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

OBJECTIVE Carbohydrate-responsive element–binding protein (ChREBP) is a key transcription factor that mediates the effects of glucose on glycolytic and lipogenic genes in the liver. We have previously reported that liver-specific inhibition of ChREBP prevents hepatic steatosis in ob/ob mice by specifically decreasing lipogenic rates in vivo. To better understand the regulation of ChREBP activity in the liver, we investigated the implication of O-linked β-N-acetylglucosamine (O-GlcNAc or O-GlcNAcylation), an important glucose-dependent posttranslational modification playing multiple roles in transcription, protein stabilization, nuclear localization, and signal transduction. RESEARCH DESIGN AND METHODS O-GlcNAcylation is highly dynamic through the action of two enzymes: the O-GlcNAc transferase (OGT), which transfers the monosaccharide to serine/threonine residues on a target protein, and the O-GlcNAcase (OGA), which hydrolyses the sugar. To modulate ChREBP^OG in vitro and in vivo, the OGT and OGA enzymes were overexpressed or inhibited via adenoviral approaches in mouse hepatocytes and in the liver of C57BL/6J or obese db/db mice. RESULTS Our study shows that ChREBP interacts with OGT and is subjected to O-GlcNAcylation in liver cells. O-GlcNAcylation stabilizes the ChREBP protein and increases its transcriptional activity toward its target glycolytic (L-PK) and lipogenic genes (ACC, FAS, and SCD1) when combined with an active glucose flux in vivo. Indeed, OGT overexpression significantly increased ChREBP^OG in liver nuclear extracts from fed C57BL/6J mice, leading in turn to enhanced lipogenic gene expression and to excessive hepatic triglyceride deposition. In the livers of hyperglycemic obese db/db mice, ChREBP^OG levels were elevated compared with controls. Interestingly, reducing ChREBP^OG levels via OGA overexpression decreased lipogenic protein content (ACC, FAS), prevented hepatic steatosis, and improved the lipidic profile of OGA-treated db/db mice. CONCLUSIONS Taken together, our results reveal that O-GlcNAcylation represents an important novel regulation of ChREBP activity in the liver under both physiological and pathophysiological conditions.

Dysfunctions in Wnt signaling increase β‐catenin stability and are associated with cancers, including colorectal cancer. In addition, β‐catenin degradation is decreased by nutrient‐dependent O ‐GlcNAcylation. Human colon tumors and colons from mice fed high‐carbohydrate diets exhibited higher amounts of β‐catenin and O ‐GlcNAc relative to healthy tissues and mice fed a standard diet, respectively. Administration of the O ‐GlcNAcase inhibitor thiamet G to mice also increased colonic expression of β‐catenin. By ETD‐MS/MS, we identified 4 O ‐GlcNAcylation sites at the N terminus of β‐catenin (S23/T40/T41/T112). Furthermore, mutation of serine and threonine residues within the D box of β‐catenin reduced O ‐GlcNAcylation by 75%. Interestingly, elevating O ‐GlcNAcylation in human colon cell lines drastically reduced phosphorylation at T41, a key residue of the D box responsible for β‐catenin stability. Analyses of β‐catenin O ‐GlcNAcylation mutants reinforced T41 as the most crucial residue that controls the β‐catenin degradation rate. Finally, inhibiting O ‐GlcNAcylation decreased the β‐catenin/α‐catenin interaction necessary for mucosa integrity, whereas O ‐GlcNAcase silencing improved this interaction. These results suggest that O ‐GlcNAcylation regulates not only the stability of β‐catenin, but also affects its localization at the level of adherens junctions. Accordingly, we propose that O ‐GlcNAcylation of β‐catenin is a missing link between the glucose metabolism deregulation observed in metabolic disorders and the development of cancer.—Olivier‐Van Stichelen, S., Dehennaut, V., Buzy, A., Zachayus, J.‐L., Guinez, C., Mir, A‐M., El Yazidi‐Belkoura, I., Copin, M.‐C., Boureme, D., Loyaux, D., Ferrara, P., Lefebvre, T. O ‐GlcNAcylation stabilizes β‐catenin through direct competition with phosphorylation at threonine 41. FASEB J. 28, 3325–3338 (2014). www.fasebj.org

An accelerated de novo lipogenesis (DNL) flux is a common characteristic of cancer cells required to sustain a high proliferation rate. The DNL enzyme fatty acid synthase (FASN) is overexpressed in many cancers and is pivotal for the increased production of fatty acids. There is increasing evidences of the involvement of FASN in several hallmarks of cancer linked to its ability to promote cell proliferation via membranes biosynthesis. In this review we discuss about the implication of FASN in the resistance to cell death and in the deregulation of cellular energetics by increasing nucleic acids, protein and lipid synthesis. FASN also promotes cell proliferation, cell invasion, metastasis and angiogenesis by enabling the building of lipid rafts and consequently to the localization of oncogenic receptors such as HER2 and c-Met in membrane microdomains. Finally, FASN is involved in immune escape by repressing the activation of pro-inflammatory cells and promoting the recruitment of M2 macrophages and T regulatory cells in the tumor microenvironment. Here, we provide an overview of the involvement of the pro-oncogenic enzyme in the hallmarks of cancer making FASN a promising target in anti-cancer therapy to circumvent resistance to chemotherapies.