Highly lipophilic 3-epi-betulinic acid derivatives as potent and selective TGR5 agonists with improved cellular efficacy

TGR5 Activation by Dietary Bioactives and Related Improvement in Mitochondrial Function for Alleviating Diabetes and Associated Complications

Takeda G protein-coupled receptor 5 (TGR5), also known as G protein-coupled bile acid receptor 1 (GPBAR1), is a cell surface receptor involved in key physiological processes, including glucose homeostasis and energy metabolism. Recent research has focused on the role of TGR5 activation in preventing or treating diabetes while also highlighting its potential impact on the progression of diabetic complications. Functional foods and edible plants have emerged as valuable sources of natural compounds that can activate TGR5, offering potential therapeutic benefits for diabetes management. Despite growing interest, studies on the activation of TGR5 by dietary bioactive compounds remain scattered. This Review aims to provide a comprehensive analysis of how dietary bioactives act as potential agents for TGR5 activation in managing diabetes and its complications. It explores the mechanisms of TGR5 activation through both direct agonistic effects and indirect pathways via modulation of the gut microbiota and bile acid metabolism.

The role of botanical triterpenoids and steroids in bile acid metabolism, transport, and signaling: Pharmacological and toxicological implications

Bile acids (BAs) are synthesized by the host liver from cholesterol and are delivered to the intestine, where they undergo further metabolism by gut microbes and circulate between the liver and intestines through various transporters. They serve to emulsify dietary lipids and act as signaling molecules, regulating the host's metabolism and immune homeostasis through specific receptors. Therefore, disruptions in BA metabolism, transport, and signaling are closely associated with cholestasis, metabolic disorders, autoimmune diseases, and others. Botanical triterpenoids and steroids share structural similarities with BAs, and they have been found to modulate BA metabolism, transport, and signaling, potentially exerting pharmacological or toxicological effects. Here, we have updated the research progress on BA, with a particular emphasis on new-found microbial BAs. Additionally, the latest advancements in targeting BA metabolism and signaling for disease treatment are highlighted. Subsequently, the roles of botanical triterpenoids in BA metabolism, transport, and signaling are examined, analyzing their potential pharmacological, toxicological, or drug interaction effects through these mechanisms. Finally, a research paradigm is proposed that utilizes the gut microbiota as a link to interpret the role of these important natural products in BA signaling.

Update on the development of TGR5 agonists for human diseases

The G protein-coupled bile acid receptor 1 (GPBAR1) or TGR5 is widely distributed across organs, including the small intestine, stomach, liver, spleen, and gallbladder. Many studies have established strong correlations between TGR5 and glucose homeostasis, energy metabolism, immune-inflammatory responses, and gastrointestinal functions. These results indicate that TGR5 has a significant impact on the progression of tumor development and metabolic disorders such as diabetes mellitus and obesity. Targeting TGR5 represents an encouraging therapeutic approach for treating associated human ailments. Notably, the GLP-1 receptor has shown exceptional efficacy in clinical settings for diabetes management and weight loss promotion. Currently, numerous TGR5 agonists have been identified through natural product-based approaches and virtual screening methods, with some successfully progressing to clinical trials. This review summarizes the intricate relationships between TGR5 and various diseases emphasizing recent advancements in research on TGR5 agonists, including their structural characteristics, design tactics, and biological activities. We anticipate that this meticulous review could facilitate the expedited discovery and optimization of novel TGR5 agonists.

Discovery of betulinic acid derivatives as gut-restricted TGR5 agonists: Balancing the potency and physicochemical properties

The pleiotropic effects of TGR5 make it an appealing target for intervention of metabolic and inflammatory disorders, but systemic activation of TGR5 faces challenges of on-target side effects, especially gallbladder filling. Gut-restricted agonists were proved to be sufficient to circumvent these side effects, but extremely low systemic exposure may not be effective in activating TGR5 since it is located on the basolateral membrane. Herein, to balance potency and physicochemical properties, a series of gut-restricted TGR5 agonists with diversified kinetophores had been designed and synthesized. Compound 22-Na exhibited significant antidiabetic effect, and showed favorable gallbladder safety after 7 days of oral administration in humanized TGR5H88Y mice, confirming that gut-restricted agonism of TGR5 is a viable strategy to alleviate systemic target-related effects.

Discovery of 9,11-Seco-Cholesterol Derivatives as Novel FXR Antagonists

The farnesoid X receptor (FXR) plays an important role in the regulation of bile acid, lipid, and glucose homeostasis. Recent findings have shown that the inhibition of FXR is beneficial to improvement of related metabolic diseases and cholestasis. In the present work, 9,11-seco-cholesterol derivatives were designed and synthesized by cleaving the C ring of cholesterol and were identified as novel structures of FXR antagonists. Compound 9a displayed the best FXR antagonistic activity at the cellular level (IC₅₀ = 4.6 μM) and decreased the expression of the target genes of FXR in vivo.

