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

Transforming growth factor-alpha prevents detachment-induced inhibition of c-Src kinase activity, Bcl-XL down-regulation, and apoptosis of intestinal epithelial cells

Detachment of epithelial cells from the extracellular matrix (ECM) results in apoptosis, a phenomenon often referred to as anoikis. Acquisition of anoikis resistance is now thought to be a prerequisite for the progression of carcinomas. Colorectal cancer cells frequently secrete epidermal growth factor receptor (EGFR) ligands, which are known to have anti-apoptotic activity. However, whether these ligands have the ability to inhibit anoikis of intestinal epithelial cells is unclear, since at least in some cell types efficient EGFR signaling requires cell-ECM adhesion. Here we report that transforming growth factor-alpha (TGF-alpha), an EGFR ligand that is frequently secreted by colorectal cancer cells, strongly inhibits anoikis of the non-malignant rat intestinal epithelial cell lines, IEC-18 and RIE-1. TGF-alpha exerts its anti-anoikis effect by preventing detachment-induced inhibition of c-Src kinase activity. We also show that Fas activation, a molecular event known to play a critical role in anoikis, is not suppressed by TGF-alpha. On the other hand, this growth factor strongly inhibits the detachment-induced down-regulation of Bcl-X(L), another change that is involved in the induction of anoikis. We further demonstrate that this inhibition occurs in a c-Src-dependent manner. We conclude that TGF-alpha has the ability to suppress anoikis of intestinal epithelial cells, at least in part, by reverting the loss of c-Src activity and Bcl-X(L) expression induced by detachment from the ECM.

Anaplastic and atypical meningiomas express high levels of Fas and undergo apoptosis in response to Fas ligation

In this study we characterized the expression of Fas and Fas ligand in different types of meningiomas and examined the effect of Fas ligation on the death of meningioma cells in culture. Using Western blot analysis, we found that extracts derived from anaplastic and atypical meningiomas expressed high levels of Fas, whereas benign meningiomas did not express detectable levels of this protein. All of the meningiomas examined expressed low levels of Fas ligand. Cultures of anaplastic meningiomas also expressed Fas and treatment of these cells with anti-Fas antibody induced cell death. The results of this study indicate that Fas is preferentially expressed in atypical and anaplastic meningiomas and suggest that it is involved in the increased apoptosis observed in these tumors.

Inhibition of TRAIL-induced apoptosis and forced internalization of TRAIL receptor 1 by adenovirus proteins

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis through two receptors, TRAIL-R1 (also known as death receptor 4) and TRAIL-R2 (also known as death receptor 5), that are members of the TNF receptor superfamily of death domain-containing receptors. We show that human adenovirus type 5 encodes three proteins, named RID (previously named E3-10.4K/14.5K), E3-14.7K, and E1B-19K, that independently inhibit TRAIL-induced apoptosis of infected human cells. This conclusion was derived from studies using wild-type adenovirus, adenovirus replication-competent mutants that lack one or more of the RID, E3-14.7K, and E1B-19K genes, and adenovirus E1-minus replication-defective vectors that express all E3 genes, RID plus E3-14.7K only, RID only, or E3-14.7K only. RID inhibits TRAIL-induced apoptosis when cells are sensitized to TRAIL either by adenovirus infection or treatment with cycloheximide. RID induces the internalization of TRAIL-R1 from the cell surface, as shown by flow cytometry and indirect immunofluorescence for TRAIL-R1. TRAIL-R1 was internalized in distinct vesicles which are very likely to be endosomes and lysosomes. TRAIL-R1 is degraded, as indicated by the disappearance of the TRAIL-R1 immunofluorescence signal. Degradation was inhibited by bafilomycin A1, a drug that prevents acidification of vesicles and the sorting of receptors from late endosomes to lysosomes, implying that degradation occurs in lysosomes. RID was also shown previously to internalize and degrade another death domain receptor, Fas, and to prevent apoptosis through Fas and the TNF receptor. RID was shown previously to force the internalization and degradation of the epidermal growth factor receptor. E1B-19K was shown previously to block apoptosis through Fas, and both E1B-19K and E3-14.7K were found to prevent apoptosis through the TNF receptor. These findings suggest that the receptors for TRAIL, Fas ligand, and TNF play a role in limiting virus infections. The ability of adenovirus to inhibit killing through these receptors may prolong acute and persistent infections.

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

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

Synergistic induction of apoptosis in primary CD4(+) T cells by macrophage-tropic HIV-1 and TGF-beta1

Depletion of CD4(+) T lymphocytes is a central immunological characteristic of HIV-1 infection. Although the mechanism of such CD4(+) cell loss following macrophage-tropic (R5) HIV-1 infection remains unclear, interactions between viral and host cell factors are thought to play an important role in the pathogenesis of HIV-1 disease. Based on the observation that TGF-beta1 enhanced expression of HIV chemokine coreceptors, the role of this host factor in virus effects was investigated using PBLs cultured in a nonmitogen-added system in the absence or presence of TGF-beta1. Most CD4 cells in such cultures had the phenotype CD25(-)CD69(-)DR(-)Ki67(-) and were CD45RO(bright)CD45RA(dim). Cultured cells had increased expression of CCR5 and CXCR4 and supported both HIV-1 entry and completion of viral reverse transcription. Virus production by cells cultured in the presence of IL-2 was inhibited by TGF-beta1, and this inhibition was accompanied by a loss of T cells from the culture and an increase in CD4(+) T cell apoptosis. Whereas R5X4 and X4 HIV-1 infection was sufficient to induce T cell apoptosis, R5 HIV-1 failed to induce apoptosis of PBLs in the absence of TGF-beta1 despite the fact that R5 HIV-1 depletes CD4(+) T cells in vivo. Increased apoptosis with HIV and TGF-beta1 was associated with reduced levels of Bcl-2 and increased expression of apoptosis-inducing factor, caspase-3, and cleavage of BID, c-IAP-1, and X-linked inhibitor of apoptosis. These results show that TGF-beta1 promotes depletion of CD4(+) T cells after R5 HIV-1 infection by inducing apoptosis and suggest that TGF-beta1 might contribute to the pathogenesis of HIV-1 infection in vivo.

