Entry of tumor cells into the blood stream is a critical step in cancer metastasis. Although significant progress has been made in visualizing tumor cell motility in vivo, the underlying mechanism of cancer cell intravasation remains largely unknown. We developed a microfluidic-based assay to recreate the tumor-vascular interface in three-dimensions, allowing for high resolution, real-time imaging, and precise quantification of endothelial barrier function. Studies are aimed at testing the hypothesis that carcinoma cell intravasation is regulated by biochemical factors from the interacting cells and cellular interactions with macrophages. We developed a method to measure spatially resolved endothelial permeability and show that signaling with macrophages via secretion of tumor necrosis factor alpha results in endothelial barrier impairment. Under these conditions intravasation rates were increased as validated with live imaging. To further investigate tumor-endothelial (TC-EC) signaling, we used highly invasive fibrosarcoma cells and quantified tumor cell migration dynamics and TC-EC interactions under control and perturbed (with tumor necrosis factor alpha) barrier conditions. We found that endothelial barrier impairment was associated with a higher number and faster dynamics of TC-EC interactions, in agreement with our carcinoma intravasation results. Taken together our results provide evidence that the endothelium poses a barrier to tumor cell intravasation that can be regulated by factors present in the tumor microenvironment.T umor-endothelial cell interactions are critical in multiple steps during cancer metastasis, ranging from cancer angiogenesis to colonization. Cancer cell intravasation is a rate-limiting step in metastasis that regulates the number of circulating tumor cells and thus presents high risk for the formation of secondary tumors (1, 2). During the metastatic process tumor cells migrate out of the primary tumor (3), navigate into a complex tumor microenvironment, and enter into blood vessels (4). Cell-cell communication and chemotaxis (5) are key to this process and can occur via paracrine signals and/or direct contact between different cell types during tumor cell invasion (6) and metastatic colonization (7). Studies using multiphoton imaging in animal models have demonstrated that the ability of tumor cells to enter into the blood stream can be controlled both by tumor cell intrinsic factors (8-11) and other cells present in the tumor microenvironment, such as macrophages (12) and neutrophils (13). However, because of the lack of physiologically relevant in vitro models and the challenges of investigating cell-cell interactions in vivo, the underlying mechanism of intravasation remains poorly understood (14). In particular, a number of fundamental questions remain as to whether intravasation is an active or passive process (15) and whether tumor cells cross the endothelial barrier through cell-cell junctions (paracellular) or through the endothelial cell body [transcellular (16)]. Therefor...

A key aspect of cancer metastases is the tendency for specific cancer cells to home to defined subsets of secondary organs. Despite these known tendencies, the underlying mechanisms remain poorly understood. Here we develop a microfluidic 3D in vitro model to analyze organ-specific human breast cancer cell extravasation into bone- and muscle-mimicking microenvironments through a microvascular network concentrically wrapped with mural cells. Extravasation rates and microvasculature permeabilities were significantly different in the bone-mimicking microenvironment compared with unconditioned or myoblast containing matrices. Blocking breast cancer cell A3 adenosine receptors resulted in higher extravasation rates of cancer cells into themyoblast-containingmatrices compared with untreated cells, suggesting a role for adenosine in reducing extravasation. These results demonstrate the efficacy of our model as a drug screening platform and a promising tool to investigate specific molecular pathways involved in cancer biology, with potential applications to personalized medicine

