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

Ornithine decarboxylase is a mediator of c-Myc-induced apoptosis

c-Myc plays a central role in the regulation of cell cycle progression, differentiation, and apoptosis. However, the proteins which mediate c-Myc function(s) remain to be determined. Enforced c-myc expression rapidly induces apoptosis in interleukin-3 (IL-3)-dependent 32D.3 murine myeloid cells following IL-3 withdrawal, and this is associated with the constitutive, growth factor-independent expression of ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis. Here we have examined the role of ODC in c-Myc-induced apoptosis. Enforced expression of ODC, like c-myc, is sufficient to induce accelerated death following IL-3 withdrawal. ODC induced cell death in a dose-dependent fashion, and alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, effectively blocked ODC-induced cell death. ODC-induced cell death was due to the induction of apoptosis. We also demonstrate that ODC is a mediator of c-Myc-induced apoptosis. 32D.3-derived c-myc clones have augmented levels of ODC enzyme activity, and their rates of death were also a function of their ODC enzyme levels. Importantly, the rates of death of c-myc clones were inhibited by treatment with DFMO. These findings demonstrate that ODC is an important mediator of c-Myc-induced apoptosis and suggest that ODC mediates other c-Myc functions.

Molecular interactions between G-actin, DNase I and the beta-thymosins in apoptosis: a hypothesis

The beta-thymosins are a family of < 5kDa (MW), mostly acidic, proteins which were originally defined in the immune system. Recently, specific members of this family of cytoplasmic polypeptides, namely beta-4 and beta-10, were shown to bind monomeric G-actin both in vitro and in vivo. Whilst many aspects of programmed cell death or 'apoptosis' remain to be defined, the Ca2+/Mg(2+)-dependent endonuclease, DNase I does feature in this process. Monomeric G-actin binds to and inhibits the DNA-degrading activity of DNase I. Given that the intracellular abundance of thymosins beta-4 and beta-10 is related to cell division and differentiation and that anticancer/morphogenic agents such as retinoic acid (RA) and cyclic AMP modulate expression of their respective genes, it is possible that these G-actin sequestering proteins play significant roles in apoptosis perhaps mediated via DNase I.

Insights from transgenic mice regarding the role of bcl-2 in normal and neoplastic lymphoid cells

The bcl-2 gene was first discovered by molecular analysis of the 14;18 chromosome translocation which is the hallmark of most cases of human follicular lymphoma. To date, it is unique among proto-oncogenes because, rather than promoting cell proliferation, it fosters cell survival. This review summarizes the impact of constitutive bcl-2 expression on the development and function of lymphocytes as well as their malignant transformation. Expression of a bcl-2 transgene in the B lymphoid compartment profoundly perturbed homeostasis and, depending on the genetic background, predisposed to a severe autoimmune disease resembling human systemic lupus erythematosus. T lymphoid cells from bcl-2 transgenic mice were remarkably resistant to diverse cytotoxic agents. Nevertheless, T lymphoid homeostasis was unaffected and tolerance to self was maintained. Expression of high levels of Bcl-2 facilitated the development of B lymphoid tumours but at relatively low frequency and with long latency. Co-expression of myc and bcl-2, on the other hand, promoted the rapid onset of novel tumours which appeared to derive from a lympho-myeloid stem or progenitor cell. Introduction of the bcl-2 transgene into scid mice facilitated the survival and differentiation of pro-B but not pro-T cells, suggesting that a function necessary to supplement or complement the action of Bcl-2 is expressed later in the T than the B lineage. Crosses of the bcl-2 transgenic mice with p53-/- mice have addressed whether loss of p53 function and gain of bcl-2 function are synergistic for lymphoid cell survival.

Antisense oligonucleotide inhibition of acetylcholinesterase gene expression induces progenitor cell expansion and suppresses hematopoietic apoptosis ex vivo

To examine the role of acetylcholinesterase (EC 3.1.1.7) in hematopoietic cell proliferation and differentiation, we administered a 15-mer phosphorothioate oligonucleotide, antisense to the corresponding ACHE gene (AS-ACHE), to primary mouse bone marrow cultures. Within 2 hr of AS-ACHE addition to the culture, ACHE mRNA levels dropped by approximately 90%, as compared with those in cells treated with the "sense" oligomer, S-ACHE. Four days after AS-ACHE treatment, ACHE mRNA increased to levels 10-fold higher than in S-ACHE cultures or in fresh bone marrow. At this later time point, differential PCR display revealed significant differences between cellular mRNA transcripts in bone marrow and those in AS-ACHE- or S-ACHE-treated cultures. These oligonucleotide-triggered effects underlay considerable alterations at the cellular level: AS-ACHE but not S-ACHE increased cell counts, reflecting enhanced proliferation. In the presence of erythropoietin it also enhanced colony counts, reflecting expansion of progenitors. AS-ACHE further suppressed apoptosis-related fragmentation of cellular DNA in the progeny cells, and it diverted hematopoiesis toward production of primitive blasts and macrophages in a dose-dependent manner promoted by erythropoietin. These findings suggest that the hematopoietic role of acetylcholinesterase, anticipated to be inverse to the observed antisense effects, is to reduce proliferation of the multipotent stem cells committed to erythropoiesis and megakaryocytopoiesis and macrophage production and to promote apoptosis in their progeny. Moreover, these findings may explain the tumorigenic association of perturbations in ACHE gene expression with leukemia.

