Dependent Mitochondrial Permeability Transition Research Articles

Heat Shock Protein 60 Regulation of the Mitochondrial Permeability Transition Pore in Tumor Cells

Mitochondrial apoptosis plays a critical role in tumor maintenance and dictates the response to therapy in vivo; however, the regulators of this process are still largely elusive. Here, we show that the molecular chaperone heat shock protein 60 (Hsp60) directly associates with cyclophilin D (CypD), a component of the mitochondrial permeability transition pore. This interaction occurs in a multichaperone complex comprising Hsp60, Hsp90, and tumor necrosis factor receptor-associated protein-1, selectively assembled in tumor but not in normal mitochondria. Genetic targeting of Hsp60 by siRNA triggers CypD-dependent mitochondrial permeability transition, caspase-dependent apoptosis, and suppression of intracranial glioblastoma growth in vivo. Therefore, Hsp60 is a novel regulator of mitochondrial permeability transition, contributing to a cytoprotective chaperone network that antagonizes CypD-dependent cell death in tumors.

Nimesulide aggravates redox imbalance and calcium dependent mitochondrial permeability transition leading to dysfunction in vitro

Nimesulide (selective cyclooxygenase-2 inhibitor) is a nonsteroidal anti-inflammatory drug for the symptomatic treatment of painful conditions like osteoarthritis, spondilitis and primary dysmenorrhoea. Nimesulide induced liver damage is a serious side effect of this otherwise popular drug. The mechanism involved in nimesulide induced hepatotoxicity is still not fully elucidated. However, both mitochondrial dysfunction and oxidative stress have been implicated in contributing to liver injury in susceptible patients. Mitochondria besides being the primary source of energy, act as a hub of signals responsible for initiating cell death, irrespective of the pathway, i.e. apoptosis or necrosis. The present study was aimed to explore the role of compounding stress, i.e. Ca 2+ overload and GSH depletion in nimesulide induced mitochondrial toxicity and dysfunction. Our study showed that, nimesulide (100 μM) treatment resulted into rapid depletion of GSH (60%) in isolated rat liver mitochondria and significant Ca 2+ dependent MPT changed. Enhanced ROS generation (DCF fluorescence) was also observed in mitochondria treated with nimesulide. An important finding was that the concentration at which nimesulide oxidized reduced pyridine nucleotides (autofluorescence of NAD(P)H), it affected mitochondrial electron flow (MTT activity decreased by 75%) and enhanced mitochondrial depolarization significantly as assessed by Rhodamine 123 fluorescent probe. Therefore, nimesulide was found to aggravate redox imbalance and affect Ca 2+ dependent mitochondrial membrane permeability transition leading to dysfunction and ultimately cell death.

Mitochondria generated nitric oxide protects against permeability transition via formation of membrane protein S-nitrosothiols

Mitochondria generated nitric oxide (NO) regulates several cell functions including energy metabolism, cell cycling, and cell death. Here we report that the NO synthase inhibitors (L-NAME, L-NNA and L-NMMA) administered either in vitro or in vivo induce Ca2+-dependent mitochondrial permeability transition (MPT) in rat liver mitochondria via a mechanism independent on changes in the energy state of the organelle. MPT was determined by the occurrence of cyclosporin A sensitive mitochondrial membrane potential disruption followed by mitochondrial swelling and Ca2+ release. In in vitro experiments, the effect of NOS inhibitors was dose-dependent (1 to 50µM). In addition to cyclosporin A, L-NAME-induced MPT was sensitive to Mg2+ plus ATP, EGTA, and to a lower degree, to catalase and dithiothreitol. In contrast to L-NAME, its isomer D-NAME did not induce MPT. L-NAME-induced MPT was associated with a significant decrease in both the rate of NO generation and the content of mitochondrial S-nitrosothiol. Acute and chronic in vivo treatment with L-NAME also promoted MPT and decreased the content of mitochondrial S-nitrosothiol. SNAP (a NO donor) prevented L-NAME mediated MPT and reversed the decrease in the rate of NO generation and in the content of S-nitrosothiol. We propose that S-nitrosylation of critical membrane protein thiols by NO protects against MPT.

