Versicolorin A enhances the genotoxicity of aflatoxin B1 in human liver cells by inducing the transactivation of the Ah-receptor.

Aflatoxins are a group of mycotoxins that have major adverse effects on human health. Aflatoxin B1 (AFB1) is the most important aflatoxin and a potent carcinogen once converted into a DNA-reactive form by cytochrome P450 enzymes (CYP450). AFB1 biosynthesis involves the formation of Versicolorin A (VerA) which shares structural similarities with AFB1 and can be found in contaminated commodities, often co-occurring with AFB1. This study investigated and compared the toxicity of VerA and AFB1, alone or in combination, in HepG2 human liver cells. Our results show that both toxins have similar cytotoxic effects and are genotoxic although, unlike AFB1, the main genotoxic mechanism of VerA does not involve the formation of DNA double-strand breaks. Additionally, we show that VerA activates the aryl hydrocarbon receptor (AhR) and significantly induce the expression of the CYP450-1A1 (CYP1A1) while AFB1 did not induce AhR-dependent CYP1A1 activation. Combination of VerA with AFB1 resulted in enhanced genotoxic effects, suggesting that AhR-activation by VerA influences AFB1 genotoxicity by promoting its bioactivation by CYP450s to a highly DNA-reactive metabolite. Our results emphasize the need for expanding the toxicological knowledge regarding mycotoxin biosynthetic precursors to identify those who may pose, directly or indirectly, a threat to human health.

The in vitro assessment of the toxicity of volatile, oxidisable, redox-cycling compounds: phenols as an example.

Phenols are regarded as highly toxic chemicals. Their effects are difficult to study in in vitro systems because of their ambiguous fate (degradation, auto-oxidation and volatility). In the course of in vitro studies of a series of redox-cycling phenols, we found evidences of cross-contamination in several in vitro high-throughput test systems, in particular by trimethylbenzene-1, 4-diol/trimethylhydroquinone (TMHQ) and 2,6-di-tertbutyl-4-ethylphenol (DTBEP), and investigated in detail the physicochemical basis for such phenomenon and how to prevent it. TMHQ has fast degradation kinetics followed by significant diffusion rates of the resulting quinone to adjacent wells, other degradation products being able to air-diffuse as well. DTBEP showed lower degradation kinetics, but a higher diffusion rate. In both cases the in vitro toxicity was underestimated because of a decrease in concentration, in addition to cross-contamination to neighbouring wells. We identified four degradation products for TMHQ and five for DTBEP indicating that the current effects measured on cells are not only attributable to the parent phenolic compound. To overcome these drawbacks, we investigated in detail the physicochemical changes occurring in the course of the incubation and made use of gas-permeable and non-permeable plastic seals to prevent it. Diffusion was greatly prevented by the use of both plastic seals, as revealed by GC-MS analysis. Gas non-permeable plastic seals, reduced to a minimum compounds diffusion as well oxidation and did not affect the biological performance of cultured cells. Hence, no toxicological cross-contamination was observed in neighbouring wells, thus allowing a more reliable in vitro assessment of phenol-induced toxicity.

Antagonistic activity towards the androgen receptor independent from natural sex hormones in human milk samples from the Norwegian HUMIS cohort.

In this paper, we investigated the possible presence of endocrine disrupting chemicals (EDCs) based on measuring the total estrogenic and androgenic activity in human milk samples. We used specific bioassays for analysis of the endocrine activity of estrogens and estrogen-like EDCs and androgens and androgen-like EDCs and developed a separation method to evaluate the contribution from natural hormones in comparison to that of EDCs to total endocrine activities. We extracted ten random samples originating from the Norwegian HUMIS biobank of human milk and analyzed their agonistic or antagonistic activity using the ERα- and AR CALUX® bioassays. The study showed antagonistic activity towards the androgen receptor in 8 out of 10 of the assessed human milk samples, while 2 out of 10 samples showed agonistic activity for the ERα. Further investigations demonstrated anti-androgenic activity in the polar fraction of 9 out of 10 samples while no apolar extracts scored positive. The culprit chemicals causing the measured antagonistic activity in AR CALUX was investigated through liquid chromatography fractionation coupled to bioanalysis and non-target screening involving UHPLC-Q-TOF-MS/MS, using a pooled polar extract. The analysis revealed that the measured anti-androgenic biological activity could not be explained by the presence of endogenous hormones nor their metabolites. We have demonstrated that human milk of Norwegian mothers contained anti-androgenic activity which is most likely associated with the presence of anthropogenic polar EDCs without direct interferences from natural sex hormones. These findings warrant a larger scale investigation into endocrine biological activity in human milk, as well as exploring the chemical sources of the activity and their potential effects on health of the developing infant.

