Promises and Failures of Death Receptors and Ligands in Cancer Therapy PT Daniel1,2, T Wieder1, I Sturm1,2 and K Schulze-Osthoff3

The kiss of death: promises and failures of death receptors and ligands in cancer therapy PT Daniel1,2, T Wieder1, I Sturm1,2 and K Schulze-Osthoff3

1Department of Hematology, Oncology, and Tumor Immunology, University Medical Center Charite´, Campus Berlin-Buch; 2Max Delbru¨ck Center for Molecular Medicine, Berlin and; 3Department of Immunology and Cell Biology, University of Mu¨nster, Germany

Death receptors and their ligands exert important regulatory 6 (DR6) (Figure 1). While the role of TRAMP and DR6 is functions in the maintenance of tissue homeostasis and the not well known, both the biology and the pathophysiology physiological regulation of programmed cell death. Currently, six different death receptors are known including tumor of the TNF and TRAIL receptors and CD95/Fas have been necrosis factor (TNF) receptor-1, CD95 (Fas/APO-1), TNF recep- extensively investigated. tor-related apoptosis-mediating protein (TRAMP), TNF-related The cognate death ligands can be expressed by various cell apoptosis-inducing ligand (TRAIL) receptor-1 and -2, and death types. CD95 ligand (CD95L) and TNF are mainly found in receptor-6 (DR6). The signaling pathways by which these activated lymphocytes and macrophages, respectively, receptors induce apoptosis are similar and rely on oligomeriz- whereas TRAIL is expressed by various cell types. A high ation of the receptor by death ligand binding, recruitment of an adapter protein through homophilic interaction of cytoplasmic constitutive expression of CD95L is detected in Sertoli cells of domains, and subsequent activation of an inducer caspase the testis and epithelial cells of the anterior eye chamber, which initiates execution of the cell death programme. The leading to the proposal that CD95L is responsible for the ability of these receptors and their ligands to kill malignant maintenance of immune-privileged sites. The triggering of cells was discovered early and helped to coin the term ‘tumor CD95 by agonistic antibodies or CD95L may prevent rejection necrosis factor’ for the first identified death ligand. This review of transplanted tissues or attenuate autoimmune diseases. Our summarizes the current and rapidly expanding knowledge about the signaling pathways triggered by death understanding of the function of CD95 and its ligand has been receptor/ligand systems, their potency in experimental cancer mainly elucidated by the finding that both molecules are therapy, and their therapeutic limitations, especially regarding mutated in mouse strains suffering from autoimmune diseases. their toxicity for non-malignant cells. Leukemia (2001) 15, Lpr mice which lack a functional CD95 receptor as well as 1022–1032. gld mice which bear a mutant CD95 ligand exhibit various Keywords: apoptosis; toxicity; signaling; TNF; CD95/Fas; TRAIL autoimmune phenomena resembling systemic lupus ery- thematosus. Defects in the CD95/CD95L system strongly dis- turb the immune system and result in the massive expansion Introduction of resting lymphoid cells accompanied by lymphadenopathy, splenomegaly, hypergammaglobulinemia and other signs of Members of the TNF receptor superfamily form a group of autoimmune disorders. In humans, a similar disease, called currently more than 20 proteins with a wide range of different ALPS (autoimmune lymphoproliferative syndrome) or Canale- functions, such as regulation of cell survival, differentiation, Smith syndrome, has been found. ALPS can be subdivided immune regulation and control of gene expression. Their into (1) type I ALPS with mutant CD95 receptor (type Ia2,3)or common structural feature is the presence of cysteine-rich CD95 ligand (type Ib4) which impair recruitment of the FADD domains which occur in two to six repeats in their extracellu- adapter protein,5 (2) type II ALPS with inactivation of signaling lar region. Among the TNF receptor-related molecules, a sub- molecules such as caspase-10,6 and (3) type III ALPS (also group has been defined. These receptors are referred to as termed ALD for autoimmune lymphoproliferative disease)7 death receptors since one of their most prominent function is with yet to be defined molecular aberrations, presumably in to induce programmed cell death.1 Death receptors are struc- further downstream signaling modules.4 ALPS may be cured turally characterized by a similar intracellular region of about by bone marrow transplantation.8 Interestingly, sporadic 80 amino acids, designated the death domain (DD), because development of lymphomas has been reported in some famil- it is required for the transmission of the cytotoxic signal. Hom- ies of ALPS patients. Nevertheless, the development of malig- ology searches in expressed sequence tag databases led to the nant disease is not a clear-cut feature of this syndrome, indi- identification of a number of DD-containing receptors. Cur- cating that dysregulation of the CD95 system alone is not rently, six different such death receptors (DRs) are known, sufficient for development of malignancies. The contribution including CD95 (also known as Fas or APO-1), TNF receptor- of other death receptors and their signal components remains 1, TNF receptor-related apoptosis-mediating protein (TRAMP), to be elucidated. two receptors that bind TNF-related apoptosis-inducing ligand Deregulation of death receptors and their signaling path- (TRAIL), called TRAIL-R1 and TRAIL-R2, and death receptor- ways may also contribute to other diseases as suggested for myelodysplastic syndromes (MDS)9,10 where CD95/Fas signaling in myeloid bone marrow precursors is disturbed. Most interestingly, increased CD95/FasL expression can be detected in myeloid blasts, erythroblasts, maturing myeloid cells, Correspondence: PT Daniel, Department of Hematology, Oncology, megakaryocytes and dysplastic cells from MDS patients, and Tumor Immunology, University Medical Center Charite´, Campus Berlin-Buch, Humboldt University, Lindenberger Weg 80, D-13125 whereas in AML, expression may be observed in the blasts. Berlin, Germany; Fax: +49–30–9417–1378 or -1109 CD95/Fas ligand expression correlated in one study with the Received 19 January 2001; accepted 26 March 2001 FAB subtype groups of MDS, and also correlated directly with Mini-review PT Daniel et al 1023

Figure 1 Schematic representation of death receptors and their neutralizing decoy receptors. Members of this subgroup of the TNF receptor superfamily are characterized by the intracellular death domain, depicted as a gray box. The two to four boxes in the extracellular part represent homologous cysteine-rich domains which mediate ligand binding and trimerization of the receptors. The TRAIL decoy receptors DcR1 and DcR2 lack an intracellular domain or contain a truncated non-functional death domain. Osteoprotegerin (OPG) and DcR3 are secreted. The commonly used terms for the different death receptors and decoy receptors are given. the percentage of abnormal metaphases found by cytogenetic raised when cultured normal human hepatocytes and certain analysis. In addition, the CD95/Fas ligand expressed in MDS other cell types were shown to be susceptible for TRAIL (see bone marrow was shown to inhibit the growth of clonogenic below). Nevertheless, the toxicity profile of TRAIL might be hematopoietic progenitors which could be blocked specifi- not as dangerous as in the CD95 system where apoptosis was cally by disrupting the CD95/Fas receptor/ligand interaction seen in most tissues.1,19,20 Thus, whether the TRAIL pathway by the use of blocking CD95/Fas-Fc fusion protein constructs. represents the long sought after means to selectively kill tumor This suggests that CD95/Fas is a key regulator of apoptosis cells remains to be shown. during myelopoiesis and that the characteristic dramatic increase in apoptosis of bone marrow precursors in MDS11 is, at least in part, due to overactivity of death ligands and their Structure and function of death receptors receptors in this cellular compartment. Furthermore, FasL expression in MDS bone marrow was suggested to be higher Although a principal feature of death receptors is to induce in patients who are more anemic and to correlate with red apoptosis, for most receptors functions unrelated to cell death cell transfusion requirements over the subsequent course of have been found. The (patho)-physiology of the prototype the disease10 although additional apoptosis pathways may death receptors TNF-R1 and CD95 has been intensively stud- contribute to disease pathogenesis.11,12 ied and described in several reviews.1,19–22 In contrast, the The impressive apoptosis-inducing properties of death physiological relevance of the new death receptors is largely receptors and their ligands promoted their application in unknown. TRAMP, also called DR3, WSL-1, APO-3 or LARD, experimental cancer therapy. In the early 1990s, experiments and the recently characterized death receptor-6 (DR6) are with agonistic human CD95 antibody showed very promising