Up-Regulation of UDP-Glucuronosyltransferase (UGT) 1A4 by 17β-Estradiol: A Potential Mechanism of Increased Lamotrigine Elimination in Pregnancy

Oral clearance of lamotrigine, an antiepileptic drug commonly used in pregnant women, is increased in pregnancy by unknown mechanisms. In this study, we show that 17 beta-estradiol (E-2) up-regulates expression of UDP glucuronosyltransferase (UGT) 1A4, the major enzyme responsible for elimination of lamotrigine. Endogenous mRNA expression levels of UGT1A4 in estrogen receptor (ER) alpha-negative HepG2 cells were induced 2.3-fold by E-2 treatment in the presence of ER alpha expression. E-2 enhanced transcriptional activity of UGT1A4 in a concentration-dependent manner in HepG2 cells when ER alpha was cotransfected. Induction of UGT1A4 transcriptional activity by E-2 was also observed in ER alpha-positive MCF7 cells, which was abrogated by pretreatment with the antiestrogen fulvestrant (ICI 182,780). Analysis of UGT1A4 upstream regions using luciferase reporter assays identified a putative specificity protein-1 (Sp1) binding site (-1906 to -1901 base pairs) that is critical for the induction of UGT1A4 transcriptional activity by E-2. Deletion of the Sp1 binding sequence abolished the UGT1A4 up-regulation by E-2, and Sp1 bound to the putative Sp1 binding site as determined by a electrophoretic mobility shift assay. Analysis of ER alpha domains using ER alpha mutants revealed that the activation function (AF) 1 and AF2 domains but not the DNA binding domain of ER alpha are required for UGT1A4 induction by E-2 in HepG2 cells. Finally, E-2 treatment increased lamotrigine glucuronidation in ER alpha-transfected HepG2 cells. Together, our data indicate that up-regulation of UGT1A4 expression by E-2 is mediated by both ER alpha and Sp1 and is a potential mechanism contributing to the enhanced elimination of lamotrigine in pregnancy.