GalNAc-T15 in gastric adenocarcinoma: Characterization according to tissue architecture and cellular location

Gastric cancer (GC) is the second most common cause of cancer-related deaths in the world. This study aims to investigate the differential tissue expression of ppGalNAc-T15 and to evaluate its possible association with clinical-pathological parameters and outcome of gastric adenocarcinoma patients. For these 70 patients were evaluated the expression by immunohistochemistry to ppGalNAc-T15. Our results showed that 33 (47.1%) patients were ppGalNAC-T15+ positive and 37 (52.9%) negative. Positive staining for ppGalNAc-T15 was significantly present in patients older than 60 years (P=0.0306) and submitted to total gastrectomy (P=0.0087). Also, some results remained at the limit of significance as surgical standing (P=0.0562) and histological grade (P=0.0549). Therefore, the ppGalNAc-T15 immunoreactivity can be useful to understand the prognosis of patients with gastric cancer.

Molecular cloning, expression, and characterization of UDP N-acetyl-α-d-galactosamine: Polypeptide N-acetylgalactosaminyltransferase 4 from Cryptosporidium parvum

Cryptosporidium spp. are the causative agents of diarrheal disease worldwide, but effective treatments are lacking. Cryptosporidium employs mucin-like glycoproteins with O-glycans to attach to and infect host intestinal epithelial cells. The Tn antigen (GalNAcα1-Ser/Thr) is an O-glycan essential for these processes, as Tn-specific lectins and a Tn-specific monoclonal antibody block attachment to and infection of host cells in vitro. The enzymes in Cryptosporidium catalyzing their synthesis, however, have not been studied. Previously, we identified four genes encoding putative UDP N-acetyl-α-d-galactosamine:polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) in the genomes of three Cryptosporidium spp. Here we report the in silico analysis, cloning, expression, purification, and characterization of one of the four enzymes Cryptosporidium parvum (Cp)-ppGalNAc-T4. This enzyme contains the characteristic domains and motifs conserved in ppGalNAc-Ts and is expressed at multiple time points during in vitro infection. Recombinant soluble Cp-ppGalNAc-T4 was enzymatically active against an unmodified EA2 peptide suggesting that it may function as an "initiating" ppGalNAc-T. Cp-ppGalNAc-T4 also exhibited a strong preference for UDP-GalNAc over other nucleotide sugar donors and was active against unmodified and O-glycosylated versions of the C. parvum gp40-derived peptide, with a preference for the former, suggesting it may play a role in modifying this glycoprotein in vivo. Given the importance of mucin-type O-glycosylation in Cryptosporidium spp., the enzymes that catalyze their synthesis may serve as potential therapeutic targets.

Inhibitory neuron-specific Cre-dependent red fluorescent labeling using VGAT BAC-based transgenic mouse lines with identified transgene integration sites

Inhibitory neurons are crucial for shaping and regulating the dynamics of the entire network, and disturbances in these neurons contribute to brain disorders. Despite the recent progress in genetic labeling techniques, the heterogeneity of inhibitory neurons requires the development of highly characterized tools that allow accurate, convenient, and versatile visualization of inhibitory neurons in the mouse brain. Here, we report a novel genetic technique to visualize the vast majority and/or sparse subsets of inhibitory neurons in the mouse brain without using techniques that require advanced skills. We developed several lines of Cre-dependent tdTomato reporter mice based on the vesicular GABA transporter (VGAT)-BAC, named VGAT-stop-tdTomato mice. The most useful line (line #54) was selected for further analysis based on two characteristics: the inhibitory neuron-specificity of tdTomato expression and the transgene integration site, which confers efficient breeding and fewer adverse effects resulting from transgene integration-related genomic disruption. Robust and inhibitory neuron-specific expression of tdTomato was observed in a wide range of developmental and cellular contexts. By breeding the VGAT-stop-tdTomato mouse (line #54) with a novel Cre driver mouse line, Galntl4-CreER, sparse labeling of inhibitory neurons was achieved following tamoxifen administration. Furthermore, another interesting line (line #58) was generated through the unexpected integration of the transgene into the X-chromosome and will be used to map X-chromosome inactivation of inhibitory neurons. Taken together, our studies provide new, well-characterized tools with which multiple aspects of inhibitory neurons can be studied in the mouse.

GM1 ganglioside-independent intoxication by Cholera toxin

Cholera toxin (CT) enters and intoxicates host cells after binding cell surface receptors via its B subunit (CTB). We have recently shown that in addition to the previously described binding partner ganglioside GM1, CTB binds to fucosylated proteins. Using flow cytometric analysis of primary human jejunal epithelial cells and granulocytes, we now show that CTB binding correlates with expression of the fucosylated Lewis X (LeX) glycan. This binding is competitively blocked by fucosylated oligosaccharides and fucose-binding lectins. CTB binds the LeX glycan in vitro when this moiety is linked to proteins but not to ceramides, and this binding can be blocked by mAb to LeX. Inhibition of glycosphingolipid synthesis or sialylation in GM1-deficient C6 rat glioma cells results in sensitization to CT-mediated intoxication. Finally, CT gavage produces an intact diarrheal response in knockout mice lacking GM1 even after additional reduction of glycosphingolipids. Hence our results show that CT can induce toxicity in the absence of GM1 and support a role for host glycoproteins in CT intoxication. These findings open up new avenues for therapies to block CT action and for design of detoxified enterotoxin-based adjuvants.

Identification of a Golgi GPI-N-acetylgalactosamine transferase with tandem transmembrane regions in the catalytic domain

Many eukaryotic proteins are anchored to the cell surface via the glycolipid glycosylphosphatidylinositol (GPI). Mammalian GPIs have a conserved core but exhibit diverse N-acetylgalactosamine (GalNAc) modifications, which are added via a yet unresolved process. Here we identify the Golgi-resident GPI-GalNAc transferase PGAP4 and show by mass spectrometry that PGAP4 knockout cells lose GPI-GalNAc structures. Furthermore, we demonstrate that PGAP4, in contrast to known Golgi glycosyltransferases, is not a single-pass membrane protein but contains three transmembrane domains, including a tandem transmembrane domain insertion into its glycosyltransferase-A fold as indicated by comparative modeling. Mutational analysis reveals a catalytic site, a DXD-like motif for UDP-GalNAc donor binding, and several residues potentially involved in acceptor binding. We suggest that a juxtamembrane region of PGAP4 accommodates various GPI-anchored proteins, presenting their acceptor residue toward the catalytic center. In summary, we present insights into the structure of PGAP4 and elucidate the initial step of GPI-GalNAc biosynthesis.

Site-specific O-glycosylation of N-terminal serine residues by polypeptide GalNAc-transferase 2 modulates human δ-opioid receptor turnover at the plasma membrane

G protein-coupled receptors (GPCRs) are an important protein family of signalling receptors that govern a wide variety of physiological functions. The capacity to transmit extracellular signals and the extent of cellular response are largely determined by the amount of functional receptors at the cell surface that is subject to complex and fine-tuned regulation. Here, we demonstrate that the cell surface expression level of an inhibitory GPCR, the human δ-opioid receptor (hδOR) involved in pain and mood regulation, is modulated by site-specific N-acetylgalactosamine (GalNAc) -type O-glycosylation. Importantly, we identified one out of the 20 polypeptide GalNAc-transferase isoforms, GalNAc-T2, as the specific regulator of O-glycosylation of Ser6, Ser25 and Ser29 in the N-terminal ectodomain of the receptor. This was demonstrated by in vitro glycosylation assays using peptides corresponding to the hδOR N-terminus, Vicia villosa lectin affinity purification of receptors expressed in HEK293 SimpleCells capable of synthesizing only truncated O-glycans, GalNAc-T edited cell line model systems, and site-directed mutagenesis of the putative O-glycosylation sites. Interestingly, a single-nucleotide polymorphism, at residue 27 (F27C), was found to alter O-glycosylation of the receptor in efficiency as well as in glycosite usage. Furthermore, flow cytometry and cell surface biotinylation assays using O-glycan deficient CHO-ldlD cells revealed that the absence of O-glycans results in decreased receptor levels at the plasma membrane due to enhanced turnover. In addition, mutation of the identified O-glycosylation sites led to a decrease in the number of ligand-binding competent receptors and impaired agonist-mediated inhibition of cyclic AMP accumulation in HEK293 cells. Thus, site-specific O-glycosylation by a selected GalNAc-T isoform can increase the stability of a GPCR, in a process that modulates the constitutive turnover and steady-state levels of functional receptors at the cell surface.

