MK2 Phosphorylates RIPK1 to Prevent TNF-Induced Cell Death

Abstract

TNF is an inflammatory cytokine that upon binding to its receptor, TNFR1, can drive cytokine production, cell survival, or cell death. TNFR1 stimulation causes activation of NF-κB, p38α, and its downstream effector kinase MK2, thereby promoting transcription, mRNA stabilization, and translation of target genes. Here we show that TNF-induced activation of MK2 results in global RIPK1 phosphorylation. MK2 directly phosphorylates RIPK1 at residue S321, which inhibits its ability to bind FADD/caspase-8 and induce RIPK1-kinase-dependent apoptosis and necroptosis. Consistently, a phospho-mimetic S321D RIPK1 mutation limits TNF-induced death. Mechanistically, we find that phosphorylation of S321 inhibits RIPK1 kinase activation. We further show that cytosolic RIPK1 contributes to complex-II-mediated cell death, independent of its recruitment to complex-I, suggesting that complex-II originates from both RIPK1 in complex-I and cytosolic RIPK1. Thus, MK2-mediated phosphorylation of RIPK1 serves as a checkpoint within the TNF signaling pathway that integrates cell survival and cytokine production.

Keywords: IAPs; MK2; RIPK1; TNF; caspase-8; cell death; complex-II; cytokine; necroptosis; p38.

Graphical abstract

Figure 1

MK2 Protects from TNF-Induced Cell Death (A) Quantification of PI-positive primary BMDMs treated with the indicated reagents for 3 hr. An early time point was chosen to avoid complications due to autocrine production of TNF. Cells were pre-treated with MK2i (1 μM) for 30 min. (B) DEVDase activity analysis of primary MEFs treated with the indicated reagents for 4 hr. Cells were pre-treated with DMSO, MK2i (1 μM), or RIPK1i (100 nM) for 30 min. (C) Quantification of PI-positive primary MEFs treated with the indicated reagents for 7 hr. Cells were pre-treated with MK2i or RIPK1i for 30 min. (D–H) The indicated cells were treated with the respective reagents, and PI-positive (D–F and H) cells or DEVDase activity (G) was quantified. Cells were pre-treated with MK2i and/or RIPK1i for 30 min. Graphs show mean ± SEM, n = 3–5 independent repeats. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001. See also Figure S1.

Figure 2

MK2 Directly Phosphorylates RIPK1 at S320/S321 in Response to TNF Stimulation (A) Schematic depicting the evolutionarily conserved MK2 phosphorylation consensus sequence of RIPK1. Color scheme emphasizes sequence conservation within the motif. (B) Western blot analysis of cell lysates separated on a 4%–12% gradient gel from primary MEFs using the indicated antibodies. Cells were pre-treated with DMSO or MK2i (1 μM, 30 min), followed by treatment with TNF (10 ng/mL) for the indicated time points. (C) Western blot analysis of cell lysates separated on a Tris-glycine 8% gel from primary WT or Mk2^−/− MEFs using the indicated antibodies. Cells were pre-treated with DMSO or p38i (1 μM, 30 min), followed by a 10 min treatment with TNF (10 ng/mL). (D) Western blot analysis of cell lysates from WT or Mk2^−/− BMDMs using the indicated antibodies. Cells were treated ± TNF (10 ng/mL) for the indicated times. (E) Western blot analysis of protein lysates from MDA-MB-468 cells using the indicated antibodies. Cells were treated ± TNF (10 ng/mL) for the indicated times. (F) Western blot analysis of cell lysates separated on Tris-glycine 8% gel BMDMs using the indicated antibodies. Cells were stimulated with the indicated reagents. (G) In vitro kinase assay using purified proteins. Recombinant active human MK2 was incubated with mouse and human RIPK1 in the presence of DMSO or MK2i and the reactions separated on a Tris-glycine 8% acrylamide gel. The presence of phosphorylated S321/320 RIPK1 and MK2 was evaluated using the indicated antibodies. See also Figure S2.

