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R J Preston

Publications and source records attributed to R J Preston.

At least 19 recordsLinked to original sources
PubMed2000

Increased sensitivity to chromatid aberration induction by bleomycin and neocarzinostatin results from alterations in a DNA damage response pathway.

DNA damage response pathways coordinate the cellular response to DNA damage. To investigate the roles of tumor suppressor genes in these pathways, human lymphoblastoid cells (wild-type, p53-/-, ATM-/-) were treated for 1 h with 0-3 microg/ml of the radiomimetic compound bleomycin (BLM), and cells treated in G(2) were analyzed for chromatid aberrations. BLM-induced aberration frequencies were significantly increased, to the greatest extent in the ATM-/- cells and, to a lesser extent, in the p53-/- cells compared to wild-type cells. These observations are consistent with p53 and ATM acting in a damage response pathway activated by DNA strand breaks. The consequences of disrupting this pathway were further investigated by studies using wortmannin, a PI-3 kinase and DNA repair inhibitor. Wortmannin significantly increased the BLM-induced aberration frequencies in all but the ATM-/- cells, elevating the sensitivity of p53-/- cells to ATM-/- levels and that of wild-type cells to intermediate levels. These differential sensitivities suggest that the ATM phenotype is the result of dual cellular defects, one involving p53 and the other a wortmannin-sensitive component. Similar studies in Brca1+/- and Brca2+/- human lymphoblasts showed no increased sensitization to BLM in the absence of inhibitor, and differential sensitization by wortmannin. To determine if there was any substrate specificity for p53- and ATM-mediated DNA damage responses, chromatid aberrations were assessed in wild-type, p53-/-, and ATM-/- cells exposed to 0-0.4 microg/ml neocarzinostatin (NCS) for 1 h. In contrast to results with BLM, the p53-/- cells exhibited a low sensitivity to NCS-induced aberrations, similar to wild-type, while ATM-/- cells remained highly sensitive. This suggests that the response to BLM- and NCS-induced lesions involves different mechanisms.

Ataxia Telangiectasia Mutated Proteins↗
PubMed2000

A comparison of the roles of p53 mutation and AraC inhibition in the enhancement of bleomycin-induced chromatid aberrations in mouse and human cells.

Previous studies have shown that p53 is involved in the repair of bleomycin-induced DNA damage, and that the frequency of bleomycin-induced chromatid aberrations is elevated in G(2)-treated p53 null transgenic mouse embryo fibroblasts (MEF) as compared to isogenic controls. To further characterize p53-mediated DNA repair, we studied the effect of p53 status on the ability of the DNA repair inhibitor 1-ss-D-arabinofuranosylcytosine (AraC) to sensitize MEF to bleomycin-induced chromatid aberrations. Both p53+/+ and p53-/- MEF were treated in G(2) with 0 to 7.5 microg/ml bleomycin in the presence or absence of AraC (5x10(-5) M). The frequency of bleomycin-induced chromatid aberrations was significantly higher in p53-/- cells than wild-type cells in the absence of AraC. AraC treatment significantly increased the frequency of bleomycin-induced chromatid aberrations in p53+/+ MEF to the levels in p53-/- (no AraC) but had no effect in p53-/- MEF. These results suggest that an AraC-sensitive DNA repair component is altered or absent in p53-/- cells. Similar results were observed in p53-mutant WTK1 and wild-type TK6 human lymphoblast cells exposed to 0 to 3 microg/ml bleomycin in G(2). However, AraC did cause a small increase in bleomycin sensitivity in WTK1 cells. This difference from the p53-/- MEF response may be due to differences in p53-mutant phenotype. To determine whether mutation of p53 alters DNA replication fidelity, p53+/+ and p53-/- MEF were exposed to 0 to 1 microg/ml mitomycin C (MMC). MMC did not induce chromosome aberrations in either cell line treated in G(2) but did with the same effectiveness in both cell lines treated in S-phase. Thus, p53 deficiency does not affect DNA replication fidelity or the repair of MMC-induced DNA damage.

