O-GlcNAc modification of proteins affects volume regulation in Jurkat cells

Emerging roles of O-GlcNAcylation in protein trafficking and secretion

The emerging roles of O-GlcNAcylation, a distinctive post-translational modification, are increasingly recognized for their involvement in the intricate processes of protein trafficking and secretion. This modification exerts its influence on both conventional and unconventional secretory pathways. Under healthy and stress conditions, such as during diseases, it orchestrates the transport of proteins within cells, ensuring timely delivery to their intended destinations. O-GlcNAcylation occurs on key factors, like coat protein complexes (COPI and COPII), clathrin, SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), and GRASP55 (Golgi reassembly stacking protein of 55 kDa) that control vesicle budding and fusion in anterograde and retrograde trafficking and unconventional secretion. The understanding of O-GlcNAcylation offers valuable insights into its critical functions in cellular physiology and the progression of diseases, including neurodegeneration, cancer, and metabolic disorders. In this review, we summarize and discuss the latest findings elucidating the involvement of O-GlcNAc in protein trafficking and its significance in various human disorders.

O-GlcNAcylation Suppresses the Ion Current IClswell by Preventing the Binding of the Protein ICln to α-Integrin

O-GlcNAcylation is a post-translational modification of proteins that controls a variety of cellular processes, is chronically elevated in diabetes mellitus, and may contribute to the progression of diabetic complications, including diabetic nephropathy. Our previous work showed that increases in the O-GlcNAcylation of cellular proteins impair the homeostatic reaction of the regulatory volume decrease (RVD) after cell swelling by an unknown mechanism. The activation of the swelling-induced chloride current IClswell is a key step in RVD, and ICln, a ubiquitous protein involved in the activation of IClswell, is O-GlcNAcylated. Here, we show that experimentally increased O-GlcNAcylation of cellular proteins inhibited the endogenous as well as the ICln-induced IClswell current and prevented RVD in a human renal cell line, while decreases in O-GlcNAcylation augmented the current magnitude. In parallel, increases or decreases in O-GlcNAcylation, respectively, weakened or stabilized the binding of ICln to the intracellular domain of α-integrin, a process that is essential for the activation of IClswell. Mutation of the putative YinOYang site at Ser67 rendered the ICln-induced IClswell current unresponsive to O-GlcNAc variations, and the ICln interaction with α-integrin insensitive to O-GlcNAcylation. In addition, exposure of cells to a hypotonic solution reduced the O-GlcNAcylation of cellular proteins. Together, these findings show that O-GlcNAcylation affects RVD by influencing IClswell and further indicate that hypotonicity may activate IClswell by reducing the O-GlcNAcylation of ICln at Ser67, therefore permitting its binding to α-integrin. We propose that disturbances in the regulation of cellular volume may contribute to disease in settings of chronically elevated O-GlcNAcylation, including diabetic nephropathy.

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

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

MicroRNA-15b Suppresses Th17 Differentiation and Is Associated with Pathogenesis of Multiple Sclerosis by Targeting O-GlcNAc Transferase

IL-17-producing Th17 cells have gradually become considered as key factors in the pathogenesis of many autoimmune diseases, including multiple sclerosis (MS). Although the involvement of certain microRNAs in the development of MS has been reported, their role in Th17-driven autoimmunity is still poorly understood. In this study, we identified microRNA (miR)-15b as an important factor in Th17-associated effects and determined that the expression of miR-15b is significantly downregulated in MS patients and in mice with experimental autoimmune encephalomyelitis. Overexpression of miR-15b alleviated experimental autoimmune encephalomyelitis, whereas knockdown of miR-15b aggravated it. We demonstrated that miR-15b suppressed Th17 differentiation both in vivo and in vitro. We also found that O-linked N-acetylglucosamine transferase is a potential target of miR-15b, enabling it to affect the transcriptional regulation of retinoic acid-related orphan receptor γT through O-linked N-acetylglucosamine glycosylation of NF-κB. These results contribute to the importance of miR-15b in Th17 differentiation and the pathogenesis of MS.

Lithium induces ER stress and N-glycan modification in galactose-grown Jurkat cells

We previously reported that lithium had a significant impact on Ca(2+) regulation and induced unfolded protein response (UPR) in yeast cells grown on galactose due to inhibition of phosphoglucomutase (PGM), however the exact mechanism has not been established yet. In this study, we analysed lithium's effect in galactose-fed cells to clarify whether these ER-related changes are the result of a relative hypoglycemic state. Furthermore, we investigated whether the alterations in galactose metabolism impact protein post-translational modifications. Thus, Jurkat cells were incubated in glucose or galactose containing media with or without lithium treatment. We found that galactose-fed and lithium treated cells showed better survivability than fasting cells. We also found higher UDP-Hexose and glycogen levels in these cells compared to fasting cells. On the other hand, the UPR (X-box binding protein 1 mRNA levels) of galactose-fed and lithium treated cells was even greater than in fasting cells. We also found increased amount of proteins that contained N-linked N-acetyl-glucosamine, similar to what was reported in fasting cells by a recent study. Our results demonstrate that lithium treatment of galactose-fed cells can induce stress responses similar to hypoglycemia, however cell survival is still secured by alternative pathways. We propose that clarifying this process might be an important addition toward the better understanding of the molecular mechanisms that regulate ER-associated stress response.

Cell-penetrant, nanomolar O-GlcNAcase inhibitors selective against lysosomal hexosaminidases

Posttranslational modification of metazoan nucleocytoplasmic proteins with N-acetylglucosamine (O-GlcNAc) is essential, dynamic, and inducible and can compete with protein phosphorylation in signal transduction. Inhibitors of O-GlcNAcase, the enzyme removing O-GlcNAc, are useful tools for studying the role of O-GlcNAc in a range of cellular processes. We report the discovery of nanomolar OGA inhibitors that are up to 900,000-fold selective over the related lysosomal hexosaminidases. When applied at nanomolar concentrations on live cells, these cell-penetrant molecules shift the O-GlcNAc equilibrium toward hyper-O-GlcNAcylation with EC₅₀ values down to 3 nM and are thus invaluable tools for the study of O-GlcNAc cell biology.

Inhibition of O-GlcNAcase using a potent and cell-permeable inhibitor does not induce insulin resistance in 3T3-L1 adipocytes

To probe increased O-GlcNAc levels as an independent mechanism governing insulin resistance in 3T3-L1 adipocytes, a new class of O-GlcNAcase (OGA) inhibitor was studied. 6-Acetamido-6-deoxy-castanospermine (6-Ac-Cas) is a potent inhibitor of OGA. The structure of 6-Ac-Cas bound in the active site of an OGA homolog reveals structural features contributing to its potency. Treatment of 3T3-L1 adipocytes with 6-Ac-Cas increases O-GlcNAc levels in a dose-dependent manner. These increases in O-GlcNAc levels do not induce insulin resistance functionally, measured using a 2-deoxyglucose (2-DOG) uptake assay, or at the molecular level, determined by evaluating levels of phosphorylated IRS-1 and Akt. These results, and others described, provide a structural blueprint for improved inhibitors and collectively suggest that increased O-GlcNAc levels, brought about by inhibition of OGA, does not by itself cause insulin resistance in 3T3-L1 adipocytes.