As one of the cellular death mechanisms, apoptosis, also known as programmed cell death, can be defined as the process of a proper death of any cell under certain or necessary conditions. Apoptosis is controlled by the interactions between several molecules and responsible for the elimination of unwanted cells from the body.

Many biochemical events and a series of morphological changes occur at the early stage and increasingly continue till the end of apoptosis process. Morphological event cascade including cytoplasmic filament aggregation, nuclear condensation, cellular fragmentation, and plasma membrane blebbing finally results in the formation of apoptotic bodies. Several biochemical changes such as protein modifications/degradations, DNA and chromatin deteriorations, and synthesis of cell surface markers form morphological process during apoptosis.

Apoptosis can be stimulated by two different pathways: (1) intrinsic pathway (or mitochondria pathway) that mainly occurs via release of cytochrome c from the mitochondria and (2) extrinsic pathway when Fas death receptor is activated by a signal coming from the outside of the cell.

Different gene families such as caspases, inhibitor of apoptosis proteins, B cell lymphoma (Bcl)-2 family, tumor necrosis factor (TNF) receptor gene superfamily, or p53 gene are involved and/or collaborate in the process of apoptosis.

Caspase family comprises conserved cysteine aspartic-specific proteases, and members of caspase family are considerably crucial in the regulation of apoptosis. There are 14 different caspases in mammals, and they are basically classified as the initiators including caspase-2, -8, -9, and -10; and the effectors including caspase-3, -6, -7, and -14; and also the cytokine activators including caspase-1, -4, -5, -11, -12, and -13. In vertebrates, caspase-dependent apoptosis occurs through two main interconnected pathways which are intrinsic and extrinsic pathways. The intrinsic or mitochondrial apoptosis pathway can be activated through various cellular stresses that lead to cytochrome c release from the mitochondria and the formation of the apoptosome, comprised of APAF1, cytochrome c, ATP, and caspase-9, resulting in the activation of caspase-9. Active caspase-9 then initiates apoptosis by cleaving and thereby activating executioner caspases. The extrinsic apoptosis pathway is activated through the binding of a ligand to a death receptor, which in turn leads, with the help of the adapter proteins (FADD/TRADD), to recruitment, dimerization, and activation of caspase-8 (or 10). Active caspase-8 (or 10) then either initiates apoptosis directly by cleaving and thereby activating executioner caspase (-3, -6, -7), or activates the intrinsic apoptotic pathway through cleavage of BID to induce efficient cell death. In a heat shock-induced death, caspase-2 induces apoptosis via cleavage of Bid.

Bcl-2 family members are divided into three subfamilies including (i) pro-survival subfamily members (Bcl-2, Bcl-xl, Bcl-W, MCL1, and BFL1/A1), (ii) BH3-only subfamily members (Bad, Bim, Noxa, and Puma9), and (iii) pro-apoptotic mediator subfamily members (Bax and Bak). Following activation of the intrinsic pathway by cellular stress, pro‑apoptotic BCL‑2 homology 3 (BH3)‑only proteins inhibit the anti‑apoptotic proteins Bcl‑2, Bcl-xl, Bcl‑W and MCL1. The subsequent activation and oligomerization of the Bak and Bax result in mitochondrial outer membrane permeabilization (MOMP). This results in the release of cytochrome c and SMAC from the mitochondria. Cytochrome c forms a complex with caspase-9 and APAF1, which leads to the activation of caspase-9. Caspase-9 then activates caspase-3 and caspase-7, resulting in cell death. Inhibition of this process by anti‑apoptotic Bcl‑2 proteins occurs via sequestration of pro‑apoptotic proteins through binding to their BH3 motifs.

One of the most important ways of triggering apoptosis is mediated through death receptors (DRs), which are classified in TNF superfamily. There exist six DRs: DR1 (also called TNFR1); DR2 (also called Fas); DR3, to which VEGI binds; DR4 and DR5, to which TRAIL binds; and DR6, no ligand has yet been identified that binds to DR6. The induction of apoptosis by TNF ligands is initiated by binding to their specific DRs, such as TNFα/TNFR1, FasL /Fas (CD95, DR2), TRAIL (Apo2L)/DR4 (TRAIL-R1) or DR5 (TRAIL-R2). When TNF-α binds to TNFR1, it recruits a protein called TNFR-associated death domain (TRADD) through its death domain (DD). TRADD then recruits a protein called Fas-associated protein with death domain (FADD), which then sequentially activates caspase-8 and caspase-3, and thus apoptosis. Alternatively, TNF-α can activate mitochondria to sequentially release ROS, cytochrome c, and Bax, leading to activation of caspase-9 and caspase-3 and thus apoptosis. Some of the miRNAs can inhibit apoptosis by targeting the death-receptor pathway including miR-21, miR-24, and miR-200c.

p53 has the ability to activate intrinsic and extrinsic pathways of apoptosis by inducing transcription of several proteins like Puma, Bid, Bax, TRAIL-R2, and CD95.

Some inhibitors of apoptosis proteins (IAPs) can inhibit apoptosis indirectly (such as cIAP1/BIRC2, cIAP2/BIRC3) or inhibit caspase directly, such as XIAP/BIRC4 (inhibits caspase-3, -7, -9), and Bruce/BIRC6 (inhibits caspase-3, -6, -7, -8, -9). 

Any alterations or abnormalities occurring in apoptotic processes contribute to development of human diseases and malignancies especially cancer.

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What is Apoptosis? The Apoptotic Pathways and the Caspase Cascade