Estrogen enhances induction of cytochrome P-4501A1 by 2,3,7, 8-tetrachlorodibenzo-p-dioxin in liver of female Long-Evans rats

Isoform-specific regulation of cytochrome P450 expression and activity by estradiol in female rats

Estradiol (E2) is the major endogenous estrogen, and its plasma concentration increases up to 100-fold during pregnancy in humans. Accumulating evidence suggests that an elevated level of E2 may influence hepatic drug metabolism, potentially being responsible for altered drug metabolism during pregnancy. We characterized effects of E2 on expression and activities of cytochrome P450 enzymes (CYPs) in an in vivo system using rats. To this end, female rats were treated with estradiol benzoate (EB) or known CYP inducers. Liver tissues were obtained after 5 days of treatment, and mRNA and protein expression levels as well as activities of major hepatic CYPs were determined by qRT-PCR, immunoblot, and microsomal assay. E2 increased CYP1A2 expression and activity to a smaller extent than β-naphthoflavone did. E2 also enhanced CYP2C expression (CYP2C6, CYP2C7, and CYP2C12) to levels comparable to those observed by phenobarbital. E2 upregulated CYP3A9 expression, while expression of CYP3A1 was downregulated. Expression of hepatic nuclear receptors (PXR and CAR) and the obligate redox partner of CYPs (POR) was downregulated in EB-treated rats, suggesting their potential involvement in regulation of CYP expression and activity by E2. In summary, in female rats E2 regulates expression of hepatic CYPs in a CYP isoform-specific manner although the directional changes are different from those clinically observed during human pregnancy. Further study is warranted to determine whether the changes in drug metabolism during human pregnancy are attributable to involvement of hormones other than E2.

Cytochrome P450 CYP1A1: wider roles in cancer progression and prevention

CYP1A1 is one of the main cytochrome P450 enzymes, examined extensively for its capacity to activate compounds with carcinogenic properties. Continuous exposure to inhalation chemicals and environmental carcinogens is thought to increase the level of CYP1A1 expression in extrahepatic tissues, through the aryl hydrocarbon receptor (AhR). Although the latter has long been recognized as a ligand-induced transcription factor, which is responsible for the xenobiotic activating pathway of several phase I and phase II metabolizing enzymes, recent evidence suggests that the AhR is involved in various cell signaling pathways critical to cell cycle regulation and normal homeostasis. Disregulation of these pathways is implicated in tumor progression. In addition, it is becoming increasingly evident that CYP1A1 plays an important role in the detoxication of environmental carcinogens, as well as in the metabolic activation of dietary compounds with cancer preventative activity. Ultimately the contribution of CYP1A1 to cancer progression or prevention may depend on the balance of procarcinogen activation/detoxication and dietary natural product extrahepatic metabolism.

Activation of the aryl hydrocarbon receptor during different critical windows in pregnancy alters mammary epithelial cell proliferation and differentiation

Exposure to the aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during pregnancy causes severe defects in mammary gland development and function; however, the underlying mechanism remains unclear. Alterations in epithelial cell proliferation, differentiation, and apoptosis during pregnancy-related mammary development can lead to failed lactogenesis. To determine which of these processes are affected and at what time periods, we examined proliferation, differentiation and apoptosis in mammary glands following exposure to TCDD during early, mid or throughout pregnancy. Although AhR activation throughout pregnancy did not cause early involution, there was a 50% decrease in cell proliferation, which was observed as early as the sixth day of pregnancy (DP). TCDD treatment on the day of impregnation only reduced development and proliferation in early and mid-pregnancy, followed by partial recovery by DP17. However, when AhR activation was delayed to DP7, developmental impairment was not observed in mid-pregnancy, but became evident by DP17, whereas proliferation was reduced at all times. Thus, early exposure to TCDD was neither necessary nor sufficient to cause persistent defects in lactogenesis. These varying outcomes in mammary development due to exposure at different times in pregnancy suggest there are critical windows during which AhR activation impairs mammary epithelial cell proliferation and differentiation.

3-Methylcholanthrene and other aryl hydrocarbon receptor agonists directly activate estrogen receptor alpha

3-Methylcholanthrene (3MC) is an aryl hydrocarbon receptor (AhR) agonist, and it has been reported that 3MC induces estrogenic activity through AhR-estrogen receptor alpha (ER alpha) interactions. In this study, we used 3MC and 3,3',4,4',5-pentachlorobiphenyl (PCB) as prototypical AhR ligands, and both compounds activated estrogen-responsive reporter genes/gene products (cathepsin D) in MCF-7 breast cancer cells. The estrogenic responses induced by these AhR ligands were inhibited by the antiestrogen ICI 182780 and by the transfection of a small inhibitory RNA for ER alpha but were not affected by the small inhibitory RNA for AhR. These results suggest that 3MC and PCB directly activate ER alpha, and this was confirmed in a competitive ER alpha binding assay and in a fluorescence resonance energy transfer experiment in which PCB and 3MC induced CFP-ER alpha/YFP-ER alpha interactions. In a chromatin immunoprecipitation assay, PCB and 3MC enhanced ER alpha (but not AhR) association with the estrogen-responsive region of the pS2 gene promoter. Moreover, in AhR knockout mice, 3MC increased uterine weights and induced expression of cyclin D1 mRNA levels. These results show that PCB and 3MC directly activate ER alpha-dependent transactivation and extend the number of ligands that activate both AhR and ER alpha.

Aryl hydrocarbon receptor-mediated transcription: ligand-dependent recruitment of estrogen receptor alpha to 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive promoters

Using chromatin immunoprecipitation assays, we studied the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated recruitment of the aryl hydrocarbon receptor (AhR) and several co-regulators to the CYP1A1 promoter. AhR displayed a time-dependent recruitment, reaching a peak at 75 min and maintaining promoter occupancy for the remainder of the time course. Recruitment of AhR was followed by TIF2/SRC2, which preceded CBP, histone H3 acetylation, and RNA polymerase II (RNAPII). Simultaneous recruitment to the enhancer and the TATA box region suggests the formation of a large multiprotein complex bridging the two promoter regions. Interestingly, estrogen receptor alpha (ERalpha) displayed a TCDD- and time-dependent recruitment to the CYP1A1 promoter, which was increased by co-treatment with estradiol. Transfection in HuH7 human liver cells confirmed previously reported ERalpha enhancement of AhR activity. In contrast, TCDD did not induce the recruitment of ERalpha to the estrogen-responsive pS2 promoter, and after 120 min of co-treatment with estradiol, ERalpha is still present on the CYP1A1 promoter but no longer at pS2. RNA interference studies with T47D cells support a role for ERalpha in TCDD-dependent CYP1A1 expression. Our data suggest that ERalpha acts as a coregulator of AhR-mediated transcriptional activation and that the recruitment of ERalpha by AhR represents a novel mechanism AhR-ERalpha cross talk.