Structure of an ancestral mammalian family 1B1 cytochrome P450 with increased thermostability

Mammalian cytochrome P450 enzymes often metabolize many pharmaceuticals and other xenobiotics, a feature that is valuable in a biotechnology setting. However, extant P450 enzymes are typically relatively unstable, with T50 values of ∼30-40 °C. Reconstructed ancestral cytochrome P450 enzymes tend to have variable substrate selectivity compared with related extant forms, but they also have higher thermostability and therefore may be excellent tools for commercial biosynthesis of important intermediates, final drug molecules, or drug metabolites. The mammalian ancestor of the cytochrome P450 1B subfamily was herein characterized structurally and functionally, revealing differences from the extant human CYP1B1 in ligand binding, metabolism, and potential molecular contributors to its thermostability. Whereas extant human CYP1B1 has one molecule of α-naphthoflavone in a closed active site, we observed that subtle amino acid substitutions outside the active site in the ancestor CYP1B enzyme yielded an open active site with four ligand copies. A structure of the ancestor with 17β-estradiol revealed only one molecule in the active site, which still had the same open conformation. Detailed comparisons between the extant and ancestor forms revealed increases in electrostatic and aromatic interactions between distinct secondary structure elements in the ancestral forms that may contribute to their thermostability. To the best of our knowledge, this represents the first structural evaluation of a reconstructed ancestral cytochrome P450, revealing key features that appear to contribute to its thermostability.

Structural characterization of the complex between alpha-naphthoflavone and human cytochrome P450 1B1

The atomic structure of human P450 1B1 was determined by x-ray crystallography to 2.7 Å resolution with α-naphthoflavone (ANF) bound in the active site cavity. Although the amino acid sequences of human P450s 1B1 and 1A2 have diverged significantly, both enzymes exhibit narrow active site cavities, which underlie similarities in their substrate profiles. Helix I residues adopt a relatively flat conformation in both enzymes, and a characteristic distortion of helix F places Phe(231) in 1B1 and Phe(226) in 1A2 in similar positions for π-π stacking with ANF. ANF binds in a distinctly different orientation in P450 1B1 from that observed for 1A2. This reflects, in part, divergent conformations of the helix B'-C loop that are stabilized by different hydrogen-bonding interactions in the two enzymes. Additionally, differences between the two enzymes for other amino acids that line the edges of the cavity contribute to distinct orientations of ANF in the two active sites. Thus, the narrow cavity is conserved in both P450 subfamily 1A and P450 subfamily 1B with sequence divergence around the edges of the cavity that modify substrate and inhibitor binding. The conservation of these P450 1B1 active site amino acid residues across vertebrate species suggests that these structural features are conserved.

The binding of lignans, flavonoids and coumestrol to CYP450 aromatase: a molecular modelling study

Androgens are transformed into aromatic estrogens by CYP450 aromatase in a three-step reaction consuming three equivalents of oxygen and three equivalents of NADPH. Estrogens are substrates for nuclear estrogen receptors (ERs) and play a key role in estrogen-dependent tumour cell formation and proliferation. Natural phytoestrogens are proved to be competitive inhibitors of aromatase enzyme at IC(50) values in micromolar levels. In order to understand the mechanisms involved in the binding of various phytoestrogens, we used our model of CYP450 aromatase to study the binding of phytoestrogens using molecular dynamics simulations with a bound phytoestrogen. The simulation trajectory was analysed to find the essential interactions which take place upon binding and a representative structure of the trajectory was minimized for docking studies. Sets of phytoestrogens, such as lignans, flavonoids/isoflavonoids and coumestrol, were docked into the aromatase active site and the binding modes were studied.

Modulation of cytochrome P450 enzyme system by selected flavonoids

OBJECTIVES

The aim of this study was to assess the effect of various flavonoids on the NADPH:cytochrome P450 oxidoreductase (CYPOR) activity in respect of the reduction of different electron acceptors as well as to study an impact of flavonoids on monooxygenation of a model substrate of cytochrome P450 (CYP).

DESIGN

The modulation of CYPOR activity was determined spectrophotometrically based on the time course of the reduction of different electron acceptors. The CYP reduction was monitored via its complex formation with CO, having pronounced the absorption maximum at 450 nm. Finally, effect of CYPOR stimulation by 7,8benzoflavone (ANF) on 7pentoxyresorufin Odepentylation was assayed in the microsomal monooxygenation system using the fluorimetric detection of formed resorufin.

RESULTS

The stimulation of CYPOR activity via ANF was found to be associated with following electron acceptors: cytochrome c, potassium ferricyanide, cytochrome b5, but not with CYP. Surprisingly, 5,6benzoflavone, a position isomer of ANF, was ineffective in the CYPOR stimulation as well as the other flavonoids tested. In microsomal preparations, ANF did not markedly enhance the reaction rate of monooxygenation of CYP2B4 model substrate.

CONCLUSION

Our results document that among all of the tested flavonoids only ANF is able to stimulate CYPOR activity, however, the ANF-mediated stimulation of CYPOR has no impact on the oxidative metabolism catalyzed by CYP system.

Autoradiographic localization of aromatic hydrocarbon receptor (AHR) in rhesus monkey ovary

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic congener of a large class of manmade pollutants that persist in the environment. TCDD exerts its toxic effects, in part, by binding to its receptor known as the aromatic hydrocarbon receptor (AHR). TCDD is estrogen modulatory and in some systems its receptor associates directly with estrogen receptors via co-activator molecules. TCDD inhibits steroid synthesis in human ovarian granulosa cells and AHR is found in these cells. We have previously shown that AHR is found in whole rhesus monkey ovary, but have yet to establish its location. In the present study, we set out to show that radiolabeled TCDD binds to monkey ovarian follicles and that this binding is receptor mediated. Ovaries from Macaca mulatta were sectioned on a cryostat at 10 micro m; and sections were incubated with either control vehicle, (3)H-TCDD, or (3)H-TCDD plus alpha-naphthoflavone (ANF), a known receptor-blocking agent. Here, we show for the first time specific binding of TCDD to the granulosa cells of antral follicles and other regions of the rhesus monkey ovary. Our data indicate a 60-fold increase in binding with (3)H-TCDD over that of control, and that this binding is reduced to the levels seen in controls with the addition of the competitive antagonist ANF. These findings support the hypothesis that TCDD directly affects primate ovarian function via the AHR.

Characterization of a 7,8-benzoflavone double effect on CFTR Cl(-) channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of adenosine triphosphate (ATP)-binding cassette (ABC) transporter ATPases. This protein forms a Cl(-) channel with a complex regulation; gene mutations cause cystic fibrosis disease. We investigated the interaction between the protein and the flavone UCCF-029 using the patch-clamp technique in the excised inside-out configuration in order to study the molecular mechanism of action for this potentiator on completely phosphorylated channel (25 U/ml protein kinase A) and a relatively low level of ATP (0.3 mM: ). Low concentrations of UCCF-029 (<50 nM: ) increase the open probability (p (o)), favoring the channel transition to an activated state, while high UCCF-029 (>50 nM: ) levels determine inhibition of the CFTR by a reduction of the total open time. Our data suggest that this drug can potentiate CFTR by binding to a specific site on the nucleotide binding domain, promoting dimer formation. The response of CFTR to variable concentrations of ATP is not modified by application of the potentiator UCCF-029 at either low, activatory, concentration or high, inhibitory, levels. Hence, we conclude that the potentiator may not interfere with binding of ATP but probably acts at an independent site in the protein, interacting directly with CFTR to modulate channel activity.

Adaptations for the Oxidation of Polycyclic Aromatic Hydrocarbons Exhibited by the Structure of Human P450 1A2

Microsomal cytochrome P450 family 1 enzymes play prominent roles in xenobiotic detoxication and procarcinogen activation. P450 1A2 is the principal cytochrome P450 family 1 enzyme expressed in human liver and participates extensively in drug oxidations. This enzyme is also of great importance in the bioactivation of mutagens, including the N-hydroxylation of arylamines. P450-catalyzed reactions involve a wide range of substrates, and this versatility is reflected in a structural diversity evident in the active sites of available P450 structures. Here, we present the structure of human P450 1A2 in complex with the inhibitor alpha-naphthoflavone, determined to a resolution of 1.95 A. alpha-Naphthoflavone is bound in the active site above the distal surface of the heme prosthetic group. The structure reveals a compact, closed active site cavity that is highly adapted for the positioning and oxidation of relatively large, planar substrates. This unique topology is clearly distinct from known active site architectures of P450 family 2 and 3 enzymes and demonstrates how P450 family 1 enzymes have evolved to catalyze efficiently polycyclic aromatic hydrocarbon oxidation. This report provides the first structure of a microsomal P450 from family 1 and offers a template to study further structure-function relationships of alternative substrates and other cytochrome P450 family 1 members.

