Mechanisms in the pathogenesis of asbestosis and silicosis

Free radical generation in the toxicity of inhaled mineral particles: the role of iron speciation at the surface of asbestos and silica

Free radical generation at the particle/biological fluid interface is one of the chemical processes that contributes to pathogenicity. In order to investigate the role played by iron, fibres of crocidolite asbestos have been modified by thermal treatments to alter their surface iron content. Two radical mechanisms, HO* from H2O2 and cleavage of a C-H bond, which are both active on the original fibres, have been tested on the modified fibres. C-H cleavage is dependent on Fe(II) abundance and location and is suppressed by surface oxidation while HO* release appears independent of the oxidation state of iron. Quartz specimens with different levels of iron impurities have been tested in a similar manner. A commercially available quartz (Min-U-Sil 5) containing trace levels of iron is also active in both tests, but reactivity is not fully suppressed by treatment with desferrioxamine, which should remove/inactivate iron. The radical yield attained is close to the level produced by a pure quartz dust, suggesting the presence of active sites other than iron. Ascorbic acid reacts with both crocidolite and quartz, with subsequent depletion of the level of antioxidant defences when particle deposition occurs in the lung lining layer. Following treatment with ascorbic acid the radical yield increases with quartz, but decreases with asbestos. Selective removal of iron and silicon from the surface may account for the differences in behaviour of the two particulates.

Polymorphisms in the tumor necrosis factor-alpha gene promoter may predispose to severe silicosis in black South African miners

Susceptibility to silicosis is in part genetically determined. Polymorphisms in the promoter region of tumor necrosis factor (TNF)-alpha, a cytokine with a central role in the pathophysiology of silicosis, have been associated with predisposition to several infectious and inflammatory diseases. Polymorphisms at positions -308, -238, and -376 in the TNF-alpha promoter region were compared in nine patients with severe silicosis with International Labour Office (ILO) grade 3 nodularity, 112 patients with less severe silicosis (ILO grades 1/1 to 2/2), and 120 black South African gold miners without silicosis (ILO grades 0/0) in an age-frequency-matched case- control study. There were no significant differences between miners with less severe silicosis and controls at any loci in the TNF-alpha promoter region, but miners with severe silicosis were significantly more likely than controls to have -238A (33% versus 6%, Fisher's exact p value = 0.022) and -376A (33% versus 5%, Fisher's exact p value = 0.016). These alleles were in linkage disequilibrium (p < 0.001), and so were not independent. The association remained significant (Fisher's exact p values = 0.011 and 0.011, respectively) when analysis was limited to the majority tribe (Basotho), which included all subjects with severe silicosis. Subjects with severe silicosis were also significantly more likely to have the -308A allele (Fisher's exact p value = 0.034), but this result was confounded by ethnicity and was not significant within Basotho tribe members (Fisher's exact p value = 0.15). TNF-alpha promoter polymorphisms are associated with severe, but not less severe, silicosis in this population. A predominant effect on disease severity, rather than on disease frequency, appears to be a general feature of promoter polymorphism in diseases in which TNF-alpha has a critical role.

Ultrafine airborne particles cause increases in protooncogene expression and proliferation in alveolar epithelial cells

Exposure to ambient particulate matter (PM) is linked to increases in respiratory morbidity and exacerbation of cardiopulmonary diseases. However, the important components of PM and their mechanisms of action in lung disease are unclear. We demonstrate the development of dose-related proliferation and apoptosis after exposure of an alveolar epithelial cell line (C10) to PM or to ultrafine carbon black (ufCB), a component of PM. Ribonuclease protection assays demonstrated that increases in mRNA levels of the early response protooncogenes c-jun, junB, fra-1, and fra-2 accompanied cell proliferation at low concentrations of PM whereas apoptotic concentrations of PM caused transient increases in expression of fos and jun family members and dose responsive increases in mRNA levels of receptor-interacting protein, Fas-associated death domain, and caspase-8. Significant increases in steady-state mRNA levels of protooncogenes and apoptosis-associated genes, TNFR-associated death domain, and Fas were also observed after exposure of epithelial cells to ufCB, but not fine carbon black or glass beads, respectively, suggesting that the ultrafine particulate component of PM is critical to its biological activity.

