Selective killing of cancer cells by a small molecule targeting the stress response to ROS

PGC1α expression defines a subset of human melanoma tumors with increased mitochondrial capacity and resistance to oxidative stress

Cancer cells reprogram their metabolism using different strategies to meet energy and anabolic demands to maintain growth and survival. Understanding the molecular and genetic determinants of these metabolic programs is critical to successfully exploit them for therapy. Here, we report that the oncogenic melanocyte lineage-specification transcription factor MITF drives PGC1α (PPARGC1A) overexpression in a subset of human melanomas and derived cell lines. Functionally, PGC1α positive melanoma cells exhibit increased mitochondrial energy metabolism and reactive oxygen species (ROS) detoxification capacities that enable survival under oxidative stress conditions. Conversely, PGC1α negative melanoma cells are more glycolytic and sensitive to ROS-inducing drugs. These results demonstrate that differences in PGC1α levels in melanoma tumors have a profound impact in their metabolism, biology, and drug sensitivity.

Ovarian cancer emerging subtypes: role of oxidative stress and fibrosis in tumour development and response to treatment

Epithelial ovarian cancer is a silent disease of usually late diagnosis and poor prognosis. Currently treatment options are limited and mainly consist of surgery followed by taxol- and platinum-based chemotherapy. Patient response to treatment is difficult to predict and there is a serious need for anticipating tumour response and orientating medical choices. In that aim, recent researches have focused on molecular aspects of ovarian tumours that could help patient stratification. We review here published discoveries in that field. We emphasize that signatures, defined by combining miRNA and transcriptomic data, enlighten important aspects of ovarian cancer biology and reliably stratify patients. The miR-200-dependent "Oxidative stress" and "Fibrosis" signatures are promising in patient stratification for defining oriented therapeutic strategies. Indeed, the "Stress" patients survive longer than the "Fibrosis" patients, who exhibit partial debulking and incomplete response to chemotherapy. Interestingly, these two subgroups might benefit from specifically targeted therapeutic approaches, as discussed here.

Non-enzymatic post-translational protein modifications and proteostasis network deregulation in carcinogenesis

Organisms are constantly challenged by stressors and thus the maintenance of biomolecules functionality is essential for the assurance of cellular homeostasis. Proteins carry out the vast majority of cellular functions by mostly participating in multimeric protein assemblies that operate as protein machines. Cells have evolved a complex proteome quality control network for the rescue, when possible, or the degradation of damaged polypeptides. Nevertheless, despite these proteostasis ensuring mechanisms, new protein synthesis, and the replication-mediated dilution of proteome damage in mitotic cells, the gradual accumulation of stressors during aging (or due to lifestyle) results in increasingly damaged proteome. Non-enzymatic post-translational protein modifications mostly arise by unbalanced redox homeostasis and/or high glucose levels and may cause disruption of proteostasis as they can alter protein function. This outcome may then increase genomic instability due to reduced fidelity in processes like DNA replication or repair. Herein, we present a synopsis of the major non-enzymatic post-translation protein modifications and of the proteostasis network deregulation in carcinogenesis. We propose that activation of the proteostasis ensuring mechanisms in premalignant cells has tumor-preventive effects, whereas considering that over-activation of these mechanisms represents a hallmark of advanced tumors, their inhibition provides a strategy for the development of anti-tumor therapies. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Targeting cancer cells with the natural compound obtusaquinone

BACKGROUND

Tumor cells present high levels of oxidative stress. Cancer therapeutics exploiting such biochemical changes by increasing reactive oxygen species (ROS) production or decreasing intracellular ROS scavengers could provide a powerful treatment strategy.

METHODS

To test the effect of our compound, obtusaquinone (OBT), we used several cell viability assays on seven different glioblastoma (GBM) cell lines and primary cells and on 12 different cell lines representing various cancer types in culture as well as on subcutaneous (n = 7 mice per group) and two intracranial GBM (n = 6-8 mice per group) and breast cancer (n = 6 mice per group) tumor models in vivo. Immunoblotting, immunostaining, flow cytometry, and biochemical assays were used to investigate the OBT mechanism of action. Histopathological analysis (n = 2 mice per group) and blood chemistry (n = 2 mice per group) were used to test for any compound-related toxicity. Statistical tests were two-sided.

RESULTS

OBT induced rapid increase in intracellular ROS levels, downregulation of cellular glutathione levels and increase in its oxidized form, and activation of cellular stress pathways and DNA damage, subsequently leading to apoptosis. Oxidative stress is believed to be the main mechanism through which this compounds targets cancer cells. OBT was well tolerated in mice, slowed tumor growth, and statistically prolonged survival in GBM tumor models. The ratio of median survival in U251 intracranial model in OBT vs control was 1.367 (95% confidence interval [CI] of ratio = 1.031 to 1.367, P = .008). Tumor growth inhibition was also observed in a mouse breast cancer model (average tumor volume per mouse, OBT vs control: 36.3 vs 200.4mm(3), difference = 164.1mm(3), 95% CI =72.6 to 255.6mm(3), P = .005).

CONCLUSIONS

Given its properties and efficacy in cancer killing, our results suggest that OBT is a promising cancer therapeutic.

Cytosine-based nucleoside analogs are selectively lethal to DNA mismatch repair-deficient tumour cells by enhancing levels of intracellular oxidative stress

BACKGROUND

DNA mismatch repair deficiency is present in a significant proportion of a number of solid tumours and is associated with distinct clinical behaviour.

METHODS

To identify the therapeutic agents that might show selectivity for mismatch repair-deficient tumour cells, we screened a pair of isogenic MLH1-deficient and MLH1-proficient tumour cell lines with a library of clinically used drugs. To test the generality of hits in the screen, selective agents were retested in cells deficient in the MSH2 mismatch repair gene.

RESULTS

We identified cytarabine and other related cytosine-based nucleoside analogues as being selectively toxic to MLH1 and MSH2-deficient tumour cells. The selective cytotoxicity we observed was likely caused by increased levels of cellular oxidative stress, as it could be abrogated by antioxidants.

CONCLUSION

We propose that cytarabine-based chemotherapy regimens may represent a tumour-selective treatment strategy for mismatch repair-deficient cancers.