The role of apoptosis in regulating hematopoiesis and hematopoietic stem cells

Apoptosis, or the ability of cells to die in an orderly and highly regulated manner, is essential for normal development and homeostasis of multicellular organisms. Diseases in which deregulation of this process is implicated include autoimmune diseases, cancer and Alzheimer's disease. The importance of apoptosis for the development and function of lymphoid cells has been extensively investigated. Less clear is the role apoptosis plays in regulating early progenitor and stem cell compartments. This question is being investigated using a transgenic mouse model, H2K-BCL-2, in which all hematopoietic cells have an increased resistance to apoptosis. The same transgenic model is also being used to address the question whether protection against apoptosis can increase system-wide resistance to lethal challenges such as irradiation and chemotherapeutic agents.

Modulation of tumor necrosis factor apoptosis-inducing ligand- induced NF-kappa B activation by inhibition of apical caspases

Tumor necrosis factor (TNF) apoptosis-inducing ligand (TRAIL), a member of the TNF family, induces apoptosis in many transformed cells. We report TRAIL-induced NF-kappaB activation, concomitant with production of the pro-inflammatory cytokine Interleukin-8 in the relatively TRAIL-insensitive cell line, HEK293. In contrast, TRAIL-induced NF-kappaB activation occurred in HeLa cells only upon pretreatment with the caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone (z-VAD.fmk), indicating that this was due to a caspase-sensitive component of TRAIL-induced NF-kappaB activation. NF-kappaB activation was mediated by the death receptors, TRAIL-R1 and -R2, but not by TRAIL-R3 or -R4 and was only observed in HeLa cells in the presence of z-VAD.fmk. Receptor-interacting protein, an obligatory component of TNF-alpha-induced NF-kappaB activation, was cleaved during TRAIL-induced apoptosis. We show that receptor-interacting protein is recruited to the native TRAIL death-inducing signaling complex (DISC) and that recruitment is enhanced in the presence of z-VAD.fmk, thus providing an explanation for the potentiation of TRAIL-induced NF-kappaB activation by z-VAD.fmk in TRAIL-sensitive cell lines. Examination of the TRAIL DISC in sensitive and resistant cells suggests that a high ratio of c-FLIP to caspase-8 may partially explain cellular resistance to TRAIL-induced apoptosis. Sensitivity to TRAIL-induced apoptosis was also modulated by inhibition or activation of NF-kappaB. Thus, in some contexts, modulation of NF-kappaB activation possibly at the level of apical caspase activation at the DISC may be a key determinant of sensitivity to TRAIL-induced apoptosis.

c-IAP1 blocks TNFalpha-mediated cytotoxicity upstream of caspase-dependent and -independent mitochondrial events in human leukemic cells

Tumor necrosis factor-alpha (TNFalpha) mediates cytochrome c release from mitochondria, loss of mitochondrial membrane potential (DeltaPsim) and apoptosis in sensitive leukemic cells. In the present study, by using the human leukemic U937 cell line, we demonstrate that the cytochrome c release is caspase-8-dependent and can be blocked by an inhibitor of caspase-8, Z-Ile-Glu (OMe)-Thr-Asp(OMe)-fluoromethyl ketone (Z-IETD.fmk), or a pan caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD.fmk). However, TNFalpha-mediated loss of DeltaPsim was not inhibited by caspase inhibitors. The apoptotic process was blocked by either Z-IETD.fmk or Z-VAD.fmk in cells with lower DeltaPsim. U937 cells with stable transfection of the cellular inhibitor of apoptosis protein 1 (c-IAP1) are resistant to TNFalpha-induced activation of caspases, Bid cleavage, cytochrome c release and DeltaPsim collapse. In addition, both c-IAP1 and XIAP were not up-regulated upon prolonged exposure to TNFalpha. In contrast, there was a caspase-dependent cleavage of XIAP, but not c-IAP1, during treatment with TNFalpha for 7 days. These results demonstrate that c-IAP1 blocks TNFalpha signaling at a level controlling both activation of caspase-8 and a signal to cause loss of DeltaPsim. The sensitive U937 cell line failed to acquire resistance and gain a self-protecting advantage against apoptosis, upon induction of c-IAP1 expression.

