Promotion and inhibition of activation-induced apoptosis in T-cell hybridomas by oncogenes and related signals

Versatility of using major histocompatibility complex class II dextramers for derivation and characterization of antigen-specific, autoreactive T cell hybridomas

Antigen-specific, T cell hybridomas are useful to study the cellular, molecular and functional events, but their generation is a lengthy process. Thus, there is a need to develop robust methods to generate the hybridoma clones rapidly in a short period of time. To this end, we have demonstrated a novel approach using major histocompatibility complex (MHC) class II dextramers to generate T cell hybridomas for an autoantigen, proteolipid protein (PLP) 139-151. Using MHC class II dextramers assembled with PLP 139-151 as screening and sorting tools, we successfully obtained mono antigen-specific clones within seven to eight weeks. In conjunction with other T cell markers, dextramers permitted phenotypic characterization of hybridoma clones for their antigen specificity in a single step by flow cytometry. Importantly, we achieved successful fusions using dextramer(+) cells sorted by flow cytometry as a starting population, resulting in direct identification of multiple antigen-specific clones. Characterization of selected clones led us to identify chemokine receptor, CCR4(+) to be expressed consistently, but their cytokine-producing ability was variable. Our work provides a proof-of principle that the antigen-specific, CD4 T cell hybridoma clones can be generated directly using MHC class II dextramers. The availability of hybridoma clones that bind dextramers may serve as useful tools for various in vitro and in vivo applications.

Cell-Cycle Gene Expression in Lovastatin-Induced Medulloblastoma Apoptosis

Background:

3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is a key rate-limiting enzyme in the mevalonate pathway, which generates precursors both for cholesterol biosynthesis and for the production of nonsteroidal mevalonate derivatives that are involved in a number of growth-regulatory processes. We have reported that lovastatin, a competitive inhibitor of HMG-CoA reductase, not only inhibits medulloblastoma proliferation in vitro, but also induces near-complete cell death via apoptosis. The mechanism of this phenomenon is unclear. Possible involvement of changes in expression of certain cell-cycle related genes led us to study some of them in more detail.

Methods:

Medulloblastoma cell lines were exposed in vitro to lovastatin, and the effects of gene expression changes were studied using RT-PCR, antisense oligonucleotide, DNA electrophoresis and Western blotting analysis.

Results:

1) Levels of total Ras gene mRNA and individual Ras gene mRNA are stable in lovastatin treatment in all examined medulloblastoma cell lines. 2) Blocking c-myc gene over-expression does not enhance medulloblastoma cell sensitivity to lovastatin. 3) Following lovastatin treatment, p16 expression exhibits no change, but pronounced increases of p27KIP1 protein are observed in all examined cell lines. Lovastatin induces pronounced increases of p21WAF1 protein only in Daoy and UW228, but not in D283 Med and D341 Med. 4) Following lovastatin treatment, increased p53 protein is detected only in D341 Med, and bax protein is unchanged in all cell lines.

Conclusion:

Lovastatin-induced growth inhibition and apoptosis in medulloblastoma are not dependent on the regulation of Ras and c-myc gene expression, but may be mediated by p27KIP1 gene expression. Lovastatin-induced apoptosis in medulloblastoma is probably p53 independent, but p53 and p21WAF1 gene expression may also mediate anti-proliferative effects of lovastatin on specific medulloblastoma cell lines.

Pramanicin analog induces apoptosis in human colon cancer cells: critical roles for Bcl-2, Bim, and p38 MAPK signaling

Pramanicin (PMC) is an antifungal agent that was previously demonstrated to exhibit antiangiogenic and anticancer properties in a few in vitro studies. We initially screened a number of PMC analogs for their cytotoxic effects on HCT116 human colon cancer cells. PMC-A, the analog with the most potent antiproliferative effect was chosen to further interrogate the underlying mechanism of action. PMC-A led to apoptosis through activation of caspase-9 and -3. The apoptotic nature of cell death was confirmed by abrogation of cell death with pretreatment with specific caspase inhibitors. Stress-related MAPKs JNK and p38 were both activated concomittantly with the intrinsic apoptotic pathway. Moreover, pharmacological inhibition of p38 proved to attenuate the cell death induction while pretreatment with JNK inhibitor did not exhibit a protective effect. Resistance of Bax −/− cells and the protective nature of caspase-9 inhibition indicate that mitochondria play a central role in PMC-A induced apoptosis. Early post-exposure elevation of cellular Bim and Bax was followed by a marginal Bcl-2 depletion and Bid cleavage. Further analysis revealed that Bcl-2 downregulation occurs at the mRNA level and is critical to mediate PMC-A induced apoptosis, as ectopic Bcl-2 expression substantially spared the cells from death. Conversely, forced expression of Bim proved to significantly increase cell death. In addition, analyses of p53−/− cells demonstrated that Bcl-2/Bim/Bax modulation and MAPK activations take place independently of p53 expression. Taken together, p53-independent transcriptional Bcl-2 downregulation and p38 signaling appear to be the key modulatory events in PMC-A induced apoptosis.

Prognostic significance of PUMA in pancreatic ductal adenocarcinoma

OBJECTIVES

To investigate retrospectively whether alterations of p53 upregulated mediator of apoptosis (PUMA) protein levels and somatic mutations of the PUMA gene are characteristic of pancreatic ductal adenocarcinoma (PDAC).

METHODS

Immunohistochemical analyses of PUMA were performed in pancreatic tumour tissue samples, and paired normal pancreatic tissue samples, from patients with PDAC. Apoptosis was detected using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling assay.

