Regulation on the expression and N-glycosylation of integrins by N-acetylglucosaminyltransferase V

Studies on primary uveal and cutaneous melanoma cell interaction with vitronectin

We have examined the diversity between primary uveal (92-1 and Mel202) and cutaneous (FM55P and IGR-39) melanoma cells in their interaction with vitronectin, and established the effect of integrins and β1,6-branched N-oligosaccharides on this process. The adhesion level of uveal melanoma cells to vitronectin was at least twice lower than that of cutaneous ones, but all cells tested repaired scratch wounds on vitronectin-coated surfaces with similar speed. Swainsonine treatment, by reducing the amount of β1,6-branches, significantly decreased cell attachment in all cases, but reduction of wound healing efficiency was compromised only in cutaneous melanoma cell. Functional blocking antibodies used in adhesion and migration assays revealed that integrin αvβ3 was strongly involved in adhesion and migration only in cutaneous melanoma cells, but its role here was less pronounced than that of integrin αvβ5. However, in uveal melanoma the specific anti-αvβ5 integrin antibody had no impact on migration speed. Therefore, the anti-α3β1 integrin antibody was used in order to explain the nature of uveal melanoma interaction with vitronectin, which caused a mild decrease in adhesion efficiency and reduced their motility. Expression of αvβ5 integrin differed between the cell lines, but there was no distinct pattern to distinguish uveal melanoma from cutaneous melanoma. In conclusion, αvβ5, but not αvβ3 integrin is heavily involved in uveal melanoma cell interaction with vitronectin. The role of β1,6-branched N-glycans in the adhesion, but not during migration, of all cells to vitronectin has been confirmed.

Upregulation of alpha and beta integrin subunits in metastatic macrophage-melanoma fusion hybrids

Fusion of cancer cells with migratory bone-marrow-derived cells such as macrophages can produce cancer cells with increased metastatic potential. To study this, we fused mouse macrophages with weakly metastatic mouse melanoma cells and generated a panel of hybrid clones. About half of these showed increased metastatic potential in mice. These hybrids expressed traits and molecules that were known indicators of tumor progression in melanoma (chemotaxis toward fibronectin, melanogenesis, autophagy, cMet, MCR1, SPARC, cell surface LAMP-1, GnT-V and β1,6-branched oligosaccharides). Here, we investigated integrin subunit expression in selected hybrids. Integrins, especially those that are substrates for the glycosyltransferase GnT-V and carriers of β1,6-branched oligosaccharides, play an important role in cell migration. We report increased expression of the integrin subunits α3, α5, α6, αv, β1, and β3 in metastatic hybrids compared with parental melanoma cells and a weakly metastatic hybrid. Notably, each of these subunits is also a substrate for GnT-V. Integrin subunit expression was further increased by inducers of cyclic AMP. Expression of these integrin subunits is a characteristic of macrophages and also associated with progression in melanoma and other cancers. In summary, our studies of macrophage-melanoma hybrids show that several α and β integrin subunits are upregulated in the metastatic lines. This adds further support for the theory that generation of a metastatic phenotype may be initiated through a single event: fusion of migratory bone marrow-derived cells with cancer cells.

Golgi bypass: skirting around the heart of classical secretion

Classical secretion consists of the delivery of transmembrane and soluble proteins to the plasma membrane and the extracellular medium, respectively, and is mediated by the organelles of the secretory pathway, the Endoplasmic Reticulum (ER), the ER exit sites, and the Golgi, as described by the Nobel Prize winner George Palade (Palade 1975). At the center of this transport route, the Golgi stack has a major role in modifying, processing, sorting, and dispatching newly synthesized proteins to their final destinations. More recently, however, it has become clear that an increasing number of transmembrane proteins reach the plasma membrane unconventionally, either by exiting the ER in non-COPII vesicles or by bypassing the Golgi. Here, we discuss the evidence for Golgi bypass and the possible physiological benefits of it. Intriguingly, at least during Drosophila development, Golgi bypass seems to be mediated by a Golgi protein, dGRASP, which is found ectopically localized to the plasma membrane.

