Negative regulation of the Apaf-1 apoptosome by Hsp70.

Release of cytochrome c from mitochondria by apoptotic signals induces ATP/dATP-dependent formation of the oligomeric Apaf-1-caspase-9 apoptosome. Here we show that the documented anti-apoptotic effect of the principal heat-shock protein, Hsp70, is mediated through its direct association with the caspase-recruitment domain (CARD) of Apaf-1 and through inhibition of apoptosome formation. The interaction between Hsp70 and Apaf-1 prevents oligomerization of Apaf-1 and association of Apaf-1 with procaspase-9. On the basis of these results, we propose that resistance to apoptosis exhibited by stressed cells and some tumours, which constitutively express high levels of Hsp70, may be due in part to modulation of Apaf-1 function by Hsp70.

GRIM-19 associates with the serine protease HtrA2 for promoting cell death.

We have isolated a novel interferon (IFN)-retinoid regulated cell death regulatory protein genes associated with retinoid-IFN-induced mortality (GRIM)-19 earlier. To understand its mechanism of action, we have employed a yeast-two-hybrid screen and identified serine protease HtrA2 as its binding partner. GRIM-19 physically interacts with HtrA2 and augments cell death in an IFN/all-trans retinoic acid (RA)-dependent manner. In the presence of GRIM-19, the HtrA2-driven destruction of the antiapoptotic protein X-linked inhibitor of apoptosis (XIAP) is augmented. These interactions were disrupted by an human herpes virus-8 (HHV-8)-coded oncoprotein, vIRF1, and conferred resistance to IFN/RA-induced cell death. These data show a critical role of HtrA2 in a cytokine-induced cell death response for the first time and its inhibition by a viral protein.

Caspase cleavage enhances the apoptosis-inducing effects of BAD.

The function of BAD, a proapoptotic member of the Bcl-2 family, is regulated primarily by rapid changes in phosphorylation that modulate its protein-protein interactions and subcellular localization. We show here that, during interleukin-3 (IL-3) deprivation-induced apoptosis of 32Dcl3 murine myeloid precursor cells, BAD is cleaved by a caspase(s) at its N terminus to generate a 15-kDa truncated protein. The 15-kDa truncated BAD is a more potent inducer of apoptosis than the wild-type protein, whereas a mutant BAD resistant to caspase 3 cleavage is a weak apoptosis inducer. Truncated BAD is detectable only in the mitochondrial fraction, interacts with BCL-X(L) at least as effectively as the wild-type protein, and is more potent than wild-type BAD in inducing cytochrome c release. Human BAD, which is 43 amino acids shorter than its mouse counterpart, is also cleaved by a caspase(s) upon exposure of Jurkat T cells to anti-FAS antibody, tumor necrosis factor alpha (TNF-alpha), or TRAIL. Moreover, a truncated form of human BAD lacking the N-terminal 28 amino acids is more potent than wild-type BAD in inducing apoptosis. The generation of truncated BAD was blocked by Bcl-2 in IL-3-deprived 32Dcl3 cells but not in Jurkat T cells exposed to anti-FAS antibody, TNF-alpha, or TRAIL. Together, these findings point to a novel and important role for BAD in maintaining the apoptotic phenotype in response to various apoptosis inducers.

Identification and characterization of murine caspase-14, a new member of the caspase family.

We report here the identification and characterization of a new member of the mouse caspase family, named caspase-14. Northern blot analysis of mRNA from various tissues with caspase-14-specific probe showed a major transcript size of approximately 2.4 kb and variant transcripts of 2.0 kb and 1.5 kb. The major transcript is detected mainly in the liver and to a lesser extent in the brain and kidney. Caspase-14 cDNA encodes a 257-amino acid-long protein that has significant homology to other members of the caspase family. Like other caspases, caspase-14 has a conserved active site, pentapeptide QACRG. However, it lacks an NH2-terminal prodomain or a caspase recruitment domain, suggesting that it could be a downstream caspase that depends on other initiator caspases for activation. Consistent with this, procaspase-14 can be processed in vitro by the death receptor-associated caspase-8 and caspase-10 but not other caspases, and in vivo after stimulation of cells with anti-Fas agonist antibody or Tumor Necrosis Factor-Related Apoptosis Inducing Ligand. Furthermore, procaspase-14 can be cleaved by granzyme B. These observations suggest that caspase-14 may play a role in death receptor and granzyme B-induced apoptosis.

