title: Regulation of hepatic insulin-like growth factor-binding protein-1 gene expression by insulin: central role for mammalian target of rapamycin independent of forkhead box O proteins.
abstract: The expression of IGF-binding protein-1 (IGFBP-1) is induced in rat liver by dexamethasone and glucagon and is completely inhibited by 100 nM insulin. Various studies have implicated phosphatidylinositol 3-kinase, protein kinase B (Akt), phosphorylation of the transcription factors forkhead in rhabdomyosarcoma 1 (Foxo1)/Foxo3, and the mammalian target of rapamycin (mTOR) in insulin's effect. In this study we examined insulin regulation of IGFBP-1 in both subconfluent and confluent hepatocytes. In subconfluent hepatocytes, insulin inhibition of IGFBP-1 mRNA levels was blocked by inhibiting PI3 kinase activation, and there was a corresponding inhibition of Foxo1/Foxo3 phosphorylation. In these same cells, inhibition of the insulin effect by rapamycin occurred in the presence of insulin-induced Foxo1/Foxo3 phosphorylation. In confluent hepatocytes, insulin could not activate the phosphatidylinositol 3-kinase (PI3 kinase)-Akt-Foxo1/Foxo3 pathway, but still inhibited IGFBP-1 gene expression in an mTOR-dependent manner. In subconfluent hepatocytes, the serine/threonine phosphatase inhibitor okadaic acid (100 nM) partially inhibited IGFBP-1 gene expression by 40%, but did not produce phosphorylation of either Akt or Foxo proteins. In contrast, 1 nm insulin inhibited the IGFBP-1 mRNA level by 40% and correspondingly activated Akt and Foxo1/Foxo3 phosphorylation to a level comparable to that observed with 100 nM insulin. These results suggest a potential role for a serine/threonine phosphatase(s) in the regulation of IGFBP-1 gene transcription, which is not downstream of mTOR and is independent of Akt. In conclusion, we have found that in rat liver, insulin inhibition of IGFBP-1 mRNA levels can occur in the absence of the phosphorylation of Foxo1/Foxo3, whereas activation of the mTOR pathway is both necessary and sufficient.
authors.affiliation: Polypeptide Hormone Laboratory, Faculty of Medicine, McGill University, Montréal, Quebéc, Canada.
authors.forename: Catherine
authors.initials: C
authors.lastname: Mounier
authors.forename: Victor
authors.initials: V
authors.lastname: Dumas
authors.forename: Barry I
authors.initials: BI
authors.lastname: Posner
mesh_terms: D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D003907:Dexamethasone / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000378:metabolism; D005786:Gene Expression Regulation; D005799:Genes, Dominant; D005934:Glucagon / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism; D007328:Insulin / Q000378:metabolism*; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000096:biosynthesis* / Q000235:genetics* / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008297:Male; D009419:Nerve Tissue Proteins / Q000378:metabolism; D019319:Okadaic Acid / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D013552:Swine; D058570:TOR Serine-Threonine Kinases; D013997:Time Factors; D000077191:Wortmannin; D000071161:Forkhead Box Protein O1
chemical_list: D000730:Androstadienes; D003907:Dexamethasone; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D005934:Glucagon; D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D009419:Nerve Tissue Proteins; D019319:Okadaic Acid; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D051057:Proto-Oncogene Proteins c-akt; D012313:RNA; D012333:RNA, Messenger; D058570:TOR Serine-Threonine Kinases; D000077191:Wortmannin; C499515:FOXO3 protein, rat; C074639:Foxo1 protein, rat; D000071161:Forkhead Box Protein O1; C546845:mTOR protein, rat