Bifunctional electrophiles cross-link thioredoxins with redox relay partners in cells

Thioredoxin protects cells against oxidative damage by reducing disulfide bonds in improperly oxidized proteins. Previously, we found that the baker's yeast cytosolic thioredoxin Trx2 undergoes cross-linking to form several protein-protein complexes in cells treated with the bifunctional electrophile divinyl sulfone (DVSF). Here, we report that the peroxiredoxin Tsa1 and the thioredoxin reductase Trr1, both of which function in a redox relay network with thioredoxin, become cross-linked in complexes with Trx2 upon DVSF treatment. Treatment of yeast with other bifunctional electrophiles, including diethyl acetylenedicarboxylate (DAD), mechlorethamine (HN2), and 1,2,3,4-diepoxybutane (DEB), resulted in the formation of similar cross-linked complexes. Cross-linking of Trx2 and Tsa1 to other proteins by DVSF and DAD is dependent on modification of the active site Cys residues within these proteins. In addition, the human cytosolic thioredoxin, cytosolic thioredoxin reductase, and peroxiredoxin 2 form cross-linked complexes to other proteins in the presence of DVSF, although each protein shows different susceptibilities to modification by DAD, HN2, and DEB. Taken together, our results indicate that bifunctional electrophiles potentially disrupt redox homeostasis in yeast and human cells by forming cross-linked complexes between thioredoxins and their redox partners.

Stra6, a retinoic acid-responsive gene, participates in p53-induced apoptosis after DNA damage

Stra6 is the retinoic acid (RA)-inducible gene encoding the cellular receptor for holo-retinol binding protein. This transmembrane protein mediates the internalization of retinol, which then upregulates RA-responsive genes in target cells. Here, we show that Stra6 can be upregulated by DNA damage in a p53-dependent manner, and it has an important role in cell death responses. Stra6 expression induced significant amounts of apoptosis in normal and cancer cells, and it was also able to influence p53-mediated cell fate decisions by turning an initial arrest response into cell death. Moreover, inhibition of Stra6 severely compromised p53-induced apoptosis. We also found that Stra6 induced mitochondria depolarization and accumulation of reactive oxygen species, and that it was present not only at the cellular membrane but also in the cytosol. Finally, we show that these novel functions of Stra6 did not require downstream activation of RA signalling. Our results present a previously unknown link between the RA and p53 pathways and provide a rationale to use retinoids to upregulate Stra6, and thus enhance the tumour suppressor functions of p53. This may have implications for the role of vitamin A metabolites in cancer prevention and treatment.

Target identification for small bioactive molecules: finding the needle in the haystack

Identification and confirmation of bioactive small-molecule targets is a crucial, often decisive step both in academic and pharmaceutical research. Through the development and availability of several new experimental techniques, target identification is, in principle, feasible, and the number of successful examples steadily grows. However, a generic methodology that can successfully be applied in the majority of the cases has not yet been established. Herein we summarize current methods for target identification of small molecules, primarily for a chemistry audience but also the biological community, for example, the chemist or biologist attempting to identify the target of a given bioactive compound. We describe the most frequently employed experimental approaches for target identification and provide several representative examples illustrating the state-of-the-art. Among the techniques currently available, protein affinity isolation using suitable small-molecule probes (pulldown) and subsequent mass spectrometric analysis of the isolated proteins appears to be most powerful and most frequently applied. To provide guidance for rapid entry into the field and based on our own experience we propose a typical workflow for target identification, which centers on the application of chemical proteomics as the key step to generate hypotheses for potential target proteins.

Overview of the therapeutic potential of piplartine (piperlongumine)

Piplartine (piperlongumine, 5,6-dihydro-1-[(2E)-1-oxo-3-(3,4,5-trimethoxyphenyl)-2-propenyl]-2(1H)-pyridinone) is a biologically active alkaloid/amide from peppers, as from long pepper (Piper longum L. - Piperaceae). Long pepper is one of the most widely used in Ayurvedic medicine, which is used to treat many diseases, including tumors. The purpose of the current paper is to address to the chemical structure establishment and to systematically survey the published articles and highlight recent advances in the knowledge of the therapeutic potential of piplartine, establishing new goals for future research. The reported pharmacological activities of piplartine include cytotoxic, genotoxic, antitumor, antiangiogenic, antimetastatic, antiplatelet aggregation, antinociceptive, anxiolytic, antidepressant, anti-atherosclerotic, antidiabetic, antibacterial, antifungal, leishmanicidal, trypanocidal, and schistosomicidal activities. Among the multiple pharmacological effects of piplartine, its anticancer property is the most promising. Therefore, the preclinical anticancer potential of piplartine has been extensively investigated, which recently resulted in one patent. This compound is selectively cytotoxic against cancer cells by induction of oxidative stress, induces genotoxicity, as an alternative strategy to killing tumor cells, has excellent oral bioavailability in mice, inhibits tumor growth in mice, and presents only weak systemic toxicity. In summary, we conclude that piplartine is effective for use in cancer therapy and its safety using chronic toxicological studies should be addressed to support the viability of clinical trials.

Piperlongumine induces inhibition of the ubiquitin-proteasome system in cancer cells

Piperlongumine, a natural product from the plant Piperlongum, has demonstrated selective cytotoxicity to tumor cells and to show anti-tumor activity in animal models [1]. Cytotoxicity of piperlongumine has been attributed to increase in reactive oxygen species (ROS) in cancer cells. We here report that piperlongumine is an inhibitor of the ubiquitin-proteasome system (UPS). Exposure of tumor cells to piperlongumine resulted in accumulation of a reporter substrate known to be rapidly degraded by the proteasome, and of accumulation of ubiquitin conjugated proteins. However, no inhibition of 20S proteolytic activity or 19S deubiquitinating activity was observed at concentrations inducing cytotoxicity. Consistent with previous reports, piperlongumine induced strong ROS activation which correlated closely with UPS inhibition and cytotoxicity. Proteasomal blocking could not be mimicked by agents that induce oxidative stress. Our results suggest that the anti-cancer activity of piperlongumine involves inhibition of the UPS at a pre-proteasomal step, prior to deubiquitination of malfolded protein substrates at the proteasome, and that the previously reported induction of ROS is a consequence of this inhibition.

Annexin A2: the importance of being redox sensitive

Hydrogen peroxide (H₂O₂) is an important second messenger in cellular signal transduction. H₂O₂-dependent signalling regulates many cellular processes, such as proliferation, differentiation, migration and apoptosis. Nevertheless, H₂O₂ is an oxidant and a major contributor to DNA damage, protein oxidation and lipid peroxidation, which can ultimately result in cell death and/or tumourigenesis. For this reason, cells have developed complex antioxidant systems to scavenge ROS. Recently, our laboratory identified the protein, annexin A2, as a novel cellular redox regulatory protein. Annexin A2 possesses a reactive cysteine residue (Cys-8) that is readily oxidized by H₂O₂ and subsequently reduced by the thioredoxin system, thereby enabling annexin A2 to participate in multiple redox cycles. Thus, a single molecule of annexin A2 can inactivate several molecules of H₂O₂. In this report, we will review the studies detailing the reactivity of annexin A2 thiols and the importance of these reactive cysteine(s) in regulating annexin A2 structure and function. We will also focus on the recent reports that establish novel functions for annexin A2, namely as a protein reductase and as a cellular redox regulatory protein. We will further discuss the importance of annexin A2 redox regulatory function in disease, with a particular focus on tumour progression.

