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(Circulation Research. 2003;92:630.)
© 2003 American Heart Association, Inc.

Molecular Medicine

The Antiinflammatory Endothelial Tyrosine Kinase Tie2 Interacts With a Novel Nuclear Factor-B Inhibitor ABIN-2

David P. Hughes, Marie B. Marron, Nicholas P.J. Brindle

From the Cardiovascular Research Institute and Department Surgery, University of Leicester, UK.

Correspondence to Nicholas P.J. Brindle, University of Leicester, Dept of Surgery, RKCSB, PO Box 65, Leicester LE2 7LX UK. E-mail npjb1{at}leicester.ac.uk

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
Tie2 is a receptor tyrosine kinase expressed predominantly in endothelial cells and is essential for blood vessel formation and maintenance. The receptor has potent antiinflammatory effects on endothelial cells, suppressing vascular endothelial growth factor– and tumor necrosis factor–induced expression of leukocyte adhesion molecules and procoagulant tissue factor and inhibiting vascular leakage. To delineate the signaling pathways utilized by Tie2, we performed yeast two-hybrid screening of a human endothelial cell cDNA library and identified a novel protein interacting with the intracellular domain of the receptor. This protein was found to be human A20 binding inhibitor of NF-B activation-2, ABIN-2, an inhibitor of NF-B–mediated inflammatory gene expression. Coexpression of Tie2 and ABIN-2 in CHO cells confirmed the interaction occurs in mammalian cells. In contrast, Tie1 did not interact with ABIN-2 in the yeast two-hybrid system or mammalian cells. Deletion analysis identified the Tie2 binding motif to be encompassed between residues 171 and 272 in ABIN-2. Interaction was dependent on Tie2 autophosphorylation but ABIN-2 was not tyrosine phosphorylated by Tie2. Furthermore, in endothelial cells the interaction was stimulated by the Tie2 ligand angiopoietin-1. Expression of ABIN-2 deletion mutants in endothelial cells suppressed the ability of angiopoietin-1 to inhibit phorbol ester–stimulated NF-B–dependent reporter gene activity. These findings provide the first direct link between Tie2 and a key regulator of inflammatory responses in endothelial cells. Interaction between Tie2 and ABIN-2 may be important in the vascular protective antiinflammatory actions of Tie2.

Key Words: Tie2 • endothelial cells • inflammation • angiogenesis

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
The Tie family of receptor tyrosine kinases comprises two members, Tie1 and Tie2. These receptors are expressed predominantly in endothelial cells and are essential for vessel formation where they are required for the later stages of angiogenesis and vessel maintenance.¹ Tie2 has roles in control of microvessel sprouting, integrity, and maturation. Mice deficient in this receptor exhibit decreased microvascular integrity, reduced vessel sprouting, and endothelial cell loss.^2,3 A number of ligands, designated the angiopoietins, have been identified for Tie2.^4–6 Unusually, this family of ligands includes both activators and antagonists; angiopoietin-1 (Ang1) and angiopoietin-4 (Ang4) are able to stimulate Tie2 autophosphorylation and this can be inhibited by angiopoietin-2 (Ang2) and angiopoietin-3 (Ang3). In accord with the sprouting defects seen in Tie2-deficient mice, activation of Tie2, by Ang1 or a derivative form of the agonist called Ang1*, has been shown to stimulate endothelial migration and sprouting of endothelial cords in collagen gels.^7–11 Ang1 inhibits apoptosis of endothelial cells after serum deprivation,^10,12–14 irradiation, and mannitol treatment.¹⁵ Significant advances have been made in delineating the signaling pathways by which Tie2 regulates endothelial migration and survival. The p85 subunit of phosphatidylinositol 3-kinase (PI-3K) interacts with Tie2 and mediates the effects of the receptor on Akt activation and cell survival as well as being partly involved in Tie2-regulated migration.^10,13,16 Tie2 has also been shown to bind to a docking protein, Dok-R, that itself recruits downstream signaling proteins including Nck and rasGAP.¹⁷ Tie2 binding with Dok-R mediates the effects of the receptor on endothelial migration.¹⁸ The adapter proteins Grb2, 7, 14, and tyrosine phosphatase Shp2 have also been found to interact with Tie2.^10,19

