Lipid peroxidation in microsomes induced by crocidolite fibres

Particle toxicology and health - where are we?

BACKGROUND

Particles and fibres affect human health as a function of their properties such as chemical composition, size and shape but also depending on complex interactions in an organism that occur at various levels between particle uptake and target organ responses. While particulate pollution is one of the leading contributors to the global burden of disease, particles are also increasingly used for medical purposes. Over the past decades we have gained considerable experience in how particle properties and particle-bio interactions are linked to human health. This insight is useful for improved risk management in the case of unwanted health effects but also for developing novel medical therapies. The concepts that help us better understand particles' and fibres' risks include the fate of particles in the body; exposure, dosimetry and dose-metrics and the 5 Bs: bioavailability, biopersistence, bioprocessing, biomodification and bioclearance of (nano)particles. This includes the role of the biomolecule corona, immunity and systemic responses, non-specific effects in the lungs and other body parts, particle effects and the developing body, and the link from the natural environment to human health. The importance of these different concepts for the human health risk depends not only on the properties of the particles and fibres, but is also strongly influenced by production, use and disposal scenarios.

CONCLUSIONS

Lessons learned from the past can prove helpful for the future of the field, notably for understanding novel particles and fibres and for defining appropriate risk management and governance approaches.

Multiple aspects of the interaction of biomacromolecules with inorganic surfaces

The understanding of the mechanisms involved in the interaction of biological systems with inorganic materials is of interest in both fundamental and applied disciplines. The adsorption of proteins modulates the formation of biofilms onto surfaces, a process important in infections associated to medical implants, in dental caries, in environmental technologies. The interaction with biomacromolecules is crucial to determine the beneficial/adverse response of cells to foreign inorganic materials as implants, engineered or accidentally produced inorganic nanoparticles. A detailed knowledge of the surface/biological fluids interface processes is needed for the design of new biocompatible materials. Researchers involved in the different disciplines face up with similar difficulties in describing and predicting phenomena occurring at the interface between solid phases and biological fluids. This review represents an attempt to integrate the knowledge from different research areas by focussing on the search for determinants driving the interaction of inorganic surfaces with biological matter.

Nonpulmonary outcomes of asbestos exposure

The adverse pulmonary effects of asbestos are well accepted in scientific circles. However, the extrapulmonary consequences of asbestos exposure are not as clearly defined. In this review the potential for asbestos to produce diseases of the peritoneum, immune, gastrointestinal (GIT), and reproductive systems are explored as evidenced in published, peer-reviewed literature. Several hundred epidemiological, in vivo, and in vitro publications analyzing the extrapulmonary effects of asbestos were used as sources to arrive at the conclusions and to establish areas needing further study. In order to be considered, each study had to monitor extrapulmonary outcomes following exposure to asbestos. The literature supports a strong association between asbestos exposure and peritoneal neoplasms. Correlations between asbestos exposure and immune-related disease are less conclusive; nevertheless, it was concluded from the combined autoimmune studies that there is a possibility for a higher-than-expected risk of systemic autoimmune disease among asbestos-exposed populations. In general, the GIT effects of asbestos exposure appear to be minimal, with the most likely outcome being development of stomach cancer. However, IARC recently concluded the evidence to support asbestos-induced stomach cancer to be "limited." The strongest evidence for reproductive disease due to asbestos is in regard to ovarian cancer. Unfortunately, effects on fertility and the developing fetus are under-studied. The possibility of other asbestos-induced health effects does exist. These include brain-related tumors, blood disorders due to the mutagenic and hemolytic properties of asbestos, and peritoneal fibrosis. It is clear from the literature that the adverse properties of asbestos are not confined to the pulmonary system.

