Proteolytic regulation of epithelial sodium channels by urokinase plasminogen activator: cutting edge and cleavage sites

Plasminogen activator inhibitor 1 (PAI-1) level is extremely elevated in the edematous fluid of acutely injured lungs and pleurae. Elevated PAI-1 specifically inactivates pulmonary urokinase-type (uPA) and tissue-type plasminogen activators (tPA). We hypothesized that plasminogen activation and fibrinolysis may alter epithelial sodium channel (ENaC) activity, a key player in clearing edematous fluid. Two-chain urokinase (tcuPA) has been found to strongly stimulate heterologous human αβγ ENaC activity in a dose- and time-dependent manner. This activity of tcuPA was completely ablated by PAI-1. Furthermore, a mutation (S195A) of the active site of the enzyme also prevented ENaC activation. By comparison, three truncation mutants of the amino-terminal fragment of tcuPA still activated ENaC. uPA enzymatic activity was positively correlated with ENaC current amplitude prior to reaching the maximal level. In sharp contrast to uPA, neither single-chain tPA nor derivatives, including two-chain tPA and tenecteplase, affected ENaC activity. Furthermore, γ but not α subunit of ENaC was proteolytically cleaved at ((177)GR↓KR(180)) by tcuPA. In summary, the underlying mechanisms of urokinase-mediated activation of ENaC include release of self-inhibition, proteolysis of γ ENaC, incremental increase in opening rate, and activation of closed (electrically "silent") channels. This study for the first time demonstrates multifaceted mechanisms for uPA-mediated up-regulation of ENaC, which form the cellular and molecular rationale for the beneficial effects of urokinase in mitigating mortal pulmonary edema and pleural effusions.