Inhibition of myoblast fusion by the glucosidase inhibitor N-methyl-1-deoxynojirimycin, but not by the mannosidase inhibitor 1-deoxymannojirimycin

Human carcinoembryonic antigen, an intercellular adhesion molecule, blocks fusion and differentiation of rat myoblasts

Human carcinoembryonic antigen (CEA), a widely used tumor marker, is a member of a family of cell surface glycoproteins that are overexpressed in many carcinomas. CEA has been shown to function in vitro as a homotypic intercellular adhesion molecule. This correlation of overproduction of an adhesion molecule with neoplastic transformation provoked a test of the effect of CEA on cell differentiation. Using stable CEA transfectants of the rat L6 myoblast cell line as a model system of differentiation, we show that fusion into myotubes and, in fact, the entire molecular program of differentiation, including creatine phosphokinase upregulation, myogenin upregulation, and beta-actin downregulation are completely abrogated by the ectopic expression of CEA. The blocking of the upregulation of myogenin, a transcriptional regulator responsible for the execution of the entire myogenic differentiation program, indicates that CEA expression intercepts the process at a very early stage. The adhesion function of CEA is essential for this effect since an adhesion-defective N domain deletion mutant of CEA was ineffective in blocking fusion and CEA transfectants treated with adhesion-blocking peptides fused normally. Furthermore, CEA transfectants maintain their high division potential, whereas control transfectants lose division potential with differentiation similarly to the parental cell line. Thus the expression of functional CEA on the surface of cells can block terminal differentiation and maintain proliferative potential.

Inhibitors of glycoprotein processing act at an early stage of myogenesis

The glycoprotein processing inhibitors bromoconduritol and N-methyl-1-deoxynojirimycin inhibit myoblast fusion and differentiation, suggesting the critical involvement of one or more glycoproteins in the control of skeletal myogenesis. In the present study we have examined the effect of inhibitors of glycoprotein processing on the expression of the muscle-specific regulatory factor myogenin. Glucosidase inhibitors, but not the mannosidase inhibitor 1-deoxymannojirimycin, inhibited the accumulation of myogenin mRNA in myoblasts, and immunoblotting confirmed that this was reflected in reduced accumulation of myogenin protein. The results indicate that the glycoprotein(s) critically involved in the control of myoblast differentiation act at an early stage in this process by modulating expression of the myogenic regulatory factor myogenin.

Glycosylation, activity and secretion of lipoprotein lipase in cultured brown adipocytes of newborn mice. Effect of tunicamycin, monensin, 1-deoxymannojirimycin and swainsonine

The effect of inhibitors on the glycosylation, activity and secretion of lipoprotein lipase was studied in brown adipocytes cultured from newborn mice. Such cells synthesized and secreted active lipoprotein lipase. It is generally accepted that active lipoprotein lipase is a homodimer. Glycosylation of lipoprotein lipase was analysed by PAGE of endoglycosidase H (endo H)-digested subunits of lipoprotein lipase immunoprecipitated from cells incubated for 1-2 h with [35S]methionine. The most prevalent 35S-labelled lipase subunit (Mr 57,000-58,000) in these cells contained endo H-resistant oligosaccharide chains, the next most prevalent contained totally endo H-sensitive chains, and the least prevalent subunit contained partially endo H-sensitive chains. Complete blocking of the glycosylation of lipoprotein lipase with tunicamycin (1 microgram/ml) for 24 h resulted in synthesis of an inactive non-secretable form of lipase with a smaller subunit (Mr 51,000-52,000). Immunofluorescent studies showed that unglycosylated lipase in tunicamycin-treated cells was retained in the endoplasmic reticulum. Cells treated with 1 microM-monensin, an intra-Golgi transport inhibitor, synthesized an active form of lipase which was not secreted, but was retained in the Golgi. The lipase in monensin-treated cells contained only partially or totally endo H-sensitive chains. Blocking either Golgi mannosidase I with 4 mM-1-deoxymannojirimycin or Golgi mannosidase II with 10 microM-swainsonine resulted in production of a form of lipoprotein lipase which was active and secreted, and which contained only endo H-sensitive chains. Our findings demonstrate that core glycosylation of lipoprotein lipase in the endoplasmic reticulum is required for lipase activity and transport from the reticulum, whereas processing of the oligosaccharide chains to endo H-resistant (complex) type chains in the Golgi is not required for either the activity or the secretion of lipoprotein lipase.

