Expression of the UDP-glucuronosyltransferase 1A Locus in Human Colon IDENTIFICATION AND CHARACTERIZATION OF THE NOVEL EXTRAHEPATIC UGT1A8

Christian P. Strassburg; Michael P. Manns; Robert H. Tukey

doi:10.1074/jbc.273.15.8719

Expression of the UDP-glucuronosyltransferase 1A Locus in Human Colon

IDENTIFICATION AND CHARACTERIZATION OF THE NOVEL EXTRAHEPATIC UGT1A8*

From the Department of Pharmacology, UCSD Cancer Center, University of California, San Diego, La Jolla, California 92093 and ^§Department of Gastroenterology and Hepatology, Medizinische Hochschule Hannover, 30625 Hannover, Germany

Abstract

UDP-glucuronosyltransferases (UGT) catalyze the conjugation of lipophilic exobiotic and endobiotic compounds, which leads to the excretion of hydrophilic glucuronides via bile or urine. By a mechanism of exon sharing, the transcripts of individual first exon cassettes located at the 5′ end of the human UGT1Alocus 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 in extrahepatic tissue, leading to the identification of UGT1A7 and UGT1A10 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 expression was analyzed in 5 colonic, 16 hepatic, 4 biliary, and 13 gastric human tissue specimens by quantitative duplex reverse transcription-polymerase chain reaction and Western blot analysis, demonstrating lower UGT1A mRNA in the extrahepatic tissues. The precise analysis of unique UGT1A transcripts by exon 1-specific duplex reverse transcription-polymerase chain reaction revealed the expression of UGT1A1, UGT1A3, UGT1A4, 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 UGT1A7 with a 93.8% identity of first exon sequences. Expressed UGT1A7 and UGT1A10 protein showed unique catalytic activity profiles, while UGT1A8 was not active with the substrates tested. The ability of UGT1A10 to glucuronidate estrone represents only the second example of a human estrone UGT. The highly related human UGT1A7–1A10 cluster is expressed in a tissue-specific fashion and underlines the role and diversity of physiological glucuronidation at the distal end of the digestive tract.

Footnotes

↵* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This work was conducted in part from support from United States Public Health Service Grant GM49135 (to R. H. T.) and the Deutsche Forschungsgemeinschaft Grant Str493/2-1 (to C. P. S.).

{emblft}AF030310.
↵‡ Recipient of the Pete Lopiccola Fellowship Award, UCSD, for Cancer Research.
↵¶ To whom correspondence should be addressed: Dept. of Pharmacology, 0636, UCSD Cancer Center, 9500 Gilman Dr., La Jolla, CA 92093-0636. Tel.: 619-822-0288; Fax: 619-822-0363; E-mail:rtukey{at}ucsd.edu.