Patent Application
20080248460
This invention relates to compositions and methods for identifying modulators of immune responses and more particularly to compositions and methods for identifying modulators of immune responses using members of the lymphotoxin-beta receptor (LTβR) complex.
Lymphotoxin-β receptor (LTβR), a non-death domain containing receptor of the tumor necrosis factor receptor (TNFR) family, is reported to play an important role in organogenesis of secondary lymphoid tissues. Two types of ligands for LTβR have been described. These have been identified as the heterotrimer LTα1β2 and the homotrimer LIGHT.
LTβR signaling is believed to be mediated by signaling molecules recruited to the intracellular domain of the receptor upon binding of a ligand. Like other members in the TNFR family, multiple signaling pathways are activated by the ligand-LTβR complex, including apoptosis, activation of NFκB and JNK pathway.
Apoptosis, or programmed cell death, is a normal cell suicide function. There are two major signaling pathways of apoptosis, the death receptor pathway and the mitochondrial pathway. Signals from both pathways lead to the activation of a common cascade of caspases, which subsequently cleave a large number of cellular proteins resulting in cell death. Most of the death domain-containing receptors induce apoptosis by recruitment of FADD and activation of caspase 8. In contrast, LTβR lacks a death domain sequence and thus does not interact with FADD. Moreover, the cell death induced by LTβR has a characteristic of a slow apoptosis process and is dependent on interferon γ. Therefore, LTβR-induced apoptosis is thought to be activated by mechanism that is distinct from a death domain-dependent mechanism.
The invention is based in part on the discovery of polypeptides associated with the lymphotoxin β receptor (LTβR) complex signaling pathway. Included in these newly identified components of the LTβR are the polypeptides Smac, cIAP1, and TRAF2, and enhancer of rudimentary (ERH). Also identified in the complex is TRAF3, which has previously been shown to associate with the LTβR complex. Smac has been reported to be implicated in a mitochondrial mediated apoptosis pathway.
Accordingly, in one aspect, the invention provides a purified complex comprising a (LTβR) polypeptide and a Smac polypeptide. In a preferred embodiment, the LTβR polypeptide is bound to a LTβR ligand. The LTβR ligand can be either a LIGHT polypeptide or an Ltα1β2 polypeptide. The complex can also include a cIAP1 polypeptide, a TRAF3 polypeptide, and a TRAF2 polypeptide. The amino acid sequences from these polypeptides can be from a mammal, e.g., a human, non-human primate, rodent, or another eukaryote such as Drosophila melanogaster. In a preferred embodiment, these polypeptides have human amino acid sequences.
In another aspect, the invention includes a purified complex comprising a LTβR polypeptide, a LTβR ligand, a TRAF2 polypeptide, a TRAF3 polypeptide, a cIAP1 polypeptide, and a Smac polypeptide.
In another aspect, the invention includes a method of identifying a modulator of a Tumor Necrosis Factor Receptor (TNFR) family member signaling pathway by contacting a cell expressing a TNFR family member with a test agent and determining whether the test agent modulates mitochondrial-mediated apoptosis in the cell. In a preferred embodiment, TNFR family member does not contain a death domain. In another embodiment, the TNFR family member is a LTβR polypeptide. In another preferred embodiment, the LTβR complex comprises LβTR, TRAF3, TRAF2, cIAP1, and Smac polypeptides. The test agent can either inhibit or activate LTβR signaling activity. In a preferred embodiment, the modulator is identified by determining activity or expression of a Smac polypeptide in the cell or by determining the interaction of cIAP1 polypeptide or a TRAF2 polypeptide with a Smac polypeptide.
The invention additionally provides a method of identifying a modulator of a lymphotoxin beta receptor (LTβR) complex signaling pathway by contacting a cell expressing an LTβR polypeptide with a test agent and determining whether the test agent modulates activity or expression of a Smac polypeptide in the cell.
