Inhibition of mitochondrial protein translation sensitizes melanoma cells to arsenic trioxide cytotoxicity via a reactive oxygen species dependent mechanism

Thiostrepton induces oxidative stress, mitochondrial dysfunction and ferroptosis in HaCaT cells

TST has been mainly studied for its anti-tumor proliferation and antimicrobial effects, but not widely used in dermatological diseases. The mechanism of cellular damage by TST in response to H2O2-mediated oxidative stress was investigated in human skin immortalized keratinocytes (HaCaT) as an in vitro model. The findings reveal that TST treatment leads to increased oxidative stress in the cells by reducing levels of superoxide dismutase (SOD), glutathione (GSH), and catalase (CAT). This effect is further supported by an upsurge in the expression of malondialdehyde (MDA, a pivotal marker of lipid peroxidation). Additionally, dysregulation of FoxM1 at both gene and protein levels corroborates its involvement TST associated effects. Analysis of ferroptosis-related genes confirms dysregulation following TST treatment in HaCaT cells. Furthermore, TST treatment exhibits effects on mitochondrial morphology and function, affirming its induction of apoptosis in the cells through heightened oxidative stress due to mitochondrial damage and dysregulation of mitochondrial membrane potential.

Unique Cellular and Biochemical Features of Human Mitochondrial Peroxiredoxin 3 Establish the Molecular Basis for Its Specific Reaction with Thiostrepton

A central hallmark of tumorigenesis is metabolic alterations that increase mitochondrial reactive oxygen species (mROS). In response, cancer cells upregulate their antioxidant capacity and redox-responsive signaling pathways. A promising chemotherapeutic approach is to increase ROS to levels incompatible with tumor cell survival. Mitochondrial peroxiredoxin 3 (PRX3) plays a significant role in detoxifying hydrogen peroxide (H₂O₂). PRX3 is a molecular target of thiostrepton (TS), a natural product and FDA-approved antibiotic. TS inactivates PRX3 by covalently adducting its two catalytic cysteine residues and crosslinking the homodimer. Using cellular models of malignant mesothelioma, we show here that PRX3 expression and mROS levels in cells correlate with sensitivity to TS and that TS reacts selectively with PRX3 relative to other PRX isoforms. Using recombinant PRXs 1–5, we demonstrate that TS preferentially reacts with a reduced thiolate in the PRX3 dimer at mitochondrial pH. We also show that partially oxidized PRX3 fully dissociates to dimers, while partially oxidized PRX1 and PRX2 remain largely decameric. The ability of TS to react with engineered dimers of PRX1 and PRX2 at mitochondrial pH, but inefficiently with wild-type decameric protein at cytoplasmic pH, supports a novel mechanism of action and explains the specificity of TS for PRX3. Thus, the unique structure and propensity of PRX3 to form dimers contribute to its increased sensitivity to TS-mediated inactivation, making PRX3 a promising target for prooxidant cancer therapy.

Environmental arsenic exposure: From genetic susceptibility to pathogenesis

More than 200 million people in 70 countries are exposed to arsenic through drinking water. Chronic exposure to this metalloid has been associated with the onset of many diseases, including cancer. Epidemiological evidence supports its carcinogenic potential, however, detailed molecular mechanisms remain to be elucidated. Despite the global magnitude of this problem, not all individuals face the same risk. Susceptibility to the toxic effects of arsenic is influenced by alterations in genes involved in arsenic metabolism, as well as biological factors, such as age, gender and nutrition. Moreover, chronic arsenic exposure results in several genotoxic and epigenetic alterations tightly associated with the arsenic biotransformation process, resulting in an increased cancer risk. In this review, we: 1) review the roles of inter-individual DNA-level variations influencing the susceptibility to arsenic-induced carcinogenesis; 2) discuss the contribution of arsenic biotransformation to cancer initiation; 3) provide insights into emerging research areas and the challenges in the field; and 4) compile a resource of publicly available arsenic-related DNA-level variations, transcriptome and methylation data. Understanding the molecular mechanisms of arsenic exposure and its subsequent health effects will support efforts to reduce the worldwide health burden and encourage the development of strategies for managing arsenic-related diseases in the era of personalized medicine.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Mitophagy inhibits proliferation by decreasing cyclooxygenase-2 (COX-2) in arsenic trioxide-treated HepG2 cells

