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J Ashby

Publications and source records attributed to J Ashby.

At least 109 records · Page 6Linked to original sources
PubMed1997

The challenge posed by endocrine-disrupting chemicals.

Rapid regulatory developments in the area of environmental endocrine disruption present a series of potential problems that are identified and illustrated with examples taken from the recent literature. A list of priorities is provided, including the need for additional epidemiological and wildlife studies, the derivation of a coordinated testing strategy, agreement on the toxicities expected of endocrine disrupting agents, and acceptance that whole animal assays will be uniquely critical in this area of toxicology. The intrinsic difficulty of attempting to simultaneously study all aspects of endocrine disruption indicates the need to reduce the scope of the problem, which can be achieved by first studying toxicities mediated by sex hormone receptors.

Androgen Receptor Antagonists↗
PubMed1996

Smoke signals.

Explore the source record for details and available documents.

Diet↗
PubMed1996

Mutagenicity to the mouse bone marrow by the mouse germ cell mutagen N-propyl-N-nitrosourea.

N-Propyl-N-nitrosourea (PNU) is shown to be active in male mouse bone marrow micronucleus assays when dosed at either 100 or 200 mg/kg in saline. Activity was observed following either intraperitoneal (i.p.) injection or oral gavage. This observation is consistent with the demonstration by Murota and Shibuya of the specific-locus mutagenicity caused by PNU in male mouse spermatogonia when dosed at 200 mg/kg by i.p. injection. These data strengthen further the observation that rodent germ cell mutagens are also mutagenic to rodent somatic cells.

Animals↗
PubMed1996

Fate of unfertilized ova in male rodent dominant lethal assays: extension of the studies by Kratochvilova.

Kratochvilova has described a technique whereby ova can be recovered from mated mice and their stage of division determined. This is of value to determine if reduced total implantations in a male dominant lethal (DL) germ cell mutation assay are due to pre-implantation loss of embryos, a presumed mutagenic event, or to chemically induced male infertility. Kratochvilova was not specific about the fate of unfertilized ova, but it was implied that they undergo a process of fragmentation that might be confused with the regular cleavage of fertilized ova. It became important for us to draw a firm distinction between ova fragmentation and regular ova cleavage in the rat. We therefore repeated the ova analyses of female mice mated with males exposed to iso-propyl methanesulphonate (iPMS), as described by Kratochvilova. Following that calibration study the technique was extended to the rat via ova cleavage analysis in mated female rats, coupled to a study of the normal decay of ova in virgin rats. Unfertilized ova are shown to undergo irregular fragmentation that can be clearly distinguished from normal cell division. It is concluded that the individual or combined incidences of single celled ova and fragmented ova (dependent on the cleavage stage of the concurrent control embryos) can provide a measure of male infertility as it relates to reduced implantations in DL assays. This ability to regard two morphological classifications of unfertilized ova as providing evidence for male infertility will simplify the conduct of ova analyses in both the mouse and the rat.

Animals↗
PubMed1996

Nongenotoxic carcinogens: development of detection methods based on mechanisms: a European project.

