Towards novel paradigms for cancer therapy

Chemical approaches to targeting drug resistance in cancer stem cells

Cancer stem cells (CSCs) are a subpopulation of cancer cells with high clonogenic capacity and ability to reform parental tumors upon transplantation. Resistance to therapy has been shown for several types of CSC and, therefore, they have been proposed as the cause of tumor relapse. Consequently, much effort has been made to design molecules that can target CSCs specifically and sensitize them to therapy. In this review, we summarize the mechanisms underlying CSC resistance, the potential biological targets to overcome resistance and the chemical compounds showing activity against different types of CSC. The chemical compounds discussed here have been divided according to their origin: natural, natural-derived and synthetic compounds.

Plasminogen activator urokinase expression reveals TRAIL responsiveness and supports fractional survival of cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/TNFSF10/Apo2L) holds promise for cancer therapy as it induces apoptosis in a large variety of cancer cells while exerting negligible toxicity in normal ones. However, TRAIL can also induce proliferative and migratory signaling in cancer cells resistant to apoptosis induced by this cytokine. In that regard, the molecular mechanisms underlying the tumor selectivity of TRAIL and those balancing apoptosis versus survival remain largely elusive. We show here that high mRNA levels of PLAU, which encodes urokinase plasminogen activator (uPA), are characteristic of cancer cells with functional TRAIL signaling. Notably, decreasing uPA levels sensitized cancer cells to TRAIL, leading to markedly increased apoptosis. Mechanistic analyses revealed three molecular events taking place in uPA-depleted cells: reduced basal ERK1/2 prosurvival signaling, decreased preligand decoy receptor 2 (DcR2)-death receptor 5 (DR5) interaction and attenuated recruitment of DcR2 to the death-inducing signaling complex upon TRAIL challenge. These phenomena were accompanied by increased FADD and procaspase-8 recruitment and processing, thus guiding cells toward a caspase-dependent cell death that is largely independent of the intrinsic apoptosis pathway. Collectively, our results unveil PLAU mRNA levels as marker for the identification of TRAIL-responsive tumor cells and highlight a key role of uPA signaling in ‘apoptosis versus survival' decision-making processes upon TRAIL challenge.

Mir-655 up-regulation suppresses cell invasion by targeting pituitary tumor-transforming gene-1 in esophageal squamous cell carcinoma

Background

MicroRNAs (miRNAs) can act as either oncogenes or tumor suppressor genes under different conditions and thus can play a significant role in cancer development. We investigated miR-655 expression in a cohort of esophageal squamous cell carcinoma (ESCC) to assess the impact of this miRNA on ESCC cell invasion and metastasis.

Methods

A qRT-PCR assay was used to quantify miR-655 expression levels in 34 paired ESCC samples and adjacent non-tumor tissues. Wound healing and transwell assays were used to evaluate the effects of miR-655 expression on the invasiveness of ESCC cells. Luciferase reporter and western blot assays were used to determine whether the mRNA encoding pituitary tumor-transforming gene-1 (PTTG1) is a major target of miR-655.

Results

The expression level of miR-655 in ESCC tissues was found to be lower than in adjacent non-tumor tissues (P < 0.05). This relatively low expression level was significantly associated with the occurrence of lymph node metastases (P < 0.05). Migration rates were significantly lower for two ESCC-derived cell lines (EC9706 and KYSE150) transfected with miR-429 mimics (P < 0.05). Subsequent western blot and luciferase reporter assays demonstrated that miR-655 could bind to putative binding sites within the PTTG1 mRNA 3’-untranslated region (3’-UTR) and thus reduce the expression.

Conclusions

miR-655 is expressed at low levels in primary ESCC tissues, and up-regulation of miR-655 inhibits ESCC cell invasiveness by targeting PTTG1. Our findings suggest that PTTG1 may act as a major target of miR-655. This study improves our understanding of the mechanisms underlying ESCC pathogenesis and may promote the development of novel targeted therapies.

