The role of diazepam binding inhibitor in the regulation of steroidogenesis

Molecular cloning and chromosomal localization of a pseudogene related to the human acyl-CoA binding protein/diazepam binding inhibitor

The acyl-CoA binding protein (ACBP) and the diazepam binding inhibitor (DBI) or endozepine are independent isolates of a single 86-amino-acid, 10-kDa protein. ACBP/DBI is highly conserved between species and has been identified in several diverse organisms, including human, cow, rat, frog, duck, insects, plants, and yeast. Although the genomic locus has not yet been cloned in humans, complementary DNA clones with different 5' ends have been isolated and characterized. These cDNA clones appear to be encoded by a single gene. However, Southern blot analyses, in situ hybridizations, and somatic cell hybrid chromosomal mapping all suggest that there are multiple ACBP/DBI-related sequences in the genome. To identify potential members of this gene family, degenerate oligonucleotides corresponding to highly conserved regions of ACBP/DBI were used to screen a human genomic DNA library using the polymerase chain reaction. A novel gene, DBIP1, that is closely related to ACBP/DBI but is clearly distinct was identified. DBIP1 bears extensive sequence homology to ACBP/DBI but lacks the introns predicted by rat and duck genomic sequence studies. A 1-base deletion in the coding region results in a frameshift and, along with the absence of introns and the lack of a detectable transcript, suggests that DBIP1 is a pseudogene. ACBP/DBI has previously been mapped to chromosome 2, although this was recently disputed, and a chromosome 6 location was suggested. We show that ACBP/DBI is correctly placed on chromosome 2 and that the gene identified on chromosome 6 is DBIP1.

Turnover of acyl-CoA-binding protein in four different cell lines measured by using two-dimensional polyacrylamide-gel electrophoresis

Acyl-CoA-binding protein (ACBP), also named diazepam-binding inhibitor or endozepine, is a 10 kDa protein for which a surprisingly large number of biological activities has been suggested. Some of these would seem to require a rapid intracellular turnover of the protein. In this paper we report on the turnover of ACBP in cell lines derived from mouse, rat and man. ACBP was identified in two-dimensional gels by using specific antibodies. Cells were labelled with [35S]methionine and chased for various periods of time. Total protein was extracted, subjected to two-dimensional PAGE, and radioactivity in the spot containing ACBP was determined by liquid-scintillation counting. ACBP half-life was determined, and varied from 25 to 53 h depending on the cell line and the growth conditions. In all cases, radioactivity in ACBP was lost slightly faster than radioactivity in total protein. These results are discussed in relation to the possible function suggested for ACBP.

Cloning and tissue-specific functional characterization of the promoter of the rat diazepam binding inhibitor, a peptide with multiple biological actions

Diazepam binding inhibitor (DBI) is a 10-kDa polypeptide that regulates mitochondrial steroidogenesis, glucose-induced insulin secretion, metabolism of acyl-CoA esters, and the action of gamma-aminobutyrate on GABAA receptors. To investigate the regulation of DBI gene expression, three positive clones were isolated from a rat genomic library. One of them contained a DBI genomic DNA fragment encompassing 4 kb of the 5' untranslated region, the first two exons, and part of the second intron of the DBI gene. Two other overlapping clones contained a processed DBI pseudogene. Several transcription initiation sites were detected by RNase protection and primer extension assays. Different tissues exhibited clear differences in the efficiencies of transcription startpoint usage. Transient expression experiments using DNA fragments of different length from the 5' untranslated region of the DBI gene showed that basal promoter activity required 146 bp of the proximal DBI sequence, whereas full activation was achieved with 423 bp of the 5' untranslated region. DNase I protection experiments with liver nuclear proteins demonstrated three protected regions at nt -387 to -333, -295 to -271, and -176 to -139 relative to the ATG initiation codon; in other tissues the pattern of protection was different. In gel shift assays the most proximal region (-176 to -139) was found to bind several general transcription factors as well as cell type-restricted nuclear proteins which may be related to specific regulatory patterns in different tissues. Thus, the DBI gene possesses some features of a housekeeping gene but also includes a variable regulation which appears to change with the function that it subserves in different cell types.

