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Biomedical subjects

R W Hart

Publications and source records attributed to R W Hart.

At least 109 records · Page 6Linked to original sources

A possible mechanism of chrysotile asbestos toxicity.

In vitro evaluations of heat treated and/or irradiated chrysotile asbestos indicate the importance of electron transfer in determining the biological outcome of asbestos exposure. Regardless of the assay system used, all show the same trends. Namely, heat treatment reduces cytotoxicity while heat treatment in combination with irradiation increases activity over heat treatment alone. Furthermore, studies of BSA and DNA binding consistently indicate a lower molecular adsorption for heated asbestos. BSA and DNA adsorption are similar for untreated, irradiated and heated or irradiated samples. Physical and chemical analyses indicate that the size and elemental composition are unaffected by the treatment processes. X-ray diffraction indicates no change in the crystal structure of chrysotile asbestos. IR spectroscopy suggests that heat treatment affects the external hydroxyl group population while heat treatment and irradiation may repopulate this functional group. A proposed mechanism consistent with the biological and physical characterization is based upon electron transfer from the chrysotile asbestos matrix to biological receptors.

Adsorption↗

Effect of age and torpor on the circadian rhythms of body temperature, activity, and body weight in the mouse (Peromyscus leucopus).

Age-related alterations in homeostatic mechanisms, as measured by changes in circadian rhythms, have been reported in the literature. The purpose of the present study was to determine if changes in the circadian rhythms of motor activity, body temperature, and body weight occur with age in the long-lived mouse, Peromyscus leucopus, and to determine if the incidence of torpor (hypothermia) was age-dependent. Young and old mice were entrained to an LD 12:12 cycle, and then physiological and behavioral performance was monitored by a data-acquisition system. The major differences between the 6-month and 60-72-month age groups were the following. Significant episodes of torpor occurred in a high percentage of old mice at 0600 hr on consecutive days, but young mice showed no evidence of torpor when food was provided ab libitum. Torpor in young mice was found to occur only during starvation conditions. Old animals phase in and out of torpor over an interval of several days; high-amplitude rhythms and good synchronization of activity and temperature occurred when torpor was expressed, and low-amplitude rhythms, poor synchronization between temperature and activity, and a phase shift of body-weight acrophase occurred on days when torpor was not expressed. Significant changes in acrophase and amplitude occurred between individual animals and days in the old group; no significant change in acrophase and amplitude was observed in the young group. Population-mean cosinor analysis revealed a significant age-related decrease in amplitude and mesor, but no significant change in acrophase was found.

Aging↗

Final report of the Color Additive Scientific Review Panel.

The Color Additives Scientific Review Panel considered whether there was information sufficient to perform a carcinogenic risk assessment on the colors D&C Red No. 19 (R-19), D&C Red No. 37 (R-37), D&C Orange No. 17 (O-17), D&C Red No. 9 (R-9), D&C Red No. 8 (R-8) and FD&C Red No. 3 (R-3) and to evaluate the assessments sent to FDA as part of the petitions for use of the colors for drug and external uses by the Cosmetic, Toiletry and Fragrance Association (CTFA). There is a lack of human data concerning the colors for making a human health assessment, so the assessments are based upon the extrapolation of animal data. The risk assessments are determined for exposure to single chemicals. Excluded from consideration are possible effects from exposure to multiple chemicals, such as co-carcinogenesis, promotion, synergism, antagonism, etc. In the light of recent efforts in establishing a consensus in risk assessment, the Panel has determined that the CTFA assessments for R-10, O-17, and R-9 are consistent with present acceptable usages, although it questions some of the assumptions used in the assessments. The Panel identified a number of general assumptions made, and discusses their validity, their impact on total uncertainty, and the potential options to address the gaps in understanding that necessitate the assumption. The Panel also derived revised risk estimates using more "reasonable" assumptions than "worst-case" situations, for 90th percentile and average exposure. For those assumptions that are easily quantifiable, the Panel's estimates are less than an order of magnitude lower than the CTFA risk estimates, indicating that the underestimates and overestimates of the CTFA risk estimates tend to balance each other. The impact of most of the assumptions is not quantifiable. The assessment for R-3 is complicated by the fact that there is no good skin penetrance study for this color. It was assumed that the penetrance is similar to that of another water-soluble xanthene color, R-19. It is expected that the absorption of the color is not likely to exceed that of the smaller molecule, R-19. Therefore, the risk estimates are similar to the CTFA estimates, but with different reasoning. The estimates for R-8 and R-37 are different from the others in that there is a lack of any exposure or toxicological information on these colors.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

Longevity-dependent organ-specific accumulation of DNA damage in two closely related murine species.

