Urokinase was discovered in the late nineteenth century, as an enzymatic principle in urine, that initiates the dissolution of blood clots. The basis of this phenomenon was recognized more than fifty years ago as the activation of plasminogen, the precursor of a tryptic protease, then known as profibrinolysin. Despite this long history, detailed data on the biochemistry of plasminogen activation have only become available recently. Urokinase (now designated urokinase-type plasminogen activator : u-PA) is synthesized and secreted as a single chain polypeptide (Mr-: 53,000) by many cell types. Single chain u-PA (scu-PA) is with equal justification called prourokinase (pro-u-PA), notwithstanding its low catalytic activity for synthetic peptide substrates and plasminogen, as most proenzymes of proteases display a certain degree of activity. The structure of pro-u-PA has been elucidated by protein and cDNA sequencing. It consists of three domains, exhibiting characteristic homology to other proteins: a serine protease domain, homologous to trypsin, chymotrypsin and elastase; a kringle domain, likewise found in prothrombin, plasminogen, tissue-type plasminogen activator (t-PA) and Factor XII; and an epidermal growth factor (EGF)-like domain, found in many other proteins, including certain clotting factors. Pro-u-PA is activated by the cleavage of its LYS158-Ile159 h1 bY either plasmin or kallikrein. This cleavage leads to a high increase of Kcat values with respect to both plasminogen and synthetic peptide substrates, but apparently to a reduction of its affinity to plasminogen. Thrartoin inactivates pro-u-PA irreversibly by the cleavage of the Arg156-Phe157 bond. U-PA but not pro-u-PA rapidly forms ccnplexes with plasminogen activator inhibitors (PAI)-l and PAI-2: second order rate constants Kass are respectively > 107 and 0.9xl06 (M-11sec-1). Unknown enzymes process pro-u-PA and u-PA to low molecular weight (LMW) pro-u-PA and LMW u-PA (Mr: 33,000) by cutting off a fragment consisting of the kr ingle and the EGF—like region. Pro—u—PA mediated plasminogen activation is fibrin dependent in vivo, and to a certain degree in vitro. Hie biochemical basis of this fibrin specificity is at present uncertain, although there are reports indicating that it may require polyvalent cations. Through its EGF-like region HMW pro-u-PA and HMW u-PA are capable of binding to specific membrane protein receptors which are found on many cells. Thus, u-PA activity may be restricted to the cell surface. According to a recent report, binding of u—PA to the receptor may also mediate signal transduction in auto- or paracrine growth control. In cells permissive for the respective pathways, pro-u-PA gene transcription is stimulated by mechanisms of signal transduction, that include the cAMP, the tyrosine specific kinase and the protein kinase C dependent pathways. Glucocorticoid hormones downregulate pro-u-PA gene transcription in cells where the gene is canstitutively expressed. Although different cells vary greatly in their response to agents that stimulate urokinase biosynthesis, growth factors and other mitogens are in many cases effective inducers. Significantly elevated levels of u-PA are also found in many malignant tissues. These findings and many others suggest that plasminogen activation by u-PA provides localized extracellular matrix degradation which is required for invasive growth, cell migration and other forms of tissue remodelling. Fibrin represents in this view only a variant of an extracellular matrix, which is provided through the clotting system in the case of an emergency.
Plasminogen Activation Initiated by Single-chain Urokinase-type Plasminogen Activator
