treatmentwIth TCDD displayedequivalentbindingto oligonucleotide

In this report, we present a characterization of the cell-specific expres slon of two human cytochrome P450 genes, CYPIAJ and CYP1BJ, by 2,3,7,S-tetrachlorodibenzo-p-dloxin (TCDD). The TCDD-dependent In ductionof CYPJAJhas beenstudiedextensivelyand servesas the proto type response for a TCDD-signaling pathway initiated by the reversible binding of TCDD to an Intracellular receptor [designated the aryl hydro carbon (Ah) receptor@ CYPJAJ Is Induced by TCDD to high levels (45-fold Increase) In the human hepatoblastoma line HepG2 as compared with the human renal adenocarcinoma line ACHN. In contrast, CYPJBJ Is Induced selectively in ACHN cells. Cell-specific Induction ofCYP1A1 and CYP1B1 mRNA correlates with comparable changes In the corresponding proteins and results, at least in part, from transcriptional activation. Character Izatlon of the mechanism(s) for the differential regulation of CYPJA1 was carried out. Nuclear extracts obtained from either cell line following treatmentwIth TCDD displayedequivalentbindingto oligonucleotide probes for two dioxin-responsive elements located 5'-ward of the CYPJA1 promoter. This result obtained with broken cell fractions was confirmed by an intact cell DNA protection assay. Possible Involvement of negative regulators Is suggested by the presence ofa negative regulatory element in the 5' flanldng region of the CYPJAJ gene and the observed superinduc don of CYP1A1mRNAby cycloheximidein TCDD-treatedHepG2cells. Electromobility shift analysis using negative regulatory element probes, however,did notdetectquantitativedifferencesin the bindingof nuclear extract proteins obtained from either HepG2 or ACHN cells treated with TCDD.Thesefindingsindicatethat the ligand-dependent activationand dioxin-responsive element binding of the Ah receptor required for CYP1AJ induction in HepG2 cells also can occur in ACHN cells. We conclude that the repression of TCDD-dependent CYPJAJ induction In ACHNcellsoccursat the levelof transactivatlonin theAh receptorsignal transductionpathway.