The plasminogen system plays a crucial role in the repair of a variety of tissues, including skeletal muscle. We hypothesized that urokinase-type plasminogen activator (uPA) promotes muscle regeneration by activating hepatocyte growth factor (HGF), which, in turn, stimulates proliferation of myoblasts required for regeneration. In our studies, levels of active HGF and phosphorylation of the HGF receptor c-met were increased after muscle injury in wild-type mice. Compared with wild-type animals, mice deficient in uPA (uPA ؊/؊ ) had markedly reduced HGF levels and c-met activation after muscle damage. This reduced HGF activity in uPA ؊/؊ animals was associated with decreased cell proliferation, myoblast accumulation, and new muscle fiber formation. On the other hand, HGF activity was enhanced at early time points in PAI-1 ؊/؊ mice compared with wild-type mice and the PAI-1 ؊/؊ animals exhibited accelerated muscle fiber regeneration. Furthermore, administration of exogenous uPA rescued HGF levels and muscle regeneration in uPA ؊/؊ mice, and an HGF-blocking antibody reduced HGF activity and muscle regeneration in wildtype mice. We also found that uPA promotes myoblast proliferation in vitro through its proteolytic activity, and this process was inhibited by an HGFblocking antibody. Together, our findings demonstrate that uPA promotes muscle regeneration through HGF activation and subsequent myoblast proliferation.
IntroductionThe plasminogen system plays a crucial role in the repair of a variety of tissues. This extracellular serine protease cascade includes the urokinase-type plasminogen activator (uPA) and its primary inhibitor, plasminogen activator inhibitor-1 (PAI-1). The classic function of uPA is to cleave plasminogen to form active plasmin, a broad-spectrum protease with activity toward a variety of extracellular matrix molecules. 1,2 Genetic manipulations that inhibit plasminogen activation lead to impaired healing of lung, liver, kidney, and skin. [3][4][5][6] Similarly, mice deficient in uPA or plasminogen demonstrate markedly impaired muscle regeneration. [7][8][9] In contrast, mice lacking PAI-1 exhibit accelerated muscle regeneration. 7 Despite its robust effect on skeletal muscle regeneration, the molecular mechanisms by which the plasminogen system regulates this process remain largely undefined. Clearance of the provisional fibrin matrix that is deposited after muscle injury may be one mechanism by which uPA promotes regeneration. 8 However, uPA may also promote tissue repair through other pathways, such as the proteolytic activation of growth factors, including hepatocyte growth factor (HGF). HGF is synthesized and secreted as an inactive single-chain molecule, and proteolytic cleavage results in formation of the active 2-chain form, which has affinity for the c-met receptor. [10][11][12] uPA is one of a select group of proteases known to activate HGF, 11,12 the others being tissue-type plasminogen activator, factor XIa, factor XIIa, matriptase, and HGF activator. [13][14][15] In addition, uPA app...