Oncolytic virotherapy and immunogenic cancer cell death: sharpening the sword for improved cancer treatment strategies

Oncolytic viruses are novel immunotherapeutics with increasingly promising outcomes in cancer patient clinical trials. Preclinical and clinical studies have uncovered the importance of virus-induced activation of antitumor immune responses for optimal therapeutic efficacy. Recently, several classes of chemotherapeutics have been shown to cause immunogenic cancer cell death characterized by the release of immunomodulatory molecules that activate antigen-presenting cells and thus trigger the induction of more potent anticancer adaptive immune responses. In preclinical models, several oncolytic viruses induce immunogenic cell death, which is associated with increased cross-priming of tumor-associated antigens. In this review, we discuss the recent advances in immunogenic cancer cell death as induced by chemotherapeutic treatments, including the roles of relevant danger-associated molecular patterns and signaling pathways, and highlighting the significance of the endoplasmic reticulum (ER) stress response. As virtually all viruses modulate both ER stress and cell death responses, we provide perspectives on future research directions that can be explored to optimize oncolytic viruses, alone or in combination with targeted drug therapies, as potent immunogenic cancer cell death-inducing agents. We propose that such optimized virus-drug synergistic strategies will improve the therapeutic outcomes for many currently intractable cancers.

Cancer drug resistance: an evolving paradigm

Resistance to chemotherapy and molecularly targeted therapies is a major problem facing current cancer research. The mechanisms of resistance to 'classical' cytotoxic chemotherapeutics and to therapies that are designed to be selective for specific molecular targets share many features, such as alterations in the drug target, activation of prosurvival pathways and ineffective induction of cell death. With the increasing arsenal of anticancer agents, improving preclinical models and the advent of powerful high-throughput screening techniques, there are now unprecedented opportunities to understand and overcome drug resistance through the clinical assessment of rational therapeutic drug combinations and the use of predictive biomarkers to enable patient stratification.

Structure-based approach to the design of BakBH3 mimetic peptides with increased helical propensity

The Bcl-2 family of proteins are well-characterized regulators of the intrinsic apoptotic pathway. Proteins within this family can be classified as either prosurvival or prodeath members and the balance between them present at the mitochondrial membrane is what determines if the cell lives or dies. Specific interactions among Bcl-2 family proteins play a crucial role in regulating programmed cell death. Structural studies have established a conserved interaction pattern among Bcl-2 family members. This interaction is mediated by the binding of the hydrophobic face of the amphipathic α-helical BH3 domain into a conserved hydrophobic groove on the prosurvival partners. It has been reported that an increase in the helical content of BH3 mimetic peptides considerably improves the binding affinity. In this context, this work states for designing peptides derived from the BH3 domain of the proapoptotic protein Bak by substitution of some non-interacting residues by the helical inducing residue Aib. Different synthetic peptides preserving BakBH3 relevant interactions were proposed and simulated presenting a better predicted binding energy and higher helical content than the wild type Bak peptide.

P-glycoprotein-dependent resistance of cancer cells toward the extrinsic TRAIL apoptosis signaling pathway

The TNF-related apoptosis-inducing ligand (TRAIL or Apo2L) preferentially cause apoptosis of malignant cells in vitro and in vivo without severe toxicity. Therefore, TRAIL or agonist antibodies to the TRAIL DR4 and DR5 receptors are used in cancer therapy. However, many malignant cells are intrinsically resistant or acquire resistance to TRAIL. It has been previously proposed that the multidrug transporter P-glycoprotein (Pgp) might play a role in resistance of cells to intrinsic apoptotic pathways by interfering with components of ceramide metabolism or by modulating the electrochemical gradient across the plasma membrane. In this study we investigated whether Pgp also confers resistance toward extrinsic death ligands of the TNF family. To this end we focused our study on HeLa cells carrying a tetracycline-repressible plasmid system which shuts down Pgp expression in the presence of tetracycline. Our findings demonstrate that expression of Pgp is a significant factor conferring resistance to TRAIL administration, but not to other death ligands such as TNF-α and Fas ligand. Moreover, blocking Pgp transport activity sensitizes the malignant cells toward TRAIL. Therefore, Pgp transport function is required to confer resistance to TRAIL. Although the resistance to TRAIL-induced apoptosis is Pgp specific, TRAIL itself is not a direct substrate of Pgp. Pgp expression has no effect on the level of the TRAIL receptors DR4 and DR5. These findings might have clinical implications since the combination of TRAIL therapy with administration of Pgp modulators might sensitize TRAIL resistant tumors.

