Early growth response-1 is a regulator of DR5-induced apoptosis in colon cancer cells

Background:

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) induces tumour cell apoptosis by binding to death receptor 4 (DR4) and DR5. DR4 and DR5 activation however can also induce inflammatory and pro-survival signalling. It is not known how these different cellular responses are regulated and what the individual role of DR4 vs DR5 is in these processes.

Methods:

DNA microarray study was carried out to identify genes differentially expressed after DR4 and DR5 activation. RT–PCR and western blotting was used to examine the expression of early growth response gene-1 (Egr-1) and the proteins of the TRAIL signalling pathway. The function of Egr-1 was studied by siRNA-mediated knockdown and overexpression of a dominant-negative version of Egr-1.

Results:

We show that the immediate early gene, Egr-1, regulates TRAIL sensitivity. Egr-1 is constitutively expressed in colon cancer cells and further induced upon activation of DR4 or DR5. Our results also show that DR4 mediates a type II, mitochondrion-dependent apoptotic pathway, whereas DR5 induces a mitochondrion-independent, type I apoptosis in HCT15 colon carcinoma cells. Egr-1 drives c-FLIP expression and the short splice variant of c-FLIP (c-FLIP_S) specifically inhibits DR5 activation.

Conclusion:

Selective knockdown of c-FLIP_S sensitises cells to DR5-induced but not DR4-induced apoptosis and Egr-1 exerts an effect as an inhibitor of the DR5-induced apoptotic pathway, possibly by regulating the expression of c-FLIP_S.     

Increased death receptor 5 expression by chemotherapeutic agents in human gliomas causes synergistic cytotoxicity with tumor necrosis factor-related apoptosis-inducing ligand in vitro and in vivo

The intractability of malignant gliomas to multimodality treatments plays a large part in their extremely poor prognosis. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a novel member of the tumor necrosis factor (TNF) family that induces apoptosis preferentially in tumor cells through binding to its cognate death receptors, DR4 and DR5. Here we show that the DNA-damaging chemotherapeutic drugs, cis-diamminedichloroplatinum(II) (CDDP) and etoposide, elicited increased expression of DR5 in human glioma cells. Exposure of such cells in vitro to soluble human TRAIL in combination with CDDP or etoposide resulted in synergistic cell death that could be blocked by soluble TRAIL-neutralizing DR5-Fc or the caspase inhibitors, Z-Asp-CH2-DCB and CrmA. Moreover, systemic in vivo administration of TRAIL with CDDP synergistically suppressed both tumor formation and growth of established s.c. human glioblastoma xenografts in nude mice by inducing apoptosis without causing significant general toxicity. The combination treatment resulted in complete and durable remission in 29% of mice with the established s.c. xenografts and also significantly extended the survival of mice bearing intracerebral xenografts. These results provide preclinical proof-of-principle for a novel therapeutic strategy in which the death ligand, TRAIL, is safely combined with conventional DNA-damaging chemotherapy.     

Melatonin sensitizes human malignant glioma cells against TRAIL-induced cell death

Despite the common expression of death receptors, many types of cancer including gliomas are resistant to the death receptor ligand (TRAIL). Melatonin antitumoral actions have been extensively described, including oncostatic properties on several tumor types and improvement of chemotherapeutic regimens. Here, we found that melatonin effectively increase cell sensitivity to TRAIL-induced cell apoptosis in A172 and U87 human glioma cells. The effect seems to be related to a modulation of PKC activity which in turns decreases Akt activation leading to an increase in death receptor 5 (DR5) levels and a decrease in the antiapoptotic proteins survivin and bcl-2 levels.     

