Florian R. Greten

Immunity, inflammation, and cancer.

Inflammatory responses play decisive roles at different stages of tumor development, including initiation, promotion, malignant conversion, invasion, and metastasis. Inflammation also affects immune surveillance and responses to therapy. Immune cells that infiltrate tumors engage in an extensive and dynamic crosstalk with cancer cells, and some of the molecular events that mediate this dialog have been revealed. This review outlines the principal mechanisms that govern the effects of inflammation and immunity on tumor development and discusses attractive new targets for cancer therapy and prevention.

NF-kappaB: linking inflammation and immunity to cancer development and progression.

There has been much effort recently to probe the long-recognized relationship between the pathological processes of infection, inflammation and cancer. For example, epidemiological studies have shown that ∼15% of human deaths from cancer are associated with chronic viral or bacterial infections. This Review focuses on the molecular mechanisms that connect infection, inflammation and cancer, and it puts forward the hypothesis that activation of nuclear factor-κB (NF-κB) by the classical, IKK-β (inhibitor-of-NF-κB kinase-β)-dependent pathway is a crucial mediator of inflammation-induced tumour growth and progression, as well as an important modulator of tumour surveillance and rejection.

NF-κB in cancer: from innocent bystander to major culprit

Nuclear factor of κB (NF-κB) is a sequence-specific transcription factor that is known to be involved in the inflammatory and innate immune responses. Although the importance of NF-κB in immunity is undisputed, recent evidence indicates that NF-κB and the signalling pathways that are involved in its activation are also important for tumour development. NF-κB should therefore receive as much attention from cancer researchers as it has already from immunologists.

IKKβ Links Inflammation and Tumorigenesis in a Mouse Model of Colitis-Associated Cancer

A link between inflammation and cancer has long been suspected, but its molecular nature remained ill defined. A key player in inflammation is transcription factor NF-kappaB whose activity is triggered in response to infectious agents and proinflammatory cytokines via the IkappaB kinase (IKK) complex. Using a colitis-associated cancer model, we show that although deletion of IKKbeta in intestinal epithelial cells does not decrease inflammation, it leads to a dramatic decrease in tumor incidence without affecting tumor size. This is linked to increased epithelial apoptosis during tumor promotion. Deleting IKKbeta in myeloid cells, however, results in a significant decrease in tumor size. This deletion diminishes expression of proinflammatory cytokines that may serve as tumor growth factors, without affecting apoptosis. Thus, specific inactivation of the IKK/NF-kappaB pathway in two different cell types can attenuate formation of inflammation-associated tumors. In addition to suppressing apoptosis in advanced tumors, IKKbeta may link inflammation to cancer.

IKK-beta links inflammation to obesity-induced insulin resistance.

Inflammation may underlie the metabolic disorders of insulin resistance and type 2 diabetes. IκB kinase β (IKK-β, encoded by Ikbkb) is a central coordinator of inflammatory responses through activation of NF-κB. To understand the role of IKK-β in insulin resistance, we used mice lacking this enzyme in hepatocytes (IkbkbΔhep) or myeloid cells (IkbkbΔmye). IkbkbΔhep mice retain liver insulin responsiveness, but develop insulin resistance in muscle and fat in response to high fat diet, obesity or aging. In contrast, IkbkbΔmye mice retain global insulin sensitivity and are protected from insulin resistance. Thus, IKK-β acts locally in liver and systemically in myeloid cells, where NF-κB activation induces inflammatory mediators that cause insulin resistance. These findings demonstrate the importance of liver cell IKK-β in hepatic insulin resistance and the central role of myeloid cells in development of systemic insulin resistance. We suggest that inhibition of IKK-β, especially in myeloid cells, may be used to treat insulin resistance.

gp130-Mediated Stat3 Activation in Enterocytes Regulates Cell Survival and Cell-Cycle Progression during Colitis-Associated Tumorigenesis

Although gastrointestinal cancers are frequently associated with chronic inflammation, the underlying molecular links have not been comprehensively deciphered. Using loss- and gain-of-function mice in a colitis-associated cancer model, we establish here a link comprising the gp130/Stat3 transcription factor signaling axis. Mutagen-induced tumor growth and multiplicity are reduced following intestinal epithelial cell (IEC)-specific Stat3 ablation, while its hyperactivation promotes tumor incidence and growth. Conversely, IEC-specific Stat3 deficiency enhances susceptibility to chemically induced epithelial damage and subsequent mucosal inflammation, while excessive Stat3 activation confers resistance to colitis. Stat3 has the capacity to mediate IL-6- and IL-11-dependent IEC survival and to promote proliferation through G1 and G2/M cell-cycle progression as the common tumor cell-autonomous mechanism that bridges chronic inflammation to tumor promotion.

