Pharmacology of Free Fatty Acid Receptors and Their Allosteric Modulators

Abstract

The physiological function of free fatty acids (FFAs) has long been regarded as indirect in terms of their activities as educts and products in metabolic pathways. The observation that FFAs can also act as signaling molecules at FFA receptors (FFARs), a family of G protein-coupled receptors (GPCRs), has changed the understanding of the interplay of metabolites and host responses. Free fatty acids of different chain lengths and saturation statuses activate FFARs as endogenous agonists via binding at the orthosteric receptor site. After FFAR deorphanization, researchers from the pharmaceutical industry as well as academia have identified several ligands targeting allosteric sites of FFARs with the aim of developing drugs to treat various diseases such as metabolic, (auto)inflammatory, infectious, endocrinological, cardiovascular, and renal disorders. GPCRs are the largest group of transmembrane proteins and constitute the most successful drug targets in medical history. To leverage the rich biology of this target class, the drug industry seeks alternative approaches to address GPCR signaling. Allosteric GPCR ligands are recognized as attractive modalities because of their auspicious pharmacological profiles compared to orthosteric ligands. While the majority of marketed GPCR drugs interact exclusively with the orthosteric binding site, allosteric mechanisms in GPCR biology stay medically underexploited, with only several allosteric ligands currently approved. This review summarizes the current knowledge on the biology of FFAR1 (GPR40), FFAR2 (GPR43), FFAR3 (GPR41), FFAR4 (GPR120), and GPR84, including structural aspects of FFAR1, and discusses the molecular pharmacology of FFAR allosteric ligands as well as the opportunities and challenges in research from the perspective of drug discovery.

Keywords: FFAR; Free fatty acid receptor; GPCR; allosteric modulator; drug discovery.

Figure 1

Overview of different modes of allosteric modulation. Agonist dose–response curves in absence of an allosteric ligand are shown in orange, while presence of an allosteric ligand is depicted in blue. (A) A positive allosteric modulator (PAM) increases affinity and/or efficacy of an orthosteric ligand, and thus has positive co-operativity factors α and/or β. (B) A negative allosteric modulator (NAM) reduces affinity and/or efficacy of an orthosteric ligand and has a negative α- and/or β-factor. (C) An ago-PAM has intrinsic efficacy (τ > 0) and is also a PAM for an orthosteric ligand. (D) An ago-NAM is a negative allosteric modulator for an orthosteric ligand that increases the activity state of the receptor (τ > 0) itself. (E) A PAM-antagonist decreases efficacy of an orthosteric agonist and thus functions as an antagonist (β < 0) and simultaneously increases affinity of the orthosteric ligand (α > 0). (F) A negative allosteric ligand (NAL) or silent allosteric modulator (SAM) has no effect on the affinity or efficacy of an orthosteric ligand (α,β = 0) but occupies the allosteric binding site, and thus competes with other allosteric ligands for this site.

Figure 2

Superimposition of all published FFAR1 crystallographic structures. The partial allosteric agonists TAK-875 and MK-8666 share the same binding site, denoted here as site A. Full allosteric agonists (AP8 and compound 1) are found in the second allosteric site, denoted here as site B. The respective PDB IDs are: 4PHU, 5TZY, 5TZR, and 5KW2. This figure was prepared using The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC (New York, NY, USA).

Figure 3

(A) The partial agonist TAK-875 seems to enter its allosteric binding pocket (site A) in an unusual way, most likely through the lipid layer, sneaking into the receptor transmembrane region. (B) Detailed view on binding pose of TAK-875 with the head group buried in the receptor transmembrane region. Hydrogen bonding pattern of TAK-875 with key residues in its allosteric binding pocket shown as dashed lines. This figure was prepared using The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC (New York, NY, USA).

Figure 4

An interhelical sliding of TM5 in relation to TM4 leads to a translocation of Leu190^5.46. This generates a deep hydrophobic pocket to accommodate the CF3 moiety of the full allosteric agonist AP8 binding to the second allosteric binding pocket (site B). Binary complex of FFAR1 and MK-8666 (PDB ID: 5TZR) shown in grey and ternary complex, consisting of FFAR1, MK-8666, and AP8, (PDB ID: 5TZY) is depicted in blue. This figure was prepared using The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC (New York, NY, USA).

Figure 5

The intracellular loop 2 (ICL2) of FFAR1 at the bottom of the receptor–ligand model is stabilized via a hydrogen bond (dashed line) of full allosteric agonist AP8 to Tyr114. This figure was prepared using The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC.