Peripheral-type benzodiazepine receptors in human cerebral cortex, kidney, and colon

Translocator Protein 18 kDa (TSPO): A Promising Molecular Target for Image-Guided Surgery of Solid Cancers

The translocator protein 18-kDa (TSPO) is a mitochondrial membrane protein that is previously identified as the peripheral benzodiazepine receptor (PBR). Furthermore, it plays a significant role in a diverse range of biochemical processes, including steroidogenesis, mitochondrial cholesterol transport, cell survival and death, cell proliferation, and carcinogenesis. Several investigations also reported its roles in various types of cancers, including colorectal, brain, breast, prostate, and lung cancers, as well as melanoma. According to a previous study, the expression of TSPO was upregulated in cancer cells, which corresponds to an aggressive phenotype and/or poor prognosis. Consequently, the potential for crafting diagnostic and prognostic tools with a focus on TSPO holds great potential. In this context, several radioligands designed to target this protein have been identified, and some of the candidates have advanced to clinical trials. In recent years, the use of hybrid probes with radioactive and fluorescence molecules for image-guided surgery has exhibited promising results in animal and human studies. This indicates that the approach can serve as a valuable surgical navigator during cancer surgery. The current hybrid probes are built from various molecular platforms, including small molecules, nanoparticles, and antibodies. Although several TSPO-targeted imaging probes have been developed, their development for image-guided surgery of cancers is still limited. Therefore, this review aims to highlight recent findings on the involvement of TSPO in carcinogenesis, as well as provide a new perspective on the potential application of TSPO-targeted hybrid probes for image-guided surgery.

Concentration, distribution, and influence of aging on the 18 kDa translocator protein in human brain: Implications for brain imaging studies

Positron emission tomography (PET) imaging of the translocator protein (TSPO) is widely used as a biomarker of microglial activation. However, TSPO protein concentration in human brain has not been optimally quantified nor has its regional distribution been compared to TSPO binding. We determined TSPO protein concentration, change with age, and regional distribution by quantitative immunoblotting in autopsied human brain. Brain TSPO protein concentration (>0.1 ng/µg protein) was higher than those reported by in vitro binding assays by at least 2 to 70 fold. TSPO protein distributed widely in both gray and white matter regions, with distribution in major gray matter areas ranked generally similar to that of PET binding in second-generation radiotracer studies. TSPO protein concentration in frontal cortex was high at birth, declined precipitously during the first three months, and increased modestly during adulthood/senescence (10%/decade; vs. 30% for comparison astrocytic marker GFAP). As expected, TSPO protein levels were significantly increased (+114%) in degenerating putamen in multiple system atrophy, providing further circumstantial support for TSPO as a gliosis marker. Overall, findings show some similarities between TSPO protein and PET binding characteristics in the human brain but also suggest that part of the TSPO protein pool might be less available for radioligand binding.

Translocator Protein (TSPO) as a Potential Biomarker in Human Cancers

TSPO is a receptor involved in the regulation of cellular proliferation, apoptosis and mitochondrial functions. Previous studies showed that the expression of TSPO protein correlated positively with tumour malignancy and negatively with patient survival. The aim of this study was to determine the transcription of Tspo mRNA in various types of normal and cancer tissues. In situ hybridization was performed to localise the Tspo mRNA in various human normal and cancer tissues. The relative level of Tspo mRNA was quantified using fluorescent intensity and visual estimation of colorimetric staining. RT-PCR was used to confirm these mRNA levels in normal lung, lung cancer, liver cancer, and cervical cancer cell lines. There was a significant increase in the level of transcription in liver, prostate, kidney, and brain cancers while a significant decrease was observed in cancers of the colon and lung. Quantitative RT-PCR confirmed that the mRNA levels of Tspo are higher in a normal lung cell line than in a lung cancer cell line. An increase in the expression levels of Tspo mRNA is not necessarily a good diagnostic biomarker in most cancers with changes not being large enough to be significantly different when detected by in situ hybridisation.

