DR4-selective tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) variants obtained by structure-based design

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potential anticancer agent that selectively induces apoptosis in a variety of cancer cells by interacting with death receptors DR4 and DR5. TRAIL can also bind to decoy receptors (DcR1, DcR2, and osteoprotegerin receptor) that cannot induce apoptosis. Different tumor types respond either to DR4 or to DR5 activation, and chemotherapeutic drugs can increase the expression of DR4 or DR5 in cancer cells. Thus, DR4 or DR5 receptor-specific TRAIL variants would permit new and tumor-selective therapies. Previous success in generating a DR5-selective TRAIL mutant using computer-assisted protein design prompted us to make a DR4-selective TRAIL variant. Technically, the design of DR4 receptor-selective TRAIL variants is considerably more challenging compared with DR5 receptor-selective variants, because of the lack of a crystal structure of the TRAIL-DR4 complex. A single amino acid substitution of Asp at residue position 218 of TRAIL to His or Tyr was predicted to have a favorable effect on DR4 binding specificity. Surface plasmon resonance-based receptor binding tests showed a lowered DR5 affinity in concert with increased DR4 specificity for the designed variants, D218H and D218Y. Binding to DcR1, DcR2, and osteoprotegerin was also decreased. Cell line assays confirmed that the variants could not induce apoptosis in DR5-responsive Jurkat and A2780 cells but were able to induce apoptosis in DR4-responsive EM-2 and ML-1 cells.     

Hypoxia inhibition of apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)

Hypoxia is a common environmental stress. Particularly, the center of rapidly growing solid tumors is easily exposed to hypoxic conditions. Thus, tumor cell response to hypoxia plays an important role in tumor progression as well as tumor therapy. However, little is known about hypoxic effect on apoptotic cell death. To examine the effects of hypoxia on TRAIL-induced apoptosis, human lung carcinoma A549 cells were exposed to hypoxia and treated with TRAIL protein. Hypoxia significantly protected A549 cells from apoptosis induced by TRAIL. Western blotting analysis demonstrated that hypoxia increased expression of antiapoptotic proteins such as Bcl-2, Bcl-XL, and IAP family members. The increase of these antiapoptotic molecules is believed to play an hypoxia-mediated protective role in TRAIL-induced apoptosis. Our findings suggest that an increase of antiapoptotic proteins induced by hypoxia may regulate the therapeutic activity of TRAIL protein in cancer therapy.     

Deficient tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptor transport to the cell surface in human colon cancer cells selected for resistance to TRAIL-induced apoptosis

Many tumor cell types are sensitive to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. Incubation of TRAIL-sensitive cells with TRAIL invariably leads to resistant survivors even when high doses of TRAIL are used. Because the emergence of resistance to apoptosis is a major concern in successful treatment of cancer, and TRAIL survivors may contribute to therapeutic failure, we investigated potential resistance mechanisms. We selected TRAIL-resistant SW480 human colon adenocarcinoma cells by repeatedly treating them with high and/or low doses of TRAIL. The resulting TRAIL-resistant clones were not cross-resistant to Fas or paclitaxel. Expression of modulators of apoptosis was not changed in the resistant cells, including TRAIL receptors, cFLIP, Bax, Bid, or IAP proteins. Surprisingly, we found that DISC formation was deficient in multiple selected TRAIL-resistant clones. DR4 was not recruited to the DISC upon TRAIL treatment, and caspase-8 was not activated at the DISC. Although total cellular DR4 mRNA and protein were virtually identical in TRAIL-sensitive parental and TRAIL-resistant clones, DR4 protein expression on the cell surface was essentially undetectable in the TRAIL-resistant clones. Moreover, exogenous DR4 and KILLER/DR5 were not properly transported to the cell surface in the TRAIL-resistant cells. Interestingly, TRAIL-resistant cells were resensitized to TRAIL by tunicamycin pretreatment, which increased cell surface expression of DR4 and KILLER/DR5. Our data suggest that tumor cells may become resistant to TRAIL through regulation of the death receptor cell surface transport and that resistance to TRAIL may be overcome by the glycosylation inhibitor/endoplasmic reticulum stress-inducing agent tunicamycin.     

