Adipocyte protein carbonylation and oxidative stress in obesity-linked mitochondrial dysfunction and insulin resistance.

Abstract

Carbonylation is the covalent, non-reversible modification of the side chains of cysteine, histidine and lysine residues by lipid peroxidation end products such as 4-hydroxy- and 4-oxononenal. The antioxidant enzyme glutathione S-transferase A4 (GSTA4) catalyzes a major detoxification pathway for such reactive lipids but its expression was selectively down regulated in the obese, insulin resistant adipocyte resulting in increased protein carbonylation. The effects of such modifications are associated with increased oxidative stress and metabolic dysregulation centered on mitochondrial energy metabolism. Mitochondrial functions in adipocytes of lean or obese GSTA4 null mice were significantly compromised compared to wild type controls and were accompanied by an increase in superoxide anion. Silencing GSTA4 mRNA in cultured adipocytes resulted in increased protein carbonylation, increased mitochondrial ROS, dysfunctional state 3 respiration and altered glucose transport and lipolysis. To address the role of protein carbonylation in the pathogenesis of mitochondrial dysfunction quantitative proteomics was employed to identify specific targets of carbonylation in GSTA4-silenced or overexpressing 3T3-L1 adipocytes. GSTA4- silenced adipocytes displayed elevated carbonylation of several key mitochondrial proteins including the phosphate carrier protein, NADH dehydrogenase 1 alpha subcomplexes 2 and 3, translocase of inner mitochondrial membrane 50, and valyl-tRNA synthetase. Elevated protein carbonylation is accompanied by diminished complex I activity, impaired respiration, increased superoxide production and a reduction in membrane potential without changes in mitochondrial number, area or density. These results suggest protein carbonylation plays a major instigating role in mitochondrial dysfunction and may be a linked to the development of insulin resistance in the adipocyte.