Mechanism of interactions of alpha-naphthoflavone with cytochrome P450 3A4 explored with an engineered enzyme bearing a fluorescent probe

Design of a partially cysteine-depleted C98S/C239S/C377S/C468A cytochrome P450 3A4 mutant designated CYP3A4(C58,C64) allowed site-directed incorporation of thiol-reactive fluorescent probes into alpha-helix A. The site of modification was identified as Cys-64 with the help of CYP3A4(C58) and CYP3A4(C64), each bearing only one accessible cysteine. Changes in the fluorescence of CYP3A4(C58,C64) labeled with 6-(bromoacetyl)-2-(dimethylamino)naphthalene (BADAN), 7-(diethylamino)-3-(4'-maleimidylphenyl)-4-methylcoumarin (CPM), or monobromobimane (mBBr) were used to study the interactions with bromocriptine (BCT), 1-pyrenebutanol (1-PB), testosterone (TST), and alpha-naphthoflavone (ANF). Of these substrates only ANF has a specific effect, causing a considerable decrease in fluorescence intensity of BADAN and CPM and increasing the fluorescence of mBBr. This ANF-binding event in the case of the BADAN-modified enzyme is characterized by an S50 of 18.2 +/- 0.7, compared with the value of 2.2 +/- 0.3 for the ANF-induced spin transition, thus revealing an additional low-affinity binding site. Studies of the effect of TST, 1-PB, and BCT on the interactions of ANF monitored by changes in fluorescence of CYP3A4(C58,C64)-BADAN or by the ANF-induced spin transition revealed no competition by these substrates. Investigation of the kinetics of fluorescence increase upon H2O2-dependent heme depletion suggests that labeled CYP3A4(C58,C64) is represented by two conformers, one of which has the fluorescence of the BADAN and CPM labels completely quenched, presumably by photoinduced electron transfer from the neighboring Trp-72 and/or Tyr-68 residues. The binding of ANF to the newly discovered binding site appears to affect the interactions of the label with the above residue(s), thus modulating the fraction of the fluorescent conformer.

Cooperative Binding of Midazolam with Testosterone and α-Naphthoflavone within the CYP3A4 Active Site: A NMR T1 Paramagnetic Relaxation Study

Recent studies have indicated that CYP3A4 exhibits non-Michaelis-Menten kinetics for numerous substrates. Both homo- and heterotropic activation have been reported, and kinetic models have suggested multiple substrates within the active site. We provide some of the first physicochemical data supporting the hypothesis of allosteric substrate binding within the CYP3A4 active site. Midazolam (MDZ) is metabolized by CYP3A4 to two hydroxylated metabolites, 1'- and 4-hydroxymidazolam. Incubations using purified CYP3A4 and MDZ showed that both alpha-naphthoflavone (alpha-NF) and testosterone affect the ratio of formation rates of 1'- and 4-hydroxymidazolam. Similar to previous reports, alpha-NF was found to promote formation of 1'-hydroxymidazolam, while testosterone stimulated formation of 4-hydroxymidazolam. NMR was used to measure the closest approach of individual MDZ protons to the paramagnetic heme iron of CYP3A4 using paramagnetic T(1) relaxation measurements. Solutions of 0.2 microM CYP3A4 with 500 microM MDZ resulted in calculated distances between 7.4 and 8.3 A for all monitored MDZ protons. The distances were statistically equivalent for all protons except C3-H and were consistent with the rotation within the active site or sliding parallel to the heme plane. When 50 microM alpha-NF was added, proton-heme iron distances ranged from 7.3 to 10.0 A. Consistent with kinetics of activation, the 1' position was situated closest to the heme, while the fluorophenyl 5-H proton was the furthest. Proton-heme iron distances for MDZ with CYP3A4 and 50 microM testosterone ranged from 7.7 to 9.0 A, with the flourophenyl 5-H proton furthest from the heme iron and the C4-H closest to the heme, also consistent with kinetic observations. When titrated with CYP3A4 in the presence of MDZ, testosterone and alpha-NF resonances themselves exhibited significant broadening and enhanced relaxation rates, indicating that these effector molecules were also bound within the CYP3A4 active site near the paramagnetic heme iron. These results suggest that the effector exerts its cooperative effects on MDZ metabolism through simultaneous binding of MDZ and effector near the CYP3A4 heme.

Specific interactions of steroids, arylhydrocarbons and flavonoids with progesterone receptors from the cytosol of the fungus Rhizopus nigricans

Rhizopus nigricans (R. nigricans) transforms fungitoxic progesterone into the less toxic 11alpha-hydroxyprogesterone which is then able to exit the mycelia into the surrounding water. Hydroxylation of progesterone is an inducible process in which cytosolic progesterone receptors could be involved. In the present study, we characterised receptors with respect to ligand specificity and to their involvement in progesterone induction of hydroxylase. EC(50) values of different ligands (steroids, xenobiotic arylhydrocarbons and natural flavonoids) were determined by competition studies using 40nM ((3)H)progesterone. C21 and C19 3-oxo-4-ene steroids were good competitors (EC(50) of progesterone 2.3 +/- 0.1 x 10(-7)M, EC(50) of androsten-3,17-dione 24 +/- 2 x 10(-7)M). The presence of hydroxyl groups in steroids significantly decreased the affinity for receptors. The arylhydrocarbons alpha-naphthoflavone and ketoconazole exhibited EC(50) values of 0.3 +/- 0.01 x 10(-7)M and 27 +/- 5 x 10(-7)M, respectively, whereas beta-naphthoflavone and benzo(a)pyrene were not able to displace labelled progesterone completely. The competition curves obtained by natural flavonoids also did not reach the bottom level of non-labelled progesterone, indicating the interaction at some allosteric binding site(s) of progesterone receptors. All ligands were examined for their involvement in progesterone-hydroxylase induction. Steroid agonists induced the enzyme in a dose-dependent manner in accordance with their affinity for receptors, whereas arylhydrocarbons and natural flavonoids did not induce the enzyme. The agonistic action of steroids, together with the antagonistic action of alpha-naphthoflavone, strongly suggests the involvement of progesterone receptors in progesterone signalling resulting in the induction of progesterone-hydroxylase.

beta-Naphthoflavone disrupts cortisol production and liver glucocorticoid responsiveness in rainbow trout

We investigated the impact of aryl hydrocarbon receptor (AhR) activation on cortisol production in rainbow trout (Oncorhynchus mykiss) using beta-naphthoflavone (BNF) and alpha-naphthoflavone (ANF) as agonist and antagonist of AhR, respectively. Fish were given a single intraperitoneal injection of either corn oil alone or corn oil containing ANF (50 mg/kg body mass), BNF (50 mg/kg) or a combination of both (ANF/BNF; 50 mg/kg each) and sampled at 2- and 6-day post-injection. Higher CYP1A protein expression in the liver and mRNA abundance in the head kidney confirmed AhR activation with BNF, but ANF did not abolish this response. BNF treatment did not significantly affect mRNA abundance of key steroidogenic enzymes (StAR protein, P450 side chain cleavage and 11beta-hydroxylase) in the head kidney of trout. However, BNF depressed the in vitro ACTH- or 8-br-cAMP-mediated cortisol production and this effect was also not modified by ANF. Plasma cortisol levels were significantly higher with BNF in the present study and this BNF-mediated cortisol response was completely abolished with ANF suggesting a role for AhR and/or CYP1A in the regulation of plasma cortisol concentration. However, despite elevated plasma cortisol levels, there was no significant effect of BNF on liver glucocorticoid receptor (GR) protein expression suggesting a lack of liver responsiveness to cortisol stimulation. Also, this was reflected in the absence of any change in plasma glucose concentration in the BNF group at 2 days, and a significant drop at 6 days compared to other treatments. Altogether, our results suggest that AhR activation disrupts interrenal corticosteroidogenesis and target tissue responsiveness to glucocorticoid stimulation in rainbow trout.

3-Methylcholanthrene induces lymphocyte and phagocyte apoptosis in common carp (Cyprinus carpio L) in vitro

Polycyclic aromatic hydrocarbons (PAH) are an important class of environmental pollutant that are known to be carcinogenic and immunotoxic. In mammals it was suggested that PAH compromise the immune system in part through the induction of programed cell death (apoptosis). In fish, no study has reported the importance of this physiological process in PAH-induced immunotoxicity. We have therefore investigated the capacity of 3-methylcholanthrene to induce lymphocyte and phagocyte apoptosis in carp. By three criteria (exposition of phosphatidylserine at the outer cell membrane, chromatin condensation and fragmentation, and decreased cell size) the data indicate that 3-methylcholanthrene (3-MC) treatment (from 20 to 200 microM) during 24 h produces apoptosis in both lymphocytes and phagocytes. In order to evaluate whether 3-MC induced apoptosis is related to the metabolic activation of 3-MC or 3-MC Ah-R binding, co-exposure experiments with 3-MC and alpha-naphtoflavone (alpha-NF), a compound that inhibits metabolic activation of 3-MC and 3-MC Ah-R binding were performed. While alpha-NF did not prevent 3-MC-induced apoptosis, the compound itself was found to be a strong inducer of apoptosis. There results might indicate that metabolic activation of 3-MC or 3-MC Ah-R binding is not causally linked to apoptosis. However, since 3-MC, alpha-NF and 3-MC + alpha-NF treatments produce the same sustained increase (3 h minimum) in intracellular calcium level, it is possible that this phenomenon is implicated in the induction of programmed cell death by these hydrocarbons.

