GalNAc-alpha-O-benzyl inhibits NeuAcalpha2-3 glycosylation and blocks the intracellular transport of apical glycoproteins and mucus in differentiated HT-29 cells

Impaired digestive function of sucrase-isomaltase in a complex with the Greenlandic sucrase-isomaltase variant

Sucrase isomaltase (SI) is the most prominent disaccharidase in the small intestine. Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disorder caused by variants in the SI gene. A homozygous frameshift mutation, c.273_274delAG (p.Gly92Leufs*8), has been identified in CSID in the Greenlandic population. This variant eliminates the luminal domain of SI and results in loss of its digestive function. Surprisingly, the truncated mutant is transport-competent and localized at the cell surface; it interacts avidly with wild type SI and negatively impacts its enzymatic function. The data propose that heterozygote carriers of p.Gly92Leufs*8 may also present with CSID symptoms.

Identification of potential classes of glycoligands mediating dynamic endothelial adhesion of human tumor cells

One critical step of metastasis formation is the extravasation of circulating tumor cells from the bloodstream. This process requires the dynamic interaction of cell adhesion molecules like E-selectin on endothelial cells with carbohydrate ligands on tumor cells. To characterize these glycans in a comprehensible approach, the rolling, tethering, and firm adhesion of nine human tumor cell lines on human umbilical vein endothelial cells was analyzed using laminar flow adhesion assays. The tumor cell lines were grouped into three subsets by their canonical E-selectin ligand status (sialyl-Lewis A and X +/+, -/+, -/-) and their adhesiveness was compared after enzymatic, pharmacologic, chemical treatment or antibody blockade of the tumor cells or endothelial cells, respectively. Tumor cells were also screened regarding their glycosyltransferase expression profile. We found that although E-selectin and terminal α2,3-sialic acid largely determined firm adhesion, adhesive events did not exclusively depend on the presence of sialyl-Lewis A and/or sialyl-Lewis X. Nevertheless, two of the three sialyl-Lewis A/X-/- tumor cells additionally or fully depended on vascular cell adhesion molecule-1 for firm adhesion. The significance of O-GalNAc- and N-glycans for adhesion varied remarkably among the tumor cells. The sialyl-Lewis A/X+/+ subset showed glycoprotein-independent adhesion, suggesting a role of glycolipids as well. All sialyl-Lewis A/X-/- tumor cells lacked FUT3 and FUT7 expression as opposed to sialyl-Lewis A/X+/+ or -/+ cell lines. In summary, the glycans on tumor cells mediating endothelial adhesion are not as much restricted to sialyl-Lewis A /X as previously assumed. The present study specifically suggests α2,3-linked sialic acid, O-GalNAc glycans, glycosphingolipids, and FUT3/FUT7 products as promising targets for future studies.

Dance of The Golgi: Understanding Golgi Dynamics in Cancer Metastasis

The Golgi apparatus is at the center of protein processing and trafficking in normal cells. Under pathological conditions, such as in cancer, aberrant Golgi dynamics alter the tumor microenvironment and the immune landscape, which enhances the invasive and metastatic potential of cancer cells. Among these changes in the Golgi in cancer include altered Golgi orientation and morphology that contribute to atypical Golgi function in protein trafficking, post-translational modification, and exocytosis. Golgi-associated gene mutations are ubiquitous across most cancers and are responsible for modifying Golgi function to become pro-metastatic. The pharmacological targeting of the Golgi or its associated genes has been difficult in the clinic; thus, studying the Golgi and its role in cancer is critical to developing novel therapeutic agents that limit cancer progression and metastasis. In this review, we aim to discuss how disrupted Golgi function in cancer cells promotes invasion and metastasis.

Activation of the UPR sensor ATF6α is regulated by its redox-dependent dimerization and ER retention by ERp18

Membrane and secretory proteins are synthesized in the endoplasmic reticulum (ER). Perturbations to ER function disrupts protein folding, causing misfolded proteins to accumulate, a condition known as ER stress. Cells adapt to stress by activating the unfolded protein response (UPR), which ultimately restores proteostasis. A key player in the UPR response is ATF6α, which requires release from ER retention and modulation of its redox status during activation. Here, we report that ER stress promotes formation of a specific ATF6α dimer, which is preferentially trafficked to the Golgi for processing. We show that ERp18 regulates ATF6α by mitigating its dimerization and trafficking to the Golgi and identify redox-dependent oligomerization of ATF6α as a key mechanism regulating its function during the UPR.

The unfolded protein response (UPR) maintains cellular proteostasis during stress by activating sensors located to the endoplasmic reticulum (ER) membrane. A major sensor for this response, ATF6α, is activated by release from ER retention and trafficking to the Golgi, where it is cleaved to generate a bZIP transactivator to initiate a transcriptional response. The reduction of a disulfide in monomeric ATF6α is thought to be necessary for release from retention, trafficking, and proteolysis. Here we show that, following ER stress, ATF6α undergoes a redox switch to form a disulfide bonded dimer, which traffics to the Golgi for cleavage by the S1P protease. Additionally, we find that overexpression of ERp18 attenuates dimer formation thereby limiting Golgi trafficking. Our results provide mechanistic insight into activation of the ATF6α pathway, revealing an unexpected role for redox-dependent oligomerization prior to Golgi trafficking.

Golgi quality control and autophagy

Organelles can easily be disrupted by intracellular and extracellular factors. Studies on ER and mitochondria indicate that a wide range of responses are elicited upon organelle disruption. One response thought to be of particular importance is autophagy. Cells can target entire organelles into autophagosomes for removal. This wholesale nature makes autophagy a robust means for eliminating compromised organelles. Recently, it was demonstrated that the Golgi apparatus is a substrate of autophagy. On the other hand, various reports have shown that components traffic away from the Golgi for elimination in an autophagosome-independent manner when the Golgi apparatus is stressed. Future studies will reveal how these different pieces of machinery coordinate to drive Golgi degradation. Quantitative measurements will be needed to determine how much autophagy contributes to the maintenance of the Golgi apparatus.

Golgi stress response: A regulatory mechanism of Golgi function

The organelle of eukaryotes is a finely regulated system. Once disturbed, it activates the specific autoregulatory systems, namely, organelle autoregulation. Among which, the Golgi stress response accounts for one. When the abundance and capacity of the Golgi apparatus are insufficient compared with cellular demand, the Golgi stress response is activated to enhance the function of the Golgi apparatus. Although the molecular mechanism of the Golgi stress response has not been well characterized yet, it seems to be an important part of the mammalian stress response. In this review, we discuss the current status of research on the six pathways of the mammalian Golgi stress response (the TFE3, heat shock protein 47, CREB3, E26 transformation specific, proteoglycan, and mucin pathways), which regulate the general function of the Golgi apparatus, anti-apoptosis, pro-apoptosis, proteoglycan glycosylation, and mucin glycosylation, respectively.

Efficient inhibition of O-glycan biosynthesis using the hexosamine analog Ac5GalNTGc

There is a critical need to develop small-molecule inhibitors of mucin-type O-linked glycosylation. The best-known reagent currently is benzyl-GalNAc, but it is effective only at millimolar concentrations. This article demonstrates that Ac₅GalNTGc, a peracetylated C-2 sulfhydryl-substituted GalNAc, fulfills this unmet need. When added to cultured leukocytes, breast cells, and prostate cells, Ac₅GalNTGc increased cell-surface VVA binding by ∼10-fold, indicating truncation of O-glycan biosynthesis. Cytometry, mass spectrometry, and western blot analysis of HL-60 promyelocytes demonstrated that 50-80 μM Ac₅GalNTGc prevented elaboration of 30%-60% of the O-glycans beyond the Tn-antigen (GalNAcα1-Ser/Thr) stage. The effect of the compound on N-glycans and glycosphingolipids was small. Glycan inhibition induced by Ac₅GalNTGc resulted in 50%-80% reduction in leukocyte sialyl-Lewis X expression and L-/P-selectin-mediated rolling under flow conditions. Ac₅GalNTGc was pharmacologically active in mouse. It reduced neutrophil infiltration to sites of inflammation by ∼60%. Overall, Ac₅GalNTGc may find diverse applications as a potent inhibitor of O-glycosylation.

