| Abstract: | Previous studies indicated a reproducible pattern of altered glycosphingolipid biosynthesis accompanying late stages of liver tumorigenesis in the rat induced by the carcinogen 2-acetylaminofluorene. The sequence began with a dramatic elevation in CMP-sialic acid:lactosylceramide sialyltransferase and was followed by sequential elevations and eventual depressions in other enzymes catalyzing sugar transfers to glycolipid acceptors. The present study focused on the early events of glycolipid biosynthesis during the first 11 weeks of 2-acetylaminofluorene administration according to the same feeding schedule as used previously. Transient elevations in CMP-sialic acid synthetase and elevations in neutral glycosphingolipid precursors to gangliosides were found to precede the major elevations in CMP-sialic acid:lactosylceramide sialyltransferase (GM3 synthetase) noted earlier. Two cycles of response were observed prior to the initiation of the sustained enhancement of biosynthesis of precursor ganglioside, GM3, and/or a significant increase in total or lipid-soluble sialic acid. In vitro rates of sialyl transfer from CMP-sialic acid to endogenous protein acceptors were not altered. The results suggest that the previous observations of altered ganglioside biosynthesis following 2-acetylaminofluorene administration are not an isolated occurrence but may represent late events in a sequence or 'cascade' of biochemical change involving, as well, biosynthesis of ganglioside precursors, CMP-sialic acid and neutral glycosphingolipids. |
