Progress in research on the roles of TGR5 receptor in liver diseases

Abstract

TGR5 (G protein-coupled bile acid receptor 1, GPBAR-1) is a G protein-coupled receptor with seven transmembrane domains and is widely distributed in various organs and tissues. As an important bile acid receptor, TGR5 can be activated by primary and secondary bile acids. Increased expression of TGR5 is a risk factor for polycystic liver disease and hepatobiliary cancer. However, there is evidence that the anti-inflammatory effect of the TGR5 receptor and its regulatory effect on hydrophobic bile acid confer protective effects against most liver diseases. Recent studies have shown that TGR5 receptor activation can alleviate the development of diabetic liver fibrosis, regulate the differentiation of natural killer T cells into NKT10 cells, increase the secretion of anti-inflammatory factors, inhibit the invasion of hepatitis B virus, promote white adipose tissue browning, improve arterial vascular dynamics, maintain tight junctions between bile duct cells, and protect against apoptosis. In portal hypertension, TGR5 receptor activation can inhibit the contraction of hepatic stellate cells and improve intrahepatic microcirculation. In addition, the discovery of the regulatory relationship between the TGR5 receptor and miRNA-26a provides a new direction for further studies of the molecular mechanism underlying the effects of TGR5. In this review, we describe recent findings linking TGR5 to various liver diseases, with a focus on the mechanisms underlying its effects and potential therapeutic implications.

Keywords:

• TGR5
• liver fibrosis
• hepatitis
• liver injury
• portal hypertension
• polycystic liver disease
• cholangiocarcinoma
• miRNA-26a

Introduction

Bile acid is a water-soluble steroid synthesised in hepatocytes by cholesterol catabolism. It promotes lipid absorption and regulates glucose and energy metabolism under physiological conditions; however, bile acid is toxic to hepatocytes when overloaded [1]. The expression of genes involved in the maintenance of bile acid homeostasis is strictly regulated by nuclear receptors (NRs), mainly farnesol X receptor (FXR). NRs function by sensing the intracellular bile acid concentration and altering the expression and activity of bile acid transporters on the plasma membrane via post-transcriptional regulation [2]. Unlike FXR, which directly regulates gene expression, TGR5 (G protein-coupled bile acid receptor1, GPBAR-1) is a plasma membrane-bound bile acid receptor that indirectly affects gene expression by initiating specific downstream signalling pathways [3].

TGR5 is a G protein-coupled receptor with seven transmembrane domains encoded by a single-exon gene (TGR5) located on chromosome 2q35 in humans and chromosome 1 (38.53 cM) in mice [4]. The open reading frame of the human TGR5 gene is 993 bp and encodes a protein of 330 amino acid residues, distributed in the liver, biliary tract, gastrointestinal tract, lung, pancreas, and other organs and tissues. It is most highly expressed in the gallbladder and adipose tissue, followed by the placenta and spleen [4]. In the liver and gallbladder, TGR5 is localised in sinusoidal endothelial cells, Kupffer cells, bile duct cells, gallbladder epithelial cells, gallbladder smooth muscle cells, and activated hepatic stellate cells (HSCs) [5,6].

As a vital bile acid receptor, TGR5 plays an indispensable role in maintaining bile acid homeostasis. Previous studies have revealed that TGR5-deficient mice show a smaller total bile acid pool with similar levels of bile acid excretion in faeces compared with those of wild-type (WT) mice [7]. TGR5-deficient mice also show changes in the proportion of bile acid and decreases in the levels of tauro-α-muricholic acid (TαMCA) and tauro-β-muricholic acid (TβMCA). Nonetheless, taurocholic acid (TCA) and taurodeoxycholic acid levels increased, directly triggering an increase in the bile acid hydrophobicity index in TGR5^–/– mice (WT mice: −0.418; TGR5^–/– mice: −0.22) [8,9]. The regulation of hydrophobic bile acid by TGR5 confers a protective effect on hepatocytes. However, compared with the nuclear receptor FXR, the roles of TGR5 in liver diseases are less well understood. Therefore, this article summarises research progress on the roles of TGR5 in various common liver diseases and related work in the past 2 years.

