TRAIL-R2: a novel apoptosis-mediating receptor for TRAIL.

TRAIL is a member of the tumor necrosis factor (TNF) family of cytokines and induces apoptosis in a wide variety of cells. Based on homology searching of a private database, a receptor for TRAIL (DR4 or TRAIL-R1) was recently identified. Here we report the identification of a distinct receptor for TRAIL, TRAIL-R2, by ligand-based affinity purification and subsequent molecular cloning. TRAIL-R2 was purified independently as the only receptor for TRAIL detectable on the surface of two different human cell lines that undergo apoptosis upon stimulation with TRAIL. TRAIL-R2 contains two extracellular cysteine-rich repeats, typical for TNF receptor (TNFR) family members, and a cytoplasmic death domain. TRAIL binds to recombinant cell-surface-expressed TRAIL-R2, and TRAIL-induced apoptosis is inhibited by a TRAIL-R2-Fc fusion protein. TRAIL-R2 mRNA is widely expressed and the gene encoding TRAIL-R2 is located on human chromosome 8p22-21. Like TRAIL-R1, TRAIL-R2 engages a caspase-dependent apoptotic pathway but, in contrast to TRAIL-R1, TRAIL-R2 mediates apoptosis via the intracellular adaptor molecule FADD/MORT1. The existence of two distinct receptors for the same ligand suggests an unexpected complexity to TRAIL biology, reminiscent of dual receptors for TNF, the canonical member of this family.     

Tumor-infiltrating CD4+ T lymphocytes express APO2 ligand (APO2L)/TRAIL upon specific stimulation with autologous lung carcinoma cells: role of IFN-alpha on APO2L/TRAIL expression and -mediated cytotoxicity

In the present report, we have investigated TRAIL/APO2 ligand (APO2L) expression, regulation, and function in human lung carcinoma tumor-infiltrating lymphocytes. Using a panel of non-small cell lung carcinoma cell lines, we first showed that most of them expressed TRAIL-R1/DR4, TRAIL-R2/DR5, but not TRAIL-R3/DcR1 and TRAIL-R4/DcR2, and were susceptible to APO2L/TRAIL-induced cell death. Two APO2L/TRAIL-sensitive tumor cell lines (MHC class I(+)/II(+) or I(+)/II(-)) were selected and specific CD4(+) HLA-DR- or CD8(+) HLA-A2-restricted CTL clones were respectively isolated from autologous tumor-infiltrating lymphocytes. Interestingly, although the established T cell clones did not constitutively express detectable levels of APO2L/TRAIL, engagement of their TCR via activation with specific tumor cells selectively induced profound APO2L/TRAIL expression on the CD4(+), but not on the CD8(+), CTL clones. Furthermore, as opposed to the CD8(+) CTL clone which mainly used granule exocytosis pathway, the CD4(+) CTL clone lysed the specific target via both perforin/granzymes and APO2L/TRAIL-mediated mechanisms. The latter cytotoxicity correlated with APO2L/TRAIL expression and was significantly enhanced in the presence of IFN-alpha. More interestingly, in vivo studies performed in SCID/nonobese diabetic mice transplanted with autologous tumor and transferred with the specific CD4(+) CTL clone in combination with IFN-alpha resulted in an important APO2L/TRAIL-mediated tumor growth inhibition, which was prohibited by soluble TRAIL-R2. Our findings suggest that APO2L/TRAIL, specifically induced by autologous tumor and up-regulated by IFN-alpha, may be a key mediator of tumor-specific CD4(+) CTL-mediated cell death and point to a potent role of this T cell subset in tumor growth control.     

Arsenic trioxide selectively induces early and extensive apoptosis via the APO2/caspase-8 pathway engaging the mitochondrial pathway in myeloma cells with mutant p53

