The small molecule luteolin inhibits N-acetyl-α-galactosaminyltransferases and reduces mucin-type O-glycosylation of amyloid precursor protein

Synergistic effects of mutation and glycosylation on disease progression

Glycosylation, a post-translational modification, plays a crucial role in proper localization and function of proteins. It is regulated by multiple glycosyltransferases and can be influenced by various factors. Inherited missense mutations in glycosylated proteins such as NOTCH3, Low-density lipoprotein receptor (LDLR), and Amyloid precursor protein (APP) could affect their glycosylation states, leading to cerebral small vessel disease, hypercholesterolemia, and Alzheimer's disease, respectively. Additionally, physiological states and aging-related conditions can affect the expression levels of glycosyltransferases. However, the interplay between mutations in glycosylated proteins and changes in their glycosylation levels remains poorly understood. This mini-review summarizes the effects of glycosylation on transmembrane proteins with pathogenic mutations, including NOTCH3, LDLR, and APP. We highlight the synergistic contributions of missense amino acids in the mutant proteins and alterations in their glycosylation states to their molecular pathogenesis.

Therapeutic Expedition of Luteolin against Brain-related Disorders: An Updated Review

Brain-related disorders include neuroinflammation, neurodegenerative disorders, and demyelination, which ultimately affect the quality of life of patients. Currently, brain-related disorders represent the most challenging health problem worldwide due to complex pathogenesis and limited availability of drugs for their management. Further, the available pharmacotherapy accompanies serious side effects, therefore, much attention has been directed toward the development of alternative therapy derived from natural sources to treat such disorders. Recently, flavonoids, natural phytochemicals, have been reported as a treatment option for preventing brain aging and disorders related to this. Among these flavonoids, dietary luteolin, a flavone, is found in many plant products such as broccoli, chamomile tea, and honeysuckle bloom having several pharmacological properties including neuroprotective activities. Therefore, the objective of this paper is to compile the available literature regarding the neuroprotective potential of luteolin and its mechanism of action. Luteolin exerts notable anti-inflammatory, antioxidant, and antiapoptotic activity suggesting its therapeutic efficacy in different neurological disorders. Numerous in-vivo and in-vitro experiments have revealed that luteolin exhibits neuroprotective potential via up-regulating the ER/ERK, PI3AKT, Nrf2 pathways and down-regulating the MAPK/JAK2STAT and NFκB pathways. Taking into account of available facts regarding the neuroprotective efficacy of luteolin, the current study highlights the beneficial effects of luteolin for the prevention, management, and treatment of different neurological disorders. Thus, luteolin can be considered an alternative for the development of new pharmacophores against various brain-related disorders.

Rational Design of Dual-Domain Binding Inhibitors for N-Acetylgalactosamine Transferase 2 with Improved Selectivity over the T1 and T3 Isoforms

The GalNAc-transferase (GalNAc-T) family, consisting of 20 isoenzymes, regulates the O-glycosylation process of mucin glycopeptides by transferring GalNAc units to serine/threonine residues. Dysregulation of specific GalNAc-Ts is associated with various diseases, making these enzymes attractive targets for drug development. The development of inhibitors is key to understanding the implications of GalNAc-Ts in human diseases. However, developing selective inhibitors for individual GalNAc-Ts represents a major challenge due to shared structural similarities among the isoenzymes and some degree of redundancy among the natural substrates. Herein, we report the development of a GalNAc-T2 inhibitor with higher potency compared to those of the T1 and T3 isoforms. The most promising candidate features bivalent GalNAc and thiophene moieties on a peptide chain, enabling binding to both the lectin and catalytic domains of the enzyme. The binding mode was confirmed by competitive saturation transfer difference NMR experiments and validated through molecular dynamics simulations. The inhibitor demonstrated an IC50 of 21.4 μM for GalNAc-T2, with 8- and 32-fold higher selectivity over the T3 and T1 isoforms, respectively, representing a significant step forward in the synthesis of specific GalNAc-T inhibitors tailored to the unique structural features of the targeted isoform.

Insights into the role of glycosyltransferase in the targeted treatment of gastric cancer

Gastric cancer is a remarkably heterogeneous tumor. Despite some advances in the diagnosis and treatment of gastric cancer in recent years, the precise treatment and curative outcomes remain unsatisfactory. Poor prognosis continues to pose a major challenge in gastric cancer. Therefore, it is imperative to identify effective targets to improve the treatment and prognosis of gastric cancer patients. It should be noted that glycosylation, a novel form of posttranslational modification, is a process capable of regulating protein function and influencing cellular activities. Currently, numerous studies have shown that glycosylation plays vital roles in the occurrence and progression of gastric cancer. As crucial enzymes that regulate glycan synthesis in glycosylation processes, glycosyltransferases are potential targets for treating GC. Hence, investigating the regulation of glycosyltransferases and the expression of associated proteins in gastric cancer cells is highly important. In this review, the related glycosyltransferases and their related signaling pathways in gastric cancer, as well as the existing inhibitors of glycosyltransferases, provide more possibilities for targeted therapies for gastric cancer.

Luteolin for neurodegenerative diseases: a review

Neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and multiple sclerosis affect millions of people around the world. In addition to age, which is a key factor contributing to the development of all neurodegenerative diseases, genetic and environmental components are also important risk factors. Current methods of treating neurodegenerative diseases are mostly symptomatic and do not eliminate the cause of the disease. Many studies focus on searching for natural substances with neuroprotective properties that could be used as an adjuvant therapy in the inhibition of the neurodegeneration process. These compounds include flavonoids, such as luteolin, showing significant anti-inflammatory, antioxidant, and neuroprotective activity. Increasing evidence suggests that luteolin may confer protection against neurodegeneration. In this review, we summarize the scientific reports from preclinical in vitro and in vivo studies regarding the beneficial effects of luteolin in neurodegenerative diseases. Luteolin was studied most extensively in various models of Alzheimer's disease but there are also several reports showing its neuroprotective effects in models of Parkinson's disease. Though very limited, studies on possible protective effects of luteolin against Huntington's disease and multiple sclerosis are also discussed here. Overall, although preclinical studies show the potential benefits of luteolin in neurodegenerative disorders, clinical evidence on its therapeutic efficacy is still deficient.

