cDNA cloning, sequence analysis, and induction by aryl hydrocarbons of a murine cytochrome P450 gene, Cyp1b1

Structure-Based Design and Synthesis of New Estrane-Pyridine Derivatives as Cytochrome P450 (CYP) 1B1 Inhibitors

Inhibition of cytochrome P450 (CYP) 1B1 is a promising therapeutic strategy, as such an inhibitor could modulate the bioactivation of procarcinogens while reducing drug resistance. Based on docking studies, the synthesis of 12 estra-1,3,5(10)-triene derivatives containing a pyridin-3-/4-yl moiety at position C2, C3, or C4 was performed, and we measured their inhibitory activity on CYP1B1 using the ethoxyresorufin-O-deethylase (EROD) assay. The position of the nitrogen atom in the aromatic ring has little influence on their inhibition potency, but compounds with a pyridinyl at C2 of the steroid nucleus are more potent CYP1B1 inhibitors than those with a pyridinyl at C3 or C4. Estradiol derivatives (OH at C17β) are also 10-fold more potent inhibitors than estrone derivatives (carbonyl at C17). Thus, 2-(pyridin-3-yl)-estradiol (4a) is the best CYP1B1 inhibitor (IC₅₀ = 0.011 μM) from this series of compounds, and the best steroid inhibitor reported until now. It is also 7.5-fold more potent than the well-known nonsteroidal CYP1B1 inhibitor α-naphthoflavone (IC₅₀ = 0.083 μM).

Targeting cytochrome P450 (CYP) 1B1 with steroid derivatives

Inhibition of cytochrome P450 1B1 (CYP1B1) represents a promising therapeutic strategy, because it would enable action at three different levels: (1) by inhibiting the formation of mutagenic 4-hydroxy-estradiol, (2) by inhibiting the bioactivation of procarcinogens, and (3) by reducing drug-resistance. Surprisingly, few steroids were reported as inhibitors of CYP1B1. From a screening performed with 90 steroid derivatives, we identified thioestrone (B19) as an inhibitor (IC₅₀=3.4μM) of CYP1B1. Molecular modeling studies showed that the 3-SH group of B19 is closer (3.36Å) to the iron atom of the heme system than the 3-OH group of enzyme substrates estrone and estradiol (4.26Å and 3.58Å, respectively). B19 also produced a better docking GOLD score that correlated with the inhibitory results obtained. The estrane derivative B19 represents an interesting lead compound that can be easily modified to extend the structure-activity relationship study and to provide a next generation of more powerful CYP1B1 inhibitors.

Aryl hydrocarbon receptor knockout rats are insensitive to the pathological effects of repeated oral exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Sustained activation of the aryl hydrocarbon receptor (AHR) is believed to be the initial key event in AHR receptor-mediated tumorigenesis in the rat liver. The role of AHR in mediating pathological changes in the liver prior to tumor formation was investigated in a 4-week, repeated-dose study using adult female wild-type (WT) and AHR knockout (AHR-KO) rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Beginning at 8 weeks of age, AHR-KO and WT rats were dosed by oral gavage with varying concentrations of TCDD (0, 3, 22, 100, 300 and 1000 ng kg(-1)  day(-1) ). Lung, liver and thymus histopathology, hematology, serum chemistry and the distribution of TCDD in liver and adipose tissue were examined. Treatment-related increases in the severity of liver and thymus pathology were observed in WT, but not AHR-KO rats. In the liver, these included hepatocellular hypertrophy, bile duct hyperplasia, multinucleated hepatocytes and inflammatory cell foci. A loss of cellularity in the thymic cortex and thymic atrophy was observed. Treatment-related changes in serum chemistry parameters were also observed in WT, but not AHR-KO rats. Finally, dose-dependent accumulation of TCDD was observed primarily in the liver of WT rats and primarily in the adipose tissue of AHR-KO rats. The results suggest that AHR activation is the initial key event underlying the progression of histological effects leading to liver tumorigenesis following TCDD treatment. Copyright © 2015 John Wiley & Sons, Ltd.

Knockout of the aryl hydrocarbon receptor results in distinct hepatic and renal phenotypes in rats and mice

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor which plays a role in the development of multiple tissues and is activated by a large number of ligands, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to examine the roles of the AHR in both normal biological development and response to environmental chemicals, an AHR knockout (AHR-KO) rat model was created and compared with an existing AHR-KO mouse. AHR-KO rats harboring either 2-bp or 29-bp deletion mutation in exon 2 of the AHR were created on the Sprague-Dawley genetic background using zinc-finger nuclease (ZFN) technology. Rats harboring either mutation type lacked expression of AHR protein in the liver. AHR-KO rats were also insensitive to thymic involution, increased hepatic weight and the induction of AHR-responsive genes (Cyp1a1, Cyp1a2, Cyp1b1, Ahrr) following acute exposure to 25 μg/kg TCDD. AHR-KO rats had lower basal expression of transcripts for these genes and also accumulated ~30-45-fold less TCDD in the liver at 7 days post-exposure. In untreated animals, AHR-KO mice, but not AHR-KO rats, had alterations in serum analytes indicative of compromised hepatic function, patent ductus venosus of the liver and persistent hyaloid arteries in the eye. AHR-KO rats, but not AHR-KO mice, displayed pathological alterations to the urinary tract: bilateral renal dilation (hydronephrosis), secondary medullary tubular and uroepithelial degenerative changes and bilateral ureter dilation (hydroureter). The present data indicate that the AHR may play significantly different roles in tissue development and homeostasis and toxicity across rodent species.

Effects of triclocarban on the transcription of estrogen, androgen and aryl hydrocarbon receptor responsive genes in human breast cancer cells

Triclocarban (TCC) is an antimicrobial agent that is used in detergents, soaps and other personal hygiene products. Similarly to triclosan the widespread use of TCC has raised concerns about its endocrine potential. In luciferase-based reporter assays TCC has been shown to enhance estrogenic and androgenic activities following cellular coexposure with estrogen or dihydrotestosterone, respectively. The present study demonstrates that although coexposure with TCC enhances the estrogenic and androgenic readout of luciferase-based reporter cell lines such as HeLa9908 and MDA-kb2, it fails to act as a xenoandrogen on transcriptional level, nor does it induce cell proliferation in the estrogen sensitive E-screen. In addition TCC did not alter the expression of estrogen responsive genes in human mammary carcinoma MCF-7 cells exposed to 17β-estradiol, bisphenol A, butylparaben or genistein. However, TCC was shown to interfere with the regulon of the aryl hydrocarbon receptor (AhR) as TCC showed a costimulatory effect on transcription of CYP1A1 and CYP1B1, effectively lowering the transcriptional threshold for both genes in the presence of estrogens. It thus seems, that while the induction of the respective luciferase reporter assays by TCC is an unspecific false positive signal caused by luciferase stabilisation, TCC has the potential to interfere with the regulatory crosstalk of the estrogen receptor (ER) and the AhR regulon.

