Alternative definition : KEGG: Apoptosis is a genetically controlled mechanism of cell death involved in the
regulation of tissue homeostasis. The 2 major pathways of apoptosis are the extrinsic
(Fas and other TNFR superfamily members and ligands) and the intrinsic (mitochondria-associated)
pathways, both of which are found in the cytoplasm. The extrinsic pathway is triggered
by death receptor engagement, which initiates a signaling cascade mediated by caspase-8
activation. Caspase-8 both feeds directly into caspase-3 activation and stimulates
the release of cytochrome c by the mitochondria. Caspase-3 activation leads to the
degradation of cellular proteins necessary to maintain cell survival and integrity.
The intrinsic pathway occurs when various apoptotic stimuli trigger the release of
cytochrome c from the mitochondria (independently of caspase-8 activation). Cytochrome
c interacts with Apaf-1 and caspase-9 to promote the activation of caspase-3. Recent
studies point to the ER as a third subcellular compartment implicated in apoptotic
execution. Alterations in Ca2 homeostasis and accumulation of misfolded proteins
in the ER cause ER stress. Prolonged ER stress can result in the activation of BAD
and/or caspase-12, and execute apoptosis.;