Hormone-stimulated steroidogenesis is coupled to mitochondrial benzodiazepine receptors. Tropic hormone action on steroid biosynthesis is inhibited by flunitrazepam

Metabolism of exogenous cholesterol by rat adrenal mitochondria is stimulated equally by physiological levels of free Ca2+ and by GTP

Adrenal mitochondria metabolize cholesterol at inner membrane (IM) cytochrome P450scc. Exogenous and outer membrane (OM) cholesterol are metabolized more slowly due to a limiting transfer of cholesterol from OM to IM. This process is stimulated by in vivo ACTH treatment and inhibited by cycloheximide (CX)-induced depletion of labile regulatory proteins. In isolated rat adrenal mitochondria, GTP enhances the metabolism of exogenous cholesterol, consistent with enhanced intermembrane cholesterol transfer (Xu et al. (1989) J. Biol Chem. 264, 17674), but metabolism of 20 alpha-hydroxycholesterol, which readily traverses mitochondrial membranes, is not affected. The non-hydrolyzable analog, GTP gamma S, completely inhibits the activation of cholesterol metabolism by GTP, suggesting a requirement for GTP hydrolysis. Low concentrations of Ca2+ (0.4-4 microM) stimulate two independent cholesterol transport processes. For exogenous cholesterol, a Ca(2+)-mediated process can replace GTP since each produces comparable stimulation and the combination produces little additional activity. This Ca2+ stimulation is insensitive to GTP gamma S and also to Ruthenium Red (RR), which prevents Ca2+ entry into the matrix. Ca2+ also enhances availability to P450 scc of endogenous OM cholesterol, which accumulates during in vivo CX-inhibition. This stimulation is, however, distinguished by insensitivity to GTP and complete inhibition by RR. Ca2+, therefore, enhances intermembrane transfer of exogenous cholesterol from OM without entry into the matrix through a process which is independently stimulated by GTP. Ca2+ induces transfer of endogenous OM cholesterol through a completely different mechanism involving RR-inhibited matrix changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the hippocampal GABAA receptor system in kindled rats by autoradiographic and in situ hybridization techniques: contingent tolerance to carbamazepine

Tolerance to the anticonvulsant effects of carbamazepine (CBZ) in the amygdala kindling paradigm is a contingent process, since it only develops in rats treated with CBZ before the kindling stimulation and not in those animals treated after the stimulation. The present study was designed to investigate the GABAA receptor system in CBZ contingent tolerance. Receptor autoradiography utilizing various radioligands that bind to different components of the GABAA receptor system and in situ hybridization with oligonucleotides that recognize different subunits of the GABAA receptor were performed. Kindling increased binding to benzodiazepine, picrotoxin, and GABA recognition sites selectively in the dentate gyrus of the hippocampus. Kindling also increased levels of mRNA for the alpha 4, beta 1, and beta 3 subunits but did not change alpha 1, alpha 2, or gamma 2 subunit levels. Rats tolerant to CBZ showed decreased [3H]muscimol binding, diazepam-insensitive [3H]Ro 15-4513 binding, and decreased alpha 4 subunit mRNA content compared to non-tolerant rats, whereas [3H]flunitrazepam binding, [35S]TBPS binding, and the levels of beta 1, and beta 3 subunit mRNAs remained elevated. The data suggest an indirect interaction of CBZ with the GABAA receptor system, since CBZ reportedly does not bind to this receptor system.

Topography of the Leydig cell mitochondrial peripheral-type benzodiazepine receptor

Native MA-10 mouse Leydig tumor cell mitochondrial preparations were examined by transmission electron (TEM) and atomic force (AFM) microscopic procedures in order to investigate the topography and organization of the peripheral-type benzodiazepine receptor (PBR). Mitochondria were immunolabeled with an anti-PBR antiserum coupled to gold-labeled secondary antibodies. Results obtained indicate that the 18,000 MW PBR protein is organized in clusters of 4-6 molecules. Moreover, on many occasions, the interrelationship among the PBR molecules was found to favor the formation of a single pore. Taking into account recent observations that the 18,000 MW PBR protein is functionally associated with the pore forming 34,000 MW voltage-dependent anion channel (VDAC) these results suggest that (i) the mitochondrial PBR complex could function as a pore, thus allowing the translocation of cholesterol and other molecules to the inner mitochondrial membrane, and (ii) the native receptor is a multimeric complex of an approximate 140,000 MW composed on an average of five 18,000 PBR subunits, one 34,000 VDAC subunit, and associated lipids.

