TRAIL-receptor antibodies as a potential cancer treatment

TRAIL-Sensitizing Effects of Flavonoids in Cancer

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) represents a promising anticancer agent, as it selectively induces apoptosis in transformed cells without altering the cellular machinery of healthy cells. Unfortunately, the presence of TRAIL resistance mechanisms in a variety of cancer types represents a major hurdle, thus limiting the use of TRAIL as a single agent. Accumulating studies have shown that TRAIL-mediated apoptosis can be facilitated in resistant tumors by combined treatment with antitumor agents, ranging from synthetic molecules to natural products. Among the latter, flavonoids, the most prevalent polyphenols in plants, have shown remarkable competence in improving TRAIL-driven apoptosis in resistant cell lines as well as tumor-bearing mice with minimal side effects. Here, we summarize the molecular mechanisms, such as the upregulation of death receptor (DR)4 and DR5 and downregulation of key anti-apoptotic proteins [e.g., cellular FLICE-inhibitory protein (c-FLIP), X-linked inhibitor of apoptosis protein (XIAP), survivin], underlying the TRAIL-sensitizing properties of different classes of flavonoids (e.g., flavones, flavonols, isoflavones, chalcones, prenylflavonoids). Finally, we discuss limitations, mainly related to bioavailability issues, and future perspectives regarding the clinical use of flavonoids as adjuvant agents in TRAIL-based therapies.

The combination of TRAIL and MG-132 induces apoptosis in both TRAIL-sensitive and TRAIL-resistant human follicular lymphoma cells

We have previously shown that the human follicular lymphoma cell line, HF28GFP, is sensitive to TRAIL-mediated apoptosis. Nevertheless, when the same cells overexpress anti-apoptotic Bcl-2 family protein, Bcl-xL (HF28Bcl-xL), they become resistant to TRAIL. Thus, these cell lines help us to investigate the action of novel apoptosis inducing candidate drugs. In the present study, we examined the effects of MG-132 (a proteasome inhibitor), LiCl (a glycogen synthase kinase-3 inhibitor) and/or TRAIL on pro-apoptotic Bcl-2 family proteins such as Bim and Bid. Here we demonstrate that the combination of MG-132 and TRAIL induced significant apoptotic cell death in both cell lines, HF28GFP and HF28BclxL. Apoptosis correlated with a decrease of phospho-ERK1/2, the accumulation of Bim and translocation of truncated Bid (tBid) and jBid. In addition, the combination of MG-132 and TRAIL seemed to target other apoptotic factors, which led to the accumulation of active capsase-3. Furthermore, co-stimulation of LiCl and TRAIL induced apoptosis in HF28GFP cells. However, HF28Bcl-xL cells were far less sensitive to the combinatorial effects of LiCl and TRAIL. Interestingly, we observed that LiCl did not target Bim and Bid proteins. In conclusion, these data show that targeting of pro-apoptotic Bcl-2 family proteins simultaneously through a selective proteasome inhibition might help to overcome TRAIL resistance caused by overexpression of anti-apoptotic Bcl-2 family proteins. Moreover, the data may provide new strategies to develop targeted therapies against lymphomas.

Expression of Death Receptor 4 Is Positively Regulated by MEK/ERK/AP-1 Signaling and Suppressed upon MEK Inhibition

Death receptor 4 (DR4) is a cell surface receptor for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and triggers apoptosis upon ligation with TRAIL or aggregation. MEK/ERK signaling is a well known and the best-studied effector pathway downstream of Ras and Raf. This study focuses on determining the impact of pharmacological MEK inhibition on DR4 expression and elucidating the underlying mechanism. We found that several MEK inhibitors including MEK162, AZD6244, and PD0325901 effectively decreased DR4 protein levels including cell surface DR4 in different cancer cell lines. Accordingly, pre-treatment of TRAIL-sensitive cancer cell lines with a MEK inhibitor desensitized them to TRAIL-induced apoptosis. These results indicate that MEK inhibition negatively regulates DR4 expression and cell response to TRAIL-induced apoptosis. MEK inhibitors did not alter DR4 protein stability, rather decreased its mRNA levels, suggesting a transcriptional regulation. In contrast, enforced activation of MEK/ERK signaling by expressing ectopic B-Raf (V600E) or constitutively activated MEK1 (MEK1-CA) or MEK2 (MEK2-CA) activated ERK and increased DR4 expression; these effects were inhibited when a MEK inhibitor was present. Promoter analysis through deletion and mutation identified the AP-1 binding site as an essential response element for enhancing DR4 transactivation by MEK1-CA. Furthermore, inhibition of AP-1 by c-Jun knockdown abrogated the ability of MEK1-CA to increase DR4 promoter activity and DR4 expression. These results suggest an essential role of AP-1 in mediating MEK/ERK activation-induced DR4 expression. Our findings together highlight a previously undiscovered mechanism that positively regulates DR4 expression through activation of the MEK/ERK/AP-1 signaling pathway.

