Regulation of the single-chain urokinase-urokinase receptor complex activity by plasminogen and fibrin: novel mechanism of fibrin specificity.
Activation of plasminogen by urokinase plasminogen activator (uPA) plays important roles in several physiologic and pathologic conditions. Cells secrete uPA as a single-chain molecule (scuPA). scuPA can be activated by proteolytic cleavage to a 2-chain enzyme (tcuPA). scuPA is also activated when it binds to its receptor (uPAR). The mechanism by which the enzymatic activity of the scuPA/suPAR complex is regulated is only partially understood. We now report that the plasminogen activator activity of the scuPA/suPAR complex is inhibited by Glu- and Lys-plasminogen, but not by mini-plasminogen. In contrast, neither Glunor Lys-plasminogen inhibits the activation of plasminogen by 2-chain uPA. Inhibition of scuPA/suPAR activity was evident at a Glu-plasminogen concentration of approximately 100 nM, and at physiologic plasma concentrations inhibition was nearly complete. A plasminogen fragment containing kringles 1-3 inhibited the enzymatic activity of scuPA/suPAR with an inhibition constant (Ki) equal to 1.9 microM, increased the Michaelis constant (Km) of scuPA/suPAR from 18 nM to 49 nM, and decreased the catalytic constant (Kcat) approximately 3-fold from 0.035 sec(-1) to 0.011 sec(-1). Inhibition of scuPA/suPAR by plasminogen was completely abolished in the presence of fibrin clots. These studies provide insight into the regulation of uPA-mediated plasminogen activation and identify a novel mechanism for its fibrin specificity.
Higazi
Abd Al-Roof
AA
Department of Pathology and Laboratory Medicine, 513A Stellar-Chance, 422 Curie Blvd, University of Pennsylvania, Philadelphia, PA, 19104, USA. higazi@mail.med.upenn.edu
Ajawi
Feras
F
Akkawi
Sa'ed
S
Hess
Edna
E
Kuo
Alice
A
Cines
Douglas B
DB
D000818:Animals; D017931:DNA Primers; D004789:Enzyme Activation; D005337:Fibrin / Q000502:physiology*; D051379:Mice; D010958:Plasminogen / Q000502:physiology*; D016133:Polymerase Chain Reaction; D020641:Polymorphism, Single Nucleotide; D011956:Receptors, Cell Surface / Q000378:metabolism*; D055293:Receptors, Urokinase Plasminogen Activator; D011994:Recombinant Proteins / Q000378:metabolism; D014568:Urokinase-Type Plasminogen Activator / Q000378:metabolism*
D017931:DNA Primers; C524899:Plaur protein, mouse; D011956:Receptors, Cell Surface; D055293:Receptors, Urokinase Plasminogen Activator; D011994:Recombinant Proteins; D005337:Fibrin; D010958:Plasminogen; D014568:Urokinase-Type Plasminogen Activator