Benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone (Z-VAD.FMK) inhibits apoptosis by blocking the processing of CPP32

Inhibition of human caspases by peptide-based and macromolecular inhibitors

Studies with peptide-based and macromolecular inhibitors of the caspase family of cysteine proteases have helped to define a central role for these enzymes in inflammation and mammalian apoptosis. A clear interpretation of these studies has been compromised by an incomplete understanding of the selectivity of these molecules. Here we describe the selectivity of several peptide-based inhibitors and the coxpox serpin CrmA against 10 human caspases. The peptide aldehydes that were examined (Ac-WEHD-CHO, Ac-DEVD-CHO, Ac-YVAD-CHO, t-butoxycarbonyl-IETD-CHO, and t-butoxycarbonyl-AEVD-CHO) included several that contain the optimal tetrapeptide recognition motif for various caspases. These aldehydes display a wide range of selectivities and potencies against these enzymes, with dissociation constants ranging from 75 pM to >10 microM. The halomethyl ketone benzyloxycarbonyl-VAD fluoromethyl ketone is a broad specificity irreversible caspase inhibitor, with second-order inactivation rates that range from 2.9 x 10(2) M-1 s-1 for caspase-2 to 2.8 x 10(5) M-1 s-1 for caspase-1. The results obtained with peptide-based inhibitors are in accord with those predicted from the substrate specificity studies described earlier. The cowpox serpin CrmA is a potent (Ki < 20 nM) and selective inhibitor of Group I caspases (caspase-1, -4, and -5) and most Group III caspases (caspase-8, -9, and -10), suggesting that this virus facilitates infection through inhibition of both apoptosis and the host inflammatory response.

Role of caspases in immunotoxin-induced apoptosis of cancer cells

Immunotoxins composed of antibodies linked to plant or bacterial toxins are being evaluated in the treatment of cancer. It is known that the toxin moieties of immunotoxins, including Pseudomonasexotoxin A (PE), diphtheria toxin, and ricin, are capable of inducing apoptosis. Since the efficiency of induction of apoptosis and the apoptosis pathway may have direct effects on the therapeutic usefulness of immunotoxins, we have studied how B3(Fv)-PE38, a genetically engineered immunotoxin in which the Fv fragment of an antibody is fused to a mutated form of PE, induces apoptosis of the MCF-7 breast cancer cell line. We show for the first time that a PE-containing immunotoxin activates ICE/ced-3 proteases, now termed caspases, and causes characteristic cleavage of the "death substrate" poly(ADP)-ribose polymerase (PARP) to an 89 kDa fragment with a time course of cleavage comparable to that induced by TNFalpha. Also the fluorescent substrate, DEVD-AFC, is cleaved 2-4-fold more rapidly by lysates from B3(Fv)-PE38 treated MCF-7 cells than untreated control cells, suggesting that a CPP32-like caspase is involved in B3(Fv)-PE38-mediated apoptosis. B3(Fv)-PE38-induced PARP cleavage is inhibited by several protease inhibitors known to inhibit caspases (zVAD-fmk, zDEVD-fmk, zIETD-fmk) as well as by overexpression of Bcl-2 providing additional evidence for caspase involvement. zVAD-fmk, a broad spectrum inhibitor of most mammalian caspases, prevents the early morphological changes and loss of cell membrane integrity produced by B3(Fv)-PE38, but not its ability to inhibit protein synthesis, arrest cell growth, and subsequently kill cells. Despite inhibition of apoptosis, the immunotoxin is still capable of selective cell killing, which indicates that B3(Fv)-PE38 kills cells by two mechanisms: one requires caspase activation, and the other is due to the arrest of protein synthesis caused by inactivation of elongation factor 2. The fact that an immunotoxin can specifically kill tumor cells without the need of inducing apoptosis makes such agents especially valuable for the treatment of cancers that are protected against apoptosis, e.g., by overexpression of Bcl-2.

Caspase activation and specific cleavage of substrates after coxsackievirus B3-induced cytopathic effect in HeLa cells

Coxsackievirus B3 (CVB3), an enterovirus in the family Picornaviridae, induces cytopathic changes in cell culture systems and directly injures multiple susceptible organs and tissues in vivo, including the myocardium, early after infection. Biochemical analysis of the cell death pathway in CVB3-infected HeLa cells demonstrated that the 32-kDa proform of caspase 3 is cleaved subsequent to the degenerative morphological changes seen in infected HeLa cells. Caspase activation assays confirm that the cleaved caspase 3 is proteolytically active. The caspase 3 substrates poly(ADP-ribose) polymerase, a DNA repair enzyme, and DNA fragmentation factor, a cytoplasmic inhibitor of an endonuclease responsible for DNA fragmentation, were degraded at 9 h following infection, yielding their characteristic cleavage fragments. Inhibition of caspase activation by benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (ZVAD.fmk) did not inhibit the virus-induced cytopathic effect, while inhibition of caspase activation by ZVAD.fmk in control apoptotic cells induced by treatment with the porphyrin photosensitizer benzoporphyrin derivative monoacid ring A and visible light inhibited the apoptotic phenotype. Caspase activation and cleavage of substrates may not be responsible for the characteristic cytopathic effect produced by picornavirus infection yet may be related to late-stage alterations of cellular homeostatic processes and structural integrity.

Involvement of APO2 ligand/TRAIL in activation-induced death of Jurkat and human peripheral blood T cells

The interaction of Fas with Fas ligand (FasL) mediates activation-induced cell death (AICD) of T hybridomas and of mature T lymphocytes. The TNF/TNF receptor system also plays a significant role in AICD of mature T cells and in the maintenance of peripheral tolerance. We previously demonstrated that in human Jurkat leukemia cells, AICD is triggered mainly by the rapid release of preformed FasL upon TCR stimulation. In the present work, we show that the cytotoxic cytokine APO2 ligand (APO2L; also known as TRAIL) is constitutively expressed as an intracytoplasmic protein in Jurkat T cells and derived sublines. APO2L is also detected in fresh human peripheral blood mononuclear cells (PBMC) from a significant number of donors, and the amount of both FasL and APO2L substantially increases upon blast generation. A neutralizing anti-APO2L monoclonal antibody (mAb) partially suppresses the cytotoxicity induced by supernatants of phytohemagglutinin (PHA)-prestimulated Jurkat or human PBMC on non-activated Jurkat cells, indicating that APO2L is released by these cells and contributes to AICD. A combination of neutralizing anti-APO2L and anti-Fas mAb blocks around 60 % of the toxicity associated with supernatants from PHA-activated human PBMC. These results show that FasL and APO2L account for the majority of cytotoxic activity released during AICD, and suggest that additional uncharacterized factors may also contribute to this process.

Granulysin-Induced Apoptosis. I. Involvement of at Least Two Distinct Pathways

Granulysin is a newly described cytolytic molecule released by CTL and NK cells via granule-mediated exocytosis. It shares homology with saposin-like proteins, including NK-lysin and amoebapores, and has been implicated in the lysis of tumor cells and microbes. In the present study we show that recombinant granulysin alone induces apoptosis of Jurkat cells. This apoptosis is associated with a sixfold increase in the ceramide/sphingomyelin ratio, implicating the activation of sphingomyelinases. Granulysin- and ceramide-induced apoptosis are similar in that they both are only minimally inhibited by the more selective cysteine protease p32 (caspase 3)-like caspase inhibitor N-acetyl-Asp-Glu-Val-Asp aldehyde, while they are significantly inhibited by the more general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk). Nevertheless, while Z-VAD-fmk almost completely inhibits ceramide-induced apoptosis, a Z-VAD-fmk-resistant component was observed using granulysin. Granulysin also causes apoptosis in cells depleted of sphingomyelin by prolonged treatment with the ceramide synthase inhibitor fumonisin B1. These data indicate that granulysin induces target cell death by both ceramide- and caspase-dependent and -independent pathways.