Rabbit CYP4B1 engineered for high-level expression in Escherichia coli: ligand stabilization and processing of the N-terminus and heme prosthetic group

Modifications at the N-terminus of the rabbit CYP4B1 gene resulted in expression levels in Escherichia coli of up to 660 nmol/L. Solubilization of the enzyme from bacterial membranes led to substantial conversion to cytochrome P420 unless alpha-naphthoflavone was added as a stabilizing ligand. Mass spectrometry analysis and Edman sequencing of purified enzyme preparations revealed differential N-terminal post-translational processing of the various constructs expressed. Notably, bacterial expression of CYP4B1 produced a holoenzyme with >98.5% of its heme prosthetic group covalently linked to the protein backbone. The near fully covalently linked hemoproteins exhibited similar rates and regioselectivities of lauric acid hydroxylation to that observed previously for the partially heme processed enzyme expressed in insect cells. These studies shed new light on the consequences of covalent heme processing in CYP4B1 and provide a facile system for future mechanistic and structural studies with the enzyme.

Tight-binding inhibition by alpha-naphthoflavone of human cytochrome P450 1A2

Human cytochrome P450 (P450) enzymes exhibit remarkable diversity in their substrate specificities, participating in oxidation reactions of a wide range of xenobiotic drugs. Previously, we reported that alpha-naphthoflavone (ANF) is bound to the recombinant P450 1A2 tightly and stabilizes an overall enzyme conformation. The present study is designed to determine the type of P450 1A2 inhibition exerted by ANF, using two different substrates of P450 1A2, 7-ethoxycoumarin (EOC) and 7-ethoxyresorufin (EOR). ANF is generally known as a competitive inhibitor of the enzyme. However, in our tight-binding enzyme kinetics study, ANF acts as noncompetitive inhibitor in 7-ethoxycoumarin O-deethylation (ECOD) (K(i)=55.0 nM), but as competitive inhibitor in 7-ethoxyresorufin O-deethylation (EROD) (K(i)=1.4 nM). Based on homology modeling studies, ANF is positioned to bind to a hydrophobic cavity next to the active site where it may cause a direct effect on substrate binding. It is agreed with the predicted binding site of ANF in P450 3A4, in which ANF is rather known as a stimulating modulator. Our results suggest that ANF binds near the active site of P450 1A2 and exhibits differential inhibition mechanisms, possibly depending on the molecular structure of the substrate.

A study on CYP1A inhibitory action of E-2-(4'-methoxybenzylidene)-1-benzosuberone and some related chalcones and cyclic chalcone analogues

In vivo investigation of E-2-(4'-methoxybenzylidene)-1-benzosuberone (4a) on the 7,12-dimethylbenz[a]anthracene (DMBA)-induced onco/tumor suppressor gene expressions suggested that inhibition of metabolic activation of DMBA might play a role in the observed activity of the compound. In order to explore this possible biological action we have investigated whether 4a and some of its structurally related analogues had inhibitory effects on the CYP1A enzymes. During our study 7-ethoxyresorufin O-dealkylation activity of CYP1A isoenzymes was measured in liver microsomes prepared from 3-methylcholanthrene treated male rats. Inhibition constants (K(i) values) were determined by using different concentrations of 7-ethoxyresorufin and the investigated chalcones (1), E-2-benzylidene-1-indanones (2), -tetralones (3) and -benzosuberones (4). Each compound was found to be a strong competitive inhibitor of the CYP1A enzymes. Their inhibitory activity was comparable with or even higher than that of 7,8-benzoflavone, the known strong CYP1A inhibitor used as reference substance. By proper selection of the substituents on the benzylidene moiety we investigated how the inhibitory activity (K(i) value) of 1-4 varied as a function of the ring size (n=0, 5, 6, 7) carbon atoms, and the nature as well as the position of the substituents. To test applicability of the previously set structural requirements for binding of xenobiotics to the CYP1A enzymes we compared some topological, physico-chemical and quantum mechanical parameters of 1-4 with 7-ethoxyresorufin and 7,8-benzoflavone, the investigated CYP1A substrate and inhibitor, respectively.

Aryl hydrocarbon receptor ligands repress T-cadherin expression in vascular smooth muscle cells

T-cadherin, a glycosylphosphatidylinositol-modified cadherin subtype, is highly expressed in cardiac and vascular tissues. Neither the functions nor regulation of T-cadherin in these tissues is understood. We have cloned rat T-cadherin cDNA encoding the full length amino acid sequence. The 5(') untranslated nucleotide sequences of rat, mouse, and human T-cadherin contain a conserved GCGTG motif which constitutes the invariant core sequence of dioxin- or xenobiotic-regulatory elements. These elements function as target sites for aryl hydrocarbon receptor/aryl hydrocarbon nuclear translocator (AhR/ARNT) in genes regulated by this transcription factor. Using cultures of rat aortic smooth muscle cells this study presents data revealing T-cadherin as a putative target gene for negative regulation of expression through AHR signalling. Prototypic AHR agonists benzo[a]pyrene (BaP) or 7,12-dimethylbenzanthracene (DMBA) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) repressed T-cadherin mRNA levels. Repression was antagonized by the cognate AHR antagonist alpha-naphthoflavone (alpha-NF). Repression was insensitive to inhibitors of gene transcription (actinomycin D) or de novo protein synthesis (cycloheximide), suggesting AHR/ARNT functions directly in transcriptional repression of T-cad. Regulation of adhesion proteins through the AHR pathway may represent a novel mechanism of action by atherogenic polycyclic aromatic hydrocarbons.

Role of specific cytochrome P450 enzymes in the N-oxidation of the antiarrhythmic agent mexiletine

1. Mexiletine is extensively metabolized in man by C- and N-oxidation and the aim of the present study was to characterize major cytochrome P450 enzyme(s) involved in the formation of N-hydroxymexiletine. 2. Incubations with genetically engineered microsomes indicated that the formation rate of N-hydroxymexiletine was highest in the presence of microsomes expressing high levels of either CYP1A2 or CYP2E1 and the formation of N-hydroxymexiletine by human liver microsomes was inhibited about 40% by antibodies directed against CYP1A1/1A2 or CYP2E1. Additional incubations demonstrated that formation of N-hydroxymexiletine was decreased 47 and 51% by furafylline, 40 microm and 120 microm, respectively, and decreased 55 and 67% by alpha-naphthoflavone, 1 microm and 3 microm, respectively (all p < 0.05 versus control). 3. The formation rate of N-hydroxymexiletine in human liver microsomes was highly correlated with CYP2B6 (RS-mexiletine, r = 0.7827; R-(-)-enantiomer, r = 0.7034; S-(+)-enantiomer, r = 0.7495), CYP2E1 (S-(+)-enantiomer, r = 0.7057) and CYP1A2 (RS-mexiletine, r = 0.5334; S-(+)-enantiomer, r = 0.6035). 4. In conclusion, we have demonstrated that CYP1A2 is a major human cytochrome P450 enzyme involved in the formation of N-hydroxymexiletine. However, other cytochrome P450 enzymes (CYP2E1 and CYP2B6) also appear to play a role in the N-oxidation of this drug.

Characterization of human CYP1A1/1A2 induction by DNA microarray and alpha-naphthoflavone

DNA microarrays and real time PCR were used to analyze the mechanism of gene induction by CYP1A1 inducers, beta-naphthoflavone, and omeprazole, in the human hepatocellular carcinoma HepG2 cells. Reproducible and significant inductions were observed in a limited number of genes including CYP1A1 and CYP1A2. Genes induced by omeprazole included several protein tyrosine kinase targets. This result confirmed that omeprazole could modulate gene expressions through protein tyrosine kinase-mediated pathway. Induction ratios were considerably different from CYP1A1 and CYP1A2 (>10-fold) to other induced genes (<5-fold). alpha-Naphthoflavone, which is known as an antagonist to 2,3,7,8-tetrachlorodibenzo-p-dioxin, inhibited the inductions of heme oxygenase 1, glutamate-cysteine ligase (modifier unit), and thioredoxin reductase by beta-naphthoflavone but not those of CYP1A1 and CYP1A2. It unexpectedly enhanced the beta-naphthoflavone-mediated CYP1A1 and CYP1A2 induction. These results suggest that the CYP1A1 and CYP1A2 genes, which share their 5(') enhancer regions, are regulated differently from the other genes.

alpha-Naphthoflavone acts as activator and reversible or irreversible inhibitor of rabbit microsomal CYP3A6

