Differential hepatocyte toxicity of recombinant Apo2L/TRAIL versions

Hepatic DR5 induces apoptosis and limits adenovirus gene therapy product expression in the liver

A major limitation of adenovirus (Ad) gene therapy product expression in the liver is subsequent elimination of the hepatocytes expressing the gene therapy product. This elimination is caused by both necrosis and apoptosis related to the innate and cell-mediated immune response to the Ad. Apoptosis of hepatocytes can be induced by the innate immune response by signaling through death domain receptors on hepatocytes including the tumor necrosis factor alpha (TNF-alpha) receptor (TNFR), Fas, and death domain receptors DR4 and DR5. We have previously shown that blocking signaling through TNFR enhances and prolongs gene therapy product expression in the liver. In the present study, we constructed an Ad that produces a soluble DR5-Fc (AdsDR5), which is capable of neutralizing TNF-related apoptosis-inducing ligand (TRAIL). AdsDR5 prevents TRAIL-mediated apoptosis of CD3-activated T cells and decreases hepatocyte apoptosis after AdCMVLacZ administration and enhances the level and duration of lacZ transgene expression in the liver. In addition to blocking TRAIL and directly inhibiting apoptosis, AdsDR5 decreases production of gamma interferon (IFN-gamma) and TNF-alpha and decreases NK cell activation, all of which limit Ad-mediated transgene expression in the liver. These results indicate that (i) AdsDR5 produces a DR5-Fc capable of neutralizing TRAIL, (ii) AdsDR5 can reduce activation of NK cells and reduce induction of IFN-gamma and TNF-alpha after Ad administration, and (iii) administration of AdsDR5 can enhance Ad gene therapy in the liver.

On the TRAIL to apoptosis

Apoptosis in mammalian cells can be initiated through two major interrelated pathways, one involving engagement of the TNF family of death receptors, the other involving the release of cytochrome c from mitochondria. Unlike other members of the TNF ligand family, TNF-related apoptosis-inducing ligand (TRAIL) preferentially induces apoptosis in tumor cell lines, but not in normal cells, suggesting that TRAIL could potentially represent a powerful cancer therapeutic. Recent experiments have revealed that one of the key regulators of TRAIL expression in lymphocytes is the NF-kappa B transcription factors. Several TRAIL receptors have been identified: two of these receptors TRAIL-R1/DR4 and TRAIL-R2/DR5 contain cytoplasmic death domains and signal apoptosis, while two other decoy receptors, TRAIL-R3/DcR1 and TRAIL-R4/DcR2 lack a functional death domain and do not mediate apoptosis. Many cancer cell lines preferentially express TRAIL-R1 and TRAIL-R2, suggesting differential regulation of the death and decoy receptors. Further knowledge of the regulation and physiological role of TRAIL and TRAIL receptors may aid in the rational design of regimens that utilize the TRAIL signaling pathway to eliminate tumor cells.

Death ligand TRAIL induces no apoptosis but inhibits activation of human (auto)antigen-specific T cells

TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily, induces apoptosis in susceptible cells, which can be both malignant and nontransformed. Despite homologies among the death ligands, there are great differences between the TRAIL system on the one hand and the TNF and CD95 systems on the other hand. In particular, TRAIL-induced apoptosis differs between rodents and man. Studies on animal models of autoimmune diseases suggested an influence of TRAIL on T cell growth and effector functions. Because we previously demonstrated that TRAIL does not induce apoptosis in human (auto)antigen-specific T cells, we now asked whether TRAIL exhibits other immunoregulatory properties in these cells. Active TRAIL inhibited calcium influx through store-operated calcium release-activated calcium channels, IFN-gamma/IL-4 production, and proliferation. These effects were independent of APC, Ag specificity, and Th differentiation, and no differences were detected between healthy donors and multiple sclerosis patients. TRAIL affected neither the expression of the cell cycling inhibitor p27(Kip1) nor the capacity of T cells to produce IL-2 upon Ag rechallenge, indicating that signaling via TRAIL receptor does not induce T cell anergy. Instead, the TRAIL-induced hypoproliferation could be attributed to the down-regulation of the cyclin-dependent kinase 4, indicating a G(1) arrest of the cell cycle. Thus, although it does not contribute to mechanisms of peripheral T cell tolerance such as clonal anergy or deletion by apoptosis, TRAIL can directly inhibit activation of human T cells via blockade of calcium influx.

