Effect of hypophysectomy and growth hormone replacement on the modulation of p450 expression after treatment with the aromatic hydrocarbon ethylbenzene

Cytochrome P450 2C11 5'-flanking region and promoter mediate in vivo suppression by 3-methylcholanthrene

Aromatic hydrocarbons such as 3-methylcholanthrene (MC) elicit toxic and adaptive responses through the aryl hydrocarbon receptor (AHR). Aromatic hydrocarbons act via an unknown mechanism to suppress the transcription of CYP2C11, a growth hormone-regulated gene encoding the male-specific rat hepatic cytochrome P450 2C11. We hypothesize that suppression of CYP2C11 by aromatic hydrocarbons is mediated by the gene's promoter and 5'-flank. Using hydrodynamics-based injections to deliver plasmid DNA to the liver of live rats, we studied the MC responsiveness of luciferase constructs containing 10.1, 5.6, and 2.4 kilobases (kb) of the CYP2C11 5'-flank. MC suppressed CYP2C11-luciferase activity of the 10.1- and 5.6-kb constructs to less than 50% of vehicle levels by 24 and 72 h. Luciferase activity of the 2.4-kb CYP2C11 construct was decreased to 63% of vehicle levels 24 h after MC treatment, but no suppression was detected by 72 h. Negative regulatory element(s) responsible for CYP2C11 reporter suppression by MC exist in the proximal 2.4 kb of the 5'-flank; however, additional cis-acting elements located between -5.6 and -2.4 kb mediate persistent reporter suppression. As a positive control for AHR activation, MC dramatically induced the luciferase activity of a Cyp1a1-driven luciferase plasmid under AHR control. Modulation of reporter gene activity by MC was accompanied by induction of endogenous CYP1A1 and suppression of endogenous CYP2C11 mRNA/protein. This is the first demonstration of aromatic hydrocarbon-mediated suppression of a CYP2C11-luciferase construct, and this finding suggests that the 5'-flanking region and promoter mediate down-regulation of this gene in the intact rat.

Cytochrome P450 2C11 5'-flanking region and promoter: regulation by aromatic hydrocarbons in vitro

Aromatic hydrocarbons elicit toxic and adaptive responses via the aryl hydrocarbon receptor (AHR). Aromatic hydrocarbons suppress the transcription of the growth hormone-regulated, male-specific rat hepatic cytochrome P450 2C11 gene (CYP2C11) in vivo via an unknown mechanism. We hypothesize that the suppression of CYP2C11 by aromatic hydrocarbons is mediated by the gene's promoter and 5'-flanking region. Following bioinformatic analysis of putative transcription factor (TF) binding sites, we cloned extended lengths of the CYP2C11 5'-flanking region into a promoterless luciferase plasmid. Suppression of CYP2C11 constructs was not observed upon treatment of transfected rat 5L, BP8 or mouse Hepa-1 cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3-methylcholanthrene. In human HepG2 cells, the 10.1-kb construct displayed a pronounced 6- to 8-fold induction by TCDD. Deletion analysis localized the paradoxical induction response to a region between -1.8 kb and -1.3 kb, which contains a dioxin-responsive element (DRE) previously shown by us to be capable of binding activated AHR. This was confirmed by site-directed mutagenesis of the DRE. Induction of the 10.1-kb construct by TCDD in HepG2 cells was blocked by alpha-naphthoflavone, an AHR antagonist/partial agonist. The AHR is likely involved in the induction of CYP2C11-luciferase activity by TCDD in HepG2 cells and this response is at least partly DRE-mediated. Although CYP2C11 is suppressed by aromatic hydrocarbons in vivo, CYP2C11-luciferase constructs display a potentially misleading paradoxical induction in vitro that is cell-specific. Regulation of CYP2C11-luciferase plasmids is being studied in vivo in rat liver, where an intact endocrine system and the full complement of TFs needed for CYP2C11 suppression are present.

