Two cellular and subcellular locations for the peripheral-type benzodiazepine receptor in rat liver

TSPO protein binding partners in bacteria, animals, and plants

The ancient membrane protein TSPO is phylogenetically widespread from archaea and bacteria to insects, vertebrates, plants, and fungi. TSPO’s primary amino acid sequence is only modestly conserved between diverse species, although its five transmembrane helical structure appears mainly conserved. Its cellular location and orientation in membranes have been reported to vary between species and tissues, with implications for potential diverse binding partners and function. Most TSPO functions relate to stress-induced changes in metabolism, but in many cases it is unclear how TSPO itself functions—whether as a receptor, a sensor, a transporter, or a translocator. Much evidence suggests that TSPO acts indirectly by association with various protein binding partners or with endogenous or exogenous ligands. In this review, we focus on proteins that have most commonly been invoked as TSPO binding partners. We suggest that TSPO was originally a bacterial receptor/stress sensor associated with porphyrin binding as its most ancestral function and that it later developed additional stress-related roles in eukaryotes as its ability to bind new partners evolved.

Sifting through the surfeit of neuroinflammation tracers

The first phase of molecular brain imaging of microglial activation in neuroinflammatory conditions began some 20 years ago with the introduction of [11C]-( R)-PK11195, the prototype isoquinoline ligand for translocator protein (18 kDa) (TSPO). Investigations by positron emission tomography (PET) revealed microgliosis in numerous brain diseases, despite the rather low specific binding signal imparted by [11C]-( R)-PK11195. There has since been enormous expansion of the repertoire of TSPO tracers, many with higher specific binding, albeit complicated by allelic dependence of the affinity. However, the specificity of TSPO PET for revealing microglial activation not been fully established, and it has been difficult to judge the relative merits of the competing tracers and analysis methods with respect to their sensitivity for detecting microglial activation. We therefore present a systematic comparison of 13 TSPO PET and single photon computed tomography (SPECT) tracers belonging to five structural classes, each of which has been investigated by compartmental analysis in healthy human brain relative to a metabolite-corrected arterial input. We emphasize the need to establish the non-displaceable binding component for each ligand and conclude with five recommendations for a standard approach to define the cellular distribution of TSPO signals, and to characterize the properties of candidate TSPO tracers.

Subcellular distribution of the 18kDa translocator protein and transcript variant PBR-S in human cells

Despite continued interest in the 18kDa translocator protein (PBR/TSPO) as a biomarker and a therapeutic target for a range of diseases, its functional role, such as in the steroid synthesis pathway and energy metabolism has either become contentious or remains to be described more precisely. The PBR/TSPO gene consists of four exons, while a shorter isoform termed PBR-S lacks exon 2. The PBR-S 102-codon open reading frame differs to that of PBR/TSPO, resulting in a protein that is completely unrelated to PBR/TSPO. To our knowledge, PBR-S protein has never been described and has no known or proposed function. To obtain possible clues on the role of this uncharacterised protein, we compared the subcellular distribution of PBR-S to that of PBR/TSPO. By expressing fluorescently tagged PBR/TSPO, we confirmed that full-length PBR/TSPO co-localises with mitochondria in HeLa, HEK-293, MDA-MB-231, BJ and U87-MG human cell lines. Unlike the strictly mitochondrial localisation of PBR/TSPO, PBR-S has a punctate distribution throughout the cytosol that co-localises with lysosomes in HeLa, HEK-293, MDA-MB-231, BJ and U87-MG cells. In summary, within the cell lines examined we confirm mitochondria rather than occasionally reported other localisations, such as the cell nucleus, to be the only site where PBR/TSPO resides. Due to the lack of any shared protein sequences and the different subcellular locations, we suggest that the previously uncharacterised PBR-S protein variant of the PBR/TSPO gene is likely to serve a different yet to be discovered function compared to PBR/TSPO.

Targeting the 18-kDa translocator protein: recent perspectives for neuroprotection

The translocator protein (TSPO, 18 kDa), mainly localized in the outer mitochondrial membrane of steroidogenic tissues, is involved in several cellular functions. TSPO level alterations have been reported in a number of human disorders, particularly in cancer, psychiatric and neurological diseases. In the central nervous system (CNS), TSPO is usually expressed in glial cells, but also in some neuronal cell types. Interestingly, the expression of TSPO on glial cells rises after brain injury and increased TSPO expression is often observed in neurological disorders, gliomas, encephalitis and traumatic injury. Since TSPO is up-regulated in brain diseases, several structurally different classes of ligands targeting TSPO have been described as potential diagnostic or therapeutic agents. Recent researches have reported that TSPO ligands might be valuable in the treatment of brain diseases. This review focuses on currently available TSPO ligands, as useful tools for the treatment of neurodegeneration, neuro-inflammation and neurotrauma.

Structure-to-function relationships of bacterial translocator protein (TSPO): a focus on Pseudomonas

The translocator protein (TSPO), which was previously designated as the peripheral-type benzodiazepine receptor, is a 3.5 billion year-old evolutionarily conserved protein expressed by most Eukarya, Archae and Bacteria, but its organization and functions differ remarkably. By taking advantage of the genomic data available on TSPO, we focused on bacterial TSPO and attempted to define functions of TSPO in Pseudomonas via in silico approaches. A tspo ortholog has been identified in several fluorescent Pseudomonas. This protein presents putative binding motifs for cholesterol and PK 11195, which is a specific drug ligand of mitochondrial TSPO. While it is a common surface distribution, the sense of insertion and membrane localization differ between α- and γ-proteobacteria. Experimental published data and STRING analysis of common TSPO partners in fluorescent Pseudomonas indicate a potential role of TSPO in the oxidative stress response, iron homeostasis and virulence expression. In these bacteria, TSPO could also take part in signal transduction and in the preservation of membrane integrity.

