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

Requirement of phospholipase C-gamma 2 activation in surface immunoglobulin M-induced B cell apoptosis

Surface IgM (sIgM) stimulation induces the tyrosine phosphorylation of multiple cellular substrates, including phospholipase C (PLC)-gamma 2, which is involved in the activation of phosphatidylinositol pathway. DT40 B cells underwent apoptotic cell death when activated through sIgM, a phenomenon that is related to elimination of self-reactive B cells. To examine the roles of PLC-gamma 2 in sIgM signaling, we have generated DT40 cells deficient in PLC-gamma 2 Cross-linking of sIgM on PLC-gamma 2-deficient cells evoked neither inositol 1,4,5-trisphosphate nor calcium mobilization. In PLC-gamma 2- or Syk-deficient DT40 cells, the induction of apoptosis was blocked, but was still observed in Lyn-deficient cells. Src homology 2 domains of PLC-gamma 2 were essential for both its activation and sIgM-induced apoptosis. Since tyrosine phosphorylation of PLC-gamma 2 is mediated by Syk, these results indicate that activation of PLC-gamma 2 through Syk is required for sIgM-induced apoptosis.

Selective neuronal vulnerability in the hippocampus--a role for gene expression?

Proposed mechanisms of neurodegeneration focus generally on the triggering of toxic biochemical pathways by an increased intracellular concentration of Ca2+. Recent evidence also suggests that Ca2+ causes transcriptional activation of so-called 'cell-death genes'. Efforts to elucidate the basis of selective vulnerability have relied on animal models of delayed neuronal death in the hippocampus. Biochemical and morphological data indicate that delayed neuronal death is a form of programmed cell death, or apoptosis. Observations that specific genes are activated transcriptionally for prolonged times in neuronal populations that are undergoing delayed death suggest that active gene expression is part of the neuronal-death cascade. Although a direct causal role remains to be proven, evidence implicates certain genes in neuronal-death pathways.

Potential roles of apoptosis in viral pathogenesis

Apoptosis is a physiologic mechanism of cell death that is induced during development or reversible tissue expansion or after tissue damage to eliminate unwanted cells. Recently, viral gene products have been shown to either specifically induce or specifically inhibit apoptosis. In this review, our current understanding of the regulation of apoptosis is outlined. The mechanisms by which viruses can inhibit or induce apoptosis are described. The role of apoptosis in modulating the immune response to viral infection is discussed. Finally, the hypothesis is advanced that the balance between induction and inhibition of apoptosis is modulated by viruses to optimize the course of viral infection.

Inhibition of apoptosis by antioxidants in the human HL-60 leukemia cell line

Cell death via apoptosis is an important event involved in a number of immunological processes. Recently, apoptosis has been associated with oxidative stress in a number of cell systems. Here we assessed the inhibitory capacity of different antioxidants on UV- and drug-induced apoptosis in the human leukemic cell line, HL-60. We found that the oxygen radical scavenger, BHA, the radioprotector cysteamine and the metal chelators, pyrrolidinedithiocarbamate (PDTC), diethyldithiocarbamate (DEDTC), and dimethyldithiocarbamate (DMDTC), were able to significantly inhibit nuclear fragmentation and reduce the formation of apoptotic bodies in UV-irradiated human leukemic cells. Both BHA and PDTC were found to reduce DNA fragmentation as assessed by in situ DNA nick-end labelling and quantification thereof using fluorescence flow cytometry. In addition to inhibiting UV-induced apoptosis, PDTC was also capable of reducing the amount of apoptosis induced by a range of cytotoxic drugs, such as actinomycin-D, camptothecin, etoposide, and melphalan, whereas BHA and cysteamine were not as effective in these cases after more than four hours in culture when compared to PDTC. To further elucidate the working mechanism of PDTC, we have looked at the effect of PDTC on DNA fragmentation in isolated nuclei, under conditions that promote activation of endogenous endonuclease involved in apoptosis. In contrast to ZnCl2, a potent inhibitor of endonuclease activity, PDTC was unable to inhibit DNA-ladder formation in this assay. Taken together, these results indicate that oxygen radicals may have a central role to play in the induction of apoptosis and that dithiocarbamates can serve as potent inhibitors of apoptosis induced by a wide variety of stimuli.

Regulation of the Fas apoptotic cell death pathway by Abl

Relatively little is known about oncogene involvement in the regulation of Fas-mediated apoptosis. Inhibition of Fas-induced cell death by the bcl-2 oncogene has been demonstrated to be only partial. In light of a growing body of evidence for the Abl kinase as a negative regulator of cell death, we sought to determine whether Abl expression could protect against Fas-mediated cell death. To address this question, we utilized two separate strategies. In the first, we expressed human Fas in K562, a chronic myelogenous leukemia cell line, which constitutively expresses bcr-abl and examined the effects of Fas ligation in these cells. Fas-positive K562 transformants (K562.Fas) were found to be protected against Fas-mediated cell death. However, down-regulation of Bcr-Abl protein levels in K562.Fas cells using antisense oligonucleotides targeted to bcr-abl mRNA rendered these cells highly susceptible to Fas-induced death. In the second approach we utilized a Fas-positive HL-60 cell line, which we transfected with a temperature-sensitive mutant of v-Abl. HL-60.v-Ablts transfectants were found to be protected from Fas-induced apoptosis at the permissive but not the restrictive temperature for the Abl kinase. Taken together, these observations identify the Abl kinase as a negative regulator of Fas-mediated cell death. Since Abl was also found to block apoptosis mediated by ceramide, a recently proposed downstream effector of the apoptotic pathway initiated by Fas, we propose that Abl exerts its protective effects downstream of the early Fas-initiated signaling events.

Activation-induced apoptosis in Jurkat cells through a myc-independent mechanism

Apoptosis can be induced in the human leukemic T-cell line Jurkat when these cells are stimulated by several known T-cell activators. Among the stimuli that activate Jurkat cells, calcium ionophore A23187, phytohemagglutinin and cross-linked monoclonal antibody anti-CD3 are the best inducers of apoptosis. The activator of protein kinase C phorbol-12,13-dibutyrate causes a rapid but transient stimulation of DNA fragmentation and cell death, and soluble monoclonal antibody anti-CD3 has no effect. Furthermore, apoptosis is delayed when Jurkat cells are stimulated simultaneously with calcium ionophore and phorbol ester. We have analysed the mechanism underlying the induction of apoptosis by these agents and found that down-regulation of c-myc expression by pretreatment with the DNA binding antibiotic mithramycin, an inhibitor of c-myc transcription, does not prevent the activation of the apoptotic process. In the light of these results we can conclude that in these leukemic T-cells, activation of apoptosis may occur by a mechanism independent of c-myc expression.