Arsenic trioxide (ATO) is effective in the treatment of acute promyelocytic leukemia (APL) and induces apoptosis in APL cells and in a great variety of other cancer cells. We have previously shown that ATO induces apoptosis in myeloma cells in two different modes depending on p53 status in the cells. In cells expressing mutated p53, ATO induced, G2/M arrest and activation caspase 8 and 3 and rapid and extensive apoptosis. Myeloma cells expressing w.t. p53, ATO induced G1 arrest and delayed apoptosis with activation of caspase 9 and 3. APO2/TRAIL receptor expression was induced in both cell types and APO2/TRAIL synergized with ATO in the induction of apoptosis. Here we tested the effect of ATO on mitochondrial membrane potential (MMP) in myeloma cells expressing mutated or w.t. p53. In myeloma cells expressing mutated p53, depolarization of MMP occurred early, concomitant with induction of APO2/TRAIL, activation of BID and release of AIF, preceding apoptosis. However, in cells expressing w.t. p53, APO2/TRAIL is not induced, BID is not cleaved and depolarization of MMP occurs concurrently with cytochrome c release and apoptosis. These results explain the greater sensitivity to ATO of cells with mutated p53 and suggest perhaps a general mechanism for ATO-induced apoptosis.     

Arsenic Trioxide Selectively Induces Early and Extensive Apoptosis via the APO2/Caspase-8 Pathway Engaging the Mitochondrial Pathway in Myeloma Cells with Mutant p53

Arsenic trioxide (ATO) is effective in the treatment of acute promyelocytic leukemia (APL) and induces apoptosis in APL cells and in a great variety of other cancer cells. We have previously shown that ATO induces apoptosis in myeloma cells in two different modes depending on p53 status in the cells. In cells expressing mutated p53, ATO induced, G2/M arrest and activation caspase 8 and 3 and rapid and extensive apoptosis. Myeloma cells expressing w.t. p53, ATO induced G1 arrest and delayed apoptosis with activation of caspase 9 and 3. APO2/TRAIL receptor expression was induced in both cell types and APO2/TRAIL synergized with ATO in the induction of apoptosis. Here we tested the effect of ATO on mitochondrial membrane potential (MMP) in myeloma cells expressing mutated or w.t. p53. In myeloma cells expressing mutated p53, depolarization of MMP occurred early, concomitant with induction of APO2/TRAIL, activation of BID and release of AIF, preceding apoptosis. However, in cells expressing w.t. p53, APO2/TRAIL is not induced, BID is not cleaved and depolarization of MMP occurs concurrently with cytochrome c release and apoptosis. These results explain the greater sensitivity to ATO of cells with mutated p53 and suggest perhaps a general mechanism for ATO-induced apoptosis.     

Low-dose UV-radiation sensitizes keratinocytes to TRAIL-induced apoptosis

The impact of low-dose ultraviolet light (UV-light) on apoptotic susceptibility of keratinocytes (KCs) induced by TRAIL is unclear. Skin expresses a functional form of TRAIL, and while sun exposure influences TRAIL death receptors, a role for decoy receptors has not been evaluated. Unraveling mechanisms involving apoptotic sensitivity of KCs is important because skin is the first target of UV-light, and a site for commonly occurring cancers. Since apoptosis is a homeostatic process eliminating UV-light induced DNA damaged cells, elucidating molecular events regulating apoptosis enhances understanding of cutaneous photocarcinogenesis. Here we demonstrate low-dose UV-light enhances susceptibility of KCs to TRAIL-induced apoptosis. Low-dose UV-light selectively reduces decoy receptors, without influencing death receptor levels. UV-induced enhanced apoptotic susceptibility was reduced by over-expression of decoy receptor TRAIL-R4, but not TRAIL-R3; or treatment with thiol compound pyrrolidine dithiocarbamate (PDTC), which also enhanced TRAIL-R4 levels. Besides influencing decoy receptors, low-dose UV-light plus TRAIL also synergistically promoted cytochrome c and Smac release from mitochondria. Inhibitors directed against caspases 2, 3, 8, and 9 reduced the synergistic apoptotic response following low-dose UV-light plus TRAIL exposure; as did forced over-expression of Bcl-x and dominant negative (DN) constructs of FADD and caspase 9. Thus, relative levels of decoy receptors significantly influence susceptibility of KCs to TRAIL-induced apoptosis with concomitant low-dose UV-light exposure; in addition to the apoptotic pathway mediated by mitochondrial permeabilization.     

