Apoptosis Meets Signal Transduction: Elimination of a BAD Influence

Abstract

The mechanism by which Bcl-2 and Bcl-xL promote cell survival is not yet understood. However, the establishment of functional links to signal transduction events has provided several new perspective on this question. One viewpoint, supported by their intracellular localization, is that Bcl-2-related proteins may serve as signaling receptors for intracellular organelles (Figure 1Figure 1). Serine kinases activated by pro-apoptotic stimuli would lead to the phosphorylation of Bcl-2, thus inactivating its ability to serve as a survival receptor. These kinases would be counterbalanced by the activity of specific phosphatases.A second perspective is that phosphorylation of Bad may provide a key survival-promoting decision point. A kinase activated by growth factors might maintain phosphorylation of Bad, resulting in persistent association of Bad with 14-3-3 in the cytosol (Figure 2Figure 2A). Bad would be unavailable for interaction with Bcl-2 and Bcl-xL, allowing the latter proteins to execute their anti-apoptotic function. A variation on this model emerges when one takes into consideration a potential role for Raf-1. By analogy to the function of Ras at the inner plasma membrane, which in its active state recruits binding of Raf-1 (Morrison 1995xMorrison, D.K. Mol. Reprod., and Develop. 1995; 42: 507–514Crossref | PubMed | Scopus (58)See all ReferencesMorrison 1995), Bcl-2 may anchor a signaling complex at the outer mitochondrial membrane (Figure 2Figure 2B). Ligation of growth factor receptors could promote Raf-1 translocation to the outer mitochondrial membrane via association with Bcl-2, where Raf-1 comes in contact with a new set of substrates and initiates a local signaling cascade. One of those substrates is Bad, which when phosphorylated, preferentially binds 14-3-3 and translocates to the cytosol.A third potential mechanism by which Bcl-2 proteins may function is suggested by the three-dimensional structure of Bcl-xL, analysis of which has revealed two central hydrophobic α-helices surrounded by five amphipathic helices, as well as a 60 residue flexible loop (Muchmore et al. 1996xMuchmore, S.W, Sattler, M, Liang, H, Meadows, R.P, Harlan, J.E, Yoon, H.S, Nettesheim, D, Chang, B.S, Thompson, C.B, Wong, S.L, Ng, S.C, and Fesik, S.W. Nature. 1996; 381: 335–341Crossref | PubMedSee all ReferencesMuchmore et al. 1996). This tertiary structure has homology with the membrane translocation domain of colicins and of diphtheria toxin. The latter is thought to dimerize and form a membrane pore that is regulated by pH. By analogy, Bcl-xL and Bcl-2 may form a pore involved in the homeostasis of cellular organelles, particularly of the mitochondria, that may protect cells against electrochemical changes that occur under stressful physiologic conditions. Bad might interfere with the assembly or the function of such pores, thus eliminating the ability of Bcl-2 and Bcl-xL to protect cells against death.In summary, although protection of cells against apoptotic death can be influenced by the level of expression of Bcl-2 and Bcl-xL, post-translational modification of Bcl-2 family members appears to be important as well. In the situations reported to date, phosphorylation appears to be inhibitory. Phosphorylation of Bcl-2 on serine residues appears to inhibit its anti-apoptotic function. Serine phosphorylation of Bad results in its binding to 14-3-3 rather than to Bcl-xL, thus blocking the pro-apoptotic function of Bad. Further investigation will be required to identify the kinases and phosphatases that directly modulate Bcl-2 and Bad, the signaling pathways that regulate those enzymes, and the role of these signaling events in the control of cell survival in response to diverse extracellular ligands.