Protease activation during apoptosis: death by a thousand cuts?

The apoptosis pathway triggered by the interferon-induced protein kinase PKR requires the third basic domain, initiates upstream of Bcl-2, and involves ICE-like proteases

The interferon-induced double-stranded RNA-dependent protein kinase (PKR) is a serine/threonine kinase which exerts antiviral and anticellular functions. The antiviral effect of PKR is mediated by the phosphorylation of the alpha subunit of the translational initiation factor elF-2 alpha, while it is not known whether the anticellular effect is due to phosphorylation of elF-2 alpha, l kappa B, or other unknown substrates. We have previously shown that activation of PKR during infection of cells with a vaccinia virus recombinant expressing the wild-type kinase resulted in a complete inhibition of viral and cellular protein synthesis and in the induction of apoptosis. Here, we report that expression of the human proto-oncogene bcl-2 blocks PKR-induced apoptosis but not PKR-induced inhibition of translation. In addition, PKR-induced apoptosis resulted in a cleavage of the death substrate poly(ADP-ribose) polymerase (PARP). Moreover, induction of apoptosis by PKR was not observed with a mutant lacking the third basic region (aa 234-272). Taken together, these results suggest that the third basic region of PKR is required for PKR-induced apoptosis, the process is initiated upstream of bcl-2 and involves activation of a cellular protease, CPP32, or its family members that cleave PARP.

Specific cleavage of the large subunit of replication factor C in apoptosis is mediated by CPP32-like protease

Recent evidence suggests that the growing family of cysteine proteases related to the interleukin-1beta-converting enzyme (ICE) is of central importance in mediating apoptosis. Proteolytic cleavage of a small group of cellular substrates by these enzymes in association with the onset of apoptosis has been reported. In the present study, we searched a protein data base for potential death substrates possessing the CPP32 cleavage site, DEVD, and identified several candidates including RFC140, the large subunit of replication factor C, which we subsequently demonstrated to be specifically cleaved in a variety of cell types undergoing apoptosis in response to different cytotoxic agents, whereas no degradation is observed in a cell line resistant to etoposide-induced apoptosis. The abrogation of RFC140 cleavage in apoptotic extracts by Ac-DEVD-CHO, a potent inhibitor of CPP32, together with the finding that a CPP32 consensus cleavage sequence, DEVD, exists in RFC140, suggests that CPP32 or a close relative is responsible for RFC140 degradation in apoptosis.

Activation of CPP32 during apoptosis of neurons and astrocytes

Members of the interleukin-1 beta-converting enzyme (ICE)/CED-3 protease family have been implicated in apoptosis in both vertebrates and invertebrates. Using primary culture methods, we report that neurons and astrocytes require the activity of the ICE/CED-3 family of proteases to undergo apoptosis induced by staurosporine, ceramide, and serum-free media. We show that specific inhibitors of ICE/CED-3 proteases can inhibit apoptosis and that cytosolic fractions from apoptosing neurons, but not healthy cells, induced apoptosis in a cell-free system. Cell extracts from neurons induced to undergo apoptosis contained ICE/ CED-3 protease activity. To determine which member of the ICE/CED-3 family was activated in neurons and astrocytes during apoptosis, we developed a novel affinity-labeling technique that labeled the active site cysteine and identified a 17-kDa subunit of the activated protease. The affinity-labeled 17-kDa protease subunit shares antigenic and molecular mass identity with the processed form of CPP32 on immunoblots, suggesting that CPP32 may be the principal effector in the apoptotic pathway in neurons and astrocytes. In time-course experiments, the activation of CPP32 preceded the detection of PARP cleavage and DNA laddering, suggesting that processing of CPP32 is a very early event in apoptosis of neurons and astrocytes and may be involved in the proteolytic action on specific cellular targets. The affinity-labeling technique developed and used in this report with neural cells allows for the sensitive detection, purification, and identification of ICE/CED-3 proteases that may be activated in other cells types under a variety of conditions, including certain diseased states.

Degradation of retinoblastoma protein in tumor necrosis factor- and CD95-induced cell death

The product of the retinoblastoma susceptibility gene, RB, is a negative regulator of cell proliferation. Inactivation of RB does not interfere with embryonic growth or differentiation. However, Rb-deficient embryos show abnormal degeneration of neurons and lens fiber cells through apoptosis, suggesting that RB may protect against programmed cell death. Consistent with this notion, RB is found to be degraded in tumor necrosis factor (TNF)- and CD95-induced death. A consensus caspase cleavage site at the C terminus of RB is cleaved in vitro and in vivo by proteases related to CPP32 (caspase 3). Mutation of the consensus cleavage site generates a cleavage-resistant RB which is not degraded during cell death. Expression of this non-degradable RB is found to antagonize the cytotoxic effects of TNF in Rb-/- 3T3 cells, but this mutant RB cannot attenuate the rapid death induced by anti-CD95 in Jurkat/T cells. These results show that RB is a target of the caspase family of proteases during cell death and suggest that the failure to degrade RB can attenuate the death response toward some but not all death inducers.

Caspase-1 processes IFN-gamma-inducing factor and regulates LPS-induced IFN-gamma production

Interferon-gamma-inducing factor (IGIF, interleukin-18) is a recently described cytokine that shares structural features with the interleukin-1 (IL-1) family of proteins and functional properties with IL-12. Like IL-12, IGIF is a potent inducer of interferon (IFN)-gamma from T cells and natural killer cells. IGIF is synthesized as a biologically inactive precursor molecule (proIGIF). The cellular production of IL-1beta, a cytokine implicated in a variety of inflammatory diseases, requires cleavage of its precursor (proIL-1beta) at an Asp-X site by interleukin-1beta-converting enzyme (ICE, recently termed caspase-1). The Asp-X sequence at the putative processing site in proIGIF suggests that a protease such as caspase-1 might be involved in the maturation of IGIF. Here we demonstrate that caspase-1 processes proIGIF and proIL-1beta with equivalent efficiencies in vitro. A selective caspase-1 inhibitor blocks both lipopolysaccharide-induced IL-1beta and IFN-gamma production from human mononuclear cells. Furthermore, caspase-1-deficient mice are defective in lipopolysaccharide-induced IFN-gamma production. Our results thus implicate caspase-1 in the physiological production of IGIF and demonstrate that it plays a critical role in the regulation of multiple proinflammatory cytokines. Specific caspase-1 inhibitors would provide a new class of anti-inflammatory drugs with multipotent action.

