Long-chain ceramide is a potent inhibitor of the mitochondrial permeability transition pore

The sphingolipid ceramide has been implicated in mediating cell death that is accompanied by mitochondrial functional alterations. Moreover, ceramide has been shown to accumulate in mitochondria upon induction of apoptotic processes. In this study, we sought to evaluate the effects of natural, highly hydrophobic long-chain ceramides on mitochondrial function in vitro. Ceramide in a dodecane/ethanol delivery system inhibited the opening of the mitochondrial permeability transition pore (PTP) induced by either oxidative stress, SH group cross-linking, or high Ca(2+) load, suggesting that the inhibitory point is at a level at which major PTP regulatory pathways converge. Moreover, ceramide had no effect on well known mitochondrial components that modulate PTP activity, such as cyclophilin D, voltage-dependent anion channel, adenine nucleotide transporter, and ATP synthase. The inhibitory effect of ceramide on PTP was not stereospecific, nor was there a preference for ceramide over dihydroceramide. However, the effect of ceramide on PTP was significantly influenced by the fatty acid moiety chain length. These studies are the first to show that long-chain ceramide can influence PTP at physiologically relevant concentrations, suggesting that it is the only known potent natural inhibitor of PTP. These results suggest a novel mechanism of ceramide regulation of mitochondrial function.     

Inorganic polyphosphate is a potent activator of the mitochondrial permeability transition pore in cardiac myocytes

Mitochondrial dysfunction caused by excessive Ca2+ accumulation is a major contributor to cardiac cell and tissue damage during myocardial infarction and ischemia-reperfusion injury (IRI). At the molecular level, mitochondrial dysfunction is induced by Ca2+-dependent opening of the mitochondrial permeability transition pore (mPTP) in the inner mitochondrial membrane, which leads to the dissipation of mitochondrial membrane potential (ΔΨm), disruption of adenosine triphosphate production, and ultimately cell death. Although the role of Ca2+ for induction of mPTP opening is established, the exact molecular mechanism of this process is not understood. The aim of the present study was to test the hypothesis that the adverse effect of mitochondrial Ca2+ accumulation is mediated by its interaction with inorganic polyphosphate (polyP), a polymer of orthophosphates linked by phosphoanhydride bonds. We found that cardiac mitochondria contained significant amounts (280±60 pmol/mg of protein) of short-chain polyP with an average length of 25 orthophosphates. To test the role of polyP for mPTP activity, we investigated kinetics of Ca2+ uptake and release, ΔΨm and Ca2+-induced mPTP opening in polyP-depleted mitochondria. polyP depletion was achieved by mitochondria-targeted expression of a polyP-hydrolyzing enzyme. Depletion of polyP in mitochondria of rabbit ventricular myocytes led to significant inhibition of mPTP opening without affecting mitochondrial Ca2+ concentration by itself. This effect was observed when mitochondrial Ca2+ uptake was stimulated by increasing cytosolic [Ca2+] in permeabilized myocytes mimicking mitochondrial Ca2+ overload observed during IRI. Our findings suggest that inorganic polyP is a previously unrecognized major activator of mPTP. We propose that the adverse effect of polyphosphate might be caused by its ability to form stable complexes with Ca2+ and directly contribute to inner mitochondrial membrane permeabilization.     

Cyclosporin A is a potent inhibitor of the inner membrane permeability transition in liver mitochondria

The immunosuppressive peptide cyclosporin A is a powerful inhibitor of the Ca2+-dependent permeability transition in rat liver mitochondria. When swelling is used to monitor the transition, the inhibitor is effective regardless of whether N-ethylmaleimide, Hg2+, WY-14643, t-butyl hydroperoxide, oxalacetate, rhein, phosphate, phosphoenolpyruvate, or ruthenium red plus uncoupler is used as the inducing agent. Twenty-five to fifty pmol/mg protein of cyclosporin A reduces the swelling response by 50% with complete inhibition obtained at about 150 pmol/mg protein. The compound, which does not inhibit Ca2+ uptake or mitochondrial phospholipase A2, is effective when added before or after the transition promoting agent. These findings, together with the shape of the inhibition dose-response curve, suggest that cyclosporin A essentially titrates a mitochondrial component which is present at 80-90 pmol/mg protein. It is proposed that this component is a solute unselective, regulated pore or a factor involved in controlling such a structure.     