Multi-specific antibodies for cancer immunotherapy

Targeted treatment of cancer with monoclonal antibodies has added to the beneficial outcome of patients. In an attempt to improve anti-tumor activity of monoclonal antibodies, multi-specific antibodies have entered the research arena. To date, only a few multi-specific constructs have entered phase III clinical trials, in contrast to classical monoclonal antibodies, which are the standard first-line therapy in several tumor entities. In this review, we will assess selected multi-specific antibodies in pre-clinical and clinical development that may be new treatment options for cancer patients in the very near future. We will further evaluate therapy modalities including the timely distribution or the combination of various therapeutic approaches and assess the potential role of multi-specific antibodies in cancer treatment.

Approaches to improve tumor accumulation and interactions between monoclonal antibodies and immune cells

Monoclonal antibodies (mAb) have become a mainstay in tumor therapy. Clinical responses to mAb therapy, however, are far from optimal, with many patients presenting native or acquired resistance or suboptimal responses to a mAb therapy. MAbs exert antitumor activity through different mechanisms of action and we propose here a classification of these mechanisms. In many cases mAbs need to interact with immune cells to exert antitumor activity. We summarize evidence showing that interactions between mAbs and immune cells may be inadequate for optimal antitumor activity. This may be due to insufficient tumor accumulation of mAbs or immune cells, or to low-affinity interactions between these components. The possibilities to improve tumor accumulation of mAbs and immune cells, and to improve the affinity of the interactions between these components are reviewed. We also discuss future directions of research that might further improve the therapeutic efficacy of antitumor mAbs.

Synergistic TRAIL sensitizers from Barleria alluaudii and Diospyros maritima

Barleria alluaudii and Diospyros maritima were both investigated as part of an ongoing search for synergistic TRAIL (tumor necrosis factor-α-related apoptosis-inducing ligand) sensitizers. As a result of this study, two naphthoquinone epoxides, 2,3-epoxy-2,3-dihydrolapachol (1) and 2,3-epoxy-2,3-dihydro-8-hydroxylapachol (2), both not previously isolated from natural sources, and the known 2-methylanthraquinone (3) were identified from B. alluaudii. Time-dependent density functional theory (TD-DFT) calculations of electronic circular dichroism (ECD) spectra were utilized to establish the absolute configuration of 1 and 2. Additionally, five known naphthoquinone derivatives, maritinone (4), elliptinone (5), plumbagin (6), (+)-cis-isoshinanolone (7), and ethylidene-6,6'-biplumbagin (8), were isolated from D. maritima. Compounds 1, 2, and 4-6 showed varying levels of synergy with TRAIL. Maritinone (4) and elliptinone (5) showed the highest synergistic effect, with more than a 3-fold increase in activity observed with TRAIL than with compound alone.

Inhibition of TACE activity enhances the susceptibility of myeloma cells to TRAIL

BACKGROUND

TNF-related apoptosis-inducing ligand/Apo2 ligand (TRAIL/Apo2L) selectively induces apoptosis in various cancer cells including myeloma (MM) cells. However, the susceptibility of MM cells to TRAIL is largely low in most of MM cells by yet largely unknown mechanisms. Because TNF-α converting enzyme (TACE) can cleave some TNF receptor family members, in the present study we explored the roles of proteolytic modulation by TACE in TRAIL receptor expression and TRAIL-mediated cytotoxicity in MM cells.

METHODOLOGY/PRINCIPAL FINDINGS

MM cells preferentially expressed death receptor 4 (DR4) but not DR5 on their surface along with TACE. Conditioned media from RPMI8226 and U266 cells contained a soluble form of DR4. The DR4 levels in these conditioned media were reduced by TACE inhibition by the TACE inhibitor TAPI-0 as well as TACE siRNA. Conversely, the TACE inhibition restored surface levels of DR4 but not DR5 in these cells without affecting DR4 mRNA levels. The TACE inhibition was able to restore cell surface DR4 expression in MM cells even in the presence of bone marrow stromal cells or osteoclasts, and enhanced the cytotoxic effects of recombinant TRAIL and an agonistic antibody against DR4 on MM cells.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that MM cells post-translationally down-modulate the cell surface expression of DR4 through ectodomain shedding by endogenous TACE, and that TACE inhibition is able to restore cell surface DR4 levels and the susceptibility of MM cells to TRAIL or an agonistic antibody against DR4. Thus, TACE may protect MM cells from TRAIL-mediated death through down-modulation of cell-surface DR4. It can be envisaged that TACE inhibition augments clinical efficacy of TRAIL-based immunotherapy against MM, which eventually becomes resistant to the present therapeutic modalities.