Visualization of acute liver damage induced by cycloheximide in rats using PET with [(18)F]FEDAC, a radiotracer for translocator protein (18 kDa)

Liver damage induced by drug toxicity is an important concern for both medical doctors and patients. The aim of this study was to noninvasively visualize acute liver damage using positron emission tomography (PET) with N-benzyl-N-methyl-2-[7,8-dihydro-7-(2-[¹⁸F]fluoroethyl)-8-oxo-2-phenyl-9H-purin-9-yl]acetamide ([¹⁸F]FEDAC), a radiotracer specific for translocator protein (18 kDa, TSPO) as a biomarker for inflammation, and to determine cellular sources enriching TSPO expression in the liver. A mild acute liver damage model was prepared by a single intraperitoneal injection of cycloheximide (CHX) into rats. Treatment with CHX induced apoptosis and necrotic changes in hepatocytes with slight neutrophil infiltration. The uptake of radioactivity in the rat livers was measured with PET after injection of [¹⁸F]FEDAC. The uptake of [¹⁸F]FEDAC increased in livers damaged from treatment with CHX compared to the controls. Presence of TSPO was examined in the liver tissue using quantitative reverse transcriptase-polymerase chain reaction and immunohistochemical assays. mRNA expression of TSPO was elevated in the damaged livers compared to the controls, and the level was correlated with the [¹⁸F]FEDAC uptake and severity of damage. TSPO expression in the damaged liver sections was mainly found in macrophages (Kupffer cells) and neutrophils, but not in hepatocytes. The elevation of TSPO mRNA expression was derived from the increase of the number of macrophages with TSPO and neutrophils with TSPO in damaged livers. From this study we considered that PET imaging with [¹⁸F]FEDAC represented the mild liver damage through the enhanced TSPO signal in inflammatory cells. We conclude that this method may be a useful tool for diagnosis in early stage of acute liver damage.

Organelle plasticity and interactions in cholesterol transport and steroid biosynthesis

Steroid biosynthesis is a multi-step process controlled by pituitary hormones, which, via cAMP-dependent signaling pathways, drive tissue-specific steroid formation. Steroidogenesis begins with the transport of the substrate, cholesterol, from intracellular stores into the inner mitochondrial membrane, where the steroidogenic enzyme CYP11A1 converts cholesterol to pregnenolone. This process is accelerated by hormones and involves a number of proteins and protein-protein interactions. Indeed, cholesterol, stored in lipid droplets and membranes, is transferred through a hormone-induced complex of proteins derived from the cytosol, mitochondria, and other organelles termed the transduceosome to the outer mitochondrial membrane. From there, cholesterol reaches CYP11A1 through outer/inner membrane contact sites. Thus, cholesterol transfer is likely achieved through a hormone-dependent reorganization of organelles and protein distribution and interactions. The findings reviewed herein suggest the presence of a hormone-dependent organelle communication network mediated by protein-protein interactions and inter-organelle trafficking, resulting in the efficient and timely delivery of cholesterol into mitochondria for steroid synthesis.

Stratified medicine in psychiatry: a worrying example or new opportunity in the treatment of anxiety?

Stratified medicine is a new term that figures highly in current MRC and NHS strategy. It has developed from the earlier terms individualised or personalised medicine and refers to the use of genetic and/or endophenotypic measures to allow better targeting of treatments. The best exemplar is HER2 positivity in breast cancer to determine the efficacy of Herceptin. Clinical trials of this anti-cancer drug were initially unpromising, but once the HER2 positive subgroup was identified it was found, in this subgroup only, to be highly effective. It is presumed that similar subgroups will be found for many common disorders not just cancers, and that these will lead to much better targeted treatments. Such an advance may be necessary to develop new treatments in certain fields where the development of broad-spectrum/blockbuster treatments appears to have reached the end of the road; a particular example of this is in psychiatry. In this paper we discuss this issue in relation to psychiatry using a new and interesting example of how genotyping might help rescue an apparently failed novel treatment in anxiety disorders.

Translocator protein (Tspo) gene promoter-driven green fluorescent protein synthesis in transgenic mice: an in vivo model to study Tspo transcription

Translocator protein (TSPO), previously known as the peripheral-type benzodiazepine receptor, is a ubiquitous drug- and cholesterol-binding protein primarily found in the outer mitochondrial membrane as part of a mitochondrial cholesterol transport complex. TSPO is present at higher levels in steroid-synthesizing and rapidly proliferating tissues and its biological role has been mainly linked to mitochondrial function, steroidogenesis and cell proliferation/apoptosis. Aberrant TSPO levels have been linked to multiple diseases, including cancer, endocrine disorders, brain injury, neurodegeneration, ischemia-reperfusion injury and inflammatory diseases. Investigation of the functions of this protein in vitro and in vivo have been mainly carried out using high-affinity drug ligands, such as isoquinoline carboxamides and benzodiazepines and more recently, gene silencing methods. To establish a model to study the regulation of Tspo transcription in vivo, we generated a transgenic mouse model expressing green fluorescent protein (GFP) from Aequorea coerulescens under control of the Tspo promoter region (Tspo-AcGFP). The expression profiles of Tspo-AcGFP, endogenous TSPO and Tspo mRNA were found to be well-correlated. Tspo-AcGFP synthesis in the transgenic mice was seen in almost every tissue examined and as with TSPO in wild-type mice, Tspo-AcGFP was highly expressed in steroidogenic cells of the endocrine and reproductive systems, epithelial cells of the digestive system, skeletal muscle and other organs. In summary, this transgenic Tspo-AcGFP mouse model recapitulates endogenous Tspo expression patterns and could be a useful, tractable tool for monitoring the transcriptional regulation and function of Tspo in live animal experiments.