Systematic analysis of the human tumor cell binding to human vs. murine E- and P-selectin under static vs. dynamic conditions

Endothelial E- and P-selectins promote metastasis formation by interacting with sialyl-Lewis X and A (sLeX/sLeA) on circulating tumor cells. This interaction precedes extravasation and can take place under dynamic and static conditions. Metastasis formation is often studied in xenograft models. However, it is unclear whether species differences exist in the ligand specificity of human (h) vs. murine (m) selectins and whether different ligands are functional under dynamic vs. static conditions. We systematically compared the h vs. m E- and P-selectin (ESel/PSel) binding of a range of human tumor cells under dynamic vs. static conditions. The tumor cells were categorized by their sLeA/X status (sLeA+/sLeX+, sLeA−/sLeX+ and sLeA−/sLeX−). The general biological nature of the tumor–selectin interaction was analyzed by applying several tumor cell treatments (anti-sLeA/X blockade, neuraminidase, pronase and inhibition of O/N-glycosylation). We observed remarkable differences in the static vs. dynamic interaction of tumor cells with h vs. m ESel/PSel depending on their sLeA/X status. The tumor cell treatments mostly affected either static or dynamic as well as either h- or m-selectin interaction. mESel showed a higher diversity of potential ligands than hESel. Inhibition of O-GalNAc-glycosylation also affected glycosphingolipid synthesis. Summarized, different ligands on human tumor cells are functional under static vs. dynamic conditions and for the interaction with human vs. murine ESel/PSel. Non-canonical selectin ligands lacking the sLeA/X glycan epitopes exist on human tumor cells. These findings have important implications for the current development of glycomimetic, antimetastatic drugs and encourage the development of immunodeficient mice with humanized selectins.

N-Glycosylation of the voltage-gated sodium channel β2 subunit is required for efficient trafficking of NaV1.5/β2 to the plasma membrane

The voltage-gated sodium channel is critical for cardiomyocyte function and consists of a protein complex comprising a pore-forming α subunit and two associated β subunits. It has been shown previously that the associated β2 subunits promote cell surface expression of the α subunit. The major α isoform in the adult human heart is Na_V1.5, and germline mutations in the Na_V1.5-encoding gene, sodium voltage-gated channel α subunit 5 (SCN5A), often cause inherited arrhythmias. Here, we investigated the mechanisms that regulate β2 trafficking and how they may determine proper Na_V1.5 cell surface localization. Using heterologous expression in polarized Madin-Darby canine kidney cells, we show that β2 is N-glycosylated in vivo and in vitro at residues 42, 66, and 74, becoming sialylated only at Asn-42. We found that fully nonglycosylated β2 was mostly retained in the endoplasmic reticulum, indicating that N-linked glycosylation is required for efficient β2 trafficking to the apical plasma membrane. The nonglycosylated variant reached the cell surface by bypassing the Golgi compartment at a rate of only approximately one-third of that of WT β2. YFP-tagged, nonglycosylated β2 displayed mobility kinetics in the plane of the membrane similar to that of WT β2. However, it was defective in promoting surface localization of Na_V1.5. Interestingly, β2 with a single intact glycosylation site was as effective as the WT in promoting Na_V1.5 surface localization. In conclusion, our results indicate that N-linked glycosylation of β2 is required for surface localization of Na_V1.5, a property that is often defective in inherited cardiac arrhythmias.

Thioglycosides Are Efficient Metabolic Decoys of Glycosylation that Reduce Selectin Dependent Leukocyte Adhesion

Metabolic decoys are synthetic analogs of naturally occurring biosynthetic acceptors. These compounds divert cellular biosynthetic pathways by acting as artificial substrates that usurp the activity of natural enzymes. While O-linked glycosides are common, they are only partially effective even at millimolar concentrations. In contrast, we report that N-acetylglucosamine (GlcNAc) incorporated into various thioglycosides robustly truncate cell surface N- and O-linked glycan biosynthesis at 10-100 μM concentrations. The >10-fold greater inhibition is in part due to the resistance of thioglycosides to hydrolysis by intracellular hexosaminidases. The thioglycosides reduce β-galactose incorporation into lactosamine chains, cell surface sialyl Lewis-X expression, and leukocyte rolling on selectin substrates including inflamed endothelial cells under fluid shear. Treatment of granulocytes with thioglycosides prior to infusion into mouse inhibited neutrophil homing to sites of acute inflammation and bone marrow by ∼80%-90%. Overall, thioglycosides represent an easy to synthesize class of efficient metabolic inhibitors or decoys. They reduce N-/O-linked glycan biosynthesis and inflammatory leukocyte accumulation.

Conformation Analysis of GalNAc-Appended Sugar Amino Acid Foldamers as Glycopeptide Mimics

Sugar amino acid (SAA)-based foldamers with well-defined secondary structures were appended with N-acetylgalactosamine (GalNAc) sugars to access sequence-defined, multidentate glycoconjugates with full control over number, spacing and position. Conformation analysis of these glycopeptides by extensive NMR spectroscopic studies revealed that the appended GalNAc units had a profound influence on the native conformational behaviour of the SAA foldamers. Whereas the 2,5-cis glycoconjugate showed a helical structure in water, comprising of two consecutive 16-membered hydrogen bonds, its 2,5-trans congener displayed an unprecedented 16/10-mixed turn structure not seen before in any glycopeptide foldamer.

Benzyl-2-Acetamido-2-Deoxy-α-d-Galactopyranoside Increases Human Immunodeficiency Virus Replication and Viral Outgrowth Efficacy In Vitro

Glycosylation of host and viral proteins is an important posttranslational modification needed to ensure correct function of glycoproteins. For this reason, we asked whether inhibition of O-glycosylation during human immunodeficiency virus (HIV) in vitro replication could affect HIV infectivity and replication rates. We used benzyl-2-acetamido-2-deoxy-α-d-galactopyranoside (BAGN), a compound that has been widely used to inhibit O-glycosylation in several cell lines. Pretreatment and culture of PHA-blast target cells with BAGN increased the percentage of HIV-infected cells (7.6-fold, p = 0.0115), the per-cell amount of HIV p24 protein (1.3-fold, p = 0.2475), and the viral particles in culture supernatants (7.1-fold, p = 0.0029) compared to BAGN-free cultures. Initiating infection with virus previously grown in the presence of BAGN further increased percentage of infected cells (30-fold, p < 0.0001), intracellular p24 (1.5-fold, p = 0.0433), and secreted viral particles (74-fold, p < 0.0001). BAGN-treated target cells showed less CD25 and CCR5 expression, but increased HLA-DR surface expression, which positively correlated with the number of infected cells. Importantly, BAGN improved viral outgrowth kinetics in 66% of the samples tested, including samples from HIV controllers and subjects in whom no virus could be expanded in the absence of BAGN. Sequencing of the isolated virus indicated no skewing of viral quasi-species populations when compared to BAGN-free culture conditions. BAGN also increased virus production in the ACH2 latency model when used together with latency-reversing agents. Taken together, our results identify BAGN treatment as a simple strategy to improve viral outgrowth in vitro and may provide novel insights into host restriction mechanisms and O-glycosylation-related therapeutic targets for HIV control strategies.

Sialylation regulates myofibroblast differentiation of human skin fibroblasts

Background

Fibroblasts are key players in maintaining skin homeostasis and in orchestrating physiological tissue repair and skin regeneration. Dysfunctions in fibroblasts that occur with aging and the senescent process lead to the delayed healing observed in elderly people. The molecular mechanisms leading to fibroblast dysfunction during aging and the senescent process have not yet been clarified. Previously, changes in patterns of glycosylation were observed in fibroblasts in aging and the senescent process, but the effect of these changes on the function of fibroblasts has not been well documented. Here, we investigated whether changes in glycosylation during the process to senescence may have functional effects on fibroblasts.

Methods

The changes in cell surface glycans on skin fibroblasts during the process to senescence were examined in early-passage (EP) and late-passage (LP) skin fibroblasts by fluorescence-activated cell sorting analysis using lectins. The contributors to the changes in cell surface glycans were examined by real-time polymerase chain reaction or Western blot analysis. The effects of changes in glycosylation on proliferation, migration, induction of cellular senescence, and myofibroblast differentiation induced by transforming growth factor (TGF)-β1 stimulation were examined in EP fibroblasts. The changes in glycosylation were performed by GalNAc-α-O-benzyl or sialidase treatment.