both structurally and functionally similar to TNF-R1, because results. A single injection of an agonistic anti-CD95 antibody their overexpression leads to nuclear factor-kappa B (NF-κB) could mediate long-term eradication of xenotransplanted activation and apoptosis.23–28 TRAMP is abundantly expressed human tumors in mice.13–15 It became, however, soon clear in thymocytes and lymphocytes and may play a role in lym- that such antibodies did not crossreact with the murine CD95 phocyte development, whereas DR6 is expressed in most receptor. Thus, the considerable toxicity of CD95 triggering human tissues. The cloning of the different TRAIL receptors on non-malignant tumor cells was overlooked, but became revealed a new apoptosis system with apparently high com- evident when a murine-specific anti-CD95 antibody was plexity. Two receptors, TRAIL-R1 (DR4, APO-2) and TRAIL- injected into mice: these animals rapidly succumbed due to R2 (DR5) contain a functional death domain and can therefore the massive liver toxicity caused by induction of apoptosis signal for apoptosis.29–34 In addition, the TRAIL system con- in hepatocytes.16 sists of three TRAIL-neutralizing decoy receptors, called DcR1 New optimism came from data obtained with a novel death (TRAIL-R3, TRID, LIT), DcR2 (TRAIL-R4) and osteoprotegerin ligand, called TRAIL (for TNF receptor-related apoptosis- (OPG, TRAIL-R5)29,35–38 (Figure 1). DcR1 contains the external inducing ligand). TRAIL has the ability to rapidly induce TRAIL-binding region as well as a region that anchors the apoptosis in a wide variety of transformed human cell lines receptor to the membrane. But unlike the other receptors, in vitro, yet no cytotoxicity is observed with most normal non- DcR1 lacks an intracellular tail needed to spark the death malignant cells.17,18 The potential therapeutic activity has pathway. DcR2 is also able to bind TRAIL but contains a been demonstrated in vivo against both human breast and truncated non-functional death domain. Finally, OPG has colon tumor xenografts grown in mice. Multiple injections of been found to bind and to neutralize TRAIL.36 The molecule TRAIL suppressed the growth of these tumors and induced was originally detected as a secreted receptor dimer involved complete tumor remission in a high proportion of tumor-bear- in inhibition of osteoclast differentiation.38 In addition, OPG ing mice. However, very recently, serious concerns were appears to be a decoy receptor for the TRANCE/RANK system

Leukemia Mini-review PT Daniel et al 1024 which not only regulates osteoclast differentiation but also mediates T cell/dendritic cell (DC) costimulatory interactions and shares functional similarities with the CD40/CD154 system. The receptor RANK (receptor activator of NF-κB) is expressed on myeloid osteoclast precursors, mature osteoclasts and DCs, and the ligand, RANKL (TRANCE/OPGL/ODF), is found on T cells.39–41 Like CD40, this receptor system lacks a death domain and has so far not been implicated in the induction of apoptosis. In contrast, it may deliver not only activation and differentiation but also survival signals to DCs, osteoclasts and their precursors. The two signaling TRAIL receptors, DR4/TRAIL-R1 and DR5/TRAIL-R2, induce apoptosis in various cancer cells, whereas the decoy receptors, which are mainly expressed on normal cells, confer resistance against TRAIL. Furthermore, a CD95 ligand-neutralizing decoy receptor DcR3 (TR6) has been identified.42 DcR3 lacks an apparent transmembrane sequence and is secreted. In contrast to the decoy receptors of the TRAIL system, DcR3 is expressed in tumor cells. It binds specifically two cellular ligands, CD95L and LIGHT (herpes virus entry mediator-L).43 Thus, certain tumors may escape CD95L- and LIGHT-dependent immune-cytotoxic attack by expression of DcR3.42,43 It is not surprising that, in addition to the receptors, the ligands also display structural homologies, which are reflected Figure 2 Cross-communication and putative amplification loops by similar mechanisms of receptor recognition and triggering. of apoptotic pathways. (a) One pathway (left) is induced by death Among the TNF-related cytokines, five ligands bind to death receptor ligands, such as CD95L. A death-inducing signaling complex receptors, such as CD95L, TNF, lymphotoxin-α (LTα), TRAIL consisting of CD95L, the CD95 receptor, the adapter protein FADD and TWEAK, which was recently identified as a ligand for and pro-caspase-8 is formed at the cell surface leading to activation 44–46 of the initiator caspase-8. Caspase-8 in turn cleaves and activates the DR3/TRAMP. As suggested by structural and biochemical effector caspase-3. This pathway can either be amplified by caspase- data, all active ligands consist of three identical subunits and 8-mediated cleavage of Bid (1) and by the downstream, caspase-6- activate their receptors by oligomerization.14,47 Another com- mediated cleavage of caspase-8 (19) which connect this pathway to mon feature is that, with the exception of LTα, all are type II the mitochondrial apoptosome. (b) Another, death receptor-inde- transmembrane proteins. Although the ligands are synthesized pendent pathway (right) is triggered by many apoptotic stimuli, such as membrane-bound molecules, they also exist as soluble as cytotoxic drugs, ceramide and irradiation, and leads to the loss of the mitochondrial membrane potential and release of cytochrome c forms generated by rather specific metalloproteases. There is into the cytosol, presumably via a Bax-dependent mechanism. Cyto- some evidence indicating that the membrane-bound and chrome c, together with (d)ATP, Apaf-1 and pro-caspase-9, then forms soluble forms of the ligands exert different activities and the mitochondrial apoptosome67 leading to activation of the initiator cellular functions. caspase-9 followed by activation of the effector caspase-3, -6, and -7. This pathway can either be amplified by downstream, caspase- 6-mediated cleavage of pro-caspase-8 (2) or by caspase-3-mediated cleavage of Bid (29). DD, death domain; DED, death effector domain; Signaling CARD, caspase recruitment domain. A major progress in the understanding of death receptor signaling came from the definition of protein motifs that propa- is more crucial for TNF-mediated NF-κB activation. In a gate the cytotoxic signals elicited upon receptor ligation mutant cell line deficient in RIP, CD95 signaling was not (Figure 2). One proximal motif is the so-called death domain affected, whereas TNF-R1-mediated NF-κB activation was (DD), which is crucial for triggering cell death. Two other blocked.54 Overexpression of most DD proteins causes cell important motifs include the death effector domain (DED) and death indicating that these molecules are involved in a less well-defined motif, termed the caspase recruitment apoptosis signaling. In the case of FADD, transient expression domain (CARD). The DD of death receptors is required for of the N-terminal region was sufficient to cause apoptosis.51 formation of a receptor signaling complex by homophilic This part of FADD was therefore termed the death effector recruitment of adapter proteins that also contain a DD motif. domain (DED). In contrast, overexpression of the C-terminal As revealed by NMR spectroscopy, the DD consists of a series DD-containing part lacking the DED protected cells from of α-helices with many exposed charged amino acids that CD95-mediated apoptosis and functioned as a dominant- mediate self-association of the DD following ligand binding negative mutant. and receptor oligomerization.48 Most information regarding death pathways has been Important DD proteins involved in death receptor signaling obtained from two-hybrid assays. In contrast, for CD95 the include the adapters FADD (MORT-1), RIP and TRADD. signaling complexes have been also identified in vivo. Treat- FADD has a high affinity to CD95, whereas it weakly binds ment of cells with agonistic anti-CD95 antibodies and sub- to the DD in TNF-R1.49–51 TNF-R1, in contrast, binds preferen- sequent co-immunoprecipitation led to the identification of tially to TRADD, which itself can then recruit FADD to the receptor-associated proteins in 2D protein gel electrophoresis. vicinity of TNF-R1.52 TRADD, in addition, recruits the serine Together with the receptor these proteins form the death- protein kinase RIP to TNF-R1.53 Although RIP was initially inducing signaling complex (DISC).55 Two spots were ident- suggested to play an important role in apoptotic signaling, it ified as differentially phosphorylated forms of FADD. Sequen-