Dimovski A. Natural antioxidants in cancer prevention

It is well known that oxidative stress is an inevitable event in aerobic life. When our cells use oxygen to create energy, a variety of reactive oxygen (ROS) and nitrogen species (RNS) are generated. These species could attack DNA directly and form mutagenic lesions afterwards. According to the oxidative stress hypothesis of aging, the oxidative damage to critical molecules accumulates over the life period and could ultimately impair the body’s function. Moreover, severe oxidative stress causes mutations of tumor suppressor genes, known as one of the initial events in carcinogenesis. Furthermore, it could also play a crucial role in the promotion of the multi-step carcinogenesis. On the other hand, the human body possesses a number of mechanisms that counteract oxidative stress by producing antioxidants in situ, or externally supplied them through foods and/or supplements. Indeed, a considerable amount of laboratory evidence from chemical, cell culture, and animal studies indicates that antioxidants may slow down or possibly prevent the cancer development. Yet, the information from recent cohort, case-control and/or ecological studies is less clear. Therefore, the objectives of this review are to compile a compendium of studies, and to identify effective and promising natural antioxidant interventions.

Design, synthesis and evaluation of small molecule reactive oxygen species generators as selective Mycobacterium tuberculosis inhibitors

Here, we report 5-hydroxy-1,2,3,4,4a,9a-hexahydro-1,4-ethano-9,10-anthraquinone (13), a small molecule generating reactive oxygen species (ROS) in pH 7.4 buffer under ambient aerobic conditions that has selective and potent Mycobacterium tuberculosis growth inhibitory activity.

Piperlongumine inhibits proliferation and survival of Burkitt lymphoma in vitro

Piperlongumine (PL), a pepper plant alkaloid from Piper longum, kills solid tumor cells in a highly selective, potent fashion. To evaluate whether PL may have similar effects on malignant blood cells, we determined the efficacy with which PL inhibits the B-lymphocyte derived neoplasm, Burkitt lymphoma (BL). Low micromolar concentrations of PL (IC(50) = 2.8 μM × 8.5 μM) curbed growth and survival of two EBV(+) BL cell lines (Daudi, Raji) and two EBV BL cell lines (Ramos, DG-75), but left normal peripheral blood B-lymphocytes unharmed. PL-dependent cytotoxicity was effected in part by reduced NF-κB and MYC activity, with the former being caused by inhibition of IκBα degradation, nuclear translocation of p65, and binding of NF-κB dimers to cognate DNA sequences in gene promoters. In 4 of 4 BL cell lines, the NF-κB/MYC-regulated cellular target genes, E2F1 and MYB, were down regulated, while the stress sensor gene, GADD45B, was up regulated. The EBV-encoded oncogene, LMP-1, was suppressed in Daudi and Raji cells. Considering that NF-κB, MYC and LMP-1 play a crucial role in the biology of many blood cancers including BL, our results provide a strong preclinical rationale for considering PL in new intervention approaches for patients with hematologic malignancies.

Redox regulation in stem-like cancer cells by CD44 variant isoforms

Increasing evidence indicates that several types of solid tumor are hierarchically organized and sustained by a distinct population of cancer stem cells (CSCs). CSCs possess enhanced mechanisms of protection from stress induced by reactive oxygen species (ROS) that render them resistant to chemo- and radiotherapy. Expression of CD44, especially variant isoforms (CD44v) of this major CSC marker, contributes to ROS defense through upregulation of the synthesis of reduced glutathione (GSH), the primary intracellular antioxidant. CD44v interacts with and stabilizes xCT, a subunit of the cystine-glutamate transporter xc(-), and thereby promotes cystine uptake for GSH synthesis. Given that cancer cells are often exposed to high levels of ROS during tumor progression, the ability to avoid the consequences of such exposure is required for cancer cell survival and propagation in vivo. CSCs, in which defense against ROS is enhanced by CD44v are thus thought to drive tumor growth, chemoresistance and metastasis. Therapy targeted to the CD44v-xCT system may therefore impair the ROS defense ability of CSCs and thereby sensitize them to currently available treatments.

xCT inhibition depletes CD44v-expressing tumor cells that are resistant to EGFR-targeted therapy in head and neck squamous cell carcinoma

The targeting of antioxidant systems that allow stem-like cancer cells to avoid the adverse consequences of oxidative stress might be expected to improve the efficacy of cancer treatment. Here, we show that head and neck squamous cell carcinoma (HNSCC) cells that express variant isoforms of CD44 (CD44v) rely on the activity of the cystine transporter subunit xCT for control of their redox status. xCT inhibition selectively induces apoptosis in CD44v-expressing tumor cells without affecting CD44v-negative differentiated cells in the same tumor. In contrast to CD44v-expressing undifferentiated cells, CD44v-negative differentiated cells manifest EGF receptor (EGFR) activation and rely on EGFR activity for their survival. Combined treatment with inhibitors of xCT-dependent cystine transport and of EGFR resulted in a synergistic reduction of EGFR-expressing HNSCC tumor growth. Thus, xCT-targeted therapy may deplete CD44v-expressing undifferentiated HNSCC cells and concurrently sensitize the remaining differentiating cells to available treatments including EGFR-targeted therapy.

Oxidants, antioxidants and the current incurability of metastatic cancers

The vast majority of all agents used to directly kill cancer cells (ionizing radiation, most chemotherapeutic agents and some targeted therapies) work through either directly or indirectly generating reactive oxygen species that block key steps in the cell cycle. As mesenchymal cancers evolve from their epithelial cell progenitors, they almost inevitably possess much-heightened amounts of antioxidants that effectively block otherwise highly effective oxidant therapies. Also key to better understanding is why and how the anti-diabetic drug metformin (the world's most prescribed pharmaceutical product) preferentially kills oxidant-deficient mesenchymal p53^{− −}cells. A much faster timetable should be adopted towards developing more new drugs effective against p53^{− −} cancers.

Oxidative stress and cancer: an overview

Reactive species, which mainly include reactive oxygen species (ROS), are products generated as a consequence of metabolic reactions in the mitochondria of eukaryotic cells. In normal cells, low-level concentrations of these compounds are required for signal transduction before their elimination. However, cancer cells, which exhibit an accelerated metabolism, demand high ROS concentrations to maintain their high proliferation rate. Different ways of developing ROS resistance include the execution of alternative pathways, which can avoid large amounts of ROS accumulation without compromising the energy demand required by cancer cells. Examples of these processes include the guidance of the glycolytic pathway into the pentose phosphate pathway (PPP) and/or the generation of lactate instead of employing aerobic respiration in the mitochondria. Importantly, ROS levels can be used as a thermostat to monitor the damage that cells can bear. The implications for ROS regulation are highly significant for cancer therapy because commonly used radio- and chemotherapeutic drugs influence tumor outcome through ROS modulation. Moreover, the discovery of novel biomarkers that are able to predict the clinical response to pro-oxidant therapies is a crucial challenge to overcome to allow for the personalization of cancer therapies.

