Specific binding of benzodiazepines to human breast cancer cell lines

TSPO ligands prevent the proliferation of vascular smooth muscle cells and attenuate neointima formation through AMPK activation

Abnormal growth of the intimal layer of blood vessels (neointima formation) contributes to the progression of atherosclerosis and in-stent restenosis. Recent evidence shows that the 18-kDa translocator protein (TSPO), a mitochondrial membrane protein, is involved in diverse cardiovascular diseases. In this study we investigated the role of endogenous TSPO in neointima formation after angioplasty in vitro and in vivo. We established a vascular injury model in vitro by using platelet-derived growth factor-BB (PDGF-BB) to stimulate rat thoracic aortic smooth muscle cells (A10 cells). We found that treatment with PDGF-BB (1–20 ng/mL) dose-dependently increased TSPO expression in A10 cells, which was blocked in the presence of PKC inhibitor or MAPK inhibitor. Overexpression of TSPO significantly promoted the proliferation and migration in A10 cells, whereas downregulation of TSPO expression by siRNA or treatment with TSPO ligands PK11195 or Ro5-4864 (10⁴ nM) produced the opposite effects. Furthermore, we found that PK11195 (10−10⁴ nM) dose-dependently activated AMPK in A10 cells. PK11195-induced inhibition on the proliferation and migration of PDGF-BB-treated A10 cells were abolished by compound C (an AMPK-specific inhibitor, 10³ nM). In rats with balloon-injured carotid arteries, TSPO expression was markedly upregulated in the carotid arteries. Administration of PK11195 (3 mg/kg every 3 days, ip), starting from the initial balloon injury and lasting for 2 weeks, greatly attenuated carotid neointima formation by suppressing balloon injury-induced phenotype switching of VSMCs (increased α-SMA expression). These results suggest that TSPO is a vascular injury-response molecule that promotes VSMC proliferation and migration and is responsible for the neointima formation after vascular injury, which provides a novel therapeutic target for various cardiovascular diseases including atherosclerosis and restenosis.

The Prognostic Value of Peripheral Benzodiazepine Receptor in Patients with Esophageal Squamous Cell Carcinoma

Background: The peripheral benzodiazepine receptor (PBR) has previously been reported as an oncogene in prostate, breast and colorectal cancers, but its prognostic value, biological behavior and function in esophageal squamous cell carcinoma (ESCC) has not been investigated. Methods: qRT-PCR, western blotting and immunohistochemistry (IHC) were used to detect PBR expression in ESCC and matched non-cancerous tissues. Based on all of the significantly independent factors, a nomogram was established to predict the prognosis of ESCC patients. In addition, we performed comprehensive in vitro experiments to study the functions of PBR in cell growth, colony formation, and migration ability, as well as its relationship with epithelial-mesenchymal transition (EMT) related proteins in ESCC cells. Results: The mRNA and protein expression levels of PBR in ESCC were higher than those in adjacent non-tumor esophageal epithelial tissues. The IHC results demonstrated that PBR expression was an independent prognostic factor in ESCC survival, patients with higher PBR expression had a poorer survival than those with low expression, and PBR expression was significantly associated with lymphoid nodal status. Furthermore, a nomogram was established to reliably predict the probability of death in ESCC patients, with a Harrell's c-index of 0.696. In the vitro experiments, knocking down the expression of PBR inhibited proliferation, colony formation and migration of ESCC cells, and regulated EMT-associated proteins (up-regulation of E-cadherin, ZO-1 and β-catenin and concomitant with down-regulation of Fibronectin and N-cadherin). Conclusions: PBR is an independent prognostic factor in ESCC, and it promotes ESCC progression and metastasis. Basing on PBR expression level, a nomogram is established and performs a well in predicting survival of ESCC patients.

Prognostic role of translocator protein and oxidative stress markers in chronic lymphocytic leukemia patients treated with bendamustine plus rituximab

Principally located in the outer mitochondrial membrane, the translocator protein (TSPO) is an 18-kDa transmembrane protein that is a key component of the mitochondrial permeability transition pore. TSPO is associated with a number of biological processes, including apoptosis, the regulation of cellular proliferation, porphyrin transport and heme biosynthesis, immunomodulation, anion transport and the regulation of steroidogenesis. Thus, numerous studies have proposed TSPO as a promising target for novel therapeutic agents, particularly for the treatment of cancer. In the present study, the response of 30 consecutive chronic lymphocytic leukemia (CLL) patients to bendamustine and rituximab treatment was evaluated according to TSPO expression levels. Furthermore, thiobarbituric acid reactive substances (TBARS) and nitric oxide (NO) levels, as well as caspase-3 activity were determined. Compared with the lymphocytes of healthy donors, the 30 consecutive CLL patients exhibited increased TSPO expression levels, decreased TBARS and NO levels and reduced caspase-3 activity. Six months after the treatment commenced, the TSPO/mitochondria ratio resembled that of the healthy controls in 24/30 CLL patients. In addition, an increase in TBARS and NO levels, two markers of oxidative stress, and a potentiation of caspase-3 activity in all responder patients was observed. Notably, the six patients who appeared to be resistant to treatment also displayed higher TSPO levels, and lower caspase-3 activity and TBARS levels. These data indicate that TSPO expression may be a molecular prognostic factor in CLL patients.

