Modulation of MDR1 and CYP3A expression by dexamethasone: evidence for an inverse regulation in adrenals.

A strong overlap exists between gp170 and CYP3A substrates and inducers. In order to investigate a putative coregulation of MDR and CYPA gene expression, we measured their transcripts in human liver and after dexamethasone treatment in HepG2 cells or in different mouse tissues. In human liver, we observed no correlation between MDR1 and CYP3A4 expression, whereas these genes were coinduced by dexamethasone in HepG2 cells. In mouse liver treated with dexamethasone, mdr1b and Cyp3a were induced (5- and 2-fold, respectively). In adrenals, the main expressing gp170 tissue, Cyp3a, was increased while mdr1b was repressed (-51%). The expression of mdr1b increased in heart, brain, and colon and decreased in lung and kidney but Cyp3a was not detectable. In conclusion, human hepatic CYP3A4 and MDR1 are not corregulated but are coinducible. In vivo murine mdr1b and Cyp3a are coregulated by dexamethasone in liver and inversely regulated in adrenals.