Role for c-myc in activation-induced apoptotic cell death in T cell hybridomas

A matter of life and cell death

In multicellular organisms, mutations in somatic cells affecting critical genes that regulate cell proliferation and survival cause fatal cancers. Repair of the damage is one obvious option, although the relative inconsequence of individual cells in metazoans means that it is often a "safer" strategy to ablate the offending cell. Not surprisingly, corruption of the machinery that senses or implements DNA damage greatly predisposes to cancer. Nonetheless, even when oncogenic mutations do occur, there exist potent mechanisms that limit the expansion of affected cells by suppressing their proliferation or triggering their suicide. Growing understanding of these innate mechanisms is suggesting novel therapeutic strategies for cancer.

Sodium butyrate suppresses apoptosis in human Burkitt lymphomas and murine plasmacytomas bearing c-myc translocations

We report that sodium butyrate, a natural product of fiber degradation by colonic bacteria, markedly suppresses c-Myc-mediated apoptosis induced in murine plasmacytomas and human Burkitt lymphomas by growth factor deprivation, but not in cell lines devoid of c-myc translocations. Attenuation of cell death is associated with downregulation of the rearranged c-myc and activation of pRb via its dephosphorylation. We suggest that in vivo sodium butyrate may play an important role in plasmacytomagenesis by supporting the survival of cells with c-myc translocations, which otherwise would be eliminated by the lack of growth factors.

Activation of c-jun and c-fos genes in dNTP imbalance cell death induced with 5-fluoro-2'-deoxyuridine in mouse mammary tumor FM3A cell line

5-Fluoro-2' -deoxyuridine (FdUrd)-induced death of mouse mammary FM3A cells was found to be associated with an increased expression of cellular c-jun and c-fos genes. The increase in these gene expressions was mediated through the protein kinase C-dependent pathway. Blockage of the expressions with the use of antisense oligodeoxynucleotide for c-jun delayed the cell death. These findings suggest that the activation of c-jun and c-fos genes, which encode transcription factors participating in cell proliferation, plays a role in FdUrd-induced cell death.

Trace elements and apoptosis

It is known that apoptosis is considered to be responsible for selective deletion of cells during embryogenesis, the homeostasis of cell populations in continuously renewing tissues (i.e., serving as a counterbalance to mitosis), and tissue involution in response to chemical or physical stimuli. There are many publications on these questions. On the other hand, the intracellular processes that contribute to apoptosis are incompletely understood. Therefore, the role of apoptosis in the intracellular accumulation and outflow of minerals is of considerable importance in light of both their essential functions and toxic effects.

Cloning, characterization, and mapping of human homolog of mouse T-cell death-associated gene

To establish immunologic autotolerance, self-reactive immature thymocytes are eliminated by negative selection during T-cell development in the thymus. Self-reactive clones undergo apoptosis after stimulation via the T-cell receptor (TCR). The process of cell selection is determined by the dedication of the TCR for tolerogenic antigen/major histocompatibility complex. We have cloned a novel human gene that is highly homologous in the transmembrane and G protein-coupling domains to mouse T-cell death-associated gene 8 (TDAG8). The gene, human TDAG8 (hTDAG8), which belongs to the G protein-couple receptor superfamily, encodes a protein of 337 amino acids. An expressed sequence tag (EST) corresponding to hTDAG8 was identified from a human thyroid cDNA library and subsequently used to isolate a full-length genomic clone. Northern blot analysis revealed that the hTDAG8 gene is expressed predominantly in lymphoid tissues, including peripheral blood leukocytes, spleen, lymph nodes, and thymus. Stably transfected mammalian CHO cells were generated, and heterologous expression of hTDAG8 was confirmed by Northern blot analysis. Fluorescent in situ hybridization (FISH) revealed that hTDAG8 maps to human chromosome 14q31-32.1, a region in which abnormalities associated with human T-cell lymphoma or leukemia are found. Taken together, these data implicate the hTDAG8 gene in T-cell-associated diseases in humans, but its actual physiological and pathological role in the human immune system needs further investigation.

Lymphokine regulation of activation-induced apoptosis in T cells of IL-2 and IL-2R beta knockout mice

Recent studies using IL-2R alpha knockout mice have generated conflicting results regarding the hypothesis that IL-2/IL-2R interaction is obligatory for the development of AICD, which plays a central and pivotal role in maintaining peripheral tolerance. A relevant consequence of AICD defect is the demonstrated development of autoimmune lymphoproliferative disease in IL-2, IL-2R alpha, and IL-2R beta knockout mice, but not in IL-4, IL-7, or IL-7R knockout mice. Whether IL-4, IL-7, or IL-15 can provide the required signal for AICD development is addressed here using IL-2 and IL-2R beta knockout mice. Lymph node T cells from knockout mice were stimulated with Con A plus rIL-1 for 3 days and then maintained in high concentrations of rIL-4, rIL-7, or rIL-15 for an additional 3 days before they were subjected to AICD analysis. Our study demonstrates that IL-4, IL-7, and IL-15 can transduce signals critical for AICD development in the absence of IL-2-mediated signals. The requirement for relatively high concentrations of these lymphokines suggests their limited role in maintaining peripheral T cell tolerance, thus explaining the differential expression of autoimmune lymphoproliferative disease in the targeted mutant strains described above.

