Targeting the LXR/mTOR Signaling Axis to Modulate Autophagy in Diabetic Kidney Disease: A Novel Therapeutic Approach

Abstract

Introduction: Diabetes Mellitus (DM) is a metabolic disease that induces kidney injury and is the most common cause of end-stage kidney disease. Accumulating evidence suggests that autophagy plays an important role in many critical aspects of normal and disease states of the kidney, including diabetic kidney disease (DKD). Between different kidney cells, studies have shown that podocytes rely heavily on autophagy for their survival even under basal conditions. Yet the mechanisms regulating autophagy in DKD are not well elucidated. The mTOR complexes and oxidative stress have emerged as potential key players in mediating diabetes-induced autophagy imbalance. The role of the Liver-X-Receptor (LXR) in DKD has been mildly highlighted, but its role in autophagy and its crosstalk with key mechanistic pathways in DKD remains unclear. In this study, we investigated the role of the LXR/mTOR axis in autophagy and its possible link to podocyte injury in type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM).

Methods: In this study, in vivo experiments were performed on control and STZ-induced T1DM or HFD/STZ-induced T2DM mice treated with mTORC1 (rapamycin), mTORC2 (JR-AB2-011), or dual mTORC1/2 (PP242) inhibitors, or with LXR activator (T0 or DMHCA). Autophagy inhibition was assessed using hydroxychloroquine in control mice. Findings were validated in cultured human podocytes (in vitro studies) transfected with siRNA targeting LXRα/β, Raptor, Rictor, or LC3B. Under high glucose conditions, podocytes were also treated with rapamycin, JR-AB2-011, PP242, or T0, enabling us to directly assess the effects of altering the LXR/mTOR pathway on podocyte function and stress responses.

Results: Our results show that DM induces autophagy deregulation, accompanied by decreased LXR expression and increased mTORC1 and mTORC2 activity, leading to kidney injury. These effects are mediated by an increase in NOX4 expression and NADPH oxidase activity, triggering ROS production. Activation of the LXR pathway, using T0 or DMHCA, restores the homeostatic functional renal levels by reducing proteinuria, restoring histological and phenotypical changes to near-normal levels, and inhibiting NOX4 expression. Furthermore, LXR activation ameliorates diabetes-induced autophagy homeostatic deregulation by restoring the expression of LC3B and p62. Additionally, LXR activation reduces both mTORC1 and mTORC2 activation. Inactivation of the mTORC1, mTORC2, or mTORC1/2 pathways mimics the effect of LXR activation on ROS production and renal injury but does not alter the LXR pathway. Treatment of control mice with hydroxychloroquine mimicked the effect of diabetes on kidneys' functional, phenotypic, histological, and molecular alterations. These in vivo findings were corroborated in cultured human podocytes, where silencing LXRα/β induced cellular damage, while knockdown of Raptor or Rictor conferred protection under high glucose conditions but not under normal glucose levels.

Conclusion: This study provides evidence for a novel function of LXR/mTOR in regulating oxidative stress and autophagy in the onset and progression of DKD.