Loss of the mucosal barrier alters the progenitor cell niche via Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling

The mucous barrier of our digestive tract is the first line of defense against pathogens and damage. Disruptions in this barrier are associated with diseases such as Crohn's disease, colitis, and colon cancer, but mechanistic insights into these processes and diseases are limited. We have previously shown that loss of a conserved O-glycosyltransferase (PGANT4) in Drosophila results in aberrant secretion of components of the peritrophic/mucous membrane in the larval digestive tract. Here, we show that loss of PGANT4 disrupts the mucosal barrier, resulting in epithelial expression of the IL-6-like cytokine Upd3, leading to activation of JAK/STAT signaling, differentiation of cells that form the progenitor cell niche, and abnormal proliferation of progenitor cells. This niche disruption could be recapitulated by overexpressing upd3 and rescued by deleting upd3, highlighting a crucial role for this cytokine. Moreover, niche integrity and cell proliferation in pgant4-deficient animals could be rescued by overexpression of the conserved cargo receptor Tango1 and partially rescued by supplementation with exogenous mucins or treatment with antibiotics. Our findings help elucidate the paracrine signaling events activated by a compromised mucosal barrier and provide a novel in vivo screening platform for mucin mimetics and other strategies to treat diseases of the oral mucosa and digestive tract.

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

Mucin-type O-glycosylation is the most abundant type of O-glycosylation. It is initiated by the members of the polypeptide N-acetyl-α-galactosaminyltransferase (ppGalNAc-T) family and closely associated with both physiological and pathological conditions, such as coronary artery disease or Alzheimer's disease. The lack of direct and selective inhibitors of ppGalNAc-Ts has largely impeded research progress in understanding the molecular events in mucin-type O-glycosylation. Here, we report that a small molecule, the plant flavonoid luteolin, selectively inhibits ppGalNAc-Ts in vitro and in cells. We found that luteolin inhibits ppGalNAc-T2 in a peptide/protein-competitive manner but not promiscuously (e.g. via aggregation-based activity). X-ray structural analysis revealed that luteolin binds to the PXP motif-binding site found in most protein substrates, which was further validated by comparing the interactions of luteolin with wild-type enzyme and with mutants using 1H NMR-based binding experiments. Functional studies disclosed that luteolin at least partially reduced production of β-amyloid protein by selectively inhibiting the activity of ppGalNAc-T isoforms. In conclusion, our study provides key structural and functional details on luteolin inhibiting ppGalNAc-T activity, opening up the way for further optimization of more potent and specific ppGalNAc-T inhibitors. Moreover, our findings may inform future investigations into site-specific O-GalNAc glycosylation and into the molecular mechanism of luteolin-mediated ppGalNAc-T inhibition.

B3GALNT2 mutations associated with non-syndromic autosomal recessive intellectual disability reveal a lack of genotype-phenotype associations in the muscular dystrophy-dystroglycanopathies

BACKGROUND

The phenotypic severity of congenital muscular dystrophy-dystroglycanopathy (MDDG) syndromes associated with aberrant glycosylation of α-dystroglycan ranges from the severe Walker-Warburg syndrome or muscle-eye-brain disease to mild, late-onset, isolated limb-girdle muscular dystrophy without neural involvement. However, muscular dystrophy is invariably found across the spectrum of MDDG patients.

METHODS

Using linkage mapping and whole-exome sequencing in two families with an unexplained neurodevelopmental disorder, we have identified homozygous and compound heterozygous mutations in B3GALNT2.

RESULTS

The first family comprises two brothers of Dutch non-consanguineous parents presenting with mild ID and behavioral problems. Immunohistochemical analysis of muscle biopsy revealed no significant aberrations, in line with the absence of a muscular phenotype in the affected siblings. The second family includes five affected individuals from an Iranian consanguineous kindred with mild-to-moderate intellectual disability (ID) and epilepsy without any notable neuroimaging, muscle, or eye abnormalities. Complementation assays of the compound heterozygous mutations identified in the two brothers had a comparable effect on the O-glycosylation of α-dystroglycan as previously reported mutations that are associated with severe muscular phenotypes.

CONCLUSIONS

In conclusion, we show that mutations in B3GALNT2 can give rise to a novel MDDG syndrome presentation, characterized by ID associated variably with seizure, but without any apparent muscular involvement. Importantly, B3GALNT2 activity does not fully correlate with the severity of the phenotype as assessed by the complementation assay.

The interdomain flexible linker of the polypeptide GalNAc transferases dictates their long-range glycosylation preferences

The polypeptide GalNAc-transferases (GalNAc-Ts), that initiate mucin-type O-glycosylation, consist of a catalytic and a lectin domain connected by a flexible linker. In addition to recognizing polypeptide sequence, the GalNAc-Ts exhibit unique long-range N- and/or C-terminal prior glycosylation (GalNAc-O-Ser/Thr) preferences modulated by the lectin domain. Here we report studies on GalNAc-T4 that reveal the origins of its unique N-terminal long-range glycopeptide specificity, which is the opposite of GalNAc-T2. The GalNAc-T4 structure bound to a monoglycopeptide shows that the GalNAc-binding site of its lectin domain is rotated relative to the homologous GalNAc-T2 structure, explaining their different long-range preferences. Kinetics and molecular dynamics simulations on several GalNAc-T2 flexible linker constructs show altered remote prior glycosylation preferences, confirming that the flexible linker dictates the rotation of the lectin domain, thus modulating the GalNAc-Ts' long-range preferences. This work for the first time provides the structural basis for the different remote prior glycosylation preferences of the GalNAc-Ts.

ANGPTL3 is part of the machinery causing dyslipidemia majorily via LPL inhibition in mastitis mice

Previous investigations have shown that inflammation induces changes in lipid and lipoprotein metabolism, and increased expression of angiopoietin-like protein 3 (ANGPTL3) contributes to the development of dyslipidemia. Here we investigated whether there is a correlation between increased ANGPTL3 expression and dyslipidemia in mastitis mice. Thirty mice were divided into two groups: control group and Staphylococcus aureus (S. aureus)-induced mastitis mice group. Changes in the levels of blood lipids [total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C)]; activity of myeloperoxidase (MPO); concentrations of plasma inflammation biomarkers [interferon-γ (IFNγ), tumor necrosis factor α (TNFα), and interleukin-1α (IL-1α)]; concentration of plasma ANGPTL3 protein; lipoprotein lipase (LPL) activities in postheparin plasma; expressions of hepatic N-acetylgalactosaminyltransferase 2 (GALNT2), hepatic ANGPTL3 and adipose LPL were determined. The major results indicated specific pathological mammary tissue changes, elevated MPO activity, reduced GALNT2 mRNA expression, elevated ANGPTL3 mRNA and protein expression and reduced LPL mRNA and protein expression. In plasma samples the S.aureus infused mice displayed elevated ANGPTL3 protein concentration, TG, TC and LDL-C levels, and reduced postheparin LPL activities and HDL-C level. The data suggests that ANGPTL3 is part of the machinery causing dyslipidemia majorily via LPL inhibition in mastitis mice.

Organelle Specific O-Glycosylation Drives MMP14 Activation, Tumor Growth, and Metastasis

Cancers grow within tissues through molecular mechanisms still unclear. Invasiveness correlates with perturbed O-glycosylation, a covalent modification of cell-surface proteins. Here, we show that, in human and mouse liver cancers, initiation of O-glycosylation by the GALNT glycosyl-transferases increases and shifts from the Golgi to the endoplasmic reticulum (ER). In a mouse liver cancer model, expressing an ER-targeted GALNT1 (ER-G1) massively increased tumor expansion, with median survival reduced from 23 to 10 weeks. In vitro cell growth was unaffected, but ER-G1 strongly enabled matrix degradation and tissue invasion. Unlike its Golgi-localized counterpart, ER-G1 glycosylates the matrix metalloproteinase MMP14, a process required for tumor expansion. Together, our results indicate that GALNTs strongly promote liver tumor growth after relocating to the ER.