Results

A decrease in sialylation of glycoproteins and an increase in sialidase NEU1 were observed in LP fibroblasts. The reduction of sialylation did not have any effect on proliferation, migration, or induction of cellular senescence. On the other hand, myofibroblast differentiation was inhibited by the reduction of sialylation, indicating that sialylation is important for myofibroblast differentiation. The localization of CD44 in lipid rafts, which is required for myofibroblast differentiation, was inhibited by the reduction of sialylation. Furthermore, reduced myofibroblast differentiation in LP fibroblasts was restored by a sialidase inhibitor.

Conclusions

Desialylation of CD44 with increased sialidase during the process to senescence reduced the localization of CD44 in lipid rafts after TGF-β1 stimulation, leading to the inhibition of myofibroblast differentiation. Thus, regulation of sialylation may be an attractive strategy for the prevention and regenerative therapy of age-related skin diseases, cosmetic skin alterations, and chronic wounds caused by delayed healing in elderly people.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0534-1) contains supplementary material, which is available to authorized users.

Simultaneous analyses of N-linked and O-linked glycans of ovarian cancer cells using solid-phase chemoenzymatic method

BACKGROUND

Glycans play critical roles in a number of biological activities. Two common types of glycans, N-linked and O-linked, have been extensively analyzed in the last decades. N-glycans are typically released from glycoproteins by enzymes, while O-glycans are released from glycoproteins by chemical methods. It is important to identify and quantify both N- and O-linked glycans of glycoproteins to determine the changes of glycans.

METHODS

The effort has been dedicated to study glycans from ovarian cancer cells treated with O-linked glycosylation inhibitor qualitatively and quantitatively. We used a solid-phase chemoenzymatic approach to systematically identify and quantify N-glycans and O-glycans in the ovarian cancer cells. It consists of three steps: (1) immobilization of proteins from cells and derivatization of glycans to protect sialic acids; (2) release of N-glycans by PNGase F and quantification of N-glycans by isobaric tags; (3) release and quantification of O-glycans by β-elimination in the presence of 1-phenyl-3-methyl-5-pyrazolone (PMP).

RESULTS

We used ovarian cancer cell lines to study effect of O-linked glycosylation inhibitor on protein glycosylation. Results suggested that the inhibition of O-linked glycosylation reduced the levels of O-glycans. Interestingly, it appeared to increase N-glycan level in a lower dose of the O-linked glycosylation inhibitor. The sequential release and analyses of N-linked and O-linked glycans using chemoenzymatic approach are a platform for studying N-glycans and O-glycans in complex biological samples.

CONCLUSION

The solid-phase chemoenzymatic method was used to analyze both N-linked and O-linked glycans sequentially released from the ovarian cancer cells. The biological studies on O-linked glycosylation inhibition indicate the effects of O-glycosylation inhibition to glycan changes in both O-linked and N-linked glycan expression.

Highly Substituted Cyclopentane-CMP Conjugates as Potent Sialyltransferase Inhibitors

Sialylconjugates on cell surfaces are involved in many biological events such as cellular recognition, signal transduction, and immune response. It has been reported that aberrant sialylation at the nonreducing end of glycoconjugates and overexpression of sialyltransferases (STs) in cells are correlated with the malignance, invasion, and metastasis of tumors. Therefore, inhibitors of STs would provide valuable leads for the discovery of antitumor drugs. On the basis of the transition state of the enzyme-catalyzed sialylation reaction, we proposed that the cyclopentane skeleton in its two puckered conformations might mimic the planar structure of the donor (CMP-Neu5Ac) in the transition state. A series of cyclopentane-containing compounds were designed and synthesized by coupling different cyclopentane α-hydroxyphosphonates with cytidine phosphoramidite. Their inhibitory activities against recombinant human ST6Gal-I were assayed, and a potent inhibitor 48l with a Ki of 0.028 ± 0.006 μM was identified. The results show that the cyclopentanoid-type compounds could become a new type of sialyltransferase inhibitors as biological probes or drug leads.

Core 2 Mucin-Type O-Glycan Is Related to EPEC and EHEC O157:H7 Adherence to Human Colon Carcinoma HT-29 Epithelial Cells

BACKGROUND AND AIM

The roles of host glycosylation in interactions with EPEC and EHEC O157:H7 are largely unclear; this study examined whether O-glycans are involved in EPEC and EHEC O157:H7 adherence to HT-29 cells.

METHODS

Bacterial adherence to the cultured cells was determined using the direct co-staining of adherent bacteria and host cells, the adherent bacteria plating, and/or the direct fluorescent observation of the adherent GFP-labeled bacteria.

RESULTS

A comparison of the adherence of EPEC and EHEC O157:H7 to HT-29-Gal and HT-29 cells indicated that the differentiation of HT-29 cells led to a reduction in the adherence of EPEC and EHEC O157:H7. EPEC and EHEC O157:H7 adhesion decreased after the abrogation of O-glycan biosynthesis mediated by benzyl-α-GalNAc treatment. Core 2 O-glycan-deficient HT-29 cells induced by C2GnT2 knockdown had a significant reduction in EPEC and EHEC O157:H7 adhesion in C2GnT2-sh2/HT-29 cells compared with HT-29 and shRNA-Ctr/HT-29 cells. MUC2 expression in benzyl-α-GalNAc-treated HT-29 cells was significantly reduced but unchanged in C2GnT2-deficient HT-29 cells. EPEC or EHEC O157:H7 infection in C2GnT2-deficient HT-29 cells deteriorated the epithelial barrier function. The occludin expression in the shRNA-Ctr/HT-29 and C2GnT2-sh2/HT-29 cells after infection with EPEC or EHEC O157:H7 was pyknic and discontinuous at the cell surface compared with its continuous distribution of control cells. These data indicate that EPEC and EHEC O157:H7 adherence to HT-29 cells is related to mucin-type core 2 O-glycan.

CONCLUSIONS

This study provides the concepts toward the design of carbohydrate-dependent inhibition of EPEC and EHEC O157:H7 adhesion to human intestinal epithelial cells.

Intracellular protein O-GlcNAc modification integrates nutrient status with transcriptional and metabolic regulation

The inducible, nutrient-sensitive posttranslational modification of protein Ser/Thr residues with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs on histones, transcriptional regulators, metabolic enzymes, oncogenes, tumor suppressors, and many critical intermediates of growth factor signaling. Cycling of O-GlcNAc modification on and off of protein substrates is catalyzed by the actions of O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. To date, there are less than 150 publications addressing the role of O-GlcNAc modification in cancer and over half were published in the last 2 years. These studies have clearly established that increased expression of OGT and hyper-O-GlcNAcylation is common to human cancers of breast, prostate, colon, lung, and pancreas. Furthermore, attenuating OGT activity reduces tumor growth in vitro and metastasis in vivo. This chapter discusses the structure and function of the O-GlcNAc cycling enzymes, mechanisms by which protein O-GlcNAc modification sense changes in nutrient status, the influence of O-GlcNAc cycling enzymes on glucose metabolism, and provides an overview of recent observations regarding the role of O-GlcNAcylation in cancer.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

The endoplasmic reticulum-resident chaperone heat shock protein 47 protects the Golgi apparatus from the effects of O-glycosylation inhibition

The Golgi apparatus is important for the transport of secretory cargo. Glycosylation is a major post-translational event. Recognition of O-glycans on proteins is necessary for glycoprotein trafficking. In this study, specific inhibition of O-glycosylation (Golgi stress) induced the expression of endoplasmic reticulum (ER)-resident heat shock protein (HSP) 47 in NIH3T3 cells, although cell death was not induced by Golgi stress alone. When HSP47 expression was downregulated by siRNA, inhibition of O-glycosylation caused cell death. Three days after the induction of Golgi stress, the Golgi apparatus was disassembled, many vacuoles appeared near the Golgi apparatus and extended into the cytoplasm, the nuclei had split, and cell death assay-positive cells appeared. Six hours after the induction of Golgi stress, HSP47-knockdown cells exhibited increased cleavage of Golgi-resident caspase-2. Furthermore, activation of mitochondrial caspase-9 and ER-resident unfolded protein response (UPR)-related molecules and efflux of cytochrome c from the mitochondria to the cytoplasm was observed in HSP47-knockdown cells 24 h after the induction of Golgi stress. These findings indicate that (i) the ER-resident chaperon HSP47 protected cells from Golgi stress, and (ii) Golgi stress-induced cell death caused by the inhibition of HSP47 expression resulted from caspase-2 activation in the Golgi apparatus, extending to the ER and mitochondria.

