Nonresponsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin of transforming growth factor beta1 and CYP 1A1 gene expression in rat liver fat-storing cells.

Exposure to the potent tumor promoter 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) or related agonists of the aryl hydrocarbon receptor (AhR) can result in hepatocarcinogenesis in rodents. Changes in the expression and/or the level of growth factors may be a critical event in TCDD-mediated hepatocarcinogenesis. In this study, the influence of TCDD, the most potent AhR agonist, on the expression of transforming growth factor beta1 (TGF beta1), an inhibitor of hepatocellular proliferation synthesized in fat-storing cells (FSC) of the liver, was investigated in Wistar rats of both sexes in vitro and ex vivo. FSC were isolated from rat liver, cultured, and treated with 10(-10) and 10(-8) M TCDD, respectively, and TGF beta1 gene expression was determined at the levels of mRNA and protein. Furthermore, adult rats were treated with TCDD (10 micrograms/kg body wt, given by a single ip injection), FSC were isolated, and TGF beta1 gene expression was analyzed at different time points. Exposure to TCDD had no effect on the expression of TGF beta1 either at the RNA or at the protein level. Surprisingly, expression of CYP1A1, an AhR-regulated gene, was also not detectable either in untreated FSC or after TCDD treatment in vitro or ex vivo. Western blot analysis revealed that the lack of TCDD responsiveness of CYP1A1 is due to the absence of detectable amounts of the AhR in FSC. Based on these results we conclude that FCS may be the only liver cell type that lacks AhR-dependent inducibility of drug metabolism.