Metastasis, the spread of cancer cells from a primary tumor to distant sites within the body, is the leading cause of mortality among cancer patients. Understanding what causes cells to move out of a tumor and into new tissues is a fundamental unanswered question in cancer biology. A hallmark trait of metastatic cancer cells is their enhanced sensitivity to growth factors and increased cell movement. It was recently shown that metastatic cancer cells that migrated out of a primary tumor in response to Epidermal Growth Factor (EGF) had increased expression levels of Mena, one of three members of the Ena/VASP family. This family of proteins is known to directly regulate the polymerization of actin filaments change the shape and dynamics of membrane protrusions, and influence cell movement. The overall goal of the current proposal is to study how Mena function contributes to carcinoma cell invasiveness. A potential mechanism for the enhanced EGF response observed in cells with increased Mena levels was suggested by the recent discovery in our lab that Mena is able to bind certain proteins when they contain phosphorylated tyrosine residues. Growth factors activate kinase signaling cascades that phosphorylate large numbers of downstream targets;if a subset of these phosphorylation events modify or create Mena binding sites, they could modulate Mena activity, leading to changes in actin network assembly and cell behavior. Thus, Mena is ideally situated to translate signals from growth factor signaling pathways into changes in cell shape and movement. One major goal of this proposal is to study the molecular mechanism underlying the ability of Mena to bind phosphorylated ligands. Phosphorylation is a critical regulator of protein-protein interactions, and so the study of how phosphorylation regulates the interaction of Mena with other proteins is particularly relevant to understanding how chemotactic cues stimulate tumor cell movement. A second goal for the current proposal will be to identify proteins whose association with Mena is regulated by phosphorylation. Recent studies have demonstrated that metastatic cancer cells have elevated Mena expression levels when compared to non-metastatic tumor cells, and that invasive cancer cells are more responsive to growth factors such as EGF. While much work has been done to identify the target proteins that are phosphorylated in response to growth factor stimulation, we currently do not know which of these phosphorylated proteins interacts with Mena. Identifying Mena ligands that are bound in a phosphorylation-dependent manner will offer insight into how chemical signals produce changes in cell motility. To investigate this possibility, we plan on studying the interaction of Mena with cellular receptors to determine if ligand phosphorylation promotes or inhibits carcinoma metastasis. These studies will provide valuable insight into the mechanisms regulating cell motility in metastatic cancer cells.
Drugs that block EGF signaling are widely used to treat a variety of cancers, and function by inhibiting protein phosphorylation. By identifying which proteins interact with Mena in a phosphorylation-dependent manner, valuable insight into how certain cancers become resistant to therapies that attenuate EGFR signaling will be gained.