GPR109A (HCAR2)

A Gαi/o-coupled receptor that functions as a molecular integrator of three distinct metabolic signals: niacin (high-affinity pharmacological ligand), D-β-hydroxybutyrate (BHB; endogenous ketone body), and butyrate (low-affinity; gut-derived SCFA). Expressed on adipocytes, immune cells (macrophages, microglia, neutrophils), and colonic epithelium. Connects nutrient-sensing cues from fasting, ketogenic diet, and gut microbiome to anti-inflammatory and metabolic programs — making it a mechanistic link between several longevity-associated interventions and their downstream anti-inflammaging effects.

Identity

UniProt: Q8TDS4 (HCAR2_HUMAN)
NCBI Gene: 338442
HGNC symbol: HCAR2
Ensembl: ENSG00000182782
Mouse ortholog: Hcar2 (historically called Puma-g in deorphanization literature)
Length: 363 amino acids (canonical isoform); 7 transmembrane helices (class A GPCR)

Structure and signaling

GPR109A is a rhodopsin-class (class A) GPCR. Upon ligand binding, it couples to Gαi/o, suppressing adenylyl cyclase activity and reducing intracellular cAMP ¹. Reduced cAMP:

In adipocytes: inhibits protein kinase A (PKA) → reduces phosphorylation and activation of hormone-sensitive lipase (HSL) → anti-lipolytic effect. This is the canonical mechanism underlying niacin’s triglyceride-lowering action.
In immune cells: downstream effects are more complex, involving inhibition of NF-κB and NLRP3 inflammasome activity through mechanisms that partially overlap with but are distinct from direct BHB-NLRP3 inhibition (see below).

A key regulatory phosphorylation site is at position Ser328; the disulfide bond between Cys100–Cys177 stabilizes the extracellular loop structure required for ligand docking.

Ligands

Ligand	Affinity (EC50)	Source	Physiological context
Nicotinic acid (niacin)	~0.1 µM (HM74A, cAMP assay) ²; ~1 µM (human HM74) / ~3 µM (mouse PUMA-G) in Ca²⁺ mobilization assay ¹	Exogenous / diet	Pharmacological dyslipidemia treatment; trace dietary amounts insufficient for receptor occupancy
D-β-hydroxybutyrate (BHB)	~0.7 mM	Endogenous ketone body	Fasting, prolonged exercise, ketogenic diet; plasma concentrations reach 1–5 mM in fasting/ketosis
Butyrate	~1–2 mM	Microbial fermentation	Colonic lumen concentrations 1–10 mM in fiber-replete microbiome

The receptor was initially designated an “orphan” GPCR. Two groups concurrently deorphanized it in 2003, identifying nicotinic acid as its high-affinity ligand ¹². BHB was identified two years later as an endogenous low-to-mid-affinity agonist ³. Butyrate was characterized as an additional low-affinity colonic agonist in the Thangaraju 2009 work ⁴.

Expression and tissue context

Expression is highest in adipose tissue and spleen (UniProt annotation). Additional expression confirmed in:

Mature neutrophils (but not immature neutrophils or eosinophils) — receptor activation triggers cAMP reduction and promotes neutrophil apoptosis
Macrophages and microglia — anti-inflammatory signaling (see below)
Colonic epithelium — mediates butyrate’s anti-proliferative and tumor-suppressive effects

Expression is notably absent from most other gastrointestinal epithelial cell types and from most circulating lymphocytes. needs-human-replication — most expression data derive from mouse studies; human tissue atlas data should be cross-checked against GTEx/Human Protein Atlas.

