Life Sciences, Vol. 49, pp. 325-344 Printed in the U.S.A.
Pergamon Press
MINIREVIEW DIAZEPAM BINDING INHIBITOR (DBI): A PEPTIDE WITH MULTIPLE BIOLOGICAL ACTIONS
Erminio Costa and Alessandro Guidotti F i d i a - G e o r g e t o w n Institute for the N e u r o s c i e n c e s Georgetown University School of M e d i c i n e Washington, D.C. 20007 (Received in final form May 29, 1991) SUMMARY
Diazepam binding inhibitor (DBI) is a 9-kD p o l y p e p t i d e that was first isolated in 1983 from rat brain by m o n i t o r i n g its ability to displace diazepam from the b e n z o d i a z e p i n e (BZD) recognition site located on the e x t r a c e l l u l a r domain of the type A receptor for 7aminobutyric acid (GABA A receptor) and from the m i t o c h o n d r i a l BZD receptor (MBR) located on the outer m i t o c h o n d r i a l membrane. In brain, DBI and its two major p r o c e s s i n g products [DBI 33-50, or o c t a d e c a n e u r o p e p t i d e (ODN) and DBI 17-50, or t r i a k o n t a t e t r a n e u r o p e p t i d e (TTN)] are unevenly distributed in neurons, with the highest c o n c e n t r a t i o n s of DBI (i0 to 50 ~M) being present in the hypothalamus, amygdala, cerebellum, and discrete areas of the thalamus, hippocampus, and cortex. DBI is also present in specialized glial cells (astroglia and Bergmann glia) and in peripheral tissues. In the periphery, the highest concentration of DBI occurs in cells of the zona glomerulosa and fasciculata of the adrenal cortex and in Leydig cells of the testis; interestingly, these are the same cell types in which MBRs are highly concentrated. Stimulation of MBRs by appropriate ligands (including DBI and TTN) facilitates cholesterol influx into mitochondria and the subsequent formation of pregnenolone, the parent molecule for endogenous steroid production; this facilitation occurs not only in peripheral steroidogenic tissues, but also in glial cells, the steroidogenic cells of the brain. Some of the steroids (pregnenolone sulfate, dehydroepiandrosterone sul fate, 3~-hydroxy-5~-pregnan-20one, and 3~, 21-dihydroxy-5~-pregnan-20-one) produced in brain (neurosteroids) function as potent (with effects in the nanomolar concentration range) positive or negative allosteric modulators of GABA A receptor function. Thus, accumulating evidence suggests that the various neurobiological actions of DBI and its p r o c e s s i n g products may be attributable to the ability of these peptides either to bind to BZD recognition sites associated with GABA A receptors or to bind to glial cell MBRs and modulate the rate and quality of n e u r o s t e r o i d o 0024-3205/91 $3.00 +.00 Copyright (c) 1991 Pergamon Press plc
326
Multiple Actions of DBI
Vol. 49, No. 5, 1991
genesis. The n e u r o b i o l o g i c a l effects of DBI and its p r o c e s s i n g products in p h y s i o l o g i c a l and p a t h o l o g i c a l conditions (hepatic encephlopaty, depression, panic) c o n c e n t r a t i o n s may therefore be e x p l a i n e d by interactions with d i f f e r e n t types of BZD r e c o g n i t i o n site. In addition, recent reports that DBI and some of its fragments inhibit (in nanomolar concentrations) glucoseinduced insulin release from p a n c r e a t i c islets and bind a c y l - c o e n z y m e A with high affinity support the h y p o t h e s i s that DBI is a precursor of b i o l o g i c a l l y active peptides with m u l t i p l e actions in the brain and in p e r i p h e r a l tissues.
