ABSTRACT
Norursodeoxycholic acid (norUDCA) exhibits efficient anti-cholestatic properties in an animal model of sclerosing cholangitis. norUDCA is eliminated as a C(23)-ester glucuronide (norUDCA-23G) in humans. The present study aimed at identifying the human UDP-glucuronosyltransferase (UGT) enzyme(s) involved in hepatic norUDCA glucuronidation and at evaluating the consequences of single nucleotide polymorphisms in the coding region of UGT genes on norUDCA-23G formation. The effects of norUDCA on the formation of the cholestatic lithocholic acid-glucuronide derivative and of rifampicin on hepatic norUDCA glucuronidation were also explored. In vitro glucuronidation assays were performed with microsomes from human tissues (liver and intestine) and HEK293 cells expressing human UGT enzymes and variant allozymes. UGT1A3 was identified as the major hepatic UGT enzyme catalyzing the formation of norUDCA-23G. Correlation studies using samples from a human liver bank (n = 16) indicated that the level of UGT1A3 protein is a strong determinant of in vitro norUDCA glucuronidation. Analyses of the norUDCA-conjugating activity by 11 UGT1A3 variant allozymes identified three phenotypes with high, low, and intermediate capacity. norUDCA is also identified as a competitive inhibitor for the hepatic formation of the pro-cholestatic lithocholic acid-glucuronide derivative, whereas norUDCA glucuronidation is weakly stimulated by rifampicin. This study identifies human UGT1A3 as the major enzyme for the hepatic norUDCA glucuronidation and supports that some coding polymorphisms affecting the conjugating activity of UGT1A3 in vitro may alter the pharmacokinetic properties of norUDCA in cholestasis treatment.
Subject(s)
Cholic Acids/chemistry , Glucuronides/chemistry , Glucuronosyltransferase/chemistry , Microsomes, Liver/enzymology , Norsteroids/chemistry , Animals , Cell Line , Cholangitis, Sclerosing/drug therapy , Cholangitis, Sclerosing/enzymology , Cholangitis, Sclerosing/genetics , Cholic Acids/therapeutic use , Disease Models, Animal , Esters/chemistry , Esters/metabolism , Glucuronides/biosynthesis , Glucuronosyltransferase/genetics , Glucuronosyltransferase/metabolism , Humans , Isoenzymes/chemistry , Isoenzymes/genetics , Isoenzymes/metabolism , Norsteroids/therapeutic use , Polymorphism, Genetic , Rifampin/chemistryABSTRACT
Chenodeoxycholic acid (CDCA) is a liver-formed detergent and plays an important role in the control of cholesterol homeostasis. During cholestasis, toxic bile acids (BA) accumulate in hepatocytes causing damage and consequent impairment of their function. Glucuronidation, a conjugation reaction catalyzed by UDP-glucuronosyltransferase (UGT) enzymes, is considered an important metabolic pathway for hepatic BA. This study identifies the human UGT1A3 enzyme as the major enzyme responsible for the hepatic formation of the acyl CDCA-24glucuronide (CDCA-24G). Kinetic analyses revealed that human liver and UGT1A3 catalyze the formation of CDCA-24G with similar K(m) values of 10.6 to 18.6 mumol/L, respectively. In addition, electrophoretic mobility shift assays and transient transfection experiments revealed that glucuronidation reduces the ability of CDCA to act as an activator of the nuclear farnesoid X-receptor (FXR). Finally, we observed that treatment of human hepatocytes with fibrates increases the expression and activity of UGT1A3, whereas CDCA has no effect. In conclusion, UGT1A3 is the main UGT enzyme for the hepatic formation of CDCA-24G and glucuronidation inhibits the ability of CDCA to act as an FXR activator. In vitro data also suggest that fibrates may favor the formation of bile acid glucuronides in cholestatic patients.
Subject(s)
Chenodeoxycholic Acid/metabolism , Glucuronosyltransferase/metabolism , Liver/enzymology , Adult , Cell Line , Clofibric Acid/pharmacology , DNA-Binding Proteins/metabolism , Female , Hepatocytes/metabolism , Humans , Male , Microbodies , Microsomes, Liver/metabolism , Middle Aged , PPAR alpha/metabolism , Pyrimidines/metabolism , Receptors, Cytoplasmic and Nuclear/metabolism , Transcription Factors/metabolismABSTRACT
Six NAT2 single-nucleotide polymorphisms (SNPs) were analysed in 105 unrelated Ngawbe and 136 unrelated Embera Amerindians (482 chromosomes) by SNP-specific polymerase chain reaction analysis. 282C>T was the most common synonymous mutation, while 857G>A was the most frequent nonsynonymous inactivating exchange. The allelic frequency of the NAT2*5 series (containing the 341T>C exchange) was 2.4% and 9.9% for Ngawbe and Embera, respectively, five- to 20-times lower than that in Caucasians. The NAT2*6 series (590G>A) showed allelic frequencies of 0% and 3.7%, eight- to 30-times lower than in Caucasians. On the other hand, the NAT2*7 series, characterized by mutation 857G>A, had allelic frequencies (23.3% and 22.8%) that were 10-20-times higher in Amerindians than in Caucasians. Amerindians are characterized by decreased genetic diversity because they display a low number of mutated alleles (four and five for Ngawbe and Embera, respectively) that are present at low proportions (27.6% and 39%), reduced genotypic variability (seven out of 15 and 12 out of 21 possible genotypes) and low heterozygosity (40% and 55.1%) at the NAT2 locus. The NAT2 phenotype was evaluated with caffeine in a subset of 72 Embera. There were no disagreements between genotype and phenotype among rapid and slow acetylators (13/72, 18%). We conclude that, in the Embera, the analysis of three inactivating mutations was sufficient in predicting the phenotype in more than 99.5% of these subjects. NAT2 would appear to be of a selectively neutral character given that there is no evidence of adaptation to the prevailing ecology in Amerindians.