In another aspect, the invention includes a method of identifying a modulator of a LTβR complex signaling pathway by contacting a cell expressing an LTβR with a test agent and determining whether the test agent modulates activity or expression of a cIAP1 polypeptide in the cell.
In another aspect, the invention provides a method of identifying a modulator of a LTβR complex signaling pathway by contacting a cell expressing an LTβR with a test agent and determining whether the test agent modulates activity or expression of a TRAF2 polypeptide in the cell.
In another aspect, the invention provides a method for identifying a modulator of an LTβR activity by contacting a test agent with a TRAF2, cIAP1, or Smac polypeptide and determining whether the test agent modulates activity of the polypeptide, thereby identifying a modulator of LTβR activity. In a preferred embodiment, the method further comprises determining whether the test agent binds directly to the polypeptide and whether the test agent affects activity or expression of the polypeptide.
In another aspect, the invention includes a method for identifying a modulator of a LTβR complex signaling pathway by contacting a Smac polypeptide with a test agent; and determining whether the test agent inhibits activity of the Smac polypeptide, thereby identifying an inhibitor of an LTβR complex signaling pathway.
In a another aspect, the invention includes a method for identifying an agent for treating or preventing an immune disorder by contacting a cell expressing a Tumor Necrosis Factor Receptor (TNFR) TNFR family member with a test agent and determining whether the test agent modulates mitochondrial-mediated apoptosis in the cell, thereby identifying an agent for treating or preventing an immune disorder. In a preferred embodiment, the TNFR family member does not contain a death domain and is a lymphotoxin β receptor (LTβR) polypeptide. This method can be used to identify an agent useful for treating an immune disorder selected from but not limited to, rheumatoid arthritis, systemic lupus erythematosus, Goodpasture's syndrome, Grave's disease, Hashimoto's thyroiditis, pemphigus vulgaris, myasthenia gravis, scleroderma, autoimmune hemolytic anemia, autoimmune thrombocytopenic purpura, polymyositis and dermatomyositis, pernicious anemia, Sjögren's syndrome, ankylosing spondylitis, vasculitis or Type I diabetes mellitus.
In a further aspect, the invention provides for a method for identifying an agent for treating or preventing cancer contacting a cell expressing a TNFR family member with a test agent and determining whether the test agent modulates mitochondrial-mediated apoptosis in the cell, thereby identifying an agent for treating or preventing cancer.
Also within the invention are modulators identified by the above-referenced screening methods, and methods of using these inhibitors to modulate an immune response.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In the case of conflict, the present Specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description and claims.
The invention provides methods for identifying lymphotoxin-beta receptor (LTβR) modulators, which can activate or inhibit LTβR signaling. The LTβR, or members of the complex, including those newly identified herein, can be used as an agent, or serve as a target for agents, that can be used to inhibit or stimulate LTβR mediated inhibition of mitochondrial apoptosis, for example to block abnormal cell growth or to extend cell growth in culture. The modulators identified herein can be used to treat a variety of indications, including immune conditions and cancer. A preferred indication is rheumatoid arthritis
The following terms are intended to have the following general meanings as they are used herein:
The term “apoptosis” refers to a process of programmed cell death.
The term “cytokine” refers to a molecule which mediates interactions between cells. A “lymphokine” is a cytokine released by lymphocytes.
The term “LTβR modulator” refers to any agent which can activate or inhibit ligand binding to LTβR, cell surface LTβR clustering or LTβR signaling, or which can influence how the LTβR signal is interpreted within the cell. Examples of LTβR activating agents include, IFN-α, IFN-γ, TNF, soluble anti-LTβR antibodies, cross-linked anti-LTβR antibodies and multivalent anti-LTβR antibodies, soluble LIGHT polypeptide or soluble Ltα1β2 polypeptide.
The term “LTβR signaling” refers to molecular reactions associated with the LTβR pathway and subsequent molecular reactions that result therefrom.