Transcriptomic and glycomic analyses highlight pathway-specific glycosylation alterations unique to Alzheimer's disease

Glycosylation has been found to be altered in the brains of individuals with Alzheimer's disease (AD). However, it is unknown which specific glycosylation-related pathways are altered in AD dementia. Using publicly available RNA-seq datasets covering seven brain regions and including 1724 samples, we identified glycosylation-related genes ubiquitously changed in individuals with AD. Several differentially expressed glycosyltransferases found by RNA-seq were confirmed by qPCR in a different set of human medial temporal cortex (MTC) samples (n = 20 AD vs. 20 controls). N-glycan-related changes predicted by expression changes in these glycosyltransferases were confirmed by mass spectrometry (MS)-based N-glycan analysis in the MTC (n = 9 AD vs. 6 controls). About 80% of glycosylation-related genes were differentially expressed in at least one brain region of AD participants (adjusted p-values < 0.05). Upregulation of MGAT1 and B4GALT1 involved in complex N-linked glycan formation and galactosylation, respectively, were reflected by increased concentrations of corresponding N-glycans. Isozyme-specific changes were observed in expression of the polypeptide N-acetylgalactosaminyltransferase (GALNT) family and the alpha-N-acetylgalactosaminide alpha-2,6-sialyltransferase (ST6GALNAC) family of enzymes. Several glycolipid-specific genes (UGT8, PIGM) were upregulated. The critical transcription factors regulating the expression of N-glycosylation and elongation genes were predicted and found to include STAT1 and HSF5. The miRNA predicted to be involved in regulating N-glycosylation and elongation glycosyltransferases were has-miR-1-3p and has-miR-16-5p, respectively. Our findings provide an overview of glycosylation pathways affected by AD and potential regulators of glycosyltransferase expression that deserve further validation and suggest that glycosylation changes occurring in the brains of AD dementia individuals are highly pathway-specific and unique to AD.

Identification of global inhibitors of cellular glycosylation

Small molecule inhibitors of glycosylation enzymes are valuable tools for dissecting glycan functions and potential drug candidates. Screening for inhibitors of glycosyltransferases are mainly performed by in vitro enzyme assays with difficulties moving candidates to cells and animals. Here, we circumvent this by employing a cell-based screening assay using glycoengineered cells expressing tailored reporter glycoproteins. We focused on GalNAc-type O-glycosylation and selected the GalNAc-T11 isoenzyme that selectively glycosylates endocytic low-density lipoprotein receptor (LDLR)-related proteins as targets. Our screen of a limited small molecule compound library did not identify selective inhibitors of GalNAc-T11, however, we identify two compounds that broadly inhibited Golgi-localized glycosylation processes. These compounds mediate the reversible fragmentation of the Golgi system without affecting secretion. We demonstrate how these inhibitors can be used to manipulate glycosylation in cells to induce expression of truncated O-glycans and augment binding of cancer-specific Tn-glycoprotein antibodies and to inhibit expression of heparan sulfate and binding and infection of SARS-CoV-2.

Integrated DNA Methylation/RNA Profiling in Middle Temporal Gyrus of Alzheimer's Disease

Alzheimer's disease is a neurodegenerative disorder clinically defined by gradual cognitive impairment and alteration in executive function. We conducted an epigenome-wide association study (EWAS) of a clinically and neuropathologically characterized cohort of 296 brains, including Alzheimer's disease (AD) and non-demented controls (ND), exploring the relationship with the RNA expression from matched donors. We detected 5246 CpGs and 832 regions differentially methylated, finding overlap with previous EWAS but also new associations. CpGs previously identified in ANK1, MYOC, and RHBDF2 were differentially methylated, and one of our top hits (GPR56) was not previously detected. ANK1 was differentially methylated at the region level, along with APOE and RHBDF2. Only a small number of genes showed a correlation between DNA methylation and RNA expression statistically significant. Multiblock partial least-squares discriminant analysis showed several CpG sites and RNAs discriminating AD and ND (AUC = 0.908) and strongly correlated with each other. Furthermore, the CpG site cg25038311 was negatively correlated with the expression of 22 genes. Finally, with the functional epigenetic module analysis, we identified a protein-protein network characterized by inverse RNA/DNA methylation correlation and enriched for "Regulation of insulin-like growth factor transport", with IGF1 as the hub gene. Our results confirm and extend the previous EWAS, providing new information about a brain region not previously explored in AD DNA methylation studies. The relationship between DNA methylation and gene expression is not significant for most of the genes in our sample, consistently with the complexities in the gene expression regulation.

Glycoproteomics Landscape of Asymptomatic and Symptomatic Human Alzheimer's Disease Brain

Molecular changes in the brain of individuals afflicted with Alzheimer's disease (AD) are an intense area of study. Little is known about the role of protein abundance and posttranslational modifications in AD progression and treatment, in particular large-scale intact N-linked glycoproteomics analysis. To elucidate the N-glycoproteome landscape, we developed an approach based on multi-lectin affinity enrichment, hydrophilic interaction chromatography, and LC-MS-based glycoproteomics. We analyzed brain tissue from 10 persons with no cognitive impairment or AD, 10 with asymptomatic AD, and 10 with symptomatic AD, detecting over 300 glycoproteins and 1900 glycoforms across the samples. The majority of glycoproteins have N-glycans that are high-mannosidic or complex chains that are fucosylated and bisected. The Man5 N-glycan was found to occur most frequently at >20% of the total glycoforms. Unlike the glycoproteomes of other tissues, sialylation is a minor feature of the brain N-glycoproteome, occurring at <9% among the glycoforms. We observed AD-associated differences in the number of antennae, frequency of fucosylation, bisection, and other monosaccharides at individual glycosylation sites among samples from our three groups. Further analysis revealed glycosylation differences in subcellular compartments across disease stage, including glycoproteins in the lysosome frequently modified with paucimannosidic glycans. These results illustrate the N-glycoproteomics landscape across the spectrum of AD clinical and pathologic severity and will facilitate a deeper understanding of progression and treatment development.

Novel luteolin@pro-phytomicelles: In vitro characterization and in vivo evaluation of protection against drug-induced hepatotoxicity

A novel nanoformulation with the small molecule phytochemical dipotassium glycyrrhizinate as a nanomaterial was developed for the oral delivery of luteolin (Lut), a widely used phytochemical, but it suffered from poor water solubility and low oral bioavailability. This novel nanoformulation, named Lut@pro-phytomicelles, can be fabricated with a simple process. Lut@pro-phytomicelles can instantly dissolve into aqueous mediums and formulate through self-assembly a clear phytomicelle solution with a Lut encapsulation efficiency of 99.16 ± 0.90%, a small micelle size of 30.32 ± 0.12 nm, and a narrow polydispersity index of 0.138 ± 0.024. The optimized formulation demonstrated that Lut had solubility in up to 50 mg/ml of water as a result of its encapsulation within DG phytomicelles. Lut@pro-phytomicelles exhibited excellent characteristics, including good storage stability, a fast in vitro release profile, improvement in in vitro antioxidant activity, and high safety potential. In the oral bioavailability evaluation, a shorter Tmax, increased Cmax, and improved AUC0-t were obtained with Lut@pro-phytomicelles when compared to bare Lut. The distribution evaluation further showed that Lut@pro-phytomicelles could effectively increase the concentrations of Lut in all the tested organs and gastrointestinal segments. In the protection efficacy evaluation, 100 mg/kg Lut@pro-phytomicelles demonstrated strong effects against acetaminophen-induced hepatotoxicity. The mechanisms of inhibiting high-mobility group box 1 signaling and suppressing oxidative stress were involved in this strong treatment effect. These results showed that simple but novel Lut@pro-phytomicelles provided a new, promising nano-delivery system for Lut with a significantly improved in vivo profile.

