Inactivation of multiple targets by nitric oxide in CD95-triggered apoptosis

Nitric oxide (NO) plays an important anti-apoptotic role by inactivating both upstream and downstream apoptotic molecules. We now report that exogenously supplied NO protected Jurkat T cells from anti-CD95-stimulated apoptosis. We have recently shown that nitrosation of the activator protein-1 (AP-1) transcriptional factor is crucial for NO-mediated inhibition of cell death triggered by etoposide or ceramide. Since the inhibition of apoptosis by NO has been reported to involve AP-1, we evaluated its involvement in in CD95-mediated cell death. Cross-linking of CD95 enhanced AP-1 DNA binding activity and AP-1-dependent CD95L transactivation, which were both significantly reduced by different NO-donors compounds. However, AP-1 induction does not seem to significantly contribute to anti-CD95-triggered apoptosis, as cell death could not be prevented by using the recombinant Fas-Fc fusion protein which inhibits the CD95/CD95L interaction. We observed that caspase 3-like activity was negatively modulated by several NO-donors in vitro and that titratable thiol groups of purified caspases 3, 7, and 9 decreased in the presence of NO-releasing compounds. In conclusion, we demonstrated that NO-mediated inhibition of other targets, possibly caspases, but not AP-1, is a crucial event responsible for protection against anti-CD95-stimulated apoptosis. Even though NO affects multiple molecular mechanisms, the relevant target for exerting the cellular effects, may vary among different models.