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The transmembrane G protein-coupled receptor TGR5 binds to and is activated by bile acids.1 ,2 TGR5 is widely distributed, in animals and humans, in the gallbladder, liver, brown adipose tissue, muscle, intestine, kidney, placenta and brain. In the liver, TGR5 appears in sinusoidal endothelial cells, bile duct epithelial cells and Kupffer cells. Interestingly, the ubiquitous TGR5 expression positions previously unsuspected tissues such as muscle, brain and adipose tissue, and cells such as macrophages and endothelial cells, as potential targets of bile acid signalling. Bile acid binding to TGR5 activates multiple signalling routes involved in a multitude of processes thus potentially affecting metabolism, inflammation, fibrosis and carcinogenesis. Lithocholic acid is the strongest natural ligand of TGR5,1 ,2 while INT-777 is a semisynthetic TGR5 agonist.3 Further development of semisynthetic bile acid analogues that target TGR5 or intracellular nuclear receptors, such as the Farnesoid X receptor (FXR), holds great expectations to treat chronic liver disease, hepatocellular cancer and extrahepatic inflammatory and metabolic diseases. Randomised, placebo controlled clinical trials for the treatment of non-alcoholic steatohepatitis and primary biliary cirrhosis with FXR agonist obeticholic acid are the first initiated trials.
Cholangiocarcinoma (CCA) is the second most common primary liver tumour and accounts for ∼3% of all GI cancers. The incidence and mortality rates of CCA are increasing worldwide, and the lack of specific biomarkers and therapeutic options contributes to poor prognosis. These observations support the urgent need in understanding the molecular mechanisms involved in disease pathogenesis and in improving the treatment strategies for patients with CCA. Current concepts are based on exploring key signalling pathways involved in proliferation, survival, apoptosis and migration.
In the current issue of Gut, Reich et al 4 unveiled the contribution of TGR5 to bile acid-dependent cholangiocyte proliferation in vivo as well as its potential role in CCA progression (figure 1). Cholangiocyte proliferation is a hallmark of cholestatic liver diseases in humans, and bile acids have been associated with CCA progression in animal models and cell culture studies.5 In turn, rodent models for cholangiocyte proliferation are linked with elevated levels of cyclic AMP and activation of MAP-kinases (eg, Erk) in cholangiocytes.
Their approach was to use TGR5 knockout and wild type mice following cholic acid feeding and common bile duct ligation to dissect bile acid-dependent cholangiocyte proliferation. TGR5-dependent proliferation and protection from apoptosis was also studied in isolated cholangiocytes and CCA cell lines following stimulation with TGR5 ligands and kinase inhibitors. First, the authors linked TGR5 expression with bile acid-induced cholangiocyte proliferation in animal models of cholestasis as well as in isolated biliary epithelial cells. Second, using isolated cholangiocytes, they demonstrated that TGR5 promotes cell proliferation through elevation of reactive oxygen species (ROS) and subsequent activation of cholangiocyte proliferation through a TGR5-ROS-Src-MMP-EGFR-ERK-dependent signalling pathway. Furthermore, this pathway can be activated by TGR5 ligands in CCA cell lines. Interestingly, activation of TGR5 induces antiapoptotic effects through serine phosphorylation of the CD95 death receptor in murine cholangiocytes and CCA cell lines.
Ligand binding to TGR5 results in stimulation of adenylate cyclase, with increased cAMP, triggering subsequent protein kinase A activation.6 While Reich et al support the role of cAMP in promoting cholangiocyte proliferation, they convincingly show that stimulation of TGR5 triggers cell proliferation independent of adenylate cyclase activation but through epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase (ERK) phosphorylation. Bile acids have been identified to interact with EGFR in cholangiocytes.7 Similarly, ERK activation has previously been identified in rat cholangiocytes as downstream signal of bile acid-induced cholangiocyte proliferation.8 However, recent evidence showed that TGR5-dependent ERK activation inhibits cell proliferation in the human cholangiocyte cell line H69.9 Interestingly, previous work linked interleukin (IL) 6-mediated ERK activation to proliferation and not growth arrest in H69 cells.10 Thus, ongoing discussion on the role of ERK for cell proliferation in H69 cells exist in the literature. The apparent discrepancy with regards to TGR5-mediated ERK activation between H69 cells and rodent cholangiocytes, which respond to bile acids with increased ERK phosphorylation, may be due to interspecies or cell type variability (transformed vs non-transformed cells). Of note, TGR5 is localised to multiple, diverse subcellular compartments of the cholangiocyte, including primary cilia, plasma membrane and subapical compartment, which result in different biological effects of TGR activation. The ciliary localisation is associated with lower cAMP levels and limits ciliated cholangiocyte proliferation. This is in clear contrast with the effects in non-ciliated cholangiocytes.9
The key question is how the findings reported by Reich et al may impact on potential treatment to specific human pathology. Decreased pruritus in cholestatic liver disease, improvement of insulin resistance in type II diabetes, protection against obesity and anti-inflammatory effects in intestinal bowel disease are potential therapeutic outcomes based on TGR5 activation. Here, the authors took the initiative of analysing TGR5 expression in human CCA tissue. Importantly, TGR5 was highly expressed in the tumour cells of human CCA. In this tissue elevated ERK1/2 phosphorylation and CD95 receptor serine phosphorylation were detected, indicating that the TGR5-dependent pathways identified in cholangiocytes may play a role in CCA cells. In this regard, development of TGR5 inhibitors could represent a novel therapeutic approach in CCA. Of note, TGR5 mediates proliferative and antiapoptotic effects in cholangiocytes and TGR5 agonists may therefore be a valuable treatment also for cholestatic liver diseases affecting the biliary tree, such as primary biliary cirrhosis. The dedicated bile acid receptor TGR5 has been a prime target for drug development. TGR5 agonists undergo enterohepatic cycling and are expected to be most effective in the liver and the intestine. Nevertheless, in view of the pleiotropic spectrum of TGR5 actions, unexpected and off-target effects might be anticipated, which may complicate drug development.
Contributors CMPR and HM wrote the commentary.
Competing interests None declared.
Provenance and peer review Commissioned; internally peer reviewed.
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