Cell-density-dependent changes in cell-surface glycopeptides and in adhesion of cultured intestinal epithelial cells

Human small-intestinal apolipoprotein B-48 oligosaccharide chains

Hepatic apolipoprotein (apo) B-100 isolated from human plasma is known to contain N-linked oligosaccharides of high-mannose-type and complex-type structures. Sequencing data have revealed that apo B-48 of small-intestinal origin, which represents about 48% of apo B-100 polypeptide from the N-terminus, possesses six potential sites for N-linked oligosaccharides, of which five are likely to be glycosylated. The characterization of the carbohydrate moiety of apo B-48 is the focus of this study. Apo B-48 was labelled with L-[35S]methionine and D-[3H]glucosamine in organ culture of human small-intestinal explants. N-Glycanase treatment resulted in loss of radioactivity from D-[3H]glucosamine-labelled but not L-[35S]methionine-labelled apo B-48 secreted into the medium, and caused no distinct change in mobility of apo B-48 upon electrophoresis on 5% polyacrylamide gel. Analysis of monosaccharide content revealed the presence of 16.8, 17.8, 13.4, 3.4, 2.4 and 2.3 residues of N-acetylglucosamine, mannose, galactose, fucose, xylose and N-acetylgalactosamine respectively. Small-intestinal apo B-48 from human lymph chylomicrons bound to [14C]concanavalin A, and the binding could be inhibited with methyl alpha-D-mannoside. In addition, wheat-germ, peanut, Limulus, soya-bean and Ulex lectins bound apo B-48 specifically. To characterize the carbohydrate moiety further, N-linked oligosaccharides were released by N-Glycanase treatment and reduced with NaB3H4. Labelled oligosaccharides were separated on a concanavalin A-Sepharose column. The majority (78%) were biantennary complex-type structures, 16% were high-mannose type and 6% (not retained by the column) most probably represented higher-branched oligosaccharides. These results suggest the presence of one high-mannose-type and four biantennary complex-type oligosaccharides, as well as probable O-linked sugars in apo B-48. By the use of h.p.l.c., exoglycosidase treatments and ion-exchange chromatography, a mixture of high-mannose-type species with predominant Man8GlcNAc2 as well as monosialylated, desialylated and fucosylated forms of complex-type oligosaccharides were detected.

Glycosylation in intestinal epithelium

This chapter reviews the glycosylation reactions in the intestinal epithelium. The intestinal epithelium represents a good model system in which the glycosylation process can be studied. The intestinal epithelium is composed of two basic epithelial cell types: the absorptive enterocyte and the mucus-producing goblet cell. Gastrointestinal epithelial renewal ensues through the processes of cell proliferation, migration, and differentiation. This renewal occurs in discrete proliferative zones along the gastrointestinal tract. In the small intestine, this proliferative zone is restricted to the base of the crypts, whereas in the large intestine it is less restrictive, occurring in the basal two thirds of the crypt. A longitudinal section along the crypt-to-surface axis, cells in various degrees of differentiation is observed, providing a unique in vivo system in which to investigate differentiation-related glycosylation events. The glycoconjugate repertoire displayed by a given cell reflects its endogenous expression of glycosyltransferases. The role played by terminal oligosaccharide structures in cell–cell recognition phenomena and the expression of glycosyltransferases occupy a key position in the post-translational processing of glycoconjugates and thus influence cellular function.

Processing of asparagine-linked oligosaccharides is an early biochemical marker of the enterocytic differentiation of HT-29 cells