The term “substantially pure” polypeptide means a polypeptide or polypeptide complex separated from components that naturally accompany it. Typically, the polypeptide or polypeptide complex is substantially pure when is at least 60%, by weight, free from the proteins and naturally-occurring organic molecules with which it is naturally associated. Preferably, the purity of the preparation is at least 75%, more preferably at least 90%, and most preferably at least 99%, by weight.
The invention includes polypeptides that have been identified as components of the Lymphotoxin-β Receptor complex. These newly identified members of the LTbR complex include TRAF2, cIAP1, and Smac.
Smac is a mitochondrial protein but is released to the cytosol concurrent with the release of cytochrome c during apoptosis. The cytosolic Smac is then recruited to the receptor through the interaction with cIAP1 and relieves the inhibition of apoptosis by cIAP1. cIAP1 has been reported to inhibit apoptosis by inhibiting the activity of caspases.
The amino acid sequences for these polypeptides are provided below. For TRAF2, and Smac nucleic acid sequences are additionally provided.
The amino acid sequence of the TRAF2 polypeptide is provided below:
A nucleic acid sequence encoding the disclosed TRAF2 amino acid sequence is provided below:
The amino acid sequence of the IAP1 polypeptide is shown below:
The amino acid sequence of the Smac polypeptide is shown below:
A nucleic acid sequence encoding the disclosed Smac polypeptide is shown below:
The amino acid sequence of an enhancer of rudimentary homologue (ERH) polypeptide found is shown below:
The nucleotide sequence encoding the disclosed ERH polypeptide is shown below:
Protein complexes of the invention including two or more of the above-disclosed polypeptides along with, in various embodiments, a LTβR receptor, a LTβR ligand (such as LIGHT), and TRAF3 polypeptide. A preferred LTβR complex includes a LTβR receptor, LIGHT, TRAF2, cIAP1, ERH, and TRAF3. A complex can be obtained, for example, by extraction from a natural source, by expression of recombinant nucleic acids encoding the members of the complex, by expression of a polypeptide fragment fusion proteins, or by chemical synthesis. A chemically synthesized polypeptide or a polypeptide produced in a cellular system different from the cell from which it naturally occurs is, by definition, substantially free from components that naturally accompany it. Accordingly, substantially pure polypeptides include those derived from eukaryotic organisms but synthesized in recombinant cells of E. coli or other prokaryotes. Purity can be measured by any appropriate methods, e.g., column chromatography, polyacrylamide gel electrophoresis, or by HPLC analysis.
The invention additionally provides methods of identifying agents that modulate lymphoxin β receptor signaling, including methods that rely on the presence and or activity of one or more of the LTβR complex polypeptides disclosed above.
LTβR complexes are members of the Tumor Necrosis Factor (TNF)-receptors. TNF-related cytokines have emerged as a large family of pleiotropic mediators of host defense and immune regulation. Members of this family exist in membrane-bound forms which act locally through cell-cell contact, or as secreted proteins which can act on distant targets. A parallel family of TNF-related receptors react with these cytokines and trigger a variety of pathways including cell death, cell proliferation, tissue differentiation and pro-inflammatory responses.
In non-tumorigenic cells, TNF and many of the TNF family ligand-receptor interactions influence immune system development and responses to various immune challenges. The LTβR, a member of the TNF family of receptors, specifically binds to surface LT ligands. Signaling by LTβR may play a role in peripheral lymphoid organ development and in humoral immune responses. LTβR signaling, like TNF-R signaling, also has anti-proliferative effects and can be cytotoxic to tumor cells. LTβR mRNAs are found in human spleen, thymus and other major organs LTβR expression patterns show that LTβR is lacking in peripheral blood T cells and T cell lines. Accordingly, agents identified in the screening methods described herein can be used to treat a variety of indications.
In one aspect, agents that can modulate LTβR signaling are selected based on their ability by block mitochondrial-mediated apoptosis. A modulator of a LTβR signaling pathway is identified by contacting a cell expressing a LTβR with a test agent and determining whether the test agent modulates mitochondrial-mediated apoptosis in the cell. Alternatively, an agent can be selected by determining whether it modulates activity or expression of a cIAP1, TRAF2, or Smac polypeptide in the cell.
