In vitro model of tumor cell extravasation

Abstract

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.

Figure 1. General schematic of the device.

Microfluidic system consisting of three independently addressable media channels, separated by chambers into which an ECM-mimicking gel can be injected (a). Figure 1b shows the inside view of the device with endothelial monolayer (blue) covering the center channel. This channel acts as cell channel where both endothelial cells and cancer cells are introduced to form monolayer and transmigrate respectively (b). The green region indicates the 3D space filled with collagen gel and the pink regions indicate the channel filled with medium. Cancer cells which adhere to endothelial monolayer can extravasate into the collagen gel region as shown in (c).

Figure 2. Confirmation of endothelial monolayer integrity.

The integrity of the endothelial monolayer was confirmed by both fluorescence imaging of the dextran distribution and confocal microscopy of fixed and labeled cells. An intact endothelial monolayer gives rise to an abrupt intensity drop between the channel and the gel region once the fluorescently-labeled dextran is introduced. Three hours after dextran injection, a sharp drop in fluorescence intensity is seen across the endothelial layer demonstrating its function as a barrier to macromolecules (a). Fluorescence intensity is quantified using Matlab (b). The dashed arrow in (a) the location and direction for the quantification.The intensity value drops to 15% of is peak value due to the barrier effect. The endothelial monolayer is located near the 400 µm point on the plot (shown with dashed line). Samples fixed on the third day after cell seeding and stained for VE-cadherin and nuclei (DAPI-blue) exhibit well-defined junctions with no apparent gaps in the confluent monolayer (c). The confocal image shows the front view of the microfluidic device.

Figure 3. Optimization of tumor cell seeding density.

The tumor cell seeding density was optimized to have only a limited number of tumor cells in ROI while maintaining as many experimental ROIs as possible that contain at least one tumor cell so tumor cell events can be observed. Histograms of number of total tumor cells present in each ROI (250 µm×250 µm×120 µm) show different trends in distribution of tumor cells for three different tumor seeding densities: 20,000 cells/ml, 50,000 cells/ml, and 200,000 cells/ml (a). The average value and the histogram can be used for choosing the optimal tumor seeding condition (b). Seeding density of 50,000 cells/ml was chosen as a compromise between mimicking the low number of tumor cells of the in vivo of extravasation condition and increasing the chance to have at least one tumor cell to analyze in any given ROI. The statistical significance was tested with one way ANOVA (p<0.05).

Figure 4. Observation of extravasation and permeability of endothelium.

The extravasation event is observed in a sample, which is fixed 1 day after tumor cells are introduced. The region of interest is captured in one confocal image scan and shows one cancer cell (green) that has transmigrated across the endothelium (denoted by VE-cadherin staining in red) and extravasated into the gel region (a). The surface view of the confocal scan shows three different possible locations of tumor cells: 1) extravasated and in gel, 2) adhered and on endothelium adjacent to gel region, and 3) in channel not near the gel. The sectional view of the same confocal scan confirms the different location of the tumor cells (b). The graph shows how many tumor cells have extravasated (dot) among the total tumor cells present (bar) for each region of interest analyzed (c). The tumor cells are categorized as extravasated only when the tumor cells have clearly passed the endothelial monolayer into the gel region. The permeability of endothelial monolayer changes significantly with addition of tumor cells (d). Fluorescently-labeled dextran was introduced on day 3 after endothelial seeding to measure the permeability before tumor and again to same samples on day 4 to see the after tumor seeding effects. The tumor cells are introduced on day 3 after day 3 permeability measurements are taken. The statistical significance was tested with paired t-test (p<0.05).

Figure 5. Beyond extravasation.

The tumor cell extravasation is observed for up to 3 days after tumor cell seeding and compared to the ones fixed and analyzed on day 1. The total number of tumor cells present in region of interest (ROI) increases significantly from 8 cells on day 1 to 13.5 cells on day 3 while the tumor seeding density as well as other experimental condition remained the same between devices (a). The total number of tumor cells are further subdivided into 2 groups depending on their location, either 1) extravasated and in the gel or 2) adherent to the endothelium adjacent to gel (b). 72% of ROIs exhibited at least 1 extravasated cancer cell (denoted % extravasation occurrence) by day 1 after introducing tumor cells, and 79% of ROIs included extravasation event by day 3, which the difference is not significant (c). The images show number of tumor cell increase (d). The phase contrast image shows the top view of the region of interest on day 1 after tumor seeding. The tumor cells (green) have proliferated from day 1 to day 3 (shown by arrows). The confocal image shows both the tumor cells and endothelium lining. All images are from the same ROI (VE-cadherin: red, nucleus: DAPI-blue, tumor cell: GFP-green).