Inactivation of adrenal cytochromes P450 by 1-aminobenzotriazole. Divergence of in vivo and in vitro actions

Impact of Extended and Combined Exposure of Bisphenol Compounds on Their Chromosome-Damaging Effect─Increased Potency and Shifted Mode of Action

Bisphenol (BP) compounds are important environmental pollutants and endocrine disruptors. BPs are capable of inducing DNA/chromosome breaks (clastogenesis, involved in carcinogenesis), which requires activation by human CYP1A1. We hypothesized that combined BPs and extended (from the standard two-cell cycle) exposure may enhance their genotoxicity via modulating CYP enzymes. In this study, individual and combined BPA/BPF/BPS/BPAF and a human hepatoma (HepG2) cell line were used for testing several genotoxicity end points. Exposing for a two-cell cycle period (48 h), each BP alone (0.625-10 μM) was negative in the micronucleus test, while micronucleus was formed under three- (72 h) and four-cell cycle (96 h) exposure; BP combinations further elevated the potency (with nanomolar thresholds). Immunofluorescence analysis of the centromere with formed micronucleus indicated that 48 h exposure produced centromere-negative micronucleus and phosphorylated histone H2AX (γ-H2AX) (evidencing clastogenesis), while extended (72 and 96 h) exposure formed centromere-positive micronucleus and phosphorylated histone H3 (p-H3) (indicating chromosome loss, i.e., aneugenesis); moreover, 1-aminotriabenzotriazole (CYP inhibitor) selectively blocked the formation of centromere-negative micronucleus and γ-H2AX, without affecting that of centromere-positive micronucleus and p-H3. This study suggests that the genotoxicity of BPs is potentiated by combined and extended exposure, the latter being specific for aneuploidy formation, a CYP activity-independent effect.

Influence of Bisphenol Compounds at Nanomolar Concentrations on Chromosome Damage Induced by Metabolically Activated Carcinogens in HepG2 Cells

Bisphenol (BP) compounds are endocrine-disrupting organic pollutants. BPs may increase the messenger RNA (mRNA) transcripts of nuclear receptors (NRs) regulating the expression of xenobiotic-metabolizing cytochrome P450 (CYP) enzymes. Their impact on the genotoxicity of metabolically activated carcinogens, however, remains unknown. In this study, effects of the bisphenols A, F, S, and AF on the expression of the aryl hydrocarbon receptor (AhR), the pregnane X receptor (PXR), the constitutive androstane receptor, and individual xenobiotic-metabolizing CYP enzymes in a human hepatoma (HepG2) cell line were investigated, along with in silico binding studies of BPs to each receptor. The results indicated that each BP at 1 to 100 nM concentrations increased the mRNA transcripts and protein levels of AhR, PXR, CYP1A1, 1A2, 1B1, 2E1, and 3A4. The predicted affinities of the BPs for binding AhR were comparable to those of potent agonists. Pretreatment of HepG2 cells with each BP potentiated the induction of micronuclei by benzo[a]pyrene, aflatoxin B1, benzene, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone; this effect was abolished/reduced by inhibitors of NRs and/or CYPs. Our study suggests that BPs at human exposure levels may aggravate chromosome damage by several impactful carcinogens in human cells by inducing relevant CYP enzymes, mostly via NR modulation.

Short-term toxicity study of 1-aminobenzotraizole, a CYP inhibitor, in Wistar rats

1-Aminobenzotriazole (ABT) is a pan-specific, mechanism-based inhibitor of CYP P450 enzymes, often used as co-treatment to investigate the metabolism-dependent toxicity of drugs or chemicals. To assess the confounding effects of ABT in such kind of mechanistic studies, a repeated dose toxicity study with ABT following 7 days oral administration at 0, 25, 50 and 100 mg/kg/day was performed in Wistar rats (5 rats/sex/group). Wistar rat is selected as a model being one of the well characterized rodent species, widely used for toxicity and toxicokinetics studies. The standard parameters of general toxicity study viz. clinical signs, body weight, feed consumption, clinical, gross and histopathology were evaluated. The ABT was tolerated up to the highest tested dose of 100 mg/kg/day. No clinical signs, mortality or effect on feed consumption at any dose. Slight increase in body weight gain was noted in ABT treated females. Increased reticulocyte, and decreased triglycerides, BUN, A/G ratio and plasma potassium; increased weight of liver, kidneys, adrenals and thyroid was noted in ABT treated animals. Microscopically, hypertrophic findings were noted in liver, thyroid, adrenal glands, pituitary and uterus. Some of these changes were observed at as low as 25 mg/kg/day, therefore, NOEL could not be established. Based on this study, it is concluded that ABT is tolerable up to 100 mg/kg/day with some variations in clinical pathology, organ weight and histopathology; these changes should be considered during the assessment of any mechanistic study with ABT. Findings of this manuscript were presented at 58th meeting of the Society of Toxicology, Baltimore, 11 March 2019.