死亡受体5与肝癌细胞凋亡的相关性

目的探讨人死亡受体5(DR5)及其配体(TRAIL)和竞争性单克隆抗体(DR5mAb)对肝癌细胞凋亡的影响。方法采用半定量RT-PCR及Western blot法检测肝癌细胞株HepG2、SMMC7721和正常人肝细胞株LO2的DR5在mRNA和蛋白水平的表达含量。应用四甲基偶氮唑盐法测试细胞的生长曲线,以观察TRAIL和DR5mAb对肝癌细胞生长的抑制作用,并用流式细胞仪测定细胞的凋亡率。结果肝癌细胞株HepG2、SMMC7721的DR5高表达,正常人肝细胞株LO2的DR5低表达,差异有统计学意义(P<0.05)。肝癌细胞的增殖率随着TRAIL浓度的增加而逐渐降低,但当TRAIL超过1000ng/ml时,肝癌细胞株的增殖率就不再下降;而肝癌细胞的增殖率随着DR5mAb浓度的增加而逐渐降低,呈现明显的剂量依赖关系。1000ng/ml TRAIL处理24h时,HepG2的凋亡率在14.74%±0.48%,继续增加TRAIL浓度,其凋亡率无显著改变;而同样剂量的DR5mAb处理24h时,HepG2的凋亡率高达52.45%±0.57%,明显高于前者(P<0.05)。同时发现,正常人肝细胞株LO2的增殖率随着TRAIL浓度的增加呈下降趋势,与PBS阴性对照组差异有统计学意义(P<0.05);而DR5mAb的浓度变化对人正常肝细胞株LO2无明显影响。结论死亡受体DR5在诱导肝癌细胞凋亡中起重要的作用;DR5mAb选择性地诱导肝癌细胞凋亡,对人正常肝细胞无诱导凋亡作用。     

Induction and intracellular regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) mediated apotosis in human malignant glioma cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) preferentially triggers apoptosis in tumor cells versus normal cells, thus providing a therapeutic potential. In this study, we examined a large panel of human malignant glioma cell lines and primary cultures of normal human astrocytes for their sensitivity to TRAIL. Of 13 glioma cell lines, 3 were sensitive (80-100% death), 4 were partially resistant (30-79% death), and 6 were resistant (< 30% death). Normal astrocytes were also resistant. TRAIL-induced cell death was characterized by activation of caspase-8 and -3, poly(ADP-ribose) polymerase cleavage, and DNA fragmentation. Decoy receptor (DcR1 and DcR2) expression was limited in the glioma cell lines and did not correlate with TRAIL sensitivity. Both sensitive and resistant cell lines expressed TRAIL death receptor (DR5), adapter protein Fas-associated death domain (FADD), and caspase-8; but resistant cell lines expressed 2-fold higher levels of the apoptosis inhibitor phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes-15 kDa (PED/PEA-15). In contrast, cellular FADD-like IL-1beta-converting enzyme-like inhibitory protein (cFLIP) expression was similar in sensitive and resistant cells. Transfection of sense PED/PEA-15 cDNA in sensitive cells resulted in cell resistance, whereas transfection of antisense in resistant cells rendered them sensitive. Inhibition of protein kinase C (PKC) activity restored TRAIL sensitivity in resistant cells, suggesting that PED/ PEA-15 function might be dependent on PKC-mediated phosphorylation. In summary, TRAIL induces apoptosis in > 50% of glioma cell lines, and this killing occurs through activation of the DR pathway. This caspase-8-induced apoptotic cascade is regulated by intracellular PED/PEA-15, but not by cFLIP or decoy receptors. This pathway may be exploitable for glioma and possibly for other cancer therapies.     

TRAIL induces apoptosis in oral squamous carcinoma cells: a crosstalk with oncogenic Ras regulated cell surface expression of death receptor 5

TNF-related apoptosis inducing ligand (TRAIL) induces apoptosis through its death receptors (DRs) 4 and/or 5 expressed on the surface of target cells. The selectivity of TRAIL towards cancer cells has promoted clinical evaluation of recombinant human TRAIL (rhTRAIL) and its agonistic antibodies in treating several major human cancers including colon and non-Hodgkin''s lymphoma. However, little is known about their ability in killing oral squamous cell carcinoma (OSCC) cells. In this study, we tested the apoptotic responses of a panel of seven human OSCC cell lines (HN31, HN30, HN12, HN6, HN4, Cal27, and OSCC3) to rhTRAIL and monoclonal antibodies against DR4 or DR5. We found that rhTRAIL is a potent inducer of apoptosis in most of the oral cancer cell lines tested both in vitro and in vivo. We also showed that DR5 was expressed on the surface of the tested cell lines which correlated with the cellular susceptibility to apoptosis induced by rhTRAIL and anti-DR5 antibody. By contrast, little or no DR4 was detected on the surface of OSCC3 and HN6 cells rendering cellular resistance to DR4 antibody and a reduced sensitivity to rhTRAIL. Notably, the overall TRAIL sensitivity correlated well with the levels of endogenous active Ras in the cell lines tested. Expression of a constitutively active Ras mutant (RasV12) in OSCC3 cells selectively upregulated surface expression of DR5, but not DR4, and restored TRAIL sensitivity. Our findings could have implications for the use of TRAIL receptor targeted therapies in the treatment of human OSCC tumors particularly the ones harboring constitutively active Ras mutant.     

