Isogloboside Biosynthesis in Metastatic R3230AC Cells Results from a Decreased GM3 Synthase Activity

We have determined that the production of a metastasis-associated neutral glycosphingolipid, isogloboside (iGb4Cer, GalNAcβ1-3Galα1-3Galβ1-4Glcβ1-O-ceramide) is associated with the loss of GM3 synthase activity. Assays for neutral glycosphingolipid-forming glycosyltransferases in cells producing various levels of iGb4Cer revealed no consistent differences that could account for the difference in iGb4Cer biosynthesis. However, comparison of the activity of GM3 synthase in homogenates of these two cell types revealed that cells that did not synthesize iGb4Cer had activity significantly greater than that of cells possessing this antigen. Furthermore, somatic cell hybrids generated using clones of the iGb4Cer -producing and nonproducing cell lines lacked iGb4Cer while possessing high levels of GM3 synthase activity. When iGb4Cer-producing cells were transfected with a GM3 synthase expression vector, all of the resultant clones were negative for iGb4Cer production. The results of these studies clearly show that the presence of GM3 synthase prevents the formation of iGb4Cer in these cells.