Background and aims: The liver represents one of the major sites of human glucuronidation. Many therapeutic drugs are substrates for UDP-glucuronosyltransferases (UGT) leading to the formation of usually inactive glucuronides. Hepatic glucuronidation undergoes significant changes during fetal and neonatal development requiring age adapted drug therapy. Regulation of individual UGT genes during hepatic development has not been defined. Subjects and methods: Expression of 13 UGT genes and glucuronidation activities were analysed in 16 paediatric liver samples (aged 7-24 months), two fetal samples, and 12 adult liver samples (aged 25-75 years) using duplex reverse transcription-polymerase chain reaction, western blot, and specific catalytic UGT activity assays. Results: No UGT transcripts were detected in fetal liver at 20 weeks' gestation. In contrast, UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, and UGT2B15 transcripts were present without variation in all 28 hepatic samples after six months of age. Significantly lower expression of UGT1A9 and UGT2B4 mRNA was identified in paediatric liver. Hepatic glucuronidation activity in children aged 13-24 months was found to be lower than in adults for ibuprofen (24-fold), amitriptyline (16-fold), 4-tert-butylphenol (40-fold), estrone (15-fold), and buprenorphine (12-fold). Conclusions: An early phase characterised by the appearance of UGT gene transcripts and a later phase characterised by upregulation of UGT expression is demonstrated during human hepatic development. The differential regulation of UGT1A9 and UGT2B4 expression extends beyond two years of age and is capable of influencing hepatic glucuronidation of common therapeutic drugs in children. The development of hepatic UGT activities is significant for paediatric drug therapy and the prevention of adverse drug effects.

UDP-glucuronosyltransferases (UGTs) convert dietary constituents, drugs, and environmental mutagens to inactive hydrophilic glucuronides. Recent studies have shown that the expression of the UGT1 and UGT2 gene families is regulated in a tissue-specific fashion. Human small intestine represents a major site of resorption of dietary constituents and orally administered drugs and plays an important role in extrahepatic UGT directed metabolism. Expression of 13 UGT1A and UGT2B genes coupled with functional and catalytic analyses were studied using 18 small intestinal and 16 hepatic human tissue samples. Hepatic expression of UGT gene transcripts was without interindividual variation. In contrast, a polymorphic expression pattern of all the UGT genes was demonstrated in duodenal, jejunal, and ileal mucosa, with the exception of UGT1A10. To complement these studies, interindividual expression of UGT proteins and catalytic activities were also demonstrated. Hyodeoxycholic acid glucuronidation, catalyzed primarily by UGT2B4 and UGT2B7, showed a 7-fold interindividual variation in small intestinal duodenal samples, in contrast to limited variation in the presence of 4-methylumbelliferone, a substrate glucuronidated by most UGT1A and UGT2B gene products. Linkage of RNA expression patterns to protein abundance were also made with several mono-specific antibodies to the UGTs. These results are in contrast to a total absence of polymorphic variation in gene expression, protein abundance, and catalytic activity in liver. In addition, the small intestine exhibits considerable catalytic activity toward most of the different classes of substrates accepted for glucuronidation by the UGTs, which is supported by immunofluorescence analysis of UGT1A protein in the mucosal cell layer of the small intestine. Thus, tissue-specific and interindividual polymorphic regulation of UGT1A and UGT2B genes in small intestine is identified and implicated as molecular biological determinant contributing to interindividual prehepatic drug and xenobiotic metabolism in humans.

No abstract

No abstract

No abstract