Requirement for O-linked N-acetylglucosaminyltransferase in lymphocytes activation

O-GlcNAc inhibits interaction between Sp1 and Elf-1 transcription factors

The novel protein modification, O-linked N-acetylglucosamine (O-GlcNAc), plays an important role in various aspects of cell regulation. Although most of nuclear transcription regulatory factors are modified by O-GlcNAc, O-GlcNAc effects on transcription remain largely undefined yet. In this study, we show that O-GlcNAc inhibits a physical interaction between Sp1 and Elf-1 transcription factors, and negatively regulates transcription of placenta and embryonic expression oncofetal protein gene (Pem). These findings suggest that O-GlcNAc inhibits Sp1-mediated gene transcription possibly by interrupting Sp1 interaction with its cooperative factor.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

O-GlcNAc modification of transcription factors, glucose sensing and glucotoxicity

Regulation of proteins by O-GlcNAc modification is becoming a major area of research. This reversible modification depends on glucose concentrations and, therefore, constitutes a powerful mechanism to regulate protein activities according to glucose availability. Its importance in glucose-dependent gene transcription is underlined by its role in pancreatic insulin biosynthesis (through PDX-1 and NeuroD1 O-GlcNAc modifications) and leptin synthesis in adipose tissue (through Sp1 O-GlcNAc modification). Moreover, in chronic hyperglycaemia, O-GlcNAc modifications of Sp1, p53 and NFkappaB participate in glucotoxicity, resulting in cardiovascular and renal alterations. The recent discovery by two independent groups that FoxO1 is regulated by O-GlcNAc modification provides a potential mechanism by which hyperglycaemia promotes gluconeogenesis and worsening of glucose intolerance, opening new research perspectives in the field.

NFkappaB activation is associated with its O-GlcNAcylation state under hyperglycemic conditions

The transcription factor NFkappaB is activated by phosphorylation and acetylation and plays important roles in inflammatory and immune responses in the cell. Additionally, posttranslational modification of the NFkappaB p65 subunit by O-linked N-acetylglucosamine (O-GlcNAc) has been reported, but the modification site of O-GlcNAc on NFkappaB p65 and its exact function have not been elucidated. In this work, we show that O-GlcNAcylation of NFkappaB p65 decreases binding to IkappaB alpha and increases transcriptional activity under hyperglycemic conditions. Also, we demonstrate that both Thr-322 and Thr-352 of NFkappaB p65 can be modified with O-GlcNAc, but modification on Thr-352, not Thr-322, is important for transcriptional activation. Our findings suggest that site-specific O-GlcNAcylation may be a reason why NFkappaB activity increases continuously under hyperglycemic conditions.

The O-linked N-acetylglucosamine modification in cellular signalling and the immune system. 'Protein modifications: beyond the usual suspects' review series

The intracellular modification of proteins by the addition of a single O-linked N-acetylglucosamine (O-GlcNAc) molecule is a ubiquitous post-translational modification in eukaryotic cells. It is catalysed by O-linked N-acetylglucosaminyltransferase, which attaches O-GlcNAc to serine/threonine residues, and it is counter-regulated by beta-N-acetylglucosaminidase, which is the antagonistic glycosidase that removes the O-GlcNAc group. O-GlcNAc modification competes with phosphorylation by protein kinases at similar sites, thereby affecting important signalling nodes. Accumulating evidence supports a central role for O-GlcNAc modifications and the corresponding enzymes in the regulation of immune cells, particularly in the activation processes of T and B lymphocytes. Here, we discuss recent advances in the field of O-GlcNAc modifications, focusing on the cells of the immune system.

Chemical approaches to understanding O-GlcNAc glycosylation in the brain

O-GlcNAc glycosylation is a unique, dynamic form of glycosylation found on intracellular proteins of all multicellular organisms. Studies suggest that O-GlcNAc represents a key regulatory modification in the brain, contributing to transcriptional regulation, neuronal communication and neurodegenerative disease. Recently, several new chemical tools have been developed to detect and study the modification, including chemoenzymatic tagging methods, quantitative proteomics strategies and small-molecule inhibitors of O-GlcNAc enzymes. Here we highlight some of the emerging roles for O-GlcNAc in the nervous system and describe how chemical tools have significantly advanced our understanding of the scope, functional significance and cellular dynamics of this modification.