Nitric oxide inhibits caspase activation and apoptotic morphology but does not rescue neuronal death

Nitric oxide (NO) has been shown to inhibit apoptotic cell death by S-nitrosylation of the catalytic-site cysteine residue of caspases. However, it is not clear whether in neurons NO-mediated caspase inactivation leads to improved cell survival. To address this issue, we studied the effect of NO donors on caspase activity and cell survival in cortical neuronal culture treated with the apoptosis inducer staurosporine (STS) and camptothecin. In parallel, cell viability was assessed by the MTS assay and MAP2 staining. We found that NO donors ((+/-)-S-nitroso-N-acetylpenicillamine, S-nitrosoglutathione, and NONOates) dose-dependently inhibited caspase-3 and -9 activity induced by STS and camptothecin. The reduction in caspase-3 activity was, in large part, because of the blockage of the proteolytic conversion of pro-caspase-3 to active caspase-3. NO donors also inhibited the appearance of the classical apoptotic nuclear morphology. However, inhibition of both caspase activity and apoptotic morphology was not associated with enhancement of cell viability. Thus, inhibition of caspase and apoptotic morphology by NO donors does not improve neuronal survival. The data suggest that inhibition of caspase by NO unmasks a caspase-independent form of cell death. A better understanding of this form of cell death may provide new strategies for neuroprotection in neuropathologies, such as ischemic brain injury, associated with apoptosis.