Tissue-Specific Expression of the Mouse Dioxin-Inducible P₁450 and P₃450 Genes: Differential Transcriptional Activation and mRNA Stability in Liver and Extrahepatic Tissues

Abstract

Expression of the P₁450 and P₃450 genes was examined in liver and five extrahepatic tissues of mice after they were treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3-methylcholanthrene. All six tissues were shown to have increased P₁450 and P₃450 mRNA concentrations after treatment with these inducers. P₃450 mRNA induction was more sensitive than P₁450 mRNA induction to small doses of TCDD in liver, kidney, and lung. When transcription run-on assays were compared with mRNA prevalence, control P₃450 mRNA in liver, kidney, and lung was shown to be 20 to 30 times more stable than control P₁450 mRNA. After TCDD treatment the increases in mRNA concentrations did not necessarily parallel the increases in transcriptional rate. Thus, the inducer appeared to enhance mRNA stability in some instances. This was evident for liver P₁450 mRNA, in which an 8-fold rise in transcription was associated with a 27-fold increase in mRNA content, and for kidney P₃450 mRNA, in which a 2-fold rise in transcription was accompanied by a 12-fold increase in mRNA content. In the kidney and lung of control and TCDD-treated mice, transcriptional rates of the P₃450 gene were at least 10-fold less than those of the P₁450 gene. These data indicate that even though both genes are controlled by the same receptor, striking tissue-specific differences in transcription and mRNA stabilization affect the final mRNA concentrations.