Nuclear c-Myc plays an important role in the cytotoxicity of tumor necrosis factor alpha in tumor cells

The role of the ubiquitin-editing enzyme A20 in diseases of the central nervous system and other pathological processes

In recent years, the ubiquitin-editing enzyme A20 has been shown to control a large set of molecular pathways involved in the regulation of protective as well as self-directed immune responses. Here, we assess the current and putative roles of A20 in inflammatory, vascular and degenerative diseases of the central nervous system and explore future directions of research.

Identification of the NF-κB inhibitor A20 as a key regulator for human adipogenesis

The zinc-finger protein A20 is a key player in the negative feedback regulation of the nuclear factor kappa-light-chain-enhancer of activated B-cell (NF-κB) pathway in response to multiple stimuli. Tumor necrosis factor alpha (TNFα), a cytokine with pleiotropic effects on cellular proliferation and differentiation, dramatically increases A20 expression in all tissues. As TNFα inhibits adipocyte differentiation, we have determined the contribution of A20 to the adipogenic capacity of human mesenchymal stromal cells (MSCs). Here we show that A20 is constitutively expressed in MSCs, which previously has been observed only in cells that are either tumor or immune cells (T/B lymphocytes). TNFα stimulation induced a rapid degradation of A20 protein mediated exclusively by the proteasome in MSCs and not by caspases. This degradation is concomitant to the induction of its own mRNA, which suggests that a tight regulation of NF-κB signaling in MSCs is fundamental. On one hand, we demonstrate that the knockdown of A20-mediated transcript dramatically decreases the adipogenic capacity of MSCs, which correlates with the phenotype observed in the presence of TNFα. On the other hand, A20 overexpression blocks NF-κB activation and drives to increased adipogenesis, even in the presence of TNFα treatment. In conclusion, our data demonstrate that the presence of A20 allows MSCs to differentiate into adipocytes by maintaining NF-κB signaling at a basal state.

Synthesis of hyaluronan in oesophageal cancer cells is uncoupled from the prostaglandin-cAMP pathway

BACKGROUND AND PURPOSE

Cyclooxygenase-2 (COX2) and hyaluronic acid (HA) are common in tumours and both independently promote tumour progression. Furthermore, COX2-dependent synthesis of prostaglandins (PGs) stimulates HA synthase-1 (HAS1) and HAS2 mRNA expression, together with HA synthesis via the cAMP/protein kinase A pathway in vascular smooth muscle cells. Therefore, the aim of the present study was to elucidate whether COX2-mediated PGs induce transcription of HAS isoforms in cancer cells as well.

EXPERIMENTAL APPROACH

Human oesophageal squamous cell (OSC) carcinoma specimens were characterized with respect to HA, COX2 and CD44 expression by immunohistochemistry. OSC cell lines (OSC1, OSC2) and HeLa cell lines (D98, H21) were exposed to exogenous PG analoques (100 nmol.L(-1)), etoricoxib (10 micromol.L(-1)) and forskolin (10 micromol.L(-1)). Subsequently, cAMP levels, HA secretion and HAS isoform expression were determined by elisa and real-time RT-PCR (reverse transcriptase polymerase chain reaction) respectively.

KEY RESULTS

COX2, HA and CD44 were detected immunohistochemically in >90% of human oesophageal tumour samples. Under basal conditions, OSC1 and OSC2 cells express HAS2 and HAS3, COX2 and Galpha(s)-coupled EP(2) and EP(4) PG receptors. Neither stimulation with the PGI(2) analogue, iloprost, addition of exogenous PGE(2) nor forskolin induced HAS1 or HAS2 mRNA expression in OSC1 and OSC2 cells. Furthermore, in HeLa cells after induction of COX2 by tumour necrosis factor alpha and subsequent PGE(2) release, inhibition of COX2 by etoricoxib did not affect HAS expression or HA secretion.