Amygdaloid central nucleus lesions and cholinergic blockade attenuate the response of the renal peripheral benzodiazepine receptor to stress

Previous research has demonstrated that the density of peripheral benzodiazepine receptors (PBR) in rat kidney rapidly drops following exposure to 80 min of stress. The present experiments examined the contribution of the central and autonomic nervous systems in mediating this effect. Ibotenic acid lesions of the amygdaloid central nucleus (ACe), but not the lateral and basolateral amygdala, diminished the magnitude of the reduction in renal PBR binding caused by stress. Pretreating rats with methyl-scopolamine also inhibited the response of the PBR to stress. Adrenergic blockade with nadolol was ineffective. In order to test whether the PBR was under direct or indirect neural control during stress, unilateral renal denervation was performed. The stress-induced reduction in PBR binding persisted in denervated kidneys revealing that any neural control over the PBR that might exist must be indirect. Together the results suggest that the CNS may be involved in regulating the PBR during stress through the activation of intermediate, possibly hormonal, factors. The involvement of the central nervous system in the modulation of the PBR indicates the relevance of the PBR to physiological adaptations to stress.

A diazepam binding inhibitor (DBI) homolog from the tobacco hornworm, Manduca sexta

A cDNA clone encoding a protein with 42-51% identity to the mammalian diazepam binding inhibitors (DBIs) has been isolated and sequenced from a midgut cDNA library of the tobacco hornworm, Manduca sexta. The putative M. sexta DBI is 90 residues in length and shares the predicted internal a-helices common to the mammalian DBIs. Sequence alignments indicate that the M. sexta DBI contains three potential proteolytic cleavage site lysines in the same positions as the mammalian DBIs. High DBI mRNA levels were found in larval midgut, larval fat body, and adult ovary. DBI mRNA was also detected in larval prothoracic glands, larval nerve cord, and adult testis. These results suggest that the putative M. sexta DBI is an important gene product with a high degree of identity to the known mammalian DBIs. The M. sexta DBI may therefore be the functional homolog to the mammalian DBIs.

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.

Acyl-CoA-binding protein/diazepam-binding inhibitor gene and pseudogenes. A typical housekeeping gene family

Acyl-CoA-binding protein (ACBP) is a 10 kDa protein isolated from bovine liver by virtue of its ability to bind and induce the synthesis of medium-chain acyl-CoA esters. Surprisingly, it turned out to be identical to a protein named diazepam-binding Inhibitor (DBI) claimed to be an endogenous modulator of the GABAA receptor in brain membranes. ACBP/DBI, or proteolytically derived polypeptides of ACBP/DBI, have also been implicated in the control of steroidogenesis in mitochondria and glucose-stimulated insulin secretion. Thus, it appears that ACBP/DBI is a remarkable, versatile protein. Now we have molecularly cloned and characterized the ACBP/DBI gene family in rat. The rat ACBP/DBI gene family comprises one expressed gene and four processed pseudogenes of which one was shown to exist in two allelic forms. The expressed gene is organized into four exons and three introns. There is a remarkable correspondence between the structural modules of ACBP/DBI as determined by 1H nuclear magnetic resonance spectroscopy and the exon-intron architecture of the ACBP/DBI gene. Detailed analyses of transcription of the ACBP/DBI gene in brain and liver were performed to map transcription initiation sites and to examine if transcripts from the ACBP/DBI gene were subject to alternative processing. In both brain and liver, transcription is initiated from two major and multiple minor initiation sites. No evidence for alternative splicing was obtained. The promoter region of the ACBP/DBI gene is located in a CpG island and lacks a canonical TATA box. Thus, the ACDB/DBI gene exhibits all the hallmarks of a typical housekeeping gene.

