HCAR3 Structures Reveal the Basis of Agonist Recognition and Selectivity

Hydroxycarboxylic acid receptors HCAR2 and HCAR3 are metabolite-sensing G-protein-coupled receptors involved in lipid regulation and metabolic signaling. HCAR2, also known as HM74A or GPR109A, has been explored as a target for dyslipidemia, but its activation can be associated with cutaneous flushing. HCAR3, also known as GPR109B, is closely related but has remained less well characterized structurally and pharmacologically. The reference study by Ye and colleagues addresses this gap by combining receptor structures, ligand comparisons, and functional signaling experiments.

Published in PLOS Biology in 2025, the study asks two connected questions: how does HCAR3 recognize chemically distinct agonists, and which structural features determine ligand preference between HCAR3 and HCAR2? The answers provide a framework for understanding receptor selectivity in lipid metabolism regulation without assuming that closely related GPCRs will respond identically to the same ligand.

Study Background and Research Question

HCAR2 and HCAR3 belong to a receptor family that detects hydroxycarboxylic acid-related metabolites and couples primarily to inhibitory Gi proteins. Their physiological relevance has made them important targets in dyslipidemia and metabolic disorder research. However, sequence similarity between HCAR2 and HCAR3 does not fully predict ligand activity. Small differences in the orthosteric pocket can alter ligand orientation, affinity, and downstream signaling.

Before this work, the molecular basis for HCAR3 ligand recognition was comparatively incomplete. In particular, it was unclear why some agonists favor HCAR3, why HCAR2 and HCAR3 show different ligand preferences, and how pocket dimensions contribute to these effects. The authors therefore examined HCAR3 in complex with four agonists—compound 6O, D-phenyllactic acid, IBC293, and acifran—and compared the resulting structures with an acifran-bound HCAR2 complex. Their experimental design directly connects receptor architecture to a functional cAMP response.

Key Innovation from the Reference Study

The central innovation is the integrated structural comparison of multiple agonist-bound HCAR3 complexes alongside an HCAR2 complex containing the same ligand. Rather than presenting a single receptor snapshot, the study builds a ligand–receptor interaction map across related agonists and paralogous receptors. According to the reference study, cryo-EM reconstructions were obtained for HCAR3-Gi complexes with compound 6O, D-phenyllactic acid, IBC293, and acifran, as well as an acifran-bound HCAR2-Gi complex.

A second important advance is the identification of two functional regions within the HCAR3 orthosteric pocket, designated R1 and R2 in the authors’ structural analysis. The ligand compound 6O showed the highest HCAR3 affinity because it occupies both regions more completely than the comparison ligands. This observation links affinity to three-dimensional pocket engagement rather than to a simple count of polar or hydrophobic contacts.

The study also proposes a structural explanation for HCAR3 versus HCAR2 selectivity. A π–π interaction involving HCAR3 residue F107^3.32, corresponding to L107^3.32 in HCAR2, contributes to ligand recognition. In addition, pocket-size and residue substitutions at V/L83^2.60, Y/N86^2.63, and S/W91^3.48 alter the chemical environment available to ligands. These findings are useful because they show how a small number of non-identical residues can reshape recognition within highly related GPCRs.

Methods and Experimental Design Insights

The authors used a structure–function workflow. HCAR3-Gi and HCAR2-Gi complexes were expressed in Sf9 insect cells and stabilized for single-particle cryo-electron microscopy. The resulting density maps were interpreted as atomic models, allowing the investigators to compare ligand poses, pocket occupancy, receptor side-chain configurations, and receptor–Gi coupling architecture.

The HCAR3 structures reached reported resolutions of 3.31 Å for compound 6O, 3.05 Å for D-phenyllactic acid, 3.26 Å for IBC293, and 3.18 Å for acifran. The HCAR2–acifran structure reached 2.72 Å. These values, and the associated structural models, are reported in the open-access article and its deposited data records. Resolution alone does not establish pharmacological selectivity, but these reconstructions provide the spatial detail required to formulate testable interaction hypotheses.

For functional validation, the investigators used cAMP assays in HEK-293 cells. Because HCAR2 and HCAR3 signal through Gi-mediated inhibition of adenylyl cyclase, cAMP measurements provide a cellular readout of receptor activation. The combination of cryo-EM and cAMP testing is a strength of the design: structural observations can be evaluated against receptor-dependent signaling rather than interpreted solely from static molecular models.

