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J. Biol. Chem., Vol. 279, Issue 33, 34323-34331, August 13, 2004
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Kinase
-NF
B Essential Modulator*






From the
BioGeM Consortium, **Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università degli Studi di Napoli "Federico II," Via S. Pansini 5, 80131 Napoli, Italy, ¶Centro di Endocrinologia ed Oncologia Sperimentale, 80131 Napoli, Italy, ||Dipartimento Scienze e Tecnologie Biologiche ed Ambientali, Università degli Studi di Lecce, 73100 Lecce, Italy, and
Dipartimento Scienze Biologiche ed Ambientali, Università degli Studi del Sannio di Benevento, Via Port'Arsa 11, 82100 Benevento, Italy
Received for publication, March 1, 2004 , and in revised form, June 4, 2004.
| ABSTRACT |
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B transcription factor following antigen receptor stimulation of both B and T lymphocytes. However, the biochemical mechanism by which CARMA1 regulates activation of NF
B remains to be determined. Here we have shown that CARMA1 and CARMA3 physically associate with I
kinase
/NF
B essential modulator (I
K
-NEMO) in lymphoid and non-lymphoid cells. CARMA1 participates to an inducible large molecular complex that contains I
K
/NEMO, Bcl10, and I
K
/
kinases. Expression of the NEMO-binding region of CARMA3 exerts a dominant negative effect on Bcl10-mediated activation of NF
B. Thus, our results provide direct evidence for physical and functional interaction between CARMA and the I
K complex and offer a biochemical framework to understand the molecular activities controlled by CARMA-1, -2, and 3 and Bcl10. | INTRODUCTION |
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B signaling pathway and the transcription factors that it activates have emerged as critical regulators of normal immune and inflammatory response, cell proliferation, differentiation, apoptosis, and oncogenesis (13). A key event in the canonical NF
B cascade is the activation of the I
kinase (I
K)1 complex, which is composed of three subunits: I
K
, I
K
, and I
K
/NF
B essential modulator (NEMO) (4). Whereas I
K
and I
K
have catalytic kinase activity, NEMO is an important regulatory subunit, and deficiency of this protein results in complete lack of NF
B activation (5). Once activated, the I
K complex is responsible for the phosphorylation and subsequent proteasome-mediated degradation of the inhibitory proteins I
Bs (4). Degradation of I
Bs frees NF
B and allows its translocation in the nucleus, where it activates transcription of target genes (4).
In T cells, activation of the I
K complex following antigen receptor stimulation requires the activity of PKC
(68) and the function of the caspase recruitment domain (CARD)-containing proteins Bcl10 (9) and CARMA1 (1016). CARD is a protein-protein interaction motif, originally identified as a conserved sequence present in various molecules involved in regulation of apoptosis, such as RAIDD, several caspases, and the Caenorhabditis elegans genes ced-3 and ced-4 (17). However, a number of CARD-containing proteins are not implicated in apoptotic signaling but participate in signal transduction pathways that regulate the activation state of NF
B (17). Bcl10 was initially identified in a subset of MALT B cell lymphomas with t (1, 14)(p22;q32) (18, 19). This 233-amino acid protein is ubiquitously expressed and contains an amino-terminal CARD domain. Bcl10/ lymphocytes show absence of NF
B activation following antigen receptor stimulation or PMA/ionomycin-induced cell activation (9, 2931). In addition to immunological deficiencies, Bcl10-null murine embryos develop exencephaly that leads to embryonic lethality, indicating that Bcl10 plays pleiotropic roles in the embryonic and adult lives of mammalian organisms (9).
The CARD-containing proteins CARMA1 (also known as CARD11/Bimp2), CARMA2 (CARD14/Bimp3), and CARMA3 (CARD10/Bimp1) share high degrees of sequence, structure, and functional homology (2023). On the other hand, the three CARMA proteins display tissue-specific distribution, suggesting that they may sub-serve distinct biochemical functions in different cell types (2023). CARMA proteins belong to the membrane-associated guanylate kinase-like (MAGUK) family of proteins that can function as molecular scaffolds that assist recruitment and assembly of signal transduction molecules and contain an SH3 domain, one or several PDZ domains, and a GUK domain (24).
Similar to Bcl10/ lymphocytes, CARMA1-deficient lymphocytes exhibit reduced activation of NF
B in response to antigen receptor cross-linking (12). However, the mechanism by which Bcl10 and CARMA proteins activate the I
K complex is still undefined. Here we have shown that CARMA1 and CARMA3 physically associate with NEMO, thereby regulating activation of the NF
B transcription factor.
| MATERIALS AND METHODS |
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Cell Culture and AntibodiesHEK293 cells were cultured in Dulbecco's modified Eagle's medium supplemented with 10% FCS and transfected by calcium phosphate precipitation using 10 µg of plasmidic DNA.