Interleukin 1 beta-converting enzyme related proteases/caspases are involved in TRAIL-induced apoptosis of myeloma and leukemia cells

The Fas/APO-1/CD95 ligand (CD95L) and the recently cloned TRAIL ligand belong to the TNF-family and share the ability to induce apoptosis in sensitive target cells. Little information is available on the degree of functional redundancy between these two ligands in terms of target selectivity and intracellular signalling pathway(s). To address these issues, we have expressed and characterized recombinant mouse TRAIL. Specific detection with newly developed rabbit anti-TRAIL antibodies showed that the functional TRAIL molecule released into the supernatant of recombinant baculovirus-infected Sf9 cells is very similar to that associated with the membrane fraction of Sf9 cells. CD95L resistant myeloma cells were found to be sensitive to TRAIL, displaying apoptotic features similar to those of the CD95L- and TRAIL-sensitive T leukemia cells Jurkat. To assess if IL-1beta-converting enzyme (ICE) and/or ICE-related proteases (IRPs) (caspases) are involved in TRAIL-induced apoptosis of both cell types, peptide inhibition experiments were performed. The irreversible IRP/caspase-inhibitor Ac-YVAD-cmk and the reversible IRP/caspase-inhibitor Ac-DEVD-CHO blocked the morphological changes, disorganization of plasma membrane phospholipids, DNA fragmentation, and loss of cell viability associated with TRAIL-induced apoptosis. In addition, cells undergoing TRAIL-mediated apoptosis displayed cleavage of poly(ADP)-ribose polymerase (PARP) that was completely blocked by Ac-DEVD-CHO. These results indicate that TRAIL seems to complement the activity of the CD95 system as it allows cells, otherwise resistant, to undergo apoptosis triggered by specific extracellular ligands. Conversely, however, induction of apoptosis in sensitive cells by TRAIL involves IRPs/caspases in a fashion similar to CD95L. Thus, differential sensitivity to CD95L and TRAIL seems to map to the proximal signaling events associated with receptor triggering.

Bik and Bak induce apoptosis downstream of CrmA but upstream of inhibitor of apoptosis

Recent studies have identified a number of cell death pathway components. In this study, we describe the role that two such components, Bik and Bak, play in initiating the apoptotic program. These Bcl-2 family members engage the death pathway downstream of the block imposed by the serpin CrmA, but upstream of the block initiated by cellular inhibitors of apoptosis, which are a family of molecules characterized by a conserved baculovirus inhibitor of apoptosis repeat motif. Distal death pathway components activated by Bik and Bak are similar to those activated by the CD-95 (Fas/Apo1) and tumor necrosis factor death receptors.

The significance of shed membrane particles during programmed cell death in vitro, and in vivo, in HIV-1 infection

The plasma membrane remodeling, including the early transverse redistribution of phosphatidylserine, is a general feature occurring in cells in which a death program has been induced. In most cases, studies of this kind have focused mainly on cells. In this study, we report a clear correlation between the degree of apoptosis induced by a variety of agents in several types of cultured cells and the amount of shed membrane microparticles captured in the corresponding supernatants by insolubilized annexin V, a protein showing a strong affinity for phosphatidylserine. Such particles carry membrane antigens specific of the cells they stem from, and through which capture is also feasible. Homologous circulating microparticles were captured in peripheral blood from individuals with HIV-1 infection. A substantial proportion bore CD4 antigen. In some cases, CD4+ particles could be detected even in the absence of circulating CD4+ T cells, testifying to the presence of such resident cells in lymphoid tissues. These results suggest that shed membrane particles are one of the hallmarks of programmed cell death, of particular interest when the corresponding cells are hardly accessible.

Is programmed cell death required for neural tube closure?

Programmed cell death (PCD) plays an important part in animal development. It is responsible for eliminating the cells between developing digits, for example, and is involved in hollowing out solid structures to create cavities (reviewed in [1] [2]). There are many cases, however, where PCD occurs in developing tissues but its function is unknown. Important examples are seen during the folding, pinching off, and fusion of epithelial sheets during vertebrate morphogenesis, as in the formation of the neural tube and lens vesicle [2]; PCD is an invariable accompaniment to these processes, but it is unclear whether it is required for the processes to occur or is just an unavoidable consequence of them. There is increasing evidence that PCD in animals is mediated by a family of cysteine proteases, known as caspases, which are thought to act in a proteolytic cascade, cleaving one another and key intracellular proteins to kill the cell in a controlled way [3] [4]. Inhibitors of caspases are, therefore, potential tools for studying the roles of PCD during animal development [5] [6]. Here, we show that peptide caspase inhibitors block neural tube closure in explanted chick embryos, suggesting that PCD is required for this crucial developmental process.

Actin is cleaved during constitutive apoptosis

Proteases play an important role in the programme of cell death by apoptosis but little is known of the substrates cleaved, particularly in constitutive models of this type of cell death. Neutrophils spontaneously undergo apoptosis in culture without requiring external stimuli. During this process we found biochemical and immunochemical evidence for the cleavage of membrane-associated actin, a component of the cytoskeleton that links polymerized actin to the plasma membrane. Cleavage occurred at a single site at the N-terminus, between residues Val43-Met44, a site devoid of a consensus motif for cleavage by cysteine proteases of the interleukin-1beta-converting enzyme (ICE)-family. Whereas actin cleavage and nuclear/cell surface markers of apoptosis were co-ordinately diminished by zVAD-fmk, an inhibitor of the ICE-like family of proteases, only acetyl-leucyl-leucylnormethional, an inhibitor of calpains, was capable of completely inhibiting actin cleavage. Our results suggest that actin is not a direct substrate for the ICE-like family of proteases. By disabling the cytoskeleton, actin cleavage may be an important component in the capacity of apoptosis to reduce the injurious potential of neutrophils.

