The CD95 (APO-1/Fas) and the TRAIL (APO-2L) apoptosis systems

Heat shock protein 70 (hsp70) is a stress-inducible protein that prevents apoptosis induced by a wide range of cytotoxic agents by an as yet undefined mechanism. The caspase family of cysteine proteases have been attributed a central role in the execution of apoptosis. However, several cases of caspase-independent apoptosis have been recently reported, suggesting that caspases may not be necessary for apoptosis in all cells. This study examines the protective role of hsp70 in both caspase-dependent and -independent apoptosis. Hydrogen peroxide (H2O2) used at low and high concentrations in Jurkat T cells induces caspase-dependent and -independent apoptosis, respectively. A hsp70-transfected Jurkat clone was used to observe the protection mediated by hsp70 during these two forms of apoptosis. Results reveal that hsp70 inhibits both caspase-dependent and -independent apoptosis. Furthermore, measurement of caspase-3 activity during caspase-dependent apoptosis revealed that caspase activation was inhibited in hsp70 transfectants. Early apoptotic events, such as mitochondrial depolarization, cytochrome c release, and increased intracellular calcium, were demonstrated to be common to both caspase-dependent and -independent H2O2-induced apoptosis. The inhibition of these events by hsp70 suggests that hsp70 may be an important anti-apoptotic regulator, functioning at a very early stage in the apoptotic pathway.     

TRAIL/Apo-2 ligand induces primary plasma cell apoptosis

Apoptosis constitutes the primary mechanism by which noncycling plasma cells are eliminated after the secretion of Ag-specific Abs in a humoral immune response. The underlying mechanism is not known. Here, we demonstrate that the expression of both TRAIL/Apo-2 ligand and the death receptors (DR) DR5 and DR4, but not Fas, are sustained in IL-6-differentiated Ig-secreting human plasma cells as well as primary mouse plasma cells generated in a T-dependent immune response. Plasma cell apoptosis is induced by both endogenous and exogenous TRAIL ex vivo, suggesting that TRAIL-mediated killing may, in part, be plasma cell autonomous. By contrast, resting and activated B cells are resistant to TRAIL killing despite comparable expression of TRAIL and DRs. The preferential killing of plasma cells by TRAIL correlates with decreased expression of CD40 and inactivation of NF-kappaB. These results provide the first evidence that primary plasma cells synthesize TRAIL and are direct targets of TRAIL-mediated apoptosis, which may relate to the inactivation of the NF-kappaB survival pathway.     

IFN-beta pretreatment sensitizes human melanoma cells to TRAIL/Apo2 ligand-induced apoptosis

All human melanoma cell lines (assessed by annexin V and TUNEL assays) were resistant to apoptosis induction by TRAIL/Apo2L protein. TRAIL/Apo2L activated caspase-8 and caspase-3, but subsequent apoptotic events such as poly(ADP-ribose) polymerase cleavage and DNA fragmentation were not observed. To probe the molecular mechanisms of cellular resistance to apoptosis, melanoma cell lines were analyzed for expression of apoptosis regulators (apoptotic protease-associated factor-1, FLIP, caspase-8, caspase-9, caspase-3, cellular inhibitor of apoptosis, Bcl-2, or Bax); no correlation was observed. TRAIL/Apo2L was induced in melanoma cell lines by IFN-beta and had been correlated with apoptosis induction. Because IFN-beta induced other gene products that have been associated with apoptosis, it was postulated that one or more IFN-stimulated genes might sensitize cells to TRAIL/Apo2L. Melanoma cell lines were treated with IFN-beta for 16-24 h before treatment with TRAIL/Apo2L. Regardless of their sensitivity to either cytokine alone, >30% of cells underwent apoptosis in response to the combined treatment. Induction of apoptosis by IFN-beta and TRAIL/Apo2L in combination correlated with synergistic activation of caspase-9, a decrease in mitochondrial potential, and cleavage of poly(ADP-ribose) polymerase. Cleavage of X-linked inhibitor of apoptosis following IFN-beta and TRAIL/Apo2L treatment was observed in sensitive WM9, A375, or WM3211 cells but not in resistant WM35 or WM164 cells. Thus, in vitro IFN-beta and TRAIL/Apo2L combination treatment had more potent apoptotic and anti-growth effects when compared with either cytokine alone in melanoma cells lines.     