The translocator protein (TSPO): a novel target for cancer chemotherapy

The translocator protein (TSPO) is an 18 kDa transmembrane protein primarily found in the outer mitochondrial membrane where it forms a key part of the mitochondrial permeability transition pore (MPTP). Omnipresent in almost all tissues, TSPO up-regulation has been connected to neuronal damage and inflammation, making the protein an important bio-imaging marker for disease progression. More recently, TSPO has attracted attention as a possible molecular target for tumour imaging and chemotherapy. In this review we summarize TSPO's molecular characteristics and highlight research progress in recent years in the field of TSPO-targeted cancer diagnostics and treatments.

Antiproliferative effect of peripheral benzodiazepine receptor antagonist PK11195 in rat mammary tumor cells

This study aims to establish the antiproliferative effects of PK11195, a peripheral benzodiazepine receptor antagonist (PBR) in rat mammary tumor cells. Breast tumors were induced by administration of a carcinogen, dimethylbenz[a]anthracene to 50-day-old female rats maintained on a standard AIN-76A diet with casein as the protein source. The tumors were developed approximately after 120 days. The tumors were of grade I (20%), grade II (60%), and grade III (20%). The tumors were isolated and cultured in DMEM/F12 media with supplements. We characterized the properties of the isolated cells and study the effect of PK11195 on those cells. We were successful in growing breast tumor cells up to 30 passages for cellular characterization. These cells had high reactivity with Ki-67 and PCNA antibodies suggesting high proliferation rate. These cells were highly invasive as evident by matrigel invading ability. Furthermore, these cells acquired a positive response for CD-31 and VEGF antibodies suggesting angiogenic potential, and also possessed migrating ability/motility as evident by the wound healing properties. These cells expressed elevated levels of PBR, a cancer promoting gene. The proliferation, invasion and migration appear to decrease when treated with PK11195, a PBR antagonist. Furthermore, PK11195 treatment caused an increase in apoptosis as evident by increase in the levels of annexin V. However, the inhibition of cell proliferation by PK11195 was counteracted by Ro5-4864, a PBR agonist. Thus, PBR antagonist may be a potential therapeutic agent for the control of aggressiveness of breast cancer.

Translocator protein blockade reduces prostate tumor growth

PURPOSE

The transmembrane molecule, translocator protein (TSPO), has been implicated in the progression of epithelial tumors. TSPO gene expression is high in tissues involved in steroid biosynthesis, neurodegenerative disease, and in cancer, and overexpression has been shown to contribute to pathologic conditions including cancer progression in several different models. The goal of our study was to examine the expression and biological relevance of TSPO in prostate cancer and show that the commonly prescribed benzodiazepine lorazepam, a ligand for TSPO, exhibits anticancer properties.

EXPERIMENTAL DESIGN

Immunohistochemical analysis using tissue microarrays was used to determine the expression profile of TSPO in human prostate cancer tissues. To show the effect of TSPO ligands (lorazepam and PK11195) in prostate cancer, we used cell proliferation assays, apoptosis ELISA, prostate cancer xenograft study, and immunohistochemistry.

RESULTS

TSPO expression is increased in prostatic intraepithelial neoplasia, primary prostate cancer, and metastases compared with normal prostate tissue and benign prostatic hyperplasia. Furthermore, TSPO expression correlates with disease progression, as TSPO levels increased with increasing Gleason sum and stage with prostate cancer metastases demonstrating the highest level of expression among all tissues examined. Functionally, we have shown that lorazepam has antiproliferative and proapoptotic properties in vitro and in vivo. Additionally, we have shown that TSPO overexpression in nontumorigenic cells conferred susceptibility to lorazepam-induced growth inhibition.

CONCLUSION

These data suggest that blocking TSPO function in tumor cells induces cell death and denotes a survival role for TSPO in prostate cancer and provides the first evidence for the use of benzodiazepines in prostate cancer therapeutics.

Melatonin and vitamin C administration ameliorate diazepam-induced oxidative stress and cell proliferation in the liver of rats

OBJECTIVE

Oxidative stress is a likely molecular mechanism in long-term diazepam administration. The benefits of antioxidants (melatonin and vitamin C) against diazepam-induced cell proliferation, DNA synthesis and oxidative damage were investigated in this study.

MATERIALS & METHODS

Four equal-sized groups of male rats [control, diazepam (3 mg/kg), diazepam plus melatonin (5 mg/kg) and diazepam plus vitamin C (50 mg/kg)] were used. Levels of lipid peroxides (LPO), superoxide dismutase (SOD) activity and glutathione (GSH) concentration were measured in tissue homogenates. Cell proliferation and rate of DNA synthesis were detected by autoradiography.

RESULTS

Results documented increased labelling index, (3)H-thymidine incorporation (DNA synthesis), LPO plus decrease in GSH levels and SOD activity in livers of diazepam-administered rats versus those of controls. When melatonin and vitamin C were given to diazepam-administered rats, they almost attenuated the increase of labelling index, DNA synthesis and LPO, and restored the levels of GSH and SOD activity.