Animals↗
PubMed1999

Cytogenetic effects of ethylene oxide, with an emphasis on population monitoring.

Cytogenetic assays are an integral component of the battery of short-term assays that are used for the hazard identification component of a cancer risk assessment. The protocol for the conduct of such assays for maximal sensitivity for detecting clastogenicity has to be attendant to the mechanism of induction of the endpoint being assessed and the fact that several aberration types are cell lethal necessitates that analysis be for cells at their first posttreatment metaphase. Cytogenetic assays for human populating monitoring have been used for predicting potential for carcinogenicity in humans. However, the assays as typically conducted are not appropriate for chronic exposures because nontransmissible alterations are assessed. The use of fluorescent in situ hybridization (FISH) techniques for the assessment of transmissible changes such as reciprocal translocations are required to make population monitoring studies interpretable, and for removing some of the concern over the influence of confounders on outcome. The database for the cytogenetic effects of ethylene oxide in vitro and in vivo, with an emphasis on human population monitoring, has been critically reviewed. Based on the endpoints studied, the size of the study groups, the information on exposure, the nature of any exposure response data, and the possible influence of confounders (i.e., control matching), it is concluded that acute, high exposures to ethylene oxide with sampling shortly (a few days) after exposure can be detected by increases in chromosome aberrations or SCE in peripheral lymphocytes. Such increases are indicators of exposure to a genotoxic chemical and not predictors of subsequent adverse health effects to individuals. The effect of chronic and/or low level (less than about 25 ppm) exposures cannot be reliably evaluated using current methods. The use of FISH, for example, for assessing reciprocal translocation frequencies (as a measure of transmissible events) will greatly improve the ability to detect chronic exposures to clastogenic chemicals.

Animals↗
PubMed1999

Effects of pregnancy and chronic exercise on maternal cardiac structure and function.

This study examined the interactive effects of pregnancy and aerobic conditioning on maternal cardiac structure and function. Effects of closely monitored cycle ergometer conditioning were studied during the second (TM2) and third trimesters (TM3) in 22 previously sedentary pregnant women (exercised group, EG) and a nonexercising pregnant control group with similar characteristics (CG, n = 19). Subjects were studied in the resting state by two-dimensional echocardiography and during cycle ergometer exercise at three steady-state power outputs at the start of TM2 (ENTRY), at the end of TM2 and TM3 (postconditioning), and 3-4 months postpartum (NPR, nonpregnant reference, CG only). Aerobic conditioning did not increase left ventricular dimensions beyond those attributable to pregnancy itself. In addition, in contrast with previous studies of nonpregnant women, physical conditioning during pregnancy did not reduce heart rate (HR) in the resting state. During exercise, the slope of the HR versus oxygen uptake (VO2) regression decreased significantly between preconditioning and the end of TM3 in the EG, suggesting that training-induced reductions in HR become more evident with increasing exercise intensity. Also, significant reductions in oxygen pulse (VO2/HR) were observed at all three work rates in the CG, but not in the EG. These findings support the hypothesis that the cardiovascular effects of aerobic conditioning are obscured by more powerful effects of pregnancy in the resting state but become "unmasked" during strenuous exercise.

Adult↗
PubMed1998

Attenuation of G1 checkpoint function by the non-genotoxic carcinogen phenobarbital.

