Functional Study of O-GlcNAcylation on ULK1 under glucose deprivation

Abstract

O-GlcNAcylation is a dynamic posttranslational modification that occurs in the serine or threonine residues of nuclear and cytosolic proteins. O-GlcNAcylation regulates various cellular events, such as a nutrient sensing, epigenetic regulation, translational regulation, and cell proliferation, as well as disease-related signal pathways. UDP-GlcNAc, a precursor of O-GlcNAcylation, is synthesized from glucose by the hexosamine biosynthetic pathway (HBP). The level of UDP-GlcNAc depends on the cellular glucose condition. For this reason, O-GlcNAcylation is called a nutrient sensor. Although O-GlcNAcylation plays a role in nutrient sensing and metabolism, the relationship between O-GlcNAcylation and autophagy remains unknown. In previous studies, decreased O-GlcNAcylation by OGT siRNA induced autophagy without starvation. In this study, we observed that a unc-51 like autophagy activating kinase 1(ULK1), a regulator of autophagy, is O-GlcNAcylated. Under glucose deprivation, O-GlcNAcylated ULk1 is increased, and increased O-GlcNAcylation induces ULK1 S757 phosphorylation, which leads to ULK1 inactivation. The ULK1 S757A mutant decreases O-GlcNAcylation on ULK1 and binding affinity with OGT compared with ULK1 wild type. These data imply that O-GlcNAcylation on ULK1 decreases ULK1 kinase activity by increasing S757 phosphorylation and reduces autophagy induction.