Mitochondrial damage can trigger mitophagy and eventually suppress proliferation. However, the effect of mitophagy on proliferation remains unclear. In this study, HepG2 cells were used to assess mitophagy and proliferation arrest in response to As2O3 exposure. The stimulatory effect of As2O3 on mitophagy was investigated by assessing morphology (mitophagosome and mitolysosome) and relevant proteins (PINK1, LC3 II/I, and COX IV). Additionally, the relationship of mitophagy and proliferation was explored through the use of mitophagy inhibitors (CsA, Mdivi-1). Interestingly, the inhibition of mitophagy rescued proliferation arrest by restoring COX-2 protein level and countered the elimination of mitochondria-located COX-2 and up-regulated the COX-2 mRNA level. Taken together, our findings indicated that mitophagy can be induced and can inhibit proliferation by reducing COX-2 in HepG2 cells during As2O3 treatment.

Mitochondrial biogenesis is required for the anchorage-independent survival and propagation of stem-like cancer cells

Here, we show that new mitochondrial biogenesis is required for the anchorage independent survival and propagation of cancer stem-like cells (CSCs). More specifically, we used the drug XCT790 as an investigational tool, as it functions as a specific inhibitor of the ERRα-PGC1 signaling pathway, which governs mitochondrial biogenesis. Interestingly, our results directly demonstrate that XCT790 efficiently blocks both the survival and propagation of tumor initiating stem-like cells (TICs), using the MCF7 cell line as a model system. Mechanistically, we show that XCT790 suppresses the activity of several independent signaling pathways that are normally required for the survival of CSCs, such as Sonic hedgehog, TGFβ-SMAD, STAT3, and Wnt signaling. We also show that XCT790 markedly reduces oxidative mitochondrial metabolism (OXPHOS) and that XCT790-mediated inhibition of CSC propagation can be prevented or reversed by Acetyl-L-Carnitine (ALCAR), a mitochondrial fuel. Consistent with our findings, over-expression of ERRα significantly enhances the efficiency of mammosphere formation, which can be blocked by treatment with mitochondrial inhibitors. Similarly, mammosphere formation augmented by FOXM1, a downstream target of Wnt/β-catenin signaling, can also be blocked by treatment with three different classes of mitochondrial inhibitors (XCT790, oligomycin A, or doxycycline). In this context, our unbiased proteomics analysis reveals that FOXM1 drives the expression of >90 protein targets associated with mitochondrial biogenesis, glycolysis, the EMT and protein synthesis in MCF7 cells, processes which are characteristic of an anabolic CSC phenotype. Finally, doxycycline is an FDA-approved antibiotic, which is very well-tolerated in patients. As such, doxycycline could be re-purposed clinically as a 'safe' mitochondrial inhibitor, to target FOXM1 and mitochondrial biogenesis in CSCs, to prevent tumor recurrence and distant metastasis, thereby avoiding patient relapse.

Thiostrepton interacts covalently with Rpt subunits of the 19S proteasome and proteasome substrates

