Bs, Signaling by the B-Cell and T-Cell Receptors, Tumor Necrosis Factor Receptor 1, and Toll-Like Receptors 2 and 4
and Vishva M. Dixit*
Molecular Oncology Department, Genentech, Inc., South San Francisco, California 94080
Received 29 August 2003/ Accepted 18 November 2003
| ABSTRACT |
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B signaling, but whether RIP3 promotes or attenuates activation of the NF-
B family of transcription factors has been controversial. We have generated RIP3-deficient mice by gene targeting and find RIP3 to be dispensable for normal mouse development. RIP3-deficient cells showed normal sensitivity to a variety of apoptotic stimuli and were indistinguishable from wild-type cells in their ability to activate NF-
B signaling in response to the following: human tumor necrosis factor (TNF), which selectively engages mouse TNF receptor 1; cross-linking of the B- or T-cell antigen receptors; peptidoglycan, which activates Toll-like receptor 2; and lipopolysaccharide (LPS), which stimulates Toll-like receptor 4. Consistent with these observations, RIP3-deficient mice exhibited normal antibody production after immunization with a T-dependent antigen and normal interleukin-1ß (IL-1ß), IL-6, and TNF production after LPS treatment. Thus, we can exclude RIP3 as an essential modulator of NF-
B signaling downstream of several receptor systems. | INTRODUCTION |
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B kinase (IKK) complex and is antiapoptotic (1, 6). IKK-mediated phosphorylation of the I
Bs promotes their ubiquitination and degradation, which allow the NF-
B transcription factors that I
Bs sequester in the cytosol to move into the nucleus (3, 4). Consistent with a role for RIP in activation of NF-
B, RIP is a potent activator of NF-
B-dependent reporter genes in transient-overexpression studies. RIP3, by comparison, is a poor activator of such reporter genes and will even inhibit RIP- or TNF-induced NF-
B activation (5, 11, 17, 20). Nevertheless, RIP3 was isolated recently from a mouse thymus expression library in a screen for proteins that could activate an NF-
B-dependent reporter gene (13).
Mutagenesis experiments have indicated that interactions between RIP and RIP3 are mediated by a RIP homotypic interaction motif located toward the C terminus of RIP3 and between the kinase and death domains of RIP (18). Disruption of either RIP homotypic interaction motif prevented RIP3 from phosphorylating RIP. It also negated the inhibitory effect of RIP3 on RIP- or TNF-induced NF-
B activation. Phosphorylation of RIP by RIP3 was crucial to the inhibitory action of RIP3 because a kinase-dead form of RIP3 failed to block RIP- or TNF-induced NF-
B activation (18). These results suggested that RIP3, similar to the protein A20 (8), might play a role in limiting the NF-
B-dependent gene transcription that occurs in response to TNF engaging TNF receptor 1 (TNFR-1). To determine the importance of RIP3 to signaling in the whole animal, we have generated RIP3-deficient mice by gene targeting. Mice lacking RIP3 were healthy and fertile and showed no signs of deregulated NF-
B signaling downstream of TNFR-1 or Toll-like receptors 2 and 4 (TLR-2 and -4).
| MATERIALS AND METHODS |
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LPS treatment and cytokine measurements. Mice from line 13 A12 were backcrossed to a C57BL/6N genetic background for 4 generations, and females aged 9 weeks were given 30 µg of lipopolysaccharides (LPS) from Escherichia coli 055:B5 (Sigma) by intraperitoneal injection. Serum was collected after 2 h. Absolute levels of interleukin 1ß (IL-1ß), IL-6, and TNF were determined by using mouse Quantikine ELISA kits (R&D Systems).
Immunofluorescence staining, cell sorting, and flow cytometry.
