Search PubMed⌕ Search

Biomedical subjects

J A Swenberg

Publications and source records attributed to J A Swenberg.

At least 73 records · Page 4Linked to original sources
PubMed1996

The effects of exposure route on DNA adduct formation and cellular proliferation by 1,2,3-trichloropropane.

1,2,3-Trichloropropane (TCP) induces high incidences of tumors at multiple sites in mice and rats when administered chronically by gavage. The animal tumor data are being used to predict human risk from potential exposure to TCP in drinking water. Risk assessment may be affected by differences in the route of exposure. Gavage administration, which results in high bolus concentrations compared to drinking water exposure, may quantitatively affect toxicokinetics, cytotoxicity, and genotoxicity. We have examined the effects of TCP exposure by the two routes on the formation of DNA adducts and the induction of cellular proliferation. Male B6C3F1 mice were administered [14C]TCP for 1 week by gavage or in drinking water at the low dose (6 mg/kg) used in the NTP carcinogenesis bioassay. Two target organs (forestomach and liver) and two nontarget organs (glandular stomach and kidney) were examined for DNA adduct formation. Adducts were hydrolyzed from DNA, isolated by HPLC, and quantitated by measuring HPLC fractions for radioactivity. In the forestomach, liver, and kidney, gavage administration of TCP resulted in 1.4-to 2.4-fold greater yields of the major DNA adduct, previously identified as S-[1-(hydroxymethyl)-2-(N7-guanyl)ethyl]glutathione. Significant differences in cell proliferation, as determined by incorporation of bromodeoxyuridine into DNA, were also observed for the two routes. Gavage administration of TCP for 2 weeks resulted in up to a threefold greater cell proliferation rate relative to administration in drinking water. Our findings of exposure-related differences in TCP-induced DNA adduct formation and cell proliferation suggest that a risk assessment based on the existing gavage study may overestimate human risk.

Administration, Oral↗
PubMed1996

DNA adducts: biological markers of exposure and potential applications to risk assessment.

DNA adducts have been investigated extensively during the past decade. This research has been advanced, in part, by the development of ultrasensitive analytical methods, such as 32P-postlabeling and mass spectrometry, that enable detection of DNA adducts at concentrations as low as one adduct per 10(9) to 10(10) normal nucleotides. Studies of mutations in activated oncogenes such as ras, inactivated tumor suppressor genes such as p53, and surrogate genes such as hprt provide linkage between DNA adducts and carcinogenesis. The measurement of DNA adducts, or molecular dosimetry, has important applications for cancer risk assessment. Cancer risk assessment currently involves estimating the probable effects of carcinogens in humans based on results of animal bioassays. Estimates of risk are then derived from mathematical models that fit data of tumor incidence at the high animal exposures and extrapolate to probable human exposures that may be orders of magnitude lower. Molecular dosimetry could extend the observable range of mechanistic data several orders of magnitude lower than can be achieved in carcinogenesis bioassays. This measurement also compensates automatically for individual and species differences in toxicokinetic factors, as well as any nonlinearities that affect the quantitative relationships between exposure and molecular dose. As a result, molecular dosimetry can provide a basis for conducting high- to low-dose, route-to-route, and interspecies extrapolations. The incorporation of such data into risk assessment promises to reduce uncertainties and produce more accurate estimates of risk compared to current methods.

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

Bioassay design and MTD setting: old methods and new approaches.

The proper design of carcinogenicity bioassays is critical to the interpretation and use of the data that emerge upon its completion. This paper explores some of the scientific underpinnings that need to be considered when designing bioassays. It also discusses new approaches that can be used to improve dose setting and interpretation of bioassay results. Critical information that determines the shape of the dose-response curve include the molecular dose of the agent that binds to DNA, cell proliferation, and cell death. Such data can be used to replace default assumptions and improve the extrapolation of risk from animal studies to humans.

Animals↗
PubMed1995

Dose-response relationships for carcinogens.