CONCLUSION

These results suggest long-term hazard in use of drugs such as diazepam; they may be toxic and damage terminates in complex liver damage. Furthermore, melatonin and vitamin C may be useful in combating free radical-induced liver injury resulting from hazard and/or repeated diazepam administration.

Identification, characterization and potent antitumor activity of ECO-4601, a novel peripheral benzodiazepine receptor ligand

PURPOSE

ECO-4601 is a structurally novel farnesylated dibenzodiazepinone discovered through DECIPHER technology, Thallion's proprietary drug discovery platform. The compound was shown to have a broad cytotoxic activity in the low micromolar range when tested in the NCI 60 cell line panel. In the work presented here, ECO-4601 was further evaluated against brain tumor cell lines. Preliminary mechanistic studies as well as in vivo antitumor evaluation were performed.

METHODS

Since ECO-4601 has a benzodiazepinone moiety, we first investigated if it binds the central and/or peripheral benzodiazepine receptors. ECO-4601 was tested in radioligand binding assays on benzodiazepine receptors obtained from rat hearts. The ability of ECO-4601 to inhibit the growth of CNS cancers was evaluated on a panel of mouse, rat and human glioma cell lines using a standard MTT assay. Antitumor efficacy studies were performed on gliomas (rat and human), human breast and human prostate mouse tumor xenografts. Antitumor activity and pharmacokinetic analysis of ECO-4601 was evaluated following intravenous (i.v.), subcutaneous (s.c.), and intraperitoneal (i.p.) bolus administrations.

RESULTS

ECO-4601 was shown to bind the peripheral but not the central benzodiazepine receptor and inhibited the growth of CNS tumor cell lines. Bolus s.c. and i.p. administration gave rise to low but sustained drug exposure, and resulted in moderate to significant antitumor activity at doses that were well tolerated. In a rat glioma (C6) xenograft model, ECO-4601 produced up to 70% tumor growth inhibition (TGI) while in a human glioma (U-87MG) xenograft, TGI was 34%. Antitumor activity was highly significant in both human hormone-independent breast (MDA-MB-231) and prostate (PC-3) xenografts, resulting in TGI of 72 and 100%, respectively. On the other hand, i.v. dosing was followed by rapid elimination of the drug and was ineffective.

CONCLUSIONS

Antitumor efficacy of ECO-4601 appears to be associated with the exposure parameter AUC and/or sustained drug levels rather than C (max). These in vivo data constitute a rationale for clinical studies testing prolonged continuous administration of ECO-4601.

Effects of peripheral-type benzodiazepine receptor ligands on Ehrlich tumor cell proliferation

Peripheral-type benzodiazepine receptors have been found throughout the body, and particularly, in high numbers, in neoplastic tissues such as the ovary, liver, colon, breast, prostate and brain cancer. Peripheral-type benzodiazepine receptor expression has been associated with tumor malignity, and its subcellular localization is important to define its function in tumor cells. We investigated the presence of peripheral-type benzodiazepine receptors in Ehrlich tumor cells, and the in vitro effects of peripheral-type benzodiazepine receptors ligands on tumor cell proliferation. Our results demonstrate the presence of peripheral-type benzodiazepine receptor in the nucleus of Ehrlich tumor cells (85.53+/-12.60%). They also show that diazepam and Ro5-4864 (peripheral-type benzodiazepine receptor agonists) but not clonazepam (a molecule with low affinity for the peripheral-type benzodiazepine receptor) decreased the percentage of tumor cells in G0-G1 phases and increased that of cells in S-G2-M phases. The effects of those agonists were prevented by PK11195 (a peripheral-type benzodiazepine receptor antagonist) that did not produce effects by itself. Altogether, these data suggest that the presence of peripheral-type benzodiazepine receptor within the nucleus of Ehrlich tumor cells is associated with tumor malignity and proliferation capacity.

Peripheral-type benzodiazepine receptor overexpression and knockdown in human breast cancer cells indicate its prominent role in tumor cell proliferation

The peripheral-type benzodiazepine receptor (PBR), an 18-kDa high affinity drug and cholesterol binding protein, is expressed at high levels in various cancers. Its expression is positively correlated with aggressive metastatic behavior in human breast cancer cells. To determine the role of PBR in tumor progression, two human mammary carcinoma cell lines were utilized: the non-aggressive MCF-7 cell line, which expresses extremely low PBR levels, and the highly aggressive MDA-MB-231 cell line, which has much higher PBR levels. We have generated stably transfected lines of the tetracycline-repressible MCF-7 cell line (MCF-7 Tet-Off) with inducible human PBR cDNA. Induction of PBR expression in MCF-7 Tet-Off cells increased PBR ligand binding and cell proliferation. Transfection of MDA-MB-231 cells with multiple siRNAs complementary to PBR (PBR-siRNAs) led to different levels of PBR mRNA knockdown. Lentiviral-mediated PBR RNA interference in MDA-MB-231 cells decreased PBR levels by 50%. Decreased PBR expression was associated with cell cycle arrest at G2 phase, decreased cell proliferation, and significant increases in the protein levels of the cyclin-dependent kinase inhibitor p21(WAF/CIP1). These changes were accompanied by p53 activation seen as increased p53 phosphorylation (Ser15). In parallel, increased proteolytic activation of caspase-3 was also observed. Taken together these results suggest that PBR protein expression is directly involved in regulating cell survival and proliferation in human breast cancer cells by influencing signaling mechanisms involved in cell cycle control and apoptosis.