Staining tumor cells with biotinylated ACL-I, a lectin isolated from the marine sponge, Axinella corrugata

Axinella corrugata lectin 1 (ACL-1) was purified from aqueous extracts of the marine sponge, Axinella corrugata. ACL-1 strongly agglutinates native rabbit erythrocytes. The hemagglutination is inhibited by N-acetyl derivatives, particularly N, N', N"-triacetylchitotriose, N-acetyl-D-glucosamine, N-acetyl-D-mannosamine and N-acetyl-D-galactosamine. We investigated the capacity of biotinylated ACL-1 to stain several transformed cell lines including breast (T-47D, MCF7), colon (HT-29), lung (H460), ovary (OVCAR-3) and bladder (T24). ACL-I may bind to both monosaccharides and oligosaccharides of tumor cells, N-acetyl-D-galactosamine, and N-acetyl-D- glucosamine glycan types. The lectins are useful, not only as markers and diagnostic parameters, but also for tissue mapping in suspicious neoplasms. In addition, they provide a better understanding of neoplasms at the cytological and molecular levels. Furthermore, the use of potential metastatic markers such as lectins is crucial for developing successful tools for therapy against cancer. We observed that biotinylated ACL-I stains tumor cells and may hold potential as a probe for identifying transformed cells and for studying glycan structures synthesized by such cells.

Sialylation of N-linked glycans mediates apical delivery of endolyn in MDCK cells via a galectin-9-dependent mechanism

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Apical delivery of endolyn requires recognition of sialic acids on its N-glycans possibly (or likely) mediated by galectin-9.

The sialomucin endolyn is implicated in adhesion, migration, and differentiation of various cell types. Along rat kidney tubules, endolyn is variously localized to the apical surface and endosomal/lysosomal compartments. Apical delivery of newly synthesized rat endolyn predominates over direct lysosomal delivery in polarized Madin–Darby canine kidney cells. Apical sorting depends on terminal processing of a subset of lumenal N-glycans. Here we dissect the requirements of N-glycan processing for apical targeting and investigate the underlying mechanism. Modulation of glycan branching and subsequent polylactosamine elongation by knockdown of N-acetylglucosaminyltransferase III or V had no effect on apical delivery of endolyn. In contrast, combined but not individual knockdown of sialyltransferases ST3Gal-III, ST3Gal-IV, and ST6Gal-I, which together are responsible for addition of α2,3- and α2,6-linked sialic acids on N-glycans, dramatically decreased endolyn surface polarity. Endolyn synthesized in the presence of kifunensine, which blocks terminal N-glycan processing, reduced its interaction with several recombinant canine galectins, and knockdown of galectin-9 (but not galectin-3, -4, or -8) selectively disrupted endolyn polarity. Our data suggest that sialylation enables recognition of endolyn by galectin-9 to mediate efficient apical sorting. They raise the intriguing possibility that changes in glycosyltransferase expression patterns and/or galectin-9 distribution may acutely modulate endolyn trafficking in the kidney.

Core-glycosylated mucin-like repeats from MUC1 are an apical targeting signal

MUC1 is efficiently delivered to the apical surface of polarized Madin-Darby canine kidney (MDCK) cells by transit through apical recycling endosomes, a route associated with delivery of apical proteins with glycan-dependent targeting signals. However, a role for glycans in MUC1 sorting has not been established. A key feature of MUC1 is a heavily O-glycosylated mucin-like domain with a variable number of nearly perfect tandem repeats and adjacent imperfect repeats. Metabolic labeling, cell surface biotinylation, immobilized lectins, and confocal immunofluorescence microscopy were used to characterize the polarized delivery of MUC1 mutants and chimeras in MDCK cells to identify the apical targeting signal. Both the interleukin-2 receptor α subunit (Tac) and a chimera where the Tac ectodomain replaced that of MUC1 were delivered primarily to the basolateral surface. Attachment of the MUC1 mucin-like domain to the N terminus of Tac enhanced apical but not basolateral delivery when compared with Tac. Conversely, deletions within the mucin-like domain in MUC1 reduced apical but not basolateral delivery when compared with MUC1. In pull-down assays with lectins, we found a notable difference in the presence of core 1 O-glycans, but not poly-N-acetyllactosamine, in apically targeted MUC1 and chimeras when compared with Tac. Consistent with these data, we found no effect on MUC1 targeting when galectin-3, with preference for poly-N-acetyllactosamine, was depleted from polarized MDCK cells. However, we did block the apical targeting activity of the mucin-like repeats when we overexpressed CMP-Neu5Ac:GalNAc-Rα2,6-sialyltransferase-1 to block core O-glycan synthesis. The cumulative data indicate that the core-glycosylated mucin-like repeats of MUC1 constitute an apical targeting signal.

Regulation of sialyl Lewis antigen expression in colon cancer cells by sialidase NEU4

Sialyl Lewis antigens, sialyl Lewis a and sialyl Lewis x, are utilized as tumor markers, and their increase in cancer is associated with tumor progression by enhancement of cancer cell adhesion to endothelial E-selectin. However, regulation mechanisms are not fully understood. We previously demonstrated that NEU4 is the only sialidase efficiently acting on mucins and it is down-regulated in colon cancer. To elucidate the significance of NEU4 down-regulation, we investigated sialyl Lewis antigens as endogenous substrates for the sialidase. NEU4 was found to hydrolyze the antigens in vitro and decrease cell surface levels much more effectively than other sialidases. Western blot, thin layer chromatography, and metabolic inhibition studies of desialylation products revealed NEU4 to preferentially catalyze sialyl Lewis antigens expressed on O-glycans. Cell adhesion to and motility and growth on E-selectin were significantly reduced by NEU4. E-selectin stimulation of colon cancer cells enhanced cell motility through activation of the p38/Hsp27/actin reorganization pathway, whereas NEU4 attenuated the signaling. On immunocytochemical analysis, some NEU4 molecules were localized at cell surfaces. Under hypoxia conditions whereby the antigens were increased concomitantly with several sialyl- and fucosyltransferases, NEU4 expression was markedly decreased. These results suggest that NEU4 plays an important role in control of sialyl Lewis antigen expression and its impairment in colon cancer.

Anti-MUC1 antibody inhibits EGF receptor signaling in cancer cells

MUC1 is a type I transmembrane glycoprotein aberrantly overexpressed in various cancer cells. High expression of MUC1 is closely associated with cancer progression and metastasis, leading to poor prognosis. We previously reported that MUC1 is internalized by the binding of the anti-MUC1 antibody, from the cell surface to the intracellular region via the macropinocytotic pathway. Since MUC1 is closely associated with ErbBs, such as EGF receptor (EGFR) in cancer cells, we examined the effect of the anti-MUC1 antibody on EGFR trafficking. Our results show that: (1) anti-MUC1 antibody GP1.4, but not another anti-MUC1 antibody C595, triggered the internalization of EGFR in pancreatic cancer cells; (2) internalization of EGFR by GP1.4 resulted in the inhibition of ERK phosphorylation by EGF stimulation, in a MUC1 dependent manner; (3) inhibition of ERK phosphorylation by GP1.4 resulted in the suppression of proliferation and migration of pancreatic cancer cells. We conclude that the internalization of EGFR by anti-MUC1 antibody GP1.4 inhibits the progression of cancer cells via the inhibition of EGFR signaling.

