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

Abstract

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/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 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 ΔIEC ). TG or vehicle was administered intrarectally, and the effect on tumor burden and β-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 ΔIEC mice, demonstrating that anti-inflammatory effects of locally delivered TG are autophagy-dependent. However, TG inhibited CAC in both wild-type and Atg7 ΔIEC 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 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 chemoprophylactic approaches.

Rho GTPases as Key Molecular Players within Intestinal Mucosa and GI Diseases

“…RAC1 appears as an attractive candidate in the context of malignancies. Taking advantage of in vivo experiments, it was shown that thiopurines treatments (TG) inhibited CAC in the AOM/DSS mouse model, via decreased β-catenin in a RAC1-dependent mechanism [ 144 ]. In the context of epithelial cell targeting, EMT and metastatic potential might be inhibited upon activation of RAC1; for instance, upon CSRP2 treatment (Hippo-ERK-PAK/LIMK/cortactin) [ 145 ], miR-142-3p transfection [ 146 ], upregulation of SSH3 (LIMK) [ 147 ] or PLS1 (ERK1/2) [ 148 ], downregulation of DMTN [ 149 ], or IRF1 suppression [ 150 ].…”

The Ugly Duckling of Thiopurines Becomes the Beautiful Swan of Colitis-associated Cancer Management

“…In addition, in vitro silencing experiments showed that TG suppression of b-catenin was dependent on Rac-1 at lower physiological concentrations, whereas it was Rac-1 independent at supraphysiological higher concentrations. 13 As a member of the Ras superfamily of Rho GTPases, GTP-bound Rac1 mediates a plethora of cellular processes including actin reorganization and gene transcription. It is possible that the TG Rac1 interaction may result in other cellular effects independent of b-catenin transcriptional activity, to inhibit CAC development, which warrants further investigation in future work.…”

“…12 Blockade of Rac1 signaling by thiopurines results in suppression of proinflammatory T cell responses, mainly by induction of T cell apoptosis and impairment of synapse formation between T cells and antigen-presenting cells. 9,10 In this issue of Cellular and Molecular Gastroenterology and Hepatology, Sheng et al 13 investigate the role of TG in CRC, exploring how TG might alleviate colitis-associated cancer (CAC). Chronic inflammation that occurs in poorly controlled ulcerative colitis can promote CAC.…”

“…In this issue of Cellular and Molecular Gastroenterology and Hepatology , Sheng et al 13 investigate the role of TG in CRC, exploring how TG might alleviate colitis-associated cancer (CAC). Chronic inflammation that occurs in poorly controlled ulcerative colitis can promote CAC.…”

“…Using in vitro and in vivo approaches, Sheng et al 13 define a pathway, independent from autophagy and inflammation, in which physiological concentrations of TG inhibit Wnt/β-catenin signaling dependent on Rac1, reducing proliferation of epithelial cells and protumorigenic genes transcription, therefore reducing the risk of CAC. The present work sheds light on mechanism of action of this less characterized thiopurine, building on observations of its role in the context of altered microbiome-dependent colitis.…”