Bcl-2 and Fas/APO-1 regulate distinct pathways to lymphocyte apoptosis.

Activation of the cell surface receptor Fas/APO-1 (CD95) induces apoptosis in lymphocytes and regulates immune responses. The cytoplasmic membrane protein Bcl-2 inhibits lymphocyte killing by diverse cytotoxic agents, but we found it provided little protection against Fas/APO-1-transduced apoptosis in B lymphoid cell lines, thymocytes and activated T cells. In contrast, the cowpox virus protease inhibitor CrmA blocked Fas/APO-1-transduced apoptosis, but did not affect cell death induced by gamma-radiation or serum deprivation. Signalling through Fas/APO-1 did not down-regulate Bcl-2 or induce its antagonists Bax and Bcl-xS. In Fas/APO-1-deficient lpr mice, Bcl-2 transgenes markedly augmented the survival of antigen-activated T cells and the abnormal accumulation of lymphocytes (although they did not interfere with deletion of auto-reactive cells in the thymus). These data raise the possibility that Bcl-2 and Fas/APO-1 regulate distinct pathways to lymphocyte apoptosis.     

Differential gene expression in anticancer drug- and TRAIL-mediated apoptosis in renal cell carcinomas

Renal cell carcinomas (RCC) exhibit marked differences in susceptibility towards anticancer drug- and TRAIL-induced apoptosis. However, the underlying mechanisms determining apoptosis-sensitivity or -resistance are not well understood. The purpose of this study was to compare gene expression patterns induced by DNA-damage- and death receptor-induced apoptosis and to detect differentially expressed genes responsible for differences in apoptosis-susceptibility. Therefore, we performed a comparative cDNA-array analysis in an apoptosis-resistant and an apoptosis-sensitive RCC cell line. In the sensitive cell line an upregulation of multiple E2F1- and p53-inducible proapaptotic and cell-cycle regulating target genes by Topotecan as well as TRAIL was observed. Interestingly, several antiapoptotic NFκB-dependent target genes were also induced. In the resistant cell line, however, only a small number of E2F1-, p53- and NFκB-dependent target genes were differentially regulated. Conclusively, anticancer drug- as well as TRAIL-sensitivity go along with an upregulation of multiple proapoptotic genes. In contrast, the mechanisms of apoptosis-resistance are—at least in part—located upstream of gene induction and seem not to depend upon upregulation of de-novo-synthesized antiapoptotic genes. Conclusively, the proapoptotic stimuli are confronted with a cellular context which allows apoptosis to be conducted—in the sensitive cell line—or not—in the resistant cell line. Electronic Supplementary Material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. Sebastian Heikaus and Ercan Casliskan contributed equally to this work. This work was supported by the ’Deutsche Forschungsgemeinschaft (DFG).     

Apo2L/TRAIL is an indirect mediator of apoptosis induced by interferon-alpha in human myeloma cells

Interferon-alpha (IFN-alpha) is currently used for the therapy of multiple myeloma (MM) though it is only effective in some patients. IFN-alpha induces apoptosis in some MM cell lines and it has been proposed to occur through an autocrine loop involving Apo2L/TRAIL. We have analysed the sensitivity to IFN-alpha and Apo2L/TRAIL of five MM cell lines and found no correlation between the apoptosis inducing ability of both cytokines. IFN-alpha-induced apoptosis in MM cells was not prevented by a caspase-8 selective inhibitor (Z-IETD-fmk) or blocking Apo2L/TRAIL. However, human monocytes treated with IFN-alpha release bioactive Apo2L/TRAIL to culture media which was cytotoxic for MM cells resistant to IFN-alpha. We propose that Apo2L/TRAIL released from IFN-alpha-stimulated blood monocytes would be a major mediator of the anti-myeloma effect of IFN-alpha in vivo.     

