Synergistic cytotoxicity through the activation of multiple apoptosis pathways in human glioma cells induced by combined treatment with ionizing radiation and tumor necrosis factor–related apoptosis-inducing ligand

Novel Apoptosis-Inducing Agents for the Treatment of Cancer, a New Arsenal in the Toolbox

Evasion from apoptosis is an important hallmark of cancer cells. Alterations of apoptosis pathways are especially critical as they confer resistance to conventional anti-cancer therapeutics, e.g., chemotherapy, radiotherapy, and targeted therapeutics. Thus, successful induction of apoptosis using novel therapeutics may be a key strategy for preventing recurrence and metastasis. Inhibitors of anti-apoptotic molecules and enhancers of pro-apoptotic molecules are being actively developed for hematologic malignancies and solid tumors in particular over the last decade. However, due to the complicated apoptosis process caused by a multifaceted connection with cross-talk pathways, protein–protein interaction, and diverse resistance mechanisms, drug development within the category has been extremely challenging. Careful design and development of clinical trials incorporating predictive biomarkers along with novel apoptosis-inducing agents based on rational combination strategies are needed to ensure the successful development of these molecules. Here, we review the landscape of currently available direct apoptosis-targeting agents in clinical development for cancer treatment and update the related biomarker advancement to detect and validate the efficacy of apoptosis-targeted therapies, along with strategies to combine them with other agents.

Role of nanotechnology and gene delivery systems in TRAIL-based therapies

Since its identification as a member of the tumour necrosis factor (TNF) family, TRAIL (TNF-related apoptosis-inducing ligand) has emerged as a new avenue in apoptosis-inducing cancer therapies. Its ability to circumvent the chemoresistance of conventional therapeutics and to interact with cancer stem cells (CSCs) self-renewal pathways, amplified its potential as a cancer apoptotic agent. Many recombinant preparations of this death ligand and monoclonal antibodies targeting its death receptors have been tested in monotherapy and combinational clinical trials. Gene therapy is a new approach for cancer treatment which implies viral or non-viral functional transgene induction of apoptosis in cancer cells or repair of the underlying genetic abnormality on a molecular level. The role of this approach in overcoming the traditional barriers of radiation and chemotherapeutics systemic toxicity, risk of recurrence, and metastasis made it a promising platform for cancer treatment. The recent first Food Drug Administration (FDA) approved oncolytic herpes virus for melanoma treatment brings forth the potency of the cancer gene therapy approach in the future. Many gene delivery systems have been studied for intratumoural TRAIL gene delivery alone or in combination with chemotherapeutic agents to produce synergistic cancer cytotoxicity. However, there still remain many obstacles to be conquered for this different gene delivery systems. Nanomedicine on the other hand offers a new frontier for clinical trials and biomedical research. The FDA approved nanodrugs motivates horizon exploration for other nanoscale designed particles’ implications in gene delivery. In this review we aim to highlight the molecular role of TRAIL in apoptosis and interaction with cancer stem cells (CSCs) self-renewal pathways. Finally, we also aim to discuss the different roles of gene delivery systems, mesenchymal cells, and nanotechnology designs in TRAIL gene delivery.

Ionizing radiations induce apoptosis in TRAIL resistant cancer cells: in vivo and in vitro analysis

Increasingly it is being realized that despite considerable advancements in therapeutic interventions related to treatment of cancer, satisfactory results are still difficult to achieve. Rapidly accumulating evidence has started to shed light on the fact that cancer cells escape from death via constitutive activation of pro-survival signaling cascades. Cell biology and genetics have extensively enhanced our current understanding of the molecular mechanisms that underlie loss of apoptosis in cancer cells. This review is focused on ionizing radiation mediated restoration of TRAIL mediated apoptosis as evidenced by cell culture and animal model studies. Moreover, we also bring to the limelight radiation induced expression of miRNAs and how miRNAs further control response of cancer cells to radiation.

