Mitotic arrest with nocodazole induces selective changes in the level of O-linked N-acetylglucosamine and accumulation of incompletely processed N-glycans on proteins from HT29 cells

Growing and dividing: how O-GlcNAcylation leads the way

Cell cycle errors can lead to mutations, chromosomal instability, or death; thus, the precise control of cell cycle progression is essential for viability. The nutrient-sensing posttranslational modification, O-GlcNAc, regulates the cell cycle allowing one central control point directing progression of the cell cycle. O-GlcNAc is a single N-acetylglucosamine sugar modification to intracellular proteins that is dynamically added and removed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. These enzymes act as a rheostat to fine-tune protein function in response to a plethora of stimuli from nutrients to hormones. O-GlcNAc modulates mitogenic growth signaling, senses nutrient flux through the hexosamine biosynthetic pathway, and coordinates with other nutrient-sensing enzymes to progress cells through Gap phase 1 (G1). At the G1/S transition, O-GlcNAc modulates checkpoint control, while in S Phase, O-GlcNAcylation coordinates the replication fork. DNA replication errors activate O-GlcNAcylation to control the function of the tumor-suppressor p53 at Gap Phase 2 (G2). Finally, in mitosis (M phase), O-GlcNAc controls M phase progression and the organization of the mitotic spindle and midbody. Critical for M phase control is the interplay between OGT and OGA with mitotic kinases. Importantly, disruptions in OGT and OGA activity induce M phase defects and aneuploidy. These data point to an essential role for the O-GlcNAc rheostat in regulating cell division. In this review, we highlight O-GlcNAc nutrient sensing regulating G1, O-GlcNAc control of DNA replication and repair, and finally, O-GlcNAc organization of mitotic progression and spindle dynamics.

Gigaxonin glycosylation regulates intermediate filament turnover and may impact giant axonal neuropathy etiology or treatment

Gigaxonin (also known as KLHL16) is an E3 ligase adaptor protein that promotes the ubiquitination and degradation of intermediate filament (IF) proteins. Mutations in human gigaxonin cause the fatal neurodegenerative disease giant axonal neuropathy (GAN), in which IF proteins accumulate and aggregate in axons throughout the nervous system, impairing neuronal function and viability. Despite this pathophysiological significance, the upstream regulation and downstream effects of normal and aberrant gigaxonin function remain incompletely understood. Here, we report that gigaxonin is modified by <italic>O</italic>-linked β-<italic>N</italic>-acetylglucosamine (O-GlcNAc), a prevalent form of intracellular glycosylation, in a nutrient- and growth factor–dependent manner. MS analyses of human gigaxonin revealed 9 candidate sites of O-GlcNAcylation, 2 of which — serine 272 and threonine 277 — are required for its ability to mediate IF turnover in gigaxonin-deficient human cell models that we created. Taken together, the results suggest that nutrient-responsive gigaxonin O-GlcNAcylation forms a regulatory link between metabolism and IF proteostasis. Our work may have significant implications for understanding the nongenetic modifiers of GAN phenotypes and for the optimization of gene therapy for this disease.

O-Linked N-Acetylglucosamine Transiently Elevates in HeLa Cells during Mitosis

O-linked N-acetylglucosamine (O-GlcNAc) is a dynamic post-translational modification of serine and threonine residues on nuclear and cytoplasmic proteins. O-GlcNAc modification influences many cellular mechanisms, including carbohydrate metabolism, signal transduction and protein degradation. Multiple studies also showed that cell cycle might be modulated by O-GlcNAc. Although the role of O-GlcNAc in the regulation of some cell cycle processes such as mitotic spindle organization or histone phosphorylation is well established, the general behaviour of O-GlcNAc regulation during cell cycle is still controversial. In this study, we analysed the dynamic changes of overall O-GlcNAc levels in HeLa cells using double thymidine block. O-GlcNAc levels in G₁, S, G₂ and M phase were measured. We observed that O-GlcNAc levels are significantly increased during mitosis in comparison to the other cell cycle phases. However, this change could only be detected when mitotic cells were enriched by harvesting round shaped cells from the G₂/M fraction of the synchronized cells. Our data verify that O-GlcNAc is elevated during mitosis, but also emphasize that O-GlcNAc levels can significantly change in a short period of time. Thus, selection and collection of cells at specific cell-cycle checkpoints is a challenging, but necessary requirement for O-GlcNAc studies.

O-GlcNAc in cancer: An Oncometabolism-fueled vicious cycle

Cancer cells exhibit unregulated growth, altered metabolism, enhanced metastatic potential and altered cell surface glycans. Fueled by oncometabolism and elevated uptake of glucose and glutamine, the hexosamine biosynthetic pathway (HBP) sustains glycosylation in the endomembrane system. In addition, the elevated pools of UDP-GlcNAc drives the O-GlcNAc modification of key targets in the cytoplasm, nucleus and mitochondrion. These targets include transcription factors, kinases, key cytoplasmic enzymes of intermediary metabolism, and electron transport chain complexes. O-GlcNAcylation can thereby alter epigenetics, transcription, signaling, proteostasis, and bioenergetics, key 'hallmarks of cancer'. In this review, we summarize accumulating evidence that many cancer hallmarks are linked to dysregulation of O-GlcNAc cycling on cancer-relevant targets. We argue that onconutrient and oncometabolite-fueled elevation increases HBP flux and triggers O-GlcNAcylation of key regulatory enzymes in glycolysis, Kreb's cycle, pentose-phosphate pathway, and the HBP itself. The resulting rerouting of glucose metabolites leads to elevated O-GlcNAcylation of oncogenes and tumor suppressors further escalating elevation in HBP flux creating a 'vicious cycle'. Downstream, elevated O-GlcNAcylation alters DNA repair and cellular stress pathways which influence oncogenesis. The elevated steady-state levels of O-GlcNAcylated targets found in many cancers may also provide these cells with a selective advantage for sustained growth, enhanced metastatic potential, and immune evasion in the tumor microenvironment.

