Essential versus accessory aspects of cell death: recommendations of the NCCD 2015

Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death' (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death' (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.

Genome evolution in yeast reveals connections between rare mutations in human cancers

Cancer cells are riddled with mutations. Less than one percent of these are thought to be mutations that drive cancer phenotypes. However, a recent study conducted on the yeast knockout collections by Teng et al. [Mol. Cell (2013) 52: 485-494] provides hard evidence that single gene deletions/mutations in most non-essential genes can drive the selection for cancer-like mutations.

Gene-dependent cell death in yeast

Caspase-dependent apoptotic cell death has been extensively studied in cultured cells and during embryonic development, but the existence of analogous molecular pathways in single-cell species is uncertain. This has reduced enthusiasm for applying the advanced genetic tools available for yeast to study cell death regulation. However, partial characterization in mammals of additional genetically encoded cell death mechanisms, which lead to a range of dying cell morphologies and necrosis, suggests potential applications for yeast genetics. In this light, we revisited the topic of gene-dependent cell death in yeast to determine the prevalence of yeast genes with the capacity to contribute to cell-autonomous death. We developed a rigorous strategy by allowing sufficient time for gene-dependent events to occur, but insufficient time to evolve new populations, and applied this strategy to the Saccharomyces cerevisiae gene knockout collection. Unlike sudden heat shock, a ramped heat stimulus delivered over several minutes with a thermocycler, coupled with assessment of viability by automated counting of microscopic colonies revealed highly reproducible gene-specific survival phenotypes, which typically persist under alternative conditions. Unexpectedly, we identified over 800 yeast knockout strains that exhibit significantly increased survival following insult, implying that these genes can contribute to cell death. Although these death mechanisms are yet uncharacterized, this study facilitates further exploration.

Mitochondrial fission and fusion dynamics: the long and short of it

Maintenance of functional mitochondria requires fusion and fission of these dynamic organelles. The proteins that regulate mitochondrial dynamics are now associated with a broad range of cellular functions. Mitochondrial fission and fusion are often viewed as a finely tuned balance within cells, yet an integrated and quantitative understanding of how these processes interact with each other and with other mitochondrial and cellular processes is not well formulated. Direct visual observation of mitochondrial fission and fusion events, together with computational approaches promise to provide new insight.

Mitochondrial factors with dual roles in death and survival

At least in mammals, we have some understanding of how caspases facilitate mitochondria-mediated cell death, but the biochemical mechanisms by which other factors promote or inhibit programmed cell death are not understood. Moreover, most of these factors are only studied after treating cells with a death stimulus. A growing body of new evidence suggests that cell death regulators also have 'day jobs' in healthy cells. Even caspases, mitochondrial fission proteins and pro-death Bcl-2 family proteins appear to have normal cellular functions that promote cell survival. Here, we review some of the supporting evidence and stretch beyond the evidence to seek an understanding of the remaining questions.

Enhancement of suicidal DNA vaccine potency by delaying suicidal DNA-induced cell death

DNA-based alphaviral RNA replicon vectors, also called suicidal DNA vectors, alleviate the concerns of integration or transformation related to conventional DNA vectors since suicidal DNA vectors eventually cause apoptosis of transfected cells. However, the expression of inserted genes in these vectors is transient and the potency of suicidal DNA vaccines may be compromised because of apoptotic cell death. Therefore, to enhance the immune response to the human papillomavirus type 16 (HPV-16) E7 antigen, we generated a DNA-based Semliki Forest virus vector, pSCA1, encoding E7 fused with BCL-xL, an antiapoptotic member of the BCL-2 family. Our results indicated that pSCA1 encoding E7/BCL-xL fusion protein delayed cell death in the pSCA1-transfected dendritic cell line and generated significantly higher E7-specific CD8(+) T-cell-mediated immune responses and better antitumor effects than pSCA1 encoding wild-type E7 gene in vaccinated mice. The antiapoptotic function of BCL-xL is important for the enhancement of antigen-specific CD8(+) T-cell responses in vaccinated mice, because a point mutant of BCL-xL lacking antiapoptotic function was ineffective. These results suggest that strategies to delay suicidal DNA-induced cell death using antiapoptotic proteins may greatly enhance the potency of suicidal DNA.

Regulation of apoptosis by viruses that infect insects

Orthobunyviruses and alphaviruses cause encephalitis, neuronal apoptosis and mortality in mammals, but fail to kill the mosquitoes that transmit these viruses. Therefore, host cell factors, as well as viral factors, regulate the outcome of infection. Drosophila Reaper is a pro-death factor in the Drosophila nervous system during development but homologues were not previously known to exist outside flies. Recent discovery of a Reaper protein encoded by orthobunyaviruses provides interesting insight into the mechanisms by which viruses modulate the death pathway in both vertebrate and invertebrate hosts.

Viral versus cellular BCL-2 proteins

All gamma herpesviruses and a few other viruses encode at least one homologue of the mammalian cell death inhibitor BCL-2. Gamma herpesviruses are associated with human and animal lymphoid and epithelial tumours. However, the role of these viral BCL-2 homologues in the virus replication cycle or in human disease is not known, though recent developments show progress in this area. The structure of viral BCL-2 family protein, KSBcl-2, is similar to that of cellular family members, but viral BCL-2 proteins differ functionally from the cellular proteins, apparently escaping the regulatory mechanisms to which their cellular counterparts are subjected. Thus, exploring the biochemical and biological functions of the viral BCL-2 family proteins will increase our understanding of their role in virus infections and will undoubtedly teach us something about their cellular kin.

Inhibition of drug-induced Fas ligand transcription and apoptosis by Bcl-XL

Fas/Fas ligand system triggers apoptosis in many cell types. Bcl-XL overexpresion antagonizes Fas/Fas ligand-mediated cell death. The mechanism by which Bcl-XL influences Fas-mediated cell death is unclear. We have found that microtubule-damaging drugs (e.g. Paclitaxel) induce apoptosis in a Fas/FasL-dependent manner. Inhibition of Fas/FasL pathway by anti-FasL antibody, mutant Fas or a dominant negative FADD blocks paclitaxel-induced apoptosis. Paclitaxel induced apoptosis through activation of both caspase-8 and caspase-3. Overexpression of Bcl-XL leads to inhibition of paclitaxel-induced FasL expression and apoptosis. Bcl-XL prevents the nuclear translocation of NFAT (nuclear factor of activated T lymphocytes) by inhibiting the activation of calcineurin, a calcium-dependent phosphatase that must dephosphorylate NFAT for it to move to the nucleus. The loop domain in Bcl-XL can suppress the anti-apoptotic function of Bcl-XL and may be a target for regulatory post-translational modifications. Upon phosphorylation, Bcl-XL loses its ability to bind with calcineurin. Without NFAT nuclear translocation, the FasL gene is not transcribed. Thus, paclitaxel and other drugs that disturb microtubule function kill cells, at least in part, through the induction of FasL, and Bcl-XL-mediated resistance to these agents is related to failure to induce FasL expression.

