Novel regulatory events that control expression of the UGT1A1 gene

点击翻译按钮获取中文摘要

The human UDP-glucuronosyltransferase 1A1 (UGT1A1) gene is regulated at the transcriptional level by a host of xenobiotic nuclear receptors (XNRs), including PXR, CAR, LXRα/β, PPARα in addition to the Ah receptor and the antioxidant response factor Nrf2. Exposure of specific ligands that targets any of these receptors will lead to induction of the UGT1A1 gene. UGT1A1 is expressed in many tissues, but predominantly in the liver and gastrointestinal (GI) tract in adults. In addition, the UGT1A1 gene is developmentally regulated, with greatly reduced expression in the liver and GI tract during neonatal development. Reduced expression of UGT1A1 during development plays an important physiological role because UGT1A1 is the sole glucuronosyltransferase responsible for the metabolism of serum bilirubin. During neonatal development, reduced UGT1A1 expression results in a build-up of serum bilirubin that is presented as either moderate or severe hyperbilirubinemia. While usually benign, severe neonatal hyperbilirubinemia (SNH) can lead to acute and chronic encephalopathy, abnormal behavior, opisthotonus, seizures, cerebellar hypoplasia, with potential linkages to autism spectrum disorders. Thus, we hypothesize that in those children that are at heightened risk for bilirubin induced neurotoxicity, controlling or accelerating the metabolism of bilirubin and reducing total serum bilirubin (TSB) levels would prevent neurotoxicity. We have recently generated humanized UGT1 (hUGT1) mice where the murine Ugt1 locus was replaced with the human UGT1 locus, including the human UGT1A1 gene. The human UGT1A1 gene is regulated in a tissue specific and developmental fashion that is concordant with its expression in human tissues. Importantly, hUGT1 mice develop SNH during the neonatal stage, providing us with a unique animal model to examine the regulatory properties of the UGT1A1 gene. We have recently established that SNH in hUGT1 mice can lead to seizures, cerebellar hypoplasia, with significant myelination defects, all of which can be reversed by inducing either liver or GI tract UGT1A1 gene expression. With this background, three significant discoveries, all identified with in the last 1-2 years and linking regulation of the UGT1A1 gene to SNH will be examined in this proposal. First, the liver UGT1A1 gene is actively repressed during the neonatal period by the corepressor protein SMRT (silencing mediatory of retinoic acid and thyroid hormone receptor). Second, oral administration of isothiocyanates to neonatal hUGT1 mice, which are known to induce oxidative stress, dramatically induce liver UGT1A1 gene expression by activating liver CAR. Third, regulation of intestinal UGT1A1 expression during development is controlled by the corepressor protein NCoR1 (nuclear repressor corepressor). We have linked repression of intestinal UGT1A1 gene expression by NCoR1 with IKKβ activity and oxidative stress. The regulatory events that we have outlined, each of which can reduce the risk of SNH, will be examined to unravel these novel mechanisms leading to expression of the human UGT1A1 gene.