Apoptosis pathways are automatic biological systems that an organism uses to destroy genetically damaged or altered cells through a process called programmed cell death. To accomplish apoptosis, which means, literally, “falling off,” an organism uses one of two known apoptosis pathways: intrinsic and extrinsic. Once activated by a damaged cell, both of these pathways use enzymes to break down the cell’s protein structure, disassembling the cell at a chemical level. Using apoptosis pathways, an organism can rid itself of damaged cells before necrosis can set in or, in the case of certain cancer cells, before the cell can spread, causing further damage.
Pathways of intrinsic apoptosis can be thought of as a cell’s self-destruct system. Intrinsic apoptosis initiates from within a cell when it becomes physically damaged or critically stressed in some other way, such as from hypoxia or disease. When this occurs, certain normally stable proteins are released by the damage, become unstable, and trigger the apoptosis process by activating genes in the cell’s DNA that create apoptosis enzymes. These enzymes then attack the cell’s mitochondrial DNA, destroying it completely and releasing yet more proteins and enzymes that further degrade the cell. Without its DNA, the cell can no longer reproduce or perform any functions other than breaking itself down chemically.
Extrinsic apoptosis pathways are activated outside a cell when other systems in an organism have determined that the cell must die, usually to make way for new or different cell growth. There are two ways that extrinsic apoptosis can be triggered: by a chemical activation or by certain stresses such as exposure to radiation. In chemical activation, the organism releases chemical compounds that attach to the cell, using receptors on the cell wall called “death receptors.” The activation of these receptors then triggers the cell’s internal apoptosis process to destroy the cell. In stress-related extrinsic apoptosis, radiation and chemicals, such as used in treating certain cancers, make the mitochondrial DNA permeable, allowing the apoptosis pathway proteins and enzymes to be released, destroying the cell.
Although apoptosis pathways are designed to benefit an organism, some other organisms, such as viruses, have adapted themselves to make use of them. Many viruses can invade a cell and then deactivate the cell’s intrinsic apoptosis pathways while they reproduce inside of the cell safely hidden from the organism’s immune system. Other viruses, such as HIV, can create chemicals that weaken or trigger apoptosis in immune system cells, thus destroying them and preventing them from combating the virus.