Inhibition of activation-induced apoptosis of thymocytes by all-trans- and 9-cis-retinoic acid is mediated via retinoic acid receptor alpha

Thymocytes can be induced to undergo apoptotic cell death by activation through the T-cell receptor (TCR). This process requires macromolecular synthesis and has been shown to be inhibited by retinoic acids (RAs). Two groups of nuclear receptors for RAs have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). All-trans-RA is the high-affinity ligand for RARs, and 9-cis-RA additionally binds to RXRs with high affinity. Because 9-cis-RA is much more potent in inhibiting TCR-mediated death than all-trans-RA, it was suggested that RXRs participate in the process. In the present study various synthetic retinoid analogues were used to address this question further. The results presented suggest that the inhibitory effect of RAs on activation-induced death of thymocytes is mediated via RARalpha, because (1) it can be reproduced by various RARalpha analogues both in vitro and in vivo, (2) the effect of RAs can be inhibited by the addition of an RARalpha antagonist, (3) CD4+CD8+thymocytes, which die on TCR stimulation, express RARalpha. Stimulation of RARgamma, in contrast, enhances the activation-induced death of thymocytes and inhibits its prevention by RARalpha stimulation. RXR co-stimulation suspends this inhibitory effect of RARgamma and permits the preventive function of RARalpha on activation-induced death. Our results suggest a complex interaction between the various isoforms of retinoid receptors and demonstrate that low (physiological) concentrations of all-trans-RA do not affect the activation-induced death of thymocytes because the RARalpha-mediated inhibitory and the RARgamma-mediated enhancing pathways are in balance, whereas if 9-cis-RA is formed, additional stimulation of RXRs permits the inhibitory action of RARalpha.

The functions of Myc in cell cycle progression and apoptosis

c-myc has emerged as one of the central regulators of mammalian cell proliferation. The gene encodes a transcription factor of the HLH/leucine zipper family of proteins that activates transcription as part of a heteromeric complex with a protein termed Max. In mammalian fibroblasts, Myc acts as an upstream regulator of cyclin-dependent kinases and functionally antagonises the action of at least one cdk inhibitor, p27. Myc also induces cells to undergo apoptosis, and the relationship between Myc-induced cell cycle entry and apoptosis is discussed.

The many roles of c-Myc in apoptosis

The proto-oncogene c-myc encodes a transcription factor c-Myc, which is of great importance in controlling cell growth and vitality. The quantity of c-Myc is carefully controlled by many mechanisms, and its actions to induce and repress genes are modulated by interactions with other regulatory proteins. Understanding the kinetic and quantitative relationships that determine how and what genes c-Myc regulates is essential to understanding how Myc is involved in apoptosis. Reduction of c-myc expression and its inappropriate expression can be associated with cellular apoptosis. This review outlines the nature and regulation of the c-myc gene and of c-Myc and presents the systems and conditions in which Myc-related apoptotic events occur. Hypotheses of the mechanisms by which expression and repression of c-myc lead to apoptosis are discussed.

Buprenorphine hydrochloride induces apoptosis in NG108-15 nerve cells

A morphine alkaloid derivative, buprenorphine hydrochloride, induces apoptosis in NG108-15 cells. Apoptosis was detected mainly by apoptosis-specific DNA fragmentation and morphological changes. This apoptosis was dose-dependent and the time-course experiment indicated that DNA fragmentation occurred within 4 h after administration of buprenorphine hydrochloride. Specific inhibitors of the previously characterized apoptotic signal cascade as well as antagonists for opioid receptors were tested. Zn2+, herbimycin A, caspase inhibitors YVAD (Ac-Tyr-Val-Ala-Asp-CHO) and DEVD (Ac-Asp-Glu-Val-Asp-CHO), naloxone and naltrindole had no effect on apoptosis-specific DNA fragmentation. The serine protease inhibitor TPCK (N-tosyl-L-phenylalanyl chloromethyl ketone) specifically inhibited apoptosis-specific DNA fragmentation induced by buprenorphine hydrochloride; however, cell viability measurements revealed that cell death still occurred in NG108-15 cells. Thus TPCK pretreatment before buprenorphine hydrochloride administration induced apoptosis-independent cell death, presumably necrosis, in NG108-15 cells. This suggests that an unidentified serine protease, presumably functioning in the buprenorphine hydrochloride-specific death-signal cascade, could be pivotal for the rapid apoptosis observed in NG108-15 cells upon treatment with buprenorphine hydrochloride.

cAMP increasing agents attenuate the generation of apoptosis by etoposide in promonocytic leukemia cells

Treatment of U-937 promonocytic cells with the DNA topoisomerase II inhibitor etoposide rapidly caused death by apoptosis, as determined by changes in chromatin structure, production of DNA breaks, nucleosome-sized DNA degradation, decrease in mitochondrial membrane potential and phosphatidyl serine translocation in the plasma membrane, and at the same time induced intracellular acidification. Both the execution of the apoptotic process and the intracellular acidification were reduced by the addition of forskolin plus theophylline or other cAMP increasing agents. These agents also attenuated the induction of apoptosis by camptothecin, heat-shock, cadmium chloride and X-radiation. Although etoposide slightly increased the production of reactive oxygen intermediates, this increase was not prevented by forskolin plus theophylline, and the addition of antioxidant agents failed to inhibit apoptosis. Etoposide caused a great increase in NF-(kappa)B binding activity, which was not prevented by forskolin plus theophylline, while AP-1 binding was little affected by the topoisomerase inhibitor. The treatments did not significantly alter the levels of Bcl-2 and Bax. By contrast, the expression of c-myc, which was very high in untreated U-937 cells and only partially inhibited by etoposide, was rapidly and almost totally abolished by the cAMP increasing agents. Finally, it was observed that etoposide caused a transient dephosphorylation of retinoblastoma (Rb), which was associated with cleavage of poly(ADP-ribose) polymerase (PARP). Both Rb dephosphorylation and PARP cleavage were inhibited by forskolin plus theophylline. The inhibition of Rb (type I) phosphatase and ICE/CED-3-like protease activities, and the abrogation of c-myc expression, are mechanisms which could explain the anti-apoptotic action of cAMP increasing agents in myeloid cells.

