/n Pharmacology & Therapeutics 187 (2018) 71-87 Contents lists available at ScienceDirect Pharmacology & Therapeutics ELSEVIER Associate editor: S. Kimura-S journal homepage: www.elsevier.com/locate/pharmthera Modulation of CYP1A1 metabolism: From adverse health effects to chemoprevention and therapeutic options * Melina Mescher, Thomas Haarmann-Stemmann IUF-Leibniz-Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany Pharmacology Therapeutics Check for updates ARTICLE INFO Available online 17 February 2018 Keywords: Aryl hydrocarbon receptor Chemoprevention Cancer therapy Cytochrome P450 Polycyclic aromatic hydrocarbons Tumor promotion ABSTRACT The human cytochrome P450 (CYP) 1A1 gene encodes a monooxygenase that metabolizes multiple exogenous and endogenous substrates. CYP1A1 has become infamous for its oxidative metabolism of benzo[a]pyrene and re- lated polycyclic aromatic hydrocarbons, converting these chemicals into very potent human carcinogens. CYP1A1 expression is mainly controlled by the aryl hydrocarbon receptor (AHR), a transcription factor whose activation is induced by binding of persistent organic pollutants, including polycyclic aromatic hydrocarbons and dioxins. Ac- cordingly, induction of CYP1A1 expression and activity serves as a biomarker of AHR activation and associated xe- nobiotic metabolism as well as toxicity in diverse animal species and humans. Determination of CYP1A1 activity is integrated into modern toxicological concepts and testing guidelines, emphasizing the tremendous importance of this enzyme for risk assessment and regulation of chemicals. Further, CYP1A1 serves as a molecular target for che- moprevention of chemical carcinogenesis, although present literature is controversial on whether its inhibition or induction exerts beneficial effects. Regarding therapeutic applications, first anti-cancer prodrugs are available, which require a metabolic activation by CYP1A1, and thus enable a specific elimination of CYP1A1-positive tumors. However, the application range of these drugs may be limited due to the frequently observed downregulation of CYP1A1 in various human cancers, probably leading to a reduced metabolism of endogenous AHR ligands and a sustained activation of AHR and associated tumor-promoting responses. We here summarize the current knowl- edge on CYP1A1 as a key player in the metabolism of exogenous and endogenous substrates and as a promising target molecule for prevention and treatment of human malignancies. © 2018 Elsevier Inc. All rights reserved. 1. Introduction The xenobiotic-metabolizing monooxygenase cytochrome P450 (CYP) 1A1 is widely regarded as the prototype target of the aryl Abbreviations: AFB, aflatoxin B1; AH, aryl hydroxylase; AHR, aryl hydrocarbon receptor; AHRR, aryl hydrocarbon receptor repressor; AOP, adverse outcome pathway; ArA, arachidonic acid; ARNT, aryl hydrocarbon receptor nuclear translocator; BaP, benzo[a]pyrene; COX, cyclooxygenase; CYP, cytochrome P450; DMBA, 7,12-dimethylbenz[a]anthracene; EGFR, epidermal growth factor receptor; ER, estrogen receptor; FICZ, 6-formylindolo[3,2-b]carbazole; GST, glutathione S- transferase; HETE, hydroxyeicosatetraenoic acid; 13C, indole-3-carbinol; IDO, indoleamine-2,3-dioxygenase; miRNA and miR, microRNA; NQ01, NAD(P)H:quinone oxidoreductase 1; NRF2, nuclear factor erythroid 2-related factor 2; PAH, polycyclic ar- omatic hydrocarbon; PCB, polychlorinated biphenyl; PPAR, peroxisome proliferator- activated receptor; PUFA, polyunsaturated fatty acid; ROS, reactive oxygen species; TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin; TDO, tryptophan-2,3-dioxygenase; 3'UTR, 3'-untranslated region; XRE, xenobiotic-responsive element. Corresponding author. E-mail address: Thomas.haarmann-stemmann@IUF-duesseldorf.de (T. Haarmann-Stemmann). hydrocarbon receptor (AHR), also known as the dioxin receptor, and therefore