Peripheral benzodiazepine receptor: structure and function in health and disease

In vitro studies using biochemical, pharmacological and molecular approaches demonstrated that the peripheral-type benzodiazepine receptor (PBR) is a mitochondrial protein, involved in the regulation of cholesterol transport from the outer to the inner mitochondrial membrane, the rate-determining step in steroid biosynthesis. In vivo animal models and ontogeny studies validated the role of PBR in steroidogenesis. Targeted disruption of the PBR gene in Leydig cells resulted in the arrest of cholesterol transport into mitochondria and steroid formation. Molecular modeling of PBR suggested that it might function as a channel for cholesterol. Indeed, cholesterol uptake and transport by bacteria cells was induced upon PBR expression. Amino acid deletion and site-directed mutagenesis studies identified a cholesterol recognition/interaction amino acid consensus sequence in the cytoplasmic carboxy-terminus of the receptor. In vitro reconstitution experiments demonstrated that the 18 kDa PBR protein binds with high affinity both drug ligands and cholesterol, suggesting that this protein might serve numerous functions considering the critical role of cholesterol in membrane biogenesis and human pathology. In this context, PBR expression correlated with the quality of kidney preservation, indicating that it might serve as an index of kidney and mitochondrial viability during ischemia-reperfusion injury. PBR overexpression was also found to be a prognostic indicator of the aggressive phenotype in breast, colorectal and prostate cancers. Moreover, in Alzheimer's disease brain specimens, PBR levels were increased and paralleled the elevated neurosteroid synthesis observed in specific brain areas. The role for PBR in these pathological conditions remains to be elucidated. paralleled the elevated neurosteroid synthesis observed in specific brain areas. The role for PBR in these pathological conditions remains to be elucidated.

Adamantinoma of tibia: a case of late local recurrence along with lung metastases

Adamantinomas of long bones are rare primary low-grade malignant tumours composed of cells with epithelial and fibrous characteristics. Local recurrence, though scarce, occurs 5-15 years after the onset of diagnosis. We report a case of local recurrence of an adamantinoma localised in tibia, along with the presence of two lung metastases, 24 years after diagnosis and surgical therapy of the primary tumour. The local recurrence and the lung metastases were removed surgically. The patient remains free of the disease for 3 years.

PBR, StAR, and PKA: partners in cholesterol transport in steroidogenic cells

Acute stimulation of cholesterol transport into mitochondria involves the cAMP-dependent protein kinase (PKA), peripheral-type benzodiazepine receptor (PBR), and the steroidogenesis acute regulatory (StAR) proteins. We investigated the respective role of these proteins in hormone-induced steroidogenesis. Oligonucleotides antisense, but not sense, to PBR and StAR reduced their respective levels in steroidogenic cells and inhibited hormone-stimulated steroid formation in MA-10 mouse Leydig tumor cells. In search of the proteins regulating PBR we identified a protein, PAP7, which interacts with PBR and the PKA regulatory subunit RIalpha, is present in adrenal and gonadal cells and is found in mitochondria. Overexpression of the full length PAP7 increased the hormone-induced steroid production. However, inhibition of PAP7 expression reduced the gonadotropin-induced steroid formation. In search of a PBR functional antagonist that would facilitate the studies on the biological function of PBR, we screened a phage display library. A 7-mer competitive PBR peptide antagonist was identified, which when transduced into Leydig cells inhibited the benzodiazepine and hormone-stimulated steroid production suggesting that the endogenous PBR agonist/receptor interaction is critical for the hormone-dependent steroidogenesis. These data indicate that hormone-induced cholesterol transport and the subsequent steroid formation is a dynamic multistep process involving protein-protein interactions.