This report describes the effect of alpha-naphthoflavone (alpha-NF), a known substrate, inhibitor and activator of several cytochromes P450 (CYP), on rabbit CYP3A6. Hepatic microsomes of rabbit pretreated with rifampicine (RIF), enriched with CYP3A6, as well as purified CYP3A6 reconstituted with isolated NADPH:CYP reductase were used as enzymatic systems in this study. The data from difference spectroscopy experiments showed that alpha-NF does yield a type I binding spectrum. This compound is oxidized by microsomal CYP3A6 into two metabolites (5,6-epoxide and trans-7,8-dihydrodiol). While alpha-NF is a substrate of CYP3A6, it also acts as an enzyme modulator. Under the conditions used, stimulation of 17beta-estradiol 2-hydroxylation by alpha-NF was observed. In contrast, this compound reversibly inhibited N-demethylation of erythromycin and tamoxifen, competitively with respect to these substrates, having the K(i) values of 51.5 and 18.0 microM, respectively. Moreover, alpha-NF was found to be an effective inactivator of progesterone and testosterone 6beta-hydroxylation catalyzed by CYP3A6 in RIF-microsomes. In addition, time- and concentration-dependent inactivation of human CYP3A4-mediated 6beta-hydroxylation of testosterone by alpha-NF, was determined. The inactivation of CYP3A6 followed pseudo-first-order kinetics and was dependent on both NADPH and alpha-NF. The concentrations required for half-maximal inactivation (K(i)) were 80.1 and 108.5 microM and the times required for half of the enzyme to be inactivated were 10.0 and 11.9 min for 6beta-hydroxylation of progesterone and testosterone, respectively. The loss of the enzyme activity was not recovered following dialysis, while 90% of the ability to form a reduced CO complex remained. This indicates the binding of alpha-NF to a CYP apoprotein molecule rather than to a heme moiety. Protection from inactivation was seen in the presence of all tested CYP3A substrates. Progesterone and testosterone protected CYP3A6 against inactivation competitively with respect to inactivator, erythromycin non-competitively and 17beta-estradiol showed a mixed type of protection. Here, we described for the first time that alpha-NF is capable of irreversible inhibition of microsomal rabbit CYP3A6 and human CYP3A4. The obtained results strongly suggest that the CYP3A active center contains at least two and probably three distinct binding sites for substrates.

A kinetic model for the metabolic interaction of two substrates at the active site of cytochrome P450 3A4

In many cases, CYP3A4 exhibits unusual kinetic characteristics that result from the metabolism of multiple substrates that coexist at the active site. In the present study, we observed that alpha-naphthoflavone (alpha-NF) exhibited a differential effect on CYP3A4-mediated product formation as shown by an increase and decrease, respectively, of the carboxylic acid (P(2)) and omega-3-hydroxylated (P(1)) metabolites of losartan, while losartan was found to be an inhibitor of the formation of the 5,6-epoxide of alpha-NF. Thus, to address this problem, a kinetic model was developed on the assumption that CYP3A4 can accommodate two distinct and independent binding domains for the substrates within the active site, and the resulting velocity equations were employed to predict the kinetic parameters for all possible enzyme-substrate species. Our results indicate that the predicted values had a good fit with the experimental observations. Therefore, the kinetic constants can be used to adequately describe the nature of the metabolic interaction between the two substrates. Applications of the model provide some new insights into the mechanism of drug-drug interactions at the level of CYP3A4.

Identification of cytochrome P450 isoform involved in the metabolism of YM992, a novel selective serotonin re-uptake inhibitor, in human liver microsomes

1. In vitro studies were conducted to identify the hepatic cytochrome P450 isoform involved in the metabolism of YM992, ((S)-2-[[(fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride), a novel serotonin re-uptake inhibitor, in human liver microsomes. 2. Microsomes prepared from yeast expressing CYP1A1, CYP1A2 and CYP2D6 effectively metabolized YM992. A significant correlation was observed between the rate of YM992 metabolism and 7-ethoxyresorufin O-deethylation, CYP1A1/2 specific activity, in liver microsomes from 16 individual donors (r2 = 0.628, p < 0.001). Alpha-naphtoflavone and isosafrole, CYP1A1/2 inhibitors, suppressed the metabolism of YM992 in human liver microsomes in a concentration-dependent manner. 3. The metabolism of YM992 in human liver microsomes was inhibited by approximately 95% by antibodies which recognize both CYP1A1 and CYP1A2 whereas antibodies specific for CYP1A1 did not show inhibitory effects. 4. The same major metabolites, M6 and M7, were generated from YM992 after incubation with human liver microsomes and recombinant human CYP1A2. 5. These results suggest that the metabolism of YM992 in human liver microsomes is mainly catalysed by CYP1A2, and that YM992 might increase plasma concentration of concomitant drugs metabolized by CYP1A2 due to competitive inhibition.

Dual role of human cytochrome P450 3A4 residue Phe-304 in substrate specificity and cooperativity

The structural basis of cooperativity of progesterone hydroxylation catalyzed by human cytochrome P450 3A4 has been investigated. A recent study suggested that substitution of larger side chains at positions Leu-211 and Asp-214 partially mimics the action of effector by reducing the size of the active site. Based on predictions from molecular modeling that Phe-304 in the highly conserved I helix is involved in both effector and substrate binding, a tryptophan residue was substituted at this position. The purified F304W mutant displayed hyperbolic progesterone hydroxylase kinetics, indicating a lack of homotropic cooperativity. However, the mutant remained responsive to stimulation by alpha-naphthoflavone, exhibiting a 2-fold decrease in the K(m) value for progesterone 6beta-hydroxylation in the presence of 25 microM effector. Combining substitutions to yield the triple mutant L211F/D214E/F304W maintained the V(max) and decreased the K(m) for progesterone 6beta-hydroxylation, minimized stimulation by alpha-naphthoflavone, and decreased the rate of alpha-naphthoflavone oxidation to one-eighth of the wild type. Interestingly, the DeltaA(max) for spectral binding of alpha-naphthoflavone was unaltered in L211F/D214E/F304W. Overall, the results suggest that progesterone and alpha-naphthoflavone are oxidized at separate locations within the P450 3A4 binding pocket, although both substrates appear to have equal access to the reactive oxygen.

Suppression of cell cycle progression by flavonoids: dependence on the aryl hydrocarbon receptor

Some flavonoids are ligands of the aryl hydrocarbon receptor (AHR) and cause cell cycle arrest. The dependency of the cytostatic effects of five flavonoids (flavone, alpha-naphthoflavone, apigenin, 3'-methoxy-4'-nitroflavone and 2'-amino-3'-methoxyflavone) on a functional AHR was examined in AHR-containing rat hepatoma 5L cells and an AHR-deficient cell line (BP8) derived from the 5L line. The potent AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was cytostatic to the 5L line due to the induction of a G(1) arrest and dramatically elevated steady-state levels of CYP1A1 mRNA. TCDD affected neither the proliferation nor CYP1A1 mRNA contents of BP8 cells. With the exception of apigenin, the flavonoids under study induced G(1) arrest in both 5L and BP8 cells when used at concentrations at which they functioned as AHR agonists, but not antagonists. Apigenin-treated 5L and BP8 cultures primarily arrested in G(2)/M. The AHR-containing murine hepatoma cell line 1c1c7 arrested following exposure to AHR agonist concentrations of flavone and alpha-naphthoflavone, but not TCDD. Unlike the G(1) arrest observed in 5L cultures, the latter two flavonoids caused principally G(2)/M arrest in 1c1c7 cells. These studies demonstrate that the cytostatic activities of flavonoids do not require the AHR and the site of checkpoint arrest with a specific flavonoid can vary with cell type.

Evidence for O-dealkylation of 7-pentoxyresorufin by cytochrome P450 2B1/2B2 isoenzymes in brain

O-dealkylation of 7-pentoxyresorufin (PR) was studied in rat brain to characterise the functional activity specific for cytochrome P450 2B1/2B2 isoenzymes in brain microsomes. Brain microsomes catalyzed the O-dealkylation of PR in the presence of NADPH. Pretreatment with phenobarbital (PB; 80 mg/kg body wt, i.p. x 5 days) resulted in 3-4 fold induction of pentoxyresorufin-O-dealkylase (PROD) activity while 3-methylcholanthrene (MC; 30 mg/kg body wt, i.p. x 5 days) did not produce any significant increase in enzyme activity. Kinetic studies revealed that the rate of velocity (Vmax) for the O-dealkylation of PR was significantly increased to 2.9 times higher in brain microsomes isolated from PB pretreated rats. In vitro studies using metyrapone, an inhibitor of P450 2B1/2B2 catalyzed reactions and antibody for hepatic PB inducible P450s (P450 2B1/2B2) significantly inhibited the activity of PROD in cerebral microsomes prepared from PB pretreated animals. These studies suggest that PB inducible isoenzymes of P450, i.e. P450 2B1/2B2 specifically catalyze the O-dealkylation of PR in brain microsomes.

