Bcl-2 resistant mitochondrial toxicity mediated by the isoquinoline carboxamide PK11195 involves de novo generation of reactive oxygen species

Targeting the Translocator Protein (18 kDa) in Cardiac Diseases: State of the Art and Future Opportunities

Mitochondria dysfunctions are typical hallmarks of cardiac disorders (CDs). The multiple tasks of this energy-producing organelle are well documented, but its pathophysiologic involvement in several manifestations of heart diseases, such as altered electromechanical coupling, excitability, and arrhythmias, is still under investigation. The human 18 kDa translocator protein (TSPO) is a protein located on the outer mitochondrial membrane whose expression is altered in different pathological conditions, including CDs, making it an attractive therapeutic and diagnostic target. Currently, only a few TSPO ligands are employed in CDs and cardiac imaging. In this Perspective, we report an overview of the emerging role of TSPO at the heart level, focusing on the recent literature concerning the development of TSPO ligands used for fighting and imaging heart-related disease conditions. Accordingly, targeting TSPO might represent a successful strategy to achieve novel therapeutic and diagnostic strategies to unravel the fundamental mechanisms and to provide solutions to still unanswered questions in CDs.

Targeting Mitochondria in Tumor-Associated Macrophages using a Dendrimer-Conjugated TSPO Ligand that Stimulates Antitumor Signaling in Glioblastoma

Mitochondria mediate critical cellular processes, including proliferation, apoptosis, and immune responses; as such, their dysfunction is pathogenic in many neurodegenerative disorders and cancers. In glioblastoma, targeted delivery of mitochondria-focused anticancer therapies has failed to translate into clinical success due to the nonspecific cellular localization, heterogeneity of receptor expression across patients, poor transport across biological barriers to reach the brain, tumor, and mitochondria, and systemic side effects. Strategies that can overcome brain and solid tumor barriers and selectively target mitochondria within specific cell types may lead to improvements in glioblastoma treatment. Developments in dendrimer-mediated nanomedicines have shown promise targeting tumor-associated macrophages (TAMs) in glioblastoma, following systemic administration. Here, we present a novel dendrimer conjugated to the translocator protein (18 kDa) (TSPO) ligand 5,7-dimethylpyrazolo[1,5-α]pyrimidin-3-ylacetamide (DPA). We developed a clickable DPA for conjugation on the dendrimer surface and demonstrated in vitro that the dendrimer-DPA conjugate (D-DPA) significantly increases dendrimer colocalization with mitochondria. Compared to free TSPO ligand PK11195, D-DPA stimulates greater antitumor immune signaling. In vivo, we show that D-DPA targets mitochondria specifically within TAMs following systemic administration. Our results demonstrate that dendrimers can achieve TAM-specific targeting in glioblastoma and can be further modified to target specific intracellular compartments for organelle-specific drug delivery.

Induction of cell-cycle arrest and apoptosis in human cholangiocarcinoma cells by pristimerin

Pristimerin, a triterpenoid isolated from Celastraceae and Hippocrateaceae, is known to induce cytotoxicity in several cancer cell lines. However, whether pristimerin can induce apoptosis in cholangiocarcinoma cells and the underlying mechanism remain unexplored. We assessed the function of human cholangiocarcinoma QBC and RBE cell lines using various experimental methods such as the cell viability assay to elucidate the viability of cells, flow cytometry to detect the death rate of cells, and Western blot analysis to evaluate the expression of cell cycle-related proteins and autophagy-related proteins. Human cholangiocarcinoma QBC cells were transplanted to nude mice to establish an animal model, and the effect of pristimerin on tumor growth in this model was observed. QBC and RBE cell lines treated with pristimerin (0, 5, 10, and 20 μmol/L) demonstrated the induction of apoptosis in a dose-dependent manner. The cell viability assay revealed a reduction in the cell viability with an increase in the pristimerin concentration. Similarly, flow cytometry revealed a gradual increase in the cell death rate with an increase in the pristimerin concentration. In addition, pristimerin significantly lowered the expression of apoptosis-related proteins (Bcl-2, Bcl-xL, and procaspase-3), but increased the Bax expression. Furthermore, pristimerin resulted in the G0/G1 cell-cycle arrest, reducing the expression of cell cycle-related proteins (cyclin E, CDK2, and CDK4), and increased the expression of autophagy-related proteins (LC3) in QBC cell line. Treatment with pristimerin could inhibit tumor growth in the nude mouse model. Overall, this study suggests the potential effect of pristimerin on the cell-cycle arrest and apoptosis in human cholangiocarcinoma cells.

Reduction of Traumatic Brain Damage by Tspo Ligand Etifoxine

Experimental studies have shown that ligands of the 18 kDa translocator protein can reduce neuronal damage induced by traumatic brain injury by protecting mitochondria and preventing metabolic crisis. Etifoxine, an anxiolytic drug and 18 kDa translocator protein ligand, has shown beneficial effects in the models of peripheral nerve neuropathy. The present study investigates the potential effect of etifoxine as a neuroprotective agent in traumatic brain injury (TBI). For this purpose, the effect of etifoxine on lesion volume and modified neurological severity score at 4 weeks was tested in Sprague-Dawley adult male rats submitted to cortical impact contusion. Effects of etifoxine treatment on neuronal survival and apoptosis were also assessed by immune stains in the perilesional area. Etifoxine induced a significant reduction in the lesion volume compared to nontreated animals in a dose-dependent fashion with a similar effect on neurological outcome at four weeks that correlated with enhanced neuron survival and reduced apoptotic activity. These results are consistent with the neuroprotective effect of etifoxine in TBI that may justify further translational research.

The TSPO Ligands 2-Cl-MGV-1, MGV-1, and PK11195 Differentially Suppress the Inflammatory Response of BV-2 Microglial Cell to LPS

The 18 kDa Translocator Protein (TSPO) is a marker for microglial activation as its expression is enhanced in activated microglia during neuroinflammation. TSPO ligands can attenuate neuroinflammation and neurotoxicity. In the present study, we examined the efficacy of new TSPO ligands designed by our laboratory, MGV-1 and 2-Cl-MGV-1, in mitigating an in vitro neuroinflammatory process compared to the classic TSPO ligand, PK 11195. We exposed BV-2 microglial cells to lipopolysaccharide (LPS) for 24 h to induce inflammatory response and added the three TSPO ligands: (1) one hour before LPS treatment (pretreatment), (2) simultaneously with LPS (cotreatment), and (3) one hour after LPS exposure (post-treatment). We evaluated the capability of TSPO ligands to reduce the levels of three glial inflammatory markers: cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and nitric oxide (NO). We compared the effects of the two novel ligands to PK 11195. Both 2-Cl-MGV-1 and MGV-1 reduced the levels of glial COX-2, iNOS, and NO in LPS-treated BV-2 cells more efficiently than PK 11195. Notably, even when added after exposure to LPS, all ligands were able to suppress the inflammatory response. Due to their pronounced anti-inflammatory activity, 2-Cl-MGV-1 and MGV-1 may serve as potential therapeutics in neuroinflammatory and neurodegenerative diseases.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Oxalicumone A, a new dihydrothiophene-condensed sulfur chromone induces apoptosis in leukemia cells through endoplasmic reticulum stress pathway