Studies on the phosphorylation of the 18 kDa mitochondrial benzodiazepine receptor protein

Steroid biosynthesis activated by pituitary tropic hormones is known to be acutely regulated by cAMP acting via Protein kinase A. Because the mitochondrial benzodiazepine receptor (MBR) has been suggested to play a role in the activation of steroidogenesis, the present study investigates whether various protein kinases phosphorylate MBR. In rat and bovine adrenal mitochondrial preparations Protein kinase A, but not other purified protein kinases, was found to phosphorylate the 18 kDa MBR protein. In digitonin-permeabilized MA-10 Leydig tumor cells incubated with [gamma-32P]ATP, phosphorylation of MBR was detectable during treatment of the cells with dibutyryl cAMP. In conclusion, these data show that the MBR protein is an in vitro and in situ substrate of Protein kinase A, but the role of this phosphorylation in the regulation of steroidogenesis remains to be established.

Modulation of peripheral benzodiazepine receptors in female rat genital organs by various gonadal steroids

Peripheral benzodiazepine receptors (PBR) in the ovary, oviduct, uterus, and kidney of immature rats were studied under short- and long-term treatment with testosterone (T), progesterone (P4), and diethylstilbestrol (DES). A significant increase in PBR specific binding was observed after 4 days' treatment with T in the ovary (1.6-fold), oviduct (2.0-fold), and uterus (1.4-fold) compared with intact rats. Four days' treatment with P4 increased PBR specific binding in the ovary (1.5-fold), but no changes were detected in the oviduct or uterus. In contrast, PBR specific binding was significantly reduced by 10 days' treatment with T or P4: 40 and 12%, respectively, in the ovary and 35 and 40%, respectively, in the oviduct. Ten days' treatment with T reduced PBR specific binding in the uterus by 25%, but the same interval of treatment with P4 did not alter specific binding in the uterus. Four or 10 days' treatment with DES significantly increased PBR specific binding in the ovary (1.5-fold), oviduct (2.4-fold), and uterus (1.9-fold). Scatchard analysis revealed that the changes in the PBR specific binding were due to a change in PBR density values rather than PBR affinity values. No change in PBR specific binding was found in the kidney following any of these treatments. Taken together, it is suggested that PBR density in the ovary is altered by exogenously administered steroids that usually are biosynthesized in the ovary. Additionally, the altered PBR density in the oviduct and uterus via the various steroids employed may imply that changes occurring in ovarian steroidogenesis should affect PBR density in these organs.

A 3D model of the peripheral benzodiazepine receptor and its implication in intra mitochondrial cholesterol transport

A three-dimensional (3D) model of the peripheral benzodiazepine receptor (PBR) has been built using molecular dynamics simulations. The transmembrane domain of the receptor has been modeled as five alpha-helices, which are not long enough to cross the entire bilayer membrane but correspond approximately to only one phospholipid layer. The receptor model has also been tested as a cholesterol carrier, and molecular dynamics simulations have shown that it could indeed accommodate a cholesterol molecule within the five helices. All three known PBR sequences have been modeled, and no significant difference has been found between them.

