Ligand-directed signalling at beta-adrenoceptors

Abstract

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

Figure 1

Effector pathways stimulated by β-ARs. Each panel shows pathways that have been demonstrated experimentally across different cell types. Some effector mechanisms are observed in multiple systems, whereas others are more restricted or opposite between cell types. For example, β₃-AR-stimulated activation of p38 MAPK is mediated by a cAMP–PKA pathway in adipocytes, whereas in CHO-K1 cells expressing the β₃-AR, increasing levels of cAMP cause inhibition of p38 MAPK phosphorylation (Sato et al., 2007). Note that for the β₂-AR, arrestin-mediated Erk1/2 phosphorylation can be dependent on EGFR transactivation (e.g. Maudsley et al., 2000a), but in many studies the involvement of EGFR has not been investigated. G* denotes activated forms of Gs or Gi/o, and R*, R‡, R^# and R′ represent active receptor conformations. In light of recent evidence, G*αsG*βγ is shown as an intact heterotrimer, while G*αi/o and G*βγ are shown as dissociated subunits (Digby et al., 2006). References: ¹ Leblais et al., 2004; ² Morisco et al., 2005; ³ Galandrin et al., 2008; Kim et al., 2008; ⁴ Kim et al., 2008; ⁵ Schmitt and Stork, 2002b; ⁶ Zheng et al., 2000; ⁷ Gong et al., 2008; ⁸ Ma et al., 2000; ⁹ Ciccarelli et al., 2007; ¹⁰ Yamauchi et al., 1997; ¹¹ Yano et al., 2007; ¹² Shenoy et al., 2006; ¹³ Maudsley et al., 2000a; ¹⁴ Luttrell et al., 1999; ¹⁵ Sun et al., 2007; ¹⁶ Lindquist et al., 2000; ¹⁷ Cao et al., 2001; ¹⁸ Westphal et al., 2008; ¹⁹ Gerhardt et al., 1999; Soeder et al., 1999; ²¹ Sato et al., 2008; ²² Cao et al., 2000; Hutchinson et al., 2002.

Figure 2

Two approaches to achieving selectivity of action at G protein-coupled receptors. In (A), three drugs, A, B and C, interact with three signalling pathways, E1–E3, that display different efficiencies of coupling to their functional response. For effector pathway E1, drug A is a full agonist, but B and C also have high efficacy; in E2 where the coupling efficiency is lower, only A is a full agonist, B is a low-efficacy partial agonist and C has no agonist properties; in E3 which is poorly coupled, only A has any agonist properties. Although some selectivity has been achieved as B and C will be agonists in some tissues and likely antagonists in others, this does not amount to ligand-directed signalling. However, in (B), the two drugs, A and B, display reversal of efficacy. For effectors E1 and E3, drug A acts as an agonist, whereas B is a low-efficacy agonist or has no effect. However, for effector E2, drug B is a full agonist, whereas A has no effect. The reversal of efficacy seen for E1 versus E2, and E2 versus E3 strongly suggests ligand-directed signalling.

Figure 3

The web of efficacy. A series of seven β-AR ligands were compared using four reporter gene assays, activator protein-1 (AP-1; JNK); cAMP response element (CRE; PKA and JNK/p38MAPK); nuclear factor of κB (NF-κB); and serum response element (SRE; MAPK/JNK). Note the similar profile exhibited by carvedilol and nebivolol, and the different profile shown by the prototypical β-AR antagonist propranolol. Bupranolol was a neutral or inverse agonist in the reporter gene assays. The human selective β₃-AR ligands L755507 and L748337 had similar profiles apart from that in the AP-1 reporter gene assay.