O-glycosylation of nuclear and cytoplasmic proteins: regulation analogous to phosphorylation?

Total serum N-glycans mark visceral leishmaniasis in human infections with Leishmania infantum

Visceral leishmaniasis (VL) is a clinical form of leishmaniasis with high mortality rates when not treated. Diagnosis suffers from invasive techniques and sub-optimal sensitivities. The current (affordable) treatment with pentavalent antimony as advised by the WHO is possibly harmful to the patient. There is need for an improved diagnosis to prevent possibly unnecessary treatment. N-glycan analysis may aid in diagnosis. We evaluated the N-glycan profiles from active VL, asymptomatic infections (ASYMP) and controls from non-endemic (NC) and endemic (EC) areas. Active VL has a distinct N-glycome profile that associates with disease severity. Our study suggests that the observed glycan signatures could be a valuable additive to diagnosis and assist in identifying possible markers of disease and understanding the pathogenesis of VL. Further studies are warranted to assess a possible future role of blood glycome analysis in active VL diagnosis and should aim at disease specificity.

Metabolic control of gene transcription in non-alcoholic fatty liver disease: the role of the epigenome

Non-alcoholic fatty liver disease (NAFLD) is estimated to affect 24% of the global adult population. NAFLD is a major risk factor for the development of cirrhosis and hepatocellular carcinoma, as well as being strongly associated with type 2 diabetes and cardiovascular disease. It has been proposed that up to 88% of obese adults have NAFLD, and with global obesity rates increasing, this disease is set to become even more prevalent. Despite intense research in this field, the molecular processes underlying the pathology of NAFLD remain poorly understood. Hepatic intracellular lipid accumulation may lead to dysregulated tricarboxylic acid (TCA) cycle activity and associated alterations in metabolite levels. The TCA cycle metabolites alpha-ketoglutarate, succinate and fumarate are allosteric regulators of the alpha-ketoglutarate-dependent dioxygenase family of enzymes. The enzymes within this family have multiple targets, including DNA and chromatin, and thus may be capable of modulating gene transcription in response to intracellular lipid accumulation through alteration of the epigenome. In this review, we discuss what is currently understood in the field and suggest areas for future research which may lead to the development of novel preventative or therapeutic interventions for NAFLD.

Nutrient regulation of signaling and transcription

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.

N-glycomic changes in serum proteins in type 2 diabetes mellitus correlate with complications and with metabolic syndrome parameters

Background

Glycosylation, i.e the enzymatic addition of oligosaccharides (or glycans) to proteins and lipids, known as glycosylation, is one of the most common co-/posttranslational modifications of proteins. Many important biological roles of glycoproteins are modulated by N-linked oligosaccharides. As glucose levels can affect the pathways leading to glycosylation of proteins, we investigated whether metabolic syndrome (MS) and type 2 diabetes mellitus (T2DM), pathological conditions characterized by altered glucose levels, are associated with specific modifications in serum N-glycome.

Methods

We enrolled in the study 562 patients with Type 2 Diabetes Mellitus (T2DM) (mean age 65.6±8.2 years) and 599 healthy control subjects (CTRs) (mean age, 58.5±12.4 years). N-glycome was evaluated in serum glycoproteins.

Results

We found significant changes in N-glycan composition in the sera of T2DM patients. In particular, α(1,6)-linked arm monogalactosylated, core-fucosylated diantennary N-glycans (NG1(6)A2F) were significantly reduced in T2DM compared with CTR subjects. Importantly, they were equally reduced in diabetic patients with and without complications (P<0.001) compared with CTRs. Macro vascular-complications were found to be related with decreased levels of NG1(6)A2F. In addition, NG1(6)A2F and NG1(3)A2F, identifying, respectively, monogalactosylated N-glycans with α(1,6)- and α(1,3)-antennary galactosylation, resulted strongly correlated with most MS parameters. The plasmatic levels of these two glycans were lower in T2DM as compared to healthy controls, and even lower in patients with complications and MS, that is the extreme “unhealthy” phenotype (T2DM+ with MS).

Conclusions

Imbalance of glycosyltransferases, glycosidases and sugar nucleotide donor levels is able to cause the structural changes evidenced by our findings. Serum N-glycan profiles are thus sensitive to the presence of diabetes and MS. Serum N-glycan levels could therefore provide a non-invasive alternative marker for T2DM and MS.

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is a ubiquitous and dynamic post-translational modification known to modify over 3,000 nuclear, cytoplasmic, and mitochondrial eukaryotic proteins. Addition of O-GlcNAc to proteins is catalyzed by the O-GlcNAc transferase and is removed by a neutral-N-acetyl-β-glucosaminidase (O-GlcNAcase). O-GlcNAc is thought to regulate proteins in a manner analogous to protein phosphorylation, and the cycling of this carbohydrate modification regulates many cellular functions such as the cellular stress response. Diverse forms of cellular stress and tissue injury result in enhanced O-GlcNAc modification, or O-GlcNAcylation, of numerous intracellular proteins. Stress-induced O-GlcNAcylation appears to promote cell/tissue survival by regulating a multitude of biological processes including: the phosphoinositide 3-kinase/Akt pathway, heat shock protein expression, calcium homeostasis, levels of reactive oxygen species, ER stress, protein stability, mitochondrial dynamics, and inflammation. Here, we will discuss the regulation of these processes by O-GlcNAc and the impact of such regulation on survival in models of ischemia reperfusion injury and trauma hemorrhage. We will also discuss the misregulation of O-GlcNAc in diseases commonly associated with the stress response, namely Alzheimer's and Parkinson's diseases. Finally, we will highlight recent advancements in the tools and technologies used to study the O-GlcNAc modification.

O-GlcNAcylation and oxidation of proteins: is signalling in the cardiovascular system becoming sweeter?

O-GlcNAcylation is an unusual form of protein glycosylation, where a single-sugar [GlcNAc (N-acetylglucosamine)] is added (via β-attachment) to the hydroxyl moiety of serine and threonine residues of nuclear and cytoplasmic proteins. A complex and extensive interplay exists between O-GlcNAcylation and phosphorylation. Many phosphorylation sites are also known glycosylation sites, and this reciprocal occupancy may produce different activities or alter the stability in a target protein. The interplay between these two post-translational modifications is not always reciprocal, as some proteins can be concomitantly phosphorylated and O-GlcNAcylated, and the adjacent phosphorylation or O-GlcNAcylation can regulate the addition of either moiety. Increased cardiovascular production of ROS (reactive oxygen species), termed oxidative stress, has been consistently reported in various chronic diseases and in conditions where O-GlcNAcylation has been implicated as a contributing mechanism for the associated organ injury/protection (for example, diabetes, Alzheimer's disease, arterial hypertension, aging and ischaemia). In the present review, we will briefly comment on general aspects of O-GlcNAcylation and provide an overview of what has been reported for this post-translational modification in the cardiovascular system. We will then specifically address whether signalling molecules involved in redox signalling can be modified by O-GlcNAc (O-linked GlcNAc) and will discuss the critical interplay between O-GlcNAcylation and ROS generation. Experimental evidence indicates that the interactions between O-GlcNAcylation and oxidation of proteins are important not only for cell regulation in physiological conditions, but also under pathological states where the interplay may become dysfunctional and thereby exacerbate cellular injury.