Leukemia Mini-review PT Daniel et al 1025 cing of the other proteins resulted in the identification of a with Apaf-1 (apoptotic protease-activating factor-1), the mam- molecule which contained two DEDs at its N-terminus that malian homologue of the C. elegans cell death regulator Ced- associate with the DED of FADD.56 At its C-terminus, it had 469 (Figure 2). A second cofactor required for Apaf-1 function the typical catalytic domain of an ICE (interleukin-1β con- is (d)ATP. Binding of these two components presumably leads verting enzyme)-like protease and was therefore termed FLICE to a conformational change in Apaf-1 and exposes an interac- (FADD-like ICE). It belongs to the cysteine proteases of the tion motif, the caspase recruitment domain (CARD). This caspase family and is now referred to as caspase-8.57 Identifi- region serves as protein interface by binding to caspases that cation of caspase-8 as part of the DISC therefore connected have a similar domain at their N-terminus. In particular, cas- two different levels in apoptosis pathways, the receptor with pase-9 is recruited and activated by Apaf-1.70 The release of the executioner caspases mediating the terminal effector cytochrome c into the cytosol is induced by a variety of pro- phase of apoptosis. apoptotic conditions. Since relocalization of cytochrome c is Overexpression of functionally inactive FADD and caspase- observed during TNF-R1 and CD95-mediated apoptosis,71 the 8 did not only block CD95, but also inhibited TNF-R1- Apaf-1-controlled pathway may also be functional in apop- induced signaling. This suggested that both receptors are totic pathways triggered by death receptors. This process pre- coupled to a similar signaling complex. As mentioned above, sumably involves a Bid-dependent pathway leading to a con- FADD, however, does not directly bind to TNF-R1 but formational change in the pro-apoptotic mitochondrial becomes associated upon binding of the DD-containing pro- activator Bax.67,72,73 tein TRADD. Similarly to TNF-R1, TRAMP and DR6 can induce apoptosis and activate NF-κB and bind to TRADD.23,24,27 Additionally, TRAMP has been reported to Initiator and effector caspases and their targets interact with TRAF2, FADD and caspase-8.23,24 It was also found that TRAIL activates caspase-8 and the structurally simi- Caspases74 are synthesized as zymogens that contain an N- lar caspase-10 (FLICE-2).29 Whether apoptosis triggered by the terminal prodomain followed by sequences encoding a large two TRAIL-receptors is FADD-dependent, has been contro- and a small subunit with the catalytic domain. The mature versial. It was initially suggested that TRAIL-R1 and TRAIL-R2 and active form of all caspases is a heterotetrameric complex do not require FADD interaction, while other reports showed composed of two large subunits and two small subunits.75,76 that overexpression of dominant-negative FADD attenuated Caspases with large prodomains, eg caspase-2, -8 and -9, are TRAIL-triggered apoptosis.29,31,32,34 Very recently, endogenous thought to be involved in the initiation of the apoptotic FADD and caspase-8 have indeed been shown to be compo- response and are therefore called initiator caspases. Initiator nents in the native DISC of both TRAIL receptors.58–60 caspases are directly linked to different death-inducing sig- It should be noted that death receptors can recruit naling complexes, such as the CD95/Fas signaling complex additional adapter proteins, such as RAIDD and RICK, which (pro-caspase-8) or the mitochondrial apoptosome (pro-cas- bind to death receptors and induce apoptosis upon over- pase-9), by protein interaction motifs in their prodomains. expression. RAIDD carries a DD at its C-terminus, and at its Two general interactions have been described. Pro-caspase-8 N-terminus it displays homologies with the prodomain of cas- contains two tandem death effector domains and is recruited pase-2. Thus, it was suggested that RAIDD would engage to CD95 receptor via the adapter protein FADD,56 whereas TNF-R1 via RIP and induce caspase-2 activation.61 Since cas- pro-caspase-9 contains a caspase recruitment domain and is pase-2(−/−) mice, however, have no defects in death receptor specifically localized to the mitochondrial death-inducing signaling,62 the contribution of this pathway to TNF-R1 and signaling complex via the adapter protein Apaf-1.77 CD95-mediated cytotoxicity remains unclear. It was further Caspases harboring a short prodomain, eg caspase-3, -6 demonstrated that death receptors can directly trigger other and -7, are activated by initiator caspases and are mainly signaling pathways leading to reactive oxygen formation, cer- responsible for the cleavage of downstream cellular targets. amide generation and activation of protein kinase cascades. Hence, these caspases are called effector caspases. Members In addition, the C-terminal portion of the adapter protein of the caspase family build up the core complex of the death DAXX was shown to interact with the CD95 death domain, machinery and are involved in the signaling processes as well while a different region activates both the JNK kinase and as in the execution processes of apoptotic cell death. There apoptosis.63 More recent studies showed that, although are several cellular structures which have to be targeted dur- specifically enhancing CD95 function, human DAXX does not ing the execution of apoptotic cell death. In principle, cas- bind directly to CD95 but instead is found in the nucleus pase-mediated cleavage should result in different effects, such where it localizes to PML oncogenic domains (PODs).64 as (1) a halt of cell cycle progression, (2) disabling of repair Another report confirmed that DAXX may activate JNK sig- mechanisms, (3) disassembly of molecular structures, (4) cell naling but challenged the view that DAXX is functionally detachment, and (5) tagging of the apoptotic cell for involved in CD95 signaling under physiological circum- engulfment by phagocytes.78 stances.65 Thus, a single receptor is able to trigger multiple Experimental data showed that not all roads to cell death pathways which, in addition, can participate in induction of depend on the activation of caspases. Caspase-independent cell death. Despite the key role of the DD, the possibility that cell death includes necrosis but also some forms related to proteins associated with other intracellular regions of the apoptosis. For example, treatment of Molt-4 cells with low receptors contribute to the overall pattern of apoptosis cannot concentrations of membrane-permeable ceramide analogues be excluded. clearly induced apoptotic cell death which was accompanied Recent studies have shown that, besides DISC-induced cas- by cleavage of PARP and inhibited by Bcl-2. In contrast, high pase-8 activation, mitochondria play an important role in the concentrations of these synthetic ceramides induced necrosis. regulation of apoptosis. An early event in this process is the This form of cell death was not blocked by Bcl-2 over- translocation of cytochrome c from the mitochondria into the expression and PARP cleavage was not observed.79 There are cytosol, which is inhibited by anti-apoptotic Bcl-2 and hom- also reports that death receptors cannot only induce apoptosis ologous proteins.66–68 In the cytosol, cytochrome c interacts but, in addition, necrotic cell death80–82 which might be as

Leukemia Mini-review PT Daniel et al 1026 important as the induction of apoptosis, especially with regard 75 kDa receptor TNF-R2. Binding of TNF activates the tran- to cancer therapy.83 Experiments with cells from Apaf-1 and scription factor NF-κB, which in turn suppresses apoptosis97 caspase-3 knockout mice revealed the existence of two dis- and may antagonize TNF receptor-triggered cell death.98 tinct pathways leading to nuclear apoptosis.84 One caspase- Further experiments identified TNF-R-associated factor 1 dependent pathway targets caspase-activated DNase (CAD), (TRAF1), TRAF2 and the inhibitor of apoptosis proteins cIAP1 while another pathway is caspase-independent and mediated and cIAP2 as gene targets of NF-κB transcriptional activity.99 by the mitochondrial apoptosis-inducing factor (AIF).85 In this Interestingly, cIAPs are able to function as both upstream and line, it has been recently demonstrated that release of AIF into downstream effectors of TNF-induced survival pathways. Evi- the cytosol of activated blood T lymphocytes may represent dence was provided that c-IAP1 and c-IAP2 are recruited to a caspase-independent, reversible commitment phase to the TNFR2-TRAF signaling complex and thereby promote NF- apoptosis.86 κB activation.100,101 In addition, it is generally accepted that Furthermore, a new apoptosis signaling pathway originating some members of the IAP family block apoptotic events by at the endoplasmic reticulum and involving caspase-12 has direct inhibition of distinct caspases.102,103 In endothelial cells, been described.87 However, this form of cell death seems to the Bcl-2 family member A1 is induced by TNF in a NF-κB- be independent from death receptors and is activated under dependent manner and functions as a cytoprotective factor relatively rare circumstances, such as the massive disturbance that maintains mitochondrial viability and function.104 Thus, of protein synthesis or aberrant protein expression.67 Its TNF-induced survival pathways obviously interfere with cell relevance still has to be determined in more detail. death signaling at different levels of the apoptotic machinery, The two principal signaling pathways