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Vol. 51, Issue 4, 629-650, December 1999
Rappaport Family Institute for Research in the Medical Sciences, Haifa, Israel (M.G.); Department of Pharmacology (M.G., I.B., R.S., L.V.) and Laboratory for Research in Anesthesia, Pain and Neuroscience (Y.K.), The Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel; Department of Anesthesiology, Ha'Emek Medical Center, Afula, Israel (Y.K.); Endocrinology Institute, Tel Aviv Sourasky Medical Center, Tel Aviv, Israel (G.W.); Geha Mental Health Center, Felsenstein Medical Research Center, Rabin Medical Center, Beilinson Campus, Petah Tiqva, Israel (A.W.); and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel (A.W.)
I. Introduction
II. The Peripheral Benzodiazepine Receptor: Molecular Identity
III. Endogenous Ligands for Peripheral Benzodiazepine Receptors
IV. Role of Peripheral Benzodiazepine Receptors in Cellular Respiration
V. Peripheral Benzodiazepine Receptors in Endocrine System and Steroid Regulation
A. Regulation of Steroidogenesis by Peripheral Benzodiazepine Receptors
B. Peripheral Benzodiazepine Receptors in Female Genital Tract
1. Ovary.
2. Uterus and Oviduct.
3. Adrenal.
4. Mammary Gland.
C. Peripheral Benzodiazepine Receptors in Male Genital Tract
VI. Peripheral Benzodiazepine Receptors under Pathological Conditions
A. Stress Response
1. Relevance of Peripheral Benzodiazepine Receptors to Stress.
2. Peripheral Benzodiazepine Receptor Ligands and Behavior.
3. Involvement of Peripheral Benzodiazepine Receptors in Acute Stress.
4. Involvement of Peripheral Benzodiazepine Receptors in Chronic Stress.
5. Putative Mechanisms Involved in Peripheral Benzodiazepine Receptor Response to Stress.
B. Anxiety Disorders
1. Generalized Anxiety Disorder.
2. Panic Disorder.
3. Generalized Social Phobia.
4. Post-Traumatic Stress Disorder (PTSD).
5. Obsessive-Compulsive Disorder (OCD).
C. Mood Disorders
D. Neurodegenerative Disorders
1. Parkinson's Disease.
2. Alzheimer's Disease.
E. Peripheral Benzodiazepine Receptors and Brain Damage
1. Peripheral Benzodiazepine Receptors and Neurotoxic Brain Damage.
2. Peripheral Benzodiazepine Receptors and Traumatic-Ischemic Brain Damage.
VII. Peripheral Benzodiazepine Receptors in Cancer and in Immune Function
VIII. Summary and Future Prospects
Acknowledgments
References
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I. Introduction |
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Benzodiazepines
(BZs)2 are used
clinically as muscle relaxants, anticonvulsants, anxiolytics, and
sedative-hypnotics. These effects are mediated primarily via the
central BZ receptors (CBRs) located in the central nervous system (CNS;
Braestrup and Squires, 1977
; Möhler and Okada, 1977; Tallman et
al., 1980). CBRs are part of a macromolecular complex that also
contains a 
-aminobutyric acid (GABA) receptor site and a chloride
ion channel (DeLorey and Olsen, 1992). This complex has been purified,
its cDNA has been cloned, and its functional receptors have been
expressed in Xenopus oocytes (Schofield et al., 1987). The
GABA/BZ receptor complex is a hetero-oligomer composed of five
subunits: 
-, 
-, -, 
-, and 
-polypeptides. BZs bind to the
-subunit and facilitate the inhibitory effect obtained by GABA
(DeLorey and Olsen, 1992).
BZs also bind to other receptors, located mainly in peripheral
tissues and glial cells in the brain (Fig.
1), called peripheral BZ receptors (PBRs;
Verma and Snyder, 1989; Gavish et al., 1992). In rats, PBRs differ from
CBRs in their drug specificity: for example, the BZ clonazepam binds to
CBRs with high affinity, whereas the BZ Ro 5-4864 (4'-chlorodiazepam)
as well as the non-BZ ligand PK 11195 (an isoquinoline carboxamide
derivative) bind to CBRs with negligible affinity. The reverse is true
with regard to PBRs (Benavides et al., 1983a,b). Imidazopyridines such
as alpidem bind with high affinity to both CBRs and PBRs (Langer and
Arbilla, 1988). FGIN-1 (2-aryl-3-indoleacetamide) binds with high
affinity to PBRs but not to CBRs (Romeo et al., 1992). The focus of the current review is the PBR.
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The binding of PK 11195 and Ro 5-4864 has been studied in various
species (Anholt et al., 1985b; Basile et al., 1986; Cymerman et al.,
1986; Awad and Gavish, 1987, 1991; Parola et al., 1991). The binding of
Ro 5-4864 has been found to differ among species (Awad and Gavish,
1987; Table 1). The order of potency of
Ro 5-4864 in displacement of PK 11195 binding is rat = guinea
pig > cat = dog > rabbit > calf. In calf, Ro
5-4864 is three orders of magnitude less potent than PK 11195 in
binding to membranal PBRs (Basile et al., 1986; Awad and Gavish, 1987;
Parola et al., 1991). In humans, it was found that PK 11195 is about
two orders of magnitude more potent than Ro 5-4864 in binding to PBRs
(Awad and Gavish, 1991). PBR species differences obtained in the
membrane-bound state are retained in the soluble state and after
purification and probably are attributable to variations in the
molecular structure of PBRs rather than to differences in the membrane
environment (Awad and Gavish, 1989b; Sprengel et al., 1989; Parola et
al., 1991; Riond et al., 1991).
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Protoporphyrin IX labels PBRs with nanomolar affinity and has
been suggested to be an endogenous ligand for PBRs (Snyder et al.,
1987). Subcellular studies have indicated that PBRs are mitochondrial in the rat brain (Basile and Skolnick, 1986), in the rat adrenal (Anholt et al., 1986b), and in the rat testis, lung, kidney, heart, skeletal muscle, and liver (Antkiewicz-Michaluk et al., 1988a). The use
of confocal microscopy further confirmed the abundant mitochondrial
localization of PBRs (Garnier et al., 1993). Other groups detected PBR
binding in nuclei (O'Beirne et al., 1990; Hardwick et al., 1999),
Golgi apparatus, lysosomes, peroxisomes (O'Beirne et al., 1990), and
plasma membrane (Oke et al., 1992). It has also been reported that PBRs
are present in mature human red blood cells, which lack mitochondria
(Olson et al., 1988a). Titration of isolated rat adrenal mitochondria
with digitonin demonstrated that PBRs are typically located on the
outer membrane of the mitochondria (Anholt et al., 1986b). Another
study showed that PBRs are also located on the inner membrane of the
rat lung mitochondria (Mukherjee and Das, 1989). Hence, although PBRs
are located mainly on the outer membrane of the mitochondria, other localizations are also possible.
Purification of PBRs contributed to an understanding of their function.
The first step for purification requires a suitable detergent that will
not destroy the binding activity. PBRs have been solubilized, using
various detergents, without major impairment of binding activity
(Martini et al., 1983; Benavides et al., 1985; Doble et al., 1985;
Gavish and Fares, 1985; Anholt et al., 1986a; Awad and Gavish, 1989a).
Antkiewicz-Michaluk et al. (1988b) purified one subunit (of 18-kDa
molecular mass) of the PBR from the rat adrenal through
utilization of photoaffinity labeling of this protein subunit with the
isoquinoline carboxamide derivative [3H]PK
14105 and subsequent solubilization of the membranes with the detergent
digitonin, followed by purification with ion-exchange chromatography
and reverse-phase HPLC.
PBRs are composed of at least three subunits: the binding site for
isoquinolines, with a molecular mass of 18 kDa; the voltage-dependent anion channel (VDAC), with a molecular mass of 32 kDa, which binds BZs;
and the adenine nucleotide carrier, with a molecular mass of 30 kDa,
which also binds BZs (McEnery et al., 1992). Although isoquinolines can
bind to the 18-kDa subunit alone, PBR-specific BZs require the
interaction of all three subunits for binding (Garnier et al., 1994b).
This complex is located on the outer and inner mitochondrial membrane
contact sites (Papadopoulos et al., 1994).
The full-length cDNA for the 18-kDa subunit has been cloned from rat
(Sprengel et al., 1989), humans (Riond et al., 1991), bovine (Parola et
al., 1991), and mouse (Garnier et al., 1994b). Many functions have been
attributed to PBRs, including a role in cell proliferation (Wang et
al., 1984; Carmel et al., 1999), steroidogenesis (Papadopoulos et al.,
1990; Papadopoulos, 1993; Kelly-Hershkovitz et al., 1998), calcium flow
(Cantor et al., 1984; Python et al., 1993), cellular respiration
(Hirsch et al., 1989), cellular immunity (Lenfant et al., 1986), and
malignancy (Starosta-Rubinstein et al., 1987; Katz et al., 1990b,c;
Alenfall and Batra, 1995).
PBRs are found in many tissues. In the current review, we survey the available data on the location, molecular characteristics, functions, and clinical implications of PBRs in these various tissues.
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II. The Peripheral Benzodiazepine Receptor: Molecular Identity |
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The PBRs were first described (more than 20 years ago) as BZ
binding sites in non-neuronal tissue (Braestrup and Squires, 1977).
Nevertheless, PBRs are found not only in peripheral tissue but also in
non-neuronal brain tissue (Fig. 1; Verma and Snyder, 1989; Gavish et
al., 1992). Interestingly, PBR densities are high in steroidogenic
tissues, in particular in the adrenal gland (Fig. 1). PBR densities in
tissues such as the kidney, heart, testis, ovary, and uterus are
approximately five times as low as that in the adrenal but are still
one order of magnitude higher than in other tissues such as the brain
(Fig. 1).
The PBR appears to be a heteromeric complex of at least three different
subunits, including an isoquinoline binding subunit (18 kDa), a VDAC
(32 kDa), and an adenine nucleotide carrier (30 kDa; Snyder et al.,
1990; McEnery et al., 1992; Garnier et al., 1994b). Papadopoulos et al.
