Apoptotic cell death and nitric oxide: activating and antagonistic transducing pathways.

Nitric oxide (NO) is a unique diffusible molecular messenger that occupies central roles in mammalian pathophysiology. Overproduction of NO is important for nonspecific "host" defense, helping to kill tumors and intracellular pathogens. Cytotoxicity as a result of long-lasting NO generation is now established to initiate apoptosis. Apoptotic cell death defines morphological alterations and distinctive biochemical events that lead to cell demise. NO-mediated apoptosis comprises upregulation of the tumor suppressor protein p53, activation of proteases known as caspases, chromatin condensation, DNA laddering, and is associated with alterations in the expression of apoptotic associated proteins that belong to the Bc1-2 family. An active role of NO was established by blocking adverse effects by NO-synthase inhibitors. Overexpression of the classical antiapoptotic protein Bc1-2 rescued cells from apoptosis by attenuating signaling downstream of p53 and upstream of caspase activation. Accumulating evidence suggests that transducing mechanisms can intersect and therefore a cell response to a given stimulus may alter significantly. As a result, transducing pathways of NO are not only adapted to cytotoxicity but also refer to cell protection. Protection from NO-elicited apoptosis may result as a consequence of a diffusion controlled NO/O2- (superoxide) interaction. The NO/O2- interaction redirects the apoptotic initiating activity of radicals (NO or O2-) towards protection as long as reduced glutathione compensates the resultant oxidative stress. Further, NO-mediated protective principles are understood on the basis of gene transcription of protective proteins such as heat shock proteins, hemeoxygenase-1, or cyclooxygenase-2 that attenuate cell injury in a cell specific way. The crosstalk between destructive and protective principles as a result of NO formation will determine the role of NO in cell injury. The balance between pro- and anti-apoptotic signaling mechanisms, their activation or deactivation as a result of NO formation, will allow cells to cope with NO or to exit into apoptosis.