Diazepam effects on non-syndromic cleft lip with or without palate: epidemiological studies, clinical findings, genes and extracellular matrix

IMPORTANCE OF THE FIELD

This review analyses international studies investigating the combined genetic and environmental causes of cleft lip with or without cleft palate (CL/P) and describes successes and limitations in identifying underlying genetic and environmental factors. CL/P, the most common congenital facial malformation, is a major public health burden in terms of medical costs and emotional stress to patients and families. Because genetic and environmental factors determine risk of occurrence, CL/P has a complex, multifactor aetiology.

AREAS COVERED IN THIS REVIEW

English language reports from 1980 to 2010 were searched for in Medline, PubMed, Science Citation Index, textbooks and review articles on drugs and pregnancy. Key words were diazepam or benzodiazepine(s) combined with cleft lip, cleft palate, oral malformations, prenatal exposure, GABA, gene expression and extracellular matrix.

WHAT THE READER WILL GAIN

This review presents an updated assessment of the mutagenic and genotoxic effects of diazepam (DZ), one of the most commonly used benzodiazepines, on CL/P occurrence.

TAKE HOME MESSAGE

Data are divergent; more studies are needed for an in-depth picture of the effects of DZ during gestation on the child's development, particularly on orofacial clefts.

Cholesterol transport in steroid biosynthesis: role of protein-protein interactions and implications in disease states

The transfer of cholesterol from the outer to the inner mitochondrial membrane is the rate-limiting step in hormone-induced steroid formation. To ensure that this step is achieved efficiently, free cholesterol must accumulate in excess at the outer mitochondrial membrane and then be transferred to the inner membrane. This is accomplished through a series of steps that involve various intracellular organelles, including lysosomes and lipid droplets, and proteins such as the translocator protein (18 kDa, TSPO) and steroidogenic acute regulatory (StAR) proteins. TSPO, previously known as the peripheral-type benzodiazepine receptor, is a high-affinity drug- and cholesterol-binding mitochondrial protein. StAR is a hormone-induced mitochondria-targeted protein that has been shown to initiate cholesterol transfer into mitochondria. Through the assistance of proteins such as the cAMP-dependent protein kinase regulatory subunit Ialpha (PKA-RIalpha) and the PKA-RIalpha- and TSPO-associated acyl-coenzyme A binding domain containing 3 (ACBD3) protein, PAP7, cholesterol is transferred to and docked at the outer mitochondrial membrane. The TSPO-dependent import of StAR into mitochondria, and the association of TSPO with the outer/inner mitochondrial membrane contact sites, drives the intramitochondrial cholesterol transfer and subsequent steroid formation. The focus of this review is on (i) the intracellular pathways and protein-protein interactions involved in cholesterol transport and steroid biosynthesis and (ii) the roles and interactions of these proteins in endocrine pathologies and neurological diseases where steroid synthesis plays a critical role.

Natural endogenous ligands for benzodiazepine receptors in hepatic encephalopathy

Benzodiazepines of natural origin (NBZDs) have been found in human blood and brains as well as in medicinal plants and foods. In plasma and brain tissue there are i.e. diazepam and nordiazepam equal to commercial drugs but there are also other benzodiazepine-like compounds termed "endozepines", which act as agonists at the benzodiazepine receptors of central type (CBR). A synthetic pathway for the production of NBZDs has not yet been found, but it has been suggested that micro-organisms may synthesize molecules with benzodiazepine-like structures. Hence NBZDs could be of both endogenous and exogenous source and be considered as natural anxyolitic and sedative. Interestingly there are also natural compounds, such as the polypeptide Diazepam Binding Inhibitor (DBI) acting as an "inversive agonist" implicated in fair and panic disorders. It has been suggested that NBZDs may play a role in the pathogenesis of hepatic encephalopathy (HE). Multidirectional studies evaluated NBZDs levels (1) in the blood of normal subjects, of cirrhotic with or without HE and in commercial benzodiazepine consumers; (2) in the blood of cirrhotic treated or not with a non-absorbable antibiotic; (3) in several constituents of our diet. In conclusion, NBZDs increase sometime in cirrhotics with or without HE but they reach concentrations not higher than those found in commercial benzodiazepines consumers. Hence NBZDs must be considered as occasional precipitating factor of HE and benzodiazepine antagonists only symptomatic drugs. The finding that NBZDs may be in part synthesized by intestinal bacterial flora and in part constituent of our diet underlines the importance to feed cirrhotic patients with selected food.

Melatonin and vitamin C administration ameliorate diazepam-induced oxidative stress and cell proliferation in the liver of rats

OBJECTIVE

Oxidative stress is a likely molecular mechanism in long-term diazepam administration. The benefits of antioxidants (melatonin and vitamin C) against diazepam-induced cell proliferation, DNA synthesis and oxidative damage were investigated in this study.

MATERIALS & METHODS

Four equal-sized groups of male rats [control, diazepam (3 mg/kg), diazepam plus melatonin (5 mg/kg) and diazepam plus vitamin C (50 mg/kg)] were used. Levels of lipid peroxides (LPO), superoxide dismutase (SOD) activity and glutathione (GSH) concentration were measured in tissue homogenates. Cell proliferation and rate of DNA synthesis were detected by autoradiography.

RESULTS

Results documented increased labelling index, (3)H-thymidine incorporation (DNA synthesis), LPO plus decrease in GSH levels and SOD activity in livers of diazepam-administered rats versus those of controls. When melatonin and vitamin C were given to diazepam-administered rats, they almost attenuated the increase of labelling index, DNA synthesis and LPO, and restored the levels of GSH and SOD activity.