Calcium/calmodulin-dependent protein kinase IV is cleaved by caspase-3 and calpain in SH-SY5Y human neuroblastoma cells undergoing apoptosis

We have previously demonstrated cleavage of alpha-spectrin by caspase-3 and calpain during apoptosis in SH-SY5Y neuroblastoma cells (Nath, R., Raser, K. J., Stafford, D., Hajimohammadreza, I., Posner, A., Allen, H., Talanian, R. V., Yuen, P., Gilbertsen, R. B., and Wang, K. K. (1996) Biochem. J. 319, 683-690). We demonstrate here that calcium/calmodulin-dependent protein kinase IV (CaMK IV) is cleaved during apoptosis by caspase-3 and calpain. We challenged SH-SY5Y cells with the pro-apoptotic agent thapsigargin. Western blot analysis revealed major CaMK IV breakdown products of 40, 38, and 33 kDa. Digestion of control SH-SY5Y lysate with purified caspase-3 produced a 38-kDa CaMK IV fragment; digestion with purified calpain produced a major fragment of 40 kDa. Pretreatment with carbobenzoxy-Asp-CH2OC(O)-2,6-dichlorobenzene or Z-Val-Ala-Asp-fluoromethylketone was able to block the caspase-3-mediated production of the 38-kDa fragment both in situ and in vitro. Calpain inhibitor II similarly blocked formation of the calpain-mediated 40-kDa fragment both in situ and in vitro. Digestion of recombinant CaMK IV by other caspase family members revealed that only caspase-3 produces a fragmentation pattern consistent to that seen in situ. The major caspase-3 and calpain cleavage sites are respectively identified as PAPD176*A and CG201*A, both within the CaMK IV catalytic domain. Furthermore, calmodulin-stimulated protein kinase activity decreases within 6 h in thapsigargin-treated SH-SY5Y. The loss of activity precedes cell death.

Inhibition of CPP32-like proteases rescues axotomized retinal ganglion cells from secondary cell death in vivo

The majority of retinal ganglion cells (RGCs) degenerate and die after transection of the optic nerve (ON) in the adult rat. This secondary cell death can primarily be ascribed to apoptosis. Recent work strongly suggests a decisive role for a family of cysteine proteases, termed caspases, as mediators of neuronal apoptosis. In this study, we investigated whether activation of caspases contributes to delayed death of RGCs after axotomy. Intraocular application of various caspase inhibitors rescued up to 34% of RGCs that would otherwise have died 14 d after ON transection. Using a modified affinity-labeling technique, we detected a 17 kDa protease subunit upregulated after axotomy. Upregulation was prevented by caspase inhibitor treatment. The 17 kDa protein was identified as a CPP32-like protease by Western blot analysis and affinity labeling with biotinylated acetyl-Asp-Glu-Val-Asp-aldehyde, which specifically inhibits CPP32-like caspases. In vivo application of the irreversible caspase inhibitor benzyloxycarbonyl-Asp-Glu-Val-Asp-chloromethylketone revealed CPP32-like proteases to be major mediators of caspase-induced apoptosis in axotomized RGCs, because this inhibitor showed an even higher neuroprotective potential than the irreversible wide-range inhibitor benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. In summary, the data presented here provide further insight into the mechanisms of injury-induced neuronal apoptosis and could give rise to more effective therapeutic intervention strategies in CNS trauma and neurodegenerative diseases.

Proteasome Activation Occurs at an Early, Premitochondrial Step of Thymocyte Apoptosis

Proteasomes and mitochondrial membrane changes are involved in thymocyte apoptosis. The hierarchical relationship between protease activation and mitochondrial alterations has been elusive. Here we show that inhibition of proteasomes by two specific agents, lactacystin or MG132, prevents all manifestations of thymocyte apoptosis induced by the glucocorticoid receptor agonist dexamethasone or by the topoisomerase II inhibitor etoposide. Lactacystin and MG132 prevent the early disruption of the mitochondrial transmembrane potential (ΔΨm), which precedes caspase activation, exposure of phosphatidylserine, and nuclear DNA fragmentation. In contrast, stabilization of the ΔΨm using the permeability transition pore inhibitor bongkrekic acid or inhibition of caspases by N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone does not prevent the activation of proteasomes, as determined with the fluorogenic substrate N-succinyl-l-leucyl-l-leucyl-l-valyl-l-tyrosine-7-amido-4-methylcoumarin. Thus, proteasome activation occurs upstream from mitochondrial changes and caspase activation. Whereas the proteasome-specific agents lactacystin and MG132 truly maintain thymocyte viability, a number of protease inhibitors that inhibit nuclear DNA fragmentation (acetyl-Asp-Glu-Val-Asp-fluoromethylketone; N-Boc-Asp(OMe)-fluoromethylketone; N-tosyl-l-Phe-chloromethylketone) do not prevent the cytolysis induced by DEX or etoposide. These latter agents fail to interfere with the preapoptotic ΔΨm disruption. Altogether, our data indicate that different proteases may be involved in the pre- or postmitochondrial phase of apoptosis. Only those protease inhibitors that interrupt the apoptotic process at the premitochondrial stage can actually preserve cell viability.

Inhibition of CPP32-like proteases rescues axotomized retinal ganglion cells from secondary cell death in vivo

The majority of retinal ganglion cells (RGCs) degenerate and die after transection of the optic nerve (ON) in the adult rat. This secondary cell death can primarily be ascribed to apoptosis. Recent work strongly suggests a decisive role for a family of cysteine proteases, termed caspases, as mediators of neuronal apoptosis. In this study, we investigated whether activation of caspases contributes to delayed death of RGCs after axotomy. Intraocular application of various caspase inhibitors rescued up to 34% of RGCs that would otherwise have died 14 d after ON transection. Using a modified affinity-labeling technique, we detected a 17 kDa protease subunit upregulated after axotomy. Upregulation was prevented by caspase inhibitor treatment. The 17 kDa protein was identified as a CPP32-like protease by Western blot analysis and affinity labeling with biotinylated acetyl-Asp-Glu-Val-Asp-aldehyde, which specifically inhibits CPP32-like caspases. In vivo application of the irreversible caspase inhibitor benzyloxycarbonyl-Asp-Glu-Val-Asp-chloromethylketone revealed CPP32-like proteases to be major mediators of caspase-induced apoptosis in axotomized RGCs, because this inhibitor showed an even higher neuroprotective potential than the irreversible wide-range inhibitor benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. In summary, the data presented here provide further insight into the mechanisms of injury-induced neuronal apoptosis and could give rise to more effective therapeutic intervention strategies in CNS trauma and neurodegenerative diseases.

Fas-mediated apoptosis in mouse hepatocytes involves the processing and activation of caspases

The mechanism of Fas antigen-induced hepatocyte apoptosis was investigated. Using a monoclonal antibody directed against the Fas antigen, apoptosis was induced in freshly isolated murine hepatocytes within 90 minutes of antibody addition as assessed by plasma membrane bleb formation, chromatin condensation, and DNA fragmentation. Pretreatment of the cells with the caspase inhibitors, N-acetyl-Asp-Glu-Val-Asp aldehyde (Ac-DEVD-CHO), benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone (Z-VAD-FMK), or Z-Asp-2,6-dichlorobenzoyloxymethylketone inhibited anti-Fas-mediated apoptosis. Likewise, the serine protease inhibitors, N-tosyl-L-phenyl chloromethyl ketone (TPCK) and 3,4-dichloroisocoumarin (DCI), prevented apoptosis, whereas N-tosyl-L-lysine chloromethyl ketone (TLCK), Ac-Leu-Leu-L-norleucinal, Ac-Leu-Leu-L-methional, and trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane were without effect. Examination of CED-3/caspase-3-related caspases revealed that pro-caspases-3 (CPP32) and -7 (Mch-3alpha) were rapidly processed after Fas antigen stimulation. Caspase-7 was further cleaved to form the catalytically active subunits. In contrast, the p17 subunit of caspase-3 was not detected, indicating slow formation or rapid degradation. The activation of CED-3-related caspases was further confirmed by an increase in the rate of Z-DEVD-7-amino-4-trifluoromethylcoumarin (Z-DEVD-AFC) hydrolysis that was sensitive to Ac-DEVD-CHO and was inhibited by pretreatment of the cells with TPCK but not by DCI. In contrast, no increase in the rates of hydrolysis of Z-YVAD-AFC, a substrate for caspase-1, was detected. Investigation of the in situ proteolytic cleavage of the CED-3 related caspases substrate, poly(ADP-ribose) polymerase, revealed that this protein was not degraded in hepatocytes undergoing Fas-mediated apoptosis. Taken together, our results show that processing of caspases, in particular, caspases-7 and -3, occurs during Fas-induced apoptosis of mouse hepatocytes and suggest a role of these proteases as well as serine protease(s) in the apoptotic response.

