Apoptosis execution

Since in all these cases we used oxidants to induce apoptosis, a relatively massive background oxidation of all PnA-labeled phospholipid classes was observed and masked apoptosis-specific PS oxidation. Therefore, we next determined whether a non-oxidant-induced apoptosis model could be used to reveal more explicitly the selective mode of PS oxidation inherent to the apoptosis execution program. To this end, we used agonistic anti-Fas antibody to induce apoptosis in Jurkat B cells according to a commonly used protocol (Fadeel et al., 1999). Importantly, we foundthat anti-Fas-triggered apoptosis in Jurkat cells was characterized by early and selective oxidaiton of cA-PnA-PS (Kagan et al., 2001) without any significant involvement of other classes of phospholipids. 

Once apoptosis is triggered, a stereotyped sequence of premitochondrial events occurs that executes the death process 609... 

Once apoptosis is triggered, a stereotyped sequence of premitochondrial events occurs that executes the cell death process. In many cases proteins and/or lipid mediators that induce changes in mitochondrial membrane permeability and calcium regulation are produced or activated. For example, the pro-apoptotic Bcl-2 family members Bax, Bad and Bid may associate with the mitochondrial membrane and modify its permeability. Membrane-derived lipid mediators such as ceramide and 4-hydroxynonenal can also induce mitochondrial membrane alterations that are critical for the execution of apoptosis. 

It is now well estahlished that activation of the caspase cascade is an indispensable and sufficient process in the execution phase of apoptosis (Nunez et al, 1998). As for mitochondria-mediated apoptosis, cytochrome c released from the mitochondrial inner membrane is well known to play an important role in the activation of caspase 9, one of the upstream proteases in the cascade (Zou et al, 1997). For activation of caspase 9, cytochrome c or apoptotic protease activating factor 2 (Apaf 2) induces the formation of the complex between Apaf 1 and caspase 9. The resultant activated caspase 9 then activates caspase 3, which in turn leads to the genomic DNA fragmentation and apoptotic cell death. 

Tepper, A.D., Ruurs, P., Wiedmer, T, Sims, PJ., Borst, J. and van Bhtterswijk, W.J., 2000, Sphingomyelin hydrolysis to ceramide during the execution phase of apoptosis results from phospholipid scrambling and alters cell-surface morphology. J. Cell Biol., 150 F5-7. 

The sensitivity to ionizing radiation is maximal in those cells able to activate a co-ordinate program of cell death (apoptosis) primed by the radiation-induced oxidative stress. Albeit apoptosis is a nuclear event and radiation-induced DNA damage is probably the most relevant mechanism of initiation of apoptosis, the control of the execution phase (and sometime also the initiation) takes place at the mitochondrial level. Radioresistance occurs... 

The central dogma of apoptosis is that all the initiating pro-apoptotic stimuli converge on the mitochondrial compartment. Thus, although apoptosis can be initiated elsewhere, the execution phase of apoptosis induced by ionizing radiation needs mitochondria. How do DNA lesions trigger mitochondria Several metabolic pathways coimect mitochondria to the nucleus. 

Figure 4. Outline of the cellular respose to ionizing radiation. 1.Early changes induced by radiation. 2. Adaptive response. 3. Initiation of apoptosis. 4. Execution of apoptosis. S. Late phases of apoptosis. The point of no return from death is the transition hum 3 to 4.

Changes in mitochondrial stmcture are very relevant during X-ray induced apoptosis. A few hours after irradiation, a hyperpolarisation of A /m is noticed. This likely represents the attempt to restore the depleted ATP levels, stimulating the oxidative burst of surviving mitochondria. If this secondary oxidative stress overcomes the threshold given by mitochondrial thiols, mitochondrial cardiolipin is oxidized and mitochondrial inner membrane allows the leakage of A /m with the consequent initiation of the execution phase. 

Signals of apoptosis lead to the activation of a family of intracellular cysteinyl aspartic acid proteases (caspases see Section 3.3) to play a pivotal role in the initiation and execution of apoptosis induced by various stimuli. Fourteen caspases in mammalian cell have been identified (W13). 

During the process of apoptosis, several mitochondrial proteins are released into the cytoplasm, including AIF and cytochrome C for the activation of proteases (L4). AIF, a tlavoprotein, can induce apoptotic morphological changes of the nucleus in a caspase-independent manner (M7, S8). Cytochrome C probably executes apoptosis by interaction with cytoplasmic protein Apaf-1 and direct activation of caspases (L2). Since the release of AIF and cytochrome C is regulated by the proteins of the Bcl-2 family, Bcl-2 can inhibit apoptosis by retention of cytochrome C in the mitochondria (T6). 

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. 

Fozard JR, Pfannkuche HJ, Schuurman HJ (1996) Mast cell degranulation following adenosine A3 receptor activation in rats. Eur J Pharmacol 298(3) 293-297 Freude B, Masters TN, Robicsek F, Fokin A, Kostin S, Zimmermann R, Ullmann C, Lorenz-Meyer S, Schaper J (2000) Apoptosis is initiated by myocardial ischemia and executed during reperfusion. J Mol Cell Cardiol 32(2) 197-208... 

Biochemical execution of cell death depends on three major components caspases, mitochondrial factors, and the Bcl-2 family of proteins (Green 2005 Yuan 2006). Caspases are a group of cysteine proteases acting as the central effectors of apoptosis. Caspases are normally suppressed by the inhibitor of apoptosis (IAP) in... 

There is a delicate balance between cellular membrane permeability and intracellular calcium homeostasis during CVB3 infection. It has been well-documented that sustained elevation of calcium levels in the cytosol precedes Cyt c release from the mitochondria, and that the small amount of released Cyt c interacts with the inositol triphosphate receptor (IP3R) on the endoplasmic reticulum (ER) and prevents inhibition of ER calcium release. The overall increase of calcium leads to a massive release of Cyt c to maintain ER calcium release through interaction with the IP3Rs in a positive feedback loop, and to activate downstream caspases to execute apoptosis of damaged cells. 

The apoptotic process is mediated by the caspase family of cysteine protease. Caspases are implicated both in the induction and execution of the death sentence. Apoptosis can be induced by (1) activation of death receptors, such as Fas, which recruits procaspase 8 via adaptor proteins and promotes its autocatalytic activation, and (2) the release of cytochrome C from mitochondria by DNA damaging drugs and other chemotherapeutic agents (Figure 7.11). 

Figure 7.11 Activation of the caspase proteases during apoptosis. Caspases are implicated in both the induction and execution of the apoptotic process. Following the apoptotic stimuli, initiator caspases (caspase 8 or 9) are activated by autocatalysis. The initiator caspases then activate the effector caspases (caspase 3, 6, and 7), which are responsible for most of the protein cleavage during apoptosis.

A. The phases of apoptosis in mammalian cells Initiation Effector Execution... 

Fig. 5. The execution phase of apoptosis. An apoptotic stimulus causes the release of cytochrome c from mitochondria. The first box contains the components required to activate caspase 9 card refers to the caspase-recruitment domain. Caspase 9 then activates caspase 3 (second box), which in turn activates caspase 6 (last box). The amino-acid sequences at the cleavage sites are shown. Caspases 3 and 6 also have a prodomain that is not present in the active protease in caspase 6 an additional cleavage removes a small portion of the middle of the protein. Various substrates of the caspases are shown, including the pathway by which caspase-activated deoxyribonuclease (CAD) is activated which in turn leads to DNA...

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