UC San Diego

During metastasis, carcinoma cells acquire invasive abilities to invade locally and to spread to distant organs. The ability to degrade extracellular matrix (ECM) to breakthrough the basement membrane, undergo local invasion, intravasation, and extravasation is crucial for tumor cells to metastasize. Tumor cells acquire ECM degradation ability through the formation of membrane protrusions termed invadopodia to localize various proteases to cell-matrix contact points. Invadopodia are specialized actin-rich cell protrusions that contain a number of actin regulatory proteins including cortactin, Arp2/3, and N-WASP. The matrix degradation activity of invadopodia has been associated with a number of proteases, including MMPs and membrane type MMPs. Inhibiting essential invadopodial components, including cortactin, Tks5, or MT1-MMP, inhibited breast cancer metastasis in vivo, supporting a critical role for invadopodia in tumor metastasis. Our lab previously discovered transcription factor TWIST1 promotes tumor metastasis by inducing invadopodia-mediated matrix degradation and tumor invasion. Additionally, TWIST1 induces the expression of Fibroblast Activation Protein (FAP), a membrane-associated serine protease that have been associated with invadopodia. Furthermore, FAP is essential for focal ECM degradation and is required for TWIST1-mediated breast cancer metastasis in vivo, however, the protease activity of FAP is not required for regulating invadopodia function. This dissertation characterized the role of FAP in invadopodia function and metastasis in two new cell lines and found that FAP deficient cells have less percentage of cells forming invadopodia. Further study suggests that FAP is not required for invadopodia initiation but may be required for invadopodia stabilization. To better understand the role of FAP at invadopodia, an immunoprecipitation followed by mass spectrometry was performed to identify FAP interacting proteins. We found FNDC3B, a transient interactor of FAP, that localizes to invadopodia and regulates invadopodia function. Despite FNDC3B deficient cells sharing similar degradation defect as that of FAP, FNDC3B is not required for invadopodia formation. The findings suggest FAP and FNDC3B have separate functions at invadopodia. We believe that FNDC3B helps promote MT1-MMP vesicle transport to invadopodia while FAP proper MT1-MMP localization once it is at invadopodia. Further studies must be undertaken to better understand the dynamics of FAP and FNDC3B at invadopodia.