Replacing dietary glucose with fructose increases ChREBP activity and SREBP-1 protein in rat liver nucleus

doi:10.1016/j.bbrc.2009.09.109

Biochemical and Biophysical Research Communications

Volume 390, Issue 2, 11 December 2009, Pages 285-289

https://doi.org/10.1016/j.bbrc.2009.09.109 Get rights and content

Abstract

Diets high in fructose cause hypertriglyceridemia and insulin resistance in part due to simultaneous induction of gluconeogenic and lipogenic genes in liver. We investigated the mechanism underlying the unique pattern of gene induction by dietary fructose. Male Sprague–Dawley rats (n = 6 per group) were meal-fed (4 h/d) either 63% (w/w) glucose or 63% fructose diet. After two weeks, animals were killed at the end of the last meal. Nuclear SREBP-1 was 2.2 times higher in fructose-fed rats than glucose-fed rats. Nuclear FoxO1 was elevated 1.7 times in fructose group, but did not reach significance (P = 0.08). Unexpectedly, no difference was observed in nuclear ChREBP between two groups. However, ChREBP DNA binding was 3.9× higher in fructose-fed animals without an increase in xylulose-5-phospate, a proposed ChREBP activator. In conclusion, the gene induction by dietary fructose is likely to be mediated in part by simultaneously increased ChREBP activity, SREBP-1 and possibly FoxO1 protein in nucleus.

Introduction

A diet high in fructose induces metabolic syndrome including insulin resistance, hypertriglyceridemia and hypertension in animal models [1], and shows similar effects in humans [2]. Liver is the major site of fructose metabolism [3]. Because fructose enters the glycolytic/gluconeogenic pathway as trioses in liver, metabolizing fructose requires simultaneous activation of part of glycolysis, de novo lipogenesis, part of gluconeogenesis and glycogen synthesis. In our previous study, we have shown that this unique metabolic condition in high fructose intake is achieved in part by modulation of gene expressions involved in carbohydrate and lipid metabolism, including increased pyruvate kinase (PK), fatty acid synthase (FAS) and glucose-6-phosphatase (G6Pase), but not phosphoenolpyruvate carboxykinase (PEPCK) [4]. This simultaneous induction of lipogenic genes and G6Pase is a salient feature when dietary glucose is replaced with fructose. However, the molecular mechanism of how a diet high in fructose exerts these metabolic adaptations is not well understood.

Sterol regulatory element-binding protein-1c (SREBP-1c) induces an entire program of de novo lipogenesis, and is considered to mediate insulin action in response to carbohydrate ingestion [5]. Moreover, SREBP-1c decreases PEPCK transcription by displacing Sp1from the PEPCK promoter in response to insulin [6]. This induction of lipogenic genes and suppression of PEPCK suggest that SREBP-1c might mediate fructose effects on gene expression.

An increase in both insulin and glucose concentrations is required for most glycolytic/lipogenic genes to be fully expressed in the liver. Carbohydrate response element-binding protein (ChREBP) is activated in high glucose and binds to carbohydrate response element (ChRE) [7]. ChREBP upregulates lipogenic genes such as PK [7], FAS [8], acetyl-CoA carboxylase (ACC) [9] and possibly stearoyl-CoA desaturase 1 (SCD1) [10] in response to high glucose. Paradoxically, G6Pase gene also has ChRE [11], and is up-regulated by high glucose in cultured hepatocytes [12]. In our previous study, dietary fructose induced PK, FAS and G6Pase genes [4], all of which possess ChRE in their promoter region, suggesting ChREBP may mediate the fructose effects on these genes. Furthermore, mice with ChREBP gene deletion displayed severe sucrose/fructose intolerance, suggesting a critical role of ChREBP in fructose metabolism [13].

A transcription factor, forkhead box O1 (FoxO1) activates both PEPCK and G6Pase genes by binding to an insulin response element (IRE) [14]. FoxO1 is phosphorylated by Akt and translocated to the cytoplasm in response to insulin, resulting in inactivation of FoxO1 [14]. A study with hamsters, suggested that a diet high in fructose may increase FoxO1 in liver although diet and feeding were not well controlled in the study [15]. Thus, FoxO1 may also mediate effects of dietary fructose on hepatic gene expression.

The present study was undertaken to elucidate the mechanism underlying the unique effects of dietary fructose on gene expression in liver by determining mRNA, proteins and activity of three key transcription factors, SREBP, ChREBP, and FoxO1 under well defined dietary and feeding conditions.

Section snippets

Materials and methods

Animals and diets. Twelve Sprague–Dawley male rats (∼90 g) were purchased from Harlan (Indianapolis, IN). The animals were housed in individual cages and maintained under standard conditions with controlled temperature (22 ± 2 °C) and a 12-h light/dark cycle. Water was provided ad libitum. Before treatment, rats were acclimated for 2 days and then were adapted to a 4-h (8 AM to 12 PM) meal feeding protocol for one week with a standard laboratory diet. The meal feeding protocol was used to

Dietary fructose increased mRNAs of lipogenic enzymes and G6Pase

ChREBP and SREBP-1 are known to activate transcription of various lipogenic enzymes, while FoxO1 is a key inducer of both G6Pase and PEPCK. We quantified the expression of mRNAs encoding lipogenic enzymes that are targets of ChREBP and/or SREBP-1 (Fig. 1A). Consistent with our previous study [4], the fructose group showed higher gene expression of lipogenic enzymes than the glucose group after two weeks (Fig. 1A). Fig. 1B shows differential expression of G6Pase and PEPCK by dietary fructose

Discussion

The results of this study indicate that dietary fructose when replaced for glucose increases activity of three transcription factors, SREBP-1, ChREBP, and FoxO1 although the nuclear abundance of FoxO1 did not reach statistical significance. Insulin is known to increase SREBP-1c activity [5], whereas it inhibits FoxO1 activity [23]. Thus, the simultaneous activation of SREBP-1 and FoxO1 by fructose suggests that the insulin effect on FoxO1 was specifically attenuated by fructose feeding, while

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Replacing dietary glucose with fructose increases ChREBP activity and SREBP-1 protein in rat liver nucleus

Abstract

Introduction

Section snippets

Materials and methods

Dietary fructose increased mRNAs of lipogenic enzymes and G6Pase

Discussion

Biochim. Biophys. Acta

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Lipid. Res.

J. Biol. Chem.

J. Nutr.

J. Biochem. Biophys. Methods

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Fructose-induced insulin resistance and hypertension in rats

Hypertension