Permeability Transition Pore Complex, Apoptosis

Marzo I, Brenner C, Zamzami N, Susin SA, Beutner G, Brdiczka D, Remy R, Xie ZH, Reed JC, Kroemer G (1998), The permeability transition pore complex a target for apoptosis regulation by caspases and bcl-2-related proteins, J. Exp. Med 187 1261-1271. 

A key event in preventing apoptosis is thus the retention of cytochrome c within mitochondria. The permeability transition pore complex is formed between the inner and outer mitochondrial membranes and is reported to control protein release from the intermembrane space. The permeabihty transition pore complex comprises the adenine nucleotide transporter, the voltage-dependent anion channel and possibly other proteins such as the benzodiazepine receptor and cyclophilin D [65]. Thus, cells possess specialised systems and processes for retaining cytochrome c within mitochondria to ensure survival, as well as systems that can rapidly mobilise this molecule when the apoptotic pathway is triggered. 

Fig. 1. Modulation of apoptosis by v-FLIP and v-Bcl-2. v-FLIPs specifically inhibit apoptosis mediated by death receptors. v-lCA specifically targets caspase-8 and inhibits its activation. v-Bcl-2 and vMIA inhibit those apoptotic pathways that are signaled through mitochondrial release of cytochrome c. FADD, Fas-associated death domain FLICE, FADD-like interleukin-converting enzyme CARD, cas-pase-recruiting domain PTPC. permeability transition pore complex FLIP, FLICE-inhibitory protein vie A, viral inhibitor of caspase 8-induced-apoptosis MIA. viral mitochondrial inhibitor of apoptosis Apcif-l, apoptotic protease-activating factor 1...

Cyclosporine binds to cyclophilin within T cells. The CsA-cyclophilin complex then binds to calcinenrin and inhibits calcinenrin s activity reqnired for the dephosphorylation of regnlatory proteins necessary for the transcription and prodnction of proinflammatory cytokines (IL-2, IL-4, IFN-y, and tnmor necrosis factor-a) from T-helper cells. CsA prevents pathologic apoptosis of the tear-secreting epithelia by preventing the ability of the mitochondrial permeability transition pore to open, a reqnired step in the apoptotic process. 

Fig. 21.14. The mitochondrial permeability transition pore (MPTP). In the MPTP, ANT is thought to complex with VDAC. The conformation of ANT is regulated by cyclophilin D (CD), and Ca. VDACs bind a number of proteins, including Bcl2 and Bax, which regulate apoptosis. The change to an open pxrre is activated by Ca, depletion of adenine nucleotides, and oxygen radicals (ROS) that alter SH groups. It is inhibited by the electrochemical potential gradient (Ap), by cytosolic ATP, and by a low cytosolic pH.

Figure 3. Schematic architecture of mitochondrial protein complexes. A transmembrane channel, called the permeability transition pore (FTP), is formed at the contaa sites between the inner and outer mitochondrial membrane (OM) of the mitochondria. The core components of PTP are the voltage-dependent anion channel (VDAC) in the outer membrane and the adenine nucleotide translocator (ANT) in the inner membrane (IM). VDAC allows diilusion of small molecules (<5 kDa), however ANT is only permeable to a few selected ions and metabolites and is responsible for maintaining the proton concentration gradient (pH) and the membrane elearic potential (A P,J. PTP is sometimes connected to destruction of permeability barrier and loss of the inner membrane potential and eventually results in mitochondrial membrane permeability transition during apoptosis and other specialized forms of cell death. Bax, Bak, Bc1-Xl and Bcl-2 locate in the outer membrane and may regulate the outer membrane permeability. The translocase of the outer membrane (TOM) and the translocase of the inner membrane (TlM) mediate protein import pathway in the mitochondria. Cy-D, cyclophilin D PBR, peripheral benzodiazepine receptor HK, hexokinase mtHSP70, mitochondrial heat shock protein 70.

The Bcl-2 family of oncoproteins is known to play an important role in apoptosis through their ability to regulate cytochrome c release from mitochondria [11,15]. The antiapoptotic proteins Bcl-2 and Bc1-Xl prevent cytochrome c release, whereas the proapoptotic family members (e.g.. Bad, Bid, Bik, Bax) facilitate cytochrome c efflux or block the protective effects of Bcl-2 and Bc1-Xl. The mechanism involved is unclear the Bcl-2 family proteins may interact directly with the MTP (mitochondrial permeability transition) protein complex (the PTPC) or form independent ionic pores in the outer mitochondrial membrane (Fig. 3). Nonetheless, cytochrome c-depen-dent caspase-3 activation and changes in the expression or phosphorylation state of Bcl-2 family proteins are taken as indicative of mitochondria-dependent apoptotic pathways. It is important to remember that other apoptogenic proteins are also present in the mitochondrial intermembrane space, including smac/ DIABLO and flavoprotein (AIF) [10,11,60]. The release stimuli for the latter factors, which are currently being elucidated, may also involve the permeability transition or the Bcl-2 family proteins [37]. 

Accumulated evidence from recent research indicates that mitochondria-derived factors, such as cytochrome c, have an important role in the apoptosis of some cells. Previous reports show that cytochrome c and caspase-9 participate in Apafl apoptosome, a complex important for caspase-3 activation. Cytochrome c was released from mitochondria of HL-60 cells during treatment with carnosic acid, carnosol or ursolic acid. Previous evidence showed that mitochondria permeability transition (MPT) coupled with depolarization of the membrane potential induces the release of cytochrome c (31-33). It also has been reported that cytochrome c released from mitochondria can precede dissipation of the voltage gradient (the mitochondrial transmembrane potential vi/ ) across the membrane, suggesting that the escape of cytochrome c from mitochondria occurs prior to permeability transition pore opening (loss of mitochondrial transmembrane potential) (34,35). Here we observed the depolarization of the mitochondrial membrane potential in HL-60 cells by treatment with carnosic acid, carnosol, or ursolic acid for Ih. Cytochrome c was released after 3h treatment of carnosic acid, carnosol, or ursolic acid. 

---