Role in aging

Anti-inflammaging via BHB and butyrate signaling

Chronic low-grade inflammation (chronic-inflammation) is one of the most reproducible correlates of mammalian aging (inflammaging). GPR109A is positioned as a receptor that converts fasting- and microbiome-derived signals into anti-inflammatory outputs:

BHB → GPR109A → macrophage suppression: Fasting and caloric restriction raise plasma BHB to concentrations sufficient for GPR109A occupancy. In macrophages and microglia, GPR109A activation suppresses NF-κB-dependent inflammatory gene expression. Note: BHB also inhibits NLRP3 inflammasome via a separate, receptor-independent mechanism (potassium efflux inhibition) ⁵; dissecting the GPR109A-dependent vs independent contributions of BHB to inflammasome suppression requires GPR109A-null models. needs-replication
Butyrate → colonic Treg homeostasis: Singh et al. 2014 (Immunity) showed that GPR109A is required for butyrate-induced colonic regulatory T cell (Treg) differentiation and IL-18 production in colonocytes ⁶. GPR109A-knockout mice had reduced colonic Tregs and increased susceptibility to colitis. This places GPR109A downstream of the gut microbiome’s butyrate output and upstream of colonic immune tolerance — a key inflammaging-relevant circuit. needs-human-replication

Dimension	Status	Notes
Pathway conserved in humans?	yes	Human HCAR2 shares ~87% amino acid identity with mouse Hcar2; functional conservation demonstrated for niacin and BHB signaling
Phenotype conserved in humans?	partial	Niacin’s anti-lipolytic effect is well-documented in humans; BHB/butyrate immune-modulatory effects have limited human evidence
Replicated in humans?	no	Singh 2014 Treg data are mouse-only; BHB anti-inflammatory human trials exist but GPR109A contribution unresolved

Neuroinflammation and Alzheimer’s disease

GPR109A (HCAR2) is expressed by microglia in the brain. In a 5xFAD mouse model:

HCAR2 expression increased in plaque-associated microglia, suggesting receptor upregulation in response to amyloid pathology ⁷
Genetic deletion of HCAR2 impaired microglial phagocytic responses and worsened amyloid burden and cognitive deficits
Pharmacological activation with niacin (Niaspan) reduced amyloid plaques, attenuated neuronal loss, and restored working memory performance
Epidemiological correlation: higher dietary niacin intake associated with reduced AD risk in the analyzed cohort (observational; not shown to be GPR109A-mediated)

This work positions HCAR2 as part of the microglial homeostatic response to amyloid — relevant to chronic-inflammation and potentially to the neurodegenerative component of brain aging. needs-replication — single lab; independent replication needed.

Dimension	Status	Notes
Pathway conserved in humans?	partial	Human microglia express HCAR2; amyloid-driven upregulation not yet confirmed in human brain
Phenotype conserved in humans?	partial	Epidemiological niacin-AD correlation is observational; no interventional human data
Replicated in humans?	no	5xFAD model data only

Niacin pharmacology and the AIM-HIGH/HPS2-THRIVE context

Niacin is FDA-approved for dyslipidemia. Its primary mechanisms include:

Anti-lipolytic action in adipocytes via GPR109A → reduced hepatic VLDL synthesis → lower triglycerides, raised HDL
Direct effects on hepatic lipid metabolism (GPR109A-independent in liver, since liver expression is low)

However, large RCTs failed to show cardiovascular benefit from niacin added to statin therapy:

AIM-HIGH (n=3,414): niacin added to simvastatin did not reduce cardiovascular events vs simvastatin alone ⁸ contradictory-evidence
HPS2-THRIVE (n=25,673): extended-release niacin + laropiprant showed no benefit and significantly increased adverse events including myopathy, infection, and bleeding

These failures suggest that raising HDL via niacin does not reduce CV risk in statin-treated patients, and that the flushing/inflammation associated with high-dose niacin (mediated partly through prostaglandin D2 release downstream of GPR109A activation in Langerhans cells) may offset benefits. GPR109A as an anti-aging target should be evaluated separately from the failed niacin-dyslipidemia indication — the relevant biology for aging involves BHB and butyrate signaling at physiological receptor occupancy, not pharmacological niacin dosing.

