Dioxin-responsive Genes: Examination of Dose-Response Relationships Using Quantitative Reverse Transcriptase-Polymerase Chain Reaction

The purpose of the present experiments was to examine dose-response relationships for induction of hepatic mRNA following a single administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to rats. The induction of cytochrome P450-1A1 (CYP1A1) mRNA is compared to other “dioxin-responsive” genes including UDP-glucuronosyltransferase I, plasminogen activator inhibitor 2, and transforming growth factor a using a sensitive reverse transcriptase-polymerase chain reaction-based method. Sample-to-sample variability in amplification is a concern in using polymerase chain reaction to quantitate biological responses. However, in the present study recombinant RNA templates were synthesized to use as internal standards in both the reverse transcription and the polymerase chain reaction steps. The induction of CYP1A1 mRNA was extremely sensitive to TCDD treatment with increases observed at doses as low as 1 ng/kg body weight. The induction of CYP1A1 mRNA correlated highly (R2 > 0.90) with an increase in ethoxyresorufin-o-deethylase activity, a CYPlAl-associated enzyme activity. However, induction of CYP1A1 mRNA levels was detected at lower TCDD doses than was ethoxyresorufin-o-deethylase activity, reflecting the greater sensitivity of the reverse transcription-polymerase chain reaction approach to detect transcriptional activation of the CYP1A1 gene. UDP-glucuronosyltransferase I mRNA was increased over control (5-fold) but required 1000-times higher TCDD doses (1 jig/kg) to result in a significant increase than did CYP1A1. Plasminogen activator inhibitor 2 and transforming growth factor a mRNA, both previously shown to be induced by TCDD in human keratinocytes, were not increased in rat liver. Hence, these studies reaffirm that TCDD acts through classical receptor mechanisms with gene-to-gene differences in responsiveness. The reverse transcription-polymerase chain reaction method developed to measure mRNA for dioxin-reponsive genes in rat liver will allow for measuring multigene and tissue responses to TCDD and other xenobiotics with high sensitivity, reproducibility, and adaptability and should increase our understanding of various dose-response relationships.