CONCLUSION

These results suggest long-term hazard in use of drugs such as diazepam; they may be toxic and damage terminates in complex liver damage. Furthermore, melatonin and vitamin C may be useful in combating free radical-induced liver injury resulting from hazard and/or repeated diazepam administration.

Translocator protein 18 kDa (TSPO): molecular sensor of brain injury and repair

For over 15 years, the peripheral benzodiazepine receptor (PBR), recently named translocator protein 18 kDa (TSPO) has been studied as a biomarker of reactive gliosis and inflammation associated with a variety of neuropathological conditions. Early studies documented that in the brain parenchyma, TSPO is exclusively localized in glial cells. Under normal physiological conditions, TSPO levels are low in the brain neuropil but they markedly increase at sites of brain injury and inflammation making it uniquely suited for assessing active gliosis. This research has generated significant efforts from multiple research groups throughout the world to apply TSPO as a marker of "active" brain pathology using in vivo imaging modalities such as Positron Emission Tomography (PET) in experimental animals and humans. Further, in the last few years, there has been an increased interest in understanding the molecular and cellular function(s) of TSPO in glial cells. The latest evidence suggests that TSPO may not only serve as a biomarker of active brain disease but also the use of TSPO-specific ligands may have therapeutic implications in brain injury and repair. This review presents an overview of the history and function of TSPO focusing on studies related to its use as a sensor of active brain disease in experimental animals and in human studies.

Downregulation of the 18-kDa translocator protein: effects on the ammonia-induced mitochondrial permeability transition and cell swelling in cultured astrocytes

Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver disease. While the pathogenesis of HE is unclear, elevated blood and brain ammonia levels are believed to be major etiological factors, and astrocytes appear to be the primary target of its toxicity. A notable feature of ammonia neurotoxicity is an upregulation of the 18-kDa translocator protein (TSPO) (formerly referred to as the peripheral benzodiazepine receptor or PBR), which is found on the outer mitochondrial membrane. However, the precise significance of this upregulation is unclear. To examine its potential role in ammonia-induced astrocyte dysfunction, we downregulated the TSPO using antisense oligonucleotides, and examined whether such downregulation could alter two prominent features of ammonia gliotoxicity, namely, the mitochondrial permeability transition (MPT) and astrocyte swelling. Nontransfected cultures treated with NH4Cl (5 mM; 48 h) showed a significant increase in astrocyte cell volume (37.5%). In cultured astrocytes transfected with TSPO antisense oligonucleotides, such cell swelling was reduced to 17%, but this change was not significantly different from control cell volume. Similarly, nontransfected cultures treated with NH4Cl (5 mM; 24 h) exhibited a 40% decline in the cyclosporin A-sensitive mitochondrial inner membrane potential (DeltaPsi(m)) (P < 0.01) (a measure of the MPT). By contrast, cells transfected with TSPO antisense oligonucleotides did not display a significant loss of the DeltaPsi(m) following ammonia exposure. Our findings highlight the important role of the TSPO in the mechanism of ammonia neurotoxicity.

PK11195, a peripheral benzodiazepine receptor (pBR) ligand, broadly blocks drug efflux to chemosensitize leukemia and myeloma cells by a pBR-independent, direct transporter-modulating mechanism

The peripheral benzodiazepine receptor (pBR) ligand, PK11195, promotes mitochondrial apoptosis and blocks P-glycoprotein (Pgp)-mediated drug efflux to chemosensitize cancer cells at least as well or better than the Pgp modulator, cyclosporine A (CSA). We now show that PK11195 broadly inhibits adenosine triphosphate (ATP)-binding cassette (ABC) transporters in hematologic cancer cell lines and primary leukemia-cell samples, including multidrug resistance protein (MRP), breast cancer resistance protein (BCRP), and/or Pgp. Ectopic expression models confirmed that pBR can directly mediate chemosensitizing by PK11195, presumably via mitochondrial activities, but showed that pBR expression is unnecessary to PK11195-mediated efflux inhibition. PK11195 binds plasma-membrane sites in Pgp-expressing cells, stimulates Pgp-associated adenosine triphosphatase (ATPase) activity, and causes conformational changes in Pgp, suggesting that PK11195 modulates Pgp-mediated efflux by direct transporter interaction(s). PK11195 and CSA bind noncompetitively in Pgp-expressing cells, indicating that PK11195 interacts with Pgp at sites that are distinct from CSA-binding sites. Importantly, PK11195 concentrations that were effective in these in vitro assays can be safely achieved in patients. Because PK11195 promotes chemotherapy-induced apoptosis by a pBR-dependent mitochondrial mechanism and broadly blocks drug efflux by an apparently pBR-independent, ABC transporter-dependent mechanism, PK11195 may be a useful clinical chemosensitizer in cancer patients.

Expression of the peripheral benzodiazepine receptor is decreased in skin cancers in comparison with normal skin

BACKGROUND

The peripheral benzodiazepine receptor (PBR) is an 18-kDa protein receptor mainly found on the outer mitochondrial membrane of cells. The PBR plays a role in several cellular functions including haem synthesis, steroidogenesis, DNA synthesis, cell growth and differentiation, and apoptosis. PBR expression in normal skin correlates with proliferating, secretory and differentiated cellular structures. Increased or aberrant expression of PBR has been associated with aggressive behaviour in several tumour types including ovarian, colon and breast adenocarcinomas and glioblastoma.

OBJECTIVES

To determine whether changes in normal PBR distribution would be useful as markers for skin cancers or possible target sites for therapies such as photodynamic therapy (PDT), we used immunohistochemistry to evaluate PBR expression and distribution in normal and photodamaged skin (actinic keratoses), skin cancers (in situ and invasive squamous cell carcinomas and superficial, nodular, morphoeiform and mixed pattern basal cell carcinomas) and several benign epithelial proliferations.