ESR investigation of the oxidative damage in lungs caused by asbestos and air pollution particles

Exposure to asbestos and air pollution particles can be associated with increased human morbidity and mortality. However, the molecular mechanism of lung injuries remains unknown. It has been postulated that the in vivo toxicity results from the catalysis of free radical generation. Using electron spin resonance (ESR) in conjunction with the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) we previously investigated in vivo free radical production by rats treated with intratracheal instillation of asbestos (crocidolite fibers) and an emission source air pollution particle (oil fly ash). In this report we compare the effect of two different exposures on the type of free radicals they induce in in vivo animal model. Twenty-four hours after the exposure, ESR spectroscopy of the chloroform extract from lungs of animals exposed to either asbestos or oil fly ash gave a spectrum consistent with a carbon-centered radical adduct (aN = 15.01 G and aH = 2.46 G). To test whether free radical formation occurred in vivo and not in vitro, a number of control experiments were performed. Combinations (both individually and together) of asbestos or oil fly ash and 4-POBN were added to lung homogenate of unexposed rats prior to chloroform extraction. No detectable ESR signal resulted. To exclude the possibility of ex vivo free radical generation, asbestos or oil fly ash was added to lung homogenate of an animal treated with 4-POBN. Also, 4-POBN was added to lung homogenate from rats instilled with asbestos or oil fly ash. Neither system produced radical adducts, indicating that the ESR signal detected in the lung extracts of the treated animals must be produced in vivo and not ex vivo or in vitro. In conclusion, ESR analysis of lung tissue demonstrated that both exposures produce lipid-derived radical metabolites despite their different composition and structure. Analogously, both exposures provide evidence of in vivo enhanced lipid peroxidation. Furthermore, it is concluded that without the presence of a spin-trapping agent, no free radical metabolites could be detected directly by ESR in either exposure.

In vivo evidence of free radical formation after asbestos instillation: an ESR spin trapping investigation

It has been postulated that the in vivo toxicity of asbestos results from its catalysis of free radical generation. We examined in vivo radical production using electron spin resonance (ESR) coupled with the spin trap alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN); 180 day-old rats were intratracheally instilled with either 500 microg crocidolite or saline. Twenty-four hours later, histologic examination revealed a neutrophilic inflammatory response. ESR spectroscopy of the chloroform extract from lungs exposed to asbestos gave a spectrum consistent with a carbon-centered radical adduct, while those spectra from lungs instilled with saline revealed a much weaker signal. This same radical formation persisted and, even one month after instillation, could be detected in the lungs of rats exposed to asbestos. The 4-POBN adducts detected by ESR are very similar to, if not identical with, ethyl and pentyl radical adducts, providing evidence of in vivo lipid peroxidation resulting from asbestos exposure. We conclude that, after instillation of crocidolite in the rat, ESR analysis of lung tissue demonstrates in vivo free radical production.

Role of iron in the reactivity of mineral fibers

Any foreign body containing iron may be (or become) highly toxic in vivo. If its solubility in water is poor, surface chemistry governs the reactivity at the solid-liquid interface. Iron toxicity thus increases with the extent of exposed surface. Iron of endogenous origin may also be deposited on the particle surface and be activated under particular circumstances. The chemical processes that implicate surface iron as a primary cause of toxicity are: free radical release, mobilization by chelators, iron-catalyzed reactions. Three kinds of solids are compared: (i) well-known toxic materials, for example asbestos; (ii) non-toxic iron oxides; and (iii) model solids with surface exposed iron prepared for investigations on the reactivity of iron in biological media. The iron content of the solid is not directly related to the biological response: only a small fraction of ions, in a well-defined coordination and redox state, appears involved in the toxicity of the mineral dust.

