OGT restrains the expansion of DNA damage signaling

RAD18 O-GlcNAcylation promotes translesion DNA synthesis and homologous recombination repair

RAD18, an important ubiquitin E3 ligase, plays a dual role in translesion DNA synthesis (TLS) and homologous recombination (HR) repair. However, whether and how the regulatory mechanism of O-linked N-acetylglucosamine (O-GlcNAc) modification governing RAD18 and its function during these processes remains unknown. Here, we report that human RAD18, can undergo O-GlcNAcylation at Ser130/Ser164/Thr468, which is important for optimal RAD18 accumulation at DNA damage sites. Mechanistically, abrogation of RAD18 O-GlcNAcylation limits CDC7-dependent RAD18 Ser434 phosphorylation, which in turn significantly reduces damage-induced PCNA monoubiquitination, impairs Polη focus formation and enhances UV sensitivity. Moreover, the ubiquitin and RAD51C binding ability of RAD18 at DNA double-strand breaks (DSBs) is O-GlcNAcylation-dependent. O-GlcNAcylated RAD18 promotes the binding of RAD51 to damaged DNA during HR and decreases CPT hypersensitivity. Our findings demonstrate a novel role of RAD18 O-GlcNAcylation in TLS and HR regulation, establishing a new rationale to improve chemotherapeutic treatment.

O-GlcNAcylation: the sweet side of epigenetics

Histones display a wide variety of post-translational modifications, including acetylation, methylation, and phosphorylation. These epigenetic modifications can influence chromatin structure and function without altering the DNA sequence. Histones can also undergo post-translational O-GlcNAcylation, a rather understudied modification that plays critical roles in almost all biological processes and is added and removed by O-linked N-acetylglucosamine transferase and O-GlcNAcase, respectively. This review provides a current overview of our knowledge of how O-GlcNAcylation impacts the histone code both directly and by regulating other chromatin modifying enzymes. This highlights the pivotal emerging role of O-GlcNAcylation as an essential epigenetic marker.

H4S47 O-GlcNAcylation regulates the activation of mammalian replication origins

The transmission and maintenance of genetic information in eukaryotic cells relies on the faithful duplication of the entire genome. In each round of division, excessive replication origins are licensed, with only a fraction activated to give rise to bi-directional replication forks in the context of chromatin. However, it remains elusive how eukaryotic replication origins are selectively activated. Here we demonstrate that O-GlcNAc transferase (OGT) enhances replication initiation by catalyzing H4S47 O-GlcNAcylation. Mutation of H4S47 impairs DBF4-dependent protein kinase (DDK) recruitment on chromatin, causing reduced phosphorylation of the replicative helicase mini-chromosome maintenance (MCM) complex and compromised DNA unwinding. Our short nascent-strand sequencing results further confirm the importance of H4S47 O-GlcNAcylation in origin activation. We propose that H4S47 O-GlcNAcylation directs origin activation through facilitating MCM phosphorylation, and this may shed light on the control of replication efficiency by chromatin environment.

LncRNA EBLN3P attributes methotrexate resistance in osteosarcoma cells through miR-200a-3p/O-GlcNAc transferase pathway

BACKGROUND

Osteosarcoma is highly malignant. The migration, invasion, and chemoresistance contribute to poor prognosis of osteosarcoma. Research reported that endogenous bornavirus-like nucleoprotein 3 pseudogene (EBLN3P) promotes the progression of osteosarcoma.

METHODS

In this study, the expression of EBLN3P in osteosarcoma tissue with different methotrexate (MTX) treatment responses was measured. Osteosarcoma cell lines with MTX resistance were constructed, and bioinformatic analysis was performed to explore the potential involved targets and pathways.

RESULTS

Higher EBLN3P was associated with MTX resistance. Downregulation of LncEBLN3P decreased the MTX resistance of osteosarcoma cells by sponging miR-200a-3p, an important microRNA that affects epithelial-mesenchymal transition (EMT). The decreased miR-200a-3p resulted in the upregulation of its target gene O-GlcNAc transferase (OGT), which in turn promoted the EMT process of osteosarcoma cells. Further analysis confirmed that the loss of OGT and over-expression of miR-200a-3p could partly abolish the MTX resistance induced by LncEBLN3P.

