Role of hyaluronan and CD44 in reactive oxygen species-induced mucus hypersecretion

Mucus hypersecretion is an important manifestation in patients with chronic inflammatory airway diseases. Mucin 5AC (MUC5AC) is a major component of airway mucus. MUC5AC expression is regulated by epidermal growth factor receptor (EGFR) which can be activated by reactive oxygen species (ROS). Hyaluronan (HA), a linear glycosaminoglycan with molecular weights ranging from 2 x 10(5) to 1 x 10(7), is expressed in airway epithelium and can be depolymerized by ROS into hyaluronan fragments. The mechanisms through which fragmented HA exerts its biologic functions have been elucidated by interactions with its receptor CD44. The aim of our study was to examine the role of HA and CD44 in ROS-induced EGFR activation and MUC5AC expression. We exposed NCI-H292 cells to ROS generated by xanthine/xanthine oxidase (X/XO). ROS-induced EGFR phosphorylation, which was activated by tissue kallekrein (TK) activation and EGF release. We found ROS promoted CD44 co-immunoprecipitation with EGFR and MUC5AC up-regulation. These effects were mimicked by hyaluronan fragments. All the effects were inhibited by blocking CD44 or EGFR, suggesting that CD44 plays a critical role in ROS-induced MUC5AC up-regulation. These results show that ROS depolymerizes hyaluronan into fragments, and these fragments bind their receptor CD44 to induce TK activation, which cleaves EGF precursors into mature EGF to activate its receptor EGFR. Furthermore, we provide evidence that hyaluronan fragments are sufficient to induce CD44/EGFR interaction and EGFR signaling which lead to MUC5AC expression. The results indicate that the regulation of ROS-induced MUC5AC expression by hyaluronan and CD44 may provide important insights in the mechanism of mucus hypersecretion.