commonly associated with the toxicity and carcinogenicity of dioxins, dioxin-like compounds (dibenzofurans, non-ortho poly- chlorinated biphenyls (PCBs)) and polycyclic aromatic hydrocarbons (PAHs) (Whitlock, 1999). Accordingly, induction of its transcription and enzyme activity has served as a sensitive indicator of AHR activation to screen a variety of compounds, mixtures and environmental matrices (Behnisch, Hosoe, & Sakai, 2001; Hahn, 2002). Moreover, CYP1A1 levels have served as biomarker for environmental and occupational exposure towards PAHs and organochlorines (Cosma, Toniolo, Currie, Pasternack, & Garte, 1992; Lagueux, Pereg, Ayotte, Dewailly, & Poirier, 1999; Lucier, Sunahara, & Wong, 1990; Tang et al., 2008; Vanden Heuvel et al., 1993). Despite of both, the large interindividual variability in its constitutive and inducible expression and the identification of AHR-independent pathways regulating its expression (Delescluse, Lemaire, de Sousa, & Rahmani, 2000; Hu, Sorrentino, Denison, Kolaja, & Fielden, 2007; van Duursen, Sanderson, & van den Berg, 2005), an induction of CYP1A1 is still seen synonymous to AHR activation by the vast majority of investi- gators in both academia and industry. Accordingly, the adverse outcome https://doi.org/10.1016/j.pharmthera.2018.02.012 0163-7258/© 2018 Elsevier Inc. All rights reserved. 72 M. Mescher, T. Haarmann-Stemmann / Pharmacology & Therapeutics 187 (2018) 71–87 pathways (AOPs) “AHR activation leading to hepatic steatosis”¹ and "AHR activation leading to embryo toxicity in fish"¹ define the induction of CYP1A1 as a key event. In the AOP "Rodent liver tumor promotion by sustained activation of AHR"¹ CYP1A1 induction serves as a surrogate marker for the key event, i.e. sustained AHR activation. Moreover, as recommended in the “OECD Guideline for the Testing of Chemicals 417 - Toxicokinetics" from 2010,² two draft guidelines describing the use of standardized in vitro models assessing changes in the expression and activity of CYP enzymes, including CYP1A1 and CYP1A2, to enable a better prediction of the toxicokinetics of chemicals in vivo are currently under evaluation by the OECD.³ Thus, knowledge about the capability of chemicals and drugs to modulate the expression and function of CYP1A1 is of tremendous importance for modern risk assessment and regulatory purposes. Besides its significance for toxicological and phar- macological testing, there is an increasing body of evidence depicting CYP1A1 as a pivotal regulator of physiological processes and as a prom- ising target for disease prevention and therapy. In the following, we summarize the current knowledge on the regu- lation of human CYP1A1 expression and discuss CYP1A1's critical role in detoxification and toxification of endogenous substrates and environ- mental chemicals. The suitability of the AHR/CYP1A1 axis as a target for cancer prevention, the use of CYP1A1 substrates for targeted cancer chemotherapy as well as the frequently observed phenomenon of CYP1A1 silencing in response to inflammatory and oncogenic signals are discussed. 2. Drug-metabolizing CYP monooxygenases The oxidation of lipophilic substrates is a central element of mam- malian drug metabolism. Some of the most important enzymes performing the so-called phase I reactions belong to the CYP superfam- ily of heme-containing monooxygenases (Ioannides & Lewis, 2004; Luch, 2005; Nebert, Shi, Galvez-Peralta, Uno, & Dragin, 2013; Nebert, Wikvall, & Miller, 2013). These enzymes catalyze the NADPH- dependent transfer of one oxygen equivalent to a broad range of exogenous and endogenous substrates thereby enhancing their water solubility. The resulting increase in polarity enables the conjugation of the respective metabolites to hydrophilic moieties, such as activated sugar, sulfate or glutathione, by respective phase II enzymes (Ioannides & Lewis, 2004; Luch, 