Functional control of polypeptide GalNAc-transferase 3 through an acetylation site in the C-terminal lectin domain

O-GalNAc glycans are important structures in cellular homeostasis. Their biosynthesis is initiated by members of the polypeptide GalNAc-transferase (ppGalNAc-T) enzyme family. Mutations in ppGalNAc-T3 isoform cause diseases (congenital disorders of glycosylation) in humans. The K626 residue located in the C-terminal β-trefoil fold of ppGalNAc-T3 was predicted to be a site with high likelihood of acetylation by CBP/p300 acetyltransferase. We used a site-directed mutagenesis approach to evaluate the role of this acetylation site in biological properties of the enzyme. Two K626 mutants of ppGalNAc-T3 (T3K626Q and T3K626A) had GalNAc-T activities lower than that of wild-type enzyme. Direct and competitive interaction assays revealed that GalNAc recognition by the lectin domain was altered in the mutants. The presence of GlcNAc glycosides affected the interaction of the three enzymes with mucin-derived peptides. In GalNAc-T activity assays, the presence of GlcNAc glycosides significantly inhibited activity of the mutant (T3K626Q) that mimicked acetylation. Our findings, taken together, reveal the crucial role of the K626 residue in the C-terminal β-trefoil fold in biological properties of human ppGalNAc-T3. We propose that acetylated residues on ppGalNAc-T3 function as control points for enzyme activity, and high level of GlcNAc glycosides promote a synergistic regulatory mechanism, leading to a metabolically disordered state.

miR‑4262 inhibits colon cancer cell proliferation via targeting of GALNT4

MicroRNAs (miRs) have been demonstrated to be important in the establishment and progression of colon cancer. However the underlying molecular mechanisms remain to be fully elucidated. Polypeptide N‑acetylgalactosaminyltransferase4 (GALNT4) participates in numerous cellular processes, including tumorigenesis. The present study used reverse transcription‑quantitative polymerase chain reaction and western blotting to investigate the expression levels of miR‑4262 and GALNT4 in tissues and cells. In addition, MTS and colony formation assays, and cell cycle analysis were performed to evaluate the effect of miR‑4262 on cell proliferation and the cell cycle. The findings demonstrated that miR‑4262 was a direct target of GALNT4 mRNA. Overexpression of miR‑4262 was demonstrated to decrease GALNT4 mRNA and protein expression levels, and thereby suppressed cell viability, growth and cell‑cycle progression in SW480 and SW620 colon cancer cells. In addition, knockdown of miR‑4262 significantly increased the cell viability, growth, and cell‑cycle progression of SW480 and SW620 cells. The expression level of miR‑4262 was observed to be downregulated as the expression of GALNT4 was upregulated in colon cancer tissues and cell lines. In conclusion, the results demonstrated that miR‑4262 may be involved in the development of colon cancer via targeting of GALNT4. The miR‑4262/GALNT4 axis may be a novel target for diagnosing and understanding the underlying molecular mechanism of colon cancer.

Human Immunodeficiency Virus and Simian Immunodeficiency Virus Maintain High Levels of Infectivity in the Complete Absence of Mucin-Type O-Glycosylation

A highly conserved threonine near the C terminus of gp120 of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) was investigated for its contributions to envelope protein function and virion infectivity. When this highly conserved Thr residue was substituted with anything other than serine (the other amino acid that can accept O-glycosylation), the resulting virus was noninfectious. We found that this Thr was critical for the association of gp120 with the virion and that amino acid substitution increased the amount of dissociated gp120 in the cell culture supernatant. When HIV virions were generated in cells overexpressing polypeptide N-acetylgalactosaminyltransferase 1 (GalNAcT1), viral infectivity was increased 2.5-fold compared to that of virus produced in wild-type HEK293T cells; infectivity was increased 8-fold when the Thr499Ser mutant was used. These infectivity enhancements were not observed when GalNAcT3 was used. Using HEK293T knockout cell lines totally devoid of the ability to perform O-linked glycosylation, we demonstrated production of normal levels of virions and normal levels of infectivity in the complete absence of O-linked carbohydrate. Our data indicate that O-glycosylation is not necessary for the natural replication cycle of HIV and SIV. Nonetheless, it remains theoretically possible that the repertoire of GalNAc transferase isoforms in natural target cells for HIV and SIV in vivo could result in O-glycosylation of the threonine residue in question and that this could boost the infectivity of virions beyond the levels seen in the absence of such O-glycosylation.IMPORTANCE Approximately 50% of the mass of the gp120 envelope glycoprotein of both HIV and SIV is N-linked carbohydrate. One of the contributions of this N-linked carbohydrate is to shield conserved peptide sequences from recognition by humoral immunity. This N-linked glycosylation is one of the reasons that primary isolates of HIV and SIV are so heavily resistant to antibody-mediated neutralization. Much less studied is any potential contribution from O-linked glycosylation. The literature on this topic to date is somewhat confusing and ambiguous. Our studies described in this report demonstrate unambiguously that O-linked glycosylation is not necessary for the natural replication cycle of HIV and SIV. However, the door is not totally closed because of the diversity of numerous GalNAc transferase enzymes that initiate O-linked carbohydrate attachment and the theoretical possibility that natural target cells for HIV and SIV in vivo could potentially complete such O-linked carbohydrate attachment to further increase infectivity.

The GalNAc-T Activation Pathway (GALA) is not a general mechanism for regulating mucin-type O-glycosylation

Mucin-type O-glycosylation is initiated by the UDP-GalNAc polypeptide:N-acetylgalactosaminyltransferase (GalNAc-T) family of enzymes. Their activity results in the GalNAc α1-O-Thr/Ser structure, termed the Tn antigen, which is further decorated with additional sugars. In neoplastic cells, the Tn antigen is often overexpressed. Because O-glycosylation is controlled by the activity of GalNAc-Ts, their regulation is of great interest. Previous reports suggest that growth factors, EGF or PDGF, induce Golgi complex-to-endoplasmic reticulum (ER) relocation of both GalNAc-Ts and Tn antigen in HeLa cells, offering a mechanism for Tn antigen overexpression termed "GALA". However, we were unable to reproduce these findings. Upon treatment of HeLa cells with either EGF or PDGF we observed no change in the co-localization of endogenous GalNAc-T1, GalNAc-T2 or Tn antigen with the Golgi complex marker TGN46. There was also no enhancement of localization with the ER marker calnexin. We conclude that growth factors do not cause redistribution of GalNAc-Ts from the Golgi complex to the ER in HeLa cells.

Variant in GALNT3 Gene Linked with Reduced Coronary Artery Disease Risk in Chinese Population

Our previous study found expression of GALNT3 gene was reduced in coronary artery disease (CAD) patients, and it contributed to endothelial injury by regulating apoptosis and matrix metalloproteinase (MMP) expression. GALNT3 gene may be a potential target for future therapeutic intervention of CAD. However, none reports linking the GALNT3 gene to susceptibility of CAD. This study investigated the variant associations of GALNT3 gene and CAD. Thirteen single nucleotide polymorphism (SNP) in and around the GALNT3 gene were tagged and analyzed in CAD patients (n = 1515) and control individuals (n = 5019), and the SNPs with CAD were tested with multiple logistic regression analysis in an additive genetic model (with one degree of freedom) after adjusting for age and sex. Expression of GALNT3 gene was detected by real-time PCR and Western blot. Luciferase reporter assays were used to detect the allele-specific effect of rs4621175 on transcriptional activity. Two GALNT3 markers, rs13427924 and rs4621175, were significantly associated with CAD (odds ratio [OR] = 0.87, p = 1.01 × 10-3 and OR = 0.75, p = 2.51 × 10-4, respectively), and the risk A allele of rs4621175 was associated with lower GALNT3 expression in both mRNA and protein level; also, A allele showed decreased reporter activity. In addition, we found the level of GALNT3 negatively correlated with MMP-2 gene expression. This study identified GALNT3 as a novel gene that rendered patients susceptible to CAD, and the A allele of a disease-associated variant rs4621175 linked reduced CAD risk through decreased GALNT3 expression. These results confirmed the role of GALNT3 gene in CAD and provided new insights into the genetic regulation of the GALNT3 gene with respect to the pathogenesis of CAD.

Expression of Polypeptide N-Acetylgalactosaminyltransferase-6 in Epithelial Ovarian Carcinoma

BACKGROUND

The family of polypeptide N-acetylgalactosanimyltransferases (GalNAc-Ts) are important factors in glycosylation in carcinomas. The purpose of this study was to investigate the clinical significance of GalNAc-T6 and its correlation with the prognosis of epithelial ovarian carcinoma.

MATERIALS AND METHODS

A total of 150 patients with epithelial ovarian carcinoma were enrolled and the relationship between GalNAc-T6 expression by immunohistochemistry and long-term survival was evaluated.

RESULTS

The expression of GalNAc-T6 was positive in 57.6% (34/59) of those with serous carcinoma, 85.3% (29/34) in mucinous carcinoma, 15.6% (5/27) in clear cell carcinoma, and 44% (14/25) in endometrioid carcinoma. In a Kaplan-Meier analysis of patients with grade 1 or 2 serous carcinoma, the 10-year overall survival rates were 47.4% in the GalNAc-T6-positive and 9.1% in the GalNAc-T6-negative groups (p=0.047).

CONCLUSION

GalNAc-T6 expression in epithelial ovarian carcinoma was different according to pathological type. In low-grade serous carcinoma, GalNAc-T6 expression may contribute to improved long-term survival.

