Global cerebral ischemia activates nuclear factor-κB prior to evidence of DNA fragmentation
Abstract The oxidative stress responsive transcription factor nuclear factor-κB (NF-κB) consists of a p50 (50 kDa) and p65/RelA (65 kDa) component and can be activated in vitro by TNFα, IL1β, hydrogen peroxide and oxygen radicals. All of the above factors are also known to be elevated at certain times after transient global ischemia. The present study was performed to determine if NF-κB was activated in vivo by transient global forebrain ischemia. Adult male rats were subjected to 30 min of 4-vessel occlusion (4-VO) and sacrificed at selected post-ischemic time points. Levels of NF-κB p50 and p65 subunits were determined by immunocytochemistry, Western blot and electrophoretic mobility-shift analysis. The enhancer complex was also confirmed by immuno-gel-shift analysis. Specific labeling of DNA strand breaks and DNA fragmentation was examined in situ by means of the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) method. Western blot analysis of hippocampus showed induction of p50 and p65. A time course of NF-κB induction in hippocampus showed a p50-specific band at 6 h that increased in intensity over 12, 48 h and then decreased by 96 h post-ischemia. Immunocytochemistry revealed at 24 h post-ischemia that p65 and p50 immunoreactivity was present in neuronal nuclei of hippocampal CA1 neurons as well as all other hippocampal regions and several other forebrain regions which were not vulnerable to transient forebrain ischemia. At 72 h post-ischemia, nuclear NF-κB immunoreactivity had disappeared in all brain areas except in hippocampal CA1 neurons which were degenerating. No evidence for DNA fragmentation as revealed by TUNEL staining could be observed at 24 h. However, at 72 h, hippocampal CA1 neurons were heavily labeled. The results of this study demonstrate that global forebrain ischemia causes a transient activation of NF-κB in many forebrain regions. NF-κB remains persistently activated in the vulnerable hippocampal CA1 sector. Because of the persistent activation of NF-κB in these neurons, the possibility exists that NF-κB has a role in programmed cell death in hippocampal CA1 neurons.