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. 2017 Dec 19;47(6):1083-1099.e6.
doi: 10.1016/j.immuni.2017.11.016. Epub 2017 Dec 12.

Oncogenic RAS Signaling Promotes Tumor Immunoresistance by Stabilizing PD-L1 mRNA

Matthew A Coelho  1 , Sophie de Carné Trécesson  1 , Sareena Rana  2 , Davide Zecchin  1 , Christopher Moore  1 , Miriam Molina-Arcas  1 , Philip East  3 , Bradley Spencer-Dene  4 , Emma Nye  4 , Karin Barnouin  5 , Ambrosius P Snijders  5 , Wi S Lai  6 , Perry J Blackshear  7 , Julian Downward  8
Affiliations

Oncogenic RAS Signaling Promotes Tumor Immunoresistance by Stabilizing PD-L1 mRNA

Matthew A Coelho et al. Immunity. .

Abstract

The immunosuppressive protein PD-L1 is upregulated in many cancers and contributes to evasion of the host immune system. The relative importance of the tumor microenvironment and cancer cell-intrinsic signaling in the regulation of PD-L1 expression remains unclear. We report that oncogenic RAS signaling can upregulate tumor cell PD-L1 expression through a mechanism involving increases in PD-L1 mRNA stability via modulation of the AU-rich element-binding protein tristetraprolin (TTP). TTP negatively regulates PD-L1 expression through AU-rich elements in the 3' UTR of PD-L1 mRNA. MEK signaling downstream of RAS leads to phosphorylation and inhibition of TTP by the kinase MK2. In human lung and colorectal tumors, RAS pathway activation is associated with elevated PD-L1 expression. In vivo, restoration of TTP expression enhances anti-tumor immunity dependent on degradation of PD-L1 mRNA. We demonstrate that RAS can drive cell-intrinsic PD-L1 expression, thus presenting therapeutic opportunities to reverse the innately immunoresistant phenotype of RAS mutant cancers.

Keywords: KRAS; PD-L1; RAS; TTP; immunotherapy; tristetraprolin.