The inability of HT-29 cells to undergo an enterocytic differentiation when grown in a glucose-containing (Glc+) medium has been recently correlated to an overall impairment of N-glycan processing. These results were obtained using confluent HT-29 cells in which the differentiation characteristics are fully expressed under differentiation permissive conditions (glucose-deprived medium, Glc-). Whether these changes of N-glycan processing appear progressively during the cell growth or are already present from the beginning of the culture was investigated in this work by comparing the actual status of N-glycan processing in both exponentially growing Glc+ and Glc- HT-29 cells. Under these conditions, HT-29 cells do not express any characteristics of enterocytic differentiation, even when grown in differentiation permissive conditions. We show here that the conversion of high-mannose to complex glycoproteins is, however, severely reduced in HT-29 cells grown in differentiation non-permissive conditions (HT-29 Glc+) whatever the phase of growth studied. In contrast, HT-29 cells grown in differentiation permissive conditions (HT-29 Glc-) display a normal pattern of N-glycan processing in both the exponential and the stationary phase of growth. We also show that both growing and confluent HT-29 Glc+ cells accumulate Man GlcNAc2 species, thus suggesting that there is an important regulatory point at this level. We therefore conclude that the N-glycan processing may be used as an early biochemical probe for the enterocytic differentiation of HT-29 cells. Whether these early changes result from an early metabolic regulation or are the consequence of a genetic control remains to be studied.

Biosynthesis, in calf pancreas microsomes, of three lipid-linked oligosaccharide diphosphates from a synthetic dolichyl diphosphate tetrasaccharide

Incubation of calf pancreas microsomes with synthetic alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-beta-D-GlcpNAc-(1 ----4)-alpha-D- GlcpNAc-PP-Dol and GDP-D-[14C]-mannose gave three major lipid-linked oligosaccharide diphosphates. After release of the phospholipid residue by mild acid hydrolysis, the corresponding [14C]oligosaccharides were analyzed by gel-filtration, liquid chromatography, degradation by endo-N-acetyl-beta-D-glucosaminidases D and H, by jack bean alpha-D-mannosidase and Aspergillus oryzae (1----2)-alpha-D-mannosidase, acetolysis, and binding to concanavalin A-Sepharose. From the results it could be inferred that the following reaction took place in calf pancreas microsomes: alpha-D-Manp-(1----6)-beta-D-Manp-(1----4)-beta-D-GlcpNAc-(1 ----4)-alpha-D- GlcpNAc-PP-Dol + GDP-D-Man gave GDP + alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)- beta-D-GlcpNAc-(1----4)-alpha-D-GlcpNAc-PP-Dol. The next products to be formed were alpha-D-Manp-(1----2)-alpha-D-Manp-(1----3)-[alpha-D-Manp -(1----6)]-beta-D- Manp-(1----4)-beta-D-GlcpNAc-(1----4)-alpha-D-GlcpNAc-PP-Dol, followed by alpha-D-Manp-(1----2)-alpha-D-Manp-(1----2)-alpha-D-Manp+ ++-(1----3)- [alpha-D-Manp-(1----6)]-beta-D-Manp-(1----4)-beta-D-GlcpNAc- (1----4)-alpha- D-GlcpNAc-PP-Dol. The mannose incorporation was enhanced by Triton X-100 and inhibited by Mn2+, and it occurred in the presence of either Mg2+ or EDTA. It is likely that the mannose donor was GDP-mannose since, under the conditions used, the formation of dolichyl mannosyl phosphate was negligible and the dolichyl heptasaccharide diphosphate accumulated.

Postnatal changes in N-linked oligosaccharides of glycoproteins in rat liver

[3H]Mannose-labelled glycopeptides in the slices of livers from neonatal and 1-, 2-, 3- and 5-week-old rats were characterized by column chromatographies on Sephadex G-50 and concanavalin A-Sepharose and by endo-beta-N-acetylglucosaminidase H digestion. The proportion of complex-type glycopeptides was increased with time until 2 weeks post partum and then returned to the neonatal level. This was mainly due to the increased proportion of concanavalin A-bound (biantennary) species. These changes were accompanied by consistent changes in the activities of processing enzymes in liver microsomal fraction, especially of N-acetylglucosaminyltransferase I. Complex-type glycopeptides from neonatal and 2- and 5-week-old rat livers were further characterized by column chromatographies on Bio-Gel P-6 and DE 52 DEAE-cellulose in combination with neuraminidase digestion. No significant difference was found between concanavalin A-bound species from neonatal liver and those from liver 5 weeks post partum, most of which were sialylated. Concanavalin A-bound species 2 weeks post partum were comparatively smaller in size and less sialylated. On the other hand, there was no significant difference among concanavalin A-unbound species from the three different sources, most of which were sialylated. Since glycoproteins from regenerating rat liver also contain a higher proportion of complex-type oligosaccharides, as previously reported, such changes in N-linked oligosaccharides of glycoproteins may be related to control of the growth of liver cells.