Role of the peripheral-type benzodiazepine receptor in adrenal and brain steroidogenesis

The peripheral-type benzodiazepine receptor (PBR) has been demonstrated to be critical for steroidogenesis in all steroid-producing tissues. Here, we review the identification and characterization of the PBR, the evidence pointing to its function as a cholesterol pore involved in transporting cholesterol from the cytoplasm of steroid-producing cells into the inner mitochondrial membrane where it is metabolized, and the known mechanisms regulating its function. We present data on the functions of the PBR in the adrenal gland, a classical steroidogenic tissue, and in the brain, which has only recently been proven to be steroidogenic. Finally, we discuss other potential roles for the PBR in pathological conditions, including cancer, neurodegeneration, and neurotoxicity, and a broader role for the PBR in mediating intracellular cholesterol transport/compartmentalization, which may or may not be linked to steroid biosynthesis.

Identification, localization, and function in steroidogenesis of PAP7: a peripheral-type benzodiazepine receptor- and PKA (RIalpha)-associated protein

Peptide hormones and cAMP acutely stimulate steroid biosynthesis by accelerating the transport of cholesterol into the mitochondria. The peripheral-type benzodiazepine receptor (PBR) has been shown to be an indispensable element of the cholesterol transport machinery. Using the yeast two-hybrid system and PBR as bait, we identified a protein that interacts with PBR, the PBR-associated protein PAP7. Using the regulatory subunit RIalpha of PKA as bait, we also isolated PAP7. Glutathione-S-transferase -PAP7 interacted with both the mitochondrial PBR and cytosolic PKA-RIalpha in MA-10 Leydig cells. PAP7 is a novel 52-kDa protein present in mouse, rat, and human tissues, and it has a major 3-kb mRNA transcript in all tissues examined. Immunohistochemical and in situ hybridization studies indicated that PAP7 is highly expressed in the gonads, adrenal, hippocampus, and distinct brain neuronal and glial populations. Overexpression of the full length PAP7 increased the hCG-induced steroid production. However, overexpression of a partial PAP7, which includes the PBR- and PKA-RIalpha-binding domains, inhibited the hormone-stimulated cholesterol transport and steroid synthesis. Treatment of MA-10 cells with oligonucleotides antisense to PAP7 also inhibited the hCG-stimulated steroid formation, suggesting that PAP7 is a functional element of the hormone-induced signal transduction cascade leading to steroidogenesis. PAP7 may function by targeting the PKA isoenzyme to organelles rich in PBR, i.e. mitochondria, where phosphorylation of specific protein substrates may induce the reorganization of PBR topography and function.

Peripheral-type benzodiazepine receptor levels correlate with the ability of human breast cancer MDA-MB-231 cell line to grow in SCID mice

MDA-MB-231 (MDA-231) human breast cancer cells have a high proliferation rate, lack the estrogen receptor, express the intermediate filament vimentin, the hyaluronan receptor CD44, and are able to form tumors in nude mice. The MDA-231 cell line has been used in our laboratory to examine the role of the peripheral-type benzodiazepine receptor (PBR) in the progression of cancer. During these studies 2 populations of MDA-231 cells were subcloned based on the levels of PBR. The subclones proliferated at approximately the same rate, lacked the estrogen receptor, expressed vimentin and CD44, and had the same in vitro chemoinvasive and chemotactic potential. Both restriction fragment length polymorphism and comparative genomic hybridization analyses of genomic DNA from these cells indicated that both subclones are of the same genetic lineage. Only the subclone with high PBR levels, however, was able to form tumors when injected in SCID mice. These data suggest that the ability of MDA-231 cells to form tumors in vivo may depend on the amount of PBR present in the cells.

Structure, function and regulation of the mitochondrial peripheral-type benzodiazepine receptor

Steroid biosynthesis begins with the transfer of cholesterol from intracellular stores into mitochondria. Through in vitro and in vivo studies using various steroidogenic cell models and with the help of pharmacological, biochemical, morphological and molecular approaches we demonstrated that the peripheral-type benzodiazepine receptor (PBR) is an 18 kDa mitochondrial protein that interacts with other proteins in the outer mitochondrial membrane to form a multimeric complex. PBR is required for the binding, uptake and release, upon ligand activation, of the substrate cholesterol. Thus, cholesterol becomes available to the inner mitochondrial membrane P450scc where steroid biosynthesis begins. The presence of mitochondrial PBR is also critical in maintaining outer mitochondrial membrane stability and in preventing apoptosis. Considering these functions of PBR and the fact that PBR is a ubiquitous protein, it is suggested that this drug receptor may serve as a target to control various mitochondrial and cell functions and to protect against experimentally or pathologically induced mitochondrial and cell toxicity.

