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1. Introduction 

Nuclear receptors are a superfamily of ligand‑dependent transcription factors, with typical domain structures and conserved sequences. There are 48 members of the human nuclear receptor superfamily which perform crucial roles in a variety of processes of physiology and pathophysiology, such as , , , . Therefore, they always attract attention from basic scientists, clinicians, and the pharmaceutical industry, and account for 3% of all human drug targets so far [1].

Nuclear receptor farnesoid X receptor (FXR) was first isolated from a rat liver cDNA library and named after its weak activation via supraphysiological farnesol, an intermediate in the mevalonate biosynthetic pathway, in 1995 [2,3]. Subsequently, bile acids were identified as endogenous FXR ligands and were given another name: bile acid receptors. As a bile acid sensor, FXR is a key modulator of enterohepatic circulation of bile acids, controlling transcription of key regulatory genes in bile acid synthesis, biliary bile acid secretion and trans‑intestinal bile acid transport to the liver via portal blood circulation. FXR also serves as a metabolic regulator of glucose, lipid and energy metabolism. Because of its master functions in bile acid and lipid metabolism, FXR is considered a promising drug target for the therapy of bile‑acid‑related liver diseases. In 2016, obeticholic acid (OCA), a highly selective agonist of FXR, was approved by the U.S. Food and Drug Administration for the treatment of primary biliary cholangitis (PBC), a chronic cholestatic liver disease. Currently, FXR agonists are tested in clinical trials for the treatment of metabolic and type 2 diabetes (T2DM),

2. The Structure and Expression of FXR 

Two FXR protein families have been identified in animals. Derived from the nuclear receptor subfamily 1, group H, member(NR1H) 4 and NR1H5 genes, they are named FXR and FXR , respectively. From fish to humans, FXR is evolutionarily conserved among species. Due to the activation of two different promoters and the use of alternative splicing, the FXR gene encodes four isoforms (FXR 1‑ 4) [4,5]. A recent study identified four novel splice variants (FXR 5–8) in human hepatocytes, which resulted from previously undetected exon-skipping events. The possible roles of these novel isoforms in the human liver require further investigation [6]. However, the FXR coding gene is a pseudogene in humans and primates [7]. FXR is highly expressed in the liver, intestine, kidney, and adrenal gland, with low levels of expression in adipose tissue and heart. The expression of FXR 1‑ 4 in different tissues remains unclear. Using Southern blot analysis, Zhang et al. found that mouse FXR 1‑ 4 had the highest expression in liver, and the expression levels of each isoform are similar. In addition, FXR 1 and FXR 2 are moderately expressed in the ileum and adrenal gland; FXR 3 and FXR 4 are highly expressed in the ileum and moderately expressed in the kidney [8]. More efforts are needed to analyze the tissue distribution of different FXR isoforms in the future to help elucidate their functions (Table 1).

Table 1. FXR isoforms

FXR has a typical domain structure of a nuclear receptor (NR), including an N‑terminal domain (NTD), a DNA‑binding domain (DBD), a hinge region and a ligand‑binding domain (LBD). The N‑terminal domain (NTD) contains a ligand‑independent transcriptional activation domain (AF1) which can interact with other coregulator proteins such as peroxisome proliferator‑activated receptor gamma (PPAR ) coactivator 1‑alpha (PGC1 ), E1A‑ binding protein p300 (P300) and nuclear receptor co‑repressor 2 (NCOR2/SMRT) [9], and many post‑translational modifications (PTM) including phosphorylation, SUMOylation, GlcNAcylation, acetylation and methylation can occur in this region [10]. PGC‑1 functions as a transcriptional coactivator of FXR that directly interacts with FXR and enhances its transactivation activity [11]. Phosphorylation of FXR by protein kinase C promotes its transcriptional activity [12]. FXR O‑GlcNAcylation at S62 results in increased FXR activity [13]. Some coactivators have intrinsic enzymatic activity and can modify NRs themselves. Acetylation of FXR by P300 increases its stability but reduces FXR‑RXR het‑ heterodimerization, leading to a reduction in FXR target gene expression [14]. The zinc finger DNA‑binding domain (DBD) is the most conserved domain, containing two zinc finger motifs that recognize specific DNA sequences [15]. The flexible hinge region is a linker between the DBD and the LBD, which is also a site for the regulation of PTMs such as the NTD [10]. Ligand‑binding domain (LBD) is composed of 11 ‑helices and four ‑strands that fold into three parallel layers to form an alpha-helical sandwich. This fold forms a hydrophobic C‑terminal ligand binding pocket (LBP) and AF‑2. LBD can bind to ligands and AF‑2 can recruit co‑activators [16–18]. When ligands are present, FXR binds to specific DNA sequences in the promoter of target genes in the form of a monomer or a heterodimer with retinoid X receptor (RXR), the common partner for NRs, to regulate gene transcription [19]. 

