The use of human hepatocyte cultures to study the induction of cytochrome P-450

We have previously reported that paclitaxel (Taxol) is a potent inducer of cytochrome P-450 (CYP) 3A protein and CYP3A mRNA in human hepatocyte cultur es. Here we report that Taxol increased CYP3A-dependent testosterone GP-hyd roxylation in intact hepatocytes, This effect was concentration-dependent, with maximal increase in enzyme activity being observed at 10 mu M Taxol, T reatment of hepatocyte cultures with concentrations of Taxol higher than 10 mu M caused a dose-dependent decrease in testosterone GP-hydroxylase activ ity, amount of CYP3A protein, and total protein synthesis. The maximal CYP3 A activity detected after treatment with Taxol or rifampicin was similar in six separate human hepatocyte cultures, suggesting that the cultures have achieved a limit of maximally inducible CYP3A, The fold increase in enzyme activity, however, was different and was inversely related to the level of expression in untreated hepatocytes, with the greatest increases being obse rved in the hepatocytes that expressed the lowest basal level of CYP3A, Pre treatment of hepatocytes with triacetyloleandomycin resulted in a 90% inhib ition of testosterone 6 beta-hydroxylase activity. Our results demonstrate the use of human hepatocyte cultures to investigate the induction of cytoch rome P-450 by xenobiotics in intact cells and stress the importance of larg e dose-response studies as well as the need to assess toxicity in these inv estigations. The response to inducers of CYP3A activity were very consisten t among different hepatocyte donors. Absolute values of testosterone 6 beta -hydroxylase activity did not vary more than 2- and 5-fold in induced and u ntreated hepatocytes, respectively.