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Background:Selective platelet release of pro- or anti-angiogenic factors distinctly regulated angiogenesis. We hypothesised that selective release of platelet angiogenic factors could differently regulate tumour growth.Methods:Breast cancer cell proliferation, cancer cell-induced endothelial tube formation in vitro, and tumour growth in vivo were studied in the presence of protease-activated receptor 1-stimulated platelet releasate (PAR1-PR; rich in pro-angiogenic factors) or PAR4-PR (rich in anti-angiogenic factors).Results:The PAR1-PR and PAR4-PR supplementation (10%) similarly enhanced cell proliferation of MCF-7 and MDA-MB-231 breast cancer cells. The cancer cells triggered capillary-like tube formation of endothelial cells that was further enhanced by pro-angiogenic factor-rich PAR1-PR. The VEGF, but not SDF-1α, receptor blockade abolished PAR1-PR/PAR4-PR-enhanced cancer cell proliferation. Integrin blockade by RGDS had identical effects as VEGF inhibition. The Src and ERK inhibition diminished, whereas PI3K and PKC blockade abolished platelet releasate-enhanced cancer cell proliferation. Using a model of subcutaneous implantation of MDA-MB-231 cells in nude mice, PAR1-PR enhanced tumour growth more markedly than PAR4-PR, and seemed to achieve the exaggeration by promoting more profound tumour angiogenesis.Conclusions:Platelet releasate increases breast cancer cell proliferation through VEGF–integrin cooperative signalling. Pro-angiogenic factor-rich platelet releasate enhances cancer cell-induced angiogenesis more markedly, and thus exaggerates tumour growth in vivo.