1-Aminobenzotriazole: A Mechanism-Based Cytochrome P450 Inhibitor and Probe of Cytochrome P450 Biology

1-Aminobenzotriazole (1-ABT) is a pan-specific, mechanism-based inactivator of the xenobiotic metabolizing forms of cytochrome P450 in animals, plants, insects, and microorganisms. It has been widely used to investigate the biological roles of cytochrome P450 enzymes, their participation in the metabolism of both endobiotics and xenobiotics, and their contributions to the metabolism-dependent toxicity of drugs and chemicals. This review is a comprehensive evaluation of the chemistry, discovery, and use of 1-aminobenzotriazole in these contexts from its introduction in 1981 to the present.

Conditional Deletion of Cytochrome P450 Oxidoreductase in the Liver and Gastrointestinal Tract: A New Model for Studying the Functions of the P450 System

We have previously described a mouse model, where hepatic cytochrome P450 oxidoreductase (POR) expression has been deleted, resulting in almost complete ablation of hepatic P450 function [Hepatic P450 Reductase Null (HRN)]. HRN mice grow normally but develop fatty livers, and they have increased cytochrome P450 levels. Associated with the hepatic lipid accumulation are significant changes in the expression of genes controlling lipid homeostasis. We have characterized this model extensively and demonstrated its value in drug efficiency testing, in toxicokinetics, and in evaluating the role of the hepatic P450 system in drug pharmacokinetics. To extend the deletion of POR, and P450 inactivation, to other tissues, and to develop the utility of this model, we have generated a mouse where POR can be deleted conditionally in the liver and gastrointestinal tract using the rat cytochrome P450 CYP1A1 promoter to drive Cre recombinase expression. Administration of the CYP1A1 inducers tetrachlorodibenzo-p-dioxin or beta-naphthoflavone resulted in both hepatic and gastrointestinal deletion of POR, whereas administration of 3-methylcholanthrene resulted specifically in loss of hepatic POR expression. In all cases, the resulting hepatic phenotype seemed identical to that of the HRN model, including increased cytochrome P450 expression. Hepatic deletion of POR and the subsequent increase in P450 expression were dependent on inducer dose, with maximal POR deletion occurring at a single dose of 3-methylcholanthrene of 40 mg/kg. This model provides a powerful approach for studying the functions of POR as well as in the evaluation of the role of hepatic and gastrointestinal P450s in drug deposition and chemical toxicity.

The use of the suicide CYP450 inhibitor ABT for distinguishing absorption and metabolism processes in in-vivo pharmacokinetic screens

Since drug candidates with low oral systemic exposure may be due to either or both absorption and metabolism factors, determining what factors limit the oral systemic exposure is not always obvious in a single in-vivo pharmacokinetic (PK) assay. A rapid rat in-vivo PK screen where the oxidative drug metabolism has been attenuated using the suicide CYP450 inhibitor aminobenzotriazole (ABT) is described. We have shown that the roles of absorption and metabolism for drug candidates with low oral systemic exposure can be determined by comparing the PK parameters of drug candidates orally administered to non-treated and ABT-treated rats. Propranolol, metoprolol and climetidine are used as model drugs. Propranolol and metoprolol have low oral systemic exposures in rats primarily due to metabolism factors while the oral systemic exposure of climetidine is high in rats. For propranolol and metoprolol, large increases in the systemic exposure of these drugs were observed between non-treated and ABT-treated rats. ABT appeared not to increase or decrease significantly the rate and extent of absorption or metabolism of cimetidine since that oral systemic exposure of non-treated and ABT-treated rats did not significantly change. These experiments suggest that for drug candidates with low systemic exposures in rats an observation of no change in the oral systemic exposure in ABT-treated rats when compared to the non-treated rats imply that absorption (or formulation) factors limit the systemic exposure of the drug while an increase in the systemic exposure in ABT-treated rats imply that metabolism factors limit the systemic exposure. Due to the ease of preparing and interpreting PK data from ABT-treated rats, is suggested that this assay could be used as an alternative to in vivo cannulation assays.

