Cell autonomous apoptosis defects in acid sphingomyelinase knockout fibroblasts

A body of evidence suggests that stress-induced sphingomyelin hydrolysis to the second messenger ceramide initiates apoptosis in some cells. Although studies using lymphoblasts from Niemann-Pick disease patients or acid sphin gomyelinase (ASMase)-deficient mice have provided genetic support for this hypothesis, these models have not been universally accepted as definitive. Here, we show that mouse embryonic fibroblasts (MEFs) prepared from osmose mice manifest cell autonomous defects in apoptosis in response to several s tresses. In particular, asmase(-/-) MEFs failed to generate ceramide and we re totally resistant to radiation-induced apoptosis but remained sensitive to staurosporine, which did not induce ceramide, asmase(-/-) MEFs were also partially resistant to tumor necrosis factor alpha/ actinomycin D and seru m withdrawal. Thus, resistance to apoptosis in asmase(-/-) MEFs was not glo bal but rather stress type specific. Most importantly, the sensitivity to s tress could be restored in the asmase(-/-) MEFs by administration of natura l ceramide, Overcoming apoptosis resistance by natural ceramide is evidence that it is the lack of ceramide, not ASMase, that determines apoptosis sen sitivity. The ability to rescue the apoptotic phenotype without reversing t he genotype by the product of the enzymatic deficiency provides proof that ceramide is obligate for apoptosis induction in response to some stresses.