Smac / Diablo proteins

Members of the lAP family inhibit caspases 3, 7 and 9 and interact with inhibitory Smac/DIABLO protein that is released from the mitochondria during apoptosis. They are over-expressed in some cancers. Peptide inhibitors of lAPs were then designed based on the Smac/DIABLO protein. The N-terminal sequence (AVPI) of Smac/DIABLO reacts with BIR3 of X-linked lAP (XIAP) and its N-terminal heptapetide promote procaspase-3 activation [241]. The... 

Quercetin and rutin increased the frequency of apoptotic cells in the male F344 rat model with chemically induced colon tumors [141]. Resveratrol enhanced upregulation of proapoptotic Smac/DIABLO protein and downregu-lated survivin in UVB exposure-mediated skin tumors [142]. Apigenin inhibits apoptosis of human prostate carcinoma tumor xenograft in athymic nude mice [143]. Caffeic acid phenethyl ester increased apoptosis and... 

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. 

There is also crosstalk between the two pathways above the mitochondria. The BH3-only protein BID is cleaved by caspase-8 and -10 which yields truncated BID (tBED), the active pro-apoptotic fragment of BID. Thereby, even in cells in which the direct apoptosis pathway which result from death receptor crosslinking is blocked, e.g. by high expression levels ofthex-linked IAP (XIAP), the activity of tBED on mitochondria can result in the activation of caspase-3 because the IAP-imposed block on full caspase-3 activation and caspase-9 activity at the apoptosome is released by Smac/ DIABLO. 

Besides cytochrome c and procaspase 9, other important apoptogenic factors are also released from the mitochondria, such as SMAC/DIABLO, Omi/HrtA2, AIF, and endonuclease G. The function of SMAC/DIABLO and Omi/HrtA2 is to activate caspase by suppressing the caspase inhibitory activity of IAP (Du et al., 2000 Verhagen et al., 2000). The protein... 

Smac/DIABLO Second mitochondrial activator of caspases/direct IAP-binding protein with low PI None... 

Smac/DIABLO is not the only regulator of lAP activity. Several studies in mammalian cells have demonstrated the presence of additional molecules that suppress lAP activity in a similar fashion to Smac/DIABLO. The best-studied example is Omi/HtrA2 (Suzuki et al., 2001 Hegde et al., 2002 Martins et al., 2002 van Loo et al., 2002). This protein exhibits, as does Smac/DIABLO, mitochondrial localization with cytoplasmic release upon stimulation. 

It is worth mentioning that in death receptor-mediated apoptosis, cells can be divided into two groups depending on the requirement for mitochondria to induce a complete apoptotic response. Type I cells do not require the mitochondrial pathway because the recruitment of procaspase-8 into the DISC complex is enough to fully activate caspase-8 which then activates effector caspases. However, Type II cells are characterized by an incomplete apoptotic response unless mitochondria are involved (Scaffidi et al., 1999). In this type of cell, efficient activation of effector caspases requires the mitochondrial amplification loop (Fig. 5). Caspase-8 cleaves cytosolic Bid, a BH3-only protein, which when cleaved to tBid is able to translocate to the mitochondria and trigger release of the proapoptotic factors cytochrome c and Smac/DIABLO (Li et al., 1998 Deng et al., 2002). The release of cytochrome c triggers apoptosome formation, subsequent caspase-9 activation, and finally the activation of effector caspases such as caspase-3. 

Smac/Diablo a mitochondrial protein that promotes some forms of apoptosis by neutralizing one or more members of the lAP family of apoptosis inhibitory proteins. 

Depending on the cell type, two different downstream pathways are triggered. In type I cells, processed caspase-8 produced in large amounts directly activates a caspase cascade. Among the caspases activated are caspase-3, which cleaves other caspases or vital substrates of the cell and thus paves the way for the execution phase of apoptosis. In type II cells, proper activation of effector caspases requires amplification via the mitochondrial pathway of apoptosis. Here, smaller amounts of active caspase-3 are produced which cleave the pro-apoptotic Bcl-2 family member Bid. The truncated form of Bid activates mitochondria by an unknown mechanism, which now release pro-apoptotic proteins like cytochrome c and Smac/Diablo (see Section 15.5). Cytochrome c release triggers the formation of the apoptosome, resulting in the activation of caspase-9 and subsequently caspase-3, which in turn can activate caspase-8 outside the Fas-DISC. 

In some cellular systems, cytochrome c is necessary, but not sufficient, for cell death. In these systems, a second mitochondrial activation of caspases is promoted by a protein with the dual name of Smac/DIABLO [28,29], which is released from the mitochondria with... 

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