Requirement for the CD95 receptor-ligand pathway in c-Myc-induced apoptosis

c-MYC: more than just a matter of life and death

Deregulated expression of c-MYC occurs in a broad range of human cancers and is often associated with poor prognosis, indicating a key role for this oncogene in tumour progression. However, as established human tumours often bear multiple genetic lesions, it is difficult to determine whether c-MYC is instrumental in the initiation/progression of the tumour, or indeed whether inactivating c-MYC would lead to tumour regression. Regulatable transgenic mouse models of oncogenesis have shed light on these issues and provide hope for effective cancer therapies.

c-Myc sensitizes cells to tumor necrosis factor-mediated apoptosis by inhibiting nuclear factor kappa B transactivation

Nuclear factor kappaB (NF-kappaB) plays a key role in suppression of tumor necrosis factor (TNF)-mediated apoptosis by inducing a variety of anti-apoptotic genes. Expression of c-Myc has been shown to sensitize cells to TNF-mediated apoptosis by inhibiting NF-kappaB activation. However, the precise step in the NF-kappaB signaling pathway and apoptosis modified by c-Myc has not been identified. Using the inducible c-MycER system and c-Myc null fibroblasts, we found that expression of c-Myc inhibited NF-kappaB activation by interfering with RelA/p65 transactivation but not nuclear translocation of NF-kappaB. Activation of c-Myc promoted TNF-induced release of cytochrome c from mitochondria to the cytosol because of the inhibition of NF-kappaB. Furthermore, we found that NF-kappaB-inducible gene A1 was attenuated by expression of c-Myc and that the restoration of A1 expression suppressed c-Myc-induced TNF sensitization. Our results elucidate the molecular mechanisms by which c-Myc increases cell susceptibility to TNF-mediated apoptosis, indicating that c-Myc may exhibit its pro-apoptotic activities by repression of cell survival genes.

c-Myc functionally cooperates with Bax to induce apoptosis

c-Myc promotes apoptosis by destabilizing mitochondrial integrity, leading to the release of proapoptotic effectors including holocytochrome c. Candidate mediators of c-Myc in this process are the proapoptotic members of the Bcl-2 family. We show here that fibroblasts lacking Bak remain susceptible to c-Myc-induced apoptosis whereas bax-deficient fibroblasts are resistant. However, despite this requirement for Bax, c-Myc activation exerts no detectable effects on Bax expression, localization, or conformation. Moreover, susceptibility to c-Myc-induced apoptosis can be restored in bax-deficient cells by ectopic expression of Bax or by microinjection of a peptide comprising a minimal BH3 domain. Microinjection of BH3 peptide also restores sensitivity to c-Myc-induced apoptosis in p53-deficient primary fibroblasts that are otherwise resistant. By contrast, there is no synergy between BH3 peptide and c-Myc in fibroblasts deficient in both Bax and Bak. We conclude that c-Myc triggers a proapoptotic mitochondrial destabilizing activity that cooperates with proapoptotic members of the Bcl-2 family.

The oncogenic potential of Kaposi's sarcoma-associated herpesvirus cyclin is exposed by p53 loss in vitro and in vivo

Expression of the Kaposi's sarcoma-associated herpesvirus (KSHV) cyclin D homolog, K cyclin, is thought to contribute to viral oncogenesis. We show that K cyclin expression in primary cells sensitizes to apoptosis and induces growth arrest, both of which are dependent on p53 but independent of E2F1 or p19(ARF). DNA synthesis, but not cytokinesis, continues in K cyclin-expressing cells, leading to multinucleation and polyploidy. Such polyploid cells exhibit pronounced centrosome amplification and consequent aneuploidy. Our data suggest that K cyclin expression leads to cytokinesis defects and polyploidy, which activates p53. However, in the absence of p53, such cells survive and expand as an aneuploid population. Corroborating these findings, in vivo Emu; K cyclin expression cooperates with p53 loss in the induction of lymphomas.

The protective effect of phorbol esters on Fas-mediated apoptosis in T cells. Transcriptional and postranscriptional regulation

Phorbol esters are tumor promoters that bind and activate both conventional and new Protein kinase C (PKC) isoforms. In various circumstances, PKC-dependent signaling pathways can promote cell survival and protect against cell death. This was first analysed in Jurkat T cells where Phorbol Myristate Acetate (PMA) was found to inhibit Fas-mediated apoptosis as judged by DiOC6(3) staining, caspase activation and DNA fragmentation, indicating that PMA exerts its protective effect upstream or at the mitochondrial level in these cells. PMA activated most of the main kinase pathways in T cells such as PKCs, p42/44MAPK, p38MAPK and p90Rsk but not JNK and Akt. A pharmacological approach allowed us to identify that nPKCs are both necessary and likely sufficient to promote T cell survival. Besides this post-transcriptional regulation, nPKCs may also regulate apoptosis at the transcriptional level. cDNA arrays were used to identify a set of genes whose expression was modulated in death versus survival conditions. Following PMA treatment, expression of Mcl-1 and Bcl-x increased while that of c-Myc was significantly reduced. Moreover, survivin expression decreased upon CH11 or PMA treatment. c-Myc, survivin and Bcl-x modulation seems to be regulated at the transcriptional level while decrease in Mcl-1 protein in CH11-treated cells resulted especially from a caspase-dependent proteolysis. Taken together, our data demonstrate that PMA-mediated inhibition of apoptosis is a complex process that is integrated at both the transcriptional and post-transcriptional level and point out to the potential role of Mcl-1, Bcl-x, c-Myc and survivin in this process.

Role of Fas/Fas ligand interaction in ischemia-induced collateral vessel growth

Fas ligand (FasL) is a death factor that induces apoptosis in cells bearing its receptor, Fas. Accumulating evidence indicates that the Fas/FasL system is involved not only in apoptosis but also in cell-activation signals. Recently, it was reported that local stimulation of Fas in vivo using an agonistic antibody triggers inflammatory cell infiltration and neoangiogenesis independently of apoptosis. On the other hand, Fas/FasL interaction has been proposed to control the growth and development of new subretinal vessels. Here, we evaluated the potential involvement of Fas/FasL interaction in collateral development in response to tissue ischemia. Hindlimb ischemia was induced in C57BL/6J (wild-type), B6-gld(FasL -/-), and B6-lpr(Fas -/-) mice by resection of the right femoral artery. The blood flow recovery of FasL -/- or Fas -/- mice was similar to that of wild-type mice, as determined using a laser Doppler imaging system. There was no significant difference in capillary density of the ischemic calf muscle among the mice, as determined by anti-CD31 immunostaining. We did not find any difference in the number of infiltrating inflammatory cells or in vascular endothelial growth factor expression. These results indicate that postnatal angiogenesis in response to acute ischemia can occur independently of the endogenous Fas/FasL interaction.

Wnt signaling promotes oncogenic transformation by inhibiting c-Myc-induced apoptosis

Aberrant activation of the Wnt/beta-catenin signaling pathway is associated with numerous human cancers and often correlates with the overexpression or amplification of the c-myc oncogene. Paradoxical to the cellular transformation potential of c-Myc is its ability to also induce apoptosis. Using an inducible c-MycER expression system, we found that Wnt/beta-catenin signaling suppressed apoptosis by inhibiting c-Myc-induced release of cytochrome c and caspase activation. Both cyclooxygenase 2 and WISP-1 were identified as effectors of the Wnt-mediated antiapoptotic signal. Soft agar assays showed that neither c-Myc nor Wnt-1 alone was sufficient to induce cellular transformation, but that Wnt and c-Myc coordinated in inducing transformation. Furthermore, coexpression of Wnt-1 and c-Myc induced high-frequency and rapid tumor growth in nude mice. Extensive apoptotic bodies were characteristic of c-Myc-induced tumors, but not tumors induced by coactivation of c-Myc and Wnt-1, indicating that the antiapoptotic function of Wnt-1 plays a critical role in the synergetic action between c-Myc and Wnt-1. These results elucidate the molecular mechanisms by which Wnt/beta-catenin inhibits apoptosis and provide new insight into Wnt signaling-mediated oncogenesis.