Pro-apoptotic cleavage products of Bcl-xL form cytochrome c-conducting pores in pure lipid membranes

During apoptotic cell death, cells usually release apoptogenic proteins such as cytochrome c from the mitochondrial intermembrane space. If Bcl-2 family proteins induce such release by increasing outer mitochondrial membrane permeability, then the pro-apoptotic, but not anti-apoptotic activity of these proteins should correlate with their permeabilization of membranes to cytochrome c. Here, we tested this hypothesis using pro-survival full-length Bcl-x(L) and pro-death Bcl-x(L) cleavage products (DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L)). Unlike Bcl-x(L), DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L) caused the release of cytochrome c from mitochondria in vivo and in vitro. Recombinant DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L), as well as Bcl-x(L), cleaved in situ by caspase 3-possessed intrinsic pore-forming activity as demonstrated by their ability to efficiently permeabilize pure lipid vesicles. Furthermore, only DeltaN61Bcl-x(L) and DeltaN76Bcl-x(L), but not Bcl-x(L), formed pores large enough to release cytochrome c and to destabilize planar lipid bilayer membranes through reduction of pore line tension. Because Bcl-x(L) and its C-terminal cleavage products bound similarly to lipid membranes and formed oligomers of the same size, neither lipid affinity nor protein-protein interactions appear to be solely responsible for the increased membrane-perturbing activity elicited by Bcl-x(L) cleavage. Taken together, these data are consistent with the hypothesis that Bax-like proteins oligomerize to form lipid-containing pores in the outer mitochondrial membrane, thereby releasing intermembrane apoptogenic factors into the cytosol.

Comparison of Bcl-2 to a Bcl-2 deletion mutant for mammalian cells exposed to culture insults

Apoptosis has been found to occur in bioreactors as a result of environmental stresses. The overexpression of bcl-2 is a widely used strategy to limit the induction of apoptosis in mammalian cell cultures. In this study, the effectiveness of wild-type Bcl-2 was compared to a Bcl-2 mutant lacking the nonstructured loop domain in two commercially prominent cell lines, Chinese hamster ovary (CHO) and baby hamster kidney (BHK) cells. The generation of a DNA "ladder" and condensation of chromatin indicated that apoptosis occurred in these cell lines following Sindbis virus infection and serum deprivation. When cells were engineered to overexpress the bcl-2 mutant, cell death due to Sindbis virus was inhibited in a concentration-dependent manner. Furthermore, the Bcl-2 mutant provided increased protection as compared to wild-type Bcl-2 following two model insults, Sindbis virus infection and serum deprivation. Total production for a heterologous protein encoded on the Sindbis virus was increased in cell lines expressing the Bcl-2 variants compared to the parental cell line. In order to understand the reasons for the improved anti-apoptosis properties of the mutant, wild-type Bcl-2 and mutant Bcl-2 were examined by Western blot following each model insult. Wild-type Bcl-2 was observed to degrade into a 23 kDa fragment following both Sindbis virus infection and serum withdrawal in both cell lines, while the mutant Bcl-2 protein was not degraded during the same period. The processing of Bcl-2 was found to correlate with reduced cell viabilities following the two external insults to suggest that Bcl-2 degradation may limit its ability to inhibit apoptosis. These studies indicate that the cells regulate anti-apoptosis protein levels and these processing events can limit the effectiveness of cell death inhibition strategies in mammalian cell culture systems.

c-IAP1 is cleaved by caspases to produce a proapoptotic C-terminal fragment

Although human c-IAP1 and c-IAP2 have been reported to possess antiapoptotic activity against a variety of stimuli in several mammalian cell types, we observed that full-length c-IAP1 and c-IAP2 failed to protect cells from apoptosis induced by Bax overexpression, tumor necrosis factor alpha treatment or Sindbis virus infection. However, deletion of the C-terminal RING domains of c-IAP1 and c-IAP2 restored antiapoptotic activity, indicating that this region negatively regulates the antiapoptotic function of the N-terminal BIR domain. This finding is consistent with the observation by others that the spacer region and RING domain of c-IAP1 functions as an E3 ligase, promoting autoubiquitination and degradation of c-IAP1. In addition, we found that c-IAP1 is cleaved during apoptosis to 52- and 35-kDa fragments. Both fragments contain the C-terminal end of c-IAP1 including the RING finger. In vitro cleavage of c-IAP1 with apoptotic cell extracts or with purified recombinant caspase-3 produced similar fragments. Furthermore, transfection of cells with the spacer-RING domain alone suppressed the antiapoptotic function of the N-terminal BIR domain of c-IAP1 and induced apoptosis. Optimal death-inducing activity of the spacer-RING required both the spacer region and the zinc-binding RING domain of c-IAP1 but did not require the caspase recruitment domain located within the spacer region. To the contrary, deletion of the caspase recruitment domain increased proapoptotic activity, apparently by stabilizing the C-terminal fragment.

Survival motor neuron protein modulates neuron-specific apoptosis

Spinal muscular atrophy (SMA) is attributed to mutations in the SMN1 gene, leading to loss of spinal cord motor neurons. The neurotropic Sindbis virus vector system was used to investigate a role for the survival motor neuron (SMN) protein in regulating neuronal apoptosis. Here we show that SMN protects primary neurons and differentiated neuron-like stem cells, but not cultured cell lines from virus-induced apoptotic death. SMN also protects neurons in vivo and increases survival of virus-infected mice. SMN mutants (SMNDelta7 and SMN-Y272C) found in patients with SMA not only lack antiapoptotic activity but also are potently proapoptotic, causing increased neuronal apoptosis and animal mortality. Full-length SMN is proteolytically processed in brains undergoing apoptosis or after ischemic injury. Mutation of an Asp-252 of SMN abolished cleavage of SMN and increased the antiapoptotic function of full-length SMN in neurons. Taken together, deletions or mutations of the C terminus of SMN that result from proteolysis, splicing (SMNDelta7), or germ-line mutations (e.g., Y272C), produce a proapoptotic form of SMN that may contribute to neuronal death in SMA and perhaps other neurodegenerative disorders.

Aven, a novel inhibitor of caspase activation, binds Bcl-xL and Apaf-1

Bcl-x(L), an antiapoptotic Bcl-2 family member, is postulated to function at multiple stages in the cell death pathway. The possibility that Bcl-x(L) inhibits cell death at a late (postmitochondrial) step in the death pathway is supported by this report of a novel apoptosis inhibitor, Aven, which binds to both Bcl-x(L) and the caspase regulator, Apaf-1. Identified in a yeast two-hybrid screen, Aven is broadly expressed and is conserved in other mammalian species. Only those mutants of Bcl-x(L)that retain their antiapoptotic activity are capable of binding Aven. Aven interferes with the ability of Apaf-1 to self-associate, suggesting that Aven impairs Apaf-1-mediated activation of caspases. Consistent with this idea, Aven inhibited the proteolytic activation of caspases in a cell-free extract and suppressed apoptosis induced by Apaf-1 plus caspase-9. Thus, Aven represents a new class of cell death regulator.