CONCLUSIONS AND IMPLICATIONS

We conclude that in oesophageal and HeLa cancer cells, HAS1/2 expression was not responsive to the PG/cAMP pathway.

BAR the door: cancer suppression by amphiphysin-like genes

The evolutionarily conserved amphiphysin-like genes Bin1 and Bin3 function in membrane and actin dynamics, cell polarity, and stress signaling. Recent genetic studies in mice discriminate non-essential roles in endocytic processes commonly ascribed to amphiphysins from essential roles in cancer suppression. Bin1 acts in default pathways of apoptosis and senescence that are triggered by the Myc and Raf oncogenes in primary cells, and Bin1 gene products display a 'moonlighting function' in the nucleus where a variety of other 'endocytic' proteins are also found. Together, genetic investigations in yeast, flies, and mice suggest that amphiphysin-like adapter proteins may suppress cancer by helping integrate cell polarity signals generated by actin and vesicle dynamics with central regulators of cell cycle arrest, apoptosis, and immune surveillance.

The proteasome is required for rapid initiation of death receptor-induced apoptosis

Due to their tremendous apoptosis-inducing potential, proteasomal inhibitors (PIs) have recently entered clinical trials. Here we show, however, that various PIs rescued proliferating tumor cells from death receptor-induced apoptosis. This protection correlated with the stabilization of X-linked IAP (XIAP) and c-FLIP and the inhibition of caspase activation. Together with the observation that PIs could not protect cells expressing XIAP or c-FLIP short interfering RNAs (siRNAs) from death receptor-induced apoptosis, our results demonstrate that PIs mediate their protective effect via the stabilization of these antiapoptotic proteins. Furthermore, we show that once these proteins were eliminated, either by long-term treatment with death receptor ligands or by siRNA-mediated suppression, active caspases accumulated to an even larger extent in the presence of PIs. Together, our data support a biphasic role for the proteasome in apoptosis, as they show that its constitutive activity is crucial for the rapid initiation of the death program by eliminating antiapoptotic proteins, whereas at later stages, the proteasome acts in an antiapoptotic manner due to the proteolysis of caspases. Thus, for a successful PI-based tumor therapy, it is crucial to carefully evaluate basal proteasomal activity and the status of antiapoptotic proteins, as their PI-mediated prolonged stability might even cause adverse effects, leading to the survival of a tumor.

Caspase-10 sensitizes breast carcinoma cells to TRAIL-induced but not tumor necrosis factor-induced apoptosis in a caspase-3-dependent manner

Although signaling by death receptors involves the recruitment of common components into their death-inducing signaling complexes (DISCs), apoptosis susceptibility of various tumor cells to each individual receptor differs quite dramatically. Recently it was shown that, besides caspase-8, caspase-10 is also recruited to the DISCs, but its function in death receptor signaling remains unknown. Here we show that expression of caspase-10 sensitizes MCF-7 breast carcinoma cells to TRAIL- but not tumor necrosis factor (TNF)-induced apoptosis. This sensitization is most obvious at low TRAIL concentrations or when apoptosis is assessed at early time points. Caspase-10-mediated sensitization for TRAIL-induced apoptosis appears to be dependent on caspase-3, as expression of caspase-10 in MCF-7/casp-3 cells but not in caspase-3-deficient MCF-7 cells overcomes TRAIL resistance. Interestingly, neutralization of TRAIL receptor 2 (TRAIL-R2), but not TRAIL-R1, impaired apoptosis in a caspase-10-dependent manner, indicating that caspase-10 enhances TRAIL-R2-induced cell death. Furthermore, whereas processing of caspase-10 was delayed in TNF-treated cells, TRAIL triggered a very rapid activation of caspase-10 and -3. Therefore, we propose a model in which caspase-10 is a crucial component during TRAIL-mediated apoptosis that in addition actively requires caspase-3. This might be especially important in systems where only low TRAIL concentrations are supplied that are not sufficient for the fast recruitment of caspase-8 to the DISC.

