Chemotaxis of tumour cells and stromal cells in the surrounding microenvironment is an essential component of tumour dissemination during progression and metastasis. This Review summarizes how chemotaxis directs the different behaviours of tumour cells and stromal cells in vivo, how molecular pathways regulate chemotaxis in tumour cells and how chemotaxis choreographs cell behaviour to shape the tumour microenvironment and to determine metastatic spread. The central importance of chemotaxis in cancer progression is highlighted by discussion of the use of chemotaxis as a prognostic marker, a treatment end point and a target of therapeutic intervention.

To examine the role of breast cancer stem cells (BCSCs) in metastasis, we generated human-in-mouse breast cancer orthotopic models using patient tumor specimens, labeled with optical reporter fusion genes. These models recapitulate human cancer features not captured with previous models, including spontaneous metastasis in particular, and provide a useful platform for studies of breast tumor initiation and progression. With noninvasive imaging approaches, as few as 10 cells of stably labeled BCSCs could be tracked in vivo, enabling studies of early tumor growth and spontaneous metastasis. These advances in BCSC imaging revealed that CD44 + cells from both primary tumors and lung metastases are highly enriched for tumor-initiating cells. Our metastatic cancer models, combined with noninvasive imaging techniques, constitute an integrated approach that could be applied to dissect the molecular mechanisms underlying the dissemination of metastatic CSCs (MCSCs) and to explore therapeutic strategies targeting MCSCs in general or to evaluate individual patient tumor cells and predict response to therapy.breast cancer | human-in-mouse cancer models | fused optical reporters | bioluminescence imaging C ancer stem cells (CSCs) were first identified in human leukemia (1, 2) and exhibited capacity to form tumors in immunodeficient mice. Because CSCs are characterized from various types of cancers, CD44 has been a useful marker for enriching CSCs not only for breast tumors but also a variety of other epithelial tumor models (3-17). We and others have previously reported that CSCs are more resistant to traditional cancer therapies (4,18,19). There is circumstantial evidence that CSCs may be involved in metastasis of solid tumors, including breast cancer. Breast CSCs (BCSCs) possess an "invasiveness" gene signature that correlates with poor overall survival and shortened metastasis-free survival in cancer patients (20). Importantly, BCSCs are enriched for cells that can undergo epithelial-mesenchymal cell transition (EMT), which likely plays a critical role in metastases in at least some tumors (21). The observation that microRNAs in normal breast stem cells and BCSCs can regulate both EMT and self-renewal further suggests that CSCs might somehow play a role in metastasis (22). Nonetheless, there remains uncertainty surrounding the contributions of CSCs to metastasis.Understanding the role of CSCs in metastasis requires a reliable, noninvasive measure of BCSC outgrowth and dissemination in representative and predictive models of human metastatic disease. Because of genetic differences in mouse tumors or genetic changes that occur with establishment of cell lines, the commonly used models to study metastases, including those involving human cancer cell lines, mouse tumor models, and/or metastatic tumor models via bloodstream injections, do not fully recapitulate human disease (9,(23)(24)(25). Here, by implanting patient tumors or BCSCs into mouse mammary fat pads and using noninvasive imaging strategies, we established represen...

Circulating tumor cells (CTCs) seed cancer metastases; however, the underlying cellular and molecular mechanisms remain unclear. CTC clusters were less frequently detected but more metastatic than single CTCs of triple negative breast cancer patients and representative patient-derived-xenograft (PDX) models. Using intravital multiphoton microscopic imaging, we found that clustered tumor cells in migration and circulation resulted from aggregation of individual tumor cells rather than collective migration and cohesive shedding. Aggregated tumor cells exhibited enriched expression of the breast cancer stem cell marker CD44 and promoted tumorigenesis and polyclonal metastasis. Depletion of CD44 effectively prevented tumor cell aggregation and decreased PAK2 levels. The intercellular CD44-CD44 homophilic interactions directed multicellular aggregation, requiring its N-terminal domain, and initiated CD44-PAK2 interactions for further activation of FAK signaling. Our studies highlight that CD44+ CTC clusters, whose presence is correlated with a poor prognosis of breast cancer patients, can serve as novel therapeutic targets of polyclonal metastasis.

Colony-stimulating factor-1 (CSF-1) and its receptor (CSF-1R) have been implicated in the pathogenesis and progression of various types of cancer, including breast cancer. This is based on high levels of circulating CSF-1 in patient sera with aggressive disease and increased CSF-1R staining in the tumor tissues. However, there have been no direct in vivo studies to determine whether a CSF-1 autocrine signaling loop functions in human breast cancer cells in vivo and whether it contributes to invasion. Recently, in mouse and rat models, it has been shown that invasion and metastasis are driven by an epidermal growth factor (EGF)/CSF-1 paracrine loop between tumor cells and host macrophages. In this macrophagedependent invasion, tumor cells secrete CSF-1 and sense EGF, whereas the macrophages secrete EGF and sense CSF-1. Here, we test the hypothesis that in human breast tumors, the expression of both the CSF-1 ligand and its receptor in tumor cells leads to a CSF-1/CSF-1R autocrine loop which contributes to the aggressive phenotype of human breast tumors. Using MDA-MB-231 cell-derived mammary tumors in severe combined immunodeficiency mice, we show here for the first time in vivo that invasion in a human mammary tumor model is dependent on both paracrine signaling with host macrophages as well as autocrine signaling involving the tumor cells themselves. In particular, we show that the autocrine contribution to invasion is specifically amplified in vivo through a tumor microenvironment-induced upregulation of CSF-1R expression via the transforming growth factor-β1. [Cancer Res 2009;69(24):9498-506]