Non-genotoxic chemical carcinogens are capable of inducing tumors in rodents without interacting with or directly altering the genetic material. Since a preponderance of evidence suggests that cancer results from the accumulation of genetic alterations, the mechanisms by which many non-genotoxic carcinogens induce genotoxic events remain unclear. The present study investigated whether the mitogenic, non-genotoxic carcinogen phenobarbital (PB) could alter cell-cycle checkpoint controls, thereby indirectly leading to the accumulation of genetic damage. Initial studies involved characterizing cell-cycle checkpoint responses to DNA damage in freshly isolated B6C3F1 mouse hepatocytes. These cells responded to bleomycin-induced DNA damage by arresting in G1 and G2. Cell-cycle arrest was coupled with p53 protein induction; however, p21WAF1 protein levels remained unchanged. Studies that utilized hepatocytes isolated from C57BL p53-/- mice showed that the DNA damage-induced G1 cell-cycle arrest was dependent on p53 function, but cell-cycle arrest in G2 was not affected by loss of p53. PB was able to delay and attenuate the G1 checkpoint response without altering G2 checkpoint function. A reduction in p53 protein, but not transcript levels, was observed in hepatocytes exposed to PB. Additionally, PB delayed and attenuated p53 protein induction during DNA damage, which suggests that changes in the p53 protein may be contributing to the attenuated G1 checkpoint response caused by PB. Altered G1 checkpoint function represents an epigenetic mechanism by which phenobarbital may prevent the detection and repair of DNA damage and indirectly increase the frequency of genotoxic events above that occurring spontaneously. Abrogation of checkpoint controls may, thus, play an important mechanistic role in mitogenic, non-genotoxic chemical carcinogenesis.

Animals↗
PubMed1998

Acid-base regulation and control of ventilation in human pregnancy.

The purposes of this review were twofold: to apply modern physicochemical principles to explain changes in acid-base regulation and the control of ventilation in human pregnancy; and to demonstrate the value of pregnancy as a model for the study of endocrine effects on physiological control systems. Application of P.A. Stewart's approach (P.A. Stewart. Can. J. Physiol. Pharmacol. 61: 1444-1461, 1983) shows that lower values of plasma hydrogen ion concentration ([H+]) observed at rest and in association with exercise in pregnancy are the result of lower values for carbon dioxide tension (Pco2) and total weak acid ([A(tot)]). This effect is partly offset by a lower strong ion difference ([SID]). The ability to predict plasma [H+] at rest and following strenuous exercise in pregnancy (J.G. Kemp, F.A. Greer, and L.A. Wolfe. J. Appl. Physiol. 83: 644-651, 1997) supports the validity of Stewart's approach. Jennings and associates (D.B. Jennings. Can. J. Physiol. Pharmacol. 72: 1499-1512, 1994) have further demonstrated in animal models the involvement of plasma osmolality and circulating levels of angiotensin II (ANG II) and arginine vasopressin (AVP) in the chemical control of ventilation. We hypothesize that pregnancy-induced increases in respiratory sensitivity to carbon dioxide are the combined result of reduced plasma osmolality, reduced cerebrospinal fluid [SID], and augmented circulating levels of progesterone, ANG II, and AVP.

Acid-Base Equilibrium↗
PubMed1997

Amplification of the DNA repair gene O6-methylguanine-DNA methyltransferase associated with resistance to alkylating drugs in a mammalian cell line.

The cytotoxic action of such alkylating chemotherapeutic drugs as 2-chloroethyl-N-nitrosourea (CNU) derivatives is countered by the repair protein O6-methylguanine-DNA methyltransferase (MGMT), which removes O6-alkylguanine induced in the DNA by these agents. Resistance to these drugs is often correlated with the MGMT levels in normal and tumor cells of human and rodent origin. Exposure of mouse 3T3 cells to increasing concentrations of CNU, and subsequent selection of resistant cells, led to the isolation of clones with 5-10 times higher levels of MGMT activity than in the control. The increased MGMT expression at both mRNA and protein levels resulted from 5- to 10-fold amplification of the Mgmt gene. Amplification of this gene was not associated with concomitant amplification of another alkylation damage repair gene, N-methylpurine-DNA glycosylase. No amplification of at least three other genes on chromosome 7 (which contains the Mgmt gene) was observed in the drug-resistant cells. Furthermore, the amplified Mgmt sequence was not associated with a homogeneously staining region, or double minute chromosomes, nor present as episomal DNA. In situ hybridization of metaphase chromosomes of the drug-resistant cells indicated both translocation and localized amplification of the Mgmt gene.

