A long-acting β(2)-adrenergic agonist increases the expression of muscarine cholinergic subtype-3 receptors by activating the β(2)-adrenoceptor cyclic adenosine monophosphate signaling pathway in airway smooth muscle cells

The persistent administration of β(2)-adrenergic (β(2)AR) agonists has been demonstrated to increase the risk of severe asthma, partly due to the induction of tolerance to bronchoprotection via undefined mechanisms. The present study investigated the potential effect of the long-acting β(2)-adrenergic agonist, formoterol, on the expression of muscarinic M3 receptor (M(3)R) in rat airway smooth muscle cells (ASMCs). Primary rat ASMCs were isolated and characterized following immunostaining with anti-α-smooth muscle actin antibodies. The protein expression levels of M(3)R and phospholipase C-β(1) (PLCβ(1)) were characterized by western blot analysis and the production of inositol 1,4,5-trisphosphate (IP(3)) was determined using an enzyme-linked immunosorbent assay. Formoterol increased the protein expression of M(3)R in rat ASMCs in a time- and dose-dependent manner, which was significantly inhibited by the β(2)AR antagonist, ICI118,551 and the cyclic adenosine monophosphate (cAMP) inhibitor, SQ22,536. The increased protein expression of M(3)R was positively correlated with increased production of PLCβ(1) and IP(3). Furthermore, treatment with the glucocorticoid, budesonide, and the PLC inhibitor, U73,122, significantly suppressed the formoterol-induced upregulated protein expression levels of M(3)R and PLCβ(1) and production of IP(3). The present study demonstrated that formoterol mediated the upregulation of M(3)R in the rat ASMCs by activating the β(2)AR-cAMP signaling pathway, resulting in increased expression levels of PLCβ(1) and IP(3), which are key to inducing bronchoprotection tolerance. Administration of glucocorticoids or a PLC antagonist prevented formoterol-induced bronchoprotection tolerance by suppressing the protein expression of M(3)R.