ACIDIC SPHINGOMYELINASE (ASM) IS NECESSARY FOR FAS-INDUCED GD3 GANGLIOSIDE ACCUMULATION AND EFFICIENT APOPTOSIS OF LYMPHOID-CELLS

Ceramides deriving from sphingomyelin hydrolysis are important mediato rs of apoptotic signals originating from Fas (APO-1/CD95). However, de finitive evidence for the role played by individual sphingomyelinases is still lacking. We have analyzed lymphoblastoid cell lines derived f rom patients affected by Niemann Pick disease (NPD), an autosomal rece ssive disorder caused by loss-oi-function mutations within the acidic sphingomyelinase (ASM) gene. NPD lymphoblasts, which display normal ne utral sphingomyelinase activity, fail to activate ASM in response to F as cross-linking, unlike normal lymphoblasts. NPD lymphoblasts also fa il to accumulate GD3 ganglioside, a downstream mediator of ceramide-in duced cell death (De Maria, R., L. Lenti, F. Malisan, F. D'Agostino, B . Tomassini, A. Zeuner, M.R. Rippo, R. Testi. 1997. Science. 277:1652- 1655), and display a substantially inefficient apoptosis after-Fas cro ss-linking. Inefficient apoptosis is due to lack of ASM activity, beca use proximal signaling from Fas in NPD lymphoblasts is not impaired an d apoptosis can be efficiently triggered by passing the ASM defect wit h exogenous ceramides. Moreover, mannose receptor-mediated transfer of ASM into NPD lymphoblasts rescues their ability to transiently activa te ASM, accumulate GD3, and rapidly undergo apoptosis after Fas cross- linking. These results provide definitive genetic evidence for the rol e of ASM in the progression of apoptotic signals originating from Fas.