Functional Selectivity of GPCR Ligand Stereoisomers: New Pharmacological Opportunities

Abstract

It is now well established that any given ligand for a G-proteincoupled receptor (GPCR) does not simply possess a single defined efficacy. Rather, a ligand possesses multiple efficacies, depending on the specific down-stream signal transduction pathway analyzed. This diversity may be based on ligandspecific GPCR conformations and is often referred to as "functional selectivity." It has been known for a century that stereoisomers of catecholamines differ in their potency and, in some systems, also in their efficacy. However, the molecular basis for efficacy differences of GPCR ligand stereoisomers has remained poorly defined. In an elegant study published in this issue of Molecular Pharmacology, Woo et al. (p. 158) show that stereoisomers of the ␤ 2 -adrenoceptor selective agonist fenoterol differentially activates G s -and G i -proteins in native rat cardiomyocytes. This study is so important because it is the first report to show that even the subtle structural differences within a ligand stereoisomer pair are sufficient to discriminate between GPCR conformations with distinct G-protein coupling properties. The study highlights of how important it is to examine the "more active" (eutomer) and the "less active" (distomer) stereoisomer to understand the mechanisms of action and the cellular effects of GPCR ligands. The study by Woo et al. will ignite a renaissance of the analysis of ligand stereoisomers, using sensitive pharmacological and biophysical assays. The available literature supports the notion that meticulous analysis of ligand stereoisomers is a goldmine for understanding mechanisms of GPCR activation, analysis of signal transduction pathways, development of new therapies for important diseases, and drug safety.

Thermodynamics and Docking of Agonists to the β2-Adrenoceptor Determined Using [3H](R,R′)-4-Methoxyfenoterol as the Marker Ligand

“…Because all of the Fen stereoisomers were full ␤ 2 -AR agonists in the HEK-␤ 2 -AR cells, the results were inconsistent with the principle of "thermodynamic agonist-antagonist discrimination." In the discussion of this inconsistency, we suggested that the results of our study might reflect the fact that the ␤ 2 -AR exists in an inactive (R) conformation and one or more ligand-specific active conformations (R* n ) (Seifert and Dove, 2009) and that displacement binding studies using [ 3 H]CGP-12117, a high-affinity neutral antagonist (Baker et al, 2008), may reflect the relative affinity of the Fen stereoisomers for the inactive receptor state. We also suggested that a potential approach to clarifying these interactions was to conduct the displacement binding studies with the ␤ 2 -AR agonist [ The data from the current study indicate that the binding properties of […”

Cannabinoid Receptor Activation Correlates with the Proapoptotic Action of the β2-Adrenergic Agonist (R,R′)-4-Methoxy-1-Naphthylfenoterol in HepG2 Hepatocarcinoma Cells

“…Because the Fen and NF analogs used in this study are potent and selective ␤ 2 -AR agonists, a potential explanation for the effect produced by replacing a phenyl ring with a naphthyl ring is "ligand-directed signaling" or "biased agonism" (Seifert and Dove, 2009). It has been demonstrated that ␤ 2 -AR binds ligands in multiple conformations and bind- ing to different receptor conformations can lead to differences in signal transduction (Wisler et al, 2007;Seifert and Dove, 2009).…”

Discovery of Regulators of Receptor Internalization with High-Throughput Flow Cytometry

“…After ligand activation, ␤ 2 ARs couple to G␣ s or G␣ i proteins and subsequently regulate both AC and mitogen-activated protein kinase (MAPK) pathways. Although most ligands have balanced efficacies for the two pathways, many are reported to have different efficacies (Kenakin, 2002;Azzi et al, 2003;Seifert and Dove, 2009;Vilardaga et al, 2009;Evans et al, 2010). Table 2 compares the efficacy patterns of selected PCL compounds for AC and ERK1/2 (a subfamily of MAPK) pathways, as well as total and surface receptor levels and the ability to induce surface receptor internalization.…”