Mouse vaginal opening is an apoptosis-dependent process which can be prevented by the overexpression of Bcl2

In the mouse, opening of the vaginal cavity to the skin is a late event, occurring around the fifth week of life; it can be induced in sexually immature mice by beta-estradiol injections. We have generated two lines of transgenic mice expressing the human Bcl2 protein in a variety of tissues. The vaginal cavity of the transgenic females remained permanently closed, a condition completely resistant to beta-estradiol injections; this was accompanied by a considerable distension of the genital tract. Histologic studies of vaginal sections at the time of opening to the skin in normal mice showed, by the TUNEL method which detects nuclei with fragmented DNA characteristic of apoptosis, that this event coincides with extensive apoptosis in the lower part of the vaginal mucosa, a process prevented in the bcl2 transgenic mice, which express Bcl2 in suprabasal epithelial cells and in subepithelial cells of the vaginal mucosa. In contrast, two lines of mice bearing a Bcl2 transgene placed under the control of a K10 keratin promoter, whose expression is restricted to the suprabasal layers of the epidermis, had a normal phenotype. Eyelids' formation and opening of the external ear canals, which also occur after birth in the mouse, were not altered in any of these transgenic lines; histological study of eye and ear sections at the time of these events failed to detect apoptosis. In conclusion, the tissue remodeling required to complete maturation of the mouse female genital tract at the time of puberty is an hormonally triggered apoptosis-dependent process.

Differentiation to an NGF-dependent state and apoptosis following NGF removal both occur asynchronously in cultures of PC12 cells

Long-term timelapse videomicroscopy was used to investigate the relationships and transitions between mitosis, differentiation, and apoptosis in cultures of NGF-differentiated PC12 cells. After 4 days in NGF, cultures were at an early stage of neuronal differentiation. Removal of NGF led to an appreciable increase in apoptosis with no effect on the relatively high mitotic rate. After 7 days in NGF, cells were more neuronal; NGF withdrawal again resulted in no change in the low mitotic rate but an even greater increase in apoptosis, eventually leading to considerable net loss of cells. After 10 days, cells were terminally differentiated; removal of NGF did not affect the negligible mitotic rate but induced a dramatic increase in apoptosis resulting in death of most of the cells. Apoptosis in the fraction of cells that had become NGF-dependent followed a similar timecourse and was characterized by the same morphology at all three differentiation states. Thus, acquisition of NGF-dependence in PC12 cultures seemed to be the result of a steadily increasing percentage of cells that had each undergone a relatively rapid transition to a postmitotic, NGF-sensitive state. These studies were also helpful for elucidating the timing of apoptosis. Onset of apoptosis was markedly asynchronous within a culture, but the active, blebbing phase, once initiated, always lasted about 45 min, regardless of differentiation state or time spent without NGF. Thus, the active phase might represent a conserved sequence of events that every cell must ultimately undergo before apoptotic death.

Activation of multiple interleukin-1beta converting enzyme homologues in cytosol and nuclei of HL-60 cells during etoposide-induced apoptosis

Recent genetic and biochemical studies have implicated cysteine-dependent aspartate-directed proteases (caspases) in the active phase of apoptosis. In the present study, three complementary techniques were utilized to follow caspase activation during the course of etoposide-induced apoptosis in HL-60 human leukemia cells. Immunoblotting revealed that levels of procaspase-2 did not change during etoposide-induced apoptosis, whereas levels of procaspase-3 diminished markedly 2-3 h after etoposide addition. At the same time, cytosolic peptidase activities that cleaved DEVD-aminotrifluoromethylcoumarin and VEID-aminomethylcoumarin increased 100- and 20-fold, respectively; but there was only a 1. 5-fold increase in YVAD-aminotrifluoromethylcoumarin cleavage activity. Affinity labeling with N-(Nalpha-benzyloxycarbonylglutamyl-Nepsilon-biotin yllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy]methyl ketone indicated that multiple active caspase species sequentially appeared in the cytosol during the first 6 h after the addition of etoposide. Analysis on one- and two-dimensional gels revealed that two species comigrated with caspase-6 and three comigrated with active caspase-3 species, suggesting that several splice or modification variants of these enzymes are active during apoptosis. Polypeptides that comigrate with the cytosolic caspases were also labeled in nuclei of apoptotic HL-60 cells. These results not only indicate that etoposide-induced apoptosis in HL-60 cells is accompanied by the selective activation of multiple caspases in cytosol and nuclei, but also suggest that other caspase precursors such as procaspase-2 are present but not activated during apoptosis.

Isolation and identification of a proteinase from calf thymus that cleaves poly(ADP-ribose) polymerase and histone H1

A proteinase was isolated from calf thymus that degraded pADPRT, histone H1 and alpha-casein in a Ca(2+)-dependent manner. In a five-step procedure, a homogenous proteinase was obtained with a subunit structure of 80 and 30 kDa. The amino-acid homology of an internal sequence as well as kinetic and inhibitor assays identified the proteinase as calpain I. It is suggested that even though the general substrate alpha-casein is widely used for the assaying of calpains, more appropriately physiological cellular components (pADPRT and histone H1) specify the thymus proteinase.

Stress-induced apoptosis and the sphingomyelin pathway

The sphingomyelin pathway is a ubiquitous, evolutionarily conserved signaling system initiated by hydrolysis of the plasma membrane phospholipid sphingomyelin to generate the second messenger ceramide. Sphingomyelin degradation is catalyzed by acid and neutral sphingomyelinase (SMase) isoforms. Most, if not all mammalian cells, appear capable of signaling though the sphingomyelin pathway. Diverse receptor types and environmental stresses utilize the sphingomyelin pathway as a downstream effector system. In some cellular systems, ceramide initiates differentiation or cell proliferation, while in other systems, ceramide signals apoptosis. Recent investigations link the activation of neutral SMase to the extracellular signal regulated kinase (ERK) cascade and pro-inflammatory responses, and acid SMase to the stress-activated protein kinase/c-jun kinase (SAPK/JNK) cascade and apoptotic responses. Environmental stresses act directly on membrane to activate acid pH-dependent sphingomyelinase (ASMase), whereas cytokine receptors signal ASMase activation through motifs termed death domains. The present review focuses on mechanisms of activation of ASMase and on ceramide signaling of the apoptotic response.

