| Co-Investigator(Kenkyū-buntansha) |
ARAKI Eiichi KUMAMOTO UNIVERSITY, FACULTY OF MRDICAL AND PHARMACEUTICAL SCIENCES, PROFESSOR, 大学院医学薬学研究部, 教授 (10253733)
FURUKAWA Noboru K KUMAMOTO UNIVERSITY, UNIVERSITY HOSPITAL, RESEARCH ASSOCIATE, 医学部附属病院, 医員 (90335795)
田口 哲也 熊本大学, 医学部附属病院, 医員 (30398200)
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| Budget Amount *help |
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 2006: ¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 2005: ¥2,200,000 (Direct Cost: ¥2,200,000)
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| Research Abstract |
Insulin is essential for maintaining glucose homeostasis. Insulin binds to the insulin receptor and activates its endogenous tyrosine kinase, which in turn phosphorylates insulin receptor substrates (IRSs), and transmits various downstream signals. The liver is one of the major target organs of insulin in which the expression of the insulin receptor is abundant. It is suggested that the expression of insulin receptor is regulated by tissue specific mechanism. However, the detail is still uncertain. On the other hand, AMP-activated protein kinase (AMPK) has been known to be important in the regulation of glucose and lipid metabolism in liver. The aim of this study is to investigate the effect of AMPK on the insulin receptor, which has a key role in the insulin action, in liver cells that is the major target organ of both AMPK and insulin. We investigated the effect of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), which is an activator of AMPK, on the expression of insulin
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receptor in a human hepatoma cell line, HepG2 cells. AICAR treatment for 48 hours significantly decreased the expression of insulin receptor protein in a dose-dependent manner in HepG2 cells. On the other hand, the inhibitory effect of AICAR was not observed in either 3T3-L1 adipocytes or CHO cells. The expression of insulin receptor mRNA was also significantly decreased with AICAR treatment in a dose-dependent manner. In addition, transcriptional activity of the insulin receptor gene promoter was also down-regulated with AICAR treatment. The inhibitors of AICAR blocked the effects of AICAR on the down-regulation of insulin receptor protein, mRNA and promoter activity. According to the investigation with a deletion mutant of the insulin receptor gene promoter, it was suggested that cis-elements responsible for the AICAR-induced down-regulation existed within 0.6 kb upstream from the ATG codon in the insulin receptor gene. We have found five consensus sequences of insulin response element (IRE-1〜5) in the human insulin receptor gene promoter. A transcription factor Foxol was suggested to bind to IRE-4 and IRE-5, which exist within 0.6 kb of AICAR response region, and the binding to each IREs was decreased with AICAR treatment. This study demonstrated for the first time that AMPK activation reduced the expression of insulin receptor, at least in part, by a down-regulation of the gene transcription, and that this effect may be specific in liver cells. In addition, it was suggested that Foxol was involved in the transcriptional regulation of insulin receptor gene in liver cells. Less
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