Structural and functional study of reconstituted peripheral benzodiazepine receptor

Recombinant mouse 18 kDa peripheral-type benzodiazepine receptor (PBR) protein was overexpressed in Escherichia coli and isolated using a His. Bind metal chelation resin. Recombinant PBR protein was purified with sodium dodecyl sulfate and reincorporated into liposomes using Bio-Beads SM2 as a detergent removing agent. Negative staining of the reconstituted PBR samples, examined by electron microscopy, showed the formation of proteoliposomes. Freeze-fracture of these proteoliposomes revealed the presence of transmembranous particles of an average size of 3.5 +/- 0.25 nm, consistent with the presence of a monomeric form of the recombinant PBR protein. The reconstituted protein exhibited the ability to bind both the PBR drug ligand isoquinoline carboxamide PK 11195 and cholesterol with nanomolar affinities. These data suggest that a PBR monomer is the minimal functional unit, binding drug ligands and cholesterol.

Effect of mono-ethylhexyl phthalate on MA-10 Leydig tumor cells

We examined the effect of mono-ethylhexyl phthalate (MEHP) on MA-10 Leydig tumor cell structure and function. Cells were exposed to various concentrations of MEHP for 24 h and then stimulated with saturating concentrations of hCG for 2.5 h. Progesterone production, cell viability, and protein content were moderately inhibited by low concentrations and severely inhibited by high concentrations of MEHP. Electron microscopy showed a variety of alterations in the MEHP-treated cells, increasing in severity with increasing concentrations of MEHP. Lipid droplets were profoundly affected in the cells treated with MEHP and morphologic evidence that metabolism of lipid storage droplets ceases at approximately the same time progesterone synthesis stops was seen. Morphometric studies indicated that the number of lipid droplets appeared to be increased 2.5-fold over control levels at MEHP concentrations of 10(-6) to 10(-3) M whereas mitochondrial volume fraction decreased. These results suggest that MEHP in Leydig cells may act as a mitochondrial toxicant and lipid metabolism disrupter.

Cholesterol binding at the cholesterol recognition/ interaction amino acid consensus (CRAC) of the peripheral-type benzodiazepine receptor and inhibition of steroidogenesis by an HIV TAT-CRAC peptide

We previously defined a cholesterol recognition/interaction amino acid consensus (CRAC; ATVLNYYVWRDNS) in the carboxyl terminus of the peripheral-type benzodiazepine receptor (PBR), an outer mitochondrial membrane protein involved in the regulation of cholesterol transport into the mitochondria, the rate-determining step in steroid biosynthesis. We examined (i) the PBR-cholesterol interaction by UV crosslinking of the C17 side-chain containing progestin, promegestone, and (ii) the role of the CRAC domain of PBR in Leydig cell steroidogenesis by using a transducible peptide composed of the TAT domain of HIV and the CRAC domain of PBR. [(3)H]Promegestone photoincorporated into recombinant PBR, and this labeling was displaced by cholesterol. [(3)H]Promegestone also photoincorporated into the TAT-CRAC peptide. [(3)H]Promegestone crosslinking to TAT-CRAC could be displaced by cholesterol and promegestone, with IC50 values of 1 and 200 microM, respectively. TAT-CRAC efficiently transduced into MA-10 Leydig cells and inhibited the hCG- and cAMP-stimulated steroid production in a dose-dependent manner. TAT-CRAC did not affect the hCG-induced cAMP synthesis and the 22R-hydroxycholesterol-supported steroidogenesis. Mutated TAT-CRAC lost its ability to bind [(3)H]promegestone and to inhibit the hCG-stimulated steroidogenesis. These results show that TAT-CRAC binds cholesterol and competes for cholesterol interaction with endogenous PBR, suggesting that the cytosolic carboxyl-terminal domain of PBR is responsible for taking up and bringing steroidogenic cholesterol into the mitochondria.