Effects of Doxazosin on Alpha 1-Adrenergic Receptors in Prostates with Benign Prostatic Hyperplasia

OBJECTIVES

The present study aimed to evaluate changes in mRNA and protein expression levels of α1-AR before and after doxazosin treatment.

METHODS

This 12-month, prospective study included males aged 50 or older who had lower urinary tract symptoms (LUTS) (International Prostate Symptom Score [IPSS] ≥ 8) with benign prostatic hyperplasia (BPH). All patients underwent transrectal ultrasound-guided prostate biopsy before and after doxazosin 4 mg medication for 12 months. The mRNA and protein expression of prostate α1-AR were analyzed using real-time quantitative reverse transcription-polymerase chain and Western blotting, respectively, before and after treatment. The clinical efficacy of doxazosin was evaluated according to changes in prostate volume, serum prostate-specific antigen (PSA) level, IPSS, quality of life (QoL) index, maximum flow rate, parameters in a voiding diary, and a Patient's Perception of Bladder Condition (PPBC) questionnaire.

RESULTS

Twenty patients aged 50-72 (median age 66) with LUTS secondary to BPH completed this study. Administering doxazosin for 12 months significantly increased α1-AR protein expression in the prostate. α1-AR mRNA expression did not change significantly after doxazosin administration. IPSS, QoL index, and PPBC scores significantly improved after 12 months of doxazosin treatment. Maximal flow rate, postvoid residual urine volume (PVR), prostate volume and the parameters from the voiding diary did not change significantly after 12 months. The change of IPSS total score and LUTS were maintained until 12 months after starting treatment with doxazosin.

CONCLUSION

Doxazosin treatment was able to increase α1-AR protein expression in the prostate. Despite increased α1-AR expression, doxazosin provides sustained, significant relief of LUTS for up to one year without a decrease in efficacy.

Human linker histones: interplay between phosphorylation and O-β-GlcNAc to mediate chromatin structural modifications

Eukaryotic chromatin is a combination of DNA and histone proteins. It is established fact that epigenetic mechanisms are associated with DNA and histones. Initial studies emphasize on core histones association with DNA, however later studies prove the importance of linker histone H1 epigenetic. There are many types of linker histone H1 found in mammals. These subtypes are cell specific and their amount in different types of cells varies as the cell functions. Many types of post-translational modifications which occur on different residues in each subtype of linker histone H1 induce conformational changes and allow the different subtypes of linker histone H1 to interact with chromatin at different stages during cell cycle which results in the regulation of transcription and gene expression. Proposed O-glycosylation of linker histone H1 promotes condensation of chromatin while phosphorylation of linker histone H1 is known to activate transcription and gene regulation by decondensation of chromatin. Interplay between phosphorylation and O-β-GlcNAc modification on Ser and Thr residues in each subtype of linker histone H1 in Homo sapiens during cell cycle may result in diverse functional regulation of proteins. This in silico study describes the potential phosphorylation, o-glycosylation and their possible interplay sites on conserved Ser/Thr residues in various subtypes of linker histone H1 in Homo sapiens.

Claudin-1 required for HCV virus entry has high potential for phosphorylation and O-glycosylation

HCV is a leading cause of hepatocellular carcinoma and cirrhosis all over the world. Claudins belong to family of tight junction's proteins that are responsible for establishing barriers for controlling the flow of molecules around cells. For therapeutic strategies, regulation of viral entry into the host cells holds a lot of promise. During HCV infection claudin-1 is highly expressed in liver and believed to be associated with HCV virus entry after HCV binding with or without co-receptor CD81. The claudin-1 assembly with tight junctions is regulated by post translational modifications. During claudins assembly and disassembly with tight junctions, phosphorylation is required at C-terminal tail. In cellular proteins, interplay between phosphorylation and O-β-GlcNAc modification is believed to be functional switch, but it is very difficult to monitor these functional and vibrant changes in vivo. Netphos 2.0 and Disphos 1.3 programs were used for potential phosphorylation; NetPhosK 1.0 and KinasePhos for kinase prediction; and YinOYang 1.2 and OGPET to predict possible O-glycosylation sites. We also identified Yin Yang sites that may have potential for O-β-GlcNAc and phosphorylation interplay at same Ser/Thr residues. We for the first time proposed that alternate phosphorylation and O-β-GlcNAc modification on Ser 192, Ser 205, Ser 206; and Thr 191 may provide an on/off switch to regulate assembly of claudin-1 at tight junctions. In addition these phosphorylation sites may be targeted by novel chemotherapeutic agents to prevent phosphorylation lead by HCV viral entry complex.

Serine 204 phosphorylation and O-β-GlcNAC interplay of IGFBP-6 as therapeutic indicator to regulate IGF-II functions in viral mediated hepatocellular carcinoma

Hepatocellular carcinoma is mainly associated with viral hepatitis B and C. Activation of cell growth stimulator IGF-II gene is observed in tumor formation especially in viral associated hepatocellular carcinoma. Elevated IGF-II levels are indicator of increased risk for cholangiocellular and hepatocellular carcinomas through over saturation of IGF-II binding capacities with IGF receptors leading to cellular dedifferentiation. In HCV, core protein is believed to trans-activate host IGF-II receptor through PKC pathway and the inhibition of tumor cell growth can be achieved by blocking IGF-II pathway either at transcriptional level or increasing its binding with IGFBPs (Insulin like growth factor proteins) at C-terminal, so that it is not available in free form. IGFBP-6 is a specific inhibitor of IGF-II actions. Affinity of IGFBPs with IGFs is controlled by post-translational modifications. Phosphorylation of IGFBPs inhibits IGFs action on target cells while O-glycosylation prevents binding of IGFBP-6 to glycosaminoglycans and cell membranes and resulting in a 10-fold higher affinity for IGF-II. O-glycosylation and phosphorylation operate the functional expression of cellular proteins, this switching on and off the protein expression is difficult to monitor in vivo. By using neural network based prediction methods, we propose that alternate O-β-GlcNAc modification and phosphorylation on Ser 204 control the binding of IGFBP-6 with IGF-II. This information may be used for developing new therapies by regulating IGFBP-6 assembly with IGF-II to minimize the risk of viral associated hepatocellular carcinoma. We can conclude that during HCV/HBV infection, O-β-GlcNAc of IGFBP-6 at Ser 204 diminish their binding with IGF-II, increase IGF-II cellular expression and promote cancer progression which can lead to hepatocellular carcinoma. Furthermore, this site can be used for developing new therapies to control the IGF-II actions during viral infection to minimize the risk of hepatocellular carcinoma.

