Localization of peripheral benzodiazepine binding sites and diazepam-binding inhibitor (DBI) mRNA in mammary glands and dimethylbenz(a)antracene (DMBA)-induced mammary tumors in the rat

Current status and future perspectives: TSPO in steroid neuroendocrinology

The mitochondrial translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), has received significant attention both as a diagnostic biomarker and as a therapeutic target for different neuronal disease pathologies. Recently, its functional basis believed to be mediating mitochondrial cholesterol import for steroid hormone production has been refuted by studies examining both in vivo and in vitro genetic Tspo-deficient models. As a result, there now exists a fundamental gap in the understanding of TSPO function in the nervous system, and its putative pharmacology in neurosteroid production. In this review, we discuss several recent findings in steroidogenic cells that are in direct contradiction to previous studies, and necessitate a re-examination of the purported role for TSPO in de novo neurosteroid biosynthesis. We critically examine the pharmacological effects of different TSPO-binding drugs with particular focus on studies that measure neurosteroid levels. We highlight the basis of key misconceptions regarding TSPO that continue to pervade the literature, and the need for interpretation with caution to avoid negative impacts. We also summarize the emerging perspectives that point to new directions that need to be investigated for understanding the molecular function of TSPO, only after which the true potential of this therapeutic target in medicine may be realized.

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.

Morphological alterations induced by loud noise in the myocardium: the role of benzodiazepine receptors

Noise represents an environmental stress factor affecting several organs and apparati, including the cardiovascular system. In experimental animals undergoing noise exposure, subcellular myocardial changes have been reported, especially at mitochondrial level; in particular, after 6 hours of exposure only the atrium exhibited significant mitochondrial alterations, whereas after 12 hours as well as subchronic exposure both atrium and ventricle were damaged. The first part of the present article overviews the experimental evidence on effects of noise on the myocardium. In the second part, the review analyzes the role of benzodiazepine receptors and the potential efficacy of benzodiazepine ligands in preventing the mitochondrial damage induced by noise exposure. Drugs acting at both central and peripheral benzodiazepine receptors significantly prevent this damage. Differences in the amount and the duration of the protective effect might depend on variability in the potency and pharmacokinetics of the specific drug. The effects of the combined treatment with selective and non-selective peripheral benzodiazepine ligands on noise stimulation are discussed at biochemical level reviewing studies on the effects of noise exposure on mitochondrial fractions.

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.

Effect of noise exposure on rat cardiac peripheral benzodiazepine receptors

Noise is an environmental physical agent, which is regarded as a stressful stimulus: impairment and modifications in biological functions are reported, after loud noise exposure, at several levels in human and animal organs and apparatuses, as well as in the endocrine, cardiovascular and nervous system. In the present study equilibrium binding parameters of peripheral benzodiazepine receptors (PBRs) labelled by the specific radioligand [3H]PK 11195, were evaluated in cardiac tissue of rats submitted to 6 or 12 h noise exposure and of rats treated "in vivo" with PBR ligands such as PK 11195, Ro54864, diazepam and then noise-exposed. Results revealed a statistically significant decrease in the maximum number of binding sites (Bmax) of [3H]PK 11195 in atrial membranes of 6 or 12 h noise exposed rats, compared with sham-exposed animals, without any change in the dissociation constant (Kd). The "in vivo" PBR ligand pre-treatment counteracted the noise-induced modifications of PBR density. As PBRs are mainly located on mitochondria we also investigated whether noise exposure can affect the [3H]PK 11195 binding parameters in isolated cardiac mitochondrial fractions. Results indicated a significant Bmax value decrease in right atrial mitochondrial fractions of rats 6 or 12 h noise-exposed. Furthermore, as PBR has been suggested to be a supramolecular complex that might coincide with the not-yet-established structure of the mitochondrial permeability transition (MPT)-pore, the status of the MPT-pore in isolated heart mitochondria was investigated in noise- and sham-exposed rats. The loss of absorbance associated with the calcium-induced MPT-pore opening was greater in mitochondria isolated from hearts of 6 h noise- than those of sham-exposed rats. In conclusion, these findings represent a further instance for PBR density decrease in response to a stressful stimulus, like noise; in addition they revealed that "in vivo" administration of PBR ligands significantly prevents this decrease. Finally, our data also suggest the involvement of MPT in the response of an organism to noise stress.