The nitric oxide donor sodium nitroprusside requires the 18 kDa Translocator Protein to induce cell death

Various studies have shown that several lethal agents induce cell death via the mitochondrial 18 kDa Translocator Protein (TSPO). In this study we tested the possibility that nitric oxide (NO) is the signaling component inducing the TSPO to initiate cell death process. Cell viability assays included Trypan blue uptake, propidium iodide uptake, lactate dehydrogenase release, and DNA fragmentation. These assays showed that application of the specific TSPO ligand PK 11195 reduced these parameters for the lethal effects of the NO donor sodium nitroprusside (SNP) by 41, 27, 40, and 42 %, respectively. TSPO silencing by siRNA also reduced the measured lethal effects of SNP by 50 % for all of these four assays. With 2,3-bis[2-methoxy-4-nitro-5-sulphophenyl]-2H-tetrazolium-5-carboxyanilide (XTT) changes in metabolic activity were detected. PK 11195 and TSPO knockdown fully prevented the reductions in XTT signal otherwise induced by SNP. Collapse of the mitochondrial membrane potential was studied with the aid of JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolylcarbocyanine chloride). PK 11195 and TSPO knockdown reduced, respectively by 36 and 100 %, the incidence of collapse of the mitochondrial membrane potential otherwise induced by SNP. 10-N-Nonyl-Acridine Orange (NAO) was used to detect mitochondrial reactive oxygen species generation due to SNP. PK 11195 and TSPO knockdown reduced this effect of SNP by 65 and 100 %, respectively. SNP did not affect TSPO protein expression and binding characteristics, and also did not cause TSPO S-nitrosylation. However, β-actin and various other proteins (not further defined) were S-nitrosylated. In conclusion, TSPO is required for the lethal and metabolic effects of the NO donor SNP, but TSPO itself is not S-nitrosylated.

Molecular imaging of microglia/macrophages in the brain

Neuroinflammation perpetuates neuronal damage in many neurological disorders. Activation of resident microglia and infiltration of monocytes/macrophages contributes to neuronal injury and synaptic damage. Noninvasive imaging of these cells in vivo provides a means to monitor progression of disease as well as assess efficacies of potential therapeutics. This review provides an overview of positron emission tomography (PET) and magnetic resonance (MR) imaging of microglia/macrophages in the brain. We describe the rationale behind PET imaging of microglia/macrophages with ligands that bind to translocator protein-18 kDa (TSPO). We discuss the prototype TSPO radioligand [(11)C]PK11195, its limitations, and the development of newer TSPO ligands as PET imaging agents. PET imaging agents for targets other than TSPO are emerging, and we outline the potential of these agents for imaging brain microglia/macrophage activity in vivo. Finally, we briefly summarize advances in MR imaging of microglia/macrophages using iron oxide nanoparticles and ultra-small super paramagnetic particles that are phagocytosed. Despite many technical advances, more sensitive agents are required to be useful indicators of neuroinflammation in brain.

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

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

Peripheral benzodiazepine receptor: characterization in human T-lymphoma Jurkat cells

Peripheral benzodiazepine receptor (PBR) has been considered a promising drug target for cancer therapy, and several ligands have been developed for this purpose. Human T-lymphoma Jurkat cells have been considered as lacking PBR and are often used as negative control to prove the specificity of PBR ligands effects. It is surprising that we evidenced PBR protein expression in this cell line by means of Western blotting and immunocytochemistry assays using specific anti-PBR antibodies. PBR intracellular localization was evidenced in mitochondria and nuclei, as demonstrated by confocal and electron microscopy. The binding of the [(3)H]4'-chloro derivative of diazepam [(3)H]7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864) and the isoquinoline carboxamide derivative [(3)H]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3 isoquinolinecarboxamide (PK11195) evidenced a single class of binding sites with an unusual affinity constant (K(d)) of 1.77 +/- 0.30 and 2.20 +/- 0.20 microM, respectively. The pharmacological profile of the classic ligands showed that PK11195 was the most potent inhibitor in the radioligand binding assays followed by Ro5-4864 and diazepam, whereas clonazepam, a specific ligand for the central-type receptor, showed a K(i) >1.0 x 10(-4) M. By a combined strategy of reverse transcriptase-polymerase chain reaction and Southern blot experiments, we succeeded in isolating and cloning the full-length Jurkat PBR cDNA, called JuPBR. The JuPBR gene showed two single-nucleotide polymorphisms resulting in the two substitutions, Ala147 --> threonine and His162 --> arginine, of PBR amino acidic sequence. In conclusion, for the first time, we demonstrated PBR expression in Jurkat cells: the protein bound classic PBR ligands with micromolar affinity constants and presented a modified amino acidic sequence consequent to the detection of two gene polymorphisms.

Microglial imaging with positron emission tomography and atrophy measurements with magnetic resonance imaging in multiple sclerosis: a correlative study

OBJECTIVE

The objectives of the present study were to assess brain atrophy in multiple sclerosis (MS) patients during different disease stages and to investigate by PET and [11C]PK11195, a marker of microglial activation, the relationship between inflammation, atrophy and clinically relevant measures.