Identification of an endogenous dominant-negative short isoform of caspase-9 that can regulate apoptosis.

Alternatively spliced isoforms of certain apoptosis regulators, such as Bcl-x, Ced-4, and Ich-1, have been shown to play opposing roles in regulating apoptosis. Here, we describe the identification of an endogenous alternatively spliced isoform of caspase-9, named caspase-9b, which lacks the central large subunit caspase domain. Caspase-9b is detectable in many cell lines by PCR and at the mRNA and protein levels. Caspase-9b can interact with the caspase recruitment domain of Apaf-1, and like the active site mutant of caspase-9, it can inhibit multiple forms of apoptosis, including those triggered by oligomerization of death receptors. It can also block activation of caspase-9 and -3 by Apaf-1 in an in vitro cytochrome c-dependent caspase activation assay. These results suggest that caspase-9b functions as an endogenous apoptosis inhibitory molecule by interfering with the formation of a functional Apaf-1-caspase-9 complex.

CRADD, a novel human apoptotic adaptor molecule for caspase-2, and FasL/tumor necrosis factor receptor-interacting protein RIP.

FADD/MORT1 is a death domain (DD)-containing adaptor/signaling molecule that interacts with the intracellular DD of FAS/APO-I (CD95) and tumor necrosis factor receptor 1 and the prodomain of caspase-8 (Mch5/MACH/FLICE). FADD engagement of caspase-8 presumably activates this caspase and leads to apoptosis. Another DD-containing adaptor/signaling molecule, CRADD, was identified and was shown to induce apoptosis. CRADD has a dual-domain structure similar to that of FADD. It has an NH2-terminal caspase homology domain that interacts with caspase-2 and a COOH-terminal DD that interacts with RIP. CRADD is constitutively expressed in many tissues and thus could play a role in regulating apoptosis in mammalian cells.

Treatment of malignant glioma cells with the transfer of constitutively active caspase-6 using the human telomerase catalytic subunit (human telomerase reverse transcriptase) gene promoter.

Because the apoptotic pathway is often disrupted in tumor cells, its genetic restoration is a very attractive approach for the treatment of tumors. To treat malignant gliomas with this approach, it would be preferred to restrict induction of apoptosis to tumor cells by establishing a tumor-specific expression system. Telomerase is an attractive target because the vast majority of malignant gliomas have telomerase activity whereas normal brain cells do not. Activation of telomerase is tightly regulated at the transcriptional level of the telomerase catalytic subunit [human telomerase reverse transcriptase, (hTERT)]. Therefore, we hypothesized that using a hTERT promoter-driven vector system, an apoptosis-inducible gene may be preferentially restricted to telomerase- or hTERT-positive tumor cells. In this study, we constructed an expression vector consisting of the constitutively active caspase-6 (rev-caspase-6) under the hTERT promoter (hTERT/rev-caspase-6) and then investigated its antitumor effect on malignant glioma cells. The rationale for using the rev-caspase-6 gene is because it induces apoptosis independent of the initiator caspases. We demonstrated that the hTERT/rev-caspase-6 construct induced apoptosis in hTERT-positive malignant glioma cells, but not in hTERT-negative astrocytes, fibroblasts, and alternative lengthening of telomeres cells. In addition, the growth of s.c. tumors in nude mice was significantly suppressed by the treatment with hTERT/rev-caspase-6 construct. The present results strongly suggest that the telomerase-specific transfer of the rev-caspase-6 gene under the hTERT promoter is a novel targeting approach for the treatment of malignant gliomas.

Bax translocation is crucial for the sensitivity of leukaemic cells to etoposide-induced apoptosis.

