Bile Acid Receptors FXR and TGR5 Emerge as Master Switches in Liver Disease

Bile acids have long been known as detergents that help the gut digest fat, but a sweeping new review published in Pharmacology Research & Perspectives makes the case that they are far more than digestive helpers. The comprehensive analysis, which synthesizes hundreds of studies, argues that two bile acid sensors—the nuclear receptor FXR and the membrane-bound G protein-coupled receptor TGR5—sit at the very center of liver health, governing everything from fat accumulation and scarring to inflammation and cancer. As metabolic liver disease surges worldwide and drug candidates stumble in late-stage trials, the review offers both a unifying explanation of why these receptors matter so much and a sobering account of how difficult it has been to turn them into medicines.

FXR, the farnesoid X receptor, is expressed predominantly in the liver, intestine, and kidneys, and functions as the body’s principal bile acid sensor. Its endogenous ligands are bile acids themselves, with binding potency following a clear hierarchy: chenodeoxycholic acid is the strongest activator, followed by deoxycholic acid, lithocholic acid, and finally cholic acid. Once activated, FXR operates a sophisticated transcriptional network that keeps bile acid levels in check. In the liver, it ramps up production of the small heterodimer partner, a transcriptional repressor that shuts down CYP7A1 and CYP8B1, the two enzymes that drive bile acid synthesis. In the intestine, FXR triggers the release of fibroblast growth factor 15 in rodents and 19 in humans, which travels through the portal vein to the liver and binds the FGFR4 receptor complex to suppress bile acid production through a second, independent route. The review emphasizes that this dual feedback system is most powerful in the liver, with weaker but meaningful effects in the small intestine and kidneys.

Beyond bile acid chemistry, FXR acts as a metabolic conductor. Activation of the receptor suppresses SREBP-1c, the master switch for fatty acid and triglyceride synthesis, through SHP-dependent signaling, while simultaneously boosting PPARα activity to upregulate carnitine palmitoyl transferase 1 and accelerate fat burning through beta-oxidation. FXR also reshapes the fatty acid profile of the liver, lowering both monounsaturated and polyunsaturated fatty acid levels through distinct mechanisms. Its influence extends to glucose: protein kinase A-mediated phosphorylation of FXR activates gluconeogenic gene expression, while intestinal FXR stimulates secretion of glucagon-like peptide-1, the incretin hormone made famous by blockbuster diabetes and obesity drugs, thereby promoting insulin release and improving glucose tolerance. The receptor additionally damps inflammation by suppressing NF-κB signaling, reducing interleukin-6 and interleukin-1β, and by directly blocking the assembly of the NLRP3 inflammasome through SHP, preventing caspase-1 activation and the pyroptotic cell death that fuels hepatic inflammation.

The clinical implications of these mechanisms are enormous. In cholestatic liver diseases such as primary biliary cholangitis and primary sclerosing cholangitis, where toxic bile acids accumulate and destroy tissue, FXR agonists have been the leading therapeutic strategy for years. Obeticholic acid, a derivative of chenodeoxycholic acid, became the first approved FXR agonist for primary biliary cholangitis in 2016 and showed promise in metabolic dysfunction-associated steatohepatitis as well. Yet the review delivers a striking update: amid persistent safety concerns and disappointing results in the COBALT trial, the manufacturer voluntarily withdrew obeticholic acid from the United States market in September 2025. Hepatotoxicity, severe pruritus, and unfavorable lipid changes had outweighed its benefits. The withdrawal casts a long shadow over the field and underscores why newer agents such as cilofexor, tropifexor, and vonafexor—each designed for better selectivity and tolerability—remain in various stages of clinical development, some already halted for lack of efficacy and others still advancing.

In fatty liver disease, the review describes FXR as a reliable intervention point operating through multiple converging pathways. The receptor inhibits fat production via the FXR-SHP-SREBP1c axis, promotes fat oxidation through PPARα, and induces detoxification enzymes such as CYP3A4 and sulfotransferase 2A1 to enhance bile acid clearance. Intriguingly, FXR also participates in a reciprocal positive feedback loop with SIRT1, the longevity-associated histone deacetylase: FXR induces SHP, which suppresses microRNA-34a and thereby lifts the brake on SIRT1 expression, while SIRT1 in turn deacetylates FXR, stabilizing it and promoting its nuclear translocation. Intestinal FXR adds another layer of protection by maintaining epithelial tight junctions, preserving the gut vascular barrier through Wnt/beta-catenin signaling, and upregulating antimicrobial defenses. In liver cancer, the evidence is equally compelling. Mice lacking FXR spontaneously develop hepatocellular carcinoma with bile acid accumulation and elevated beta-catenin target genes, and FXR activation directly disrupts the beta-catenin-TCF4 transcriptional complex while also suppressing tumor progression through the JAK2-STAT3 pathway and the FGF15/19-FGFR4 axis.

