| Budget Amount *help |
¥3,400,000 (Direct Cost: ¥3,400,000)
Fiscal Year 2003: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2002: ¥2,400,000 (Direct Cost: ¥2,400,000)
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| Research Abstract |
We investigated the effect of PTP1B overexpression for insulin signaling in 3T3L1 adipocytes. 0verexpression of wild-type PTP1B in L1 adipocytes led to the profound decrease in the insulin-stimulated phosphorylation of mitogen-activated protein (MAP) kinase as well as in L6 myocytes. Even though the identical decrease in IRS-1 phosphorylation as in L6 myocytes, wild-type PTP1B overexpression in L1 adipocytes led to a small inhibition in the insulin-stimulated Akt phosphorylation as accompanied with a small, but significant attenuation in the insulin-stimulated glucose uptake, when compared with phosphatase-negative mutant. Regarding the relatively small effect on Akt phosphorylation, we found the identical findings in rat 1 fibroblasts overexpressing human insulin receptor, suggesting that the higher expression levels of insulin receptor and IRS-1 might be responsible. With regard to the large effect on MAP kinase phosphorylation, we found that PTP1B overexpression led to the impairmen
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ts of both phosphorylation of IRS-1 and Shc, resulting in a decrease in their association with Grb2. Furthermore, platelet derived growth factor stimulated Shc phosphorylation was also attenuated, without any change in its receptors, suggesting that PTP1B directly attacked Shc. These data demonstrate that PTP1B negatively regulates insulin signaling, and MAP kinase cascade is much sensitive as compared with Akt pathway in some cell lines, especially in L1 adipocytes.
Insulin-resistant rats fed a diet high in fructose showed an increased protein-tyrosine phosphatase 1B (PTP1B) content with strong expression of sterol regulatory element binding protein (SREBP)-1 mRNA in the liver. To clarify the association of PTP1B with SREBP-1 gene expression, we overexpressed PTP1B in rat hepatocytes, which led to increased mRNA content and promoter activity of SREBP-1a and -1c, including increased nuclear Sp1 binding. Since PTP1B overexpression increased phosphatase 2A (PP2A) activity, we inhibited PP2A activity by expression of its selective inhibitor, SV40 small t antigen, and found that this normalized the PTP1B-enhanced SREBP-1a and -1c mRNA expressions through activation of Sp1 site. Furthermore, a transient liver-specific overexpression of PTP1B protein led to increased serum triglyceride levels with enhanced SREBP-1a and -1c gene expressions in mouse liver. Finally, administration of a new PTP1B inhibitor (JTP-55773) to db/db mice led to amelioration of both postprandial hypertriglyceridemia and fatty liver with decreased SREBP-1 mRNA expression. These results indicate that PTP1B may regulate gene expression of SREBP-1 via enhancing PP2A activity, thus mediating hepatic lipogenesis and postprandial hypertriglyceridemia. Therefore, PTP1B is a newly identified therapeutic target for the amelioration of postprandial hypertriglyceridemia, as well as for the reduction of resistance to insulin. Less
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