Metabolism of ceramide-containing endocytotic vesicles in human diploid fibroblasts.

Two isoenzymes of ceramidase, including lysosomal acid ceramidase and nonlysosomal alkaline ceramidase, catalyze the degradation of ceramide in cultured human skin fibroblasts. A genetically determined disorder of sphingolipid metabolism (Farber's-disease) is characterized by the deficiency of acid ceramidase and by the pathological accumulation of ceramide. In this report, we take advantage of this genetic deficiency to study the intracellular transport of reconstituted low density lipoprotein (LDL) containing ceramide and of ceramide-containing liposomes into lysosomes. Our findings show that there is decreased incorporation of LDL in Farber's diseased fibroblasts, presumably related to the lack of lysosomal degradation of ceramide. In turnover experiments, the deficiency of lysosomal acid ceramidase in Farber's disease fibroblasts leads to the complete lack of degradation of ceramide internalized through the LDL uptake process. In contrast, this genetic defect does not affect either the uptake or turnover of ceramide-containing phosphatidylcholine liposomes. Comparison of these results suggests that in cultured skin fibroblasts the coated vesicles containing ceramide-LDL are designated for lysosomal delivery and are clearly distinguished from endocytotic vesicles involved in the uptake of ceramide-containing liposomes, which may be destined to be transported into subcellular organelles other than lysosomes.