Diseases caused by silica: mechanisms of injury and disease development

While silica particles are considered to be fibrogenic and carcinogenic agents, the mechanisms responsible are not well understood. This article summarizes literature on silica-induced accelerated silicosis, chronic silicosis, silico-tuberculosis, bronchogenic carcinoma, and immune-mediated diseases. This article also discusses the generation of reactive oxygen species (ROS) that occurs directly from the interaction of silica with aqueous medium and from silica-stimulated cells, the molecular mechanisms of silica-induced lung injuries with focus on silica-induced NF-kappaB activation, including its mechanisms, possible attenuation and relationship to silica-induced generation of cyclooxygenase II and TNF-alpha. Silica-induced AP-1 activation, protooncogene expression, and the role of ROS in these processes are also briefly discussed.

The pathogenesis of mesothelioma

About 80% of malignant mesotheliomas (MM) in the Western World develop in individuals with higher than background exposure to asbestos. Only a fraction of those exposed to asbestos develop mesothelioma, indicating that additional factors play a role. Simian virus 40 (SV40), a DNA tumor virus that preferentially causes mesothelioma in hamsters, has been detected in several human mesotheliomas. The expression of the SV40 large tumor antigen in mesothelioma cells, and not in nearby stromal cells, and the capacity of antisense T-antigen treatment to arrest mesothelioma cell growth in vitro suggest that SV40 contributes to tumor development. The capacity of T-antigen to bind and inhibit cellular p53 and retinoblastoma (Rb)-family proteins in mesothelioma, together with the very high susceptibility of human mesothelial cells to SV40-mediated transformation in vitro, supports a causative role of SV40 in the pathogenesis of mesothelioma. Asbestos appears to increase SV40-mediated transformation of human mesothelial cells in vitro, suggesting that asbestos and SV40 may be cocarcinogens. p53 mutations are rarely found in mesothelioma; p16, p14ARF, and NF2 mutations/losses are frequent. Recent studies revealed the existence of a genetic factor that predisposes affected individuals to mesothelioma in the villages of Karain and Tuzkoy, in Anatolia, Turkey. Erionite, a type of zeolite, may be a cofactor in these same villages, where 50% of deaths are caused by mesothelioma. Mesothelioma appears to have a complex etiology in which environmental carcinogens (asbestos and erionite), ionizing radiation, viruses, and genetic factors act alone or in concert to cause malignancy.

Diseases caused by asbestos: mechanisms of injury and disease development

Asbestos is a ubiquitous, naturally occurring fiber that has been linked to the development of malignant and fibrotic diseases of the lung and pleura. These diseases may be initiated by injury to epithelial cells and mesothelial cells by asbestos fibers through the formation of reactive oxygen intermediates. Elaboration of oxidants are also a consequence of inflammation, a hallmark of exposure to asbestos after inhalation or injection of asbestos fibers into animals. The type, size, and durability of asbestos fibers may be important in toxicity and pathogenicity of asbestos types. This review discusses the pathways of oxidant generation by asbestos fibers, cell-cell interaction that may initiate and perpetuate inflammation, cytokine release and proliferative responses to asbestos, and cell signaling pathways implicated in these events.

Mineral composition other than quartz is a critical determinant of the particle inflammatory potential

The aim of this study was to examine the inflammatory potential of stone quarry particles with differing mineral and metal composition and if the effects could be related to the leaching of metals from the particles and if antioxidants would reduce the cytokine release. After intratracheal instillation of rats with a type of mylonite (median size 8 microns) we found a stronger inflammatory potential of mylonite than of quartz at 20 h after treatment. In isolated rat type 2 cells and human epithelial lung cells (A549) mylonite induced a much greater release of MIP-2/IL-8 than quartz or a type of basalt and a feldspar. The mylonite particles were more potent even when compared to smaller size fractions of quartz. Thus mineral composition can be more important than size in eliciting acute inflammatory responses. The content of metals in basalt and mylonite showed minor variations with somewhat more metals present in basalt. The release of metals from the two particle types varied, but in general more metals were released from basalt than from mylonite particles. However, metal release was not related to the differences in proinflammatory effect. Antioxidants seemed to decrease the release of cytokines induced by mylonite particles, but a suppression of basal cytokine release by antioxidants was also observed, questioning the involvement of oxygen radicals in the mylonite-induced effects.

