O‐GlcNAcylation Links ChREBP to Glucose Sensing in Liver and Pancreatic β Cells

ChREBP ( Carbohydrate Responsive Element Binding Protein ) has emerged as a major molecular determinant of lipogenic gene expression in response to glucose in liver cells. However, the mechanisms of ChREBP activation by glucose remain complex. We identified that ChREBP is modified by post‐translational modifications including acetylation and O‐ GlcNAcylation in response to glucose. O‐GlcNAcylation is a reversible post‐translational modification that depends on glucose concentration and on its flux through the hexosamine biosynthetic pathway (HBP). O‐GlcNAcylation is a highly dynamic process, controlled by the action of two enzymes: the O‐GlcNAc transferase (OGT), which transfers the monosaccharide to serine/threonine residues on a target protein, and the O‐GlcNAcase (OGA), which hydrolyses the sugar. We provided evidence that transcriptional activity of ChREBP is regulated by O‐GlcNAcylation in liver cells. We observed that O‐GlcNAcylation of ChREBP in liver stabilizes the ChREBP protein and increases its transcriptional activity toward its target glycolytic and lipogenic genes. Interestingly, reducing ChREBP O‐GlcNAcylation levels via OGA overexpression decreased lipogenic protein content, prevented hepatic steatosis and improved the lipidic profile of obese db/db mice. Our results revealed that in liver, O‐GlcNAcylation of ChREBP represents a novel and important regulatory mechanism of hepatic metabolism under physiological and pathological situations. Results we recently obtained suggest that ChREBP is also O‐GlcNAcylated in pancreatic β‐cells in response to high‐glucose concentrations, suggesting a potential role of this transcription factor in pancreatic dysfunctions associated with chronic hyperglycemia. Several lines of evidence suggest that O‐GlcNAcylation may also participate to pancreatic glucotoxicity. β‐pancreatic alterations in diabetes involve marked oxidative stress due to the fact that β‐pancreatic cells are poorly equipped in anti‐oxidant defence. One major mediator of the deleterious effects of hyperglycemia and oxidative stress in β‐pancreatic cells is TxNIP (Thioredoxin‐interacting protein). TxNIP favours the production of ROS through its inhibitory action of thioredoxin (TRX), a key protein in the anti‐oxidative response. By interacting with NLRP3, TxNIP stimulates a cascade of activation of the inflammasome, resulting in the secretion of the pro‐inflammatory cytokine IL1β. Overexpression of TxNIP in INS1 rat pancreatic β‐cells increases their susceptibility to apoptosis whereas TxNIP deficient islets are protected from glucose‐induced apoptosis. Interestingly, TxNIP is a direct target of ChREBP. Our data indicate that O‐GlcNAcylation of ChREBP in pancreatic β cells increases its transcriptional activity toward TxNIP. In addition, TxNIP is itself modified by O‐GlcNAcylation in response to high glucose concentrations, resulting in a modification of its interaction with NLRP3. In this context, our results suggest that O‐GlcNAcylation of ChREBP and TxNIP in β cells may contribute to glucotoxicity, inflammasome activation and β‐cell apoptosis.