Cyclooxygenase-2 (COX-2) inhibitors sensitize tumor cells specifically to death receptor-induced apoptosis independently of COX-2 inhibition

Cyclooxygenase-2 (COX-2) is involved in diverse processes such as inflammation, carcinogenesis and apoptosis. As COX-2 inhibitors interfere with these processes, inhibition of COX-2 has been suggested as a promising anticancer treatment. However, the role of COX-2 in modulation of apoptosis as well as the death pathways affected by COX-2 inhibitors are poorly characterized. Here we demonstrate that the selective COX-2 inhibitors NS-398 and nimesulide increased TNF sensitivity of TNF-resistant HeLa H21 and TNF-sensitive HeLa D98 cells, although this cytokine induced significant COX-2 activity, as judged by prostaglandin E(2) (PGE(2)) production, only in H21 cells. TNF did also not induce PGE(2) production in MCF-7/casp-3 cells stably expressing COX-2; however, nimesulide strongly enhanced TNF-induced apoptosis in these cells. Furthermore, COX-2 activity in HeLa H21 cells could be inhibited by NS-398 concentrations that were 10 000-fold lower compared to those required for the induction of cell death. Most intriguingly, sensibilization to apoptosis was specifically observed in response to activation of death receptors. Not only TNF-induced cell death but also apoptosis triggered by the CD95 and TRAIL receptors was enhanced by nimesulide. In contrast, apoptosis induced by the anticancer drugs doxorubicine and etoposide that target the mitochondrial death pathway remained unaffected. Together, our data suggest that COX-2 inhibitors overcome apoptosis resistance and selectively sensitize tumor cells to the extrinsic death receptor-induced apoptotic pathway independently of COX-2.

Inhibition of retinoblastoma protein degradation by interaction with the serpin plasminogen activator inhibitor 2 via a novel consensus motif

Plasminogen activator inhibitor-2 (PAI-2) is well documented as an inhibitor of the extracellular serine proteinase urokinase-type plasminogen activator (uPA) and is expressed in activated monocytes and macrophages, differentiating keratinocytes, and many tumors. Here we show that PAI-2 has a novel intracellular function as a retinoblastoma protein (Rb)-binding protein. PAI-2 colocalized with Rb in the nucleus and inhibited the turnover of Rb, which led to increases in Rb protein levels and Rb-mediated activities. Although PAI-2 contains an LXCXE motif, Rb binding was primarily mediated by the C-D interhelical region of PAI-2, which was found to bind to the C pocket of Rb. The C-D interhelical region of PAI-2 contained a novel Rb-binding motif, termed the PENF homology motif, which is shared by many cellular and viral Rb-binding proteins. PAI-2 expression also protected Rb from the accelerated degradation mediated by human papillomavirus (HPV) E7, leading to recovery of Rb and inhibition of E6/E7 mRNA expression. Protection of Rb by PAI-2 begins to explain many of the diverse, uPA-independent phenotypes conferred by PAI-2 expression. These results indicate that PAI-2 may enhance Rb's tumor suppressor activity and suggest a potential therapeutic role for PAI-2 against HPV-transformed lesions.

Neuronal differentiation and protection from nitric oxide-induced apoptosis require c-Jun-dependent expression of NCAM140