Evidence that the human death receptor 4 is regulated by activator protein 1

Death receptor 4 (DR4; also called TRAIL-R1), a member of the tumor necrosis factor receptor superfamily, is a cell surface receptor that triggers the apoptotic machinery upon binding to its ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Although several chemotherapeutic agents were reported to induce DR4 expression, the mechanism of this effect remains largely unknown. To begin to understand its regulation, we cloned a 1.8 Kb 5'-flanking region of the human DR4 gene and identified several putative binding sites for transcription factors including activator protein 1 (AP-1). Among the three putative AP-1 binding sites, the site located at -350/-344 is functionally active as evidenced by a combination of electrophoretic mobility shift and luciferase reporter assays. The AP-1 activator phorbol 12-myristate 13-acetate (TPA) enhanced the binding of this DR4 AP-1 binding site to protein(s) in a nuclear extract from TPA-treated cells, increased luciferase activity of a reporter construct containing this site and induced DR4 expression at the transcription level. These results indicate that AP-1 regulates DR4 expression via the AP-1 binding site located at -350/-344. AP-1 has been implicated in many critical cellular processes including apoptosis, and is a major target of the c-Jun NH(3)-terminal kinase signaling pathway that is activated by many anticancer drugs. Therefore, our findings may increase the understanding of the mechanisms underlying AP-1-mediated apoptosis as well as drug-induced apoptosis.

Ectopic overexpression of second mitochondria-derived activator of caspases (Smac/DIABLO) or cotreatment with N-terminus of Smac/DIABLO peptide potentiates epothilone B derivative-(BMS 247550) and Apo-2L/TRAIL-induced apoptosis

Second mitochondria-derived activator of caspases (Smac)/DIABLO is a mitochondrial protein that is released into the cytosol along with cytochrome c (cyt c) during the execution of the intrinsic pathway of apoptosis. Smac/DIABLO promotes apoptosis by neutralizing the inhibitory effect of the inhibitor of apoptosis (IAP) family of proteins on the processing and activities of the effector caspases. Present studies demonstrate that, upon engagement of the mitochondrial pathway of apoptosis, epothilone (Epo) B derivative BMS 247550, a novel nontaxane antimicrotubule agent, as well as the death ligand Apo-2L/TRAIL (tumor necrosis factor-alpha-related apoptosis-inducing ligand) induce the mitochondrial release and cytosolic accumulation of Smac/DIABLO, along with cyt c, in human acute leukemia Jurkat T cells. While it had no activity alone, ectopic overexpression of Smac/DIABLO or treatment with the N-terminus heptapeptide (Smac-7) or tetrapeptide (Smac-4) of Smac/DIABLO significantly increased Epo B- or Apo-2L/TRAIL-induced processing and PARP cleavage activity of caspase-3. This produced a significant increase in apoptosis of Jurkat cells (P <.05). Increased apoptosis was also associated with the down-regulation of XIAP, cIAP1, and survivin. Along with the increased activity of caspase-3, ectopic overexpression of Smac/DIABLO or cotreatment with Smac-4 also increased Epo B- or Apo-2L/TRAIL-induced processing of caspase-8 and Bid, resulting in enhanced cytosolic accumulation of cyt c. This was not due to increased assembly and activity of Apo-2L/TRAIL-induced DISC (death-inducing signaling complex) but dependent on the feedback activity of caspase-3. These findings demonstrate that cotreatment with the N-terminus Smac/DIABLO peptide is an effective strategy to enhance apoptosis triggered by the death receptor or mitochondrial pathway and may improve the antitumor activity of Apo-2L/TRAIL and Epo B.

Inhibition of TRAIL-induced apoptosis by Bcl-2 overexpression

Primary or acquired resistance to current treatment protocols remains a major concern in clinical oncology and may be caused by defects in apoptosis programs. Since recent data suggest that TRAIL can bypass apoptosis resistance caused by Bcl-2, we further investigated the role of Bcl-2 in TRAIL-induced apoptosis. Here we report that overexpression of Bcl-2 conferred protection against TRAIL in neuroblastoma, glioblastoma or breast carcinoma cell lines. Bcl-2 overexpression reduced TRAIL-induced cleavage of caspase-8 and Bid indicating that caspase-8 was activated upstream and also downstream of mitochondria in a feedback amplification loop. Importantly, Bcl-2 blocked cleavage of caspases-9, -7 and -3 into active subunits and cleavage of the caspase substrates DFF45 or PARP. Also, Bcl-2 blocked cleavage of XIAP and overexpression of XIAP conferred resistance against TRAIL indicating that apoptosis was also amplified through a feedforward loop between caspases and XIAP. In contrast, in SKW lymphoblastoid cells, TRAIL-induced activation of caspase-8 directly translated into full activation of caspases, cleavage of XIAP, DFF45 or PARP and apoptosis independent of Bcl-2 overexpression, although Bcl-2 similarly inhibited loss of mitochondrial membrane potential and the release of cytochrome c, AIF and Smac from mitochondria in all cell types. By demonstrating a cell type dependent regulation of the TRAIL signaling pathway at different level, e.g. by Bcl-2 and by XIAP, these findings may have important clinical implication. Thus, strategies targeting the molecular basis of resistance towards TRAIL may be necessary in some tumors for cancer therapy with TRAIL.