Ang1 is a powerful inhibitor of vascular endothelial growth factor (VEGF)– or inflammation-induced vascular leakage in vivo,^20,21 and decreases permeability of endothelial monolayers.²² Tie2 activation also exerts potent antiinflammatory effects on endothelial cells, decreasing VEGF- and tumor necrosis factor (TNF)-–induced leukocyte:endothelial adhesion and suppressing TNF-–stimulated leukocyte transmigration across endothelial monolayers.²² Furthermore, stimulation of Tie2 inhibits expression of the NF-B–responsive genes intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin induced by VEGF,²³ as well as tissue factor induced by TNF- and VEGF.²⁴ Tie2 suppression of endothelial activation and promotion of vessel integrity are likely to be critical in the prostabilizing effects proposed for the receptor in vascular maturation and maintenance.¹ Furthermore, the unique profile of activities regulated by this receptor make it a potential therapeutic target for suppressing edema, vascular inflammation, and leakage associated with a number of diseases.

In an attempt to understand the mechanisms by which Tie2 regulates endothelial function, we have sought to identify proteins from endothelial cells that specifically bind to the intracellular domain of the receptor. We find Tie2 directly interacts with a potent regulator of inflammatory gene expression, A20 binding inhibitor of NF-B activation-2 (ABIN-2). This interaction may be important in mediating the inhibitory effects of Tie2 on NF-B–responsive genes and thereby have a key role in promoting neovessel quiescence and maturation.

	Materials and Methods

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The antibodies recognizing Tie1 and Tie2 were obtained from R&D Systems and Santa Cruz Biotechnology Inc, and the monoclonal antibody against phosphotyrosine was from BD Transduction Laboratories. cDNAs encoding human Ang1 and Tie2 were obtained from the American Tissue Culture Collection and the latter subcloned into the expression vector pCR3 (Invitrogen Life Technologies). Tie1 cDNA has previously been described.²⁵ Endothelial cells were obtained and cultured as previously described.²⁵ Chinese hamster ovary (CHO) cells were obtained from the European Collection of Cell Cultures and maintained in DMEM supplemented with 10% fetal calf serum (FCS), 100 µg/mL streptomycin, and 100 U/mL penicillin under 5% CO₂/95% air in a humidified incubator at 37°C. Ang1* is a recombinant form of Ang1, with improved solubility and stability, in which amino acid residues 1 to 77 of Ang1 are replaced with residues 1 to 73 of Ang2 and Cys265 of Ang1 substituted with Ser.^5,26 cDNA encoding Ang1*, incorporating the carboxy-terminal human IgG1 Fc extension (obtained from human placental cDNA), was generated by site directed mutagenesis and PCR from human Ang1 cDNA, sequenced and cloned into pcDNA3.1. The plasmid was transfected into CHO cells and stable Ang1* expressing clones selected using G418 (0.5 mg/mL). Recombinant Ang1* was isolated by affinity chromatography on protein G or protein A sepharose from medium conditioned by CHO cells. The yield of functionally active product was measured by BIAcore, using immobilized human Tie2/Fc chimera and a standard of Ang2 (both from R&D Systems). All other reagents were from Sigma-Aldrich Company or sources described previously.²⁵

