Liver X receptors (LXRs), LXRα and LXRβ, are nuclear receptors that regulate the metabolism of cholesterol and bile acids and are activated by oxysterols. Humanized UGT1 (hUGT1) mice express the 9-human UGT1A genes associated with the UGT1 locus in a Ugt1-null background. The expression of UGT1A1 is developmentally delayed in the liver and intestines, resulting in the accumulation of serum bilirubin during the neonatal period. Induction of UGT1A1 in newborn hUGT1 mice leads to rapid reduction in total serum bilirubin (TSB) levels, a phenotype measurement that allows for an accurate prediction on UGT1A1 expression. When neonatal hUGT1 mice were treated by oral gavage with the LXR agonist T0901317, TSB levels were dramatically reduced. To determine the LXR contribution to the induction of UGT1A1 and the lowering of TSB levels, experiments were conducted in neonatal hUGT1/Lxrα ^-/- , hUGT1/Lxrβ ^-/- , and hUGT1/Lxrαβ ^-/- mice treated with T0901317. Induction of liver UGT1A1 was dependent upon LXRα, with the induction pattern paralleling induction of LXRα-specific stearoyl CoA desaturase 1. However, the actions of T0901317 were also shown to display a lack of specificity for LXR, with the induction of liver UGT1A1 in hUGT1/Lxrαβ ^-/- mice, a result associated with activation of both pregnane X receptor and constitutive androstane receptor. However, the LXR agonist GW3965 was highly selective toward LXRα, showing no impact on lowering TSB values or inducing UGT1A1 in hUGT1/Lxrα ^-/- mice. An LXR-specific enhancer site on the UGT1A1 gene was identified, along with convincing evidence that LXRα is crucial in maintaining constitutive expression of UGT1A1 in adult hUGT1 mice. SIGNIFICANCE STATEMENT: It has been established that activation of LXRα, and not LXRβ, is responsible for the induction of liver UGT1A1 and metabolism of serum bilirubin in neonatal hUGT1 mice. Although induction of the human UGT1A1 gene is initiated at a newly characterized LXR enhancer site, allelic deletion of the Lxrα gene drastically reduces the constitutive expression of liver UGT1A1 in adult hUGT1 mice. Combined, these findings indicate that LXRα is critical for the developmental expression of UGT1A1.

The gastrointestinal tract is enriched with xenobiotic processing proteins that play important roles in xenobiotic bioactivation, metabolism, and detoxification. The application of genetically modified mouse models has been instrumental in characterizing the function of xenobiotic processing genes (XPG) and their proteins in drug metabolism. Here, we report the utilization of three-dimensional crypt organoid cultures from these animal models to study intestinal drug metabolism and toxicity. With the successful culturing of crypt organoids, we profiled the abundance of Phase I and Phase II XPG expression, drug transporter gene expression, and xenobiotic nuclear receptor (XNR) gene expression. Functions of XNRs were examined by treating crypt cells with XNR prototypical agonists. Real-time quantitative polymerase chain reaction demonstrated that the representative downstream target genes were induced. These findings were validated from cultures developed from XNR-null mice. In crypt cultures isolated from Pxr^-/- mice, pregnenolone 16α-carbonitrile failed to induce Cyp3a11 gene expression; similarly, WY14643 failed to induce Cyp4a10 in the Pparα^-/- crypts. Crypt cultures from control (Ugt1^F/F ) and intestinal epithelial cell (IEC) specific Ugt1 null mice (Ugt1^ΔIEC ) were treated with camptothecin-11, an anticancer prodrug with severe intestinal toxicity that originates from insufficient UGT1A1-dependent glucuronidation of its active metabolite SN-38. In the absence of Ugt1 gene expression, Ugt1^ΔIEC crypt cultures exhibit very limited production of SN-38 glucuronide, concordant with increased apoptosis in comparison with Ugt1^F/F crypt cultures. This study suggests crypt organoid cultures as an effective in vitro model for studying intestinal drug metabolism and toxicity.