PTTG1 promotes migration and invasion of human non-small cell lung cancer cells and is modulated by miR-186

Deeper mechanistic understanding of non-small cell lung cancer (NSCLC), a leading cause of total cancer-related deaths, may facilitate the establishment of more effective therapeutic strategies. In this study, pituitary tumor transforming gene (PTTG1) expression was associated with lymph node and distant metastasis in patients with NSCLC and was correlated with patient survival. Reduction of PTTG1 by small interfering RNA (siRNA) inhibits the migration and invasion of NSCLC cells by mediating matrix metalloproteinases expression. To the best of our knowledge, this study is the first to report that PTTG1 promotes epidermal growth factor (EGF) induced the phosphorylation of LIN-11, Isl1 and MEC-3 protein domain kinase and cofilin, a critical step in cofilin recycling and actin polymerization. Additionally, EGF-induced Akt phosphorylation was suppressed through knockdown of PTTG1. Interestingly, miR-186 can modulate PTTG1 protein expression. As observed from the animal experiment in this study, knockdown of PTTG1 through siRNA and overexpression of miR-186 inhibited invasive activity of NSCLC cells toward the SCID mice lung. In summary, our in vitro and in vivo results indicate that PTTG1 modulated by miR-186 has an important function in NSCLC invasion/metastasis. This study identified both PTTG1 and miR-186 as potential anti-invasion targets for therapeutic intervention in NSCLC.

Transcriptional repression of AIB1 by FoxG1 leads to apoptosis in breast cancer cells

The oncogene nuclear receptor coactivator amplified in breast cancer 1 (AIB1) is a transcriptional coactivator that is overexpressed in various types of human cancers. However, the molecular mechanisms controlling AIB1 expression in the majority of cancers remain unclear. In this study, we identified a novel interacting protein of AIB1, forkhead-box protein G1 (FoxG1), which is an evolutionarily conserved forkhead-box transcriptional corepressor. We show that FoxG1 expression is low in breast cancer cell lines and that low levels of FoxG1 are correlated with a worse prognosis in breast cancer. We also demonstrate that transient overexpression of FoxG1 can suppress endogenous levels of AIB1 mRNA and protein in MCF-7 breast cancer cells. Exogenously expressed FoxG1 in MCF-7 cells also leads to apoptosis that can be rescued in part by AIB1 overexpression. Using chromatin immunoprecipitation, we determined that FoxG1 is recruited to a region of the AIB1 gene promoter previously characterized to be responsible for AIB1-induced, positive autoregulation of transcription through the recruitment of an activating, multiprotein complex, involving AIB1, E2F transcription factor 1, and specificity protein 1. Increased FoxG1 expression significantly reduces the recruitment of AIB1, E2F transcription factor 1 and E1A-binding protein p300 to this region of the endogenous AIB1 gene promoter. Our data imply that FoxG1 can function as a pro-apoptotic factor in part through suppression of AIB1 coactivator transcription complex formation, thereby reducing the expression of the AIB1 oncogene.

Identification of 5-Iodotubercidin as a genotoxic drug with anti-cancer potential

Tumor suppressor p53, which is activated by various stress and oncogene activation, is a target for anti-cancer drug development. In this study, by screening panels of protein kinase inhibitors and protein phosphatase inhibitors, we identified 5-Iodotubercidin as a strong p53 activator. 5-Iodotubercidin is purine derivative and is used as an inhibitor for various kinases including adenosine kinase. We found that 5-Iodotubercidin could cause DNA damage, verified by induction of DNA breaks and nuclear foci positive for γH2AX and TopBP1, activation of Atm and Chk2, and S15 phosphorylation and up-regulation of p53. As such, 5-Iodotubercidin induces G2 cell cycle arrest in a p53-dependent manner. Itu also induces cell death in p53-dependent and -independent manners. DNA breaks were likely generated by incorporation of 5-Iodotubercidin metabolite into DNA. Moreover, 5-Iodotubercidin showed anti-tumor activity as it could reduce the tumor size in carcinoma xenograft mouse models in p53-dependent and -independent manners. These findings reveal 5-Iodotubercidin as a novel genotoxic drug that has chemotherapeutic potential.

Synthesis and cytotoxic activity of metallic complexes of lawsone

In the present study, a series of metallic complexes of the 1,4-naphthoquinone lawsone (2-6) were synthesized and evaluated for potential cytotoxicity in a mouse leukemic macrophagic RAW 264.7 cell line. Cell viability was determined by the MTT assay. Significant growth inhibition was observed for the copper complex (4) with an IC(50) value of 2.5 μM. This compound was selected for further evaluation of cytotoxic activity on several human cancer cells including HT-29 (human colorectal adenocarcinoma), HepG2 (human hepatocellular carcinoma) and HeLa, (human cervical adenocarcinoma cells). Significant cell viability decrease was also observed in HepG2 cells. The apoptotic potential of this complex was evaluated in these cells. Compound 4 induced apoptosis by a mechanism that involves the activation of caspases 3, 8 and 9 and modulation of apoptotic-related proteins such as Bax, Bad, and p53. These results indicate that metal complexes of lawsone derivatives, in particular compound 4, might be used for the design of new antitumoral agents.

