Differential splicing of the lectin domain of an O-glycosyltransferase modulates both peptide and glycopeptide preferences.

Mucin-type O-glycosylation is an essential post-translational modification required for protein secretion, extracellular matrix formation, and organ growth. O-Glycosylation is initiated by a large family of enzymes (GALNTs in mammals and PGANTs in Drosophila) that catalyze the addition of GalNAc onto the hydroxyl groups of serines or threonines in protein substrates. These enzymes contain two functional domains: a catalytic domain and a C-terminal ricin-like lectin domain comprised of three potential GalNAc recognition repeats termed α, ß, and γ. The catalytic domain is responsible for binding donor and acceptor substrates and catalyzing transfer of GalNAc, whereas the lectin domain recognizes more distant extant GalNAc on previously glycosylated substrates. We previously demonstrated a novel role for the α repeat of lectin domain in influencing charged peptide preferences. Here, we further interrogate how the differentially spliced α repeat of the PGANT9A and PGANT9B O-glycosyltransferases confers distinct preferences for a variety of endogenous substrates. Through biochemical analyses and in silico modeling using preferred substrates, we find that a combination of charged residues within the α repeat and charged residues in the flexible gating loop of the catalytic domain distinctively influence the peptide substrate preferences of each splice variant. Moreover, PGANT9A and PGANT9B also display unique glycopeptide preferences. These data illustrate how changes within the noncatalytic lectin domain can alter the recognition of both peptide and glycopeptide substrates. Overall, our results elucidate a novel mechanism for modulating substrate preferences of O-glycosyltransferases via alternative splicing within specific subregions of functional domains.

Snail1 is stabilized by O-GlcNAc modification in hyperglycaemic condition.

Protein O-phosphorylation often occurs reciprocally with O-GlcNAc modification and represents a regulatory principle for proteins. O-phosphorylation of serine by glycogen synthase kinase-3ß on Snail1, a transcriptional repressor of E-cadherin and a key regulator of the epithelial-mesenchymal transition (EMT) programme, results in its proteasomal degradation. We show that by suppressing O-phosphorylation-mediated degradation, O-GlcNAc at serine112 stabilizes Snail1 and thus increases its repressor function, which in turn attenuates E-cadherin mRNA expression. Hyperglycaemic condition enhances O-GlcNAc modification and initiates EMT by transcriptional suppression of E-cadherin through Snail1. Thus, dynamic reciprocal O-phosphorylation and O-GlcNAc modification of Snail1 constitute a molecular link between cellular glucose metabolism and the control of EMT.

O-GlcNAcylation of RIPK1 rescues red blood cells from necroptosis.

Necroptosis is a type of cell death with excessive inflammation and organ damage in various human diseases. Although abnormal necroptosis is common in patients with neurodegenerative, cardiovascular, and infectious diseases, the mechanisms by which O-GlcNAcylation contributes to the regulation of necroptotic cell death are poorly understood. In this study, we reveal that O-GlcNAcylation of RIPK1 (receptor-interacting protein kinase1) was decreased in erythrocytes of the mouse injected with lipopolysaccharide, resulting in the acceleration of erythrocyte necroptosis through increased formation of RIPK1-RIPK3 complex. Mechanistically, we discovered that O-GlcNAcylation of RIPK1 at serine 331 in human (corresponding to serine 332 in mouse) inhibits phosphorylation of RIPK1 at serine 166, which is necessary for the necroptotic activity of RIPK1 and suppresses the formation of the RIPK1-RIPK3 complex in Ripk1 -/- MEFs. Thus, our study demonstrates that RIPK1 O-GlcNAcylation serves as a checkpoint to suppress necroptotic signaling in erythrocytes.

O-GlcNAc modification of PPARγ reduces its transcriptional activity.