Inhibition of transcription of the human c-myc protooncogene by intermolecular triplex

Triplex-forming oligonucleotides (TFOs) have been shown to inhibit both transcription in vitro and the expression of target genes in cell culture by binding to polypurine/polypyrimidine sequences in several human gene promoters. The c-myc protooncogene is overexpressed in a variety of human cancers and appears to play an important role in the proliferation of these cells. In an attempt to assay the ability of triplex-forming oligonucleotides to inhibit expression of a target gene in vivo, we have developed a cellular system involving transfection of a c-myc promoter-driven luciferase reporter plasmid with triplex-forming oligonucleotides targeted to the human c-myc protooncogene. To increase the stability of the TFO, we have used modified phosphorothioate oligonucleotides. Triplex formation with a modified phosphorothioate oligonucleotide occurs with approximately equal binding affinity as that seen using a phosphodiester oligonucleotide. Phosphorothioate-modified TFOs targeted to c-myc inhibit transcription of the c-myc promoter in HeLa cells as demonstrated by a decrease in luciferase expression from a luciferase reporter gene construct. These results suggests that triplex formation may represent a gene-specific means of inhibiting specific protooncogene expression.

Biphasic c-Myc protein expression during gossypol-induced apoptosis in rat spermatocytes

The presence of protooncogene products such as c-Myc proteins in rat spermatocytes has been quantitatively detected by Western immunoblot and a computer-controlled Spotdenso-program with an IS-1000 digital imaging system. Cellular levels of c-Myc proteins in response to gossypol were measured in spermatocytes during the process of gossypol-induced apoptosis. Within 0.5 to 2 h of the addition of gossypol, levels of c-Myc proteins fall dramatically and remain at a low level for the next several hours. The reduction in c-Myc proteins occurs 4.5-6 h before the apoptosis of spermatocytes in the presence of gossypol. Between 3 and 5 h after exposure to gossypol, the c-Myc protein content returns to preexposure (or higher) levels. In addition, the increase in c-Myc proteins occurs 1.5-4 h before the apoptotic death of spermatocytes. An identical pattern of c-Myc protein response to gossypol was also found in total testicular tissue in vitro. These results suggest that spermatocyte apoptosis induced by gossypol is correlated with biphasic c-Myc protein expression. This article present some hypothetical models with which to explain c-Myc protein-mediated apoptosis.

Apoptosis and cell culture technology

The importance of apoptosis in cell culture has now been widely recognised. Recent work has shown that this process is widely manifested during the in vitro cultivation of commercially important mammalian cell lines. In this review I summarise what is now known of the characteristics, significance and regulatory mechanisms of apoptosis. As the process of cell proliferation and cell death are now considered intimately related, particular attention is paid to highlight the progress and opportunities in the field of cell culture engineering. The strategies that have been undertaken to prevent the induction of apoptosis in cell culture and those which have been suggested as possibilities to improve culture productivity through the apoptosis route are discussed with given examples.

Contribution of gene-specific lesions, DNA-replication-associated damage, and subsequent transcriptional inhibition in topoisomerase inhibitor-mediated apoptosis in lymphoma cells

Lymphoid lineage tumor cells differ widely in their relative sensitivity or resistance to the induction of apoptosis by a variety of chemotherapeutic drugs. We used a model system of virally transformed B- and T-lymphoma cell lines to show that avian T-lymphoma cells are highly resistant, whereas B-lymphoma cells are highly sensitive, to the induction of apoptosis by a wide spectrum of chemotherapeutic drugs that induce different types of lesions in DNA. Among the various drugs examined, the topoisomerase inhibitors, camptothecin, actinomycin D, and etoposide, were the most potent inducers of apoptosis. Examination of the relative contribution of DNA replication and transcriptional inhibition to the differential induction of apoptosis by the topoisomerase inhibitors revealed that the signals initiating the apoptotic response vary, even among compounds with similar cellular targets. Specifically, DNA replication plays a major role in the induction of camptothecin-induced apoptosis, and a lesser role in the induction of apoptosis by etoposide. In contrast, DNA replication is not involved in the induction of apoptosis by actinomycin D. Transcriptional inhibition may provide the major cellular signal for apoptosis induction by this compound. In addition, we determined that the extent of topoisomerase I-cleavable complex inhibition is similar even in genes that are transcribed at different levels and by different RNA polymerases. An overexpressed c-myc gene is no more vulnerable to topoisomerase inhibition than its normally expressed counterpart. In contrast, even under conditions yielding similar amounts of topoisomerase inhibition, rRNA genes are more sensitive to transcriptional inhibition than are the c-myc genes.

Gender and androgen treatment influence the expression of proto-oncogenes and apoptotic factors in lacrimal and salivary tissues of MRL/lpr mice

The objectives of this study were to (1) determine whether Fas antigen, Fas ligand, p53, and proto-oncogene mRNAs may be detected in lacrimal and submandibular glands of the MRL/lpr mouse model of Sjögren's syndrome, and (2) examine whether gender and androgen or cyclophosphamide therapy influence the mRNA expression of these apoptotic factors. Tissues were obtained from treated or untreated MRL/lpr mice after the onset of disease and processed for the analysis of mRNAs by RT-PCR and Southern blot hybridization. Our results demonstrated that (1) Fas antigen (exons 1-->2 or 3-->7+), Fas ligand, c-myb, c-myc, bcl-2, Bax, p53, and androgen receptor (AR) mRNAs are present in exocrine tissues of MRL/lpr mice; (2) the amounts of c-myb, c-myc, bcl-2, p53, and AR mRNA are higher (P < 0.05) and the level of Fas antigen (exons 1-->2) mRNA is lower (P < 0.05) in lacrimal glands of female compared to male mice. In contrast, the content of c-myb and p53 mRNA is greater (P < 0.05) in submandibular tissues of female relative to those of male mice; and (3) testosterone or cyclophosphamide treatment led to a significant (P < 0.05) decline in the mRNA levels of c-myb, bcl-2, and/or AR, but an increase (P < 0.05) in the mRNA amount of Bax, in lacrimal, but not in salivary, glands of female mice. These findings demonstrate that gender-associated differences exist in the expression of apoptotic factor mRNAs in exocrine tissues of autoimmune mice and that some of these differences appear to be due to the influence of androgens.

