TGR5 in cholangiocytes

Serum bile acids associate with liver volume in polycystic liver disease and decrease upon treatment with lanreotide

BACKGROUND

Polycystic liver disease (PLD) is a common extrarenal manifestation of autosomal dominant polycystic kidney disease (ADPKD). Bile acids may play a role in PLD pathogenesis. We performed a post-hoc exploratory analysis of bile acids in ADPKD patients, who had participated in a trial on the effect of a somatostatin analogue. Our hypothesis was that serum bile acid levels increase in PLD, and that lanreotide, which reduces liver growth, may also reduce bile acid levels. Furthermore, in PLD, urinary excretion of bile acids might contribute to renal disease.

METHODS

With liquid chromatography-mass spectrometry, 11 bile acids in serum and 6 in urine were quantified in 105 PLD ADPKD patients and 52 age-, sex-, mutation- and eGFR-matched non-PLD ADPKD patients. Sampling was done at baseline and after 120 weeks of either lanreotide or standard care.

RESULTS

Baseline serum levels of taurine- and glycine-conjugated bile acids were higher in patients with larger livers. In PLD patients, multiple bile acids decreased upon treatment with lanreotide but remained stable in untreated subjects. Changes over time did not correlate with changes in liver volume. Urine bile acid levels did not change and did not correlate with renal disease progression.

CONCLUSION

In ADPKD patients with PLD, baseline serum bile acids were associated with liver volume. Lanreotide reduced bile acid levels and has previously been shown to reduce liver volume. However, in this study, the decrease in bile acids was not associated with the change in liver volume.

Primary biliary cholangitis: molecular pathogenesis perspectives and therapeutic potential of natural products

Primary biliary cirrhosis (PBC) is a chronic cholestatic immune liver disease characterized by persistent cholestasis, interlobular bile duct damage, portal inflammation, liver fibrosis, eventual cirrhosis, and death. Existing clinical and animal studies have made a good progress in bile acid metabolism, intestinal flora disorder inflammatory response, bile duct cell damage, and autoimmune response mechanisms. However, the pathogenesis of PBC has not been clearly elucidated. We focus on the pathological mechanism and new drug research and development of PBC in clinical and laboratory in the recent 20 years, to discuss the latest understanding of the pathological mechanism, treatment options, and drug discovery of PBC. Current clinical treatment mode and symptomatic drug support obviously cannot meet the urgent demand of patients with PBC, especially for the patients who do not respond to the current treatment drugs. New treatment methods are urgently needed. Drug candidates targeting reported targets or signals of PBC are emerging, albeit with some success and some failure. Single-target drugs cannot achieve ideal clinical efficacy. Multitarget drugs are the trend of future research and development of PBC drugs.

Bile Acids and Biliary Fibrosis

Biliary fibrosis is the driving pathological process in cholangiopathies such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC). Cholangiopathies are also associated with cholestasis, which is the retention of biliary components, including bile acids, in the liver and blood. Cholestasis may worsen with biliary fibrosis. Furthermore, bile acid levels, composition and homeostasis are dysregulated in PBC and PSC. In fact, mounting data from animal models and human cholangiopathies suggest that bile acids play a crucial role in the pathogenesis and progression of biliary fibrosis. The identification of bile acid receptors has advanced our understanding of various signaling pathways involved in regulating cholangiocyte functions and the potential impact on biliary fibrosis. We will also briefly review recent findings linking these receptors with epigenetic regulatory mechanisms. Further detailed understanding of bile acid signaling in the pathogenesis of biliary fibrosis will uncover additional therapeutic avenues for cholangiopathies.

Bile Acids Alter the Autophagy and Mitogenesis in Skeletal Muscle Cells

Muscle atrophy decreases muscle mass with the subsequent loss of muscle function. Among the mechanisms that trigger sarcopenia is mitochondrial dysfunction. Mitochondria, whose primary function is to produce ATP, are dynamic organelles that present the process of formation (mitogenesis) and elimination (mitophagy). Failure of any of these processes contributes to mitochondrial malfunction. Mitogenesis is mainly controlled by Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α), a transcriptional coactivator that regulates the expression of TFAM, which participates in mitogenesis. Mitophagy is a process of selective autophagy. Autophagy corresponds to a degradative pathway of protein complexes and organelles. Liver disease caused sarcopenia and increased bile acids in the blood. We demonstrated that the treatment with cholic (CA) or deoxycholic (DCA) bile acids generates mitochondrial dysfunction and loss of biomass. This work assessed whether CA and DCA alter autophagy and mitogenesis. For this, western blot evaluated the autophagy process by determining the protein levels of the LC3II/LC3I ratio. In addition, we assessed mitogenesis using a luciferase-coupled plasmid reporter for the PGC-1α promoter and the protein levels of TFAM by western blot. Our results indicate that treatment with CA or DCA induces autophagy, represented by an increase in the LC3II/LC3I ratio. In addition, a decreased autophagic flux was observed. On the other hand, when treated with CA or DCA, a decrease in the activity of the PGC-1α promoter was observed. However, the levels of TFAM increased in myotubes incubated with CA and DCA. Our results demonstrate that CA and DCA modulate autophagy ad mitogenesis in C₂C₁₂ myotubes.