METHODS

A rabbit polyclonal antibody to a synthetic peptide fragment of the PBR was developed and characterized by enzyme-linked immunosorbent assay and Western blot analysis. The antibody was used to stain formalin-fixed and paraffin-embedded tissue samples (n = 157) by a routine avidin-biotin immunohistochemical technique. Sections were evaluated for antibody localization, distribution (0-4+) and reaction intensity (negative to strong).

RESULTS

Normal skin stained with a strong homogeneous positive reaction (3-4+) in the spinous and granular layers (with a gradient corresponding to increasing differentiation), the pilosebaceous units, eccrine gland ducts, endothelial cells and pilar muscle. In cutaneous neoplasms and other skin diseases, a heterogeneous pattern (0-4+) of PBR expression at lower intensity was seen depending on tumour type and degree of differentiation. PBR expression was greatest in well-differentiated tumours, synonymous with the PBR expression gradient seen in normal skin; and least in poorly differentiated and infiltrative tumour types.

CONCLUSIONS

The haem biosynthetic pathway has been harnessed for PDT of skin carcinomas by application of exogenous aminolaevulinic acid to generate the endogenous photosensitizer protoporphyrin IX (PpIX). Owing to the role of PBR as a transporter of haem precursors in haem synthesis, PBR density and distribution in skin cancers could be a predictor of the capacity for PpIX production and subsequent response to PDT in skin cancers.

Aging reduces glycerol-3-phosphate acyltransferase activity in activated rat splenic T-lymphocytes

T-lymphocyte proliferation declines with age. Phosphatidic acid (PA) is the precursor to all glycerophospholipids, which serve as important membrane structural components and signaling molecules. Therefore, we tested the hypothesis that aged T-lymphocyte proliferation may be reduced, in part, suppressing phosphatidic acid (PA) biosynthesis. We showed, for the first time, that anti-CD3 stimulation in rat splenic T-lymphocytes selectively increased mitochondrial glycerol-3-phosphate acyltransferase (GPAT) activity. GPAT activity could be further increased by the addition of recombinant acyl-CoA binding protein (rACBP), but the amplification of GPAT activity was blunted by aging. This is important because PA is the precursor lipid for phospholipid synthesis and GPAT is the rate-limiting enzyme in PA biosynthesis. The mechanism by which stimulation and rACBP increased GPAT activity may involve phosphorylation since incubating Jurkat T-lymphocyte mitochondria with casein kinase 2 in vitro significantly increased GPAT activity. The data presented here suggest a novel mechanism by which aging may reduce activation-dependent mitochondrial GPAT activity. This age-induced alteration would result in reduced PA biosynthesis and could explain, in part, the diminished phospholipid content of the membrane and subsequent loss of proliferative capacity in the aged T-lymphocyte.

GABAA/central benzodiazepine receptor and peripheral benzodiazepine receptor ligands as inducers of phenobarbital-inducible CYP2B and CYP3A

A sequence critical for phenobarbital (PB) induction, the PB response unit (PBRU), situated upstream of the rat CYP2B1 and CYP2B2 genes, includes two nuclear receptor binding sites, NR1 and NR2. When NR1 and NR2 are mutated PB responsiveness is abolished. While no nuclear receptor for which PB is an agonist ligand has yet been identified, PB is a ligand of GABA(A) receptors and it can displace [(3)H] 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195) from its binding site on the peripheral benzodiazepine receptor (PBR). We assessed CYP2B levels in primary rat hepatocytes following treatment with 10 ligands of either or both of these receptors. All compounds tested were found to be CYP2B1/CYP2B2 inducers and most were CYP3A inducers. Five had not previously been described as CYP2B1/CYP2B2 inducers: bicuculline, flunitrazepam, 4'-chlorodiazepam (Ro5-4864), N,N-dihexyl-2-(4-fluorophenyl)indole-3-acetamide (FGIN 1-27) and 7-(dimethylcarbamoyloxy)-6-phenylpyrrolo-[2,1-d][1,5]benzothiazepine (DCPPBT). Reporter gene analysis demonstrated that CYP2B induction by these agents and other PBR or GABA(A) receptor ligands is mediated through the PBRU and the NR1/NR2 sites, suggesting a molecular mechanism similar to that for PB induction. The potencies for PBRU-dependent induction by 11 ligands of PBR or the GABA(A) receptor was evaluated. FGIN-127, DCPPBT and PK 11195 exhibited EC(50) values for PBRU-dependent transcription activation about three orders of magnitude higher than the reported affinities of the PBR for these agents, arguing against the involvement of the PBR in PB induction. However the EC(50) values found for the agents tested encourage further investigation on the possible involvement of the GABA(A) receptor in PB induction.

