Complexity of agonist- and cyclic AMP-mediated downregulation of the human beta 1-adrenergic receptor: role of internalization, degradation, and mRNA destabilization

• Published in: Biochemistry (Easton) 2002-06, Vol.41 (25), p.8019-8030
• Main Authors: Dunigan, Cheryl D, Hoang, Quang, Curran, Patricia K, Fishman, Peter H
• Format: Article
• Language: English
• Subjects: Adrenergic beta-1 Receptor Antagonists; Adrenergic beta-2 Receptor Antagonists; Adrenergic beta-Agonists - pharmacology; Amino Acid Sequence; Animals; Blotting, Western; Cell Line; Cricetinae; Cyclic AMP - analogs & derivatives; Cyclic AMP - pharmacology; Cyclic AMP - physiology; Down-Regulation - drug effects; Humans; Isoproterenol - pharmacology; Leupeptins - pharmacology; Molecular Sequence Data; Pepstatins - pharmacology; Protease Inhibitors - pharmacology; Receptors, Adrenergic, beta-1 - genetics; Receptors, Adrenergic, beta-1 - metabolism; Receptors, Adrenergic, beta-1 - physiology; Receptors, Adrenergic, beta-2 - metabolism; RNA Stability - drug effects; RNA, Messenger - antagonists & inhibitors; RNA, Messenger - metabolism; Thionucleotides - pharmacology
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi025538r

Prolonged agonist exposure often induces downregulation of G protein-coupled receptors (GPCRs). Although downregulation of the prototypical beta(2)-adrenergic receptor (beta(2)AR) has been extensively studied, the underlying mechanisms have yet to be resolved. As even less is known about the beta(1)-subtype, we investigated the downregulation of human beta(1)AR stably expressed in Chinese hamster fibroblasts in response to the agonist isoproterenol or the cell-permeable, chlorophenylthio-cAMP (CPT-cAMP). While either effector mediated decreases in both beta(1)AR binding activity and steady-state beta(1)AR mRNA levels, there were significant differences in their actions. Whereas agonist-mediated downregulation of beta(1)AR followed first-order kinetics, that induced by CPT-cAMP was delayed for several hours and approximately 50% of the former. Furthermore, agonist but not CPT-cAMP induced beta(1)AR internalization, and inhibiting internalization also suppressed agonist-mediated downregulation. The latter, however, was more sensitive than the former to agonist concentration (EC(50) of 0.3 vs 48 nM). Thus, at < or =1 nM agonist, downregulation occurred without internalization and with a pattern similar to that mediated by CPT-cAMP. The amounts of beta(1)AR downregulated or internalized were proportional to initial receptor levels but reached saturation at approximately 2 and 3 pmol/mg of protein, respectively. The fate of beta(1)AR protein during downregulation was determined by immunoblotting with anti-C-terminal antibodies. In agonist-treated cells, beta(1)AR protein disappeared with time and without any immunoreactive degradation products. Agonist-mediated downregulation of the human beta(1)AR appears to be a complex process that consists of both agonist- and cAMP-specific components. The former involves both receptor internalization and degradation whereas the latter involves a reduction in receptor mRNA.