Human Cholangiocytes Form a Polarized and Functional Bile Duct on Hollow Fiber Membranes

Liver diseases affect hundreds of millions of people worldwide; most often the hepatocytes or cholangiocytes are damaged. Diseases of the biliary tract cause severe patient burden, and cholangiocytes, the cells lining the biliary tract, are sensitive to numerous drugs. Therefore, investigations into proper cholangiocyte functions are of utmost importance, which is restricted, in vitro, by the lack of primary human cholangiocytes allowing such screening. To investigate biliary function, including transepithelial transport, cholangiocytes must be cultured as three-dimensional (3D) ductular structures. We previously established murine intrahepatic cholangiocyte organoid-derived cholangiocyte-like cells (CLCs) and cultured them onto polyethersulfone hollow fiber membranes (HFMs) to generate 3D duct structures that resemble native bile ducts at the structural and functional level. Here, we established an efficient, stepwise method for directed differentiation of human intrahepatic cholangiocyte organoids (ICOs) into CLCs. Human ICO-derived CLCs showed key characteristics of cholangiocytes, such as the expression of structural and functional markers, formation of primary cilia, and P-glycoprotein-mediated transport in a polarized fashion. The organoid cultures exhibit farnesoid X receptor (FXR)-dependent functions that are vital to liver bile acid homeostasis in vivo. Furthermore, human ICO-derived CLCs cultured on HFMs in a differentiation medium form tubular architecture with some tight, confluent, and polarized monolayers that better mimic native bile duct characteristics than differentiated cultures in standard 2D or Matrigel-based 3D culture plates. Together, our optimized differentiation protocol to obtain CLC organoids, when applied on HFMs to form bioengineered bile ducts, will facilitate studying cholangiopathies and allow developing therapeutic strategies.

TGR5 protects against cholestatic liver disease via suppressing the NF-κB pathway and activating the Nrf2/HO-1 pathway

Background

Characterized by the presence of inflammation, fibrosis, and bile duct proliferation, cholestatic liver disease (CLD) affects people of all age groups. Takeda G-protein-coupled receptor (TGR5) has been implicated in the suppression of inflammation via toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB). Kupffer cells and their M1 polarization play important roles in inflammation and cholestatic liver injury via production of pro-inflammatory cytokines. Nevertheless, the function of TGR5 signaling in CLD is largely unknown.

Methods

We conducted liver tissue experiments, animal experiments, serum marker testing, liver histology analysis, Kupffer cell experiments, RNA extraction and Real-time PCR, western blotting, evaluation of ROS production by flow cytometry and statistical differences were analyzed by student t-test using GraphPad Prism.

Results

We found that serum bile acid (BA) and TGR5 levels were elevated in patients with cholestasis cirrhosis. Knockout of TGR5 in animals significantly increased bile duct ligation (BDL)-caused liver injury through increasing oxidative stress, promoting M1-predominant polarization of Kupffer cells, and elevating the serum levels of inflammatory cytokines. In contrast, TGR5 activation inhibited ROS production, secretion of pro-inflammatory cytokines, and M1-predominant polarization of Kupffer cells. Moreover, results showed that TGR5 exerted its effects via suppressing NF-κB signaling and activating nuclear factor 2 (Nrf2)/HO-1 signaling. Finally, the effect of TGR5 on cholestatic liver damage was also confirmed in vivo.

Conclusions

TGR5 activation protected against BDL-induced CLD by both suppressing inflammation via inhibiting the NF-κB pathway and reducing ROS production via activation of Nrf2/HO-1 signaling. These findings show the importance of TGR5 in CLD and provide new insight into therapeutic strategies for CLD.

FGF19 and FGFR4 promotes the progression of gallbladder carcinoma in an autocrine pathway dependent on GPBAR1-cAMP-EGR1 axis

Treatment options for gallbladder carcinoma (GBC) are limited and GBC prognosis remains poor. There is no well-accepted targeted therapy to date, so effective biomarkers of GBC are urgently needed. Here we investigated the expression and correlations of fibroblast growth factor receptors (FGFR1-4) and 18 fibroblast growth factors (FGFs) in two independent patient cohorts and evaluated their prognostic significance. Consequently, we demonstrated that both FGF19 and FGFR4 were unfavorable prognostic biomarkers, and their co-expression was a more sensitive predictor. By analyzing the correlations between all 18 FGFs and FGFR4, we showed that FGF19 expression was significantly associated with FGFR4 and promoted GBC progression via stimulating FGFR4. With experiments using GBC cells, GPBAR1-/- mice models, and human subjects, we demonstrated that elevated bile acids (BAs) could increase the transcription and expression of FGF19 and FGFR4 by activating GPBAR1-cAMP-EGR1 pathway. FGF19 secreted from GBC cells promoted GBC progression by stimulating FGFR4 and downstream ERK in an autocrine manner with bile as a potential carrier. Patients with GBC had significantly higher FGF19 in serum and bile, compared to patients with cholelithiasis. BLU9931 inhibited FGFR4 and attenuated its oncogenic effects in GBC cell line. In conclusion, upregulation of BAs elevated co-expression of FGF19 and FGFR4 by activating GPBAR1-cAMP-EGR1 pathway. Co-expression of FGF19 and FGFR4 was a sensitive and unfavorable prognostic marker. GBC cells secreted FGF19 and facilitated progression by activating FGFR4 with bile as a potential carrier in an autocrine pathway.