Salmonella typhimurium mediated delivery of Apoptin in human laryngeal cancer

An effective cancer therapeutic should target tumours specifically with limited systemic toxicity. Here, we transformed an attenuated Salmonella typhimurium (S. typhimurium) with an Apoptin expressing plasmid into a human laryngeal carcinoma cell line. The expression of the inserted gene was measured using fluorescence and immunoblotting assays. The attenuated S. typhimurium-mediated Apoptin significantly decreased cytotoxicity and strongly increased cell apoptosis through the activation of caspase-3. The process was mediated by Bax, cytochrome c and caspase-9. A syngeneic nude murine tumour model was used to determine the anti-tumour effects of the recombinant bacteria in vivo. Systemic injection of the recombinant bacteria with and without re-dosing caused significant tumour growth delay and reduced tumour microvessel density, thereby extending host survival. Our findings indicated that the use of recombinant Salmonella typhimurium as an Apoptin expression vector has potential cancer therapeutic benefits.

TRPM8 ion channels differentially modulate proliferation and cell cycle distribution of normal and cancer prostate cells

Overexpression of the cation-permeable channel TRPM8 in prostate cancers might represent a novel opportunity for their treatment. Inhibitors of TRPM8 reduce the growth of prostate cancer cells. We have used two recently described and highly specific blockers, AMTB and JNJ41876666, and RNAi to determine the relevance of TRPM8 expression in the proliferation of non-tumor and tumor cells. Inhibition of the expression or function of the channel reduces proliferation rates and proliferative fraction in all tumor cells tested, but not of non-tumor prostate cells. We observed no consistent acceleration of growth after stimulation of the channel with menthol or icilin, indicating that basal TRPM8 expression is enough to sustain growth of prostate cancer cells.

New frontiers in promoting tumour cell death: targeting apoptosis, necroptosis and autophagy

Cancer is a multifaceted disease comprising a combination of genetic, metabolic and signalling aberrations, which severely disrupt the normal homeostasis of cell growth and death. Many oncogenic events while promoting tumour development also increase the sensitivity of cells to cell death stimuli including chemotherapeutic drugs. As a result, tumour cells often acquire the ability to evade death by inactivating cell death pathways that normally function to eliminate damaged and harmful cells. The impairment of cell death function is also often the reason for the development of chemotherapeutic resistance encountered during treatment. It is therefore necessary to achieve a comprehensive understanding of existing cell death pathways and the relevant regulatory components involved, with the intention of identifying new strategies to kill cancer cells. This review provides an insightful overview of the common forms of cell death signalling pathways, the interactions between these pathways and the ways in which these pathways are deregulated in cancer. We also discuss the emerging therapies targeted at activating or restoring cell death pathways to induce tumour cell death, which are currently being tested in clinical trials.

Chromosome instability modulated by BMI1-AURKA signaling drives progression in head and neck cancer

Chromosomal instability (CIN) is widely considered a hallmark of cancer, but its precise roles in cancer stem cells (CSC) and malignant progression remain uncertain. BMI1 is a member of the Polycomb group of chromatin-modifier proteins that is essential for stem cell self-renewal. In human cancers, BMI1 overexpression drives stem-like properties associated with induction of epithelial-mesenchymal transition (EMT) that promotes invasion, metastasis, and poor prognosis. Here, we report that BMI1 mediates its diverse effects through upregulation of the mitotic kinase Aurora A, which is encoded by the AURKA gene. Two mechanisms were found to be responsible for BMI1-induced AURKA expression. First, BMI1 activated the Akt pathway, thereby upregulating AURKA expression through activation of the β-catenin/TCF4 transcription factor complex. Second, BMI1 repressed miRNA let-7i through a Polycomb complex-dependent mechanism, thereby relieving AURKA expression from let-7i suppression. AURKA upregulation by BMI1 exerts several effects, including centrosomal amplification and aneuploidy, antiapoptosis, and cell-cycle progression through p53 degradation and EMT through stabilization of Snail. Inhibiting Aurora A kinase activity attenuated BMI1-induced tumor growth in vivo. In clinical specimens of head and neck cancer, we found that coamplification of BMI1 and AURKA correlated with poorer prognosis. Together, our results link CSCs, EMT, and CIN through the BMI1-AURKA axis and suggest therapeutic use from inhibiting Aurora A in head and neck cancers, which overexpress BMI1.

let-7b and miR-126 are down-regulated in tumor tissue and correlate with microvessel density and survival outcomes in non--small--cell lung cancer

