Functional integrins from normal and glycosylation-deficient baby hamster kidney cells. Terminal processing of asparagine-linked oligosaccharides is not correlated with fibronectin-binding activity

The role of protein disulfide isomerase in the post-ligation phase of β3 integrin-dependent cell adhesion

INTRODUCTION

Protein disulfide isomerase (PDI) catalyzes disulfide bond exchange. It is crucial for integrin-mediated platelet adhesion and aggregation and disulfide bond exchange is necessary for αIIbβ3 and αvβ3 activation. However, the role of disulfide bond exchange and PDI in the post-ligation phase of αIIbβ3 and αvβ3 mediated cell adhesion has yet to be determined.

METHODS

To investigate a possible such role, we expressed wild type (WT) human αIIb and either WT human β3, or β3 harboring single or double cysteine to serine substitutions disrupting Cys473-Cys503 or Cys523-Cys544 bonds, in baby hamster kidney (BHK) cells, leading to expression of both human αIIbβ3 and a chimeric hamster/human αvβ3. Adhesion to fibrinogen-coated wells was studied in the presence or absence of bacitracin, a PDI inhibitor, with and without an αvβ3 blocker.

RESULTS

Flow cytometry showed WT and mutant αIIbβ3 expression in BHK cells and indicated that mutated αIIbβ3 receptors were constitutively active while WT αIIbβ3 was inactive. Both αIIbβ3 and αvβ3 integrins, WT and mutants, mediated adhesion to fibrinogen as shown by reduced but still substantial adhesion following treatment with the αvβ3 blocker. Mutated αIIbβ3 integrins disrupted in the Cys523-Cys544 bond still depended on PDI for adhesion as shown by the inhibitory effect of bacitracin in the presence of the αvβ3 blocker. Mutated integrins disrupted in the Cys473-Cys503 bond showed a similar trend.

CONCLUSIONS

PDI-mediated disulfide bond exchange plays a pivotal role in the post-ligation phase of αIIbβ3-mediated adhesion to fibrinogen, while this step in αvβ3-mediated adhesion is independent of disulfide exchange.

Different adhesion and migration properties of human HCV29 non-malignant urothelial and T24 bladder cancer cells: role of glycosylation

In tumour cells, alterations in cellular glycosylation may play a key role in their metastatic behaviour. This study used cell lines having very different behaviour in vivo: HCV29 non-malignant transitional epithelium and T24 bladder transitional cell carcinoma. These differences in behaviour might be due in part to differences in cellular glycosylation patterns. Glycan chain analysis of their glycoproteins was performed with the use of specific lectins. The functional role of carbohydrates was studied by treating these cells with swainsonine, an inhibitor of Golgi alpha-mannosidase II, and in vitro adhesion and migration assays. The adhesion of swainsonine-treated HCV29 and T24 cells was increased on fibronectin and type IV collagen by 1.5- and 2-fold, respectively, whereas adhesion on laminin was virtually unchanged after swainsonine-treatment in HCV29 cells and was increased in T24 cells. Swainsonine treatment reduced the rate of T24 cell migration by 20%. We concluded that beta1-6 branched tri- and tetraantennary complex-type glycans have an important function in adhesion and migration in the studied cell lines. These data support the view that oligosaccharides are involved in several steps of the metastatic process.

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.

Role of cell surface glycosylation in mediating repair of human airway epithelial cell monolayers

Our laboratory recently demonstrated the pattern of cell surface glycosylation of nonsecretory central airway epithelium (Dorscheid DR, Conforti AE, Hamann KJ, Rabe KF, and White SR. Histochem J 31: 145-151, 1999), but the role of glycosylation in airway epithelial cell migration and repair is unknown. We examined the functional role of cell surface carbohydrates in wound repair after mechanical injury of 1HAEo(-) human airway epithelial and primary bronchial epithelial monolayers. Wound repair stimulated by epidermal growth factor was substantially attenuated by 10(-7) M tunicamycin (TM), an N-glycosylation inhibitor, but not by the inhibitors deoxymannojirimycin or castanospermine. Wound repair of 1HAEo(-) and primary airway epithelial cells was blocked completely by removal of cell surface terminal fucose residues by alpha-fucosidase. Cell adhesion to collagen matrix was prevented by TM but was only reduced ~20% from control values with prior alpha-fucosidase treatment. Cell migration in Blind Well chambers stimulated by epidermal growth factor was blocked by pretreatment with TM but alpha-fucosidase pretreatment produced no difference from control values. These data suggest that cell surface N-glycosylation has a functional role in airway epithelial cell adhesion and migration and that N-glycosylation with terminal fucosylation plays a role in the complex process of repair by coordination of certain cell-cell functions.