Fas-associated death domain protein interleukin-1beta-converting enzyme 2 (FLICE2), an ICE/Ced-3 homologue, is proximally involved in CD95- and p55-mediated death signaling

The pivotal discovery that Fas-associated death domain protein (FADD) interleukin-1beta-converting enzyme (FLICE)/MACH was recruited to the CD95 signaling complex by virtue of its ability to bind the adapter molecule FADD established that this protease has a role in initiating the death pathway (Boldin, M. P., Goncharov, T. M. , Goltsev, Y. V., and Wallach, D. (1996) Cell 85, 803-815; Muzio, M., Chinnaiyan, A. M., Kischkel, K. C., O'Rourke, K., Shevchenko, A., Ni, J., Scaffidi, C., Bretz, J. D., Zhang, M., Gentz, R., Mann, M., Krammer, P. H., Peter, M. E., and Dixit, V. M. (1996) Cell 85, 817-827). In this report, we describe the cloning and characterization of a new member of the caspase family, a homologue of FLICE/MACH, and Mch4. Since the overall architecture and function of this molecule is similar to that of FLICE, it has been designated FLICE2. Importantly, the carboxyl-terminal half of the small catalytic subunit that includes amino acids predicted to be involved in substrate binding is distinct. We show that the pro-domain of FLICE2 encodes a functional death effector domain that binds to the corresponding domain in the adapter molecule FADD. Consistent with this finding, FLICE2 is recruited to both the CD95 and p55 tumor necrosis factor receptor signaling complexes in a FADD-dependent manner. A functional role for FLICE2 is suggested by the finding that an active site mutant of FLICE2 inhibits CD95 and tumor necrosis factor receptor-mediated apoptosis. FLICE2 is therefore involved in CD95 and p55 signal transduction.

Granzyme B (GraB) autonomously crosses the cell membrane and perforin initiates apoptosis and GraB nuclear localization

Granzyme B (GraB) induces apoptosis in the presence of perforin. Perforin polymerizes in the cell membrane to form a nonspecific ion pore, but it is not known where GraB acts to initiate the events that ultimately lead to apoptosis. It has been hypothesized that GraB enters the target cell through a perforin channel and then initiates apoptosis by cleaving and activating members of the ICE/Ced-3 family of cell death proteases. To determine if GraB can enter the cell, we treated YAC-1 or HeLa cells with FITC-labeled GraB and measured intracellular fluorescence with a high sensitivity CCD camera and image analyzer. GraB was internalized and found diffusely dispersed in the cell cytoplasm within 10 min. Uptake was inhibited at low temperature (4 degrees C) and by pretreatment with metabolic inhibitors, NaF and DNP, or cytochalasin B, a drug that both blocks microfilament formation, and FITC-GraB remained on the cell membrane localized in patches. With the simultaneous addition of perforin and FITC-GraB, no significant increase in cytoplasmic fluorescence was observed over that found in cells treated only with FITC-GraB. However, FITC-GraB was now detected in the nucleus of apoptotic cells labeling apoptotic bodies and localized areas within and along the nuclear membrane. The ability of GraB to enter cells in the absence of perforin was reexamined using anti-GraB antibody immunogold staining of ultrathin cryosections of cells incubated with GraB. Within 15 min, gold particles were detected both on the plasma membrane and in the cytoplasm of cells with some gold staining adjacent to the nuclear envelope but not in the nucleus. Cells internalizing GraB in the absence of perforin appeared morphologically normal by Hoechst staining and electron microscopy. GraB directly microinjected into the cytoplasm of B16 melanoma cells induced transient plasma membrane blebbing and nuclear coarsening but the cells did not become frankly apoptotic unless perforin was added. We conclude that GraB can enter cells autonomously but that perforin initiates the apoptotic process and the entry of GraB into the nucleus.

Sensitization to Fas-mediated apoptosis by hepatitis C virus core protein

We have characterized viral-cell interactions of hepatitis C virus (HCV) and liver cells to study the pathogenesis of HCV infection. HepG2 cells constitutively expressing HCV core protein showed apoptotic changes in response to stimulation with anti-Fas monoclonal antibody. Cells treated with the antibody showed extensive cell rounding, shrinkage, and cytoplasmic blebbing and finally detached from plates. Fragmentation of the chromatin was observed in the nucleus and DNA ladders were detected. In contrast, cells expressing HCV envelope, nonstructural proteins or normal HepG2 cells did not exhibit such Fas-mediated apoptosis. However, expression of Fas receptor was not upregulated on the surface of the cells expressing HCV core protein. Apoptotic cell death was prevented by pretreatment with a specific inhibitor of the cysteine protease CPP32, while the specific inhibitor of interleukin-1 beta-converting enzyme did not show the preventive effect. The results suggest (i) that intracellular expression of HCV core protein makes cells prone to apoptotic death without upregulation of surface Fas expression and (ii) that the CPP32 protease plays a part in the apoptosis effector pathway of HCV core-expressing cells. HCV core protein may have a role in immune-mediated liver cell injury.

Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-alpha (TNF-alpha). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.

Inhibition of ICE-related proteases (caspases) and nuclear apoptosis by phenylarsine oxide

Biochemical analyses of nuclear apoptosis in vitro have revealed the existence of multiple active interleukin-1beta-converting enzyme-related proteases (caspases) with distinct substrate recognition properties in extracts of preapoptotic chicken DU249 cells (S/M extracts). Previously we demonstrated that the activity of a caspase that cleaves lamins is required for the disintegration of nuclei in the late stages of apoptosis, despite the presence of a second active caspase that cleaves poly(ADP-ribose) polymerase (PARP). One simple explanation for this observation was that the lamin-cleaving caspase is sufficient to drive the nuclear events of apoptotic execution. Here, we report that phenylarsine oxide (PAO) inhibits the protease activities of recombinant human caspases as well as endogenous chicken caspases that are active in S/M extracts. PAO at 100 microM blocks the morphological changes of nuclear apoptosis in vitro and internucleosomal DNA fragmentation in S/M extracts without interfering with PARP or lamin A cleavage. Thus, lamin cleavage is not sufficient to drive the changes in nuclear morphology characteristic of apoptosis. Affinity labeling with YV(bio)KD-aomk shows that the degree of sensitivity to PAO differs among active caspases in S/M extracts. These results suggest that a PAO-sensitive caspase that is distinct from the PARP- or lamin-cleaving enzymes is required for the initiation of apoptotic morphological changes and for the activation of endonuclease(s). Taken together, our results suggest that two or more caspases are required for proteolytic events that are essential for the initiation and completion of nuclear apoptotic changes. The observation that PAO is an inhibitor of caspases and nuclear apoptotic events should be useful for the biochemical dissection of apoptosis in vitro and in vivo.