Antiapoptotic herpesvirus Bcl-2 homologs escape caspase-mediated conversion to proapoptotic proteins

The antiapoptotic Bcl-2 and Bcl-x(L) proteins of mammals are converted into potent proapoptotic factors when they are cleaved by caspases, a family of apoptosis-inducing proteases (E. H.-Y. Cheng, D. G. Kirsch, R. J. Clem, R. Ravi, M. B. Kastan, A. Bedi, K. Ueno, and J. M. Hardwick, Science 278:1966-1968, 1997; R. J. Clem, E. H.-Y. Cheng, C. L. Karp, D. G. Kirsch, K. Ueno, A. Takahashi, M. B. Kastan, D. E. Griffin, W. C. Earnshaw, M. A. Veliuona, and J. M. Hardwick, Proc. Natl. Acad. Sci. USA 95:554-559, 1998). Gamma herpesviruses also encode homologs of the Bcl-2 family. All tested herpesvirus Bcl-2 homologs possess antiapoptotic activity, including the more distantly related homologs encoded by murine gammaherpesvirus 68 (gammaHV68) and bovine herpesvirus 4 (BHV4), as described here. To determine if viral Bcl-2 proteins can be converted into death factors, similar to their cellular counterparts, five herpesvirus Bcl-2 homologs from five different viruses were tested for their susceptibility to caspases. Only the viral Bcl-2 protein encoded by gammaHV68 was susceptible to caspase digestion. However, unlike the caspase cleavage products of cellular Bcl-2, Bcl-x(L), and Bid, which are potent inducers of apoptosis, the cleavage product of gammaHV68 Bcl-2 lacked proapoptotic activity. KSBcl-2, encoded by the Kaposi's sarcoma-associated herpesvirus, was the only viral Bcl-2 homolog that was capable of killing cells when expressed as an N-terminal truncation. However, because KSBcl-2 was not cleavable by caspases, the latent proapoptotic activity of KSBcl-2 apparently cannot be released. The Bcl-2 homologs encoded by herpesvirus saimiri, Epstein-Barr virus, and BHV4 were not cleaved by apoptotic cell extracts and did not possess latent proapoptotic activities. Thus, herpesvirus Bcl-2 homologs escape negative regulation by retaining their antiapoptotic activities and/or failing to be converted into proapoptotic proteins by caspases during programmed cell death.

Part II. Overexpression of bcl-2 family members enhances survival of mammalian cells in response to various culture insults

A number of bioreactor configurations have been developed for the manufacture of products from mammalian cell hosts. Even in the most efficient of these, however, problems such as nutrient exhaustion, growth factor deprivation, and toxin accumulations may arise. Consequently, the current effort focused on the feasibility of overexpressing anti-apoptosis genes in baby hamster kidney (BHK) and Chinese hamster ovary (CHO) cells as a means of limiting cell death upon exposure to three such insults. Extended periods of glucose deprivation, serum withdrawal, and treatment with ammonium chloride each caused significant damage, often apoptotic in nature, to BHK and CHO cells, typically rendering cultures completely nonviable. The overexpression of bcl-2 and bcl-x(L), however, was able to abrogate the cell death in BHK cultures, though to varying degrees. For instance, the presence of Bcl-2, which did little to suppress apoptosis upon glucose deprivation, significantly improved the viabilities of these cells during serum withdrawal. In contrast, bcl-x(L) overexpression provided BHK cells with enhanced protection in the absence of glucose, allowing cultures to remain viable throughout the entire three week study. CHO cultures, on the other hand, displayed similar trends in survival in response to both glucose and serum deprivation. During these studies, Bcl-x(L) was consistently able to afford cells the highest degree of protection, though Bcl-2 also enhanced culture viabilities and viable numbers. Death suppression following exposure to 50 mM ammonium chloride was observed to a limited extent in both BHK and CHO cells overexpressing bcl-2 and bcl-x(L). However, even during such harsh treatment, Bcl-x(L) was able to enhance the survival of both cultures, providing CHO cells with viable numbers that were nearly 20-fold that of the controls after five days of exposure. Furthermore, the extensions in cell survival provided by the anti-apoptosis gene products enabled the recovery of many of the cultures during rescue attempts in which the death-inducing stimulus was removed. Clearly, engineering cells to better withstand and recover from the insults common during the large scale cultivation of mammalian cells has a number of potential applications in the biopharmaceutical industries where cell death can limit culture productivities.

Part I. Bcl-2 and Bcl-x(L) limit apoptosis upon infection with alphavirus vectors

Viral expression systems offer the ability to generate high levels of a particular protein within a relatively short period of time. In particular, alphavirus constructs based on Sindbis virus (SV) and Semliki Forest virus (SFV) are promising vehicles as they are cytoplasmic vectors with the potential for high expression levels. Two such alphavirus vectors were utilized during the current study to infect two commercially relevant cell lines, baby hamster kidney (BHK) and Chinese hamster ovary (CHO); the first was a fully competent SV derivative carrying the gene for chloramphenicol acetyltransferase (dsSV-CAT), while the second was a replication deficient SFV construct containing the human interleukin-12 (IL-12) p35 and p40 genes (SFV-IL-12). Since infection with these vectors induced apoptosis in both cell lines, the present effort was dedicated to determining the ability of anti-apoptosis genes to limit the cell death associated with these virus constructs. Infection with the dsSV-CAT vector resulted in the rapid death of BHK and CHO cells within 4 days, a phenomenon which was considerably delayed by stably overexpressing bcl-2 or bcl-x(L). In fact, cellular lifespans were doubled in both BHK-bcl2 and CHO-bclx(L) cells relative to the parental cell lines. Furthermore, the presence of these gene products provided increases of up to 2-fold in recombinant CAT production. Overexpression of bcl-2 and bcl-x(L) also altered the response of these cells upon infection with SFV-IL-12. While the parental cell lines were completely nonviable within 1 week, the BHK-bcl2, BHK-bclx(L), and CHO-bclx(L) cells each recovered from the infection, resuming exponential growth and regaining viabilities of over 90% by 9 days post-infection. Total IL-12 productivities were nearly doubled by Bcl-2 and Bcl-x(L) in the CHO cells, although this effect was apparently cell-line specific, as the native BHK cells were able to secrete more IL-12 than either of its transfected derivatives. Regardless, the presence of the anti-apoptosis genes allowed the production of IL-12 to be maintained, albeit at low levels, from each of the cell lines for the duration of the culture process. Therefore, overexpression of bcl-2 family members can have a significant impact on culture viabilities and recombinant protein production during alphavirus infections of mammalian cells.

