Amongst the other genes, Ube4b was shown to be responsive to TCDD across all four rat strains as well as the two lines, LnA and LnC. Ube4b encodes for an ubiquitination factor E4B, which binds to the ubiquitin moieties and accelerates ubiquitin chain assembly in synchrony with factors E1, E2, and E3, which subsequently tags aberrant proteins for degradation ( Koegl et al., 1999). We found that Ube4b is consistently dysregulated by TCDD treatment (2-fold

induction). It is unclear what role it plays in dioxin toxicity but it could be a protective mechanism that is elicited in response to exposure to xenobiotics. Interestingly, the AHR was recently shown to act as a ligand-dependent ubiquitin E3 ligase targeting e.g. sex hormone receptors and β-catenin for proteasomal degradation ( Ohtake and Kato, 2011). Glrx1, another gene whose mRNA abundances were statistically different between the treated and Doxorubicin untreated rats across all four rat strains, is a glutaredoxin that catalyzes deglutathionylation of protein-SS-glutathione mixed disulfides.

Glrx1 was induced more than 2-fold across all rat strains and lines. It is involved in protecting cells against oxidative stress ( Terada et al., 2010); up-regulation of Glrx1 may be a protective mechanism Etoposide concentration since other studies have also suggested its potential role in regulating apoptosis in cardiomyocytes ( Gallogly et al., 2010) and controlling autocrine and paracrine proinflammatory responses in retinal glial cells ( Shelton et al., 2009). Since L-E rats, which are much more sensitive to TCDD-induced liver tumor promotion than H/W rats ( Viluksela et al., 2000), exhibited an upward

trend in Glrx1 expression at the latest time-points analyzed ( Fig. 7), dysregulation of Glrx1 might have a role in the hepatocarcinogenicity of TCDD in rats. On the other hand, the enhanced Glrx1 expression coincides with aggravation of lipid peroxidation (an index of oxidative stress) in lethally TCDD-treated L-E rats ( Pohjanvirta et al., 1990). Another trend that was also consistent with our previous finding is that outside of the set of “classic” AHR-responsive genes, genes vary significantly in their responses to TCDD across the different rat strains. We identified a set of genes whose expression was significantly altered by TCDD in the sensitive rat strains but Dynein not the resistant H/W rats. These genes may represent predisposing genes that give rise to the observed toxicities to TCDD as mentioned above in the sensitive strains. For example, Slc37a4 encodes a transporter protein that transports glucose-6-phosphate to the microsomal lumen where hexose-6-phosphate dehydrogenase hydrolyses it to glucose and inorganic phosphate (Pi) ( Marcolongo et al., 2007). Deficiencies in the protein have been associated with disturbed glucose homeostasis and glycogen storage diseases ( Chou et al., 2002 and Pan et al., 2009).