Serum alters the uptake and relative potencies of halogenated aromatic hydrocarbons in cell culture bioassays

The effects of many chemicals on cellular processes are governed by their a bility to enter the cell, which is in turn a function of the composition of the cell's external environment. To examine this relationship, the effect of serum in cell culture medium on the bioavailability of cytochrome P450 1 A (CYP1A)-inducing compounds was determined in PLHC-1 (Poeciliopsis lucida hepatocellular carcinoma) cells. The presence of 10% calf serum in the medi um increased the EC50 (effective concentration to achieve 50% maximal respo nse) for induction of ethoxyresorufin O-deethylase (EROD) activity by 2,3,7 ,8-tetrachlorodibenzo-p-dioxin (TCDD) 20-fold as compared to treatment in s erum-free medium. Measurement of [H-3]TCDD uptake and Ah receptor binding i ndicated that the apparent difference in potencies was a result of decrease d bioavailability in the presence of serum, effectively reducing the concen tration of TCDD within the cells. Induction of EROD and CYP1A protein in re sponse to treatment with each of three coplanar polychlorinated biphenyls ( PCB congeners 77, 126, and 169) was similarly affected by serum, although t he magnitude varied among inducers and assays. Relative potencies (calculat ed as EC50(TCDD) / EC50(PCB)) for EROD induction by the three PCBs were sig nificantly higher in the absence of serum. However, serum showed no signifi cant effect on the relative potencies for CYP1A protein induction. These re sults demonstrate that measured inducing potencies, and relative potencies for EROD induction, by halogenated aromatic hydrocarbons are strongly depen dent on the composition of culture medium, which can lead to artificial dif ferences in comparisons among cell types.