2005; Nebert, Shi, et al., 2013; Nebert, Wikvall, et al., 2013). In mammals, CYP enzymes are bound to mem- branes, in particular the inner mitochondrial membrane and the endo- plasmic reticulum. There are 18 families of CYP enzymes present in mammals, which encode for 57 individual CYPs in the human genome. CYP enzymes carrying >40% homology in their amino acid sequence are classified as family and designated by Arabic numbers (e.g. CYP2). A se- quence homology of >55% further subdivides the members of a family into subfamilies, which is indicated by a capital letter (e.g. CYP2D). The subsequent Arabic number designates the isoenzymes of a subfam- ily (e.g. CYP2D6) (Ioannides & Lewis, 2004; Nebert, Shi, et al., 2013; Nebert, Wikvall, et al., 2013). The expression of several CYP enzymes is regulated by different transcription factors and thus is inducible by numerous endogenous and exogenous compounds. In addition, a vast number of allelic variants of CYP-encoding genes exists, which may affect CYP expression and function. A respective overview is given on the website of the Human Cytochrome P450 Allele Nomenclature Committee (Sim & Ingelman- Sundberg, 2006). Along with the CYP2 and CYP3 families, CYP1 family enzymes are mainly responsible for the oxidative metabolism of xenobiotic 2 1 www.aopwiki.org, AOPS 21, 41 and 57, last accessed in November 2017 www.oecd-ilibrary.org/environment/test-no-417-toxicokinetics_9789264070882- en, last accessed in November 2017 3 www.oecd.org/env/ehs/testing/section4-health-effects.htm, last accessed in November 2017 compounds. The CYP1 family consists of the three isoenzymes CYP1A1, CYP1A2 and CYP1B1 (Nebert, Dalton, Okey, & Gonzalez, 2004). Whereas basal expression of CYP1A2 is restricted to the liver, it is inducible in the brain, the gastrointestinal tract and hepatic tissue (Nebert et al., 2004). By contrast, CYP1A1 and CYP1B1 are both extrahepatically expressed and inducible (Nebert et al., 2004). The spec- trum of exogenous substrates for CYP1 isoenzymes includes but is not limited to PAHs, heterocyclic amines, aflatoxins, caffeine and pharma- ceutical drugs, such as acetaminophen (paracetamol), granisetron, clo- zapine and R-warfarin (Brown, Reisfeld, & Mayeno, 2008). In addition, CYP1 isoenzymes metabolize several endogenous substances, for in- stance melatonin, steroid hormones and polyunsaturated fatty acids (PUFAs) (Brown et al., 2008; Nebert et al., 2004). 3. Regulation of human CYP1A1 expression 3.1. Transcriptional regulation of CYP1A1 The human CYP1A locus is located on chromosome 15q22 (Jaiswal, Nebert, McBride, & Gonzalez, 1987). The CYP1A1 and CYP1A2 genes consist of 7 exons encoding proteins of 512 amino acids and 516 amino acids, respectively. The two CYP1A genes are arranged in head- to-head orientation with a DNA spacer of about 23 kb in between. This spacer region contains common regulatory elements, including a cluster of up to 15 xenobiotic-responsive elements (XRE) (Corchero, Pimprale, Kimura, & Gonzalez, 2001; Jorge-Nebert et al., 2010; Kress, Reichert, & Schwarz, 1998; Nukaya & Bradfield, 2009; Nukaya, Moran, & Bradfield, 2009; Ueda et al., 2006), emphasizing the pivotal role of the AHR in CYP1A gene regulation (Fig. 1). The AHR is a ligand-activated transcription factor belonging to the basic-Helix-Loop-Helix/Per-ARNT-Sim protein superfamily, whose members regulate gene expression in response to environmental and physiological signals (Bersten, Sullivan, Peet, & Whitelaw, 2013). Origi- nally discovered as a key regulator of xenobiotic metabolism that trig- gers the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related environmental pollutants, there is now ample evidence that the AHR is an important regulator of numerous physiological as well as pathophysiological processes (Abel & Haarmann-Stemmann, 2010; Bersten et al., 2013; Denison, Soshilov, He, DeGroot, & Zhao, 