In general, any compound can be used as a test agent. Suitable test agents include, e.g., proteins, nucleic acids, carbohydrates, or small molecules. For example, the test agents can be obtained using any of the numerous approaches in combinatorial library methods known in the art, including: biological libraries; spatially addressable parallel solid phase or solution phase libraries; synthetic library methods requiring deconvolution; the “one-bead one-compound” library method; and synthetic library methods using affinity chromatography selection. The biological library approach is limited to peptide libraries, while the other four approaches are applicable to peptide, non-peptide oligomer or small molecule libraries of compounds (Lam (1997) Anticancer Drug Des 12:145).
Examples of methods for the synthesis of molecular libraries can be found in the art, for example in: DeWitt et al. (1993) Proc Natl Acad Sci U.S.A. 90:6909; Erb et al. (1994) Proc Natl Acad Sci U.S.A. 91:11422; Zuckermann et al. (1994) J Med Chem 37:2678; Cho et al. (1993) Science 261:1303; Carrell et al. (1994) Angew Chem Int Ed Engl 33:2059; Carell et al. (1994) Angew Chem Int Ed Engl 33:2061; and Gallop et al. (1994) J Med Chem 37:1233.
Libraries of compounds may be presented in solution (e.g., Houghten (1992) BioTechniques 13:412-421), or on beads (Lam (1991) Nature 354:82-84), on chips (Fodor (1993) Nature 364:555-556), bacteria (Ladner U.S. Pat. No. 5,223,409), spores (Ladner U.S. Pat. No. '409), plasmids (Cull et al. (1992) Proc Natl Acad Sci USA 89:1865-1869) or on phage (Scott and Smith (1990) Science 249:386-390; Devlin (1990) Science 249:404-406; Cwirla et al. (1990) Proc Natl Acad Sci U.S.A. 87:6378-6382; Felici (1991) J Mol Biol 222:301-310; Ladner above.).
In general, assays recognized in the art can be used to assess mitochondrial-mediated apoptosis. Such assays are described in, e.g., U.S. Pat. No. 5,935,937. One example is the following: (1) tumor cells such as HT29 or MCF-7 cells are cultured for three to four days in a series of tissue culture wells containing media and at least one LTβR activating agent in the presence or absence of serial dilutions of the agent being tested; (2) a vital dye stain which measures mitochondrial function such as MTT is added to the tumor cell mixture and reacted for several hours; (3) the optical density of the mixture in each well is quantified. The optical density is proportional to the number of tumor cells remaining in the presence of the LTβR activating agent and the test LTβR blocking agent in each well.
The invention can additionally use known or putative LTβR modulating activating agents. Agents that induce LTβR signaling (such as activating soluble LTβR fragments or anti-LTβR monoclonal antibodies, and modified forms thereof) can be selected based on their ability, alone or in combination with other agents, for example the interaction of Smac with cIAP1, to potentiate tumor cell cytotoxicity through mitochondrial-mediated apoptosis using the tumor cell assay described above.
Another method for selecting an LTβR modulation agent is to monitor the ability of the putative agent to directly interfere with LTβR-ligand binding. Any of a number of assays that measure the strength of ligand-receptor binding can be used to perform competition assays with putative LTβR blocking agents. The strength of the binding between a receptor and ligand can be measured using an enzyme-linked immunoadsorption assay (ELISA) or a radio-immunoassay (RIA). Specific binding may also be measured by fluorescently labelling antibody-antigen complexes and performing fluorescence-activated cell sorting (FACS) analysis, or by performing other such immunodetection methods, all of which are techniques well known in the art.
The ligand-receptor binding interaction may also be measured with the BIACORE™ instrument (Pharmacia Biosensor) which exploits plasmon resonance detection (Zhou et al., Biochemistry, 32, pp. 8193-98 (1993); Faegerstram and O'Shannessy, “Surface plasmon resonance detection in affinity technologies”, in Handbook of Affinity Chromatography, pp. 229-52, Marcel Dekker, Inc., New York (1993)).