Activation of CPP32 and Mch3 alpha in wild-type p53-induced apoptosis

DNA-damaging agents induce apoptosis primarily by a p53-dependent pathway. LTR6 cells containing a temperature-sensitive p53 were used to dissect further the mechanisms of p53-induced apoptosis. Apoptosis was accompanied by the processing and activation of CPP32 and Mch3 alpha, together with the cleavage of poly(ADP-ribose) polymerase and lamin B1. These results demonstrate a critical role for the activation of interleukin-1 beta-converting enzyme-like proteases in p53-induced apoptosis.

Down-regulation of actin genes precedes microfilament network disruption and actin cleavage during p53-mediated apoptosis

Inactivation of Simian Virus 40 large T antigen, in cells immortalized with conditional mutants, leads to activation of p53 and apoptosis. We used the mRNA differential display method to identify genes differentially expressed during this process. We found that steady-state levels of mRNA for cytoplasmic actins decreased early during apoptosis. We also showed that, although the steady-state level of the corresponding proteins is not profoundly affected, they are substrates for an interleukin 1-beta converting enzyme (ICE)-like protease activated during the process. However, only a very small fraction of actin is proteolysed during the early stages of apoptosis. The microfilament network is affected and non polymerized actin accumulates in apoptotic bodies after the decrease of mRNA levels, but before a significant amount of actin is cleaved. This suggests that down-regulation of actin genes may be involved in microfilament rearrangements during p53-mediated apoptosis.

c-Jun NH2-terminal kinase-mediated activation of interleukin-1beta converting enzyme/CED-3-like protease during anticancer drug-induced apoptosis

Upon treatment with various anticancer drugs, myeloid leukemia U937 cells undergo apoptosis. In this study, we found that either etoposide (VP-16) or camptothecin (CPT) activated c-Jun N-terminal kinase 1/stress-activated protein kinase (JNK1/SAPK), transient c-jun expression, and ICE (interleukin-1beta converting enzyme)/CED-3-like proteases in U937 cells. Phorbol ester-resistant U937 variant, UT16 cells, displayed a decreased susceptibility to apoptosis induced by these drugs. The drugs did not cause JNK1 activation, c-jun expression, nor activation of ICE/CED-3-like proteases in UT16 cells. As reported previously, benzyloxycarbonyl-Asp-CH2OC(O)-2,6-dichlorobenzene (Z-Asp), a preferential inhibitor of ICE/CED-3-like proteases, blocked the apoptosis of U937 cells. Interestingly, however, Z-Asp did not inhibit JNK1 activation in either VP-16- or CPT-treated U937 cells. The JNK1 antisense oligonucleotides diminished protein expression of JNK1 and inhibited drug-induced apoptosis of U937 cells, whereas sense control oligonucleotides did not. Consistent with this observation, the antisense oligonucleotide-treated cells did not respond to VP-16 or CPT with Z-Asp-sensitive proteases. These results indicate that JNK1 triggers the DNA damaging drug-induced apoptosis of U937 cells by activating Z-Asp-sensitive ICE/CED-3-like proteases.

Attenuation of TGF-beta-induced apoptosis in primary cultures of hepatocytes by calpain inhibitors

Apoptosis induced in primary cultures of rat hepatocytes by transforming growth factor beta 1 (TGF-beta 1) was greatly attenuated by inhibitors (5 microM) of calpain I and calpain II, respectively. Both inhibitors prevented the TGF-beta-elicited increase of nucleosomal DNA fragments and the occurrence of DNA-breaks in the TUNEL reaction. The detrimental effect of TGF-beta on cell viability measured by the WST-1 test was strongly reduced by calpain inhibitors. Calpain II > I inhibitors suppressed spontaneous DNA cleavage in hepatocytes during culture and prevented the appearance of immunocytochemically visible TGF-beta (APAAP staining), which occurs in untreated parenchymal cell cultures. The data show that inactivation of calpains attenuates both the TGF-beta-elicited and the spontaneous apoptosis of cultured hepatocytes; the latter effect is likely due to the suppression of endogenous TGF-beta activation. It is suggested that calpains participate in these processes.

Cytokine response modifier A (CrmA) inhibits ceramide formation in response to tumor necrosis factor (TNF)-alpha: CrmA and Bcl-2 target distinct components in the apoptotic pathway

Proteases are now firmly established as major regulators of the "execution" phase of apoptosis. Here, we examine the role of proteases and their relationship to ceramide, a proposed mediator of apoptosis, in the tumor necrosis factor-alpha (TNF-alpha)-induced pathway of cell death. Ceramide induced activation of prICE, the protease that cleaves the death substrate poly(ADP-ribose) polymerase. Bcl-2 inhibited ceramide-induced death, but not ceramide generation. In contrast, Cytokine response modifier A (CrmA), a potent inhibitor of Interleukin-1 beta converting enzyme and related proteases, inhibited ceramide generation and prevented TNF-alpha-induced death. Exogenous ceramide could overcome the CrmA block to cell death, but not the Bcl-2 block. CrmA, however, did not inhibit the activation of nuclear factor (NF)-kappa B by TNF-alpha, demonstrating that other signaling functions of TNF-alpha remain intact and that ceramide does not play a role in the activation of NF-kappa B. These studies support a distinct role for proteases in the signaling/activation phase of apoptosis acting upstream of ceramide formation.

