2,3,7,8-Tetrachlorodibenzo-p-dioxin as growth modulator in mouse hepatocytes with high and low affinity Ah receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a highly potent tumor promoter in rodent liver, has been shown to increase epidermal growth factor (EGF)-stimulated DNA synthesis in rat hepatocytes in primary culture, a comitogenic effect possibly linked to the mechanism of liver tumor promotion. In hepatocytes isolated from male congenic mice (C57BL/6J) with high-affinity (AhbAhb) or low-affinity (AhdAhd) Ah receptor the association of the comitogenic action of TCDD with the Ah receptor was investigated. The potency of TCDD as inducer of CYP1A-catalyzed 7-ethoxyresorufin O-deethylase (EROD) activity was approximately 10-fold lower in AhdAhd compared to AhbAhb cells. In both cell types, TCDD did not stimulate DNA synthesis in the absence of EGF. When added together with EGF, TCDD exhibited two opposing effects on DNA synthesis, measured as incorporation of [3H]thymidine into DNA: (i) At 3 x 10(-14) M, TCDD increased EGF-stimulated DNA synthesis approximately 1.4-fold in AhbAhb but not in AhdAhd cells at a plating density of 35,000 cells/cm2. In hepatocytes from AhdAhd mice, 3 x 10(-12) M TCDD was required to elicit a similar comitogenic response. (ii) At a density of 10,000 cells/cm2, 3 x 10(-12) M TCDD had a pronounced inhibitory effect on EGF-stimulated DNA synthesis in AhbAhb but not in AhdAhd cells. Essentially similar results were obtained by counting of 5-bromo-2'-deoxyuridine-labeled nuclei. These findings demonstrate that TCDD can enhance or antagonize EGF-stimulated DNA synthesis in mouse hepatocytes modified by the cell density. The different concentration-response relationships in hepatocytes from both strains suggest that the Ah receptor regulates these responses.