Sphingolipid metabolism in cultured fibroblasts: microscopic and biochemical studies employing a fluorescent ceramide analogue.

A fluorescent analogue of ceramide, N-7-(4-nitrobenzo-2-oxa-1,3-diazole)-epsilon-aminocaproyl sphingosine (C6-NBD-ceramide), was used to investigate sphingolipid metabolism in Chinese hamster fibroblasts. C6-NBD-ceramide was incorporated into small unilamellar dioleoyl phosphatidylcholine vesicles and incubated with cells in monolayer culture at 2 degrees C, resulting in rapid and preferential transfer of the labeled ceramide from vesicles to cells. The cells were then washed and subsequently incubated at 37 degrees C for various intervals. The metabolism of C6-NBD-ceramide was monitored by lipid extraction and analysis, and the intracellular distribution of the labeled molecule was followed by fluorescence microscopy. Initially, fluorescence was detected almost exclusively in mitochondria, with over 90% of the extractable lipid fluorescence due to C6-NBD-ceramide. After 30 min at 37 degrees C, intense fluorescence appeared in the Golgi apparatus. This organelle was identified by colocalization of NBD fluorescence with a Golgi-apparatus-specific stain. At later times the plasma membrane became visibly labeled as well, at which point 90% of the cell-associated fluorescence was recovered as NBD-labeled sphingomyelin and NBD-labeled cerebroside. These metabolites were identified by enzymatic and biochemical analysis and by thin-layer chromatography of the fluorescent lipid extracts. The finding that C6-NBD-ceramide is used by these cells in standard pathways of sphingolipid biosynthesis suggests that this fluorescent precursor will be a valuable tool for correlating the metabolism of sphingolipids with their intracellular distribution and translocation. In addition, during its metabolism by Chinese hamster fibroblasts, this compound acts as a vital stain for the Golgi apparatus.