Intracellular regulation of tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis in human multiple myeloma cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL, Apo2 ligand) effectively kills multiple myeloma (MM) cells in vitro irrespective of refractoriness to dexamethasone and chemotherapy. Because clinical trials with this anticancer agent are expected shortly, we investigated the signaling pathway of TRAIL-induced apoptosis in MM. We detected rapid cleavage of caspases-8, -9, -3, and -6, as well as the caspase substrates poly(ADP-ribose) polymerase (PARP) and DNA fragmentation factor-45 (DFF45), but not caspase-10, upon TRAIL treatment in sensitive MM cells, pointing to caspase-8 as the apical caspase of TRAIL signaling in MM cells. These phenomena were not observed or were significantly delayed in TRAIL-resistant MM cells, suggesting that resistance may arise from inhibition at the level of caspase-8 activation. Higher levels of expression for various apoptosis inhibitors, including FLICE-inhibitory protein (FLIP), and lower procaspase-8 levels were present in TRAIL-resistant cells and sensitivity was restored by the protein synthesis inhibitor cycloheximide (CHX) and the protein kinase C (PKC) inhibitor bisindolylmaleimide (BIM), which both lowered FLIP and cellular inhibitor of apoptosis protein-2 (cIAP-2) protein levels. Forced expression of procaspase-8 or FLIP antisense oligonucleotides also sensitized TRAIL-resistant cells to TRAIL. Moreover, the cell permeable nuclear factor (NF)-kappaB inhibitor SN50, which sensitizes TRAIL-resistant cells to TRAIL, also inhibited cIAP2 protein expression. Finally, CHX, BIM, and SN50 facilitated the cleavage and activation of procaspase-8 in TRAIL-resistant cells, confirming that inhibition of TRAIL-induced apoptosis occurs at this level and that these agents sensitize MM cells by relieving this block. Our data set a framework for the clinical use of approaches that sensitize MM cells to TRAIL by agents that inhibit FLIP and cIAP-2 expression or augment caspase-8 activity.

Tumor-cell resistance to death receptor--induced apoptosis through mutational inactivation of the proapoptotic Bcl-2 homolog Bax

The importance of Bax for induction of tumor apoptosis through death receptors remains unclear. Here we show that Bax can be essential for death receptor--mediated apoptosis in cancer cells. Bax-deficient human colon carcinoma cells were resistant to death-receptor ligands, whereas Bax-expressing sister clones were sensitive. Bax was dispensable for apical death-receptor signaling events including caspase-8 activation, but crucial for mitochondrial changes and downstream caspase activation. Treatment of colon tumor cells deficient in DNA mismatch repair with the death-receptor ligand apo2 ligand (Apo2L)/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selected in vitro or in vivo for refractory subclones with Bax frameshift mutations including deletions at a novel site. Chemotherapeutic agents upregulated expression of the Apo2L/TRAIL receptor DR5 and the Bax homolog Bak in Baxminus sign/minus sign cells, and restored Apo2L/TRAIL sensitivity in vitro and in vivo. Thus, Bax mutation in mismatch repair--deficient tumors can cause resistance to death receptor--targeted therapy, but pre-exposure to chemotherapy rescues tumor sensitivity.

Inflammatory cytokine regulation of TRAIL-mediated apoptosis in thyroid epithelial cells

Death receptor-mediated apoptosis has been implicated in target organ destruction in chronic autoimmune thyroiditis. Depending on the circumstances, inflammatory cytokines such as IL-1, TNF and IFNgamma have been shown to contribute to either the induction, progression or inhibition of this disease. Here we demonstrate that the death ligand TRAIL can induce apoptosis in primary, normal, thyroid epithelial cells under physiologically relevant conditions, specifically, treatment with the combination of inflammatory cytokines IL-1beta and TNFalpha. In contrast, IFNgamma is capable of blocking TRAIL-induced apoptosis in these cells. This regulation of TRAIL-mediated apoptosis by inflammatory cytokines appears to be due to alterations of cell surface expression of TRAIL receptor DR5 and not DR4. We also show the in vivo presence of TRAIL and TRAIL receptors DR5 and DcR1 in both normal and inflamed thyroids. Our data suggests TRAIL-mediated apoptosis may contribute to target organ destruction in chronic autoimmune thyroiditis.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) triggers apoptosis in normal prostate epithelial cells

TRAIL is a pro-apoptotic cytokine believed to selectively kill cancer cells without harming normal ones. However, we found that in normal human prostate epithelial cells (PrEC) TRAIL is capable of inducing apoptosis as efficiently as in some tumor cell lines. At the same time, TRAIL did not cause apoptosis in several other human primary cell lines: aorta smooth muscle cells, foreskin fibroblasts, and umbilical vein endothelial cells. Compared to these primary cells, PrEC were found to contain significantly fewer TRAIL receptors DcR1 and DcR2 which are not capable of conducting the apoptotic signal. This result suggests that the unusual sensitivity of PrEC to TRAIL may result from their deficiency in anti-apoptotic decoy receptors. The protein synthesis inhibitor cycloheximide significantly enhanced TRAIL toxicity toward PrEC as measured by tetrazolium conversion but had little or no effect on other TRAIL-induced apoptotic responses. Although cycloheximide did not further accelerate the processing of caspases 3 and 8, it significantly enhanced cleavage of the caspase 3 substrate gelsolin, indicating that in PrEC a protein(s) with a short half-life may inhibit the activity of the executioner caspases toward specific substrates. As the majority of prostate cancers are derived from epithelial cells, our data suggest the possibility that TRAIL could be a useful treatment for the early stages of prostate cancer.