alpha1,3 fucosyltransferase-VII up-regulates the mRNA of alpha5 integrin and its biological function

After transfection of alpha1,3fucosyltransferase (FucT)-VII cDNA into H7721 human hepatocarcinoma cells, the expression of alpha5, but not beta1 integrin was significantly up-regulated. This was evidenced by the increase of alpha5 integrin on cell surface as well as the increase of alpha5 mRNA and protein in the cells. However, the expressions of sialyl Lewis X (SLe(x), the product of alpha1,3FucT-VII) on both alpha5 and beta1 integrin subunits were unchanged. Concomitantly, the tyrosine autophosphorylated FAK and dephosphorylated Src (FAK and Src involve in the signal transduction of integrin alpha5beta1) were up-regulated, while the Tyr-527 phosphorylated Src was down-regulated. The above-mentioned alterations were correlated to the expressions of alpha1,3FucT-VII in different alpha1,3FucT-VII transfected H7721 cell lines. In addition, after alpha1,3FucT-VII transfection, cell adhesion to fibronectin (Fn) and chemotaxic cell migration were obviously promoted. The cell adhesion could be blocked by alpha5 integrin antibody, and cell migration was obviously attenuated by the antibodies to both alpha5 integrin and SLe(x). These findings suggest that the increased surface alpha5 integrin caused by the up-regulation of alpha5 mRNA promotes the cell adhesion to Fn, cell migratiom, and Fn-induced signaling of alpha5beta1 integrin. The up-regulation of surface SLe(x) originated from the over expression of alpha1,3FucT-VII also led to the stimulation of cell migration. This is the first time to report that alpha1,3FucT-VII can regulate the mRNA expression of integrin.

dGRASP-mediated noncanonical integrin secretion is required for Drosophila epithelial remodeling

Integral plasma membrane proteins are typically transported in the secretory pathway from the endoplasmic reticulum and the Golgi complex. Here we show that at specific stages of Drosophila development corresponding to morphological changes in epithelia, apposed basolateral membranes separate slightly, allowing new plasma membrane contacts with basal extracellular matrix. At these sites, newly synthesized integrin alpha subunits are deposited via a mechanism that appears to bypass the Golgi. We show that the Drosophila Golgi resident protein dGRASP localizes to these membrane domains and that, in the absence of dGRASP, the integrin subunit is retained intracellularly in both follicular and wing epithelia that are found disrupted. We propose that this dGRASP-mediated noncanonical secretion route allows for developmental regulation of integrin function upon epithelial remodeling. We speculate that this mechanism might be used during development as a means of targeting a specific subset of transmembrane proteins to the plasma membrane.

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.

Down-regulation of N-acetylglucosaminyltransferase-V induces ER stress by changing glycosylation and function of GLUT1

N-Acetylglucosaminyltransferase-V (GnT-V) is a key enzyme in the processing of N-glycans during synthesis of glycoproteins. We have reported that down-regulating GnT-V could induce endoplasmic reticulum stress (ER stress) in 7721 cells, a human hepatocarcinoma cell line. In a search for mechanisms of ER stress, we found that there was a prominent decline of glucose uptake in antisense GnT-V transfectant, furthermore, a decrease of tri- or tetra-antannary sugar chain of glucose transporter 1 (GLUT1). However, distribution of GLUT1 in antisense GnT-V transfectant was not affected. Glucose deprivation has been known to activate ER stress in tumor cells. Therefore, the data presented in this study indicate that the glycosylation change and decrease of transport activity of GLUT1 may be one possible mechanism of ER stress induced by down-regulating GnT-V, and GnT-V may contribute to the regulation of glucose uptake by modifying glycosylation of GLUT1 in some tumor cells.