Role of early and late replication events in induction of apoptosis by baculoviruses

Autographa californica nuclear polyhedrosis virus (AcMNPV) mutants that lack the apoptotic suppressor gene p35 cause apoptosis in Spodoptera frugiperda SF21 cells. To identify a viral signal(s) that induces programmed cell death, we first defined the timing of apoptotic events during infection. Activation of a P35-inhibitable caspase, intracellular fragmentation of host and AcMNPV DNA, and cell membrane blebbing coincided with the initiation of viral DNA synthesis between 9 and 12 h after infection and thus suggested that apoptotic signaling begins at or before this time. Virus entry was required since binding of budded virus to host cell receptors alone was insufficient to induce apoptosis. To therefore determine the contribution of early and late replication events to apoptotic signaling, we used the AcMNPV mutant ts8 with a temperature-sensitive lesion in the putative helicase gene p143. At the nonpermissive temperature at which viral DNA synthesis was conditionally blocked, ts8 caused extensive apoptosis of the SF21 cell line p3576D, which dominantly interferes with anti-apoptotic function of viral P35. Confirming that apoptosis can be induced in the absence of normal viral DNA synthesis, parental SF21 cells also underwent apoptosis when infected with a ts8 p35 deletion mutant at the nonpermissive temperature. However, maximum levels of ts8 p35 deletion mutant-induced apoptosis required a temperature-sensitive event(s) that included the initiation of viral DNA synthesis. Collectively, these data suggested that baculovirus-induced apoptosis can be triggered by distinct early (pre-DNA synthesis) and late replicative events, including viral DNA synthesis or late gene expression.

Interaction of an adenovirus 14.7-kilodalton protein inhibitor of tumor necrosis factor alpha cytolysis with a new member of the GTPase superfamily of signal transducers

The adenovirus (Ad) 14.7-kDa E3 protein (E3-14.7K), which can inhibit tumor necrosis factor alpha (TNF-alpha) cytolysis, was used to screen HeLa cell cDNA libraries for interacting proteins in the yeast two-hybrid system. A new member of the low-molecular-weight (LMW) GTP-binding protein family with Ras and ADP-ribosylation factor homology was discovered by this selection and has been named FIP-1 (14.7K-interacting protein). FIP-1 colocalized with Ad E3-14.7K in the cytoplasm especially near the nuclear membrane and in discrete foci on or near the plasma membrane. Its interaction with E3-14.7K was dependent on the FIP-1 GTP-binding domain. The stable expression of FIP-1 antisense message partially protected the cells from TNF-alpha cytolysis. FIP-1 was associated transiently with several unknown phosphorylated cellular proteins within 15 min after treatment with TNF-alpha. FIP-1 mRNA was expressed ubiquitously but at higher levels in human skeletal muscle, heart, and brain. In addition to homology to other LMW GTP-binding proteins, FIP-1 has regions of homology to two prokaryotic metalloproteases. However, there was no homology between FIP-1 and any of the recently isolated death proteins in the TNF-alpha or Fas/APO1 cytolytic pathway and no interaction with several members of the Bcl-2 family of inhibitors of apoptosis. These data suggest that FIP-1, as a cellular target for Ad E3-14.7K, is either a new intermediate on a previously described pathway or part of a novel TNF-alpha-induced cell death pathway. FIP-1 has two consensus sequences for myristoylation which would be expected to facilitate membrane association and also has sequences for Ser/Thr as well as Tyr phosphorylation that could affect its function.

Apoptosis of skeletal and cardiac muscles and physical exercise

Besides the well-known reciprocal influences of skeletal muscle and heart during and after physical exercise, a new perspective is emerging on the short- and long-term effects of exercise-induced damage, in particular the pathogenic role of inappropriate apoptosis in skeletal and cardiac muscle. Cells from multicellular organisms self-destruct when they are no longer needed, or have become damaged; they do this by activating a genetically controlled cell suicide machinery that leads to programmed cell death (PCD), or apoptosis. Apoptosis is a specific form of programmed cell death that plays an important role in development, growth regulation and disease. Skeletal muscles in adult animals are fully differentiated syncytial cells. Apoptosis, which is known to be present in tissues that modulate their cellular homeostasis under the influence of growth and/or hormonal factors, has been recently described in early stages of myocardial infarct, and in dystrophic skeletal muscle. The role and the cellular and molecular aspects of muscle cell death and apoptosis are far from clear, particularly following several types of muscle damage (genetic defects, exercise-induced damage, oxidative stress, etc.). It can be predicted that apoptosis plays a major role in regulating myoblast proliferation during muscle regeneration, and in the progression of dystrophinopathies. A particularly important area has recently developed concerning cardiac muscle and reperfusion injury after ischemia; in this case as well, a major role of apoptosis is emerging.

Retinoblastoma protein in growth suppression and death protection

Puzzling new information indicates an inadequacy in our understanding of the retinoblastoma protein (RB). RB and the transcription factor E2F appear to be collaborators. RB-E2F interaction is necessary but not sufficient for growth suppression. Unbecoming of a tumor suppressor, RB has an active role in antagonizing the death response. How RB integrates its multiple functions into a tumor suppression program is still an open issue.

IpaB, a Shigella flexneri invasin, colocalizes with interleukin-1 beta-converting enzyme in the cytoplasm of macrophages

Shigellae are the most prevalent etiological agents of dysentery. A crucial step in shigella pathogenesis is the induction of macrophage apoptosis. The invasion plasmid antigen B (IpaB) is necessary and sufficient to induce macrophage programmed cell death. IpaB activates apoptosis by binding to interleukin-1 beta (IL-1 beta)-converting enzyme (ICE) or a highly homologous protease. Here, we show that IpaB is disseminated throughout the cytoplasm of shigella-infected macrophages as detected by both immunofluorescence and immunoelectron microscopy. The cytoplasmic distribution of IpaB requires phagosome escape, and it is specific to IpaB, since lipopolysaccharide, used here as a bacterial marker, remains closely associated with the bacteria. In double-labeling experiments, we show that IpaB and ICE colocalize in the cytoplasm of the macrophage, suggesting that soon after secretion, IpaB binds to ICE to initiate apoptosis and to promote the cleavage of IL-1 beta.

