Proposed mechanisms underlying the regulatory control of fibrosis development by the PAI-1/uPA balance in aged-associated muscle dystrophy progression. PAI-1 is the physiological inhibitor of the protease uPA, which, through the generation of active plasmin, processes latent TGF-β into its active form. (left) When PAI-1 levels are high (at early stages of dystrophic disease), there is a controlled activation of uPA and production of active TGF-β, which stimulates collagen and TIMP-1 expression in fibroblasts through Smad2-mediated transcriptional activation, thus resulting in altered ECM metabolism and slow development of fibrosis and muscle dystrophy progression. (right) When PAI-1 levels are low (at advanced stages of dystrophic disease), there is a hyperactivation of uPA, leading to increased production of active TGF-β and Smad2 signaling (yet, additional proteolytic TGF-β–activating pathways in fibrotic muscle cannot be discarded). Most importantly, there is a derepression of fibroblast AKT signaling and abnormal fibroblast proliferation through miR-21 (via TGF-β–dependent miR-21 biogenesis), which inhibits the expression of PTEN phosphatase, an inhibitor of AKT activation. The increase in AKT activity results in enhanced muscle fibroblast proliferation, which in turn causes further fibrosis and worsens muscle dystrophy. Interestingly, high miR-21 expression in dystrophic muscle also leads to Smad7 (an inhibitory Smad) down-regulation, thus providing a TGF-β activity–amplifying loop, resulting in increased fibrosis. Whether other additional mechanisms of miR-21 activation, in addition to TGF-β, are operational in this context awaits further investigation. Overall, our data validate uPA as a disease target in muscular dystrophy (as demonstrated by the efficacy of amiloride and genetic uPA interference) and introduces miR-21 as a novel therapeutic candidate.