The interruption of PKC-ι signaling and TRAIL combination therapy against glioblastoma cells

Glioblastoma is a highly aggressive type of brain cancer which currently has limited options for treatment. It is imperative to develop combination therapies that could cause apoptosis in glioblastoma. The aim of this study was to characterize the affect of modified ICA-1, a PKC-iota inhibitor, on the growth pattern of various glioblastoma cell lines. T98G and U87 glioblastoma cells were treated with ICA-1 alone and the absolute cell numbers of each group were determined for cell growth expansion analysis, cell viability analysis, and cell death analysis. Low dose ICA-1 treatment alone significantly inhibited cell growth expansion of high density glioblastoma cells without inducing cell death. However, the high dose ICA-1 treatment regimen provided significant apoptosis for glioblastoma cells. Furthermore, this study was conducted to use a two layer molecular level approach for treating glioblastoma cells with ICA-1 plus an apoptosis agent, tumor-necrosis factor-related apoptosis-inducing ligand (TRAIL), to induce apoptosis in such chemo-refractory cancer cells. Following ICA-1 plus TRAIL treatment, apoptosis was detected in glioblastoma cells via the TUNEL assay and via flow cytometric analysis using Annexin-V FITC/PI. This study offers the first evidence for ICA-1 alone to inhibit glioblastoma cell proliferation as well as the novel combination of ICA-1 with TRAIL to cause robust apoptosis in a caspase-3 mediated mechanism. Furthermore, ICA-1 plus TRAIL simultaneously modulates down-regulation of PKC-iota and c-Jun.

Ursolic acid sensitizes prostate cancer cells to TRAIL-mediated apoptosis

Prostate cancer is one of the most commonly occurring malignancies in men, and because existing treatments are not able to manage this neoplasm adequately, novel approaches are needed. Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has strong antitumor activity via the induction of apoptotic cell death in a wide range of tumor cell types and has negligible toxicity to most normal cells, some prostate carcinoma cells are resistant to the apoptotic effects of TRAIL. Therefore, combinatorial approaches with TRAIL and different chemotherapeutic agents have been developed to overcome the resistance of cancer cells to TRAIL. Here, we investigated the sensitizing effects of ursolic acid (UA), a pentacyclic triterpenoid found in many plants, on TRAIL-induced prostate cancer cell apoptosis. We found TRAIL-induced prostate cancer cells apoptosis was significantly enhanced by UA, and that UA induced CHOP-dependent DR5 up-regulation. This study shows the use of UA as a sensitizer for TRAIL-induced apoptotic cell death offers a promising means of enhancing the efficacy of TRAIL-based prostate cancer treatments.

TRAIL conjugated to nanoparticles exhibits increased anti-tumor activities in glioma cells and glioma stem cells in vitro and in vivo

Glioblastomas (GBM) are characterized by resistance to chemotherapy and radiotherapy, and therefore, alternative therapeutic approaches are needed. TRAIL induces apoptosis in cancer but not in normal cells and is considered to be a promising anti-tumor agent. However, its short in vivo half-life and lack of efficient administration modes are serious impediments to its therapeutic efficacy. Nanoparticles (NP) have been used as effective delivery tools for various anticancer drugs. TRAIL was conjugated to magnetic ferric oxide NP by binding the TRAIL primary amino groups to activated double bonds on the surface of the NP. The effect of NP-TRAIL was examined on the apoptosis of glioma cells and self-renewal of glioma stem cells (GSCs). In addition, the ability of the NP-TRAIL to track U251 cell-derived glioma xenografts and to affect cell apoptosis, tumor volume, and survival among xenografted rats was also examined. Conjugation of TRAIL to NP increased its apoptotic activity against different human glioma cells and GSCs, as compared with free recombinant TRAIL. Combined treatment with NP-TRAIL and γ-radiation or bortezomib sensitized TRAIL-resistant GSCs to NP-TRAIL. Using rhodamine-labeled NP and U251 glioma cell-derived xenografts, we demonstrated that the NP-TRAIL were found in the tumor site and induced a significant increase in glioma cell apoptosis, a decrease in tumor volume, and increased animal survival. In summary, conjugation of TRAIL to NP increased its apoptotic activity both in vitro and in vivo. Therefore, NP-TRAIL represents a targeted anticancer agent with more efficient action for the treatment of GBM and the eradication of GSCs.

