Relationship between GFR, intact PTH, oxidized PTH, non-oxidized PTH as well as FGF23 in patients with CKD

Fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) are regulators of renal phosphate excretion and vitamin D metabolism. In chronic kidney disease (CKD), circulating FGF23 and PTH concentrations progressively increase as renal function declines. Oxidation of PTH at two methionine residues (positions 8 and 18) causes a loss of function. The impact of n-oxPTH and oxPTH on FGF23 synthesis, however, and how n-oxPTH and oxPTH concentrations are affected by CKD, is yet unknown. The effects of oxidized and non-oxidized PTH 1-34 on Fgf23 gene expression were analyzed in UMR106 osteoblast-like cells. Furthermore, we investigated the relationship between n-oxPTH and oxPTH, respectively, with FGF23 in two independent patients' cohorts (620 children with CKD and 600 kidney transplant recipients). While n-oxPTH stimulated Fgf23 mRNA synthesis in vitro, oxidation of PTH in particular at Met8 led to a markedly weaker stimulation of Fgf23. The effect was even stronger when both Met8 and Met18 were oxidized. In both clinical cohorts, n-oxPTH-but not oxPTH-was significantly associated with FGF23 concentrations, independent of known confounding factors. Moreover, with progressive deterioration of kidney function, intact PTH (iPTH) and oxPTH increased substantially, whereas n-oxPTH increased only moderately. In conclusion, n-oxPTH, but not oxPTH, stimulates Fgf23 gene expression. The increase in PTH with decreasing GFR is mainly due to an increase in oxPTH in more advanced stages of CKD.