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.

Structure and function of normal and transformed murine acyl-CoA binding proteins

Acyl-CoA binding protein (ACBP) is a ubiquitous cytosolic protein found in high levels in tumorigenic cells. However, the molecular basis for the elevated levels of ACBP in malignant cells, ligand binding characteristics, and function in microsomal phospholipid synthesis have not been resolved. To address whether tumorigenic ACBP differs from the native protein, ACBP was purified from LM cells, a tumorigenic subline of mouse L-929 fibroblasts, and its primary structure was examined by delayed-extraction MALDI-linear TOF mass spectrometry. Proteolytic digestion and peptide sequence analysis confirmed that ACBP from LM cells was identical to native mouse ACBP (based on cDNA-derived amino acid sequence) with no amino acid substitutions, deletions, or posttranslational modifications. A fluorescent binding assay revealed that mouse ACBP bound cis-parinaroyl-CoA with high affinity, Kd 7.6 +/- 2.3 nM, at a single binding site. Furthermore, mouse ACBP enhanced microsomal phosphatidic acid formation from oleoyl-CoA 2.3-fold. Mouse ACBP also inhibited microsomal phospholipid acyl chain remodeling of choline-containing phospholipids, phosphatidylcholine and sphingomyelin, by 50 and 64%, respectively. These effects were specific compared to those of native rat liver or recombinant rat ACBP. Mouse and rat ACBPs differed by three amino acid substitutions at positions 4, 68, and 78. Although these small differences in amino acid sequence did not alter binding affinity for cis-parinaroyl-CoA, rat liver ACBP stimulated utilization of oleoyl-CoA 3.8-fold by microsomal glycerol-3-phosphate acyltransferase, significantly higher than that observed with mouse ACBP, but did not alter microsomal phospholipid acyl chain remodeling from oleoyl-CoA. In addition, these ACBPs protected oleoyl-CoA against hydrolysis. Finally, both mouse and rat ACBP shifted the incorporation of oleoyl-CoA from microsomal phospholipid acyl chain remodeling to phosphatidic acid biosynthesis. These data for the first time show a role for ACBP in stimulating microsomal phosphatidic acid biosynthesis and acyl chain remodeling in vitro. While ACBP from tumorigenic cells did not differ from normal, ACBPs from different murine species displayed subtle differences in their effects on microsomal phospholipid metabolism in vitro.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

Expression of the neuropeptide Y Y1 receptor in human embryonic kidney 293 cells: ligand binding characteristics, in situ hybridization and receptor autoradiography

The human neuropeptide Y Y1 receptor cDNA was transfected into human embryonic kidney 293 (HEK 293) cells and used to determine the selectivity of newly developed Y1 and Y2 radioligands in a model which expresses a single NPY receptor subtype. The Y1 receptor probe, [125I][Leu31,Pro34]PYY, binds with high afinity (KD of 0.4-0.6 nM) to Y1-transfected HEK 293 cells whereas the Y2 radioligand, [125I]PPY3-36 failed to demonstrate any significant labelling. Only non-selective (PYY) or selective Y1 receptor agonists behaved as potent competitors for [125I][Leu31,Pro34]PYY binding in transfected cells. Additionally, the efficacy of the transfection method used was evaluated at both the transcriptional and translational levels. In situ hybridization revealed the heterogeneous distribution of the NPY Y1 receptor mRNA expressed in transfected HEK 293 cells. Similarly, the levels of NPY Y1 binding sites per transfected cell varied as shown using [125I][Leu31,Pro34]PYY receptor autoradiography. Taken together, these results demonstrate that HEK 293 cells transfected with the NPY Y1 receptor cDNA expressed both the related receptor mRNA and protein albeit at different levels depending upon each transfected cell. Additionally, these data further establish the selectivity of the newly developed Y1 and Y2 radioligands.