Radiotherapy and TRAIL for cancer therapy

The use of radiotherapy and concomitant chemotherapy substantially improved cure rates in patients with different malignant tumours. However, it is unlikely that further improvements based on conventional chemotherapy may be achieved in the future since increased rates of acute side effects already limit the value of these approaches. Additionally, the increased local control rates are counterweighted by still high rates of distant failures resulting in low net gains for the patients. Thus, there is a currently unmet need for the integration of target-specific drugs improving local control as well distant control into radiation based treatment protocols. In this regard, the death-receptor ligand TNF-α-related apoptosis-inducing ligand (TRAIL/Apo2L) and TRAIL-receptor agonistic antibodies were shown to display a high selectivity for tumour cells and act synergistically with conventional chemotherapy drugs and radiation. Up to now it has been shown that radiation strongly sensitises malignant cells to TRAIL and TRAIL-agonistic antibodies. Synergistic induction of apoptosis was demonstrated in a majority of malignant cell types and xenograft models. Especially in those cells types displaying only weak responses to either treatment alone, strong sensitising effects were described. Moreover, in merely all normal cells and tissues no synergistic effects were found. Depending on cell type and experimental setting, the efficacy of combined treatment is determined by the p53-status, the balance between pro- and anti-apoptotic Bcl-2 proteins and modulation of TRAIL-receptor signal transduction.

Irradiation enhances the tumor tropism and therapeutic potential of tumor necrosis factor-related apoptosis-inducing ligand-secreting human umbilical cord blood-derived mesenchymal stem cells in glioma therapy

Irradiation is a standard therapy for gliomas and many other cancers. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is one of the most promising candidates for cancer gene therapy. Here, we show that tumor irradiation enhances the tumor tropism of human umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs) and the therapeutic effect of TRAIL delivered by UCB-MSCs. The sequential treatment with irradiation followed by TRAIL-secreting UCB-MSCs (MSC-TRAIL) synergistically enhanced apoptosis in either TRAIL-sensitive or TRAIL-resistant glioma cells by upregulating the death receptor 5 and by inducing caspase activation. Migration assays showed greater MSC migration toward irradiated glioma cells and the tumor site in glioma-bearing mice compared with unirradiated tumors. Irradiated glioma cells had increased expression of interleukin-8 (IL-8), which leads to the upregulation of the IL-8 receptor on MSCs. This upregulation, which is involved in the migratory capacity of UCB-MSCs, was confirmed by siRNA inhibition and an antibody-neutralizing assay. In vivo survival experiments in orthotopic xenografted mice showed that MSC-based TRAIL gene delivery to irradiated tumors had greater therapeutic efficacy than a single treatment. These results suggest that clinically relevant tumor irradiation increases the therapeutic efficacy of MSC-TRAIL by increasing tropism of MSCs and TRAIL-induced apoptosis, which may be a more useful strategy for cancer gene therapy.

Ursolic acid, a pentacyclin triterpene, potentiates TRAIL-induced apoptosis through p53-independent up-regulation of death receptors: evidence for the role of reactive oxygen species and JNK

Discovery of the molecular targets of traditional medicine and its chemical footprints can validate the use of such medicine. In the present report, we investigated the effect of ursolic acid (UA), a pentacyclic triterpenoid found in rosemary and holy basil, on apoptosis induced by TRAIL. We found that UA potentiated TRAIL-induced apoptosis in cancer cells. In addition, UA also sensitized TRAIL-resistant cancer cells to the cytokine. When we investigated the mechanism, we found that UA down-regulated cell survival proteins and induced the cell surface expression of both TRAIL receptors, death receptors 4 and 5 (DR4 and -5). Induction of receptors by UA occurred independently of cell type. Gene silencing of either receptor by small interfering RNA reduced the apoptosis induced by UA and the effect of TRAIL. In addition, UA also decreased the expression of decoy receptor 2 (DcR2) but not DcR1. Induction of DRs was independent of p53 because UA induced DR4 and DR5 in HCT116 p53(-/-) cells. Induction of DRs, however, was dependent on JNK because UA induced JNK, and its pharmacologic inhibition abolished the induction of the receptors. The down-regulation of survival proteins and up-regulation of the DRs required reactive oxygen species (ROS) because UA induced ROS, and its quenching abolished the effect of the terpene. Also, potentiation of TRAIL-induced apoptosis by UA was significantly reduced by both ROS quenchers and JNK inhibitor. In addition, UA was also found to induce the expression of DRs, down-regulate cell survival proteins, and activate JNK in orthotopically implanted human colorectal cancer in a nude mouse model. Overall, our results showed that UA potentiates TRAIL-induced apoptosis through activation of ROS and JNK-mediated up-regulation of DRs and down-regulation of DcR2 and cell survival proteins.