Transmissible gastroenteritis coronavirus induces programmed cell death in infected cells through a caspase-dependent pathway

In this report, we show that apoptosis (or programmed cell death) is induced in different cell lines infected with a coronavirus, the porcine transmissible gastroenteritis virus (TGEV). Kinetic analysis of internucleosomal DNA cleavage by agarose gel electrophoresis and flow cytometry or cytometric monitoring of the mitochondrial transmembrane potential showed that, for ST cells infected with TGEV, the first overt signs of apoptosis appeared from 10 to 12 h postinfection on. They preceded morphological changes characteristic of cells undergoing apoptosis, as observed by light and electron microscopy. The tripeptide pan-ICE (caspase) inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone blocked TGEV-induced apoptosis with no effect on virus production. The thiol agent pyrrolidine dithiocarbamate inhibited apoptosis, suggesting that TGEV infection may lead to apoptosis via cellular oxidative stress. The effect of TGEV infection on activation of NF-kappaB, a transcription factor known to be activated by oxidative stress, was examined. NF-kappaB DNA binding was shown to be strongly and quickly induced by TGEV infection. However, transcription factor decoy experiments showed that NF-kappaB activation is not critical for TGEV-induced apoptosis.

Dismantling in cell death: molecular mechanisms and relationship to caspase activation

The notion of a cell death programme was introduced in view of the reproducibility of its occurrence in time and space (e.g. in the developing embryo) and of its genetic determination. Programmed cell death can be schematically subdivided into three steps: a signalling phase, an execution phase and a dismantling phase. This review focuses on the latter. Apoptosis is the most studied form of dismantling of animal cells. The molecular pathways leading to certain apoptotic lesions appear to be dependent on the proteolytic activity of caspases. Death itself can, however, be caspase-independent. Also, non-apoptotic forms of cell death exist, even in animal cells; their molecular bases are still unknown. The relationship between cell death, apoptosis and caspases is discussed.

Apoptosis induction by the glucocorticoid hormone dexamethasone and the calcium-ATPase inhibitor thapsigargin involves Bc1-2 regulated caspase activation

The requirement for caspases (ICE-like proteases) were investigated in mediating apoptosis of WEHI7.2 mouse lymphoma cells in response to two death inducers with different mechanisms of action, the glucocorticoid hormone dexamethasone (DX) and the calcium-ATPase inhibitor thapsigargin (TG). Apoptosis induction by these agents followed different kinetics, and was closely correlated with in vivo activation of caspase-3 (CPP32/Yama/Apopain) and cleavage of the caspase target protein poly(ADP-ribose) polymerase (PARP). Caspase activation and PARP cleavage were inhibited by Bcl-2 overexpression. Cell extracts from DX- and TG-treated cells cleaved the in vitro synthesized baculovirus p35 ICE-like protease target, producing 25 and 10 kDa fragments. p35 cleavage was inhibited by mutating the active site aspartic acid to alanine, and by a panel of protease inhibitors that inhibit caspase-3-like proteases, including iodoacetamide, N-ethylmaleimide, and Ac-DEVD-cho. Treatment of cells in vivo with two cell permeant peptide fluoromethylketone inhibitors of caspase activity, Z-VAD-fmk and Z-DEVD-fmk, inhibited DX- and TG-induced apoptotic nuclear changes and maintained plasma membrane integrity, whereas the cathepsin inhibitor, Z-FA-fmk, and two calpain inhibitors failed to inhibit apoptosis. An unexpected observation was that due to the delayed time course of DX-induced apoptosis, optimal preservation of plasma membrane integrity was achieved by adding caspase inhibitors beginning 8 h after DX addition. In summary, the findings indicate that two diverse apoptosis-inducing signals converge into a common Bcl-2-regulated pathway that leads to caspase activation and apoptosis.

Apoptosis and increased expression of inducible nitric oxide synthase in human allograft rejection

BACKGROUND

The mechanisms of myocyte death during cardiac allograft rejection are incompletely understood. In a previous study using a rat heterotopic cardiac allograft model, we showed that cardiac myocyte apoptosis, inducible nitric oxide synthase (iNOS) mRNA, protein and enzyme activity, and nitrotyrosine increased simultaneously during cardiac allograft rejection. This study was designed to investigate whether apoptosis and expression of iNOS occur in human cardiac allograft rejection.

METHODS

Right ventricular endomyocardial biopsies from 30 cases of allograft rejection (International Society of Heart and Lung Transplantation grade 3A/B) were compared with 12 biopsies with no rejection (International Society of Heart and Lung Transplantation grade 0). Samples were co-labeled for apoptosis and muscle actin. Serial sections were stained for iNOS, nitrotyrosine, and the leukocyte markers CD3, CD4, CD8, and CD68 to identify T-cell subpopulations and macrophages.

RESULTS

Biopsies with cardiac allograft rejection showed a 30-fold increase of apoptotic cells when compared with controls. Most apoptotic cardiac myocytes were found in proximity to macrophage (CD68+)-rich inflammatory infiltrates. iNOS immunoreactivity was strongest in macrophages and adjacent myocytes, which also showed high levels of nitrotyrosine, representing damage by peroxynitrite.

CONCLUSIONS

Apoptosis is a major form of myocyte death during human cardiac allograft rejection. Cardiac myocyte apoptosis is closely associated with expression of iNOS in macrophages and myocytes and with nitration of myocyte proteins by peroxynitrite.

Alphaviruses induce apoptosis in Bcl-2-overexpressing cells: evidence for a caspase-mediated, proteolytic inactivation of Bcl-2

Bcl-2 oncogene expression plays a role in the establishment of persistent viral infection by blocking virus-induced apoptosis. This might be achieved by preventing virus-induced activation of caspase-3, an IL-1beta-converting enzyme (ICE)-like cysteine protease that has been implicated in the death effector phase of apoptosis. Contrary to this model, we show that three cell types highly overexpressing functional Bcl-2 displayed caspase-3 activation and underwent apoptosis in response to infection with alphaviruses Semliki Forest and Sindbis as efficiently as vector control counterparts. In all three cell types, overexpressed 26 kDa Bcl-2 was cleaved into a 23 kDa protein. Antibody epitope mapping revealed that cleavage occurred at one or two target sites for caspases within the amino acid region YEWD31 (downward arrow) AGD34 (downward arrow) A, removing the N-terminal BH4 region known to be essential for the death-protective activity of Bcl-2. Preincubation of cells with the caspase inhibitor Z-VAD prevented Bcl-2 cleavage and partially restored the protective activity of Bcl-2 against virus-induced apoptosis. Moreover, a murine Bcl-2 mutant having Asp31, Asp34 and Asp36 substituted by Glu was resistant to proteolytic cleavage and abrogated apoptosis following virus infection. These findings indicate that alphaviruses can trigger a caspase-mediated inactivation of Bcl-2 in order to evade the death protection imposed by this survival factor.

Caspases are activated in a branched protease cascade and control distinct downstream processes in Fas-induced apoptosis

Two novel synthetic tetrapeptides, VEID-CHO and DMQD-CHO, could selectively inhibit caspase-6 and caspase-3, respectively. We used these inhibitors to dissect the pathway of caspase activation in Fas-stimulated Jurkat cells and identify the roles of each active caspase in apoptotic processes. Affinity labeling techniques revealed a branched protease cascade in which caspase-8 activates caspase-3 and -7, and caspase-3, in turn, activates caspase-6. Both caspase-6 and -3 have major roles in nuclear apoptosis. Caspase-6 cleaves nuclear mitotic apparatus protein (NuMA) and mediates the shrinkage and fragmentation of nuclei. Caspase-3 cleaves NuMA at sites distinct from caspase-6, and mediates DNA fragmentation and chromatin condensation. It is also involved in extranuclear apoptotic events: cleavage of PAK2, formation of apoptotic bodies, and exposure of phosphatidylserine on the cell surface. In contrast, a caspase(s) distinct from caspase-3 or -6 mediates the disruption of mitochondrial membrane potential (permeability transition) and the shrinkage of cytoplasm. These findings demonstrate that caspases are organized in a protease cascade, and that each activated caspase plays a distinct role(s) in the execution of Fas-induced cell death.

E4orf4, a novel adenovirus death factor that induces p53-independent apoptosis by a pathway that is not inhibited by zVAD-fmk

In the absence of E1B, the 289-amino acid product of human adenovirus type 5 13S E1A induces p53-independent apoptosis by a mechanism that requires viral E4 gene products (Marcellus, R.C., J.C. Teodoro, T. Wu, D.E. Brough, G. Ketner, G.C. Shore, and P.E. Branton. 1996. J. Virol. 70:6207-6215) and involves a mechanism that includes activation of caspases (Boulakia, C.A., G. Chen, F.W. Ng, J. G. Teodoro, P.E. Branton, D.W. Nicholson, G.G. Poirier, and G.C. Shore. 1996. Oncogene. 12:529-535). Here, we show that one of the E4 products, E4orf4, is highly toxic upon expression in rodent cells regardless of the p53 status, and that this cytotoxicity is significantly overcome by coexpression with either Bcl-2 or Bcl-XL. Conditional expression of E4orf4 induces a cell death process that is characterized by apoptotic hallmark features, such as externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytoplasmic vacuolation, condensation of chromatin, and internucleosomal DNA degradation. However, the wide-spectrum inhibitor of caspases, tetrapeptide zVAD-fmk, does not affect any of these apoptogenic manifestations, and does not alter the kinetics of E4orf4-induced cell death. Moreover, E4orf4 expression does not result in activation of the downstream effector caspase common to most apoptosis-inducing events, caspase-3 (CPP32). We conclude, therefore, that in the absence of E1A, E4orf4 is sufficient by itself to trigger a p53-independent apoptosis pathway that may operate independently of the known zVAD-inhibitable caspases, and that may involve an as yet uncharacterized mechanism.

