Genetic polymorphisms in the 5'-flanking region of human UDP-glucuronosyltransferase 2B7 affect the Nrf2-dependent transcriptional regulation.

Objectives: Human uridine diphosphate-glucuronosyltransferase 2B7 (UGT2B7) plays important roles in the metabolism of some clinical drugs, carcinogens, and steroid hormones. The molecular mechanisms of the inducible expression of UGT2B7 in response to xenobiotics have not been fully clarified. We sought to investigate whether the UGT2B7 is under the control of NF-E2 p45-related factor 2 (Nrf2), a key transcriptional factor regulating the expression of cytoprotective enzymes.

Methods: HepG2, HuH7, HLE, and Caco-2 cells were treated with sulforaphane (SFN), and the UGT2B7 mRNA levels were determined by real-time reverse transcriptase PCR. These cells were genotyped for the UGT2B7*2 (H268Y) allele using the PCR-restriction fragment length polymorphism method. Luciferase analyses and gel shift analyses were performed to identify the responsive regions for Nrf2 signaling.

Results: The UGT2B7 mRNA was induced by SFN in HepG2 and HuH7 genotyped as UGT2B7*1/*1, but not in HLE and Caco-2 cells genotyped as UGT2B7*2/*2. In HepG2 cells, the UGT2B7 protein level and morphine glucuronosyltransferase activity were also significantly induced by SFN. The induction was prominently decreased with small interfering RNA for Nrf2. In the 5'-flanking region (-2.5 kb) of the UGT2B7*2 allele, a 324-base pair insertion at -2067 and 12 single nucleotide polymorphisms simultaneously existed. Luciferase analyses and gel shift analyses revealed that an antioxidant responsive element at -1170 was responsible for the transactivation by Nrf2. In addition, a region from -990 to -858 on the UGT2B7*1 allele was also responsible for the transactivation by Nrf2. Abrogation of the Nrf2-dependent transactivation of the UGT2B7*2 allele was owing to the single nucleotide polymorphism -900A>G.

Conclusion: UGT2B7 is transcriptionally regulated by Nrf2, but the mechanism is hindered by polymorphisms in the promoter region of UGT2B7*2. The allele-specific mechanism may cause variability of the glucuronidation in response to oxidative stress.