Oxalicumone A (POA1), a novel dihydrothiophene-condensed sulfur chromone isolated from the marine fungus Penicillium oxalicum SCSGAF 0023, showed cytotoxicity against several cancer cells previously. In this study, its anti-cancer activity and underlying mechanism of this action were investigated in leukemia cells like KG-1a, HL60, U937, and K562. The results showed that POA1 inhibited dose-/time-dependently cell growth and induced apoptosis in leukemia cells. Also, POA1 caused cleavages of caspase-3, 8, 9 and PARP1, loss of mitochondrial membrane potential, up-regulations of phosphorylated p38 and JNK, and activation of endoplasmic reticulum stress (ER stress). Furthermore, 4-PBA (an ER stress inhibitor) but not SP600125 and SB203580 (JNK and p38 inhibitor, respectively) could largely inhibit POA1-induced growth suppression. Additionally, 4-PBA obstructed mitochondrial depolarization and cleavage of PARP1. These data suggested that ER stress pathway might be an important mediator in POA1-induced apoptosis. In conclusion, POA1 may have antitumor effects in leukemia cells through the induction of ER stress pathway.

Translocator Protein (TSPO) Affects Mitochondrial Fatty Acid Oxidation in Steroidogenic Cells

Translocator protein (TSPO), also known as the peripheral benzodiazepine receptor, is a highly conserved outer mitochondrial membrane protein present in specific subpopulations of cells within different tissues. In recent studies, the presumptive model depicting mammalian TSPO as a critical cholesterol transporter for steroidogenesis has been refuted by studies examining effects of Tspo gene deletion in vivo and in vitro, biochemical testing of TSPO cholesterol transport function, and specificity of TSPO-mediated pharmacological responses. Nevertheless, high TSPO expression in steroid-producing cells seemed to indicate an alternate function for this protein in steroidogenic mitochondria. To seek an explanation, we used CRISPR/Cas9-mediated TSPO knockout steroidogenic MA-10 Leydig cell (MA-10:TspoΔ/Δ) clones to examine changes to core mitochondrial functions resulting from TSPO deficiency. We observed that 1) MA-10:TspoΔ/Δ cells had a shift in substrate utilization for energy production from glucose to fatty acids with significantly higher mitochondrial fatty acid oxidation (FAO), and increased reactive oxygen species production; and 2) oxygen consumption rate, mitochondrial membrane potential, and proton leak were not different between MA-10:TspoΔ/Δ and MA-10:Tspo+/+ control cells. Consistent with this finding, TSPO-deficient adrenal glands from global TSPO knockout (Tspo(-/-)) mice also showed up-regulation of genes involved in FAO compared with the TSPO floxed (Tspo(fl/fl)) controls. These results demonstrate the first experimental evidence that TSPO can affect mitochondrial energy homeostasis through modulation of FAO, a function that appears to be consistent with high levels of TSPO expression observed in cell types active in lipid storage/metabolism.

Mitochondrial translocator protein (TSPO): From physiology to cardioprotection

The mitochondrial translocator protein (TSPO) is a high affinity cholesterol binding protein which is primarily located in the outer mitochondrial membrane where it has been shown to interact with proteins implicated in mitochondrial permeability transition pore (mPTP) formation. TSPO is found in different species and is expressed at high levels in tissues that synthesize steroids but is also present in other peripheral tissues especially in the heart. TSPO has been involved in the import of cholesterol into mitochondria, a key step in steroidogenesis. This constitutes the main established function of the protein which was recently challenged by genetic studies. TSPO has also been associated directly or indirectly with a wide range of cellular functions such as apoptosis, cell proliferation, differentiation, regulation of mitochondrial function or porphyrin transport. In the heart the role of TSPO remains undefined but a growing body of evidence suggests that TSPO plays a critical role in regulating physiological cardiac function and that TSPO ligands may represent interesting drugs to protect the heart under pathological conditions. This article briefly reviews current knowledge regarding TSPO and discusses its role in the cardiovascular system under physiological and pathologic conditions. More particularly, it provides evidence that TSPO can represent an alternative strategy to develop new pharmacological agents to protect the myocardium against ischemia-reperfusion injury.

Lost in translocation: the functions of the 18-kD translocator protein

Research spanning nearly four decades has assigned to the translocator protein (18 kDa) (TSPO) a critical role, among others, in the mitochondrial import of cholesterol, the subsequent steps of (neuro)steroid production, and systemic endocrine regulation, with implications for the pathophysiology of immune, inflammatory, neurodegenerative, and psychiatric as well as neoplastic diseases. Recent knockout studies in mice unexpectedly report normal or latent phenotypes, raising doubts about the protein's role in steroidogenesis and other previously postulated functions and challenging the validity of earlier data on the selectivity of TSPO-binding drugs. Here we provide a synthesis of the current debate from a structural and molecular biology perspective, discuss the limits of inference in loss-of-function (gene knockout) studies, and suggest new functions of TSPO.

The translocator protein (TSPO) ligand PK11195 induces apoptosis and cell cycle arrest and sensitizes to chemotherapy treatment in pre- and post-relapse neuroblastoma cell lines

High-risk neuroblastoma (NB) has a poor prognosis. Even with intensive myeloablative chemotherapy, relapse is common and almost uniformly fatal, and new treatments are needed. Translocator protein 18kDa (TSPO) ligands have been studied as potential new therapeutic agents in many cancers, but not in NB. We studied the effects of TSPO ligands on cell proliferation, cell cycle progression and apoptosis using paired cell lines derived from the same patient at the time of initial surgery and again after development of progressive disease or relapse post-chemotherapy. We found that TSPO expression was significantly increased 2- to 10-fold in post-relapse cell lines compared with pre-treatment lines derived from the same individual. Subsequently, these cell lines were treated with the specific TSPO ligand 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195) (0-160µM) as a single agent, with cytotoxic chemotherapy agents alone (carboplatin, etoposide or melphalan), or with combinations of PK11195 and chemotherapy drugs. We found that PK11195 inhibited proliferation in a dose-dependent manner, induced apoptosis and caused G 1/S cell cycle arrest in all tested NB cell lines at micromolar concentrations. In addition, PK11195 significantly decreased mRNA expression of the chemotherapy resistance efflux pumps ABCA3, ABCB1 and ABCC1 in two post-relapse NB cell lines. We also found that pre-treatment with PK11195 sensitized these cell lines to treatment with cytotoxic chemotherapy agents. These results suggest that PK11195 alone or in combination with standard chemotherapeutic drugs warrants further study for the treatment of neuroblastoma.

