Inhibition of the NF-kappa B transcription factor increases Bax expressionin cancer cell lines

The NF-kappaB transcription factor has been shown to inhibit apoptosis in s everal experimental systems. We therefore investigated whether the expressi on of the Bar proapoptotic protein could be influenced by NF-kappaB activit y. Increased Bar protein expression was detected in HCT116, OVCAR-3 and MCF 7 cells stably expressing a mutated unresponsive I kappaB-alpha inhibitory protein that blocks NF-kappaB activity. Northern blots showed that bar mRNA expression was increased as a consequence of mutated I kappaB-alpha expres sion in HCT116 cells. A careful examination of the human bar gene promoter sequence showed three putative binding sites for NF-kappaB, and the kappa B 2 site at position -687 could indeed bind NF-kappaB complexes in vitro. Tra nsient transfection of a bar promoter luciferase construct in HCT116 cells showed that NF-kappaB proteins could partially inhibit the transactivation of the bar promoter by p53, Mutations or deletions of the kappaB sites, inc luding kappa B2, indicated that this NF-kappaB-dependent inhibitory effect did not require NF-kappaB DNA-binding, and was thus an indirect effect. How ever, cotransfection of expression vectors for several known cofactors fail ed to identify a competition between p53 and NF-kappaB for a transcription coactivator. Our findings thus demonstrate for the first time that NF-kappa B regulates, through an indirect pathway, the bar gene expression.