Hypoxia upregulates the expression of the O-linked N-acetylglucosamine containing epitope H in human ependymal cells

Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by mouse IgM monoclonal antibody H (mabH). Epitope H is present in several types of cells and in several polypeptides outside the CNS. Previous results have shown that in the adult human brains, epitope H is confined mostly to a minority of fibrous astrocytes, and it is greatly upregulated in the reactive astrocytes. Post-translational modification with O-GlcNAc occurs on many proteins involved in several cell processes, such as cell cycle progression, apoptosis, proteasome degradation pathways, and modulation of cellular function in response to nutrition and stress. Hypoxia is one of the major causes of cellular stress. Therefore, in this study, we used the mAbH and the indirect immunoperoxidase method to investigate the expression of epitope H in ependymal cells in brains of persons who died with signs of hypoxic encephalopathy. The results of the present study showed that practically all ependymal cells showed cytoplasmic staining for epitope H in supranuclear cytoplasm in the brain of two premature neonates and in ten infants who died with signs of hypoxic encephalopathy. However, the overwhelming majority of ependymal cells of the nine human embryos taken from legal abortions, ranging from 26 days until 13 weeks of gestational age, and of the ten infants' brains without any sign of hypoxic encephalopathy remained negative. Only occasionally did the ependymal cells show weak cytoplasmic staining in some foci. In addition, the reactive astrocytes in the hypoxic brains showed strong cytoplasmic staining, confirming previous results.

The alpha,alpha-difluorinated phosphonate L-pSer-analogue: an accessible chemical tool for studying kinase-dependent signal transduction

This overview focuses on the (alpha,alpha-difluoromethylene)phosphonate mimic of phosphoserine (pCF(2)Ser) and its application to the study of kinase-mediated signal transduction-pathways of great interest to drug development. The most versatile modes of access to these chemical biological tools are discussed, organized by method of PCF(2)-C bond formation. The pCF(2)-Ser mimic may be site-specifically incorporated into peptides (SPPS) and proteins (expressed protein ligation). This isopolar, dianionic pSer mimic results in a "constitutive phosphorylation" phenotype and is seen to support native protein-protein interactions that depend on serine phosphorylation. Signal transduction pathways studied with this chemical biological approach include the regulation of p53 tumor suppressor protein activity and of melatonin production. Given these successes, the future is bright for the use of such "teflon phospho-amino acid mimics" to map kinase-based signaling pathways.

Biochemistry of Tau in Alzheimer's disease and related neurological disorders

Microtubule-associated Tau proteins belong to a family of factors that polymerize tubulin dimers and stabilize microtubules. Tau is strongly expressed in neurons, localized in the axon and is essential for neuronal plasticity and network. From the very beginning of Tau discovery, proteomics methods have been essential to the knowledge of Tau biochemistry and biology. In this review, we have summarized the main contributions of several proteomic methods in the understanding of Tau, including expression, post-translational modifications and structure, in both physiological and pathophysiological aspects. Finally, recent advances in proteomics technology are essential to develop further therapeutic targets and early predictive and discriminative diagnostic assays for Alzheimer's disease and related disorders.

Transcription factor p53 exhibits increased binding to the α2-macroglobulin gene promoter and decreased glycosylation in fetal and adult rat liver during the acute-phase response

The binding affinity of p53 for the MG promoter was assessed by DNA-affinity chromatography with the extended ?2-macroglobulin (MG) gene promoter (-852/+12) and immunoblot analysis. During the increased MG gene transcription observed in the fetus and the acute-phase (AP) response in both the fetus and the adult, p53 exhibited increased binding to the MG promoter. This increase was accompanied by decreased O-linked N-acetyl glucosamine glycosylation of p53. We suggest that the enzymatic removal of sugar moieties in vivo serves to activate the MG gene promoter binding potential of p53 and its participation in upregulated MG gene transcription.

Tauopathies: A Distinct Class of Neurodegenerative Diseases

Neurodegenerative diseases are characterized by neuronal loss and intraneuronal accumulation of fibrillary materials, of which, neurofibrillary tangles (NFT) are the most common. Neurofibrillary tangles also occur in normal aging and contain the hyperphosphorylated microtubule-associated protein tau. A detailed presentation is made of the molecular bases of Alzheimer's disease (AD), postencephalitic parkinsonism, amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC) of Guam, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick's disease, frontotemporal dementia (FTD), Down's syndrome, myotonic dystrophy (DM) and Niemann-Pick Type C (NPC) disease, which are considered to be common tauopathies. The unique human tau gene extends over 100 kb of the long arm of chromosome 17 and contains 16 exons. The human brain contains six tau isoforms that contain from 352 to 441 amino acids. To date, 34 pathogenic tau mutations have been described among 101 families affected by FTD with parkinsonism linked to chromosome 17 (FTDP-17). These mutations may involve alternative splicing of exon 10 that lead to changes in the proportion of 4-repeat- and 3-repeat-tau isoforms, or may modify tau interactions with microtubules. Tau aggregates differ in degree of phosphorylation and in content of tau isoforms. Five classes of tauopathies have been defined depending on the type of tau aggregates. The key event in tauopathies is the disorganization of the cytoskeleton, which is based on mutations/polymorphisms in the tau gene and lead to nerve cell degeneration. In this review, tauopathies as a distinct class of neurodegenerative diseases are discussed with emphasis on their molecular pathology and genetics.

Expression of the O-linked N-acetylglucosamine containing epitope H in normal myometrium and uterine smooth muscle cell tumors

Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by monoclonal antibody H (mAbH). We have previously shown that epitope H is present in more than one polypeptide and in various types of normal and pathological cells. In the present study, we focused on uterine smooth muscle cell tumors and their adjacent normal myometrium to gain further insight into the expression patterns of epitope H in human tissues. The indirect immunoperoxidase method was applied using the mAbH and the monoclonal anti-cytokeratin 8 antibody (AbCK8) in 50 cases of typical uterine leiomyomas and in five cases of uterine leiomyosarcomas, with four cases belonging to Group II A and one to Group III according to Bell et al. [6]. Western immunoblotting was applied using mAbH and AbCK8 in five cases of uterine leiomyomas and their adjacent myometrium. The main results were as follows: (1) epitope H showed intense immunohistochemical expression in 46% (23/50) and moderate expression in 54% (27/50) of uterine leiomyomas, (2) epitope H showed intense immunohistochemical expression in 40% (2/5) and moderate expression in 60% (3/5) of uterine leiomyosarcomas, (3) epitope H showed no difference in the immunohistochemical expression between leiomyomas and their adjacent myometrium and between leiomyosarcomas and their adjacent myometrium, (4) immunohistochemical expression of cytokeratin 8 was not detected in the normal and neoplastic smooth muscle cells, (5) Western immunoblotting showed that in the smooth muscle cells of the myometrium and leiomyomas, epitope H is localized in four polypeptides with molecular weights of 100, 61, 59, and 54 kDa, and (6) Western immunoblotting did not detect cytokeratin 8 in the normal and neoplastic smooth muscle cells. The present results indicate fluctuations of the epitope expression levels in uterine smooth muscle cell tumors and their adjacent myometrium. These fluctuations may be of interest for gaining insight into the pathogenesis of uterine smooth muscle cell tumors, since O-GlcNAc glycosylation is involved in cell cycle and apoptosis pathways and may modify proteins involved in oncogenesis (tumor suppressor proteins and oncoproteins) and proteins with important biological functions such as cytoskeletal proteins, transcription factors, and heat-shock proteins. Furthermore, the present results indicate that cytokeratin 8, without being present in the cells of the myometrium, leiomyomas and leiomyosarcomas, shares its epitope H, which contains its unique sugar O-N-acetylglucosamine residue, with four other unrelated polypeptides produced by the normal and neoplastic smooth muscle cells. This should be considered when using anti-cytokeratin 8 antibodies in immunohistochemistry against smooth muscle cell tumors to avoid false positive immunohistochemical results.