Biotransformation of chemical carcinogens involves both metabolic activation and detoxication. The molecular dose present on DNA as adducts represents a balance between these two pathways (formation) and DNA repair. All of these are enzymatic processes subject to saturation. When none of the pathways is saturated, linear molecular dosimetry is expected, whereas if metabolic activation is saturated, a supralinear response occurs. If detoxication or DNA repair is saturated, a sublinear response occurs. With chronic exposure, steady-state concentrations of DNA adducts develop and these follow the same patterns. With several alkylating agents, multiple adducts are formed. The extent of formation is chemically defined, but different DNA repair pathways can be involved for different adducts. By understanding the molecular dose and biology of each adduct and comparing these to the dose-response for tumor induction, it may be possible to identify the most appropriate biomarkers for risk assessment. Recently, endogenous DNA adducts identical to those induced by known human carcinogens have been identified. These endogenously formed adducts may play an important role in human carcinogenesis.

Animals↗
PubMed1995

DNA adduct formation in B6C3F1 mice and Fischer-344 rats exposed to 1,2,3-trichloropropane.

1,2,3-Trichloropropane (TCP) is a multispecies, multisite carcinogen which has been found to be an environmental contaminant. In this study, we have characterized and measured DNA adducts formed in vivo following exposure to TCP. [14C]TCP was administered to male B6C3F1 mice and Fischer-344 rats by gavage at doses used in the NTP carcinogenesis bioassay. Both target and nontarget organs were examined for the formation of DNA adducts. Adducts were hydrolyzed from DNA by neutral thermal or mild acid hydrolysis, isolated by HPLC, and detected and quantitated by measurement of radioactivity. The HPLC elution profile of radioactivity suggested that one major DNA adduct was formed. To characterize this adduct, larger yields were induced in rats by intraperitoneal administration of TCP (300 mg/kg). The DNA adduct was isolated by HPLC based on coelution with the radiolabeled adduct, and compared to previously identified adducts. The isolated adduct coeluted with S-[1-(hydroxymethyl)-2-(N7-guanyl)-ethyl]glutathione, an adduct derived from the structurally related carcinogen 1,2-dibromo-3-chloropropane (DBCP). Analysis by electrospray mass spectrometry suggested that the TCP-induced adduct and the DBCP-derived adduct were identical. The 14C-labeled DNA adduct was distributed widely among the organs examined. Adduct levels varied depending on species, organ, and dose. In rat organs, adduct concentrations for the low dose ranged from 0.8 to 6.6 mumol per mol guanine and from 7.1 to 47.6 mumol per mol guanine for the high dose. In the mouse, adduct yields ranged from 0.32 to 28.1 mumol per mol guanine for the low dose and from 12.2 to 208.1 mumol per mol guanine for the high dose. The relationship between DNA adduct formation and organ-specific tumorigenesis was unclear. Although relatively high concentrations of DNA adducts were detected in target organs, several nontarget sites also contained high adduct levels. Our data suggest that factors in addition to adduct formation may be important in TCP-induced carcinogenesis.

Animals↗
PubMed1994

Retrospective assessment of liver cell proliferation via PCNA: a comparison with tritiated thymidine.

Cell proliferation (S phase response) in archival liver tissues of partially hepatectomized rats was determined via proliferating cell nuclear antigen (PCNA) immunohistochemistry. These results were compared with the S phase response assessed previously in the same tissues via tritiated thymidine (Tdr) autoradiography. The effect of prolonged tissue fixation on PCNA immunohistochemistry was compared in two studies: study A, the liver was fixed for a maximum of 7 days and then embedded in paraffin and stored for approximately 18 months, while in study B, the liver was fixed in formalin for 7 years and then embedded in paraffin and stored for approximately 18 months until sectioning and immunostaining. PCNA immunostaining was successful in the liver sections of both studies, irrespective of the length of formalin fixation. Furthermore, the S phase labeling indices (LI) determined via PCNA and Tdr were comparable, although not identical, in the two studies. Therefore, use of PCNA immunohistochemistry should allow retrospective staining of rodent tissues for the assessment of cell proliferative activity in formalin-fixed organs from previously conducted toxicity and carcinogenicity studies.