Ca2+ influx-mediated dilation of the endoplasmic reticulum and c-FLIPL downregulation trigger CDDO-Me–induced apoptosis in breast cancer cells

The synthetic triterpenoid 2-cyano-3, 12-dioxooleana-1, 9(11)-dien-C28-methyl ester (CDDO-Me) is considered a promising anti-tumorigenic compound. In this study, we show that treatment with CDDO-Me induces progressive endoplasmic reticulum (ER)-derived vacuolation in various breast cancer cells and ultimately kills these cells by inducing apoptosis. We found that CDDO-Me–induced increases in intracellular Ca²⁺ levels, reflecting influx from the extracellular milieu, make a critical contribution to ER-derived vacuolation and subsequent cell death. In parallel with increasing Ca²⁺ levels, CDDO-Me markedly increased the generation of reactive oxygen species (ROS). Interestingly, there exists a reciprocal positive-regulatory loop between Ca²⁺ influx and ROS generation that triggers ER stress and ER dilation in response to CDDO-Me. In addition, CDDO-Me rapidly reduced the protein levels of c-FLIP_L (cellular FLICE-inhibitory protein) and overexpression of c-FLIP_L blocked CDDO-Me–induced cell death, but not vacuolation. These results suggest that c-FLIP_L downregulation is a key contributor to CDDO-Me–induced apoptotic cell death, independent of ER-derived vacuolation. Taken together, our results show that ER-derived vacuolation via Ca²⁺ influx and ROS generation as well as caspase activation via c-FLIP_L downregulation are responsible for the potent anticancer effects of CDDO-Me on breast cancer cells.     

Prediction of proapoptotic anticancer therapeutic response in vivo based on cell death visualization and TRAIL death ligand-receptor interaction

Tumor growth is often associated with insufficient apoptosis. The Tumor Necrosis Factor (TNF)-Related Apoptosis-Inducing Ligand (TRAIL) and its proapoptotic receptors death receptor 4 (DR4) and DR5 agonistic monoclonal antibodies are being developed as targeted therapeutics because they kill cancer cells while sparing normal cells. A challenge to targeted therapeutics is the selection of patients who are most likely to benefit from targeted drugs because of the heterogeneity of cancer. Molecular imaging may be useful in targeted drug development by assessing the target expression and drug-target interaction, and for predicting therapeutic response. We hypothesized that the cell surface expression level of DR4/5 may predict the proapoptotic targeted therapeutic response if the signaling pathway downstream is intact. The goal of this proof-of-concept study was to develop a molecular imaging strategy to predict proapoptotic anti-cancer therapy response at an early stage of treatment. TRAIL and the DR5 agonistic monoclonal antibody HGS-ETR2 (Lexatumumab, TRM-2) were labeled with a near-infrared dye and these were used to image the TRAIL receptors on cultured TRAIL sensitive and TRAIL resistant human tumor cells as well as tumor xenografts. Imaging of cells and tumor-bearing animals was conducted with near infrared fluorescence imagers and apoptosis in cells was assessed by western blots of PARP-cleavage and flow cytometry of sub-G1 content. Apoptosis in tumors was evaluated by imaging near-infrared dye-labeled Annexin V and tumor tissue activated caspase-3 staining. Both in vitro and in vivo studies showed that imaging of death inducing ligand-receptor interaction was consistent with the apoptosis readout. Thus TRAIL sensitive tumors that express TRAIL receptors underwent cell death following treatment whereas tumors lacking TRAIL receptor expression were shown to be TRAIL resistant. In vivo molecular imaging of TRAIL receptor expression correlated with response to TRAIL therapy and an apoptotic response in vivo.     

TRAIL, FasL and a blocking anti-DR5 antibody augment paclitaxel-induced apoptosis in human non-small-cell lung cancer.