TGR5 and liver fibrosis

Liver fibrosis is a prevalent pathological process in various chronic liver diseases, including viral hepatitis, immune hepatitis, non-alcoholic fatty liver disease (NAFLD), cholestatic hepatitis, and liver cancer. Extracellular matrix deposition is the primary pathological manifestation of hepatic fibrosis. Irrespective of the trigger, liver injury causes changes in hepatic sinusoidal endothelial cells and HSCs, eventually leading to the progression of hepatic fibrosis and microvascular dysfunction [10–13]. In the past, hepatic fibrosis was considered irreversible because it was accompanied by the destruction of the liver parenchyma and extracellular matrix accumulation. Nevertheless, since the discovery of HSCs, hepatic fibrosis has become a key focus of research. In the 1990s, Hammel et al. confirmed that even late hepatic fibrosis could be reversed by tracking patients with chronic choledochal stricture caused by chronic pancreatitis [14,15]. Notably, the activation of HSCs is a major cellular mechanism underlying hepatic fibrosis (Figure 1).

Figure 1. Overview of the role of HSCs in liver fibrosis. ① Liver cell damage caused by various pathogenic factors causes nearby hepatic sinusoidal endothelial cells, Kupffer cells, and platelets to secrete platelet-derived growth factor (PDGF), angiotensin 1 (ET-1), transforming growth factor β (TGF-β), and other cytokines, which activate the proliferation of hepatic stellate cells, promote the transformation to the myofibroblast phenotype, and increase extracellular matrix synthesis. Activated hepatic stellate cells can secrete PDGF, ET-1, TGF-β, and other cytokines to continue activation [41,42]. ② High glucose and high insulin levels caused by insulin resistance can significantly increase the proliferation ability of activated hepatic stellate cells [29].

During acute liver injury, HSCs are activated and differentiated into growth factor-producing cells, myofibroblasts, and extracellular matrix, which repair the liver by promoting hepatocyte proliferation and replacing necrotic hepatic parenchyma cells [16,17].

Nonetheless, in many chronic liver diseases, the continuous activation of HSCs results in excessive extracellular matrix deposition, thereby replacing normal hepatocytes and destroying the inherent structure and function of the liver. This further progresses to bridging fibrosis and eventually to liver cirrhosis and even liver failure [18]. Thus, inhibiting the continuous activation of HSCs is one of the primary measures for the prevention and treatment of hepatic fibrosis.

The activation of HSCs is regulated by various cytokines, among which platelet-derived growth factor B (PDGF-B) is the strongest known activator of HSCs and exerts a strong chemotactic effect [19]. A previous study has revealed that serum PDGF-B levels in patients with early liver fibrosis with different aetiologies are higher than those in non-fibrotic liver tissues [20]. Moreover, the inhibition of the expression and function of PDGF-B has yielded promising results for the treatment of liver fibrosis [20–25]. The functional inhibition of PDGF-B is achieved by blocking the binding of PDGF-B to its receptor platelet-derived growth factor receptor-β (PDGFR-β) or by inhibiting the expression of PDGFR-β [22,25]. Unlike other cells, activated HSCs demonstrate elevated levels of PDGFR-β, and the platelet-derived growth factor B/platelet-derived growth factor receptor β (PDGF-B/PDGFR-β) axis plays an important role in the activation of HSCs and the formation of hepatic fibrosis [26,27]. In a study of the TGR5-mediated regulation of portal pressure via the inhibition of endothelin-1 (ET-1), Klindt et al. found that after long-term treatment with lithocholic acid (LCA), mRNA expression levels of PDGFR-β are significantly higher in TGR5 knockout mice than in WT mice [10]. However, it is possible that cholestasis causes more serious liver damage in TGR5 knockout mice, and the upregulation of PDGFR-β continuously activates HSCs, thereby promoting extracellular matrix production for liver repair, which also creates conditions that promote the occurrence of liver fibrosis. Although these findings are insufficient to confirm the regulatory effects of TGR5 on PDGFR-β in liver fibrosis, the loss of TGR5 disrupts the balance of bile acids and increases TCA. In contrast, TCA promotes cholesterol absorption and increases liver oxidative stress to accelerate the progression of liver fibrosis [9,10,28].

