Nutrient regulation of signaling and transcription

Abstract

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

Keywords: Alzheimer's disease; O-GlcNAcase; O-GlcNAcylation; O-linked N-acetylglucosamine (O-GlcNAc); O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT); cancer; diabetes; kinases; neurodegeneration; phosphorylation.

Figure 1.

The HBP links flux through major metabolic pathways, allowing O-GlcNAcylation to serve as a “rheostat” that modulates most cellular processes in response to nutrients. The biosynthesis of UDP-GlcNAc, the donor for the OGT, is directly coupled to flux through glucose, amino acid, fatty acid, and nucleotide metabolic pathways. OGT is highly sensitive to UDP-GlcNAc concentrations, both in terms of activity and selectivity. O-GlcNAcylation has extensive cross-talk with phosphorylation. Shown is the universal symbol for a rheostat, indicating that unlike phosphorylation, which is more analogous to a switch, O-GlcNAc serves in a more analog fashion as a rheostat to modulate processes in response to nutrients and stress. GFAT, glutamine:fructose-6-phosphate amidotransferase. Modified from Refs. and . This research was originally published in Annual Review of Biochemistry. Hart, G. W., Slawson, C., Ramirez-Correa, G., and Lagerlof, O. Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease. Annu. Rev. Biochem. 2011; 80:825–858 © Annual Reviews and Nature. Hart, G. W., Housley, M. P., and Slawson, C. Cycling of O-linked β-N-acetylglucosamine on nucleocytoplasmic proteins. Nature 2007; 446:1017–1022 © Springer Nature.

Figure 2.

O-GlcNAcylation serves as a nutrient sensor to modulate nearly every step in transcription. Nearly every transcription factor is O-GlcNAcylated, often at multiple sites. OGT is a polycomb gene. Assembly of the preinitiation complex requires O-GlcNAcylation of RNA polymerase II. Elongation of mRNA requires removal of O-GlcNAc from RNA polymerase II. O-GlcNAc is part of the histone code. O-GlcNAc regulates ubiquitinylation and methylation of histones. O-GlcNAc regulates DNA methylation by the TET proteins. TATA-binding protein's residence time at promoters is regulated by O-GlcNAcylation. HDAC, histone deacetylase; Pol, polymerase; TF, transcription factor; H3K4me3, histone H3 Lys-4 trimethylation. Modified from Ref. . This research was originally published in Cell Metabolism. Hardivillé, S., and Hart, G. W. Nutrient regulation of signaling, transcription, and cell physiology by O-GlcNAcylation. Cell Metab. 2014; 20:208–213. © Cell Press.

Figure 3.

Nutrients regulate cytokinesis and the cell cycle by O-GlcNAcylation. A, OGT is highly concentrated at the mid-body during the late stages of cytokinesis. B, O-GlcNAcylated proteins are enriched at the midbody and at the nascent nuclear envelope during the late stages of cytokinesis. C, overexpression of OGT causes defective cytokinesis, resulting in polyploidy. D, during the late stages of cytokinesis, OGT, OGA, protein phosphatase I (PP1c), Polo-like kinase (PLK1), and Aurora kinase B (among other proteins) are in a transient molecular complex that modifies proteins involved in cell division.

Figure 4.

O-GlcNAcylation is directly involved in etiologies of chronic diseases associated with aging. Prolonged elevation of O-GlcNAcylation contributes directly to glucose toxicity, insulin resistance, and β-cell dysfunctions in diabetes. Every cancer type studied to date has elevated O-GlcNAc cycling, and blocking O-GlcNAcylation prevents cancer progression. Decreased O-GlcNAcylation in the brain is associated with both Alzheimer's disease and Parkinson's disease.