Iron deprivation-induced apoptosis in HL-60 cells

Iron deprivation of HL-60 cells with deferoxamine B mesylate (DFO) induced apoptosis. DNA fragmentation became apparent with 10(-6) M DFO after 48 h treatment. The apoptosis peak according to the DNA histogram on flow cytometry and typical nuclear collapse and were observed microscopically after 48 h treatment with 10(-4) M DFO. Cells treated with 10(-4) M DFO for as little as 24 h were shown to be committed to apoptosis, as chromatin condensation progressed gradually thereafter.

Apoptosis induced at different dose rates: implication for the shoulder region of cell survival curves

Apoptosis is induced by radiation, administered at different dose rates of 3-60 Gy/h, in rat embryo cells transfected with a c-myc oncogene (REC:myc(ch1)) or with a c-Ha-ras oncogene (REC:ras(ch1)). Apoptosis is evaluated in terms of altered morphology, chromatin condensation and DNA fragmentation. The apoptotic dose response of REC:myc(ch1) rises steeply at low doses (to about 40% at 5 Gy), and reaches a plateau at high doses (of about 60% at > 15 Gy). In comparison with REC:myc(ch1), the REC:ras(ch1) is much less susceptible, with a maximum apoptotic fraction of about 10%. Interestingly, radiation-induced apoptosis is nearly dose-rate independent. In parallel, we assessed radiation-induced cell-killing as assayed by colony-formation. In contrast to that observed for apoptosis, the dose response of colony-formation is strongly dependent on dose rate. Cell surviving fraction measured at 3 Gy/h decreases exponentially with dose, with REC:myc(ch1) exhibiting a steeper slope than REC:ras(ch1). Thus, the different low-dose-rate radiosensitivity of the two cell lines may in part be due to their different susceptibility to radiation-induced apoptosis. Taken together, these findings suggest that radiation-induced apoptosis contributes significantly to the initial (shoulder) region of acute dose-rate survival curves of susceptible cells, and may have implications for fractionated and low dose rate radiotherapy.

Loss of cell cycle controls in apoptotic lymphoblasts of the bursa of Fabricius

Lymphoblasts of the normal embryonic follicles of the chicken bursa of Fabricius undergo rapid apoptosis when exposed to gamma-radiation or when cell-cell contacts are disrupted by mechanical dispersion in short term culture. We have observed previously that overexpression of v-myc sensitizes preneoplastic bursal lymphoblasts to induction of cell death, whereas resistance to induced cell death is acquired during progression to neoplasia. In this study we observed extensive DNA degradation in the large majority of the lymphoblast population within the first hour after dispersion-induced apoptosis. Paradoxically these cells continued to progress into S-phase with the bulk of DNA cleavage and death occurring in S-phase cells (i.e., in cells with more than 2C and less than 4C DNA content). We confirmed the S phase status of apoptotic cells by determining that detection of nuclear cyclin A in individual cells also corresponded with detection of DNA breakage. Levels of cyclin E, cyclin E-dependent H1 histone kinase, and p53 proteins were maintained during dispersion-induced DNA cleavage. gamma-radiation failed either to inhibit cell cycle progression or to raise p53 levels in dispersed bursal lymphoblasts. In intact bursal follicles low doses of gamma-radiation induced p53 whereas higher, apoptosis-inducing doses failed to induce p53 or prevent G1 to S-phase progression. These results suggest that normal DNA damage-induced cell cycle checkpoint controls are lost or overridden when apoptosis is induced in bursal lymphoblasts.

c-Myc binds to 5' flanking sequence motifs of the dihydrofolate reductase gene in cellular extracts: role in proliferation

The dihydrofolate reductase is a key enzyme of the folate metabolism which supplies the cell with dTTPs for DNA synthesis. Using cellular extracts, we demonstrate the formation of c-Myc/Max heterodimers at the dihydrofolate reductase (DHFR) 5' flanking CANNTG (E-box) motifs. The presence of these complexes correlates with c-Myc levels and active cellular proliferation.

Programmed cell death: implications for neuropsychiatric disorders

Programmed cell death, sometimes referred to as apoptosis, occurs through an active process requiring new gene transcription, in contrast to the passive cell death produced by metabolic toxins. Programmed cell death is an essential part of normal development, particularly in the nervous system. Spatial, temporal, or quantitative errors in the stimuli that initiate programmed cell death, or errors within the programmed cell death pathway itself, can result in an abnormal number of neurons and pathological neural development. Excesses and deficits in neuronal numbers have now been observed not only in typical neurodegenerative disorders such as Alzheimer's and Huntington's diseases, but also in several neurodevelopmental disorders, including schizophrenia and autism. Recent investigations into the mechanisms of cell death during C. elegans neurodevelopment thymocyte negative selection, and withdrawal of sympathetic ganglion cells trophic support provides intriguing clues to the etiology and pathophysiology of these neuropsychiatric disorders.

