The metabolism of
sphingomyelin (SPM) was investigated in Epstein-Barr virus-transformed lymphoid cell lines from normal individuals and from patients with
Niemann-Pick disease Type A (deficient in the
acid, lysosomal
sphingomyelinase) and
familial hypercholesterolemia (lacking the
low density lipoprotein receptor). Cells were incubated with the following radioactive or fluorescent SPMs: [
choline-methyl-14C] SPM, [oleoyl-3H]SPM,
pyrene-propenoyl-SPM (P3:1-SPM),
pyrene-butanoyl-SPM (P4-SPM),
pyrene-dodecanoyl-SPM (P12-SPM), and
pyrene-sulfonylamino-undecanoyl-SPM (PSA11-SPM). Several pathways of uptake and subsequent metabolism of SPM in the lymphoblastoid cells were identified. [
choline-methyl-14C]SPM and the P12-analog, administered to the cells in the presence of
lipoproteins, were taken up through the
apoB/E receptor-dependent pathway of endocytosis and degraded solely by the lysosomal
sphingomyelinase. Under similar conditions, the other
sphingomyelins, i.e. [oleoyl-3H]SPM, P3:1-SPM, P4-SPM, and
PSA11-SPM, were taken up by a
low density lipoprotein receptor-independent pathway and degraded mostly by a nonlysosomal
sphingomyelinase which also catalyzed their hydrolysis in Niemann-Pick cells. In the absence of serum, all
sphingomyelins were taken up by an
apoB/E receptor-independent pathway and hydrolyzed by a nonlysosomal
sphingomyelinase. Indeed, in vitro assays demonstrated the presence, in lymphoblastoid cells, of the neutral
magnesium-activated
sphingomyelinase, which was also fully active in the Niemann-Pick cells. In conclusion, our observations are consistent with: (i) the existence in lymphoblastoid cells of several pathways for the uptake and subsequent utilization of SPM; (ii) a major role of
lipoproteins for the metabolic routing of the SPM; and (iii) the effect of the structure of the fatty acyl residue of SPM on its possible association with
lipoproteins and/or cell membranes.