Innate Immunity: Resistance and Disease-Promoting Principles

G. Hartmann, H. Wagner

• Publisher: S. Karger
• Year: 2013
• ISBN:
  9783318023480
• Topic: Medicine
• Subtopic: Endocrinology & Metabolism

Our understanding of the complex innate immune response is increasing rapidly. Its role in the protection against viral or bacterial pathogens is essential for the survival of an organism. However, it is equally important to avoid unregulated inflammation because innate immune responses can cause or promote chronic autoinflammatory diseases such as gout, atherosclerosis, type 2 diabetes or certain aspects of the metabolic syndrome. In this book leading international experts in the field of innate immunity share their findings, define the 'state of the art' in this field and evaluate how insight into the molecular basis of these diseases could help in the design of new therapies. A tremendous amount of work on the innate immune response has been done over the last fifteen years, culminating in the 2011 Nobel Prize in Physiology/Medicine awarded for the discoveries of Toll genes in immunity in flies, membrane-bound Toll-like receptors in mammals, and dendritic cells as initiators of adaptive immunity.

Chapter 1: Innate Immunity and Inflammation

Hartmann G, Wagner H (eds): Innate Immunity: Resistance and Disease-Promoting Principles.
Else Kröner-Fresenius Symp. Basel, Karger, 2013, vol 4, pp 4-10 (DOI: 10.1159/000346526)

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Innate Immunity, Inflammation and Colorectal Cancer

Kepeng Wang · Sergei Grivennikov · Michael Karin

Laboratory of Gene Regulation and Signal Transduction, Department of Pharmacology and Pathology, School of Medicine, University of California, San Diego, La Jolla, Calif., USA

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Abstract

Up to 20% of cancers arise from chronic inflammation and persistent infections. Moreover, most solid tumors contain immune infiltrates. Tumor-associated inflammatory cells play broad roles in different stages of cancer development, including initiation, promotion, progression and metastasis. Cytokines are important mediators of the inflammatory effect on tumorigenesis. Our work showed that IL-6 is an important tumor promoter in early colitis-associated cancer (CAC). IL-6 is mainly produced by tumor-infiltrating myeloid cells under the control of NF-κB. IL-6 promotes proliferation of tumor-initiating cells derived from the intestinal epithelium and protects them from apoptosis during colitis. The pro-survival and proliferative effects of IL-6 are mainly mediated by STAT3, whose ablation in intestinal epithelial cells significantly reduces CAC tumorigenesis and augments the host susceptibility to experimental colitis. Taken together, the NF-κB-IL-6-STAT3 pathway links innate immunity to tumorigenesis by promoting the survival and proliferation of tumor-initiating cells. IL-6 and other cytokines may also enhance cancer progression.

Copyright © 2013 S. Karger AG, Basel

A link between inflammation and cancer has been suspected for a long time since Rudolf Virchow observed infiltration of leukocytes in malignant tumors [1]. Yet, experimental evidence demonstrating the important role of inflammation in tumorigenesis has become available only in the last decade [2]. About 15-20% of cancers arise in the context of preceding chronic inflammation. For instance, Helicobacter pylori infection is associated with gastric cancer, while HBV and HCV infections and hepatitis are associated with hepatocellular carcinoma. Chronic colonic inflammation manifested in ulcerative colitis and other inflammatory bowel diseases (IBD) significantly increases the probability of colorectal cancer (CRC) development [3-6]. While it is already known that chronic inflammation induces and promotes cancer and that the usage of non-steroidal anti-inflammatory drugs, such as aspirin, decreases the life-long risk of cancer death [7, 8], less is known about molecular and cellular mechanisms connecting inflammation and cancer. Recent studies have begun to decipher such links, focusing on signaling within and interactions between myeloid, immune and epi-thelial cells [6, 9-12]. Inflammatory cytokines are the key mediators and regulators of these interactions.

CRC is the third most common and deadly cancer around the globe [13]. Patients with ulcerative colitis have significantly higher risk of CRC as approximately 20% of IBD patients develop colitis-associated cancer (CAC) within 30 years of disease emergence [14, 15]. CAC is a classical inflammation-driven cancer, which can be relatively easily modeled in mice by three cycles of dextran sodium sulfate (DSS) in the drinking water subsequent to a single dose of the pro-carcinogen azoxymethane (AOM) [16, 17].

It has been previously shown by our lab that an overall reduction of the DSS-evoked intestinal inflammatory response by inactivation of IKKß in myeloid cells leads to decreased tumor size, with concomitant reduction in the expression of multiple proinflammatory cytokines that may serve as tumor growth factors [18]. Importantly, inactivation of NF-κB signaling in intestinal epithelial cells (IEC) by cell type-specific disruption of the Ikkß gene reduced tumor count and increased apoptosis in tumors and injured tissue [18]. That study first suggested that cytokines produced by inflammatory cells in an NF-κB -dependent manner can act on premalignant cells derived from IEC to activate the NF-κB-dependent pro-survival gene expression program. One of the cytokines which is induced by NF-κB and can activate NF-κB is tumor necrosis factor (TNF), whose role in CAC has been demonstrated. Mice lacking TNFR1 showed reduced mucosal damage, reduced infiltration of macrophages and neutrophils, and attenuated formation of colon tumors [19]. Likely, TNF exerts its tumorigenic properties by acting both on immune and epithelial cells. While adoptive transfer experiments showed that TNFR1 signaling is particularly important in the radiosensitive compartment [19], other studies demonstrated the potential importance of TNFR2 signaling in IEC [20].