Arsenic trioxide-induced apoptosis and its enhancement by buthionine sulfoximine in hepatocellular carcinoma cell lines

We treated four hepatocellular carcinoma cell lines, HLE, HLF, HuH7, and HepG2 with ATO and demonstrated that arsenic trioxide (ATO) at low doses (1--3 muM) induced a concentration-dependent suppression of cell growth in HLE, HLF, and HuH7. HLE cells underwent apoptosis at 2 microM ATO, which was executed by the activation of caspase-3 through the mitochondrial pathway mediated by caspase-8 activation and Bid truncation. When these cell lines were exposed to ATO in combination with l-S,R-buthionine sulfoximine (BSO) which inhibits GSH synthesis, a synergistic growth suppression was induced, even in HepG2 showing a lower sensitivity to ATO than other cell lines tested. The intracellular GSH levels after the treatment with ATO plus BSO were considerably decreased in HLE cells compared with those after the treatment with ATO or BSO alone. The production of reactive oxygen species (ROS) which was examined by 2' ,7' -dichlorodihydrofluorescein diacetate, increased significantly after the treatment with ATO plus BSO in HLE cells. These findings indicate that ATO at low concentrations induces growth inhibition and apoptosis, and furthermore that the ATO-BSO combination treatment enhances apoptosis through increased production of ROS in hepatocellular carcinoma cells.     

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.     

TRAIL pathway components and their putative role in granulosa cell apoptosis in the human ovary

Extensive apoptotic oocyte reduction occurs during fetal ovarian development. The regulatory pathways responsible for oocyte selection to programmed cell death are, however, poorly understood. The aim of this study was to investigate the potential involvement of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its death receptors TRAIL-R1/DR4 and TRAIL-R2/DR5 and decoy receptors TRAIL-R3/DcR1 and TRAIL-R4/DcR2 in the apoptotic process characterizing human fetal and adult ovaries. For this purpose, in situ hybridization and immunohistochemistry were applied to human fetal and adult ovarian samples to study the mRNA and protein expression of TRAIL pathway components, and a human granulosa cell tumor-derived cell line (KGN) was used to elucidate functional effects of TRAIL on apoptosis. TRAIL was expressed in human fetal ovary from the 11th week until term. The pro-apoptotic TRAIL-R2/DR5 and the anti-apoptotic TRAIL-R4/DcR2 were also expressed in human ovaries throughout the fetal period. Among the different ovarian cell types, these TRAIL pathway components were mainly localized in the oocytes, and their expression increased towards term. Expression of TRAIL-R1/DR4 and TRAIL-R3/DcR1 was negligible in all of the fetal ovaries studied. Adult ovaries expressed TRAIL, TRAIL-R2/DR5, TRAIL-R3/DcR1 and TRAIL-R4/DcR2 in granulosa cells and oocytes of small primary/secondary follicles as well as in granulosa and theca cells of more developed antral follicles. In KGN cells, TRAIL efficiently induced apoptosis in a dose-dependent manner, and this was blocked by a caspase inhibitor. The results indicate a role of the TRAIL pathway components in the regulation of granulosa cell apoptosis in in vitro and suggest that these factors may have a role in regulating ovarian apoptosis also in vivo.     