Cell death/apoptosis: normal, chemically induced, and teratogenic effect

Cell death is an integral part of a variety of biological processes including cell proliferation, differentiation, and morphogenesis. We review here the morphological and biochemical nature as well as the genetic basis for cell death during normal and abnormal development. Most often referred to in normal development as programmed cell death, this controlled process determines the size, patterning, and function of many tissues. The importance of its proper genetic regulation is demonstrated by the discovery of cell death-specific genes and the several disorders including cancer and teratogenesis that result from repression or enhancement of cell death. In our studies we employed the developing mouse limb, which provides a defined window of active cell death, to elucidate mechanisms of cell death. We have developed markers that reveal in the developing normal limb an apoptotic morphology with phagocytosis and DNA fragmentation. In the limb deformity mutant Hammertoe there is a defective (restricted) cell death pattern, but the morphology remains apoptotic. By the use of these markers, we were able to observe that the teratogen retinoic acid produced enhanced apoptotic cell death. Most interestingly, retinoic acid-induced cell death in the Hammertoe mutant resulted in correction of the mutant phenotype. Future studies will determine the relationship between exogenous agents and endogenous signaling pathways as well as indicate how these interactions can alter the fate of a given cell and potentially ameliorate a genetic abnormality.

A new dexamethasone-induced gene of the leucine zipper family protects T lymphocytes from TCR/CD3-activated cell death

By comparing mRNA species expressed in dexamethasone (DEX)-treated and untreated murine thymocytes, we have identified a gene, glucocorticoid-induced leucine zipper (GILZ), encoding a new member of the leucine zipper family. GILZ was found expressed in normal lymphocytes from thymus, spleen, and lymph nodes, whereas low or no expression was detected in other nonlymphoid tissues, including brain, kidney, and liver. In thymocytes and peripheral T cells, GILZ gene expression is induced by DEX. Furthermore, GILZ expression selectively protects T cells from apoptosis induced by treatment with anti-CD3 monoclonal antibody but not by treatment with other apoptotic stimuli. This antiapoptotic effect correlates with inhibition of Fas and Fas ligand expression. Thus, GILZ is a candidate transcription factor involved in the regulation of apoptosis of T cells.

Radioprotective and antileukemic effects of growth factors in regenerating hematopoietic tissue

We have examined the role of growth factors present in regenerating hematopoietic tissue in the prevention of some radiation-induced effects. These factors, extracted from calf spleen undergoing reparative regeneration, increased the 30-day survival of irradiated mice and partially decreased the incidence of leukemia in surviving mice. The growth factors modified the properties of leukemia cells in vitro, and could suppress residual leukemia cells in vivo. The antileukemic activity of regenerating hematopoietic tissue can be purified to a homogeneous state. The radioprotective activity is associated with the production of regulatory molecules that have been partially characterized. These findings provide evidence that the natural resistance of regenerating hematopoietic tissue (which has an increased number of cell targets for radiation and other damaging agents) results from concurrent local production of a battery of defensive regulatory molecules.

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

Induction of apoptosis by oncogenes like c-myc may be important in restraining the emergence of neoplasia. However, the mechanism by which c-myc induces apoptosis is unknown. CD95 (also termed Fas or APO-1) is a cell surface transmembrane receptor of the tumor necrosis factor receptor family that activates an intrinsic apoptotic suicide program in cells upon binding either its ligand CD95L or antibody. c-myc-induced apoptosis was shown to require interaction on the cell surface between CD95 and its ligand. c-Myc acts downstream of the CD95 receptor by sensitizing cells to the CD95 death signal. Moreover, IGF-I signaling and Bcl-2 suppress c-myc-induced apoptosis by also acting downstream of CD95. These findings link two apoptotic pathways previously thought to be independent and establish the dependency of Myc on CD95 signaling for its killing activity.

Effects of 3-deazaadenosine on apoptosis-related gene transcripts in HL-60 cells

The effect of the transmethylation inhibitor 3-deazaadenosine on transcription levels of genes associated with apoptosis was investigated in HL-60 cells. After incubation of HL-60 cells with 100 microM 3-deazaadenosine for 45 min., a schedule known to perturb transmethylation metabolites and initiate apoptosis in these cells, a 50% decrease in c-myc and a 50% increase in bcl-2 RNA steady-state levels compared to control cells were observed. Transcription levels of c-myc continued to decrease after extended exposure to 3-deazaadenosine, while bcl-2 mRNA levels dropped to 25% and 30% below those in control cells after 1.5 hr and 3 hr, respectively. The expression levels of the bcl-2 related bax gene, showed a similar pattern as bcl-2; a 60% increase was initially measured, but after 1.5 and 3 hr, bax transcripts were 80% and 70% respectively, of those found in untreated cells. Another bcl-2 related gene, bcl-x, was previously reported to generate two transcripts in human cells. The long variant bcl-x1 acts as bcl-2, while the short form bcl-xs induces apoptosis. We were unable to detect bcl-xs transcripts in untreated and 3-deazaadenosine treated cells by the highly sensitive reverse transcriptase polymerase chain reaction method. This suggests that this gene product may not be involved in 3-deazaadenosine induced apoptosis in HL-60 cells. Bcl-x1 mRNA levels, however, slowly decreased with about 50% after 1.5 or 3 hr 3-deazaadenosine treatment. It is concluded that 3-deazaadenosine initiated apoptosis affects c-myc, bcl-2, bax and bcl-x1 mRNA levels.

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.

