© 2005 The Japanese Biochemical Society
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Transcriptional Enhancement of UDP-Glucuronosyltransferase Form 1A2 (UGT1A2) by Nuclear Factor I-A (NFI-A) in Rat Hepatocytes
Graduate School of Life Science, University of Hyogo, Harima Science Park City, Hyogo 678-1297
* To whom correspondence should be addressed. Phone: +81-791-58-0208, Fax: +81-791-58-0132, E-mail: emys{at}sci.u-hyogo.ac.jp
In cultured primary hepatocytes UDP-glucuronosyltransferase form 1A2 (UGT1A2) mRNA level is 80 times higher than that found in rat liver. We previously identified an enhancer sequence in the UGT1A2 promoter, and designated it as culture-associated expression responsive enhancer module (CEREM). Affinity chromatography with DNA fragments containing CEREM allowed enrichment of nuclear factor I (NFI) proteins from cultured hepatocytes. The NFI family is encoded by four distinct genes, NFI-A, NFI-B, NFI-C, and NFI-X. Immunoblot analysis with isoform-specific antibodies showed that NFI-A1 existed as a major component in rat liver and cultured hepatocytes. By contrast, NFI-C1 was present in rat liver but disappeared immediately upon cultivation of hepatocytes. Only trace amounts of NFI-B and NFI-X were detectable in rat liver and cultured hepatocytes. NFI-A1 elevated expression of the reporter gene that is under the control of CEREM, while NFI-C1 had an inhibitory effect. Co-expression of a constant amount of NFI-A1 with an increasing amount of NFI-C1 led to a concentration-dependent decrease in the expression of the CEREM-controlled reporter gene mediated by NFI-A1. Activation of UGT1A2 expression by NFI-A1 is suppressed by the coexistence of NFI-C1 in the liver, and culture-associated expression of UGT1A2 is triggered by the rapid disappearance of NFI-C1 in cultured hepatocytes.
1 The UGT1 gene is a single locus with multiple first exons followed by one set of commonly used exons (II, III, IV, and V), and each UGT1 isoform is generated through alternative splicing (3–5). We tentatively divided the UGT1 isoforms into a phenol cluster (cluster A) and a bilirubin cluster (cluster B) according to their sequence homology and preferential substrates (5). According to the recommended nomenclature system proposed by Mackenzie et al. (2), the number of each UGT1 isoform indicates the exact location of the first exon from the commonly used exons in the UGT1 locus. Thus, UGT1A2 is formed by combination of exon A2 with the common exons.
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