Viral inhibitors of apoptosis

Avian reoviruses cause apoptosis in cultured cells: viral uncoating, but not viral gene expression, is required for apoptosis induction

The cytopathic effect evidenced by cells infected with avian reovirus S1133 suggests that this virus may induce apoptosis in primary cultures of chicken embryo fibroblasts. In this report we present evidence that avian reovirus infection of cultured cells causes activation of the intracellular apoptotic program and that this activation takes place during an early stage of the viral life cycle. The ability of avian reoviruses to induce apoptosis is not restricted to a particular virus strain or to a specific cell type, since different avian reovirus isolates were able to induce apoptosis in several avian and mammalian cell lines. Apoptosis was also provoked in ribavirin-treated avian reovirus-infected cells and in cells infected with UV-irradiated reovirions, indicating that viral mRNA synthesis and subsequent steps in viral replication are not needed for apoptosis induction in avian reovirus-infected cells and that the number of inoculated virus particles, not their infectivity, is the critical factor for apoptosis induction by avian reovirus. Our finding that apoptosis is no longer induced when intracellular viral uncoating is blocked indicates that intraendosomal virion disassembly is required for apoptosis induction and that attachment and uptake of parental reovirions are not sufficient to cause apoptosis. Taken together, our results suggest that apoptosis is triggered from within the infected cell by viral products generated after intraendosomal uncoating of parental reovirions.

Direct cleavage of the human DNA fragmentation factor-45 by granzyme B induces caspase-activated DNase release and DNA fragmentation

The protease granzyme B (GrB) plays a key role in the cytocidal activity during cytotoxic T lymphocyte (CTL)-mediated programmed cell death. Multiple caspases have been identified as direct substrates for GrB, suggesting that the activation of caspases constitutes an important event during CTL-induced cell death. However, recent studies have provided evidence for caspase-independent pathway(s) during CTL-mediated apoptosis. In this study, we demonstrate caspase-independent and direct cleavage of the 45 kDa unit of DNA fragmentation factor (DFF45) by GrB both in vitro and in vivo. Using a novel and selective caspase-3 inhibitor, we show the ability of GrB to process DFF45 directly and mediate DNA fragmentation in the absence of caspase-3 activity. Furthermore, studies with DFF45 mutants reveal that both caspase-3 and GrB share a common cleavage site, which is necessary and sufficient to induce DNA fragmentation in target cells during apoptosis. Together, our data suggest that CTLs possess alternative mechanism(s) for inducing DNA fragmentation without the requirement for caspases.

Role of programmed cell death in development

Programmed cell death (PCD) is an integral part of both animal and plant development. In animals, model systems such as Caenorhabditis elegans, Drosophila melanogaster, and mice have shown a general cell death profile of induction, caspase mediation, cell death, and phagocytosis. Tremendous strides have been made in cell death research in animals in the past decade. The ordering of the C. elegans genes Ced-3, 4 and 9, identification of caspase-activated DNase that degrades nuclear DNA during PCD, identification of signal transduction modules involving caspases as well as the caspase-independent pathway, and the involvement of mitochondria are some of the findings of immense value in understanding animal PCDs. Similarly, the caspase inactivation mechanisms of infecting viruses to stall host cell death give a new dimension to the viral infection process. However, plant cell death profiles provide an entirely different scenario. The presence of a cell wall that cannot be phagocytosed, absence of the hallmarks of animal PCDs such as DNA laddering, formation of apoptotic bodies, a cell-death-specific nuclease, a biochemical machinery of killer enzymes such as caspases all point to novel ways of cell elimination. Large gaps in our understanding of plant cell death have prompted speculative inferences and comparisons with animal cell death mechanisms. This paper deals with both animals and plants for a holistic view on cell death in eukaryotes.

[Apoptosis--what is it? Significance in coronary heart disease and myocardial infarct]

Apoptosis is a physiological, highly conserved program of cellular suicide, characterized by nuclear condensation with DNA-fragmentation, by alterations in the distribution of cell membrane phospholipids, and by cellular shrink-age. Apoptotic cellular remnants engulfed by cell membranes are phagocytized largely without activation of inflammatory reactions. The apoptotic program is executed by a cascade of highly specific caspases, activated by complexation of initiatorcaspases in cytosolic signalling complexes at receptors of the TNF family or at impaired mitochondria. In many forms of cellular stress with damage of nuclear DNA and mitochondria, mixed forms of cell death are triggered with regulated activation of the apoptotic program and concomitantly, with induction of catastrophic necrosis. Such a mixed form of myocyte death is observed in myocardial ischemia and reperfusion. Antiapoptotic interventions can delay ischemic myocardial damage in experiments. Therefore, those interventions appear conceivable as future strategy for acutely enhancing the available time interval for therapeutic reperfusion. However, chronic inhibition of apoptosis for ongoing prevention of myocardial ischemic damage may not become a plausible strategy because of disturbances of the immune system, because of putatively infavorable effects on arteriosclerotic lesions and because of likely disturbances in the physiologic elimination of damaged mitochondria.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

STAT1 is inactivated by a caspase

Apoptosis involves the activation of a cascade of interleukin-1beta converting enzyme-like proteases (caspases), a group of cysteine proteases related to the prototype interleukin-1beta-converting enzyme (caspase-1). These proteases cleave specific intracellular targets such as poly(ADP-ribose) polymerase, DNA-dependent protein kinase, and nuclear lamins. We show here that apoptosis can be induced by double-stranded RNA. The induction of apoptosis by double-stranded RNA and other agents leads to the cleavage by a caspase of the signal transducer and activator of transcription factor, STAT1 which is pivotal in the signal transduction pathways of the interferons and many other cytokines and growth factors. The product of this cleavage is no longer able to mediate interferon-activated signal transduction and the cleavage event may play a role in regulating the apoptosis response itself.