Functional characterization and expression of PBR in rat gastric mucosa: stimulation of chloride secretion by PBR ligands

Previous studies have demonstrated that gastric mucosa contained high levels of the polypeptide diazepam binding inhibitor, the endogenous ligand of the peripheral-type benzodiazepine receptor (PBR). However, the expression and function of this receptor protein in these tissues have not been investigated. Immunohistochemistry identified an intense PBR immunoreactivity in the mucous and parietal cells of rat gastric fundus and in the mucous cells of antrum. Immunoelectron microscopy revealed the mitochondrial localization of PBR in these cells. Binding of isoquinoline PK 11195 and benzodiazepine Ro5-4864 to gastric membranes showed that fundus had more PBR-binding sites than antrum, displaying higher affinity for PK 11195 than Ro5-4864. In a Ussing chamber, PK 11195 and Ro5-4864 increased short-circuit current (I(sc)) in fundic and antral mucosa in a concentration-dependent manner in the presence of GABA(A) and central benzodiazepine receptor (CBR) blockers. This increase in I(sc) was abolished after external Cl(-) substitution and was sensitive to chloride channels or transporter inhibitors. PK 11195-induced chloride secretion was also 1) sensitive to verapamil and extracellular calcium depletion, 2) blocked by thapsigargin and intracellular calcium depletion, and 3) abolished by the mitochondrial pore transition complex inhibitor cyclosporine A. PK 11195 had no direct effect on H(+) secretion, indicating that it stimulates a component of Cl(-) secretion independent of acid secretion in fundic mucosa. These data demonstrate that mucous and parietal cells of the gastric mucosa express mitochondrial PBR functionally coupled to Ca(2+)-dependent Cl(-) secretion, possibly involved in the gastric mucosa protection.

Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis

Cholesterol transport from the outer to the inner mitochondrial membrane is the rate-determining step in steroid and bile acid biosyntheses. Biochemical, pharmacological and molecular studies have demonstrated that the peripheral-type benzodiazepine receptor (PBR) is a five transmembrane domain mitochondrial protein involved in the regulation of cholesterol transport. PBR gene disruption in Leydig cells completely blocked cholesterol transport into mitochondria and steroid formation, while PBR expression in bacteria, devoid of endogenous PBR and cholesterol, induced cholesterol uptake and transport. Molecular modeling of PBR suggested that cholesterol might cross the membrane through the five helices of the receptor and that synthetic and endogenous ligands might bind to common sites in the cytoplasmic loops. A cholesterol recognition/interaction amino acid consensus (CRAC) sequence in the cytoplasmic carboxy-terminus of the PBR was identified by mutagenesis studies. In vitro reconstitution of PBR into proteoliposomes demonstrated that PBR binds both drug ligands and cholesterol with high affinity. In vivo polymeric forms of PBR were observed and polymer formation was reproduced in vitro, using recombinant PBR protein reconstituted into proteoliposomes, associated with an increase in drug ligand binding and reduction of cholesterol-binding capacity. This suggests that the various polymeric states of PBR might be part of a cycle mediating cholesterol uptake and release into the mitochondria, with PBR functioning as a cholesterol exchanger against steroid product(s) arising from cytochrome P450 action. Taking into account the widespread presence of PBR in many tissues, a more general role of PBR in intracellular cholesterol transport and compartmentalization might be considered.

The peripheral benzodiazepine receptor in photodynamic therapy with the phthalocyanine photosensitizer Pc 4

The peripheral benzodiazepine receptor (PBR) is an 18 kDa protein of the outer mitochondrial membrane that interacts with the voltage-dependent anion channel and may participate in formation of the permeability transition pore. The physiological role of PBR is reflected in the high-affinity binding of endogenous ligands that are metabolites of both cholesterol and heme. Certain porphyrin precursors of heme can be photosensitizers for photodynamic therapy (PDT), which depends on visible light activation of porphyrin-related macrocycles. Because the apparent binding affinity of a series of porphyrin analogs for PBR paralleled their ability to photoinactivate cells, PBR has been proposed as the molecular target for porphyrin-derived photocytotoxicity. The phthalocyanine (Pc) photosensitizer Pc 4 accumulates in mitochondria and structurally resembles porphyrins. Therefore, we tested the relevance of PBR binding on Pc 4-PDT. Binding affinity was measured by competition with 3H-PK11195, a high-affinity ligand of PBR, for binding to rat kidney mitochondria (RKM) or intact Chinese hamster ovary (CHO) cells. To assess the binding of the Pc directly, we synthesized 14C-labeled Pc 4 and found that whereas Pc 4 was a competitive inhibitor of 3H-PK11195 binding to the PBR, PK11195 did not inhibit the binding of 14C-Pc 4 to RKM. Further, 14C-Pc 4 binding to RKM showed no evidence of saturation up to 10 microM. Finally, when Pc 4-loaded CHO cells were exposed to activating red light, apoptosis was induced; Pc 4-PDT was less effective in causing apoptosis in a companion cell line overexpressing the antiapoptotic protein Bcl-2. For both cell lines, PK11195 inhibited PDT-induced apoptosis; however, the inhibition was transient and did not extend to overall cell death, as determined by clonogenic assay. The results demonstrate (1) the presence of low-affinity binding sites for Pc 4 on PBR; (2) the presence of multiple binding sites for Pc 4 in RKM and CHO cells other than those that influence PK11195 binding; and (3) the ability of high supersaturating levels of PK11195 to transiently inhibit apoptosis initiated by Pc 4-PDT, with less influence on overall cell killing. We conclude that the binding of Pc 4 to PBR is less relevant to the photocytotoxicity of Pc 4-PDT than are other mitochondrial events, such as photodamage to Bcl-2 and that the observed inhibition of Pc 4-PDT-induced apoptosis by PK11195 likely occurs through a mechanism independent of PBR.

Expression of the peripheral-type benzodiazepine receptor and apoptosis induction in hepatic stellate cells

BACKGROUND AND AIMS

Hepatic stellate cell (HSC) transformation and proliferation play an important role in liver fibrogenesis, and HSC apoptosis may be involved in the termination of this response.

METHODS

Expression of the peripheral benzodiazepine receptor (PBR) and PBR-ligand-induced apoptosis were studied in cultured rat liver HSC.