Specific T-cell tolerance may be preceded by a primary response

We have evaluated the ability of ovalbumin to induce T-cell-specific tolerance in SJL mice. A significant decrease of interleukin 2 in lymph-node culture supernatants from tolerant mice upon antigen stimulation was seen. Oral tolerization was less effective than i.p.- or s.c.-tolerization protocols. Transfer experiments of either splenic or lymph-node T cells from tolerant mice to naive mice definitely ruled out suppression as a mechanism involved in tolerant mice. Surprisingly, we found that, before the establishment of specific T-cell tolerance to ovalbumin, T cells from mice that will display tolerance were responsive and synthesized interleukin 2 upon antigen challenge in vitro. Thus, we concluded that anergy cannot account solely for the T-cell unresponsiveness in tolerant mice. Furthermore, although we cannot rule out the hypothesis that the T-cell unresponsiveness in tolerant mice can be explained by programmed cell death of ovalbumin-specific T cells, these data led us to speculate that T-cell "refractoriness" could explain the drop of interleukin 2 production in lymph-node T-cell culture supernatant from tolerant mice.

Activation-induced apoptosis in lymphocytes

Activation-induced apoptosis has been proposed as a mechanism for purging the immune repertoire of anti-self specificities, not only in development but also during the generation of somatic mutation in germinal centers. The pathways involved in driving immature and mature T and B cells to programmed cell death are reviewed with respect to two hypotheses, the pre-emptive death model, in which certain signals are obligatory for programmed cell death, and the two signal: death/survival model. Depending on the system, some data support the former pathway, in which certain signals are obligatory for programmed cell death, whereas other data are consistent with the two signal hypothesis. Moreover, recent data suggests that the c-myc protein plays a pivotal role in controlling this process. Finally conflicting roles of protein kinases, bcl-2 and p53 are reviewed and contrasted for involvement in activation-induced cell death in T and B lymphocytes.

Induction of Apoptosis and c-myc in L1210 Lymphocytic Leukemia Cells by Adenosine

High concentrations of adenosine (Ado), when added to L1210 lymphocytic leukemia cells, resulted in apoptosis or programmed cell death. The apoptotic process was accompanied by distinct morphological changes including chromatin condensation and blebbing of plasma membranes. Extensive DNA fragmentation was correlated with Ado concentrations. Furthermore, apoptosis in these cells was preceded by an early but transient expression of c-myc proto-oncogene, and was not influenced by homocysteine thiolactone added to the cells. Since severe combined immunodeficiency (SCID) is associated with a deficiency of adenosine deaminase, leading to defects in both cellular and humoral immunity, Ado-induced apoptosis may thus be a contributing factor in the pathology of SCID. Copyright 1994 S. Karger AG, Basel

Protein kinase C inhibitors induce apoptosis in human malignant glioma cell lines

Previous work has demonstrated the importance of the protein kinase C (PKC) system in regulating glioma growth, and has led to clinical trials utilizing PKC inhibitors as adjuncts in the therapy of patients harboring malignant gliomas. This study was performed to explore the possibility that inhibition of PKC in gliomas was triggering an apoptosis signal. Glioma cell lines were treated with PKC inhibitors staurosporine (10 nM), and tamoxifen (10 microM). DNA from cells treated with each of these drugs exhibited a 'ladder' pattern of oligonucleosome-sized fragments characteristic of apoptosis, thus suggesting that in glioma cells, these drugs may be cytocidal in action.

The human T-cell leukemia virus type I Tax protein induces apoptosis which is blocked by the Bcl-2 protein

The Tax protein of human T-cell leukemia virus type I activates transcription of cellular and viral genes and can immortalize primary T lymphocytes. We have previously reported that the Tax protein transforms Rat-1 cells. Here we show that Tax-transformed Rat-1 cells detach from plates to undergo apoptotic cell death by serum deprivation. These cells exhibit DNA fragmentation into oligonucleosomal fragments and chromatin condensation. Constitutive expression of a proto-oncogene, bcl-2, effectively blocks Tax-mediated apoptosis caused by serum deprivation without affecting the levels of Tax expression and the transformed phenotype of the cells.

