Diazepam-binding inhibitor (DBI)-processing products, acting at the mitochondrial DBI receptor, mediate adrenocorticotropic hormone-induced steroidogenesis in rat adrenal gland.

Abstract

Diazepam-binding inhibitor (DBI) is a 9-kDa polypeptide that colocalizes in glial, adrenocortical, and Leydig cells with the mitochondrial DBI receptor (MDR). By binding with high affinity to the MDR, DBI and one of its processing products-DBI-(17-50)-regulate pregnenolone synthesis and have been suggested to participate in the immediate activation of adrenal steroidogenesis by adrenocorticotropic hormone (ACTH). In adrenals of hypophysectomized rats (1 day after surgery), ACTH failed to acutely affect the amount of adrenal DBI and the density of MDR but increased the rate of DBI processing, as determined by the HPLC proffle of DBI-(17-50)-like immunoreactivity. The similar latency times for this effect and for ACTH stimulation of adrenal steroidogenesis suggest that the two processes are related. The ACTH-induced increase in both adrenal steroidogenesis and rate of DBI processing were completely inhibited by cycloheximide; this result suggests the requirement for the de novo synthesis of a protein with a short half-life, probably an endopeptidase. This enzyme, under the influence of ACTH, may activate formation of a DBI-processing product that stimulates steroidogenesis via the MDR. In support of this hypothesis is the demonstration that in hypophysectomized rats the MDR antagonist PK 11195 1-(2-chlorophenyl)-N-methyl-N-(l-methylpropyl)-3-isoquinolinecarboxamide completely inhibited the adrenal steroldogenesis stimulated by ACTH and by the high-affinity MDR lgand 4'-chlorodiazepam.The rate-limiting step in steroid biosynthesis is the conversion of cholesterol to pregnenolone. This reaction is catalyzed by the side-chain cleavage enzyme cytochrome P-450 [P-450SC; cholesterol, reduced-adrenal-ferredoxin:oxygen oxidoreductase (side-chain-cleaving), EC 1.14.15.6], which is located on the inner mitochondrial membrane (1). Several laboratories have established that it is the rate at which cholesterol is transported to the inner mitochondrial membrane, and not P-450SCC activity itself, that limits the rate of pregnenolone synthesis (2-5). The transfer of cholesterol to P-450,c, therefore, must be facilitated to achieve maximal rates of pregnenolone synthesis. The protein synthesis inhibitor cycloheximide has been shown to block the induction of steroidogenesis by adrenocorticotropic hormone (ACTH) (6)(7)(8), and this block appears to occur at the level of cholesterol transport from the outer to the inner mitochondrial membrane. It is believed that one or more proteins with a rapid turnover rate are required to effect cholesterol translocation within mitochondria and that ACTH may either increase the concentration of these proteins or activate them posttranslationally.A receptor that is located on the outer mitochondrial membrane and that is enriched in the mitochondria of steroidogenic cells participates in the regulation of intramitochondrial cholesterol transport and may mediate the immediate action of steroidogenic hormones, such as ACTH and human chorionic gonadotropin (9-12). Because diazepambinding ...

Evidence for a diazepam-binding inhibitor (DBI) benzodiazepine receptor-like mechanism in ecdysteroidogenesis by the insect prothoracic gland

“…Both RNA and protein synthesis are absolutely required for gland activation (Keightly et al 1990), leading to an increase in the appearence of a number of proteins (Rybczynski and Gilbert 1994). Inhibition of protein synthesis in mammalian steroidogenic tissues blocks cholesterol transfer to mitochondria, possibly by disruption of DBI processing steps (Cavallaro et al 1992). Likewise, the involvement of cytoskeletal proteins in the PTTH-stimulation of ecdysteroid production was suggested following exposure of PGs to mictotubule inhibitors (Watson et al 1996).…”

Rodent Leydig Cell Tumorigenesis: A Review of the Physiology, Pathology, Mechanisms, and Relevance to Humans

“…It appears that LH regulates posttranslational activation of DBI to stimulate steroidogenesis (Papadopoulos et al, 1991;Cavallaro et al, 1992). It also has been speculated that this peptide accelerates C 27 -side chain cleavage enzyme activity directly by loading the enzyme with substrate (Hall, 1994).…”

Translocator protein: pharmacology and steroidogenesis

“…These findings suggest that TSPO-ligand binding has systemspecific and experiment-specific properties, dependent on the local microenvironment and molecular associations. This may account for disparate findings, such as PK 11195 agonist functionality in adrenocortical tumour cell steroidogenesis [35] with antagonist adrenocortical functionality in rats [68], or PK 11195 agonist activity [34] or lack of function [50] in Leydig tumour cells separated by a quarter century of culture.…”