Effects of tunicamycin, N-methyl-1-deoxynojirimycin, and manno-1-deoxynojirimycin on the biosynthesis of lactosaminoglycans in F9 teratocarcinoma cells

F9 teratocarcinoma cells were incubated with D-[2-3H]mannose or D-[6-3H]galactose, and the labeled glycopeptides obtained after exhaustive digestion by pronase were fractionated on Bio-Gel P-6 before and after treatment by endo-beta-N-acetylglucosaminidase H. Tunicamycin almost completely inhibited the synthesis of lactosaminoglycans found in excluded glycopeptides of large molecular weight. Manno-1-deoxynojirimycin greatly inhibited the incorporation of labeled mannose into both lactosaminoglycan and complex oligosaccharides, while it greatly increased that into Man8GlcNAc and Man9GlcNAc oligosaccharides. In contrast, N-methyl-1-deoxynojirimycin only partially inhibited the incorporation into lactosaminoglycan and complex oligosaccharides, and caused the accumulation of Glc3Man7-9GlcNAc oligosaccharides. These results demonstrate that, in these cells, lactosaminoglycans are N-linked, and suggest that there is transfer of both glucosylated and nonglucosylated oligosaccharides from lipid to protein.

Synthetic activities of mass cultures and clones of human gingival fibroblasts

The percentage of synthesis dedicated to collagen is elevated in low-density cultures of human gingival fibroblasts, as is per-cell total protein synthetic activity and glycosaminoglycan accumulation. These observations can be explained, in part, by a decrease in membrane transport of precursor substance in high-density cultures. Synthetic activity by human fibroblasts can be reliably assayed in vitro using as few as 500 cells sparsely seeded. Such low-cell number assay is essential for study of single-cell clones, where replicative life span is limited.

Castanospermine inhibits glucosidase I and glycoprotein secretion in human hepatoma cells

We studied the effect of the plant alkaloid castanospermine on the biosynthesis and secretion of human hepatoma glycoproteins. The HepG-2 cells, grown in the presence or absence of the alkaloid, were labelled with [2-3H]mannose and then the labelled glycopeptides were prepared by Pronase digestion. This material was analysed by gel filtration on Bio-Gel P-4 before and after treatment with endo-beta-N-acetylglucosaminidase H. Castanospermine caused an accumulation of high-mannose oligosaccharides, by 70-75% over control. The major accumulated product, which could also be labelled with [3H]galactose and was only partially susceptible to alpha-mannosidase digestion, was identified by h.p.l.c. as a Glc3Man9GlcNAc. Thus the alkaloid inhibits glucosidase I in the human hepatoma cells. Analysis of total glycoproteins secreted by the cells into the medium revealed the presence of only complex oligosaccharides in both control and treated cultures, and the amount of the oligosaccharides labelled with radioactive mannose, galactose or N-acetylmannosamine, secreted by treated cells, was decreased by about 60%. The rate of secretion of total protein labelled with [35S]methionine and precipitated from the medium with trichloroacetic acid was inhibited by up to 40% in the presence of castanospermine. Pulse-chase studies utilizing [35S]methionine labelling were performed to study the effect of the alkaloid on secretion of individual plasma proteins. Immunoprecipitation at different chase times with monospecific antisera showed that castanospermine markedly decreased the secretion rates of alpha 1-antitrypsin, caeruloplasmin and, to a lesser extent, that of antithrombin-III. Secretions of apolipoprotein E, a glycoprotein containing only O-linked oligosaccharide(s), and albumin, a non-glycosylated protein, were not affected by the drug. It is suggested that castanospermine inhibits secretion of at least some glycoproteins containing N-linked oligosaccharides, owing to the inhibition of oligosaccharide processing.

