Caspase effector caspases

Active caspases 8, 9 and 10 can convert caspase-3, the most abundant effector caspase from its pro-form to its active cleaved form. Cleavage of a number of different substrates by caspase-3 and also by caspase-6 and -7 which are two other executioner caspases besides caspase-3 then results in the typical morphology which is characteristic of apoptosis. Yet, the activation of caspase-3 and also of caspase-9 can be counteracted by IAPs, so called inhibitor of apoptosis proteins. However, concomitantly with cytochrome C also other proteins are released from mitochondria, including Smac/DIABLO. Smac/DIABLO and potentially other factors can interact with IAPs and thereby neutralize their caspase-inhibitory activity. This releases the breaks on the cell death program and allows apoptosis to ensue. 

Two main apoptotic pathways have been identified in mammalian cells the extrinsic pathway that is activated by the binding of ligands to cell-surface death receptors, and the intrinsic pathway that involves the mitochondrial release of cytochrome cP The activation of extrinsic and intrinsic apoptotic pathways promotes the cleavage into the active form of the pro-caspase-8 and pro-caspase-9, respectively, that mainly determine the activation of effector caspase-3. ° The intrinsic pathway is the main apoptotic pathway activated by chemotherapeutic drugs, while the cytotoxic drug-induced activation of the extrinsic pathway is a more controversial issue. ... 

When a cell is infected with a virus, the latter utilises the metabolic machinery within the host cell to generate viral proteins, RNA and DNA to produce more virus particles which then escape to infect other cells. The process is stopped by death of the host cells so that generation of new viruses is halted. The major mechanism that results in death of the host cell is apoptosis. The cells that are responsible for the death of the infected cells are either cytotoxic lymphocytes or natural killer cells. Death is caused either by release of toxic biochemicals and/or proteolytic enzymes or by binding to a death receptor, which is present on many cells. The entry of proteolytic enzymes or binding to the death receptor results in activation of initiator caspases. These activate effector caspases that cause damage to the cell which results in death due to apoptosis (Chapter 17 Figures 17.28, 29 and 30). 

Figure 20.35 Mechanisms by which external or internal stress leads to cell damage resulting in apoptosis. The stress leads to activation of initiator proteolytic enzymes (caspases) that initiate activation of effector caspases. These enzymes cause proteolytic damage to the cytoskeleton, plasma membrane and DNA. The activation of DNAases in the nucleus results in cleavage of DNA chains between histones that produces a specific pattern of DNA damage which, upon electrophoresis, gives a specific pattern of DNA fragments. The major endproduct of apoptosis are the apoptolic bodies which are removed by the phagocytes.

Caspase-8 activates the effector caspases either directly, or indirectly by promoting the cytochrome c (see p. 140) from mitochondria. Once in the cytoplasm, cytochrome c binds to and activates the protein Apaf-1 (not shown) and thus triggers the caspase cascade. Apoptotic signals can also come from the cell nucleus. If irreparable DNA damage is present, the p53 protein (see p. 394)—the product of a tumor suppressor gene—promotes apoptosis and thus helps eliminate the defective cell. 

Figure 14-8. Overview of pathways that regulate programmed cell death. Apoptosis may occur in response to signaling through either the extrinsic pathway or the intrinsic pathway. In each case, proteolytic cleavage activates an initiator caspase, caspase 8 or 9, either of which can cleave an effector caspase such as caspase 3. Apaf-1 is part of a large complex called the apoptosome that mediates the intrinsic pathway. Binding of an extracellular death ligand to its cell-surface receptor activates the extrinsic pathway.

Based on their function in apoptosis, the caspases are divided into two classes (see Fig. 15.6) the initiator caspases (caspases 8 and 9) and the effector caspases (caspases 3, 6 and 7). 

Tlie initiator caspases receive proapoptotic signals and initiate the activation of a caspase cascade. Tliey are activated by an interaction with a transmembrane receptor or by cytotoxic influences. A complex is thus formed known as the apoptosome (see 15.4). The effector caspases are activated by an upstream caspase via a cascade mechanism. They are the component that executes apoptosis, initiates degradation of central proteins and directs the cell to death. 

Based on the triggering stimulus and the nature of the components involved, at least two apoptotic signaling pathways can be differentiated that lead to activation of the effector caspases. On the one hand, receptor systems may be involved on the other hand, activation may be triggered by cytotoxic stress. The two pathways differ in the mechanism of activation of the initiator caspase but use the same effector caspase at least partially. 

In the case of the death receptor Fas (see 15.4), activation of the caspase involves a protein that interacts with the cytoplasmic part of the receptor and is known as FADD protein (Fas-associated death domain). The FADD protein has distinct structural motives that mediate specific interactions with other proteins. It interacts via the death domain with the receptor and via the death effector domain with the corresponding caspase (here caspase 8). 

In this model, cellular stress mediates the release of cytochrome C from the mitochondrion. The proapoptotic proteins Bax and BH3 proteins support the release of cytochrome C, while the antiapoptotic Bcl2 protein has an inhibitory effect. Cytosolic cytochrome C binds to the cofactor Apaf 1, which then associates via its CARD motif with procaspase 9 to a complex termed apopto-some. In this complex, procaspase 9 is processed to the mature caspase which subsequently activates downstream effector caspases and commits the cell to death. 

Upon recruitment by FADD, caspase 8 oligomerizes, triggering activation by selfcleavage. Caspase 8 then proteolytically activates downstream effector caspases, such as caspase 9, and commits the cell to death. 

Binding of the ligand of the Fas receptor triggers clustering of the receptor and association of the cofactor FADD (fas-assodated protein with death domain) which interacts with the receptor via its death domain (DD). Procaspase 8 binds to FADD via a common DED (death effector domain) motif and is thereby also recruited into the Fas-receptor associated complex. Due to the clustering of the proteins, proximity-induced cleavage of procaspase 8 to the mature initiator caspase 8 takes place. This activates the effector caspases and triggers cell death. 

Fig. 15.10. Pathways of DNA damage-mediated and p53-mediated apoptosis The presence of DNA lesions activates the ATM kinase and leads to an increase in p53 concentration. In a transcription-dependent pathway, p53 functions as a transcription activator of the bax gene. The increase in Bax protein facilitates release of cytochrome C from mitochondria and this serves as a trigger for activation of initiator and effector caspases. p53 also influences apoptosis by less well characterized ways, some of which are transcription independent.

Caspases involved in the apoptotic process can be subclassified as initiator or effector caspases, depending on the structure of their prodomain. This also reflects their different roles in the apoptotic cascade (Earnshaw et al., 1999). 

Effector caspases (caspases 3, 6, and 7) may possess a small inactive prodomain or may lack it completely, since they do not need to form... 

Effector caspases are activated by a transactivation mechanism, which is characterized by the catalytic action of a mature caspase on a procaspase (Thornberry et al., 1997 Earnshaw et al., 1999 Slee et al., 1999). Nevertheless, their activation can also occur by the action of other proteases. Granzyme B, a serine-protease, also has proteolytic specificity for aspartic acid residues. It is able to cleave and directly activate caspase 3 (Darmon et al., 1995). Cathepsin B, a lysosomal protease, cleaves and activates procaspase 11 (Schotte et al., 1998). 

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