CONCLUSION

LncEBLN3P is upregulated in osteosarcoma and increases the MTX resistance in osteosarcoma cells through downregulating miR-200a-3p, which in turn promoted the EMT process of osteosarcoma cells by increasing the OGT.

Targeting O-GlcNAcylation to overcome resistance to anti-cancer therapies

In cancer cells, metabolic reprogramming is associated with an alteration of the O-GlcNAcylation homeostasis. This post-translational modification (PTM) that attaches O-GlcNAc moiety to intracellular proteins is dynamically and finely regulated by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA). It is now established that O-GlcNAcylation participates in many features of cancer cells including a high rate of cell growth, invasion, and metastasis but little is known about its impact on the response to therapies. The purpose of this review is to highlight the role of O-GlcNAc protein modification in cancer resistance to therapies. We summarize the current knowledge about the crosstalk between O-GlcNAcylation and molecular mechanisms underlying tumor sensitivity/resistance to targeted therapies, chemotherapies, immunotherapy, and radiotherapy. We also discuss potential benefits and strategies of targeting O-GlcNAcylation to overcome cancer resistance.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

OGA is associated with deglycosylation of NONO and the KU complex during DNA damage repair

Accumulated evidence shows that OGT-mediated O-GlcNAcylation plays an important role in response to DNA damage repair. However, it is unclear if the “eraser” O-GlcNAcase (OGA) participates in this cellular process. Here, we examined the molecular mechanisms and biological functions of OGA in DNA damage repair, and found that OGA was recruited to the sites of DNA damage and mediated deglycosylation following DNA damage. The recruitment of OGA to DNA lesions is mediated by O-GlcNAcylation events. Moreover, we have dissected OGA using deletion mutants and found that C-terminal truncated OGA including the pseudo HAT domain was required for the recruitment of OGA to DNA lesions. Using unbiased protein affinity purification, we found that the pseudo HAT domain was associated with DNA repair factors including NONO and the Ku70/80 complex. Following DNA damage, both NONO and the Ku70/80 complex were O-GlcNAcylated by OGT. The pseudo HAT domain was required to recognize NONO and the Ku70/80 complex for their deglycosylation. Suppression of the deglycosylation prolonged the retention of NONO at DNA lesions and delayed NONO degradation on the chromatin, which impaired non-homologus end joining (NHEJ). Collectively, our study reveals that OGA-mediated deglycosylation plays a key role in DNA damage repair.

O-GlcNAcylation Affects the Pathway Choice of DNA Double-Strand Break Repair

Exposing cells to DNA damaging agents, such as ionizing radiation (IR) or cytotoxic chemicals, can cause DNA double-strand breaks (DSBs), which are crucial to repair to maintain genetic integrity. O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is a post-translational modification (PTM), which has been reported to be involved in the DNA damage response (DDR) and chromatin remodeling. Here, we investigated the impact of O-GlcNAcylation on the DDR, DSB repair and chromatin status in more detail. We also applied charged particle irradiation to analyze differences of O-GlcNAcylation and its impact on DSB repair in respect of spatial dose deposition and radiation quality. Various techniques were used, such as the γH2AX foci assay, live cell microscopy and Fluorescence Lifetime Microscopy (FLIM) to detect DSB rejoining, protein accumulation and chromatin states after treating the cells with O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA) inhibitors. We confirmed that O-GlcNAcylation of MDC1 is increased upon irradiation and identified additional repair factors related to Homologous Recombination (HR), CtIP and BRCA1, which were increasingly O-GlcNAcyated upon irradiation. This is consistent with our findings that the function of HR is affected by OGT inhibition. Besides, we found that OGT and OGA activity modulate chromatin compaction states, providing a potential additional level of DNA-repair regulation.