Antiapoptosis chemicals prolong productive lifetimes of mammalian cells upon Sindbis virus vector infection

Viral expression systems allow for the rapid production of large amounts of recombinant protein in cell culture. In particular, Sindbis virus vectors now exist that make possible the expression of a variety of heterologous proteins in mammalian culture systems. Unfortunately, infection of cultured cells with Sindbis virus vectors typically results in apoptotic cell death, as demonstrated in the current study by DNA laddering and fluorescence microscopy. Fortunately, it has recently been demonstrated that apoptosis can be inhibited in vitro by certain chemical reagents that are capable of blocking specific steps during the cell death cascade. In this study, a rat prostate carcinomal cell line, AT3-neo, was infected with a Sindbis virus vector containing the gene for chloramphenicol acetyltransferase (dsSV-CAT) in the presence of several representative antiapoptotic chemicals and analyzed for cell viability as well as recombinant protein production. N-acetylcysteine (NAC), pyrrolidine dithiocarbamate (PDTC), bongkrekic acid (BA), and N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk) all exhibited the capacity to limit apoptosis in the infected cells. In fact, after just 1 day, percentage viabilities of the cells exposed to chemical reagents were between 72% and 91%, compared with 44% for the untreated controls. Furthermore, cells maintained on these agents were able to survive the infection from 1 to 3 days longer than the control samples. In addition to providing gains in cell viability, chemical treatment allowed for higher levels of recombinant protein production in most cases. Maximum chloramphenicol acetyltransferase (CAT) productivities in cells maintained on BA, NAC, and Z-VAD.fmk were 1.7-, 2.2-, and 3.9-fold higher than those obtained from the untreated cultures. Consequently, the addition of chemical reagents to culture media as a means of inhibiting apoptosis may be a valuable tool in the cell culture industry, where cell death severely limits productivity levels and adds significantly to production costs.

Pro-B-cell-specific transcription and proapoptotic function of protein kinase Ceta

Using a subtractive cloning scheme on cDNA prepared from primary pro-B and pre-B cells, we identified several genes whose products regulate apoptosis. We further characterized one of these genes, encoding protein kinase Ceta (PKCeta). PKCeta transcripts were readily detected in pro-B cells but were absent in pre-B cells. Although both a full-length and a truncated form of PKCeta were detectable in bone marrow pro-B cells, transition to the pre-B-cell stage was associated with increased relative levels of truncated PKCeta. We found that PKCeta is proteolyzed in apoptotic lymphocytes, generating a kinase-active fragment identical to the truncated form which is capable of inducing apoptosis when expressed in a pro-B cell line. Caspase-3 can generate an identical PKCeta cleavage product in vitro, and caspase inhibitors prevent the generation of this product during apoptosis in transfected cell lines. Inducible overexpression of either the full-length or truncated form of PKCeta results in cell cycle arrest at the G(1)/S transition. These results suggest that the expression and proteolytic activation of PKCeta play an important role in the regulation of cell division and cell death during early B-cell development.

Caspase-3-dependent cleavage of Bcl-2 promotes release of cytochrome c

Caspases are cysteine proteases that mediate apoptosis by proteolysis of specific substrates. Although many caspase substrates have been identified, for most substrates the physiologic caspase(s) required for cleavage is unknown. The Bcl-2 protein, which inhibits apoptosis, is cleaved at Asp-34 by caspases during apoptosis and by recombinant caspase-3 in vitro. In the present study, we show that endogenous caspase-3 is a physiologic caspase for Bcl-2. Apoptotic extracts from 293 cells cleave Bcl-2 but not Bax, even though Bax is cleaved to an 18-kDa fragment in SK-NSH cells treated with ionizing radiation. In contrast to Bcl-2, cleavage of Bax was only partially blocked by caspase inhibitors. Inhibitor profiles indicate that Bax may be cleaved by more than one type of noncaspase protease. Immunodepletion of caspase-3 from 293 extracts abolished cleavage of Bcl-2 and caspase-7, whereas immunodepletion of caspase-7 had no effect on Bcl-2 cleavage. Furthermore, MCF-7 cells, which lack caspase-3 expression, do not cleave Bcl-2 following staurosporine-induced cell death. However, transient transfection of caspase-3 into MCF-7 cells restores Bcl-2 cleavage after staurosporine treatment. These results demonstrate that in these models of apoptosis, specific cleavage of Bcl-2 requires activation of caspase-3. When the pro-apoptotic caspase cleavage fragment of Bcl-2 is transfected into baby hamster kidney cells, it localizes to mitochondria and causes the release of cytochrome c into the cytosol. Therefore, caspase-3-dependent cleavage of Bcl-2 appears to promote further caspase activation as part of a positive feedback loop for executing the cell.

Bcl-2-mediated drug resistance: inhibition of apoptosis by blocking nuclear factor of activated T lymphocytes (NFAT)-induced Fas ligand transcription

Bcl-2 inhibits apoptosis induced by a variety of stimuli, including chemotherapy drugs and glucocorticoids. It is generally accepted that Bcl-2 exerts its antiapoptotic effects mainly by dimerizing with proapoptotic members of the Bcl-2 family such as Bax and Bad. However, the mechanism of the antiapoptotic effects is unclear. Paclitaxel and other drugs that disturb microtubule dynamics kill cells in a Fas/Fas ligand (FasL)-dependent manner; antibody to FasL inhibits paclitaxel-induced apoptosis. We have found that Bcl-2 overexpression leads to the prevention of chemotherapy (paclitaxel)-induced expression of FasL and blocks paclitaxel-induced apoptosis. The mechanism of this effect is that Bcl-2 prevents the nuclear translocation of NFAT (nuclear factor of activated T lymphocytes, a transcription factor activated by microtubule damage) by binding and sequestering calcineurin, a calcium-dependent phosphatase that must dephosphorylate NFAT to move to the nucleus. Without NFAT nuclear translocation, the FasL gene is not transcribed. Thus, it appears that paclitaxel and other drugs that disturb microtubule function kill cells at least in part through the induction of FasL. Furthermore, Bcl-2 antagonizes drug-induced apoptosis by inhibiting calcineurin activation, blocking NFAT nuclear translocation, and preventing FasL expression. The effects of Bcl-2 can be overcome, at least partially, through phosphorylation of Bcl-2. Phosphorylated Bcl-2 cannot bind calcineurin, and NFAT activation, FasL expression, and apoptosis can occur after Bcl-2 phosphorylation.

