Regulation of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in Burkitt's lymphoma cell lines

Chemical synthetic approaches to mimic the TRAIL: promising cancer therapeutics

Apoptosis is programmed cell death that eliminates undesired cells to maintain homeostasis in metazoan. Aberration of this process may lead to cancer genesis. The tumor necrosis factor related apoptosis inducing ligand (TRAIL) induces apoptosis in cancer cells after ligation with death receptors (DR4/DR5) while sparing most normal cells. Therefore, strategies to induce apoptosis in cancer cells by mimicking the TRAIL emerge as a promising therapeutic tool. Hence, approaches are taken to develop TRAIL/DR-based cancer therapeutics. The recombinant soluble TRAIL (rhTRAIL) and death receptor agonistic antibodies were produced and tested pre-clinically and clinically. Pre-clinical and clinical trial data demonstrate that these therapeutics are safe and relatively well tolerated. But some of these therapeutics failed to exert adequate efficacy in clinical settings. Besides these biotechnologically derived therapeutics, a few chemically synthesized therapeutics are reported. Some of these therapeutics exert considerable efficacy in vitro and in vivo. In this review, we will discuss chemically synthesized TRAIL/DR-based therapeutics, their chemical and biological behaviour, design concepts and strategies that may contribute to further improvement of TRAIL/DR-based therapeutics.

ONC201 induces cell death in pediatric non-Hodgkin's lymphoma cells

ONC201/TIC10 is a small molecule initially discovered by its ability to coordinately induce and activate the TRAIL pathway selectively in tumor cells and has recently entered clinical trials in adult advanced cancers. The anti-tumor activity of ONC201 has previously been demonstrated in several preclinical models of cancer, including refractory solid tumors and a transgenic lymphoma mouse model. Based on the need for new safe and effective therapies in pediatric non-Hodgkin's lymphoma (NHL) and the non-toxic preclinical profile of ONC201, we investigated the in vitro efficacy of ONC201 in non-Hodgkin's lymphoma (NHL) cell lines to evaluate its therapeutic potential for this disease. ONC201 caused a dose-dependent reduction in the cell viability of NHL cell lines that resulted from induction of apoptosis. As expected from prior observations, induction of TRAIL and its receptor DR5 was also observed in these cell lines. Furthermore, dual induction of TRAIL and DR5 appeared to drive the observed apoptosis and TRAIL expression was correlated linearly with sub-G1 DNA content, suggesting its potential role as a biomarker of tumor response to ONC201-treated lymphoma cells. We further investigated combinations of ONC201 with approved chemotherapeutic agents used to treat lymphoma. ONC201 exhibited synergy in combination with the anti-metabolic agent cytarabine in vitro, in addition to cooperating with other therapies. Together these findings indicate that ONC201 is an effective TRAIL pathway-inducer as a monoagent that can be combined with chemotherapy to enhance therapeutic responses in pediatric NHL.

Trailing TRAIL Resistance: Novel Targets for TRAIL Sensitization in Cancer Cells

Resistance to chemotherapeutic drugs is the major hindrance in the successful cancer therapy. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor (TNF) family of ligands, which initiates apoptosis in cancer cells through interaction with the death receptors DR4 and DR5. TRAIL is perceived as an attractive chemotherapeutic agent as it specifically targets cancer cells while sparing the normal cells. However, TRAIL therapy has a major limitation as a large number of the cancer develop resistance toward TRAIL and escape from the destruction by the immune system. Therefore, elucidation of the molecular targets and signaling pathways responsible for TRAIL resistance is imperative for devising effective therapeutic strategies for TRAIL resistant cancers. Although, various molecular targets leading to TRAIL resistance are well-studied, recent studies have implicated that the contribution of some key cellular processes toward TRAIL resistance need to be fully elucidated. These processes primarily include aberrant protein synthesis, protein misfolding, ubiquitin regulated death receptor expression, metabolic pathways, epigenetic deregulation, and metastasis. Novel synthetic/natural compounds that could inhibit these defective cellular processes may restore the TRAIL sensitivity and combination therapies with such compounds may resensitize TRAIL resistant cancer cells toward TRAIL-induced apoptosis. In this review, we have summarized the key cellular processes associated with TRAIL resistance and their status as therapeutic targets for novel TRAIL-sensitizing agents.

