Protective effects of a glucocorticoid on downregulation of pulmonary β2-adrenergic receptors in vivo

Abstract

We investigated the in vivo effects of a glucocorticoid on β-agonist- induced downregulation of β1- and β2-adrenergic receptors (determined by [125I]iodocyanopindolol binding), mRNA expression (assessed by Northern blotting), and gene transcription (using nuclear run-on assays) in rat lung. Dexamethasone (Dex) (0.2 mg/kg/d, days 1-8) increased β1- and β2-receptor numbers by 70 and 69% above control, respectively, but did not change their mRNA expression. Isoproterenol (Iso) (0.96 mg/kg/d, days 2-8) decreased β1- and β2-receptor numbers by 48 and 51%, respectively, and also reduced mRNA expression by 69 and 57%, respectively. The combination of Dex and Iso resulted in no net change in β2-receptor number and its mRNA expression, although there was a significant reduction in β1-receptor number and mRNA expression. The mapping of β1- and β2-receptors by receptor autoradiography confirmed these findings over alveoli, epithelium, endothelium, and airway and vascular smooth muscle. We also measured the activation of the transcription factor, cyclic AMP response element binding protein (CREB) using an electrophoretic mobility shift assay. CREB-like DNA- binding activity was decreased after Iso treatment but this decrease was prevented after treatment with Dex. Nuclear run-on assays revealed that the transcription rate of the β1-receptor gene did not alter after Dex treatment, but was reduced after Iso treatment. The transcription rate of the β2-receptor gene was increased after Dex treatment by approximately twofold, but there was no change after Iso treatment. We conclude that glucocorticoids can prevent homologous downregulation of β2-receptor number and mRNA expression at the transcriptional level without affecting β1- receptors and that the transcription factor CREB may be involved in this phenomenon. Such an effect may have clinical implications for preventing the development of tolerance to β2-agonists in asthmatic patients treated with β-agonist bronchodilators.