E. Gaafar P, Mousa MR, Fayez AM, Elsheikh MA. Novel Luteolin-Loaded Chitosan Decorated Nanoparticles for Brain-Targeting Delivery in a Sporadic Alzheimer's Disease Mouse Model: Focus on Antioxidant, Anti-Inflammatory, and Amyloidogenic Pathways

Preparation and evaluation of a non-invasive intranasal luteolin delivery for the management of cognitive dysfunction in Alzheimer's disease (AD) using novel chitosan decorated nanoparticles. Development of luteolin-loaded chitosomes was followed by full in vitro characterization. In vivo efficacy was evaluated using a sporadic Alzheimer's disease (SAD) animal model via intracerebroventricular injection of 3 mg/kg streptozotocin (ICV-STZ). Treatment groups of luteolin suspension and chitosomes (50 mg/kg) were then intranasally administered after 5 h of ICV-STZ followed by everyday administration for 21 consecutive days. Behavioral, histological, immunohistochemical, and biochemical studies were conducted. Chitosomes yielded promising quality attributes in terms of particle size (PS) (412.8 ± 3.28 nm), polydispersity index (PDI) (0.378 ± 0.07), Zeta potential (ZP) (37.4 ± 2.13 mv), and percentage entrapment efficiency (EE%) (86.6 ± 2.05%). Behavioral findings showed obvious improvement in the acquisition of short-term and long-term spatial memory. Furthermore, histological evaluation revealed an increased neuronal survival rate with a reduction in the number of amyloid plaques. Biochemical results showed improved antioxidant effects and reduced pro-inflammatory mediators' levels. In addition, a suppression by half was observed in the levels of both Aβ aggregation and hyperphosphorylated-tau protein in comparison to the model control group which in turn confirmed the capability of luteolin-loaded chitosomes (LUT-CHS) in attenuating the pathological changes of AD. The prepared nanoparticles are considered a promising safe, effective, and non-invasive nanodelivery system that improves cognitive function in SAD albino mice as opposed to luteolin suspension.

Mucin-Type O-Glycosylation Proximal to β-Secretase Cleavage Site Affects APP Processing and Aggregation Fate

The amyloid-β precursor protein (APP) undergoes proteolysis by β- and γ-secretases to form amyloid-β peptides (Aβ), which is a hallmark of Alzheimer's disease (AD). Recent findings suggest a possible role of O-glycosylation on APP's proteolytic processing and subsequent fate for AD-related pathology. We have previously reported that Tyr681-O-glycosylation and the Swedish mutation accelerate cleavage of APP model glycopeptides by β-secretase (amyloidogenic pathway) more than α-secretase (non-amyloidogenic pathway). Therefore, to further our studies, we have synthesized additional native and Swedish-mutated (glyco)peptides with O-GalNAc moiety on Thr663 and/or Ser667 to explore the role of glycosylation on conformation, secretase activity, and aggregation kinetics of Aβ40. Our results show that conformation is strongly dependent on external conditions such as buffer ions and solvent polarity as well as internal modifications of (glyco)peptides such as length, O-glycosylation, and Swedish mutation. Furthermore, the level of β-secretase activity significantly increases for the glycopeptides containing the Swedish mutation compared to their nonglycosylated and native counterparts. Lastly, the glycopeptides impact the kinetics of Aβ40 aggregation by significantly increasing the lag phase and delaying aggregation onset, however, this effect is less pronounced for its Swedish-mutated counterparts. In conclusion, our results confirm that the Swedish mutation and/or O-glycosylation can render APP model glycopeptides more susceptible to cleavage by β-secretase. In addition, this study sheds new light on the possible role of glycosylation and/or glycan density on the rate of Aβ40 aggregation.

Small molecule inhibitors of mammalian glycosylation

Glycans are one of the fundamental biopolymers encountered in living systems. Compared to polynucleotide and polypeptide biosynthesis, polysaccharide biosynthesis is a uniquely combinatorial process to which interdependent enzymes with seemingly broad specificities contribute. The resulting intracellular cell surface, and secreted glycans play key roles in health and disease, from embryogenesis to cancer progression. The study and modulation of glycans in cell and organismal biology is aided by small molecule inhibitors of the enzymes involved in glycan biosynthesis. In this review, we survey the arsenal of currently available inhibitors, focusing on agents which have been independently validated in diverse systems. We highlight the utility of these inhibitors and drawbacks to their use, emphasizing the need for innovation for basic research as well as for therapeutic applications.

Novel Quinic Acid Glycerates from Tussilago farfara Inhibit Polypeptide GalNAc-Transferase

The discovery of a bioactive inhibitor tool for human polypeptide N-acetylgalactosaminyl transferases (GalNAc-Ts), the initiating enzyme for mucin-type O-glycosylation, remains challenging. In the present study, we identified an array of quinic acid derivatives, including four new glycerates (1-4) from Tussilago farfara, a traditional Chinese medicinal plant, as active inhibitors of GalNAc-T2 using a combined screening approach with a cell-based T2-specific sensor and purified enzyme assay. These inhibitors dose-dependently inhibited human GalNAc-T2 but did not affect O-linked N-acetylglucosamine transferase (OGT), the other type of glycosyltransferase. Importantly, they are not cytotoxic and retain inhibitory activity in cells lacking elongated O-glycans, which are eliminated by the CRISPR/Cas9 gene editing tool. A structure-activity relationship study unveiled a novel quinic acid-caffeic acid conjugate pharmacophore that directs inhibition. Overall, these new natural product inhibitors could serve as a basis for developing an inhibitor tool for GalNAc-T2.