The Role of Stress-Induced O-GlcNAc Protein Modification in the Regulation of Membrane Transport

O-linked N-acetylglucosamine (O-GlcNAc) is a posttranslational modification that is increasingly recognized as a signal transduction mechanism. Unlike other glycans, O-GlcNAc is a highly dynamic and reversible process that involves the addition and removal of a single N-acetylglucosamine molecule to Ser/Thr residues of proteins. UDP-GlcNAc—the direct substrate for O-GlcNAc modification—is controlled by the rate of cellular metabolism, and thus O-GlcNAc is dependent on substrate availability. Serving as a feedback mechanism, O-GlcNAc influences the regulation of insulin signaling and glucose transport. Besides nutrient sensing, O-GlcNAc was also implicated in the regulation of various physiological and pathophysiological processes. Due to improvements of mass spectrometry techniques, more than one thousand proteins were detected to carry the O-GlcNAc moiety; many of them are known to participate in the regulation of metabolites, ions, or protein transport across biological membranes. Recent studies also indicated that O-GlcNAc is involved in stress adaptation; overwhelming evidences suggest that O-GlcNAc levels increase upon stress. O-GlcNAc elevation is generally considered to be beneficial during stress, although the exact nature of its protective effect is not understood. In this review, we summarize the current data regarding the oxidative stress-related changes of O-GlcNAc levels and discuss the implications related to membrane trafficking.

Design of glycosyltransferase inhibitors targeting human O-GlcNAc transferase (OGT)

Inhibition of glycosyltransferases requires the design of neutral inhibitors to allow cell permeation as mimicks of their natural dianionic substrates.

O-GlcNAcylation: A New Cancer Hallmark?

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a reversible post-translational modification consisting in the addition of a sugar moiety to serine/threonine residues of cytosolic or nuclear proteins. Catalyzed by O-GlcNAc-transferase (OGT) and removed by O-GlcNAcase, this dynamic modification is dependent on environmental glucose concentration. O-GlcNAcylation regulates the activities of a wide panel of proteins involved in almost all aspects of cell biology. As a nutrient sensor, O-GlcNAcylation can relay the effects of excessive nutritional intake, an important cancer risk factor, on protein activities and cellular functions. Indeed, O-GlcNAcylation has been shown to play a significant role in cancer development through different mechanisms. O-GlcNAcylation and OGT levels are increased in different cancers (breast, prostate, colon…) and vary during cell cycle progression. Modulating their expression or activity can alter cancer cell proliferation and/or invasion. Interestingly, major oncogenic factors have been shown to be directly O-GlcNAcylated (p53, MYC, NFκB, β-catenin…). Furthermore, chromatin dynamics is modulated by O-GlcNAc. DNA methylation enzymes of the Tet family, involved epigenetic alterations associated with cancer, were recently found to interact with and target OGT to multi-molecular chromatin-remodeling complexes. Consistently, histones are subjected to O-GlcNAc modifications which regulate their function. Increasing number of evidences point out the central involvement of O-GlcNAcylation in tumorigenesis, justifying the attention received as a potential new approach for cancer treatment. However, comprehension of the underlying mechanism remains at its beginnings. Future challenge will be to address directly the role of O-GlcNAc-modified residues in oncogenic-related proteins to eventually propose novel strategies to alter cancer development and/or progression.

Characterization of O-GlcNAc cycling and proteomic identification of differentially O-GlcNAcylated proteins during G1/S transition

BACKGROUND

DNA replication represents a critical step of the cell cycle which requires highly controlled and ordered regulatory mechanisms to ensure the integrity of genome duplication. Among a plethora of elements, post-translational modifications (PTMs) ensure the spatiotemporal regulation of pivotal proteins orchestrating cell division. Despite increasing evidences showing that O-GlcNAcylation regulates mitotic events, the impact of this PTM in the early steps of the cell cycle remains poorly understood.

METHODS AND RESULTS

Quiescent MCF7 cells were stimulated by serum mitogens and cell cycle progression was determined by flow cytometry. The levels of O-GlcNAc modified proteins, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA) were examined by Western blotting and OGA activity was measured during the progression of cells towards S phase. A global decrease in O-GlcNAcylation was observed at S phase entry, concomitantly to an increase in the activity of OGA. A combination of two-dimensional electrophoresis, Western blotting and mass spectrometry was then used to detect and identify cell cycle-dependent putative O-GlcNAcylated proteins. 58 cytoplasmic and nuclear proteins differentially O-GlcNAcylated through G1/S transition were identified and the O-GlcNAc variations of Cytokeratin 8, hnRNP K, Caprin-1, Minichromosome Maintenance proteins MCM3, MCM6 and MCM7 were validated by immunoprecipitation.

CONCLUSIONS

The dynamics of O-GlcNAc is regulated during G1/S transition and observed on key proteins involved in the cytoskeleton networks, mRNA processing, translation, protein folding and DNA replication.

GENERAL SIGNIFICANCE

Our results led us to propose that O-GlcNAcylation joins the PTMs that take part in the regulation of DNA replication initiation.