The Ginkgo biloba extract EGb 761 rescues the PC12 neuronal cells from beta-amyloid-induced cell death by inhibiting the formation of beta-amyloid-derived diffusible neurotoxic ligands

beta Amyloid (Abeta) treatment induced free radical production and increased glucose uptake, apoptosis and cell death in PC12 nerve cells. Addition of the standardized extract of Ginkgo biloba leaves, EGb 761 together with the Abeta protein prevented, in a dose-dependent manner, the Abeta-induced free radical production, increased glucose uptake, apoptosis and cell death. However, pretreatment of the cells with EGb 761 did not rescue the cells from the Abeta-induced toxicity although it prevented the Abeta-induced reactive oxygen species generation. Moreover, the terpene and flavonoid-free EGb 761 extract, HE 208, although inhibited the Abeta-induced increased glucose uptake, it failed to protect the cells from apoptosis and cytotoxicity induced by Abeta. In conclusion, these results indicate that the terpenoid and flavonoid constituents of EGb 761, acting probably in combination with components present in HE 208, are responsible for rescuing the neuronal cells from Abeta-induced apoptosis and cell death; their mechanism of action being distinct of their antioxidant properties. Because pre- and post-treatment with EGb 761 did not protect the cells from Abeta-induced neurotoxicity, we examined whether EGb 761 interacts directly with Abeta. Indeed, in vitro reconstitution studies demonstrated that EGb 761 inhibits, in a dose-dependent manner, the formation of beta-amyloid-derived diffusible neurotoxic soluble ligands (ADDLs), suggested to be involved in the pathogenesis of Alzheimer's disease.

Pathways of dehydroepiandrosterone formation in rat brain glia

In peripheral steroidogenic tissues, dehydroepiandrosterone (D) is formed from pregnenolone (P) by the microsomal cytochrome P450c17 enzyme. Although some steroidogenic P450s have been found in brain tissue, no enzyme has been shown to possess P450c17 activity. We recently demonstrated the presence of an alternative, Fe(2+)-dependent pathway responsible for D formation from alternative precursors in rat glioma cells. We and others could not find P450c17 mRNA and protein in rat brain, but demonstrate herein the presence of Fe(2+)-dependent alternative pathway for D formation in rat brain cortex microsomes. Using primary cultures of differentiating rat glial cells, we observed that P450c17 mRNA and protein were present in O-2A oligodendrocyte precursors and mature oligodendrocytes. In the presence of P, O-2A and mature oligodendrocytes formed D. Addition of Fe(2+) together with submaximal concentrations of P increased D formation by these cells. Treatment of oligodendrocytes with the P450c17 inhibitor SU 10603 in the presence or absence of P failed to inhibit D production. These data suggest that D formation in oligodendrocytes occurs independently of the P450c17 protein present in the cells. In isolated type I astrocytes we did not find neither P450c17 mRNA nor protein. These cells responded to Fe(2+) by producing D and addition of P together with Fe(2+) further increased D synthesis. SU 10603 failed to inhibit D formation by astrocytes. Taken together these results suggest that in differentiating rat brain oligodendrocytes and astrocytes D is formed via a P450c17-independent and oxidative stress-dependent alternative pathway.

Drug-induced inhibition of the peripheral-type benzodiazepine receptor expression and cell proliferation in human breast cancer cells