TGR5 Regulates Macrophage Inflammation in Nonalcoholic Steatohepatitis by Modulating NLRP3 Inflammasome Activation

Nonalcoholic steatohepatitis (NASH) is a chronic liver disease associated with dysregulation of liver metabolism and inflammation. G-protein coupled bile acid receptor 1 (TGR5) is a cell surface receptor that is involved in multiple metabolic pathways. However, the functions of TGR5 in regulating macrophage innate immune activation in NASH remain unclear. Here, we found that TGR5 expression was decreased in liver tissues from humans and mice with NASH. Compared to wild type (WT) mice, TGR5-knockout (TGR5^−/−) mice exhibited exacerbated liver damage, increased levels of proinflammatory factors, and enhanced M1 macrophage polarization. Moreover, TGR5 deficiency facilitated M1 macrophage polarization by promoting NLRP3 inflammasome activation and caspase-1 cleavage. Taken together, our findings revealed that TGR5 signaling attenuated liver steatosis and inflammation and inhibited NLRP3-mediated M1 macrophage polarization in NASH.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Bile Acid-Activated Receptors: GPBAR1 (TGR5) and Other G Protein-Coupled Receptors

The BA-responsive GPCRs S1PR2 and TGR5 are almost ubiquitously expressed in human and rodent tissues. In the liver, S1PR2 is expressed in all cell types, while TGR5 is predominately found in non-parenchymal cells. In contrast to S1PR2, which is mainly activated by conjugated bile acids (BAs), all BAs serve as ligands for TGR5 irrespective of their conjugation state and substitution pattern.Mice with targeted deletion of either S1PR2 or TGR5 are viable and develop no overt phenotype. In liver injury models, S1PR2 exerts pro-inflammatory and pro-fibrotic effects and thus aggravates liver damage, while TGR5 mediates anti-inflammatory, anti-cholestatic, and anti-fibrotic effects. Thus, inhibitors of S1PR2 signaling and agonists for TGR5 have been employed to attenuate liver injury in rodent models for cholestasis, nonalcoholic steatohepatitis, and fibrosis/cirrhosis.In biliary epithelial cells, both receptors activate a similar signaling cascade resulting in ERK1/2 phosphorylation and cell proliferation. Overexpression of both S1PR2 and TGR5 was found in human cholangiocarcinoma tissue as well as in CCA cell lines, where stimulation of both GPCRs resulted in transactivation of the epidermal growth factor receptor and triggered cell proliferation as well as increased cell migration and invasiveness.This chapter will focus on the function of S1PR2 and TGR5 in different liver cell types and summarizes current knowledge on the role of these receptors in liver disease models.

Neuroprotective effects of INT-777 against Aβ1-42-induced cognitive impairment, neuroinflammation, apoptosis, and synaptic dysfunction in mice

Increasing evidence demonstrates that the neurotoxicity of amyloid-beta (Aβ) deposition plays a causative role in Alzheimer's disease (AD). Herein, we evaluated the neuroprotective effects of 6α-ethyl-23(S)-methylcholic acid (S-EMCA, INT-777), a specific G-protein coupled bile acid receptor 1 (TGR5) agonist, in the Aβ_1-42-treated mouse model of acute neurotoxicity. Single intracerebroventricular (i.c.v.) injection of aggregated Aβ_1-42 (410 pmol/mouse; 5 μl) into the mouse brain induced cognitive impairment, neuroinflammation, apoptosis, and synaptic dysfunction. In contrast, INT-777 (1.5 or 3.0 μg/mouse, i.c.v.) significantly improved Aβ_1-42-induced cognitive impairment, as reflected by better performance in memory tests. Importantly, INT-777 treatment reversed Aβ_1-42-induced TGR5 down-regulation, suppressed the increase of nuclear NF-κB p65, and mitigated neuroinflammation, as evidenced by lower proinflammatory cytokines and less Iba1-positive cells in the hippocampus and frontal cortex. INT-777 treatment also pronouncedly suppressed apoptosis through the reduction of TUNEL-positive cells, decreased caspase-3 activation, increased the ratio of Bcl-2/Bax, and ameliorated synaptic dysfunction by promoting dendritic spine generation with the upregulation of postsynaptic and presynaptic proteins (PSD95 and synaptophysin) in Aβ_1-42-treated mice. Our results indicate that INT-777 has potent neuroprotective effects against Aβ_1-42-induced neurotoxicity. Taken together, these findings suggest that the activation of TGR5 could be a novel and promising strategy for the treatment of AD.

Cell fate decisions of human iPSC-derived bipotential hepatoblasts depend on cell density

During embryonic development bipotential hepatoblasts differentiate into hepatocytes and cholangiocytes- the two main cell types within the liver. Cell fate decision depends on elaborate interactions between distinct signalling pathways, namely Notch, WNT, TGFβ, and Hedgehog. Several in vitro protocols have been established to differentiate human pluripotent stem cells into either hepatocyte or cholangiocyte like cells (HLC/CLC) to enable disease modelling or drug screening. During HLC differentiation we observed the occurrence of epithelial cells with a phenotype divergent from the typical hepatic polygonal shape- we refer to these as endoderm derived epithelial cells (EDECs). These cells do not express the mature hepatocyte marker ALB or the progenitor marker AFP. However they express the cholangiocyte markers SOX9, OPN, CFTR as well as HNF4α, CK18 and CK19. Interestingly, they express both E Cadherin and Vimentin, two markers that are mutually exclusive, except for cancer cells. EDECs grow spontaneously under low density cell culture conditions and their occurrence was unaffected by interfering with the above mentioned signalling pathways.

Therapeutic Targets in Polycystic Liver Disease

Polycystic liver diseases (PLD) are a group of genetic disorders initiated by mutations in several PLD-related genes and characterized by the presence of multiple cholangiocyte-derived hepatic cysts that progressively replace liver tissue. PLD co-exists with Autosomal Dominant Polycystic Kidney Disease (ADPKD) and Autosomal Recessive PKD as well as occurs alone (i.e., Autosomal Dominant Polycystic Liver Disease [ADPLD]). PLD associated with ADPKD and ARPKD belong to a group of disorders known as cholangiociliopathies since many disease-causative and disease-related proteins are expressed in primary cilia of cholangiocytes. Aberrant expression of these proteins in primary cilia affects their structures and functions promoting cystogenesis. Current medical therapies for PLD include symptomatic management and surgical interventions. To date, the only available drug treatment for PLD patients that halt disease progression and improve quality of life are somatostatin analogs. However, the modest clinical benefits, need for long-term maintenance therapy, and the high cost of treatment justify the necessity for more effective treatment options. Substantial evidence suggests that experimental manipulations with components of the signaling pathways that influence cyst development (e.g., cAMP, intracellular calcium, receptor tyrosine kinase, transient receptor potential cation channel subfamily V member 4 (TRPV4) channel, mechanistic target of rapamycin (mTOR), histone deacetylase (HDAC6), Cdc25A phosphatase, miRNAs and metalloproteinases) attenuate growth of hepatic cysts. Many of these targets have been evaluated in pre-clinical trials suggesting their value as potential new therapies. This review outlines the current clinical and preclinical treatment strategies for PLD.

The cholangiocyte primary cilium in health and disease

Cholangiocytes, like most cells, express primary cilia extending from their membranes. These organelles function as antennae which detect stimuli from bile and transmit the information into cells regulating several signaling pathways involved in secretion, proliferation and apoptosis. The ability of primary cilia to detect different signals is provided by ciliary associated proteins which are expressed in its membrane. Defects in the structure and/or function of these organelles lead to cholangiociliopathies that result in cholangiocyte hyperproliferation, altered fluid secretion and absorption. Since primary cilia dysfunction has been observed in several epithelial tumors, including cholangiocarcinoma (CCA), primary cilia have been proposed as tumor suppressor organelles. In addition, the loss of cilia is associated with dysregulation of several molecular pathways resulting in CCA development and progression. Thus, restoration of the primary cilia may be a potential therapeutic approach for several ciliopathies and CCA.

TGR5 contributes to hepatic cystogenesis in rodents with polycystic liver diseases through cyclic adenosine monophosphate/Gαs signaling