Identification of Betulinic Acid Derivatives as Potent TGR5 Agonists with Antidiabetic Effects via Humanized TGR5H88Y Mutant Mice

Takeda G protein-coupled receptor 5 (TGR5) is a promising target for treating metabolic syndrome and inflammatory diseases. Herein, we identified a new series of betulinic acid derivatives as potent TGR5 agonists, which show remarkable activity on human (h) and canine (c) TGR5 but exhibit unpromising activity on murine (m) TGR5. Species difference was also observed with many other reported TGR5 agonists. Therefore, we screened 29 amino acids which were conserved in hTGR5 and cTGR5 but different in mTGR5 and found a key amino acid, H88 in mTGR5 (Y89 in hTGR5), which contributed to the species difference. With the CRISPR/Cas9 system, the mTGR5H88Y mutation was introduced into mice, and the optimized compound 11d-Na displayed a significant glucose-lowering effect and stimulated GLP-1 and insulin secretion in TGR5H88Y mice but not in wild-type animals. Taken together, our study provides a useful tool to bridge the gap of species difference and discovers a potent TGR5 agonist for further investigation.

Acankoreagenin and acankoreosides, a family of lupane triterpenoids with anti-inflammatory properties: an overview

Acankoreagenin (ACK, also known as acankoreanogenin and HLEDA) and impressic acid are two lupane-type triterpenes that can be isolated from various Acanthopanax and Schefflera species. They efficiently block activation of the NF-κB signaling pathway and the release of proinflammatory cytokines and/or the action of inflammation mediators (HMGB1, iNOS, and NO). These effects are the basis for the antiviral and anticancer activities reported with these pentacyclic compounds or their various glycoside derivatives. More than 15 acankoreosides (Ack-A to -O, and -R) and a few other mono- and bidesmosidic saponins (acantrifoside A and acangraciliside S) derive from the ACK aglycone. Compounds like Ack-A and -B are remarkable anti-inflammatory agents, inhibiting cytokine release from activated macrophages. Despite their effectiveness, ACK and impressic acid are far much less known and studied than the structurally related compounds betulinic acid and 23-hydroxybetulinic acid (anemosapogenin). The structural differences (notably the R/S stereoisomerism of the 3-hydroxyl group) and functional similarities of these compounds are discussed. The complete series of acankoreosides is presented for the first time. These natural products deserve further attention as anti-inflammatory agents, and ACK is recommended as a template for the design of new anticancer and antiviral drugs.

A novel bile acid analog, A17, ameliorated non-alcoholic steatohepatitis in high-fat diet-fed hamsters

Being endocrine signaling molecules that regulate lipid metabolism and affect energy balance, bile acids are potential drug candidates for non-alcoholic steatohepatitis (NASH). Obeticholic acid (OCA) could improve NASH accompanied by significant side effects. Therefore, it is worthwhile to develop safer and more effective bile acid analogs. In this study, a new bile acid analog A17 was synthesized and its potential anti-NASH effects were assessed in vitro and in vivo. The impact of A17 on steatosis was investigated in the rat primary hepatocytes challenged with oleic acid. It was found that A17 alleviated lipid accumulation by reducing fatty acid (FA) uptake and promoting FA oxidation. The reduction of FA uptake came from inhibiting fatty acid translocase (Cd36) expression. The promotion of FA oxidation came from stimulating the phosphorylation of adenosine monophosphate (AMP)-activated protein kinase alpha (AMPKα). In addition, A17 reduced lipopolysaccharide-induced inflammation in Raw264.7 cells by activating Takeda G protein-coupled receptor 5 (TGR5). In in vivo study, male Golden Syrian hamsters were fed with high fat (HF) diet and then treated with 50 mg/kg/d A17 for 6 weeks. A17 lowered the lipid profiles and liver enzyme levels in serum and improved liver pathological conditions with less side effects compared with OCA. Further studies confirmed that the molecular mechanisms of A17 in vivo were similar to those in vitro. In conclusion, a novel bile acid analog A17 was identified to ameliorate NASH in HF-fed hamsters. The potential mechanisms could be contributed to reducing FA uptake, stimulating FA oxidation and relieving inflammation.