Cytochrome P450-catalyzed brassinosteroid pathway activation through synthesis of castasterone and brassinolide in Phaseolus vulgaris

The last reaction in the biosynthesis of brassinolide has been examined enzymatically. A microsomal enzyme preparation from cultured cells of Phaseolus vulgaris catalyzed a conversion from castasterone to brassinolide, indicating that castasterone 6-oxidase (brassinolide synthase) is membrane associated. This enzyme preparation also catalyzed the conversions of 6-deoxocastasterone and typhasterol to castasterone which have been reported to be catalyzed by cytochrome P450s, CYP85A1 of tomato and CYP92A6 of pea, respectively. The activities of these enzymes require molecular oxygen as well as NADPH as a cofactor. The enzyme activities were strongly inhibited by carbon monoxide, an inhibitor of cytochrome P450, and this inhibition was recovered by blue light irradiation in the presence of oxygen. Commercial cytochrome P450 inhibitors including cytochrome c, SKF 525A, 1-aminobenzotriazole and ketoconazole also inhibited the enzyme activities. The present work presents unanimous enzymological evidence that cytochrome P450s are responsible for the synthesis of brassinolide from castasterone as well as of castasterone from typhasterol and 6-deoxocastasterone, which have been deemed activation steps of BRs.

Taxol biosynthesis: differential transformations of taxadien-5 alpha-ol and its acetate ester by cytochrome P450 hydroxylases from Taxus suspension cells

The biosynthesis of the diterpenoid antineoplastic drug Taxol in Taxus species involves the cyclization of the ubiquitous isoprenoid intermediate geranylgeranyl diphosphate to taxa-4(5),11(12)-diene followed by cytochrome P450-mediated hydroxylation (with allylic rearrangement) of this olefin precursor to taxa-4(20),11(12)-dien-5 alpha-ol, and further oxygenation and acylation reactions. Based on the abundances of naturally occurring taxoids, the subsequent order of oxygenation of the taxane core is considered to occur at C10, then C2 and C9, followed by C13, and finally C7 and C1. Circumstantial evidence suggests that the acetylation of taxadien-5 alpha-ol may constitute the third specific step of Taxol biosynthesis. To determine whether taxadienol or the corresponding acetate ester serves as the direct precursor of subsequent oxygenation reactions, microsomal preparations isolated from induced Taxus cells and optimized for cytochrome P450 catalysis were incubated with each potential substrate. Both taxadienol and taxadienyl acetate were oxygenated to the level of a diol and to higher polyols at comparable rates by cytochrome P450 enzymes of the microsomal preparation. Preparative-scale incubation allowed the isolation of sufficient quantities of the diol derived from taxadienol to permit the NMR-based structural elucidation of this metabolite as taxa-4(20),11(12)-dien-5 alpha,13 alpha-diol, which may represent an alternate route of taxoid metabolism in induced cells. GC-MS-based structural definition of the diol monoacetate derived in microsomes from taxadienyl acetate confirmed this metabolite as taxa-4(20),11(12)-dien-5 alpha-acetoxy-10 beta-ol, thereby indicating that acetylation at C5 of taxadienol precedes the cytochrome P450-mediated insertion of the C10-beta-hydroxyl group of Taxol.

Cytochromes P-450 from cassava (Manihot esculenta Crantz) catalyzing the first steps in the biosynthesis of the cyanogenic glucosides linamarin and lotaustralin. Cloning, functional expression in Pichia pastoris, and substrate specificity of the isolated recombinant enzymes

