FVIII is an important cofactor in the
tenase coagulation factor complex, lack of FVIII causes severe
bleeding, whereas high FVIII levels seem to be associated with venous and arterial
thromboembolism. Resting platelets do not bind FVIII, but activated platelets bind unactivated FVIII if vWF is not present. We investigated a possible influence of platelet bound FVIII on platelet function itself as it is unclear if there is a direct effect of FVIII on platelet function. The influence of FVIII on platelet function was investigated by flow cytometric analysis of
P-selectin expression (CD62P) and PAC-1 binding before and after submaximal stimulation with
TRAP-6 (5 microM final concentration), by confocal microscopy and by platelet aggregometry. For flow cytometry and confocal microscopy, washed platelets were incubated with human recombinant FVIII for 5 min at 37 degrees C. Analysis of platelet surface area was measured by computerized image analysis. Treatment with FVIII only caused no changes in
P-selectin expression or PAC-1 binding, respectively. Stimulation of platelets with
TRAP-6 increased the expression of
P-selectin (445%) and PAC-1 binding (934%) as expected. These effects were further increased when platelets were stimulated with
TRAP-6 and FVIII (
P-selectin 499%, difference not significant; PAC-1 1626%, P < 0.05. Values were expressed in%, related to unstimulated,
buffer treated platelets). Platelet spreading on
fibrinogen was significantly increased when platelets were treated with FVIII and
TRAP-6 compared to
TRAP-6 alone (368 vs. 307 average pixel/platelet, P<0.05). In addition platelet aggregation was enhanced when platelets were stimulated with FVIII and
TRAP-6 compared to
TRAP-6 alone. FVIII can act as a positive regulator of platelet function in TRAP-co-stimulated platelets. We hypothesize that FVIII induced increase in platelet activation might contribute to venous and even arterial
thrombus formation in patients with high FVIII levels.