Role of apoptosis in HIV disease pathogenesis

After approximately one and a half decades of intensive studies, the exact mechanisms to explain HIV-mediated cytopathicity are still enigmatic and need closer scrutiny. There has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing a killing of infected cells by HIV-1 and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis. Accumulating evidence points to the notion that apoptosis might be a major contributor to the depletion of CD4 T-cells in HIV infection. This review summarizes current information about the regulatory mechanisms of T-cell apoptosis and the role of apoptosis in HIV pathogenesis with the goal of providing an integrated view of HIV cytopathicity.

Apoptosis overview emphasizing the role of oxidative stress, DNA damage and signal-transduction pathways

Apoptosis (programmed cell death) is a central protective response to excess oxidative damage (especially DNA damage), and is also essential to embryogenesis, morphogenesis and normal immune function. An understanding of the cellular events leading to apoptosis is important for the design of new chemotherapeutic agents directed against the types of leukemias and lymphomas that are resistant to currently used chemotherapeutic protocols. We present here a review of the characteristic features of apoptosis, the cell types and situations in which it occurs, the types of oxidative stress that induce apoptosis, the signal-transduction pathways that either induce or prevent apoptosis, the biologic significance of apoptosis, the role of apoptosis in cancer, and an evaluation of the methodologies used to identify apoptotic cells. Two accompanying articles, demonstrating classic apoptosis and non-classic apoptosis in the same Epstein-Barr virus-transformed lymphoid cell line, are used to illustrate the value of employing multiple criteria to determine the type of cell death occurring in a given experimental system. Aspects of apoptosis and programmed cell death that are not covered in this review include histochemistry, details of cell deletion processes in the sculpting of tissues and organs in embryogenesis and morphogenesis, and the specific pathways leading to apoptosis in specific cell types. The readers should refer to the excellent books and reviews on the morphology, biochemistry and molecular biology of apoptosis already published on these topics. Emphasis is placed, in this review, on a proposed common pathway of apoptosis that may be relevant to all cell types.

Augmentation by aphidicolin of 1-beta-D-arabinofuranosylcytosine-induced c-jun and NF-kappa B activation in a human myeloid leukemia cell line: correlation with apoptosis

1-beta-D-arabinofuranosylcytosine (ara-C) (2 microM) can induce apoptosis in a human myeloid leukemia cell line, U937, after 4 h of incubation. Pretreatment of cells with aphidicolin (2 microM) augments ara-C-induced apoptosis, since it was first observed at 0.4 microM ara-C and became more intense at 2 and 10 microM. Although aphidicolin itself had a marginal effect on c-jun expression, it significantly augmented ara-C induced c-jun upregulation by shortening the lag time and lowering ara-C concentrations necessary for the induction of detectable c-jun transcripts. Aphidicolin and ara-C acted synergistically to increase NF-kappa B DNA binding activity as determined by an electrophoretic mobility shift assay. Expression of c-myc was slightly increased through the DNA degradative phase, and was then downregulated. Thus, the activation of NF-kappa B and c-jun expression seems to be well correlated with the potentiation by aphidicolin of ara-C-induced apoptosis.

Apoptosis of stomach carcinoma cells induced by a human monoclonal antibody

BACKGROUND

Although stomach carcinoma is estimated to be one of the most frequent cancers worldwide, still little is known about the immunity of patients with stomach cancer. Humoral tumor immunity has been studied by isolating B cells of patients with cancer to characterize the activity of such antibodies on tumor cells. Apoptosis, programmed cell death, explains the suicide of cells by fragmentation of DNA, cell shrinkage and dilation of endoplasmatic reticulum, followed by cell fragmentation and formation of membrane vesicles called apoptotic bodies. Apoptosis serves to remove unwanted, damaged, or dangerous, e.g., precancerous cells.

METHODS

The human monoclonal antibody SC-1 was isolated from a patient with a signet ring cell carcinoma of the stomach by fusion of spleen lymphocytes to the heteromyeloma SPM4-0. The antibody was tested for growth-inhibiting effects in vitro in soft agar assays, in 3-H thymidine uptake experiments, and in a mitochondrial enzymatic activity assay. In vivo intraperitoneal tumor growth was investigated in nu-nu mice.

RESULTS

The immunoglobulin M (lambda) antibody identifies a molecule with a molecular weight of approximately 49 kilodaltons in stomach carcinoma cells. No reactivity was observed with carcinomas of other origins, melanomas, lymphomas, or normal tissue. When tested in vitro, the antibody inhibited tumor cell growth in cell culture and soft agar. In vivo growth of stomach carcinoma cells in nu-nu mice was reduced when the antibody was injected after the tumor cells. Ultrastructural and functional studies revealed that the SC-1 antibody induced apoptosis of tumor cells.

CONCLUSION

The human monoclonal antibody SC-1 described here inhibited growth of stomach carcinoma cells in vitro and in vivo by inducing apoptosis, and may, therefore, be valuable for treating patients with stomach carcinoma.

Apoptosis in gastric carcinomas and its association with cell proliferation and differentiation

The significance of apoptosis in human gastric carcinomas was investigated in comparison with proliferative activity and p53 accumulation, using an in situ DNA nick end labeling method and immunohistochemistry for both Ki-67 antigen and p53 protein. Apoptotic labeling indices (LI) of 51 differentiated carcinomas (21 of early and 22 of advanced stage) were significantly lower than for 33 undifferentiated tumors (9 of early and 24 of advanced stage) (P < 0.05). In both types, apoptotic LI of advanced stage lesions were significantly higher than for the early stage cases (P < 0.005, P < 0.03). The distribution of apoptotic cells was different from that of Ki-67-positive cells, generally exhibiting an inverse correlation for areas of predominance. In contrast, there was no significant correlation between p53 immunoreactivity and either apoptotic LI or Ki-67 LI. It is concluded that in human gastric carcinomas the susceptibility to apoptosis is related to tumor cell differentiation and depth of invasion, and may play a role in selection of clonal subpopulations with high growth potential.