Jurkat cells were cultured in RPMI supplemented with 10% FCS. Sources of antisera and monoclonal antibodies were the following: anti-CARMA1, Apotech; anti-FLAG, Sigma; anti-NEMO, anti-c-myc, anti-HA, Santa Cruz Biotechnology, BD Pharmingen; anti-I
B, anti-phospho-I
B, New England Biolabs. Anti-Bcl-10 antibody has been described elsewhere (26). Rabbit antisera to CARMA3 were generated in our animal facility using recombinant CARMA3 as an antigen.
Immunoblot Analysis and CoprecipitationCell lysates were made in lysis buffer (150 mM NaCl, 20 mM Hepes, pH 7.4, 1% Triton X-100, 10% glycerol, and a mixture of protease inhibitors). Proteins were separated by SDS-PAGE, transferred onto nitrocellulose membrane, and incubated with primary antibodies followed by horseradish peroxidase-conjugated secondary antibodies (Amersham Biosciences). Blots were developed using the ECL system (Amersham Biosciences). For coimmunoprecipitation experiments, cells were lysed in lysis buffer and immunocomplexes were bound to protein A/G, resolved by SDS-PAGE, and analyzed by immunoblot assay.
Recombinant Protein Production and in Vitro Binding AssaysHistidine-tagged CARMA3 polypeptides were expressed from pET 28 vector (Novagen) in BL21 bacterial strain. Cell lysates from 5 x 106 Jurkat cells were made in lysis buffer, and 300 µl of lysate were mixed with 50 µl of the indicated recombinant protein. Samples were incubated at 4 °C, washed several times by pulse centrifugation in the same buffer, and resuspended in 50 µl of sample buffer. 10 µl of the reaction were loaded for SDS-PAGE and Western blot analysis.
Gel Filtration AnalysisFifty million Jurkat cells were washed in phosphate-buffered saline and lysed in 500 µl of lysis buffer supplemented with a mixture of proteases and phosphatases inhibitors. Lysates were centrifuged for 60 min at 15,000 rpm, and the supernatant was recovered and loaded on a Superdex 200 gel filtration chromatography column (Amersham Biosciences) precalibrated with tryptophane (0.267 kDa), TTF1 homeodomain (15 kDa), bovine serum albumin (67 kDa), IgG (150 kDa), and thyroglobulin (669 kDa). 300-ml fractions were recovered and analyzed by Western blotting or subjected to immunoprecipitation.
Luciferase AssayTo assess for NF
B activation, HEK293 cells were transfected with the indicated plasmidic DNAs together with pNF
B-luc (Clontech) in 6-well plates. 24 h later, luciferase activity was determined with the luciferase assay system (Promega). A plasmid expressing
-galactosidase was added to the transfection mixture for normalization of the efficiency of transfection.
| RESULTS AND DISCUSSION |
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-galactosidase reporter gene. Sequence analysis revealed that one of the isolated plasmids encoded for amino acids 4971032 of CARMA3. Further assays were then performed with this library clone in the reporter yeast strain to identify the regions of CARMA3 and NEMO involved in the interaction. As summarized in Fig. 1, the CARMA34971032 library clone interacted with GAL4BD·NEMO and with truncated forms of NEMO containing the Leu51-Ala100 region of the protein. Thus, we concluded that the region Leu51-Ala100 of NEMO is essential for binding to CARMA3 in yeast. A reciprocal analysis, using CARMA3 deletion mutants, indicated that the region Ile600-Leu800 of CARMA 3, containing a PDZ and an SH3 domain, is required for association with NEMO in yeast (Fig. 1).
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Stimulation-dependent Association of CARMA3 with NEMO To confirm association of CARMA3 and NEMO in a different experimental system, CARMA3 polypeptides were expressed as His-tagged proteins in bacteria and tested for binding to NEMO endogenously expressed in Jurkat cells. As shown in Fig. 3A, recombinant polypeptides containing the region Ile600-Leu800 of CARMA3 pull down NEMO from Jurkat lysates. Similar to the results obtained in HEK293 cells, stimulation of Jurkat cells with PMA and ionomycin facilitates association of NEMO with CARMA3. Time course experiments indicated that in this experimental system association of CARMA3 and NEMO peaks at 30 min following stimulation (Fig. 3B). All tested CARMA3 polypeptides lacking the region Ile600-Leu800 failed to pull down NEMO from Jurkat cells lysates (Fig. 3A and data not shown).
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B transcription factor in lymphocytes, we tested whether CARMA1 also binds to NEMO. To this end, lysates from Jurkat cells left untreated or stimulated with PMA/ionomycin were immunoprecipitated with anti-NEMO or an isotype-matched antibody and coimmunoprecipitated proteins were tested for the presence of CARMA1 by immunoblot assay. As shown in Fig. 3C, CARMA1 coprecipitates with NEMO when Jurkat cells were stimulated with PMA and ionomycin.