Phenylarsine oxide interferes with the death inducing signaling complex and inhibits tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induced apoptosis

The mechanism by which tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces death is the subject of intense scrutiny due to its preferential targeting of transformed cells for deletion. Based on recent findings that the TRAIL-dependent death inducing signaling complex (DISC) forms and signals at the plasma membrane without being internalized, we investigated the possibility that agents that prevent endocytosis may stabilize the surface bound DISC and thereby enhance TRAIL-dependent signaling. We utilized phenylarsine oxide (PAO), a trivalent arsenical that has been reported to inhibit endocytosis and to induce mitochondrial permeability transition. Therefore PAO could, by two separate and independent activities, enhance TRAIL-induced killing. Paradoxically, we found that rather than synergizing with TRAIL, PAO was an effective inhibitor of TRAIL-induced killing. Recruitment of FADD and caspase-8 to the TRAIL-dependent DISC was diminished in a concentration-dependent manner in cells exposed to PAO. The effects of PAO could not be reversed by washing cells under non-reducing conditions, suggesting covalent linkage of PAO with its cellular target(s); however, 2,3-dimercaptoethanol effectively overcame the inhibitory action of PAO and restored sensitivity to TRAIL-induced apoptosis. PAO inhibited formation of the TRAIL-dependent DISC and therefore prevented all subsequent apoptotic events.     

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-induced apoptosis is dependent on activation of cysteine and serine proteases

We examined the role of caspases and serine protease(s) in cell death induced by tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). After incubation of adenocarcinoma cells with TRAIL, caspase-3, -8 were activated and the cleavage of Bid induced the release of cytochrome c, from the mitochondria to the cytosol. Tetrapeptide inhibitors of caspase-1, -2, -3, and -8 suppressed DNA fragmentation and attenuated the release of cytochrome c, whereas inhibitors of caspase-5 did not. Interestingly, the general serine protease(s) inhibitor 4-(2-aminoethyl)benzylsulfonyl fluoride (AEBSF) resulted in the arrest of apoptosis. However, the AEBSF did not prevent the release of mitochondrial cytochrome c during TRAIL-induced apoptosis. From these results, we postulate that serine protease(s) may be involved in post-mitochondrial apoptotic events, that lead to the activation of the initiator, caspase-9.     

Receptor-selective mutants of apoptosis-inducing ligand 2/tumor necrosis factor-related apoptosis-inducing ligand reveal a greater contribution of death receptor (DR) 5 than DR4 to apoptosis signaling

Apoptosis-inducing ligand 2 (Apo2L), also called tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), triggers programmed cell death in various types of cancer cells but not in most normal cells. Apo2L/TRAIL is a homotrimeric protein that interacts with five receptors: death receptor 4 (DR4) and DR5 mediate apoptosis activation, whereas decoy receptor 1 (DcR1), DcR2, and osteoprotegerin counteract this function. Many cancer cell lines express both DR4 and DR5, and each of these receptors can initiate apoptosis independently of the other. However, the relative contribution of DR4 and DR5 to ligand-induced apoptosis is unknown. To investigate this question, we generated death receptor-selective Apo2L/TRAIL variants using a novel approach that enables phage display of mutated trimeric proteins. Selective binding to DR4 or DR5 was achieved with three to six-ligand amino acid substitutions. The DR4-selective Apo2L/TRAIL variants examined in this study showed a markedly reduced ability to trigger apoptosis, whereas the DR5-selective variants had minimally decreased or slightly increased apoptosis-inducing activity. These results suggest that DR5 may contribute more than DR4 to Apo2L/TRAIL-induced apoptosis in cancer cells that express both death receptors.     