(1997a) reported that isoquinolines that bind specifically to PBRs
interact specifically with the 18-kDa subunit, whereas PBR-specific BZ
ligands bind to a site consisting of both VDAC and the 18-kDa PBR
subunits (Snyder et al., 1990). In studies on the topography of PBRs in
the MA-10 Leydig cell mitochondrial membrane, Papadopoulos et al.
(1994, 1997a) also showed that the 18-kDa PBR protein is organized in clusters of four to six molecules associated with one VDAC molecule in
such a way as to favor the formation of what they called "single pores".
The cDNA for the 850-nucleotide PBR mRNA has been cloned for a number
of species, including humans (Riond et al., 1991), rat (Sprengel et
al., 1989; Krueger et al., 1990), mouse (Garnier et al., 1994b), and
cows (Parola et al., 1991). More recently, the genes for two of these
species [humans (Lin et al., 1993) and rat (Casalotti et al., 1992)]
have also been partially cloned and characterized. This approximately
13-kbp gene (Lin et al., 1993) was found as a single copy in the human
genome and located on chromosome 22 in the 22q13.31 band (Riond et al.,
1991; Chang et al., 1992). Figure 2 shows
that this gene is composed of four exons, with the first exon and half
of the fourth exon being untranslated. This gene has one transcription
initiation site (Casalotti et al., 1992; Lin et al., 1993). Lin et al.
(1993) also reported on an alternatively spliced PBR mRNA found in
human tissue. In that case, compared with the full-size form, about
10-fold more of the smaller PBR mRNA form, in which its exon 2 sequences had been spliced out, was present in a congenital lipoid
adrenal hyperplasia patient. Because this shorter mRNA is unable to
translate PBR protein and because it is present in humans at greater
levels than the full-length PBR mRNA species in some human conditions, it will be important to establish any novel function arising from any
of its resultant translated products. It is of interest that the PBR
promoter in both rat and humans does not contain a TATAA box but does
contain multiple Sp1 boxes (Casalotti et al., 1992; Lin et al., 1993).
This is also an indication that the product of this gene has a
"housekeeping gene" function. To date, no additional transcription
factor regulatory sites have been identified in this promoter.
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The protein domain of exon 2 has been linked to the isoquinoline
binding site and part of the PBR-specific BZ binding site (Farges et
al., 1994; Papadopoulos et al., 1997a). Furthermore, a cholesterol
recognition/interaction site has been characterized at the carboxyl end
of the protein (Li and Papadopoulos, 1998). The 18-kDa PBR subunit is
highly conserved between the four species cloned (Fig. 2; Table 1).
When querying GenBank for other homologies throughout evolution, we
also found that this gene is quite well conserved (59% over exons
2-4), even in bacteria (Fig. 2). Lummis et al. (1991) reported the
presence of BZ binding sites in the bacterium Escherichia
coli. On an initial pharmacological characterization of these
bacterial receptors, they claimed that these BZ receptors are distinct
from both mammalian PBRs and CBRs and appear to be more closely related
to those of insects. It may be that the gene sequence we found at
GenBank is related to the receptor reported by Lummis and colleagues.
If this is the case, its nucleotide sequence may be more related to PBR
than its pharmacology. Furthermore, the fact that BZ receptors are so
well conserved throughout evolution may imply that the functions of
this gene must be fundamental for the cell.
Although numerous molecular studies have characterized the PBR
proteins, much still needs to be undertaken to determine their basic
(cellular) functions. A detailed discussion of their putative functions
follows. Here, we mention a few molecular approaches that have been
attempted. Efforts to generate a PBR-negative knockout mouse did not
work out in that the animals died at an early embryonic stage
(Papadopoulos et al., 1997b). Unfortunately, the specific reason for
the early death of these fetal mice was not resolved. Nevertheless,
this result does reinforce the notion that PBRs are involved in a basic
cellular function that is necessary during murine fetal development.
Two studies examined the effects of knockout of the 18-kDa PBR subunit
gene (Papadopoulos et al., 1997b) or of 18-kDa PBR mRNA
(Kelly-Hershkovitz et al., 1998) in cultured Leydig cells. Although
both studies suggested the direct involvement of PBRs in Leydig cell
steroidogenesis, the latter study ruled out its involvement in Leydig
cell proliferation. At the level of steroidogenesis, Papadopoulos et
al. (1997b) suggested that their study indicated PBR as the
rate-limiting step in cholesterol transport into the mitochondria. This
was not found in the antisense knockout study, which suggested that the
involvement of PBR in steroidogenesis occurs later in the pathway. The
reasons for the differences between the studies may reflect the
different cell lines used (MA-10 versus R2C) or the different knockout
approaches applied. Specifically, although PBR gene knockout
totally abrogated PBR 18-kDa expression (Papadopoulos et al., 1997b),
antisense RNA knockout in cultured Leydig cells resulted in only an
approximately 50% reduction in basal PBR ligand binding
(Kelly-Hershkovitz et al., 1998). Hence, observations generated from
the antisense knockout approach, although suggestive, are not yet
conclusive. Ultimately, the resolution of these apparently conflicting
data and how this relates to the fundamental function of this gene
product that could result in the abrogation of early fetal mouse
development remains an open question.
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III. Endogenous Ligands for Peripheral Benzodiazepine Receptors |
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The PBR is designated a receptor because the binding of specific
ligands to it has been implicated in several physiological functions,
including steroidogenesis (Papadopoulos et al., 1990), cell growth and
differentiation (Wang et al., 1984), chemotaxis (Ruff et al., 1985),
and mitochondrial respiratory control (Hirsch et al., 1989). However,
the quest for an endogenous ligand for this receptor is still under
way. When Beaumont et al. (1983) assayed ultrafiltrates of serum and
urine collected from uremic patients, as well as from normal plasma,
they isolated BZ-like molecules that inhibited
[3H]Ro 5-4864 binding to PBR. The affinity of
these molecules for PBR was 125-fold greater than that for the CBR. The
molecular mass ranged between 0.5 and 1.0 kDa, but the exact
identity of the inhibitor was not specified.
Mantione et al. (1988) reported the presence of both high (molecular
mass > 10 kDa) and low (molecular mass < 2 kDa) materials, isolated in crude form from rat antral stomach, that inhibited the
specific binding of [3H]Ro 5-4864 to PBRs. The
high-molecular mass material was further purified to yield a 16 kDa
protein called anthralin. This protein also inhibited the
binding of [3H]nitrendipine to the
dihydropyridine Ca2+ channel. The protein was
heat and pronase sensitive and partially sensitive to trypsin, whereas
its activity was enhanced by Ca2+ ions in a
concentration-dependent fashion. It was also found that the protein had
enzymatic properties similar to the phospholipase A2 isoenzyme. These authors proposed that
anthralin may be an endogenous ligand of combined interaction,
with both PBR and dihydropyridine binding sites (Mantione et al.,
1988). This conclusion is consistent with pharmacological and
electrophysiological observations of functional coupling between the
two receptor systems (Mestre et al., 1984).
Corda et al. (1984) reported the isolation and purification to
homogeneity of a 104-amino-acid-residue neuropeptide that inhibited diazepam binding to the BZ binding site. Tryptic digestion of this
peptide [termed diazepam-binding inhibitor (DBI)] yielded an
octadecaneuropeptide (ODN) that could compete for
[3H]diazepam binding, elicit proconflict
responses in rodents, and antagonize the anticonflict actions of BZs
(Alho et al., 1985). The proconflict effect of ODN was inhibited by Ro
15-1788 (flumazenil; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazole-(1,5-a)-(1,4)benzodiazepine-3-carboxylate), a CBR-specific ligand (Ferrero et al., 1986b). DBI has been detected in
human brain, and DBI-like immunoreactivity has been found in the
cerebrospinal fluid of human volunteers (Ferrero et al., 1986a). This
immunoreactivity is increased in dementia with normal-pressure hydrocephalus but not in Alzheimer's disease, multi-infarct
dementia, or dementia with Parkinson's disease (Ferrero et al., 1988).
DBI has also been located in peripheral tissues rich in PBRs, such as
adrenal gland, testis, and kidney (Gray et al., 1986).
Papadopoulos and Brown (1995) have shown that the PBR is the key
element in the regulation of cholesterol transport from intracellular stores to the inner mitochondrial membrane and that the presence of DBI
is vital for steroidogenesis and stimulated cholesterol transport
(Boujrad et al., 1993). In addition, DBI directly promotes loading of
cholesterol to the inner mitochondrial membrane side chain cleavage
cytochrome P-450 (P-450scc) enzyme and thus
starts the metabolism of cholesterol to pregnenolone (the initial step in steroidogenesis). Increased expression of DBI has been found in
brain tumors (astrocytoma, glioblastoma), with the highest levels in
the most neoplastic tumors (Alho et al., 1995). A close association
between DBI and PBRs in ovary further emphasizes their important role
in the regulation of steroid production (Alho et al., 1994). Chemical
cross-linking studies of purified metabolically radiolabeled DBI to
mitochondria of R2C rat Leydig tumor cells provided direct evidence
that DBI specifically binds to the 18-kDa PBR protein and plays a key
role in maintaining R2C constitutive steroidogenesis (Garnier et al.,
1994a). Furthermore, the close localization of PBRs and DBI at the
outer mitochondrial membrane has also been demonstrated by
immunoelectron microscopy (Schultz et al., 1992). It can be concluded
that DBI is an important candidate for an endogenous ligand to PBRs.
Other candidates for endogenous ligands to PBRs are porphyrins.
Porphyrins are known to modulate enzymatic activity of several enzymes,
including tryptophan pyrrolase, guanylate cyclase, and glutathione-5-transferase, and are involved with several mitochondrial proteins (Verma and Snyder, 1988). The major physiological porphyrins, protoporphyrin IX and heme, display inhibition constant
(Ki) values of 20 to 50 nM for the PBR
and 1000 times less at the CBR. This receptor specificity implies a
physiological role for the PBR. Iron substitution of hemin with zinc
results in an 8- to 10-fold reduction in the affinity for PBRs.
