A relatively low glucose promotes the proliferation of vascular endothelial cells by suppressing VEGFR2 O-GlcNAcylation and its proteasome degradation.

PURPOSE: Vascular endothelial growth factor receptors (VEGFRs) have been demonstrated to play a critical role in ischemic retinal diseases, as VEGFRs mediate hypoxia-induced neovascularization. Not only hypoxia, ischemia also induces the deficiency of glucose, yet its effects on VEGFR signal and neovascularization have seldom been studied. Bioinformatics analysis predicted that VEGFRs may be regulated by O-GlcNAcylation, while glucose deficiency influences the O-GlcNAcylation. METHODS: In this study, we treated human retinal microvascular endothelial cells with low glucose (LG) alone or in combination with low oxygen (oxygen and glucose deprivation, OGD). Cell viability and apoptosis rate were used to evaluate cell growth characters. RESULTS: LG (2.8 mmol/L) treatment induced mRNA and protein levels of VEGFR1, 2, 3 even in the presence of the protein synthesis inhibitor, cycloheximide (CHX), suggesting that the increase in VEGFR proteins is partially associated with post-translational modifications. Immunoprecipitation analysis showed that O-GlcNAc level was decreased by LG in both VEGFR1, 2, but a de-O-GlcNAc glycosylase inhibitor restored the O-GlcNAc levels. This inhibitor also abolished the LG-induced increase in VEGFR2 protein, whereas this effect was not disappeared in the presence of the proteasome inhibitor, MG132. Similar results were also observed under OGD condition. VEGFR2 knockdown more significantly retarded the growth of hRMECs and HUVECs than VEGFR1, 3 knockdown under LG and OGD conditions. CONCLUSIONS: A relatively low glucose suppressed O-GlcNAcylation in VEGFR2, whereby inhibiting its proteasome degradation; up-regulated VEGFR2 promoted the proliferation of vascular endothelial cells under ischemic condition.

[Effect of knockdown of O-Glcnac transferase on hepatocyte fat synthesis].

Objective: To investigate the effect of knockdown of O-GlcNAc transferase (OGT) on hepatocyte fat synthesis. Methods: Liver cell line L02 were used to established the model of hepatic steatosis. The levels of OGT and O-GlcNAc protein were detected by Western blot. The OGT knockdown cell line of L02 cells was established, and its lipid formation ability was detected after induction of oleic acid (OA). Real-time quantitative PCR (qRT-PCR) and Western blot were used to detect mRNA and protein expression of enzymes related to fat synthesis. An independent sample t test was used. Results: Western blot showed that the expression of OGT and O-GlcNAc was increased in L02 cells after adipogenesis (P < 0.05). After shOGT lentivirus infects L02 cells, OGT mRNA levels were down-regulated (P < 0.01). Oil red O staining showed that the lipid in L02 shOGT cells decreased, qRT-PCR showed that the mRNA expressions of fat synthase (ACC1), (FASN) and (SCD1) were decreased, the difference was statistically significant (P < 0.05), protein Expression is consistent with mRNA expression. Conclusion: Knockdown of OGT can inhibit hepatocyte fat synthesis by reducing O-GlcNAc levels.

O-GlcNAc Glycosylation of nNOS Promotes Neuronal Apoptosis Following Glutamate Excitotoxicity.

Ischemic stroke is a dominant health problem with extremely high rates of mortality and disability. The main mechanism of neuronal injury after stroke is excitotoxicity, during which the activation of neuronal nitric oxide synthase (nNOS) exerts a vital role. However, directly blocking N-methyl-D-aspartate receptors or nNOS can lead to severe undesirable effects since they have crucial physiological functions in the central nervous system. Here, we report that nNOS undergoes O-linked-ß-N-acetylglucosamine (O-GlcNAc) modification via interacting with O-GlcNAc transferase, and the O-GlcNAcylation of nNOS remarkably increases during glutamate-induced excitotoxicity. In addition, eliminating the O-GlcNAcylation of nNOS protects neurons from apoptosis during glutamate stimulation by decreasing the formation of nNOS-postsynaptic density protein 95 complexes. Taken together, our data suggest a novel function of the O-GlcNAcylation of nNOS in neuronal apoptosis during glutamate excitotoxicity, suggesting a novel therapy strategy for ischemic stroke.

Global increase in O-linked N-acetylglucosamine modification promotes osteoblast differentiation.

