Nephrin plays a key role in maintaining the structure of the slit diaphragm in the glomerular filtration barrier. Our previous studies have demonstrated potent renoprotective activity for 1,25-dihydroxyvitamin D (1,25(OH)2D3). Here we showed that in podocytes 1,25(OH)2D3 markedly stimulated nephrin mRNA and protein expression. ChIP scan of the 6-kb 5′ upstream region of the mouse nephrin gene identified several putative vitamin D response elements (VDREs), and EMSA confirmed that the VDRE at −312 (a DR4-type VDRE) could be bound by vitamin D receptor (VDR)/retinoid X receptor. Luciferase reporter assays of the proximal nephrin promoter fragment (−427 to +173) showed strong induction of luciferase activity upon 1,25(OH)2D3 treatment, and the induction was abolished by mutations within −312VDRE. ChIP assays showed that, upon 1,25(OH)2D3 activation, VDR bound to this VDRE leading to recruitment of DRIP205 and RNA polymerase II and histone 4 acetylation. Treatment of mice with a vitamin D analog induced nephrin mRNA and protein in the kidney, accompanied by increased VDR binding to the −312VDRE and histone 4 acetylation. 1,25(OH)2D3 reversed high glucose-induced nephrin reduction in podocytes, and vitamin D analogs prevented nephrin decline in both type 1 and 2 diabetic mice. Together these data demonstrate that 1,25(OH)2D3 stimulates nephrin expression in podocytes by acting on a VDRE in the proximal nephrin promoter. Nephrin up-regulation likely accounts for part of the renoprotective activity of vitamin D.
This work was supported, in whole or in part, by National Institutes of Health Grant R01HL085793. This work was also supported by a Genzyme renal research fellowship.
