Use of established hepatoma cell lines in biotoxicology

A reporter gene assay to assess the molecular mechanisms of xenobiotic-dependent induction of the human CYP3A4 gene in vitro

1. A plasmid containing 1 kb of the CYP3A4 regulatory (promoter) region coupled to a reporter gene for secretary placental alkaline phosphatase (SPAP) was transfected into HepG2 cells. Transfected cells were dosed with several known inducers of CYP3A4 and the levels of SPAP were measured. The effect of co-transfecting a plasmid encoding the human glucocorticoid receptor on reporter gene activity was also examined. 2. Dexamethasone induced CYP3A4-dependent reporter gene expression in a concentration-dependent manner and induction was approximately doubled in the presence of the glucocorticoid receptor. Dexamethasone-dependent induction was blocked by RU-486 (a glucocorticoid receptor antagonist), in the presence of the co-transfected glucocorticoid receptor. 3. Induction of CYP3A4-dependent reporter gene expression and enhancement of the induction by the glucocorticoid receptor was also observed with pregnenolone-16alpha-carbonitrile (PCN), rifampicin, phenytoin, carbamazepine, phenylbutazone and phenobarbitone, all known in vivo inducers of CYP3A4 in man. 4. Metyrapone and sulfinpyrazone induced CYP3A4-dependent reporter gene expression, but induction was not enhanced by the glucocorticoid receptor. 5. Clotrimazole, erythromycin and triacetyloleandomycin (TAO) did not induce CYP3A4-dependent reporter gene expression, consistent with the observation that these inducers act through post-transcriptional mechanisms. 6. These results highlight differences in the molecular mechanisms of induction of CYP3A4 by the xenobiotics studied and indicate that the glucocorticoid receptor is involved in the induction of the CYP3A4 gene by some, but not all, CYP3A4 inducers. 7. We propose that the approach described here provides a useful in vitro approach for the identification of transcriptional regulators of the CYP3A4 gene.

The use of in vitro metabolism techniques in the planning and interpretation of drug safety studies

An important issue in toxicology is the suitability of the data obtained with experimental animals for human risk assessment. Because it is not possible to use humans in long-term toxicological studies, the use of animals will continue. However, the data obtained in animal studies can be better extrapolated to the patient by utilizing bridging studies with in vitro models of human drug metabolism. There are 2 basic categories of in vitro methods for the examination of human liver drug metabolism. The first group of in vitro methods consists of the cellular models, which include primary hepatocytes, liver slices, and cell lines. The second group is the use of preparations of the drug-metabolizing enzymes such as tissue homogenates, subcellular fractions, and isolated enzymes. Studies modeling both the human and experimental animal metabolism of a drug are useful in the design of toxicological studies. In vitro studies can identify metabolites, species-specific metabolic routes, and the experimental animal model that best reflects the potential human exposure to the drug and its metabolites. This information can also be useful in the design of the clinical studies by identifying human metabolites, the enzymes responsible for the metabolic clearance of the drug, the effects of genetics and other host factors on the metabolism of the drug, and potential drug-drug interactions. An example of how such information is generated and interpreted is presented.