Superinduction of Oxidized Tryptophan-Inducible Cytochrome P450 1A1 by Cycloheximide in Hepa lclc7 Cells

Previous studies from this laboratory have shown that L-tryptophan, after oxidation by either ultraviolet (UV) irradiation or ozone, causes induction of cytochrome P450 (CYP)1A1 mRNA, protein, and the corresponding 7-ethoxyresorufin O-deethylase (EROD) activity in wild type mouse hepatoma cells, Hepa lclc7 (Hepa-1), through the aryl hydrocarbon receptor (AhR). In the present study, we have examined the effect of temporary inhibition of protein synthesis by cycloheximide on oxidized tryptophan inducible CYP1A1 mRNA, protein, and EROD activity in Hepa-1 cells. The results demonstrate that combined exposure of wild-type Hepa-1 cells to either UV- or ozone-oxidized tryptophan and cycloheximide causes an increase in CYP1A1 mRNA, protein, and EROD activity, which is greater than the sum of the increases that were observed by exposure to each compound alone. The increase in EROD activity is dependent upon the dose and duration of cycloheximide treatment and is prolonged by actinomycin D when the latter compound was administered after removal of cycloheximide. Studies carried out to investigate the mechanism of this superinduction using various mutants of Hepa-1 cells, which are defective in either the AhR or AhR nuclear translocator protein indicated that the superinduction of oxidized tryptophan inducible EROD activity by cycloheximide occurs through the AhR. This is the first demonstration that oxidized tryptophan, in the presence of cycloheximide, causes superinduction of transcription of the Cyp1a1 gene with concomitant increase of CYP1A1 protein and EROD activity in Hepa-1 cells.