TGR5, the review’s second protagonist, tells a complementary story through entirely different cell biology. Rather than sitting in the nucleus, TGR5 is embedded in the plasma membrane, where bile acid binding triggers cyclic AMP production within seconds. It is distributed widely across the liver, gallbladder, intestine, kidney, spleen, brain, skeletal muscle, and brown adipose tissue, with ligand potency ranking lithocholic acid highest, followed by deoxycholic acid, chenodeoxycholic acid, and cholic acid. In metabolic terms, TGR5 activation promotes GLP-1 secretion from intestinal endocrine cells, enhances insulin sensitivity, and induces the browning of white adipose tissue. The review details two thermogenic mechanisms: cAMP-PKA signaling enhances the conversion of thyroid hormone T4 to the active T3, upregulating uncoupling protein 1 and driving mitochondrial heat production, while a parallel pathway induces mitochondrial creatine kinase 2 to establish a UCP1-independent futile creatine cycle. Together these actions increase energy expenditure and counteract obesity.

TGR5 also functions as an immunological brake. Its activation inhibits NF-κB and NLRP3 inflammasome signaling through the cAMP-PKA axis, steering macrophages away from the pro-inflammatory M1 phenotype and toward the reparative M2 state. In models of cholestasis induced by bile duct ligation, TGR5 knockout worsened oxidative stress and Kupffer cell polarization, while TGR5 agonists reduced damage by inhibiting NF-κB and activating the Nrf2/HO-1 pathway. In cancer, TGR5-deficient mice proved more susceptible to chemically induced liver cancer, and receptor activation suppressed tumor development through STAT3 inhibition. Agonist development, however, remains early: the semi-synthetic bile acid analog INT-777 has never entered human trials, while the next-generation candidate RDX8940 shows potent activity against insulin resistance and hepatic steatosis in preclinical models but still lacks human safety data. Antagonists such as SBI-115 and SBI-364, meanwhile, have revealed unexpected value in polycystic kidney and liver disease, where excessive TGR5 signaling drives aberrant bile duct proliferation.

Perhaps the review’s most important contribution is its portrait of FXR and TGR5 as an interlocking system rather than two isolated targets. The two receptors coordinate bile acid homeostasis, with FXR providing the slow transcriptional feedback and TGR5 handling rapid metabolic responses. Along the gut-liver axis, bile acids activate FXR in enterocytes to release FGF15/19 while simultaneously engaging TGR5 on intestinal L cells to secrete GLP-1, and gut microbes remodel bile acid structures into conjugates that can activate both receptors at once. During liver regeneration, knockout of either receptor delays recovery, indicating partial functional compensation that is not fully interchangeable. In the intestinal epithelium the two receptors actually pull in opposite directions—FXR restrains proliferation through cell-cycle inhibitors like p21 while TGR5 promotes it through cAMP-PKA and EGFR crosstalk—creating a delicate balance that maintains tissue homeostasis.

The review closes with a candid assessment of the obstacles ahead. Only a small fraction of FXR- and TGR5-targeted drugs have reached clinical use, and most candidates suffer from low bioavailability, short duration of action, or side effects such as pruritus, dyslipidemia, and hepatotoxicity. FXR biology is strikingly tissue-dependent: only about 11 percent of FXR binding sites are shared between mouse liver and intestine, and while single-organ knockout produces mild phenotypes, double knockout causes severe cholestatic injury, revealing that hepatic and intestinal FXR cannot substitute for one another. TGR5’s role in primary sclerosing cholangitis remains genuinely contested, with evidence both for cholangiocyte-protective and disease-promoting functions depending on context. The authors call for deeper mechanistic mapping of cell-type-specific receptor functions, next-generation dual-target modulators, combination strategies with GLP-1 receptor agonists, and rigorously designed trials. For a field still reeling from obeticholic acid’s withdrawal, the message is clear: bile acid receptors remain among the most promising targets in hepatology, but realizing that promise will require matching the elegance of the biology with equally sophisticated pharmacology.

Subject of Research: The roles of the bile acid receptors FXR and TGR5 in hepatic steatosis, fibrosis, cholestatic disease, and hepatocellular carcinoma

Article Title: The Bile Acid Signaling Axis: Deciphering the Roles of FXR and TGR5 in Hepatic Steatosis, Fibrosis, and Cancer

Article References: Liu, J., Sun, K., Liu, H., Zhu, M., Li, C., He, Y., & Qin, L. (2026). The Bile Acid Signaling Axis: Deciphering the Roles of FXR and TGR5 in Hepatic Steatosis, Fibrosis, and Cancer. Pharmacology Research & Perspectives, 14(5), Article e70318. https://doi.org/10.1002/prp2.70318

Image Credits: AI Generated

DOI: 10.1002/prp2.70318

Keywords: bile acids, FXR, TGR5, liver fibrosis, hepatocellular carcinoma, MASLD, MASH, cholestasis, obeticholic acid, GLP-1, NF-kB, gut-liver axis