Multiple pathways of neuronal death induced by DNA-damaging agents, NGF deprivation, and oxidative stress

Here, we compare the pathways by which DNA-damaging agents, NGF deprivation, and superoxide dismutase 1 (SOD1) depletion evoke apoptosis of sympathetic neurons. Previous work raised the hypothesis that cell cycle signaling plays a required role in neuronal apoptosis elicited by NGF deprivation and the DNA-damaging agent camptothecin. To test this hypothesis, we extended our investigation of DNA-damaging agents to cytosine arabinoside (AraC) and UV irradiation. As with NGF deprivation and camptothecin treatment, the cyclin-dependent kinase inhibitors flavopiridol and olomoucine protected neurons from apoptosis induced by AraC and UV treatment. These observations support the model that camptothecin, AraC, and UV treatment cause DNA damage, which leads to apoptosis by a mechanism that, as in the case of NGF deprivation, includes activation of cell cycle components. Flavopiridol and olomoucine, however, had no effect on death induced by SOD1 depletion, suggesting that CDKs do not play a role in this paradigm of neuronal death. To compare further the mechanisms of death evoked by NGF withdrawal, SOD1 depletion, and DNA-damaging agents, we investigated their responses to inhibitors of cysteine aspartases, elements of apoptotic pathways. The V-ICEinh and BAF, two peptide inhibitors of cysteine aspartases, protected neurons in all three death paradigms. In contrast, the cysteine aspartase inhibitory peptide zVAD-fmk conferred protection from NGF withdrawal and SOD1 depletion, but not DNA-damaging agents, whereas acYVAD-cmk protected only from SOD1 depletion. Taken together, these findings indicate that three different apoptotic stimuli activate separate pathways of death in the same neuron type.

Multiple pathways of neuronal death induced by DNA-damaging agents, NGF deprivation, and oxidative stress

Here, we compare the pathways by which DNA-damaging agents, NGF deprivation, and superoxide dismutase 1 (SOD1) depletion evoke apoptosis of sympathetic neurons. Previous work raised the hypothesis that cell cycle signaling plays a required role in neuronal apoptosis elicited by NGF deprivation and the DNA-damaging agent camptothecin. To test this hypothesis, we extended our investigation of DNA-damaging agents to cytosine arabinoside (AraC) and UV irradiation. As with NGF deprivation and camptothecin treatment, the cyclin-dependent kinase inhibitors flavopiridol and olomoucine protected neurons from apoptosis induced by AraC and UV treatment. These observations support the model that camptothecin, AraC, and UV treatment cause DNA damage, which leads to apoptosis by a mechanism that, as in the case of NGF deprivation, includes activation of cell cycle components. Flavopiridol and olomoucine, however, had no effect on death induced by SOD1 depletion, suggesting that CDKs do not play a role in this paradigm of neuronal death. To compare further the mechanisms of death evoked by NGF withdrawal, SOD1 depletion, and DNA-damaging agents, we investigated their responses to inhibitors of cysteine aspartases, elements of apoptotic pathways. The V-ICEinh and BAF, two peptide inhibitors of cysteine aspartases, protected neurons in all three death paradigms. In contrast, the cysteine aspartase inhibitory peptide zVAD-fmk conferred protection from NGF withdrawal and SOD1 depletion, but not DNA-damaging agents, whereas acYVAD-cmk protected only from SOD1 depletion. Taken together, these findings indicate that three different apoptotic stimuli activate separate pathways of death in the same neuron type.

Resistance to apoptosis correlates with a highly proliferative phenotype and loss of Fas and CPP32 (caspase-3) expression in human leukemia cells

Apoptosis induced by effector cells of the immune system or by cytotoxic drugs is a main mechanism mediating the prevention or elimination of tumoral cells. For instance, the human T-cell leukemia Jurkat is sensitive to Fas-induced apoptosis and to activation-induced cell death (AICD), and the promonocytic leukemia U937 is sensitive to Fas- and TNF-induced apoptosis. In this work, we have analyzed the mechanisms of resistance to physiological or pharmacological apoptosis in human leukemia by generating highly proliferative (hp) sub-lines derived from Jurkat and U937 cells. These hp sub-lines were resistant to Fas- and TNF-induced apoptosis, as well as to AICD. This was due to the complete loss of Fas and TNFR surface expression and, in the case of Jurkat-derived sub-lines, also of CD3, CD2 and CD59 molecules. The sub-lines also completely lacked the expression of the apoptotic protease CPP32, present in parental cells. Moreover, these sub-lines were no longer sensitive to doxorubicin-induced apoptosis, which was efficiently blocked by the general caspase inhibitor Z-VAD-fmk in the parental cell lines. These data suggest a molecular mechanism for the development of resistance of leukemic cells to physiological and pharmacological apoptosis inducers, giving rise to highly proliferative tumoral phenotypes. These results also indicate that Fas and CPP32 could be useful prognostic markers for the progression and/or therapy outcome of human leukemias.

The c-Jun N-Terminal Kinase Cascade Plays a Role in Stress-Induced Apoptosis in Jurkat Cells by Up-Regulating Fas Ligand Expression

T lymphocytes undergo apoptosis in response to cellular stress, including UV exposure and gamma irradiation. However, the mechanism by which stress stimuli induce apoptosis is not well understood. While stress stimuli induce the activation of the c-Jun N-terminal kinase (JNK) pathway, it is not clear whether the JNK cascade is activated as a result of cell death or whether the cascade participates in inducing apoptosis. Using a Jurkat T cell line transfected with dominant active (DA)-mitogen-activated protein kinase kinase kinase (MEKK1) in a tetracycline-regulated expression system, we found that expression of DA-MEKK1 results in the apoptosis of Jurkat cells in parallel with prolonged JNK activation. Moreover, DA-MEKK1 induced Fas ligand (FasL) cell surface and mRNA expression, as well as FasL promoter activation. Interference with Fas/FasL interaction prevented DA-MEKK1-mediated apoptosis. In comparing the effect of different stress stimuli to DA-MEKK1, we found that UV, gamma irradiation, and anisomycin prolonged JNK activation in parallel with FasL expression and onset of cell death. In addition, these stimuli also enhance cell surface expression of FasL. Interference with Fas/FasL interactions inhibited anisomycin but not UV- or gamma irradiation-induced apoptosis. Our data show that while the JNK pathway contributes to stress-induced apoptosis in T lymphocytes by regulating FasL expression, not all stress stimuli use the same cell death pathway.

Doxorubicin-induced apoptosis in human T-cell leukemia is mediated by caspase-3 activation in a Fas-independent way

It has recently been proposed that doxorubicin (DOX) can induce apoptosis in human T-leukemia cells via the Fas/FasL system in an autocrine/paracrine way. We show here that treatment of Jurkat cells with either anti-Fas antibodies, anthracyclin drugs or actinomycin D induces the activation of CPP32 (caspase-3) and apoptosis. However, DOX treatment did not induce the expression of membrane FasL or the release of soluble FasL and co-incubation with blocking anti-Fas antibodies prevented Fas-induced but not DOX-induced apoptosis. All the morphological and biochemical signs of apoptosis induced by anti-Fas or DOX can be prevented by Z-VAD-fmk, a general caspase inhibitor. DEVD-cho, a specific inhibitor of CPP32-like caspases which completely blocks Fas-mediated apoptosis, prevented drug-induced nuclear apoptosis but not cell death. We conclude that: (i) DOX-induced apoptosis in human T-leukemia/lymphoma is Fas-independent and (ii) caspase-3 is responsible of DOX-induced nuclear apoptosis but other Z-VAD-sensitive caspases are implicated in cell death.