Regulation of cell survival in Burkitt lymphoma: implications from studies of apoptosis following cold-shock treatment

Burkitt lymphoma (BL) tumour-cell populations are known to display high rates both of proliferation and of apoptosis in vivo, but the mechanisms which determine whether a BL cell continues to cycle or engages its cell-death programme are not understood. Group-I BL-derived cell lines, which retain in vitro the proliferative and apoptotic capacities of the parental cells, selectively entered apoptosis when returned to 37 degrees C after a brief period at low temperature (1 degree C). The induction of apoptosis by cold treatment, as determined by morphological characteristics and DNA fragmentation, was readily detectable within the first 1 to 2 hr of re-incubation at 37 degrees C, reaching a maximum at 4 to 6 hours. Commitment to enter apoptosis occurred after as little as 20 to 30 min at 1 degree C. Significant cell death at 1 degree C occurred only during prolonged incubation in the cold and displayed the characteristics of necrosis. Both bcl-2-dependent and -independent survival pathways were found to provide protection from cold-induced apoptosis, but only if engaged before cold-shock treatment. These results indicate that continued cycling of group-I BL cells is dependent upon their capacity to inhibit or circumvent their normally constitutively active apoptotic programme, and are consistent with the notion that the synthesis of one or more critical "survival" proteins of short half-life is necessary to guarantee successful passage through the cell cycle. Notably, high levels of apoptosis were also inducible in group-I BL cells by inhibitors of RNA and protein synthesis.

The novel primary response gene MyD118 and the proto-oncogenes myb, myc, and bcl-2 modulate transforming growth factor beta 1-induced apoptosis of myeloid leukemia cells

Cell numbers are regulated by a balance among proliferation, growth arrest, and programmed cell death. A profound example of cell homeostasis, controlled throughout life, is the complex process of blood cell development, yet little is understood about the intracellular mechanisms that regulate blood cell growth arrest and programmed cell death. In this work, using transforming growth factor beta 1 (TGF beta 1)-treated M1 myeloid leukemia cells and genetically engineered M1 cell variants, the regulation of growth arrest and apoptosis was dissected. Blocking of early expression of MyD118, a novel differentiation primary response gene also shown to be a primary response gene induced by TGF beta 1, delayed TGF beta 1-induced apoptosis, demonstrating that MyD118 is a positive modulator of TGF beta 1-mediated cell death. Elevated expression of bcl-2 blocked the TGF beta 1-induced apoptotic pathway but not growth arrest induced by TGF beta 1. Deregulated expression of either c-myc or c-myb inhibited growth arrest and accelerated apoptosis, demonstrating for the first time that c-myb plays a role in regulating apoptosis. In all cases, the apoptotic response was correlated with the level of MyD118 expression. Taken together, these findings demonstrate that the primary response gene MyD118 and the c-myc, c-myb, and bcl-2 proto-oncogenes interact to modulate growth arrest and apoptosis of myeloid cells.

Apoptosis. Its significance in cancer and cancer therapy

Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

Binding and suppression of the Myc transcriptional activation domain by p107

An amino-terminal transactivation domain is required for Myc to function as a transcription factor controlling cell proliferation, differentiation, and apoptosis. A complementary DNA expression library was screened with a Myc fusion protein to identify proteins interacting with this domain, and a clone encoding the Rb-related p107 protein was isolated. The p107 protein was shown to associate with Myc in vivo and to suppress the activity of the Myc transactivation domain. However, mutant forms of Myc from Burkitt lymphoma cells, which contain sequence alterations in the transactivation domain, were resistant to p107-mediated suppression. Thus, disruption of a regulatory interaction between Myc and p107 may be important in tumorigenesis.

The novel primary response gene MyD118 and the proto-oncogenes myb, myc, and bcl-2 modulate transforming growth factor beta 1-induced apoptosis of myeloid leukemia cells

Cell numbers are regulated by a balance among proliferation, growth arrest, and programmed cell death. A profound example of cell homeostasis, controlled throughout life, is the complex process of blood cell development, yet little is understood about the intracellular mechanisms that regulate blood cell growth arrest and programmed cell death. In this work, using transforming growth factor beta 1 (TGF beta 1)-treated M1 myeloid leukemia cells and genetically engineered M1 cell variants, the regulation of growth arrest and apoptosis was dissected. Blocking of early expression of MyD118, a novel differentiation primary response gene also shown to be a primary response gene induced by TGF beta 1, delayed TGF beta 1-induced apoptosis, demonstrating that MyD118 is a positive modulator of TGF beta 1-mediated cell death. Elevated expression of bcl-2 blocked the TGF beta 1-induced apoptotic pathway but not growth arrest induced by TGF beta 1. Deregulated expression of either c-myc or c-myb inhibited growth arrest and accelerated apoptosis, demonstrating for the first time that c-myb plays a role in regulating apoptosis. In all cases, the apoptotic response was correlated with the level of MyD118 expression. Taken together, these findings demonstrate that the primary response gene MyD118 and the c-myc, c-myb, and bcl-2 proto-oncogenes interact to modulate growth arrest and apoptosis of myeloid cells.