Steric shielding of surface epitopes and impaired immune recognition induced by the ebola virus glycoprotein

Many viruses alter expression of proteins on the surface of infected cells including molecules important for immune recognition, such as the major histocompatibility complex (MHC) class I and II molecules. Virus-induced downregulation of surface proteins has been observed to occur by a variety of mechanisms including impaired transcription, blocks to synthesis, and increased turnover. Viral infection or transient expression of the Ebola virus (EBOV) glycoprotein (GP) was previously shown to result in loss of staining of various host cell surface proteins including MHC1 and β1 integrin; however, the mechanism responsible for this effect has not been delineated. In the present study we demonstrate that EBOV GP does not decrease surface levels of β1 integrin or MHC1, but rather impedes recognition by steric occlusion of these proteins on the cell surface. Furthermore, steric occlusion also occurs for epitopes on the EBOV glycoprotein itself. The occluded epitopes in host proteins and EBOV GP can be revealed by removal of the surface subunit of GP or by removal of surface N- and O- linked glycans, resulting in increased surface staining by flow cytometry. Importantly, expression of EBOV GP impairs CD8 T-cell recognition of MHC1 on antigen presenting cells. Glycan-mediated steric shielding of host cell surface proteins by EBOV GP represents a novel mechanism for a virus to affect host cell function, thereby escaping immune detection.

The Ebola virus (EBOV) is a highly pathogenic virus that infects humans and non-human primates, causing severe disease or death in the majority of these cases. The interaction of this virus with its host on a cellular level is only just beginning to be understood. EBOV, like many viruses, affects the expression or function of several cell surface proteins, including adhesion factors and protein complexes responsible for allowing the immune system to recognize infected cells. Our group and others have previously shown that expression of the main viral glycoprotein of EBOV in cultured cells is sufficient to cause this disruption. Here we have identified the mechanism by which this disruption occurs. Heavily glycosylated domains of the EBOV glycoprotein form a steric shield over proteins at the cell surface. This steric interference blocks the detection of affected surface proteins using antibody reagents, but also has the functional effect of abrogating cell adhesion and preventing interactions with CD8 T cells. The results from this study highlight a novel mechanism for viral disruption of host cell surface protein functions and give insight to interactions between the Ebola virus and its host.

Dissecting the biological role of mucin-type O-glycosylation using RNA interference in Drosophila cell culture

Mucin type O-glycosylation is a highly conserved form of post-translational modification initiated by the family of enzymes known as the polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAcTs in mammals and PGANTs in Drosophila). To address the cellular functions of the many PGANT family members, RNA interference (RNAi) to each pgant gene was performed in two independent Drosophila cell culture lines. We demonstrate that RNAi to individual pgant genes results in specific reduction in gene expression without affecting the expression of other family members. Cells with reduced expression of individual pgant genes were then examined for changes in viability, morphology, adhesion, and secretion to assess the contribution of each family member to these cellular functions. Here we find that RNAi to pgant3, pgant6, or pgant7 resulted in reduced secretion, further supporting a role for O-glycosylation in proper secretion. Additionally, RNAi to pgant3 or pgant6 resulted in altered Golgi organization, suggesting a role for each in establishing or maintaining proper secretory apparatus structure. Other subcellular effects observed included multinucleated cells seen after RNAi to either pgant2 or pgant35A, suggesting a role for these genes in the completion of cytokinesis. These studies demonstrate the efficient and specific knockdown of pgant gene expression in two Drosophila cell culture systems, resulting in specific morphological and functional effects. Our work provides new information regarding the biological roles of O-glycosylation and illustrates a new platform for interrogating the cellular and subcellular effects of this form of post-translational modification.

An O-glycosyltransferase promotes cell adhesion during development by influencing secretion of an extracellular matrix integrin ligand

Protein secretion and localization are crucial during eukaryotic development, establishing local cell environments as well as mediating cell interactions, signaling, and adhesion. In this study, we demonstrate that the glycosyltransferase, pgant3, specifically modulates integrin-mediated cell adhesion by influencing the secretion and localization of the integrin ligand, Tiggrin. We demonstrate that Tiggrin is normally O-glycosylated and localized to the basal matrix where the dorsal and ventral cell layers adhere in wild type Drosophila wings. In pgant3 mutants, Tiggrin is no longer O-glycosylated and fails to be properly secreted to this basal cell layer interface, resulting in disruption of integrin-mediated cell adhesion in the wing. pgant3-mediated effects are dependent on enzymatic activity, as mutations that form a stable protein yet abrogate O-glycosyltransferase activity result in Tiggrin accumulation within the dorsal and ventral cells comprising the wing. Our results provide the first in vivo evidence for the role of O-glycosylation in the secretion of specific extracellular matrix proteins, thus altering the composition of the cellular "microenvironment" and thereby modulating developmentally regulated cell adhesion events. As alterations in cell adhesion are a hallmark of cancer progression, this work provides insight into the long-standing association between aberrant O-glycosylation and tumorigenesis.

Glycosylation pattern of brush border-associated glycoproteins in enterocyte-like cells: involvement of complex-type N-glycans in apical trafficking

We have previously reported that galectin-4, a tandem repeat-type galectin, regulates the raft-dependent delivery of glycoproteins to the apical brush border membrane of enterocyte-like HT-29 cells. N-Acetyllactos-amine-containing glycans, known as galectin ligands, were found enriched in detergent-resistant membranes. Here, we analyzed the potential contribution of N- and/or O-glycans in this mechanism. Structural studies were carried out on the brush border membrane-enriched fraction using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and nano-ESI-QTOF-MS/MS. The pattern of N-glycans was very heterogeneous, with the presence of high mannose- and hybrid-type glycans as well as a multitude of complex-type glycans. In contrast, the pattern of O-glycans was very simple with the presence of two major core type 1 O-glycans, sialylated and bisialylated T-antigen structures [Neu5Acalpha2-3Galbeta1-3GalNAc-ol and Neu5Acalpha2- 3Galbeta1-3(Neu5Acalpha2-6)GalNAc-ol]. Thus, N-glycans rather than O-glycans contain the N-acetyllactosamine recognition signals for the lipid raft-based galectin-4-dependent apical delivery. In the presence of 1-deoxymannojirimycin, a drug which inhibits the generation of hybrid-type or complex type N-glycans, the extensively O-glycosylated mucin-like MUC1 glycoprotein was not delivered to the apical brush border but accumulated inside the cells. Altogether, our data demonstrate the crucial role of complex N-glycans in the galectin-4-dependent delivery of glycoproteins to the apical brush border membrane of enterocytic HT-29 cells.

MUC1 traverses apical recycling endosomes along the biosynthetic pathway in polarized MDCK cells

MUC1 is a heavily glycosylated transmembrane protein localized at the apical surface of polarized epithelial cells. Here, we examined the biosynthetic route of newly synthesized MUC1 in polarized Madin-Darby canine kidney (MDCK) cells. Apically and basolaterally destined cargo are sorted at the trans-Golgi network into distinct vesicles, and proteins with lipid raft-dependent apical targeting signals and glycan-dependent apical targeting signals appear to specifically transit apical early endosomes (AEEs) and apical recycling endosomes (AREs), respectively. Using metabolic labeling we found that MUC1 is efficiently targeted to the apical surface of polarized MDCK cells with a t(1/2) of 45 min. Apical delivery was not altered by inactivation of AEEs by treatment with hydrogen peroxide and diaminobenzidine treatment after apical loading of endosomes with horseradish peroxidase-conjugated wheat germ agglutinin. However, expression of a GFP-tagged myosin Vb tail fragment (GFP-MyoVbT) that disrupts export from the ARE significantly reduced MUC1 apical expression. Moreover, MUC1 expressed for brief periods in MDCK cells co-localized with GFP-MyoVbT. We conclude that MUC1 traffics to the apical surface via AREs in polarized renal epithelial cells.

