Transient induction of 7-ethoxyresorufin-O-deethylase (EROD) activity by medium change in the rainbow trout liver cell line, RTL-W1

Cytochrome P4501A (CYP1A) metabolizes a wide array of lipophilic xenobiotics. In fish liver, CYP1A is constitutively expressed at low levels, but xenobiotics can strongly induce CYP1A expression via a receptor-mediated pathway. While induction of hepatic CYP1A in teleosts by xenobiotics is well investigated, very little is known on the regulation of constitutive CYP1A expression and its induction by factors other than xenobiotics. In the present study we show that in the rainbow trout liver cell line, RTL-W1, CYP1A-catalyzed 7-ethoxyresorufin-O-deethylase (EROD) activity can be induced by a change of the culture medium, in the absence of xenobiotics. The increase in cellular EROD levels is of transient nature. Experiments with cell incubation solutions supplemented with various medium components indicate that photooxidized tryptophan is the agent causing the increase of EROD activity after medium change.