To measure directly the accumulation of DNA damage with age, and to understand better the effect of modulators of DNA damage in vivo, the DNA of brain, liver, and kidney of two mice from different families, Mus musculus and Peromyscus leucopus, have been examined for age-dependent accumulation of single-strand breaks plus alkali-labile bonds, by the alkaline sucrose sedimentation method. These two species of small rodents are closely related taxonomically, yet differ significantly in maximum achievable lifespan. Using the reciprocal of the number average molecular weight for estimation of DNA size, these analyses indicate that: (a) DNA damage does not measurably accumulate in brain tissue; (b) the accumulation of DNA damage was more pronounced in hepatic DNA than other tissue DNA; and (c) the rate of accumulation of DNA damage in liver and kidney cells with age was greater in the shorter-lived species (M. musculus) and was inversely proportional to maximum achievable lifespan. There are suggestions that a similar threshold might exist for tolerance of DNA damage in the two species in specific organs, and that these species differ in the rate at which this threshold is reached as a function of maximum achievable lifespan.

Animals↗

Stage-specific induction of sister-chromatid exchanges in utero.

In order to correlate the induction of sister-chromatid exchanges (SCE) to biological endpoints, and elucidate aspects of this relationships, 7,12-dimethylbenz[a]anthracene (DMBA) and methyl methanesulfonate (MMS), chemicals with different biological actions at different stages in development, have been evaluated for their ability to induce SCE at different gestational ages in the Sprague-Dawley rat. Transplacental exposure to these agents was accomplished by a recently developed intraperitoneal infusion technique to replenish metabolically degraded 5-bromo-2'-deoxyuridine used for SCE visualization. Maternal bone marrow and whole fetal tissue, fetal liver and fetal brain were compared. Day-9 embryo was found to be very sensitive to the effects of both agents, with the ability to induce SCE declining in development in whole fetus and fetal organs. The embryotoxic effects of the agents seem to be ones best correlated with the capacity of the agents to induce SCE. Also, fetal liver is more sensitive than fetal brain to the effects of DMBA compared with MMS, suggesting fetal metabolic activation may have occurred. Measurement of the amount of radiolabelled DMBA reaching the fetal tissue used to estimate SCE indicates that the amount of chemical reaching the fetus does not account for the increased sensitivity, especially at Day 9. Some factor(s) in development, such as differentiation stage, rather than the fetal accessibility to chemical, seem to be important in the induction of SCE in utero.

9,10-Dimethyl-1,2-benzanthracene↗

Correlation between ultraviolet-induced DNA repair in primate lymphocytes and fibroblasts and species maximum achievable life span.

The maximum achievable life span of animals appears to be species specific and has been suggested to be related to DNA damage. The rate of accumulation of such damage may be altered by repair mechanisms. The excision repair of ultraviolet (UV)-induced DNA damage in fibroblasts has been shown to correlate well with maximum achievable life spans in a number of placental mammals. Evidence is presented here that the correlation between life span and UV-induced excision repair of DNA damage holds within a more closely related group, the primates, in not only primary fibroblasts cell cultures, but also nonstimulated lymphocytes.

Animals↗

Induction of single-strand breaks plus alkali-labile bonds by N-nitrosoureas in rat tissues in vivo: ethylnitrosourea versus benzylnitrosourea.

Alkaline sucrose sedimentation procedures were used to quantitate the amount of single-strand breaks plus alkali-labile bonds (SSB + ALB) induced and repaired following a single intraperitoneal injection of the neurocarcinogen N-ethyl-N-nitrosourea (ENU) and its non-neurocarcinogenic analog N-benzyl-N-nitrosourea (BNU) in the brain, liver and kidney of female Sprague-Dawley rats. SSB + ALB were measured and used as an indicator of apurinic/apyrimidinic sites, phosphotriesters and in situ breaks. ENU induced a dose-dependent increase in the number of SSB + ALB at the doses studied (0, 0.39, 0.77, 1.54 mmoles/kg) in all 3 tissues. At 1 h postinjection with 0.77 mmoles/kg of these compounds there were 50-70% fewer breaks induced by BNU than ENU. The SSB + ALB induced by ENU persisted over a 7-day period, while those induced by BNU did not. Thus, these studies showed that 2 homologues of nitrosoureas, ENU and BNU, exhibited different potentials to induce and to persist SSB + ALB in vivo.

DNA↗

The measurement of sister chromatid exchanges induced in utero utilizing intraperitoneal infusion of BrdU: a novel technique.