Mucin-Type O-Glycosylation in the Drosophila Nervous System

Mucin-type O-glycosylation, a predominant type of O-glycosylation, is an evolutionarily conserved posttranslational modification in animals. Mucin-type O-glycans are often found on mucins in the mucous membranes of the digestive tract. These glycan structures are also expressed in other cell types, such as blood cells and nephrocytes, and have crucial physiological functions. Altered expression of mucin-type O-glycans is known to be associated with several human disorders, including Tn syndrome and cancer; however, the physiological roles of mucin-type O-glycans in the mammalian brain remains largely unknown. The functions of mucin-type O-glycans have been studied in the fruit fly, Drosophila melanogaster. The basic structures of mucin-type O-glycans, including Tn antigen (GalNAcα1-Ser/Thr) and T antigen (Galβ1–3GalNAcα1-Ser/Thr), as well as the glycosyltransferases that synthesize them, are conserved between Drosophila and mammals. These mucin-type O-glycans are expressed in the Drosophila nervous system, including the central nervous system (CNS) and neuromuscular junctions (NMJs). In primary cultured neurons of Drosophila, mucin-type O-glycans show a characteristic localization pattern in axons. Phenotypic analyses using mutants of glycosyltransferase genes have revealed that mucin-type O-glycans are required for CNS development, NMJ morphogenesis, and synaptic functions of NMJs in Drosophila. In this review, we describe the roles of mucin-type O-glycans in the Drosophila nervous system. These findings will provide insight into the functions of mucin-type O-glycans in the mammalian brain.

Comprehensive analysis of O-glycosylation of amyloid precursor protein (APP) using targeted and multi-fragmentation MS strategy

BACKGROUND

The aberrant proteolytic processing of amyloid precursor protein (APP) into amyloid β peptide (Aβ) in brain is a critical step in the pathogenesis of Alzheimer's disease (AD). As an O-glycosylated protein, O-glycosylation of APP is considered to be related to Aβ generation. Therefore, comprehensive analysis of APP O-glycosylation is important for understanding its functions.

METHODS

We developed a Targeted MS approach with Multi-Fragmentation techniques (TMMF strategy), and successfully characterized O-glycosylation profiling of APP695 expressed in HEK-293 T cells. We calculated relative abundance of glycopeptides with various O-glycosites and O-glycans, and further investigated the alteration of APP O-glycosylation upon TNF-α treatment.

RESULTS

A total of 14 O-glycosites were identified on three glycopeptides of APP, and at least four O-glycans including GalNAc (Tn antigen), core 1, and mono-/di-sialylated core 1 glycans were determinant at the residues of Thr576 and Thr577. We found a dense cluster of truncated O-glycans on the region nearby beginning of E2 domain and high abundance of sialylated O-glycans on the region close to β-cleavage site. Moreover, we also observed that TNF-α could upregulate the expression of APP and the truncated O-glycans on APP in HEK-293 T cell.

CONCLUSION

Our study established an intact O-glycopeptide MS analysis strategy for APP O-glycopeptide identification with enhanced fragmentation efficiency and detection sensitivity. These results provide a comprehensive O-glycosylation map of APP expressed in HEK-293 T cell.

GENERAL SIGNIFICANCE

The accurate O-glycosites and O-glycan structures on APP may lead to a better understanding of the roles O-glycosylation plays in the processing and functions of APP.

Neuroinflammation in Alzheimer's disease and beneficial action of luteolin

Alzheimer's disease (AD), already the world's most common form of dementia, is projected to continue increasing in prevalence over the next several decades. The current lack of understanding of the pathogenesis of AD has hampered the development of effective treatments. Historically, AD research has been predicated on the amyloid cascade hypothesis (ACH), which attributes disease progression to the build-up of amyloid protein. However, multiple clinical studies of drugs interfering with ACH have failed to show any benefit demonstrating that AD etiology is more complex than previously thought. Here we review the current literature on the emerging key role of neuroinflammation, especially activation of microglia, in AD pathogenesis. Moreover, we provide compelling evidence that certain flavonoids, especially luteolin formulated in olive pomace oil together with hydroxytyrosol, offers a reasonable prophylactic treatment approach due to its many beneficial actions.

The Role of APP O-Glycosylation in Alzheimer's Disease

The number of people with dementia is increasing rapidly due to the increase in the aging population. Alzheimer’s disease (AD) is a type of neurodegenerative dementia caused by the accumulation of abnormal proteins. Genetic mutations, smoking, and several other factors have been reported as causes of AD, but alterations in glycans have recently been demonstrated to play a role in AD. Amyloid-β (Aβ), a cleaved fragment of APP, is the source of senile plaque, a pathological feature of AD. APP has been reported to undergo N- and O-glycosylation, and several Polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) have been shown to have catalytic activity for the transfer of GalNAc to APP. Since O-glycosylation in the proximity of a cleavage site in many proteins has been reported to be involved in protein processing, O-glycans may affect the cleavage of APP during the Aβ production process. In this report, we describe new findings on the O-glycosylation of APP and Aβ production.

Ser and Thr acceptor preferences of the GalNAc-Ts vary among isoenzymes to modulate mucin-type O-glycosylation

A family of polypeptide GalNAc-transferases (GalNAc-Ts) initiates mucin-type O-glycosylation, transferring GalNAc onto hydroxyl groups of Ser and Thr residues of target substrates. The 20 GalNAc-T isoenzymes in humans are classified into nine subfamilies according to sequence similarity. GalNAc-Ts select their sites of glycosylation based on weak and overlapping peptide sequence motifs, as well prior substrate O-GalNAc glycosylation at sites both remote (long-range) and neighboring (short-range) the acceptor. Together, these preferences vary among GalNAc-Ts imparting each isoenzyme with its own unique specificity. Studies on the first identified GalNAc-Ts showed Thr acceptors were preferred over Ser acceptors; however studies comparing Thr vs. Ser glycosylation across the GalNAc-T family are lacking. Using a series of identical random peptide substrates, with single Thr or Ser acceptor sites, we determined the rate differences (Thr/Ser rate ratio) between Thr and Ser substrate glycosylation for 12 isoenzymes (representing 7 GalNAc-T subfamilies). These Thr/Ser rate ratios varied across subfamilies, ranging from ~2 to ~18 (for GalNAc-T4/GalNAc-T12 and GalNAc-T3/GalNAc-T6, respectively), while nearly identical Thr/Ser rate ratios were observed for isoenzymes within subfamilies. Furthermore, the Thr/Ser rate ratios did not appreciably vary over a series of fixed sequence substrates of different relative activities, suggesting the ratio is a constant for each isoenzyme against single acceptor substrates. Finally, based on GalNAc-T structures, the different Thr/Ser rate ratios likely reflect differences in the strengths of the Thr acceptor methyl group binding to the active site pocket. With this work, another activity that further differentiates substrate specificity among the GalNAc-Ts has been identified.

