Activated thyroid hormone receptor modulates dioxin-inducible aryl hydrocarbon receptor-mediated CYP1A1 induction in human hepatocytes but not in human hepatocarcinoma HepG2 cells

Aryl hydrocarbon receptor (AhR) is a transcription factor, the activity of which is modulated by hormones including glucocorticoids and estrogens. In this study, we examined the effects of triiodothyronine (T3), a ligand and activator of thyroid hormone receptor (TR), on transcriptional activity of AhR and the expression of its target gene CYP1A1. Study was carried out in human hepatocellular carcinoma cells HepG2 and primary cultures of human hepatocytes (HH). Gene reporter assay in stably transfected AZ-AhR cells revealed that T3 dose dependently augmented 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible AhR-dependent luciferase activity. In contrast, T3 had no effect on TCDD-inducible expression of CYP1A1 mRNA, protein and catalytic activity. Incubation of human hepatocytes with T3 had modulatory and inter-individual (7 cell cultures from 7 different liver donors) effects on both basal and dioxin-inducible CYP1A1/2. Since there was no correlation between T3 effects on CYP1A expression and T3-dependent expression of Spot14 mRNA, the involvement of additional factors besides TR is supposed. Overall, the co-incubation of normal and cancer human hepatic cells with TCDD and T3 suggested transcriptional cross-talk between AhR and TR, which may have physiological and toxicological implications.