Targeting apoptosis pathways in glioblastoma

The treatment of glioblastoma remains a major challenge for clinicians since these highly aggressive brain tumors are relatively resistant towards radio- and chemotherapy. The pathways that control apoptosis are altered in glioblastoma cells leading to resistance towards apoptotic stimuli in general. In this review we describe the alterations affecting the p53 pathway, the BCL-2 protein family, the inhibitor of apoptosis proteins and several growth factor pathways involved in the regulation of programmed cell death and define possible targets for new therapies within these apoptotic pathways in glioblastomas. Moreover, we review strategies to target death receptor pathways, most notably to render the glioblastoma cells more susceptible towards this approach without enhancing toxicity in general. Most of the strategies targeting apoptosis in glioblastomas presented here are in a pre-clinical stage of development, however, they all share the ultimative goal to improve the outcome for glioblastoma patients.

FasL and FADD delivery by a glioma-specific and cell cycle-dependent HSV-1 amplicon virus enhanced apoptosis in primary human brain tumors

BACKGROUND

Glioblastoma multiforme is the most malignant cancer of the brain and is notoriously difficult to treat due to the highly proliferative and infiltrative nature of the cells. Herein, we explored the combination treatment of pre-established human glioma xenograft using multiple therapeutic genes whereby the gene expression is regulated by both cell-type and cell cycle-dependent transcriptional regulatory mechanism conferred by recombinant HSV-1 amplicon vectors.

RESULTS

We demonstrated for the first time that Ki67-positive proliferating primary human glioma cells cultured from biopsy samples were effectively induced into cell death by the dual-specific function of the pG8-FasL amplicon vectors. These vectors were relatively stable and exhibited minimal cytotoxicity in vivo. Intracranial implantation of pre-transduced glioma cells resulted in better survival outcome when compared with viral vectors inoculated one week post-implantation of tumor cells, indicating that therapeutic efficacy is dependent on the viral spread and mode of viral vectors administration. We further showed that pG8-FasL amplicon vectors are functional in the presence of commonly used treatment regimens for human brain cancer. In fact, the combined therapies of pG8-FasL and pG8-FADD in the presence of temozolomide significantly improved the survival of mice bearing intracranial high-grade gliomas.

CONCLUSION

Taken together, our results showed that the glioma-specific and cell cycle-dependent HSV-1 amplicon vector is potentially useful as an adjuvant therapy to complement the current gene therapy strategy for gliomas.

Cisplatin-enhanced sensitivity of glioblastoma multiforme U251 cells to adenovirus-delivered TRAIL in vitro

TRAIL is a novel therapeutic agent for potential use in glioblastoma multiforme therapy; however, glioblastoma multiforme cells exhibit resistance to TRAIL-induced apoptosis. To evaluate the effects of cisplatin on sensitivity of human glioma cell line U251 to Ad-TRAIL and to investigate the potential mechanism, U251 cells were transfected with Ad-TRAIL and then exposed to cisplatin. The proliferation inhibition of the treated cells was studied by the method of MTT. The cell apoptosis was analyzed by Hoechst33342 staining and by flow cytometry with propidium iodide staining. Semi-quantitative RT-PCR was introduced to detect the mRNA expression of TRAIL, DR4, DR5, Caspase 3, and survivin. Protein expression of DR5 and cleaved Caspase 3 was detected by Western blot assay. The results showed that the combination treatment of cisplatin and Ad-TRAIL could inhibit the proliferation of U251 cells significantly compared with the alone treatment (P < 0.01), which was chiefly attributed to the induction of obvious apoptosis. The enhancement of Ad-TRAIL by cisplatin was due to the up-regulation of DR5 but not DR4 expression, and followed by the down-regulation of survivin and activation of Caspase 3. In conclusion, cisplatin could enhance the apoptosis induction of U251 cells to adenovirous vector carried TRAIL.