Nutrient regulation of the flow of genetic information by O-GlcNAcylation

O-linked-β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification (PTM) that is actively added to and removed from thousands of intracellular proteins. As a PTM, O-GlcNAcylation tunes the functions of a protein in various ways, such as enzymatic activity, transcriptional activity, subcellular localization, intermolecular interactions, and degradation. Its regulatory roles often interplay with the phosphorylation of the same protein. Governed by 'the Central Dogma', the flow of genetic information is central to all cellular activities. Many proteins regulating this flow are O-GlcNAc modified, and their functions are tuned by the cycling sugar. Herein, we review the regulatory roles of O-GlcNAcylation on the epigenome, in DNA replication and repair, in transcription and in RNA processing, in protein translation and in protein turnover.

O-GlcNAc: Regulator of Signaling and Epigenetics Linked to X-linked Intellectual Disability

Cellular identity in multicellular organisms is maintained by characteristic transcriptional networks, nutrient consumption, energy production and metabolite utilization. Integrating these cell-specific programs are epigenetic modifiers, whose activity is often dependent on nutrients and their metabolites to function as substrates and co-factors. Emerging data has highlighted the role of the nutrient-sensing enzyme O-GlcNAc transferase (OGT) as an epigenetic modifier essential in coordinating cellular transcriptional programs and metabolic homeostasis. OGT utilizes the end-product of the hexosamine biosynthetic pathway to modify proteins with O-linked β-D-N-acetylglucosamine (O-GlcNAc). The levels of the modification are held in check by the O-GlcNAcase (OGA). Studies from model organisms and human disease underscore the conserved function these two enzymes of O-GlcNAc cycling play in transcriptional regulation, cellular plasticity and mitochondrial reprogramming. Here, we review these findings and present an integrated view of how O-GlcNAc cycling may contribute to cellular memory and transgenerational inheritance of responses to parental stress. We focus on a rare human genetic disorder where mutant forms of OGT are inherited or acquired de novo. Ongoing analysis of this disorder, OGT- X-linked intellectual disability (OGT-XLID), provides a window into how epigenetic factors linked to O-GlcNAc cycling may influence neurodevelopment.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Fueling the fire: emerging role of the hexosamine biosynthetic pathway in cancer

Altered metabolism and deregulated cellular energetics are now considered a hallmark of all cancers. Glucose, glutamine, fatty acids, and amino acids are the primary drivers of tumor growth and act as substrates for the hexosamine biosynthetic pathway (HBP). The HBP culminates in the production of an amino sugar uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) that, along with other charged nucleotide sugars, serves as the basis for biosynthesis of glycoproteins and other glycoconjugates. These nutrient-driven post-translational modifications are highly altered in cancer and regulate protein functions in various cancer-associated processes. In this review, we discuss recent progress in understanding the mechanistic relationship between the HBP and cancer.

Post-Translational Modifications of the Mini-Chromosome Maintenance Proteins in DNA Replication

The eukaryotic mini-chromosome maintenance (MCM) complex, composed of MCM proteins 2-7, is the core component of the replisome that acts as the DNA replicative helicase to unwind duplex DNA and initiate DNA replication. MCM10 tightly binds the cell division control protein 45 homolog (CDC45)/MCM2-7/ DNA replication complex Go-Ichi-Ni-San (GINS) (CMG) complex that stimulates CMG helicase activity. The MCM8-MCM9 complex may have a non-essential role in activating the pre-replicative complex in the gap 1 (G1) phase by recruiting cell division cycle 6 (CDC6) to the origin recognition complex (ORC). Each MCM subunit has a distinct function achieved by differential post-translational modifications (PTMs) in both DNA replication process and response to replication stress. Such PTMs include phosphorylation, ubiquitination, small ubiquitin-like modifier (SUMO)ylation, O-N-acetyl-D-glucosamine (GlcNAc)ylation, and acetylation. These PTMs have an important role in controlling replication progress and genome stability. Because MCM proteins are associated with various human diseases, they are regarded as potential targets for therapeutic development. In this review, we summarize the different PTMs of the MCM proteins, their involvement in DNA replication and disease development, and the potential therapeutic implications.

