AJP - Lung Cellular and Molecular Physiology, Vol 272, Issue 5 916-L924, Copyright © 1997 by American Physiological Society

ARTICLES

Chronic effects of catecholamines on the beta 2-adrenoreceptor system in cultured human airway epithelial cells

S. G. Kelsen, O. O. Anakwe, M. O. Aksoy, P. J. Reddy, N. Dhanesekaran, R. Penn and J. L. Benovic
Department of Medicine, Temple University School of Medicine, Philadelphia 19140, USA.

Chronic catecholamine treatment induces beta-adrenergic receptor (beta AR) downregulation, i.e., a loss of total cell receptors. In the human respiratory tract, the mechanism(s) underlying beta AR downregulation remains poorly understood. The present study, therefore, examined the effects of 24 h of exposure to isoproterenol (Iso; 10 nM or 1 microM) on beta AR density and the rate of beta AR degradation, steady-state beta 2-adrenergic receptor (beta 2 AR) mRNA levels, and the content of Gs alpha and Gi alpha proteins in cultured human bronchial epithelial cells (i.e., the BEAS-2B cell line). beta AR density assessed by binding with [125I]iodopindolol decreased in a dose-dependent fashion with 24 h of Iso exposure. With Iso (1 microM), beta AR density decreased by approximately 82%. In contrast, forskolin (100 microM) and dibutyryl adenosine 3',5'-cyclic monophosphate (1 mM), agents that also increase adenosine 3',5'-cyclic monophosphate (cAMP) levels, had no significant effect on beta AR density. Iso exposure also elicited a concomitant decrease in Iso-stimulated cAMP but had no significant effect on the content of the G proteins G alpha i2 and Gs alpha assessed by immunoblotting and toxin-catalyzed ADP ribosylation. Of note, Iso exposure (1 microM) had no effect on steady-state levels of beta 2 AR mRNA measured both by Northern analysis and by reverse transcriptase-polymerase chain reaction. However, beta AR half-life assessed in the presence of the protein synthesis inhibitor cycloheximide was reduced by approximately 60% in Iso-treated cells (i.e., from 37 h in control to 16 h in 1 microM Iso). These results suggest that, in human airway epithelial cells, beta 2 AR downregulation 1) is not primarily driven by intracellular cAMP levels, 2) is not associated with significant decreases in steady-state levels of beta 2 AR mRNA, and 3) is largely posttranslationally regulated by increases in the rate of receptor protein degradation.

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