5α-Bile alcohols function as farnesoid X receptor antagonists

Farnesoid X receptor: From Structure to Function and Its Pharmacology in Liver Fibrosis

The farnesoid X receptor (FXR), a ligand-activated transcription factor, plays a crucial role in regulating bile acid metabolism within the enterohepatic circulation. Beyond its involvement in metabolic disorders and immune imbalances affecting various tissues, FXR is implicated in microbiota modulation, gut-to-brain communication, and liver disease. The liver, as a pivotal metabolic and detoxification organ, is susceptible to damage from factors such as alcohol, viruses, drugs, and high-fat diets. Chronic or recurrent liver injury can culminate in liver fibrosis, which, if left untreated, may progress to cirrhosis and even liver cancer, posing significant health risks. However, therapeutic options for liver fibrosis remain limited in terms of FDA-approved drugs. Recent insights into the structure of FXR, coupled with animal and clinical investigations, have shed light on its potential pharmacological role in hepatic fibrosis. Progress has been achieved in both fundamental research and clinical applications. This review critically examines recent advancements in FXR research, highlighting challenges and potential mechanisms underlying its role in liver fibrosis treatment.

Discovery of novel and selective farnesoid X receptor antagonists through structure-based virtual screening, preliminary structure-activity relationship study, and biological evaluation

Farnesoid X receptor (FXR) is a bile acids receptor and plays a crucial role in regulating bile acids, lipids, and glucose metabolism. Previous research suggests that inhibiting FXR activation can be beneficial in reducing cholesterol and low-density lipoprotein cholesterol (LDL-C) levels, offering potential treatment options for metabolic syndrome with lipid disorders. Herein, we report p-acetylaminobenzene sulfonate derivatives as a novel scaffold of FXR antagonists by multistage screening. Among these derivatives, compound F44-A13 exhibited a half-maximal inhibitory concentration of 1.1 μM. Furthermore, compound F44-A13 demonstrated effective inhibition of FXR activation in cellular assays and exhibited high selectivity over eleven other nuclear receptors. Besides, compound F44-A13 significantly suppressed the regulation of FXR target genes Shp, Besp, and Cyp7a1, while reducing cholesterol levels in human hepatoma HepG2 cells. Pharmacological studies conducted on C57BL/6 mice further confirmed that compound F44-A13 had beneficial effects in reducing cholesterol, triglycerides, and LDL-C levels. These findings highlight that F44-A13 is a highly selective FXR antagonist that might serve as a useful molecule for further FXR studies as well as the development of FXR antagonists for the potential treatment of metabolic diseases with lipid disorders.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Farnesoid X receptor: a potential therapeutic target in multiple organs

Farnesoid X receptor (FXR), a member of the nuclear receptor family, is a common receptor found in the intestine and liver, and helps to maintain systemic metabolic homeostasis through regulating bile acid, glucose, lipid metabolism, and energy homeostatsis. In addition, FXR regulates the functions of various organs, such as liver, intestine, kidney, breast, pancreas, cardiovascular system and brain. FXR also plays a key role in regulation of gut-microbiota through mediating the various signaling pathways. Accordingly, FXR has become an attractive therapeutic target in a variety of diseases. This review combines classical and recent research reports to introduce the basic information about FXR and its important roles in various organs of the body.

Discovery, Structural Refinement and Therapeutic Potential of Farnesoid X Receptor Activators

Farnesoid X receptor acts as bile acid sensing transcription factor and has been identified as valuable molecular drug target to treat severe liver disorders, such as non-alcoholic steatohepatitis (NASH). Preclinical and clinical data indicate anti-fibrotic effects obtained with FXR activation that also appear promising for other fibrotic diseases beyond NASH. Strong efforts in FXR ligand discovery have yielded potent steroidal and non-steroidal FXR activators, some of which have been studied in clinical trials. While the structure–activity relationship of some FXR agonist frameworks have been studied extensively, the structural diversity of potent FXR activator chemotypes is still limited to a handful of well-studied compound classes. Together with safety concerns related to full therapeutic activation of FXR, this indicates the need for novel innovative FXR ligands with selective modulatory properties. This chapter evaluates FXR's value as drug target with emphasis on fibrotic diseases, analyses FXR ligand recognition and requirements and focuses on the discovery and structural refinement of leading FXR activator chemotypes.