Elevation of O-GlcNAc and GFAT expression by nicotine exposure promotes epithelial-mesenchymal transition and invasion in breast cancer cells

Cigarette smoking has been shown to be a carcinogenic factor in breast cancer. Nicotine (Nic), an active component of tobacco, has been found to induce epithelial-mesenchymal transition (EMT) in breast cancer cells. However, the alterations in protein O-GlcNAcylation in Nic-mediated tumorigenesis and malignization mechanisms are less well studied. Herein, we found that cellular O-GlcNAcylation dramatically increased in human breast cancer cells with EMT activation induced by Nic. Elevated O-GlcNAcylation subsequently promoted Nic-induced EMT activation and increased cell migratory abbility. In addition, we demonstrated that a differentiation factor for the mammary epithelium, CCAAT/enhancer-binding protein B (CEBPB), was involved in Nic-induced hyper-O-GlcNAcylation via transcriptional regulation of the expression of the key enzyme glutamine: fructose-6-phosphate amidotransferase (GFAT) and thus increased the flux through the hexosamine biosynthetic pathway (HBP). Finally, elevated O-GlcNAcylation of the transcriptional repressor C/EBP homologous protein (CHOP) suppressed its heterodimerization with CEBPB and facilitated the DNA-binding activity of CEBPB, further generating positive feedback that enhanced EMT upon Nic stimulation. In conclusion, our results have revealed a new regulatory mechanism involving CEBPB/GFAT-induced hyper-O-GlcNAcylation that plays a key role in EMT and smoking-mediated breast cancer progression.

Proteomic analysis of the OGT interactome: novel links to epithelial-mesenchymal transition and metastasis of cervical cancer

The role of O-GlcNAc transferase (OGT) in gene regulation and tumor invasion is poorly understood. Here, we have identified several previously undiscovered OGT-interacting proteins, including the PRMT5/WDR77 complex, the PRC2 complex, the ten-eleven translocation (TET) family, the CRL4B complex and the nucleosome remodeling and deacetylase (NuRD) complex. Genome-wide analysis of target genes responsive to OGT resulted in identification of a cohort of genes including SNAI1 and ING4 that are critically involved in cell epithelial–mesenchymal transition and invasion/metastasis. We have demonstrated that OGT promotes carcinogenesis and metastasis of cervical cancer cells. OGT’s expression is significantly upregulated in cervical cancer, and low OGT level is correlated with improved prognosis. Our study has thus revealed a mechanistic link between OGT and tumor progression, providing potential prognostic indicators and targets for cancer therapy.

O-GlcNAc Transferase Inhibition Differentially Affects Breast Cancer Subtypes

Post-translational modification of intracellular proteins with a single N-acetylglucosamine sugar (O-GlcNAcylation) regulates signaling, proliferation, metabolism and protein stability. In breast cancer, expression of the enzyme that catalyzes O-GlcNAcylation – O-GlcNAc-transferase (OGT), and the extent of protein O-GlcNAcylation, are upregulated in tumor tissue, and correlate with cancer progression. Here we compare the significance of O-GlcNAcylation in a panel of breast cancer cells of different phenotypes. We find a greater dependency on OGT among triple-negative breast cancer (TNBC) cell lines, which respond to OGT inhibition by undergoing cell cycle arrest and apoptosis. Searching for the cause of this response, we evaluate the changes in the proteome that occur after OGT inhibition or knock-down, employing a reverse-phase protein array (RPPA). We identify transcriptional repressor - hairy and enhancer of split-1 (HES1) - as a mediator of the OGT inhibition response in the TNBC cells. Inhibition of OGT as well as the loss of HES1 results in potent cytotoxicity and apoptosis. The study raises a possibility of using OGT inhibition to potentiate DNA damage in the TNBC cells.