The caspase-cleaved DAP5 protein supports internal ribosome entry site-mediated translation of death proteins

Apoptosis is characterized by a translation switch from cap-dependent to internal ribosome entry site (IRES)-mediated protein translation. During apoptosis, several members of the eukaryotic initiation factor (eIF)4G family are cleaved specifically by caspases. Here we investigated which of the caspase-cleaved eIF4G family members could support cap-independent translation through IRES elements that retain activity in the dying cell. We focused on two major fragments arising from the cleavage of eIF4GI and death-associated protein 5 (DAP5) proteins (eIF4GI M-FAG/p76 and DAP5/p86, respectively), because they are the only potential candidates to preserve the minimal scaffold function needed to mediate translation. Transfection-based experiments in cell cultures indicated that expression of DAP5/p86 in cells stimulated protein translation from the IRESs of c-Myc, Apaf-1, DAP5, and XIAP. In contrast, these IRESs were refractory to the ectopically expressed eIF4GI M-FAG/p76. Furthermore, our study provides in vivo evidence that the caspase-mediated removal of the C-terminal tail of DAP5/p97 relieves an inhibitory effect on the protein's ability to support cap-independent translation through the DAP5 IRES. Altogether, the data suggest that DAP5 is a caspase-activated translation factor that mediates translation through a repertoire of IRES elements, supporting the translation of apoptosis-related proteins.

Deregulated c-Myc prematurely recruits both Type I and II CD95/Fas apoptotic pathways associated with terminal myeloid differentiation

Previously we have reported that deregulated expression of c-myc in normal and leukemic myeloid cells blocked differentiation and, concomitantly, induced p53-independent apoptosis. Here, we show that this morbidity was due to premature recruitment of the Fas/CD95 cell death pathway which normally operates to induce apoptosis at the end of the terminal myeloid differentiation program. Analysis of the regulated components of this pathway revealed that IL6-mediated induction of differentiation resulted in rapid cell surface expression of CD95 receptor. Deregulated c-myc prevented the downregulation of CD95 ligand by maintaining its transcription, but caused premature downregulation of c-FLIP. First, the Type II (mitochondria-dependent, bcl-2-sensitive) and, then, the Type I (mitochondria-independent, bcl-2-insensitive) pathway were activated. Stable exogenous c-FLIP expression completely rescued the apoptotic phenotype. Furthermore, when the deregulated c-myc transgene was stably transduced into bone marrow cells from Fas(lpr/lpr) (CD95 receptor mutant) and FasL(gld/gld) (CD95 ligand mutant) mice, cell death was significantly suppressed relative to c-myc-transduced wild type bone marrow cells upon induction of differentiation. These data indicate that c-myc-mediated apoptosis associated with blocks in myeloid differentiation is dependent on the Fas/CD95 pathway. Our findings offer important new insights into understanding how deregulated c-myc alters normal blood cell homeostasis, and how additional mutations might promote leukemogenesis.

A matter of life and death

We propose that deregulation of proliferation, together with a reduction in apoptosis, creates a platform that is both necessary and can be sufficient for cancer. The secondary traits of diverse neoplasms are a consequence of cell proliferation, tissue expansion, and other outcomes of this platform.

Expression of Apo-3 and Apo-3L in primitive neuroectodermal tumours of the central and peripheral nervous system

Deregulation of apoptosis has been implicated in the pathogenesis, spontaneous regression and treatment resistance of neuroblastoma. A newly recognised member of the tumour necrosis factor (TNF)-family of death receptors known as Apo-3 has been mapped to human chromosome 1p36.3, a region commonly deleted in aggressive neuroblastoma. Based on its localisation and function, Apo-3 is a candidate for the putative neuroblastoma tumour suppressor gene. Therefore we analysed mRNA expression of the Apo-3 receptor/ligand (Apo-3/Apo-3L) system in a representative panel of 18 neuroblastoma cell lines, 41 primary neuroblastoma and 13 ganglioneuromas/ganglioneuroblastomas by semi-quantitative RT-PCR. We compared the level of expression with the well-established prognostic factors age, stage, histology, MYCN-amplification and TrkA expression, as well as outcome. For comparison, we studied Apo-3/Apo-3L expression in 27 central nervous system (CNS) primitive neuroectodermal tumours/medulloblastomas (PNET/medulloblastoma) and in six normal brain samples. Neuroblastoma cell lines with 1p deletion and MYCN-amplification expressed significantly lower levels of Apo-3 (P=0.009 and P=0.03, respectively) compared with neuroblastoma cell lines without 1p deletion or MYCN-amplification. The mean expression level of Apo-3L was significantly higher in ganglioneuromas/ganglioneuroblastomas compared with neuroblastomas (P=0.001) and in normal brain compared with PNET/medulloblastoma (P<0.0001). Expression of Apo-3L was significantly associated with survival in neuroblastomas (P<0.049) and in PNET/medulloblastomas (P=0.01). Expression of Apo-3 was significantly associated with survival in PNET/medulloblastomas (P=0.03). Thus, the Apo-3 receptor/ligand system might be involved in the regulation of apoptosis in neuroblastomas and PNET.

Induction of apoptosis in cancer: new therapeutic opportunities

Autonomous cell proliferation is one of the hallmarks of cancer cells, driven by activated growth-promoting oncogenes. However, deregulated activation of these oncogenes also triggers apoptosis via multiple pathways. Among them, the ARF-p53 pathway appears to play a major role in mediating oncogene-induced apoptosis. Consequently, suppression of apoptosis by inactivation of p53 and other tumor suppressors is central to tumor development. These findings have broad implications in understanding cancer genetics and therapy. They help define the roles for oncogenes and tumor suppressor genes in tumorigenesis. Furthermore, the notion that cancer cells often carry specific defects in apoptotic pathways but are inherently sensitive to apoptosis as a result of deregulated proliferation, offers numerous opportunities for manipulating apoptosis in directions of clinical application.

Suppression of Akt signaling induces Fas ligand expression: involvement of caspase and Jun kinase activation in Akt-mediated Fas ligand regulation

Fas and Fas ligand (FasL) expression has been detected in chronic vascular lesions, and Fas-mediated apoptosis of vascular smooth muscle cells (VSMC) may influence the integrity of the atherosclerotic plaque. Here we report that FasL is not expressed by normal VSMC, but its expression is upregulated by stresses that induce apoptosis, including serum deprivation, exposure to the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, and ablation of Akt signaling. Conversely, constitutive activation of Akt signaling diminished FasL expression in VSMC cultures exposed to low-mitogen media or wortmannin. Under conditions of suppressed PI 3-kinase/Akt signaling, VSMC apoptosis was partially inhibited by treatment with neutralizing antibody against FasL. Suppression of Akt signaling increased the activity of c-Jun N-terminal kinase, and transduction of dominant-negative c-Jun inhibited FasL induction under these conditions. Diminished Akt signaling promoted the cleavage of caspase 3, and both caspase 3 cleavage and FasL induction were inhibited by transduction of dominant-negative caspase 9 or the caspase 8 inhibitor CrmA. Similarly, induction of FasL by the Akt-regulated forkhead transcription factor FKHRL1 was dependent upon caspase and c-Jun activation. Taken together, these results indicate that the sequential activation of caspase 3 and c-Jun participates in the induction of FasL under conditions of suppressed Akt signaling or FKHRL1 activation and that FasL participates in a positive-feedback loop to promote cell death under conditions of cellular stress.