Transcriptional suppression of cytochrome P450 genes by endogenous and exogenous chemicals

This article is an invited report of a symposium sponsored by the Division for Drug Metabolism of the American Society for Pharmacology and Experimental Therapeutics held at Experimental Biology 2003 in San Diego, California, April 11-15, 2003. Several members of the cytochrome P450 (P450) superfamily are induced after exposure to a variety of chemical signals, and we have gained considerable mechanistic insight into these processes over the past four decades. In addition, the expression of many P450s is suppressed in response to various endogenous and exogenous chemicals; however, relatively little is known about the molecular mechanisms involved. The goal of this symposium was to critically examine our current understanding of molecular mechanisms involved in transcriptional suppression of CYP genes by endogenous and exogenous chemicals. Specific examples were drawn from the following chemical categories: polycyclic and halogenated aromatic hydrocarbon environmental toxicants, inflammatory mediators, the endogenous sterol dehydroepiandrosterone and peroxisome proliferators, and bile acids. Multiple molecular mechanisms are involved in transcriptional suppression, and these processes often involve rather complex cascades of transcription factors and other regulatory proteins. Mechanistic studies of CYP gene suppression can enhance our understanding of how organisms respond to xenobiotics as well as to perturbations in endogenous chemicals involved in maintaining homeostasis.

The 2001 Veylien Henderson Award of the Society of Toxicology of Canada. Positive and negative transcriptional regulation of cytochromes P450 by polycyclic aromatic hydrocarbons

Most responses to aromatic hydrocarbons such as 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin are mediated by the aromatic hydrocarbon receptor (AHR). The AHR regulates induction of drug-metabolizing enzymes such as cytochrome P450 1A1. However, the expression of several genes of biological significance is decreased by these chemicals. We are examining the mechanisms by which aromatic hydrocarbons suppress constitutive hepatic cytochromes P450, especially the male-specific rat liver cytochrome P450 2C11 (CYP2C11), which is regulated by pulsatile growth hormone (GH) secretion. Aromatic hydrocarbons suppress CYP2C11 via a transcriptional mechanism both in vivo and in cultured hepatocytes, and the AHR appears to be involved; however, studies of protein-DNA interactions and reporter genes driven by the CYP2C11 5'-flanking region have not provided a definitive mechanism for this response. MC attenuates the ability of GH to stimulate hepatic CYP2C11 expression in hypophysectomized (hypx) male rats, and this prompted studies of effects of aromatic hydrocarbons on hepatic GH signaling pathways as a novel aspect of endocrine disruption. Our studies with hypx rats also suggest that the hepatic AHR protein is regulated by a pituitary factor(s). The goal of these molecular mechanistic studies is to improve our understanding of how environmental contaminants modulate the expression of genes coding for xenobiotic- and hormone-metabolizing enzymes.

Altered ethylbenzene-mediated hepatic CYP2E1 expression in growth hormone-deficient dwarf rats

Ethylbenzene (EB) effectively induces several hepatic P450 enzymes including CYP2E1 and CYP2B. Hypophysectomy diminishes the magnitude of EB-mediated induction of CYP2B. Although growth hormone (GH) plays a key role in sexual dimorphism of CYP2C11, its impact on EB-mediated P450 expression is still unknown. Because hypophysectomy leads to a depletion of multiple pituitary hormones besides GH, a study was designed to investigate the possible involvement of GH in EB-mediated hepatic P450 expression using GH-deficient dwarf rats as a more specific animal model. In these rats, pituitary GH was selectively reduced to about 10% of normal levels and other pituitary trophic hormones including thyroid-stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, and prolactin are largely unchanged. Male control and HsdOla:DWARF-dw-4 (Harlan, UK) rats were subjected to a single ip injection of EB (10 mmol/kg). CYP2E1- and CYP2B-dependent activities, protein, and RNA levels were measured 10 and 24 h afterward. The results indicated that dwarf rats without EB exposure expressed higher CYP2E1. Although EB treatment induced CYP2E1 activity, protein, and mRNA both in controls and dwarf rats, the magnitude of the response to EB exposure was greater 10 h after the treatment in dwarf rats. Hypophysectomy also increased CYP2E1 protein induction by EB compared to intact rats. This effect was reversed by GH supplementation to hypophysectomized rats. Overall, responses of CYP2B to EB exposure in dwarf rats did not display basic differences from controls. In conclusion, the results demonstrate that (1) the suppression of CYP2B induction found in the multi-hormone-deficient HX rats is not found in the more specific GH-deficient rat model, confirming that GH does not have a major influence on CYP2B expression and (2) both hypophysectomized and GH-deficient rats show an altered inducibility of CYP2E1 after EB treatment.