Yeast Two-Hybrid Analysis
The yeast strains PJ69-4A and PJ69-4 were generously supplied by Dr Philip James (University of Wisconsin, Madison, Wis).²⁷ Yeast two-hybrid screening was performed using a library, kindly provided by Drs Emma Knight and Sally Prigent (University of Leicester, Leicester, UK), in which the pVP16 activation domain (AD) was ligated to 350 to 700 bp fragments of human dermal microvascular endothelial cDNA.²⁸ The library was transformed into the PJ69-4A yeast strain expressing the cytoplasmic domain of Tie2 (nt 2525-3540) or Tie1 (nt 2509-3452) in pBD-GAL4 vector (Stratagene) using the lithium acetate-base method of transformation. Proteins interacting with Tie2 were detected by their ability to reconstitute the Gal4 transcriptional regulator leading to induction of HIS3, LacZ, and Ade2 reporter genes. Transformants were plated on synthetic medium lacking histidine, leucine, and tryptophan, supplemented with 5 mmol/L 3-amino 1,2,4-triazole (3-AT). Growing colonies were replated onto synthetic medium lacking histidine, leucine, and tryptophan and supplemented with 10 mmol/L 3-AT. ß-Galactosidase activity of growing colonies was tested using colony nitrocellulose filter lift assays. His^- and ß-galactosidase positive transformants were further screened for Ade2 reporter gene activity by replating onto synthetic medium lacking adenine, leucine, and tryptophan supplemented with histidine. Library plasmids whose interactions with bait resulted in transcriptional activation of all three reporter genes were recovered into Escherichia coli HB 101 cells plated onto medium lacking leucine and analyzed further by DNA sequencing and additional interaction analysis. Interactions were further tested by mating yeast PJ69-4A and PJ694- strains containing either an AD or BD-plasmid, as indicated in Results, and analyzing for histidine, adenine, and ß-galactosidase reporter activity as before.

Cloning of Full-Length Human 9.1
The sequence of full-length 9.1 was determined using RACE cDNA amplification (Clontech). 5'- and 3'-RACE reactions were performed using cDNA templates synthesized from human placental RNA and the forward priming gene specific primer 3GSP9.1 (5'-TGGCCAAGTGT- CTGGATGAACGACAGC-3') and the reverse primer 5GSP9.1 (5'-CGCTGCCACTTGGCATTGAGGTCTTC-3'). To facilitate insertion of full-length 9.1 into the pFLAG-CMV-2 vector the coding sequence was amplified. DNA sequencing was performed on both strands by Cytomyx. (Cambridge).

Expression in Mammalian Cells
For expression of full-length and deleted versions of 9.1, the relevant cDNA sequences were subcloned into pFLAG-CMV2 expression vectors (Sigma) in-frame with the amino-terminal FLAG-epitope tag. Deletion mutants were constructed using existing restriction sites within the full-length sequence. Site directed mutagenesis of Tie2 was performed using QuikChange Site Directed Mutagenesis Kit (Stratagene) and the specific mutation confirmed by DNA sequencing. Expression vectors were transfected into CHO and endothelial cells using Superfect transfection reagent (Qiagen) or Targefect F-2 (Targeting Systems).

RT-PCR
Isolation of total RNA from human umbilical vein endothelial cells (HUVECs), HeLa, Jurkat, and 293 cells was performed using the RNeasy mini kit (Qiagen), and cDNA synthesis was performed using Omniscript reverse transcriptase (Qiagen). cDNA templates were screened for the presence of 9.1 by PCR using 5GSP9.1 and 3GSP9.1 primers and human glyceraldehydes 3-phosphate dehydrogenase primers (Stratagene). Reaction products were visualized by ethidium bromide staining after

Immunoprecipitation and Immunoblotting
Cells were washed in PBS and lysed at 4°C in ice-cold lysis buffer (50 mmol/L Tris, pH 7.4, containing 50 mmol/L NaCl, 1% Triton X-100, 1 mmol/L sodium fluoride, 1 mmol/L EGTA, 1 mmol/L aminoethylbenzenesulfonic acid, 1 µg/mL leupeptin, 1 µg/mL aprotinin, 1 µg/mL pepstatin). Lysates were cleared by centrifugation at 12 000g for 10 minutes and then incubated for 2 hours or overnight at 4°C with 4 µg/mL of the appropriate antibody and protein G-agarose beads. Immune complexes were washed extensively with lysis buffer and proteins eluted by boiling in Laemmli sample buffer in the presence of 100 mmol/L DTT. Proteins were separated by SDS polyacrylamide gel electrophoresis, transferred to nitrocellulose membranes, and probed with appropriate antibodies. Immunoreactive proteins were visualized with a peroxidase conjugated secondary antibody and a chemiluminescent detection system.²⁹

