Effect of inhibition of glutathione synthesis on insulin action: in vivo and in vitro studies using buthionine sulfoximine

Decreased cellular GSH content is a common finding in experimental and huma n diabetes, in which increased oxidative stress appears to occur. Oxidative stress has been suggested to play a causative role in the development of i mpaired insulin action on adipose tissue and skeletal muscle. In this study we undertook to investigate the potential of GSH depletion to induce insul in resistance, by utilizing the GSH synthesis inhibitor, L-buthionine-[S,R] -sulfoximine (BSO), GSH depletion (20-80% in various tissues), was achieved in vivo by treating rats for 20 days with BSO, and in vitro (80%) by treat ing 3T3-L1 adipocytes with BSO for 18 h. No demonstrable change in the GSH/ GSSG ratio was observed following BSO treatment. GSH depletion was progress ively associated with abnormal glucose tolerance test, which could not be a ttributed to impaired insulin secretion. Skeletal muscle insulin responsive ness was unaffected by GSH depletion, based on normal glucose response to e xogenous insulin, 2-deoxyglucose uptake measurements in isolated soleus mus cle, and on normal skeletal muscle expression of GLUT4 protein. Adipocyte i nsulin responsiveness in vitro was assessed in 3T3-L1 adipocytes, which dis played decreased insulin-stimulated tyrosine phosphorylation of insulin-rec eptor-substrate proteins and of the insulin receptor, but exaggerated prote in kinase B phosphorylation. However, insulin-stimulated glucose uptake was unaffected by GSH depletion. In accordance, normal adipose tissue insulin sensitivity was observed in BSO-treated rats in vivo, as demonstrated by no rmal inhibition of circulating non-esterified fatty acid levels by endogeno us insulin secretion. In conclusion, GSH depletion by BSO results in impair ed glucose tolerance, but preserved adipocyte and skeletal muscle insulin r esponsiveness. This suggests that alternative oxidation-borne factors media te the induction of peripheral insulin resistance by oxidative stress.