Additionally, tin and cobalt replacement produces derivatives that are
1000 times less affinitive than those with iron. Cytochromes, which
contain porphyrins, are selectively associated with the inner
mitochondrial membrane and are involved in steroidogenesis. The
striking association between steroid-forming tissues such as the
adrenal gland and testis, which exhibit high PBR and porphyrin levels,
also suggests a physiological role for the interaction of porphyrins
with PBRs (Verma et al., 1987).
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IV. Role of Peripheral Benzodiazepine Receptors in Cellular Respiration |
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The PBRs are found primarily on the outer mitochondrial membrane
(O'Beirne and Williams, 1988; Anholt et al., 1986b). Adrenal mitochondria possess the highest density of PBR sites, kidney mitochondria have an intermediate number, and liver has very low levels
(Le Fur et al., 1983; De Souza et al., 1985). The parallels between the
differential binding affinities of various PBR-specific versus
CBR-specific ligands in these three tissues concerning their effect on
mitochondrial respiration in the same tissues have led a few
investigators to propose a role for PBRs in the regulation of the
mitochondrial respiratory chain (Hirsch et al., 1989;
Moreno-Sánchez et al., 1991). Hirsch et al. (1989) interpreted their data to suggest that the PBR ligands they used increased state IV
and decreased state III respiration rates, which resulted in a
significant decrease in the respiratory control ratio. Another group
suggested that part of this inhibition of respiration was a specific
effect, whereas part was nonspecific (Moreno-Sánchez et al.,
1991). In particular, these authors claimed that the inhibition of
reduced nicotinamide-adenine dinucleotide oxidase and the hydrolysis of
ATP by PBR ligands was not specific, whereas oxidizable substrate transport was specifically inhibited by the PBR ligand AHN 086 (Moreno-Sánchez et al., 1991). In contrast, Zisterer et al.
(1992) suggested that the results of those previous studies were due to
nonspecific effects of PBR ligands and were not mediated through PBRs.
It seems that at the present, no study definitively allows us to
conclude how PBRs are actually involved in cellular respiration. If it
should turn out that the PBR-specific ligands do affect mitochondrial
respiration independent of PBRs, it would be essential to determine how
they do this. It may be that some "alternate" peripheral-type BZ
binding sites contribute to other functions or actions previously
attributed to PBRs.
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V. Peripheral Benzodiazepine Receptors in Endocrine System and Steroid Regulation |
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The biosynthesis of steroids in all steroidogenic tissues begins
with the enzymatic conversion of the precursor cholesterol to form
pregnenolone. This reaction is catalyzed by the enzyme P-450scc, which is located on the matrix side of
the inner mitochondrial membrane. The rate-limiting step in this
process is the transport of cholesterol from the cellular stores across
the aqueous intermembrane space of the mitochondria to the inner
mitochondrial membrane and the P-450scc. PBRs
have been suggested to play a major role in this mitochondrial
cholesterol transport (Krueger and Papadopoulos, 1990).
It has been reported that the steroidogenic acute regulatory protein
(StAR) is involved in the acute trophic hormone regulation of steroid
synthesis (Miller, 1995; Stocco and Clark, 1996a,b); however, the
question as to how the transfer of cholesterol occurs remains open, and
it is very likely that other proteins are involved in this process. In
fact, a recent report suggests that PBR and StAR work together in
cholesterol transport into the mitochondria (Sridaran et al., 1999).
Two important observations suggest that PBRs are likely to play a role
in steroidogenesis. First, PBRs are found primarily on the outer
mitochondrial membrane (Anholt et al., 1986b; Mukherjee and Das, 1989;
O'Beirne et al., 1990); second, PBRs are extremely abundant in
steroidogenic endocrine tissues (Benavides et al., 1983a; De Souza et
al., 1985).
A. Regulation of Steroidogenesis by Peripheral Benzodiazepine Receptors
It has been demonstrated that PBR-specific ligands indeed regulate
mitochondrial steroidogenesis. DBI, a potential endogenous ligand for
PBRs with a molecular mass of 10 kDa regulates steroidogenesis activated by corticotropin (ACTH) and luteinizing hormone via binding
to PBRs and thus control mitochondrial cholesterol transport (Hall,
1991; Papadopoulos et al., 1991a,b; Brown et al., 1992). The addition
of DBI to a cholesterol side chain cleavage reconstituted enzyme system
was able to stimulate the conversion of cholesterol into pregnenolone
(Brown and Hall, 1991), whereas the depletion of DBI from Leydig tumor
cells resulted in a loss of trophic hormone-stimulated steroid
production in these cells (Boujrad et al., 1993). Flunitrazepam, a
partial BZ agonist for PBRs, inhibits hormone-stimulated steroid biosynthesis in Y-1 adrenocortical and MA-10 Leydig cells, via its
binding to PBRs (Papadopoulos et al., 1991c). Ligands for the PBR
enhance steroid production in Y-1 adrenocortical and MA-10 Leydig cell
lines. These effects are attributable to their binding to PBRs (Mukhin
et al., 1989; Papadopoulos et al., 1990). Ligands with high affinity to
PBRs stimulate pregnenolone synthesis in brain mitochondrial
preparation (Guarneri et al., 1992; Papadopoulos et al., 1992; McCauley
et al., 1995). It has been suggested that PBRs play a role in the
translocation of cholesterol from the outer to the inner mitochondrial
membrane, which is the rate-limiting step in steroidogenesis (Krueger
and Papadopoulos, 1990; Papadopoulos et al., 1990). The above findings
demonstrate that PBRs play a significant role in steroid biosynthesis
in various tissues.
B. Peripheral Benzodiazepine Receptors in Female Genital Tract
PBRs have been characterized in the genital tract of mature female
rats (Fares et al., 1987).
1. Ovary.
By means of immunocytochemical, in situ
hybridization, and autoradiographic methods, it has been demonstrated
that the PBR and its putative endogenous ligand (DBI) are present in
all ovarian steroid-secreting cells (Toranzo et al., 1994). In the
immature rat ovary, PBR density increases with age (Fares et al.,
1987), but hypophysectomy reduced this PBR density by 68% (Bar-Ami et al., 1989; Table 2). This effect was
abolished either by 2 days' treatment with pregnant mare serum
gonadotropin (PMSG) or 4 days' treatment with diethylstilbestrol
(DES), a synthetic derivative of estradiol (Bar-Ami et al., 1989; Table
2). [3H]PK 11195 binding in the ovary of
hypophysectomized rats was increased by 3.4- and 4-fold after PMSG or
DES treatment, respectively (Bar-Ami et al., 1989; Table 2). In the
intact immature rat, PMSG or DES also significantly increases PBR
density (Fares et al., 1987; Table 2). PMSG, which contains both
follicle-stimulating and luteinizing hormone activity (Moore and Ward,
1980), acts directly on the ovary to induce multiple follicular growth
and development in the immature rat and elicits a significant increase in ovarian estradiol biosynthesis (Braw and Tsafriri, 1980).
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). Long-term (10-day) treatment
with testosterone and progesterone significantly reduced PBR density
(40 and 14%, respectively), whereas a 2-fold increase in PBR binding
capacity was obtained after long-term treatment with DES (Bar-Ami et
al., 1994). These results indicate that PBR density in the ovary is
altered by exogenously administrated steroids that are usually
biosynthesized in the ovary and that PBRs may play an important role in
the differentiation and development of the ovary.
In the adult rat, ovarian PBR density increases to a maximum on the day
of proestrus (Fares et al., 1988). An increase in rat ovarian PBR
density has been observed during pregnancy (Weizman et al., 1997a;
Sridaran et al., 1999). Thus, it seems that the hormonal changes
occurring during the estrous cycle and pregnancy play a role in the
regulation of ovarian PBR.
2. Uterus and Oviduct.
Autoradiographic studies have shown
high PBR density in the epithelium and glands of the uterus and lower
PBR density in uterine smooth muscle (Verma and Snyder, 1989). In the
immature rat, PBR density in the oviduct and uterus increases with age
(Fares et al., 1987). In a cycling adult rat, PBR density in both
organs increases to a maximal value on the day of proestrus and has
been found to correlate with the increase in serum estradiol levels throughout the 4-day cycle (Fares et al., 1988). These data suggest that PBR density in the uterus and oviduct is under hormonal regulation.
; Table 2).
[3H]PK 11195 binding increased 3.6- and
2.9-fold in DES- and PMSG-treated uterus, respectively, compared with
untreated hypophysectomized rats (Bar-Ami et al., 1989; Table 2). DES
or PMSG treatment of intact immature rats increased uterine PBR density
by approximately 2-fold but did not affect renal PBR density (Fares et
al., 1987; Table 2).
Short-term (4-day) treatment with testosterone, but not with
progesterone, increased PBR density in the oviduct and uterus of
immature rats by 2.0- and 1.4-fold, respectively, whereas long-term (10-day) treatment with testosterone or progesterone resulted in a
significant decrease (>35%) in PBR density in the oviduct. Uterine
PBR densities were significantly reduced (25%) after long-term treatment with testosterone but not with progesterone. Short-term but
not long-term administration of testosterone was also associated with
significant increases in estradiol levels (Bar-Ami et al., 1994). Thus,
we suggest that the increase in PBR density in DES and in short-term
testosterone treatment could be at least partially mediated by
estrogenic action on the uterus and oviduct. Renal PBR binding was not
affected by these endocrinological manipulations, perhaps due to the
fact that the kidney, in contrast to the ovary and uterus, is not a
target for gonadotropins and estradiol. These latter results suggest
that PBRs may play an important role in the differentiation and
development of the uterus and oviduct.
3. Adrenal.
The highest concentration of PBRs in peripheral
tissues is found in the adrenal tissue (De Souza et al., 1985; Fig. 1).
Autoradiographic studies have shown PBRs to be present in the adrenal
cortex and absent from the adrenal medulla (Benavides et al., 1983a; De
Souza et al., 1985). Immunostaining and confocal microscopy on adrenal PBR by Oke et al. (1992) confirmed these findings. The adrenal medulla,
however, appears to be enriched with GABAA
receptors that possess PBRs (Kitayama et al., 1989, 1990). The ontogeny of both adrenal mitochondrial PBR binding density and immunoreactivity directly parallels that of the onset of steroidogenesis in the adrenal
cortex (Zilz et al., 1999). This is consistent with the notion that
PBRs are a prerequisite for adrenocortical steroidogenesis (Zilz et
al., 1999).
