Membrane-tethered MUC1 Mucin is Phosphorylated by Epidermal Growth Factor Receptor in Airway Epithelial Cells and Associates with TLR5 to Inhibit Recruitment of MyD88

MUC1 is a membrane-tethered mucin glycoprotein expressed on the apical surface of mucosal epithelial cells. Previous in vivo and in vitro studies established that MUC1 counterregulates airway inflammation by suppressing TLR signaling. In this article, we elucidate the mechanism by which MUC1 inhibits TLR5 signaling. Overexpression of MUC1 in HEK293 cells dramatically reduced Pseudomonas aeruginosa-stimulated IL-8 expression and decreased the activation of NF-κB and MAPK compared with cells not expressing MUC1. However, overexpression of MUC1 in HEK293 cells did not affect NF-κB or MAPK activation in response to TNF-α. Overexpression of MyD88 abrogated the ability of MUC1 to inhibit NF-κB activation, and MUC1 overexpression inhibited flagellin-induced association of TLR5/MyD88 compared with controls. The MUC1 cytoplasmic tail associated with TLR5 in all cells tested, including HEK293T cells, human lung adenocarcinoma cell line A549 cells, and human and mouse primary airway epithelial cells. Activation of epidermal growth factor receptor tyrosine kinase with TGF-α induced phosphorylation of the MUC1 cytoplasmic tail at the Y46EKV sequence and increased association of MUC1/TLR5. Finally, in vivo experiments demonstrated increased immunofluorescence colocalization of Muc1/TLR5 and Muc1/phosphotyrosine staining patterns in mouse airway epithelium and increased Muc1 tyrosine phosphorylation in mouse lung homogenates following P. aeruginosa infection. In conclusion, epidermal growth factor receptor tyrosine phosphorylates MUC1, leading to an increase in its association with TLR5, thereby competitively and reversibly inhibiting recruitment of MyD88 to TLR5 and downstream signaling events. This unique ability of MUC1 to control TLR5 signaling suggests its potential role in the pathogenesis of chronic inflammatory lung diseases.