Reduced Renal α-Klotho Expression in CKD Patients and Its Effect on Renal Phosphate Handling and Vitamin D Metabolism

Renal alpha-Klotho (alpha-KL) plays a fundamental role as a co-receptor for fibroblast growth factor 23 (FGF23), a phosphaturic hormone and regulator of 1,25(OH)(2) vitamin D-3 (1,25VitD(3)). Disruption of FGF23-alpha-KL signaling is thought to be an early hallmark of chronic kidney disease (CKD) involving reduced renal alpha-KL expression and a reciprocal rise in serum FGF23. It remains unclear, however, whether the rise in FGF23 is related to the loss of renal alpha-KL. We evaluated alpha-KL expression in renal biopsy samples and measured levels of several parameters of mineral metabolism, as well as soluble alpha-KL (sKL), in serum and urinary samples from CKD patients (n = 236). We found that although renal alpha-KL levels were significantly reduced and serum FGF23 levels were significantly elevated in early and intermediate CKD, serum phosphate levels remained within the normal range. Multiple regression analysis showed that the increases in FGF23 were significantly associated with reduced renal function and elevated serum phosphate, but were not associated with loss of renal alpha-KL. Moreover, despite falling renal alpha-KL levels, the increase in FGF23 enhanced urinary fractional excretion of phosphate and reduced serum 1,25VitD(3) levels in early and intermediate CKD, though not in advanced CKD. Serum sKL levels also fell significantly over the course of CKD, and renal alpha-KL was a significant independent determinant of sKL. These results demonstrate that FGF23 levels rise to compensate for renal failure-related phosphate retention in early and intermediate CKD. This enables FGF23-alpha-KL signaling and a neutral phosphate balance to be maintained despite the reduction in alpha-KL. In advanced CKD, however, renal alpha-KL declines further. This disrupts FGF23 signaling, and serum phosphate levels significantly increase, stimulating greater FGF23 secretion. Our results also suggest the serum sKL concentration may be a useful marker of renal alpha-KL expression levels.