# Urokinase System in Pathogenesis of Pulmonary Fibrosis: A Hidden Threat of COVID-19
## 1 Introduction  
December 2019 was marked by an outbreak of a new strain of coronavirus (SARS-CoV-2) in Wuhan, China, which triggered a pandemic of acute respiratory disease coronavirus 2019 (COVID-19) across the globe [1]. The unprecedented spread of COVID-19 posed a severe healthcare challenge, with long-term sequelae of severe COVID-19 cases and deaths to be encountered even after two years of the outbreak [2]. One of the most common and severe post-COVID-19 complications is pulmonary fibrosis: early evidence suggested that more than a third of COVID-19 patients discharged from hospital afterrecovery developed fibrotic lung abnormalities, and this percentage increased with disease severity and duration [3; 4; 5; 6]. A reliable incidence of COVID-19-induced pulmonary fibrosis, however, can only be assessed after decades of post-COVID-19 observational studies [7; 8]. Epithelial and endothelial cell damage, excessive fibrin deposition, unregulated release of cytokines and proteases, and abnormal immune system activation [9; 10; 11; 12] ultimately result in pulmonary fibrosis with limited treatment options. The mechanism of SARS-CoV-2 fusion with the host cell membrane requires proteolytic cleavage of the viral spike protein (S-protein) by serine proteases: primarily the transmembrane serine protease TMPRSS2 and potentially plasmin [13; 14; 15]. In the case of the transmembrane protease TMPRSS2, effective infection with SARS-CoV-2 requires angiotensin-converting enzyme 2 (ACE2) and TMPRSS2 to be co-expressed on the same cell membrane. While in the case of the fibrinolytic plasminogen activator system, there is still no clear picture regarding its mechanistic underpinnings in COVID-19-related pathologies. It is well established that the plasminogen activator system plays a crucial role in the pathogenesis of lung injury and pulmonary fibrosis. The system comprises tissue plasminogen activator (tPA), urokinase (uPA), its receptor uPAR and plasminogen activator inhibitors (PAI-1 and PAI-2) [16; 17; 18]. Lung fibrinolysis is driven by uPA as opposed to intravascular fibrinolysis controlled by tPA [16; 18]. uPA (encoded by the _PLAU_ gene) is a serine protease that cleaves inactive plasminogen, converting it into a protease plasmin with a wide range of substrate specificity (fibrin, ECM components, matrix metalloproteinases) [19]. uPA interaction with uPAR (encoded by the _PLAU_ gene) on the cell surface accelerates its catalytic activation and can trigger intracellular signal transduction [19; 20; 21; 22]. uPAR-bound uPA cleaves plasminogen \(\sim\) six times more efficiently [23]. uPA can also be reciprocally activated by its ligand plasmin [24; 25; 26]. The absence of uPAR or plasminogen significantly impairs uPA activation [24]. Plasmin activation by uPA is regulated by its endogenous inhibitor, PAI-1 [18]. Lung epithelial cells express uPA, uPAR and PAI-1; alveolar macrophages and lung fibroblasts can also serve as a source of uPA in lung tissue [16]. The plasminogen activator system has been widely studied in relation to acute lung injury (for a review, see [16]); however, its role in COVID-19-induced pulmonary fibrosis remains underexplored. Previously, it was reported that the reduced serum levels of plasminogen and PAI-1 were significantly associated with low survival in severely ill COVID-19 patients [27; 28], while high soluble uPAR serum level was shown to be an early predictor of clinical severity and poor outcome in patients with COVID-19 [29; 30; 31; 32; 33; 34]. These data indicate that the plasminogen activator system is demonstrably involved in COVID-19 pathogenesis. Further research is warranted to understand the function of the plasminogen activator system in post-COVID-19 lung fibrosis. In the current study, we aimed to define the role of uPA and uPAR in COVID-19-induced lung fibrosis with putative mechanisms of their action.