Caspase cellular mechanisms

Figure 20.5 Proposed model for the cellular mechanisms of gold(lll) porphyrin 1a induced apoptosis in HONE1 ceiis. Goid (III) porphyrin la directiy caused depietion of A P. ieading to the aiteration of Bci-2 famiiy proteins, AIF nucleus translocation, and cytochrome c release, which further activated caspase-9 and caspase-3, and subsequently caused PARP-1 cleavage. ROS were also generated. The altered cellular oxidative state affected cytotoxicity of gold(lll) porphyrin la by regulating mitochondrial permeabilization.

Palombo and colleagues (40) investigated the effects of different types and concentrations of CLA and linoleic acid on colorectal and prostate cancer (PC3) cell proliferation and cytotoxicity. Linoleic add was without effect at all concentrations and in all cells, which appears to be a consensus in all reports. PC3 cells were the least sensitive to CLA treatment of the cells studied. These authors also found an induction of a homogeneous caspase activity (caspases 2,3,6-10) indicative of an induction of apoptosis, but they did not investigate other underlying cellular mechanisms to explain their findings. 

Curcumin was found to inhibit cellular proliferation and enhance apoptosis in a variety of lymphoma cell lines in vitro [Skommer et al., 2006 Thompson et al., 2004 Wu et al., 2002]. The proposed mechanism of curcumin s action in the majority of these studies involves the suppression of the expression of NF-KB-regulated gene products. One study suggested a novel function for curcumin as a suppressor of JAK-1 and STAT3 activation in primary effusion lymphoma cells, a function that would lead to the inhibition of proliferation and the induction of caspase-dependent apoptosis [Uddin et al., 2005],... 

Intrinsic (mitochondrial) pathway of caspase activation is initiated by the permeabilization of the mitochondrial outer membrane by proapoptotic members of the Bcl-2 family, resulting in a release of cytochrome c and other proteins from the intermembrane space of mitochondria into the cytosol. Cytochrome c translocation to the cytosol may follow a number of possible mechanisms. However, once in the cytosol, cytochrome c binds to apoptosis protease activating factor (Apaf-1) and in the presence of dATP or ATP facilitates Apaf-1 oligomerization and the recruitment of procaspase-9. The formation of this caspase-activating complex, termed the apoptosome, results in the activation of procaspase-9, and this in turn cleaves and activates the effector caspase-3 and -7. Activated effector caspases cleave key substrates in the cell and produce the cellular and biochemical events characteristic for apoptosis [33-35]. 

Activation of caspases is irreversible, because it involves peptide-bond cleav e. This is unlike most other protein modifications which play a role in cellular regulation. Therefore, proteolysis is involved only in unidirectional, irreversible processes, such as the cell cycle and cell death. But, the possibilities to regulate irreversible reactions are rather limited. In a cascade of proteolytic reactions, the first enzyme in the chain is the most likely point of control. This is the initiator caspase. The signals controlling initiator caspases vary, there are both external and internal signals (Fig. 13.5). Several mechanisms control the irreversible activation of caspases, including phosphorylation, separation, and compartmentalization of pro-caspases and positive and n ative regulators. 

To elucidate the mechanisms of NO-induced cellular stress, the effects of SIN-1 (32) on neuroblastoma cells were examined. SIN-1 induced a transient decline in ATP levels and the delayed loss of cell viability, with no significant increase in caspase-3 activity or DNA laddering. NO was suggested to be a potent toxin independent of peroxynitrite formation [41]. 

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