Angiogenesis is a critical event in the development, progression, and spread of various human cancers, including lung cancer. Molecular mechanisms that underlie the complex regulation of angiogenic processes are poorly understood. However, an increasing body of evidence indicates miRNAs as important regulators of tumor angiogenesis. Forceps biopsies were collected from tumor tissue, surrounding tissue, and non-tumor tissue from 50 NSCLC patients. Lung tissue samples from individuals with no clinical evidence of a cancerous disease served as controls. Immunohistochemical staining for Factor VIII was used to evaluate microvessel density (MVD). TaqMan® primer-probe sets were used in quantitative real-time RT-PCR reactions to determine expression levels of let-7b, miR-126, miR-9, and miR-19a. We demonstrated significantly higher MVD and decreased expression levels of let-7b and miR-126 in tumor tissue and surrounding tissue in comparison to corresponding non-tumor tissue or lung tissue from the control group. In addition, no differences in MVD and expression levels of both miRNAs between tumor tissue and surrounding tissue from NSCLC patients were observed. Low expression of both miRNAs correlated with high MVD and worse progression-free survival and overall survival. These observations strongly suggest similar molecular alternations within tumor tissue and surrounding tissue that comprise a specific microenvironment. Low expression of let-7b and miR-126 seems to have a possible anti-angiogenic role in lung tumor tissue and significantly correlates with worse survival outcomes for lung cancer patients. Moreover, the regulation of let-7b and miR-126 expression could have therapeutic potential because it could reduce tumor angiogenesis and therefore suppress tumor growth in lung cancer patients.

A novel pro-apoptotic effector lactaptin inhibits tumor growth in mice models

Lactaptin, a human milk-derived protein, induces apoptosis in cultured tumor cells. We designed a recombinant analog of lactaptin (RL2) and tested its antitumor activity. The sensitivity of hepatocarcinoma A-1 (HA-1), Lewis lung carcinoma, and Ehrlich carcinoma to RL2 were tested to determine the most reliable in vitro animal model. HA-1 cells, which had the highest sensitivity to RL2, were transplanted into A/Sn mice to investigate RL2 antitumor activity in vivo. Investigation of the molecular effects of RL2 shows that RL2 induces apoptotic transformation of HA-1 cells in vitro: phosphatidylserine translocation from inner side of the lipid bilayer to the outer one and dissipation of the mitochondrial membrane potential. Repetitive injections of RL2 (5-50 mg/kg) for 3-5 days effectively inhibited ascites and solid tumor transplant growth when administered intravenously or intraperitoneally, without obvious side effects. The solid tumor inhibitory effect of RL2 (5 i.v. injections, cumulative dose 125 mg/kg) was comparable with that of cyclophosphamide at a therapeutic dose (5 i.v. injections, cumulative dose 150 mg/kg). In combination therapy with cyclophosphamide, RL2 had an additive antitumor effect for ascites-producing tumors. Histomorphometric analysis indicated a three-fold reduction of spontaneous metastases in the liver of RL2-treated mice with solid tumor transplants in comparison with control animals. Repeated RL2 treatment substantially prolonged the lifespan of mice with intravenously injected tumor cells. Recombinant analog of lactaptin effectively induced apoptosis of tumor cells in vitro and suppressed the growth of sensitive tumors and metastases in vivo.

Therapeutic efficacy of an hTERT promoter-driven oncolytic adenovirus that expresses apoptin in gastric carcinoma

The efficacy and specificity of treatment are the major challenges for cancer gene therapy. Oncolytic virotherapy is an attractive drug delivery platform of cancer gene therapy. Previous studies have determined that apoptin is a p53-independent, Bcl-2-insensitive apoptotic protein that has the ability to induce apoptosis specifically in tumor cells. In this study, we show that the administration of a dual cancer-specific oncolytic adenovirus construct, Ad-hTERT-E1a-apoptin [in which the adenovirus early region 1a (E1a) gene is driven by the cancer-specific promoter of human telomerase reverse transcriptase (hTERT) and that expresses apoptin simultaneously], suppresses tumor growth in gastric carcinoma cells in vitro and reduces the tumor burden in vivo in xenografted nude mice. The observation that infection with the Ad-hTERT-E1a-apoptin construct significantly inhibited the growth of gastric cancer cells and protected normal human gastric epithelium from growth inhibition confirmed the induction of cancer cell-selective adenovirus replication, growth inhibition and apoptosis by this therapeutic approach. In vivo assays were performed using BALB/c nude mice that had established primary tumors. Subcutaneous primary tumor volume was reduced not only in the intratumoral injection group but also in the systemic delivery mice following treatment with Ad-hTERT-E1a-apoptin. Furthermore, treatment of primary models with Ad-hTERT-E1a-apoptin increased the mouse survival time. These data reinforce previous research and highlight the potential therapeutic application of Ad-hTERT-E1a-apoptin for the treatment of neoplastic diseases in clinical trials.