; Bar-Ami et al., 1989; Table 2). This effect is
manifested mainly in the zona fasciculata and zona reticularis, whereas
no effect has been noted in zona glomerulosa density (Anholt et al.,
1985a). Hormone replacement by i.p. injections of ACTH resulted in a
1.7-fold increase in PBR density compared with that in intact rats,
whereas hormone replacement by PMSG and DES failed to restore adrenal
PBR density (Fares et al., 1989; Table 2).
In hypophysectomized rats, the amount of DBI-like immunoreactivity in
adrenal glands decreased dramatically (80%), although the
administration of ACTH reduced this decrease, and there was a positive
correlation between the amount of adrenal DBI-like immunoreactivity and
plasma corticosterone concentrations (Massotti et al., 1991). These
findings suggest that de novo synthesis of DBI in the adrenal could be
an important factor in the mediation of ACTH-induced steroidogenesis.
Various PBR-specific ligands may alter steroidogenic activity in the
adrenal cortex. The effect of these ligands can be seen over the entire
hypothalamic-pituitary-adrenal (HPA) axis. Thus, Ro 5-4864 directly
stimulates the release of corticotropin-releasing hormone (CRH) but not
ACTH, whereas PK 11195 directly stimulates the secretion of ACTH
(Calogero et al., 1990). In various models of induced stress in rats,
the increase in corticosterone and ACTH was attenuated by treatment
with diazepam (Copland and Balfour, 1987). Furthermore, in human
subjects with panic disorder (PD) or generalized anxiety disorder
(GAD), treatment with diazepam causes a significant reduction in ACTH
and cortisol (Roy-Byrne et al., 1991). Diazepam has the capacity to
bind to both CBRs and PBRs (Gobbi et al., 1987). Furthermore, Holloway
et al. (1989) have shown that the CBR-specific ligand midazolam and the
mixed PBR/CBR ligand diazepam inhibit cortisol and aldosterone
synthesis in bovine adrenal cells in vitro. Finally, alprazolam, a
CBR-specific ligand, is capable of suppressing the HPA axis in primates
(Kalogeras et al., 1990). Thus, it seems that PBR-specific ligands
stimulate adrenocortical steroidogenic activity by acting at a number
of loci in the HPA axis and that CBR-specific ligands produce the opposite effect by suppression of the hypothalamic secretion of CRH.
In adrenal glomerulosa cell culture, the addition of various
PBR-specific ligands increased angiotensin II-induced aldosterone secretion. The effect of these various ligands coincides with the rank
of their potential to displace [3H]PK 11195 from adrenal glomerulosa cells, which suggests a receptor-mediated action (Song and Zhou, 1989).
Adrenal PBR density is down-regulated during rat pregnancy and
lactation (Weizman et al., 1997a). In addition, a significant decrease
(31%) in adrenal PBR density was observed after the surgical castration of male rats (Weizman et al., 1992; Table
3). Long-term administration of
testosterone acetate prevented this castration-induced PBR depletion.
Moreover, long-term treatment of male rats with testosterone acetate
induced an increase in adrenal PBR density, whereas cyproterone acetate
induced a decrease in adrenal PBR density (Amiri et al., 1991; Table
3). Cyproterone acetate possesses glucocorticoid activity, which
suppresses ACTH release via negative feedback. The decrease in the
trophic effect of ACTH may lead to a marked loss in adrenal weight and
to depletion of adrenal PBR. Testosterone acetate, however, has a
marked anabolic effect, which is reflected by the augmentation of PBR
density in the adrenal glands (Amiri et al., 1991).
|
). Furthermore, ex vivo treatment of rat adrenocortical cells with
EGb 761 and ginkgolide B for 2 days inhibited the synthesis of the PBR.
In addition, these treatments resulted in 50 and 80% reduction,
respectively, of ACTH-stimulated corticosterone production (Amri et
al., 1997). These latter data provide further evidence that PBRs are
essential for steroidogenesis in the adrenal.
4. Mammary Gland.
Although high PBR density is found in
acinar cells in normal mammary gland and in
7,12-dimethylbenz[a]anthracene-induced tumors (Tong et
al., 1991), PBR density does not change in response to lactation.
Synthesis of DBI has been shown to occur in mammary acinar cells by in
situ hybridization (Tong et al., 1991). Whether PBR density in the
mammary gland is under hormonal regulation and whether PBRs are
involved in mammary cell functions such as casein biosynthesis have yet
to be explored.
C. Peripheral Benzodiazepine Receptors in Male Genital Tract
PBRs have been identified in the testis (Anholt et al., 1985a; De
Souza et al., 1985; Mercer et al., 1992), as well as in the vas
deferens, prostate, seminal vesicle, and Cowper's gland (Katz et al.,
1990a). The hierarchical order of PBR density in the rat genital tract
is expressed as follows: testis > Cowper's gland > prostate > vas deferens > seminal vesicle (Katz et al., 1990a). PBRs have been immunolocalized in the rat testis and were found
to be present exclusively in the interstitial Leydig cells (Garnier et
al., 1993). A 3-fold increase in PBR density was demonstrated in rat
testis during maturation (Mercer et al., 1992). These results might
reflect critical interactions between PBRs and gonadal hormone activity
during development, because testicular PBRs are putatively involved in
testosterone production. PBR density in the rat testis is dependent on
the trophic influence of pituitary hormones: although hypophysectomy
induces depletion of testicular PBR (Anholt et al., 1985a), ligands
specific for PBRs increase the in vitro human chorionic
gonadotropin-stimulated testosterone secretion of decapsulated rat
testis and interstitial cell suspension (Ritta et al., 1987; Ritta and
Calandra, 1989).
Treatment with estradiol decreases rat testicular PBR density (Anholt
et al., 1985a; Gavish et al., 1986a). The effect obtained with
estradiol treatment might be due to its antiandrogenic impact (Gavish
et al., 1986a). Cyproterone acetate induces a decrease in rat
testicular PBR density (Amiri et al., 1991; Table 3). This effect might
be due to its antiandrogenic activity at the target tissue level (Murad
and Haynes, 1985). The reduction in rat testicular PBR density after
testosterone administration might be due to the suppressive effect of
the exogenous androgen on the production of endogenous male hormone via
negative feedback (Amiri et al., 1991).
The increase in adrenal PBR density after testosterone administration
(Amiri et al., 1991) might be due its anabolic effect, whereas the
decrease in adrenal PBR density after cyproterone administration (Amiri
et al., 1991) might be due to its antiandrogenic and/or glucocorticoid
activity, which suppresses pituitary ACTH secretion and can lead to a
loss of adrenal weight (Poyet and Labrie, 1985). The decrease in
adrenal PBR density after testicular removal accords with the trend
toward diminution proposed by Anholt et al. (1985a). PBR density in the
heart was not affected by either cyproterone acetate or testosterone
administration (Amiri et al., 1991; Table 3).
Cowper's glands are accessory sex organs that produce the coagulating
components of semen and are dependent on the trophic influence of
testosterone. Testosterone administration to rats induced an increase
in PBR density in Cowper's gland (Amiri et al., 1991; Table 3). PBR
density in Cowper's gland was decreased (71%) after testicular
removal, whereas testosterone administration prevented this
castration-induced PBR depletion (Weizman et al., 1992). PBR depletion
in this organ after castration, although prevented by testosterone,
indicates that PBRs in this organ are localized on cells whose
integrity depends on the trophic influence of testosterone. No
significant changes were obtained in PBR density in the heart after
testicular removal (Weizman et al., 1992), suggesting that
testosterone-related PBR changes may be restricted to the male genital tract.
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VI. Peripheral Benzodiazepine Receptors under Pathological Conditions |
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A. Stress Response
1. Relevance of Peripheral Benzodiazepine Receptors to
Stress.
PBRs seem to be involved in the regulation of several
major stress systems: 1) the HPA axis, 2) the sympathetic nervous
system, 3) the renin-angiotensin axis, and 4) the neuroendocrine-immune axis. The localization of PBRs on the outer and inner mitochondrial membranes suggests that they may be involved in basic cellular metabolic processes, in addition to their role in the regulation of
biosynthesis of steroids. This assumption is supported by the observation of high PBR density in tissues that display high levels of
cytochrome oxidase activity and use oxidative phosphorylation for their
metabolic needs (Anholt et al., 1986a,b; Hirsch et al., 1989).; Cahard et al., 1994). These
plasma membrane receptors might be involved in neuroendocrine-immune function, in contrast to the mitochondrial receptors, which seem to be
involved in the transfer of specific molecules into the mitochondria
(Berkovich et al., 1993). The involvement of PBRs in the regulation of
basic biological processes that are pertinent to physiological response
to stress, such as cellular metabolism, neuroendocrine activity, and
immune functioning, points to their possible role in the mediation of
the organism's adaptation to stress.
2. Peripheral Benzodiazepine Receptor Ligands and
Behavior.
Behavioral studies have demonstrated that Ro 5-4864 possesses anxiogenic and convulsant properties, whereas PK 11195 has
been found to be anxiolytic and anticonvulsant (Weissman et al., 1983, 1984a,b; Bénavidès et al., 1984; Pellow and File, 1984;
Mizoule et al., 1985; Massotti and Lucantoni, 1986; Hariton et al.,
1988). PK 11195 reverses acute anxiety-induced (forced swimming stress) increase in platelet aggregation, indicating a possible anxiolytic effect of the drug (Serrano et al., 1988). Using Vogel's conflict model of anxiety (thirsty-lick test) in rats, Mizoule et al. (1985) reported that Ro 5-4864 decreased, whereas PK 11195 increased, punished
drinking in a dose-dependent fashion. Furthermore, the punishment
effects of Ro 5-4864 were antagonized by low doses of PK 11195. These
authors showed in this model that Ro 5-4864 and PK 11195 act similarly
to CBR inverse agonists and partial agonists, respectively, in a
conflict situation. It is of note that neither the Ro 5-4864 nor the PK
11195 behavioral effects were blocked by the CBR antagonist Ro 15-1788, suggesting that the action of Ro 5-4864, at least in Vogel's conflict
model of anxiety, is independent of the CBR. Other experimental models support the anxiogenic activity of Ro 5-4864, although some have failed
to antagonize this effect with PK 11195 (Pellow and File, 1984; File
and Pellow, 1985).