The peripheral-type benzodiazepine receptor (PBR) expression and localization correlate with human breast cancer cell proliferation and aggressive phenotype expression. The standardized extract of Ginkgo biloba leaves (EGb 761) and isolated ginkgolide B (GKB) were shown to decrease PBR mRNA expression in adrenal cells. We examined the effect of EGb 761 and GKB on PBR expression and cell proliferation in human breast cancer cells. EGb 761 and GKB decreased in a time- and dose-dependent manner PBR expression and cell proliferation in the highly aggressive, rich in PBR, human breast cancer cell line MDA-231 whereas they did not affect the proliferation of the non-aggressive human breast cancer cell line MCF-7, which contains very low PBR levels. This effect was reversible and not due to the antioxidant properties of the compounds tested. Using a human cDNA expression array we determined that EGb 761 treatment altered, in addition to PBR, the expression of 36 gene products involved in various pathways regulating cell proliferation. These in vitro data were further validated in an in vivo model where EGb 761 and GKB significantly inhibited the nuclear PBR expression and growth of MDA-231 cell xenografts in nude mice. Taken together, these data suggest that the manipulation of PBR expression could be used to control tumor growth and that EGb 761 and GKB, under the conditions used, exert cytostatic properties.

The peroxisome proliferator perfluorodecanoic acid inhibits the peripheral-type benzodiazepine receptor (PBR) expression and hormone-stimulated mitochondrial cholesterol transport and steroid formation in Leydig cells

The peroxisome proliferator perfluordecanoic acid (PFDA) has been shown to exert an antiandrogenic effect in vivo by acting directly on the interstitial Leydig cells of the testis. The objective of this study was to examine the in vitro effects of PFDA and identify its site of action in steroidogenesis using as model systems the mouse tumor MA-10 and isolated rat Leydig cells. PFDA inhibited in a time- and dose-dependent manner the hCG-stimulated Leydig cell steroidogenesis. This effect was localized at the level of cholesterol transport into the mitochondria. PFDA did not affect either the total cell protein synthesis or the mitochondrial integrity. Moreover, it did not induce any DNA damage. Morphological studies indicated that PFDA induced lipid accumulation in the cells, probably due to the fact that cholesterol mobilized by hCG did not enter the mitochondria to be used for steroidogenesis. In search of the target of PFDA, we examined its effect on key regulatory mechanisms of steroidogenesis. PFDA did not affect the hCG-induced steroidogenic acute regulatory protein (StAR) levels. However, it was found to inhibit the mitochondrial peripheral-type benzodiazepine receptor (PBR) ligand binding capacity, 18-kDa protein, and messenger RNA (mRNA) levels. Further studies indicated that PFDA did not affect PBR transcription, but it rather accelerated PBR mRNA decay. Taken together, these data suggest that PFDA inhibits the Leydig cell steroidogenesis by affecting PBR mRNA stability, thus inhibiting PBR expression, cholesterol transport into the mitochondria, and the subsequent steroid formation. Moreover, this action of PFDA on PBR mRNA stability indicates a new mechanism of action of peroxisome proliferators distinct from the classic transcription-mediated regulation of target genes.

Pathways of neurosteroid biosynthesis in cell lines from human brain: regulation of dehydroepiandrosterone formation by oxidative stress and beta-amyloid peptide

Neurosteroids in rodents can originate from peripheral tissues or be locally synthesized in specific brain areas. There is, as yet, no information about the synthesis and regulation of neurosteroids in human brain. We examined the ability of human brain cells to synthesize steroids from a radiolabeled precursor and the mRNA and protein expression of key components of peripheral steroidogenic machinery. Oligodendrocytes are the source of pregnenolone in human brain. Human astrocytes do not synthesize radiolabeled pregnenolone, nor do human neurons. There is potential for all three cell types to metabolize pregnenolone to other neurosteroids, including dehydroepiandrosterone. mRNA and protein for cytochrome P450 17alpha-hydroxylase were found in all cell types, although no activity could be demonstrated. We examined the ability of the cells to make dehydroepiandrosterone via an alternative pathway induced by treatment with Fe2+. Oligodendrocytes and astrocytes make dehydroepiandrosterone via this pathway, but neurons do not. In searching for a natural regulator of dehydroepiandrosterone formation, we observed that treating oligodendrocytes with beta-amyloid, which increases reactive oxygen species, also increased dehydroepiandrosterone formation. These effects of beta-amyloid were blocked by vitamin E. These results indicate that human brain makes steroids in a cell-specific manner and suggest that dehydroepiandrosterone synthesis can be regulated by intracellular free radicals.

