Releasates from platelets treated with 50 µM 4-OH or vehicle control were assayed for angiogenic potential using capillary tube formation assays. Capillary tube formation was quantified as the average number of branch points per field of view (B), with representative images shown (A). To determine the metastatic potential of 4-OH-treated platelets, releasates were generated as previously described and used in standard transwell invasion assays. The ability of MCF-7 tumor cells to invade through Matrigel in response to platelet releasates was quantified (D) and representative images are shown (D). Model: tamoxifen and its metabolites directly inhibit platelet activation in response to breast tumor cells, leading to a decreased release of tumor-supporting pro-angiogenic and pro-metastatic factors from platelets (E). Bars indicate SEM. P<**0.01, ***0.001 by ANOVA, n=3 independent replicates per treatment group.