RESULTS

Transformation of HSC led to a transient expression of PBR at the messenger RNA and protein level, which was maximal after about 3 and 7 days of culture, respectively, and declined thereafter. Immunoreactive PBR showed a punctate staining and colocalized with mitochondrial manganese-dependent superoxide dismutase and adenine nucleotide translocator 1. The selective PBR ligands 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195) and 4' chlorodiazepam (Ro5-4864), but not the centrally acting benzodiazepine ligand clonazepam, induced dose-dependent apoptosis in HSC. The apoptotic potency of PK11195 paralleled the level of PBR expression. PK11195 induced dephosphorylation of protein kinase B/Akt and Bad and a downregulation of Bcl-2. Collapse of the mitochondrial membrane potential preceeded PBR-ligand-induced apoptosis. No apoptosis was induced by PK11195 in parenchymal cells, despite the presence of PBR, and PK11195 had no effect in these cells on Bad phosphorylation and Bcl-2 expression.

CONCLUSIONS

Transformation of HSC leads to a transient expression of PBR and renders the cells sensitive to PBR-ligand-induced apoptosis, involving protein kinase B/Akt and Bad-dependent mechanisms.

The peripheral benzodiazepine receptors: a review

Peripheral benzodiazepine receptors (PBRs) have been identified in various peripheral tissues as well as in glial cells in the brain. This review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. It deals with the pharmacological, structural and molecular characterization of the PBR, the proteins associated with the receptor (VDAC, ANC, PRAX-1) and their roles in cell growth and differentiation, cancer, steroid biosynthesis, and other physiological roles.

Relation of cell proliferation to expression of peripheral benzodiazepine receptors in human breast cancer cell lines

Peripheral benzodiazepine receptor (PBR) agonist [(3)H]Ro5-4864 has been shown to bind with high affinity to the human breast cancer cell line BT-20. Therefore, we investigated different human breast cancer cell lines with regard to binding to [(3)H]Ro5-4864 and staining with the PBR-specific monoclonal antibody 8D7. Results were correlated with cell proliferation characteristics. In flow cytometric analysis, the estrogen receptor (ER)-negative breast cancer cell lines BT-20, MDA-MB-435-S, and SK-BR-3 showed significantly higher PBR expression (relative fluorescence intensity) than the ER-positive cells T47-D, MCF-7 and BT-474 (P<0.05). Accordingly, BT-20 and MDA-MB-435-S had the highest capacity for binding [(3)H]-Ro5-4864, while the ER-positive cells exhibited only low binding of the benzodiazepine. PBR expression correlated inversely with cell doubling time (r = 0.78) and positively with Ki-67 expression (r = 0.77). The amount of mitochondria was significantly higher in cells with high PBR expression. As PBR could be demonstrated only after permeabilization of cells, PBR is suggested to be localized within the cytoplasm. Moreover, colocalization of PBR and mitochondria was shown by confocal microscopy analysis. The highest amounts of both PBR and mitochondria were found in cell lines with high mitotic activity. Therefore, it is concluded that the level of PBR is dependent on the number of mitochondria. PBR and its putative endogenous ligand diazepam-binding inhibitor are possibly involved in the regulation of cell proliferation of human breast cancer cell lines.

Pharmacological characterization of an 18-kDa protein associated with the peripheral-type benzodiazepine receptor in salivary glands

Pharmacological characterization of peripheral type benzodiazepine receptors in rat, rabbit, mouse and human salivary glands was determined by receptor binding and photoaffinity labeling analysis using [3H]PK14105 (1-(2-fluoro-5-nitrophenyl)-3-isoquinolinecarboxylic acid). [3H]PK14105 bound to the membranes of salivary glands in rats, rabbits, mice and humans with high affinity at the nanomolar level. The rank order of receptor density in submandibular glands among several species was as follows: human > or = rat > or = mouse > rabbit. Competitive potency of receptor ligands against [3H]PK14105 was as follows: PK1195 > or = Ro5-4864 > diazepam > clonazepam > Ro15-1788. The rank order of potency against calcium channel ligands and co-transport inhibitors was as follows: nitrendipine > BAY K 8644 > bumetanide > furosemide. Pretreatment with nitrendipine or BAY K 8644 decreased the affinity of [3H]PK14105 binding to rat parotid gland membranes, without changing the density. The photoaffinity labeling with [3H]PK14105 indicated the presence of the 18-kDa protein in all salivary glands of our experiment. The inhibition of photolabeling by some receptor ligands was the same results as the receptor binding assay. In conclusion, the peripheral type benzodiazepine receptors include the 18-kDa protein photolabeled with [3H]PK14105 in salivary glands of rat, mouse, rabbit and human.

Increased expression of peripheral benzodiazepine receptors and diazepam binding inhibitor in human tumors sited in the liver

The peripheral benzodiazepine receptor system triggers intracellular metabolic events and has been associated with cell proliferation. Its endogenous ligand, the diazepam binding inhibitor, contributes to steroidogenesis by promoting cholesterol delivery to the inner mitochondrial membrane. The present study was undertaken to verify whether this system is altered in tumors sited in the liver. Peripheral benzodiazepine receptors and diazepam binding inhibitor were studied using immunocytochemistry and in situ hybridization in 9 human tumors sited in the liver, in liver hyperplasia, cirrhotic nodular regeneration, intestinal adenocarcinoma and in surrounding non-tumoral tissue. Immunocytochemical staining and in situ hybridization demonstrated that peripheral benzodiazepine receptors and diazepam binding inhibitor were more prominently expressed in neoplastic cells than in non-tumoral tissue. They were present in the same cells, suggesting that diazepam binding inhibitor may act in an intracrine manner in these cells. Higher peripheral benzodiazepine receptors and diazepam binding inhibitor expression in tumor cells suggest an implication of this system in the metabolism of neoplastic cells. Furthermore the evaluation of peripheral benzodiazepine receptor and diazepam binding inhibitor expression might be useful in evaluating malignancy and in diagnostic approaches of tumors in liver tissue.