Receptor-mediated endocytosis is not required for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is selectively toxic to tumor compared with normal cells. Other members of the TNF family of death ligands (TNF, CD95L) engage their respective receptors (TNF-R1 and CD95), resulting in internalization of receptor and ligand and recruitment of adaptor proteins to the caspase activation platform known as the death-inducing signaling complex (DISC). Recently, TNF-R1 and CD95 have been shown to induce apoptosis with an absolute requirement for internalization of their corresponding receptors in the formation of a DISC. We show that TRAIL and its receptors are rapidly endocytosed in a time- and concentration-dependent manner. Blockade of receptor internalization with hyperosmotic sucrose did not inhibit TRAIL-induced apoptosis but, rather, amplified the apoptotic signaling of TRAIL. Plate-bound and soluble TRAIL induced similar levels of apoptosis. Together these results suggest that neither ligand nor receptor internalization is required for TRAIL-induced apoptosis. Internalization of TRAIL is mediated primarily by clathrin-dependent endocytosis and also by clathrin-independent pathways. Inhibition of clathrin-dependent internalization by overexpression of dominant negative forms of dynamin or AP180 did not inhibit TRAIL-induced apoptosis. Consistent with the finding that neither internalization of TRAIL nor its receptors is required for transmission of its apoptotic signal, recruitment of FADD (Fas-associated death domain) and procaspase-8 to form the TRAIL-associated DISC occurred at 4 degrees C, independent of endocytosis. Our findings demonstrate that TRAIL and TRAIL receptor 1/2, unlike TNF-TNF-R1 or CD95L-CD95, do not require internalization for formation of the DISC, activation of caspase-8, or transmission of an apoptotic signal in BJAB type I cells.     

Impact of DNA methyltransferases on the epigenetic regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor expression in malignant melanoma

Aberrant promoter methylation and resultant silencing of TRAIL decoy receptors were reported in a variety of cancers, but to date little is known about the relevance of this epigenetic modification in melanoma. In this study, we examined the methylation and the expression status of TRAIL receptor genes in cutaneous and uveal melanoma cell lines and specimens and their interaction with DNA methyltransferases (DNMTs) DNMT1, DNMT3a, and DNMT3b. DR4 and DR5 methylation was not frequent in cutaneous melanoma but on the contrary it was very frequent in uveal melanoma. No correlation between methylation status of DR4 and DR5 and gene expression was found. DcR1 and DcR2 were hypermethylated with very high frequency in both cutaneous and uveal melanoma. The concordance between methylation and loss of gene expression ranged from 91% to 97%. Here we showed that DNMT1 was crucial for DcR2 hypermethylation and that DNMT1 and DNMT3a coregulate the methylation status of DcR1. Our work also revealed the critical relevance of DcR1 and DcR2 expression in cell growth and apoptosis either in cutaneous or uveal melanoma. In conclusion, the results presented here claim for a relevant impact of aberrant methylation of decoy receptors in melanoma and allow to understand how the silencing of DcR1 and DcR2 is related to melanomagenesis.     

Pigment epithelium-derived factor (PEDF) promotes tumor cell death by inducing macrophage membrane tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)

Pigment epithelium-derived factor (PEDF) is an intrinsic anti-angiogenic factor and a potential anti-tumor agent. The tumoricidal mechanism of PEDF, however, has not been fully elucidated. Here we report that PEDF induces the apoptosis of TC-1 and SK-Hep-1 tumor cells when they are cocultured with bone marrow-derived macrophages (BMDMs). This macrophage-mediated tumor killing is prevented by blockage of TNF-related apoptosis-inducing ligand (TRAIL) following treatment with the soluble TRAIL receptor. PEDF also increases the amount of membrane-bound TRAIL on cultured mouse BMDMs and on macrophages surrounding subcutaneous tumors. PEDF-induced tumor killing and TRAIL induction are abrogated by peroxisome proliferator-activated receptor γ (PPARγ) antagonists or small interfering RNAs targeting PPARγ. PEDF also induces PPARγ in BMDMs. Furthermore, the activity of the TRAIL promoter in human macrophages is increased by PEDF stimulation. Chromatin immunoprecipitation and DNA pull-down assays confirmed that endogenous PPARγ binds to a functional PPAR-response element (PPRE) in the TRAIL promoter, and mutation of this PPRE abolishes the binding of the PPARγ-RXRα heterodimer. Also, PPARγ-dependent transactivation and PPARγ-RXRα binding to this PPRE are prevented by PPARγ antagonists. Our results provide a novel mechanism for the tumoricidal activity of PEDF, which involves tumor cell killing via PPARγ-mediated TRAIL induction in macrophages.     