The role of free radicals in asbestos-induced diseases

Asbestos exposure causes pulmonary fibrosis and malignant neoplasms by mechanisms that remain uncertain. In this review, we explore the evidence supporting the hypothesis that free radicals and other reactive oxygen species (ROS) are an important mechanism by which asbestos mediates tissue damage. There appears to be at least two principal mechanisms by which asbestos can induce ROS production; one operates in cell-free systems and the other involves mediation by phagocytic cells. Asbestos and other synthetic mineral fibers can generate free radicals in cell-free systems containing atmospheric oxygen. In particular, the hydroxyl radical often appears to be involved, and the iron content of the fibers has an important role in the generation of this reactive radical. However, asbestos also appears to catalyze electron transfer reactions that do not require iron. Iron chelators either inhibit or augment asbestos-catalyzed generation of the hydroxyl radical and/or pathological changes, depending on the chelator and the nature of the asbestos sample used. The second principal mechanism for asbestos-induced ROS generation involves the activation of phagocytic cells. A variety of mineral fibers have been shown to augment the release of reactive oxygen intermediates from phagocytic cells such as neutrophils and alveolar macrophages. The molecular mechanisms involved are unclear but may involve incomplete phagocytosis with subsequent oxidant release, stimulation of the phospholipase C pathway, and/or IgG-fragment receptor activation. Reactive oxygen species are important mediators of asbestos-induced toxicity to a number of pulmonary cells including alveolar macrophages, epithelial cells, mesothelial cells, and endothelial cells. Reactive oxygen species may contribute to the well-known synergistic effects of asbestos and cigarette smoke on the lung, and the reasons for this synergy are discussed. We conclude that there is strong evidence supporting the premise that reactive oxygen species and/or free radicals contribute to asbestos-induced and cigarette smoke/asbestos-induced lung injury and that strategies aimed at reducing the oxidant stress on pulmonary cells may attenuate the deleterious effects of asbestos.

Blood superoxide dismutase and plasma malondialdehyde among workers exposed to asbestos

Blood superoxide dismutase (SOD) and plasma malondialdehyde (MDA) (an indicator of lipid peroxidation [LPO]) were determined in 97 randomly selected asbestos exposed workers (age range: 25-60 years, mean duration of exposures 19.8 +/- 8.3 years) and in 42 healthy male controls. MDA, SOD, and MDA/SOD ratio in asbestos exposed workers were significantly higher than in controls. Among both the controls and exposed workers neither age nor smoking was related to SOD or MDA levels. SOD was significantly positively correlated with MDA among the exposed workers. Such correlation was not observed among the controls. SOD but not MDA was significantly positively correlated with the duration of exposure to asbestos. Mean levels of SOD or MDA in exposed workers with radiographic signs of lung fibrosis or pleural thickening did not differ significantly from those without such signs. The results confirm the possible involvement of LPO and development of anti-oxidant mechanism(s) of prolonged exposure to asbestos in humans. However, SOD seems not to be the essential anti-asbestos-induced LPO. Relation between these factors and lung fibrosis is still unclear.

Effects of glutathione depletion, chelation and diuresis on iron nitrilotriacetate-induced lipid peroxidation in rats and mice

1. Rats and mice dosed with iron nitrilotriacetate (FeNTA) i.p. (2-12 mg Fe/kg) showed evidence of lipid peroxidation as indicated by increased exhalation of ethane and increased malondialdehyde formation in liver and kidney. 2. Buthionine sulphoximine (BSO) administered i.p. to rats and mice decreased the total glutathione (GSH) content of liver and kidney. When the rodents were pretreated i.p. with BSO prior to injection of FeNTA the increases in ethane exhalation, and in liver and kidney malondialdehyde production, were greater than with FeNTA alone, and the total GSH of liver and kidney were decreased. 3. Diuresis produced by i.p. administration of furosemide to mice substantially decreased the ethane exhalation resulting from FeNTA administration, had a lowering effect on kidney MDA, but had no significant effect on liver MDA production. 4. Similarly, desferrioxamine beta-mesylate administered i.p. to mice markedly decreased the ethane exhalation and kidney MDA production resulting from FeNTA administration.