Bcl-2 family members and functional electron transport chain regulate oxygen deprivation-induced cell death

The mechanisms underlying cell death during oxygen deprivation are unknown. We report here a model for oxygen deprivation-induced apoptosis. The death observed during oxygen deprivation involves a decrease in the mitochondrial membrane potential, followed by the release of cytochrome c and the activation of caspase-9. Bcl-X(L) prevented oxygen deprivation-induced cell death by inhibiting the release of cytochrome c and caspase-9 activation. The ability of Bcl-X(L) to prevent cell death was dependent on allowing the import of glycolytic ATP into the mitochondria to generate an inner mitochondrial membrane potential through the F(1)F(0)-ATP synthase. In contrast, although activated Akt has been shown to inhibit apoptosis induced by a variety of apoptotic stimuli, it did not prevent cell death during oxygen deprivation. In addition to Bcl-X(L), cells devoid of mitochondrial DNA (rho degrees cells) that lack a functional electron transport chain were resistant to oxygen deprivation. Further, murine embryonic fibroblasts from bax(-/-) bak(-/-) mice did not die in response to oxygen deprivation. These data suggest that when subjected to oxygen deprivation, cells die as a result of an inability to maintain a mitochondrial membrane potential through the import of glycolytic ATP. Proapoptotic Bcl-2 family members and a functional electron transport chain are required to initiate cell death in response to oxygen deprivation.

Acute and chronic smooth muscle cell apoptosis after mechanical vascular injury can occur independently of the Fas-death pathway

Vascular smooth muscle cell (VSMC) apoptosis has been demonstrated in vascular lesions, such as atherosclerotic and postangioplasty restenotic lesions. Balloon injury also induces VSMC apoptosis. Fas is a death factor that mediates apoptosis when it is activated by its ligand, FasL. Fas-mediated apoptosis was found to be implicated in the pathogenesis of vascular diseases in which Fas/FasL expression was detected. We investigated whether the Fas/FasL interaction mediated acute and chronic VSMC apoptosis and lesion formation in a vascular injury model that may resemble balloon angioplasty. A large spring wire was inserted into the femoral artery of C3H/HeJ (wild-type), C3H-gld (Fas ligand-/-), and C3H-lpr (Fas-/-) mice. The wire was left in place for 1 minute to denude and expand the artery. Massive apoptosis was observed in medial VSMCs from 1 to 7 hours later. There was no difference in the number of apoptotic cells among the 3 groups of mice 4 hours after injury. At 4 weeks, the injured arteries presented signs of concentric neointimal hyperplasia composed exclusively of VSMCs. There was no difference in the degree of neointima hyperplasia (intima/media ratios were as follows: wild type 1.4+/-0.3, gld 1.0+/-0.2, and lpr 1.3+/-0.2) or in the number of apoptotic nuclei among the 3 groups. These findings suggest the existence of other signaling pathways for acute and chronic VSMC apoptosis, at least that induced by mechanical vascular injury.

Caspase 8 activation independent of Fas (CD95/APO-1) signaling may mediate killing of B-chronic lymphocytic leukemia cells by cytotoxic drugs or gamma radiation

Ligation of the cell-surface Fas molecule by its ligand (Fas-L) or agonistic anti-Fas monoclonal antibodies results in the cleavage and activation of the cysteine protease procaspase 8 followed by the activation of procaspase 3 and by apoptosis. In some leukemia cell lines, cytotoxic drugs induce expression of Fas-L, which may contribute to cell killing through the ligation of Fas. The involvement of Fas, Fas-L, and caspase 8 was studied in the killing of B-cell chronic lymphocytic leukemia (B-CLL) cells by chlorambucil, fludarabine, or gamma radiation. Spontaneous apoptosis was observed at 24-hour incubation, with additional apoptosis induced by each of the cytotoxic treatments. Although Fas mRNA expression was elevated after exposure to chlorambucil, fludarabine, or gamma radiation, Fas protein levels only increased after irradiation. Therefore, Fas expression may be regulated by multiple mechanisms that allow the translation of Fas mRNA only in response to restricted cytotoxic stimuli. None of the cytotoxic stimuli studied here induced Fas-L expression. An agonistic anti-Fas monoclonal antibody (CH-11) did not significantly augment apoptosis induction by any of the death stimuli. A Fas-blocking antibody (ZB4) did not inhibit spontaneous, chlorambucil-, fludarabine-, or radiation-induced apoptosis. However, procaspase 8 processing was induced by all cytotoxic stimuli. These data suggest that the Fas/Fas-L signaling system does not play a major role in the induction of apoptosis in B-CLL cells treated with cytotoxic drugs or radiation. However, Fas-independent activation of caspase 8 may play a crucial role in the regulation of apoptosis in these cells.

A role for c-myc in DNA damage-induced apoptosis in a human TP53-mutant small-cell lung cancer cell line

Based on the role of p53 in the control of apoptosis following DNA damage, the status of the TP53 gene has been implicated as a major determinant of tumour responsiveness to cytotoxic therapies. In spite of the high frequency of TP53 mutations, small-cell lung cancer (SCLC) is recognised as one of the most chemoresponsive solid tumours. Since the relevance of the TP53 gene status in the modulation of tumour responsiveness is dependent on the molecular/biological context, in the present study, we have examined the relationship between chemosensitivity and susceptibility to apoptosis of a TP53-mutant human SCLC cell line. The cell line, in spite of TP53 mutation, retained an efficient response to genotoxic stress as documented by cells ability to modulate the p53 protein, arrest in the G1 and G2 phases of the cell cycle and its marked susceptibility to apoptosis following treatment with DNA damaging agents. Exposure to DNA-damaging agents caused an increase of c-Myc, a DNA damage-responsive transcription factor. An analysis of damage-induced apoptosis in the presence of an anti-Fas/CD95 inhibitory antibody indicated that Fas/CD95 was not required for the apoptotic response. The results support an implication of c-myc in sensitising cells to apoptosis, since inhibition of c-Myc expression with an antisense oligodeoxynucleotide (AS-ODN) almost abolished the drug-induced apoptotic response. In conclusion, the present results support a role for c-myc in the induction of apoptosis by genotoxic stress in the absence of a functional p53 and provide new insights into the mechanisms that may influence apoptosis in TP53-mutant cells. Elucidation of this pathway and of the possible cooperation with p53-dependent mechanisms may provide a basis for therapeutic intervention.

