Physiological Actions of Fibroblast Growth Factor-23

The roles of Orai and Stim in bone health and disease

Orai and Stim proteins are the mediators of calcium release-activated calcium signaling and are important in the regulation of bone homeostasis and disease. This includes separate regulatory systems controlling mesenchymal stem cell differentiation to form osteoblasts, which make bone, and differentiation and regulation of osteoclasts, which resorb bone. These systems will be described separately, and their integration and relation to other systems, including Orai and Stim in teeth, will be briefly discussed at the end of this review.

Renal ischemia-reperfusion injury impairs renal calcium, magnesium, and phosphate handling in mice

Fibroblast growth factor 23 (FGF23) levels are elevated in patients with acute kidney injury (AKI). The consequences on renal Ca²⁺, Mg²⁺, and P_i regulatory mechanisms are unknown. We hypothesized that renal ischemia-reperfusion (I/R) injury alters the expression of important renal Ca²⁺, Mg²⁺, and P_i transport proteins. I/R injury was induced in male C57BL/6 mice by clamping both renal arteries for 27 min. Mice were investigated 18 h later. The mRNA and protein levels of renal Ca²⁺, Mg²⁺, and P_i transport proteins were measured by RT-qPCR and western blot analysis. I/R injury-induced hyperphosphatemia and hypermagnesemia were paralleled by a decrease in glomerular filtration rate and an increase in the fractional excretion of Ca²⁺, Mg²⁺, and P_i. I/R injury affected the fibroblast growth factor 23 (FGF23)-klotho-vitamin D axis by increasing plasma levels of FGF23 and downregulation of renal klotho expression. Plasma levels of PTH and 1,25-dihydroxyvitamin D₃ were unchanged. Further, downregulation of key genes for paracellular reabsorption of Ca²⁺ and Mg²⁺ (claudin (Cldn)2, Cldn10b, Cldn16, Cldn19) and for active transcellular transport of Ca²⁺, Mg²⁺, and P_i (calbindin-D_28K, Ncx1, Pmca4, Cnnm2, Trpm7, NaP_i-2a, and NaP_i-2c) was observed. However, renal expression of Trpv5 and Trpv6 was increased. In vitro studies support a direct effect of proinflammatory cytokines on the mRNA expression of Cldn16, Cldn19, and Trpv6. Our findings indicate that renal I/R injury increases FGF23 blood levels independent of PTH and 1,25-dihydroxyvitamin D₃. This increase is associated with hypermagnesemia, hyperphosphatemia, and increased or decreased expression of specific renal Ca²⁺, Mg²⁺, and P_i transporters, respectively.

Role of PKB/SGK-dependent phosphorylation of GSK-3α/β in vascular calcification during cholecalciferol overload in mice

Medial vascular calcification is a highly regulated process involving osteo-/chondrogenic transdifferentiation of vascular smooth muscle cells. Both, protein kinase B (PKB) and serum- and glucocorticoid-inducible kinase 1 (SGK1) are involved in the intracellular signaling of vascular calcification and both phosphorylate and inactivate glycogen synthase kinase 3 (GSK-3). The present study explored whether PKB/SGK-dependent phosphorylation of GSK-3α/β is involved in vascular calcification. Experiments were performed in Gsk-3α/β double knockin mice lacking functional PKB/SGK phosphorylation sites (gsk-3^KI) and corresponding wild-type mice (gsk-3^WT) following high-dosed cholecalciferol treatment as well as ex vivo in aortic ring explants from gsk-3^KI and gsk-3^WT mice treated without and with phosphate. In gsk-3^WT mice, high-dosed cholecalciferol induced vascular calcification and aortic osteo-/chondrogenic signaling, shown by increased expression of osteogenic markers Msx2, Cbfa1 and tissue-nonspecific alkaline phosphatase (Alpl). All these effects were suppressed in aortic tissue from gsk-3^KI mice. Cholecalciferol decreased aortic Gsk-3α/β phosphorylation (Ser^21/9) in gsk-3^WT mice, while no phosphorylation was observed in gsk-3^KI mice. Moreover, the mRNA expression of type III sodium-dependent phosphate transporter (Pit1) and plasminogen activator inhibitor 1 (Pai1) was increased following cholecalciferol treatment in aortic tissue of gsk-3^WT mice, effects again blunted in gsk-3^KI mice. In addition, phosphate treatment induced mineral deposition and osteogenic markers expression in aortic ring explants from gsk-3^WT mice, effects reduced in aortic ring explants from gsk-3^KI mice. In conclusion, vascular PKB/SGK-dependent phosphorylation of GSK-3α/β contributes to the osteoinductive signaling leading to vascular calcification.