Effect of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (PK11195), a specific ligand of the peripheral benzodiazepine receptor, on the lipid fluidity of mitochondria in human glioma cells

When human glioma cells were incubated for 24 hr in serum-free medium with nanomolar concentrations of 1-(2-chlorophenyl)-N-methyl-N(1-methylpropyl)-3-isoquinoline carboxamide (PK11195), a specific ligand of the peripheral benzodiazepine receptor (PBR), a significant increase in the membrane fluidity of mitochondria isolated from these cells was registered. These effects were not observed with a shorter incubation time (2 hr) of the cells with PK11195 nor in the presence of serum. Other significant associated changes were observed: a significant increase of 16+/-4% of [3H]thymidine incorporation into DNA was detected in cells in the presence of PK11195 in serum-free medium, and an increase of 33+/-5% as compared to controls in nonyl acridine orange uptake, as indicator of mitochondrial mass, was also registered in cells treated with 10 nM PK11195. [3H]PK11195 binding was decreased in cells incubated with PK11195; a 45% decrease compared to controls was obtained. In view of the effect of PBR ligands on DNA synthesis, changes in mitochondrial lipid metabolism through interaction with PBRs might lead to biogenesis of mitochondria to support the increased metabolic requirements for cell division, which is even higher in malignant cells.

Ontogeny of diazepam binding inhibitor/acyl-CoA binding protein mRNA and peripheral benzodiazepine receptor mRNA expression in the rat

The Diazepam Binding Inhibitor/Acyl-CoA Binding Protein (DBI/ACBP) has been implicated in different functions, as acyl-CoA transporter and as an endogenous ligand at the GABA(A) receptor and the peripheral benzodiazepine receptor (PBR). The latter is thought to be involved in control of steroidogenesis. We studied the ontogeny of DBI/ACBP and PBR mRNA expression in embryos and offspring of time-pregnant Long Evans rats by in-situ hybridization with 33P-endlabelled oligonucleotides. Both mRNAs were present in embryo and placenta at gestational day (G)11, the earliest stage studied. DBI/ACBP mRNA was strongly expressed from embryonic through mid-foetal stages in central nervous system (maximum in neuroepithelium), cranial and sympathetic ganglia, anterior pituitary, adrenal cortex, thyroid, thymus, liver and (late foetal) brown adipose tissue, moderately in testis, heart, lung and kidney. In brain, a late foetal decrease of DBI/ACBP mRNA was followed by an increase at postnatal day 6. Peripheral benzodiazepine receptor mRNA expression started very low and increased to moderate levels in adrenal cortex and medulla, testis, thyroid, brown adipose tissue, liver, heart, lung, salivary gland at mid- to late-foetal stages. Data suggest a significant role of DBI/ACBP at early developmental stages. Both proteins may be involved in the control of foetal steroidogenesis. However, differences in developmental patterns indicate that additional functions may be equally important during ontogeny, such as the involvement in lipid metabolism in the case of DBI/ACBP.

Characterization of central- and peripheral-type benzodiazepine receptors in rat salivary glands

Benzodiazepines have been shown to inhibit salivary secretion from the rat salivary gland. This action is mediated by specific benzodiazepine binding sites in the glands. The presence and characteristics of central- and peripheral-type benzodiazepine receptors in rat parotid and submandibular glands were examined employing [3H]Ro15-1788 and [3H]PK11195 as radioligands. [3H]Ro15-1788 and [3H]PK11195 bound with high affinity for both salivary glands ([3H]Ro15-1788: 24.5 and 37.4 mM, [3H]PK11195: 1.37 and 1.88 nM, for parotid and submandibular glands, respectively). [3H]Ro15-1788 binding sites occupied only 0.22 to 0.43% of the total binding for benzodiazepine receptors in the glands. The rank order of the competing potency of [3H]Ro15-1788 binding (Ro15-1788 = clonazepam > diazepam > flunitrazepam > PK11195 > Ro5-4864) and [3H]PK11195 binding (Ro5-4864 = PK11195 > diazepam = flunitrazepam > clonazepam) demonstrated that [3H]Ro15-1788 and [3H]PK11195 binding sites were characteristic of the central and peripheral type, respectively. These studies show that both central- and peripheral-type benzodiazepine receptors exist in rat parotid and submandibular glands.

Peripheral-type benzodiazepine receptors

1. The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the gamma-aminobutyric acid A receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues and is therefore called the peripheral type benzodiazepine receptor (PBR). 2. The first section of this review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. 3. The next section deals with the pharmacological, structural and molecular characterization of the PBR that has taken place in the past few years. 4. The final section describes the possible physiological role(s) of the PBR and identifies future work that would help deepen our understanding of the PBR and its function.

Multiple forms and locations for the peripheral-type benzodiazepine receptor

The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the neuronal gamma-aminobutyric acidA (GABAA) receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues, in blood cells, and in glial cells in the brain, but its functions remain unclear. Although these peripheral-type benzodiazepine binding sites (PBBS) have been localized to the mitochondrial outer membrane in many tissues, a growing body of evidence suggests that they may also exist on the plasma membrane. Plasma membrane PBBS have been described in heart, liver, adrenal, and testis and on hemopoietic cells. In rat liver, the two subcellular forms of PBBS are found separately in two different subpopulations of cells. The discovery of a plasma membrane fraction of PBBS clearly has implications for some of its putative functions, including steroidogenesis, mitochondrial respiration, heme metabolism, calcium channel modulation, cell growth, and immunomodulation. This commentary reviews the evidence for two locations for the PBBS and discusses the relevance of mitochondrial and plasma membrane forms with regard to structure, molecular biology, and proposed roles.