). Behavioral
and electroencephalographic studies have shown that Ro 5-4864 induces
convulsions in mice, rats, and guinea pigs and facilitates audiogenic
seizures in genetically susceptible mice in a fashion similar to CBR
inverse agonists and GABA-coupled chloride channel blockers (Weissman
et al., 1983; Bénavidès et al., 1984; Rastogi and Ticku,
1985). The Ro 5-4864-induced convulsions can be antagonized by PK
11195, but not by Ro 15-1788 (Weissman et al., 1983;
Bénavidès et al., 1984). These data support the notion that
Ro 5-4864 convulsant activity is not mediated by CBRs. However, it has
been shown that Ro 5-4864-induced supraspinal convulsions can be
antagonized by the selective CBR antagonist Ro 15-1788 (Massotti and
Lucantoni, 1986). Furthermore, it has been demonstrated that
flurazepam-potentiated muscimol responses in a cuneate nucleus slice
preparation can be attenuated by Ro 5-4864 (Simmonds, 1985). It has
been suggested that anxiogenic and convulsant effects of high doses of
Ro 5-4864 are mediated via a low-affinity binding site at the
GABA-gated chloride ion channel (Gee, 1987), indicating a possible link
between PBRs and CBRs. The relevance of the pharmacological data on PBR
ligands to anxiety and response to stress is still unclear.
3. Involvement of Peripheral Benzodiazepine Receptors in Acute
Stress.
a. Animal Studies.
The sensitivity of PBRs to acute stress
has been demonstrated in various animal models. Drugan et al. (1986)
were the first to show the involvement of PBRs in the physiological
response to stress, using inescapable tail shocks as an animal model of stress. Five shocks induced a significant increase in the density of
renal but not cerebral cortex PBRs. The exposure of mice to acute
maximal electroshock induced rapid up-regulation of PBR density in
mouse cerebral cortex and cardiac ventricles, measured 30 min after the
procedure (Basile et al., 1987).
-endorphin (Noel et al., 1972; Cohen et al., 1981).
Abdominal wall surgery in rats was associated with a significant elevation in the density of cerebral and renal PBRs as well as cerebral
CBRs on days 1 and 3 after the surgical procedure but not on day 7. The
up-regulation of PBRs and CBRs was not accompanied by any alteration in
[3H]quinuclidinyl benzilate binding to
cholinergic muscarinic receptors and monoamine oxidase A and B
activity, indicating that BZ receptors are selectively sensitive to
acute surgical stress. It is noteworthy that the elevation and later
normalization of the BZ receptors correspond to the stages of repair of
surgical wounds (Okun et al., 1988).
Acute single forced swimming stress in rats is associated with a
significant increase in PBR density in cerebral cortex, olfactory bulb,
kidney, platelets, and lymphocytes, as well as a parallel increase in
cerebral CBRs (Novas et al., 1987; Rago et al., 1989a,b). Up-regulation
of forebrain PBR has also been demonstrated in chicks submitted to
acute swimming stress (Martijena et al., 1992; Marin et al., 1996). In
addition, low subsolubilizing concentrations of Triton X-100 caused a
significant increase in measurable forebrain PBR density in the
unstressed chicks but not in the stressed chicks. These results
indicate that both acute stress and Triton X-100 enhance
[3H]Ro 5-4864 accessibility to a pool of
receptors not detected before stress or in the absence of the
detergent. Thus, it appears that the in vitro increase caused by stress
could not be enhanced further by Triton X-100. It was suggested that
the acute stress-induced increase in forebrain PBR could be explained
by recruitment of receptors, via translocation of receptors coming from
another subcellular pool, but not by immediate increase in the receptor protein biosynthesis (Marin et al., 1996). It is of note that a similar
recruitment of CBRs was demonstrated in chicks exposed to acute stress
(Martijena et al., 1992).
Exposing handling-habituated rats to acute noise stress resulted in an
80% increase in PBR density in the cerebral cortex and a decrease of
30 to 40% in CBR density in the same tissue, demonstrating that the
stress-induced change in PBRs is not necessarily associated with a
parallel change in CBRs (Mennini et al., 1989). Noise stress in rats
also induced an increase in adrenal PBR and DBI-like immunoreactivity
in adrenal gland and hippocampus (but not in the cerebral cortex,
striatum, hypothalamus, or cerebellum), as well as in plasma
corticosterone (Ferrarese et al., 1991). It has been suggested that the
stress-induced up-regulation of adrenal DBI and PBR may play a role in
the activation of adrenal release of steroids and/or neurosteroids and
could thereby be responsible for changes in brain BZ receptors and
hippocampal DBI. In turn, the increase in hippocampal DBI in stressed
rats could possibly lead to increased biosynthesis of neurosteroids via
hippocampal glial cells, and these neurosteroids might act as GABA
agonists (Majewska et al., 1986; Deutsch et al., 1992) and have a
negative feedback action, inactivating the HPA axis (Mennini, 1993).
b. Human Studies.
Platelet PBRs were assessed in psychiatric
residents who were completing the written part of the Israeli
Board-certification examination in comparison with age- and sex-matched
controls. The platelet PBR density was significantly elevated
immediately after the examination and showed a trend toward a decrease
to normal range 10 days later. Similar changes were detected in anxiety levels, as measured by the appropriate rating scale (Karp et al., 1989). Thus, as shown in animal studies, acute stress in humans can
lead to an increase in PBRs.
4. Involvement of Peripheral Benzodiazepine Receptors in Chronic
Stress.
a. Animal Studies.
As described above, exposure of rats to
five inescapable tail shocks induced a significant increase in renal
PBR density (Drugan et al., 1986). In contrast, 80 repeated tail shocks
resulted in a significant decrease of PBR density in kidney, heart,
pituitary gland, and cerebral cortex. This biphasic effect of
inescapable shock may indicate differences in PBR response to short-
versus long-term exposure to stress. This assumption is supported by the finding that repeated swimming stress (daily forced swimming for 21 days) was associated with a significant reduction in renal PBR, in
contrast to the up-regulatory effect of single swimming stress on PBR
density in this organ (Burgin et al., 1996). Another model
demonstrating the association between down-regulation of renal PBR and
long-term stress is that of Maudsley reactive rats, which have been
bred for high levels of fearfulness. Maudsley reactive rats exhibited
decreased renal PBR density compared with Maudsley nonreactive rats
(Drugan et al., 1987).
). Food
deprivation is known to cause adrenal hyperfunction (Young et al.,
1987); thus, some of the receptor alterations could be attributable to
the suppressive effect of hypercortisolemia or to the metabolic changes
associated with starvation.
b. Human Studies.
Platelet PBRs were studied in Israeli
soldiers at the beginning of a parachute training course, after 6 days
of preparatory exercises, and after the fourth actual parachute jump.
Reduced (26%) density of PBRs was observed immediately after the
repeated actual jumps, indicating that repeated stress is associated
with a down-regulation of PBR. The PBR half-life is approximately 3 days; thus, the binding measure after the fourth parachute jump (8 days
after the baseline measurement) was due to platelets not present at the
time of the first sampling. It is possible that the repeated
stress-induced receptor changes occur at the precursor cells
(megakaryocytes) and not at the peripheral blood platelets. The
decrease in PBR density was accompanied by a parallel decrease in
systolic blood pressure. Both effects of repeated stress may be related
to an habituation process that involves an adaptive reduction in
sympathetic responsivity (Dar et al., 1991).
The density of platelet PBRs was also assessed in Israeli civilians
exposed to missile attacks during the Persian Gulf War. PBR density was
shown to be 22 and 15% lower before and during the war, respectively,
than at 4 weeks after the end of the war. Thus, it appears that relief
of stress, as assessed by the Hamilton Anxiety Rating Scale, led to an
increase in PBR density, which correlated with the relief of anxiety
(Weizman et al., 1994). It is of note that the human studies showed the
same results as the animal studies (i.e., up-regulation and
down-regulation of PBRs in response to acute and chronic stress, respectively).
5. Putative Mechanisms Involved in Peripheral Benzodiazepine
Receptor Response to Stress.
The exact mechanisms that play a
role in modulation of PBR expression in response to stress are not well
established. PBRs are localized predominantly on the outer
mitochondrial membrane, are very dense in peripheral organs that are
highly activated during stress (heart, kidney, adrenal, and lung), and
have been suggested to be involved in oxidative metabolism and
steroidogenesis. Activation of the PBR during acute exposure to a
stressor may provide neural and metabolic preparation for better coping
with the stress. Furthermore, in some situations, the changes in PBRs are accompanied by a simultaneous increase in CBR activity, a combination that enables efficient psychological and physical fitness
(Drugan and Holmes, 1991). Renal PBR activation during acute stress may
be relevant to stress-induced hypertension via activation of the
renin-angiotensin system (Holmes and Drugan, 1993; Drugan, 1996). In
steroidogenic tissues, PBR ligands can affect the translocation of
cholesterol from the outer to the inner mitochondrial membrane,
although the absolute rate changes are limited (Krueger and
Papadopoulos, 1990). Because stress is accompanied by an increase in
glucocorticoid synthesis and release, it is possible that this receptor
plays a pivotal role in the neuroendocrine response to stress.
). Furthermore, recovery from the stress
is accompanied by restoration of PBR density to a normal range (Weizman
et al., 1994). However, it should be noted that there are insufficient
data regarding the impact of chronic stress on adrenal PBR and that
regarding PBR, only a small fraction of intracellular cholesterol is
available for steroid synthesis; uptake of cholesterol from plasma is
also directly stimulated by ACTH (Krueger and Papadopoulos, 1990). Furthermore, it seems that there are no unequivocal data demonstrating that PBRs are a necessary and sufficient condition for steroidogenesis (Stocco and Clark, 1996a,b; Weizman et al., 1997b), but according to
Zilz et al. (1999), they are an absolute prerequisite. The decrease in
PBRs in steroidogenic and nonsteroidogenic peripheral organs after
chronic stress may reflect a mechanism with an aim to diminish the
sensitivity of these receptors to cope with increased sympathetic tone
and hypercortisolemia. The suppression of renal PBRs during repeated
stress may prevent overactivity of the renin-angiotensin system, which
might lead to stress-induced hypertension (Holmes and Drugan, 1994;
Drugan, 1996). It is noteworthy that the chronic stress-induced
reduction in PBRs is more pronounced in males than in females, in both
rats and humans (Drugan et al., 1991; Weizman et al., 1994); however,
the cause of this gender-related difference is unclear. Thus, the
present suggested mechanisms for the involvement of PBRs in stress are
largely hypothetical.
