Targeting TRAIL towards the clinic

Tumor necrosis factor-related apoptosis-inducing ligand or Apo2 ligand (TRAIL/Apo2L) is a member of the tumor necrosis factor (TNF) superfamily that induces apoptosis upon binding to its death domain-containing transmembrane receptors. The preferential toxicity of TRAIL to cancer cells and the sparing of normal cells make it an ideal cancer therapeutic agent. TRAIL induces apoptosis via the extrinsic death receptor apoptotic pathway and activates the JNK, ERK, Akt and NF-κB signaling cascades. However, not all cancer cells are sensitive to TRAIL therapy. This may limit its efficacy in the clinic, although ways have already been identified to overcome resistance by combining TRAIL with chemotherapeutic and other biological agents. This review focuses on TRAIL receptor-targeting as anticancer therapy, the apoptotic signaling pathways induced by TRAIL receptors, the prognostic implications of TRAIL receptor expression and modulation by combination therapies. The mechanisms of TRAIL resistance and strategies to overcome drug resistance will also be addressed. Finally, the progress of TRAIL and DR4/DR5-specific agonistic antibodies in clinical trials and the development of new receptor-selective TRAIL variants are discussed including future directions for apoptosis inducing therapy.     

Inostamycin enhanced TRAIL-induced apoptosis through DR5 upregulation on the cell surface

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been considered as a possible therapeutic agent for cancer treatment. This is because of its selective cytotoxicity against various cancer cells without a detrimental effect on normal cells. However, recent studies have reported that the potential application of TRAIL in cancer therapy is limited, as many cancer cells have been found to be resistant to TRAIL. Therefore, small molecule compounds that potentiate the cytotoxicity of TRAIL would be strategic candidates for therapeutic applications in combination with TRAIL. Here we found that a combined treatment of inostamycin and TRAIL synergistically induced caspase-dependent apoptosis in HCT116 cells. Inostamycin upregulated DR5, and a knockdown of DR5 suppressed the apoptosis that was synergistically induced by co-treatment with inostamycin and TRAIL. Moreover, inostamycin increased the expression of DR5 on the cell surface. Therefore, inostamycin-increased cell surface expression of DR5 may have contributed to the enhancement of TRAIL-induced apoptosis. Our study suggests that combined treatment with inostamycin and TRAIL may offer a strategy to overcome TRAIL resistance in tumor cells.     

Targeting androgen receptor and trail:a novel treatment paradigm for breast cancer

OBJECTIVE TNF-related apoptosis-inducing ligand(TRAIL)is a promising cancer therapeutic agent due to its minimal toxicity to normal tissues and remarkable apoptotic activity in tumors.However,most breast cancer cells are resistant to TRAIL-induced apoptosis.Our objectives are to investigate the underlying molecular mechanisms and to develop strategies to overcome such resistance.METHODS To identify modulators of TRAIL-induced apoptosis,we carried out a genome wide si RNA screen.To validate the screening result,we either silenced or overexpressed the identified genes in various breast cancer cells and changes in growth and TRAIL-induced cell apoptosis were determined in vitro and in an orthotopic xenograft mouse model.Finally,we investigated whether small molecules targeting the identified genes improve the effectiveness of TRAIL-therapy.RESULTS We unexpectedly identified androgen receptor(AR)to be responsible for TRAIL resistance.While AR is classically viewed as the key factor in prostate cancer progression,we found that AR expression levels were markedly elevated in human invasive breast cancer specimens including triple-negative breast cancers(TNBC)that are highly aggressive with poor prognosis.Importantly,breast cancer cell lines express different levels of AR that correlated with their TRAIL resistance.AR overexpression in MDA-MB-231 and MDA-MB-436 cells suppressed the TRAIL sensitivity whereas knockdown of AR rendered MCF-7 and MDA-MB-453 cells sensitive to TRAIL-induced apoptosis.AR overexpression also induced TRAIL resistance in breast tumors in vivo.Further,we observed an upregulation of the TRAIL receptor,death receptor 5(DR5)in breast cancer cells,following the removal or inhibition of AR by its antagonists Casodex and MDV3100.Treatment with AR antagonists also enhanced TRAIL-induced breast cancer cell apoptosis.CONCLUSION AR signaling suppresses TRAIL-induced breast cancer cell apoptosis,in part,by suppressing DR5 expression,and a combination of AR antagonists together with TRAIL may be a novel and effective therapy for TNBC.     