The balance between bone formation and bone resorption is maintained by osteoblasts and osteoclasts, and an imbalance in this bone metabolism leads to osteoporosis. Here, we found that osteoblast differentiation in MC3T3-E1 cells is promoted by the inactivation of O-linked ß-N-acetylglucosaminidase (O-GlcNAcase) and suppressed by the inactivation of O-GlcNAc transferase, as indicated by extracellular matrix calcification. The expression of osteogenic genes such as alp, ocn, and bsp during osteoblast differentiation was positively regulated in a O-GlcNAc glycosylation-dependent manner. Because it was confirmed that Ets1 and Runx2 are the two key transcription factors responsible for the expression of these osteogenic genes, their transcriptional activity might therefore be regulated by O-GlcNAc glycosylation. However, osteoclast differentiation of RAW264 cells, as indicated by the expression and activity of tartrate-resistant acid phosphatase, was unaffected by the inactivation of either O-GlcNAcase or O-GlcNAc transferase. Our findings suggest that an approach to manipulate O-GlcNAc glycosylation could be useful for developing the therapeutics for osteoporosis.

Regulatory O-GlcNAcylation sites on FoxO1 are yet to be identified.

O-GlcNAcylation is a reversible post-translational modification that regulates cytosolic and nuclear proteins. We and others previously demonstrated that FoxO1 is O-GlcNAcylated in different cell types, resulting in an increase in its transcriptional activity. Four O-GlcNAcylation sites were identified in human FOXO1 but directed mutagenesis of each site individually had modest (T317) or no effect (S550, T648, S654) on its O-GlcNAcylation status and transcriptional activity. Moreover, the consequences of mutating all four sites had not been investigated. In the present work, we mutated these sites in the mouse Foxo1 and found that mutation of all four sites did not decrease Foxo1 O-GlcNAcylation status and transcriptional activity, and would even tend to increase them. In an attempt to identify other O-GlcNAcylation sites, we immunoprecipitated wild-type O-GlcNAcylated Foxo1 and analysed the tryptic digest peptides by mass spectrometry using High-energy Collisional Dissociation. We identified T646 as a new O-GlcNAcylation site on Foxo1. However, site directed mutagenesis of this site individually or together with all four previously identified residues did not impair Foxo1 O-GlcNAcylation and transcriptional activity. These results suggest that residues important for the control of Foxo1 activity by O-GlcNAcylation still remain to be identified.

Circ8199 encodes a protein that inhibits the activity of OGT by JAK2-STAT3 pathway in esophageal squamous cell carcinoma.

Esophageal squamous cell carcinoma (ESCC) is an invasive malignant tumor with a high incidence rate and mortality. It is imperative to study its tumorigenesis and development for better treatment. CircRNA has been proven to play an important role in various cancers. Our previous studies found that the circ8199 gene is associated with tumor prognosis. To further clarify the role of circ8199 in ESCC, we performed functional experiments and found that overexpression of circ8199 significantly inhibited the proliferation of ESCC cells and the activity of O-linked N-acetylglucosamine transferase (OGT) simultaneously. Further experiments demonstrated that circ8199 could interact with OGT, leading to a decrease in OGT's activity. The reduction of circ8199 expression stimulated the binding activity between OGT and its downstream gene JAK2, promoting the O-GlcNAc glycosylation modification of JAK2 and activating the JAK2-STAT3 pathway. Our study indicated that circ8199 regulates the JAK2-STAT3 pathway through OGT, providing a candidate mechanism for drug discovery and development.

Variation of ewe olfactory secretome during a ram effect.

Introduction: Under temperate latitudes, reproduction in Ovis aries displays a marked seasonality, governed by the photoperiod. In natural conditions, the transition between sexual rest and sexual activity in both sexes is induced by the decrease of day light. Meanwhile, specific odors emitted by a sexually active male are able to reactivate the gonadotropic axis of anovulatory ewes. This physiological effect is called "male effect", precisely ram effect in the ovine species. We have previously shown that the secreted proteins, namely Olfactory Binding Proteins (OBP), contained in the nasal mucus constitute the olfactory secretome (OS), the composition of which is determined by the status of oestrus cycle of females and differs between sexual rest and sexual activity periods. The objective of this study was to test the hypothesis that exposure to sexually active male can also modify the composition of ewes olfactory secretome during a male effect, as well as hormones produced by the reactivation of the oestrus cycle in sexual activity period under natural conditions. Methods: We have set up a new non-invasive protocol of nasal mucus sampling and collected it from 12 ewes at different times during a ram effect. We analyzed the composition of their olfactory secretome by proteomics, mainly SDS-PAGE and MALDI-TOF mass spectrometry. As post-translational modifications of OBPs were a hallmark of ewes' sexual activity period, we were looking for glycosylation by western-blot and mass spectrometry. Results: The efficiency of male effect was low in stimulated ewes as only 3 females displayed elevated progesterone levels in their blood. Besides, half of control ewes (non-stimulated ones) were cycled. We noticed a common OS profile in ewes in anoestrus, versus OS of cycled ones. A very clear and important result was the apparition of O-GlcNAcylation, previously detected only in sexual activity, after only 30 min of male introduction into the flock. Discussion: This exploratory study paves the way for further experiments with larger flock to confirm and reinforce these results, and for eventually exploiting the nasal mucus as an indicator of females' receptivity to male odors.