Hepatic cystogenesis in polycystic liver disease is associated with increased levels of cyclic adenosine monophosphate (cAMP) in cholangiocytes lining liver cysts. Takeda G protein receptor 5 (TGR5), a G protein-coupled bile acid receptor, is linked to cAMP and expressed in cholangiocytes. Therefore, we hypothesized that TGR5 might contribute to disease progression. We examined expression of TGR5 and Gα proteins in cultured cholangiocytes and in livers of animal models and humans with polycystic liver disease. In vitro, we assessed cholangiocyte proliferation, cAMP levels, and cyst growth in response to (1) TGR5 agonists (taurolithocholic acid, oleanolic acid [OA], and two synthetic compounds), (2) a novel TGR5 antagonist (m-tolyl 5-chloro-2-[ethylsulfonyl] pyrimidine-4-carboxylate [SBI-115]), and (3) a combination of SBI-115 and pasireotide, a somatostatin receptor analogue. In vivo, we examined hepatic cystogenesis in OA-treated polycystic kidney rats and after genetic elimination of TGR5 in double mutant TGR5^-/- ;Pkhd1^del2/del2 mice. Compared to control, expression of TGR5 and Gα_s (but not Gα_i and Gα_q ) proteins was increased 2-fold to 3-fold in cystic cholangiocytes in vitro and in vivo. In vitro, TGR5 stimulation enhanced cAMP production, cell proliferation, and cyst growth by ∼40%; these effects were abolished after TGR5 reduction by short hairpin RNA. OA increased cystogenesis in polycystic kidney rats by 35%; in contrast, hepatic cystic areas were decreased by 45% in TGR5-deficient TGR5^-/- ;Pkhd1^del2/del2 mice. TGR5 expression and its colocalization with Gα_s were increased ∼2-fold upon OA treatment. Levels of cAMP, cell proliferation, and cyst growth in vitro were decreased by ∼30% in cystic cholangiocytes after treatment with SBI-115 alone and by ∼50% when SBI-115 was combined with pasireotide.

CONCLUSION

TGR5 contributes to hepatic cystogenesis by increasing cAMP and enhancing cholangiocyte proliferation; our data suggest that a TGR5 antagonist alone or concurrently with somatostatin receptor agonists represents a potential therapeutic approach in polycystic liver disease. (Hepatology 2017;66:1197-1218).

Bile acid receptors in the biliary tree: TGR5 in physiology and disease

Bile salts represent signalling molecules with a variety of endocrine functions. Bile salt effects are mediated by different receptor molecules, comprising ligand-activated nuclear transcription factors as well as G protein-coupled membrane-bound receptors. The farnesoid X receptor (FXR) and the plasma membrane-bound G protein-coupled receptor TGR5 (Gpbar-1) are prototypic bile salt receptors of both classes and are highly expressed in the liver including the biliary tree as well as in the intestine. In liver, TGR5 is localized in different non-parenchymal cells such as sinusoidal endothelial cells, Kupffer cells, hepatic stellate cells and small and large cholangiocytes. Through TGR5 bile salts can mediate choleretic, cell-protective as well as proliferative effects in cholangiocytes. A disturbance of these signalling mechanisms can contribute to the development of biliary diseases. In line with the important role of TGR5 for bile salt signalling, TGR5 knockout mice are more susceptible to cholestatic liver damage. Furthermore, in absence of TGR5 cholangiocyte proliferation in response to cholestasis is attenuated and intrahepatic and extrahepatic bile ducts show increased cell damage, underscoring the role of the receptor for biliary physiology. Decreased TGR5 expression may also contribute to the development or progression of cholangiopathies like primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) since reduced TGR5-dependent cell-protective mechanisms such as bicarbonate secretion renders cholangiocytes more vulnerable towards bile salt toxicity. Nevertheless, TGR5 overexpression or constant stimulation of the receptor can promote cholangiocyte proliferation leading to cyst growth in polycystic liver disease or even progression of cholangiocarcinoma. Not only the stimulation of TGR5-mediated pathways by suitable TGR5 agonists but also the inhibition of TGR5 signalling by the use of antagonists represent potential therapeutic approaches for different types of biliary diseases. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Mechanisms of bile acid mediated inflammation in the liver

Bile acids are synthesized in the liver and are the major component in bile. Impaired bile flow leads to cholestasis that is characterized by elevated levels of bile acid in the liver and serum, followed by hepatocyte and biliary injury. Although the causes of cholestasis have been extensively studied, the molecular mechanisms as to how bile acids initiate liver injury remain controversial. In this chapter, we summarize recent advances in the pathogenesis of bile acid induced liver injury. These include bile acid signaling pathways in hepatocytes as well as the response of cholangiocytes and innate immune cells in the liver in both patients with cholestasis and cholestatic animal models. We focus on how bile acids trigger the production of molecular mediators of neutrophil recruitment and the role of the inflammatory response in this pathological process. These advances point to a number of novel targets where drugs might be judged to be effective therapies for cholestatic liver injury.

Hepatostat: Liver regeneration and normal liver tissue maintenance

In contrast to all other organs, liver-to-body-weight ratio needs to be maintained always at 100% of what is required for body homeostasis. Adjustment of liver size to 100% of what is required for homeostasis has been called "hepatostat." Removal of a portion of any other organ is followed with local regeneration of a limited degree, but it never attempts to reach 100% of the original size. The complex mechanisms involved in this uniquely hepatic process encompass a variety of regenerative pathways that are specific to different types of injury. The most studied form of liver regeneration (LR) is that occurring after loss of hepatocytes in a single acute injury, such as rodent LR after two-thirds partial hepatectomy or administration of damaging chemicals (CCl₄ , acetaminophen, etc.). Alternative regenerative pathways become activated when normal regeneration is thwarted and trigger the appearance of "progenitor" cells. Chronic loss of hepatocytes is associated with regenerative efforts characterized by continual hepatocyte proliferation and often has adverse consequences (development of cirrhosis or liver cancer). Even though a very few hepatocytes proliferate at any given time in normal liver, the mechanisms involved in the maintenance of liver weight by this slow process in the absence of liver injury are not as well understood. (Hepatology 2017;65:1384-1392).