Increased expression of peripheral benzodiazepine receptor (PBR) in dimethylbenz[a]anthracene-induced mammary tumors in rats

Expression of peripheral benzodiazepine receptors (PBR) has been found in every tissue examined; however, it is most abundant in steroid-producing tissues. Although the primary function of PBR is the regulation of steroidogenesis, its existence in nonsteroidogenic tissues as well as in other cellular compartments including the nucleus suggests that there may be other roles for PBR. Our laboratory reported earlier a significant increase of PBR density in the nucleus of DMBA-induced malignant submandibular glands of rats, suggesting a role of PBR in nuclear events of peripheral tissues. Since then numerous studies have demonstrated the abundance of PBR in tumors. Numerous studies implicate a role for cholesterol in the mechanisms underlying cell proliferation and cancer progression. Based on studies with a battery of human breast cancer cell lines and several human tissue biopsies, Hardwick et al. suggested 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. The purpose of the present study is to confirm this hypothesis by developing an animal breast cancer model and correlating the above events with the breast cancer. Weanling rats were maintained on a diet containing animal protein (casein) for 30 days and then a single dose of DMBA in sesame oil (80 mg/kg) was administered by gavage to the animals. Control animals received the vehicle only. After 122 days of DMBA administration, the animals were sacrificed. All tumors were detected by palpation. B(max) of PBRs was 52.6% and 128.4% higher in the non-aggressive and aggressive cancer tissues, respectively, than that in normal tissues. Cholesterol uptake into isolated nuclei was found to be higher in both non-aggressive and aggressive tumor breast tissue than that in control tissue. There was also corresponding increase in B(max) of PBRs in the nucleus of cancer tissues. Furthermore, the nuclear nucleoside triphosphatase (NTPase) activity was found to be higher in aggressive tumor tissues than that in non-aggressive tumor tissues. In conclusion, these data suggest that PBR ligand binding, and PBR-mediated cholesterol transport into the nucleus may be involved in the development of mammary gland adenocarcinoma, thus participating in the advancement of the disease.

Peripheral benzodiazepine receptor: characterization in human T-lymphoma Jurkat cells

Peripheral benzodiazepine receptor (PBR) has been considered a promising drug target for cancer therapy, and several ligands have been developed for this purpose. Human T-lymphoma Jurkat cells have been considered as lacking PBR and are often used as negative control to prove the specificity of PBR ligands effects. It is surprising that we evidenced PBR protein expression in this cell line by means of Western blotting and immunocytochemistry assays using specific anti-PBR antibodies. PBR intracellular localization was evidenced in mitochondria and nuclei, as demonstrated by confocal and electron microscopy. The binding of the [(3)H]4'-chloro derivative of diazepam [(3)H]7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin-2-one (Ro5-4864) and the isoquinoline carboxamide derivative [(3)H]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3 isoquinolinecarboxamide (PK11195) evidenced a single class of binding sites with an unusual affinity constant (K(d)) of 1.77 +/- 0.30 and 2.20 +/- 0.20 microM, respectively. The pharmacological profile of the classic ligands showed that PK11195 was the most potent inhibitor in the radioligand binding assays followed by Ro5-4864 and diazepam, whereas clonazepam, a specific ligand for the central-type receptor, showed a K(i) >1.0 x 10(-4) M. By a combined strategy of reverse transcriptase-polymerase chain reaction and Southern blot experiments, we succeeded in isolating and cloning the full-length Jurkat PBR cDNA, called JuPBR. The JuPBR gene showed two single-nucleotide polymorphisms resulting in the two substitutions, Ala147 --> threonine and His162 --> arginine, of PBR amino acidic sequence. In conclusion, for the first time, we demonstrated PBR expression in Jurkat cells: the protein bound classic PBR ligands with micromolar affinity constants and presented a modified amino acidic sequence consequent to the detection of two gene polymorphisms.

Mechanisms of mitochondrial apoptosis induced by peripheral benzodiazepine receptor ligands in human colorectal cancer cells

Specific ligands of the peripheral benzodiazepine receptor (PBR) have been shown to induce apoptosis in gastrointestinal cancers. The aim of this study was to characterize the signaling pathways of PBR ligand-induced apoptosis. FGIN-1-27 but not PK 11195-induced apoptosis was associated with a decrease of mitochondrial membrane potential and an increase of mitochondrial volume in HT29 colorectal cancer cells. However, PK 11195-elicited apoptosis was associated with a downregulation of Bcl-2, translocation of Bax to the mitochondria including subsequent oligomerization, and activation of caspase-9, indicating the involvement of mitochondria in PK 11195-induced apoptosis. Moreover, PK 11195-induced apoptosis was associated with the generation of reactive oxygen species. This study demonstrates a novel mechanism of PK 11195-induced mitochondrial apoptosis without alteration of the mitochondrial membrane potential. The characterization of signaling pathways associated with PBR ligand-induced apoptosis will build the base for a future use of these ligands in anti-neoplastic therapeutic approaches.