This r e v i e w presents our current u n d e r s t a n d i n g of the p o s s i b l e b i o l o g i c a l actions of the peptide d i a z e p a m b i n d i n g inhibitor (DBI). DBI and its natural p r o c e s s i n g products are p a r t i c u l a r l y abundant in brain and certain peripheral organs and have a d i s t r i b u t i o n p a t t e r n that corresponds to that of b e n z o d i a z e p i n e (BZD) r e c o g n i t i o n sites. These peptides represent a family of p u t a t i v e endogenous ligands for the BZD r e c o g n i t i o n sites located at the a l l o s t e r i c center of the type A r e c e p t o r for 7a m i n o b u t y r i c acid (GABA A receptor) and on m i t o c h o n d r i a of brain and p e r i p h e r a l tissues. BENZODIAZEPINE RECOGNITION SITES In brain, there are two types of BZD r e c o g n i t i o n site (Fig. i), each thought to be a s s o c i a t e d with a specific protein. The "central" BZD r e c o g n i t i o n site is p r e v a l e n t in neurons and is associated with GABA A receptors which are ligand-gated Cl channels (1-7). The "peripheral" BZD r e c o g n i t i o n site (8,9) is p r e v a l e n t in glia (9,10) and is p r e f e r e n t i a l l y a s s o c i a t e d with a protein in the outer mitochondrial membrane (11,12) that regulates the access of cholesterol to the inner m i t o c h o n d r i a l membrane, where a specific type of c y t o c h r o m e P450 catalyzes the c o n v e r s i o n of cholesterol into p r e g n e n o l o n e (13-15). In addition to brain (9,10,12,15) the latter BZD r e c o g n i t i o n site -- here termed the m i t o c h o n d r i a l BZD receptor (MBR) -- is also abundant in p e r i p h e r a l s t e r o i d o g e n i c cells (adrenal gland and testis), epithelial cells of the loop of Henle in the kidney, and h e p a t o c y t e s (9). A) BZD b i n d i n g sites on GABA A receptors An interaction of a n x i o l y t i c BZDs with GABAA r e c e p t o r s was first p r o p o s e d in 1975 as a result of studies on the mode of action of these drugs (i). The d e m o n s t r a t i o n of such an interaction has made p o s s i b l e many of the recent advances in our u n d e r s t a n d i n g of the structure and function of GABA A receptors (1-7). Studies of r e c o m b i n a n t GABA A receptors expressed in a kidney tumor cell line after t r a n s f e c t i o n with a combination of ~, B, 7 subunit cDNAs have shown that there are actually at least two d i f f e r e n t a l l o s t e r i c m o d u l a t o r y sites for BZDs in the e x t r a c e l l u l a r domain of the GABA A receptor (Table I). One site requires the presence in the r e c e p t o r complex of specific ~ (16) and ~ subunits and is the site at which the anxiolytic BZDs (for example, diazepam), the a n x i o g e n i c B - c a r b o l i n e carboxylate esters (BCs) [for example, B - c a r b o l i n e - 3 - c a r b o x y l i c acid ethyl ester methyl amide (FG 7142)] and the BZD antagonist flumazenil act (4-7). A second site is s t i m u l a t e d by 4'-chlorodiazepam, a convulsant BZD ligand (9), which d o w n - r e g u l a t e s GABAA receptors (17,18) in a m a n n e r that is
Vol. 49, No. 5, 1991
Multiple Actions of DBI
327
dependent on the presence of receptor 7 subunits but resistant to flumazenil inhibition (6).
Binding Protein..qll~-DBI ~
m
~w/(TrN)
s
Cholesterol~ n ~
~,..
",,
".
/
/
aitochondrial "'""~.'.'.~ DB'nd'ngSite / I NeUl~°tra;~om itter
GLIA / rane3aOH~gE~e.~oOneS O ~ THDOC
Receptor
Pergamon Press
MINIREVIEW DIAZEPAM BINDING INHIBITOR (DBI): A PEPTIDE WITH MULTIPLE BIOLOGICAL ACTIONS
Erminio Costa and Alessandro Guidotti F i d i a - G e o r g e t o w n Institute for the N e u r o s c i e n c e s Georgetown University School of M e d i c i n e Washington, D.C. 20007 (Received in final form May 29, 1991) SUMMARY
Diazepam binding inhibitor (DBI) is a 9-kD p o l y p e p t i d e that was first isolated in 1983 from rat brain by m o n i t o r i n g its ability to displace diazepam from the b e n z o d i a z e p i n e (BZD) recognition site located on the e x t r a c e l l u l a r domain of the type A receptor for 7aminobutyric acid (GABA A receptor) and from the m i t o c h o n d r i a l BZD receptor (MBR) located on the outer m i t o c h o n d r i a l membrane. In brain, DBI and its two major p r o c e s s i n g products [DBI 33-50, or o c t a d e c a n e u r o p e p t i d e (ODN) and DBI 17-50, or t r i a k o n t a t e t r a n e u r o p e p t i d e (TTN)] are unevenly distributed in neurons, with the highest c o n c e n t r a t i o n s of DBI (i0 to 50 ~M) being present in the hypothalamus, amygdala, cerebellum, and discrete areas of the thalamus, hippocampus, and cortex. DBI is also present in specialized glial cells (astroglia and Bergmann glia) and in peripheral tissues. In the periphery, the highest concentration of DBI occurs in cells of the zona glomerulosa and fasciculata of the adrenal cortex and in Leydig cells of the testis; interestingly, these are the same cell types in which MBRs are highly concentrated. Stimulation of MBRs by appropriate ligands (including DBI and TTN) facilitates cholesterol influx into mitochondria and the subsequent formation of pregnenolone, the parent molecule for endogenous steroid production; this facilitation occurs not only in peripheral steroidogenic tissues, but also in glial cells, the steroidogenic cells of the brain. Some of the steroids (pregnenolone sulfate, dehydroepiandrosterone sul fate, 3~-hydroxy-5~-pregnan-20one, and 3~, 21-dihydroxy-5~-pregnan-20-one) produced in brain (neurosteroids) function as potent (with effects in the nanomolar concentration range) positive or negative allosteric modulators of GABA A receptor function. Thus, accumulating evidence suggests that the various neurobiological actions of DBI and its p r o c e s s i n g products may be attributable to the ability of these peptides either to bind to BZD recognition sites associated with GABA A receptors or to bind to glial cell MBRs and modulate the rate and quality of n e u r o s t e r o i d o 0024-3205/91 $3.00 +.00 Copyright (c) 1991 Pergamon Press plc
326
Multiple Actions of DBI
Vol. 49, No. 5, 1991
genesis. The n e u r o b i o l o g i c a l effects of DBI and its p r o c e s s i n g products in p h y s i o l o g i c a l and p a t h o l o g i c a l conditions (hepatic encephlopaty, depression, panic) c o n c e n t r a t i o n s may therefore be e x p l a i n e d by interactions with d i f f e r e n t types of BZD r e c o g n i t i o n site. In addition, recent reports that DBI and some of its fragments inhibit (in nanomolar concentrations) glucoseinduced insulin release from p a n c r e a t i c islets and bind a c y l - c o e n z y m e A with high affinity support the h y p o t h e s i s that DBI is a precursor of b i o l o g i c a l l y active peptides with m u l t i p l e actions in the brain and in p e r i p h e r a l tissues.