Hypericin photo-induced apoptosis involves the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and activation of caspase-8

Hypericin (HYP) is a photosensitizing pigment from Hypericum perforatum that displays cytotoxic effects in neoplastic cell lines. Therefore, HYP is presently under consideration as a new anticancer drug in photodynamic therapy. Here, we investigated the mechanism of action of HYP photo-induced apoptosis of Jurkat cells compared to the cytostatic drug paclitaxel (PXL). Both photoactivated HYP and PXL similarly increased the activity of caspase-8 and caspase-3, and drug-induced apoptosis of Jurkat cells was completely blocked by inhibitors of caspase-8 (Z-IETD-FMK) and caspase-3 (Z-DEVD-FMK). The involvement of death receptors was analyzed using neutralizing monoclonal antibodies against Fas (SM1/23), FasL (NOK-2) and TNF-R1 (MAB225), and a polyclonal rabbit anti-human TNF-related apoptosis-inducing ligand (TRAIL) antiserum. TRAIL antibody blocked TRAIL-induced and HYP photo-induced, but not PXL-induced apoptosis of Jurkat cells. In contrast, PXL-induced, but not HYP-induced apoptosis was blocked by the SM1/23 and NOK-2 antibodies. Anti-TNF-R1 antibody had no effect. These findings suggest that HYP photo-induced apoptosis of Jurkat cells is mediated in part by the TRAIL/TRAIL-receptor system and subsequent activation of upstream caspases.     

Association of human tumor necrosis factor-related apoptosis inducing ligand with membrane upon acidification.

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) has been known to induce tumor-specific apoptosis and to share the structural and functional characteristics with the proteins of TNF family. Recently, the crystal structure of human TRAIL showed that TRAIL is a homotrimeric protein whose subunits contain mainly beta-sheets. We characterized the structural changes of recombinant human TRAIL induced by acidification and the biological implication of the structural characteristics at acidic pH in the interaction with the lipid bilayer. At acidic pH below pH 4.5, TRAIL resulted in substantial structural changes to a molten globule (MG)-like state. Far-UV CD spectrum of TRAIL indicated that the acidification induced alpha-helices that are absent in the native state. TRAIL at acidic pH exhibited significant change of tertiary structures as reflected in the near-UV CD spectrum. Thermal transition curve indicated that there was less cooperation at acidic pH than at neutral pH in the thermal denaturation of TRAIL. Moreover, TRAIL at the MG-like state not only enhanced the binding ability to liposomes, but also increased the release rate of a fluorescent dye, calcein, encapsulated in liposomes. The binding assay with anilinonaphthalene-8-sulfonic acid revealed that the surface hydrophobicity of TRAIL was increased while tryptophan residues became more exposed to solvent as judged by blue shift of the maximum fluorescence wavelength. Taken together, our results demonstrate that the acidification of human TRAIL induces the MG-like state in vitro and makes the membrane permeable through the favorable interaction of TRAIL with the membrane, implicating that general intrinsic properties such as TRAIL, TNF-alpha and lymphotoxin are shared by TNF family members.     

TNF-related apoptosis inducing ligand (TRAIL) and its receptors in tumor surveillance and cancer therapy

TNF-related apoptosis-inducing ligand (TRAIL/APO-2L) is a typical member of the TNF ligand family that induces apoptosis by activating the death receptors TRAIL-R1 and TRAIL-R2. TRAIL has attracted great attention in recent years as a promising anti cancer reagent because recombinant soluble TRAIL derivatives induce apoptosis in a broad range of tumor cells but not or only rarely in non-transformed cells. In this review we will address the putative role of TRAIL in cancer treatment in the light of the emerging importance of TRAIL in tumor surveillance and discuss the molecular basis of the cooperation of TRAIL and chemotherapeutic drugs. In particular, we debate controversial data in the literature concerning the cytotoxicity of different TRAIL derivatives on primary human cells.     