Vertebrate protein glycosylation: diversity, synthesis and function

Protein glycosylation is a ubiquitous post-translational modification found in all domains of life. Despite their significant complexity in animal systems, glycan structures have crucial biological and physiological roles, from contributions in protein folding and quality control to involvement in a large number of biological recognition events. As a result, they impart an additional level of 'information content' to underlying polypeptide structures. Improvements in analytical methodologies for dissecting glycan structural diversity, along with recent developments in biochemical and genetic approaches for studying glycan biosynthesis and catabolism, have provided a greater understanding of the biological contributions of these complex structures in vertebrates.

The roles of O-linked β-N-acetylglucosamine in cardiovascular physiology and disease

More than 1,000 proteins of the nucleus, cytoplasm, and mitochondria are dynamically modified by O-linked β-N-acetylglucosamine (O-GlcNAc), an essential post-translational modification of metazoans. O-GlcNAc, which modifies Ser/Thr residues, is thought to regulate protein function in a manner analogous to protein phosphorylation and, on a subset of proteins, appears to have a reciprocal relationship with phosphorylation. Like phosphorylation, O-GlcNAc levels change dynamically in response to numerous signals including hyperglycemia and cellular injury. Recent data suggests that O-GlcNAc appears to be a key regulator of the cellular stress response, the augmentation of which is protective in models of acute vascular injury, trauma hemorrhage, and ischemia-reperfusion injury. In contrast to these studies, O-GlcNAc has also been implicated in the development of hypertension and type II diabetes, leading to vascular and cardiac dysfunction. Here we summarize the current understanding of the roles of O-GlcNAc in the heart and vasculature.

In vitro metabonomic study detects increases in UDP-GlcNAc and UDP-GalNAc, as early phase markers of cisplatin treatment response in brain tumor cells

O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) is important in a number of biological processes and diseases including transcription, cell stress, diabetes, and neurodegeneration and may be a marker of tumor metastasis. Uridine diphospho-N-acetylglucosamine (UDP-GlcNAc), the donor molecule in O-GlcNAcylation, can be detected by (1)H nuclear magnetic resonance spectroscopy ((1)H NMR), giving the potential to measure its level noninvasively, providing a novel biomarker of prognosis and treatment monitoring. In this in vitro metabonomic study, four brain cancer cell lines were exposed to cisplatin and studied for metabolic responses using (1)H NMR. The Alamar blue assay and DAPI staining were used to assess cell sensitivity to cisplatin treatment and to confirm cell death. It is shown that in the cisplatin responding cells, UDP-GlcNAc and uridine diphospho-N-acetylgalactosamine (UDP-GalNAc), in parallel with (1)H NMR detected lipids, increased with cisplatin exposure before or at the onset of the microscopic signs of evolving cell death. The changes in UDP-GlcNAc and UDP-GalNAc were not detected in the nonresponders. These glycosylated UDP compounds, the key substrates for glycosylation of proteins and lipids, are commonly implicated in cancer proliferation and malignant transformation. However, the present study mechanistically links UDP-GlcNAc and UDP-GalNAc to cancer cell death following chemotherapeutic treatment.

Cytoskeletal keratin glycosylation protects epithelial tissue from injury

Keratins 8 and 18 (K8 and K18) are heteropolymeric intermediate filament phosphoglycoproteins of simple-type epithelia. Mutations in K8 and K18 predispose the affected individual to liver disease as they protect hepatocytes from apoptosis. K18 undergoes dynamic O-linked N-acetylglucosamine glycosylation at Ser 30, 31 and 49. We investigated the function of K18 glycosylation by generating mice that overexpress human K18 S30/31/49A substitution mutants that cannot be glycosylated (K18-Gly(-)), and compared the susceptibility of these mice to injury with wild-type and other keratin-mutant mice. K18-Gly(-) mice are more susceptible to liver and pancreatic injury and apoptosis induced by streptozotocin or to liver injury by combined N-acetyl-D-glucosaminidase inhibition and Fas administration. The enhanced apoptosis in the livers of mice that express K18-Gly(-) involves the inactivation of Akt1 and protein kinase Ctheta as a result of their site-specific hypophosphorylation. Akt1 binds to K8, which probably contributes to the reciprocal hyperglycosylation and hypophosphorylation of Akt1 that occurs on K18 hypoglycosylation, and leads to decreased Akt1 kinase activity. Therefore, K18 glycosylation provides a unique protective role in epithelial injury by promoting the phosphorylation and activation of cell-survival kinases.

O-GlcNAcylation determines the solubility, filament organization, and stability of keratins 8 and 18

Keratins 8 and 18 (K8/18) are intermediate filament proteins expressed specifically in simple epithelial tissues. Dynamic equilibrium of these phosphoglycoproteins in the soluble and filament pool is an important determinant of their cellular functions, and it is known to be regulated by site-specific phosphorylation. However, little is known about the role of dynamic O-GlcNAcylation on this keratin pair. Here, by comparing immortalized (Chang) and transformed hepatocyte (HepG2) cell lines, we have demonstrated that O-GlcNAcylation of K8/18 exhibits a positive correlation with their solubility (Nonidet P-40 extractability). Heat stress, which increases K8/18 solubility, resulted in a simultaneous increase in O-GlcNAc on these proteins. Conversely, increasing O-GlcNAc levels were associated with a concurrent increase in their solubility. This was also associated with a notable decrease in total cellular levels of K8/18. Unaltered levels of transcripts and the reduced half-life of K8 and K18 indicated their decreased stability on increasing O-GlcNAcylation. On the contrary, the K18 glycosylation mutant (K18 S29A/S30A/S48A) was notably more stable than the wild type K18 in Chang cells. The K18-O-GlcNAc mutant accumulated as aggregates upon stable expression, which possibly altered endogenous filament architecture. These results strongly indicate the involvement of O-GlcNAc on K8/18 in regulating their solubility and stability, which may have a bearing on the functions of these keratins.