Reduction of the antiapoptotic protein cFLIP enhances the susceptibility of human renal cancer cells to TRAIL apoptosis

Human renal carcinoma cells (RCCs) were sensitized to the apoptotic effects of tumor necrosis factor (TNF)–related apoptosis-inducing ligand (TRAIL), by treatment with cycloheximide (CHX). In contrast to a previous study, a rapid and dramatic decrease in levels of cellular FLICE (Fas-associated death domain–like IL-1–converting enzyme) inhibitory protein (cFLIP) following cycloheximide treatment was observed in all RCCs studied. The unambiguous detection of this decrease in cFLIP was dependent on the quality of the particular antibody preparation used to detect cFLIP. Cycloheximide treatment caused no major change in levels of pro-caspase-8 or cell surface expression of TRAIL receptors. Therefore, cycloheximide treatment resulted in an increase in the pro-caspase-8 to cFLIP ratio, which correlated with sensitization to TRAIL-mediated apoptosis. Furthermore, treatment of human RCCs with small interfering oligoribonucleotides (siRNA) for cFLIP caused a reduction of cFLIP protein and sensitized cells to TRAIL-mediated apoptosis. We concluded that in the presence of an intact TRAIL signaling pathway, a significant reduction of cFLIP alone is sufficient to sensitize human RCCs to TRAIL apoptosis.The publisher or recipient acknowledges the right of the US Government to retain a nonexclusive, royalty-free license in and to any copyright covering the article.     

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.     

Sensitivity of myofibroblasts to H2O2-mediated apoptosis and their antioxidant cell network

During wound healing, the transition from granulation to scar tissue shows a decrease in myofibroblast cellularity. Previous results have correlated the disappearance of these cells with the induction of apoptotic cell death by some unknown stimuli. In contrast, hypertrophic scar appearance after wound healing is thought to be linked to a disorder of apoptotic function which induces myofibroblast persistence in granulation tissue. Oxidative stress being an important mediator of apoptosis, we have evaluated the apoptotic response of normal and pathological wound myofibroblasts (WMyo and HMyo respectively) in their interaction with two oxidative stress inducers: hydrogen peroxide, using a high concentration as a single dose, and sodium ascorbate which induced a continuous release of H2O2 at a low concentration. Our results showed that, according to the H2O2 treatment type, HMyo were more sensitive (after ascorbate treatment) or less sensitive (after H2O2 treatment) when compared to WMyo and Fb. We next assessed the presence of several molecules known to be involved in the antioxidant network protecting cells against H2O2 injury and found HMyo to have a higher level of activity of glutathione peroxidase and a lower level of activity of catalase than WMyo. These results can help explain the contradictory responses of myofibroblasts according to the oxidative stress treatment. This is the first study linking refractory oxidative stress mediated cell death to cellular phenotype in hypertrophic myofibroblasts, and indicates a pivotal role for the antioxidant enzyme system in this type of resistance.     

Induction of necrotic tumor cell death by TRAIL/Apo-2L

A great deal of enthusiasm is being generated for TRAIL (TNF-related apoptosis-inducing ligand)/Apo-2L as a tumor therapeutic agent because it is cytotoxic to a variety of tumor cell types but not normal cells. Moreover, it is well documented that TRAIL/Apo-2L-induced tumor cell death is a caspase-dependent apoptotic process. Through the use of a transfected cell line expressing murine TRAIL/Apo-2L and a recombinant adenovirus encoding the murine TRAIL/Apo-2L cDNA (Ad5-mTRAIL) against two murine tumor cell lines [TRAMP-C2 (prostate adenocarcinoma) and Renca (renal adenocarcinoma)], we found that mTRAIL/Apo-2L also can kill tumor cells by inducing necrosis. Specifically, we observed the default method of mTRAIL/Apo-2L-induced death in TRAMP-C2 cells was via a necrotic process, characterized by the complete lack of an annexin V⁺/PI^– population, SAPK/JNK phosphorylation, caspase activation, Bid cleavage, or cytochrome c release. Moreover, the inclusion of zVAD-fmk, an inhibitor of caspase activation, markedly enhanced mTRAIL/Apo-2L-mediated killing of TRAMP-C2. In contrast, apoptosis was induced in TRAMP-C2 using TNF, as measured by the criteria listed above, as was Renca by mTRAIL/Apo-2L. These results demonstrate the natural occurrence of both TRAIL/Apo-2L-induced apoptotic and necrotic signaling mechanisms within tumor cells.     

