UDP-glucuronosyltransferase (UGT) 1A1 glucuronidates endogenous metabolites, such as bilirubin, and exogenous substances, and plays a critical role in their detoxification and excretion. In a previous article, we described the phenobarbital response activity to a 290-base pair (bp) distal enhancer sequence (-3499/-3210) of the human UGT1A1 gene that is activated by the constitutive androstane receptor (CAR). Here, we show that dexamethasone at submicromolar concentrations enhances the pregnane X receptor (PXR) activator-mediated expression of the UGT1A1 gene and protein in HepG2 cells. We investigated the molecular mechanism of UGT1A1 induction by glucocorticoids at submicromolar concentrations and PXR activators and the functional cross-talk between the glucocorticoid receptor (GR) and CAR/PXR. The glucocorticoid- response element (GRE) was characterized by cotransfection experiments, site-directed mutagenesis, and electrophoretic mobility shift assays. Analysis of the human UGT1A1 promoter revealed GREs at -3404/-3389 and -3251/-3236 close to the CAR/PXR response element gtNR1 (-3382/ -3367). Furthermore, in an in vitro reporter gene assay, dexamethasone effectively enhanced CAR/PXR-mediated transactivation of the 290-bp distal enhancer module in HepG2 cells and CV-1 cells in the presence of exogenously expressed GR and glucocorticoid receptor-interacting protein 1 (GRIP1). In glutathione S-transferase pull-down experiments, CAR and PXR interacted with GRIP1. Together, these results demonstrate a rational mechanistic basis for UGT1A1 induction by glucocorticoids and PXR activators, showing that activated GR enhances CAR/PXR-mediated UGT1A1 regulation with the transcriptional cofactor GRIP1 and that GR may be involved synergistically in the xenobiotic-responsive regulation of UGT1A1 by CAR/PXR.
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