Bax translocation from cytosol to mitochondria is believed to be a crucial step for triggering cytochrome c release from mitochondria. However, it is unclear whether Bax translocation is associated with Bax induction by DNA damaging agents. The induction of Bax in response to DNA damaging agents has been considered to be linked with p53. In this study, we used the p53 negative human chronic myeloid leukaemia K562 cell line. Bax up-regulation occurred at the whole cell level after DNA damage induced by etoposide. However, after incubation with etoposide, Bax failed to translocate to mitochondria and as a result, the apoptotic process was blocked. A Bax stable transfectant, the K/Bax cell line, expressed more Bax protein in the cytosol, mitochondria and nuclei. This Bax overexpression induced cytochrome c release, a reduction of cytochrome c oxidase activity and mitochondrial membrane potential (Delta(Psi)m). However, Bax-induced apoptosis was blocked downstream of mitochondria in K562 cells. The increased levels of mitochondrial Bax sensitized cells to etoposide-induced activation of caspases-2, -3 and -9 and apoptosis. However, after transient transfection with the Apaf-1 gene, K/Bax cells were sensitized to etoposide-induced caspase activation and apoptosis to a larger extent compared with Bax or Apaf-1 transfection alone. We therefore conclude that two mechanisms contribute to the resistance of K562 cells to etoposide-induced apoptosis; firstly failure of Bax targeting to mitochondria and, secondly, deficiency of Apaf-1. Uncoupling of Bax translocation from Bax induction can occur in response to etoposide-induced DNA damage.

Regulation of IL-1beta generation by Pseudo-ICE and ICEBERG, two dominant negative caspase recruitment domain proteins.

We report here the identification and functional characterization of two new human caspase recruitment domain (CARD) molecules, termed Pseudo-interleukin-1beta converting enzyme (ICE) and ICEBERG. Both proteins share a high degree of homology, reaching 92% and 53% identity, respectively, to the prodomain of caspase-1/ICE. Interestingly, both Pseudo-ICE and ICEBERG are mapped to chromosome 11q22 that bears caspases-1, -4- and -5 genes, all involved in cytokine production rather than in apoptosis. We demonstrate that Pseudo-ICE and ICEBERG interact physically with caspase-1 and block, in a monocytic cell line, the interferon-gamma and lipopolysaccharide-induced secretion of interleukin-1beta which is a well-known consequence of caspase-1 activation. Moreover, Pseudo-ICE, but not ICEBERG, interacts with the CARD-containing kinase RICK/RIP2/CARDIAK and activates NF-kappaB. Our data suggest that Pseudo-ICE and ICEBERG are intracellular regulators of caspase-1 activation and could play a role in the regulation of IL-1beta secretion and NF-kappaB activation during the pro-inflammatory cytokine response.

Functional characterization of mitochondria in neutrophils: a role restricted to apoptosis.

Mitochondria are known to combine life-supporting functions with participation in apoptosis by controlling caspase activity. Here, we report that in human blood neutrophils the mitochondria are different, because they preserve mainly death-mediating abilities. Neutrophil mitochondria hardly participate in ATP synthesis, and have a very low activity of the tested marker enzymes. The presence of mitochondria in neutrophils was confirmed by quantification of mitochondrial DNA copy number, by detection of mitochondrial porin, and by JC-1 measurement of Deltapsi(m). During neutrophilic differentiation, HL-60 cells demonstrated a profound cytochrome c depletion and mitochondrial shape change reminiscent of neutrophils. However, blood neutrophils containing extremely low amounts of cytochrome c displayed strong caspase-9 activation during apoptosis, which was also observed in apoptotic neutrophil-derived cytoplasts lacking any detectable cytochrome c. We suggest that other proapoptotic factors such as Smac/DIABLO and HtrA2/Omi, which are massively released from the mitochondria, have an important role in neutrophil apoptosis.

Death effector domain-containing proteins DEDD and FLAME-3 form nuclear complexes with the TFIIIC102 subunit of human transcription factor IIIC.

