Drug-mediated regulation of mRNA expression of all members of the cytochrome P450 3A (CYP3A) subfamily has been measured by reverse transcription-polymerase chain reaction (RT-PCR) in the human hepatocellular carcinoma cell line, HepG2. Transcriptional regulation was proved by inhibition of induction with actinomycin D. Besides the positive control dexamethasone, the H(+)/K(+)-ATPase inhibitors omeprazole, lansoprazole, pantoprazole, and rabeprazole, and the herbal antidepressant St. John's wort (Hypericum extract) were studied. All CYP3A mRNAs were induced by dexamethasone. CYP3A4 was the only CYP3A isoform that was induced by all of the four benzimidazole derivatives, while CYP3A5, CYP3A7, and CYP3A43 were unaffected or even slightly downregulated by these drugs. St. John's wort also increased CYP3A4 mRNA exclusively, leaving CYP3A5 and CYP3A43 unaffected, whereas CYP3A7 was decreased. Depending on the inducer, expression of CYP3A4 is differently regulated from CYP3A5, CYP3A7, and CYP3A43.
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