Platelets promote metastasis by upregulating tumor cell IL-8 secretion. The invasion of MDA-MB-231 cells in response to releasate from resting or activated platelets was assessed in the presence of either 1 μg/mL of an IL-8 neutralizing antibody or 1 ng/mL exogenous recombinant human IL-8 (rhIL-8). Representative images are shown in panel A, and quantification is shown in panel B. One value for TRAP-APR was considered an outlier (17.2-fold, with standard deviation from the mean >2.5) and removed from graphical representation and statistical analysis. The dashed black line represents invasion on incubation with resting platelet releasate. (C) MBA-MB-231 cells with an IL-8 knockdown or a scrambled control were exposed to platelets for 24 hours, and IL-8 within the supernatant was measured by ELISA. Cell invasion was quantified in the IL-8 knockdown MBA-MB-231 cell line and a scrambled control exposed to APR, with images and quantification shown in panels D and E, respectively. P values for panels B-C or E were determined by separate unpaired 2-tailed Student t tests or 2-way ANOVA with Sidak’s multiple comparisons, respectively. n = 3 individual replicates per treatment group for panel B, n = 6 for panel C, and n = 4 for panel E. Scale bars represent 100 μm.