RESULTS

A total of 70 patients with PDAC had samples available; 49 cases (70.0%) had high PUMA protein levels. PUMA was not detected in paired normal tissue samples. Significantly higher levels of PUMA protein were detected in low-grade tumours (tumour -node-metastasis stages I and II), compared with higher grade (stage III) tumours. Of the PDAC cases, the mean apoptosis index value for PUMA-positive specimens was significantly higher than that for PUMA-negative specimens. Overall survival was significantly associated with PUMA immunoreactivity.

CONCLUSIONS

High levels of PUMA in PDAC tumour cells suggest that PUMA expression may play a role in pancreatic tumourigenesis.

Divergent effects of oncostatin M on astroglioma cells: influence on cell proliferation, invasion, and expression of matrix metalloproteinases

Oncostatin M (OSM), a cytokine of the interleukin-6 (IL-6) family, can either promote or inhibit cell growth in various normal and tumor cells. We addressed the effects of exogenous OSM on the proliferation and invasion of human astroglioma cells. In addition, we investigated one of the possible mechanisms involved: modulation of matrix metalloproteinase (MMP) expression and enzymatic activity. We found that OSM inhibited the proliferation of two human astroglioma cell lines (CH235-MG and U87-MG), and that this effect was not due to apoptosis. The inhibitory effect of OSM on proliferation was mediated through the gp130/OSMRbeta receptor complex. To extend these findings, we analyzed the effects of OSM on primary tumor cells from glioblastoma patients. OSM suppressed the proliferation of primary glioblastoma cells, but not that of normal astrocytes. Interestingly, OSM did not suppress astroglioma cell invasion. This may be due to the differential regulation of MMPs by OSM. We found that OSM inhibited the constitutive expression of MMP-2, while MMP-9 expression was enhanced in astroglioma cell lines. We conclude that OSM inhibits proliferation of human astroglioma cells and primary glioblastoma cells via the gp130/OSMRbeta receptor complex. However, OSM does not affect the invasive capacity of the astroglioma cells, which may be due to the divergent effects of OSM on MMP-2 and MMP-9 expression. Collectively, these findings suggest a complex role for OSM in astroglioma biology.

Apoptosis: a novel therapeutic tool?

Apoptosis is a genetically programmed cell death mechanism that appears to occur in all multicellular organisms. It is a normal process that serves to maintain cellular homeostasis. However, in many diseases there is a disruption in the equilibrium between cell proliferation and cell death that contributes directly to the disease. In these cases, a possible therapeutic intervention would be to restore the skewed equilibrium by pushing it in the desired direction through the use of pharmacological agents or genetic approaches. These observations have instigated substantial research in the field of apoptosis, resulting in an increasingly detailed analysis of the molecular mechanisms and the sequence of events that occur in this cell death pathway. In addition, by trying to understand this pathway, several potential therapeutic agents have arisen from those used in chemo-, radio-, and cytokine therapy. While these agents have been relatively successful, it is rare that their effect is complete. Thus, the search continues for a strategy to conquer those cells that are resistant to these regimens. Genetic approaches are novel and have been shown to be quite successful in several in vitro and animal models. They also tend to have low toxicity. It is believed that using a more traditional front-line approach of therapy, supplemented by appropriate genetic intervention, will allow substantial increases in the efficacy of treatment, while at the same time introducing little or no additional toxicity.

How do cultured CD8(+) murine T cell clones survive repeated ligation of the TCR?

Many murine T cell clones grow continuously in culture despite weekly ligation of their TCR by antigen. To learn how the cultured cells avoid or minimize antigen-induced cell death (AICD), we compared Fas and tumor necrosis factor (TNF) receptors (TNFR) on several long-term cultured CD8(+) T cell clones with those on naive and activated naive cells expressing the same TCR (2C). In contrast to the naive cells, Fas was absent on the cultured clones and the TNFR-II receptor, present initially at high levels on the cultured cells, was rapidly down-modulated in response to TCR ligation and had virtually disappeared by 2 h, when only approximately 10% of the cloned cells had been induced to express TNF-alpha. The extent of AICD of the cultured clones in response to cognate peptide-MHC on the presenting cells used for routine stimulation of the cultures was also considerably less than the massive cell death of the clones following exposure to anti-CD3 antibody plate-bound at high density.

Differential regulation of apoptosis in AK-5 tumor cells by the proto-oncogene Bcl-2: presence of Bcl-2 dependent and independent pathways

The anti-apoptotic protein Bcl-2 functions as a crucial negative regulator of apoptosis. Bcl-2 has been shown to prevent the efflux of apoptogenic factors from mitochondria to cytosol, thus inhibiting cell death. Here, we show the susceptibility of a spontaneously regressing, rat histiocytic tumor cell line, AK-5, to the apoptotic effects of diverse stimuli and the ability of Bcl-2 overexpression to block cell death. Bcl-2 overexpression selectively inhibits apoptosis induced by ceramide and serum factor from AK-5 tumor regressing animals but not actinomycin D and curcumin, whereas the pancaspase inhibitor z-Val-Ala-Asp fluoromethylketone completely blocks apoptosis, irrespective of the inducer used. The ability of Bcl-2 overexpression to block cell death does not depend on its ability to prevent cytochrome c release but correlates with its ability to prevent the dissipation of mitochondrial transmembrane potential. The results demonstrate that there are inducer dependent redundant activation pathways in a single cell, which may either be Bcl-2 dependent or independent.

