Several studies showed that Sf-9 cells can synthesize the galactosylated N-linked oligosaccharides if beta-1,4-galactosyltransferase (beta-1,4-GalT) is supplied. The full-length human beta-1,4-GalT I, II, III, IV, V, and VI cDNAs were independently transfected into Sf-9 cells, and the galactosylation of endogenous membrane glycoproteins was examined by lectin blot analysis using Ricinus communis agglutinin-I (RCA-I), which preferentially interacts with oligosaccharides terminated with Galbeta1-->4GlcNAc group. Several RCA-I-reactive bands appeared in all of the gene-transfected cells, and disappeared on treatment of blots with beta-1,4-galactosidase or N-glycanase prior to incubation with lectin. Introduction of the antisense beta-1,4-GalT II and V cDNAs separately into human colorectal adenocarcinoma SW480 cells, in which beta-1,4-GalT I, II, and V genes were expressed, resulted in the reduction of RCA-I binding toward N-linked oligosaccharides of the membrane glycoproteins. Differences were found in their K(m) values toward UDP-Gal and GlcNAcbeta-S-pNP and in their acceptor specificities toward oligosaccharides with the GlcNAcbeta1-->4(GlcNAcbeta1-->2)Man branch and with the GlcNAcbeta1-->6(GlcNAcbeta1-->2)Man branch. These results indicate that beta-1,4-GalTs II, III, IV, V, and VI are involved in the N-linked oligosaccharide biosynthesis cooperatively but not in a redundant manner with beta-1,4-GalT I within cells.
Our previous studies showed that mouse β4-galactosyltransferase 5 (β4GalT5) is a lactosylceramide (Lac-Cer) synthase, and that its gene expression increases by 2- to 3-fold upon malignant transformation of cells. In the present study, we examined whether or not the tumorigenic and metastatic potentials of B16-F10 mouse melanoma cells can be suppressed by reducing the expression of the β4GalT5 gene. We isolated a stable clone named E5 whose β4GalT5 gene expression level was reduced to 35% that of a control clone C1 by transfection of its antisense cDNA. Thin-layer chromatography analysis of glycosphingolipids showed that the amounts of Lac-Cer and ganglioside GM3 are significantly less in clone E5 than in clone C1. Clone C1 and E5 cells were each transplanted subcutaneously or injected intravenously into C57BL/6 mice, and the sizes of tumors and numbers of colonies formed in the lungs were determined. The average tumor size and average number of colonies formed with clone E5 were decreased to 44 and 49%, respectively, of those formed with clone C1. Furthermore, the numbers and sizes of colonies formed in the soft agarose gels, and the volumes of tumors formed in athymic mice with fibroblasts from wild type, heterozygous and homozygous β4GalT5-knockout mouse embryos upon transformation with the polyoma virus oncogene correlated with the β4GalT5 gene dosage. These results strongly indicate that the amounts of Lac-Cer synthesized by β4GalT5 correlate with the tumorigenic potentials of malignantly transformed cells.
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