Modulation of cholinephosphotransferase activity in breast cancer cell lines by Ro5-4864, a peripheral benzodiazepine receptor agonist

Changes in phospholipid and fatty acid profile are hallmarks of cancer progression. Increase in peripheral benzodiazepine receptor expression has been implicated in breast cancer. The benzodiazepine, Ro5-4864, increases cell proliferation in some breast cancer cell lines. Biosynthesis of phosphatidylcholine (PC) has been identified as a marker for cells proliferating at high rates. Cholinephosphotransferase (CPT) is the terminal enzyme for the de novo biosynthesis of PC. We have addressed here whether Ro5-4864 facilitates some cancer causing mechanisms in breast cancer. We report that cell proliferation increases exponentially in aggressive breast cancer cell lines 11-9-1-4 and BT-549 when treated with nanomolar concentrations of Ro5-4864. This increase is seen within 24 h of treatment, consistent with the cell doubling time in these cells. Ro5-4864 also upregulates c-fos expression in breast cancer cell lines 11-9-1-4 and BT-549, while expression in non-tumorigenic cell line MCF-12A was either basal or slightly downregulated. We further examined the expression of the CPT gene in breast cancer (11-9-1-4, BT-549) and non-tumorigenic cell lines (MCF-12A, MCF-12F). We found that the CPT gene is overexpressed in breast cancer cell lines compared to the non-tumorigenic cell lines. Furthermore, the activity of CPT in forming PC is increased in the breast cancer cell lines cultured for 24 h. Additionally, we examined the CPT activity in the presence of nanomolar concentrations of Ro5-4864. Biosynthesis of PC was increased in breast cancer cell lines upon treatment. We therefore propose that Ro5-4864 facilitates PC formation, a process important in membrane biogenesis for proliferating cells.

Cell cycle-related signaling pathways modulated by peripheral benzodiazepine receptor ligands in colorectal cancer cells

Specific ligands of the peripheral benzodiazepine receptor (PBR) have been shown to induce both apoptosis and G1/G0 cell cycle arrest in colorectal cancers. The signaling pathways leading to cell cycle arrest are still unknown. Using cDNA array technology, we identified signaling molecules involved in cell cycle arrest induced by the PBR ligands FGIN-1-27 and PK 11195. Differential gene expression was confirmed by semi-quantitative RT-PCR or Western blot analysis of gene products. The PBR ligand-mediated signaling involved the upregulation of the cyclin-dependent kinase inhibitors p21WAF1/CIP1 and p27Kip1, cdc16, and the cell cycle inhibitors gadd45 and gadd153, the downregulation of the cyclins D1 and B1, as well as the inactivation of ERK1/2. The p21-deficient colorectal cancer cell line HCT116 p21-/- was significantly less sensitive to PBR ligands than the parental HCT116 wild-type cells, demonstrating the functional involvement of p21WAF1/CIP1 in PBR ligand-mediated G1 arrest. This study thus revealed PBR ligand-triggered signaling pathways leading to cell cycle arrest. Moreover, we showed the functional implication and interaction of differentially expressed gene products and provided a model of signaling pathways involved in PBR ligand-induced G1 arrest. These results form the basis for future PBR ligand-mediated therapeutic approaches.

Peripheral type benzodiazepine receptor in human parathyroid glands: up-regulation in adenoma

In this study we report the presence of peripheral benzodiazepine receptors (PBRs) in human parathyroid glands and describe the effect of their benzodiazepine type ligands on parathyroid cell function. PBR binding features in normal parathyroid tissue were characterized and compared to parathyroid adenoma, using a specific and selective ligand for PBR, [3H] 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline-carboxamide ([3H]PK11195). Affinity and density of [3H]PK11195 binding sites in homogenate membrane preparations from adenomatous and normal tissues were determined. Parathyroid adenoma showed a statistically significant 2.2 fold increase of [3H]PK11195 binding sites, while the affinity remained unchanged. Our results represent the first evidence of PBRs in parathyroid glands and suggest for them a role in influencing PTH release. A clear trend of PBR up-regulation in parathyroid adenoma was also found.

