Induction of benzo[a]pyrene metabolism by 2,3,7,8-tetrachlorodibenzo-p-dioxin in primary cultures of adult rat hepatocytes

Primary hepatocyte cultures prepared from adult male Sprague-Dawley rats were extremely sensitive to induction of benzo[a]pyrene (BaP) metabolism by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), added to the cells in culture. A 48-hr exposure to 1 pmol TCDD/10⁶ cells resulted in a 4-fold induction of BaP metabolism. Significant induction was produced by a 48-hr exposure to a dose as low as 0.01 pmol (3 pg) TCDD/10⁶ cells. Cells exposed to TCDD from 7 to 55 hr in culture and then assayed for BaP metabolism responded to much lower doses of TCDD than did cells assayed at 36 hr in culture after being exposed to TCDD from 7 to 36 hr in culture. The doses of TCDD required for half-maximal induction of BaP metabolism were 24.8 and 164 fmol TCDD/10⁶ cells for cells assayed at 55 and 36 hr, respectively. Hepatocytes treated with TCDD starting at later times in culture showed a much more rapid time course of induction than did cells treated at earlier times in culture. A 2.6-fold induction occurred in cells treated from 49 to 55 hr in culture, compared to a 1.2-fold induction from 31 to 37 hr in culture. In contrast to cells exposed to TCDD from 7 to 55 hr in culture, cells exposed from 49 to 55 hr in culture were nearly as sensitive to inhibition of BaP metabolism by SKF 525-A as were uninduced control cells. An apparent derepression of the induction of BaP metabolism is occurring in primary hepatocyte cultures, resulting in an increase in the intitial rate of induction by TCDD and a decrease in the dose of TCDD required to obtain the half-maximum response.