Protease activation during apoptosis: death by a thousand cuts?

An apoptotic endonuclease activated either by decreasing pH or by increasing calcium

DNA fragmentation in isolated nuclei from the murine IL3-dependent bone marrow cell line BAF3 could be stimulated either by decreasing pH below 6.5 or by adding microM calcium at neutral pH. An endonuclease which could also be stimulated either by a decrease in pH, to 6.5, or by the presence of microM calcium at neutral pH, was purified 10(4)-fold from nuclei of BAF3 cells. Digestion of DNA with the purified enzyme resulted in 5′-terminal hydroxyl and 3′-terminal phosphate ends. These characteristics are distinct from those described for other mammalian endonucleases. The possible role of this enzyme in genome digestion during apoptosis is discussed.

UV irradiation-induced apoptosis leads to activation of a 36-kDa myelin basic protein kinase in HL-60 cells

UV irradiation induces apoptosis (or programmed cell death) in HL-60 promyelocytic leukemia cells within 3 h. UV-induced apoptosis is accompanied by activation of a 36-kDa myelin basic protein kinase (p36 MBP kinase). This kinase is also activated by okadaic acid and retinoic acid-induced apoptosis. Irrespective of the inducing agent, p36 MBP kinase activation is restricted to the subpopulation of cells actually undergoing apoptosis. Activation of p36 MBP kinase occurs in enucleated cytoplasts, indicating no requirement for a nucleus or fragmented DNA in signaling. We also demonstrate the activation of p36 kinase in tumor necrosis factor-alpha- and serum starvation-induced cell death using the human prostatic tumor cell line LNCap and NIH 3T3 fibroblasts, respectively. We postulate that p36 MBP kinase is a common component in diverse signaling pathways leading to apoptosis.

Cleavage of lamin A by Mch2 alpha but not CPP32: multiple interleukin 1 beta-converting enzyme-related proteases with distinct substrate recognition properties are active in apoptosis

Although proteases related to the interleukin 1 beta-converting enzyme (ICE) are known to be essential for apoptotic execution, the number of enzymes involved, their substrate specificities, and their specific roles in the characteristic biochemical and morphological changes of apoptosis are currently unknown. These questions were addressed using cloned recombinant ICE-related proteases (IRPs) and a cell-free model system for apoptosis (S/M extracts). First, we compared the substrate specificities of two recombinant human IRPs, CPP32 and Mch2 alpha. Both enzymes cleaved poly-(ADP-ribose) polymerase, albeit with different efficiencies. Mch2 alpha also cleaved recombinant and nuclear lamin A at a conserved VEID decreases NG sequence located in the middle of the coiled-coil rod domain, producing a fragment that was indistinguishable from the lamin A fragment observed in S/M extracts and in apoptotic cells. In contrast, CPP32 did not cleave lamin A. The cleavage of lamin A by Mch2 alpha and by S/M extracts was inhibited by millimolar concentrations of Zn2+, which had a minimal effect on cleavage of poly (ADP-ribose) polymerase by CPP32 and by S/M extracts. We also found that N-(acetyltyrosinylvalinyl-N epsilon-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy]methyl ketone, which derivatizes the larger subunit of active ICE, can affinity label up to five active IRPs in S/M extracts. Together, these observations indicate that the processing of nuclear proteins in apoptosis involves multiple IRPs having distinct preferences for their apoptosis-associated substrates.

Early detachment of colon carcinoma cells during CD95(APO-1/Fas)-mediated apoptosis. I. De-adhesion from hyaluronate by shedding of CD44

Ligation of CD95 (APO-1/Fas) cell surface receptors induces death in apoptosis-sensitive cells. Induction of apoptosis in adherent gamma interferon-stimulated HT-29 and COLO 205 colon carcinoma cells by cross-linking CD95 with anti-APO-1 monoclonal antibody resulted in detachment of the cells from hyaluronate starting about 1 h after antibody exposure. Loss of adhesion was paralleled by a substantial reduction of the multifunctional cell surface adhesion molecule CD44. As evidenced by cycloheximide treatment, this effect was not caused by impaired protein synthesis. Depletion of surface CD44 was also not due to membrane blebbing, since cytochalasin B failed to inhibit ascension from hyaluronate. Instead, ELISA and time kinetics showed increasing amounts of soluble CD44 in the supernatant of CD95-triggered cells. SDS-PAGE revealed that soluble CD44 had an apparent molecular mass of about 20 kD less than CD44 immunoprecipitated from intact cells. Thus, CD95-triggering induced shedding of CD44. Shedding is a novel mechanism operative in early steps of CD95-mediated apoptosis. Shedding surface molecules like CD44 might contribute to the active disintegration of dying epithelial cells in vivo.

The adenovirus E3-14.7K protein and the E3-10.4K/14.5K complex of proteins, which independently inhibit tumor necrosis factor (TNF)-induced apoptosis, also independently inhibit TNF-induced release of arachidonic acid

