Response of human keratinocytes to extremely low concentrations of N-methyl-N'-nitro-N-nitrosoguanidine

Micronuclei in neonates and children: effects of environmental, genetic, demographic and disease variables

Children may be more susceptible to the effects of the environmental exposure and medical treatments than adults; however, limited information is available about the differences in genotoxic effects in children by age, sex and health status. Micronucleus (MN) assay is a well established method of monitoring genotoxicity, and this approach is thoroughly validated for adult lymphocytes by the Human Micronucleus Biomonitoring project (HUMN.org). Similar international undertaking is in progress for exfoliated buccal cells. Most of the MN studies in children are focused on analyses of lymphocytes but in the recent years, more investigators are interested in using exfoliated cells from the oral cavity and other cell types that can be collected non-invasively, which is particularly important in paediatric cohorts. The baseline MN frequency is relatively low in newborns and its assessment requires large cohorts and cell sample counts. Available results are mostly consistent in conclusion that environmental pollutants and radiation exposures lead to the increase in the MN frequency in children. Effects of medical treatments are less clear, and more studies are needed to optimise the doses and minimise genotoxicity without compromising therapy outcomes. Despite the recent progress in MN assay in children, more studies are warranted to establish the relationship between MN in lymphocytes and exfoliated cells, to clarify sex, age and genotype differences in baseline MN levels and the changes in response to genotoxicants. One of the most important types of MN studies in children are prospective cohorts that will help to clarify the predictive value of MN and other cytome end points for cancer and other chronic diseases of childhood and adulthood. Emerging 'omic' and other novel molecular technologies may shed light on the molecular mechanisms and biological pathways associated with the MN levels in children.

Comparison of genotoxic potency of styrene 7,8-oxide with gamma radiation and human cancer risk estimation of styrene using the rad-equivalence approach

Styrene is suspected to cause lympho-hematopoietic malignancies through the formation of styrene 7,8-oxide. However, we are still unable to calculate the cancer risk for workers exposed to styrene using epidemiological data. The aims of this study were to determine the blood dose after styrene exposure and to compare the genotoxic potency of styrene 7,8-oxide and gamma radiation in order to calculate the cancer risk by means of the rad-equivalence approach. Leucocytes of 20 individuals were exposed to 0, 0.1, 0.2 or 0.3 mM styrene 7,8-oxide (1 h) or 1, 2 or 3 gray (=100, 200, 300 rad) gamma radiation. Genotoxicity was evaluated with the cytokinesis-block micronucleus assay. Comparison of the two slopes of the regression lines between micronuclei and dose revealed a genotoxic potency for styrene 7,8-oxide of 37 rad/mMh, corresponding with a median value derived from mutagenicity studies (1, 37, 208 rad/mMh). At exposure levels of 1 ppm styrene, a blood styrene 7,8-oxide concentration between 0.03 x 10(-)(6) and 0.42 x 10(-)(6) mM is to be expected using data of toxicokinetic models and human exposure studies. With the cancer risk per unit dose of gamma radiation as benchmark, we calculated a lifetime risk of acquiring a fatal lympho-hematopoietic cancer of 0.17 in 10(3) workers (between 0.037 x 10(-)(3) and 5.0 x 10(-)(3)) exposed to 20 ppm styrene during 40 years.

Environmental lead exposure increases micronuclei in children

The objective of this pilot study was to investigate the contribution of environmental exposures to lead in the development of cytogenetic damage detected as the frequency of micronuclei (MN) in children. The other aim was to apply the MN assay in combination with fluorescence in situ hybridization (FISH) using a pan-centromeric chromosome probe to elucidate the formation mechanism of induced MN. The examined population was composed of 9-year-old children (n = 92), living in the region where non-ferrous ores are extracted and processed. The non-exposed group consisted of 49 children of the same age from an unexposed recreational area. Exposure to lead was assessed by determination of lead concentrations in blood (PbB) by atomic absorption spectrophotometry, whereas the level of selenium (Se) in serum was detected by using graphite furnace atomic-absorption spectrometry. The frequency of MN was determined by the cytokinesis-block MN assay and fluorescence in situ hybridization performed using a specific pan-centromeric probe. Environmental exposure to lead resulted in significantly increased levels of PbB (5.29 +/- 2.09 versus 3.45 +/- 1.20 microg/dl in controls), although the average level was much below the value of the biological exposure limit = 10 microg/dl. A negative correlation between lead in blood and Se in serum concentrations (P = 0.006) was found for the pooled study population. The results showed a significant difference (P < 0.0001) in the level of MN between the exposed and control group (standard MN test: 2.96 +/- 2.36 versus 1.16 +/- 1.28; FISH technique: 3.57 +/- 3.02 versus 1.43 +/- 1.69, respectively). The frequencies of both centromere-positive (C+MN) and centromere-negative (C-MN) micronuclei were significantly increased in exposed children; however, the contribution of C+MN in the total number of MN in peripheral blood lymphocytes of exposed children was significantly higher than in the controls what may suggest a pro-aneugenic effect of the exposure to lead. The results of multiple regression analysis indicated that the exposure to lead was an important factor affecting the increase in MN frequency what was confirmed by significant correlation between the PbB and MN levels. In conclusion, our results suggest that the exposure to lead may be associated with an increased frequency of MN, especially of C+MN; however, the influence of other factors (e.g. vitamins and minerals in the diet) cannot be excluded.