Animals↗
PubMed1994

Monitoring exposure to acrylonitrile using adducts with N-terminal valine in hemoglobin.

Human exposure to acrylonitrile (ACN), a carcinogen in rats, may occur in industrial settings, through waste water and tobacco smoke. ACN is an electrophilic compound and binds covalently to nucleophilic sites in macromolecules. Measurements of adducts with hemoglobin could be utilized for improved exposure assessments. In this study, a method for quantification of N-(2-cyanoethyl)valine (CEVal), the product of reaction of ACN with N-terminal valine in hemoglobin has been developed. The method is based on the N-alkyl Edman procedure, which involves derivatization of the globin with pentafluorophenyl isothiocyanate and gas chromatographic-mass spectrometric analysis of the resulting thiohydantoin. An internal standard was prepared by reacting valylglycylglycine with [2H3]ACN, spiked with [14C]ACN to a known sp. act. Levels of CEVal were measured in globin from rats exposed to 3-300 p.p.m. ACN in drinking water for 105 days and from humans (four smokers and four non-smokers). CEVal was detected at all exposure levels in the drinking water study. The relationship between adduct level and water concentration was linear at concentrations of 10 p.p.m. (corresponding to an average daily uptake of c. 0.74 mg ACN/kg body wt during the 65 days prior to sacrifice) and below, with a slope of 37.7 pmol CEVal/g globin/p.p.m. At higher concentrations, adduct levels increased sublinearly, indicating saturation of a metabolic process for elimination of ACN. Comparison of adduct formation with the estimated dose (mg/kg/day) of ACN indicated that at low dose (0-10 p.p.m.) CEVal = 0.508 x ACN dose + 0.048 and at high dose (35-300 p.p.m.) CEVal = 1.142 x ACN dose - 1.098. Globin from the smokers (10-20 cigarettes/day) contained about 90 pmol CEVal/g, whereas the adduct levels in globin from non-smokers were below the detection limit. The analytical sensitivity should be sufficient to allow monitoring of occupationally exposed workers at levels well below the current Occupational Safety and Health Administration standard of 2 p.p.m.

Acrylonitrile↗
PubMed1994

Biochemical events during initiation of rat hepatocarcinogenesis.

Carcinogenesis is a multistep, multistage process that begins with irreversible, but heritable damage to a single cell. The partial hepatectomy/diethylnitrosamine (DEN) model of rat hepatocarcinogenesis has been well characterized and many aspects of the stage of initiation are known. Recently, it has been suggested that hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) may be one population of initiated cells. Male Fischer rats were subjected to a 70% partial hepatectomy and at the peak of cell proliferation 24 h later were administered either the solvent trioctanoin, or 10 mg DEN/kg. The rats were administered 100 mg bromodeoxyuridine (BrdU)/kg 1 h prior to death at various times after DEN administration. Since initiation of the carcinogenesis process requires the division of cells containing DNA damage to induce mutations, we examined the concentration of alkylated adducts and the labeling index at various times after DEN administration. In addition, the time course of hepatic PGST expression was determined concurrent with the adduct concentration and labeling index. During the first day after DEN or solvent administration to a rat subjected to a 70% partial hepatectomy, a diurnal variation in labeling index was observed. A recovery to postsurgical labeling index levels was demonstrated for both the solvent- and DEN-treated groups by 7 days. The concentration of three promutagenic lesions was maximal at 6 h after DEN administration. The detectable level of the O6EG adduct was negligible by 24 h after DEN administration, while the two O-alkylpyrimidines, O2ET and O4ET, were retained for much longer periods. Single hepatocytes expressing PGST were observed by 2 days after DEN administration, while small foci of PGST-expressing hepatocytes could be reliably detected by 2 weeks. Two phases of PGST expression in single hepatocytes were observed. The first phase was maximal at day 3 and complete by day 6, while the second reached a plateau by day 8 and was maintained for the 28 days of the study. The presence of the three O-alkylation adducts during a time of enhanced cellular proliferation suggests that all three promutagenic adducts may contribute to the initiation that results in the partial hepatectomy/DEN model of rat hepatocarcinogenesis.