Ectopic expression of specific GA2 oxidase mutants promotes yield and stress tolerance in rice

A major challenge of modern agricultural biotechnology is the optimization of plant architecture for enhanced productivity, stress tolerance and water use efficiency (WUE). To optimize plant height and tillering that directly link to grain yield in cereals and are known to be tightly regulated by gibberellins (GAs), we attenuated the endogenous levels of GAs in rice via its degradation. GA 2-oxidase (GA2ox) is a key enzyme that inactivates endogenous GAs and their precursors. We identified three conserved domains in a unique class of C₂₀ GA2ox, GA2ox6, which is known to regulate the architecture and function of rice plants. We mutated nine specific amino acids in these conserved domains and observed a gradient of effects on plant height. Ectopic expression of some of these GA2ox6 mutants moderately lowered GA levels and reprogrammed transcriptional networks, leading to reduced plant height, more productive tillers, expanded root system, higher WUE and photosynthesis rate, and elevated abiotic and biotic stress tolerance in transgenic rice. Combinations of these beneficial traits conferred not only drought and disease tolerance but also increased grain yield by 10-30% in field trials. Our studies hold the promise of manipulating GA levels to substantially improve plant architecture, stress tolerance and grain yield in rice and possibly in other major crops.

Expression of N-Acetylgalactosaminyltransferase-6 Is Related to Expression of Cell Adhesion Molecules in Endometrial Cancer

BACKGROUND

The aberrant glycosylation of mucin type O-glycans is thought to be associated with functional alteration of cancer cells, including adhesive properties, as well as their potential for invasion and metastasis. Positive expression of N-acetylgalactosaminyltransferase-6 (GalNAc-T6) may also be a marker for aberrant O-glycans in carcinogenesis. We previously reported that over-expression of GalNAc-T6 had a strong association with endometrial cell invasion ability in vitro.

MATERIALS AND METHODS

This study investigated the relationship between GalNAc-T6 expression and cell adhesion molecules in 218 endometrial carcinomas by immunohistochemistry.

RESULTS

Expression of GalNAc-T6 was found to be significantly related to expression of E-cadherin. Positive expression of GalNAc-T6 was significantly associated with better histological grade and good clinical prognosis of patients, but positive E-cadherin and β-catenin expression were not significantly associated with improved overall survival.

CONCLUSION

GalNAc-T6 might be related to cell-cell adhesion in the early phase of cancer invasion in endometrial carcinoma.

Characterization of GALNTL5 gene sequence and expression in ovine testes and sperm

The polypeptide N-acetylgalactosaminyltransferase-like protein 5 (GALNTL5), which belongs to the polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) gene family, is a newly identified gene that is specifically expressed in testis and involved in spermatogenesis. However, there is no data showing the existence of GALNTL5 in ram testis at various developmental stages and its influence on sperm motility. Therefore, the objectives of the present study were to evaluate the presence of GALNTL5 in the testis of 3-24 months (M) ram and to investigate the expression of GALNTL5 in spermatozoa with different motilities. We detected a 1602 bp cDNA fragment of GALNTL5 that included a 1326 bp coding sequence, encoding 441 amino acids and 90 and 185 bp of the 5' and 3' untranslated regions, respectively. The GALNTL5 amino acid sequence showed 51.87-83.48% identity with the sequences of proteins from other species. It was detected exclusively in the testis and the levels of both the mRNA and protein were progressively increased with age. Immunohistochemistry further revealed that GALNTL5 specifically localized in the elongating spermatids and spermatozoa, and it was demonstrated to be strongly concentrated in the head, neck, and mid-piece region of spermatozoa by immunocytochemistry. The sperm density and the percentage of live sperm in the high motility group (≥80%) were significantly higher than in the low motility group (≤50%), and the reverse trend was observed with the abnormal sperm. Western blot analysis showed that the protein expression of PGK2, ALDOA, and GALNTL5 were significantly higher in the high motility group than in the low motility group. Overall, the data suggest that GALNTL5 is an important functional molecule during spermatogenesis. Moreover, it is the first to suggest that the expression level of GALNTL5 is positively correlated with the sperm motility.

Correlation of polypeptide N-acetylgalactosamine transferases-3 and -6 to different stages of endometriosis

PURPOSE

To investigate the expression patterns of N-acetyl galactosamine transferases (GalNAc-Ts)-3 and GalNAc-T6 in clinicopathologically characterized endometriosis (EMS), and to explore their clinical significance.

METHODS

Ectopic and eutopic endometrial tissue samples were obtained and confirmed with CD-10 immunohistochemistry in patients with EMS (n = 12), whereas normal control endometrium was obtained from patients with uterine septum (n = 12). The mRNA and protein levels of GalNAc-T3 and GalNAc-T6 were detected in these samples using quantitative real-time PCR, immunohistochemistry, and western blotting.

RESULTS

GalNAc-T3 and GalNAc-T6 were expressed in the endometrium of all groups, with no significant changes observed during the menstrual cycle. The expression of GalNAc-T3 and GalNAc-T6 in ectopic endometrium was significantly lower than that in eutopic (P < 0.05) or control endometrium (P < 0.05), whereas there were no significant differences (P > 0.05) between eutopic and control endometria. Furthermore, the expression of GalNAc-T3 and GalNAc-T6 was significantly lower in patients with stage III/IV EMS compared to patients with stage I/II (P < 0.05).

CONCLUSIONS

Both GalNAc-T3 and GalNAc-T6 expression levels were downregulated in ectopic endometrium, which may increase the adhesion and invasion of endometrial cells and contribute to the development of EMS. Moreover, we found a strong correlation between the expression of GalNAc-T3 and GalNAc-T6 and different stages of EMS.

Systematic identification of the protein substrates of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase-T1/T2/T3 using a human proteome microarray

O-GalNAc glycosylation is the initial step of the mucin-type O-glycosylation. In humans, it is catalyzed by a family of 20 homologous UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts). So far, there is very limited information on their protein substrate specificities. In this study, we developed an on-chip ppGalNAc-Ts assay that could rapidly and systematically identify the protein substrates of each ppGalNAc-T. In detail, we utilized a human proteome microarray as the protein substrates and UDP-GalNAz as the nucleotide sugar donor for click chemistry detection. From a total of 16 368 human proteins, we identified 570 potential substrates of ppGalNAc-T1, T2, and T3. Among them, 128 substrates were overlapped, while the rest were isoform specific. Further cluster analysis of these substrates showed that the substrates of ppGalNAc-T1 had a closer phylogenetic relationship with that of ppGalNAc-T3 compared with ppGalNAc-T2, which was consistent with the topology of the phylogenetic tree of these ppGalNAc-Ts. Taken together, our microarray-based enzymatic assay comprehensively reveals the substrate profile of the ppGalNAc-T1, T2, and T3, which not only provides a plausible explanation for their partial functional redundancy as reported, but clearly implies some specialized roles of each enzyme in different biological processes.

GALNT2 effect on HDL-cholesterol and triglycerides levels in humans: Evidence of pleiotropy?

A wide range of studies both in humans and animal models point GALNT2 as a shaper of serum HDL-C and TG levels. Available data in humans indicate that, while under conditions of extreme GALNT2 loss-of-function HDL-C is the main target, a fine-tuning of GALNT2 changes is mostly associated with TG levels. Understanding whether different degrees of GALNT2 change do modulate different serum lipid fractions and, if so, addressing the mechanisms underlying such pleiotropic effects has the potential not only to improve our understanding of HDL-C and TG metabolism, but also to make GALNT2 becoming a target for treating atherogenic dyslipidemia and related clinical events.

Site-specific O-Glycosylation by Polypeptide N-Acetylgalactosaminyltransferase 2 (GalNAc-transferase T2) Co-regulates β1-Adrenergic Receptor N-terminal Cleavage

The β1-adrenergic receptor (β1AR) is a G protein-coupled receptor (GPCR) and the predominant adrenergic receptor subtype in the heart, where it mediates cardiac contractility and the force of contraction. Although it is the most important target for β-adrenergic antagonists, such as β-blockers, relatively little is yet known about its regulation. We have shown previously that β1AR undergoes constitutive and regulated N-terminal cleavage participating in receptor down-regulation and, moreover, that the receptor is modified by O-glycosylation. Here we demonstrate that the polypeptide GalNAc-transferase 2 (GalNAc-T2) specifically O-glycosylates β1AR at five residues in the extracellular N terminus, including the Ser-49 residue at the location of the common S49G single-nucleotide polymorphism. Using in vitro O-glycosylation and proteolytic cleavage assays, a cell line deficient in O-glycosylation, GalNAc-T-edited cell line model systems, and a GalNAc-T2 knock-out rat model, we show that GalNAc-T2 co-regulates the metalloproteinase-mediated limited proteolysis of β1AR. Furthermore, we demonstrate that impaired O-glycosylation and enhanced proteolysis lead to attenuated receptor signaling, because the maximal response elicited by the βAR agonist isoproterenol and its potency in a cAMP accumulation assay were decreased in HEK293 cells lacking GalNAc-T2. Our findings reveal, for the first time, a GPCR as a target for co-regulatory functions of site-specific O-glycosylation mediated by a unique GalNAc-T isoform. The results provide a new level of β1AR regulation that may open up possibilities for new therapeutic strategies for cardiovascular diseases.

