COSTIMULATORY EFFECT OF NITRIC-OXIDE ON ENDOTHELIAL NF-KAPPA-B IMPLIES A PHYSIOLOGICAL SELF-AMPLIFYING MECHANISM

Here we investigated the effects of the second messenger molecule NO a t various concentrations on the activation of transcription factor NF- kappa B, I kappa B-alpha kinase (IKK-alpha), Jun N-terminal kinase (JN K) and apoptosis in murine endothelial cells. Low concentrations of NO alone failed to activate NF-kappa B, IKK-alpha and JNK. When NF-kappa B was prestimulated by TNF-alpha or phorbol 12-myristate 13-acetate, the addition of NO at low concentrations enhanced the activation of NF -kappa B. This provides a mechanism for a self-amplifying signal in th e inflammatory response, since the inducible NO synthase in endothelia l cells is regulated by NF-kappa B. The co-stimulatory effect of NO on NF-kappa B activation was also evident from IKK-alpha kinase assays a nd reporter gene experiments in endothelial cells. High doses of NO im paired the TNF-alpha-induced DNA-binding activity of NF-kappa B. Accor dingly, these high amounts of NO also repressed the TNF-alpha-induced transactivation by NF-kappa B as efficient as dexamethasone. The doses of NO required for the inhibition of NF-kappa B are not cytotoxic for the endothelial cells, enabling the establishment of an autoregulator y loop for NF-kappa B signaling.