Hepatic uptake of organic anions affects the plasma bilirubin level in subjects with Gilbert's syndrome mutations in UGT1A1

Abstract

Although in Gilbert's syndrome (GS), bilirubin glucuronidation is impaired due to an extra TA in the TATA box of the promoter of the gene for bilirubin UDP-glucuronosyltransferase 1 (UGT1A1), many GS homozygotes lack unconjugated hyperbilirubinemia. Accordingly, an additional defect in bilirubin transport might be required for phenotypic expression. Plasma bilirubin and the early fractional hepatic uptake rate (BSP K 1 ) of a low dose of tetrabromosulfophthalein (0.59 mol/kg) were determined in (1) 15 unrelated patients with unconjugated hyperbilirubinemia plus 12 random controls; (2) 4 unrelated GS probands and 15 of their first-degree relatives; (3) 7 unrelated patients with hemolysis due to ␤-Thalassemia minor. Subjects were classified by DNA sequencing of the promoter region of both UGT1A1 alleles. In group 1, GS homozygotes showed a highly significant negative linear correlation between plasma bilirubin levels and BSP K 1 . BSP K 1 values overlapped considerably between GS and normal subjects, whereas, in group 2, they were clustered within, and sharply segregated among, families. Patients with hemolysis, despite elevated plasma bilirubin levels, had mean BSP K 1 values similar to the normal subjects. Within each GS subgroup with defined UGT1A1 mutations, the plasma bilirubin level is in part determined by the organic anion uptake rate, assessed by early plasma disappearance of low-dose BSP. The lower BSP uptake in GS is not secondary to the hyperbilirubinemia, but probably caused by (an) independent, genetically determined defect ( Gilbert's Syndrome (GS) is an inherited form of mild, chronic, unconjugated hyperbilirubinemia, 1-3 which is associated with an extra TA in the promoter region of both alleles for bilirubin UDP-glucuronosyltransferase 1 (UGT1A1). [4][5][6][7][8] The resultant 65% decrease in transcription of the (TA) 7 TAA mutant alleles explains the impaired conjugation of bilirubin found in all GS patients. 9,10 The abnormal (TA) 7 TAA allele has a reported prevalence of 34% to 40% in white adults 4,5 and Sephardic Jewish neonates. 11 Thus, 12% to 16% should be and are homozygous for this abnormality, 4-8 yet only 3% to 9% of such populations, or less than half the expected proportion of homozygotes, have an increased level of unconjugated bilirubin (UCB) in plasma. 7,8,[12][13][14] This suggests that additional steps in bilirubin metabolism and/or transport must be impaired for hyperbilirubinemia to be manifested in (TA) 7 TAA homozygotes with reduced expression of UGT1A1.Thus far, both increased bilirubin production (from hemolysis, 1-3,11,15,16 which may not be overt [16][17][18] ) and/or decreased hepatic uptake of UCB have been documented in many presumed GS subjects. Some also exhibit decreased plasma clearance of tetrabromosulfophthalein (BSP) 20-22 and other organic anions, 20,23,24 including several therapeutic agents, [24][25][26] which appear to share hepatic uptake mechanisms with UCB. 27 With BSP, impaired uptake may be unmasked in GS patients if the administered dose is reduced ...

Hepatocellular Organic Anion–Transporting Polypeptides (OATPs) and Multidrug Resistance–Associated Protein 2 (MRP2) Are Inhibited by Silibinin

“…In contrast, unconjugated hyperbilirubinemia is caused by the complete absence of UGT1A1 (Crigler-Najjar syndrome type 1) or decreased activity (10% activity in Crigler-Najjar type 2 and 30% activity in Gilbert syndrome) of this enzyme Bosma et al, 1995). In addition to deficient UGT-mediated glucuronidation, decreased uptake from the blood into the liver was also suggested to play a role in unconjugated hyperbilirubinemia (Martin et al, 1976;Persico et al, 2001), which has been shown for OATP1B1, where polymorphisms in this transporter's gene are associated with elevated serum bilirubin levels (Roth et al, 2012). Furthermore, drug-induced unconjugated hyperbilirubinemia by rifamycin SV, indinavir, and cyclosporin A was reported to be due to an inhibition of this OATP family member (Campbell et al, 2004).…”

“…After UDP-glucuronosyltransferase (UGT) 1A1-mediated glucuronidation, bilirubin monoglucuronides and diglucuronides are exported from hepatocytes into bile via the ATP-binding cassette (ABC) transporter multidrug resistanceassociated protein (MRP) 2 (Kamisako et al, 1999). Patients with mild unconjugated hyperbilirubinemia resulting from Gilbert syndrome show deficient UGT activity but also a decreased bilirubin uptake into the liver (Martin et al, 1976;Persico et al, 2001). Therefore, a contribution of hepatocellular bilirubin uptake by transport processes to unconjugated hyperbilirubinemia has been suggested.…”

Polymorphisms in OATP-C

“…Although, homozygous mutations in the promoter of bilirubin UDP-glucuronosyltransferase appear to be necessary for the syndrome to manifest itself (20), a genetically determined polymorphism(s) in a bilirubin uptake carrier is also thought to contribute to the pathogenesis of this disorder (21,22). Such a role for OATP-C is plausible because no other human OATPs have yet been shown to be capable of transporting unconjugated bilirubin (22). However, a role for OATP-C mutations as a component of Gilbert's syndrome awaits confirmation.…”

Jaundice: a basic review

“…This gene encodes for UDPGlucronyltransferase and thus the defective conjugating enzyme leads to the hepatic jaundice. [22][23][24] Any defect in the hepatic excretory mechanism of bilirubin can also cause hepatic jaundice. The excretory mechanisms involve hepatocytic bile acid-independent secretion, hepatocytic bile acid-dependent secretion and bile ductular secretion.…”