Incorporation of metabolic enzymes to improve predictivity of reporter gene assay results for estrogenic and anti-androgenic activity.

Identification and monitoring of so-called endocrine-disrupting compounds has received ample attention; both the OECD and the United States Environmental Protection Agency (US EPA) have designed tiered testing approaches, involving in vitro bioassays to prioritize and partly replace traditional animal experiments. Since the estrogen (ER) and androgen (AR) receptor are frequent targets of endocrine disrupting chemicals, bioassays detecting interaction with these receptors have a high potential to be of use in risk assessment of endocrine active compounds. However, in many bioassays in vivo hepatic metabolism is not accounted for, which hampers extrapolation to the in vivo situation. In the present study, we have developed a metabolic module using rat liver S9 as an add-on to human cell-based reporter gene assays. The method was applied to reporter gene assays for detection of (anti-) estrogens and (anti-) androgens, but can be extended to cell-based reporter gene assays covering a variety of endpoints related to endocrine disruption.

Evaluation of an alternative in vitro test battery for detecting reproductive toxicants in a grouping context.

Previously we showed a battery consisting of CALUX transcriptional activation assays, the ReProGlo assay, and the embryonic stem cell test, and zebrafish embryotoxicity assay as 'apical' tests to correctly predict developmental toxicity for 11 out of 12 compounds, and to explain the one false negative [7]. Here we report on applying this battery within the context of grouping and read across, put forward as a potential tool to fill data gaps and avoid animal testing, to distinguish in vivo non- or weak developmental toxicants from potent developmental toxicants within groups of structural analogs. The battery correctly distinguished 2-methylhexanoic acid, monomethyl phthalate, and monobutyltin trichloride as non- or weak developmental toxicants from structurally related developmental toxicants valproic acid, mono-ethylhexyl phthalate, and tributyltin chloride, respectively, and, therefore, holds promise as a biological verification model in grouping and read across approaches. The relevance of toxicokinetic information is indicated.

Activation of the anticancer drugs cyclophosphamide and ifosfamide by cytochrome P450 BM3 mutants.

Cyclophosphamide (CPA) and ifosfamide (IFA) are widely used anticancer agents that require metabolic activation by cytochrome P450 (CYP) enzymes. While 4-hydroxylation yields DNA-alkylating and cytotoxic metabolites, N-dechloroethylation results in the generation of neuro- and nephrotoxic byproducts. Gene-directed enzyme prodrug therapies (GDEPT) have been suggested to facilitate local CPA and IFA bioactivation by expressing CYP enzymes within the tumor cells, thereby increasing efficacy. We screened bacterial CYP BM3 mutants, previously engineered to metabolize drug-like compounds, for their ability to catalyze 4-hydroxylation of CPA and IFA. Two CYP BM3 mutants showed very rapid initial bioactivation of CPA and IFA, followed by a slower phase of product formation. N-dechloroethylation by these mutants was very low (IFA) to undetectable (CPA). Using purified CYP BM3 as an extracellular bioactivation tool, cytotoxicity of CPA and IFA metabolism was confirmed in U2OS cells. This novel application of CYP BM3 possibly provides a clean and catalytically efficient alternative to liver microsomes or S9 for the study of CYP-mediated drug toxicity. To our knowledge, the observed rate of CPA and IFA 4-hydroxylation by these CYP BM3 mutants is the fastest reported to date, and might be of potential interest for CPA and IFA GDEPT.

Application of drug metabolising mutants of cytochrome P450 BM3 (CYP102A1) as biocatalysts for the generation of reactive metabolites.

Recently, several mutants of cytochrome P450 BM3 (CYP102A1) with high activity toward drugs have been obtained by a combination of site-directed and random mutagenesis. In the present study, the applicability of these mutants as biocatalysts in the production of reactive metabolites from the drugs clozapine, diclofenac and acetaminophen was investigated. We showed that the four CYP102A1 mutants used in this study formed the same metabolites as human and rat liver microsomes, with an activity up to 70-fold higher compared to human enzymes. Using these CYP102A1 mutants, three novels GSH adducts of diclofenac were discovered which were also formed in incubations with human liver microsomes. This work shows that CYP102A1 mutants are very useful tools for the generation of high levels of reference metabolites and reactive intermediates of drugs. Producing high levels of those reactive metabolites, that might play a role in adverse drug reactions (ADRs) in humans, will facilitate their isolation, structural elucidation, and could be very useful for the toxicological characterization of novel drugs and/or drug candidates.