O-GlcNAcylation Enhances Double-Strand Break Repair, Promotes Cancer Cell Proliferation, and Prevents Therapy-Induced Senescence in Irradiated Tumors

The metabolic reprogramming associated with characteristic increases in glucose and glutamine metabolism in advanced cancer is often ascribed to answering a higher demand for metabolic intermediates required for rapid tumor cell growth. Instead, recent discoveries have pointed to an alternative role for glucose and glutamine metabolites as cofactors for chromatin modifiers and other protein posttranslational modification enzymes in cancer cells. Beyond epigenetic mechanisms regulating gene expression, many chromatin modifiers also modulate DNA repair, raising the question whether cancer metabolic reprogramming may mediate resistance to genotoxic therapy and genomic instability. Our prior work had implicated N-acetyl-glucosamine (GlcNAc) formation by the hexosamine biosynthetic pathway (HBP) and resulting protein O-GlcNAcylation as a common means by which increased glucose and glutamine metabolism can drive double-strand break (DSB) repair and resistance to therapy-induced senescence in cancer cells. We have examined the effects of modulating O-GlcNAcylation on the DNA damage response (DDR) in MCF7 human mammary carcinoma in vitro and in xenograft tumors. Proteomic profiling revealed deregulated DDR pathways in cells with altered O-GlcNAcylation. Promoting protein O-GlcNAc modification by targeting O-GlcNAcase or simply treating animals with GlcNAc protected tumor xenografts against radiation. In turn, suppressing protein O-GlcNAcylation by blocking O-GlcNAc transferase activity led to delayed DSB repair, reduced cell proliferation, and increased cell senescence in vivo. Taken together, these findings confirm critical connections between cancer metabolic reprogramming, DDR, and senescence and provide a rationale to evaluate agents targeting O-GlcNAcylation in patients as a means to restore tumor sensitivity to radiotherapy. IMPLICATIONS: The finding that the HBP, via its impact on protein O-GlcNAcylation, is a key determinant of the DDR in cancer provides a mechanistic link between metabolic reprogramming, genomic instability, and therapeutic response and suggests novel therapeutic approaches for tumor radiosensitization.

High-throughput Evaluation of Protein Migration and Localization after Laser Micro-Irradiation

DNA- and histone-related research frequently comprises the quantitative analysis of protein modifications, such as histone phosphorylation. Analysis of accumulation and disappearance of protein foci are used to monitor DNA damage and repair kinetics. If the protein of interest doesn't accumulate in foci, laser micro-irradiation of single nuclei provides an alternative method to monitor DNA repair proteins and histone dynamics at the DNA damage site. We have developed an automated evaluation tool for standardized, high-throughput analysis of micro-irradiated cells featuring single cell background subtraction and detection across multiple fluorescence channels, allowing for robust statistics.

O-GlcNAc transferase associates with the MCM2-7 complex and its silencing destabilizes MCM-MCM interactions

O-GlcNAcylation of proteins is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). The homeostasis of O-GlcNAc cycling is regulated during cell cycle progression and is essential for proper cellular division. We previously reported the O-GlcNAcylation of the minichromosome maintenance proteins MCM2, MCM3, MCM6 and MCM7. These proteins belong to the MCM2-7 complex which is crucial for the initiation of DNA replication through its DNA helicase activity. Here we show that the six subunits of MCM2-7 are O-GlcNAcylated and that O-GlcNAcylation of MCM proteins mainly occurs in the chromatin-bound fraction of synchronized human cells. Moreover, we identify stable interaction between OGT and several MCM subunits. We also show that down-regulation of OGT decreases the chromatin binding of MCM2, MCM6 and MCM7 without affecting their steady-state level. Finally, OGT silencing or OGA inhibition destabilizes MCM2/6 and MCM4/7 interactions in the chromatin-enriched fraction. In conclusion, OGT is a new partner of the MCM2-7 complex and O-GlcNAcylation homeostasis might regulate MCM2-7 complex by regulating the chromatin loading of MCM6 and MCM7 and stabilizing MCM/MCM interactions.

O-GlcNAc Modification During Pregnancy: Focus on Placental Environment

Successful placentation is a key event for fetal development, which commences following embryo implantation into the uterine wall, eliciting decidualization, placentation, and remodeling of blood vessels to provide physiological exchange between embryo-fetus and mother. Several signaling pathways are recruited to modulate such important processes and specific proteins that regulate placental function are a target for the glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAc), or O-GlcNAcylation. This is a reversible post-translational modification on nuclear and cytoplasmic proteins, mainly controlled by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation has been implicated as a modulator of proteins, both in physiological and pathological conditions and, more recently, O-GlcNAc has also been shown to be an important modulator in placental tissue. In this mini-review, the interplay between O-GlcNAcylation of proteins and placental function will be addressed, discussing the possible implications of this post-translational modification through placental development and pregnancy.