A mitotic GlcNAcylation/phosphorylation signaling complex alters the posttranslational state of the cytoskeletal protein vimentin

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic intracellular protein modification responsive to stress, hormones, nutrients, and cell cycle stage. Alterations in O-GlcNAc addition or removal (cycling) impair cell cycle progression and cytokinesis, but the mechanisms are not well understood. Here, we demonstrate that the enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are in a transient complex at M phase with the mitotic kinase Aurora B and protein phosphatase 1. OGT colocalized to the midbody during telophase with Aurora B. Furthermore, these proteins coprecipitated with each other in a late mitotic extract. The complex was stable under Aurora inhibition; however, the total cellular levels of O-GlcNAc were increased and the localization of OGT was decreased at the midbody after Aurora inhibition. Vimentin, an intermediate filament protein, is an M phase substrate for both Aurora B and OGT. Overexpression of OGT or OGA led to defects in mitotic phosphorylation on multiple sites, whereas OGT overexpression increased mitotic GlcNAcylation of vimentin. OGA inhibition caused a decrease in vimentin late mitotic phosphorylation but increased GlcNAcylation. Together, these data demonstrate that the O-GlcNAc cycling enzymes associate with kinases and phosphatases at M phase to regulate the posttranslational status of vimentin.

Post-translational modification by O-GlcNAc: another way to change protein function

Modification of intracellular proteins by the beta-linkage of the monosaccharide, N-acetylglucosamine to serine or threonine hydroxyls (O-GlcNAc) is abundant and reversible. Although many proteins bear this post-translational covalent modification, the changes in function of the proteins as a result of this modification are only starting to be understood. In this article, we describe how aspects of the flux from the glucose backbone to this modification are modified and how the cellular activity and content of the GC-box binding transcription factor, Sp1, is altered by O-glycosylation. The association of the enzyme that puts on the O-GlcNAc modification with the bi-functional enzyme that removes this modification is discussed relative to the transition between transcriptional repression and activation.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

Influence of serine O-glycosylation or O-phosphorylation close to the vJun nuclear localisation sequence on nuclear import

Nuclear import triggered by the nuclear-localisation sequence (NLS) of the viral Jun (vJun) protein is mediated by phosphorylation of a serine close to the NLS. Since phosphorylation and glycosylation of serine residues are often in a reciprocal "yin-yang" relationship, we investigated whether glycosylation of this serine with O-linked N-acetylglucosamine (O-GlcNAc) would also regulate nuclear import via the vJun NLS. Peptides containing the vJun NLS with an adjacent O-phosphorylated, O-GlcNAc-functionalised or unmodified serine, and equipped with an N-terminal biotin or a 7-nitrobenz-2-oxa-1,3-diazolyl (NBD) fluorescent label, were synthesised on the solid phase by means of an Fmoc/Boc strategy and a Pd0-sensitive HYCRON linker. Fluorescence-polarisation measurements on the NBD-labelled peptides indicated that modification with phosphate or O-GlcNAc leads to a decrease in affinity to the import-mediating adapter protein, importin alpha, of about one order of magnitude compared to the unmodified NLS. Microinjection of biotinylated NLS peptide conjugated with fluorescently labelled avidin into NIH/3T3 and MDCK cells, revealed that avidin-unmodified-NLS peptide was rapidly imported into the nucleus. However, either phosphate or O-GlcNAc next to the NLS caused almost complete exclusion of the protein conjugate from nuclear import. These findings indicate that nuclear import by the vJun NLS might not be regulated by a "yin-yang" modification of an adjacent serine with phosphate or O-GlcNAc. Rather, negative regulation of binding between the polybasic NLS and importin by a negatively charged or a bulky, uncharged residue appears likely.

Stabilization of plakoglobin and enhanced keratinocyte cell-cell adhesion by intracellular O-glycosylation

O-Glycosylation modifies and regulates a variety of intracellular proteins. Plakoglobin, which functions in both cell-cell adhesion and signal transduction, is modified by O-glycosylation; however, the significance is unknown. To investigate the functional consequence of plakoglobin O-glycosylation, we cloned and overexpressed in keratinocytes murine O-GlcNAc transferase (mOGT). Over expression of mOGT in murine keratinocytes resulted in (i) glycosylation of plakoglobin and (ii) increased levels of plakoglobin due to post-translational stabilization of plakoglobin. Additionally, overexpression of mOGT in keratinocytes correlated with increased staining for cell-cell adhesion proteins and greater cell-cell adhesion. These observations suggest that O-glycosylation functions to regulate the post-translational stability of plakoglobin and keratinocyte cell-cell adhesion.

Methods for analysis of O-linked modifications on epidermal growth factor-like and thrombospondin type 1 repeats

The identification of novel forms of O-linked glycosylation on epidermal growth factor and thrombospondin type 1 repeats, and their emerging functional significance, require the development of new methods for their analysis. This chapter describes detailed methods to analyze both the structure and the site of modification of O-fucose and O-glucose glycans on proteins. These methods use both traditional biochemical methods of carbohydrate composition analysis and electrospray ionization-mass spectrometry of glycopeptides.