Overall, TNF-IKKß-NF-κB signaling activates myeloid cells to produce proinflammatory cytokines, which in turn serve as growth factors that increase malignant cell proliferation. NF-κB signaling in epithelial cells mainly prevents premalignant cell from undergoing apoptosis; therefore, in its absence, we observe fewer and smaller tumors being formed upon AOM treatment combined with chronic colitis [18].

Inactivation of NF-κB in myeloid cells led to reduced production of IL-6 during DSS-evoked intestinal inflammation [21]. IL-6 is a multifunctional cytokine that plays important roles in immune responses, cell survival and proliferation [22]. IL-6 binds to soluble or membrane-bound IL-6Rα, and cell surface gp130, and activates intracellular signaling mediated by STAT3, Ras and PI3K-Akt [22]. In immunity, IL-6 is important for T cell survival and differentiation, and therefore plays a pivotal role in the pathogenesis of autoimmune disorders [23]. Blocking IL-6 signaling by neutralizing antibody against IL-6R or by gp130-Fc fusion protein caused suppression of colitis in a mouse model of Crohn's disease [24]. IL-6 also plays an important role in tissue homeostasis and regeneration, suggesting that it may directly promote tumorigenesis and malignant cell survival [25, 26]. Moreover, IL-6 mRNA is upregulated in multiple human cancers, including breast, lung, prostate, liver and colon cancer [27], and its expression levels often correlate with tumor mass and poor prognosis. IL-6 promotes proliferation of human colon carcinoma cells in vitro via activation of STAT3 [28]. In the settings of CAC, IL-6 may perpetuate chronic inflammation and maintain production of proinflammatory cytokines responsible for growth and survival of malignant cells. Blocking IL-6 signaling by injection of chimeric gp130-Fc protein which blocks IL-6 transsignaling resulted in reduced CAC tumor burden in mice [29]. These lines of evidence suggest that IL-6 may play an oncogenic role in CAC by acting on both immune cells and malignant epithelial cells.

By using genetic tools to disrupt IL-6 signaling in vivo, we demonstrated that IL-6 regulates survival and proliferation of IEC and preneoplastic cells during induction of CAC and that the effects of IL-6 are in part mediated by its ability to activate transcription factor STAT3 in premalignant cells [30]. These data were further corroborated by genetic inactivation of STAT3 in IECs, which inhibits CAC induction and growth [30, 31]. Therefore, cytokines produced at high levels during chronic inflammatory responses can also act on premalignant and cancerous cells to enhance tumor promotion and progression.

Results

IL-6 Is Required for CAC Tumorigenesis

It is generally believed that activation of NF-κB in myeloid cells promotes neoplastic growth of CAC in mice in part by induction of proinflammatory cytokines and growth factors [2, 21]. IL-6 fulfills the criteria of being an NF-κB target and an important regulator of inflammation on one hand and a potent growth factor for epithelial and malignant cells on the other. Since levels of IL-6 are typically upregulated in many cancers and chronic inflammatory conditions, we sought to examine the effect of complete IL-6 deficiency on CAC development. CAC was induced in wildtype (WT) and Il6^-/- mice by injection of a single dose of AOM followed by 3 cycles of DSS in the drinking water [16, 17]. As expected, colonic IL-6 was upregulated upon DSS treatment. Ablation of IL-6 resulted in reduced tumor number, size and total tumor load in mice. These data indicate that IL-6 is important for both tumor development and growth in CAC. Differences in tumor multiplicity and size may be explained by altered cancer cell apoptosis and/or proliferation. Reduced tumor number in Il6^-/- mice suggested that IL-6 may contribute to cancer cell survival and/or proliferation. To test this hypothesis, we examined apoptosis and cell proliferation in WT or Il6^-/- mice subjected to acute colitis. DSS-exposed Il6^-/- mice exhibited elevated apoptosis in IECs. Consistently, we observed downregulation of antiapoptotic protein BCL-XL in IECs of Il6^-/- mice undergoing acute colitis. We also observed a significant decrease in the proliferation of basal crypt cells of Il6^-/- mice subjected to intestinal injury caused by DSS exposure. Therefore, IL-6 promotes both proliferation and survival of IECs during acute colitis. Presumably, IL-6 exerts the same effect on malignant IECs; it protects them from apoptosis, and promotes their proliferation in CAC.

To determine the origin of IL-6 in CAC, we performed reciprocal adoptive transfer experiments by introducing WT or Il6^-/- bone marrow into WT or Il6^-/- lethally irradiated recipient mice. Reduction in tumor number, size and load in mice deficient in IL-6 in hematopoietic cells suggested the importance of immune cells in overall IL-6 production during CAC tumorigenesis. To further delineate the source of IL-6 in CAC, we purified different myeloid and immune cells from CAC adenomas by FACS sorting. Analysis of IL-6 mRNA by q-PCR revealed that IL-6 is mainly produced by dendritic cells and macrophages, followed by T cells. In patients suffering from ulcerative colitis and CAC, IL-6 is expressed in colonic...

Table of contents

• Cover Page
• Front Matter
• Introduction
• Chapter 1: Innate Immunity and Inflammation
• Chapter 2: Innate Immunity and Transplantation
• Chapter 3: Innate Immunity and Intestinal Microbiota
• Chapter 4: Innate Immunity and Disease Promotion
• Chapter 5: Drosophila and Immunity
• Chapter 6: Innate Signaling and Adaptive Immunity
• Chapter 7: Speakers at the Symposium
• Author Index
• Subject Index