Tumor necrosis factor (TNF) is an inflammatory cytokine that inhibits the replication of many viruses in cultured cells. We have reported that adenovirus (Ad) infection of TNF-resistant mouse cells renders them susceptible to lysis by TNF and that two sets of proteins encoded by the E3 transcription unit block TNF cytolysis. The E3 protein sets are named E3-14.7K (14,700 kDa) and E3-10.4K/14.5K (a complex of two proteins of 10,400 and 14,500 kDa). TNF activation of the 85-kDa cytosolic phospholipase A2 (cPLA2) is thought to be essential for TNF cytolysis (i.e.,TNF-induced apoptosis). Here we provide evidence that cPLA2 is important in the response of Ad-infected cells to TNF and that the mechanism by which E3-14.7K and E3-10.4K/14.5K inhibit TNF cytolysis is by inhibiting TNF activation of cPLA2. cPLA2 cleaves arachidonic acid (AA) specifically from membrane phospholipids; therefore, cPLA2 activity was measured by the release of 3H-AA from cells prelabeled with 3H-AA. Uninfected cells or cells infected with wild-type Ad were not lysed and did not release 3H-AA in response to TNF. In contrast, TNF treatment induced cytolysis and 3H-AA release in uninfected cells sensitized to TNF by treatment with cycloheximide and also in infected cells sensitized to TNF by expression of E1A. In C127 cells, in which either E3-14.7K or E3-10.4K/14.5K inhibits TNF cytolysis, either set of proteins inhibited TNF-induced release of 3H-AA. In C3HA cells, in which E3-14.7K but not E3-10.4K/14.5K prevents TNF cytolysis, E3-14.7K but not E3-10.4K/14.5K prevented TNF-induced release of 3H-AA. When five virus mutants with lesions in E3-14.7K were examined, there was a perfect correlation between a mutant's ability to inhibit both TNF-induced cytolysis and release of 3H-AA. E3-14.7K expressed in two stably transfected C127 cell lines prevented both TNF-cycloheximide-induced cytolysis and release of 3H-AA. The E3 proteins also prevented TNF-induced cytolysis and release of 3H-AA in mouse L929 cells, which are spontaneously sensitive to TNF. TNF cytolysis was blocked by dexamethasone, an inhibitor of PLA2 activity, and by nordihydroquaiaretic acid, which inhibits the metabolism of AA to the leukotrienes. Indomethacin, which blocks the formation of prostaglandins from AA, did not inhibit TNF cytolysis. The leukotrienes and prostaglandins are amplifiers of the inflammatory response. We propose that E3-14.7K and E3-10.4K/14.5K function independently in Ad infection to inhibit both cytolysis and inflammation induced by TNF.

Interleukin-1 beta-converting enzyme-like protease cleaves DNA-dependent protein kinase in cytotoxic T cell killing

Cytotoxic T cells (CTL) represent the major defense mechanism against the spread of virus infection. It is believed that the pore-forming protein, perforin, facilitates the entry of a series of serine proteases (particularly granzyme B) into the target cell which ultimately leads to DNA fragmentation and apoptosis. We demonstrate here that during CTL-mediated cytolysis the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), an enzyme implicated in the repair of double strand breaks in DNA, is specifically cleaved by an interleukin (IL)-1 beta-converting enzyme (ICE)-like protease. A serine protease inhibitor, 3,4-dichloroisocoumarin (DCl), which is known to block granzyme B activity, inhibited CTL-induced apoptosis and prevented the degradation of DNA-PKcs in cells but failed to prevent the degradation of purified DNA-PKcs by CTL extracts. However, Tyr-Val-Ala-Asp-CH2Cl (YVAD-CMK) and other cysteine protease inhibitors prevented the degradation of purified DNA-PKcs by CTL extracts. Furthermore, incubation of DNA-PKcs with granzyme B did not produce the same cleavage pattern observed in cells undergoing apoptosis and when this substrate was incubated with either CTL extracts or the ICE-like protease, CPP32. Sequence analysis revealed that the cleavage site in DNA-PKcs during CTL killing was the same as that when this substrate was exposed to CPP32. This study demonstrates for the first time that the cleavage of DNA-PKcs in this intact cell system is exclusively due to an ICE-like protease.

Cleavage of huntingtin by apopain, a proapoptotic cysteine protease, is modulated by the polyglutamine tract

Apoptosis has recently been recognized as a mode of cell death in Huntington disease (HD). Apopain, a human counterpart of the nematode cysteine protease death-gene product, CED-3, has a key role in proteolytic events leading to apoptosis. Here we show that apoptotic extracts and apopain itself specifically cleave the HD gene product, huntingtin. The rate of cleavage increases with the length of the huntingtin polyglutamine tract, providing an explanation for the gain-of-function associated with CAG expansion. Our results show that huntingtin is cleaved by cysteine proteases and suggest that HD might be a disorder of inappropriate apoptosis.

Bcl-2 regulates activation of apoptotic proteases in a cell-free system

BACKGROUND

Apoptosis plays an important role in the normal development and homeostasis of metazoans. Aberrations in this process have been implicated in several major human diseases, but its molecular mechanism is poorly understood. In animals as diverse as humans and nematodes, the Bcl-2 oncoprotein prevents or delays apoptosis, whereas proteases of the interleukin-1beta-converting enzyme (ICE) family are required, suggesting that they are components of a conserved mechanism controlling the onset of apoptosis.

RESULTS

A cell-free system produced from Xenopus laevis eggs reproduces nuclear events characteristic of apoptosis after a lag phase. We have used this system to define the temporal sequence of biochemical events and to examine the relationship between Bcl-2 and apoptotic proteases. Bcl-2 prevents apoptotic chromatin condensation and DNA cleavage, but only when added prior to the activation of a protease which has characteristics similar to the Ced-3 sub-family of ICE-like proteases and which cleaves poly(ADP-ribose) polymerase (PARP). Bcl-2 attenuates activation of this protease, an effect that does not require de novo protein synthesis or the presence of intact nuclei. The Ced-3-related protease CPP-32 is cleaved during the late stages of apoptosis in this system and after PARP cleavage. Generation of CPP-32-cleaving activity is inhibited by Bcl-2.