Semisynthetic bile acids: a new therapeutic option for metabolic syndrome

Bile acids are endogenous emulsifiers synthesized from cholesterol having a peculiar amphiphilic structure. Appreciation of their beneficial effects on human health, recognized since ancient times, has expanded enormously since the discovery of their role as signaling molecules. Activation of farnesoid X receptor (FXR) and Takeda G-protein receptor-5 (TGR5) signaling pathways by bile acids, regulating glucose, lipid and energy metabolism, have become attractive avenue for metabolic syndrome treatment. Therefore, extensive effort has been directed into the research and synthesis of bile acid derivatives with improved pharmacokinetic properties and high potency and selectivity for these receptors. Minor modifications in the structure of bile acids and their derivatives may result in fine-tuning modulation of their biological functions, and most importantly, in an evasion of undesired effect. A great number of semisynthetic bile acid analogues have been designed and put in preclinical and clinical settings. Obeticholic acid (INT-747) has achieved the biggest clinical success so far being in use for the treatment of primary biliary cholangitis. This review summarizes and critically evaluates the key chemical modifications of bile acids resulting in development of novel semisynthetic derivatives as well as the current status of their preclinical and clinical evaluation in the treatment of metabolic syndrome, an aspect that is so far lacking in the scientific literature. Taking into account the balance between therapeutic benefits and potential adverse effects associated with specific structure and mechanism of action, recommendations for future studies are proposed.

FXR modulators for enterohepatic and metabolic diseases

INTRODUCTION

Farnesoid X receptor (FXR), a nuclear receptor mainly expressed in enterohepatic tissues, is a master for bile acid, lipid and glucose homeostasis. Additionally, it acts as a cell protector with unclear mechanism but may be implicated in combating against inflammation, fibrosis and cancers. FXR is thus accepted as a promising target particularly for the enterohepatic diseases, and numerous FXR modulators have been patented and developed.

AREAS COVERED

This review provides an update on the development of FXR modulators for enterohepatic diseases and offers an in-depth perspective on new strategies for the development of novel FXR modulators.

EXPERT OPINION

Despite the development of numerous FXR modulators, which culminated in the successful launch of obeticholic acid (OCA), it remains a matter of debate on how the function of FXR should be exploited for therapeutic purposes. The improvement for obesity achieved by either FXR agonists or antagonists is still in confusion. Whether the side effect of pruritus induced by OCA could be exempted for non-steroidal FXR agonists needs further validation. Apart from the development of conventional FXR ligands, emerging evidence support that restoration of FXR protein level may represent a new strategy in targeting FXR for enterohepatic and metabolic diseases.

Discovery and SAR study of 3-(tert-butyl)-4-hydroxyphenyl benzoate and benzamide derivatives as novel farnesoid X receptor (FXR) antagonists

3-(tert-Butyl)-4-hydroxyphenyl 2,4-dichlorobenzoate (1) was discovered in our in-house high throughput screening as a moderate FXR antagonist. To improve the potency and the stability of the hit 1, forty derivatives were synthesized and SAR was systematically explored. The results turn out that replacing the 2,4-dichlorophenyl with 2,6-dichloro-4-amidophenyl shows great improvement in potency, replacing the benzoate with benzamide shows improvement in stability and slight declining of potency and 3-(tert-butyl)-4-hydroxyphenyl unit is essential in obtaining the FXR antagonistic activity.

Steroidal scaffolds as FXR and GPBAR1 ligands: from chemistry to therapeutical application

Bile acids (BAs) are experiencing a new life. Next to their ancestral roles in lipid digestion and solubilization, BAs are today recognized signaling molecules involved in many physiological functions. These signaling pathways involve the activation of metabolic nuclear receptors, mainly the BA sensor FXR, and the dedicated membrane G protein-coupled receptor, GPBAR1 (TGR5). As a consequence, the discovery of GPBAR1/FXR selective or dual modulators represents an important answer to the urgent demand of new pharmacological opportunity for several human diseases including dyslipidemia, cholestasis, nonalcoholic steatohepatitis, Type 2 diabetes and inflammation. Targeted oriented discovery of natural compounds and medicinal chemistry manipulation have allowed the development of promising drug candidates.