Steroid hormone secretion by testicular tissue from African catfish, Clarias gariepinus, in primary culture: identification and quantification by gas chromatography - mass spectrometry

With the final aim of identifying the testicular steroids involved in the feedback mechanism of the hypothalamus-pituitary-gonad axis, steroid secretion by the testis of African catfish, Clarias gariepinus, was studied in vitro, by means of gas chromatography followed by mass spectrometry. Testicular fragments of sexually mature catfish raised in captivity were incubated in L-15 medium with and without catfish pituitary extract (cfPE). Without adding cfPE, 22 steroids could be identified, amongst which 11β-hydroxyandrostenedione, 11β-hydroxytestosterone, 11-ketotestosterone and 11-ketoandrostenedione were dominating. After incubation in the presence of cfPE, the concentrations of the four 11-oxygenated steroids were increased about 4-fold. The amounts of pregnane derivatives in the incubation medium showed the largest increases in the presence of cfPE. 5β-Pregnane-triol levels, for example, were 60-fold higher than in the medium from control incubations. The secretion of 5α- and 5β-androstanes was also stimulated by cfPE.The stimulation was not equal for all steroids, indicating that cfPE not only stimulates total steroidogenesis by increasing the availability of cholesterol, but also by influencing specific steroid converting enzyme activities.

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.

Inhibitory effect of diazepam on the activity of the hypothalamic-pituitary-adrenal axis in female rats

The acute intraperitoneal administration of anxiolytic diazepam (2 mg/kg) inhibits the activity of the hypothalamic-pituitary-adrenal (HPA) axis, i.e., it decreases the concentration of adrenocorticotropic hormone (ACTH) and corticosterone in female rats. This fall of ACTH and corticosterone levels was reversed by an antagonist of central benzodiazepine receptors-flumazenil. The antagonist of peripheral benzodiazepine receptors-PK 11195, failed to affect diazepam-induced decrement of plasma ACTH and corticosterone levels. The suppressed HPA function obtained after diazepam administration was also antagonized by bicuculline, an antagonist of GABA recognition sites, and by picrotoxin, a drug that blocks the GABA-A receptor associated chloride channel. These results suggest that central benzodiazepine receptors, the part of GABA-A macromolecular complex, are involved in diazepam-induced inhibition of the activity of the HPA axis.

Adrenal mitochondrial benzodiazepine receptors are sensitive to agents active at the dopamine receptor

Male rats were treated for 21 days with drugs known to affect prolactin secretion, in order to assess the effects of these drugs on mitochondrial benzodiazepine receptors (MBRs). Sulpiride, a selective dopamine D2 receptor antagonist and hyperprolactinemic agent, decreased MBR density in the adrenal gland (49%; P < 0.005), whereas metoclopramide, another dopamine antagonist with a preference for dopamine D2 receptors, increased adrenal gland MBR density (31%; P < 0.05). Bromocriptine, a specific dopamine agonist, increased MBR density in this organ (87%; P < 0.001). None of the three agents influenced kidney or testicular MBRs. These data indicate that the mechanism of organ-specific alterations in MBRs seems to be prolactin independent.

ACTH-induced mitochondrial DBI receptor (MDR) and diazepam binding inhibitor (DBI) expression in adrenals of hypophysectomized rats is not cause-effect related to its immediate steroidogenic action

Diazepam binding inhibitor (DBI) is a 10-kDa polypeptide that is enriched in steroidogenic cells such as adrenocortical, Leydig, and glial cells. In these cells, DBI and some of its processing products bind to the mitochondrial DBI receptor (MDR), located on the outer mitochondrial membrane, and stimulate pregnenolone formation by facilitating cholesterol access to the inner mitochondrial membrane where the cytochrome P-450 side chain cleavage enzyme is located. To determine whether the ACTH-induced increase in adrenal steroidogenesis occurs via changes in DBI and MDR expression the adrenal content of DBI-like immunoreactivity (DBI-LI), the MDR density, and the expression of mRNAs encoding for DBI and MDR were studied in hypophysectomized rats treated with vehicle or ACTH. After 9 days from the hypophysectomy, the levels of DBI-like immunoreactivity (DBI-LI) and DBI-mRNA declined to approximately 20% of their normal value; in contrast MDR-density and MDR-mRNA levels were reduced by 50-60% and were associated to a similar decrease in the activity of type A monoamine oxidase, a marker for mitochondrial proteins. Prolonged administration of ACTH-R (ACTH in saline containing 16% gelatin, 15 U/kg/day, from day 7 after surgery) to hypophysectomized rats, completely restored DBI and MDR adrenal expression to values similar to those of sham-operated rats. Our results indicate that ACTH, probably acting at the transcriptional level, is required for the normal expression of DBI and MDR in adrenal cortex. Changes in DBI and MDR expression after ACTH administration were not temporally related to the immediate steroidogenesis induced by ACTH, and may reflect its long-term trophic action on adrenocortical cells.