Here, we report a novel mechanism of proteasome inhibition mediated by Thiostrepton (Thsp), which interacts covalently with Rpt subunits of the 19S proteasome and proteasome substrates. We identified Thsp in a cell-based high-throughput screen using a fluorescent reporter sensitive to degradation by the ubiquitin-proteasome pathway. Thiostrepton behaves as a proteasome inhibitor in several paradigms, including cell-based reporters, detection of global ubiquitination status, and proteasome-mediated labile protein degradation. In vitro, Thsp does not block the chymotrypsin activity of the 26S proteasome. In a cell-based IκBα degradation assay, Thsp is a slow inhibitor and 4 hrs of treatment achieves the same effects as MG-132 at 30 min. We show that Thsp forms covalent adducts with proteins in human cells and demonstrate their nature by mass spectrometry. Furthermore, the ability of Thsp to interact covalently with the cysteine residues is essential for its proteasome inhibitory function. We further show that a Thsp modified peptide cannot be degraded by proteasomes in vitro. Importantly, we demonstrate that Thsp binds covalently to Rpt subunits of the 19S regulatory particle and forms bridges with a proteasome substrate. Taken together, our results uncover an important role of Thsp in 19S proteasome inhibition.

Disabling Mitochondrial Peroxide Metabolism via Combinatorial Targeting of Peroxiredoxin 3 as an Effective Therapeutic Approach for Malignant Mesothelioma

Dysregulation of signaling pathways and energy metabolism in cancer cells enhances production of mitochondrial hydrogen peroxide that supports tumorigenesis through multiple mechanisms. To counteract the adverse effects of mitochondrial peroxide many solid tumor types up-regulate the mitochondrial thioredoxin reductase 2 - thioredoxin 2 (TRX2) - peroxiredoxin 3 (PRX3) antioxidant network. Using malignant mesothelioma cells as a model, we show that thiostrepton (TS) irreversibly disables PRX3 via covalent crosslinking of peroxidatic and resolving cysteine residues in homodimers, and that targeting the oxidoreductase TRX2 with the triphenylmethane gentian violet (GV) potentiates adduction by increasing levels of disulfide-bonded PRX3 dimers. Due to the fact that activity of the PRX3 catalytic cycle dictates the rate of adduction by TS, immortalized and primary human mesothelial cells are significantly less sensitive to both compounds. Moreover, stable knockdown of PRX3 reduces mesothelioma cell proliferation and sensitivity to TS. Expression of catalase in shPRX3 mesothelioma cells restores defects in cell proliferation but not sensitivity to TS. In a SCID mouse xenograft model of human mesothelioma, administration of TS and GV together reduced tumor burden more effectively than either agent alone. Because increased production of mitochondrial hydrogen peroxide is a common phenotype of malignant cells, and TS and GV are well tolerated in mammals, we propose that targeting PRX3 is a feasible redox-dependent strategy for managing mesothelioma and other intractable human malignancies.

The redox-active nanomaterial toolbox for cancer therapy

Advances in nanomaterials science contributed in recent years to develop new devices and systems in the micro and nanoscale for improving the diagnosis and treatment of cancer. Substantial evidences associate cancer cells and tumor microenvironment with reactive oxygen species (ROS), while conventional cancer treatments and particularly radiotherapy, are often mediated by ROS increase. However, the poor selectivity and the toxicity of these therapies encourage researchers to focus efforts in order to enhance delivery and to decrease side effects. Thus, the development of redox-active nanomaterials is an interesting approach to improve selectivity and outcome of cancer treatments. Herein, we describe an overview of recent advances in redox nanomaterials in the context of current and emerging strategies for cancer therapy based on ROS modulation.

Combined arsenic trioxide-cisplatin treatment enhances apoptosis in oral squamous cell carcinoma cells

BACKGROUND

Oral squamous cell carcinoma (OSCC) accounts for the majority of oral cancers. Despite recent advances in OSCC diagnostics and therapeutics, the overall survival rate still remains low. Here, we assessed the efficacy of a combinatorial arsenic trioxide (ATO) and cisplatin (CDDP) treatment in human OSCC cells.

METHODS

The combinatorial effect of ATO/CDDP on the growth and apoptosis of OSCC cell lines HSC-2, HSC-3, and HSC-4 was evaluated using MTT and annexin V assays, respectively. Chou-Talalay analyses were preformed to evaluate the combinatorial effects of ATO/CDDP on the dose-reduction index (DRI). To clarify the mechanism underlying the ATO/CDDP anticancer effect, we also examined the involvement of reactive oxygen species (ROS) in ATO/CDDP-induced apoptosis.