Myoelectric assessment of large bowel viability: an experiment in dogs

OBJECTIVE

To design a device for myoelectric assessment of intestinal ischaemia and compare it with every day surgical experience and Doppler signals recorded on the bowel wall.

DESIGN

Experimental study.

SETTING

Thessaloniki university hospital, Greece.

MATERIAL

12 adult mongrel dogs.

INTERVENTIONS

On the first day the large intestine was devascularised for a length of 20 cm, 5 cm away from the ileocaecal valve, and the threshold of the electric stimulus (mA) required to produce a contraction of the normal large bowel was recorded. On the second day, measurements were made on the ischaemic segment of the large bowel at 0.5 cm intervals. Bowel resection and anastomoses were done at the stimulus level of 40 mA.

RESULTS

The mean (SD) stimulus threshold of the normal large intestine was 12.2 mA. The necrotic intestine demanded current stimulus of 100 mA or failed to contract. On the eighth postoperative day the animals were killed to assess anastomotic healing. Of the 12 anastomoses made at the 40 mA stimulus point, only one ruptured. The 40 mA limit of the stimulus level seems to be of value in assessments of bowel viability in vivo.

CONCLUSION

The use of a personal computer as a read out device makes myoelectric analysis easier and more reliable in the assessment of intestinal viability. This method may have a clinical application.

Free radicals and lipid peroxidation do not mediate beta-amyloid-induced neuronal cell death

"beta Amyloid (Abeta)-induced free radical-mediated neurotoxicity" is a leading hypothesis as a cause of Alzheimer's disease (AD). Abeta increased free radical production and lipid peroxidation in PC12 nerve cells, leading to increased 4-hydroxy-2-nonenal (HNE) production and modification of specific mitochondrial target proteins, apoptosis and cell death. Pretreatment of the cells with isolated ginkgolides, the anti-oxidant component of Ginkgo biloba leaves, or vitamin E, prevented the Abeta-induced increase of reactive oxygen species (ROS). Ginkgolides, but not vitamin E, inhibited the Abeta-induced HNE modification of mitochondrial proteins. However, treatment with these anti-oxidants did not rescue the cells from Abeta-induced apoptosis and cell death. These results indicate that free radicals and lipid peroxidation may not mediate Abeta-induced neurotoxicity.

Mitochondrial peripheral-type benzodiazepine receptor expression. Correlation with gonadotropin-releasing hormone (GnRH) agonist-induced apoptosis in the corpus luteum

We have demonstrated that continuous administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) decreases the expression of the mitochondrial peripheral-type benzodiazepine receptor (PBR) and increases the rate of DNA degradation in a time-dependent manner in the corpora lutea of pregnant rats. In the present study, we show in situ the GnRH-Ag-induced DNA fragmentation and correlate the increase of the rate of DNA degradation with the decrease in mitochondrial PBR ligand binding (r = 0.89). The GnRH-Ag-induced decrease in the 18-kDa PBR protein also correlated with the reduction in the Bcl-X(L), but not Bcl-2 (cell survival), gene product levels and the increase in the Bax (cell death) gene product expression in the luteal mitochondrial preparations. Considering the function of PBR in cholesterol uptake and intramitochondrial movement, we propose that decreased PBR expression may lead to reduced levels of mitochondrial membrane cholesterol, which, together with the ability of Bcl-X(L) and Bax to form ion channels, produces breaks in the outer membranes allowing the exit of cytochrome c, thus triggering apoptosis. Alternatively, PBR may exert an as yet unidentified anti-apoptotic function.