Iron mobilization from asbestos by chelators and ascorbic acid

The ability of chelators and ascorbic acid to mobilize iron from crocidolite, amosite, medium- and short-fiber chrysotile, and tremolite was investigated. Ferrozine, a strong Fe(II) chelator, mobilized Fe(II) from crocidolite (6.6 nmol/mg asbestos/h) and amosite (0.4 nmol/mg/h) in 50 mM NaCl, pH 7.5. Inclusion of ascorbate increased these rates to 11.4 and 4.9 nmol/mg/h, respectively. Ferrozine mobilized Fe(II) from medium-fiber chrysotile (0.6 nmol/mg/h) only in the presence of ascorbate. Citrate and ADP mobilized iron (ferrous and/or ferric) from crocidolite at rates of 4.2 and 0.3 nmol/mg/h, respectively, which increased to 4.8 and 1.0 nmol/mg/h in the presence of ascorbate. Since ascorbate alone mobilized iron from crocidolite (0.5 nmol/mg/h), the increase appeared to result from additional chelation by ascorbate. Citrate also mobilized iron from amosite (1.4 nmol/mg/h) and medium-fiber chrysotile (1.6 nmol/mg/h). Mobilization of iron from asbestos appeared to be a function not only of the chelator, but also of the surface area, crystalline structure, and iron content of the asbestos. These results suggest that iron can be mobilized from asbestos in the cell by low-molecular-weight chelators. If this occurs, it may have deleterious effects since this could result in deregulation of normal iron metabolism by proteins within the cell resulting in iron-catalyzed oxidation of biomolecules.

Induction of mesothelioma by intraperitoneal injections of ferric saccharate in male Wistar rats

Iron appears to play a major role in catalysing free radical production, leading to lipid peroxidation and DNA damage. We, therefore, investigated the effect of colloidal iron deposited in the peritoneum. Wistar male rats were given either ferric saccharate, ferric saccharate and nitrilotriacetic acid (NTA), NTA or saline. NTA was shown previously to 'free' iron to promote lipid peroxidation and an iron chelate of NTA is known to be carcinogenic to the kidney. Iron at a dose of 5 mg kg-1 day-1, and saline at a dose of 0.5 ml day-1 were injected i.p. for 3 months. NTA at a dose of 83.5 mg kg-1 day-1 was give i.p. for 5 months. All the rats were killed about a year later for histological examination. In nine of the 19 rats treated with ferric saccharate, mesothelial tumors were induced in the serosa of the tunica vaginalis or the length of the spermatic cord. Among rats treated with ferric saccharate and NTA, seven had localised mesotheliomas in the above locations and six had wide-spread peritoneal mesotheliomas. No mesothelial tumors developed in either NTA treated or saline treated rats. No pleural mesotheliomas were found in any group. These findings add to the evidence that iron is involved in some carcinogenic processes.

Mineral fiber-induced malondialdehyde formation and effects of oxidant scavengers in phagocytic cells

Malondialdehyde (MDA) is a product of free-radical reaction with lipids and has been implicated in a variety of pathological processes including inflammation and carcinogenesis. In order to document the toxic reactions related to the pathogenic mechanisms of mineral fibers, asbestos and other mineral dusts were examined for their potency to produce lipid peroxidation using the thiobarbital method for MDA measurement. Human peripheral blood-derived neutrophils (PMN), guinea pig peritoneal macrophages, and guinea pig alveolar lavage cells produced MDA when treated with crocidolite asbestos. Of the various mineral dusts tested, only crocidolite showed a significant increase of MDA production. The amount of MDA produced by PMN treated with crocidolite increased with milling the fiber and with the incubation time. Both superoxide dismutase (SOD) and catalase were examined for their ability to inhibit MDA formation. At concentrations of up to 50 micrograms/10(6) cells, SOD did not inhibit the MDA formation in macrophages. However, catalase at the same concentration inhibited MDA formation in macrophages completely. A possible mechanism of MDA formation and its relationship with superoxide production are discussed.

The role of catalytic iron in asbestos induced lipid peroxidation and DNA-strand breakage in C3H10T1/2 cells

The involvement of catalytic iron in the vitro activities of crocidolite asbestos has been investigated. Exposure of C3H10T1/2 cells to either the UICC crocidolite standard reference sample or a non fibrous (milled) derivative resulted in an increase of thiobarbituric acid reactive substances. This catalytic activity was inhibited by pretreatment with the iron chelator desferrioxamine. The effect of this activity on cellular DNA was measured in an assay based on the production of DNA-strand breaks. Increased levels of DNA-strand breaks were detected in cultures treated with both the milled and UICC crocidolite. Inclusion of desferrioxamine with the asbestos inhibited DNA-strand breakage. It is concluded the catalytic iron present on the dust is capable of damaging both lipid and DNA and that this could be an important mechanism in asbestos pathogenicity.