Death effector domain-containing proteins are involved in important cellular processes such as death-receptor induced apoptosis, NF-kappaB activation and ERK activation. Here we report the identification of a novel nuclear DED-containing protein, FLAME-3. FLAME-3 shares significant sequence (46.6% identical) and structural homology to another DED-containing protein, DEDD. FLAME-3 interacts with DEDD and c-FLIP (FLAME-1) but not with the other DED-containing proteins FADD, caspase-8 or caspase-10. FLAME-3 translocates to, and sequesters c-FLIP in the nucleus upon overexpression in human cell lines. Using the yeast two-hybrid system to identify DEDD-interacting proteins, the TFIIIC102 subunit of human transcription factor TFIIIC was identified as a DEDD- and FLAME-3-specific interacting protein. Co-expression of either DEDD or FLAME-3 with hTFIIIC102 in MCF-7 cells induces the translocation from the cytoplasm and sequestration of hTFIIIC102 in the nucleus, indicating that DEDD and FLAME-3 form strong heterocomplexes with hTFIIIC102 and might be important regulators of the activity of the hTFIIIC transcriptional complex. Consistent with this, overexpression of DEDD or FLAME-3 in 293 cells inhibited the expression of a luciferase-reporter gene under the control of the NF-kappaB promoter. Our data provide the first direct evidence for the involvement of DED-containing proteins in the regulation of components of the general transcription machinery in the nucleus.

Control of erythroid cell production via caspase-mediated cleavage of transcription factor SCL/Tal-1.

SCL/Tal-1 is a helix-loop-helix (HLH) transcription factor required for blood cell development, whose abnormal expression is responsible for induction of T-cell acute lymphoblastic leukemia. We show here that SCL/Tal-1 is a key target of caspases in developing erythroblasts. SCL/Tal-1 degradation occurred rapidly after caspase activation and preceded the cleavage of the major erythroid transcription factor GATA-1. Expression of a caspase-resistant SCL/Tal-1 in erythroid progenitors was able to prevent amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis induced by growth factor deprivation or death receptor triggering. The potent proerythropoietic activity of uncleavable SCL/Tal-1 was clearly evident in the absence of erythropoietin, a condition that did not allow survival of normal erythroid cells or expansion of erythroblasts expressing caspase-resistant GATA-1. In the absence of erythropoietin, cells expressing caspase-resistant SCL/Tal-1 maintain high levels of Bcl-X(L), which inhibits amplification of the caspase cascade and mediates protection from apoptosis. Thus, SCL/TAL-1 is a survival factor for erythroid cells, whereas caspase-mediated cleavage of SCL/Tal-1 results in amplification of caspase activation, GATA-1 degradation and impaired erythropoiesis.

Expression and transcriptional regulation of caspase-14 in simple and complex epithelia.

Caspase-14 is a recent addition to the caspase family of aspartate proteases involved in apoptotic processes. Human caspase-14 appears to be only weakly processed during apoptosis, and it does not cleave classical caspase substrates. Post partum, caspase-14 is prominently expressed by human keratinocytes and reportedly participates in terminal differentiation of complex epithelia. Here we provide evidence challenging the view that caspase-14 expression or processing is linked exclusively to terminal keratinocyte differentiation. We demonstrate that caspase-14 expression extended to multiple cell lines derived from simple epithelia of the breast, prostate, and stomach. In keratinocytes and breast epithelial cells, caspase-14 expression was upregulated in high-density cultures and during forced suspension culture. These effects were primarily due to transcriptional activation as indicated by reporter gene assays using a 2 kb caspase-14 promoter fragment. Importantly, caspase-14 was not cleaved during forced suspension culture of either cell type although this treatment induced caspase-dependent apoptosis (anoikis). Forced expression of caspase-14 in immortalized human keratinocytes had no effect on cell death in forced suspension nor was the transfected caspase-14 processed in this setting. In contrast to postconfluent and forced suspension culture, terminal differentiation of keratinocytes induced in vitro by Ca2+ treatment was not associated with increased caspase-14 expression or promoter activity. Our results indicate that (1) caspase-14 is expressed not only in complex but also simple epithelia; (2) cells derived from complex and simple epithelia upregulate caspase-14 expression in conditions of high cell density or lack of matrix interaction and; (3) in both cell types this phenomenon is due to transcriptional regulation.

The serine protease Omi/HtrA2 is released from mitochondria during apoptosis. Omi interacts with caspase-inhibitor XIAP and induces enhanced caspase activity.

