We have demonstrated that Wnt signaling is not only crucial for macrophage-induced malignant invasion in the primary tumor, but also for colonization of distant tissues by regulating the interactions of tumor cells with microglia, the resident brain macrophages. We have characterized a new type of microglia-assisted invasion, dependent on alternative Wnt signaling. This is inhibited by the Toll-like receptor (TLR)-4 agonist bacterial lipopolysacharide (LPS), suggesting crosstalk between these pathways. Now we plan to further dissect the involved Wnt cascades mediating the pro-invasive effect of different types of macrophages (peripheral blood- or bone marrow-derived, resident) by use of Wnt inhibitors, modulators of the sub-pathways and reporter assays. In vitro findings will be confirmed by immunohistochemistry of human brain metastases and in a newly established brain slice coculture system using different mouse models (Wnt 5a +/-, Wnt 5a -/-, Wnt 6 -/-). The in vivo relevance of these results will be analyzed in a syngeneic murine model of brain metastasis by intracerebral inoculation of breast cancer cells and modulation of metastatic progression via local injection of Wnt and other (ant)agonists as well as coinjection of ex vivo manipulated migroglia. We will also investigate the links between Wnt and TLR signaling by characterizing common targets, such as the chemokine receptor CXCR4, identified in our microglia gene expression data. This will further clarify the antiinvasive function of TLR signaling and may provide a target for antimetastatic strategies.
