Nucleotide binding to CARD12 and its role in CARD12-mediated caspase-1 activation

CARD12 (Ipaf/Clan) is an important regulator of caspase-1 activation. It belongs to the family of the nucleotide-binding site and leucine-rich repeat (NBS-LRR) proteins. The NBS domain of the NBS-LRR proteins contains putative ATP/GTPase-specific P-loop and Mg2+-binding site motifs. However, the nucleotide-binding properties and the function of the NBS domain are unknown. We developed a nucleotide-binding assay and investigated nucleotide binding to CARD12. We find that the NBS domain of CARD12 contains a nucleotide-binding pocket with specificity for ATP/dATP. A point mutation in the P-loop (K175R) of the NBS domain abolishes ATP/dATP binding. We further demonstrate that the nucleotide-binding site is required for CARD12-mediated caspase-1 activation. CARD12 self-association and association with procaspase-1 in transfected cells were markedly decreased by the P-loop mutation K175R. Furthermore, the P-loop mutation greatly reduced caspase-1 activation-dependent proIL-1beta processing. Thus, CARD12 function is dependent on the nucleotide-binding site. Our data provide insights into the molecular mechanisms of CARD12-mediated caspase-1 activation.     

Nod1, an Apaf-1-like activator of caspase-9 and nuclear factor-kappaB.

Ced-4 and Apaf-1 belong to a major class of apoptosis regulators that contain caspase-recruitment (CARD) and nucleotide-binding oligomerization domains. Nod1, a protein with an NH2-terminal CARD-linked to a nucleotide-binding domain and a COOH-terminal segment with multiple leucine-rich repeats, was identified. Nod-1 was found to bind to multiple caspases with long prodomains, but specifically activated caspase-9 and promoted caspase-9-induced apoptosis. As reported for Apaf-1, Nod1 required both the CARD and P-loop for function. Unlike Apaf-1, Nod1 induced activation of nuclear factor-kappa-B (NF-kappaB) and bound RICK, a CARD-containing kinase that also induces NF-kappaB activation. Nod1 mutants inhibited NF-kappaB activity induced by RICK, but not that resulting from tumor necrosis factor-alpha stimulation. Thus, Nod1 is a leucine-rich repeat-containing Apaf-1-like molecule that can regulate both apoptosis and NF-kappaB activation pathways.     

Regulation of the Apaf-1/caspase-9 apoptosome by caspase-3 and XIAP

The apoptosome is a multiprotein complex comprising Apaf-1, cytochrome c, and caspase-9 that functions to activate caspase-3 downstream of mitochondria in response to apoptotic signals. Binding of cytochrome c and dATP to Apaf-1 in the cytosol leads to the assembly of a heptameric complex in which each Apaf-1 subunit is bound noncovalently to a procaspase-9 subunit via their respective CARD domains. Assembly of the apoptosome results in the proteolytic cleavage of procaspase-9 at the cleavage site PEPD(315) to yield the large (p35) and small (p12) caspase-9 subunits. In addition to the PEPD site, caspase-9 contains a caspase-3 cleavage site (DQLD(330)), which when cleaved, produces a smaller p10 subunit in which the NH(2)-terminal 15 amino acids of p12, including the XIAP BIR3 binding motif, are removed. Using purified proteins in a reconstituted reaction in vitro, we have assessed the relative impact of Asp(315) and Asp(330) cleavage on caspase-9 activity within the apoptosome. In addition, we characterized the effect of caspase-3 feedback cleavage of caspase-9 on the rate of caspase-3 activation, and the potential ramifications of Asp(330) cleavage on XIAP-mediated inhibition of the apoptosome. We have found that cleavage of procaspase-9 at Asp(330) to generate p35, p10 or p37, p10 forms resulted in a significant increase (up to 8-fold) in apoptosome activity compared with p35/p12. The significance of this increase was demonstrated by the near complete loss of apoptosome-mediated caspase-3 activity when a point mutant (D330A) of procaspase-9 was substituted for wild-type procaspase-9 in the apoptosome. In addition, cleavage at Asp(330) exposed a novel p10 NH(2)-terminal peptide motif (AISS) that retained the ability to mediate XIAP inhibition of caspase-9. Thus, whereas feedback cleavage of caspase-9 by caspase-3 significantly increases the activity of the apoptosome, it does little to attenuate its sensitivity to inhibition by XIAP.     

