Release of Ca2+ from heart and kidney mitochondria by peripheral-type benzodiazepine receptor ligands

Molecular mechanism for the selective impairment of cancer mitochondrial function by a mitochondrially targeted vitamin E analogue

The effects of α-tocopheryl succinate (α-TOS), α-tocopheryl acetyl ether (α-TEA) and triphenylphosphonium-tagged vitamin E succinate (mitochondrially targeted vitamin E succinate; MitoVES) on energy-related mitochondrial functions were determined in mitochondria isolated from AS-30D hepatoma and rat liver, bovine heart sub-mitochondrial particles (SMPs), and in rodent and human carcinoma cell lines and rat hepatocytes. In isolated mitochondria, MitoVES stimulated basal respiration and ATP hydrolysis, but inhibited net state 3 (ADP-stimulated) respiration and Ca(2+) uptake, by collapsing the membrane potential at low doses (1-10μM). Uncoupled mitochondrial respiration and basal respiration of SMPs were inhibited by the three drugs at concentrations at least one order of magnitude higher and with different efficacy: MitoVES>α-TEA>α-TOS. At high doses (>10μM), the respiratory complex II (CII) was the most sensitive MitoVES target. Acting as an uncoupler at low doses, this agent stimulated total O(2) uptake, collapsed ∆ψ(m), inhibited oxidative phosphorylation and induced ATP depletion in rodent and human cancer cells more potently than in normal rat hepatocytes. These findings revealed that in situ tumor mitochondria are preferred targets of the drug, indicating its clinical relevance.

Roles of mitochondrial benzodiazepine receptor in the heart

Mitochondrial benzodiazepine receptor (mBzR) is a type of peripheral benzodiazepine receptor that is located in the outer membrane of mitochondria. It is an 18-kDa protein that can form a multimeric complex with voltage-dependent anion channel (32 kDa) and adenine nucleotide translocator (30 kDa). mBzR is found in various species and abundantly distributed in peripheral tissues, including the cardiovascular system. The mitochondria are well known as the site of energy production, and the heart is the organ that highly requires this energy supply. In the past decades, it has been shown that mBzR plays a critical role in regulating mitochondrial and heart functions. A growing body of evidence demonstrates that mBzR is associated with regulation of mitochondrial respiration, mitochondrial membrane potential, apoptosis, and reactive oxygen species production. Moreover, mBzR has been suggested to play a role in alteration of physiological effects in the heart such as contractility and heart rate. mBzR is involved in the pathologic condition such as ischemia/reperfusion injury, responses to stress, and changes in electrophysiological properties and arrhythmogenesis. In this review, evidence of the roles of mBzR in the heart under both physiological and pathologic conditions is presented. Clinical studies regarding the use of pharmacologic intervention involving mBzR in the heart are also discussed as a possible target for the treatment of electrical and mechanical dysfunction in the heart.

Translocator protein (18 kDa) ligand PK 11195 induces transient mitochondrial Ca2+ release leading to transepithelial Cl- secretion in HT-29 human colon cancer cells

BACKGROUND INFORMATION

TSPO (translocator protein), known previously as PBR (peripheral-type benzodiazepine receptor), is a 18 kDa protein expressed in the mitochondrial membrane of a variety of tissues. TSPO has been reported to be over-expressed in human colorectal tumours and cancer cell lines, but its function is not well characterized.

RESULTS

We investigated the expression and function of TSPO in the human colon cancer cells HT-29. Immunohistochemical studies revealed that TSPO is localized in mitochondria, and its endogenous ligand, the polypeptide diazepam-binding inhibitor, in the cytosol. Radioligand binding studies using the specific high-affinity drug ligand [(3)H]PK 11195 and membrane fraction demonstrated saturable binding, with K(d) and B(max) values of 13.5+/-1.5 nM and 10.1+/-1.0 pmol/mg respectively. PK 11195 induced a rapid and transient dose-dependent rise in intracellular [Ca(2+)], which was unaffected by extracellular Ca(2+), but was blocked by the PTP (permeability transition pore) inhibitor, cyclosporin A, and by the TSPO partial agonist, flunitrazepam. Using HT-29 clone 19A cell line, which forms cell monolayers, we demonstrated that TSPO ligand stimulated a Ca(2+)-dependent transepithelial Cl(-) secretion. This secretion was inhibited: (i) after removal of extracellular Cl(-); (ii) by apical addition of the Cl(-) channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate]; and (iii) by basolateral addition of the Na(+)-K(+)-2Cl(-) co-transporter inhibitor bumetanide. Furthermore, the intracellular Ca(2+) chelator BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)] and cyclosporin A abolished the rise in PK 11195-induced Cl(-) secretion.