B. Anxiety Disorders
1. Generalized Anxiety Disorder.
GAD is characterized by
excessive anxiety and worry occurring frequently for at least 6 months.
The anxiety, worry, or physical symptoms cause clinically significant
distress and impairment in social and occupational functioning
(American Psychiatric Association, 1994). Reduced (24%)
[3H]PK 11195 binding to platelet membranes was
observed in GAD patients compared with age- and sex-matched normal
controls. Four weeks of diazepam treatment induced a reduction in the
anxiety, accompanied by an elevation (69%) in PBR density. One week of
diazepam withdrawal resulted in a slight decrease (16%) in PBR density
compared with the level during the drug treatment (Weizman et al.,
1987b). Similar results are reported in lymphocyte PBRs, and the
decrease was restored to a normal value after long-term treatment with
BZs (Ferrarese et al., 1990; Rocca et al., 1991). It has been suggested that endogenous neuropeptides such as DBI may be released in anxiety and down-regulate PBR expression (Ferrarese et al., 1990). It is of
note that Ro 5-4864 possesses proconflict and anxiogenic effects that
can be antagonized by PK 11195 (Mizoule et al., 1985). Furthermore, PK
11195 has been reported to possess anxiolytic properties in psychiatric
patients (Papart et al., 1988; Ansseau et al., 1991).). It seems that the elevation of PBRs induced by chronic diazepam
treatment, as observed in GAD patients, can also be achieved in
unstressed rats. Thus, the stress-induced low PBR levels are not a
prerequisite for the up-regulatory effect of diazepam. The increased
PBR density in the heart after diazepam treatment may reflect a
drug-induced reduction in the sympathetic tone due to the nonspecific
general sedative effect of diazepam treatment (Basile and Skolnick,
1988).
2. Panic Disorder.
PD is characterized by recurrent
unexpected panic attacks, which lead to persistent concern and worry
about having additional attacks and to impairment in daily functioning
(American Psychiatric Association, 1994). Platelet PBR density was
found to be reduced (29%) in PD patients compared with normal controls
(Marazziti et al., 1994), although an earlier study failed to
demonstrate such an alteration in lymphocyte membranes (Rocca et al.,
1991).
3. Generalized Social Phobia.
Patients with GSP have a
persistent and excessive fear of one or more social or performance
situations in which the person is exposed to unfamiliar people or to
possible scrutiny by others. Exposure to the feared social situation
provokes anxiety, which may be similar to situationally predisposed
panic attack (American Psychiatric Association, 1994). An abnormally
low number of platelet PBRs (36%) have been demonstrated in GSP
patients (Johnson et al., 1998).
4. Post-Traumatic Stress Disorder (PTSD).
PTSD is a severe
anxiety disorder that may occur after the exposure to a traumatic
event. The traumatic event is persistently reexperienced, and
persistent avoidance of stimuli associated with the trauma, numbing of
general responsiveness, and symptoms of increased arousal are present
(American Psychiatric Association, 1994). Platelet PBRs were assessed
in drug-free PTSD patients. All were Israeli citizens who had been
exposed to repeated missile attacks during the Persian Gulf War. The
PTSD symptoms persisted for 2 years. Civilians from the same
geographical area who were exposed to the same attacks composed the
control group. Decreased platelet PBR density (62%) was observed in
the PTSD patients compared with the control group. The reduction in PBR
density in the PTSD patients was more prominent in men than in women
(Gavish et al., 1996).
5. Obsessive-Compulsive Disorder (OCD).
OCD is characterized
by recurrent obsessions and compulsions that cause marked distress,
interfere with the patient's daily function, and are commonly
associated with depression (American Psychiatric Association, 1994).
Two studies have demonstrated a lack of alteration in platelet PBR in
OCD patients (Weizman et al., 1993a; Marazziti et al., 1994), whereas
one study has demonstrated decreased lymphocyte membrane PBR density
(25%) in OCD (Rocca et al., 1991).
, 1993; Weizman
and Gavish, 1993).
It is of note that ablation of the amygdaloid central nucleus
attenuates stress-induced changes in PBR density (Holmes and Drugan,
1993), indicating that PBR down-regulation may be specifically associated with anxiety disorders characterized by overactivity of this
nucleus (Johnson and Lydiard, 1995). The receptor down-regulation may
reflect an adaptive response that prevents chronic overproduction of
glucocorticoids in hyperarousal anxiety states. OCD, in contrast to the
other anxiety disorders, is not associated with down-regulation of
PBRs. OCD is not ameliorated by BZs but rather by serotonin reuptake
inhibitors. Furthermore, this disorder is not associated with
overactivation of the amygdala (Johnson and Lydiard, 1995). Thus, it
seems that PBRs are selectively suppressed by anxiety disorders
associated with long-term repeated hyperactivity of the autonomic
nervous system. It is not clear whether the altered PBRs are also
accompanied by functional alterations in CBR activity, yet it is
possible that there is a CBR/PBR/endogenous ligand/steroid/immune network that becomes operational on the exposure of an organism to
stress or anxiety (Dodd and Lenfant, 1993).
C. Mood Disorders
To evaluate the relationship between depression and PBRs, platelet
PBRs were measured in untreated depressed patients in comparison with
normal controls. The density and the dissociation constant of PBRs on
platelets of those patients did not differ from those of controls.
Furthermore, no correlation was found between PBR density and the
severity of the depression in these patients (Weizman et al., 1995). It
seems that depression, like OCD and in contrast to stress and some
anxiety disorders, is not associated with suppression of PBR density.
In accordance with this, the amygdala, which seems to play a role
in stress-induced depletion of PBR, is not overactive in depression or
OCD (Holmes and Drugan, 1993).
It is of note that chronic treatment with antidepressants has been
demonstrated to modulate adrenal and hepatic PBRs in rats, a phenomenon
that seems to be irrelevant to the antidepressive activity of the drugs
but relevant to their effects on the function of these peripheral
organs (Weizman et al., 1993b). Moreover, electroconvulsive therapy
down-regulates platelet PBR density in medication-resistant depressed
patients, a phenomenon that appears to be related to the repeated
stress associated with the treatment and not to its antidepressive
properties (Weizman et al., 1996).
The mood stabilizer carbamazepine has an up-regulatory effect on
platelet PBRs; however, the significance of this activity with regard
to the antiepileptic and/or mood-stabilizing properties of this agent
is unclear (Weizman et al., 1987a).
D. Neurodegenerative Disorders
1. Parkinson's Disease.
Diminished platelet PBR density has
been detected in Parkinson's disease patients (Bonuccelli et al.,
1991 2. Alzheimer's Disease.
Postmortem brain studies have
demonstrated increased PBR density in the temporal lobe (Owen et al.,
1983). The depletion of PBRs was not dependent on antiparkinsonian
treatment, although animal studies have demonstrated a tissue-specific
modulatory effect of dopamine agonists on PBR expression (Amiri et al.,
1993). The decreased PBR expression in mitochondrial receptors involved in respiratory control may reflect an impairment in cellular
respiratory function (Hirsch et al., 1989). It is of note that reduced
PBR density on platelets has also been found in schizophrenic patients under antidopaminergic treatment; however, the relevance of this finding to extrapyramidal syndromes is unclear (Gavish et al., 1986b;
Weizman et al., 1986).; Diorio et al., 1991), Broca's area, and the precentral and
postcentral gyri (McGeer et al., 1988) in Alzheimer's disease. Such
alterations were not detected in multi-infarct dementia, indicating
that these changes may be specific for Alzheimer's disease. Because
PBRs in the brain are localized mainly in glial cells, it is possible
that the increased PBR density reflects brain damage and gliosis. Ex
vivo study has demonstrated increased platelet PBR density in
Alzheimer's disease patients compared with multi-infarct demented
patients and age- and sex-matched normal controls (Bidder et al.,
1990). These results are consistent with the postmortem brain studies.
Because Alzheimer's disease is associated with enhanced HPA axis
activity (Christie et al., 1987), it is possible that the increased
endocrine steroid activity plays a role in PBR up-regulation.).
). The lesion was elicited by
intracortical injection of the photosensitive dye rose bengal and
exposure to an intense green light shone on the brain surface.
In another study, it was shown that ischemic brain damage can be
attenuated by anti-ischemic drugs belonging to several chemical classes: NMDA receptor antagonists, calcium channel blockers, platelet-activating factor antagonists, adenosine receptor antagonists, and opioid receptor antagonists (Gotti et al., 1990). Limited protection was provided by adenosine receptor antagonist, and effective
protection was achieved with NMDA receptor antagonist. Lesions and
sparing effects were assessed using PBR as the biomarker (Gotti et al.,
1990).
Another model used as a biomarker of neurotoxicity is systemic
application of the neurotoxin domoic acid (Kuhlmann and Guilarte, 1997). This agent causes amnesia, seizures, coma, and death, as well as
extensive forebrain damage. PBRs have been used as an indicator of
neurotoxicity and compared with functional and histological parameters.
PBR expression is increased in the limbic structures, striatum, and
substantia nigra 5 days after domoic acid treatment. The histological
changes are accompanied by spatial learning and memory impairments. The
strong correlation between receptor and behavioral changes shows the
extent and effect of brain injury elicited by the acute administration
of the excitotoxin (Kuhlmann and Guilarte, 1997).
In summary, PBR determination is a highly effective tool for
quantification of lesions caused by excitatory amino acids and by a
variety of neurotoxic agents leading to brain damage in which the
principal pathology is expressed by inflammation or gliosis.
2. Peripheral Benzodiazepine Receptors and Traumatic-Ischemic Brain
Damage.
