Programmed cell death (also called apoptosis) is a normal physiological form of cell death that plays a key role in both the maintenance of adult tissues and in embryonic development.
In adults, programmed cell death is responsible for balancing cell proliferation and maintaining constant cell numbers in tissues undergoing cell turnover. In addition, programmed cell death provides a defense mechanism by which damaged and potentially dangerous cells can be eliminated for the good of the organism as a whole. Virus-infected cells frequently undergo programmed cell death, thereby preventing the production of new virus particles and limiting spread of the virus through the host organism. Other types of insults, such as DNA damage, also induce programmed cell death. In the case of DNA damage, programmed cell death may eliminate cells carrying potentially harmful mutations, including cells with mutations that might lead to the development of cancer. The survival of many types of cells in animals is dependent on growth factors or contacts with neighboring cells or the extracellular matrix, so programmed cell death is thought to play an important role in regulating the associations between cells in tissues. Regulation of programmed cell death is mediated by the integrated activity of a variety of signaling pathways, some acting to induce cell death and others to promote cell survival [13].
1.4.1 Characterization
Programmed cell death is an active process characterized by a distinct morphological change. During apoptosis, chromosomal DNA is usually fragmented as a result of cleavage between nucleosomes. The chromatin condenses and the nucleus then breaks up into small pieces. Finally, the cell itself shrinks and breaks up into membrane-enclosed fragments called apoptotic bodies. Such apoptotic cells and cell fragments are readily recognized and phagocytosed by both macrophages and neighboring cells, so cells that die by apoptosis are
efficiently removed from tissues and won’t cause inflammation [13, 22, 23].
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(B)
Figure1. 11 Aptptosis (A) Light micrographs of normal (left) and apoptotic (right) human leukemia cells, illustrating chromatin condensation and nuclear fragmentation during apoptosis. (B) Diagrammatic representation of the events of apoptosis [13].
1.4.2 caspase family
The caspases are named because thy have cysteine (C) residues at their active sites and cleave after aspartic acid (Asp) residues in their substrate proteins. The caspases are the ultimate effectors or executioners of programmed cell death, bringing about the events of apoptosis by cleaving nearly 100 different cell target proteins. The caspases are synthesized as inactive precursors that are usually converted to the active form by proteolytic acleavage, catalyzed by other caspases. The activation of an initiator caspase therefore starts off a chain reaction leading to activation of additional downstream caspases and death of the cell.
Regulation of caspases is thu central to determining cell survival [24].
There are several pathways of apoptosis, and caspases play a key role to execute cell death.
For example, mammal cells which are triggered apoptosis would form a complex within themselves. This complex is a multisubunit called apoptosome which consists of Apaf-1, caspase-9 and cytochrome c released from injured mitochondria. Once caspase-9 is activated, it would cleave and activate other downstream effector caspases, such as caspase-3, eventually resulting in cell death [24, 25].
Other pathway involves cell death receptors and also induces activation of caspases. The cell death signals are polypeptides belonging to the tumor necrosis factor (TNF) family. One of the best characterized members of this family is the cell surface receptor called Fas, which plays important roles in controlling cell death in the immune system. TNF and related family members consist of three identical polypeptide chains, and their binding induces receptor trimerization. The cytoplasmic portions of the receptors bind adaptor molecules that in turn bind an upstream caspase called caspase-8. This leads to activation of caspase-8 as a result of self-cleavage, and the activated molecules of caspase-8 can then activate other downstream caspases, thereby initiating a caspase cascade that results in death of the cell [13, 26].
Figure1. 12 Regulators and effectors of apoptosis. In mammalian cells, many cell death signals induce apoptosis as a result of damage to mitochondria, resulting in the release of cytochrome c and other pro-apoptotic molecules. Release of cytochrome c from mitochrondrial leads the formation of complexes (apoptosomes) containing Apaf-1 and caspase-9. [13]
1.4.3 Singnaling cell survival
There are other signaling pathways acted in the opposite direction to promote cell survival by inhibiting apoptosis. These signal pathways control the fate of a variety of cells whose survival is dependent on extracellular growth factors or cell-cell interactions. One of the major intracellular signaling pathways responsible for promoting cell survival is initiated by the enzyme Phosphatidylinositol 3-kinase (PI 3-kinase), which is activated cell survival is activated by either protein-tyrosine kinases of G protein-coupled receptors. PI 3-kinase
phosphorylates the membrane phospholipids PIP2 to form PIP3, which activates the protein-serine /threonine kinase Akt. Akt then phosphorlyates a number of proteins that regulate apoptosis. Overall speaking, activation of the PI 3-kinase pathway is critical for the survival of many cell types [13, 17, 27].
Figure1. 13 The PI 3-kinase pathway and cell survival. Survival factors such as NGF activate receptor protein-tryosine kinase, leading to activation of PI 3-kinase and formation of PIP3. PIP3 recruits the protein kinase Akt to the plasma membrane where it is activated as a result of phosphorylation by PDK1. Akt then appears to phosphorylate a number of proteins that contribute to cell survival. [13]
1.4.4 Anoikis
Fibroblasts and certain other cell types dissociated from their extracellular matrix undergo reversible growth arrest, thereby revealing their anchorage dependence. The ECM plays a role to physically support cell growth. The well linkage to ECM through integrins helps to transmit survival signals and to activate PI 3-kinase. PI 3-kinase and the AKT oncoprotein may mediate the anoikis-suppressing effects of focal adhesion kinase. The new term “anoikis”
-- the ancient Greek word for “homelessness”--was coined to denote the apoptosis that occurs in cells that are detached from matrix (or that are attached via the wrong molecules). The existence of anoikis implies that integrin signaling regulates critical components of the apoptotic machinery [28, 29]. Some papers suggested that primary embryonic fibroblasts are very susceptible to apoptosis (anoikis) when held in suspension. Embryonic fibroblasts were similar to epithelial cells and endothelial cells in that apoptosis is a default pathway: active suppression of apoptosis is essential for cell survival [30].