A functional role for death proteases in s-Myc- and c-Myc-mediated apoptosis

Upon activation, cell surface death receptors, Fas/APO-1/CD95 and tumor necrosis factor receptor-1 (TNFR-1), are attached to cytosolic adaptor proteins, which in turn recruit caspase-8 (MACH/FLICE/Mch5) to activate the interleukin-1 beta-converting enzyme (ICE)/CED-3 family protease (caspase) cascade. However, it remains unknown whether these apoptotic proteases are generally involved in apoptosis triggered by other stimuli such as Myc and p53. In this study, we provide lines of evidence that a death protease cascade consisting of caspases and serine proteases plays an essential role in Myc-mediated apoptosis. When Rat-1 fibroblasts stably expressing either s-Myc or c-Myc were induced to undergo apoptosis by serum deprivation, a caspase-3 (CPP32)-like protease activity that cleaves a specific peptide substrate, Ac-DEVD-MCA, appeared in the cell lysates. Induction of s-Myc- and c-Myc-mediated apoptotic cell death was effectively prevented by caspase inhibitors such as Z-Asp-CH2-DCB and Ac-DEVD-CHO. Furthermore, exposing the cells to a serine protease inhibitor, 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), also significantly inhibited s-Myc- and c-Myc-mediated apoptosis and the appearance of the caspase-3-like protease activity in vivo. However, AEBSF did not directly inhibit caspase-3-like protease activity in the apoptotic cell lysates in vitro. Together, these results indicate that caspase-3-like proteases play a critical role in both s-Myc- and c-Myc-mediated apoptosis and that caspase-3-like proteases function downstream of the AEBSF-sensitive step in the signaling pathway of Myc-mediated apoptosis.

Mechanisms of cycloheximide-induced apoptosis in liver cells

Cycloheximide in sublethal doses caused apoptosis in liver cells in vivo, inducing c-myc, c-fos, c-jun and p53 genes and accumulation of sphingosine, a toxic product of the sphingomyelin cycle. These data support the hypothesis that continuous synthesis of labile protective proteins is required to restrain apoptosis in liver; sphingosine might be important in mediating cycloheximide-induced apoptosis as an endogenous modulator of protein kinase C activity.

Role for CD40-mediated activation of c-Rel and maintenance of c-myc RNA levels in mitigating anti-IgM-induced growth arrest

CD40 crosslinking on B cells activates NF-kappaB and stress-activated protein kinase (SAPK) pathways. Since CD40 crosslinking rescues WEHI 231 B cells from anti-IgM-induced apoptosis, those pathways were likely candidates to be involved. Indeed, both signaling cascades predominated in anti-IgM-treated WEHI 231 cells, treated concurrently with anti-CD40 to rescue them from apoptosis. Crosslinking of CD40 activated the NF-kappaB proteins c-Rel and p50, but had no influence on their cytoplasmic steady state level. However, in contrast to-and even in the presence of-anti-IgM-mediated signals, engagement of CD40 resulted in a prolonged nuclear translocation of c-Rel, thereby allowing the formation of active NF-kappaB complexes. Consistent with this, the upstream regulatory element of the c-myc promoter, known to be regulated by NF-kappaB, was differently regulated after BCR ligation vs BCR plus CD40 crosslinking. The level of c-myc RNA was rapidly downregulated after BCR engagement, but persistent in the presence of CD40 signaling.

Role of c-myc and p27 in anti-IgM induced B-lymphoma apoptosis

Crosslinking membrane IgM receptors on a set of murine B cell lymphomas leads to a rapid increase in c-myc, followed by a decrease in its expression to undetectable levels by 8-24 hours. These cells die soon thereafter via apoptosis. IgD receptor crosslinking also leads to an increase in c-myc expression, but it remains above baseline levels for more than 24 hours; these cells continue to proliferate and do not die. We previously reported that antisense oligonucleotides for c-myc prevented growth arrest and cell death in these lymphomas, independent of the presence of mitogenic CpG motifs. Indeed, antisense for c-myc actually led to a stabilization of c-myc message and protein. Growth arrest in these cells is dependent on the increased synthesis of the p27 cyclin kinase inhibitor (Kip1) normally induced after anti-IgM crosslinking. Consistent with its biologic effects, anti-IgD does not cause an increase in p27. Since dexamethasone causes a loss of myc and synergizes with the anti-IgM signal, we suggest that accelerated cell death with this steroid in the presence of anti-IgM is due to a more rapid degradation of this oncogene product. Finally, we propose that c-myc drives the transcription or activation of an inhibitor of the p27 Kip (Kipi). Hence, loss of c-myc in response to anti-IgM signals in these B-cell lymphomas leads to upregulation of p27, growth arrest and apoptosis. It follows that maintenance of c-myc in these B-cell lymphomas should lead to survival and no increase in p27.

Etorphine inhibits cell growth and induces apoptosis in SK-N-SH cells: involvement of pertussis toxin-sensitive G proteins

Opiates have been used extensively in the treatment of pain but with the severe side effect of addiction, which is believed to be related to opiates' direct (primary) or indirect (secondary) neurotoxicity. In this study, the effects of opioids on cell growth and apoptosis have been examined in human neuroblastoma cell line SK-N-SH. Etorphine, a wide-spectrum and potent agonist of opioid receptors, was found to significantly inhibit cell growth and to induce apoptosis. The inhibitory and apoptotic activities of etorphine followed a dose- and time-dependent manner. The more specific agonists of opioid receptors such as morphine, [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAGO), [D-Pen2, D-Pen5]-enkephalin (DPDPE), dynorphin A and nociceptin/orphanin FQ did not show similar toxic activities under the same conditions. In addition, the effects of etorphine could not be blocked by the opioid receptor antagonist naloxone, suggesting that the effects of etorphine might not be mediated by a classical opioid receptor. However, pretreatment of SK-N-SH cells with pertussis toxin (PTX) blocked the inhibition of cell growth and apoptosis induced by etorphine, indicating the involvement of PTX-sensitive G proteins in the processes. It was also shown that etorphine-induced apoptosis was prevented by actinomycin D (AD) and interleukin-1beta converting enzyme inhibitor I. Interestingly, etorphine was similarly potent to inhibit growth of pheochromocytoma (PC12) cells but less effective in SH-SY5Y neuroblastoma cells and C6 glioma cells. We propose that inhibition of cell growth and induction of apoptosis may be one mechanism of opioid neurotoxicity.