Animals↗
PubMed1994

A comparison of two ultrasensitive methods for measuring 1,N6-etheno-2'-deoxyadenosine and 3,N4-etheno-2'-deoxycytidine in cellular DNA.

1,N6-Ethenodeoxyadenosine (edA) and 3,N4-ethenodeoxycytidine (edC) are two mutagenic adducts associated with exposure to ethyl carbamate (urethane) and vinyl chloride. We have recently developed two ultrasensitive methods for determining the molecular dose of these adducts in cellular DNA. In both methods, purified DNA was first enzymatically digested to 2c-deoxyribonucleotide 3c-monophosphates. Etheno-modified nucleotides were then separated from normal nucleotides in one of two ways: either by reverse phase, ion-pair HPLC coupled with 260 nm UV detection, or by immunoaffinity chromatography using reusable microcolumns containing specific monoclonal antibodies coupled to Protein A-Sepharose. Fractions enriched for the adducted nucleotides were labeled using T4 poly-nucleotide kinase and [32P]ATP, and individual nucleotides were subsequently resolved by two-dimensional TLC, visualized by autoradiography, and quantified by liquid scintillation counting. When used to analyze the same sample of etheno-modified calf thymus DNA, both assays produced similar results. However, when both methods were used to analyze rat liver DNA 'spiked' with known amounts of etheno nucleotide standards, the immuno-affinity/32P TLC procedure proved to be more sensitive and more reproducible than the HPLC/32P TLC method: while the detection limit of the immunoaffinity/32P TLC technique was < 4 etheno adducts/10(9) parent deoxynucleotides, the HPLC/32P TLC method often failed to detect adducts at concentrations < 2/10(8). In other experiments, the immunoaffinity/32P TLC method was used to demonstrate formation of edA and edC in cells treated with vinyl chloride monomer. Because of its exquisite sensitivity, the immunoaffinity/32P TLC method promises to be extremely useful for measuring both background and induced levels of etheno adducts, making it possible to examine the role of these adducts in inducing mutations and/or carcinogenesis.

Cell Line↗
PubMed1994

Review of styrene and styrene oxide long-term animal studies.

Eleven long-term toxicity studies were reviewed on styrene and five on styrene oxide in an effort to evaluate the potential carcinogenic activity of these chemicals in animals. The styrene studies included inhalation exposure (rats, mice, guinea pigs, and rabbits), intragastric gavage (rats and mice), drinking water (rats), and intraperitoneal injection (rats), while styrene oxide exposure was via intragastric gavage (rats and mice) or skin painting (mice). Each study was reviewed and evaluated for details and adequacy of design, adequacy of reported data, and interpretation. The results of this review are 1. There was no convincing evidence of carcinogenic activity of styrene in animals, although many of the studies were considered inadequate. 2. Styrene oxide was carcinogenic to the forestomach of both sexes of rats and mice after gavage exposure and was associated with an increase in liver neoplasms in male mice in one study. No carcinogenic activity was observed in mice after dermal exposure (skin paint). 3. None of the studies of styrene or styrene oxide reported here are well suited for extrapolating potential carcinogenic activity of either compound to humans because all have deficiencies in design, conduct, interpretation, or utilized a less than ideal route of exposure. A chronic state-of-the-art inhalation study is needed to evaluate this aspect of hazard assessment.

Animals↗
PubMed1993

Repair of O6-methylguanine and O4-methylthymidine in F344 rat liver following treatment with 1,2-dimethylhydrazine and O6-benzylguanine.

