An SH2 domain-containing 5 ' inositolphosphatase inhibits insulin-induced GLUT4 translocation and growth factor-induced actin filament rearrangement

Tyrosine kinase receptors lead to rapid activation of phosphatidylinositol 3-kinase (PI3 kinase) and the subsequent formation of phosphatidylinositide s (PtdIns) 3,4-P2 and PtdIns 3,4,5-P3, which are thought to be involved in signaling for glucose transporter GLUT4 translocation, cytoskeletal rearran gement, and DNA synthesis. However, the specific role of each of these PtdI ns in insulin and growth factor signaling is still mainly unknown. Therefor e, we assessed, in the current study, the effect of SH2-containing inositol phosphatase (SHIP) expression on these biological effects. SHIP is a 5' ph osphatase that decreases the intracellular levels of PtdIns 3,4,5-P3. Expre ssion of SHIP after nuclear microinjection in 3T3-L1 adipocytes inhibited i nsulin-induced GLUT4 translocation by 100 +/- 21% (mean +/- the standard er ror) at submaximal (3 ng/ml) and 64 +/- 5% at maximal (10 ng/ml) insulin co ncentrations (P < 0.05 and P < 0.001, respectively). A catalytically inacti ve mutant of SHIP had no effect on insulin-induced GLUT4 translocation. Fur thermore, SHIP also abolished GLUT4 translocation induced by a membrane-tar geted catalytic subunit of PI3 kinase. In addition, insnlin-, insulin-like growth factor I (IGF-I)-, and platelet derived growth factor-induced cytosk eletal rearrangement, i.e., membrane ruffling, was significantly inhibited (78 +/- 10, 64 +/- 3, and 62 +/- 5%, respectively; P < 0.05 for all) in 3T3 -L1 adipocytes. In a rat fibroblast cell line overexpressing the human insu lin receptor (HIRc-B), SHIP inhibited membrane ruffling induced by insulin and IGF I by 76 +/- 3% (P < 0.001) and 68 +/- 5% (P < 0.005), respectively. However, growth factor-induced stress fiber breakdown was not affected by SHIP expression. Finally, SHIP decreased significantly growth factor-induce d mitogen-activated protein kinase activation and DNA synthesis. Expression of the catalytically inactive mutant had no effect on these cellular respo nses. In summary, our results show that expression of SHIP inhibits insulin -induced GLUT4 translocation, growth factor-induced membrane ruffling, and DNA synthesis, indicating that PtdIns 3,4,5-P3 is the key phospholipid prod uct mediating these biological actions.