HbOThrace trait, HbOThrace hemoglobinopathy and HbOThrace/Hb beta zero hemoglobinopathy: a retrospective study of 118 cases

HbOThrace is an abnormal hemoglobin which is constructed by defective beta chains (Glu121-->Lys121). It mostly characterises the Muslim minority of the Thrace region. During the last 6 years, our department detected HbOThrace in 118 cases. Four groups were formed, the first consisted of cases of HbOThrace trait with normal iron and ferritin levels, the second of cases of HbOThrace hemoglobinopathy, the third of cases of HbOThrace/Hb beta zero hemoglobinopathy and the fourth of cases of HbOThrace trait with low iron and ferritin levels. The second, third and fourth group were compared with the first one. The conclusion is that the presence of HbOThrace leads to a mild hypochromic anemia, with no clinical findings, but the coexistence of beta zero gene or iron deficiency produces more pronounced laboratory findings and even clinically evident anemia.

In vitro studies on the role of the peripheral-type benzodiazepine receptor in steroidogenesis

In vitro studies using isolated cells, mitochondria and submitochondrial fractions demonstrated that in steroid synthesizing cells, the peripheral-type benzodiazepine receptor (PBR) is an outer mitochondrial membrane protein, preferentially located in the outer/inner membrane contact sites, involved in the regulation of cholesterol transport from the outer to the inner mitochondrial membrane, the rate-determining step in steroid biosynthesis. Mitochondrial PBR ligand binding characteristics and topography are sensitive to hormone treatment suggesting a role of PBR in the regulation of hormone-mediated steroidogenesis. Targeted disruption of the PBR gene in Leydig cells in vitro resulted in the arrest of cholesterol transport into mitochondria and steroid formation; transfection of the mutant cells with a PBR cDNA rescued steroidogenesis demonstrating an obligatory role for PBR in cholesterol transport. Molecular modeling of PBR suggested that it might function as a channel for cholesterol. This hypothesis was tested in a bacterial system devoid of PBR and cholesterol. Cholesterol uptake and transport by these cells was induced upon PBR expression. Amino acid deletion followed by site-directed mutagenesis studies and expression of mutant PBRs demonstrated the presence in the cytoplasmic carboxy-terminus of the receptor of a cholesterol recognition/interaction amino acid consensus sequence. This amino acid sequence may help for recruiting the cholesterol coming from intracellular sites to the mitochondria.

Peripheral-type benzodiazepine receptor (PBR) in human breast cancer: correlation of breast cancer cell aggressive phenotype with PBR expression, nuclear localization, and PBR-mediated cell proliferation and nuclear transport of cholesterol

Aberrant cell proliferation and increased invasive and metastatic behavior are hallmarks of the advancement of breast cancer. Numerous studies implicate a role for cholesterol in the mechanisms underlying cell proliferation and cancer progression. The peripheral-type benzodiazepine receptor (PBR) is an Mr 18,000 protein primarily localized to the mitochondria. PBR mediates cholesterol transport across the mitochondrial membranes in steroidogenic cells. A role for PBR in the regulation of tumor cell proliferation has also been shown. In this study, we examined the expression, characteristics, localization, and function of PBR in a battery of human breast cancer cell lines differing in their invasive and chemotactic potential as well as in several human tissue biopsies. Expression of PBR ligand binding and mRNA was dramatically increased in the highly aggressive cell lines, such as MDA-231, relative to nonaggressive cell lines, such as MCF-7. PBR was also found to be expressed at high levels in aggressive metastatic human breast tumor biopsies compared with normal breast tissues. Subcellular localization with both antibodies and a fluorescent PBR drug ligand revealed that PBR from the MDA-231 cell line as well as from aggressive metastatic human breast tumor biopsies localized primarily in and around the nucleus. This localization is in direct contrast to the largely cytoplasmic localization seen in MCF-7 cells, normal breast tissue, and to the typical mitochondrial localization seen in mouse tumor Leydig cells. Pharmacological characterization of the receptor and partial nucleotide sequencing of PBR cDNA revealed that the MDA-231 PBR is similar, although not identical, to previously described PBR. Addition of high affinity PBR drug ligands to MDA-231 cells increased the incorporation of bromodeoxyuridine into the cells in a dose-dependent manner, suggesting a role for PBR in the regulation of MDA-231 cell proliferation. Cholesterol uptake into isolated MDA-231 nuclei was found to be 30% greater than into MCF-7 nuclei. High-affinity PBR drug ligands regulated the levels of cholesterol present in MDA-231 nuclei but not in MCF-7. In addition, the PBR-dependent MDA-231 cell proliferation was found to highly correlate (r = -0.99) with the PBR-mediated changes in nuclear membrane cholesterol levels. In conclusion, these data suggest that PBR expression, nuclear localization, and PBR-mediated cholesterol transport into the nucleus are involved in human breast cancer cell proliferation and aggressive phenotype expression, thus participating in the advancement of the disease.