2,3,7,8-Tetrachlorodibenzo-p-dioxin activates the aryl hydrocarbon receptor and alters sex steroid hormone secretion without affecting growth of mouse antral follicles in vitro

The persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an ovarian toxicant. These studies were designed to characterize the actions of TCDD on steroidogenesis and growth of intact mouse antral follicles in vitro. Specifically, these studies tested the hypothesis that TCDD exposure leads to decreased sex hormone production/secretion by antral follicles as well as decreased growth of antral follicles in vitro. Since TCDD acts through binding to the aryl hydrocarbon receptor (AHR), and the AHR has been identified as an important factor in ovarian function, we also conducted experiments to confirm the presence and activation of the AHR in our tissue culture system. To do so, we exposed mouse antral follicles for 96 h to a series of TCDD doses previously shown to have effects on ovarian tissues and cells in culture, which also encompass environmentally relevant and pharmacological exposures (0.1-100 nM), to determine a dose response for TCDD in our culture system for growth, hormone production, and expression of the Ahr and Cyp1b1. The results indicate that TCDD decreases progesterone, androstenedione, testosterone, and estradiol levels in a non-monotonic dose response manner without altering growth of antral follicles. The addition of pregnenolone substrate (10 μM) restores hormone levels to control levels. Additionally, Cyp1b1 levels were increased by 3-4 fold regardless of the dose of TCDD exposure, evidence of AHR activation. Overall, these data indicate that TCDD may act prior to pregnenolone formation and through AHR transcriptional control of Cyp1b1, leading to decreased hormone levels without affecting growth of antral follicles.

Cytochrome P450 1C1 complementary DNA cloning, sequence analysis and constitutive expression induced by benzo-a-pyrene in Nile tilapia (Oreochromis niloticus)

CYP1C is the newest member of the CYP1 family of P450s; however, its physiological significance, inducers, and metabolic functions are unknown. In this study, a new complementary DNA of the CYP1C subfamily encoding CYP1C1 was isolated from Nile tilapia (Oreochromis niloticus) liver after intracoelomic injection with benzo-a-pyrene (BaP). The full-length cDNA was 2223 base pair (bp) long and contained an open reading frame of 1581 bp encoding a protein of 526 amino acids and a stop codon. The sequence exhibited 3' non-coding region of 642 bp. The deduced amino acid sequence of O. niloticus CYP1C1 shows similarities of 86, 82.5, 79.7, 78.7, 77.8, 75.5, 69.6 and 61.3% with scup CYP1C1, killifish CYP1C1,1C2, Japanese eel CYP1C1, zebra fish CYP1C1, common carp CYP1C1, scup CYP1C2, common carp CYP1C2 and zebra fish CYP1C2, respectively. Phylogenetic tree based on the amino acids sequences clearly shows tilapia CYP1C1 and scup CYP1C1 to be more closely related to each other than to CYP1C genes from other species. Furthermore, for measuring BaP induction of CYP1C1 mRNA in different organs of tilapia (O. niloticus), β-actin gene as internal control was selected based on previous studies to assess their expression variability. Real time RCR results revealed that there was a large increase in CYP1C1 mRNA in liver (43.1), intestine (5.1) and muscle (2.4).

Transcriptional factor aryl hydrocarbon receptor (Ahr) controls cardiovascular and respiratory functions by regulating the expression of the Vav3 proto-oncogene

Aryl hydrocarbon receptor (Ahr) is a transcriptional factor involved in detoxification responses to pollutants and in intrinsic biological processes of multicellular organisms. We recently described that Vav3, an activator of Rho/Rac GTPases, is an Ahr transcriptional target in embryonic fibroblasts. These results prompted us to compare the Ahr(-/-) and Vav3(-/-) mouse phenotypes to investigate the implications of this functional interaction in vivo. Here, we show that Ahr is important for Vav3 expression in kidney, lung, heart, liver, and brainstem regions. This process is not affected by the administration of potent Ahr ligands such as benzo[a]pyrene. We also report that Ahr- and Vav3-deficient mice display hypertension, tachypnea, and sympathoexcitation. The Ahr gene deficiency also induces the GABAergic transmission defects present in the Vav3(-/-) ventrolateral medulla, a main cardiorespiratory brainstem center. However, Ahr(-/-) mice, unlike Vav3-deficient animals, display additional defects in fertility, perinatal growth, liver size and function, closure, spleen size, and peripheral lymphocytes. These results demonstrate that Vav3 is a bona fide Ahr target that is in charge of a limited subset of the developmental and physiological functions controlled by this transcriptional factor. Our data also reveal the presence of sympathoexcitation and new cardiorespiratory defects in Ahr(-/-) mice.

Differential effect of 7,12-dimethylbenz[a]anthracene on human and mouse CYP1B1 from livers of castrated transgenic mice

Humanized transgenic mice coexpressing tetracycline-controlled transactivator (tTA) and human cytochrome P450 1B1 (CYP1B1) (hCYP1B1) have been created by this group. The aims of this study was to determine if 7,12-dimethylbenz[a]anthracene (DMBA) functions as testosterone or doxycycline in its ability to induce or reduce expression of hCYP1B1 or endogenous mouse CYP1B1 (mCYP1B1). This was tested in the livers by treating castrated transgenic males and hCYP1B1/luciferase-transfected cells with DMBA. Herein, DMBA-treated group exhibited (i) gradual reduction of hCYP1B1 expression at the transcript, protein, and activity levels but gradually induced its transcript level during DMBA release; (ii) gradual reduction of hCYP1B1 at the transcript and protein levels, as in the case of doxycycline or testosterone; (iii) gradual induction of mCYP1B1 expression at the transcript and protein levels but gradually reduced its transcript level during DMBA release. In parallel, DMBA-treated transfected cells exhibited gradual increase in luciferase activity in a time-and dose-dependent manner. Thus, castrated transgenic males or in vitro system could be useful as models for the detection of polycyclic aromatic hydrocarbons (PAHs) or environmental toxicants by measuring either hCYP1B1 or mCYP1B1 expressions.