Peripheral-type benzodiazepine receptor (PBR) and PBR drug ligands in fibroblast and fibrosarcoma cell proliferation: role of ERK, c-Jun and ligand-activated PBR-independent pathways

Peripheral-type benzodiazepine receptor (PBR) is a 18-kDa high-affinity drug and cholesterol binding protein, that has been implicated in several physiological processes, such as cholesterol transport and mitochondrial respiration. Specific PBR ligands regulate cell proliferation, although their action is controversial and probably cell-type specific. The aim of the present study was to examine the expression of PBR in cells of mesenchymal origin, i.e. human fibroblasts and fibrosarcoma cells, as well as its role in the regulation of their proliferation. Both mesenchymal cell types express high levels of PBR, localized exclusively in mitochondria. PBR-specific drug ligands, the isoquinoline carboxamide PK 11195 and the benzodiazepine Ro5-4864, at relative high concentrations (10(-4)M), exert a strong inhibitory effect on cell proliferation by arresting the cells at the G0/G1 phase of the cell cycle, while no apoptotic cell death was observed. In normal fibroblasts, this inhibition was correlated with a decrease in the activation of the cell cycle markers ERK and c-Jun. PBR knockdown by RNA inhibition did not affect the proliferation of either cell type and did not influence the inhibitory effect of PK 11195 and Ro5-4864 on cell growth. These data suggest that in fibroblasts and fibrosarcoma cells PBR drug ligands inhibit cell proliferation in a PBR-independent manner. These results are in contrast to data reported on cells of epithelial origin, suggesting that the origin of the cells is crucial in defining the role of PBR in their proliferation, and raise caution in the commonly made assumption that PBR mediates cell functions affected by PBR drug ligands.

Binding characteristics of SSR180575, a potent and selective peripheral benzodiazepine ligand

The peripheral benzodiazepine receptor (PBR), has been recently shown to play a key role in the regulation of the mitochondrial process leading to apoptosis, which occurs during cardiac ischemia. The present work shows that SSR180575, a novel PBR ligand of potential interest in pathological cardiovascular indications, irreversibly and specifically binds with high affinity on both rat heart mitochondria and on a cell line transfected with the human PBR (K(d)=1.95+/-0.22 and 4.58+/-0.83nM, respectively). In conclusion, SSR180575 is a specific and potent PBR ligand which irreversible binding to PBR appears of high interest in various therapeutic indications where apoptosis occurs.

Ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in oesophageal cancer cells: involvement of the p38MAPK signalling pathway

Specific ligands of the peripheral benzodiazepine receptor (PBR) are known to induce apoptosis and cell cycle arrest in oesophageal cancer cells. However, the underlying mechanisms are still unknown. Here, we investigated the transcriptional alterations and activation of protein kinases in response to PBR-specific ligands. Using cDNA arrays, we examined the transcriptional effects of the PBR-specific ligand FGIN-1-27 in two oesophageal cancer cell lines, KYSE-140 (squamous cell carcinoma) and OE-33 (adenocarcinoma). In oesophageal cancer cells, FGIN-1-27 induced extensive changes in the expression of genes involved in the regulation of apoptosis and cell cycle. Both in oesophageal cancer cell lines (KYSE-140, OE-33) we observed a strong upregulation of the growth arrest and DNA-damage-inducible genes, gadd45 and gadd153, in response to PBR ligands. gadd genes are known to be induced by p38MAPK activation. Using Western blotting we detected a time- and dose-dependent phosphorylation of p38MAPK, which was found to be functionally involved in gadd induction, apoptosis, and cell cycle arrest. In conclusion, our data indicate that PBR-specific ligands cause apoptosis and cell cycle arrest by activation of the p38MAPK pathway and induction of gadd45 and gadd153.

Effects of peripheral benzodiazepine receptor ligands on proliferation and differentiation of human mesenchymal stem cells

The peripheral benzodiazepine receptor (PBR) has been known to have many functions such as a role in cell proliferation, cell differentiation, steroidogenesis, calcium flow, cellular respiration, cellular immunity, malignancy, and apoptosis. However, the presence of PBR has not been examined in mesenchymal stem cells. In this study, we demonstrated the expression of PBR in human bone marrow stromal cells (hBMSCs) and human adipose stromal cells (hATSCs) by RT-PCR and immunocytochemistry. To determine the roles of PBR in cellular functions of human mesenchymal stem cells (hMSCs), effects of diazepam, PK11195, and Ro5-4864 were examined. Adipose differentiation of hMSCs was decreased by high concentration of PBR ligands (50 microM), whereas it was increased by low concentrations of PBR ligands (<10 microM). PBR ligands showed a biphasic effect on glycerol-3-phosphate dehydrogenase (GPDH) activity. High concentration of PBR ligands (from 25 to 75 microM) inhibited proliferation of hMSCs. However, clonazepam, which does not have an affinity to PBR, did not affect adipose differentiation and proliferation of hMSCs. The PBR ligands did not induce cell death in hMSCs. PK11195 (50 microM) and Ro5-5864 (50 microM) induced cell cycle arrest in the G(2)/M phase. These results indicate that PBR ligands play roles in adipose differentiation and proliferation of hMSCs.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human esophageal cancer cells

Esophageal cancer is the most markedly increasing tumor entity in Western countries. Due to very poor 5-year-survival, new therapeutic approaches are mandatory. Peripheral benzodiazepine receptors (PBR) have been implicated in growth control of various tumor models, but they have not been studied yet in esophageal cancer. We used esophageal cancer cell lines and primary cell cultures of human esophageal cancers and evaluated (i) expression and localization of PBR; (ii) PBR-ligand-induced inhibition of cell growth; (iii) induction of apoptosis; and (iv) alterations in cell cycle. Expression of PBR was detected both in cell lines and in primary cell cultures of human esophageal cancers. PBR was localized in the mitochondria. The PBR-specific ligands FGIN-1-27 and PK 11195, but not the centrally acting benzodiazepine clonazepam or the indolacetamide FGIN-1-52, neither of which displaying any affinity to the PBR, inhibited cell proliferation. FGIN-1-27 and PK 11195, but not clonazepam, potently induced apoptosis. FGIN-1-27 was shown to sequentially decrease the mitochondrial membrane potential, then to activate caspase-3 and finally to cause DNA fragmentation. In addition, PBR-specific ligands induced cell cycle arrest in the G1/G0 phase. Our data qualify PBR-specific ligands as innovative proapoptotic and antiproliferative substances. They might prove suitable for the treatment of esophageal cancer.