Predominant antitumor effects by fully human anti-TRAIL-receptor 2 (DR5) monoclonal antibodies in human glioma cells in vitro and in vivo

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2 L) preferentially induces apoptosis in human tumor cells through its cognate death receptors DR4 or DR5, thereby being investigated as a potential agent for cancer therapy. Here, we applied fully human anti-human TRAIL receptor monoclonal antibodies (mAbs) to specifically target one of death receptors for TRAIL in human glioma cells, which could also reduce potential TRAIL-induced toxicity in humans. Twelve human glioma cell lines treated with several fully human anti-human TRAIL receptor mAbs were sensitive to only anti-DR5 mAbs, whereas they were totally insensitive to anti-DR4 mAb. Treatment with anti-DR5 mAbs exerted rapid cytotoxicity and lead to apoptosis induction. The cellular sensitivity was closely associated with cell-surface expression of DR5. Expression of c-FLIP(L), Akt, and Cyclin D1 significantly correlated with sensitivity to anti-DR5 mAbs. Primary cultures of glioma cells were also relatively resistant to anti-DR5 mAbs, exhibiting both lower DR5 and higher c-FLIP(L) expression. Downregulation of c-FLIP(L) expression resulted in the sensitization of human glioma cells to anti-DR5 mAbs, whereas overexpression of c-FLIP(L) conferred resistance to anti-DR5 mAb. Treatment of tumor-burden nude mice with the direct agonist anti-DR5 mAb KMTR2 significantly suppressed growth of subcutaneous glioma xenografts leading to complete regression. Similarly, treatment of nude mice bearing intracerebral glioma xenografts with KMTR2 significantly elongated lifespan without tumor recurrence. These results suggest that DR5 is the predominant TRAIL receptor mediating apoptotic signals in human glioma cells, and sensitivity to anti-DR5 mAbs was determined at least in part by the expression level of c-FLIP(L) and Akt. Specific targeting of death receptor pathway through DR5 using fully human mAbs might provide a novel therapeutic strategy for intractable malignant gliomas.

Review: on TRAIL for malignant glioma therapy?

Glioblastoma (GBM) is a devastating cancer with a median survival of around 15 months. Significant advances in treatment have not been achieved yet, even with a host of new therapeutics under investigation. Therefore, the quest for a cure for GBM remains as intense as ever. Of particular interest for GBM therapy is the selective induction of apoptosis using the pro-apoptotic tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL signals apoptosis via its two agonistic receptors TRAIL-R1 and TRAIL-R2. TRAIL is normally present as homotrimeric transmembrane protein, but can also be processed into a soluble trimeric form (sTRAIL). Recombinant sTRAIL has strong tumouricidal activity towards GBM cells, with no or minimal toxicity towards normal human cells. Unfortunately, GBM is a very heterogeneous tumour, with multiple genetically aberrant clones within one tumour. Consequently, any single agent therapy is likely to be not effective enough. However, the anti-GBM activity of TRAIL can be synergistically enhanced by a variety of conventional and novel targeted therapies, making TRAIL an ideal candidate for combinatorial strategies. Here we will, after briefly detailing the biology of TRAIL/TRAIL receptor signalling, focus on the promises and pitfalls of recombinant TRAIL as a therapeutic agent alone and in combinatorial therapeutic approaches for GBM.

Withaferin A sensitizes TRAIL-induced apoptosis through reactive oxygen species-mediated up-regulation of death receptor 5 and down-regulation of c-FLIP

Withaferin A (Wit A) has reportedly shown cytotoxicity in a variety of tumor cell lines. Here, we show that cotreatment with subtoxic doses of Wit A and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in human renal cancer cells, Caki cells, but not in human normal mesangial cells. Moreover, the combined treatment with Wit A and TRAIL dramatically induces apoptosis in various cancer cell types, suggesting that this combined treatment might offer an attractive strategy for safely treating human cancers. Treatment of Caki cells with Wit A up-regulated death receptor 5 (DR5) in a C/EBP homologous protein (CHOP)-dependent manner. Interestingly, a Wit A-induced increase in ROS levels preceded the up-regulation of CHOP and DR5. The involvement of ROS in CHOP-mediated DR5 up-regulation was confirmed by the result that pretreatment with an antioxidant, NAC or catalase, inhibited Wit A-induced up-regulation of both CHOP and DR5. We also found that Wit A treatment down-regulated c-FLIP via NF-kappaB-mediated transcriptional control as well as ROS signaling pathways. Taken together, our results show that DR5 up-regulation and c-FLIP down-regulation contribute to the sensitizing effect of Wit A on TRAIL-mediated apoptosis in cancer cells.

