Protein kinase B/Akt mediates effects of insulin on hepatic insulin-like growth factor-binding protein-1 gene expression through a conserved insulin response sequence.
Insulin regulates the expression of multiple hepatic genes through a conserved insulin response sequence (IRS) (CAAAAC/TAA) by an as yet undetermined mechanism. Protein kinase B/Akt (PKB/Akt), a member of the PKA/PKC serine/threonine kinase family, functions downstream from phosphatidylinositol 3'-kinase (PI3K) in mediating effects of insulin on glucose transport and glycogen synthesis. We asked whether PKB/Akt mediates sequence-specific effects of insulin on hepatic gene expression using the model of the insulin-like growth factor binding protein-1 (IGFBP-1) promoter. Insulin lowers IGFBP-1 mRNA levels, inhibits IGFBP-1 promoter activity, and activates PKB/Akt in HepG2 hepatoma cells through a PI3K-dependent, rapamycin-insensitive mechanism. Constitutively active PI3K and PKB/Akt are each sufficient to mediate effects of insulin on the IGFBP-1 promoter in a nonadditive fashion. Dominant negative K179 PKB/Akt disrupts the ability of insulin and PI3K to activate PKB/Akt and to inhibit promoter activity. The IGFBP-1 promoter contains two IRSs each of which is sufficient to mediate sequence-specific effects of insulin, PI3K, and PKB/Akt on promoter activity. Highly related IRSs from the phosphoenolpyruvate carboxykinase and apolipoprotein CIII genes also are effective in this setting. These results indicate that PKB/Akt functions downstream from PI3K in mediating sequence-specific effects of insulin on the expression of IGFBP-1 and perhaps multiple hepatic genes through a conserved IRS.
Cichy
S B
SB
Department of Medicine, University of Illinois College of Medicine at Chicago, Chicago, Illinois 60612, USA.
Uddin
S
S
Danilkovich
A
A
Guo
S
S
Klippel
A
A
Unterman
T G
TG
D001483:Base Sequence; D017124:Conserved Sequence; D005786:Gene Expression Regulation; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000096:biosynthesis* / Q000235:genetics; D008099:Liver / Q000187:drug effects*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000032:analysis
D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; C494918:AKT1 protein, human