Polyphosphate Expression by Cancer Cell Extracellular Vesicles Mediates Binding of Factor XII and Contact Activation.

Extracellular vesicles (EV) have been implicated in diverse biological processes, including intracellular communication, transport of nucleic acids, and regulation of vascular function. Levels of EV are elevated in cancer, and studies suggest that EV may stimulate thrombosis in cancer patients through expression of tissue factor. However, limited data also implicates EV in activation of the contact pathway of coagulation through activation of factor XII (FXII) to factor XIIa (FXIIa). To better define the ability of EV to initiate contact activation, we compared the ability of EV derived from different cancer cell lines to activate FXII. EV from all cell lines activated FXII, with those derived from pancreatic and lung cancer cell lines demonstrating the most potent activity. Concordant with activation of FXII, EV induced the cleavage of high molecular weight kininogen to cleaved kininogen. We also observed that EV from cancer patients stimulated FXII activation and HK cleavage. To define the mechanisms of FXII activation by EV, EV were treated with calf intestinal alkaline phosphatase or E. coli exopolyphosphatase to degrade polyphosphate; this treatment blocked binding of FXII to EV and the ability of EV to mediate FXII activation. In vivo, EV induced pulmonary thrombosis in wild-type mice, with protection conferred by deficiency of FXII, HK, or prekallikrein. Moreover, pre-treatment of EV with calf intestinal alkaline phosphatase inhibited their prothrombotic effect. These results indicate that polyphosphate mediates binding of contact factors to EV, and that EV-associated polyphosphate may contribute to the prothrombotic effects of EV in cancer.