Polyunsaturated Fatty Acyl Coenzyme A Suppress the Glucose-6-phosphatase Promoter Activity by Modulating the DNA Binding of Hepatocyte Nuclear Factor 4α

Abstract

Glucose-6-phosphatase confers on gluconeogenic tissues the capacity to release endogenous glucose in blood. The expression of its gene is modulated by nutritional mechanisms dependent on dietary fatty acids, with specific inhibitory effects of polyunsaturated fatty acids (PUFA). The presence of consensus binding sites of hepatocyte nuclear factor 4 (HNF4) in the ؊1640/؉60 bp region of the rat glucose-6-phosphatase gene has led us to consider the hypothesis that HNF4␣ could be involved in the regulation of glucose-6-phosphatase gene transcription by long chain fatty acid (LCFA). Our results have shown that the glucose-6-phosphatase promoter activity is specifically inhibited in the presence of PUFA in HepG2 hepatoma cells, whereas saturated LCFA have no effect. In HeLa cells, the glucose-6-phosphatase promoter activity is induced by the co-expression of HNF4␣ or HNF1␣. PUFA repress the promoter activity only in HNF4␣-cotransfected HeLa cells, whereas they have no effects on the promoter activity in HNF1␣-cotransfected HeLa cells. From gel shift mobility assays, deletion, and mutagenesis experiments, two specific binding sequences have been identified that appear able to account for both transactivation by HNF4␣ and regulation by LCFA in cells. The binding of HNF4␣ to its cognate sites is specifically inhibited by polyunsaturated fatty acyl coenzyme A in vitro. These data strongly suggest that the mechanism by which PUFA suppress the glucose-6-phosphatase gene transcription involves an inhibition of the binding of HNF4␣ to its cognate sites in the presence of polyunsaturated fatty acyl-CoA thioesters.Glucose-6-phosphatase (Glc6Pase 1 ; EC 3.1.3.9) confers on gluconeogenic tissues, i.e. the liver, the kidney, and the small intestine, the capacity to release endogenous glucose in blood (1, 2). The expression of its gene is increased during diabetes and fasting and normalized upon insulin treatment and refeeding, respectively, in all three gluconeogenic tissues (3,4). An increase in the Glc6Pase flux (5, 6) and maximal velocity (7) has also been strongly suggested to account for increased glucose production and hepatic insulin resistance in type 2 diabetes mellitus.The Glc6Pase gene expression is also modulated by nutritional mechanisms dependent on dietary fatty acids. In the liver of rats, Glc6Pase mRNA and protein contents are increased upon high fat feeding (8) and upon elevation in plasma fatty acid levels (9). Under these nutritional conditions, the suppression of hepatic glucose production by insulin is impaired (10). This suggests that a high plasma fatty acid level may contribute increased production of glucose via increased expression of Glc6Pase, resulting in the development of liver insulin resistance (9,11,12). In vitro, the treatment of fetal hepatocytes with a high concentration (500 M) of long chain fatty acids (LCFA), such as oleic and linoleic acids, increases the Glc6Pase mRNA content (13). We have shown that the likely mechanism involves a stabilizing effect on Glc6Pase mRNA (13). On the other han...

Signalling cross-talk between hepatocyte nuclear factor 4α and growth-hormone-activated STAT5b

“…ligandindependent) nuclear receptor. Structural studies have revealed the presence of a previously unrecognized fatty acid docked in the ligand-binding domain of HNF4α [31,32], consistent with the finding that fatty acids regulate HNF4α-dependent gene expression [52,53]. Fatty acids may also modulate STAT5 activity [54,55].…”

Global gene expression analysis in liver of obese diabetic db/db mice treated with metformin

“…G6pc gene expression was observed in an in vivo study to be modulated by polyunsaturated fatty acids [37]. Interestingly, as shown in Table 1, a transcription regulatory protein, Nr0b2, that regulates gluconeogenic genes [31], was down-regulated by metformin treatment, as observed in the case of G6pc.…”

“…We also observed that similar serial administration of metformin caused changes in similar metabolic pathways including glycolysis-gluconeogenesis and fatty acid metabolism (data not shown). Changes in the fatty acid and glycerolipid metabolism pathways may contribute to an improvement of lipid metabolism by repeated treatment with metformin, while some fatty acids might be important in modulating mRNA expression of genes such as G6pc [37]. Interestingly, changes of expression in the bile acid biosynthesis, but not the sterol biosynthesis pathway, were detected; indeed, coordinated transcriptional regulation between bile acids and gluconeogenesis [39,40], as well as suppression of gluconeogenenic genes by bile acid [31], have been reported recently.…”

n-3 long chain polyunsaturated fatty acids: a nutritional tool to prevent insulin resistance associated to type 2 diabetes and obesity?

“…When fish oil is substituted for saturated or polyunsaturated n-6 in the high fat diet, liver insulin resistance does not occur. N-3 LC-PUFA decrease the activity and the expression of glucose-6-phosphatase, the last enzyme responsible for liver glucose output [39,40]. Such an inhibiting effect of n-3 LC-PUFA on G6Pase activity could overcome its overactivity and help to restore normal inhibition of hepatic glucose production.…”