Prenatal diazepam induced persisting downregulation of peripheral (omega 3) benzodiazepine receptors on rat splenic macrophages

Prenatal treatment with a low dose of diazepam (1.25 mg/kg/d, gestational day 14-20) has previously been found to cause longterm impairment of cellular immune responses in rat offspring. A possible site of action of diazepam, i.e., the peripheral-type (omega 3) benzodiazepine receptor, was characterized on splenic macrophages as well as on a rat splenic cell preparation containing mainly lymphocytes. In membranes of both preparations, [3H]PK 11195 bound to a single site which in competition experiments exhibited characteristics of the (omega 3) site. Prenatal exposure to diazepam was followed, at 8 weeks of life, by a marked decrease in maximal binding capacity (Bmax) of spleen macrophage membranes in offspring of both sexes, while membranes of the splenic cell preparation exhibited an increase of the dissociation constant of [3H]PK 11195 at 2 and 8 weeks of life. Both types of delayed effects may reduce the metabolic capacity of these immune cells, in which we have also observed deficits of cytokine release.

Distribution profile and properties of peripheral-type benzodiazepine receptors on human hemopoietic cells

The cellular localization of peripheral-type benzodiazepine receptors (PBRs) was characterized in several human blood cell subpopulations including erythrocytes, platelets, monocytes and polymorphonuclear neutrophils (PMN), B, NK, T8 and T4-cells. Pharmacological properties of the PBR were established by binding studies and PBR mRNA expression was measured by quantitative polymerase chain reaction based method. These data clearly indicate 1) the PBR is pharmacologically homogeneous in the various types of blood cells, 2) the rank order of PBR cell density is monocytes = PMN > lymphocytes >> platelets > erythrocytes, 3) the PBR appears to be transcriptionally regulated since mRNA levels are roughly correlated with PBR density.

Regulation of cholesterol movement to mitochondrial cytochrome P450scc in steroid hormone synthesis

Transfer of cholesterol to cytochrome P450scc is generally the rate-limiting step in steroid synthesis. Depending on the steroidogenic cell, cholesterol is supplied from low or high density lipoproteins (LDL or HDL) or de novo synthesis. ACTH and gonadotropins stimulate this cholesterol transfer prior to activation of gene transcription, both through increasing the availability of cytosolic free cholesterol and through enhanced cholesterol transfer between the outer and inner mitochondrial membranes. Cytosolic free cholesterol from LDL or HDL is primarily increased through enhanced cholesterol ester hydrolysis and suppressed esterification, but increased de novo synthesis can be significant. Elements of the cytoskeleton, probably in conjunction with sterol carrier protein(2) (SCP(2)), mediate cholesterol transfer to the mitochondrial outer membranes. Several factors contribute to the transfer of cholesterol between mitochondrial membranes; steroidogenesis activator peptide acts synergistically with GTP and is supplemented by SCP(2). 5-Hydroperoxyeicosatrienoic acid, endozepine (at peripheral benzodiazepine receptors), and rapid changes in outer membrane phospholipid content may also contribute stimulatory effects at this step. It is suggested that hormonal activation, through these factors, alters membrane structure around mitochondrial intermembrane contact sites, which also function to transfer ADP, phospholipids, and proteins to the inner mitochondria. Cholesterol transfer may occur following a labile fusion of inner and outer membranes, stimulated through involvement of cardiolipin and phosphatidylethanolamine in hexagonal phase membrane domains. Ligand binding to benzodiazepine receptors and the mitochondrial uptake of 37 kDa phosphoproteins that uniquely characterize steroidogenic mitochondria could possibly facilitate these changes. ACTH activation of rat adrenals increases the susceptibility of mitochondrial outer membranes to digitonin solubilization, suggesting increased cholesterol availability. Proteins associated with contact sites were not solubilized, indicating that this part of the outer membrane is resistant to this treatment. Two pools of reactive cholesterol within adrenal mitochondria have been distinguished by different isocitrate- and succinate-supported metabolism. These pools appear to be differentially affected in vitro by the above stimulatory factors.