CONCLUSIONS

These experiments provide direct biochemical evidence that Bcl-2 protects against apoptosis, at least in part, by regulating the activation of a series of apoptotic proteases that cleave PARP and other substrates. This cell-free system provides a useful biochemical model for analyzing the molecular mechanism controlling the activation of these proteases.

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.

Activation of the CPP32 apoptotic protease by distinct signaling pathways with differential sensitivity to Bcl-xL

In the absence of growth factors, many types of mammalian cells undergo apoptosis. We and others have shown recently that growth factors promote cell survival by activating phosphatidylinositol 3-kinase (PI 3-kinase) in several cell types. In the present study, we have compared downstream elements of the apoptotic pathways induced by PI 3-kinase inhibitors and other stimuli. In U937 cells, both PI 3-kinase inhibitors (wortmannin and LY294002) and etoposide activated the CPP32 apoptotic protease by cleavage to active p17 subunits. In contrast, treatment with tumor necrosis factor alpha (TNFalpha) resulted in the accumulation of a distinct active CPP32 subunit, p20. Furthermore, overexpression of Bcl-xL blocked DNA fragmentation, CPP32 activation and cleavage of poly(ADP-ribose) polymerase in U937 cells treated with both PI 3-kinase inhibitors and etoposide, but not in cells treated with TNFalpha. Distinct patterns of CPP32 activation and differential sensitivities to Bcl-xL thus distinguish the cell death pathways activated by PI 3-kinase inhibition and DNA damage from that activated by TNFalpha.

Apoptosis induction resulting from proteasome inhibition

Proteases are known to be involved in the apoptotic pathway. We report here that benzyloxycarbonyl (Z)-Leu-Leu-leucinal(ZLLLal), a leupeptin analogue, can induce apoptosis in MOLT-4 and L5178Y cells. ZLLLal is a cell-permeant inhibitor of proteasome. Among the protease inhibitors tested, only calpain inhibitor I (acetyl-Leu-Leu-norleucinal) and ZLLLal caused a marked induction of apoptosis in MOLT-4 cells. In contrast Z-Leu-leucinal, a specific inhibitor of calpain, did not induce apoptosis. When MOLT-4 cells were incubated in the presence of ZLLLal, p53 accumulated in the cells. These results strongly suggest that inhibition of proteasome induces p53-dependent apoptosis and that proteasome can protect cell from apoptosis.

ICE-LAP6, a novel member of the ICE/Ced-3 gene family, is activated by the cytotoxic T cell protease granzyme B

Members of the ICE/Ced-3 gene family are likely effector components of the cell death machinery. Here, we characterize a novel member of this family designated ICE-LAP6. By phylogenetic analysis, ICE-LAP6 is classified into the Ced-3 subfamily which includes Ced-3, Yama/CPP32/apopain, Mch2, and ICE-LAP3/Mch3/CMH-1. Interestingly, ICE-LAP6 contains an active site QACGG pentapeptide, rather than the QACRG pentapeptide shared by other family members. Overexpression of ICE-LAP6 induces apoptosis in MCF7 breast carcinoma cells. More importantly, ICE-LAP6 is proteolytically processed into an active cysteine protease by granzyme B, an important component of cytotoxic T cell-mediated apoptosis. Once activated, ICE-LAP6 is able to cleave the death substrate poly(ADP-ribose) polymerase into signature apoptotic fragments.

Events in apoptosis. Acidification is downstream of protease activation and BCL-2 protection

Cytoplasmic acidification is now recognized as a feature of apoptosis in a variety of systems. However, its relation to other events in the process of apoptosis is not yet characterized. In this work, we examined the effect of BCL-2 overexpression on acidification mediated by cycloheximide treatment or Fas ligation in Jurkat T-lymphoblasts. We find that BCL-2 overexpression attenuates cytoplasmic acidification and apoptosis detected by annexin V labeling. Acidification and phosphatidylserine externalization were found to occur concurrently. We also examined the requirement for protease activation for cytoplasmic acidification to occur and found that inhibition of interleukin-1beta converting enzyme/CED-3 family proteases (using carbobenzoxy-Val-Ala-Asp-fluoromethylketone, an inhibitor of these proteases) prevents acidification and apoptosis mediated by Fas ligation. These studies suggest that BCL-2 acts at a point upstream of acidification and that protease activation is also upstream of acidification.

Cytotoxic T-cell-derived granzyme B activates the apoptotic protease ICE-LAP3

Cytotoxic T lymphocytes (CTLs) and natural killers (NK) cells provide immune surveillance against viruses and neoplasms, and play a central role in the pathogenesis of autoimmune disease, AIDS and graft rejection. Thus, it is important to understand the precise molecular mechanism(s) whereby cytotoxic lymphocytes destroy susceptible target cells. Granule-mediated cytotoxicity requires a combination of both perforin and granzyme B. Perforin polymerizes to form transmembrane channels and presumably allows granzyme B access to target cell substrates, which until recently, were unknown. One clue to the identity of the physiological substrate(s) activated by granzyme B comes from its unusual specificity for cleaving synthetic substrates after aspartate residues. Members of the ICE/CED-3 family of cysteine proteases are prime candidates as they are important apoptotic effectors and are expressed as zymogens, which can be processed to form active heterodimeric enzymes after cleavage at specific aspartate residues. Previous studies have shown that granzyme B proteolytically activates the cell death effector Yama/CPP32/apopain (referred to here as Yama). Here we report that granzyme B also activates ICE-LAP3/Mch3/CMH-1 (referred to here as ICE-LAP3), which, along with Yama and Mch2, forms a subset of the ICE/CED-3 family of cysteine proteases most closely related to the Caenorhabditis elegans cell death gene, CED-3. Importantly, Jurkat T cells incubated with granzyme B and a sublytic concentration of perforin undergo apoptosis, which is preceded by the activation of endogenous ICE-LAP3. Thus, we propose that granzyme B mediates apoptosis by directly engaging the target cell's death effector machinery, which is probably composed of an arsenal of intracellular, CED-3-like cysteine proteases.

