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Submitted on November 20, 2003
Accepted on March 18, 2004
First Department of Internal Medicine and Department of Clinical Pharmacology, Toyama Medical & Pharmaceutical University, 2630 Sugitani, Toyama 930-0194, Japan and Sainou Hospital, Toyama 930-0887, Japan.
* To whom correspondence should be addressed. E-mail: tsasaoka-tym{at}umin.ac.jp.
SH2-containing inositol 5'-phosphatase 2 (SHIP2) possesses 5'-phosphatase activity to specifically hydrolyze the PI3-kinase product PI(3,4,5)P3 in the regulation of insulin signaling. In the present study, we examined the impact of SHIP2 on the regulation of insulin signaling leading to protein synthesis in 3T3-L1 adipocytes cultured with standard and excess concentrations of amino acids. Insulin-induced translocation of PDK1 to the plasma membrane, phosphorylation of Akt and p70S6-kinase and ribosomal protein S6, increase in the amount of 4E-BP1 
-form, association of eIF4E with eIF4G, and protein synthesis were decreased by overexpression of wild-type SHIP2 by adenovirus-mediated gene transfer. The effect of SHIP2 overexpression on the regulation of insulin-induced phosphorylation of Akt and p70S6-kinase was somewhat augmented by the incubation with 5-fold excess concentrations of amino acids for 30 min. In contrast, the impact of SHIP2 expression was diminished in insulin-induced phosphorylation of p70S6-kinase and S6, but not of Akt, after the incubation for 16 h. Interestingly, incubation with the excess concentrations of amino acids for 30 min induced activation of PI3-kinase and phosphorylation of Akt, whereas phosphorylation of p70S6-kinase and S6 was decreased. Furthermore, although the exposure for longer time periods up to 24 h did not elicit phosphorylation of Akt, it markedly induced phosphorylation of p70S6-kinase and S6. These results indicate that SHIP2 plays an important role in the negative regulation of insulin signaling for the protein synthesis, and that the impact of SHIP2 is altered dependent on the acute or chronic exposure of excess concentrations of amino acids in culture.
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