Piperlongumine induces rapid depletion of the androgen receptor in human prostate cancer cells

BACKGROUND

Androgen receptor (AR) signaling is regarded as the driving force in prostate carcinogenesis, and its modulation represents a logical target for prostate cancer (PC) prevention and treatment. Natural products are the most consistent source of small molecules for drug development. In this study, we investigate the functional impact of piperlongumine (PL), a naturally occurring alkaloid present in the Long pepper (Piper longum), on AR expression in PC cells and delineate its mechanism of action.

METHODS

Expression and transcriptional activity of AR was examined by western blotting and luciferase reporter assay, respectively. CellTiter Blue assay was utilized to quantify cell proliferation. Reactive oxygen species (ROS) generation was examined by staining cells with a ROS indicator CM-H(2) DCFDA, followed by flow cytometry analysis.

RESULTS

The results of our experiments demonstrate that PL rapidly reduces AR protein levels in PC cells via proteasome-mediated ROS-dependent mechanism. Moreover, PL effectively depletes a modified AR lacking the ligand-binding domain, shedding light on a new paradigm in the treatment approach to prostatic carcinoma that expresses mutated constitutively active AR. Importantly, PL effectively depletes AR in PC cells at low micromolar concentrations, while concurrently exerting a significant inhibitory effect on AR transcriptional activity and proliferation of PC cells.

CONCLUSIONS

Our investigation demonstrates for the first time that PL induces rapid depletion of the AR in PC cells. As such, PL may afford novel opportunities for both prevention and treatment of prostatic malignancy.

Hace1 controls ROS generation of vertebrate Rac1-dependent NADPH oxidase complexes

The Hace1-HECT E3 ligase is a tumor suppressor that ubiquitylates the activated GTP-bound form of the Rho family GTPase Rac1, leading to Rac1 proteasomal degradation. Here we show that, in vertebrates, Hace1 targets Rac1 for degradation when Rac1 is localized to the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase holoenzyme. This event blocks de novo reactive oxygen species generation by Rac1-dependent NADPH oxidases, and thereby confers cellular protection from reactive oxygen species-induced DNA damage and cyclin D1-driven hyper-proliferation. Genetic inactivation of Hace1 in mice or zebrafish, as well as Hace1 loss in human tumor cell lines or primary murine or human tumors, leads to chronic NADPH oxidase-dependent reactive oxygen species elevation, DNA damage responses and enhanced cyclin D1 expression. Our data reveal a conserved ubiquitin-dependent molecular mechanism that controls the activity of Rac1-dependent NADPH oxidase complexes, and thus constitutes the first known example of a tumor suppressor protein that directly regulates reactive oxygen species production in vertebrates.

T63, a new 4-arylidene curcumin analogue, induces cell cycle arrest and apoptosis through activation of the reactive oxygen species-FOXO3a pathway in lung cancer cells

Curcumin (diferuloylmethane) is a natural polyphenol product of the plant Curcuma longa and has a diversity of antitumor activities. T63, a new 4-arylidene curcumin analogue, was reported to inhibit proliferation of lung cancer cells. However, its precise molecular antitumor mechanisms have not been well elucidated. Here, we showed that T63 could significantly inhibit the proliferation of A549 and H460 human lung cell lines via induction of G0/G1 cell cycle arrest and apoptosis. We found that the reactive oxygen species (ROS)-activated FOXO3a cascade plays a central role in T63-induced cell proliferation inhibition. Mechanistically, enhancement of ROS production by T63 induced FOXO3a expression and nuclear translocation through activation of p38MAPK and inhibition of AKT, subsequently elevating the expression of FOXO3a target genes, including p21, p27, and Bim, and then increased the levels of activated caspase-3 and decreased the levels of cyclin D1. Moreover, the antioxidant N-acetylcysteine markedly blocked the above effects, and small interfering RNA-mediated knockdown of FOXO3a also significantly decreased T63-induced cell cycle arrest and apoptosis. In vivo experiments showed that T63 significantly suppressed the growth of A549 lung cancer xenograft tumors, associated with proliferation suppression and apoptosis induction in tumor tissues, without inducing any notable major organ-related toxicity. These data indicated that the novel curcumin analogue T63 is a potent antitumor agent that induces cell cycle arrest and apoptosis and has significant therapeutic potential for lung cancer.

Starvation-induced activation of ATM/Chk2/p53 signaling sensitizes cancer cells to cisplatin

Background

Optimizing the safety and efficacy of standard chemotherapeutic agents such as cisplatin (CDDP) is of clinical relevance. Serum starvation in vitro and short-term food starvation in vivo both stress cells by the sudden depletion of paracrine growth stimulation.

Methods

The effects of serum starvation on CDDP toxicity were investigated in normal and cancer cells by assessing proliferation, cell cycle distribution and activation of DNA-damage response and of AMPK, and were compared to effects observed in cells grown in serum-containing medium. The effects of short-term food starvation on CDDP chemotherapy were assessed in xenografts-bearing mice and were compared to effects on tumor growth and/or regression determined in mice with no diet alteration.

Results

We observed that serum starvation in vitro sensitizes cancer cells to CDDP while protecting normal cells. In detail, in normal cells, serum starvation resulted in a complete arrest of cellular proliferation, i.e. depletion of BrdU-incorporation during S-phase and accumulation of the cells in the G0/G1-phase of the cell cycle. Further analysis revealed that proliferation arrest in normal cells is due to p53/p21 activation, which is AMPK-dependent and ATM-independent. In cancer cells, serum starvation also decreased the fraction of S-phase cells but to a minor extent. In contrast to normal cells, serum starvation-induced p53 activation in cancer cells is both AMPK- and ATM-dependent. Combination of CDDP with serum starvation in vitro increased the activation of ATM/Chk2/p53 signaling pathway compared to either treatment alone resulting in an enhanced sensitization of cancer cells to CDDP. Finally, short-term food starvation dramatically increased the sensitivity of human tumor xenografts to cisplatin as indicated not only by a significant growth delay, but also by the induction of complete remission in 60% of the animals bearing mesothelioma xenografts, and in 40% of the animals with lung carcinoma xenografts.

Conclusion

In normal cells, serum starvation in vitro induces a cell cycle arrest and protects from CDDP induced toxicity. In contrast, proliferation of cancer cells is only moderately reduced by serum starvation whereas CDDP toxicity is enhanced. The combination of CDDP treatment with short term food starvation improved outcome in vivo. Therefore, starvation has the potential to enhance the therapeutic index of cisplatin-based therapy.