Antamanide, a derivative of Amanita phalloides, is a novel inhibitor of the mitochondrial permeability transition pore

Antamanide is a cyclic decapeptide derived from the fungus Amanita phalloides. Here we show that antamanide inhibits the mitochondrial permeability transition pore, a central effector of cell death induction, by targeting the pore regulator cyclophilin D. Indeed, (i) permeability transition pore inhibition by antamanide is not additive with the cyclophilin D-binding drug cyclosporin A, (ii) the inhibitory action of antamanide on the pore requires phosphate, as previously shown for cyclosporin A; (iii) antamanide is ineffective in mitochondria or cells derived from cyclophilin D null animals, and (iv) abolishes CyP-D peptidyl-prolyl cis-trans isomerase activity. Permeability transition pore inhibition by antamanide needs two critical residues in the peptide ring, Phe6 and Phe9, and is additive with ubiquinone 0, which acts on the pore in a cyclophilin D-independent fashion. Antamanide also abrogates mitochondrial depolarization and the ensuing cell death caused by two well-characterized pore inducers, clotrimazole and a hexokinase II N-terminal peptide. Our findings have implications for the comprehension of cyclophilin D activity on the permeability transition pore and for the development of novel pore-targeting drugs exploitable as cell death inhibitors.     

The mitochondrial permeability transition pore is a dispensable element for mitochondrial calcium efflux

The mitochondrial permeability transition pore (mPTP) has long been known to have a role in mitochondrial calcium (Ca²⁺) homeostasis under pathological conditions as a mediator of the mitochondrial permeability transition and the activation of the consequent cell death mechanism. However, its role in the context of mitochondrial Ca²⁺ homeostasis is not yet clear. Several studies that were based on PPIF inhibition or knock out suggested that mPTP is involved in the Ca²⁺ efflux mechanism, while other observations have revealed the opposite result.     

4-Hydroxy-oxyphenbutazone is a potent inhibitor of cytokine production

4-Hydroxy-oxyphenbutazone (4OH-OPB), is currently in phase II trials for its immunosuppressive effect in patients with rheumatoid arthritis. 4OH-OPB and other compounds related to phenylbutazone were tested for their effect on in vitro cytokine production by monocytes and lymphocytes present in peripheral mononuclear cells (PBMC) or whole blood (WB) cultures, and compared against phenylbutazone and oxyphenbutazone, two known anti-inflammatory drugs. In PBMC cultures, 4OH-OPB was by far the most potent inhibitor, and both monokines and Th1 and Th2 lymphokines were efficiently inhibited at low concentrations. In WB cultures, 4OH-OPB was less effective than in PBMC cultures, but was still the best inhibitor of lymphokine production and, furthermore, was the only inhibitor of monokine production. The increase in 4OH-OPB concentration needed to induce the same inhibition of cytokine production in WB as in PBMC culture could be mimicked by the addition of erythrocytes to the PBMC cultures. Experiments with radioactively-labeled 4OH-OPB suggest that 4OH-OPB is taken up very rapidly into erythrocytes and is secreted by the erythrocytes with much slower kinetics via a multidrug-resistance-associated protein. The secreted compound is most likely structurally different from 4OH-OPB, as in PBMC and WB cultures, the inhibition of cytokine production seems to be caused by a different mechanism. In PBMC cultures, the inhibition of cytokine production is accompanied by a loss of cell viability, while this is not the case when 4OH-OPB inhibits cytokine production in WB. Our data suggest that 4OH-OPB may be useful as an immunosuppressive drug for patients with inflammatory diseases.     