3. The Ligands of FXR

Bile acids are important endogenous agonists of FXR. Cholic acid (CA) and chenodeoxycholic acid (CDCA) are two primary bile acids synthesized in the liver. The secondary bile acids lithocholic acid (LCA) and deoxycholic acid (DCA) are generated from CA and CDCA. The potency of bile acids in activating FXR is ranked as: CDCA > DCA > LCA > CA [20]. OCA is a semi‑synthetic derivative of CDCA, also known as 6‑ethyl‑ CDCA and INT‑747, which is the first FXR agonist to enter clinical research [21]. However, OCA has side effects such as dose‑dependent pruritis, which can lead to treatment discontinuation in ~1–10% of patients. EDP‑305 is another steroid FXR agonist studied for the treatment of NASH and PBC [22]. In order to reduce side effects and improve the therapeutic effect, non‑steroidal FXR agonists have been gradually synthesized. These include GW4064 [23], Cilofexor [24], Tropifexor [25], Nidufexor [26], and others [27–34]. Therefore, FXR agonists are actively used to treat a variety of metabolic diseases in the clinics [35] (Table 2). 

Table 2. FXR agonist clinical trials. 

Some bile acids are considered FXR Antagonists. Tauro‑ ‑muricholic acid (T‑ ‑MCA), an endogenous FXR antagonist, was reported to inhibit activation of FXR [36]. It is reported that glycine‑ ‑muricholic acid (Gly‑MCA) inhibited FXR signaling exclusively in the intestine, resulting in a decrease in the serum and intestine ceramide level and an improvement in metabolic dysfunction in obese mice [37]. Ursodeoxycholic acid (UDCA) exerts FXR‑ FXR-antagonistic effects on bile acid and lipid metabolism in morbid obesity, although it is a commonly used therapeutic agent in cholestatic liver disease [38]. More recently, Brevini et al. found that UDCA reduces angiotensin‑converting enzyme 2 (ACE2) expression by inhibiting FXR activity, resulting in reduced susceptibility to SARS‑CoV‑2 infection [39]. Therefore, FXR antagonists are also actively used to treat multiple diseases in the clinics (Table 3).