2011; Esser & Rannug, 2015; Murray, Patterson, & Perdew, 2014). In its inactive state, the AHR rests in a cytosolic multiprotein com- plex consisting of two heatshock protein 90 molecules, the AHR- interacting protein and the co-chaperone p23 (Abel & Haarmann- Stemmann, 2010; Bersten et al., 2013; Denison et al., 2011). Upon ligand-binding, the AHR undergoes conformational changes initiating the dissociation of the multiprotein complex and the exposure of a nuclear localization sequence. Subsequently, the AHR translocates into the nucleus and dimerizes with its partner molecule AHR nuclear translocator (ARNT). The AHR/ARNT complex binds to XRES (5'- GCGTG-3') in the enhancer region of target genes, such as CYP1A1, and sequentially recruits transcriptional co-activators and RNA poly- merase II to induce their transcription (Fig. 1) (Abel & Haarmann- Stemmann, 2010; Bersten et al., 2013; Denison et al., 2011). The AHR gene battery encodes for CYP1 isoenzymes and various other drug- metabolizing enzymes, such as NAD(P)H:quinone oxidoreductase 1 (NQO1), glutathione S-transferase (GST) A2, aldehyde dehydrogenase 3A1 and family 1 UDP-glucuronosyltransferases. In addition, the AHR gene battery encodes for proteins controlling cell division, differentia- tion and apoptosis (e.g. Bax, c-jun, c-myc, Hes-1, IL-2, junD, p21CIP1 and p27KIP1) as well as for a negative feedback inhibitor of AHR signal- ing, the AHR repressor (AHRR) (Bock & Köhle, 2006). Depending on cell-type and tissue, the AHRR may compete with AHR for both ARNT- and XRE-binding thereby attenuating the AHR-driven expression of CYP1A1 and other target genes (Vogel & Haarmann-Stemmann, 2017). AHR ligands include ubiquitous environmental pollutants, such as PAHs, dibenzo-p-dioxins, dibenzofurans and dioxin-like PCBs, as HSP90 00 00 AHR HSP90 p23 AIP p23 AIP D AHR AHR ARNT XRE M. Mescher, T. Haarmann-Stemmann / Pharmacology & Therapeutics 187 (2018) 71-87 ARNT AHRR CYP1A1 CYP1B1 ... Fig. 1. Regulation of CYP1A1 expression through the AHR signaling pathway. In its inactive state, the AHR rests in a cytosolic multiprotein complex. Upon ligand-binding, this complex dissociates and the AHR shuttles into the nucleus and heterodimerizes with ARNT. The AHR/ARNT complex binds to XRES in the enhancer region of target genes, for instance encoding CYP1A1 and AHRR, to induce their transcription. well as plant- and bacteria-derived polyphenols and indoles (Abel & Haarmann-Stemmann, 2010; Denison et al., 2011; Hubbard et al., 2015; Murray et al., 2014; Stejskalova, Dvorak, & Pavek, 2011). In addi- tion, various endogenous compounds have been identified to serve as AHR agonists, including tryptophan metabolites, such as kynurenine, kynurenic acid and xanthurenic acid, indole derivatives, such as 6- formylindolo[3,2-b] carbazole (FICZ), and arachidonic acid (ArA) me- tabolites, such as lipoxin A4 (Abel & Haarmann-Stemmann, 2010; Denison et al., 2011; Murray et al., 2014; Stejskalova et al., 2011). Notably, remarkable species-specific differences in the AHR ligand spectrum and affinity exist, which have to be carefully considered for regulatory purposes, for instance when data from animal studies are being extrapolated to define human threshold values. A good example for chemicals exhibiting species-specific differences in AHR binding af- finity are PCBs: Whereas several dioxin-like PCBs activate the rat AHR, PCB 126 seems to be the only congener that is capable of stimulating human AHR activity (Larsson et al., 2015). In addition, comparisons of inducible CYP1A enzyme activity and XRE-driven reporter gene activity in rat and human hepatic cells have shown that PCB 126 as well as TCDD is dramatically less affine to the human than to the rat AHR protein (Brennan et al., 2015; Larsson et al., 2015; Silkworth et al., 2005). Besides the canonical AHR/ARNT pathway, the ligand-activated