3T3 Cells↗
PubMed1997

Telomeres, telomerase and chromosome stability.

Telomeres in most species consist of repeat units of a small number of nucleotides that together with secondary structures and associated proteins stabilize the linear chromosomal DNA molecule. Chromosomes lose a small amount of telomeric DNA after each cell replication. It has been proposed that when telomeres shorten below a critical length, a DNA damage response pathway is activated and induces cell cycle arrest. In cells such as stem cells that maintain a proliferative capacity, telomere length is maintained by the reverse transcriptase, telomerase. In addition, telomerase activity is present in 90% of primary human tumors, suggesting a role for telomerase in providing a proliferative capacity to cells, which is a requirement in progression to malignancy. Telomerase activity can be involved in chromosome healing, although telomerase-independent processes also appear to be capable of capping broken chromosome ends. This review describes the structure and maintenance of telomeres, the importance of a critical telomere length to cell proliferation and the telomeric status of broken chromosome ends produced during development or by spontaneous or induced DNA damages.

Animals↗
PubMed1996

Interindividual variations in susceptibility and sensitivity: linking risk assessment and risk management.

In the past few years, our knowledge of mammalian genomes has increased enormously. Our understanding of the molecular basis of the normal cellular processes of DNA replication and repair and cell cycle control, together with how their fidelity malfunctions as part of tumor development, has increased in parallel. This has led to a clearer appreciation that there are subpopulations that have been generically described as being genetically or otherwise susceptible to the induction of cancer or birth defects. The term susceptibility is a default option, since there clearly will be a very broad range of sensitivities among the so-called susceptible populations, dependent upon the specific underlying mechanism. This could lead to the conduct of risk assessments for each specific situation, involving both genotypes of individuals and agents of concern. This would ideally take into account the effects on response of various modifying factors, genetic and other. One advantage to be gained from this approach is the ability to determine if a particular susceptibility places subpopulations at extreme risk as compared to the overall normal distribution of risk in the population, or whether such a susceptible population presents a slight extension of the upper bound of the risk distribution or lies within the normal distribution. In addition, the specific mechanism of the susceptibility as related to exposure scenarios and the magnitude and demographics of the susceptible populations need to be taken into account. Thus, the management of risk has to be linked to the specific risk assessment. For many of the so-called susceptible populations an uncertainty factor of less than 10, even including 1, would be predicted to bring the risk within the normal distribution. It is hoped that as more mechanistic information on susceptibility becomes available and a specific risk can be defined, the practice of risk management will be considerably improved.

Animals↗
PubMed1996

Aneuploidy in germ cells: disruption of chromosome mover components.

The task of the Workgroup on "Disruption of Chromosome Mover Components" was to establish what cellular structures are involved in chromosome segregation and how disruption of these could occur. Recent research on the mechanism of action of the cellular components that segregate chromosomes accurately during mitosis or meiosis has served to highlight the number of potential targets for disruption. The process of chromosome segregation represents an orchestrated chain of events centered on the activities of cellular motors, kinesins and dyneins. These motors are involved in arranging chromosomes at the metaphase plate, providing the spindle tension necessary for progression, and the actual segregation of the chromosomes to the poles. The Workgroup determined that there is a lack of information on the effects of chemical exposure to cell motors and other chromosome mover components, and that there is a clear need for further research. This article describes the discussions of the Workgroup and highlights areas of future research into chromosome movement, particularly in human meiotic and mitotic cells. The Workgroup emphasized that obtaining mechanistic data on the induction of aneuploidy will allow for extrapolation of the dose response curves for chemical exposures below the level of observation and for using aneuploidy data for quantitative risk assessment for adverse health effects.

Age Factors↗
PubMed1996

Chromosome aberrations, micronuclei, aneuploidy, sister chromatid exchanges, and cancer risk assessment.

