The p75(NTR)-induced apoptotic program develops through a ceramide-caspasepathway negatively regulated by nitric oxide

SK-N-BE neuroblastoma cell clones transfected with p75(NTR) and lacking Trk neurotrophin receptors, previously reported to undergo extensive spontaneo us apoptosis and to be protected by nerve growth factor (NGF) (Bunone, G., Mariotti, A., Compagni, A., Morandi, E., and Della Valle, G:. (1997) Oncoge ne 14, 1463-1470), are shown to exhibit (i) increased levels of the pro-apo ptotic lipid metabolite ceramide and (ii) high activity of caspases, the pr oteases of the cell death cascade. In the p75(NTR) expressing cells, these parameters were partially normalized by prolonged NGF treatment, which, in addition, decreased apoptosis, similar to caspase blockers. Conversely, exo genous ceramide increased caspase activity and apoptosis in both wild-type and p75(NTR)-expressing cells. A new p75(NTR)-expressing clone characterize d by low spontaneous apoptosis exhibited high endogenous ceramide and low c aspase levels. A marked difference between the apoptotic and resistant clon es concerned the very low and high activities of nitric-oxide (NO) synthase , respectively. Protection from apoptosis by NO was confirmed by results wi th the NO donor S-nitrosoacetylpenicillamine and the NO-trapping agent hemo globin, We conclude that the p75NTR receptor, while free of NGF, triggers a cascade leading to apoptosis; the cascade includes generation of ceramide and increased caspase activity; and the protective role of NO occurs at ste p(s) in between the latter events.