Prognostic value of bcl-2 expression in invasive breast cancer

Expression of the bcl-2 proto-oncogene was studied immunohistochemically in 251 invasive ductal breast carcinomas (median follow-up time 91 months, range 24-186 months) and the results were correlated with clinicopathological data and prognostic variables. Sixty-three (25%) tumours were scored bcl-2 negative and 188 (75%) tumours were bcl-2 positive. No relationship could be observed between bcl-2 status and tumour grade, pTNM staging or menopausal status. A strong positive relationship was demonstrated between bcl-2 immunoreactivity and oestrogen receptor status (P < 0.001) and progesterone receptor status (P < 0.001). No prognostic value was demonstrated for bcl-2 expression on disease-free survival and overall survival in axillary node-negative breast cancer patients. However, in axillary node-positive breast cancer patients multivariate analysis demonstrated absence of bcl-2 expression to be independently related to shortened disease-free survival (P = 0.003) and shortened overall survival (P < 0.001). Our results suggest a potential important role for bcl-2 expression as a modulator of response to adjuvant therapy in breast cancer.

ASTRO Research Fellowship: apoptosis as a predictor of tumor response to radiation in stage IB cervical carcinoma. American Society for Therapeutic Radiology and Oncology

PURPOSE

Levels of apoptosis predict for tumor responsiveness to radiation in various animal systems. To investigate the potential role of apoptosis as a predictor of response in human tumors, a retrospective review was undertaken of patients with adenocarcinoma of the cervix whose primary lesion at presentation measured at least 4 cm and who underwent definitive radiation therapy. A previous report had indicated that roughly half this group of patients should have a long-term relapse free survival.

METHODS AND MATERIALS

Pretreatment biopsy specimens of 44 patients with Stage IB adenocarcinoma of the cervix, whose primary lesion at presentation measured at least 4 cm in greatest dimension, were scored for apoptosis by two independent investigators without knowledge of the treatment outcome, and the results were averaged. Actuarial methods were used to assess overall survival, disease-free survival, determinate survival, and local control as a function of the baseline level of apoptosis. Patients ranged in age from 21 to 87 years and were treated with definitive radiotherapy between 1964 and 1989. Follow-up for the surviving patients ranged from 1 to 278 months, with a mean of 101 months.

RESULTS

Patients whose tumors had a baseline level of apoptosis above the median value (2%) had a better overall survival than those with lower levels of apoptosis (p = 0.056). A similar trend for disease-free survival (p = 0.32) and determinate survival (p = 0.27) did not reach statistical significance, perhaps because of the small number of patients. Because only 6 of the 44 patients (13%) had a local tumor failure, it was not possible to establish a correlation between the pretreatment level of apoptosis and the local tumor control by radiation.

CONCLUSION

The baseline level of apoptosis predicted for survival in patients with Stage IB cervical adenocarcinoma. Further investigation of the measurement of apoptosis as a potential predictive assay is warranted in other human tumor systems.

Unimpaired thymic and peripheral T cell death in mice lacking the nuclear receptor NGFI-B (Nur77)

T cell hybridomas require the immediate-early gene NGFI-B (nur77) for T cell receptor (TCR)-mediated apoptosis, a model for negative selection of self-reactive T cells. TCR-mediated death was examined in mice bearing an NGFI-B loss-of-function mutation, either by administration of antibodies to CD3 (anti-CD3) or in two well-characterized transgenic models expressing self-reactive TCRs. Both the extent and the rate of thymocyte death were unimpaired. Anti-CD3-induced death was normal in CD4+ peripheral T cells, in which death is mediated predominantly by the Fas signaling pathway. Thus, no unique requirement for NGFI-B is observed for thymic or peripheral T cell death.

Oncogene regulation of endonuclease activation in apoptosis

Recent work has demonstrated that oncogenes and tumor suppressor proteins regulate apoptotic cell death. Current efforts are therefore directed at determining how they exert their effects. It appears that oncogenes can control activation of the apoptosis endonuclease(s) either by regulating the signal(s) that trigger the process or via effects on expression and/or activation of key components of the effector machinery, including the endogenous endonuclease itself. This article summarizes the available information on the biochemical mechanisms underlying the effects of oncogenes on endogenous endonuclease activation and presents models to explain the observation that they also regulate alternative cell fates such as cell proliferation and differentiation.

Cell death regulation during multistep lymphomagenesis

The three most common genetic abnormalities occurring in malignant lymphomas involve alterations resulting in the deregulated expression of the c-myc and bcl-2 oncogenes and the inactivation of the p53 tumor suppressor gene. Relevant strains of genetically engineered mice, including bcl-2-Ig and E mu-myc transgenic mice and p53 knockout mice, have been used to prospectively examine the regulation of apoptotic cell death by these genes, individually and in combination, and their contribution to in vivo lymphomagenesis. The potential importance of the therapeutic induction of apoptosis is discussed.

Apoptosis of a fibrosarcoma induced by protein-free culture involves DNA cleavage to large fragments but not internucleosomal fragmentation

A murine fibrosarcoma clone, Gc-4 SD, grows depending on fetal calf serum. In MTT assay, protein-free cultivation resulted in a reduction of the viable cell number time-dependently. Electron-microscopic and flow-cytometric analyses revealed that the reduction in growth was accompanied by the appearance of apoptotic cells. However, no internucleosomal fragmentation was observed even after SI-nuclease treatment. On the other hand, pulse field gel electrophoresis revealed that cleavage of DNA into high-molecular-weight fragments estimated as 50 to 150 kilobase pairs (kbp), with a peak of 100 kbp, was found in the serum-deprived cells. Large fragments disappeared from the DNA extracts when the smaller cells with high blue fluorescence with Hoechst 33342 were removed by flow cytometry, suggesting direct correlation between the large DNA fragmentation and apoptosis. The addition of aurintricarboxylic acid neither abolished the large DNA fragmentation nor inhibited the reduction in the number of viable cells. Both cycloheximide and actinomycin D enhanced the reduction in the number of viable cells as well as the large DNA fragmentation. These results suggest that apoptosis of a fibrosarcoma induced by protein-free culture involves a specific endogenous endonuclease, which may be distinct from and independent of the ATA-sensitive endonuclease producing internucleosomal DNA fragmentation.