of apoptosis via death eg at the death-inducing signaling complex, the mitochondria receptors and mitochondria have been well established and and the effector caspases. intensive experimental efforts over the past years have elucidated their molecular components.88–90 Although both pathways use distinct signaling components they converge at the Death ligands in cancer therapy level of the effector caspase-3 (Figure 2). Several lines of evidence demonstrated an interconnection of the pathways. Bid, Due to their ability to kill cancer cells and to eradicate xeno- a BH3 domain-containing proapoptotic Bcl-2 family member, transplanted tumors, some of the death receptors have been is a substrate of caspase-8 in the CD95 signaling pathway.91 considered as potential targets in cancer therapy. In addition, Following caspase cleavage, truncated Bid translocates to treatment of autoimmune disorders or prevention of transplant mitochondria and induces the release of cytochrome c, ther- rejection by death receptor targeting is under investigation. In eby amplifying the apoptotic signal (Figure 2). Furthermore, the late 1980s, clinical phase I and II trials were performed by reconstituting the apoptosome in a cell-free system it was with TNF for the treatment of various cancers, eg hairy cell shown that caspase-8 activation may occur downstream of leukemia and solid tumors. The results, however, were disap- caspase-3 in the cytochrome c-initiated mitochondrial cas- pointing and showed no significant antitumor activity, but pase cascade.92 Thus, caspase-3 can participate in a feedback were associated with a high grade of systemic toxicity (eg amplification loop, at least in vitro. Results from our labora- cytokine release syndrome, hepatotoxicity, thrombo- tory confirmed these in vitro data in intact cells. We could cytopenia).105,106 As a consequence, the systemic application demonstrate CD95-independent processing and activation of of TNF was abandoned in cancer therapy. caspase-8 in drug-treated B lymphoid cells. Time course At the end of the 1980s, the impressive antitumor activity experiments revealed that processing of caspase-8 was pre- of agonistic monoclonal antibodies against CD95 in murine ceded by loss of the mitochondrial membrane potential and xenotransplant models of human tumors established that activation of caspase-3. Activation of caspase-8 was blocked apoptosis induction might be employed for cancer therapy.13–15 by preincubation of the B lymphoid cells with the caspase-3 A single injection of antibodies against human CD95 resulted specific inhibitor zDEVD-fmk.93 Thus, in drug-induced in cure from xenotransplanted human tumors in immuno- apoptosis in B lymphoid cells the molecular order is rep- deficient nude and SCID mice. Thus, stimulating the CD95 resented by the hierarchical activation of mitochondria, cas- receptor with agonistic monoclonal antibodies or CD95L was pase-3 and downstream caspase-8. In this system caspase-8 evaluated as a novel approach for antitumor therapy. The use functions as an amplifying executioner rather than as an of a species-specific monoclonal anti-murine-CD95 antibody initiator caspase (Figure 2). Interestingly, a short cut of the revealed, however, considerable systemic toxicity, in parti- drug-induced amplification loop has been described which is cular hepatotoxicity.16 Efforts are still being made to reduce triggered by caspase-3-mediated cleavage of Bid. This sug- the systemic toxicity, for instance by reducing CD95 gests that Bid can also be cleaved by caspases other than cas- expression in liver cells prior to CD95 agonistic therapy.107 pase-8.94,95 Taken together, it seems that a tight cross-com- CD95 triggering either by agonistic antibodies or its ligand munication between the main death pathways exists under revealed that a broad variety of non-malignant cells are sus- physiological as well as under pathological conditions. The ceptible to CD95-triggered apoptosis.1,19,20 This may lead to usage of such additional pathways and the multiple amplifi- immune suppression, since physiological signaling via CD95 cation loops by death receptors remains to be elucidated but is involved in immune tolerance both in the T and B cell com- may play a decisive role in determining the final outcome of partment.108–111 Interestingly, co-stimulatory signals such as death receptor signaling. via B7/CD28112,113 or CD2/CD58, eg by dendritic cells,114 can overcome this deleterious effect of immune cell depletion by death receptor signaling. Most studies examining non-malig- Survival pathways nant cells have been performed by the use of agonistic antibodies, and rather rarely recombinant CD95L has been Under certain circumstances, ligation of death receptors after employed. For the trimerization and activation of CD95 challenge with death ligands might also lead to induction of downstream signaling, this difference might not be trivial. A survival pathways.96 This is best documented for TNF and its number of reports showed that the death-inducing activity of

Leukemia Mini-review PT Daniel et al 1027 such antibodies is different from the action of the natural Smac/Diablo, has been identified that obviously plays a criti- ligand. In addition, the soluble form of CD95L that is pro- cal role in antagonizing the caspase-inhibitory effect of duced by proteolytic cleavage is a less potent inducer of IAPs.130,131 The role of Smac in tumor biology is as yet unclear apoptosis than its membrane-bound form.115 but inactivation of this novel player could also result in a More recently, TRAIL/APO-2L was shown to be selectively resistant phenotype. Thus, death receptor-targeted therapeutic cytotoxic for tumor cells but exerted only minimal or no tox- approaches may encounter similar problems as conventional icity in normal tissues of mice and primates. Therefore, TRAIL chemo- or radiotherapy. Nevertheless, the resistance profile was considered as a new agent with great potential for its in may be quite different and cross-resistance may be a rare vivo anticancer effect, either alone or in combination with event.132 drugs. It has been demonstrated that tumor cells that are resistant to TRAIL can be sensitized by subtoxic concentrations of drugs and cytokines, and the sensitized tumor cells are sig- Death receptors in radio- and chemotherapy nificantly killed by TRAIL.17,116 Although TRAIL is expressed on a wide range of tissues, co-expression of the decoy recep- An exciting finding is that death receptors of the TNF, CD95 tor proteins DcR1 (TRAIL-R3) and DcR2 (TRAIL-R4) inhibits and TRAIL systems appear to cooperate with radiation- and cell death due to their absent or truncated cytoplasmic death drug-induced cell death.116,133–137 Radiation and chemo- domains. Initially, expression of these decoy receptors was therapy upregulate CD95 in different tumor cells via a p53- held responsible for the observed tumor cell-specific killing dependent mechanism.138 The TRAIL receptor KILLER/DR5 is by TRAIL. induced by DNA damage and also appears to be regulated by Although TRAIL has been found to kill a wide variety of the tumor suppressor gene p53,139 which may at the same tumor cells, very recently, serious concerns were raised about time be counteracted by negative feedback induction of the the use of TRAIL in cancer therapy.117 Its safety has been chal- TRAIL decoy receptors by p53.140,141 Both CD95 and lenged by a report that showed induction of apoptosis by KILLER/DR5 therefore provide potential links between DNA TRAIL in cultured human hepatocytes.118 The study compared damage-mediated activation of p53 and the caspase cas- the sensitivity of hepatocytes from rats, mice, rhesus monkey cade.142 There is a considerable amount of crosstalk between and humans. It was found that while the animal hepatocytes the two signaling pathways (death receptor- vs cytotoxic drug- were not affected by the drug, TRAIL did cause significant induced apoptosis; see also Figure 2). Therefore, the pretreat- death in normal liver cells of humans. The reason for the spec- ment of cancer cells with TNFα, CD95L or TRAIL may sensi- ies specificity of TRAIL liver toxicity is difficult to explain, and tize tumor cells to chemotherapy or radiation and vice it is possible that hepatocytes of mice and monkeys produce versa.135 Unfortunately, deregulation of the p53/p14ARF-path- a protective factor that is absent in human cells. Certainly, way which is a frequent event in cancer143–145 may therefore these results are a warning and may suggest that substantial also contribute not only to drug resistance but may also lead liver toxicity might result if TRAIL is used as a systemic drug to impaired expression of death receptors and their signaling in clinical trials. It has also been observed that recombinant pathways,145 which may result in additional defects in the TRAIL induces apoptosis in normal living brain slices and regulation of cell death and survival. erythropoietic cells,119,120 while another report claims that This striking interaction between drug- and