How beta-catenin expands Foxp3+RORγt+ Pro-inflammatory T-regulatory cells

In human colon cancer and in mice with polyposis T-regulatory cells (Tregs) tend to promote rather than suppress inflammation. This reversal of properties is attributed to the expansion of a Treg subset that expresses the signature transcriptional factor of Th17 cells RORγt. RORγt is a transcriptional target of β-catenin. β-Catenin is abundantly expressed in lamina propria Tregs and is further elevated in Tregs during polyposis. As the central effector of Wnt signaling β-catenin is better known for supporting self renewal of stem and progenitor cells, and for enhancing the survival of double positive (DP) thymocytes in part through transcriptional activation of RORγt. B-catenin has been also implicated both in the survival of extrathymic induced Tregs and in the differentiation of Th17 cells. It is therefore plausible that persistently elevated levels of β-catenin support survival of cells with both Treg and Th17 properties. In agreement with this, we found that induction of high β-catenin levels in DP thymocytes by targeted mutagenesis (in CD4CreCtnnb1ex3 mice) produced Foxp3+ lymphocytes with Th17 characteristics. This, we related to increased DNA accessibility through histone-3 acetylation (H3KAc) in sites proximal to Tcf-1 binding and sustained RORγt expression. The net result was T-cell dependent colitis and cancer. These findings suggest that high β-catenin levels prevent epigenetic closing of a group of Tcf-1 target genes including RORγt and thereby enable their sustained expression. This is in line with the finding that that Tcf/Lef binding motifs tend to lose chromatin accessibility in Tregs, and an earlier suggestion that β-catenin enhances DNA accessibility and activates transcription by binding to Tcf/Lef factors. Based on these observations we propose a model whereby pro-inflammatory cues in polyposis and colon cancer stabilize -catenin in Tregs, which in turn reprograms Treg to become pro-inflammatory by introducing global changes in chromatin landscape and gene expression. This drives a vicious cycle whereby inflammatory cues induce pro-inflammatory Tregs, which in turn support tumor growth. To test this model we will 1: Determine epigenetic and transcriptional mechanisms by which β-catenin renders Tregs pro-inflammatory. We will test the hypothesis that β-catenin enhances H3KAc marks of open chromatin near Tcf-1 sites and thereby controls the expression of key target genes that define Treg properties. 2. Establish the requirement for β-catenin and Tcf-1 in generation of pro-inflammatory Tregs during polyposis. We will test the hypothesis that during polyposis β-catenin is upregulated in Tregs by inflammatory cues and has a non-redundant role in activating RORγt and generating pro-inflammatory Tregs. The proposed studies are expected to reveal the modus operandi of the β-catenin/Tcf-1 pathway in Treg function and to determine how epigenetic mechanisms regulate Treg plasticity in colon cancer. We expect our findings to be paradigm shifting both in terms of the concept of Treg plasticity and therapeutic approaches for control of inflammation in chronic inflammatory bowel diseases and colon cancer.