The use of mass spectrometry for the proteomic analysis of glycosylation

Of all protein PTMs, glycosylation is by far the most common, and is a target for proteomic research. Glycosylation plays key roles in controlling various cellular processes and the modifications of the glycan structures in diseases highlight the clinical importance of this PTM. Glycosylation analysis remains a difficult task. MS, in combination with modern separation methodologies, is one of the most powerful and versatile techniques for the structural analysis of glycoconjugates. This review describes methodologies based on MS for detailed characterization of glycoconjugates in complex biological samples at the sensitivity required for proteomic work.

Nuclear localization of cytokeratin 8 and the O-linked N-acetylglucosamine-containing epitope H in epithelial cells of infiltrating ductal breast carcinomas: a combination of immunogold and EDTA regressive staining methods

In a previous study, the authors have shown cytokeratin 8 (CK8) and epitope H ultrastructural localization in breast cancer cell nuclei. Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by monoclonal antibody H. In this study, double immunogold labeling of CK8 and epitope H combined with the EDTA regressive staining method was applied in biopsy material from infiltrating ductal breast carcinomas and fibroadenomas, to localize both antigens in correlation to RNPs distribution in the nuclear subcompartments of cancer cells. CK8 and epitope H were localized mostly over condensed chromatin, whereas staining was weaker over interchromatin granule clusters and perichromatin fibers. These results revealed, the distribution of CK8 in the nucleus as MAR-binding protein, contributing in the organization of the nuclear DNA in the neoplastic cell, as well as the distribution of O-GlcNAc glycosylated polypeptides bearing the epitope H. The latter finding indicates that these polypeptides might play a significant role in the neoplastic behavior of breast cancer cells because they colocalize in the same nuclear subcompartments with proteins modified by O-GlcNAc, such as hnRNPs G and A1, RNA polymerase II, its transcription factors, and the oncogene product of c-myc. These proteins are known to participate in coordinated transcription/RNA processing events, contributing in the neoplastic behavior of breast cancer cells.

Partially overlapping distribution of epsin1 and HIP1 at the synapse: analysis by immunoelectron microscopy

Synapses of neurons use clathrin-mediated endocytic pathways for recycling of synaptic vesicles and trafficking of neurotransmitter receptors. Epsin 1 and huntingtin-interacting protein 1 (HIP1) are endocytic accessory proteins. Both proteins interact with clathrin and the AP2 adaptor complex and also bind to the phosphoinositide-containing plasma membrane via an epsin/AP180 N-terminal homology (ENTH/ANTH) domain. Epsin1 and HIP1 are found in neurons; however, their precise roles in synapses remain largely unknown. Using immunogold electron microscopy, we examine and compare the synaptic distribution of epsin1 and HIP1 in rat CA1 hippocampal synapse. We find that epsin1 is located across both sides of the synapse, whereas HIP1 displays a preference for the postsynaptic compartment. Within the synaptic compartments, espin1 is distributed similarly throughout, whereas postsynaptic HIP1 is concentrated near the plasma membrane. Our results suggest a dual role for epsin1 and HIP1 in the synapse: as broadly required factors for promoting clathrin assembly and as adaptors for specific endocytic pathways.

Tuning the conformation properties of a peptide by glycosylation and phosphorylation

We have deployed the alpha-helical hairpin peptide (alpha-helix/turn/alpha-helix) and used it as a model system to explore how glycosylation and phosphorylation might affect the conformational properties of the peptide. The native conformations of the modified peptides in buffer solution have been compared with that of the wild-type peptide by nuclear magnetic resonance spectroscopy. Circular dichroism spectroscopy was used to probe the effects of an O-linked beta-GlcNAc and a phosphate group on the overall folding stability of the peptide. Finally, the rate of fibrillogenesis was used to infer the effects of these chemical modifications on the alpha-to-beta transition as well as the rate of nucleation of amyloidogenesis.

Recombinant O-GlcNAc transferase isoforms: identification of O-GlcNAcase, yes tyrosine kinase, and tau as isoform-specific substrates

O-linked N-acetylglucosaminyltransferase (OGT) catalyzes the transfer of O-linked GlcNAc to serine or threonine residues of a variety of substrate proteins, including nuclear pore proteins, transcription factors, and proteins implicated in diabetes and neurodegenerative disorders. We have identified two nucleocytoplasmic isoforms of OGT (ncOGT and sOGT) and one isoform that localizes to the mitochondria (mOGT). These three isoforms contain identical catalytic regions but differ in the number of tetratricopeptide repeat motifs found at the N-terminus of each enzyme. We expressed each of these OGT isoforms in a soluble form in Escherichia coli and have used them to identify novel targets including the Src-family tyrosine kinase yes and O-GlcNAc-ase. We demonstrate that some substrate proteins, such as Nup62 and casein kinase II, are glycosylated by both ncOGT and mOGT, while others such as O-GlcNAcase and tau are specifically modified by ncOGT. The yes kinase was specifically modified by mOGT. The short isoform of OGT (sOGT) did not glycosylate any of the substrates tested, although it retains a potentially active catalytic domain. Our findings demonstrate the potential utility of recombinant OGT in identifying new targets and illustrate the necessity to examine all active isoforms of the enzyme. The identification of a tyrosine kinase and O-GlcNAcase as OGT targets suggests the potential for OGT participation in numerous signal transduction cascades.

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.