Single-cell suspensions prepared from thymus, spleen, bone marrow, and lymph nodes were surface stained with monoclonal antibodies conjugated to fluorescein isothiocyanate (FITC) and R-phycoerythrin (PE). Viable cells not stained by propidium iodide were analyzed in a FACScan flow cytometer (Becton Dickinson). The monoclonal antibodies used were H129.19 anti-CD4, 53-6.7 anti-CD8, 145-2C11 anti-CD3
, M1/70 anti-Mac-1, RA3-6B2 anti-B220, R6-60.2 anti-IgM, 11-26c.2a anti-IgD, RB6-8C5 anti-Gr-1, DX5 anti-CD49b, and TER-119 anti-erythroid cells (BD Biosciences). For proliferation assays, B and T cells were negatively sorted from spleen and lymph nodes, respectively. Viable, small cells not stained with a cocktail of antibodies specific for macrophages, granulocytes, erythroid cells (anti-Mac-1, TER-119, and Gr-1), and either B (anti-B220, IgM, and IgD) or T cells (anti-CD3, CD4, and CD8) were sorted in a MoFlo (Cytomation). Apoptosis of thymocytes treated with 2 ng of phorbol myristate acetate (Sigma)/ml, 1 µg of ionomycin (Sigma)/ml, 1 µM dexamethasone (Sigma), 10 ng of recombinant human TNF (Genentech) ± 20 µg of cycloheximide per ml, or 100 ng of Flag-tagged human FasL (Alexis)/ml cross-linked with 1 µg of M2 anti-FLAG antibodies (Sigma)/ml was assayed by staining cells with propidium iodide and FITC-conjugated annexin V (BD Biosciences).
Proliferation assays.
Purified B and T lymphocytes (typically > 95% pure as judged by flow cytometric analysis of cells stained with anti-B220 or anti-CD4 and CD8 antibodies) were cultured in the high-glucose version of Dulbecco's modified Eagle's medium supplemented with 250 µM L-asparagine, 50 µM 2-mercaptoethanol, and 10% fetal calf serum. T cells (105 cells/ml, starting concentration) were stimulated with plate-bound 145-2C11 anti-CD3
and 37.51 anti-CD28 antibodies (10 µg of each antibody/ml in the coating solution; BD Biosciences). B cells (3 x 105 cells/ml, starting concentration) were stimulated with 20 µg of F(ab')2 goat anti-mouse IgM (Jackson ImmunoResearch)/ml, or 20 µg of LPS from Salmonella enterica serovar Abortus Equi (Sigma)/ml. One-hundred-microliter cultures were pulsed for 6 h with 0.5 µCi of [methyl-3H]thymidine (Amersham).
Electrophoretic mobility shift assays.
Nuclear extracts were prepared from lymph node cells essentially as described earlier (7). Two micrograms of protein extract and 1 µg of poly(dI-dC) (Amersham) were incubated at 4°C for 10 min in 10 mM Tris-HCl, pH 7.5, 100 mM NaCl, 1 mM EDTA, 5% glycerol, 0.1% NP-40, and 1 mM dithiothreitol. A 32P-labeled probe containing the
B3 site from the murine rel promoter was added, and complexes were allowed to form at room temperature for 30 min. The products were resolved in 6% polyacrylamide DNA retardation gels (Invitrogen).
Embryo fibroblasts and macrophages. Fibroblasts derived from embryos at 13.5 to 14.5 days postcoitum by trypsinization were cultured for 2 to 4 passages on gelatin-coated plates in the high-glucose version of Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum. Macrophages were derived from bone marrow cells cultured for 4 to 6 days in the high-glucose version of Dulbecco's modified Eagle's medium supplemented with 250 µM L-asparagine, 50 µM 2-mercaptoethanol, 10% fetal calf serum, and 25 ng of macrophage colony-stimulating factor (R&D Systems)/ml. Nonadherent cells were washed away, and the adherent macrophages were stimulated with 20 ng of recombinant human TNF/ml, 10 µg of peptidoglycan from Staphylococcus aureus (Sigma)/ml, or 1 µg of LPS from S. enterica serovar Abortus Equi (Sigma)/ml.
Western blotting.
Cell lysates were prepared in 20 mM Tris-HCl, pH 7.5, 135 mM NaCl, 1.5 mM MgCl2, 2 mM EGTA, 1% Triton X-100, 10% glycerol, 1 mM Na3VO4, 10 mM NaF, 1 mM sodium pyrophosphate, and 0.1 mM ß-glycerophosphate supplemented with a complete protease inhibitor cocktail (Roche). Proteins were resolved in 4 to 20% gradient Tris-glycine polyacrylamide gels (Invitrogen) and were transferred to nitrocellulose membranes (Invitrogen). Blots were probed with the following antibodies: rabbit anti-mouse RIP3 (MoBiTec), AC-15 anti-ß-actin (Abcam), rabbit anti-I
B
(Cell Signaling), and rabbit anti-phospho-I
B
(Ser32) (Cell Signaling).