One of the inherent difficulties associated with the accurate detection of sister chromatid exchanges (SCE's) in vivo is the maintenance of a continuous supply of 5-bromo-2'-deoxyuridine (BrdU) for differential staining of sister chromatids. We have developed a simple, reproducible intraperitoneal technique which has the advantages of continuous BrdU infusion and is less stressful than presently available methods. Additionally, a gentle trypsinization methodology was developed to obtain relatively large numbers of metaphases from fetal tissues. Using these techniques, we examined the induction of SCE by 7,12-dimethylbenz(a)anthracene (DMBA) when injected into pregnant CD rats on days 11, 13, 15, and 16 of gestation. The levels of SCE induced were slightly higher in fetal than maternal tissue, although not significantly.

9,10-Dimethyl-1,2-benzanthracene↗

Characterization of cytotoxic effect of asbestos on normal human fibroblasts.

Using viable cell counting and the incorporation of radioactive thymidine in cell TCA precipitate, we have attempted to separate the effects of asbestos on cell growth inhibition (reproductive failure) vs. cell killing. NIEHS crocidolite and NIEHS intermediate-sized chrysotile at a concentration of 10 micrograms/mL were studied. Cytotoxic effects were demonstrated with both types of asbestos. Thymidine incorporation expressed as cpm/viable cell was enhanced in the presence of either form of asbestos. It is concluded that both negatively charged crocidolite and positively charged chrysotile are toxic to human fibroblast cells in culture and exert their cell killing but not growth inhibition effect concurrently with a stimulation of DNA synthesis.

Asbestos↗

Effects of chrysotile co-exposure on BaP binding in normal human fibroblasts.

The effects of chrysotile and benzo(a)pyrene (BaP) co-exposure on uptake, metabolism and BaP binding to nucleic acids were investigated in normal human fibroblasts. A significant enhancement in these parameters was found when co-exposure was carried out under serum-free conditions. In the presence of serum, only a slight enhancement of BaP uptake was detected, while metabolism and binding to nucleic acids were not affected over the 8-hr incubation period studied.

Asbestos↗

Thermal modification of chrysotile asbestos: evidence for decreased cytotoxicity.

Many asbestiform minerals exhibit temperature-dependent thermoluminescence. Since thermoluminescence involves electronic transitions within crystalline materials, the effect of temperature on asbestos cytotoxicity was evaluated. Heat pretreatment of Canadian chrysotile asbestos reduces its cytotoxicity towards cultured human fibroblasts and bovine alveolar macrophages. When monitored 44 hr after the addition of either 200 degrees C or 400 degrees C heat-pretreated asbestos, alveolar macrophage viability was approximately 40% higher than comparable amounts of unheated asbestos. Similarly, asbestos toxicity, expressed as fibroblast growth inhibition, was inversely related to the asbestos pretreatment temperature in the following manner, 70 degrees C greater than 200 degrees C greater than 400 degrees C = unexposed fibroblast controls. Pretreatment of chrysotile asbestos to 400 degrees C reduced its adsorptive capacity for bovine serum albumin by 25%. Furthermore, asbestos heated to 200 degrees C followed by irradiation with 4 MeV X-rays (4500 rads) resulted in reactivation of asbestos cytotoxicity. Scanning electron microscopy indicated that the ratios of free to fiber-associated alveolar macrophages and the fiber fragment size distributions were unaffected by either heat pretreatment or X-ray irradiation. These observations strongly suggest that the surface charge characteristics and electronic state of asbestos fibers may be responsible for its biological activity.

Adsorption↗

A sequence specific endonuclease from Micrococcus radiodurans.

A new sequence specific endonuclease, Mra I has been purified from Micrococcus radiodurans. This enzyme cleaves bacteriophage lambda DNA at three sites, adenovirus type 2 DNA at more than 12 sites and has a unique site on phi X174 DNA. It has no sites on SV40, PM2 and pBR322 DNA. The three sites on phage lambda DNA are different from those cleaved by Sma I, Xma I and Xor II. The sites of cleavage are located at 0.424, 0.447 and 0.834 fractional lengths on the physical map of lambda DNA. Mra I is shown to be an isoschizomer of Sac II and Sst II recognizing the palindromic nucleotide sequence '5-CCGC reduced GG-3'. The enzyme shows an absolute requirement of Mg2+, but is active in the absence of added 2-mercaptoethanol. The enzyme shows activity at a broad range of temperature and pH with an optimum at 45 degrees C and pH 7.0. Mra I represents the first restriction enzyme from a bacterium whose DNA lacks modified methylated bases.