death receptor- CFU-GM or BFU-E colony formation in vitro is unaffected by induced apoptosis led to the hypothesis that these receptors TRAIL.121 In the brain, the susceptible cells were identified as may be directly involved in drug-induced apoptosis.146–148 neurons, oligodendrocytes, astrocytes and microglial cells.119 Death ligands and receptors may be induced during drug- Altogether, the current data suggest that a potential use of triggered apoptosis (see above). TRAIL in cancer therapy has to be considered with caution A number of reports, however, showed that cell death and awaits further in vivo studies. induced by anti-cancer drugs or irradiation is completely Apart from toxicity, another problem arises regarding resist- independent from death receptor signaling.93,149–151 This also ance mechanisms in cancer cells. Tumor cells can be resistant holds true for the activation of caspase-8 during drug-induced to death receptor-induced apoptosis, either due to the apoptosis.152 In contrast to the death-receptor/FADD-depen- expression of decoy receptors or the inactivation or loss of dent pathway, caspase-8 activation is triggered in this situ- the targeted receptor or distal signaling cascades. Mutations ation by a receptor-independent, post-mitochondrial mech- in CD95 have been described in T-lineage acute leuke- anism downstream of caspase-3 and functions as an mias122,123 or Burkitt lymphoma and loss of caspase-8 amplifying executioner caspase during drug-induced expression was found in childhood neuroblastomas.124 In apoptosis.93 Neither does it depend on receptor-independent addition, clonal variability in CD95/Fas expression has been FADD-mediated signaling, as recently suggested, for ganciclo- demonstrated to be a major determinant in the development vir-induced cell death, since overexpression of a dominant of resistance against death signaling via the CD95/Fas recep- negative FADD adapter protein lacking the DED domain for tor.125 Furthermore, homozygous deletion of death receptor caspase-8 recruitment cannot block drug or irradiation- DR4 gene may lead to TRAIL resistance.126 Another mech- induced cell death.93,151,152 Thus, drug-induced apoptosis anism for cellular resistance towards death receptor-induced definitively does not depend on CD95/Fas or other death apoptosis is the activation of survival signaling pathways, for receptors. The caspase-8 activation observed in various sys- instance via upregulation of NF-κB or members of the IAP tems of anticancer therapy-induced cell death93,149,152–154 family or FLIP proteins.127 Other possible causes include the therefore represents a downstream signaling event which may, deregulation of the mitochondrial apoptosome,67 especially nevertheless, be involved in the amplification of apoptosis the members of the Bcl-2 family which may show consider- execution. able intra- and interdisease variability, eg in lymphoma,128 As described above, death receptor signaling may contrib- and may shift to an apoptosis-refractory profile when resist- ute to drug-induced death and the pathways may cross-sensit- ance develops.129 Recently, an IAP-neutralizing and therefore ize for each other. However, inactivation of death receptor apoptosis-promoting mitochondrial protein, termed signaling cannot prevent drug-induced apoptosis. Thus, path-

Leukemia Mini-review PT Daniel et al 1028 ways apart from death receptors such as the mitochondrial hematopoeitic progenitors, faulty marrow microenvironment, signaling cascade are certainly the decisive steps for the inherited MDS/AML and Fas-mediated apoptosis. Leukemia execution of cell death in cancer therapy. 2000; 14: 965 (Abstr.). 10 Gupta P, Niehans GA, LeRoy SC, Gupta K, Morrison VA, Schultz C, Knapp DJ, Kratzke RA. Fas ligand expression in the bone marrow in myelodysplastic syndromes correlates with FAB subtype Conclusions and anemia, and predicts survival. Leukemia 1999; 13: 44–53. 11 Blagosklonny M. The dilemma of apoptosis in myelodysplasia Death receptor signaling plays an important role in mediating and leukemia: a new promise of therapeutic intervention? Leuke- apoptosis of non-malignant cells to maintain tissue homeo- mia 2000; 14: 2017–2018. stasis. This has been demonstrated most convincingly in 12 Tsoplou P, Kouraklis-Symeonidis A, Thanopoulou E, Zikos P, Orphanos V, Zoumbos NC. Apoptosis in patients with myelo- immune-competent cells and may explain the toxicity profile dysplastic syndromes: differential involvement of marrow cells in when death ligands, such as CD95L or TRAIL, are employed ‘good’ vs ‘poor’ prognosis patients and correlation with in cancer therapy. The potency of such death ligands to kill apoptosis-related genes. Leukemia 1999; 13: 1554–1563. cancer cells is impressive but the danger to elicit deleterious 13 Coney LR, Daniel PT, Sanborn D, Dhein J, Debatin KM, Krammer systemic toxicity cannot be neglected. The ability of TRAIL to PH, Zurawski VR Jr. Apoptotic cell death induced by a mouse- induce apoptosis in normal primary hepatocytes, cells of the human anti-APO-1 chimeric antibody leads to tumor regression. CNS, lymphocytes and erythropoietic cells requires special Int J Cancer 1994; 58: 562–567. 14 Dhein J, Daniel PT, Trauth BC, Oehm A, Moller P, Krammer PH. attention and should warn researchers to proceed with great Induction of apoptosis by monoclonal antibody anti-APO-1 class caution into TRAIL clinical trials. However, there remains switch variants is dependent on cross-linking of APO-1 cell sur- optimism: apoptosis by death receptors can be modulated and face antigens. J Immunol 1992; 149: 3166–3173. the ongoing progress in elucidating their signaling cascades 15 Lucking-Famira KM, Daniel PT, Moller P, Krammer PH, Debatin may help to define survival signals which may be utilized to KM. APO-1 (CD95) mediated apoptosis in human T-ALL save normal cells from toxic side-effects, while selectively engrafted in SCID mice. Leukemia 1994; 8: 1825–1833. targeting cancer cells to the ‘magic bullets’ of death ligands. 16 Ogasawara J, Watanabe-Fukunaga R, Adachi M, Matsuzawa A, Kasugai T, Kitamura Y, Itoh N, Suda T, Nagata S. Lethal effect of the anti-Fas antibody in mice. Nature 1993; 364: 806–809. 17 Ashkenazi A, Pai RC, Fong S, Leung S, Lawrence DA, Marsters Acknowledgements SA, Blackie C, Chang L, McMurtrey AE, Hebert A, DeForge L, Koumenis IL, Lewis D, Harris L, Bussiere J, Koeppen H, Shahrokh Work from the authors cited in this review was supported Z, Schwall RH. Safety and antitumor activity of recombinant sol- by the European Union Biomed2 and TMR programmes, uble Apo2 ligand. J Clin Invest 1999; 104: 155–162. the Deutsche Forschungsgemeinschaft and the Deutsche 18 Walczak H, Miller RE, Ariail K, Gliniak B, Griffith TS, Kubin M, Chin W, Jones J, Woodward A, Le T, Smith C, Smolak P, Good- Krebshilfe. win RG, Rauch CT, Schuh JC, Lynch DH. Tumoricidal activity of tumor necrosis factor-related apoptosis-inducing ligand in vivo. Nat Med 1999; 5: 157–163. References 19 Krammer PH, Behrmann I, Daniel P, Dhein J, Debatin KM. Regu- lation of apoptosis in the immune system. Curr Opin Immunol 1 Schulze-Osthoff K, Ferrari D, Los M, Wesselborg S, Peter ME. 1994; 6: 279–289. Apoptosis signaling by death receptors. Eur J Biochem 1998; 254: 20 Krammer PH. CD95’s deadly mission in the immune system. Nat- 439–459. ure 2000; 407: 789–795. 2 Drappa J, Vaishnaw AK, Sullivan KE, Chu JL, Elkon KB. Fas gene 21 Nagata S. Apoptosis by death factor. Cell 1997; 88: 355–365. mutations in the Canale–Smith syndrome, an inherited lympho- 22 Wallach D, Varfolomeev EE, Malinin NL, Goltsev YV, Kovalenko proliferative disorder associated with autoimmunity. N Engl J AV, Boldin MP. Tumor necrosis factor receptor and Fas signaling Med 1996; 335: 1643–1649. mechanisms. Annu Rev Immunol 1999; 17: 331–367. 3 Martin DA, Zheng L, Siegel RM, Huang B, Fisher GH, Wang J, 23 Bodmer JL, Burns K, Schneider P, Hofmann K, Steiner V, Thome Jackson CE, Puck JM, Dale J, Straus SE, Peter ME, Krammer PH, M, Bornand T, Hahne M, Schroter M, Becker K, Wilson A, French Fesik S, Lenardo MJ. Defective CD95/APO-1/Fas signal complex LE, Browning JL, MacDonald HR, Tschopp J. TRAMP, a novel formation in the human autoimmune lymphoproliferative syn- apoptosis-mediating receptor with sequence homology to tumor drome, type Ia. Proc Natl Acad Sci USA 1999; 96: 4552–4557. necrosis factor receptor 1 and Fas(Apo-1/CD95). Immunity 1997; 4 Jackson CE, Puck JM. Autoimmune lymphoproliferative syn- 6: 79–88. drome, a disorder of apoptosis. Curr Opin Pediatr 1999; 11: 24 Chinnaiyan AM, O’Rourke K, Yu GL, Lyons RH, Garg M, Duan 521–527. DR, Xing L, Gentz R, Ni J, Dixit VM. Signal transduction by DR3, 5 Vaishnaw AK, Orlinick JR, Chu JL, Krammer PH, Chao MV, Elkon a death domain-containing receptor related to TNFR-1 and KB. The molecular basis for apoptotic defects in patients with CD95. Science 1996; 274: 990–992. CD95 (Fas/Apo-1) mutations. J Clin Invest 1999; 103: 355–363. 25 Kitson J, Raven T, Jiang YP, Goeddel DV, Giles KM, Pun KT, 6 Wang J, Zheng L, Lobito A, Chan FK, Dale J, Sneller M, Yao Grinham CJ, Brown R, Farrow SN. A death-domain-containing X, Puck JM, Straus SE, Lenardo MJ. Inherited human caspase 10 mutations underlie defective lymphocyte and dendritic cell receptor that mediates apoptosis. Nature 1996; 384: 372–375. apoptosis in autoimmune lymphoproliferative syndrome type II. 26 Marsters SA, Sheridan JP, Donahue CJ, Pitti RM, Gray CL, God- Cell 1999; 98: 47–58. dard AD, Bauer KD, Ashkenazi A. Apo-3, a new member of the 7 Ramenghi U, Bonissoni S, Migliaretti G, DeFranco S, Bottarel F, tumor necrosis factor receptor family, contains a death domain Gambaruto C, DiFranco D, Priori R, Conti F, Dianzani I, Valesini and activates apoptosis and NF-kappa B. Curr Biol 1996; 6: G, Merletti F, Dianzani U. Deficiency of the Fas apoptosis path- 1669–1676. way without Fas gene mutations is a familial trait predisposing 27 Pan G, Bauer JH, Haridas V, Wang S, Liu D, Yu G, Vincenz to development of autoimmune diseases and cancer. Blood C, Aggarwal BB, Ni J, Dixit VM. Identification and functional 2000; 95: 3176–3182. characterization of DR6, a novel death domain-containing TNF 8 Sleight BJ, Prasad VS, DeLaat C, Steele P, Ballard E, Arceci RJ, receptor. FEBS Lett 1998; 431: 351–356. Sidman CL. Correction of autoimmune lymphoproliferative syn- 28 Pan G, Ni J, Yu G, Wei YF, Dixit VM. TRUNDD, a new member drome by bone marrow transplantation. Bone Marrow Transplant of the TRAIL receptor family that antagonizes TRAIL signalling. 1998; 22: 375–380. FEBS Lett 1998; 424: 41–45. 9 Freedman M, Dror Y. Shwachman–Diamond syndrome: aberrant 29 Pan G, Ni J, Wei YF, Yu G, Gentz R, Dixit VM. An antagonist

Leukemia Mini-review PT Daniel et al 1029 decoy receptor and a death domain-containing receptor for NMR structure and mutagenesis of the Fas (APO-1/CD95) death TRAIL. Science 1997; 277: 815–818. domain. Nature 1996; 384: 638–641. 30 Pan G, O’Rourke K, Chinnaiyan AM, Gentz R, Ebner R, Ni J, 49 Boldin MP, Mett IL, Wallach D. A protein related to a proteaso- Dixit VM. The receptor for the cytotoxic ligand TRAIL. Science mal subunit binds to the intracellular domain of the p55 TNF 1997; 276: 111–113. receptor upstream to its ‘death domain’. FEBS Lett 1995; 367: 31 Schneider P, Thome M, Burns K, Bodmer JL, Hofmann K, Kataoka 39–44. T, Holler N, Tschopp J. TRAIL receptors 1 (DR4) and 2 (DR5) 50 Boldin MP, Varfolomeev EE, Pancer Z, Mett IL, Camonis JH, Wal- signal FADD-dependent apoptosis and activate NF-kappaB. lach D. A novel protein that interacts with the death domain of Immunity 1997; 7: 831–836. Fas/APO1 contains a sequence motif related to the death domain. 32 Schneider P, Bodmer JL, Thome M, Hofmann K, Holler N, J Biol Chem 1995; 270: 7795–7798. Tschopp J. Characterization of two receptors for TRAIL. FEBS Lett 51 Chinnaiyan AM, O’Rourke K, Tewari M, Dixit VM. FADD, a 1997; 416: 329–334. novel death domain-containing protein, interacts with the death 33 Sheridan JP, Marsters SA, Pitti RM, Gurney A, Skubatch M, Bald- domain of Fas and initiates apoptosis. Cell 1995; 81: 505–512. win D, Ramakrishnan L, Gray CL, Baker K, Wood WI, Goddard 52 Hsu H, Shu HB, Pan MG, Goeddel DV. TRADD-TRAF2 and AD, Godowski P, Ashkenazi A. Control of TRAIL-induced TRADD-FADD interactions define two distinct TNF receptor 1 apoptosis by a family of signaling and decoy receptors. Science signal transduction pathways. Cell 1996; 84: 299–308. 1997; 277: 818–821. 53 Hsu H, Huang J, Shu HB, Baichwal V, Goeddel DV. TNF-depen- 34 Walczak H, Degli-Esposti MA, Johnson RS, Smolak PJ, Waugh dent recruitment of the protein kinase RIP to the TNF receptor- JY, Boiani N, Timour MS, Gerhart MJ, Schooley KA, Smith CA, 1 signaling complex. Immunity 1996; 4: 387–396. Goodwin RG, Rauch CT. TRAIL-R2: a novel apoptosis-mediating 54 Ting AT, Pimentel-Muinos FX, Seed B. RIP mediates tumor receptor for TRAIL. EMBO J 1997; 16: 5386–5397. necrosis factor receptor 1 activation of NF-kappaB but not 35 Degli-Esposti MA, Smolak PJ, Walczak H, Waugh J, Huang CP, Fas/APO-1-initiated apoptosis. EMBO J 1996; 15: 6189–6196. DuBose RF, Goodwin RG, Smith CA. Cloning and characteriz- 55 Kischkel FC, Hellbardt S, Behrmann I, Germer M, Pawlita M, ation of TRAIL-R3, a novel member of the emerging TRAIL recep- Krammer PH, Peter ME. Cytotoxicity-dependent APO-1 tor family. J Exp Med 1997; 186: 1165–1170. (Fas/CD95)-associated proteins form a death-inducing signaling 36 Emery JG, McDonnell P, Burke MB, Deen KC, Lyn S, Silverman complex (DISC) with the receptor. EMBO J 1995; 14: 5579– C, Dul E, Appelbaum ER, Eichman C, DiPrinzio R, Dodds RA, 5588. James IE, Rosenberg M, Lee JC, Young PR. Osteoprotegerin is a 56 Muzio M, Chinnaiyan AM, Kischkel FC, O’Rourke K, Shevch- receptor for the cytotoxic ligand TRAIL. J Biol Chem 1998; 273: enko A, Ni J, Scaffidi C, Bretz JD, Zhang M, Gentz R, Mann 14363–14367. M, Krammer PH, Peter ME, Dixit VM. FLICE, a novel FADD- 37 Marsters SA, Sheridan JP, Pitti RM, Huang A, Skubatch M, Bald- homologous ICE/CED-3-like protease, is recruited to the CD95 win D, Yuan J, Gurney A, Goddard AD, Godowski P, Ashkenazi (Fas/APO-1) death-inducing signaling complex. Cell 1996; 85: A. A novel receptor for Apo2L/TRAIL contains a truncated death 817–827. domain. Curr Biol 1997; 7: 1003–1006. 57 Alnemri ES, Livingston DJ, Nicholson DW, Salvesen G, 38 Simonet WS, Lacey DL, Dunstan CR, Kelley M, Chang MS, Luthy Thornberry NA, Wong WW, Yuan J. Human ICE/CED-3 protease R, Nguyen HQ, Wooden S, Bennett L, Boone T, Shimamoto G, nomenclature. Cell 1996; 87: 171. DeRose M, Elliott R, Colombero A, Tan HL, Trail G, Sullivan J, 58 Bodmer JL, Holler N, Reynard S, Vinciguerra P, Schneider P, Juo Davy E, Bucay N, Renshaw-Gegg L, Hughes TM, Hill D, Pattison P, Blenis J, Tschopp J. TRAIL receptor-2 signals apoptosis through W, Campbell P, Boyle WJ. Osteoprotegerin: a novel secreted pro- FADD and caspase-8. Nat Cell Biol 2000; 2: 241–243. tein involved in the regulation of bone density. Cell 1997; 89: 59 Kischkel FC, Lawrence DA, Chuntharapai A, Schow P, Kim KJ, 309–319. Ashkenazi A. Apo2L/TRAIL-dependent recruitment of endogen- 39 Burgess TL, Qian Y, Kaufman S, Ring BD, Van G, Capparelli C, ous FADD and caspase-8 to death receptors 4 and 5. Immunity Kelley M, Hsu H, Boyle WJ, Dunstan CR, Hu S, Lacey DL. The 2000; 12: 611–620. ligand for osteoprotegerin (OPGL) directly activates mature 60 Sprick MR, Weigand MA, Rieser E, Rauch CT, Juo P, Blenis J, osteoclasts. J Cell Biol 1999; 145: 527–538. Krammer PH, Walczak H. FADD/MORT1 and caspase-8 are 40 Green EA, Flavell RA. TRANCE-RANK, a new signal pathway recruited to TRAIL receptors 1 and 2 and are essential for involved in lymphocyte development and T cell activation. J Exp apoptosis mediated by TRAIL receptor 2. Immunity 2000; 12: Med 1999; 189: 1017–1020. 599–609. 41 Teitelbaum SL. Bone resorption by osteoclasts. Science 2000; 61 Duan H, Dixit VM. RAIDD is a new ‘death’ adaptor molecule. 289: 1504–1508. Nature 1997; 385: 86–89. 42 Pitti RM, Marsters SA, Lawrence DA, Roy M, Kischkel FC, Dowd 62 Bergeron L, Perez GI, Macdonald G, Shi L, Sun Y, Jurisicova A, P, Huang A, Donahue CJ, Sherwood SW, Baldwin DT, Godowski Varmuza S, Latham KE, Flaws JA, Salter JC, Hara H, Moskowitz PJ, Wood WI, Gurney AL, Hillan KJ, Cohen RL, Goddard AD, MA, Li E, Greenberg A, Tilly JL, Yuan J. Defects in regulation Botstein D, Ashkenazi A. Genomic amplification of a decoy of apoptosis in caspase-2-deficient mice. Genes Dev 1998; 12: receptor for Fas ligand in lung and colon cancer. Nature 1998; 1304–1314. 396: 699–703. 63 Yang X, Khosravi-Far R, Chang HY, Baltimore D. Daxx, a novel 43 Yu KY, Kwon B, Ni J, Zhai Y, Ebner R, Kwon BS. A newly ident- Fas-binding protein that activates JNK and apoptosis. Cell 1997; ified member of tumor necrosis factor receptor superfamily (TR6) 89: 1067–1076. suppresses LIGHT-mediated apoptosis. J Biol Chem 1999; 274: 64 Torii S, Egan DA, Evans RA, Reed JC. Human Daxx regulates Fas- 13733–13736. induced apoptosis from nuclear PML oncogenic domains 44 Beutler B, van Huffel C. Unraveling function in the TNF ligand (PODs). EMBO J 1999; 18: 6037–6049. and receptor families. Science 1994; 264: 667–668. 65 Villunger A, Huang DC, Holler N, Tschopp J, Strasser A. Fas 45 Chicheportiche Y, Bourdon PR, Xu H, Hsu YM, Scott H, Hession ligand-induced c-Jun kinase activation in lymphoid cells requires C, Garcia I, Browning JL. TWEAK, a new secreted ligand in the extensive receptor aggregation but is independent of DAXX, and tumor necrosis factor family that weakly induces apoptosis. J Biol Fas-mediated cell death does not involve DAXX, RIP, or RAIDD. Chem 1997; 272: 32401–32410. J Immunol 2000; 165: 1337–1343. 46 Wiley SR, Schooley K, Smolak PJ, Din WS, Huang CP, Nicholl 66 Adams JM, Cory S. The Bcl-2 protein family: arbiters of cell sur- JK, Sutherland GR, Smith TD, Rauch C, Smith CA. Identification vival. Science 1998; 281: 1322–1326. and characterization of a new member of the TNF family that 67 Daniel PT. Dissecting the pathways to death. Leukemia 2000; induces apoptosis. Immunity 1995; 3: 673–682. 14: 2035–2044. 47 Banner DW, D’Arcy A, Janes W, Gentz R, Schoenfeld HJ, Broger 68 Van der Heiden MG, Thompson CB. Bcl-2 proteins: regulators C, Loetscher H, Lesslauer W. Crystal structure of the soluble of apoptosis or of mitochondrial homeostasis? Nat Cell Biol human 55 kd TNF receptor-human TNF beta complex: impli- 1999; 1: E209–E216. cations for TNF receptor activation. Cell 1993; 73: 431–445. 69 Zou H, Henzel WJ, Liu X, Lutschg A, Wang X. Apaf-1, a human 48 Huang B, Eberstadt M, Olejniczak ET, Meadows RP, Fesik SW. protein homologous to C. elegans CED-4, participates in cyto-

