Xenobiotic metabolism, cancer chemoprevention and cancer biomarkers

Division Of Basic Sciences - Nci

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Abstract

The role of PPARalpha in bile acid (BA) homeostasis is beginning to emerge. Ppara-null and hepatocyte-specific Ppara-null (Ppara-dHep) as well as the respective wild-type mice were treated with the potent PPARalpha agonist Wy-14,643 (Wy) and global metabolomics performed to determine the role of hepatocyte PPARalpha in the regulation of BA homeostasis revealing a new level of control of BA homeostasis beyond FXR. Levels of all serum BA were markedly elevated in Wy-treated wild-type mice but not in Ppara-null and Ppara-dHep mice. Gene expression analysis showed that PPARalpha activation (1) down-regulated the expression of sodium-taurocholate acid transporting polypeptide and organic ion transporting polypeptide 1 and 4, responsible for the uptake of BAs into the liver; (2) decreased the expression of bile salt export pump transporting BA from hepatocytes into the bile canaliculus; (3) upregulated the expression of multidrug resistance-associated protein 3 and 4 transporting BA from hepatocytes into the portal vein. Moreover, there was a notable increase in the compositions of serum, hepatic and biliary cholic acid and taurocholic acid following Wy treatment, which correlated with the upregulated expression of the Cyp8b1 gene encoding sterol 12alpha-hydroxylase. The effects of Wy were identical between the Ppara-dHep and Ppara-null mice. Hepatocyte PPARalpha controlled BA synthesis and transport not only via direct transcriptional regulation but also via crosstalk with hepatic FXR signaling. These findings underscore a key role for hepatocyte PPARalpha in the control of BA homeostasis. BA are also involved in hepatotoxicity induced by certain drugs, Rutaecarpine (RUT), evodiamine (EOD), and dehydroevodiamine (DHED) are the three main bioactive indoloquinazoline alkaloids isolated from Euodia rutaecarpa, a widely prescribed traditional Chinese medicine. Here, the structure-activity relationships of these analogs for aryl hydrocarbon receptor (AHR) activation were explored by use of Ahr-deficient (Ahr-null) mice, primary hepatocyte cultures, luciferase reporter gene assays, in silico ligand-docking studies, and metabolomics. In vitro, both mRNA analysis of AHR target genes in mouse primary hepatocytes and luciferase reporter assays in hepatocarcinoma cell lines demonstrated that RUT, EOD, and DHED significantly activated AHR, with an efficacy order of RUT DHED EOD. Ligand-docking analysis predicted that the methyl substitute at the N-14 atom was a key factor affecting AHR activation. In vivo, EOD was poorly orally absorbed and failed to activate AHR, whereas RUT and DHED markedly upregulated expression of the hepatic AHR gene battery in wild-type mice, but not in Ahr-null mice. Furthermore, RUT, EOD, and DHED were not hepatotoxic at the doses used; however, RUT and DHED disrupted BA homeostasis in an AHR-dependent manner. These findings revealed that the methyl group at the N-14 atom of these analogs and their pharmacokinetic behaviors were the main determinants for AHR activation, and suggest that attention should be given to monitoring BA metabolism in the clinical use of E. rutaecarpa. Metabolomics was used to study chemically-induced liver toxicity. Triptolide, a major active constitute of Tripterygium wilfordii Hook. F, is prescribed for the treatment of autoimmune diseases in China. One of its most severe adverse effects observed in the clinical use is hepatotoxicity, but the mechanism remained unknown. LC/MS-based metabolomic analysis was employed to characterize the metabolic changes in serum and liver induced by triptolide in mice. Mice were administered triptolide by gavage to establish the acute liver injury model, and serum biochemical and liver histological analyses applied to assess the degree of toxicity. Multivariate data analyses were performed to investigate the metabolic alterations. Potential metabolites were identified using variable importance in the projection values and Student's t-test. A total of 30 metabolites were observed that were significantly changed by triptolide treatment and the abundance of 29 metabolites was correlated with the severity of toxicity. Pathway analysis indicated that the mechanism of triptolide-induced hepatotoxicity was related to alterations in multiple metabolic pathways, including glutathione metabolism, tricarboxylic acid cycle, purine metabolism, glycerophospholipid metabolism, taurine and hypotaurine metabolism, pantothenate and CoA biosynthesis, pyrimidine metabolism and amino acid metabolism. The current study provides new mechanistic insights into the metabolic alterations that lead to triptolide-induced hepatotoxicity.