RAS and PD-L1: A Masters’ Liaison in Cancer Immune Evasion
2017; Cell Press; Volume: 47; Issue: 6 Linguagem: Inglês
10.1016/j.immuni.2017.12.001
ISSN1097-4180
AutoresNicole Glodde, Michael Hölzel,
Tópico(s)Immune cells in cancer
ResumoMutant RAS is a major oncoprotein in human cancer and PD-L1 is a key driver of cancer immune evasion. In this issue of Immunity, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar demonstrate that oncogenic RAS signaling promotes tumor immune escape by stabilizing PD-L1 mRNA. Mutant RAS is a major oncoprotein in human cancer and PD-L1 is a key driver of cancer immune evasion. In this issue of Immunity, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar demonstrate that oncogenic RAS signaling promotes tumor immune escape by stabilizing PD-L1 mRNA. The immune system can effectively restrain tumor growth, but unfortunately tumors can escape from immune surveillance by many mechanisms. The clinical success of immune checkpoint blockers has demonstrated that the PD-1:PD-L1 axis is a major driver of immune escape in human tumors including non-small cell lung cancer (NSCLC) (Garon et al., 2015Garon E.B. Rizvi N.A. Hui R. Leighl N. Balmanoukian A.S. Eder J.P. Patnaik A. Aggarwal C. Gubens M. Horn L. et al.KEYNOTE-001 InvestigatorsPembrolizumab for the treatment of non-small-cell lung cancer.N. Engl. J. Med. 2015; 372: 2018-2028Crossref PubMed Scopus (4368) Google Scholar). Binding of PD-L1 (CD274) to the PD-1 receptor on T cells confers a state of "exhaustion," which impairs T cells in their capacity to kill the cancer cells. Tumors exploit induction of PD-L1 in order to prevent their elimination through the immune system and this has strongly fostered interest in understanding the regulation of PD-L1 expression. Interferon-γ (IFN-γ) is the best-characterized signal from the tumor microenvironment that leads to rapid transcriptional upregulation of PD-L1 mRNA and protein level. Recent unbiased genetic screens have identified the so far uncharacterized proteins CMTM6 and CMTM4 as positive regulators of PD-L1 protein stability with remarkable selectivity (Mezzadra et al., 2017Mezzadra R. Sun C. Jae L.T. Gomez-Eerland R. de Vries E. Wu W. Logtenberg M.E.W. Slagter M. Rozeman E.A. Hofland I. et al.Identification of CMTM6 and CMTM4 as PD-L1 protein regulators.Nature. 2017; 549: 106-110Crossref PubMed Scopus (373) Google Scholar). Both factors enhance PD-L1 expression on tumor cells and thereby impair tumor-specific T cell activity. Interestingly, the PD-L1 gene itself is also subject to genomic alterations in different types of cancers (Kataoka et al., 2016Kataoka K. Shiraishi Y. Takeda Y. Sakata S. Matsumoto M. Nagano S. Maeda T. Nagata Y. Kitanaka A. Mizuno S. et al.Aberrant PD-L1 expression through 3′-UTR disruption in multiple cancers.Nature. 2016; 534: 402-406Crossref PubMed Scopus (426) Google Scholar). Rearrangements in the 3′ untranslated region (UTR) result in truncated PD-L1 transcripts with enhanced stability, arguing that increased PD-L1 levels confer a selective growth advantage to tumor cells. Furthermore, the data also show that the 3′ UTR of PD-L1 mRNA critically contributes to post-transcriptional regulation of PD-L1 protein expression. In this issue of Immunity, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar unveil a so far unknown link between oncogenic RAS signaling in cancer cells and increased PD-L1 mRNA stability, which leads to elevated PD-L1 surface expression and impaired tumor immune surveillance. There are three RAS genes (HRAS, KRAS, and NRAS) and among those, KRAS is the most frequently mutated oncogene in human cancer. RAS mutations abrogate intrinsic GTPase activity and lock mutant RAS proteins in the activated GTP-bound state leading to constitutive downstream signaling, which includes the RAF-MEK-ERK and the Pi3K-AKT pathways. Based on a recent clinical trial that reported increased PD-L1 expression in NSCLC with KRAS mutations (Garon et al., 2015Garon E.B. Rizvi N.A. Hui R. Leighl N. Balmanoukian A.S. Eder J.P. Patnaik A. Aggarwal C. Gubens M. Horn L. et al.KEYNOTE-001 InvestigatorsPembrolizumab for the treatment of non-small-cell lung cancer.N. Engl. J. Med. 2015; 372: 2018-2028Crossref PubMed Scopus (4368) Google Scholar), Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar asked whether oncogenic RAS signaling directly regulates PD-L1 expression. Using a non-transformed pneumocyte cell line expressing a tamoxifen-regulatable ER-KRASG12V fusion protein, the authors found rapid and profound upregulation of PD-L1 mRNA and surface protein level upon activation of oncogenic RAS. In contrast to IFN-γ, induction of PD-L1 by activated RAS did not involve a transcriptional response, but it was mediated by an increased half-life of the PD-L1 mRNA. Subsequent experiments with small molecule inhibitors identified the RAF-MEK-ERK cascade as a critical regulator of PD-L1 mRNA stability. Importantly, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar corroborated their findings by showing downregulation of PD-L1 in KRASG12C mutant human