Synergistic induction of apoptosis in human leukemia cells (U937) exposed to bryostatin 1 and the proteasome inhibitor lactacystin involves dysregulation of the PKC/MAPK cascade

Cotreatment with a minimally toxic concentration of the protein kinase C (PKC) activator (and down-regulator) bryostatin 1 (BRY) induced a marked increase in mitochondrial dysfunction and apoptosis in U937 monocytic leukemia cells exposed to the proteasome inhibitor lactacystin (LC). This effect was blocked by cycloheximide, but not by alpha-amanitin or actinomycin D. Qualitatively similar interactions were observed with other PKC activators (eg, phorbol 12-myristate 13-acetate and mezerein), but not phospholipase C, which does not down-regulate the enzyme. These events were examined in relationship to functional alterations in stress (eg, SAPK, JNK) and survival (eg, MAPK, ERK) signaling pathways. The observations that LC/BRY treatment failed to trigger JNK activation and that cell death was unaffected by a dominant-interfering form of c-JUN (TAM67) or by pretreatment with either curcumin or the p38/RK inhibitor, SB203580, suggested that the SAPK pathway was not involved in potentiation of apoptosis. In marked contrast, perturbations in the PKC/Raf/MAPK pathway played an integral role in LC/BRY-mediated cell death based on evidence that pretreatment of cells with bisindolylmaleimide I, a selective PKC inhibitor, or geldanamycin, a benzoquinone ansamycin, which destabilizes and depletes Raf-1, markedly suppressed apoptosis. Furthermore, ERK phosphorylation was substantially prolonged in LC/BRY-treated cells compared to those exposed to BRY alone, and pretreatment with the highly specific MEK inhibitors, PD98059, U0126, and SL327, opposed ERK activation while protecting cells from LC/BRY-induced lethality. Together, these findings suggest a role for activation and/or dysregulation of the PKC/MAPK cascade in modulation of leukemic cell apoptosis following exposure to the proteasome inhibitor LC. (Blood. 2001;97:2105-2114)

Regeneration and tolerance factor's potential role in T-cell activation and apoptosis

Regeneration and tolerance factor (RTF) is a novel membrane protein that has a diverse expression pattern and immunoregulatory properties. RTF is expressed in vivo on the surface of individuals with B cell chronic lymphocytic leukemia and on activated T lymphocytes of HIV infected individuals as determined by their coexpression with CD38 and HLA-DR. The unique expression patterns of this protein in vivo lead us to investigate its expression in vitro. The activation of human PBMCs through the TCR, using anti-CD3 antibody and PMA, upregulated cell surface expression of RTF from 2. 3% to 91.2% (mean channel fluorescence [MCF] increased threefold). The activation of Jurkat T cells through the TCR upregulated surface expression of RTF from 8.3% (MCF-1.3) to 58.7% (MCF-13.1). The Jurkat T-cell line was used as a model system to explore RTF's role in cellular activation. Using the Jurkat T-cell model, we found anti-RTF antibody induces apoptosis. The addition of anti-RTF antibody increased annexin V binding by threefold compared with the IgG1 kappa isotype control antibody (p < 0.00002) and activated caspase 3. These data indicate that RTF is expressed during T-cell activation and may be associated with apoptosis.

Mevalonate prevents lovastatin-induced apoptosis in medulloblastoma cell lines

BACKGROUND

3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) is a key rate-limiting enzyme in the mevalonate pathway, which generates precursors for cholesterol biosynthesis and the production of non-steroidal mevalonate derivatives that are involved in a number of growth-regulatory processes. We have reported that lovastatin, a competitive inhibitor of HMG-CoA reductase, not only inhibits medulloblastoma proliferation in vitro, but also induces near-complete cell death via apoptosis. The present study explores some of the pathways which may be involved in lovastatin-induced apoptosis.

METHODS

Medulloblastoma cell lines were exposed in vitro to lovastatin with or without mevalonate, and document the effects using morphology, flow cytometry. DNA electrophoresis and Northern analysis.

RESULTS

1) Mevalonate prevents apoptosis when co-incubated with lovastatin, or when administered to lovastatin-pretreated cells. 2) Mevalonate restores the lovastatin-arrested cell cycle, allowing S phase entry. 3) Mevalonate does not prevent lovastatin-induced apoptosis after a critical duration of lovastatin pretreatment. For cell lines Daoy and UW228 this was 24 hours, and for D283 Med and D341 Med it was 48 hours. 4) Increases in HMG-CoA reductase mRNA levels induced by lovastatin are abrogated by co-incubation with lovastatin and mevalonate.

CONCLUSIONS

These results confirm that lovastatin inhibition of this enzyme results in blockage of the mevalonate pathway, and that such a block is a critical step in the mechanism of lovastatin-induced apoptosis.

Allergens induce apoptosis in lymphocytes from atopic patients

Repeated stimulation of immune cells may induce an "activation-induced cell death" (AICD) program. Allergy is characterized by the cyclic activation of allergen-reactive immune cells. To study the effects of allergen stimulation in cell proliferation and apoptosis in atopic subjects, peripheral blood mononuclear cells (PBL) from 40 atopic patients with positive reactivity to the allergens Olea Europaea (OE) and Lollium Perenne (LP) (20 without immunotherapy and 20 with specific immunotherapy) and 10 normal subjects were cultured with the allergens OE and LP. PBL from atopic patients proliferate more vigorously than cells from normal subjects after culture in vitro with both allergens, although PBL from atopic subjects without immunotherapy proliferate more than PBL from atopic subjects with immunotherapy. The study of cell proliferation shows that in atopic patients PBL mainly exhibit the CD4/CD45RO phenotype. This preferential proliferation is more evident in PBL from atopic patients treated without immunotherapy. Cell culture with specific allergens induces apoptosis in PBL from atopic patients. The percentage of apoptosis increased when atopic patients had been previously treated with immunotherapy. In addition to the observed increase in cell proliferation, apoptosis mainly occurs in the CD45RO cells that support the involvement of these cells in allergy. Furthermore, results obtained in cells from immunized patients suggest that an AICD process may partly at least explain the mechanism of action of allergen immunotherapy.