Concentrations of O6-methylguanine, O4-methylthymidine, and N-7-methylguanine were measured in the livers of Fischer 344 rats following treatment with 1,2-dimethylhydrazine (20 mg/kg, s.c.) alone or in combination with the O6-alkylguanine transferase inhibitor O6-benzylguanine (100 mg/kg, i.p., daily). Animals were sacrificed at 12, 24, 36, or 48 h following 1,2-dimethylhydrazine exposure. Direct measurement of alkyltransferase demonstrated that daily treatment with O6-benzylguanine completely eliminated detectable alkyltransferase activity in the livers of treated rats. Adducts in liver DNA were quantitated by high performance liquid chromatography separation followed by fluorescence detection, UV absorbance, and/or specific radioimmunological assays. In animals exposed to 1,2-dimethylhydrazine alone O6-methylguanine concentrations declined rapidly, whereas animals exposed to both O6-benzylguanine and 1,2-dimethylhydrazine showed less removal of O6-methylguanine, with significant differences between the two populations appearing at 36 and 48 h. O4-Methylthymidine removal also differed significantly between the two groups, with O6-benzylguanine-treated animals exhibiting higher concentrations of adducts at 36 and 48 h. O6-Benzylguanine treatment had no effect on the removal of N-7-methylguanine. These results show that the rate of disappearance of both O6-methylguanine and O4-methylthymidine is slower following alkyltransferase depletion, suggesting that mammalian alkyltransferase is involved in the removal of O4-methylthymidine lesions as well as O6-methylguanine lesions.

1,2-Dimethylhydrazine↗
PubMed1993

Characterization of p53 mutations in methylene chloride-induced lung tumors from B6C3F1 mice.

Mutations of the p53 tumor suppressor gene are the most common defined genetic alterations seen in a wide variety of human cancers. In contrast, little is known about the importance of the p53 gene in chemically induced tumors of rodents, which are widely used as models for the evaluation of human health risks. In this study we examined 54 methylene chloride-induced and seven spontaneously arising lung tumors from female B6C3F1 mice for losses of heterozygosity (LOH) at markers near the p53 gene on chromosome 11. LOH was detected in seven methylene chloride-induced lung carcinomas by Southern analysis of a restriction fragment length polymorphism and PCR analysis of five simple sequence length polymorphisms. In each case allele loss was observed at all six markers; thus, these chromosomal alterations were likely to have resulted from mitotic nondisjunction. In contrast, LOH was not detected in 20 liver tumors from methylene chloride-treated mice at the Acrb locus, which is tightly linked to the p53 gene on chromosome 11. In addition single strand conformation polymorphism analysis was performed to screen for mutations in the most conserved regions of the p53 gene (exons 5 to 8). Consequently, potential mutations identified by direct sequencing, were only detected in four of the seven tumor samples with LOH, but not in any of the remaining lung tumors. Overexpression of the p53 protein by immunohistochemical staining was detected only in the four tumors that contained p53 point mutations and in a focal area of another tumor. Finally, using a simple sequence length polymorphism within the retinoblastoma tumor suppressor gene, LOH on mouse chromosome 14 was also detected in three lung carcinomas and one liver tumor. Inactivation of p53 and possibly the retinoblastoma tumor suppressor gene appear to be infrequent events in lung and liver tumors from methylene chloride treated mice.

Animals↗
PubMed1993

Alpha 2u-globulin nephropathy: review of the cellular and molecular mechanisms involved and their implications for human risk assessment.

This paper reviews what is known about the induction of alpha 2u-globulin nephropathy and carcinogenesis. This unique male-rat-specific disease is associated with exposure to an ever-increasing number of chemicals. The processes leading to nephropathy and renal cancer are among the best-understood mechanisms for nongenotoxic chemicals and strongly support that it is a male-rat-specific process that is not relevant for human risk assessment. Nevertheless, the data available for individual chemicals vary greatly. This necessitates a case-by-case analysis of the available data when determining the relevance for humans of this chemically induced renal disease in male rats.