This paper describes the four cytogenetic endpoints most frequently used in hazard identification assays as the first step in the risk assessment process. These are structural chromosome aberrations, micronuclei, aneuploidy, and sister chromatid exchanges. The biological mechanisms involved in the formation of the alterations observed in each assay are briefly discussed. Variations in and recent improvements to each assay are described, with an emphasis on the use of molecular techniques to improve the sensitivity of the assay, and to allow for detection of specific alterations that are, or could be, associated with cancer induction. This, in turn, will make the data obtained in the cytogenetic assays more useful in cancer and genetic risk assessment. Thus, the aim of this paper is to encourage cytogeneticists to design their experiments in such a way that the data obtained will be of maximum possible benefit for characterizing and quantifying adverse human health effects, particularly cancer.

Aneuploidy↗
PubMed1996

The relationship between p53 status, DNA repair and chromatid aberration induction in G2 mouse embryo fibroblast cells treated with bleomycin.

The involvement of p53 in the formation of chromosome aberrations was assessed by analyzing bleomycin-induced, chromatid-type aberrations in G2 phase fibroblasts derived from embryos from wild-type and p53 knock-out mice. Cells that were p53+/- or p53-/- were more sensitive to the induction of aberrations than the p53+/+ cells, particularly at concentrations of 7.5 and 10.0 microg/ml. The p53-deficient cells also showed an overdispersed distribution of bleomycin-induced chromatid aberrations, a greater amount of overall genomic instability and a possible loss of a cell death pathway. These data are interpreted as indicating a role for p53 in DNA repair in the G2 phase, with a loss of p53 leading to an increased frequency of deletions (incomplete repair) and interchanges (misrepair). The specific role remains to be elucidated. The mitotic index decreased with increasing bleomycin concentration to a similar extent in all three cell lines, indicating that the loss of a G2 checkpoint in p53-/- and p53+/- cells was not an explanation for the increased sensitivities in these cells compared with the p53+/+.

Animals↗
PubMed1995

Reconsideration of the genetic risk assessment for ethylene oxide exposures.

The US Environmental Protection Agency (EPA) developed a genetic risk assessment model for exposures to ethylene oxide utilizing data on the induction of reciprocal translocations in male germ cells [Rhomberg et al. 1990]. This particular approach served as a reasonable initial attempt, albeit somewhat limited with regard to endpoint and only partially attentive to the mechanisms of induction of genetic alterations and the behavior of chromosomes during meiosis. The present paper discusses the scientific basis for a reassessment of the EPA model, providing data and hypotheses related to effective dose to the target cells and shape of the dose-response relationship at low doses, and dose rates. While the present genetic risk assessment approach is discussed in terms of ethylene oxide, it would be applicable to most mutagenic chemicals. The outcome of the discussion is that the genetic risk for exposed males from reciprocal translocation induction will be negligible at low doses since the dose-response curve is likely to be a function of the square of the dose. In addition, the proportion of genetically unbalanced live born offspring in humans arising from reciprocal translocation carriers is less than 10% of the frequency formed through meiotic segregation and fertilization for such carriers. Simply from a consideration of mechanism--namely, the very high probability of DNA repair prior to the next S-phase for a resting oocyte--it would be predicted that there would be a very low to negligible frequency of translocations in female germ cells from ethylene oxide exposure. It is further stressed that additional components of a genetic risk model require a consideration of all germ cell stages in the male, and the inclusion of calculations for point and deletion mutations. Some indications of likely response are presented with these points in mind.

Animals↗
PubMed1995

Ethylene oxide: evaluation of genotoxicity data and an exploratory assessment of genetic risk.

