Biosynthesis and shedding of murine lymphoma gangliosides

Ganglioside biosynthesis and subsequent shedding are a potential mechanism contributing to tumor cell escape from the host immune response. As a first step in identifying active molecular species, structural characterization and quantification of the purified individual cellular and shed gangliosides of YAC-1 murine lymphoma cells were undertaken. These studies uncovered three striking changes in ganglioside metabolism in cells passaged in vivo, compared with cells cultured in vitro. (i) Marked inhibition of GalNAcGM1b synthesis: GM1b was present in an equal proportion to its biosynthetic product GalNAcGM1b in vitro, but was present in a 6-fold higher concentration in vivo. (ii) Marked inhibition of NeuGc synthesis: NeuGc, present in vitro in an up to 7-fold higher concentration than its biosynthetic precursor NeuAc, was decreased in relative concentration in vivo (1:1). (iii) Selectivity of shedding: ganglioside shedding in vitro was generalized with respect to both carbohydrate structure and ceramide structure (mainly d18:1-C24:1 and d18:1-C16:0), while in vivo, there was selective shedding of gangliosides containing NeuGc and the shorter chain fatty acid. The reduced synthesis of NeuGc and of GalNAcGM1b in vivo, and the selective shedding of more polar ganglioside species, also in vivo, show that the extracellular environment can markedly affect cellular ganglioside metabolism.