A role for TRAIL/TRAIL-R2 in radiation-induced apoptosis and radiation-induced bystander response of human neural stem cells

Adult neurons, which are terminally differentiated cells, demonstrate substantial radioresistance. In contrast, human neural stem cells (NSC), which have a significant proliferative capacity, are highly sensitive to ionizing radiation. Cranial irradiation that is widely used for treatment of brain tumors may induce death of NSC and further cause substantial cognitive deficits such as impairing learning and memory. The main goal of our study was to determine a mechanism of NSC radiosensitivity. We observed a constitutive high-level expression of TRAIL-R2 in human NSC. On the other hand, ionizing radiation through generation of reactive oxygen species targeted cell signaling pathways and dramatically changed the pattern of gene expression, including upregulation of TRAIL. A significant increase of endogenous expression and secretion of TRAIL could induce autocrine/paracrine stimulation of the TRAIL-R2-mediated signaling cascade with activation of caspase-3-driven apoptosis. Furthermore, paracrine stimulation could initiate bystander response of non-targeted NSC that is driven by death ligands produced by directly irradiated NSC. Experiments with media transfer from directly irradiated NSC to non-targeted (bystander) NSC confirmed a role of secreted TRAIL for induction of a death signaling cascade in non-targeted NSC. Subsequently, TRAIL production through elimination of bystander TRAIL-R-positive NSC might substantially restrict a final yield of differentiating young neurons. Radiation-induced TRAIL-mediated apoptosis could be partially suppressed by anti-TRAIL antibody added to the cell media. Interestingly, direct gamma-irradiation of SK-N-SH human neuroblastoma cells using clinical doses (2–5 Gy) resulted in low levels of apoptosis in cancer cells that was accompanied however by induction of a strong bystander response in non-targeted NSC. Numerous protective mechanisms were involved in the maintenance of radioresistance of neuroblastoma cells, including constitutive PI3K-AKT over-activation and endogenous synthesis of TGFβ1. Specific blockage of these survival pathways was accompanied by a dramatic increase in radiosensitivity of neuroblastoma cells. Intercellular communication between cancer cells and NSC could potentially be involved in amplification of cancer pathology in the brain.     

Potential mechanisms of resistance to TRAIL/Apo2L-induced apoptosis in human promyelocytic leukemia HL-60 cells during granulocytic differentiation

Human promyelocytic leukemia HL-60 cells are well known to differentiate into granulocytes or monocytes in the presence of some agents such as DMSO or PMA, respectively. Differentiated HL-60 cells become resistant to some apoptotic stimuli including anticancer drugs or irradiation though undifferentiated cells significantly respond to these stimuli. TRAIL (TNF-related apoptosis-inducing ligand) which is also known as Apo2 ligand (Apo2L), a new member of TNF family, can induce apoptosis in some tumor cells but not in many normal cells. We show here that apoptosis is well induced in HL-60 cells by TRAIL, but susceptibility to TRAIL is reduced during granulocytic differentiation by DMSO. We also suggest some possible mechanisms by which granulocytic differentiated cells become resistant to TRAIL-induced apoptosis. First, in granulocytic differentiated cells, expression of antagonistic decoy receptors for TRAIL (TRAIL-R3/TRID/DcR1/LIT and TRAIL-R4/TRUNDD/DcR2) were enhanced. In addition, expression of Toso, a cell surface apoptosis regulator, seemed to block activation of caspase-8 by TRAIL via enhanced expression of FLIPL in granulocytic differentiated cells. These findings suggest that differentiated cells are resistant using plural mechanisms against various apoptosis-inducing stimuli rather than undifferentiated cells.     

