Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 34

If animal cells are injured mechanically, by wounding, heat or deep temperatures, or toxic chemicals, they often die spontaneously in a process called necrosis. Necrotic cells release their content in the environment and usually elicit an inflammatory response.

Cells do not live forever; they can show senescence (see Section 4.1.2 on telomeres and telomerase) and can eventually die in a programmed cell death, termed apoptosis. Cells then shrink, the cytoskeleton collapses, the nuclear envelope disassembles, and the chromosomes become fragmented. DNA shows a typical ladder when studied by electrophoresis. Cells break into several smaller compartments, the apoptotic bodies, which are eventually degraded by macrophages through phagocytosis. Thus, apoptotic cells do no leave a corpse behind. Apoptosis occurs during the development of tissues and organs but also in aging tissues and organs. Also some chemicals that disturb membranes or DNA can induce apoptosis.

Apoptosis (Figure 3.29) is triggered by a group of intracellular proteases, termed caspases (c for cysteine and asp for aspartic acid). The caspases are produced as inactive precursors and are activated only during apoptosis. Two classes of caspases are distinguished: initiator caspasesand executioner caspases. The initiator caspases (caspase 8 and 9) occur as inactive monomers. Activated by an apoptotic signal, the monomers assemble to complexes. Dimers are formed, which cleave each other in the protease domain, resulting in an active enzyme. The activated initiator caspases then activate executioner caspases (caspases 3, 6, and 7) by cleaving their protease domain. As a consequence, the activated executioner caspases then start to hydrolyze all cellular proteins, which will lead to cell death. Also a DNA degrading endonuclease becomes activated, which then starts to degrade the chromosomes.

Figure 3.29 Schematic outline of apoptotic pathways.

The initial signal for apoptosis can derive from the extrinsic pathway or the intrinsic (mitochondrial pathway). The extrinsic pathway becomes activated, when certain signal proteins bind to surface death receptors, which belong to the tumor necrosis factor (TNF) receptor family. The death receptor consists of a TNF receptor plus a Fas death receptor. When a killer lymphocyte with Fas ligands binds to the Fas death receptor, intracellular adaptor proteins bind to initiator caspases (caspase 8) forming a death‐inducing signaling complex(DISC). DISC actives downstream the executioner caspases.

The intrinsic pathway, which is activated by DNA or membrane damage, is more complicated: in principle, mitochondrial proteins (such as cytochrome c) are released into the cytosol where they activate the caspase cascade. Cytochrome c binds to the adaptor protein Apaf1 (apoptotic protease activating factor), which oligomerizes to the apoptosome. Apaf1 proteins activate initiator caspases (caspase 9), which then stimulate the executioner caspases downstream. The release of mitochondrial proteins is tightly controlled by proteins of the Bcl2 family. In the Bcl2 family, we distinguish between pro‐apoptotic (Bax, Bak, Bad, Bim, Bid, Puma, Noxa) and anti‐apoptotic (Bcl2, BclX_L) proteins. If DNA is strongly damaged, the tumor suppressor gene p53 accumulates. p53 stimulates the expression of pro‐apoptotic proteins Puma and Noxa and then initiates the intrinsic pathway.