Alterations in the cadherin/catenin/LEF adhesion and signaling system are common in many cancers. LEF/TCF-regulated genes are also important in embryogenesis. Clearly, factors that regulate this pathway are likely to be important in the broad areas of development and carcinogenesis. Preliminary results demonstrate a direct role for retinoic acid (RA) in the regulation of beta-catenin/LEF signaling. Retinoic acid receptor alpha (RAR-alpha) but not retinoid X-receptor alpha (RXR-alpha) interacts directly with beta-catenin in the presence of RA to inhibit beta-catenin/TCF/LEF signaling. Beta-catenin can also act as a co-activator for RAR-regulated genes.
The first aim i s directed at a detailed investigation of the molecular basis of beta-catenin-RAR interactions. Specifically we will directly test the role of one or more of the five LXXLL motifs found in the beta-catenin armadillo repeats and the activation function-2 (AF-2) domain of RAR-alpha.
In aim 2 we propose to study the molecular mechanisms of beta-catenin/retinoid cross regulation and will investigate the role of common coactivators and repressors, and of wnt- 1. Chromatin immunoprecipitation and mass spectroscopy will be used to identify components of the endogenous beta-catenin/RAR complexes. The newly discovered oncogenic effects of beta-catenin/TCF signaling and the well known cancer preventive actions of RA clearly establish the clinical significance of these findings but do not show that the interaction is important in the natural history of neoplasia or in embryogenesis.
Our third aim i s to directly investigate the significance of beta-catenin and retinoid pathway cross-regulation in developmental and cancer model systems. To do this we will first test the effects of RA on beta-catenin-induced organizer gene expression and axis duplication in Xenopus. Secondly we will investigate the role of RA on beta-catenin regulated cyclin D1 expression and growth of colon cancer cells. In both models we will elucidate the relative contribution of RA-mediated inhibition of AP-1 and beta-catenin/TCF activation, RA mediated stimulation of RAR-responsive genes and co-activator and co-repressor competition. Thirdly, we will investigate the effects of RA on the incidence and growth of breast tumors in a new transgenic animal model in which beta-catenin is over expressed in the mammary gland. In general the work proposed tests the molecular mechanisms, and significance of, cross regulation of beta-catenin/TCF retinoid signaling pathways.