Evaluating the association of multiple single nucleotide polymorphisms with response to gemcitabine and platinum combination chemotherapy in urothelial carcinoma of the bladder

OBJECTIVE

To examine germline single nucleotide polymorphisms (SNPs) as markers of response to gemcitabine platinum (GP) combination chemotherapy in urothelial carcinoma (UC).

METHODS

Saliva or blood was prospectively collected from 216 patients treated with GP for UC of the bladder between 1991 and 2011. Based on reported associations with gemcitabine and cisplatin response or putative mechanisms of gemcitabine or cisplatin/carboplatin activity, we selected SNPs of interest and were able to genotype 59 SNPs (using the SequenomMass ARRAYiPLEX platform) in 261 patients randomly split 2/3 into a training set (n = 174) and 1/3 into a test set (n = 87). Logistic regression was used to test the association between response to GP and SNPs.

RESULTS

The median age at diagnosis was 64 years (range: 28 - 85) for the discovery set and 67 years (range: 30 - 84) for the validation set. Males composed 76% and 69%, and white non-Hispanics composed 88% and 91% of the training and test validation sets, respectively. Three SNPs on GALNTL4 (rs7937567, rs12278731, and rs9988868) and one intergenic SNP (rs1321391) were significantly associated with response to GP in the training set and were used to build a SNP score. However, when assessed in the test set, the SNP score was not significantly associated with response.

CONCLUSION

Multiple SNPs selected from previous studies failed to predict response to GP in this cohort. Larger studies capable of accounting for population-based allele frequency heterogeneity may be required for replication of genetic alterations important to pharmacogenomics.
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Prognostic role of N-Acetylgalactosaminyltransferase 10 in metastatic renal cell carcinoma

BACKGROUND AND PURPOSE

A previous study demonstrated that GALNT10 affects the sensitivity of cancer cells to tyrosine kinase inhibitor (TKI) therapy. The aim of this study was to assess whether GALNT10 holds a prognostic role in metastatic renal cell carcinoma (mRCC) patients treated with TKI agents.

RESULTS

GALNT10 had no statistical correlation with any other clinicopathological parameters except for route of gaining samples (P = 0.001) and Heng's risk stratification (P = 0.011). Patients with high level of GALNT10 had significantly shorter overall survival (OS) (P < 0.001) and progression-free survival (PFS) (P = 0.002). Importantly, this relationship existed in OS and PFS analyses in sunitinib-treated patients and in OS analyses in sorafenib-treated patients (P = 0.024). In contrast to sorafenib group, percentage of partial response (PR) and stable disease (SD) were higher in sunitinib group, while percentage of progression disease (PD) was much lower. Univariate and multivariate analyses identified that GALNT10 was an independent prognostic factor for OS (HR = 1.938, P = 0.014), not for PFS (HR = 1.532, P = 0.065), in mRCC. Incorporating it into Heng's risk model could sharpen its efficacy in distinguishing patients with potential higher risk.

MATERIALS AND METHODS

We retrospectively enrolled 138 mRCC patients treated with sunitinib or sorafenib at Zhongshan Hospital, Shanghai, China. A total of 111 valid cases were finally applied for analyses.

CONCLUSIONS

These findings suggest that GALNT10 could be applied as a prognostic marker for OS in mRCC patients.

Crosstalk between ABO and Forssman (FORS) blood group systems: FORS1 antigen synthesis by ABO gene-encoded glycosyltransferases

A and B alleles at the ABO genetic locus specify A and B glycosyltransferases that catalyze the biosynthesis of A and B oligosaccharide antigens, respectively, of blood group ABO system which is important in transfusion and transplantation medicine. GBGT1 gene encodes Forssman glycolipid synthase (FS), another glycosyltransferase that produces Forssman antigen (FORS1). Humans are considered to be Forssman antigen-negative species without functional FS. However, rare individuals exhibiting Apae phenotype carry a dominant active GBGT1 gene and express Forssman antigen on RBCs. Accordingly, FORS system was recognized as the 31st blood group system. Mouse ABO gene encodes a cis-AB transferase capable of producing both A and B antigens. This murine enzyme contains the same GlyGlyAla tripeptide sequence as FSs at the position important for the determination of sugar specificity. We, therefore, transfected the expression construct into appropriate recipient cells and examined whether mouse cis-AB transferase may also exhibit FS activity. The result was positive, confirming the crosstalk between the ABO and FORS systems. Further experiments have revealed that the introduction of this tripeptide sequence to human A transferase conferred some, although weak, FS activity, suggesting that it is also involved in the recognition/binding of acceptor substrates, in addition to donor nucleotide-sugars.

Isoform-specific regulation of osteogenic factors by polypeptide N-Acetylgalactosaminyltransferases 1 and 4

The family of UDP-GalNAc polypeptide: N-Acetylgalactosaminlytransfersases (ppGalNAcTs) catalyzes the initial step of O-linked protein glycosylation. Mucin-type O-glycoproteins are abundant in the bone and may play an important role in osteogenesis. Herein, we examined the effects of ppGalNAc-T isoforms on osteogenesis of MC3T3-E1 pre-osteoblasts. We found that ppGalNAc-T1 and -T4 isoforms were highly expressed during osteogenesis of MC3T3-E1 and their knockdown by short hairpin RNA (shRNA) decreased osteoblast formation and bone mineralization. Knockdown of ppGalNAc-T1 or -T4 decreased mRNA and protein levels of bone sialoprotein (BSP). Knockdown of ppGalNAc-T1decreased mRNA levels of osteocalcin (OC), osteoprotegerin (OPG). Knockdown ofppGalNAc-T4 isoform decreased mRNA levels of OC, OPG and vitamin D receptor (VDR). While knockdown of T1 or T4 isoforms did not change the expression of osteopontin (OPN), COLLI, receptor activator for nuclear factor-κB ligand (RANKL) and transforming growth factor-β (TGF-β). Our results demonstrated that the ppGalNAc-T4 was highly expressed in MC3T3-E1 cells during osteogenesis for the first time. We also found that ppGalNAc-T1 and -T4 affected the expression of different osteogenic factors, suggesting distinct roles ppGalNAc-T isoformsplay in regulating osteogenesis in vitro.

Making Home Sweet and Sturdy: Toxoplasma gondii ppGalNAc-Ts Glycosylate in Hierarchical Order and Confer Cyst Wall Rigidity

The protozoan intracellular parasite Toxoplasma gondii forms latent cysts in the central nervous system (CNS) and persists for the lifetime of the host. This cyst is cloaked with a glycosylated structure called the cyst wall. Previously, we demonstrated that a mucin-like glycoprotein, CST1, localizes to the cyst wall and confers structural rigidity on brain cysts in a mucin-like domain-dependent manner. The mucin-like domain of CST1 is composed of 20 units of threonine-rich tandem repeats that are O-GalNAc glycosylated. A family of enzymes termed polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) initiates O-GalNAc glycosylation. To identify which isoforms of ppGalNAc-Ts are responsible for the glycosylation of the CST1 mucin-like domain and to evaluate the function of each ppGalNAc-T in the overall glycosylation of the cyst wall, all five ppGalNAc-T isoforms were deleted individually from the T. gondii genome. The ppGalNAc-T2 and -T3 deletion mutants produced various glycosylation defects on the cyst wall, implying that many cyst wall glycoproteins are glycosylated by T2 and T3. Both T2 and T3 glycosylate the CST1 mucin-like domain, and this glycosylation is necessary for CST1 to confer structural rigidity on the cyst wall. We established that T2 is required for the initial glycosylation of the mucin-like domain and that T3 is responsible for the sequential glycosylation on neighboring acceptor sites, demonstrating hierarchical glycosylation by two distinct initiating and filling-in ppGalNAc-Ts in an intact organism.

IMPORTANCE

Toxoplasma gondii is an obligate intracellular parasite that infects a third of the world's population. It can cause severe congenital disease and devastating encephalitis in immunocompromised individuals. We identified two glycosyltransferases, ppGalNAc-T2 and -T3, which are responsible for glycosylating cyst wall proteins in a hierarchical fashion. This glycosylation confers structural rigidity on the brain cyst. Our studies provide new insights into the mechanisms of O-GalNAc glycosylation in T. gondii.