Occurrence of oligosialic acids on integrin alpha 5 subunit and their involvement in cell adhesion to fibronectin

Integrin alpha(5)beta(1), a major fibronectin receptor, functions in a wide variety of biological phenomena. We have found that alpha 2-8-linked oligosialic acids with 5 < or = degree of polymerization (DP) < or = 7 occur on integrin alpha(5) subunit of the human melanoma cell line G361. The integrin alpha(5) subunit immunoprecipitated with anti-integrin alpha(5) antibody reacted with the monoclonal antibody 12E3, which recognizes oligo/polysialic acid with DP > or = 5 but not with the polyclonal antibody H.46 recognizing oligo/polysialic acid with DP > or = 8. The occurrence of oligosialic acids was further demonstrated by fluorometric C(7)/C(9) analysis on the immunopurified integrin alpha(5) subunit. Oligosialic acids were also found in the alpha(5) subunit of several other human cells such as foreskin fibroblast and chronic erythroleukemia K562 cells. These results suggest the ubiquitous modification with unique oligosialic acids occurs on the alpha(5) subunit of integrin alpha(5)beta(1). The adhesion of human melanoma G361 cells to fibronectin was mainly mediated by integrin alpha(5)beta(1). Treatment of cells with sialidase from Arthrobacter ureafaciens cleaving alpha 2-3-, alpha 2-6-, and alpha 2-8-linked sialic acids inhibited adhesion to fibronectin. On the other hand, N-acetylneuraminidase II, which cleaves alpha 2-3 and alpha 2-6 but not alpha 2-8 linkages, showed no inhibitory activity. After the loss of oligosialic acids, integrin alpha(5)beta(1) failed to bind to fibronectin-conjugated Sepharose, indicating that the oligosialic acid on the alpha(5) subunit of integrin alpha(5)beta(1) plays important roles in cell adhesion to fibronectin.

Carbohydrate-carbohydrate binding of ganglioside to integrin alpha(5) modulates alpha(5)beta(1) function

Gangliosides GT1b and GD3, components of keratinocyte membranes, inhibit keratinocyte adhesion to fibronectin. Although ganglioside sialylation is known to be important, the mechanism of inhibition is unknown. Using purified insect recombinant alpha(5) and beta(1) proteins and alpha(5)beta(1) integrin from lysed keratinocyte-derived SCC12 cells, we have shown that GT1b and GD3 inhibit the binding of alpha(5)beta(1) to fibronectin. Co-immunoprecipitation of GT1b and alpha(5)beta(1) from SCC12 cells and direct binding of GT1b and GD3 to affinity-purified alpha(5)beta(1) from SCC12 cells and insect recombinant alpha(5)beta(1), particularly the alpha(5) subunit, further suggest interaction between ganglioside and alpha(5)beta(1). The carbohydrate moieties of integrin appear to be critical since gangliosides are unable to bind deglycosylated forms of alpha(5)beta(1) from SCC12 and insect cells or poorly glycosylated recombinant alpha(5)beta(1) from Escherichia coli cells. The GT1b-alpha(5)beta(1) interaction is inhibited by concanavalin A, suggesting that GT1b binds to mannose structures in alpha(5)beta(1). The preferential binding of GT1b to high mannose rather than reduced mannose ovalbumin further implicates the binding of GT1b to mannose structures. These data provide evidence that highly sialylated gangliosides regulate alpha(5)beta(1)-mediated adhesion of epithelial cells to fibronectin through carbohydrate-carbohydrate interactions between GT1b and the alpha(5) subunit of alpha(5)beta(1) integrin.