The nerve growth factor-responsive PC12 cell line does not express the Myc dimerization partner Max

Heterodimerization of Max with the nuclear oncoprotein Myc and the differentiation-associated proteins Mad and Mxi1 enables these factors to bind E-box sites in DNA and control genes implicated in cell proliferation and differentiation. We show that in the PC12 pheochromocytoma tumor cell line, functional Max protein is not expressed because of the synthesis of a mutant max transcript. This transcript encodes a protein incapable of homo- or heterodimerization. Furthermore, the mutant Max protein, unlike wild-type Max, is incapable of repressing transcription from an E-box element. Synthesis of mutant max transcripts appears to be due to a homozygous chromosomal alteration within the max gene. Reintroduction of max into PC12 cells results in repression of E-box-dependent transcription and a reduction in growth rate, which may explain the loss of Max expression either during the growth of the pheochromocytoma or in subsequent passage of the PC12 cell line in vitro. Finally, the ability of these cells to divide, differentiate, and apoptose in the absence of Max demonstrates for the first time that these processes can occur via Max- and possibly Myc-independent mechanisms.

BCR-ABL: an anti-apoptosis gene in chronic myelogenous leukemia

The expression of bcr-abl in chronic myelogenous leukemia leads to a large increase in the generation of mature myeloid cells. The key biochemical alteration in this disease is an increased Abl kinase activity. This up-regulation in activity is mediated through the binding of a portion of the Bcr molecule to the SH2 regulatory domain of the Abl protein. One effect of this alteration is a marked increase in resistance to drug induced cell death by apoptosis. This resistance can be overcome with the use of appropriate antisense oligonucleotides to the bcr-abl gene. The role and contribution of apoptosis to the development of the disease and the prospect of using antisense oligonucleotides as therapeutic agents is discussed.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Tumorigenic activity of the BCR-ABL oncogenes is mediated by BCL2

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the c-abl protein-tyrosine kinase gene on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, p210BCR-ABL and p190BCR-ABL, are produced that are characteristic of chronic myelogenous leukemia and acute lymphoblastic leukemia, respectively. Their role in the etiology of human leukemia remains to be defined. Transformed murine hematopoietic cells can be used as a model of BCR-ABL function since these cells can be made growth factor independent and tumorigenic by the action of the BCR-ABL oncogene. We show that the BCR-ABL oncogenes prevent apoptotic death in these cells by inducing a Bcl-2 expression pathway. Furthermore, BCR-ABL-expressing cells revert to factor dependence and nontumorigenicity after Bcl-2 expression is suppressed. These results help to explain the ability of BCR-ABL oncogenes to synergize with c-myc in cell transformation.

Apoptosis and the cell cycle

In this review, we consider apoptosis as a process intimately linked to the cell cycle. There are several reasons for thinking of apoptosis as a cell cycle phenomenon. First, within the organism, apoptosis is almost exclusively found in proliferating tissues. Second, artificial manipulation of the cell cycle can either prevent or potentiate apoptosis, depending on the point of arrest. Data from such studies have suggested that molecules acting late in G1 are required for apoptosis. Since passage through late G1 into S phase in mammalian cells is known to be regulated by p53 and by activation of cyclin-dependent kinases, we also examine recent studies linking these molecules to the apoptotic pathway.

Apoptosis and hepatobiliary disease

In this review we have attempted to summarize recent information on the role of apoptosis in normal liver biology and as a pathophysiological mechanism of cell death during hepatobiliary disease. Apoptosis is an important mechanism of cell injury and death in pathogenic settings as diverse as cell-mediated cytotoxicity, drug and chemical toxicity, carcinogenesis, viral disease, and autoimmune disease. Several factors account for the widespread attention and flurry of investigative activity into the phenomenon of apoptosis including the following: (1) realization that apoptosis is an important component of the behavior of all cells; (2) increasing evidence supporting a role for apoptotic cell death in diverse pathological settings; (3) elucidation of the biochemical pathways and insight into the genetic and molecular regulation of the process; and (4) development of new techniques to identify cells undergoing apoptosis. Recognition of the importance and role of apoptosis in hepatobiliary disease should stimulate interest and promote basic research toward understanding the mechanisms involved. Subsequent investigation into defining the cellular regulatory mechanisms and biochemical processes involved will be fertile ground for future research that may ultimately lead to novel therapeutic approaches for autoimmune diseases, malignancies, and viral disease.

Apoptosis: molecular mechanisms and implications for human disease

Apoptosis is a highly regulated process of cell death with characteristic morphological changes that are distinct from necrosis. The biochemical machinery responsible for apoptotic cell death appears to be constitutively expressed in most, if not all, cells and can be triggered by a variety of signals, including sustained increases in the intracellular Ca2+ level. Apoptosis is the main mechanism of cell deletion during development, normal cell turnover, hormone-induced tissue atrophy, and pathological processes such as T-cell depletion in HIV/AIDS and neurodegenerative disease. The aim of this review is to briefly summarize current knowledge of the molecular mechanisms of apoptosis and its role in human disease.

Cell cycle and apoptosis: common pathways to life and death

Programmed cell death, or apoptosis, is a highly regulated process used to eliminate unwanted or damaged cells from multicellular organisms. The morphology of cells undergoing apoptosis is similar to cells undergoing both normal mitosis and an aberrant form of mitosis called mitotic catastrophe. During each of these processes, cells release substrate attachments, lose cell volume, condense their chromatin, and disassemble the nuclear lamina. The morphological similarities among cells undergoing these processes suggest that the underlying biochemical changes also may be related. The susceptibility of cells to apoptosis frequently depends on the differentiation state of the cell. Additionally, cell cycle checkpoints appear to link the cell cycle to apoptosis. Deregulation of the cell cycle components has been shown to induce mitotic catastrophe and also may be involved in triggering apoptosis. Some apoptotic cells express abnormal levels of cell cycle proteins and often contain active Cdc2, the primary kinase active during mitosis. Although cell cycle components may not be involved in all forms of apoptosis, in many instances cell proliferation and cell death may share common pathways.

Relationship between bcl-2 and myc gene expression in human neuroblastoma

We have investigated the relationship between the expression of bcl-2 and myc family genes in primary human neuroblastoma (NB) tumors and cell lines. Of 20 NB tumors examined, bcl-2 transcripts were expressed at variable levels in 16 tumors of all clinical stages. Of the 2 tumors with N-myc amplification, one expressed bcl-2 at a high level, whereas the other did so at a low level. In contrast, all NB tumors showed the expression of c-myc and/or N-myc transcripts. Similarly, of 9 NB cell lines with N-myc amplification examined, 6 expressed bcl-2 at high levels, whereas the other 3 expressed it at barely detectable levels. The 3 cell lines without N-myc amplification also expressed bcl-2 protein at high levels. All NB cell lines tested expressed either c-myc or N-myc protein. These data suggest that in NB, there is no significant association between bcl-2 expression and advanced tumor stages or N-myc amplification. The data also show that bcl-2 expression does not always coincide with myc expression in NB, suggesting that bcl-2- independent mechanisms may exist in the bcl-2-negative NB tumor cells in order to suppress the cell death promoting action of high myc expression.

Effects of interferon-alpha on human B cells: repression of apoptosis and prevention of cell growth are independent responses of Burkitt lymphoma lines

We have previously shown that interferon-alpha (IFN-alpha) can repress apoptosis in Burkitt lymphoma (BL) cells. In this study, we have compared this protective response with a further, well-established effect of IFN-alpha on BL cells, that of growth arrest. Of a panel of BL lines comprising (i) EBV-positive and -negative lines that retain the phenotype of the parental tumour cells and (ii) the prototype IFN-alpha-growth-inhibited line, Daudi, only Daudi cells were found to undergo substantial growth inhibition in response to the cytokine. By contrast, all lines, with the notable exception of Daudi, were protected by IFN-alpha from high-rate apoptosis initiated by the Ca2+ ionophore ionomycin. Ionomycin failed to elicit an IFN-alpha-repressible apoptotic response in either wild-type Daudi cells or IFN-resistant sublines that were refractory to the growth-arresting effects of the cytokine. Analysis of c-myc protein levels confirmed previous observations that repression of apoptosis in IFN-alpha-rescuable BL cells was associated with an early inhibition of myc that was followed by a return to high-level expression. Significantly, ionomycin alone induced a comparable transient inhibition of myc protein in Daudi cells. In Daudi cells, but not in IFN-alpha-rescuable BL cells, renewed expression of myc observed after the early, transient down-regulation was followed by sustained down-regulation of the protein, which paralleled growth arrest. Our results indicate that long-term growth arrest and repression of apoptosis in BL are distinct cellular responses to IFN-alpha.

Towards understanding the molecular basis of thyroid cancer

Cancer is a multistep phenomenon and multiple genetic lesions are involved in the emergence of the cancerous lesion. This has best been demonstrated in colonic cancer. The authors review their work and that of others highlighting what is known about thyroid cancer. They implicate ras mutations predominantly in follicular carcinoma, rearrangement of the ret proto-oncogene in papillary carcinoma and the tumor suppressor genes p53 and retinoblastoma gene product in all stages of thyroid carcinoma. They find a low rate of ret proto-oncogene rearrangement in the Saudi population (>5%) as compared to elsewhere in the world (20%). They find TSH receptor message abundance to be predictive of prognosis in thyroid cancer patients. Lastly, they examine whether the abundance of the anti-metastatic gene nm23 message abundance negatively correlated with the tendency of thyroid tumors to metastasize and find that not to be the case in thyroid carcinoma. The study of oncogenes and tumor suppressor genes in the pathogenesis of thyroid cancer is in its infancy; however, rapid progress is being made in identifying genes participating in malignant thyroid cell transformation.

Three distinct IL-2 signaling pathways mediated by bcl-2, c-myc, and lck cooperate in hematopoietic cell proliferation

Two interleukin-2 receptor-dependent signaling pathways have thus far been identified: the c-fos/c-jun induction pathway mediated by src family protein-tyrosine kinases and the c-myc induction pathway. Here, we provide evidence for the existence of a third, rapamycin-sensitive pathway, which results in the induction of another proto-oncogene, bcl-2. In the hematopoietic cell line BAF-B03, the expression of any two of lckF505 (an active form of p56lck), Bcl-2, or c-Myc is sufficient to promote transit of the cell cycle, regardless of the activation state of the third pathway. We also provide evidence that epidermal growth factor receptor signaling may act through the same pathway that involves p56lck. These studies demonstrate a novel approach to dissecting signaling pathways regulating cellular proliferation.

Overexpression of the c-Myc oncoprotein blocks the growth-inhibitory response but is required for the mitogenic effects of transforming growth factor beta 1

One of the more intriguing aspects of transforming growth factor beta 1 (TGF beta 1) is its ability to function as both a mitogenic factor for certain mesenchymal cells and a potent growth inhibitor of lymphoid, endothelial, and epithelial cells. Data are presented indicating that c-myc may play a pivotal role in both the mitogenic and antiproliferative actions of TGF beta 1. In agreement with previous studies using C3H/10T1/2 fibroblasts constitutively expressing an exogenous c-myc cDNA, we show that AKR-2B fibroblasts expressing a chimeric estrogen-inducible form of c-myc (mycER) are able to form colonies in soft agar in the presence of TGF beta 1 only when c-myc is activated by hormone. Whereas these findings support a synergistic role for c-myc in mitogenic responses to TGF beta 1, we also find that c-myc can antagonize the growth-inhibitory response to TGF beta 1. Mouse keratinocytes (BALB/MK), which are normally growth-arrested by TGF beta 1, are rendered insensitive to the growth-inhibitory effects of TGF beta 1 upon mycER activation. This ability of mycER activation to block TGF beta 1-induced growth arrest was found to occur only when the fusion protein was induced with hormone in the early part of G1. Addition of estradiol late in G1 had no suppressive effect on TGF beta 1-induced growth inhibition.

Superoxide and hydrogen peroxide in relation to mammalian cell proliferation

A wide variety of normal and malignant cell types generate and release superoxide or hydrogen peroxide in vitro either in response to specific cytokine/growth factor stimulus or constitutively in the case of tumour cells. These species at submicromolar levels appear to act as novel intra and intercellular "messengers" capable of promoting growth responses in culture. The mechanisms may involve direct interaction with specific receptors or oxidation of growth signal transduction molecules such as protein kinases, protein phosphatases, transcription factors, or transcription factor inhibitors. It is also possible that hydrogen peroxide may modulate the redox state and activity of these important signal transduction proteins indirectly through changes in cellular levels of GSH and GSSG. Critical balances appear to exist in relation to cell proliferation on one hand and lipid peroxidation and cell death on the other. Progression to a more prooxidant state whilst initially leading to enhanced proliferative responses results subsequently in increased cell death.