Glutamine deprivation-mediated cell shrinkage induces ligand-independent CD95 receptor signaling and apoptosis

Cell shrinkage and loss of cell viability by apoptosis have been examined in cultured CD95(Fas/Apo-1)-expressing leukemia-derived CEM and HL-60 cells subjected to acute deprivation of glutamine, a major compatible osmolyte engaged in cell volume control. Glutamine deprivation-mediated cell shrinkage promoted a ligand-independent activation of the CD95-mediated apoptotic pathway. Cell transfection with plasmids expressing FADD-DN or v-Flip viral proteins pointed to a functional clustering of CD95 receptors at the cell surface with activation of the 'extrinsic pathway' caspase cascade. Accordingly, cell shrinkage did not induce apoptosis in CD95 receptor-negative lymphoma L1210 cells. Replacement of glutamine with surrogate compatible osmolytes counteracted cell volume decrement and protected the CD95-expressing cells from apoptosis. A glutamine deprivation-dependent cell shrinkage with activation of the CD95-mediated pathway was also observed when asparaginase was added to the medium. Asparagine depletion had no role in this process. The cell-size shrinkage-dependent apoptosis induced by glutamine restriction in CD95-expressing leukemic cells may therefore be of clinical relevance in amidohydrolase enzyme therapies.

Apoptosis regulators and their role in tumorigenesis

It has become clear that, together with deregulated growth, inhibition of programmed cell death (PCD) plays a pivotal role in tumorigenesis. In this review, we present an overview of the genes and mechanisms involved in PCD. We then summarize the evidence that impaired PCD is a prerequisite for tumorigenesis, as indicated by the fact that more and more neoplastic mutations appear to act by interfering with PCD. This has made the idea of restoration of corrupted 'death programs' an intriguing new area for potential cancer therapy.

c-myc box II mutations in Burkitt's lymphoma-derived alleles reduce cell-transformation activity and lower response to broad apoptotic stimuli

In addition to c-myc rearrangement, over 50% of Burkitt's lymphoma cases present clustered mutations in exon 2, where many of the functional activities of c-Myc protein are based. This report describes the functional consequences induced by tumour-derived c-myc mutations located in c-myc box II. Two mutated alleles were studied, focusing on the P138C mutation, and compared to wild-type c-myc. The c-Myc transformation, transactivation and apoptosis activities were explored based on cells over-expressing c-Myc. While the transcriptional activation activity was not affected, our experiments exploring the anchorage-independent growth capacity of c-Myc-transfected Rat1a cells showed that c-Myc box II mutants were less potent than wild-type c-Myc in promoting cell transformation. Considering the possibility that these mutations could be interfering with the ability of c-Myc to promote apoptosis, we tested c-Myc-transfected Rat1a fibroblasts under several conditions: serum deprivation-, staurosporine- and TNFalpha-induced cell death. Interestingly, the mutated alleles were characterized by an overall decrease in ability to mediate apoptosis. Our study indicates that point mutations located in c-Myc box II can decrease the ability of the protein to promote both transformation and apoptosis without modifying its transactivating activity.

Translocations involving c-myc and c-myc function

c-MYC is the prototype for oncogene activation by chromosomal translocation. In contrast to the tightly regulated expression of c-myc in normal cells, c-myc is frequently deregulated in human cancers. Herein, aspects of c-myc gene activation and the function of the c-Myc protein are reviewed. The c-myc gene produces an oncogenic transcription factor that affects diverse cellular processes involved in cell growth, cell proliferation, apoptosis and cellular metabolism. Complete removal of c-myc results in slowed cell growth and proliferation, suggesting that while c-myc is not required for cell proliferation, it acts as an integrator and accelerator of cellular metabolism and proliferation.

Differential sensitivity of human papillomavirus type 16(+) and type 18(+) cervical carcinoma cells to CD95-mediated apoptosis

When cervical carcinoma cells were monitored for apoptotic signals, HPV18(+) lines were found to be highly sensitive to agonistic CD95 antibodies or recombinant CD95 ligands after co-exposure with CHX (CD95(S)). In contrast, HPV16(+) cervical carcinoma cells and HPV16-immortalized non-malignant human keratinocytes were CD95-resistant (CD95(R)) under equivalent conditions. Somatic cell hybridization between CD95(S) and CD95(R) cervical carcinoma cell lines revealed that CD95 sensitivity was a dominant trait, which could be correlated with abundant c-Myc and low Bcl-X(L) expression. Although CD95(R) cervical carcinoma cells expressed even higher levels of p53 and CD95 receptor at the surface, resistance could be attributed to the inability to form a functional DISC, necessary for successful transmission of the apoptogenic response. These data indicate that resistance to apoptotic stimuli represents an important immunological escape mechanism during virus-induced carcinogenesis.

Proteasome inhibitors induce Fas-mediated apoptosis by c-Myc accumulation and subsequent induction of FasL message in human glioma cells

Proteasome inhibitors were shown previously to induce mitochondria-independent and caspase-3-dependent apoptosis in human glioma cell lines by unknown mechanisms. Here, we showed that treatment with proteasome inhibitors, lactacystin or acetyl-leucinyl-leucinyl-norleucinal, led to elevation of the steady-state c-Myc protein but not c-myc mRNA, suggesting the accumulation of c-Myc protein by proteasome inhibitors. In addition, the marked association of c-Myc protein with ubiquitin by treatment with proteasome inhibitors indicated the involvement of proteasome in c-Myc proteolysis and the stabilization of c-Myc protein by proteasome inhibitors in vivo. The expression of Fas (also termed CD95 or APO-1) mRNA, if analyzed by reverse transcriptase polymerase chain reaction assay, was found to occur constitutively, and increased slightly by the treatment with proteasome inhibitors. In contrast, the expression of Fas ligand (FasL) mRNA was markedly induced temporarily before the activation of caspase-3 by the treatment. Agonistic anti-Fas antibody (CH11) induced apoptotic cell death, suggesting the presence of a functional Fas receptor. In addition, proteasome inhibitor-induced apoptosis was prevented by the addition of antagonistic anti-FasL antibody (4A5) or z-IETD.fmk, a potent inhibitor of caspase-8, indicating the involvement of the Fas receptor-ligand apoptotic signaling system in proteasome inhibitor-mediated apoptosis. Thus, it is suggested that proteasome inhibitors cause the accumulation of c-Myc protein which induces transiently FasL message to stimulate the Fas receptor-ligand apoptotic signaling pathway.

Myc potentiates apoptosis by stimulating Bax activity at the mitochondria

The ability of the c-Myc oncoprotein to potentiate apoptosis has been well documented; however, the mechanism of action remains ill defined. We have previously identified spatially distinct apoptotic pathways within the same cell that are differentially inhibited by Bcl-2 targeted to either the mitochondria (Bcl-acta) or the endoplasmic reticulum (Bcl-cb5). We show here that in Rat1 cells expressing an exogenous c-myc allele, distinct apoptotic pathways can be inhibited by Bcl-2 or Bcl-acta yet be distinguished by their sensitivity to Bcl-cb5 as either susceptible (serum withdrawal, taxol, and ceramide) or refractory (etoposide and doxorubicin). Myc expression and apoptosis were universally associated with Bcl-acta and not Bcl-cb5, suggesting that Myc acts downstream at a point common to these distinct apoptotic signaling cascades. Analysis of Rat1 c-myc null cells shows these same death stimuli induce apoptosis with characteristic features of nuclear condensation, membrane blebbing, poly (ADP-ribose) polymerase cleavage, and DNA fragmentation; however, this Myc-independent apoptosis is not inhibited by Bcl-2. During apoptosis, Bax translocation to the mitochondria occurs in the presence or absence of Myc expression. Moreover, Bax mRNA and protein expression remain unchanged in the presence or absence of Myc. However, in the absence of Myc, Bax is not activated and cytochrome c is not released into the cytoplasm. Reintroduction of Myc into the c-myc null cells restores Bax activation, cytochrome c release, and inhibition of apoptosis by Bcl-2. These results demonstrate a role for Myc in the regulation of Bax activation during apoptosis. Moreover, apoptosis that can be triggered in the absence of Myc provides evidence that signaling pathways exist which circumvent Bax activation and cytochrome c release to trigger caspase activation. Thus, Myc increases the cellular competence to die by enhancing disparate apoptotic signals at a common mitochondrial amplification step involving Bax activation and cytochrome c release.