MEK1/2 inhibitors promote Ara-C-induced apoptosis but not loss of Deltapsi(m) in HL-60 cells

The effects of pharmacologic MEK1/2 inhibitors on ara-C-mediated mitochondrial injury, caspase activation, and apoptosis have been examined in HL-60 leukemic cells. Coadministration of subtoxic concentrations of the MEK1/2 inhibitors U0126 (20 microM), PD98059 (40 microM), or PD184352 (10 microM) with 10-100 microM ara-C (6 h) potentiated apoptosis (i.e., by approx twofold), and pro-caspase 3, pro-caspase 8, Bid, and PARP cleavage. Unexpectedly, MEK1/2 inhibitors failed to enhance ara-C-mediated loss of mitochondrial membrane potential (DeltaPsi(m)), but instead induced substantial increases in cytosolic release of cytochrome c and Smac/DIABLO. U0126/ara-C-mediated apoptosis and pro-caspase 3 activation, but not cytochrome c or Smac/DIABLO release, were blocked by the pan-caspase inhibitor ZVAD-fmk. Together, these findings indicate that potentiation of ara-C-mediated lethality in HL-60 cells by MEK1/2 inhibitors involves enhanced cytosolic release of cytochrome c and Smac/DIABLO but not discharge of DeltaPsi(m), implicating activation of an apoptotic pathway that differs, at least with respect to the nature of the accompanying mitochondrial injury, from that triggered by ara-C alone.

Apoptosis induced by human Fas-associated factor 1, hFAF1, requires its ubiquitin homologous domain, but not the Fas-binding domain

FAF1 is a Fas-binding protein without typical death domain. Instead, FAF1 has several domains found in proteins of ubiquitination pathway. Transient overexpression of hFAF1 in BOSC23 cells caused membrane blebbing and cell body condensation which were characteristics of apoptosis. Subsequent analysis revealed that overexpression of hFAF1 induced nuclear condensation, appearance of phosphatidylserines in the outer leaflet of the cellular membrane, and caspase 3 activation. The apoptotic potential of hFAF1 required downstream ubiquitin homologous domain (UB2) and adjacent nuclear localization signal but not the Fas-binding domain. Our data showed that mere intrinsic overexpression of hFAF1 initiated apoptosis in the absence of any extrinsic death signal in BOSC23 cells.

Fas ligand and TRAIL augment the effect of photodynamic therapy on the induction of apoptosis in JURKAT cells

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) and Fas ligand (FasL) trigger apoptosis by stimulating the formation of a death inducing signaling complex at the cytoplasmic terminus of their respective receptors. Photodynamic therapy (PDT) is an approved treatment for several types of cancer as well as for age-related macular degeneration and is under investigation for different cancer, ocular, autoimmune and cardiovascular indications. The effect of low dose PDT in combination with TRAIL and FasL on Jurkat lymphoma cell apoptosis was examined. Individually, TRAIL, FasL, and PDT could induce apoptosis in these cells. However, at suboptimal levels of PDT, the number of cells undergoing apoptosis was increased when recombinant FasL and/or TRAIL were added. Additive effects of these treatments were evident for different apoptosis parameters including DNA fragmentation, caspase processing and activity and caspase substrate degradation. Overall, these results provide evidence that PDT-treated cells may be more likely to undergo apoptosis when also exposed to receptor-mediated signals delivered by factors such as TRAIL or FasL. For PDT, immune cell-mediated death receptor ligation may represent a way whereby tumor cells that have withstood the direct effects of photosensitization may be eliminated.

Pharmacologic modulation of the immune interaction between cytotoxic lymphocytes and ventricular myocytes

Numerous studies have demonstrated that immune effector mechanisms cause serious heart diseases, among which are heart transplant rejection, myocarditis, and the resulting dilated cardiomyopathy, as well as Chagas' disease. Whereas different effectors of the immune system can affect cardiac function, this review primarily focuses on the immune damage caused by cytotoxic T lymphocytes. The immune attack staged by cytotoxic T lymphocytes is carried out by one of two distinct modes of lymphocytotoxicity: (a) secretion of lytic granules containing the pore-forming protein perforin and a family of serine proteases (i.e., granzymes) and (b) interaction between the lymphocyte Fas ligand and the target cell Fas receptor. Ventricular myocytes challenged by the immune system sustain diverse intracellular changes, among which the rise in intracellular calcium ([Ca2+]i) constitutes an important contributor to myocyte dysfunction. Hence, this [Ca2+]i rise, which does not necessarily result in apoptosis, can affect cardiac function directly and indirectly. Importantly, the final outcomes of these perturbations vary markedly and depend on intracellular circumstances such as the magnitude of the absolute rise in [Ca2+]i and its temporal and spatial determinants, the metabolic status of the myocyte, as well as a fine balance between pro-apoptotic and anti-apoptotic factors. In view of the central role of [Ca2+]i rise in immune-mediated myocyte dysfunction and possibly cell death, this review addresses three topics related to the immune assault on the heart: (a) [Ca2+]i rise in affected myocytes; (b) the source for the [Ca2+]i rise; and (c) pharmacologic modification of the immune-mediated [Ca2+]i rise.

Potential and caveats of TRAIL in cancer therapy

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

CD95-induced JNK activation signals are transmitted by the death-inducing signaling complex (DISC), but not by Daxx

Here we investigated CD95-mediated JNK activation pathways and their physiological relevance by employing a variety of cell lines with deficiencies in individual signal transmitting proteins. JNK activation was completely dependent on the activation of caspases in type I and type II cells, as revealed by the inhibitory effects of the caspase inhibitors zVAD-fmk or the cowpoxvirus-encoded CrmA protein. Jurkat cells deficient in caspase-8 or expressing a dominant negative (DN) form of FADD were unable to induce JNK in response to CD95 ligation, indicating that these death-inducing signaling complex (DISC) proteins are required for signal transmission. Activation of caspases, JNK and apoptosis occurred with a markedly slower kinetics in cells expressing a DN version of ASK1, revealing an important contribution of ASK1 for these processes. A C-terminally truncated version of Daxx impaired CD95-mediated apoptosis without affecting the JNK signal. DN forms of FADD, MKK4 and MKK7 completely inhibited CD95-mediated JNK activation but remained without impact on cell killing, indicating that JNK activation is not required for the execution process of CD95-mediated cell killing.