Expression of choline acetyltransferase activity in a co-culture of spinal cord and skeletal muscle cells is inhibited by myogenic differentiation inhibitors

The effect of myogenic differentiation on the expression of choline acetyltransferase (ChAT) activity in co-cultured spinal cord neurons was studied. ChAT activity in spinal cord cells dissociated from 14-day mouse embryos was markedly increased when co-cultured with skeletal myotubes from 20-day embryos. This enhancement of ChAT activity was not observed in the presence of concanavalin A (ConA) or N-methyl-1-deoxynojirimycin (MDJN) which inhibits myoblast fusion, creatine phosphokinase and acetylcholinesterase activities in muscle cells. ChAT activity in spinal cord neurons cultured alone was unaffected by these agents. The inhibitory effect of ConA and MDJN was reversible, with an almost full recovery of ChAT activity following removal of the agents. Addition of ConA or MDJN after myotube formation exerted little inhibitory effect on ChAT activity. The effects of ConA and MDJN on ChAT activity in co-cultures were comparable to those on creatine phosphokinase and acetylcholinesterase. These observations indicate that the neurotrophic effects of skeletal muscle cells on spinal cord neurons are dependent on the differentiation state of the muscle cells.

Inhibition of glycoprotein processing blocks assembly of spicules during development of the sea urchin embryo

Previous studies have implicated an 130-kD glycoprotein containing complex, N-linked oligosaccharide chain(s) in the process of spicule formation in sea urchin embryos. To ascertain whether the processing of high mannose oligosaccharides to complex oligosaccharides is necessary for spiculogenesis, intact embryos and cultures of spicule-forming primary mesenchyme cells were treated with glycoprotein processing inhibitors. In both the embryonic and cell culture systems 1-deoxymannojirimycin (1-MMN) and, to a lesser extent, 1-deoxynojirimycin (1-DNJ) inhibited spicule formation. These inhibitors did not affect gastrulation in whole embryos or filopodial network formation in cell cultures. Swainsonine (SWSN) and castanospermine (CSTP) had no effect in either system. Further analysis revealed the following: (a) 1-MMN entered the embryos and blocked glycoprotein processing in the 24-h period before spicule formation as assessed by a twofold increase in endoglycosidase H sensitivity among newly synthesized glycoproteins upon addition of 1-MMN; (b) 1-MMN did not affect general protein synthesis until after its effects on spicule formation were observed; (c) Immunoblot analysis with an antibody directed towards the polypeptide chain of the 130-kD protein (mAb A3) demonstrated that 1-MMN did not affect the level of the polypeptide that is known to be synthesized just before spicule formation; (d) 1-MMN and 1-DNJ almost completely abolished (greater than 95%) the appearance of mAb 1223 reactive complex oligosaccharide moiety associated with the 130-kD glycoprotein; CSTP and SWSN had much less of an effect on expression of this epitope. These results indicate that the conversion of high mannose oligosaccharides to complex oligosaccharides is required for spiculogenesis in sea urchin embryos and they suggest that the 130-kD protein is one of these essential complex glycoproteins.

Acquisition of a lysosomal enzyme by myoblasts in tissue culture

Skeletal muscle myoblasts from different sources acquired high levels of the lysosomal enzyme beta-glucuronidase, when they were cultured together with mitogen-activated lymphocytes. Immunofluorescent staining, thermal stability, and electrophoretic mobility showed that the increase in enzyme activity in the myoblasts was due to the presence of the lymphocyte form of the enzyme. Although myoblasts were able to take up exogenous beta-glucuronidase from the culture medium by mannose 6-phosphate receptor-mediated endocytosis, enzyme acquisition during co-culture with lymphocytes was independent of this pathway. Enzyme transfer from the lymphocytes was found to require direct cell-cell contact with the muscle cells, and was accompanied by an increase in beta-glucuronidase activity in the lymphocytes themselves. Since this additional activity was also due to the presence of the lymphocyte form of the enzyme, these results indicate that interaction with the muscle cells induced the de novo synthesis of beta-glucuronidase in the lymphocytes.

Studies on the effect of ketoconazole on the fusion of L6 myoblasts

The effect of ketoconazole on the fusion of L6 myoblasts was studied. Ketoconazole was a potent inhibitor of myoblast fusion at concentrations as low as 0.1 microM, but fusion was restored when the inhibitor was removed. The inhibitor resulted in decreased binding of conA and WGA to cell surface oligosaccharides showing that it was inhibiting N-linked cell surface glycoproteins. Inhibition of fusion by ketoconazole was accompanied by reduced creatine phosphokinase activities showing that it is affecting biochemical differentiation. Incorporation of labelled mannose from GDP-mannose into lipid-sugar and lipid-oligosaccharide complexes involved in the synthesis of N-linked oligosaccharides was also inhibited by ketoconazole, but the inhibition was reversed by addition of exogenous dolichol phosphate to the incorporation mixture. The main conclusion from these studies was that ketoconazole inhibited fusion of L6 myoblasts by affecting the synthesis of dolichol-phosphate required for the synthesis of lipid-oligosaccharides needed for the synthesis of fusogenic cell surface N-linked glycoproteins.