CONCLUSIONS

These findings indicate that TSPO is located in mitochondrial membranes of HT-29 and reveal that its activation induces a rise in cytosolic Ca(2+), leading to the stimulation of Cl(-) secretion.

The peripheral-type benzodiazepine receptor and the cardiovascular system. Implications for drug development

Peripheral-type benzodiazepine receptors (PBRs) are abundant in the cardiovascular system. In the cardiovascular lumen, PBRs are present in platelets, erythrocytes, lymphocytes, and mononuclear cells. In the walls of the cardiovascular system, PBR can be found in the endothelium, the striated cardiac muscle, the vascular smooth muscles, and the mast cells. The subcellular location of PBR is primarily in mitochondria. The PBR complex includes the isoquinoline binding protein (IBP), voltage-dependent anion channel (VDAC), and adenine nucleotide transporter (ANT). Putative endogenous ligands for PBR include protoporphyrin IX, diazepam binding inhibitor (DBI), triakontatetraneuropeptide (TTN), and phospholipase A2 (PLA2). Classical synthetic ligands for PBR are the isoquinoline 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinolinecarboxamide (PK 11195) and the benzodiazepine 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5 4864). Novel PBR ligands include N,N-di-n-hexyl 2-(4-fluorophenyl)indole-3-acetamide (FGIN-1-27) and 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575), both possessing steroidogenic properties, but while FGIN-1-27 is pro-apoptotic, SSR180575 is anti-apoptotic. Putative PBR functions include regulation of steroidogenesis, apoptosis, cell proliferation, the mitochondrial membrane potential, the mitochondrial respiratory chain, voltage-dependent calcium channels, responses to stress, and microglial activation. PBRs in blood vessel walls appear to take part in responses to trauma such as ischemia. The irreversible PBR antagonist, SSR180575, was found to reduce damage correlated with ischemia. Stress, anxiety disorders, and neurological disorders, as well as their treatment, can affect PBR levels in blood cells. PBRs in blood cells appear to play roles in several aspects of the immune response, such as phagocytosis and the secretion of interleukin-2, interleukin-3, and immunoglobulin A (IgA). Thus, alterations in PBR density in blood cells may have immunological consequences in the affected person. In conclusion, PBR in the cardiovascular system may represent a new target for drug development.

Inhibitory effect of diazepam on muscarinic receptor-stimulated inositol 1,4,5-trisphosphate production in rat parotid acinar cells

1. This study examined the effect of diazepam (DZP) on phosphoinositide turnover, which plays an important role in the regulation of salivary secretion, in rat parotid acinar cells. 2. DZP (10(-9) M to 10(-5) M), a potent agonist of both central- and peripheral-type benzodiazepine receptors, dose-dependently decreased inositol 1,4,5-trisphosphate IP3 production stimulated by carbachol, a muscarinic receptor agonist, in the cells. 3. DZP produced a maximum inhibitory response at a concentration of 10(-5) M, with IP3 production decreased to 63% of maximal levels. The concentration inducing half maximal inhibition of IP3 production was approximately 3.5 x 10 (-8) M. 4. An inhibitory response to DZP was produced by a short-term pretreatment (<3 min) of the cells and prevented by antagonist and competing ligand for the central- and peripheral-type benzodiazepine receptors, flumazenil and PK 11195, respectively. 5. DZP showed a non-competitive inhibition of carbachol-stimulated IP3 production. It did not directly inhibit the activities of GTP-binding regulatory proteins and phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PLC) in the parotid gland membranes, though choline chloride inhibited PLC activity. 6. DZP (10(-5) M) attenuated the increase in the intracellular Ca2+ concentration ([Ca(2+)](i)) in the cells following stimulation of the muscarinic and alpha(1)-adrenoceptors. 7. These results suggest that in the parotid acinar cells, DZP inhibits muscarinic receptor-stimulated IP3 production through benzodiazepine receptors and that PLC activity which produces IP3 is inhibited by chloride. The decreases in IP3 and [Ca(2+)](i) in the cells may be connected with the suppression of salivary secretion induced by DZP.