Mitochondria-rich normal, metaplastic, and neoplastic cells show overexpression of the epitope H recognized by the monoclonal antibody H

The epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by monoclonal antibody H (mAbH). We have previously shown that the epitope H is present in more than one polypeptide and in various types of normal and pathological cells. In the present study, we focused on mitochondria-rich normal, metaplastic, and neoplastic cells, prompted by our recent immuno-electron microscopy findings that mAbH clearly stains the mitochondria of breast epithelial cells in infiltrating ductal breast carcinomas and fibroadenomas. The indirect immunoperoxidase method was applied using the mAbH to investigate the distribution of the epitope H in mitochondria-rich normal cells and in metaplastic and neoplastic oncocytic cells. Immunohistochemical staining for the mAbH was observed in oxyphil cells of parathyroid glands, in striated duct cells of parotid glands, in urinary tubules of kidneys, in parietal cells of gastric body mucosa, in oxyphil cells of Hashimoto's thyroiditis, in epithelial cells of Warthin's tumors of the parotid gland, in neoplastic cells of oxyphil adenomas and carcinomas (Hürthle's tumors) of the thyroid gland, and in neoplastic cells of oncocytomas of the kidneys. The present study shows that the epitope H is strongly expressed in mitochondria-rich normal cells, as well as in metaplastic and neoplastic oncocytic cells, which are known to have cytoplasms packed with mitochondria. Since mAbH recognizes an O-GlcNAc residue, our findings indicate that O-GlcNAc-glycosylated polypeptides are present at mitochondria where the components of the respiratory chain and energy transduction are localized. These findings may be of interest for gaining insight into the histophysiology of mitochondria-rich normal cells and into the pathogenesis of oncocytic lesions, since O-GlcNAc glycosylation may modify proteins involved in oncogenesis such as tumor suppressor proteins and oncoproteins, as well as proteins with important biological functions such as cytoskeletal proteins, transcription factors, heat-shock proteins, and chromatin proteins.

Mutational analysis of the catalytic domain of O-linked N-acetylglucosaminyl transferase

O-Linked N-acetylglucosaminyltransferase (OGT) catalyzes the transfer of O-linked GlcNAc to serine/threonine residues of a variety of target proteins, many of which have been implicated in such diseases as diabetes and neurodegeneration. The addition of O-GlcNAc to proteins occurs in response to fluctuations in cellular concentrations of UDP-GlcNAc, which result from nutrients entering the hexosamine biosynthetic pathway. However, the molecular mechanisms involved in sugar nucleotide recognition and transfer to protein are poorly understood. We employed site-directed mutagenesis to target potentially important amino acid residues within the two conserved catalytic domains of OGT (CD I and CD II), followed by an in vitro glycosylation assay to evaluate N-acetylglucosaminyltransferase activity after bacterial expression. Although many of the amino acid substitutions caused inactivation of the enzyme, we identified three amino acid residues (two in CD I and one in CD II) that produced viable enzymes when mutated. Structure-based homology modeling revealed that these permissive mutants may be either in or near the sugar nucleotide-binding site. Our findings suggest a model in which the two conserved regions of the catalytic domain, CD I and CD II, contribute to the formation of a UDP-GlcNAc-binding pocket that catalyzes the transfer of O-GlcNAc to substrate proteins. Identification of viable OGT mutants may facilitate examination of its role in nutrient sensing and signal transduction cascades.

Turnover and characterization of UDP-N-acetylglucosaminyl transferase in a stably transfected HeLa cell line

To estimate the turnover of UDP-N-acetylglucosaminyl transferase (OGT), we exposed stably transfected HeLa cells to tetracycline for 16h to induce OGT gene expression and increase cytosolic enzyme levels. Removal of tetracycline led to a progressive decrease in OGT activity (after a 6h lag period), yielding an estimated OGT half-life of 13h. A similar half-life (12h) was obtained by measuring the loss of biosynthetically labeled OGT ([35S]methionine pulse-chase experiments). Since OGT turnover was relatively slow, it is unlikely that changes in OGT gene expression or protein expression play a role in the short-term regulatory actions mediated by the hexosamine signaling pathway. We also found that the overexpressed 110kDa murine OGT subunit (recombinant enzyme) was enzymatically similar to the endogenous holoenzyme derived from rat brain tissue. Thus, stably transfected HeLa cells provide an abundant source of enzyme that can be used to study the structure, function, and regulation of OGT.

O-glycosylation of the tail domain of neurofilament protein M in human neurons and in spinal cord tissue of a rat model of amyotrophic lateral sclerosis (ALS)

Mammalian neurofilaments (NFs) are modified by post-translational modifications that are thought to regulate NF assembly and organization. Whereas phosphorylation has been intensely studied, the role of another common modification, the attachment of O-linked N-acetylglucosamine (GlcNAc) to individual serine and threonine residues, is hardly understood. We generated a novel monoclonal antibody that specifically recognizes an O-glycosylated epitope in the tail domain of NF-M and allows determination of the glycosylation state at this residue. The antibody displays strong species preference for human NF-M, shows some reactivity with rat but not with mouse or bovine NF-M. By immunohistochemistry and Western blot analysis of biopsy-derived human temporal lobe tissue we show that immunoreactivity is highly enriched in axons parallel to hyperphosphorylated NFs. Treatment of cultured neurons with the GlcNAcase inhibitor PUGNAc causes a 40% increase in immunoreactivity within 1 h, which is completely reversible and parallels the total increase in cellular O-GlcNAc modification. Treatment with the mitogen-activated protein kinase kinase inhibitor PD-98059 leads to a similar increase in immunoreactivity. In spinal cord tissue of a transgenic rat model for amyotrophic lateral sclerosis, immunoreactivity is strongly decreased compared with wild-type animals while phosphorylation is increased. The data suggest that hyperphosphorylation and tail domain O-glycosylation of NFs are synchronously regulated in axons of human neurons in situ and that O-glycosylation of NF-M is highly dynamic and closely interweaved with phosphorylation cascades and may have a pathophysiological role.

Adhesion properties of adhesion-regulating molecule 1 protein on endothelial cells

Numerous adhesion molecules have been described, and the molecular mechanisms of lymphocyte trafficking across the endothelium is starting to be elucidated. Identification of the molecules involved in the organoselectivity of this process would help in the targeting of drug therapy to specific tissues. Adhesion-regulating molecule-1 (ARM-1) is an adhesion-regulating molecule previously identified on T cells. It does not belong to any known families of adhesion molecules. In this study, we show the presence of ARM-1 in endothelial cells, the adhesion partners of lymphocytes. ARM-1 mRNA was found to be differentially expressed in endothelial cell lines of various tissue origin and lymphocyte cell lines. Interestingly, ARM-1 is absent from skin endothelial cells. In our assay, skin endothelial cells display a distinct capacity to mediate adhesion of activated T lymphocytes. Overexpression of ARM-1 in skin endothelial cells increased adhesion of CEMT4 and NK lymphocytes, confirming that ARM-1 also regulates adhesion in endothelial cells. We also show that ARM-1 is a cytosolic protein associated with the plasma membrane. However, no cell surface expression of the protein was observed. These results suggest an indirect role of ARM-1 in adhesion rather than a direct role as an adhesion molecule itself.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. Recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

Differential effects of vanadate on UDP-N-acetylglucosaminyl transferase activity derived from cytosol and nucleosol

UDP-N-acetylglucosaminyl transferase (OGT) is a key enzyme of a novel signal transduction pathway that regulates protein function through O-linked glycosylation. In the current study, we found that sodium vanadate potently inhibits OGT activity in brain cytosol (IC50 = 55 microM) and nucleosol (IC50 = 150 microM), but fails to alter activity of a related enzyme (UDP-galactosyltransferase). Vanadate also inhibits OGT activity in cytosol (IC50 of 2.3 microM) and nucleosol (IC50 of 130) derived from a stable HeLa cell line that overexpresses OGT. When HeLa cytosol was immunopurified to separate OGT from other cellular proteins, vanadate still inhibited OGT activity (IC50 = 2 microM). We conclude that OGT derived from cytosol exhibits greater vanadate sensitivity than nucleosol OGT and that a large difference exists (25-fold) in vanadate sensitivity when comparing OGT activity in different cell types (IC50 of 55 microM for brain cytosol vs. 2.3 microM for HeLa cytosol). Understanding the mechanism(s) by which a tyrosine phosphatase inhibitor differentially reduces OGT activity should lead to new insights into OGT function and regulation.