Bax- and Bak-induced cell death in the fission yeast Schizosaccharomyces pombe

The effects of the expression of the human Bcl-2 family proteins Bax, Bak, Bcl-2, and Bcl-XL were examined in the fission yeast Schizosaccharomyces pombe and compared with Bax-induced cell death in mammalian cells. Expression of the proapoptotic proteins Bax and Bak conferred a lethal phenotype in this yeast, which was strongly suppressed by coexpression of the anti-apoptotic protein Bcl-XL. Bcl-2 also partially abrogated Bax-mediated cytotoxicity in S. pombe, whereas a mutant of Bcl-2 (Gly145Ala) that fails to heterodimerize with Bax or block apoptosis in mammalian cells was inactive. However, other features distinguished Bax- and Bak-induced death in S. pombe from animal cell apoptosis. Electron microscopic analysis of S. pombe cells dying in response to Bax or Bak expression demonstrated massive cytosolic vacuolization and multifocal nuclear chromatin condensation, thus distinguishing this form of cell death from the classical morphological features of apoptosis seen in animal cells. Unlike Bax-induced apoptosis in 293 cells that led to the induction of interleukin-1 beta-converting enzyme (ICE)/CED-3-like protease activity, Bax- and Bak-induced cell death in S. pombe was accompanied neither by internucleosomal DNA fragmentation nor by activation of proteases with specificities similar to the ICE/CED-3 family. In addition, the baculovirus protease inhibitor p35, which is a potent inhibitor of ICE/CED-3 family proteases and a blocker of apoptosis in animal cells, failed to prevent cell death induction by Bax or Bak in fission yeast, whereas p35 inhibited Bax-induced cell death in mammalian cells. Taken together, these findings suggest that Bcl-2 family proteins may retain an evolutionarily conserved ability to regulate cell survival and death but also indicate differences in the downstream events that are activated by overexpression of Bax or Bak in divergent cell types.

Bcl-x(L) forms an ion channel in synthetic lipid membranes

Bcl-2-related proteins are critical regulators of cell survival that are localized to the outer mitochondrial, outer nuclear and endoplasmic reticulum membranes. Despite their physiological importance, the biochemical function of Bcl-2-related proteins has remained elusive. The three-dimensional structure of Bcl-xL, an inhibitor of apoptosis, was recently shown to be similar to the structures of the pore-forming domains of bacterial toxins. A key feature of these pore-forming domains is the ability to form ion channels in biological membranes. Here we demonstrate that Bcl-xL shares this functional feature. Like the bacterial toxins, Bcl-xL can insert into either synthetic lipid vesicles or planar lipid bilayers and form an ion-conducting channel. This channel is pH-sensitive and becomes cation-selective at physiological pH. The ion-conducting channel(s) formed by Bcl-xL display multiple conductance states that have identical ion selectivity. Together, these data suggest that Bcl-xL may maintain cell survival by regulating the permeability of the intracellular membranes to which it is distributed.

Spodoptera frugiperda caspase-1, a novel insect death protease that cleaves the nuclear immunophilin FKBP46, is the target of the baculovirus antiapoptotic protein p35

Employing the degenerate primer-dependent polymerase chain reaction approach used recently to clone human Mch2, we have identified and cloned the insect Spodoptera frugiperda target of the baculovirus antiapoptotic protein p35. This protein named Sf caspase-1 belongs to the family of caspases and is highly related to human Mch3 and CPP32 in sequence and specific activity. The proenzyme of Sf caspase-1 is 299 amino acids in length and can undergo autocatalytic processing in Escherichia coli to an active enzyme heterocomplex. Autoprocessing occurs at Asp-28, Asp-184, and Asp-195 to generate the large p19/p18 and small p12 subunits. Sf caspase-1 is able to induce apoptosis in Sf9 cells and is capable of cleaving p35 to similar sized fragments as observed with extracts from p35 null mutant baculovirus-infected Sf9 cells. Sf caspase-1 activity is potently inhibited by p35, suggesting that it is an important target of this antiapoptotic protein. Finally, the Sf9 nuclear immunophilin FKBP46 was identified as a death-associated substrate for Sf caspase-1.

Activation of a CrmA-insensitive, p35-sensitive pathway in ionizing radiation-induced apoptosis

The response of eukaryotic cells to ionizing radiation (IR) includes induction of apoptosis. However, the signals that regulate this response are unknown. The present studies demonstrate that IR treatment of U-937 cells is associated with: (i) internucleosomal DNA fragmentation; (ii) cleavage of poly(ADP-ribose) polymerase; (iii) cleavage of protein kinase C delta; and (iv) induction of an Ac-DEVD-p-nitroanilide cleaving activity. Overexpression of the cowpox protein CrmA blocked tumor necrosis factor (TNF)-induced apoptosis but had no effect on IR-induced DNA fragmentation or cleavage of poly(ADP-ribose) polymerase and protein kinase C delta. By contrast, overexpression of the baculovirus p35 protein blocked both IR- and TNF-induced apoptosis. The results further demonstrate that the IR-induced proteolytic activity is directly inhibited by the addition of purified recombinant p35, but not by CrmA. We show that the CPP32 protease is sensitive to p35 and not CrmA. We also show that IR induces activation of CPP32 and that this event, like induction of apoptosis, is sensitive to overexpression of p35 and not CrmA. These findings indicate that IR-induced apoptosis involves activation of CPP32 and that this CrmA-insensitive apoptotic pathway is distinct from those induced by TNF and certain other stimuli.

A Cysteine Protease Inhibitor Prevents Activation-Induced T-Cell Apoptosis and Death of Peripheral Blood Cells From Human Immunodeficiency Virus-Infected Individuals by Inhibiting Upregulation of Fas Ligand

Activation of T-cell hybridomas, preactivated normal T cells, and peripheral blood lymphocytes (PBL) from human immunodeficiency virus (HIV)-infected individuals results in apoptosis. In the first two cases, apoptosis is caused by the upregulation of Fas ligand (FasL) and its subsequent interaction with Fas; the mechanism for the spontaneous and activation-induced death of lymph node cells and PBL from HIV+ blood is not known. A number of protease inhibitors have been shown to prevent T-cell apoptosis under all of these circumstances, but the mechanism of action has not been determined. Here we show that the cysteine protease inhibitor E64d prevents activation-induced T hybridoma cell death by inhibiting the upregulation of FasL. Quantitative polymerase chain reaction (PCR) demonstrated that mRNA for FasL is expressed at low levels in fresh PBL from HIV-infected blood, but increases in cultured PBL from both uninfected and HIV-infected donors. The ex vivo apoptosis of PBL from HIV+ donors was prevented by adding the soluble extracellular domain of Fas, demonstrating a requisite role for Fas/FasL interactions in this form of cell death. Furthermore, while having no effect on the death of PBL from HIV-infected blood stimulated directly via Fas, E64d inhibited FasL upregulation. Thus, aberrant apoptosis of cultured PBL from HIV-infected individuals is mediated by FasL and Fas, and E64d blocks this apoptosis by inhibiting the upregulation of FasL. These results are consistent with the hypothesis that the abnormal expression of Fas and the inducible expression of FasL contributes to the immunodeficiency of patients with acquired immune deficiency syndrome and suggest that modulation of FasL expression could be an effective target for therapeutic intervention.