Apoptosis in HL-60 cells induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX)

The potent bacterial mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), which is formed during chlorination of drinking water, has been studied with respect to induction of cell death in promyelocytic leukemic HL-60 cells. Cells exposed to MX for 1 h and further incubated for 3 h, revealed no significant increase in the proportion of cells with compromised plasma membrane damage as judged by trypan blue or propidium iodide exclusion. However, flow cytometric studies and microscopic analysis of HL-60 cells after staining with Giemsa or Hoechst 33342, revealed that more than 30% of the cells exposed to 30-100 microM of MX, showed the characteristic morphology and biochemical markers of apoptosis. On the other hand, in cultures exposed to 300 microM MX, less than 5% of the cells appeared to be apoptotic (< G1 DNA) 3 h after treatment, which is similar to control values. Microscopic analysis of Hoechst 33342-stained cells revealed that they were 'arrested' in the early stages of chromatin condensation, but these cells eventually became necrotic. Some decrease in the percentage of cells in S-phase was observed 3 h after exposure to MX (10, 30 and 100 microM), but the induced cell death was not markedly cell stage specific. The characteristic ladder pattern of apoptotic cells was observed when DNA isolated from MX-exposed HL-60 cells was electrophoresed in agarose. The apoptotic process could also be detected by analysis with alkaline filter elution (AE), as a decrease in the total DNA recovered; and by single cell gel electrophoresis, as a decrease in the average number of cells/comets observable on each slide. With the protocols used no apparent increase in values in the normalized area above the curve (NAAC) (alkaline elution) or tail moments (single cell gel electrophoresis (SCGE)) were detected, indicating that apoptotic cells are not necessarily a confounding factor when assaying for genotoxicity with these techniques.

Potentiation of ara-C-induced apoptosis by the protein kinase C activator bryostatin 1 in human leukemia cells (HL-60) involves a process dependent upon c-Myc

The role of the nuclear phosphoprotein c-Myc has been examined with respect to the regulation of 1-beta-D-arabinofuranosylcytosine (ara-C)-induced apoptosis in human leukemia cells exposed to bryostatin 1 and other pharmacologic protein kinase C (PKC) activators. Pretreatment of HL-60 cells for 24 hr with 10 nM bryostatin 1 significantly potentiated the ability of ara-C (10 microM; 6 hr) to induce apoptosis without reducing the expression of c-Myc protein. In contrast, equivalent exposure to the stage 2 tumor-promoting PKC activator mezerein (10 nM) in conjunction with ara-C reduced c-Myc levels by 87% and failed to potentiate apoptosis. Co-administration of bryostatin 1 with mezerein before ara-C prevented down-regulation of c-Myc and augmented cell death, whereas co-treatment with the calcium ionophore A23187 (250 nM) and bryostatin 1 reduced c-Myc levels by 80% and abrogated the increase in ara-C-induced apoptosis. When cells were exposed for 24 hr to a c-myc antisense oligonucleotide (AS-ODN;10 microM) but not to a scrambled sequence ODN (SS-ODN) prior to ara-C, c-Myc expression was reduced by 81%, and apoptosis and cell viability were unperturbed. However, AS-ODN (but not SS-ODN) reduced c-Myc protein in cells pre-exposed to bryostatin 1 by 74% and abrogated potentiation of ara-C-induced apoptosis. The actions of c-myc AS-ODN did not stem from proximal G1 arrest/differentiation or biochemical events, since they were not associated with a reduction in the S-phase cell fraction, p21(WAF1/CIP1) induction, pRb hypophosphorylation, or alterations in ara-C metabolism. Together, these findings indicate that HL-60 cell apoptosis proceeds by both c-Myc-dependent and -independent pathways, and that only the former are involved in the potentiation of ara-C-mediated cell death by bryostatin 1.

Proteasome regulation of Fas ligand cytotoxicity

The role of NF-kappa B in regulating FasL-mediated cytotoxicity was investigated by using lactacystin. Lactacystin is a microbial metabolite known to inhibit only the protease activity of the proteasome, which is required for NF-kappa B translocation. When activated by immobilized anti-CD3 monoclonal antibody, hybridoma T cells (5D5) degraded I kappa B beta, translocated NF-kappa B into the nucleus, transcribed immediate-early genes and the Fas ligand (FasL) gene, and expressed FasL-mediated cytotoxicity. Lactacystin strongly blocked I kappa B beta degradation and the translocation of NF-kappa B (p50/RelA heterodimer), but had little effect on the expression of the transcription factors, Oct-1 and AP-1. Moreover, lactacystin did not inhibit the nuclear translocation of NF-ATp whereas cyclosporin A inhibited the translocation of both NF-kappa B and NF-ATp. The expression of c-myc and nur77, two immediate-early genes implicated in FasL gene activation, was blocked by lactacystin. Subsequently, the expression of FasL gene and FasL-mediated cytotoxicity was inhibited. LLnL, a well-known peptide aldehyde which inhibits the protease activities of the proteasome and cysteine proteases, also inhibited NF-kappa B translocation and FasL-mediated cytotoxicity. However, these events were not inhibited by the highly specific cysteine protease inhibitor E64. These observations provide further evidence that FasL cytotoxicity is regulated by the proteasome. Furthermore, lactacystin must be added early in order to efficiently inhibit the induction of FasL cytotoxicity, indicating that the early events are critical for FasL gene activation. Our study integrates the proteasome-dependent I kappa B degradation and NF-kappa B translocation into a T cell activation cascade which results in FasL gene activation and the expression of FasL-mediated cytotoxicity.