c-Jun, a crucial component of the dimeric transcription factor activating protein 1 (AP-1), can regulate apoptosis induced by oxidative stress and has been implicated in neuronal differentiation, but the mechanisms are largely unknown. We found that specific inhibition of transcription or stable transfection with cDNA encoding dominant-negative c-Jun sensitized SH-SY5Y neuroblastoma cells (TAM-67 cells) to apoptosis induced by the nitric oxide (NO) donor sodium nitroprusside or SIN-1. TAM-67 cells also became refractory to nerve growth factor (NGF)-induced neuronal differentiation. Dominant-negative c-Jun abolished expression of a 140-kDa neural cell adhesion molecule (NCAM140) and dramatically enhanced the expression of NCAM180 in TAM-67 cells. Inhibition of c-Jun in TAM-67 cells also resulted in a corresponding decrease in the amount of NCAM140 mRNA and an increase in the amount of NCAM180 mRNA. Reexpression of NCAM140 in TAM-67 cells restored NGF-induced neuronal differentiation and resistance to NO-induced apoptosis. Our results show that c-Jun/AP-1, through up-regulation of NCAM140, plays an important role in both NGF-induced neuronal differentiation and resistance to apoptosis induced by NO in neuroblastoma cells. As NCAM140 and NCAM180 are translated from differentially spliced mRNAs transcribed from the same gene, alternative splicing of NCAM pre-mRNA (and consequently the synthesis of the smaller NCAM140 species) appears to be regulated by c-Jun/AP-1.

Ionizing radiation but not anticancer drugs causes cell cycle arrest and failure to activate the mitochondrial death pathway in MCF-7 breast carcinoma cells

There is considerable evidence that ionizing radiation (IR) and chemotherapeutic drugs mediate apoptosis through the intrinsic death pathway via the release of mitochondrial cytochrome c and activation of caspases -9 and -3. Here we show that MCF-7 cells that lack caspase-3 undergo a caspase-dependent apoptotic cell death in the absence of DNA fragmentation and alpha-fodrin cleavage following treatment with etoposide or doxorubicin, but not after exposure to IR. Re-expression of caspase-3 restored DNA fragmentation and alpha-fodrin cleavage following drug treatment, but it did not alter the radiation-resistant phenotype of these cells. In contrast to the anticancer drugs, IR failed to induce the intrinsic death pathway in MCF-7/casp-3 cells, an event readily observed in IR-induced apoptosis of HeLa cells. Although IR-induced DNA double-strand breaks were repaired with similar efficiencies in all cell lines, cell cycle analyses revealed a persistent G2/M arrest in the two MCF-7 cell lines, but not in HeLa cells. Together, our data demonstrate that caspase-3 is required for DNA fragmentation and alpha-fodrin cleavage in drug-induced apoptosis and that the intrinsic death pathway is fully functional in MCF-7 cells. Furthermore, they show that the radiation-resistant phenotype of MCF-7 cells is not due to the lack of caspase-3, but is caused by the failure of IR to activate the intrinsic death pathway. We propose (1) different signaling pathways are induced by anticancer drugs and IR, and (2) IR-induced G2/M arrest prevents the generation of an apoptotic signal required for the activation of the intrinsic death 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.

Unusual resistance of ALR/Lt mouse beta cells to autoimmune destruction: role for beta cell-expressed resistance determinants

Genetic analysis of autoimmune insulin-dependent diabetes mellitus (IDDM) has focused on genes controlling immune functions, with little investigation of innate susceptibility determinants expressed at the level of target beta cells. The Alloxan (AL) Resistant (R) Leiter (Lt) mouse strain, closely related to the IDDM-prone nonobese diabetic (NOD)/Lt strain, demonstrates the importance of such determinants. ALR mice are unusual in their high constitutive expression of molecules associated with dissipation of free-radical stress systemically and at the beta-cell level. ALR islets were found to be remarkably resistant to two different combinations of beta-cytotoxic cytokines (IL-1beta, tumor necrosis factor alpha, and IFN-gamma) that destroyed islets from the related NOD and alloxan-susceptible strains. The close MHC relatedness between the NOD and ALR strains (H2-Kd and H2-Ag7 identical) allowed us to examine whether ALR islet cells could survive autoimmune destruction by NOD-derived Kd-restricted diabetogenic cytotoxic T lymphocyte clones (AI4 and the insulin-reactive G9C8 clones). Both clones killed islet cells from all Kd-expressing strains except ALR. ALR resistance to diabetogenic immune systems was determined in vivo by means of adoptive transfer of the G9C8 clone or by chimerizing lethally irradiated ALR or reciprocal (ALR x NOD)F1 recipients with NOD bone marrow. In all in vivo systems, ALR and F1 female recipients of NOD marrow remained IDDM free; in contrast, all of the NOD recipients became diabetic. In conclusion, the ALR mouse presents a unique opportunity to identify dominant IDDM resistance determinants expressed at the beta cell level.