Revisiting the human polypeptide GalNAc-T1 and T13 paralogs

Polypeptide GalNAc-transferases (GalNAc-Ts) constitute a family of 20 human glycosyltransferases (comprising 9 subfamilies), which initiate mucin-type O-glycosylation. The O-glycoproteome is thought to be differentially regulated via the different substrate specificities and expression patterns of each GalNAc-T isoforms. Here, we present a comprehensive in vitro analysis of the peptide substrate specificity of GalNAc-T13, showing that it essentially overlaps with the ubiquitous expressed GalNAc-T1 isoform found in the same subfamily as T13. We have also identified and partially characterized nine splice variants of GalNAc-T13, which add further complexity to the GalNAc-T family. Two variants with changes in their lectin domains were characterized by in vitro glycosylation assays, and one (Δ39Ex9) was inactive while the second one (Ex10b) had essentially unaltered activity. We used reverse transcription-polymerase chain reaction analysis of human neuroblastoma cell lines, normal brain and a small panel of neuroblastoma tumors to demonstrate that several splice variants (Ex10b, ΔEx9, ΔEx2-7 and ΔEx6/8-39bpEx9) were highly expressed in tumor cell lines compared with normal brain, although the functional implications remain to be unveiled. In summary, the GalNAc-T13 isoform is predicted to function similarly to GalNAc-T1 against peptide substrates in vivo, in contrast to a prior report, but is unique by being selectively expressed in the brain.

GALNT6 Stabilizes GRP78 Protein by O-glycosylation and Enhances its Activity to Suppress Apoptosis Under Stress Condition

We previously reported that overexpression of an O-type glycosyltransferase, GALNT6 (polypeptide N-acetylgalactosaminyltransferase 6) played critical roles in mammary carcinogenesis. To further investigate the biological function of GALNT6, we screened a substrate protein(s) of GALNT6 using a VVA (Vicia villosa agglutinin) lectin (specific to GalNAc-Ser/Thr) pull-down method followed by mass spectrometry analysis. Here we report GRP78 (glucose-regulated protein 78, also known as HSPA5, heat shock 70 kDa protein 5), which is highly expressed in cancer cells and indicated to play important roles in various cellular processes including ER (endoplasmic reticulum) stress and autophagy, as a novel substrate of GALNT6. We found that GALNT6-induced O-glycosylation is critical for the stability of GRP78, its subcellular localization in ER, and its anti-apoptotic function. Furthermore, we demonstrated that overexpression of GRP78 could be important for Golgi-to-ER relocation of GALNT6. Collectively, our study revealed biological significances of O-glycosylation of GRP78 protein, which might play significant roles in the survival of cancer cells, and thus provided a new insight in cancer cell death and useful information for development of anti-cancer treatment targeting the GALNT6-GRP78 pathway.

GALNT14 promotes lung-specific breast cancer metastasis by modulating self-renewal and interaction with the lung microenvironment

Some polypeptide N-acetyl-galactosaminyltransferases (GALNTs) are associated with cancer, but their function in organ-specific metastasis remains unclear. Here, we report that GALNT14 promotes breast cancer metastasis to the lung by enhancing the initiation of metastatic colonies as well as their subsequent growth into overt metastases. Our results suggest that GALNT14 augments the self-renewal properties of breast cancer cells (BCCs). Furthermore, GALNT14 overcomes the inhibitory effect of lung-derived bone morphogenetic proteins (BMPs) on self-renewal and therefore facilitates metastasis initiation within the lung microenvironment. In addition, GALNT14 supports continuous growth of BCCs in the lung by not only inducing macrophage infiltration but also exploiting macrophage-derived fibroblast growth factors (FGFs). Finally, we identify KRAS-PI3K-c-JUN signalling as an upstream pathway that accounts for the elevated expression of GALNT14 in lung-metastatic BCCs. Collectively, our findings uncover an unprecedented role for GALNT14 in the pulmonary metastasis of breast cancer and elucidate the underlying molecular mechanisms.

Extrinsic Functions of Lectin Domains in O-N-Acetylgalactosamine Glycan Biosynthesis

Glycan biosynthesis occurs mainly in Golgi. Molecular organization and functional regulation of this process are not well understood. We evaluated the extrinsic effect of lectin domains (β-trefoil fold) of polypeptide GalNAc-transferases (ppGalNAc-Ts) on catalytic activity of glycosyltransferases during O-GalNAc glycan biosynthesis. The presence of lectin domain T3lec or T4lec during ppGalNAc-T2 and ppGalNAc-T3 catalytic reaction had a clear inhibitory effect on GalNAc-T activity. Interaction of T3lec or T4lec with ppGalNAc-T2 catalytic domain was not mediated by carbohydrate. T3lec, but not T2lec and T4lec, had a clear activating effect on Drosophila melanogaster core 1 galactosyltransferase enzyme activity and a predominant inhibitory effect on in vivo human core 1 glycan biosynthesis. The regulatory role of the β-trefoil fold of ppGalNAc-Ts in enzymatic activity of glycosyltransferases involved in the O-glycan biosynthesis pathway, described here for the first time, helps clarify the mechanism of biosynthesis of complex biopolymers (such as glycans) that is not template-driven.

miR-214 inhibits invasion and migration via downregulating GALNT7 in esophageal squamous cell cancer

Previous studies verified that miR-214 is of great significance in the invasion and migration of a variety of cancers. It has been demonstrated that UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 7(GALNT7) is a putative target of miR-214. We performed this study to figure out how miR-214 and GALNT7 play their roles in the invasion and migration of esophageal squamous cell carcinoma (ESCC). The expression of miR-214 was significantly downregulated in tumors compared to the corresponding non-tumor tissues while GALNT7 showed an opposite tendency. The low expression of miR-214 and the high expression of GALNT7 were found positively correlated with poor tumor differentiation (P = 0.004), tumor invasion (P = 0.013), and lymph node metastasis (P = 0.012) in ESCC patients. Functional study demonstrated that overexpression of miR-214 or knockdown of GALNT7 could weaken invasive and migratory ability in Eca109, TE1, and KYSE150. Moreover, tumorigenicity assay showed us mice injected with cells containing miR-214 mimic or GALNT7 small interfering RNA formed substantially smaller tumors than that in miR-214 inhibitor group. Consequently, we concluded that miR-214 shows potential to be a diagnostic marker and therapeutic target in ESCC.

Pro-survival effects by NF-κB, Akt and ERK(1/2) and anti-apoptosis actions by Six1 disrupt apoptotic functions of TRAIL-Dr4/5 pathway in ovarian cancer

Apoptotic signaling provoked by death receptors, DR4 and DR5, are generally considered to promote cell death and chemosensitivity in multiple cancers, but this view is being thrown into doubt with recent findings that up-regulated DR4 and DR5 in advanced stages of ovarian cancer are associated with the poor prognosis. For this conflict, two reasonable explanations have been proposed: one is that DR4 and DR5 not exclusively mediate apoptotic pathway, but also favor survival signal; another is that apoptotic signals by DR4 and DR5 are disrupted by some regulators. This study identified these two speculations in TRAIL-resistant (SKOV-3ip1 and A2780) or sensitive (OVCAR-3) ovarian cancer cells. Activation of DR4 and DR5 using their specific ligand, TRAIL, activated pro-survival factors including NF-κB, Akt and ERK(1/2) in ovarian cancer SKOV-3ip1 and A2780 cells. Pharmacological inhibition of their activities potentiated TRAIL cytotoxicity, reducing cell viability and increasing apoptosis. Six1, a homeobox transcription factor, had higher expression in SKOV-3ip1 and A2780 cells than in OVCAR-3 cells. Silencing Six1 raised levels of apoptotic factors including cleaved Bid, caspase-8 and caspase-3, and overrode the TRAIL-resistance. Co-treatment with Six1 knockdown and peptidyl O-glycosyltransferase 14 overexpression showed additive effects on apoptosis signal, leading to increased apoptosis in SKOV-3ip1 and A2780 cells. This study demonstrated that pro-survival effects by NF-κB, Akt and ERK(1/2) and anti-apoptosis actions by Six1 disrupt apoptotic functions of TRAIL-Dr4/5 pathway in ovarian cancer, which may explain why up-regulated DR4 and DR5 in ovarian cancer are associated with poor prognosis and low survival ratio of the patients.