AHR frequently interacts with other major cellular signaling pathways, in- cluding epidermal growth factor receptor (EGFR), hypoxia-inducible factor, Wnt/ẞ-catenin, nuclear factor erythroid 2-related factor 2 (NRF2) and NF-κB signaling, which also may have an impact on 73 CYP1A1 expression (Abel & Haarmann-Stemmann, 2010; Denison et al., 2011; Haarmann-Stemmann & Abel, 2012; Ma, Kinneer, Bi, Chan, & Kan, 2004; Schulthess et al., 2015). In addition to the XRE cluster, several binding sites for other tran- scription factors, including peroxisome proliferator-activated receptor (PPAR)-a, constitutive androstane receptor and p53, have been identi- fied in the regulatory sequence of the human CYP1A loci (Table 1), which may modulate CYP1A1 expression in a cell-, tissue- and develop- mental stage-specific manner in response to various physiological and pathophysiological signals. 3.2. Post-transcriptional regulation of CYP1A1 by microRNAs MicroRNAs (miRNAs) are conserved small non-coding RNAs of ~23 nucleotides that resemble the RNA-silencing function of small- interfering RNAs. They assemble with Argonaute proteins and form miRNA-induced silencing complexes to interact with complementary mRNAs, preferentially in the 3'-untranslated region (3'UTR), to induce their degradation and inhibit their translation (He & Hannon, 2004). By using different online prediction tools, several studies have identified miRNAs that may target the human CYP1A1 mRNA. However, probably due to their extremely variable expression pattern, the identified miRNAs do not overlap. For instance, one study has identified one or more putative binding sites for miR-125b-2, miR-488, miR-511, miR- 626, miR-657 and miR-892a in the 3'UTR of the human CYP1A1 tran- script (Jorge-Nebert et al., 2010). Another study found a correlation between the expression of miR-21, miR-34a, miR-132, miR-132-3p, miR-148b, miR-200a and miR-200b and a reduced CYP1A1 protein level (Rieger, Klein, Winter, & Zanger, 2013). In addition, miR-21-3p, miR-125b-5p, miR-150 and miR-892a were shown to repress CYP1A1 in human tissues and cells (Burgess et al., 2015; Choi et al., 2012; Lo et al., 2017; Sturchio et al., 2014). Finally, an in silico approach using five different prediction platforms identified 2, 85, 264 and two- times 14 unique miRNAs targeting the human CYP1A1 mRNA (Ramamoorthy & Skaar, 2011), illustrating a huge grade of diversity and uncertainty in miRNA prediction. Taken together, these findings indicate that CYP1A1 is post- transcriptionally regulated by miRNAs. Due to the limited experimental evidence, it is hardly predictable to which extent miRNAs influence the activity and function of CYP1A1. The correlation of miRNA expression patterns with a reduced expression of CYP1A1 is probably also compro- mised by miRNAs that target AHR or ARNT and thus indirectly abrogate CYP1A1 expression. Interestingly, several single nucleotide polymor- phisms in the CYP1A1 gene have been described to delete or create po- tential binding sites for miRNAs in the 3'UTR of the respective mRNA molecule (Jorge-Nebert et al., 2010; Ramamoorthy & Skaar, 2011). Table 1 Transcription factors having one or more binding-sites in the human CYP1A1 promoter/ enhancer. Transcription factor Aryl hydrocarbon receptor Basic transcription element binding protein 3 Basic transcription element binding protein 4 Constitutive androstane receptor Liver X receptor alpha Nuclear factor-I p53 Reference Prototype ligand TCDD / Kress et al. (1998) Kaczynski et al. (2002) Phenobarbital Oxysterols Peroxisome proliferator-activated Arachidonic acid receptor alpha Retinoic acid receptor alpha Thyroid hormone receptor alpha Vitamin D receptor 9-cis-Retinoic acid Triiodothyroine Calcitrol Kaczynski et al. (2002) Yoshinari, Yoda, Toriyabe, and Yamazoe (2010) Shibahara et al. (2011) Morel and Barouki (1998) Wohak et al. (2016) Seree