While the accumulation of genetic changes in a somatic cell is considered essential for the genesis of a cancer, it has become clear that not all carcinogens are genotoxic, suggesting that some carcinogens indirectly participate in the generation of genetic changes during carcinogenesis. A European project funded by the European Community was thus conceived to study mechanisms of nongenotoxic aspects of carcinogenesis. Two main strategical approaches were adapted: (i) to study whether and how Syrian hamster embryo (SHE), Syrian hamster dermal (SHD) and BALB/c 3T3 cell transformation systems simulate in vivo carcinogenesis, and to examine whether they can detect nongenotoxic carcinogens; (ii) to study, refine and validate mechanisms-based end-points for detection of nongenotoxic carcinogens. For mechanisms-based research, the proposed end-points included gap junctional intercellular communication (GJIC) inhibition, altered expression of critical genes, immortalization and aberrant cell proliferation. We also selected model compounds commonly usable for various endpoints. Our major results can be summarized as follows: (1) SHE and BALB/c 3T3 transformation systems reflect both genotoxic and nongenotoxic carcinogenic events; they detect not only genotoxic but also many although not all, nongenotoxic carcinogens. This is further supported by the fact that both genotoxic and nongenotoxic carcinogens were able to immortalize SHD cells. (2) Many nongenotoxic carcinogens, although not all, inhibit GJIC in vitro as well as in vivo. Mechanistic studies suggest an important role of blocked GJIC in carcinogenesis and that different mechanisms are involved in inhibition of the communication by different agents used. However, inhibition of GJIC is not a prerequisite for the enhancement (or induction) of transformation of SHE or BALB/c 3T3 cells. (3) Among compounds examined, there was a good correlation between induction of micronuclei and cell transformation in SHE cells while no such correlation was found between the induction of cell transformation and ornithine decarboxylase activity. (4) Two transgenic mouse mutation assays (lacI and lacZ) were established and validated. The genotoxin dimethylnitrosamine was shown to be mutagenic to the liver in both assays. Ortho-anisidine, a bladder-specific carcinogen that was inactive in standard rodent genetic toxicity assays was uniquely mutagenic to the bladder of the transgenic mice. The peroxisome proliferator methyl clofenipate was established as nonmutagenic to the liver of both transgenic mice. That eliminated DNA damage as a cause of the liver tumours produced by this chemical and weakened the idea that induced cell division leads to mutation induction. (5) With an in vitro DNA replication model, it was found that DNA damage induced by genotoxic agents can be responsible for inhibition of DNA replication, while certain nongenotoxic agents such as phorbol esters increase DNA replication. (6) An attempt to use structure-activity relationship for subfamilies of nongenotoxic carcinogens, e.g., receptor-mediated carcinogens, has been initiated with some promising results. Our results support the idea that there are multiple nongenotoxic mechanisms in carcinogenesis, and that working hypothesis-oriented approaches are encouraged rather than simple screening of chemicals in developing test systems for the detection of nongenotoxic carcinogens.

3T3 Cells↗
PubMed1996

IPCS harmonization of methods for the prediction and quantification of human carcinogenic/mutagenic hazard, and for indicating the probable mechanism of action of carcinogens.

A flow chart is presented as a recommended sequence of tests to predict the carcinogenic hazard, and to predict and quantify the mutagenic hazard to germ cells of chemicals to humans. Ten associated principles of testing for these endpoints are also suggested. These recommendations are the result of a meeting convened under the auspices of the International Programme on Chemical Safety (IPCS), as part of their project on 'Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals'. The meeting was held at Carshalton, Surrey, from 13-17 February 1995.

Animals↗
PubMed1996

The rodent bone marrow micronucleus assay: contrast between its sensitivity to human carcinogens and its insensitivity to NTP rodent carcinogens.

The rodent bone marrow micronucleus (MN) assay occupies a critical position in the accompanying schemes to detect potential human carcinogens and germ cell mutagens (Shelby, 1996; Ashby et al., 1996: for reviews of the MN assay see Heddle et al., 1983; Schlegel and MacGregor, 1984; CSGMT, 1990; Mavournin et al., 1990; Tinwell, 1990; Gatehouse, 1994; Asanami et al., 1995). The intention of this article is to note two perceptional problems currently associated with the MN assay. The first concerns how it should be used--as a screening assay, or as a means to evaluate the genetic toxicity in vivo of genotoxins defined in vitro. The second relates to its sensitivity to the rodent carcinogens defined by the US National Toxicology Program (NTP).

Animals↗
PubMed1996

Activity of iPMS and nPMS in mouse bone marrow micronucleus assays: comparison with mouse dominant lethal assay data.

isoPropyl methanesulphonate (iPMS) and its n-propyl analogue (nPMS) are shown to be active in mouse bone marrow micronucleus assays using male CBA, male and female (C3H/El X 102/E1)Fl and male and female Muta Mouse mice. iPMS was significantly more active than nPMS. No significant strain or gender differences were observed. These findings reflect the differences reported earlier for these two chemicals in mouse dominant lethal mutation assays. The earlier described dominant lethal assay data are represented schematically and discussed.

Alkylating Agents↗
PubMed1996

Unexpected genetic toxicity to rodents of the N',N'-dimethyl analogues of MNU and ENU.