Autoradiographic distribution of peripheral benzodiazepine receptors in the retina of the albino rabbit, Lepus cunicula

The distribution of peripheral benzodiazepine receptors (PBRs) in the retina of the albino rabbit, Lepus cunicula, was studied by autoradiography using [3H]-PK11195, a isoquinoline carboxamide, as a tracer. Autoradiograms obtained by directly placing the slides containing the retina sections on tritium-sensitive film provide evidence for the presence of PBRs in rabbit retina. Furthermore, the dark field examination of photomicrographs taken from autoradiograms showed two dense horizontal bands corresponding to the outer and inner photoreceptor segments, and to the inner plexiform layer. The retinal regions where [3H]-PK11195 binding was more dense are rich in mitochondria, suggesting that as in other neuronal tissues, retinal PBRs are involved in the mitochondrial activity.

First-degree atrioventricular block in alprazolam overdose reversed by flumazenil

This report describes a case of alprazolam overdose associated with marked first-degree atrioventricular block reversed by flumazenil. Animal and human evidence suggests activity of certain benzodiazepines at peripheral benzodiazepine receptors in the myocardium and elsewhere. Myocardial benzodiazepine receptor ligands appear to affect calcium-channel activity, which may explain the clinical findings. Benzodiazepines may behave like weak calcium-channel blockers. This case raises the possibility of a potential role for flumazenil as an adjunct in the management of calcium-channel blocker toxicity.

Inhibitory regulation of amylase release in rat parotid acinar cells by benzodiazepine receptors

This study examined the influence of benzodiazepine receptors on amylase release from rat parotid acinar cells. Diazepam (10(-8)-10(-6) M), which is a potent agonist of both central- and peripheral-type benzodiazepine receptors, dose dependently decreased amylase release induced by isoprenaline and carbachol, which are beta-adrenoceptor and muscarinic receptor agonists, respectively. The maximum inhibitory response was obtained with 10(-6) M diazepam: amylase release was decreased to 57% (isoprenaline) and 39% (carbachol) of maximal levels, while these responses were completely inhibited by propranolol and atropine, respectively. Clonazepam and 7-chloro-1,3-dihydro-1-methyl-5-p-chlorophenyl)-2H-1,4-benzodiazepine-2- one (Ro 5-4864), which are selective agonists of central- and peripheral-type benzodiazepine receptors, respectively, also produced a significant and dose-dependent decrease in isoprenaline-induced amylase release. The inhibitory potency was diazepam > clonazepam > Ro 5-4864. Flumazenil and 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (PK 11195), which are selective antagonists of central- and peripheral-type benzodiazepine receptors, respectively, dose dependently blocked the inhibition of isoprenaline-induced amylase release by diazepam. At a concentration of 10(-5) M, flumazenil and PK 11195 restored amylase release to approximately 75% of that in the presence of isoprenaline alone. The combination of both antagonists completely prevented the inhibition by diazepam. Similarly, the inhibitory responses of clonazepam and Ro 5-4864 were completely blocked by flumazenil and PK 11195, respectively. These results suggest that, in rat parotid acinar cells, benzodiazepines inhibit beta-adrenoceptor and muscarinic receptor-stimulated amylase release and that both central- and peripheral-type benzodiazepine receptors contribute to this inhibitory regulation.

Mitochondrial permeability transition as induced by cross-linking of the adenine nucleotide translocase

Mitochondrial permeability transition is caused by the opening of a transmembrane pore whose chemical nature has not been well established yet. The present work was aimed to further contribute to the knowledge of the membrane entity comprised in the formation of the non-specific channel. The increased permeability was established by analyzing the inability of rat kidney mitochondria to take up and accumulate Ca2+, as well as their failure to build up a transmembrane potential, after the cross-linking of membrane proteins by copper plus ortho-phenanthroline. To identify the cross-linked proteins, polyacrylamide gel electrophoresis was performed. The results are representative of at least three separate experiments. It is indicated that 30 microM Cu2+ induced the release of 4.3 nmol Ca2+ per mg protein. However, in the presence of 100 microM ortho-phenanthroline only 2 microM Cu2+ was required to attain the total release of the accumulated Ca2+; it should be noted that such a reaction is not inhibited by cyclosporin. The increased permeability corresponds to cross-linking of membrane proteins in which approximately 4 nmol thiol groups per mg protein appear to be involved. Such a linking process is inhibited by carboxyatractyloside. By using the fluorescent probe eosin-5-maleimide the label was found in a cross-linking 60 kDa dimer of two 30 kDa monomers. From the data presented it is concluded that copper-o-phenanthroline induces the intermolecular cross-linking of the adenine nucleotide translocase which in turn is converted to non-specific pore.

