Bile acids in nonalcoholic steatohepatitis: Pathophysiological driving force or innocent bystanders?
Authors: Daniel Jahn, Andreas Geier
Journal: Hepatology (2017)

Potential conflict of interest: Dr. Geier advises for AbbVie, Alexion, BMS, Gilead, Intercept, Novartis, and Sequana, is on the speakers' bureau for AbbVie, Alexion, BMS, Falk, Gilead, Intercept, Novartis, and Sequana, and received research grants from Intercept and Novartis. See Article On Page 534 Nonalcoholic fatty liver disease (NAFLD) is one of the most prevalent chronic liver diseases and is estimated to become the primary cause of liver transplantation within the next 20 years. The natural course of NAFLD starts with bland liver steatosis (nonalcoholic fatty liver; NAFL), which per se is a relatively benign state. However, NAFL can progress to nonalcoholic steatohepatitis (NASH), which is characterized by distinct histopathological changes including ballooning, inflammation, and fibrosis. Besides their function as intestinal "soaps" bile acids (BAs) act as specific signaling molecules regulating a broad range of metabolic pathways via activation of the BA receptors, farnesoid X receptor (FXR) and Takeda G‐protein‐coupled receptor 5. Whereas the pathophysiology of NAFLD is incompletely understood, several lines of evidence suggest an involvement of impaired BA homeostasis. This includes a general increase of total serum and fecal BA levels in NASH1 and a positive correlation of elevated total serum BAs with measures of disease severity (e.g., NAFLD activity).3 However, it is currently unclear whether and how BAs directly contribute to the pathomechanisms of NAFLD. In this issue of Hepatology, Puri et al.4 report that the presence and severity of NASH are associated with specific changes in serum BA composition. Using liquid chromatography/mass spectrometry–based relative quantification in a cohort of 62 NAFLD patients, the investigators show that the progression from NAFL to NASH is characterized by increased ratios of primary/secondary as well as conjugated/unconjugated BAs. More subtle analyses of individual primary BAs moreover suggest that NASH patients had an increased ratio of total (i.e., unconjugated, glyco‐conjugated, and tauro‐conjugated) cholate (CA) to total chenodeoxycholate (CDCA) compared to either healthy controls (significant) or patients with NAFL (trend). In logistic regression models, certain CA conjugates were also positively correlated with the histological features of NASH, including steatosis (taurocholate; TCA), lobular inflammation (glycocholate; GCA), and ballooning (TCA), as well as with overall NAFLD activity (conjugated CA and TCA). Moreover, the abundance of conjugated CA was positively associated with an increased likelihood for significant liver fibrosis. Although it is currently unclear why BA composition is altered in NAFLD, it is equally intriguing to think about the putative downstream consequences of these alterations. In this regard, Puri et al. suggest that the gradual shift from CDCA to CA in NAFL and NASH may be accompanied by a reduction of FXR signaling attributed to the "dilution" of the potent FXR activator, CDCA, by CA (a weak agonist of human FXR). In line with this hypothesis, the investigators show that the expression of SHP (positive FXR target) is markedly down‐regulated in livers of patients with NAFL or NASH, whereas cholesterol 7 alpha‐hydroxylase (CYP7A1; negatively regulated by FXR through small heterodimer partner [SHP]) is strongly induced. Consistently, serum 7‐alpha‐hydroxy‐4‐cholesten‐3‐one levels are increased in NASH.2 Whether CDCA‐mediated induction of FXR transcriptional activity is, in fact, modulated by CA has, however, not been formally tested on the molecular level. Could reduced FXR signaling attributed to changes in BA composition be a common feature of other studies in NAFLD? Because of large variations of the primary readouts and the way results are reported, this question cannot be conclusively answered at present. However, some indications may point in this direction. Using global metabolomics plasma profiling in 25 NAFLD subjects, Kalhan et al.5 found a 4‐fold increase in plasma levels of GCA and TCA and a 2‐fold increase in glycochenodeoxycholate (GCDCA) in patients with NASH compared to healthy controls. Although not specifically reported, this suggests a relative increase of CA and its