Resistance of prostate cancer cells to soluble TNF-related apoptosis-inducing ligand (TRAIL/Apo2L) can be overcome by doxorubicin or adenoviral delivery of full-length TRAIL

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) has been shown to induce apoptosis in malignant cells without harming normal cells. To determine the antitumor potential of TRAIL against prostate cells, we undertook a comprehensive study that included eight prostate cancer cells lines (CWR22Rv1, Du145, DuPro, JCA-1, LNCaP, PC-3, PPC-1, and TsuPr1) and primary cultures of normal prostate epithelial cells (PrEC). Cells were tested for susceptibility to soluble TRAIL in the presence or absence of the chemotherapeutic agent doxorubicin. TRAIL was also delivered by an adenoviral vector. Our results reveal that Du145, DuPro, LNCap, TsuPr1, and PrEC were resistant to 100 ng/mL TRAIL. JCA-1 and PPC-1 were slightly sensitive (20% killing) and PC-3 and CWR22Rv1 exhibited the highest sensitivity to TRAIL (30% and 50% killing, respectively). The combination of 10 ng/mL TRAIL with doxorubicin resulted in 60-80% cytotoxicity in seven of eight prostate cancer cells. TRAIL-mediated apoptosis involved cleavage of Bid, caspase-3, and PARP, and required caspase-8 and -9 activity. Full-length TRAIL delivered by an adenoviral vector (AdTRAIL-IRES-GFP) killed prostate cancer cell lines and PrEC without requisite doxorubicin cotreatment. Therefore, expression of the transgene from a tissue-specific promotor would make gene therapy with AdTRAIL-IRES-GFP a possibility.

Targeting apoptosis in cancer chemotherapy

The aim of cancer biology is for a better understanding of the molecular basis of cancer, with the expectation that this will result in therapeutic advances and improved outcomes for patients. The discovery of apoptosis has contributed much to our understanding of the mechanisms of cell death, in both normal and neoplastic cells, and it has led to changes in the way that chemotherapy has been viewed. It is now increasingly accepted that part of the efficacy of conventional chemotherapeutic drugs is due to their ability to induce apoptosis, although this area is not without controversy. This has allowed advances in the fundamental understanding of apoptosis to have similar impacts upon cancer biology. It is now possible to construct a framework where cellular decisions about life and death can be seen as the result of a balance of pro- and anti-apoptotic signals, enacted by protein members of the Bcl-2 family, controlling mitochondrial cytochrome c release. This framework has allowed the importance of providing death signals and abrogating survival signals to both be appreciated. A range of novel approaches to the induction of apoptosis by downregulating survival signalling are described. In addition, many alternative strategies aimed at targeting particular molecular abnormalities of neoplastic cells as a means of inducing apoptosis are also under investigation and several of these are discussed. The mechanistic understanding of cell death will have profound impacts upon the practice of oncology and outlook for many patients.

Electroporation-mediated tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo2L gene therapy for hepatocellular carcinoma

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo2 ligand (Apo2L) has been identified as important in promoting programmed cell death in breast and colon cancer xenografts. More importantly, normal liver tissue appears not to be susceptible to the cytotoxic effects of TRAIL/Apo2L, although activation of the related Fas ligand receptor system is known to promote massive liver apoptosis terminating in fulminant hepatitis. In the present study, we investigated the therapeutic potential of TRAIL/Apo2L gene therapy in hepatocellular carcinoma (HCC) and evaluated its side effects in an immune-competent mouse model. Intratumoral administration of the TRAIL/Apo2L vector by electroporation elevated serum TRAIL/Apo2L through at least day 28 after gene therapy and significantly inhibited the growth not only of the HCC directly administered TRAIL/Apo2L vector, but also of distant subcutaneous HCC. In addition, intratumoral administration of the TRAIL/Apo2L vector inhibited spontaneous lung metastasis. Serum alanine aminotransferase was mildly elevated by TRAIL/Apo2L gene therapy, but without showing such histological signs as TUNEL staining. These results demonstrate that TRAIL/Apo2L gene therapy for HCC by electroporation in vivo is efficient without significant side effects, and is thus promising for use in future clinical trials.