NF-B Reporter Assays
Human umbilical vein endothelial cells (HUVECs) were grown to 80% confluence in 35-mm dishes and transfected with 1 µg/dish of DNA, comprising 500 ng of the NF-B–responsive luciferase reporter pNF-B-Luc (Stratagene), 100 ng of the pRL-TK reporter (Promega), in which Renilla luciferase activity is constitutively expressed under the control of the moderate herpes simplex virus thymidine kinase promoter, and 400 ng of control or test plasmid. After 24 hours, cells were washed with PBS and medium replaced with serum-free medium. Cells were activated by phorbol 12-myristate 13-actetate (PMA) at a final concentration of 5 ng/mL for 4 hours. Where Ang1* was present, it was added to a final concentration of 400 ng/mL 15 minutes before PMA. Luciferase reporter gene activities were measured in cell lysates using the Dual Luciferase Reporter Assay (Promega) and NF-B–activated reporter activity was normalized to Renilla luciferase activity to correct for any differences in transfection efficiency.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
Yeast Two-Hybrid Screen
In order to gain insight into the signaling pathways utilized by Tie2, we undertook a yeast two-hybrid screen to identify proteins that interact with the intracellular domain of this receptor. A cDNA library derived from human dermal microvascular endothelial cells was interrogated with a fusion protein comprising the cytoplasmic domain of Tie2 fused in-frame with a Gal4 DNA binding domain. This Tie2 fusion protein was found to exhibit constitutive tyrosine phosphorylation when expressed in yeast (Figure 1A). A number of clones were identified during the screen as encoding proteins that interact with Tie2 and these included a novel partial cDNA sequence designated clone 9.1. This clone consistently and strongly activated histidine, adenine, and ß-galactosidase reporters in the yeast two-hybrid system (Figure 1B). Neither clone 9.1 nor the Tie2-binding domain activated reporters when expressed alone. The interaction with Tie2 was specific in that the Tie1 cytoplasmic domain failed to interact with clone 9.1 (Figure 1B). One reason for screening a microvascular endothelial cell library was the possibility that such a library may contain cell-specific interacting proteins for this endothelial receptor tyrosine kinase. To examine whether clone 9.1 was expressed specifically in endothelial cells mRNA from several cell lines, namely the cervical adenocarcinoma line HeLa, lymphocytic Jurkat line, and kidney epithelial 293 cells, as well as HUVECs, were probed by RT-PCR. This demonstrated expression was not specific for endothelial cells (Figure 1C).

View larger version (23K): [in this window] [in a new window]   Figure 1. Interaction between Tie2 and 9.1 in yeast two-hybrid system. A, Immunoblotting of whole-cell lysates from yeast transformed with Gal4 BD or intracellular domain of Tie2 fused to Gal4 BD with antibodies recognizing phosphotyrosine (-PY) and Tie2 (-Tie2) demonstrates the Tie2 fusion protein is expressed and tyrosine phosphorylated. Arrowhead indicates the position of Tie2. B, Summary of histidine, adenine, and ß-galactosidase reporter activation for Tie2 and Tie1 intracellular domains with clone 9.1 in the yeast two-hybrid system. EV denotes Gal4-BD or Gal4-AD vector as indicated. + Denotes growth; ++ coloration appeared within 30 minutes; and - lack of growth or lack of coloration. C, Expression of 9.1 and GAPDH as detected by RT/PCR of mRNA isolated from endothelial and nonendothelial cells.

Interaction in Mammalian Cells
To confirm the protein encoded by clone 9.1 interacts with Tie2 in mammalian cells, the open reading frame of 9.1 was cloned downstream of a FLAG-epitope tag and coexpressed in CHO cells with full-length Tie2. After expression cells were lysed and FLAG-9.1 immunoprecipitated via the epitope tag. Immunoprecipitated proteins were resolved by SDS-PAGE and probed for the presence of Tie2 (Figure 2). Tie2 was coimmunoprecipitated with 9.1 confirming the interaction can occur in mammalian cells. Tie2 was not detected in immunoprecipitates from cells transfected with Tie2 and FLAG vector. The specificity of interaction of 9.1 with members of the Tie family was tested in mammalian cells by cotransfection of FLAG-9.1 with full-length Tie1 in CHO cells and immunoprecipitation of epitope-tagged 9.1. Consistent with the findings in the yeast two-hybrid system, only background levels of Tie1 coimmunoprecipitated with FLAG-9.1, indicating a lack of interaction between these two molecules in mammalian cells (Figure 2).

