Chronic effects of catecholamines on the beta 2-adrenoreceptor system in cultured human airway epithelial cells.
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.
Kelsen
S G
SG
Department of Medicine, Temple University School of Medicine, Philadelphia 19140, USA.
Anakwe
O O
OO
Aksoy
M O
MO
Reddy
P J
PJ
Dhanesekaran
N
N
Penn
R
R
Benovic
J L
JL
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D001980:Bronchi / Q000166:cytology / Q000378:metabolism*; D002460:Cell Line; D000242:Cyclic AMP / Q000096:biosynthesis; D015536:Down-Regulation; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology*; D012333:RNA, Messenger / Q000378:metabolism; D011943:Receptors, Adrenergic, beta / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism*; D013997:Time Factors
D000318:Adrenergic beta-Agonists; D012333:RNA, Messenger; D011943:Receptors, Adrenergic, beta; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D007545:Isoproterenol