Resistance of actin to cleavage during apoptosis

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1beta-converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation.

Macromolecular substrates for the ICE-like proteases during apoptosis

The interleukin-1 beta-converting enzyme (ICE) family of proteases is an important component of the mechanism of the apoptotic process, but the physiologic roles of the different homologs during apoptosis remain unclear. Significant information about the roles of proteolysis in apoptosis will be gained through identification of the distal substrates through which these proteases achieve their pro-apoptotic effects. Identification of these substrates therefore remains an important challenge. A subset of autoantibodies from patients with systemic lupus erythematosus (SLE) recognize molecules that are specifically cleaved early during apoptosis. Several of the identified autoantigens are nuclear proteins (PARP, U1-70 kDa, and DNA-PKcs) that are substrates for CPP32 in vitro and in apoptotic cells. Of note, these substrates are catalytic proteins involved in homeostatic pathways, suggesting that abolition of homeostasis is one fundamental feature ensuring the rapid irreversibility of the apoptotic process. Identification of the other substrates for this protease family will provide the tools to assess the roles of the different proteases in apoptotic death.

Ultraviolet light-induced keratinocyte apoptosis: a potential mechanism for the induction of skin lesions and autoantibody production in LE

Systemic lupus erythematosus (SLE) is a prototype systemic autoimmune disease which is characterized clinically by pleiotropy and periodicity. The immune features which accompany the characteristic flares of the disease have strongly suggested that the autoimmune response is driven by self antigen, and is T cell-dependent. These features have prompted the search for potential initiating process(es) which induce the release of self-antigens in a form which causes T cell tolerance to those self molecules to be broken. We review here several recent observations which implicate apoptotic cells as an important potential source of clustered and concentrated autoantigens in SLE, and present our current model whereby the novel autoantigen fragments generated in apoptotic surface blebs initiate and drive the autoimmune response in this disease.

Apoptosis in neurodegenerative disorders

Although the exact mechanism of nigral cell death in Parkinson's disease (PD) is not known, increasing evidence suggests the presence of apoptotic cell death in PD. When we applied the TUNEL method to detect DNA fragmentation, four out of seven late onset sporadic patients with PD showed TUNEL-positive neurons. The percentages of those neurons among the remaining melanin containing neurons were 0.6 to 4.8% (average 2.1%). But TUNEL-positive neurons could not be detected in control subjects as well as four patients with young onset (under 40 years of the age) PD. Numbers of nigral toxins such as MPTP, complex I inhibitors, and mitochondrial respiratory inhibitors have been reported to induced apoptotic cell death. These findings suggest that apoptosis is involved in nigral cell cleath in PD at least in part and warrant further studies on apoptosis-related substances in PD.

Liver apoptosis

Apoptosis, also called programmed cell death, is a peculiar form of cell death different from cell necrosis in many morphological and biochemical aspects. Like mitosis or differentiation, apoptosis is a normal cell phenomenon which depends on the expression of genes capable of inducing or inhibiting this type of cell destruction. But apoptosis can also be triggered by many external factors and has been described in many diseases. The very different conditions where programmed cell death occurs suggest that the mechanisms leading to the activation of apoptosis-controlling genes are variable. As in other cells, apoptosis occurs in the liver cells, first in the normal state during liver development and then in the adult liver, respectively for liver organogenesis and the renewal of hepatocytes. But apoptosis is also present in various viral, immunological, malignant or drug-induced human liver diseases. In addition, in the animal, hepatocyte apoptosis can be triggered either in vivo or in vitro by many toxic agents. In contrast to other cells, the mechanisms leading to liver cell apoptosis remain poorly investigated. However, two proteins could play an important role in this field, the fas/apo-1 protein present at the surface of hepatocytes and the bcl-2 protein localized in biliary cells. Analysis of the genes controlling the expression of these two proteins could provide essential information on the mechanisms of liver apoptosis.

Fas activation of the p38 mitogen-activated protein kinase signalling pathway requires ICE/CED-3 family proteases

The Fas receptor mediates a signalling cascade resulting in programmed cell death (apoptosis) within hours of receptor cross-linking. In this study Fas activated the stress-responsive mitogen-activated protein kinases, p38 and JNK, within 2 h in Jurkat T lymphocytes but not the mitogen-responsive kinase ERK1 or pp70S6k. Fas activation of p38 correlated temporally with the onset of apoptosis, and transfection of constitutively active MKK3 (glu), an upstream regulator of p38, potentiated Fas-induced cell death, suggesting a potential involvement of the MKK3/p38 activation pathway in Fas-mediated apoptosis. Fas has been shown to require ICE (interleukin-1 beta-converting enzyme) family proteases to induce apoptosis from studies utilizing the cowpox ICE inhibitor protein CrmA, the synthetic tetrapeptide ICE inhibitor YVAD-CMK, and the tripeptide pan-ICE inhibitor Z-VAD-FMK. In this study, crmA antagonized, and YVAD-CMK and Z-VAD-FMK completely inhibited, Fas activation of p38 kinase activity, demonstrating that Fas-dependent activation of p38 requires ICE/CED-3 family members and conversely that the MKK3/p38 activation cascade represents a downstream target for the ICE/CED-3 family proteases. Intriguingly, p38 activation by sorbitol and etoposide was resistant to YVAD-CMK and Z-VAD-FMK, suggesting the existence of an additional mechanism(s) of p38 regulation. The ICE/CED-3 family-p38 regulatory relationship described in the current work indicates that in addition to the previously described destructive cleavage of substrates such as poly(ADP ribose) polymerase, lamins, and topoisomerase, the apoptotic cysteine proteases also function to regulate stress kinase signalling cascades.