Increased bleeding in TrampC1 and AT-3 primary tumors in GPVI-deficient animals and upon GPVI blockade. TrampC1 prostate (A) and AT-3 breast cancer (B) tumors were grown in GPVI-deficient (Gp6−/−) mice. Tumor growth was measured every second day for 31 (A) or 21 (B) days postimplantation (n = 8; mean ± SEM). (C-E) The relative hemoglobin (rel. Hb) content was measured 31 or 21 days postimplantation (n = 8; mean ± SD). Effect of platelet depletion (R300 antibody against GPIbα) and GPVI blockade (JAQ1 F(ab′)2) on TrampC1 (F) and AT-3 (G) primary tumors grown in C57BL/6 animals 18 hours after antibody treatment. Relative quantification of the hemoglobin content in TrampC1 (F; n = 15) and AT-3 tumors (G; n = 10) (mean ± SD). Representative pictures (H) and hematoxylin and eosin staining (I) of the tumors. Arrows indicate accumulation of red blood cells in the tumor tissues. Scale bars, 100 µm. ***P < .001, **P < .01, and *P < .05; Kruskal-Wallis with Dunn’s multiple comparisons test. WT, wild-type.