Role of von Willebrand factor in tumour cell-induced platelet aggregation: differential regulation by NO and prostacyclin

1 We have studied the effects of a novel agonist, solid-phase von Willebrand Factor (sVWF), on tumour cell-induced platelet aggregation (TCIPA). 2 Washed platelet suspensions were obtained from human blood and the effects of HT-1080 human fibrosarcoma cells and sVWF on platelets were studied using aggregometry, phase-contrast microscopy, and flow cytometry. 3 Incubation of platelets with sVWF (1.2 mug ml(-1)) and HT-1080 cells (5 x 10(3) ml(-1)) resulted in a two-phased reaction characterized first by the adhesion of platelets to sVWF, then by aggregation. 4 TCIPA in the presence of sVWF was inhibited by S-nitroso-glutathione (GSNO, 100 muM) and prostacyclin (PCI2, 30 nm). 5 Platelet activation in the presence of tumour cells and sVWF resulted in the decreased surface expression of platelet glycoprotein (GP)Ib and up-regulation of GPIIb/IIIa receptors. 6 Pre-incubation of platelets with PGI(2) (30 nm) resulted in inhibition of sVWF-tumour cell-stimulated platelet surface expression of GPIIb/IIIa as measured by flow cytometry using antibodies directed against both non-activated and activated receptor. In contrast, GSNO (100 muM) did not affect sVWF-tumour cell-stimulated platelet surface expression of GPIIb/IIIa. 7 Flow cytometry performed with PAC-1 antibodies that bind only to the activated GPIIb/IIIa revealed that GSNO (100 muM) caused inhibition of activation of GPIIb/IIIa. 8 The inhibitors exerted no significant effects on TCIPA-mediated changes in GPIb. 9 Thus, sVWF potentiates the platelet-aggregatory activity of HT-1080 cells and these effects appear to be mediated via up-regulation of platelet GPIIb/IIIa. 10 Prostacyclin and NO inhibit TCIPA-sVWF-mediated platelet aggregation. The mechanisms of inhibition of this aggregation by PGI(2) differ from those of NO.