Alpha 1,3-fucosyltransferase-VII regulates the signaling molecules of the insulin receptor pathway

Two H7721 human hepatocarcinoma cell lines showing moderate and high expression of alpha1,3-fucosyltransferase (FucT)-VII cDNA were established and designated FucTVII-M and FucTVII-H, respectively. In alpha1,3-FucT-VII-transfected cells, expression of insulin receptor (InR) alpha- and beta subunits and epidermal growth factor receptor (EGFR) on the cell surface and in cells, as well as the sialyl Lewis X (SLe(x), the product of alpha1,3-FucT-VII) content of the EGFR were unchanged. However the level of SLe(x) on the InR alpha subunit (InR-alpha) was increased dramatically. Tyrosine autophosphorylation of InR-beta , but not EGFR, was elevated. Concomitantly, tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), Ser/Thr phosphorylation of protein kinase B (PKB; Akt), p42/44 mitogen-activated protein kinase (MAPK), MAPK kinase (MEK), and the protein of some other signaling molecules, such as phosphoinositide-dependent kinase-1 (PDK-1), novel protein kinase (PKN), c-Raf-1 and beta-catenin were also upregulated. The activities of PKB and transcription factor TCF were concomitantly stimulated. Upregulation of InR signaling molecules and their phosphorylation was correlated with the level of SLe(x) on InR-alpha and alpha1,3-FucT-VII expression in cells. In addition, the phosphorylation intensity and difference in phosphorylation intensity between cells with different levels of alpha1,3-FucT-VII expression were attenuated significantly by the inhibitor of InR tyrosine kinase and by the mAb to SLe(x). Furthermore, insulin-induced signaling was facilitated in alpha1,3-FucT-VII-transfected cells, particularly FucTVII-H. These findings provide strong evidence that alpha1,3-FucT-VII may affect insulin signaling by upregulating the phosphorylation and expression of some signaling molecules involved in the InR-signaling pathway. These effects are likely mediated by its product, SLe(x), on the glycans of the InR. This is the first study to report that changes in the terminal structure of glycans on a surface receptor can modify cell signaling.

Blocking of N-acetylglucosaminyltransferase V induces cellular endoplasmic reticulum stress in human hepatocarcinoma 7,721 cells

N-acetylglucosaminyltransferase V (GnT-V) is an important tumorigenesis and metastasis-associated enzyme. To study its biofunction, the GnT-V stably suppressed cell line (GnT-V-AS/7,721) was constructed from 7,721 hepatocarcinoma cells in previous study. In this study, cDNA array gene expression profiles were compared between GnT-V-AS/7,721 and parental 7,721 cells. The data indicated that GnT-V-AS/7,721 showed a characteristic expression pattern consistent with the ER stress. The molecular mechanism of the ER stress was explored in GnT-V-AS/7,721 by the analysis on key molecules in both two unfolded protein response (UPR) pathways. For ATF6 and Ire1/XBP-1 pathway, it was evidenced by the up-regulation of BIP at mRNA and protein level, and the appearance of the spliced form of XBP-1. As for PERK/eIF2alpha pathway, the activation of ER eIF2alpha kinase PERK was observed. To confirm the results from GnT-V-AS/7,721 cells, the key molecules in the UPR were examined again in 7,721 cells interfered with the GnT-V by the specific RNAi treatment. The results were similar with those from GnT-V-AS/7721, indicating that blocking of GnT-V can specifically activate ER stress in 7,721 cells. Rate of (3)H-Man incorporation corrected with rate of (3)H-Leu incorporation in GnT-V-AS/7,721 was down-regulated greatly compared with the control, which demonstrated the deficient function of the enzyme synthesizing N-glycans after GnT-V blocking. Moreover, the faster migrating form of chaperone GRP94 associated with the underglycosylation, and the extensively changed N-glycans structures of intracellular glycoproteins were also detected in GnT-V-AS/7,721. These results supported the mechanism that blocking of GnT-V expression impaired functions of chaperones and N-glycan-synthesizing enzymes, which caused UPR in vivo.