METHODS

Eight healthy subjects and 22 MS patients were included. Semiquantitative [11C]PK11195 uptake values, with normalization on cortical grey matter, were measured for magnetic resonance imaging T2- and T1-lesions and normal appearing white matter (NAWM). As atrophy index we used the ratio of the amount of white and grey matter divided by the ventricular size, using an optimized a priori based segmentation algorithm (SPM99).

RESULTS

Atrophy was significantly greater in MS patients compared to age-matched controls. A significant correlation was found between brain atrophy and both disease duration and disability, as measured with the Expanded Disability Status Scale. For NAWM, [11C]PK11195 uptake increased with the amount of atrophy, while T2-lesional [11C]PK11195 uptake values decreased according to increasing brain atrophy.

CONCLUSIONS

The present study suggests that brain atrophy, correlating with disease duration and disability, is directly related to NAWM and T2-lesional inflammation as measured by microglial activation.

Peripheral type benzodiazepine receptor in human parathyroid glands: up-regulation in adenoma

In this study we report the presence of peripheral benzodiazepine receptors (PBRs) in human parathyroid glands and describe the effect of their benzodiazepine type ligands on parathyroid cell function. PBR binding features in normal parathyroid tissue were characterized and compared to parathyroid adenoma, using a specific and selective ligand for PBR, [3H] 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline-carboxamide ([3H]PK11195). Affinity and density of [3H]PK11195 binding sites in homogenate membrane preparations from adenomatous and normal tissues were determined. Parathyroid adenoma showed a statistically significant 2.2 fold increase of [3H]PK11195 binding sites, while the affinity remained unchanged. Our results represent the first evidence of PBRs in parathyroid glands and suggest for them a role in influencing PTH release. A clear trend of PBR up-regulation in parathyroid adenoma was also found.

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.

Specific binding of benzodiazepines to human breast cancer cell lines

Binding of [3H]Ro5-4864, a peripheral benzodiazepine receptor (PBR) agonist, to BT-20 human, estrogen- (ER) and progesterone- (PR) receptor negative breast cancer cells was characterized. It was found to be specific, dose-dependent and saturable with a single population of binding sites. Dissociation constant (K(D)) was 8.5 nM, maximal binding capacity (Bmax) 339 fM/10(6) cells. Ro5-4864 (IC50 17.3 nM) and PK 11195 (IC50 12.3 nM) were able to compete with [3H]Ro5-4864 for binding, indicating specificity of interaction with PBR. Diazepam was able to displace [3H]Ro5-4864 from binding only at high concentrations (>1 microM), while ODN did not compete for PBR binding. Thymidine-uptake assay showed a biphasic response of cell proliferation. While low concentrations (100 nM) of Ro5-4864, PK 11195 and diazepam increased cell growth by 10 to 20%, higher concentrations (10-100 microM) significantly inhibited cell proliferation. PK 11195, a potent PBR ligand, was able to attenuate growth of BT-20 cells stimulated by 100 nM Ro5-4864 and to reverse growth reduction caused by 1 and 10 microM Ro5-4864, but not by 50 microM and 100 microM. This indicates that the antimitotic activity of higher concentrations of Ro5-4864 is independent of PBR binding. It is suggested, that PBR are involved in growth regulation of certain human breast cancer cell lines, possibly by supplying proliferating cells with energy, as their endogenous ligand is a polypeptide transporting Acyl-CoA.

Peripheral-type benzodiazepine receptors in the regulation of proliferation of MCF-7 human breast carcinoma cell line

Peripheral-type benzodiazepine receptors (PBR) have been implicated in cell proliferation. The aim of the present study was to test the effect of the PBR ligands PK 11195 and Ro 5-4864 and the central-type benzodiazepine receptor ligand clonazepam on breast carcinoma cell proliferation, using [3H] thymidine incorporation. We then carried out a study to identify where the PBR-specific ligands Ro 5-4864 and PK 11195 act in the cell cycle, using flow cytometric analysis. We found PBR expression in the malignant breast cancer tumors, representing various levels of estrogen and/or progesterone receptors, as well as in the MCF-7 breast carcinoma cell line. PK 11195 and Ro 5-4864 inhibited cell proliferation at concentrations of 10(-5) to 10(-4) M, while clonazepam (the central-type benzodiazepine receptor-specific ligand) had no effect. In this same concentration range, PK 11195 and Ro 5-4864, in contrast to clonazepam, induced an accumulation of MCF-7 cells in both the G0-G1 and G2-M phases of the cell cycle. The present study demonstrates that PBR ligands play a role in regulating cell proliferation in the human breast carcinoma cell line MCF-7.

