The effects of acute and chronic nadolol treatment on β2AR signaling in HEK293 cells.

Nadolol (NAD) is a β-adrenergic receptor blocker with inverse agonist activity at βARs. Previous studies in our laboratory showed that chronic treatment with NAD decreased airway resistance response (R (aw)) to the muscarinic agonist methacholine in a murine model of asthma while acute treatment with NAD increased R (aw) (Callaerts-Vegh et al., Proc Natl Acad Sci U S A 101:4948-4953, 2004). Chronic treatment with NAD also caused decreased airway inflammation and mucin content in a murine asthma model (Nguyen et al., Am J Respir Cell Mol Biol 38:256-262, 2008). In this study, we examined the effects of nadolol on β(2)AR levels and signaling components downstream of the β(2)AR using a line of HEK293 cells expressing human β(2)ARs. Chronic treatment with NAD increased β(2)AR protein levels and decreased receptor degradation, consistent with receptor stabilization by the inverse agonist. Basal cAMP levels decreased after 5 min of treatment with NAD but increased after a 24-h treatment. A 5-min treatment with NAD decreased forskolin-stimulated phosphorylation at the β(2)AR PKA site Ser 262 while a 24-h treatment with NAD increased it. In contrast, chronic treatment with NAD had no effect on phosphorylation of the β(2)AR GRK site at Ser 355, 356. Chronic treatment with NAD upregulated cellular levels of G(α)s but had no effect on G(α)i. Chronic NAD treatment therefore increases cellular cAMP levels by mechanisms that include the upregulation of β(2)AR and G(α)s. This effect may explain in part the beneficial effects of chronic nadolol treatment on airway contractility.