Specific changes in beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation.
Agonist-activated beta2-adrenoceptors rapidly internalize and then recycle to the cell surface, however chronic agonist eventually causes receptor downregulation. To characterize beta2-adrenoceptor trafficking kinetics and intracellular sorting during downregulation, human embryonic kidney cells expressing epitope-tagged receptors were examined by radioligand binding with (+/-)-[3H]4-(3-tertiarybutylamino-2-hydroxypropoxy)-benzimidazole- 2-on hydrochloride ([3H]CGP12177) and immunofluorescence microscopy. The first-order receptor recycling rate constant declined after 18 h of agonist compared with 15 min (0.05 min(-1) vs. 0.12 min(-1)), thus increasing the intracellular transit time (20.0 min vs. 8.3 min). There was also a reduction in the rate of receptor endocytosis and a decline in the total number of receptors. Although the intracellular receptor fraction did not increase between 15 min and 18 h of agonist, some receptors moved irreversibly into a protease-containing compartment while retaining radioligand binding activity. Our results indicate that beta2-adrenoceptor downregulation is associated principally with an increased intracellular transit time during recycling. This could promote the diversion of receptors into protease-containing compartments, where there is an irreversible commitment to downregulation prior to loss of radioligand binding activity.
Moore
R H
RH
Department of Pediatrics (Pulmonary), Baylor College of Medicine, Houston, TX 77030, USA.
Hall
H S
HS
Rosenfeld
J L
JL
Dai
W
W
Knoll
B J
BJ
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000378:metabolism / Q000494:pharmacology; D001692:Biological Transport; D002460:Cell Line; D015536:Down-Regulation; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D011412:Propanolamines / Q000378:metabolism / Q000494:pharmacology; D011480:Protease Inhibitors / Q000494:pharmacology; D011869:Radioligand Assay; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects / Q000502:physiology*; D012680:Sensitivity and Specificity; D013997:Time Factors; D014316:Tritium
D000318:Adrenergic beta-Agonists; D000319:Adrenergic beta-Antagonists; D011412:Propanolamines; D011480:Protease Inhibitors; D018343:Receptors, Adrenergic, beta-2; D014316:Tritium; D007545:Isoproterenol; C033394:CGP 12177