Differential activation of TRAIL-R1 and -2 by soluble and membrane TRAIL allows selective surface antigen-directed activation of TRAIL-R2 by a soluble TRAIL derivative

TNF-related apoptosis-inducing ligand (TRAIL) is a typical member of the tumor necrosis factor (TNF) ligand family that is expressed as a type II membrane protein (memTRAIL) and signals apoptosis via the death domain-containing receptors TRAIL-R1 and -2. Soluble recombinant derivatives of TRAIL (sTRAIL) are considered as novel tumors therapeutics because of their selective apoptosis inducing activity in a variety of human tumors but not in normal cells. Using antagonistic antigen-binding fragment (Fab) preparations of TRAIL-R1- and TRAIL-R2-specific antibodies, we demonstrate in this study that TRAIL-R1 becomes activated by both the soluble and the membrane-bound form of the ligand, whereas TRAIL-R2 becomes only activated by memTRAIL or soluble TRAIL secondarily cross-linked by antibodies. Furthermore, we show that the restricted signal capacity of sTRAIL can be readily converted into a fully signal competent memTRAIL-like molecule, i.e. a TRAIL-R2 stimulating ligand, by genetic fusion to an antibody derivative that allows antigen-dependent 'immobilization' of the fusion protein to cell surfaces. We conclude that antibody targeting-dependent activation can be used to design selective therapeutics derived of those ligands of the TNF family that are biologically inactive in their soluble form.

Ultraviolet light downregulates CD95 ligand and TRAIL receptor expression facilitating actinic keratosis and squamous cell carcinoma formation

Long-term ultraviolet light exposure of human skin epidermis in Caucasians is associated with an increased risk for the development of melanoma and nonmelanoma skin cancers. Ultraviolet radiation not only induces DNA damage in epidermal cells, it also interferes with skin homeostasis, which is maintained by a unique distribution pattern of apoptosis-inducing and apoptosis-preventing molecules. We demonstrate that, beside CD95 ligand, TRAIL and TRAIL receptors also function as important sensors in the human epidermis preserving skin integrity and preventing cell transformation. Ultraviolet irradiation extensively changes the expression pattern of some of these molecules, diminishing their sensor function. In particular, CD95 ligand and to a somewhat lesser extent TRAIL receptors are downregulated upon ultraviolet light exposure. CD95 ligand downregulation is not due to protein degradation as in situ hybridization experiments strongly support a transcriptional regulation. The downregulation of these molecules with sensor function increases the risk that aberrant cells are less efficiently eliminated. This concept is supported by the fact that the expression of these molecules is also low or absent in actinic keratosis, a precancerous state that has developed as the consequence of long-term ultraviolet exposure. Progression to invasive neoplasms is then accompanied by an upregulation of CD95 ligand and a downregulation of CD95 and of the TRAIL receptors. The high expression of CD95 ligand, TRAIL, and FLIP in squamous cell carcinoma may then contribute to the immune escape of the tumor, whereas the lack of expression of CD95 and TRAIL receptors prevents autolysis of the tumor.

NF-kappaB inducers upregulate cFLIP, a cycloheximide-sensitive inhibitor of death receptor signaling

The caspase 8 homologue FLICE-inhibitory protein (cFLIP) is a potent negative regulator of death receptor-induced apoptosis. We found that cFLIP can be upregulated in some cell lines under critical involvement of the NF-kappaB pathway, but NF-kappaB activation was clearly not sufficient for cFLIP induction in all cell lines. Treatment of SV80 cells with the proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG-132) or geldanamycin, a drug interfering with tumor necrosis factor (TNF)-induced NF-kappaB activation, inhibited TNF-induced upregulation of cFLIP. Overexpression of a nondegradable IkappaBalpha mutant (IkappaBalpha-SR) or lack of IkappaB kinase gamma expression completely prevented phorbol myristate acetate-induced upregulation of cFLIP mRNA in Jurkat cells. These data point to an important role for NF-kappaB in the regulation of the cFLIP gene. SV80 cells normally show resistance to TNF-related apoptosis-inducing ligand (TRAIL) and TNF, as apoptosis can be induced only in the presence of low concentrations of cycloheximide (CHX). However, overexpression of IkappaBalpha-SR rendered SV80 cells sensitive to TRAIL-induced apoptosis in the absence of CHX, and cFLIP expression was able to reverse the proapoptotic effect of NF-kappaB inhibition. Western blot analysis further revealed that cFLIP, but not TRAF1, A20, and cIAP2, expression levels rapidly decrease upon CHX treatment. In conclusion, these data suggest a key role for cFLIP in the antiapoptotic response of NF-kappaB activation.

Proliferation, cell cycle and apoptosis in cancer

Beneath the complexity and idiopathy of every cancer lies a limited number of 'mission critical' events that have propelled the tumour cell and its progeny into uncontrolled expansion and invasion. One of these is deregulated cell proliferation, which, together with the obligate compensatory suppression of apoptosis needed to support it, provides a minimal 'platform' necessary to support further neoplastic progression. Adroit targeting of these critical events should have potent and specific therapeutic consequences.