Changes in c-myc expression and the kinetics of dexamethasone-induced programmed cell death (apoptosis) in human lymphoid leukaemia cells

The kinetics of dexamethasone-induced death of CCRF CEM clone C7A human lymphoblastic leukaemia cells was determined with respect to changes in the expression of the c-myc protein. Cell death was characterised as being by apoptosis: cells with an intact plasma membrane had condensed chromatin and were characterised as having approximately 300 kbp fragments when DNA integrity was analysed by pulsed-field electrophoresis. Onset of apoptosis required a minimum of 36 h exposure to 5 microM dexamethasone; before this time no apoptotic cells were observed. This 36 h incubation period appeared to be necessary to prime the cells for subsequent death by apoptosis. In the continued presence of dexamethasone the percentage of apoptotic cells increased to 60% apoptotic cells by 54 h. Investigation of changes in c-myc protein showed that it was undetectable after 12 h of incubation with dexamethasone, although cells were not committed to die at this time. Cells were treated with dexamethasone for 54 h and for various pulsed periods with a non-toxic concentration of cycloheximide (200 nM). When cycloheximide was present during the first 36 h priming period of dexamethasone treatment, there was an immediate loss of c-myc protein and apoptosis at 54 h was completely inhibited. In contrast, there was no inhibition of apoptosis when dexamethasone-treated cells were incubated with an 18 h pulse of cycloheximide added after 36 h. Cells exposed to dexamethasone for 36 h ('primed') were given various periods of dexamethasone-free incubation before readdition of dexamethasone for a further 18 h. The longer the cells were free of drug after priming, the less susceptible they became to apoptosis, suggesting a slow decay of their 'memory' of the initial 36 h period of exposure. Cycloheximide inhibited the decay of this memory. Removal of dexamethasone after a 36 h exposure was characterised by a subsequent 24 h suppression of c-myc protein expression. Despite this, 90% of cells became refractory to apoptosis before the reappearance of c-myc protein. The evidence does not support the hypothesis that changes in c-myc expression are required for the engagement of apoptosis of CEM cells.

Regulation of apoptosis in the immune system

Apoptosis in T and B lymphocytes is involved in all fundamental processes in the immune system. It is a mechanism to regulate the course of an immune response and to establish immunological memory as well as central and peripheral tolerance. Apoptosis in lymphocytes is regulated by gene products that induce or block this process. Elucidating the molecular basis for sensitivity and resistance towards induction of apoptosis is the key to the understanding of the development of the immune system, basic immune reactions and the pathogenesis of autoimmune diseases, AIDS and cancer.

Apoptosis: programmed cell death in health and disease

Apoptosis is a normal physiological cell death process of eliminating unwanted cells from living organisms during embryonic and adult development. Apoptotic cells are characterised by fragmentation of nuclear DNA and formation of apoptotic bodies. Genetic analysis revealed the involvement of many death and survival genes in apoptosis which are regulated by extracellular factors. There are multiple inducers and inhibitors of apoptosis which interact with target cell specific surface receptors and transduce the signal by second messengers to programme cell death. The regulation of apoptosis is elusive, but defective regulation leads to aetiology of various ailments. Understanding the molecular mechanism of apoptosis including death genes, death signals, surface receptors and signal pathways will provide new insights in developing strategies to regulate the cell survival/death. The current knowledge on the molecular events of apoptotic cell death and their significance in health and disease is reviewed.

Photodynamic therapy with photofrin II induces programmed cell death in carcinoma cell lines

The mode of cell death following photodynamic therapy was investigated from the perspective of programmed cell death or apoptosis. Human prostate carcinoma cells (PC3), human non-small cell lung carcinoma (H322a) and rat mammary carcinoma (MTF7) were treated by photodynamic therapy. An examination of extracted cellular DNA by gel electrophoresis showed the characteristic DNA ladder indicative of internucleosomal cleavage of DNA during apoptosis. The magnitude of the response and the photodynamic therapy dosage required to induce DNA fragmentation were different in PC3 and MTF7. The MTF7 cells responded with rapid apoptosis at the dose of light and drug that yielded 50% cell death (LD50). In contrast, PC3 showed only marginal response at the LD50 but had a marked response at the LD85. Thus, apoptosis did not ensue as quickly in PC3 as in MTF7. The H322a cells were killed by photodynamic therapy but failed to exhibit any apoptotic response. The results also suggested that apoptosis in these cell lines has a minor requirement for de novo protein synthesis and no requirement for de novo RNA synthesis. This study indicates that although apoptosis can occur during photodynamic therapy-induced cell death, this response is not universal for all cancer cell lines.