Probing the active site of cytochrome P450 2B1: metabolism of 7-alkoxycoumarins by the wild type and five site-directed mutants

A series of 7-alkoxycoumarins (chain length of 1-7 carbon atoms) was utilized as active site probes of purified Escherichia coli-expressed cytochrome P450 2B1 wild type and five site-directed mutants (I114V, F206L, V363A, V363L, and G478S). The production of 7-hydroxycoumarin, the O-dealkylation product, by the wild-type enzyme exhibited a rank order of C2 > C4 > C3 > C1 > C5 > C6 = C7. The pattern observed for the P450 I114V mutant was similar to that of the wild-type enzyme, whereas with F206L and G478S mutants, the rate of O-dealkylation was low with all the compounds. In contrast, with V363A, the highest rate of product formation was observed with 7-butoxycoumarin. The V363L mutant preferentially catalyzed the O-dealkylation of 7-methoxy- and 7-ethoxycoumarin, and a further increase in the length of the alkyl chain led to a marked decrease in product formation. The stoichiometry of 7-butoxycoumarin oxidation by V363L suggested that products other than 7-hydroxycoumarin were also formed. HPLC and GC-EIMS analyses revealed that P450 2B1 V363L produced 7-(3-hydroxybutoxy)coumarin and 7-(4-hydroxybutoxy)coumarin as major oxidation products, while the V363A mutant mainly catalyzed the O-dealkylation of 7-butoxycoumarin. Docking of alkoxycoumarins into the active site of a P450 2B1 homology model confirmed the importance of the studied residues in substrate dealkylation and explained the formation of novel 7-butoxycoumarin products by the V363L mutant.

Identification of three key residues in substrate recognition site 5 of human cytochrome P450 3A4 by cassette and site-directed mutagenesis

Cassette mutagenesis and site-directed mutagenesis were used to investigate the importance of individual amino acid residues at positions 364-377 of cytochrome P450 3A4 in determining steroid hydroxylation or stimulation by alpha-naphthoflavone. The mutants were expressed in an Escherichia coli system, and solubilized membranes were prepared. All mutants except R365G and R365K exhibited anti-3A immunoreactivity on Western blotting, although R372S and R375K were not detected as the Fe2+-CO complex. Replacement of Arg-372 by Lys yielded a typical P450 spectrum. The results indicate that the highly conserved Arg residues at positions 365 and 375 may play a role in stabilizing the tertiary structure or in heme binding. Catalytic activities of 12 mutants were examined using progesterone and testosterone as substrates, and residues 369, 370, and 373 were found to play an important role in determining substrate specificity. Although the three mutants hydroxylated progesterone and testosterone primarily at the 6beta-position like the wild-type, replacement of Ile-369 by Val suppressed progesterone 16alpha-hydroxylase activity, whereas substitution of Ala-370 with Val enhanced progesterone 16alpha-hydroxylation. Interestingly, substitution of Leu-373 with His resulted in production of a new metabolite from both steroids. Moreover, the mutants at positions 369 and 373 were more and less responsive, respectively, than the wild-type to alpha-naphthoflavone stimulation. Alterations in activities or expression of several mutants were interpreted using a three-dimensional model of P450 3A4. The results suggest that analogy with mammalian family 2 and bacterial cytochromes P450 can be used to predict P450 3A residues that contribute to regiospecific steroid hydroxylation.

Characterization of a fusion protein between human cytochrome P450 1A1 and rat NADPH-P450 oxidoreductase in Escherichia coli

A cDNA of fusion protein between human cytochrome P450 1A1 and rat NADPH-P450 reductase was genetically engineered and expressed in Escherichia coli DH5alpha cells under the control of an inducible tac promoter (Y. J. Chun, T. Shimada, and F. P. Guengerich, (1996) Arch. Biochem. Biophys. 330, 48-58). E. coli membranes of transformed cells showed much higher P450 1Al-dependent monooxygenase and NADPH-P450 reductase activities than pCW control vector or P450 1A1 expression vector-transformed cells. Ethoxyresorufin O-deethylase and methoxyresorufin O-demethylase were 22-fold and 11-fold higher than the control activity, respectively. alpha-Naphthoflavone and beta-naphthoflavone strongly inhibited P450 1A1 activity of the fusion protein, with alpha-naphthoflavone being more potent than beta-naphthoflavone. Divalent cations (e.g. Ca2+ and Mg2+) increased P450 1A1 activity as well as NADPH-P450 reductase activity. These results demonstrate that this fusion protein in E. coli membrane may be a useful model for elucidating details of protein-protein interactions between P450 and NADPH-P450 reductase in the endoplasmic reticulum of mammalian cells.

2-Butanol metabolism by rat hepatic and pulmonary cytochromes P450

Three microsomal enzyme inducers, ethanol, phenobarbital (PB), and beta-naphthoflavone (beta NF), were compared for their effects on butanol oxidase activity in rat hepatic and pulmonary microsomes. Four concentrations of 2-butanol (1.0, 5.0, 10, and 33 mM) were used to determine if the effects of induction on 2-butanol metabolism were substrate concentration dependent. Ethanol induced at all substrate concentrations in the liver while PB induced at only the high substrate concentrations (5.0, 10, and 33 mM). beta NF did not induce at any substrate concentration. 2-Butanol oxidation in the lung was not induced by any of the treatments. Thus, both ethanol and phenobarbital induce hepatic enzymes capable of 2-butanol oxidation, and the isozyme(s) induced by the latter has a somewhat lesser affinity for this alcohol.

Human antioxidant-response-element-mediated regulation of type 1 NAD(P)H:quinone oxidoreductase gene expression. Effect of sulfhydryl modifying agents

Human antioxidant-response element (hARE) containing two copies of the AP1/AP1-like elements arranged as inverse repeat is known to mediate basal and beta-naphthoflavone-induced transcription of the type 1 NAD(P)H:quinone oxidoreductase (NQO1) gene. Band-shift assays revealed that beta-naphthoflavone increased binding of nuclear proteins at the hARE. Super shift assays identified Jun-D and c-Fos proteins in the band-shift complexes observed with control and beta-naphthoflavone-treated Hepa-1 nuclear extracts. Hepa-1 cells stably transformed with hARE-tk-chloramphenicol acetyl transferase (CAT) recombinant plasmid were used to demonstrate that, in addition to beta-naphthoflavone, a variety of antioxidants, tumor promoters and hydrogen peroxide (H2O2) also increased expression of hARE-mediated CAT gene. beta-naphthoflavone induction of the CAT gene expression in Hepa-1 cells was found insensitive to inhibitors of protein kinase C and tyrosine kinases. However, binding of regulatory proteins at the hARE and the CAT gene expression in Hepa-1 cells were increased by dithiothreitol, 2-mercaptoethanol and diamide. Treatment of the Hepa-1 cells with N-ethylmaleimide reduced binding of proteins at the hARE and interfered with expression and beta-naphthoflavone induction of the CAT gene. These results suggested a role of sulfhydryl modification of hARE binding (Jun and Fos) proteins which mediate basal and induced expression of the NQO1 gene. We also report that in-vitro-translated products of the proto-oncogenes, Jun and Fos, bind to the hARE in band-shift assays. The incubation of Jun and Fos proteins with small amounts of nuclear extract from dimethylsulfoxide-treated (control) or beta-naphthoflavone treated Hepa-1 cells prior to band-shift assays increased the binding of Jun and Fos proteins to the hARE. Interestingly, the increase in binding of Jun and Fos proteins to the hARE was more prominent with beta-naphthoflavone-treated nuclear extract as compared to the control nuclear extract. In addition, incubation of control nuclear extract with beta-naphthoflavone, microsomes and NADPH increased the binding of Jun and Fos proteins to the hARE. Evidence from in vitro studies indicate the presence of unknown nuclear factor(s) that receive signals from metabolites of beta-naphthoflavone and modulate Jun and Fos binding to the AP1 site contained within the hARE.

Involvement of P4503A in the metabolism of 7,8-benzoflavone by human liver microsomes

1. The metabolism of 7,8-benzoflavone (ANF) was studied in human liver microsomal fractions. Five metabolites were generated and tentatively identified as ANF 5,6-oxide, 5,6-dihydrodiol, 7,8-dihydrodiol, 6-hydroxy ANF, and 7-hydroxy ANF based on UV and mass spectral analysis. 2. The involvement of P4503A in the metabolism of ANF was confirmed by the following observations: (1) correlation of ANF 5,6-oxide formation was noted with testosterone 6 beta-hydroxylation, an indicator of P4503A, in human liver microsomal fractions; (2) metabolism of ANF was inhibited by various selective P4503A enzyme inhibitors; (3) rabbit antibody raised against P4503A4 inhibited the ANF metabolism by > 80%; and (4) microsomal fractions that specifically expressed P4503A4 metabolized ANF to oxidized metabolites. 3. These results indicate that P4503A isoform(s) mainly metabolize ANF to oxidized derivatives in human liver microsomal fractions.

