Glucose Regulation of the Acetyl-CoA Carboxylase Promoter PI in Rat Hepatocytes

Abstract

The rat acetyl-CoA carboxylase (ACC) ␣ gene is transcribed from two promoters, denoted PI and PII, that direct regulated expression in a tissue-specific manner. Induction of ACC, the rate-controlling enzyme of fatty acid biosynthesis, occurs in the liver in response to feeding of a high carbohydrate, low fat diet, conditions that favor enhanced lipogenesis. This induction is mainly due to increases in PI promoter activity. We have used primary cultured hepatocytes from the rat to investigate glucose regulation of ACC expression. Glucose and insulin synergistically activated expression of ACC mRNAs transcribed from the PI promoter with little or no effect on PII mRNAs. Glucose treatment stimulated PI promoter activity in transfection assays and a glucose-regulated element was identified (؊126/؊102), homologous to those previously described in other responsive genes, including L-type pyruvate kinase, S 14 and fatty acid synthase. Mutation of this element eliminated the response to glucose. This region of the ACC PI promoter was able to bind a liver nuclear factor designated ChoRF that interacts with other conserved glucose-regulated elements. This ACC PI element is also capable of conferring a strong response to glucose when linked to a heterologous promoter. We conclude that induction of ACC gene expression under lipogenic conditions in hepatocytes is mediated in part by the activation of a glucose-regulated transcription factor, ChoRF, which stimulates transcription from the PI promoter. Similar mechanisms operate on related genes permitting the coordinate induction of the lipogenic pathway.

Direct Role of ChREBP·Mlx in Regulating Hepatic Glucose-responsive Genes

“…After a 3-h attachment to 35-mm Primaria plates, cells were transduced with recombinant adenovirus for 2 h in Williams' E medium containing 5.5 mM glucose, 23 mM HEPES, 0.01 M dexamethasone, 2 mM glutamine, 50 units/ml penicillin, 50 g/ml streptomycin, 26 mM sodium bicarbonate, and 0.1 unit/ml insulin (low glucose medium). Cells were then transfected using F1 reagent (Targeting Systems, San Diego, CA) with a mixture of a firefly luciferase reporter driven by an ACC ChoREcontaining promoter region (18) and an internal control plasmid encoding Renilla luciferase (pRL-CMV, Promega). In some experiments expression plasmids for ChREBP, Mlx, or MondoA were cotransfected with the reporter plasmid mixture.…”

“…To test their effect on a glucose-responsive promoter, increasing amounts of either dominant negative Mlx virus were transduced into hepatocytes. Subsequently, cells were transfected with a reporter containing multiple copies of the ChoRE from the ACC promoter region (18). Because this reporter plasmid contains only the ChoRE element and the basal promoter, it provides a direct readout for the glucose-responsive transcription factor.…”

“…However, the intracellular mechanism of the glucose-signaling pathway is not fully understood. A carbohydrate response element (ChoRE) that mediates the transcriptional response to glucose has been mapped within the promoter regions in several lipogenic enzyme genes (15)(16)(17)(18)(19). This element is composed of two E-box (CACGTG) or E-box-like sequences that are separated by 5 bp.…”

Hepatic Glucokinase Is Required for the Synergistic Action of ChREBP and SREBP-1c on Glycolytic and Lipogenic Gene Expression

“…Indeed, SREBP-1c, which is induced by insulin in primary cultures of hepatocytes (13), is able to induce lipogenic genes by its capacity to bind to the sterol regulatory element (SRE) present in their promoters (14 -16). Since a glucose-or carbohydrate-response element (ChoRE) was also identified in both glycolytic and lipogenic genes through promoter mapping analysis (17)(18)(19), it remains to be determined whether glucose metabolism, which could account by itself for the induction of these genes, is necessary for the transcriptional effect of SREBP-1c. In addition, the recent identification of a glucoseresponsive transcription factor named ChREBP (carbohydrate responsive element-binding protein) (20) has shed light on the possible mechanism whereby glucose affects gene transcription and further supports the hypothesis that glucose acts through a distinct, but synergic, signaling pathway through insulin to regulate glycolytic and lipogenic gene expression in liver.…”

Hormones and Nutrients Regulate Acetyl-CoA Carboxylase Promoter I in Rat Primary Hepatocytes

“…The major signal controlling ACC gene expression has been reported to be glucose (11,15). Although an intrinsic repressor element has been described (1), the role of this element in the regulation of the PI promoter is unknown.…”