Loss of Function of GALNT2 Lowers High-Density Lipoproteins in Humans, Nonhuman Primates, and Rodents

Human genetics studies have implicated GALNT2, encoding GalNAc-T2, as a regulator of high-density lipoprotein cholesterol (HDL-C) metabolism, but the mechanisms relating GALNT2 to HDL-C remain unclear. We investigated the impact of homozygous GALNT2 deficiency on HDL-C in humans and mammalian models. We identified two humans homozygous for loss-of-function mutations in GALNT2 who demonstrated low HDL-C. We also found that GALNT2 loss of function in mice, rats, and nonhuman primates decreased HDL-C. O-glycoproteomics studies of a human GALNT2-deficient subject validated ANGPTL3 and ApoC-III as GalNAc-T2 targets. Additional glycoproteomics in rodents identified targets influencing HDL-C, including phospholipid transfer protein (PLTP). GALNT2 deficiency reduced plasma PLTP activity in humans and rodents, and in mice this was rescued by reconstitution of hepatic Galnt2. We also found that GALNT2 GWAS SNPs associated with reduced HDL-C also correlate with lower hepatic GALNT2 expression. These results posit GALNT2 as a direct modulator of HDL metabolism across mammals.

Loss of N-acetylgalactosaminyltransferase 3 in poorly differentiated pancreatic cancer: augmented aggressiveness and aberrant ErbB family glycosylation

BACKGROUND

Aberrant glycosylation of several proteins underlie pancreatic ductal adenocarcinoma (PDAC) progression and metastasis. O-glycosylation is initiated by a family of enzymes known as polypeptide N-acetylgalactosaminyl transferases (GalNAc-Ts/GALNTs). In this study, we investigated the role of the O-glycosyltransferase GALNT3 in PDAC.

METHODS

Immunohistochemistry staining of GALNT3 was performed on normal, inflammatory and neoplastic pancreatic tissues. Several in vitro functional assays such as proliferation, colony formation, migration and tumour-endothelium adhesion assay were conducted in GALNT3 knockdown PDAC cells to investigate its role in disease aggressiveness. Expression of signalling molecules involved in growth and motility was evaluated using western blotting. Effect of GALNT3 knockdown on glycosylation was examined by lectin pull-down assay.

RESULTS

N-acetylgalactosaminyl transferase 3 expression is significantly decreased in poorly differentiated PDAC cells and tissues as compared with well/moderately differentiated PDAC. Further, knockdown of GALNT3 resulted in increased expression of poorly differentiated PDAC markers, augmented growth, motility and tumour-endothelium adhesion. Pull-down assay revealed that O-glycans (Tn and T) on EGFR and Her2 were altered in PDAC cells, which was accompanied by their increased phosphorylation.

CONCLUSIONS

Our study indicates that loss of GALNT3 occurs in poorly differentiated PDAC, which is associated with the increased aggressiveness and altered glycosylation of ErbB family proteins.

Glycomimetics Targeting Glycosyltransferases: Synthetic, Computational and Structural Studies of Less-Polar Conjugates

The Leloir donors are nucleotide sugars essential for a variety of glycosyltransferases (GTs) involved in the transfer of a carbohydrate to an acceptor substrate, typically a protein or an oligosaccharide. A series of less-polar nucleotide sugar analogues derived from uridine have been prepared by replacing one phosphate unit with an alkyl chain. The methodology is based on the radical hydrophosphonylation of alkenes, which allows coupling of allyl glycosyl compounds with a phosphate unit suitable for conjugation to uridine. Two of these compounds, the GalNAc and galactose derivatives, were further tested on a model GT, such as GalNAc-T2 (an important GT widely distributed in human tissues), to probe that both compounds bound in the medium-high micromolar range. The crystal structure of GalNAc-T2 with the galactose derivative traps the enzyme in an inactive form; this suggests that compounds only containing the β-phosphate could be efficient ligands for the enzyme. Computational studies with GalNAc-T2 corroborate these findings and provide further insights into the mechanism of the catalytic cycle of this family of enzymes.

Role of N-acetylgalactosaminyltransferase 6 in early tumorigenesis and formation of metastasis

Glycosylation is one of the most important posttranslational modifications of proteins and lipids that contributes to the structural diversity of cellular molecules. Enzymes of the glycosyltransferase class are responsible for altering glycosylation patterns by adding carbohydrate chains to the respective acceptor molecules. It is well known that glycosylation is commonly altered in cancerous tissue. Therefore, the present study aimed to determine the incidence of N‑acetylgalactosaminyltransferase 6 (GALNT6), a prominent member of the glycosyltransferase class, in breast cancer tissue of different developmental stages by immunohistochemistry. Although no correlation was identified between tumour characteristics and GALNT6 staining intensity, to the best of our knowledge, this is the first study to demonstrate that tissue from carcinoma in situ‑tumours and metastases were more heavily stained than late‑stage breast cancers. This may indicate an important role of glycosylation aberration in escaping the immune system at early phases of tumour development. The present study also hypothesised that nascent or early metastasizing tumours are normally recognized by the immune system of the patient, but glycosylation pattern changes may facilitate tumor escape from immune recognition. In follow‑up studies, our group will aim to confirm and consolidate these results in a larger patient cohort that may give greater insight into breast cancer characterization as well as tumour treatment.

Development and Validation of a Universal High-Throughput UDP-Glycosyltransferase Assay with a Time-Resolved FRET Signal

Glycosyltransferase enzymes play diverse metabolic and regulatory roles by catalyzing the transfer of sugar molecules to protein, lipid, and carbohydrate acceptors, and they are increasingly of interest as therapeutic targets in a number of diseases, including metabolic disorders, cancer, and infectious diseases. The glycosyltransferases are a challenging target class from an assay development perspective because of the diversity of both donor and acceptor substrates and the lack of suitable glycan detection methods. However, many glycosyltransferases use uridine 5'-diphosphate (UDP) sugars as donor substrates, and detection of the free UDP reaction product provides a generic approach for measuring the activity of those enzymes. To exploit this approach for a broadly applicable high-throughput screening (HTS) assay for discovery of glycosyltransferase inhibitors, we developed a Transcreener(®) assay for immunodetection of UDP with a time-resolved Förster resonance energy transfer (TR-FRET) signal. We optimized the assay for detection of glycosyltransferase activity with nucleotide diphosphate (NDP) sugars at concentrations from 10 μM to 1 mM, achieving Z' values of 0.6 or higher. The assay was validated by orthogonal pooled screening with 8,000 compounds using polypeptide N-acetylgalactosaminyltransferase T3 as the target, and the hits were confirmed using an orthogonal readout. The reagents and signal were both stable for more than 8 h at room temperature, insuring robust performance in automated HTS environments. The TR-FRET-based UDP detection assay provides a broadly applicable approach for screening glycosyltransferases that use a UDP-sugar donor.

Ganglioside GM2 mediates migration of tumor cells by interacting with integrin and modulating the downstream signaling pathway

The definitive role of ganglioside GM2 in mediating tumor-induced growth and progression is still unknown. Here we report a novel role of ganglioside GM2 in mediating tumor cell migration and uncovered its mechanism. Data shows differential expression levels of GM2-synthase as well as GM2 in different human cancer cells. siRNA mediated knockdown of GM2-synthase in CCF52, A549 and SK-RC-26B cells resulted in significant inhibition of tumor cell migration as well as invasion in vitro without affecting cellular proliferation. Over-expression of GM2-synthase in low-GM2 expressing SK-RC-45 cells resulted in a consequent increase in migration thus confirming the potential role GM2 and its downstream partners play in tumor cell migration and motility. Further, treatment of SK-RC-45 cells with exogenous GM2 resulted in a dramatic increase in migratory and invasive capacity with no change in proliferative capacity, thereby confirming the role of GM2 in tumorigenesis specifically by mediating tumor migration and invasion. Gene expression profiling of GM2-synthase silenced cells revealed altered expression of several genes involved in cell migration primarily those controlling the integrin mediated signaling. GM2-synthase knockdown resulted in decreased phosphorylation of FAK, Src as well as Erk, while over-expression and/or exogenous GM2 treatment caused increased FAK and Erk phosphorylation respectively. Again, GM2 mediated invasion and Erk phosphorylation is blocked in integrin knockdown SK-RC-45 cells, thus confirming that GM2 mediated migration and phosphorylation of Erk is integrin dependent. Finally, confocal microscopy suggested co-localization while co-immunoprecipitation and surface plasmon resonance (SPR) confirmed direct interaction of membrane bound ganglioside, GM2 with the integrin receptor.

GALNT7, a target of miR-494, participates in the oncogenesis of nasopharyngeal carcinoma

GalNAc-transferase-7 (GALNT7) is essential for the regulation of cell proliferation and has been implicated in tumorigenesis. However, the role of GALNT7 in the development and progression of nasopharyngeal carcinoma (NPC) remains unclear. Our previous study showed that GALNT7 was a putative target of miR-494, which was confirmed by luciferase reporter assay. In the present study, we demonstrated that in vitro knockdown of GALNT7 significantly inhibited the proliferation, colony formation, migration, and invasion of NPC-derived cells. In vivo tumorigenicity assay showed that miR-494 and GALNT7-small interfering RNA (siRNA) reduced tumor growth in nude mice. Taken together, our results provided new evidence for an oncogenic role of GALNT7 in NPC.