The PBR and its specific ligands have been found to
participate in traumatic-ischemic brain damage. In the rat, a
mechanical brain injury was elicited by lateral fluid percussion over
the parietal cortex (Toulmond et al., 1993). The percussion was a short, constant impact pressure that resulted in accumulation of blood
in the subarachnoid space and cortical edema in the hyperacute post-traumatic phase. In the acute (4- to 24-h post-traumatic) phase,
phagocytes invaded the injured brain areas. Three to 7 days after
injury, complete neuronal loss was observed around the impact areas.
These pathophysiological changes were similar to those described
clinically in cortical and subcortical injury. A spatial correlation
between the histological alterations and PBR binding was shown that
reached highest values at 7 days after injury, at which time the lesion
was consolidated and astroglial cells replaced neuronal cells (Toulmond
et al., 1993).
). These mediators increased in the
acute post-traumatic phase. The administration of Ro 5-4864 enhanced
the increase of both mediators, suggesting the ability of PBRs to
modulate the inflammatory reaction at the site of brain injury (Taupin
et al., 1993). A conflicting observation was reported by Zavala et al.
(1990). They found that i.p. injection of Ro 5-4864 inhibited the
capacity of macrophages to produce IL-1, IL-6, and TNF. The cause of
dissimilar effects of Ro 5-4864 on the abilities of brain astroglia and
peritoneal macrophages to produce cytokines is not known.
Encephalopathy of various causes, such as hepatic insufficiency or
thiamine deficiency, leads to an increase in PBRs as well as coinciding
with gliosis in the latter (Lavoie et al., 1990; Leong et al., 1994).
Portacaval anastomosis, a model of hepatic encephalopathy, also
produced an increase in brain PBR, suggesting a putative role for
neurosteroids (not inactivated by the liver) in modulation of PBRs
(Giguère et al., 1992). Further studies will show whether PBRs
can be used clinically for the assessment of the severity of brain
lesions and encephalopathy.
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VII. Peripheral Benzodiazepine Receptors in Cancer and in Immune Function |
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Attention has focused on the involvement of the PBRs in cell proliferation and differentiation for two main reasons: 1) the growing number of neoplastic tissues reported to show altered binding characteristics of PBRs (Table 4) and 2) the effects that ligands for PBRs have on differentiation and proliferation of normal and malignant cells in vitro (Table 5). Evidence of the involvement of ligands for PBRs in the regulation of cellular processes is accumulating and suggests a few pathways, depending on the tissue concerned.
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During the 1980s, data regarding the change of PBR binding
characteristics in neoplastic tissues accumulated and suggested a
possible involvement in cancer. Ovarian carcinoma is a malignant disease that is usually diagnosed at late stages, making overall therapy less effective. Increased PBR density in human epithelial ovarian carcinoma was noticed in comparison to benign tumors (5-fold) and normal ovaries (3-fold; Katz et al., 1990b; Table 4). In binding
experiments with [3H]PK 11195, Katz et al.
(1990c) demonstrated the presence of PBRs in normal colonic tissue and
a 3.2-fold increase in PBR density for adenocarcinoma of the colon
(Table 4). It has been shown for guinea pig ileum that diazepam
inhibits the longitudinal smooth muscle contraction, coinduced by
intracellular release of Ca2+ in a
concentration-dependent manner. This finding may strengthen the theory
that the inhibition of smooth muscle contraction and the reduction in
digestive tract motility lead to a longer time of exposure of the
intestinal mucosa to carcinogenic factors, which will consequently
induce a higher occurrence of bowel cancer (Sugarbaker et al., 1982).
Several studies have demonstrated increased binding site densities for
BZ ligands in various brain tumors (Ferrarese et al., 1989; Black et
al., 1990; Ikezaki et al., 1990; Table 4). In particular, one study of
PBR density in several types of brain tumors showed marked increases in
high-grade astrocytoma and glioblastoma cells in comparison with normal
brain parenchyma, whereas low-grade gliomas and meningiomas exhibited
much lower elevations in PBR binding site densities (Cornu et al.,
1992; Table 4). Olson et al. (1988b) examined PBR ligand binding in
postmortem glioma samples and found high densities of PBR-specific
binding in intact glioma tumor cells compared with cells of normal
cerebral cortex or necrotic areas of the tumor. The clinical relevance
is obvious, because PBRs may accurately delineate glioma borders with
the use of positron emission tomography in preference to other, less
accurate, imaging techniques (Starosta-Rubinstein et al., 1987; Van
Dort et al., 1988). The PBR itself may be not only a proliferation
marker or gliosis border delineator but also the route by which cancer
cells may be specifically targeted. Verma et al. (1998) showed that the
high affinity of porphyrins to PBRs, and their elevated levels in
cancer cells allowed selective retention and cell-specific cytotoxicity
through light irradiation (photodynamic therapy).
One study demonstrated that patients expressing high levels of
PBR-immunoreactive cells were correlated with shorter life expectancies
(Miettinen et al., 1995). In addition, in a recent study focusing on
human breast cancer, PBRs were found to be highly expressed in
aggressive metastatic human breast tumor biopsy samples compared with
normal breast tissues (Hardwick et al., 1999). Moreover, it was found
that the more aggressive the breast cancer cell lines, the more
abundant were PBR ligand binding and mRNA. The same study went on to
characterize the change in cellular location of PBR protein when the
more aggressive and the less aggressive breast cancer cell lines were
compared. The more aggressive cell lines showed a nuclear localization
for PBR, as opposed to the "normal" or the less aggressive tumor
mitochondrial location (Hardwick et al., 1999).
Matthew et al. (1981) showed that tyrosinase activity and melanin
synthesis (differentiation markers) were induced by diazepam in B16/C3
mouse melanoma cells (Table 5). Furthermore, BZs with higher affinities
to PBR showed higher potencies in melanogenesis induction. In human
prostatic nodular hyperplasia, the density of PBR in response to Ro
5-4864 did not alter, but the affinity was lowered 15-fold in
comparison with normal prostate (Escubedo et al., 1993; Table 4). Black
et al. (1994) demonstrated that PK 11195 and Ro 5-4864 stimulated
mitochondrial proliferation (1.6-fold) and growth hormone stimulation
(2.4-fold) in pituitary tumor GH3 cells (exposure
to central-type BZ ligands did not have any effect; Table 5).
A strong and positive correlation has been shown between the affinity
of PBR ligands and the antiproliferative activity of mouse thymoma
cells (Wang et al., 1984). Such a correlation was not found for CBR
ligands. This finding reinforces the notion of PBR involvement in
growth control and cellular proliferation. The effects that BZs have on
different cells are not always similar, as they have been reported to
inhibit mitogenesis in Swiss 3T3 cells while inducing mitogenesis in
Friend erythroleukemia cells (Clarke and Ryan, 1980). Pawlikowski et
al. (1988) noted concentration-dependent inhibition of cellular
proliferation in mouse spleen lymphocytes by diazepam and Ro 5-4864 (Table 5). Increased mitochondrial proliferation and morphological
changes in glioma cells were shown to take place after the application
of PK 11195 and Ro 5-4864 (Shiraishi et al., 1991; Table 5). Carmel et
al. (1999) found that PK 11195 and Ro 5-4864 inhibited MCF-7 breast
carcinoma cell line proliferation at concentrations of
10
5 to 104 M, whereas
clonazepam (CBR-specific ligand) had no effect (Table 5). Ikezaki and
Black (1990) showed that for C6 glioma cells, growth rate and thymidine
incorporation increased 20 to 30% after PK 11195 stimulation in the
nanomolar range. They found a correlation between the affinity of
peripheral BZs for PBRs in the nanomolar range and their potency for
inhibiting proliferation (in the micromolar range), although it is
difficult to explain inhibition through high-affinity receptors for
BZs. This three-orders-of-magnitude discrepancy does not permit
unequivocal proof of antiproliferative action through PBR. Furthermore,
Gorman et al. (1989) showed that rat NCTC epithelial cells and mouse
Sp2/0-Ag14 hybridoma cells, which do not possess any detectable levels
of PBR, were inhibited by PK 11195 and Ro 5-4864 (Table 5). This result
suggests the possibility of another pathway for the inhibition of
cellular growth.
The association between PBRs and Ca2+ channels
(see above) suggests a role in PBR-related proliferation and
differentiation. Increased cytoplasmic levels of
Ca2+ are a prerequisite for mitosis in eukaryotic
cells. Studies on the effect of PBR ligands on
Ca2+ channels showed that at high concentrations
(micromolar range), these ligands inhibited Ca2+
flow through plasma membranes by modulating voltage-sensitive Ca2+ channels (Cantor et al., 1984). In addition,
the same group found that a Ca2+ ion channel
blocker was displaced by the PBR ligand Ro 5-4864 in cardiac, renal,
and cerebral membranes. Similar results were obtained by Python et al.
(1993), who reported that PBR ligands may block
Ca2+ ion flow through voltage-activated channels
in glomerulosa cells of the adrenal gland.
The evidence of PBR involvement in the regulation of cellular
proliferation suggests a few pathways, one of which might be modulation
of the capability of the immune system in elimination of neoplasms.
Stepien et al. (1988) demonstrated that different BZs modulate thymus
cell proliferation, thus regulating immune system function (Table 5).
Numerous observations indicate that macrophages play a significant role
in the immune response to tumors. For example, macrophages are often
observed to cluster around tumors, and their presence is often
correlated with tumor regression. BZs have been found to bind to
specific receptors on macrophages and to modulate in vitro their
metabolic oxidative responsiveness (Lenfant et al., 1985). Zavala et
al. (1990) showed that the capacity of macrophages to produce IL-1,
TNF, and IL-6 was inhibited by i.p. injection of Ro 5-4864 (with no
effect by the central-type BZ clonazepam). This result demonstrated in
vivo immunosuppressive properties of PBRs and mixed ligands, but not of
CBR ligands, affecting characteristic phagocyte functions involved in
host-defense mechanisms.