Translocator protein (18 kDa) mediates the pro-growth effects of diazepam on Ehrlich tumor cells in vivo

The Translocator Protein (TSPO), previously known as the peripheral-type benzodiazepine receptor, is a ubiquitous drug- and cholesterol-binding protein that is up regulated in several types of cancer cells. TSPO drug ligands (e.g., diazepam) induce or inhibit tumor cell proliferation, depending on the dose and tissue origin. We have previously shown that TSPO is expressed in Ehrlich tumor cells and that diazepam increases proliferation of these cells in vitro. Here, we investigated the in vivo effects of diazepam on Ehrlich tumor growth and the role of TSPO in mediating this process. Oral administration of diazepam to mice (3.0mg/kg/day for 7 days) produced plasma and ascitic fluid drug concentrations of 83.83 and 54.12 nM, respectively. Diazepam increased Ehrlich tumor growth, likely due to its ability to increase tumor cell proliferation and Reactive Oxygen Species production. Radioligand binding assays and nucleotide sequencing revealed that Ehrlich tumor cell TSPO had the same pharmacological and biochemical properties as TSPO described in other tumor cells. The estimated K(d) for PK 11195 in Ehrlich tumor cells was 0.44 nM and 8.70 nM (low and high binding site, respectively). Structurally diverse TSPO drug ligands with exclusive affinity for TSPO (i.e., 4-chlordiazepam, Ro5-4864, and isoquinoline-carboxamide PK 11195) also increased Ehrlich tumor growth. However, clonazepam, a GABA(A)-specific ligand with no affinity for TSPO, failed to do so. Taken together, these data suggest that diazepam induces in vivo Ehrlich tumor growth in a TSPO-dependent manner.

Depleted folate pool and dysfunctional mitochondria associated with defective mitochondrial folate proteins sensitize Chinese ovary cell mutants to tert-butylhydroperoxide-induced oxidative stress and apoptosis

The functional role of mitochondrial (mt) folate-associated proteins in mammalian cells is not clearly understood. We investigated the respiratory function and apoptosis phenotype of Chinese hamster ovary (CHO) mutant cells with defective mt serine hydroxymethyltransferase (SHMT) activities (glyA) or with defective mt folate transporter (glyB) in the absence/presence of oxidant challenge. The mechanisms underlying their aberrant phenotypes were explored. Compared with CHOK1 wild-type cells, both mutants carried dysfunctional mitochondria with reduced respiratory complex IV activity and depolarized mt membrane potential (P<.05). Elevated superoxide levels and accumulated mtDNA large deletions were observed in glyB in association with a depleted compartmental folate pool (P<.05). tert-Butylhydroperoxide (tBH) treatment at 50 microM for 72 h significantly depleted mt and cytosolic folate levels, impaired antioxidant defenses, and aggravated mt oxidative dysfunction in both mutants (P<.05), more severely in glyB. Only tBH-treated glyB cells displayed an elevated ratio of mt Bax/Bcl-2, activation of procaspases 9 and 3, and apoptosis promotion. The apoptotic phenotype of tBH-treated glyB could be partially corrected by folate supplementation (10-1000 microM), which enriched compartmental folate levels, restored antioxidant defenses, eliminated mt oxidative injuries, and normalized mt membrane function. Our data identify previously unrecognized roles of mt folate-associated proteins in the protection of mitochondria against oxidative insults. Defective mt folate transporter sensitized glyB cells to elevated oxidative stress and tBH-induced apoptosis, partly mediated by depleted compartmental folate and mt dysfunction. Defective mt SHMT sensitized glyA to respiratory dysfunction and tBH-induced oxidative injury without apoptosis promotion.

Effect of peripheral benzodiazepine receptor (PBR/TSPO) ligands on opening of Ca2+-induced pore and phosphorylation of 3.5-kDa polypeptide in rat brain mitochondria

The effect of nanomolar concentrations of PBR/TSPO ligands--Ro 5-4864, PK11195, and PPIX--on Ca2+-induced permeability transition pore (PTP) opening in isolated rat brain mitochondria was investigated. PBR/TSPO agonist Ro 5-4864 (100 nM) and endogenous ligand PPIX (1 microM) were shown to stimulate PTP opening, while antagonist PK11195 (100 nM) suppressed this process. Correlation between PBR ligand action on PTP opening and phosphorylation of a 3.5 kDa polypeptide was investigated. In intact brain mitochondria, incorporation of [gamma-(32)P]ATP into 3.5 kDa peptide was decreased in the presence of Ro 5-4864 and PPIX and increased in the presence of PK11195. At threshold Ca2+ concentrations leading to PTP opening, PBR/TSPO ligands were found to stimulate dephosphorylation of the 3.5 kDa peptide. Specific anti-PBR/TSPO antibody prevented both PTP opening and dephosphorylation of the 3.5-kDa peptide. The peptide was identified as subunit c of F(o)F(1)-ATPase by Western blot using specific anti-subunit c antibody. The results suggest that subunit c of F(o)F(1)-ATPase could be an additional target for PBR/TSPO ligands action, is subjected to Ca2+- and TSPO-dependent phosphorylation/dephosphorylation, and is involved in PTP operation in mitochondria.

Effects of Vinpocetine on mitochondrial function and neuroprotection in primary cortical neurons

Vinpocetine (ethyl apovincaminate), a synthetic derivative of the Vinca minor alkaloid vincamine, is widely used for the treatment of cerebrovascular-related diseases. One of the proposed mechanisms underlying its action is to protect against the cytotoxic effects of glutamate overexposure. Glutamate excitotoxicity leads to the disregulation of mitochondrial function and neuronal metabolism. As Vinpocetine has a binding affinity to the peripheral-type benzodiazepine receptor (PBR) involved in the mitochondrial transition pore complex, we investigated whether neuroprotection can be at least partially due to Vinpocetine's effects on PBRs. Neuroprotective effects of PK11195 and Ro5-4864, two drugs with selective and high affinity to PBR, were compared to Vinpocetine in glutamate excitotoxicity assays on primary cortical neuronal cultures. Vinpocetine exerted a neuroprotective action in a 1-50microM concentration range while PK11195 and Ro5-4864 were only slightly neuroprotective, especially in high (>25microM) concentrations. Combined pretreatment of neuronal cultures with Vinpocetine and PK11195 or Ro5-4864 showed increased neuroprotection in a dose-dependent manner, indicating that the different drugs may have different targets. To test this hypothesis, mitochondrial membrane potential (MMP) of cultured neurons was measured by flow cytometry. 25microM Vinpocetine reduced the decrease of mitochondrial inner membrane potential induced by glutamate exposure, but Ro5-4864 in itself was found to be more potent to block glutamate-evoked changes in MMP. Combination of Ro5-4864 and Vinpocetine treatment was found to be even more effective. In summary, the present results indicate that the neuroprotective action of vinpocetine in culture can not be explained by its effect on neuronal PBRs alone and that additional drug targets are involved.