c-Myc down-regulation increases susceptibility to cisplatin through reactive oxygen species-mediated apoptosis in M14 human melanoma cells

Our aim in this work was to define the role of c-Myc in the susceptibility to cisplatin [cis-diamminedichloroplatinum(II) (CDDP)] in human melanoma cells. Two M14 melanoma cell clones obtained by transfection and expressing six to ten times lower c-Myc protein levels than the parental cells and the control clone were employed. Analysis of survival curves demonstrates an increase in CDDP sensitivity in c-Myc low-expressing clones if compared with the control clone and the parental line. The enhanced sensitivity is unrelated to the impairment in enzymatic DNA repair activity. Cell cycle analysis demonstrates that although the control clone is able to completely recover from the CDDP-induced S-G(2)/M block, this arrest is prolonged in c-Myc low-expressing clones and a fraction of cells undergoes apoptosis. Although no changes in P53, Bax, Bcl-2, and Bcl-x(L/S) protein levels are observed, apoptosis is associated with the formation of reactive oxygen species (ROS), activation of caspase-1, caspase-3 and cleavage of the specific caspase substrate poly-ADP-ribose polymerase. The use of the antioxidant N-acetyl cysteine and caspase inhibitors prevents CDDP-induced apoptosis in c-Myc low-expressing clones, demonstrating that ROS, caspase-1, and caspase-3 are required for apoptotic cell death. Moreover, ROS generation depends on caspase-1-like activation because the Ac-YVAD-cho inhibitor abrogates CDDP-induced ROS in the c-Myc low-expressing clones.

An apoptosis signaling pathway induced by the death domain of FADD selectively kills normal but not cancerous prostate epithelial cells

The adaptor protein FADD directly, or indirectly via another adaptor called TRADD, recruits caspase 8 to death receptors of the tumor necrosis factor receptor family. Consequentially, a dominant-negative mutant (FADD-DN, which consists only of the FADD death domain) that binds to receptors but cannot recruit caspase 8 has been widely used to inhibit apoptosis by various stimuli that work via death receptors. Here, we show that FADD-DN also has another cell type- and cancer-dependent activity because it induces apoptosis of normal human prostate epithelial cells but not normal prostate stromal cells or prostate cancer cells. This activity is independent of FADD-DN's ability to bind to three known interacting proteins, Fas, TRADD or RIP suggesting that it is distinct from FADD's functions at activated death receptors. FADD-DN induces caspase activation in normal epithelial cells as demonstrated using a Fluorescence Resonance Energy Transfer assay that measures caspase activity in individual living cells. However, caspase-independent pathways are also implicated in FADD-DN-induced apoptosis because caspase inhibitors were inefficient at preventing prostate cell death. Therefore, the death domain of FADD has a previously unrecognized role in cell survival that is epithelial-specific and defective in cancer cells. This FADD-dependent signaling pathway may be important in prostate carcinogenesis.

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.

Epithelial membrane protein 2, a 4-transmembrane protein that suppresses B-cell lymphoma tumorigenicity

A murine homologue of the epithelial membrane protein 2 (EMP2) gene was identified in a search for genes associated with B-cell lymphoma tumorigenicity by using suppression subtractive hybridization. Expression of EMP2 messenger RNA in primary mouse tissues was limited to certain epithelial cell types and the peritoneal lymphoid compartment. EMP2 was expressed in the poorly tumorigenic DAC B-lymphoma cell line but was significantly down-regulated in a subline selected for in vivo tumor formation in Balb/c mice. Recombinant restoration of EMP2 expression in the subline suppressed its tumorigenicity, suggesting that loss of EMP2 was a causal factor in the malignant phenotype. Recombinant overexpression of EMP2 was studied in B lymphoma and NIH3T3 cells. EMP2 in both cell types induced cell death on serum deprivation. EMP2-induced cell death correlated with the expression level of EMP2 protein and was prevented by caspase inhibitors Z-VAD and Z-DEVD. These findings for the first time describe an apoptotic effect of a GAS3 family gene in lymphocytes. They also suggest that EMP2 may influence B-lymphoma tumorigenicity through a functional tumor suppressor phenotype. (Blood. 2001;97:3890-3895)

Tumor necrosis factor-alpha induces neuronal death by silencing survival signals generated by the type I insulin-like growth factor receptor

Within the central nervous system, the proinflammatory cytokine tumor necrosis factor (TNF)-alpha is best characterized by its ability to directly foment signals of death. However, recent evidence suggests that TNF-alpha also promotes neurodegeneration through inhibition of a vital survival signal, insulin-like growth factor-I (IGF-I). By inhibiting essential components of the IGF-I survival response, such as phosphatidylinositol 3'-kinase (PI 3-kinase), low nontoxic concentrations of TNF-alpha indirectly trigger the death of neurons. We suggest that this inhibition of survival signaling is a pathophysiologically relevant action of TNF-alpha in the brain. This type of cross-talk by which vastly different receptors utilize shared intracellular substrates is potentially applicable to a broad number of receptors that are coexpressed on the same cell. The use of neuronal growth factors in the treatment of neurodegenerative diseases, such as cerebral ischemia and the AIDS dementia complex, may prove much more effective if the elevated expression of TNF-alpha in these disorders is neutralized.

Presence and significance of CD-95 (Fas/APO1) expression after spinal cord injury

OBJECT

A glycoprotein, CD95 (Fas/APO1) is widely considered to be implicated in the development of apoptosis in a number of tissues. Based on the hypothesis that apoptosis is related to cell death after spinal cord injury (SCI), the authors studied the presence and distribution of CD95 (Fas/APO1)-positive cells in injured spinal cord tissue for the purpose of determining the significance of this protein during the early phases of SCI.

METHODS

The presence and distribution of cells showing positive immunostaining for CD95 (Fas/APO1) were studied 1, 4, 8, 24, 48, and 72 hours and 1, 2, and 4 weeks after induction of experimental SCI in rats. Studies were conducted using a monoclonal antibody to the CD95 (Fas/APO1) protein. Positivity for CD95 (Fas/APO1) was observed in apoptotic cells, mainly in the gray matter, 1 hour after trauma, and the number of immunostained cells increased for the first 8 hours, at which time the protein was expressed in both gray and white matter. From 24 to 72 hours postinjury, the number of immunostained cells decreased in the gray matter, but increased in the white matter. From then on, there were fewer CD95 (Fas/APO1)-positive cells, but some cells in the white matter still exhibited positive immunostaining 1 and 2 weeks after injury. At 4 weeks, there remained no CD95 (Fas/APO1)-positive cells in injured spinal cord.