Synergistic Effect of N-Acetylglucosamine and Retinoids on Hyaluronan Production in Human Keratinocytes

Hyaluronan (HA) is well known to reside in the extracellular matrix as a water-sorbed macromolecule. The aims of this study were twofold: to investigate the regulation of HA synthesis in keratinocytes, and to develop a method to modulate this regulatory process. We found that N-acetylglucosamine (NAG) increased the production of HA by cultured keratinocytes dose dependently, but had no effect on the production by skin fibroblasts. The effect of NAG in keratinocytes was found to be specific for HA production, as there was no change in sulfated glycosaminoglycan formation. The copresence of NAG with either of two retinoids, retinoic acid (RA) or retinol, exerted a synergistic effect on HA production. To investigate whether human HA synthase (HAS) genes were regulated by NAG or retinoids, total RNA extracted from cells treated with these agents was subjected to Northern blot analysis. We observed that RA and retinol markedly induced the expression of HA synthase-3 (HAS3) mRNA. Moreover, beta-carotene, a provitamin A, influenced HA production and HAS3 gene expression in a manner similar to the retinoids. Conversely, NAG had no effect on the expression of HAS3 transcripts. Pretreatment of cells with RA stimulated the activity of membrane-associated HAS, whereas pretreatment with NAG did not. These results suggest that HA production is regulated by at least two pathways: one involving the regulation of HAS gene expression, and the other independent of such a regulatory effect. Taken together, our findings suggest that NAG is a new modulator of HA synthesis.

UDP-N-acetylglucosaminyl transferase (OGT) in brain tissue: temperature sensitivity and subcellular distribution of cytosolic and nuclear enzyme

In brain tissue, UDP-N-acetylglucosaminyl transferase (OGT) is known to catalyze the addition of a single N-acetylglucosamine moiety (GlcNAc) onto two proteins linked to the etiology of neurodegenerative disease--beta-amyloid associated protein and tau. Hyperphosphorylation of tau appears to cause neurofibrillary tangles and cell death, and a functional relationship appears to exist between phosphorylation and glycosylation. Since a greater understanding of brain OGT may provide new insights into the pathogenesis of Alzheimer's disease, we examined the characteristics and subcellular distribution of OGT protein and OGT activity and its relationship to O-linked glycosylation. We found that cytosolic OGT activity is 10 times more abundant in brain tissue compared with muscle, adipose, heart, and liver tissue. Temperature studies demonstrated that cytosolic OGT activity was stable at 24 degrees C but was rapidly inactivated at 37 degrees C (T1/2 = 20 min). Proteases were probably not involved because OGT immunopurified from cytosol retained temperature sensitivity. Subcellular distribution studies showed abundant OGT protein in the nucleus that was enzymatically active. Nuclear OGT activity exhibited a high affinity for UDP-GlcNAc and a salt sensitivity that was similar to cytosolic OGT; however, nuclear OGT was not inactivated at 37 degrees C, as was the cytosolic enzyme. Two methods were used to measure O-linked glycoproteins in brain cytosol and nucleosol -[3H]galactose labeling and western blotting using antibodies against O-linked glycoproteins. Both methods revealed a greater abundance of O-linked glycoproteins in the nucleus compared to cytosol.

Enhanced expression of uridine diphosphate-N-acetylglucosaminyl transferase (OGT) in a stable, tetracycline-inducible HeLa cell line using histone deacetylase inhibitors: kinetics of cytosolic OGT accumulation and nuclear translocation

We have created a stable, tetracycline-inducible HeLa cell line that overexpresses murine uridine diphosphate-N-acetylglucosaminyl transferase (OGT). Tetracycline increased cytosolic OGT activity about 4-fold in a dose-dependent manner (ED(50)=0.03 microg/ml) with enhanced activity observable at 8h and maximal activity observable by 40h. Enhanced OGT activity was due to overexpression of OGT protein as determined by Western analysis. Trichostatin A (TSA), a potent and specific histone deacetylase inhibitor (HDI), markedly enhanced tetracycline-induced OGT gene expression, resulting in a >10-fold increase in OGT activity (>50-fold compared to that of uninduced cells). Other HDIs such as butyrate (ED(50)=1.6mM) and propionate (ED(50)=8mM) were similarly effective, but less potent than TSA (ED(50)=120 nM). We next examined the appearance of recombinant OGT in cytosol and nucleosol at various times (10 min to 6h) after inducing OGT gene. Within 2h, recombinant OGT was detected by Western analysis in both cytosol and nucleosol. This indicates rapid biosynthesis and accumulation of recombinant OGT in the cytosol and subsequent nuclear translocation. Entry of OGT into the nucleus was closely correlated with enhanced O-linked glycosylation of nuclear proteins, indicating that recombinant OGT was enzymatically active. The ability to rapidly induce OGT expression in a stable cell line provides an excellent model system to study the mechanism(s) underlying OGT nuclear translocation and a useful system to elucidate the cascade of signaling events related to O-linked glycosylation.

Measurement of UDP-N-acetylglucosaminyl transferase (OGT) in brain cytosol and characterization of anti-OGT antibodies

UDP-N-acetylglucosaminyl transferase (OGT) catalyzes O-linked glycosylation of cytosolic and nuclear proteins, but enzyme studies have been hampered by the lack of a rapid, sensitive, and economical OGT assay. Employed assay methods typically involved the use of HPLC, formic acid, and large amounts of expensive radiolabeled [3H]UDP-N-acetylglucosaminyl ([3H]UDP-GlcNAc). In the current study, we have developed an OGT assay that circumvents many of these problems through four critical assay improvements: (1) identification of an abundant and enriched source of OGT enzyme (rat brain tissue), (2) utilization of a rapid method for efficiently removing salts and sugar nucleotides from cytosol (polyethylene glycol precipitation of active enzyme), (3) expression of a recombinant p62 acceptor substrate designed to facilitate purification (polyhistidine metal-chelation site), and (4) development of two alternative methods to rapidly separate free [3H]UDP-GlcNAc from 3H-p62ST acceptor peptide (trichloroacetic acid precipitation and metal-chelation affinity purification). To study the enzymology of OGT, independent of potential regulatory proteins within cytosol, we also developed and characterized an alternate OGT assay that uses antibody-purified OGT as the enzyme source. The major advantage of this assay lies in the ability to measure OGT in the absence of other cytosolic proteins.