Low glucose-enhanced TRAIL cytotoxicity is mediated through the ceramide-Akt-FLIP pathway

To examine whether the tumor microenvironment alters cytokine-induced cytotoxicity, human prostate adenocarcinoma DU-145 cells were exposed to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and/or glucose deprivation, a common characteristic of the tumor microenvironment. TRAIL alone reduced cell survival in a dose-dependent manner. Glucose deprivation alone induced no cytotoxicity within 4 h. However, the combination of TRAIL (50 ng/ml) and glucose deprivation for 4 h increased cell death and PARP cleavage by promoting activation of caspase-8 and caspase-3, relative to that of TRAIL alone. Similar results were observed in human colorectal carcinoma CX-1 cells. Data from immunoblotting analysis reveal that glucose deprivation-enhanced TRAIL cytotoxicity is inversely related to the intracellular level of FLICE inhibitory protein (FLIP) but not that of death receptor 5 (DR5). Results from mass spectrometry show that glucose deprivation elevates ceramide. The elevation of ceramide may cause dephosphorylation of Akt and maintain dephosphorylation of Akt in the presence of TRAIL and then subsequently down-regulate the expression of FLIP. Taken together, the present studies suggest that glucose deprivation enhances TRAIL-induced cytotoxicity through the ceramide-Akt-FLIP pathway.

Molecular cross-talk between the TRAIL and interferon signaling pathways

TRAIL/APO-2L induces apoptosis in a variety of transformed cells and has potential as an anti-cancer therapeutic. The physiologic role of TRAIL is presumably more complex than merely activating caspase-mediated cell death. To shed light into TRAIL-mediated signaling, we used DNA microarrays to profile gene expression mediated by TRAIL in breast carcinoma cells. Primary response genes induced by TRAIL included a number of known NF-kappaB-dependent genes such as cIAP2, A20, and E-selectin. Remarkably, global transcriptome analysis revealed that TRAIL also induced a cohort of genes related to the interferon-signaling pathway. Assessing interferon-induced gene expression suggested various points of interaction with the TRAIL signaling pathway. Interestingly, while we observed interferon-mediated up-regulation of TRAIL, we also demonstrated a concomitant TRAIL-mediated induction of interferon-beta. Combining TRAIL and interferon in vitro, synergistically induced apoptosis and caspase activation in breast cancer cells. Together, these data indicate multiple levels of molecular cross-talk between the two diverse cytokines with anti-tumor properties.

Loss of caspase-8 mRNA expression is common in childhood primitive neuroectodermal brain tumour/medulloblastoma

Upon binding of tumour necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), the agonistic TRAIL receptors DR4 and DR5 activate caspase-8 leading to apoptosis. In primitive neuroectodermal brain tumour (PNET) cell lines, TRAIL-induced apoptosis was recently shown to correlate with caspase-8 mRNA expression (Grotzer MA, Eggert A, Zuzak TJ, et al. Oncogene 2000, 19, 4604-4610). In this study, we analysed the expression of the TRAIL death pathway in 27 primary PNET/medulloblastoma. As shown by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), all PNET/medulloblastoma evaluated expressed DR5, the adapter protein FADD and caspase-3, but only 48% expressed caspase-8. The mRNA expression of caspase-8 was significantly lower in primary PNET/medulloblastoma compared with normal brain samples. PCR revealed >75% methylation of the caspase-8 promoter region in three of seven PNET cell lines and in 55% of the primary PNET/medulloblastoma evaluated. In the PNET cell lines, the methylation status correlated with the caspase-8 mRNA expression. We conclude that loss of caspase-8 gene expression is common in PNET/medulloblastoma suggesting that suppression of death receptor induced apoptosis may play an important role in the pathogenesis of this common childhood brain tumour.

The molecular architecture of the TNF superfamily

Ligands of the TNF (tumour necrosis factor) superfamily have pivotal roles in the organization and function of the immune system, and are implicated in the aetiology of several acquired and genetic diseases. TNF ligands share a common structural motif, the TNF homology domain (THD), which binds to cysteine-rich domains (CRDs) of TNF receptors. CRDs are composed of structural modules, whose variation in number and type confers heterogeneity upon the family. Protein folds reminiscent of the THD and CRD are also found in other protein families, raising the possibility that the mode of interaction between TNF and TNF receptors might be conserved in other contexts.

Death receptor ligands in tumors

Activation of apoptosis via death receptors is a tightly regulated event, and the death pathway itself is open to interference on the part of soluble or membrane-bound decoy receptors. The aggregation state of the death-inducing ligand is a crucial factor, particularly when these molecules are used as recombinant drugs against tumors. Whether tumors are sensitive to such ligands is determined by both the net abundance of death receptors versus decoy receptors and the balance between intracellular apoptotic and antiapoptotic mechanisms. This means that in vivo elimination of tumor cells by effector arms such as T lymphocytes, natural killer cells, macrophages, and dendritic cells is dependent on both the function of activated lymphoid cells and the genetic properties of tumor cells. Death receptor ligands, however, may be a double-edged sword. When expressed on cytotoxic T lymphocytes, natural killer cells, monocytes, and dendritic cells, they induce the apoptosis of many tumor cells, whereas their expression on tumor cells induces the apoptosis of killer cells. The in vivo result is influenced by the number of infiltrating cells, their state of activation, the cytokine repertoire in the tumor microenvironment, and the ability of the tumor to produce soluble factors inhibiting their cytolytic functions.