NSCLC cell lines treated with selective KRASG12C or MEK inhibitors. In order to elucidate the molecular mechanism of how RAS signaling regulates PD-L1 mRNA stability, the authors analyzed the 3′ UTR region and identified several AU-rich elements (AREs). AREs are known to confer mRNA instability through binding of AU-rich element binding proteins (AUBPs), which recruit protein complexes for targeted mRNA decay. Using their panel of NSCLC cell lines, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar performed genetic gain- and loss-of-function experiments for several known AUBPs and obtained the most consistent results for tristetraprolin (TTP, also known as ZFP36). TTP physically bound to PD-L1 mRNA and depletion or overexpression of TTP reciprocally increased or decreased PD-L1 mRNA and protein level in an ARE-dependent manner. Taken together, the authors identified the RNA-binding protein TTP as a suppressor of PD-L1 expression by enhancing the degradation of PD-L1 mRNA. ERK is known to phosphorylate TTP and to impair its activity. This would have been an obvious mechanism linking the RAS-RAF-MEK-ERK cascade with the regulation of PD-L1 mRNA stability via TTP. Unexpectedly, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar unraveled a different pathway downstream of ERK that critically controlled TTP activity in the context of oncogenic RAS signaling. The authors found that ERK-driven reactive oxygen species (ROS) production activated the p38-MK2 stress kinase cascade, causing phosphorylation of TTP at two newly identified serine phosphorylation sites (S68/S186 in human TTP). Importantly, phosphorylation of TTP at these two relevant residues reduced TTP-mediated instability of PD-L1 mRNA (Figure 1). Finally, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar addressed how TTP influences tumor growth and immune surveillance using the CT26 and MC38 syngeneic colon cancer models. Enforced TTP expression reduced PD-L1 surface level, promoted T cell infiltration, and restrained tumor growth. This effect was absent in mice depleted for CD4+ and CD8+ T cells as well as in immunodeficient mice, which confirmed the predominant immunological mechanism. With regard to selectivity for PD-L1, co-expression of full-length PD-L1 lacking the 3′ UTR region completely abrogated the tumor growth inhibitory effects mediated by TTP overexpression. The importance of the work by Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar lies in the fact that it connects RAS and PD-L1, two master drivers of oncogenesis and cancer immune evasion, respectively. The authors also unveiled a phosphorylation-dependent mechanism of how oncogenic RAS signaling impairs the activity of the RNA-binding protein TTP, causing PD-L1 mRNA stabilization. From a translational perspective, the study delineates several opportunities for rational combinations of small molecule inhibitors with current cancer immunotherapies. Based on Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar, mutant RAS inhibitors promise to foster anti-tumor immunity in addition to targeting the cancer cells' dependency on oncogenic RAS signaling. In particular, patients with KRAS-mutated NSCLC could benefit from this strategy, as the high neoantigen load in these cancers favors clinical responses to immunotherapy. Current clinical trials evaluate MEK inhibitors in combination with different immune checkpoint blockers, which will provide a unique opportunity to confirm the findings by Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar in a clinical setting. With regard to potential companion diagnostics, antibodies recognizing human TTP phosphorylated at S68 and S186 would be helpful tools in order to correlate TTP activity with PD-L1 expression in tissue specimens from patients. Regulation of mRNA stability by the TTP-ARE axis is not unique to PD-L1, as many transcripts encoding immunoregulatory molecules contain AREs in their 3′ UTR (Kratochvill et al., 2011Kratochvill F. Machacek C. Vogl C. Ebner F. Sedlyarov V. Gruber A.R. Hartweger H. Vielnascher R. Karaghiosoff M. Rülicke T. et al.Tristetraprolin-driven regulatory circuit controls quality and timing of mRNA decay in inflammation.Mol. Syst. Biol. 2011; 7: 560Crossref PubMed Scopus (89) Google Scholar). Indeed, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar show that MEK inhibition also enhances degradation of the PTGS2 mRNA similar to PD-L1 mRNA. Cyclooxygenase-2 (PTGS2, COX-2) is an enzyme that generates prostaglandin E2 (PGE2), which was recently shown to drive tumor-promoting inflammation and to restrain anti-tumor immunity (Zelenay et al., 2015Zelenay S. van der Veen A.G. Böttcher J.P. Snelgrove K.J. Rogers N. Acton S.E. Chakravarty P. Girotti M.R. Marais R. Quezada S.A. et al.Cyclooxygenase-dependent tumor growth through evasion of immunity.Cell. 2015; 162: 1257-1270Abstract Full Text Full Text PDF PubMed Scopus (645) Google Scholar). In their study, Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar did not investigate the contribution of COX-2 and PGE2 to tumor immune escape, but