Selective inhibition of FLICE-like inhibitory protein expression with small interfering RNA oligonucleotides is sufficient to sensitize tumor cells for TRAIL-induced apoptosis

BACKGROUND: Most tumors express death receptors and their activation represents a potential selective approach in cancer treatment. The most promising candidate for tumor selective death receptor-activation is tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo2L, which activates the death receptors TRAIL-R1 and TRAIL-R2, and induces apoptosis preferentially in tumor cells but not in normal tissues. However, many cancer cells are not or only moderately sensitive towards TRAIL and require cotreatment with irradiation or chemotherapy to yield a therapeutically reasonable apoptotic response. Because chemotherapy can have a broad range of unwanted side effects, more specific means for sensitizing tumor cells for TRAIL are desirable. The expression of the cellular FLICE-like inhibitory protein (cFLIP) is regarded as a major cause of TRAIL resistance. We therefore analyzed the usefulness of targeting FLIP to sensitize tumor cells for TRAIL-induced apoptosis. MATERIALS AND METHODS: To selectively interfere with expression of cFLIP short double-stranded RNA oligonucleotides (small interfering RNAs [siRNAs]) were introduced in the human cell lines SV80 and KB by electroporation. Effects of siRNA on FLIP expression were analyzed by Western blotting and RNase protection assay and correlated with TRAIL sensitivity upon stimulation with recombinant soluble TRAIL and TRAIL-R1- and TRAIL-R2-specific agonistic antibodies. RESULTS: FLIP expression can be inhibited by RNA interference using siRNAs, evident from reduced levels of FLIP-mRNA and FLIP protein. Inhibition of cFLIP expression sensitizes cells for apoptosis induction by TRAIL and other death ligands. In accordance with the presumed function of FLIP as an inhibitor of death receptor-induced caspase-8 activation, down-regulation of FLIP by siRNAs enhanced TRAIL-induced caspase-8 activation. CONCLUSION: Inhibition of FLIP expression was sufficient to sensitize tumor cells for TRAIL-induced apoptosis. The combination of TRAIL and FLIP-targeting siRNA could therefore be a useful strategy to attack cancer cells, which are resistant to TRAIL alone.     

Human normal hepatocytes are susceptible to apoptosis signal mediated by both TRAIL-R1 and TRAIL-R2

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) triggers apoptosis in tumor cells without toxicity to normal cells, but some recombinant versions of TRAIL caused hepatocyte death. We generated fully human monoclonal antibodies (mAbs) that bind specifically to TRAIL receptor 1 (TRAIL-R1) and TRAIL receptor 2 (TRAIL-R2), which mediate apoptosis signal when they ligate with TRAIL, to investigate the contribution of each receptor to induce tumor cell apoptosis and hepatocyte toxicity. All of mAbs to TRAIL-R1 and TRAIL-R2 induced cell death in several cancer cell lines susceptible to TRAIL but not in human umbilical vein endothelial cells in vitro. Both anti-TRAIL-R1 mAbs and anti-TRAIL-R2mAbs also caused cell death in hepatocytes. However, a subset of mAbs to TRAIL-R2, which was characterized by the TRAIL blocking activity, did not show strong hepatocyte toxicity. These results indicate that human normal hepatocytes are susceptible to both TRAIL-R1- and TRAIL-R2-mediated apoptosis signal.Cell Death and Differentiation (2004) 11, 203-207. doi:10.1038/sj.cdd.4401331 Published online 24 October 2003     

Chemotherapy overcomes TRAIL-R4-mediated TRAIL resistance at the DISC level

TNF-related apoptosis-inducing ligand or Apo2L (Apo2L/TRAIL) is a promising anti-cancer drug owing to its ability to trigger apoptosis by binding to TRAIL-R1 or TRAIL-R2, two membrane-bound receptors that are often expressed by tumor cells. TRAIL can also bind non-functional receptors such as TRAIL-R4, but controversies still exist regarding their potential to inhibit TRAIL-induced apoptosis. We show here that TRAIL-R4, expressed either endogenously or ectopically, inhibits TRAIL-induced apoptosis. Interestingly, the combination of chemotherapeutic drugs with TRAIL restores tumor cell sensitivity to apoptosis in TRAIL-R4-expressing cells. This sensitization, which mainly occurs at the death-inducing signaling complex (DISC) level, through enhanced caspase-8 recruitment and activation, is compromised by c-FLIP expression and is independent of the mitochondria. Importantly, TRAIL-R4 expression prevents TRAIL-induced tumor regression in nude mice, but tumor regression induced by TRAIL can be restored with chemotherapy. Our results clearly support a negative regulatory function for TRAIL-R4 in controlling TRAIL signaling, and unveil the ability of TRAIL-R4 to cooperate with c-FLIP to inhibit TRAIL-induced cell death.     