Solution structure and mutagenesis of the caspase recruitment domain (CARD) from Apaf-1

Activation of procaspase-9, a key component of the apoptosis mechanism, requires the interaction of its caspase recruitment domain (CARD) with the CARD in the adaptor protein Apaf-1. Using nuclear magnetic resonance spectroscopy and mutagenesis we have determined the structure of the CARD from Apaf-1 and the residues important for binding the CARD in procaspase-9. Apaf-1's CARD contains seven short alpha-helices with the core six helices arranged in an antiparallel manner. Residues in helix 2 have a central role in mediating interaction with procaspase-9 CARD. This interaction surface is distinct from that proposed based on the structure of the CARD from RAIDD, but is coincident with that of the structurally similar FADD death effector domain and the Apaf-1 CARD interface identified by crystallographic studies.     

The adapter protein apoptotic protease-activating factor-1 (Apaf-1) is proteolytically processed during apoptosis

Apoptotic protease-activating factor-1 (Apaf-1), a key regulator of the mitochondrial apoptosis pathway, consists of three functional regions: (i) an N-terminal caspase recruitment domain (CARD) that can bind to procaspase-9, (ii) a CED-4-like region enabling self-oligomerization, and (iii) a regulatory C terminus with WD-40 repeats masking the CARD and CED-4 region. During apoptosis, cytochrome c and dATP can relieve the inhibitory action of the WD-40 repeats and thus enable the oligomerization of Apaf-1 and the subsequent recruitment and activation of procaspase-9. Here, we report that different apoptotic stimuli induced the caspase-mediated cleavage of Apaf-1 into an 84-kDa fragment. The same Apaf-1 fragment was obtained in vitro by incubation of cell lysates with either cytochrome c/dATP or caspase-3 but not with caspase-6 or caspase-8. Apaf-1 was cleaved at the N terminus, leading to the removal of its CARD H1 helix. An additional cleavage site was located within the WD-40 repeats and enabled the oligomerization of p84 into a approximately 440-kDa Apaf-1 multimer even in the absence of cytochrome c. Due to the partial loss of its CARD, the p84 multimer was devoid of caspase-9 or other caspase activity. Thus, our data indicate that Apaf-1 cleavage causes the release of caspases from the apoptosome in the course of apoptosis.     

Expression and functional analysis of Apaf-1 isoforms. Extra Wd-40 repeat is required for cytochrome c binding and regulated activation of procaspase-9

Apaf-1 is an important apoptotic signaling molecule that can activate procaspase-9 in a cytochrome c/dATP-dependent fashion. Alternative splicing can create an NH(2)-terminal 11-amino acid insert between the caspase recruitment domain and ATPase domains or an additional COOH-terminal WD-40 repeat. Recently, several Apaf-1 isoforms have been identified in tumor cell lines, but their expression in tissues and ability to activate procaspase-9 remain poorly characterized. We performed analysis of normal tissue mRNAs to examine the relative expression of the Apaf-1 forms and identified Apaf-1XL, containing both the NH(2)-terminal and COOH-terminal inserts, as the major RNA form expressed in all tissues tested. We also identified another expressed isoform, Apaf-1LN, containing the NH(2)-terminal insert, but lacking the additional WD-40 repeat. Functional analysis of all identified Apaf-1 isoforms demonstrated that only those with the additional WD-40 repeat activated procaspase 9 in vitro in response to cytochrome c and dATP, while the NH(2)-terminal insert was not required for this activity. Consistent with this result, in vitro binding assays demonstrated that the additional WD-40 repeat was also required for binding of cytochrome c, subsequent Apaf-1 self-association, binding to procaspase-9, and formation of active Apaf-1 oligomers. These experiments demonstrate the expression of multiple Apaf-1 isoforms and show that only those containing the additional WD-40 repeat bind and activate procaspase-9 in response to cytochrome c and dATP.     

Cytochrome c promotes caspase-9 activation by inducing nucleotide binding to Apaf-1

We report here the biochemical analysis of the reconstituted de novo procaspase-9 activation using highly purified cytochrome c, recombinant apoptotic protease-activating factor-1 (Apaf-1), and recombinant procaspase-9. Using a nucleotide binding assay, we found that Apaf-1 alone bound dATP poorly and the nucleotide binding to Apaf-1 was significantly stimulated by cytochrome c. The binding of dATP to Apaf-1 induces the formation of a multimeric Apaf-1. cytochrome c complex, apoptosome. Procaspase-9 also synergistically promotes dATP binding to Apaf-1 in a cytochrome c-dependent manner. The dATP bound to apoptosome remained as dATP, not dADP. A nonhydrolyzable ATP analog, ADPCP (beta,gamma-methylene adenosine 5'-triphosphate), was able to support apoptosome formation and caspase activation in place of dATP or ATP. These data indicate that the key event in Apaf-1-mediated caspase-9 activation is cytochrome c-induced dATP binding to Apaf-1.     