CYP1C1 messenger RNA expression is inducible by benzo[a]pyrene in Fundulus heteroclitus embryos and adults

CYP1C is the newest member of the CYP1 family of P450s; however, its physiological significance, inducers, and metabolic functions are unknown. Two full-length alleles of Fundulus heteroclitus CYP1C1 complementary DNA were cloned. The 529 amino acid protein shared the highest amino acid identity with Stenotomus chrysops CYP1C1 (81%). To investigate whether the carcinogen benzo[a]pyrene (BaP) was a CYP1C1 inducer, we used real-time PCR to quantitatively measure tissue- and sex-specific expression of both CYP1C1 and CYP1A messenger RNAs (mRNAs) in BaP-exposed adult fish. CYP1C1 mRNA expression was constitutively higher than CYP1A in brain, spleen, eye, and gonad, while CYP1A was higher in gastrointestinal tract (GI), heart, gill, and liver. Kidney had equal but high expression of both CYP1s. There were sex differences in constitutive CYP1 expression in the GI, liver, gill, and eye. BaP exposure caused induction of CYP1C1 expression in female and male heart (31- and 17-fold), gill (seven- and four-fold), and liver (six- and five-fold), respectively. Embryo CYP1 expression was constitutively highest at 2 weeks posthatch, and whole embryos expressed 3- to 15-fold more CYP1C1 mRNA compared to CYP1A. BaP, 10 microg/l for 10 days, caused induction of both genes at 120 and 240 h postfertilization. Our results suggest that teleost CYP1C, in addition to CYP1A, is inducible by BaP, has a broad tissue distribution, and should be further investigated for its role in carcinogen bioactivation.

Lineage-specific effects of polychlorinated biphenyls (PCB) on gene expression in the rabbit blastocyst

The ubiquitous toxic environmental contaminants polychlorinated biphenyls (PCBs) change gene expression in preimplantation embryos. Cell lineage-specific effects of PCB are not known. Rabbit day 6 blastocysts were exposed in vitro to low (0.1 ng/congener/mL medium) and high (1 microg) PCB levels of coplanar (PCB 77, 126, and 169) or non-coplanar PCBs (PCB 28, 52, 101, 118, 138, 153, and 180). Embryoblast (ICM) and trophoblast cells (TE) were separated and analyzed for transcriptional changes of PCB-and implantation-associated genes by semiquantitative RT-PCR. PCBs increased CYP 1A1 mRNA only in the ICM. CYP 1B1, VEGFR2, and COX-2 transcript numbers were elevated in both ICM and TE. Transcripts for HIF-1alpha were decreased in the ICM. No obvious differences in gene expression following exposure to coplanar and non-coplanar PCBs were detected. Our results show that transcriptional responses to PCBs differ between the cell lineages of the rabbit blastocyst, indicating that PCBs can influence the highly sensitive process of early mammalian development.

The mutagenic effects of 7,12-dimethylbenz[a]anthacene, 3-methylcholanthrene and benzo[a]pyrene to the developing Syrian hamster fetus measured by an in vivo/in vitro mutation assay

The transplacental mutagenicity of three polycylic aromatic hydrocarbons, 7,12-dimethylbenz[a]anthacene (DMBA), 3-methylcholanthrene (MC) and benzo[a]pyrene (BP), was measured by an in vivo/in vitro mutation assay. Fetal sensitivity and dose-response characteristics with regard to transplacental mutagenesis by these compounds have never been quantified. In the current experiment, pregnant Syrian hamsters were exposed to these compounds at day 12 of gestation. Twenty-four hours later the fetuses were removed and their cells were allowed a 5-day expression time in culture. They were then seeded for colony formation and also for mutation selection by diphtheria toxin. DMBA at 0.2 mmol/kg (51.3 mg/kg) had an induced mutant frequency of 1.56 x 10(-4) mutants per surviving cell. This was 598 times the historical control. DMBA at 0.2 mmol/kg was 3.6 times more potent than the highly mutagenic positive control, ethylnitrosourea, at 1 mmol/kg. DMBA also caused a dose-dependent increase in cloning efficiency, which was highly correlated with mutation rate. BP and MC were less effective than DMBA, causing increased mutations that were 31.6 and 17.7 times the historical control, respectively, and for neither was there any correlation of mutation rate with cloning efficiency. The special effectiveness of DMBA as a transplacental mutagen may relate to its ability to cause increased cell division and fixation of DNA lesions as mutations.

Molecular cues to implantation

Successful implantation is the result of reciprocal interactions between the implantation-competent blastocyst and receptive uterus. Although various cellular aspects and molecular pathways of this dialogue have been identified, a comprehensive understanding of the implantation process is still missing. The receptive state of the uterus, which lasts for a limited period, is defined as the time when the uterine environment is conducive to blastocyst acceptance and implantation. A better understanding of the molecular signals that regulate uterine receptivity and implantation competency of the blastocyst is of clinical relevance because unraveling the nature of these signals may lead to strategies to correct implantation failure and improve pregnancy rates. Gene expression studies and genetically engineered mouse models have provided valuable clues to the implantation process with respect to specific growth factors, cytokines, lipid mediators, adhesion molecules, and transcription factors. However, a staggering amount of information from microarray experiments is also being generated at a rapid pace. If properly annotated and explored, this information will expand our knowledge regarding yet-to-be-identified unique, complementary, and/or redundant molecular pathways in implantation. It is hoped that the forthcoming information will generate new ideas and concepts for a process that is essential for maintaining procreation and solving major reproductive health issues in women.

Discovery of cytochrome P450 1B1 inhibitors as new promising anti-cancer agents

Human cytochrome P450 (CYP)1B1 is a major enzyme for carcinogenic estrogen metabolism and involved in the metabolic activation of procarcinogens of the polycyclic aromatic hydrocarbons (PAHs). CYP1B1 is known to be expressed at a high frequency in various human cancers, but not in normal tissues. It also plays an important role in the metabolism of various anti-cancer drugs. These findings suggest inhibition of CYP1B1 as a new oncological therapeutic strategy. Several natural and synthetic compounds have been studied in an effort to find the isoform-specific inhibitors of the CYP1 subfamily. A survey of the inhibitors of CYP1B1 and other related inhibitors of the CYP1 subfamily is provided in this review.

