Cracking the O-GlcNAc code in metabolism

Abstract

Nuclear, cytoplasmic, and mitochondrial proteins are extensively modified by O-linked β-N-acetylglucosamine (O-GlcNAc) moieties. This sugar modification regulates fundamental cellular processes in response to diverse nutritional and hormonal cues. The enzymes O-GlcNAc transferase (OGT) and O-linked β-N-acetylglucosaminase (O-GlcNAcase) mediate the addition and removal of O-GlcNAc, respectively. Aberrant O-GlcNAcylation has been implicated in a plethora of human diseases, including diabetes, cancer, aging, cardiovascular disease, and neurodegenerative disease. Because metabolic dysregulation is a vital component of these diseases, unraveling the roles of O-GlcNAc in metabolism is of emerging importance. Here, we review the current understanding of the functions of O-GlcNAc in cell signaling and gene transcription involved in metabolism, and focus on its relevance to diabetes, cancer, circadian rhythm, and mitochondrial function.

Figure 1. Schematic diagram of OGT and OGA isoforms

The ogt gene produces multiple OGT isoforms through alternative splicing and multiple start codons [109]. The longest and shortest isoforms (ncOGT and sOGT) are found in both the nucleus and the cytoplasm, whereas an atypical isoform (mOGT) targets the inner mitochondrial membrane [96]. These isoforms are distinct in the number of TPR at the N-terminus. They have a common C-terminal region comprised of the catalytic domains I&II (CDI&II) and the phosphoinositide-binding domain (PPO). The canonical OGA is a nucleocytoplasmic enzyme that contains the N-terminal O-GlcNAc cleavage domain, the OGT-binding region, and the C-terminal histone acetyltransferase (HAT)-like domain. A splicing variant lacking the HAT-like domain appears to reside in the endoplasmic reticulum and lipid droplets [110].

Figure 2. O-GlcNAc regulation of insulin signaling

On binding to insulin, the auto-phosphorylated IR catalyzes tyrosine phosphorylation of IRS proteins, which results in the docking and activation of phosphatidylinositol-3-OH kinase (PI3K). PI3K produces phosphatidylinositol 3,4,5-triphosphate (PIP₃), which recruits PDK1 and AKT to the plasma membrane. AKT activated by PDK1 phosphorylates numerous substrates to mediate physiological functions. Subsequently, PIP₃-binding OGT attenuates insulin signaling by O-GlcNAcylating IR, IRS, PDK1, and AKT. AKT phosphorylates AS160, a Rab GAP, to mediate the translocation of the glucose transporter GLUT4 to the membrane. Insulin also activates PKCζ/λ to stimulate the trafficking of GLUT4 vesicles by actin remodeling. O-GlcNAcylation suppresses GLUT4 trafficking by inhibiting Munc18c and possibly regulating PKCζ/λ and actin. O-GlcNAcylation antagonizes insulin's suppression of gluconeogenesis by activating transcription factor and cofactors such as FOXO1, PGC-1α, and CRTC2. SREBP-1c and ChREBP are two key transcription factors that induce expression of lipogenic genes. O-GlcNAcylation regulates lipogenesis by directly stabilizing ChREBP and promoting Srebp-1c transcription through LXR activation. O-GlcNAcylation of GS suppresses glycogen synthesis. The possible role of O-GlcNAcylated GSK3β in glycogen storage has not been explored. In pancreatic β-cells, O-GlcNAcylated Pdx-1 and NeuroD1 promote transcription of the insulin gene. Red and black hexagons containing letter G indicate positive and negative regulation of the proteins by O-GlcNAcylation, respectively. Grey hexagons note that the role of O-GlcNAcylation is unknown.

Figure 3. Regulation of cancer metabolism by the hexosamine/O-GlcNAc pathway

In normal cells, p53 suppresses glycolysis by inducing expression of TIGAR and inhibiting expression of glycolytic genes such as hexokinase II, phosphoglycerate mutase, and glucose transporter 3 (GLUT3). p53 also inhibits the diversion of glucose flux to the anabolic pentose phosphate pathway (PPP) by binding to and inhibiting glucose-6-phosphate dehydrogenase (G6PD). Loss-of-function of p53 in tumor cells increases glucose uptake, aerobic glycolysis, and PPP flux. PPP flux produces reducing agents for macromolecule biosynthesis and reactive oxygen species (ROS) neutralization. 2∼5% glucose is shunted to the HBP that produce UDP-GlcNAc, a substrate for OGT. OGT inhibits ubiquitin-mediated degradation of p53 by direct O-GlcNAcylation. OGT also potentiates NF-κB signaling by O-GlcNAcylation and activation of IKKβ, which triggers phosphorylation of IκBα and releasing of NF-κB to the nucleus to promote transcription of glucose transporters and HIF-1. HIF-1 increases expression of glycolytic genes to promote tumor growth. Finally, O-GlcNAcylation of the critical glycolytic enzyme PFK-1 reduces its enzyme activity (red arrow) and redirects glucose flux through the PPP, thereby conferring a selective growth advantage on cancer cells.