Tumor cell-induced platelet aggregation represents a critical process both for successful metastatic spread of the tumor and for the development of thrombotic complications in cancer patients. To get further insights into this process, we investigated and compared the molecular mechanisms of platelet aggregation induced by two different breast cancer cell lines (MDA-MB-231 and MCF7) and a colorectal cancer cell line (Caco-2). All the three types of cancer cells were able to induce comparable platelet aggregation, which, however, was observed exclusively in the presence of CaCl₂ and autologous plasma. Aggregation was supported both by fibrinogen binding to integrin αIIbβ3 as well as by fibrin formation, and was completely prevented by the serine protease inhibitor PPACK. Platelet aggregation was preceded by generation of low amounts of thrombin, possibly through tumor cells-expressed tissue factor, and was supported by platelet activation, as revealed by stimulation of phospholipase C, intracellular Ca²⁺ increase and activation of Rap1b GTPase. Pharmacological inhibition of phospholipase C, but not of phosphatidylinositol 3-kinase or Src family kinases prevented tumor cell-induced platelet aggregation. Tumor cells also induced dense granule secretion, and the stimulation of the P2Y12 receptor by released ADP was found to be necessary for complete platelet aggregation. By contrast, prevention of thromboxane A₂ synthesis by aspirin did not alter the ability of all the cancer cell lines analyzed to induce platelet aggregation. These results indicate that tumor cell-induced platelet aggregation is not related to the type of the cancer cells or to their metastatic potential, and is triggered by platelet activation and secretion driven by the generation of small amount of thrombin from plasma and supported by the positive feedback signaling through secreted ADP.

肿瘤细胞诱导的血小板聚集代表了肿瘤成功转移扩散和癌症患者血栓形成并发症发展的关键过程。为了进一步了解这一过程，我们研究并比较了两种不同的乳腺癌细胞系（MDA-MB-231和MCF7）和结直肠癌细胞系（Caco-2）诱导的血小板聚集的分子机制。三种类型的癌细胞均能够诱导相当的血小板凝集，但是，仅在CaCl ₂存在下才能观察到和自体血浆。纤维蛋白原与整联蛋白αIIbβ3的结合以及纤维蛋白的形成均支持聚集，丝氨酸蛋白酶抑制剂PPACK完全阻止了聚集。血小板凝集之前可能会通过肿瘤细胞表达的组织因子产生少量凝血酶，并受到血小板活化的支持（如磷脂酶C，细胞内Ca ^2+的刺激所揭示）增加和激活Rap1b GTPase。药理学上抑制磷脂酶C，但不抑制磷脂酰肌醇3-激酶或Src家族激酶阻止了肿瘤细胞诱导的血小板聚集。肿瘤细胞也诱导致密的颗粒分泌，并且发现释放的ADP刺激P2Y12受体对于完整的血小板聚集是必需的。相比之下，血栓烷A _2的预防阿司匹林的合成并未改变所有分析的癌细胞系诱导血小板聚集的能力。这些结果表明，肿瘤细胞诱导的血小板聚集与癌细胞的类型或其转移潜能无关，而是由血小板活化和分泌触发的，血小板活化和分泌是由血浆中少量凝血酶的产生驱动，并受到阳性反应的支持。通过分泌的ADP反馈信令。