Table 3. FXR antagonist clinical trials

4. The General Function of FXR 

4.1. FXR and Bile Acid Metabolism 

FXR plays a critical role in bile acid homeostasis [46]. FXR inhibits bile acid synthesis: in the liver, FXR decreases the expression of cytochrome P450 (CYP)7A1 and CYP8A1, which are rate‑limiting enzymes in bile acid biosynthesis of cholesterol [47,48]. In the intestinal enterocytes, FXR induces the expression of fibrotic growth factor 15 (FGF15; FGF19, the orthologue humans of FGF15), which travels through the portal vein to the liver and activates the FGF receptor 4 (FGFR4)/ ‑Klotho complex, thereby inhibiting transcription of CYP7A1 and CYP8A1 [49]. FXR reduces bile acid accumulation in hepatocytes and enterocytes: in the liver, FXR can inhibit the expression of Na+ ‑taurocholate cotransport‑ ing polypeptide (NTCP) and organic‑anion‑transporting polypeptides (OATP) at the sinusoidal membrane to reduce hepatocytes' reabsorption of bile acids in the portal vein [50]. FXR promotes the excretion of bile acids into bile by activating transporters on the apical membrane surface, including multidrug‑resistance‑associated protein (MRP)2/3 [51], bile salt export pump (BSEP) [52] and multidrug resistance (MDR)2/3 [53]. FXR also promotes bile acid efflux into blood circulation by inducing expression of MRP4 and organic solute transporter (OST) /OST in the basolateral membrane [54]. In the intestine, FXR inhibits apical sodium‑dependent bile acid transporter (ASBT) at the apical membrane surface [55], thereby reducing the reabsorption of bile acids in the intestinal epithelium, upregulating ileal bile acid‑binding protein (IBABP) and promoting its movement from the apical membrane to the basolateral membrane [56]. Moreover, FXR promotes the transportation of bile acids to the portal vein and their subsequent return to the liver via upregulation of the expression of OST and OST on the basolateral membrane surface of the enterocyte [57]. 

4.2. FXR and Glucose Metabolism 

FXR plays diverse roles in glucose metabolism. The activation of FXR increases glycogen synthesis by inhibiting glycogen synthase kinase‑3 beta (GSK3 ) gene expression, which phosphorylates and subsequently inactivates glycogen synthase [58,59], reduces glycolysis by suppressing the transcriptional activity of ChREBP [60] and decreases the expression of multiple gluconeogenic genes, including phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6‑phosphatase (G‑6‑Pase) [58,61], resulting in a decrease in gluconeogenesis and serum glucose. However, several studies found that FXR antagonists exhibited a beneficial effect on glucose metabolism in T2DM mice, although the exact mechanism remains unclear [62]. Furthermore, FXR increases glucose‑stimulated insulin secretion in islets [63] by inducing the expression of glucose‑regulated transcription factor Krueppel‑like factor 11 (KLF11) [64], adenylyl cyclase 8 (ADCY8) [65] and the transient receptor potential ankyrin 1 (TRPA1) channel [66]. 

4.3. FXR and Lipid Metabolism 

FXR reduces the plasma low‑density lipoprotein cholesterol (LDL‑C) level [67] by inducing the internalization and degradation of the LDL particle [68]. Moreover, it was found that FXR knockout mice displayed elevated plasma high‑density lipoprotein cholesterol (HDL‑C) due to the reduced expression of reverse cholesterol transport gene, scavenger receptor class B member 1 (SCARB1), and ATP‑binding cassette (ABC) transporters G5 (ABCG5) and G8 (ABCG8), which facilitated the removal of HDL‑C from the blood [69,70].

Finally, FXR can also reduce the accumulation of cholesterol in hepatocytes and renal epithelial cells. This can be achieved by decreasing cholesterol synthesis via inhibition of the expression of sterol‑regulatory element‑binding protein 2 (SREBP‑2) and ‑Hydroxy ‑methylglutaryl‑CoA (HMG‑CoA) [71] and promoting cholesterol efflux via increasing expression of ATP‑binding cassette transporterA1 (ABCA1) [72]. On the other hand, FXR lowered hepatic and renal triglyceride accumulation and plasma triglyceride levels in insulin resistance models, such as ob/ob and KK‑Ay mice [59], by reducing the expression of fatty acid synthase (FAS) and acetyl CoA carboxylase (ACC). This was achieved by in-inhibiting SREBP‑1c and carbohydrate response element binding protein (ChREBP) [73–75], promoting triglycerides clearance by increasing fatty acids oxidation via PPAR / ‑ carnitine palmitoyltransferase I (CPT1) axis [76,77] and reducing fatty acid uptake by reducing the expression of CD36 [78]. In adipocytes, FXR induces brown adipose tissue (BAT) whiten‑ ing, presenting with large intracellular lipid droplets and extracellular collagen deposition as a result of activation of stearoyl‑coenzyme A desaturase (SCD) expression via PPAR activation [79–82] (Figure 1).

Figure 1. The general function of FXR. ㊉ indicates stimulation; ㊀ indicates inhibition

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