In addition, high glucose and high insulin levels significantly increase the proliferation of activated HSCs in the state of insulin resistance (IR) [29]. This might explain the aggravation of liver fibrosis in patients diagnosed with type 2 diabetes mellitus. TGR5 has been shown to play a positive role in the regulation of various metabolic diseases, including type 2 diabetes [30,31]. The activation of TGR5 stimulates the secretion of glucagon-like peptide 1 (GLP-1), which promotes insulin secretion, regulates glucose homeostasis, and exerts antidiabetic effects. GLP-1 activates GLP-1 receptors (GLP-1R) distributed in islet β cells, α cells, and D cells. GLP-1R activation inhibits the secretion of glucagon by α cells and increases the secretion of somatostatin in D cells. GLP-1 promotes the release of insulin under hyperglycaemic conditions, thereby stabilising blood glucose levels [32–34]. Thus, TGR5 is an important target for the treatment of diabetic liver fibrosis. A recent study has shown that the TGR5 agonist oleanolic acid (OA) in combination with the DPP-4 inhibitor anagliptin (ANA), another antidiabetic drug, can effectively inhibit porcine serum-induced liver fibrosis in male diabetic rats [35]. Although a high dose of OA affects bile acid metabolism with a risk of cholestatic liver injury, OA below the minimum toxic dose can still have anti-diabetes and anti-fibrotic effects [35,36]. Notably, although TGR5 is highly expressed in activated HSCs, its effect on these cells is not clear [6]. OA does not have a significant effect on the proliferation of HSCs or the expression of the profibrogenic markers Acta2 and Col1a1 [35]. The improved effect of OA in combination with ANA for the treatment of diabetic liver fibrosis is primarily achieved by the activation of TGR5 in the intestine to stimulate intestinal L cells, which secrete GLP-1, in addition to the anti-diabetic and antioxidative stress effects of ANA [35].

To date, effective clinical treatments for liver fibrosis are lacking. Therapeutic strategies for early liver fibrosis primarily relieve liver injury and inflammation by improving basic diseases; however, anti-fibrotic therapies are essential at the progressive stage of liver fibrosis and cirrhosis. Chronic inflammation caused by various triggers is the premise and driving factor for liver fibrosis, and the inhibition of inflammation, protection of hepatocytes, and antioxidation are the major therapeutic strategies for fibrosis [37,38]. TGR5 significantly inhibits NF-κB-mediated inflammation. After the injection of LPS into TGR5 gene-deficient mice, several inflammatory cells infiltrated the liver, accompanied by a significant increase in inflammatory factors (such as iNOS, MCP-1, and IL-1β). The treatment of WT mice with a TGR5 agonist significantly inhibited NF-κB-mediated inflammatory genes [39], reflecting conditions in diabetic renal fibrosis [40]. Although the mechanism underlying the effects of TGR5 in HSCs is unclear, its strong anti-inflammatory effect makes it an important candidate for the prevention and treatment of liver fibrosis.

TGR5 and autoimmune hepatitis and hepatitis B

Immune responses are essentially defence responses to pathogenic microorganisms or mutant cells; however, an excessive immune response can result in self-damage [43]. Notably, immunoreactive natural killer T (NKT) cells are pathogenic cells in autoimmune liver disease [44–47]. In common models of immune hepatitis, NKT cells contribute to immune liver injury induced by ConA. In the absence of NKT cells, ConA loses its damaging effect on the liver, and the activation of NKT cells is accompanied by α-GalCer-induced acute hepatitis [48,49]. Recently, Biagioli et al. detected the expression of TGR5 in NKT cells. They also found that TGR5 regulates NKT cell typing and inhibits inflammation in a mouse model of hepatitis induced by ConA and α-GalCer [50]. In particular, TGR5 activation promotes type I NKT cell differentiation into NKT10 cells, significantly increases the secretion of anti-inflammatory IL-10, and reduces the production of pro-inflammatory factors, including IFN-γ and IL-4 [50]. Additionally, type II NKT cells are less frequent than type I NKT cells in the liver; however, the activation of TGR5 could increase the number of type II NKT cells and regulate phenotypic differentiation to IL-10-secreting phenotypes [50]. These results indicate that TGR5 regulates the phenotypic differentiation of type I and type II NKT cells in the liver immune system, providing a novel basis for the treatment of autoimmune liver diseases.