Phosphorylation and O-GlcNAcylation of the PHF-1 Epitope of Tau Protein Induce Local Conformational Changes of the C-Terminus and Modulate Tau Self-Assembly Into Fibrillar Aggregates.

Phosphorylation of the neuronal microtubule-associated Tau protein plays a critical role in the aggregation process leading to the formation of insoluble intraneuronal fibrils within Alzheimer's disease (AD) brains. In recent years, other posttranslational modifications (PTMs) have been highlighted in the regulation of Tau (dys)functions. Among these PTMs, the O-ß-linked N-acetylglucosaminylation (O-GlcNAcylation) modulates Tau phosphorylation and aggregation. We here focus on the role of the PHF-1 phospho-epitope of Tau C-terminal domain that is hyperphosphorylated in AD (at pS396/pS404) and encompasses S400 as the major O-GlcNAc site of Tau while two additional O-GlcNAc sites were found in the extreme C-terminus at S412 and S413. Using high resolution NMR spectroscopy, we showed that the O-GlcNAc glycosylation reduces phosphorylation of PHF-1 epitope by GSK3ß alone or after priming by CDK2/cyclin A. Furthermore, investigations of the impact of PTMs on local conformation performed in small peptides highlight the role of S404 phosphorylation in inducing helical propensity in the region downstream pS404 that is exacerbated by other phosphorylations of PHF-1 epitope at S396 and S400, or O-GlcNAcylation of S400. Finally, the role of phosphorylation and O-GlcNAcylation of PHF-1 epitope was probed in in-vitro fibrillization assays in which O-GlcNAcylation slows down the rate of fibrillar assembly while GSK3ß phosphorylation stimulates aggregation counteracting the effect of glycosylation.

Research Progress on O-GLcNAc Glycosylation Modification in Neurodegenerative Diseases / 中山大学学报(医学科学版)

O-linked N-acetylglucosamine glycosylation （O-GlcNAcylation） is a protein post- translational protein formed by O-linked acetylglucosamine and serine/threonine residues on intracellular proteins. O-GlcNAc glycosylation modification is ubiquitous in the brain and is closely related to transcription， translation and protein homeostasis. O-GlcNAc glycosylation modification is involved in the occurrence and development of various neurological degenerative diseases， but its regulatory mechanism remains unclear. This paper reviews the relationship between O-GlcNAc glycosylation modification and neurological degenerative diseases.

Deficient O-GlcNAc Glycosylation Impairs Regulatory T Cell Differentiation and Notch Signaling in Autoimmune Hepatitis.

Post-translational modifications such as glycosylation play an important role in the functions of homeostatic proteins, and are critical driving factors of several diseases; however, the role of glycosylation in autoimmune hepatitis is poorly understood. Here, we established an O-GlcNAc glycosylation-deficient rat model by knocking out the Eogt gene by TALEN-mediated gene targeting. O-GlcNAc glycosylation deficiency overtly aggravated liver injury in concanavalin-A induced autoimmune hepatitis, and delayed self-recovery of the liver. Furthermore, flow cytometry analysis revealed increased CD4+ T cell infiltration in the liver of rats with O-GlcNAc glycosylation deficiency, and normal differentiation of regulatory T cells (Tregs) in the liver to inhibit T cell infiltration could not be activated. Moreover, in vitro experiments showed that O-GlcNAc glycosylation deficiency impaired Treg differentiation to inhibit the Notch signaling pathway in CD4+ T cells. These finding indicate that O-GlcNAc glycosylation plays a critical role in the activation of Notch signaling, which could promote Treg differentiation in the liver to inhibit T cell infiltration and control disease development in autoimmune hepatitis. Therefore, this study reveals a regulatory role for glycosylation in the pathogenesis of autoimmune hepatitis, and highlights glycosylation as a potential treatment target.

