ABSTRACT
Pro-carcinogenic bacteria have the potential to initiate and/or promote colon cancer, in part via immune mechanisms that are incompletely understood. Using ApcMin mice colonized with the human pathobiont enterotoxigenic Bacteroides fragilis (ETBF) as a model of microbe-induced colon tumorigenesis, we show that the Bacteroides fragilis toxin (BFT) triggers a pro-carcinogenic, multi-step inflammatory cascade requiring IL-17R, NF-κB, and Stat3 signaling in colonic epithelial cells (CECs). Although necessary, Stat3 activation in CECs is not sufficient to trigger ETBF colon tumorigenesis. Notably, IL-17-dependent NF-κB activation in CECs induces a proximal to distal mucosal gradient of C-X-C chemokines, including CXCL1, that mediates the recruitment of CXCR2-expressing polymorphonuclear immature myeloid cells with parallel onset of ETBF-mediated distal colon tumorigenesis. Thus, BFT induces a pro-carcinogenic signaling relay from the CEC to a mucosal Th17 response that results in selective NF-κB activation in distal colon CECs, which collectively triggers myeloid-cell-dependent distal colon tumorigenesis.
Subject(s)
Bacterial Toxins/immunology , Bacteroides fragilis/immunology , Carcinogenesis/pathology , Colon/immunology , Colorectal Neoplasms/etiology , Epithelial Cells/immunology , Interleukin-17/immunology , Metalloendopeptidases/immunology , Transcription Factor RelA/metabolism , Adenomatous Polyposis Coli Protein/genetics , Animals , Bacterial Toxins/metabolism , Bacteroides fragilis/pathogenicity , Cell Line, Tumor , Colon/cytology , Colon/microbiology , Colorectal Neoplasms/microbiology , Colorectal Neoplasms/pathology , Enzyme Activation/immunology , Female , Gene Deletion , HT29 Cells , Humans , Inflammation/immunology , Inflammation/microbiology , Interleukin-17/genetics , Male , Metalloendopeptidases/metabolism , Mice , Mice, Inbred C57BL , Mice, Knockout , Myeloid Cells/immunology , Receptors, Interleukin-17/genetics , Receptors, Interleukin-17/immunology , Receptors, Interleukin-8B/genetics , STAT3 Transcription Factor/metabolismABSTRACT
BACKGROUND: Chronic inflammation and composition of the colon microbiota have been associated with colorectal cancer in humans. The human commensal enterotoxigenic Bacteroides fragilis (ETBF) is linked to both inflammatory bowel disease and colorectal cancer and, in our murine model, causes interleukin 17A (IL-17A)-dependent colon tumors. In these studies, we hypothesized that persistent colonization by ETBF is required for tumorigenesis. METHODS: We established a method for clearing ETBF in mice, using the antibiotic cefoxitin. Multiple intestinal neoplasia mice were colonized with ETBF for the experiment duration or were cleared of infection after 5 or 14 days. Gross tumors and/or microadenomas were then evaluated. In parallel, IL-17A expression was evaluated in wild-type littermates. RESULTS: Cefoxitin treatment resulted in complete and durable clearance of ETBF colonization. We observed a stepwise increase in median colon tumor numbers as the duration of ETBF colonization increased before cefoxitin treatment. ETBF eradication also significantly decreased mucosal IL-17A expression. CONCLUSIONS: The timing of ETBF clearance profoundly influences colon adenoma formation, defining a period during which the colon is susceptible to IL-17A-dependent tumorigenesis in this murine model. This model system can be used to study the microbiota-dependent and molecular mechanisms contributing to IL-17A-dependent colon tumor initiation.