Bile salt receptor TGR5 is highly expressed in esophageal adenocarcinoma and precancerous lesions with significantly worse overall survival and gender differences

Bile acid reflux in the esophagus plays an important role in the carcinogenesis of esophageal adenocarcinoma (EAC). The G-protein coupled bile acid receptor (TGR5) has been associated with the development of gastrointestinal cancer. However, little is known regarding the role of TGR5 in esophageal carcinoma and precancerous lesions. We analyzed genomic DNA from 116 EACs for copy number aberrations via Affymetrix SNP6.0 microarrays. The TGR5 gene locus was amplified in 12.7% (14/116) of the EACs. The TGR5 protein expression was also assessed using immunohistochemistry from tissue microarrays, including Barrett’s esophagus (BE), low-(LGD) and high-grade dysplasia (HGD), columnar cell metaplasia (CM), squamous epithelium (SE), EAC and squamous cell carcinoma. The TGR5 protein was highly expressed in 71% of EAC (75/106), 100% of HGD (11/11), 72% of LGD (13/18), 66% of BE (23/35), 84% of CM (52/62), and 36% of SE (30/83). The patients with high expression of TGR5 exhibited significantly worse overall survival compared to the patients with nonhigh expression. TGR5 high expression was significantly increased in the males compared to the females in all cases with an odds ratio of 1.9 times. The vitamin D receptor (VDR) was significantly correlated with TGR5 expression. Our findings indicated that TGR5 may play an important role in the development and prognosis of EAC through a bile acid ligand. Gender differences in TGR5 and VDR expression may explain why males have a higher incidence of EAC compared to females.

TGR5 is essential for bile acid-dependent cholangiocyte proliferation in vivo and in vitro

OBJECTIVE

Cholestatic liver diseases in humans as well as bile acid (BA)-feeding and common bile duct ligation (CBDL) in rodents trigger hyperplasia of cholangiocytes within the portal fields. Furthermore, elevation of BA levels enhances proliferation and invasiveness of cholangiocarcinoma (CCA) cells in animal models, thus promoting tumour progression. TGR5 is a G-protein coupled BA receptor, which is highly expressed in cholangiocytes and postulated to mediate the proliferative effects of BA.

DESIGN

BA-dependent cholangiocyte proliferation was examined in TGR5-knockout and wild type mice following cholic acid (CA)-feeding and CBDL. TGR5-dependent proliferation and protection from apoptosis was studied in isolated cholangiocytes and CCA cell lines following stimulation with TGR5 ligands and kinase inhibitors. TGR5 expression was analysed in human CCA tissue.

RESULTS

Cholangiocyte proliferation was significantly reduced in TGR5-knockout mice in response to CA-feeding and CBDL. Taurolithocholic acid and TGR5-selective agonists induced cholangiocyte proliferation through elevation of reactive oxygen species and cSrc mediated epidermal growth factor receptor transactivation and subsequent Erk1/2 phosphorylation only in wild type but not in TGR5-knockout-derived cells. In human CCA tissue TGR5 was overexpressed and the pathway of TGR5-dependent proliferation via epidermal growth factor receptor and extracellular signal-regulated kinase (ERK)1/2 activation also translated to CCA cell lines. Furthermore, apoptosis was inhibited by TGR5-dependent CD95 receptor serine phosphorylation.

CONCLUSIONS

TGR5 is an important mediator of BA-induced cholangiocyte proliferation in vivo and in vitro. Furthermore, TGR5 protects cholangiocytes from death receptor-mediated apoptosis. These mechanisms may protect cholangiocytes from BA toxicity under cholestatic conditions, however, they may trigger proliferation and apoptosis resistance in malignantly transformed cholangiocytes, thus promoting CCA progression.

TGR5: pathogenetic role and/or therapeutic target in fibrosing cholangitis?

Primary sclerosing cholangitis (PSC) is a chronic inflammatory disease affecting the intrahepatic and extrahepatic biliary tree leading to bile duct strictures, progressive cholestasis, and development of liver fibrosis and cirrhosis. The pathogenesis of PSC is still elusive; however, both an immune-mediated injury of the bile ducts as well as increased recruitment of intestinal-primed T lymphocytes to the biliary tracts seem to contribute to disease development and progression. TGR5 (Gpbar-1) is a G-protein-coupled receptor responsive to bile acids, which is expressed in cholangiocytes, intestinal epithelial cells, and macrophages of the liver and intestine as well as in CD14-positive monocytes of the peripheral blood. Activation of TGR5 in biliary epithelial cells promotes chloride and bicarbonate secretion, triggers cell proliferation, and prevents apoptotic cell death. In immune cells, stimulation of TGR5 inhibits cytokine expression and secretion, thus reducing systemic as well as hepatic and intestinal inflammation. The expression pattern of TGR5 in the liver and intestine as well as the potential protective functions of TGR5 suggest a role for this receptor in the pathogenesis of PSC. While mutations in the coding region of the TGR5 gene are too rare to contribute to overall disease susceptibility, the expression and localization of the receptor have not been studied in PSC livers. Pharmacological activation of TGR5 in mice promotes protective mechanisms in biliary epithelial cells and reduces hepatic and systemic inflammation; however, it also provokes pruritus. Further studies are needed to predict the potential benefits as well as side effects of TGR5 agonist treatment in PSC patients.