Neuroprotective effect of Ro5-4864 following brain injury

The 18 kDa translocator protein (TSPO) is a protein complex located at the outer mitochondrial membrane and interacting with the mitochondrial permeability transition pore (mPTP), indicating its involvement in the control of mPTP opening. We intended to explore the effect of TSPO ligands, PK 11195 and Ro5-4864 on apoptosis in a rat model of cortical injury. Sprague-Dawley rats received a daily intraperitoneal injection of dimethylsulfoxide (vehicle), PK 11195, or Ro5-4864, starting 2 days prior the injury and a third injection after the injury. At 6 weeks, immunohistochemistry analysis showed that Ro5-4864 resulted in a significant increase in the number of surviving neurons and in the density of the neurofilament network in the perilesional cortex in comparison with animals of the vehicle and PK 11195 groups. In tissue samples dissected from the injured area, Ro5-4864 caused a significant reduction in activation of caspases 3 and 9 but not of caspase 8 in comparison with the vehicle and PK 11195 groups. In addition, measurements of transmembrane mitochondrial potential of mitochondria (Deltapsi(M)) isolated from normal rat brain showed that loss of Deltapsi(M) induced by recombinant Bax could be significantly reduced by Ro5-4864 in a concentration-dependent manner. Our findings indicate that the neuroprotective effect shown by Ro5-4864 in the present model of brain injury involves the mitochondrial pathway of apoptosis modulation of mPTP.

Comparative anticancer effects of flavonoids and diazepam in cultured cancer cells

This study examined the comparative anticancer effects of flavonoids and diazepam in the cultured cancer cells. In the SNU-C4 colorectal and MDA-MB-231 breast adenocarcinoma cells, apigenin and fisetin, flavonoids, and diazepam inhibited cancer cell survival concentration and incubation-time dependently. Diazepam consistently inhibited FAS activity, a known anticancer mechanism of flavonoids, in a concentration dependent manner. Unlike diazepam, in highly aggressive breast MDA-MB-231 cells known to have a nuclear/perinuclear located PBR, PK11195, a specific PBR ligand enhanced the proliferation of cells, and the proliferative effect of PK11195 was reversed by an addition of lovastatin, a HMG-CoA reductase inhibitor. Diazepam- and flavonoids-induced cytotoxic activity in both cancer cell lines was not reduced by the addition of 5-fluorouracil (5-FU), a chemotherapeutic agent. Like flavonoids, diazepam inhibited the release of vascular endothelial growth factor (VEGF) and granulocyte-macrophage-colony stimulating factor (GM-CSF) into supernatants of cultured in the SNU-C4 and MDA-MB-231 cells. In conclusion, this study provided in vitro information on the safe use of sedative in oncologic patients.

Intracellular cholesterol changes induced by translocator protein (18 kDa) TSPO/PBR ligands

One of the main functions of the translocator protein (18 kDa) or TSPO, previously known as peripheral-type benzodiazepine receptor, is the regulation of cholesterol import into mitochondria for steroid biosynthesis. In this paper we show that TSPO ligands induce changes in the distribution of intracellular cholesterol in astrocytes and fibroblasts. NBD-cholesterol, a fluorescent analog of cholesterol, was rapidly removed from membranes and accumulated into lipid droplets. This change was followed by a block of cholesterol esterification, but not by modification of intracellular cholesterol synthesis. NBD-cholesterol droplets were in part released in the medium, and increased cholesterol efflux was observed in [(3)H]cholesterol-prelabeled cells. TSPO ligands also induced a prominent shrinkage and depolarization of mitochondria and depletion of acidic vesicles with cytoplasmic acidification. Consistent with NBD-cholesterol changes, MTT assay showed enhanced accumulation of formazan into lipid droplets and inhibition of formazan exocytosis after treatment with TSPO ligands. The effects of specific TSPO ligands PK 11195 and Ro5-4864 were reproduced by diazepam, which binds with high affinity both TSPO and central benzodiazepine receptors, but not by clonazepam, which binds exclusively to GABA receptor, and other amphiphilic substances such as DIDS and propranolol. All these effects and the parallel immunocytochemical detection of TSPO in potentially steroidogenic cells (astrocytes) and non-steroidogenic cells (fibroblasts) suggest that TSPO is involved in the regulation and trafficking of intracellular cholesterol by means of mechanisms not necessarily related to steroid biosynthesis.

Strategy for reversing resistance to a single anticancer agent in human prostate and pancreatic carcinomas

Effective therapies for most solid cancers, especially those that have progressed to metastasis, remain elusive because of inherent and acquired resistance of tumor cells to conventional treatments. Additionally, the effective therapeutic window for many protocols can be very narrow, frequently resulting in toxicity. The present study explores an anticancer strategy that effectively eliminates resistant cancer cells without exerting deleterious effects on normal cells. This approach employs melanoma differentiation-induced gene-7/interleukin-24 (mda-7/IL-24), a cancer-specific, apoptosis-inducing cytokine, in combination with nontoxic doses of a chemical compound from the endoperoxide class that decomposes in water generating singlet oxygen. This combinatorial regimen specifically induced in vitro apoptosis in prostate carcinoma cells, with innate resistance to chemotherapy or engineered resistance to mda-7/IL-24, as well as pancreatic carcinoma cells inherently resistant to any treatment modality, including mda-7/IL-24. Apoptosis induction correlated with increased cellular reactive oxygen species production and was prevented by general antioxidants, such as N-acetyl-l-cysteine or Tiron. Induction of apoptosis in combination-treated cancer cells correlated with a reduction in the antiapoptotic protein BCL-x(L). In contrast, both normal prostate and pancreatic epithelial cells were unaffected by the single or combination treatment. These provocative findings suggest that this combinatorial strategy might provide a platform for developing effective treatments for therapy-resistant cancers.

Upregulation of PBR mRNA expression in human neuroblastoma cells by flavonoids

To investigate the putative mediation of peripheral benzodiazepine receptor (PBR) in the cytotoxicity of flavonoids, in this study, modulatory effects of several flavonoids on the lipid peroxide (LPO) production and PBR mRNA expression of human neuroblastoma cells were observed. Elevated levels of peroxidated products in cancer cells may activate pro-apoptotic and anti-proliferative signaling pathways. Treatment of 10(-6) M 4'-chlorodiazepam and PK 11195 ligands of the PBR for 6 days enhanced the generation of LPO of the human neuroblastoma cells. Several flavonoids, well-known cytotoxic substances, potentiated the enhancement of LPO production by PBR ligands. Treatment of 10(-6) M flavonoids for 6 days elevated the expression of PBR mRNA in cells. These findings indicate that the potential of flavonoids to induce apoptosis in cancer cells is strongly associated with their PBR-inducing properties, thereby providing a new mechanism by which polyphenolic compounds may exert their cancer-preventive and anti-neoplastic effects.