Deregulated expression of the c-myc oncogene abolishes inhibition of proliferation of rat vascular smooth muscle cells by serum reduction, interferon-gamma, heparin, and cyclic nucleotide analogues and induces apoptosis

We have investigated the requirement for c-myc downregulation in the growth arrest of vascular smooth muscle cells (VSMCs). Rat VSMCs were infected with a retrovirus vector directing constitutive expression of either the complete human c-Myc protein (VSM-myc cells) or the c-Myc deletion mutant D106-143, which is inactive in cotransformation and autosuppression assays (VSM-D106-143 myc cells). Clones of transfected VSM-myc cells were isolated that constitutively expressed a range of levels of c-Myc protein from that observed in normal proliferating VSMCs to approximately seven times normal. The growth rates of these clones and their responses to growth inhibitors were then assessed. VSM-myc clones possessed a shorter mean intermitotic time than normal cells, which was inversely correlated (P < .05) with the level of c-Myc protein expressed. VSM-myc cells also expressed lower levels of alpha-smooth muscle actin mRNA and protein and exhibited an altered morphology. The proliferation of normal VSMCs and VSM-D106-143 myc cells was inhibited by serum reduction (0.5% fetal calf serum) and also by treatment with interferon-gamma (100 IU/mL), heparin (50 micrograms/mL), 8-bromo-cAMP (0.1 mmol/L), or 8-bromo-cGMP (0.1 mmol/L). In contrast, proliferation of VSM-myc cells was not inhibited by any of these agents, even if present at 10-fold higher concentrations. However, approximately 75% of VSM-myc cells expressing levels of c-Myc protein seen in normal proliferating VSMCs underwent apoptosis after 4 days of serum reduction or treatment with interferon-gamma. The results show that constitutive c-myc expression induces continuous cell proliferation, reduction in alpha-smooth muscle actin expression and apoptosis in VSMCs. We conclude that downregulation of c-myc is a prerequisite for growth arrest and subsequent survival of VSMCs. Conversely, deregulated c-myc expression may be important in the proliferation and death of VSMCs--characteristics of the pathogenesis of atherosclerosis.

A sequence-specific, single-strand binding protein activates the far upstream element of c-myc and defines a new DNA-binding motif

The far upstream element (FUSE) of the human c-myc proto-oncogene stimulates expression in undifferentiated cells. A FUSE-binding protein (FBP) is present in undifferentiated but not differentiated cells. Peptide sequences from the purified protein allowed cloning of cDNAs encoding FBP. Expression of FBP mRNA declined upon differentiation, suggesting transcriptional regulation of FBP. Features in the FBP cDNA suggest that FBP is also regulated by RNA processing, translation, and post-translational mechanisms. Both cellular and recombinant FBP form sequence-specific complexes with a single strand of FUSE. Transfection of FBP into human leukemia cells stimulated c-myc-promoter-driven expression from a reporter plasmid in a FUSE-dependent manner. Deletion and insertion mutagenesis of FBP defined a novel single-strand DNA-binding domain. Analysis of the primary and predicted secondary structure of the amino acid sequence reveals four copies of a reiterated unit comprised of a 30-residue direct repeat and an amphipathic alpha-helix separated by an 18- to 21-residue spacer. The third and fourth copies of this repeat-helix unit constitute the minimum single-stranded DNA-binding domain. To determine whether the FUSE site, in vivo, possesses single-strand conformation, and therefore could be bound by FBP, cells were treated with potassium permanganate (KMnO4) to modify unpaired bases. Modification of genomic DNA in vivo revealed hyperreactivity associated with single-stranded DNA in the FUSE sequence and protection on the strand that binds FBP in vitro. The role of single-stranded DNA and single-strand binding proteins in c-myc regulation is discussed.

The phosphoprotein phosphatase calcineurin controls calcium-dependent apoptosis in B cell lines

Group I Burkitt's lymphoma cell lines and the B104 lymphoma cell line which expresses a phenotype of immature B cells undergo apoptosis after cross-linking of their surface immunoglobulin (Ig) receptors or after exposure to a calcium ionophore, while protein kinase C (PKC)-activating phorbol esters prevent such apoptosis. We show here that blockade of the phosphoprotein phosphatase calcineurin or phosphatase 2B by cyclosporin A (CsA) also protects these B cell lines against Ca(2+)-dependent apoptosis but not against apoptosis triggered by the PKC inhibitor chelerythrine or by serum deprivation. Okadaic acid, an inhibitor of phosphatases 1, 2A and 2C was ineffective. Among a series of human cytokines tested, only interferon-alpha and tumor necrosis factor-alpha were shown to protect against Ca(2+)-dependent apoptosis when used alone or in combination with CsA. In contrast to phorbol esters which block the progression into the S/G2 phases of the cell cycle, CsA partially restored the proliferation of cells exposed to the calcium ionophore. Altogether these data provide indirect evidence for the control of B cell apoptosis by the serine/threonine phosphorylation status of yet undefined key cellular substrates.

Growth factors as survival factors: regulation of apoptosis

Apoptosis is now widely recognized as a common form of cell death and represents a mechanism of cell clearance in many physiological situations where deletion of cells is required. Peptide growth factors, initially characterised as stimulators of cell proliferation, have now been shown to inhibit death in many cell types. Deprivation of growth factors leads to the induction of apoptosis, i.e. condensation of chromatin and degradation in oligonucleosome-sized fragments, formation of plasma and nuclear membrane blebs and cell fragmentation into apoptotic bodies which can be taken up by neighbouring cells. Here we discuss the mechanism(s) by which growth factors may inhibit apoptosis.