Differentiation and mineralization in chick chondrocytes maintained in a high cell density culture: a model for endochondral ossification

Chondrocytes isolated from the proliferative and differentiating zones of 3-wk-old chick growth plates were cultured in the presence of 10% fetal bovine serum (FBS) and ascorbic acid for up to 21 d in a high cell density culture within Eppendorf tubes. The proliferative, differentiating, and calcification properties of the chondrocytes were examined by immunolocalization and by enzyme histochemical and biochemical methods. The cells maintained a chondrocyte phenotype throughout culture: they were round in shape and synthesized both collagen type II and proteoglycans. The expression of a hypertrophic phenotype was evident by Day 3 of culture and from this time onwards characteristics of terminal differentiation were observed. The cells were positive for both alkaline phosphatase (ALP) activity and c-myc protein and the surrounding matrix stained strongly for collagen type X. Small foci of mineralization associated with individual chondrocytes were first evident by Day 6 and more widespread areas of mineralization occupying large areas of matrix were present by Day 15. Mineralization occurred without the addition of exogenous phosphate to the medium. This culture system displays characteristics that are similar in both morphological and developmental terms to that of chick chondrocyte differentiation and calcification in vivo and therefore offers an excellent in vitro model for endochondral ossification.

FAS-induced apoptosis is mediated via a ceramide-initiated RAS signaling pathway

Fas receptor-induced apoptosis plays critical roles in immune homeostasis. However, most of the signal transduction events distal to Fas ligation have not been elucidated. Here, we show that Ras is activated following ligation of Fas on lymphoid lines. The activation of Ras is a critical component of this apoptotic pathway, since inhibition of Ras by neutralizing antibody or a dominant-negative Ras mutant interfered with Fas-induced apoptosis. Furthermore, ligation of Fas also resulted in stimulation of the sphingomyelin signalling pathway to produce ceramides, which, in turn, are capable of inducing both Ras activation and apoptosis. This suggests that ceramides acts as second messengers in Fas signaling via Ras. Thus, ligation of the Fas molecule on lymphocyte lines induces activation of Ras via the action of ceramide, and this activation is necessary, but not sufficient, for subsequent apoptosis.

Proto-oncogenes and oncogenes in epidermal neoplasia

This review briefly will focus on the role of selected proto-oncogenes and their activated forms during the regulation of cell proliferation, cell death and tumor formation in the epidermis. In addition, the multiple and complex functions of these proteins in normal as well as transformed cells will be discussed.

Cell death (apoptosis) in cell culture systems

In this review we describe the process of cell death known as apoptosis, outlining the key morphological and biochemical features. We also identify the stress elements in bioreactors that drive cells to their death. Finally, we outline methods that might be used to enhance the life span and increase the robustness of cells in culture.

Proteolysis of fodrin (non-erythroid spectrin) during apoptosis

Several recent studies have implicated proteases as important triggers of apoptosis. Thus far, substrates that are cleaved during apoptosis have been elusive. In this report we demonstrate that cleavage of alpha-fodrin (non-erythroid spectrin) accompanies apoptosis, induced by activation via the CD3/T cell receptor complex in a murine T cell hybridoma, ligation of the Fas (CD95) molecule on a human T cell lymphoma line and other Fas-expressing cells, or treatment of cells with staurosporine, dexamethasone, or synthetic ceramide. Furthermore, inhibition of activation-induced apoptosis by pretreatment of T hybridoma cells with antisense oligonucleotides directed against c-myc also inhibited fodrin proteolysis, confirming that this cleavage process is tightly coupled to apoptosis. Fodrin cleavage during apoptosis may have implications for the membrane blebbing seen during this process.

A simple assay for examining the effect of transiently expressed genes on programmed cell death

Programmed cell death (PCD) has been observed in a wide variety of cell types in response to physiologic signals or types of stress. How these stimuli trigger PCD, and whether there is a common PCD signal transduction pathway, is not clear. As more genes are described that may participate in or regulate PCD, an assay system in which gene products can easily be introduced and/or modulated would be of great value. To avoid the generation and screening of multiple individual stable cell transfectants, a simple transient transfection death assay has been developed. 2B4.11, a murine T cell hybridoma, was transfected by electroporation with a constitutively active beta-galactosidase reporter gene and the cells were incubated in culture medium or with a PCD-inducing stimulus. The amount of beta-galactosidase activity remaining in the intact cells at the end of the culture period represented only viable transfected cells. Bcl-2 was chosen to examine whether this system would be useful to study the effect of transiently transfected genes since it blocks PCD in a number of experimental systems. Consistent with data obtained using stable transfectants, transient expression of Bcl-2 in 2B4.11 completely protected cells from glucocorticoid- and cytotoxic agent-induced PCD. This protection from death was confirmed at the individual cell level by the transient co-expression of a class I Ld surface antigen and flow cytometric analysis. Some of the advantages of the transient transfection death assay described here are; (1) the simple and sensitive beta-galactosidase assay, (2) the rapidity of the assay, (3) the ability to perform conventional viability assays to monitor treatment-induced cytotoxicity, (4) multiple gene products can be tested alone, and in combination, (5) antisense or dominant negative approaches can be used, and (6) the adaptability of this assay system to other cell types, transfection techniques, or reporter and expression vectors. The transient transfection death assay should make it easier to identify and order important steps in the PCD signal transduction pathways.

Functional consequences of APO-1/Fas (CD95) antigen expression by normal and neoplastic hematopoietic cells

