Natural biased signaling of hydroxycarboxylic acid receptor 3 and G protein-coupled receptor 84.

Background: Medium-chain fatty acids and their 3-hydroxy derivatives are metabolites endogenously produced in humans, food-derived or originating from bacteria. They activate G protein-coupled receptors, including GPR84 and HCA, which regulate metabolism and immune functions. Although both receptors are coupled to G proteins, share at least one agonist and show overlapping tissue expression, GPR84 exerts pro-inflammatory effects whereas HCA is involved in anti-inflammatory responses. Here, we analyzed signaling kinetics of both HCA and GPR84, to unravel signal transduction components that may explain their physiological differences.

Methods: To study the signaling kinetics and components involved in signal transduction of both receptors we applied the label-free dynamic mass redistribution technology in combination with classical cAMP, ERK signaling and β-arrestin-2 recruitment assays. For phenotypical analyses, we used spheroid cell culture models.

Results: We present strong evidence for a natural biased signaling of structurally highly similar agonists at HCA and GPR84. We show that HCA signaling and trafficking depends on dynamin-2 function. Activation of HCA by 3-hydroxyoctanoic acid but not 3-hydroxydecanoic acid leads to β-arrestin-2 recruitment, which is relevant for cell-cell adhesion. GPR84 stimulation with 3-hydroxydecanoic acid causes a sustained ERK activation but activation of GPR84 is not followed by β-arrestin-2 recruitment.

Conclusions: In summary, our results highlight that biased agonism is a physiological property of HCA and GPR84 with relevance for innate immune functions potentially to differentiate between endogenous, non-pathogenic compounds and compounds originating from e.g. pathogenic bacteria. Video Abstract.