A phase I safety and pharmacokinetic study of the death receptor 5 agonistic antibody PRO95780 in patients with advanced malignancies

PURPOSE

PRO95780 is a fully human IgG1 monoclonal antibody that triggers the extrinsic apoptosis pathway through death receptor 5. This first-in-human study assessed the safety, tolerability, pharmacokinetics, and any early evidence of efficacy of PRO95780 in patients with advanced malignancies.

EXPERIMENTAL DESIGN

Target concentrations were predicted to occur at 10 mg/kg. Patients received up to eight cycles of PRO95780 i.v. using a 3+3 dose escalation design at 1 to 20 mg/kg every 14 days (every 28 days in cycle 1; stage 1), with cohort expansion at either the maximum tolerated dose or 10 mg/kg, whichever was lower (stage 2). Patients were evaluated for response every other cycle.

RESULTS

The maximum tolerated dose was not reached within this study. Four (8%) of 50 patients reported adverse events of greater than grade 2 at least possibly related to PRO95780, including 2 patients with reversible grade 3 transaminase elevation. The mean terminal half-life was 8.8 to 19.3 days, with dose-dependent increases in exposure (peak plasma concentration and area under the concentration) across 1 to 15 mg/kg. Most patients treated with 10 mg/kg or above achieved trough concentration above the target efficacious concentration at day 15 with moderate accumulation after multiple doses. No objective responses occurred, although three minor responses were observed in patients with colorectal and granulosa cell ovarian cancers (each treated with 4 mg/kg) and chondrosarcoma (10 mg/kg).

CONCLUSIONS

PRO95780 is safe and well tolerated at doses up to 20 mg/kg. Evidence of activity was noted in several different tumor types at 4 and 10 mg/kg. Pharmacokinetic analysis supports a dosing regimen of 10 to 15 mg/kg every 2 to 3 weeks.

The TRAIL to viral pathogenesis: the good, the bad and the ugly

Since the discovery of Tumor Necrosis Factor-Related Apoptosis Inducing Ligand (TRAIL) in 1995, much has been learned about the protein, its receptors and signaling cascade to induce apoptosis and the regulation of its expression. However, the physiologic role or roles that TRAIL may play in vivo are still being explored. The expression of TRAIL on effector T cells and the ability of TRAIL to induce apoptosis in virally infected cells provided early clues that TRAIL may play an active role in the immune defense against viral infections. However, increasing evidence is emerging that TRAIL may have a dual function in the immune system, both as a means to kill virally infected cells and in the regulation of cytokine production. TRAIL has been implicated in the immune response to viral infections (good), and in the pathogenesis of multiple viral infections (bad). Furthermore, several viruses have evolved mechanisms to manipulate TRAIL signaling to increase viral replication (ugly). It is likely that whether TRAIL ultimately has a proviral or antiviral effect will be dependent on the specific virus and the overall cytokine milieu of the host. Knowledge of the factors that determine whether TRAIL is proviral or antiviral is important because the TRAIL system may become a target for development of novel antiviral therapies.

Ligand-based targeting of apoptosis in cancer: the potential of recombinant human apoptosis ligand 2/Tumor necrosis factor-related apoptosis-inducing ligand (rhApo2L/TRAIL)

Cancer is a leading cause of premature human death and commands considerable research attention. Apoptosis (type 1 programmed cell death) is critical in maintaining tissue homeostasis in metazoan organisms, and its dysregulation underpins the initiation and progression of cancer. Conventional chemotherapy and radiotherapy can induce apoptosis as a secondary consequence of inflicting cell damage. However, more direct and selective strategies to manipulate the apoptotic process in cancer cells are emerging as potential therapeutic tools. Genetic and biochemical understanding of the cellular signaling mechanisms that control apoptosis has increased substantially during the last decade. These advances provide a strong scientific framework for developing several types of targeted proapoptotic anticancer therapies. One promising class of agents is the proapoptotic receptor agonists. Of these, recombinant human apoptosis ligand 2/tumor necrosis factor-related apoptosis-inducing ligand (rhApo2L/TRAIL)-an optimized soluble form of an endogenous apoptosis-inducing ligand-is unique in that it activates two related proapoptotic receptors, DR4 and DR5. Preclinical data indicate that rhApo2L/TRAIL can induce apoptosis in a broad range of human cancer cell lines while sparing most normal cell types. In vitro, and in various in vivo tumor xenograft models, rhApo2L/TRAIL exhibits single-agent antitumor activity and/or cooperation with certain conventional and targeted therapies. Preclinical safety studies in nonhuman primates show rhApo2L/TRAIL to be well tolerated. Moreover, early clinical trial data suggest that rhApo2L/TRAIL is generally safe and provide preliminary evidence for potential antitumor activity. Clinical studies are ongoing to assess the safety and efficacy of this novel agent in combination with established anticancer therapies.

TRAIL-R deficiency in mice promotes susceptibility to chronic inflammation and tumorigenesis

Preclinical data support the potential of the death-signaling receptors for TRAIL as targets for cancer therapy. However, it is unclear whether these death-signaling receptors suppress the emergence and growth of malignant tumors in vivo. Herein we show that TNF-related apoptosis-inducing ligand receptor (TRAIL-R), the only proapoptotic death-signaling receptor for TRAIL in the mouse, suppresses inflammation and tumorigenesis. Loss of a single TRAIL-R allele on the lymphoma-prone Emu-myc genetic background significantly reduced median lymphoma-free survival. TRAIL-R-deficient lymphomas developed with equal frequency irrespective of mono- or biallelic loss of TRAIL-R, had increased metastatic potential, and showed apoptotic defects relative to WT littermates. In addition, TRAIL-R-/- mice showed decreased long-term survival following a sublethal dose of ionizing radiation. Histological evaluation of moribund irradiated TRAIL-R-/- animals showed hallmarks of bronchopneumonia as well as tumor formation with increased NF-kappaB p65 expression. TRAIL-R also suppressed diethylnitrosamine-induced (DEN-induced) hepatocarcinogenesis, as an increased number of large tumors with apoptotic defects developed in the livers of DEN-treated TRAIL-R-/- mice. Thus TRAIL-R may function as an inflammation and tumor suppressor in multiple tissues in vivo.

Cooperation of the proapoptotic receptor agonist rhApo2L/TRAIL with the CD20 antibody rituximab against non-Hodgkin lymphoma xenografts

Recombinant human rhApo2L/TRAIL selectively stimulates apoptosis in various cancer cells through its receptors DR4 and DR5, and is currently in clinical trials. Preclinical studies have established antitumor activity of rhApo2L/TRAIL in models of epithelial cancers; however, efficacy in non-Hodgkin lymphoma (NHL) models is not well studied. Of 7 NHL cell lines tested in vitro, rhApo2L/TRAIL stimulated apoptosis in BJAB, Ramos RA1, and DoHH-2 cells. Rituximab, a CD20 antibody used to treat certain types of NHL, augmented rhApo2L/TRAIL-induced caspase activation in Ramos RA1 and DoHH2 but not BJAB or SC-1 cells, through modulation of intrinsic rather than extrinsic apoptosis signaling. In vivo, rhApo2L/TRAIL and rituximab cooperated to attenuate or reverse growth of tumor xenografts of all 4 of these cell lines. Depletion of natural killer (NK) cells or serum complement substantially reduced combined efficacy against Ramos RA1 tumors, suggesting involvement of antibodydependent cell- and complement-mediated cytotoxicity. Both agents exhibited greater activity against disseminated than subcutaneous BJAB xenografts, and worked together to inhibit or abolish disseminated tumors and increase survival. Moreover, rhApo2L/TRAIL helped circumvent acquired rituximab resistance of a Ramos variant. These findings provide a strong rationale for clinical investigation of rhApo2L/TRAIL in combination with rituximab as a novel strategy for NHL therapy.

Targeting TRAIL agonistic receptors for cancer therapy

Based on preclinical studies demonstrating that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exerts a potent and cancer cell-specific proapoptotic activity, recombinant TRAIL as well as agonistic anti-TRAIL-R1 and anti-TRAIL-R2 antibodies recently entered clinical trials. Additionally, gene therapy approaches using TRAIL-encoding adenovirus (Ad-TRAIL) are currently being developed to overcome the limitations inherent to TRAIL receptor targeting, i.e., pharmacokinetic of soluble TRAIL, pattern of receptor expression, and tumor cell resistance. To optimize gene therapy approaches, CD34+ cells transduced with Ad-TRAIL (CD34-TRAIL+) have been investigated as cellular vehicles for TRAIL delivery. Transduced cells exhibit a potent tumor killing activity on a variety of tumor cell types both in vitro and in vivo and are also cytotoxic against tumor cells resistant to soluble TRAIL. Studies in tumor-bearing nonobese diabetic/severe combined immunodeficient mice suggest that the antitumor effect of CD34-TRAIL+ cells is mediated by both direct tumor cell killing due to apoptosis and indirect tumor cell killing due to vascular-disrupting mechanisms. The clinical translation of cell and gene therapy approaches represent a challenging strategy that might achieve systemic tumor targeting and increased intratumor delivery of the therapeutic agent.

Antitumor activity of human CD34+ cells expressing membrane-bound tumor necrosis factor-related apoptosis-inducing ligand

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis in a variety of transformed cells while sparing normal cells. To enhance the therapeutic index of soluble (s)TRAIL, we used CD34+ cells transduced with a replication-deficient adenovirus encoding the human TRAIL gene (CD34-TRAIL+) for the systemic delivery of membrane-bound (m)TRAIL to lymphoid tumors. CD34-TRAIL+ cells were evaluated for their activity in vitro and in vivo in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice xenografted with sTRAIL-sensitive and -resistant tumors. In vitro, coculturing CD34-TRAIL+ cells with sTRAIL-sensitive or -resistant lymphoma cell lines induced significant levels of caspase-dependent tumor cell death. In vivo, CD34-TRAIL+ cells significantly increased the survival of NOD/SCID mice bearing sTRAIL-sensitive or -resistant lymphoid tumors at an early or advanced stage of disease. No obvious toxicity was observed on administration of CD34-TRAIL+ cells. Histological analysis revealed high-level expression of the agonistic receptor TRAIL-R2 by tumor endothelial cells, and efficient tumor homing of transduced cells. Injection of CD34-TRAIL+ cells resulted in extensive damage of tumor vasculature followed by hemorrhagic necrosis exhibiting a perivascular distribution. These results show that CD34-TRAIL+ cells might be an efficient vehicle for mTRAIL delivery to tumors, where they exert a potent antitumor effect possibly mediated by both direct tumor cell killing and indirect vascular-disrupting mechanisms.

EBV can protect latently infected B cell lymphomas from death receptor-induced apoptosis

The relationship between EBV infection and sensitivity to death receptor (DR)-induced apoptosis is poorly understood. Using EBV- and EBV+ BJAB cells, we provide the first evidence that EBV can protect latently infected B cell lymphomas from apoptosis triggered through Fas or TRAIL receptors. Caspase 8 activation was impaired and cellular FLIP recruitment was enriched in death-inducing signaling complexes formed in EBV-infected BJAB cells relative to parent BJAB cells. Furthermore, latent membrane protein 1 expression alone could reduce caspase activation and confer partial resistance to DR apoptosis in BJAB cells. This protective effect was dependent on C-terminal activating region 2-driven NF-kappaB activation, which in turn up-regulated cellular FLIP expression in latent membrane protein 1+ BJAB cells. Thus, the ability of latent EBV to block DR apoptosis may help to ensure the survival of host cells during B cell differentiation, and contribute to the development of B cell lymphomas, especially in immunocompromised individuals.

EBV B lymphoma cell lines from patients with post-transplant lymphoproliferative disease are resistant to TRAIL-induced apoptosis

Lymphomas associated with post-transplant lymphoproliferative disease (PTLD) represent a significant complication of immunosuppression in transplant recipients. In immunocompetent individuals, EBV-specific cytotoxic T lymphocytes (CTL) prevent the outgrowth of activated B lymphoblasts through apoptosis induction. Soluble versions of TNF-related apoptosis-inducing ligand/Apo2 ligand (TRAIL) can induce apoptosis in numerous tumor cell types. Given the therapeutic potential of TRAIL, we examined the sensitivity of EBV+ spontaneous lymphoblastoid cell lines (SLCL) derived from patients with PTLD to treatment with soluble TRAIL. Despite abundant expression of TRAIL receptors (TRAIL-R), resistance to TRAIL-induced apoptosis was observed in all SLCL examined. This resistance could not be overcome by concomitant treatment with several pharmacological agents. Unlike BJAB positive control cells, for each SLCL tested, cleavage and activation of caspase 8 was inhibited due to failed recruitment of FADD and caspase 8 to TRAIL receptors upon stimulation. Further indicative of a proximal defect, TRAIL receptor aggregation could not be detected on the cell surface of SLCL following ligand engagement. These results suggest that the use of TRAIL for eliminating PTLD-associated tumors may be of limited clinical utility, and illustrate another mechanism by which EBV+ B lymphoma cells can evade tumor surveillance at the level of death receptor signaling.

Lymphomas are sensitive to perforin-dependent cytotoxic pathways despite expression of PI-9 and overexpression of bcl-2

There is considerable interest in immunotherapeutic approaches for lymphoma. The expression of proteinase inhibitor 9 (PI-9), a molecule that inactivates granzyme B, is considered an immune escape mechanism in lymphoma. Further, lymphomas frequently overexpress the antiapoptotic molecule bcl-2, which is able to inhibit perforin-dependent cytotoxic pathways. In this study, the impact of PI-9 and bcl-2 expression on the sensitivity of lymphomas to T- and natural killer (NK) cell-mediated cytotoxicity was analyzed. We found PI-9 expression in 10 of 18 lymphoma cell lines and in 9 of 14 primary lymphomas. Overexpression of bcl-2 was found in 8 of 18 cell lines and in 12 of 14 primary lymphomas. All lymphoma cells were sensitive to cytolysis by specific T cells and cytokine-activated NK cells, and no difference in sensitivity was observed with respect to PI-9 or bcl-2 expression. Cytolysis was mediated predominantly through perforin-dependent pathways despite expression of PI-9 and bcl-2. Interestingly, the majority of lymphoma cells were resistant to cytolysis by resting allogeneic NK cells. This was due to the failure of lymphomas to induce degranulation of resting NK cells. These results show that resistance to perforin-dependent pathways is not a relevant immune escape mechanism in lymphoma and therefore is unlikely to impair clinical outcome of immunotherapeutic approaches.

Expression of TRAIL and TRAIL receptors in normal and malignant tissues

TRAIL, tumor necrosis factor-related apoptosis-inducing ligand, is a member of the TNF family of proteins. Tumour cells were initially found to have increased sensitivity to TRAIL compared with normal cells, raising hopes that TRAIL would prove useful as an anti-tumor agent. The production of reliable monoclonal antibodies against TRAIL and its receptors that can stain fixed specimens will allow a thorough analysis of their expression on normal and malignant tissues. Here we report the generation of monoclonal antibodies against TRAIL and its four membrane-bound receptors (TR1-4), which have been used to stain a range of normal and malignant cells, as routinely fixed specimens. Low levels of TRAIL expression were found to be limited mostly to smooth muscle in lung and spleen as well as glial cells in the cerebellum and follicular cells in the thyroid. Expression of the TRAIL decoy receptors (TR3 and 4) was not as widespread as indicated by Northern blotting, suggesting that they may be less important for the control of TRAIL cytotoxicity than previously thought. TR1 and TR2 expression increases significantly in a number of malignant tissues, but in some common malignancies their expression was low, or patchy, which may limit the therapeutic role of TRAIL. Taken together, we have a panel of monoclonal antibodies that will allow a better assessment of the normal role of TRAIL and allow assessment of biopsy material, possibly allowing the identification of tumors that may be amenable to TRAIL therapy.

Sensitivity of adult T-cell leukaemia lymphoma cells to tumour necrosis factor-related apoptosis-inducing ligand

Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis in many transformed cells, but not in normal cells, and hence TRAIL has recently emerged as a novel anti-cancer agent. Adult T-cell leukaemia lymphoma (ATLL) is a neoplasm of T-lymphocyte origin aetiologically associated with human T-lymphotropic virus type 1 (HTLV-I), and is resistant to standard anti-cancer therapy. We thus characterized the sensitivity of ATLL cells to TRAIL in this study. Although most primary ATLL cells and cell lines expressed TRAIL death receptors on their surface, they showed only restricted sensitivity to TRAIL. Among the 10 ATLL cell lines examined, one was sensitive, but two had insufficient death-receptor expression, two had an unknown resistant mechanism with abrogation of the death signal upstream of caspase-8, and the remaining five showed attenuation of the signal in both extrinsic and intrinsic pathways by X-linked inhibitor of apoptosis and Bcl-2/Bcl-xL respectively. Furthermore, the level of HTLV-I tax expression was significantly correlated to TRAIL resistance. Interestingly, ATLL cells themselves expressed TRAIL on the cell surface. Constitutive production of TRAIL may offer resistance, thus allowing the development of TRAIL-resistant ATLL cells. Consequently, the resistant mechanism in ATLL cells against TRAIL was assigned to multiple factors and was not explained by a definitive single agent.

Direct repression of FLIP expression by c-myc is a major determinant of TRAIL sensitivity

Tumor necrosis factor alpha (TNF-alpha)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF-alpha family of death receptor ligands and holds great therapeutic potential as a tumor cell-specific cytotoxic agent. Using a panel of established tumor cell lines and normal cells, we found a significant difference between the number of TRAIL-sensitive cells expressing high levels of c-myc and TRAIL-resistant cells expressing low levels of c-myc (P < 0.05, n = 19). We also found a direct linear correlation between c-myc levels and TRAIL sensitivity in TRAIL-sensitive cell lines (r = 0.94, n = 6). Overexpression of c-myc or activation of a myc-estrogen receptor (ER) fusion sensitized TRAIL-resistant cells to TRAIL. Conversely, small interfering RNA (siRNA)-mediated knockdown of c-myc significantly reduced both c-myc expression and TRAIL-induced apoptosis. The gene encoding the inhibitor of caspase activation, FLICE inhibitory protein (FLIP), appears to be a direct target of c-myc-mediated transcriptional repression. Overexpression of c-myc or activation of myc-estrogen receptor (ER) decreased FLIP levels both in cell culture and in mouse models of c-myc-induced tumorigenesis, while knocking down c-myc using siRNA increased FLIP expression. Chromatin immunoprecipitation and luciferase reporter analyses showed that c-myc binds and represses the human FLIP promoter. c-myc expression enhanced TRAIL-induced caspase 8 cleavage and FLIP cleavage at the death-inducing signaling complex. Combined siRNA-mediated knockdown of FLIP and c-myc resensitized cells to TRAIL. Therefore, c-myc down-regulation of FLIP expression provides a universal mechanism to explain the ability of c-myc to sensitize cells to death receptor stimuli. In addition, identification of c-myc as a major determinant of TRAIL sensitivity provides a potentially important screening tool for identification of TRAIL-sensitive tumors.

PI-3-kinase/NF-kappaB mediated response of Jurkat T leukemic cells to two different chemotherapeutic drugs, etoposide and TRAIL

Jurkat T leukemic cells respond to Etoposide, antineoplastic agent which targets the DNA unwinding enzyme, Topoisomerase II, and TNF-Related-Apoptosis-Inducing-Ligand (TRAIL), 34 kDa transmembrane protein, which displays minimal or no toxicity on normal cells and tissues, not only disclosing the occurrence of apoptosis but also a kind of resistance. A similar rate of viability upon the exposure to these two drugs up to 24 h has been evidenced, followed by the occurrence of a rescue process against TRAIL, not performed against Etoposide, along with an higher number of dead cells upon Etoposide exposure, in comparison with TRAIL treatment. These preliminary results let us to speculate on the possible involvement of PI-3-kinase in TRAIL resistance disclosed by surviving cells (20%), may be phosphorylating Akt-1 and, in parallel, IkappaB alpha on both serine and tyrosine residues. On the other hand, in Etoposide Jurkat exposed cells Ser 32-36 phosphorylation of IkappaB alpha is not sufficient to overbalance the apoptotic fate of the cells, since Bax increase, IAP decrease, and caspase-3 activation determine the persistence of the apoptotic state along with the occurrence of cell death by necrosis. Thus, the existence of a balance between apoptotic and rescue response in 20% of cells surviving to TRAIL suggests the possibility of pushing it in favor of cell death in order to improve the yield of pharmacological strategies.

