Deletion of core fucosylation on alpha3beta1 integrin down-regulates its functions

Galectin-3 promotes lamellipodia formation in epithelial cells by interacting with complex N-glycans on alpha3beta1 integrin

Recent studies have shown that galectin-3 (Gal-3; also known as LGALS3), a beta-galactoside-binding lectin, promotes cell migration during re-epithelialization of corneal wounds. The goal of this study was to characterize the molecular mechanism by which Gal-3 stimulates cell migration. We demonstrate here that exogenous Gal-3, but not Gal-1 or Gal-8, promotes cell scattering and formation of lamellipodia in human corneal epithelial cells in a beta-lactose-inhibitable manner. alpha3beta1 integrin was identified as the major Gal-3-binding protein in corneal epithelial cells by affinity chromatography of cell lysates on a Gal-3-Sepharose column. Preincubation of cells with anti-alpha3 integrin function-blocking antibody significantly inhibited the induction of lamellipodia by Gal-3. Furthermore, exogenous Gal-3 activated both focal adhesion kinase, a key regulator of integrin-dependent intracellular signaling, and Rac1 GTPase, a member of the family of Rho GTPases, well known for its role in the reorganization of the actin cytoskeleton and formation of lamellipodial extensions. Experiments involving knockdown of beta-1,6-N-acetylglucosaminytransferase V, an enzyme that synthesizes high-affinity glycan ligands for Gal-3, revealed that carbohydrate-mediated interaction between Gal-3 and complex N-glycans on alpha3beta1 integrin plays a key role in Gal-3-induced lamellipodia formation. We propose that Gal-3 promotes epithelial cell migration by cross-linking MGAT5-modified complex N-glycans on alpha3beta1 integrin and subsequently activating alpha3beta1-integrin-Rac1 signaling to promote lamellipodia formation.

Core fucosylation of E-cadherin enhances cell-cell adhesion in human colon carcinoma WiDr cells

Alpha1,6-fucosyltransferase (Fut8), an enzyme that catalyzes the introduction of alpha1,6 core fucose to the innermost N-acetylglucosamine residue of the N-glycan, has been implicated in the development, immune system, and tumorigenesis. We found that alpha1,6-fucosyltransferase and E-cadherin expression levels are significantly elevated in primary colorectal cancer samples. Interestingly, low molecular weight population of E-cadherin appeared as well as normal sized E-cadherin in cancer samples. To investigate the correlation between alpha1,6-fucosyltransferase and E-cadherin expression, we introduced alpha1,6-fucosyltransferase in WiDr human colon carcinoma cells. It was revealed that the low molecular weight population of E-cadherin was significantly increased in alpha1,6-fucosyltransferase-transfected WiDr cells in dense culture, which resulted in an enhancement in cell-cell adhesion. The transfection of mutated alpha1,6-fucosyltransferase with no enzymatic activity had no effect on E-cadherin expression, indicating that core fucosylation is involved in the phenomena. In alpha1,6-fucosyltransferase knock down mouse pancreatic acinar cell carcinoma TGP49 cells, the expression of E-cadherin and E-cadherin dependent cell-cell adhesion was decreased. The introduction of alpha1,6-fucosyltransferase into kidney epithelial cells from alpha1,6-fucosyltransferase(-/-) mice restored the expression of E-cadherin and E-cadherin-dependent cell-cell adhesion. Based on the results of lectin blotting, peptide N-glycosidase F treatment, and pulse-chase studies, it was demonstrated that the low molecular weight population of E-cadherin contains peptide N-glycosidase F insensitive sugar chains, and the turnover rate of E-cadherin was reduced in alpha1,6-Fucosyltransferase transfectants. Thus, it was suggested that core fucosylation regulates the processing of oligosaccharides and turnover of E-cadherin. These results suggest a possible role of core fucosylation in the regulation of cell-cell adhesion in cancer.