Hepatitis B virus (HBV) remains a serious global public health issue. Recent research has shown that the TGR5-specific agonist INT-777 can inhibit HBV infection in HepG2-hNTCP-C4 cells [51]. Notably, in acute chronic hepatitis B liver failure caused by HBV, TGR5 promoter methylation is associated with a significantly increased mortality rate [52]. Thus, the TGR5 receptor has potential value for the prevention of HBV infection, and further studies of its mechanisms of action will provide a new approach for the treatment of hepatitis B.

TGR5 and non-alcoholic steatohepatitis

Excessive fat, fructose intake, and metabolic diseases lead to NAFLD induced by lipotoxicity. The prevalence of NAFLD increases with changes in lifestyle and diet, including shifts to sedentary lifestyles and high-fat diets. A recent large-scale study reported that the global prevalence of NAFLD is approximately 25 and is increasing annually. Hyperlipidaemia and obesity have the highest rates of metabolic complications, followed by metabolic syndrome, hypertension, and type 2 diabetes [53]. Unlike non-alcoholic fatty liver (NAFL), the pathological changes non-alcoholic steatohepatitis (NASH) primarily involve the fat accumulation, inflammatory cell accumulation in the liver parenchyma, macrophage activation, and HSC activation, which are risk factors for severe liver fibrosis, cirrhosis, and hepatocellular carcinoma [54]. As mentioned earlier, the activation of TGR5 promotes the secretion of GLP-1 by intestinal L cells, with antidiabetic effects, and GLP-1 reduces steatosis, inflammation, and fibrosis of the liver in rodent models of NASH [55–57]. In a recent study, quercetin restored bile acid homeostasis by activating the FXR/TGR5 signalling pathway, thereby improving dyslipidaemia and liver fat accumulation in db/db mice [58].

Carino et al. found that the TGR5-specific ligand BAR501 activates TGR5 in an HFD-F mouse model (liver and blood vessel injury model induced by a high-fat and high-fructose diet) could promote WAT browning, as evidenced by an increase in browning markers, such as UCP1, PGC-1α, PPARα, FGF21, and Prdm16, and increased BAT thermogenesis to protect the liver [59]. PGC1-α is a vital metabolic regulatory gene in WAT, BAT, and muscle tissues. It coordinates the expression of genes related to glucose, fatty acid, and cholesterol homeostasis, increases the mitochondrial content of WAT, and induces the expression of UCP-1, thereby increasing the thermogenesis of WAT [60]. In vitro experiments have shown that the regulation of PGC-1 α by TGR5 is mediated by the recruitment of CREB to the PGC1-α promoter, consistent with other previous studies showing that CREB induces PGC1 mRNA expression [59,61]. As such, TGR5 is an important target for the treatment of NASH.

It is generally agreed that the severity of steatosis, inflammation, and fibrosis of the liver is closely related to vascular injury, and the occurrence of NASH also increases the risk of cardiovascular complications [62–69]. In 2018, Carino et al. found that in an HFD-F mouse model, BAR501 reverses hepatic fat deposition in NASH and inhibits the increase in aortic intima-media thickness and the expression of inflammatory genes (IL-6, TNF-α, iNOS, and F4amp80) as well as adhesion molecules (VCAM, intercellular adhesion molecule1, and endothelial selectin) [70]. BAR501 also increased the production of the vasodilator NO and hydrogen sulphide and reversed vascular injury, such as the change in the vascular motor activity of the aortic ring, to a certain extent [70]. These findings support the use of TGR5 agonists for the treatment of NASH and cardiovascular complications.

