Prostaglandin E2-EP2 signaling as a node of chronic inflammation in the colon tumor microenvironment
© The Author(s) 2017
Received: 23 August 2016
Accepted: 6 January 2017
Published: 1 March 2017
Colorectal cancer is the third most common cancer. Involvement of prostaglandin (PG) system in the pathogenesis of colorectal cancer has been suggested from clinical studies demonstrating therapeutic effect of NSAIDs including aspirin or selective COX-2 inhibitors. However, mechanisms on how PG regulates inflammatory responses leading to colorectal cancer development remain obscure. Further, careful attention is needed to use these drugs for a long time because of adverse effects due to non-specific inhibition of physiological PG production in addition to pathological one, making the development of alternatives to aspirin important. Recent studies using mouse model of colitis-associated colon cancer, azoxymethane (AOM)-dextran sodium sulfate (DSS) model, have revealed some of the mechanisms on how PG regulates inflammation in lesions and proposed PG receptor as a therapeutic target.
Main body of abstract
Among each PG receptor subtype examined, prostaglandin E receptor 2 (EP2) signaling specifically contributes to colorectal cancer formation and inflammation in lesions of AOM-DSS model. EP2 is expressed in neutrophils, infiltrated major inflammatory cells, and tumor-associated fibroblasts (TAFs) in the tumor stroma of this mouse model and also in clinical specimen from ulcerative colitis-associated colorectal cancer. Bone marrow transfer experiments between wild-type and EP2-deficient mice have confirmed the involvement of EP2 signaling in these two types of cells in the pathogenesis of the disease. EP2 signaling in both types of cells regulates the transition to and maintenance of inflammation in multiple steps to shape the tumor microenvironment which contributes to trigger and promote colorectal cancer. In this process, PGE2-EP2 signaling synergizes with TNF-α to amplify TNF-α-induced inflammatory responses, forms a positive feedback loop involving COX-2-PGE2-EP2 signaling to exacerbate PG-mediated inflammation once triggered, and alternates active cell populations participating in inflammation through forming self-amplification loop among neutrophils. Thus, EP2 signaling functions as a node of inflammatory responses in the tumor microenvironment. Based on such a notion, EP2 can become a strong candidate for therapeutic target of colorectal cancer treatment. Indeed, in AOM-DSS model, a selective EP2 antagonist, PF-04418948, potently suppresses colorectal tumor formation.
PGE2-EP2 signaling functions as a node of chronic inflammation which shapes the tumor microenvironment and thus is a strong candidate of target for the chemoprevention of colorectal cancer.
KeywordsProstaglandin EP2 Inflammation Microenvironment Colon cancer Neutrophil Fibroblast CXCL1 TNF-α COX-2
Prostaglandins (PGs) including PGD2, PGE2, PGF2α, PGI2, and thromboxane (TX) A2 are arachidonic acid metabolites formed by sequential actions of cyclooxygenase (COX) and respective synthases for each PG and exert their actions by acting on their cognate G-protein-coupled receptors (GPCRs) . PGs are traditionally recognized as a major mediator of acute inflammatory responses because non-steroidal anti-inflammatory drugs (NSAIDs), which inhibit the activity of COX and shut off PG production, effectively suppress symptoms of acute inflammation: fever, reddish, swelling, and pain. Interestingly, recent experimental studies mainly using mice deficient in each PG receptor subtype subjected to animal disease models have revealed the involvement of PG system and its receptor signaling in the pathogenesis of various diseases with chronic course such as cancer, vascular diseases, or neurodegenerative diseases and thereby suggested the regulation of not only acute inflammation but also chronicity of inflammation by PGs .
Colorectal cancer is the third common cancer . One of the major risk factors of colorectal cancer is inflammatory bowel diseases such as ulcerative colitis , indicating the involvement of inflammatory responses in the pathogenesis of colorectal cancer. Indeed, in 1988, reduction of the risk of colorectal cancer development in aspirin users was reported . Subsequently, some clinical studies reported reduction of the risk and mortality of colorectal cancer by the use of NSAIDs including aspirin [6–8], suggesting the close association of the pathogenesis of colorectal cancer with PG-mediated inflammation. Up to now, contribution of PG system to colon cancer cells has been extensively studied mainly using cancer cell lines, i.e., prostaglandin E receptor 2 (EP2) signaling promotes growth of colon cancer cells via driving a Gs-axin-b-catenin axis in vitro . Although inflammation in the tumor microenvironment, where many types of cells interact with tumor cells, is essential to promote their development and growth, studies addressing how PG system regulates this inflammation in the tumor microenvironment of colorectal cancer in detail are quite limited [10, 11].
In this short review, we explain and interpret our recent experimental findings regarding the regulation of chronic inflammatory responses in the tumor microenvironment of colorectal cancer by PGE2-EP2 signaling cascade .
