Factor seven activating protease (FSAP): does it activate factor VII?

Background: Factor seven activating protease (FSAP) was initially reported as an activator of single‐chain urokinase‐type plasminogen activator (scuPA) and factor VII (FVII). Subsequently, numerous additional substrates have been identified, and multiple other biological effects have been reported. Due to the apparent lack of specificity, the physiological role of FSAP has become increasingly unclear. Rigorous studies have been limited by the difficulty of obtaining intact FSAP from blood or recombinant sources. Objectives: Our aim was to produce intact recombinant human FSAP, and to assess its role as a trigger of coagulation and fibrinolysis. Results: Expression of wild‐type FSAP in various mammalian cells invariably resulted in the accumulation of degraded FSAP due to autoactivation and degradation. To overcome this problem, we constructed a variant in which Arg313 at the natural activation site was replaced by Gln, creating a cleavage site for the bacterial protease thermolysin. HEK293 cells produced FSAPR313Q in its intact form. Thermolysin‐activated FSAP displayed the same reactivity toward the substrate S‐2288 as plasma‐derived FSAP, and retained its ability to activate scuPA. Polyphosphate and heparin increased Vmax by 2–3‐fold, without affecting Km (62 nm) of scuPA activation. Surprisingly, FVII activation by activated FSAP proved negligible, even in the presence of calcium ions, phospholipid vesicles and recombinant soluble tissue factor. On membranes of 100% cardiolipin FVII cleavage did occur, but this resulted in transient activation and rapid degradation. Conclusions: While FSAP indeed activates scuPA, FVII appears remarkably resistant to activation. Therefore, reappraisal of the putative role of FSAP in hemostasis seems appropriate.