TGR5 and cholestatic liver injury

Cholestatic liver injury is primarily caused by disturbances in bile production, secretion, and excretion with various underlying causes. This in turn leads to the accumulation of hydrophobic bile acids in the liver [71]. Of note, an appropriate amount of bile acid promotes the repair of the injured liver; however, if the liver injury is serious, the accumulation of bile acid in the liver becomes toxic [72–74]. This is especially apparent after hepatectomy, probably because when the liver is mostly removed or a large number of hepatocytes are damaged, the remaining liver is more likely to have an overload of bile acid. Hence, the negative effect of bile acid is far greater than its positive effect [75–77]. After partial hepatectomy, TGR5 protects the liver from cholestasis by regulating the secretion of hydrophobic bile acids and cytokines [78]. After partial hepatectomy, the secretion of pro-inflammatory cytokines was higher, hydrophobic cholestasis was worse, hepatocyte necrosis was greater, and liver regeneration was lower in mice with TGR5 gene deletion than in WT mice [78,79]. TGR5 is highly expressed in bile duct cells; it promotes the formation of the bicarbonate umbrella, inhibits apoptosis, and maintains the integrity of tight junctions (TJs) to avoid bile acid toxicity in bile duct cells [80–82]. Sequencing analyses of primary sclerosing cholangitis and healthy controls by Hov et al. have revealed a high frequency of single nucleotide polymorphisms and missense mutations (including W83R, V178M, A217P, S272G, and Q296X5) in TGR5. These missense mutations substantially reduce the expression of the TGR5 receptor, decrease its activity, and affect the transduction of intracellular signalling molecules. Based on the protective effect of TGR5 on bile duct cells, the loss and functional changes of the TGR5 receptor may increase the susceptibility of bile duct cells to damage caused by cholestasis [83].

Mice with a γ-catenin and β-catenin double-knockout show severe cholestatic liver injury due to changes in TJs between bile duct cells [84]. The change in TJs causes bile leakage and bile reflux in hepatic parenchyma cells, resulting in cholestatic liver injury, specifically in neonatal sclerosing cholangitis syndrome [85–89]. The absence of TGR5 leads to a change in TJs between bile duct epithelial cells. When TGR5 is deleted, junctional adhesion molecule (JAM)-A, one of the earliest TJ proteins during cell contact, is significantly reduced and phosphorylated, causing a change in the permeability of the bile duct epithelium and destruction of the blood–bile barrier [80,90,91]. TGR5 fundamentally maintains the blood–bile barrier, prevents bile reflux under physiological conditions, and regulates the innate immune response and macrophage migration in mice with cholestatic liver injury induced by bile duct ligation. It has been documented that the main downstream effector of Wnt signalling, β-catenin, plays a vital role in liver development, metabolism, and regeneration, chronic inflammation, and innate and acquired immunity [92–94]. Rao et al. found that the interaction between TGR5 and GSK3 interferes with the β-catenin–destruction complex, activating β-catenin signal transduction, promoting the activity of the PI3K/Akt signalling pathway, inhibiting the TLR4/NF-kB signalling pathway, and reducing the secretion of inflammatory factors, such as TNF-α and IL-6 [95]. Moreover, the activation of TGR5 in a cholestatic liver injury model downregulates the expression of the chemokines CXCL-10 and MCP-1, which regulate the immune response and reduce the aggregation of macrophages in the liver [95,96]. In summary, TGR5 plays a key role in maintaining TJs of bile duct epithelial cells. When cholestasis occurs in the liver, TGR5 inhibits the secretion of inflammatory factors and reduces the accumulation of macrophages via various signalling pathways, thereby reducing cholestatic hepatitis.

TGR5 and hepatic ischaemia–reperfusion injury

Hepatic ischaemia–reperfusion injury (HIRI) is a pathological phenomenon in which the blood supply to the liver is restored after it is blocked for a long period, aggravating liver dysfunction and structural injury. HIRI is an important factor influencing the success of liver transplantation, accounting for approximately 10% of early transplant failures [97–99].

