Insulin inhibits the activation of transcription by a C-terminal fragment of the forkhead transcription factor FKHR. A mechanism for insulin inhibition of insulin-like growth factor-binding protein-1 transcription.
The forkhead rhabdomyosarcoma transcription factor (FKHR) is a promising candidate to be the transcription factor that binds to the insulin response element of the insulin-like growth factor-binding protein-1 (IGFBP-1) promoter and mediates insulin inhibition of IGFBP-1 promoter activity. Cotransfection of mouse FKHR increased IGFBP-1 promoter activity 2-3-fold in H4IIE rat hepatoma cells; insulin inhibited FKHR-stimulated promoter activity approximately 70%. A C-terminal fragment of mouse FKHR (residues 208-652) that contains the transcription activation domain fused to a Gal4 DNA binding domain potently stimulated Gal4 promoter activity. Insulin inhibited FKHR fragment-stimulated promoter activity by approximately 70%. Inhibition was abolished by coincubation with the phosphatidylinositol-3 kinase inhibitor, LY294002. The FKHR 208-652 fragment contains two consensus sites for phosphorylation by protein kinase B (PKB)/Akt, Ser-253 and Ser-316. Neither site is required for insulin inhibition of promoter activity stimulated by the FKHR fragment, and overexpression of Akt does not inhibit FKHR fragment-stimulated Gal4 promoter activity. These results suggest that insulin- and phosphatidylinositol-3 kinase-dependent phosphorylation of another site in the fragment by a kinase different from PKB/Akt inhibits transcription activation by the fragment. Phosphorylation of this site also may be involved in insulin inhibition of transcription activation by full-length FKHR, but only after phosphorylation of Ser-253 by PKB/Akt.
Tomizawa
M
M
Growth and Development Section, Clinical Endocrinology Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA.
Kumar
A
A
Perrot
V
V
Nakae
J
J
Accili
D
D
Rechler
M M
MM
Kumaro
A
A
D000818:Animals; D004268:DNA-Binding Proteins / Q000494:pharmacology*; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D007328:Insulin / Q000494:pharmacology*; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000037:antagonists & inhibitors* / Q000235:genetics; D051379:Mice; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments / Q000494:pharmacology*; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000502:physiology; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D014157:Transcription Factors / Q000494:pharmacology*; D015533:Transcriptional Activation / Q000187:drug effects*
D004268:DNA-Binding Proteins; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; D014157:Transcription Factors; C467200:Foxo1 protein, mouse; C074639:Foxo1 protein, rat; C494920:Akt1 protein, rat