Polychlorobiphenyls are potent inducers of hepatic
cytochrome P450 in various species. Until now, no model based on cultured cells can be considered as a universal surrogate for in vivo metabolism. In this respect, cultured rat hepatocytes, quail hepatocytes, and human
hepatoma (HepG2) cells were used to study the effects of
3,3',4,4'-tetrachlorobiphenyl (3,3',4,4'-TCB) and
Aroclor 1254 on
drug-metabolizing
enzymes. The presence of
dexamethasone in the culture medium allows the expression and the induction of several
cytochrome P450 isoenzymes found in adult cells. Induction of ethoxycoumarin-(ECOD) and
ethoxyresorufin-O-deethylase (
EROD), activities were measured. Induced P450s were identified by immunoblotting and Northern blotting.
Aroclor 1254 induced ECOD activity in all three cell types, but the effect was much stronger in fetal rat hepatocytes than in human or quail cells.
Aroclor failed to induce
EROD activity in quail cells, had a slight inducer effect in HepG2 cells, and a marked effect in rat hepatocytes. 3,3',4,4'-TCB had no effect in HepG2 cells but significantly increased
EROD and ECOD activities, especially the latter, in rat and quail cells. On the immunoblots, specific
antibodies revealed essentially
CYP1A1 in fetal rat hepatocytes,
CYP2B1/2 in quail hepatocytes and CYP3A1 in HepG2 cells. Analysis of Northern blots showed an hybridization with
CYP1A1, 2B1 and 3A1
mRNA in fetal rat hepatocytes,
CYP3A and 1A
mRNA in HepG2 cells, and a form of CYP2
mRNA in fetal quail hepatocytes closely related to homolog rat CYP2E or
CYP2C. In quail hepatocytes, induction did not increase proportionally with the concentration of inducer in the culture medium. Instead, the dose-response curves (for
EROD activity especially) peaked sharply at 1 muM
Aroclor 1254, an effect attributed to changes in membrane fluidity or
lipid content. Our results highlight the advantage of using several types of cultured hepatocytes to investigate fundamental aspects of
drug-metabolism-linked toxicity, the balance between
xenobiotic bioactivation and detoxication being differently affected by
PCBs in different animal species.