A 66-base-pair enhancer module activates the expression of a distinct isoform of UDP-glucuronosyltransferase family 1 (UGT1A2) in primary hepatocytes

Ciprofibrate regulation of rat hepatic bilirubin glucuronidation and UDP-glucuronosyltransferases expression

Synthetic fibrates are hypolipidemic drugs known to stimulate hepatic peroxisome proliferation and bilirubin glucuronidation. This study was designed to estimate the effects of ciprofibrate simultaneously on rat hepatic bilirubin glucuronoconjugation and on hepatic expression of UGT1A1, UGT1A2 and UGT1A5, all of which belong to the bilirubin cluster. Hepatic bilirubin glucuronidation activity and UDP-glucuronosyltransferase expression (RT-PCR and Western blotting) were measured after a single-dose ciprofibrate treatment (5 mg/kg by gastric intubation) in 36-h time course experiments. Ciprofibrate regulation of PPARα and UGT1A5 mRNA expression was also investigated in rat hepatocytes. Bilirubin conjugation activity was induced by ciprofibrate, reaching a maximum level (2.4×) 24 h after the treatment. UGT1A1 and UGT1A5 mRNA expression was induced 1.5 times by ciprofibrate, with UGT1A5 reaching the basal level of UGT1A1. Although UGT1A2 mRNA was induced approximately threefold by ciprofibrate, its expression level remained low in comparison with basal or induced levels of UGT1A1 and UGT1A5 mRNA. In the 36-h time course experiment, bilirubin conjugation activity as well as UGT1A5 and PPARα mRNA expression presented a biphasic induction profile. Although a similar level of induction was observed in primary cultured hepatocyte experiments, such biphasic variation was not observed for both UGT1A5 and PPARα, and the induction of UGT1A5 mRNA expression by ciprofibrate required de novo protein synthesis. A single dose of ciprofibrate significantly induces rat liver bilirubin conjugation as well as UGT1A1, UGT1A5 and PPARα expression. The induction mechanism may involve PPARα, at least regarding UGT1A5 regulation.

Takashi Iyanagi: UGT1 gene complex: from Gunn rat to human

UDP-glucuronosyltransferases (UGT) comprise a large gene superfamily that can be classified, based on the degree of amino-acid similarity between isoforms, into several gene families. Among these gene families, the UDP-glucuronosyltransferase family 1 (UGT1) gene is a unique gene complex organized to generate enzymes that share a common carboxyl terminal portion and are unique in the variable amino terminal region. Each variable exon I is preceded by a regulatory 5'-region and, in response to a specific signal, transcription processing splices mRNA from each unique exon 1 to the four common exons ( 2, 3, 4, and 5) to provide a template for synthesis of the individual isoforms. A novel clue to elucidate the gene structure of mammalian UGT1 was cDNA cloning of rat UGT1A6 from the hyperbilirubinemic Gunn rat by Professor Takashi Iyanagi Ph.D. The elucidation of the structure of the rat UGT1 gene complex has led to a greater understanding of the genetic basis of Crigler-Najjar and Gilbert's syndromes. Now, examination of the UGT1 gene structure in hyperbilirubinemic patients has revealed more than 100 different genetic defects in Crigler-Najjar syndromes and one genetic alternation that accounts for the majority of Gilbert's syndrome cases. This review of a chapter in UGT history will focus on the extensive research of Iyanagi and coworkers with the rat UGT1 gene complex and advancing to the study of the human gene.

Human UGT1A8 and UGT1A10 mRNA are expressed in primary human hepatocytes

It is widely believed that the UGT1A isoforms, UGT1A8 and -1A10, are expressed exclusively in extrahepatic tissues. In this work, human primary hepatocytes from six donors were analyzed for UGT1A8 and -1A10 mRNA expression by semi-quantitative RT-PCR. New primers to amplify UGT1A8 mRNA were designed and found to differ from those previously published. We demonstrated that UGT1A8 and -1A10 mRNA are expressed in hepatocytes. Although basal UGT mRNA levels were detected in untreated hepatocytes, significant up-regulation of the levels of mRNA for these isoforms were seen after treatment with 3-methylcholanthrene (3-MC) and rifampicin (Rif). RT-PCR products for all UGTs were sequenced and unambiguously identified as matching the corresponding cDNA. The discovery of these isoforms in hepatocytes is a novel discovery and will stimulate studies on the potential role for these isoforms in hepatic detoxification.

9-cis-Retinoic acid regulation of four UGT isoforms in hepatocytes from rats with various thyroid states

PURPOSE

To investigate the influence of thyroid hormone status on the regulation of UGTs expression by 9-cis-retinoic acid in cultured rat primary hepatocytes.

METHODS

Hepatocytes from rats with various thyroid states were isolated and treated with 9-cis retinoic acid (1 x 10(-6) M). mRNA was amplified by reverse transcription and polymerase chain reaction (RT-PCR) and quantified by UV light densitometry. Variations in the expression levels of four different UGT isoforms (UGT1A1, 1A2, 1A5, and 1A6) that are involved in the glucuronication of bilirubin and phenols were determined by comparison with those of an internal standard, beta-actin, which is known to be insensitive to nutritional and hormonal conditions.

RESULTS

Primary hepatocyte cultures from rats with various thyroid states present similar metabolite characteristics to those from hypo- or hyperthyroid animals. The treatment of hepatocytes from hypothyroid rats with 9-cis-retinoic acid (1 x 10(-6) M) did not significantly modify bilirubin and phenol-UGT isoform expression. In contrast, in hepatocytes from normal and specially hyperthyroid rats treated with 9-cis-retinoic acid, UGT mRNA levels were modified. This suggests that the effect of retinoic acid on UGT mRNA expression requires the presence of thyroid hormone. This was confirmed by the treatment of cultured hepatocytes from hypothyroid rats with both retinoic acid and L-T3.

CONCLUSIONS

This study demonstrates that in cultured hepatocytes, the thyroid status can differentially modulate the expression of four UGT isoforms, and the regulation of their expression can be affected by 9-cis-retinoic acid.

Accelerated degradation of mislocalized UDP-glucuronosyltransferase family 1 (UGT1) proteins in Gunn rat hepatocytes

Gunn rat is a hyperbilirubinemic rat strain that is inherently deficient in the activity of UDP-glucuronosyltransferase form 1A1 (UGT1A1). A premature termination codon is predicted to produce truncated UGT1 proteins that lack the COOH-terminal 116 amino acids in Gunn rat. Pulse-chase experiments using primary cell cultures showed that the truncated UGT1A1 protein in Gunn rat hepatocytes was synthesized similarly to wild-type UGT1A1 protein in normal Wistar rat hepatocytes. However, the truncated UGT1A1 protein was degraded rapidly with a half-life of about 50 min, whereas the wild-type UGT1A1 protein had a much longer half-life of about 10 h. The rapid degradation of truncated UGT1A1 protein was inhibited partially but not completely by treating Gunn rat hepatocytes with proteasome inhibitors such as carbobenzoxy-Leu-Leu-leucinal and lactacystin. By contrast, neither the lysosomal cysteine protease inhibitor nor the calpain inhibitor slowed the degradation. Our findings show that the absence of UGT1 protein from Gunn rat hepatocytes is due to rapid degradation of the truncated UGT1 protein by the proteasome and elucidate the molecular basis underlying the deficiency in bilirubin glucuronidation.