Effect of inhibitors of glycoprotein processing on integrin and the adhesion of myoblasts to extracellular matrix proteins

The effect of the glucosidase inhibitors N-methyl-1-deoxynojirimycin (MDJN) and bromoconduritol on the adhesion of chick myoblasts and rat L6 myoblasts to fibronectin and laminin was compared with that of the mannosidase I inhibitor, 1-deoxymannojirimycin (ManDJN). Chick and rat L6 myoblasts treated with glucosidase inhibitors showed impaired binding to fibronectin. Glucosidase inhibitor-treated chick, but not rat L6, myoblasts also showed impaired binding to laminin. In contrast ManDJN had no significant effect on the adhesion of rat or chick cells to either substrate, suggesting that complex oligosaccharides are not required for normal biosynthesis of myoblast fibronectin or laminin receptors. Binding of monoclonal antibody JG22 to glucosidase-inhibitor-treated myoblasts revealed a marked decrease in the number of integrin molecules available at the cell surface. We suggest that the previously reported inhibitory effects of glucosidase inhibitors on the terminal differentiation of myoblasts may be mediated, at least in part, through their effect on integrin accumulation.

Glycoprotein glycans may not be necessary for the differentiation of skeletal myoblasts

Previous work using glycosylation inhibitors has suggested that high-mannose type but not complex type oligosaccharides on the surface of cells may play a role in the differentiation of skeletal myoblasts. Earlier, we had shown that a concanavalin A-resistant mutant derived from an L6 myoblast line fails to differentiate in a medium containing 10% horse serum. Here we show that one such concanavalin A-resistant mutant (D-1) which was reported to have oligosaccharides of the type Man(3-5)G1cNAc2, shows significant fusion ability when grown in media containing 1% horse serum. Lowering the serum concentration did not alter the dolichol-phosphate mannosyltransferase activity in D-1 which remained at low levels compared to L6. The incorporation of [3H]mannose in D-1 was found to be 60% of L6 in 10% serum whereas in 1% serum the incorporation into D-1 was further reduced to 30% of L6. [3H]mannose-labeled ConA-binding proteins isolated from L6 were quantitatively and qualitatively similar in cells grown in either 10 or 1% serum. However, in D-1 cells a further decrease in the ConA-binding ability of these glycoproteins was observed. Biochemical differentiation also occurs in D-1 upon fusion in 1% serum as seen by the increase in mRNA levels of the muscle-specific markers myosin light chain and troponin T. These results suggest the high-mannose type of oligosaccharides may not be involved in myoblast differentiation.

Studies on glycosyltransferases in fusion-defective conA-resistant L6 rat myoblast cell lines

1. Sialyl- and galactosyl-transferase activities were determined in wild type and conA-resistant L6 rat myoblasts with substrates derived from fetuin, alpha 1-acid glycoprotein and bovine submaxillary mucin; fetuin was the best acceptor for both enzyme activities, whereas the mucin did not act as an acceptor. 2. The optimum pH for sialyltransferase was 6.6 in both cell lines. 3. The optimum pH for galactosyltransferase in the wild type cell line was 6.2 which was slightly higher than the value of 5.8 found for the conA-resistant cells. 4. Values for Km for both enzyme activities increased five to ten-fold in the variant cell line with both acceptors. 5. The main sialyltransferase activity was the Gal beta 1----4GlcNAc alpha 2----3sialyltransferase for N-linked chains. The galactosyltransferase was most likely the enzyme that is responsible for the synthesis of the Gal beta 1----4GlcNAc structure.

Effects of inhibitors of glycoprotein processing on oligodendroglial differentiation in primary cultures of embryonic rat brain cells

The effects of N-linked oligosaccharide processing inhibitors on oligodendroglial differentiation were examined in cultures of embryonic rat brain cells. The glucosidase inhibitors, 1-deoxynojirimycin (dNM and castanospermine, were found to have marked inhibitory effects on the developmental expression of oligodendroglial properties, i.e., sulfogalactolipid synthesis and 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNP). On the other hand, the mannosidase inhibitors, 1-deoxymannojirimycin (dMM) and swainsonine, had relatively little effects. Since both classes of inhibitors block the formation of complex-type oligosaccharide chains as revealed by concanavalin A-Sepharose affinity chromatography of the glycopeptides, complex oligosaccharides do not seem to play a role in oligodendroglial differentiation. The results indicate instead that the early trimming reactions involving the removal of glucose residues by processing glucosidases may be critical for the functioning of specific glycoprotein(s) essential to oligodendroglial differentiation.