Inhibition of mouse mast cell proliferation and proinflammatory mediator release by benzodiazepines

Mast cell (MC) activation may occur in vitro and in vivo following stimulation with various immunologic or nonimmunologic agents. Such activation leads to the release of several biological mediators, including vasoactive amines, nitric oxide and cytokines, which account for the adverse effects observed during allergic reactions. While high affinity binding sites for benzodiazepines (BZDs) have been reported on MC, the effects of the ligation of these receptors on the proliferation of, and the mediator release from, these cells are poorly documented. In the present work, we have examined the effects of midazolam and of diazepam on the proliferation of mucosal (MMC)-like and of serosal (CTMC)-like mouse MC. In addition, we have studied the effects of these BZDs on beta-hexosaminidase, TNF-alpha and nitrite release induced from mouse mast cells through IgE receptor activation. We demonstrated that each of the two BZDs studied inhibited the proliferation of MMC- and CTMC-like elements in a dose-dependent fashion (10 to 100 microM). Furthermore, the BZDs inhibited the IgE-mediated release of beta-hexosaminidase, TNF-alpha and nitrites from MMC- or CTMC-like cells. Altogether, these data provide new insights into the pharmacological regulation of MC activation and may lead to the discovery of new and potent antiallergic compounds.

Characterization of binding sites for the omega3 receptor ligands [3H]PK11195 and [3H]RO5-4864 in human brain

The kinetics and pharmacology of the isoquinoline and benzodiazepine binding sites of the omega3 or peripheral-type benzodiazepine receptors were studied using the specific ligands [3H] 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin -2-one ([3H]PK11195) and [3H]1-(2-Chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarb oxamide ([3H]RO5-4864), respectively. Binding of both ligands was saturable, reversible, displayed nanomolar affinity, and best fit to a single site model. Occipital cortex and cerebellum displayed highest and lowest densities of binding sites respectively; for both ligands. Bmax values of [3H]PK11195 were several-fold higher than that of [3H]RO5-4864 in all regions studied consistent with their binding to distinct subunits of the human peripheral-type benzodiazepine receptor heteromeric complex. However, the isoquinoline and benzodiazepine ligands were found to be mutually competitive at nanomolar concentrations suggesting allosteric interactions between these two sites. Competition binding experiments showed that the binding of both ligands was displaced by diazepam with Ki values in the nM range, and by clonazepam in the microM range. The novel peripheral-type benzodiazepine receptor ligand 2-(4-fluorophenyl)-N,N-di-n-hexyl-1H-indole-3-acetamide (FGIN1-27) displaced only [3H]PK11195 binding with high potency. Heterogeneity of the two sites is observed, manifested by their differential susceptibility towards detergents and alcohols. Histidine residue modification by diethylpyrocarbonate treatment abolished only [3H]PK11195 binding but had no effect on [3H]RO5-4864 binding. These studies demonstrate that the isoquinoline and benzodiazepine sites on the peripheral-type benzodiazepine receptor in human brain manifest many pharmacological characteristics that are distinct from each other and from rodent brain peripheral-type benzodiazepine receptors.

Characterization of peripheral benzodiazepine receptors in purified large mammal pancreatic islets

In this work, we evaluated the biochemical properties of peripheral benzodiazepine receptors (PBRs) in the porcine endocrine pancreas and their role in insulin release. Binding of [3H]1-(2-chlorophenyl-N-methyl-1-methyl-propyl)-3-isoquinolinecarboxa mide ([3H]PK-11195), a specific ligand of PBRs, to islet membranes was saturable and Scatchard's analysis of saturation curve demonstrated the presence of a single population of binding sites, with a dissociation constant (Kd) value of 4.75 +/- 0.70 nM and a maximum amount of specifically bound ligand (Bmax) of 4505 +/- 502 fmol/mg of proteins. The pharmacological profile of PBRs was determined as the ability of PK-11195 and several benzodiazepine compounds to displace [3H]PK-11195 from these binding sites. The rank order of potency yielded the following affinity results: PK-11195 > 7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4-benzodiazepine-2 -on (Ro 5-4864) > diazepam > or = flunitrazepam >> flumazenil. Secretion studies demonstrated that PK-11195 (1 and 10 microM) and Ro 5-4864 (10 and 50 microM) significantly potentiated insulin secretion from freshly isolated porcine islets at 3.3 mM glucose. This potentiating effect was not observed at 16.7 mM glucose concentration nor by the addition of clonazepam. These results show the presence of PBRs in purified porcine pancreatic islets and suggest an implication of PBRs in the mechanisms of insulin release.

