Summary
The natural anticoagulant protein S contains a so-called thrombin-sensitive region
(TSR), which is susceptible to proteolytic cleavage. We have previously shown that
a platelet-associated protease is able to cleave protein S under physiological plasma
conditions in vitro. The aim of the present study was to investigate the relation between platelet-associated
protein S cleaving activity and in vivo protein S cleavage, and to evaluate the impact of in vivo protein S cleavage on its anticoagulant activity. Protein S cleavage in healthy subjects
and in thrombocytopenic and thrombocythaemic patients was evaluated by immunological
techniques. Concentration of cleaved and intact protein S was correlated to levels
of activated protein C (APC)-dependent and APC-independent protein S anticoagulant
activity. In plasma from healthy volunteers 25% of protein S is cleaved in the TSR.
While in plasma there was a clear positive correlation between levels of intact protein
S and both APC-dependent and APC-independent protein S anticoagulant activities, these
correlations were absent for cleaved protein S. Protein S cleavage was significantly
increased in patients with essential thrombocythaemia (ET) and significantly reduced
in patients with chemotherapy-induced thrombocytopenia. In ET patients on cytoreductive
therapy, both platelet count and protein S cleavage returned to normal values. Accordingly,
platelet transfusion restored cleavage of protein S to normal values in patients with
chemotherapy-induced thrombocytopenia. In conclusion, proteases from platelets seem
to contribute to the presence of cleaved protein S in the circulation and may enhance
the coagulation response in vivo by down regulating the anticoagulant activity of protein S.
Keywords
Coagulation - thrombin generation - protein S - cellular proteases - platelets