Aging-context tier-1 rationale. Niacin (nicotinic acid) is FDA-approved for dyslipidemia (high-affinity pharmacological agonist of GPR109A), not for an aging-rejuvenation indication, and the AIM-HIGH and HPS2-THRIVE outcome trials failed to show added cardiovascular benefit on top of statins. The aging-context tier-1 designation reflects GPR109A’s role as the receptor that integrates fasting-derived BHB and microbiome-derived butyrate into anti-inflammaging signals (macrophage / microglial NF-κB suppression, colonic Treg homeostasis), connecting caloric-restriction / ketogenic-diet / dietary fiber to chronic-inflammation modulation. Strict Open Targets Approved Drug = true for an aging indication does not apply. no-mechanism — the precise downstream effectors coupling GPR109A → NF-κB suppression in macrophages are not fully established.

Connection to longevity interventions

GPR109A sits at the intersection of several interventions with documented pro-longevity effects:

caloric-restriction and intermittent-fasting: both raise plasma BHB into the 0.5–5 mM range needed for GPR109A occupancy, providing a mechanistic route from caloric restriction → BHB → GPR109A → anti-inflammaging
ketogenic-diet: raises BHB to the highest sustained concentrations among dietary interventions; most direct pharmacological challenge of GPR109A
Gut microbiome / dietary fiber: butyrate produced by colonic fermentation (by Faecalibacterium prausnitzii, Roseburia spp., Butyricicoccus, among others) activates GPR109A in colonocytes and macrophages — one mechanistic route from microbiome diversity → colonic immune homeostasis

Note: GPR109A-mediated signaling is functionally distinct from FFAR2 (gpr43) and FFAR3 (gpr41) — both of which bind short-chain fatty acids (acetate, propionate, butyrate) at micromolar-to-millimolar affinity. GPR109A has no affinity for acetate or propionate and binds butyrate with lower affinity than FFAR2. The SCFA receptor cluster should be evaluated as complementary rather than redundant.

Pathway membership

scfa-signaling — canonical GPR109A/FFAR2/FFAR3 axis
ketogenesis-pathway — BHB as endogenous ligand; fasting-induced ketogenesis feeds receptor
g-protein-coupled-receptor-pathway — Gαi/o coupling; adenylyl cyclase inhibition; cAMP reduction

Limitations and gaps

#gap/needs-human-replication — Singh 2014 colonic Treg homeostasis data are mouse-only; human colonic biopsy studies showing GPR109A-dependent Treg induction are lacking
#gap/needs-human-replication — Moutinho 2022 microglia/amyloid findings are 5xFAD mouse model; human microglial HCAR2 upregulation in AD not confirmed in single-cell atlases at publication
#gap/needs-replication — BHB anti-inflammatory effect through GPR109A (as opposed to GPR109A-independent NLRP3 inhibition) has not been genetically dissected in multiple independent labs
#gap/no-mechanism — precise signaling cascade coupling GPR109A → NF-κB suppression in macrophages not fully established; Gβγ vs Gαi contributions unclear
#gap/dose-response-unclear — threshold plasma BHB required for meaningful GPR109A occupancy in human macrophages/microglia in vivo is unknown; in vitro EC50 (~0.7 mM) may not predict in vivo tissue concentrations
#gap/long-term-unknown — chronic GPR109A agonism effects (beyond short-term anti-lipolytic / anti-inflammatory) not studied in long-lived cohorts
#gap/contradictory-evidence — niacin (high-affinity agonist) failed CV outcome trials, while low-affinity endogenous agonists BHB/butyrate are associated with positive metabolic outcomes; the pharmacology at different receptor occupancies may produce qualitatively different downstream outputs
gtex-aging-correlation: not populated — requires GTEx v2 API query per sops/finding-tissue-expression.md unsourced
mr-causal-evidence: not-tested — no published Mendelian randomization studies using HCAR2 as instrument for aging/inflammatory phenotypes as of 2026-05

