Human fibroblasts, rat neurons, and murine neuroblastoma cells, cultured in the presence of fetal calf serum, were fed with [1-3H]sphingosine to radiolabel sphingolipids. The fate of cell sphingolipids, the release of sphingolipids in the culture medium, the interaction of sphingolipids with the proteins and lipoproteins of fetal calf serum, and the fate of sphingolipids taken up by the cells were investigated. For this latter purpose, the culture medium containing radioactive sphingolipids was delivered to nonlabeled cells. The presence of tritium at position 1 of sphingosine allowed us to follow the extent of sphingolipid catabolism by measuring the production of radioactive phosphatidylethanolamine and proteins by recycling the radioactive ethanolamine formed during sphingosine catabolism and the production of tritiated water. We confirmed that in cells the recycling of sphingosine occurred to a high extent and that only a minor portion of cell sphingolipids was catabolized to the small fragments of ethanolamine and water. Cell sphingolipids were released in the culture medium, where they formed large lipoproteic aggregates at a rate of about 12% per day. Released sphingolipids were taken up by the cells and catabolized to the sphingosine and then to ethanolamine, and recycling of sphingosine was not observed. This suggests that in the presence of fetal calf serum in the culture medium, exogenous sphingolipids directly reach the lysosomes, were they are entirely catabolized. Thus, the trafficking of sphingolipids from cells to the extracellular environment and from this to other cells does not allow the modification of the plasma membrane composition.
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