Comparative study of the levels of sialyltransferases responsible for the formation of sugar chains in glycoproteins and gangliosides in rat liver and hepatomas

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.

Clinicopathologic evaluation of CDw75 antigen expression in colorectal adenocarcinomas

CDw75, a B lymphocyte surface antigen, is a sialylated carbohydrate epitope, which is generated by the enzyme beta galactosyl alpha 2,6 sialyltransferase (Sia-T1). In colon carcinomas, although higher levels of Sia-T1 has been described and found to be correlated with metastatic potential of tumor cells, the expression of CDw75 antigen still remains unknown. To address this issue, we investigated immunohistochemically CDw75 antigen expression in 195 colorectal adenocarcinomas and their nodal metastases. The correlation between CDw75 antigen expression with selected clinicopathologic variables was analyzed by using Chi-square and Fisher s exact tests. Positive staining was observed in 101 cases. Non-neoplastic mucosa was negative consistently. The frequency of positivity was decreased according to the degree of differentiation (p<0.001). Antigen expression was found to be associated with deeper penetration (p<0.006), positive lymph nodes (p<0.001), distant metastases (p<0.006) and advanced stage (p<0.001). Same relationships were detected in well and moderately differentiated tumors when CDw75 immunoreactivity was evaluated in each histologic grade separately. Our findings indicate that CDw75 antigen expression may be a good indicator of the biological aggressiveness of colorectal adenocarcinomas especially in tumors with well and moderately differentiated morphology.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Beta-galactoside alpha2,6 sialyltransferase in human colon cancer: contribution of multiple transcripts to regulation of enzyme activity and reactivity with Sambucus nigra agglutinin

Colon cancer tissues display an increased activity of beta-galactoside alpha2,6 sialyltransferase (ST6Gal.I) and an increased reactivity with the lectin from Sambucus nigra (SNA), specific for alpha2,6-sialyl-linkages. Experimental and clinical studies indicate a contribution of these alterations to tumor progression, but their molecular bases are largely unknown. In many tissues, ST6Gal.I is transcriptionally regulated through the usage of different promoters that originate mRNAs diverging in the 5;-untranslated regions. RT-PCR analysis of 14 carcinoma samples, all expressing an increased ST6Gal.I enzyme activity, and of the corresponding normal mucosa revealed the presence of at least 2 transcripts. One, containing the 5;-untranslated exons, Y+Z, is thought to represent the "housekeeping" expression, and another previously described in hepatic tissues. Both the Y+Z and the hepatic transcripts were detectable in normal and cancer tissues but that latter form had a marked tendency to accumulate in cancer. The extent of alpha2,6-sialylation of glycoconjugates, as determined by SNA-dot blot analysis, was markedly enhanced in all cancer specimens, but the level of reactivity only partially correlated with the level of enzyme expression. Western blot analysis revealed a strikingly heterogeneous pattern of SNA reactivity among cancer tissues. These data indicate that: i) during neoplastic transformation of colonic cells, ST6Gal.I expression may be modulated through a differential promoter usage; ii) the extent of alpha2,6-sialylation of cancer cell membranes is not a direct function of the ST6Gal.I activity, strongly suggesting the existence of other, more complex mechanisms of regulation.

Gangliosides inhibit PDGF-induced signal transduction events in U-1242 MG human glioma cells

In this study we investigated the responses of intracellular calcium ([Ca2+]i) and protein kinase C (PKC) to PDGF in U-1242 MG cells. PDGF-BB stimulated [3H]PDBu binding approximately 2-3 fold. This response was inhibited by preincubating the cells with an inhibitor of phospholipase C (PLC), U73122, suggesting that PLC mediates the induction of PKC translocation by PDGF. PDGF also increased the concentration of [Ca2+]i that was attenuated in a calcium-free medium. This indicates that PDGF-induced elevation of [Ca2+]i is mainly due to influx of extracellular calcium. PDGF-stimulated translocation of PKC was inhibited by the intracellular calcium buffer BAPTA/AM. All gangliosides studied except GM3 inhibited these responses with similar efficacy. Collectively, these results indicate that the signal transduction pathway initiated by PDGF leading to PKC translocation in U-1242 MG cells is intact, and this pathway is inhibited by several gangliosides.

Metastatic potential of transformed rat 3Y1 cell lines is inversely correlated with lysosomal-type sialidase activity

We have investigated sialidase activities in transformed rat 3Y1 cells of different metastatic potential. Only lysosome-type sialidase was apparent in the particulate fractions of 3Y1 cells and their transformants. As compared with control 3Y1 cells, src-transformed cells exhibited decreased sialidase activity, and v-fos transfer to these latter induced even more severe decrease in the sialidase activity with acquisition of high lung metastatic ability. Various lysosomal enzymes other than sialidase were hardly affected by the transformation. Sialic acid transfer to N-linked glycoproteins was slightly elevated in the transformants, but not in parallel with their metastatic potential.

