Expression of P4501A1 in a primary culture of rainbow trout hepatocytes exposed to beta-naphthoflavone or 2,3,7,8-tetrachlorodibenzo-p-dioxin

Primary cultures of rainbow trout hepatocytes were used to study the expression of CYP1A1 mRNA, its protein product (P4501A1), and catalytic activities (7-ethoxy-resorufin-O-deethylase, EROD) during a 96-h period after exposure of cells to 3-naphthoflavone (BNF) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Hepatocytes were isolated from immature rainbow trout by a two-step perfusion method and incubated at 10 degrees C. Cells were exposed to the inducers for 48 h after 24 h of preculturing. The EROD activity of BNF-treated hepatocytes was higher than that of the control hepatocytes 12 h after addition of the inducer. Activities peaked after 48 h and had declined to control levels after 72 h. EROD activity in TCDD-treated cells was significantly elevated after 12 h, but in contrast to BNF-exposure, activities continued to increase during the experimental period. The content of P4501A1 protein, measured with an indirect ELISA technique (using anti-codP4501A1 IgG), increased linearly during the first 12 h and remained constant thereafter. In TCDD-exposed cells the immunochemically determined P4501A1 levels changed in parallel with EROD activity. CYP1A1 mRNA levels, determined by Northern blot and slot-blot analyses (using the trout P4501A1 cDNA pSg15 probe), were hardly detectable in control cells. In BNF- and TCDD-treated cells a 2.8-kb mRNA band was detected by the probe 6 h before the protein and catalytic activities became detectable. The elevated levels of CYP1A1 mRNA were sustained more effectively by TCDD than by BNF. In addition, a second mRNA band at 1.9 kb was seen. The results suggest that transcriptional activation is probably the prime factor even though post-transcriptional events may be involved in the regulation of P4501A1 induction by PAHs in rainbow trout hepatocytes.