Polypeptide modulators of caspase recruitment domain (CARD)-CARD-mediated protein-protein interactions

The caspase recruitment domain (CARD) is present in a large number of proteins. Initially, the CARD was recognized as part of the caspase activation machinery. CARD-CARD interactions play a role in apoptosis and are responsible for the Apaf-1-mediated activation of procaspase-9 in the apoptosome. CARD-containing proteins mediate the inflammasome-dependent activation of proinflammatory caspase-1. More recently, new roles for CARD-containing proteins have been reported in signaling pathways associated with immune responses. The functional role of CARD-containing proteins and CARDs in coordinating apoptosis and inflammatory and immune responses is not completely understood. We have explored the putative cross-talk between apoptosis and inflammation by analyzing the modulatory activity on both the Apaf-1/procaspase-9 interaction and the inflammasome-mediated procaspase-1 activation of CARD-derived polypeptides. To this end, we analyzed the activity of individual recombinant CARDs, rationally designed CARD-derived peptides, and peptides derived from phage display.     

Increased expression of Apaf-1 and procaspase-3 and the functionality of intrinsic apoptosis apparatus in non-small cell lung carcinoma

The intrinsic apoptosis apparatus plays a significant role in generating and amplifying cell death signals. In this study we examined whether there are differences in the expression of its components and in its functioning in non-small cell lung carcinoma (NSCLC) and the lung. We show that NSCLC cell lines express Apaf-1 and procaspase-9 and -3 proteins and that the expression of Apaf-1 and procaspase-3, but not of procaspase-9 and -7, is frequently up-regulated in NSCLC tissues as compared to the lung. NSCLC tissues and lungs and some NSCLC cell lines expressed also caspase-9S(b) and displayed a high caspase-9S(b)/procaspase-9 expression ratio. Procaspase-3 from NSCLCs and lungs was readily processed to caspase-3 by granzyme B or caspase-8, and the granzyme B-generated caspase-3-like activity was significantly higher in tumor tissues and cells than in lungs. By contrast, cytochrome c plus dATP could induce a significant increase of caspase-3-like activity in cytosol only in some NSCLC cell lines and in subsets of studied NSCLC tissues and lungs, while procaspase-3 and -7 were detectably processed only in NSCLC tissues which showed a high (cytochrome c+dATP)-induced caspase-3-like activity. Taken together, the present study provides evidence that the expression of Apaf-1 and procaspase-3 is up-regulated in NSCLCs and indicates that the tumors have a capability to suppress the apoptosome-driven caspase activation in their cytosol.     

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.     

Molecular cloning and characterization of DEFCAP-L and -S, two isoforms of a novel member of the mammalian Ced-4 family of apoptosis proteins

We report the deduced amino acid sequences of two alternately spliced isoforms, designated DEFCAP-L and -S, that differ in 44 amino acids and encode a novel member of the mammalian Ced-4 family of apoptosis proteins. Similar to the other mammalian Ced-4 proteins (Apaf-1 and Nod1), DEFCAP contains a caspase recruitment domain (CARD) and a putative nucleotide binding domain, signified by a consensus Walker's A box (P-loop) and B box (Mg(2+)-binding site). Like Nod1, but different from Apaf-1, DEFCAP contains a putative regulatory domain containing multiple leucine-rich repeats (LRR). However, a distinguishing feature of the primary sequence of DEFCAP is that DEFCAP contains at its NH(2) terminus a pyrin-like motif and a proline-rich sequence, possibly involved in protein-protein interactions with Src homology domain 3-containing proteins. By using in vitro coimmunoprecipitation experiments, both long and short isoforms were capable of strongly interacting with caspase-2 and exhibited a weaker interaction with caspase-9. Transient overexpression of full-length DEFCAP-L, but not DEFCAP-S, in breast adenocarcinoma cells MCF7 resulted in significant levels of apoptosis. In vitro death assays with transient overexpression of deletion constructs of both isoforms using beta-galactosidase as a reporter gene in MCF7 cells suggest the following: 1) the nucleotide binding domain may act as a negative regulator of the killing activity of DEFCAP; 2) the LRR/CARD represents a putative constitutively active inducer of apoptosis; 3) the killing activity of LRR/CARD is inhibitable by benzyloxycarbonyl-Val-Ala-Asp (OMe)-fluoromethyl ketone and to a lesser extent by Asp-Glu-Val-Asp (OMe)-fluoromethyl ketone; and 4) the CARD is critical for killing activity of DEFCAP. These results suggest that DEFCAP is a novel member of the mammalian Ced-4 family of proteins capable of inducing apoptosis, and understanding its regulation may elucidate the complex nature of the mammalian apoptosis-promoting machinery.     

