A nucleotide analog prevents colitis associated cancer via Β-catenin independently of inflammation and autophagy.

Abstract: BACKGROUND & AIMS Chronic bowel inflammation increases the risk of colon cancer: colitis associated cancer (CAC). Thiopurine use is associated with a reduction in dysplasia and CAC in inflammatory bowel disease (IBD). Abnormal Wnt/β-catenin signalling is characteristic of >90% of colorectal cancers. Immunosuppression by thiopurines is via Rac1 GTPase, which also affects Wnt/β-catenin signalling. Autophagy is implicated in colonic tumours, 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 harbouring intestinal epithelial cell (IEC)-specific deletion of autophagy related 7 gene (Atg7ΔIEC). TG or vehicle were administered intrarectally, and the effect on tumour burden and β-catenin activity assessed. The mechanisms of action of TG were investigated in vitro and in vivo. RESULTS TG ameliorated DSS colitis in wild type (WT) but not Atg7ΔIEC mice, demonstrating that anti-inflammatory effects of locally delivered TG are autophagy-dependent. However, TG inhibited CAC in both WT and Atg7ΔIEC mice. This was associated with decreased β-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 demonstrated by siRNA knock-down and overexpression of constitutively active Rac1. CONCLUSIONS Our findings provide evidence for a new mechanism that could be exploited to improve CAC chemoprophylactic approaches.