Proteolytic cleavage during chemotherapy-induced apoptosis

Treatment with anticancer drugs sets into motion a morphologically and biochemically distinct type of cell death called apoptosis. Recent genetic and biochemical studies have suggested that proteases play a prominent role in the active phase of apoptotic cell death. Ongoing studies are aimed at identifying the proteases involved, the substrates that are cleaved, and the means by which the proteolytic process is regulated in nonapoptotic and apoptotic cells. The possibility that these findings will suggest new approaches to treating cancer and other diseases is discussed.

Rapid and specific efflux of reduced glutathione during apoptosis induced by anti-Fas/APO-1 antibody

Although human JURKAT T lymphocytes induced to undergo apoptosis with anti-Fas/APO-1 antibody were observed to rapidly lose reduced glutathione (GSH), increased concentrations of oxidized products were not detectable. Unexpectedly, the reduced tripeptide was instead quantitatively recovered in the incubation medium of the cells. As GSH loss was blocked by bromosulfophthalein and dibromosulfophthalein, known inhibitors of hepatocyte GSH transport, a specific export rather than nonspecific leakiness through plasma membranes is proposed to be responsible. Apoptosis was delayed when GSH-diethylesters were used to elevate intracellular GSH, although the high capacity of the activated efflux system quickly negated the benefit of this treatment. Stimulation of GSH efflux provides a novel mechanism whereby Fas/APO-1 ligation can deplete GSH. We speculate that it enhances the oxidative tonus of a responding cell without requiring an increase in the production of reactive oxygen species.

Constitutive expression of the machinery for programmed cell death

In the presence of cycloheximide (CHX) to inhibit protein synthesis, a high concentration of staurosporine (STS) induces almost all cells in explant cultures of 8/8 types of newborn mouse organs and 3/3 types of adult mouse organs to die with the characteristic features of apoptosis. Eggs and blastomeres also die in this way when treated with STS and CHX, although they are less sensitive to this treatment than trophectoderm or inner cell mass cells whose sensitivity resembles that of other developing cells. Human red blood cells are exceptional in being completely resistant to treatment with STS and CHX. As (STS plus CHX)-induced cell deaths have been shown to display the characteristic features of programmed cell death (PCD), we conclude that all mammalian nucleated cells are capable of undergoing PCD and constitutively express all the proteins required to do so. It seems that the machinery for PCD is in place and ready to run, even though its activation often depends on new RNA and protein synthesis.

Activation of apoptosis by Apo-2 ligand is independent of FADD but blocked by CrmA

A new member of the tumor necrosis factor (TNF) cytokine family, designated Apo-2 ligand (Apo-21) [1] or TRAIL [2], has been shown recently to induce apoptosis in various tumor cell lines; however, its biological role is unknown. Here, we show that Apo-21, activated apoptosis in T-cell-enriched cultures of peripheral blood lymphocytes stimulated by interleukin-2 (IL-2), but not in unstimulated cells. This finding suggests that, like Fas/Apo-1 ligand and TNF [3-5], Apo-2L may play a role in regulating post-stimulation apoptosis of mature lymphocytes. Studies on the mechanism of Apo-2L action demonstrated marked membrane blebbing, a hallmark of apoptosis, within a few minutes of the addition of Apo-2L to tumor cells. Ectopic expression of a dominant negative mutant of FADD, a cytoplasmic protein that mediates death signalling by Fas/Apo-1 and by TNF receptor type 1 (TNFR1) [6-9], inhibited the induction of apoptosis by anti-Fas/Apo-1 antibody, but had little effect on Apo-2L function. In contrast, expression of CrmA, a cowpox virus-derived inhibitor of the Ced-2-like proteases ICE [10] and CPP32/Yama [11,12], blocked the induction of apoptosis by either Apo-2L or anti-Fas/Apo-1 antibody. These results suggest that Apo-2L activates a rapid, FADD-independent pathway to trigger a cell-death programme that requires the function of cysteine proteases such as ICE or CPP32/Yama.

Role of Ced-3/ICE-family proteases in staurosporine-induced programmed cell death

In the accompanying paper by Weil et al. (1996) we show that staurosporine (STS), in the presence of cycloheximide (CHX) to inhibit protein synthesis, induces apoptotic cell death in a large variety of nucleated mammalian cell types, suggesting that all nucleated mammalian cells constitutively express all of the proteins required to undergo programmed cell death (PCD). The reliability of that conclusion depends on the evidence that STS-induced, and (STS + CHS)-induced, cell deaths are bona fide examples of PCD. There is rapidly accumulating evidence that some members of the Ced-3/Interleukin-1 beta converting enzyme (ICE) family of cysteine proteases are part of the basic machinery of PCD. Here we show that Z-Val-Ala-Asp-fluoromethylketone (zVAD-fmk), a cell-permeable, irreversible, tripeptide inhibitor of some of these proteases, suppresses STS-induced and (STS + CHX)-induced cell death in a wide variety of mammalian cell types, including anucleate cytoplasts, providing strong evidence that these are all bona fide examples of PCD. We show that the Ced-3/ICE family member CPP32 becomes activated in STS-induced PCD, and that Bcl-2 inhibits this activation. Most important, we show that, in some cells at least, one or more CPP32-family members, but not ICE itself, is required for STS-induced PCD. Finally, we show that zVAD-fmk suppresses PCD in the interdigital webs in developing mouse paws and blocks the removal of web tissue during digit development, suggesting that this inhibition will be a useful tool for investigating the roles of PCD in various developmental processes.

