Differential NF-kappaB regulation of Bcl-x gene expression in hippocampus and basal forebrain in response to hypoxia

Cell death often occurs after hypoxic/ischemic injury to the central nervous system. Changes in levels of the anti-apoptotic Bcl-X_L protein may be a determining factor in hypoxia-induced neuronal apoptosis. The transcription factor NF-κB regulates bcl-x gene expression. The goal here was to examine the role of NF-κB in the regulation of bcl-x transcription in hypoxia-induced cell death. Rat hippocampus and basal forebrain tissues were collected at different time points after hypoxia (7%O₂, 93%N₂ for 10 or 20 minutes). The results showed that hypoxia induced apoptosis in the hippocampus and basal forebrain are both time- and tissue-dependent. The hippocampal nuclear NF-κB dimers c-Rel/p50 and p50/p50 bound to the bcl-x promoter NF-κB sequence (CS4), but only nuclear p50/p50 bound to the CS4 sequence in the basal forebrain. Hypoxia-induced differential binding patterns of c-Rel/p50 and p50/p50 correlated with the bcl-x expression pattern in the hippocampus suggesting their regulatory role in bcl-x gene expression. The hypoxia-induced patterns of binding of c-Rel/p50 to the bcl-x promoter CS4 sequence were different from those to the IgG-κB enhancer sequence, while those of p50/p50 were similar for both sequences. Nuclear protein levels of c-Rel, but not p50, correlated with the cRel/p50 DNA binding patterns to the bcl-x CS4 site. Similar sequence-specific nuclear binding changes were observed in thoracic cord after spinal cord contusion injury and in LPS-treated Hela cells. There were differential responses to hypoxia among the different NF-κB nuclear subunits in the hippocampus. These results suggest that there is a tissue-specific and gene-specific regulation of bcl-x gene expression by NF-κB in hypoxia-induced cell death in the hippocampus. The absence of these regulating features in the basal forebrain may account for the early appearance of apoptosis in response to hypoxia in that brain region as compared with that in hippocampus. These results support the hypothesis that NF-κB has tissue- and gene-specific regulatory effects and these effects account for NF-κB's pro- or anti-apoptotic role in different apoptotic processes.