PTHrP Overexpression Increases Sensitivity of Breast Cancer Cells to Apo2L/TRAIL

Parathyroid hormone-related protein (PTHrP) is a key component in breast development and breast tumour biology. PTHrP has been discovered as a causative agent of hypercalcaemia of malignancy and is also one of the main factors implicated in breast cancer mediated osteolysis. Clinical studies have determined that PTHrP expression by primary breast cancers was an independent predictor of improved prognosis. Furthermore, PTHrP has been demonstrated to cause tumour cell death both in vitro and in vivo. Apo2L/TRAIL is a promising new anti-cancer agent, due to its ability to selectively induce apoptosis in cancer cells whilst sparing most normal cells. However, some cancer cells are resistant to Apo2L/TRAIL-induced apoptosis thus limiting its therapeutic efficacy. The effects of PTHrP on cell death signalling pathways initiated by Apo2L/TRAIL were investigated in breast cancer cells. Expression of PTHrP in Apo2L/TRAIL resistant cell line MCF-7 sensitised these cells to Apo2L/TRAIL-induced apoptosis. The actions of PTHrP resulted from intracellular effects, since exogenous treatment of PTHrP had no effect on Apo2L/TRAIL-induced apoptosis. Apo2L/TRAIL-induced apoptosis in PTHrP expressing cells occurred through the activation of caspase-10 resulting in caspase-9 activation and induction of apoptosis through the effector caspases, caspase-6 and -7. PTHrP increased cell surface expression of Apo2L/TRAIL death receptors, TRAIL-R1 and TRAIL-R2. Antagonistic antibodies against the death receptors demonstrated that Apo2L/TRAIL mediated its apoptotic signals through activation of the TRAIL-R2 in PTHrP expressing breast cancer cells. These studies reveal a novel role for PTHrP with Apo2L/TRAIL that maybe important for future diagnosis and treatment of breast cancer.     

Sensitivity to TRAIL/APO-2L-mediated apoptosis in human renal cell carcinomas and its enhancement by topotecan

TRAIL (APO-2L) is a newly identified member of the TNF family and induces apoptosis in cancer cells without affecting most non-neoplastic cells, both in vitro and in vivo. Our study focused on the expression and function of TRAIL and its receptors in renal cell carcinoma (RCC) cell lines of all major histological types. Here, we demonstrate that all RCC cell lines express TRAIL as well as the death-inducing receptors TRAIL-R1 (DR4) and TRAIL-R2 (Killer/DR5). Exposure to TRAIL induced apoptosis in 10 of 16 RCC cell lines. Remarkably, five of six TRAIL-resistant RCC cell lines exhibited high levels of TRAIL expression. Topotecan, a novel topoisomerase I inhibitor, induced upregulation of TRAIL-R2 as well as downregulation of TRAIL. Neutralization of TRAIL with recombinant soluble TRAIL-R1-Fc and TRAIL-R2-Fc failed to inhibit topotecan-induced apoptosis indicating that topotecan-induced cell death can occur in a TRAIL-independent fashion. However, exposure to topotecan resulted in an enhancement of TRAIL-induced apoptosis in all primarily TRAIL-resistant RCC cell lines. This synergistic effect of cotreatment with Topotecan and TRAIL may provide the basis for a new therapeutic approach to induce apoptosis in otherwise unresponsive RCC.     

Delivery of sTRAIL variants by MSCs in combination with cytotoxic drug treatment leads to p53-independent enhanced antitumor effects

