Factor VII-activating
protease (FSAP) is a novel plasma-derived
serine protease structurally homologous to tissue-type and
urokinase-type
plasminogen activators. We demonstrate that
plasminogen activator inhibitor-1 (PAI-1), the predominant inhibitor of tissue-type and
urokinase-type
plasminogen activators in plasma and tissues, is an inhibitor of FSAP as well. We detected PAI-1.FSAP complexes in addition to high levels of extracellular
RNA, an important FSAP cofactor, in bronchoalveolar lavage fluids from patients with
acute respiratory distress syndrome. Hydrolytic activity of FSAP was inhibited by
PAI-1 with a second-order inhibition rate constant (K(a)) of 3.38 +/- 1.12 x 10(5) m(-1).s(-1). Residue Arg(346) was a critical recognition
element on
PAI-1 for interaction with FSAP.
RNA, but not
DNA, fragments (>400
nucleotides in length) dramatically enhanced the reactivity of
PAI-1 with FSAP, and 4 microg.ml(-1)
RNA increased the K(a) to 1.61 +/- 0.94 x 10(6) m(-1).s(-1).
RNA also stabilized the active conformation of
PAI-1, increasing the half-life for spontaneous conversion of active to latent
PAI-1 from 48.4 +/- 8 min to 114.6 +/- 5 min. In contrast, little effect of
DNA on
PAI-1 stability was apparent. Residues Arg(76) and Lys(80) in
PAI-1 were key elements mediating binding of
nucleic acids to
PAI-1. FSAP-driven inhibition of vascular smooth muscle cell proliferation was antagonized by
PAI-1, suggesting functional consequences for the FSAP-PAI-1 interaction. These data indicate that extracellular
RNA and
PAI-1 can regulate FSAP activity, thereby playing a potentially important role in hemostasis and cell functions under various pathophysiological conditions, such as
acute respiratory distress syndrome.