Inhibition of virus-induced neuronal apoptosis by Bax

The Bax protein is widely known as a pro-apoptotic Bcl-2 family member that when overexpressed can trigger apoptosis in multiple cell types and is important for the developmental cell death of neurons. However, Bax was found here to be a potent inhibitor of neuronal cell death in mice infected with Sindbis virus. Newborn mice, which are highly susceptible to a fatal infection with neurotropic Sindbis virus, were significantly protected from neuronal apoptosis and fatal disease when infected with a recombinant Sindbis virus encoding Bax. Deletion of the N terminus of Bax, which mimics cleaved Bax, converted Bax into a pro-apoptotic factor in vivo. As mice mature during the first week after birth, they acquire resistance to a fatal Sindbis virus infection. However, Bax-deficient mice remained very sensitive to fatal disease compared with their control littermates, indicating that endogenous Bax functions as a survival factor and contributes to age-dependent resistance to Sindbis virus-induced mortality. The protective effects of Bax were reproduced in cultured hippocampal neurons but not in cultured dorsal root ganglia neurons. These findings indicate that cell-specific factors determine the anti-apoptotic versus pro-apoptotic function of Bax.

Decreased intracellular superoxide levels activate Sindbis virus-induced apoptosis

Infection of many cultured cell types with Sindbis virus (SV), an alphavirus, triggers apoptosis through a commonly utilized caspase activation pathway. However, the upstream signals by which SV activates downstream apoptotic effectors, including caspases, remain unclear. Here we report that in AT-3 prostate carcinoma cells, SV infection decreases superoxide (O-2) levels within minutes of infection as monitored by an aconitase activity assay. This SV-induced decrease in O-2 levels appears to activate or modulate cell death, as a recombinant SV expressing the O-2 scavenging enzyme, copper/zinc superoxide dismutase (SOD), potentiates SV-induced apoptosis. A recombinant SV expressing a mutant form of SOD, which has reduced SOD activity, has no effect. The potentiation of SV-induced apoptosis by wild type SOD is because of its ability to scavenge intracellular O-2 rather than its ability to promote the generation of hydrogen peroxide. Pyruvate, a peroxide scavenger, does not affect the ability of wild type SOD to potentiate cell death; and increasing the intracellular catalase activity via a recombinant SV vector has no effect on SV-induced apoptosis. Moreover, increasing intracellular O-2 by treatment of 3T3 cells with paraquat protects them from SV-induced death. Altogether, our results suggest that SV may activate apoptosis by reducing intracellular superoxide levels and define a novel redox signaling pathway by which viruses can trigger cell death.

Perspective: virus infections and the death of neurons

Neuronotropic viruses induce apoptosis in neurons, and Bcl-2-related anti-apoptotic proteins and caspase inhibitors decrease mortality from acute viral encephalitis. Infected neurons develop cytoplasmic blebbing characteristic of apoptosis, but a paucity of apoptotic nuclear changes potentially indicates unique aspects of virus-induced neuronal apoptosis that remain to be discovered.

Deletion of the loop region of Bcl-2 completely blocks paclitaxel-induced apoptosis

At high concentrations, the tubule poison paclitaxel is able to kill cancer cells that express Bcl-2; it inhibits the antiapoptotic activity of Bcl-2 by inducing its phosphorylation. To localize the site on Bcl-2 regulated by phosphorylation, mutant forms of Bcl-2 were constructed. Mutant forms of Bcl-2 with an alteration in serine at amino acid 70 (S70A) or with deletion of a 60-aa loop region between the alpha1 and alpha2 helices (Deltaloop Bcl-2, which also deletes amino acid 70) were unable to be phosphorylated by paclitaxel treatment of MDA-MB-231 cells into which the genes for the mutant proteins were transfected. The Deltaloop mutant completely inhibited paclitaxel-induced apoptosis. In cells expressing the S70A mutant, paclitaxel induced about one-third the level of apoptosis seen with wild-type Bcl-2. To evaluate the role of mitogen-activated protein kinases (MAPKs) in Bcl-2 phosphorylation, the activation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p38 was examined. Paclitaxel-induced apoptosis was associated with phosphorylation of Bcl-2 and activation of ERK and JNK MAPKs. If JNK activation was blocked by transfections with either a stress-activated protein kinase kinase dominant-negative (K-->R) gene (which prevents the activation of a kinase upstream of JNK) or MAPK phosphatase-1 gene (which dephosphorylates and inactivates JNK), Bcl-2 phosphorylation did not occur, and the cells were not killed by paclitaxel. By contrast, neither an ERK inhibitor (PD098059) nor p38 inhibitors (SB203580 and SB202190) had an effect on Bcl-2 phosphorylation. Thus, our data show that the antiapoptotic effects of Bcl-2 can be overcome by phosphorylation of Ser-70; forms of Bcl-2 lacking the loop region are much more effective at preventing apoptosis than wild-type Bcl-2 because they cannot be phosphorylated. JNK, but not ERK or p38 MAPK, appear to be involved in the phosphorylation of Bcl-2 induced by paclitaxel.

Suppression of steady-state, but not stimulus-induced NF-kappaB activity inhibits alphavirus-induced apoptosis

Recent studies have established cell type- specific, proapoptotic, or antiapoptotic functions for the transcription factor NF-kappaB. In each of these studies, inhibitors of NF-kappaB activity have been present before the apoptotic stimulus, and so the role of stimulus- induced NF-kappaB activation in enhancing or inhibiting survival could not be directly assessed. Sindbis virus, an alphavirus, induces NF-kappaB activation and apoptosis in cultured cell lines. To address whether Sindbis virus- induced NF-kappaB activation is required for apoptosis, we used a chimeric Sindbis virus that expresses a superrepressor of NF-kappaB activity. Complete suppression of virus-induced NF-kappaB activity neither prevents nor potentiates Sindbis virus-induced apoptosis. In contrast, inhibition of NF-kappaB activity before infection inhibits Sindbis virus-induced apoptosis. Our results demonstrate that suppression of steady-state, but not stimulus-induced NF-kappaB activity, regulates expression of gene products required for Sindbis virus-induced death. Furthermore, we show that in the same cell line, NF-kappaB can be proapoptotic or antiapoptotic depending on the death stimulus. We propose that the role of NF-kappaB in regulating apoptosis is determined by the death stimulus and by the timing of modulating NF-kappaB activity relative to the death stimulus.

Viral interference with apoptosis

Viruses have exploited a fascinating array of mechanisms to inhibit programed cell death. Different viruses tamper with cell suicide pathways by terminating cell death receptor signalling, mimicking cellular Bc1-2 functions, encoding protease inhibitors and other tactics. These stealth strategies are likely to be essential for the preservation of some viruses as it appears nearly impossible for a virus to enter a cell and set up housekeeping without setting off the alarm system that triggers activation of the cell death pathway. Thus, many viruses have developed strategies to subvert the cellular selfdestruct mechanisms, thereby providing suitable environments for long-term persistence, the establishment of latency or for progeny virus production. Investigation of these viral strategies has significantly advanced our knowledge of cellular death pathways and has impacted on our understanding of viral pathogenesis.

