β 2 -Adrenoceptor agonist profiling reveals biased signalling phenotypes for the β 2 -adrenoceptor with possible implications for the treatment of asthma.

Background and Purpose: β-Adrenoceptor agonists relieve airflow obstruction by activating β ₂ -adrenoceptors, which are G protein-coupled receptors (GPCRs) expressed on human airway smooth muscle (HASM) cells. The currently available β-adrenoceptor agonists are balanced agonists, however, and signal through both the stimulatory G protein (G _s )- and β-arrestin-mediated pathways. While G _s signalling is beneficial and promotes HASM relaxation, β-arrestin activation is associated with reduced G _s efficacy. In this context, biased ligands that selectively promote β ₂ -adrenoceptor coupling to G _s signalling represent a promising strategy to treat asthma. Here, we examined several β-adrenoceptor agonists to identify G _s -biased ligands devoid of β-arrestin-mediated effects.
Experimental Approach: G _s -biased ligands for the β ₂ -adrenoceptor were identified by high-throughput screening and then evaluated for G _s interaction, G _i interaction, cAMP production, β-arrestin interaction, GPCR kinase (GRK) phosphorylation of the receptor, receptor trafficking, ERK activation, and functional desensitization of the β ₂ -adrenoceptor.
Key Results: We identified ractopamine, dobutamine, and higenamine as G _s -biased agonists that activate the G _s /cAMP pathway upon β ₂ -adrenoceptor stimulation while showing minimal G _i or β-arrestin interaction. Furthermore, these compounds did not induce any receptor trafficking and had reduced GRK5-mediated phosphorylation of the β ₂ -adrenoceptor. Finally, we observed minimal physiological desensitization of the β ₂ -adrenoceptor in primary HASM cells upon treatment with biased agonists.
Conclusion and Implications: Our work demonstrates that G _s -biased signalling through the β ₂ -adrenoceptor may prove to be an effective strategy to promote HASM relaxation in the treatment of asthma. Such biased compounds may also be useful in identifying the molecular mechanisms that determine biased signalling and in design of safer drugs.
(© 2022 British Pharmacological Society.)