Arsenite induces apoptosis via a direct effect on the mitochondrial permeability transition pore

The molecular mode of action of arsenic, a therapeutic agent employed in the treatment of acute promyelocytic leukemia, has been elusive. Here we provide evidence that arsenic compounds may act on mitochondria to induce apoptosis. Arsenite induces apoptosis accompanied by a loss of the mitochondrial transmembrane potential (Delta Psim). Inhibition of caspases prevents the arsenite-induced nuclear DNA loss, but has no effect on the Delta Psim dissipation and cytolysis induced by arsenite. In contrast, Bcl-2 expression induced by gene transfer prevents all hallmarks of arsenite-induced cell death, including the Delta Psim collapse. PK11195, a ligand of the mitochondrial benzodiazepine receptor, neutralizes this Bcl-2 effect. Mitochondria are required in a cell-free system to mediate arsenite-induced nuclear apoptosis. Arsenite causes the release of an apoptosis-inducing factor (AIF) from the mitochondrial intermembrane space. This effect is prevented by the permeability transition (PT) pore inhibitor cyclosporin A, as well as by Bcl-2, which is known to function as an endogenous PT pore antagonist. Arsenite also opens the purified, reconstituted PT pore in vitro in a cyclosporin A- and Bcl-2-inhibitible fashion. Altogether these data suggest that arsenite can induce apoptosis via a direct effect on the mitochondrial PT pore.

Mitochondrial release of caspase-2 and -9 during the apoptotic process

The barrier function of mitochondrial membranes is perturbed early during the apoptotic process. Here we show that the mitochondria contain a caspase-like enzymatic activity cleaving the caspase substrate Z-VAD.afc, in addition to three biological activities previously suggested to participate in the apoptotic process: (a) cytochrome c; (b) an apoptosis-inducing factor (AIF) which causes isolated nuclei to undergo apoptosis in vitro; and (c) a DNAse activity. All of these factors, which are biochemically distinct, are released upon opening of the permeability transition (PT) pore in a coordinate, Bcl-2-inhibitable fashion. Caspase inhibitors fully neutralize the Z-VAD.afc-cleaving activity, have a limited effect on the AIF activity, and have no effect at all on the DNase activities. Purification of proteins reacting with the biotinylated caspase substrate Z-VAD, immunodetection, and immunodepletion experiments reveal the presence of procaspase-2 and -9 in mitochondria. Upon induction of PT pore opening, these procaspases are released from purified mitochondria and become activated. Similarly, upon induction of apoptosis, both procaspases redistribute from the mitochondrion to the cytosol and are processed to generate enzymatically active caspases. This redistribution is inhibited by Bcl-2. Recombinant caspase-2 and -9 suffice to provoke full-blown apoptosis upon microinjection into cells. Altogether, these data suggest that caspase-2 and -9 zymogens are essentially localized in mitochondria and that the disruption of the outer mitochondrial membrane occurring early during apoptosis may be critical for their subcellular redistribution and activation.

Transforming growth factor beta1 inhibits Fas ligand expression and subsequent activation-induced cell death in T cells via downregulation of c-Myc

Activation-induced cell death (AICD) is a process that regulates the size and the duration of the primary immune T cell response. In this report, we investigated the mechanisms involved in the regulation of AICD by transforming growth factor beta1 (TGF-beta1). We found that TGF-beta1 decreased apoptosis of human T cells or T cell hybridomas after activation by anti-CD3. This decrease was associated with inhibition of Fas (Apo-1/CD95) ligand (FasL) expression, whereas Fas signaling was not affected by TGF-beta1. In parallel, TGF-beta1 inhibited c-Myc expression in T cell hybridomas, and ectopic expression of a chimeric molecule composed of c-Myc and the steroid binding domain of the estrogen receptor (Myc-ER) blocked both the inhibition of FasL and the decrease of AICD induced by TGF-beta1, providing that 4-hydroxytamoxifen was present. These results identify one mechanism by which TGF-beta1 blocks AICD to allow the clonal expansion of effector T cells and the generation of memory T cells during immune responses.

A novel immunosuppressant, FTY720, increases the efficiency of a superantigen-induced peripheral T-cell deletion whilst inhibiting negative selection in the thymus