Down-Regulation of Survivin by Nemadipine-A Sensitizes Cancer Cells to TRAIL-Induced Apoptosis

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor family of cytokines. TRAIL selectively induces apoptotic cell death in various tumors and cancer cells, but it has little or no toxicity in normal cells. Agonism of TRAIL receptors has been considered to be a valuable cancer-therapeutic strategy. However, more than 85% of primary tumors are resistant to TRAIL, emphasizing the importance of investigating how to overcome TRAIL resistance. In this report, we have found that nemadipine-A, a cell-permeable L-type calcium channel inhibitor, sensitizes TRAIL-resistant cancer cells to this ligand. Combination treatments using TRAIL with nemadipine-A synergistically induced both the caspase cascade and apoptotic cell death, which were blocked by a pan caspase inhibitor (zVAD) but not by autophagy or a necrosis inhibitor. We further found that nemadipine-A, either alone or in combination with TRAIL, notably reduced the expression of survivin, an inhibitor of the apoptosis protein (IAP) family of proteins. Depletion of survivin by small RNA interference (siRNA) resulted in increased cell death and caspase activation by TRAIL treatment. These results suggest that nemadipine-A potentiates TRAIL-induced apoptosis by down-regulation of survivin expression in TRAIL resistant cells. Thus, combination of TRAIL with nemadipine-A may serve a new therapeutic scheme for the treatment of TRAIL resistant cancer cells, suggesting that a detailed study of this combination would be useful.     

17-Allylamino-17-demethoxygeldanamycin overcomes TRAIL resistance in colon cancer cell lines

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) is a promising candidate for treatment of cancer, but displays variable cytotoxicity in cell lines. The mechanisms of sensitivity and resistance have not been fully elucidated; both AKT and NF-kappaB pathways may modulate cytotoxic responses. We have shown that the Hsp90 inhibitor 17-AAG enhances the cytotoxicity of oxaliplatin in colon cancer cell lines through inhibition of NF-kappaB. We analyzed the effects of TRAIL and 17-AAG in combination in a series of nine colon cancer cell lines and characterized activation of the pathways to apoptosis. IC(50) values for a 72 h exposure to TRAIL ranged from 30 to 4000 ng/ml. Cytotoxicity assays demonstrated additivity or synergism of the TRAIL/17-AAG combination in all cell lines, with combination indices at IC(50) ranging from 0.53 to 1. The sensitizing effect of 17-AAG was greater in the TRAIL-resistant cell lines. In TRAIL-resistant cell lines, the combination of 17-AAG and TRAIL resulted in activation of both extrinsic and intrinsic apoptotic pathways, though with quantitative differences between HT29 and RKO cells: differential effects of 17-AAG on AKT and NF-kappaB characterized these cell lines. In both cell lines, the combination also led to down-regulation of X-linked inhibitor of apoptosis protein (XIAP) and enhanced activation of caspase-3. We conclude that either AKT or NF-kappaB may promote resistance to TRAIL in colon cancer cells, and that the ability of 17-AAG to target multiple putative determinants of TRAIL sensitivity warrants their further investigation in combination.     

Bortezomib treatment of ovarian cancer cells mediates endoplasmic reticulum stress,cell cycle arrest,and apoptosis