A GTP-binding adapter protein couples TRAIL receptors to apoptosis-inducing proteins

Apoptosis-inducing tumor necrosis factor (TNF) family receptors recruit the proforms of caspase family cell death proteases to ligand-receptor complexes through interactions with intracellular adapter proteins. We have found that the GTP-binding protein DAP3 binds directly (with high affinity) to the death domain of TNF-related apoptosis-inducing ligand (TRAIL) receptors, and is required for TRAIL-induced apoptosis. DAP3 also associates with the pro-caspase-8--binding adapter protein Fas-associated death domain (FADD), and links FADD to the TRAIL receptors DR4 and DR5. We have also found that binding of DAP3 to FADD and activation of pro-caspase-8 in an in vitro reconstituted system is GTP-dependent. Elucidation of this mechanism suggests GTP-binding proteins as potential targets for pharmacological intervention in TRAIL-induced apoptosis.

Different roles of spermine in glucocorticoid- and Fas-induced apoptosis

Two experimental systems representative of the mitochondrial and death receptor apoptotic pathways are the dexamethasone-induced programmed cell death in mouse thymocytes and the antibody-mediated cross-ligation of the Fas receptor in the human leukemic T-cell line Jurkat, respectively. In both cell systems, caspase-9, -8, and -3 were activated upon induction of apoptosis and a sub-G(1) peak appeared as a sign of ongoing DNA fragmentation. Addition of 1 mM spermine together with dexamethasone inhibited caspase activation and the appearance of the sub-G(1) peak in mouse thymocytes. In contrast, Fas-induced cell death was totally unaffected by spermine addition. Spermine addition significantly elevated the spermine concentration in both thymocytes and Jurkat cells. Thus, spermine per se did not inhibit the caspases but rather their activation. The fact that spermine inhibited caspase activation only in the thymocytes implies that spermine inhibited dexamethasone-induced apoptosis upstream of caspase-9 activation.

Intracellular mechanisms of TRAIL: apoptosis through mitochondrial-dependent and -independent pathways

Tumor necrosis (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines that promotes apoptosis. TRAIL induces apoptosis via death receptors (DR4 and DR5) in a wide variety of tumor cells but not in normal cells. The objectives of this study are to investigate the intracellular mechanisms by which TRAIL induces apoptosis. The death receptor Fas, upon ligand binding, trimerizes and recruits the adaptor protein FADD through the cytoplasmic death domain of Fas. FADD then binds and activates procaspase-8. It is unclear whether FADD is required for TRAIL-induced apoptosis. Here we show that the signaling complex of DR4/DR5 is assembled in response to TRAIL binding. FADD and caspase-8, but not caspase-10, are recruited to the receptor, and cells deficient in either FADD or caspase-8 blocked TRAIL-induced apoptosis. In addition, TRAIL initiates the activation of caspases, the loss of mitochondrial transmembrane potential (Deltapsi(m)), the cleavage of BID, and the redistribution of mitochondrial cytochrome c. Treatment of Jurkat cells with cyclosporin A delayed TRAIL-induced Deltapsi(m), caspase-3 activation and apoptosis. Similarly, Overexpression of Bcl-2 or Bcl-X(L) delayed, but did not inhibit, TRAIL-induced Deltapsi(m) and apoptosis. In contrast, XIAP, cowpox virus CrmA and baculovirus p35 inhibited TRAIL-induced apoptosis. These data suggest that death receptors (DR4 and DR5) and Fas receptors induced apoptosis through identical signaling pathway, and TRAIL-induced apoptosis via both mitochondrial-dependent and -independent pathways.

Effects of smoking on activation markers, Fas expression and apoptosis of peripheral blood lymphocytes

BACKGROUND

Smoking influences numbers and function of peripheral blood lymphocytes (PBL) by a process that is badly understood. We conducted this study to evaluate whether the immune impairment of smoking might be related to changes in the expression or functionality of Fas, a cell surface molecule that plays a central role in immune homeostasis and cytotoxic activity.

METHODS

PBL from 10 smoking and 10 nonsmoking healthy volunteers were isolated. Flow cytometry was performed to measure the state of activation, Fas expression and apoptosis of PBL. Functionality of Fas was tested by assessing apoptosis after incubation of isolated lymphocytes with agonistic anti-Fas antibodies in four smoking and four nonsmoking individuals.

RESULTS

Smoking was associated with an increase in the percentage of Fas-expressing CD4+ T and B lymphocytes. A decrease in the percentage of activated (CD38+) B cells was observed. In vitro Fas-induced apoptosis did not appear different between smokers and nonsmokers. No differences in the percentages of circulating apoptotic lymphocytes could be demonstrated between smoking and nonsmoking individuals. Conclusion Smoking is associated with increased Fas expression on PBL in general, and on B cells in particular. This might render these cells more susceptible for apoptosis. As Fas is functionally intact this may also explain the reduced percentage of activated (CD38+) B cells found in smoking individuals. The latter may contribute to the reduced humoral immune response observed in smokers.

