D ₃ , and urinary vanillylmandelic acid (VMA) were measured by ELISA, and serum and urinary phosphate and calcium were measured by photometry in gsk3 ^KI and gsk3 ^WT mice, before and after 1 wk of oral treatment with the β‐blocker propranolol. Urinary VMA excretion, serum FGF23, and renal phosphate and calcium excretion were significantly higher, and serum 1,25(OH) ₂ D ₃ and phosphate concentrations were lower in gsk3 ^KI mice than in gsk3 ^WT mice. Propranolol treatment decreased serum FGF23 and loss of renal calcium and phosphate and increased serum phosphate concentration in gsk3 ^KI mice. We conclude that Akt/PKB/SGK‐sensitive GSK3 inhibition participates in the regulation of FGF23 release, 1,25(OH) ₂ D ₃ formation, and thus mineral metabolism, by controlling the activity of the sympathetic nervous system.—Fajol, A., Chen, H., Umbach, A. T., Quarles, L. D., Lang, F., Föller, M. Enhanced FGF23 production in mice expressing PI3K‐insensitive GSK3 is normalized by β‐blocker treatment. FASEB J. 30, 994–1001 (2016). www.fasebj.org