Abnormalities of lysosomes in human diploid fibroblasts from patients with Farber's disease

An accumulation of ceramide associated with the deficiency of acid ceramidase has been demonstrated in cultured diploid skin fibroblasts from a patient with Farber's disease. We extend this observation to investigate the lysosomal localization of accumulated ceramide and the abnormalities of lysosomes caused by this ceramide accumulation in Farber's diseased fibroblasts. We have found that the lysosomal fraction isolated from Farber's diseased fibroblasts by a subcellular fractionation procedure is markedly low in density compared with that of normal fibroblasts and is separated from other subcellular organelles. Ultrastructural studies of the isolated lysosomal fraction from Farber's diseased fibroblasts showed a mixed population of intact and swollen vesicles with a lysosomal appearance. Examination under high magnification clearly demonstrated lysosomal inclusions which contain lamellar and curvilinear membranes and resembled those seen in the intact fibroblasts. Subcellular localization of Farber's fibroblasts showed that the accumulated [3H]ceramide from the culture medium was predominantly localized in the lysosomal fraction with a markedly low density and very little was found to be associated with other cellular membranes. Our finding that ceramide is accumulated in the lysosomal fraction of Farber's fibroblasts and that these cells also show membranous inclusions strongly suggests that the accumulation of ceramide is directly involved in the formation of lysosomal inclusions.

Substrate-specificities of acid and alkaline ceramidases in fibroblasts from patients with Farber disease and controls

The specific activity of acid ceramidase (N-acylsphingosine deacylase, EC 3.5.1.23) was measured at pH4.5 in normal fibroblasts and in fibroblasts from patients with Farber disease and obligate heterozygotes. Greater activity was found when the synthetically made ceramide substrates contained shorter-chain fatty acids or higher content of double bonds. Acid ceramidase activities towards N-lauroyl- (C(12:0)), N-myristoyl- (C(14:0)) and N-palmitoyl- (C(16:0)) sphingosine (C(18:1)) were respectively about 38, 26 and 6 times higher than the activity towards the N-stearoyl (C(18:0)) substrate. The activity towards N-linolenoylsphingosine (C(18:3)/C(18:1)), N-linoleoylsphingosine (C(18:2)/C(18:1)) and N-oleoylsphingosine (C(18:1)/C(18:1)) were respectively about 5, 4 and 3 times higher than the activity towards N-stearoylsphingosine (C(18:0)/C(18:1)). The activity towards N-stearoyldihydrosphingosine (C(18:0)/C(18:0)) was about 40% of that towards N-stearoylsphingosine. Fibroblast alkaline ceramidase possessed significant activity only towards ceramides of unsaturated fatty acids, with a pH optimum of about 9.0. Deficiency of acid ceramidase activity in fibroblasts from patients with Farber disease and intermediate activities in obligate heterozygotes were demonstrated with all ceramides examined except for N-hexanoylsphingosine (C(6:0)/C(18:1)), whereas alkaline ceramidase activity was unaffected. Comparative kinetic studies of acid ceramidase activity with N-lauroylsphingosine and N-oleoylsphingosine demonstrated about 5 (2-12)-fold and 7 (4-17)-fold higher K(m) values in fibroblasts from patients with Farber disease as compared with normal controls. N-Lauroylsphingosine, towards which acid ceramidase activity in control fibroblasts was about 10 times higher than that towards N-oleoylsphingosine, may serve as a better substrate for enzymic diagnosis of Farber disease as well as for further characterization of the catalytically defective acid ceramidase.

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.