Different sialyltransferase activities in human colorectal carcinoma cells from surgical specimens detected by specific glycoprotein and glycolipid acceptors

The amount and type of sialylation of tumor cell membranes depends on the activity of a number of different sialyltransferase enzymes. For the detection of specific activities in human colorectal carcinoma tissue several glycoprotein and glycolipid acceptors were used: desialylated fetuin, alpha 1-acid glycoprotein, beta 2-glycoprotein I, ovine submaxillaris mucin, and the gangliosides GM1, GM2, GM3 and GD1a. Because of their possible relevance for metastasis, precursors of Le(a) and Le(x) antigens, too, were employed, namely neoglycolipids produced by coupling LcOse4 or NeoLcOse4 oligosaccharides to L-alpha-phosphatidyl-ethanol-amine-dipalmitoyl. Our data indicate that human colorectal tumor tissue contains two highly active sialyltransferase enzymes, which are only weakly expressed in normal mucosa. These are a N-glycan-specific alpha 2,6-sialyltransferase, which was significantly increased in metastasizing tumors, and a Gal beta 1,3Gal-NAc-specific sialyltransferase, which was increased in tumors of early stages. A shift to enhanced alpha 2,6-sialylation of membrane glycoproteins during carcinogenesis was demonstrated by lectin ELISA analysis of magneto-bead separated tumor cells. Quantitative determination of specific sialyltransferase activities may be a sensitive tool for detection and monitoring of colon carcinoma.

Gangliosides inhibit platelet-derived growth factor-stimulated receptor dimerization in human glioma U-1242MG and Swiss 3T3 cells

We previously showed that gangliosides inhibit DNA synthesis in Swiss 3T3 cells stimulated with platelet-derived growth factor (PDGF) in a dose-responsive manner. This correlated with the inhibitory effects of several gangliosides (except GM3) on tyrosine phosphorylation of the PDGF receptor (PDGFR). [35S]Methionine-labeled Swiss 3T3 cells were incubated either with or without gangliosides and stimulated with PDGF, and proteins were cross-linked with bis(sulfosuccinimidyl) suberate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that two protein bands (170 and 350 kDa) were specifically immunoprecipitated with an anti-PDGFR antibody. Using both Swiss 3T3 and human glioma U-1242MG cells, western blots with anti-PDGFR and anti-phosphotyrosine antibodies confirmed that these bands were the PDGFR monomer and dimer, respectively, and that phosphotyrosine was present in these bands only after cells were stimulated with PDGF. Of the gangliosides tested, GM1, GM2, GD1a, GD1b, GD3, and GT1b, but not GM3, inhibited the formation of the 350-kDa band. These results demonstrate that all gangliosides tested, except GM3, probably inhibit PDGF-mediated growth by preventing dimerization of PDGFR monomers. Loss of more complex gangliosides in human gliomas would permit unregulated activation of the PDGFR, contributing to uncontrolled growth stimulation. We propose that ganglioside inhibition of receptor dimerization is a novel mechanism for regulating and coordinating several trophic factor-mediated cell functions.

Postnatal changes in sialylation of glycoproteins in rat liver

Glycoproteins containing N-linked oligosaccharides were prepared from plasma and liver microsomes of rats aged 0-5 weeks, and galactose and sialic acid content were determined. The sialic acid/galactose ratios in plasma membrane N-glycans remained at about 1 throughout the postnatal period, suggesting that most of the galactose residues are sialylated. In the same way, it was suggested that most of the galactose residues of microsomal N-glycans were sialylated at 0, 4 and 5 weeks of age, but that the degree of sialylation was lower at the other ages, with a minimum at 2 weeks. When the activities of sialyltransferase and galactosyltransferase in liver Golgi membranes were determined, age-dependent changes were found, not only in the specific activities of the enzymes, but also in the Golgi membrane content per g of liver. The activity of galactosyltransferase per g of liver increased immediately after birth, whereas that of sialyltransferase remained at a low level for 2 weeks and then increased to a constant level at 4 weeks. It is probable that this delayed increase in the activity of sialyltransferase results in the decreased sialylation of microsomal N-glycans at 1, 2 and 3 weeks. Sialyltransferase was solubilized from the liver microsomes of rats aged 2, 3 and 4 weeks and characterized. Phosphocellulose column chromatography separated the activity into two subfractions, designated transferase I and transferase II in the order of elution. The increase in total sialyltransferase activity during this period was caused mainly by an increase in transferase I. Rechromatography of each transferase from 3-week-old rats after neuraminidase treatment showed that transferase I but not transferase II contained sialic acid residue(s) and that desialylated transferase I was eluted in a similar way as transferase II. Although the apparent Km value for CMP-N-acetylneuraminic acid and the heat stability of transferase I were different from those of transferase II, the difference was abolished by treating transferase I with neuraminidase, suggesting that transferase II may be a desialylated form of transferase I. These changes in the sialylation of membrane glycoproteins, including sialyltransferase, may be related to the control of liver growth during postnatal development.