Inhibition of the mitochondrial F1F0-ATPase by ligands of the peripheral benzodiazepine receptor

Although PK11195 binds to the peripheral benzodiazepine receptor with nanomolar affinity, significant data exist which suggest that it has another cellular target distinct from the PBR. Here we demonstrate that PK11195 inhibits F(1)F(0)-ATPase activity in an OSCP-dependent manner, similar to the pro-apoptotic benzodiazepine Bz-423. Importantly, our data indicate that cellular responses observed with micromolar concentrations of PK11195, which are commonly attributed to modulation of the PBR, are likely a direct result of mitochondrial F(1)F(0)-ATPase inhibition.

The peripheral-type benzodiazepine receptor is involved in control of Ca2+-induced permeability transition pore opening in rat brain mitochondria

The peripheral-type benzodiazepine receptor (PBR) is an 18 kDa mitochondrial membrane protein with still elusive function in cell death. Here, we studied whether PBR is involved in Ca2+-induced permeability transition pore (PTP) opening in isolated rat brain mitochondria (RBM). PTP opening is important in mitochondrial events leading to programmed cell death. Immunoblots revealed a single 18 kDa anti-PBR antibody-immunoreactive band in purified RBM. Adenine nucleotide transporter, a key PTP component, was found in the PBR-immunoprecipitate. In isolated intact RBM, addition of a specific anti-PBR antibody [H. Li, Z. Yao, B. Degenhardt, G. Teper, V. Papadopoulos, Cholesterol binding at the cholesterol recognition/interaction amino acid consensus (CRAC) of the peripheral-type benzodiazepine receptor and inhibition of steroidogenesis by an HIV TAT-CRAC peptide, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 1267-1272] delayed Ca2+-induced dissipation of membrane potential (psi(m)) and diminished cyclosporine A-sensitive Ca2+ efflux, which are both indicative for the suppression of PTP opening. Moreover, anti-PBR antibody caused partial retention of Ca2+ in the mitochondrial matrix in spite of psi(m) dissipation, and reduced activation of respiratory rate at Ca2+-induced PTP opening. A release of pro-apoptotic factors, AIF and cytochrome c, from RBM was shown at threshold Ca2+ load. Anti-PBR antibody blocked the release of AIF but did not affect the cytochrome c release. Addition of ATP was able to initiate PTP closing, associated with psi(m) restoration and Ca2+ re-accumulation. At the same time mitochondrial protein phosphorylation (incorporation of 32P from [gamma-32P]ATP) occurred and anti-PBR antibody was able to inhibit phosphorylation of these proteins. The endogenous PBR ligand, protoporphyrin IX, facilitated PTP opening and phosphorylation of the mitochondrial proteins, thus, inducing effects opposite to anti-PBR antibody. This study provides evidence for PBR involvement in PTP opening, controlling the Ca2+-induced Ca2+ efflux, and AIF release from mitochondria, important stages of initiation of programmed cell death.

High-affinity peripheral benzodiazepine receptor ligand, PK11195, regulates protein phosphorylation in rat brain mitochondria under control of Ca(2+)

The effects of PK11195, a high-affinity peripheral benzodiazepine receptor (PBR) ligand, on protein phosphorylation in isolated purified rat brain mitochondria were investigated. The isoquinoline carboxamide ligand of PBR, PK11195, but not the benzodiazepine ligand Ro5-4864, in the nanomolar concentration range strongly increased the phosphorylation of 3.5 and 17 kDa polypeptides. The effect of PK11195 was seen in the presence of elevated Ca(2+) levels (3 x 10(-7) to 10(-6) m), but not at very low Ca(2+) levels (10(-8) to 3 x 10(-8) m). This indicates that PBR involves Ca(2+) as a second messenger in the regulation of protein phosphorylation. Staurosporine, an inhibitor of protein kinase activity was able to suppress the PK11195-promoted protein phosphorylation. When the permeability transition pore (PTP) was opened by threshold Ca(2+) load, phosphorylation of the 3.5-kDa polypeptide was diminished, but strong phosphorylation of the 43-kDa protein was revealed. The 43-kDa protein appears to be a PTP-specific phosphoprotein. If PTP was opened, PK11195 did not increase the phosphorylation of the 3.5 and 17-kDa proteins but suppressed the phosphorylation of the PTP-specific 43-kDa phosphoprotein. The ability of PK11195 to increase the protein phosphorylation, which was lost under Ca(2+)-induced PTP opening, was restored again in the presence of calmidazolium, an antagonist of calmodulin and inhibitor of protein phosphatase PP2B. These results show a tight interaction of PBR with the PTP complex in rat brain mitochondria. In conclusion, a novel function of PBR in brain mitochondria has been revealed, and the PBR-mediated protein phosphorylation has to be considered an important element of the PBR-associated signal transducing cascades in mitochondria and cells.

PIGA (N,N-Di-n-butyl-5-chloro-2-(4-chlorophenyl)indol-3-ylglyoxylamide), a new mitochondrial benzodiazepine-receptor ligand, induces apoptosis in C6 glioma cells

Mitochondrial benzodiazepine-receptor (mBzR) ligands constitute a heterogeneous class of compounds that show a pleiotropic spectrum of effects within the cells, including the modulation of apoptosis. In this paper, a novel synthetic 2-phenylindol-3-ylglyoxylamide derivative, N,N-di-n-butyl-5-chloro-2-(4-chlorophenyl)indol-3-ylglyoxylamide (PIGA), which shows high affinity and selectivity for the mBzR, is demonstrated to induce apoptosis in rat C6 glioma cells. PIGA was able to dissipate mitochondrial transmembrane potential (DeltaPsim) and to cause a significant cytosolic accumulation of cytochrome c. Moreover, typical features of apoptotic cell death, such as caspase-3 activation and DNA fragmentation, were also detected in PIGA-treated cells. Our data expand the knowledge on mBzR ligand-mediated apoptosis and suggest PIGA as a novel proapoptotic compound with therapeutic potential against glial tumours, in which apoptosis resistance has been reported to be involved in carcinogenesis.

Mechanisms of mitochondrial apoptosis induced by peripheral benzodiazepine receptor ligands in human colorectal cancer cells

Specific ligands of the peripheral benzodiazepine receptor (PBR) have been shown to induce apoptosis in gastrointestinal cancers. The aim of this study was to characterize the signaling pathways of PBR ligand-induced apoptosis. FGIN-1-27 but not PK 11195-induced apoptosis was associated with a decrease of mitochondrial membrane potential and an increase of mitochondrial volume in HT29 colorectal cancer cells. However, PK 11195-elicited apoptosis was associated with a downregulation of Bcl-2, translocation of Bax to the mitochondria including subsequent oligomerization, and activation of caspase-9, indicating the involvement of mitochondria in PK 11195-induced apoptosis. Moreover, PK 11195-induced apoptosis was associated with the generation of reactive oxygen species. This study demonstrates a novel mechanism of PK 11195-induced mitochondrial apoptosis without alteration of the mitochondrial membrane potential. The characterization of signaling pathways associated with PBR ligand-induced apoptosis will build the base for a future use of these ligands in anti-neoplastic therapeutic approaches.