Bufadienolides and polyhydroxycholestane derivatives from Bufo bufo gargarizans

A new polyhydroxycholestane sulfate ester, 3α,12β,25,26-tetrahydroxy-7-oxo-5β-cholestane 26-O-sulfate (1), was isolated from dried skin of Bufo bufo gargarizans Cantor and its structure was elucidated on the basis of extensive 1D and 2D NMR as well as HR-ESI-MS analysis. A comparison of steroidal metabolite profiles, based on HPLC and LC-MS analyses, indicates that the chemical compositions of the various parts of toads, such as venom, skin and stratum corneum, are significantly different.

FXR signaling in the enterohepatic system

Enterohepatic circulation serves to capture bile acids and other steroid metabolites produced in the liver and secreted to the intestine, for reabsorption back into the circulation and reuptake to the liver. This process is under tight regulation by nuclear receptor signaling. Bile acids, produced from cholesterol, can alter gene expression in the liver and small intestine via activating the nuclear receptors farnesoid X receptor (FXR; NR1H4), pregnane X receptor (PXR; NR1I2), vitamin D receptor (VDR; NR1I1), G protein coupled receptor TGR5, and other cell signaling pathways (JNK1/2, AKT and ERK1/2). Among these controls, FXR is known to be a major bile acid-responsive ligand-activated transcription factor and a crucial control element for maintaining bile acid homeostasis. FXR has a high affinity for several major endogenous bile acids, notably cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid. By responding to excess bile acids, FXR is a bridge between the liver and small intestine to control bile acid levels and regulate bile acid synthesis and enterohepatic flow. FXR is highly expressed in the liver and gut, relative to other tissues, and contributes to the maintenance of cholesterol/bile acid homeostasis by regulating a variety of metabolic enzymes and transporters. FXR activation also affects lipid and glucose metabolism, and can influence drug metabolism.

Structure-activity relationship of bile alcohols as human farnesoid X receptor agonist

FXR (farnesoid X receptor) is a bile acid-activated nuclear receptor that regulates not only the biosynthesis and enterohepatic circulation of bile acids, but also triglyceride, cholesterol and glucose metabolism. FXR-mediated signaling pathways have become promising novel drug targets for the treatment of common metabolic and hepatic diseases. With the aim of uncovering novel modulators of FXR and further elucidating the molecular basis of FXR activation, we investigated the structure-activity relationships of a variety of naturally occurring sterols structurally related to bile acids in terms of their FXR agonist activity. Here, we report that the ability of bile alcohols to activate FXR varied with the position and number of hydroxyl groups existing in the steroid side chain of bile alcohols. In addition, we showed that the shortening of the steroid side chain of bile acids as well as bile alcohols resulted in a decline of the ability of these agents to activate FXR. Thus, we provide new insights into the structure-activity relationships of bile acids and bile alcohols as FXR agonists.

Bile alcohols function as the ligands of membrane-type bile acid-activated G protein-coupled receptor

TGR5 is a G protein-coupled receptor that is activated by bile acids, resulting in an increase in cAMP levels and the subsequent modulation of energy expenditure in brown adipose tissue and muscle. Therefore, the development of a TGR5-specific agonist could lead to the prevention and treatment of various metabolic disorders related to obesity. In the present study, we evaluated the ability of bile alcohols, which are structurally and physiologically similar to bile acids and are produced as the end products of cholesterol catabolism in evolutionarily primitive vertebrates, to act as TGR5 agonists. In a cell-based reporter assay and a cAMP production assay performed in vitro, most bile alcohols with a side chain containing hydroxyl group(s) were highly efficacious agonists for TGR5 comparable to its most potent ligand in the naturally occurring bile acid, lithocholic acid. However, the abilities of the bile alcohols to activate TGR5 varied with the position and number of the hydroxyl substituent in the side chain. Additionally, the conformation of the steroidal nucleus of bile alcohols is also important for its activity as a TGR5 agonist. Thus, we have provided new insights into the structure-activity relationships of bile alcohols as TGR5 agonists.