Identification of novel sites of O-N-acetylglucosamine modification of serum response factor using quadrupole time-of-flight mass spectrometry

The addition of a single N-acetylglucosamine moiety O-linked to serine and threonine residues of nuclear and cytoplasmic proteins is a widespread post-translational modification. The conventional method for detecting and locating sites of modification is through a multistep radioactivity-based approach. We have recently shown that sites of O-GlcNAc modification can be determined using quadrupole time-of-flight tandem mass spectrometry (Chalkley, R. J., and Burlingame, A. L. (2001) Identification of GlcNAcylation sites of peptides and alpha-crystallin using Q-TOF mass spectrometry. J. Am. Soc. Mass Spectrom. 12, 1106-1113). In this work utilization of this new approach has revealed previously undetected sites of O-GlcNAc modification of the transcription factor serum response factor.

One O-linked sugar can affect the coil-to-beta structural transition of the prion peptide

It has been known that the structural transition from PrP(C) to PrP(Sc) leads to the prion formation. This putative conformational change challenges the central dogma of the protein folding theory-"one sequence, one structure." Generally, scientists believe that there must be either a posttranslational modification or environmental factors involved in this event. However, all of the efforts to solve the mystery of the PrP(C) to PrP(Sc) transition have ended in vain so far. Here we provide evidence linking O-linked glycosylation to the structural transition based on prion peptide studies. We find that the O-linked alpha-GalNAc at Ser-135 suppresses the formation of amyloid fibril formation of the prion peptide at physiological salt concentrations, whereas the peptide with the same sugar at Ser-132 shows the opposite effect. Moreover, this effect is sugar specific. Replacing alpha-GalNAc with beta-GlcNAc does not yield the same effect.

Elevated nucleocytoplasmic glycosylation by O-GlcNAc results in insulin resistance associated with defects in Akt activation in 3T3-L1 adipocytes

Increased flux of glucose through the hexosamine biosynthetic pathway (HSP) is believed to mediate hyperglycemia-induced insulin resistance in diabetes. The end product of the HSP, UDP beta-N-acetylglucosamine (GlcNAc), is a donor sugar nucleotide for complex glycosylation in the secretory pathway and for O-linked GlcNAc (O-GlcNAc) addition to nucleocytoplasmic proteins. Cycling of the O-GlcNAc posttranslational modification was blocked by pharmacological inhibition of O-GlcNAcase, the enzyme that catalyzes O-GlcNAc removal from proteins, with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc). PUGNAc treatment increased levels of O-GlcNAc and caused insulin resistance in 3T3-L1 adipocytes. Insulin resistance induced through the HSP by glucosamine and chronic insulin treatment correlated with increased O-GlcNAc levels on nucleocytoplasmic proteins. Whereas insulin receptor autophosphorylation and insulin receptor substrate 2 tyrosine phosphorylation were not affected by PUGNAc inhibition of O-GlcNAcase, downstream phosphorylation of Akt at Thr-308 and glycogen synthase kinase 3 beta at Ser-9 was inhibited. PUGNAc-induced insulin resistance was associated with increased O-GlcNAc modification of several proteins including insulin receptor substrate 1 and beta-catenin, two important effectors of insulin signaling. These results suggest that elevation of O-GlcNAc levels attenuate insulin signaling and contribute to the mechanism by which increased flux through the HSP leads to insulin resistance in adipocytes.

Stimulation of insulin secretion and associated nuclear accumulation of iPLA(2)beta in INS-1 insulinoma cells

Accumulating evidence suggests that the cytosolic calcium-independent phospholipase A(2) (iPLA(2)beta) manifests a signaling role in insulin-secreting (INS-1) beta-cells. Earlier, we reported that insulin-secretory responses to cAMP-elevating agents are amplified in iPLA(2)beta-overexpressing INS-1 cells (Ma Z, Ramanadham S, Bohrer A, Wohltmann M, Zhang S, and Turk J. J Biol Chem 276: 13198-13208, 2001). Here, immunofluorescence, immunoaffinity, and enzymatic activity analyses are used to examine distribution of iPLA(2)beta in stimulated INS-1 cells in greater detail. Overexpression of iPLA(2)beta in INS-1 cells leads to increased accumulation of iPLA(2)beta in the nuclear fraction. Increasing glucose concentrations alone results in modest increases in insulin secretion, relative to parental cells, and in nuclear accumulation of the iPLA(2)beta protein. In contrast, cAMP-elevating agents induce robust increases in insulin secretion and in time-dependent nuclear accumulation of iPLA(2)beta fluorescence, which is reflected by increases in nuclear iPLA(2)beta protein content and specific enzymatic activity. The stimulated effects are significantly attenuated in the presence of cell-permeable inhibitors of protein phosphorylation and glycosylation. These findings suggest that conditions that amplify insulin secretion promote translocation of beta-cell iPLA(2)beta to the nuclei, where it may serve a crucial signaling role.

Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds

Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.

Dynamic O-glycosylation of nuclear and cytosolic proteins: further characterization of the nucleocytoplasmic beta-N-acetylglucosaminidase, O-GlcNAcase

beta-O-linked N-acetylglucosamine (O-GlcNAc) is an abundant and dynamic post-translational modification implicated in protein regulation that appears to be functionally more similar to phosphorylation than to classical glycosylation. There are nucleocytoplasmic enzymes for the attachment and removal of O-GlcNAc. Here, we further characterize the recently cloned beta-N-acetylglucosaminidase, O-GlcNAcase. Both recombinant and purified endogenous O-GlcNAcase rapidly release free GlcNAc from O-GlcNAc-modified peptide substrates. The recombinant enzyme functions as a monomer and has kinetic parameters (K(m) = 1.1 mm for paranitrophenyl-GlcNAc, k(cat) = 1 s(-1)) that are similar to those of lysosomal hexosaminidases. The endogenous O-GlcNAcase appears to be in a complex with other proteins and is predominantly localized to the cytosol. Overexpression of the enzyme in living cells results in decreased O-GlcNAc modification of nucleocytoplasmic proteins. Finally, we show that the enzyme is a substrate for caspase-3 but, surprisingly, the cleavage has no effect on in vitro O-GlcNAcase activity. These studies support the identification of this protein as an O-GlcNAcase and identify important interactions and modifications that may regulate the enzyme and O-GlcNAc cycling.