Differential effects of filipin and methyl-beta-cyclodextrin on B cell receptor signaling

The CED4/Apaf-1 family of proteins functions as critical regulators of apoptosis and NF-kappaB signaling pathways. A novel human member of this family, called CARD12, was identified that induces apoptosis when expressed in cells. CARD12 is most similar in structure to the CED4/Apaf-1 family member CARD4, and is comprised of an N-terminal caspase recruitment domain (CARD), a central nucleotide-binding site (NBS), and a C-terminal domain of leucine-rich repeats (LRR). The CARD domain of CARD12 interacts selectively with the CARD domain of ASC, a recently identified proapoptotic protein. In addition, CARD12 coprecipitates caspase-1, a caspase that participates in both apoptotic signaling and cytokine processing. CARD12 may assemble with proapoptotic CARD proteins to coordinate the activation of downstream apoptotic and inflammatory signaling pathways.     

Pro-apoptotic proteins released from the mitochondria regulate the protein composition and caspase-processing activity of the native Apaf-1/caspase-9 apoptosome complex

The apoptosome is a large caspase-activating ( approximately 700-1400 kDa) complex, which is assembled from Apaf-1 and caspase-9 when cytochrome c is released during mitochondrial-dependent apoptotic cell death. Apaf-1 the core scaffold protein is approximately 135 kDa and contains CARD (caspase recruitment domain), CED-4, and multiple (13) WD40 repeat domains, which can potentially interact with a variety of unknown regulatory proteins. To identify such proteins we activated THP.1 lysates with dATP/cytochrome c and used sucrose density centrifugation and affinity-based methods to purify the apoptosome for analysis by MALDI-TOF mass spectrometry. First, we used a glutathione S-transferase (GST) fusion protein (GST-casp9(1-130)) containing the CARD domain of caspase-9-(1-130), which binds to the CARD domain of Apaf-1 when it is in the apoptosome and blocks recruitment/activation of caspase-9. This affinity-purified apoptosome complex contained only Apaf-1XL and GST-casp9(1-130), demonstrating that the WD40 and CED-4 domains of Apaf-1 do not stably bind other cytosolic proteins. Next we used a monoclonal antibody to caspase-9 to immunopurify the native active apoptosome complex from cell lysates, containing negligible levels of cytochrome c, second mitochondria-derived activator of caspase (Smac), or Omi/HtrA2. This apoptosome complex exhibited low caspase-processing activity and contained four stably associated proteins, namely Apaf-1, pro-p35/34 forms of caspase-9, pro-p20 forms of caspase-3, X-linked inhibitor of apoptosis (XIAP), and cytochrome c, which was only bound transiently to the complex. However, in lysates containing Smac and Omi/HtrA2, the caspase-processing activity of the purified apoptosome complex increased 6-8-fold and contained only Apaf-1 and the p35/p34-processed subunits of caspase-9. During apoptosis, Smac, Omi/HtrA2, and cytochrome c are released simultaneously from mitochondria, and thus it is likely that the functional apoptosome complex in apoptotic cells consists primarily of Apaf-1 and processed caspase-9.     