Down-regulation of the expression of β1,4-galactosyltransferase V promotes integrin β1 maturation

In previous study, we have shown that beta1,4-galactosyltransferase V (GalT V) functions as a positive growth regulator in glioma. Here, we reported that down-regulation of the expression of GalT V in SHG44 cells by transfection with antisense cDNA specifically up-regulated the expression of cell surface integrin beta1 without the change of its mRNA, and with integrin beta1 125 kDa mature form increased and 105 kDa precursor form decreased. It is well known that the N-glycans of integrins modulate the location and functions of integrins. The SHG44 cells transfected with antisense cDNA of GalT V demonstrated decreased Golgi localization of integrin beta1, strengthened the interaction between integrin alpha5 and beta1 subunit, and enhanced the adhesion ability to fibronectin and the level of focal adhesion kinase phosphorylation. Our results suggested that the down-regulation of the expression of GalT V could promote the expression of cell surface integrin beta1 and subsequently inhibit glioma malignant phenotype.

Does glycosylation of melanoma cells influence their interactions with fibronectin?

Cell surface integrins, especially those binding to fibronectin (FN), participate in processes of tumor cell invasion and metastasis. Changes in glycosylation of cell surface adhesion proteins are often associated with malignant transformation of cells. In this study we examined the influence of swainsonine (SW) on adhesion, wound healing and haptotactic migration on FN, comparing the responses of different human melanoma cell lines: primary WM35 and metastatic WM9, WM239 and A375. We also examined the role of alpha subunits in adhesion to FN. All of the antibodies inhibited adhesion to FN but with different efficiencies depending on the cell line. Adhesion was mediated mainly by integrin alpha(5)beta(1) (WM9, A375), alpha(3)beta(1) (WM35, A375, WM239). Scratch wound repair was significantly faster on FN-coated wells than on plastic for all cells except for WM9. A375 and WM9 had the greatest migration ability, both expressing the highest level of alpha(5)beta(1) integrin. It seems very likely that adhesion to FN can be accomplished by many different integrins, but for effective migration alpha(5)beta(1) integrin is responsible. Only A375 and WM239 cell lines reacted to SW treatment. In the presence of SW WM239 and A375 cells had 70% and 40% increased adhesion to FN, and their migration was decreased 40% and 50%, respectively. Interestingly, although most of the cell lines share a common profile of integrins, each line interacted with FN differently. They differed mainly in the repertoire of integrins used for adhesion, and in the manner in which glycosylation affected these processes. The influence of SW was observed in two metastatic cell lines indicating the contribution of glycosylation status to the progression of melanoma. The lack of reaction to SW in WM9 cells may suggest that there is a threshold in the expression level of the highly branched N-glycans that may influence the adhesion and migration properties of the cell.

Down regulation of N-acetylglucosaminyltransferase V facilitates all-transretinoic acid to induce apoptosis of human hepatocarcinoma cells

After N-acetylglucosaminyltransferase V (GnT-V) activity was down-regulated by the transfection of its antisense cDNA(GnTV-AS), apoptosis of H7721 cells was appeared and the apoptosis induced by 80 microM all-transretinoic acid (ATRA) was facilitated, while ATRA itself could not induce apparent apoptosis in mock cells transfected with the vector. In the study of the molecular mechanism of this phenomenon, it was found that GnTV-AS reduced the expressions of anti-apoptotic proteins, such as phosphorylated protein kinase B and phosphorylated Bad as well as Bcl-2 and Bcl-X (L), and elevated those of pro-apoptotic proteins, including Bax, full length caspase-3 and its activated fragments as well as anti-oncoprotein p53. In the contrast, ATRA up regulated the expressions of Bax and activated caspase-3 fragments only. After the GnTV-AS transfected cells were treated with ATRA, phosphorylated PKB and Bad were further decreased, while Bax and activated caspase-3 fragment were further increased, leading to the enhanced apoptosis in flow-cytometry analysis when compared with GnTV-AS cells not treated with ATRA. It was speculated that the decreased phospho-Bad resulted from the reduced phospho-PKB and the up regulation of p53 caused the elevated activity of Bax. The increased active caspase-3 was the consequence of the elevated Bax/ Bcl-2(Bcl-X(L)) activity ratio in the cells.