The mechanism underlying HIRI is complex, involving the activation of T cells and macrophages, thereby increasing the secretion of proinflammatory cytokines and chemokines, specifically cytokines involved in the innate immune response (e.g., TNF-α, IL-6, and IL-10). Endoplasmic reticulum stress triggers the unfolded protein response to induce apoptosis mediated by C/EBP homologous protein (CHOP) [100–104]. The role of TGR5 in acute inflammation and hepatocyte apoptosis in HIRI is largely underexplored. Nonetheless, Zhuang et al. found that the protective effect of ischaemic preconditioning on HIRI might be achieved by the activation of the anti-apoptotic effect of TGR5 [105]. IP activates the TGR5 receptor, reaching a peak at 6 h, consistent with the results of the ischaemia–reperfusion model administered the TGR5 agonist INT777. The downregulation of the pro-apoptotic protein caspase-3 and the upregulation of the anti-apoptotic protein Bcl-2 were in agreement with the outcomes of the ischaemia–reperfusion model treated with the TGR5 agonist INT777 [105]. In addition, the anti-apoptotic effect of TGR5 in HIRI was found in subsequent studies. Yang et al. reported that in an IR mouse model, there were fewer apoptotic cells in the TGR5^+/+ group than in the TGR5^–/– group [106]. Additional studies have shown that TGR5 inhibits TLR4-mediated Caspase8 activation to prevent hepatocyte apoptosis, upregulates the expression of IκBα, and inhibits NF-κB phosphorylation; TGR5 significantly inhibits macrophage pro-inflammatory activation by inhibiting the TLR4/NF-κB signalling pathway [106]. Therefore, the activation of TGR5 can effectively reduce hepatocyte apoptosis in HIRI and inhibit the immune-inflammatory reaction. As such, TGR5 is expected to become a crucial target for the treatment of hepatic ischaemia–reperfusion injury.

TGR5 and portal hypertension

Portal hypertension is one of the most important complications of chronic liver disease caused by increased intrahepatic vascular resistance in hepatic fibrosis, microvascular thrombosis, hepatic sinusoid endothelial cell dysfunction, HSC activation, and platelet dysfunction [13]. The manifestations of limbal epithelial stem cell (LESC) dysfunction include decreased permeability and decreased nitric oxide (NO) secretion. In the physiological state, LESCs have high permeability, whereas in chronic liver disease, liver fibrosis causes LESCs to lose fenestration and form a basement membrane, causing a decrease in permeability. As an important vasodilator, a decrease in NO secretion significantly increases intravascular pressure. Notably, TGR5 is highly expressed in LSECs. When TGR5 is activated, it induces the expression and activation of endothelial nitric oxide synthase (eNOS) in endothelial cells, and NO secretion mainly depends on the activity and expression of eNOS [107]. In addition, the activation of TGR5 can trigger the expression of cystathionine-gamma-lyase (CSE) and the serine phosphorylation of eNOS, leading to the production of hydrogen sulphide (H₂S) and NO, respectively. Consequently, this inhibits the expression and secretion of the strong vasoconstrictor ET-1 in LSECs (Figure 2) [107–111].

Figure 2. TGR5 receptor alleviates portal hypertension by promoting the release of NO and H₂S and inhibiting the production of ET-1 and ETAR. ① TGR5 activates eNOS by increasing the phosphorylation of AKT at S473 via PI3K [112,113]. ② TGR5 promotes the release of Ca⁺ in the endoplasmic reticulum and the entry of extracellular Ca + into the cell [113,114]. ③ TGR5 increases eNOS Ser1177 phosphorylation and inhibits eNOS Thr495 phosphorylation by the cAMP/PKA signalling pathway [107,115]. ④ TGR5 increases the phosphorylation of S377 in CSE via the cAMP/PKA and PI3K/AKT pathways, thereby promoting the generation of H₂S [108,109]. ⑤ TGR5 promotes the phosphorylation of FOXOA1 via the PI3K/AKT pathway and then inhibits endothelin-1 (ET-1) [109]. ⑥ The activation of TGR5 increases cAMP levels in HSCs, leading to the internalisation of ETAR [10].