The peroxisome proliferator-activated receptor γ (PPARγ), a member of the nuclear receptor superfamily, is a key regulator of adipogenesis and is important for the homeostasis of the adipose tissue. The ß-O-linked N-acetylglucosamine (O-GlcNAc) modification, a posttranslational modification on various nuclear and cytoplasmic proteins, is involved in the regulation of protein function. Here, we report that PPARγ is modified by O-GlcNAc in 3T3-L1 adipocytes. Mass spectrometric analysis and mutant studies revealed that the threonine 54 of the N-terminal AF-1 domain of PPARγ is the major O-GlcNAc site. Transcriptional activity of wild type PPARγ was decreased 30% by treatment with the specific O-GlcNAcase (OGA) inhibitor, but the T54A mutant of PPARγ did not respond to inhibitor treatment. In 3T3-L1 cells, an increase in O-GlcNAc modification by OGA inhibitor reduced PPARγ transcriptional activity and terminal adipocyte differentiation. Our results suggest that the O-GlcNAc state of PPARγ influences its transcriptional activity and is involved in adipocyte differentiation.

O-GlcNAcylation of tubulin inhibits its polymerization.

The attachment of O-linked ß-N-acetylglucosamine (O-GlcNAc) to proteins is an abundant and reversible modification that involves many cellular processes including transcription, translation, cell proliferation, apoptosis, and signal transduction. Here, we found that the O-GlcNAc modification pattern was altered during all-trans retinoic acid (tRA)-induced neurite outgrowth in the MN9D neuronal cell line. We identified several O-GlcNAcylated proteins using mass spectrometric analysis, including α- and ß-tubulin. Further analysis of α- and ß-tubulin revealed that O-GlcNAcylated peptides mapped between residues 173 and 185 of α-tubulin and between residues 216 and 238 of ß-tubulin, respectively. We found that an increase in α-tubulin O-GlcNAcylation reduced heterodimerization and that O-GlcNAcylated tubulin did not polymerize into microtubules. Consequently, when O-GlcNAcase inhibitors were co-incubated with tRA, the extent of neurite outgrowth was decreased by 20% compared to control. Thus, our data indicate that the O-GlcNAcylation of tubulin negatively regulates microtubule formation.

O-GlcNAc protein modification in cancer cells increases in response to glucose deprivation through glycogen degradation.

When cellular glucose concentrations fall below normal levels, in general the extent of protein O-GlcNAc modification (O-GlcNAcylation) decreases. However, recent reports demonstrated increased O-GlcNAcylation by glucose deprivation in HepG2 and Neuro-2a cells. Here, we report increased O-GlcNAcylation in non-small cell lung carcinoma A549 cells and various other cells in response to glucose deprivation. Although the level of O-GlcNAc transferase was unchanged, the enzyme contained less O-GlcNAc, and its activity was increased. Moreover, O-GlcNAcase activity was reduced. The studied cells contain glycogen, and we show that its degradation in response to glucose deprivation provides a source for UDP-GlcNAc required for increased O-GlcNAcylation under this condition. This required active glycogen phosphorylase and resulted in increased glutamine:fructose-6-phosphate amidotransferase, the first and rate-limiting enzyme in the hexosamine biosynthetic pathway. Interestingly, glucose deprivation reduced the amount of phosphofructokinase 1, a regulatory glycolytic enzyme, and blocked ATP synthesis. These findings suggest that glycogen is the source for increased O-GlcNAcylation but not for generating ATP in response to glucose deprivation and that this may be useful for cancer cells to survive.

O-GlcNAcylation regulates hyperglycemia-induced GPX1 activation.

Hyperglycemia induces activation of glutathione peroxidase 1 (GPX1), an anti-oxidant enzyme essential for cell survival during oxidative stress. However, the mechanism of GPX1 activation is unclear. Here, we report that hyperglycemia-induced protein glycosylation by O-linked N-acetylglucosamine (O-GlcNAc) is crucial for activation of GPX1 and for its binding to c-Abl and Arg kinases. GPX1 itself is modified with O-GlcNAc on its C-terminus. We also demonstrate that pharmacological injection of the O-GlcNAcase inhibitor NTZ induces GPX1 activation in the mouse liver. Our findings suggest a crucial role for GPX1 and its O-GlcNAc modification in hyperglycemia and diabetes mellitus.