Targeting DNA repair mechanisms in cancer

Preservation of genomic integrity is an essential process for cell homeostasis. DNA-damage response (DDR) promotes faithful transmission of genomes in dividing cells by reversing the extrinsic and intrinsic DNA damage, and is required for cell survival during replication. Radiation and genotoxic drugs have been widely used in the clinic for years to treat cancer but DNA repair mechanisms are often associated with chemo- and radio-resistance. To increase the efficacy of these treatments, inhibitors of the major components of the DDR such as ATM (ataxia telangiectasia mutated), ATR (ATM and Rad3-related), DNA-PK (DNA-dependent protein kinase, catalytic subunit), Chk1 (checkpoint protein 1) and Chk2 (checkpoint protein 2) have been used to confer radio- and/or chemosensitivity upon cancer cells. The elucidation of the molecular mechanisms of DNA repair and the discovery that tumors are frequently repair-deficient provide a therapeutic opportunity to selectively target this deficiency. Genetic mutations in the DNA repair genes constitute not only the initiating event of the cancer cell but also its weakness since the mutated gene is often needed by the cancer cell to maintain its own survival. This weakness has been exploited to specifically kill the tumor cells while sparing the normal ones, a concept known as 'synthetic lethality'. Recent efforts in the design of cancer therapies are directed towards exploiting synthetic lethal interactions with cancer-associated mutations in the DDR. In this review, we will discuss the latest concepts in targeting DNA repair mechanisms in cancer and the novel and promising compounds currently in clinical trials.

How close are we to targeting the leukemia stem cell?

There are a number of approaches for selective targeting of leukemic stem cells (LSCs). These include targeting stem-cell properties, such as self-renewal, inducing cycling of quiescent LSCs to sensitize them to conventional agents, employing or inducing immune-based mechanisms, and targeting tumor-specific physiology. Agents such as parthenolide inhibit the ability of leukemic stem cells to respond to oxidative stress and make leukemic stem cells and bulk leukemic cells susceptible to cell death, while normal stem cells remain relatively unharmed by these agents. The major mechanism of action of these small molecules appears to revolve around the aberrant glutathione metabolism pathway found in leukemic cells.

Piperlongumine inhibits atherosclerotic plaque formation and vascular smooth muscle cell proliferation by suppressing PDGF receptor signaling

Piperlongumine (piplartine, PL) is an alkaloid found in the long pepper (Piper longum L.) and has well-documented anti-platelet aggregation, anti-inflammatory, and anti-cancer properties; however, the role of PL in prevention of atherosclerosis is unknown. We evaluated the anti-atherosclerotic potential of PL in an in vivo murine model of accelerated atherosclerosis and defined its mechanism of action in aortic vascular smooth muscle cells (VSMCs) in vitro. Local treatment with PL significantly reduced atherosclerotic plaque formation as well as proliferation and nuclear factor-kappa B (NF-κB) activation in an in vivo setting. PL treatment in VSMCs in vitro showed inhibition of migration and platelet-derived growth factor BB (PDGF-BB)-induced proliferation to the in vivo findings. We further identified that PL inhibited PDGF-BB-induced PDGF receptor beta activation and suppressed downstream signaling molecules such as phospholipase Cγ1, extracellular signal-regulated kinases 1 and 2 and Akt. Lastly, PL significantly attenuated activation of NF-κB-a downstream transcriptional regulator in PDGF receptor signaling, in response to PDGF-BB stimulation. In conclusion, our findings demonstrate a novel, therapeutic mechanism by which PL suppresses atherosclerosis plaque formation in vivo.

The mitochondrial citrate transporter, CIC, is essential for mitochondrial homeostasis

Dysregulation of the pathways that preserve mitochondrial integrity hallmarks many human diseases including diabetes, neurodegeration, aging and cancer. The mitochondrial citrate transporter gene, SLC25A1 or CIC, maps on chromosome 22q11.21, a region amplified in some tumors and deleted in developmental disorders known as velo-cardio-facial- and DiGeorge syndromes. We report here that in tumor cells CIC maintains mitochondrial integrity and bioenergetics, protects from mitochondrial damage and circumvents mitochondrial depletion via autophagy, hence promoting proliferation. CIC levels are increased in human cancers and its inhibition has anti-tumor activity, albeit with no toxicity on adult normal tissues. The knock-down of the CIC gene in zebrafish leads to mitochondria depletion and to proliferation defects that recapitulate features of human velo-cardio-facial syndrome, a phenotype rescued by blocking autophagy. Our findings reveal that CIC maintains mitochondrial homeostasis in metabolically active, high proliferating tissues and imply that this protein is a therapeutic target in cancer and likely, in other human diseases.

Inhibitor mediated protein degradation

The discovery of drugs that cause the degradation of their target proteins has been largely serendipitous. Here we report that the tert-butyl carbamate-protected arginine (Boc(3)Arg) moiety provides a general strategy for the design of degradation-inducing inhibitors. The covalent inactivators ethacrynic acid and thiobenzofurazan cause the specific degradation of glutathione-S-transferase when linked to Boc(3)Arg. Similarly, the degradation of dihydrofolate reductase is induced when cells are treated with the noncovalent inhibitor trimethoprim linked to Boc(3)Arg. Degradation is rapid and robust, with 30%-80% of these abundant target proteins consumed within 1.3-5 hr. The proteasome is required for Boc(3)Arg-mediated degradation, but ATP is not necessary and the ubiquitin pathways do not appear to be involved. These results suggest that the Boc(3)Arg moiety may provide a general strategy to construct inhibitors that induce targeted protein degradation.

Synthesis, cellular evaluation, and mechanism of action of piperlongumine analogs

Piperlongumine is a naturally occurring small molecule recently identified to be toxic selectively to cancer cells in vitro and in vivo. This compound was found to elevate cellular levels of reactive oxygen species (ROS) selectively in cancer cell lines. The synthesis of 80 piperlongumine analogs has revealed structural modifications that retain, enhance, and ablate key piperlongumine-associated effects on cells, including elevation of ROS, cancer cell death, and selectivity for cancer cells over nontransformed cell types. Structure/activity relationships suggest that the electrophilicity of the C2-C3 olefin is critical for the observed effects on cells. Furthermore, we show that analogs lacking a reactive C7-C8 olefin can elevate ROS to levels observed with piperlongumine but show markedly reduced cell death, suggesting that ROS-independent mechanisms, including cellular cross-linking events, may also contribute to piperlongumine's induction of apoptosis. In particular, we have identified irreversible protein glutathionylation as a process associated with cellular toxicity. We propose a mechanism of action for piperlongumine that may be relevant to other small molecules having two sites of reactivity, one with greater and the other with lesser electrophilicity.

