Control of mitochondrial apoptosis by the Bcl-2 family

Abstract

Programmed cell death, or apoptosis, is important for the development and homeostasis of tissues. Too little cell death can result in autoimmune diseases or cancer, whereas excessive cell death can lead to debilitating degenerative diseases of the heart or nervous system. The realization that apoptosis was genetically controlled first arose when it was observed that certain mutants of the model organism Caenorhabditis elegans caused failure of apoptosis in cells that normally undergo this process during development (Hengartner et al., 1992). Subsequently, it was found that proteins that are encoded by the mutant genes discovered in C. elegans shared homology with mammalian proteins, including B-cell CLL/lymphoma 2 (Bcl-2) (Hengartner and Horvitz, 1994). Further study in mammals revealed that there is an intrinsic apoptotic pathway that involves the mitochondria and an extrinsic apoptotic pathway that involves death receptors. The mitochondrial pathway of apoptosis in mammals, on which this poster article is focused, is regulated by members of the Bcl-2 family of proteins.Proteins of the Bcl-2 family have either pro- or anti-apoptotic activities and regulate the mitochondrial pathway of apoptosis by controlling the permeabilization of the outer mitochondrial membrane. In response to many types of stress or damage, certain members of the Bcl-2 family, known as BH3-only proteins (see below), are activated. Certain BH3-only proteins cause the activation of the pro-apoptotic proteins Bcl-2-associated X protein (Bax) or Bcl-2 antagonist/killer-1 (Bak) at the mitochondrion. Activated Bax and Bak homo-oligomerize and participate in the formation of pores in the outer mitochondrial membrane through which pro-apoptotic molecules escape, including second mitochondria-derived activator of caspase (Smac) (also known as Diablo) and cytochrome c. Release of cytochrome c leads to the activation of caspases, which are proteases that cleave key cellular proteins. This leads to many of the morphological characteristics of apoptosis, including condensed nuclei, DNA laddering and exposure of phosphatidylserine to the outer leaflet of the plasma membrane.Expression of Bcl-2 or other related anti-apoptotic proteins, including myeloid cell leukemia-1 (Mcl-1), Bcl-2-like 1 (Bcl-XL), Bcl-2-like 2 (Bcl-w) and Bcl-2-related protein A1 (Bfl-1), block cell death in response to many varieties of insult by preventing the activation and homo-oligomerization of both Bax and Bak. Anti-apoptotic proteins perform their anti-death function by sequestering BH3-only proteins or activated, monomeric Bax and Bak. Cells that survive continuous, permanent death signaling owing to the presence of Bcl-2 are dependent on Bcl-2 for their survival. It is known that certain cancer cells depend upon Bcl-2 and other anti-apoptotic proteins for survival. We have found that dependence on anti-apoptotic proteins can be identified in cancer cells using a strategy that we call BH3 profiling (see below and Box 1). In cancer cells that are dependent on Bcl-2, the Bcl-2 protein binds pro-apoptotic BH3-only proteins such as Bcl-2-interacting mediator of cell death (Bim). We describe such cells as being `primed for death'. Molecular therapies that are targeted to anti-apoptotic proteins such as Bcl-2 can induce apoptosis in primed cancer cells; one such Bcl-2 antagonist, Abbott (ABT)-737, has shown impressive success in killing leukemia and lymphoma cells (Table 1).In this brief Cell Science at a Glance poster article, we discuss how proteins of the Bcl-2 family control the crucial event in the commitment to apoptosis – the permeabilization of the mitochondrial outer membrane. In addition, we discuss recently developed methods for probing how cancer cells manipulate members of the Bcl-2 family to block apoptosis. Understanding these principles is key to understanding how certain insults and derangements commit selected cells to death, but spare others.The Bcl-2 family can be divided into pro-apoptotic and anti-apoptotic proteins. These proteins contain one or more Bcl-2 homology (BH) domains, which share sequence homology and are important for heterodimeric interactions among members of the Bcl-2 family (Chittenden et al., 1995; Danial and Korsmeyer, 2004). Most anti-apoptotic proteins contain BH domains 1-4. Pro-apoptotic proteins can be divided into two groups according to function and the number of BH domains possessed. Bax and Bak are pro-apoptotic proteins that contain BH domains 1-3 and are known as multidomain pro-apoptotic or effector proteins. The remaining pro-apoptotic proteins contain only the third BH domain and are known as BH3-only proteins. BH3-only proteins act as upstream sentinels of cellular damage and derangement. They can be activated by many noxious stimuli – including DNA damage, growth-factor withdrawal and oncogene activation – via mechanisms that include transcriptional upregulation, subcellular localization and/or post-translational modific