P124 THE UBIQUITIN EDITING ENZYME A20 LINKS INFLAMMATION AND COLITIS ASSOCIATED CANCER

Abstract

Inflammatory bowel disease (IBD) patients are at increased risk of colorectal cancer (CRC)1. Tumor initiating events in colitis-associated cancers (CACs) differ significantly from sporadic CRC2. In CACs, hyperactivity of wnt/beta-catenin signaling typically occur without adenomatous polyposis coli gene mutations commonly seen in sporadic colon cancers. This suggests distinct mechanisms cause dysregulation of wnt signaling in CACs. A20 (TNFAIP3) is a ubiquitin-editing enzyme, linked to IBD, with roles in dampening inflammation3. We found that A20 is a tumor suppressor in the murine colon directly inhibiting beta-catenin signaling4. We now report regulation of wnt signaling by A20 depends on interaction with receptor interacting protein kinase 4 (RIPK4). Interestingly, A20’s function downstream of inflammatory signaling depends upon the closely related proteins RIPK1 and RIPK2 suggesting a conserved interaction between A20 and this family of proteins. Using a mammalian yeast-2-hybrid system we found A20 and RIPK4 interact (Fig. 1A). This association was confirmed via immunoprecipitation using endogenous proteins and was dependent upon wnt stimulation (Fig. 1B). Using a beta-catenin luciferase reporter assay, we showed siRNA knockdown of RIPK4 partially abrogates enhanced wnt signaling in absence of A20 (Fig. 1C), suggesting A20 negatively regulates wnt signaling via RIPK4. Mechanistically, we found A20 normally supports K48-linked ubiquitination of RIPK4 (Fig. 1D). We then determined whether A20 plays a role in regulating CACs with the AOM-DSS model of CAC in mice with a targeted deletion of A20 in the intestinal epithelium. Preliminary data suggests A20FL/FL Villin-Cre mice treated with AOM-DSS develop larger tumors than control mice (Figs. 2A,2B). Overall, we propose that A20 restricts CACs by inhibiting the activity of RIPK4. 1. Gut. 2001;48(4):526–35. 2. Cancer Res. 1995;55(10):2035–38. 3. Nat. Rev. Immunol. 2012;12:774–85. 4. PLoS One. 2013;8(5):e62223.