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Regulation of "a" and "asialo" series ganglioside biosynthesis was analyzed. COS-1 cells expressing only GD1a showed high synthetic activities of GM3, GM2, GM1a, and GD1a, but little activity for GA2 synthesis. However, IL-3-dependent murine NFS60-I7, which has GM1b and GD1 alpha, exhibited high synthetic activities of GM2, GM1a, and GD1a, but GM3 synthase was only 1/6 of COS-1 and GA2 synthetic activity was low. By contrast, IL-3 gene-transfected subline NFS60-H7 expressing GD1a in addition to GM1b and GD1 alpha displayed up-regulated GM3 synthase and GA2 synthase activities, while GM2, GM1a, and GD1a synthase activities were in the same levels as in NFS60-I7 cells. Since GA2 synthetic activities were not parallel with GM2 synthase in the investigated machinery of ganglioside biosynthesis, it is strongly suggested that biosynthesis of "a" and "asialo" series gangliosides is regulated differentially from each other in murine NFS60 cell lines.

Previous studies (Y. Nagata, S. Yamashiro, J. Yodoi, K. O. Lloyd, H. Shiku, and K. Furukawa (1992) J. Biol. Chem. 267, 12082-12089) had isolated a putative cDNA coding for beta 1,4 N-acetylgalactosaminyltransferase (GM2/GD2 synthase) and demonstrated the presence of GM2 and/or GD2 gangliosides in melanoma cell lines stably transfected with this gene. We have now measured the levels of glycosyltransferase activities and mRNA levels in five transfected cell lines in comparison with their parent cell lines (mouse melanoma B16 and human melanoma MeWo). Membrane preparations from the transfected cell lines demonstrated de novo synthesis of GM2 or GD2 in in vitro assays using GM3 or GD3, respectively, as ganglioside acceptors. The enzyme levels, however, varied considerably among the different transfectants, as did the mRNA levels for the beta 1,4 Gal-NAc-transferase. The effect of the transfected gene on levels of preexisting glycosyltransferases involved in ganglioside biosynthesis was also measured and the ganglioside composition of the cell lines was determined. The level of beta 1,4 GalNAc-transferase expressed in the different cell lines was found to dramatically influence the overall ganglioside composition of the cell. In the transfected cell line with the highest levels of the transferase, for example, biosynthesis was almost completely redirected away from the b pathway to the a pathway with the resulting expression of only GM2. These data from this family of related cell lines clearly demonstrate the primary roles that relative glycosyltransferase levels play in determining the ganglioside composition of cells.

(1-->4)-N-Acetyl-beta-D-galactosaminyltransferase (GalNAcT-1) and (1-->3)-N-acetyl-beta-D-galactosaminyltransferase (GalNAcT-2), which are involved in the in vitro biosynthesis of GM2 and GbOse4Cer glycosphingolipids, respectively, have been solubilized and separated by differential detergent extraction from a membrane preparation of 19-day-old embryonic chicken brain. The separated GalNAcT-1 activity had a pH optima of 7.8-8.0, and the separated GalNAcT-2 activity a single pH optimum of 7.2. Furthermore, the partially purified GalNAcT-2 preparation catalyzed the transfer of N-acetylgalactosamine from UDP-D-[3H]GalNAc to only GbOse3Cer and nLcOse5Cer. Both GalNAcT-1 and GalNAcT-2 activities were purified to approximately 316- and 428-fold, respectively, by use of UDP-hexanolamine-Sepharose 4B affinity-column chromatography. However, the partially purified GalNAcT-1 preparation appeared to be active only with GM3, lactosylceramide, and lactotriaosylceramide. The proposed linkage of the N-acetylgalactosamine unit incorporated into GM3 is beta-D-GalpNAc-(1-->4)-GM3 from the isolation of [3H]threitol after hydrolysis of the desialylated, lead tetraacetate-treated, enzymic product, beta-D-GalpNAc-(1-->4)-beta-D-[6-3H]Galp-(1-->4)-beta-D-Glcp-(1-->1)-Cer . In addition, beta-D-GalpNAc-(1-->3)-GbOse3Cer was produced, as shown by the identification of 2,4,6-tri-O-methyl-galactose after permethylation and hydrolysis of the GalNAcT-2 enzymic product, GalpNAc-[6-3H]Galp--->Gal-->Glc-->Cer.