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Figures

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Graphical abstract
Figure 1
Figure 1
Cell-Intrinsic Upregulation of PD-L1 through Oncogenic RAS Signaling (A) Western blotting analysis of ER-KRASG12V type II pneumocytes treated with 4-OHT in starvation medium. Phospho-ERK and phospho-AKT was measured over time to monitor RAS pathway activation. Data are representative of two independent experiments. (B) qPCR analysis of ER-KRASG12V type II pneumocytes treated with 4-OHT or IFN-γ in starvation medium. Mean ± SEM of biological duplicates (n = 2) from the experiment described in (A). (C) Representative flow cytometry histogram of PD-L1 surface protein expression in ER-KRASG12V type II pneumocytes treated in starvation medium for 48 hr. Data are representative of two independent experiments. (D) Western blotting analysis of RAS signaling following 5 hr treatment with the KRASG12C inhibitor ARS853. Phospho-ERK and phospho-AKT signal reflect RAS pathway activity. Data are representative of two independent experiments. (E) qPCR analysis following 5 hr treatment with the KRASG12C inhibitor ARS853 (10 μM). Mean ± SEM of biological duplicates (n = 2) from the experiment described in (D). (F) Flow cytometry analysis of PD-L1 surface protein expression in H358 cells treated with ARS853 (10 μM) for 48 hr. Mean ± SEM of biological triplicates. (G) Flow cytometry analysis of PD-L1 surface protein expression in ER-KRASG12V type II pneumocytes treated in starvation medium for 24 hr. Mean ± SEM of two independent experiments. (H) qPCR analysis from the experiment described in (G). Mean ± SEM of biological triplicates pooled from two independent experiments. (I) qPCR analysis of H358 cells treated for 24 hr. Mean ± SEM of two independent experiments. (J) qPCR analysis of H358 cells treated with PMA for 3 hr following a 30 min pre-treatment with DMSO or MEK inhibitor. Mean ± SD of two independent experiments. Abbreviations and quantities are as follows: MFI, mean fluorescence intensity; EtOH, ethanol vehicle; 4-OHT, 100 nM; IFN-γ, 20 ng/mL; MEK inhibitor GSK1120212, 25 nM; PI3K inhibitor GDC-0941, 500 nM; PMA, 200 nM. ∗∗∗∗p < 0.0001, ∗∗∗p < 0.001, ∗∗p < 0.01, ∗p < 0.05, n.s., not significant. Unpaired, two-tailed Student’s t tests. See also Figure S1.
Figure 2
Figure 2
RAS Signaling Increases PD-L1 mRNA Stability through AU-Rich Elements in the 3′ UTR (A) qPCR analysis of PD-L1 mRNA stability in ER-KRASG12V type II pneumocytes after the concomitant addition of actinomycin D (5 μg/mL or 10 μg/mL) and 4-OHT or vehicle added at time = 0 hr in starvation medium. Mean ± SEM of two independent experiments. ∗∗∗p < 0.0005; two-way ANOVA. (B) qPCR analysis of PD-L1 mRNA stability in KPB6 cells after the addition of actinomycin D (5 μg/mL) and DMSO or MEK inhibitor. Cells were pre-treated with DMSO or MEK inhibitor for 30 min before actinomycin D addition. Mean ± SEM of two independent experiments. ∗∗∗p < 0.0005; two-way ANOVA. (C) qPCR analysis of PD-L1 mRNA stability after the addition of actinomycin D (5 μg/mL) and DMSO or ARS853. Cells were pre-treated with DMSO or ARS853 for 35 min before actinomycin D addition. Mean ± SEM of two independent experiments. ∗∗∗p < 0.0005; two-way ANOVA. (D) Sequence alignment of conserved AU-rich element ATTTA pentamer sequences (highlighted in red) in the mouse and human CD274 3′ UTR. (E) Normalized luciferase signal in ER-HRASG12V MCF10A cells from wild-type (ATTTA x 6) or mutant (ATGTA x 6) PD-L1 3′ UTR reporters, 24 hr after treatment in starvation medium. Mean ± SEM of three independent experiments. (F) Normalized luciferase signal in H358 cells from wild-type (ATTTA x 6) or mutant (ATGTA x 6) PD-L1 3′ UTR reporters, 6 hr after treatment. Mean ± SEM of three independent experiments. Abbreviations and quantities: 4-OHT, 100 nM; MEK inhibitor GSK1120212, 25 nM; PMA, 200 nM. ∗∗∗p < 0.0005, ∗∗p < 0.005, ∗p < 0.05, n.s., not significant. Unpaired, two-tailed Student’s t tests. See also Figure S2.
Figure 3
Figure 3
AU-Rich Element Binding Proteins TTP and KSRP Are Negative Regulators of PD-L1 Expression (A–C) qPCR analysis 48 hr after transfection with siRNAs targeting AU-rich element binding proteins (AU-BPs) relative to siScrambled (siSc) control. Mean ± SD of biological triplicates. (D) qPCR and western blotting analysis of H358 cells 24 hr after transfection. qPCR data represent the mean ± SD of biological triplicates and are representative of two independent experiments. ∗, non-specific band. (E) Normalized luciferase signal from the wild-type, PD-L1 3′ UTR reporter 24 hr after co-transfection with the indicated constructs. Mean ± SEM of two independent experiments. (F) qPCR analysis after serum stimulation in serum-starved TTP WT or TTP KO MEFs. Mean ± SEM of two independent experiments. (G) qPCR analysis of PD-L1 mRNA stability after the addition of actinomycin D (5 μg/mL) in TTP WT or TTP KO MEFs. Mean ± SEM of two independent experiments. (H) Normalized luciferase signal in KPB6 TTP (tet-ON) cells wild-type (ATTTA x 6) or mutant (ATGTA x 6) PD-L1 3′ UTR reporters, 7 hr after treatment. Data represent the mean ± SEM of biological triplicates and are representative of two independent experiments. Abbreviations and quantities: MEK inhibitor, GSK1120212, 25 nM; Dox., doxycycline 1 μg/mL. ∗∗∗∗p < 0.0001, ∗∗∗p < 0.001, ∗∗p < 0.01. Unpaired, two-tailed Student’s t tests. See also Figure S3.
Figure 4
Figure 4