Leukemia Mini-review PT Daniel et al 1030 chrome c-dependent activation of caspase-3. Cell 1997; 90: 89 Green DR, Reed JC. Mitochondria and apoptosis. Science 1998; 405–413. 281: 1309–1312. 70 Li P, Nijhawan D, Budihardjo I, Srinivasula SM, Ahmad M, 90 Kroemer G, Reed JC. Mitochondrial control of cell death. Nat Alnemri ES, Wang X. Cytochrome c and dATP-dependent forma- Med 2000; 6: 513–519. tion of Apaf-1/Caspase-9 complex initiates an apoptotic protease 91 Li H, Zhu H, Xu CJ, Yuan J. Cleavage of BID by caspase 8 cascade. Cell 1997; 91: 479–489. mediates the mitochondrial damage in the Fas pathway of 71 Krippner A, Matsuno-Yagi A, Gottlieb RA, Babior BM. Loss of apoptosis. Cell 1998; 94: 491–501. function of cytochrome c in Jurkat cells undergoing fas-mediated 92 Slee EA, Harte MT, Kluck RM, Wolf BB, Casiano CA, Newmeyer apoptosis. J Biol Chem 1996; 271: 21629–21636. DD, Wang HG, Reed JC, Nicholson DW, Alnemri ES, Green DR, 72 Desagher S, Osen-Sand A, Nichols A, Eskes R, Montessuit S, Martin SJ. Ordering the cytochrome c-initiated caspase cascade: Lauper S, Maundrell K, Antonsson B, Martinou J. Bid-induced hierarchical activation of caspases-2, -3, -6, -7, -8, and -10 in a conformational change of Bax is responsible for mitochondrial caspase-9-dependent manner. J Cell Biol 1999; 144: 281–292. cytochrome c release during apoptosis. J Cell Biol 1999; 144: 93 Wieder T, Essmann F, Prokop A, Schmelz K, Schulze-Osthoff K, 891–901. Beyaert R, Do¨rken B, Daniel PT. Activation of caspase-8 in drug- 73 Eskes R, Desagher S, Antonsson B, Martinou JC. Bid induces the induced apoptosis of B-lymphoid cells is independent of oligomerization and insertion of Bax into the outer mitochondrial CD95/Fas receptor ligand interaction and occurs downstream of membrane. Mol Cell Biol 2000; 20: 929–935. caspase-3. Blood 2001; 97: 1378–1387. 74 Fadeel B, Orrenius S, Zhivotovsky B. The most unkindest cut of 94 Engels IH, Stepczynska A, Stroh C, Lauber K, Berg C, Schwenzer all: on the multiple roles of mammalian caspases. Leukemia R, Wajant H, Janicke RU, Porter AG, Belka C, Gregor M, 2000; 14: 1514–1525. Schulze-Osthoff K, Wesselborg S. Caspase-8/FLICE functions as 75 Walker NP, Talanian RV, Brady KD, Dang LC, Bump NJ, Ferenz an executioner caspase in anticancer drug-induced apoptosis. CR, Franklin S, Ghayur T, Hackett MC, Hammill LD. Crystal Oncogene 2000; 19: 4563–4573. structure of the cysteine protease interleukin-1 beta-converting 95 Slee EA, Keogh SA, Martin SJ. Cleavage of BID during cytotoxic enzyme: a (p20/p10)2 homodimer. Cell 1994; 78: 343–352. drug and UV radiation-induced apoptosis occurs downstream of 76 Rotonda J, Nicholson DW, Fazil KM, Gallant M, Gareau Y, Lab- the point of Bcl-2 action and is catalysed by caspase-3: a poten- elle M, Peterson EP, Rasper DM, Ruel R, Vaillancourt JP, tial feedback loop for amplification of apoptosis-associated mito- Thornberry NA, Becker JW. The three-dimensional structure of chondrial cytochrome c release. Cell Death Differ 2000; 7: apopain/CPP32, a key mediator of apoptosis. Nat Struct Biol 556–565. 1996; 3: 619–625. 96 Okuma E, Saeki K, Shimura M, Ishizaka Y, Yasugi E, Yuo A. 77 Li P, Nijhawan D, Budihardjo I, Srinivasula SM, Ahmad M, Induction of apoptosis in human hematopoietic U937 cells by Alnemri ES, Wang X. Cytochrome c and dATP-dependent forma- granulocyte–macrophage colony-stimulating factor: possible tion of Apaf-1/caspase-9 complex initiates an apoptotic protease existence of caspase 3-like pathway. Leukemia 2000; 14: 612– cascade. Cell 1997; 91: 479–489. 619. 78 Stroh C, Schulze-Osthoff K. Death by a thousand cuts: an ever 97 Chu ZL, McKinsey TA, Liu L, Gentry JJ, Malim MH, Ballard DW. increasing list of caspase substrates. Cell Death Differ 1998; 5: Suppression of tumor necrosis factor-induced cell death by 997–1000. inhibitor of apoptosis c-IAP2 is under NF-kappaB control. Proc 79 Wieder T, Geilen CC, Kolter T, Sadeghlar F, Sandhoff K, Brossmer Natl Acad Sci USA 1997; 94: 10057–10062. R, Ihrig P, Perry D, Orfanos CE, Hannun YA. Bcl-2 antagonizes 98 Antwerp DJ, Martin SJ, Kafri T, Green DR, Verma IM. Suppression apoptotic cell death induced by two new ceramide analogues. of TNF-α-induced apoptosis by NF-κB. Science 1996; 274: FEBS Lett 1997; 411: 260–264. 787–789. 80 Kawahara A, Ohsawa Y, Matsumura H, Uchiyama Y, Nagata S. 99 Wang CY, Mayo MW, Korneluk RG, Goeddel DV, Baldwin AS Caspase-independent cell killing by Fas-associated protein with Jr. NF-kappaB antiapoptosis: induction of TRAF1 and TRAF2 and death domain. J Cell Biol 1998; 143: 1353–1360. c-IAP1 and c-IAP2 to suppress caspase-8 activation. Science 81 Schulze-Osthoff K, Krammer PH, Droge W. Divergent signalling 1998; 281: 1680–1683. via APO-1/Fas and the TNF receptor, two homologous molecules 100 Rothe M, Pan MG, Henzel WJ, Ayres TM, Goeddel DV. The involved in physiological cell death. EMBO J 1994; 13: 4587– TNFR2-TRAF signaling complex contains two novel proteins 4596. related to baculoviral inhibitor of apoptosis proteins. Cell 1995; 82 Vercammen D, Beyaert R, Denecker G, Goossens V, Van Loo 83: 1243–1252. G, Declercq W, Grooten J, Fiers W, Vandenabeele P. Inhibition 101 Rothe M, Sarma V, Dixit VM, Goeddel DV. TRAF2-mediated of caspases increases the sensitivity of L929 cells to necrosis activation of NF-kappa B by TNF receptor 2 and CD40. Science mediated by tumor necrosis factor. J Exp Med 1998; 187: 1995; 269: 1424–1427. 1477–1485. 102 Deveraux QL, Roy N, Stennicke HR, Van Arsdale T, Zhou Q, 83 Blagosklonny MV. Cell death beyond apoptosis. Leukemia 2000; Srinivasula SM, Alnemri ES, Salvesen GS, Reed JC. IAPs block 14: 1502–1508. apoptotic events induced by caspase-8 and cytochrome c by 84 Susin SA, Daugas E, Ravagnan L, Samejima K, Zamzami N, direct inhibition of distinct caspases. EMBO J 1998; 17: 2215– Loeffler M, Costantini P, Ferri KF, Irinopoulou T, Prevost MC, 2223. Brothers G, Mak TW, Penninger J, Earnshaw WC, Kroemer G. 103 Roy N, Deveraux QL, Takahashi R, Salvesen GS, Reed JC. The Two distinct pathways leading to nuclear apoptosis. J Exp Med c-IAP-1 and c-IAP-2 proteins are direct inhibitors of specific 2000; 192: 571–580. caspases. EMBO J 1997; 16: 6914–6925. 85 Susin SA, Lorenzo HK, Zamzami N, Marzo I, Snow BE, Brothers 104 Duriez PJ, Wong F, Dorovini-Zis K, Shahidi R, Karsan A. A1 func- GM, Mangion J, Jacotot E, Costantini P, Loeffler M, Larochette tions at the mitochondria to delay endothelial apoptosis in N, Goodlett DR, Aebersold R, Siderovski DP, Penninger JM, Kro- response to tumor necrosis factor. J Biol Chem 2000; 275: emer G. Molecular characterization of mitochondrial apoptosis- 18099–18107. inducing factor. Nature 1999; 397: 441–446. 105 Brown TD, Goodman P, Fleming T, Macdonald JS, Hersh EM, 86 Dumont C, Durrbach A, Bidere N, Rouleau M, Kroemer G, Braun TJ. A phase II trial of recombinant tumor necrosis factor Bernard G, Hirsch F, Charpentier B, Susin SA, Senik A. Caspase- in patients with adenocarcinoma of the pancreas: a Southwest independent commitment phase to apoptosis in activated blood Oncology Group study. J Immunother 1991; 10: 376–378. T lymphocytes: reversibility at low apoptotic insult. Blood 2000; 106 Kemeny N, Childs B, Larchian W, Rosado K, Kelsen D. A phase 96: 1030–1038. II trial of recombinant tumor necrosis factor in patients with 87 Nakagawa T, Zhu H, Morishima N, Li E, Xu J, Yankner BA, advanced colorectal carcinoma. Cancer 1990; 66: 659–663. Yuan J. Caspase-12 mediates endoplasmic-reticulum-specific 107 Zhang H, Cook J, Nickel J, Yu R, Stecker K, Myers K, Dean NM. apoptosis and cytotoxicity by amyloid-beta. Nature 2000; 403: Reduction of liver Fas expression by an antisense oligonucleotide 98–103. protects mice from fulminant hepatitis. Nat Biotechnol 2000; 18: 88 Ashkenazi A, Dixit VM. Death receptors: signaling and modu- 862–867. lation. Science 1998; 281: 1305–1308. 108 Daniel PT, Krammer PH. Activation induces sensitivity toward