View larger version (32K): [in this window] [in a new window]   Figure 2. Tie2 and 9.1 interact in mammalian cells. Chinese hamster ovary cells were cotransfected with FLAG-epitope–tagged 9.1 and full-length Tie2 or Tie1, or FLAG-vector and each of the receptors, as indicated. Twenty-four hours later, lysates were prepared and 9.1 recovered by immunoprecipitation with an antibody recognizing the epitope tag. Proteins in immunoprecipitates (Ip) and whole-cell lysates (Wcl) were resolved by SDS-PAGE, and 9.1, Tie2, and Tie1 detected by immunoblotting. Mobility of molecular mass markers is indicated in kDa to the left of blots.

Rapid amplification of cDNA ends (RACE) was used to determine the 5' and 3' ends of clone 9.1. This resulted in an approximately 2-kb sequence containing a putative initiation methionine and an open reading frame of 1287 bp, stop codon, and 3'-untranslated region (Figure 3A). Full-length 9.1 encodes a predicted protein of 429 amino acids and calculated relative molecular mass of approximately 49 kDa. The protein contains two regions, between residues 84 to 114 and 255 to 305, with a high predicted probability of forming coiled-coil domains,³⁰ suggesting the possibility that it may form oligomers. In the course of the present investigation, a mouse sequence designated A20-binding inhibitor of nuclear factor-B activation-2 (ABIN-2) was reported.³¹ Full-length 9.1 exhibits 78% amino acid identity with mouse ABIN-2 and is presumed to be the human homologue of this protein. Indeed, the predicted amino acid sequence of full-length 9.1 is identical to the predicted amino acid sequence derived from two human expressed sequence tag clones conceptually translated and designated human ABIN-2.³¹ Hence, full-length 9.1 will be referred to as human ABIN-2.

View larger version (59K): [in this window] [in a new window]   Figure 3. Deduced amino acid sequence of full-length 9.1 and interaction with Tie2. A, Nucleotide sequence of full-length 9.1 was obtained by rapid amplification of cDNA ends from mRNA isolated from human placenta. Deduced amino acid sequence is shown aligned with mouse ABIN-2 (GenBank accession No. AJ304865). Sequence of the original 9.1 clone found to interact with Tie2 is overlined. B, Schematic representation of 9.1 constructs and their designations. Amino acid residue numbers of the full-length and deleted versions are indicated. C, Indicated constructs with amino-terminal FLAG-tags were coexpressed with Tie2 in CHO cells. Twenty-four hours later, lysates were prepared and expressed full-length (human ABIN-2) and deleted 9.1 recovered by immunoprecipitation with an antibody recognizing the epitope tag. Proteins in immunoprecipitates (Ip) and whole-cell lysates (Wcl) were resolved by SDS-PAGE and ABIN-2, 9.1, C, and Tie2 detected by immunoblotting.

Deletion Analysis of Tie2 Binding Motif
Human ABIN-2 was analyzed for its ability to interact with Tie2. In addition, deletion mutants were constructed in an attempt to define the region within 9.1 responsible for interaction with Tie2. Epitope-tagged human ABIN-2 and deletion mutants or empty FLAG-vector were cotransfected into CHO cells with Tie2. Twenty-four hours after transfection full-length and deleted proteins were recovered by immunoprecipitation from lysates with an antibody against the FLAG-epitope. Tie2 coimmunoprecipitated with human ABIN-2, indicating the interaction domain originally identified in the 9.1 fragment (amino acids 171 to 272) was available to interact with the receptor tyrosine kinase. Deletions of residues 229 to 272 of the 171 to 272 fragment resulted in loss of interaction with Tie2. Thus, the C-terminal 44 amino acid residues of 171 to 272 are required for interaction with Tie2 (Figure 3C).