With any of these or other techniques for measuring receptor-ligand interactions, one can evaluate the ability of a LTβR blocking agent, alone or in combination with other agents, to inhibit binding of surface or soluble LT ligands to surface or soluble LTβR molecules. Such assays may also be used to test LTβR blocking agents or derivatives of such agents (e.g. fusions, chimeras, mutants, and chemically altered forms), alone or in combination, to optimize the ability of that altered agent to block LTβR activation.
The invention will be further illustrated in the following non-limiting examples.
The materials and methods used in the examples below included the following.
U937, HEK293 and MCF7 cells were obtained from American Type Culture Collection (ATCC) and cultured in RPMI 1640 (Gibco BRL), DME (Gibco BRL) and EMEM (ATCC) with 0.01% insulin, respectively. All media were supplemented with 10% FBS. TRAF2 and TRAF3 antibodies were purchased from Santa Cruz Biotechnology. cIAP1 antibody was obtained from R & D Systems. Anti-Flag (M2) antibody and affinity beads were obtained from Sigma. HA antibody (3F10) was purchased from Roche Molecular Chemicals. Smac antibodies were purchased from Alexis Biochemicals and Cell Signaling Technology. All chemical reagents otherwise specified were purchased from Sigma.
N-terminal FLAG-tagged full length LT□R in pFLAG-CMV2 vector (Eastman Kodak, Co.) was a kind gift from Dr. Shie-Liang Hsieh (see also Wu et al., J. Biol. Chem. 274:11868-73, 199). The full-length and Δ76 deletion mutant of Smac with a C-terminal HA-tag were amplified by PCR reaction from a human ovary cDNA library. The PCR fragments were then cloned into pcDNA3.1 (+) at NdeI and XhoI sites.
1×1010 U937 cells were washed twice with warm PBS (37° C.) and resuspended at a concentration of 1×107 cells/ml. Cells were either treated or left untreated with 20 ng/ml of Flag-LIGHT (Alexis) for 10 min at 37° C. Cells were then lysed in 50 ml of lysis buffer (20 mM Tris.HCl, pH 7.2, 150 mM NaCl, 1% Triton X-100, 1 mM EDTA, 30 mM NaF, 1 mM NaVO4 and protease inhibitor cocktails (Roche)), and gently rocked at 4° C. for 30 min. Cell debris was removed by centrifugation twice at 10,000 g for 30 min. Lysate was preclarified by incubation with Gamma Binding beads (Pharmacia) for 1 hour. The resulting lysate was applied twice to a mini-column (BioRad) of 0.2 ml M2-affinity beads (Sigma). The beads were washed twice with high salt (1M NaCl) lysis buffer, three times more with lysis buffer and then transferred into an eppendorf tube. The immuno-complex was first eluted with Flag peptide (Sigma) at a concentration of 2 mg/ml. The residual binding proteins were then further eluted with 8M Urea. One half of the peptide-eluted proteins or 1/10 of the 8M Urea-eluted proteins were separated on the 4-12% SDS-PAGE gel and transferred to nitrocellulose membrane for Western blotting using TRAF3 antibody. These samples were also separated on the 4-12% SDS-PAGE gel and visualized by silver staining.
Protein bands of interest were manually excised from the gel, reduced and alkylated with iodoacetamide, and then digested in situ with trypsin using an automated digestion robot (ABIMED, Germany) as described by Houthaeve et al., J. Protein Chemistry 16:343-48, 1997. The peptide digests were then sequenced using a high-throughput tandem mass spectrometer (ThermoQuest LCQ-DECA, San Jose Calif.) equipped with a micro-electrospray reversed phase liquid chromatography interface. Data were acquired in automated MS/MS mode using the data acquisition software provided with the LCQ to detect and sequence each peptide as it eluted from the column. The dynamic exclusion and isotope exclusion functions were employed to increase the number of peptide ions that were analyzed. During the LS-MS/MS run, typically >1000 fragmentation spectra were collected from each sample and matched against the nonredundant databases (NCBI) using the Sequest software package (ThermoQuest).