Clerodane diterpenes from Casearia arguta that act as synergistic TRAIL sensitizers

Casearia arguta was investigated as part of the ongoing search for synergistic TRAIL (tumor necrosis factor-α-related apoptosis-inducing ligand) sensitizers. As a result of this study, argutins A-H, eight new highly oxygenated clerodane diterpenes, were isolated from the plant Casearia arguta collected in Guatemala. The modified Mosher ester method was utilized to establish the absolute configuration of argutins A and F. Each of the argutins showed varying levels of synergy with TRAIL. Argutin B showed the highest TRAIL sensitization; the synergistic effect of argutin B and TRAIL together was 3-fold greater than argutin B alone.

Lipid rafts and clusters of apoptotic signaling molecule-enriched rafts in cancer therapy

There is an imperative need to overcome the rather poor outcomes of current cancer chemotherapy with further development of novel targets and drugs. Direct activation of the cancer cell apoptotic machinery constitutes an appealing approach. Recent findings have shown that apoptosis can be triggered by co-aggregation of lipid rafts with death receptors and downstream signaling molecules, thus facilitating their interactions to convey apoptotic signals. We postulate this cluster of apoptotic signaling molecule-enriched rafts (CASMER) as a novel supramolecular entity that modulates apoptosis, representing a new target in cancer therapy. The development of drugs that target lipid rafts leading to the formation of clusters of apoptotic signaling molecule-enriched rafts offers new opportunities for therapeutic intervention in cancer therapy.

A Novel High-Throughput Technique for Identifying Monoclonal Antibodies Capable of Death Receptor Induced Apoptosis

The study of death receptor family induced apoptosis has gained momentum in recent years with the knowledge that therapeutic antibodies targeting DR4 and DR5 (death receptor's 4 and 5) have proved efficacious in multiple clinical trials. The therapeutic rationale is based on targeting and amplifying a tumour tissues normal cell death programme (apoptosis). While advances in the targeting of DR4 and DR5 have been successful the search for an agonistic antibody to another family member, the Fas receptor, has proven more elusive. This is partly due to the differing in vitro and in vivo characteristics of individual antibodies. In order to induce Fas targeted cell death an antibody must be capable of binding to and trimerising the receptor. It has been shown that antibodies capable of performing this function in vivo, with the assistance of tumour associated cells, do not always induce apoptosis in vitro. As a result the use of current methodologies to detect functional antibodies in vitro may have dismissed potential therapeutic candidates ('false negative'). Here we report a novel high throughput screening technique which artificially cross-links antibodies bound to the Fas receptor. By combining this process with Annexin-V and Prodidium Iodide (PI) staining we can select for antibodies which have the potential to induce apoptosis in vivo.

A cell-based high-throughput screen to identify synergistic TRAIL sensitizers

We have developed a high-throughput screen (HTS) to search for novel molecules that can synergize with TRAIL, thus promoting apoptosis of ACHN renal tumor cells in a combinatorial fashion. The HTS detects synthetic compounds and pure natural products that can pre-sensitize the cancer cells to TRAIL-mediated apoptosis, yet have limited toxicity on their own. We have taken into account the individual effects of the single agents, versus the combination, and have identified hits that are synergistic, synergistic-toxic, or additive when combined with TRAIL in promoting tumor cell death. Preliminary mechanistic studies indicate that a subset of the synergistic TRAIL sensitizers act very rapidly to promote cleavage and activation of caspase-8 following TRAIL binding. Caspase-8 is an apical enzyme that initiates programmed cell death via the extrinsic apoptotic pathway. Thus, these TRAIL sensitizers may potentially reduce resistance of tumor cells to TRAIL-mediated apoptosis. Two representative sensitizers were found to increase levels of p53 but did not inhibit the proteasome, suggesting that early DNA damage-sensing pathways may be involved in their mechanisms of action.