Promoter structure and transcription initiation sites of the human death receptor 5/TRAIL-R2 gene

The death receptor 5 (DR5) is a receptor for tumor necrosis factor-related apoptosis-inducing ligand and is able to induce apoptosis in various tumor cells. The expression of DR5 is up-regulated at the transcriptional level by p53, genotoxic stress and so on. To investigate the structure of the DR5 gene promoter, we screened and sequenced a genomic clone containing the 5'-flanking region of the DR5 gene. RNase protection assays showed two major transcription start sites around -122 and -137 upstream of the translation initiation codon ATG. Transient transfections with serial 5'-deletion mutants identified the minimal promoter element spanning -198 to -116. Site-directed mutagenesis demonstrated that the DR5 gene promoter has no typical TATA-box, but has two Sp1 sites responsible for the basal transcription activity of the DR5 gene promoter.

TRAIL/Apo-2L: mechanisms and clinical applications in cancer

TNF-related apoptosis-inducing ligand (TRAIL/APO-2L) is a member of the TNF family that promotes apoptosis by binding to the transmembrane receptors TRAIL-R1/DR4 and TRAIL-R2/DR5. Its cytotoxic activity is relatively selective to the human tumor cell lines without much effect on the normal cells. Hence, it exerts an antitumor activity without causing toxicity, as apparent by studies with several xenograft models. This review discusses the intracellular mechanisms by which TRAIL induces apoptosis. The major pathway of its action proceeds through the formation of DISC and activation of caspase-8. The apoptotic processes, therefore, follow two signaling pathways, namely the mitochondrial-independent activation of caspase-3, and mitochondrial-dependent apoptosis due to cleavage of BID by caspase-8, the formation of apoptosomes, and activation of caspase-9 and the downstream caspases. Bcl-2 and Bcl-X(L) have no effect on TRAIL-induced apoptosis in lymphoid cells, whereas these genes block or delay apoptosis in nonlymphoid cancer cells. TRAIL participates in cytotoxicity mediated by activated NK cells, monocytes, and some cytotoxic T cells. Hence, TRAIL may prove to be an effective antitumor agent. In addition, it may enhance the effectiveness of treatment with chemotherapeutic drugs and irradiation. Nontagged Apo-2L/TRAIL does not cause hepatotoxicity in monkeys and chimpanzees and in normal human hepatocytes. Thus, nontagged Apo-2L/TRAIL appears to be a promising new candidate for use in the treatment of cancer.

Induction of cell death in human immunodeficiency virus-infected macrophages and resting memory CD4 T cells by TRAIL/Apo2l

Because the persistence of human immunodeficiency virus (HIV) in cellular reservoirs presents an obstacle to viral eradication, we evaluated whether tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) induces apoptosis in such reservoirs. Lymphocytes and monocyte-derived macrophages (MDM) from uninfected donors do not die following treatment with either leucine zipper human TRAIL (LZhuTRAIL) or agonistic anti-TRAIL receptor antibodies. By contrast, such treatment induces apoptosis of in vitro HIV-infected MDM as well as peripheral blood lymphocytes from HIV-infected patients, including CD4(+) CD45RO(+) HLA-DR(-) lymphocytes. In addition, LZhuTRAIL-treated cells produce less viral RNA and p24 antigen than untreated controls. Whereas untreated cultures produce large amounts of HIV RNA and p24 antigen, of seven treated CD4(+) CD45RO(+) HLA-DR(-) cell cultures, viral RNA production was undetectable in all, p24 antigen was undetectable in six, and proviral DNA was undetectable in four. These data demonstrate that TRAIL induces death of cells from HIV-infected patients, including cell types which harbor latent HIV reservoirs.

Insights into cancer therapeutic design based on p53 and TRAIL receptor signaling

Knowledge of the emerging pathways of cell death downstream of the p53 tumor suppressor and the TRAIL death-inducing ligand is suggesting ways to improve therapeutic design in cancer. In contrast to its unique G1 cell cycle arresting mechanism that is maintained by p21(WAF1), there are signals transduced by p53 to multiple apoptotic effectors perhaps due to the importance of apoptosis in suppressing tumors. There is evidence for cytoplasmic as well as mitochondrial activation of caspases downstream of p53, although in some cell lineages the signal ultimately involves the mitochondria. The TRAIL signaling pathway appears promising for therapeutic development despite sharing some similarities with the toxic Fas and TNF pathways, in terms of effector molecules and downstream signals. One of the key findings is the tissue specificity of cell death responses, a feature that could be exploited in strategies to widen the therapeutic window of combination cancer therapies. Efforts continue to develop p53-targeted cancer therapy, and novel clues to enhance or block specific effectors may improve therapeutic design.

