Mechanisms of cytochrome c release from mitochondria

Abstract

In healthy cells, cytochrome c (Cyt c) is located in the mitochondrial intermembrane/intercristae spaces, where it functions as an electron shuttle in the respiratory chain and interacts with cardiolipin (CL). Several proapoptotic stimuli induce the permeabilization of the outer membrane, facilitate the communication between intermembrane and intercristae spaces and promote the mobilization of Cyt c from CL, allowing for Cyt c release. In the cytosol, Cyt c mediates the allosteric activation of apoptosis-protease activating factor 1, which is required for the proteolytic maturation of caspase-9 and caspase-3. Activated caspases ultimately lead to apoptotic cell dismantling. Nevertheless, cytosolic Cyt c has been associated also to vital cell functions (i.e. differentiation), suggesting that its release not always occurs in an all-or-nothing fashion and that mitochondrial outer membrane permeabilization may not invariably lead to cell death. This review deals with the events involved in Cyt c release from mitochondria, with special attention to its regulation and final consequences.

Methods for the assessment of mitochondrial membrane permeabilization in apoptosis

“…the nucleus, lysosomes, the endoplasmic reticulum, or the cytosol) converge on mitochondria where they favor mitochondrial membrane permeabilization (MMP) [7][8][9]. Upon permeabilization of the mitochondrial outer membrane (OM), intermembrane space (IMS) proteins, that include caspase activators such as cytochrome c (Cyt c) [10], Omi/HtrA2 (Omi stress-regulated endoprotease/High temperature requirement protein A 2) [11,12] and Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP binding protein with a low pI) [13,14], as well as caspase-independent death effectors like apoptosis-inducing factor (AIF) [15,16] and endonuclease G (EndoG) [17], are released into the cytosol. Cyt c promotes the activation of the initiator caspase-9 in a direct fashion via the assembly of the apoptosome (together with the apoptosis protease activating factor-1, i.e.…”

“…Moreover, small amounts of Cyt c released through a partial OM permeabilization may participate in one or more feed-forward mechanisms that amplify MMP (for a review see Ref. [10]). The heterogeneity in the kinetics of IMS protein release may yield false negative (and more rarely false positive) results.…”

Cdk2 acts upstream of mitochondrial permeability transition during paclitaxel-induced apoptosis

“…In the first, ligandbound membrane receptors such as CD95/Fas/APO-1 recruit and oligomerize the adaptor protein and procaspase-8, followed by caspase-8 processing (Salvesen and Dixit, 1999). In the second pathway, apoptosis is triggered by a change in mitochondrial permeability that results in the release of cytochrome c Garrido et al, 2006;Caroppi et al, 2009). Once cytochrome c binds its cytosolic partner Apaf-1, the human homologue of the apoptotic protein CED-4, to form an oligomeric Apaf-1-cytochrome c complex in an ATP/dATP-dependent manner.…”

“…Pro-apoptotic Bcl-2 proteins induce the mitochondrial cytochrome c release whereas overexpressed anti-apoptotic Bcl-2 proteins prevent it (Placzek et al, 2010). Secondly, the process of apoptosome formation and caspase-9 autoactivation can be negatively regulated by Hsp-70 and Hsp-90, which prevents the oligomerization of Apaf-1 and its association with procaspase-9 (Beere et al, 2000;Pandey et al, 2000;Saleh et al, 2000;Garrido et al, 2006). Thirdly, activations of caspase-9, -3 and -7 are negatively regulated by inhibitor of apoptosis (IAP) family proteins (Liston et al, 2003;Chang and Schimmer, 2007), which directly bind and inhibit these caspases.…”

Cardiolipin: Setting the beat of apoptosis

“…Mitochondrial membrane permeabilization is tightly regulated by proteins of the Bcl-2 family and is often considered the point of no return in the apoptotic signalling cascade, leading to several events such as DNA degradation in the nucleus and exposure of phosphatidylserine (PS) on the outer leaflet of the plasma membrane [8,9].…”