Procaspase

NO may react with superoxide to yield the highly reactive peroxynitrite, ONOO-. Superoxide may also be converted into H202 and the reactive hydroxyl radical, OH. In this way excessive activation of glutamate receptors leads to oxidative damage. The calcium influx has a major effect on mitochondria and causes them to depolarize and swell. This leads to a pore being formed in the outer mitochondrial membrane, which allows the escape of cytochrome c and procaspases from the mitochondria into the cytosol. Cytochrome c activates the caspase cascade, which leads to apoptotic cell death (Ch. 35). 

Saleh, A., Srinivasula, S. M., Acharya, S., Fishel, R., and Alnemri, E. S., 1999, Cytochrome c and dATP-mediated oligomerization of Apaf-1 is a prerequisite for procaspase-9 activation. JBiol Chem 274 17941-17945. 

Fig. 2. Activation mechanism of caspases. Receptors convey activating signals to adapters that facilitate oligomerization and subsequent autoactivation of long prodomain caspases (I). Caspases transactivate other procaspases upon activation (11).

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. 

Activation of the caspases requires the help of a number of cofactors that are also known as activators or adaptors. Different cofactors are involved depending on the trigger mechanism of caspase activation. A central function of the cofactors is to bring about aggregation and thus activation of the procaspases. This occurs by specific protein-protein interactions with the help of common structural motives. Examples of such motives are the death domains (DD), death effector domains (DED) and the caspase recruitment domains (CARD), which all have a similar structure of six a-heh-ces. 

In addition to receptor-mediated apoptosis (see 15.5), there is another main pathway activated by various forms of cellular stress. Examples of stress effects that can induce apoptosis are y- and UV-radiation, treatment with cytotoxic drugs such as actinomycin D and removal of cytokines. As a consequence of stress, procaspase 8 is activated by a... 

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. 

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. 

An important transcriptional target of the p53 protein that can induce apoptosis is the bax gene. The Bax protein belongs to the family of Bcl-2 proteins (see 15.3.2) and has a proapototic effect. There is speculation that the p53-induced increase in Bax concentration leads to formation of ion pores in mitochondria and that cytochrome c is released into the cytosol via these pores. Cytochrome c then functions as a cofactor which, together with Apafl protein, activates procaspase 8 and initiates the apoptotic program. 

Bcl-2 and related cytoplasmic proteins are key regulators of apoptosis [26], Anti-apoptotic proteins such as Bcl-2 and Bc1-Xl prevent apoptosis in response to numerous stimuli. During the apoptotic process, cytochrome c is released from mitochondria, but the release can be inhibited by the presence of Bcl-2 on the organelles [27]. The released cytochrome c forms an essential part of die apoptosome, which is composed of cytochrome c, Apaf-1, and procaspase-9 [28]. The complex formation results in activation of caspase-9, which leads to the stimulation of caspase-3. Bcl-XL has recently been reported to bind to Apaf-1 [29], It may inhibit the association of Apaf-1 with procaspase-9 and thereby prevent caspase activation. 

Gao X, Deeb D, Jiang H, Liu YB, Dulchavsky SA, Gautam SC. 2005. Curcumin differentially sensitizes malignant glioma cells to TRAIL/Apo2L-mediated apoptosis through activation of procaspases and release of cytochrome c from mitochondria. J Exp Ther Oncol 5 39 48. 

Caspase family. This scheme illustrates the domain structures, internal cleavage sites, preferred peptide substrate sequences, and biological function of caspases. Each procaspase consists of a large and small domain and may also possess DED (death effector domain) and CARD (caspase recruitment domain) (adapted from Hill etai, 2003). 

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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