Retinoic acid-induced apoptosis in leukemia cells is mediated by paracrine action of tumor-selective death ligand TRAIL

The therapeutic and preventive activities of retinoids in cancer are due to their ability to modulate the growth, differentiation, and survival or apoptosis of cancer cells. Here we show that in NB4 acute promyelocytic leukemia cells, retinoids selective for retinoic-acid receptor-alpha induced an autoregulatory circuitry of survival programs followed by expression of the membrane-bound tumor-selective death ligand, TRAIL (tumor necrosis factor-related apoptosis-inducing ligand, also called Apo-2L). In a paracrine mode of action, TRAIL killed NB4 as well as heterologous and retinoic-acid-resistant cells. In the leukemic blasts of freshly diagnosed acute promyelocytic leukemia patients, retinoic-acid-induced expression of TRAIL most likely caused blast apoptosis. Thus, induction of TRAIL-mediated death signaling appears to contribute to the therapeutic value of retinoids.

Bcl-2 oncoprotein protects the human prostatic carcinoma cell line PC3 from TRAIL-mediated apoptosis

The role of Bcl-2 in TRAIL-induced apoptosis has been investigated in lymphoid cells. Here we show that the human prostatic carcinoma cell line PC3 was sensitive to TRAIL treatment whereas PC3 overexpressing of Bcl-2 was resistant. TRAIL receptors ligation in PC3 activated caspases -2, -3, -7, -8, and -9, induced Bid processing, dissipation of mitochondrial transmembrane potential (Delta Psi(m)), and cytochrome c release. We have detected caspases -8 and -3 only in the cytosolic fraction of cells, but caspases -2, -7, and -9 were found both in cytosolic and mitochondrial fractions. Bcl-2 overexpression did not affect caspase-8 activation although it did change the processing pattern of caspase-3. At the same time, Bcl-2 overexpression inhibited the activation of mitochondrial localized caspases -2, -7, and -9. Bcl-2 also abrogated TRAIL-induced cytochrome c release and dissipation of Delta Psi(m). These findings suggest that TRAIL-induced apoptosis in the epithelial cell line PC3 depends both on mitochondrial integrity and caspase activation.

Death domain mutagenesis of KILLER/DR5 reveals residues critical for apoptotic signaling

The Fas/tumor necrosis factor (TNF)/TRAIL receptors signal death through a cytoplasmic death domain (DD) containing six alpha-helices with positively charged helix 2 interacting with negatively charged helix 3 of another DD. DD mutation occurs in head/neck and lung cancer (TRAIL receptor KILLER/DR5) and in lpr mice (Fas). We examined the apoptotic potential of known KILLER/DR5 lung tumor-derived mutants (n = 6) and DD mutants (n = 18) generated based on conservation with DR4, Fas, Fas-associated death domain (FADD), and tumor necrosis factor receptor 1 (TNFR1). With the exception of Arg-330 required in Fas or FADD for aggregation or for TNFR1 cytotoxicity, surprisingly major loss-of-function KILLER/DR5 alleles (W325A, L334A (lpr-like), I339A, and W360A) contained hydrophobic residues. Loss-of-function of I339A (highly conserved) has not been reported in DDs. Charged residue mutagenesis revealed the following points. 1) E326A, conserved in DR4, is dispensable for death; the homologous residue is positively charged in Fas, TNFR1, and FADD and is critical for DD interactions. 2) K331A, D336A, E338A, K340A, K343A, and D351A have partial loss-of-function suggesting multiple charges stabilize receptor-adapter interactions. Analysis of the tumor-derived KILLER/DR5 mutants revealed the following. 1) L334F has partial loss-of-function versus L334A, whereas E338K has major loss-of-function versus E338A, examples where alanine and tumor-specific substitutions have divergent phenotypes. 2) Unexpectedly, S324F, E326K, K386N, and D407Y have no loss-of-function with tumor-specific or alanine substitutions. Loss-of-function KILLER/DR5 mutants were deficient in recruitment of FADD and caspase 8 to TRAIL death-inducing signaling complexes. The results reveal determinants within KILLER/DR5 for death signaling and drug design.

TRAIL-Induced Apoptosis in Type I Leukemic Cells Is Not Enhanced by Overexpression of Bax

We have previously shown that Bax translocation was crucial in TNFalpha or etoposide-induced apoptosis. Overexpression of Bax sensitized chronic myeloid leukemic K562 cells to etoposide-induced apoptosis. Treatment with TNF-related apoptosis-inducing ligand (TRAIL) induces a loss of mitochondrial membrane potential (DeltaPsim), cytochrome c release from mitochondria, activation of caspases-8, -9, and -3, and cleavage of Bid in the K562 cell line. Bax failed to sensitize K562 cells to TRAIL-induced apoptosis. TRAIL did not induce Bax expression and/or translocation from cytosol to mitochondria in the K562 cell line. However, 100 microM Z-VAD.fmk, a pan caspase inhibitor, completely blocked TRAIL-initiated mitochondrial alterations and cleavages of caspases and Bid. We propose that TRAIL-induced apoptosis in K562 cells is via Type I apoptotic signal pathway. Bax translocation is not essential for TRAIL-induced cytochrome c release and DeltaPsim collapse in the Type I cells.

