Induction of CYP2C genes in human hepatocytes in primary culture

The expression and inducibility of four CYP2C genes, including CYP2C8, -2C9 , -2C18, and -2C19, was investigated in primary cultures of human hepatocyt es. By the use of RNase protection assay and specific antibodies, each CYP2 C mRNA and protein were quantified unequivocally. The four CYP2C mRNAs were expressed in human livers and cultured primary hepatocytes, but only the C YP2C18 protein was not detected. Compounds known to activate the pregnane X receptor (PXR) such as rifampicin, or the constitutively activated recepto r (CAR) such as phenobarbital, induced CYP2C8, CYP2C9, and to a lesser exte nt CYP2C19 mRNAs and proteins. CYP2C18 mRNA was expressed but not inducible . The concentration dependence of CYP2C8 and CYP2C9 mRNAs in response to ri fampicin and phenobarbital paralleled that of CYP3A4 and CYP2B6, the maximu m accumulation being reached with 10 muM rifampicin and 100 muM phenobarbit al. In contrast, dexamethasone produced maximum induction of CYP2C8 and CYP 2C9 mRNAs at 0.1 muM while in these conditions neither CYP3A4 nor CYP2B6 wa s significantly induced. Moreover, the concentration dependence of CYP2C8 a nd CYP2C9 mRNAs in response to dexamethasone paralleled that of tyrosine am inotransferase. Furthermore, dexamethasone, which has been recently shown t o up-regulate PXR and CAR expression through the glucocorticoid receptor, p otentiated CYP2C8 and CYP2C9 mRNA induction in response to rifampicin and p henobarbital. Collectively, these results suggest the possible implication of at least three receptors in the regulation of CYP2C8 and CYP2C9 expressi on, i.e., glucocorticoid receptor, PXR, and/or CAR.