Lung carcinoma is one of the most frequent causes of malignancy-related mortality in the world. Paclitaxel (PA) is an antineoplastic agent used in the treatment of non-small-cell lung cancer (NSCLC) and possesses a single-agent response rate approaching 25%. PA kills tumor cells by inducing both cellular necrosis and apoptosis. Fas and Trail receptors (DR4 and DR5) are TNF family members and act as death signal transduction proteins in the apoptosis cascade. Despite the importance of PA in lung cancer treatment, the function of Fas, DR4 and DR5 in PA-induced apoptosis, as well as the effect of their respective ligands FasL and TRAIL alone or in combination with PA, remains poorly understood. We show here that 10 microM PA induces a significant 10- to 57-fold increase in primary lung cancer cell apoptosis and is associated with 20-215% increases in caspase-3 activity in various NSCLC cell types. All the lung cancer cells express Fas, FasL, DR4 and DR5; however PA did not significantly modify their levels. We provide here the first time evidence that TRAIL is a potent inducer of apoptosis in multiple NSCLC cell lines. Noticeably, CH11, the Fas receptor cross-linking and the antagonistic anti-DR5 antibody enhance considerably the spontaneous apoptotic rate in 3 out of 5 cell types. The combination treatments, FasL+PA, TRAIL+PA or PA+anti-DR5 antibody, greatly enhance PA-apoptotic effect in most cell lines. These data suggest that the use of new combination treatment with PA and ligands targeting Fas or TRAIL receptors would be particularly efficacious.     

重组人TRAIL蛋白联合顺铂对人卵巢癌细胞生长及凋亡的影响

背景与目的：研究发现肿瘤坏死因子的相关凋亡诱导配体(tumor necrosis factor-related apoptosis inducing ligand,TRAIL)可以增强化疗药物对肿瘤细胞的杀伤作用。本研究旨在探讨TRAIL与顺铂联合应用对体外培养的卵巢癌细胞SKOV3和OVCAR3生长凋亡的影响及可能的诱导机制。方法：利用MTT法和流式细胞仪检测在顺铂和重组人TRAIL蛋白共同作用下,SKOV3和OVCAR3细胞的增殖抑制效应及细胞凋亡程度;并应用实时荧光定量聚合酶链反应(real-time fluorescent quantitative polymerase chain reaction,RTFQ-PCR)检测药物处理后TRAIL死亡受体DR4、DR5的mRNA表达水平;同时用蛋白[质]印迹法(Western blot)检测DR4、DR5的蛋白表达水平。结果：SKOV3和OVCAR3细胞均对TRAIL蛋白敏感,随着TRAIL蛋白浓度的升高,细胞的生长抑制率可达64%;而TRAIL与顺铂联合用药对两种细胞的抑制率均达到92%以上,对细胞的增殖抑制呈现高效协同作用,与单独用药组比较差异有统计学意义(P<0.05);TRAIL和顺铂联合组两种细胞凋亡率分别为(31.50±0.79)%和(36.60±1.31)%,显著高于单独用药组;RTFQ-PCR和Western blot检测结果显示,SKOV3和OVCAR3细胞在TRAIL与顺铂联合用药后,死亡受体DR4、DR5表达水平均显著上调。结论：在体外,TRAIL与化疗药物顺铂联用能明显抑制卵巢癌细胞增殖,诱导肿瘤细胞凋亡。TRAIL能明显增强顺铂对卵巢癌细胞的敏感性,其诱导机制可能与死亡受体DR4、DR5表达水平上调有关。     

Recurrence-free survival in prostate cancer is related to increased stromal TRAIL expression

BACKGROUND:

TRAIL (tumor necrosis factor related apoptosis‐inducing ligand) is involved in tumor immune surveillance and, thus, may be a potential cancer therapy. TRAIL expression in the tumor microenvironment has been shown to impact cancer survival in multiple tumor types, including ovarian cancer. We studied TRAIL expression and outcomes in patients with prostate cancer.

METHODS:

A tissue microarray (TMA) of 200 prostate cancer patients and benign prostate tissue controls was used to assess the epithelial and stromal protein expression of TRAIL, death receptors (DR4 and DR5), decoy receptors (DcR1 and DcR2), and the FLICE inhibitory protein (FLIP_L). We correlated these expression patterns with clinicopathological parameters and determined its impact on recurrence‐free survival.

RESULTS:

Nearly all (99.5%) prostate cancer tissues examined displayed either decreased expression of pro‐apoptotic TRAIL receptors, increased FLIP_L expression, or both. We observed elevated death receptor, decoy receptor, FLIP_L, and epithelial TRAIL expression in prostate cancer epithelium. TRAIL expression in the stromal tumor microenvironment surrounding the prostate cancer was markedly lower. Elevated TRAIL expression in the tumor microenvironment was also significantly associated with increased recurrence‐free survival (P = .014), after controlling for other prognostic markers. In contrast, epithelial expression of TRAIL did not have an effect on overall survival.

