The dopamine D1 receptor is expressed and induces CREB phosphorylation and MUC5AC expression in human airway epithelium

Background: Dopamine receptors comprise two subgroups, G(s) protein-coupled D-1-like receptors (D-1, D-5) and G(1)-coupled D-2-like receptors (D-2, D-3, D-4). In airways, both dopamine D-1 and D-2 receptors are expressed on airway smooth muscle and regulate airway smooth muscle force. However, functional expression of the dopamine D-1 receptor has never been identified on airway epithelium. Activation of G(s)-coupled receptors stimulate adenylyl cyclase leading to cyclic AMP (cAMP) production, which is known to induce mucus overproduction through the cAMP response element binding protein (CREB) in airway epithelial cells. We questioned whether the dopamine D1 receptor is expressed on airway epithelium, and whether it promotes CREB phosphorylation and MUC5AC expression. Methods: We evaluated the protein expression of the dopamine D-1 receptor on native human airway epithelium and three sources of cultured human airway epithelial cells including primary cultured airway epithelial cells, the bronchial epithelial cell line (16HBE14o-), and the pulmonary mucoepidermoid carcinoma cell line (NCI-H292) using immunohistochemistry and immunoblotting. To characterize the stimulation of cAMP through the dopamine D-1 receptor, 16HBE14o- cells and NCI-H292 cells were treated with dopamine or the dopamine D-1 receptor agonists (SKF38393 or A68930) before cAMP measurements. The phosphorylation of CREB by A68930 in both 16HBE14o- and NCI-H292 cells was measured by immunoblot. The effect of dopamine or A68930 on the expression of MUC5AC mRNA and protein in NCI-H292 cells was evaluated by real-time PCR and immunofluorescence staining, respectively. Results: The dopamine D-1 receptor protein was detected in native human airway epithelium and three sources of cultured human airway epithelial cells. Dopamine or the dopamine D-1-like receptor agonists stimulated cAMP production in 16HBE14o- cells and NCI-H292 cells, which was reversed by the selective dopamine D-1-like receptor antagonists (SCH23390 or SCH39166). A68930 significantly increased phosphorylation of CREB in both 16HBE14o- and NCI-H292 cells, which was attenuated by the inhibitors of PKA (H89) and MEK (U0126). Expression of MUC5AC mRNA and protein were also increased by either dopamine or A68930 in NCI-H292 cells. Conclusions: These results suggest that the activation of the dopamine D-1 receptor on human airway epithelium could induce mucus overproduction, which could worsen airway obstructive symptoms.