Hormesis and dose-response-mediated mechanisms in carcinogenesis: evidence for a threshold in carcinogenicity of non-genotoxic carcinogens

Recently the idea of hormesis, a biphasic dose-response relationship in which a chemical exerts opposite effects dependent on the dose, has attracted interest in the field of carcinogenesis. With non-genotoxic agents there is considerable experimental evidence in support of hormesis and the present review highlights current knowledge of dose-response effects. In particular, several in vivo studies have provided support for the idea that non-genotoxic carcinogens may inhibit hepatocarcinogenesis at low doses. Here, we survey the examples and discuss possible mechanisms of hormesis using phenobarbital, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), alpha-benzene hexachloride (alpha-BHC) and other non-genotoxins. Furthermore, the effects of low and high doses of non-genotoxic and genotoxic compounds on carcinogenesis are compared, with especial attention to differences in mechanisms of action in animals and possible application of the dose-response concept to cancer risk assessment in humans. Epigenetic processes differentially can be affected by agents that impinge on oxidative stress, DNA repair, cell proliferation, apoptosis, intracellular communication and cell signaling. Non-genotoxic carcinogens may target nuclear receptors, cause aberrant DNA methylation at the genomic level and induce post-translational modifications at the protein level, thereby impacting on the stability or activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Genotoxic agents, in contrast, cause genetic change by directly attacking DNA and inducing mutations, in addition to temporarily modulating the gene activity. Carcinogens can elicit a variety of changes via multiple genetic and epigenetic lesions, contributing to cellular carcinogenesis.

Value of GST-P positive preneoplastic hepatic foci in dose-response studies of hepatocarcinogenesis: evidence for practical thresholds with both genotoxic and nongenotoxic carcinogens. A review of recent work

Recent low-dose carcinogenesis studies, including major group projects are reviewed. The prevailing paradigm is that carcinogens, particularly genotoxic compounds, have no threshold in exerting their potential for cancer induction. However, the nonthreshold hypothesis can be challenged for cancer risk assessment in humans. A recent very large-scale cooperative effort in Japan furthermore showed that the genotoxic hepatocarcinogen, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, forms DNA adducts and 8-hydroxy-2'-deoxyguanosine at low doses, but does not induce glutathione S-transferase placental form (GST-P) positive foci as preneoplastic lesions in rat liver (< or = 10 ppm in diet). Moreover, very low doses of a N-nitroso compound. diethylnitrosamine (DEN), were also found not to significantly induce GST-P positive foci in rat liver (< or = 0.01 ppm in drinking water). Given the direct correlation between induction of the preneoplastic lesions in the short-term and carcinomas in the longer term with different carcinogens, the results imply a practical nonobserved effect level for hepatocarcinogenicity. Similar results were also observed with so-called nongenotoxic carcinogens such as phenobarbital (PB) and p,p-dichlorodiphenyltrichloroethane (DDT), which do not exert positive effects on lesion development at very low doses. Furthermore, experiments with application of PB and DDT after treatment with DEN indicate that at very low doses (< or = 2 ppm in diet), they may even inhibit the development of GST-P positive foci. The data reviewed provide evidence that preneoplastic foci in the liver can be employed as end-point lesions in place of tumors and that exposure to very low levels of carcinogens, typical of those found in the human environment, does not necessarily present as a risk factor.

Lack of a dose-response relationship for carcinogenicity in the rat liver with low doses of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline or N-nitrosodiethylamine

For a long period, it has been generally considered that carcinogens, particularly genotoxic ones, have no threshold in exerting their potential for cancer induction. However, the non-threshold theory can be challenged with regard to assessment of cancer risk to humans. Here we show that a food-derived, genotoxic hepatocarcinogen, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, forms DNA adducts at low doses, but does not induce glutathione S-transferase placental form (GST-P)-positive foci (considered to be preneoplastic lesions) or 8-hydroxy-2'-deoxyguanosine in rat liver. Moreover a N-nitroso compound, N-nitrosodiethylamine, at low doses was also found not to induce GST-P-positive foci in rat liver. These results imply that there is a no-observed effect level for hepatocarcinogenesis by these genotoxic carcinogens.