CONCLUSIONS:

Expression of the components of the pro‐apoptotic TRAIL pathway is altered in prostate cancer. Moreover, TRAIL expression in the tumor microenvironment may affect recurrence‐free survival rate of prostate cancer patients. Consequently, these results may be useful in devising future therapeutic strategies targeting the TRAIL pathway in prostate cancer. Cancer 2011. © 2010 American Cancer Society.     

Induction of proapoptotic antibodies to triple-negative breast cancer by vaccination with TRAIL death receptor DR5 DNA

TNF-related apoptosis-inducing ligand receptor 2 [TRAIL-R2 or death receptor 5 (DR5)] is expressed at elevated levels in a broad range of solid tumors to mediate apoptotic signals from TRAIL or agonist antibodies. We tested the hypothesis that DR5 DNA vaccination will induce proapoptotic antibody to trigger apoptosis of tumor cells. BALB/c mice were electrovaccinated with DNA-encoding wild-type human DR5 (phDR5) or its derivatives. Resulting immune serum or purified immune IgG induced apoptosis in triple-negative breast cancer (TNBC) cells, which were also TRAIL sensitive. The proapoptotic activity of immune serum at dilutions of 0.5-2% was comparable to that of 1-2 μg/ml of TRAIL. Apoptotic activity of immune serum was enhanced by antibody crosslinking. Apoptotic cell death induced by anti-DR5 antibody was shown by the cleavage of PARP and caspase-3. In contrast, immune serum had no effect on the proliferation of activated human T cells, which expressed low levels of DR5. In vivo, hDR5 reactive immune serum prevented growth of SUM159 TNBC cells in severe combined immune-deficient mice. DR5-specific IFN-γ-secreting T cells were also induced by DNA vaccination. Furthermore, the feasibility to overcome immune tolerance to self DR5 was shown by the induction of mouse DR5-binding antibody after electrovaccination of BALB/c mice with pmDR5ectm-Td1 encoding a fusion protein of mouse DR5 and an immunogenic fragment of tetanus toxin. These findings support DR5 as a promising vaccine target for controlling TNBC and other DR5-positive cancers.     

Sulforaphane enhances TRAIL-induced apoptosis through the induction of DR5 expression in human osteosarcoma cells

Sulforaphane (SFN), a naturally occurring isothiocyanate, is an attractive agent because of its potent anticancer effects. SFN suppresses the proliferation of various cancer cells in vitro and in vivo. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is also one of the most promising candidates for cancer therapeutics owing to its ability to selectively induce apoptosis in tumor cells. In this study, we report that SFN enhances TRAIL-induced apoptosis in human osteosarcoma cells, Saos2 and MG63. The apoptosis induced by co-treatment with SFN and TRAIL was markedly blocked by a dominant negative form of the TRAIL receptor or caspase inhibitors. The combined use of SFN and TRAIL effectively induced Bid cleavage and the activation of caspases 8, 10, 9 and 3 at ineffective concentrations for each agent. SFN upregulated the expression of death receptor 5 (DR5), a receptor for TRAIL, at mRNA and protein levels in a dose-dependent manner. In addition, the SFN-mediated sensitization to TRAIL was reduced by DR5 siRNA, suggesting that the sensitization was at least partially mediated through the induction of DR5 expression. Furthermore, SFN sensitized TRAIL-induced apoptosis in a p53-independent manner. On the other hand, SFN neither induced DR5 protein expression or enhanced TRAIL-induced apoptosis in normal human peripheral blood mononuclear cells. Thus, combined treatment with SFN and TRAIL might be a promising therapy for osteosarcoma.     

Arsenic trioxide sensitizes human glioma cells, but not normal astrocytes, to TRAIL-induced apoptosis via CCAAT/enhancer-binding protein homologous protein-dependent DR5 up-regulation