Human isolates of dengue type 1 virus induce apoptosis in mouse neuroblastoma cells

Human isolates of dengue (DEN) type 1 viruses FGA/89 and BR/90 differ in their membrane fusion properties in mosquito cell lines (P. Desprès et al., Virology 196:209-216, 1993). FGA/89 and BR/90 were assayed for their neurovirulence in newborn mice, and neurons were the major target cells for both DEN-1 virus strains within the central nervous system. To study the susceptibility of neurons to DEN virus infection, DEN virus replication was analyzed in the murine neuroblastoma cell line Neuro 2a. Infection of Neuro 2a cells with FGA/89 or BR/90 induced apoptotic DNA degradation after 25 h of infection. Studies of DEN protein synthesis revealed that accumulation of viral proteins leads to apoptotic cell death. The apoptotic process progressed more rapidly following BR/90 infection than it did after FGA/89 infection. The higher cytotoxicity of BR/90 for Neuro 2a cells was linked to an incomplete maturation of the envelope proteins, resulting in abortive virus assembly. Accumulation of viral proteins in the endoplasmic reticulum may induce stress and thereby activate the apoptotic pathway in mouse neuroblastoma cells.

Ligation of CD40 rescues Ramos-Burkitt lymphoma B cells from calcium ionophore- and antigen receptor-triggered apoptosis by inhibiting activation of the cysteine protease CPP32/Yama and cleavage of its substrate PARP

The new and growing family of interleukin-1beta-converting enzyme (ICE) cysteine proteases are now recognised to be major effectors of cellular death by apoptosis. Like other members of this family, the CPP32/Yama proform is activated by processing to its active heterodimeric enzyme or apopain when it likely contributes to the process of apoptosis by cleaving poly(ADP-ribose) polymerase (PARP) and thereby inhibiting much of its DNA repair activity. Apoptosis plays a fundamental role in the regulation of the immune system where it is involved in the selection of both T and B lymphocytes bearing antigen receptor (AgR) for non-self. Cells of the Ramos Epstein-Barr virus (EBV)-genome-negative Burkitt lymphoma (BL) B cell line (Ramos-BL) can be triggered into growth arrest and apoptosis by treating with the calcium ionophore ionomycin or by crosslinking their surface AgR with antibodies directed against immunoglobulin (Ig)M (anti-IgM). Ionomycin- and AgR-triggered growth arrest and apoptosis are arrested by signals transduced through the surface CD40 of Ramos-BL B cells. Both ionomycin and anti-IgM trigger activation of CPP32 and cleavage of PARP prior to the onset of apoptosis; this process is abrogated by treatment with anti-CD40 and is independent of Bcl-2 expression. A tripeptide inhibitor of ICE family cysteine proteases, Z-Val-Ala-Asp-fluoromethylketone (zVAD-fmk) inhibits ionomycin- and AgR-triggered CPP32 activation, PARP cleavage and apoptosis, but not growth arrest, in Ramos-BL B cells. Thus, in this report we demonstrate that in a physiological system, activation of endogenous members of the ICE family, including CPP32, and cleavage of the death substrate PARP act as major effectors of apoptotic death.

Processing and activation of CMH-1 by granzyme B

Granzyme B plays an essential role in cytotoxic T lymphocyte (CTL)-mediated cell killing. Recent studies suggest that granzyme B may exert its effect by cleaving and activating CPP32, a member of the interleukin-1 beta-converting enzyme/Ced-3 family of cysteine proteases. We have examined the processing and activation of CMH-1, a close homologue of CPP32, by granzyme B in vitro. We have found that granzyme B specifically cleaves CMH-1 at Asp198-Ser199 between the p20 and p12 and activates the cysteine protease. Cleavage between p20 and the prosequence of CMH-1 at Asp23-Ala24 is autocatalytic and is not required for CMH-1 activity in vitro. The cleavage and activation of CMH-1 by granzyme B in vitro sugge st that, in addition to CPP32, CMH-1 may also play a role in CTL-mediated cell killing.

The Caenorhabditis elegans cell-death protein CED-3 is a cysteine protease with substrate specificities similar to those of the human CPP32 protease

The Caenorhabditis elegans cell-death gene ced-3 encodes a protein similar to mammalian interleukin-1beta-converting enzyme (ICE), a cysteine protease implicated in mammalian apoptosis. We show that the full-length CED-3 protein undergoes proteolytic activation to generate a CED-3 cysteine protease and that CED-3 protease activity is required for killing cells by programmed cell death in C. elegans. We developed an easy and general method for the purification of CED-3/ICE-like proteases and used this method to facilitate a comparison of the substrate specificities of four different purified cysteine proteases. We found that in its substrate preferences CED-3 was more similar to the mammalian CPP32 protease than to mammalian ICE or NEDD2/ICH-1 protease. Our results suggest that different mammalian CED-3/ICE-like proteases may have distinct roles in mammalian apoptosis and that CPP32 is a candidate for being a mammalian functional equivalent of CED-3.

The cell-death machine

Apoptosis, or programmed cell death, is the physiological process whereby individual cells are deliberately eliminated to achieve homeostasis and proper metazoan development. Numerous genes have recently been identified that are involved in apoptosis: some are believed to encode death effectors, whereas others encode positive or negative regulators of the cell-death machine. Precisely how these various proteins interact in the molecular mechanism of apoptosis remains to be discovered.

