Mitochondrial Benzodiazepine Receptors Regulate Steroid
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Proc. Nati. Acad. Sci. USA Vol. 86, pp. 9813-9816, December 1989 Biochemistry Mitochondrial benzodiazepine receptors regulate steroid biosynthesis (peripheral-type benzodiazepine receptor function/cholesterol transport/adrenocortical cells) ALEXEY G. MUKHIN*, VASSILIOS PAPADOPOULOSt, ERMINIO COSTA*t, AND KARL E. KRUEGER*t *Fidia43eorgetown Institute for the Neurosciences and tDepartment of Anatomy and Cell Biology, Georgetown University School of Medicine, 3900 Reservoir Road, Washington, DC 20007 Contributed by Erminio Costa, October 5, 1989 ABSTRACT Recent observations on the steroid synthetic steroid biosynthetic pathway, occurs in mitochondria where capability within the brain open the possibility that benzodi- PBRs are associated with the outer mitochondrial membrane azepines may influence steroid synthesis in nervous tissue (15, 16). Consequently, several groups have demonstrated through interactions with peripheral-type benzodiazepine rec- that benzodiazepines can influence steroid production in ognition sites, which are highly expressed in steroidogenic cells several steroidogenic systems (17-19). and associated with the outer mitochondrial membrane. To To investigate whether PBRs play a central role in steroid examine this possibility nine molecules that exhibit a greater production, we have used the well-characterized Y-1 adreno- than 10,000-fold difference in their affinities for peripheral- cortical cell line as a model system (20, 21). The data type benzodiazepine binding sites were tested for their effects presented here strongly support a role for PBR in the regu- on a well-established steroidogenic model system, the Y-1 lation of steroidogenesis. One might infer that the complex mouse adrenal tumor cell line. 4'-Chlorodiazepam, PK 11195, central actions of various benzodiazepines could include and PK 14067 stimulated steroid production by 2-fold in Y-1 effects on neurosteroid production mediated through cen- cells, whereas diazepam, flunitrazepam, zolpidem, and PK trally located mitochondrial PBRs. 14068 displayed a lower (1.2- to 1.5-fold) maximal stimulation. In contrast, clonazepam and flumazenil did not stimulate steroid synthesis. The potencies of these compounds to inhibit METHODS 3H-labeled PK 11195 binding to peripheral-type benzodi- Cells. The Y-1 mouse adrenal tumor cell line used in these azepine recognition sites correlated (r = 0.985) with their studies was obtained from American Type Culture Collec- potencies to stimulate steroid production. Similar rindings were tion. Stock cultures were grown in modified Waymouth's observed in bovine and rat adrenocortical cell preparations. MB752/1 medium containing 20 mM Hepes, NaHCO3 (1.2 These results suggest that ligands of the peripheral-type ben- g/liter), 15% (vol/vol) horse serum, and 2.5% (vol/vol) fetal zodiazepine recognition site acting on this mitochondrial re- calf serum (pH 7.4). Before use, the cells were washed for ceptor can enhance steroid production. This action may con- three 30-min periods with serum-free medium to eliminate tribute specificity to the pharmacological profile of drugs serum components that may interfere with the assays and preferentially acting on the benzodiazepine recognition site then incubated as indicated in the presence of drugs under associated with the outer membrane of certain mitochondrial investigation. Bovine fasciculata-reticularis and adrenocor- populations. tical cells from adult Sprague-Dawley rats (300 g) were prepared as described (22). Benzodiazepines elicit their primary therapeutic actions in Radioligand Binding Assays. Y-1 cell cultures were scraped the central nervous system through specific binding sites on from the flasks (75 mm2) in 5 ml of Kreb's buffer (8, 29), -y-aminobutyric acid (GABA)-gated chloride channels (1). In dispersed by repetitive pipetting, and centrifuged at 1200 x addition to these sites another class of binding sites for in Kreb's benzodiazepines was identified in peripheral tissues (2) and g for 5 min. The cell pellets were resuspended has been referred to as peripheral-type benzodiazepine rec- buffer and larger cellular aggregates were allowed to settle for ognition sites (PBRs). PBRs are also abundant in neural tissue 5 min. The finely dispersed cell suspensions were taken for (3, 4) where they appear to be preferentially associated with further experimentation. astroglial cells (5). Since the physiological role ofPBRs is not Binding assays were performed in a total volume of 250- presently known, the effects resulting from benzodiazepine 400 ,l of Kreb's buffer containing 3H-labeled PK 11195 [1-(2- interactions with PBRs in peripheral tissues and in the brain chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline- are poorly characterized. Ligands that can bind to the PBR carboxamide] (0.07-20 nM) with 5 ,g of cellular protein. with high affinity have been reported to induce convulsions Incubations were conducted at 37°C for 30 min and bound (6) and exhibit proconflict actions (7, 8). The