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

ABSTRACT: Decreased cellular GSH content is a common finding in experimental and human diabetes, in which increased oxidative stress appears to occur. Oxidative stress has been suggested to play a causative role in the development of impaired insulin action on adipose tissue and skeletal muscle. In this study we undertook to investigate the potential of GSH depletion to induce insulin 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 treating 3T3-L1 adipocytes with BSO for 18 h. No demonstrable change in the GSH/GSSG ratio was observed following BSO treatment. GSH depletion was progressively associated with abnormal glucose tolerance test, which could not be attributed to impaired insulin secretion. Skeletal muscle insulin responsiveness was unaffected by GSH depletion, based on normal glucose response to exogenous insulin, 2-deoxyglucose uptake measurements in isolated soleus muscle, and on normal skeletal muscle expression of GLUT4 protein. Adipocyte insulin responsiveness in vitro was assessed in 3T3-L1 adipocytes, which displayed decreased insulin-stimulated tyrosine phosphorylation of insulin-receptor-substrate proteins and of the insulin receptor, but exaggerated protein 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 normal inhibition of circulating non-esterified fatty acid levels by endogenous insulin secretion. In conclusion, GSH depletion by BSO results in impaired glucose tolerance, but preserved adipocyte and skeletal muscle insulin responsiveness. This suggests that alternative oxidation-borne factors mediate the induction of peripheral insulin resistance by oxidative stress.

SUBMITTER: Khamaisi M

PROVIDER: S-EPMC1221181 | biostudies-other | 2000 Jul

REPOSITORIES: biostudies-other

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Project description:The aim of this study was to deeper investigate the mechanisms through which ENPP1, a negative modulator of insulin receptor (IR) activation, plays a role on insulin signaling, insulin secretion and eventually glucose metabolism. ENPP1 cDNA (carrying either K121 or Q121 variant) was transfected in HepG2 liver-, L6 skeletal muscle- and INS1E beta-cells. Insulin-induced IR-autophosphorylation (HepG2, L6, INS1E), Akt-Ser(473), ERK1/2-Thr(202)/Tyr(204) and GSK3-beta Ser(9) phosphorylation (HepG2, L6), PEPCK mRNA levels (HepG2) and 2-deoxy-D-glucose uptake (L6) was studied. GLUT 4 mRNA (L6), insulin secretion and caspase-3 activation (INS1E) were also investigated. Insulin-induced IR-autophosphorylation was decreased in HepG2-K, L6-K, INS1E-K (20%, 52% and 11% reduction vs. untransfected cells) and twice as much in HepG2-Q, L6-Q, INS1E-Q (44%, 92% and 30%). Similar data were obtained with Akt-Ser(473), ERK1/2-Thr(202)/Tyr(204) and GSK3-beta Ser(9) in HepG2 and L6. Insulin-induced reduction of PEPCK mRNA was progressively lower in untransfected, HepG2-K and HepG2-Q cells (65%, 54%, 23%). Insulin-induced glucose uptake in untransfected L6 (60% increase over basal), was totally abolished in L6-K and L6-Q cells. GLUT 4 mRNA was slightly reduced in L6-K and twice as much in L6-Q (13% and 25% reduction vs. untransfected cells). Glucose-induced insulin secretion was 60% reduced in INS1E-K and almost abolished in INS1E-Q. Serum deficiency activated caspase-3 by two, three and four folds in untransfected INS1E, INS1E-K and INS1E-Q. Glyburide-induced insulin secretion was reduced by 50% in isolated human islets from homozygous QQ donors as compared to those from KK and KQ individuals. Our data clearly indicate that ENPP1, especially when the Q121 variant is operating, affects insulin signaling and glucose metabolism in skeletal muscle- and liver-cells and both function and survival of insulin secreting beta-cells, thus representing a strong pathogenic factor predisposing to insulin resistance, defective insulin secretion and glucose metabolism abnormalities.

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

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