TRAIL-R3-related apoptosis: epigenetic and expression analyses in women with ovarian neoplasia

OBJECTIVE

To assess the expression of TRAIL-R3 and the methylation of a CpG island within the TRAIL-R3 promoter both in cystadenoma tumors and primary and metastatic epithelial ovarian carcinoma (EOC).

METHODS

RNA was obtained from women with normal ovarian (NO) tissues (n=18), ovarian serous cystadenoma tumors (n=11) and EOC (n=16) using Trizol. Quantitative PCR (qRT-PCR) was performed to quantify the relative levels of TRAIL-R3. The methylation frequency of the CpG island in the TRAIL-R3 promoter was assessed using the methylation-specific PCR (MSP) assay after DNA bisulfite conversion. The differences between the groups were evaluated using the chi-square, Student's t, ANOVA, Mann-Whitney U, Wilcoxon or Kruskal-Wallis tests as indicated. The survival rates were calculated using the Kaplan-Meier method.

RESULTS

Cystadenoma and metastatic EOC tumors expressed significantly more TRAIL-R3 mRNA than primary EOC tumors. Methylation of the TRAIL-R3 promoter was absent in NO tissues, while hemimethylation of the TRAIL-R3 promoter was frequently found in the neoplasia samples with 45.4% of the cystadenoma tumors, 8.3% of the primary EOC samples and 11.1% of the metastatic EOC samples showing at least partial methylation (p=0.018). Neither the expression of TRAIL-R3 nor alterations in the methylation profile were associated to cumulative progression-free survival or the overall survival in EOC patients.

CONCLUSIONS

Primary EOC is associated to a lower TRAIL-R3 expression, which leads to a better understanding of the complex disease and highlighting potential therapeutic targets. Promoter DNA methylation was not related to this finding, suggesting the presence of other mechanisms to transcriptional control.

An NQO1 substrate with potent antitumor activity that selectively kills by PARP1-induced programmed necrosis

Agents, such as β-lapachone, that target the redox enzyme, NAD(P)H:quinone oxidoreductase 1 (NQO1), to induce programmed necrosis in solid tumors have shown great promise, but more potent tumor-selective compounds are needed. Here, we report that deoxynyboquinone kills a wide spectrum of cancer cells in an NQO1-dependent manner with greater potency than β-lapachone. Deoxynyboquinone lethality relies on NQO1-dependent futile redox cycling that consumes oxygen and generates extensive reactive oxygen species (ROS). Elevated ROS levels cause extensive DNA lesions, PARP1 hyperactivation, and severe NAD+ /ATP depletion that stimulate Ca2+ -dependent programmed necrosis, unique to this new class of NQO1 "bioactivated" drugs. Short-term exposure of NQO1+ cells to deoxynyboquinone was sufficient to trigger cell death, although genetically matched NQO1- cells were unaffected. Moreover, siRNA-mediated NQO1 or PARP1 knockdown spared NQO1+ cells from short-term lethality. Pretreatment of cells with BAPTA-AM (a cytosolic Ca2+ chelator) or catalase (enzymatic H2O2 scavenger) was sufficient to rescue deoxynyboquinone-induced lethality, as noted with β-lapachone. Investigations in vivo showed equivalent antitumor efficacy of deoxynyboquinone to β-lapachone, but at a 6-fold greater potency. PARP1 hyperactivation and dramatic ATP loss were noted in the tumor, but not in the associated normal lung tissue. Our findings offer preclinical proof-of-concept for deoxynyboquinone as a potent chemotherapeutic agent for treatment of a wide spectrum of therapeutically challenging solid tumors, such as pancreatic and lung cancers.