Bid mediates apoptotic synergy between tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and DNA damage

The death ligand, TRAIL (tumor necrosis factor-related apoptosis-inducing ligand), has shown great promise for inducing apoptosis selectively in tumors. Although many tumor cells are resistant to TRAIL-induced apoptosis alone, they can often be sensitized by co-treatment with DNA-damaging agents such as etoposide. However, the molecular mechanism underlying this therapeutically important synergy is unknown. We explored the mechanism mediating TRAIL-DNA damage apoptotic synergy in human mesothelioma cells, a tumor type particularly refractory to existing therapies. We show that Bid, a cytoplasmic Bcl-2 homology domain 3-containing protein activated by caspase 8 in response to TRAIL ligation, is essential for TRAIL-etoposide apo-ptotic synergy and, furthermore, that exposure to DNA damage primes cells to induction of apoptosis by otherwise sublethal levels of activated Bid. Finally, we show that the extensive caspase 8 cleavage seen during TRAIL-etoposide synergy is a consequence and not a cause of the apoptotic cascade activated downstream of Bid. These data indicate that TRAIL-etoposide apoptotic synergy arises because DNA damage increases the inherent sensitivity of cells to levels of TRAIL-activated Bid that would otherwise be insufficient for apoptosis. Such studies indicate how the adroit combination of differing proapoptotic and sublethal signals can provide an effective strategy for treating refractory tumors.     

Expression and characterization of human tumor necrosis factor-related apoptosis inducing ligand in methylotrophic yeast Pichia pastoris

Recombinant human TRAIL was successfully expressed in a secreted form in methyltrophic yeast Pichia pastoris induced by methanol. The expressive product was immunoreactive to TRAIL antibody. The addition of the expressive product into cultured human lung cancer cell A549 showed moderate cell death, typical morphological characterization of apoptotic bodies, and DNA fragmentation. This result provided a new method to produce recombinant human TRAIL as a cancer therapeutic drug.     

Biological and physicochemical evaluation of the conformational stability of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)

Tumor necrosis factor (TNF)-related, apoptosis-inducing ligand (Apo2L/TRAIL) has a unique homotrimeric structure, and its conformational stability is essential for its apoptotic activity. The conformational stability of a modified version of TRAIL(114–281) with two additional domains of histidine tag and isoleucine zipper [His-ILZ-TRAIL(114–281)] was evaluated in various pH environments according to three different biological or physicochemical considerations: cytotoxicity, antibody-binding affinity, and tertiary structure. The biological properties of His-ILZ-TRAIL(114–281) were the most stably maintained at pH 6.0. The physicochemical analyses (circular dichroism and fluorescence spectroscopy) demonstrate that its bioactivity loss by pH challenge was originated from its structural collapse as a homotrimer.     

Calyculin A causes sensitization to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis by ROS-mediated down-regulation of cellular FLICE-inhibiting protein (c-FLIP) and by enhancing death receptor 4 mRNA stabilization

Calyculin A (Cal A) is a serine/threonine phosphatase inhibitor that is capable of inducing apoptosis in cancer cells. In this study, we examined whether Cal A could modulate TRAIL-induced apoptosis in human renal carcinoma-derived Caki cells. Our results show that Cal A is capable of sensitizing Caki cells to TRAIL-induced apoptosis, as well as U2OS human osteosarcoma cells and A549 human lung adenocarcinoma epithelial cells. Cal A increases intracellular ROS production and down-regulates c-FLIP(L) expression. Interestingly, the down-regulation of protein phosphatase 1 (PP1) by PP1 siRNA also reduced c-FLIP(L) expression via reactive oxygen species production. Furthermore, Cal A induced death receptor 4 (DR4) mRNA and protein expression by enhancing DR4 mRNA stability. We also found that PP4 siRNA up-regulated DR4 mRNA and protein expression. Collectively, our results suggest that Cal A could enhance TRAIL-mediated apoptosis via the down-regulation of c-FLIP(L) and the up-regulation of DR4 in human renal cell carcinoma cell line Caki.