Homology between O-linked GlcNAc transferases and proteins of the glycogen phosphorylase superfamily

The O-linked GlcNAc transferases (OGTs) are a recently characterized group of largely eukaryotic enzymes that add a single beta-N-acetylglucosamine moiety to specific serine or threonine hydroxyls. In humans, this process may be part of a sugar regulation mechanism or cellular signaling pathway that is involved in many important diseases, such as diabetes, cancer, and neurodegeneration. However, no structural information about the human OGT exists, except for the identification of tetratricopeptide repeats (TPR) at the N terminus. The locations of substrate binding sites are unknown and the structural basis for this enzyme's function is not clear. Here, remote homology is reported between the OGTs and a large group of diverse sugar processing enzymes, including proteins with known structure such as glycogen phosphorylase, UDP-GlcNAc 2-epimerase, and the glycosyl transferase MurG. This relationship, in conjunction with amino acid similarity spanning the entire length of the sequence, implies that the fold of the human OGT consists of two Rossmann-like domains C-terminal to the TPR region. A conserved motif in the second Rossmann domain points to the UDP-GlcNAc donor binding site. This conclusion is supported by a combination of statistically significant PSI-BLAST hits, consensus secondary structure predictions, and a fold recognition hit to MurG. Additionally, iterative PSI-BLAST database searches reveal that proteins homologous to the OGTs form a large and diverse superfamily that is termed GPGTF (glycogen phosphorylase/glycosyl transferase). Up to one-third of the 51 functional families in the CAZY database, a glycosyl transferase classification scheme based on catalytic residue and sequence homology considerations, can be unified through this common predicted fold. GPGTF homologs constitute a substantial fraction of known proteins: 0.4% of all non-redundant sequences and about 1% of proteins in the Escherichia coli genome are found to belong to the GPGTF superfamily.

Identification of GlcNAcylation sites of peptides and alpha-crystallin using Q-TOF mass spectrometry

The addition of a single N-acetylglucosamine residue O-linked to serine and threonine residues of nuclear and cytoplasmic proteins is a widespread modification throughout all eukaryotes. The conventional method for detecting and locating sites of modification is a multi-step radioactivity-based protocol. In this paper we show that using quadrupole time-of-flight (Q-TOF) mass spectrometry, modification sites can be identified at a significantly higher sensitivity than previous approaches. This is the first demonstration that sites of O-GlcNAcylation can be identified directly using mass spectrometry.

Hyperglycemia activates p53 and p53-regulated genes leading to myocyte cell death

To determine whether enzymatic p53 glycosylation leads to angiotensin II formation followed by p53 phosphorylation, prolonged activation of the renin-angiotensin system, and apoptosis, ventricular myocytes were exposed to levels of glucose mimicking diabetic hyperglycemia. At a high glucose concentration, O-glycosylation of p53 occurred between 10 and 20 min, reached its peak at 1 h, and then decreased with time. Angiotensin II synthesis increased at 45 min and 1 h, resulting in p38 mitogen-activated protein (MAP) kinase-driven p53 phosphorylation at Ser 390. p53 phosphorylation was absent at the early time points, becoming evident at 1 h, and increasing progressively from 3 h to 4 days. Phosphorylated p53 at Ser 18 and activated c-Jun NH(2)-terminal kinases were identified with hyperglycemia, whereas extracellular signal-regulated kinase was not phosphorylated. Upregulation of p53 was associated with an accumulation of angiotensinogen and AT(1) and enhanced production of angiotensin II. Bax quantity also increased. These multiple adaptations paralleled the concentrations of glucose in the medium and the duration of the culture. Myocyte death by apoptosis directly correlated with glucose and angiotensin II levels. Inhibition of O-glycosylation prevented the initial synthesis of angiotensin II, p53, and p38-MAP kinase (MAPK) phosphorylation and apoptosis. AT(1) blockade had no influence on O-glycosylation of p53, but it interfered with p53 phosphorylation; losartan also prevented phosphorylation of p38-MAPK by angiotensin II. Inhibition of p38-MAPK mimicked at a more distal level the consequences of losartan. In conclusion, these in vitro results support the notion that hyperglycemia with diabetes promotes myocyte apoptosis mediated by activation of p53 and effector responses involving the local renin-angiotensin system.

Increased N-acetyl-beta-glucosaminidase activity in primary breast carcinomas corresponds to a decrease in N-acetylglucosamine containing proteins

N-acetylglucosamine (O-GlcNAc) modification on serine or threonine residues of cytoplasmic and nuclear proteins has become a more recognized intracellular covalent modification. Removal of this modification is carried out by N-acetyl-beta-glucosaminidase (O-GlcNAcase). Since little information exists on monoglycosylation and O-GlcNAcase activity in mitogenic systems, we investigated O-GlcNAcase activity in primary breast tumors compared to matched normal adjacent breast tissue and examined enzymatic activity in relationship to the level of protein monoglycosylation. Using a variation of the acidic hexosaminidase activity assay, we demonstrated an increase in both O-GlcNAcase and lysosomal hexosaminidase activity in breast tumor tissue compared to matched adjacent tissue. Although no clear correlation with tumor grade or type was apparent among the samples examined (12 matched pairs), the increase in O-GlcNAcase and lysosomal hexosaminidase activity in tumor tissue was consistently elevated and statistically significant (P<0.05). Protein monoglycosylation was evaluated using immunoblotting, affinity blotting, and radioactive labeling. While the variety of modified proteins was greater in tumor tissue compared to adjacent tissue, the total amount of O-GlcNAc monoglycosylation was significantly decreased in the tumor tissue especially on proteins in the molecular mass range of 45-65 kDa. O-GlcNAcase may be involved in the selective removal of O-GlcNAc on certain proteins in breast tumor tissue.

Characterization of a mouse monoclonal antibody specific for O-linked N-acetylglucosamine

beta-O-linked N-acetylglucosamine (O-GlcNAc) is an abundant posttranslational modification of resident nuclear and cytoplasmic proteins in eukaryotes. Increasing evidence suggests that O-GlcNAc plays a regulatory role in numerous cellular processes. Here we report on the production and characterization of a highly specific mouse monoclonal antibody, MAb CTD110.6, that specifically reacts with O-GlcNAc. The antibody recognizes O-GlcNAc in beta-O-glycosidic linkage to both serine and threonine. We could detect no cross-reactivity with alpha-linked Ser/Thr-O-GlcNAc, alpha-linked Ser-O-linked N-acetylgalactosamine (O-GalNAc), or N-linked oligosaccharides on ovalbumin and immunoglobulin G. The monosaccharide GlcNAc, but not GalNAc, abolishes immunoreactivity, further demonstrating specificity toward O-GlcNAc. Furthermore, galactose capping of O-GlcNAc sites also inhibits CTD110.6 immunoreactivity. Enrichment of GlcNAc-containing glycoproteins using the lectin wheat germ agglutinin dramatically enriches for CTD110.6-reactive proteins. The antibody reacts with a large number of proteins from cytoplasmic and nuclear extracts and readily detects in vivo changes in O-GlcNAc modification. These studies demonstrate that CTD110.6 is highly specific toward O-GlcNAc, with no cross-reactivity toward similar carbohydrate antigens or toward peptide determinants.