Human DBI (endozepine): relationship to a homologous membrane associated protein (MA-DBI)

Human endozepine, an 86 amino acid polypeptide, was originally isolated from human brain tissue as a putative ligand of the benzodiazepine receptor. Complete amino acid sequencing of the human and bovine proteins revealed significant homology with the partial sequence of diazepam binding inhibitor (DBI), a protein from rat brain. Both endozepine and DBI have been shown to elicit behavioral effects, suggesting that they function as pharmacologically-active ligands of the GABA (gamma-aminobutyric acid) receptor complex. Subsequent cDNA cloning of human and bovine endozepine, rat DBI and human DBI has shown that these proteins are encoded by the same gene. A related cDNA, encoding a transmembrane protein of 533 amino acids with a domain homologous to DBI, has also been cloned from bovine brain.

Role of DBI in brain and its posttranslational processing products in normal and abnormal behavior

Because diazepam binding inhibitor (DBI) and its processing products coexist with gamma-aminobutyric acid (GABA) in several axon terminals, DBI immunoreactivity was measured in the cerebrospinal fluid (CSF) of individuals suffering from various neuropsychiatric disorders, that are believe to be associated with abnormalities of GABAergic transmission. Increased amounts of DBI-like immunoreactivity were found in the CSF of patients suffering from severe depression with a severe anxiety component (Barbaccia, Costa, Ferrero, Guidotti, Roy, Sunderland, Pickar, Paul and Goodwin, 1986). Moreover, the amount of DBI and its processing products was found to be increased in the CSF of patients with hepatic encephalopathy (HE) (Rothstein, McKhann, Guarneri, Barbaccia, Guidotti and Costa, 1989; Guarneri, Berkovich, Guidotti and Costa, 1990). The clinical rating of HE correlated with the extent of the increase in DBI in CSF. Other lines of research suggest that DBI and DBI processing products may be important factors in behavioral adaptation to stress, acting via benzodiazepine (BZD) binding sites, located on mitochondria. DBI and its processing products, ODN and TTN, are present in high concentrations in the hypothalamus and in the amygdala, two areas of the brain that are important in regulating behavioral patterns associated with conflict situations, anxiety and stress. In CSF, the content of DBI changes in association with corticotropin releasing factor (CRF) (Roy, Pickar, Gold, Barbaccia, Guidotti, Costa and Linnoila, 1989). Finally DBI is preferentially concentrated in steroidogenic tissues and cells (adrenal cortical cells, Leydig cells of the testes and glial cells of the brain).(ABSTRACT TRUNCATED AT 250 WORDS)

Diazepam binding inhibitor peptide: cloning and gene expression

Diazepam binding inhibitor (DBI) is a peptide, initially identified for its ability of displacing the binding of diazepam. The screening of lambda gt 10 cDNA libraries from rat brain with a 47merdeoxyoligonucleotide probe, complementary to a small portion of DBI coding region, allowed the isolation of cDNA clones encoding the entire aminoacid sequence of DBI. This sequence, when compared to that of mouse, human and bovine, revealed that DBI is a well conserved peptide, suggesting a similar function in different species. In order to characterize the function of DBI, studies on the regulation of DBI gene expression were undertaken. The expression of DBI mRNA occurs unevenly in the brain, as well as in peripheral tissues. Moreover, the biosynthesis of DBI is up-regulated in the cerebellum and cerebral cortex of rats made tolerant to diazepam, suggesting that changes in the biosynthesis of DBI might be one of the mechanisms eliciting tolerance to benzodiazepine. In peripheral tissues, the expression of DBI mRNA changes during development. In liver, the content of DBI mRNA was found maximal at postnatal day 1. In contrast, in kidney and heart a linear increase in levels of DBI mRNA was observed from postnatal day 1 to the adult stage, where it reached its maximum level. The tissue specific regulation of DBI mRNA expression, both pharmacologically or developmentally, leads to the hypothesis that DBI might have different functions in different tissues. This would be in line with recent findings that DBI might be also involved in the regulation of an important step of cell metabolism.