Overview of cell death

Genomic instability is a characteristic of most cancers. In hereditary cancers, genomic instability results from mutations in DNA repair genes and drives cancer development. In precancerous lesions that typically retain wild-type (WT) p53 function, the oncogene-induced DNA damage elicits p53-dependent apoptosis and/or senescence, which limits growth of the lesion. Activation of DNA damage checkpoint pathway can serve to remove potential DNA-damaged cells by apoptosis induction to block carcinogenesis. It has been proved that apoptotic signals help to protect genomic integrity (Kerr et al., 1972; Negrini et al., 2010).

Apoptosis is a form of programmed cell death that occurs in multicellular organism.

Biological events lead to characteristic cell changes morphology and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, chromosomal DNA fragmentation, and global mRNA decay (Elmore, 2007). Apoptosis is considered an important component of various processes including normal cell turnover, proper development and functioning of the immune system, hormone-dependent atrophy, embryonic development and chemical-induced cell death.

Normally, apoptosis occurs during development and aging, and as a homeostatic mechanism to maintain cell populations in tissues. It also occurs as a defense mechanism, for example, in immune reactions or when cells are damaged by disease or noxious agents (Norbury and Hickson, 2001). But inappropriate apoptosis (too little or too much) is a factor in many human conditions including neurodegenerative diseases, ischemic damage, autoimmune disorders and many types of cancer (Elmore, 2007).

stimulus. Irradiation or drugs used for cancer chemotherapy results in DNA damage in some cells, which can lead to apoptotic death through a p53-dependent pathway. Some hormones, for example corticosteroids, results apoptotic death in some thymocytes, however other cells are unaffected or even stimulated.

The most characteristic feature of apoptosis is pyknosis, which is induced by chromatin condensation. During the early process of apoptosis, cell shrinkage and pyknosis are visible by light microscopy, with cell shrinkage, the cells are becoming smaller in size, the cytoplasm is dense and the organelles are more tightly packed. On histologic examination with hematoxylin and eosin stain, apoptotic cell appears as a round or oval mass with dark eosinophilic cytoplasm and dense purple nuclear chromatin fragments (Figure 1) (Elmore, 2007).

Figure 1. Figure 1A is a photomicrograph of a section of exocrine pancreas from a B6C3F1 mouse.

The arrows indicate apoptotic cells that are shrunken with condensed cytoplasm. The nuclei are pyknotic and fragmented. Figure 1B is an image of myocardium from a 14 week-old rat treated with ephedrine (25 mg/kg) and caffeine (30 mg/kg). Within the interstitial space, there are apoptotic cells with condensed cytoplasm, condensed and hyperchromatic chromatin and fragmented nuclei (long arrows). Admixed with the apoptotic bodies are macrophages, some with engulfed apoptotic bodies (arrowheads) (Elmore, 2007).

Another mode of cell death is necrosis, that, unlike apoptosis, is an alternative uncontrolled form of cell death that is induced by external injury, such as hypoxia or inflammation (Elmore, 2007). Necrosis is alternative to apoptotic cell death, and it is considered a toxic process where cell is a passive victim and follows an energy-independent mode of death. This process often involves upregulation of various pro-inflammatory proteins and compounds, such as nuclear factor nuclear factor kappa B (NF-kB), resulting in the rupture of the cell membrane causing spillage of the cell contents into surrounding areas, resulting in a cascade of inflammation and tissue damage (D'Arcy, 2019). In contrast to apoptosis, necrosis is an energy independent form of cell death, where the cell is damaged so severely by a sudden shock (radiation, heat, chemicals, hypoxia, etc.) that it is unable to function. The cell usually responds by swelling as it fails to maintain homoeostasis with its environment (Table 1). Necrosis is usually observed as an endpoint state in cell culture by the presence of cellular fragments in the media, what is described in many cases in a cell culture setting as necrosis is often simply the remains of late apoptotic cells, the apoptotic bodies of which have lost integrity (Nikoletopoulou et al., 2013). Since necrosis refers to the degradative processes that occur after cell death, it is considered by some to be an inappropriate term to describe a mechanism of cell death (Elmore, 2007).

Apoptosis Necrosis

Single cells or small clusters of cells Often contiguous cells Cell shrinkage and convolution Cell swelling

Pyknosis and karyorrhexis Karyolysis, pyknosis, and karyorrhexis Intact cell membrane Disrupted cell membrane Cytoplasm retained in apoptotic

bodies Cytoplasm released

No inflammation Inflammation usually present

Autophagy is another cell death process where cellular components such as macroproteins or even whole organelles are sequestered into lysosomes for degradation (Mizushima et al., 2008; Shintani and Klionsky, 2004). The lysosomes are then able to digest these substrates, the components of which can either be recycled to create new cellular structures and/or organelles or alternatively can be further processed and used as a source of energy. Autophagy can be initiated by a variety of stressors, most notably by nutrient deprivation (caloric restriction) or can result from signals present during cellular differentiation and embryogenesis and on the surface of damaged organelles (Mizushima et al., 2008). Recently, accumulating evidence reveals that autophagy and apoptosis can cooperate, antagonize or assist each other, thus influencing differentially the fate of the cell (Nikoletopoulou et al., 2013).

In summary, cell death may result from a number of distinct and highly regulated energy-dependent processes (apoptosis and autophagy) or from energy independent processes (necrosis). These distinct processes can be distinguished from one another based on their morphologic and biochemical behavior. Cell death can be separated into primarily non-inflammatory and pro-inflammatory categories (Figure 2). Among them, apoptosis is generally non-inflammatory and results in the orderly removal of damaged cells from tissues without inducing collateral damage to surrounding cells (Elmore, 2007).

Fig 2. Types of cell death and their morphological hallmarks. Diagrammatic classification of different types of cell death. PCD: programmed cell death. Morphological features of a) a healthy cell, b) a necrotic cell, c) an apoptotic cell and d) an autophagic cell (Nikoletopoulou et al., 2013).