Modulation of cell death by Bcl-XL through caspase interaction

The caspases are cysteine proteases that have been implicated in the execution of programmed cell death in organisms ranging from nematodes to humans. Many members of the Bcl-2 family, including Bcl-XL, are potent inhibitors of programmed cell death and inhibit activation of caspases in cells. Here, we report a direct interaction between caspases and Bcl-XL. The loop domain of Bcl-XL is cleaved by caspases in vitro and in cells induced to undergo apoptotic death after Sindbis virus infection or interleukin 3 withdrawal. Mutation of the caspase cleavage site in Bcl-XL in conjunction with a mutation in the BH1 homology domain impairs the death-inhibitory activity of Bcl-XL, suggesting that interaction of Bcl-XL with caspases may be an important mechanism of inhibiting cell death. However, once Bcl-XL is cleaved, the C-terminal fragment of Bcl-XL potently induces apoptosis. Taken together, these findings indicate that the recognition/cleavage site of Bcl-XL may facilitate protection against cell death by acting at the level of caspase activation and that cleavage of Bcl-XL during the execution phase of cell death converts Bcl-XL from a protective to a lethal protein.

Sindbis virus induces apoptosis through a caspase-dependent, CrmA-sensitive pathway

Sindbis virus infection of cultured cells and of neurons in mouse brains leads to programmed cell death exhibiting the classical characteristics of apoptosis. Although the mechanism by which Sindbis virus activates the cell suicide program is not known, we demonstrate here that Sindbis virus activates caspases, a family of death-inducing proteases, resulting in cleavage of several cellular substrates. To study the role of caspases in virus-induced apoptosis, we determined the effects of specific caspase inhibitors on Sindbis virus-induced cell death. CrmA (a serpin from cowpox virus) and zVAD-FMK (N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone) inhibited Sindbis virus-induced cell death, suggesting that cellular caspases facilitate apoptosis induced by Sindbis virus. Furthermore, CrmA significantly increased the rate of survival of infected mice. These inhibitors appear to protect cells by inhibiting the cellular death pathway rather than impairing virus replication or by inhibiting the nsP2 and capsid viral proteases. The specificity of CrmA indicates that the Sindbis virus-induced death pathway is similar to that induced by Fas or tumor necrosis factor alpha rather than being like the death pathway induced by DNA damage. Taken together, these data suggest a central role for caspases in Sindbis virus-induced apoptosis.

Conversion of Bcl-2 to a Bax-like death effector by caspases

Caspases are a family of cysteine proteases implicated in the biochemical and morphological changes that occur during apoptosis (programmed cell death). The loop domain of Bcl-2 is cleaved at Asp34 by caspase-3 (CPP32) in vitro, in cells overexpressing caspase-3, and after induction of apoptosis by Fas ligation and interleukin-3 withdrawal. The carboxyl-terminal Bcl-2 cleavage product triggered cell death and accelerated Sindbis virus-induced apoptosis, which was dependent on the BH3 homology and transmembrane domains of Bcl-2. Inhibitor studies indicated that cleavage of Bcl-2 may further activate downstream caspases and contribute to amplification of the caspase cascade. Cleavage-resistant mutants of Bcl-2 had increased protection from interleukin-3 withdrawal and Sindbis virus-induced apoptosis. Thus, cleavage of Bcl-2 by caspases may ensure the inevitability of cell death.

Regulators of apoptosis on the road to persistent alphavirus infection

Alphavirus infection can trigger the host cell to activate its genetically programmed cell death pathway, leading to the morphological features of apoptosis. The ability to activate this death pathway is dependent on both viral and cellular determinants. The more virulent strains of alphavirus induce apoptosis with increased efficiency both in animal models and in some cultured cells. Although the immune system clearly plays a central role in clearing virus, the importance of other cellular factors in determining the outcome of virus infections are evident from the observation that mature neurons are better able to resist alphavirus-induced apoptosis than immature neurons are, both in culture and in mouse brains. These findings are consistent with the age-dependent susceptibility to disease seen in animals. Cellular genes that are known to regulate the cell death pathway can modulate the outcome of alphavirus infection in cultured cells and perhaps in animals. The cellular bax and bak genes, which are known to accelerate cell death, also accelerate virus-induced apoptosis. In contrast, inhibitors of apoptotic cell death such as bcl-2 suppress virus-induced apoptosis, which can facilitate a persistent virus infection. Thus, the balance of cellular factors that regulate cell death may be critical in virus infections. Additional viral factors also contribute to this balance. The more virulent strains of alphavirus have acquired the ability to induce apoptosis in mature neurons, while mature neurons are resistant to cell death upon infection with less virulent strains. Here we discuss a variety of cellular and viral factors that modulate the outcome of virus infection.

A single amino acid change in the E2 glycoprotein of Sindbis virus confers neurovirulence by altering an early step of virus replication

Amino acid changes in the envelope glycoproteins of Sindbis virus have been linked to neurovirulence; however, the molecular mechanisms by which these amino acid changes alter neurovirulence are not known. Recombinant-virus studies have mapped an important determinant of neurovirulence in adult mice to a single amino acid change, glutamine to histidine, at position 55 of the E2 glycoprotein (P. C. Tucker, E. G. Strauss, R. J. Kuhn, J. H. Strauss, and D. E. Griffin, J. Virol. 67:4605-4610, 1993). To investigate how histidine confers neurovirulence, we examined the various stages of the virus life cycle in neural (N18) and nonneural (BHK) cells. In BHK cells, recombinant viruses 633 (E255Q) and TE (E255H) replicated similarly. In contrast, in N18 neuroblastoma cells, TE established infection more efficiently, replicated faster, and achieved higher rates of virus release than did 633. Viral structural protein synthesis was similar in 633- and TE-infected BHK cells, while in N18 cells, structural protein synthesis was detected only in TE-infected cells at 6 h and remained higher for at least 16 h postinfection. Viral RNA synthesis was initiated more rapidly and was up to fivefold greater in TE- versus 633-infected N18 cells. Taken together with other data demonstrating minimal effects on virus binding and entry (P. C. Tucker, S. H. Lee, N. Bui, D. Martinie, and D. E. Griffin, J. Virol. 71:6106-6112, 1997), these data suggest that E2 position 55 plays an important role at early stages of infection of neural cells, thereby facilitating neurovirulence.