Metabolic precision labeling enables selective probing of O-linked N-acetylgalactosamine glycosylation

Protein glycosylation events that happen early in the secretory pathway are often dysregulated during tumorigenesis. These events can be probed, in principle, by monosaccharides with bioorthogonal tags that would ideally be specific for distinct glycan subtypes. However, metabolic interconversion into other monosaccharides drastically reduces such specificity in the living cell. Here, we use a structure-based design process to develop the monosaccharide probe N-(S)-azidopropionylgalactosamine (GalNAzMe) that is specific for cancer-relevant Ser/Thr(O)-linked N-acetylgalactosamine (GalNAc) glycosylation. By virtue of a branched N-acylamide side chain, GalNAzMe is not interconverted by epimerization to the corresponding N-acetylglucosamine analog by the epimerase N-acetylgalactosamine-4-epimerase (GALE) like conventional GalNAc-based probes. GalNAzMe enters O-GalNAc glycosylation but does not enter other major cell surface glycan types including Asn(N)-linked glycans. We transfect cells with the engineered pyrophosphorylase mut-AGX1 to biosynthesize the nucleotide-sugar donor uridine diphosphate (UDP)-GalNAzMe from a sugar-1-phosphate precursor. Tagged with a bioorthogonal azide group, GalNAzMe serves as an O-glycan-specific reporter in superresolution microscopy, chemical glycoproteomics, a genome-wide CRISPR-knockout (CRISPR-KO) screen, and imaging of intestinal organoids. Additional ectopic expression of an engineered glycosyltransferase, "bump-and-hole" (BH)-GalNAc-T2, boosts labeling in a programmable fashion by increasing incorporation of GalNAzMe into the cell surface glycoproteome. Alleviating the need for GALE-KO cells in metabolic labeling experiments, GalNAzMe is a precision tool that allows a detailed view into the biology of a major type of cancer-relevant protein glycosylation.

Metabolic precision labeling enables selective probing of O-linked N-acetylgalactosamine glycosylation

Protein glycosylation events that happen early in the secretory pathway are often dysregulated during tumorigenesis. These events can be probed, in principle, by monosaccharides with bioorthogonal tags that would ideally be specific for distinct glycan subtypes. However, metabolic interconversion into other monosaccharides drastically reduces such specificity in the living cell. Here, we use a structure-based design process to develop the monosaccharide probe N-(S)-azidopropionylgalactosamine (GalNAzMe) that is specific for cancer-relevant Ser/Thr(O)-linked N-acetylgalactosamine (GalNAc) glycosylation. By virtue of a branched N-acylamide side chain, GalNAzMe is not interconverted by epimerization to the corresponding N-acetylglucosamine analog by the epimerase N-acetylgalactosamine-4-epimerase (GALE) like conventional GalNAc-based probes. GalNAzMe enters O-GalNAc glycosylation but does not enter other major cell surface glycan types including Asn(N)-linked glycans. We transfect cells with the engineered pyrophosphorylase mut-AGX1 to biosynthesize the nucleotide-sugar donor uridine diphosphate (UDP)-GalNAzMe from a sugar-1-phosphate precursor. Tagged with a bioorthogonal azide group, GalNAzMe serves as an O-glycan-specific reporter in superresolution microscopy, chemical glycoproteomics, a genome-wide CRISPR-knockout (CRISPR-KO) screen, and imaging of intestinal organoids. Additional ectopic expression of an engineered glycosyltransferase, "bump-and-hole" (BH)-GalNAc-T2, boosts labeling in a programmable fashion by increasing incorporation of GalNAzMe into the cell surface glycoproteome. Alleviating the need for GALE-KO cells in metabolic labeling experiments, GalNAzMe is a precision tool that allows a detailed view into the biology of a major type of cancer-relevant protein glycosylation.

Shigella flexneri Targets Human Colonic Goblet Cells by O Antigen Binding to Sialyl-Tn and Tn Antigens via Glycan-Glycan Interactions

Shigella flexneri targets colonic cells in humans to initiate invasive infection processes that lead to dysentery, and direct interactions between their lipopolysaccharide O antigens and blood group A related glycans are involved in the cell adherence interactions. Here, we show that treatment with Tn and sialyl-Tn glycans, monoclonal antibodies and lectins reactive to Tn/sialyl-Tn, and luteolin (a Tn antigen synthesis inhibitor) all significantly inhibited S. flexneri adherence and invasion of cells in vitro. Surface plasmon resonance analysis showed that lipopolysaccharide O antigen had a high affinity interaction with Tn/sialyl-Tn. Immunofluorescence probing of human colon tissue with antibodies detected expression of Tn/sialyl-Tn by MUC2 producing goblet cells (GCs), and S. flexneri incubated with human colon tissue colocalized with GCs. Our findings demonstrate that S. flexneri targets GCs in the human colonic crypts via glycan-glycan interactions, establishing new insight into the infection process in humans.

miR-452 Reverses Abnormal Glycosylation Modification of ERα and Estrogen Resistance in TNBC (Triple-Negative Breast Cancer) Through Targeting UGT1A1

Background: The breast epithelial cells in patients with triple-negative breast cancer (TNBC) actually have specific estrogen receptor (ER) expression, and the abnormal glycosylation of UGT1A1 in TNBC cells resulted in abnormal expression and function of ERα through regulating the modification of ERα. Therefore, our study targets the role of UGT1A1 expression, then glycosylation modification of ERα (estrogen receptor α) and estrogen resistance in development of TNBC.

Methods: The differential expression of mRNA and miRNA in TNBC tissues was tested. Luciferase activity was analyzed in TNBC cells treated with miR-452. Moreover, the human mammary gland and TNBC cell lines were dealt with estrogen and miR-452 or its inhibitors, then proliferation ability was further determined. Moreover, the role of interaction between UGT1A1 and ERα in the glycosylation modification of ERα and UGT activity, and metabolism of estrogen were assessed. The effects of miR-452 on TNBC by improving abnormal glycosylation modification of ERα by targeting UGT1A1 and estrogen resistance were studied in vitro and in vivo.

Results: The expression level of UGT1A1 in TNBC tumor tissues was higher than its matched para-tumorous tissues, but the miR-452 expression was opposite. The glycosylation modification site of ERα expressed in TNBC cells was different from that of normal mammary epithelial cells. The estrogen 17β-estradiol (E2) significantly promoted mitotic entry of TNBC cells. The interaction between UGT1A1 and ERα affected the expression level of each other, as well as the UGT enzyme activity and proliferation of TNBC cells. UGT1A1 induced production of intracellular estrogens and TNBC proliferation, but it could be reversed by overexpression of ERα. Upregulation of ERα caused the downregulation of UGT1A1 and marked decrease of intracellular estrogen products, and then suppressed TNBC proliferation. Moreover, UGT1A1 was the target gene of miR-452; miR-452 antagomir restrained TNBC xenograft.

