Inhibition of the NF-κB transcription factor increases Bax expression in cancer cell lines

The NF-kappaB transcription factor has been shown to inhibit apoptosis in several experimental systems. We therefore investigated whether the expression of the Bax proapoptotic protein could be influenced by NF-kappaB activity. Increased Bax protein expression was detected in HCT116, OVCAR-3 and MCF7 cells stably expressing a mutated unresponsive I kappaB-alpha inhibitory protein that blocks NF-kappaB activity. Northern blots showed that bax mRNA expression was increased as a consequence of mutated I kappaB-alpha expression in HCT116 cells. A careful examination of the human bax gene promoter sequence showed three putative binding sites for NF-kappaB, and the kappa B2 site at position -687 could indeed bind NF-kappaB complexes in vitro. Transient transfection of a bax promoter luciferase construct in HCT116 cells showed that NF-kappaB proteins could partially inhibit the transactivation of the bax promoter by p53, Mutations or deletions of the kappaB sites, including kappa B2, indicated that this NF-kappaB-dependent inhibitory effect did not require NF-kappaB DNA-binding, and was thus an indirect effect. However, cotransfection of expression vectors for several known cofactors failed to identify a competition between p53 and NF-kappaB for a transcription coactivator. Our findings thus demonstrate for the first time that NF-kappaB regulates, through an indirect pathway, the bax gene expression.