Cholangiocarcinoma development: The resurgence of bile acids
Authors: Luca Maroni, Gianfranco Alpini, Marco Marzioni
Journal: Hepatology (2014)

Cholangiocarcinoma (CCA) is a rare malignant cancer arising from cholangiocytes, the epithelial cells lining the biliary epithelium. Despite being overall a rare neoplasm (3% of all gastrointestinal tumors worldwide), CCA is the second most common primary hepatic neoplasm, after hepatocellular carcinoma (HCC). CCA still represents a challenge for patients and clinicians.1 While the incidence of the disease appears to have increased over the past 30 years, prognosis of untreated patients has remained substantially unchanged and dismal. Treatment options are limited: surgical resection of the tumor remains the sole curative approach, but it can be achieved only in a minority of patients. Liver transplantation with neoadjuvant chemoradiation is currently reserved only for a few patients affected by perihilar CCA who meet stringent inclusion criteria.2 In this complex scenario, a deeper understanding of the mechanisms leading to CCA formation is eagerly sought, since it holds the hope for the generation of new and effective therapeutic options to treat such a deadly disease. In the present issue of Hepatology, Liu et al.3 report that conjugated bile acids are able to sustain CCA growth and invasiveness through the activation of the sphingosine 1-phosphate receptor 2 (S1PR2), a new bile acid receptor identified recently.4 The authors demonstrate that S1PR2 is the predominant sphingosine 1-phosphate (S1P) receptor expressed in rat and human CCA cell lines. Importantly, human CCA tissues showed significantly higher S1PR2 levels as compared to nontumor samples obtained from the same patients. In line with what was previously shown for rat hepatocytes,4 conjugated bile acid stimulation of S1PR2 induced the activation of ERK1/2 and AKT. The increased nuclear p-ERK1/2 levels were blunted by the incubation with JTE-013, a S1PR2 selective antagonist. Moreover, by using two different in vitro models the authors prove that taurocholic acid (TCA) exerts proproliferative effects on rat and human cell lines that are blocked by treatment with JTE-013 or by shRNA knockdown of S1PR2. In fact, treatment with JTE-013 alone reduced CCA cell proliferation compared to baseline, suggesting stimulatory autocrine/paracrine functions of the S1P secreted by the cells. Finally, Liu et al. show that TCA-mediated activation of S1PR2 is also involved in CCA cells invasiveness. Indeed, TCA stimulated cell migration both in the Matrigel invasion assay and in scratch assays, which were again inhibited by S1PR2 antagonism (Fig. 1). Schematic representation of bile acid-activated intracellular pathways involved in the sustenance of cholangiocarcinoma growth and invasiveness. Despite that the majority of CCA are sporadic, several conditions associated with biliary inflammation and cholestasis, such as primary sclerosing cholangitis (PSC), hepatolithiasis, hepatobiliary flukes, and bile duct cysts are well-known predisposing factors for the development of the tumor.2 Retention of biliary constituents may play a role in CCA development. In particular, a possible involvement of bile acids in the pathogenesis of CCA first attracted the interest of scientists 20 years ago. The earliest studies demonstrated that bile acids have proproliferative effects on biliary cells.5 In human cholangiocarcinoma cells, bile acids activate the epidermal growth factor receptor (EGFR) in a transforming growth factor-α (TGF-α)-dependent manner.6 The activation of EGFR by bile acids resulted in increased expression of cyclooxygenase-2 (COX-2),7 an enzyme involved in the pathogenesis of numerous cancers. Bile acids can also increase the levels of Mcl-1, a potent antiapoptotic member of the Bcl-2 family.8 Mice lacking the bile acid farnesoid X receptor (FXR) spontaneously develop hepatobiliary malignancies.9 Conjugated bile acids have been shown to decrease FXR expression in vitro and to promote CCA growth in vivo, while the concomitant treatment with the FXR agonist GW4064 could block both processes. Notably, FXR expression was found to be down-regulated in human CCA tissues compared to control.10 Conjugated bile acids appear to influence CCA cell growth also by modulating the nuclear factor kappa B (NF-κB) pathway, thorough an up-regulation of the phosphorylated IkB11 (Fig. 1). Indirect effects of bile acids may also promote CCA development by enhancing the expression of interleukin-6.12 To this extent, a proinflammatory biliary microenvironment has been consistently linked with the formation of nitric oxide and thereby with increased DNA damage and inhibited apoptosis.13 Sphingosine 1-phosphate receptors are G-protein-coupled receptors expressed in a variety of tissues. The family is composed of five different members (S1PR1-5) and S1P is the natural ligand of the receptor. The role of S1PR2 in tumor growth and progression is debated in the literature and appears to be cell-type-specific. Indeed, both proproliferative and antiproliferative effects of S1PR2 have been reported in several cancers.14, 15 The reason for such opposite effects has not been fully elucidated yet; however, differences in the specific cell response to S1PR2 activation might involve the recruitment of cell-specific intracellular pathways. In liver cells, S1P has been shown to sustain the proliferation and inhibit apoptosis of rat hepatoma cells transfected with S1PR1 and S1PR2.16 Moreover, the mRNA levels of sphingosine kinase 1 (SphK1), one of the enzymes responsible for S1P formation, are up-regulated in human HCC. Consistently, the down-regulation of S1PR1 by FTY720, a sphingosine 1-phosphate analog, has proven effective in reducing the recurrence of HCC in rats after liver transplantation.17 The present study is in part limited by the fact that the role of S1PR2 was not tested in an in vivo experimental context. However, it has the merit to awake the potential role of bile acids as promoters of CCA growth/progression, an aspect that has been neglected for a long time. The further definition of the mechanisms by which bile acids participate in CCA cell biology broadens the possibilities to identify candidate molecules to counteract such a dramatic malignancy. To this regard, S1PR2 appears particularly interesting due to its specific overexpression in human CCA. The point is now to understand what it is that causes S1PR2 up-regulation and activity in CCA cells. Are the changes in the biliary microenvironment or a peculiar event associated with malignant transformation of cholangiocytes? An answer to such a dilemma would have a major impact: it will serve as guidance to verify whether the counteraction of the bile acid/S1PR2 signal may sit within the strategies of chemoprevention or within the candidate molecular target for the cure of biliary malignancies. Luca Maroni, M.D., Ph.D.1 Gianfranco Alpini, Ph.D.2,3,4 Marco Marzioni, M.D.1 1Clinic of Gastroenterology and Hepatology, Università Politecnica delle Marche, Ancona, Italy 2Research, Central Texas Veterans Health Care System, Waco, TX 3Scott & White Digestive Disease Research Center, Scott & White Memorial Hospital, Bell County, TX4Department of Medicine, Division Gastroenterology, S&W and Texas A&M System Health Science Center, College of Medicine Temple, TX