Abstract
Human HepG2 hepatoma cells are used widely as a replicating cell system that expresses numerous liver-specific functions, including those associated with the biotransformation of foreign chemicals. In the work presented here the ability of this cell line to reproduce the in vivo modulation of CYP3A4, CYP3A5, CYP3A7, hGRa and hPXR gene expression by classical xenobiotic cytochrome P450 3A isoenzyme inducers was studied. Transfection experiments using a plasmid construct containing 1 kb of the proximal region of CYP3A4 driving an alkaline phosphatase reporter cDNA did not reliably reproduce the in vivo activation of this promoter by dexamethasone or rifampicin even after co-transfection with hGRa or hPXR expression vectors. RT-PCR experiments were performed as an alternative technique for direct investigation of CYP3A and hPXR mRNA induction. The results showed that HepG2 cells constitutively express hGRa, hPXR, RXR, SRC-1, HNF-4, CYP3A5 and CYP3A7 but not CYP3A4 mRNAs. This is the first time that the expression of CYP3A5 mRNA has been identified in HepG2 cells. The results also demonstrated that the expression of hPXR, CYP3A5 and CYP3A7 was induced by dexamethasone or rifampicin in cells at passages 5 and 10 (after resuscitation from storage) but not at passages 15 and 20, unless the cells were dosed in serum-free medium. The results are in agreement with induction experiments performed by other authors and show that HepG2 cells can be used as a suitable in vitro system for the study of CYP3A gene induction by xenobiotics. However, the pattern of gene expression more closely resembles that in the human foetal liver and thus HepG2 cells could be of most use as an in vitro model of foetal drug metabolism and gene modulation.