Bile acids are not just emulsifiers — they are hormones acting through two distinct receptor classes. (1) FXR (farnesoid X receptor, NR1H4) is a nuclear receptor activated principally by CDCA > LCA > DCA > CA. FXR-RXR heterodimer drives transcription of: FGF15/19 (intestinal — feedback signal to liver via FGFR4/βKlotho → ↓CYP7A1 → ↓de novo bile acid synthesis); SHP (small heterodimer partner — represses LRH-1/HNF4α); BSEP (canalicular bile acid efflux); OST-α/β (basolateral efflux); IBABP (intestinal binding); apoC-II (lipoprotein metabolism). FXR also represses NF-κB → anti-inflammatory in liver/gut. (2) TGR5 (GPBAR1) is a Gs-coupled GPCR responding to LCA > DCA > CDCA → cAMP → PKA. Tissue effects: brown adipose / muscle (D2 → T3 → thermogenesis); intestinal L-cells (↑GLP-1 secretion — incretin axis crosstalk); macrophages (anti-inflammatory); biliary epithelium (proliferation + chloride secretion); gallbladder relaxation (postprandial). Microbiota–FXR crosstalk: gut bacteria deconjugate + 7α-dehydroxylate bile acids → secondary BAs (DCA, LCA) → tip the FXR/TGR5 balance; tauro-β-muricholic acid is a natural FXR antagonist (Sayin 2013). Therapeutics: obeticholic acid (OCA, INT-747 — semi-synthetic CDCA derivative, potent FXR agonist) for PBC + cholestatic liver disease, NASH trials; cilofexor + tropifexor (FXR agonists in development); UDCA + tauroUDCA (TUDCA) (PXR/FXR partial modulators — cytoprotective in cholestasis); bile-acid sequestrants (colesevelam, cholestyramine, colestipol) lower bile-acid pool → ↑FXR signal release → glucose effects (metformin-like). Cross-links: bile acid synthesis (upstream), insulin glucose homeostasis (TGR5 → GLP-1), gi endocrine peptides misc (FGF15/19 endocrine).
Organ Systems
digestive
endocrine
Pathway Steps
cholesterol → primary bile acids (cholic + chenodeoxycholic) — via CYP7A1 hepatic — rate-limiting; opposed by FXR via SHP feedback. Bile-acid synthesis is the major route of cholesterol catabolism: the liver converts cholesterol to the primary bile acids cholic and chenodeoxycholic acid, the rate-limiting enzyme being CYP7A1 (classic pathway). This is quantitatively the body’s main way to dispose of cholesterol, and it is feedback-regulated by FXR.
primary BAs (taurine/glycine conjugated) → secondary BAs (DCA, LCA, UDCA) — via gut microbiota deconjugation + 7α-dehydroxylation (Clostridium / Eubacterium). Conjugated to taurine or glycine for solubility, primary bile acids are deconjugated and dehydroxylated by gut bacteria into secondary bile acids (deoxycholic, lithocholic, ursodeoxycholic). This microbial transformation shapes the bile-acid pool — making the microbiome a regulator of host bile-acid signaling, and a route by which antibiotics alter it.
CDCA (or OCA) binding FXR-LBD → FXR-RXR heterodimer on FXRE — via corepressor displacement; coactivator recruitment. Bile acids are signaling molecules, not just detergents: chenodeoxycholic acid (and the drug obeticholic acid, OCA) is the endogenous agonist of the nuclear receptor FXR, which heterodimerizes with RXR on FXR response elements. FXR is the master sensor that feedback-regulates bile-acid synthesis, transport, and metabolism.
FXR active (intestine) → FGF15/19 secretion → liver FGFR4/βKlotho — via → ERK → ↓CYP7A1 → ↓bile acid synthesis (feedback). In the intestine, FXR induces FGF15 (mouse)/FGF19 (human), a hormone that travels to the liver and signals through FGFR4/βKlotho to repress CYP7A1 — the endocrine arm of bile-acid feedback. FGF19 analogs are in trials for cholestatic and metabolic liver disease.
FXR active (liver) → SHP induction → LRH-1 / HNF4α repression — via → ↓CYP7A1; ↑BSEP for canalicular efflux. In the liver, FXR induces the atypical nuclear receptor SHP, which represses LRH-1 and HNF4α to shut down CYP7A1 — the intracellular arm of negative feedback. Together the intestinal FGF19 and hepatic SHP arms keep bile-acid synthesis matched to need; FXR agonists exploit this in cholestasis and MASH.
LCA / DCA binding TGR5 → Gs → cAMP → PKA — via BAT/muscle thermogenesis; L-cell GLP-1; gallbladder relaxation. Bile acids also act through a membrane GPCR, TGR5 (GPBAR1), preferentially bound by the secondary bile acids lithocholic and deoxycholic acid. TGR5 couples to Gs to raise cAMP and activate PKA — a non-genomic bile-acid signal distinct from FXR, mediating metabolic and anti-inflammatory effects across tissues.
TGR5 (L-cell) → GLP-1 secretion — via incretin axis crosstalk; partial mechanism of bile-acid sequestrant glycemic effect. A key TGR5 action is on intestinal L-cells, where bile acids stimulate secretion of the incretin GLP-1 — linking bile-acid delivery after a meal to insulin secretion and appetite. This is one mechanism by which bile-acid signaling (and bariatric surgery, which raises bile acids) improves glucose metabolism.
Known Modulators
colesevelam (activator) — FXR signal release (sequesters BAs → ↑BA synthesis → glycemic benefit). FDA-approved for both LDL and T2D — uniquely dual indication
cholestyramine (activator) — FXR signal release (sequestrant)
metformin (activator) — FXR (indirect — alters BA pool composition + microbiota)