Urokinase

Urokinase, also known as urokinase-type plasminogen activator (uPA), is a serine protease present in humans and other animals. The human urokinase protein was discovered, but not named, by McFarlane and Pilling in 1947. Urokinase was originally isolated from human urine, and it is also present in the blood and in the extracellular matrix of many tissues. The primary physiological substrate of this enzyme is plasminogen, which is an inactive form (zymogen) of the serine protease plasmin. Activation of plasmin triggers a proteolytic cascade that, depending on the physiological environment, participates in thrombolysis or extracellular matrix degradation. This cascade had been involved in vascular diseases and cancer progression.

Urokinase is encoded in humans by the PLAU gene, which stands for "plasminogen activator, urokinase". The same symbol represents the gene in other animal species.

The PLAU gene encodes a serine protease (EC 3.4.21.73) involved in degradation of the extracellular matrix and possibly tumor cell migration and proliferation. A specific polymorphism in this gene may be associated with late-onset Alzheimer disease and also with decreased affinity for fibrin-binding. The protein encoded by this gene converts plasminogen to plasmin by specific cleavage of an Arg-Val bond in plasminogen. This gene's proprotein is cleaved at a Lys-Ile bond by plasmin to form a two-chain derivative in which a single disulfide bond connects the amino-terminal A-chain to the catalytically active, carboxy-terminal B-chain. This two-chain derivative is also called HMW-uPA (high molecular weight uPA). HMW-uPA can be further processed into LMW-uPA (low molecular weight uPA) by cleavage of chain A into a short chain A (A1) and an amino-terminal fragment. LMW-uPA is proteolytically active but does not bind to the uPA receptor.

Urokinase is a 411-residue protein, consisting of three domains: the serine protease domain (consisting of residues 159–411), the kringle domain (consisting of residues 50-131), and the EGF-like domain (consisting of residues 1-49). The kringle domain and the serine protease domain are connected by an interdomain linker or connecting peptide (consisting of residues 132–158). Urokinase is synthesized as a zymogen form (prourokinase or single-chain urokinase), and is activated by proteolytic cleavage between Lys158 and Ile159. The two resulting chains are kept together by a disulfide bond between Cys148 and Cys279.

In comparison to the mammalian system, zebrafish (Danio rerio) contains two orthologs of urokinase which have been characterised as zfuPA-a and zfuPA-b. zfuPA-a differs from the mammalian uPA by lacking an exon sequence encoding for the uPAR (urokinase receptor) binding domain; while the zfuPA-b lacks two cysteines of the epidermal growth factor-like domain. zfuPA-b also has no binding activity in fish white blood cells or fish cell lines. The uPAR binding in mammalian system is essential for the activity of urokinase and uPAR as it also functions as an adhesion receptor due to its affinity to vitronectin, integrins and other proteases like PAI-1. The lack of the uPAR binding region in zebrafish uPA, suggests that zebrafish uPA functions without uPAR binding.

The most important inhibitors of urokinase are the serpins plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator inhibitor-2 (PAI-2), which inhibit the protease activity irreversibly. In the extracellular matrix, urokinase is tethered to the cell membrane by its interaction to the urokinase receptor.

uPa also interacts with protein C inhibitor.

zfuPA-a and zfuPA-b are poor activators of human plasminogen, while human uPA is a poor activator of salmon plasminogen. With the primary difference between the zebrafish uPA and human uPA being in the EGF domain.