Contribution of sialidase NEU1 to suppression of metastasis of human colon cancer cells through desialylation of integrin beta4

We previously found an inverse relationship between sialidase Neu1 expression and metastatic potential of murine cancer cells. To elucidate the mechanism underlying the cellular events, the human sialidase gene NEU1 was overexpressed or silenced in colon cancer HT-29 cells. When NEU1-overexpressing cells were injected transsplenically into mice, in vivo liver metastasis was significantly reduced. NEU1 suppressed cell migration, invasion and adhesion in vitro, whereas the silencing resulted in the opposite. One of the major molecular changes by NEU1 was decreased sialylation of integrin beta4, assessed by PNA- and MAL-II-lectin blotting of immunoprecipitates with anti-integrin beta4 antibody. The desialylation was accompanied by decreased phosphorylation of the integrin followed by attenuation of focal adhesion kinase and Erk1/2 pathway. Moreover, NEU1 caused downregulation of matrix metalloproteinase-7, overexpression of which is associated with cancer metastasis. Treatment of the cells with GalNAc-alpha-O-benzyl, an inhibitor of O-glycosylation, showed increased PNA-positive integrin beta4 with its decreased phosphorylation, indicating that sialic acid removal from the integrin O-glycans results in the decreased phosphorylation. Biotinylation and immunofluorescence staining exhibited some NEU1 molecules to be at the cell surface accessible to the integrin. These results suggest that NEU1 is important in regulation of integrin beta4-mediated signaling, leading to suppression of metastasis.

Cell surface O-glycans limit Staphylococcus aureus adherence to corneal epithelial cells

The mucin-rich environment of the intact corneal epithelium is thought to contribute to the prevention of Staphylococcus aureus infection. This study examined whether O-glycans, which constitute the majority of the mucin mass of epithelial cell glycocalyces, prevented bacterial adhesion and growth. Abrogation of mucin O glycosylation using the chemical primer benzyl-alpha-GalNAc resulted in increased adherence of parental strain RN6390 to apical human corneal-limbal epithelial (HCLE) cells and to biotinylated cell surface protein in static and liquid phase adhesion assays, consistent with a role of mucin O-glycans in preventing bacterial adhesion. Comparable results were found with ALC135, an isogenic mutant strain defective in the accessory gene regulators agr and sar, indicating that the agr- and/or sar-regulated virulence factors did not play a major role in mediating adhesion to the corneal cell surface after mucin O-glycan truncation. In exoglycosidase digestion studies, treatment with sialidase from Arthrobacter ureafaciens--which hydrolyzed mucin-associated O-acetyl sialic acid--but not from Clostridium perfringens resulted in an increase in RN6390 and ALC135 adhesion. Abrogation of mucin O glycosylation in HCLE cell cultures did not affect bacterial growth. Overall, these data indicate that mucin O-glycans contribute to the prevention of bacterial adherence to the apical surface of corneal epithelial cells and suggest that alteration of cell surface glycosylation from disease or trauma, including that stemming from contact lens wear, could contribute to a higher risk of infection.

Recent insights into the biological roles of mucin-type O-glycosylation

In this special issue of the Glycoconjugate Journal focusing on glycosciences and development, we summarize recent advances in our understanding of the role of mucin-type O-glycans in development and disease. The presence of this widespread protein modification has been known for decades, yet identification of its biological functions has been hampered by the redundancy and complexity of the enzyme family controlling the initiation of O-glycosylation, as well as the diversity of extensions of the core sugar. Recent studies in organisms as diverse as mammals and Drosophila have yielded insights into the function of this highly abundant and evolutionarily-conserved protein modification. Gaining an understanding of mucin-type O-glycans in these diverse systems will elucidate crucial conserved processes underlying many aspects of development and homeostasis.

Galectin-1 signaling in leukocytes requires expression of complex-type N-glycans

Dimeric galectin-1 (dGal-1) is a homodimeric lectin with multiple proposed functions. Although dGal-1 binds to diverse glycans, it is unclear whether dGal-1 preferentially binds to specific subsets of glycans on cell surfaces to transmit signals. To explore this question, we selectively inhibited major glycan biosynthetic pathways in human HL60, Molt-4, and Jurkat cells. Inhibition of N-glycan processing blocked surface binding of dGal-1 and prevented dGal-1-induced Ca(2+) mobilization and phosphatidylserine exposure. By contrast, inhibition of O-glycan or glycosphingolipid biosynthesis did not affect dGal-1 binding or dGal-1-induced Ca(2+) mobilization and phosphatidylserine exposure. These results demonstrate that dGal-1 preferentially binds to and signals through glycoproteins containing complex-type N-glycans in at least some leukocyte subsets.

Antiadhesive character of mucin O-glycans at the apical surface of corneal epithelial cells

PURPOSE

Prolonged contact of opposite mucosal surfaces, which occurs on the ocular surface, oral cavity, reproductive tract, and gut, requires a specialized apical cell surface that prevents adhesion. The purpose of this study was to evaluate the contribution of mucin O-glycans to the antiadhesive character of human corneal-limbal epithelial (HCLE) cells.

METHODS

Mucin O-glycan biosynthesis in HCLE cells was disrupted by metabolic interference with benzyl-alpha-GalNAc. The cell surface mucin MUC16 and its carbohydrate epitope H185 were detected by immunofluorescence and Western blot. HCLE cell surface features were assessed by field emission scanning electron microscopy. Cell-cell adhesion assays were performed under static conditions and in a parallel plate laminar flow chamber.

RESULTS

Benzyl-alpha-GalNAc disrupted the biosynthesis of O-glycans without affecting apomucin biosynthesis or cell surface morphology. Static adhesion assays showed that the apical surface of differentiated HCLE cells expressing MUC16 and H185 was more antiadhesive than undifferentiated HCLE cells, which lacked MUC16. Abrogation of mucin O-glycosylation in differentiated cultures with benzyl-alpha-GalNAc resulted in increased adhesion of applied corneal epithelial cells and corneal fibroblasts. The antiadhesive effect of mucin O-glycans was further demonstrated by fluorescence video microscopy in dynamic flow adhesion assays. Cationized ferritin labeling of the cell surface indicated that anionic repulsion did not contribute to the antiadhesive character of the apical surface.

CONCLUSIONS

These results indicate that epithelial O-glycans contribute to the antiadhesive properties of cell surface-associated mucins in corneal epithelial cells and suggest that alterations in mucin O-glycosylation are involved in the pathology of drying mucosal diseases (e.g., dry eye).

TNF trafficking to human mast cell granules: mature chain-dependent endocytosis

Mast cells play a crucial role at the early stages of immune response against bacteria and parasites where their functionality is based on their capability of releasing highly bioactive compounds, among them TNF. Mast cells are considered the only cells storing preformed TNF, which allows for the immediate release of this cytokine upon contact with pathogens. We approached the question of mechanisms and amino acid motifs directing newly synthesized TNF for storage in cytoplasmic granules by analyzing the trafficking of a series of TNF-enhanced GFP fusion proteins in human mast cell lines HMC-1 and LAD2. Protein covering the full TNF sequence was successfully sorted into secretory granules in a process involving transient exposure on the outer membrane and re-endocytosis. In human cells, contrary to results previously obtained in a rodent model, TNF seems not to be glycosylated and, thus, trafficking is carbohydrate independent. In an effort to localize the amino acid motif responsible for granule targeting, we constructed additional fusion proteins and analyzed their trafficking, concluding that granule-targeting sequences are localized in the mature chain of TNF and that the cytoplasmic tail is expendable for endocytotic sorting of this cytokine, thus excluding direct interactions with intracellular adaptor proteins.

Tumor suppressor p16INK4a--modulator of glycomic profile and galectin-1 expression to increase susceptibility to carbohydrate-dependent induction of anoikis in pancreatic carcinoma cells

Expression of the tumor suppressor p16(INK4a) after stable transfection can restore the susceptibility of epithelial tumor cells to anoikis. This property is linked to increases in the expression and cell-surface presence of the fibronectin receptor. Considering its glycan chains as pivotal signals, we assumed an effect of p16(INK4a) on glycosylation. To test this hypothesis for human Capan-1 pancreatic carcinoma cells, we combined microarray for selected glycosyltransferase genes with 2D chromatographic glycan profiling and plant lectin binding. Major differences between p16-positive and control cells were detected. They concerned expression of beta1,4-galactosyltransferases (down-regulation of beta1,4-galactosyltransferases-I/V and up-regulation of beta1,4-galactosyltransferase-IV) as well as decreased alpha2,3-sialylation of O-glycans and alpha2,6-sialylation of N-glycans. The changes are compatible with increased beta(1)-integrin maturation, subunit assembly and binding activity of the alpha(5)beta(1)-integrin. Of further functional relevance in line with our hypothesis, we revealed differential reactivity towards endogenous lectins, especially galectin-1. As a result of reduced sialylation, the cells' capacity to bind galectin-1 was enhanced. In parallel, the level of transcription of the galectin-1 gene increased conspicuously in p16(INK4a)-positive cells, and even figured prominently in a microarray on 1996 tumor-associated genes and in proteomic analysis. The cells therefore gain optimal responsiveness. The correlation between genetically modulated galectin-1 levels and anoikis rates in engineered transfectants inferred functional significance. To connect these findings to the fibronectin receptor, galectin-1 was shown to be co-immunoprecipitated. We conclude that p16(INK4a) orchestrates distinct aspects of glycosylation that are relevant for integrin maturation and reactivity to an endogenous effector as well as the effector's expression. This mechanism establishes a new aspect of p16(INK4a) functionality.

A UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase is required for epithelial tube formation

Epithelial tubes are essential for the proper function of a diverse array of eukaryotic organs. Here we present a novel class of genes required for maintaining epithelial cell shape, polarity, and paracellular barrier function in the Drosophila embryonic tracheal system. Mutations in one member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family (pgant35A) are recessive lethal and result in tracheal tubes that are irregular in diameter and morphology. Further analysis of the pgant35A mutants reveals diminished levels of the apical determinant Crbs and the luminal marker 2A12, concomitant with increased staining in cytoplasmic vesicles within tracheal cells. GalNAc-containing glycoproteins are severely diminished along the apical region of the tracheal system as well. Tracheal cells become irregular in size and shape, and septate junction proteins are mislocalized to a more apical position. Most notably, paracellular barrier function is lost in the tracheal system of the mutants. Overexpression of wild type pgant35A under control of the trachea-specific breathless (btl) promoter results in partial rescue of the lethality. We propose a model where pgant35A is required to establish proper apical composition of tracheal cells by influencing apical delivery of proteins/glycoproteins. Disruption of the normal apical content results in altered cell morphology and loss of paracellular barrier function. These studies demonstrate a previously unrecognized requirement for mucin-type O-glycosylation in epithelial tube integrity and have obvious implications for epithelial morphogenesis in higher eukaryotes, since a unique ortholog to pgant35A exists in mammals.

Pleomorphic extra-renal manifestation of the glomerular podocyte marker podocalyxin in tissues of normal beagle dogs

Podocalyxin (PC) was initially identified as a major sialoprotein on the apical surface of glomerular podocytes to perform the filtration barrier function. Later, it was reported to be expressed in endothelial cells, megakaryotes/platelets, and hemangioblasts, the common progenitor cells of the hematopoietic and endothelial cells. Recently, increasing numbers of reports have indicated that PC is not merely a molecule restricted at renal glomerulus, angiogenic or hematopoietic system. To further elucidate the expression pattern and address the possible physiological role of PC in adult mammals, we conducted an extensive study by immunohistochemistry and immunofluorescence staining on various tissues of healthy adult beagle dogs. By combinatory usage of two different anti-podocalyxin antibodies recognizing distinct epitopes in PC, we have demonstrated that (1) PC is expressed in renal tubules, mesothelium, myocardium, striated muscles in tongue, esophagus and extraocular region, myoepithelial cells in esophagus and salivary glands, neurons, and ependyma, etc.; (2) there are at least three forms of PC proteins, depending upon the accessibility of two different PC antibodies, expressed in different organs/systems; and (3) a particular form of PC is distributed in a vesicle-like compartment in certain organs/systems, such as the central nervous system.

Chondrocyte secreted CRTAC1: a glycosylated extracellular matrix molecule of human articular cartilage

Cartilage acidic protein 1 (CRTAC1), a novel human marker which allowed discrimination of human chondrocytes from osteoblasts and mesenchymal stem cells in culture was so far studied only on the RNA-level. We here describe its genomic organisation and detect a new brain expressed (CRTAC1-B) isoform resulting from alternate last exon usage which is highly conserved in vertebrates. In humans, we identify an exon sharing process with the neighbouring tail-to-tail orientated gene leading to CRTAC1-A. This isoform is produced by cultured human chondrocytes, localized in the extracellular matrix of articular cartilage and its secretion can be stimulated by BMP4. Of five putative O-glycosylation motifs in the last exon of CRTAC1-A, the most C-terminal one is modified according to exposure of serial C-terminal deletion mutants to the O-glycosylation inhibitor Benzyl-alpha-GalNAc. Both isoforms contain four FG-GAP repeat domains and an RGD integrin binding motif, suggesting cell-cell or cell-matrix interaction potential. In summary, CRTAC1 acquired an alternate last exon from the tail-to-tail oriented neighbouring gene in humans resulting in the glycosylated isoform CRTAC1-A which represents a new extracellular matrix molecule of articular cartilage.

Role of N- and O-glycans in polarized biosynthetic sorting

The maintenance of proper epithelial function requires efficient sorting of newly synthesized and recycling proteins to the apical and basolateral surfaces of differentiated cells. Whereas basolateral protein sorting signals are generally confined to their cytoplasmic regions, apical targeting signals have been identified that localize to luminal, transmembrane, and cytoplasmic aspects of proteins. In the past few years, both N- and O-linked glycans have been identified as apical sorting determinants. Glycan structures are extraordinarily diverse and have tremendous information potential. Moreover, because the oligosaccharides added to a given protein can change depending on cell type and developmental stage, the potential exists for altering sorting pathways by modulation of the expression pattern of enzymes involved in glycan synthesis. In this review, we discuss the evidence for glycan-mediated apical sorting along the biosynthetic pathway and present possible mechanisms by which these common and heterogeneous posttranslational modifications might function as specific sorting signals.

The chemistry and biology of mucin-type O-linked glycosylation

Mucin-type O-linked glycosylation is a fundamental post-translational modification that is involved in a variety of important biological processes. However, the lack of chemical tools to study mucin-type O-linked glycosylation has hindered our molecular understanding of O-linked glycans in many biological contexts. The review discusses the significance of mucin-type O-linked glycosylation initiated by the polypeptide N-acetylgalactosaminyltransferases in biology and development of chemical tools to study these enzymes and their substrates.

Galectin-4 and sulfatides in apical membrane trafficking in enterocyte-like cells

We have previously reported that 1-benzyl-2-acetamido-2-deoxy-alpha-D-galactopyranoside (GalNAc alpha-O-bn), an inhibitor of glycosylation, perturbed apical biosynthetic trafficking in polarized HT-29 cells suggesting an involvement of a lectin-based mechanism. Here, we have identified galectin-4 as one of the major components of detergent-resistant membranes (DRMs) isolated from HT-29 5M12 cells. Galectin-4 was also found in post-Golgi carrier vesicles. The functional role of galectin-4 in polarized trafficking in HT-29 5M12 cells was studied by using a retrovirus-mediated RNA interference. In galectin-4-depleted HT-29 5M12 cells apical membrane markers accumulated intracellularly. In contrast, basolateral membrane markers were not affected. Moreover, galectin-4 depletion altered the DRM association characteristics of apical proteins. Sulfatides with long chain-hydroxylated fatty acids, which were also enriched in DRMs, were identified as high-affinity ligands for galectin-4. Together, our data propose that interaction between galectin-4 and sulfatides plays a functional role in the clustering of lipid rafts for apical delivery.

Potent glycan inhibitors of myelin-associated glycoprotein enhance axon outgrowth in vitro

Myelin-associated glycoprotein (MAG, Siglec-4) is one of several endogenous axon regeneration inhibitors that limit recovery from central nervous system injury and disease. Molecules that block such inhibitors may enhance axon regeneration and functional recovery. MAG, a member of the Siglec family of sialic acid-binding lectins, binds to sialoglycoconjugates on axons and particularly to gangliosides GD1a and GT1b, which may mediate some of the inhibitory effects of MAG. In a prior study, we identified potent monovalent sialoside inhibitors of MAG using a novel screening platform. In the current study, the most potent of these were tested for their ability to reverse MAG-mediated inhibition of axon outgrowth from rat cerebellar granule neurons in vitro. Monovalent sialoglycans enhanced axon regeneration in proportion to their MAG binding affinities. The most potent glycoside was disialyl T antigen (NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc-R), followed by 3-sialyl T antigen (NeuAcalpha2-3Galbeta1-3GalNAc-R), structures expressed on O-linked glycoproteins as well as on gangliosides. Prior studies indicated that blocking gangliosides reversed MAG inhibition. In the current study, blocking O-linked glycoprotein sialylation with benzyl-alpha-GalNAc had no effect. The ability to reverse MAG inhibition with monovalent glycosides encourages further exploration of glycans and glycan mimetics as blockers of MAG-mediated axon outgrowth inhibition.

