Superoxide Destabilization of β-Catenin Augments Apoptosis of High-Glucose-Stressed Mesangial Cells

Abstract

Intense mesangial cell apoptosis contributes to the pathogenesis of diabetic nephropathy. Although reactive oxygen radicals and Wnt signaling components are potent regulators that modulate renal tissue remodeling and morphogenesis, cross-talk between oxidative stress and Wnt/β-catenin signaling in controlling high-glucose-impaired mesangial cell survival and renal function have not been tested. In this study, high glucose induced Ras and Rac1 activation, superoxide burst, and Wnt5a/β-catenin destabilization and subsequently promoted caspase-3 and poly (ADP-ribose) polymerase cleavage and apoptosis in mesangial cell cultures. The pharmacological and genetic suppression of superoxide synthesis by superoxide dismutase and diphenyloniodium, dominant-negative Ras (S17N), and dominant-negative Rac1 (T17N) abrogated high-glucose-induced glycogen synthase kinase (GSK-3β) activation and caspase-3 and poly (ADP-ribose) polymerase degradation. Inactivation of Ras and Racl also reversed Wnt/β-catenin expression and survival of mesangial cells. Stabilization of β-catenin by the transfection of stable β-catenin (Δ45) and kinase-inactive GSK-3β attenuated high-glucose-mediated mesangial cell apoptosis. Exogenous superoxide dismutase administration attenuated urinary protein secretion in diabetic rats and abrogated diabetes-mediated reactive oxygen radical synthesis in renal glomeruli. Immunohistological observation revealed that superoxide dismutase treatment abrogated diabetes-induced caspase-3 cleavage and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling (TUNEL) and increased Wnt5a/β-catenin expression in renal glomeruli. Taken together, high glucose induced oxidative stress and apoptosis in mesangial cells. The Ras and Rac1 regulation of superoxide appeared to raise apoptotic activity by activating GSK-3β and inhibiting Wnt5a/β-catenin signaling. Controlling oxidative stress and Wnt/β-catenin signaling has potential for protecting renal tissue against the deleterious effect of high glucose.