By some estimates, tumor metastasis is responsible for over 90% of cancer mortality. At the cellular level, epithelial to mesenchymal transition (EMT)is the initiating event in tumor metastasis. We have been investigating the contribution of translational control to TGF-ß-induced EMT in a breast epithelial model. Our preliminary data unambiguously demonstrate that the CELF1 RNA binding protein, most well-known for its role in Type 1 Myotonic Dystrophy, is both necessary and sufficient for EMT in this system. In epithelial cells, the CELF1 protein is actively degraded via the proteasome. However, upon TGF-ß addition and EMT, this protein is stabilized and appears to increase the translation of several target genes, some of which have already been defined as positive regulators of EMT. To our knowledge this is the first data directly demonstrating the role of this gene product in EMT. Our overall objective is to further elucidate the mechanism by which TGF-ß induces EMT via the CELF1 protein. We predict that a more developed understanding of the cellular mechanisms at play in this pathway will reveal enzymatic activities playing critical roles in EMT, and that eventual therapeutic targeting of these activities may be viable therapeutic targets for the prevention of metastasis in early stage cancers. We hope to achieve our objective by testing our core hypothesis that induced stability of CELF1 protein drives EMT via translational activation of distinct downstream effector proteins. We propose doing this via three Specific Aims. First, we will identify the downstream effectors by which CELF1 promotes EMT and tumor metastasis in experimental models via a candidate approach. Using classical biochemical approaches, we will next establish the mechanism by which TGF-ß treatment leads to an increase in CELF1 protein levels. Finally, we will determine to what extent misexpression of CELF1 impacts tumor colonization and metastasis in in vivo models, and survey a broad range of human breast cancers to determine whether CELF1 is misexpressed in these tumors as compared to normal tissue. The proposed work is directly relevant to cancer because EMT is a core mechanism underlying tumor metastasis. Identification of the enzymatic activities by which CELF1 stability and function are controlled in the context of EMT will reveal candidate therapeutic targets for theprevention of metastasis of early-stage cancers. Downstream effectors of the CELF1 protein may also prove to be potential therapeutic targets, but are certain to provide insight into the cellular mechanisms underlying EMT that may be translatable to a broad range of solid tumors.