IκBα inhibits apoptosis at the outer mitochondrial membrane through a novel, NF-κB-independent, interaction with VDAC1

The inducible transcription factor NF-κB is tightly regulated by the inhibitory IκB-family of proteins that associate with the transcription factor and act in response to stress stimuli. The best studied inhibitory protein is IκBα which resides in the cytosol where it retains NF-κB. Our study shows that IκBα also associates with the outer mitochondrial membrane (OMM) and exerts an unexpected novel anti-apoptotic function, independent of NF-κB inhibition. IκBα-/- cells become refractory to apoptosis when IκBα is specifically reconstituted at the OMM. We found that cancer cells with constitutively active NF-κB accumulate IκBα at the OMM and when its expression is down-regulated these cells are sensitised to apoptosis. At the OMM IκBα associates with VDAC1 and hexokinase II (HKII). Our findings show that IκBα inhibits the dissociation of HKII from VDAC1 and prevents Bax-mediated cytochrome c release. Deletion mutants of IκBα reveal a domain necessary for apoptosis inhibition that is different from the domain for NF-κB retention, thereby separating the two functions. These results reveal an unexpected activity of IκΒα in guarding the integrity of the OMM against apoptosis induction and open possibilities for more specific interference in diseases involving deregulated NF-κB.