The antitumor activity of macrophages is probably mediated by several
macrophage products. Activation of macrophages with interferon
(IFN)- and macrophage-activating factor increases not only their
secretion of various products but also their cytotoxicity to tumor
cells. Peripheral BZ ligands have been found to potentiate the
antiproliferative activity of recombinant human IFNs (Solowey et al.,
1990). Activated macrophages secrete large amounts of lytic enzymes,
which can reach high levels around a tumor, especially if antitumor
antibodies bind to Fc receptors on macrophages and serve to bridge the
macrophages to the tumor. Diazepam (both a CBR and a PBR ligand) and Ro
5-4864 (a PBR ligand) have been found to inhibit neutrophil chemotaxis
and superoxide production in a stimulus-dependent way, thus possibly
affecting antitumor activity (Finnerty et al., 1991). Macrophages also
secrete a cytokine called TNF- that has potent antitumor properties.
When cloned, TNF- was injected into tumor-bearing animals; it
induced hemorrhage and necrosis of the tumor. Interestingly, in
response to TNF- and IL-2, PBR density increased in both a dose-
and time-dependent manner in cultured polygonal astrocytes (Oh et al.,
1992). Supporting results were obtained by Matsumoto et al. (1994), who
showed that PBR modulates lipopolysaccharide-induced TNF activity in
mouse macrophages.
Prenatal exposure to low doses of BZs can result in long-lasting
alterations of the cytokine network, as indicated by the reduced
release of TNF-, IL-1, IL-6, IL-2, and IFN- (Schlumpf et al.,
1993). The concomitant reduction of PBR on macrophages has been
suggested as a possible link between prenatal treatment and postnatal
disturbed immune function.
Rosenberg (1984) injected FBL-3 lymphoma into the footpad of syngeneic
mice and found that in the presence of high concentrations of cloned
IL-2, large numbers of activated lymphoid cells were generated that
could kill fresh tumor cells but not normal cells. Rosenberg called
these cells lymphokine-activated killer cells. These cells appear to be
a heterogeneous population of lymphoid cells that include natural
killer (NK) cells and natural cytotoxic cells. Bessler et al. (1997)
demonstrated that Ro 5-4864 and PK 11195 significantly and specifically
suppressed NK cell activity; this result was completely reversed by the
addition of human recombinant IL-2 or human leukocyte IFN. This
suppression of NK cells may connect the involvement of PBRs in reducing
the immunological capacity for eliminating malignant growths. On the
other hand, Solowey et al. (1990) demonstrated that peripheral-acting
BZs were found to potentiate the antiproliferative action of
recombinant human IFNs, which have been found to inhibit cell division
of both normal and malignantly transformed cells in vitro, perhaps through modulation of major histocompatibility complex (MHC)
expression, whereas IFN- has been shown to increase the expression
of class II MHC on macrophages.
These findings are significant if a better understanding of the specific modes of PBR involvement in the tumoricidal activity of the immune system, which might be relevant to the control of cell growth and tumorigenesis, is to be reached.
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VIII. Summary and Future Prospects |
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The combined facts that the PBR gene is conserved throughout evolution from bacteria to humans and that this gene appears to have the hallmarks of a typical housekeeping gene suggest that this gene's product has a basic cellular function. Even though many functions have been attributed to this gene product, its primary roles remain an open question. Hence, a resolution of the primary functions of the PBR gene products must be a central theme for future work with PBRs.
One of the often-suggested putative roles attributed to PBRs is as the rate-limiting step in steroidogenesis. The relevance of PBRs for steroidogenesis is reflected by their abundance in the adrenal gland and in male and female gonadal tissues. It is explained that this role is mediated through regulation of cholesterol transport from the cytoplasm to the mitochondrial matrix. Because another protein, StAR, has the same attribute, either a biochemical redundancy is apparent or some interaction between these proteins may mediate mitochondrial cholesterol transport.
Another putative function for PBRs involves a regulatory role in cell
proliferation. This role may be closely associated with some type of
regulatory function in cellular respiration, which also is one of the
many assigned roles for the PBR gene product. Hence, it has
been argued that the dysregulation of PBRs in some tissues may lead to
diseases of cellular proliferation, including cancer. A number of
reports have, in fact, found PBR overexpression in particular tumor
types and some transformed cell lines. In all of these cases, a
causative pathology for PBR overexpression has never been shown, hence
not ruling out a passive coincident dysregulation of the expression of
PBR genes in these tumors. In support of this, we recently
reported that on antisense knockout of PBRs in a mouse Leydig tumor
cell line, no apparent changes in the cell proliferation or cell cycle
were measurable (Kelly-Hershkovitz et al., 1998). We acknowledge that
this may be unique to these specific cells, and hence a more extensive
study must still be undertaken. Furthermore, other as-yet-undefined
effects of PBR overexpression may be important in malignancy. In this
respect, the modulatory role of PBRs on immune system function should
also be taken into consideration.
In addition, at behavioral levels, PBRs appear to be involved in the biological coping with stress and anxiety disorders. It has been suggested that PBRs play a role in the regulation of several stress systems such as the HPA axis, the sympathetic nervous system, the renin-angiotensin axis, and the neuroendocrine-immune axis. In these systems, acute stress typically leads to increases in PBR density, whereas chronic stress typically leads to decreases in PBR density. Furthermore, in GAD, PD, GSP, and PTSD, PBR density is typically decreased in platelets. In the brain, where PBRs are associated with glial cells, PBRs are increased in specific brain areas in neurodegenerative disorders and also after neurotoxic and traumatic-ischemic brain damage. These accumulating data indicate a possible role for PBR in adaptation of the organism to stress and brain damage.
Because PBRs appear to be involved in a large variety of physical diseases, mental disorders, and responses to stress, clinical benefit may be attainable by the increasing pharmacological knowledge surrounding these receptors. Nevertheless, there still is much to be learned about the structure of PBRs as well as their cellular location, regulation of gene expression of the PBR subunits, and the interaction between PBR subunits. As mentioned above, other proteins like StAR may also be involved in this complex. A molecular understanding of the protein component subunits, as well as how they fit together and function as a whole, still requires much work. The 18-kDa isoquinoline-binding PBR subunit is not only found on the mitochondrial membrane; it is also found to a lesser extent on the plasma membrane of the cell, as well as on the membrane of other cellular organelles. Its role, structure, and other interacting subunits in these other cellular locations also must be addressed.
Much has been learned in the decade since the 18-kDa PBR subunit mRNA was cloned and still much must be learned from and about the genes encoding PBR subunits. This will involve many levels of study. It may be that once we understand why evolution so carefully preserves the sequence of the 18-kDa PBR subunit gene, we will be able to uncover new biochemical pathways that will link the various putative PBR functions now being discussed. Only the future can tell.
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Acknowledgments |
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The Center for Absorption in Science, Ministry of Immigrant Absorption, State of Israel, is acknowledged for their support to L.V. and G.W. We thank Ruth Singer for editing this manuscript.
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Footnotes |
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1 Address for correspondence: Dr. Moshe Gavish, Department of Pharmacology, The Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, P.O.B. 9649, 31096 Haifa, Israel. E-mail: mgavish{at}tx.technion.ac.il
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Abbreviations |
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BZ, benzodiazepine;
ACTH, corticotropin;
CBR, central benzodiazepine receptor;
CNS, central
nervous system;
CRH, corticotropin-releasing hormone;
DBI, diazepam-binding inhibitor;
DES, diethylstilbestrol;
GABA, -aminobutyric acid;
GAD, generalized anxiety disorder;
GSP, generalized social phobia;
HPA, hypothalamic-pituitary-adrenal;
IFN, interferon;
IL, interleukin;
MHC, major histocompatibility complex;
NK, natural killer;
NMDA, N-methyl-D-aspartate;
OCD, obsessive-compulsive disorder;
ODN, octadecaneuropeptide;
P-450scc, side-chain cleavage cytochrome P-450;
PBR, peripheral benzodiazepine receptor;
PD, panic disorder;
PMSG, pregnant
mare serum gonadotropin;
PTSD, post-traumatic stress disorder;
StAR, steroidogenic acute regulatory protein;
TNF, tumor necrosis factor;
VDAC, voltage-dependent anion channel.
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PHARMACOLOGICAL REVIEWS
Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics
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 H. Klonowski-Stumpe, R. Fischer, R. Reinehr, R. Luthen, and D. Haussinger Apoptosis in activated rat pancreatic stellate cells Am J Physiol Gastrointest Liver Physiol, September 1, 2002; 283(3): G819 - G826. [Abstract] [Full Text] [PDF]
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 K. Fukuda, T. Shoda, H. Mima, and H. Uga Midazolam Induces Expression of c-Fos and EGR-1 by a Non-GABAergic Mechanism Anesth. Analg., August 1, 2002; 95(2): 373 - 378. [Abstract] [Full Text] [PDF]
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 A. Szewczyk and L. Wojtczak Mitochondria as a Pharmacological Target Pharmacol. Rev., March 1, 2002; 54(1): 101 - 127. [Abstract] [Full Text] [PDF]
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 A. Mazzone, C. Cusa, I. Mazzucchelli, M. Vezzoli, E. Ottini, R. Pacifici, P. Zuccaro, and C. Falcone Increased production of inflammatory cytokines in patients with silent myocardial ischemia J. Am. Coll. Cardiol., December 1, 2001; 38(7): 1895 - 1901. [Abstract] [Full Text] [PDF]
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J. J. Rady and J. M. Fujimoto Confluence of Antianalgesic Action of Diverse Agents through Brain Interleukin1beta in Mice J. Pharmacol. Exp. Ther., November 1, 2001; 299(2): 659 - 665. [Abstract] [Full Text] [PDF]
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 C. Johnston, W. Jiang, T. Chu, and B. Levine Identification of Genes Involved in the Host Response to Neurovirulent Alphavirus Infection J. Virol., November 1, 2001; 75(21): 10431 - 10445. [Abstract] [Full Text] [PDF]
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 A. Cagnin, R. Myers, R. N. Gunn, A. D. Lawrence, T. Stevens, G. W. Kreutzberg, T. Jones, and R. B. Banati In vivo visualization of activated glia by[11C] (R)-PK11195-PET following herpes encephalitis reveals projected neuronal damage beyond the primary focal lesion Brain, October 1, 2001; 124(10): 2014 - 2027. [Abstract] [Full Text] [PDF]
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