Structure of the rat gene encoding the mitochondrial benzodiazepine receptor

The gene encoding the rat mitochondrial benzodiazepine receptor (MBR) was cloned and characterized. Hybridization of a previously cloned cDNA for MBR to genomic Southern blots indicated that the gene was probably present at one copy per haploid genome. Rapid amplification of cDNA ends with rat adrenal RNA was used to obtain 47 nt of additional sequence upstream from our previously cloned MBR cDNA proving to be a crucial step in cloning the first exon of this gene. The MBR gene is comprised of four exons spanning approx. 10 kb. The first intron, contained within a 8-kb stretch of this gene, is located within the 5'-untranslated sequence, whereas the remaining two introns are much shorter (641 and 854 bp) and interrupt the coding sequence. The third intron contains sequences homologous to rodent B1 repetitive elements and a novel sequence closely resembling part of a repetitive element belonging to the Alu family in humans. The transcription start point was mapped by S1 nuclease protection assays suggesting that the first exon is just 56 bp in length. The sequence upstream from this region contains three GC boxes but lacks other known consensus recognition sites for sequence-specific transcription factors.

Further evidence that the mitochondrial proteins induced by hormone stimulation in MA-10 mouse Leydig tumor cells are involved in the acute regulation of steroidogenesis

In previous studies we and others have described several mitochondrial proteins which are synthesized in response to acute hormone stimulation in several steroidogenic tissues. In both MA-10 mouse Leydig tumor cells and primary cultures of rat adrenal cortex cells, these proteins consist of a family of 37 kilodalton (kDa) and 32 kDa precursor forms and fully processed forms which are 30 kDa in molecular weight. The nature of the appearance of these proteins and their subcellular localization to the mitochondria, the site of the rate limiting step in steroidogenesis, has led to the speculation that they may be involved in the acute regulation of steroidogenesis. In the present study we have taken advantage of another steroidogenic cell, the R2C rat Leydig tumor cell, to perform studies which further indicate that these mitochondrial proteins are involved in the regulation of steroidogenesis. Unlike the MA-10 cell which requires hormone stimulation for steroid production, the R2C cell is a constitutive progesterone producer whose steroid production cannot be further increased with hormone stimulation. We have shown that the R2C cell line is less sensitive to the inhibition of steroid production by the metal chelator orthophenanthroline (OP) than is the MA-10 cell. We have demonstrated that progesterone production and the 30 kDa mitochondrial proteins remain present in the R2C cells at a concentration of OP which completely inhibits progesterone production and totally eliminates the 30 kDa proteins in MA-10 cells. As further evidence for the role of these proteins in steroidogenic regulation, we have isolated several revertants of the R2C parent (P) cell line which have lost the ability to synthesize progesterone constitutively, but which can be stimulated to synthesize this steroid by trophic hormone and cAMP analog. In these revertants, designated (R), the normally constitutively present 30 kDa proteins are greatly decreased compared to controls, but reappear in large amounts following hormone stimulation. Taken together, these data provide further evidence that the 30 kDa mitochondrial proteins are involved in the acute regulation of steroidogenesis in Leydig cells.

Cyclic AMP-dependent increase of steroidogenesis in brain cortical minces

Rat brain cortical minces were incubated with forskolin and dibutyryl-cyclic AMP for 60 and 30 min, respectively. The concentrations of pregnenolone, progesterone and desoxycorticosterone in this preparation were significantly increased by both substances. The results indicate that, in brain tissue, steroidogenesis appears to be regulated by receptor transduction mechanisms that operate through adenylate cyclase.