Mesenchymal stem cells (MSCs) are able to infiltrate tumor tissues and thereby effectively deliver gene therapeutic payloads. Here, we engineered murine MSCs (mMSCs) to express a secreted form of the TNF-related apoptosis-inducing ligand (TRAIL), which is a potent inducer of apoptosis in tumor cells, and tested these MSCs, termed MSC.sTRAIL, in combination with conventional chemotherapeutic drug treatment in colon cancer models. When we pretreated human colorectal cancer HCT116 cells with low doses of 5-fluorouracil (5-FU) and added MSC.sTRAIL, we found significantly increased apoptosis as compared with single-agent treatment. Moreover, HCT116 xenografts, which were cotreated with 5-FU and systemically delivered MSC.sTRAIL, went into remission. Noteworthy, this effect was protein 53 (p53) independent and was mediated by TRAIL-receptor 2 (TRAIL-R2) upregulation, demonstrating the applicability of this approach in p53-defective tumors. Consequently, when we generated MSCs that secreted TRAIL-R2-specific variants of soluble TRAIL (sTRAIL), we found that such engineered MSCs, labeled MSC.sTRAIL^DR5, had enhanced antitumor activity in combination with 5-FU when compared with MSC.sTRAIL. In contrast, TRAIL-resistant pancreatic carcinoma PancTu1 cells responded better to MSC.sTRAIL^DR4 when the antiapoptotic protein XIAP (X-linked inhibitor of apoptosis protein) was silenced concomitantly. Taken together, our results demonstrate that TRAIL-receptor selective variants can potentially enhance the therapeutic efficacy of MSC-delivered TRAIL as part of individualized and tumor-specific combination treatments.     

Playing the DISC: Turning on TRAIL death receptor-mediated apoptosis in cancer

Formation of the pro-apoptotic death-inducing signaling complex (DISC) can be initiated in cancer cells via binding of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to its two pro-apoptotic receptors, TRAIL receptor 1 (TRAIL-R1) and TRAIL-R2. Primary components of the DISC are trimerized TRAIL-R1/-R2, FADD, caspase 8 and caspase 10. The anti-apoptotic protein FLIP can also be recruited to the DISC to replace caspase 8 and form an inactive complex. Caspase 8/10 processing at the DISC triggers the caspase cascade, which eventually leads to apoptotic cell death. Besides TRAIL, TRAIL-R1- or TRAIL-R2-selective variants of TRAIL and agonistic antibodies have been designed. These ligands are of interest as anti-cancer agents since they selectively kill tumor cells. To increase tumor sensitivity to TRAIL death receptor-mediated apoptosis and to overcome drug resistance, TRAIL receptor ligands have already been combined with various therapies in preclinical models. In this review, we discuss factors influencing the initial steps of the TRAIL apoptosis signaling pathway, focusing on mechanisms modulating DISC assembly and caspase activation at the DISC. These insights will direct rational design of drug combinations with TRAIL receptor ligands to maximize DISC signaling.     

Involvement of TRAIL/TRAIL-receptors in human intestinal cell differentiation

Despite the fact that tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) and its receptors (TRAIL-Rs) are expressed in intestinal mucosa, little is known about the biological role of this system in intestinal cell physiology. The expression of surface TRAIL and TRAIL-R1, -R2, -R3, -R4 were examined by flow cytometry in the immortalized human cell line tsFHI under culture conditions promoting growth or growth arrest and expression of differentiated traits. A progressive increase of surface TRAIL expression paralleled tsFHI differentiation, consistently with immunohistochemistry analysis showing an increase of TRAIL immunostaining along the crypt-villus axis in normal jejuneal mucosa. In spite of the presence of TRAIL-R1 and TRAIL-R2 "death receptors," recombinant TRAIL was not cytotoxic for tsFHI cells. Exposure of tsFHI to recombinant TRAIL rather increased/anticipated the expression levels of the cyclin-dependent kinase inhibitors p21 and p27, which mediate the induction of growth arrest and the stabilization of differentiated traits, respectively, as well as of the canonical differentiation marker DPPIV. The differentiation inducing activity of TRAIL was abolished by pre-incubation with a Fc-TRAIL-R2 chimera. On the other hand, TRAIL did not significantly modulate the levels of osteoprotegerin (OPG), CXCL8/IL-8, CXCL9/MIG, and CXCL10/IP10 spontaneously released or induced by inflammatory cytokines. Taken together, these data suggest that TRAIL might act as a paracrine trophic cytokine on intestinal epithelium, promoting intestinal cell differentiation.     