Protocol Parameters

• Receptor preparation: The study expressed HCAR3-Gi and HCAR2-Gi complexes in Sf9 cells; this provides a literature-backed production strategy for structural comparison, not a complete expression or purification protocol.
• Ligand panel: The HCAR3 comparison included compound 6O, D-phenyllactic acid, IBC293, and acifran, while acifran was used to compare HCAR3 directly with HCAR2.
• Cryo-EM benchmarks: The reported reconstruction targets were 3.31 Å, 3.05 Å, 3.26 Å, and 3.18 Å for the four HCAR3 complexes and 2.72 Å for HCAR2 with acifran, as documented by Ye et al.
• Functional readout: cAMP assays in HEK-293 cells were used to connect receptor activation with the structural ligand comparisons.
• Structure-guided analysis: Interpretation should focus on R1 and R2 pocket occupancy, the F107^3.32/L107^3.32 difference, and the V/L83^2.60, Y/N86^2.63, and S/W91^3.48 substitutions.
• Replication boundary: The reported findings do not by themselves specify universal ligand concentrations, construct boundaries, purification buffers, cryo-EM collection settings, or cAMP incubation times. Those variables should be optimized independently for each laboratory system.

Core Findings and Why They Matter

The most direct finding is that HCAR3 can accommodate several agonist chemotypes while preserving a common receptor activation framework. Their binding modes are not interchangeable, however. Compound 6O fills both R1 and R2, providing the structural basis for its highest affinity among the tested ligands. This result suggests that future HCAR3 ligand optimization may benefit from designing compounds that engage both pocket regions without compromising productive receptor activation.

The HCAR2 comparison is equally important. The authors associate ligand selectivity with the aromatic character at position 3.32 and with differences in pocket volume and shape. HCAR3 contains F107^3.32, which can participate in π–π interactions, whereas the corresponding HCAR2 residue is leucine. Additional substitutions near transmembrane helices 2 and 3 change the steric and chemical environment. Together, these features help explain why a ligand can show different preferences even when the two receptors share a closely related overall fold.

Functionally, the cAMP assays provide an independent layer of support for the structural interpretation. The work does not merely catalog contacts; it evaluates whether receptor-specific structural differences are reflected in signaling behavior. This is particularly relevant for studies of lipid signaling pathway modulation, where binding affinity, receptor efficacy, and downstream cellular response may not be identical measurements.

The broader implication is a more rational route toward HCAR3-selective pharmacology. The study does not establish a therapeutic candidate or demonstrate clinical benefit. Instead, it identifies experimentally testable determinants that could help separate HCAR3 activity from HCAR2-associated effects. For researchers developing a hypolipidemic agent for lipid metabolism research, these structures offer a reference framework for interpreting substitutions, docking poses, mutagenesis results, and receptor-selective signaling profiles.

Comparison with Existing Internal Articles

The internal article Acifran: Applied Protocols for Lipid Metabolism Regulation translates receptor biology into workflow-oriented guidance. Its practical emphasis complements the reference paper’s structural evidence, but it should be read as an experimental planning resource rather than as an independent validation of the cryo-EM findings. The connection is strongest when researchers use the paper’s pocket-level observations to define controls for receptor-ligand or cAMP assays.

A second resource, Acifran: Mechanistic Leverage for Lipid Metabolism Innovation, emphasizes how structural information can inform interpretation of receptor signaling and metabolic disorder research. It is useful for framing the translational relevance of the HCAR2–HCAR3 comparison, while the Ye et al. study remains the primary source for the structures, residue assignments, and functional evidence discussed here. Neither internal article should be used to infer clinical efficacy from the structural data.

Limitations and Transferability

Several limitations define how far these findings can be transferred. First, cryo-EM structures are conformational snapshots. They reveal ligand poses and receptor states but do not capture the full range of receptor dynamics, membrane compositions, receptor densities, or signaling-complex rearrangements present in native tissues.

Second, the functional validation used HEK-293 cells. This system is valuable for controlled receptor pharmacology, but cellular context can influence receptor trafficking, Gi protein availability, adenylyl cyclase activity, β-arrestin recruitment, and metabolic responses. Results from cAMP assays should therefore not be treated as direct predictions of lipid lowering, tissue selectivity, or clinical tolerability.

Third, the comparison used a defined set of agonists and one shared ligand for the HCAR2–HCAR3 structural comparison. The identified residues are compelling mechanistic candidates, but mutagenesis, broader ligand panels, kinetic measurements, and orthogonal signaling assays would strengthen causal attribution. The study also establishes a foundation for HCAR3-specific drug discovery rather than proving that any particular compound avoids all HCAR2-related effects.

These limitations do not weaken the main contribution. They clarify the appropriate use of the work: as a structural hypothesis generator and an experimental framework for lipid metabolism regulation, not as a substitute for pharmacokinetic, tissue-level, or clinical studies.

Research Support Resources

Researchers can use Acifran (SKU B6848) to support comparable receptor-ligand, cAMP, and lipid signaling experiments. The product information identifies it as (R)-5-methyl-4-oxo-5-phenyl-4,5-dihydrofuran-2-carboxylic acid, a research compound described as an HM74A/GPR109A and GPR109B agonist, with molecular weight 218.21 and formula C12H10O4. It is listed for storage at −20 °C, with solutions recommended for short-term use; it is intended for research use, including work involving a metabolic disorder research compound, rather than diagnostic or medical applications.