Modulation of O-GlcNAc glycosylation during Xenopus oocyte maturation

O-linked N-acetylglucosamine (O-GlcNAc) glycosylation is a post-translational modification, which is believed antagonises phosphorylation. We have studied the O-GlcNAc level during Xenopus oocyte meiotic resumption, taking advantage of the high synchrony of this model which is dependent upon a burst of phosphorylation. Stimulation of immature stage VI oocytes using progesterone was followed by a 4.51 +/- 0.32 fold increase in the GlcNAc content, concomitantly to an increase in phosphorylation, notably on two cytoplasmic proteins of 66 and 97 kDa. The increase of O-GlcNAc for the 97 kDa protein, which we identified as beta-catenin was partly related to its accumulation during maturation, as was demonstrated by the use of the protein synthesis inhibitor--cycloheximide. Microinjection of free GlcNAc, which inhibits O-glycosylated proteins-lectins interactions, delayed the progesterone-induced maturation without affecting the O-GlcNAc content. Our results suggest that O-GlcNAc glycosylation could regulate protein-protein interactions required for the cell cycle kinetic.

Perturbations in O-linked beta-N-acetylglucosamine protein modification cause severe defects in mitotic progression and cytokinesis

The dynamic modification of nuclear and cytoplasmic proteins with O-linked beta-N-acetylglucosamine (O-GlcNAc) is a regulatory post-translational modification that is rapidly responsive to morphogens, hormones, nutrients, and cellular stress. Here we show that O-GlcNAc is an important regulator of the cell cycle. Increased O-GlcNAc (pharmacologically or genetically) results in growth defects linked to delays in G2/M progression, altered mitotic phosphorylation, and cyclin expression. Overexpression of O-GlcNAcase, the enzyme that removes O-GlcNAc, induces a mitotic exit phenotype accompanied by a delay in mitotic phosphorylation, altered cyclin expression, and pronounced disruption in nuclear organization. Overexpression of the O-GlcNAc transferase, the enzyme that adds O-GlcNAc, results in a polyploid phenotype with faulty cytokinesis. Notably, O-GlcNAc transferase is concentrated at the mitotic spindle and midbody at M phase. These data suggest that dynamic O-GlcNAc processing is a pivotal regulatory component of the cell cycle, controlling cell cycle progression by regulating mitotic phosphorylation, cyclin expression, and cell division.

Proteomic approaches to analyze the dynamic relationships between nucleocytoplasmic protein glycosylation and phosphorylation

O-linked beta-N-acetylglucosamine (O-GlcNAc) is both an abundant and dynamic posttranslational modification similar to phosphorylation that occurs on serine and threonine residues of cytosolic and nuclear proteins in all metazoans and cell types examined, including cardiovascular tissue. Since the discovery of O-GlcNAc more than 20 years ago, the elucidation of O-GlcNAc as a posttranslational modification has been slow, albeit similar to the rate of acceptance of phosphorylation, because of the lack of tools available for its study. Identifying O-GlcNAc posttranslational modifications on proteins is a major challenge to proteomics. The recent development of mild beta-elimination followed by Michael addition with dithiothreitol has significantly improved the site mapping of both O-GlcNAc and O-phosphate in functional proteomics. beta-Elimination followed by Michael addition with dithiothreitol facilitates the study of the labile O-GlcNAc modification in the etiology of disease states. We discuss how recent technological innovations will expand our present understanding of O-GlcNAc and what the implications are for diabetes and cardiovascular complications.

Plakoglobin is O-glycosylated close to the N-terminal destruction box

Plakoglobin provides a key linkage in protein chains that connect desmosomal and classical cadherins to the cytoskeleton. It is also present in a significant cytosolic pool that has the capacity to impact on canonical Wnt signaling by competing for interaction with partner proteins of beta-catenin. The closely related protein, beta-catenin, is rapidly targeted for proteasomal degradation by phosphorylation of a "destruction box" within the N-terminal domain. Inhibition of this process forms the basis of Wnt signaling. This destruction box is also found in the N-terminal domain of plakoglobin. We report that plakoglobin is modified by the addition of O-GlcNAc at a single site in close proximity to the destruction box. O-GlcNAc modification has been proposed to counteract phosphorylation, provide protection from proteasomal degradation, mediate signal transduction, silence transcription, and regulate multimolecular protein assembly. This finding has potential implications for understanding the roles of plakoglobin.

Glycan-dependent signaling: O-linked N-acetylglucosamine

The addition of O-linked N-acetylglucosamine (O-GlcNAc) to target proteins may serve as a signaling modification analogous to protein phosphorylation. Like phosphorylation, O-GlcNAc is a dynamic modification occurring in the nucleus and cytoplasm. Various analytical methods have been developed to detect O-GlcNAc and distinguish it from glycosylation in the endomembrane system. Many target molecules have been identified; these targets are typically components of supramolecular complexes such as transcription factors, nuclear pore proteins, or cytoskeletal components. The enzymes responsible for O-GlcNAc addition and removal are highly conserved molecules having molecular features consistent with a signaling role. The O-GlcNAc transferase and O-GlcNAcase are likely to act in consort with kinases and phosphatases generating various isoforms of physiological substrates. These isoforms may differ in such properties as protein-protein interactions, protein stability, and enzymatic activity. Since O-GlcNAc plays a critical role in the regulation of signaling pathways of higher plants, the glycan modification is likely to perform similar signaling functions in mammalian cells. Glucose and amino acid metabolism generates hexosamine precursors that may be key regulators of a nutrient sensing pathway involving O-GlcNAc signaling. Altered O-linked GlcNAc metabolism may also occur in human diseases including neurodegenerative disorders, diabetes mellitus and cancer.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.

