Colorectal cancer: the facts in the case of the microbiota

Abstract

The importance of the microbiota in the development of colorectal cancer (CRC) is increasingly evident, but identifying specific microbial features that influence CRC initiation and progression remains a central task for investigators. Studies determining the microbial mechanisms that directly contribute to CRC development or progression are revealing bacterial factors such as toxins that contribute to colorectal carcinogenesis. However, even when investigators have identified bacteria that express toxins, questions remain about the host determinants of a toxin's cancer-potentiating effects. For other cancer-correlating bacteria that lack toxins, the challenge is to define cancer-relevant virulence factors. Herein, we evaluate three CRC-correlating bacteria, colibactin-producing Escherichia coli, enterotoxigenic Bacteroides fragilis, and Fusobacterium nucleatum, for their virulence features relevant to CRC. We also consider the beneficial bioactivity of gut microbes by highlighting a microbial metabolite that may enhance CRC antitumor immunity. In doing so, we aim to elucidate unique and shared mechanisms underlying the microbiota's contributions to CRC and to accelerate investigation from target validation to CRC therapeutic discovery.

Figure 1. Potential mutagenic effects of pks⁺ E. coli.

E. coli strains can produce harmful toxins. Top right: The polyketide synthase (pks) island encodes the genes required for the synthesis of colibactin, a well-known genotoxin. Recent studies showed that polyphosphate kinase (PPK) activity is essential for ClbB function and colibactin metabolism. The ulcerative colitis medication mesalamine reduces PPK activity and colibactin production. Bottom right: Colibactin binding to DNA forms DNA cross-links and interstrand breaks that dysregulate cell division and increase mutagenesis. Importantly, a colibactin-specific mutational signature, characterized by single-base substitutions, deletions, and insertions at T sites, is enriched in CRC. indel, insertion-deletion; del, deletion of nucleotide; ins, insertion of nucleotide.

Figure 2. Enterotoxigenic Bacteroides fragilis promotes tumorigenesis by distinct mechanisms.

(A) B. fragilis toxins (BFTs) activate the Ras/mTOR and p38 mitogen-activated protein kinase (p38) intracellular signaling pathways. BFTs induce inhibitor of apoptosis protein-2 (IAP2) expression, resulting in increased tumor growth and inhibition of apoptosis. BFTs also increase intestinal cell proliferation and permeability by inducing c-myc expression after E-cadherin cleavage and β-catenin nuclear localization, in a process that was recently shown to involve G protein–coupled receptor 35 (GPR35). (B) Enterotoxigenic B. fragilis (ETBF) promotes epigenetic alterations with the potential to cause DNA damage by inducing DNA methyltransferase 1 (DNMT1) recruitment and inducing JmjC domain–containing histone demethylase 2B (JMJD2B) in CRC cells. ETBF-produced BFTs also induce DNA damage by increasing ROS generation. (C) ETBF and BFTs induce a proinflammatory environment that contributes to carcinogenesis. BFTs induce activation of the transcription factors STAT3 and NF-κB, increasing intestinal permeability and production of inflammatory cytokines. In a multistep process, ETBF induces phosphorylation (“P” in yellow circles) of STAT3 and IL-17–producing Th17 and γδ T cells. Both processes promote the recruitment of pro-tumorigenic myeloid cells that suppress cytotoxic antitumor immunity.

Figure 3. Potential mechanisms of Fusobacterium nucleatum activity in CRC.

(A) Fusobacterium adhesin A (FadA) binding to E-cadherin increases β-catenin and WNT signaling and upregulates annexin A1 that drives epithelial cell proliferation. FadA also has amyloid-like properties that enhance F. nucleatum (Fn) adhesion to cancer cells. (B) Fusobacterium autotransporter protein 2 (Fap2) binds d-galactose-β(1-3)-N-acetyl-d-galactosamine (Gal-GalNAc) on cancer cells and recruits Fn to tumors. Fap2 also binds to T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT) and impairs T and NK cell function, reduces cytotoxicity, and promotes immune cell death, resulting in tumor escape from immunosurveillance. Fap2⁺ Fn activates epithelial and myeloid cells and induces a pro-tumorigenic inflammatory response. (C) Fn LPS induces the expression of microRNA-21 in colon epithelial cells in a TLR4-dependent manner, which results in dysregulated cell proliferation and tumor growth. This same pathway also increases cancer cell autophagy, which enhances resistance to chemotherapy-induced cell death.