The stimulatory effects of asbestos on NADPH-dependent lipid peroxidation in rat liver microsomes

Lipid peroxidation in rat liver microsomes induced by asbestos fibres, crocidolite and chrysotile, is greatly increased in the presence of NADPH, leading to malondialdehyde levels comparable with those induced by CCl4, a very strong inducer of lipid peroxidation. This synergic effect only occurs during the first minutes and could be explained by an increase or a regeneration of the ferrous active sites of asbestos by NADPH, which in turn could rapidly be prevented by the adsorption of microsomal proteins on the surface of the fibres. It is not inhibited by superoxide dismutase, catalase and mannitol, indicating that oxygen radicals are not involved in the reaction. It is also not inhibited by desferrioxamine, indicating that it is not due to a release of free iron ions in solution from the fibres. Lipid peroxidation in NADPH-supplemented microsomes is also greatly increased upon addition of magnetite. This could be linked to the presence of ferrous ions in this solid iron oxide, since the ferric oxides haematite and goethite are completely inactive.

Hydroxyl radical production in the presence of fibres by a Fenton-type reaction

Glass fibres are considered to be inert and therefore thought to present no real hazard to the health of people who inhale them. Results in the present study however indicate that these fibres are able to produce hydroxyl radicals in the presence of hydrogen peroxide by a Fenton-type reaction. Since hydroxyl radical is implicated in lipid peroxidation, single-strand DNA breaks and carcinogenesis, care should be exercised when dealing with glass fibres.

Effect of asbestos fibers on aryl hydrocarbon hydroxylase and aminopyrine N-demethylase activities of rat liver microsomes

Chrysotile asbestos fibers impair the activities of rat liver microsomal aryl hydrocarbon hydroxylase (AHH), aminopyrine (AP) N-demethylase and dimethylnitrosamine (DMN) demethylase in vitro. This inhibition is concentration-dependent. Preincubation of 3-methylcholanthrene (3-MC)-pretreated rat liver microsomes with chrysotile depresses the overall metabolism of [G-3H]benzo[a]pyrene (BaP). Various forms of asbestos employed inhibit AHH activity to the same extent. However, other types of asbestos are not as effective as chrysotile in diminishing AP demethylase activity. Chrysotile and crocidolite fibers are not found to significantly change the apparent Km of AHH activity, from 3-MC-pretreated rat liver microsomes, for BaP. Increasing the microsomal protein concentration partially abolishes the inhibition of AHH activity caused by chrysotile fibers. Inhibition of AP demethylase and AHH activities is attenuated by bovine serum albumin (BSA) or ferritin. Depression of AHH activity by crocidolite is significantly reversed by ferritin. Since polymers such as ferritin override enzyme inhibition by chrysotile as well as crocidolite, surface chemical groups of the fibers may be involved in enzyme modification.

The importance of free radicals and catalytic metal ions in human diseases

The study of free radical reactions is not an isolated and esoteric branch of science. A knowledge of free radical chemistry and biochemistry is relevant to an understanding of all diseases and the mode of action of all toxins, if only because diseased or damaged tissues undergo radical reactions more readily than do normal tissues. However it does not follow that because radical reactions can be demonstrated, they are important in any particular instance. We hope that the careful techniques needed to assess the biological role of free radicals will become more widely used.

Role of oxygen radicals in tumor promotion

Tumor promoters provoke the elaboration of oxygen radicals by direct chemical generation and through the indirect activation or alteration of cellular sources including membrane oxidases, peroxisomes, and electron transport chains in mitochondria and endoplasmic reticulum. Although direct measurement of amplified oxygen radical production in response to tumor promoters in target tissues remains problematic, studies with scavengers of reactive oxygen species demonstrate inhibition of biochemical and biological sequelae of tumor promoter exposure and provide strong presumptive evidence for oxygen radical involvement in this late stage of carcinogenesis. The critical macromolecular targets for these oxygen radicals remain undefined; however, they may include lipids, DNA, DNA repair systems, and other enzymes.