Use of galactosyltransferase to assess the biological function of O-linked N-acetyl-d-glucosamine: a potential role for O-GlcNAc during cell division

Many cytosolic and nuclear proteins are modified by monomeric O-linked N-acetyl-d-glucosamine (O-GlcNAc). The biological functions of this form of glycosylation are unclear but evidence suggests that it heightens regulation of protein function. To assess the biological function of O-GlcNAc addition, we examined the biological effects of galactosyltransferase (GalT) microinjected into the cytoplasm of Xenopus ovarian oocytes. GalT, which catalyzes beta1-4-galactose addition to O-GlcNAc, should inhibit deglycosylation and lectin-like interactions requiring unmodified O-GlcNAc residues. Although GalT injection into diplotene-arrested oocytes has no detectable effects on cell viability, it is toxic to oocytes entering meiosis. Cell-cycle-specific toxicity is recapitulated in vitro as GalT inhibits formation of nuclei and microtubule asters from cell-free extracts of ovulated frog eggs. These observations suggest that regulation of O-GlcNAc is important for cell cycle progression and may be important in diseases in which O-GlcNAc metabolism is abnormal. The methods described here outline a viable experimental scheme for ascribing a biological function to this form of glycosylation.

A nuclear transforming factor that converts the goat uterine nonactivated estrogen receptor to nuclear estrogen receptor II

A 62-kDa nuclear protein that transforms the goat uterine nonactivated estrogen receptor (naER) to nuclear estrogen receptor II (nER II) has been isolated and purified. This is being identified as the naER-transforming factor (naER-TF). The transformation is achieved through deglycosylation of the naER. It is observed that the naER-TF action on the naER introduces significant changes in the structural and functional features of the naER. The capacity of the naER to bind estradiol increases 8- to 10-fold, while its hormone binding affinity reduces to a considerable extent following its exposure to naER TF. There is a critical ratio in the concentration of the two proteins, the TF and the naER, that would ensure an optimum transformation process. The transformed naER is incapable of dimerization with the estrogen receptor activation factor (E-RAF).

Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules

Notch is a large cell-surface receptor known to be an essential player in a wide variety of developmental cascades. Here we show that Notch1 endogenously expressed in Chinese hamster ovary cells is modified with O-linked fucose and O-linked glucose saccharides, two unusual forms of O-linked glycosylation found on epidermal growth factor-like (EGF) modules. Interestingly, both modifications occur as monosaccharide and oligosaccharide species. Through exoglycosidase digestions we determined that the O-linked fucose oligosaccharide is a tetrasaccharide with a structure identical to that found on human clotting factor IX: Sia-alpha2,3-Gal-beta1, 4-GlcNAc-beta1,3-Fuc-alpha1-O-Ser/Thr. The elongated form of O-linked glucose appears to be a trisaccharide. Notch1 is the first membrane-associated protein identified with either O-linked fucose or O-linked glucose modifications. It also represents the second protein discovered with an elongated form of O-linked fucose. The sites of glycosylation, which fall within the multiple EGF modules of Notch, are highly conserved across species and within Notch homologs. Since Notch is known to interact with its ligands through subsets of EGF modules, these results suggest that the O-linked carbohydrate modifications of these modules may influence receptor-ligand interactions.

Cell cycle-dependent regulation of N-acetylglucosaminyltransferase-III in a human colon cancer cell line, Colo201

The mechanism for cell-cycle-dependent regulation of N-acetylglucosaminyltransferase III (GnT-III) activity was investigated using synchronized culture of Colo201, a human colon cancer cell line. In the synchronized culture, it was found that GnT-III activity rapidly increased in the M phase and the maximal activity was five times higher than the basal level found in the G1 phase. Northern blot and Western blot analyses revealed that the increase in the activity is due not to an increase in expression level of its mRNA but, rather, to the level of protein. Furthermore, it was shown by a pulse-chase experiment that the increased protein level of GnT-III is the result of its prolonged turnover rate. Lectin blotting with erythroagglutinating phytohemagglutinin showed that the content of bisecting N-acetylglucosamine structure in glycoproteins was transiently increased during the M phase in conjunction with the increased activity of GnT-III. These results suggest that GnT-III activity undergoes a cell-cycle-dependent regulation and thereby oligosaccharide structures of N-glycans vary specifically during the M phase of the cell cycle. Thus, it is possible that the cell-cycle-dependent alteration of N-glycans by GnT-III might play a role in biological events, such as the progression of cell cycle and cell division.

Effect of okadaic acid on O-linked N-acetylglucosamine levels in a neuroblastoma cell line

O-Linked N-Acetylglucosamine (O-GlcNAc) is a major form of post-translational modification found in nuclear and cytoplasmic proteins. Several authors have advanced the hypothesis according to which phosphorylation and O-GlcNAc glycosylation are reciprocally related to one another [1,2]. In order to test this hypothesis we have investigated the effect of a broad spectrum phosphatase inhibitor, okadaic acid (OA), generally used to induce protein hyperphosphorylation, on the GlcNAc content of cellular glycoproteins. We demonstrate that in neuronal cells lines OA decreases the level of O-GlcNAc in both nuclear and cytoplasmic proteins with a greater effect in the nuclear fraction. This phenomenon was demonstrated by the use of three different procedures for the detection of O-GlcNAc in conjunction with a systematic treatment with PNGase F. O-Linked GlcNAc was characterized using respectively lectin staining with WGA, galactosyltransferase labeling and metabolic labeling of cultured cells with [3H]glucosamine. Although the effects on individual proteins varied, a less pronounced effect was observed on HeLa or COS cell total homogenates. When Kelly cells were treated with OA, the major observation was a decrease in O-GlcNAc content of nuclear proteins. The measurement of the UDP-GlcNAc level clearly demonstrates that the decrease on the O-GlcNAc level in the neuroblastoma cell line after treatment with okadaic acid is not a consequence of the modification of the UDP-GlcNAc pool.

