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.

S.T Okino,U.R Pendurthi,R.H Tukey

doi:10.1016/s0021-9258(19)50526-9

S.T Okino, U.R Pendurthi + Show 1 more

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https://doi.org/10.1016/s0021-9258(19)50526-9

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Abstract

Tetradecanoyl phorbol acetate (TPA) has been shown to inhibit 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced mouse P450IA1 benzo[a]pyrene 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-on assays were conducted, maximal levels of transcriptional activation were achieved for both liver Cyp1a-1 and Cyp1a-2 with 1 micrograms/kg (approximately equal to 3.0 nmol/kg) TCDD. TCDD elicited a dose-dependent increase in the rates of gene transcription, which paralleled the induction of P450IA1 and P450IA2 mRNA. Only Cyp1a-1 gene transcription was elevated in kidney. When these experiments were repeated following the co-administration of TPA with TCDD, the levels of TCDD-mediated transcriptional increases in liver Cyp1a-1 and Cyp1a-2 and P450IA1 and P450IA2 mRNAs were dramatically inhibited. The reduction in Cyp1a gene transcription by TPA could be accounted for by reduced DNA binding of the dioxin receptor to the xenobiotic-responsive element (XRE) sequences, as measured by gel-retardation analysis. Analysis of nuclear [3H]TCDD dioxin receptor by sucrose density gradients demonstrated that the inhibition of Cyp1a gene transcription and DNA binding by TPA resulted from a reduction in nuclear dioxin receptor concentration.

Highlights

Phorbol Esters Inhibit the Dioxin Receptor-mediated Transcriptional Activation of the Mouse Cypla - 1 and Cypla - 2 Genes by 29397,8-Tetrachlorodibenzo-p-dioxin*
In untreated mice where P450IA2 mRNA is expressed constitutively [38], Tetradecanoyplhorbol acetate (TPA) had noeffect on the levels of RNA expression, indicating that in these experiments TwPaAs inhibiting the selective induction of gene expression through a TCDD
When micewere treated with TPA for 12 h prior to the administration of TCDD, the cellular or molecular actions of TPA completely interfered with nuclear dioxin receptor binding to DNA following a 1-h treatmentwith TCDD.These results indicate that treatment with TCDD leads to a rapid influx of dioxin receptor, followed by the establishment of lower steady state concentrations of the nuclear dioxin receptor

Summary

Introduction

Phorbol Esters Inhibit the Dioxin Receptor-mediated Transcriptional Activation of the Mouse Cypla - 1 and Cypla - 2 Genes by 29397,8-Tetrachlorodibenzo-p-dioxin*. Tetradecanoyplhorbol acetate (TPA) has been P450s that constitute this “Super” gene family, two cytoshown to inhibit 2,3,7,8-tetrachlorodibenzo-p-dioxin chrome P450s, P450IA1 and P450IA2, are induced by these (TCDD)-induced mouse P46OIAl benzo[a]pyrene hy- agents. Exposure to these environmental contaminants and droxylase activity The induction of P450IA1and P450IA2 may play an importantrole in chemical induced carcinogenesis [3,4,5,6]. Tivation were achieved for both liver Cypla-1 and An intracellular protein, the dioxin receptor

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