Mechanism of Oxidative DNA damage in diabetes: tuberin inactivation and downregulation of DNA repair enzyme OGG1

Abstract

Objective: To investigate potential mechanisms of oxidative DNA damage in a rat model of type I diabetes and in murine proximal tubular epithelial cells (MCT) and primary culture of rat proximal tubular epithelial cells (RPTE).

Research Design and Methods: Phosphorylation of Akt and tuberin, 8-oxodG levels and OGG1 expression were measured in kidney cortical tissue of control and type I diabetic animals as well as in proximal tubular cells incubated with normal or high glucose.

Results: In the renal cortex of diabetic rats, the increase in Akt phosphorylation is associated with enhanced phosphorylation of tuberin, decreased OGG1 protein expression and 8-oxodG accumulation. Exposure of proximal tubular epithelial cells to high glucose (HG) causes a rapid increase in ROS generation that correlates with the increase in Akt and tuberin phosphorylation. HG also resulted in downregulation of OGG1 protein expression, paralleling its effect on Akt and tuberin. Inhibition of PI-3K/Akt significantly reduced HG-induced tuberin phosphorylation and restored OGG1 expression. Hydrogen peroxide stimulates Akt and tuberin phosphorylation and decreases OGG1 protein expression. The antioxidant, N-acetylcysteine, significantly inhibited ROS generation, Akt/PKB and tuberin phosphorylation and resulted in deceased 8-oxodG accumulation and upregulation of OGG1 protein expression.

Conclusions: Hyperglycemia in type I diabetes and treatment of proximal tubular epithelial cells with HG leads to phosphorylation/inactivation of tuberin and downregulation of OGG1 via a redox-dependent activation of Akt in renal tubular epithelial cells. This signaling cascade provides a mechanism of oxidative stress-mediated DNA damage in diabetes.