Polychlorinated biphenyls affect gene expression in the rabbit preimplantation embryo

Polychlorinated biphenyls (PCBs) have been shown to be embryotoxic. The mechanism(s) of action is not clearly understood. The toxic effects could be either direct or indirect. Furthermore, PCB congeners vary in their toxic potential. They can be classified in coplanar PCBs binding to the transcription factor aryl hydrocarbon receptor (AhR), which induce subsequent changes in gene expression, and noncoplanar PCBs exhibiting AhR-independent effects. In order to investigate possible mechanisms, 5 and 6 days old preimplantation rabbit embryos were exposed in vitro to low levels of coplanar (PCB 77, 126, and 169) or noncoplanar PCBs (PCB 28, 52, 101, 118, 138, 153, and 180). The PCB effects were studied by semiquantitative RT-PCR analysis of AhR target genes (cytochrome P450 (CYP) 1A1, 1A2, UDP-glucuronosyl transferase 1, glutathione S-transferase pi1 and aldehyde dehydrogenase) and dioxin-responsive genes (IL 1beta, PAI 2, Cox 2, TGFalpha, EGF, erbB 1-4, c-fos, c-jun, HSP 90, cyclophilin 40), and by differential display (DD) RT-PCR. CYP 1B1 mRNA and AhR protein were localized by in situ hybridization and immunohistochemistry, respectively. From the AhR target genes studied only CYP 1B1, and cyclooxygenase 2 showed an increase in mRNA levels after coplanar and noncoplanar PCB. Interleukin 1beta and plasminogen activator inhibitor 2 were downregulated. CYP 1B1 mRNA showed a stage specific inducibility at day 6, but not at day 5. By DD RT-PCR we identified six new genes previously not reported to be regulated by PCBs. The mRNAs encoding the subunits 1, 2, 4, and 5 of the NADH ubiquinone oxidoreductase and beta-globin showed a decrease, whereas trichohyalin mRNA was increased after PCB exposure. Coplanar and noncoplanar PCB congeners elicited similar responses on the mRNA levels of the studied genes. Exposure to coplanar PCBs did not result in the AhR being translocated to the nucleus. Our results show that (i). PCBs induce changes in gene expression in rabbit day 5 and 6 preimplantation embryos and imply (ii). that the transcriptional changes observed were not mediated by the nuclear AhR.

Comparison of the genotoxic activities of the K-region dihydrodiol of benzo[a]pyrene with benzo[a]pyrene in mammalian cells: morphological cell transformation; DNA damage; and stable covalent DNA adducts

Benzo[a]pyrene (B[a]P) is the most thoroughly studied polycyclic aromatic hydrocarbon (PAH). Many mechanisms have been suggested to explain its carcinogenic activity, yet many questions still remain. K-region dihydrodiols of PAHs are metabolic intermediates depending on the specific cytochrome P450 and had been thought to be detoxification products. However, K-region dihydrodiols of several PAHs have recently been shown to morphologically transform mouse embryo C3H10T1/2CL8 cells (C3H10T1/2 cells). Because K-region dihydrodiols are not metabolically formed from PAHs by C3H10T1/2 cells, these cells provide a useful tool to independently study the mechanisms of action of PAHs and their K-region dihydrodiols. Here, we compare the morphological cell transforming, DNA damaging, and DNA adducting activities of the K-region dihydrodiol of B[a]P, trans-B[a]P-4,5-diol with B[a]P. Both trans-B[a]P-4,5-diol and B[a]P morphologically transformed C3H10T1/2 cells by producing both Types II and III transformed foci. The morphological cell transforming and cytotoxicity dose response curves for trans-B[a]P-4,5-diol and B[a]P were indistinguishable. Since morphological cell transformation is strongly associated with mutation and/or larger scale DNA damage in C3H10T1/2 cells, the identification of DNA damage induced in these cells by trans-B[a]P-4,5-diol was sought. Both trans-B[a]P-4,5-diol and B[a]P exhibited significant DNA damaging activity without significant concurrent cytotoxicity using the comet assay, but with different dose responses and comet tail distributions. DNA adduct patterns from C3H10T1/2 cells were examined after trans-B[a]P-4,5-diol or B[a]P treatment using 32P-postlabeling techniques and improved TLC elution systems designed to separate polar DNA adducts. While B[a]P treatment produced one major DNA adduct identified as anti-trans-B[a]P-7,8-diol-9,10-epoxide-deoxyguanosine, no stable covalent DNA adducts were detected in the DNA of trans-B[a]P-4,5-diol-treated cells. In summary, this study provides evidence for the DNA damaging and morphological cell transforming activities of the K-region dihydrodiol of B[a]P, in the absence of covalent stable DNA adducts. While trans-B[a]P-4,5-diol and B[a]P both induce morphological cell transformation, their activities as DNA damaging agents differ, both qualitatively and quantitatively. In concert with the morphological cell transformation activities of other K-region dihydrodiols of PAHs, these data suggest a new mechanism/pathway for the morphological cell transforming activities of B[a]P and its metabolites.

Molecules in blastocyst implantation: uterine and embryonic perspectives

Synchronized development of the embryo to the active stage of the blastocyst, differentiation of the uterus to the receptive state, and a "cross talk" between the blastocyst and uterine luminal epithelium are essential to the process of implantation. In spite of considerable accumulation of information and the present state of the knowledge, our understanding of the definitive mechanisms that regulate these events remains elusive. Although there are species variations in the process of implantation, many basic similarities do exist among various species. This review focuses on specific aspects of the implantation process in mice with the hope that many of the findings will be relevant to the process in humans. To establish signaling mechanisms of embryo-uterine interactions during implantation, studies on both embryonic and uterine consequences are required to generate more meaningful information. Due to ethical restriction and experimental limitation, it is difficult to generate such information in humans. This review has attempted to provide a comprehensive, but not complete, narration of a number of embryonic and uterine factors that are involved in the process of implantation in autocrine, paracrine, and/or juxtacrine manners in mice at the physiological, cellular, molecular, and genetic levels.

Effect of chemopreventive agents on DNA adduction induced by the potent mammary carcinogen dibenzo[a,l]pyrene in the human breast cells MCF-7

Over 1500 structurally diverse chemicals have been identified which have potential cancer chemopreventive properties. The efficacy and mechanisms of this growing list of chemoprotective agents may be studied using short-term bioassays that employ relevant end-points of the carcinogenic process. In this study, we have examined the effects of eight potential chemopreventive agents, N-acetylcysteine (NAC), benzylisocyanate (BIC), chlorophyllin, curcumin, 1,2-dithiole-3-thione (D3T), ellagic acid, genistein, and oltipraz, on DNA adduction of the potent mammary carcinogen dibenzo[a,l]pyrene (DBP) using the human breast cell line MCF-7. Bioactivation of DBP by MCF-7 cells resulted in the formation of one predominant (55%) dA-derived and several other dA- or dG-derived DNA adducts. Three test agents, oltipraz, D3T, and chlorophyllin substantially (>65%) inhibited DBP-DNA adduction at the highest dose tested (30 microM). These agents also significantly inhibited DBP adduct levels at a lower dose of 15 microM, while oltipraz was effective even at the lowest dose of 5 microM. Two other agents, genistein and ellagic acid were moderate (45%) DBP-DNA adduct inhibitors at the highest dose tested, while NAC, curcumin, and BIC were ineffective. These studies indicate that the MCF-7 cell line is an applicable model to study the efficacy of cancer chemopreventive agents in a human setting. Moreover, this model may also provide information regarding the effect of the test agents on carcinogen bioactivation and detoxification enzymes.

