Influence of common variants in the pharmacokinetic genes (OATP-C, UGT1A1, and MRP2) on serum bilirubin levels in healthy subjects

Abstract

To assess the contribution of OATP-C to the hepatobiliary transport of bilirubin, a pharmacogenomic evaluation with regard to polymorphisms of three candidate genes, OATP-C, MRP2, and UGT1A1, was performed. Serum total and direct (conjugated) bilirubin levels were used as phenotypic indexes. Pharmacokinetic variables of pravastatin, a typical substrate for OATP-C, were obtained from our previous study. Among 23 volunteers, two variants (Val417Ile and Ser789Phe) were observed in the MRP2 gene. While there was no apparent effect of these two variants and the UGT1A1*28 on direct bilirubin levels, the OATP-C variants were associated with differences in unconjugated bilirubin levels. Subjects with the OATP-C*15 allele had higher bilirubin levels; unconjugated bilirubin levels in *1b/*1b (n = 3), *1b/*15 (n= 7), and *15/*15 (n = 1) subjects were 0.40 ± 0.10, 0.77 ± 0.35, and 0.70 (mg/dL), respectively. In addition, the correlation between unconjugated bilirubin levels and pharmacokinetic parameters of pravastatin revealed that the subjects with higher bilirubin levels had lower non-renal clearance values, and then higher serum concentrations of pravastatin. Large clinical studies are needed to confirm a role of OATP-C in the carrier-mediated uptake of bilirubin in the human liver.

Introduction

After rapidly and selectively being taken up from blood into the liver [1], bilirubin is conjugated with glucuronate by uridine diphosphate glucuronosyltransferase (UGT) isoform 1A1 (UGT1A1) [2], [3], and then secreted into bile across the canalicular membrane of hepatocytes by multidrug resistance-associated protein 2 (MRP2, symbol ABCC2) [4], [5]. Although the mechanisms responsible for the uptake of bilirubin by hepatocytes have not yet been well defined in humans, passive diffusion and/or carrier-mediated transport by specific membrane proteins have been proposed [6], [7], [8]. Among known transporters expressed in the basolateral membrane of human hepatocytes [9], [10], a recent in vitro study indicated that organic anion transporting polypeptide C (OATP-C, also known as OATP2 and LST-1; symbol SLC21A6) may play a key role in the hepatocellular transport of unconjugated bilirubin [11]. We have recently studied in vivo the functional significance of genetic polymorphisms of the human OATP-C gene to the disposition kinetics of pravastatin (a substrate for OATP-C) and indicated that certain variants, such as Val174Ala (OATP-C*15), are likely to be associated with a reduction in hepatocellular uptake of pravastatin, resulting in less non-renal clearance and higher serum concentrations [12]. Thus, it is hypothesized that polymorphisms of the OATP-C gene could modulate the hepatic transport of unconjugated bilirubin. In contrast to the uptake process-mediated predominantly by OATPs, MRP2 is known to be involved in the hepatobiliary excretion of various organic anions. A deficiency of MRP2 is known to cause Dubin–Johnson syndrome (DJS), a rare autosomal recessive liver disorder characterized by chronic conjugated hyperbilirubinemia [13], [14], [15]. Although various causative mutations for DJS have been identified in the MRP2 gene [13], [14], [15], [16], no study has examined the contribution of naturally occurring and relatively frequently observed variants to bilirubin levels. In contrast to transporter-mediated DJS, the molecular defect in Gilbert's syndrome, a benign unconjugated hyperbilirubinemia, is reported to be the presence of an additional TA repeat [(TA)₆TAA → (TA)₇TAA] in the TATA sequence of the UGT1A1 gene [17], designated as UGT1A1*28 [18]. Thus, both transport and metabolism processes should be taken into consideration, when examining the hepatobiliary kinetics of bilirubin.

It is useful to accumulate findings with regard to the factors responsible for large inter-individual differences in hepatic uptake and efflux processes of endogenous and xenobiotic compounds not only for individualized drug therapy but also for understanding the pathogenesis of liver disease. In the present study, in order to gain further insight into the relationship between OATP-C polymorphism and hepatic bilirubin transport, we have re-evaluated our previously published study [12], but have focused on the bilirubin transport capability. In addition, we examined the effects of certain mutations of the MRP2 gene on direct bilirubin levels in serum. This is the first in vivo preliminary report that unconjugated bilirubin is likely to be taken up into the liver by OATP-C.