Regulation of the UGT1A1 bilirubin-conjugating pathway: Role of a new splicing event at the UGT1A locus

Authors

  • Eric Lévesque,

    1. Laboratory of Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research Center and Faculty of Pharmacy, Laval University, Québec, Canada
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    • Eric Lévesque and Hugo Girard contributed equally to this study.

  • Hugo Girard,

    1. Laboratory of Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research Center and Faculty of Pharmacy, Laval University, Québec, Canada
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    • Eric Lévesque and Hugo Girard contributed equally to this study.

  • Kim Journault,

    1. Laboratory of Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research Center and Faculty of Pharmacy, Laval University, Québec, Canada
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  • Johanie Lépine,

    1. Laboratory of Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research Center and Faculty of Pharmacy, Laval University, Québec, Canada
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  • Chantal Guillemette

    Corresponding author
    1. Laboratory of Pharmacogenomics, Oncology and Molecular Endocrinology Research Center, CHUL Research Center and Faculty of Pharmacy, Laval University, Québec, Canada
    • Laboratory of Pharmacogenomics, CHUL Research Center, T3-48, 2705 Boulevard Laurier, Québec, Canada, G1V 4G2
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    • fax: 418-654-2761

    • Chantal Guillemette is the chairholder of the Canada Research Chair in Pharmacogenomics.


  • The naming of the UGT1A1 isoform 2 (UGT1A1_i2 for protein and UGT1A1_v2 for gene product) was done according to the Human Gene Nomenclature Guidelines and was approved by the UGT nomenclature committee.

  • Potential conflict of interest: Nothing to report.

Abstract

UDP-glucuronosyltransferase 1A1 (UGT1A1) is involved in a wide range of biological and pharmacological processes because of its critical role in the conjugation of a diverse array of endogenous and exogenous compounds. We now describe a new UGT1A1 isoform, referred to as isoform 2 (UGT1A1_i2), encoded by a 1495-bp complementary DNA isolated from human liver and generated by an alternative splicing event involving an additional exon found at the 3′ end of the UGT1A locus. The N-terminal portion of the 45-kd UGT1A1_i2 protein is identical to UGT1A1 (55 kd, UGT1A1_i1); however, UGT1A1_i2 contains a unique 10-residue sequence instead of the 99–amino acid C-terminal domain of UGT1A1_i1. RT-PCR and Western blot analyses with a specific antibody against UGT1A1 indicate that isoform 2 is differentially expressed in liver, kidney, colon, and small intestine at levels that reach or exceed, for some tissues, those of isoform 1. Western blots of different cell fractions and immunofluorescence experiments indicate that UGT1A1_i1 and UGT1A1_i2 colocalize in microsomes. Functional enzymatic data indicate that UGT1A1_i2, which lacks transferase activity when stably expressed alone in HEK293 cells, acts as a negative modulator of UGT1A1_i1, decreasing its activity by up to 78%. Coimmunoprecipitation of UGT1A1_i1 and UGT1A1_i2 suggests that this repression may occur via direct protein–protein interactions. Conclusion: Our results indicate that this newly discovered alternative splicing mechanism at the UGT1A locus amplifies the structural diversity of human UGT proteins and describes the identification of an additional posttranscriptional regulatory mechanism of the glucuronidation pathway. (HEPATOLOGY 2007;45:128–138.)

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