Interstitial flow is the convective transport of fluid through tissue extracellular matrix. This creeping fluid flow has been shown to affect the morphology and migration of cells such as fibroblasts, cancer cells, endothelial cells, and mesenchymal stem cells. A microfluidic cell culture system was designed to apply stable pressure gradients and fluid flow and allow direct visualization of transient responses of cells seeded in a 3D collagen type I scaffold. We used this system to examine the effects of interstitial flow on cancer cell morphology and migration and to extend previous studies showing that interstitial flow increases the metastatic potential of MDA-MB-435S melanoma cells [Shields J, et al. (2007) Cancer Cell 11:526-538]. Using a breast carcinoma line (MDA-MB-231) we also observed cell migration along streamlines in the presence of flow; however, we further demonstrated that the strength of the flow as well as the cell density determined directional bias of migration along the streamline. In particular, we found that cells either at high seeding density or with the CCR-7 receptor inhibited migration against, rather than with the flow. We provide further evidence that CCR7-dependent autologous chemotaxis is the mechanism that leads to migration with the flow, but also demonstrate a competing CCR7-independent mechanism that causes migration against the flow. Data from experiments investigating the effects of cell concentration, interstitial flow rate, receptor activity, and focal adhesion kinase phosphorylation support our hypothesis that the competing stimulus is integrin mediated. This mechanism may play an important role in development of metastatic disease. mechanobiology | computational model | signaling | cell mechanics T issues are composed of cells residing in an extracellular matrix (ECM) containing interstitial fluid that transports nutrients and signaling molecules (1, 2). Osmotic and hydrostatic pressure gradients across tissues resulting from physiologic processes such as drainage toward lymphatics, inflammation, locally elevated pressures due to tumor growth or leaky microvessels, and muscle contraction each drive fluid flow through the ECM (2, 3). This fluid flow is termed interstitial flow and has long been recognized to be instrumental in tissue transport and physiology (1, 4, 5). Chary and Jain used fluorescence recovery after photobleaching to directly observe fluid flow in the tissue interstitium and determined typical flow velocities to be on the order of 0.1-2.0 μm/s, and more recent studies have demonstrated that flow can reach velocities as high as 4.0 μm/s (6, 7).Interstitial flow is particularly important in driving transport in the vicinity of tumors, as neoplastic tissue is often characterized by localized increases in interstitial pressure, leading to high interstitial pressure gradients at the tumor margin (8). Interstitial flow has hence emerged as a possible stimulus for guiding tumor cell migration in the formation of metastases (9-12).Shields et al. observed increase...

Cancer metastases arise following extravasation of circulating tumor cells with certain tumors exhibiting high organ specificity. Here, we developed a 3D microfluidic model to analyze the specificity of human breast cancer metastases to bone, recreating a vascularized bone-mimicking microenvironment with human osteo-differentiated bone marrow-derived mesenchymal stem cells and endothelial cells. The tri-culture system allowed us to study the transendothelial migration of highly metastatic breast cancer cells and to monitor their behavior within the bone-like matrix. Extravasation, quantified 24h after cancer cell injection, was significantly higher in the osteo-cell conditioned microenvironment compared to collagen gel-only matrices (77.5±3.7% vs. 37.6±7.3%), and the migration distance was also significantly greater (50.8±6.2µm vs. 31.8±5.0µm). Extravasated cells proliferated to form micrometastases of various sizes containing 4 to more than 60 cells by day 5. We demonstrated that the breast cancer cell receptor CXCR2 and the bone-secreted chemokine CXCL5 play a major role in the extravasation process, influencing extravasation rate and travelled distance. Our study provides a novel 3D in vitro quantitative data on extravasation and micrometastasis generation of breast cancer cells within a bone-mimicking microenvironment and demonstrates the potential value of microfluidic systems to better understand cancer biology and screen for new therapeutics.

Tumor cells that disseminate from the primary tumor and survive the vascular system can eventually extravasate across the endothelium to metastasize at a secondary site. In this study, we developed a microfluidic system to mimic tumor cell extravasation where cancer cells can transmigrate across an endothelial monolayer into a hydrogel that models the extracellular space. The experimental protocol is optimized to ensure the formation of an intact endothelium prior to the introduction of tumor cells and also to observe tumor cell extravasation by having a suitable tumor seeding density. Extravasation is observed for 38.8% of the tumor cells in contact with the endothelium within 1 day after their introduction. Permeability of the EC monolayer as measured by the diffusion of fluorescently-labeled dextran across the monolayer increased 3.8 fold 24 hours after introducing tumor cells, suggesting that the presence of tumor cells increases endothelial permeability. The percent of tumor cells extravasated remained nearly constant from1 to 3 days after tumor seeding, indicating extravasation in our system generally occurs within the first 24 hours of tumor cell contact with the endothelium.