Regulation of lipogenic enzyme and phospho enol pyruvate carboxykinase gene expression in cultured white adipose tissue

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.

Section: Fig 4 Putative Chore Of Acc Pi Promoter Binds To Chorfmentioning

confidence: 56%

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

O’Callaghan

Koo

et al. 2001

“…We have already shown that activation of PKA in adipose tissue counteracted the glucose effect on FAS gene expression (26). Similarly, activation of PKA inhibited glucose-induced expression of the S14 (27) and L-PK (28) genes, although for the former, it has been attributed to an effect on glucose metabolism (9) and for the latter to an effect on glucokinase activity.…”

Section: Resultsmentioning

confidence: 99%

AMP-activated Protein Kinase Inhibits the Glucose-activated Expression of Fatty Acid Synthase Gene in Rat Hepatocytes

Foretz¹,

Carling²,

Guichard³

et al. 1998

Self Cite

232

172

Although it is now clearly established that a number of genes involved in glucose and lipid metabolism are up-regulated by high glucose concentrations in both liver and adipose tissue, the signaling pathway arising from glucose to the transcriptional machinery is still poorly understood. We have analyzed the regulation of fatty acid synthase gene expression by glucose in cultured rat hepatocytes. Glucose (25 mM) induces an activation of the transcription of the fatty acid synthase gene, and this effect is markedly reduced by incubation of the cells with okadaic acid, an inhibitor of protein phosphatases 1 and 2A. A similar reduction in glucoseactivated fatty acid synthase gene expression is obtained by incubation with 5-amino-imidazolecarboxamide riboside, a cell-permeable activator of the AMPactivated protein kinase. Taken together, these results indicate that the glucose-induced expression of the fatty acid synthase gene involves a phosphorylation/dephosphorylation mechanism and suggest that the AMP-activated protein kinase plays an important role in this process. This is the first evidence that implicates the AMP-activated protein kinase in the regulation of gene expression. AMP-activated protein kinase is the mammalian analog of SNF1, a kinase involved in yeast in the transcriptional regulation of genes by glucose.Although it is now clearly established that a number of genes involved in glucose and lipid metabolism are up-regulated by high glucose concentrations in both liver and adipose tissue (for review, see Refs. 1 and 2), the signaling pathway arising from glucose to the transcriptional machinery is still poorly understood. There is general agreement on the fact that glucose has to be metabolized to stimulate the transcription of lipogenicrelated genes such as L-pyruvate kinase, fatty acid synthase (FAS), 1 acetyl-CoA carboxylase (ACC), and S14 (3-5). In previous papers, we have proposed that glucose-6-phosphate could be the signal metabolite for the glucose-induced FAS transcription (3, 5, 6) and therefore for other genes belonging to the same class. This hypothesis was challenged by Doiron et al. (7) who proposed that the signal metabolite was xylulose-5-phosphate. However, whatever the metabolite, the link between the glucose signal and the activation of gene transcription remains unknown.Phosphorylation/dephosphorylation processes are one of the major mechanisms involved in the regulation of glucose and lipid metabolism both at the cellular and molecular levels in eukaryotic cells. It is now well established that many transcription factors have their activity regulated by phosphorylation through a modification of their DNA binding activity, their transactivating capacity, or their subcellular localization (8).In cultured hepatocytes, it has been shown that okadaic acid, an inhibitor of phosphatases 1 and 2A led to the inhibition of glucose stimulation of S14 gene transcription (9). Further experiments with calyculin, a much more potent inhibitor of protein phosphatase 1 than okadaic acid, suggest th...

“…The expression of the genes for several other proteins is regulated by glucose; these include genes for L-type pyruvate kinase (13,14), fatty-acid synthase (15,16), PEPCK (17,18), and the type 2 glucose transporter (GLUT-2) (19,20). The mechanism by which glucose regulates the expression of these genes remains largely unknown.…”

Section: From the Department Of Nutrition Case Western Reserve Univementioning

confidence: 99%

Regulation of the Glucose-6-phosphatase Gene by Glucose Occurs by Transcriptional and Post-transcriptional Mechanisms

Massillon¹

2001

To understand how glucose regulates the expression of the glucose-6-phosphatase gene, the effect of glucose was studied in primary cultures of rat hepatocytes. Glucose-6-phosphatase mRNA levels increased about 10-fold when hepatocytes were incubated with 20 mM glucose. The rate of transcription of the glucose-6-phosphatase gene increased about 3-fold in hepatocytes incubated with glucose. The half-life of glucose-6-phosphatase mRNA was estimated to be 90 min in the absence of glucose and 3 h in its presence. Inhibition of the oxidative and the nonoxidative branches of the pentose phosphate pathway blocked the stimulation of glucose-6-phosphatase expression by glucose but not by xylitol or carbohydrates that enter the glycolytic/gluconeogenic pathways at the level of the triose phosphates. These results indicate that (i) the glucose induction of the mRNA for the catalytic unit of glucose-6-phosphatase occurs by transcriptional and post-transcriptional mechanisms and that (ii) xylitol and glucose increase the expression of this gene through different signaling pathways.Glc-6-Pase 1 (EC 3.1.3.9) is a multicomponent protein complex comprising catalytic and transporting entities (1-5). The complex is tightly associated with the endoplasmic reticulum, and the enzymatic component catalyzes the hydrolysis of glucose 6-phosphate to glucose, a final common step to both the pathways of glycogenolysis and gluconeogenesis. Hepatic Glc-6-Pase activity is effectively regulated by hormonal and nutritional status. For example, fasting and hormones that increase cAMP concentration stimulate its gene expression while refeeding and insulin decrease it (1, 2, 6 -12).The expression of the genes for several other proteins is regulated by glucose; these include genes for L-type pyruvate kinase (13, 14), fatty-acid synthase (15, 16), PEPCK (17, 18), and the type 2 glucose transporter (GLUT-2) (19,20). The mechanism by which glucose regulates the expression of these genes remains largely unknown. Recently, Kahn and colleagues (21, 22) have proposed a signaling pathway model to explain the molecular mechanism by which glucose regulates the expression of the L-type pyruvate kinase. This model consists of the following details: (i) the presence of glucose is sensed in the cell; (ii) this information is transduced by intracellular messengers; (iii) a second messenger, presumably xylulose 5-phosphate, rises and then modulates the activity of protein kinase and protein phosphatase involved in a cascade of phosphorylation/dephosphorylation. This cascade then leads to a modification of the phosphorylation state of the glucoseresponsive complex, followed by an increase in the transcriptional rate of the target gene. In this model, the presence of an active glucokinase that phosphorylates glucose to glucose 6-phosphate (Glc-6-P) is paramount.The molecular mechanism by which glucose regulates the expression of the Glc-6-Pase gene is currently unknown. Here we show that glucose regulation of the expression of this gene involves metabolism of glucose...