Low concentration N-methyl-N'-nitro-N-nitrosoguanidine activates DNA polymerase-beta expression via cyclic-AMP-protein kinase A-cAMP response element binding protein pathway

Ultraviolet light (UV), ionizing-radiation or alkylating agents are known as carcinogens, mostly because of their ability to damage DNA directly. However, they may also play a diverse role in activating the signal pathways and altering the gene expression. We have shown previously that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) of 0.2 microM can increase the transcription of DNA polymerase-beta gene, which has a cyclic AMP response element (CRE) motif in its promoter. Using the CRE report vector, we show here, such treatment can stimulate the CRE-driven gene expression by approximately 1.5-fold compared with control. Consistent with it, the proportion of ser-133 phosphorylated CRE binding protein (CREB), the related transcription factor was 2.08-fold higher versus control in vero cells after 60 min of MNNG treatment. Although CREB has many putative kinases for its phosphorylation, such as p38 mitogen-activated protein kinase (p38 MAPK), Ca(2+)/calmodulin-dependent protein kinase Pi (CaMK Pi) and protein kinase C (PKC), we found the protein kinase A (PKA) was activated and its activation peaked when cells were treated for 60 min (with arbitrary activity unit of 11.03+/-2.80 and 0.86+/-0.43 in treatment and control, respectively), this phasic character was similar to that of the CREB phosphorylation. We also determined the intracellular cyclic AMP (cAMP) levels and it was found that the cAMP concentration was elevated after 60 min treatment (1.53-fold higher). However, to our surprise, we did not find any accompanying cAMP elevation in cells treated by MNNG for 30 min, in which PKA was activated significantly. These findings, together with other observations, suggest that cAMP-PKA-CREB signaling pathway mediates the low concentration MNNG induced pol-beta expression. In addition to elevated cAMP, there might exist a cAMP-independent PKA activation manner in this course.

Can the concept of hormesis Be generalized to carcinogenesis?

The concept of hormesis (i.e., low-dose stimulation/high-dose inhibition) has been shown to be widely generalizable with respect to chemical class, animal model, gender, and biological end point. The public health implication of this lack of linearity in the low-dose area of the dose-response curve raises the question of whether low doses of carcinogens will reduce cancer risk. Articles relating to the process of carcinogenesis (i.e., initiation, promotion, tumor development, and progression) were obtained from a recently developed chemical hormesis database and evaluated for their evidence of hormesis. Numerous examples in well-designed studies indicate that U- or J-shaped dose-response relationships exist with respect to various biomarkers of carcinogenesis in different animal models of both sexes. Examples of such J-shaped dose-response relationships in each stage of the process of carcinogenesis were selected for detailed toxicological examination. These results have important implications for both the hazard assessment of carcinogens and cancer risk assessment procedures.

Substrate and mispairing properties of 5-formyl-2'-deoxyuridine 5'-triphosphate assessed by in vitro DNA polymerase reactions

5-Formyluracil (fU) is one of the thymine lesions produced by reactive oxygen radicals in DNA and its constituents. In this work, 5-formyl-2'-deoxyuridine 5'-triphosphate (fdUTP) was chemically synthesized and extensively purified by HPLC. The electron withdrawing 5-formyl group facilitated ionization of fU. Thus, p K a of the base unit of fdUTP was 8.6, significantly lower than that of parent thymine (p K a = 10.0 as dTMP). fdUTP efficiently replaced dTTP during DNA replication catalyzed by Escherichia coli DNA polymerase I (Klenow fragment), T7 DNA polymerase (3'-5'exonuclease free) and Taq DNA polymerase. fU-specific cleavage of the replication products by piperidine revealed that when incorporated as T, incorporation of fU was virtually uniform, suggesting minor sequence context effects on the incorporation frequency of fdUTP. fdUTP also replaced dCTP, but with much lower efficiency than that for dTTP. The substitution efficiency for dCTP increased with increasing pH from 7.2 to 9.0. The parallel correlation between ionization of the base unit of fdUTP (p K a = 8.6) and the substitution efficiency for dCTP strongly suggests that the base-ionized form of fdUTP is involved in mispairing with template G. These data indicate that fU can be specifically introduced into DNA as unique lesions by in vitro DNA polymerase reactions. In addition, fU is potentially mutagenic since this lesion is much more prone to form mispairing with G than parent thymine.

The role of poly(ADP-ribosyl)ation in the adaptive response

An involvement of the poly(ADP-ribosyl)ation system in the expression of the adaptive response has been demonstrated with inhibitors of the nuclear enzyme poly(ADP-ribose) polymerase. This enzyme is a key component of a reaction cycle in chromatin, involving dynamic synthesis and degradation of variably sized ADP-ribose polymers in response to DNA strand breaks. The present report reviews recent work focussing on the response of the poly(ADP-ribosyl)ation system in low dose adaptation. The results suggest that adaptation of human cells to minute concentrations of an alkylating agent involves a different activation mechanism for poly(ADP-ribose) polymerase than DNA break-mediated stimulation after high dose treatment. Moreover, adaptation induces the formation of branched polymers with a very high binding affinity for histone tails and selected other proteins. High dose challenge treatment of adapted cells further enhances formation of branched polymers. We propose that apart from sensing DNA nicks, poly(ADP-ribose) polymerase may be part of pathway protecting cells from downstream events of DNA damage.