Nucleotide sequence and genomic structure of duck acyl-CoA binding protein/diazepam-binding inhibitor: co-localization with S-acyl fatty acid synthase thioesterase

A computer-aided homology search of the GenBank nucleotide database using the amino acid sequence of human acyl CoA-binding protein (ACBP)/diazepam-binding inhibitor (DBI)-endozepine as a probe revealed that a genomic fragment containing the gene encoding the mallard duck (Anas platyrhynchos) S-acyl fatty acid synthase thioesterase also contains sequences which encode the duck homolog of ACBP/DBI. The duck ACBP/DBI gene is positioned downstream of the thioesterase gene in a tail-to-tail orientation separated from the 3' end of the thioesterase gene by only several hundred nucleotides. Three exons were identified that have strong homology to the published cDNA sequences of human and bovine ACBP/DBI. These exons define all of the coding region except for the amino-terminal domain, which was subsequently cloned by polymerase chain reaction (PCR) amplification. The encoded amino acid sequence of the duck ACBP/DBI is 62-68% homologous to mammalian ACBP/DBI sequences. While the mammalian ACBP/DBI is expressed mainly in the liver, with smaller amounts in the brain and heart, mRNA transcripts of duck ACBP/DBI were detected only in the brain with no evidence for expression in the liver or heart. The close proximity of the genes for ACBP/DBI and S-acyl fatty acid synthase thioesterase raises the possibility of co-regulation of expression.

The function of acyl-CoA-binding protein (ACBP)/diazepam binding inhibitor (DBI)

Acyl-CoA-binding protein has been isolated independently by five different groups based on its ability to (1) displace diazepam from the GABAA receptor, (2) affect cell growth, (3) induce medium-chain acyl-CoA-ester synthesis, (4) stimulate steroid hormone synthesis, and (5) affect glucose-induced insulin secretion. In this survey evidence is presented to show that ACBP is able to act as an intracellular acyl-CoA transporter and acyl-CoA pool former. The rat ACBP genomic gene consists of 4 exons and is actively expressed in all tissues tested with highest concentration being found in liver. ACBP consists of 86 amino acid residues and contains 4 alpha-helices which are folded into a boomerang type of structure with alpha-helices 1, 2 and 4 in the one arm and alpha-helix 3 and an open loop in the other arm of the boomerang. ACBP is able to stimulate mitochondrial acyl-CoA synthetase by removing acyl-CoA esters from the enzyme. ACBP is also able to desorb acyl-CoA esters from immobilized membranes and transport and deliver these for mitochondrial beta-oxidation. ACBP efficiently protects acetyl-CoA carboxylase and the mitochondrial ADP/ATP translocase against acyl-CoA inhibition. Finally, ACBP is shown to be able to act as an intracellular acyl-CoA pool former by overexpression in yeast. The possible role of ACBP in lipid metabolism is discussed.

Localization of diazepam-binding inhibitor (DBI) mRNA in the rat brain by high resolution in situ hybridization

An endogenous peptide, named diazepam-binding inhibitor (DBI) capable of displacing benzodiazepines from binding sites has been recently fully characterized. In order to clearly identify the cell types responsible for the biosynthesis of DBI in the rat central nervous system, we have performed high resolution in situ hybridization in the area postrema, hypothalamus and cerebellum, using a [35S]-labeled single stranded RNA probe. Hybridization signal was detected in both semithin and ultrathin sections. In all the brain areas examined, specific labeling was exclusively observed in non-neuronal cells including ependymal and subependymal cells bordering the third ventricle. The results obtained clearly establish that DBI is synthesized by non-neuronal cells in the rat brain.