Abstract
The β1-adrenergic receptor (β1AR) is the predominant βAR in the heart and is the main target for β-adrenergic antagonists, widely used in the treatment of cardiovascular diseases. Previously, we have shown that the human (h) β1AR is cleaved in its N terminus by a metalloproteinase, both constitutively and in a receptor activation–dependent manner. In this study, we investigated the specific events involved in β1AR regulation, focusing on the effects of long-term treatment with β-adrenergic ligands on receptor processing in stably transfected human embryonic kidney 293i cells. The key findings were verified using the transiently transfected hβ1AR and the endogenously expressed receptor in neonatal rat cardiomyocytes. By using flow cytometry and Western blotting, we demonstrated that isoproterenol, S-propranolol, CGP-12177 [4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one], pindolol, and timolol, which displayed agonistic properties toward the β1AR in either the adenylyl cyclase or the mitogen-activated protein kinase signaling pathways, induced cleavage of the mature cell-surface receptor. In contrast, metoprolol, bisoprolol, and CGP-20712 [1-[2-((3-carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol], which showed no agonistic activity, had only a marginal or no effect. Importantly, the agonists also stabilized intracellular receptor precursors, possibly via their pharmacological chaperone action, and they stabilized the receptor in vitro. The opposing effects on the two receptor forms thus led to an increase in the amount of cleaved receptor fragments at the plasma membrane. The results underscore the pluridimensionality of β-adrenergic ligands and extend this property from receptor activation and signaling to the regulation of β1AR levels. This phenomenon may contribute to the exceptional resistance of β1ARs to downregulation and tendency toward upregulation following long-term ligand treatments.
Footnotes
This work was supported by the Sigrid Jusélius Foundation and a grant (127199) from the Academy of Finland. A.E.H was supported by the Finnish Foundation for Cardiovascular Research, the Finnish Medical Foundation, the Finnish-Norwegian Medical Foundation, the Emil Aaltonen Foundation, and the Ida Montin Foundation. J.J.L. is a Ph.D. student of the Finnish Glycoscience Graduate School.
↵This article has supplemental material available at molpharm.aspetjournals.org.
- Received June 11, 2012.
- Accepted October 11, 2012.
- Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics
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