Cytochrome c-dependent activation of caspase-3 by tumor necrosis factor requires induction of the mitochondrial permeability transition

The killing of L929 mouse fibroblasts by tumor necrosis factor-alpha (TNF-alpha) in the presence of 0.5 microg/ml actinomycin D (Act D) is prevented by inhibition of the mitochondrial permeability transition (MPT) with cyclosporin A (CyA) in combination with the phospholipase A(2) inhibitor aristolochic acid (ArA). The MPT is accompanied by the release of cytochrome c from the mitochondria, caspase-8 and caspase-3 activation in the cytosol, cleavage of the nuclear enzyme poly(ADP-ribose)polymerase (PARP), and DNA fragmentation, all of which were inhibited by CyA plus ArA. The caspase-3 inhibitor z-Asp-Glu-Val-aspartic acid fluoromethyl-ketone (Z-DEVD-FMK) did not prevent the loss of viability or the redistribution of cytochrome c, but it did prevent caspase-3 activation, PARP cleavage, and DNA fragmentation. Inhibition of the MPT reduced the activation of caspase-8 to the level occurring with TNF-alpha alone (no ActD). The caspase-8 inhibitor z-Ile-Glu(OMe)-Thr-Asp(OMe) fluoromethylketone (Z-IETD-FMK) did not prevent the cell killing and decreased only slightly the translocation of Bid to the mitochondria. These data indicate that induction of the MTP by TNF-alpha causes a release of cytochrome c, caspase-3 activation with PARP cleavage and DNA fragmentation. The loss of viability is dependent on the MPT but independent of the activation of caspase-3. The activation of caspase-8 is not dependent on the MPT. There is no evidence linking this enzyme to the loss of viability. Thus, the killing of L929 fibroblasts by TNF-alpha can occur in the absence of either caspase-3 or caspase-8 activity. Alternatively, cell death can be prevented despite an activation of caspase-8.

c-Myc does not prevent glucocorticoid-induced apoptosis of human leukemic lymphoblasts

Due to their growth arrest- and apoptosis-inducing ability, glucocorticoids (GC) are widely used in the therapy of various lymphoid malignancies. The signal transduction pathways leading to this clinically-relevant form of apoptosis have, however, not been sufficiently elucidated. GC bind to their specific receptor, a ligand-activated transcription factor of the Zn-finger type, that activates or represses transcription of GC-responsive genes. Previous studies in leukemia cells suggested that transcriptional repression of c-myc expression might be the crucial event in GC-induced apoptosis, although in other systems, c-Myc apparently increased the sensitivity to cell-death inducers. To address this controversy, we stably transfected the GC-sensitive human T-ALL cell line CEM-C7H2 with constructs allowing tetracycline-regulated expression of c-Myc. Subsequent analyses of these cell lines showed that overexpression of c-Myc per se had little, if any, effect on cell viability, although it rendered the cells more sensitive to apoptosis induced by low serum, confirming the functionality of the expressed transgene. More importantly, however, when the cells were treated with GC in the presence of exogenous c-Myc, they underwent apoptosis exceeding that in cells treated in the absence of transgenic c-Myc. The data indicate that c-myc downregulation is not critical for induction of cell-death by GC in this system, and support the notion that c-Myc sensitizes cells to apoptosis-inducing agents.

