Co‐stimulatory effect of nitric oxide on endothelial NF‐κB implies a physiological self‐amplifying mechanism

Here we investigated the effects of the second messenger molecule NO at various concentrations on the activation of transcription factor NF‐κB, IκB‐α kinase (IKK‐α), Jun N‐terminal kinase (JNK) and apoptosis in murine endothelial cells. Low concentrations of NO alone failed to activate NF‐κB, IKK‐α and JNK. When NF‐κB was prestimulated by TNF‐α or phorbol 12‐myristate 13‐acetate, the addition of NO at low concentrations enhanced the activation of NF‐κB. This provides a mechanism for a self‐amplifying signal in the inflammatory response, since the inducible NO synthase in endothelial cells is regulated by NF‐κB. The co‐stimulatory effect of NO on NF‐κB activation was also evident from IKK‐α kinase assays and reporter gene experiments in endothelial cells. High doses of NO impaired the TNF‐α‐induced DNA‐binding activity of NF‐κB. Accordingly, these high amounts of NO also repressed the TNF‐α‐induced transactivation by NF‐κB as efficient as dexamethasone. The doses of NO required for the inhibition of NF‐κB are not cytotoxic for the endothelial cells, enabling the establishment of an autoregulatory loop for NF‐κB signaling.