Hepatic bile acid uptake in humans and mice: Multiple pathways and expanding potential role for gut‐liver signaling
Authors: Paul A. Dawson
Journal: Hepatology (2017)

Potential conflict of interest: Nothing to report. Supported by NIH research grant DK047987. See Article On Page 1631 Hepatic bile formation depends upon the synthesis, canalicular secretion, and return of bile acids (BAs) in the enterohepatic circulation. Hepatocellular uptake of BAs from sinusoidal blood is mediated by a combination of transporters including the Na+‐Taurocholate cotransporting polypeptide (NTCP; gene symbol SLC10A1) and members of the Organic Anion Transporting Polypeptide (OATP/SLCO) family of facilitative carriers such as OATP1B1 and OATP1B3 in humans and Oatp1b2 in mice. Although the identity of these carriers has been known for many years, recent studies of patients and mouse models are providing important new insights to their relative quantitative significance, as well as the role of gut‐liver signaling in their regulation. Based upon its expression and transport properties, the NTCP was long presumed the major mechanism for hepatic clearance of conjugated BAs. This tenet was borne out in 2015 when a pediatric patient was described with an inherited functional defect in NTCP and highly elevated serum levels of conjugated BAs.1 Notably, this patient exhibited no jaundice, pruritus, or other signs of liver disease, raising important questions regarding the long‐term pathophysiological consequences of reduced hepatic BA clearance. Since then, two additional reports of patients with NTCP deficiency have appeared, which are beginning to answer those questions.2 The first report describes a Chinese pediatric patient with transient jaundice and a marked sustained hypercholanemia, where plasma BAs reached >700 μmol/L (normal, 0‐10) over the first 30 months of life.2 Similar to the initial case report, there was no obvious clinical phenotype and the child had normal anthropomorphical indices without signs of delayed motor milestones or growth retardation. The patient is homozygous for a c.800C>T nonsynonymous single‐nucleotide polymorphisms (rs2296651; p.Ser267Phe), which is prevalent in Asian populations (minor allele frequency, >5%).4 In cells, this amino acid change leads to near complete loss of BA transport activity and also reduces hepatitis B virus infection.4 Screening of an additional group of healthy individuals in Guangzhou, China, identified more heterozygous carriers, as well as a homozygous asymptomatic 30‐year‐old female subject with only mildly elevated serum BAs (19.3 μmol/L). The second new report describes a 30‐year‐old female patient of Thai origin with moderately elevated serum BA levels (values less than 90 μmol/L). The patient is a compound heterozygote, carrying the c.800C>T variation (p.Ser267Phe) and a frameshift mutation (c.615‐618del; pSer206Profs*12) on the other allele, and showed normal weight, height, and cognitive development. The patient is apparently asymptomatic except for recurrent urticarial rash, but was identified as a result of late fetal loss. In light of the association of high BAs and adverse pregnancy outcomes in intrahepatic cholestasis of pregnancy, it is possible that the patient's hypercholanemia may have contributed to the late fetal demise. Although the number of subjects is very small, the finding of only mild‐to‐moderate serum BA elevations in NTCP‐deficient adults suggests potential adaptations, which study of the NTCP‐deficient mouse is beginning to address. In 2015, Slijepcevic et al. reported on the phenotypic characterization of an NTCP (Slc10a1) knockout mouse.6 Surprisingly, more than half of the adult NTCP knockout mice maintained normal plasma levels of conjugated BAs, suggesting the presence of alternate hepatic bile acid clearance mechanisms. Notably, the hypercholanemic mice expressed lower hepatic levels of several Oatps, particularly Oatp1a1. In an encore performance, the investigators used a combination of genetic and pharmacological approaches to elucidate those alternative clearance mechanisms.7 In humans, mutations in the adjacent OATP genes, SLCO1B1 and SLCO1B3, are responsible for Rotor syndrome, a benign conjugated hyperbilirubinemia. Similarly, knockout mice lacking the entire family of Oatp1a/Oatp1b (Slco1a/1b) transporters exhibit a marked conjugated hyperbilirubinemia and elevated plasma unconjugated, but not