Tpl-2 induces apoptosis by promoting the assembly of protein complexes that contain caspase-9, the adapter protein Tvl-1, and procaspase-3

The Tpl-2 proto-oncoprotein promotes cellular proliferation when overexpressed in a variety of tumor cell lines. Here, we present evidence that when overexpressed in immortalized non-transformed cells, Tpl-2 induces apoptosis by promoting the activation of caspase-3 via a caspase-9-dependent mechanism, and that apoptosis is enhanced when Tpl-2 is co-expressed with the newly identified ankyrin repeat protein Tvl-1. The activation of caspase-3 by caspase-9 is known to depend on the assembly of a multimolecular complex that includes Apaf-1 and caspase-9. Data presented here show that co-expression of Tpl-2 with Tvl-1 promotes the assembly of a complex that involves several proteins that bind Apaf-1 including Tvl-1, itself, Tpl-2 and phosphorylated procaspase-9. More important, procaspase-3, which under normal growth conditions is not associated with the complex, binds Tvl-1 conditionally in response to Tpl-2-generated apoptotic signals. The conditional association of procaspase-3 with Tvl-1 promotes the in vivo proteolytic maturation of procaspase-3 by caspase-9, a process casually linked to apoptosis.     

Cloning of a novel human caspase-9 splice variant containing only the CARD domain

Caspase-9 plays a key role in the intrinsic apoptotic pathway and currently two splice variants (caspase-9-alpha and -beta) have been identified. The present study cloned and characterized a novel caspase-9 splice variant, hereby designated Casp9-gamma. Casp9-gamma is generated from an additional alternative 3' splice site in the fourth exon of caspase-9, resulting in a 58-nucleotide fragment insertion compared with the full-length caspase-9-alpha. The fragment introduces an in-frame stop codon, and the resulting open reading frame (ORF) is preterminated. The Casp9-gamma comprises the deduced 154 amino acid residues containing only the caspase recruitment domain (CARD) and does not contain the large and small subunits. The Casp9-gamma does not promote apoptosis when overexpressed in mammalian cells. Moreover, it inhibits the cleavage of procaspase-3 mediated by proapoptotic member Bax or apoptosis inductor staurosporine. Therefore, Casp9-gamma may function as an endogenous apoptotic inhibitor by interfering with the CARD-CARD interaction between Apaf-1 (apoptotic protease activating factor-1) and procaspase-9. In addition, Casp9-gamma does not enhance NF-kappaB activation in transfected 293T cells, conflicting with previous evidence that the isolated CARD of caspase-9 activates NF-kappaB in ND7 cells. This suggests that the procaspase-9-mediated NF-kappaB activation in response to cellular stresses is cell type-specific through an unidentified mechanism.     

The PYRIN-CARD protein ASC is an activating adaptor for caspase-1

The PYRIN and CARD domains are members of the six-helix bundle death domain-fold superfamily that mediates assembly of large signaling complexes in the apoptotic and inflammatory signaling pathways. Here we show that the PYRIN-CARD protein ASC functions as a caspase-1-activating adaptor. ASC interacted specifically with procaspase-1 via CARD-CARD interactions and induced its oligomerization. Consistent with these results ectopic expression of full-length ASC, but not its isolated CARD or PYRIN domain, with procaspase-1 induced activation of procaspase-1 and processing of pro-interleukin-1beta in transfected cells. Substitution of the PYRIN domain of ASC with an inducible FKBP12 oligomerization domain produced a molecule that can induce caspase-1 activation in response to stimulation with the oligomerization drug AP20187, suggesting that the PYRIN domain functions as an oligomerization domain, whereas the CARD domain functions as the effector domain in the caspase-1 activation pathway. Furthermore stable expression of an isolated CARD of ASC in THP-1 cells diminished interleukin-1beta generation in response to pro-inflammatory cytokines. These results indicate that ASC is involved in the caspase-1 signaling pathway by mediating the assembly of a caspase-1-inflammasome signaling complex in response to pro-inflammatory cytokine stimulation.     

Apaf-1XL is an inactive isoform compared with Apaf-1L

Apaf-1 plays a crucial role in the cytochrome c/dATP-dependent activation of caspase-9 and -3. We found that the human myeloid leukemic K562 cells were more resistant to cytochrome c-induced activation of caspase-9 and -3 in a cell-free system compared with the human T-lymphoblastic subclone CEM/VLB(100) cells. Apaf-1 cDNA sequencing revealed an additional insert of 11 aa between the CARD and CED-4 (ATPase) domains in K562 cells, which was identical to the sequence of Apaf-1XL. Immunoprecipitation of Apaf-1 with caspase-9 after a cell-free reaction demonstrated that Apaf-1XL in the K562 cell line showed a lower binding ability to caspase-9 compared with Apaf-1L protein. The resistance of K562 cells to cytochrome c-dependent apoptosis may be partly due to this Apaf-1XL form. These results suggest that the additional insert between CARD and CED-4 domains might affect Apaf-1 recruitment of caspase-9 during apoptosis.     

