EXPRESSION OF THE UDP-GLUCURONOSYLTRANSFERASE 1A LOCUS IN HUMAN COLON- IDENTIFICATION AND CHARACTERIZATION OF THE NOVEL EXTRAHEPATIC UGT1A8

UDP-glucuronosyltransferases (UGT) catalyze the conjugation of lipophi lic exobiotic and endobiotic compounds, which leads to the excretion o f hydrophilic glucuronides via bile or urine, By a mechanism of exon s haring, the transcripts of individual first exon cassettes located at the 5' end of the human UGT1A locus are spliced to exons 2-5, leading to the expression of at least nine individual UGT genes. Recently, the tissue-specific expression of the UGT1A locus has been demonstrated i n extrahepatic tissue, leading to the identification of UGT1A7 and UGT 1A10 mRNA (Strassburg, C. P., Oldhafer, K., Manns, M. P., and Tukey, R H. (1997) Mol. Pharmacol 52, 212), However, UGT1A expression has not been defined in human colon, which is a metabolically active, external surface organ and a common route of drug administration. UGT1A expres sion was analyzed in 5 colonic, 16 hepatic, 4 biliary, and 13 gastric human tissue specimens by quantitative duplex reverse transcription-po lymerase chain reaction and Western blot analysis, demonstrating lower UGT1A mRNA in the extrahepatic tissues. The precise analysis of uniqu e UGT1A transcripts by exon 1-specific duplex reverse transcription-po lymerase chain reaction revealed the expression of UGT1A1, UGT1A3, UGT 1A4, UGT1A6, and UGT1A9 in the colon, which are also present in human liver, In addition, the expression of extrahepatic UGT1A10 and UGT1A8 was demonstrated. UGT1A8 was found to be closely related to gastric UG T1A7 with a 93.8% identity of first exon sequences. Expressed UGT1A7 a nd UGT1A10 protein showed unique catalytic activity profiles, while UG T1A8 was not active with the substrates tested, The ability of UGT1A10 to glucuronidate estrone represents only the second example of a huma n estrone UGT. The highly related human UGT1A7-1A10 cluster is express ed in a tissue-specific fashion and underlines the role and diversity of physiological glucuronidation at the distal end of the digestive tr act.