Background: Plasma Tissue Factor Pathway Inhibitor (TFPI) circulates bound to factor V (fV) and Protein S (PS). Estrogen therapy decreases plasma TFPI and PS. TFPI, fV, and PS circulate within platelets, and are released upon activation to modulate thrombus formation.
Objective: Identify factors affecting the concentrations of plasma and platelet TFPI, fV, and PS.
Methods: Blood samples were obtained from 435 healthy individuals. Plasma total TFPI, TFPIɑ, fV, and PS, and platelet TFPI, fV, and PS were quantified. Correlations between these protein concentrations and age, gender, race, and estrogen use were established.
Results: In males, only plasma fV increased with age, while in females, all plasma analytes increased with age. Males had higher plasma total TFPI, TFPIα, and PS than females. The platelet proteins in either sex remained relatively stable with increasing age. Platelet TFPI and PS were comparable in both sexes, while platelet fV was higher in females. Estrogen use was associated with decreased plasma total TFPI and TFPIα, and platelet PS, but not with platelet TFPI concentration. Racial differences in plasma and platelet proteins were observed, some of which were larger than inter-individual differences observed within racial groups. TFPI, fV and PS concentrations correlated in plasma, while only fV and PS correlated in platelets.
Conclusions: Plasma and platelet TFPI, fV and PS differ in their: (i) in vivo association; (ii) demographic correlates; and (iii) alteration by estrogen therapies. Therefore, the plasma and platelet pools of these proteins may modulate hemostasis and thrombosis via different biochemical pathways.
Keywords: TFPI; factor V; hemophilia; platelets; protein.