Targeting death-receptors in radiation therapy

The development of apoptosis resistance is a crucial step during the pathogenesis of malignant tumors. Thus, any treatment approach overcoming apoptosis resistance may be a valuable tool in oncology. Although a variety of treatments induce apoptosis, only very few specifically trigger programmed cell death. In this regard, the class of apoptosis inducing ligands may turn out to have a considerable potential in oncology. TNF-alpha-related apoptosis-inducing ligand (TRAIL/Apo2L) is the most promising candidate, either alone or in combination with established cancer therapies, since it induces apoptosis in a wide range of malignant cells while sparing most normal tissues. Since death-receptor induced apoptosis is mainly mediated via nonmitochondrial death pathways, it is possible to induce apoptosis in cancer cell systems which mainly harbor defects within the mitochondrial death cascades. Even more so it has been shown that conventional DNA damaging approaches reduced the killing threshold for receptor induced apoptosis, making TRAIL an ideal candidate for combined approaches. Thus, combined treatments might offer the chance to enhance therapeutic efficiency and overcome resistance. In combination, additive or synergistic apoptotic responses and substantially enhanced clonogenic cell kill has been documented. Furthermore, in several settings it has been shown that combined modality teatments were effective in malignant cells, which are highly resistant to either treatment, alone. Ionizing radiation is one of the most effective modalities in oncology. Thus, it is reasonable to test, how far combinations of TRAIL with ionizing radiation may increase the efficacy. Indeed, the combination of TRAIL with ionizing radiation in several in vitro settings as well as xenograft models resulted in highly increased rates of cell kill and long-term tumor control. No increase in the rate and severity of side effects has been documented, indicating that the combination really increases the therapeutic ratio. It is important to note that TRAIL and TRAIL receptor agonistic antibodies, either as single

Rosiglitazone promotes tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis by reactive oxygen species-mediated up-regulation of death receptor 5 and down-regulation of c-FLIP

Death receptor 5 (DR5/TRAIL-R2) is an apoptosis-inducing membrane receptor for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study, we show that rosiglitazone sensitizes human renal cancer cells to TRAIL-mediated apoptosis, but not normal human mesangial cells. Furthermore, because rosiglitazone-enhanced TRAIL-mediated apoptosis is induced in various types of cancer cells but is not interrupted by Bcl-2 overexpression, this combinatory treatment may provide an attractive strategy for cancer treatment. We found that treatment with rosiglitazone significantly induces DR5 expression at both its mRNA and its protein levels, accompanying the generation of reactive oxygen species (ROS). Both treatment with DR5/Fc chimeric protein and silencing of DR5 expression using small interfering RNAs attenuated rosiglitazone plus TRAIL-induced apoptosis, showing the critical role of DR5 in this cell death. Pretreatment with GSH significantly inhibited rosiglitazone-induced DR5 up-regulation and the cell death induced by the combined treatment with rosiglitazone and TRAIL, suggesting that ROS mediate rosiglitazone-induced DR5 up-regulation, contributing to TRAIL-mediated apoptosis. However, both DR5 up-regulation and sensitization of TRAIL-mediated apoptosis induced by rosiglitazone are likely PPARgamma-independent, because a dominant-negative mutant of PPARgamma and a potent PPARgamma inhibitor, GW9662, failed to block DR5 induction and apoptosis. Interestingly, we also found that rosiglitazone treatment induced down-regulation of cellular FLICE-inhibitory protein (c-FLIPs), and ectopic expression of c-FLIPs attenuated rosiglitazone plus TRAIL-mediated apoptosis, demonstrating the involvement of c-FLIPs in this apoptosis. Taken together, the results of this study demonstrate that rosiglitazone enhances TRAIL-induced apoptosis in various cancer cells by ROS-mediated DR5 up-regulation and down-regulation of c-FLIPs.