Bnip3-mediated mitochondrial autophagy is independent of the mitochondrial permeability transition pore

Bnip3 is a pro-apoptotic BH3-only protein which is associated with mitochondrial dysfunction and cell death. Bnip3 is also a potent inducer of autophagy in many cells. In this study, we have investigated the mechanism by which Bnip3 induces autophagy in adult cardiac myocytes. Overexpression of Bnip3 induced extensive autophagy in adult cardiac myocytes. Fluorescent microscopy studies and ultrastructural analysis revealed selective degradation of mitochondria by autophagy in myocytes overexpressing Bnip3. Oxidative stress and increased levels of intracellular Ca²⁺ have been reported by others to induce autophagy, but Bnip3-induced autophagy was not abolished by antioxidant treatment or the Ca²⁺ chelator BAPTA-AM. We also investigated the role of the mitochondrial permeability transition pore (mPTP) in Bnip3-induced autophagy. Although the mPTP has previously been implicated in the induction of autophagy and selective removal of damaged mitochondria by autophagosomes, mitochondria sequestered by autophagosomes in Bnip3-treated cardiac myocytes had not undergone permeability transition, and treatment with the mPTP inhibitor cyclosporine A did not inhibit mitochondrial autophagy in cardiac myocytes. Moreover, cyclophilin D (cypD) is an essential component of the mPTP and Bnip3 induced autophagy to the same extent in embryonic fibroblasts isolated from wild-type and cypD-deficient mice. These results support a model where Bnip3 induces selective removal of the mitochondria in cardiac myocytes, and that Bnip3 triggers induction of autophagy independent of Ca²⁺, ROS generation, and mPTP opening.     

Bnip3-mediated mitochondrial autophagy is independent of the mitochondrial permeability transition pore

Bnip3 is a pro-apoptotic BH3-only protein which is associated with mitochondrial dysfunction and cell death. Bnip3 is also a potent inducer of autophagy in many cells. In this study, we have investigated the mechanism by which Bnip3 induces autophagy in adult cardiac myocytes. Overexpression of Bnip3 induced extensive autophagy in adult cardiac myocytes. Fluorescent microscopy studies and ultrastructural analysis revealed selective degradation of mitochondria by autophagy in myocytes overexpressing Bnip3. Oxidative stress and increased levels of intracellular Ca²⁺ have been reported by others to induce autophagy, but Bnip3-induced autophagy was not abolished by antioxidant treatment or the Ca²⁺ chelator BAPTA-AM. We also investigated the role of the mitochondrial permeability transition pore (mPTP) in Bnip3-induced autophagy. Although the mPTP has previously been implicated in the induction of autophagy and selective removal of damaged mitochondria by autophagosomes, mitochondria sequestered by autophagosomes in Bnip3-treated cardiac myocytes had not undergone permeability transition and treatment with the mPTP inhibitor cyclosporine A did not inhibit mitochondrial autophagy in cardiac myocytes. Moreover, cyclophilin D (cypD) is an essential component of the mPTP and Bnip3 induced autophagy to the same extent in embryonic fibroblasts isolated from wild-type and cypD-deficient mice. These results support a model where Bnip3 induces selective removal of the mitochondria in cardiac myocytes and that Bnip3 triggers induction of autophagy independent of Ca²⁺, ROS generation and mPTP opening.Key words: Bnip3, autophagy, cardiac myocytes, mitochondria, permeability transition pore, cyclophilin D     

Chenodeoxycholate is a potent inducer of the permeability transition pore in rat liver mitochondria

Several reports support the concept that bile acids may be cytotoxic during cholestatic disease process by causing mitochondrial dysfunction. Here we report additional data and findings aimed at a better understanding of the involvement of the permeability transition pore (PTP) opening in bile acids toxicity. The mitochondrial PTP is implicated as a mediator of cell injury and death in many situations. In the presence of calcium and phosphate, chenodeoxycholic acid (CDCA) induced a permeability transition in freshly isolated rat liver mitochondria, characterized by membrane depolarization, release of matrix calcium, and osmotic swelling. All these events were blocked by cyclosporine A (CyA) and the calcium uniporter inhibitor ruthenium red (RR). The results suggest that CDCA increases the sensitivity of isolated mitochondria in vitro to the calcium-dependent induction of the PTP.     