A novel immunosuppressant, FTY720, was generated by chemical modification of ISP-I, an immunosuppressive compound purified from culture filtrates of Isaria sinclairii. FTY720 directly induces apoptotic cell death in lymphocytes, which is believed to be the mechanism by which this drug exerts its immunosuppressive effect. We examined the effect of FTY720 treatment on antigen-induced apoptotic cell death in peripheral T cells and thymocytes. A superantigen, staphylococcus enterotoxin B (SEB), induces T-cell antigen receptor (TCR) Vbeta-specific apoptotic cell death in mature T cells in vivo. In this well-documented experimental system, FTY720 administration significantly enhanced the efficiency of superantigen-induced T-cell deletion. We also determined that apoptotic cell death with DNA fragmentation induced in T-hybridoma cells after stimulation in vitro with anti-TCR antibodies was enhanced in the presence of non-cytolytic doses of FTY720. In sharp contrast, negative selection of T cells in the thymus, another example of antigen-induced apoptosis, was found to be inhibited by FTY720 treatment. A rescue effect was observed on clonal deletion in the H-Y-specific TCRalpha beta transgenic male thymus. In a chicken egg albumin (OVA)-specific TCRalphabeta transgenic system, OVA-induced apoptotic cell death of CD4+CD8+ thymocytes was also inhibited by FTY720 injection. Thus, FTY720 increased the susceptibility of mature T cells to TCR-mediated apoptosis but decreased that of immature thymocytes. The results in this report suggest that the potent immunosuppressive effect of FTY720 is, in part, a result of the augmentation of effects on antigen-induced apoptosis in mature T cells, and that two distinct apoptotic cell death pathways are operating in mature and immature T cells.

PK11195, a ligand of the mitochondrial benzodiazepine receptor, facilitates the induction of apoptosis and reverses Bcl-2-mediated cytoprotection

One critical step of the apoptotic process is the opening of the mitochondrial permeability transition (PT) pore leading to the disruption of mitochondrial membrane integrity and to the dissipation of the inner transmembrane proton gradient (Delta Psim). The mitochondrial PT pore is a polyprotein structure which is inhibited by the apoptosis-inhibitory oncoprotein Bcl-2 and which is closely associated with the mitochondrial benzodiazepine receptor (mBzR). Here we show that PK11195, a prototypic ligand of the 18-kDa mBzR, facilitates the induction of Delta Psim disruption and subsequent apoptosis by a number of different agents,including agonists of the glucocorticoid receptor,chemotherapeutic agents (etoposide, doxorubicin),gamma irradiation, and the proapoptotic second messenger ceramide. Whereas PK11195 itself has no cytotoxic effect, it enhances apoptosis induction by these agents. This effect is not observed for benzodiazepine diazepam, whose binding site in the mBzR differs from PK11195. PK11195 partially reverses Bcl-2 mediated inhibition of apoptosis in two different cell lines. Thus, transfection-enforced Bcl-2 overexpression confers protection against glucocorticoids and chemotherapeutic agents, and this protection is largely reversed by the addition of PK11195. This effect is observed at the level of Delta Psim dissipation as well as at the level of nuclear apoptosis. To gain insights into the site of action of PK11195, we performed experiments on isolated organelles. PK11195 reverses the Bcl-2-mediated mitochondrial retention of apoptogenic factors which cause isolated nuclei to undergo apoptosis in a cell-free system. Mitochondria from control cells, but not mitochondria from Bcl-2-overexpressing cells, readily release such apoptogenic factors in response to atractyloside, a ligand of the adenine nucleotide translocator. However, control and Bcl-2-overexpressing mitochondria respond equally well to a combination of atractyloside and PK11195. Altogether, these findings indicate that PK11195 abolishes apoptosis inhibition by Bcl-2 via a direct effect on mitochondria. Moreover, they suggest a novel strategy for enhancing the susceptibility of cells to apoptosis induction and, concomitantly, for reversing Bcl-2-mediated cytoprotection.

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.

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.

Comparison of Sindbis virus-induced pathology in mosquito and vertebrate cell cultures

We have compared Sindbis virus-induced cytopathology in vertebrate and mosquito (Aedes albopictus) cell cultures. It has been shown that vertebrate cells undergo apoptosis when infected by Sindbis virus and this was confirmed here using hamster cells (BHK). The occurrence of cell death in Sindbis virus-infected A. albopictus cells is a cell clone-specific phenomenon and, unlike in BHK cell cultures, mosquito cell death does not correlate with a large induction of apoptosis, as determined by assays testing for DNA fragmentation or reduced cellular DNA content. Cell cycle distribution changes were observed in Sindbis virus-infected BHK and C7-10 cell cultures, and the changes are distinct, both in the time of induction and the types of perturbations. In Sindbis virus-infected BHK cells, the major cell cycle profile change is the early accumulation of cells with sub-G1 DNA content and a corresponding reduction in the proportion of cells in G1 and G2/M. For Sindbis virus-infected C7-10 cells, the major perturbations are an increased proportion of cells showing G2/M or polyploid DNA content and a reduction in the proportion of G1 and S phase cells. These data suggest that the pathology induced in mosquito cell cultures by Sindbis virus infection may be distinct from the pathology which appears in vertebrate cell cultures.

CD4 regulates the efficiency of an endogenous superantigen-induced clonal deletion of TCRV beta 11+ cells in the periphery

Peripheral T-cell antigen receptor V beta (TCRV beta) repertoire is influenced by clonal deletion both in the thymus and periphery. Developing thymocytes expressing certain TCRV beta are deleted by endogenous superantigens presented on major histocompatibility complex (MHC) molecules in the thymus. Likewise, mature T cells bearing particular TCRV beta chains can be clonally deleted by superantigens in the periphery. The efficiency with which T cells expressing particular V beta subunits are deleted differs depending upon which coreceptor is expressed. Indeed, while deletion of V beta 11+ splenic T cells in CBA/J (Mls-1, a I-E, + MTV 9+) mice is quite efficient for CD4+ spleen T cells, it is much less efficient for CD8+ splenic T cells. If the difference in the efficiency of deletion is due solely to the coreceptor expressed, then a transgene encoding CD4 should increase the efficiency with which CD8+ cells are deleted. To address this question, we have produced CD4 transgenic (TG) mice that express physiologic levels of CD4 on all thymocytes and peripheral CD8 T cells. CD4 molecules expressed on CD8+ splenic T cells were associated with P56lck tyrosine kinase, and were functional as evidenced by their ability to facilitate class II alloreactivity. Furthermore, we found that ectopic expression of TG CD4 molecules on CD8+ cells was able to affect the efficiency of deletion in response to superantigen stimulation. In particular, deletion of TCRV beta 11+ T cells was much less efficient for CD8+ than for CD4+ T-cell subpopulations in (CBA/J x B6) F1 mice. However, expression of the CD4 transgene on CD8+ splenic T cells from these mice increased the efficiency of deletion in the CD8+ V beta 11 T cells. Interestingly, this effect was not observed in a mature CD8+ thymocyte subpopulation. The results in this report demonstrate that CD4 molecules are involved in peripheral deletion of TCRV beta 11+ T cells in (CBA/J x B6) F1 mice, and that the TCRV beta repertoire can be altered by ectopic expression of CD4 on all T-lineage cells.