Herpesvirus saimiri encodes a functional homolog of the human bcl-2 oncogene

Here we demonstrate that open reading frame 16 (ORF16) of the oncogenic herpesvirus saimiri protects cells from heterologous virus-induced apoptosis. The BH1 and BH2 homology domains are highly conserved in ORF16, and ORF16 heterodimerizes with Bcl-2 family members Bax and Bak. However, ORF16 lacks the core sequence of the conserved BH3 homology domain, suggesting that this region is not essential for anti-apoptotic activity. Conservation of a functional bcl-2 homolog among gammaherpesviruses suggests that inhibition of programmed cell death is important in the biology of these viruses.

A Bcl-2 homolog encoded by Kaposi sarcoma-associated virus, human herpesvirus 8, inhibits apoptosis but does not heterodimerize with Bax or Bak

The Bcl-2 protein family is characterized by the ability to modulate cell death, and members of this family share two highly conserved domains called Bcl-2 homology 1 (BH1) and 2 (BH2) which have been shown to be critical for the death-repressor activity of Bcl-2 and Bcl-xL. Through sequence analysis we identified a novel viral Bcl-2 homolog, designated KSbcl-2, from human herpesvirus 8 (HHV8) or Kaposi sarcoma-associated herpesvirus. The overall amino acid sequence identity between KSbcl-2 and other Bcl-2 homologs is low (15-20%) but concentrated within the BH1 and BH2 regions. Overexpression of KSbcl-2 blocked apoptosis as efficiently as Bcl-2, Bcl-xL, or another viral Bcl-2 homolog encoded by Epstein-Barr virus, BHRF1. Interestingly, KS-bcl-2 neither homodimerizes nor heterodimerizes with other Bcl-2 family members, suggesting that KSbcl-2 may have evolved to escape any negative regulatory effects of the cellular Bax and Bak proteins. Furthermore, the herpesvirus Bcl-2 homologs including KSbcl-2, BHRF1, and ORF16 of herpesvirus saimiri contain poorly conserved Bcl-2 homology 3 (BH3) domains compared with other mammalian Bcl-2 homologs, implying that BH3 may not be essential for anti-apoptotic function. This is consistent with our observation that amino acid substitutions within the BH3 domain of Bcl-xL had no effect on its death-suppressor activity.

A conserved family of cellular genes related to the baculovirus iap gene and encoding apoptosis inhibitors

The baculovirus inhibitor of apoptosis gene, iap, can impede cell death in insect cells. Here we show that iap can also prevent cell death in mammalian cells. The ability of iap to regulate programmed cell death in widely divergent species raised the possibility that cellular homologs of iap might exist. Consistent with this hypothesis, we have isolated Drosophila and human genes which encode IAP-like proteins (dILP and hILP). Like IAP, both dILP and hILP contain amino-terminal baculovirus IAP repeats (BIRs) and carboxy-terminal RING finger domains. Human ilp encodes a widely expressed cytoplasmic protein that can suppress apoptosis in transfected cells. An analysis of the expressed sequence tag database suggests that hilp is one of several human genes related to iap. Together these data suggest that iap and related cellular genes play an evolutionarily conserved role in the regulation of apoptosis.

Bc1-2 protects mice against fatal alphavirus encephalitis

Virus-induced apoptosis has been well characterized in vitro, but the role of apoptosis in viral pathogenesis is not well understood. The suicide of a cell in response to viral infection is postulated to be an important host defense for the organism, leading to a reduction in its total viral burden. However, virus-induced death of nonregenerating cells in the central nervous system may be detrimental to the host. Therefore, to investigate the role of apoptosis in the pathogenesis of fatal encephalitis, we constructed a recombinant alphavirus chimera that expresses the antiapoptotic gene, bcl-2, in virally infected neural cells. Infection of neonatal mice with the alphavirus chimera expressing human bcl-2 [Sindbis virus (SIN)/bcl-2] resulted in a significantly lower mortality rate (7.5%) as compared with infection with control chimeric viruses containing a chloramphenicol acetyltransferase (CAT) reporter gene (SIN/CAT) (78.1%) or bcl-2 containing a premature stop codon (SIN/bcl-2stop) (72.1%) (P < 0.001). Viral titers were reduced 5-fold 1 day after infection and 10-fold 6 days after infection in the brains of SIN/bcl-2-infected mice as compared to SIN/CAT or SIN/bcl-2stop-infected mice. In situ end labeling to detect apoptotic nuclei demonstrated a reduction in the number of foci of apoptotic cells in the brains of mice infected with SIN/bcl-2 as compared with SIN/bcl-2stop. The reduction in apoptosis was associated with a reduction in the number of foci of cells expressing alphavirus RNA. Thus, the antiapoptotic gene, bcl-2, suppresses viral replication and protects against a lethal viral disease, suggesting an interaction between cellular genetic control of viral replication and cell death.

Alphavirus-induced apoptosis in mouse brains correlates with neurovirulence

Sindbis virus induces apoptotic cell death in cultured cell lines, raising the possibility that apoptosis of infected neurons and other target cells in vivo may contribute to the resulting disease and mortality. To investigate the role of apoptosis in Sindbis virus pathogenesis, infected mouse brains were assayed by the in situ terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling technique and for DNA ladder formation. Infection with recombinant Sindbis virus strain 633 resulted in widespread apoptosis in newborn mouse brains and spinal cords, but few apoptotic cells were observed following infection of 2-week-old animals. This finding correlates with the age-dependent mortality observed in mice. The more neurovirulent virus TE, which differs from 633 by a single amino acid in the E2 glycoprotein, induced significant apoptosis in brains and spinal cords of 2-week-old animals, consistent with its ability to cause fatal disease in older animals. Double-labeling experiments demonstrated that the apoptotic cells were also infected with Sindbis virus. Thus, Sindbis virus-induced apoptosis appears to be a result of virus infection and is likely to reflect pathogenic mechanisms for other viruses.

Bax-independent inhibition of apoptosis by Bcl-XL

The Bcl-2-related protein, Bcl-XL, has been shown to block apoptosis induced by a variety of stimuli and to be a stronger protector against apoptosis than Bcl-2 under certain circumstances. Using site-specific mutagenesis, we show here that the amino-acid residues critical for protection of cells by Bcl-XL against Sindbis virus-induced apoptosis are clustered within the Bcl-2-homology regions 1 and 2 (BH1 and BH2 regions). The residues necessary for Bcl-XL function are not identical to those required for Bcl-2 function. Although it has been suggested that heterodimerization between Bcl-XL and Bax is essential for the anti-death activity of Bcl-XL (refs 7,8), our results suggest that the interaction with Bax is not required for Bcl-XL to exert its death-repressing activity. Specific mutations that disrupt the ability of Bcl-XL to interact with Bax or Bak still preserve 70-80% of the anti-death activity of wild-type Bcl-XL.

