Objective:To investigate the expression level of vitamin D receptor (VDR) in podocytes of diabetic kidney disease (DKD) and its role in podocyte injury and proteinuria.Methods:(1) Sixty-five patients who had been diagnosed with type 2 diabetes mellitus (with or without albuminuria) were enrolled in this study and 25 age-and sex-matched healthy control subjects were enrolled. According to the ratio of urinary excretion of albumin/creatinine (ACR), the type 2 diabetes mellitus patients were classified into without proteinuria group (ACR<30 mg/g, n=24), microalbuminuria group (ACR 30-300 mg/g, n=18) and clinical proteinuria group (ACR>300 mg/g, n=23). Another 25 patients with DKD confirmed by renal biopsy were selected as the DKD group. Normal kidney tissue samples were taken from the same period of urinary surgical department for 10 cases of renal tumors in patients with renal resection. The test indicators in each group were compared. The VDR expression in blood, urine samples and kidney tissues of patients was detected by real-time quantitative PCR, ELISA and immunohistochemistry, and then were compared among different groups. The correlation between VDR and ACR was analyzed by Pearson correlation analysis. (2) Male db/db mice with genetic background of C57BLKs/J and db/m mice born in littermates were randomly divided into normal control group (group A), DKD control group (group B), DKD treated with dimethyl sulfoxide group (group C), DKD treated with paricalcitol (VDR agonist) group (group D). The C and D groups were treated by continuous intraperitoneal injection for 8 weeks, and the group A and B were not treated. The mice were started to intervene continuously for 8 weeks at the age of 10 weeks. At 22 weeks of age (12 weeks after starting intervention), the biochemical indexes of the mice's body weight, blood and urine were compared. The changes of β-catenin and VDR were detected by Western blotting. The expressions of podocyte marker protein podocin and podocyte injury protein α-SMA were observed by immunofluorescence. Results:(1) Compared with the normal healthy control group, the plasma levels of VDR mRNA and protein in diabetic patients without proteinuria, microalbuminuria and clinical proteinuria were lower (all P<0.05). Compared with the diabetic patients without proteinuria, the plasma levels of VDR mRNA and protein in diabetic patients with microalbuminuria and clinical proteinuria were lower (all P<0.05). (2) Compared with the normal healthy control group, the plasma levels of VDR mRNA and protein in diabetic patients without proteinuria and DKD patients were lower (all P<0.05). Compared with diabetic patients without proteinuria, the plasma levels of VDR mRNA and protein in the DKD group were also lower (both P<0.05). (3) Immunohistochemical results showed that the expression of VDR in kidney tissue of DKD group was significantly lower than that of normal control group. (4) The relative level of VDR mRNA in plasma of patients with DKD was negatively correlated with ACR ( r=-0.342, P<0.05). (5) The levels of VDR in urine supernatant of each group showed opposite trends with the plasma levels. (6) Western blotting results showed that the expression of β-catenin protein in groups B and C was higher than that in group D (both P<0.05), and the expression of VDR protein was lower than that in group D (both P<0.05). Immunofluorescence results showed that the expression of podocin in groups B and C was lower than that in group D (both P<0.05), and the expression of α-SMA was higher than that in group D (both P<0.05). Conclusion:VDR overexpression relieves podocyte injury and proteinuria in DKD.