Specific activation of the cysteine protease CPP32 during the negative selection of T cells in the thymus

Cysteine proteases of the CED-3 and ICE family have been recently proposed as the ultimate executioners in several mammalian cell death pathways. Among them, the cysteine protease CPP32 has been shown to participate in programmed cell death (PCD), or apoptosis, affecting lymphoid cells in vitro. In the thymus, negative selection is a mechanism through which developing thymocytes expressing a TcR with high affinity for self peptide-MHC complexes are eliminated by PCD. In order to investigate the role of CPP32 in thymic apoptosis, isolated thymocytes were submitted to cell surface CD3 crosslinking by immobilized anti-CD3 mAb or to dexamethasone treatment. Although apoptosis occurred in the absence or after crosslinking with anti-CD3 mAb, specific activation of CPP32, as assessed by the extent of proteolytic cleavage of the p32 zymogen, was only detected in thymocytes cultured in the presence of the immobilized antibody or dexamethasone. This activation was a very early event during apoptosis as it occurred before the exposure of phosphatidyl serine to the upper side of the cell membrane. This was observed both in anti-CD3- and dexamethasone-induced apoptosis. Moreover, using mice transgenic for pigeon cytochrome C (PCC)-specific TcR, we were able to show that, after injection of PCC, the activation of CPP32 and cleavage of its substrate occurred in thymocytes obtained from mice expressing a permissive MHC haplotype for PCC presentation (H-2k). Moreover, PCC induced apoptosis was blocked by the caspase inhibitor zVAD. While spontaneous apoptosis was not accompanied by detectable levels of CPP32 processing, it was characterized by the proteolysis of poly(ADP-ribose) polymerase (PARP) and was blocked by the cysteine protease inhibitor, zVAD-CH2F. Taken together, these results support the concept that CPP32 is among the earliest effectors of the pathway leading to negative selection of autoreactive thymocytes. Our results also suggest the involvement of a distinct CPP32-like cysteine protease in spontaneous apoptosis of thymocytes.

Internucleosomal DNA cleavage triggered by plasma membrane damage during necrotic cell death. Involvement of serine but not cysteine proteases

Autolytic DNA breakdown, detected as smears in electrophoretic gels, is a late event in necrosis. On the other hand, internucleosomal DNA cleavage, visualized as ladders, is thought to be a hallmark of apoptosis. We now report that this specific form of DNA fragmentation also occurs during necrosis and is an early event but appears to be triggered by proteolytic mechanisms significantly different from those documented in apoptosis. Treatment of MDCK cells with a mitochondrial uncoupler and a Ca2+ ionophore led to ATP depletion, necrotic morphology, and progressive fragmentation of DNA in an internucleosomal or ladder pattern. DNA breakdown was immediately preceded by increased permeability of the plasma membrane to macromolecules. Provision of glycine along with the noxious agents did not modify the extent of ATP depletion, but prevented plasma membrane damage. This was accompanied by complete inhibition of DNA fragmentation. Internucleosomal DNA cleavage was observed also during necrosis after rapid permeabilization of plasma membranes by detergents or streptolysin-O in hepatocytes, thymocytes, and P19, Jurkat, and MDCK cells. DNA fragmentation associated with necrosis was Ca2+/Mg2+ dependent, was suppressed by endonuclease inhibitors, and was abolished by serine protease inhibitors but not by inhibitors of interleukin-1 beta converting enzyme (ICE)-related proteases or caspases. Moreover, unlike apoptosis, it was not accompanied by caspase-mediated proteolysis. On the other hand, the cleavage-site-directed chymotryptic inhibitor N-tosyl-L-phenylalanyl-chloromethyl ketone (TPCK) suppressed DNA fragmentation not only in necrotic cells but also during Fas-mediated apoptosis, without inhibiting caspase-related proteolysis. The results suggest a novel pathway of endonuclease activation during necrosis not involving the participation of caspases. In addition, they indicate that techniques based on double-strand DNA breaks may not reliably differentiate between apoptosis and necrosis.

Interleukin-1 beta converting enzyme-like protease involvement in Fas-induced and activation-induced peripheral blood T cell apoptosis in HIV infection. TNF-related apoptosis-inducing ligand can mediate activation-induced T cell death in HIV infection

Apoptosis of peripheral blood T cells has been suggested to play an important role in the pathogenesis of human immunodeficiency virus (HIV) infection. Spontaneous, Fas (CD95)-induced and activation-induced T cell apoptosis have all been described in peripheral blood mononuclear cell cultures of HIV-infected individuals. We have previously shown that activation-induced T cell apoptosis is Fas independent in peripheral blood T cells from HIV+ individuals. In this study, we extend and confirm these observations by using an inhibitor of interleukin-1 beta converting enzyme (ICE) homologues. We show that z-VAD-fmk, a tripeptide inhibitor of ICE homologues, can inhibit Fas-induced apoptosis of peripheral blood CD4(+) and CD8+ T cells from asymptomatic HIV+ individuals. z-VAD-fmk also inhibited activation (anti-CD3)- induced CD4+ and CD8+ T cell apoptosis (AICD) in some but not all asymptomatic HIV+ individuals. Apoptosis was measured by multiparameter flow cytometry. The z-VAD-fmk inhibitor also enhanced survival of T cells in anti-Fas or anti-CD3 antibody-treated cultures and inhibited DNA fragmentation. AICD that could be inhibited by z-VAD-fmk was Fas independent and could be inhibited with a blocking monoclonal antibody to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a recently described member of the TNF/nerve growth factor ligand family. The above findings show that Fas-induced T cell apoptosis is ICE dependent in HIV infection. AICD can be blocked by ICE inhibitors in some patients, and this AICD is mediated by TRAIL. These results show that TRAIL can be a mediator of AICD in T cells. These different mechanisms of peripheral blood T cell apoptosis may play different roles in the pathogenesis of HIV infection.

drICE is an essential caspase required for apoptotic activity in Drosophila cells

Caspases are involved in the execution of cell death in all multicellular organisms so far studied, including the nematode worm, fruit fly and vertebrates. While Caenorhabditis elegans has only a single identified caspase, CED-3, whose activity is absolutely required for all developmental programmed cell deaths, most mammalian cell types express multiple caspases with varying specificities. The fruit fly Drosophila melanogaster is genetically tractable, less complex than vertebrates and possesses two known caspases, DCP-1 and drICE. The fly may therefore provide a good model system for examining the hierarchy and relative roles of individual caspases in the execution of apoptosis. We have examined the role of drICE in in vitro apoptosis of the D.melanogaster cell line S2. We show that cytoplasmic lysates made from S2 cells undergoing apoptosis induced by either reaper (rpr) expression or cycloheximide treatment contain a caspase activity with DEVD specificity which can cleave p35, lamin DmO, drICE and DCP-1 in vitro, and which can trigger chromatin condensation in isolated nuclei. Using antibodies specific to drICE, we show that immunodepletion of drICE from these lysates is sufficient to remove most measurable in vitro apoptotic activity, and that re-addition of exogenous drICE to such immunodepleted lysates restores apoptotic activity. We conclude that, at least in S2 cells, drICE can be the sole caspase effector of apoptosis.

Caspase-3-generated fragment of gelsolin: effector of morphological change in apoptosis

The caspase-3 (CPP32, apopain, YAMA) family of cysteinyl proteases has been implicated as key mediators of apoptosis in mammalian cells. Gelsolin was identified as a substrate for caspase-3 by screening the translation products of small complementary DNA pools for sensitivity to cleavage by caspase-3. Gelsolin was cleaved in vivo in a caspase-dependent manner in cells stimulated by Fas. Caspase-cleaved gelsolin severed actin filaments in vitro in a Ca2+-independent manner. Expression of the gelsolin cleavage product in multiple cell types caused the cells to round up, detach from the plate, and undergo nuclear fragmentation. Neutrophils isolated from mice lacking gelsolin had delayed onset of both blebbing and DNA fragmentation, following apoptosis induction, compared with wild-type neutrophils. Thus, cleaved gelsolin may be one physiological effector of morphologic change during apoptosis.

Caspases: the executioners of apoptosis

Apoptosis is a major form of cell death, characterized initially by a series of stereotypic morphological changes. In the nematode Caenorhabditis elegans, the gene ced-3 encodes a protein required for developmental cell death. Since the recognition that CED-3 has sequence identity with the mammalian cysteine protease interleukin-1 beta-converting enzyme (ICE), a family of at least 10 related cysteine proteases has been identified. These proteins are characterized by almost absolute specificity for aspartic acid in the P1 position. All the caspases (ICE-like proteases) contain a conserved QACXG (where X is R, Q or G) pentapeptide active-site motif. Capases are synthesized as inactive proenzymes comprising an N-terminal peptide (prodomain) together with one large and one small subunit. The crystal structures of both caspase-1 and caspase-3 show that the active enzyme is a heterotetramer, containing two small and two large subunits. Activation of caspases during apoptosis results in the cleavage of critical cellular substrates, including poly(ADP-ribose) polymerase and lamins, so precipitating the dramatic morphological changes of apoptosis. Apoptosis induced by CD95 (Fas/APO-1) and tumour necrosis factor activates caspase-8 (MACH/FLICE/Mch5), which contains an N-terminus with FADD (Fas-associating protein with death domain)-like death effector domains, so providing a direct link between cell death receptors and the caspases. The importance of caspase prodomains in the regulation of apoptosis is further highlighted by the recognition of adapter molecules, such as RAIDD [receptor-interacting protein (RIP)-associated ICH-1/CED-3-homologous protein with a death domain]/CRADD (caspase and RIP adapter with death domain), which binds to the prodomain of caspase-2 and recruits it to the signalling complex. Cells undergoing apoptosis following triggering of death receptors execute the death programme by activating a hierarchy of caspases, with caspase-8 and possibly caspase-10 being at or near the apex of this apoptotic cascade.

