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Role of the Src Homology 2 (SH2) Domain and C-Terminus Tyrosine Phosphorylation Sites of SH2-Containing Inositol Phosphatase (SHIP) in the Regulation of Insulin-Induced Mitogenesis¹

First Department of Medicine, Toyama Medical and Pharmaceutical University, Toyama, 930-0194 Japan

Address all correspondence and requests for reprints to: Toshiyasu Sasaoka, M.D., Ph.D., First Department of Medicine, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama, 930-0194, Japan. E-mail: tsasaoka-tym{at}umin.ac.jp

To examine the role of SHIP in insulin-induced mitogenic signaling, we used a truncated SHIP lacking the SH2 domain (SH2-SHIP) and a Y917/1020F-SHIP (2F-SHIP) in which two tyrosines contributing to Shc binding were mutated to phenylalanine. Wild-type (WT)-, SH2-, and 2F-SHIP were transiently transfected into Rat1 fibroblasts overexpressing insulin receptors (HIRc). Insulin-stimulated tyrosine phosphorylation of WT-SHIP and SH2-SHIP, whereas tyrosine phosphorylation of 2F-SHIP was not detectable, indicating that 917/1020-Tyr are key phosphorylation sites on SHIP. Although SHIP can bind via its 917/1020-Tyr residues and SH2 domain to Shc PTB domain and 317-Tyr residue, respectively, insulin-induced SHIP association with Shc was more greatly decreased in 2F-SHIP cells than that in SH2-SHIP cells. Insulin stimulation of Shc association with Grb2, which is important for p21ras-MAP kinase activation, was decreased by overexpression of WT- and 2F-SHIP. Importantly, insulin-induced Shc·Grb2 association was not detectably reduced in SH2-SHIP cells. In accordance with the extent of Shc association with Grb2, insulin-induced MAP kinase activation was relatively decreased in both WT-SHIP and 2F-SHIP cells, but not in SH2-SHIP cells. To examine the functional role of SHIP in insulin’s biological action, insulin-induced mitogenesis was compared among these transfected cells. Insulin stimulation of thymidine incorporation and bromodeoxyuridine incorporation was decreased in WT-SHIP cells compared with that of control HIRc cells. Expression of 2F-SHIP also significantly reduced insulin-induced mitogenesis, whereas it was only slightly affected by overexpression of SH2-SHIP. Furthermore, the reduction of insulin-induced mitogenesis in WT-SHIP cells was partly compensated by coexpression of Shc. These results indicate that SHIP plays a negative regulatory role in insulin-induced mitogenesis and that the SH2 domain of SHIP is important for its negative regulatory function.

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