Lupane-Type Triterpenes of Phoradendron vernicosum

Three new lupane-type triterpenes, 3α,24-dihydroxylup-20(29)-en-28-oic acid (1), 3α,23-dihydroxy-30-oxolup-20(29)-en-28-oic acid (2), and 3α,23-O-isopropylidenyl-3α,23-dihydroxylup-20(29)-en-28-oic acid (3), together with eight known compounds (4-11) were isolated from a methanol extract of Phoradendron vernicosum aerial parts. The chemical structures of 1-3 were determined on the basis of spectroscopic data interpretation. The isolated compounds were tested against seven human cancer cell lines and two normal cell lines.

Ursolic acid activates the TGR5 receptor to enhance GLP-1 secretion in type 1-like diabetic rats

Endogenous Takeda G-protein-coupled receptor 5 (TGR5), G-protein-coupled bile acid receptor 1 (GPBAR1), regulates glucose metabolism. In animals, TGR5 activation by a chemical agonist may increase incretin secretion and reduce the blood sugar level. Recently, betulinic acid has been suggested to activate TGR5. Ursolic acid is a well-known pentacyclic triterpenoid that is similar to betulinic acid. It is of special interest to determine the potential effect of ursolic acid on TGR5. Therefore, we transfected cultured Chinese hamster ovary (CHO-K1) cells with the TGR5 gene. The functions of the transfected cells were confirmed via glucose uptake using a fluorescent indicator. Moreover, NCI-H716 cells that secreted incretin were also investigated, and the glucagon-like peptide (GLP-1) levels were quantified using ELISA kits. In addition, streptozotocin (STZ)-induced type 1-like diabetic rats were used to identify the effect of ursolic acid in vivo. Ursolic acid concentration dependently increased glucose uptake in CHO-K1 cells expressing TGR5. In NCI-H716 cells, ursolic acid induced a concentration-dependent elevation in GLP-1 secretion, which was inhibited by triamterene at the effective concentrations to block TGR5. Ursolic acid also increased the plasma GLP-1 level via TGR5 activation, which was further characterized in vivo with type 1-like diabetic rats. Moreover, ursolic acid is more effective than betulinic acid in reduction of hyperglycemia and increase of GLP-1 secretion. Therefore, we demonstrated that ursolic acid can activate TGR5, enhancing GLP-1 secretion in vitro and in vivo. Therefore, ursolic acid is suitable for use in TGR5 activation.

Glycyrrhizic acid increases glucagon like peptide-1 secretion via TGR5 activation in type 1-like diabetic rats

Glycyrrhizic acid (GA) is belonged to triterpenoid saponin that is contained in the root of licorice and is known to affect metabolic regulation. Recently, glucagon like peptide-1 (GLP-1) has widely been applied in diabetes therapeutics. However, the role of GLP-1 in GA-induced anti-diabetic effects is still unknown. Therefore, we are interested in understanding the association of GLP-1 with GA-induced effects. In type 1-like diabetic rats induced by streptozotocin (STZ-treated rats), GA increased the level of plasma GLP-1, which was blocked by triamterene at a dose sufficient to inhibit Takeda G-protein-coupled receptor 5 (TGR5). The direct effect of GA on TGR5 has been identified using the cultured Chinese hamster ovary cells (CHO-K1 cells) transfected TGR5 gene. Moreover, in intestinal NCI-H716 cells that secreted GLP-1, GA promoted GLP-1 secretion with a marked elevation of calcium levels. However, both effects of GA were reduced by ablation of TGR5 with siRNA in NCI-H716 cells. Therefore, we demonstrated that GA can enhance GLP-1 secretion through TGR5 activation.

The Dietary Furocoumarin Imperatorin Increases Plasma GLP-1 Levels in Type 1-Like Diabetic Rats

Imperatorin, a dietary furocoumarin, is found not only in medicinal plants, but also in popular culinary herbs, such as parsley and fennel. Recently, imperatorin has been shown to activate GPR119 in cells. Another GPR, GPR131, also called TGR5 or G-protein-coupled bile acid receptor 1 (GPBAR1), is known to regulate glucose metabolism. Additionally, TGR5 activation increases glucagon-like peptide (GLP-1) secretion to lower blood sugar levels in animals. Therefore, the present study aims to determine whether the effects of imperatorin on GLP-1 secretion are mediated by TGR5. First, we transfected cultured Chinese hamster ovary cells (CHO-K1 cells) with the TGR5 gene. Glucose uptake was confirmed in the transfected cells using a fluorescent indicator. Moreover, NCI-H716 cells, which secrete GLP-1, were used to investigate the changes in calcium concentrations and GLP-1 levels. In addition, streptozotocin (STZ)-induced type 1-like diabetic rats were used to identify the effects of imperatorin in vivo. Imperatorin dose-dependently increased glucose uptake in CHO-K1 cells expressing TGR5. In STZ diabetic rats, similar to the results in NCI-H716 cells, imperatorin induced a marked increase of GLP-1 secretion that was reduced, but not totally abolished, by a dose of triamterene that inhibited TGR5. Moreover, increases in GLP-1 secretion induced by imperatorin and GPR119 activation were shown in NCI-H716 cells. We demonstrated that imperatorin induced GLP-1 secretion via activating TGR5 and GPR119. Therefore, imperatorin shall be considered as a TGR5 and GPR119 agonist.