Regulation, function, and tissue-specific expression of cytochrome P450 CYP1B1

Cytochrome P450 CYP1B1 is a relatively recently identified member of the CYP1 gene family. The purpose of this commentary is to review the regulatory mechanisms, metabolic specificity, and tissue-specific expression of this cytochrome P450 and to highlight its unique properties. The regulation of CYP1B1 involves a variety of both transcriptional and post-transcriptional mechanisms. CYP1B1 can metabolize a range of toxic and carcinogenic chemicals in vitro but in some cases with a unique stereoselectivity. Estradiol 4-hydroxylation appears to be a characteristic reaction catalyzed by human CYP1B1. However, there are considerable species differences regarding the regulation, metabolic specificity, and tissue-specific expression of this P450. In humans CYP1B1 is overexpressed in tumor cells, and this has important implications for tumor development and progression and the development of anticancer drugs specifically activated by CYP1B1.

A cytochrome P4501B gene from a fish, Pleuronectes platessa

Tetrapod cytochrome P4501 family (CYP1A1, CYP1A2 and CYP1B1) enzymes are most active in hydroxylating a variety of environmental contaminants including polyaromatic hydrocarbons (PAH), planar polychlorinated biphenyls and arylamines and thus play a pivotal role in the toxicology of these compounds. Mammalian CYP1A1 and CYP1A2 genes appear to have diverged after the evolutionary emergence of mammals, whereas fish species apparently possess only one CYP1A family gene, and fish CYP1A enzymes exhibit properties of both of the mammalian isoforms. We have isolated a further CYP1 family gene from a marine flatfish (plaice; Pleuronectes platessa), which, on the basis of exon organisation and sequence similarity, can be assigned as a piscine CYP1B. Its deduced amino acid sequence shows the closest (54%) identity to mammalian CYP1B1 proteins and, on the basis of molecular modeling studies, shows a high degree of positional and structural conservation of the substrate contacting amino acid residues in its putative active site when compared to other CYP1 enzymes. Phylogenetic analysis of fish and mammalian CYP1 family sequences indicates that the plaice CYP1B and mammalian CYP1B1 genes share a common ancestry. Plaice CYP1B has a more restricted tissue expression profile than the previously isolated plaice CYP1A, only being detectable, by Northern blotting, in gill tissue. In contrast to CYP1A, which shows extensive PAH-dependent induction in a variety of tissues, plaice CYP1B appears unresponsive to treatment with a prototypical PAH-type inducer, beta-naphthoflavone (BNF).

8,9-dihydroxy-8,9-dihydrodibenzo[a,l]pyrene is a potent morphological cell-transforming agent in C3H10T(1)/(2)Cl8 mouse embryo fibroblasts in the absence of detectable stable covalent DNA adducts

The comparative genotoxic effects of racemic trans-8,9-dihydroxy-8, 9-dihydrodibenzo[a,l]pyrene (trans-DB[a,l]P-8,9-diol), the metabolic K-region dihydrodiol of dibenzo[a,l] pyrene (DB[a,l]P) (dibenzo[def, p]chrysene) and DB[a,l]P in transformable mouse embryo C3H10T(1)/(2)Cl8 (C3H10T(1)/(2)) fibroblasts was investigated. The C3H10T(1)/(2) mouse embryo morphological cell-transforming activities of these polycyclic aromatic hydrocarbons (PAHs) were assayed using concentration-response studies. At concentrations of 33 nM and above both trans-DB[a,l]P-8,9-diol and DB[a,l]P produced significant (and similar) numbers of type II and III foci per dish and numbers of dishes with type II and II foci. Concomitant cytotoxicity studies revealed a reduction in colony survival of approximately 25% up to 198 nM for both PAHs. DNA adducts of trans-DB[a,l]P-8,9-diol and DB[a,l]P in C3H10T(1)/(2) cells were analyzed by a (32)P-post-labeling TLC/HPLC method. No adducts were observed in the DNA of C3H10T(1)/(2) cells treated with trans-DB[a, l]P-8,9-diol at concentrations that induced morphological cell transformation. Under the same exposure and chromatographic conditions, DNA adducts of deoxyadenosine and deoxyguanosine derived from the fjord region anti-DB[a,l]P-11,12-diol-13,14-epoxide and syn-DB[a,l]P-11,12-diol-13,14-epoxide were observed in the DNA of DB[a,l]P-treated cells. These results indicate that trans-DB[a,l]P-8, 9-diol has intrinsic genotoxic activity equal to that of DB[a,l]P, based on morphological cell transformation of mouse embryo fibroblasts. The activity of trans-DB[a,l]P-8,9-diol is apparently not associated with the formation of observable stable covalent DNA adducts. These results suggest that under appropriate conditions, trans-DB[a,l]P-8,9-diol may serve as an intermediate in the genotoxicity of DB[a,l]P.

Expression of dioxin-related transactivating factors and target genes in human eutopic endometrial and endometriotic tissues

OBJECTIVE

Although an association between dioxin exposure and endometriosis has been proposed, the effects of this environmental toxin on human endometriosis are not known. To understand the potential underlying molecular mechanisms we studied the expressions of cytochrome P-450 genes (CYP1A1, CYP1A2, and CYP1B1 ), which are induced by dioxin, and the expressions of cytosolic receptor for dioxin, aryl hydrocarbon receptor, and its nuclear translocator, aryl hydrocarbon receptor nuclear translocator protein, in endometriotic and eutopic endometrial tissues.

STUDY DESIGN

Levels of transcripts of CYP1A1, CYP1A2, CYP1B1, aryl hydrocarbon receptor, and aryl hydrocarbon receptor nuclear translocator protein were determined by a quantitative reverse transcriptase-polymerase chain reaction and Southern blot assay in total ribonucleic acid samples from endometriotic and eutopic endometrial tissues. Eutopic endometrial tissue samples (n = 33) and endometriotic tissue samples (n = 10) were obtained at the time of uterine curettage and laparoscopy from disease-free women and from patients with endometriosis. Portions of these eutopic endometrial and endometriotic tissues were obtained simultaneously from the same patients (n = 8 pairs of samples). Levels of transcripts of CYP1A1, CYP1A2, CYP1B1, aryl hydrocarbon receptor, and aryl hydrocarbon receptor nuclear translocator protein were determined in endometrial and endometriotic tissues during follicular and luteal phases of the cycle and in cultured endometriotic stromal cells treated with forskolin, phorbol diacetate, medroxyprogesterone acetate,and serum.