Bortezomib, an approved drug for the treatment of certain haematological neoplasms, is currently being tested in clinical trials as a potential therapeutic agent against several types of solid cancer, including ovarian cancer. We have analyzed the effect of bortezomib on ovarian cancer cells and tissue explants either as a single agent or in combination with carboplatin, taxol, or TRAIL (tumor necrosis factor-related apoptosis-inducing ligand). Bortezomib alone efficiently induced apoptosis in ovarian cancer cells. Apoptosis was preceded by an upregulation of the endoplasmic reticulum stress sensor ATF3, and increased the expression of cytoplasmic heat shock proteins. Bortezomib enhanced the sensitivity of ovarian cancer cells and tissue explants to an apoptosis-inducing TRAIL receptor antibody by upregulating the TRAIL receptor DR5. In contrast to the synergistic effect observed for TRAIL, the efficacy of the taxol treatment was reduced by bortezomib, and bortezomib inhibited the G2/M phase accumulation of ovarian cancer cells treated with taxol. Bortezomib alone or in combination with taxol induced a cell cycle arrest within the S phase, and downregulation of cdk1, a cyclin-dependent kinase that is necessary for the entry into the M phase. Thus, bortezomib can be regarded as a promising agent for the treatment of ovarian cancer and could either be administered as a single agent or in combination with TRAIL. However, a combination treatment with taxanes may not be beneficial and may even be less effective.     

P-glycoprotein enhances TRAIL-triggered apoptosis in multidrug resistant cancer cells by interacting with the death receptor DR5

The death-inducing cytokine TRAIL is a promising agent for anticancer therapy since it preferentially kills cancer versus normal cells; however, some cancer cells are TRAIL-resistant. We initially explored whether overexpression of the MDR1 gene product P-glycoprotein (P-gp), which causes multidrug resistance (MDR) in cancer cells, also contributes to TRAIL-resistance. Surprisingly, our results revealed that P-gp-overexpression enhances TRAIL-induced apoptosis not only in neoplastic cells transfected with the MDR1 gene but also in MDR variants selected with cytotoxic anticancer agents. Mechanistic analysis of TRAIL-induced apoptosis in the MDR1-transfected MCF-7 breast cancer cell line BC-19 revealed that TRAIL-triggered significantly more apoptosis in these cells compared with parental MCF-7 cells by binding to the TRAIL receptor DR5. DR5 but not DR4 engagement by TRAIL attenuated cellular ATP levels by robustly stimulating P-gp ATPase activity, and thus triggered P-gp-dependent apoptosis by depletion of the cellular ATP pool. In addition to hyperactive P-gp-mediated ATP hydrolysis, TRAIL-induced, P-gp-potentiated apoptosis was associated with activation of caspases-6, -7, -8, and -9; Bid cleavage; and mitochondrial depolarization. P-gp interacted with the TRAIL receptors DR4, DR5, and DcR1 in plasma membranes and enhanced TRAIL binding to DR5. Interestingly, the decreased level of the decoy TRAIL receptor, DcR1, in BC-19 cells further sensitized these cells to TRAIL. Therefore, both extrinsic and intrinsic apoptosis pathways are involved in this process. These findings for the first time reveal that TRAIL treatment preferentially causes apoptosis in P-gp-overexpressing MDR cells, and suggests significant clinical implications for the use of TRAIL in treating neoplasms that have failed chemotherapy.     

Galangin and TRAIL cooperate to suppress A549 lung cancer proliferation via apoptosis and p38 MAPK activation

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) is a promising antitumor therapy against lung cancer which is currently undergoing a phase III clinical trial in China. Unfortunately some cancer patients in the clinical trial displayed resistance to TRAIL treatment. In investigating ways to overcome this resistance, we evaluated the inhibitory effect of galangin on TRAIL resistant A549 human lung adenocarcinoma cells. Here we report that, in comparison with the single agents, the combination of galangin and TRAIL markedly suppressed proliferation of A549 cells and induced apoptosis as shown by DAPI and JC-1 staining. The combination of galangin and TRAIL induced PARP cleavage, activation of caspase-8 and p38 MAPK (mitogen-activated protein kinases). These findings indicate that the combination of galangin and TRAIL may constitute a promising strategy for the treatment of lung cancer.     

