Oxidized Phosphatidylcholines Inhibit Airway Smooth Muscle β2-Adrenergic Receptors via Protein Kinase C

Understanding mechanisms for β2 adrenergic receptor (β2AR) insensitivity in asthmatics is incomplete. We discovered that the accumulation of oxidized phosphatidylcholines (OxPC) in the lung correlates with airway hyperresponsiveness, and OxPC induce contraction and cytokine synthesis in human airway smooth muscle (ASM) cells. Here, we test whether OxPC impair β2AR agonist bronchodilator responses and related mechanism(s) for β2AR insensitivity. Using tracheal rings from BALB/c mice, we assayed the effects of oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (OxPAPC) on β2AR agonist (isoproterenol (Iso))-induced relaxation of methacholine (MCh)-contracted airways. OxPAPC pre-exposure attenuated Iso-induced airway relaxation, increasing Iso EC50 4.3-fold and reducing maximum relaxation by 12.2%. OxPAPC did not affect adenylyl cyclase (AC) mediated relaxation induced by forskolin. OxPAPC significantly inhibited Iso-induced bronchodilation in murine precision cut lung slices (PCLS). Intranasal OxPAPC pre-challenge significantly inhibited albuterol-mediated suppression of MCh-induced respiratory resistance in BALB/c mice. In cultured human ASM, OxPAPC dose-dependently inhibited Iso- (but not forskolin) induced phosphorylation of the Protein Kinase A substrate VASP by up to 53%. Live cell cADDis assay for intracellular cAMP confirmed that OxPAPC dose-dependently reduces Iso-elicited cAMP generation up to 50%. OxPAPC did not reduce the abundance of cell surface HA-tagged human β2AR in HEK-293 cells. However, inhibition of Protein Kinase C prevented the suppressive effects of OxPAPC on both Iso-induced relaxation of murine tracheal rings, and cAMP signaling in cultured HASM cells. In summary, OxPAPC impairs β2AR agonist induced bronchodilation by PKC-dependent suppression of β2AR-mediated cAMP signaling upstream of AC in ASM cells. These observations reveal a new mechanism for bronchodilator insensitivity.