Prostasin expression is regulated by airway surface liquid volume and is increased in cystic fibrosis

Abstract

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.

SPLUNC1 regulates airway surface liquid volume by protecting ENaC from proteolytic cleavage

“…2 A). However, the serine proteases that activate ENaC are up-regulated in CF airway epithelia (10,40) and neutrophil elastase, which also activates ENaC, is increased in CF airways (41)(42)(43). Thus, it is possible that the excessive protease up-regulation seen in CF airways (10,40) interferes with the normal regulation of ENaC by SPLUNC1 and other potential ENaC regulators, and thus, shifts the balance from anti-proteases and less ENaC activity to a protease-replete state with more ENaC activity, overwhelming the ability of SPLUNC1 to inactivate ENaC and contributing to CF airway surface liquid volume depletion.…”

α1-Antitrypsin Inhibits Epithelial Na+ Transport In Vitro and In Vivo

“…In contrast, our data show that A1AT down-regulates both I amil values in vitro and Na 1 -dependent AFC across the distal lung epithelia of anesthetized mice in vivo. Recently, Myerburg and colleagues (45) reported that protease inhibitor, PN-1, another member of the SERPIN family, inhibited Na 1 currents across human airway epithelial cells isolated from CF lungs (45). Protease inhibitor PN-1, like A1AT, inhibits matriptase (45).…”

Reduced Sodium Transport With Nasal Administration of the Prostasin Inhibitor Camostat in Subjects With Cystic Fibrosis

“…9 Prostasin, a trypsin-like serine protease, is a major channel-activating protease (CAP) of ENaC-mediated sodium currents in CF epithelia. [10][11][12][13][14] Attenuation of ENaC function by CAP inhibition is predicted to improve mucociliary clearance with potential downstream effects on pulmonary obstruction and clinical stability. Camostat is an inhibitor of prostasin that has been shown to inhibit ENaC function in vitro 15 and in vivo.…”