A novel modulator of cellular invasion and metastasis in endocrine cancer

Metastasis is a multistep process responsible for the majority of endocrine cancer deaths. Central to the ability of cells to move is the recruitment of actin fibres at the periphery of the cell by key proteins, especially the cortical actin binding protein cortactin. A full understanding of cortactin function is required in order to address metastatic cell activity within endocrine cancer. We used IP-MS to discover protein binding partners, and now identify the proto-oncogene PBF as a new functional binding partner of cortactin, whose expression has recently been correlated with thyroid and breast cancer metastasis, and with colon cancer extra-mural vascular invasion. We show that cortactin and PBF interact and co-localise through immunofluorescence and Proximity Ligation Assays, and that this occurs within or close to the plasma membrane, and preferentially at the leading edge of migrating cells. Oncogenic expression of PBF induced potent cell invasion and migration in thyroid TPC-1 (P=0.01), breast MCF-7 (P<0.001) and colorectal HCT116 cells (P<0.001), which was entirely abrogated by the knockdown of cortactin expression. In n=43 matched papillary thyroid cancers, cortactin was significantly upregulated at the mRNA (P=0.022) and protein (P=0.045) levels, particularly in more aggressive tumours, and significantly correlated with PBF expression. We also demonstrate the interaction between PBF and cortactin through co-immunoprecipitation assays and reveal that artificially targeting PBF to the plasma membrane results in increased cortactin binding, entirely blocking endogenous cellular invasion. Thus, we identify a new modulator of cortactin function, and show for the first time that cortactin is over-expressed in differentiated thyroid cancer. We propose that modulation of PBF subcellular localisation may present a novel mechanism of addressing in vivo tumour cell invasion and migration.