The zinc finger protein A20 interacts with a novel anti-apoptotic protein which is cleaved by specific caspases

A20 is a Cys2/Cys2 zinc finger protein which is induced by a variety of inflammatory stimuli and which has been characterized as an inhibitor of cell death by a yet unknown mechanism. In order to clarify its molecular mechanism of action, we used the yeast two-hybrid system to screen for proteins that interact with A20. A cDNA fragment was isolated which encoded a portion of a novel protein (TXBP151), which was recently found to be a human T-cell leukemia virus type-I (HTLV-I) Tax-binding protein. The full-length 2386 bp TXBP151 mRNA encodes a protein of 86 kDa. Like A20, overexpression of TXBP151 could inhibit apoptosis induced by tumour necrosis factor (TNF) in NIH3T3 cells. Moreover, transfection of antisense TXBP151 partially abolished the anti-apoptotic effect of A20. Furthermore, apoptosis induced by TNF or CD95 (Fas/APO-1) was associated with proteolysis of TXBP151. This degradation could be inhibited by the broad-spectrum caspase inhibitor zVAD-fmk or by expression of the cowpox virus-derived inhibitor CrmA, suggesting that TXBP151 is a novel substrate for caspase family members. TXBP151 was indeed found to be specifically cleaved in vitro by members of the caspase-3-like subfamily, viz. caspase-3, caspase-6 and caspase-7. Thus TXBP151 appears to be a novel A20-binding protein which might mediate the anti-apoptotic activity of A20, and which can be processed by specific caspases.

Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the tumor necrosis factor alpha signalling pathway

The human promonocytic cell line U937 undergoes apoptosis upon treatment with tumor necrosis factor alpha (TNF-alpha). This cell line has previously been shown to be very sensitive to the lytic effect of the autonomous parvovirus H-1. Parvovirus infection leads to the activation of the CPP32 ICE-like cysteine protease which cleaves the enzyme poly(ADP-ribose)polymerase and induces morphologic changes that are characteristic of apoptosis in a way that is similar to TNF-alpha treatment. This effect is also observed when the U937 cells are infected with a recombinant H-1 virus which expresses the nonstructural (NS) proteins but in which the capsid genes are replaced by a reporter gene, indicating that the induction of apoptosis can be assigned to the cytotoxic nonstructural proteins in this cell system. The c-Myc protein, which is overexpressed in U937 cells, is rapidly downregulated during infection, in keeping with a possible role of this product in mediating the apoptotic cell death induced by H-1 virus infection. Interestingly, four clones (designated RU) derived from the U937 cell line and selected for their resistance to H-1 virus (J. A. Lopez-Guerrero et al., Blood 89:1642-1653, 1997) failed to decrease c-Myc expression upon treatment with differentiation agents and also resisted the induction of cell death after TNF-alpha treatment. Our data suggest that the RU clones have developed defense strategies against apoptosis, either by their failure to downregulate c-Myc and/or by activating antiapoptotic factors.

Autocrine self-elimination of cultured ovarian cancer cells by tumour necrosis factor alpha (TNF-alpha)

Human ovarian adenocarcinoma cells N.1 secrete an autocrine activity that stimulates active cell death under serum-reduced conditions. To substitute the autocrine activity by a single physiological component, 28 cytokines, growth factors and biomodulators were tested [interleukin 1alpha (IL-1alpha), IL-1beta, IL-2, IL-3, IL-4, IL-6, IL-10, IL-11, stem cell factor (SCF), platelet-derived growth factor (PDGF), acid fibroblast growth factor (aFGF), basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF-1), IGF-2, insulin, macrophage colony-stimulating factor (M-CSF), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), oncostatin, RANTES (regulated on activation normal T cell expressed and secreted), angiogenin, leukaemia inhibitory factor (LIF), erythropoietin (EPO), interferon alpha (INF-alpha), INF-gamma, transferrin, tumour necrosis factor alpha (TNF-alpha, TNF-beta and bovine serum albumin for control reasons]. In these experiments, only TNF-alpha and TNF-beta rapidly induced apoptosis. TNF-alpha and TNF-receptor 1 were expressed by N.1 cells, and the secretion of TNF-alpha was verified by enzyme-linked immunosorbent assay (ELISA). Autocrine factor-triggered apoptosis was inhibited when conditioned supernatant was preincubated with anti-TNF-alpha antibody. These findings suggested that the apoptosis-inducing component of the N.1 autocrine activity was TNF-alpha. In the presence of antisense c-myc oligonucleotides, induction of cell death by autocrine factor was partly inhibited. Autocrine factor and TNF-alpha stimulated transcription of the invasiveness-related protease plasminogen activator/urokinase mRNA (upa) with similar kinetics. When N.1 cells were exposed to purified plasminogen activator/urokinase protein (uPA), cell matrix contact was disrupted. Thus, uPA might serve a physiological role during TNF-induced apoptosis by affecting the interactions between cells and the basal membrane, thereby facilitating anoikis. This mechanistic study, which was restricted to a single human ovarian carcinoma model cell line (N.1), provides evidence that N.1 maintains the capacity to undergo c-myc-dependent apoptosis by the TNF-TNF-receptor pathway, and no additional pharmacological stimuli for induction of apoptosis are required.

