PHORBOL ESTERS INHIBIT THE DIOXIN RECEPTOR-MEDIATED TRANSCRIPTIONAL ACTIVATION OF THE MOUSE CYP1A-1 AND CYP1A-2 GENES BY 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN

Tetradecanoyl phorbol acetate (TPA) has been shown to inhibit 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD)-induced mouse P450IA1 benzo[a]pyren e hydroxylase activity (Raunio, H., and Pelkonen, O. (1983) Cancer Res . 43, 782-786). When we co-administered TPA and TCDD to C57BL/6 mice, the accumulation of TCDD-inducible liver P450IA1 and P450IA2 mRNA, as well as kidney P450IA1 mRNA, was greatly inhibited. When nuclear run-o n assays were conducted, maximal levels of transcriptional activation were achieved for both liver Cyp1-alpha-1 and Cyp1-alpha-2 with 1-mu-g /kg (almost-equal-to 3.0 nmol/kg) TCDD. TCDD elicited a dose-dependent increase in the rates of gene transcription, which paralleled the ind uction of P450IA1 and P450IA2 mRNA. Only Cyp1-alpha-1 gene transcripti on was elevated in kidney. When these experiments were repeated follow ing the co-administration of TPA with TCDD, the levels of TCDD-mediate d transcriptional increases in liver Cyp1-alpha-1 and Cyp1-alpha-2 and P450IA1 and P450IA2 mRNAs were dramatically inhibited. The reduction in Cyp1-alpha gene transcription by TPA could be accounted for by redu ced DNA binding of the dioxin receptor to the xenobiotic-responsive el ement (XRE) sequences, as measured by gel-retardation analysis. Analys is of nuclear [H-3]TCDD dioxin receptor by sucrose density gradients d emonstrated that the inhibition in Cyp1-alpha gene transcription and D NA binding by TPA resulted from a reduction in nuclear dioxin receptor concentration.