A Nucleotide Analog Prevents Colitis-Associated Cancer via Beta-Catenin Independently of Inflammation and Autophagy

Colitis increases the risk of bowel cancer. We show in mouse models that a thioguanine nucleotide can inhibit Wnl/bcalenin signaling via Rac1 in colonic epithelial cells to prevent carcinogenesis. The novel mechanism is independent of inflammation and autophagy. BACKGROUND & AIMS: Chronic bowel inflammation increases the risk of colon cancer; colitis-associated cancer (CAC). Thiopurine treatments are associated with a reduction in dysplasia and CAC in inflammatory bowel disease (IBD). Abnormal Wnt/beta-catenin signalling is characteristic of >90% of colorectal cancers. Immunosuppression by thiopurines is via Rac1 GTPase, which also affects Wnt/beta-catenin signalling. Autophagy is implicated in colonic tumors, and topical delivery of the thiopurine thioguanine (TG) is known to alleviate colitis and augment autophagy. This study investigated the effects of TG in a murine model of CAC and potential mechanisms. METHODS: Colonic dysplasia was induced by exposure to azoxymethane (AOM) and dextran sodium sulfate (DSS) in wild-type (WT) mice and mice harboring intestinal epithelial cell-specific deletion of autophagy related 7 gene (Atg7(Delta IEC)). TG or vehicle was administered intrarectally, and the effect on tumor burden and beta-catenin activity was assessed. The mechanisms of action of TG were investigated in vitro and in vivo. RESULTS: TG ameliorated DSS colitis in wild-type but not Atg7(Delta IEC) mice, demonstrating that anti-inflammatory effects of locally delivered TG are autophagy-dependent. However, TG inhibited CAC in both wild-type and Atg7(Delta IEC) mice. This was associated with decreased beta-catenin activation/nuclear translocation demonstrating that TG's inhibition of tumorigenesis occurred independently of anti-inflammatory and pro-autophagic actions. These results were confirmed in cell lines, and the dependency on Rac1 GTPase was demonstrated by siRNA knockdown and overexpression of constitutively active Rac1. CONCLUSIONS: Our findings provide evidence for a new mechanism that could be exploited to improve CAC chemo-prophylactic approaches.

Automated summary

The study demonstrates that thioguanine nucleotide can inhibit Wnt/beta-catenin signaling via Rac1 to prevent carcinogenesis in colonic epithelial cells, independently of inflammation and autophagy.