RESULTS

Combination index (CI) analyses revealed that a synergistic interaction of ATO and CDDP elicits a wide range of effects in HSC-2 cells, with CI values ranging from 0.78 to 0.90, where CI < 1 defines synergism. The CI values in HSC-3 and HSC-4 cells ranged from 0.34 to 0.45 and from 0.60 to 0.92, respectively. In addition, ATO/CDDP yielded favorable DRI values ranging from 1.6-fold to 7.71-fold dose reduction. Compared to mono-therapy, ATO/CDDP combinatorial therapy significantly augmented the loss of mitochondrial potential, caspase-3/7 activity and subsequent apoptosis. These changes were all abrogated by the antioxidant N-acetylcysteine.

CONCLUSIONS

This study provides the first evidence for a synergistic ATO/CDDP anticancer (apoptotic) activity in OSCC cells with a favorable DRI, thereby highlighting its potential as a combinational therapeutic regime in OSCC.

Thiopeptide antibiotics: retrospective and recent advances

Thiopeptides, or thiazolyl peptides, are a relatively new family of antibiotics that already counts with more than one hundred different entities. Although they are mainly isolated from soil bacteria, during the last decade, new members have been isolated from marine samples. Far from being limited to their innate antibacterial activity, thiopeptides have been found to possess a wide range of biological properties, including anticancer, antiplasmodial, immunosuppressive, etc. In spite of their ribosomal origin, these highly posttranslationally processed peptides have posed a fascinating synthetic challenge, prompting the development of various methodologies and strategies. Regardless of their limited solubility, intensive investigations are bringing thiopeptide derivatives closer to the clinic, where they are likely to show their veritable therapeutic potential.

Expression of AFP and STAT3 is involved in arsenic trioxide-induced apoptosis and inhibition of proliferation in AFP-producing gastric cancer cells

Alpha-fetoprotein (AFP)-producing gastric cancer (AFPGC), represented by the production of AFP, has a more aggressive behavior than common gastric cancer. The underlying mechanisms are not well understood. Arsenic trioxide (As₂O₃) is used clinically to treat acute promyelocytic leukemia(APL) and has activity in vitro against several solid tumor cell lines, with induction of apoptosis and inhibition of proliferation the prime effects. Signal transducer and activator of transcription 3 (STAT3) has an important role in tumorigenesis of various primary cancers and cancer cell by upregulating cell-survival and downregulating tumor suppressor proteins. Here, we found decreased expression of AFP and STAT3 after induction of apoptosis by As₂O₃ in the AFPGC FU97 cells. Also, the level of the STAT3 target oncogene Bcl-2 was decreased with As₂O₃, and that of the tumor suppressor Bax was increased. Furthermore, STAT3 expression and depth of invasion and lymph node metastasis were associated. Survival of patients with gastric cancer was lower with AFP and STAT3 double overexpression than with overexpression of either alone. Downregulation of AFP and STAT3 expression plays an important role in As₂O₃-induced apoptosis of AFPGC cells, which suggests a new mechanism of As₂O₃-induced cell apoptosis. As₂O₃ may be a possible agent for AFPGC treatment.

Factors determining sensitivity and resistance of tumor cells to arsenic trioxide

Previously, arsenic trioxide showed impressive regression rates of acute promyelocytic leukemia. Here, we investigated molecular determinants of sensitivity and resistance of cell lines of different tumor types towards arsenic trioxide. Arsenic trioxide was the most cytotoxic compound among 8 arsenicals investigated in the NCI cell line panel. We correlated transcriptome-wide microarray-based mRNA expression to the IC(50) values for arsenic trioxide by bioinformatic approaches (COMPARE and hierarchical cluster analyses, Ingenuity signaling pathway analysis). Among the identified pathways were signaling routes for p53, integrin-linked kinase, and actin cytoskeleton. Genes from these pathways significantly predicted cellular response to arsenic trioxide. Then, we analyzed whether classical drug resistance factors may also play a role for arsenic trioxide. Cell lines transfected with cDNAs for catalase, thioredoxin, or the anti-apoptotic bcl-2 gene were more resistant to arsenic trioxide than mock vector transfected cells. Multidrug-resistant cells overexpressing the MDR1, MRP1 or BCRP genes were not cross-resistant to arsenic trioxide. Our approach revealed that response of tumor cells towards arsenic trioxide is multi-factorial.