Alterations in fucosyl oligosaccharides of glycoproteins during rat liver regeneration

[3H]Fucose-labelled glycopeptides in the slices of liver 24h after partial hepatectomy were fractionated on Sephadex G-50. Glycopeptides from regenerating liver contained a higher proportion of lower-Mr components than did controls. Regenerating liver contained a higher proportion of glycopeptides that were bound to concanavalin A-Sepharose and were subsequently eluted with 20mM-methyl alpha-D-glucopyranoside than did controls. Concanavalin A-bound glycopeptides from each source were entirely bound to a lentil lectin-Sepharose column. Both the concanavalin A-bound and -unbound fractions from regenerating liver were indistinguishable from the respective controls by Bio-Gel P6 column chromatography and neuraminidase digestion. These results show that fucosyl glycopeptides from regenerating liver contain a higher proportion of biantennary species with core fucose residues than do controls. Glycopeptides from regenerating livers 12h, 72h and 144h after partial hepatectomy were also examined; however, the difference was not significant. These observations suggest that the alterations in fucosyl glycopeptides may be related to rapid growth of hepatocytes 24h after partial hepatectomy. No significant difference was found in either [3H]mannose- or [3H]fucose-labelled glycoproteins from regenerating liver and from controls by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, suggesting that the alteration in glycopeptides should depend on some differences in the late stage of oligosaccharide processing.

Modification of the N-linked oligosaccharides in cell surface glycoproteins during chick embryo development. A using lectin affinity and a high resolution chromatography study

Important differences in asparagine-linked glycopeptides were observed in vitro cultured fibroblasts derived from chick embryo at different stages of development. Cells from 8-day and 16-day embryos were labeled metabolically with [3H]mannose. Cell surface glycopeptides obtained after mild trypsin treatment were extensively digested with pronase and then chromatographed on concanavalin-A-Sepharose and other immobilized lectins. The most important changes concerned the complex type chains. The ratio between triantennary plus tetraantennary and biantennary chains increased about 2.5-fold from the 8th to the 16th day of development. In the same way, complex chains with bisecting N-acetylglucosamine increased from 8-day to 16-day cells as shown by Phaseolus-vulgaris-erythroagglutinin--agarose chromatography. In 16-day cells, the majority of triantennary chains (60%) with alpha-linked mannose substituted at C2 and C6 positions and biantennary chains (50%) were shown to contain fucosyl (alpha 1----6)N-acetylglucosaminyl structure in the core region by their ability to bind to a lentil lectin affinity column. Similarly, in 8-day cells, triantennary chains (50%) were more fucosylated than biantennary chains (35%). Thus, complex structures exhibited an increased fucosylation of their invariable core from the 8th to the 16th day of development, except for fucosylated triantennary chains which were retained on Phaseolus vulgaris Leucoagglutin and on lentil lectin. These latter structures were present at the surface of 8-day cells and absent at the surface of 16-day cells. After chromatography on Bio-Gel P6 and treatment with endo-beta-N-acetylglucosaminidase H, the [3H]-mannose-labeled glycopeptides were separated by high resolution chromatography into glycopeptides with complex chains and glycopeptides with high-mannose chains. Analysis of the high-mannose oligosaccharides released after endo-beta-N-acetylglucosaminidase H treatment by chromatography on Bio-Gel P4 indicated that the same type of high-mannose chains were present at the surface of 8-day and 16-day cells. Quantification of mannose, galactose and sialic acid residues using gas liquid chromatography was consistent with a decrease of the relative amount of oligomannose chains and an increase of the relative amount of complex type chains in 16-day cells compared to 8-day cells. Thus N-linked oligosaccharides derived from cell surface glycoproteins undergo changes during embryo development resulting in greater complexity of carbohydrate chains.