For immunoprecipitation, 1×108 U937 cells were treated with FLAG-LIGHT at 20 ng/ml for different time or left untreated. Cells were then harvested and lysed in 4 ml of lysis buffer (see above). Cell debris was removed by centrifugation at 14,000×g for 10 min and resulting lysate was pre-cleared with Gamma Binding beads (Pharmacia Biotech) for 1 hour at 4° C. Then 20 □l of M2 beads were added to cell lysate and incubated at 4° C. for 3 hours. After binding, beads were washed five times with lysis buffer. Immune complexes bound to the beads were eluted with sample buffer, resolve on 4-12% SDS-PAGE gels, transferred to PVDF membrane and probed with TRAF2, TRAF3 or cIAP1 antibody. Signals were detected with HRP-conjugated secondary antibody and ECL detection kits (Amersham Pharmacia Biotech). For immunoprecipitation in HEK 293 cells, 4 μg of pFlag-CMV2-LTβR, pcDNA3-Smac-HA or pcDNA3-Δ76Smac-HA were transfected into cells on a 100 mm dish using Fugene 6 (Roche Molecular Biochemicals) according to the manufacture's instruction. Forty-eight hours after transfection, cells were collected with cell lifters and lysed in 0.5 ml of lysis buffer. Immunoprecipitation was performed in the same fashion as in U937 cells, with either M2 beads for LT□R or with HA monoclonal antibody for Smac or Δ76 Smac. The presence of Smac, LTβR, TRAF2 and cIAP1 in the immune complex were then analyzed by Western blots.
MCF7 cells (5×105 cells /well) were seeded on cover slides in 6-well plates one day before transfection. Cells in each well were transfected with 1 μg of pcDNA3 vector, pcDNA3-Smac-HA or pcDNA3-Δ76Smac-HA expression constructs together with pEMC-βGal using Fugene 6 (Roche Molecular Chemicals). Twenty-four hours after transfection, cells were treated with PBS (control), LIGHT (20 ng/ml) or LTα1β2 (20 ng/ml) for 6 hours, then fixed and stained with X-gal (Sigma). Apoptosis was assessed by morphological analysis and expressed as a percentage of apoptotic (round and detached) cells in the total of transfected blue cells.
LIGHT binds to both LTβR and TR2/HVEM. In U937 cells, LTβR is constitutively expressed while the expression of TR2/HVEM is induced by differentiating agents. This phenomenon was exploited to form a specific ligand-LTβR complex. Undifferentiated U937 cells were treated with Flag-tagged LIGHT for 10 minutes to form a LIGHT-LTβR complex. Endogenous receptor complex was affinity-purified with anti-Flag antibody (M2)-conjugated beads and then eluted with Flag peptide. The eluted proteins were resolved on 4-12% SDS-PAGE gels and visualized by silver staining.
Approximately eight protein bands were found to be present only in the LIGHT-treated sample but not in the control. TRAF3 was detected in the LIGHT-treated sample using a polyclonal antibody against TRAF3.
In order to identify the additional proteins in this complex, eight bands, which were assigned numbers 1-8, were excised from the gel and analyzed by liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). As expected, LIGHT was detected in band 6 (see also Table 1). Several peptides derived from the polypeptideTRAF2 were detected in band 3. Proteins in other bands either could not be determined due to the poor quality of the spectrum or later turned out to be non-specific binding proteins, such as Hsp 90 at band 1 and actin at band 4.
Peptides corresponding to TRAF3 at expected position of band 3 could not be detected, nor could peptides corresponding to receptor polypeptides. One possibility was that the amount of TRAF3 and receptors, if any, was below the detection limit of mass spectrometry. As indicated by the Western blot of TRAF3, Flag peptides only eluted one tenth of the total TRAF3 protein on the beads. This low efficiency was probably due to the multimeric and high affinity interactions between antibody and ligand-receptor complex.
