Apoptosis by the intrinsic pathway is triggered by the release of cytochrome c from mitochondria, which triggers the activation of caspases. Once in the cytoplasm, cytochrome c binds to a protein called Apaf-1 (apoptotic protease activating factor-1), stimulating its oligomerization to form the apoptosome. The apoptosome then recruits an initiator caspase, pro-caspase 9, aggregation of which promotes its self-cleavage and frees its catalytic domain to activate effector caspases (Fig. 9.42).
The release of cytochrome c is controlled by interactions between members of the Bcl-2 family of proteins. The Bcl-2 family of proteins is defined by the presence of one or more Bcl-2 homology (BH) domains and can be divided into two general groups: members that promote apoptosis, and members that
When programmed cell death is induced, the mitochondria swell and leak, releasing cytochrome c, which binds to and induces a conformational change in Apaf-1 caspase 3 cleaved ICAD Apaf-1:cytochrome ccomplex self assembles into an apoptosome, which recruits and activates multiple copies of pro-caspase 9, which in turn activates pro-caspase 3 Caspase 3 cleaves ICAD, releasing CAD to enter the nucleus and cleave DNA pro-caspase 3 pro-caspase 3 cleavage CAD Apaf-1
Fig. 9.42 In the intrinsic pathway, cytochrome c release from apaf-1. The resultant conformational change that ensues in apaf-1 mitochondria induces formation of the apoptosome, which induces self-assembly of the multimeric apoptosome, which recruits activates pro-caspase 9 to initiate programmed cell death. pro-caspase 9 (third panel). Clustering of pro-caspase 9 by the In normal cells, cytochrome c is confined to the mitochondria apoptosome activates it, allowing it to cleave downstream caspases, (first panel). however, during stimulation of the intrinsic pathway, such as caspase 3; this results in the activation of enzymes such as the mitochondria swell, allowing the cytochrome c to leak out ICad, which can cleave dna (fourth panel). into the cytosol (second panel), where cytochrome c is bound by Fig. 9.43 General scheme of intrinsic pathway regulation by the Bcl-2 family of proteins. extracellular apoptotic stimuli activate a group of pro-apoptotic (sentinel) proteins. sentinel proteins can function either to block the protection provided by pro-survival, protector proteins or to directly activate pro-apoptotic, executioner proteins. In mammalian cells, apoptosis is mediated by the executioner proteins Bax, Bak, and Bok. In normal cells, these proteins are prevented from acting by the protector proteins (Bcl-2, Bcl-XL, and Bcl-W). The release of activated executioner proteins causes the release of cytochrome c and subsequent cell death, as shown in Fig. 9.42.
Apoptotic stimuli Sentinels Bad, Bil, Bid, Bim, p53, PUMA, NOXA Death Protectors Bcl-2, Bcl-XL, Bcl-W Executioners Bax, Bak, Bok inhibit apoptosis (Fig. 9.43). Pro-apoptotic Bcl-2 family members, such as Bax, Bak, and Bok (referred to as executioners), bind to mitochondrial membranes and can directly cause cytochrome c release. How they do this is still not known, but they may form pores in the membranes.