Anti-cancer activity of a novel small molecule compound that simultaneously activates p53 and inhibits NF-κB signaling

The p53 and NF-κB pathways play important roles in diverse cellular functions, including cell growth, apoptosis, and tumorigenesis. Mutations that inactivate the p53 gene and constitutive NF-κB pathway activation are common occurrences in human cancers. Although many drugs are being developed that selectively activate p53 or inhibit NF-κB, there are few drug candidates that can do both. Simultaneous activation of p53 and inhibition of the NF-κB pathway is therefore a prime target for new cancer drug development. This study is the first report of a high-throughput approach with mass compounds that concurrently target both pathways. Using a cell-based screening assay and a library of 200,000 synthetic compounds, we identified 9 small molecules that simultaneously inhibit NF-κB and activate p53. One of these compounds, N-2, increased the expression of p53 target genes, including p21 and GADD45a. In addition, N-2 inhibited the transcriptional activity of NF-κB, concomitantly repressing interleukin-6 and monocyte chemotactic protein-1 (MCP-1) expression. When cell lines derived from a diverse range of cancers were treated in vitro with N-2, we observed increased cell death. N-2 also significantly inhibited allograft growth in murine models of melanoma and lung carcinoma. Our findings suggest that N-2 may act as a bivalent anti-cancer agent through simultaneous modulation of NF-κB and p53 activities.

Transcriptional regulation of the GPX1 gene by TFAP2C and aberrant CpG methylation in human breast cancer

The complexity of gene regulation has created obstacles to defining mechanisms that establish the patterns of gene expression characteristic of the different clinical phenotypes of breast cancer. TFAP2C is a transcription factor that has a critical role in the regulation of both estrogen receptor-alpha (ERα) and c-ErbB2/HER2 (Her2). Herein, we performed chromatin immunoprecipitation and direct sequencing (ChIP-seq) for TFAP2C in four breast cancer cell lines. Comparing the genomic binding sites for TFAP2C, we identified that glutathione peroxidase (GPX1) is regulated by TFAP2C through an AP-2 regulatory region in the promoter of the GPX1 gene. Knockdown of TFAP2C, but not the related factor TFAP2A, resulted in an abrogation of GPX1 expression. Selenium-dependent GPX activity correlated with endogenous GPX1 expression and overexpression of exogenous GPX1 induced GPX activity and significantly increased resistance to tert-butyl hydroperoxide. Methylation of the CpG island encompassing the AP-2 regulatory region was identified in cell lines where TFAP2C failed to bind the GPX1 promoter and GPX1 expression was unresponsive to TFAP2C. Furthermore, in cell lines where GPX1 promoter methylation was associated with gene silencing, treatment with 5'-aza-2-deoxycytidine (5'-aza-dC) (an inhibitor of DNA methylation) allowed TFAP2C to bind to the GPX1 promoter resulting in the activation of GPX1 RNA and protein expression. Methylation of the GPX1 promoter was identified in ∼20% of primary breast cancers and a highly significant correlation between the TFAP2C and GPX1 expression was confirmed when considering only those tumors with an unmethylated promoter, whereas the related factor, TFAP2A, failed to demonstrate a correlation. The results demonstrate that TFAP2C regulates the expression of GPX1, which influences the redox state and sensitivity to oxidative stress induced by peroxides. Given the established role of GPX1 in breast cancer, the results provide an important mechanism for TFAP2C to further influence oncogenesis and progression of breast carcinoma cells.

The tyrphostin AG1478 augments oridonin-induced A431 cell apoptosis by blockage of JNK MAPK and enhancement of oxidative stress

Oridonin, a diterpenoid compound, extracted and purified from Rabdosia rubescen has been reported to have cytotoxic effect on tumour cells through apoptosis, and tyrosine kinase pathways are involved in these processes. A specific epidermal growth factor receptor (EGFR) inhibitor AG1478 was used to examine the relationship between EGFR signal pathways and oridonin-induced apoptosis and autophagy in EGFR abundant human epidermoid carcinoma A431 cells. Inhibition of EGFRaugmented oridonin-induced A431 cell apoptosis, while the changes of expression of downstream proteins, Bcl-2, Bcl-xL, Bax, cytochrome c, pro-caspase-3, Fas, FADD and pro-caspase-8 suggested that both the intrinsic and extrinsic apoptotic pathways are involved in these processes. Pretreatment with AG1478 aggravated oridonin-induced loss of mitochondrial membrane potential (MMP) and increased ROS generation in A431 cells, while a ROS scavenger, N-acetylcysteine (NAC) completely reversed oridonin- and AG1478-induced ROS generation and apoptosis. Therefore, AG1478 augmented oridonin-induced apoptosis by enhancing oxidative stress. Pretreatment with AG1478 decreased the expression of downstream MAPK proteins ERK, JNK and P38 and their phosphorylated forms to varying degrees compared with oridonin alone treatment. Then after administration of ERK, JNK and P38 inhibitors, only JNK inhibitor SP600125 effectively augmented oridonin-induced apoptosis and ROS generation. Therefore, in EGFR downstream pathways, JNK played a major role in preventing oridonin-induced apoptosis. Autophagy antagonised apoptosis and exerted a protective effect in A431 cells, and both AG1478 and SP600125 decreased oridonin-induced autophagy. Inhibition of EGFR augmented oridonin-induced apoptosis and this was caused by enhanced oxidative stress, and JNK played a major protective role by increasing autophagy, leading to antagonising apoptosis and ROS generation.

Pathway of cytotoxicity induced by folic acid modified selenium nanoparticles in MCF-7 cells

Selenium nanoparticles (Se NPs) have been recognized as promising materials for biomedical applications. To prepare Se NPs which contained cancer targeting methods and to clarify the cellular localization and cytotoxicity mechanisms of these Se NPs against cancer cells, folic acid protected/modified selenium nanoparticles (FA-Se NPs) were first prepared by a one-step method. Some morphologic and spectroscopic methods were obtained to prove the successfully formation of FA-Se NPs while free folate competitive inhibition assay, microscope, and several biological methods were used to determine the in vitro uptake, subcellular localization, and cytotoxicity mechanism of FA-Se NPs in MCF-7 cells. The results indicated that the 70-nm FA-Se NPs were internalized by MCF-7 cells through folate receptor-mediated endocytosis and targeted to mitochondria located regions through endocytic vesicles transporting. Then, the FA-Se NPs entered into mitochondria; triggered the mitochondria-dependent apoptosis of MCF-7 cells which involved oxidative stress, Ca(2)+ stress changes, and mitochondrial dysfunction; and finally caused the damage of mitochondria. FA-Se NPs released from broken mitochondria were transported into nucleus and further into nucleolus which then induced MCF-7 cell cycle arrest. In addition, FA-Se NPs could induce cytoskeleton disorganization and induce MCF-7 cell membrane morphology alterations. These results collectively suggested that FA-Se NPs could be served as potential therapeutic agents and organelle-targeted drug carriers in cancer therapy.