Cell surface localization of the peripheral-type benzodiazepine receptor (PBR) in adrenal cortex

Previous studies demonstrated that the mitochondrial peripheral-type benzodiazepine receptor (PBR) regulates steroid biosynthesis. In this study we investigate further PBR action by examining its subcellular localization in mouse adrenal gland using anti-peptide PBR antiserum and employing biotin-streptavidin peroxidase immunocytochemistry. Results demonstrated PBR immunostaining exclusively in the cortex. Within this region, however, PBR staining was homogeneously distributed in cells of the zona glomerulosa, whereas in cells of the zona fasciculata both cytoplasmic and prominent plasma membrane immunostaining was evident. Next, PBR distribution was examined using confocal microscopy. Confocal optical sections were obtained, 3-D reconstructions of these sections generated, and vertical, z-sections of the 3-D reconstruction recreated. The immunostaining pattern observed was consistent with a cell surface distribution of PBR. The demonstration of a subset of PBR at the plasma membrane may account for actions of PBR ligands not related to mitochondrial function.

Impact of psychological dynamics of stress on the peripheral benzodiazepine receptor

In an attempt to dissociate the relative impact of psychological vs. physiological concomitants of stress on the peripheral benzodiazepine receptor (PBR), the influence of stressor controllability and predictability was investigated in rats. In addition, the effect of a purely psychological stressor, contextually conditioned fear, was examined. The response of the PBR in rats confronted with a naturalistic threat, a cat, was also tested. Various peripheral and CNS tissues were analyzed. Specific binding of [3H]Ro 5-4864 was significantly reduced in the kidneys of subjects receiving either controllable or uncontrollable shock. Similar changes were seen in the kidneys of subjects receiving either predictable or unpredictable shock. Mean [3H]Ro 5-4864 binding in lung was reduced following both predictable and unpredictable shock, but only the reduction in the predictable shock group reached significance. Controllability appeared to protect against the stress-induced reduction in [3H]Ro 5-4864 binding in lung. Contextually conditioned fear only affected PBR in the olfactory bulb, and exposure to a cat was without effect. These data suggest that the PBR responds only to potent stressors, and psychological influences on the PBR are tissue specific.

Immunomodulatory effect of peripheral benzodiazepine receptor ligands on human mononuclear cells

Immunomodulatory effect of ligands active at the peripheral benzodiazepine receptor (PBR) was examined in human peripheral blood mononuclear cells (PBMC). Ro5-4864, PK11195 and diazepam suppressed phytohemagglutinin (PHA) and concanavalin A (ConA) induced proliferation of PBMC. All three ligands inhibited interleukin-3-like activity (IL-3-LA) secretion, while the production of interleukin-2 (IL-2) was inhibited by Ro5-4864 and diazepam only. The selective central benzodiazepine ligand clonazepam did not affect the cellular immune functions examined. Our results indicate an in-vitro immuno-suppressive activity of peripheral and mixed, but not central type benzodiazepine ligands.

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

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

Orchiectomy upregulates rabbit prostate peripheral benzodiazepine receptors

The effect of orchiectomy on peripheral benzodiazepine receptors (PBZr) in the rabbit prostate was studied. The mean PBZr density in the mitochondrial fraction isolated from prostates of intact (non castrated) rabbits was 4066 fmol/mg protein which following castration increased to 7236 fmol/mg protein (p less than 0.005). The apparent dissociation constant (KD) of prostatic PBZr was higher in castrated rabbits (4.2 nM) than in intact animals (2.7 nM). These data suggest a role of androgen in the regulation of prostatic PBZr.

Peripheral-type mitochondrial binding sites for benzodiazepines in GH3 pituitary cells