Developmental expression of the peripheral-type benzodiazepine receptor and the advent of steroidogenesis in rat adrenal glands

Although the precise mechanism whereby cholesterol is transported across the outer mitochondrial membrane is uncertain, a multimeric receptor complex termed the peripheral-type benzodiazepine receptor (PBR) appears essential for this process. We therefore predicted that adrenal cells at different developmental stages would express PBR coincidentally with the advent of steroidogenesis. Adrenals of neonatal rats demonstrate greatly reduced sensitivity to ACTH that gradually increases after the first 2 weeks of life. Thus, neonates have lower circulating corticosterone levels following exposure to stress. We examined mitochondrial PBR ligand binding activity, immunoreactive (ir) PBR content, and adrenal sensitivity to ACTH in vivo and in vitro. Ontogeny of both mitochondrial PBR ligand binding capacity and irPBR directly paralleled that of ACTH-inducible steroidogenesis in isolated rat adrenal cells and in rats injected with ACTH. In addition, neonatal PBR had approximately 2-fold higher affinity for PK11195, a synthetic ligand that binds with high affinity to PBR. No correlation was observed during neonatal life between ir-steroidogenic acute regulatory (StAR) protein content and steroidogenesis. These results are consistent with the hypothesis that PBR is an absolute prerequisite for adrenocortical steroidogenesis, and suggest that the stress hyporesponsive period of neonatal rats may result from decreased PBR expression. In addition, the higher affinity of neonatal PBR and the relatively high basal expression of StAR protein in neonatal adrenals may partly explain the high constitutive steroidogenesis characteristic of neonatal rat adrenal cells.

GnRH agonist treatment decreases progesterone synthesis, luteal peripheral benzodiazepine receptor mRNA, ligand binding and steroidogenic acute regulatory protein expression during pregnancy

We have demonstrated that continuous administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) suppresses luteal steroidogenesis in the pregnant rat. We further demonstrated that the peripheral-type benzodiazepine receptor (PBR) and the steroidogenic acute regulatory protein (StAR) play key roles in cholesterol transport leading to steroidogenesis. The purpose of this study was to understand the cellular and molecular mechanisms involved in the suppression of luteal steroidogenesis leading to a fall in serum progesterone levels in GnRH-Ag-treated rats during early pregnancy. Pregnant rats were treated individually starting on day 8 of pregnancy with 5 microgram/day GnRH-Ag using an osmotic minipump. Sham-operated control rats received no treatment. At 0, 4, 8 and 24 h after initiation of the treatment, rats were killed and corpora lutea (CL) were removed for PBR mRNA, protein and radioligand binding analyses, immunoblot 1-D gel analysis of StAR, P450 scc and 3beta-hydroxysteroid dehydrogenase as well as 2-D gel analysis of StAR. The treatment decreased the luteal PBR mRNA expression at all time periods starting at 4 h compared with that in corresponding sham controls. GnRH-Ag also reduced, in the CL, the PBR protein/ligand binding, the StAR protein and P450 scc protein and its activity as early as 8 h after the treatment and they remained low compared with those in corresponding sham controls. The data from 2-D gel studies suggest that the majority of the decrease in StAR protein appears to be in the phosphorylated forms of StAR. Thus, we have demonstrated, for the first time, the presence of PBR and StAR in the pregnant rat CL and that the coordinated suppression of these proteins involved in the mitochondrial cholesterol transport along with P450 scc by GnRH-Ag leads to reduced ovarian steroidogenesis.