Proteome analysis of supernatant of isolated mitochondria exposed to recombinant tBid, a proapoptotic Bcl-2 member, revealed the presence of the serine protease Omi, also called HtrA2. This release was prevented in mitochondria derived from Bcl-2-transgenic mice. Release of Omi under apoptotic conditions was confirmed in vivo in livers from mice injected with agonistic anti-Fas antibodies and was prevented in livers from Bcl-2 transgenic mice. Omi release also occurs in apoptotic dying but not in necrotic dying fibrosarcoma L929 cells, treated with anti-Fas antibodies and TNF, respectively. The amino acid sequence reveals the presence of an XIAP interaction motif at the N-terminus of mature Omi. We demonstrate an interaction between endogeneous Omi and recombinant XIAP. Furthermore we show that endogenous Omi is involved in enhanced activation of caspases in cytosolic extracts.

FLIP is expressed in mouse testis and protects germ cells from apoptosis.

Apoptosis control in adult testis is crucial to achieve normal spermatogenesis. In this study c-FLIP, an apoptosis-modulating protein, was investigated. In Western blot and immunohistochemical analyses, the 55 KDa c-FLIP long isoform (c-FLIP(L)) was found to be expressed strongly in spermatocytes and spermatids, at low levels in spermatogonia and at almost undetectable levels in Sertoli cells. This expression pattern was confirmed by Northern blot analyses. Further experiments carried out on GC-1spg germ cell line revealed that reducing c-FLIP(L) expression increases Fas-dependent apoptosis. Conversely, restoring c-FLIP(L) expression reduces this response to control levels. Caspase-10 expression was found to match c-FLIP(L) expression pattern; further, caspase-10 activation upon anti-Fas treatment inversely correlated with c-FLIP(L) expression. Finally, TUNEL staining of seminiferous tubules incubated with anti-Fas antibody showed that apoptosis occurs mostly in basally located germ cells, indicating that such cells, expressing low levels of c-FLIP(L), are sensitive to Fas-mediated apoptosis. These data indicate for the first time that c-FLIP(L) might control germ cell apoptosis and caspase activity in the adult testis.

Synthesis of a bacteriophage MB78 late protein by novel ribosomal frameshifting.

MB78 is a virulent phage of Salmonella typhimurium that possesses a number of interesting features, making it a suitable organism to study the regulation of gene expression. A detailed physical map of this phage genome has been constructed and is being extensively studied at the molecular level. Here, we demonstrate the expression of two late proteins of bacteriophage MB78 derived from the same gene as a result of possible ribosomal frameshifting. In vitro transcription-translation yields a major protein that migrates as 28kDa, whereas in vivo expression using pET expression vectors yields two equally expressed proteins of molecular sizes 28 and 26kDa. A putative slippery sequence TTTAAAG and a pseudoknot structure, two essential cis elements required for the classical ribosomal frameshifting, are identified in the reading frame. Mutations created at the slippery sequence resulted in a single 28kDa protein and completely abolished the expression of 26kDa protein. Thus, we have produced the first evidence that ribosomal frameshifting occurs in bacteriophage MB78 of Salmonella typhimurium.

Structural modification of serum vitamin D3-binding protein and immunosuppression in AIDS patients.

A serum glycoprotein, vitamin D3-binding protein (Gc protein), can be converted by beta-galactosidase of stimulated B lymphocytes and sialidase of T lymphocytes to a potent macrophage-activating factor (MAF), a protein with N-acetylgalactosamine as the remaining sugar moiety. Thus, Gc protein is a precursor for MAF. Treatment of purified Gc protein with immobilized beta-galactosidase and sialidase generates an extremely high-titered MAF (GcMAF). When peripheral blood monocytes/macrophages of 46 HIV-infected patients were treated with GcMAF (100 pg/ml), the monocytes/macrophages of all patients were efficiently activated. However, the MAF precursor activity of plasma Gc protein was low in 16 (35%) of of these patients. Loss of the MAF precursor activity appeared to be due to deglycosylation of plasma Gc protein by alpha-N-acetylgalactosaminidase found in the patient blood stream. Levels of plasma alpha-N-acetylgalactosaminidase activity in individual patients had an inverse correlation with the MAF precursor activity of their plasma Gc protein. Thus, precursor activity of Gc protein and alpha-N-acetylgalactosaminidase activity in patient blood can serve as diagnostic and prognostic indices.