Serum protein N-glycans profiling for the discovery of potential biomarkers for nonalcoholic steatohepatitis

The hepatic histology in nonalcoholic fatty liver disease can vary from isolated hepatic steatosis to steatohepatitis can progress to cirrhosis and liver-related death. The aim was to evaluate the use of blood serum N-glycan fingerprinting as a tool for differential diagnosis of nonalcoholic steatohepatitis from steatosis. A group of 47 patients with NAFLD was diagnosed by clinical laboratory analysis and ultrasonography, and was studied histologically using the Brunt's scoring system. The control group included 13 healthy individuals. N-glycan profiles of serum proteins were determined by DNA sequencer-based carbohydrate analytical profiling. We have found that the concentrations of two glycans (NGA2F and NA2) and their logarithm ratio of NGA2F versus NA2 (named GlycoNashTest) were associated with the degree of NASH-related fibrosis, but had no correlation with the grade of inflammation nor steatosis severity. When used to screen NAFLD patients, GlycoNashTest could identify advanced NASH-related fibrosis (F3-F4) with the diagnosis sensitivity of 89.5% and specificity of 71.4%. The serum N-glycan profile is a promising noninvasive method for detecting NASH or NASH-related fibrosis in NAFLD patients, which could be a valuable supplement to other markers currently used in diagnosis of NASH.

Requirement of Fut8 for the expression of vascular endothelial growth factor receptor-2: a new mechanism for the emphysema-like changes observed in Fut8-deficient mice

alpha1,6-Fucosylation plays key roles in many biological functions, as evidenced by the study of alpha1,6-fucosyltransferase (Fut8) knockout (Fut8(-/-)) mice. Phenotypically, Fut8(-/-) mice exhibit emphysema-like changes in the lung, and severe growth retardation. Fut8(-/-) cells also show marked dysregulation of the TGF-beta1 receptor, EGF receptor, integrin activation and intracellular signalling, all of which can be rescued by reintroduction of Fut8. The results of the present study demonstrated that vascular endothelial growth factor receptor-2 (VEGFR-2) expression was significantly suppressed in Fut8(-/-) mice, suggesting that Fut8 was required for VEGFR-2 expression. The expression of VEGFR-2 mRNA and protein was consistently down-regulated by knockdown of the Fut8 gene with small interference RNA in A549 cells, as well as in TGP49 cells, suggesting that suppression occurs at the level of transcription. In contrast, the expression level of ceramide, an inducer of cell apoptosis, was increased in the lungs of Fut8(-/-) mice. The terminal transferase dUTP nick end-labelling (TUNEL) assay was used to identify apoptotic cells. The number of TUNEL-positive septal epithelia and endothelia cells was significantly increased in the alveolar septa of lungs from Fut8(-/-) mice when in comparison with lungs from wild-type mice. It is well known that, in emphysema, ceramide expression can be greatly enhanced by blockade of the VEGFR-2. Thus, suppression of VEGFR-2 expression may provide a novel explanation for the emphysema-like changes in Fut8(-/-) mice.

Tetraspanin CD151 regulates glycosylation of (alpha)3(beta)1 integrin

The tetraspanin CD151 forms a stoichiometric complex with integrin alpha3beta1 and regulates its endocytosis. We observed that down-regulation of CD151 in various epithelial cell lines changed glycosylation of alpha3beta1. In contrast, glycosylation of other transmembrane proteins, including those associated with CD151 (e.g. alpha6beta1, CD82, CD63, and emmprin/CD147) was not affected. The detailed analysis has shown that depletion of CD151 resulted in the reduction of Fucalpha1-2Gal and bisecting GlcNAc-beta(1-->4) linkage on N-glycans of the alpha3 integrin subunit. The modulatory activity of CD151 toward alpha3beta1 was specific, because stable knockdown of three other tetraspanins (i.e. CD9, CD63, and CD81) did not affect glycosylation of the integrin. Analysis of alpha3 glycosylation in CD151-depleted breast cancer cells with reconstituted expression of various CD151 mutants has shown that a direct contact with integrin is required but not sufficient for the modulatory activity of the tetraspanin toward alpha3beta1. We also found that glycosylation of CD151 is also critical; Asn(159) --> Gln mutation in the large extracellular loop did not affect interactions of CD151 with other tetraspanins or alpha3beta1 but negated its modulatory function. Changes in the glycosylation pattern of alpha3beta1 observed in CD151-depleted cells correlated with a dramatic decrease in cell migration toward laminin-332. Migration toward fibronectin or static adhesion of cells to extracellular matrix ligands was not affected. Importantly, reconstituted expression of the wild-type CD151 but not glycosylation-deficient mutant restored the migratory potential of the cells. These results demonstrate that CD151 plays an important role in post-translation modification of alpha3beta1 integrin and strongly suggest that changes in integrin glycosylation are critical for the promigratory activity of this tetraspanin.