Prostaglandin system as a node of inflammation in tumor environment and its contribution to colon tumor formation
In summary, PGE2-EP2 signaling plays a role as a node of inflammation in the tumor microenvironment to amplify inflammatory responses which support tumor development and progression. In this process, this EP2 signaling contributes to the chronicity of inflammation in multiple steps (Fig. 1), i.e., (1) the formation of a positive feedback loop to exacerbate PG-mediated inflammatory responses, (2) the function as a “cytokine amplifier,” and (3) the alternation of active cell populations in situ partially through forming a self-amplification loop among neutrophils. Such a role of EP2 or PG receptor signaling on chronicity of inflammation can be observed also in the microenvironment of various inflammatory settings, and thus, PG system contributes to the pathogenesis of various diseases as a node of inflammatory responses. Although such roles of PGs should be verified in more detail, understanding of PG system as a major regulator of inflammation in not only acute but also chronic one is conceptually important for our understanding of the pathogenesis underlying various diseases and also as a translational research to develop novel therapeutic drugs targeting inflammatory diseases.
Potential of EP2 as a therapeutic target to treat colorectal cancer
As described above, EP2 signaling plays a crucial role in colon tumorigenesis as a node of inflammation in the tumor microenvironment. Thereby, EP2 may be a strong candidate as a therapeutic target for the treatment of colorectal cancer especially with active inflammatory responses in the stroma. The potential of pharmacological inhibition of EP2 signaling as a therapeutic measure for colorectal cancer treatment is therefore examined in AOM-DSS model of mice . A selective EP2 antagonist, PF-04418948 [14, 15], is administered to mice subjected to AOM-DSS model, and colon tumor formation in this model is then examined. As a result, PF-04418948 dose dependently (1–100 mg/kg) suppresses the number of colon tumor, and notably at the highest dose of this compound (100 mg/kg), any colon tumor is induced . Consistent with the EP2-mediated formation of a self-amplification loop among neutrophils via secretion of their chemoattractant CXCL1, infiltration of inflammatory cells and expression of CXCL1 in the lesion are both remarkably suppressed in PF-04418948-treated mice . A selective EP2 antagonist can therefore become a strong candidate for drugs to treat colorectal cancer or prevent the initiation/recurrence of it via acting on inflammatory responses in situ. Here, it should be carefully discussed that NSAIDs and COX-2 inhibitors have significant adverse effects such as gastrointestinal hemorrhage and cardiovascular accidents  derived from a non-specific inhibition of PG receptors, some of which mediates a physiological function to maintain homeostasis of organs, and impaired balance between PGI2 and TXA2, respectively. Indeed, deficiency in COX-1 or COX-2 potentiates injury in the colon epithelium under treatment with DSS in mice . Also, other anti-inflammatory drugs such as those targeting IL-6 and TNF-α have a potential risk of development of infectious and other types of cancers. Considered with normal development of EP2-deficient mice except for impaired fertilization  and the induced expression of EP2 selectively in lesions, a selective EP2 antagonist can be a safer and potent alternative to aspirin and other anti-inflammatory drugs in the chemoprevention of colorectal cancer. Further, because the use of aspirin reduces the risk of death from all cancers including colorectal cancer, lung cancer, and esophageal cancer  and high expression of EP2 in tumor lesions have a positive correlation with poor outcome in some cancers [20, 21], a selective EP2 antagonist can also be a strong candidate as a therapeutic drug to treat a variety of cancers.
Colorectal cancer is considered as a PG-mediated pathology through various clinical studies which have demonstrated the therapeutic effect of NSAIDs or selective COX-2 inhibitors on this disease. We addressed mechanisms on how PG system regulates colon tumorigenesis, and our recent experimental findings using AOM-DSS model in mice have demonstrated the involvement of PGE2-EP2 system in the pathogenesis of colorectal cancer. EP2 expresses in infiltrated neutrophils and TAFs in the tumor microenvironment, and this signaling plays a role to maintain inflammatory responses in the colon to form a microenvironment supporting the initiation and promotion of cancer cells. The processes EP2 signaling regulates such as the maintenance of inflammation include the formation of a positive feedback loop to amplify PG-mediated inflammation, the amplification of inflammation synergistically with other cytokines, and the alternation of cell populations participating in situ. Further, we propose the potential of a selective EP2 agonist as a safer alternative to aspirin.
Dextran sodium sulfate
Prostaglandin E receptor 2
We would like to express our sincere gratitude to all the researchers, collaborators, technical assistants, and secretaries for contributing to the researches cited in the present manuscript. We also thank the grants supporting our research work.
There is no funding support regarding this review article.
Availability of data and materials
Not applicable. This review article included only already published data.
