Mitochondrial regulation of apoptotic cell death.

Although it has long been known that impairment of mitochondrial function may lead to ATP depletion and necrotic cell death, recent work has revealed that these organelles also play an important role in the regulation of apoptotic cell death by mechanisms which have been conserved through evolution. Thus, it seems that a number of toxicants target the mitochondria and promote their release of cytochrome c and other pro-apoptotic proteins, which can trigger caspase activation and other parts of the apoptotic process. Cytochrome c release is governed by the Bcl-2 family of proteins, whereas subsequent caspase activation is modulated by other proteins, including inhibitor of apoptosis proteins (IAPs) and heat shock proteins. Recent findings indicate that cytochrome c extrusion occurs by a two-step process, which is initiated by a disruption of the association of the hemoprotein with cardiolipin, the phospholipid that anchors it to the outer surface of the inner mitochondrial membrane. Release of the solubilized pool of cytochrome c into the cytosol may then occur by permeabilization of the outer mitochondrial membrane mediated by pro-apoptotic Bcl-2 family proteins, notably Bax and Bak, or by Ca2+-triggered mitochondrial permeability transition. Taken together, these findings have placed the mitochondria in the focus of apoptosis research and further underlined the important function of these organelles in cell life and death.