The first committed steps in the biosynthesis of the two cyanogenic glucosides linamarin and lotaustralin in cassava are the conversion of L-valine and L-isoleucine, respectively, to the corresponding oximes. Two full-length cDNA clones that encode cytochromes P-450 catalyzing these reactions have been isolated. The two cassava cytochromes P-450 are 85% identical, share 54% sequence identity to CYP79A1 from sorghum, and have been assigned CYP79D1 and CYP79D2. Functional expression has been achieved using the methylotrophic yeast, Pichia pastoris. The amount of CYP79D1 isolated from 1 liter of P. pastoris culture exceeds the amounts that putatively could be isolated from 22,000 grown-up cassava plants. Each cytochrome P-450 metabolizes L-valine as well as L-isoleucine consistent with the co-occurrence of linamarin and lotaustralin in cassava. CYP79D1 was isolated from P. pastoris. Reconstitution in lipid micelles showed that CYP79D1 has a higher k(c) value with L-valine as substrate than with L-isoleucine, which is consistent with linamarin being the major cyanogenic glucoside in cassava. Both CYP79D1 and CYP79D2 are present in the genome of cassava cultivar MCol22 in agreement with cassava being allotetraploid. CYP79D1 and CYP79D2 are actively transcribed, and production of acyanogenic cassava plants would therefore require down-regulation of both genes.

Inhibition of testicular steroid metabolism by administration of 1-aminobenzotriazole to rats

Effects of 1-aminobenzotriazole (ABT) on testicular steroid metabolism were evaluated in rats. Administration of ABT to adult male rats caused dose-dependent decreases in testicular microsomal and mitochondrial cytochrome P450 concentrations. Significant losses of P450 occurred within 8 h of ABT treatment. Accompanying the declines in testicular P450 content were decreases in microsomal 17 alpha-hydroxylase and mitochondrial cholesterol sidechain cleavage activities. Incubation of testicular microsomes or mitochondria in vitro with ABT plus an NADPH-generating system had no effect on P450 concentrations or on rates of steroid metabolism. By contrast, incubation of hepatic microsomes with ABT under the same conditions decreased P450 levels and xenobiotic-metabolizing activity. The results indicate that ABT in vivo causes inactivation of steroidogenic P450 isozymes in the testis, but the mechanism of inactivation differs from that on xenobiotic-metabolizing isozymes.

Myoglobin toxicity in proximal human kidney cells: roles of Fe, Ca2+, H2O2, and terminal mitochondrial electron transport

The purpose of this study was to gain direct insights into mechanisms by which myoglobin induces proximal tubular cell death. To avoid confounding systemic and hemodynamic influences, an in vitro model of myoglobin cytotoxicity was employed. Human proximal tubular (HK-2) cells were incubated with 10 mg/ml myoglobin, and after 24 hours the lethal cell injury was assessed (vital dye uptake; LDH release). The roles played by heme oxygenase (HO), cytochrome p450, free iron, intracellular Ca2+, nitric oxide, H2O2, hydroxyl radical (-OH), and mitochondrial electron transport were assessed. HO inhibition (Sn protoporphyrin) conferred almost complete protection against myoglobin cytotoxicity (92% vs. 22% cell viability). This benefit was fully reproduced by iron chelation therapy (deferoxamine). Conversely, divergent cytochrome p450 inhibitors (cimetidine, aminobenzotriazole, troleandomycin) were without effect Catalase induced dose dependent cytoprotection, virtually complete, at a 5000 U/ml dose. Conversely, -OH scavengers (benzoate, DMTU, mannitol), xanthine oxidase inhibition (oxypurinol), superoxide dismutase, and manipulators of nitric oxide expression (L-NAME, L-arginine) were without effect. Intracellular (but not extracellular) calcium chelation (BAPTA-AM) caused approximately 50% reductions in myoglobin-induced cell death. The ability of Ca2+ (plus iron) to drive H2O2 production (phenol red assay) suggests one potential mechanism. Blockade of site 2 (antimycin) and site 3 (azide), but not site 1 (rotenone), mitochondrial electron transport significantly reduced myoglobin cytotoxicity. Inhibition of Na, K-ATPase driven respiration (ouabain) produced a similar protective effect. We conclude that: (1) HO-generated iron release initiates myoglobin toxicity in HK-2 cells; (2) myoglobin, rather than cytochrome p450, appears to be the more likely source of toxic iron release; (3) H2O2 generation, perhaps facilitated by intracellular Ca2+/iron, appears to play a critical role; and (4) cellular respiration/terminal mitochondrial electron transport ultimately helps mediate myoglobin's cytotoxic effect. Formation of poorly characterized toxic iron/H2O2-based reactive intermediates at this site seems likely to be involved.