Abstract
BackgroundHyperactivity of the epithelial sodium (Na+) channel (ENaC) and increased Na+ absorption by airway epithelial cells leading to airway surface liquid dehydration and impaired mucociliary clearance are thought to play an important role in the pathogenesis of cystic fibrosis (CF) pulmonary disease. In airway epithelial cells, ENaC is constitutively activated by endogenous trypsin-like serine proteases such as Channel-Activating Proteases (CAPs). It was recently reported that ENaC activity could also be stimulated by apical treatment with human neutrophil elastase (hNE) in a human airway epithelial cell line, suggesting that hNE inhibition could represent a novel therapeutic approach for CF lung disease. However, whether hNE can also activate Na+ reabsorption in primary human nasal epithelial cells (HNEC) from control or CF patients is currently unknown.MethodsWe evaluated by short-circuit current (Isc) measurements the effects of hNE and EPI-hNE4, a specific hNE inhibitor, on ENaC activity in primary cultures of HNEC obtained from control (9) and CF (4) patients.ResultsNeither hNE nor EPI-hNE4 treatments did modify Isc in control and CF HNEC. Incubation with aprotinin, a Kunitz-type serine protease inhibitor that blocks the activity of endogenous CAPs, decreased Isc by 27.6% and 54% in control and CF HNEC, respectively. In control and CF HNEC pretreated with aprotinin, hNE did significantly stimulate Isc, an effect which was blocked by EPI-hNE4.ConclusionsThese results indicate that hNE does activate ENaC and transepithelial Na+ transport in both normal and CF HNEC, on condition that the activity of endogenous CAPs is first inhibited. The potent inhibitory effect of EPI-hNE4 on hNE-mediated ENaC activation observed in our experiments highlights that the use of EPI-hNE4 could be of interest to reduce ENaC hyperactivity in CF airways.
Highlights
Abnormalities in cyclic AMP-dependent chloride secretion and excessive sodium (Na+) reuptake by airway epithelial cells related to cystic fibrosis transmembrane conductance regulator (CFTR) deficiency are thought to alter fluid homeostasis at the airway surface liquid leading to dehydration, impaired mucociliary clearance, and infection [1]
Our results showed that neither human neutrophil elastase (hNE) nor trypsin treatment did modify Isc in normal and CF human nasal epithelial cells (HNEC), suggesting that epithelial sodium (Na+) channel (ENaC) at cell surface was already fully activated by endogenous serine proteases such as epithelial Channel-Activating Proteases (CAPs)
Inhibition of endogenous CAPs with aprotinin induced a sustained decrease in Isc in both normal and CF HNEC, supporting this hypothesis
Summary
Abnormalities in cyclic AMP-dependent chloride secretion and excessive sodium (Na+) reuptake by airway epithelial cells related to cystic fibrosis transmembrane conductance regulator (CFTR) deficiency are thought to alter fluid homeostasis at the airway surface liquid leading to dehydration, impaired mucociliary clearance, and infection [1]. In 1997, using functional complementation assays to detect increases in ENaC activity in the Xenopus kidney A6 renal cell line, Vallet et al (10) cloned a trypsin-like serine protease, the channel-activating protease 1 (CAP1). This glycosylphophatidylinositol-anchored protease increased amiloride-sensitive Na+ current when coexpessed ENaC in Xenopus oocytes [10,11]. The channel-activating proteases (CAP1,-2 and 3) are coexpressed with ENaC in epithelial tissues transporting Na+ like renal collecting duct, lung, and colon [12,19,20]. Whether hNE can activate Na+ reabsorption in primary human nasal epithelial cells (HNEC) from control or CF patients is currently unknown
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