Hepatocyte-specific O-GlcNAc transferase downregulation ameliorates nonalcoholic steatohepatitis by improving mitochondrial function

Objective: O-GlcNAcylation is a post-translational modification that directly couples the processes of nutrient sensing, metabolism, and signal transduction, affecting protein function and localization, since the O-linked N-acetylglucosamine moiety comes directly from the metabolism of glucose, lipids, and amino acids. The addition and removal of O-GlcNAc of target proteins are mediated by two highly conserved enzymes: O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase (OGA), respectively. Deregulation of O-GlcNAcylation has been reported to be associated with various human diseases such as cancer, diabetes, and cardiovascular diseases. The contribution of deregulated O-GlcNAcylation to the progression and pathogenesis of NAFLD remains intriguing, and a better understanding of its roles in this pathophysiological context is required to uncover novel avenues for therapeutic intervention. By using a translational approach, our aim is to describe the role of OGT and O-GlcNAcylation in the pathogenesis of NAFLD. Methods: We used primary mouse hepatocytes, human hepatic cell lines and in vivo mouse models of steatohepatitis to manipulate O-GlcNAc transferase (OGT). We also studied OGT and O-GlcNAcylation in liver samples from different cohorts of people with NAFLD. Results: O-GlcNAcylation was upregulated in the liver of people and animal models with steatohepatitis. Downregulation of OGT in NAFLD-hepatocytes improved diet-induced liver injury in both in vivo and in vitro models. Proteomics studies revealed that mitochondrial proteins were hyper-O-GlcNAcylated in the liver of mice with steatohepatitis. Inhibition of OGT is able to restore mitochondrial oxidation and decrease hepatic lipid content in in vitro and in vivo models of NAFLD. Conclusions: These results demonstrate that deregulated hyper-O-GlcNAcylation favors NAFLD progression by reducing mitochondrial oxidation and promoting hepatic lipid accumulation

Neddylation of phosphoenolpyruvate carboxykinase 1 controls glucose metabolism

Neddylation is a post-translational mechanism that adds a ubiquitin-like protein, namely neural precursor cell expressed developmentally downregulated protein 8 (NEDD8). Here, we show that neddylation in mouse liver is modulated by nutrient availability. Inhibition of neddylation in mouse liver reduces gluconeogenic capacity and the hyperglycemic actions of counter-regulatory hormones. Furthermore, people with type 2 diabetes display elevated hepatic neddylation levels. Mechanistically, fasting or caloric restriction of mice leads to neddylation of phosphoenolpyruvate carboxykinase 1 (PCK1) at three lysine residues—K278, K342, and K387. We find that mutating the three PCK1 lysines that are neddylated reduces their gluconeogenic activity rate. Molecular dynamics simulations show that neddylation of PCK1 could re-position two loops surrounding the catalytic center into an open configuration, rendering the catalytic center more accessible. Our study reveals that neddylation of PCK1 provides a finely tuned mechanism of controlling glucose metabolism by linking whole nutrient availability to metabolic homeostasis