The extent of polylactosamine glycosylation of MDCK LAMP-2 is determined by its Golgi residence time

The increased polylactosamine glycosylation of LAMP-2 in MDCK cells cultured for 1 day relative to cells cultured for 3 days has been correlated with its slower rate of Golgi transit (Nabi and Rodriguez-Boulan, 1993, Mol. Biol. Cell., 4, 627-635). To determine if the differential polylactosamine glycosylation of LAMP-2 is a consequence of glycosyltransferase expression levels, the activities of beta1-6GlcNAc-TV, beta1-3GlcNAc-T(i), beta1-2GlcNAc-TI, beta1, 4Gal-T, alpha2-6sialyl-T, and alpha2-3sialyl-T were assayed and no significant differences in the activities of these enzymes in 1 and 3 day cell extracts were detected. During MDCK epithelial polarization, the Golgi apparatus undergoes morphological changes and apiconuclear Golgi networks were more evident in 3 day cells. Treatment with nocodazole disrupted Golgi networks and generated numerous Golgi clusters in both 1 day and 3 day cells. In the presence of nocodazole the differential migration of LAMP-2 in 1 and 3 day MDCK cells was maintained and could be eliminated by treatment with endo-beta-galactosidase, indicating that gross Golgi morphology did not influence the extent of LAMP-2 polylactosamine glycosylation. Nocodazole treatment did, however, result in the faster migration of LAMP-2 which was not due to modification of core N-glycans as the precursor form of the glycoprotein migrated with an identical molecular size. Following incubation at 20 degrees C, which prevents the exit of proteins from the trans-Golgi network, the molecular size of LAMP-2 increased to a similar extent in both 1 and 3 day MDCK cells. Extending the time of incubation at 20 degrees C did not influence the size of LAMP-2, demonstrating that its glycosylation is modified not by its retention within the Golgi but rather by its equivalent slower Golgi passage at the lower temperature in both 1 and 3 day cells. An identical effect was observed in nocodazole treated cells, demonstrating that Golgi residence time determines the extent of LAMP-2 polylactosamine glycosylation, even in isolated Golgi clusters.

Reduced O-glycosylated clathrin assembly protein AP180: implication for synaptic vesicle recycling dysfunction in Alzheimer's disease

Synapse loss is one of the neuropathologies in Alzheimer's disease (AD) that may play a crucial role in the mechanism of its distinct cognitive impairment and dementia. In a previous study [18], a significant reduction of O-glycosylated clathrin assembly protein AP180 was observed in neocortex of AD. The reduction correlated with the density of neurofibrillary tangles. In this study we further determine that the O-GlcNAc/AP180 ratio is not changed, but the level of AP180 protein decreases in AD. Furthermore, whereas the level of neurofilament (NF-M) remains relatively unchanged, another clathrin assembly protein, AP-2, is also reduced in AD along with a small loss of synaptophysin. Our findings suggest that synaptic vesicle recycling dysfunction may be involved in the pathology of synapse loss in AD.

Reduction of O-linked N-acetylglucosamine-modified assembly protein-3 in Alzheimer's disease

Abnormal protein processing and modification is associated with Alzheimer's disease (AD) pathology. The role of phosphorylation in AD has been studied extensively because the presumed abnormal phosphorylation of tau protein is believed to play a role in the formation of paired helical filaments. Glycosylation with O-linked N-acetylglucosamine (O-GlcNAc) to serine and threonine residues is a dynamic protein modification of intracellular proteins, and it shares similar features with protein phosphorylation. In this study, O-GlcNAc glycosylation of proteins from autopsied human brains with confirmed AD and non-AD age-matched controls was examined. O-GlcNAcylation was demonstrated by labeling protein extracts with [3H]galactose in the presence of galactosyltransferase and subsequent analyses of saccharide-protein linkage and saccharide structure. The number of O-GlcNAc-containing proteins and the overall O-GlcNAc level do not appear to be different between AD and control brain tissues. The only significant change observed is a marked reduction of O-GlcNAcylated clathrin assembly protein-3 (AP-3) in AD. The reduction is more evident in brain neocortical regions, and there appears to be a negative correlation between O-glycosylated AP-3 and the density of neurofibrillary tangles. These data suggest a possible association between the O-glycosylated AP-3 and AD pathology.

Nuclear and cytoplasmic glycosylation

O-GlcNAcylation is a form of cytoplasmic and nuclear glycosylation that is found on many diverse proteins of the cell including RNA polymerase II and its associated transcription factors, cytoskeletal proteins, nucleoporins, viral proteins, heat shock proteins, tumor suppressors, and oncogenes. It involves the attachment of a single, unmodified N-acetylglucosaminyl residue O-glycosidically linked to the hydroxyl groups of serine and threonine moieties of proteins. It is a highly abundant and dynamic form of posttranslational modification that appears to modulate function in a manner similar to phosphorylation. All O-GlcNAc-containing proteins are phosphoproteins that are involved in the formation of multimeric complexes, suggesting that O-GlcNAc may play a role in mediating protein-protein interactions. O-GlcNAc sites resemble phosphorylation sites and in many cases the two modifications are mutually exclusive; therefore, O-GlcNAcylation may act as an antagonist of phosphorylation and help to mediate many essential functions of the cell.