BCL-2 but not its Epstein-Barr virus-encoded homologue, BHRF1, is commonly expressed in posttransplantation lymphoproliferative disorders

Posttransplantation lymphoproliferative disease (PTLD) is virtually always associated with Epstein-Barr virus (EBV) infection. BCL-2 and other proteins that confer resistance to apoptosis have been implicated in the pathogenesis of a variety of malignancies including lymphomas. One EBV protein, BHRF1, is a homologue of BCL-2, whereas another, the latency membrane protein 1 (LMP-1), upregulates BCL-2 expression in vitro. In the present study, we used immunohistochemistry to study the expression of these viral and cellular proteins as well as a variety of other EBV-encoded proteins in PTLD. BHRF1 was not detected in any PTLD specimen, whereas BCL-2 was shown in 12 of 17 lesions examined. With one exception, all LMP1-positive cases also expressed BCL-2 and the absence of LMP1 was always associated with a lack of BCL-2 expression. The results do not support a role for the EBV homologue of BCL-2 in PTLD, but they do support a role for viral induction of BCL-2 expression.

Anti-apoptotic genes of baculoviruses

Baculoviruses possess two different classes of genes with anti-apoptptic activity: p35 and iap. The p35 gene product (P35) is able to block apoptosis induced by a variety of stimuli in phylogenetically diverse organisms. P35 has recently been shown to be capable of inhibiting the ICE/ced-3 family of cysteine proteases, a family of enzymes which are implicated in cell death and which exhibit specificity for cleavage at aspartate residues. The products of the iap genes are a distinct class of proteins containing a carboxyl ring finger and tandem duplications of a unique motif known as the BIR motif. Homologues of the baculovirus iap genes have been identified in the human genome. Both classes of baculovirus anti-apoptotic genes will continue to be important tools in defining the pathways involved in apoptosis. Since our demonstration in 1991 that a baculovirus prevents host cells from undergoing apoptosis by expressing a gene known as p35(Clem et al., 1991), the study of baculovirus-induced apoptosis and the anti-apoptotic genes they possess has led to discoveries with far-reaching implications for viral pathogenesis, human disease, and the study of cell death. It is now known that a variety of eukaryotic viruses encode genes which allow them to control cellular apoptosis. Understanding the mechanism(s) by which these viral gene products act provides fundamental insights into the pathways regulating apoptosis. In this review, we discuss the inhibition of apoptosis by baculoviruses, concentrating mainly on the nature and mechanism of action of the two classes of baculovirus genes, p35 and iap, which are able to control apoptosis in a diversity ofeukaryotes.

Thiol agents and Bcl-2 identify an alphavirus-induced apoptotic pathway that requires activation of the transcription factor NF-kappa B

Oxidative stress has been proposed as a common mediator of apoptotic death. To investigate further the role of oxidants in this process we have studied the effects of antioxidants on Sindbis virus (SV)-induced apoptosis in two cell lines, AT-3 (a prostate carcinoma line) and N18 (a neuroblastoma line). The thiol antioxidant, N-acetylcysteine (NAC), at concentrations above 30 mM, completely abrogates SV-induced apoptosis in AT-3 and N18 cells. The effects of NAC cannot be attributed to inhibition of viral entry or viral replication, changes in extracellular osmolarity or to increases in cellular glutathione levels, nor can they be mimicked by chelators of trace metals, inhibitors of lipid peroxidation or peroxide scavengers. In contrast, other thiol agents including pyrrolidine dithiocarbamate (PDTC, 75 microM) are protective. Because NAC and PDTC are among the most effective inhibitors of the transcription factor NF-kappa B, we examined SV's ability to activate NF-kappa B before the onset of morphologic or biochemical evidence of apoptosis. Within hours of infection, SV induced a robust increase in nuclear NF-kappa B activity in AT-3 and N18 cells; this activation was suppressible by NAC and PDTC. Over-expression of bcl-2 in AT-3 cells, which has been shown to inhibit SV-induced apoptosis, also inhibits SV-induced NF-kappa B activation. To determine if NF-kappa B activation is necessary for SV-induced apoptosis in these cells, we used double stranded oligonucleotides with consensus NF-kappa B sequences as transcription factor decoys (TFDs) to inhibit NF-kappa B binding to native DNA sites. Wild-type, but not mutant, TFDs inhibit SV-induced apoptosis in AT-3 cells. In contrast, TFD inhibition of NF-kappa B nuclear activity in N18 cells did not prevent SV-induced apoptosis. Taken together, these observations define a cell type-specific, transcription factor signaling pathway necessary for SV-induced apoptosis. Understanding the precise mechanism by which Bcl-2 and thiol agents inhibit SV-induced nuclear NF-kappa B activity in AT-3 cells may provide insights into the pluripotent antiapoptotic actions of these agents.

Apoptosis as a cause of death in measles virus-infected cells

To determine the mechanism of measles virus-induced cell death, we studied the infection of Vero cells and monocytic cell lines with wild-type (Chicago-1) and vaccine (Edmonston) strains of measles virus. DNA fragmentation indicative of apoptosis was apparent by flow cytometry, agarose gel electrophoresis, and electron microscopy. Within syncytia, DNA strand breaks were demonstrated by end labeling with terminal transferase and then by visualization.

Visna virus Tat protein: a potent transcription factor with both activator and suppressor domains

Visna virus is a pathogenic lentivirus of sheep tat is distantly related to the primate lentiviruses, including human immunodeficiency virus type 1. The visna virus genome encodes a small regulatory protein, Tat, which is necessary for efficient viral replication and enhanced viral transcription. To investigate the mechanism of action of the visna Tat protein and to localize the protein domain(s) responsible for transcriptional activation, chimeric proteins containing visna virus Tat sequences fused to the DNA binding domain of the yeast transactivation factor GAL4 (residues 1 to 147) were made. The GAL4-Tat fusion proteins were transfected into cells and tested for the ability to activate the adenovirus E1b promoter via upstream GAL4 DNA binding sites. Full-length GAL4-Tat fusion proteins were weak transactivators in this system, giving only a two- to fourfold increase in transcription in several cell types, including HeLa and sheep choroid plexus cells. In contrast, fusion of the N-terminal region of the Tat protein to GAL4 revealed a potent activation domain. Amino acids 13 to 38 appeared to be the most critical for activation. No other region of the protein showed any activation in the GAL4 system. This N-terminal region of the visna virus Tat protein has a large number of acidic and hydrophobic residues, suggesting that Tat has an acidic activation domain common to many transcriptional transactivators. Mutations in hydrophobic and bulky aromatic residues dramatically reduced the activity of the chimeric protein. Competition experiments suggest that mechanism of the visna virus Tat activation domain may closely resemble that of the herpesvirus activator VP16 and human immunodeficiency virus Tat, a related lentivirus activator, since both significantly reduce the level of visna virus Tat activation. Finally, a domain between residues 39 and 53 was identified in the Tat protein that, in the GAL4 system, negatively regulates activation by Tat.