PK11195 potently sensitizes to apoptosis induction independently from the peripheral benzodiazepin receptor

1-(2-Chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195) is a prototypic ligand of the peripheral benzodiazepine receptor (PBR), a mitochondrial outer membrane protein. PK11195 can be used to chemosensitize tumor cells to a variety of chemotherapeutic agents, both in vitro and in vivo. PK11195 has been suggested to exert this effect via inhibition of the multiple drug resistance (MDR) pump and by direct mitochondrial effects which could be mediated by the PBR. Here, we established a model system in which PK11195 and another PBR ligand, 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864), sensitize to nutrient depletion-induced cell death. In this MDR-independent model, PK11195 and Ro5-4864 are fully active even when the PBR is knocked down by small interfering RNA. Cells that lack PBR possess low-affinity binding sites for PK11195 and Ro5-4864. The starvation-sensitizing effects of PK11195 are not due to a modulation of the adaptive response of starved cells, namely autophagy and NF-kappaB activation. Rather, it appears that the combination of PK11195 with autophagy or NF-kappaB inhibitors has a potent synergistic death-inducing effect. Starved cells treated with PK11195 exhibit characteristics of apoptosis, including loss of the mitochondrial transmembrane potential, mitochondrial cytochrome c release, caspase activation and chromatin condensation. Accordingly, stabilization of mitochondria by overexpression of Bcl-2 or expression of the viral mitochondrial inhibitor (vMIA) from cytomegalovirus inhibits cell death induced by PK11195 plus starvation. Thus, PK11195 potently sensitizes to apoptosis via a pathway that involves mitochondria, yet does not involve the PBR.

BimEL up‐regulation potentiates AIF translocation and cell death in response to MPTP

This study attempted to elucidate the signaling mechanism underlying dopaminergic cell death in the MPP+ model for Parkinson's disease. In neuronal-differentiated PC12 cells, through the regulation by activated JNK and c-jun, BimEL expression was markedly increased in response to MPP+ treatment, which led to the cell degeneration. In lieu of Smac translocation as seen in other paradigms, up-regulation of BimEL effected an increase in calpain I activity that, in turn, mediated AIF release from the mitochondria. In support, we found that knocking down BimEL expression resulted in a decrease in calpain I activity, as well as AIF release from the mitochondria and cell death. Finally, inhibition of calpain activity mitigated AIF release from the mitochondria and cell death. Under cell-free conditions, activated purified calpain I could induce the release of AIF from isolated mitochondria without the participation of BimEL or activated JNK, suggesting that AIF release is a direct consequence of calpain I activity. In concert, the results suggest a novel signaling pathway for dopaminergic cell degeneration, in which MPP+ induces the up-regulation of BimEL, which in turn potentiates an elevation in calpain I activity that mediates AIF release and cell death in a caspase-independent manner.

Effect of PK11195, a peripheral benzodiazepine receptor agonist, on insulinoma cell death and insulin secretion

Functional role of peripheral benzodiazepine receptor on mitochondrial membrane in apoptosis and insulin secretion from insulinoma cells was studied. A prototypic peripheral benzodiazepine receptor agonist PK11195 induced insulinoma cell apoptosis, while a central benzodiazepine receptor agonist did not. Death of insulinoma cells by PK11195 was inhibited by cyclosporin A, a blocker of mitochondrial permeability transition pore. Caspase inhibitors further inhibited MIN6N8 cell death. PK11195 induced dissipation of mitochondrial potential and cytochrome c translocation to cytoplasm. PK11195 induced an increase in cytoplasmic [Ca(2+)], which was reversed by cyclosporin A. Rhod-2 staining showed decreased mitochondrial [Ca(2+)] after PK11195 treatment. PK11195 potentiated glucose-induced insulin secretion probably due to the increased cytoplasmic [Ca(2+)]. Calpain was activated following Ca(2+) release, and calpain inhibitors attenuated death of insulinoma cells by PK11195. These results suggest that PK11195 induces mitochondrial potential loss, cytochrome c translocation, increased insulin secretion in conjunction with an increase in cytoplasmic [Ca(2+)] and calpain activation, which collectively leads to apoptosis of insulinoma cells.

Peripheral benzodiazepine receptor (PBR) ligand cytotoxicity unrelated to PBR expression

Some synthetic ligands of the peripheral-type benzodiazepine receptor (PBR), an 18 kDa protein of the outer mitochondrial membrane, are cytotoxic for several tumor cell lines and arise as promising chemotherapeutic candidates. However, conflicting results were reported regarding the actual effect of these drugs on cellular survival ranging from protection to toxicity. Moreover, the concentrations needed to observe such a toxicity were usually high, far above the affinity range for their receptor, hence questioning its specificity. In the present study, we have shown that micromolar concentrations of FGIN-1-27 and Ro 5-4864, two chemically unrelated PBR ligands are toxic for both PBR-expressing SK-N-BE neuroblastoma cells and PBR-deficient Jurkat lymphoma cells. We have thereby demonstrated that the cytotoxicity of these drugs is unrelated to their PBR-binding activity. Moreover, Ro 5-4864-induced cell death differed strikingly between both cell types, being apoptotic in Jurkat cells while necrotic in SK-N-BE cells. Again, this did not seem to be related to PBR expression since Ro 5-4864-induced death of PBR-transfected Jurkat cells remained apoptotic. Taken together, our results show that PBR is unlikely to mediate all the effects of these PBR ligands. They however confirm that some of these ligands are very effective cytotoxic drugs towards various cancer cells, even for reputed chemoresistant tumors such as neuroblastoma, and, surprisingly, also for PBR-lacking tumor cells.

Evidence in favour of a role for peripheral-type benzodiazepine receptor ligands in amplification of neuronal apoptosis

The mitochondrial peripheral benzodiazepine receptor (PBR) is involved in a functional structure designated as the mitochondrial permeability transition (MPT) pore, which controls apoptosis. PBR expression in nervous system has been reported in glial and immune cells. We now show expression of both PBR mRNA and protein, and the appearance of binding of a synthetic ligand fluo-FGIN-1-27 in mitochondria of rat cerebellar granule cells (CGCs). Additionally, the effect of PBR ligands on colchicine-induced apoptosis was investigated. Colchicine-induced neurotoxicity in CGCs was measured at 24 h. We show that, in vitro, PBR ligands 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195), 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4- benzodiazepin-2-one (Ro5-4864) and diazepam (25- 50 microM) enhanced apoptosis induced by colchicine, as demonstrated by viability experiments, flow cytometry and nuclear chromatin condensation. Enhancement of colchicine-induced apoptosis was characterized by an increase in mitochondrial release of cytochrome c and AIF proteins and an enhanced activation of caspase-3, suggesting mitochondrion dependent mechanism that is involved in apoptotic process. Our results indicate that exposure of neural cells to PBR ligands generates an amplification of apoptotic process induced by colchicine and that the MPT pore may be involved in this process.