Inhibition of Bcl-2 improves effect of LCL161, a SMAC mimetic, in hepatocellular carcinoma cells

In this study, we investigated the effect of LCL161, a SMAC mimetic, in hepatocellular carcinoma (HCC). LCL161 showed differential effects on apoptosis in four HCC cell lines, and the endogenous level of Bcl-2 determined the sensitivity of HCC cells to LCL161. Cytotoxicity and apoptosis were observed in sensitive PLC5 and Hep3B cells that express lower levels of Bcl-2, but not in resistant Huh-7 and SK-Hep1 cells with higher Bcl-2 expression. Down regulation of Bcl-2 by small interference RNA overcame the resistance to LCL161 in Huh-7, and the apoptotic effect was rescued in Bcl-2-expressing Hep3B. To test the hypothesis that Bcl-2 determines the sensitivity of HCC cells to LCL161, we assayed the biological effect of SC-2001, a novel Bcl-2 inhibitor derived from obatoclax, in LCL161-resistant cell lines. Huh-7 cells co-treated with LCL161 and SC-2001 showed a significant dose-dependent apoptotic effect demonstrated by sub-G1 assay and cleavage of PARP. Furthermore, the combination index (CI) of LCL161 and SC-2001 showed a convincing synergism in resistant Huh-7. In addition, the combinational therapy showed significant growth inhibition in Huh-7-bearing xenograft tumors. Notably, down regulation of Bcl-2 was observed in a tumor sample treated with LCL161 and SC-2001. In conclusion, targeting Bcl-2 with SC-2001 overcomes drug resistance to LCL161 in HCC cells thus suggesting a new anti-IAP combinational therapy for HCC.     

Butein sensitizes human leukemia cells to apoptosis induced by tumor necrosis factor-related apoptosis inducing ligand (TRAIL)

Resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) therapy is frequently encountered, requiring combined treatments with sensitizing agents. It is, therefore, important to find nontoxic drugs which can be used together with TRAIL. In this study, we investigated natural compounds that can overcome resistance to TRAIL, and found that butein, a polyphenol, exhibits significant synergism with TRAIL. Treatment with TRAIL in combination with subtoxic concentrations of butein sensitizes TRAIL-resistant human leukemia U937 cells to apoptosis. Butein increased caspase-3 activity and expression of death receptor DR5. The apoptotic cell death induced by combined treatment was significantly reduced by z-DEVD-fmk, a caspase-3 inhibitor, suggesting a critical role of caspase-3 in apoptosis. These results indicate that butein sensitizes TRAIL-resistant U937 cells to TRAIL-induced apoptosis in a caspase-3 dependent manner which might be correlated with upregulation of death receptor DR5. Our data suggests that combined treatment with butein and TRAIL may provide a safe and effective strategy for treating cancer.     

Induction and intracellular localization of Nur77 dictate fenretinide-induced apoptosis of human liver cancer cells

Fenretinide, a synthetic retinoid, is known to induce apoptosis in various cancer cells. However, the mechanism by which fenretinide induces apoptosis remains unclear. The current study examines the mechanisms of fenretinide-induced apoptosis in human hepatoma cells. The induction of Nur77 and the cytoplasmic distribution of Nur77 induced by fenretinide were positively correlated with the apoptotic effect of fenretinide in HCC cells. The sensitivity of Huh-7 cells was related to Nur77 translocation and targeting mitochondria, whereas the mechanism of resistance for HepG2 cells seemed due to Nur77 accumulating in the nucleus. The intracellular location of Nur77 was also associated with the differential capability of fenretinide-induced ROS generation in these two cell lines. In addition, the knockdown of Nur77 expression by siRNA greatly reduced fenretinide-induced apoptosis and cleaved caspase 3 in Huh-7 cells. Therefore, our findings demonstrate that fenretinide-induced apoptosis of HCC cells is Nur77 dependent and that the intracellular localization of Nur77 dictates the sensitivity of the HCC cells to fenretinide-induced apoptosis.     

