Apical hyperosmotic challenge upregulates aquaporin-3 in human airway epithelia.

BACKGROUND: The respiratory epithelium is covered by the airway surface liquid (ASL). ASL volume depletion is a hallmark of cystic fibrosis and inhaled hyperosmotic agents are supposed to restore ASL volume und improve mucociliary clearance. We previously demonstrated that aquaporin 3 (AQP3) limits isosmotic fluid transport across the airway epithelium. The present study aimed on the role of AQP3 for ASL regulation of primary nasal epithelial cells (NEC) and NCI-H441 cells upon apical exposure to hyperosmotic solutions METHODS: We used NCI-H441 cells and NEC cultivated at air-liquid-interface as epithelial cell models. Epithelia were exposed to isosmotic or hyperosmotic saline or mannitol solutions. ASL volumes were determined in time-course experiments using deuterium oxide dilution method. AQP3 was investigated by semi-quantitative RT-PCR and immunocytochemistry.

RESULTS: Exposure of epithelia to hyperosmotic solutions led to ASL volume changes with initial secretion within in the first 4 to 8 h followed by volume reabsorption in both NCI-H441 and NEC. Begin of volume resorption coincided with transient AQP3 mRNA upregulation that was followed by AQP3 protein accumulation at the lateral membranes starting after 8 and increasing to 24 h after exposure to hyperosmotic solutions. AQP3 accumulation could be prevented by inhibition of epithelial sodium channels (ENaC) and was affected by pharmacological modulators of calcium homeostasis.

CONCLUSIONS: We conclude, that apical hyperosmotic conditions up-regulate AQP3 in airway epithelia via an ENaC and calcium dependent pathway. Probably, AQP3 regulation provides a counteractive mechanism that drives fluid reabsorption after osmotically induced fluid secretion.