Cytochrome P450 3A4/5 enzymes (CYP3A4/5) collectively metabolize more drugs than any other cytochrome P450 enzymes. Our laboratory has found that the oral clearance of the HIV protease inhibitor, indinavir (a CYP3A substrate), is much greater during pregnancy than post-partum  and we have shown that CYP3A activity (as measured by midazolam oral clearance) is induced in pregnant women . The increased clearance of the protease inhibitors observed in pregnant women was reproduced in mice and was found to be due to increased Cyp3a mRNA/protein expression in the liver  and positively correlated with higher hepatic mRNA levels of the transcription factors HNF6
(estrogen receptor alpha), which are respectively major players in the growth hormone (GH) and estrogen signaling pathways. Based on the above data, we have hypothesized that, during human pregnancy, the increase in plasma concentration of placental growth hormone (PGH), which is secreted by the placenta and gradually replaces the pituitary GH, progesterone, estrogens (primarily 17β-estradiol), and corticosteroids activates a network of nuclear receptors and transcriptional factors to induce hepatic CYP3A activity. To test our hypothesis we incubated these hormones with sandwich-cultured human hepatocytes from healthy women (post- or premenopausal), using the total or 10X the maximum total plasma concentration of each hormone observed during human pregnancy. Primary cultures of human hepatocytes were incubated for 72 hrs with or without the pregnancy hormone(s). At the end of the incubation, CYP3A activity was measured using midazolam hydroxylation. Additionally we quantified using quantitative Real-Time PCR (qPCR), the mRNA expression of CYP3A4 and 3A5 and the respective hormone receptors: glucocorticoid receptor, GR
; estrogen receptor alpha, ERα
, and growth hormone receptor, GHR
. Our results show that 17β-estradiol or cortisol significantly induced CYP3A activity (~4-6 fold in premenopausal donors at the total concentration of each hormone) and CYP3A4
mRNA (~12-50 fold in postmenopausal and ~8-9 fold in premenopausal donors) expression in human hepatocytes. However, when the hormones were incubated in combination, they remarkably induced CYP3A activity (~12-fold in premenopausal donors at the total concentration of each hormone) and CYP3A4
mRNA expression (~70-fold). In addition, qPCR analysis of the mRNA levels of the hormone receptors, revealed only a modest increase in ERα
transcript levels (~3-6 fold in pre- and postmenopausal donors respectively) in the presence of estradiol, while there was no biologically significant increase in the transcript levels of the GHR or GR. These data implicate cortisol and 17β-estradiol as mediators of hepatic CYP3A induction during pregnancy and provide important leads regarding the molecular mechanisms of this induction.
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