This r e v i e w presents our current u n d e r s t a n d i n g of the p o s s i b l e b i o l o g i c a l actions of the peptide d i a z e p a m b i n d i n g inhibitor (DBI). DBI and its natural p r o c e s s i n g products are p a r t i c u l a r l y abundant in brain and certain peripheral organs and have a d i s t r i b u t i o n p a t t e r n that corresponds to that of b e n z o d i a z e p i n e (BZD) r e c o g n i t i o n sites. These peptides represent a family of p u t a t i v e endogenous ligands for the BZD r e c o g n i t i o n sites located at the a l l o s t e r i c center of the type A r e c e p t o r for 7a m i n o b u t y r i c acid (GABA A receptor) and on m i t o c h o n d r i a of brain and p e r i p h e r a l tissues. BENZODIAZEPINE RECOGNITION SITES In brain, there are two types of BZD r e c o g n i t i o n site (Fig. i), each thought to be a s s o c i a t e d with a specific protein. The "central" BZD r e c o g n i t i o n site is p r e v a l e n t in neurons and is associated with GABA A receptors which are ligand-gated Cl channels (1-7). The "peripheral" BZD r e c o g n i t i o n site (8,9) is p r e v a l e n t in glia (9,10) and is p r e f e r e n t i a l l y a s s o c i a t e d with a protein in the outer mitochondrial membrane (11,12) that regulates the access of cholesterol to the inner m i t o c h o n d r i a l membrane, where a specific type of c y t o c h r o m e P450 catalyzes the c o n v e r s i o n of cholesterol into p r e g n e n o l o n e (13-15). In addition to brain (9,10,12,15) the latter BZD r e c o g n i t i o n site -- here termed the m i t o c h o n d r i a l BZD receptor (MBR) -- is also abundant in p e r i p h e r a l s t e r o i d o g e n i c cells (adrenal gland and testis), epithelial cells of the loop of Henle in the kidney, and h e p a t o c y t e s (9). A) BZD b i n d i n g sites on GABA A receptors An interaction of a n x i o l y t i c BZDs with GABAA r e c e p t o r s was first p r o p o s e d in 1975 as a result of studies on the mode of action of these drugs (i). The d e m o n s t r a t i o n of such an interaction has made p o s s i b l e many of the recent advances in our u n d e r s t a n d i n g of the structure and function of GABA A receptors (1-7). Studies of r e c o m b i n a n t GABA A receptors expressed in a kidney tumor cell line after t r a n s f e c t i o n with a combination of ~, B, 7 subunit cDNAs have shown that there are actually at least two d i f f e r e n t a l l o s t e r i c m o d u l a t o r y sites for BZDs in the e x t r a c e l l u l a r domain of the GABA A receptor (Table I). One site requires the presence in the r e c e p t o r complex of specific ~ (16) and ~ subunits and is the site at which the anxiolytic BZDs (for example, diazepam), the a n x i o g e n i c B - c a r b o l i n e carboxylate esters (BCs) [for example, B - c a r b o l i n e - 3 - c a r b o x y l i c acid ethyl ester methyl amide (FG 7142)] and the BZD antagonist flumazenil act (4-7). A second site is s t i m u l a t e d by 4'-chlorodiazepam, a convulsant BZD ligand (9), which d o w n - r e g u l a t e s GABAA receptors (17,18) in a m a n n e r that is
Vol. 49, No. 5, 1991
Multiple Actions of DBI
327
dependent on the presence of receptor 7 subunits but resistant to flumazenil inhibition (6).
Binding Protein..qll~-DBI ~
m
~w/(TrN)
s
Cholesterol~ n ~
~,..
",,
".
/
/
aitochondrial "'""~.'.'.~ DB'nd'ngSite / I NeUl~°tra;~om itter
GLIA / rane3aOH~gE~e.~oOneS O ~ THDOC
Receptor