Murine monoclonal antibody (mAb) 7C11 binds to the same cell surface epitope as anti-APO-1 and anti-Fas and reacts specifically with cells transfected with a cDNA encoding the human Fas antigen. Furthermore, incubation with 7C11 causes death of hematopoietic cell lines that express APO-1/Fas but not APO-1/Fas-negative cell lines. 7C11 therefore recognizes the human APO-1/Fas (CD95) antigen, a 40 to 50 kDa cell surface glycoprotein that can trigger apoptosis or programmed cell death. Expression of APO-1/Fas antigen by normal and neoplastic hematopoietic cells was determined by flow cytometry using 7C11. APO-1/Fas is expressed by approximately 30 to 40% of resting peripheral blood T cells, B cells, and monocytes and by approximately 5% of resting NK cells and thymocytes. It was not detected on granulocytes, erythrocytes, or platelets. Approximately 80 to 90% of activated T cells, B cells, and thymocytes express APO-1/Fas, as do the majority of activated NK cells. Perturbation of APO-1/Fas by 7C11 does not affect the viability of resting lymphocytes or monocytes. In contrast, activated T cells and NK cells undergo apoptosis within 3 hours of exposure to 7C11. Other mAb that stimulate T cells or NK cells do not cause rapid induction of programmed cell death. APO-1/Fas antigen is expressed by many cell lines of lymphoid and myeloid lineage. However, this antigen was detected on neoplastic cells from only one of 69 patients with acute myeloid leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, chronic lymphocytic leukemia, or multiple myeloma. Only 3 out of 25 tumor samples from patients with non-Hodgkin's lymphoma were found to express APO-1/Fas. All three of these lymphomas harbored the bcl-2-Ig fusion gene associated with the chromosomal translocation t (14;18). Conversely, only 27% of lymphomas that possessed the bcl-2-Ig gene were found to express the APO-1/Fas antigen. Like normal activated lymphocytes, leukemia and lymphoma cells that expressed APO-1/Fas antigen were found to undergo apoptosis in vitro after incubation with 7C11. The APO-1/Fas antigen appears to regulate the growth of normal hematopoietic cells, and the marked upregulation of this antigen on activated normal lymphocytes contrasts sharply with the absence of APO-1/Fas on neoplastic cells of hematopoietic lineage. Defects in the apoptotic signal delivered through this antigen might contribute to the pathogenesis of hematopoietic neoplasms. Thus, the gene encoding APO-1/Fas can be considered a novel type of tumor suppressor gene, just as bcl-2 can be considered a cellular proto-oncogene.

Role of Rel-related factors in control of c-myc gene transcription in receptor-mediated apoptosis of the murine B cell WEHI 231 line

Treatment of immature murine B lymphocytes with an antiserum against their surface immunoglobulin (sIg)M results in cell death via apoptosis. The WEHI 231 B cell line (IgM, kappa) has been used extensively as a model for this anti-Ig receptor-mediated apoptosis. Anti-sIg treatment of WEHI 231 cells causes an early, transient increase in the levels of c-myc messenger RNA and gene transcription, followed by a rapid decline below control values. Given the evidence for a role of the c-myc gene in promoting apoptosis, we have characterized the nature and kinetics of changes in the binding of Rel-related factors, which modulate c-myc promoter activity. In exponentially growing WEHI 231 cells, multiple Rel-related binding activities were detectable. The major binding species was identified as p50/c-Rel heterodimers; only minor amounts of nuclear factor kappa B (NF-kappa B) (p50/p65) were detectable. Cotransfection of an inhibitor of NF-kappa B (I kappa B)-alpha expression vector reduced c-myc-promoter/upstream/exon1-CAT reporter construct activity, indicating the role of Rel factor binding in c-myc basal expression in these cells. Treatment with anti-sIg resulted in a rapid transient increase in the rate of c-myc gene transcription and in the binding of Rel factors. At later times, formation of p50 homodimer complexes occurred. In cotransfection analysis, p65 and c-Rel expression potently and modestly transactivated the c-myc promoter, respectively, whereas, overexpression of the p50 subunit caused a significant drop in its activity. The role of activation of Rel-family binding was demonstrated directly upon addition of the antioxidant pyrrolidinedithiocarbamate, which inhibited the anti-sIg-mediated activation of the endogenous c-myc gene. Similarly, induction after anti-sIg treatment of a transfected c-myc promoter was abrogated upon cotransfection of an I kappa B-alpha expression vector. These results implicate the Rel-family in Ig receptor-mediated signals controlling the activation of c-myc gene transcription in WEHI 231 cells, and suggest a role for this family in apoptosis of this line, which is mediated through a c-myc signaling pathway.

Melatonin modulation of estrogen-regulated proteins, growth factors, and proto-oncogenes in human breast cancer

The growth-inhibitory actions of the pineal hormone, melatonin, on human breast tumor cells and the possible association between this inhibition and melatonin's down-regulation of the estrogen receptor (ER) expression were examined in the ER-positive, estrogen-responsive MCF-7 human breast tumor cell line. As previously reported, melatonin dramatically inhibits the growth of these breast tumor cells and down-regulates ER levels in these cells, suggesting that the modulation of ER may be an important mechanism by which melatonin inhibits breast cancer cell growth. In the present studies, Northern blot analysis was used to examine the expression of estrogen-regulated transcripts known to be involved in estrogen's mitogenic actions. Melatonin, at a physiologic concentration (10(-9) M), rapidly, significantly, and, in some cases, transiently elevated the steady-state mRNA levels of growth stimulatory products such as TGF alpha, c-myc, and pS2, which are normally up-regulated in response to estrogen. Conversely, melatonin decreased the expression of other factors normally up-regulated by estrogen, such as progesterone receptor and c-fos. Significant stimulation of the expression of the growth-inhibitory factor TGF beta was seen with melatonin treatment, potentially supporting the concept that melatonin's growth-inhibitory activity is mediated through the breast tumor cells' estrogen-response pathway. The early regulation of many of these products by melatonin suggests that mechanisms more rapid than the down-regulation of ER are important in melatonin's modulation of their expression. However, the long-term modulation of these transcripts (12-48 hr) may be heavily influenced by melatonin's down-regulation of ER expression. These results clearly define the need for additional in depth studies to dissect the cellular events leading to melatonin-induced growth inhibition in breast tumor cells.

Fas(CD95)/FasL interactions required for programmed cell death after T-cell activation

Receptor crosslinking of T-cell hybridomas induces cell activation followed by apoptosis. This activation-induced cell death requires de novo synthesis of RNA and proteins, but the actual gene products that provide the death signal have not been identified. We show here that receptor crosslinking induces Fas ligand and upregulates Fas, and that the ensuing engagement of Fas by Fas ligand activates the cell-death programme. Cell death, but not activation, can be selectively prevented by a soluble Fas-immunoglobulin fusion protein. Thus, Fas and Fas ligand are the death-gene products, and their interaction accounts for the molecular mechanism of activation-induced T-cell death.

Cell-autonomous Fas (CD95)/Fas-ligand interaction mediates activation-induced apoptosis in T-cell hybridomas

A number of murine T-cell hybridomas undergo apoptosis within a few hours of activation by specific antigens, mitogens, antibodies against the T-cell antigen receptor, or a combination of phorbol ester and calcium ionophore. This phenomenon has been extensively studied as a model for clonal deletion in the immune system, in which potentially autoreactive T cells eliminate themselves by apoptosis after activation, either in the thymus or in the periphery. Here we show that the Fas/CD95 receptor, which can transduce a potent apoptotic signal when ligand, is rapidly expressed following activation of T-cell hybridomas, as is its functional, membrane-bound ligand. Interference with the ensuing Fas/Fas-ligand interaction inhibits activation-induced apoptosis. Because T-cell receptor ligation can induce apoptosis in a single T hybridoma cell, we suggest that the Fas/Fas-ligand interaction can induce cell death in a cell-autonomous manner.