Comparison between 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin as inhibitors of oligosaccharide processing in intestinal epithelial cells

The alpha-glucosidase inhibitor N-methyl-1-deoxynojirimycin (MDJN) inhibits the synthesis of N-linked complex oligosaccharides in rat intestinal epithelial cells to the same extent as reported previously for 1-deoxynojirimycin (DJN) [Saunier, Kilker, Tkacz, Quaroni & Herscovics (1982) J. Biol. Chem. 257, 14155-14161]. Analysis of each of the endo-beta-N-acetylglucosaminidase H (endo H)-sensitive oligosaccharides separated by h.p.l.c. with yeast glucosidase I, which specifically removes the terminal glucose residue from oligosaccharides containing three glucose residues, and with jack-bean (Canavalia ensiformis) alpha-mannosidase, indicates that both inhibitors cause the accumulation of a mixture of glucosylated oligosaccharides containing one to three glucose residues and seven to nine, and even possibly six, mannose residues. About 70% of the endo H-sensitive oligosaccharides formed in the presence of MDJN contain three glucose residues, compared with only about 20% of the corresponding oligosaccharides of the DJN treated cells. It is concluded that both compounds inhibit the formation of N-linked complex oligosaccharides by interfering with the processing glucosidases. These compounds are valuable in the study of the role of oligosaccharides in glycoproteins.

Identification of asparagine-linked oligosaccharides involved in tumor cell adhesion to laminin and type IV collagen

MDW4, a wheat germ agglutinin-resistant nonmetastatic mutant of the highly metastatic murine tumor cell line called MDAY-D2 has previously been shown to attach to fibronectin and type IV collagen, whereas MDAY-D2 and phenotypic revertants of MDW4 attached poorly to these substrates. The increased adhesiveness of the mutant cells appeared to be closely related to a lesion in cell surface carbohydrate structures. In an effort to identify the carbohydrates involved in cell attachment, glycopeptides isolated from mutant and wild-type cells as well as from purified glycoproteins were tested for their ability to inhibit the attachment of MDW4 cells to plastic surfaces coated with fibronectin, laminin, or type IV collagen. The addition of mannose-terminating glycopeptide to the adhesion assay inhibited MDW4 cell attachment to type IV collagen. In contrast, a sialylated poly N-acetyllactosamine-containing glycopeptide, isolated from wheat germ agglutinin-sensitive MDAY-D2 cells but absent in MDW4 cells, inhibited MDW4 attachment to laminin. None of the glycopeptides used in this study inhibited attachment of MDW4 cells to fibronectin-coated plastic. Peptide N-glycosidase treatment of the cells to remove surface asparagine-linked oligosaccharides inhibited MDW4 adhesion to type IV collagen, but not to laminin, and the same treatment of the wheat germ agglutinin-sensitive cells enhanced attachment to laminin. Tumor cell attachment to, and detachment from, the sublaminal matrix protein laminin and type IV collagen are thought to be important events in the metastatic process. Our results indicate that tumor cell attachment to these proteins may be partially modulated by the expression of specific oligosaccharide structures associated with the cell surface.

Specific internalization of 1,25-dihydroxyvitamin D3 by cultured intestinal epithelial cells

A rapid and sensitive method for evaluating the specific internalization of [3H]1,25-(OH)2-D3 by intact, cultured intestinal epithelial (IEC) cells was developed. The uptake of hormone was determined by incubation of suspensions of IEC cells in medium containing [3H]1,25-(OH)2-D3 and 1% fetal bovine serum. Saturation of the high affinity intracellular receptor with 1,25-(OH)2-D3 was reflected in an analysis of the saturable uptake of [3H]1,25-(OH)2-D3; the apparent dissociation constant for labelled hormone was 0.64 nM which is in the range of that reported for hormone binding to receptor extracted from known target tissues. The uptake of [3H]1,25-(OH)2-D3 was dependent on both the cellular content of receptor and the availability of unbound hormone in the extracellular medium. The latter phenomenon was demonstrated in competitive internalization studies with 25-OH-D3 and 24,25-(OH)2-D3. At extracellular concentrations of 25-OH-D3 and 24,25-(OH)2-D3 ranging from 10(-8) to 10(-7) M there was competitive inhibition of [3H]1,25-(OH)2-D3, uptake by IEC cells. However, when cells were exposed to a higher concentration (10(-6) M) of these metabolites, label uptake paradoxically increased owing to probable saturation of the sterol binding sites on the vitamin D binding protein present in the serum-supplemented incubation medium.