A risk estimate of the heritable effects of ethylene oxide exposure, using the parallelogram approach, as suggested by Frits Sobels, is described. The approach is based on available data on the ethylene oxide-induced responses for the same genetic endpoint in somatic cells of both laboratory animals and humans, and for germ cell mutations in the same laboratory animal. Human germ cell effects are estimated. The available data sets for this approach were evaluated. We consider this as complementary to the genetic risk assessment carried out by U.S. EPA scientists, in which the risk from heritable (reciprocal) translocations induced by ethylene oxide was estimated. In the present study we restricted our assessment to dominant mutations. The sensitivity factor relating mouse to man was based on ethylene oxide-induced HPRT mutant frequencies in lymphocytes in vivo. From this comparison, it could be concluded that occupational exposure for 1 year to 1 ppm ethylene oxide would lead to a risk of a dominantly inherited disease in the offspring of 4 x 10(-4) above the background level. The uncertainty interval of this figure is quite large (0.6-28) x 10(-4). The values are compatible with the existing estimates of the corresponding risk from exposure to low LET radiation when the genotoxic potency ratio of ethylene oxide and radiation is considered. This risk estimation approach has allowed us to identify additional data that are required for a more complete risk estimation of the heritable effects of ethylene oxide, or indeed any mutagenic chemical.

Animals↗
PubMed1994

Future of germ cell cytogenetics.

The celebration of the 25th Anniversary of the Environmental Mutagen Society provides an excellent opportunity to assess the status of research in a broad range of areas, with an emphasis on the directions in which they are going. This chapter concentrates on the analysis of chromosomal alterations in mammalian germ cells. The future developments in germ cell cytogenetics research will build heavily upon techniques developed over the past 25 years. With these it is possible to assess numerical and structural alterations in the male in differentiating spermatogonia, spermatocytes, and post-meiotic cells (at the first cleavage division) and for the female in oocytes and the zygote. The most predictable advances will be in the identification of specific alterations through FISH of interphase spermatozoa in humans and further improvements with the human sperm/hamster egg in vitro fertilization technique. Of particular importance is the fact that this will allow for the study of effects in human germ cells. From a more speculative viewpoint it might be possible to assess the role of particular genomic organization on genetic outcomes by direct observation; these might include genomic imprinting and the visual separation of male and female genomes. The overall aim of germ cell cytogenetic studies will remain as improving our ability to identify and estimate the true genetic risk in humans.

Animals↗
PubMed1994

A re-evaluation of the cytogenetic effects of styrene.

Results from new chromosome studies in laboratory animals, comparative investigations of styrene metabolism and pharmacokinetics in humans and animals, and several recent cytogenetic surveys of styrene-exposed workers have necessitated a comprehensive re-evaluation of the chromosome-damaging effects of this chemical. Both styrene and its genotoxic metabolite, styrene oxide, can induce chromosome aberrations (CA) and sister chromatid exchanges (SCE) in vitro, but the chromosome-damaging ability of styrene is only manifested if test conditions favour its metabolic activation over inactivation. There is no convincing evidence of styrene clastogenicity in experimental animals. Styrene oxide is clastogenic only at lethal concentrations via i.p. injection in Chinese hamsters (but not via inhalation) or after oral treatment of mice, a route considered inappropriate for investigating the chromosome-damaging potential of inhaled styrene in man. Styrene and styrene oxide can induce SCE in animals at very high concentrations. Eighteen of 52 cytogenetic studies (CA, micronuclei, SCE) on peripheral blood lymphocytes of styrene workers have reported increases in chromosome damage. The positive findings are not compatible with the conclusion that styrene is responsible for the cytogenetic effects for the following reasons. (a) The positive or negative outcome of the various investigations bears no relationship to the degree of exposure of the workers. (b) There is no convincing evidence of a positive dose response relationship. (c) The relative induction of CA and SCE in worker studies are the opposite of observations of styrene effects in cultured lymphocytes and in laboratory animals. (d) The reports of chromosome-type exchanges in some studies of styrene workers is inconsistent with observations of styrene clastogenicity in cultured lymphocytes. (e) Reports of SCE induction in workers exposed to low concentrations of styrene are not compatible with results of animal inhalation studies, particularly in view of the differences in styrene metabolism and pharmacokinetics between humans and rodents. The increases in cytogenetic effects reported in some studies on styrene workers are probably attributable to the presence of other chromosome-damaging agents in the workplace and/or to inadequate investigations.

Animals↗