Neuronal cell death: searching for the smoking gun

During the past year,several model systems have been developed to identify cellular and biochemical events involved in neuronal cell death, to investigate the role of bcl-2 in cell survival, and to characterize the relationship between cell death and the cell cycle.

Susceptibility to cytotoxic T lymphocyte-induced apoptosis is a function of the proliferative status of the target

Cytotoxic T lymphocytes (CTL) kill cells by perturbing the target's plasma membrane and by inducing the disintegration of the target cell's DNA into oligonucleosomal fragments, a process characteristic of apoptosis. We show that the DNA fragmentation event is distinct from the membrane lysis event and is dependent on the state of target cell activation or commitment into the mitotic cycle. Quiescent cells were refractory to DNA fragmentation, but not to membrane lysis. Log phase growth, transformation with c-myc, or infection of quiescent G0 targets with herpes simplex virus-1, which induces a competent state for DNA synthesis, all enhanced target cell susceptibility to CTL-induced DNA fragmentation without altering the membrane lysis. These results suggest that G0 cells are resistant to CTL-induced apoptosis, but that entry into G1 or a G1-like state by growth factors, cellular transformation, or DNA virus infection renders them competent to enter the apoptotic pathway(s).

Requirement for the orphan steroid receptor Nur77 in apoptosis of T-cell hybridomas

Apoptosis is a phenomenon observed during development of many cell types in many organisms. It is an internal, programmed cell death characterized by DNA fragmentation into nucleosome-size pieces. Anti-CD3-induced apoptosis in T-cell hybridomas and immature thymocytes requires new gene transcription and may be related to negative selection during T-cell development. Using subtractive hybridization, we isolated a complementary DNA clone encoding the orphan steroid receptor Nur77 (refs 7-9). It shows different patterns of messenger RNA induction between apoptotic and stimulated T cells. We report here the use of gel shift analysis to demonstrate that the Nur77 protein is present at high levels in apoptotic T-cell hybridomas and apoptotic thymocytes, but not in growing T cells or stimulated splenocytes. A Nur77 dominant negative protected T-cell hybridomas from activation-induced apoptosis. Hence Nur77 is necessary for induced apoptosis in T-cell hybridomas.

Apoptotic signals delivered through the T-cell receptor of a T-cell hybrid require the immediate-early gene nur77

Engagement of the T-cell antigen receptor (TCR) on immature thymic T cells induces death by apoptosis. Although several lines of evidence indicate that apoptosis requires de novo gene expression, little is known about the molecular pathways that mediate this response. Here we show that nur77 (refs 4-7), a zinc-finger transcription factor, is expressed in response to TCR engagement in immature T cells and T-cell hybrids. Antisense inhibition of nur77 expression prevents apoptosis in TCR-stimulated cells. nur77 is also expressed in response to mitogens, but in this case transcription is regulated by 5' upstream elements that are distinct from those used for induction of apoptosis. In addition, polyadenylation is only observed on nur77 transcripts found in condemned cells. These data support a role for nur77 in cell death that may be distinct from that of activation.

Mechanism of B cell antigen receptor function: transmembrane signaling and triggering of apoptosis

The antigen receptor of B lymphocytes (BCR) plays important roles in virtually every stage in the development, inactivation, or activation of B cells. The BCR is a complex of membrane immunoglobulin (mIg) and a heterodimer of two transmembrane polypeptides called Ig-alpha and Ig-beta. Site directed mutation of the mu immunoglobulin heavy chain has demonstrated that the mu transmembrane domain plays a key role in the assembly of mIgM with Ig-alpha/Ig-beta. In addition, there is a strong correlation between the ability of various mutant mIgM molecules to associate with Ig-alpha/Ig-beta and their ability to induce signal transduction reactions such as protein tyrosine phosphorylation and phosphoinositide breakdown. The cytoplasmic domains of Ig-alpha and Ig-beta share a region of limited homology with each other and with components of the T cell antigen receptor and of the Fc receptor. The presence of regions of the cytoplasmic domains of Ig-alpha or Ig-beta including this conserved amino acid sequence motif is sufficient to confer signaling function on chimeric transmembrane proteins. Both Ig-alpha and Ig-beta chimeras are capable of inducing all of the BCR signaling events tested. Based on these and related observations, we propose that the motifs act to initiate the BCR signaling reactions by binding and activating tyrosine kinases. Among the important events mediated by BCR signaling is induced expression of a series of genes referred to as early response genes. In B cells these include transcription factors and at least one component that regulates signaling events. One of these genes, c-myc, appears to play an important role in mediating apoptosis in B cells stimulated via the BCR complex.

Oxidative stress as a mediator of apoptosis

Many agents which induce apoptosis are either oxidants or stimulators of cellular oxidative metabolism. Conversely, many inhibitors of apoptosis have antioxidant activities or enhance cellular antioxidant defenses. Mammalian cells exist in a state of oxidative siege in which survival requires an appropriate balance of oxidants and antioxidants. Thomas Buttke and Paul Sandstrom suggest that eukaryotic cells may benefit from this perilous existence by invoking oxidative stress as a common mediator of apoptosis.

