Extensive splicing of transcripts encoding the bile acid-conjugating enzyme UGT2B4 modulates glucuronidation.

BACKGROUND AND AIMS

UGT2B4 is a member of the UDP-glucuronosyltransferase (UGT) superfamily, a major detoxifying system in humans. UGT2B4 is involved in bile acids metabolism and highly expressed in liver and extrahepatic tissues. The aim of this study was to uncover new molecular mechanisms underlying interindividual variability in the UGT2B4 pathway.

METHODS

We carried out a comprehensive scan for additional exons at this locus and discovered multiple alternative splicing events. We then assessed the expression profile of alternatively spliced transcripts in human tissues and the activity of the corresponding overexpressed proteins toward bile acids.

RESULTS

We discovered three previously unidentified UGT2B4 exons, increasing the total known gene length to 46 kb. Molecular analyses revealed at least eight distinct mRNAs produced by (i) alternative promoter usage, (ii) complete and partial exon skipping, and (iii) use of alternative 3' splice sites. These splice variants were predominantly expressed in liver, gastrointestinal tract, and other extrahepatic tissues. Quantitative analyses of splicing events further sustain their prevalence in the liver. UGT2B4 proteins produced from these mRNA variants had undetectable transferase activity in human cells. However, when stably co-expressed with the active UGT2B4 isoform 1, three newly identified UGT2B4 isoforms (i2, i3, and i5) were found to negatively regulate glucuronidation.

CONCLUSION

In addition to heritable genetic mutations and control of gene expression, the newly discovered diversity of UGT2B4 mRNAs may introduce variability in this glucuronidation pathway.