Chitinase activates protease-activated receptor-2 in human airway epithelial cells

Mammalian chitinase released by airway epithelia is thought to be an important mediator of disease manifestation in an experimental model of asthma. However, the intracellular signaling mechanisms engaged by exogenous chitinase in human airway epithelial cells are unknown. Here, we investigated the direct effects of exogenous chitinase from Streptomyces griseus on Ca²⁺ signaling in human airway epithelial cells. Spectrofluorometry was used to measure intracellular Ca²⁺ concentration ([Ca²⁺] _i) in fura-2-AM-loaded cells. S. griseus chitinase induced dose-dependent [Ca²⁺]_i increases in normal human bronchial epithelial cells and promoted [Ca²⁺]_i oscillations in H292 cells. Chitinase-induced [Ca²⁺]_i oscillations were independent of extracellular Ca²⁺, suggesting that the observed [Ca²⁺]_i increases were due to Ca²⁺ release from intracellular stores. Accordingly, after depleting endoplasmic reticulum (ER) Ca²⁺ with the ER Ca²⁺ ATPase inhibitor, thapsigargin, chitinase-mediated [Ca²⁺]_i increases were abolished. Treatment with the phospholipase C (PLC) inhibitor U73122 or the 1, 4, 5-trisinositolphosphate (IP₃) receptor inhibitor 2-APB attenuated chitinase-induced [Ca²⁺]_i increases. Desensitization of protease-activated receptor-2 (PAR-2) by repetitive agonist stimulation or siRNA-mediated PAR-2 knockdown revealed that chitinase-mediated [Ca ²⁺]_i increases were exclusively mediated by PAR-2 activation. Finally, chitinase was found to cleave a model peptide representing the cleavage site of PAR-2 and enhanced IL-8 production. These results indicate that exogenous chitinase is a potent proteolytic activator of PAR-2 that can directly induce PLC/IP₃-dependent Ca²⁺ signaling in human airway epithelial cells.