RESULTS

Transcripts of dioxin receptor, its nuclear translocator, and two dioxin-induced target genes (CYP1A2 and CYP1B1) were demonstrated during follicular and luteal phases of the cycle in both eutopic endometrial tissues and tissues affected by pelvic endometriosis, with no readily detectable differences between these tissues. On the other hand, levels of transcripts of another dioxin-induced gene, CYP1A1, were found to be strikingly higher in endometriotic tissues than in the eutopic endometrium. Mean levels in endometriotic tissues were 8.7 times those found in eutopic endometrium. Various hormonal treatments of endometriotic stromal cells did not significantly alter these levels.

CONCLUSION

We demonstrated for the first time the expression of dioxin-related transcription factors aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein and target genes CYP1A1, CYP1A2, and CYP1B1 in endometriotic tissues and stromal cells. Strikingly elevated CYP1A1 transcripts in endometriosis may give rise to significantly increased P-4501A1 enzyme activity and thus promote the development and growth of endometriosis by either activating procarcinogens or inducing the formation of catechol estrogens or both. In fact, the proposed link between dioxin exposure and endometriosis may be explained in part by the up-regulation of the CYP1A1 gene expression in endometriotic tissues.

Association of CYP1B1 genetic polymorphism with incidence to breast and lung cancer

Cytochrome P450 1B1 (CYP1B1) participates in the metabolic activation of a number of procarcinogens including benzo[a]pyrene and the hydroxylation of 17beta-estradiol at the C-4 position. In this study, we investigated the association between CYP1B1 genetic polymorphism and breast or lung cancer incidence. The Ala-Ser polymorphism at codon 119 in presumed substrate recognition site 1 was significantly associated with the incidence of breast or squamous cell carcinoma of the lung. On the other hand, Leu-Val polymorphism at codon 432 did not show any association to the cancers. An allele containing both Ala and Leu simultaneously, comprised 75% of alleles among 315 Japanese healthy controls, was significantly inversely associated with breast cancer incidence. When expressed in a recombinant system, this CYP1B1 cDNA showed the lowest 17beta-estradiol 4-hydroxylase activity among four different variant forms of CYP1B1. Thus, inter-individual differences in activation of procarcinogens or metabolism of oestrogen originating from genetic polymorphisms of the human CYP1B1 gene may contribute to the susceptibility of human cancers.

Targeted knockout of Cyp1a1 gene does not alter hepatic constitutive expression of other genes in the mouse [Ah] battery

Using the Cre-lox system, we have generated a cytochrome P450 1A1 Cyp1a1(-/-) knockout mouse by deletion of the translated portions of the Cyp1a1 gene. These mice are viable and demonstrate no obvious phenotype, compared with wild-type littermates. As a first step toward characterizing genes that might be expected to compensate for loss of CYP1A1, constitutive expression of [Ah] gene battery members was examined. In a cultured hepatoma CYP1A1 metabolism-deficient mutant line that does not express Cyp1a2, we have previously shown that constitutive transcriptional up-regulation of other [Ah] gene battery members occurs; these results are consistent with the elevation of a putative endogenous ligand (EL) for the Ah receptor that is a substrate for CYP1A1. The [Ah] battery includes Cyp1a2, NAD(P)H:quinone oxidoreductase (Nqo1), and three other Phase II genes. Examining mRNA, protein, and enzyme activity, we demonstrate that the absence of CYP1A1 has no effect on the hepatic constitutive expression of Cyp1a2 or Nqo1. We postulate that CYP1A1 and CYP1A2 might have overlapping substrate specificity for metabolism of the EL, such that basal CYP1A2 in the liver can compensate for the loss of CYP1A1.

Immunohistochemical localization of cytochrome P450 CYP1B1 in breast cancer with monoclonal antibodies specific for CYP1B1

Cytochrome P450 CYP1B1 is a recently identified member of the CYP1 P450 family. We have shown that this P450 displays increased expression in several types of human cancer, indicating that CYP1B1 is a potential tumor biomarker. In this study we developed monoclonal antibodies (MAbs) to CYP1B1 that are effective on formalin-fixed, paraffin-embedded tissue sections and investigated the presence of CYP1B1 in a series of primary breast cancers. The MAbs were generated using a synthetic peptide coupled to carrier protein as the immunogen. The MAbs specifically recognized CYP1B1 and did not recognize either CYP1A1 or CYP1A2, related CYP1 forms. The MAbs were tested by immunohistochemistry and were found to be effective on formalin-fixed, paraffin-embedded tissue sections. The majority of breast cancers showed positive immunoreactivity for CYP1B1, and in each case CYP1B1 was specifically localized to tumor cells. The presence of CYP1B1 in breast cancer cells is likely to contribute to their metabolism of estradiol because CYP1B1 is a specific estradiol hydroxylase. (J Histochem Cytochem 47:1457-1464, 1999)

Cellular localization of hepatic cytochrome 1B1 expression and its regulation by aromatic hydrocarbons and inflammatory cytokines

Cytochrome P450 1B1 (CYP1B1) is an activator of several xenobiotics and is induced in the liver upon experimental exposure to aromatic hydrocarbons. Since its cellular localization and regulation are incompletely clarified, Cyp1B1 expression and inducibility by 9,10-dimethyl-1,2-benzanthracene (DMBA) and inflammatory cytokines were investigated in different rat liver cell populations in vitro and in the liver during hepatocellular injury. Expression of Cyp1B1 was studied by Northern blot analysis in hepatic stellate cells (HSCs), myofibroblasts (MFs), Kupffer cells (KCs), and hepatocytes at various time points of primary cultures and in acutely damaged rat liver (carbon tetrachloride model). Enzyme inducibility was assessed by incubation of cells with DMBA as well as, in the case of HSCs, with tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor beta1 (TGFbeta1). Cyp1B1 messengers were expressed at high levels by HSCs and MFs, whereas constitutive expression was not detectable in KCs or in hepatocytes. Cyp1B1-specific mRNA were expressed at highest levels in HSCs at an early stage of activation (2 days after plating) and were diminished upon further activation. DMBA strongly enhanced Cyp1B1 gene expression in HSCs, MFs, and in hepatocytes at day 3 of primary cultures, but not in hepatocytes at day 1, or in KCs. The inflammatory cytokine TNF-alpha enhanced the Cyp1B1 gene expression in HSCs, either when administered alone or in addition to DMBA, while TGFbeta1 did not affect Cyp1B1 expression, even after DMBA induction. We conclude that HSCs and MFs seem to be the major cellular sources of hepatic Cyp1B1 expression and that the constitutive expression of the Cyp1B1 gene and the responsiveness to DMBA stimulation differ between mesenchymal and parenchymal liver cells, indicating a cell-specific regulation of Cyp1B1 gene expression. Interestingly, TNF-alpha is a potent stimulator of the Cyp1B1 gene in HSCs and acts in concert with DMBA.