Therapeutic targets for cholestatic liver injury

INTRODUCTION

Cholestasis is a reduction in bile flow that occurs during numerous pathologies. Blockage of the biliary tracts results in hepatic accumulation of bile acids or their conjugate bile salts. The molecular mechanisms behind liver injury associated with cholestasis are extensively studied, but not well understood. Multiple models of obstructive cholestasis result in a significant inflammatory infiltrate at the sites of necrosis that characterize the injury.

AREAS COVERED

This review will focus on direct bile acid toxicity during cholestasis, bile acid signaling processes and on the development and continuation of inflammation during cholestasis, with a focus on novel proposed molecular mediators of neutrophil recruitment. While significant progress has been made on these molecular mechanisms, a continued focus on how cholestasis and the innate immune system interact is necessary to discover targetable therapeutics that might protect the liver while leaving global immunity intact.

EXPERT OPINION

While bile acid toxicity likely occurs in humans and other mammals when toxic bile acids accumulate, persistent inflammation is likely responsible for continued liver injury during obstructive cholestasis. Targeting molecular mediators of inflammation may help prevent liver injury during acute cholestasis both in murine models and human patients.

Bile acid effects are mediated by ATP release and purinergic signalling in exocrine pancreatic cells

Background

In many cells, bile acids (BAs) have a multitude of effects, some of which may be mediated by specific receptors such the TGR5 or FXR receptors. In pancreas systemic BAs, as well as intra-ductal BAs from bile reflux, can affect pancreatic secretion. Extracellular ATP and purinergic signalling are other important regulators of similar secretory mechanisms in pancreas. The aim of our study was to elucidate whether there is interplay between ATP and BA signalling.

Results

Here we show that CDCA (chenodeoxycholic acid) caused fast and concentration-dependent ATP release from acini (AR42J) and duct cells (Capan-1). Taurine and glycine conjugated forms of CDCA had smaller effects on ATP release in Capan-1 cells. In duct monolayers, CDCA stimulated ATP release mainly from the luminal membrane; the releasing mechanisms involved both vesicular and non-vesicular secretion pathways. Duct cells were not depleted of intracellular ATP with CDCA, but acinar cells lost some ATP, as detected by several methods including ATP sensor AT1.03^YEMK. In duct cells, CDCA caused reversible increase in the intracellular Ca²⁺ concentration [Ca^{2 +}]_i, which could be significantly inhibited by antagonists of purinergic receptors. The TGR5 receptor, expressed on the luminal side of pancreatic ducts, was not involved in ATP release and Ca²⁺ signals, but could stimulate Na⁺/Ca²⁺ exchange in some conditions.

Conclusions

CDCA evokes significant ATP release that can stimulate purinergic receptors, which in turn increase [Ca²⁺]_i. The TGR5 receptor is not involved in these processes but can play a protective role at high intracellular Ca²⁺ conditions. We propose that purinergic signalling could be taken into consideration in other cells/organs, and thereby potentially explain some of the multifaceted effects of BAs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-015-0107-9) contains supplementary material, which is available to authorized users.

TGR5 in the Cholangiociliopathies

A plasma membrane-bound G protein-coupled receptor, TGR5, that transmits bile acid signaling into a cellular response primarily via the cAMP pathway is expressed in human and rodent cholangiocytes and is localized to multiple, diverse subcellular compartments, including primary cilia. Ciliary-associated TGR5 plays an important role in cholangiocyte physiology and may contribute to a group of liver diseases referred to as the 'cholangiociliopathies', which include polycystic liver disease (PLD) and, possibly, cholangiocarcinoma and primary sclerosing cholangitis. Based on our observations that (1) ciliated and nonciliated cholangiocytes respond to TGR5 activation differently (i.e. the level of cAMP increases in nonciliated cholangiocytes but decreases in ciliated cells) and (2) hepatic cysts are derived from cholangiocytes that are characterized by both malformed cilia and increased cAMP levels, we hypothesized that TGR5-mediated cAMP signaling in cystic cholangiocytes contributes to hepatic cystogenesis. Indeed, our studies show that TGR5 is overexpressed and mislocalized in cystic cholangiocytes, and when activated by ligands, results in increased intracellular cAMP levels, cholangiocyte hyperproliferation and cyst growth. Our studies also show that genetic elimination of TGR5 in an animal model of PLD inhibits hepatic cystogenesis. Collectively, these data suggest the involvement of TGR5 in PLD and that TGR5 targeting in cystic cholangiocytes may have therapeutic potential.

