ROLE OF THE GUT MICROBIOTA-ARYL HYDROCARBON RECEPTOR AXIS IN COLORECTAL CANCER PROGRESSION

The gut microbiota significantly influences host physiology through the production of bioactive metabolites that interact with hosts. However, the identities and biological activities of the majority of gut bacterial metabolites remain largely unknown. Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that can sense structurally diverse chemicals. AhR regulates the expression of genes involved in multiple physiological processes, including cancer progression. The first chapter reviews the role of AhR in colorectal cancer (CRC), focusing on how bacterial metabolites modulate cancer cell proliferation, metastasis, and immune modulation. The second chapter presents a study identifying Fusobacterium nucleatum (Fn), a bacterium enriched in CRC, as a potent producer of metabolites that activate AhR. Screening a collection of 10 representing human gut bacteria revealed that Fn elicited the strongest AhR activation, driven by three metabolites fusotrisindoline (FTIN), trisindoline (TIN), and streptindole (STIN). These Fn metabolites were characterized as transient AhR agonists in human HepG2 and murine Hepa1c1c7 hepatoma cells, with FTIN as the most potent agonist (EC50 = 43 nM in HepG2 cells). TnaA deletion in Fn abrogated the production of these AhR-activating metabolites without affecting bacterial growth and adhesion to CRC cells. Moreover, the Fn tnaA deletion mutant failed to promote the human CRC SNU-C4 cell proliferation, while wild-type Fn enhanced the cell growth, suggesting the critical role of tnaA-mediated AhR-activating metabolites production of Fn in Fn-mediated CRC cell growth. An analysis of 30 pairs of clinical CRC tissues revealed that FTIN is enriched in tumors compared to the adjacent normal colon tissues across all clinical stages. While Fusobacterium abundance was higher in the tumor compared to the normal colon, FTIN levels were not correlated with Fusobacterium abundance. This study demonstrated that physiological factors such as oxygen level and nutrient availability in the microenvironment synergistically modulate the FTIN production of Fn. Additionally, non-Fusobacterium CRC-associated bacteria also contributed to FTIN production in CRC tissues. These findings indicate that FTIN is a clinically relevant bacterial metabolite and reveal its potential as a CRC diagnostic marker.