Agonists and hydrogen peroxide mediate hyperoxidation of ß2-adrenergic receptor in airway epithelial cells: Implications for tachyphylaxis to ß2-agonists in constrictive airway disorders.

ABSTRACT

Asthma and other airway obstructive disorders are characterized by heightened inflammation and excessive airway epithelial cell reactive oxygen species (ROS), which give rise to a highly oxidative environment. After decades of use, ß2-adrenergic receptor (ß2AR) agonists remain at the forefront of treatment options for asthma, however, chronic use of ß2-agonists leads to tachyphylaxis to the bronchorelaxant effects, a phenomenon that remains mechanistically unexplained. We have previously demonstrated that ß2AR agonism increases ROS generation in airway epithelial cells, which upholds proper receptor function via feedback oxidation of ß2AR cysteine thiolates to Cys-S-sulfenic acids (Cys-SOH). Our previous results also demonstrate that prevention of normal redox cycling of this post-translational oxi-modification back to the thiol prevents proper receptor function. Given that Cys-S-sulfenic acids can be irreversibly overoxidized to Cys-S-sulfinic (Cys-SO2H) or S-sulfonic (Cys-SO3H) acids, which are incapable of further participation in redox reactions, we hypothesized that ß2-agonist tachyphylaxis may be explained by hyperoxidation of ß2AR to S-sulfinic acids. Here, using airway epithelial cell lines and primary small airway epithelial cells from healthy and asthma-diseased donors, we show that ß2AR agonism generates H2O2 in a receptor and NAPDH oxidase-dependent manner. We also demonstrate that acute and chronic receptor agonism can facilitate ß2AR S-sulfination, and that millimolar H2O2 concentrations are deleterious to ß2AR-mediated cAMP formation, an effect that can be rescued to a degree in the presence of the cysteine-donating antioxidant N-acetyl-L-cysteine. Our results reveal that the oxidative state of ß2AR may contribute to receptor functionality and may, at least in part, explain ß2-agonist tachyphylaxis.