Alterations in N-linked oligosaccharides of glycoproteins during rat liver regeneration

[3H]Mannose-labeled glycopeptides in the slices after partial hepatectomy were characterized by column chromatography using Sephadex G-50, DE-52 and Con A-Sepharose, and further by digestion with alpha-mannosidase and endo-beta-N-acetylglucosaminidase H. They contained both 'complex type' and 'high-mannose type' oligosaccharides. A higher proportion of 'complex type' oligosaccharides was contained in regenerating liver 24 h after partial hepatectomy than in control. This tendency was increased gradually with time and was most pronounced at 144 h. In our previous studies, the activities of microsomal N-acetylglucosaminyltransferase towards endogenous and exogenous acceptors at 144 h after partial hepatectomy were shown to exceed most prominently that in control. No differences in the oligosaccharides were observed at 240 h when the deficit of liver had been restored. The oligosaccharides of glycopeptides in the incubation media were mostly 'complex type' and the differences between regenerating liver and control were observed only at 144 h. These results suggest that oligosaccharide processing of glycoproteins is regulated at the transfer step of peripheral N-acetylglucosamine to core oligosaccharides 144 h after partial hepatectomy, and that these alterations in oligosaccharides of glycoproteins may be related to hypertrophy and hyperplasia of hepatic cells in liver regeneration.

Interaction of bovine epithelial lens (BEL) cells with extracellular matrix (ECM) and eye-derived growth factor (EDGF). I. Effects on short-term adhesiveness and on long-term organization of the culture

A growth factor (EDGF) derived from the retina controls the proliferation and shape of adult bovine epithelial lens (BEL) cells in vitro as well as extracellular matrix (ECM) assembly. In order to analyse this mechanism and the specificity of the interactions between BEL cells and the extracellular matrix we have investigated the adhesion and growth of BEL cells on various substrata (fibronectin, laminin, ECM). BEL cells treated with EDGF adhered more slowly to plastic Petri dishes than untreated cells, in part due to EDGF inhibition of fibronectin deposition. The untreated BEL cells spread less well on ECM or laminin than on fibronectin-coated plastic. The preferential adhesiveness of BEL cells on fibronectin vs laminin was confirmed by attachment experiments performed on replicas of SDS-PAGE of these proteins. However, in long-term cultures, 8 days after seeding, BEL cells were very differently arranged on plastic or on ECM. ECM by itself did not increase the proliferation rate but helped to restore an organized cell monolayer. BEL cells stimulated to grow on ECM by treatment with EDGF exhibited at least transiently contact inhibition producing a perfectly organized epithelium similar to the one observed in vivo. These results suggest specific interactions between ECM or ECM components with BEL cell that restrain excessive cell spreading and restore an original polarized phenotype of the cells seen in vivo.

Regulation of low-density lipoprotein metabolism by cell density and proliferative state

Stimulation of the proliferation of human skin fibroblasts by platelet-derived growth factor increased the binding and degradation of low-density lipoproteins at cell densities of 2000-30,000 cells/cm2. Binding and degradation of low-density lipoprotein was an inverse function of cell density in both proliferating and quiescent cells, indicating that the effect of cell density on the LDL receptor has proliferation-dependent and proliferation-independent components. The effect of medium conditioned by confluent fibroblasts on LDL metabolism was tested to determine if the effects of cell density on LDL metabolism might be mediated by cellular secretion products. Fibroblast-conditioned medium increased LDL metabolism, suggesting secretion products do not mediate these effects of cell density. These data indicate that regulation of the low-density lipoprotein receptor is not a simple on/off response to growth stimulation, but is responsive to extracellular cues such as cell density.