Effect of progesterone and its synthetic analogues on the activity of mitochondrial permeability transition pore in isolated rat liver mitochondria

The influence of progesterone and its synthetic analogues on the induction of the Ca 2+-dependent mitochondrial permeability transition pore (MPTP) has been studied. The novel synthetic analogue of progesterone 17a-acetoxy-3b-butanoyloxy-6-methyl-pregna-4,6-diene-20-on (buterol) was compared with progesterone and medroxyprogesterone acetate (MPA). It was found that progesterone and buterol have opposite effects on the induction of MPTP opening by calcium ions. By contrast to progesterone, which decreased the calcium ion concentration necessary for pore opening, and MPA, which also, although at a lesser extent, activated the pore induction, buterol at a concentration of 20–100 μM blocked the pore opening and increased the calcium retention capacity of mitochondria more than twofold. The action of buterol is specific to the pore since it did not affect the respiration, whereas progesterone completely inhibited NAD-dependent respiration. MPA acted similar to progesterone but less effectively. The inhibitory effect of buterol was eliminated in the presence of carboxyatractyloside, which selectively binds the thiol groups of adenylate translocase and prevents the adenine nucleotide binding. These data indicate that buterol interacts with thiol groups, which explains its inhibitory effect not only on the mitochondrial pore but also on the transport system of xenobiotics in tumor cells in which buterol reduces the multidrug resistance.

An increase in intracellular Ca2+ is required for the activation of mitochondrial calpain to release AIF during cell death

Apoptosis-inducing factor (AIF), a flavoprotein with NADH oxidase activity anchored to the mitochondrial inner membrane, is known to be involved in complex I maintenance. During apoptosis, AIF can be released from mitochondria and translocate to the nucleus, where it participates in chromatin condensation and large-scale DNA fragmentation. The mechanism of AIF release is not fully understood. Here, we show that a prolonged ( approximately 10 min) increase in intracellular Ca(2+) level is a prerequisite step for AIF processing and release during cell death. In contrast, a transient ATP-induced Ca(2+) increase, followed by rapid normalization of the Ca(2+) level, was not sufficient to trigger the proteolysis of AIF. Hence, import of extracellular Ca(2+) into staurosporine-treated cells caused the activation of a calpain, located in the intermembrane space of mitochondria. The activated calpain, in turn, cleaved membrane-bound AIF, and the soluble fragment was released from the mitochondria upon outer membrane permeabilization through Bax/Bak-mediated pores or by the induction of Ca(2+)-dependent mitochondrial permeability transition. Inhibition of calpain, or chelation of Ca(2+), but not the suppression of caspase activity, prevented processing and release of AIF. Combined, these results provide novel insights into the mechanism of AIF release during cell death.

Bax-mediated mitochondrial outer membrane permeabilization (MOMP), distinct from the mitochondrial permeability transition, is a key mechanism in diclofenac-induced hepatocyte injury: Multiple protective roles of cyclosporin A

Diclofenac, a widely used nonsteroidal anti-inflammatory drug, has been associated with rare but severe cases of clinical hepatotoxicity. Diclofenac causes concentration-dependent cell death in human hepatocytes (after 24–48 h) by mitochondrial permeabilization via poorly defined mechanisms. To explore whether the cyclophilin D (CyD)-dependent mitochondrial permeability transition (mPT) and/or the mitochondrial outer membrane permeabilization (MOMP) was primarily involved in mediating cell death, we exposed immortalized human hepatocytes (HC-04) to apoptogenic concentrations of diclofenac (> 500 μM) in the presence or absence of inhibitors of upstream mediators. The CyD inhibitor, cyclosporin A (CsA, 2 μM) fully inhibited diclofenac-induced cell injury, suggesting that mPT was involved. However, CyD gene silencing using siRNA left the cells susceptible to diclofenac toxicity, and CsA still protected the CyD-negative cells from lethal injury. Diclofenac induced early (9 h) activation of Bax and Bak and caused mitochondrial translocation of Bax, indicating that MOMP was involved in cell death. Inhibition of Bax protein expression by using siRNA significantly protected HC-04 from diclofenac-induced cell injury. Diclofenac also induced early Bid activation (tBid formation, 6 h), which is an upstream mechanism that initiates Bax activation and mitochondrial translocation. Bid activation was sensitive to the Ca 2+ chelator, BAPTA. In conclusion, we found that Bax/Bak-mediated MOMP is a key mechanism of diclofenac-induced lethal cell injury in human hepatocytes, and that CsA can prevent MOMP through inhibition of Bax activation. These data support our concept that the Ca 2+–Bid–Bax–MOMP axis is a critical pathway in diclofenac (metabolite)-induced hepatocyte injury.

