LXR-mTOR-Nox4 signaling axes : novel therapeutic targets in diabetic nephropathy

Abstract

Background. Podocyte injury has been shown to play a major role in the initiation of Diabetic Nephropathy (DN). The molecular mechanism by which hyperglycemia-diabetes induces podocyte injury is suggested to be multi-factorial and not well defined. Recent research suggests that defective autophagy may play a role in podocyte dysfunction during the onset and development of diabetes. Furthermore, our lab has previously described the importance of Nox4 in the progression of podocyte injury. However, a mechanistic link between NADPH oxidases (Nox)-induced ROS production and the alteration in autophagy has not yet been elucidated. Moreover, the Liver-X-receptor and the mTOR pathways have been recently shown to be associated with autophagic and oxidative stress responses. In this study, we aim to investigate the role of the Nox4-LXR-mTOR axis on autophagy and their possible link to podocyte depletion and injury. Methods. Both in vitro and in vivo models were used in this study. A conditionally immortalized human podocyte cell line was used for our in vitro studies. High-fat diet-low-dose streptozotocin-induced type 2 diabetic mice were used for the in vivo studies, as well as high-dose streptozotocin-induced type 1 diabetic mice. Pharmacological means were utilized to alter the expression of NADPH oxidases (GKT), LXR (T0) and the mTOR (Rapamycin) signaling pathways, while podocyte depletion-loss, autophagy alteration and glomerular injury were assessed as the corresponding biological output via Western Blotting, PCR and immunohistological assessments. Results. Our results reveal that high glucose HG-hyperglycemia induces defective autophagy in both podocytes and isolated glomeruli of type 2 diabetic mice. HG-hyperglycemia reduces LXR mRNA levels and protein expression and activates the mTORC1 pathway. These effects were mediated by an increase in Nox4 protein expression and NADPH oxidase activity, triggering ROS production. Activation of the LXR pathway was shown to decrease diabetes-induced Nox4 expression and activ