Non-helical type IV collagen polypeptides in human placenta

Our previous reports showed that cultured human cells secrete non-disulfide-bonded non-helical alpha1(IV) and alpha2(IV) chains under physiological conditions. In the present report we show that the alpha(IV) chains in non-helical form were reactive to lectin ABA (Agaricus bisporus agglutinin), whereas the alpha(IV) chains secreted in triple-helical form were not. These results indicate that ABA could be used to distinguish the two conformational isomers of type IV collagen polypeptides. An alpha1(IV) chain isolated from human placenta with an antibody-coupled column showed a positive reaction to ABA, indicating that gelatin form of the type IV collagen alpha1(IV) chain is produced and retained in the tissue in vivo. A possible significance of the gelatin form is discussed from the finding that the non-helical alpha1(IV) chain purified with EDTA-free buffer contained degraded polypeptides including NC1-size domain and showed an apparent inhibition against activated pro-MMP-9. This is the first report to show that a gelatin form of protein exists in vivo.

Glycosylation Broadens the Substrate Profile of Membrane Type 1 Matrix Metalloproteinase

Membrane type 1 matrix metalloproteinase (MT1-MMP) is a collagenolytic enzyme that has been implicated in normal development and in pathological processes such as cancer metastasis. The activity of MT1-MMP is regulated extensively at the post-translational level, and the current data support the hypothesis that MT1-MMP activity is modulated by glycosylation. Enzymatic deglycosylation, site-directed mutagenesis, and lectin precipitation assays were used to demonstrate that MT1-MMP contains O-linked complex carbohydrates on the Thr(291), Thr(299), Thr(300), and/or Ser(301) residues in the proline-rich linker region. MT1-MMP glycoforms were detected in human cancer cell lines, suggesting that MT1-MMP activity may be regulated by differential glycosylation in vivo. Although the autolytic processing and interstitial collagenase activity of MT1-MMP were not impaired in glycosylation-deficient mutants, cell surface MT1-MMP-catalyzed activation of pro-matrix metalloproteinase-2 (proMMP-2) required proper glycosylation of MT1-MMP. The inability of carbohydrate-free MT1-MMP to activate proMMP-2 was not a result of defective MT1-MMP zymogen activation, aberrant protein stability, or inability of the mature enzyme to oligomerize. Rather, our data support a mechanism whereby glycosylation affects the recruitment of tissue inhibitor of metalloproteinases-2 (TIMP-2) to the cell surface, resulting in defective formation of the MT1-MMP/TIMP-2/proMMP-2 trimeric activation complex. These data provide evidence for an additional mechanism for post-translational control of MT1-MMP activity and suggest that glycosylation of MT1-MMP may regulate its substrate targeting.

1-benzyl-2-acetamido-2-deoxy-alpha-D-galactopyranoside blocks the apical biosynthetic pathway in polarized HT-29 cells

In previous work we reported that long term treatment of polarized HT-29 cells by 1-benzyl-2-acetamido-2-deoxy-alpha-d-galactopyranoside (GalNAcalpha-O-bn) induced undersialylation and intracellular distribution of apical glycoproteins such as dipeptidyl peptidase IV (DPP-IV), and we suggested therefore that sialylation could act as an apical targeting signal. In this work, the apical direct biosynthetic route was studied after transfection of polarized enterocyte-like HT-29 5M12 cloned cells with a murine cDNA coding for a soluble form of DPP-IV, which was secreted into the apical medium. A 24-h treatment of transfected cells by GalNAcalpha-O-bn markedly inhibited the apical secretion and the sialylation of this soluble murine DPP-IV, which became blocked inside the cell. A similar short GalNAcalpha-O-bn treatment also induced an intracellular distribution of both endogenous transmembrane DPP-IV and proteins involved in the regulation of the apical trafficking such as the apical t-SNARE syntaxin-3 and the raft-associated protein annexin XIIIb, whereas the basolateral t-SNARE syntaxin-4 kept its normal localization. These apical membrane proteins moved efficiently from trans-Golgi network to apical carrier vesicles but failed to be transported from carrier vesicles to the apical plasma membrane. Isolation of membrane microdomains showed that GalNAcalpha-O-bn induced the formation of abnormal lipid-rich microdomains in comparison to normal rafts, as shown by their lower buoyant density and their depletion in annexin XIIIb. In conclusion, GalNAcalpha-O-bn blocks the anterograde traffic to the apical surface of polarized HT-29 cells at the transport level or docking/fusion level of carrier vesicles.

Aberrant O-glycosylation inhibits stable expression of dysadherin, a carcinoma-associated antigen, and facilitates cell-cell adhesion

Recently, we identified dysadherin, a novel carcinoma-associated glycoprotein, and showed that overexpression of dysadherin in human hepatocarcinoma PLC/PRF/5 cells could suppress E-cadherin-mediated cell-cell adhesion and promote tumor metastasis. The present study shows evidence that dysadherin is actually O-glycosylated. This was based on a direct carbohydrate composition analysis of a chimera protein of an extracellular domain of dysadherin fused to an Fc fragment of immunoglobulin. To assess the importance of O-glycosylation in dysadherin function, dysadherin-transfected hepatocarcinoma cells were cultured in a medium containing benzyl-alpha-GalNAc, a modulator of O-glycosylation. This treatment facilitated homotypic cell adhesion among dysadherin transfectants accompanied with morphological changes, indicating that the anti-adhesive effect of dysadherin was weakened. Modification of O-glycan synthesis also resulted in down-regulation of dysadherin expression and up-regulation of E-cadherin expression in dysadherin transfectants but did not affect E-cadherin expression in mock transfectants. Structural analysis of O-glycans released from the dysadherin chimera proteins indicated that a series of O-glycans with core 1 and 2 structures are attached to dysadherin, and their sialylation is remarkably inhibited by benzyl-alpha-GalNAc treatment. However, sialidase treatment of the cells did not affect calcium-dependent cell aggregation, which excluded the possibility that sialic acid itself is directly involved in cell-cell adhesion. We suggest that aberrant O-glycosylation in carcinoma cells inhibits stable expression of dysadherin and leads to the up-regulation of E-cadherin expression by an unknown mechanism, resulting in increased cell-cell adhesion. The carbohydrate-directed approach to the regulation of dysadherin expression might be a new strategy for cancer therapy.

The Golgi protein RCAS1 controls cell surface expression of tumor-associated O-linked glycan antigens

Tumor immunology has received a large impetus from the identification of tumor-associated antigens. Among them, a monoclonal antibody, 22.1.1, was instrumental in defining a novel tumor-associated antigen that was termed "receptor binding cancer antigen expressed on SiSo cells" (RCAS1). RCAS1 was proposed to induce growth arrest and apoptosis on activated immune cells, mediated by a putative death receptor. Structurally, RCAS1 was predicted to exist as a type II transmembrane protein and in a soluble form. Here, we analyzed occurrence, membrane topology, and subcellular localization of the RCAS1-encoded gene product. RCAS1 was shown to be a ubiquitously expressed type III transmembrane protein with a Golgi-predominant localization. Monoclonal antibody 22.1.1 failed to recognize RCAS1, as demonstrated by confocal microscopy. Instead, we showed that the cognate 22.1.1 epitope is identical with the tumor-associated O-linked glycan Tn (N-acetyl-d-galactosamine, GalNAc). Overexpression of RCAS1 in cell lines that are negative for 22.1.1 surface staining led to the generation of Tn and the closely related TF (Thomsen-Friedenreich, Galbeta1-3GalNAc) antigen, thus providing a functional link to the generation of the 22.1.1 epitope. We suggest that RCAS1 modulates surface expression of tumor-associated, normally cryptic O-linked glycan structures and contributes indirectly to the antigenicity of tumor cells.