DIOXIN INDUCES TRANSCRIPTION OF FOS AND JUN GENES BY AH RECEPTOR-DEPENDENT AND RECEPTOR-INDEPENDENT PATHWAYS

Halogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD; dioxin), and polycyclic aromatic hydrocarbons, such as benzo[a]pyrene, are environmental contaminants that cause many appare ntly unrelated toxic effects. In a previous study, we have shown that treatment of mouse hepatoma cells with TCDD or B(a)P results in an inc rease in mRNA levels of the immediate-early protooncogenes c-fos, c-ju n, junB, and junD, and the concomitant increase of the DNA-binding act ivity of the transcription factor AP-1, a dimer of FOS and JUN protein s. To analyze the mechanism of fos/jun activation by TCDD we have used electrophoretic mobility shift and transient expression assays of rep orter gene constructs containing response elements for 12-O-tetradecan oylphorbol-13-acetate (TRE), serum (SRE), cAMP (CRE), and aromatic hyd rocarbons (AhRE) from the fos and jun genes fused to the firefly lucif erase gene under the control of the SV40 minimal promoter. In mouse he patoma Hepa-l cells, which have Ah receptor (AHR) and Ah receptor nucl ear translocator (ARNT) proteins, inclusion of TRE, SRE, and the AhRE motifs from c-jun and junD, but not CRE or the AhREs from c-fos, fosB, and junB, causes a large TCDD-dependent increase in luciferase expres sion. In agreement with these results, c-jun and junD, but not c-fos, fosB, and junB AhREs, competed with a canonical Cyp1A1 AhRE for bindin g to the AHR ARNT heterodimeric complex. In African Green Monkey CV-1 cells, which lack AHR, expression plasmids with AhRE motifs require co expression of AHR and ARNT for TCDD to stimulate luciferase expression . In contrast, SRE-containing expression plasmids respond equally well to TCDD whether or not AHR and ARNT are coexpressed. These results su ggest that TCDD induces expression of the immediate-early response gen es fos and jun by activation of possibly three separate signal transdu ction pathways, at least one of which does not require a functional Ah receptor complex. (C) 1996 Academic Press, Inc.