Clara cell protein (CC16) in serum and bronchoalveolar lavage fluid of subjects exposed to asbestos

The Clara cell protein (CC16) is a small and readily diffusible protein of 16 kDa secreted by bronchiolar Clara cells in the distal airspaces. These epithelial cells are altered in several pulmonary pathological processes induced by various lung toxicants. In the search for a new biomarker of asbestos-induced lung impairment, we used a sensitive immunoassay to determine the levels of CC16 in bronchoalveolar fluid (BALF) and serum of subjects exposed to asbestos compared with a group of healthy controls. In the BALF of asbestos-exposed subjects there was an insignificant trend towards CC16 elevation compared with controls, with a (mean +/- SD of 0.81 +/- 0.65 mg l-1 for asbestos-exposed subjects (n = 23) versus 0.39 +/- 0.19 mg l-1 for controls (n = 11) (p = 0.09). In serum, CC16 concentration was significantly increased among asbestos-exposed subjects, with values of 27.2 +/- 24.0 micrograms l-1 for asbestos-exposed subjects (n = 34) versus 16.1 +/- 7.6 micrograms l-1 for controls (n = 34) (p = 0.01). Regarding the effects of smoking, there were significant differences between generally lower CC16 levels in serum and BALF (p = 0.05 and 0.001, respectively) of smokers compared with the higher levels in non-smokers. Serum CC16 levels positively correlated with those in BALF, which is consistent with a diffusional transfer of CC16 from the bronchoalveolar space into the serum. No association, however, emerged between the levels of CC16 in serum or BALF and either the duration of asbestos exposure or the severity of the lung impairment as assessed by chest X-ray. These findings suggest that exposure to asbestos elicits early changes in the local and, importantly, also the systemic levels of CC16. This pneumoprotein therefore appears as a promising non-invasive biomarker of asbestos-induced lung injury and occupational disease in both smoking and non-smoking exposed subjects.

Mortality from lung cancer among silicotic patients in Sardinia: an update study with 10 more years of follow up

OBJECTIVES

To evaluate the association between silica, silicosis and lung cancer, the mortality of 724 patients with silicosis, first diagnosed by standard chest x ray film between 1964 and 1970, has been analysed by a cohort study extended to 31 December 1997.

METHODS

Smoking and detailed occupational histories were available for each member of the cohort as well as the estimated lifetime exposure to respirable silica dust and radon daughters. Two independent readers blindly classified standard radiographs according to the 12 point International Labour Organisation (ILO) scale. Lung function tests meeting the American Thoracic Society's criteria were available for 665 patients. Standardised mortality ratios (SMRs) for selected causes of death were based on the age specific Sardinian regional death rates.

RESULTS

The mortality for all causes was significantly higher than expected (SMR 1.35, 95% confidence interval (95% CI) 1.24 to 1.46) mainly due to tuberculosis (SMR 22.0) and to non-malignant chronic respiratory diseases (NMCRD) (SMR 6.03). All cancer deaths were within the expected numbers (SMR 0.93; 95% CI 0.76 to 1.14). The SMR for lung cancer was 1.37 (95% CI 0.98 to 1.91, 34 observed), increasing to 1.65 (95% CI 0.98 to 2.77) allowing for 20 years of latency since the first diagnosis of silicosis. Although mortality from NMCRD was strongly associated to the severity of radiological silicosis and to the extent of the cumulative exposure to silica, SMR for lung cancer was weakly related to the ILO categories and to the cumulative exposure to silica dust only after 20 years of lag interval. A significant excess of deaths from lung cancer (SMR 2.35) was found among silicotic patients previously employed in underground metal mines characterised by a relatively high airborne concentration of radon daughters and among ever smokers who showed an airflow obstruction at the time of the first diagnosis of silicosis (SMR 3.29). Mortality for lung cancer related to exposure was evaluated with both the Cox's proportional hazards modelling within the entire cohort and a nested case-control study (34 cases of lung cancer and 136 matched controls). Both multivariate analyses did not show any significant association with cumulative exposure to silica or severity of silicosis, but confirmed the association between mortality for lung cancer and relatively high exposure to radon, smoking, and airflow obstruction as significant covariates.