Gel Filtration Analysis of CARMA1 and NEMOTo confirm that CARMA1 stably associates to NEMO following cellular stimulation, lysates prepared from Jurkat cells left untreated or stimulated with PMA/ionomycin were fractionated on a Superdex 200 gel filtration column and analyzed by immunoblot experiments. As previously reported (5), elution of NEMO, monitored with anti-NEMO antibody, was mostly observed in fractions containing proteins of 600800 kDa (Fig. 4A). When we looked for CARMA1 elution, a partially overlapping profile was obtained. We also examined the elution profile of the adapter protein Bcl10, which in lymphoid cells mediates activation of NF
B via interaction with CARMA1. When compared with NEMO and CARMA1, elution of Bcl10 was observed in fractions containing proteins of lower molecular mass (Fig. 4A). Remarkably, when we examined elution of these proteins in lysates of stimulated Jurkat cells, the elution peaks of CARMA1 and Bcl10 both appeared shifted toward the NEMO-containing fractions (Fig. 4A).
|
K
/
from lysates of stimulated cells, indicating that both CARMA1 and Bcl10 participate to the 600800-kDa complex containing NEMO, I
K
, and I
K
(Fig. 4B). Similar results were obtained when NEMO-containing fractions were immunoprecipitated with anti-CARMA antisera (Fig. 4C).
CARMA3 Modulates Bcl10-induced NF
B ActivationThe interaction of CARMA3 and CARMA1 with NEMO prompted us to investigate whether deletion mutants of CARMA3 may influence Bcl10-induced activation of NF
B. Indeed, Fig. 5 shows that expression of polypeptides containing the NEMO-binding region of CARMA3 reduces activation of NF
B mediated by expression of Bcl10 (Fig. 5A). This inhibition was specific for Bcl10, because no effect on activation of NF
B induced by other activators was observed (Fig. 5B).
|
B. Bcl10 functions as a positive regulator of lymphocyte proliferation and specifically links antigen receptor signaling to NF
B activation (9). In addition, during embryonic development the function of Bcl10 is necessary for the correct neural tube closure, suggesting a general requirement of Bcl10 for proper I
K regulation and NF
B signaling (9) in different cell types. The three CARMA proteins appear to be key players in mediating the tissue-specific, Bcl10-dependent, activation of NF
B. CARMA1 is expressed in a variety of adult tissues, including thymus, spleen, liver, and peripheral blood leukocytes (20, 21). CARMA2 is expressed mostly in placenta, whereas CARMA3 is expressed in fetal lung, liver, and kidney (21). Because Bcl10 is ubiquitously expressed, it may utilize the corresponding tissue-specific CARMA to regulate activation of the I
K complex.
Recent evidence indicates that Bcl10-mediated activation of NF
B requires Lys63-linked ubiquitination of NEMO, which is mediated by the paracaspase MALT1 and the ubiquitin-conjugating complex E2 (27). Together with these observations, our data allow the proposition of the following model (Fig. 6). In resting cells, Bcl10 is found preassociated with the ubiquitin-conjugating complex and MALT1 (27). Following stimulation, Bcl10 binds to CARMA via a CARD-CARD interaction, carrying the ubiquitin-conjugating complex in close proximity to NEMO, which associates to the region Ile600-Leu800 of CARMA. The close proximity of NEMO to the ubiquitin-conjugating complex results in Lys63-linked ubiquitination of NEMO. This event eventually determines activation of the I
K complex, which, in turn, triggers NF
B activation by phosphorylation of I
B.
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B activation following stimulation with PMA and ionomycin (9, 1316, 28). Second, CARMA mutants unable to recruit Bcl10 act as dominant negative inhibitors of PMA- and ionomycin-induced NF
B activation (12). Third, whereas deletions of the C-terminal GUK domain do not interfere with the ability of CARMA to induce NF
B activity, deletion of the SH3 and PDZ domains reduces the levels of NF
B activation (23). The importance of CARMA in linking Bcl10/MALT1 to NEMO may extend to cellular processes other than lymphocyte activation and central nervous development. Future studies will be aimed at exploring the potential role of this pathway in a variety of biological systems.
| FOOTNOTES |
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To whom correspondence should be addressed. Tel.: 3908-2430-5166; Fax: 3908-2423-013; E-mail: vito{at}unisannio.it.
1 The abbreviations used are: I
K, I
kinase; NEMO, NF
B essential modulator; CARD, caspase recruitment domain; PMA, phorbol 12-myristate 13-acetate; FCS, fetal calf serum; mAb, monoclonal antibody. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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