Xenobiotic Receptors Modulate PCB-induced Steatohepatitis

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 DESCRIPTION (provided by applicant): Non-alcoholic fatty liver disease (NAFLD) is intimately associated with obesity, diabetes and the metabolic syndrome. Traditionally, NAFLD has been associated with caloric excess on a susceptible genetic background, however recent studies from our laboratory and others have pointed out a role for environmental toxicants, including polychlorinated biphenyls (PCBs), in the etiology of steatosis and steatohepatitis, particularly in the context of hypercaloric diet coexposure. Our proposed study uses exposure parameters informed by our recent analysis of serum samples from a previously-described cohort of individuals residing in Anniston, AL who were exposed to high levels of PCB-containing waste. Although many of these individuals have serum levels of PCBs above that found in the general population of the US, their exposures remain well below the levels implicated in acute liver injury. Nevertheless, these individuals have significantly elevated biomarkers of liver injury as well as elevated inflammatory cytokines, elevated prevalence of diabetes, and increased BMI when compared to an unexposed population. A frustrating finding of previous toxicity studies has been the very different pathological response of mice and humans to such subacute, chronic, and environmentally relevant PCB exposures. Our preliminary data in a mouse model of subacute PCB exposure shows paradoxical elevations of liver injury biomarkers, fatty change, and inflammatory cytokines at lower exposure vs. higher exposure to a PCB mixture (20mg Aroclor 1260/kg body weight vs. 200mg/kg body weight Aroclor 1260). In two knockout mouse models that lack the constitutive androstane receptor (CAR) or the pregnane and xenobiotic receptor (PXR), our laboratory has found differential transcription of the gene targets of these receptors in response to Aroclor 1260 and high-fat diet exposure. These patterns of pathological and phenotypic responses are consistent with a transcription-factor based mechanism of PCB toxicity. The differences in disease endpoints may therefore be due to differences in ligand specificity and target gene cohort between the human and mouse forms of these receptors. We therefore propose to expand our investigation of receptor-based mechanisms of PCB toxicity with the use of a novel triple-transgenic mouse model expressing human forms of AhR, PXR, and CAR. We hypothesize that using both an acute-exposure primary hepatocyte culture, and a whole animal chronic exposure model, we will find that differences in the structure and function of these receptors will drive a differential transcriptioal response to PCBs and ultimately, differences in PCB-associated disease endpoints.