Apical hyperosmotic challenge upregulates aquaporin-3 in human airway epithelia

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.

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.

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.