No abstract

No abstract

Progressive elevations of fibroblastic growth factor 23 [FGF23] in chronic kidney disease may reduce serum 25-hydroxyvitamin D [25(OH)] and 1,25-dihydroxyvitamin D [1,25(OH)2D] levels, via stimulation of 24-hydroxylase (Cyp24A1) mediated catabolism of these vitamin D metabolites. To test this possibility, we measured serum concentrations of 24,25-dihydroxyvitamin D [24,25(OH)2D], a product of Cyp24A1 hydroxylation of 25(OH)D, in the Col4α3 knockout mouse, a model of Alport syndrome-derived chronic kidney disease, and in patients with chronic kidney disease of variable severity. There was an inverse correlation between serum FGF23 and both 25(OH)D and 1,25(OH)2D in the mouse model but no significant relationship was observed in the cross-sectional patient cohort. The FGF23-dependent increase in Cyp24a1 mRNA expression in the mouse kidneys was consistent with the possibility that FGF23 induces vitamin D catabolism. There was, however, a reduction in serum 24,25(OH)2D levels, rather than the expected elevation, in both the mice and patients with chronic kidney disease. Low 25(OH)D and elevated FGF23 and parathyroid hormone levels were correlated with the reduced serum 24,25(OH)2D concentrations of these patients. Thus, we failed to find support for FGF23-mediated catabolism of vitamin D metabolites in chronic kidney disease assessed by 24,25(OH)2D levels.

Modest elevations of serum 25(OH)D levels after cholecalciferol treatment are sufficient to induce compensatory degradative pathways in patients with sufficient renal reserves, suggesting that optimal circulating 25(OH)D levels are approximately 20 ηg/mL. In addition, catabolism of 25(OH)D may also contribute to the low circulating vitamin D levels in CKD, since elevations of FGF-23 in CKD are associated with increased 24,25(OH)2D after cholecalciferol administration.