Modulation of cytochrome P450 activities by 7,8-benzoflavone and its metabolites

7,8-Benzoflavone(ANF) is a potent in vitro inhibitor of CYP1A2 but is an in vitro activator of CYP3A4. We have investigated the inhibition of caffeine 3-demethylation by metabolites of ANF as well as ANF by human liver microsomes. ANF was the most potent among all the compounds tested. Metabolites of ANF with dihydrodiol substitution at positions 5,6 or 7,8 showed less inhibitory activity. These results suggest that ANF lies in the most appropriate orientation to the active site of CYP1A2. The activation of CYP3A4 enzyme activities by ANF and its metabolites was also investigated. Testosterone 6 beta-hydroxylation mediated by CYP3A4 was stimulated by ANF and metabolites with substitutions at positions 5,6 or 7,8. Hydroxy ANF metabolites, however, decreased the testosterone 6 beta-hydroxylation.

Evaluation of the xenobiotic biotransformation capability of six rodent hepatoma cell lines in comparison with rat hepatocytes

Phase I and II activities were examined in six rodent hepatoma cell lines and compared with those of cultured rat hepatocytes both in basal conditions and after exposure to 5 microM methylcholanthrene, 2 mM phenobarbital, and 15 microM beta-naphtoflavone. The metabolic profile of testosterone was also studied. The highest aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities were found in MH1C1 cells. Comparable values for 7-ethoxyresorufin O-deethylase activity, ranging from 21.6 to 42.9 pmol/mg x min, were observed in the hepatocytes and hepatoma cells, except the HTC cells. In contrast, only Fao cells showed 7-pentoxyresorufin O-depentylase activity at levels similar to those of hepatocytes (6.2 +/- 1.0 and 7.4 +/- 1.2 pmol/mg x min, respectively). Rat hepatocytes actively hydroxylated p-nitrophenol, but this activity was not measurable in hepatoma cells. Glutathione transferase activity was maintained in all the hepatoma cell lines at similar levels to those found in hepatocytes (684 +/- 56 nmol/mg x min). The seven hydroxylated metabolites of testosterone produced by cultured hepatocytes were negligible in hepatoma cells. Exposure of cells to inducers revealed that aryl hydrocarbon hydroxylase activity was mainly increased after treatment with 3-methylcholanthrene and beta-naphtoflavone, and the highest values were found in rat hepatocytes followed by MH1C1 and Fao cells. 3-Methylcholanthrene and naphtoflavone treatment also resulted in a marked increase in 7-ethoxyresorufin O-deethylase activity in hepatocytes as well as in H4IIC3, McA-Rh7777, MH1C1, and Fao cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of CYP3A4: evidence for the simultaneous binding of two substrates in a cytochrome P450 active site

A unique characteristic of the CYP3A subfamily of cytochrome P450 enzymes is their ability to be activated by certain compounds. It is reported that CYP3A4-catalyzed phenanthrene metabolism is activated by 7,8-benzoflavone and that 7,8-benzoflavone serves as a substrate for CYP3A4. Kinetic analyses of these two substrates show that 7,8-benzoflavone increases the Vmax of phenanthrene metabolism without changing the Km and that phenanthrene decreases the Vmax of 7,8-benzoflavone metabolism without increasing the Km. These results suggest that both substrates (or substrate and activator) are simultaneously present in the active site. Both compounds must have access to the active oxygen, since neither phenanthrene nor 7,8-benzoflavone can competitively inhibit the other substrate. These data provide the first evidence that two different molecules can be simultaneously bound to the same P450 active site. Additionally, structure-activity relationship studies were performed with derivatives of 7,8-benzoflavone structure. The effects of 13 different compounds on the regioselectivity of phenanthrene, chrysene, and benzo[a]pyrene metabolism were determined. Of the 13 compounds studied, 6 were activators, 2 were partial activators, and 5 were inhibitors. Analyses of the data suggest that (1) naphthalene substituted with a ketone in the 2-position can activate 3A4 and (2) the presence of an activator results in a narrower effective substrate binding site. Since the CYP3A enzymes are very important in drug metabolism, the possibility of activation, and autoactivation, must be considered when in vitro-in vivo correlations are made and when possible drug interactions are considered.

Activation of benzo[a]pyrene and 2-aminoanthracene to bacteria mutagens by mussel digestive gland postmitochondrial fraction

The ability of the mussel postmitochondrial fraction (S9) to activate benzo[a]pyrene (BaP) and 2-aminoanthracene (2AA) to mutagenic metabolites towards Salmonella typhimurium strain TA98 was tested. The mechanisms involved in this activation were investigated and mussel cytochrome P-450-dependent monooxygenases and its NADPH cytochrome c reductase were found to contribute to the activation of BaP. This activation was improved by treating the mussel with 4,5,4',5'-tetrachlorobiphenyl (TCB) (a 3-methylcholanthrene-type inducer of cytochrome P-450-dependent monooxygenase in marine fish) and was inhibited by alpha-naphthoflavone (ANF), a cytochrome P-450 inhibitor. However, both BaP activation and cytochrome P-450-related metabolic activities are much weaker in mussels than in vertebrates. Mussel S9 activates aromatic amines more effectively than BaP. Pretreatment of mussels with TCB or addition of ANF in the incubation medium has no effect on 2AA activation. As suggested by Kurelec (1985), aromatic amine metabolism may be supported by a flavoprotein mixed-function amine oxidase which is NADPH-dependent.

Effect of hepatic cytochrome P-450 inducing agents on mutagen activity in the host-mediated assay

Intraperitoneal treatment of female BALB/c mice with either phenobarbitone or beta-naphthoflavone led to the induction of various hepatic enzymes associated with xenobiotic metabolism and to increased abilities of hepatic S9 fractions to convert the dietary carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) to an active bacterial mutagen. In the case of another carcinogen, aflatoxin B1 an increase in in vitro hepatic activation was seen only in mice treated with phenobarbitone. In contrast, pretreatment with either phenobarbitone or beta-naphthoflavone reduced the in vivo activity of both aflatoxin B1 and MeIQx in the host mediated bacterial mutation assay. These data indicate that, for some carcinogens at least, the host-mediated assay may be used to predict the carcinogenic consequences of hepatic enzyme induction.

Alpha-naphthoflavone acts as an antagonist of 2,3,7, 8-tetrachlorodibenzo-p-dioxin by forming an inactive complex with the Ah receptor

alpha-Naphthoflavone (ANF) has previously been shown to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for binding to the Ah receptor under conditions in vitro. However, ANF also prevents TCDD-elicited cytochrome P450lA1 induction, immunosuppression, and down-regulation of the estrogen receptor in vivo and within intact isolated cells. These data suggest that ANF is a TCDD antagonist. This study investigated the ability of ANF to transform the Ah receptor contained in rat hepatic cytosol or mouse hepatoma cells to a form that recognizes the dioxin-responsive enhancer element (DRE) upstream of the cytochrome P450lA1 gene. Gel retardation analysis indicated that TCDD- or beta-naphthoflavone (BNF)-bound receptor was able to bind to the DRE, whereas essentially no receptor-DRE complexes were observed using cytosol incubated with ANF concentrations as high as 1000 nM. Furthermore, an excess of ANF, when added to cytosol just before TCDD, blocked, in a concentration-dependent manner, the ability of TCDD to transform the receptor to a form that bound to the DRE. These studies indicated that ANF binds to the receptor and confers on it a conformation that cannot recognize the DNA recognition sequence contained in the DRE. Although an excess of the agonist 2,3,7,8-tetrachlorodibenzofuran (TCDF) readily reversed the inhibitory actions of ANF, ANF was unable to reverse the effects of TCDD, TCDF, or BNF on the receptor. These studies suggested that TCDD binding, unlike that of ANF, results in a receptor conformation that has higher affinity for the ligand. Treatment of mouse hepatoma Hepa 1c1c7 cells with TCDD or BNF resulted in receptor contained in nuclear extracts that bound to the DRE. Only a very minor ligand-dependent protein-DNA complex was detected when cells were treated with ANF. These data indicated that ANF acts as an antagonist of TCDD by directly binding to the Ah receptor and eliciting a protein conformation that has very low affinity for DNA.

Metabolic effects of growth factors and polycyclic aromatic hydrocarbons on cultured human placental cells of early and late gestation

The metabolic effects of epidermal growth factor (EGF), insulin, insulin-like growth factor-I (IGF-I), and IGF-II were determined on human placental cells in monolayer culture obtained from early gestation (less than 20 weeks) and late gestation (38-42 weeks). Parameters studied were uptake of aminoisobutyric acid (AIB), uptake of 3-O-methylglucose and [3H]thymidine incorporation into cell protein. Since benzo[alpha]pyrene (BP) inhibits EGF binding and autophosphorylation in cultured human placental cells, particularly in early gestation, we also studied the effect of benzo[alpha]pyrene and other polycyclic aromatic hydrocarbons (PAHs) on EGF-mediated AIB uptake. The metabolic effects of EGF, insulin, and the IGFs in cultured human placental cells varied with gestational age and the growth factor studied. All three classes of growth factors stimulated AIB uptake in both early and late gestation at concentrations from 10-100 micrograms/L, well within a physiological range. However, insulin stimulation of AIB uptake was maximal at a high concentration (200 micrograms/L) in both early and late gestation cells, suggesting an action via type 1 IGF receptors rather than via insulin receptors. EGF stimulated 3-O-methylglucose uptake only in term placental cells. No significant stimulation of [3H]thymidine incorporation by any of the growth factors tested was seen with either early or late gestation cells. The effect of PAHs on AIB uptake by cultured placental cells was variable. BP alone stimulated AIB uptake by both very early and late gestation cells and enhanced EGF-stimulated AIB uptake. alpha-naphthoflavone alone inhibited AIB uptake at all gestational ages and inhibited EGF-stimulated AIB uptake. beta-Naphthoflavone and 3-methylcholanthrene minimally inhibited AIB uptake by early gestation cells and did not modify EGF-stimulated uptake at any gestational period. Our prior results demonstrated that BP more significantly inhibited EGF than IGF or insulin receptor binding as well as autophosphorylation in early gestation placenta, and that BP was the most potent of the PAHs tested. Thus, the direct effect of the PAHs on placental EGF receptors and amino acid transport may indicate altered function of EGF in the regulation of placental growth in smoking mothers that is developmentally regulated.