Alpha-Globulins↗
PubMed1993

Molecular dosimetry of DNA and hemoglobin adducts in mice and rats exposed to ethylene oxide.

Experiments involving ethylene oxide (ETO) have been used to support the concept of using adducts in hemoglobin as a surrogate for DNA adducts in target tissues. The relationship between repeated exposures to ETO and the formation of N-(2-hydroxyethyl)valine (HEtVal) in hemoglobin and 7-(2-hydroxyethyl)guanine (7-HEG) in DNA was investigated in male rats and mice exposed by inhalation to 0, 3, 10, 33, or 100 ppm ETO for 6 hr/day for 4 weeks, or exposed to 100 ppm (mice) or 300 ppm (rats) for 1, 3, 5, 10, or 20 days (5 days/week). HEtVal was determined by Edman degradation, and 7-HEG was quantitated by HPLC separation and fluorescence detection. HEtVal formation was linear between 3 and 33 ppm ETO and increased in slope above 33 ppm. The dose-response curves for 7-HEG in rat tissues were linear between 10 and 100 ppm ETO and increased in slope above 100 ppm. In contrast, only exposures to 100 ppm ETO resulted in significant accumulation of 7-HEG in mice. Hemoglobin adducts were lost at a greater rate than predicted by normal erythrocyte life span. The loss of 7-HEG from DNA was both species and tissue dependent, with the adduct half-lives ranging from 2.9 to 5.8 days in rat tissues (brain, kidney, liver, lung, spleen, testis) and 1.0 to 2.3 days in all mouse tissues except kidney (t1/2 = 6.9 days). The concentrations of HEtVal were similar in concurrently exposed rats and mice, whereas DNA from rats had at least 2-fold greater concentrations of 7-HEG than DNA from mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Incorporating biologically based models into assessments of risk from chemical contaminants.

The general approach to assessment of risk from chemical contaminants in drinking water involves three steps: hazard identification, exposure assessment, and dose-response assessment. Traditionally, the risks to humans associated with different levels of a chemical have been derived from the toxic responses observed in animals. It is becoming increasingly clear, however, that further information is needed if risks to humans are to be assessed accurately. Biologically based models help clarify the dose-response relationship and reduce uncertainty.

Animals↗
PubMed1993

Styrene and styrene oxide: results of studies on carcinogenicity in experimental animals.

Fourteen long-term toxicity studies were reviewed in an effort to evaluate the potential carcinogenic activity of styrene and styrene oxide in animals. Each study was reviewed and evaluated for detail and adequacy of design, adequacy of reported data and interpretation. The results of the review are: 1. There is no convincing evidence for a carcinogenic action of styrene in animals, even though it has been studied in several species and by several routes of exposure: inhalation, gavage, in the drinking-water and by intraperitoneal and subcutaneous injection. Most of the studies of styrene, however, have deficiencies in design and/or conduct. 2. Styrene oxide was carcinogenic to the forestomach of rats and mice of each sex after exposure by gavage at all doses tested, including one as low as 50 mg/kg per day. An increase in the incidence of liver neoplasms was observed in male mice in one study. No carcinogenic activity was observed in mice exposed by skin painting. The relevance to humans of the studies in which exposure was by gavage is limited because: (i) the route is less than ideal for extrapolating to human risk from exposure by inhalation or dermally; (ii) xenobiotics often cause neoplasms at this site when given at high concentrations; and (iii) neoplasms at sites distant from the site of exposure were found in only one sex of one species. 3. None of the studies of styrene or styrene oxide reported here is well suited for extrapolating to potential carcinogenic activity in humans, because all have deficiencies in design, conduct and/or interpretation. An up-to-date chronic inhalation study would have to be conducted in order to evaluate this aspect of hazard assessment.

Animals↗