High glucose impairs insulin signaling in the glomerulus: An in vitro and ex vivo approach

Objective: Chronic hyperglycaemia, as seen in type II diabetes, results in both morphological and functional impairments of podocytes in the kidney. We investigated the effects of high glucose (HG) on the insulin signaling pathway, focusing on cell survival and apoptotic markers, in immortalized human glomerular cells (HGEC; podocytes) and isolated glomeruli from healthy rats. Methods and Findings: HGEC and isolated glomeruli were cultured for various time intervals under HG concentrations in the presence or absence of insulin. Our findings indicated that exposure of HGEC to HG led to downregulation of all insulin signaling markers tested (IR, p-IR, IRS-1, p-Akt, p-Fox01,03), as well as to increased sensitivity to apoptosis (as seen by increased PARP cleavage, Casp3 activation and DNA fragmentation). Short insulin pulse caused upregulation of insulin signaling markers (IR, p-IR, p-Akt, p-Fox01,03) in a greater extent in normoglycaemic cells compared to hyperglycaemic cells and for the case of p-Akt, in a PI3K-dependent manner. IRS-1 phosphorylation of HG-treated podocytes was negatively regulated, favoring serine versus tyrosine residues. Prolonged insulin treatment caused a significant decrease of IR levels, while alterations in glucose concentrations for various time intervals demonstrated changes of IR, p-IR and p-Akt levels, suggesting that the IR signaling pathway is regulated by glucose levels. Finally, HG exerted similar effects in isolated glomeruli. Conclusions: These results suggest that HG compromises the insulin signaling pathway in the glomerulus, promoting a proapoptotic environment, with a possible critical step for this malfunction lying at the level of IRS-1 phosphorylation; thus we herein demonstrate glomerular insulin signaling as another target for investigation for the prevention and/ or treatment of diabetic nephropathy. © 2016 Katsoulieris et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Vitamin D receptor activators upregulate and rescue podocalyxin expression in high glucose-treated human podocytes

Background: Vitamin D is beneficial in human and experimental chronic kidney disease, the leading cause of which is diabetic nephropathy. Vitamin D through its receptor, VDR, provides renal protection in diabetic nephropathy, but limited data exist about its effect on podocytes. Renal podocytes form the main filtration barrier possessing a unique phenotype maintained by proteins including podocalyxin and nephrin, the expression of which is suppressed in pathological conditions. Methods: We used immortalized human podocytes (human glomerular epithelial cells, HGEC) to assess podocalyxin and nephrin expression after treatment with 1,25-dihydroxyvitamin D3 (calcitriol) and its analogue paricalcitol. The involvement of VDR was investigated by silencing with hVDR-siRNA and ChIP analysis. Results: HGEC exhibit high glucose-mediated downregulation of podocalyxin and nephrin, loss of which has been linked with loss of the permselective renal barrier and proteinuria. Calcitriol and paricalcitol reversed high glucose-induced decrease of nephrin and significantly enhanced podocalyxin expression in podocytes cultured in high glucose. HGEC express VDR and retinoid X receptor (RXR). In the presence of calcitriol and paricalcitol, VDR expression was upregulated and VDR colocalized with RXR in the nucleus. VDR knockdown abolished the protective action of calcitriol and paricalcitol on podocalyxin expression indicating that podocalyxin activation of expression is partly mediated by VDR. Furthermore, VDR specifically regulates podocalyxin expression by bounding to a site upstream of the podocalyxin promoter. Conclusion: Vitamin D analogues maintain and, furthermore, re-activate the expression of specialized components of podocytes including podocalyxin, hence they provide protection against loss of the permselective renal barrier, with molecular mechanisms elucidated herein. © 2013 S. Karger AG, Basel