Beta1,6-branched oligosaccharides are increased in lymph node metastases and predict poor outcome in breast carcinoma

PURPOSE

This study was designed to provide a comprehensive assessment on the role of beta1,6-branched oligosaccharides in the metastasis and outcome of breast carcinoma. Generation of these structures on N-glycans is initiated by beta1,6-N-acetylglucosaminyltransferase V and used by both myeloid cells and cancer cells in systemic migration.

EXPERIMENTAL DESIGN

Tissue microarrays of >700 tumors (>400 patients; 30-year follow-up data) were stained through lectin histochemistry with leukocytic phytohemagglutinin (LPHA), a selective marker for beta1,6-branched oligosaccharides. Node-negative and node-positive primary tumors and patient-matched lymph node metastases were scored by blinded observers.

RESULTS

Metastases stained at significantly greater intensities than did the patient-matched primary tumors (P < 0.0001), demonstrating for the first time that the abundance of beta1,6-branched oligosaccharides was directly associated with breast carcinoma nodal metastasis. Multivariate analyses revealed that beta1,6-branched oligosaccharides in primary tumors were a predictor of poor outcome, most notably in node-negative tumors, where an LPHA staining score of 3+ gave a risk factor of 3.3, independent of tumor size, nuclear grade, or patient age (P = 0.007).

CONCLUSIONS

The data firmly establish a role for beta1,6-N-acetylglucosaminyltransferase V activity and beta1,6-branched oligosaccharides in breast carcinoma metastasis, and reemphasize the involvement, although poorly understood, of aberrant glycosylation in tumor progression.

Evidence for the presence of a low-mass beta1 integrin on the cell surface

Although the cell line K562 reportedly expresses a single species of beta1 integrin, alpha5beta1, surface staining with monoclonal antibodies JB1A, 12G10 and B3B11 to the beta1 chain clearly demonstrated differences in the expression levels of the epitopes detected by these antibodies. The present studies were initiated to determine the basis for this molecular heterogeneity in the integrins. Cross-linking of surface integrins with B3B11 caused their selective aggregation. This distribution was similar to that observed for the alpha5 chain. In contrast, cross-linking the beta1 chains with 12G10 did not cause codistribution of alpha5, suggesting that these two species were not associated on the cell surface. Immunoprecipitates of the surface integrins of K562 cells indicated the presence of 120 and 140 kDa forms of the beta1 chain which were detected by 12G10 and B3B11, respectively. Immunological, biochemical and mass spectrometric analysis of K562 surface integrins also failed to demonstrate the presence of any alpha chain in association with the 120 kDa species of beta1 of K562 cells. Treatment of the two forms of beta1 with PGNase reduced their masses to approximately 90 kDa, suggesting that N-glycosylation was responsible for the mass differences. Collectively, these results provide evidence for a novel species of beta1 on the cell surface, which does not appear to be associated with any alpha chain. The data also suggest that differences in glycosylation may be involved in defining the association between the integrin alpha and beta chains and the functional properties of these integrins.

Hypersialylation of beta1 integrins, observed in colon adenocarcinoma, may contribute to cancer progression by up-regulating cell motility