Peripheral benzodiazepine receptor ligands: mitochondrial transmembrane potential depolarization and apoptosis induction in rat C6 glioma cells

The peripheral benzodiazepine receptor (PBR) is a component of a multiprotein complex, located at the contact site between the inner and outer mitochondrial membranes, which constitutes the mitochondrial permeability transition (MPT)-pore. The opening of the MPT-pore, leading to the transmembrane mitochondrial potential (DeltaPsi(m)) dissipation, is a critical event in the mechanism of apoptosis. In the present work, we investigated the ability of the specific PBR ligands, PK 11195 or Ro5-4864, to affect mitochondrial potential and to induce apoptotic cell death in rat C6 glioma cells. Both specific ligands inhibited cell survival in a dose- and time-dependent manner, as assessed by MTS conversion assay, whereas the non-site selective ligand Diazepam or the low-affinity benzodiazepine Clonazepam showed no significant effects. After cell exposure to PK 11195 or Ro5-4864 we evidenced typical alterations of apoptotic cell death such as DNA fragmentation and chromatin condensation assessed by flow cytometric and transmission electron microscopy (TEM) analysis, respectively. Activation of the "effector" caspase-3 confirmed the ability of specific PBR ligands to induce apoptosis. Moreover, PK 11195 and Ro5-4864 induced a decrease of DeltaPsi(m), as evidenced by JC-1 flow cytometry analysis. Our data demonstrate the pro-apoptotic effects of specific PBR ligands on rat C6 glioma cells.

Targeting mitochondria to overcome conventional and bortezomib/proteasome inhibitor PS-341 resistance in multiple myeloma (MM) cells

Bortezomib (PS-341), a selective inhibitor of proteasomes, induces apoptosis in multiple myeloma (MM) cells; however, prolonged drug exposure may result in cumulative toxicity and the development of chemoresistance. Here we show that combining PK-11195 (PK), an antagonist to mitochondrial peripheral benzodiazepine receptors (PBRs), with bortezomib triggers synergistic anti-MM activity even in doxorubicin-, melphalan-, thalidomide-, dexamethasone-, and bortezomib-resistant MM cells. No significant cytotoxicity was noted in normal lymphocytes. Low-dose combined PK and bortezomib treatment overcomes the growth, survival, and drug resistance conferred by interleukin-6 or insulin growth factor within the MM bone marrow milieu. The mechanism of PK + bortezomib-induced apoptosis includes: loss of mitochondrial membrane potential; superoxide generation; release of mitochondrial proteins cytochrome-c (cyto-c) and Smac; and activation of caspases-8/-9/-3. Furthermore, PK + bortezomib activates c-Jun NH2 terminal kinase (JNK), which translocates to mitochondria, thereby facilitating release of cyto-c and Smac from mitochondria to cytosol. Blocking JNK, by either dominant-negative mutant (DN-JNK) or cotreatment with a specific JNK inhibitor SP600125, abrogates both PK + bortezomib-induced release of cyto-c/Smac and induction of apoptosis. Together, these preclinical studies suggest that combining bortezomib with PK may enhance its clinical efficacy, reduce attendant toxicity, and overcome conventional and bortezomib resistance in patients with relapsed refractory MM.

Defective core-apoptosis signalling in diffuse malignant pleural mesothelioma: opportunities for effective drug development

Because of a lack of effective treatments, survival from diffuse pleural mesothelioma remains poor. Many people do not think that treatments for this disease are effective. The understanding of the biology of mesothelioma relevant to the apoptosis-resistant phenotype has been slow to advance. However, this is now changing, and strategies for rational therapeutic drug development are emerging that have the potential to change the natural history and improve survival in the increasing number of patients that will be diagnosed in the next two decades. This review discusses recent developments in apoptosis biology that are specific to mesothelioma and the therapeutic implications for this aggressive cancer.

Mitochondrial benzodiazepine receptors regulate oxygen homeostasis in the early mouse embryo

The peripheral benzodiazepine receptor (Bzrp) has been implicated in the control of several processes, including mitochondrial biogenesis and embryo development. The present study examined the impact that specific Bzrp ligands have on oxygen homeostasis in the early mouse embryo. Day 9 embryos at the 16-18 somite pair stage were exposed to standard (21% oxygen) and suboptimal (5% oxygen) oxygen tensions in whole embryo culture. Analysis of gene expression used relative PCR to monitor changes in nuclear respiratory factor-1 (Nrf1), mitochondrial 16S ribosomal RNA (16S rRNA), and genes for several glycolytic enzymes. Ocular development was highly sensitive to periods of hypoxia through a mechanism blocked with the potent Bzrp ligand PK11195. Hypoxia led to a decline of Nrf1 and 16S rRNA levels also through a mechanism blocked with PK11195. Similar activity was observed for FGIN-1-27 whereas Ro5-4864 had contradictory effects. Morpholino-based gene knockdown of Nrf1 (anti-NRF1) produced a sequence-specific decrease in 16S rRNA insensitive to PK11195. These functional relationships suggest that Bzrp-dependent signals regulate the Nrf1 --> Tfam1 --> mtDNA --> 16S rRNA pathway in response to oxygen levels. The activity of PK11195 most likely has a pharmacodynamic basis with regards to specific embryonic precursor target cell populations, transducing a mitochondrial signal to an Nrf1 response analogous to retrograde regulation in yeast for mitochondria-to-nucleus signaling.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human esophageal cancer cells

Esophageal cancer is the most markedly increasing tumor entity in Western countries. Due to very poor 5-year-survival, new therapeutic approaches are mandatory. Peripheral benzodiazepine receptors (PBR) have been implicated in growth control of various tumor models, but they have not been studied yet in esophageal cancer. We used esophageal cancer cell lines and primary cell cultures of human esophageal cancers and evaluated (i) expression and localization of PBR; (ii) PBR-ligand-induced inhibition of cell growth; (iii) induction of apoptosis; and (iv) alterations in cell cycle. Expression of PBR was detected both in cell lines and in primary cell cultures of human esophageal cancers. PBR was localized in the mitochondria. The PBR-specific ligands FGIN-1-27 and PK 11195, but not the centrally acting benzodiazepine clonazepam or the indolacetamide FGIN-1-52, neither of which displaying any affinity to the PBR, inhibited cell proliferation. FGIN-1-27 and PK 11195, but not clonazepam, potently induced apoptosis. FGIN-1-27 was shown to sequentially decrease the mitochondrial membrane potential, then to activate caspase-3 and finally to cause DNA fragmentation. In addition, PBR-specific ligands induced cell cycle arrest in the G1/G0 phase. Our data qualify PBR-specific ligands as innovative proapoptotic and antiproliferative substances. They might prove suitable for the treatment of esophageal cancer.