Base Sequence↗

Effects of cell density on the neutral glycolipid composition of cultured human brain and glioma cells.

Density dependent chain elongation of neutral glycosphingolipids (NGSL) is associated with contact inhibition of mitosis in several normal cultured cell lines. Transformed non-neural cell lines which have impaired contact inhibition frequently lose this biochemical response. To determine if either of these phenomena occur in human neural cells we determined NGSL compositions of cultured glioblastoma multiforme and normal fetal brain cells. Fetal cells generally had more total NGSL than the tumor cells. As a percentage of total NGSL, both cell lines at higher cell densities had larger proportions of ceramide trihexoside and globoside, but smaller proportions of cerebroside. This decrease was mainly in non-hydroxy fatty acid cerebroside of glioma cells, but in hydroxy fatty acid cerebroside of normal fetal brain cells. These results demonstrate that although glioblastoma multiforme cells have markedly impaired growth control, they still preserve density dependent chain elongation of NGSL. A role for this phenomenon in normal cellular growth control has yet to be established.

Aged↗

DNA repair assays as tests for environmental mutagens. A report of the U.S. EPA Gene-Tox Program.

A literature review was undertaken to determine the usefulness of DNA repair assays, other than unscheduled DNA synthesis, as screening techniques for mutagenic carcinogens. 92 reports were found to contain useful data for 49 chemicals using 6 techniques, namely, (1) cesium chloride equilibrium density gradients to study repair replication, (2) benzoylated naphthoylated diethylaminoethyl cellulose columns to study repair replication, (3) 313-nm irradiation of DNA containing bromodeoxyuridine to study repair replication, (4) alkaline elution to study repair of single-strand breaks and crosslinks, (5) alkaline sucrose gradients to study repair of single-strand breaks, and (6) direct assays for removal of adducts from DNA. Almost all of the 49 chemicals studied were known mutagens or carcinogens and/or known inducers of DNA repair, 9 compounds failed to elicit DNA repair by at least 1 assay technique, and at least 3 of these were not tested by the most appropriate and sensitive method. Nevertheless, although valid for studying repair phenomena in eukaryotic cells, these assays are not considered useful for screening. They are time-consuming, expensive, and/or require highly specialized skills. Despite the high frequency of positive reports, it is obvious from the literature that repair assays will fail to detect, or will detect with low efficiency, those agents whose main action is either intercalation or induction of strand breaks. For these and other reasons, DNA repair as a basis for screening for mutagenic carcinogens is not considered to be a useful concept.

Bromodeoxyuridine↗

Pisum sativum endonuclease. Studies on substrate specificity and possible use as a biochemical tool.

An endonuclease purified from germinating pea (Pisum sativum) seeds has been shown to catalyze the hydrolysis of heat-denatured single-stranded DNA. Since P. sativum endonuclease shows appreciable activity in the presence of DNA destabilizing agents and, unlike many similar endonucleases, significant activity at neutral pH, it is a potentially valuable tool for studies of the secondary structure of nucleic acids. The residual hydrolysis of duplex DNA is directed towards partially denatured, A,T-rich areas in native DNA. The rate of hydrolysis of deoxypolynucleotides was in the order poly(dT) greater than denatured DNA greater than poly(dA) greater than poly(dA-dT) = native DNA. Neither poly(dC), poly(dG) nor poly(dC).poly(dG) were attacked by the enzyme. Supercoiled, covalently closed circular phage PM2 form I DNA is converted to singly hit nicked circular form II and doubly hit linear from III duplexes. Prolonged treatment with enzyme does not further cleave the linear form III DNA. Addition of increasing concentrations of NaCl in the incubation mixture suppresses the conversion of form I to form II, but not the conversion of form II to form III, which is enhanced with the increasing ionic strength. The enzymatically relaxed circular form, I degree, obtained by unwinding of supercoiled DNA with a DNA-relaxing protein, is resistant to the action of the enzyme. Molecules with intermediate superhelix densities do not serve as substrates. The sites of cleavage of P. sativum endonuclease in PM2 DNA occur within regions that are readily denaturable in a topologically constrained superhelical molecule.

Animals↗

Evolution and longevity-assurance processes.

The role of the development of information fidelity systems in evolution is explored, with evidence that the loss in the ability to maintain the integrity of homeostasis in organisms over time, aging, is correlated with the loss in the ability to maintain integrity at the molecular biological level. Evolutionary-comparative analysis places an upper limit on the number of these systems important to the evolution of longevity in the primates, and suggests the importance of insuring the stability of information bearing macromolecules in evolution and the role of modulators of damage to these moieties in the expression of the senescent state.

Aging↗