β2 -Adrenoceptor agonist profiling reveals biased signalling phenotypes for the β2 -adrenoceptor with possible implications for the treatment of asthma

Abstract

Background and purpose: β-Adrenoceptor agonists relieve airflow obstruction by activating β₂ -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 β₂ -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 β₂ -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 β₂ -adrenoceptor.

Key results: We identified ractopamine, dobutamine, and higenamine as G_s -biased agonists that activate the G_s /cAMP pathway upon β₂ -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 β₂ -adrenoceptor. Finally, we observed minimal physiological desensitization of the β₂ -adrenoceptor in primary HASM cells upon treatment with biased agonists.

Conclusion and implications: Our work demonstrates that G_s -biased signalling through the β₂ -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.

Keywords: G protein; G protein-coupled receptor; airway smooth muscle; asthma; biased signalling; desensitization; β-arrestins; β2-adrenoceptor.