| RESULTS AND DISCUSSION |
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RIP3 is dispensable for macrophage, lymphocyte, and natural killer cell development. Both myeloid and lymphoid cell lineages express RIP3 (Fig. 1C and data not shown), so we examined the development of these cell populations in rip3-/- mice. Thymus, spleen, lymph node, and bone marrow cellularities were comparable between rip3-/- mice and their wild-type littermates (data not shown). Flow cytometric analysis of cells stained with antibodies to CD4 and CD8 revealed no difference in the frequency of CD4+8+, CD4+8-, and CD4-8+ cells in thymus, spleen, lymph nodes, and peripheral blood, suggesting that T-cell development can occur normally in the absence of RIP3 (Fig. 2A and B and data not shown). The proportion of cells expressing B220 and IgM in the bone marrow, spleen, lymph nodes, and peripheral blood also was not affected by loss of RIP3, suggesting that RIP3 is not essential for B-cell development (Fig. 2C and D and data not shown). In addition, rip3-/- mice had normal numbers of CD49b+ natural killer cells, TER119+ erythroid cells, and Mac-1+ Gr-1+ macrophages (Fig. 2C and D and data not shown).
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B transcription factors Rel, RelA (also called p65), and NF-
B1 (also called p50) exhibit defects in lymphocyte development, proliferation, and antibody production (2, 7, 12, 14, 21), our findings suggest that RIP3 is not required to transduce signals for NF-
B activation downstream of the B- and T-cell receptors. Consistent with this notion, rip3+/+ and rip3-/- lymph node T cells stimulated with cross-linking antibodies to CD3 and CD28 exhibited equivalent NF-
B DNA binding activity in electrophoretic mobility shift assays (Fig. 3C). Both rip3+/+ and rip3-/- cells contained two nuclear NF-
B complexes (C1 and C2) able to bind to the
B3 site from the murine rel promoter, and as in previous studies (7, 10), the slower-migrating C1 complex was increased following mitogenic stimulation.
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B signaling downstream of TNFR-1. In contrast to rip3-/- mice, A20-deficient mice die soon after birth from inflammatory disease, and their cells, including thymocytes, are hypersensitive to TNF-induced apoptosis (8).
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B signaling by TNFR-1, TLR-2, and TLR-4 in rip3-/- cells.
To examine the NF-
B signaling pathway downstream of TNFR-1 in rip3-/- cells directly, we determined whether I
B
is phosphorylated and then degraded in rip3-/- embryo fibroblasts and bone marrow-derived macrophages treated with human TNF. Western blot analysis with antibodies to I
B
revealed transient phosphorylation of I
B
after 5 min of TNF treatment, and this was followed by a reduction in the overall level of I
B
both in rip3+/+ and rip3-/- cells (Fig. 4C and D). I
B
is itself induced by NF-
B (16), and accordingly, I
B
in rip3+/+ and rip3-/- cells had returned to basal levels after 60 min of stimulation with TNF (Fig. 4C). In the case of A20-null cells, I
B
protein is induced but persistent IKK activity appears to prevent its accumulation (8). Thus, consistent with the healthy phenotype of rip3-/- mice, RIP3 is dispensable for TNF-induced activation of NF-
B and for later switching off of NF-
B-dependent gene transcription. Absence of a phenotype in rip3-/- cells when RIP3 inhibited TNF- and RIP-induced NF-
B transcriptional activity in overexpression studies (18) might indicate that RIP3 and another protein(s) have overlapping, redundant roles in negatively regulating TNF-induced NF-
B signaling or that the previously observed inhibitory activity of RIP3 might simply be an artifact of its ectopic expression.
We also looked at the response of rip3-/- macrophages to the gram-negative bacterial cell wall component peptidoglycan and the gram-positive bacterial cell wall component LPS, which activate NF-
B via TLR-2 and TLR-4, respectively (19). Phosphorylation and subsequent degradation of I
B
in response to these stimuli occurred to a similar extent and with similar kinetics in rip3+/+ and rip3-/- cells (Fig. 4D). Thus, macrophages, in addition to B lymphocytes (Fig. 3A), can respond to LPS without RIP3. Consistent with these findings, rip3-/- mice injected with a sublethal dose of LPS produced amounts of IL-1ß, IL-6, and TNF comparable to those produced by their wild-type littermates (Fig. 5). Therefore, despite the apparent ability of RIP3 to interact with RIP (17, 18, 20), we find no evidence of a nonredundant role for RIP3 in the regulation of NF-
B signaling by TNFR-1 or other receptors such as the B- and T-cell antigen receptors, TLR-2, and TLR-4.
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Present address: Shanghai Genomics, Inc., Shanghai 201203, People's Republic of China. ![]()
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