Dicing with death: dissecting the components of the apoptosis machinery

Apoptosis, a mode of cell death commonly observed when death is a desirable or programmed event, has several characteristic structural features. These features appear to be induced by a range of gene products which, together, supervise and participate in the controlled dismantling of the cell. In this article the molecular components of the apoptotic machinery and the proteins implicated in the regulation of this mechanism of cell death are discussed.

Genetically engineering mammalian cell lines for increased viability and productivity

The generation of new host cell lines for the production of foreign proteins can be achieved by cell engineering. This approach can be used to enhance the cell's ability to produce proteins that are properly processed and secreted at elevated levels and consequently can increase the overall productivity of an expression system. One potential target for cell engineering is the modification of the cell's protein folding capacity. The appropriate folding, assembly, localization and secretion of newly synthesized proteins is dependent upon the action of a group of proteins known as molecular chaperones. Improving the host cell's chaperoning capacity might increase the yield of properly folded recombinant proteins by preventing the formation of insoluble aggregates. Another potentially beneficial cell engineering goal is the inhibition of physiological cell death. The productivity of genetically engineered cells is dependent upon the maintenance of high levels of cell viability throughout the bioprocess period. Fluctuations in a cell's environment can trigger a deliberate form of cell death known as apoptosis. The proteins that mediate this self-destruction are currently being characterized. Regulating the expression of these death genes by cellular engineering could limit the loss of productivity that results from the physiological death of the recombinant cell line.

Lymphoma models for B cell activation and tolerance. X. Anti-mu-mediated growth arrest and apoptosis of murine B cell lymphomas is prevented by the stabilization of myc

Treatment of the WEHI-2131 or CH31 B cell lymphomas with anti-mu or transforming growth factor (TGF)-beta leads to growth inhibition and subsequent cell death via apoptosis. Since anti-mu stimulates a transient increase in c-myc and c-fos transcription in these lymphomas, we examined the role of these proteins in growth regulation using antisense oligonucleotides. Herein, we demonstrate that antisense oligonucleotides for c-myc prevent both anti-mu- and TGF-beta-mediated growth inhibition in the CH31 and WEHI-231 B cell lymphomas, whereas antisense c-fos has no effect. Furthermore, antisense c-myc promotes the appearance of phosphorylated retinoblastoma protein in the presence of anti-mu and prevents the progression to apoptosis as measured by propidium iodide staining. Northern and Western analyses show that c-myc message and the levels of multiple myc proteins were maintained in the presence of antisense c-myc, results indicating that myc species are critical for the continuation of proliferation and the prevention of apoptosis. These data implicate c-myc in the negative signaling pathway of both TGF-beta and anti-mu.

Programmed cell death, apoptosis and killer genes

A cursory examination of the literature reveals that the study of programmed cell death and apoptosis is increasing exponentially. Most contributors to this field have come either from developmental biology or immunology and view programmed cell death from different perspectives, leading both to confusion and an inability to fully appreciate the literature from other disciplines. Here, Lawrence Schwartz and Barbara Osborne define the terms and ideas relevant to the study of cell death in a way that will be accessible to investigators from all fields.

Neither macromolecular synthesis nor myc is required for cell death via the mechanism that can be controlled by Bcl-2

Expression of c-myc and macromolecular synthesis have been associated with physiological cell death. We have studied their requirement for the death of factor (interleukin-3)-dependent cells (FDC-P1) bearing an inducible bcl-2 expression construct. FDC-P1 cells expressing bcl-2 turned off expression of c-myc when deprived of interleukin-3 but remained viable as long as bcl-2 was maintained. A subsequent decline in Bcl-2 allowed the cells to undergo apoptosis directly from G0, in the absence of detectable c-myc expression. Thus c-myc expression may lead to apoptosis in some cases but is not directly involved in the mechanism of physiological cell death that can be controlled by Bcl-2. The macromolecular synthesis inhibitors actinomycin D and cycloheximide triggered rapid cell death of FDC-P1 cells in the presence of interleukin-3, but the cells could be protected by Bcl-2. Thus, the cell death machinery can exist in a quiescent state and can be activated by mechanisms that do not require synthesis of RNA or protein.

Signaling of programmed cell death induction in WEHI-231 B lymphoma cells

The murine WEHI-231 B lymphoma is highly sensitive to membrane immunoglobulin ligation which leads to programmed cell death (PCD) in this cell line. To study the molecular pathways involved in PCD induction in these cells, we derived two variants of WEHI-231 resistant to anti-Ig treatment. The level of bcl-2 mRNA was identical in the wild type and the variants, either untreated or anti-Ig treated, suggesting that PCD is not under the control of bcl-2 in WEHI-231 cells. In contrast, c-myc gene expression was markedly different in the wild type and the variants, both in the unstimulated and anti-Ig-stimulated state. Our findings are interpreted in the context of the dual capacity of c-myc to promote cell growth or cell death, in conjunction with other growth regulatory signals.