Conclusion: Our results demonstrated that estrogen was a positive factor in the proliferation of TNBC cells at onset of mitosis through accentuating the expression and enzyme activity of UGT1A1. However, miR-452 targeted to UGT1A1, then regulated glycosylation modification of ERα, estrogen metabolism, and TNBC development associated with estrogen resistance.

Bioavailability and Pharmaco-therapeutic Potential of Luteolin in Overcoming Alzheimer's Disease

Luteolin is a naturally occurring, yellow crystalline flavonoid found in numerous dietary supplements we frequently have in our meals. Studies in the last 2 decades have revealed its therapeutic potential to reduce the Alzheimer's disease (AD) symptoms in various in vitro and in vivo models. The anti-Alzheimer's potential of luteolin is attributed to its ability to suppress Aβ as well as tau aggregation or promote their disaggregation, down-regulate the expression of COX-2, NOS, MMP-9, TNF-α, interleukins and chemokines, reduce oxidative stress by scavenging ROS, modulate the activities of transcription factors CREB, cJun, Nrf-1, NF-κB, p38, p53, AP-1 and β-catenine and inhibiting the activities of various protein kinases. In several systems, luteolin has been described as a potent antioxidant and anti-inflammatory agent. In addition, we have also discussed about the bio-availability of the luteolin in the plasma. After being metabolized luteolin persists in plasma as glucuronides and sulphate-conjugates. Human clinical trials indicated no dose limiting toxicity when administered at a dose of 100 mg/day. Improvements in the formulations and drug delivery systems may further enhance the bioavailability and potency of luteolin. The current review describes in detail the data supporting these studies.

Enhancing and Complementary Mechanisms of Synergistic Action of Acori Tatarinowii Rhizoma and Codonopsis Radix for Alzheimer's Disease Based on Systems Pharmacology

Materials and Methods

In this study, a systems pharmacology-based strategy was used to elucidate the synergistic mechanism of Acori Tatarinowii Rhizoma and Codonopsis Radix for the treatment of AD. This novel systems pharmacology model consisted of component information, pharmacokinetic analysis, and pharmacological data. Additionally, the related pathways were compressed using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, and the organ distributions were determined in the BioGPS bank.

Results

Sixty-eight active ingredients with suitable pharmacokinetic profiles and biological activities were selected through ADME screening in silico. Based on 62 AD-related targets, such as APP, CHRM1, and PTGS1, systematic analysis showed that these two herbs were mainly involved in the PI3K-Akt signaling pathway, MAPK signaling pathway, neuroactive ligand-receptor interaction, and fluid shear stress and atherosclerosis, indicating that they had a synergistic effect on AD. However, ATR acted on the KDR gene, while CR acted on IGF1R, MET, IL1B, and CHUK, showing that they also had complementary effects on AD. The ingredient contribution score involved 29 ingredients contributing 90.14% of the total contribution score of this formula for AD treatment, which emphasized that the effective therapeutic effects of these herbs for AD were derived from both ATR and CR, not a single herb. Organ distribution showed that the targets of the active ingredients were mainly located in the whole blood, the brain, and the muscle, which are associated with AD.

Conclusions

In sum, our findings suggest that the systems pharmacology methods successfully revealed the synergistic and complementary mechanisms of ATR and CR for the treatment of AD.

A simple bacterial expression system for human ppGalNAc-T and used for the synthesis of O-GalNAc glycosylated interleukin 2

Mucin-type O-glycosylation (hereafter referred to as O-GalNAc glycosylation) is one of the most abundant glycosylation on proteins. It is initiated by the members of polypeptide N-acetyl-α-galactosaminyltransferases (ppGalNAc-Ts) family. The ppGalNAc-Ts could be used as tool enzymes to modify target proteins including therapeutic glycoprotein drugs with O-GalNAc glycosylation at specific glycosylated sites in vitro. Obtaining a large amount of ppGalNAc-T can greatly increase the yield of therapeutic O-glycoprotein and reduce the culture costs. In this study, we reported a simple Escherichia coli (E. coli) expression system capable of producing human ppGalNAc-Ts. By co-expressing human PDI, we could simply obtain active ppGalNAc-Ts with high efficiency. Using the E. coli expressed ppGalNAc-T2, we site-specifically synthesized O-glycosylated IL-2 at the native glycosylated site Thr23 residue. These results reveal the E. coli system we constructed is suitable to produce active ppGalNAc-Ts and thus has the potential value for basic research and production of therapeutic O-glycoproteins in vitro.

Amyloid precursor protein glycosylation is altered in the brain of patients with Alzheimer's disease

Background

The amyloid precursor protein (APP) is a transmembrane glycoprotein that undergoes alternative proteolytic processing. Its processing through the amyloidogenic pathway originates a large sAPPβ ectodomain fragment and the β-amyloid peptide, while non-amyloidogenic processing generates sAPPα and shorter non-fibrillar fragments. Hence, measuring sAPPα and sAPPβ has been proposed as a means to identify imbalances between the amyloidogenic/non-amyloidogenic pathways in the brain of Alzheimer’s disease (AD) patients. However, to date, no consistent changes in these proteolytic fragments have been identified in either the brain or cerebrospinal fluid of AD individuals.

Methods

In frontal cortex homogenates from AD patients (n = 7) and non-demented controls (NDC; n = 7), the expression of total APP mRNA and that of the APP isoforms generated by alternative splicing, APP695 and APP containing the Kunitz protease inhibitor (KPI), was analyzed by qRT-PCR using TaqMan and SYBR Green probes. The balance between the amyloidogenic/non-amyloidogenic pathways was examined in western blots estimating the sAPPα and sAPPβ fragments and their membrane-tethered C-terminal fragments CTFα and CTFβ. CHO-PS70 cells, stably over-expressing wild-type human APP, served to evaluate whether Aβ42 peptide treatment results in altered APP glycosylation. We determined the glycosylation pattern of sAPPα and sAPPβ in brain extracts and CHO-PS70 culture media by lectin-binding assays.