CONCLUSIONS

These findings indicate that CD95 (Fas/APO1) is expressed after SCI, suggesting a role for this protein in the development of apoptosis after trauma and the possibility of a new therapeutic approach to SCI based on blocking the CD95 (Fas/APO1) system.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Internal ribosome entry site regulates translation of Kaposi's sarcoma-associated herpesvirus FLICE inhibitory protein

The gammaherpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) (or human herpesvirus 8) is associated with the endothelial tumor Kaposi's sarcoma (KS) and lymphoproliferative disorders in immunocompromised individuals. Only a small number of viral proteins are expressed in B cells latently infected with KSHV; here we characterize the mechanism of expression of one of these, the viral FLICE inhibitory protein v-FLIP (K13, ORF71). The v-FLIP coding region is present in a bicistronic message, following the v-cyclin coding region. Using both in vitro translation and cell transfection assays, we have identified an internal ribosome entry site (IRES) preceding the v-FLIP start codon and overlapping the v-cyclin (ORF 72) coding region, which allows v-FLIP translation. Using an antibody against v-FLIP we have detected expression of the endogenous protein in latently infected KSHV-positive primary effusion lymphoma (PEL) cell lines. Induction of apoptosis by serum withdrawal from PEL cells results in a relative increase in v-FLIP synthesis, as previously described for some cellular proteins translated from IRES.

Cell type specific involvement of death receptor and mitochondrial pathways in drug-induced apoptosis

Apoptosis in response to cellular stress such as treatment with cytotoxic drugs is mediated by effector caspases (caspase-3) which can be activated by different initiator pathways. Here, we report on a cell type specific triggering of death receptor and/or mitochondrial pathways upon drug treatment. In type I cells (BJAB), both the receptor and the mitochondrial pathway were activated upon drug treatment, since blockade of either the receptor pathway by overexpression of dominant negative FADD (FADD-DN) or of the mitochondrial pathway by overexpression of Bcl-X(L) only partially inhibited apoptosis. Drug treatment induced formation of a FADD- and caspase-8-containing CD95 death-inducing signaling complex (DISC) in type I cells resulting in activation of caspase-8 as the most apical caspase. In contrast, in type II cells (Jurkat), apoptosis was predominantly controlled by mitochondria, since overexpression of Bcl-2 completely blocked drug-induced apoptosis, while overexpression of FADD-DN had no protective effect. In these cells, caspases including caspase-8 were activated by mitochondria-driven signaling events and no DISC was detected despite expression levels of CD95, FADD and caspase-8 proteins comparable to type I cells. Likewise, drug-induced CD95 aggregation was predominantly found in type I cells. Bid was cleaved prior to mitochondrial alterations in type I cells providing a molecular link between caspase-8 activation and mitochondrial perturbations, whereas in type II cells, Bid was cleaved downstream of mitochondria. Our findings of a cell type specific response to cytotoxic drugs have implications for the identification of molecular parameters for chemosensitivity or resistance in different tumor cells.

Glutamine-dependent antiapoptotic interaction of human glutaminyl-tRNA synthetase with apoptosis signal-regulating kinase 1

Glutamine has been known to be an apoptosis suppressor, since it blocks apoptosis induced by heat shock, irradiation, and c-Myc overexpression. Here, we demonstrated that HeLa cells were susceptible to Fas-mediated apoptosis under the condition of glutamine deprivation. Fas ligation activated apoptosis signal-regulating kinase 1 (ASK1) and c-Jun N-terminal kinase (JNK; also known as stress-activated protein kinase (SAPK)) in Gln-deprived cells but not in normal cells, suggesting that Gln might be involved in the activity control of ASK1 and JNK/SAPK. As one of the possible mechanisms for the suppressive effect of Gln on ASK1, we investigated the molecular interaction between human glutaminyl-tRNA synthetase (QRS) and ASK1 and found the Gln-dependent association of the two molecules. While their association was enhanced by the elevation of Gln concentration, they were dissociated by Fas ligation within 5 min. The association involved the catalytic domains of the two enzymes. The ASK1 activity was inhibited by the interaction with QRS as determined by in vitro kinase and transcription assays. Finally, we have shown that QRS inhibited the cell death induced by ASK1, and this antiapoptotic function of QRS was weakened by the deprivation of Gln. Thus, the antiapoptotic interaction of QRS with ASK1 is controlled positively by the cellular concentration of Gln and negatively by Fas ligation. The results of this work provide one possible explanation for the working mechanism of the antiapoptotic activity of Gln and suggest a novel function of mammalian ARSs.

Apoptosis of bovine granulosa cells after serum withdrawal is mediated by Fas antigen (CD95) and Fas ligand

Ovarian follicular atresia occurs by apoptosis of granulosa and theca cells. The Fas antigen (Fas), a cell surface receptor that triggers apoptosis when activated by Fas ligand (FasL), may be involved in this process. A possible role of the Fas pathway in mediating serum withdrawal-induced apoptosis of granulosa cells was examined. Granulosa cells collected from 5- to 10-mm bovine follicles were cultured in DMEM-F12 containing serum for 3 days, deprived of serum, and live cells were counted at various times after serum withdrawal. Cell death increased significantly 6 h after serum withdrawal (21% +/- 7%; P: < 0.05 vs. 0 h) and continued to increase until 24 h (43% +/- 6%). No further increases in cell death were observed through 72 h. Detection of the translocation of phosphatidylserine to the outer surface of the cell membrane by annexin V binding indicated that cells died by apoptosis. Quantitative reverse transcriptase-polymerase chain reaction assays showed no changes in Fas mRNA levels but a 4.7-fold increase in FasL mRNA 3 h after serum withdrawal (P: < 0.05 vs. 0 h). FasL mRNA remained elevated through 24 h and returned to basal levels at 48 h. Immunohistochemical staining showed that both Fas and FasL protein increased on the cell surface within 3 h and remained elevated through 12 h (the last time point tested). Binding of FasL to Fas was blocked with two reagents that bind to the extracellular domain of FasL: an anti-FasL antibody and Fas:Fc, a chimeric protein consisting of the Fc portion of human immunoglobulin G and the extracellular domain of human Fas. Cell death 24 h after serum withdrawal was reduced 55% +/- 10% and 34% +/- 12% by anti-FasL antibody and Fas:Fc, respectively (P: < 0.05 vs. no blocking protein). In conclusion, serum withdrawal-induced apoptosis of bovine granulosa cells is mediated at least partially by Fas/FasL interactions. These results are consistent with a potential role of Fas in an autocrine or paracrine pathway to trigger ovarian follicular atresia.

Targeted deletion of the S-phase-specific Myc antagonist Mad3 sensitizes neuronal and lymphoid cells to radiation-induced apoptosis

The Mad family comprises four basic-helix-loop-helix/leucine zipper proteins, Mad1, Mxi1, Mad3, and Mad4, which heterodimerize with Max and function as transcriptional repressors. The balance between Myc-Max and Mad-Max complexes has been postulated to influence cell proliferation and differentiation. The expression patterns of Mad family genes are complex, but in general, the induction of most family members is linked to cell cycle exit and differentiation. The expression pattern of mad3 is unusual in that mad3 mRNA and protein were found to be restricted to proliferating cells prior to differentiation. We show here that during murine development mad3 is specifically expressed in the S phase of the cell cycle in neuronal progenitor cells that are committed to differentiation. To investigate mad3 function, we disrupted the mad3 gene by homologous recombination in mice. No defect in cell cycle exit and differentiation could be detected in mad3 homozygous mutant mice. However, upon gamma irradiation, increased cell death of thymocytes and neural progenitor cells was observed, implicating mad3 in the regulation of the cellular response to DNA damage.