蛇床子素联合肿瘤坏死因子诱导凋亡配体对乳腺癌干细胞的杀伤效应

目的 探讨蛇床子素联合肿瘤坏死因子诱导凋亡配体[tumor necrosis factor （TNF）-related apoptosis inducing ligand,TRAIL]对乳腺癌干细胞的杀伤效应及机制。方法 用TRAIL及蛇床子素体外处理MCF-7乳腺癌非干细胞及MCF-7乳腺癌干细胞,MTT法检测乳腺癌细胞的细胞活力;流式细胞术检测乳腺癌细胞的凋亡;Western blot试验检测caspase-9和caspase-3的活化,凋亡酶激活因子（apoptotic protease activating facter-1,Apaf-1）的表达水平,细胞色素C的释放;免疫共沉淀法检测Apaf-1和caspase-9前体的相互作用。结果 MCF-7肿瘤干细胞对TRAIL的敏感性显著低于MCF-7非干细胞。在MCF-7肿瘤干细胞的培养体系中加入蛇床子素能显著提高TRAIL对MCF-7肿瘤干细胞的细胞活力抑制率。Western blot实验结果表明,蛇床子素能显著上调MCF-7肿瘤干细胞中Apaf-1的表达水平,但不影响细胞色素C的释放。在MCF-7肿瘤干细胞中转染Apaf-1 siRNA后,蛇床子素联合TRAIL对MCF-7肿瘤干细胞的协同杀伤活性受到显著抑制。另外,免疫共沉淀实验结果表明,蛇床子素联合TRAIL能显著诱导MCF-7肿瘤干细胞中Apaf-1与caspase-9前体的相互作用,并使之发生活化。结论 蛇床子素通过上调Apaf-1的表达促进TRAIL对乳腺癌干细胞caspase-9的活化,从而增强TRAIL对乳腺癌干细胞的凋亡诱导效应。     

Enhanced TRAIL-mediated apoptosis in prostate cancer cells by the bioactive compounds neobavaisoflavone and psoralidin isolated from Psoralea corylifolia

Numerous compounds detected in medical plants and dietary components or supplements possess chemopreventive, antitumor and immunomodulatory properties. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an important endogenous anticancer factor that induces apoptosis selectively in cancer cells. However, some tumor cells are resistant to TRAIL-mediated apoptosis. Naturally occurring agents could sensitize TRAIL-resistant cancer cells and augment their apoptotic activity.We examined the cytotoxic and apoptotic effects of neobavaisoflavone and psoralidin in combination with TRAIL on LNCaP prostate cancer cells. The cytotoxicity was evaluated by MTT and LDH assays. The apoptosis was detected using Annexin V-FITC by flow cytometry and fluorescence microscopy. The LNCaP cells were shown to be resistant to TRAIL-induced apoptosis. Our study demonstrated that neobavaisoflavone and psoralidin sensitized TRAIL-resistant cells and markedly augmented TRAIL-mediated apoptosis and cytotoxicity in prostate cancer cells. Cotreatment of LNCaP cells with 100 ng/ml TRAIL and 50 μM neobavaisoflavone or 50 μM psoralidin increased the percentage of the apoptotic cells to 77.5±0.5% or 64.4±0.5%, respectively. The data indicate the potential role of the bioactive compounds isolated from the medicinal plant Psoralea corylifolia (neobavaisoflavone and psoralidin) in prostate cancer chemoprevention through enhancement of TRAIL-mediated apoptosis.     

Concepts in the use of TRAIL/Apo2L: an emerging biotherapy for myeloma and other neoplasias

TNF-related apoptosis inducing ligand/Apo2 ligand (TRAIL/Apo2L) is a member of the TNF superfamily of death ligands that selectively induces apoptosis in tumour cells of diverse origins. In this report, we have reviewed recent studies examining TRAIL/Apo2L-induced apoptosis in multiple myeloma (MM), a B-cell malignancy which, in spite of its initial sensitivity to steroids, cytotoxic and high-dose chemotherapy, remains incurable. Recently, we demonstrated that TRAIL/Apo2L induces apoptosis of steroid- and chemotherapy-sensitive and resistant MM cell lines. Moreover, TRAIL/Apo2L selectively induced apoptosis of patient MM tumour cells while sparing non-malignant bone marrow and peripheral blood mononuclear cells. In addition, TRAIL/Apo2L inhibited the growth of human plasmacytomas xenografted into mice. Importantly, TRAIL/Apo2L-induced apoptosis was unaffected by IL-6, a potent growth and survival factor for MM cells which, as we and others have previously shown, blocks various pro-apoptotic signals including Fas ligand, which like TRAIL/Apo2L is also a member of the TNF family of ligands. In view of the potential clinical application of TRAIL/Apo2L to the treatment of MM, we have attempted to discern intracellular mechanisms of action and resistance for TRAIL/Apo2L in MM, along with strategies to increase sensitivity and overcome resistance of MM cells to TRAIL/Apo2L. These studies demonstrated that doxorubicin, an agent which is commonly used to treat MM patients, upregulated the expression of the DR5 death-signalling TRAIL receptor and synergistically enhanced the pro-apoptotic effect of TRAIL on MM cells. Moreover, NF-κB inhibitors such as SN50 (a cell permeable inhibitor of NF-κB nuclear translocation) as well as the proteasome inhibitor PS-341, which is currently in Phase II clinical trials, also enhanced the pro-apoptotic activity of TRAIL/Apo2L in MM cells. Lastly, TRAIL/Apo2L-induced apoptosis in MM cells was dependent on caspase-8 activation and inhibited by the caspase regulatory proteins FLIP and cIAP2. These studies provide a framework for the use of TRAIL/Apo2L as a single agent or as part of combination therapy for the treatment of MM.     