Role of bile acids and bile acid receptors in metabolic regulation

The incidence of the metabolic syndrome has taken epidemic proportions in the past decades, contributing to an increased risk of cardiovascular disease and diabetes. The metabolic syndrome can be defined as a cluster of cardiovascular disease risk factors including visceral obesity, insulin resistance, dyslipidemia, increased blood pressure, and hypercoagulability. The farnesoid X receptor (FXR) belongs to the superfamily of ligand-activated nuclear receptor transcription factors. FXR is activated by bile acids, and FXR-deficient (FXR(-/-)) mice display elevated serum levels of triglycerides and high-density lipoprotein cholesterol, demonstrating a critical role of FXR in lipid metabolism. In an opposite manner, activation of FXR by bile acids (BAs) or nonsteroidal synthetic FXR agonists lowers plasma triglycerides by a mechanism that may involve the repression of hepatic SREBP-1c expression and/or the modulation of glucose-induced lipogenic genes. A cross-talk between BA and glucose metabolism was recently identified, implicating both FXR-dependent and FXR-independent pathways. The first indication for a potential role of FXR in diabetes came from the observation that hepatic FXR expression is reduced in animal models of diabetes. While FXR(-/-) mice display both impaired glucose tolerance and decreased insulin sensitivity, activation of FXR improves hyperglycemia and dyslipidemia in vivo in diabetic mice. Finally, a recent report also indicates that BA may regulate energy expenditure in a FXR-independent manner in mice, via activation of the G protein-coupled receptor TGR5. Taken together, these findings suggest that modulation of FXR activity and BA metabolism may open new attractive pharmacological approaches for the treatment of the metabolic syndrome and type 2 diabetes.

Evolution of pharmacologic specificity in the pregnane X receptor

Background

The pregnane X receptor (PXR) shows the highest degree of cross-species sequence diversity of any of the vertebrate nuclear hormone receptors. In this study, we determined the pharmacophores for activation of human, mouse, rat, rabbit, chicken, and zebrafish PXRs, using a common set of sixteen ligands. In addition, we compared in detail the selectivity of human and zebrafish PXRs for steroidal compounds and xenobiotics. The ligand activation properties of the Western clawed frog (Xenopus tropicalis) PXR and that of a putative vitamin D receptor (VDR)/PXR cloned in this study from the chordate invertebrate sea squirt (Ciona intestinalis) were also investigated.

Results

Using a common set of ligands, human, mouse, and rat PXRs share structurally similar pharmacophores consisting of hydrophobic features and widely spaced excluded volumes indicative of large binding pockets. Zebrafish PXR has the most sterically constrained pharmacophore of the PXRs analyzed, suggesting a smaller ligand-binding pocket than the other PXRs. Chicken PXR possesses a symmetrical pharmacophore with four hydrophobes, a hydrogen bond acceptor, as well as excluded volumes. Comparison of human and zebrafish PXRs for a wide range of possible activators revealed that zebrafish PXR is activated by a subset of human PXR agonists. The Ciona VDR/PXR showed low sequence identity to vertebrate VDRs and PXRs in the ligand-binding domain and was preferentially activated by planar xenobiotics including 6-formylindolo-[3,2-b]carbazole. Lastly, the Western clawed frog (Xenopus tropicalis) PXR was insensitive to vitamins and steroidal compounds and was activated only by benzoates.

Conclusion

In contrast to other nuclear hormone receptors, PXRs show significant differences in ligand specificity across species. By pharmacophore analysis, certain PXRs share similar features such as human, mouse, and rat PXRs, suggesting overlap of function and perhaps common evolutionary forces. The Western clawed frog PXR, like that described for African clawed frog PXRs, has diverged considerably in ligand selectivity from fish, bird, and mammalian PXRs.

Evolution of the bile salt nuclear receptor FXR in vertebrates

Bile salts, the major end metabolites of cholesterol, vary significantly in structure across vertebrate species, suggesting that nuclear receptors binding these molecules may show adaptive evolutionary changes. We compared across species the bile salt specificity of the major transcriptional regulator of bile salt synthesis, the farnesoid X receptor (FXR). We found that FXRs have changed specificity for primary bile salts across species by altering the shape and size of the ligand binding pocket. In particular, the ligand binding pockets of sea lamprey (Petromyzon marinus) and zebrafish (Danio rerio) FXRs, as predicted by homology models, are flat and ideal for binding planar, evolutionarily early bile alcohols. In contrast, human FXR has a curved binding pocket best suited for the bent steroid ring configuration typical of evolutionarily more recent bile acids. We also found that the putative FXR from the sea squirt Ciona intestinalis, a chordate invertebrate, was completely insensitive to activation by bile salts but was activated by sulfated pregnane steroids, suggesting that the endogenous ligands of this receptor may be steroidal in nature. Our observations present an integrated picture of the coevolution of bile salt structure and of the binding pocket of their target nuclear receptor FXR.