Results

In the cortex of AD patients, we detected an increase in total APP mRNA relative to the controls, due to an increase in both the APP695 and APP-KPI variants. However, the sAPPα or sAPPβ protein levels remained unchanged, as did those of CTFα and CTFβ. We studied the glycosylation of the brain sAPPα and sAPPβ using lectins and pan-specific antibodies to discriminate between the fragments originated from neuronal APP695 and glial/KPI variants. Lectin binding identified differences in the glycosylation of sAPPβ species derived from the APP695 and APP-KPI variants, probably reflecting their distinct cellular origin. Moreover, the lectin-binding pattern differed in the sAPPα and sAPPβ originated from all the variants. Finally, when the lectin-binding pattern was compared between AD and NDC groups, significant differences were evident in sAPPα glycosylation. Lectin binding of the soluble sAPPα and sAPPβ from CHO-PS70 cells were also altered in cells treated with the Aβ peptide.

Conclusion

Our analysis of the lectin binding to sAPPα and sAPPβ suggests that glycosylation dictates the proteolytic pathway for APP processing. Differences between the demented and controls indicate that changes in glycosylation may influence the generation of the different APP fragments and, consequently, the pathological progression of AD.

O-linked N-acetylgalactosamine modification is present on the tumor suppressor p53

BACKGROUND

Mucin-type O-glycosylation (referred to as O-GalNAc glycosylation) is the most abundant O-glycosylation on membrane and secretory proteins. Recently several evidences suggest that nuclear or cytoplasmic proteins might also have O-GalNAc glycosylation. However, what nucleocytoplasmic proteins are O-GalNAc glycosylated and what the biological function of this modification in cells are still poorly understood. Previously, we reported the tumor suppressor p53 could be O-GalNAc glycosylated in vitro. To investigate the existence and function of O-GalNAc glycosylation on nucleocytoplasmic proteins in cell, p53 as a representative nucleocytoplasmic protein was studied.

METHODS

Using lectin blotting with GalNAc specific lectins, enzymatic treatments with O-GlcNAcase, core 1 β1, 3-galactosyltransferase and O-glycosidase, and metabolic labeling with un-O-acetylated GalNAz in UDP-Gal/UDP-GalNAc 4-epimerase (GALE) knockout cells, we validated the O-GalNAc glycosylation on p53. Using mass spectrometry analysis and site-directed mutagenesis, we identified the glycosylated sites and studied the functions of O-GalNAc glycosylation on p53.

RESULTS

The p53 was O-GalNAc glycosylated in cells. Ser121 residue was one of the glycosylated sites on p53. The O-GalNAc glycosylation at Ser121 was associated with the stability and activity of p53.

CONCLUSIONS

These results revealed that the O-GalNAc glycosylation was a novel modification on p53.

GENERAL SIGNIFICANCE

Our study provided a pilot evidence that the O-GalNAc glycosylation existed on nucleocytoplasmic protein.

Glycomic Signatures of Plasma IgG Improve Preoperative Prediction of the Invasiveness of Small Lung Nodules

Preoperative assessment of tumor invasiveness is essential to avoid overtreatment for patients with small-sized ground-glass nodules (GGNs) of 10 mm or less in diameter. However, it is difficult to determine the pathological state by computed tomography (CT) examination alone. Aberrant glycans has emerged as a tool to identify novel potential disease biomarkers. In this study, we used a lectin microarray-based strategy to investigate whether glycosylation changes in plasma immunoglobulin G (IgG) provide additional information about the invasiveness of small GGNs before surgery. Two independent cohorts (discovery set, n = 92; test set, n = 210) of GGN patients were used. Five of 45 lectins (Sambucus nigra agglutinin, SNA; Datura stramonium agglutinin, DSA; Galanthus nivalis agglutinin, GNA; Euonymus europaeus lectin, EEL; and Vicia villosa agglutinin, VVA) were identified as independent factors associated with pathological invasiveness of small GGNs (p < 0.01). Receiver-operating characteristic (ROC) curve analysis indicated the combination of these five lectins could significantly improve the accuracy of CT in diagnosing invasive GGNs, with an area under the curve (AUC) of 0.792 (p < 0.001), a sensitivity of 74.6%, and specificity of 74.4%, which was superior to current clinical biomarkers. These results suggest that the multilectin assay based on plasma IgG glycosylation may be a useful in vitro complementary test to enhance preoperative determination of the invasiveness of GGNs and guide surgeons to select proper clinical management to avoid overtreatment.

The structure of the colorectal cancer-associated enzyme GalNAc-T12 reveals how nonconserved residues dictate its function

Polypeptide N-acetylgalactosaminyl transferases (GalNAc-Ts) initiate mucin type O-glycosylation by catalyzing the transfer of N-acetylgalactosamine (GalNAc) to Ser or Thr on a protein substrate. Inactive and partially active variants of the isoenzyme GalNAc-T12 are present in subsets of patients with colorectal cancer, and several of these variants alter nonconserved residues with unknown functions. While previous biochemical studies have demonstrated that GalNAc-T12 selects for peptide and glycopeptide substrates through unique interactions with its catalytic and lectin domains, the molecular basis for this distinct substrate selectivity remains elusive. Here we examine the molecular basis of the activity and substrate selectivity of GalNAc-T12. The X-ray crystal structure of GalNAc-T12 in complex with a di-glycosylated peptide substrate reveals how a nonconserved GalNAc binding pocket in the GalNAc-T12 catalytic domain dictates its unique substrate selectivity. In addition, the structure provides insight into how colorectal cancer mutations disrupt the activity of GalNAc-T12 and illustrates how the rules dictating GalNAc-T12 function are distinct from those for other GalNAc-Ts.

Exploring Regulation of Protein O-Glycosylation in Isogenic Human HEK293 Cells by Differential O-Glycoproteomics

Most proteins trafficking the secretory pathway of metazoan cells will acquire GalNAc-type O-glycosylation. GalNAc-type O-glycosylation is differentially regulated in cells by the expression of a repertoire of up to twenty genes encoding polypeptide GalNAc-transferase isoforms (GalNAc-Ts) that initiate O-glycosylation. These GalNAc-Ts orchestrate the positions and patterns of O-glycans on proteins in coordinated, but poorly understood ways - guided partly by the kinetic properties and substrate specificities of their catalytic domains, as well as by modulatory effects of their unique GalNAc-binding lectin domains. Here, we provide the hereto most comprehensive characterization of nonredundant contributions of individual GalNAc-T isoforms to the O-glycoproteome of the human HEK293 cell using quantitative differential O-glycoproteomics on a panel of isogenic HEK293 cells with knockout of GalNAc-T genes (GALNT1, T2, T3, T7, T10, or T11). We confirm that a major part of the O-glycoproteome is covered by redundancy, whereas distinct O-glycosite subsets are covered by nonredundant GalNAc-T isoform-specific functions. We demonstrate that the GalNAc-T7 and T10 isoforms function in follow-up of high-density O-glycosylated regions, and that GalNAc-T11 has highly restricted functions and essentially only serves the low-density lipoprotein-related receptors in linker regions (C6XXXTC1) between the ligand-binding repeats.