Apopain/CPP32 cleaves proteins that are essential for cellular repair: a fundamental principle of apoptotic death

Proteolysis mediated by the interleukin 1 beta-converting enzyme (ICE) homologues is an important mechanism of the apoptotic process. The ICE homologue apopain/CPP-32/Yama (subsequently referred to as apopain) cleaves poly(ADP-ribose)polymerase (PARP) early during apoptosis. Additional apoptosis-specific protein cleavages have been observed in which the direct involvement of ICE-like proteases has been postulated. These substrates include the 70-kD protein component of the U1-ribonucleoprotein (U1-70kD), and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs). The present studies demonstrate that U1-70kD and DNA-PKcs are excellent substrates for apopain, with cleavage occurring at sites that are highly similar to the cleavage site within PARP. The fragments generated from isolated protein substrates by apopain are identical to those observed in intact apoptotic cells, in apoptotic cell extracts, and in normal cell extracts to which apopain has been added. Like PARP, cleavage of these substrates in apoptotic cell extracts is abolished by nanomolar concentrations of Ac-DEVD-CHO and micromolar amounts of Ac-YVAD-CHO, confirming the involvement of apopain or an apopain-like activity. We propose that a central function of apopain or similar homologues in apoptosis is the cleavage of nuclear repair proteins, thereby abolishing their critical homeostatic functions.

Nitric oxide induced poly(ADP-ribose) polymerase cleavage in RAW 264.7 macrophage apoptosis is blocked by Bcl-2

Endogenously generated or exogenously supplied nitric oxide causes cleavage of poly(ADP-ribose) polymerase (PARP) and apoptotic cell death in RAW 264.7 macrophages. With the use of NO donors such as S-nitrosoglutathione or spermine-NO we established that PARP digestion occurs in parallel with DNA fragmentation, and is preceded by accumulation of the tumor suppressor gene product p53. PARP cleavage in response to lipopolysaccharide and interferon-gamma treatment is prevented by NG-monomethyl-L-arginine, thus proving a NO requirement. Endogenous NO generation, p53 accumulation, and PARP degradation occurred prior to the detection of significant chromatin condensation. In contrast, in stable Bcl-2 transfected cells, NO-initiated PARP cleavage was almost completely blocked. Our data implicate PARP as a proteolytic substrate during NO-mediated apoptotic cell death in RAW 264.7 macrophages and establish Bcl-2 as an efficient signal terminator in this process.

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

Interleukin-1 beta converting enzyme (ICE)-like proteases, which are synthesized as inactive precursors, play a key role in the induction of apoptosis. We now demonstrate that benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone (Z-VAD.FMK), an ICE-like protease inhibitor, inhibits apoptosis by preventing the processing of CPP32 to its active form. These results suggest that novel inhibitors of apoptosis can be developed which prevent processing of proforms of ICE-like proteases.

Mitochondrial control of nuclear apoptosis

Anucleate cells can be induced to undergo programmed cell death (PCD), indicating the existence of a cytoplasmic PCD pathway that functions independently from the nucleus. Cytoplasmic structures including mitochondria have been shown to participate in the control of apoptotic nuclear disintegration. Before cells exhibit common signs of nuclear apoptosis (chromatin condensation and endonuclease-mediated DNA fragmentation), they undergo a reduction of the mitochondrial transmembrane potential (delta psi m) that may be due to the opening of mitochondrial permeability transition (PT) pores. Here, we present direct evidence indicating that mitochondrial PT constitutes a critical early event of the apoptotic process. In a cell-free system combining purified mitochondria and nuclei, mitochondria undergoing PT suffice to induce chromatin condensation and DNA fragmentation. Induction of PT by pharmacological agents augments the apoptosis-inducing potential of mitochondria. In contrast, prevention of PT by pharmacological agents impedes nuclear apoptosis, both in vitro and in vivo. Mitochondria from hepatocytes or lymphoid cells undergoing apoptosis, but not those from normal cells, induce disintegration of isolated Hela nuclei. A specific ligand of the mitochondrial adenine nucleotide translocator (ANT), bongkreik acid, inhibits PT and reduces apoptosis induction by mitochondria in a cell-free system. Moreover, it inhibits the induction of apoptosis in intact cells. Several pieces of evidence suggest that the proto-oncogene product Bcl-2 inhibits apoptosis by preventing mitochondrial PT. First, to inhibit nuclear apoptosis, Bcl-2 must be localized in mitochondrial but not nuclear membranes. Second, transfection-enforced hyperexpression of Bcl-2 directly abolishes the induction of mitochondrial PT in response to a protonophore, a pro-oxidant, as well as to the ANT ligand atractyloside, correlating with its apoptosis-inhibitory effect. In conclusion, mitochondrial PT appears to be a critical step of the apoptotic cascade.

The adenovirus death protein (E3-11.6K) is required at very late stages of infection for efficient cell lysis and release of adenovirus from infected cells

