Time-course comparison of xenobiotic activators of CAR and PPARα in mouse liver

Abstract

Constitutive androstane receptor (CAR) and peroxisome proliferator activated receptor (PPAR)α are transcription factors known to be primary mediators of liver effects, including carcinogenesis, by phenobarbital-like compounds and peroxisome proliferators, respectively, in rodents. Many similarities exist in the phenotypes elicited by these two classes of agents in rodent liver, and we hypothesized that the initial transcriptional responses to the xenobiotic activators of CAR and PPARα will exhibit distinct patterns, but at later time-points these biological pathways will converge. In order to capture the global transcriptional changes that result from activation of these nuclear receptors over a time course in the mouse liver, microarray technology was used. First, differences in basal expression of liver genes between C57Bl/6J wild-type and Car-null mice were examined and 14 significantly differentially expressed genes were identified. Next, mice were treated with phenobarbital (100 mg/kg by gavage for 24 hrs, or 0.085% w/w diet for 7 or 28 days), and liver gene expression changes with regards to both time and treatment were identified. While several pathways related to cellular proliferation and metabolism were affected by phenobarbital in wild-type mice, no significant changes in gene expression were found over time in the Car-nulls. Next, we determined commonalities and differences in the temporal response to phenobarbital and WY-14,643, a prototypical activator of PPARα. Gene expression signatures from livers of wild-type mice C57Bl6/ J mice treated with PB or WY-14,643 were compared. Similar pathways were affected by both compounds; however, considerable time-related differences were present. This study establishes common gene expression fingerprints of exposure to activators of CAR and PPARα in rodent liver and demonstrates that despite similar phenotypic changes, molecular pathways differ between classes of chemical carcinogens.

Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

“…Activation of CAR in both the mouse and rat results in changes in the expression of a large number of genes involved in phase I and II xenobiotic metabolism, cell proliferation, apoptosis and energy metabolism (Hamadeh et al, 2002; Ross et al, 2009, 2010; Tien & Negishi, 2006; Ueda et al, 2002; Waterman et al, 2010; Yoshinari et al, 2008). Studies in wild-type mice and CAR-knockout mice have demonstrated that not all genes affected by PB are CAR-dependent (Ueda et al, 2002).…”

“…Studies in mice and rats have demonstrated that while some hepatic genes are affected by both the CAR activator PB and peroxisome proliferators (which activate PPARα), many other genes are only affected by either PB or peroxisome proliferators (Hamadeh et al, 2002; Ross et al, 2009). …”

Eye-specific gene expression following embryonic ethanol exposure in zebrafish: Roles for heat shock factor 1

“…1B), an average-linkage, hierarchical clustering was performed in R, on mean-centered data (centered by differentially expressed genes, as in [46]), and the output is shown as a heatmap in Supplemental Figure 1. The two major categories of differentially expressed genes – those that are predominantly downregulated over developmental time vs. those that are predominantly upregulated over developmental time - showed only minor differences (ethanol vs. control) from 24 to 36 hpf.…”

“…Average-linkage, hierarchical clustering was performed using EDGE software, in the R programming environment. Clusters were built using mean-centered data, and with the correlation distance option, similar to the approach of [46], and displayed using the heatmap function.…”

Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

“…Consequently, some CAR ligands function as inverse agonists, i.e., reducing the constitutive activity (Forman et al, 1998). Microarray analysis studies have demonstrated that CAR ligands upregulate or repress genes with diverse functions in mouse liver, including genes involved in regulation of energy metabolism and heme synthesis (Rezen et al, 2009; Ross et al, 2009; Slatter et al, 2006; Ueda et al, 2002). Crystallographic structures of human and mouse CARs have provided insight into both the high level of constitutive activity and the ability of steroidal compounds (e.g., androstenol, 5α-androst-16-en-3α-ol) to act as inverse agonists (Shan et al, 2004; Suino et al, 2004; Xu et al, 2004).…”