Identification of UAP1L1 as a critical factor for protein O-GlcNAcylation and cell proliferation in human hepatoma cells

Aged hepatocyte-specific-Mcl-1 knockout (MKO-hep) mice are prone to develop liver tumors mimicking human hepatocellular carcinoma (HCC). Here we reported that a protein named UDP-N-acetylglucosamine pyrophosphorylase-1-like-1 (Uap1l1) is upregulated in the liver of young MKO-hep mice without any macroscopically detectable tumor nodules and is prominently expressed in the hepatic tumors developed in the aged MKO-hep mice. Intriguingly, human UAP1L1 is also significantly upregulated in a distinct subset of HCC tissues and patients with upregulated expression of UAP1L1 appeared to have poor prognosis. Overexpression of UAP1L1 significantly promoted, whereas UAP1L1 knockdown markedly reduced the proliferation of human hepatoma cells both in vitro and in vivo. UAP1L1 shows ~59% sequence identity to UDP-N-acetylglucosamine pyrophosphorylase-1 (UAP1), which is directly involved in the synthesis of the sugar donor (UDP-GlcNac) for N-acetylglucosamine modification (O-GlcNAcylation) of proteins. However, unlike UAP1, UAP1L1 harbors very limited UDP-GlcNAc synthesis activity. Moreover, although both UAP1 and UAP1L1 are required for O-GlcNAc transferase (OGT)-mediated protein O-GlcNAcylation, they appear to function distinctly from each other. UAP1L1 directly interacts with OGT, but does not seem to be an OGT substrate. In addition, UAP1L1 alone is not sufficient to activate OGT activity in vitro, suggesting that UAP1L1 may function together with other proteins to modulate OGT activity in vivo. Lastly, UAP1L1 knockdown attenuated c-MYC O-GlcNAcylation and protein stability, and overexpression of c-MYC significantly rescued the proliferation defect of UAP1L1 knockdown HepG2 cells, suggesting that c-MYC is one downstream target of UAP1L1 that contributes to UAP1L1-mediated cell proliferation, at least in HepG2 cells.

'O-GlcNAc Code' Mediated Biological Functions of Downstream Proteins

As one of the post-translational modifications, O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) often occurs on serine (Ser) and threonine (Thr) residues of specific substrate cellular proteins via the addition of O-GlcNAc group by O-GlcNAc transferase (OGT). Maintenance of normal intracellular levels of O-GlcNAcylation is controlled by OGT and glycoside hydrolase O-GlcNAcase (OGA). Unbalanced O-GlcNAcylation levels have been involved in many diseases, including diabetes, cancer, and neurodegenerative disease. Recent research data reveal that O-GlcNAcylation at histones or non-histone proteins may provide recognition platforms for subsequent protein recruitment and further initiate intracellular biological processes. Here, we review the current understanding of the 'O-GlcNAc code' mediated intracellular biological functions of downstream proteins.

Functional crosstalk among oxidative stress and O-GlcNAc signaling pathways

In metazoans, thousands of intracellular proteins are modified with O-linked β-N-acetylglucosamine (O-GlcNAc) in response to a wide range of stimuli and stresses. In particular, a complex and evolutionarily conserved interplay between O-GlcNAcylation and oxidative stress has emerged in recent years. Here, we review the current literature on the connections between O-GlcNAc and oxidative stress, with a particular emphasis on major signaling pathways, such as KEAP1/NRF2, FOXO, NFκB, p53 and cell metabolism. Taken together, this work sheds important light on the signaling functions of protein glycosylation and the mechanisms of stress responses alike and illuminates how the two are integrated in animal cell physiology.

