Airway surface liquid (ASL) absorption is initiated by Na+ entry via epithelial Na+ channels (ENaC), which establishes an osmotic gradient that drives fluid from the
luminal to serosal airway surface. We and others have recently reported that a
protease/anti-
protease balance regulates ENaC in human airway epithelial cells (HAEC) and provides a mechanism for autoregulation of ASL volume. In
cystic fibrosis (CF), this balance is disturbed, leading to constitutive proteolytic activation of ENaC and the pathological Na+ hyperabsorption characteristic of this airway disease.
Prostasin is a
glycosylphosphatidylinositol-anchored
serine protease that activates ENaC and is expressed on the surface epithelium lining the airway. In this report we present evidence that
prostasin expression is regulated by the ASL volume, allowing for increased proteolytic activation of ENaC when the ASL volume is high.
Prostasin activity is further regulated by the cognate
serpin protease nexin-1 (PN-1), which is expressed in HAEC and inhibits Na+ absorption by forming an inactive complex with
prostasin and preventing the proteolytic processing of
prostasin. Whereas these mechanisms regulate
prostasin expression in response to ASL volume in non-CF epithelia, HAEC cultured from CF patients express >50% more
prostasin on the epithelial surface. These findings suggest that a proteolytic cascade involving
prostasin, an upstream
prostasin-activating
protease, and PN-1 regulate Na+ absorption in the airway and that abnormal
prostasin expression contributes to excessive proteolytic activation of ENaC in CF patients.