Footnotes

doi:10.1038/nm824 · Tunaru S et al. · Nature Medicine 2003 · n=N/A (receptor deorphanization) · in-vitro / in-vivo (mouse) · deorphanizes PUMA-G (mouse) and HM74 (human; NB: not HM74A — that high-affinity isoform distinction is Wise 2003) as nicotinic acid receptors; EC50 ~3 µM (PUMA-G) and ~1 µM (HM74) in Ca²⁺ mobilization assay; Puma-g-KO mice lose niacin anti-lipolytic response completely; KO mice show no FFA decrease after nicotinic acid injection vs. >1h decrease in WT · model: CHO-K1 expression cells + mouse PUMA-G-KO adipocytes · locally available: ↩ ↩² ↩³
doi:10.1074/jbc.M210695200 · Wise A et al. · J Biol Chem 2003 · in-vitro · concurrent deorphanization; identifies HM74 (low-affinity) and HM74A (high-affinity, = GPR109A) as nicotinic acid receptors; EC50 ~0.1 µM for HM74A ↩ ↩²
doi:10.1074/jbc.C500213200 · Taggart AKP et al. · J Biol Chem 2005 · in-vitro + in-vivo (mouse) · identifies D-β-hydroxybutyrate as endogenous ligand for PUMA-G/GPR109A; EC50 ~0.7 mM; BHB anti-lipolytic effect abolished in Puma-g-KO mice · model: mouse adipocytes ↩
doi:10.1158/0008-5472.CAN-08-4466 · Thangaraju M et al. · Cancer Research 2009 · in-vitro + in-vivo (Apc-min mouse) · GPR109A is a butyrate receptor in colonic epithelium; receptor loss accelerates intestinal tumorigenesis in Apc-min model; butyrate-induced apoptosis requires GPR109A · model: colon epithelial cells + Apc-min/Gpr109a-KO mice ↩
doi:10.1038/nm.3804 · Youm YH et al. · Nature Medicine 2015 · in-vitro + in-vivo (mouse) · BHB suppresses NLRP3 inflammasome by blocking potassium efflux — receptor-independent mechanism; distinct from GPR109A-mediated anti-inflammatory effects · model: human monocytes + mouse gout/arthritis models · note: GPR109A contribution not tested in this study ↩
doi:10.1016/j.immuni.2013.12.007 · Singh N et al. · Immunity 2014 · in-vivo (mouse) · GPR109A (encoded by Niacr1) activation by butyrate/niacin promotes colonic Treg induction (Foxp3+ CD4+ T cells: ~25.8% WT vs ~14.4% Niacr1−/− in colonic LP, Fig. 1A; p<0.002) and IL-18 production in colonic epithelium; colonic DCs and macrophages from Niacr1−/− mice defective in Treg-inducing capacity; all Niacr1−/− mice died by day 10 of 3% DSS colitis vs. 0/N WT deaths; Niacr1−/−·ApcMin/+ mice showed markedly increased polyps vs. ApcMin/+ controls (p<0.02 colon, p<0.01 small intestine) · model: Niacr1−/− mice on C57BL/6 background (5× backcross) + DSS colitis + AOM+DSS carcinogenesis · locally available: ↩
doi:10.1126/scitranslmed.abl7634 · Moutinho M et al. · Science Translational Medicine 2022 · in-vivo (5xFAD mouse) · HCAR2 expressed on microglia; receptor upregulated by amyloid; HCAR2-KO worsens amyloid burden and cognition; niacin treatment reduces plaques and restores memory; epidemiological niacin-AD correlation in cohort data · model: 5xFAD + HCAR2-KO mice ↩
doi:10.1056/NEJMoa1107579 · AIM-HIGH Investigators · N Engl J Med 2011 · n=3,414 · rct · niacin added to simvastatin raised HDL but did not reduce cardiovascular events vs simvastatin alone; trial stopped early for futility · model: human (stable CV disease) ↩

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