Cytochrome P450 1B1 mRNA measured in blood mononuclear cells by quantitative reverse transcription-PCR

Cytochrome P450 (CYP) 1B1 activates polycyclic aromatic hydrocarbons and aryl aromatic hydrocarbons to carcinogens. We describe a competitive reverse transcription-PCR (RT-PCR) assay for the quantification of CYP1B1 mRNA in blood mononuclear cells (BMCs) by simultaneous RT and PCR amplification of cellular RNA with decreasing amounts of an internal standard. The concentration of CYP1B1 mRNA is derived from the ratio between the intensities of the bands corresponding to the amplified products. To reduce the variability of mRNA extraction efficiency, the measured amount of CYP1B1 has been calculated in relation to the β-actin gene products. We measured CYP1B1 expression in the BMCs of 75 human subjects; no significant differences in the CYP1B1:β-actin ratio were detected between women (range, 0.47–4.35; median, 2.0) and men (range, 0.72–3.85; median, 2.09). The analytical imprecision (CV) of duplicates was 14% (n = 25 pairs), and the intraindividual CV for two samples, 1 month apart, was 22% (n = 20). No significant differences were detected in smokers (n = 25; range, 0.77–3.55; median, 2.14) compared with nonsmokers (n = 50; range, 0.47–4.35; median, 2.0). The method has a wide range of linearity, good sensitivity and precision, and is suitable for studies of individual susceptibility as indicated by CYP1B1 expression in BMCs.

Coordination of differential effects of primary estrogen and catecholestrogen on two distinct targets mediates embryo implantation in the mouse

In the mouse, estrogen is essential for blastocyst implantation in the progesterone (P4)-primed uterus. The mechanism(s) by which estrogen initiates this response still remains elusive. The present investigation, using delayed implantation in the mouse, examined the differential role of estradiol-17beta (E2) and its catechol metabolite 4-hydroxy-E2 (4-OH-E2) in uterine and blastocyst activation for implantation. The conditions of delayed implantation were induced by ovariectomizing mice on day 4 (day 1 = vaginal plug) of pregnancy or pseudopregnancy and maintaining them with P4 from days 5-7. The binding of EGF to blastocysts was used as a marker for blastocyst activation. Our results show that whereas E2 fails to activate dormant blastocysts (with respect to EGF binding in vitro), 4-OH-E2, cAMP, or prostaglandin E2, is effective in this response. Further, whereas 4-OH-E2 induced-activation is not blocked by an antiestrogen, an inhibitor of PG synthesis, adenylyl cyclase or protein kinase A effectively blocks this activation. These results suggest that 4-OH-E2 effects on blastocysts are mediated by PGs, which, in turn, stimulate cAMP production and thus activation of protein kinase A. Two-fluoro-E2 is a poor substrate and an inhibitor of catecholestrogen synthesis, but it is estrogenic, with respect to uterine growth and gene expression. Using blastocyst transfer experiments, we observed that dormant blastocysts incubated with 4-OH-E2 in vitro, but not with E2, are capable of implanting in P4-treated delayed implanting mice receiving two-fluoro-E2. The results suggest that whereas E2 is necessary for preparation of the uterus, uterine-derived catecholestrogen is important for blastocyst activation for implantation. Indeed, the receptive uterus has the capacity to synthesize 4-OH-E2. Collectively, we demonstrate that the primary ovarian estrogen E2, via its interaction with nuclear estrogen receptors, participates in the preparation of the P4-primed uterus to the receptive state in an endocrine manner, whereas its metabolite 4-OH-E2, produced from E2 in the uterus, mediates blastocyst activation for implantation in a paracrine manner. Our results also establish that these target-specific effects of primary estrogen and catecholestrogen are both essential for implantation and that successful implantation occurs only when the activated stage of the blastocyst coincides with the receptive state of the uterus.

Characterization of the mouse Cyp1B1 gene. Identification of an enhancer region that directs aryl hydrocarbon receptor-mediated constitutive and induced expression

Transcriptional activation of the Cyp1B1 gene in rodents is stimulated by both polycyclic hydrocarbons and cAMP. The mouse Cyp1B1 gene structure contains three exons, of which the second nucleotide of exon 2 is the translation start site. Primer extension analysis identified a transcription start domain defining an exon 1 of 371 base pairs. The sequence 1.075 kilobases upstream of the transcription start site showed 11 xenobiotic-responsive elements (XRE) (TnGCGTG or GCGTG) that are putative aryl hydrocarbon receptor (AhR)-binding sites and three steroidogenic factor-1 motifs that are associated with cAMP-mediated transcriptional activation of genes. A transiently transfected Cyp1B1-luciferase construct, composed of exon 1 and 1.075 kilobases of 5'-flanking region, was induced by 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10.0 +/- 3.0-fold, n = 6) in C3H10T1/2 cells, which exclusively express Cyp1B1. The 90-base pair basal promoter contains two SP-1 sites, one SF-1 site, and a TATA-like box. TCDD induction and basal expression were dependent on positive regulatory elements present between -1075 and -810. Five XRE motifs localized in the enhancer region were completely conserved between mouse and human CYP1B1 sequences. Similar inductions were seen in Hepa-1 cells, which express Cyp1A1 but not Cyp1B1. However, basal Cyp1B1 promoter activities were 4-10-fold higher in C3H10T1/2 cells providing the enhancer region was present, partially reproducing the in vivo cell-specific expression of Cyp1B1. Gel shift experiments established that TCDD stimulates AhR binding to the downstream XRE in the enhancer region. However, oligonucleotides that encompass two other XREs show a high affinity complex of similar size that is evident even without TCDD treatment and that does not contain either the AhR or AhR nuclear translocator. The fourth XRE is immediately adjacent to an E-box, and this oligonucleotide formed a smaller complex that was dependent on this E-box sequence. Negative regulatory sequences have been located between the promoter and TCDD-responsive enhancer regions. Constitutive expression of the Cyp1B1 gene was lost in AhR-deficient cells and was restored by transfected AhR cDNA. Reporter constructs function in a parallel manner, demonstrating the key role of the AhR in constitutive as well as TCDD-induced expression of Cyp1B1 in mouse embryo fibroblasts.

