Abstract
Abstract Background and Aims Vitamin D receptor (VDR) loss, slit diagram (SD) to tight junction (TJ) transition and impaired autophagic flux contribute to podocyte injury in diabetic nephrology. This study aims to examine the effect and mechanism of VDR on autophagic flux and SD-TJ transition in diabetic nephropathy. Method Renal biopsy tissues from DN patients at stage IIa, IIb, III, IV and patients with minimal lesions were used to evaluate the expression of VDR, autophagic flux and SD-TJ transition glomeruli. In vitro, cultured podocytes were treated with serum starvation (SS), autophagic inhibitors (3-methyladenine 3-MA or chloroquine CQ) to determine the degradation pathway of TJ marker ZO-1. Meanwhile, db/db mice and STZ-induced rats were used to explore the therapeutic effect and mechanism of VDR agonist in diabetic nephropathy. Results SD-TJ transition between foot processes could be observed under electron microscopy in DN patients at all stages, whereas foot processes were separated by the filtration slit and appeared to be single cross-strands in the normal glomeruli. There was a trend of increasing expression of autophagic marker p62 and ZO-1 and the expression of p62 is positively correlated with the changes of ZO-1 in the glomeruli of DN patients. In vitro, inhibiting autophagy with 3-MA and CQ resulted in the accumulation of ZO-1 in cultured podocytes. In addition, Co-IP experiments further convinced the interaction between p62 and ZO-1, which was enhanced by the activation of autophagy. Podocytes apoptosis and the activity of caspase 3 and caspase 8 were significantly increased in the presence of 3-MA or CQ, while these effects were rescued by silencing p62. According to VDR gene expression data in GEO database, VDR expression was decreased in diabetic nephropathy patients compared with normal people. Knocking down VDR lowered the expression of atg3 and leaded to the blockage of autophagy, which could be reversed by over-expressing Atg3. Podocytes treated with high glucose resulted in the decrease of VDR and Atg3, impaired autophagic flux and aggravated podocytes injury. However, VDR agonist treatment partially reversed all the changes. In vivo, db/db mice and STZ-induced rats (DN animal models) exhibited SD-TJ transition, massive proteinuria, decreased expression of VDR and podocin and the increased accumulation of p62 and ZO-1, all of which could be partially reversed by VDR agonist. Conclusion VDR loss contributed to the impairment of autophagic flux and SD-TJ transition via down-regulation Atg3 in diabetic nephropathy. Here, we identified a new mechanism and evidence for VDR agonist to treat diabetic nephropathy.
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