Modulation of tamoxifen-induced apoptosis by peripheral benzodiazepine receptor ligands in breast cancer cells

The peripheral benzodiazepine receptor (PBR), an integral protein of the mitochondrial membrane, is involved in the formation of mitochondrial permeability transition (MPT) pores. The opening of the MPT-leading to the dissipation of the inner-mitochondrial transmembrane potential (deltapsi(m))-is considered to be an early apoptotic event. Therefore, we investigated the effect of the high-affinity PBR ligands Ro5-4684 and PK 11195 on tamoxifen (TAM)-induced apoptosis in MCF-7 and BT-20 breast cancer cell lines. Application of 100 nM TAM led to induction of apoptosis in both cell lines. Estrogene receptor (ER)-positive MCF-7 cells arrested in G(2/M) by TAM treatment showed no general dissipation of deltapsi(m), but reduction of deltapsi(m) was observed in a population of cells with high deltapsi(m). In ER-negative BT-20 cells TAM treatment induced no arrest of the cell cycle but dissipation of deltapsi(m). In both cell lines, nanomolar concentrations of the PBR ligands, which showed minor pro-apoptotic action themselves, reduced TAM-induced decrease of deltapsi(m) and apoptosis. In MCF-7 cells, a reduction of bcl-2 protein expression by TAM treatment was abolished by a combination of TAM with PBR ligands. Bax protein expression in BT-20 cells showed a significant increase in TAM-treated cells after 24hr but was not increased when treated with TAM and PBR ligands. From these findings, we concluded that binding of PBR ligands in nanomolar concentrations protects cells against apoptosis.

The peripheral benzodiazepine receptor in photodynamic therapy with the phthalocyanine photosensitizer Pc 4

The peripheral benzodiazepine receptor (PBR) is an 18 kDa protein of the outer mitochondrial membrane that interacts with the voltage-dependent anion channel and may participate in formation of the permeability transition pore. The physiological role of PBR is reflected in the high-affinity binding of endogenous ligands that are metabolites of both cholesterol and heme. Certain porphyrin precursors of heme can be photosensitizers for photodynamic therapy (PDT), which depends on visible light activation of porphyrin-related macrocycles. Because the apparent binding affinity of a series of porphyrin analogs for PBR paralleled their ability to photoinactivate cells, PBR has been proposed as the molecular target for porphyrin-derived photocytotoxicity. The phthalocyanine (Pc) photosensitizer Pc 4 accumulates in mitochondria and structurally resembles porphyrins. Therefore, we tested the relevance of PBR binding on Pc 4-PDT. Binding affinity was measured by competition with 3H-PK11195, a high-affinity ligand of PBR, for binding to rat kidney mitochondria (RKM) or intact Chinese hamster ovary (CHO) cells. To assess the binding of the Pc directly, we synthesized 14C-labeled Pc 4 and found that whereas Pc 4 was a competitive inhibitor of 3H-PK11195 binding to the PBR, PK11195 did not inhibit the binding of 14C-Pc 4 to RKM. Further, 14C-Pc 4 binding to RKM showed no evidence of saturation up to 10 microM. Finally, when Pc 4-loaded CHO cells were exposed to activating red light, apoptosis was induced; Pc 4-PDT was less effective in causing apoptosis in a companion cell line overexpressing the antiapoptotic protein Bcl-2. For both cell lines, PK11195 inhibited PDT-induced apoptosis; however, the inhibition was transient and did not extend to overall cell death, as determined by clonogenic assay. The results demonstrate (1) the presence of low-affinity binding sites for Pc 4 on PBR; (2) the presence of multiple binding sites for Pc 4 in RKM and CHO cells other than those that influence PK11195 binding; and (3) the ability of high supersaturating levels of PK11195 to transiently inhibit apoptosis initiated by Pc 4-PDT, with less influence on overall cell killing. We conclude that the binding of Pc 4 to PBR is less relevant to the photocytotoxicity of Pc 4-PDT than are other mitochondrial events, such as photodamage to Bcl-2 and that the observed inhibition of Pc 4-PDT-induced apoptosis by PK11195 likely occurs through a mechanism independent of PBR.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human colorectal cancer cells