Figure 3

Phosphorylation of RIPK1 at S320/321 Is Dependent on the TAK1 > p38α > MK2 Signaling Cascade but Independent of IKK (A) Western blot analysis of cell lysates of the indicated cells using the described antibodies. Cells were left untreated or pre-treated for 30 min with the indicated inhibitors, followed by TNF treatment (10 ng/mL, 10 min) (SM CompA, 500 nM; RIPK1i GSK’963, 100 nM; TAK1i (5Z)-7-O, 1 μM; IKKi TPCA-1, 5 μM; p38i, 1 μM; MK2i PF3644022, 1 μM). (B) Western blot analysis of cell lysates from BMDMs subjected to pre-treatment for 30 min with the indicated inhibitors (SM CompA, 500 nM; RIPK1i/Nec1s, 1 μM; TAK1i (5Z)-7-O, 250 nM; IKKi TPCA-1, 250 nM; p38i/LY2228820, 250 nM; MK2i/PF3644022, 2 μM), followed by treatment with TNF (100 ng/mL) for 10 min. (C) Western blot analysis of cell lysates from BMDMs subjected to pre-treatment for 30 min with the indicated inhibitors, followed by TNF treatment (10 ng/mL, 10 min), as in (A). (D) Western blot analysis of cell lysates from immortalized WT and Nemo^−/− MEFs treated with TNF for the indicated time points. (E) Western blot analysis of cell lysates from Flp-In T-REx 293 cells in which the respective genes were knocked out using CRISPR/Cas9. Cells were treated with human TNF (10 ng/mL) for 10 min. See also Figure S3.

Figure 4

MK2-Dependent Phosphorylation of RIPK1 Does Not Affect NF-κB Signaling but Suppresses RIPK1 Activation (A) Western blot analysis of cell lysates from primary MEFs using the indicated antibodies. Cells were treated with TNF (10 ng/mL) for the indicated time points. (B) Western blot analysis of cell lysates from WT or Mk2^−/− BMDMs using the indicated antibodies. Cells were treated with TNF (100 ng/mL) for the indicated time points. (C) TNF-induced complex-I immunoprecipitation. Primary MEFs were treated with FLAG-mTNF (1 μg/mL) for the indicated time points, followed by FLAG immunoprecipitation and western blot analysis. Lysates pre- (right) and post-immunoprecipitation (bottom) were also analyzed by western blot. (D) Immunoprecipitation of RIPK1 from WT and Mk2^−/− immortalized MEFs, treated ± TNF (10 ng/mL). A Tris-glycine 8% acrylamide gel was used to visualize the RIPK1 phospho-dependent mobility shift. Quantification of the intensity of the P-S166 signal, normalized to total RIPK1, is shown to the right. (E) Immunoprecipitation of RIPK1 from MEFs treated with TNF (10 ng/mL) ± MK2i (1 μM). The presence of the indicated proteins was evaluated by western blot. (F) WT and Ripk1^−/− MEFs stably expressing murine RIPK1-ΔDD were stimulated with FLAG-mTNF (1 μg/mL) for the indicated time points. Western blot analysis with the indicated antibodies is shown. See also Figure S4.

Figure 5

MK2 Limits Complex-II Formation (A) TNF-induced complex-II immunoprecipitation using anti-FADD. Western blot analysis of complex-II from WT and Mk2^−/− BMDMs using the indicated antibodies. Cells were treated with TNF (100 ng/mL) and SM (500 nM) for 1 hr and z-VAD-FMK (20 μM) to stabilize complex-II. (B) TNF-induced complex-II was immunoprecipitated with anti-FADD from BMDM lysates. Cells were treated with TSZ (as in A) for the indicated times ± Mk2i (2 μM). (C) TUBE affinity purification of lysates from primary MEFs. Cells were pretreated with DMSO or MK2i (1 μM) for 30 min, and treated ± TNF for the indicated times. The TUBE affinity-purified ubiquitylated proteome was subsequently left untreated or exposed to USP21. Western blot analysis for the indicated proteins is shown. The graph to the right depicts the quantification of non-modified RIPK1 in the USP21-treated samples. (D) Quantification of PI-positive primary WT and Ripk1^−/− MEFs reconstituted with RIPK1-ΔDD. Cells were pretreated with DMSO or MK2i (1 μM) for 30 min, followed by TS treatment for 3 hr. Graphs show mean ± SEM, n = 3 independent repeats. ∗p < 0.05, ∗∗p < 0.01, and ∗∗∗p < 0.001. (E) TNF-induced complex-II was immunoprecipitated with anti-FADD from lysates of WT MEFs reconstituted with RIPK1-ΔDD (as in D). Cells were treated with TSZ (as in A) for 3 hr. See also Figure S5.

Figure 6

MK2-Dependent Phosphorylation of RIPK1 at S321 Protects Cells from TNF-Induced Cell Death (A) Quantification of PI-positive WT and Ripk1^S321D BMDMs treated with the indicated reagents for 5 hr. (B) DEVDase activity analysis of BMDMs treated with the indicated reagents for 1 hr. (C) Quantification of PI-positive primary WT and Ripk1^S321D MEFs treated with the indicated reagents for 6 hr. (D) PLA of primary WT and Ripk1^S321D MEFs using RIPK1 and caspase-8 antibodies. Cells were stimulated with the indicated reagents for 3 hr. The panel below shows quantifications of RIPK1/caspase-8 PLA speckles. Scale bar, 10 μm. Graphs show mean ± SEM, n = 3–8 independent repeats. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001. See also Figure S6.