TA drafted the manuscript, and TA and SN revised it. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
- Hirata T, Narumiya S. Prostanoid receptors. Chem Rev. 2011;111:6209–30.View ArticlePubMedGoogle Scholar
- Aoki T, Narumiya S. Prostaglandins and chronic inflammation. Trends Pharmacol Sci. 2012;33:304–11.View ArticlePubMedGoogle Scholar
- Brenner H, Kloor M, Pox CP. Colorectal cancer. Lancet. 2014;383:1490–502.View ArticlePubMedGoogle Scholar
- Beaugerie L, Svrcek M, Seksik P, Bouvier AM, Simon T, Allez M, et al. Risk of colorectal high-grade dysplasia and cancer in a prospective observational cohort of patients with inflammatory bowel disease. Gastroenterology. 2013;145:166–75. e8.View ArticlePubMedGoogle Scholar
- Kune GA, Kune S, Watson LF. Colorectal cancer risk, chronic illnesses, operations, and medications: case control results from the Melbourne Colorectal Cancer Study. Cancer Res. 1988;48:4399–404.PubMedGoogle Scholar
- Thun MJ, Namboodiri MM, Calle EE, Flanders WD, Heath Jr CW. Aspirin use and risk of fatal cancer. Cancer Res. 1993;53:1322–7.PubMedGoogle Scholar
- Thun MJ, Namboodiri MM, Heath Jr CW. Aspirin use and reduced risk of fatal colon cancer. N Engl J Med. 1991;325:1593–6.View ArticlePubMedGoogle Scholar
- Bertagnolli MM, Eagle CJ, Zauber AG, Redston M, Solomon SD, Kim K, et al. Celecoxib for the prevention of sporadic colorectal adenomas. N Engl J Med. 2006;355:873–84.View ArticlePubMedGoogle Scholar
- Castellone MD, Teramoto H, Williams BO, Druey KM, Gutkind JS. Prostaglandin E2 promotes colon cancer cell growth through a Gs-axin-beta-catenin signaling axis. Science. 2005;310:1504–10.View ArticlePubMedGoogle Scholar
- Sonoshita M, Takaku K, Sasaki N, Sugimoto Y, Ushikubi F, Narumiya S, et al. Acceleration of intestinal polyposis through prostaglandin receptor EP2 in Apc(delta 716) knockout mice. Nat Med. 2001;7:1048–51.View ArticlePubMedGoogle Scholar
- Oshima M, Dinchuk JE, Kargman SL, Oshima H, Hancock B, Kwong E, et al. Suppression of intestinal polyposis in Apc delta716 knockout mice by inhibition of cyclooxygenase 2 (COX-2). Cell. 1996;87:803–9.View ArticlePubMedGoogle Scholar
- Ma X, Aoki T, Tsuruyama T, Narumiya S. Definition of prostaglandin E2-EP2 signals in the colon tumor microenvironment that amplify inflammation and tumor growth. Cancer Res. 2015;75:2822–32.View ArticlePubMedGoogle Scholar
- Neufert C, Becker C, Neurath MF. An inducible mouse model of colon carcinogenesis for the analysis of sporadic and inflammation-driven tumor progression. Nat Protoc. 2007;2:1998–2004.View ArticlePubMedGoogle Scholar
- af Forselles KJ, Root J, Clarke T, Davey D, Aughton K, Dack K, et al. In vitro and in vivo characterization of PF-04418948, a novel, potent and selective prostaglandin EP2 receptor antagonist. Br J Pharmacol. 2011;164:1847–56.View ArticlePubMedPubMed CentralGoogle Scholar
- Birrell MA, Maher SA, Buckley J, Dale N, Bonvini S, Raemdonck K, et al. Selectivity profiling of the novel EP2 receptor antagonist, PF-04418948, in functional bioassay systems: atypical affinity at the guinea pig EP2 receptor. Br J Pharmacol. 2013;168:129–38.View ArticlePubMedPubMed CentralGoogle Scholar
- Solomon SD, McMurray JJ, Pfeffer MA, Wittes J, Fowler R, Finn P, et al. Cardiovascular risk associated with celecoxib in a clinical trial for colorectal adenoma prevention. N Engl J Med. 2005;352:1071–80.View ArticlePubMedGoogle Scholar
- Morteau O, Morham SG, Sellon R, Dieleman LA, Langenbach R, Smithies O, et al. Impaired mucosal defense to acute colonic injury in mice lacking cyclooxygenase-1 or cyclooxygenase-2. J Clin Invest. 2000;105:469–78.View ArticlePubMedPubMed CentralGoogle Scholar
- Hizaki H, Segi E, Sugimoto Y, Hirose M, Saji T, Ushikubi F, et al. Abortive expansion of the cumulus and impaired fertility in mice lacking the prostaglandin E receptor subtype EP2. Proc Natl Acad Sci U S A. 1999;96:10501–6.View ArticlePubMedPubMed CentralGoogle Scholar
- Rothwell PM, Wilson M, Elwin CE, Norrving B, Algra A, Warlow CP, et al. Long-term effect of aspirin on colorectal cancer incidence and mortality: 20-year follow-up of five randomised trials. Lancet. 2010;376:1741–50.View ArticlePubMedGoogle Scholar
- Kuo KT, Wang HW, Chou TY, Hsu WH, Hsu HS, Lin CH, et al. Prognostic role of PGE2 receptor EP2 in esophageal squamous cell carcinoma. Ann Surg Oncol. 2009;16:352–60.View ArticlePubMedGoogle Scholar
- Miyata Y, Ohba K, Matsuo T, Watanabe S, Hayashi T, Sakai H, et al. Tumor-associated stromal cells expressing E-prostanoid 2 or 3 receptors in prostate cancer: correlation with tumor aggressiveness and outcome by angiogenesis and lymphangiogenesis. Urology. 2013;81:136–42.View ArticlePubMedGoogle Scholar