TRAIL induces apoptosis and inflammatory gene expression in human endothelial cells

Human TRAIL can efficiently kill tumor cells in vitro and kill human tumor xenografts in mice with little effect on normal mouse cells or tissues. The effects of TRAIL on normal human tissues have not been described. In this study, we report that endothelial cells (EC), isolated from human umbilical veins or human dermal microvessels, express death domain-containing TRAIL-R1 and -R2. Incubation with TRAIL for 15 h causes approximately 30% of cultured EC to die, as assessed by propidium iodide uptake. Death is apoptotic, as assessed by Annexin V staining, 4',6'-diamidino-2-phenylindole staining, and DNA fragment ELISA. EC death is increased by cotreatment with cycloheximide but significantly reduced by caspase inhibitors or transduced dominant-negative Fas-associated death domain protein. In surviving cells, TRAIL activates NF-kappaB, induces expression of E-selectin, ICAM-1, and IL-8, and promotes adhesion of leukocytes. Injection of TRAIL into human skin xenografts promotes focal EC injury accompanied by limited neutrophil infiltration. These data suggest that TRAIL is an inducer of tissue injury in humans, an outcome that may influence antitumor therapy with TRAIL.     

Apo2L/TRAIL and its death and decoy receptors

Apo2 ligand or tumour necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is one of the several members of the tumour necrosis factor (TNF) gene superfamily that induce apoptosis through engagement of death receptors (DRs). Apo2L/TRAIL interacts with an unusually complex receptor system of two DRs and three decoys. This protein has garnered intense interest as a potential candidate for cancer therapy because as a trimer it selectively induces apoptosis in many transformed cells but not in normal cells. While much of the early characterisation of Apo2L/TRAIL and its receptors relied on overexpression studies, recent work using untransfected cells has clarified how endogenous proteins transmit apoptotic signals from this ligand. In this review, we focus on the apoptotic signalling pathways stimulated by Apo2L/TRAIL and summarise what is known about its physiological role.     

A caspase-8-independent component in TRAIL/Apo-2L-induced cell death in human rhabdomyosarcoma cells

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) belongs to the Tumor necrosis factor (TNF) family of death-inducing ligands, and signaling downstream of TRAIL ligation to its receptor(s) remains to be fully elucidated. Components of the death-inducing signaling complex (DISC) and TRAIL signaling downstream of receptor activation were examined in TRAIL - sensitive and -resistant models of human rhabdomyosarcoma (RMS). TRAIL ligation induced DISC formation in TRAIL-sensitive (RD, Rh18, Rh30) and TRAIL-resistant RMS (Rh28, Rh36, Rh41), with recruitment of FADD and procaspase-8. In RD cells, overexpression of dominant-negative FADD (DNFADD) completely abolished TRAIL-induced cell death in contrast to dominant-negative caspase- 8 (DNC8), which only partially inhibited TRAIL-induced apoptosis, growth inhibition, or loss in clonogenic survival. DNC8 did not inhibit the cleavage of Bid or the activation of Bax. Overexpression of Bcl-2 or Bcl-xL inhibited TRAIL-induced apoptosis, growth inhibition, and loss in clonogenic survival. Bcl-2 and Bcl-xL, but not DNC8, inhibited TRAIL-induced Bax activation. Bcl-xL did not inhibit the early activation of caspase-8 (<4 h) but inhibited cleavage of Bid, suggesting that Bid is cleaved downstream of the mitochondria, independent of caspase-8. Exogenous addition of sphingosine also induced activation of Bax via a caspase-8-and Bid-independent mechanism. Further, inhibition of sphingosine kinase completely protected cells from TRAIL-induced apoptosis. Data demonstrate that in RMS cells, the TRAIL signaling pathway circumvents caspase-8 activation of Bid upstream of the mitochondria and that TRAIL acts at the level of the mitochondria via a mechanism that may involve components of the sphingomyelin cycle.