Abstract
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.
Highlights
The consumption of cruciferous vegetables such as broccoli, watercress, and cauliflower have been correlated through epidemiological studies with a lower incidence of cancer development[1,2,3]
We have shown previously that xenobiotic induction of either gastrointestinal (GI) tract or liver UGT1A1 will promote the reduction of total serum bilirubin (TSB) in neonatal humanized UGT1 (hUGT1) mice[20,33]
Analysis of additional potential gene expression differences that might exist between liver and small intestine (SI) following phenethyl isothiocyanate (PEITC) administration confirmed induction of the Cyp2b10 gene and protein expression in both tissues (Fig. 2), an observation that implicates a role for constitutive androstane receptor (CAR) in the induction process
Summary
The consumption of cruciferous vegetables such as broccoli, watercress, and cauliflower have been correlated through epidemiological studies with a lower incidence of cancer development[1,2,3]. The anticarcinogenic activity of ITCs has been linked to the activation of the nuclear factor erythroid 2 related factor 2 (Nrf2)-Keap[1] signaling pathway[6], which is followed by binding of the transcriptional factor Nrf[2] to antioxidant response elements (AREs)[7,8] Prominent in this induction process are the cluster of genes that encode proteins involved in limiting both endogenous and exogenous chemicals from initiating deleterious biological effects by directing their metabolism, an event that biologically inactivates the agents and in many cases, facilitates their excretion from the body. These observations led us to examine in greater detail the potential cross-talk between the PEITC generated antioxidant response and CAR activation, and the implications of ROS on activation and induction of UGT1A1
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