TGR5 downregulated the expression of ET-1, which can cause the contraction of HSCs and decrease the sensitivity of HSCs to ET-1. In particular, TGR5 activation causes an increase in cAMP, while cAMP can induce the internalisation of ETAR, a receptor of ET-1 on HSCs, as well as the sensitivity of HSCs to ET-1 [10].

TGR5 in polycystic liver disease and cholangiocarcinoma and the role of miRNA26a

In addition to promoting the proliferation of bile duct cells, TGR5 promotes the proliferation of rat and human cystic bile duct cells in polycystic liver disease, which is one of the primary mechanisms of cystic growth [116,117]. Masyuk et al. found that TGR5 promotes the development of polycystic liver disease via the cAMP/G-α pathway and identified a novel TGR5 antagonist, SBI-115, with an effective inhibitory effect on polycystic liver disease either alone or in combination with the somatostatin receptor (SSTR) analogue pisitide (a drug targeting cAMP) [117,118]. This study provides a novel treatment option for polycystic liver disease. Nonetheless, information on TGR5 and polycystic liver disease is limited, and the contributions of other mechanisms linking TGR5 to polycystic liver disease cannot be excluded.

TGR5 promotes the growth of various human cancer cells, including gastric adenocarcinoma, endometrial carcinoma, and cholangiocarcinoma [119–121].

Hepatic cholangiocarcinoma is a malignant tumour originating from bile duct epithelial cells. It is the second most prevalent primary liver cancer worldwide. However, limited information on its pathogenesis and the lack of early diagnostic biomarkers and treatment strategies contribute to a poor prognosis [121–123]. Previous studies have shown that TGR5 expression is higher in cholangiocarcinoma than in paracancerous tissues, specifically in extrahepatic and hilar cholangiocarcinoma [116,124]. In vitro experiments have shown that TCA and other TGR5 agonists could promote the proliferation of cholangiocarcinoma cell lines (EGI-1 and TFK-1) by EGFR activation and MAPK phosphorylation [111,116]. A recent study has shown that silencing the mortalin gene can attenuate the TGR5-induced increases in cholangiocarcinoma cell viability, colony formation, and proliferating cell nuclear antigen (PCNA) expression [121]. TGR5 positively regulates the expression of mortalin at both the protein and mRNA levels and inactivates the tumour suppressor p53, thereby promoting the proliferation of cholangiocarcinoma cells [121]. These results provide insight into the molecular mechanisms underlying cholangiocarcinoma; however, the mechanism underlying the relationship between TGR5 and mortalin remains to be elucidated.

Studies of patients with hepatitis B-related hepatocellular carcinoma have shown that miR-26 (including miR-26a and miR-26b) is a tumour suppressor. In addition, its silencing can reduce the survival rate of patients and improve sensitivity to interferon-α therapy [125–127]. A recent study has also confirmed that the overexpression of miRNA-26a reduces F-box protein 11 (FBXO11), which in turn inhibits the proliferation, colony formation, migration, and invasion of hepatocellular carcinoma cells [128,129]. In other chronic liver diseases, miR-26a promotes hepatocyte regeneration. This is because the increase in liver regeneration in other chronic liver diseases and after partial hepatectomy triggers the upregulation of miR26a. As a consequence, phosphatase and tensin homolog (PTEN), a target of miR-26a, is downregulated, and the p-Akt/cyclinD1 signalling pathway is upregulated, thereby promoting the proliferation of hepatocytes [129]. Moreover, in vitro studies have shown that miR-26a regulates fatty acid and cholesterol homeostasis and has a protective effect against NAFLD [130].