O-GlcNAc stabilizes SMAD4 by inhibiting GSK-3ß-mediated proteasomal degradation.

O-linked ß-N-acetylglucosamine (O-GlcNAc) is a post-translational modification which occurs on the hydroxyl group of serine or threonine residues of nucleocytoplasmic proteins. It has been reported that the presence of this single sugar motif regulates various biological events by altering the fate of target proteins, such as their function, localization, and degradation. This study identified SMAD4 as a novel O-GlcNAc-modified protein. SMAD4 is a component of the SMAD transcriptional complex, a major regulator of the signaling pathway for the transforming growth factor-ß (TGF-ß). TGF-ß is a powerful promoter of cancer EMT and metastasis. This study showed that the amount of SMAD4 proteins changes according to cellular O-GlcNAc levels in human lung cancer cells. This observation was made based on the prolonged half-life of SMAD4 proteins. The mechanism behind this interaction was that O-GlcNAc impeded interactions between SMAD4 and GSK-3ß which promote proteasomal degradation of SMAD4. In addition, O-GlcNAc modification on SMAD4 Thr63 was responsible for stabilization. As a result, defects in O-GlcNAcylation on SMAD4 Thr63 attenuated the reporter activity of luciferase, the TGF-ß-responsive SMAD binding element (SBE). This study's findings imply that cellular O-GlcNAc may regulate the TGF-ß/SMAD signaling pathway by stabilizing SMAD4.

A molecular switch orchestrates enzyme specificity and secretory granule morphology.

Regulated secretion is an essential process where molecules destined for export are directed to membranous secretory granules, where they undergo packaging and maturation. Here, we identify a gene (pgant9) that influences the structure and shape of secretory granules within the Drosophila salivary gland. Loss of pgant9, which encodes an O-glycosyltransferase, results in secretory granules with an irregular, shard-like morphology, and altered glycosylation of cargo. Interestingly, pgant9 undergoes a splicing event that acts as a molecular switch to alter the charge of a loop controlling access to the active site of the enzyme. The splice variant with the negatively charged loop glycosylates the positively charged secretory cargo and rescues secretory granule morphology. Our study highlights a mechanism for dictating substrate specificity within the O-glycosyltransferase enzyme family. Moreover, our in vitro and in vivo studies suggest that the glycosylation status of secretory cargo influences the morphology of maturing secretory granules.

Activation of c-Jun N-terminal kinase is required for neurite outgrowth of dopaminergic neuronal cells.

Recent studies indicate that activation of stress-activated protein kinases may be implicated in a broad range of biological activities including differentiation. To directly examine whether stress-activated protein kinases are involved in neuronal differentiation, we utilized retinoic acid-induced and spontaneous models of neurite outgrowth in dopaminergic neurons. Here, we show that retinoic acid-induced neurite outgrowth in MN9D dopaminergic neuronal cells was accompanied by activation of c-Jun N-terminal kinase but not p38. Consequently, cotreatment with a specific inhibitor of c-Jun N-terminal kinase or overexpression of c-Jun N-terminal kinase-binding domain of c-Jun N-terminal kinase-interacting protein-1 blocked retinoic acid-induced neurite outgrowth. In primary cultures of dopaminergic neurons, the extent of neurite outgrowth increased spontaneously in a time-dependent manner. When these cultures were treated with a specific inhibitor of c-Jun N-terminal kinase, the total extent of neurites, the primary neurite length and the number of neurites per cell were suppressed significantly. Thus, our data indicate that the c-Jun N-terminal kinase signal seems to play an important role during morphological differentiation in cultured dopaminergic neurons.