Leukemia Mini-review PT Daniel et al 1031 APO-1 (CD95)-mediated apoptosis in human B cells. J Immunol of apoptotic regulators in B cell non-Hodgkin’s lymphomas. Leu- 1994; 152: 5624–5632. kemia 1997; 11: 1868–1877. 109 Daniel PT, Oettinger U, Mapara MY, Bommert K, Bargou R, 129 Prokop A, Wieder T, Sturm I, Essmann F, Seeger K, Wuchter C, Do¨rken B. Activation and activation-induced death of human Ludwig W-D, Henze G, Do¨rken B, Daniel PT. Relapse in child- tonsillar B cells and Burkitt lymphoma cells: lack of CD95 hood acute lymphoblastic leukemia is associated with decrease (Fas/APO-1) ligand expression and function. Eur J Immunol 1997; of Bax/Bcl-2-ratio and loss of spontaneous caspase-3 processing 27: 1029–1034. in vivo. Leukemia 2000; 14: 1606–1613. 110 Dhein J, Walczak H, Baumler C, Debatin KM, Krammer PH. 130 Chai J, Du C, Wu JW, Kyin S, Wang X, Shi Y. Structural and Autocrine T-cell suicide mediated by APO-1/(Fas/CD95). Nature biochemical basis of apoptotic activation by Smac/DIABLO. Nat- 1995; 373: 438–441. ure 2000; 406: 855–862. 111 Lagresle C, Bella C, Daniel PT, Krammer PH, Defrance T. Regu- 131 Srinivasula SM, Datta P, Fan XJ, Fernandes-Alnemri T, Huang lation of germinal center B cell differentiation. Role of the human Z, Alnemri ES. Molecular determinants of the caspase-promoting APO-1/Fas (CD95) molecule. J Immunol 1995; 154: 5746–5756. activity of Smac/DIABLO and its role in the death receptor path- 112 Daniel PT, Kroidl A, Cayeux S, Bargou R, Blankenstein T, Do¨rken way. J Biol Chem 2000; 275: 36152–36157. B. Costimulatory signals through B7.1/CD28 prevent T cell 132 Walczak H, Bouchon A, Stahl H, Krammer PH. Tumor necrosis apoptosis during target cell lysis. J Immunol 1997; 159: 3808– factor-related apoptosis-inducing ligand retains its apoptosis- 3815. inducing capacity on Bcl-2- or Bcl-xL-overexpressing chemo- 113 Daniel PT, Kroidl A, Kopp J, Sturm I, Moldenhauer G, Do¨rken therapy-resistant tumor cells. Cancer Res 2000; 60: 3051–3057. B, Pezzutto A. Immunotherapy of B-cell lymphoma with CD3x19 133 Darzynkiewicz Z, Carter SP, Old LJ. Effect of recombinant tumor bispecific antibodies: costimulation via CD28 prevents ‘veto’ necrosis factor on HL-60 cells: cell-cycle specificity and syner- apoptosis of antibody-targeted cytotoxic T cells. Blood 1998; 92: gism with actinomycin D. J Cell Physiol 1987; 130: 328–335. 4750–4757. 134 Isonishi S, Jekunen AP, Hom DK, Eastman A, Edelstein PS, Thie- 114 Daniel PT, Scholz C, Essmann F, Westermann J, Pezzutto A, baut FB, Christen RD, Howell SB. Modulation of cisplatin sensi- Do¨rken B. CD95/Fas-triggered apoptosis of activated T lympho- tivity and growth rate of an ovarian carcinoma cell line by bom- cytes is prevented by dendritic cells through a CD58-dependent besin and tumor necrosis factor-alpha. J Clin Invest 1992; 90: mechanism. Exp Hematol 1999; 27: 1402–1408. 1436–1442. 115 Strasser A, O’Connor L. Fas ligand – caught between Scylla and 135 Micheau O, Solary E, Hammann A, Martin F, Dimanche-Boitrel Charybdis. Nat Med 1998; 4: 21–22. MT. Sensitization of cancer cells treated with cytotoxic drugs to 116 Bonavida B, Ng CP, Jazirehi A, Schiller G, Mizutani Y. Selectivity fas-mediated cytotoxicity. J Natl Cancer Inst 1997; 89: 783–789. of TRAIL-mediated apoptosis of cancer cells and synergy with 136 Posovszky C, Friesen C, Herr I, Debatin KM. Chemotherapeutic drugs: the trail to non-toxic cancer therapeutics. Int J Oncol drugs sensitize pre-B ALL cells for CD95- and cytotoxic T-lym- 1999; 15: 793–802. phocyte-mediated apoptosis. Leukemia 1999; 13: 400–409. 117 Nagata S. Steering anti-cancer drugs away from the TRAIL. Nat 137 Gliniak B, Le T. Tumor necrosis factor-related apoptosis-inducing Med 2000; 6: 502–503. ligand’s antitumor activity in vivo is enhanced by the chemother- 118 Jo M, Kim TH, Seol DW, Esplen JE, Dorko K, Billiar TR, Strom apeutic agent CPT-11. Cancer Res 1999; 59: 6153–6158. SC. Apoptosis induced in normal human hepatocytes by tumor 138 Owen-Schaub LB, Zhang W, Cusack JC, Angelo LS, Santee SM, necrosis factor-related apoptosis-inducing ligand. Nat Med 2000; Fujiwara T, Roth JA, Deisseroth AB, Zhang WW, Kruzel E. Wild- 6: 564–567. type human p53 and a temperature-sensitive mutant induce 119 Nitsch R, Bechmann I, Deisz RA, Haas D, Lehmann TN, Fas/APO-1 expression. Mol Cell Biol 1995; 15: 3032–3040. Wendling U, Zipp F. Human brain-cell death induced by 139 Wu GS, Burns TF, McDonald ER, 3rd, Meng RD, Kao G, Muschel tumour-necrosis-factor-related apoptosis-inducing ligand R, Yen T, el-Deiry WS. Induction of the TRAIL receptor (TRAIL). Lancet 2000; 356: 827–828. KILLER/DR5 in p53-dependent apoptosis but not growth arrest. 120 Zamai L, Secchiero P, Pierpaoli S, Bassini A, Papa S, Alnemri ES, Oncogene 1999; 18: 6411–6418. Guidotti L, Vitale M, Zauli G. TNF-related apoptosis-inducing 140 Meng RD, McDonald ER, 3rd, Sheikh MS, Fornace AJ Jr, El-Deiry ligand (TRAIL) as a negative regulator of normal human erythro- WS. The TRAIL decoy receptor TRUNDD (DcR2, TRAIL-R4) is poiesis. Blood 2000; 95: 3716–3724. induced by adenovirus-p53 overexpression and can delay 121 Gazitt Y. TRAIL is a potent inducer of apoptosis in myeloma cells TRAIL-, p53-, and KILLER/DR5-dependent colon cancer derived from multiple myeloma patients and is not cytotoxic to apoptosis. Mol Ther 2000; 1: 130–144. hematopoietic stem cells. Leukemia 1999; 13: 1817–1824. 141 Sheikh MS, Fornace AJ Jr. Death and decoy receptors and p53- 122 Beltinger C, Kurz E, Bohler T, Schrappe M, Ludwig WD, Debatin mediated apoptosis. Leukemia 2000; 14: 1509–1513. KM. CD95 (APO-1/Fas) mutations in childhood T-lineage acute 142 Wu GS, Kim K, el-Deiry WS. KILLER/DR5, a novel DNA-damage lymphoblastic leukemia. Blood 1998; 91: 3943–3951. 123 Tamiya S, Etoh K, Suzushima H, Takatsuki K, Matsuoka M. inducible death receptor gene, links the p53-tumor suppressor Mutation of CD95 (Fas/Apo-1) gene in adult T-cell leukemia to caspase activation and apoptotic death. Adv Exp Med Biol cells. Blood 1998; 91: 3935–3942. 2000; 465: 143–151. 124 Teitz T, Wei T, Valentine MB, Vanin EF, Grenet J, Valentine VA, 143 Gronbaek K, de Nully Brown P, Moller M, Nedergaard T, Ralf- Behm FG, Look AT, Lahti JM, Kidd VJ. Caspase 8 is deleted or kiaer E, Moller P, Zeuthen J, Guldberg P. Concurrent disruption silenced preferentially in childhood neuroblastomas with ampli- of p16INK4a and the ARF-p53 pathway predicts poor prognosis fication of MYCN. Nat Med 2000; 6: 529–535. in aggressive non-Hodgkin’s lymphoma. Leukemia 2000; 14: 125 Shain KH, Landowski TH, Buyuksal I, Cantor AB, Dalton WS. 1727–1735. Clonal variability in CD95 expression is the major determinant 144 Zhou M, Gu L, Abshire T, Homans A, Billett A, Yeager A, Findley in Fas-medicated, but not chemotherapy-medicated apoptosis in H. Incidence and prognostic significance of MDM2 oncoprotein the RPMI 8226 multiple myeloma cell line. Leukemia 2000; 14: overexpression in relapsed childhood acute lymphoblastic leuke- 830–840. mia. Leukemia 2000; 14: 61–67. 126 Ozoren N, Fisher MJ, Kim K, Liu CX, Genin A, Shifman Y, Dicker 145 Zhou M, Gu L, Yeager AM, Findley HW. Sensitivity to Fas- DT, Spinner NB, Lisitsyn NA, El-Deiry WS. Homozygous mediated apoptosis in pediatric acute lymphoblastic leukemia is deletion of the death receptor DR4 gene in a nasopharyngeal associated with a mutant p53 phenotype and absence of Bcl-2 cancer cell line is associated with TRAIL resistance. Int J Oncol expression. Leukemia 1998; 12: 1756–1763. 2000; 16: 917–925. 146 Findley HW, Zhou M. The clinical significance of Fas expression 127 Irisarri M, Plumas J, Bonnefoix T, Jacob M, Roucard C, Pasquier in leukemia: questions and controversies. Leukemia 1999; 13: M, Sotto J, Lajmanovich A. Resistance to CD95-mediated 147–149. apoptosis through constitutive c-FLIP expression in a non-Hodg- 147 Friesen C, Fulda S, Debatin KM. Cytotoxic drugs and the CD95 kin’s lymphoma B cell line. Leukemia 2000; 14: 2149–2158. pathway. Leukemia 1999; 13: 1854–1858. 128 Xerri L, Devilard E, Bouabdallah R, Hassoun J, Chaperot L, Birg 148 Solary E, Droin N, Bettaieb A, Corcos L, Dimanche-Boitrel MT, F, Plumas J. Quantitative analysis detects ubiquitous expression Garrido C. Positive and negative regulation of apoptotic path-

Leukemia Mini-review PT Daniel et al 1032 ways by cytotoxic agents in hematological malignancies. Leuke- FADD/MORT1 signaling. Implications for cancer therapy. J Exp mia 2000; 14: 1833–1849. Med 2000; 191: 195–200. 149 Belka C, Rudner J, Wesselborg S, Stepczynska A, Marini P, Lep- 152 Wesselborg S, Engels IH, Rossmann E, Los M, Schulze-Osthoff ple-Wienhues A, Faltin H, Bamberg M, Budach W, Schulze- K. Anticancer drugs induce caspase-8/FLICE activation and Osthoff K. Differential role of caspase-8 and BID activation dur- apoptosis in the absence of CD95 receptor/ligand interaction. ing radiation- and CD95-induced apoptosis. Oncogene 2000; Blood 1999; 93: 3053–3063. 19: 1181–1190. 153 Kitamura K, Minami Y, Yamamoto K, Akao Y, Kiyoi H, Saito H, 150 Eischen CM, Kottke TJ, Martins LM, Basi GS, Tung JS, Earnshaw Naoe T. Involvement of CD95-independent caspase-8 activation WC, Leibson PJ, Kaufmann SH. Comparison of apoptosis in wild- in arsenic trioxide-induced apoptosis. Leukemia 2000; 14: type and Fas-resistant cells: chemotherapy-induced apoptosis is 1743–1750. not dependent on Fas/Fas ligand interactions. Blood 1997; 90: 154 Nomura Y, Inanami O, Takahashi K, Matsuda A, Kuwabara M. 2- 935–943. Chloro-29-deoxyadenosine induces apoptosis through the Fas/Fas 151 Newton K, Strasser A. Ionizing radiation and chemotherapeutic ligand pathway in human leukemia cell line MOLT-4. Leukemia drugs induce apoptosis in lymphocytes in the absence of Fas or 2000; 14: 299–306.

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