DNA adduct formation in liver and kidney of male Syrian hamsters treated with estrogen and/or alpha-naphthoflavone

Chronic administration of estrogens to male Syrian hamsters induces kidney tumors. Co-administration of estrogen plus alpha-naphthoflavone (ANF) suppresses this kidney carcinogenesis but induces liver tumors instead. In an attempt to elucidate the mechanism of the switch from estrogen-induced kidney to liver carcinogenesis in response to ANF treatment, patterns of kidney and liver DNA adducts were investigated by 32P-postlabeling analysis and compared to controls. Chronic treatment of hamsters with ANF alone or in combination with estradiol resulted in a flavone-specific DNA adduct pattern in livers of these animals. These spots, adducts 1 and 2, on 32P-postlabeling maps were taken as evidence of covalent ANF-DNA modifications. The kidney-specific estrogen-induced indirect DNA adducts, observed previously in hamsters treated chronically with estrogen, occurred in renal but not hepatic DNA of animals treated with estradiol alone or in combination with ANF. Pretreatment of hamsters with ANF for 3 days decreased by 75-80% the hepatic and renal diethylstilbestrol (DES)-DNA adducts, which are formed after injection of a single large dose of DES. It is concluded from these changes in DNA adduct patterns and levels that estrogen quinone-DNA adduction may play an etiological role in estrogen-induced kidney cancer. The prevention of estrogen-induced kidney tumors by ANF co-treatment may be a consequence of the decrease in renal concentrations of these adducts in response to ANF. Hepatic concentrations of estrogen quinone-DNA adducts also decrease, but ANF-DNA adducts, observed only in liver, may assume an etiological role in the induction of hepatomas.

Involvement of cytochrome P-450c in alpha-naphthoflavone metabolism by rat liver microsomes

Metabolism of alpha-naphthoflavone (ANF) is increased markedly in rat liver microsomes by 3-methylcholanthrene (3-MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), two inducers of cytochromes P-450c and P-450d (P-450c and P-450d). Although several indirect lines of evidence in the literature suggest that ANF is metabolized by P-450c, Vyas et al. [J. Biol. Chem. 258:5649-5659 (1983)] reported that ANF metabolism by 3-MC-induced rat liver microsomes was only partially inhibited by antibodies against P-450c. Our laboratory has previously reported clastogenic effects of metabolites of ANF, and in the present study we reexamined the role of P-450c in ANF metabolism by both uninduced and TCDD-induced rat liver microsomes, using monospecific polyclonal antibodies to P-450c and P-450d. ANF metabolism was inhibited to different extents in TCDD-induced microsomes by different preparations of anti-P-450c. One lot of anti-P-450c produced only 50% inhibition of ANF metabolism in TCDD-induced microsomes, whereas another lot of anti-P-450c inhibited ANF metabolism by 80%. Anti-P-450d had no effect on ANF metabolism. Neither anti-P-450c nor anti-P-450d inhibited ANF metabolism in uninduced rat liver microsomes. In a reconstituted enzyme system, purified P-450c metabolized ANF 47 and 510 times more rapidly than P-450d and P-450b, respectively. Metabolites resulting from oxidation at 7,8- or 5,6-positions (7,8-dihydro-7,8-dihydroxy-ANF, 5,6-dihydro-5,6-dihydroxy-ANF, 5,6-oxide-ANF, and 6-hydroxy-ANF) were formed by all preparations of microsomes. An unknown toxic ANF metabolite was formed only with a reconstituted P-450c system and with 3-MC- or TCDD-induced microsomes. Our results indicate that P-450c is responsible for the majority of the metabolism of ANF in TCDD-induced microsomes, whereas other constitutive isozymes are responsible for the metabolism seen in uninduced liver microsomes. The variable inhibition of ANF metabolism with different lots of anti-P-450c probably reflects the differences in the proportion of antibodies to different epitopes important in the binding or metabolism of this substrate.

Modification of 7,8-benzoflavone metabolism in hamster liver and kidney microsomes by hepatic tumor inducing treatments

The effect of pre-treatment of male Syrian golden hamsters with 7,8-benzoflavone (BF), with diethylstilbestrol (DES) and with BF plus DES on the metabolism of [14C]BF in hepatic and renal microsomes has been studied in vitro. Whereas hepatic microsomes from DES-treated animals produced the same pattern of BF metabolites as control microsomes, a marked quantitative and qualitative alteration of BF metabolism was observed with liver microsomes from animals pre-treated with BF and with BF plus DES: the metabolic rate was increased and three new metabolites were formed which were not observed with control hepatic microsomes. These metabolites, which were tentatively identified as BF-5,6-dihydrodiol and two isomeric dihydroxy-BFs, were not detected in incubations with renal microsomes under any pre-treatment regimen. Non-extractable binding of radioactivity to hepatic and renal microsomal protein was observed in all incubations but did not exhibit as pronounced a dependence on pre-treatment as did the pattern of BF metabolites. Based on the metabolic data it is concluded that BF induces its own oxidative metabolism. Among the metabolites are reactive intermediates that bind to cellular macromolecules and may play an important role in tumor formation in the male Syrian hamster liver following prolonged treatment with BF plus DES.

Effects of bromocriptine on hepatic cytochrome P-450 monooxygenase system

We have evaluated the in vitro effects of bromocriptine (Br), on the hepatic cytochrome P-450 monooxygenase system of rats pretreated with saline phenobarbitone (PB) and beta-naphthoflavone (BNF). Br inhibited ethoxyresorufin O-dealkylase (EROD) activity in liver microsomes of rats pretreated with saline and PB but not in BNF pretreated animals. Maximum inhibition of EROD activity by Br in the microsomes of saline and PB pretreated rats were 50%-60% of the control. In contrast, a dual effect was observed on aminopyrine N-demethylase activity (APD) by Br in microsomes of saline, PB and BNF pretreated rats. At a low concentration (25 microM), Br inhibited the activity of APD to a similar extent in all pretreatment groups; however, with higher concentrations of Br (50 microM to 300 microM), enhancement of APD activity was observed. Br (300 microM) increased the APD activity to 2-3 times the control level in microsomes of rats pretreated with saline, PB or BNF. Spectral studies revealed a Type II binding of Br to cytochrome P-450 from microsomes of saline and PB pretreated rats. A reverse type I binding was observed for BNF induced microsomes. In addition, Br also enhanced NADPH cytochrome c (P-450) reductase activity to a similar extent in all pretreatment groups. These results suggest that the inhibition of EROD activity may be due to direct binding by Br to certain isozymes of cytochrome P-450 and that the enhancing effect of Br on APD activity may be in part due to the activation of the NADPH cytochrome c reductase component of the cytochrome P-450 monooxygenase system.

Effect of pretreatment with 7,8-benzoflavone and diethylstilbestrol on the hepatic metabolism of diethylstilbestrol in the male Syrian golden hamster in vivo

Liver tumors are induced in male Syrian golden hamsters by the combined treatment with diethylstilbestrol (DES) and 7,8-benzoflavone (7,8-BF), but not with either substance alone. With the aim of clarifying whether metabolic activation of DES is involved in the mechanism of tumorigenesis in this animal model, we have studied the effect of pretreatment with 7,8-BF alone, DES alone, and 7,8-BF plus DES for 2, 8, 20 and 32 weeks on the hepatic in vivo metabolism of DES, using biliary metabolites collected from bile-duct cannulated male hamsters as probe. Formation of glucuronides and sulfates was not affected by treatment with 7,8-BF nor 7,8-BF plus DES. In contrast, animals pretreated with DES alone had a decreased amount of glucuronides and an increased proportion of unconjugated material in the bile. Oxidative metabolism of DES was not significantly altered in hamsters treated with 7,8-BF for up to 20 weeks, whereas pretreatment with DES alone and with 7,8-BF plus DES caused an enhancement of oxidative DES metabolism in vivo, leading mostly to highly polar, as yet unidentified products. From a consideration of various cytochrome P-450-associated enzyme activities, it is concluded that the observed effect on biliary DES metabolites is most likely to be due to an estrogen-induced intrahepatic cholestasis. Taken together, the data do not support a role for the metabolic activation of DES in this tumor model. Alternative mechanisms are proposed.