Phenyl-n-butylborinic acid is a potent transition state analog inhibitor of lipolytic enzymes

The cholesterol esterase and lipoprotein lipase catalyzed hydrolyses of the water-soluble substrate p-nitrophenyl butyrate are competitively inhibited by butaneboronic acid and phenylboronic acid. Phenyl-n-butylborinic acid has been synthesized and characterized as an ultrapotent transition state analog inhibitor: Ki = 2.9 +/- 0.6 nM and 1.7 +/- 0.3 microM for the cholesterol esterase and lipoprotein lipase reactions, respectively. These results are interpreted in terms of transition state structure and stabilization.     

Novel insights into the mitochondrial permeability transition

Alavian and colleagues recently provided further evidence in support of the notion that the c subunit of the mitochondrial F₁F_O ATP synthase constitutes the long-sought pore-forming unit of the supramolecular complex responsible for the so-called ‘mitochondrial permeability transition’ (MPT). Besides shedding new light on the molecular mechanisms that underlie the MPT, these findings corroborate the notion that several components of the cell death machinery, including cytochrome c and the F₁F_O ATP synthase, mediate critical metabolic activities.     

Sanglifehrin A acts as a potent inhibitor of the mitochondrial permeability transition and reperfusion injury of the heart by binding to cyclophilin-D at a different site from cyclosporin A

Cyclosporin A (CsA) inhibits opening of the mitochondrial permeability transition pore (MPTP), a critical event in some forms of necrotic and apoptotic cell death, by binding to cyclophilin D (CyP-D) and inhibiting its peptidyl-prolyl cis-trans isomerase (PPIase) activity. Sanglifehrin A (SfA), like CsA, exerts its immunosuppressive action by binding to cyclophilin A but at a different site from CsA, and unlike the latter, SfA does not inhibit calcineurin activity. Here we demonstrate that SfA inhibits the PPIase activity of CyP-D (K(0.5) 2 nm) and acts as a potent inhibitor of MPTP opening under both energized and de-energized conditions. However, unlike CsA, the dose-response curve for inhibition by SfA is sigmoidal rather than hyperbolic, suggesting a multimeric structure for the MPTP with cooperativity between subunits. Furthermore, SfA does not prevent CyP-D binding to submitochondrial particles or detergent-solubilized adenine nucleotide translocase (ANT), implying that CyP-D binding to the ANT does not require PPIase activity but pore opening does. Once bound to the MPTP, SfA is not readily dissociated, and inhibition of pore opening is maintained following extensive washing. To investigate the potential of SfA as an inhibitor of cell death in vivo, we used the Langendorff perfused rat heart. SfA caused a time-dependent inhibition of the MPTP that was maintained on mitochondrial isolation to a greater extent than was CsA inhibition. We demonstrate that SfA, like CsA, improves the recovery of left ventricular developed pressure during reperfusion after 30 min of global ischemia and greatly reduces lactate dehydrogenase release, implying inhibition of necrotic damage. Because SfA does not inhibit calcineurin activity, our data suggest that it may be more desirable than CsA for protecting tissues recovering from ischemic episodes and for studying the role of the MPTP in cell death.     

Physiological roles of the mitochondrial permeability transition pore

Neurons experience high metabolic demand during such processes as synaptic vesicle recycling, membrane potential maintenance and Ca²⁺ exchange/extrusion. The energy needs of these events are met in large part by mitochondrial production of ATP through the process of oxidative phosphorylation. The job of ATP production by the mitochondria is performed by the F₁F_O ATP synthase, a multi-protein enzyme that contains a membrane-inserted portion, an extra-membranous enzymatic portion and an extensive regulatory complex. Although required for ATP production by mitochondria, recent findings have confirmed that the membrane-confined portion of the c-subunit of the ATP synthase also houses a large conductance uncoupling channel, the mitochondrial permeability transition pore (mPTP), the persistent opening of which produces osmotic dysregulation of the inner mitochondrial membrane, uncoupling of oxidative phosphorylation and cell death. Recent advances in understanding the molecular components of mPTP and its regulatory mechanisms have determined that decreased uncoupling occurs in states of enhanced mitochondrial efficiency; relative closure of mPTP therefore contributes to cellular functions as diverse as cardiac development and synaptic efficacy.     

