Sodium bisulfite pyrosequencing revealed that developmental exposure to environmental contaminant mixtures does not affect DNA methylation of DNA repeats in Sprague-Dawley rats

Integration of Epigenetic Mechanisms into Non-Genotoxic Carcinogenicity Hazard Assessment: Focus on DNA Methylation and Histone Modifications

Epigenetics involves a series of mechanisms that entail histone and DNA covalent modifications and non-coding RNAs, and that collectively contribute to programing cell functions and differentiation. Epigenetic anomalies and DNA mutations are co-drivers of cellular dysfunctions, including carcinogenesis. Alterations of the epigenetic system occur in cancers whether the initial carcinogenic events are from genotoxic (GTxC) or non-genotoxic (NGTxC) carcinogens. NGTxC are not inherently DNA reactive, they do not have a unifying mode of action and as yet there are no regulatory test guidelines addressing mechanisms of NGTxC. To fil this gap, the Test Guideline Programme of the Organisation for Economic Cooperation and Development is developing a framework for an integrated approach for the testing and assessment (IATA) of NGTxC and is considering assays that address key events of cancer hallmarks. Here, with the intent of better understanding the applicability of epigenetic assays in chemical carcinogenicity assessment, we focus on DNA methylation and histone modifications and review: (1) epigenetic mechanisms contributing to carcinogenesis, (2) epigenetic mechanisms altered following exposure to arsenic, nickel, or phenobarbital in order to identify common carcinogen-specific mechanisms, (3) characteristics of a series of epigenetic assay types, and (4) epigenetic assay validation needs in the context of chemical hazard assessment. As a key component of numerous NGTxC mechanisms of action, epigenetic assays included in IATA assay combinations can contribute to improved chemical carcinogen identification for the better protection of public health.

Endocrine-disrupting chemicals alter the neuromolecular phenotype in F2 generation adult male rats

Endocrine-disrupting chemical (EDC) exposures to the fetus have long-lasting effects on health and disease in adulthood. Such EDC exposure to the F1 fetuses also reaches the germ cells that become the F2 generation. Previously, we demonstrated that adult social and communicative behaviors such as ultrasonic vocalizations and mating behaviors were altered by EDCs in F2 rats, especially males. In the current study, we used the brains of these F2 males to ascertain the underlying molecular changes in the hypothalamus related to these behavioral outcomes. Their progenitors were Sprague-Dawley rat dams, treated on pregnancy days 8 to 18 with one of three treatments: a polychlorinated biphenyl (PCB) mixture, Aroclor 1221, selected because it is weakly estrogenic; the anti-androgenic fungicide vinclozolin (VIN); or the vehicle, 6% dimethylsulfoxide in sesame oil (VEH). In adulthood, F1 male and female offspring were bred with untreated partners to generate paternal or maternal lineages of the F2 offspring, the subjects of molecular work. Quantitative real-time PCR was conducted in the medial preoptic area (POA) and the ventromedial nucleus (VMN) of the hypothalamus, selected for their roles in social and sexual behaviors. Of the genes assessed, steroid hormone receptors (estrogen receptor α, androgen receptor, progesterone receptor) but not dopamine receptors 1 and 2 or DNA methyltransferase 3a expression were altered, particularly in the VIN males. Several significant correlations between behavior and gene expression were also detected. These results suggest that preconceptional exposure of male rats to EDCs at the germ cell stage alters the neuromolecular phenotype in adulthood in a lineage-dependent manner.

Effects of cross-fostering and developmental exposure to mixtures of environmental contaminants on hepatic gene expression in prepubertal 21 days old and adult male Sprague-Dawley rats

The notion that adverse health effects produced by exposure to environmental contaminants (EC) may be modulated by the presence of non-chemical stressors is gaining attention. Previously, our lab demonstrated that cross-fostering (adoption of a litter at birth) acted as a non-chemical stressor that amplified the influence of developmental exposure to EC on the glucocorticoid stress-response in adult rats. Using liver from the same rats, the aim of the current study was to investigate whether cross-fostering might also modulate EC-induced alterations in hepatic gene expression profiles. During pregnancy and nursing, Sprague-Dawley dams were fed cookies laced with corn oil (control, C) or a chemical mixture (M) composed of polychlorinated biphenyls (PCB), organochlorine pesticides (OCP), and methylmercury (MeHg), at 1 mg/kg/day. This mixture simulated the contaminant profile reported in maternal human blood. At birth, some control and M treated litters were cross-fostered to form two additional groups with different biological/nursing mothers (CC and MM). The hepatic transcriptome was analyzed by DNA microarray in male offspring at postnatal days 21 and 78-86. Mixture exposure altered the expression of detoxification and energy metabolism genes in both age groups, but with different sets of genes affected at day 21 and 78-86. Cross-fostering modulated the effects of M on gene expression pattern (MM vs M), as well as expression of energy metabolism genes between control groups (CC vs C). In conclusion, while describing short and long-term effects of developmental exposure to EC on hepatic transcriptomes, these cross-fostering results further support the consideration of non-chemical stressors in EC risk assessments.

In vitro functional analysis of human cytochrome P450 2A13 genetic variants: P450 2A13*2, *3, *4, and *10

Humans possess three cytochrome P450 enzymes in the 2A subfamily (2A6, 2A7, and 2A13). P450 2A13 is mainly expressed in the human trachea and lung, whereas P450 2A6 is found in human liver. The P450 2A13 enzyme may be considered as the primary enzyme responsible for metabolic activation of many tobacco-specific carcinogens. Genetic variations significantly influence the toxicological consequences attributed to tobacco smoking. The aim of this study was to examine the in vitro functional activities of five P450 2A13 genetic variations (R257C, 133_134insT, R101Q, I331T, and R257C/I331T) in P450 2A13*2, *3, *4, and *10 alleles. Mutant clones were constructed and their recombinant enzymes were expressed in Escherichia coli. P450 2A13 mutants containing R257C, 133_134insT, I331T, and R257C/I331T displayed P450 holoenzyme spectra. The R101Q mutant was not apparently expressed. P450 2A13 enzymes displayed the typical type I binding spectra to coumarin and the calculated binding affinities of R257C, R257C/I331T, and 133_134insT mutants were decreased approximately three- to sevenfold. In catalytic analyses of purified mutant enzymes for coumarin and nicotine, the R257C and I331T mutants exhibited lower kcat values with catalytic efficiencies reduced up to approximately 20%. The double mutation of R257C/I331T induced increased Km values and diminished kcat values that resulted in >50% decrease in catalytic efficiencies. For 133_134insT mutant, catalytic activities were not markedly saturated but the measured rates at the highest concentrations were significantly lower than those of the wild-type or other mutant enzymes. Functional analysis of these variations in P450 2A13 allelic variants may help to understand the consequences of P450 2A13 polymorphism in bioactivation of many tobacco-derived carcinogens.