Intestinal Tumorigenesis Initiated by Dedifferentiation and Acquisition of Stem-Cell-like Properties

Cell-type plasticity within a tumor has recently been suggested to cause a bidirectional conversion between tumor-initiating stem cells and nonstem cells triggered by an inflammatory stroma. NF-κB represents a key transcription factor within the inflammatory tumor microenvironment. However, NF-κBs function in tumor-initiating cells has not been examined yet. Using a genetic model of intestinal epithelial cell (IEC)-restricted constitutive Wnt-activation, which comprises the most common event in the initiation of colon cancer, we demonstrate that NF-κB modulates Wnt signaling and show that IEC-specific ablation of RelA/p65 retards crypt stem cell expansion. In contrast, elevated NF-κB signaling enhances Wnt activation and induces dedifferentiation of nonstem cells that acquire tumor-initiating capacity. Thus, our data support the concept of bidirectional conversion and highlight the importance of inflammatory signaling for dedifferentiation and generation of tumor-initiating cells in vivo.

Epithelial-cell-intrinsic IKK-β expression regulates intestinal immune homeostasis

Intestinal epithelial cells (IECs) provide a primary physical barrier against commensal and pathogenic microorganisms in the gastrointestinal (GI) tract, but the influence of IECs on the development and regulation of immunity to infection is unknown. Here we show that IEC-intrinsic IκB kinase (IKK)-β-dependent gene expression is a critical regulator of responses of dendritic cells and CD4+ T cells in the GI tract. Mice with an IEC-specific deletion of IKK-β show a reduced expression of the epithelial-cell-restricted cytokine thymic stromal lymphopoietin in the intestine and, after infection with the gut-dwelling parasite Trichuris, fail to develop a pathogen-specific CD4+ T helper type 2 (TH2) response and are unable to eradicate infection. Further, these animals show exacerbated production of dendritic-cell-derived interleukin-12/23p40 and tumour necrosis factor-α, increased levels of CD4+ T-cell-derived interferon-γ and interleukin-17, and develop severe intestinal inflammation. Blockade of proinflammatory cytokines during Trichuris infection ablates the requirement for IKK-β in IECs to promote CD4+ TH2 cell-dependent immunity, identifying an essential function for IECs in tissue-specific conditioning of dendritic cells and limiting type 1 cytokine production in the GI tract. These results indicate that the balance of IKK-β-dependent gene expression in the intestinal epithelium is crucial in intestinal immune homeostasis by promoting mucosal immunity and limiting chronic inflammation.

The two faces of IKK and NF-kappaB inhibition: prevention of systemic inflammation but increased local injury following intestinal ischemia-reperfusion.

We studied the role of NF-κB in acute inflammation caused by gut ischemia-reperfusion through selective ablation of IκB kinase (IKK)-β, the catalytic subunit of IKK that is essential for NF-κB activation. Ablation of IKK-β in enterocytes prevented the systemic inflammatory response, which culminates in multiple organ dysfunction syndrome (MODS) that is normally triggered by gut ischemia-reperfusion. IKK-β removal from enterocytes, however, also resulted in severe apoptotic damage to the reperfused intestinal mucosa. These results show the dual function of the NF-κB system, which is responsible for both tissue protection and systemic inflammation, and underscore the caution that should be exerted in using NF-κB and IKK inhibitors.

NF-kappaB is a negative regulator of IL-1beta secretion as revealed by genetic and pharmacological inhibition of IKKbeta.

IKKbeta-dependent NF-kappaB activation plays a key role in innate immunity and inflammation, and inhibition of IKKbeta has been considered as a likely anti-inflammatory therapy. Surprisingly, however, mice with a targeted IKKbeta deletion in myeloid cells are more susceptible to endotoxin-induced shock than control mice. Increased endotoxin susceptibility is associated with elevated plasma IL-1beta as a result of increased pro-IL-1beta processing, which was also seen upon bacterial infection. In macrophages enhanced pro-IL-1beta processing depends on caspase-1, whose activation is inhibited by NF-kappaB-dependent gene products. In neutrophils, however, IL-1beta secretion is caspase-1 independent and depends on serine proteases, whose activity is also inhibited by NF-kappaB gene products. Prolonged pharmacologic inhibition of IKKbeta also augments IL-1beta secretion upon endotoxin challenge. These results unravel an unanticipated role for IKKbeta-dependent NF-kappaB signaling in the negative control of IL-1beta production and highlight potential complications of long-term IKKbeta inhibition.