2-Aminoresorcinol is a potent alpha-glucosidase inhibitor

A series of aminoresorcinols and related compounds were tested for rat intestinal alpha-glucosidase inhibition and these results suggested that the 2-aminoresorcinol moiety of 6-amino-5,7-dihydroxyflavone (2) is important to exert the intestinal alpha-glucosidase inhibitory activity and 2-aminoresorcinol (4), itself, is a potent alpha-glucosidase inhibitor and inhibited sucrose-hydrolyzing activity of rat intestinal alpha-glucosidase uncompetitively.     

Chebulagic acid is a potent alpha-glucosidase inhibitor

Chebulagic acid, isolated form Terminalia chebula Retz, proved to be a reversible and non-competitive inhibitor of maltase with a K(i) value of 6.6 muM. The inhibitory influence of chebulagic acid on the maltase-glucoamylase complex was more potent than on the sucrase-isomaltase complex. The magnitude of alpha-glucosidase inhibition by chebulagic acid was greatly affected by its origin. These results show a use for chebulagic acid in managing type-2 diabetes.     

Melatonin is a potent inhibitor for myeloperoxidase

Myeloperoxidase (MPO) catalyzes the formation of potent oxidants that have been implicated in the pathogenesis of various diseases including atherosclerosis, asthma, arthritis, and cancer. Melatonin plays an important part in the regulation of various body functions including circadian sleep rhythms, blood pressure, oncogenesis, retinal function, seasonal reproduction, and immunity. Here, we demonstrate that melatonin serves as a potent inhibitor of MPO under physiological-like conditions. In the presence of chloride (Cl-), melatonin inactivated MPO at two points in the classic peroxidase cycle through binding to MPO to form an inactive complex, melatonin-MPO-Cl, and accelerating MPO compound II formation, an inactive form of MPO. Inactivation of MPO was mirrored by the direct conversion of MPO-Fe(III) to MPO compound II without any sign of compound I accumulation. This behavior indicates that melatonin binding modulates the formation of MPO intermediates and their decay rates. The Cl- presence enhanced the affinity of MPO toward melatonin, which switches the enzyme activity from peroxidation to catalase-like activity. In the absence of Cl-, melatonin served as a 1e- substrate for MPO compound I, but at higher concentration it limited the reaction by its dissociation from the corresponding complex. Importantly, melatonin-dependent inhibition of MPO occurred with a wide range of concentrations that span various physiological and supplemental ranges. Thus, the interplay between MPO and melatonin may have a broader implication in the function of several biological systems. This dual regulation by melatonin is unique and represents a new means through which melatonin can control MPO and its downstream inflammatory pathways.     

Thapsigargin directly induces the mitochondrial permeability transition.

High concentrations of thapsigargin (TG) have been used to study the process of necrotic cell death, which involves mitochondria in the cell rapidly undergoing the mitochondrial permeability transition (MPT). We therefore investigated the effects of TG on MPT in isolated liver and heart mitochondria. Using a matrix swelling assay in combination with a novel enzymatic method based on inner membrane permeability to citrate synthase substrates, TG induced MPT in a concentration-dependent manner, independent of extramitochondrial [Ca2+] and inhibitable by cyclosporin A. Evidence from alamethicin-permeabilized mitochondria suggests that TG induces MPT by causing Ca2+ release from mitochondrial matrix Ca2+-binding sites. These findings suggest that the MPT-inducing effect of TG may contribute to its pro-necrotic and pro-apoptotic effects in various cell types.