Expression and inducibility of the human bilirubin UDP-glucuronosyltransferase UGTLA1 in liver and cultured primary hepatocytes: Evidence for both genetic and environmental influences

In Crigler-Najjar type II patients and, recently in Crigler-Najjar type I p atients treated with human hepatocyte cell therapy, phenobarbital has been used for reducing the serum bilirubin load. Its effect is attributed to ind uction of the enzyme required for hepatic bilirubin elimination, UDP-glucur onosyltransferase, UGT1A1. This study investigated the expression and induc ibility of UGT1A1 in human donor livers and their corresponding primary hep atocyte cultures. Immunoblot analysis using a specific antibody directed ag ainst the amino terminal of the human UGT1A1 isoform showed that 5 hepatocy te donors exhibited a >50-fold difference in UGT1A1 level. UGT1A1 protein l evel correlated strongly with both liver microsomal bilirubin UGT activity and liver UGT1A1 mRNA level (r(2) = .82 and .72, respectively). Of the 4 pa tients with the lowest UGT1A1 levels, 3 were homozygotes for the UGT1A1 pro moter variant sequence associated with Gilbert's syndrome, and the fourth w as a heterozygote. The 3 donors with the highest levels had a history of ph enytoin exposure. Hepatocytes isolated from the phenytoin-exposed donors ex hibited marked declines in UGT1A1 mRNA levels during culturing. Induction s tudies using hepatocytes treated for 48 hours with phenobarbital (2 mmol/L) , oltipraz (50 (mu mol/L), or 3-methylcholanthrene (2.5 mu mol/L) revealed UGT1A1-inducing effects of phenobarbital, oltipraz, and, in particular, 3-m ethylcholanthrene. Our data suggest that both genetic and environmental fac tors play an important role in the marked interindividual variability in UG T1A1 expression. An understanding of these mechanisms could lead to advance s in the pharmacological therapy of life-threatening unconjugated hyperbili rubinemia.