Perspectives on the mitochondrial multiple conductance channel

A multiple conductance channel (MCC) with a peak conductance of over 1 nS is recorded from mitoplasts (mitochondria with the inner membrane exposed) using patch-clamp techniques. MCC shares many general characteristics with other intracellular megachannels, many of which are weakly selective, voltage-dependent, and calcium sensitive. A role in protein import is suggested by the transient blockade of MCC by peptides responsible for targeting mitochondrial precursor proteins. MCC is compared with the peptide-sensitive channel of the outer membrane because of similarities in targeting peptide blockade. The pharmacology and regulation of MCC by physiological effectors are reviewed and compared with the properties of the pore hypothesized to be responsible for the mitochondrial inner membrane permeability transition.

Inhibition by the adenosine analogue, (R-)-N6-phenylisopropyladenosine, of kainic acid neurotoxicity in rat hippocampus after systemic administration

1. Binding of the peripheral benzodiazepine receptor ligand, [3H]-PK 11195, to rat hippocampal membranes has been used to quantify the reactive gliosis resulting from neuronal death induced by intraperitoneally administered kainic acid. 2. Intraperitoneal administration of kainic acid (10 mg kg-1) caused a 350-500% increase in [3H]-PK 11195 binding measured in rat hippocampal P2 membranes 7 days later. Co-treatment with the adenosine derivative R-phenylisopropyladenosine (R-PIA) (100, 25 or 10 micrograms kg-1, i.p.) abolished this elevation. The protective action of R-PIA could itself be abolished by co-treatment with 8-phenyltheophylline (1 mg kg-1). 3. Body temperatures were recorded in the antagonist experiments and no significant changes were recorded, suggesting that the protective action of R-PIA was not mediated by hypothermia. 4. Since systemic kainic acid-induced neurotoxicity has been claimed as a good model of neuronal death in temporal lobe epilepsy, the results suggest that the systemic administration of purines in low doses may provide protection against certain neurodegenerative insults.

Characterization of peripheral-type benzodiazepine binding sites on human lymphocytes and lymphoma cell lines and their role in cell growth

Peripheral-type benzodiazepine binding sites (PBRs) are ubiquitous in mammalian tissues. However, the physiological role of PBRs has not yet been clarified. In this study we characterized a saturable and high affinity binding site for [3H]Pk 11195 (isoquinoline carboxamide derivative) on human lymphocytes and different lymphoma cell lines. Binding parameters of the human T-lymphoma cell line CCRF-CEM came closest to values for lymphocyte binding. Thus, the CCRF-CEM cell line appears to be a suitable lymphocyte cell model for further study of PBRs. To evaluate the pharmacological specificity of binding to human lymphocytes and CCRF-CEM cells we investigated the potency of different ligands to displace [3H]Pk 11195 from its binding site. Pk 11195 was found to be the most potent inhibitor followed by 4'-chlorodiazepan (Ro5-4864) and diazepam (range of inhibition constants from 6.7 x 10(-9) M to 3.6 x 10(-7) M), whereas ligands specific for the central-type receptor like clonazepam and flumazenil had no displacing potency in the tested concentration range (10(-10)-10(-4) M). Since it was assumed that PBRs might be involved in the regulation of cell growth and differentiation, we studied the influence of PBR ligands on cell growth and survival using a quantitative colorimetric assay (MTT). Ligands which bind selectively to PBRs inhibited cell multiplication in vitro. However, half-effective concentrations (EC50) were in the micromolar range and above therapeutic in vivo concentrations (range of EC50 values from 2.4 x 10(-5) M to 1.5 x 10(-4) M). Clonazepam and flumazenil had no inhibiting potency in the tested concentration range (10(-10)-10(-4) M). Although the difference between values for displacing potency and ability to inhibit cell multiplication cannot be explained as yet, it is interesting that all PBR-ligands followed the same sequence in displacing [3H]Pk 11195 and inhibiting cell multiplication and that central type ligands were ineffective in both assays. This association suggest a mediating role of PBR binding in cell growth.