To increase the recovery of proteins from the beads, the beads were treated with 8M Urea. Samples were then resolved on SDS-PAGE gels and bands at expected position of TRAF3 were excised and analyzed by mass spectrometry. Three peptides from TRAF3 (Table 1) were detected in the LIGHT-treated sample. These peptides were absent in the sample without LIGHT treatment.
All the bands in both LIGHT-treated and control samples were then treated with 8M urea, even though 8M Urea also eluted proteins non-specifically bound to the beads, resulting in an indistinguishable pattern between LIGHT-treated and control samples on SDS-PAGE gels. After protein identification, non-specific binding proteins in the control sample were subtracted from those in the LIGHT-treated sample to yield proteins that specifically bind to the LIGHT-receptor complex. Table 1 summarizes the peptides detected and their assigned proteins. A total of five sproteins were identified, including the four listed above and ERH.
As expected, LTβR was detected as the receptor in the complex (Table 1). The detection of LIGHT and LTβR in the complex further confirmed the successful isolation of LIGHT-LTβR complex. TRAF2 and TRAF3 were also detected (Table 1). In addition, four peptides derived from cIAP1 at the position of band 2 and two peptides derived from Smac at the position of band 6 were detected (Table 1).
No TRAF4, TRAF5 or NIK polypeptides were detected by either mass spectrometry or Western blot analysis even though their roles have been suggested in the LTβR signaling. This could be due to the low abundance of the proteins and the high complexity of the samples.
To confirm the association of these proteins with the LIGHT-LTβR complex, U937 cells were treated with Flag-LIGHT for various amounts of time. Cells were then immunoprecipitated with Flag antibody, followed by Western blot analysis using antibody against TRAF2, TRAF3 or cIAP1.
The recruitment of endogenous TRAF2, TRAF3 and cIAP1 to LIGHT-LTβR complex was found to be time-dependent. Recruitment of cIAP1 was gradually increased within 15 minutes. A similar pattern was observed for TRAF3. Recruitment of TRAF2 appeared to be more rapid and appeared to peak between 5 and 10 minutes. The kinetics of recruitment suggests that TRAF2 is recruited to the receptor prior to TRAF3 and cIAP1. The direct interaction of TRAF3 with the intracellular domain of LTβR has been demonstrated using purified proteins (Force et al., J. Biol. Chem. 272:30835-40, 1997), and there is no evidence of interaction between TRAF2 and TRAF3 (see, e.g., Wajant et al., Cytokine Growth Factor 10:15-26, 1999). Thus, TRAF3 is likely directly recruited to LTβR upon LIGHT treatment. In contrast, the recruitment of cIAP1 to LTβR probably occurs via its interaction with TRAF2 because cIAP1-TRAF2 interaction has been shown in vitro and there is no evidence of interaction between cIAP1 and receptor or TRAF3.
Interestingly, the cIAP1 in the complex recognized by a polyclonal antibody raised against the C-terminus of cIAP1 (R&D systems, AF818) is about 60 kDa, which is smaller than the full-length cIAP1 (about 70 kDa) in cell lysate. This 60 kDa band was not detected by another antibody raised against a peptide at the BIR1 domain of the cIAP1 (sc-1867, Santa Cruz, which recognizes 70 kDa-cIAP1 in cell lysate). These observations indicate that the N-terminus of cIAP1 in the complex is cleaved.
