Toxicogenomic Analysis of Gender, Chemical, and Dose Effects in Livers of TCDD- or Aroclor 1254–Exposed Rats Using a Multifactor Linear Model

Abstract

Chronic exposure of Sprague-Dawley (SD) rats to either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or Aroclor 1254 results in female-selective induction of hepatic tumors. The relative potency of dioxins and polychlorinated biphenyl mixtures, such as Aroclor 1254, is often estimated using the internationally endorsed toxic equivalency (TEQ) approach. Comparing the genome wide changes in gene expression in both genders following exposure to TEQ doses of these chemicals should identify critical sets of early response genes while further defining the concept of the TEQ of halogenated aromatic hydrocarbons. Aroclor 1254 at 0.6, 6.0, and 60 mg/kg body weight and TEQ doses of TCDD (0.3 and 3.0 mug/kg), calculated to match the top two Aroclor 1254 doses, were orally administered to SD rats for three consecutive days. Day 4 gene expression in hepatic tissue was determined using microarrays. A linear mixed-effects statistical model was developed to analyze the data in relation to treatment, gender, and gender * treatment (G*T) interactions. The genes most changed included 54 genes with and 51 genes without a significant model G*T term. The known aryl hydrocarbon receptor (AHR) battery genes (Cyp1a1, Cyp1a2, Cyp1b1, Aldh3a1), and novel genes, responded in a TEQ dose-dependent manner in both genders. However, an important observation was the apparent disruption of sexually dimorphic basal gene expression, particularly for female rats. Because many of these genes are involved in steroid metabolism, exposure to either TCDD or Aroclor 1254 could disrupt proliferative signals more in female rats as a possible consequence of altered estrogen metabolism. This study extends the findings of previous rodent bioassays by identifying groups of genes, other than the well-characterized AHR response genes, whose disruption may be important in the tumorigenic mechanism in this rat strain.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

“…This reduced sensitivity is partially explained by the human AhR having a tenfold reduced binding affinity to TCDD than rats (Connor and Aylward 2006). Primary hepatocyte studies corroborate this viewpoint and suggest even less human sensitivity relative to rats (Schrenk et al 1995; Silkworth et al 2008; Budinsky et al 2010; Carlson et al 2009; Black et al 2012; Le Vee et al 2010; Schrenk et al 1994). Much of the sensitivity comparison is based on CYP1A induction.…”

“…Therefore, the TEFs do not reflect the relative potency comparison across the PCDD/F AhR ligands in humans, the range of variability and uncertainty in overall relative potency of a congener or the critical risk end point of cancer. Limited studies of CYP1A induction in primary human hepatocytes suggest that the TEF values are too conservative but again, CYP1A induction may not accurately depict the gene networks associated with the tumor promotion MOA (Budinsky et al 2010, 2012; Silkworth et al 2008). Genomic data for TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF) and 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) reveal that TCDF and 4-PeCDF are 27-fold and 35-fold less potent than TCDD (Rowlands et al 2011, 2012).…”

Sex-related differences in murine hepatic transcriptional and proteomic responses to TCDD

“…But differences in sensitivities have also been identified within a single strain, between male and female animals (Pohjanvirta et al, 1993;Enan et al, 1996;Silkworth et al, 2008). In L-E rats, female animals are more sensitive to the acute lethality of TCDD (LD 50 = 9.8 μg/kg) while males are more resistant (LD 50 = 17.7 μg/kg) (Pohjanvirta et al, 1993).…”

A novel toxicogenomics-based approach to categorize (non-)genotoxic carcinogens

“…In PMH, for example, we found an overlap of 22 genes between the gene sets induced by ARO and HCE. ARO was selected as AhR agonist (Table 2), but it is known that ARO in hepatocytes also binds to other nuclear receptors like the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) (Aly and Domenech 2009;Silkworth et al 2008). Kojima et al (2011) have shown that organochlorine pesticides HCE and HCH also act as PXR agonists.…”