SH2-containing inositol phosphatase 2 negatively regulates insulin-induced glycogen synthesis in L6 myotubes.
OBJECTIVE
PI(3,4,5)P3 produced by PI3-kinase seems to be a key mediator for insulin's metabolic actions. We have recently cloned rat SHIP2 cDNA which is abundantly expressed in target tissues of insulin. Here, we clarify 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 studied by expressing wild-type (WT)-SHIP2 and a 5'-phosphatase defective (Delta IP)-SHIP2 into L6 myotubes by means of adenovirus mediated gene transfer.


RESULTS
The early events of insulin signalling including tyrosine phosphorylation of the insulin receptor and IRS-1, IRS-1 association with the p85 subunit, 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 activate a downstream molecule of PI3-kinase Akt in vitro, overexpression of WT-SHIP2 inhibited insulin-induced phosphorylation and activation of Akt. Conversely, Akt activity was increased by expression of Delta IP-SHIP2. GSK3 beta located downstream of Akt is an important molecule to further transmit insulin signal for glycogen synthesis in skeletal muscles. In accordance with the results of Akt, insulin-induced phosphorylation and inactivation of GSK3 beta, subsequent activation of glycogen synthase and glycogen synthesis were decreased by expression of WT-SHIP2, whereas these events were increased by expression of Delta IP-SHIP2.


CONCLUSIONS
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 myotubes.
Sasaoka
T
T
Department of Clinical Pharmacology, Toyama Medical and Pharmaceutical University, Toyama, Japan. tsasaoka-tym@umin.ac.jp
Hori
H
H
Wada
T
T
Ishiki
M
M
Haruta
T
T
Ishihara
H
H
Kobayashi
M
M
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002250:Carbon Radioisotopes; D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects; D015870:Gene Expression; D005822:Genetic Vectors; D005947:Glucose / Q000378:metabolism; D006003:Glycogen / Q000096:biosynthesis*; D038362:Glycogen Synthase Kinase 3; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D018482:Muscle, Skeletal / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000072183:Phosphatidylinositol-3,4,5-Trisphosphate 5-Phosphatases; D010750:Phosphoproteins / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000235:genetics / Q000502:physiology*; D010766:Phosphorylation; D019000:Phosphotyrosine / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D015398:Signal Transduction; D014162:Transfection
D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002250:Carbon Radioisotopes; D005947:Glucose; D006003:Glycogen; D038362:Glycogen Synthase Kinase 3; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D019869:Phosphatidylinositol 3-Kinases; D000072183:Phosphatidylinositol-3,4,5-Trisphosphate 5-Phosphatases; D010750:Phosphoproteins; D010744:Phosphoric Monoester Hydrolases; D019000:Phosphotyrosine; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D011972:Receptor, Insulin; C526292:IRS1 protein, human; C526296:Irs1 protein, rat; C494918:AKT1 protein, human; C494920:Akt1 protein, rat; C098706:INPPL1 protein, human