Inhibition of endogenous SHIP2 ameliorates insulin resistance caused by chronic insulin treatment in 3T3-L1 adipocytes

Aims/hypothesis: SHIP2 is a physiologically important negative regulator of insulin signalling hydrolysing the PI3- kinase product, PI( 3,4,5) P3, which also has an impact on insulin resistance. In the present study, we examined the effect of inhibiting the endogenous SHIP2 function on the insulin resistance caused by chronic insulin treatment. Methods: The endogenous function of SHIP2 was inhibited by expressing a catalytically inactive SHIP2 (DeltaIP-SHIP), and compared with the effect of treatments designed to restore the levels of IRS- 1 in insulin signalling systems of 3T3-L1 adipocytes. Results: Chronic insulin treatment induced the large (86%) down-regulation of IRS1 and the modest ( 36%) up-regulation of SHIP2. Subsequent stimulation by insulin of Akt phosphorylation, PKClambda activity, and 2-deoxyglucose (2-DOG) uptake was markedly decreased by the chronic insulin treatment. Coincubation with the mTOR inhibitor, rapamycin, effectively inhibited the proteosomal degradation of IRS- 1 caused by the chronic insulin treatment. Although the coincubation with rapamycin and advanced overexpression of IRS- 1 effectively ameliorated subsequent insulin-induced phosphorylation of Akt, insulin stimulation of PKClambda activity and 2-DOG uptake was partly restored by these treatments. Similarly, expression of DeltaIP-SHIP2 effectively ameliorated the insulin-induced phosphorylation of Akt without affecting the amount of IRS-1. Furthermore, the decreased insulininduced PKClambda activity and 2-DOG uptake following chronic insulin treatment were ameliorated by the expression of DeltaIP-SHIP2 more effectively than by the treatment with rapamycin. Conclusions/interpretation: Our results indicate that the inhibition of endogenous SHIP2 is effective in improving the state of insulin resistance caused by chronic insulin treatment.