Apoptosis and necrosis in toxicology: a continuum or distinct modes of cell death?

Mounting evidence indicates that apoptosis rather than necrosis predominates in many cytolethal toxic injuries. Associated cell death models of apoptosis and necrosis are either: (1) totally separate death modes, (2) a continuum whereby they are extremes of biochemically overlapping death pathways, or (3) essentially distinct processes with only limited molecular and cell biology overlap. We conclude that the current balance of evidence favours the third of these options. The established axiom that, even when considering the same toxicant, injury amplitude (dose) is a primary determinant of whether cells die via active cell death (apoptosis) or failure of homeostasis (necrosis) remains valid. Tissue selectivity of toxicants can stem from the apoptotic or necrotic thresholds at which different cells die, as well as targeting factors such as toxicokinetics, receptor recognition, bioactivation, and cell-specific lesions.

Inhibition of the Raf-1 kinase by cyclic AMP agonists causes apoptosis of v-abl-transformed cells

Here we investigate the role of the Raf-1 kinase in transformation by the v-abl oncogene. Raf-1 can activate a transforming signalling cascade comprising the consecutive activation of Mek and extracellular-signal-regulated kinases (Erks). In v-abl-transformed cells the endogenous Raf-1 protein was phosphorylated on tyrosine and displayed high constitutive kinase activity. The activities of the Erks were constitutively elevated in both v-raf- and v-abl-transformed cells. In both cell types the activities of Raf-1 and v-raf were almost completely suppressed after activation of the cyclic AMP-dependent kinase (protein kinase A [PKA]), whereas the v-abl kinase was not affected. Raf inhibition substantially diminished the activities of Erks in v-raf-transformed cells but not in v-abl-transformed cells, indicating that v-abl can activate Erks by a Raf-1-independent pathway. PKA activation induced apoptosis in v-abl-transformed cells while reverting v-raf transformation without severe cytopathic effects. Overexpression of Raf-1 in v-abl-transformed cells partially protected the cells from apoptosis induced by PKA activation. In contrast to PKA activators, a Mek inhibitor did not induce apoptosis. The diverse biological responses correlated with the status of c-myc gene expression. v-abl-transformed cells featured high constitutive levels of expression of c-myc, which were not reduced following PKA activation. Myc activation has been previously shown to be essential for transformation by oncogenic Abl proteins. Using estrogen-regulated c-myc and temperature-sensitive Raf-1 mutants, we found that Raf-1 activation could protect cells from c-myc-induced apoptosis. In conclusion, these results suggest (i) that Raf-1 participates in v-abl transformation via an Erk-independent pathway by providing a survival signal which complements c-myc in transformation, and (ii) that cAMP agonists might become useful for the treatment of malignancies where abl oncogenes are involved, such as chronic myeloid leukemias.

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.

Apoptosis of intestinal intraepithelial lymphocytes induced by exogenous and endogenous glucocorticoids

To investigate the effect of glucocorticoids on apoptosis in intestinal intraepithelial lymphocytes (i-IEL), we examined the changes of i-IEL followed by in vivo treatment with dexamethasone. The fragmented DNA of i-IEL were significantly increased at 15 hr after dexamethasone treatment and, subsequently, the number of total i-IEL were decreased by day 4 after treatment. Although all subsets of i-IEL including CD8 alpha/alpha(+), CD8 alpha/beta(+), CD4+ and CD4+CD8+ i-IEL were decreased after dexamethasone treatment, CD8 alpha/alpha(+) i-IEL appeared to be relatively resistant to dexamethasone-induced apoptosis. Consistent with the in vivo findings, CD8 alpha/alpha(+) i-IEL exhibited less susceptibility to dexamethasone-induced cell death in vitro than other subsets. To investigate whether this process occurs under physiological conditions, we examined the kinetics of i-IEL after treatment with 15-hr water immersion stress. In mice subjected to water immersion stress, plasma glucocorticoids were remarkably elevated soon after the 15-hr stress. The increase in the fragmented DNA of i-IEL and subsequent decrease in the number of i-IEL were observed in the stressed mice in the same kinetics as seen in the dexamethasone-treated mice. Similar to dexamethasone-induced ell death, CD8 alpha/alpha(+) i-IEL appeared to be relatively resistant to stress-induced apoptosis compared with other i-IEL subsets. The expression level of Bcl-2 was significantly higher in CD8 alpha/alpha(+) i-IEL than in CD8 alpha/beta(+) i-IEL. Our results indicate that i-IEL are subjected to cell death via apoptosis by exogenous and endogenous glucocorticoids and that different sensitivity to steroid-induced apoptosis may exist among i-IEL subsets in relation to their Bcl-2 expression.