The central executioner of apoptosis: multiple connections between protease activation and mitochondria in Fas/APO-1/CD95- and ceramide-induced apoptosis

According to current understanding, cytoplasmic events including activation of protease cascades and mitochondrial permeability transition (PT) participate in the control of nuclear apoptosis. However, the relationship between protease activation and PT has remained elusive. When apoptosis is induced by cross-linking of the Fas/APO-1/CD95 receptor, activation of interleukin-1beta converting enzyme (ICE; caspase 1) or ICE-like enzymes precedes the disruption of the mitochondrial inner transmembrane potential (DeltaPsim). In contrast, cytosolic CPP32/ Yama/Apopain/caspase 3 activation, plasma membrane phosphatidyl serine exposure, and nuclear apoptosis only occur in cells in which the DeltaPsim is fully disrupted. Transfection with the cowpox protease inhibitor crmA or culture in the presence of the synthetic ICE-specific inhibitor Ac-YVAD.cmk both prevent the DeltaPsim collapse and subsequent apoptosis. Cytosols from anti-Fas-treated human lymphoma cells accumulate an activity that induces PT in isolated mitochondria in vitro and that is neutralized by crmA or Ac-YVAD.cmk. Recombinant purified ICE suffices to cause isolated mitochondria to undergo PT-like large amplitude swelling and to disrupt their DeltaPsim. In addition, ICE-treated mitochondria release an apoptosis-inducing factor (AIF) that induces apoptotic changes (chromatin condensation and oligonucleosomal DNA fragmentation) in isolated nuclei in vitro. AIF is a protease (or protease activator) that can be inhibited by the broad spectrum apoptosis inhibitor Z-VAD.fmk and that causes the proteolytical activation of CPP32. Although Bcl-2 is a highly efficient inhibitor of mitochondrial alterations (large amplitude swelling + DeltaPsim collapse + release of AIF) induced by prooxidants or cytosols from ceramide-treated cells, it has no effect on the ICE-induced mitochondrial PT and AIF release. These data connect a protease activation pathway with the mitochondrial phase of apoptosis regulation. In addition, they provide a plausible explanation of why Bcl-2 fails to interfere with Fas-triggered apoptosis in most cell types, yet prevents ceramide- and prooxidant-induced apoptosis.

Caspase activity is required for commitment to Fas-mediated apoptosis

Recognition of the widespread importance of apoptosis has been one of the most significant changes in the biomedical sciences in the past decade. The molecular processes controlling and executing cell death through apoptosis are, however, still poorly understood. The ICE (Interleukin-1beta Converting Enzyme) family-recently named the caspases for cysteine aspartate-specific proteases-plays a central role in apoptosis and may well constitute part of the conserved core mechanism of the process. Potentially, these proteases may be of great significance, both in the pathology associated with failure of apoptosis and also as targets for therapeutic intervention where apoptosis occurs inappropriately, e.g. in degenerative disease and AIDS. However, this is only likely if caspase activity is required before commitment to mammalian cell death. Here, we have used both peptide inhibitors and crmA transfection to inhibit these proteases in intact cells. Our experiments show that selective inhibition of some caspases protects human T cells (Jurkat and CEM-C7) from Fas-induced apoptosis, dramatically increasing their survival (up to 320-fold) in a colony-forming assay. This suggests that dysfunction of some, but not all, caspases could indeed play a crucial part in the development of some cancers and autoimmune disease, and also that these proteases could be appropriate molecular targets for preventing apoptosis in degenerative disease.

Activation of protein kinase C attenuates early signals in Fas-mediated apoptosis

Activation of protein kinase C (PKC) has been reported to inhibit Fas (APO-1, CD95)-mediated apoptosis in different cellular systems. Human Jurkat leukemic T cells express the Fas antigen in the cell membrane and undergo apoptosis upon cross-linking by anti-Fas monoclonal antibodies (mAb). Cleavage of the apoptosis-associated protease CPP32 and its substrate poly(ADP-ribose)polymerase are observed after the engagement of Fas antigen with mAb. In this report, we show that all these effects are substantially inhibited by the activation of PKC with a phorbol ester. Bisindolylmaleimide, an inhibitor of PKC, prevents phorbol ester-induced down-regulation of Fas signaling. Inhibition of Fas-mediated cell death by phorbol ester is also observed in other human leukemic T cell lines. Cross-linking of Fas antigen by mAb results in the rapid increase in tyrosine phosphorylation of several protein substrates which is further elevated in the presence of the protein tyrosine phosphatase inhibitor, orthovanadate. Furthermore, orthovanadate markedly enhances the cell death response to Fas mAb in different human leukemic T cell lines and human T cell blasts. These effects of orthovanadate on early tyrosine phosphorylation and cell death are clearly diminished by PKC activation. These results strongly suggest that tyrosine phosphorylation is involved in Fas signaling in apoptosis and that PKC plays a negative role in Fas-mediated apoptosis by counteracting at a very early stage the signals generated following cross-linking of this receptor.

Identification of a Drosophila melanogaster ICE/CED-3-related protease, drICE

Cysteine proteases of the ICE/CED-3 family (caspases) are required for the execution of programmed cell death (PCD) in a wide range of multicellular organisms. Caspases are implicated in the execution of apoptosis in Drosophila melanogaster by the observation that expression of baculovirus p35, a caspase inhibitor, blocks cell death in vivo in Drosophila. We report here the identification and characterization of drICE, a D. melanogaster caspase. We show that overexpression of drICE sensitizes Drosophila cells to apoptotic stimuli and that expression of an N-terminally truncated form of drICE rapidly induces apoptosis in Drosophila cells. Induction of apoptosis by rpr overexpression or by cycloheximide or etoposide treatment of Drosophila cells results in proteolytic processing of drICE. We further show that drICE is a cysteine protease that cleaves baculovirus p35 and Drosophila lamin DmO in vitro and that drICE is expressed at all the stages of Drosophila development at which PCD can be induced. Taken together, these results strongly argue that drICE is an apoptotic caspase that acts downstream of rpr. drICE is therefore the first unequivocal link between the molecular machinery of Drosophila cell death and the conserved machinery of Caenorhabditis elegans and vertebrates. Identification of drICE should facilitate the elucidation of upstream regulators and downstream targets of caspases by genetic screening.

Processing/activation of at least four interleukin-1beta converting enzyme-like proteases occurs during the execution phase of apoptosis in human monocytic tumor cells

Identification of the processing/activation of multiple interleukin-1beta converting enzyme (ICE)-like proteases and their target substrates in the intact cell is critical to our understanding of the apoptotic process. In this study we demonstrate processing/activation of at least four ICE-like proteases during the execution phase of apoptosis in human monocytic tumor THP.1 cells. Apoptosis was accompanied by processing of Ich-1, CPP32, and Mch3alpha to their catalytically active subunits, and lysates from these cells displayed a proteolytic activity with kinetics, characteristic of CPP32/Mch3alpha but not of ICE. Fluorescence-activated cell sorting was used to obtain pure populations of normal and apoptotic cells. In apoptotic cells, extensive cleavage of Ich-1, CPP32, and Mch3alpha. was observed together with proteolysis of the ICE-like protease substrates, poly (ADP-ribose) polymerase (PARP), the 70-kD protein component of U1 small nuclear ribonucleoprotein (U1-70K), and lamins A/B. In contrast, no cleavage of CPP32, Mch3alpha or the substrates was observed in normal cells. In cells exposed to an apoptotic stimulus, some processing of Ich-1 was detected in morphologically normal cells, suggesting that cleavage of Ich-1 may occur early in the apoptotic process. The ICE-like protease inhibitor, benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethyl ketone (Z-VAD.FMK), inhibited apoptosis and cleavage of Ich-1, CPP32, Mch3alpha, Mch2alpha, PARP, U1-70K, and lamins. These results suggest that Z-VAD.FMK inhibits apoptosis by inhibiting a key effector protease upstream of Ich-1, CPP32, Mch3alpha, and Mch2alpha. Together these observations demonstrate that processing/activation of Ich-1, CPP32, Mch3alpha, and Mch2alpha accompanies the execution phase of apoptosis in THP.1 cells. This is the first demonstration of the activation of at least four ICE-like proteases in apoptotic cells, providing further evidence for a requirement for the activation of multiple ICE-like proteases during apoptosis.

