Caspase initiator caspases

Figure 17.4. Initiator and effector caspases. Initiator caspases cleave and activate effector caspases. Cleavage sites are shown.

BH3 domain) of the BH3-only proteins binds to other Bcl-2 family members thereby influencing their conformation. This interaction facilitates the release of cytochrome C and other mitochondrial proteins from the intermembrane space of mitochondria. Despite much effort the exact biochemical mechanism which governs this release is not yet fully understood. The release of cytochrome C facilitates the formation of the apoptosome, the second platform for apoptosis initiation besides the DISC. At the apoptosome which is also a multi-protein complex the initiator caspase-9 is activated. At this point the two pathways converge. 

Apoptotic initiator caspases (caspase-2, -8, -9 and -10) constitute a subgroup of the caspase family. These caspases are the first to become proteolytically active in the apoptotic cascade. Their activation takes place in multiprotein complexes initiated by pro-apoptotic stimuli, such as TNFa, a-Fas, staurosporine. Once activated, they can process their substrates, which include the apoptotic executioner caspases. 

In some studies it has been shown that ITCs can cause increases in the pro-apoptotic caspase enzymes, caspase 3, caspase 8 or caspase However, in other work, specific caspase inhibitors failed to block cell death or cell detachment from the substratum . This suggests that caspase-activation may only be a bystander event, or may only occur after the initiating event of a block in the cell cycle. Indeed, although expression of c-Jun amino-terminal kinase (INK) in response to ITCs has been linked to the pro-apoptotic process, it remains entirely possible that this is a parallel signalling pathway, more closely related to the induction of Phase 1 or Phase 2 enzymes than to the... 

Moreover, a recent study also revealed that ROS generation led to the activation of caspase-2 during p-carotene-induced apoptosis in the human leukemic T cell line Molt 4. The apoptosis progressed by simultaneous activation of caspase-8 and caspase-9, and a cross talk between these initiator caspases was mediated by the pro-apoptotic protein Bid. Inhibition of caspases 2, 8, 9, and 3 independently suppressed the caspase cascade. The cleavage of the anti-apoptotic protein BclXL was found to be another important event during P-carotene-induced apoptosis, suggesting the presence of an extensive feedback amplification loop in P-carotene-induced apoptosis (Prasad et al., 2006). 

For example, Jurkat T cells apoptose in response to serum withdrawal, addition of exogenous cell-permeable ceramide or stimulation with the apoptotic agonist Fas. However, transfection with SPHKl, measured as a 400-600-fold increase in activity, reduced apoptosis and inaeased proliferation (Ohvera et al, 1999). These effects were correlated with an inhibition of the activity of caspase 3, an initiator caspase (Olivera et al, 1999), A similar effect has recently been observed in rat pheochromocytoma PC12 cells (Edsall etal, 2(X)1). Tropbic withdrawal led to the activation of... 

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). 

Intrinsic system Apoptosis is initiated due to changes to the mitochondria during which the inner membrane becomes permeable to large molecules, probably as a result of a decrease in the membrane potential. This can result from intracellular damage (e.g. accumulation of Ca ions), lack of oxygen or fuel. This results in release of cytochrome c and other proteins from the mitochondria which stimulate apoptosis. In fact, these apoptotic proteins plus cytochrome c form a complex, the apoptosome, which activates an initiator caspase. 

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.

The caspases that are involved in apoptosis may be further classified as either initiators or effectors. Induction of apoptosis via death receptors results in the activation of an initiator caspase. Caspases-8, -9, and -10 are initiators because they initiate the cascade of biochemical events that culminates in apoptosis. Caspases-3, -6, and -7 propagate this cascade (the so-called doomsday signal), thus functioning as effectors. Although there is some overlap, caspases-1, -4, -5, -11, -12, and -13 are involved in processing cytokines, thus influencing immunoregulation. The various caspases are listed in table 8.2. 

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. 

An essential part of the apoptotic program is a caspase cascade. Apoptosis is initiated by proteolytic processing of intiator-procapsases under the influence of a variety of signals. The mature initiator caspase catalyzes the processing of a effector-procaspase to the active enzyme, which degrades specific substrates and/or activates further procaspases. In this way, caspases can be activated sequentially in a protease cascade. 

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. 

The Apafl protein has been identified as a central cofactor in cytotoxically-triggered apoptosis. The Apafl protein is homologous to Ced4 protein of C. elegans. It binds to the initiator caspase with the help of a CARD motif A CARD motif is found in the Apafl protein and in various caspases (caspases 1, 2, 4, 5 and 9). 

Procaspase 8 functions as an initiator caspase in this system, since its activation is the signal for activation of the downstream caspase cascade. The DED motif of caspase 8 is localized in its large prodomain. Similar motives are foimd in other caspases with large prodomains (caspases 2, 8 and 9). 

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. 15.9. Antiapoptotic signalling by the PI3-kinase/Akt kinase pathway The PI3 kinase/Akt kinase pathway influences apoptosis via phosphorylation of the Bad protein, which is a member of the family of Bcl-2 proteins. Activation of the PI3-kinase pathway leads to Akt-kinase-catalyzed phosphorylation of Bad protein. Bad protein in its unphosphorylated form participates in activation of initiator caspases and thus has a proapoptotic effect. Phosphorylation of Bad protein by Akt kinase (or related kinases) has an antiapoptotic effect since phosphoryla-ted Bad protein is a binding substrate of 14-3-3 proteins. Bad is thus sequestered in an inactive state and is not available for triggering of apoptosis.

When caspase 8, an initiator caspase, is activated by an apoptotic signal carried through FADD, it further self-activates by cleaving its own proenzyme form. Mitochondria are one target of active caspase 8. The protease causes the release of certain proteins contained between the inner and outer mitochondrial membranes ... 

Initiator caspases (caspases 2, 8, 9, 10, and 12) possess a functional prodomain. This prodomain is large and contains interaction domains (structural motifs) where adaptor proteins, like Apaf-1 and FADD, can bind. The interaction with adaptor proteins is important for the caspase activation process. There are two types of interaction domains DED... 

Caspase activation through the death receptor-induced pathway. The activation of initiator caspases 8 and 10 by the death receptors results in the activation of effector caspases 3, 6, and 7. In type II cell lines, the activation of these initiator caspases also results in Bid cleavage and, therefore, in the activation of the mitochondrial pathway. 

Slee EA, Harte MT, Kluck RM, Wolf BB, Casiano CA, Newmeyer DD, Wang HG, Reed JC, Nicholson DW, Alnemri ES, Green DR, Martin SJ (1999), Ordering the cytochrome C-initiated caspase cascade hierarchical activation of caspases-2, -3, -6, -7, -8 and -10 in a caspase-9-dependent manner, J. Cell Biol. 144 281-292. 

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