Label-free electrochemical biosensing of small-molecule inhibition on O-GlcNAc glycosylation.

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) plays a critical role in modulating protein function in many cellular processes and human diseases such as Alzheimer's disease and type II diabetes, and has emerged as a promising new target. Specific inhibitors of OGT could be valuable tools to probe the biological functions of O-GlcNAcylation, but a lack of robust nonradiometric assay strategies to detect glycosylation, has impeded efforts to identify such compounds. Here we have developed a novel label-free electrochemical biosensor for the detection of peptide O-GlcNAcylation using protease-protection strategy and electrocatalytic oxidation of tyrosine mediated by osmium bipyridine as a signal reporter. There is a large difference in the abilities of proteolysis of the glycosylated and the unglycosylated peptides by protease, thus providing a sensing mechanism for OGT activity. When the O-GlcNAcylation is achieved, the glycosylated peptides cannot be cleaved by proteinase K and result in a high current response on indium tin oxide (ITO) electrode. However, when the O-GlcNAcylation is successfully inhibited using a small molecule, the unglycosylated peptides can be cleaved easily and lead to low current signal. Peptide O-GlcNAcylation reaction was performed in the presence of a well-defined small-molecule OGT inhibitor. The results indicated that the biosensor could be used to screen the OGT inhibitors effectively. Our label-free electrochemical method is a promising candidate for protein glycosylation pathway research in screening small-molecule inhibitors of OGT.

High-glucose environment induced intracellular O-GlcNAc glycosylation and reduced galectin-7 expression in keratinocytes: Implications on impaired diabetic wound healing.

BACKGROUND: Diabetes is an important global health issue due to its increasing prevalence and association with various complications. Impaired wound healing is a serious complication associated with diabetes that frequently results in infection and amputation. Galectin-7 (Gal-7) has been reported to play an important role during skin wound healing. Previously, we had demonstrated that high glucose environment alters physiologic functions of keratinocytes and contributes to impaired wound healing in diabetic condition. OBJECTIVE: In this study, we hypothesized that Gal-7 expression of keratinocytes may be involved in delayed wound healing of diabetics. METHODS: Using cultured human keratinocytes and diabetic mice model, the Gal-7 expression was evaluated under high glucose environment. RESULTS: Our results demonstrated that high-glucose environment reduced Gal-7 expression, a molecule that plays an important role in keratinocyte migration. Additionally, we found that increased O-linked N-Acetyl-glucosamine (O-GlcNAc) is responsible for reduced Gal-7 expression in keratinocytes exposed to high glucose environment. CONCLUSION: Taken together, restoring the levels of Gal-7 and O-GlcNAc glycosylation may present novel therapeutic approach to promote wound healing in diabetic patients.

Functional analysis of recombinant human and Yarrowia lipolytica O-GlcNAc transferases expressed in Saccharomyces cerevisiae.

O-linked ß-N-acetylglucosamine (O-GlcNAc) glycosylation is an important post-translational modification in many cellular processes. It is mediated by O-GlcNAc transferases (OGTs), which catalyze the addition of O-GlcNAc to serine or threonine residues of the target proteins. In this study, we expressed a putative Yarrowia lipolytica OGT (YlOGT), the only homolog identified in the subphylum Saccharomycotina through bioinformatics analysis, and the human OGT (hOGT) as recombinant proteins in Saccharomyces cerevisiae, and performed their functional characterization. Immunoblotting assays using antibody against O-GlcNAc revealed that recombinant hOGT (rhOGT), but not the recombinant YlOGT (rYlOGT), undergoes auto-O-GlcNAcylation in the heterologous host S. cerevisiae. Moreover, the rhOGT expressed in S. cerevisiae showed a catalytic activity during in vitro assays using casein kinase II substrates, whereas no such activity was obtained in rYlOGT. However, the chimeric human-Y. lipolytica OGT, carrying the human tetratricopeptide repeat (TPR) domain along with the Y. lipolytica catalytic domain (CTD), mediated the transfer of O-GlcNAc moiety during the in vitro assays. Although the overexpression of full-length OGTs inhibited the growth of S. cerevisiae, no such inhibition was obtained upon overexpression of only the CTD fragment, indicating the role of TPR domain in growth inhibition. This is the first report on the functional analysis of the fungal OGT, indicating that the Y. lipolytica OGT retains its catalytic activity, although the physiological role and substrates of YlOGT remain to be elucidated.

O-GlcNAcylation and Inflammation: A Vast Territory to Explore.