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

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

Periconception onset diabetes is associated with embryopathy and fetal growth retardation, reproductive tract hyperglycosylation and impaired immune adaptation to pregnancy

Diabetes has been linked with impaired fertility but the underlying mechanisms are not well defined. Here we use a streptozotocin-induced diabetes mouse model to investigate the cellular and biochemical changes in conceptus and maternal tissues that accompany hyperglycaemia. We report that streptozotocin treatment before conception induces profound intra-cellular protein β-O-glycosylation (O-GlcNAc) in the oviduct and uterine epithelium, prominent in early pregnancy. Diabetic mice have impaired blastocyst development and reduced embryo implantation rates, and delayed mid-gestation growth and development. Peri-conception changes are accompanied by increased expression of pro-inflammatory cytokine Trail, and a trend towards increased Il1a, Tnf and Ifng in the uterus, and changes in local T-cell dynamics that skew the adaptive immune response to pregnancy, resulting in 60% fewer anti-inflammatory regulatory T-cells within the uterus-draining lymph nodes. Activation of the heat shock chaperones, a mechanism for stress deflection, was evident in the reproductive tract. Additionally, we show that the embryo exhibits elevated hyper-O-GlcNAcylation of both cytoplasmic and nuclear proteins, associated with activation of DNA damage (ɣH2AX) pathways. These results advance understanding of the impact of peri-conception diabetes, and provide a foundation for designing interventions to support healthy conception without propagation of disease legacy to offspring.

O-GlcNAc: A Sweetheart of the Cell Cycle and DNA Damage Response

The addition and removal of O-linked N-acetylglucosamine (O-GlcNAc) to and from the Ser and Thr residues of proteins is an emerging post-translational modification. Unlike phosphorylation, which requires a legion of kinases and phosphatases, O-GlcNAc is catalyzed by the sole enzyme in mammals, O-GlcNAc transferase (OGT), and reversed by the sole enzyme, O-GlcNAcase (OGA). With the advent of new technologies, identification of O-GlcNAcylated proteins, followed by pinpointing the modified residues and understanding the underlying molecular function of the modification has become the very heart of the O-GlcNAc biology. O-GlcNAc plays a multifaceted role during the unperturbed cell cycle, including regulating DNA replication, mitosis, and cytokinesis. When the cell cycle is challenged by DNA damage stresses, O-GlcNAc also protects genome integrity via modifying an array of histones, kinases as well as scaffold proteins. Here we will focus on both cell cycle progression and the DNA damage response, summarize what we have learned about the role of O-GlcNAc in these processes and envision a sweeter research future.

Polη O-GlcNAcylation governs genome integrity during translesion DNA synthesis

DNA polymerase η (Polη) facilitates translesion DNA synthesis (TLS) across ultraviolet (UV) irradiation- and cisplatin-induced DNA lesions implicated in skin carcinogenesis and chemoresistant phenotype formation, respectively. However, whether post-translational modifications of Polη are involved in these processes remains largely unknown. Here, we reported that human Polη undergoes O-GlcNAcylation at threonine 457 by O-GlcNAc transferase upon DNA damage. Abrogation of this modification results in a reduced level of CRL4^CDT2-dependent Polη polyubiquitination at lysine 462, a delayed p97-dependent removal of Polη from replication forks, and significantly enhanced UV-induced mutagenesis even though Polη focus formation and its efficacy to bypass across cyclobutane pyrimidine dimers after UV irradiation are not affected. Furthermore, the O-GlcNAc-deficient T457A mutation impairs TLS to bypass across cisplatin-induced lesions, causing increased cellular sensitivity to cisplatin. Our findings demonstrate a novel role of Polη O-GlcNAcylation in TLS regulation and genome stability maintenance and establish a new rationale to improve chemotherapeutic treatment.

H2A O-GlcNAcylation at serine 40 functions genomic protection in association with acetylated H2AZ or γH2AX

Background

We have previously reported a novel O-GlcNAc modification at serine 40 (S40) of H2A (H2AS40Gc). S40-type H2A isoforms susceptible to O-GlcNAcylation are evolutionarily new and restricted to the viviparous animals; however, the biological function of H2AS40Gc is largely unknown. H2A isoforms are consisted of S40 and alanine 40 (A40) type and this residue on H2A is located in the L1 of the globular domain, which is also known as a variable portion that distinguishes between the canonical and non-canonical H2A variants. In this study, by considering the similarity between the S40-type H2A and histone H2A variants, we explored the function of H2AS40Gc in mouse embryonic stem cells (mESCs).

