Genotoxic and epigenetic carcinogens: their identification and significance

Genome Engineering, Chemical Exposure, and the Germline: An Ethical Synthesis

Concerns about the human germline in the context of genome editing have been at the forefront of contemporary bioethics, with the clear recognition that the heritability associated with such intervention merits special moral consideration. In contrast, the question of moral responsibility for modifications of the germline genome that result from anthropogenic environmental toxicants has received little attention. Yet, whether the impact of human technological activity on enduring shifts in human heredity occurs via purposeful genetic modification or nondirected changes that undermine genome stability, the result is irreversible genetic change in future generations. This article argues that the robust ethical reflection developed by the bioethics community to address human heritable genome editing can be used as a resource to address understudied questions of moral responsibility for anthropogenic insults to the germline. Drawing on this bioethics work, the article outlines a future-oriented ethical framework for germline responsibility in a time of widespread concern about industrial chemicals and human futures.

Human Elimination of Organochlorine Pesticides: Blood, Urine, and Sweat Study

Background. Many individuals have been exposed to organochlorinated pesticides (OCPs) through food, water, air, dermal exposure, and/or vertical transmission. Due to enterohepatic reabsorption and affinity to adipose tissue, OCPs are not efficiently eliminated from the human body and may accrue in tissues. Many epidemiological studies demonstrate significant exposure-disease relationships suggesting OCPs can alter metabolic function and potentially lead to illness. There is limited study of interventions to facilitate OCP elimination from the human body. This study explored the efficacy of induced perspiration as a means to eliminate OCPs. Methods. Blood, urine, and sweat (BUS) were collected from 20 individuals. Analysis of 23 OCPs was performed using dual-column gas chromatography with electron-capture detectors. Results. Various OCPs and metabolites, including DDT, DDE, methoxychlor, endrin, and endosulfan sulfate, were excreted into perspiration. Generally, sweat samples showed more frequent OCP detection than serum or urine analysis. Many OCPs were not readily detected in blood testing while still being excreted and identified in sweat. No direct correlation was found among OCP concentrations in the blood, urine, or sweat compartments. Conclusions. Sweat analysis may be useful in detecting some accrued OCPs not found in regular serum testing. Induced perspiration may be a viable clinical tool for eliminating some OCPs.

Mechanism of 1,3-dichloropropene-induced rat liver carcinogenesis

1,3-Dichloropropene (1,3-D) is a soil fumigant used primarily for preplanting control of parasitic nematodes. In a previous chronic dietary exposure study, 1,3-D induced an increased incidence of hepatocellular adenomas in male rats at a dose of 25 mg/kg/day. Although the mechanism for tumor induction in the rat liver by 1,3-D has not been specifically elucidated, available data suggested that the observed liver tumorigenesis was through a nongenotoxic mode of action at the tumor promotion stage. Fischer 344 rats containing preneoplastic lesions were treated (via gavage) with 25 mg/kg/day 1,3-D or 80 mg/kg/day phenobarbital (PB) for 30 days and 60 days, or for 30 days followed by a 30-day recovery period (no compound exposure). Following treatment, placental form glutathione S-transferase (GSTP) positive and GSTP-negative liver focal lesions were quantitated as to size and number. 1,3-D treatment had no effect on GSTP-positive foci number or relative size but significantly increased the number, labeling index, and relative size of GSTP-negative focal lesions (identified by H and E staining) after 30 and 60 days of treatment. Following the 30-day recovery period, the number, labeling index, and relative size of the GSTP-negative lesions in 1,3-D-treated animals returned to control levels. As expected, PB treatment produced an increase in number and relative size of the GSTP-positive lesions. The results of this study are consistent with 1,3-D inducing liver carcinogenesis through a nongenotoxic mode of action by functioning as a tumor promoter specifically through induction of a non-GSTP staining focal hepatocyte population.

Is your initiator really necessary?

There is little hard evidence for the involvement of specific genotoxic initiators in the pathogenesis of the common carcinoma. Recent findings suggest that sporadic carcinogenesis is a dynamic and probabilistic process which requires a critical mass of abnormal cells for its expression, and that this requirement may distinguish the evolution of carcinomas from that of paediatric or haematologic malignancies. The proposal that specific carcinogens are neither necessary nor sufficient for tumourigenesis is consistent with the growing realization that aberrant expression of specific oncogenes is neither necessary nor sufficient for cellular transformation. These new perspectives have major implications for basic research strategy and public health policy.

Epigenetic promoting effects of butylated hydroxyanisole

Studies on the potential genotoxicity and epigenetic effects of BHA are reviewed. BHA was not DNA-reactive, but studies showed that it did have an effect on membrane systems, blocking exchange between hepatocytes and epithelial cells. The available data strongly suggest that BHA is an epigenetic carcinogen that produces forestomach neoplasms through a promoting effect. Standard food safety procedures are suggested as appropriate for setting safe levels of exposure.

Lack of induction of sister chromatid exchanges and of mutation to 6-thioguanine resistance in V79 cells by butylated hydroxyanisole with and without activation by rat or hamster hepatocytes

The antioxidant butylated hydroxyanisole (BHA) and the promutagen/carcinogen 7,12-dimethylbenz[a] anthracene (DMBA) were examined for mutagenicity and the induction of sister chromatid exchanges (SCE) in a hepatocyte-mediated mutation assay with V79 Chinese hamster lung cells. Rat and hamster hepatocytes, prepared by in situ collagenase perfusion, were compared in the mutation assay to determine whether there are species differences in the ability to activate BHA and DMBA to ultimate mutagens. At the marginally cytotoxic concentration of 1.0 microM (2.6 micrograms/ml), DMBA induced a significant increase in the frequency of SCE and in the number of mutations to 6-thioguanine resistance (6-TGR) at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) locus with either rat or hamster hepatocyte-mediated activation, but induced highest mutation frequencies with rat hepatocytes. These findings support the contention that species differences can affect mutational response in hepatocyte-mediated assays with V79 cells. BHA was strongly cytotoxic to V79 cells at dose levels in excess of 0.3 mM (54 micrograms/ml). In contrast to DMBA, BHA showed no evidence of genotoxicity at marginally cytotoxic concentrations up to and including 0.3 mM as shown by the inability of this antioxidant to increase the frequency of sister chromatid exchanges or to induce mutations to 6-thioguanine resistance when activation was provided by rat or hamster hepatocytes.