Synthetic lethal targeting of MYC by activation of the DR5 death receptor pathway

The genetic concept of synthetic lethality provides a framework for identifying genotype-selective anticancer agents. In this approach, changes in cellular physiology that arise as a consequence of oncogene activation or tumor suppressor gene loss, rather than oncoproteins themselves, are targeted to achieve tumor selectivity. Here we show that agonists of the TRAIL death receptor DR5 potently induce apoptosis in human cells overexpressing the MYC oncogene, both in vitro and as tumor xenografts in vivo. MYC sensitizes cells to DR5 in a p53-independent manner by upregulating DR5 cell surface levels and stimulating autocatalytic processing of procaspase-8. These results identify a novel mechanism by which MYC sensitizes cells to apoptosis and validate DR5 agonists as potential MYC-selective cancer therapeutics.

Upregulation of FLIP(S) by Akt, a possible inhibition mechanism of TRAIL-induced apoptosis in human gastric cancers

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in some, but not all cancer cells. To assess the regulation of TRAIL-resistance in the human gastric cancer cells, we examined TRAIL sensitivity, TRAIL receptor expression, and intracellular signaling events induced by TRAIL. All the gastric cancer cell lines tested were susceptible to TRAIL to some extent, except for SNU-216 cell line, which was completely resistant. TRAIL receptor expression was not related to the TRAIL-sensitivity. Of the cell lines tested, SNU-216 showed the highest level of constitutively active Akt and the short form of FLICE inhibitory protein (FLIP(S)). Treatment with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or with the protein synthesis inhibitor cycloheximide induced a suppression of constitutive Akt activation in SNU-216 cells and a concomitant decrease in the expression of FLIP(S). The reduction of Akt activity by LY294002 affected the transcriptional level of FLIP(S), but not the mRNA stability. As a result, LY294002 or cycloheximide significantly enhanced TRAIL-induced apoptosis. Moreover, the overexpression of constitutively active Akt in the TRAIL-sensitive cell line, SNU-668, rendered the cell line resistant to TRAIL. In addition, infection of the same cell line with retrovirus expressing FLIP(S) completely inhibited TRAIL-induced apoptosis by blocking the activation of caspase-8. Therefore, our results suggest that Akt activity promotes human gastric cancer cell survival against TRAIL-induced apoptosis via upregulation of FLIP(S), and that the cytotoxic effect of TRAIL can be enhanced by modulating the Akt/FLIP(S) pathway in human gastric cancers.

Chemical sensitization and regulation of TRAIL-induced apoptosis in a panel of B-lymphocytic leukaemia cell lines

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) effectively kills tumour cells but not normal cells. We investigated TRAIL sensitivity and the TRAIL-induced apoptosis signalling pathway in a panel of B-lymphocytic leukaemia cell lines. Depending upon TRAIL sensitivity, leukaemia cells could be divided into three groups: highly sensitive, moderately sensitive and resistant. TRAIL receptor-2 (DR5) plays an important role in transducing apoptosis signals. DR5 was internalized into the cytoplasm where it recruited FAS-associated death domain protein (FADD) under TRAIL stimulation in both sensitive and resistant cells. However, the active form of caspase-8 was recruited to FADD and only sensitive cells showed increased caspase-8 activity upon TRAIL stimulation. The caspase-8 specific inhibitor, Z-IETD, impaired caspase-8 activation and completely abrogated TRAIL-induced apoptosis. These results suggest that TRAIL resistance in B-lymphocytic leukaemia cells is due to negative regulation at the level of caspase-8 activation and that caspase-8 activation is an indispensable process in TRAIL-induced apoptosis. However, FADD-like interleukin-1 beta-converting enzyme inhibitory protein (c-FLIPL) was similarly expressed and down-regulated after TRAIL stimulation in both sensitive and resistant cells. Interestingly, in some cell lines, TRAIL sensitivity and caspase-8 activity was enhanced or restored with the treatment of cycloheximide (CHX). In addition, X-linked inhibitor of apoptosis (XIAP) levels decreased significantly and rapidly following treatment with CHX. Down-regulation of XIAP may be responsible for enhancement or restoration of TRAIL sensitivity after CHX treatment in B-lymphocytic leukaemia cells.