Inhibition by differentiation-inducing agents of wild-type p53-dependent apoptosis in HL-60 cells

The product of the p53 tumor-suppressor gene has been shown to function in apoptosis and cell cycle regulation. However, there is little information regarding the regulation of apoptosis in cell differentiation. We investigated the relationship between p53-dependent apoptosis and differentiation induction using human promyelocytic leukemia HL-60 cells transfected with pMAMneo expression vectors containing dexamethasone-inducible wild-type p53 (wt-p53) cDNA inserts. Continuous exposure of the pMAMneo/wt-p53 transfectants to 1 microM dexamethasone for more than 24 h caused overexpression of wt-p53 followed by cell death with morphological changes typical of apoptosis. Using the wt-p53-inducible HL-60 cells, we examined the effects of differentiation inducers on the wt-p53-dependent apoptosis. All-trans retinoic acid (all-trans RA) at 1 nM or granulocyte macrophage colony-stimulating factor (GM-CSF) at 35 pM inhibited the wt-p53-induced apoptosis over a 42-h treatment. The apoptosis inhibition by GM-CSF, but not all-trans RA, was abolished by specific inhibitors of protein kinase C. These results suggest that extracellular signals involved in the differentiation induction could modulate the wt-p53-dependent apoptosis through protein kinase C-dependent and independent pathways.

A potential role for apoptosis in neurodegeneration and Alzheimer's disease

Previous studies have shown that beta-amyloid (A beta) peptides are neurotoxic. Recent data suggest that neurons undergoing A beta-induced cell death exhibit characteristics that correspond to the classical features of apoptosis, suggesting that these cells may initiate a program of cell death. This chapter explores the criteria and precautions that must be applied to evaluate mechanisms of cell death in vitro and in vivo, discusses the evidence supporting an apoptotic mechanism of cell death in response to A beta in cultured neurons, and describes potential correlations for these findings in the Alzheimer's disease brain. In addition, cellular signaling pathways that may be associated with apoptosis in response to A beta are examined, and support for apoptosis as a mechanism of cell death for other neurodegeneration-inducing stimuli (e.g., oxidative injury) is described. The connection of multiple stimuli that induce neuronal cell death to an apoptotic mechanism suggests that apoptosis could play a central role in neurodegeneration in the brain.

Activation-induced T-cell death is cell cycle dependent and regulated by cyclin B

Developing thymocytes and some T-cell hybridomas undergo activation-dependent programmed cell death. Although recent studies have identified some critical regulators in programmed cell death, the role of cell cycle regulation in activation-induced cell death in T cells has not been addressed. We demonstrate that synchronized T-cell hybridomas, irrespective of the point in the cell cycle at which they are activated, stop cycling shortly after they reach G2/M. These cells exhibit the diagnostic characteristics of apoptotic cell death. Although p34cdc2 levels are not perturbed after activation of synchronously cycling T cells, cyclin B- and p34cdc2-associated histone H1 kinase activity is persistently elevated. This activation-dependent induction of H1 kinase activity in T cells is associated with a decrease in the phosphotyrosine content of p34cdc2. We also demonstrate that transient inappropriate coexpression of cyclin B with p34cdc2 induces DNA fragmentation in a heterologous cell type. Finally, in T cells, cyclin B-specific antisense oligonucleotides suppress activation-induced cell death but not cell death induced by exposure to dexamethasone. We therefore conclude that a persistent elevation of the level of cyclin B kinase is required for activation-induced programmed T-cell death.

Cell density and paradoxical transcriptional properties of c-Myc and Max in cultured mouse fibroblasts

Deregulated expression of the c-Myc oncoprotein occurs in several human malignancies. The c-Myc protein behaves as a transcription factor, and undoubtedly its role in carcinogenesis involves its ability to affect the expression of genes involved in cell growth. c-Myc has been reported to both activate and repress transcription in transient transfection experiments using reporter constructs bearing multiple copies of the c-Myc binding site, CAC (G/A) TG. We investigated these apparently paradoxical effects of c-Myc by determining if they arose from differences in the cell proliferation states of transfected cells. We found that endogenous c-Myc protein levels vary inversely with the degree of cell confluency, such that at low cell confluency, where endogenous levels of c-Myc are high and presumably endogenous levels of Max are limiting, exogenous c-Myc fails to affect basal transcription. In cells at high cell confluency, in which endogenous c-Myc levels are low, exogenous c-Myc augments transactivation by titrating the relative excess endogenous Max. These observations suggest that the apparently paradoxical behavior of c-Myc in transfection experiments is partially dependent on ambient cellular levels of c-Myc.

Apoptosis in B lymphocytes: the WEHI-231 perspective

In this review we summarize recent work on the molecular basis of apoptosis in the murine B cell lymphoma WEHI-231. WEHI-231 cells undergo apoptosis in response to antigen receptor cross-linking with anti-Ig reagents. Death is specifically triggered via surface IgM (sIgM); cross-linking sIgD, Ia or FcR has no effect. Apoptosis is preceded by growth arrest in the G0/G1 phase of the cell cycle and may not occur in all currently available WEHI-231 sublines. The continuous passage of WEHI-231 cells in different laboratories has yielded variants that differ greatly in their response to anti-Ig treatment because apoptotic cells tend to be negatively selected in culture. Resistant and susceptible variants undergo growth arrest in response to anti-Ig but only susceptible cells go on to die by apoptosis. Cells resistant to anti-Ig have intact apoptotic machinery as indicated by their susceptibility to dexamethasone, irradiation and other treatments. However, anti-Ig-resistant cells are also resistant to apoptosis induced by the immunosuppressants cyclosporin A, FK-506 and rapamycin. We discuss the experimental evidence indicating that the apoptotic machinery in WEHI-231 cells is pre-activated but under constant negative regulation by short-lived protein inhibitors. Inhibition is removed by a mediator released in response to anti-Ig treatment in susceptible sublines. The mediator of death is the sphingosine derivative, ceramide, presumably produced by membrane sphingomyelinases activated by anti-Ig. A hypothetical model on how ceramide kills WEHI-231 is presented.