Qifan Zhu

Critical Role for the DNA Sensor AIM2 in Stem Cell Proliferation and Cancer

Colorectal cancer is a leading cause of cancer-related deaths. Mutations in the innate immune sensor AIM2 are frequently identified in patients with colorectal cancer, but how AIM2 modulates colonic tumorigenesis is unknown. Here, we found that Aim2-deficient mice were hypersusceptible to colonic tumor development. Production of inflammasome-associated cytokines and other inflammatory mediators was largely intact in Aim2-deficient mice; however, intestinal stem cells were prone to uncontrolled proliferation. Aberrant Wnt signaling expanded a population of tumor-initiating stem cells in the absence of AIM2. Susceptibility of Aim2-deficient mice to colorectal tumorigenesis was enhanced by a dysbiotic gut microbiota, which was reduced by reciprocal exchange of gut microbiota with healthy wild-type mice. These findings uncover a synergy between a specific host genetic factor and gut microbiota in determining the susceptibility to colorectal cancer. Therapeutic modulation of AIM2 expression and microbiota has the potential to prevent colorectal cancer.

Cutting edge: STING mediates protection against colorectal tumorigenesis by governing the magnitude of intestinal inflammation

Stimulator of IFN genes (STING) is a cytoplasmic innate immune sensor for cyclic dinucleotides that also serves a dual role as an adaptor molecule for a number of intracellular DNA receptors. Although STING has important functions in the host defense against pathogens and autoimmune diseases, its physiological role in cancer is unknown. In this study, we show that STING-deficient mice are highly susceptible to colitis-associated colorectal cancer. Colons of STING-deficient mice exhibit significant intestinal damage and overt proliferation during early stages of tumorigenesis. Moreover, STING-deficient mice fail to restrict activation of the NF-κB– and STAT3-signaling pathways, which leads to increased levels of the proinflammatory cytokines IL-6 and KC. Therefore, our results identified an unexpected and important role for STING in mediating protection against colorectal tumorigenesis.

NLRC3 is an inhibitory sensor of PI3K–mTOR pathways in cancer

NLRs (nucleotide-binding domain and leucine-rich repeats) belong to a large family of cytoplasmic sensors that regulate an extraordinarily diverse range of biological functions. One of these functions is to contribute to immunity against infectious diseases, but dysregulation of their functional activity leads to the development of inflammatory and autoimmune diseases. Cytoplasmic innate immune sensors, including NLRs, are central regulators of intestinal homeostasis. NLRC3 (also known as CLR16.2 or NOD3) is a poorly characterized member of the NLR family and was identified in a genomic screen for genes encoding proteins bearing leucine-rich repeats (LRRs) and nucleotide-binding domains. Expression of NLRC3 is drastically reduced in the tumour tissue of patients with colorectal cancer compared to healthy tissues, highlighting an undefined potential function for this sensor in the development of cancer. Here we show that mice lacking NLRC3 are hyper-susceptible to colitis and colorectal tumorigenesis. The effect of NLRC3 is most dominant in enterocytes, in which it suppresses activation of the mTOR signalling pathways and inhibits cellular proliferation and stem-cell-derived organoid formation. NLRC3 associates with PI3Ks and blocks activation of the PI3K-dependent kinase AKT following binding of growth factor receptors or Toll-like receptor 4. These findings reveal a key role for NLRC3 as an inhibitor of the mTOR pathways, mediating protection against colorectal cancer.

Caspase-11 is expressed in the colonic mucosa and protects against dextran sodium sulfate-induced colitis

Ulcerative colitis and Crohn’s disease are major inflammatory syndromes that affect millions of patients. Caspase-11 confers protection against Gram-negative enteropathogens, but its role during colitis is unknown. Here, we show that caspase-11 was constitutively expressed in the colon, and that caspase-11-deficient (caspase-11−/−) mice were hypersusceptible to dextran sodium sulfate (DSS)-induced colitis. Notably, pro-inflammatory Prevotella species were strongly reduced in the gut microbiota of caspase-11−/− mice. Co-housing with wild-type mice leveled Prevotella contents, but failed to protect caspase-11−/− mice from increased susceptibility to DSS-induced colitis. We therefore addressed the role of caspase-11 in immune signaling. DSS-induced tissue damage, release of the pyroptosis/necroptosis marker HMGB1, and inflammatory cell infiltration in the gut were markedly increased in caspase-11−/− mice. Moreover, caspase-11−/− mice showed normal or increased production of mature interleukin (IL)-1β and IL-18, whereas IL-1β and IL-18 secretion was blunted in animals lacking both caspases 1 and 11. In conclusion, we showed that caspase-11 shapes the gut microbiota composition, and that caspase-11−/− mice are highly susceptible to DSS-induced colitis. Moreover, DSS-induced inflammasome activation relied on caspase-1, but not caspase-11. These results suggest a role for other caspase-11 effector mechanisms such as pyroptosis in protection against intestinal inflammation.

