Insulin stimulates the expression of carbohydrate response element binding protein (ChREBP) by attenuating the repressive effect of Pit-1, Oct-1/Oct-2, and Unc-86 homeodomain protein octamer transcription factor-1.
The carbohydrate response element binding protein (ChREBP) has been recognized as a key controller of hepatic lipogenesis. Whereas the function of ChREBP has been extensively investigated, mechanisms underlying its transcription remain largely unknown, although ChREBP production is elevated in a hyperinsulinemic mouse model. We located a conserved Pit-1, Oct-1/Oct-2, and Unc-86 (POU) protein binding site (ATGCTAAT) within the proximal promoter region of human ChREBP. This site interacts with the POU homeodomain protein octamer transcription factor-1 (Oct-1), as detected by gel shift and chromatin immunoprecipitation assays. Oct-1 cotransfection in the human HepG2 cell line repressed ChREBP promoter activity approximately 50-75% (P < 0.01 to P < 0.001), and this repression was dependent on the existence of the POU binding site. Furthermore, overexpression of Oct-1 repressed endogenous ChREBP mRNA and protein expression, whereas knockdown of Oct-1 expression, using a lentivirus-based small hairpin RNA approach, led to increased ChREBP mRNA and protein expression. In contrast, HepG2 cells treated with 10 or 100 nM insulin for 4 or 8 h resulted in an approximately 2-fold increase of ChREBP promoter activity (P < 0.05 to P < 0.01). Insulin (10 nM) also stimulated endogenous ChREBP expression in HepG2 and primary hamster hepatocytes. More importantly, we found that the stimulatory effect of insulin on ChREBP promoter activity was dependent on the presence of the POU binding site, and insulin treatment reduced Oct-1 expression levels. Our observations therefore identify Oct-1 as a transcriptional repressor of ChREBP and suggest that insulin stimulates ChREBP expression via attenuating the repressive effect of Oct-1.
Department of Physiology, University of Toronto, Toronto, Canada.
Adam S
AS
Sirek
Ling
L
Liu
Mark
M
Naples
Khosrow
K
Adeli
Dominic S
DS
Ng
Tianru
T
Jin
D000818:Animals; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors / Q000235:genetics*; D001665:Binding Sites / Q000235:genetics; D015153:Blotting, Western; D002460:Cell Line; D045744:Cell Line, Tumor; D047369:Chromatin Immunoprecipitation; D006224:Cricetinae; D024202:Electrophoretic Mobility Shift Assay; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology*; D016086:Lentivirus / Q000235:genetics; D050811:Octamer Transcription Factor-1 / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D050809:POU Domain Factors / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D007004:Hypoglycemic Agents; D007328:Insulin; C116385:MLXIPL protein, human; D050811:Octamer Transcription Factor-1; D050809:POU Domain Factors