RAS Regulates PD-L1 Expression through TTP (A) qPCR analysis of H358 cells following siRNA-mediated knock-down of TTP (24 hr) followed by MEK inhibition (24 hr). Mean ± SEM of two independent experiments. (B) qPCR analysis of ER-KRASG12V type II pneumocytes treated for 24 hr in starvation medium. Mean ± SEM of three independent experiments. (C) qPCR analysis of RNA-IP immunoprecipitates from H358 cells. Mean ± SEM from biological triplicates. (D) Western blotting analysis of H358 cells expressing the indicated constructs. 6.5 hr post-transfection, cells were treated with DMSO or MEK inhibitor for an additional 16 hr. Arrow indicates Myc-TTP. Data are representative of two independent experiments. (E) Western blotting analysis of immunoprecipitations from H358 cells transfected with Myc-TTP. 6.5 hr post-transfection, cells were treated with DMSO or MEK inhibitor for an additional 16 hr. Arrow indicates Myc-TTP; ∗ indicates co-precipitating protein. Data are representative of two independent experiments. (F) qPCR analysis of TTP WT or TTP KO MEFs treated with okadaic acid or DMSO for 2 hr. Mean ± SEM of two independent experiments. Abbreviations and quantities: EtOH, ethanol vehicle; 4-OHT, 100 nM; okadaic acid, OA, 1 μM; MEK inhibitor, GSK1120212, 25 nM. ∗∗∗∗p < 0.0001, ∗∗∗p < 0.001, ∗∗p < 0.01. Unpaired, two-tailed Student’s t tests. See also Figure S4.
Figure 5
Figure 5
RAS-ROS-p38 Signaling Controls TTP Activity (A) Histograms represent peak areas from extracted ion chromatograms for non-phosphorylated and phosphorylated peptides corresponding to S52 and S178 phosphosites of mouse TTP. Myc-TTP was immunoprecipitated from CT26 Myc-TTP (tet-ON) cells 1 hr after the indicated treatment. Mean ± SD of technical triplicates. Representative of two independent biological experiments. (B) qPCR analysis of ER-KRASG12V type II pneumocytes treated in starvation medium for 24 hr. Mean ± SEM of four independent experiments. (C) Representative flow cytometry histograms of PD-L1 surface protein expression in MCF10A ER-ΔMEKK3 cells treated in starvation medium for 1 day or 4 days. Data are representative of two independent experiments. (D) Flow cytometry analysis of PD-L1 surface protein expression on ER-HRASG12V MCF10A cells (24 hr) and ER-HRASG12V HKE-3 cells (48 hr) after treatment in starvation medium. Data are representative of biological duplicates. (E) qPCR analysis of CT26 cells at 2 hr or 24 hr after MK2 inhibition with PF 3644022. Mean ± SEM of two independent experiments. (F) Sequence alignments of the conserved phosphosites (highlighted red) targeted by MK2 in mouse (Mm) and human (Hs) TTP protein. (G) Western blotting of immunoprecipitations from CT26 TTP KO cells harboring tet-ON, WT, or phospho mutant, Myc-TTP constructs. Cells were treated with dox. for 24 hr before the addition of PMA or DMSO for 1 hr. Arrow indicates Myc-TTP. Data are representative of two independent experiments. (H) qPCR analysis of CT26 TTP KO cells harboring tet-ON, WT, or phospho mutant, Myc-TTP constructs, treated with dox or vehicle for 48 hr. Data represent the mean ± SEM of two independent experiments. ∗∗p < 0.005, ∗p < 0.05. Unpaired, two-tailed Student’s t test. Abbreviations and quantities: 4-OHT, 100 nM; NAC, N-acetyl-L-cysteine, 10 mM; PMA, 200 nM; MEK inhibitor, GSK1120212, 25 nM; MK2 inhibitor PF 3644022, 1 μM; MK2 inhibitor III, 1 μM; dox., doxycycline, 1 μg/mL. See also Figure S5.
Figure 6
Figure 6
RAS Pathway Activation Is Associated with PD-L1 Upregulation in Human Cancers (A) Heat-maps showing fold change in expression of T cell function related genes between high and low RAS pathway activity cohorts of lung adenocarcinoma (LUAD) and colon adenocarcinoma (COAD) TCGA samples. KRAS mutation status (codons 12, 13, and 61) is indicated for each sample. Genes are ranked in order of significance. Wald test, DESeq2. (B) Box-and-whisker plots comparing PD-L1 mRNA expression in RAS high versus low pathway activity cohorts in LUAD and COAD using two independent RAS gene expression signatures. Wald test, DESeq2. See also Figure S6.
Figure 7
Figure��7
Restoration of Tumor Cell TTP Expression Enhances Anti-tumor Immunity (A) Western blotting analysis of CT26 Myc-TTP tet-ON cells expressing either empty vector or mouse Cd274 cDNA lacking the 3′ UTR (PD-L1 Δ3′ UTR), 24 hr after treatment (Dox., 0.1 μg/mL or 1 μg/mL). Arrow indicates Myc-TTP. Data are representative of two independent experiments. (B) Representative flow cytometry histograms of PD-L1 surface protein expression in CT26 stable cells lines in (A), 72 hr after treatment (Dox., 1 μg/mL). Data are representative of three independent experiments. (C) Tumor growth curves for CT26-derived cell lines subcutaneously transplanted into BALB/c mice (n = 8 per group). (D) Tumor growth curves for MC38-derived cell lines subcutaneously transplanted into C57BL/6 mice (n = 6 per group). X denotes the loss of a doxycycline-treated mouse. (E) Tumor growth curves for CT26-derived cell lines subcutaneously transplanted into nu/nu mice (n = 6 per group). (F) Tumor growth curves for CT26-derived cell lines subcutaneously transplanted into BALB/c mice (n = 4–5 per group). For (C)–(F), data represent the mean ± SEM from individual experiments. ∗∗p < 0.01, ∗∗∗∗p < 0.0001, n.s., not significant; two-way ANOVA. (G) Histological analysis of subcutaneous tumors at the end-point from the experiment described in (C), with quantification of CD3+ cells in 5 fields of view per mouse with 5–6 mice per group. Mean ± SEM. ∗∗p < 0.01; unpaired, two-tailed Student’s t test. (H) Quantification of CD8+/Treg cell ratios and CD8+ IFN-γ+ cells from flow cytometry analysis of tumors after 18–20 days of growth. Each data point represents data from an individual mouse; mean ± SEM. ∗p < 0.05; unpaired, two-tailed Student’s t test. Data are pooled from two independent experiments. (I) Proposed molecular model. Signaling nodes that influence anti-tumor immunity and are amenable to inhibition with drugs used in this study are highlighted. S52 and S178 represent MK2 target sites and numbering corresponds to mouse TTP. OA, okadaic acid. See also Figure S7.
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