Tumor-promoting phorbol ester induces alterations of sialidase and sialyltransferase activities of JB6 cells

Sialidase and sialyltransferase activities were studied in JB6 mouse epidermal cells before and after exposure to phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), which irreversibly induces anchorage-independent growth and tumorigenicity. JB6 cells exhibited sialidase activities toward 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (4MU-NeuAc) and gangliosides at pH 4.5 in the particulate fraction but apparently not in the cytosol at pH 4.5 or 6.0. In JB6 cells exposed to TPA and in the anchorage-independent transformants, the sialidase activity toward 4MU-NeuAc was decreased and the activity toward gangliosides was increased compared with those in untreated JB6 cells. Immunological analysis with antisera against membrane-associated sialidases I and II revealed that plasma membrane-associated sialidase I was increased and lysosomal membrane-associated sialidase II was decreased under these conditions. TPA treatment also affected the sialyltransferase activities of JB6 cells: and elevation of the transfer activities toward asialo-orosomucoid and asialo-porcine submaxillary mucin but a reduction of GM3 and GD3 synthase activities were observed on exposure to TPA and in cells transformed by TPA to retain anchorage-independency. These results suggest that an increase in sialic acid bound to glycoproteins and a decrease in that bound to glycolipids may occur in JB6 cells exposed to TPA and in the anchorage-independent transformants.

Hexadecanoic and neuraminic acid incorporations in two rat colon carcinoma cell lipids: selective influence of 1-O-octadecyl 2-O-methyl-3-phosphocholine on glycerolipid and ganglioside biosynthesis

[3H] hexadecanoic and N-acetyl [14C] neuraminic acids were incorporated in glycerolipids or gangliosides of 2 rat colon carcinoma cell lines, having (PRO cells), or not (REG cells) invasive capacities when inoculated in syngeneic BD IX rats. The cells were cultured (48 h) in presence of 1-0-octadecyl-2-0-methyl-3-phosphocholine (ET 18-0-CH3) 20 or 40 microM, which, on transformed cells, inhibits the cell growth, modifies the glycerolipid biosynthesis, and activates the sialyltransferases. ET 18-0-CH3 20 microM activated, in PRO and in REG cells the incorporation of [3H] hexadecanoate in monosialogangliosides (1.45 fold compared to controls), but not in disialogangliosides and the distribution of this fatty acid between monosialo- (82%) and disialogangliosides (18%) was unchanged with controls. After [14C] neuraminic acid labelings, and for control experiments, the total radioactivities in gangliosides, in PRO cells, were twice higher than in REG cells, a difference which, probably, reflects the ganglioside content. ET 18-0-CH3 20 microM did not increase the incorporation of the [14C] neuraminic acid in PRO and in REG cells, and did not change its distribution between monosialo (70-80% for controls and experiments with ET 18-0-CH3) and disialogangliosides (20-30%). Similar results were obtained with ET 18-0-CH3 40 microM for the distribution of [14C] neuraminic acid in monosialo- and disialogangliosides. Whatever the precursor, the trisialogangliosides were never radiolabeled. Analysis of the [3H] glycerolipids (the main radiolabeled lipid classes in controls were: phosphatidylcholines, triglycerides, sphingomyelins and phosphatidyl-inositols) revealed that ET 18-0-CH3, compared to controls, did not activate the incorporation of [3H] hexadecanoate in total glycerolipids (PRO or REG cells). It activated (3 fold) its incorporation in triglyerides, inhibited it (0.5-0.6 fold) in phosphatidylcholines, sphingomyelins and phosphatidyl-inoditols and all these most noticeable differences were observed in PRO and in REG cells. These findings reflect the impossibility of ET 18-0-CH3 to activate the sialyltransferases during the ganglioside biosynthesis in colon carcinoma cells, while it modified ceramide, glycerophospholipid and neutral glycerolipid biosynthesis.

Increased CMP-NeuAc:Gal beta 1,4GlcNAc-R alpha 2,6 sialyltransferase activity in human colorectal cancer tissues

The sialyltransferase activities of 10 human colorectal specimens were compared with those of the corresponding adjacent normal mucosa. Using asialofetuin as an acceptor we found, in tumor tissues of 9 out of 10 patients, an increased sialyltransferase activity towards the N-linked chains as determined upon peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase (PNGase) treatment. On the contrary, the activity towards the O-linked chains was not significantly changed. When the specificity of the sialyltransferase acting on N-linked chains was investigated by using N-acetyl-lactosamine (Gal beta 1,4GlcNAc) as an acceptor, we found that the alpha 2,6 sialyltransferase activity expressed by both normal and tumor colorectal tissues was far higher than the alpha 2,3-activity and that alpha 2,6 was the only sialyltransferase activity increased in tumor tissues. Kinetic analysis revealed that normal and tumor alpha 2,6 sialyltransferases have the same apparent Km for the acceptor substrate (469 and 465 microns), but normal enzyme has a higher Km for CMP-NeuAc (303 microns) than the tumor enzyme (50 microns). The higher affinity of tumor enzyme for the nucleotide-sugar might partially explain its increased activity in tumor tissues. In addition, tumor tissues contain a lower amount of sialic acid despite the increase in alpha 2,6 sialyltransferase activity.