Necrosis

Morphologically, necrosis is very different from apoptosis (Fig. 6.1). Unlike the cell shrinking that occurs early in apoptosis, early stages of necrosis are characterized by "oncosis" or swelling [9]. The typical blebbing into multiple bodies observed during apoptosis is not observed in necrotic cells. Necrotic cell swelling is associated with organelle enlargement and rupture, which eventually leads to cytoplasmic destruction and nuclear shrinkage [8, 9]. In fact, a key feature that distinguishes necrosis from apoptosis is the rapid and early loss of cytoplasmic membrane integrity, concomitant with extensive cytoplasmic damage. These events are likely to be mediated by the enzymatic activity of lysosomal proteases such as the cathepsins, which are released upon lysosomal rupture [74]. Loss of cytoplasmic membrane integrity facilitates the release into the surrounding medium of intracellular proteases and other dangerous intracellular contents, including proinflammatory signals [49, 50]. Interestingly, the nuclear membrane stays relatively intact during the early stages of necrosis [8, 9, 75], which could be associated with the preservation of lamin B integrity during this cell death process [33, 34, 75]. Lamin B is important for maintaining nuclear membrane integrity, and its cleavage early during apoptosis facilitates nuclear fragmenta tion [76]. Chromatin fragmentation occurs in necrotic cells but does not appear to lead to the nucleosomal ladder typically observed in apoptotic cells [77]. There is evidence, however, that serum nucleases such as DNAse I can penetrate necrotic cells and induce nucleosomal laddering [78].

Traditionally necrosis has been considered a non-programmed, accidental mode of cell death associated mainly with pathological conditions and that develops in response to acute cell injury caused by ischemia, extreme heat, severe bacterial and viral infections, and exposure to high levels of chemicals or toxins [8, 9, 65, 79]. There is, however, growing evidence to support the notion that there might be two types of necrosis, one that is physiological and involved in programmed cell death (PCD) and another that is mainly associated with pathological conditions [11, 12, 77, 80]. Physiological necrosis, considered a type of necrosis-like PCD, has been implicated in development-associated interdigital cell death [81], development-associated regression of the human tail [82], follicular maturation during oogenesis [83, 84], normal renewal of small and large intestines [85, 86], natural killer cell-mediated cytotoxicity [87, 88], complement-mediated cell killing [89], and activation-induced cell death of T lymphocytes [90].

In both pathological and experimental conditions, necrosis often coexists with apoptosis, arising either independently or as a secondary event following apop-tosis [8, 9, 65, 79, 91-96]. Under certain pathological conditions such as ischemia, extensive necrosis in the affected area within a tissue may trigger secondary damage in surrounding areas, usually occurring through apoptosis [65, 97, 98]. It has been recognized that necrosis and apoptosis can be induced by the same insults, but the intensity of these insults, the biochemical environment, and the cell type determine which mode of cell death predominates [99].

Caspases may play a key role in guarding the cell against unwanted necrotic death [77]. This notion is supported by the observation that a caspase-indepen-dent mode of cell death with necrotic morphology usually ensues when specific cell types are exposed to apoptosis inducers in the presence of broad caspase inhibitors such as benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD-fmk) [100]. This type of cell death could be considered as a backup cellular defense system that ensures the cell's demise in the event that the caspase activation program is rendered nonfunctional, e.g., during a viral infection. Viruses are known to modulate host cell apoptosis by producing proteins that antagonize caspases and perhaps other components of the apoptotic program [101]. Agents that can induce caspase-independent cell death with necrotic morphology in the presence of caspase inhibitors include cancer drugs [102, 103], death receptor ligands [104, 105], oncogenes [106], anti-CD2 antibodies [107], stauros-porine (STS) [107], and viral proteins [108].

It is likely that in the presence of cell-death stimuli, caspase-independent ne-crotic cell death is activated as a background pathway that runs concurrently with apoptosis, leading eventually to the secondary necrosis that follows apopto-sis in the absence of phagocytosis. This background pathway might be activated by death-enhancing factors - such as reactive oxygen species (ROS) or cytochrome c - that are generated or released by mitochondria [77, 109, 110]. In the event the caspase activation program is impaired, this pathway might then be revealed or enhanced. Alternatively, caspase-independent necrotic cell death may be activated only after the cell senses that caspases are not able to respond to a specific death signal. For instance, LCC human carcinoma cells deficient in caspases die via necrosis in the presence of a zinc chelator, whereas cells expressing caspases respond to the same insult by activating the apoptosis pathway [111]. Inhibition of effector caspases by exogenous nitric oxide is also known to switch apoptosis to necrosis [112]. Moreover, necrotic cell death plays an important role in driving the development of mouse embryos in which caspases were inhibited or genetically deleted [12, 81, 113]. It should be noted that not all cas-pase-independent cell-death processes display necrotic morphology, since some display the morphological features of apoptosis [10, 11].

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