Lupulone, a hop bitter acid, activates different death pathways involving apoptotic TRAIL-receptors, in human colon tumor cells and in their derived metastatic cells

Our study aimed to compare death signalling pathways triggered by lupulone in TRAIL-sensitive human colon cancer cells (SW480) and in their derived TRAIL-resistant metastatic cells (SW620). Lupulone (40 microg/ml) up-regulated expression of TRAIL DR4/DR5 death receptors at the cell surface of both cell lines, even in the absence of exogenous TRAIL ligand. Cell death induced by lupulone was inhibited in SW480 and SW620 cells exposed to blocking anti-DR4/DR5 antibodies. In SW480 cells, lupulone triggered cell death through a cross-talk between TRAIL-DR4/DR5 and the mitochondrial (intrinsic) pathways involving caspase-8 activation and Bid protein cleavage. As a consequence mitochondrial cytochrome c was released into the cytosol and activation of caspases-9 and -3 was observed. In the metastatic SW620 cells, lupulone restored the sensibility of these cells to TRAIL ligand and activated the extrinsic apoptotic pathway via DR4/DR5 death receptors and the involvement of the caspase-8/caspase-3 cascade. The demonstration that lupulone is able to activate TRAIL-death signalling pathways even in TRAIL resistant cancer cells highlights the potential of this natural compound for cancer prevention and therapy.

Enhancement of glioma radiotherapy and chemotherapy response with targeted antibody therapy against death receptor 5

PURPOSE

TRA-8 is an agonistic mouse monoclonal antibody that binds to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptor 5, which induces apoptosis in cancer cells through a caspase-8-dependent mechanism. We investigated the ability of TRA-8 to augment the radiotherapy (RT) and chemotherapy response of human glioma cells in vitro and in vivo.

METHODS AND MATERIALS

The in vitro cytotoxicity of TRA-8 and temozolomide (Tmz) or RT was examined using adenosine triphosphate-dependent viability and clonogenic survival assays with five glioma cell lines. Death receptor 5 expression was determined by flow cytometry. In vivo studies included subcutaneous and intracranial xenograft models testing various combination treatments, including RT, Tmz, and TRA-8.

RESULTS

TRA-8, combined with Tmz or RT, produced enhanced cytotoxicity against five glioma cell lines compared with the use of the individual agents alone. Death receptor 5 upregulation occurred in response to RT. Complete tumor regression in the subcutaneous experiments was the most common in animals that received combination therapy with TRA-8/Tmz/RT. TRA-8 enhanced tumor growth delay in combination with RT or Tmz. TRA-8 alone had limited activity against intracranial tumors. In contrast, the median survival of mice treated with TRA-8/Tmz/RT was significantly greater than the control or TRA-8-alone-treated mice. The median survival of the mice treated with TRA-8/Tmz/RT or chemoradiotherapy only was significantly greater than the control or TRA-8-treated mice. A trend toward improved survival was observed between TRA-8/Tmz/RT-treated and Tmz/RT-treated mice.

CONCLUSIONS

These preliminary findings support the hypothesis that TRA-8 will augment the RT and chemotherapy response in gliomas. A humanized version of TRA-8 is being evaluated in a Phase II clinical trial.

Suppression of cFLIP is sufficient to sensitize human melanoma cells to TRAIL- and CD95L-mediated apoptosis

Death ligands such as tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and certain forms of CD95L are attractive therapeutic options for metastatic melanoma. Since knowledge about the regulation of death receptor sensitivity in melanoma is sparse, we have analysed these signaling pathways in detail. The loss of CD95 or TRAIL-R1, but not of TRAIL-R2, surface expression correlated with apoptosis sensitivity in a panel of melanoma cell lines. In contrast, the expression of proteins of the apical apoptosis signaling cascade (FADD, initiator caspases-8 and cFLIP) did not predict apoptosis sensitivity. Since both TRAIL-R1 and -R2 transmit apoptotic signals, we asked whether cFLIP, highly expressed in several of the cell lines tested, is sufficient to maintain resistance to TRAIL-R2-mediated apoptosis. Downregulation of cFLIP in TRAIL-R2-positive, TRAIL-resistant IGR cells dramatically increased TRAIL sensitivity. Conversely ectopic expression of cFLIP in TRAIL-sensitive, TRAIL-R2-expressing RPM-EP melanoma cells inhibited TRAIL- and CD95L-mediated cell death. Thus, modulation of cFLIP is sufficient to sensitize TRAIL-R2-expressing cells for TRAIL. Taken together, albeit expressing all proteins necessary for death receptor-mediated apoptosis, TRAIL-R1 negative melanoma cells cannot undergo TRAIL- or CD95L-induced apoptosis due to expression of cFLIP. Hence, cFLIP represents an attractive therapeutic target for melanoma treatment, especially in combination with TRAIL receptor agonists.