Induction of TNF-sensitive cellular phenotype by c-Myc involves p53 and impaired NF-kappaB activation

Normal fibroblasts are resistant to the cytotoxic action of tumor necrosis factor (TNF), but are rendered TNF-sensitive upon deregulation of c-Myc. To assess if oncoproteins induce the cytotoxic TNF activity by modulating TNF signaling, we investigated the TNF-elicited signaling responses in fibroblasts containing a conditionally active c-Myc protein. In association with cell death, c-Myc impaired TNF-induced activation of phospholipase A2, JNK protein kinase and cell survival-signaling-associated NF-kappaB transcription factor complex. The TNF-induced death of mouse primary fibroblasts expressing deregulated c-Myc was inhibited by transient overexpression of the p65 subunit of NF-kappaB, which increased NF-kappaB activity in the cells. Unlike other TNF-induced signals, TNF-induced accumulation of the wild-type p53 mRNA and protein was not inhibited by c-Myc. TNF, with c-Myc, induced apoptosis in mouse primary fibroblasts but only weakly in p53-deficient primary fibroblasts. The C-terminal domain of p53, which is a transacting dominant inhibitor of wild-type p53, failed to inhibit apoptosis by c-Myc and TNF, suggesting that the cell death was not dependent on the transcription-activating function of p53. Taken together, the present findings show that the cytotoxic activity of TNF towards oncoprotein-expressing cells involves p53 and an impaired signaling for survival in such cells.

A role for c-myc in chemically induced renal-cell death

A variety of genes, including c-myc, are activated by chemical toxicants in vivo and in vitro. Although enforced c-myc expression induces apoptosis after withdrawing survival factors, it is not clear if activation of the endogenous c-myc gene is an apoptotic signal after toxicant exposure. The renal tubular epithelium is a target for many toxicants. c-myc expression is activated by tubular damage. In quiescent LLC-PK1 renal epithelial cells, c-myc but not max or mad mRNA is induced by the nephrotoxicant S-(1,2-dichlorovinyl)-L-cysteine (DCVC). The kinetics of DCVC-induced c-myc expression and apoptosis suggested an association between cell death and prolonged activation of c-myc expression after toxicant exposure. Accordingly, prolonged activation of an estrogen receptor-Myc fusion construct, but not a construct in which a c-Myc transactivation domain had been deleted, was sufficient to induce apoptosis in LLC-PK1 cells. Moreover, under conditions in which necrosis was the predominant cell death pathway caused by DCVC in parental cells, overexpressing c-myc biased the cell death pathway toward apoptosis. DCVC also induced ornithine decarboxylase (odc) mRNA and activated the odc promoter. Activation of the odc promoter by DCVC required consensus c-Myc-Max binding sites in odc intron 1. Inhibiting ODC activity with alpha-difluoromethylornithine delayed DCVC-induced cell death. Therefore, odc is a target gene in the DCVC apoptotic pathway involving c-myc activation and contributes to apoptosis. Finally, a structurally related cytotoxic but nongenotoxic analog of DCVC did not induce c-myc and did not activate the odc promoter or induce apoptosis. The data support the hypothesis that activation of apoptotic cell death in quiescent renal epithelial cells involves induction of c-myc. This is the first study to demonstrate that c-myc induction by a specific nephrotoxicant leads to gene activation and cell death.

