Glycoprotein biosynthesis in intestinal epithelial cells during differentiation. Incorporation of [14C]mannose from GDP-[14C]mannose into dolichol derivatives

Biological and pathobiological aspects of the glycocalyx of the small intestinal epithelium. A review

The literature on the glycocalyx of small intestinal epithelium is reviewed. The structure, general and barrier functions, synthesis, and degradation of the glycocalyx, and pathobiological aspects of the glycocalyx in relation to its barrier function are mentioned. Topics for future research are indicated.

Treatment of human cells and interferon has no effect on the glycosylation of viral and cellular proteins

The effect of interferon on the glycosylation of viral and cellular proteins was examined in human cells. Vesicular stomatitis virus released from control and interferon-treated HeLa cells was found to be equally glycosylated. Likewise, interferon treatment of RPMI 8226 cells, a human cell line secreting immunoglobulin G, had no effect on protein glycosylation. In addition, treatment with interferon of RPMI 8226 cells and of lymphoblastoid Daudi cells had no effect on the incorporation of N-acetylglucosamine into dolichol derivatives in a cell-free assay. Similar treatment of HeLa and murine L cells showed an apparent inhibition of this glycosyltransferase activity. This effect of interferon in HeLa cells could at least in part by accounted for by a high nucleotide pyrophosphatase activity, which degraded the sugar nucleotide substrate. These results indicate that interferon does not inhibit protein glycosylation in human cells.

Evidence for the stimulation of dolichol and mannolipid synthesis by glucocorticoids in HeLa cells

The effects of addition of 1 microM-dexamethasone on the rate of transfer of mannose from GDP-mannose into mannolipid have been studied in HeLa cell cultures. Concurrent with an increase in incorporation of mannose into glycoproteins, the incorporation of mannose from GDP-mannose in vitro into mannolipid and dolichol-linked oligosaccharides was increased after dexamethasone treatment. Stimulation of mannolipid synthesis showed a correlation with the 11 beta, 17 alpha, 21-trihydroxy structure of C21 steroids. Dexamethasone treatment also resulted in an increased incorporation of acetate into dolichol and dolichyl phosphate. The results suggest that the effect of dexamethasone on the cell-surface glycoprotein accumulation is related to increased sugar-linked dolichol synthesis.

Glycoprotein biosynthesis in animal cells grown in suspension culture. Assembly of lipid-linked saccharides and formation of protein-bound 'high-mannose' oligosaccharides

Glycoprotein biosynthesis was studied with mouse L-cells grown in suspension culture. Glucose-deprived cells incorporated [3H]mannose into 'high-mannose' protein-bound oligosaccharides and a few relatively high-molecular-weight lipid-linked oligosaccharides. The latter were retained by DEAE-cellulose and turned over quite slowly during pulse--chase experiments. Increased heterogeneity in size of lipid-linked oligosaccharides developed during prolonged glucose deprivation. Sequential elongation of lipid-linked oligosaccharides was also observed, and conditions that prevented the assembly of the higher lipid-linked oligosaccharides also prevented the formation of the larger protein-bound 'high-mannose' oligosaccharides. In parallel experiments, [3H]mannose was incorporated into a total polyribosome fraction, suggesting that mannose residues were transferred co-translationally to nascent protein. Membrane preparations from these cells catalysed the assembly from UDP-N-acetyl-D-[6-3H]glucosamine and GDP-D-[U-14C]mannose of polyisoprenyl diphosphate derivatives whose oligosaccharide moieties were heterogeneous in size. Elongation of the N-acetyl-D-[6-3H]glucosamine-initiated glycolipids with mannose residues produced several higher lipid-linked oligosaccharides similar to those seen during glucose deprivation in vivo. Glucosylation of these mannose-containing oligosaccharides from UDP-D-[6-3H]glucose was restricted to those of a relatively high molecular weight. Protein-bound saccharides formed in vitro were mainly smaller in size than those assembled on the lipid acceptors. These results support the involvement of lipid-linked saccharides in the synthesis of asparagine-linked glycoproteins, but show both in vivo and in vitro that protein-bound 'high-mannose' oligosaccharide formation can occur independently of higher lipid-linked oligosaccharide synthesis.