Effects of cucurbitacins on cell morphology are associated with sensitization of renal carcinoma cells to TRAIL-induced apoptosis

Cucurbitacins B and D were among the compounds identified as sensitizers of cancer cells to TRAIL-mediated apoptosis in a high-throughput screen. Therefore a series of cucurbitacins was further investigated for TRAIL sensitization and possible mechanisms of action. A total of six cucurbitacins promoted TRAIL-induced apoptosis (B, I, E, C, D, and K) and one (P) was inactive. Sensitization of renal adenocarcinoma cells to TRAIL was apparent after as little as 1-4 h pretreatment and did not require continued presence of cucurbitacin. Active cucurbitacins induced caspase-8 activation only after subsequent TRAIL addition and caspase activation was required for apoptosis suggesting amplified proximal signaling from TRAIL death receptors. Cucurbitacin-sensitized TRAIL-induced cytotoxicity was inhibited by N-acetyl cysteine. Structure-activity relationship analysis in comparison to published studies suggests that TRAIL-sensitizing and general cytotoxic activities of cucurbitacins may be decoupled. Cucurbitacins are reported to be inhibitors of STAT3 activation. However, their TRAIL-sensitizing activity is STAT3-independent. Treatment of renal carcinoma cells with active cucurbitacins produced rapid and dramatic changes in cell morphology and cytoskeletal organization (also prevented by NAC). Therefore, cucurbitacins may be useful as tools for investigating the molecular mechanism(s) of action of TRAIL sensitizers, particularly with regard to temporal aspects of sensitization and modulation of TRAIL signaling by cell morphology, and could form the basis for future therapeutic development in combination with TRAIL death receptor agonists.

Acquired resistance to drugs targeting receptor tyrosine kinases

Development of resistance to chemotherapeutic drugs represents a significant hindrance to the effective treatment of cancer patients. The molecular mechanisms responsible have been investigated for over half a century and have revealed the lack of a single cause. Rather, a multitude of mechanisms have been delineated ranging from induction and expression of membrane transporters that pump drugs out of cells (multidrug resistance (MDR) phenotype), changes in the glutathione system and altered metabolism to name a few. Treatment of cancer patients/cancer cells with chemotherapeutic agents and/or molecularly targeted drugs is accompanied by acquisition of resistance to the treatment administered. Chemotherapeutic agent resistance was initially assumed to be due to induction of mutations leading to a resistant phenotype. This has also been true for molecularly targeted drugs. Considerable experience has been gained from the study of agents targeting the Bcr-Abl tyrosine kinase including imatinib, dasatinib and sunitinib. It is clear that mutations alone are not responsible for the many resistance mechanisms in play. Rather, additional mechanisms are involved, ranging from epigenetic changes, alternative splicing and the induction of alternative/compensatory signaling pathways. In this review, resistance to receptor tyrosine kinase inhibitors (RTKIs), RTK-directed antibodies and antibodies that inactivate ligands for RTKs are discussed. New approaches and concepts aimed at avoiding the generation of drug resistance will be examined. The recent observation that many RTKs, including the IGF-1R, are dependence receptors that induce apoptosis in a ligand-independent manner will be discussed and the implications this signaling paradigm has on therapeutic strategies will be considered.

Development and application of an in vitro apoptin kinase assay

Apoptin, a protein derived from chicken anemia virus (CAV), induces apoptosis selectively in human tumor cells as compared with normal cells. This activity depends on phosphorylation and relocation of apoptin to the nucleus of cancer cells. Here, we describe an in vitro kinase assay that allows the biochemical characterization of apoptin kinase activity in tumor cells. The kinase phosphorylates apoptin in a strictly ATP-dependent fashion and in a broad salt range. The kinase activity is present constitutively in both cytoplasm and nucleus of various human tumor cells. Q-column chromatography showed that both cytoplasmic and nuclear fractions have identical fractionation characteristics, suggesting that the same kinase is present in both cellular compartments. Kinase activity derived from positive Q-column fractions bound to amylose-maltose-binding protein (MBP)-apoptin and could be eluted with ATP only in the presence of the cofactor Mg(2+). Apparently, unphosphorylated apoptin interacts with the kinase and is released only after phosphorylation has occurred, proving that our assay recognizes the genuine apoptin kinase. This is further corroborated by the finding that apoptin is phosphorylated in vitro at positions Thr108 and Thr107, in concert with earlier in vivo observations. Our assay excludes cyclin-dependent kinase 2 (CDK2) and protein kinase C beta (PKC-β), previously nominated by two separate studies as being the genuine apoptin kinase.