Toxic effects of chemicals: difficulties in extrapolating data from animals to man

This review attempts an analysis of the major components which make it extremely difficult to extrapolate toxicological data obtained with chemicals from animals to man. A first problem concerns the use of doses to express the unit of comparison across animal species; the dose is a parameter exogenous to the body and when a chemical enters the body concentrations should be utilized. There is in fact evidence that for several chemicals equal doses in different animal species do not mean equal concentrations in blood or tissues. Concentrations of chemicals should be measured for extrapolation purposes as close as possible to the site of the toxic effect. A second problem regards the fact that several chemical are transformed in the body into other chemical species--sometimes few and sometimes many--and some of these species (active metabolites) display biological activity in some cases higher than different from or antagonistic to those of the parent compounds. Some of these metabolites are highly reactive and therefore bind to body components, particularly macromolecules such as proteins and nucleic acids. There is evidence that metabolism is quantitatively and/or qualitatively different in various animal species. A third problem concerns the difference in various animal species in the biological substrates on which chemicals exert their toxic effects. Equal concentrations of chemicals and their metabolites do not mean equal toxic effects across animal species because endogenous metabolic processes, cell permeability, enzymes, and receptors are not necessarily the same in animals and man. To overcome these difficulties there is a need for closer integration of different disciplines in the toxicological evaluation of chemicals. A scientific rather than a routine approach in toxicology is emphasized.

Absence of a promoting or sequential syncarcinogenic effect in rat liver by the carcinogenic hypolipidemic drug nafenopin given after N-2-fluorenylacetamide

The hypolipidemic agent nafenopin, (NF), has been reported to be carcinogenic to rat liver. To determine whether nafenopin exerts a promoting or syncarcinogenic effect in rat liver, its effect on liver carcinogenesis induced by N-2-fluorenylacetamide (FAA) was studied. In two separate experiments, male F344 rats were fed 0.02% FAA for either 10 or 8 weeks to induce preneoplastic liver lesions. Following a recovery period of 1 week, rats were given 0.01 or 0.02% NF in the diet for 23 weeks in one experiment and 0.05 or 0.1% for 24 weeks in the other. The final incidence of neoplasms, and their numbers, size distribution, and degrees of differentiation were not significantly different in groups given NF after FAA compared to those maintained on a basal diet after FAA. In the group treated with the highest dose level of NF following FAA, however, there was a decrease in the number of grossly visible small neoplasms. In contrast, the liver neoplasm promoter phenobarbital increased the multiplicity, although not the incidence, of liver neoplasms when given after FAA. Thus, four different dose levels of NF showed no promoting or syncarcinogenic effect on FAA-induced hepatocarcinogenesis.

Absence of genotoxicity of the carcinogenic sulfated polysaccharides carrageenan and dextran sulfate in mammalian DNA repair and bacterial mutagenicity assays

The sulfated polysaccharides degraded carrageenan and dextran sulfate sodium were studied for genotoxicity using DNA repair tests employing cultured rat hepatocytes or intestinal mucosal cells. No evidence of unscheduled DNA synthesis was obtained. In addition, both alone and in combination with the comutagen norharman, these substances were nonmutagenic in the Salmonella microsome/mutagenicity test. These findings suggest that the carcinogenicity of the sulfated polysaccharides may be due to nongenotoxic mechanisms.

Neoplastic conversion in rat liver by the antihistamine methapyrilene demonstrated by a sequential syncarcinogenic effect with N-2-fluorenylacetamide

A study was performed to determine whether the enhancing effect of the antihistamine methapyrilene (MP) in rat liver carcinogenesis represents promotion or syncarcinogenesis . The effect on hepatocarcinogenicity induced by N-2-fluorenylacetamide (FAA) of sequential administration of MP given either before or after FAA was studied in comparison with diethylnitrosamine (DEN) also given either before or after FAA. MP in either sequence with FAA enhanced liver carcinogenicity as did DEN. Moreover, MP by itself induced liver altered foci, albeit at a high dose for a prolonged interval. A single liver neoplasm occurred with exposure to MP alone. These findings suggest that MP produces neoplastic conversion of liver cells which can be summated with the genotoxic effect of FAA to produce syncarcinogenesis .

An enhancing effect of the antihistaminic drug methapyrilene on rat liver carcinogenesis by previously administered N-2-fluorenylacetamide

The effect on hepatocarcinogenesis of sequential administration of the antihistaminic drug methapyrilene (MP) or the typical liver tumor promoter, phenobarbital (PB), each given after the genotoxic liver carcinogen, N-2-fluorenylacetamide (FAA) was studied. An initial exposure to 0.02% of FAA in the diet for 8 weeks was used to produce hepatocellular altered foci. Rats maintained for an additional 24 weeks on basal diet developed a low incidence of liver neoplasms. MP at 500 and 1000 ppm in the diet for 24 weeks after FAA increased the frequency of altered foci at early stages and liver neoplasms later, as did PB. Neither MP nor PB alone produced neoplasms, but MP alone produced a significant incidence of altered foci. Therefore, the results provide evidence for a promoting action of MP, but additional effects may be involved in its carcinogenicity.