conjugates compared to CDCA, which would be consistent with the Puri et al. study and the hypothesis of reduced FXR activity. A limitation of most studies analyzing BAs in NAFLD is the current lack of knowledge to what extent levels of BAs circulating in the blood reflect BA composition within the liver. An interesting study by Aranha et al.6 analyzed BAs in liver tissue in a cohort of 15 NASH patients. Although no association was reported for the ratio of CA/CDCA with NASH severity, the investigators found a positive correlation between intrahepatic CA and severity of inflammation. Notably, the ratio of trihydroxylated/dihydroxylated BAs was likewise positively associated with degree of liver inflammation. This appears consistent with reduced FXR activity in advanced NAFLD given that dihydroxylated BAs comprise two of the most potent endogenous FXR ligands, namely CDCA and deoxycholate. In a second study measuring BAs in liver tissue of NAFLD subjects, Lake et al.7 did not find a general increase of CA and its conjugates. However, based on their data, the investigators suggested that the total hepatic BA composition in NASH was shifted to more hydrophilic metabolites. This, again, may point toward reduced FXR signaling given that the most potent FXR ligands are hydrophobic BAs. Of note, besides changes in BAs as ligand activators of FXR, a number of other regulators, including reduced adiponectin, increased free fatty acids, and inflammatory stimuli, have been shown to reduce FXR activity in NASH and may explain the postulated changes in FXR signaling3 (Fig. 1).Figure 1: Differential changes of BA homeostasis characterize NAFL and NASH. Although BA homeostasis seems to be largely intact in NAFL (left), it is severely compromised at different functional levels in NASH (right). Down‐regulation of proteins involved in BA homeostasis is indicated in black, whereas up‐regulation is colored in green. Unchanged expression is indicated in orange. Potential targets for pharmacological intervention with FXR agonists are indicated by blue arrows ("OCA"). Abbreviations: AdipoQ, adiponectin; ASBT, apical sodium–bile acid transporter; FFA, free fatty acid; FGFR4, fibroblast growth factor receptor 4.In line with decreased FXR activation, Puri et al. observed a gradual decrease of mRNA expression of the BA export pump (bile salt export pump; BSEP) from healthy liver to NASH. In accord, decreased expression also of BSEP protein has been observed in NASH patients. Multidrug resistance‐associated protein (MRP) 2, another canalicular export pump, appears to be removed from the canalicular membrane of hepatocytes in NASH.8 In the same study, expression of the basolateral rescue transporter, MRP3, was increased in line with elevated serum BAs and highlights the possibility that gene induction by increased hepatocellular BAs occurs by transcription factors other than FXR. Increased basolateral expression of the principal BA uptake transporter, sodium‐taurocholate cotransporting polypeptide (NTCP), appears compatible with an impaired negative feedback regulation and could contribute to increased intracellular BA load in NASH.3 Whether these molecular changes translate into decreased bile flow remains open, but could be postulated from rodent studies. Despite increased total fecal BAs in NAFLD patients, serum levels of fibroblast growth factor 19 (FGF19; which is an intestinal FXR target) were found to be either unaltered2 or decreased.9 This indicates that intestinal reuptake of BAs and/or activity of intestinal FXR could also be affected in these patients. Moreover, the hepatic response to FGF19 was found to be impaired in NAFLD and could additionally contribute to the deterioration of BA homeostasis (for a review of FGF19 pathway alterations in NAFLD, see Jahn et al.9). In summary, reduced FXR signaling seems to contribute to NASH pathophysiology. In fact, several drugs targeting FXR, such as obeticholic acid (OCA), are currently tested in clinical trials. In addition to its general anti‐inflammatory properties, OCA has recently been shown to activate FXR target gene expression, including SHP and BSEP, in liver tissue of bariatric patients.10 At the end, BAs and their signaling molecules are not only a pathophysiological driving force, but, at the same time, an attractive target for future NASH therapy. Author names in bold designate shared co‐first authorship.