Bcl-2 inhibits the mitochondrial release of an apoptogenic protease

Bcl-2 belongs to a family of apoptosis-regulatory proteins which incorporate into the outer mitochondrial as well as nuclear membranes. The mechanism by which the proto-oncogene product Bcl-2 inhibits apoptosis is thus far elusive. We and others have shown previously that the first biochemical alteration detectable in cells undergoing apoptosis, well before nuclear changes become manifest, is a collapse of the mitochondrial inner membrane potential (delta psi m), suggesting the involvement of mitochondrial products in the apoptotic cascade. Here we show that mitochondria contain a pre-formed approximately 50-kD protein which is released upon delta psi m disruption and which, in a cell-free in vitro system, causes isolated nuclei to undergo apoptotic changes such as chromatin condensation and internucleosomal DNA fragmentation. This apoptosis-inducing factor (AIF) is blocked by N-benzyloxycarbonyl-Val-Ala-Asp.fluoromethylketone (Z-VAD.fmk), an antagonist of interleukin-1 beta-converting enzyme (ICE)-like proteases that is also an efficient inhibitor of apoptosis in cells. We have tested the effect of Bcl-2 on the formation, release, and action of AIF. When preventing mitochondrial permeability transition (which accounts for the pre-apoptotic delta psi m disruption in cells), Bcl-2 hyperexpressed in the outer mitochondrial membrane also impedes the release of AIF from isolated mitochondria in vitro. In contrast, Bcl-2 does not affect the formation of AIF, which is contained in comparable quantities in control mitochondria and in mitochondria from Bcl-2-hyperexpressing cells. Furthermore, the presence of Bcl-2 in the nuclear membrane does not interfere with the action of AIF on the nucleus, nor does Bcl-2 hyperexpression protect cells against AIF. It thus appears that Bcl-2 prevents apoptosis by favoring the retention of an apoptogenic protease in mitochondria.

A novel gene product that couples TCR signaling to Fas(CD95) expression in activation-induced cell death

Cross-linking the TCR in T cell hybridomas induces cell apoptosis following activation. This activation-induced apoptosis has been used as a model for clonal deletion of thymocytes or peripheral T cells. Anti-TCR-induced apoptosis of T cell hybridomas requires de novo macromolecular synthesis, including up-regulation of Fas and FasL. The Fas-FasL interaction then activates the apoptosis program. To study apoptosis-specific signaling processes, we generated a mutant T cell hybridoma line defective in induction of apoptosis, but competent to induce activation, upon TCR triggering. Subsequently, we cloned the gene TDAG51, which restored activation-induced apoptosis when transfected into the mutant cell line, and showed that TDAG51 expression was required for Fas expression. Thus, TDAG51 plays an essential role in induction of apoptosis by coupling TCR stimulation to Fas expression.

Mitochondrial control of nuclear apoptosis

Anucleate cells can be induced to undergo programmed cell death (PCD), indicating the existence of a cytoplasmic PCD pathway that functions independently from the nucleus. Cytoplasmic structures including mitochondria have been shown to participate in the control of apoptotic nuclear disintegration. Before cells exhibit common signs of nuclear apoptosis (chromatin condensation and endonuclease-mediated DNA fragmentation), they undergo a reduction of the mitochondrial transmembrane potential (delta psi m) that may be due to the opening of mitochondrial permeability transition (PT) pores. Here, we present direct evidence indicating that mitochondrial PT constitutes a critical early event of the apoptotic process. In a cell-free system combining purified mitochondria and nuclei, mitochondria undergoing PT suffice to induce chromatin condensation and DNA fragmentation. Induction of PT by pharmacological agents augments the apoptosis-inducing potential of mitochondria. In contrast, prevention of PT by pharmacological agents impedes nuclear apoptosis, both in vitro and in vivo. Mitochondria from hepatocytes or lymphoid cells undergoing apoptosis, but not those from normal cells, induce disintegration of isolated Hela nuclei. A specific ligand of the mitochondrial adenine nucleotide translocator (ANT), bongkreik acid, inhibits PT and reduces apoptosis induction by mitochondria in a cell-free system. Moreover, it inhibits the induction of apoptosis in intact cells. Several pieces of evidence suggest that the proto-oncogene product Bcl-2 inhibits apoptosis by preventing mitochondrial PT. First, to inhibit nuclear apoptosis, Bcl-2 must be localized in mitochondrial but not nuclear membranes. Second, transfection-enforced hyperexpression of Bcl-2 directly abolishes the induction of mitochondrial PT in response to a protonophore, a pro-oxidant, as well as to the ANT ligand atractyloside, correlating with its apoptosis-inhibitory effect. In conclusion, mitochondrial PT appears to be a critical step of the apoptotic cascade.