Thiostrepton is an inducer of oxidative and proteotoxic stress that impairs viability of human melanoma cells but not primary melanocytes

Pharmacological induction of oxidative and proteotoxic stress has recently emerged as a promising strategy for chemotherapeutic intervention targeting cancer cells. Guided by a differential phenotypic drug screen for novel lead compounds that selectively induce melanoma cell apoptosis without compromising viability of primary human melanocytes, we have focused on the cyclic pyridinyl-polythiazolyl peptide-antimicrobial thiostrepton. Using comparative gene expression-array analysis, the early cellular stress response induced by thiostrepton was examined in human A375 metastatic melanoma cells and primary melanocytes. Thiostrepton displayed selective antimelanoma activity causing early induction of proteotoxic stress with massive upregulation of heat shock (HSPA6, HSPA1A, DNAJB4, HSPB1, HSPH1, HSPA1L, CRYAB, HSPA5, DNAJA1), oxidative stress (HMOX1, GSR, SOD1), and ER stress response (DDIT3) gene expression, confirmed by immunodetection (Hsp70, Hsp70B', HO-1, phospho-eIF2α). Moreover, upregulation of p53, proapoptotic modulation of Bcl-2 family members (Bax, Noxa, Mcl-1, Bcl-2), and induction of apoptotic cell death were observed. Thiostrepton rapidly induced cellular oxidative stress followed by inactivation of chymotrypsin-like proteasomal activity and melanoma cell-directed accumulation of ubiquitinated proteins, not observed in melanocytes that were resistant to thiostrepton-induced apoptosis. Proteotoxic and apoptogenic effects were fully antagonized by antioxidant intervention. In RPMI 8226 multiple myeloma cells, known to be exquisitely sensitive to proteasome inhibition, early proteotoxic and apoptogenic effects of thiostrepton were confirmed by array analysis indicating pronounced upregulation of heat shock response gene expression. Our findings demonstrate that thiostrepton displays dual activity as a selective prooxidant and proteotoxic chemotherapeutic, suggesting feasibility of experimental intervention targeting metastatic melanoma and other malignancies including multiple myeloma.

Biochemical and biosynthetic preparation of natural product-like cyclic peptide libraries

Natural product gene clusters are increasingly being used to compliment biochemical methods for production of cyclic peptide libraries.

The transcription factor FOXM1 is a cellular target of the natural product thiostrepton

Transcription factors are proteins that bind specifically to defined DNA sequences to promote gene expression. Targeting transcription factors with small molecules to modulate the expression of certain genes has been notoriously difficult to achieve. The natural product thiostrepton is known to reduce the transcriptional activity of FOXM1, a transcription factor involved in tumorigenesis and cancer progression. Herein we demonstrate that thiostrepton interacts directly with FOXM1 protein in the human breast cancer cells MCF-7. Biophysical analyses of the thiostrepton-FOXM1 interaction provide additional insights on the molecular mode of action of thiostrepton. In cellular experiments, we show that thiostrepton can inhibit the binding of FOXM1 to genomic target sites. These findings illustrate the potential druggability of transcription factors and provide a molecular basis for targeting the FOXM1 family with small molecules.