O-GlcNAcylation is a reversible post-translational modification that regulates the activities of cytosolic and nuclear proteins according to glucose availability. This modification appears to participate in several hyperglycemia-associated complications. An important feature of metabolic diseases such as diabetes and obesity is the presence of a low-grade chronic inflammation that causes numerous complications. Hyperglycemia associated with the metabolic syndrome is known to promote inflammatory processes through different mechanisms including oxidative stress and abnormally elevated protein O-GlcNAcylation. However, the role of O-GlcNAcylation on inflammation remains contradictory. O-GlcNAcylation associated with hyperglycemia has been shown to increase nuclear factor κB (NFκB) transcriptional activity through different mechanisms. This could contribute in inflammation-associated diabetic complications. However, in other conditions such as acute vascular injury, O-linked N-acetyl glucosamine (O-GlcNAc) also exerts anti-inflammatory effects via inhibition of the NFκB pathway, suggesting a complex regulation of inflammation by O-GlcNAc. Moreover, whereas macrophages and monocytes exposed to high glucose for a long-term period developed a pro-inflammatory phenotype, the impact of O-GlcNAcylation in these cells remains unclear. A future challenge will be to clearly establish the role of O-GlcNAcylation in pro- and anti-inflammatory functions in macrophages.

Inhibitory effect of resveratrol on fat synthesis in liver cancer HepG2 cells and its mechanism / 吉林大学学报(医学版)

Objective: To investigate the effect of resveratrol (Res) on the fat synthesis in the liver cancer HepG cells, and to elucidate its possible mechanism. Methods: The HepG cells were cultured in vitro and divided into Res group (treated with 40 umol • L-1 DMSO- diluted Res for 24 h) and control group (treated with the same concentration of DMSO for 24 h). The cell supernatant was collected, and the levels of triglyceride (TG) and total cholesterol (TO in the cells in various groups were measured by ELISA. The mRNA and protein expression levels of lipase synthase acetyl-CoA carboxylase (ACCl), fatty acid synthetase (FASN) and stearoyl-CoA desaturase (SCD1) in the cells in various groups were detected by qRT-PCR and Western blotting method. The levels of O-linked N-acetylglucosamine (O-GIcNAc) glycosylation in the cells in various groups were detected by Western blotting method. Results: Compared with control group, the levels of TG and TC in the cells in Res group were decreased, but the difference was not statistically significant (t1=1.886, P>0.05; t2=2.457,P>0.05). Compared with control group, the levels of expressions of ACCl, FASN and SCD1 mRNA and proteins in the cells in Res group were significantly decreased ( P<0.05 or P<0. 01); the O-GlcNAc glycosylation level in the cells in Res group was significantly decreased (t=2. 87, P<0.05). Conclusion: Res lias the effect of inhibiting the fat synthesis in the liver cancer HepG cells. Its mechanism may be related to the reduction of cellular O-GlcNAc glycosylation level and the reduction of the expression of FASN.

Inhibitory effect of resveratrol on fat synthesis in liver cancer HepG2 cells and its mechanism / 吉林大学学报(医学版)

Objective:To investigate the effect of resveratrol (Res) on the fat synthesis in the liver cancer HepG2cells, and to elucidate its possible mechanism.Methods:The HepG2cells were cultured in vitro and divided into Res group (treated with 40μmol·L-1 DMSO-diluted Res for 24h) and control group (treated with the same concentration of DMSO for 24h) .The cell supernatant was collected, and the levels of triglyceride (TG) and total cholesterol (TC) in the cells in various groups were measured by ELISA.The mRNA and protein expression levels of lipase synthase acetyl-CoA carboxylase (ACC1) , fatty acid synthetase (FASN) and stearoyl-CoA desaturase (SCD1) in the cells in various groups were detected by qRT-PCR and Western blotting method.The levels of O-linked N-acetylglucosamine (O-GlcNAc) glycosylation in the cells in various groups were detected by Western blotting method.Results:Compared with control group, the levels of TG and TC in the cells in Res group were decreased, but the difference was not statistically significant (t1=1.886, P＞0.05;t2=2.457, P＞0.05) .Compared with control group, the levels of expressions of ACC1, FASN and SCD1mRNA and proteins in the cells in Res group were significantly decreased (P＜0.05or P＜0.01) ;the O-GlcNAc glycosylation level in the cells in Res group was significantly decreased (t=2.87, P＜0.05) .Conclusion:Res has the effect of inhibiting the fat synthesis in the liver cancer HepG2 cells.Its mechanism may be related to the reduction of cellular O-GlcNAc glycosylation level and the reduction of the expression of FASN.