Colon adenocarcinomas are known to express elevated levels of alpha2-6 sialylation and increased activity of ST6Gal-I, the Golgi glycosyltransferase that creates alpha2-6 linkages. Elevated ST6Gal-I positively correlates with metastasis and poor survival, and therefore ST6Gal-I-mediated hypersialylation likely plays a role in colorectal tumor invasion. Previously we found that oncogenic ras (present in roughly 50% of colon adenocarcinomas) up-regulates ST6Gal-I and, in turn, increases sialylation of beta1 integrin adhesion receptors in colon epithelial cells. However, we wanted to know if this pattern held true in vivo and, if so, how beta1 hypersialylation might contribute to colon tumor progression. In the present study, we find that beta1 integrins from colon adenocarcinomas consistently carry higher levels of alpha2-6 sialic acid. To explore the effects of increased alpha2-6 sialylation on beta1-integrin function, we stably expressed ST6Gal-I in a colon epithelial cell line lacking endogenous ST6Gal-I. ST6Gal-I expressors (with alpha2-6 sialylated beta1 integrins) exhibited up-regulated attachment to collagen I and laminin and increased haptotactic migration toward collagen I, relative to parental cells (with completely unsialylated beta1 integrins). Blockade of ST6Gal-I expression with short interfering RNA reversed collagen binding back to the level of ST6Gal-I nonexpressors, confirming that alpha2-6 sialylation regulates beta1 integrin function. Finally, we show that beta1 integrins from ST6Gal-I expressors have increased association with talin, a marker for integrin activation. Collectively, these findings suggest that beta1 hypersialylation may augment colon tumor progression by altering cell preference for certain extracellular matrix milieus, as well as by stimulating cell migration.

N-glycoproteins bearing beta1-6 branched oligosaccharides from the A375 human melanoma cell line analysed by tandem mass spectrometry

Tumour-associated alterations of cell surface glycosylation play a crucial role in the adhesion and metastasis of cancer cells. It is well known that the metastatic potential is associated with increased GlcNAc beta1-6 branching in N-glycans of tumour cells specifically recognized by a lectin from Phaseolus vulgaris leukoagglutinin (PHA-L). We identified proteins bearing GlcNAc beta1-6 branched N-glycans in the A375 human melanoma cell line by affinity chromatography separation on a PHA-L agarose column, followed by immunoidentification and tandem mass spectrometry (MS/MS) analysis. Amongst the proteins identified were integrin subunits alpha2, alpha3, alpha5 and beta1, as well as N-cadherin and lysosome-associated membrane proteins (LAMP-1 and LAMP-2). In addition, L1, Mac-2 binding protein (Mac-2-BP), activated leukocyte cell adhesion molecule/CD166 (ALCAM) and melanotransferrin were shown to react with PHA-L. Some of these proteins are connected mainly with nervous tissues or the immune system and play a crucial role in cell adhesion processes. The presence of GlcNAc beta1-6 branched oligosaccharides in these proteins may influence their adhesion properties, reducing adhesion of the cells to the extracellular matrix (ECM) and thus facilitating tumour cell invasion.

Increased expression of N-acetylglucosaminyltransferase-V in human hepatoma cells by retinoic acid and 1alpha,25-dihydroxyvitamin D3

UDP-N-acetylglucosamine: alpha-6-D-mannoside beta-1,6N-acetylglucosaminyltransferase-V activities were determined in human hepatoma cell lines of Hep3B and HepG2, and also compared with those of normal liver tissues and primary hepatocytes. When GlcNAcbeta1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1-4)(Manbeta1-4GlcNAc-2-amino pyridine (GlcN,GlcN-biant-PA) and UDP-GlcNAc were used as substrates, the enzymes displayed optimum temperatures of 50 degrees C, optimum pHs of 6.5 in each case, K(m) values for UDP-GlcNAc to be 5.8 (Hep3B) and 4.5 mM (HepG2) and K(m) values for GlcN,GlcN-biant-PA (mM) to be 1.28 (Hep3B) and 2.4 (HepG2). This indicates that values of Hep3B GlcNAc-transferase-V were distinguishable with HepG2 enzyme. Furthermore, Hep3B enzyme in membrane fraction showed about 1.5-fold higher specific activity (1.423 pmol/(h mg) than that (1.066 pmol/(h mg)) of HepG2. Normal hepatocytes are characterized by very low level of GlcNAc-transferase-V activity whereas hepatoma cells contained high activities. Treatment of hepatoma cells with retinoic acid and 1alpha,2,5-dihydroxyvitamin D(3) (Vit-D(3)) resulted in an increase in GlcNAc-transferase-V activity, while treatment with dimethyl sulfoxide and cytosine-arabinoside resulted in decrease in the enzyme activity. Although retinoic acid (RA) treated cells shows a changed GlcNAc-transferase-V mRNA expression, expression of marker proteins such as alpha-fetoprotein and albumin was not changed. This is the first demonstration of GlcNAc-transferase-V activity in RA and Vit-D(3)-treated hepatoma cell lines.