Phosphotyrosine Dependence of Interaction
To determine whether 9.1 interaction with Tie2 was phosphotyrosine-dependent a mutant form of Tie2 was constructed in which the invariant lysine in the ATP binding site of the kinase was replaced with arginine. As shown in Figure 4A, when expressed in CHO cells, wild-type Tie2 was constitutively tyrosine phosphorylated. The kinase-negative Tie2^K855R, however, was not tyrosine phosphorylated. Wild-type Tie2 or Tie2^K855R were coexpressed with 9.1 in CHO cells and interaction determined by coimmunoprecipitation analysis. Wild-type Tie2 but not the unphosphorylated Tie2^K855R was coimmunoprecipitated with 9.1, indicating the interaction is phosphotyrosine-dependent. The effect of Tie2 on tyrosine phosphorylation status of human ABIN-2 was analyzed by coexpressing the protein with Tie2 in CHO cells and antiphosphotyrosine immunoblotting (Figure 4C). Human ABIN-2 showed no evidence of tyrosine phosphorylation when coexpressed with kinase active Tie2.

View larger version (19K): [in this window] [in a new window]   Figure 4. Interaction between Tie2 and 9.1 is phosphotyrosine-dependent. A, Wild-type Tie2 and kinase-negative Tie2K855R were expressed in CHO cells and immunoprecipitated. Tyrosine phosphorylation status was determined by antiphosphotyrosine (-PY) immunoblotting. B, Wild-type or kinase-negative Tie2K855R were coexpressed with 9.1 in CHO cells. Twenty-four hours later, lysates were prepared and 9.1 recovered by immunoprecipitation with an antibody recognizing the epitope tag. Proteins in immunoprecipitates (Ip) and whole-cell lysates (Wcl) were resolved by SDS-PAGE and 9.1 and Tie2 detected by immunoblotting. C, Phosphorylation status of ABIN-2 coexpressed with kinase-active Tie2. Human ABIN-2 was coexpressed with wild-type Tie2 and recovered by immunoprecipitation. Tyrosine phosphorylation status was determined by antiphosphotyrosine (-PY) immunoblotting. Mobility of molecular mass markers is indicated in kDa to the left of blots.

Regulation by Angiopoietin-1
Data from the two-hybrid screen, together with the coimmunoprecipitation analyses, suggest ABIN-2 could interact with Tie2 in endothelial cells after activation of receptor phosphorylation. Endogenous ABIN-2 is highly sensitive to proteolysis and cannot be immunoprecipitated under conditions that would preserve interactions with other proteins.³¹ Therefore, to determine whether ligand-regulated interaction could occur in endothelial cells expressing physiological levels of Tie2, HUVECs were transfected with epitope-tagged 9.1. Twenty-fours hours after transfection HUVECs were stimulated with Ang1* for 10 minutes, lysed, and subjected to immunoprecipitation with an antibody recognizing the FLAG-epitope. Ang1* is a modified version of Ang1 that is more stable and more readily purified.⁵ Ang1* stimulated the interaction between 9.1 and endogenous Tie2 in endothelial cells, as judged by the increased levels of Tie2 coimmunoprecipitated from the stimulated cells (Figure 5). As expected, this coimmunoprecipitated Tie2 was tyrosine phosphorylated. Under the conditions of this study, unactivated HUVECs exhibited a low level of tyrosine phosphorylated Tie2 that coimmunoprecipitated with 9.1.

View larger version (54K): [in this window] [in a new window]   Figure 5. Angiopoietin-1 stimulates interaction. HUVECs were transfected with epitope-tagged 9.1. Twenty-four hours later, cells were treated for 10 minutes with 400 ng/mL Ang1* or control vehicle (Cont), as indicated, and 9.1 recovered by immunoprecipitation with an antibody recognizing the epitope tag. Proteins in immunoprecipitates (Ip) and whole-cell lysates (Wcl) were resolved by SDS-PAGE and Tie2 and 9.1 detected by immunoblotting. Tyrosine phosphorylation status of Tie2 was determined by antiphosphotyrosine (PY) immunoblotting.