Characterisation of alpha3beta1 and alpha(v)beta3 integrin N-oligosaccharides in metastatic melanoma WM9 and WM239 cell lines

It is well documented that glycan synthesis is altered in some pathological processes, including cancer. The most frequently observed alterations during tumourigenesis are extensive expression of beta1,6-branched complex type N-glycans, the presence of poly-N-acetyllactosamine structures, and high sialylation of cell surface glycoproteins. This study investigated two integrins, alpha3beta1 and alpha(v)beta3, whose expression is closely related to cancer progression. Their oligosaccharide structures in two metastatic melanoma cell lines (WM9, WM239) were analysed with the use of matrix-assisted laser desorption ionisation mass spectrometry. Both examined integrins possessed heavily sialylated and fucosylated glycans, with beta1,6-branches and short polylactosamine chains. In WM9 cells, alpha3beta1 integrin was more variously glycosylated than alpha(v)beta3; in WM239 cells the situation was the reverse. Functional studies (wound healing and ELISA integrin binding assays) revealed that the N-oligosaccharide component of the tested integrins influenced melanoma cell migration on vitronectin and alpha3beta1 integrin binding to laminin-5. Additionally, more variously glycosylated integrins exerted a stronger influence on these parameters. To the best of our knowledge, this is the first report concerning structural characterisation of alpha(v)beta3 integrin glycans in melanoma or in any cancer cells.

Increased bisecting and core-fucosylated N-glycans on mutant human amyloid precursor proteins

Alteration of glycoprotein glycans often changes various properties of the glycoprotein. To understand the significance of N-glycosylation in the pathogenesis of early-onset familial Alzheimer's disease (AD) and in beta-amyloid (Abeta) production, we examined whether the mutations in the amyloid precursor protein (APP) gene found in familial AD affect the N-glycans on APP. We purified the secreted forms of wild-type and mutant human APPs (both the Swedish type and the London type) produced by transfected C17 cells and determined the N-glycan structures of these three recombinant APPs. Although the major N-glycan species of the three APPs were similar, both mutant APPs contained higher contents of bisecting N-acetylglucosamine and core-fucose residues as compared to wild-type APP. These results demonstrate that familial AD mutations in the polypeptide backbone of APP can affect processing of the attached N-glycans; however, whether these changes in N-glycosylation affect Abeta production remains to be established.

Branched N-glycans regulate the biological functions of integrins and cadherins

Glycosylation is one of the most common post-translational modifications, and approximately 50% of all proteins are presumed to be glycosylated in eukaryotes. Branched N-glycans, such as bisecting GlcNAc, beta-1,6-GlcNAc and core fucose (alpha-1,6-fucose), are enzymatic products of N-acetylglucosaminyltransferase III, N-acetylglucosaminyltransferase V and alpha-1,6-fucosyltransferase, respectively. These branched structures are highly associated with various biological functions of cell adhesion molecules, including cell adhesion and cancer metastasis. E-cadherin and integrins, bearing N-glycans, are representative adhesion molecules. Typically, both are glycosylated by N-acetylglucosaminyltransferase III, which inhibits cell migration. In contrast, integrins glycosylated by N-acetylglucosaminyltransferase V promote cell migration. Core fucosylation is essential for integrin-mediated cell migration and signal transduction. Collectively, N-glycans on adhesion molecules, especially those on E-cadherin and integrins, play key roles in cell-cell and cell-extracellular matrix interactions, thereby affecting cancer metastasis.

Biological function of fucosylation in cancer biology

Fucosylation is one of the most common modifications involving oligosaccharides on glycoproteins or glycolipids. Fucosylation comprises the attachment of a fucose residue to N-glycans, O-glycans and glycolipids. O-Fucosylation, which is a special type of fucosylation, is very important for Notch signalling. The regulatory mechanisms for fucosylation are complicated. Many kinds of fucosyltransferases, the GDP-fucose synthesis pathway and GDP-fucose transporter are involved in the regulation of fucosylation. Increased levels of fucosylation have been reported in a number of pathological conditions, including inflammation and cancer. Therefore, certain types of fucosylated glycoproteins such as AFP-L3 or several kinds of antibodies, which recognize fucosylated oligosaccharides such as sialyl Lewis a/x, have been used as tumour markers. Furthermore, fucosylation of glycoproteins regulates the biological functions of adhesion molecules and growth factor receptors. Changes in fucosylation could provide a novel strategy for cancer therapy. In this review, the biological significance of and regulatory pathway for fucosylation have been described.

