Regulation of Diazepam Binding Inhibitor in Rat Adrenal Gland by Adrenocorticotropin

Abstract

Diazepam binding inhibitor (DBI) is a 9-kDa polypeptide that was initially isolated from rat brain and subsequently found to be present in several peripheral tissues. DBI is particularly abundant in steroidogenic tissues, such as the adrenal glands and testes, which also contain a high concentration of peripheral/mitochondrial benzodiazepine receptors (MBRs). Because occupancy of adrenal MBRs with DBI results in increased steroidogenesis, we have investigated the relation between ACTH, DBI, and the MBR in the rat adrenal glands. Evidence presented here indicates that both the amount of DBI and its rate of synthesis in the adrenal cortex are under the control of ACTH. Seven and 9 days after hypophysectomy, the amount of DBI-like immunoreactivity (DBI-LI) in rat adrenal glands decreased dramatically from approximately 80 to 15 ng/mg tissue. The administration of single dose of ACTH (ACTH residues 1-39; 200 mU/kg, iv) or repeated doses of ACTH-R (ACTH in saline containing 16% gelatin; 15 U/kg, sc, twice daily) reduced the decrease in adrenal DBI-LI caused by hypophysectomy. In hypophysectomized rats (7 days after hypophysectomy) the increases in both adrenal DBI-LI and plasma corticosterone induced by ACTH 1 h after a single injection (200 mU/kg, iv) were inhibited by injection of cycloheximide (40 mg/kg, ip) 10 min after ACTH. However, cycloheximide at this dose had no effect on the ACTH-induced increase in adrenal cAMP concentration or the number of affinity of MBRs for 4'-[3H]chlorodiazepam.

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

“…Initially, we determined whether ACTH could rapidly increase DBI or MDR expression in adrenal cortex. Our results, in agreement with those already described (38,39), provide clear evidence that in the rat adrenal gland, the immediate steroidogenic action of ACTH is not associated with'changes in DBI or MDR expression. Treatment of hypophysectomized rats, 1 day after surgery, with ACTH failed to affect immediately (up to 1.5 hr) the amounts of adrenal DBI-LI or MDR or the concentrations of their mRNAs.…”

Peripheral-Type Benzodiazepine/Diazepam Binding Inhibitor Receptor: Biological Role in Steroidogenic Cell Function*

“…It has, however, been reported that trophic hormone replacement in hypophysectomized rats increased DBI levels in the adrenal (176) and in the Leydig cells (114). Increases in DBI levels in these experiments were observed only after at least 2 h subsequent to the administration of hormones.…”

Benzodiazepines and steroidogenesis

“…Recently, DBI itself and its two major process¬ ing products DBI 33-550 and DBI 17-50 have recently been found to stimulate pregnenolone formation in adrenocortical and testicular mitochondria (Papadopoulos, Berkovich, Krueger et al 1991 ). It is interesting that DBI in the adrenal appears to be under the control of ACTH, the tissue content declines after hypophysectomy and increases with ACTH treatment (Massotti, Slobodyanski, Konkel et al 1991). …”

“…Although initially this seemed a strong possibility, there are a number of facts which argue against DBI having a unique role, of which probably the most telling is that the time-course of its appearance after ACTH treatment does not correlate with steroidogenic response either in vitro or in vivo (Massotti et al 1991;Brown, Hall, Shoyab & Papadopoulos, 1992). It may be simply that process¬ ing of the molecule is required, since there is some evidence that the smaller DBI-derived peptides are more active (Papadopoulos et al 1991 ).…”