Aryl hydrocarbon receptor knockout rats are insensitive to the pathological effects of repeated oral exposure to 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin

Abstract Sustained activation of the aryl hydrocarbon receptor (AHR) is believed to be the initial key event in AHR receptor‐mediated tumorigenesis in the rat liver. The role of AHR in mediating pathological changes in the liver prior to tumor formation was investigated in a 4‐week, repeated‐dose study using adult female wild‐type (WT) and AHR knockout (AHR‐KO) rats treated with 2,3,7,8‐tetrachlorodibenzo‐ p ‐dioxin (TCDD). Beginning at 8 weeks of age, AHR‐KO and WT rats were dosed by oral gavage with varying concentrations of TCDD (0, 3, 22, 100, 300 and 1000 ng kg −1 day −1 ). Lung, liver and thymus histopathology, hematology, serum chemistry and the distribution of TCDD in liver and adipose tissue were examined. Treatment‐related increases in the severity of liver and thymus pathology were observed in WT, but not AHR‐KO rats. In the liver, these included hepatocellular hypertrophy, bile duct hyperplasia, multinucleated hepatocytes and inflammatory cell foci. A loss of cellularity in the thymic cortex and thymic atrophy was observed. Treatment‐related changes in serum chemistry parameters were also observed in WT, but not AHR‐KO rats. Finally, dose‐dependent accumulation of TCDD was observed primarily in the liver of WT rats and primarily in the adipose tissue of AHR‐KO rats. The results suggest that AHR activation is the initial key event underlying the progression of histological effects leading to liver tumorigenesis following TCDD treatment. Copyright © 2015 John Wiley & Sons, Ltd.