Analysis of N-glycan in serum glycoproteins from db/db mice and humans with type 2 diabetes

Glycosylation has an important role in regulating properties of proteins and is associated with many diseases. To examine the alteration of serum N-glycans in type 2 diabetes, we used the db/db mouse model. Serum N-glycans were fluorescence labeled and applied to HPLC. There were reproducible differences in N-glycan profiles between the db/db mouse model and the db/+ control. The structures of the oligosaccharides, which had changed in their amounts, were analyzed by a two-dimensional mapping method, matrix-assisted laser desorption ionization-time-of-flight mass spectrometry, and exoglycosidase digestion. Those analyses revealed an increase in the N-glycans possessing alpha1,6-fucose in the serum of db/db mice. The level of alpha1,6-fucosyltransferase mRNA was increased in the liver of the db/db mice. The ratio of a biantennary N-glycan with alpha1,6-fucose to that without alpha1,6-fucose in the liver tissue of the db/db mouse was increased relative to the db/+ control. Next, we analyzed the serum N-glycan profile in human subjects with type 2 diabetes and found an increased amount of a biantennary N-glycan that had an alpha1,6-fucose with a bisecting N-acetylglucosamine. In conclusion, the increase in alpha1,6-fucosylation is a striking change in the serum N-glycans of the db/db mice, whereas the change in the fucosylation in humans with type 2 diabetes was small, albeit statistically significant. It is likely that the change is caused, at least partially, by the increase in the alpha1,6-fucosyltransferase mRNA level in the liver. The increased alpha1,6-fucosylation may affect protein properties associated with the pathophysiology of type 2 diabetes.

Regulation of Integrin Functions by N-glycans

Integrins are cell surface transmembrane glycoproteins that function as adhesion receptors in cell-ECM interactions and link matrix proteins to the cytoskeleton. Integrins play an important role in cytoskeleton organization and in the transduction of intracellular signals, regulating various processes such as proliferation, differentiation, apoptosis, and cell migration. Although integrin-mediated adhesion is based on the binding of alpha and beta subunits to a defined peptide sequence, the strength of this binding is modulated by various factors including the status of glycosylation of integrin. Glycosylation reactions are catalyzed by the catalytic action of glycosyltransferases, such as N-acetylglucosaminyltransferase III, V and alpha1,6 fucosyltransferase, etc., which catalyze the formation of glycosidic bonds. In this talk we will briefly overview the N-glycan structures of integrins, such as alpha3beta1 and alpha5beta1, and their related functions arising from recent studies, which provide insight into some long-standing questions concerning N-glycosylation functions.

Crystal structure of mammalian α1,6-fucosyltransferase, FUT8

Mammalian alpha1,6-fucosyltransferase (FUT8) catalyses the transfer of a fucose residue from a donor substrate, guanosine 5'-diphosphate-beta-L-fucose to the reducing terminal N-acetylglucosamine (GlcNAc) of the core structure of an asparagine-linked oligosaccharide. Alpha1,6-fucosylation, also referred to as core fucosylation, plays an essential role in various pathophysiological events. Our group reported that FUT8 null mice showed severe growth retardation and emphysema-like lung-destruction as a result of the dysfunction of epidermal growth factor and transforming growth factor-beta receptors. To elucidate the molecular basis of FUT8 with respect to pathophysiology, the crystal structure of human FUT8 was determined at 2.6 A resolution. The overall structure of FUT8 was found to consist of three domains: an N-terminal coiled-coil domain, a catalytic domain, and a C-terminal SH3 domain. The catalytic region appears to be similar to GT-B glycosyltransferases rather than GT-A. The C-terminal part of the catalytic domain of FUT8 includes a Rossmann fold with three regions that are conserved in alpha1,6-, alpha1,2-, and protein O-fucosyltransferases. The SH3 domain of FUT8 is similar to other SH3 domain-containing proteins, although the significance of this domain remains to be elucidated. The present findings of FUT8 suggest that the conserved residues in the three conserved regions participate in the Rossmann fold and act as the donor binding site, or in catalysis, thus playing key roles in the fucose-transferring reaction.