Adenovirus (Ad) infection is concluded by assembly of virions in the cell nucleus followed by lysis of cells by an unknown mechanism. We have described an Ad nuclear membrane glycoprotein of 11,600 kDa (E3-11.6K) which is encoded by the E3 transcription unit and which is synthesized in small amounts from the E3 promoter at early stages of infection but in large amounts from the major late promoter at very late stages of infection. We now report that E3-11.6K is required for the efficient lysis (death) of Ad-infected cells, and we propose that the function of E3-11.6K is to mediate the release of Ad progeny from infected cells. We have renamed E3-11.6K the Ad death protein (ADP). Virus mutants that lack ADP replicated as well as adp+ Ad, but the cells lysed more slowly, virus release from the cell was retarded, and the plaques were small and developed slowly. Cells infected with adp+ viruses began to lyse at 2 or 3 days postinfection (p.i.) and were completely lysed by 5 or 6 days p.i. In contrast, cells infected with adp mutants did not begin significant lysis until 5 or 6 days p.i. Cell lysis and viability were determined by plaque size, extracellular virus, cell morphology, release of lactate dehydrogenase, trypan blue exclusion, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay for mitochondrial activity, RNA degradation, and DNA degradation as determined by agarose gel electrophoresis and the terminal deoxynucleotidyltransferase end labeling assay. Protein synthesis was almost nonexistent at 3 days p.i. in cells infected with adp+ Ads, but it was still increasing in cells infected with adp mutants. Host cell protein synthesis was undetectable at 1 day p.i. in cells infected with adp+ Ads or adp mutants. Cells infected with adp mutants showed Ad cytopathic effect at 1 or 2 days p.i. in that they rounded up and detached, but the cells remained metabolically active and intact for >5 days p.i. When examined by electron microscopy, the nuclei were extremely swollen and full of virus, and the nuclear membrane appeared to be intact. ADP is unrelated in sequence to other known cell death-promoting proteins.

The proteinase activated receptor-2 (PAR-2) mediates mitogenic responses in human vascular endothelial cells

Proteolytically cleaved receptors, typified by the functional thrombin receptor (TR), represent a novel class of receptors that mediate signaling events by functional coupling to G proteins. Northern blot analysis completed with a human proteinase activated receptor-2 (PAR-2) cDNA as probe demonstrated the approximately 3.5kb PAR-2 transcript in total cellular RNA from human umbilical vein endothelial cells (HUVEC). Microspectrofluorimetry using Fura2-loaded HUVEC demonstrated a dose-dependent elevation in intracellular calcium transients ([Ca2+]i) to murine PAR39-44 (SLIGRL, putative neoligand after cleavage), with an approximate EC50 of 30 microM, and evidence for homologous desensitization with complete recovery at 45 min. Xenopus oocytes microinjected with TR cRNA failed to respond to 200 microM PAR39-44, and TR-targeted antisense oligonucleotides specifically abrogated thrombin-induced but not PAR39-44-mediated [Ca2+]i, excluding the possibility that TR/PAR-2 cell-surface coexpression was structurally linked. HUVEC incubated with PAR39-44 demonstrated a dose- and time-dependent mitogenic response similar to that seen with thrombin or TR42-47 (TR-activating peptide, SFLLRN). Preactivation of HUVEC with either PAR39-44 or thrombin resulted in heterologous desensitization to the corresponding agonist, an effect that was mediated primarily by TR internalization as evaluated by immunofluorescence and quantitative ELISA. These results ascribe a previously unrecognized function to the PAR-2 receptor, imply that a natural enzyme agonist may circulate in plasma, and suggest the presence of an additional regulatory mechanism controlling receptor activation events in vascular endothelial cells.

Cyclin D1 induced apoptosis maintains the integrity of the G1/S checkpoint following ionizing radiation irradiation

Cell cycle "checkpoints" help to ensure the integrity of normal cellular functions prior to replicative DNA synthesis and/or cell division. Cell kinetic abnormalities, particularly arrests at the G1/S and G2/M cell cycle checkpoints, are induced following exposure to ionizing radiation in vitro. Following irradiation, cellular signaling pathways may lead to G1 arrest and/or apoptosis at the G1/S cell cycle transition point. Transfection of cyclin D1, a G1/S cyclin, into a rat embryo cells (REC) results in cellular populations that overexpress cyclin D1, are transformed morphologically, demonstrate an increased incidence of apoptosis, and are tumorigenic in immune-deficient mice. Despite such phenotypic changes, transfected cell populations maintain the integrity of the G1 checkpoint following ionizing radiation. The transfected cells overexpressing Cyclin D1 have a statistically significant increase in the incidence of apoptosis as compared to parental REC strains or mock-transfected REC. The work provides further evidence of Cyclin D1 playing a critical role in maintaining the integrity of the G1/S checkpoint, via the activation of apoptotic pathways following exposure to ionizing radiation in vitro.

Surface blebs on apoptotic cells are sites of enhanced procoagulant activity: implications for coagulation events and antigenic spread in systemic lupus erythematosus

The restriction of phosphatidylserine (PtdSer) to the inner surface of the plasma membrane bilayer is lost early during apoptosis. Since PtdSer is a potent surface procoagulant, and since there is an increased incidence of coagulation events in patients with systemic lupus erythematosus (SLE) who have anti-phospholipid antibodies, we addressed whether apoptotic cells are procoagulant and whether anti-phospholipid antibodies influence this. Apoptotic HeLa cells, human endothelial cells, and a murine pre-B-cell line were markedly procoagulant in a modified Russell viper venom assay. This procoagulant effect was entirely abolished by addition of the PtdSer-binding protein, annexin V, confirming that it was PtdSer-dependent. The procoagulant effect was also abolished by addition of IgG purified from the plasma of three patients with anti-phospholipid antibody syndrome, but not IgG from normal controls. Confocal microscopy of apoptotic cells stained with fluorescein-isothiocyanate-conjugated-annexin V demonstrated (Ca2+)-dependent binding to the surface of membrane blebs o apoptotic cells, but not to intracellular membranes. Recent data indicate that the surface blebs of apoptotic cells constitute an important immunogenic particle in SLE. We propose that the PtdSer exposed on the outside of these blebs can induce the production of anti-phospholipid antibodies, which might also enhance the immunogenicity of the bleb contents. When apoptosis occurs in a microenvironment in direct contact with circulating plasma, the unique procoagulant consequences of the apoptotic surface may additionally be expressed. This might explain the increased incidence of pathological intravascular coagulation events that occur in some lupus patients who have anti-phospholipid antibodies.