Lijinsky and his colleagues have reported that the N',N'-dimethyl analogues of ENU and MNU [N',N'-dimethyl-N-ethyl-N-nitrosourea (DMENU) and trimethylnitrosourea (TMNU), respectively] are carcinogenic to rats despite their extreme hydrolytic stability which would reduce or preclude generation of alkylating species analogous to those formed upon hydrolysis of ENU and MNU. Lijinsky and his colleagues were unable to rationalize those activities of DMENU and TMNU despite extensive experimentation. We therefore decided to study this problem further. Whichever mode is accepted for the generation of electrophilic/mutagenic/carcinogenic reactive species from ENU and MNU, blocking of the free-NH2 group with methyl groups (-NMe2) should ablate or abolish activity. Consistent with this DMENU and TMNU gave negative results in the NBP alkylation test while the parent compounds gave an instantaneous deep blue coloration. Studies of the rate of hydrolysis of these four compounds revealed ENU and MNU to have half-lives of 8 min, while the alkylated analogues (DMENU and TMNU) had half-lives of 25 and 41 days, respectively. Hydrolysis of ENU and MNU, to yield the alkylating species, proceeds either via proton abstraction from the -NH2 group or by attack by water on the carbon of the carbonyl group. Methylation will inhibit both of these pathways, the first absolutely (no -NH2 protons) and the second partially, via steric inhibition. The slow hydrolysis observed for DMENU and TMNU suggests that the latter route of hydrolysis is applicable. Studies with strain TA1535 of Salmonella typhimurium (without S9 mix) confirmed the potent mutagenic activity for ENU and MNU (approximately 300-fold increase in revertants at 2,000 micrograms/plate and approximately 180-fold increase in revertants at 150 micrograms/plate respectively). In contrast, the methylated analogues showed only weak mutagenic activity (approximately 3-fold) at approximately 100-fold higher dose-levels. Addition of S9 mix did not affect the mutagenicity of DMENU or TMNU. To this point, hypothesis and data coincide. ENU and MNU are potent micronucleus-inducing agents to the mouse bone marrow, and given the above data, it was expected that DMENU and TMNU would show weak or no activity in that assay. In fact, the methylated analogues were as effective as ENU and MNU as clastogens to the mouse bone marrow. Four possible reasons for this conflict of theory and data are explored. The speculative explanation we favour for these effects is that the net alkylation of bone marrow DNA is the same for all four chemicals. With ENU and MNU, most of the alkylating activity is dissipated by rapid hydrolysis. Thus, only a small fraction of the administered dose survives to alkylate the bone marrow. Due to the enhanced stability of the methyl analogues most of the delivered dose will reach the bone marrow. However, because of their lower intrinsic reactivity, only a small fraction of the target dose will alkylate the bone marrow DNA during the time window of the experiment. If these opposing influences happen to balance out, the essentially identical bone marrow genetic toxicity for the four chemicals could be explained.

Alkylating Agents↗
PubMed1996

Prediction of Salmonella mutagenicity.

The ability of a number of prediction systems was examined to determine how well they could predict Salmonella mutagenicity. The prediction systems included two computer-based systems (CASE and TOPKAT), the measurement of a physiochemical parameter (ke) and the use of structural alerts by an expert chemist. The computer-based systems operators and the chemist were supplied with the structures of 100 chemicals that had been tested for mutagenicity in the Salmonella test; the actual chemicals were needed for the physiochemical measurement. None of the participants was provided with the chemical names or Salmonella test results prior to submitting their predictions. The three systems that predicted the mutagenicity from the structure of the chemicals produced equivalent results (71-76% concordance with the Salmonella results); the physiochemical system produced a lower (60-61%) concordance.

Databases, Factual↗
PubMed1996

Methyl methanesulphonate (MMS) as a resource conserving and reliable positive control agent for male rat and male mouse dominant lethal assays.

A single i.p. injection of methyl methanesulphonate (MMS; 40 mg/kg) to male rats, followed by their sequential mating over 4-28 days post-dosing, was shown to induce dominant lethal (DL) effects in two strains of rats and in four separate experiments. Activity was evident between 4 days post-dosing (sampling treated spermatozoa) and 28 days post-dosing (sampling treated early spermatids). The highest incidences of resorptions were seen between days 15 and 21 post-dosing, being approximately 1 week later than the peak of activity in the mouse. The DL effects seen were strong and could be detected as early as 4-10 days post-dosing, at which time only small reductions in pregnancy rates and total implantation numbers were seen. Similar, but weaker, mutagenic effects have been reported by Ehling et al. for MMS in mouse DL assays. These data indicate, therefore, that MMS provides a convenient positive control agent for both male rat and male mouse dominant lethal assays, requiring only approximately five treated males, each mated to two females at around 14 days post-dosing.

Animals↗