Glucose Activates ChREBP by Increasing Its Rate of Nuclear Entry and Relieving Repression of Its Transcriptional Activity

Abstract

Carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that activates genes involved in de novo lipogenesis in mammals. The current model for glucose activation of ChREBP proposes that increased glucose metabolism triggers a cytoplasmic to nuclear translocation of ChREBP that is critical for activation. However, we find that ChREBP actively shuttles between the cytoplasm and nucleus in both low and high glucose in the glucose-sensitive ␤ cell-derived line, 832/13. Glucose stimulates a 3-fold increase in the rate of ChREBP nuclear entry, but trapping ChREBP in the nucleus by mutagenesis or with a nuclear export inhibitor does not lead to constitutive activation. In fact, mutational studies targeting the nuclear export signal of ChREBP also identified a distinct function essential for glucose-dependent transcriptional activation. From this, we conclude that an additional event independent of nuclear translocation is required for activation. The N-terminal segment of ChREBP (amino acids 1-298) has previously been shown to repress activity under basal conditions. This segment has five highly conserved regions, Mondo conserved regions 1-5 (MCR1 to -5). Based on activating mutations in MCR2 and MCR5, we propose that these two regions act coordinately to repress ChREBP in low glucose. In addition, other mutations in MCR2 and mutations in MCR3 were found to prevent glucose activation. Hence, we conclude that both relief of repression and adoption of an activating form are required for ChREBP activation.The mammalian liver plays a critical role in maintaining energy homeostasis of an organism in response to its dietary state. When food is abundant, excess dietary carbohydrates are converted to triglycerides in the liver through the pathway of de novo lipogenesis for long term energy storage. Lipogenic enzymes, such as L-type pyruvate kinase (1), acetyl-CoA carboxylase (2), fatty acid synthase (3), and stearoyl-CoA desaturase (4), involved in the conversion of glucose to triglycerides are induced upon feeding of a high carbohydrate diet. Transcriptional induction of these genes requires signals from insulin, acting through sterol response element-binding protein-1c (5-8), and a second signaling pathway initiated in response to increased metabolism of simple carbohydrates, such as glucose (9 -12). Lipogenic genes responsive to glucose contain a DNA element called the carbohydrate response element (ChoRE) 2 (13-17). The ChoRE consists of two E box sequences (CACGTG) separated by 5 base pairs and serves as the recognition site for two heterodimeric transcription factors: carbohydrate response element-binding protein (ChREBP) and Maxlike protein X (Mlx) (18 -22). Both ChREBP and Mlx are required for binding to the ChoRE, but recent evidence establishes ChREBP as the direct target of glucose signaling. ChREBP is highly expressed in glucose-responsive tissues, such as the liver, adipose, and pancreas, whereas Mlx expression is ubiquitous (23-25). High carbohydrate-fed ChREBP Ϫ/Ϫ mice...

Treatment with the 5-HT3 antagonist tropisetron modulates glucose-induced obesity in mice

“…33 Specifically, a diet rich in glucose has been shown to activate hepatic ChREBP in animal models leading to an increased expression of enzymes involved in the formation of triglycerides. 34,35 In this study, we confirmed these results and showed an elevated mRNA expression of ChREBP associated with increased levels of FAS and triglycerides in the liver of mice fed a 30% glucose solution. It is interesting that, such changes were not found in the livers of mice fed glucose concomitantly treated with tropisetron further, suggesting that tropisetron may prevent the metabolism of glucose into triglycerides in the liver.…”

Glucose-stimulated Expression of Txnip Is Mediated by Carbohydrate Response Element-binding Protein, p300, and Histone H4 Acetylation in Pancreatic Beta Cells

“…Some ChREBP binding to the txnip promoter was even observed at low glucose, but upon stimulation with high glucose, ChREBP binding increased 3-fold, which was also associated with a 2-fold increase in Pol II occupancy. These results are consistent with the finding that ChREBP shuttling from the cytoplasm to the nucleus occurs at both low and high glucose in beta cells but that high glucose leads to a 3-fold increase in the rate of its nuclear entry (33) and results in the required dephosphorylation steps (34). Glucose-induced ChREBP recruitment displayed a dose-and time-dependent pattern, and the very rapid increase of ChREBP binding to the txnip promoter observed within 15 min of exposure to high glucose (Fig.…”

Endurance Training Activates Pancreatic Islets Amp-Activated Kinase-Uncoupling Protein 2 Pathway and Reduces Insulin Secretion

“…Moreover, exerciseinduced decreased blood glucose could alter b-cell programming, favoring its function and survival. It was demonstrated that glucose activates the carbohydrate response elementbinding protein (ChREBP; Davies et al 2008), and ChREBP In addition to the endurance exercise-induced enhanced AMPK-UCP2 signaling pathway, our results demonstrated that this effect is dependent on the exercise frequency, as observed by the graduated response seen in different experimental groups. Although exercise-induced whole body insulin sensitivity changes could influence nutrient control of pancreatic islet adaptations, the observed dose-response effect may also indicate skeletal muscle and pancreatic islet crosstalk cytokine modulation.…”