Regulation of cytochrome P4501A1 expression by steroid compounds

Abstract

Cytochrome P4501A1 inducibility of estradiol and progesterone was investigated in mouse hepatoma Hepa 1c1c7 cells. Estradiol and progesterone increased cytochrome P4501A1-specific 7-ethoxyresorufin O-deethylase (EROD) activity at the concentrations higher than 1μM. This induction was dose- and time-related. At low concentrations less than 1μM, neither estradiol nor progesterone had EROD inducibility. To determine the possible involvement of steroid hormone receptors in the induction hormone receptor antagonists were treated together with steroids. Tamoxifen, an antagonist for estrogen receptor, could not recover the action of estradiol and RU486, an antagonist for progesterone receptor, also could not alleviate the action of progesterone. These results indicated that the EROD induction by estradiol and progesterone was independent of their cognate sex hormone receptors. In order to investigate the mechanism responsible for the EROD induction, immunoblot and northern blot analysis were performed. The increase in EROD activity by sex steroids was due to the increase in the amounts of corresponding cytochrome P4501A1 protein and CYP1A1 mRNA. Therefore, it was speculated that the CYP1A1 induction by sex steroids was dependent on transcriptional of CYP1A1 gene. Transient transfection assay using CYP1A1-CAT reporter plasmid containing 5``-upstream regulatory region of CYP1A1 gene showed that the inserted 482 bp fragment had sufficient regulatory sequences necessary for the CYP1A1 induction by sex steroids. To identify the minimal sequences responsible for the induction, electrophoretic mobility shift assay was performed with the oligonucleotide of dioxin responsive element (DRE), which has been known to be most important in up-regulation of CYP1A1 expression. Estradiol and progesterone stimulated the formation of DRE-aromatic hydrocarbon receptor (AhR) complexes in Hepa 1c1c7 cell cultures. AhR was shown to be directly activated by binding of estradiol and progester...