NATURAL CERAMIDE IS UNABLE TO ESCAPE THE LYSOSOME, IN CONTRAST TO A FLUORESCENT ANALOG

Since the generation upon cell stimulation of the second messenger cer amide has been reported to occur in an endosomal/lysosomal compartment , we investigated whether ceramide formed in the lysosomes can escape this compartment. The metabolic fate of radiolabelled ceramide produce d by intralysosomal hydrolysis of LDL-associated [ceramide-H-3]sphingo myelin or [stearoyl-1-C-14]sulfatide was examined in fibroblasts from control individuals and a patient with inborn lysosomal ceramidase def iciency (Farber disease), The behavior of this radioactive ceramide wa s compared to that of a fluorescent (lissamine-rhodaminyl) ceramide an alogue deriving from sulfatide degradation. While in Farber cells the natural, radiolabelled ceramide remained completely undegraded and acc umulated in the lysosomes, the fluorescent derivative was rapidly conv erted to sphingomyelin. These findings strongly suggest that, in contr ast to fluorescent derivatives, endogenous long-chain ceramide is unab le to exit from lysosomes, therefore making the lysosomal ceramide unl ikely to be a biomodulatory molecule. (C) 1998 Federation of European Biochemical Societies.