Mucin glycosylating enzyme GALNT2 suppresses malignancy in gastric adenocarcinoma by reducing MET phosphorylation

Glycosylation affects malignancy in cancer. Here, we report that N- acetylgalactosaminyltransferase 2 (GALNT2), an enzyme that mediates the initial step of mucin type-O glycosylation, suppresses malignant phenotypes in gastric adenocarcinoma (GCA) by modifying MET (Hepatocyte growth factor receptor) activity. GALNT2 mRNA and protein were downregulated in GCAs, and this reduction was associated with more advanced disease stage and shorter recurrence-free survival. Suppressing GALNT2 expression in GCA cells increased cell growth, migration, and invasion in vitro, and tumor metastasis in vivo. GALNT2 knockdown enhanced phosphorylation of MET and decreased expression of the Tn antigen on MET. Inhibiting MET activity with PHA665752 decreased the malignant phenotypes caused by GALNT2 knockdown in GCA cells. Our results indicate that GALNT2 suppresses the malignant potential of GCA cells and provide novel insights into the significance of O-glycosylation in MET activity and GCA progression.

Posttranslational processing of FGF23 in osteocytes during the osteoblast to osteocyte transition

FGF23 is an O-glycosylated circulating peptide hormone with a critical role in phosphate homeostasis; it is inactivated by cellular proprotein convertases in a pre-release degradative pathway. We have here examined the metabolism of FGF23 in a model bone cell line, IDG-SW3, prior to and following differentiation, as well as in regulated secretory cells. Labeling experiments showed that the majority of (35)S-labeled FGF23 was cleaved to smaller fragments which were constitutively secreted by all cell types. Intact FGF23 was much more efficiently stored in differentiated than in undifferentiated IDG-SW3 cells. The prohormone convertase PC2 has recently been implicated in FGF23 degradation; however, FGF23 was not targeted to forskolin-stimulatable secretory vesicles in a regulated cell line, suggesting that it lacks a targeting signal to PC2-containing compartments. In vitro, PC1/3 and PC2, but not furin, efficiently cleaved glycosylated FGF23; surprisingly, PC5/6 accomplished a small amount of conversion. FGF23 has recently been shown to be phosphorylated by the kinase FAM20C, a process which was shown to reduce FGF23 glycosylation and promote its cleavage; our in vitro data, however, show that phosphorylation does not directly impact cleavage, as both PC5/6 and furin were able to efficiently cleave unglycosylated, phosphorylated FGF23. Using qPCR, we found that the expression of FGF23 and PC5/6, but not PC2 or furin, increased substantially following osteoblast to osteocyte differentiation. Western blotting confirmed the large increase in PC5/6 expression upon differentiation. FGF23 has been linked to a variety of bone disorders ranging from autosomal dominant hypophosphatemic rickets to chronic kidney disease. A better understanding of the biosynthetic pathway of this hormone may lead to new treatments for these diseases.

Parathyroid hormone 1 receptor is essential to induce FGF23 production and maintain systemic mineral ion homeostasis

Parathyroid-hormone-type 1 receptor (PTH1R) is extensively expressed in key regulatory organs for systemic mineral ion homeostasis, including kidney and bone. We investigated the bone-specific functions of PTH1R in modulating mineral ion homeostasis by generating a novel mouse model in which PTH1R is ablated in the limb mesenchyme using Prx1Cre transgenic mice. Such ablation decreased FGF23 protein and serum levels by 50%, despite normal Fgf23 mRNA levels in long bones. Circulating calcium and PTH levels were unchanged, but inorganic phosphate and 1,25(OH)2D3 levels were significantly decreased and accompanied by elevated urinary calcium and phosphate wasting. Key renal genes for balancing mineral ion homeostasis, calbindinD28k, Klotho, and Napi2a were suppressed by 30-40%. Intermittent hPTH(1-34) injections increased Fgf23 mRNA (7.3-fold), Nurr1 mRNA (3.1-fold), and serum intact-FGF23 (1.6-fold) in controls, but failed to induce Fgf23, Nurr1 mRNA, or intact FGF23 production in mutants. Moreover, a significant elevation in serum C-terminal-FGF23 levels (4-fold) was detected in both genotypes. PTH markedly downregulated Galnt3 expression (2.7-fold) in controls but not in mutants. These results demonstrate the pivotal role of PTH1R in long bones to regulate systemic mineral ion homeostasis and the direct induction of FGF23 by PTH1R signaling.

Activity Detection of GalNAc Transferases by Protein-Based Fluorescence Sensors In Vivo

Mucin-type O-glycosylation occurring in the Golgi apparatus is an important protein posttranslational modification initiated by up to 20 GalNAc-transferase isozymes with largely distinct substrate specificities. Regulation of this enzyme family affects a vast array of proteins transiting the secretory pathway and misregulation causes human diseases. Here we describe the use of protein-based fluorescence sensors that traffic in the secretory pathway to monitor GalNAc-transferase activity in living cells. The sensors can either be "pan" or isozyme specific.

Strong expression of polypeptide N-acetylgalactosaminyltransferase 3 independently predicts shortened disease-free survival in patients with early stage oral squamous cell carcinoma

The polypeptide N-acetylgalactosaminyltransferase (GalNAc-Ts) family of enzymes regulates the critical initial steps of mucin-type O-glycosylation. Among GalNAc-Ts that may significantly influence cancer biology, thus affecting cell differentiation, adhesion, invasion, and/or metastasis, GalNAc-T3 exhibits a high expression in several human cancers, closely associated with tumor progression and a poor prognosis. However, the expression pattern of GalNAc-T3 in oral squamous cell carcinoma (OSCC) remains obscure. Since postoperative recurrence of even early stage OSCC (ESOSCC) occurs at an early phase, significantly affecting their clinical course and worse outcome, the identification of clinically significant accurate biomarkers is needed. Therefore, we investigated the correlation between the immunohistochemical GalNAc-T3 expression and various clinicopathological characteristics and recurrence using 110 paraffin-embedded tumor samples obtained from patients with surgically resected ESOSCC (T1-2N0). Recurrence was recognized in 37 of 110 (33.6 %) patients. The GalNAc-T3 expression was considered to be strongly positive when 20 % or more of the cancer cells showed positive cytoplasmic staining. Consequently, a strong expression of GalNAc-T3 was observed in 40 patients (36.4 %), showing a close relationship to poor differentiation, the presence of lymphatic and vascular invasion, and recurrence. Univariate and multivariate analyses further demonstrated that the patients with a strong GalNAc-T3+ status had markedly lower disease-free survival (DFS) rates, especially within the first 2 years postoperatively. Therefore, GalNAc-T3 might play a role in the pathogenesis of ESOSCC recurrence, and its immunohistochemical detection potentially predicts a shorter DFS and may be a useful parameter for providing clinical management against ESOSCC in the early postoperative phase.

Multiple Hepatic Regulatory Variants at the GALNT2 GWAS Locus Associated with High-Density Lipoprotein Cholesterol

Genome-wide association studies (GWASs) have identified more than 150 loci associated with blood lipid and cholesterol levels; however, the functional and molecular mechanisms for many associations are unknown. We examined the functional regulatory effects of candidate variants at the GALNT2 locus associated with high-density lipoprotein cholesterol (HDL-C). Fine-mapping and conditional analyses in the METSIM study identified a single locus harboring 25 noncoding variants (r(2) > 0.7 with the lead GWAS variants) strongly associated with total cholesterol in medium-sized HDL (e.g., rs17315646, p = 3.5 × 10(-12)). We used luciferase reporter assays in HepG2 cells to test all 25 variants for allelic differences in regulatory enhancer activity. rs2281721 showed allelic differences in transcriptional activity (75-fold [T] versus 27-fold [C] more than the empty-vector control), as did a separate 780-bp segment containing rs4846913, rs2144300, and rs6143660 (49-fold [AT(-) haplotype] versus 16-fold [CC(+) haplotype] more). Using electrophoretic mobility shift assays, we observed differential CEBPB binding to rs4846913, and we confirmed this binding in a native chromatin context by performing chromatin-immunoprecipitation (ChIP) assays in HepG2 and Huh-7 cell lines of differing genotypes. Additionally, sequence reads in HepG2 DNase-I-hypersensitivity and CEBPB ChIP-seq signals spanning rs4846913 showed significant allelic imbalance. Allelic-expression-imbalance assays performed with RNA from primary human hepatocyte samples and expression-quantitative-trait-locus (eQTL) data in human subcutaneous adipose tissue samples confirmed that alleles associated with increased HDL-C are associated with a modest increase in GALNT2 expression. Together, these data suggest that at least rs4846913 and rs2281721 play key roles in influencing GALNT2 expression at this HDL-C locus.