Imaging FRET between spectrally similar GFP molecules in single cells

Fluorescence resonance energy transfer (FRET) detection in fusion constructs consisting of green fluorescent protein (GFP) variants linked by a sequence that changes conformation upon modification by enzymes or binding of ligands has enabled detection of physiological processes such as Ca(2+) ion release, and protease and kinase activity. Current FRET microscopy techniques are limited to the use of spectrally distinct GFPs such as blue or cyan donors in combination with green or yellow acceptors. The blue or cyan GFPs have the disadvantages of less brightness and of autofluorescence. Here a FRET imaging method is presented that circumvents the need for spectral separation of the GFPs by determination of the fluorescence lifetime of the combined donor/acceptor emission by fluorescence lifetime imaging microscopy (FLIM). This technique gives a sensitive, reproducible, and intrinsically calibrated FRET measurement that can be used with the spectrally similar and bright yellow and green fluorescent proteins (EYFP/EGFP), a pair previously unusable for FRET applications. We demonstrate the benefits of this approach in the analysis of single-cell signaling by monitoring caspase activity in individual cells during apoptosis.

Apoptotic proteins. p53 and c-myc related pathways

c-Myc and p53 are two proteins that have critical roles in the regulation of apoptosis and the cell cycle. The authors review how these two proteins are thought to control the opposing events of proliferation and apoptosis and examine whether their well-documented biological roles in tumorigenesis can be applied to the vascular system.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit expression of Fas ligand in activated T lymphocytes by regulating c-Myc, NF-kappa B, NF-AT, and early growth factors 2/3

Activation-induced cell death in T cells, a major mechanism for limiting an ongoing immune response, is initiated by Ag reengagement and mediated through Fas/Fas ligand interactions. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP), two multifunctional neuropeptides, modulate innate and adaptive immunity. We reported previously that VIP/PACAP protect T cells from activation-induced cell death through down-regulation of Fas ligand (FasL). In this study, we investigate the molecular mechanisms involved in the protective effect of VIP and PACAP. VIP/PACAP reduce in a dose-dependent manner anti-CD3-induced apoptosis in 2B4.11 T cell hybridomas. The protective effect is mediated through the specific type 2 VIP receptor, and the cAMP/protein kinase A pathway. A functional study demonstrates that VIP/PACAP inhibit activation-induced FasL expression. VIP/PACAP inhibit the expression and/or DNA-binding activity of several transcriptional factors involved in FasL expression, i.e., c-myc, NF-kappaB, NF-ATp, and early growth factors (Egr) 2/3. The inhibition of NF-kappaB binding is due to the stabilization of I-kappaB (inhibitory protein that dissociates from NF-kappaB), through the inhibition of I-kappaB kinase alpha activity. Subsequently, p65 nuclear translocation is significantly reduced. The inhibition in NF-ATp binding results from a calcineurin-independent reduction in NF-ATp nuclear translocation. VIP/PACAP inhibit the expression of Egr2 and 3, but not of Egr1. The effects on the transcriptional factors are mediated through type 2 VIP receptor with cAMP as secondary messenger.

Expression of Fas (APO-1/CD95) and Fas ligand (FasL) in human neuroblastoma

BACKGROUND AND PROCEDURE

To determine the possible role of Fas/FasL system in the particularly heterogeneous behaviour of neuroblastoma (NB), we have measured the functional expression of Fas and its ligand, FasL, in primary neuroblastoma samples and cell lines by immunohistochemistry and flow cytometry.

RESULTS

Our results reveal that while Fas expression is associated with low stage and more mature tumors, heterogeneous FasL expression was mostly detected in high stage tumors, with our apparent correlation to MYCN amplification. Flow cytometric analysis of cell lines demonstrated a high expression of Fas in epithelial-type, HLA class I positive cell lines, which was lost upon activation with phorbol esters. In contrast, Fas ligand was detected in only a small subset of cell lines.

CONCLUSIONS

In some cell lines, cytotoxic assays revealed the ability of NB-associated Fas receptor to transduce an apoptotic signal upon triggering. The pattern of functional Fas/FasL expression in tumours and cell lines suggests that this system may be involved in the evasion of highly malignant neuroblastoma cells to host immune response.

Analysis of C-MYC function in normal cells via conditional gene-targeted mutation

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

Proteases for cell suicide: functions and regulation of caspases

Caspases are a large family of evolutionarily conserved proteases found from Caenorhabditis elegans to humans. Although the first caspase was identified as a processing enzyme for interleukin-1beta, genetic and biochemical data have converged to reveal that many caspases are key mediators of apoptosis, the intrinsic cell suicide program essential for development and tissue homeostasis. Each caspase is a cysteine aspartase; it employs a nucleophilic cysteine in its active site to cleave aspartic acid peptide bonds within proteins. Caspases are synthesized as inactive precursors termed procaspases; proteolytic processing of procaspase generates the tetrameric active caspase enzyme, composed of two repeating heterotypic subunits. Based on kinetic data, substrate specificity, and procaspase structure, caspases have been conceptually divided into initiators and effectors. Initiator caspases activate effector caspases in response to specific cell death signals, and effector caspases cleave various cellular proteins to trigger apoptosis. Adapter protein-mediated oligomerization of procaspases is now recognized as a universal mechanism of initiator caspase activation and underlies the control of both cell surface death receptor and mitochondrial cytochrome c-Apaf-1 apoptosis pathways. Caspase substrates have bene identified that induce each of the classic features of apoptosis, including membrane blebbing, cell body shrinkage, and DNA fragmentation. Mice deficient for caspase genes have highlighted tissue- and signal-specific pathways for apoptosis and demonstrated an independent function for caspase-1 and -11 in cytokine processing. Dysregulation of caspases features prominently in many human diseases, including cancer, autoimmunity, and neurodegenerative disorders, and increasing evidence shows that altering caspase activity can confer therapeutic benefits.

MycN sensitizes neuroblastoma cells for drug-triggered apoptosis

BACKGROUND

Amplification of the MYCN gene is found in a large proportion of neuroblastomas and is associated with a poor prognosis.

PROCEDURE

To investigate the effect of ectopic MycN expression on the susceptibility of neuroblastoma cells to cytotoxic drugs, we used a human neuroblastoma cell line with tetracycline-controlled expression of MycN.

RESULTS

Neither conditional expression of MycN alone nor low drug concentrations induced apoptosis. However, MycN and cytotoxic drugs cooperated to induce cell death. Apoptosis triggered by MycN and doxorubicin was mediated by cleavage of caspases and involved activation of the CD95 system. MycN overexpression and cytotoxic drugs also synergized to induce p53 and Bax protein expression and to trigger mitochondrial permeability transition and cytochrome c release.

CONCLUSION

In that amplification of MYCN is considered an adverse prognostic factor, these findings suggest that dysfunctions in apoptosis pathways may be a mechanism by which MycN-induced apoptosis of neuroblastoma cells is inhibited.

v-Src generates a p53-independent apoptotic signal

Evasion of apoptosis appears to be a necessary event in tumor progression. Some oncogenes, such as c-myc and E1A, induce apoptosis in the absence of survival factors. However, others, such as bcl-2 and v-src, activate antiapoptotic pathways. For v-Src, these antiapoptotic pathways are dependent on the function of Ras, phosphatidylinositol (PI) 3-kinase, and Stat3. Here we asked whether v-Src can activate a proapoptotic signal when survival signaling is inhibited. We show that when the functions of Ras and PI 3-kinase are inhibited, v-src-transformed Rat-2 fibroblasts undergo apoptosis, evidenced by loss of adherence, nuclear fragmentation, and chromosomal DNA degradation. The apoptotic response is dependent on activation of caspase 3. Under similar conditions nontransformed Rat-2 cells undergo considerably lower levels of apoptosis. Apoptosis induced by v-Src is accompanied by a loss of mitochondrial membrane potential and release of cytochrome c and is blocked by overexpression of bcl-2, indicating that it is mediated by the mitochondrial pathway. However apoptosis induced by v-Src is not accompanied by an increase in the level of p53 and is not dependent on p53 function. Thus v-Src generates a p53-independent proapoptotic signal.