The bile acid glycochenodeoxycholate induces trail-receptor 2/DR5 expression and apoptosis

Toxic bile salts induce hepatocyte apoptosis by both Fas-dependent and -independent mechanisms. In this study, we examined the cellular mechanisms responsible for Fas-independent, bile acid-mediated apoptosis. HuH-7 cells, which are known to be Fas deficient, were stably transfected with the sodium-dependent bile acid transporting polypeptide. The toxic bile acid glycochenodeoxycholate (GCDC)-induced apoptosis in these cells in a time- and concentration-dependent manner. Apoptosis and mitochondrial cytochrome c release were inhibited by transfection with dominant negative FADD, CrmA transfection, or treatment with the selective caspase 8 inhibitor IETD-CHO. These observations suggested the Fas-independent apoptosis was also death receptor mediated. Reverse transcriptase-polymerase chain reaction demonstrated tumor necrosis factor-R1, tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-R1/DR4, -R2/DR5, and TRAIL, but not tumor necrosis factor-alpha expression by these cells. GCDC treatment increased expression of TRAIL-R2/DR5 mRNA and protein 10-fold while expression of TRAIL-R1 was unchanged. Furthermore, aggregation of TRAIL-R2/DR5, but not TRAIL-R1/DR4 was observed following GCDC treatment of the cells. Induction of TRAIL-R2/DR5 expression and apoptosis by bile acids provides new insights into the mechanisms of hepatocyte apoptosis and the regulation of TRAIL-R2/DR5 expression.

Identification of inhibitors of TRAIL-induced death (ITIDs) in the TRAIL-sensitive colon carcinoma cell line SW480 using a genetic approach

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in tumor cell lines, whereas normal cells appear to be protected from its cytotoxic effects. Therefore TRAIL holds promise as a potential therapeutic agent against cancer. To elucidate some of the critical factors that contribute to TRAIL resistance, we performed a genetic screen in the human colon carcinoma cell line SW480 by infecting this TRAIL-sensitive cell line with a human placental cDNA retroviral library and isolating TRAIL-resistant clones. Characterization of the resulting clones for inhibitors of TRAIL-induced death (ITIDs) led to the isolation of c-FLIP(S), Bax inhibitor 1, and Bcl-XL as candidate suppressors of TRAIL signaling. We have demonstrated that c-FLIP(S) and Bcl-XL are sufficient when overexpressed to convey resistance to TRAIL treatment in previously sensitive cell lines. Furthermore both c-FLIP(S) and Bcl-XL protected against overexpression of the TRAIL receptors DR4 and KILLER/DR5. When c-FLIP(S) and Bcl-XL were overexpressed together in SW480 and HCT 116, an additive inhibitory effect was observed after TRAIL treatment suggesting that these two molecules function in the same pathway in the cell lines tested. Furthermore, we have demonstrated for the first time that a proapoptotic member of the Bcl-2 family, Bax, is required for TRAIL-mediated apoptosis in HCT 116 cells. Surprisingly, we have found that the serine/threonine protein kinase Akt, which is an upstream regulator of both c-FLIP(S) and Bcl-XL, is not sufficient when overexpressed to protect against TRAIL in the cell lines tested. These results suggest a key role for c-FLIP(S), Bcl-XL, and Bax in determining tumor cell sensitivity to TRAIL.

Activation of the nitric oxide synthase pathway represents a key component of tumor necrosis factor-related apoptosis-inducing ligand-mediated cytotoxicity on hematologic malignancies

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induced both cytotoxic (apoptosis) and cytostatic (cell cycle perturbation) effects on the human myeloid K562 cell line. TRAIL stimulated caspase 3 and nitric oxide synthase (NOS) activities, and both pathways cooperate in mediating inhibition of K562 survival/growth. This was demonstrated by the ability of z-VAD-fmk, a broad inhibitor of effector caspases, and N-nitro-L-arginine methyl ester (L-NAME), an NOS pharmacologic inhibitor, to completely (z-VAD-fmk) or partially (L-NAME) suppress the TRAIL-mediated inhibitory activity. Moreover, z-VAD-fmk was able to block TRAIL-mediated apoptosis and cell cycle abnormalities and increase of NOS activity. The addition of the NO donor sodium nitroprusside (SNP) to K562 cells reproduced the cytostatic effect of TRAIL without inducing apoptosis. When TRAIL was associated to SNP, a synergistic increase of apoptosis and inhibition of clonogenic activity was observed in K562 cells as well as in other myeloblastic (HEL, HL-60), lymphoblastic (Jurkat, SupT1), and multiple myeloma (RPMI 8226) cell lines. Although SNP greatly augmented TRAIL-mediated antileukemic activity also on primary leukemic blasts, normal erythroid and granulocytic cells were less sensitive to the cytotoxicity mediated by TRAIL with or without SNP. These data indicate that TRAIL promotes cytotoxicity in leukemic cells by activating effector caspases, which directly lead to apoptosis and stimulate NO production, which mediates cell cycle abnormalities. Both mechanisms seem to be essential for TRAIL-mediated cytotoxicity.