The properties of the mitochondrial megachannel in mitoplasts from human colon carcinoma cells are not influenced by Bax

This paper explores the relationship between Bax and the mitochondrial permeability transition pore (PTP). Isolated human colon tumor (HCT116) Bax − mitochondria exposed to recombinant Bax exhibited a slow, cyclosporin A-sensitive swelling, but only at [Bax] > 200 nM. The amount of Bax incorporated was much higher than that found in organelles isolated from HCT116 Bax + staurosporine- or etoposide-treated apoptotic cells, casting doubts on the significance of the putative PT induction for apoptosis. Bax did not influence the electrophysiological properties of an approximately 1 nS channel ascribed to the Ca 2+-dependent mitochondrial permeability transition pore. These observations indicate that the PTP is independent of Bax.

Thiol protecting agents and antioxidants inhibit the mitochondrial permeability transition promoted by etoposide: implications in the prevention of etoposide-induced apoptosis

Etoposide (VP-16) is known to promote cell apoptosis either in cancer or in normal cells as a side effect. This fact is preceded by the induction of several mitochondrial events, including increase in Bax/Bcl-2 ratio followed by cytochrome c release and consequent activation of caspase-9 and -3, reduction of ATP levels, depolarization of membrane potential (Δ Ψ) and rupture of the outer membrane. These events are apoptotic factors essentially associated with the induction of the mitochondrial permeability transition (MPT). VP-16 has been shown to stimulate the Ca 2+-dependent MPT induction similarly to prooxidants and to promote apoptosis by oxidative stress mechanisms, which is prevented by glutathione (GSH) and N-acetylcysteine (NAC). Therefore, the aim of this work was to study the effects of antioxidants and thiol protecting agents on MPT promoted by VP-16, attempting to identify the underlying mechanisms on VP-16-induced apoptosis. The increased sensitivity of isolated mitochondria to Ca 2+-induced swelling, Ca 2+ release, depolarization of Δ Ψ and uncoupling of respiration promoted by VP-16, which are prevented by cyclosporine A proving that VP-16 induces the MPT, are also efficiently prevented by ascorbate, the primary reductant of the phenoxyl radicals produced by VP-16. The thiol reagents GSH, dithiothreitol and N-ethylmaleimide, which have been reported to prevent the MPT induction, also protect this event promoted by VP-16. The inhibition of the VP-16-induced MPT by antioxidants agrees with the prevention of etoposide-induced apoptosis by GSH and NAC and suggests the generation of oxidant species as a potential mechanism underlying the MPT that may trigger the release of mitochondrial apoptogenic factors responsible for apoptotic cascade activation.

Thyroxine induces cyclosporin A-insensitive, Ca 2+-dependent reversible permeability transition pore in rat liver mitochondria

The effect of thyroxine on Ca 2+-dependent mitochondrial permeability transition has been examined. It is shown that 40 μM thyroxine induces high amplitude swelling and decrease in membrane potential in Ca 2+-loaded rat liver mitochondria, both in the presence and absence of cyclosporin A. Thyroxine-induced decrease in membrane potential is partially or completely reversed by addition of EGTA into the incubation medium. Nigericin and ADP are shown to prevent, or significantly delay, the effects of thyroxine on both mitochondrial swelling and membrane potential, whereas nicotinamide potentiates the permeabilisation of mitochondria. It is suggested that thyroxine induced reversible, cyclosporin A-insensitive permeability transition pore (PTP) opening in the inner mitochondrial membrane.