Hormonal regulation of peripheral-type benzodiazepine receptors

Central benzodiazepine (BZ) receptors are located only in the central nervous system and mediate the clinical effects obtained by various BZs. In addition, there is another receptor that binds BZs with different drug specificities, which is located mainly on the outer mitochondrial membrane of various peripheral tissues. Peripheral BZ receptors (PBR) are composed of three subunits: an isoquinoline binding site, a voltage-dependent anion channel, and an adenine nucleotide carrier, with molecular weights of 18, 32, and 30 kDa, respectively. Complementary DNA of the isoquinoline binding subunit has been cloned in rat, calf, and human. The major role of PBR is in the regulation of steroid biosynthesis. Various PBR ligands stimulate the conversion of cholesterol into pregnenolone and the production of steroid hormones. The naturally occurring diazepam-binding inhibitor stimulates in vivo steroidogenesis via binding to PBR. In the female, PBR density is increased in rat and human ovary proportional with greater cell maturation and differentiation. In the male, testosterone modulates PBR density in the genital tract. These results show the strong relationship between PBR and the endocrine system.

Effect of peripheral benzodiazepine receptor ligands on lipopolysaccharide-induced tumor necrosis factor activity in thioglycolate-treated mice

To investigate the effect of peripheral and central benzodiazepine receptor ligands on lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF) activity in mouse macrophages, three types of ligands, 4'-chlorodiazepam (pure peripheral), midazolam (mixed), and clonazepam (pure central), were compared. Midazolam and 4'-chlorodiazepam significantly suppressed LPS (1-microgram/ml)-induced TNF activity in thioglycolate-elicited mouse macrophages. In every concentration examined (0.001 to 100 microM), 4'-chlorodiazepam was the most effective agent, clonazepam was the least effective agent, and midazolam had an effect intermediate between those of the other two ligands. The peripheral benzodiazepine receptor ligands had a dose-dependent suppressive effect, and the 50% inhibitory concentrations were 0.01 microM for 4'-chlorodiazepam and 5 microM for midazolam. Concomitant use of PK 11195 (10 microM), an antagonist of the peripheral benzodiazepine receptor, reversed this suppressive effect with 4'-chlorodiazepam (10 microM) or midazolam (10 microM). PK 11195 showed this antagonistic effect in a dose-dependent manner. Intravenous 4'-chlorodiazepam (5 mg/kg of body weight) significantly suppressed LPS (100-micrograms)-induced TNF activity of sera (2 h postchallenge with LPS) from thioglycolate-treated mice. The present findings suggest that the peripheral benzodiazepine receptor plays an important role in modulating LPS-induced TNF activity in mouse macrophages.

Ro 5-4864 has a negative inotropic effect on human atrial muscle strips that is not antagonized by PK 11195

The effects of Ro 5-4864, a peripheral benzodiazepine receptor agonist (9 x 10(-6) M and 9 x 10(-5) M) and of PK 11195, a peripheral benzodiazepine receptor antagonist (3 x 10(-6) M and 3 x 10(-5) M), alone or together, on contraction parameters of human atrial muscle strips were studied. Atrial muscle strips were obtained from patients undergoing cardiac surgery. The strips were suspended in Krebs-Henseleit solution at 36.8 +/- 0.2 degrees C, connected to an isometric force transducer and then stimulated at 15 V above threshold and paced at 1.5 Hz. Ro 5-4864 at its higher concentration, alone or mixed with PK 11195 at both concentrations, depressed the contraction amplitude to 80% of the control value (P < 0.05). In conclusion, Ro 5-4864 had a small but significant depressant effect on the contraction amplitude of human atrial strips. Surprisingly, this effect was not reversed by the peripheral benzodiazepine receptor antagonist PK 11195.

Biochemical, physiological, and pathological aspects of the peripheral benzodiazepine receptor

The PBR is a mitochondrial protein composed of at least two subunits, an approximately 30-kDa subunit that contains the site for BZs and an approximately 18-kDa subunit that binds isoquinoline carboxamide derivatives. Porphyrins and diazepam binding inhibitor are putative endogenous ligands for these receptors, which are under neural and hormonal control. Alterations in the density of PBR seem to be a sensitive indicator of stress: up-regulation after acute stress and down-regulation induced by repeated stress. PBR-specific ligands are involved in the control of cell proliferation and differentiation, and their binding is increased in some cancer tumors. Numerous studies in various endocrine organs have revealed that PBR are located in specific regions or tissues in the organs. Furthermore, PBR densities in various organs subject to hormonal control are regulated by organotropic hormones. At least in some cases, BZ ligands do not exert a specific effect in an organ, but rather modulate the well-documented effects of that particular hormone. To the best of our knowledge, BZ ligand action in peripheral tissues is dependent on recognition of PBR, which may suggest a receptor-mediated action.