mechanisms by radioligand was collected by filtration through Whatman which these behavioral effects are generated remain unclear. GF/C filters and rapidly washed four times with 5 ml of 25 The central nervous system can synthesize steroids (9, 10). mM Tris-HCl (pH 7.4) containing 1 ,M PK 11195. Radioac- In another set of investigations, steroids were demonstrated tivity was determined by liquid scintillation counting. Non- to be potent modulators oftype A GABA (GABAA) receptors specific binding was measured in the presence of 10 ,uM (11, 12), implying a physiological role ofsteroids as regulators nonradioactive PK 11195. of GABAergic transmission. These findings are significant Measurement of Steroid Production. These experiments when one considers that PBRs are extremely abundant in were performed with the Y-1 cells plated in 12 x 22 mm wells steroidogenic cells (13, 14). Furthermore, metabolism of cholesterol to pregnenolone, the first intermediate in the Abbreviations: PBR, peripheral-type benzodiazepine recognition site; GABA, y-aminobutyric acid; PK 11195, 1-(2-chlorophenyl)- N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide; PK The publication costs of this article were defrayed in part by page charge 14067 and PK 14068, (-)- and (+)-N,N-diethyl-2-methyl-3-[4-(2- payment. This article must therefore be hereby marked "advertisement" fhenyl)quinolinyllpropanamide, respectively. in accordance with 18 U.S.C. §1734 solely to indicate this fact. To whom reprint requests should be addressed. 9813 Downloaded by guest on September 27, 2021 9814 Biochemistry: Mukhin et al. Proc. Natl. Acad. Sci. USA 86 (1989) and incubated for the times shown in a final volume of 1 ml of serum-free medium at 370C. At the end of the incubation 100 period the cell medium was saved, centrifuged at 1500 x g for 10 min, and stored at -20'C until use. Steroid production was cn determined by measuring 20a-hydroxyprogesterone, the c c main steroid product of these cells (20, 21), using a radioim- .-4 4-J m c munoassay with an antibody donated by G. Nieswender 0 (Colorado State University, Fort Collins). Cortisol and cor- u CD) ticosterone, indices for steroid production in bovine and rat adrenocortical preparations, respectively, were also mea- L) L sured by radioimmunoassay using commercial kits (Baxter n Scientific Products, McGaw Park, IL, and Endocrine Sci- ences, Tarzana, CA, respectively). Analysis of the radioim- munoassay data was performed using the IBM-PC RIA DATA a, -10 -9 -8 -7 -6 -5 -4 REDUCTION program (version 4.1) from Jaffe (Silver Spring, CD, Log [Ligand], M MD). Protein Measurements. Protein was quantitated by the method of Bradford (23) using bovine serum albumin as a 100 standard. B C 80 RESULTS 4-i i U) a Binding of PBR Ligands in Y-1 Cells. Preliminary experi- cn ments to measure 3H-labeled PK 11195 binding in Y-1 cells at CL 370C demonstrated rapid association kinetics reaching equi- I ) 60 -0 librium within 15 min, which was stable for at least 2 hr and fully reversible by adding excess PK 11195 (data not shown). 'C, Scatchard analysis of 3H-labeled PK 11195 binding in Y-1 C'\ cells revealed a single class of recognition sites with a 400 t _ 3 , dissociation constant of 1.8 nM and a Bma, of 54 pmol/mg of -10 -9 -8 -7 -6 -5 -4I protein (Fig. 1). The high density of PBRs in Y-1 total cell membranes is consistent with the abundance reported (24) in Log [Ligand], M rat adrenal mitochondrial preparations. FIG. 2. Binding specificity of PBRs and stimulation of ste- Nine ligands, exhibiting a range greater than four orders of roidogenesis in Y-1 cells. (A) Potencies of nine compounds to magnitude in their affinities for PBRs, were examined for compete against 3H-labeled PK 11195 (0.8 nM) for binding in Y-1 their potencies to inhibit 3H-labeled PK 11195 binding to Y--1 cells. (B) Secretion of 20a-hydroxyprogesterone (20a-OH- cells at 370C (Fig. 2A). These compounds demonstrated a Progesterone) was measured in cultures of Y-1 cells treated for 4 hr rank order of ligand displacement potency (PK 11195 > PK with various concentrations of ligand as indicated. A, PK 11195; *, 14067 > 4'-chlorodiazepam > zolpidem > PK 14608, diaze- PK 14067; E, 4'-chlorodiazepam; O, zolpidem; o, PK 14068; *, pam, flunitrazepam > clonazepam, flumazenil), which is diazepam; A, flunitrazepam; x, clonazepam; *, flumazenil. Curves consistent with reports (16) on PBRs from rat tissues {PK were generated by nonlinear regression analysis of the data. 14067 and PK 14068, (-)- and (+)-N,N-diethyl-2-methyl- 14068 (25). Also at 37°C, benzodiazepines, 4'-chlorodiaz- 3-[4-(2-phenyl)quinolinyl]propanamide, respectively; these epam, diazepam, and flunitrazepam showed a relatively low compounds are referred to in ref. 25 as (-)- and (+)-N,N- potency compared to PK 11195, which is in agreement with diethyl-a-methyl-2-phenyl-4-quinoline propanamide|. In par- a report (26) that demonstrated that the binding of benzodi- ticular it should be noted that PK 14067 was more than two azepines to PBRs is a temperature-sensitive enthalpy-driven orders of magnitude more potent than its stereoisomer PK process.