Biological oxidations and P450 reactions. Recombinant mouse CYP1B1 expressed in Escherichia coli exhibits selective binding by polycyclic hydrocarbons and metabolism which parallels C3H10T1/2 cell microsomes, but differs from human recombinant CYP1B1

Orthologs of a previously identified CYP1B subfamily designated CYP1B1, which are constitutively expressed in mammary, uterine, and embryonic cells, have previously been functionally linked to 7,12-dimethylbenz-a-anthracene (DMBA) metabolism. A chimeric construct of mouse CYP1B1 in which the 20 NH2-terminal amino acids have been replaced by eight residues from human CYP17 has been expressed in Escherichia coli. This recombinant mouse CYP1B1 (recCYP1B1m) exhibited DMBA metabolism accurately reproducing the characteristic product distribution and specific activity of 3.4 nmol/nmol P450/min seen in C3H10T1/2 cells from which this cDNA has been cloned. The high proportion of 10,11- and 3,4-dihydrodiols and near absence of 5,6-dihyrodiol- and 7-hydroxy-DMBA metabolites are seen only in rodent microsomes where CYP1B1 is highly expressed. This distribution of products from recCYP1B1m was highly dependent on addition of epoxide hydrolase, particularly the ratio of 3,4-dihydrodiol to 4-phenol metabolites. These characteristics in addition to inhibition by antibodies raised to recCYP1B1m establish that the CYP1B1 cDNA indeed encodes the P450 responsible for polycyclic aromatic hydrocarbon (PAH) metabolism from C3H10T1/2 cells. DMBA metabolites from cDNA-expressed human CYP1B1 (recCYP1B1h) however, exhibited a different regioselectivity toward DMBA resembling human CYP1A1 catalyzed DMBA metabolism. Reconstitution of recCYP1B1m with different concentrations of NADPH-P450 reductase indicated a high affinity interaction with an apparent Km of 3 nM. Large PAH such as benz[a]pyrene, benz[e]pyrene, benz[a]anthracene, DMBA, 3-methylcholanthrene, and 1-ethynylpyrene bound to recCYP1B1m with high affinity (Kd 0.08 to 0.22 microM) concomitant with substantial spectral shifts (40% low to high spin state change). Smaller PAHs like pyrene, phenanthrene, and naphthalene neither produced spectral changes nor inhibited the spectral change caused by benz[a]pyrene. Among tested steroids, progesterone bound weakly to recCYP1B1m (Kd > 20 microM) with a comparable spectral shift and was a weak inhibitor of DMBA metabolism, but was not metabolized. While 17beta-estradiol is a substrate for human CYP1B1 we have found no evidence for binding to mouse CYP1B1. This data establishes CYP1B1 as an important contributor to activation of PAHs, particularly in extra hepatic tissues that are susceptible to cancer where CYP1B1 in contrast to CYP1A1 is constitutively expressed.

Evidence for mitochondrial metabolism of 7,12-dimethylbenz(a)anthracene in porcine ovaries: comparison with microsomal metabolism

7,12-dimethylbenz(a)anthracene (DMBA) causes necrosis in endocrine organs. DMBA metabolism in follicles and corpora lutea from porcine ovaries was demonstrated not only in the microsomal but also in the mitochondrial fraction, in contrast to what has been found in the rat ovary. Maximal activities were present in these fractions of the corpus luteum, with specific activities of 5.9 and 2.2 pmol/min x mg protein, respectively. DMBA metabolism in mitoplasts, i.e. mitochondrial inner membranes, proved to be more than 10-fold higher than the corresponding activity in the mitochondrial fraction. The purities of the subcellular fractions were assessed by measurements of marker enzymes. 17-42% of the mitochondrial DMBA metabolism was concluded to be due to microsomal contamination. In the mitoplast fraction such contamination was only 0.18-2.8%. Ellipticine and alpha-naphthoflavone reduced the metabolism of DMBA in the luteal microsomal fraction by 95 and 77%, respectively. In mitochondria the inhibition by these agents was 63 and 30%, respectively. Indomethacine and estradiol decreased microsomal DMBA metabolism by 53 and 52%, respectively. In mitochondria the inhibition was 52 and 23%, respectively. None of these inhibitors affected the DMBA metabolism by the mitoplast fraction.

Caffeine increases its own metabolism through cytochrome P4501A induction in rats

Caffeine is one of the most widely used - and maybe abused - xenobiotic compounds in the world. If numerous pharmacological properties of caffeine have been reported, the effects of caffeine treatment on the hepatic drug-metabolizing enzyme system have been scarcely studied. Pretreatment of rats for 3 days with 150 mg/kg/day of caffeine dramatically increased P4501A and P4502B dependent catalytic activities determined in vitro. Furthermore, N-demethylations and C-8 oxidation of caffeine were increased by about 2 fold by caffeine treatment. Immunoblot analysis demonstrated that the liver contents of P4501A2 and P4502B1/2B2, known to be involved in these monooxygenase activities, increased also by about 2 fold. Cytochrome P4503A1 and 2E1 were not modified. Taken together, there data suggest that caffeine increases its own metabolism through P4501A induction.

Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor

The aryl hydrocarbon (Ah) receptor (AHR) mediates many carcinogenic and teratogenic effects of environmentally toxic chemicals such as dioxin. An AHR-deficient (Ahr-/-) mouse line was constructed by homologous recombination in embryonic stem cells. Almost half of the mice died shortly after birth, whereas survivors reached maturity and were fertile. The Ahr-/- mice showed decreased accumulation of lymphocytes in the spleen and lymph nodes, but not in the thymus. The livers of Ahr-/- mice were reduced in size by 50 percent and showed bile duct fibrosis Ahr-/- mice were also nonresponsive with regard to dioxin-mediated induction of genes encoding enzymes that catalyze the metabolism of foreign compounds. Thus, the AHR plays an important role in the development of the liver and the immune system.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

The human and mouse homologs of the yeast RAD52 gene: cDNA cloning, sequence analysis, assignment to human chromosome 12p12.2-p13, and mRNA expression in mouse tissues

The yeast Saccharomyces cerevisiae RAD52 gene is involved in DNA double-strand break repair and mitotic/meiotic recombination. The N-terminal amino acid sequence of yeast S. cerevisiae, Schizosaccharomyces pombe, and Kluyveromyces lactis and chicken is highly conserved. Using the technology of mixed oligonucleotide primed amplification of cDNA (MOPAC), two mouse RAD52 homologous cDNA fragments were amplified and sequenced. Subsequently, we have cloned the cDNA of the human and mouse homologs of yeast RAD52 gene by screening cDNA libraries using the identified mouse cDNA fragments. Sequence analysis of cDNA derived amino acid revealed a highly conserved N-terminus among human, mouse, chicken, and yeast RAD52 genes. The human RAD52 gene was assigned to chromosome 12p12.2-p13 by fluorescence in situ hybridization, R-banding, and DNA analysis of somatic cell hybrids. Unlike chicken RAD52 and mouse RAD51, no significant difference in mouse RAD52 mRNA level was found among mouse heart, brain, spleen, lung, liver, skeletal muscle, kidney, and testis. In addition to an approximately 1.9-kb RAD52 mRNA band that is present in all of the tested tissues, an extra mRNA species of approximately 0.85 kb was detectable in mouse testis.