Effects of ABIN-2 Deletion Mutants on NF-B Activity in Endothelial Cells
The potent NF-B inhibitory activity of ABIN-2³¹ suggest this molecule may be a candidate for mediating Tie2 inhibition of NF-B–dependent gene expression. Recruitment of ABIN-2 to stimulated Tie2 could, directly or indirectly, activate the NF-B inhibitory activity of ABIN-2 leading to suppression of inflammatory gene expression. If interaction between ABIN-2 and Tie2 has a role in Tie2 inhibition of NF-B, then expression of deleted forms of ABIN-2, containing the Tie2 interaction site but lacking NF-B inhibitory activity, would be expected to suppress Tie2 inhibition of NF-B. Deletion of the carboxy-terminal 90 amino acid residues of ABIN-2 removes its ability to inhibit NF-B.³¹ The possibility that deletion-mutants of human ABIN-2 could act in such a dominant-negative manner was examined in HUVECs. Endothelial cells were transfected with a control plasmid (GFP) or the ABIN-2 fragment designated 9.1 (encompassing amino acid residues 171 to 272), together with an NF-B–dependent reporter gene. Twenty-four hours later, cells were washed and pretreated with control vehicle or Ang1* for 15 minutes, then activated with phorbol ester for an additional 4 hours before measurement of reporter gene activity. PMA stimulated the activity of NF-B in control transfected cells and this was prevented by Ang1* (Figure 6). Expression of 9.1 partially inhibited the effects of Ang1*, allowing PMA-activated NF-B activity to remain significantly increased in the presence of the Tie2 ligand, although not to the same level as with PMA alone (Figure 6). In three independent experiments, expression of 9.1 caused a significant average reduction of 31.0±6.8% (P<0.05, paired t test) in Ang1* inhibition of PMA-activated NF-B activity compared with control transfected cells. There was some increase in PMA-stimulated NF-B activity in the absence of Ang1* in cells expressing 9.1. This may reflect 9.1 inhibiting a limitation on activated NF-B exerted by the low level of Tie2:ABIN-2 interaction seen in the absence of Ang1* under our experimental conditions (Figure 5), or it may be due to some unknown direct effect of 9.1 on the NF-B system. Therefore, we also examined a deletion mutant corresponding to amino acids 1 to 340 of human ABIN-2. This mutant, designated ABIN-2 (1-340), lacks only the C-terminal 90 amino acid residues and has been shown to have no direct effects on NF-B or A20-mediated NF-B inhibition.³¹ ABIN-2 (1-340) was more effective than 9.1, completely reversing the inhibitory effect of Ang1* on PMA-stimulated NF-B activity in endothelial cells (Figure 6) and significantly suppressing Ang1* inhibition by 111.7±7.4% (P<0.01, paired t test, n=3). These data suggest ABIN-2 has a functional role in regulation of activated NF-B by Tie2.

View larger version (27K): [in this window] [in a new window]   Figure 6. Effects of deletion mutants of ABIN-2 on endothelial NF-B activity. HUVEC were transfected with NF-B–responsive luciferase reporter pNF-B-Luc, together with pRL-TK reporter and GFP, 9.1, or human ABIN-2 (1-340) as indicated. Twenty-four hours later, cells were treated for 4 hours with control vehicle (), 5 ng/mL PMA (), 400 ng/mL Ang1* (), or 5 ng/mL PMA in the presence of 400 ng/mL Ang1* (). Cell lysates were assayed for luciferase activities. pNF-B-Luc activity was normalized for transfection efficiency by comparison with pRL-TK activity. NF-B reporter gene activity (relative light units) is expressed relative to PMA-stimulated cells transfected with GFP. Each bar represents mean and SEM for 3 independent experiments performed in triplicate. *P<0.05 compared with the respective control NF-B activity for GFP, 9.1, or 1-340 transfected cells (Student’s t test).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
This study demonstrates that the endothelial receptor tyrosine kinase Tie2 interacts with the NF-B regulatory protein ABIN-2. This novel interaction provides a direct link between the receptor tyrosine kinase and a key regulator of NF-B activity, providing a potential route for the potent vascular protective antiinflammatory effects of Tie2.