Effects on free radical generation by ligands of the peripheral benzodiazepine receptor in cultured neural cells

The effect of peripheral benzodiazepine receptor (PBR) ligands on free radical production was investigated in primary cultures of rat brain astrocytes and neurons as well as in BV-2 microglial cell lines using the fluorescent dye dichlorofluorescein-diacetate. Free radical production was measured at 2, 30, 60 and 120 min of treatment with the PBR ligands 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolinecarboxamide (PK11195), 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864) and protoporphyrin IX (PpIX) (all at 10 nm). In astrocytes, all ligands showed a significant increase in free radical production at 2 min. The increase was short-lived with PK11195, whereas with Ro5-4864 it persisted for at least 2 h. PpIX caused an increase at 2 and 30 min, but not at 2 h. Similar results were observed in microglial cells. In neurons, PK11195 and PpIX showed an increase in free radical production only at 2 min; Ro5-4864 had no effect. The central-type benzodiazepine receptor ligand, clonazepam, was ineffective in eliciting free radical production in all cell types. As the PBR may be a component of the mitochondrial permeability transition (MPT) pore, and free radical production may occur following induction of the MPT, we further investigated whether cyclosporin A (CsA), an inhibitor of the MPT, could prevent free radical formation by PBR ligands. CsA (1 micro m) completely blocked free radical production following treatment with PK11195 and Ro5-4864 in all cell types. CsA was also effective in blocking free radical production in astrocytes following PpIX treatment, but it failed to do so in neurons and microglia. Our results indicate that exposure of neural cells to PBR ligands generates free radicals, and that the MPT may be involved in this process.

The myxoma poxvirus protein, M11L, prevents apoptosis by direct interaction with the mitochondrial permeability transition pore

M11L, an antiapoptotic protein essential for the virulence of the myxoma poxvirus, is targeted to mitochondria and prevents the loss of mitochondrial membrane potential that accompanies cell death. In this study we show, using a cross-linking approach, that M11L physically associates with the mitochondrial peripheral benzodiazepine receptor (PBR) component of the permeability transition (PT) pore. Close association of M11L and the PBR is also indicated by fluorescence resonance energy transfer (FRET) analysis. Stable expression of M11L prevents the release of mitochondrial cytochrome c induced by staurosporine or protoporphyrin IX (PPIX), a ligand of the PBR. Transiently expressed M11L also prevents mitochondrial membrane potential loss induced by PPIX, or induced by staurosporine in combination with PK11195, another ligand of the PBR. Myxoma virus infection and the associated expression of early proteins, including M11L, protects cells from staurosporine- and Fas-mediated mitochondrial membrane potential loss and this effect is augmented by the presence of PBR. We conclude that M11L regulates the mitochondrial permeability transition pore complex, most likely by direct modulation of the PBR.

Identification of a peptide antagonist to the peripheral-type benzodiazepine receptor that inhibits hormone-stimulated leydig cell steroid formation

Peripheral-type benzodiazepine receptor (PBR) is an 18-kDa high-affinity cholesterol and drug ligand-binding protein involved in various cell functions, including cholesterol transport and steroid biosynthesis. To aid our investigation of the biological function of PBR, we have set out to identify functional antagonists. By screening phage display libraries, we have identified peptides that displace the high-affinity PBR benzodiazepine drug ligand, Ro5-4864 (4'-chlorodiazepam). Among these peptides, STPHSTP was the most potent (IC(50) = 10 microM). All of the isolated peptides showed a conserved motif STXXXXP. The role of these peptides in Leydig cell steroidogenesis was examined using a transducible peptide composed of the TAT domain of human immunodeficiency virus and the peptides under investigation. Synthesized peptides efficiently transduced into MA-10 Leydig cells, and the peptide TAT-STPHSTP inhibited Ro5-4864- and human chorionic gonadotropin-stimulated steroid production in a dose-dependent manner (ED(50) = 5 microM). TAT-STPHSTP behaved as a competitive PBR antagonist, which did not affect 22R-hydroxycholesterol-supported steroidogenesis. These results yield leads for the development of potent PBR antagonists and indicate that endogenous PBR agonist-receptor interaction is critical for hormone-induced steroidogenesis.

Modulation of tamoxifen-induced apoptosis by peripheral benzodiazepine receptor ligands in breast cancer cells

The peripheral benzodiazepine receptor (PBR), an integral protein of the mitochondrial membrane, is involved in the formation of mitochondrial permeability transition (MPT) pores. The opening of the MPT-leading to the dissipation of the inner-mitochondrial transmembrane potential (deltapsi(m))-is considered to be an early apoptotic event. Therefore, we investigated the effect of the high-affinity PBR ligands Ro5-4684 and PK 11195 on tamoxifen (TAM)-induced apoptosis in MCF-7 and BT-20 breast cancer cell lines. Application of 100 nM TAM led to induction of apoptosis in both cell lines. Estrogene receptor (ER)-positive MCF-7 cells arrested in G(2/M) by TAM treatment showed no general dissipation of deltapsi(m), but reduction of deltapsi(m) was observed in a population of cells with high deltapsi(m). In ER-negative BT-20 cells TAM treatment induced no arrest of the cell cycle but dissipation of deltapsi(m). In both cell lines, nanomolar concentrations of the PBR ligands, which showed minor pro-apoptotic action themselves, reduced TAM-induced decrease of deltapsi(m) and apoptosis. In MCF-7 cells, a reduction of bcl-2 protein expression by TAM treatment was abolished by a combination of TAM with PBR ligands. Bax protein expression in BT-20 cells showed a significant increase in TAM-treated cells after 24hr but was not increased when treated with TAM and PBR ligands. From these findings, we concluded that binding of PBR ligands in nanomolar concentrations protects cells against apoptosis.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human colorectal cancer cells

The peripheral benzodiazepine receptor (PBR) has been implicated in growth control of various tumour models. Although colorectal cancers were found to overexpress PBR, the functional role of PBR in colorectal cancer growth has not been addressed to date. Using primary cell cultures of human colorectal cancers and the human colorectal carcinoma cell lines HT29, LS174T, and Colo320 DM we studied the involvement of PBR in the growth control and apoptosis of colorectal cancers. Both mRNA and protein expression of PBR were detected by RT-PCR and flow cytometry. Using confocal laser scanning microscopy and immunohistochemistry the PBR was localized in the mitochondria. The specific PBR ligands FGIN-1-27, PK 11195, or Ro5-4864 inhibited cell proliferation dose-dependently. FGIN-1-27 decreased the mitochondrial membrane potential, which indicates an early event in apoptosis. Furthermore, FGIN-1-27, PK 11195 or Ro5-4864 increased caspase-3 activity. In addition to their apoptosis-inducing effects, PBR ligands induced cell cycle arrest in the G(1)/G(0)-phase. Thus, our data demonstrate a functional involvement of PBR in colorectal cancer growth and qualify the PBR as a possible target for innovative therapeutic approaches in colorectal cancer.