Newborns commonly develop physiological hyperbilirubinemia (also known as jaundice). With increased bilirubin levels being observed in breast-fed infants, breast-feeding has been recognized as a contributing factor for the development of neonatal hyperbilirubinemia. Bilirubin undergoes selective metabolism by UDP-glucuronosyltransferase (UGT) 1A1 and becomes a water soluble glucuronide. Although several factors such as gestational age, dehydration and weight loss, and increased enterohepatic circulation have been associated with breast milk-induced jaundice (BMJ), deficiency in UGT1A1 expression is a known cause of BMJ. It is currently believed that unconjugated bilirubin is metabolized mainly in the liver. However, recent findings support the concept that extrahepatic tissues, such as small intestine and skin, contribute to bilirubin glucuronidation during the neonatal period. We will review the recent advances made towards understanding biological and molecular events impacting BMJ, especially regarding the role of extrahepatic UGT1A1 expression.

Isothiocyanates, such as phenethyl isothiocyanate (PEITC), are formed following the consumption of cruciferous vegetables and generate reactive oxygen species (ROS) that lead to the induction of cytoprotective genes such as the UDP-glucuronosyltransferases (UGTs). The induction of ROS activates the Nrf2-Keap 1 pathway leading to the induction of genes through antioxidant response elements (AREs). UGT1A1, the sole enzyme responsible for the metabolism of bilirubin, can be induced following activation of Nrf2. When neonatal humanized UGT1 (hUGT1) mice, which exhibit severe levels of total serum bilirubin (TSB) because of a developmental delay in expression of the UGT1A1 gene, were treated with PEITC, TSB levels were reduced. Liver and intestinal UGT1A1 were induced, along with murine CYP2B10, a consensus CAR target gene. In both neonatal and adult hUGT1/Car^-/- mice, PEITC was unable to induce CYP2B10. A similar result was observed following analysis of UGT1A1 expression in liver. However, TSB levels were still reduced in hUGT1/Car^-/- neonatal mice because of ROS induction of intestinal UGT1A1. When oxidative stress was blocked by exposing mice to N-acetylcysteine, induction of liver UGT1A1 and CYP2B10 by PEITC was prevented. Thus, new findings in this report link an important role in CAR activation that is dependent upon oxidative stress.

Humanized UDP-glucuronosyltransferase (UGT)-1 (hUGT1) mice encode the UGT1 locus including the UGT1A1 gene. During neonatal development, delayed expression of the UGT1A1 gene leads to hyperbilirubinemia as determined by elevated levels of total serum bilirubin (TSB). We show in this report that the redox-sensitive NF-κB pathway is crucial for intestinal expression of the UGT1A1 gene and control of TSB levels. Targeted deletion of IKKβ in intestinal epithelial cells (hUGT1/Ikkβ(ΔIEC) mice) leads to greater neonatal accumulation of TSB than observed in control hUGT1/Ikkβ(F/F) mice. The elevation in TSB levels in hUGT1/Ikkβ(ΔIEC) mice correlates with a reduction in intestinal UGT1A1 expression. As TSB levels accumulate in hUGT1/Ikkβ(ΔIEC) mice during the neonatal period, the increase over that observed in hUGT1/Ikkβ(F/F) mice leads to weight loss, seizures and eventually death. Bilirubin accumulates in brain tissue from hUGT1/Ikkβ(ΔIEC) mice inducing an inflammatory state as shown by elevated TNFα, IL-1β and IL-6, all of which can be prevented by neonatal induction of hepatic or intestinal UGT1A1 and lowering of TSB levels. Altering the redox state of the intestines by oral administration of cadmium or arsenic to neonatal hUGT1/Ikkβ(F/F) and hUGT1/Ikkβ(ΔIEC) mice leads to induction of UGT1A1 and a dramatic reduction in TSB levels. Microarray analysis following arsenic treatment confirms upregulation of oxidation-reduction processes and lipid metabolism, indicative of membrane repair or synthesis. Our findings indicate that the redox state in intestinal epithelial cells during development is important in maintaining UGT1A1 gene expression and control of TSB levels.