Absent or reduced expression of the caspase 8 gene occurs frequently in neuroblastoma, but not commonly in Ewing sarcoma or rhabdomyosarcoma

PROCEDURE

To clarify whether the caspase 8 gene is involved in the pathogenesis of neuroblastoma (NB), we examined alterations of the caspase 8 gene in 15 NB, seven Ewing sarcoma (ES), and eight rhabdomyosarcoma (RMS) cell lines, using reverse transcription-polymerase chain reaction (RT-PCR) and RT-PCR single-strand conformation polymorphism (SSCP) analyses.

RESULTS

The caspase 8 gene was not expressed in 11 (73%) of 15 NB cell lines, it was absent in only one of seven ES cell lines, but was present in all eight RMS cell lines examined. No mutations were detected in any cell lines examined.

CONCLUSIONS

Inactivation of the caspase 8 gene is considered to be involved in the pathogenesis of NB, but not ES or RMS.

Prevention of beta cell dysfunction and apoptosis activation in human islets by adenoviral gene transfer of the insulin-like growth factor I

Interleukin-1beta is a potent pro-inflammatory cytokine that has been shown to inhibit islet beta cell function as well as to activate Fas-mediated apoptosis in a nitric oxide-dependent manner. Furthermore, this cytokine is effective in recruiting lymphocytes that mediate beta cell destruction in IDDM onset. The insulin-like growth factor I (IGF-I) has been shown to block IL-1beta actions in vitro. We hypothesized that gene transfer of the insulin-like growth factor I to intact human islets could prevent IL-1beta-induced beta cell dysfunction and sensitization to Fas-triggered apoptosis activation. Intact human islets were infected with adenoviral vectors encoding IGF-I as well as beta-galactosidase and enhanced green fluorescent protein as controls. Adenoviral gene transfer of human IGF-I prevented IL-1beta-mediated nitric oxide production from human islets in vitro as well as the suppression of beta cell function as determined by glucose-stimulated insulin production. Moreover, IGF-I gene transfer prevented IL-1beta-induced, Fas-mediated apoptosis. These results suggest that locally produced IGF-I from cultured islets may be beneficial in maintaining beta cell function and promoting islet survival before and following islet transplantation as a potential therapy for type I diabetes.

v-Jun sensitizes cells to apoptosis by a mechanism involving mitochondrial cytochrome C release

v-Jun shares the ability of the Myc, E1A, and E2F oncogenes to both sustain cell cycle progression and promote apoptosis in the absence of mitogenic stimulation. To gain an insight into the mechanism of apoptosis sensitization, we examined the possible involvement of key regulatory proteins previously implicated in oncogene-induced cell death during v-Jun-induced apoptosis triggered by serum withdrawal. We observed that ectopic expression of the anti-apoptotic Bcl-2 protein, or of two downstream effectors of growth factor signalling, v-PI 3-Kinase and v-Src, partially or completely suppressed apoptosis. Apoptosis was also observed in the presence of serum growth factors when endogenous PI3K activity was blocked using the synthetic inhibitor LY294002, further suggesting an important role for PI3-K in cell survival. Cytochrome C was released into the cytosol of apoptotic v-Jun expressing cells, and this release was inhibited by Bcl-2, suggesting an important role for mitochondrial dysfunction in v-Jun induced apoptosis. In contrast, inhibition of Fas signalling using dominant negative FADD did not inhibit apoptosis, nor was there any evidence for accumulation or activation of p53 in v-Jun transformed cells. Consistent with this latter observation, inhibition of p53 function by HPV16 E6 protein had no effect on v-Jun induced cell death. Taken together, these results suggest that mitochondrial dysfunction is an important component of the mechanism through which v-Jun sensitizes cells to apoptosis, but that the apoptotic signals elicited by v-Jun upstream of the mitochondria do not depend on increased levels of p53 activity or Fas signalling.

A 'non-canonical' DNA-binding element mediates the response of the Fas-ligand promoter to c-Myc

Cell number is regulated by maintaining a balance between cell proliferation and cell death through apoptosis. Key regulators of this balance include the oncogene product c-Myc, which promotes either entry into the cell cycle or apoptosis [1]. Although the mechanism of c-Myc-induced apoptosis remains unclear, it is susceptible to regulation by survival factors [2,3] and can proceed through the interaction of Fas ligand (FasL) with its receptor, Fas [4]. Activated T lymphocytes are eliminated by an apoptotic process known as activation-induced cell death (AICD), which requires the transcriptional induction of FasL expression [5-7] and sustained levels of c-Myc [8]. The FasL promoter can be driven by c-Myc overexpression, and functional inhibitors of Myc and its binding partner, Max, inhibit the transcriptional activity of the FasL promoter [9,10]. We identified a non-canonical binding site (ATTCTCT) for c-Myc-Max heterodimers in the FasL promoter, which, when mutated, abolished activity in response to c-Myc. Exchange of the canonical c-Myc responsive elements (CACGTG) in the ornithine decarboxylase (ODC) promoter [11] with the non-canonical sequence in the FasL promoter generated an ODC-FasL promoter that was significantly more responsive to c-Myc than the wild-type ODC promoter. Our findings identify a precise physiological role for c-Myc in the induction of apoptosis as a transcriptional regulator of the FasL gene.

The c-Myc-interacting adaptor protein Bin1 activates a caspase-independent cell death program

Cell death processes are progressively inactivated during malignant development, in part by loss of tumor suppressors that can promote cell death. The Bin1 gene encodes a nucleocytosolic adaptor protein with tumor suppressor properties, initially identified through its ability to interact with and inhibit malignant transformation by c-Myc and other oncogenes. Bin1 is frequently missing or functionally inactivated in breast and prostate cancers and in melanoma. In this study, we show that Bin1 engages a caspase-independent cell death process similar to type II apoptosis, characterized by cell shrinkage, substratum detachment, vacuolated cytoplasm, and DNA degradation. Cell death induction was relieved by mutation of the BAR domain, a putative effector domain, or by a missplicing event that occurs in melanoma and inactivates suppressor activity. Cells in all phases of the cell cycle were susceptible to death and p53 and Rb were dispensable. Notably, Bin1 did not activate caspases and the broad spectrum caspase inhibitor ZVAD.fmk did not block cell death. Consistent with the lack of caspase involvement, dying cells lacked nucleosomal DNA cleavage and nuclear lamina degradation. Moreover, neither Bcl-2 or dominant inhibition of the Fas pathway had any effect. In previous work, we showed that Bin1 could not suppress cell transformation by SV40 large T antigen. Consistent with this finding, we observed that T antigen suppressed the death program engaged by Bin1. This observation was interesting in light of emerging evidence that T antigen has roles in cell immortalization and human cell transformation beyond Rb and p53 inactivation. In support of a link to c-Myc-induced death processes, AEBSF, a serine protease inhibitor that inhibits apoptosis by c-Myc, potently suppressed DNA degradation by Bin1. Our findings suggest that the tumor suppressor activity of Bin1 reflects engagement of a unique cell death program. We propose that loss of Bin1 may promote malignancy by blunting death penalties associated with oncogene activation.