Sialidase treatment exposes the beta1-integrin active ligand binding site on HL60 cells and increases binding to fibronectin

The migration of neutrophils from the circulation to areas of inflammation is the result of the sequential activation of multiple cellular adhesion molecules. beta1-Integrins are cell surface glycoproteins and the class of adhesion molecules responsible for binding to the extracellular matrix. The goal of this study was to determine the contribution of glycosylation, specifically the presence of sialic acid, to beta1-integrin adhesion in a neutrophil model. beta1-Integrins on differentiated HL60 cells were remodeled by treatment with the exoglycosidases, sialidase and beta-galactosidase. beta1-Integrin activity was determined by measuring adherence to the extracellular matrix protein fibronectin. The expression of beta1-integrins, beta2-integrins and activated beta1-integrins was determined by flow cytometry. Remodeling of beta1-integrins by treatment with sialidase increased adhesion by greater than 1,000%. Flow cytometric analysis of remodeled beta1-integrins demonstrated an increased expression of the activated beta1-integrin, but only minor increases in the expression of total beta1- and beta2-integrins. We postulate that glycosidase treatment increases adhesion and expression of activated beta1-integrins by exposure of the normally hidden ligand-binding site. The glycosylation of beta1-integrins on neutrophils may act to hide the ligand-binding site in unstimulated cells thereby contributing to the affinity modulation observed in neutrophil beta1-integrin function.

Altered glycosylation of alpha(s)beta 1 integrins from rat colon carcinoma cells decreases their interaction with fibronectin

Malignant cell transformation is generally accompanied by changes in their interactions with environing matrix proteins in a way to facilitate their migration and generate invasion. Our results show the binding of rat colon adenocarcinoma PROb cells to fibronectin strongly reduced when compared to normal rat intestine epithelial cells. This decrease was not due to the level of alpha(s)beta 1 integrins expressed at the surface of the cell line. However, beta 1- and alpha(s)-associated subunits appeared to be structurally altered as shown by immunoprecipitation followed by electrophoresis. Pulse chase experiments using 35S methionine evidenced differences in the biosynthesis of beta 1- and alpha(s)associated integrins: normal epithelial IEC18 cells required 16 h for maximal biosynthesis of the completely mature beta 1 subunit, while PROb cells did it within 4-6 h. Studies using endoglycosidases O, H, D, and N glycanase confirmed that the molecular weight alterations were due to abnormal glycosylation and suggested that alpha(s)beta 1 integrins of PROb cells could bear both mature complex and immature high mannose types while IEC18 cells borne only mature complex type oligosaccharidic chains. Treatment of both cell types with castanospermine, an inhibitor of N-glycosylation, reduced the differences observed in their adhesion to the fibronectin without significantly affecting beta 1 receptors expression at the cell surface. These results strongly suggest a role of the glycosylation of beta 1 receptors in the adhesion of rat colon adenocarcinoma PROb cells to fibronectin substrata.

Role of alpha v beta 5 and alpha v beta 6 integrin glycosylation in the adhesion of a colonic adenocarcinoma cell line (HT29-D4)

We have previously characterized the expression of the alpha nu beta 5 and alpha nu beta 6 integrins as major receptors for the human colonic adenocarcinoma cell line (HT29-D4), on vitronectin and fibronectin, respectively [Lehmann et al. (1994): Cancer Res 54:2102-2107]. In the present work we investigated the glycosylation role of these integrins in their adhesive functions. To this end, we used glycohydrolases to show that cell surface integrins were N-glycosylated and sialylated, and that only the alpha v subunit carried some immature oligosaccharide side chains. To alter the glycosylation state of the cell surface alpha v beta 5 and alpha v beta 6 integrins, we used two oligosaccharide-processing inhibitors: 1-deoxymannojirimycin (dMNJ) and tunicamycin (TM). Following treatment of HT29-D4 cells with dMNJ, cell surface alpha v beta 5 and alpha v beta 6 carried only high-mannose-type sugar chains, while TM-treated cells expressed de-N-glycosylated integrins. Neither alpha/beta heterodimers assembly nor cell surface expression were impaired in the presence of the drugs. Finally, we established that adhesion of dMNJ- or TM-treated cells was altered on both vitronectin and fibronectin substrata, whereas the adhesion of these cells on laminin or collagen type I was virtually unchanged.