Thus, miR-26a plays a vital role in liver cancer and other chronic liver diseases. Interestingly, despite limited studies of TGR5 and miRNA interactions, Chen et al. found a regulatory relationship between TGR5 and miR-26a. When TGR5 knockout mice and WT mice were fed various TGR5 ligands (including OA, CA, and 23(S)-m-LCA), miR-26a was significantly upregulated in the livers of WT mice, with no significant changes in expression in TGR5^–/– mice. Furthermore, miR-26a was not affected by the administration of the FXR-specific ligand GW4064, excluding the effect of FXR. Therefore, TGR5 had a positive regulatory effect on miR-26a. Nonetheless, this regulatory effect was not observed in Kupffer cells with JNK knockout, indicating that TGR5 regulates miR-26a via the JNK signalling pathway [3].

As described above, miR-26a contributes to hepatocellular carcinoma, liver regeneration, and energy metabolism, and its function is similar to that of TGR5 in liver diseases. The discovery of the regulatory relationship between TGR5 and miR-26a provides a fundamental basis for research and drug development related to TGR5.

Summary and prospects

TGR5 plays an important role in regulating bile acid metabolism, maintaining energy balance, inhibiting inflammation, maintaining the blood–bile barrier, anti-apoptosis, and promoting cell proliferation. Moreover, it has complex regulatory effects in liver diseases and related complications. The loss, overexpression, or functional changes in TGR5 influence the occurrence and development of diseases. Therefore, a detailed understanding of the mechanism underlying the effects of TGR5 in liver diseases is critical.

Factors contributing to the occurrence and development of liver diseases are not independent and there is often variation in aetiologies and complications, including cardiovascular disease and NAFLD. Treatments based on common signalling mechanisms may be particularly efficient. The activation of TGR5 promotes the production of brown fat and increases heat production to protect the liver. TGR5 also inhibits the release of inflammatory cytokines, upregulates the expression of vasodilators (such as NO and H₂S), and reverses vascular injury caused by fat and fructose. In addition, in type 2 diabetes, the proliferation of continuously activated HSCs aggravates the development of liver fibrosis. TGR5 regulates energy metabolism and stimulates GP-1 secreted by intestinal L cells, exerting anti-diabetic effects and relieves fibrosis. Furthermore, the occurrence of liver fibrosis is a driving factor for portal hypertension. The activation of TGR5 improves fibrosis and reduces portal hypertension by regulating the dysfunction of LESCs, increasing vasodilator research, and reducing HSC contraction. Therefore, studies of TGR5 have revealed the relationship between different liver diseases and diseases of different systems, providing a basis for the establishment of comprehensive treatment strategies.

Although numerous TGR5 agonists have beneficial effects for the treatment of liver disease in animal models, the overactivation of TGR5 can disrupt bile acid metabolism, decrease gallbladder motility, and result in cholestasis liver injury and cholecystolithiasis. It is noteworthy that OA protects against fibrosis and diabetes, even below the minimum toxic dose, thus emphasising the importance of the early clinical application of TGR5 agonists. Owing to recent advances in high-throughput sequencing technology, the important functions of non-coding RNAs are being revealed, including the regulatory relationship between noncoding RNAs and TGR5. For example, a recent study has shown that reducing the expression of the long-chain non-coding RNASNHG5 upregulates TGR5 to protect colorectal cells after vertical sleeve gastrectomy in type 2 diabetes mellitus. The activation of TGR5 significantly upregulates miRNA-26a in Kupffer cells via the JNK signalling pathway. These studies of the relationship between noncoding RNAs and TGR5 will further advance our understanding of the mechanism by which TGR5 regulates energy metabolism, protects against apoptosis, and promotes cell proliferation.

Acknowledgments

Lijin Zhao proposed the theme of this article. Ke Ma carried out the conception, research and writing of the article. Dan Tang and Chang Yu participated in the revision of the article.

Disclosure statement

The authors report no conflict of interest regarding the publication of this paper.

Additional information

Funding

This study was supported by the National Natural Science Foundation of China [NO. 81960125]; and Major Research Projects of Innovative Groups of Guizhou Provincial Department of Education [NO. Qian Jiao He KY Zi [2016] 039].