Isolation of the mitochondrial benzodiazepine receptor: association with the voltage-dependent anion channel and the adenine nucleotide carrier

The mitochondrial benzodiazepine receptor (mBzR) has been solubilized with retention of reversible ligand binding, and the associated subunits were characterized. mBzR comprises immunologically distinct protein subunits of 18-, 30-, and 32-kDa. The 18-kDa protein is labeled by the isoquinoline carboxamide mBzR ligand [3H]PK14105, whereas the 30- and 32-kDa subunits are labeled by the benzodiazepine (Bz) ligands [3H]flunitrazepam and [3H]AHN-086. Selective antibodies and reagents identify the 32- and 30-kDa proteins as the voltage-dependent anion channel (VDAC) and the adenine nucleotide carrier (ADC), respectively. While isoquinoline carboxamide and Bz ligands target different subunits, they interact allosterically, as the binding of Bz and isoquinoline carboxamide ligands is mutually competitive at low nanomolar concentrations. Moreover, eosin-5-maleimide and mercuric chloride inhibit [3H]PK11195 binding to the intact receptor via sulfhydryl groups that are present in ADC. VDAC and ADC, outer and inner mitochondrial membrane channel proteins, respectively, together with the 18-kDa subunit, may comprise mBzR at functionally important transport sites at the junction of two mitochondrial membranes.

The mitochondrial benzodiazepine receptor: evidence for association with the voltage-dependent anion channel (VDAC)

Specific, high-affinity receptors for numerous drugs have recently been localized to mitochondrial membrane proteins. This review discusses the association of the mitochondrial receptor for benzodiazepines (mBzR) with the voltage-dependent anion channel (VDAC), indicating a possible auxiliary role for VDAC as a putative drug binding protein. The proposed subunit composition of the purified mBzR complex isolated from rat kidney mitochondria includes VDAC, which functions as a recognition site for benzodiazepines (e.g., flunitrazepam), the adenine nucleotide carrier (ADC), and an 18 kDa outer membrane protein identified by covalent labelling with the mBzR antagonists isoquinoline carboxamides (e.g., PK14105).

Requisite structural characteristics for benzodiazepine inhibition of triiodothyronine uptake into a human liver cell line

Previously, we reported potent inhibition of triiodo-L-thyronine (T3) cellular uptake into a human liver cell line (HepG2) by central and peripheral receptor specific benzodiazepine (BZ) compounds and our working hypothesis of BZ's as direct competitors for the iodothyronine transporter, displacing T3 but not acting as a substrate for transport. In this report, we list other reported uptake inhibitors and compare them to 23 benzodiazepine receptor ligands, in their potency to inhibit cellular uptake of T3. The most potent inhibitors are restricted to the benzodiazepine class. From the BZ structure-activity relationship (SAR) for inhibition, we see that the nonfused phenyl ring may be essential for activity and the strongest relationship is seen with substitution at R2' where Cl greater than F greater than H. Substitution at R4' and hydroxyl substitution at R3 enhances potency as will alkyl groups at R1 or on the imidazole group in the 1,2-annelated series. With R7 substitution, Cl is preferred over NO2 but not necessarily H when R4' = Cl; this may reflect a slightly different orientation of the molecule with large aliphatic R1 groups and/or R4' substitution. The carbonyl at R2 in the 1,4 benzodiazepine series, enhances their potency. The resultant structure-activity relationship highlights the importance of the halogen-substituted nonfused phenyl ring and seems unique relative to other described benzodiazepine sites and/or effects.

Intramitochondrial K+ as activator of carboxyatractyloside-induced Ca2+ release

The role of intramitochondrial K+ content on the increase in membrane permeability to Ca2+, as induced by carboxyatractyloside was studied. In mitochondria containing a high K+ concentration (83 nmol/mg), carboxyatractyloside induced a fast and extensive mitochondrial Ca2+ release, membrane de-energization, and swelling. Conversely, in K(+)-depleted mitochondria (11 nmol/mg), carboxyatractyloside was ineffective. The addition of 40 mM K+ to K(+)-depleted mitochondria restored the capability of atractyloside to induce an increase in membrane permeability to Ca2+ release. The determination of matrix free Ca2+ concentration showed that, at an external free-Ca2+ concentration of 0.8 microM, control mitochondria contained 3.9 microM of free Ca2+ whereas K(+)-depleted mitochondria contained 0.9 microM free Ca2+. It is proposed that intramitochondrial K+ affects the matrix free Ca2+ concentration required to induce a state of high membrane permeability.