Results

We found several similarities between the S40-type H2A isoforms and histone H2A variants such H2AZ and H2AX. mRNA of S40-type H2A isoforms (H2A1 N and H2A3) had a poly(A) tail and was produced throughout the cell cycle in contrast to that of A40-type. Importantly, H2AS40Gc level increased owing to chemical-induced DNA damage, similar to phosphorylated H2AX (γH2AX) and acetylated H2AZ (AcH2AZ). H2AS40Gc was accumulated at the restricted area (± 1.5 kb) of DNA damage sites induced by CRISPR/CAS9 system in contrast to accumulation of γH2AX, which was widely scattered. Overexpression of the wild-type (WT) H2A3, but not the S40 to A40 mutation (S40A-mutant), protected the mESC genome against chemical-induced DNA damage. Furthermore, 3 h after the DNA damage treatment, the genome was almost recovered in WT mESCs, whereas the damage advanced further in the S40A-mutant mESCs, suggesting functions of H2AS40Gc in the DNA repair mechanism. Furthermore, the S40A mutant prevented the accumulation of the DNA repair apparatus such as DNA-PKcs and Rad51 at the damage site. Co-immunoprecipitation experiment in WT and S40A-mutant mESCs revealed that H2AS40Gc physiologically bound to AcH2AZ at the initial phase upon DNA damage, followed by binding with γH2AX during the DNA damage repair process.

Conclusions

These data suggest that H2AS40Gc functions to maintain genome integrity through the DNA repair mechanism in association with AcH2AZ and γH2AX.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0157-x) contains supplementary material, which is available to authorized users.

Potential coordination role between O-GlcNAcylation and epigenetics

Dynamic changes of the post-translational O-GlcNAc modification (O-GlcNAcylation) are controlled by O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) and the glycoside hydrolase O-GlcNAcase (OGA) in cells. O-GlcNAcylation often occurs on serine (Ser) and threonine (Thr) residues of the specific substrate proteins via the addition of O-GlcNAc group by OGT. It has been known that O-GlcNAcylation is not only involved in many fundamental cellular processes, but also plays an important role in cancer development through various mechanisms. Recently, accumulating data reveal that O-GlcNAcylation at histones or non-histone proteins can lead to the start of the subsequent biological processes, suggesting that O-GlcNAcylation as ‘protein code’ or ‘histone code’ may provide recognition platforms or executive instructions for subsequent recruitment of proteins to carry out the specific functions. In this review, we summarize the interaction of O-GlcNAcylation and epigenetic changes, introduce recent research findings that link crosstalk between O-GlcNAcylation and epigenetic changes, and speculate on the potential coordination role of O-GlcNAcylation with epigenetic changes in intracellular biological processes.

O-GlcNAcylation and chromatin remodeling in mammals: an up-to-date overview

Post-translational modifications of histones and the dynamic DNA methylation cycle are finely regulated by a myriad of chromatin-binding factors and chromatin-modifying enzymes. Epigenetic modifications ensure local changes in the architecture of chromatin, thus controlling in fine the accessibility of the machinery of transcription, replication or DNA repair to the chromatin. Over the past decade, the nutrient-sensor enzyme O-GlcNAc transferase (OGT) has emerged as a modulator of chromatin remodeling. In mammals, OGT acts either directly through dynamic and reversible O-GlcNAcylation of histones and chromatin effectors, or in an indirect manner through its recruitment into chromatin-bound multiprotein complexes. In particular, there is an increasing amount of evidence of a cross-talk between OGT and the DNA dioxygenase ten-eleven translocation proteins that catalyze active DNA demethylation. Conversely, the stability of OGT itself can be controlled by the histone lysine-specific demethylase 2 (LSD2). Finally, a few studies have explored the role of O-GlcNAcase (OGA) in chromatin remodeling. In this review, we summarize the recent findings on the link between OGT, OGA and chromatin regulators in mammalian cellular models, and discuss their relevance in physiological and pathological conditions.