Dissecting the early steps of the metastatic cascade using pharmacologic and imaging approaches
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The early events in the metastatic cascade, the invasion of the surrounding stroma, interactions of tumor cells with stromal cells, motility and intravasation, present researchers with an opportunity for identifying novel targets for cancer treatment. In this study, we used pharmacologic and imaging approaches to understand the properties of tumor cells in an intact orthotopic carcinoma tumor. The ErbB family of receptor-tyrosine kinases, which for years has been linked to poor prognosis of cancer patients, has been found to contribute to motility, invasion and intravasation in mammary adenocarcinomas. However, unique contributions of individual ErbB family members have not been elucidated. We describe the use of tyrosine kinase inhibitors to distinguish the roles of ErbBs in the early events in metastasis. ERBB1 has been found to contribute to the invasive and motile properties of the cells, whereas ERBB2, while contributing to invasion and motility, is also essential for intravasation. We show that the use of clinically available inhibitors for short-term blocking tumor cell invasion and intravasation is feasible. To further improve the imaging, a new approach using an imaging window was taken that would allow for tracking tumor cells over several days, rather than several hours. The imaging window is placed on top of a growing tumor. Combined with the use of photoswitchable proteins, this new mammary tumor imaging window allows for monitoring of motility of individual cells over an extended period of time. We were also able to define two distinct microenvironments in the primary tumor, the vessel-rich microenvironment, where tumor cells are found to migrate and intravasate over the 24 hour imaging period, and a vessel-poor microenvironment, where tumor cells do not migrate as extensively. The use of these complementary approaches to study tumor cell behavior in vivo yielded an improved understanding of the contributions of the tumor cell properties and the tumor microenvironment to the invasion, intravasation and motility of tumor cells.