Cytochrome c and dATP-mediated oligomerization of Apaf-1 is a prerequisite for procaspase-9 activation.

To elucidate the mechanism of activation of procaspase-9 by Apaf-1, we produced recombinant full-length Apaf-1 and purified it to complete homogeneity. Here we show using gel filtration that full-length Apaf-1 exists as a monomer that can be transformed to an oligomeric complex made of at least eight subunits after binding to cytochrome c and dATP. Apaf-1 binds to cytochrome c in the absence of dATP but does not form the oligomeric complex. However, when dATP is added to the cytochrome c-bound Apaf-1 complex, complete oligomerization occurs, suggesting that oligomerization is driven by hydrolysis of dATP. This was supported by the observation that ATP, but not the nonhydrolyzable adenosine 5'-O-(thiotriphosphate), can induce oligomerization of the Apaf-1-cytochrome c complex. Like the spontaneously oligomerizing Apaf-530, which lacks its WD-40 domain, the oligomeric full-length Apaf-1-cytochrome c complex can bind and process procaspase-9 in the absence of additional dATP or cytochrome c. However, unlike the truncated Apaf-530 complex, the full-length Apaf-1 complex can release the mature caspase-9 after processing. Once released, mature caspase-9 can process procaspase-3, setting into motion the caspase cascade. These observations indicate that cytochrome c and dATP are required for oligomerization of Apaf-1 and suggest that the WD-40 domain plays an important role in oligomerization of full-length Apaf-1 and the release of mature caspase-9 from the Apaf-1 oligomeric complex.     

Oligomerization is a general mechanism for the activation of apoptosis initiator and inflammatory procaspases

Proteolytic activation of initiator procaspases is a crucial step in the cellular commitment to apoptosis. Alternative models have been postulated for the activation mechanism, namely the oligomerization or induced proximity model and the allosteric regulation model. While the former holds that procaspases become activated upon proper oligomerization by an adaptor protein, the latter states that the adaptor is an allosteric regulator for procaspases. The allosteric regulation model has been applied for the activation of procaspase-9 by apoptotic protease-activating factor (Apaf-1) in an oligomeric complex known as the apoptosome. Using approaches that allow for controlled oligomerization, we show here that aggregation of multiple procaspase-9 molecules can induce their activation independent of the apoptosome. Oligomerization-induced procaspase-9 activation, both within the apoptosome and in artificial systems, requires stable homophilic association of the protease domains, raising the possibility that the function of Apaf-1 is not only to oligomerize procaspase-9 but also to maintain the interaction of the caspase-9 protease domain after processing. In addition, we provide biochemical evidence that other apoptosis initiator caspases (caspase-2 and -10) as well as a procaspase involved in inflammation (murine caspase-11) are also activated by oligomerization. Thus, oligomerization of precursor molecules appears to be a general mechanism for the activation of both apoptosis initiator and inflammatory procaspases.     

ASC-mediated NF-kappaB activation leading to interleukin-8 production requires caspase-8 and is inhibited by CLARP

ASC is an adaptor molecule that mediates apoptotic and inflammatory signals from several Apaf-1-like molecules, including CARD12/Ipaf, cryopyrin/PYPAF1, PYPAF5, PYPAF7, and NALP1. To characterize the signaling pathway mediated by ASC, we established cell lines in which muramyl dipeptide, the bacterial component recognized by another Apaf-1-like molecule, Nod2, induced an interaction between a CARD12-Nod2 chimeric protein and ASC, and elicited cell autonomous NF-kappaB activation. This response required caspase-8, and was suppressed by CLARP/FLIP, an inhibitor of caspase-8. The catalytic activity of caspase-8 was required for the ASC-mediated NF-kappaB activation when caspase-8 was expressed at an endogenous level, although it was not essential when caspase-8 was overexpressed. In contrast, FADD, the adaptor protein linking Fas and caspase-8, was not required for this response. Consistently, ASC recruited caspase-8 and CLARP but not FADD and Nod2 to its speck-like aggregates in cells. Finally, muramyl dipeptide induced interleukin-8 production in MAIL8 cells. These results are the first to indicate that caspase-8 plays an important role in the ASC-mediated NF-kappaB activation, and that the ASC-mediated NF-kappaB activation actually induces physiologically relevant gene expression.