| Tumor metastasis is the leading cause of death among cancer patients. For breast cancers, metastases can develop years after primary tumor resection. While much remains to be understood regarding the molecular mechanisms that influence metastatic progression, evidence indicate that a potential target for anti-metastatic drug development is circulating tumor cells. The cytoskeletal organization of detached and circulating tumor cells is currently not well-defined; however, limited evidence reveal that it is dynamically distinct from that of attached cells. Consequently, the cytoskeleton of circulating tumor cells may provide targets for new therapies that limit metastatic tumor spread. In vivo, circulating tumor cells reattach in distant tissues via a mechanism that is tubulin-dependent and suppressed by polymerized actin. The cytoskeletal mechanisms that promote reattachment of circulating tumor cells match exactly with the mechanisms supporting tubulin microtentacles, which have been identified recently in detached breast tumor cells. Vimentin and actin cortical integrity greatly influence microtentacle formation. To test how microtubule-associated proteins affect formation of microtentacles, tau was chosen, as it has been shown to be expressed in a subset of chemotherapy-resistant breast cancers. By blindly evaluating microtentacle formation in a panel of breast tumor cell lines, it was observed that microtentacle frequency directly correlates with tau expression. Additionally, immunofluorescence and confocal microscopy revealed that tau localizes to these microtentacles. These data led to the hypothesis that tau stimulates microtentacle formation and increases the metastatic potential of detached breast tumor cells. Ectopic-expression and shRNA-mediated knockdown demonstrated that tau is both necessary and sufficient to promote the extension of microtentacles in detached breast tumor cells. When monitoring cell-substratum attachment using a real-time cell sensing device, tau-induced microtentacles increased the reattachment of suspended cells. In vivo, tau-induced microtentacles allow greater trapping and retention of cells in the lungs during experimental metastasis assays. Immunohistochemical analysis of matched primary and metastatic tumors from 102 breast cancer patients demonstrated that tau expression increases significantly during tumor metastasis. Tau enrichment in metastatic tumors and the ability of tau to promote tumor cell reattachment through microtentacle formation support a model in which tau-induced microtubule stabilization provides a selective advantage during tumor metastasis. |