et al. (2004) Vecchini et al. (1994) Vecchini et al. (1994) Matsunawa et al. (2012) 74 4. Metabolic control of AHR signaling M. Mescher, T. Haarmann-Stemmann / Pharmacology & Therapeutics 187 (2018) 71-87 Except metabolically stable ligands, such as TCDD, xenobiotics that enter the cell and activate AHR induce their own oxidative metabolism via CYP1 isoenzymes. Transient transfection experiments with CYP1A1, CYP1A2 and CYP1B1 constructs have shown that overexpression of the CYP1 enzymes reduced the basal activity of a XRE-driven reporter gene, indicating the presence of endogenous AHR agonists (Chiaro, Patel, Marcus, & Perdew, 2007). A possible candidate is FICZ (Rannug et al., 1987), which is intracellularly generated upon absorption of ultraviolet-B radiation by tryptophan (Fritsche et al., 2007) or through the oxidation of tryptophan or indole-3-pyruvate by hydrogen peroxide (Smirnova et al., 2016; Wincent et al., 2012), and known to be rapidly metabolized by CYP1A1 (Bergander et al., 2004). Accordingly, it was shown that inhibition of CYP1A1 enzyme activity elevates intracellular FICZ levels and associated AHR activity (Wincent et al., 2012, 2016). Vice versa, general as well as intestine-directed overexpression of CYP1A1 in mice resulted in an accelerated clearance of endogenous as well as food-derived AHR ligands, leading to the termination of AHR sig- naling and associated adverse health effects in the intestine, i.e. loss of certain immune cell populations and elevated susceptibility to enteric infection (Schiering et al., 2017). Just recently, CYP1A1 inhibition was reported to simultaneously increase the expression of c-kit and IL-22 and decrease the expression of IL-17 in human primary CD4+ T helper cells. Co-treatment with the ligand-selective AHR antagonist CH-223191 reversed these effects, indicating that the CYP1A1-driven metabolism of endogenous AHR ligands critically influences AHR- dependent immune reactions (Effner et al., 2017). 5. Substrates of CYP1A1 As recently described in more detail (Santes-Palacios et al., 2016; Sridhar, Goyal, Liu, & Foroozesh, 2017), the human CYP1A1 protein con- sists of four ẞ-sheets and 12 α-helices harboring the catalytically active site with the heme iron center. The structure of the human CYP1A1 en- zyme allows the binding of planar aromatic or heterocyclic molecules with a dimension of approximately ~12.3 Å × ~4.6 Å (Santes-Palacios et al., 2016). The catalytic cycle starts with a substrate binding near to the catalytic center resulting in the displacement of a loosely bound water molecule. Subsequently, the CYP reductase transfers an electron from NADPH to the iron atom leading to the association of molecular ox- ygen. The transmission of a second electron and the admission of two protons lead to cleavage of the molecular oxygen. Subsequently, one ox- ygen atom is integrated into a water molecule, whereas the other one is transmitted to the substrate. Finally, water displaces the oxidized sub- strate, thereby restoring the resting state (Santes-Palacios et al., 2016; Sridhar et al., 2017). The major catalytic reactions carried out by CYP1A1 are oxidation reactions, such as hydroxylation, epoxidation, N-hydroxylation and O-demethylation, as well as nitroreductions (Santes-Palacios et al., 2016; Sridhar et al., 2017). The structure of some aromatic and heterocyclic chemicals and their CYP1A1- generated metabolites are shown in Fig. 2. A more complete picture of CYP1A1 substrates is provided in the following review articles (Brown et al., 2008; Nebert et al., 2004; Santes-Palacios et al., 2016). 