Antiapoptotic effect of benzyloxycarbonyl-aspartyl-(beta-tertier-butyl ester)-bromomethylketone (Z-D(OtBu)-Bmk), an intermediate of interleukin-1 beta converting enzyme inhibitors

The effect of several interleukin-1 beta converting enzyme (ICE) inhibitors on apoptosis was examined. The ICE inhibitors tested were peptide aldehydes such as ethyloxycarbonyl-Ala-Tyr-Val-Ala-Asp-aldehyde (Etoco-AYVAD-CHO), acetyl-Tyr-Val-Ala-Asp-aldehyde (Ac-YVAD-CHO), benzyloxycarbonyl-Val-His-Asp-aldehyde (Z-VHD-CHO), a tetrapeptide chloromethylketone, acetyl-Tyr-Val-Ala-Asp-chloromethylketone (Ac-YVAD-Cmk) and their common intermediate benzyloxycarbonyl-Asp-(beta-tertier-butyl ester)-bromomethylketone (Z-D(OtBu)-Bmk). Apoptosis was induced with several chemical agents conventionally used for this purpose in THP-1, L929, NB-41A3 cell lines and mouse thymocytes. DNA fragmentation during apoptosis was measured by conventional gel electrophoresis and ELISA. The cell morphology was examined by hematoxylin/eosin staining method. Cell viability was also monitored by MTT assay. Contrary to expectations, the peptide aldehydes listed above and Ac-YVAD-Cmk, known as highly specific ICE inhibitors, did not inhibit the apoptosis of these cell types. However, Z-D(OtBu)-Bmk, which had no relevant inhibitory activity on ICE, potently blocked the DNA fragmentation in THP-1 cells and thymocytes whichever of the inducing agents was used. In the other two cell lines Z-D(OtBu)-Bmk was inactive. The apoptotic cell morphology was also inhibited by Z-D(OtBu)-Bmk. Nevertheless, Z-D(OtBu)-Bmk failed to prevent the loss of mitochondrial activity and the cell destruction in the late phase of apoptosis. These data suggest that ICE is not involved in the apoptotic cell death induced by chemical agents. Thus, Z-D(OtBu)-Bmk, a common intermediate of some ICE inhibitors, may be a useful antiapoptotic agent for studying the early events of apoptosis in some cell types.

Bax promotes neuronal cell death and is downregulated during the development of the nervous system

The Bcl-2 and Bcl-x proteins suppress programmed cell death, whereas Bax promotes apoptosis. We investigated the pattern of expression of Bcl-2, Bax and Bcl-x during neuronal differentiation and development. All three proteins were widely expressed in neonatal rats but, in the adult, Bax levels were 20- to 140-fold lower in the cerebral cortex, cerebellum and heart muscle, whereas Bcl-x was not downregulated in any of the tissues examined. In the cerebral cortex and cerebellum, the decrease in Bax levels occurred after the period of developmental cell death. Further, microinjection of a Bax expression vector into cultured sympathetic neurons, which depend on nerve growth factor for survival, induced apoptosis in the presence of survival factor and increased the rate of cell death after nerve growth factor withdrawal. This effect could be blocked by co-injection of an expression vector for Bcl-xL or for the baculovirus p35 protein, an inhibitor of caspases (ICE-like proteases). These results suggest that, during development, the sensitivity of neurons to signals that induce apoptosis may be regulated by modulating Bax levels and that Bax-induced death requires caspase activity.

Selective activation of caspases during apoptotic induction in HL-60 cells. Effects Of a tetrapeptide inhibitor

Apoptosis is a highly regulated biochemical process that results in the selective death of cells. Members of the caspase family of cysteine proteases play a pivotal role in the effector phase of apoptosis. We show that, in HL-60 cells, the addition of either anisomycin, a protein synthesis inhibitor, or geranylgeraniol, an intermediate in the cholesterol biosynthetic pathway, results in a rapid and en masse induction of apoptosis. The levels of actin, p42 and p44 MAPK, JNK1, JNK2, p38, and PCNA were not substantially altered during this process. Although these treatments appear to function by diverse pathways, they both result in the processing and activation of caspase-3 (CPP32beta/Yama/Apopain). In contrast, no activation of caspase-1 (interleukin-1beta converting enzyme (ICE)) was observed. Furthermore, we obtained ambiguous results regarding the activation of caspase-2 (Ich-1) depending on the antibody used. Pretreatment of the cells with benzyloxycarbonyl-Val-Ala-Asp-(OMe)-fluoromethylketone (zVAD.fmk), a tetrapeptide inhibitor of caspases, prevented the induction of apoptosis for 24 h. Even after 72 h of treatment, some cells were still alive and progressing through the cell cycle, suggesting that blockage of caspase activity is able to protect cells. These results suggest that selective activation of some caspases is necessary to induce apoptosis in HL-60 cells.

Substrate and inhibitor specificity of interleukin-1 beta-converting enzyme and related caspases

Interleukin-1beta-converting enzyme (ICE) is a novel cysteine protease responsible for the cleavage of pre-interleukin-1beta (pre-IL-1beta) to the mature cytokine and a member of a family of related proteases (the caspases) that includes the Caenorhabditis elegans cell death gene product, CED-3. In addition to their sequence homology, these cysteine proteases display an unusual substrate specificity for peptidyl sequences with a P1 aspartate residue. We have examined the kinetics of processing pre-IL-1beta to the mature form by ICE and three of its homologs, TX, CPP-32, and CMH-1. Of the ICE homologs, only TX processes pre-IL-1beta, albeit with a catalytic efficiency 250-fold less than ICE itself. We also investigated the ability of these four proteases to process poly(ADP-ribose) polymerase, a DNA repair enzyme that is cleaved within minutes of the onset of apoptosis. Every caspase examined cleaves PARP, with catalytic efficiencies ranging from 2.3 x 10(6) M-1 s-1 for CPP32 to 1.0 x 10(3) M-1 s-1 for TX. In addition, we report kinetic constants for several reversible inhibitors and irreversible inactivators, which have been used to implicate one or more caspases in the apoptotic proteolysis cascade. Ac-Asp-Glu-Val-Asp aldehyde (DEVD-CHO) is a potent inhibitor of CPP-32 with a Ki value of 0.5 nM, but is also potent as inhibitor of CMH-1 (Ki = 35 nM) and ICE (Ki = 15 nM). The x-ray crystal structure of DEVD-CHO complexed to ICE presented here reveals electrostatic interactions not present in the Ac-YVAD-CHO co-complex structure (Wilson, K. P., Black, J.-A. F., Thomson, J. A., Kim, E. E., Griffith, J. P., Navia, M. A., Murcko, M. A., Chambers, S. P., Aldape, R. A., Raybuck, S. A., and Livingston, D. J. (1994) Nature 370, 270-275), accounting for the surprising potency of this inhibitor against ICE.

Structure of recombinant human CPP32 in complex with the tetrapeptide acetyl-Asp-Val-Ala-Asp fluoromethyl ketone

The cysteine protease CPP32 has been expressed in a soluble form in Escherichia coli and purified to >95% purity. The three-dimensional structure of human CPP32 in complex with the irreversible tetrapeptide inhibitor acetyl-Asp-Val-Ala-Asp fluoromethyl ketone was determined by x-ray crystallography at a resolution of 2.3 A. The asymmetric unit contains a (p17/p12)2 tetramer, in agreement with the tetrameric structure of the protein in solution as determined by dynamic light scattering and size exclusion chromatography. The overall topology of CPP32 is very similar to that of interleukin-1beta-converting enzyme (ICE); however, differences exist at the N terminus of the p17 subunit, where the first helix found in ICE is missing in CPP32. A deletion/insertion pattern is responsible for the striking differences observed in the loops around the active site. In addition, the P1 carbonyl of the ketone inhibitor is pointing into the oxyanion hole and forms a hydrogen bond with the peptidic nitrogen of Gly-122, resulting in a different state compared with the tetrahedral intermediate observed in the structure of ICE and CPP32 in complex with an aldehyde inhibitor. The topology of the interface formed by the two p17/p12 heterodimers of CPP32 is different from that of ICE. This results in different orientations of CPP32 heterodimers compared with ICE heterodimers, which could affect substrate recognition. This structural information will be invaluable for the design of small synthetic inhibitors of CPP32 as well as for the design of CPP32 mutants.