Apoptosis: molecular regulation of cell death

The field of apoptosis is unusual in several respects. Firstly, its general importance has been widely recognised only in the past few years and its surprising significance is still being evaluated in a number of areas of biology. Secondly, although apoptosis is now accepted as a critical element in the repertoire of potential cellular responses, the picture of the intra-cellular processes involved is probably still incomplete, not just in its details, but also in the basic outline of the process as a whole. It is therefore a very interesting and active area at present and is likely to progress rapidly in the next two or three years. This review emphasises recent work on the molecular mechanisms of apoptosis and, in particular, on the intracellular interactions which control this process. This latter area is of crucial importance since dysfunction of the normal control machinery is likely to have serious pathological consequences, probably including oncogenesis, autoimmunity and degenerative disease. The genetic analysis of programmed cell death during the development of the nematode Caenorhabditis elegans has proved very useful in identifying important events in the cell death programme. Recently defined genetic connections between C. elegans cell death and mammalian apoptosis have emphasised the value of this system as a model for cell death in mammalian cells, which, inevitably, is more complex. The signals inducing apoptosis are very varied and the same signals can induce differentiation and proliferation in other situations. However, some pathways appear to be of particular significance in the control of cell death; recent analysis of the apoptosis induced through the cell-surface Fas receptor has been especially important for immunology. Two gene families are dealt with in particular detail because of their likely importance in apoptosis control. These are, first, the genes encoding the interleukin-1 beta-converting enzyme family of cysteine proteases and, second, those related to the proto-oncogene bcl-2. Both of these families are homologous to cell death genes in C. elegans. In mammalian cells the number of members of both families which have been identified is growing rapidly and considerable effort is being directed towards establishing the roles played by each member and the ways in which they interact to regulate apoptosis. Other genes with established roles in the regulation of proliferation and differentiation are also important in controlling apoptosis. Several of these are known proto-oncogenes, e.g. c-myc, or tumour suppressors, e.g. p53, an observation which is consistent with the importance of defective apoptosis in the development of cancer. Viral manipulation of the apoptosis of host cells frequently involves interactions with these cellular proteins. Finally, the biochemistry of the closely controlled cellular self-destruction which ensues when the apoptosis programme has been engaged is also very important. The biochemical changes involved in inducing phagocytosis of the apoptotic cell, for example, allow the process to be neatly integrated within the tissues, under physiological conditions. Molecular defects in this area too may have important pathological consequences.

Control of CD4 effector fate: transforming growth factor beta 1 and interleukin 2 synergize to prevent apoptosis and promote effector expansion

The signals that determine the size and duration of the primary T cell immune response are not well defined. We studied CD4 T cells at an important checkpoint in their development: when they have become effectors and are ready to rapidly mediate effector functions, both via direct interaction with antigen (Ag)-presenting cells and via cytokine production. We determined the effects of specific Ag and the cytokines interleukin (IL) 2 and transforming growth factor (TGF) beta 1 on T helper cell type 2 (Th2) effector apoptosis versus expansion. Th2-polarized effector cells were generated in vitro from naive CD4 T of T cell receptor transgenic mice, and then restimulated with or without peptide Ag plus Ag-presenting cells and cytokines. In the absence of added cytokines, effector cells cultured without Ag died of apoptosis after 4-7 d. Paradoxically, Ag both induced proliferation and high levels of cytokine synthesis and accelerated effector cell death. IL-2 directly induced proliferation of effectors, supported and prolonged Ag-induced proliferation, and partially blocked apoptosis. TGF-beta did not effect proliferation or influence cytokine secretion, but it also partially blocked apoptosis. Together, IL-2 and TGF-beta synergized to almost completely block apoptosis, resulting in prolonged effector expansion and leading to the accumulation of a large pool of specific effectors. When Ag and both cytokines were present, the effector population increased 10(4)-10(5) fold over 20 d of culture. The synergy of IL-2 and TGF-beta suggests that they interfere with programmed cell death by distinct mechanisms. Since Th2 effectors are specialized to help B cells develop into antibody-secreting plasma cells, these results suggest that the availability of Ag and of the cytokines IL-2 and TGF-beta is a key factor influencing the fate of Th2 effector cells and thus the size and duration of the primary antibody response.

Sequential reduction of mitochondrial transmembrane potential and generation of reactive oxygen species in early programmed cell death

Programmed cell death (PCD) is a physiological process commonly defined by alterations in nuclear morphology (apoptosis) and/or characteristic stepwise degradation of chromosomal DNA occurring before cytolysis. However, determined characteristics of PCD such as loss in mitochondrial reductase activity or cytolysis can be induced in enucleated cells, indicating cytoplasmic PCD control. Here we report a sequential disregulation of mitochondrial function that precedes cell shrinkage and nuclear fragmentation. A first cyclosporin A-inhibitable step of ongoing PCD is characterized by a reduction of mitochondrial transmembrane potential, as determined by specific fluorochromes (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine++ + iodide; 3,3'dihexyloxacarbocyanine iodide). Cytofluorometrically purified cells with reduced mitochondrial transmembrane potential are initially incapable of oxidizing hydroethidine (HE) into ethidium. Upon short-term in vitro culture, such cells acquire the capacity of HE oxidation, thus revealing a second step of PCD marked by mitochondrial generation of reactive oxygen species (ROS). This step can be selectively inhibited by rotenone and ruthenium red yet is not affected by cyclosporin A. Finally, cells reduce their volume, a step that is delayed by radical scavengers, indicating the implication of ROS in the apoptotic process. This sequence of alterations accompanying early PCD is found in very different models of apoptosis induction: glucocorticoid-induced death of lymphocytes, activation-induced PCD of T cell hybridomas, and tumor necrosis factor-induced death of U937 cells. Transfection with the antiapoptotic protooncogene Bcl-2 simultaneously inhibits mitochondrial alterations and apoptotic cell death triggered by steroids or ceramide. In vivo injection of fluorochromes such as 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide; 3,3'dihexyloxacarbocyanine iodide; or HE allows for the detection of cells that are programmed for death but still lack nuclear DNA fragmentation. In particular, assessment of mitochondrial ROS generation provides an accurate picture of PCD-mediated lymphocyte depletion. In conclusion, alterations of mitochondrial function constitute an important feature of early PCD.