Role of the Insulin Receptor Substrate 1 and Phosphatidylinositol 3-Kinase Signaling Pathway in Insulin-Induced Expression of Sterol Regulatory Element Binding Protein 1c and Glucokinase Genes in Rat Hepatocytes
- Michihiro Matsumoto,
- Wataru Ogawa,
- Kiyoshi Teshigawara,
- Hiroshi Inoue,
- Kazuaki Miyake,
- Hiroshi Sakaue and
- Masato Kasuga
- From the Department of Clinical Molecular Medicine, Division of Diabetes, Digestive, and Kidney Diseases, Kobe University Graduate School of Medicine, Kobe, Japan
The mechanism by which insulin induces the expression of the sterol regulatory element binding protein 1c (SREBP-1c) and glucokinase genes was investigated in cultured rat hepatocytes. Overexpression of an NH2-terminal fragment of IRS-1 that contains the pleckstrin homology and phosphotyrosine binding domains (insulin receptor substrate-1 NH2-terminal fragment [IRS-1N]) inhibited insulin-induced tyrosine phosphorylation of IRS-1 as well as the association of IRS-1 with phosphatidylinositol (PI) 3-kinase activity, whereas the tyrosine phosphorylation of IRS-2 and its association with PI 3-kinase activity were slightly enhanced. The equivalent fragment of IRS-2 (IRS-2N) prevented insulin-induced tyrosine phosphorylation of both IRS-1 and IRS-2, although that of IRS-1 was inhibited more efficiently. The insulin-induced increases in the abundance of SREBP-1c and glucokinase mRNAs, both of which were sensitive to a dominant-negative mutant of PI 3-kinase, were blocked in cells in which the insulin-induced tyrosine phosphorylation of IRS-1 was inhibited by IRS-1N or IRS-2N. A dominant-negative mutant of Akt enhanced insulin-induced tyrosine phosphorylation of IRS-1 (but not that of IRS-2) and its association with PI 3-kinase activity, suggesting that Akt contributes to negative feedback regulation of IRS-1. The Akt mutant also promoted the effects of insulin on the accumulation of SREBP-1c and glucokinase mRNAs. These results suggest that the IRS-1–PI 3-kinase pathway is essential for insulin-induced expression of SREBP-1c and glucokinase genes.
Address correspondence and reprint requests to Dr. Wataru Ogawa, Department of Clinical Molecular Medicine, Division of Diabetes, Digestive, and Kidney Diseases, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. E-mail:.
Received for publication 19 October 2001 and accepted in revised form 21 February 2002.
FBS, fetal bovine serum; GK, glucokinase; GSK, glycogen synthase kinase; HA, hemagglutinin epiptope; IRS, insulin receptor substrate; IRS-1N, IRS-1 NH2-terminal fragment; KRLB, kinase regulatory loop binding; MOI, multiplicity of infection; PFU, plaque-forming unit; PH, pleckstrin homology; PI, phosphatidylinositol; PTB, phosphotyrosine binding; SREBP-1c, sterol regulatory element binding protein 1c.