SummaryThe proapoptotic Bcl-2 protein Bax is predominantly found in the cytosol of nonapoptotic cells and is commonly thought to translocate to mitochondria following an apoptotic stimulus. The current model for Bax activation is that BH3 proteins bind to cytosolic Bax, initiating mitochondrial targeting and outer-membrane permeabilization. Here, we challenge this and show that Bax is constitutively targeted to mitochondria but in nonapoptotic cells is constantly translocated back to the cytosol. Using live-cell spinning-disk confocal imaging with a combination of FLIP, FRAP, and photoactivatable GFP-Bax, we demonstrate that disrupting adhesion-dependent survival signals slows the rate of Bax’s dissociation from mitochondria, leading to its accumulation on the outer mitochondrial membrane. The overall accumulation of mitochondrial Bax following loss of survival signaling sensitizes cells to proapoptotic BH3 proteins. Our findings show that Bax is normally in a dynamic equilibrium between cytosol and mitochondria, enabling fluctuations in survival signals to finely adjust apoptotic sensitivity.

survival signalling in cell types from distinct embryonic lineages that show differing sensitivities to anoikis. We demonstrate that both fibroblasts and epithelial cells prevent anoikis through FAK activation. We show that FAK activates multiple downstream pathways in order to suppress anoikis. However FAK regulates survival through a more restricted set of pathways in the more anoikis-sensitive epithelial cells. Furthermore, we identify a novel role for paxillin in apoptosis suppression.

Most cells undergo apoptosis through the intrinsic pathway. This is dependent on mitochondrial outer membrane permeabilisation (MOMP), which is mediated by the pro-apoptotic Bcl-2 family proteins, Bax and Bak. During apoptosis, Bax translocates from the cytosol to the outer mitochondrial membrane (OMM), wherein it contributes to the formation of pores to release cytochrome-c. However, it remains unclear whether Bax translocation is sufficient to bring about MOMP or whether Bax requires further signals on the OMM to be fully activated. We have previously shown that during mammary epithelial cell anoikis, Bax translocation does not commit cells to MOMP and detached cells are rescued if survival signals from the extracellular matrix (ECM) are restored. These findings implied that a second signal is required for mitochondrial Bax to fully activate and cause MOMP. We now identify p38MAPK (mitogen-activated protein kinase) as this necessary signal to activate Bax after its translocation to mitochondria. The inhibition of p38MAPK did not prevent Bax translocation, but its activity was required for mitochondrial Bax to bring about MOMP. p38MAPK was activated and recruited to a high molecular weight mitochondrial complex after loss of ECM attachment. Artificially targeting p38MAPK to the OMM increased the kinetics of anoikis, supporting a requirement for its mitochondrial localisation to regulate Bax activation and drive commitment to apoptosis. Metazoan cells are pre-programmed to undergo apoptosis in response to becoming damaged or losing essential survival signals. Apoptosis in response to these cellular stresses is initiated through the intrinsic, or mitochondrial, pathway. 1 During intrinsic apoptosis, mitochondrial outer membrane permeabilisation (MOMP) results in the release of proteins, such as cytochrome-c and second mitochondrial activator of caspases (Smac)/Diablo, which activate the caspases to drive cell death. The release of cytochrome-c and Smac/Diablo is an extremely rapid event, with mitochondria throughout the cell releasing their contents within a few minutes of MOMP first commencing. 2,3 The resulting activation of caspases occurs immediately and cell death is complete within a few minutes. After MOMP, cells rapidly lose viability, even if caspase activation is inhibited. Thus, MOMP represents a point at which cells cannot be rescued from an apoptotic fate.The Bcl-2 family of proteins regulates MOMP. 4 These are characterised by a number of shared regions of sequence homology, the Bcl-2 Homology (BH) domains. Pro-survival members, such as Bcl-2 and Bcl-X L , share BH-domains 1-4. In contrast, the pro-apoptotic proteins, Bax and Bak, only posses BH-domains 1, 2 and 3. A third group of Bcl-2 proteins are the BH3-only proteins, which function by sensing survival and stress signals, and transmit these to the pro-and antiapoptotic multi-domain proteins. 5 The multi-domain proapoptotic proteins, Bax and Bak, are expressed in virtually all tissues, and are absolutely required for MOMP to occur. 6 Cells deriv...