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Factors influencing biased agonism in recombinant cells expressing the human ?1A -adrenoceptor.

ABSTRACT: BACKGROUND AND PURPOSE:Agonists acting at GPCRs promote biased signalling via G? or G?? subunits, GPCR kinases and ?-arrestins. Since the demonstration of biased agonism has implications for drug discovery, it is essential to consider confounding factors contributing to bias. We have examined bias at human ?1A -adrenoceptors stably expressed at low levels in CHO-K1 cells, identifying off-target effects at endogenous receptors that contribute to ERK1/2 phosphorylation in response to the agonist oxymetazoline. EXPERIMENTAL APPROACH:Intracellular Ca2+ mobilization was monitored in a Flexstation® using Fluo 4-AM. The accumulation of cAMP and ERK1/2 phosphorylation were measured using AlphaScreen® proximity assays, and mRNA expression was measured by RT-qPCR. Ligand bias was determined using the operational model of agonism. KEY RESULTS:Noradrenaline, phenylephrine, methoxamine and A61603 increased Ca2+ mobilization, cAMP accumulation and ERK1/2 phosphorylation. However, oxymetazoline showed low efficacy for Ca+2 mobilization, no effect on cAMP generation and high efficacy for ERK1/2 phosphorylation. The apparent functional selectivity of oxymetazoline towards ERK1/2 was related to off-target effects at 5-HT1B receptors endogenously expressed in CHO-K1 cells. Phenylephrine and methoxamine showed genuine bias towards ERK1/2 phosphorylation compared to Ca2+ and cAMP pathways, whereas A61603 displayed bias towards cAMP accumulation compared to ERK1/2 phosphorylation. CONCLUSION AND IMPLICATIONS:We have shown that while adrenergic agonists display bias at human ?1A -adrenoceptors, the marked bias of oxymetazoline for ERK1/2 phosphorylation originates from off-target effects. Commonly used cell lines express a repertoire of endogenous GPCRs that may confound studies on biased agonism at recombinant receptors.

SUBMITTER: da Silva Junior ED

PROVIDER: S-EPMC5481649 | biostudies-literature | 2017 Jul

REPOSITORIES: biostudies-literature

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Factors influencing biased agonism in recombinant cells expressing the human α<sub>1A</sub> -adrenoceptor.

da Silva Junior Edilson Dantas ED Sato Masaaki M Merlin Jon J Broxton Natalie N Hutchinson Dana S DS Ventura Sabatino S Evans Bronwyn A BA Summers Roger J RJ

British journal of pharmacology 20170610 14

<h4>Background and purpose</h4>Agonists acting at GPCRs promote biased signalling via Gα or Gβγ subunits, GPCR kinases and β-arrestins. Since the demonstration of biased agonism has implications for drug discovery, it is essential to consider confounding factors contributing to bias. We have examined bias at human α<sub>1A</sub> -adrenoceptors stably expressed at low levels in CHO-K1 cells, identifying off-target effects at endogenous receptors that contribute to ERK1/2 phosphorylation in respon ...[more]

PMID: 28444738

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Project description:The ?1A-AR is thought to couple predominantly to the G?q/PLC pathway and lead to phosphoinositide hydrolysis and calcium mobilization, although certain agonists acting at this receptor have been reported to trigger activation of arachidonic acid formation and MAPK pathways. For several G protein-coupled receptors (GPCRs) agonists can manifest a bias for activation of particular effector signaling output, i.e., not all agonists of a given GPCR generate responses through utilization of the same signaling cascade(s). Previous work with G?q coupling-defective variants of ?1A-AR, as well as a combination of Ca2+ channel blockers, uncovered cross-talk between ?1A-AR and ?2-AR that leads to potentiation of a G?q-independent signaling cascade in response to ?1A-AR activation. We hypothesized that molecules exist that act as biased agonists to selectively activate this pathway. In this report, isoproterenol (Iso), typically viewed as ?-AR-selective agonist, was examined with respect to activation of ?1A-AR. ?1A-AR selective antagonists were used to specifically block Iso evoked signaling in different cellular backgrounds and confirm its action at ?1A-AR. Iso induced signaling at ?1A-AR was further interrogated by probing steps along the G?q /PLC, G?s and MAPK/ERK pathways. In HEK-293/EBNA cells transiently transduced with ?1A-AR, and CHO_?1A-AR stable cells, Iso evoked low potency ERK activity as well as Ca2+ mobilization that could be blocked by ?1A-AR selective antagonists. The kinetics of Iso induced Ca2+ transients differed from typical G?q- mediated Ca2+ mobilization, lacking both the fast IP3R mediated response and the sustained phase of Ca2+ re-entry. Moreover, no inositol phosphate (IP) accumulation could be detected in either cell line after stimulation with Iso, but activation was accompanied by receptor internalization. Data are presented that indicate that Iso represents a novel type of ?1A-AR partial agonist with signaling bias toward MAPK/ERK signaling cascade that is likely independent of coupling to G?q.

Project description:Two splice isoforms of rabbit alpha(1a)-adrenergic receptor (AR), (named alpha(1a)-OCU.2-AR and alpha(1a)-OCU.3-AR) have been isolated from the liver cDNA library in addition to the previously reported isoform (alpha(1a)-OCU.1-AR). Although they have the identical splice position with human alpha(1a)-AR isoforms, the C-terminal sequences are distinct from those of human isoforms. Among these rabbit alpha(1a)-AR isoforms, there are no significant differences in pharmacological properties: high affinity for prazosin, WB4101, KMD-3213 and YM617 and low affinity for BMY7378, using COS-7 cells expressing each isoform by radioligand binding assay. Competitive reverse transcription-polymerase chain reaction (RT - PCR) analysis revealed that mRNA of alpha(1a)-ARs was expressed in liver, thoracic aorta, brain stem and thalamus of rabbit. The splice isoforms exhibited a distinct distribution pattern in rabbit; alpha(1a)-OCU. 1-AR was expressed most abundantly in those tissues. CHO clones, stably expressing each isoforms with receptor density 740 fmol mg(-1) protein in alpha(1a)-OCU.1-AR, 1200 fmol mg(-1) in alpha(1a)-OCU.2-AR and 570 fmol mg(-1) in alpha(1a)-OCU.3-AR, respectively, showed a noradrenaline-induced increase in inositol trisphosphate which was suppressed by prazosin. Noradrenaline elicited a concentration-dependent increase in extracellular acidification rate (EAR) in the CHO clones with pEC(50) values of 6. 19 for alpha(1a)-OCU.1-AR, 6.49 for alpha(1a)-OCU.2-AR and 6.58 for alpha(1a)-OCU.3-AR, respectively. Noradrenaline caused a concentration-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)]i) in the CHO clones with pEC(50) values of 6. 14 for alpha(1a)-OCU.1-AR, 7.25 for alpha(1a)-OCU.2-AR and 7.70 for alpha(1a)-OCU.3-AR, respectively. In conclusion, the present study shows the occurrence of three splice isoforms of rabbit alpha(1a)-AR, which are unique in C-terminal sequence and in tissue distribution. They show similar pharmacological profiles in binding studies but alpha(1a)-OCU.3-AR had the highest potency of noradrenaline in functional studies in spite of the lowest receptor density. These findings suggest that the structure of C-terminus of alpha(1a)-ARs may give the characteristic functional profile.

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Factors influencing biased agonism in recombinant cells expressing the human ?1A -adrenoceptor.

Publications

Factors influencing biased agonism in recombinant cells expressing the human α<sub>1A</sub> -adrenoceptor.

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