Transforming growth factor-beta (TGF-b) plays many roles in growth and development, and aberrant TGF-b signaling contributes to the pathogenesis and progression of various diseases, including the initiation and progression of cancers toward metastasis, and fibrosis. TGF-b signaling induces epithelial cells to acquire invasive, mesenchymal characteristics, a process known as epithelial-mesenchymal transition (EMT). EMT, driven by TGF-b signaling, is now increasingly seen as a key differentiation event leading to the invasive behavior of carcinomas, and contributing to fibrosis. TGF-b signals through a complex of two type I and two type II transmembrane receptor kinases (TbRI and TbRII) at the cell surface, in which TGF-b induces the TbRII receptors to phosphorylate and activate the TbRI receptors. These in turn activate Smads through direct Ser phosphorylation, and Smad signaling leads to changes in gene expression. TGF-b also activates non-Smad pathways including the PI3K-Akt-mTOR and Erk- MAP kinase pathways, key signaling pathways that are induced by receptor tyrosine kinases and are often misregulated in disease. Recently, TbRI was shown to be phosphorylated on tyrosines, which may allow it to activate these non-Smad pathways. In addition, TGF-b receptors also phosphorylate on tyrosines, and thus function as dual specificity kinases, giving them a bigger role in signal activation than just activating Smad signaling. The roles of tyrosine phosphorylation of TbRI in the activation of TGF-b-induced Smad and non- Smad signaling pathways have not been explored. The overall goal of my project is to define the roles of the phosphorylated tyrosines in TbRI in the activation of non-Smad pathways in response to TGF-b, and in the EMT response of epithelial cells to TGF-b.
In Aim 1, I propose to identify the tyrosine phosphorylation sites in TbRI, and, using TbRI mutants, to correlate specific tyrosine phosphorylation sites with their roles in TGF-b-activated non-Smad signaling, and endosomal compartmentalization of the receptors.
In Aim 2, I aim to determine the importance of TbRI tyrosine residues in the cellular response to TGF-b, and in TGF-b-induced EMT. Possible consequences of TbRI mutations on the EMT response to TGF-b will be extended in vivo, through an evaluation of the effects on subcutaneous tumor formation and metastatic lung colonization in a mouse model.
TGF-b is known to activate several signaling pathways in addition to the better known Smad pathway, and these are known to play a significant role in cancer progression. My project aims to understand the roles of phosphorylation of the TGF-b receptor on tyrosines, on the activation of these pathways by TGF-b, and how these in turn affect the response of the cell to TGF-b. A better understanding of how TGF-b regulates these pathways through the function of its receptors will enable us to better define targets for treatment of misregulated TGF-b signaling in cancer and fibrosis.