FcγRIII-Mediated Regulation of Hematopoiesis in Murine Bone Marrow Cells by Interleukin-3 and CD95 (Fas/Apo-1)

The interleukin-3 (IL-3)–dependent murine bone marrow–derived cell line FDC-P2/185-4 (185-4) undergoes apoptosis when IL-3 is withdrawn from culture medium. Previous results from our studies indicated that a high concentration of aggregated mouse IgG prevented apoptosis of 185-4 cells through FcγRIII by an autocrine mechanism, producing IL-3. But after 24 hours, 185-4 cells expressed CD95 (Fas/Apo-1) on their surfaces on stimulation via FcγRIII. In addition, this CD95 was functional and apoptosis was induced by anti-CD95 monoclonal antibody (MoAb). We investigated how these conflicting effects were induced by FcγRIII stimulation within the context of cell survival and death. The results showed that IL-3 was induced by calcium ionophore and that the IL-3 induced by FcγRIII stimulation was blocked by EGTA or FK506, but not by staurosporine (protein kinase C [PKC] inhibitor), indicating the important role of calcium-calcineurin in this system. On the other hand, the CD95 expression induced by FcγRIII stimulation was blocked by staurosporine, but not by EGTA or FK506, and phorbol myristate acetate (PMA) induced CD95 expression in the same manner as FcγRIII, indicating the involvement of PKC in the CD95 expression induced by FcγRIII stimulation. Thus, FcγRIII-mediated stimulation even while promoting immediate survival of the bone marrow cells, also triggers mechanisms that will facilitate their eventual deletion at the end of the response. These results suggest that a balance between cell survival and death is maintained to avoid unlimited cell growth caused by FcγRIII-ligand interaction in hematopoiesis during inflammation.

Apoptosis in transitional cell carcinoma of the renal pelvis and ureter: association with proliferative activity, bcl-2 expression and prognosis

PURPOSE

We attempted to clarify the significance of apoptosis in tumor development and progression in 64 cases of transitional cell carcinoma of the renal pelvis and ureter by investigating apoptosis, proliferative activity and bcl-2 expression.

MATERIALS AND METHODS

We used in situ deoxyribonucleic acid nick end labeling for apoptotic cells and immunohistochemical staining for bcl-2 on archival tumor material.

RESULTS

The apoptotic index correlated significantly with mitotic index, histological grade and pathological stage. There was no significant relationship between apoptotic index and bcl-2 expression. Patients with an apoptotic index greater than or equal to 0.22 had a poor prognosis.

CONCLUSIONS

Apoptosis is closely related to proliferative activity, tumor differentiation and depth of invasion in transitional cell carcinoma of the renal pelvis and ureter.

Fas- or ceramide-induced apoptosis is mediated by a Rac1-regulated activation of Jun N-terminal kinase/p38 kinases and GADD153

In the present study, we show that Fas receptor ligation or cellular treatment with synthetic C6-ceramide results in activation or phosphorylation, respectively, of the small G-protein Rac1, Jun N-terminal kinase (JNK)/p38 kinases (p38-K), and the transcription factor GADD153. A signaling cascade from the Fas receptor via ceramide, Ras, Rac1, and JNK/p38-K to GADD153 is demonstrated employing transfection of transdominant inhibitory N17Ras, N17Rac1, c-Jun, or treatment with a specific p38-K inhibitor. The critical function of this signaling cascade is indicated by prevention of Fas- or C6-ceramide-induced apoptosis after inhibition of Ras, Rac1, or JNK/p38-K.

Growth inhibition and induction of apoptosis by HGF in transformed rat liver epithelial cells

Recently we demonstrated in a transgenic mouse model that hepatocyte growth factor (HGF) inhibits c-myc dependent hepatocarcinogenesis. The inhibitory effects of HGF in carcinogenesis were further characterized using a series of rat liver epithelial (RLE) cell lines which were transformed in vitro with either aflatoxin or oncogenes, or spontaneously. HGF caused a cytostatic effect and enhanced cell motility in spontaneously and aflatoxin-transformed cells. In normal RLE cells HGF was slightly stimulatory and did not induce scattering. The HGF receptor was tyrosine phosphorylated in all cell lines, indicating that it is functionally active and capable of signaling events. In the aflatoxin transformed cells HGF also induced apoptosis, associated with constitutive c-myc expression and 1 Kb bax-alpha transcripts. These findings indicate that transformed RLE cell lines may provide a useful model to further examine the mechanism(s) by which HGF and its receptor modulate neoplastic development.

Distinct roles for MAX protein isoforms in proliferation and apoptosis

MAX is a basic helix-loop-helix-leucine zipper protein that plays a central role in the transcriptional control of Myc oncoproteins. MYC-MAX heterodimers stimulate transcription, whereas MAX homodimers, or heterodimers between MAX and members of the MAD family of basic helix-loop-helix-leucine zipper proteins, repress transcription. Max exists in two major isomeric forms, MAX(L) and MAX(S), which differ from one another only by a 9-amino acid insertion/deletion. We show here that MAX(L) is much more effective at homodimeric DNA binding than MAX(S). In NIH3T3 cells, MAX(L) was able to repress a c-Myc-responsive reporter gene whereas MAX(S) either stimulated the reporter gene or had little effect on its expression. In comparison to control cell lines or those stably over-expressing MAX(S), MAX(L)-over-expressing cell lines showed reduced expression of transiently expressed or endogenous c-Myc responsive genes, grew more slowly, possessed a higher growth factor requirement, and showed accelerated apoptosis following growth factor deprivation. Differential effects on growth and apoptosis represent two previously unrecognized properties of MAX proteins. These can at least partly be explained by the differences in their DNA binding abilities and their effects on target gene expression.