Inhibition of polypeptide N-acetyl-α-galactosaminyltransferases is an underlying mechanism of dietary polyphenols preventing colorectal tumorigenesis

Ellagitannin-derived ellagic acid (EA) and colonic metabolite urolithins are functional dietary ingredients for cancer prevention, but the underlying mechanism need elucidation. Mucin-type O-glycosylation, initiated by polypeptide N-acetyl-α-galactosaminyltransferases (ppGalNAc-Ts), fine-tunes multiple biological processes and is closely associated with cancer progression. Herein, we aim to explore how specific tannin-based polyphenols affect tumor behavior of colorectal cancer cells (CRC) by modulating O-glycosylation. Utilizing HPLC-based enzyme assay, we find urolithin D (UroD), EA and gallic acid (GA) potently inhibit ppGalNAc-Ts. In particular, UroD inhibits ppGalNAc-T2 through a peptide/protein-competitive manner with nanomolar affinity. Computational simulations combined with site-directed mutagenesis further support the inhibitors' mode of action. Moreover, lectin analysis and metabolic labelling reveal that UroD can reduce cell O-glycans but not N-glycans. Transwell experiments prove that UroD inhibits migration and invasion of CRC cells. Our work proves that specific tannin-based polyphenols can potently inhibit ppGalNAc-Ts activity to reduce cell O-glycosylation and lead to lowering the migration and invasion of CRC cells, suggesting that disturbance of mucin-type O-glycosylation is an important mechanism for the function of dietary polyphenols.

Polypeptide GalNAc-Ts: from redundancy to specificity

Mucin-type O-glycosylation is a post-translational modification (PTM) that is predicted to occur in more than the 80% of the proteins that pass through the Golgi apparatus. This PTM is initiated by a family of polypeptide GalNAc-transferases (GalNAc-Ts) that modify Ser and Thr residues of proteins through the addition of a GalNAc moiety. These enzymes are type II membrane proteins that consist of a Golgi luminal catalytic domain connected by a flexible linker to a ricin type lectin domain. Together, both domains account for the different glycosylation preferences observed among isoenzymes. Although it is well accepted that most of the family members share some degree of redundancy toward their protein and glycoprotein substrates, it has been recently found that several GalNAc-Ts also possess activity toward specific targets. Despite the high similarity between isoenzymes, structural differences have recently been reported that are key to understanding the molecular basis of both their redundancy and specificity. The present review focuses on the molecular aspects of the protein substrate recognition and the different glycosylation preferences of these enzymes, which in turn will serve as a roadmap to the rational design of specific modulators of mucin-type O-glycosylation.

Site-specific glycosylation of Ebola virus glycoprotein by human polypeptide GalNAc-transferase 1 induces cell adhesion defects

The surface glycoprotein (GP) of Ebola virus causes many of the virus's pathogenic effects, including a dramatic loss of endothelial cell adhesion associated with widespread hemorrhaging during infection. Although the GP-mediated deadhesion depends on its extracellular mucin-like domain, it is unknown whether any, or all, of this domain's densely clustered O-glycosylation sites are required. It is also unknown whether any of the 20 distinct polypeptide GalNAc-transferases (ppGalNAc-Ts) that initiate mucin-type O-glycosylation in human cells are functionally required. Here, using HEK293 cell lines lacking specific glycosylation enzymes, we demonstrate that GP requires extended O-glycans to exert its deadhesion effect. We also identified ppGalNAc-T1 as largely required for the GP-mediated adhesion defects. Despite its profound effect on GP function, the absence of ppGalNAc-T1 only modestly reduced the O-glycan mass of GP, indicating that even small changes in the bulky glycodomain can cause loss of GP function. Indeed, protein-mapping studies identified a small segment of the mucin-like domain critical for function and revealed that mutation of five glycan acceptor sites within this segment are sufficient to abrogate GP function. Together, these results argue against a mechanism of Ebola GP-induced cell detachment that depends solely on ectodomain bulkiness and identify a single host-derived glycosylation enzyme, ppGalNAc-T1, as a potential target for therapeutic intervention against Ebola virus disease.

The Multiplicity of Polypeptide GalNAc-Transferase: Assays, Inhibitors, and Structures

Mucin-type O-glycosylation is the dominant form of glycosylation in eukaryotes and plays an important role in various physiological processes. The polypeptide GalNAc-transferase (GalNAc-T) catalyzes the first step in the attachment of mucin-type O-glycosylation. GalNAc-T was recently uncovered to be linked with cancer, atherogenic dyslipidemia, and X-linked hypophosphatemic rickets. Therefore, it has attracted increasing interest as a new target for exploring the underlying mechanism and developing new treatments for related diseases. Decades of studies on GalNAc-T have laid a stable foundation for understanding the catalytic mechanism, determining atom-resolution three-dimensional structures, and developing various types of biochemical assays as well as small-molecule inhibitor leads. Here, we systematically summarize this invaluable knowledge on GalNAc-T and cultivate new perspectives to foster breakthrough points for mucin-type O-glycosylation.

The polypeptide N-acetylgalactosaminyltransferase 4 exhibits stage-dependent expression in colorectal cancer and affects tumorigenesis, invasion and differentiation

The aberrant expression of mucin-type O-glycosylation plays important roles in cancer malignancy. The polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) are a family of conserved enzymes that initiate the mucin-type O-glycosylation in cells. In human, consistent up- or down-regulation of ppGalNAc-Ts expression during cancer development has been frequently reported. Here, we provide evidence that ppGalNAc-T4 shows a stage-dependent expression at the different stages of colorectal cancer (CRC) in the 62 pair-matched tumor/normal tissues. In detail, ppGalNAc-T4 expression is significantly induced at stage I and II but not at stage III and IV. Overexpression of ppGalNAc-T4 in CRC cells enhances colony formation and sphere formation suggesting an important role of ppGalNAc-T4 in tumorigenesis. Conversely, knockdown of ppGalNAc-T4 in CRC cells increases the cell migration and invasion, and leads to an epithelial-mesenchymal transition-like transition. Further analysis suggests that loss of ppGalNAc-T4 contributes to the dedifferentiation of CRC and high expression of ppGalNAc-T4 correlates to a good prognosis of patients. Taken together, our results not only demonstrate a stage-dependent expression of ppGalNAc-T4 in CRC progression, but also suggest that such stage-dependent expression may contribute to the tumorigenesis at the early stage and promote cell migration and invasion at the advanced stage.