ABIN-2 is a recently discovered binding partner for the zinc finger protein A20.³¹ NF-B activation in response to a wide range of stimuli, including TNF-, interleukin-1, and phorbol esters, is inhibited in endothelial and other cells by A20.^32,33 The A20 gene was originally identified as an early response gene in endothelial cells, induced by TNF-, interleukin-1, and lipopolysacharide.³⁴ A20 functions as a feedback inhibitor of the NF-B pathway, the zinc finger protein being both induced by, and itself inhibiting, NF-B.³⁵ Indeed, TNF-–induced NF-B activity fails to terminate in cells lacking A20.³⁵ Furthermore, transgenic mice deficient in A20 are highly responsive to TNF- and LPS and develop severe inflammation.³⁶ The precise mechanism whereby A20 inhibits NF-B activity is not yet clear. The protein is known to bind to IKK/NEMO, a key regulatory component of the IKK complex that controls activation of NF-B via phosphorylation of the IB inhibitory protein.³⁷ Interestingly, binding of A20 alone to IKK/NEMO is not sufficient to inhibit NF-B, suggesting possible involvement of additional molecules.³⁸ ABIN-2 inhibits NF-B when expressed in mammalian cells³¹ and, along with the related but structurally distinct protein ABIN-1, has been suggested as a possible effector of A20 inhibition of NF-B.^31,38,39 It is not yet known whether the NF-B inhibitory activity of ABIN-2 requires the molecule to interact with A20. The finding that a mutant form of ABIN-2 that binds A20 but lacks the NF-B inhibitory motif is unable to act as a dominant-negative for ABIN-2 or A20 indicates the complexity of the mechanism by which ABIN-2, A20, and other regulators of NF-B interact to control this transcription factor.³¹

There is increasing evidence that Tie2 activation is associated with antiinflammatory effects in endothelial cells. Activation of Tie2 suppresses TNF-–induced leukocyte transmigration through endothelial monolayers.²² Furthermore, the Tie2 agonist Ang1 inhibits VEGF-induced leukocyte adhesion to endothelial cells and VEGF-stimulated endothelial ICAM-1, VCAM-1 and E-selectin expression as well as VEGF- and TNF-stimulated tissue factor expression.^23,24 These molecules are all known to be regulated by the NF-B system. Interestingly, inhibition of Akt/PI-3K enhances the stimulatory effects of VEGF and TNF on expression of tissue factor, and VEGF on ICAM-1, VCAM-1, and E-selectin, indicating an important role for these kinases.^23,24 The inhibitory effects of ABIN-2 on NF-B activity,³¹ and the present finding that activated Tie2 can interact with ABIN-2, suggest the Tie2:ABIN-2 interaction may have a functional role in mediating Tie2 inhibition of NF-B. Ang1 activation of Tie2 leading to receptor phosphorylation would allow recruitment of ABIN-2 to the receptor permitting activation of its NFB inhibitory activity. Inhibition of NF-B could then occur by a direct action of activated ABIN-2 or via further interaction of activated ABIN-2 with A20. The mechanisms regulating ABIN-2 activity have yet to be defined but could include conformational changes resulting from protein:protein interaction or changes in serine/threonine phosphorylation. If Tie2 inhibition of NF-B involves ABIN-2, it would be expected that this could be suppressed by expression of deleted forms of ABIN-2 containing the Tie2 binding sequence but lacking NF-B inhibitory activity. In the present study, two such deleted forms of ABIN-2, the 9.1 fragment and ABIN-2 (1-340), were found to suppress the effects of Ang1* on PMA-activated NF-B activity. The 9.1 fragment only partially suppressed Ang1* inhibition of NF-B. This may reflect a lower affinity or limited ability of this short fragment to compete with endogenous ABIN-2 for binding. ABIN-2(1 to 340) however had more potent dominant-negative activity and completely suppressed the effects of Ang1* on stimulated NF-B activity. These data suggest that ABIN-2 has a functional role in mediating Tie2 inhibition of activated NF-B. Very little is known about ABIN-2 and further work will be required to determine the mechanisms by which the molecule is regulated and precisely how it controls NF-B activity.

In summary, our data demonstrate Tie2 interacts with the novel regulator of NF-B activity ABIN-2. This interaction provides a direct molecular link between the tyrosine kinase and the NF-B system that may be important in the antiinflammatory and prostabilizing actions of Tie2 in endothelial cells and microvessels.

	Acknowledgments

 
This work was supported by the Wellcome Trust (Project Grant 062645). We also thank AstraZeneca for support.

Received August 28, 2002; revision received January 27, 2003; accepted February 12, 2003.

	References

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