Cyclin D3 sensitizes tumor cells to tumor necrosis factor-induced, c-Myc-dependent apoptosis

c-Myc is an important mediator of apoptosis in cytokine- or serum-deprived cells and sensitizes various cell types to tumor necrosis factor alpha (TNF) cytotoxicity. However, downstream mediators of c-Myc-dependent apoptosis are largely unknown. In this study, we investigated whether one or more cyclins which, like c-Myc, are important regulators of the cell cycle are involved in TNF-induced apoptosis downstream of c-Myc. Cyclin D3 and c-Myc levels in HeLa and fibrosarcoma cells correlated with sensitivity of these cells to TNF-induced apoptosis, as both proteins were highly expressed in TNF-sensitive HeLa D98 cells and HT-1080 fibrosarcoma cells but not in their TNF-resistant counterparts, HeLa H21 and SS-HT-1080 cells, respectively. All other cyclins tested were equally expressed in all tumor cell lines. Reduction in the expression of c-Myc by dexamethasone or inhibition of the transcriptional activity of c-Myc by introduction of a dominant negative form of c-Myc into TNF-sensitive HeLa D98 cells strongly suppressed the expression of cyclin D3 (but none of the other cyclins) and rendered the cells resistant to TNF-induced apoptosis. Conversely, introduction of the c-myc gene into TNF-resistant, c-Myc- and cyclin D3-deficient HeLa H21 cells resulted in enhanced cyclin D3 expression and TNF killing. When cyclin D3 expression in HeLa cells was altered by sense or antisense cyclin D3 cDNA, there was a concomitant alteration in their susceptibility to TNF-induced apoptosis without any change in c-Myc levels. Overall, our results show that cyclin D3 sensitizes tumor cells to TNF-induced apoptosis and indicate that the expression of c-Myc and expression of cyclin D3 in HeLa and in HT-1080 fibrosarcoma cells are closely linked.

Inhibition of c-myc expression induces apoptosis of WEHI 231 murine B cells

Treatment of WEHI 231 immature B-lymphoma cells with an antibody against their surface immunoglobulin (anti-Ig) induces apoptosis and has been studied extensively as a model of B-cell tolerance. Anti-Ig treatment of exponentially growing WEHI 231 cells results in an early transient increase in c-myc expression that is followed by a decline to below basal levels; this decrease in c-myc expression immediately precedes the induction of cell death. Here we have modulated NF-kappaB/Rel factor activity, which regulates the rate of c-myc gene transcription, to determine whether the increase or decrease in c-Myc-levels mediates apoptosis in WEHI 231 cells. Addition of the serine/threonine protease inhibitor N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), which blocks the normally rapid turnover of the specific inhibitor of NF-kappaB/Rel IkappaBalpha in these cells, caused a drop in Rel-related factor binding. TPCK treatment resulted in decreased c-myc expression, preventing the usual increase seen following anti-Ig treatment. Whereas inhibition of the induction of c-myc expression mediated by anti-Ig failed to block apoptosis, reduction of c-myc expression in exponentially growing WEHI 231 cells induced apoptosis even in the absence of anti-Ig treatment. In WEHI 231 clones ectopically expressing c-Myc, apoptosis induced by treatment with TPCK or anti-Ig was significantly diminished and cells continued to proliferate. Furthermore, apoptosis of WEHI 231 cells ensued following enhanced expression of Mad1, which has been found to reduce functional c-Myc levels. These results indicate that the decline in c-myc expression resulting from the drop in NF-kappaB/Rel binding leads to activation of apoptosis of WEHI 231 B cells.