Effect of N-acetylglucosaminyltransferase V on the expressions of other glycosyltransferases

Transfection of sense cDNA of N-acetylglucosaminyltransferase V (GnTV) into H7721 human hepatocellular carcinoma cells resulted in the decreased expression of surface sialyl Lewis X (SLe(x)), a sialylated fucose-containing antigen. The enzymatic mechanisms were speculated to be the concomitantly decreased expression of alpha1,3-fucosyltransferase (FucT)-III, -VI, -VII and the branching enzyme of O-glycans, core 2-beta1,6-N-acetylglucosaminyltransferase (C2GnT)-I, -II. These two glycosyltransferase families were suggested to be the key enzymes in the synthesis of SLe(x). The expression of alpha2,3-sialyltransferase (ST3)-IV, but not ST3-I, -II and -III was elevated by sense GnTV. However, it did not cause the increase of SLe(x) synthesis. Transfection of antisense GnTV into H7721 cells showed entirely opposite effects on the expression of above-mentioned SLe(x) and glycosyltransferases as the sense GnTV.

Variant glycosylation: an underappreciated regulatory mechanism for β1 integrins

Although it has been known for many years that beta1 integrins undergo differential glycosylation in accordance with changes in cell phenotype, the potential role of N-glycosylation as a modulator of integrin function has received little attention. One reason for the relatively limited interest in this topic likely relates to the fact that much of the prior research was correlative in nature. However, new results now bolster the hypothesis that there is a causal relationship between variant glycosylation and altered integrin activity. In this review, the evidence for variant glycosylation as a regulatory mechanism for beta1 integrins are summarized, with particular emphasis on: (1). outlining the instances in which cell phenotypic variation is associated with differential beta1 glycosylation, (2). describing the specific alterations in glycan structure that accompany phenotypic changes and (3). presenting potential mechanisms by which variant glycosylation might regulate integrin function.

Mutations in and near the second calcium-binding domain of integrin alphaIIb affect the structure and function of integrin alphaIIbbeta3

Calcium-binding domains in the alpha-subunit of integrins contain a central loop structure. To examine the importance of the loop structure, a series of alphaIIb mutants containing changes to the calcium-liganding amino acids have been constructed. Significantly, none of the mutant alphaIIbbeta3 complexes was detected on the surface of transfected cells, but mutant pro-alphaIIb was detected in cell lysates in complex with beta3. To study the importance of the regions flanking the second calcium-binding domain for ligand-binding and ligand-binding specificity, three alphaIIb/alpha5 chimaeras containing alpha5 sequences flanking or flanking and including the second calcium-binding domain were constructed. The chimaera containing both alpha5-flanking regions was not expressed on the cell surface, but FR1 and FR2, substituting either the first or second flanking region, were expressed. FR1beta3-transfected cells lost the ability to adhere to fibrinogen and to support aggregation and had minimal fibrinogen-binding ability. The heterodimer complex was less stable than the wild-type. FR2beta3-transfected cells adhered to fibrinogen and bound soluble fibrinogen with higher affinity when compared with wild-type. In addition, the heterodimer complex was more stable than wild-type. These results indicate that the conformation of the second calcium-binding domain is critical for maturation of the alphaIIbbeta3 complex and expression on the cell surface and that the surrounding sequences are critical for alphaIIbbeta3 function.