Effect of drug transporter pharmacogenetics on cholestasis

INTRODUCTION

The liver is the central place for the metabolism of drugs and other xenobiotics. In the liver cell, oxidation and conjugation of compounds take place, and at the same time, bile formation helps in extrusion of these compounds via the biliary route. A large number of transporters are responsible for drug uptake into the liver cell and excretion into bile or efflux to the sinusoidal blood.

AREAS COVERED

Genetic variants of these transporters and their transactivators contribute to changes in drug handling and are also responsible for cholestatic syndromes of different severity. This review summarizes the current knowledge regarding the influence of these genetic changes. The review covers progressive hereditary cholestatic syndromes as well as recurrent or transient cholestatic syndromes such as drug-induced liver injury, intrahepatic cholestasis of pregnancy, and benign recurrent intrahepatic cholestasis.

EXPERT OPINION

Polymorphisms in transporter genes are frequent. For clinically relevant cholestatic syndromes, it often requires a combination of genetic variants or acquired triggers such as pregnancy or drug treatment. In combination with other pathogenetic aspects, genetic variants in drug transporters may contribute to our understanding of not only cholestatic diseases such as primary sclerosing cholangitis or primary biliary cirrhosis, but also the natural course of chronic liver disease in general.

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Review article: controversies in the management of primary biliary cirrhosis and primary sclerosing cholangitis

BACKGROUND

Despite considerable advances over the last two decades in the molecular understanding of cholestasis and cholestatic liver disease, little improvement has been made in diagnostic tools and therapeutic strategies.

AIMS

To critically review controversial aspects of the scientific basis for common clinical practice in primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) and to discuss key ongoing challenges to improve patient management.

METHODS

We performed a literature search using PubMed and by examining the reference lists of relevant review articles related to the clinical management of PBC and PSC. Articles were considered on the background of the European Association for the Study of the Liver (EASL) and the American Association for the Study of Liver Diseases (AASLD) practice guidelines and clinical experience of the authors.

RESULTS

Ongoing challenges in PBC mainly pertain to the improvement of medical therapy, particularly for patients with a suboptimal response to ursodeoxycholic acid. In PSC, development of medical therapies and sensitive screening protocols for cholangiocarcinoma represent areas of intense research. To rationally improve patient management, a better understanding of pathogenesis, including complications like pruritis and fatigue, is needed and there is a need to identify biomarker end-points for treatment effect and prognosis. Timing of liver transplantation and determining optimal regimens of immunosuppression post-liver transplantation will also benefit from better appreciation of pre-transplant disease mechanisms.

CONCLUSION

Controversies in the management of PBC and PSC relate to topics where evidence for current practice is weak and further research is needed.

Recent advances in the morphological and functional heterogeneity of the biliary epithelium

This review focuses on the recent advances related to the heterogeneity of different-sized bile ducts with regard to the morphological and phenotypical characteristics, and the differential secretory, apoptotic and proliferative responses of small and large cholangiocytes to gastrointestinal hormones/peptides, neuropeptides and toxins. We describe several in vivo and in vitro models used for evaluating biliary heterogeneity. Subsequently, we discuss the heterogeneous proliferative and apoptotic responses of small and large cholangiocytes to liver injury and the mechanisms regulating the differentiation of small into large (more differentiated) cholangiocytes. Following a discussion on the heterogeneity of stem/progenitor cells in the biliary epithelium, we outline the heterogeneity of bile ducts in human cholangiopathies. After a summary section, we discuss the future perspectives that will further advance the field of the functional heterogeneity of the biliary epithelium.

Are GPCRs Still a Source of New Targets?

G-protein–coupled receptors (GPCRs) still offer enormous scope for new therapeutic targets. Currently marketed agents are dominated by those with activity at aminergic receptors and yet they account for only ~10% of the family. Progress up until now with other subfamilies, notably orphans, Family A/peptide, Family A/lipid, Family B, Family C, and Family F, has been, at best, patchy. This may be attributable to the heterogeneous nature of GPCRs, their endogenous ligands, and consequently their binding sites. Our appreciation of receptor similarity has arguably been too simplistic, and screening collections have not necessarily been well suited to identifying leads in new areas. Despite the relative shortage of high-quality tool molecules in a number of cases, there is an emerging, and increasingly substantial, body of evidence associating many as yet “undrugged” receptors with a very wide range of diseases. Significant advances in our understanding of receptor pharmacology and technical advances in screening, protein X-ray crystallography, and ligand design methods are paving the way for new successes in the area. Exploitation of allosteric mechanisms; alternative signaling pathways such as G12/13, Gβγ, and β-arrestin; the discovery of “biased” ligands; and the emergence of GPCR-protein complexes as potential drug targets offer scope for new and much improved drugs.