Bortezomib sensitises TRAIL-resistant HPV-positive head and neck cancer cells to TRAIL through a caspase-dependent,E6-independent mechanism

Human papillomavirus (HPV) is causative for a new and increasing form of head and neck squamous cell carcinomas (HNSCCs). Although localised HPV-positive cancers have a favourable response to radio-chemotherapy (RT/CT), the impact of HPV in advanced or metastatic HNSCC remains to be defined and targeted therapeutics need to be tested for cancers resistant to RT/CT. To this end, we investigated the sensitivity of HPV-positive and -negative HNSCC cell lines to TRAIL (tumour necrosis factor-related apoptosis-inducing ligand), which induces tumour cell-specific apoptosis in various cancer types. A clear correlation was observed between HPV positivity and resistance to TRAIL compared with HPV-negative head and neck cancer cell lines. All TRAIL-resistant HPV-positive cell lines tested were sensitised to TRAIL-induced cell death by treatment with bortezomib, a clinically approved proteasome inhibitor. Bortezomib-mediated sensitisation to TRAIL was associated with enhanced activation of caspase-8, -9 and -3, elevated membrane expression levels of TRAIL-R2, cytochrome c release and G2/M arrest. Knockdown of caspase-8 significantly blocked cell death induced by the combination therapy, whereas the BH3-only protein Bid was not required for induction of apoptosis. XIAP depletion increased the sensitivity of both HPV-positive and -negative cells to TRAIL alone or in combination with bortezomib. In contrast, restoration of p53 following E6 knockdown in HPV-positive cells had no effect on their sensitivity to either single or combination therapy, suggesting a p53-independent pathway for the observed response. In summary, bortezomib-mediated proteasome inhibition sensitises previously resistant HPV-positive HNSCC cells to TRAIL-induced cell death through a mechanism involving both the extrinsic and intrinsic pathways of apoptosis. The cooperative effect of these two targeted anticancer agents therefore represents a promising treatment strategy for RT/CT-resistant HPV-associated head and neck cancers.Head and neck squamous cell carcinoma (HNSCC) represents the sixth most common cancer worldwide.¹ While the overall incidence of HNSCC, traditionally associated with tobacco or alcohol consumption, is declining, a subset of oropharyngeal cancers caused by infection with high-risk types of human papillomavirus (HPV) has risen significantly.^2,3 Transformation upon HPV infection occurs mainly because of inactivation of the p53 and retinoblastoma tumour suppressor proteins mediated by the viral oncoproteins E6 and E7, respectively.⁴HPV-positive (HPV⁺) cancers represent a distinct subset of HNSCC in terms of biology and clinical behaviour. In general, they are characterised by better overall survival and an improved response to conventional radio-chemotherapy (RT/CT) compared with HPV-negative (HPV⁻) cancers.^5,6 To further minimise treatment-related toxicity without compromising outcome, there have been suggestions of treatment de-escalation in conjunction with targeted therapies.⁷The novel anticancer agent TRAIL (tumour necrosis factor-related apoptosis-inducing ligand) selectively kills several types of malignant cell lines with little effect on normal cells.⁸ Recombinant TRAIL or monoclonal antibodies targeting TRAIL receptors (TRAIL-Rs) are currently being tested in phase I/II clinical trials for patients with advanced tumours.^9,10 TRAIL induces cell death by binding to TRAIL-R1 or TRAIL-R2, resulting in receptor oligomerisation and formation of the death-inducing signalling complex (DISC)¹¹ and activation of initiator caspase-8.¹² Caspase-8 directly activates effector caspase-3 to induce apoptosis through the type I pathway or cleaves the BH3-only protein Bid, generating tBid. This type II pathway involves an amplification loop through the intrinsic pathway of apoptosis characterised by cytochrome c release from the mitochondria, activation of initiator caspase-9 and ultimately caspase-3.¹³Despite its tumour-selective activity, various cancer cell lines remain resistant to TRAIL, limiting the clinical potential of TRAIL-based monotherapies. Many recent studies focus on combination strategies with other agents to sensitise resistant cells to TRAIL.¹⁴ The proteasome inhibitor bortezomib is an FDA-approved drug for the treatment of multiple myeloma, but has shown only little single-agent activity in solid malignancies such as HNSCC while being effective in combination with other treatment options.^{15, 16, 17} Combining bortezomib with TRAIL-R agonists produced a synergistic cytotoxic effect in various types of cancers. Potential mechanisms underlying sensitisation to TRAIL-induced apoptosis include inhibition of NF-κB signalling, stabilisation of BH3-only proteins, p53 or p21, upregulation of TRAIL-Rs and enhanced stability of caspase-8.^{18, 19, 20, 21, 22, 23, 24, 25, 26}So far, little data is available on the therapeutic potential of TRAIL alone or in combination with bortezomib in HNSCC or other HPV⁺ related cancers. Treatment with the proteasome inhibitor MG132 sensitised TRAIL-resistant HPV⁺ cervical cancer cells to TRAIL through p53-dependent upregulation of TRAIL-Rs and inactivation of XIAP.²⁷ Overexpression of E6 was shown to protect colon cancer cells from death receptor-induced apoptosis by affecting the stability of the DISC, indicating a functional link between the presence of E6 and TRAIL signalling.²⁸In this study, we tested the response of HPV⁺ and HPV⁻ HNSCC cells to treatment with TRAIL alone or combined with bortezomib, revealing a clear pattern of sensitivity to TRAIL depending on HPV status and a synergistic effect when combined with bortezomib. In addition, we identified some of the proteins and pathways involved in the response to TRAIL/bortezomib in HNSCCs.     