Requirement of an ICE-like protease for induction of apoptosis and ceramide generation by REAPER

Genetic studies indicated that the Drosophila melanogaster protein REAPER (RPR) controls apoptosis during embryo development. Induction of RPR expression in Drosophila Schneider cells rapidly stimulated apoptosis. RPR-mediated apoptosis was blocked by N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk), which suggests that an interleukin-1 beta converting enzyme (ICE)-like protease is required for RPR function. RPR-induced apoptosis was associated with increased ceramide production that was also blocked by Z-VAD-fmk, which suggests that ceramide generation requires an ICE-like protease as well. Thus, the intracellular RPR protein uses cell death signaling pathways similar to those used by the vertebrate transmembrane receptors Fas (CD95) and tumor necrosis factor receptor type 1.

New members of the Bcl-2 family and their protein partners

Any model of apoptosis must explain the mechanism of action of the Bcl-2 family of proteins. This has proved to be unusually difficult. This review concentrates on some of the newly isolated members of this growing family and attempts to provide an insight into the complexity of interactions through which the Bcl-2 proteins modulate apoptosis.

ICE-related proteases in apoptosis

Apoptotic execution involves numerous enzymatic pathways, all of which appear to be triggered by the activation of one or more ICE-related proteases (IRPs). Considerable effort is currently being expended in the identification and functional characterization of the rapidly expanding superfamily of IRPs. Important questions that remain unsolved include the identity of the vertebrate IRP that triggers the apoptotic cascade and the identities of the crucial substrates whose cleavage results in the dramatic morphological changes during apoptosis.

TRADD-TRAF2 and TRADD-FADD interactions define two distinct TNF receptor 1 signal transduction pathways

Tumor necrosis factor (TNF) can induce apoptosis and activate NF-kappa B through signaling cascades emanating from TNF receptor 1 (TNFR1). TRADD is a TNFR1-associated signal transducer that is involved in activating both pathways. Here we show that TRADD directly interacts with TRAF2 and FADD, signal transducers that activate NF-kappa B and induce apoptosis, respectively. A TRAF2 mutant lacking its N-terminal RING finger domain is a dominant-negative inhibitor of TNF-mediated NF-kappa B activation, but does not affect TNF-induced apoptosis. Conversely, a FADD mutant lacking its N-terminal 79 amino acids is a dominant-negative inhibitor of TNF-induced apoptosis, but does not inhibit NF-kappa B activation. Thus, these two TNFR1-TRADD signaling cascades appear to bifurcate at TRADD.

Identification and characterization of CPP32/Mch2 homolog 1, a novel cysteine protease similar to CPP32

We have identified and characterized a novel cysteine protease named CMH-1 that is a new member of the interleukin 1 beta converting enzyme (ICE) family of proteases with substrate specificity for Asp-X. CMH-1 has the highest similarity to CPP32 (52% amino acid identity) and MCH2 (31% identical). CMH-1 shares conserved amino acid residues that form the core structure of ICE as well as those residues involved in catalysis and in the P1 aspartate binding. Overexpression of CMH-1 in COS cells resulted in the processing of CMH-1 and the induction of apoptosis of transfected cells. Coexpression of CMH-1 with poly(ADP-ribose) polymerase (PARP) also resulted in a specific cleavage of PARP. Purified recombinant CMH-1 cleaved PARP but not interleukin 1 beta precursor in vitro.

Suppression of apoptosis in mammalian cells by NAIP and a related family of IAP genes

Dysregulation of apoptosis can result in inappropriate suppression of cell death, as occurs in the development of some cancers, or in failure to control the extent of cell death, as is believed to occur in acquired immunodeficiency and certain neurodegenerative disorders, such as spinal muscular atrophy (SMA). Recently, we isolated a candidate gene, encoding neuronal apoptosis inhibitor protein (NAIP), for SMA. This gene is homologous to two baculovirus inhibitor of apoptosis proteins (Cp-IAP and Op-IAP) and is partly deleted in individuals with type I SMA. A second SMA candidate gene encoding survival motor neuron (SMN), which is contiguous with the NAIP locus on 5q13.1, was also reported. Here we demonstrate a NAIP-mediated inhibition of apoptosis induced by a variety of signals, and have identified three additional human complementary DNAs and a Drosophila melanogaster sequence that are also homologous to the baculovirus IAPs. The four open reading frames (ORFs) possess three baculoviral inhibition of apoptosis protein repeat (BIR) domains and a carboxy-terminal RING zinc-finger. The human iap genes have a distinct but overlapping pattern of expression in fetal and adult tissues. These proteins significantly increase the number of known apoptotic suppressors.

Control of apoptosis by the cellular ATP level

Apoptosis is a physiological form of cell death. Its causes and execution mechanisms are not clearly understood. Oxidative stress, nitric oxide and its congeners, Ca2+, proteases, nucleases, and mitochondria are considered mediators of apoptosis. At present their importance and exact role are elusive but it is clear that mitochondria are both the target and the source of oxidative stress, nitric oxide, and Ca2+. The mitochondrial membrane potential (delta psi), which is the driving force for mitochondrial ATP synthesis, declines during apoptosis, and maintenance of delta psi prevents apoptosis. Since apoptosis is highly regulated and involves the activity of hydrolytic enzymes, chromatin condensation and vesicle formation apoptosis is likely to have a high energy demand. We propose that the cellular ATP level is an important determinant for cell death. This hypothesis is supported by circumstantial evidence, is consistent with the available data, has a corrolary in aging, and is amenable to direct experimental testing particularly with flow cytometry as a promising tool.