Apoptosis/apoptotic process

Hoechst 33342 [2 -(4-ethoxyphenyl)-5-(4-methyl-l-piperazinyl)-2,5 -bi-IH-benzimidazole Ho342 I], a bisbenzimidazole dye, binds to adenine/thymine-rich regions in the minor groove of DNA. This dye induces apoptosis and inhibits topo 1 activity in vivo. It has been suggested that the destruction of immunoreactive topo I and topo I-DNA complexes or cleavable complexes results in inhibition of topo I activity, a key step in the Hoechst 33342-induced apoptotic process [40]. 

Several studies show that carotenoids are able to act as apoptosis inducers by modulating different molecular pathways involved in the apoptotic process, as recently reviewed (Palozza et al., 2004a). 

In apoptosis a series of events takes place in an orderly sequence involving the activation of various proteases which are called caspases, for cysteine and aspartate proteases. Several distinct caspases act in a cascade vaguely reminiscent of the blood-clotting cascade of complement proteins. If one wishes to interfere with the apoptotic process, then one strategy would be to develop drugs that inhibit various caspases, a current effort underway in the pharmaceutical industry. 

Several genetic alterations can influence radiosensitivity of cancer cells and radioresponsive tumors are known to allow more easily radiation-induced apoptosis. Therefore, the proper functioning of the apoptotic machinery regulates radiosensitivity. As described above, a cracial role is played by p53, and its downstream genes p21 and bax, in activating the apoptotic process. Those cancers with mutations at these levels are expected to be radioresistant. 

An important trigger for apoptosis is known as the Fas system. This is used by cytotoxic Tcells, for example, which eliminate infected cells in this way (top left). Most of the body s cells have Fas receptors (CD 95) on their plasma membrane. If a T cell is activated by contact with an MHC presenting a viral peptide (see p. 296), binding of its Fas ligands occurs on the target cell s Fas receptors. Via the mediator protein FADD ( Fas-associated death domain ), this activates cas-pase-8 inside the cell, setting in motion the apoptotic process. 

A larger part of the apoptotic program exists in the cell in a latent, inactive form and it only requires an apoptotic stimulus to activate the program and to initiate apoptosis. Thus, apoptotic processes can also take place without activation of transcription. There are also forms of apoptosis that are dependent on transcription. 

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. 

At low levels of mitochondrial damage, removal of damaged mitochondria by lysosomes rescues the cell. If this is overwhelmed then apoptosis occurs triggers such as cytochrome c (see below) start the apoptotic process. If most or all mitochondria are damaged, then the depletion of ATP is so extensive that the apoptotic process is not possible (ATP is required for the process), and the cell undergoes necrosis. 

Translocation of the phosphatidylserine from the inner to the outer leaflet of the plasma membrane is an initial event related to the apoptotic process and possibly serves as a signal for the removal of apoptotic bodies by phagocytic cells (Martin et al., 1995). The exposure of this phospholipid has been largely used as a specific apoptosis marker. 

When AIF and endonuclease G are released into the cytoplasm, they directly translocate to the nucleus and induce DNA fragmentation and subsequent chromosomal condensation, a remarkable morphological feature of the apoptotic process. AIF induces chromatin digestion into large fragments of approximately 50 kb, probably by activating a nuclear DNAse. Therefore, these proteins are important for the caspase-indepen-dent apoptosis pathway. 

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.

Maruyama R, Takemura G, Aoyama T, Hayakawa K, Koda M, Kawase Y, et al. Dynamic process of apoptosis in adult rat cardiomyocytes analyzed using 48-hour videomicroscopy and electron microscopy Beating and rate are associated with the apoptotic process. Am J Pathol 2001 159 863-891. 

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