5.1. CYP1A1 and estrogen metabolism Estrogen levels critically influence cell proliferation, development and tissue homeostasis, and a dysregulation of estrogen/estrogen recep- tor (ER) signaling is associated with the development of cancer, meta- bolic and cardiovascular diseases, osteoporosis and neurodegenerative diseases (Jia, Dahlman-Wright, & Gustafsson, 2015). The oxidative metabolism of 17ẞ-estradiol is driven by several CYP enzymes in hepatic as well as extrahepatic tissues. The quantitatively dominating metabolites of 17ẞ-estradiol are 2-hydroxyestradiol and 4-hydroxyestradiol (Jefcoate et al., 2000; Tsuchiya, Nakajima, & Yokoi, 2005). The hydroxylation at position 2 is carried out by CYP1A1 (Fig. 2), CYP1A2 and CYP3A4 (Lee, Cai, Thomas, Conney, & Zhu, 2003) and is, at least in the context of carcinogenicity, regarded as a detoxifica- tion process (Tsuchiya et al., 2005). Even though, 2-hydroxyestradiol may undergo redox cycling, it is rapidly methylated by the catechol O-methyltransferase to 2-methoxyestradiol, a process that also neutralizes the mitogenic potential of 17ẞ-estradiol. Accordingly, 2- hydroxyestradiol is not carcinogenic in animal models. In contrast, the 4-hydroxylation of 17ẞ-estradiol is carried out by CYP1B1 (Hayes et al., 1996), which is highly expressed in estrogen target tis- sues (Jefcoate et al., 2000; Tsuchiya et al., 2005). 4-hydroxyestradiol is a potent redox-cycler, whose detoxification by the catechol O- methyltransferase occurs much slower as compared to 2- hydroxyestradiol. The resulting generation of reactive oxygen species (ROS) and the associated oxidative damage of DNA, along with the mitogenic action of estradiol, may contribute to the development of hormone-dependent malignancies, such as endometrial and breast cancer (Jefcoate et al., 2000; Tsuchiya et al., 2005). The CYP1A1-mediated metabolic breakdown of estrogens may contribute to the anti-estrogenic effects associated with an exposure to several environmental as well as natural AHR ligands. In fact, the anti-estrogenicity of TCDD, PCBs and structurally related persistent organic pollutants has been assigned, at least in part, to an AHR- dependent induction of CYP1A1 and CYP1B1 activities (Hayes et al., 1996; Segura-Aguilar, Castro, & Bergman, 1997; Spink et al., 1992; Spink, Lincoln, Dickerman, & Gierthy, 1990). In addition, PAHs found in tobacco smoke (Michnovicz, Hershcopf, Naganuma, Bradlow, & Fishman, 1986) as well as dietary derived compounds, such as indole-3-carbinol (I3C) and its acidic condensation product indolo [3,2-b]carbazole (Liu, Wormke, Safe, & Bjeldanes, 1994; Yuan et al., 1999), may enhance the CYP1A1-mediated oxidation of 17ẞ-estradiol to 2-hydroxyestradiol. An enforcement of steroid hormone metabolism by CYP1A1 should not only be seen in the light of endocrine disruption, as it may be of interest for the therapy of hormone-dependent cancers as well. In fact, I3C has been successfully investigated in a phase I clinical trial for its capability to enhance estrogen detoxification in a cohort of high-risk breast cancer women (Reed et al., 2005). Notably, additional CYP1-independent mechanisms have been de- scribed by which AHR modulators may manipulate endocrine systems and vice versa (Monostory, Pascussi, Kobori, & Dvorak, 2009). For instance, AHR activation may enforce the proteasomal degradation of ERQ and other steroid hormone receptors (Ohtake et al., 2007; Wormke et al., 2003) and dysregulate the expression of CYP19 aroma- tase, a key enzyme in estrogen synthesis (Baba et al., 2005). 5.2. CYP1A1 and arachidonic acid metabolism Beside steroid hormones, CYP1A1 is also critically involved in the metabolism of endogenous PUFAs, in particular arachidonic acid (ArA) and eicosanoids (Hankinson, 2016; Nebert et al., 2004; Nebert, Shi, et al., 2013; Nebert, Wikvall, et al., 2013). ArA is a 0-6 PUFA present in the phospholipids of cell membranes, which triggers inflammatory re- actions, cellular signaling and vasodilation. Phospholipases are capable of releasing ArA from the cell membranes thereby making it accessible for various oxidases. Specifically, ArA is metabolized by cyclooxygenase (COX)-1 and COX-2, arachidonate 5-, 12- and 15-lipoxygenase as well as by several CYP enzymes to a wide range of biologically active eicosa- noids, including prostacyclins, prostaglandins (PG), thromboxanes, leukotriens, epoxyeicosatrienoic acids and hydroxyeicosatetraenoic acids (HETE) (Funk, 2001; Soberman & Christmas, 2003). Eicosanoids are local hormones produced by vertebrate cells, which act in an auto- crine and/or paracrine manner. These straight-chain PUFAs act through G-protein coupled cell surface receptors, so-called prostanoid receptors, as well as nuclear receptors, in particular PPARs (Grygiel-Gorniak, 2014; Luo, Flamand, & Brock, 2006). Eicosanoids are involved in the regulation of pro-inflammatory responses, fever, pain, blood clotting, blood M. Mescher, T. Haarmann-Stemmann / Pharmacology & Therapeutics 187 (2018) 71-87 O-deethylation CH3 7-ethoxyresorufin benzo[a]pyrene N CH3 ΝΗ -NH2 2-amino-1-methyl-6-phenyl- Imidazo[4,5-b]pyridine HN epoxidation N-hydroxylation NH hydroxylation 6-formylindolo[3,2-b]carbazole H3C NH melatonin HN CH3 OH resorufin benzo[a]pyrene-7,8-epoxide CH3 OH NH NH 2-hydroxyamino-1 methyl-6-phenyl- imidazo[4,5-b]pyridine HN H NH OH 2-hydroxyindolo[3,2-b]carbazole- 6-carboxaldehyde hydroxylation H3C hydroxylation OH CH3 но NH CH3 HN 6-hydroxymelatonin OH OH CH3 75 arachidonic acid 19-hydroxy-arachidonic acid Fig. 2. Typical substrates, reactions and metabolites of CYP1A1. This figure presents an overview on different exogenous and endogenous substrates of CYP1A1 and the corresponding types of reaction and metabolites. pressure and allergic reactions (Funk, 2001; Soberman & Christmas, 2003). In addition, several eicosanoids, in particular PGE2 (Nakanishi & Rosenberg, 2013; Pang, Hurst, & Argyle, 2016) but also 12-HETE and 20-HETE (Honn et al., 1994; Pidgeon et al., 2007; Wang & DuBois, 2010), have been identified to promote cancer growth by interacting with different signal transduction pathways. For instance, 12-HETE was shown to activate protein kinase C and MAPK signaling pathways thereby altering cancer cell proliferation, motility and apoptosis suscep- tibility (Ding, Tong, & Adrian, 2001; Szekeres, Tang, Trikha, & Honn, 2000). The laboratory of Oliver Hankinson has recently performed a liquid chromatography-tandem mass spectrometry-based analysis of a large number of PUFA metabolites in tissue extracts from TCDD-treated wild-type and AHR-KO mice (Hankinson, 2016; Yang, Solaimani, Dong, Hammock, & Hankinson, 2013). The analysis revealed that TCDD treatment markedly increased the levels of several epoxides and diol metabolites of both -6 and -3 PUFA in the liver and lungs of mice in an AHR- and CYP1-dependent fashion. Another study has shown that in mouse liver microsomes, TCDD-induced CYP1A enzymes increased epoxyeicosatrienoic acid 2-fold, 19-HETE 5-fold and 16- to 18-HETE 20-fold (Lee, Lawrence, Kerkvliet, & Rifkind, 1998). Human CYP1A1 acts primarily as an ArA hydroxylase, which oxidizes ArA to 19-OH-ArA (90%) (Fig. 2) and 14,15-epoxyeicosatrienoic acids (7%) (Schwarz et al., 2004). With eicosapentaenoic acid as substrate, human CYP1A1 behaves as epoxygenase producing 17,18- epoxyeicosatrienoic acids (68%) and to a lesser extent 19-OH- eicosapentaenoic acid (31%) (Schwarz et al., 2004). In addition, in vitro studies revealed that human CYP1A1 is also capable of oxidizing ArA to HETE. Specifically, the production of 12-HETE and several C-terminal HETE (16- to 20-HETE) was shown to depend on the cata- lytic activity of CYP1A1 (Choudhary, Jansson, Stoilov, Sarfarazi, & Schenkman, 2004; Jarrar et al., 2013; Nguyen et al., 2016). Interestingly, studies on TCDD-exposed chick embryo livers revealed an induction of CYP1A4, the avian ortholog of mammalian CYP1A1, and subsequent epoxygenation of ArA not only in microsomes but also in mitochondria (Labitzke, Diani-Moore, & Rifkind, 2007).