The Contribution of Peroxisome Proliferator-Activated Receptor Alpha to the Relationship Between Toxicokinetics and Toxicodynamics of Trichloroethylene

Trichloroethylene (TCE) is classified as a human carcinogen based on convincing evidence for a positive association with (1) renal-cell carcinoma in humans and (2) tumors of multiple sites in mice and rats of both sexes (Guha et al., 2012). The epidemiological evidence for the association between TCE exposure and liver cancer in humans is limited, even though liver is a well-established target organ in mice. Activation of peroxisome proliferator-activated receptor alpha (PPARα) is one of the mechanisms thought to be involved in the pathogenesis of liver cancer in mice exposed to TCE. In humans, the role of PPARα remains as a contentious issue in hazard assessment of TCE and other agents (Corton et al., 2014; Keshava and Caldwell, 2006). The absence of functional PPARα completely abolished the hepatocarcinogenic response from the prototypical and highly potent ligand WY-14 643 in the mouse (Peters et al., 1997). Mice expressing human PPARα (hPPARα) also have diminished hepatotoxic or hepatocarcinogenic responses when exposed to the peroxisome proliferators fenofibrate (Cheung et al., 2004) or WY-14 643 (Morimura et al., 2006). A number of hypotheses have been proposed to link PPARα and liver carcinogenesis through alterations in cell proliferation and apoptosis (Peters, 2008; Peters et al., 2012). At the same time, in a mouse model of constitutive activation of this nuclear receptor in liver, cell proliferation but not liver cancer were reported, which suggests that ligand activation and recruitment of co-effector proteins may also play an important role (Yang et al., 2007). Inter-individual and inter-species differences in genomic sequence, expression patterns and signaling cascades of PPARα have been reported, further compounding the challenge of assessing the relative role of this mechanism in carcinogenesis (Rusyn and Corton, 2012). Additional mechanisms may also be operational in the pathogenesis of environmental chemicals that are weak or nonselective agonists of PPARα (Ito et al., 2012; Ren et al., 2010; Wood et al., 2014). TCE is a relatively weak activator of either human or murine PPARα, but TCE metabolites tri- and di-chloro acetic acids (TCA and DCA) were found to be more potent activators (Maloney and Waxman, 1999; Zhou and Waxman, 1998). TCE-induced peroxisome proliferation response in mouse liver and kidney is thought to be mediated exclusively by TCA and DCA (Corton, 2008; Rusyn et al., 2014). Moreover; TCE metabolism to TCA and DCA is not thought to involve PPARα-inducible cytochrome P450 enzymes (Lash et al., 2014). Several studies observed abrogated toxic effects (eg, increased peroxisomal volume and peroxisomal enzyme activity) of TCE in Pparα-null mice (Laughter et al., 2004; Nakajima et al., 2000; Ramdhan et al., 2010). In addition, in mouse exposed to TCE, strain-specific tissue levels of TCA and DCA have been shown to be highly correlated with PPARα activation in liver (Yoo et al., 2015a) and kidney (Yoo et al., 2015b). Taken together, these studies suggest a simple toxicokinetic-toxicodynamic adverse outcome pathway whereby: (1) TCE is metabolized to TCA and DCA in the liver; (2) these metabolites activate PPARα in the liver (where they are formed in situ) and kidney (where they are transported for urinary excretion); and (3) activation of PPARα leads to a cascade of hepatocellular responses that may contribute to TCE-associated hepatocarcinogenesis in mice. However, a study of TCE inhalation in wild type, Pparα-null, and hPPARα mice (Ramdhan et al., 2010) showed genotype-dependent differences in levels of urinary TCA and trichloroethanol (TCOH), suggesting that PPARα status may actually affect TCE toxicokinetics. This in turn may affect the interpretation of previous studies which exposed Pparα-null and hPPARα mice to TCE, since differences in responses may not be due solely to differences in activation of PPARα (or lack thereof) but also in the production of metabolites. To test the hypothesis that PPARα status affects TCE toxicokinetics, we measured TCE and its metabolites in serum, liver, and kidney in wild type, Ppara-null, and hPPARα mice exposed to TCE acutely and sub-chronically by oral gavage. Additionally, to assess the relative contributions of toxicokinetics or toxicodynamics to TCE-induced hepatic and renal toxicity, we measured hepatic and renal levels of PPARα-responsive genes as well as biochemical markers of toxicity. Our results demonstrate that PPARα status affects TCE toxicokinetics in the liver and kidney. Such alternations in toxicokinetics may contribute to genotype-dependent differences in toxic responses in mouse liver and kidney.