NFκB in the mechanism of ammonia-induced astrocyte swelling in culture

Astrocyte swelling and brain edema are major neuropathological findings in the acute form of hepatic encephalopathy (fulminant hepatic failure), and substantial evidence supports the view that elevated brain ammonia level is an important etiological factor in this condition. Although the mechanism by which ammonia brings about astrocyte swelling remains to be determined, oxidative/nitrosative stress and mitogen-activated protein kinases (MAPKs) have been considered as important elements in this process. One factor known to be activated by both oxidative stress and MAPKs is nuclear factor kappa B (NF kappa B), a transcription factor that activates many genes, including inducible nitric oxide synthase (iNOS). As the product of NOS, nitric oxide (NO), is known to cause astrocyte swelling, we examined the potential involvement of NF kappa B in ammonia-induced astrocyte swelling. Western blot analysis of cultured astrocytes showed a significant increase in NF kappa B nuclear translocation (a measure of NF kappa B activation) from 12 h to 2 days after treatment with NH4Cl (5 mM). Cultures treated with anti-oxidants, including superoxide dismutase, catalase, and vitamin E as well as the MAPKs inhibitors, SB239063 (an inhibitor of p38-MAPK) and SP600125 (an inhibitor of c-Jun N-terminal kinase), significantly diminished NF kappa B activation by ammonia, supporting a role of oxidative stress and MAPKs in NF kappa B activation. The activation of NF kappa B was associated with increased NOS protein expression and NO generation, and these changes were blocked by BAY 11-7082, an inhibitor of NF kappa B. Additionally, ammonia-induced astrocyte swelling was inhibited by the NF kappa B inhibitors, BAY 11-7082 and SN-50, thereby implicating NF kappa B in the mechanism of astrocyte swelling. Our studies indicate that cultured astrocytes exposed to ammonia display NF kappa B activation, which is likely to be a consequence of oxidative stress and activation of MAPKs. NF kappa B activation appears to contribute to the mechanism of ammonia-induced astrocyte swelling, apparently through its up-regulation of iNOS protein expression and the subsequent generation of NO.