Although two peptides from Smac in the LIGHT-LTβR complex were detected by mass spectrometry, an endogenous association was not detected using Smac antibodies (Alexis Biochemicals or Cell Signaling Technology). This result could be due to the low sensitivity of Smac antibodies. Therefore, the association of Smac with LTβR was investigated by overexpression in HEK293 cells that do not have endogenous LTβR and HVEM (Zhai et al., Clin. Invest. 102:1142-51, 1998). Smac was expressed as a C-terminal HA-tagged fusion protein and appeared as a doublet of 28 kDa and 23 kDa on the Western blot. These sizes corresponded to full length Smac with the N-terminal mitochondrial localization signal peptide (amino acid 1-55) and the mature Smac without its signal peptide (Du et al., Cell 102:33-42, 2000). Both forms were co-immunoprecipitated with Flag-tagged LTβR. When cytosol and mitochondria were fractioned, all the full length Smac was found to reside in the mitochondria fraction, and a significant amount (about one third) of the mature Smac was in the cytosolic fraction. Therefore, these data suggest that the cytosolic mature form of Smac is the physiological form of Smac that interacts with LTβR. The observation that the full-length Smac was co-immunoprecipitated with LTβR may be artificial, due to the disruption of the mitochondrial membrane by Triton X-100.
There was no further increase of Smac recruitment when stimulated with LIGHT (data not shown). This is likely due to the aggregation and activation of LTβR resulting from overexpression. In the reciprocal immunoprecipitation of Smac using HA antibody, LTβR was detected, which further confirms the association. In accordance with the observation in U937 cells (discussed above), endogenous TRAF2, cIAP1 (60 kDa) and TRAF3 were also found to be recruited to LTβR overexpressed in HEK 293 cells. Furthermore, endogenous TRAF2 and cIAP1 were detected in the reciprocal immunoprecipitation of Smac, indicating the formation of a complex of LTβR-TRAF2-cIAP1-Smac. Taken together, these data strongly support the physiological association of TRAF2, TRAF3, cIAP1, and Smac with LTβR.
In contrast to the full-length Smac, the deletion mutant of Smac (Δ76Smac) that lacks both the cIAP1 binding site (amino acid 56-75) and mitochondrial localization signal lost the ability to bind to LTβR. This suggests that the cIAP1 binding site of Smac is important for its recruitment to the receptor. The interaction between the N-terminus of Smac and the BIR3 domain of XIAP has been demonstrated by the X-ray crystal structure and mutational analysis (Chai et al., Nature 406:855-862, 2000; Wu et al., Nature 408:1008-12, 2000). Because known IAP polypeptides are highly homologous it is likely that the recruitment of Smac is mediated by its interaction with the BIR3 domain of cIAP1. Despite the difference between the full-length and the deletion mutant of Smac, the level of cIAP1, TRAF2, and TRAF3 on LTβR remained the same, suggesting that the recruitment of Smac occurs after the recruitment of TRAF2, TRAF3 and cIAP1.
Smac has been shown to promote apoptosis in response to several stimuli, such as UV irradiation, that trigger the mitochondria-mediated apoptosis pathway (Du et al., Cell 102:33-42, 2000). MCF7 cells were cotransfected with plasmids expressing β-galactosidase (pEMC-βgal) and Smac-HA, Δ76 Smac-HA or empty vector. After 24 hours, cells were treated with PBS, LIGHT (20 ng/ml), or LTα1β2 (20 ng/ml), respectively, for 6 hours, then fixed, and stained with X-gal. Apoptosis was assessed by the morphological analysis of the β gal-expressing cells. (see Example 1 for details).
The results are shown in
Overexpression of full-length Smac potentiated apoptosis in MCF7 cells, and stimulation of LIGHT further increased apoptosis. A similar effect was observed in the LTα1β2-stimulated cells. Interestingly, mutant Δ76 Smac, which lost the ability to recruit to the receptor, could still potentiate LTβR-mediated apoptosis to a degree similar to the full-length Smac. This result suggests that C-terminus of Smac also possesses proapoptotic activity and is independent of its interaction with cIAP1.
While not wishing to be bound by theory, a model for LTβR-induced apoptosis is diagrammed in
The descriptions given are intended to exemplify, but not limit, the scope of the invention.
This application is a continuation of U.S. Ser. No 10/361,270, filed Feb. 10, 2003, which claims priority to U.S. Ser. No. 60/355,183, filed Feb. 8, 2002, each of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60355183 | Feb 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10361270 | Feb 2003 | US |
| Child | 11825385 | US |