Anticancer activity of metal complexes: involvement of redox processes

Cells require tight regulation of the intracellular redox balance and consequently of reactive oxygen species for proper redox signaling and maintenance of metal (e.g., of iron and copper) homeostasis. In several diseases, including cancer, this balance is disturbed. Therefore, anticancer drugs targeting the redox systems, for example, glutathione and thioredoxin, have entered focus of interest. Anticancer metal complexes (platinum, gold, arsenic, ruthenium, rhodium, copper, vanadium, cobalt, manganese, gadolinium, and molybdenum) have been shown to strongly interact with or even disturb cellular redox homeostasis. In this context, especially the hypothesis of "activation by reduction" as well as the "hard and soft acids and bases" theory with respect to coordination of metal ions to cellular ligands represent important concepts to understand the molecular modes of action of anticancer metal drugs. The aim of this review is to highlight specific interactions of metal-based anticancer drugs with the cellular redox homeostasis and to explain this behavior by considering chemical properties of the respective anticancer metal complexes currently either in (pre)clinical development or in daily clinical routine in oncology.

The conformation change of Bcl-2 is involved in arsenic trioxide-induced apoptosis and inhibition of proliferation in SGC7901 human gastric cancer cells

Background

Arsenic trioxide has been established as a first-line agent for treating acute promyelocytic leukemia. Experimental data suggest that arsenic trioxide also can have a potential use as chemotherapeutic agent for other malignancies. The precise mechanisms of action of arsenic trioxide have though not been elucidated. As the role of Bcl-2 in arsenic trioxide-mediated cell apoptosis and conformation change of Bcl-2 in response to arsenic trioxide treatment has not been studied. The aim of the present study was to determine whether conformation change of Bcl-2 is involved in the action of arsenic trioxide.

Methods

Human gastric cancer SGC7901 cells were exposed to different concentrations of arsenic trioxide. Proliferation was measured by using the Kit-8 cell counting assay. Analysis of nuclear morphology was observed by DAPI staining. The apoptosis rates of cells treated with arsenic trioxide were analyzed by flow cytometry using Annexin V-FITC staining. The conformation change of Bcl-2 and Bax activation were detected by immunostaining and Western blot analysis. Total expression of Bcl-2 and Bax were examined by Western blot analysis.

Results

Arsenic trioxide inhibited the growth of human gastric cancer SGC7901 cells and induced apoptosis. There were two Bcl-2 phenotypes coexisting in SGC7901 cells and the Bcl-2 cytoprotective phenotype could change into a cytodestructive phenotype following conformational change of Bcl-2, triggered by arsenic trioxide exposure. Bax activation might also be involved in arsenic trioxide-induced Bcl-2 conformational change. Arsenic trioxide did not change levels of total Bcl-2 expression, but up-regulated total Bax expression for the treatment time ranging from 3 to 24 hours.

Conclusion

Arsenic trioxide induces apoptosis through induction of Bcl-2 conformational change, Bax activation and up-regulation of total Bax expression rather than affecting total Bcl-2 expression in human gastric cancer SGC7901 cells. The conformational change of Bcl-2 may be a novel described mechanism of arsenic trioxide-induced apoptosis in cancer cells.

Enhancement of arsenic trioxide cytotoxicity by dietary isothiocyanates in human leukemic cells via a reactive oxygen species-dependent mechanism

Although clearly effective in acute promyelocytic leukemia (APL), arsenic trioxide (ATO) demonstrates little clinical benefit as a single agent in the treatment of non-APL hematological malignancies. We screened a library of 2000 marketed drugs and naturally occurring compounds to identify agents that potentiate the cytotoxic effects of ATO in leukemic cells. Here, we report the identification of three isothiocyanates (sulforaphane, erysolin and erucin) found in cruciferous vegetables as enhancers of ATO cytotoxicity. Both erysolin and sulforaphane significantly enhanced ATO-mediated cytotoxicity and apoptosis in a panel of leukemic cell lines; erucin activity was variable among cell types. Cellular exposure to sulforaphane in combination with ATO resulted in a dramatic increase in levels of reactive oxygen species (ROS) compared to treatment with either agent alone. Sulforaphane, alone or with ATO, decreased intracellular glutathione (GSH) content. Furthermore, addition of the free radical scavenger N-acetyl-l-cysteine (NAC) rescued cells from ATO/isothiocyanate-mediated cytotoxicity. Our data suggest that isothiocyanates enhance the cytotoxic effects of ATO through a ROS-dependent mechanism. Combinatorial treatment with isothiocyanates and ATO might provide a promising therapeutic approach for a variety of myeloid malignancies.