Autophagy as a mediator of chemotherapy-induced cell death in cancer

Since the 1940s, chemotherapy has been the treatment of choice for metastatic disease. Chemotherapeutic agents target proliferating cells, inducing cell death. For most of the history of chemotherapy, apoptosis was thought to be the only mechanism of drug-induced cell death. More recently, a second type of cell death pathway has emerged: autophagy, also called type II programmed cell death. Autophagy is a tightly regulated process by which selected components of a cell are degraded. It primarily functions as a cell survival adaptive mechanism during stress conditions. However, persistent stress can also promote extensive autophagy, leading to cell death, hence its name. Alterations in the autophagy pathway have been described in cancer cells that suggest a tumor-suppressive function in early tumorigenesis, but a tumor-promoting function in established tumors. Moreover, accumulating data indicate a role for autophagy in chemotherapy-induced cancer cell death. Here, we discuss some of the evidence showing autophagy-dependent cell death induced by anti-neoplastic agents in different cancer models. On the other hand, in some other examples, autophagy dampens treatment efficacy, hence providing a therapeutic target to enhance cancer cell killing. In this paper, we propose a putative mechanism that could reconcile these two opposite observations.

Lanatoside C sensitizes glioblastoma cells to tumor necrosis factor-related apoptosis-inducing ligand and induces an alternative cell death pathway

Human glioblastoma (GBM) cells are notorious for their resistance to apoptosis-inducing therapeutics. We have identified lanatoside C as a sensitizer of GBM cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced cell death partly by upregulation of the death receptor 5. We show that lanatoside C sensitizes GBM cells to TRAIL-induced apoptosis in a GBM xenograft model in vivo. Lanatoside C on its own serves as a therapeutic agent against GBM by activating a caspase-independent cell death pathway. Cells treated with lanatoside C showed necrotic cell morphology with absence of caspase activation, low mitochondrial membrane potential, and early intracellular ATP depletion. In conclusion, lanatoside C sensitizes GBM cells to TRAIL-induced cell death and mitigates apoptosis resistance of glioblastoma cells by inducing an alternative cell death pathway. To our knowledge, this is one of the first examples of use of caspase-independent cell death inducers to trigger tumor regression in vivo. Activation of such mechanism may be a useful strategy to counter resistance of cancer cells to apoptosis.

Naturally occurring, tumor-specific, therapeutic proteins

The emerging approach to cancer treatment known as targeted therapies offers hope in improving the treatment of therapy-resistant cancers. Recent understanding of the molecular pathogenesis of cancer has led to the development of targeted novel drugs such as monoclonal antibodies, small molecule inhibitors, mimetics, antisense and small interference RNA-based strategies, among others. These compounds act on specific targets that are believed to contribute to the development and progression of cancers and resistance of tumors to conventional therapies. Delivered individually or combined with chemo- and/or radiotherapy, such novel drugs have produced significant responses in certain types of cancer. Among the most successful novel compounds are those which target tyrosine kinases (imatinib, trastuzumab, sinutinib, cetuximab). However, these compounds can cause severe side-effects as they inhibit pathways such as epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor, which are also important for normal functions in non-transformed cells. Recently, a number of proteins have been identified which show a remarkable tumor-specific cytotoxic activity. This toxicity is independent of tumor type or specific genetic changes such as p53, pRB or EGFR aberrations. These tumor-specific killer proteins are either derived from common human and animal viruses such as E1A, E4ORF4 and VP3 (apoptin) or of cellular origin, such as TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and MDA-7 (melanoma differentiation associated-7). This review aims to present a current overview of a selection of these proteins with preferential toxicity among cancer cells and will provide an insight into the possible mechanism of action, tumor specificity and their potential as novel tumor-specific cancer therapeutics.

Targeted expression of tumor necrosis factor-related apoptosis-inducing ligand TRAIL in skin protects mice against chemical carcinogenesis

Background

Gene ablation studies have revealed that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL, Apo2L, TNFSF10) plays a crucial role in tumor surveillance, as TRAIL-deficient mice exhibit an increased sensitivity to different types of tumorigenesis. In contrast, possible tumor-protective effect of increased levels of endogenous TRAIL expression in vivo has not been assessed yet. Such models will provide important information about the efficacy of TRAIL-based therapies and potential toxicity in specific tissues.

Methods

To this aim, we engineered transgenic mice selectively expressing TRAIL in the skin and subjected these mice to a two-step chemical carcinogenesis protocol that generated benign and preneoplastic lesions. We were therefore able to study the effect of increased TRAIL expression at the early steps of skin tumorigenesis.

Results

Our results showed a delay of tumor appearance in TRAIL expressing mice compared to their wild-type littermates. More importantly, the number of tumors observed in transgenic animals was significantly lower than in the control animals, and the lesions observed were mostly benign. Interestingly, Wnt/β-catenin signaling differed between tumors of wild-type and TRAIL transgenics.

Conclusion

Altogether, these data reveal that, at least in this model, TRAIL is able on its own to act on pre-transformed cells, and reduce their tumorigenic potential.