Effects of bleomycin and methylprednisolone on the biosynthesis of oligosaccharide-lipids and glycoproteins in lung

Tissues from rabbit lung were found to incorporate radioactivity from [3H]mannose into the oligosaccharide of a polar lipid soluble in chloroform/methanol/water (10:10:3, by vol.). Only one oligosaccharide-lipid was formed and the composition of the radiolabelled carbohydrate moiety was Glc3Man9(GlcNAc)2. An antitumour antibiotic, bleomycin, and an anti-inflammatory steroid, methylprednisolone, partially inhibited the incorporation of [3H]mannose into oligosaccharide-lipids and in addition resulted in the production of two new components, Man5(GlcNAc)2 and Man2(GlcNAc)2. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of the glycoproteins synthesized from the oligosaccharide-lipid intermediates showed labelled components with Mr greater than 200000, 130000, 80000 and 62000. The addition of the drugs resulted in the same pattern with the addition of a new component of Mr 36000. Endoglucosaminidase H-treatment of the glycopeptides isolated from the Pronase-digested glycoproteins indicated that the oligosaccharide units in the glycoproteins were of the high-mannose-type.

Changes in cell-surface fucose-containing glycopeptides and adhesion of cultured intestinal epithelial cells as a function of cell density

Confluent cultured intestinal epithelial cells displayed greater adhesion to the substratum than did subconfluent cells. Subconfluent and confluent cells were labelled with [3H]fucose for 24h and the cell-surface components were released by mild Pronase treatment. After extensive Pronase digestion, cell-surface and cell-residue glycopeptides were fractionated on Bio-Gel P-6. The cell surface contained a higher proportion of lower-molecular-weight glycopeptides than the residue. No significant difference in elution pattern was found between total cell-surface glycopeptides of subconfluent and confluent cells. However, confluent cells contained almost twice as much [3H]-fucose-labelled glycopeptides that were bound to concanavalin A-Sepharose and were subsequently eluted with 20mM-methyl alpha-D-glucopyranoside as subconfluent cells. When the bound glycopeptides were chromatographed on Bio-Gel P-6, it was found that confluent cells contained a larger proportion of lower-molecular-weight glycopeptides than subconfluent cells. This difference in size was eliminated after treatment of glycopeptides with sialidase. When growth of subconfluent cells was inhibited with a non-toxic concentration of retinoic acid, no significant effect on the elution pattern of [3H]fucose-labelled glycopeptides was observed on either Bio-Gel P-6 or concanavalin A-Sepharose. No significant difference was found in the total [3H]fucose-labelled glycoproteins from subconfluent and confluent cells by two-dimensional gel electrophoresis. It is suggested that the differences in [3H]fucose-labelled glycopeptides between subconfluent and confluent cells are cell-density-dependent rather than growth-dependent, and that these differences are likely to result from some changes in glycosylation mechanism(s). Furthermore, the differences in cell-surface glycopeptides may be related to the changes in the adhesion of the cells to the substratum.

Alterations in glycopeptide and glycosaminoglycan biosynthesis in bovine lens epithelial cells cultured in the presence of an eye-derived growth factor

D-[3H]glucosamine-labeled glycosaminoglycans and glycopeptides, obtained by Pronase digestion from bovine lens epithelial cells grown in the presence and in the absence of an eye-derived growth factor, were investigated comparatively in the extra- and pericellular compartments of 16-17th subcultures. Glycosaminoglycans and glycopeptides were separated on Ultrogel AcA 202 column and the retained glycopeptides were further separated by the successive application of affinity, ion-exchange and exclusion chromatographic methods. The following results were obtained: (1) the proportion of the label increased in hyaluronic acid and decreased in the glycopeptide fraction on stimulation with the growth factor: (2) the comparison of the chromatographic elution profiles of the glycopeptides from control and stimulated cells revealed that differences between the two cell groups were largely quantitative. An increase in the high molecular weight glycopeptides, as it was found in transformed cells, could not be demonstrated in eye-derived growth factor-treated cells.

Studies on the stimulation of dolichol-mediated glycosylation by dexamethasone in HeLa cells

The effect of the addition of 1 microM-dexamethasone on the temporal sequence of the glycosylation process has been studied in HeLa S3G cells. In the presence of delipidized serum, dexamethasone caused an increase in 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and an accelerated synthesis of dolichols. These events were followed by an increase in the transfer of mannose from GDP-mannose to mannolipid. An increase in the rate of synthesis of lipid-linked oligosaccharides and a stimulation of glycosylation were observed in cells grown in the presence of delipidized serum in the culture medium. The data are consistent with the view that cellular syntheses of lipids and glycoproteins are co-ordinately controlled.