Benzodiazepines (BZs) interact with two classes of high affinity binding sites, equilibrium dissociation constants in the nanomolar range, a neuronal or central-type and a non-neuronal or peripheral-type. The peripheral-type binding site has been shown to be present on the outer mitochondrial membrane and appears to be involved in regulation of cholesterol transport in steroid hormone-producing endocrine cells. In rat pituitary GH3 cells, BZs bind to receptors for thyrotropin-releasing hormone (TRH) and via interaction at a different site block Ca2+ influx through voltage-sensitive channels. These, however, are low affinity interactions occurring at micromolar BZ concentrations. Here, using [3H]Ro 5-4864, we report that GH3 cells also have high affinity peripheral-type BZ binding sites. Apparent equilibrium dissociation constants of 7.8 +/- 1.7 nM and 9.3 +/- 4.5 nM for [3H]Ro 5-4864 were measured with intact cells and isolated mitochondria, respectively. As predicted from studies of these sites in other cells, the order of potencies of BZs to displace [3H]Ro 5-4864 was Ro 5-4864 greater than diazepam (DZP) much greater than clonazepam (CIZP); chlordiazepoxide (CDE) did not affect binding. Nifedipine, a dihydropyridine antagonist of Ca2+ channels that has been shown to displace BZs from peripheral-type sites in other cells, was shown to be a competitive inhibitor of [3H]Ro 5-4864 binding with a half-effective concentration in the micromolar range. Ro 5-4864, however, had no effect on Ca2+ influx or efflux in mitochondria isolated from GH3 cells. Hence, GH3 cells exhibit mitochondrial, peripheral-type BZ binding sites but the role of these putative receptors in these neuroendocrine cells, which do not produce steroid hormones, is unclear.

Endozepine/diazepam binding inhibitor in adrenocortical and Leydig cell lines: absence of hormonal regulation

One of the many effects which have been attributed to the peptide endozepine/diazepam binding inhibitor (Ep/DBI) is the stimulation of adrenocortical and testicular Leydig cell mitochondrial steroidogenesis. We have used two cell lines (Y-1 mouse adrenal cell tumour and MA-10 mouse Leydig cell tumour), both of which exhibit hormone stimulated steroid production, to investigate the role of Ep/DBI in acute hormone stimulated steroidogenesis. The time course of incorporation of 35S-translabel into Ep/DBI and its turnover rate when the isotope was removed were examined. Cell samples were extracted and separated on Sep-Pak C18 columns and analysed using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis followed by fluorography as well as by direct scintillation counting. This allowed us to estimate the in vivo half-life of Ep/DBI and also to investigate the hormonal dependence of the peptide. Data presented here suggest that (i) Ep/DBI levels are not regulated by trophic hormones in these steroidogenic cell lines, and (ii) that the peptide has a relatively long half-life (greater than 3 h), a finding incompatible with suggestions of it having a rapid turnover. Therefore, it seems unlikely that control of Ep/DBI steroidogenic effects is via hormonal modulation of the peptide levels.

The role of diazepam binding inhibitor and its processing products at mitochondrial benzodiazepine receptors: regulation of steroid biosynthesis

The rate-limiting step in the biosynthesis of steroids is the transport of the substrate cholesterol from the outer to the inner mitochondrial membrane, where cholesterol is metabolized to pregnenolone. This transport is markedly stimulated by the action of hormones, such as adrenocorticotropic hormone (ACTH) and luteinizing hormone (LH) for adrenocortical and testicular Leydig cells, respectively. Recently, it was demonstrated that the peripheral-type or mitochondrial benzodiazepine receptor, abundant in steroidogenic tissues, is involved in the regulation of steroid biosynthesis. In search for an endogenous ligand for mitochondrial benzodiazepine receptors, regulating steroidogenesis, the effects of Diazepam Binding Inhibitor (DBI) were studied. The model systems used were the Y-1 adrenocortical and the MA-10 Leydig cell lines, previously shown to be valid steroidogenic models. Both cell lines contain significant levels of immunoreactive DBI. Purified DBI from rat brain, at high nanomolar concentrations, increased formation of pregnenolone, when added to mitochondrial preparations of both cell types; but at concentrations of DBI above 1 microM, a decrease in the stimulation was observed. Flunitrazepam, a benzodiazepine which binds to mitochondrial benzodiazepine receptors, with high nanomolar affinity, inhibited the stimulatory action of DBI on the formation of mitochondrial pregnenolone, indicating that DBI exerts its stimulatory effects through an action on mitochondrial benzodiazepine receptors. In order to determine the biologically active amino acid sequence in the DBI molecule, various fragments of DBI were synthesized and tested; also, peptides structurally unrelated to DBI were tested.(ABSTRACT TRUNCATED AT 250 WORDS)