ROS-activated anticancer prodrugs: a new strategy for tumor-specific damage

Targeting tumor cells is an important strategy to improve the selectivity of cancer therapies. With the advanced studies in cancer biology, we know that cancer cells are usually under increased oxidative stress. The high level of reactive oxygen species in cancer cells has been exploited for developing novel therapeutic strategies to preferentially kill cancer cells. Our group, amongst others, have used boronic acids/esters as triggers for developing ROS-activated anticancer prodrugs that target cancer cells. The selectivity was achieved by combining a specific reaction between boronates and H2O2, with the efficient masking of drug toxicity in the prodrug via boronates. Prodrugs activated via ferrocene-mediated oxidation have also been developed to improve the selectivity of anticancer drugs. We describe how the strategies of ROS-activation can be used for further development of new ROS-targeting prodrugs, eventually leading to novel approaches and/or combined technology for more efficient and selective treatment of cancers.

Passenger deletions generate therapeutic vulnerabilities in cancer

Inactivation of tumour-suppressor genes by homozygous deletion is a prototypic event in the cancer genome, yet such deletions often encompass neighbouring genes. We propose that homozygous deletions in such passenger genes can expose cancer-specific therapeutic vulnerabilities when the collaterally deleted gene is a member of a functionally redundant family of genes carrying out an essential function. The glycolytic gene enolase 1 (ENO1) in the 1p36 locus is deleted in glioblastoma (GBM), which is tolerated by the expression of ENO2. Here we show that short-hairpin-RNA-mediated silencing of ENO2 selectively inhibits growth, survival and the tumorigenic potential of ENO1-deleted GBM cells, and that the enolase inhibitor phosphonoacetohydroxamate is selectively toxic to ENO1-deleted GBM cells relative to ENO1-intact GBM cells or normal astrocytes. The principle of collateral vulnerability should be applicable to other passenger-deleted genes encoding functionally redundant essential activities and provide an effective treatment strategy for cancers containing such genomic events.

Suppression of peroxiredoxin 4 in glioblastoma cells increases apoptosis and reduces tumor growth

Glioblastoma multiforme (GBM), the most common and aggressive primary brain malignancy, is incurable despite the best combination of current cancer therapies. For the development of more effective therapies, discovery of novel candidate tumor drivers is urgently needed. Here, we report that peroxiredoxin 4 (PRDX4) is a putative tumor driver. PRDX4 levels were highly increased in a majority of human GBMs as well as in a mouse model of GBM. Reducing PRDX4 expression significantly decreased GBM cell growth and radiation resistance in vitro with increased levels of ROS, DNA damage, and apoptosis. In a syngenic orthotopic transplantation model, Prdx4 knockdown limited GBM infiltration and significantly prolonged mouse survival. These data suggest that PRDX4 can be a novel target for GBM therapies in the future.

Specific 5-hydroxylation of piperlongumine by Beauveria bassiana ATCC 7159

A new hydroxylated derivative was efficiently prepared by transforming the natural anti-cancer product, piperlongumine, with Beauveria bassiana ATCC 7159. Its structure was determined to be 5-hydroxylpiperlongumine on the basis of the spectroscopic data. The absolute configuration at C-5 was established as R by Mosher's method.

Quantitative microtiter fibronectin fibrillogenesis assay: use in high throughput screening for identification of inhibitor compounds

Fibronectin (FN) is a plasma glycoprotein that circulates in the near micromolar concentration range and is deposited along with locally produced FN in the extracellular matrices of many tissues. The control of FN deposition is tightly controlled by cells. Agents that modulate FN assembly may be useful therapeutically in conditions characterized by excessive FN deposition, such as fibrosis, inflammatory diseases, and malignancies. To identify such agents by high throughput screening (HTS), we developed a microtiter assay of FN deposition by human fibroblasts. The assay provides a robust read-out of FN assembly. Alexa 488-FN (A488-FN) was added to cell monolayers, and the total fluorescence intensity of deposited A488-FN was quantified. The fluorescence intensity of deposited A488-FN correlated with the presence of FN fibrils visualized by fluorescence microscopy. The assay Z' values were 0.67 or 0.54, respectively, when using background values of fluorescence either with no added A488-FN or with A488-FN added together with a known inhibitor of FN deposition. The assay was used to screen libraries comprising 4160 known bioactive compounds. Nine compounds were identified as non- or low-cytotoxic inhibitors of FN assembly. Four (ML-9, HA-100, tyrphostin and imatinib mesylate) are kinase inhibitors, a category of compounds known to inhibit FN assembly; two (piperlongumine and cantharidin) are promoters of cancer cell apoptosis; and three (maprotiline, CGS12066B, and aposcopolamine) are modulators of biogenic amine signaling. The latter six compounds have not been recognized heretofore as affecting FN assembly. The assay is straight-forward, adapts to 96- and 384-well formats, and should be useful for routine measurement of FN deposition and HTS. Screening of more diverse chemical libraries and identification of specific and efficient modulators of FN fibrillogenesis may result in therapeutics to control excessive connective tissue deposition.

Ferroptosis: an iron-dependent form of nonapoptotic cell death

Nonapoptotic forms of cell death may facilitate the selective elimination of some tumor cells or be activated in specific pathological states. The oncogenic RAS-selective lethal small molecule erastin triggers a unique iron-dependent form of nonapoptotic cell death that we term ferroptosis. Ferroptosis is dependent upon intracellular iron, but not other metals, and is morphologically, biochemically, and genetically distinct from apoptosis, necrosis, and autophagy. We identify the small molecule ferrostatin-1 as a potent inhibitor of ferroptosis in cancer cells and glutamate-induced cell death in organotypic rat brain slices, suggesting similarities between these two processes. Indeed, erastin, like glutamate, inhibits cystine uptake by the cystine/glutamate antiporter (system x(c)(-)), creating a void in the antioxidant defenses of the cell and ultimately leading to iron-dependent, oxidative death. Thus, activation of ferroptosis results in the nonapoptotic destruction of certain cancer cells, whereas inhibition of this process may protect organisms from neurodegeneration.

Ectopic ATP synthase blockade suppresses lung adenocarcinoma growth by activating the unfolded protein response

Ectopic expression of the mitochondrial F(1)F(0)-ATP synthase on the plasma membrane has been reported to occur in cancer, but whether it exerts a functional role in this setting remains unclear. Here we show that ectopic ATP synthase and the electron transfer chain exist on the plasma membrane in a punctuated distribution of lung adenocarcinoma cells, where it is critical to support cancer cell proliferation. Applying ATP synthase inhibitor citreoviridin induced cell cycle arrest and inhibited proliferation and anchorage-independent growth of lung cancer cells. Analysis of protein expression profiles after citreoviridin treatment suggested this compound induced the unfolded protein response (UPR) associated with phosphorylation the translation initiation factor 2α (eIF2α), triggering cell growth inhibition. Citreoviridin-enhanced eIF2α phosphorylation could be reversed by siRNA-mediated attenuation of the UPR kinase PKR-like endoplasmic reticulum kinase (PERK) combined with treatment with the antioxidant N-acetylcysteine, establishing that reactive oxygen species (ROS) boost UPR after citreoviridin treatment. Thus, a coordinate elevation of UPR and ROS initiates a positive feedback loop that convergently blocks cell proliferation. Our findings define a molecular function for ectopic ATP synthase at the plasma membrane in lung cancer cells and they prompt further study of its inhibition as a potential therapeutic approach.