their data suggest that PD-L1 is the key mechanism downstream of TTP. Nevertheless, it is likely that the ROS-p38-MK2-TTP cascade also controls the expression of additional key factors involved in cancer immune escape. In this case, TTP could emerge as a central signaling hub controlling the cancer and immune cell crosstalk. Furthermore, it will be interesting to see how the ROS-p38-MK2-TTP cascade modulates cytokine and chemokine expression patterns. Some chemokines enhance T cell recruitment and enforce anti-tumor immune responses, whereas others fuel tumor-promoting inflammation. Thus, TTP-dependent control of ARE-containing cytokine mRNAs could have ambiguous effects on tumor immune surveillance. As infiltrating immune cells are major sources of cytokines in the tumor microenvironment, it remains to be addressed whether the ROS-p38-MK2-TTP cascade regulates cytokine expression in immune cells, and in which subtypes. Such studies will increase our understanding how, for example, MEK inhibitors, which act upstream of the ROS-p38-MK2-TTP cascade, modulate immune cell functions and interactions. Indeed, MEK inhibitor co-treatment improves anti-tumor T cell activity (Ebert et al., 2016Ebert P.J.R. Cheung J. Yang Y. McNamara E. Hong R. Moskalenko M. Gould S.E. Maecker H. Irving B.A. Kim J.M. et al.MAP kinase inhibition promotes T cell and anti-tumor activity in combination with PD-L1 checkpoint blockade.Immunity. 2016; 44: 609-621Abstract Full Text Full Text PDF PubMed Scopus (465) Google Scholar). Interestingly, TTP deficiency in CD8+ T cells and macrophages enhances anti-tumor CD8+ responses through altered cytokine expression (Wang et al., 2017Wang Q. Ning H. Peng H. Wei L. Hou R. Hoft D.F. Liu J. Tristetraprolin inhibits macrophage IL-27-induced activation of antitumour cytotoxic T cell responses.Nat. Commun. 2017; 8: 867Crossref PubMed Scopus (22) Google Scholar). As another example, the c-MET receptor tyrosine kinase, which is an upstream activator of the RAS pathway, controls neutrophil recruitment and function in response to cancer immunotherapy (Glodde et al., 2017Glodde N. Bald T. van den Boorn-Konijnenberg D. Nakamura K. O'Donnell J.S. Szczepanski S. Brandes M. Eickhoff S. Das I. Shridhar N. et al.Reactive neutrophil responses dependent on the receptor tyrosine kinase c-MET limit cancer immunotherapy.Immunity. 2017; 47: 789-802Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). Of note, TTP-deficient neutrophils have a reduced apoptosis rate through stabilization of the mRNA encoding for the anti-apoptotic protein MCL1 (Ebner et al., 2017Ebner F. Sedlyarov V. Tasciyan S. Ivin M. Kratochvill F. Gratz N. Kenner L. Villunger A. Sixt M. Kovarik P. The RNA-binding protein tristetraprolin schedules apoptosis of pathogen-engaged neutrophils during bacterial infection.J. Clin. Invest. 2017; 127: 2051-2065Crossref PubMed Scopus (25) Google Scholar). This suggests that TTP critically regulates myeloid immune cell functions and phenotypes in the tumor microenvironment. Thus, TTP-dependent effects of oncogenic signal transduction inhibitors on immune cells can influence anti-tumor immunity, a knowledge that is needed to optimally combine these inhibitors with cancer immunotherapy. In summary, the work by Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar unveiled a novel molecular mechanism of PD-L1 regulation that is controlled by oncogenic RAS signaling. It highlights the importance of non-cancer cell-autonomous mechanisms for tumor development driven by major oncogenes like RAS. The study also opens up new avenues for the improvement of current cancer immunotherapies by targeting PD-L1 mRNA stability with small molecule inhibitors in tumors lacking genomic rearrangements of the PD-L1 3′ UTR region. As RAS mutations and PD-L1 overexpression are common in human cancer, the findings by Coelho et al., 2017Coelho M.A. de Carné Trécesson S. Rana S. Zecchin D. Moore C. Molina-Arcas M. East P. Spencer-Dene B. Nye E. Barnouin K. et al.Oncogenic RAS signalling promotes tumour immunoresistance by stabilising PD-L1 mRNA.Immunity. 2017; 47 (this issue): 1083-1099Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar delineate therapeutic strategies with broad clinical applicability. This work was supported by grants to M.H. from the Else-Kröner Fresenius Stiftung (2013_A297), the DFG (HO4281/2-1), and the DFG cluster of excellence (EXC1023). M.H. is a member of DFG cluster of excellence ImmunoSensation (EXC1023). Oncogenic RAS Signaling Promotes Tumor Immunoresistance by Stabilizing PD-L1 mRNACoelho et al.ImmunityDecember 12, 2017In BriefCoelho et al. demonstrate a post-transcriptional mechanism whereby oncogenic RAS signaling increases PD-L1 expression. Mechanistically, PD-L1 mRNA is targeted by TTP through AU-rich elements in the 3′ UTR, making it unstable. Oncogenic RAS signaling reduces TTP activity and stabilizes the PD-L1 transcript. Restoring TTP activity reduces PD-L1 expression and enhances anti-tumor immunity. Full-Text PDF Open Access
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