conjugated, BAs. In the present study, the investigators examine the contribution of the murine Oatps to hepatic conjugated BA clearance in wild‐type and Slco1a/1b knockout mice treated with the NTCP inhibitor, myrcludex B. Whereas acute inhibition of NTCP with myrcludex B had only a small effect on plasma taurocholate clearance in wild‐type mice, it essentially abrogated taurocholate clearance and biliary secretion in Slco1a/1b knockout mice. Plasma conjugated BA levels were also markedly elevated in those mice, affirming a critical compensatory role for the Oatps. The availability of Slco1a1/1b knockout mice expressing the human OATP1B1 then permitted the investigators to begin examining whether human OATPs can play a similar compensatory role. This was particularly important in light of the relatively mild hypercholanemia recently reported for adult human subjects with NTCP deficiency. Although hepatic expression of human OATP1B1 reversed the conjugated hyperbilirubinemia in Slco1a1/1b knockout mice (as reported), it was not able to compensate and block the rise in plasma conjugated BAs after inhibition of NTCP with myrcludex B. Moreover, administration of myrcludex B to healthy human subjects for 6 days significantly elevated plasma BA levels. The findings provided an explanation for why plasma BA levels were normal in most of the NTCP knockout mice. But it did not explain why hepatic Oatp expression was reduced in the subset of NTCP knockout mice developing hypercholanemia. Although inflammation can reduce hepatic Oatp1a1 expression, cytokine levels were not increased nor were these signs of hepatic inflammation in the hypercholanemic mice. But, interestingly, hepatic BA synthesis, as estimated by measuring plasma 7α‐hydroxy‐4‐cholesten‐3‐one (C4) levels, was significantly reduced in NTCP knockout mice, suggesting increased gut‐liver signaling through the ileal farnesoid X receptor (FXR)‐fibroblast growth factor 15 and 19 (FGF15/19)/hepatic FGFR4 (fibroblast growth factor receptor‐4) pathway. Indeed, ileal expression of FGF15 (human ortholog, FGF19) was increased and hepatic expression of cholesterol 7α‐hydroxylase reduced in the hypercholanemic myrcludex B–treated Slco1a/1b knockout mice. With regard to ileal FGF15 expression, the same trend was observed in hypercholanemic NTCP knockout mice and in bile duct–ligated wild‐type mice treated with myrcludex B, leading the investigators to hypothesize that BAs can be internalized by enterocytes from the basolateral (blood) side by the bidirectional transporter, organic solute transporter OSTα‐OSTβ, when BA concentrations are sufficiently elevated. By this mechanism, the ileum can function to sense plasma BA levels and respond with production of the enterokine, FGF15/19, to suppress hepatic BA synthesis. Moreover, additional evidence was presented suggesting that this gut‐liver signaling and FGF15 may be mediating the suppression of hepatic Oatp expression in the NTCP knockout mice. Overall, the results indicate that both NTCP and Oatps participate in hepatic clearance of conjugated BAs in mice. Whether the hepatic OATPs can partially compensate for loss of NTCP in humans, particularly in adults, is still unclear. Mean plasma BA levels were significantly increased in human subjects after administration of myrcludex B, but there was considerable variation in the response, with some subjects exhibiting only mild elevations. The findings also build upon the concept of the ileal enterocyte as a BA sensor. BA signaling in the gut‐liver axis is widely recognized, whereby BAs internalized from the gut lumen by the apical sodium‐dependent bile acid transporter (ASBT) act through FXR‐FGF15/19 to regulate hepatic BA synthesis and other metabolic pathways. In this new twist, the ileal enterocyte signaling pathway may also be engaged under conditions of very high plasma BA levels. Additional studies such as analysis of Slco1a1/1b knockout mice lacking OSTα‐OSTβ will be needed to directly explore those mechanisms. Finally, studies such as this work by Slijepcevic et al. will be needed to understand the mechanisms that modify the presentation of NTCP deficiency in humans and ultimately the role of BAs in health and disease.Figure 1: Working model of the phenotype associated with mouse NTCP deficiency. Abbreviations: BSEP, bile salt export pump; T‐BA, taurine‐conjugated bile acid. Author names in bold designate shared co‐first authorship.