Oxidative stress in apoptosis and cancer: an update

The oxygen paradox tells us that oxygen is both necessary for aerobic life and toxic to all life forms. Reactive oxygen species (ROS) touch every biological and medical discipline, especially those involving proliferative status, supporting the idea that active oxygen may be increased in tumor cells. In fact, metabolism of oxygen and the resulting toxic byproducts can cause cancer and death. Efforts to counteract the damage caused by ROS are gaining acceptance as a basis for novel therapeutic approaches, and the field of prevention of cancer is experiencing an upsurge of interest in medically useful antioxidants. Apoptosis is an important means of regulating cell numbers in the developing cell system, but it is so important that it must be controlled. Normal cell death in homeostasis of multicellular organisms is mediated through tightly regulated apoptotic pathways that involve oxidative stress regulation. Defective signaling through these pathways can contribute to both unbalance in apoptosis and development of cancer. Finally, in this review, we discuss new knowledge about recent tools that provide powerful antioxidant strategies, and designing methods to deliver to target cells, in the prevention and treatment of cancer.

A novel flow cytometric HTS assay reveals functional modulators of ATP binding cassette transporter ABCB6

ABCB6 is a member of the adenosine triphosphate (ATP)-binding cassette family of transporter proteins that is increasingly recognized as a relevant physiological and therapeutic target. Evaluation of modulators of ABCB6 activity would pave the way toward a more complete understanding of the significance of this transport process in tumor cell growth, proliferation and therapy-related drug resistance. In addition, this effort would improve our understanding of the function of ABCB6 in normal physiology with respect to heme biosynthesis, and cellular adaptation to metabolic demand and stress responses. To search for modulators of ABCB6, we developed a novel cell-based approach that, in combination with flow cytometric high-throughput screening (HTS), can be used to identify functional modulators of ABCB6. Accumulation of protoporphyrin, a fluorescent molecule, in wild-type ABCB6 expressing K562 cells, forms the basis of the HTS assay. Screening the Prestwick Chemical Library employing the HTS assay identified four compounds, benzethonium chloride, verteporfin, tomatine hydrochloride and piperlongumine, that reduced ABCB6 mediated cellular porphyrin levels. Validation of the identified compounds employing the hemin-agarose affinity chromatography and mitochondrial transport assays demonstrated that three out of the four compounds were capable of inhibiting ABCB6 mediated hemin transport into isolated mitochondria. However, only verteporfin and tomatine hydrochloride inhibited ABCB6's ability to compete with hemin as an ABCB6 substrate. This assay is therefore sensitive, robust, and suitable for automation in a high-throughput environment as demonstrated by our identification of selective functional modulators of ABCB6. Application of this assay to other libraries of synthetic compounds and natural products is expected to identify novel modulators of ABCB6 activity.

Annexin A2 is a novel cellular redox regulatory protein involved in tumorigenesis

Annexins are a structurally related family of calcium and phospholipid-binding proteins that are involved in the regulation of a wide range of molecular and cellular processes. Annexin A2 is unique among the annexins in that it possesses redox sensitive cysteine(s). The ubiquitous and abundant expression of ANXA2 in cells and its reactivity with hydrogen peroxide led us to hypothesize that this protein could play a role in cellular redox regulation. Here we show that ANXA2 protein levels are induced by hydrogen peroxide. Furthermore, depletion of ANXA2 resulted in the elevation of cellular reactive oxygen species (ROS) upon oxidative stress, increased activation of the ROS-induced pro-apoptotic kinases, JNK, p38 and Akt and elevated sensitivity to ROS-mediated cellular damage/death. ANXA2-null mice showed significantly elevated protein oxidation in the liver and lung tissues compared to WT mice. ANXA2 depleted cancer cells showed enhanced cellular protein oxidationconcomitant with decreased tumor growth compared to control cancer cells andboth the oxidation of cellular proteins and tumor growth deficit werereversed by the antioxidant N-acetyl cysteine, indicating that ANXA2 plays akey role in the regulation of cellular redox during tumorigenesis. Ex-vivo human cancer studies showed that up-regulation of the reduced form of ANXA2 is associated with protection of the tumor proteins from oxidation. In summary, our results indicate that ANXA2 plays an important role incellular redox regulation by protecting cells from oxidative stress, aneffect that is particularly important during tumorigenesis.

Impairment of the ubiquitin-proteasome pathway by methyl N-(6-phenylsulfanyl-1H-benzimidazol-2-yl)carbamate leads to a potent cytotoxic effect in tumor cells: a novel antiproliferative agent with a potential therapeutic implication

In recent years, there has been a great deal of interest in proteasome inhibitors as a novel class of anticancer drugs. We report that fenbendazole (FZ) (methyl N-(6-phenylsulfanyl-1H-benzimidazol-2-yl)carbamate) exhibits a potent growth-inhibitory activity against cancer cell lines but not normal cells. We show here, using fluorogenic substrates, that FZ treatment leads to the inhibition of proteasomal activity in the cells. Succinyl-Leu-Leu-Val-Tyr-methylcoumarinamide (MCA), benzyloxycarbonyl-Leu-Leu-Glu-7-amido-4-MCA, and t-butoxycarbonyl-Gln-Ala-Arg-7-amido-4-MCA fluorescent derivatives were used to assess chymotrypsin-like, post-glutamyl peptidyl-hydrolyzing, and trypsin-like protease activities, respectively. Non-small cell lung cancer cells transiently transfected with an expression plasmid encoding pd1EGFP and treated with FZ showed an accumulation of the green fluorescent protein in the cells due to an increase in its half-life. A number of apoptosis regulatory proteins that are normally degraded by the ubiquitin-proteasome pathway like cyclins, p53, and IκBα were found to be accumulated in FZ-treated cells. In addition, FZ induced distinct ER stress-associated genes like GRP78, GADD153, ATF3, IRE1α, and NOXA in these cells. Thus, treatment of human NSCLC cells with fenbendazole induced endoplasmic reticulum stress, reactive oxygen species production, decreased mitochondrial membrane potential, and cytochrome c release that eventually led to cancer cell death. This is the first report to demonstrate the inhibition of proteasome function and induction of endoplasmic reticulum stress/reactive oxygen species-dependent apoptosis in human lung cancer cell lines by fenbendazole, which may represent a new class of anticancer agents showing selective toxicity against cancer cells.