Expression of P-170 glycoprotein sensitizes lymphoblastoid CEM cells to mitochondria-mediated apoptosis

Multidrug resistance caused by P-glycoprotein (P-170) is a phenomenon by which cells exposed to a single drug acquire resistance to other structurally and functionally unrelated drugs. This is a widespread phenomenon described in vivo in the management of infectious as well as non-infectious diseases. Several in vitro models have been developed in order to evaluate physiopathological properties of P-170. Among these are P-170-expressing variants of the human T-lymphoblastoid CEM cell line called VBL100. As a general rule, drug resistance normally results in resistance to apoptosis induction. By contrast, a paradoxical activity is exerted in this cell model by the cytokine tumour necrosis factor-alpha (TNF-alpha), which is capable of inducing apoptosis in P-170-expressing variants better than in wild-type (wt) cells. In the present study we partially address the mechanisms underlying this activity. In fact, the susceptibility of VBL100 cells to TNF-alpha appears to be specifically due to the depolarization of their mitochondrial membrane, a key factor for apoptotic induction. The same was observed with staurosporine, a specific mitochondrion-mediated proapoptotic chemical probe. Conversely, other proapoptotic stimuli, such as Fas/CD95 or the anti-cancer drug etoposide, did induce significant cell death in wild type cells only. Thus, schematically, mitochondrially dependent stimuli appeared to be more effective in VBL100-cell killing, while 'physiological' stimuli showed the opposite behaviour. Importantly, under steady-state conditions, VBL100 cells displayed per se a mitochondrial membrane hyperpolarization that appeared strictly related to their high susceptibility to specific apoptotic stimuli. In conclusion, the study of a well-established cell model such as that represented by the wt/VBL CEM lymphoid cell line seems to suggest that the multidrug resistance phenotype can specifically sensitize cells towards 'unphysiological', mitochondria-associated cell death cascade or, in the same fashion, it could shift cells from type I (mainly plasma membrane-associated) towards type II (mainly mitochondrial membrane-associated) phenotype.

Tumor necrosis factor-related apoptosis-inducing ligand in T cell development: sensitivity of human thymocytes

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) is a recently identified member of the tumor necrosis factor cytokine superfamily. TRAIL has been shown to induce apoptosis in various tumor cell lines, whereas most primary cells seem to be resistant. These observations have raised considerable interest in the use of TRAIL in tumor therapy. Yet little is known about the physiological function of TRAIL. This is particularly the case in the immune system, where TRAIL has been suggested by some to be involved in target cell killing and lymphocyte death. We have developed a panel of mAbs and soluble proteins to address the role of TRAIL in lymphocyte development. These studies demonstrate activation-induced sensitization of thymocytes to TRAIL-mediated apoptosis and expression of the apoptosis-inducing TRAIL receptors. However, with the use of several model systems, our subsequent experiments rule out the possibility that TRAIL plays a major role in antigen-induced deletion of thymocytes. In contrast to thymocytes, there is no up-regulation of TRAIL receptors in peripheral T cells on activation, which remain resistant to TRAIL. Thus, susceptibility to TRAIL-induced apoptosis is controlled differently by central and peripheral T cells.

Elevated AKT activity protects the prostate cancer cell line LNCaP from TRAIL-induced apoptosis

We find that the prostate cancer cell lines ALVA-31, PC-3, and DU 145 are highly sensitive to apoptosis induced by TRAIL (tumor-necrosis factor-related apoptosis-inducing ligand), while the cell lines TSU-Pr1 and JCA-1 are moderately sensitive, and the LNCaP cell line is resistant. LNCaP cells lack active lipid phosphatase PTEN, a negative regulator of the phosphatidylinositol (PI) 3-kinase/Akt pathway, and demonstrate a high constitutive Akt activity. Inhibition of PI 3-kinase using wortmannin and LY-294002 suppressed constitutive Akt activity and sensitized LNCaP cells to TRAIL. Treatment of LNCaP cells with TRAIL alone induced cleavage of the caspase 8 and XIAP proteins. However, processing of BID, mitochondrial release of cytochrome c, activation of caspases 7 and 9, and apoptosis did not occur unless TRAIL was combined with either wortmannin, LY-294002, or cycloheximide. Blocking cytochrome c release by Bcl-2 overexpression rendered LNCaP cells resistant to TRAIL plus wortmannin treatment but did not affect caspase 8 or BID processing. This indicates that in these cells mitochondria are required for the propagation rather than the initiation of the apoptotic cascade. Infection of LNCaP cells with an adenovirus expressing a constitutively active Akt reversed the ability of wortmannin to potentiate TRAIL-induced BID cleavage. Thus, the PI 3-kinase-dependent blockage of TRAIL-induced apoptosis in LNCaP cells appears to be mediated by Akt through the inhibition of BID cleavage.

Natural ceramide reverses Fas resistance of acid sphingomyelinase(-/-) hepatocytes

The role of the second messenger ceramide in Fas-mediated death requires clarification. To address this issue, we generated hepatocytes from paired acid sphingomyelinase (ASMase; asmase)(+/+) and asmase(-/-) mice. asmase(-/-) hepatocytes, derived from 8-week-old mice, manifested normal sphingomyelin content and normal morphological, biochemical, and biologic features. Nonetheless, ASMase-deficient hepatocytes did not display rapid ceramide elevation or apoptosis in response to Jo2 anti-Fas antibody. asmase(-/-) hepatocytes were not inherently resistant to apoptosis because staurosporine, which did not induce early ceramide elevation, stimulated a normal apoptotic response. The addition of low nanomolar quantities of natural C16-ceramide, which by itself did not induce apoptosis, completely restored the apoptotic response to anti-Fas in asmase(-/-) hepatocytes. Other sphingolipids did not replace natural ceramide and restore Fas sensitivity. Overcoming resistance to Fas in asmase(-/-) hepatocytes by natural ceramide is evidence that it is the lack of ceramide and not ASMase which determines the apoptotic phenotype. The ability of natural ceramide to rescue the phenotype without reversing the genotype provides evidence that ceramide is obligate for Fas induction of apoptosis in hepatocytes.