Metabolic activation of alpha-naphthoflavone by 2,3,7,8-tetrachlorodibenzodioxin-induced rat liver microsomes

Previous studies in our laboratory had demonstrated that addition of alpha-naphthoflavone (ANF) to lymphocytes from smokers or polychlorinated biphenyls (PCB)s-exposed individuals caused an increase in sister chromatid exchange (SCE) frequency whereas lymphocytes from controls were relatively unaffected. In order to investigate the mechanism responsible, metabolism of ANF by uninduced and 2,3,7,8-tetrachlorodibenzodioxin (TCDD)-induced microsomes was studied as a function of microsomal protein concentration and incubation time. Nonpolar metabolites were analyzed and the amount of conjugated (polar) and protein-bound metabolites determined. The initial ANF-metabolism rate was 10-fold higher in TCDD-induced microsomes (4.9 +/- 0.6 nmol/min per mg TCDD-induced microsomal protein vs. 0.5 +/- 0.2 nmol/min per mg uninduced microsomal protein) than in uninduced microsomes. Moreover, uninduced microsomes no longer metabolize ANF after 30-40 min while TCDD-induced microsomes metabolize ANF for longer than 2 h or until all the ANF is gone. In addition to the metabolites formed by uninduced microsomes [7,8-dihydro-7,8-dihydroxy-ANF (7,8-dihydrodiol); 5,6-dihydro-5,6-dihydroxy-ANF (5,6-dihydrodiol); 5,6-oxide-ANF and 6-hydroxy-ANF], TCDD-induced microsomes from unidentified metabolites. When TCDD-induced microsomes and 40 microM ANF were added to Chinese hamster ovary (CHO) cells, we found a correlation between the concentration of 5,6-oxide-ANF and clastogenicity to CHO cells. However, purified 5,6-oxide-ANF did not induce SCEs in CHO cells in the absence or presence of TCDD-induced microsomes. However, a minor metabolite (identified as the 9,10-dihydro-9,10-dihydroxy-ANF by acid dehydration) formed with TCDD-induced microsomes produces clastogenicity in CHO cells. These data indicate that a minor metabolite of ANF is a potent clastogen which suggests that this metabolite may be responsible for the ANF-mediated increases in SCE frequency in lymphocytes from smokers or PCB-exposed individuals.

Increased frequencies of alpha-naphthoflavone induced sister chromatid exchanges in long-term survivors of childhood acute lymphoblastic leukemia

Cultures of whole-blood were established from 12 children (mean age = 13.5 years) surviving acute lymphoblastic leukemia (mean time of discontinued therapy = 4.5 years) and 10 age-matched controls were assessed for the level of baseline, alpha-naphthoflavone (ANF) and methyl-methane-sulphonate (MMS) induced sister chromatid exchanges (SCEs). The average baseline levels of SCEs in the patient group (7.65) and in the control (6.72) were different at a statistical marginal level (P = 0.05). No statistical difference was observed in the levels of MMS-induced SCEs between the two groups (P greater than 0.1). In contrast, highly significant differences (P = 0.003) were observed in the level of ANF-induced SCE-levels between the patients (10.78) and the controls (8.76).

alpha-Naphthoflavone metabolized by 2,3,7,8-tetrachlorodibenzo(p)dioxin-induced rat liver microsomes: a potent clastogen in Chinese hamster ovary cells

alpha-Naphthoflavone (ANF) is a widely used inhibitor of P-450-mediated metabolism. Previously, we have demonstrated that in vitro addition of ANF to human lymphocytes produced significantly greater numbers of sister chromatid exchanges (SCEs) in samples from smokers compared to nonsmokers. In order to study the mechanism of this differential induction, we investigated the clastogenic activity of ANF as a consequence of metabolism by induced and uninduced rat liver microsomes. Exponentially growing Chinese hamster ovary cells were treated with ANF for 2 h in the presence or absence of microsomes, followed by incubation for 12 (chromosome aberrations) or 24 h (SCEs). ANF induced concentration (4 to 40 microM)-dependent increases in SCEs and chromosome aberrations when coincubated with 2,3,7,8-tetrachlorodibenzo(p)dioxin-induced microsomes. At the lower concentrations of ANF, chromatid damage was most predominant, whereas at the higher concentrations, a high percentage of cells was killed. The surviving cells exhibited shattered chromosomes and multiple damage in the form of chromatid exchanges and breaks. ANF was not clastogenic nor did it induce SCEs in Chinese hamster ovary cells when incubated with microsomes from control rats or phenobarbital-treated rats. Moreover, NADPH was required for the clastogenic actions of ANF in the presence of 2,3,7,8-tetrachlorodibenzo(p)dioxin-induced microsomes. Analysis of the ANF metabolites by high-pressure liquid chromatography revealed that 2,3,7,8-tetrachlorodibenzo(p)dioxin-induced microsomes metabolized ANF to a much greater extent than control or phenobarbital-induced microsomes. Our results suggest that the clastogenic activity of ANF in Chinese hamster ovary cells is mediated by the cytochrome P-450 monooxygenase system.

Influence of cytochrome P-450 type on the pattern of conjugation of 4-hydroxybiphenyl generated from biphenyl or 4-methoxybiphenyl

The rate of production of 4-hydroxybiphenyl from 4-methoxybiphenyl in hepatocytes isolated from untreated rats was essentially identical to that from biphenyl in hepatocytes isolated from rats pretreated with beta-naphthoflavone at 40 mg/kg. Similar results were obtained using liver microsomes isolated from untreated or treated rats. The selective inhibition of these reactions by metyrapone, alpha-naphthoflavone and ethanol demonstrated that different forms of cytochrome P-450 are responsible for O-demethylation of 4-methoxybiphenyl in livers of untreated rats and 4-hydroxylation of biphenyl in livers of pretreated rats. The pattern of conjugation of 4-hydroxybiphenyl generated from biphenyl in hepatocytes isolated from pretreated rats was different from that for 4-hydroxybiphenyl generated from 4-methoxybiphenyl in hepatocytes isolated from untreated rats, there being a 60% decrease in sulphation measured after 60 min incubation with biphenyl. Glucuronidation of 4-hydroxybiphenyl was not affected. The sulphation of 4-hydroxybiphenyl added directly was significantly decreased in hepatocytes isolated from pretreated rats. The initial rate of glucuronidation of 4-hydroxybiphenyl added directly was not altered by the pretreatment, but there was a significant decrease in overall glucuronidation measured over a 60 min incubation. Similar results were obtained using liver slices. beta-Naphthoflavone added directly to liver slices significantly decreased the extent of sulphation of 4-hydroxybiphenyl but did not influence glucuronidation. There was evidence of a late decrease in biphenyl 4-hydroxylase activity in hepatocytes isolated from pretreated rats. It is concluded that the differences observed in the cellular metabolism of biphenyl and 4-methoxybiphenyl can be ascribed to competition between beta-naphthoflavone and/or its metabolites retained within the cells following pretreatment, and biphenyl and/or its metabolites for the pathways common to their metabolism. It is also concluded that the type of cytochrome P-450 involved in the generation of 4-hydroxybiphenyl does not, per se, influence the subsequent pattern of conjugation.

Inhibition of human estrogen synthetase (aromatase) by flavones

Several naturally occurring and synthetic flavones were found to inhibit the aromatization of androstenedione and testosterone to estrogens catalyzed by human placental and ovarian microsomes. These flavones include (in order of decreasing potency) 7,8-benzoflavone, chrysin, apigenin, flavone, flavanone, and quercetin; 5,6-benzoflavone was not inhibitory. 7,8-Benzoflavone and chrysin were potent competitive inhibitors and induced spectral changes in the aromatase cytochrome P-450 indicative of substrate displacement. Flavones may thus compete with steroids in their interaction with certain monooxygenases and thereby alter steroid hormone metabolism.

Metabolism of alpha-naphthoflavone by rat, mouse, rabbit, and hamster liver microsomes

The metabolism of alpha-naphthoflavone (ANF) was studied in hepatic microsomes from rats, mice, rabbits, and hamsters, species in which ANF exerts its biological activities. The major metabolites produced by all species were ANF-5,6-oxide, ANF-6-phenol, and ANF-7,8-dihydrodiol. Minor metabolites produced by all species were ANF-5,6-dihydrodiol, ANF-7-phenol, and ANF-9-phenol. In general, the total rates of metabolism were similar within all species: 22-32 nmol ANF metabolized/15 min/mg protein. Mouse liver microsomes were approximately 1.7 to 2.9 times as active as the other species on a nanomole of cytochrome P-450 basis. The major sites of enzymatic oxidation were the 5,6 and 7,8 bonds of ANF where for all species, 49-71% and 15-46% of the total metabolism occurred, respectively.