Ontogenic isoform switching of UDP–glucuronosyltransferase family 1 in rat liver

https://doi.org/10.1016/j.bbrc.2008.10.043Get rights and content

Abstract

The liver undergoes dramatic changes in function during development. The development of UDP–glucuronosyltransferase family 1 (UGT1) isoforms was studied in livers from rats at 16–20 days of gestation, at days 1, 2, 3, 4, and 7 of infancy, at days 14 and 28 of childhood, and at day 56 of young adulthood. We found developmental stage-specific switching of regulation of the rat UGT1 gene complex. UGT1A6 was expressed as a predominant component of UGT1 in fetus liver, while other UGT1 isoforms were repressed. In contrast, expression of UGT1A1 surged immediately after birth. Expression of UGT1A5 was transiently elevated in childhood. We also found age-dependent alternative usage of dual UGT1A6 promoters in rat liver. Since UGT1A1 is the only bilirubin-glucuronidating isoform, the ontogeny of UGT1A1 in liver microsomes demonstrates that inadequate UGT1A1 proteins in the early neonatal period are linked to the common etiology of idiopathic hyperbilirubinemia in the newborn infant.

Section snippets

Rat liver samples

Fetal rat livers were obtained from timed-pregnant rats, and they were obtained from Nihon SLC inc. (Shizuoka, Japan). Male Wistar rats were used in this study except for 16-day-fetuses, which cannot be distinguished by morphological sex differences. Livers were excised from rats, cut into pieces with scissors, and then quickly frozen in liquid nitrogen. For each age analyzed, a total of six to nine rat livers were pooled before use.

Preparation of total RNA and analysis of mRNA expression

Frozen liver samples (0.2 g) were crushed with a glass morter

Ontogenic expression profiles and switching of UGT1 mRNA

Our previous studies determined the genomic organization of the rat UGT1 gene complex and demonstrated drug-responsive and tissue-specific expression of each UGT1 isoform [6]. The N-terminal substrate-binding domains of each UGT1 isoform are encoded by distinct large first exons, while the C-terminal UDP-glucuronic acid-binding domain of all UGT1 isoforms is identical and is encoded by four commonly used exons (Fig. 1). The isoform-specific first exons are organized in a tandem array, and the

Concluding remarks

We have demonstrated that the UGT1A1 gene is silenced before birth while the UGT1A6 gene is active in the developing rat liver. The perinatal isoform switching, that is a shift from phenol-glucuronidating UGT1A6 to bilirubin-metabolizing UGT1A1, in hepatic microsomes occurred after birth. This transition period between fetus and neonate is linked to the inadequacy of perinatal glucuronidation of bilirubin and the common etiology of idiopathic hyperbilirubinemia in the newborn infant. We also

Acknowledgments

This work was supported in part by the Grants-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan and Grants for Scientific Research from the Foundation of Himeji Institute of Technology.

References (21)

There are more references available in the full text version of this article.
View full text