Cardiolipin deficiency releases cytochrome c from the inner mitochondrial membrane and accelerates stimuli-elicited apoptosis

Abstract

Cardiolipin (CL) is a mitochondria-specific phospholipid synthesized by CL synthase (CLS). We describe here a human gene for CLS and its analysis via RNAi knockdown on apoptotic progression. Although mitochondrial membrane potential is unchanged in cells containing only 25% of the normal amount of CL, free cytochrome c (cyt. c) is detected in the intermembrane space and the mitochondria exhibit signs of reorganized cristae. However, the release of cyt. c from the mitochondria still requires apoptotic stimulation. Increased sensitivity to apoptotic signals and accelerated rates of apoptosis are observed in CL-deficient cells, followed by elevated levels of secondary necrosis. Apoptosis is thought to progress via binding of truncated Bid (tBid) to mitochondrial CL, followed by CL oxidation which results in cyt. c release. The exaggerated and accelerated apoptosis observed in CL-deficient cells is matched by an accelerated reduction in membrane potential and increased cyt. c release, but not by decreased tBid binding. This study suggests that the CL/cyt. c relationship is important in apoptotic progression and that regulating CL oxidation or/and deacylation could represent a possible therapeutic target.

Cardiolipin or MTCH2 can serve as tBID receptors during apoptosis

“…Indeed, these mitochondria exhibited increased electron density with less and disorganized cristae (Figure 2e), which is in agreement with previously published results. [33][34][35][36][37] In addition to reduction in cyt c, we also observed a decrease in the protein level of MTCO1 (Figure 2b). A reduction in the amount of this labile protein suggests a defect in respiratory chain complex IV assembly.…”

“…26,28 Some authors even showed increased kinetics of apoptosis in cells with less CL. 35 Importantly, MTCH2, a protein of the mitochondrial carrier family inserted in the MOM, has been recently proposed to act as a receptor for tBID under apoptotic conditions. 29,41 To test the involvement of CL in apoptosis, we chose to focus on TRAIL-induced apoptosis in HCT116 cells.…”

Role of cardiolipin in cytochrome c release from mitochondria

“…4 In contrast, these findings do not support a prominent role for cardiolipin in targeting tBid to the OMM, as the association of tBid with mitochondria was unchanged by the reduced cardiolipin level. 34 Similarly, although a requirement of cardiolipin for tBid/Bax-induced cytochrome c release was demonstrated with artificial liposomes, this has not yet been found to be the case in experiments with more physiological models, such as isolated mitochondria and intact cells. The apparent discrepancy between the findings with mitochondria and liposomes suggests that additional molecules may exist in the OMM that are important for this process.…”

“…Recently, Choi et al 34 addressed the question of the role of cardiolipin in apoptosis signaling using RNAi knockdown of the mammalian cardiolipin synthase to decrease the content of this phospholipid in the mitochondria. When exposing the cells to triggers of apoptosis, such as agonistic Fas antibody or TNFa, it was found that decreasing the cardiolipin level to 25% of that present in control cells led to accelerated cell death.…”

Interplay between bax, reactive oxygen species production, and cardiolipin oxidation during apoptosis

“…CL was found to be the most effective "destabilizer" of the cyt c's structure as compared to other anionic phospholipids such as PA, PS, and PIs. It should be also noted that only marginal amounts (<1.0%) of PS, PA and PG are present in mitochondria [22,23]. The content of CL is about 10 times higher than that of PI in the inner mitochondrial membrane [22].…”