Suppression of tumor growth following intralesional therapy with TRAIL recombinant adenovirus

TRAIL is a member of the tumor necrosis factor superfamily that induces apoptosis in a variety of tumor cell types both in vitro and in vivo, while demonstrating minimal cytotoxicity toward normal tissues. One disadvantage to previous in vivo protocols was the need for large quantities of TRAIL to suppress tumor growth. Here we engineered a replication-deficient adenovirus to encode human TNFSF10 (Ad5-TRAIL) as an alternative to recombinant, soluble TRAIL protein. The results show that TRAIL-sensitive prostate tumor cell targets infected with Ad5-TRAIL undergo apoptosis through the production and expression of TRAIL protein. This activity was limited to TRAIL-sensitive tumor cells, as normal prostate epithelial cells were not killed by Ad5-TRAIL. Furthermore, in vivo administration of Ad5-TRAIL at the site of tumor implantation suppressed the outgrowth of human prostate tumor xenografts in SCID mice. Histologic examination of prostate tumors treated locally with Ad5-TRAIL revealed areas of apoptosis within 24 hours of injection. These results further define Ad5-TRAIL as a novel anti-tumor therapeutic and demonstrate its potential use as a means for treating prostate tumors, as well as other solid tumors, in vivo.

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

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

Tumoricidal activity of a novel anti-human DR5 monoclonal antibody without hepatocyte cytotoxicity

A novel anti-human DR5 monoclonal antibody, TRA-8, induces apoptosis of most tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-sensitive tumor cells both in vitro and in vivo. In contrast to both the membrane-bound form of human TRAIL, which induced severe hepatitis in mice, and the soluble form of human TRAIL, which induced apoptosis of normal human hepatocytes in vitro, TRA-8 did not induce significant cell death of normal human hepatocytes. However, both primary hepatocellular carcinoma cells and an established liver cancer cell line were highly susceptible to the killing mediated by TRA-8. We show here that elevated levels of cell-surface expression of DR5 and increased susceptibility to DR5-mediated apoptosis are characteristics of malignant tumor cells. In contrast, DR5 alone is not sufficient to trigger apoptosis of normal hepatocytes. Therefore, selective, specific targeting of DR5 with an agonistic antibody might be a safe and effective strategy for cancer therapy.

Potential and caveats of TRAIL in cancer therapy

Induction of apoptosis in tumor cells is a major goal for chemotherapy and radiation treatment strategies. However, disordered gene expression often leads to apoptosis resistance rendering tumor cells insensitive to various conventional treatments. TNF-related apoptosis-inducing ligand (TRAIL) is a recently identified cytokine of the TNF superfamily that induces apoptosis in tumor cells upon binding to different receptors. Remarkably, the majority of tumor cell lines are sensitive to TRAIL-induced apoptosis, while most nontransformed cell types are TRAIL-resistant. Furthermore, a combination treatment of TRAIL with ionizing irradiation or chemotherapeutic agents induces apoptosis in a highly synergistic manner, particularly in those cells that are otherwise resistant to a sole treatment. In contrast to other TNF members, TRAIL apparently does not exert overt systemic toxicity in murine and primate models, although unexpected concerns about a potential hepatotoxicity of TRAIL have been recently raised. While the molecular mechanisms of TRAIL sensitivity and resistance are poorly understood, TRAIL seems to be a promising biological agent for combination therapy with chemotherapeutic drugs or irradiation.

[Haptoglobin and orosomucoid in the blood in a case of congenital afibrinogenemia and in the family of the patient]

Background

TRAIL (tumor necrosis factor related apoptosis inducing ligand) is an apoptosis inducing ligand with high specificity for malignant cell systems. Combined treatment modalities using TRAIL and cytotoxic drugs revealed highly additive effects in different tumour cell lines. Little is known about the efficacy and underlying mechanistic effects of a combined therapy using TRAIL and ionising radiation in solid tumour cell systems. Additionally, little is known about the effect of TRAIL combined with radiation on normal tissues.

Methods

Tumour cell systems derived from breast- (MDA MB231), lung- (NCI H460) colorectal- (Colo 205, HCT-15) and head and neck cancer (FaDu, SCC-4) were treated with a combination of TRAIL and irradiation using two different time schedules. Normal tissue cultures from breast, prostate, renal and bronchial epithelia, small muscle cells, endothelial cells, hepatocytes and fibroblasts were tested accordingly. Apoptosis was determined by fluorescence microscopy and western blot determination of PARP processing. Upregulation of death receptors was quantified by flow cytometry.

Results

The combined treatment of TRAIL with irradiation strongly increased apoptosis induction in all treated tumour cell lines compared to treatment with TRAIL or irradiation alone. The synergistic effect was most prominent after sequential application of TRAIL after irradiation. Upregulation of TRAIL receptor DR5 after irradiation was observed in four of six tumour cell lines but did not correlate to tumour cell sensitisation to TRAIL. TRAIL did not show toxicity in normal tissue cell systems. In addition, pre-irradiation did not sensitise all nine tested human normal tissue cell cultures to TRAIL.

Conclusions

Based on the in vitro data, TRAIL represents a very promising candidate for combination with radiotherapy. Sequential application of ionising radiation followed by TRAIL is associated with an synergistic induction of cell death in a large panel of solid tumour cell lines. However, TRAIL receptor upregulation may not be the sole mechanism by which sensitation to TRAIL after irradiation is induced.