Identification and characterization of nuclear factor kappaB binding sites in the murine bcl-x promoter.

Signal transduction pathways that mediate neuronal commitment to apoptosis involve the nuclear factor kappaB (NF-kappaB) transcription factor. Bcl-X(L) is a potent regulator of apoptosis in the CNS and is highly expressed in the developing and adult brain. We identified three putative NF-kappaB DNA binding sequences clustered upstream of the brain-specific transcription start site in the upstream promoter region. Recombinant p50/p50 and NF-kappaB proteins from nuclear extracts bound to these sites as determined by electrophoretic mobility shift assay and biotin-oligonucleotide/streptavidin affinity assays. NF-kappaB overexpression, coupled with bcl-x promoter/reporter assays using a series of murine bcl-x promoter and deletion mutants, has identified the downstream 1.1 kb of the bcl-x promoter as necessary for basal promoter activity and induction by NF-kappaB. The mutagenic removal of NF-kappaB binding sites individually or in combination revealed altered response patterns to p49/p65 and p50/p65 overexpression. These results support the hypothesis that NF-kappaB can act to enhance Bcl-X(L) expression via highly selective interactions, where NF-kappaB binding and bcl-x promoter activation are dependent on both DNA binding site sequence and NF-kappaB subunit composition. Our data suggest that molecular events associated with NF-kappaB promote regulation of neuronal apoptosis in the developing or injured CNS.