The complexity of TNF-related apoptosis-inducing ligand

One of the major goals of researchers in the field of apoptosis is to understand the molecular mechanisms of the various components of the apoptotic pathways, with the hope to identify targets for novel cancer therapies. The discovery of a TNF-related, apoptosis-inducing ligand, TRAIL, that kills transformed cells with great specificity in vitro, has provided the hope that TRAIL may be used to induce cell death in tumor cells without affecting normal tissues. However, TRAIL signaling is very complex and a clear understanding of its function is necessary before it can be used in cancer therapy. Complexity of TRAIL-induced signaling is apparent from its ubiquitous expression, its ability to interact with five receptors, and its tumor-selective induction of apoptosis. The signaling events that mediate the tumor selectivity of TRAIL-induced apoptosis and the biological functions of each of the TRAIL receptors are not well characterized. This review will focus on the complexity of TRAIL and the role of c-FLIP in mediating TRAIL function.

Apoptosis and rheumatoid arthritis: past, present, and future directions

The current studies of apoptosis in rheumatoid arthritis (RA) suggest that molecules (Fas-related or TNF-related), pathways (activation of pro-apoptosis or anti-apoptosis pathway), cell types (lymphocytes or synovial fibroblast), and the mechanism that triggers apoptosis (tolerance induction-related, down-modulation of inflammation-related, or DNA damage-related) all play a fundamental role to determine the induction or prevention of RA. These series of defects at different levels and in different cells lead to hyperproliferation, defective apoptosis, or hyperapoptosis. This review summarizes the available knowledge of apoptosis and RA to help identify candidate target cells and target molecules for delivery of gene constructs or modified biological or chemical reagents to the target site for effective modification of these cells.

TNF-related apoptosis-inducing ligand-induced apoptosis of melanoma is associated with changes in mitochondrial membrane potential and perinuclear clustering of mitochondria

Past studies have shown that TNF-related apoptosis-inducing ligand (TRAIL) induced apoptosis in a high proportion of cultured melanoma by caspase-dependent mechanisms. In the present studies we have examined whether TRAIL-induced apoptosis of melanoma was mediated by direct activation of effector caspases or whether apoptosis was dependent on changes in mitochondrial membrane potential (MMP) and mitochondrial-dependent pathways of apoptosis. Changes in MMP were measured by fluorescent emission from rhodamine 123 in mitochondria. TRAIL, but not TNF-alpha or Fas ligand, was shown to induce marked changes in MMP in melanoma, which showed a high correlation with TRAIL-induced apoptosis. This was associated with activation of proapoptotic protein Bid and release of cytochrome c into the cytosol. Overexpression of B cell lymphoma gene 2 (Bcl-2) inhibited TRAIL-induced release of cytochrome c, changes in MMP, and apoptosis. The pan caspase inhibitor z-Val-Ala-Asp-fluoromethylketone (zVAD-fmk) and the inhibitor of caspase-8 (z-Ile-Glu-Thr-Asp-fluoromethylketone; zIETD-fmk) blocked changes in MMP and apoptosis, suggesting that the changes in MMP were dependent on activation of caspase-8. Activation of caspase-9 also appeared necessary for TRAIL-induced apoptosis of melanoma. In addition, TRAIL, but not TNF-alpha or Fas ligand, was shown to induce clustering of mitochondria around the nucleus. This process was not essential for apoptosis but appeared to increase the rate of apoptosis. Taken together, these results suggest that TRAIL induces apoptosis of melanoma cells by recruitment of mitochondrial pathways to apoptosis that are dependent on activation of caspase-8. Therefore, factors that regulate the mitochondrial pathway may be important determinants of TRAIL-induced apoptosis of melanoma.

Sensitivity to TRAIL/APO-2L-mediated apoptosis in human renal cell carcinomas and its enhancement by topotecan

TRAIL (APO-2L) is a newly identified member of the TNF family and induces apoptosis in cancer cells without affecting most non-neoplastic cells, both in vitro and in vivo. Our study focused on the expression and function of TRAIL and its receptors in renal cell carcinoma (RCC) cell lines of all major histological types. Here, we demonstrate that all RCC cell lines express TRAIL as well as the death-inducing receptors TRAIL-R1 (DR4) and TRAIL-R2 (Killer/DR5). Exposure to TRAIL induced apoptosis in 10 of 16 RCC cell lines. Remarkably, five of six TRAIL-resistant RCC cell lines exhibited high levels of TRAIL expression. Topotecan, a novel topoisomerase I inhibitor, induced upregulation of TRAIL-R2 as well as downregulation of TRAIL. Neutralization of TRAIL with recombinant soluble TRAIL-R1-Fc and TRAIL-R2-Fc failed to inhibit topotecan-induced apoptosis indicating that topotecan-induced cell death can occur in a TRAIL-independent fashion. However, exposure to topotecan resulted in an enhancement of TRAIL-induced apoptosis in all primarily TRAIL-resistant RCC cell lines. This synergistic effect of cotreatment with Topotecan and TRAIL may provide the basis for a new therapeutic approach to induce apoptosis in otherwise unresponsive RCC.