Proteasome regulation of activation-induced T cell death

Lactacystin, a microbial metabolite that inhibits protease activity only in the proteasome, was used to study the role of the proteasome in the activation-induced cell death (AICD) of T cells. Lactacystin induces DNA fragmentation and apoptosis in a T cell hybridoma (DO.11. 10) in a dose-dependent manner. Between 1 and 10 microM, the mildly cytotoxic lactacystin inhibited the AICD of DO.11.10 cells cultured in anti-CD3-coated wells. Degradation of IkappaBbeta and the translocation of the NF-kappaB (p50/RelA) into the nucleus, which occurred at 1.5 hr after anti-CD3 activation, were inhibited by lactacystin. Lactacystin did not inhibit the expression of nuclear transcription factor Oct-1. The activation-induced expression of the immediate-early gene, Nur77, and the T cell death genes, CD95 (Fas) and CD95 ligand (FasL), were inhibited. Functional expression of FasL cytotoxicity and the increase of cell surface Fas were also inhibited. Lactacystin must be added within 2 hr of activation to efficiently block AICD. In addition, lactacystin failed to inhibit the killing of DO.11.10 by FasL-expressing allo-specific cytotoxic effector cells. These observations strongly suggest a direct link between the proteasome-dependent degradation of IkappaBbeta and the AICD that occurs through activation of the FasL gene and up-regulation of the Fas gene.

Ras activation leads to cell proliferation or apoptotic cell death upon interleukin-2 stimulation or lymphokine deprivation, respectively

Lymphokine-dependent cells undergo apoptosis upon lymphokine withdrawal. We describe that lymphokine deprivation of the interleukin (IL)-2- or IL-4-dependent mouse T cell line TS1 alpha beta induces Ras activation which plays a role in programmed cell death, since blocking Ras activity reduces the induction of apoptosis. Induction of apoptosis by lymphokine deprivation can be prevented by expression of the Bcl-2 protein. Rescue from cell death by IL-2 also promotes Ras activation, but, in contrast to lymphokine withdrawal, stimulates Bcl-2 expression. IL-4-induced cell survival is Ras- and Bcl-2 independent. These results are compatible with a model in which cell proliferation requires the simultaneous induction of at least two pathways which act in combination to prevent cell death.

Oct-1, silencer sequence, and GC box regulate thyroid hormone receptor beta1 promoter

Thyroid hormone, acting through thyroid hormone receptors (TRs), plays a crucial role in brain development and its insufficiency results in irreversible brain damage. TR alpha mRNA is expressed continuously from early embryonic stages, but the level of TR beta1 mRNA in brain is more abundant in adult than in fetus. To identify important factors which regulate TR beta1 expression, we compared mouse fetal and adult brain nuclear extracts by DNase I footprinting and electrophoretic gel mobility shift assays (EMSA) of the TR beta1 promoter. We carried out transient transfection studies in COS 1 cells using the TR beta1 promoter fused to Luciferase gene, and used mutated promoter vectors and various expression vectors. In DNase I footprinting using the fragment -950 to -717, fetal brain nuclear extracts protected the areas -910 to -884 and -815 to -800 more than did adult extracts. In EMSA, proteins in fetal nuclear extracts bound to a silencer sequence (-924 to -916), GC box (-901 to -887), and E box (-810 to -805), more strongly than did proteins in adult brain extracts. The bands formed on GC box were not supershifted by Sp-1, Sp-2, Sp-3, Sp-4, EGR-1, or EGR-2 antibodies. Three bands were detected on the octamer binding site probe (-913 to -906) and one protein was supershifted by Oct-1 antibody. Adult brain extracts appear to contain more Oct-1 protein than do fetal extracts. The other two bands were more intense in fetal extracts than in adult extracts, but were not supershifted by either Oct-1 or Oct-2 antibodies. Mutation of the silencer response element, mutation of the GC box, and Oct-1 over expression in COS 1 cells increased TR beta1 promoter function as assayed by Luciferase reporter. Mutation of the octamer binding site, to which only Oct-1 bound in COS 1 cells, decreased Luciferase reporter activity. Thus the TR beta1 promoter was regulated negatively by the proteins bound to the silencer sequence and the GC box, and positively by Oct-1. Silencer and GC box binding proteins are more abundant in fetal brain, and Oct-1 is more abundant in adult brain. The results may be responsible for increased amounts of TR beta1 present in late fetal and adult brain.

Multiple centrosomal microtubule organising centres and increased microtubule stability are early features of VP-16-induced apoptosis in CCRF-CEM cells

Microtubular reorganisation contributing to apoptotic morphology occurs in normal and neoplastic cells undergoing apoptosis induced by cytotoxic drugs [1-3]. The aim of this study was to correlate the changes in the microtubules (MTs) with behavior of the centrosome in apoptotic cells, and to see whether post-translational changes in tubulin occurred with the emergence of apoptotic MT bands. Apoptosis was induced in the human T-cell leukaemia line (CCRF-CEM) by treatment with 17 microM etoposide over a 4 h period. The time course of changes was assessed using flow cytometry (FCM) and immunocytochemistry in cells labelled for a centrosomal antigen (CSP-alpha) or alpha-tubulins. One hour following treatment we observed multiple centrosomal microtubule organising centres (MTOCs) associated with the nucleus and the transient appearance of a subset of stable MTs detected with an antibody specific for acetylated alpha-tubulin, as the bands of MTs which lobulate the nucleus are formed. The altered properties of the MTs thus reflect changes in function as apoptosis progresses.

Pathways leading to cell death in T cells

Antigen-induced apoptosis of T cells is a highly regulated process which plays a key role in the elimination of self-reactive T cells and, thus, in the prevention of autoimmunity. It has recently become apparent that members of the tumor necrosis factor (TNF) and TNF receptor (TNFR) superfamily regulate antigen-induced T-cell death. Studies characterizing genes which control TNF/TNFR superfamily expression and how TNF/TNFR signal transducers activate cell death machinery, such as caspases, have begun to reveal the molecular control of antigen-induced T-cell death.