Inhibition of interleukin 1beta converting enzyme family proteases reduces ischemic and excitotoxic neuronal damage

The interleukin 1beta converting enzyme (ICE) family plays a pivotal role in programmed cell death and has been implicated in stroke and neurodegenerative diseases. During reperfusion after filamentous middle cerebral artery occlusion, ICE-like cleavage products and tissue immunoreactive interleukin 1beta (IL-1beta) levels increased in ischemic mouse brain. Ischemic injury decreased after intracerebroventricular injections of ICE-like protease inhibitors, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD.FMK), acetyl-Tyr-Val-Ala-Asp-chloromethylketone, or a relatively selective inhibitor of CPP32-like caspases, N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone, but not a cathepsin B inhibitor, N-benzyloxycarbonyl-Phe-Ala-fluoromethylketone. z-VAD.FMK decreased ICE-like cleavage products and tissue immunoreactive IL-1beta levels in ischemic mouse brain and reduced tissue damage when administered to rats as well. Only z-VAD.FMK and acetyl-Tyr-Val-Ala-Asp-chloromethylketone reduced brain swelling, and N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone did not attenuate the ischemia-induced increase in tissue IL-1beta levels. The three cysteine protease inhibitors significantly improved behavioral deficits, thereby showing that functional recovery of ischemic neuronal tissue can follow blockade of enzymes associated with apoptotic cell death. Finally, we examined the effect of z-VAD.FMK on excitotoxicity and found that it protected against alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-induced or to a lesser extent N-methyl-D-aspartate-induced excitotoxic brain damage. Thus, ICE-like and CPP32-like caspases contribute to mechanisms of cell death in ischemic and excitotoxic brain injury and provide therapeutic targets for stroke and neurodegenerative brain damage.

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.

FLICE induced apoptosis in a cell-free system. Cleavage of caspase zymogens

Engagement of CD95 or tumor necrosis factor 1 receptor (TNFR-1) by ligand or agonist antibodies is capable of activating the cell death program, the effector arm of which is composed of mammalian interleukin-1beta converting enzyme (ICE)-like cysteine proteases (designated caspases) that are related to the Caenorhabditis elegans death gene, CED-3. Caspases, unlike other mammalian cysteine proteases, cleave their substrates following aspartate residues. Furthermore, proteases belonging to this family exist as zymogens that in turn require cleavage at internal aspartate residues to generate the two-subunit active enzyme. As such, family members are capable of activating each other. Remarkably, both CD95 and TNFR-1 death receptors initiate apoptosis by recruiting a novel ICE/CED-3 family member, designated FLICE/MACH, to the receptor signaling complex. Therefore, FLICE/MACH represents the apical triggering protease in the cascade. Consistent with this, recombinant FLICE was found capable of proteolytically activating downstream caspases. Furthermore, CrmA, a pox virus-encoded serpin that inhibits Fas and tumor necrosis factor-induced cell death attenuates the ability of FLICE to activate downstream caspases.

Inhibition of Ced-3/ICE-related proteases does not prevent cell death induced by oncogenes, DNA damage, or the Bcl-2 homologue Bak

There is increasing evidence for a central role in mammalian apoptosis of the interleukin-1 beta-converting enzyme (ICE) family of cysteine proteases, homologues of the product of the nematode "death" gene, ced-3. Ced-3 is thought to act as an executor rather than a regulator of programmed cell death in the nematode. However, it is not known whether mammalian ICE-related proteases (IRPs) are involved in the execution or the regulation of mammalian apoptosis. Moreover, an absolute requirement for one or more IRPs for mammalian apoptosis has yet to be established. We have used two cell-permeable inhibitors of IRPs, Z-Val-Ala-Asp.fluoromethylketone (ZVAD.fmk) and t-butoxy carbonyl-Asp.fluoromethylketone (BD.fmk), to demonstrate a critical role for IRPs in mammalian apoptosis induced by several disparate mechanisms (deregulated oncogene expression, ectopic expression of the Bcl-2 relative Bak, and DNA damage-induced cell death). In all instances, ZVAD.fmk and BD.fmk treatment inhibits characteristic biochemical and morphological events associated with apoptosis, including cleavage of nuclear lamins and poly-(ADP-ribose) polymerase, chromatin condensation and nucleosome laddering, and external display of phosphatidylserine. However, neither ZVAD.fmk nor BD.fmk inhibits the onset of apoptosis, as characterized by the onset of surface blebbing; rather, both act to delay completion of the program once initiated. In complete contrast, IGF-I and Bcl-2 delay the onset of apoptosis but have no effect on the kinetics of the program once initiated. Our data indicate that IRPs constitute part of the execution machinery of mammalian apoptosis induced by deregulated oncogenes, DNA damage, or Bak but that they act after the point at which cells become committed to apoptosis or can be rescued by survival factors. Moreover, all such blocked cells have lost proliferative potential and all eventually die by a process involving cytoplasmic blebbing.

Caspase inhibition selectively reduces the apoptotic component of oxygen-glucose deprivation-induced cortical neuronal cell death

Cultured mouse cortical neurons undergo apoptosis when exposed to staurosporine. The cell-permeable caspase inhibitor Z-Val-Ala-Asp fluoromethylketone (Z-VAD.FMK) attenuated this death, without altering overall protein synthesis. Z-VAD.FMK also attenuated cortical neuronal apoptosis induced by removal of serum. However, Z-VAD.FMK did not attenuate the excitotoxic necrosis induced by 5-min exposure to 100 microM NMDA, 24-h exposure to 100 microM kainate, or 90-min exposure to oxygen-glucose deprivation. We have previously shown that blockade of the excitotoxic component of oxygen-glucose deprivation-induced neuronal death with glutamate antagonists unmasks an apoptotic death. Treatment with Z-VAD.FMK, but not the cathepsin-B protease inhibitor Z-Phe-Ala fluoromethylketone (Z-FA.FMK), also attenuated this oxygen-glucose deprivation-induced neuronal apoptosis. These data support the idea that brain caspases mediate the apoptotic component of oxygen-glucose deprivation-induced neuronal death and raise the possibility that combining caspase inhibitors with glutamate antagonists might attenuate brain damage induced by hypoxic-ischemic insults in vivo.

Systemic injection of a tripeptide inhibits the intracellular activation of CPP32-like proteases in vivo and fully protects mice against Fas-mediated fulminant liver destruction and death

Mice injected with anti-Fas antibody die within a few hours with total liver destruction due to massive apoptosis of hepatocytes. We show that this is preceded and accompanied by the sequential activation of cysteine proteases of the interleukin 1 beta-converting enzyme (ICE) and CPP32 types in the cytosol of the hepatocytes, and that proCPP32 cleavage and enzymatic activity can be prevented by intravenous injections of the tripeptide N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk), an inhibitor of ICE-like proteases. Four Z-VAD.fmk injections at 1-hour intervals abolished all signs of liver damage after anti-Fas antibody injection and resulted in 100% long-range recovery, without residual tissue damage, from a condition otherwise uniformly fatal within < 3 hours. This treatment was effective even when delayed until some liver DNA degradation was already detectable. Injections of the tetrapeptide Ac-YVAD.cmk, more specific for the ICE-like subfamily of cysteine proteases but less cell permeable, also gave protection, but at higher doses and when injections started before that of anti-Fas antibody. These observations afford a way of temporarily modulating a number of apoptotic processes in vivo and may have important therapeutic implications in some human diseases.

Sequential activation of three distinct ICE-like activities in Fas-ligated Jurkat cells

ICE family proteases have been implicated as important effectors of the apoptotic pathway, perhaps acting hierarchically in a protease cascade. Using cleavage of endogenous protease substrates as probes, three distinct tiers of ICE-like activity were observed after Fas ligation in Jurkat cells. The earliest cleavage detected (30 min) was of fodrin, and produced a 150 kDa fragment. The second phase of cleavage (50 min) involved PARP, U1-70kDa and DNA-PKcs, all substrates of the CPP32-like proteases. Lamin B cleavage was observed during the third cleavage phase (90 min). Distinct inhibition profiles obtained using a panel of peptide-based inhibitors of ICE-like proteases clearly distinguished the three different cleavage phases. These studies provide evidence for a sequence of ICE-like proteolytic activity during apoptosis. The early fodrin cleavage, producing a 150 kDa fragment, identifies an ICE-like activity proximal to CPP32 in Fas-induced Jurkat cell apoptosis.