Trichostatin A sensitizes human ovarian cancer cells to TRAIL-induced apoptosis by down-regulation of c-FLIPL via inhibition of EGFR pathway

TRAIL-resistant cancer cells can be sensitized to TRAIL by combination therapy. In this study, we investigated the effect of trichostatin A (TSA), a histone deacetylase inhibitor, to overcome the TRAIL resistance in human ovarian cancer cells. Co-treatment of human ovarian cancer cells with TSA and TRAIL synergistically inhibited cell proliferation and induced apoptosis. The combined treatment of ovarian cancer SKOV3 cells with TSA and TRAIL significantly activated caspase-8 and truncated Bid, resulting in the cytosolic accumulation of cytochrome c as well as the activation of caspase-9 and -3. Moreover, we found that down-regulation of c-FLIP_L might contribute to TSA-mediated sensitization to TRAIL-induced apoptosis in SKOV3 cells, and this result was supported by showing that down- or up-regulation of c-FLIP_L with transfection of siRNA or plasmid sensitized or made SKOV3 cells resistant to TRAIL-induced apoptosis, respectively. TSA or co-treatment with TSA alone and TRAIL also resulted in down-regulation of EGFR1/2 and dephosphorylation of its downstream targets, AKT and ERK. Treatment of SKOV3 cells with PKI-166 (EGFR1/2 inhibitor), LY294002 (AKT inhibitor), and PD98059 (ERK inhibitor) decreased c-FLIP_L expression and co-treatment with TRAIL further reduced the level of c-FLIP_L, respectively, as did TSA. Collectively, our data suggest that TSA-mediated sensitization of ovarian cancer cells to TRAIL is closely correlated with down-regulation of c-FLIP_L via inhibition of EGFR pathway, involving caspase-dependent mitochondrial apoptosis, and combination of TSA and TRAIL may be an effective strategy for treating TRAIL-resistant human ovarian cancer cells.     

Itch/AIP4-independent proteasomal degradation of cFLIP induced by the histone deacetylase inhibitor SAHA sensitizes breast tumour cells to TRAIL

The histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA, vorinostat) is undergoing clinical trials as an antitumor drug and has received regulatory approval for cancer treatment. Here, we show that pre-treatment of human breast cancer cells with SAHA makes them susceptible to apoptosis induced by TRAIL (tumour necrosis factor-related apoptosis-inducing ligand). The apoptosis of breast tumour cells induced by TRAIL is blocked at the level of apical activation of caspase-8 and SAHA enhances the TRAIL-induced processing of procaspase-8. Consequently, a TRAIL associated pathway of apoptosis operated via mitochondria is activated in cells treated with SAHA. Interestingly, degradation of cellular FLICE-inhibitory proteins (cFLIP_L and cFLIP_S) by an ubiquitin/proteasome-dependent Itch/AIP4-independent mechanism is observed upon exposure to SAHA. Targeting cFLIP_L directly with siRNA oligonucleotides also sensitizes human breast tumour cells to TRAIL-induced apoptosis. Furthermore, cFLIP_L over-expression significantly inhibits the apoptosis elicited through the combined effects of SAHA and TRAIL. Together, these results indicate that SAHA sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating the activation of early events in the apoptotic TRAIL pathway. Therefore, the combination of TRAIL and SAHA may represent a therapeutic tool to combat breast tumours.     

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.