SV40 large T antigen is modified with O-linked N-acetylglucosamine but not with other forms of glycosylation

SV40 large T antigen has been reported to be modified with several different sugars including N-acetylglucosamine, galactose, and mannose. In this report we have reexamined the glycosylation of T antigen and found that while we could detect modification with N-acetylglucosamine, we could not detect any other sugars on the protein. Surprisingly, even though [3H]galactose could be metabolically incorporated into the protein, analysis showed that all of the radioactivity in T antigen had been converted to other species. The N-acetylglucosamine was demonstrated to be linked to the protein in the form of O-linked N-acetylglucosamine, the best characterized form of nuclear and cytoplasmic glycosylation in mammalian systems. We have localized the major site of glycosylation to the amino terminal portion of the molecule. Analysis of mutated T antigen where serines 111/112 were substituted with alanine suggest that these residues constitute a glycosylation site on the protein. These two serines fall within a typical O-linked N-acetylglucosamine glycosylation site (PSS) and are also known to be phosphorylated. Thus, it is likely that competition between phosphorylation and glycosylation occurs at this site.

Modulation of O-linked N-acetylglucosamine levels on nuclear and cytoplasmic proteins in vivo using the peptide O-GlcNAc-beta-N-acetylglucosaminidase inhibitor O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate

O-Linked N-acetylglucosamine (O-GlcNAc) is a ubiquitous and abundant post-translational modification found on nuclear and cytoplasmic proteins and is thought to be a dynamically regulated modification much like phosphorylation. In this study we have demonstrated that O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbama te (PUGNAc), a potent in vitro inhibitor of the enzyme responsible for the removal of O-GlcNAc from proteins (peptide O-GlcNAc-beta-N-acetylglucosaminidase), can be used to increase O-GlcNAc levels on nuclear and cytoplasmic proteins in vivo. Overall, PUGNAc caused approximately a 2-fold increase in O-GlcNAc levels in the human colon cancer cells, HT29, although the effects on individual proteins varied. The increase appeared to be the result of the direct inhibition of the peptide O-GlcNAc-beta-N-acetylglucosaminidase since neither the O-GlcNAc transferase nor UDP-GlcNAc levels were affected by the treatment. O-GlcNAc levels in other cell lines tested (NIH 3T3, CV-1, and HeLa) were also affected by PUGNAc, although the effects on HeLa cells were minimal. At the concentrations tested, PUGNAc was non-toxic and had no affect on the growth rate of any of the cell lines examined. Interestingly, we demonstrated that an increase in O-GlcNAc levels on the transcription factor Sp1 resulted in a reciprocal decrease in its level of phosphorylation, supporting the hypothesis that O-GlcNAc competes with phosphate on some proteins. These studies demonstrate that PUGNAc is an effective inhibitor of O-GlcNAc turnover within cells and can be used to selectively alter the extent of O-GlcNAc on cellular proteins.

Interaction of the transcription factor Sp1 with the nuclear pore protein p62 requires the C-terminal domain of p62

The transcription factor Sp1 plays an important role in the expression of many cellular genes. In studies of proteins that associate with Sp1, a 62-kDa glycoprotein was found in immunoprecipitates of Sp1. This protein was detected in these immunoprecipitates by the monoclonal antibody, RL2, which was originally raised against nuclear pore proteins but was subsequently found to recognize an epitope that contains O-linked N-acetylglucosamine (O-GlcNAc). The association of this protein with Sp1 could be blocked by SDS denaturation of the protein complex. Western blot analysis of the Sp1 immunoprecipitate using antibodies to p62 nucleoporin indicated that this nuclear pore protein associates with Sp1. Furthermore, immunoprecipitation of p62 nucleoporin resulted in the coprecipitation of Sp1. Recombinant p62, expressed as a GST-fusion protein using a vaccinia virus system, also interacted with both recombinant and native Sp1. This interaction between p62 and Sp1 required the C-terminus of p62 and the C-terminus was able to bind Sp1, albeit less efficiently than native p62. A mammalian two-hybrid interaction assay was devised in which p62 was fused to the Gal4 DNA-binding domain. This system also indicated that p62, through its C-terminus, interacts with Sp1 in the living cell. We propose that this interaction of a nuclear pore protein with Sp1 may reflect the nuclear organization required to bring transcribable DNA in contact with the transcription factors.

The O-linked fucose glycosylation pathway. Evidence for protein-specific elongation of o-linked fucose in Chinese hamster ovary cells

O-Linked fucose is an unusual form of glycosylation recently shown to modify the hydroxyls of serine or threonine residues at a strict consensus site within epidermal growth factor-like domains of several serum proteins. Here we demonstrate that Chinese hamster ovary cells modify numerous proteins with O-linked fucose and that the fucose is elongated on specific proteins. We have identified at least two forms of O-linked fucose elongation in Chinese hamster ovary cells: a disaccharide (Glcbeta1,3Fuc) and a larger oligosaccharide of indeterminate structure. Interestingly, it appears that the level of monosaccharide accumulates in the cells over time whereas the disaccharide does not. Analysis of the O-linked fucose-containing saccharides on individual proteins revealed that some proteins are modified with the monosaccharide only, whereas others are modified with monosaccharide and disaccharide, or monosaccharide and oligosaccharide. These results suggest that elongation of the O-linked fucose monosaccharide is a protein-specific phenomena. The presence of elongated O-linked fucose moieties suggests that a novel glycosylation pathway exists in mammalian cells with O-linked fucose as the core.