The current study shows that treatment of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-resistant glioma cells with a combination of TRAIL and subtoxic doses of arsenic trioxide (As(2)O(3)) induces rapid apoptosis. Whereas TRAIL-mediated proteolytic processing of procaspase-3 was partially blocked in glioma cells, treatment with As(2)O(3) efficiently recovered TRAIL-induced activation of caspases. We also found that As(2)O(3) treatment of glioma cells significantly up-regulated DR5, a death receptor of TRAIL. Furthermore, suppression of DR5 expression by small interfering RNA (siRNA) inhibited As(2)O(3)/TRAIL-induced apoptosis of U87MG glioma cells, suggesting that DR5 up-regulation is critical for As(2)O(3)-induced sensitization of glioma cells to TRAIL-mediated apoptosis. Our results also indicate that an increase in CCAAT/enhancer binding protein homologous protein (CHOP) protein levels precedes As(2)O(3)-induced DR5 up-regulation. The involvement of CHOP in this process was confirmed by siRNA-mediated CHOP suppression, which not only attenuated As(2)O(3)-induced DR5 up-regulation but also inhibited the As(2)O(3)-stimulated TRAIL-induced apoptosis. These results therefore suggest that the CHOP-mediated DR5 up-regulation, brought about by As(2)O(3), stimulates the TRAIL-mediated signaling pathway. This in turn leads to complete proteolytic processing of caspase-3, which is partially primed by TRAIL in glioma cells. In contrast to human glioma cells, astrocytes were very resistant to the combined administration of As(2)O(3) and TRAIL, demonstrating the safety of this treatment. In addition, As(2)O(3)-mediated up-regulation of CHOP and DR5, as well as partial proteolytic processing of procaspase-3 by TRAIL, was not induced in astrocytes. Taken together, the present results suggest that the combined treatment of glioma cells with As(2)O(3) plus TRAIL may provide an effective and selective therapeutic strategy.     

肿瘤细胞对TRAIL敏感性与其表面DR5表达水平的相关性研究

目的　探讨肿瘤细胞表面DR5表达水平与其对肿瘤坏死因子相关的凋亡诱导配体(TRAIL)敏感性之间的关系。方法　利用抗DR5特异性单克隆抗体 ,采用流式细胞仪技术直接检测不同肿瘤细胞系表面DR5的表达水平 ,并采用TRAIL凋亡检测试剂盒检测肿瘤细胞对TRAIL诱导凋亡的敏感性 ,研究两者之间的关系。结果　不同肿瘤细胞表面DR5的表达水平分别为 :U937细胞97.9%、Jurkat细胞 95 .1%、SW4 80细胞 93.8%、HCT116细胞 86 .2 %、HL 6 0细胞 6 4 .2 %、HeLa细胞4 6 .6 %、K5 6 2细胞 13.1% ;TRAIL诱导的细胞凋亡率分别为 :U937细胞 72 .6 %、Jurkat细胞 85 .2 %、SW4 80细胞 78.6 %、HCT116细胞 70 .2 %、HL 6 0细胞 6 0 .1%、HeLa细胞 4 5 .4 %、K5 6 2细胞 12 .3%。经统计学分析 ,两者之间呈现非常明显的正相关 (r=0 .997,P <0 .0 0 1)。结论　肿瘤细胞对TRAIL的敏感性与其表面DR5表达水平有关 ,表明DR5的表达水平在TRAIL诱导细胞凋亡方面起着十分重要的作用     

DcR2 (TRAIL-R4) siRNA and adenovirus delivery of TRAIL (Ad5hTRAIL) break down in vitro tumorigenic potential of prostate carcinoma cells

High levels of decoy receptor 2 (DcR2; TRAIL-R4) expression are correlated with TRAIL resistance in prostate cancer cells. In addition, upregulation of TRAIL death receptor (DR4 and DR5) expression, either by ionizing radiation or chemotherapy, can sensitize cancer cells to TRAIL. Considering more than half of human cancers are TRAIL resistant, modulation of surface TRAIL receptor expression appears to be an attractive treatment modality to counteract TRAIL resistance. In this study, three siRNA duplexes targeting DcR2 receptor were tested. Ad5hTRAIL infections were performed to overexpress human full-length TRAIL to induce cell death, and the in vitro tumorigenic potential of prostate cancer cells was assessed using colony-forming assays on soft agar. The DU145 and LNCaP prostate cancer cell lines, which express high levels of DcR2, were resistant to Ad5hTRAIL-induced death. Downregulation of surface DcR2 expression by siRNA sensitized these prostate cancer cell lines to Ad5hTRAIL. In addition, DcR2 siRNA-mediated knockdown of DcR2, followed by Ad5hTRAIL infection, dramatically reduced the in vitro tumorigenic potential of prostate cancer cells. Collectively, our results suggest the potential for combining receptor-specific siRNA with TRAIL in the treatment of certain cancers.