IL-33 regulates the IgA-microbiota axis to restrain IL-1α–dependent colitis and tumorigenesis

Inflammatory bowel diseases (IBD) affect over 5 million individuals in the industrialized world, with an increasing incidence rate worldwide. IBD also predisposes affected individuals to development of colorectal cancer, which is a leading cause of cancer-related deaths in adults. Mutations in genes encoding molecules in the IL-33 signaling pathway are associated with colitis and colitis-associated cancer (CAC), but how IL-33 modulates gut homeostasis is unclear. Here, we have shown that Il33-deficient mice are highly susceptible to colitis and CAC. Mechanistically, we observed that IL-33 promoted IgA production from B cells, which is important for maintaining microbial homeostasis in the intestine. Il33-deficient mice developed a dysbiotic microbiota that was characterized by increased levels of mucolytic and colitogenic bacteria. In response to chemically induced colitis, this microbial landscape promoted the release of IL-1α, which acted as a critical driver of colitis and CAC. Consequently, reconstitution of symbiotic microbiota or IL-1α ablation markedly ameliorated colitis susceptibility in Il33-deficient animals. Our results demonstrate that IL-33 promotes IgA production to maintain gut microbial homoeostasis and restrain IL-1α-dependent colitis and CAC. This study therefore highlights modulation of IL-33, IgA, IL-1α, and the microbiota as a potential therapeutic approach in the treatment of IBD and CAC.

NLRC3 regulates cellular proliferation and apoptosis to attenuate the development of colorectal cancer

ABSTRACT Nucleotide-binding domain, leucine-rich-repeat–containing proteins (NLRs) are intracellular innate immune sensors of pathogen-associated and damage-associated molecular patterns. NLRs regulate diverse biologic processes such as inflammatory responses, cell proliferation and death, and gut microbiota to attenuate tumorigenesis. In a recent publication in Nature, we identified NLRC3 as a negative regulator of PI3K–mTOR signaling and characterized its potential tumor suppressor function. Enterocytes lacking NLRC3 cannot control cellular proliferation because they are unable to suppress activation of PI3K–mTOR signaling pathways. In this Extra-View, we explore possible mechanisms through which NLRC3 regulates cellular proliferation and cell death. Besides interacting with PI3K, NLRC3 associates with TRAF6 and mTOR, confirming our recent finding that NLRC3 negatively regulates the PI3K–mTOR axis. Herein, we show that NLRC3 suppresses c-Myc expression and activation of PI3K–AKT targets FoxO3a and FoxO1 in the colon of Nlrc3−/− mice, suggesting that additional signaling pathways contribute to increased cellular proliferation. Moreover, NLRC3 suppresses colorectal tumorigenesis by promoting cellular apoptosis. Genes encoding intestinal stem cell markers BMI1 and OLFM4 are upregulated in the colon of Nlrc3−/− mice. Herein, we discuss recent findings and explore mechanisms through which NLRC3 regulates PI3K–mTOR signaling. Our studies highlight the therapeutic potential of modulating NLRC3 to prevent and treat cancer.

Cutting Edge: Distinct Regulatory Mechanisms Control Proinflammatory Cytokines IL-18 and IL-1β

Interleukin-18 and IL-1β, which are cytokines of the IL-1 family, are synthesized as precursor proteins and activated by the inflammasome via proteolytic processing. IL-1β is only induced in response to inflammatory stimuli, but IL-18 is constitutively expressed. However, how IL-18 and IL-1β expression is regulated by different inflammatory signals remains poorly studied. In this study, we found that IL-18 and IL-1β are differentially regulated. Despite being constitutively expressed, IL-18 expression was increased and sustained after stimulation of TLRs. In contrast, IL-1β was induced but not sustained after chronic treatment. Furthermore, type I IFN signaling was essential for induction of IL-18 and macrophages lacking type I IFN signaling were impaired in their ability to promote IL-18 induction. Thus, our findings reveal a fundamental difference in IL-18 and IL-1β regulation and uncover novel mechanisms that are relevant to the inflammatory settings where these proinflammatory cytokines play a critical role.

DOCK2 confers immunity and intestinal colonization resistance to Citrobacter rodentium infection

Food poisoning is one of the leading causes of morbidity and mortality in the world. Citrobacter rodentium is an enteric pathogen which attaches itself to enterocytes and induces attachment and effacing (A/E) lesions. The ability of the bacterium to cause infection requires subversion of the host actin cytoskeleton. Rac-dependent actin polymerization is activated by a guanine nucleotide exchange factor known as Dedicator of cytokinesis 2 (DOCK2). However, the role of DOCK2 in infectious disease is largely unexplored. Here, we found that mice lacking DOCK2 were susceptible to C. rodentium infection. These mice harbored increased levels of C. rodentium bacteria, showed more pronounced weight loss and inflammation-associated pathology, and were prone to bacterial dissemination to the systemic organs compared with wild-type mice. We found that mice lacking DOCK2 were more susceptible to C. rodentium attachment to intestinal epithelial cells. Therefore, our results underscored an important role of DOCK2 for gastrointestinal immunity to C. rodentium infection.