The peripheral benzodiazepine receptor (PBR) has been implicated in growth control of various tumour models. Although colorectal cancers were found to overexpress PBR, the functional role of PBR in colorectal cancer growth has not been addressed to date. Using primary cell cultures of human colorectal cancers and the human colorectal carcinoma cell lines HT29, LS174T, and Colo320 DM we studied the involvement of PBR in the growth control and apoptosis of colorectal cancers. Both mRNA and protein expression of PBR were detected by RT-PCR and flow cytometry. Using confocal laser scanning microscopy and immunohistochemistry the PBR was localized in the mitochondria. The specific PBR ligands FGIN-1-27, PK 11195, or Ro5-4864 inhibited cell proliferation dose-dependently. FGIN-1-27 decreased the mitochondrial membrane potential, which indicates an early event in apoptosis. Furthermore, FGIN-1-27, PK 11195 or Ro5-4864 increased caspase-3 activity. In addition to their apoptosis-inducing effects, PBR ligands induced cell cycle arrest in the G(1)/G(0)-phase. Thus, our data demonstrate a functional involvement of PBR in colorectal cancer growth and qualify the PBR as a possible target for innovative therapeutic approaches in colorectal cancer.

Cell cycle-related expression and ligand binding of peripheral benzodiazepine receptor in human breast cancer cell lines

The aim of this study was to measure the expression of peripheral benzodiazepine receptors (PBR) as well as mitochondria content in different phases of the cell cycle of BT-20 and MCF-7 breast cancer cell lines, using two-parameter flow cytometric analyses. The PBR expression as well as mitochondria mass, were found to increase as cells pass through different stages of the cell cycle, whereas the amount of PBR in quiescent cells was very low. Binding capacity for the PBR ligand [3H]-Ro5-4864 was strongly related to the phase of the cell cycle with a positive correlation (r=0.98) with a high percentage of cells in S phase. Incubation of BT-20 cells in serum-deprived medium with nanomolar concentrations of Ro5-4864 caused an increase in S phase cells. This effect was not observed in MCF-7 cells. Using micromolar concentrations of Ro5-4864, both BT-20 and MCF-7 cells were reversibly arrested in the G(0/1) phase.

Relation of cell proliferation to expression of peripheral benzodiazepine receptors in human breast cancer cell lines

Peripheral benzodiazepine receptor (PBR) agonist [(3)H]Ro5-4864 has been shown to bind with high affinity to the human breast cancer cell line BT-20. Therefore, we investigated different human breast cancer cell lines with regard to binding to [(3)H]Ro5-4864 and staining with the PBR-specific monoclonal antibody 8D7. Results were correlated with cell proliferation characteristics. In flow cytometric analysis, the estrogen receptor (ER)-negative breast cancer cell lines BT-20, MDA-MB-435-S, and SK-BR-3 showed significantly higher PBR expression (relative fluorescence intensity) than the ER-positive cells T47-D, MCF-7 and BT-474 (P<0.05). Accordingly, BT-20 and MDA-MB-435-S had the highest capacity for binding [(3)H]-Ro5-4864, while the ER-positive cells exhibited only low binding of the benzodiazepine. PBR expression correlated inversely with cell doubling time (r = 0.78) and positively with Ki-67 expression (r = 0.77). The amount of mitochondria was significantly higher in cells with high PBR expression. As PBR could be demonstrated only after permeabilization of cells, PBR is suggested to be localized within the cytoplasm. Moreover, colocalization of PBR and mitochondria was shown by confocal microscopy analysis. The highest amounts of both PBR and mitochondria were found in cell lines with high mitotic activity. Therefore, it is concluded that the level of PBR is dependent on the number of mitochondria. PBR and its putative endogenous ligand diazepam-binding inhibitor are possibly involved in the regulation of cell proliferation of human breast cancer cell lines.

The triakontatetraneuropeptide (TTN) stimulates thymidine incorporation in rat astrocytes through peripheral-type benzodiazepine receptors

Astrocytes and astrocytoma cells actively express the diazepam-binding inhibitor (DBI) gene, suggesting that DBI-processing products may regulate glial cell activity. In the present study, we have investigated the possible effect of one of the DBI-derived peptides, the triakontatetraneuropeptide (TTN), on [(3)H]thymidine incorporation in cultured rat astrocytes. Reversed-phase HPLC analysis of incubation media indicated that TTN is the major form of DBI-derived peptides released by cultured astrocytes. At very low concentrations (10(-14)-10(-11) M), TTN induced a dose-dependent increase in [(3)H]thymidine incorporation, whereas at higher concentrations (10(-10)-10(-5) M) the effect of TTN gradually declined. In the same range of concentrations, the specific peripheral-type benzodiazepine receptor (PBR) agonist Ro 5-4864 mimicked the bell-shaped stimulatory effect of TTN on [(3)H]thymidine incorporation. The PBR antagonist PK11195 (10(-6) M) suppressed the stimulatory action of both TTN and Ro 5-4864 on [(3)H]thymidine incorporation, whereas the central-type benzodiazepine receptor antagonist flumazenil (10(-6) M) had no effect. The present study demonstrates that the endozepine TTN stimulates DNA synthesis in rat glial cells through activation of PBRs. These data strongly suggest that TTN exerts an autocrine/paracrine stimulatory effect on glial cell proliferation.