Phosphate Homeostasis, Inflammation and the Regulation of FGF-23.

Fibroblast growth factor 23 (FGF23) is released primarily from osteoblasts/osteocytes in bone. In cooperation with the transmembrane protein Klotho, FGF23 is a powerful inhibitor of 1 alpha 25OH Vitamin D Hydroxylase (Cyp27b1) and thus of the formation of 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3). As 1,25(OH)(2)D-3 up-regulates intestinal calcium and phosphate absorption, the downregulation of 1,25(OH)(2)D-3 synthesis counteracts phosphate excess and tissue calcification. FGF23 also directly inhibits renal phosphate reabsorption. Other actions of FGF23 include triggering of cardiac hypertrophy. FGF23 formation and/or release are stimulated by 1,25(OH)(2)D-3, phosphate excess, Ca2+, PTH, leptin, catecholamines, mineralocorticoids, volume depletion, lithium, high fat diet, iron deficiency, TNF alpha and TGF beta 2. The stimulating effect of 1,25(OH)(2)D-3 on FGF23 expression is dependent on RAC1/PAK1 induced actin-polymerisation. Intracellular signaling involved in the stimulation of FGF23 release also includes increases in the cytosolic Ca2+ concentration ([Ca2+]i) following intracellular Ca2+ release and store-operated Ca2+ entry (SOCE). SOCE is accomplished by the Ca2+ release-activated calcium channel protein 1 (Orai1) and its stimulator stromal interaction molecule 1 (STIM1). Expression of Orai1, SOCE and FGF23-formation are up-regulated by the proinflammatory transcription factor NF kappa B. The present brief review describes the cellular mechanisms involved in FGF23 regulation and its sensitivity to both phosphate metabolism and inflammation. The case is made that up-regulation of FGF23 by inflammatory mediators and signaling may amplify inflammation by inhibiting formation of the anti-inflammatory 1,25(OH)(2)D-3. (C) 2018 The Author(s) Published by S. Karger AG, Basel