Investigation of triamterene as an inhibitor of the TGR5 receptor: identification in cells and animals

BACKGROUND

G-protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5) has been shown to participate in glucose homeostasis. In animal models, a TGR5 agonist increases incretin secretion to reduce hyperglycemia. Many agonists have been developed for clinical use. However, the effects of TGR5 blockade have not been studied extensively, with the exception of studies using TGR5 knockout mice. Therefore, we investigated the potential effect of triamterene on TGR5.

METHODS

We transfected the TGR5 gene into cultured Chinese hamster ovary cells (CHO-K1 cells) to express TGR5. Then, we applied a fluorescent indicator to examine the glucose uptake of these transfected cells. In addition, NCI-H716 cells that secrete incretin were also evaluated. Fura-2, a fluorescence indicator, was applied to determine the changes in calcium concentrations. The levels of cyclic adenosine monophosphate (cAMP) and glucagon-like peptide (GLP-1) were estimated using enzyme-linked immunosorbent assay kits. Moreover, rats with streptozotocin (STZ)-induced type 1-like diabetes were used to investigate the effects in vivo.

RESULTS

Triamterene dose dependently inhibits the increase in glucose uptake induced by TGR5 agonists in CHO-K1 cells expressing the TGR5 gene. In cultured NCI-H716 cells, TGR5 activation also increases GLP-1 secretion by increasing calcium levels. Triamterene inhibits the increased calcium levels by TGR5 activation through competitive antagonism. Moreover, the GLP-1 secretion and increased cAMP levels induced by TGR5 activation are both dose dependently reduced by triamterene. However, treatment with KB-R7943 at a dose sufficient to block the Na+/Ca2+ exchanger (NCX) failed to modify the responses to TGR5 activation in NCI-H716 cells or CHO-K1 cells expressing TGR5. Therefore, the inhibitory effects of triamterene on TGR5 activation do not appear to be related to NCX inhibition. Blockade of TGR5 activation by triamterene was further characterized in vivo using the STZ-induced diabetic rats.

CONCLUSION

Based on the obtained data, we identified triamterene as a reliable inhibitor of TGR5. Therefore, triamterene can be developed as a clinical inhibitor of TGR5 activation in future studies.

Development of betulinic acid as an agonist of TGR5 receptor using a new in vitro assay

BACKGROUND

G-protein-coupled bile acid receptor 1, also known as TGR5 is known to be involved in glucose homeostasis. In animal models, treatment with a TGR5 agonist induces incretin secretion to reduce hyperglycemia. Betulinic acid, a triterpenoid present in the leaves of white birch, has been introduced as a selective TGR5 agonist. However, direct activation of TGR5 by betulinic acid has not yet been reported.

METHODS

Transfection of TGR5 into cultured Chinese hamster ovary (CHO-K1) cells was performed to establish the presence of TGR5. Additionally, TGR5-specific small interfering RNA was employed to silence TGR5 in cells (NCI-H716 cells) that secreted incretins. Uptake of glucose by CHO-K1 cells was evaluated using a fluorescent indicator. Amounts of cyclic adenosine monophosphate and glucagon-like peptide were quantified using enzyme-linked immunosorbent assay kits.

RESULTS

Betulinic acid dose-dependently increases glucose uptake by CHO-K1 cells transfected with TGR5 only, which can be considered an alternative method instead of radioligand binding assay. Additionally, signals coupled to TGR5 activation are also increased by betulinic acid in cells transfected with TGR5. In NCI-H716 cells, which endogenously express TGR5, betulinic acid induces glucagon-like peptide secretion via increasing calcium levels. However, the actions of betulinic acid were markedly reduced in NCI-H716 cells that received TGR5-silencing treatment. Therefore, the present study demonstrates the activation of TGR5 by betulinic acid for the first time.

CONCLUSION

Similar to the positive control lithocholic acid, which is the established agonist of TGR5, betulinic acid has been characterized as a useful agonist of TGR5 and can be used to activate TGR5 in the future.