Antifolate activity of pyrimethamine enhances temozolomide-induced cytotoxicity in melanoma cells

Most metastatic melanoma patients fail to respond to available therapy, underscoring the need to develop more effective treatments. We screened 2,000 compounds from the Spectrum Library in human melanoma cell lines to identify compounds that enhanced the cytotoxic effect of temozolomide, a drug used to treat metastatic melanoma. Screening was done with the temozolomide-resistant melanoma cell line SK-MEL-19, and six compounds were identified that had little or no inherent cytotoxicity but significantly enhanced growth-inhibition by temozolomide. These compounds were tested in five additional melanoma cell lines. Cell proliferation and death assays were used to compare the efficacy of single agent temozolomide versus combination treatments. Effects of combination treatment on levels of DNA double-strand breaks, the DNA repair protein O(6)-methylguanine-DNA-methyltransferase, apoptosis [measured by cleaved caspase-3 and poly(ADP-ribose) polymerase], and cell cycle were examined. Pyrimethamine, an antiparasitic, sensitized melanoma cells to temozolomide. Temozolomide combined with Pyrimethamine synergistically inhibited cell proliferation in melanoma cells with combination index values of 0.7 or less. In addition, combination treatment induced cell cycle arrest and increased both DNA damage and apoptosis. The increase in cell death due to combination treatment was rescued by leucovorin. Other folate antagonists were also effective enhancers of temozolomide-induced cytotoxicity, and the effects of antifolates were also evident in gliomas. Our screening approach led to the identification of Pyrimethamine, an orally available drug that efficiently crosses the blood-brain barrier, as a potent enhancer of the efficacy of temozolomide as an antineoplastic agent via inhibition of folate metabolism.

Mebendazole induces apoptosis via Bcl-2 inactivation in chemoresistant melanoma cells

Most metastatic melanoma patients fail to respond to available therapy, underscoring the need for novel approaches to identify new effective treatments. In this study, we screened 2,000 compounds from the Spectrum Library at a concentration of 1 micromol/L using two chemoresistant melanoma cell lines (M-14 and SK-Mel-19) and a spontaneously immortalized, nontumorigenic melanocyte cell line (melan-a). We identified 10 compounds that inhibited the growth of the melanoma cells yet were largely nontoxic to melanocytes. Strikingly, 4 of the 10 compounds (mebendazole, albendazole, fenbendazole, and oxybendazole) are benzimidazoles, a class of structurally related, tubulin-disrupting drugs. Mebendazole was prioritized to further characterize its mechanism of melanoma growth inhibition based on its favorable pharmacokinetic profile. Our data reveal that mebendazole inhibits melanoma growth with an average IC(50) of 0.32 micromol/L and preferentially induces apoptosis in melanoma cells compared with melanocytes. The intrinsic apoptotic response is mediated through phosphorylation of Bcl-2, which occurs rapidly after treatment with mebendazole in melanoma cells but not in melanocytes. Phosphorylation of Bcl-2 in melanoma cells prevents its interaction with proapoptotic Bax, thereby promoting apoptosis. We further show that mebendazole-resistant melanocytes can be sensitized through reduction of Bcl-2 protein levels, showing the essential role of Bcl-2 in the cellular response to mebendazole-mediated tubulin disruption. Our results suggest that this screening approach is useful for identifying agents that show promise in the treatment of even chemoresistant melanoma and identifies mebendazole as a potent, melanoma-specific cytotoxic agent.