SH2-containing inositol phosphatase 2 negatively regulates insulin-inducedglycogen synthesis in L6 myotubes

Aims/hypothesis. PI(3,4,5)P3 produced by PI3-kinase seems to be a key media tor for insulin's metabolic actions. We have recently cloned rat SHIP2 cDNA which is abundantly expressed in target tissues of insulin. Here, we clari fy the role of SHIP2 possessing 5'-phosphatase activity toward PI(3,4,5)P3 in insulin signalling in the skeletal muscle. Methods. The role of SHIP2 in insulin-induced glycogen synthesis was studie d by expressing wild-type (WT)-SHIP2 and a 5'-phosphatase defective (AIP)SH IP2 into L6 myotubes by means of adenovirus mediated gene transfer. Results. The early events of insulin signalling including tyrosine phosphor ylation of the insulin receptor and IRS-1, IRS-1 association with the p85 s ubunit, and PI3-kinase activity were not affected by expression of WT- and Delta IP-SHIP2. Although PI(3,4,5)P3 and PI(3,4)P2 are known to possibly ac tivate a downstream molecule of PI3-kinase Akt in vitro, overexpression of WT-SHIP2 inhibited insulin-induced phosphorylation and activation of Akt. C onversely, Akt activity was increased by expression of Delta IP-SHIP2. GSK3 beta located downstream of Akt is an important molecule to further transmi t insulin signal for glycogen synthesis in skeletal muscles. In accordance with the results of Akt, insulin-induced phosphorylation and inactivation o f GSK3 beta, subsequent activation of glycogen synthase and glycogen synthe sis were decreased by expression of WT-SHIP2, whereas these events were inc reased by expression of Delta IP-SHIP2. Conclusion/interpretation. Our results indicate that SHIP2 plays a negative regulatory role via the 5'-phosphatase activity in insulin signalling, and that PI(3,4,5)P3 rather than PI(3,4)P2 is important for in vivo regulation of insulin-induced Akt activation leading to glycogen synthesis in L6 myot ubes.