Fibroblast Growth Factor 23 Regulates Renal 1,25-Dihydroxy vitamin D and Phosphate Metabolism via the MAP Kinase Signaling Pathway in Hyp Mice

In X-linked hypophosphatemia (XLH) and in its murine homologue, the Hyp mouse, increased circulating concentrations of fibroblast growth factor 23 (FGF-23) are critical to the pathogenesis of disordered metabolism of phosphate (P sub(i)) and 1,25-dihydroxyvitamin D [1,25(OH) sub(2)D]. In this study, we hypothesized that in Hyp mice, FGF-23-mediated suppression of renal 1,25(OH) sub(2)D production and P sub(i) reabsorption depends on activation of mitogen-activated protein kinase (MAPK) signaling. Wild-type and Hyp mice were administered either vehicle or the MEK inhibitor PD0325901 (12.5 mg/kg) orally daily for 4 days. At baseline, the renal abundance of early growth response 1 (egrl) mRNA was approximately 2-fold greater in Hyp mice than in wild-type mice. Treatment with PD0325901 greatly suppressed egr1 mRNA abundance in both wild-type and Hyp mice. In Hyp mice, PD0325901 induced an 8-fold increase in renal 1 alpha -hydroxylase mRNA expression and a 4-fold increase in serum 1,25(OH) sub(2)D concentrations compared with vehicle-treated Hyp mice. Serum P sub(i) levels in Hyp mice increased significantly after treatment with PD0325901, and the increase was associated with increased renal Npt2a mRNA abundance and brush-border membrane Npt2a protein expression. These findings provide evidence that in Hyp mice, MAPK signaling is constitutively activated in the kidney and support the hypothesis that the FGF-23-mediated suppression of renal 1,25(OH) sub(2)D production and P sub(i), reabsorption depends on activation of MAPK signaling via MEK/ERK1/2. These findings demonstrate the physiologic importance of MAPK signaling in the actions of FGF-23 in regulating renal 1,25(OH) sub(2)D and P sub(i), metabolism.