vCLAP, a caspase-recruitment domain-containing protein of equine Herpesvirus-2, persistently activates the Ikappa B kinases through oligomerization of IKKgamma.

vCLAP, the E10 gene product of equine herpesvirus-2, is a caspase-recruitment domain (CARD)-containing protein that has been shown to induce both apoptosis and NF-kappaB activation in mammalian cells. vCLAP has a cellular counterpart, Bcl10/cCLAP, which is also an activator of apoptosis and NF-kappaB. Recent studies demonstrated that vCLAP activates NF-kappaB through an IkappaB kinase (IKK)-dependent pathway, but the underlying mechanism remains unknown. In this report, we demonstrate that vCLAP associates stably with the IKK complex through direct binding to the C-terminal region of IKKgamma. Consistent with this finding, IKKgamma was found to be essential for vCLAP-induced NF-kappaB activation, and the association between vCLAP and the IKK complex induced persistent activation of the IKKs. Moreover, enforced oligomerization of the isolated C-terminal region of vCLAP, which interacts with IKKgamma, can trigger NF-kappaB activation. Finally, substitution of the C-terminal region of IKKgamma, which interacts with vCLAP, with the CARD of vCLAP or Bcl10 produced a molecule that was able to activate NF-kappaB when ectopically expressed in IKKgamma-deficient cells. These data suggest that vCLAP-induced oligomerization of IKKgamma, which is mediated by the CARD of vCLAP, could be the mechanism by which vCLAP induces activation of NF-kappaB.

Molecular ordering of the Fas-apoptotic pathway: the Fas/APO-1 protease Mch5 is a CrmA-inhibitable protease that activates multiple Ced-3/ICE-like cysteine proteases.

The Fas/APO-1-receptor associated cysteine protease Mch5 (MACH/FLICE) is believed to be the enzyme responsible for activating a protease cascade after Fas-receptor ligation, leading to cell death. The Fas-apoptotic pathway is potently inhibited by the cowpox serpin CrmA, suggesting that Mch5 could be the target of this serpin. Bacterial expression of proMch5 generated a mature enzyme composed of two subunits, which are derived from the pre-cursor proenzyme by processing at Asp-227, Asp-233, Asp-391, and Asp-401. We demonstrate that recombinant Mch5 is able to process/activate all known ICE/Ced-3-like cysteine proteases and is potently inhibited by CrmA. This contrasts with the observation that Mch4, the second FADD-related cysteine protease that is also able to process/activate all known ICE/Ced-3-like cysteine proteases, is poorly inhibited by CrmA. These data suggest that Mch5 is the most upstream protease that receives the activation signal from the Fas-receptor to initiate the apoptotic protease cascade that leads to activation of ICE-like proteases (TX, ICE, and ICE-relIII), Ced-3-like proteases (CPP32, Mch2, Mch3, Mch4, and Mch6), and the ICH-1 protease. On the other hand, Mch4 could be a second upstream protease that is responsible for activation of the same protease cascade in CrmA-insensitive apoptotic pathways.

Autoactivation of procaspase-9 by Apaf-1-mediated oligomerization.

Activation of procaspase-9 by Apaf-1 in the cytochrome c/dATP-dependent pathway requires proteolytic cleavage to generate the mature caspase molecule. To elucidate the mechanism of activation of procaspase-9 by Apaf-1, we designed an in vitro Apaf-1-procaspase-9 activation system using recombinant components. Here, we show that deletion of the Apaf-1 WD-40 repeats makes Apaf-1 constitutively active and capable of processing procaspase-9 independent of cytochrome c an dATP. Apaf-1-mediated processing of procaspase-9 occurs at Asp-315 by an intrinsic autocatalytic activity of procaspase-9 itself. We provide evidence that Apaf-1 can form oligomers and may facilitate procaspase-9 autoactivation by oligomerizing its precursor molecules. Once activated, caspase-9 can initiate a caspase cascade involving the downstream executioners caspase-3, -6, and -7.