Vitamin D and renal outcome: the fourth outcome of CKD-MBD? Oshima Award Address 2015.

Bone fracture, cardiovascular events, and mortality are three outcomes of chronic kidney disease-mineral and bone disorder (CKD-MBD), and the umbrella concept originally described for dialysis patients. The reported association of serum phosphorus or fibroblast growth factor 23 (FGF23) levels with renal outcome suggests that the fourth relevant outcome of CKD-MBD in predialysis patients is renal outcome. We found that proteinuria of 2+ or greater with a dipstick test was associated with low vitamin D status due to urinary loss of 25-hydroxyvitamin D (25D). Moreover, active vitamin D or its analogues decrease proteinuria. Given our finding that maxacalcitol does not repress renin, the reduction of proteinuria by this agent is likely due to direct upregulation of the nephrin and podocin in podocytes. Moreover, this agent downregulates the mesenchymal marker desmin in podocytes and blocks transforming growth factor-beta autoinduction, leading to attenuation of renal fibrosis in a unilateral ureteral obstructive (UUO) model. These facts are reminiscent of the suppression of epithelial-mesenchymal transition (EMT) by vitamin D. EMT blockage may explain our finding that vitamin D prescription in renal transplant recipients is associated with a lower incidence of cancer. We also reported that low vitamin D status and high FGF23 levels predict a worse renal outcome. However, administration of massive doses of 25D exacerbates renal fibrosis in UUO kidneys in 1alpha-hydroxylase knockout mice. Moreover, FGF23 inhibits 1alpha-hydroxylase in proximal tubules and monocytes. Taken together, local 1,25(OH)2 D in the kidney tissue but not 25D seems to protect the kidney.