Distinct function of monoclonal antibody to TRAIL-R2 as potentiator or inhibitor of the ligand TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) specifically induces apoptosis in tumor cells but may be toxic to human hepatocytes. Although hepatocytes are susceptible to apoptotic signals mediated by TRAIL-receptor 2 (TRAIL-R2), we previously reported that some anti-TRAIL-R2 monoclonal antibodies (mAbs) produce little hepatocyte toxicity. Those mAbs neutralized the cytotoxic activity of TRAIL by inhibiting receptor-ligand binding. The hepatocyte-toxic mAbs did not compete with TRAIL for binding to TRAIL-R2, and potentiated ligand activity in both cancer cells and hepatocytes. A neutralizing antibody to TRAIL inhibited hepatocyte death by anti-TRAIL-R2 mAbs, suggesting that the toxicity may reflect their ability to potentiate membrane-bound TRAIL on hepatocytes.     

Post-translational modification of TRAIL receptor type 1 on various tumor cells and the susceptibility of tumors to TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its receptors (TRAIL-R1 and TRAIL-R2) are promising targets for tumor therapy. However, their clinical use is limited because some tumors show resistance to TRAIL-treatment. Here, we analyzed epitopes of nine TRAIL-R1-specific human monoclonal antibodies and demonstrated at least five tentative epitopes on human TRAIL-R1. We found that some of the five were post-translationally modified on some tumor cell lines. Interestingly, one of them, an epitope of TR1-272 antibody (TR1-272-epitope) disappeared on the tumor cells that are more susceptible to TRAIL-induced apoptosis compared to TR1-272-epitope positive cells. Treatment of TR1-272-epitope negative cells with TRAIL induced large cluster formation of TRAIL-R1, while treatment of TR1-272-epiope positive cells with TRAIL did not. These results suggest that TR1-272-antibody might distinguish the TRAIL-R1 conformation that could deliver stronger death signals. Further analysis of epitope-appearance and sensitivity to TRAIL should clarify the mechanisms of TRAIL-induced apoptosis of tumor cells and would provide useful information about tumor therapy using TRAIL and TRAIL-R signaling.     

Apoptosis-induced by TRAIL AND TNF-alpha in human multiple myeloma cells is not blocked by BCL-2

TRAIL, the ligand for the newly discovered DR-4 and DR-5 receptor is a member of the tumour necrosis factor (TNF) family of death signal tranduction proteins with a mechanism of cell death, similar to the Fas and Fas ligand (Fas-L) system. Here, we provide first time evidence that TRAIL and TNF-alpha are potent inducers of apoptosis in multiple myeloma (MM) cell lines and freshly isolated myeloma cells. TRAIL effectively induced extensive apoptosis in 8226 and ARP-1 MM cells in a time- and dose-dependent manner reaching 80% within 48 h of treatment with a dose of 160 ng/ml. Bcl-2 transfected 8226 and ARP-1 cells were equally sensitive to apoptosis by TRAIL. Apoptosis with TNFalpha, reached >60% within 48 h of treatment with a dose of 160 ng/ml. In addition to MM cell lines, freshly isolated, flow-sorted myeloma cells from 8 different MM patients expressing variable levels of bcl-2 were equally sensitive to both TRAIL and TNF-alpha. We have previously shown that anti-Fas-induced apoptosis is not blocked by endogenous or ectopic bcl-2 in MM cell lines. Here we extend our observation with Fas to include TNF-alpha and TRAIL to the apoptotic signals that are not be blocked by bcl-2, in MM cells.