An Interconnected Hierarchical Model of Cell Death Regulation by the BCL-2 Family

Multidomain proapoptotic BAX and BAK, once activated, permeabilize mitochondria to trigger apoptosis, whereas antiapoptotic BCL-2 members preserve mitochondrial integrity. The BH3-only molecules (BH3s) promote apoptosis by either activating BAX-BAK or inactivating antiapoptotic members. Here, we present biochemical and genetic evidence that NOXA is a bona fide activator BH3. Using combinatorial gain-of-function and loss-of-function approaches in Bid(−/−)Bim(−/−)Puma(−/−)Noxa(−/−) and Bax(−/−)Bak(−/−) cells, we have constructed an interconnected hierarchical model that accommodates and explains how the intricate interplays between the BCL-2 members dictate cellular survival versus death. BID, BIM, PUMA and NOXA directly induce stepwise, bimodal activation of BAX-BAK. BCL-2, BCL-X(L) and MCL-1 inhibit both modes of BAX-BAK activation by sequestering activator BH3s and “BH3-exposed” monomers of BAX-BAK, respectively. Furthermore, autoactivation of BAX and BAK can occur independently of activator BH3s through downregulation of BCL-2, BCL-X(L) and MCL-1. Our studies lay a foundation on targeting the BCL-2 family for treating diseases with dysregulated apoptosis.