CONCLUSIONS

The findings indicate that the slightly increased mortality for lung cancer in this cohort of silicotic patients was significantly associated with other risk factors-such as cigarette smoking, airflow obstruction, and estimated exposure to radon daughters in underground mines-rather than to the severity of radiological silicosis or to the cumulative exposure to crystalline silica dust itself.

Potential pulmonary effects of man-made organic fiber (MMOF) dusts

In the first half of the twentieth century epidemiologic evidence linked elevated incidences of pulmonary fibrosis and cancer with inhalation of chrysotile and crocidolite asbestos, a family of naturally occurring inorganic fibrous materials. As the serpentine and amphibole forms of asbestos were phased out, synthetic vitreous fibers (SVFs; fiber glass, mineral wool, and refractory fiber) became increasingly utilized, and concerns were raised that they too might cause adverse health effects. Extensive toxicological research on SVFs has demonstrated that their pulmonary effects are directly related to fiber dose in the lung over time. This is the result of deposition (thin fibers deposit in the lower lung more efficiently than thick fibers) and lung-persistence ("biopersistence" is directly related to fiber length and inversely related to dissolution and fragmentation rates). In rat inhalation studies, asbestos was determined to be 7- to 10-fold more biopersistent in the lung than SVFs. Other than its effect on biopersistence, fiber composition did not appear to play a direct role in the biological activity of SVFs. Recently, the utilization of man-made organic fibers (MMOFs) (also referred to by some as synthetic organic fibers) has increased rapidly for a variety of applications. In contrast to SVFs, research on the potential pulmonary effects of MMOFs is relatively limited, because traditionally MMOFs were manufactured in diameters too thick to be respirable (inhalable into the lower lung). However, new developments in the MMOF industry have resulted in the production of increasingly fine-diameter fibers for special applications, and certain post-manufacturing processes (e.g., chopping) generate respirable-sized MMOF dust. Until the mid-1990s, there was no consistent evidence of human health affects attributed to occupational exposure to MMOFs. Very recently, however, a unique form of interstitial lung disease has been reported in nylon flock workers in three different plants, and respirable-sized nylon shreds (including fibers) were identified in workplace air samples. Whether nylon dust or other occupational exposures are responsible for the development of lung disease in these workers remains to be determined. It is also unknown whether the biological mechanisms that determine the respirability and toxicity of SVFs apply to MMOFs. Thus, it is appropriate and timely to review the current data regarding MMOF workplace exposure and pulmonary health effects, including the database on epidemiological, exposure assessment, and toxicology studies.

Putting tumours in context

The interactions between cancer cells and their micro- and macroenvironment create a context that promotes tumour growth and protects it from immune attack. The functional association of cancer cells with their surrounding tissues forms a new 'organ' that changes as malignancy progresses. Investigation of this process might provide new insights into the mechanisms of tumorigenesis and could also lead to new therapeutic targets.

Ultrastructural effects of silicic acid on primary lung fibroblasts in tissue culture

Transmission and scanning electron microscopic examination of primary lung fibroblasts exposed in tissue culture to polymeric silicic acid (PSA) revealed profound cellular changes in the cell surface membranes, resulting in rapid endocytosis of affected membranes and formation of multivesicular bodies. Exposure to monomeric silicic acid did not appear to exhibit any immediate adverse effects. Appearance of numerous cytoplasmic vacuoles within 1 h of PSA exposure was easily visible by light microscopy. Electron microscopy revealed that PSA exposure caused formation of an 'osmiophilic' cell surface membrane. Numerous osmiophilic cytoplasmic blebs on the surface and subsequent endocytotic vesicles appeared to collapse and aggregate into multivesicular bodies. This study provides ultrastructural evidence of the direct interaction between lung fibroblasts and polymeric silicic acid, which has a dramatic effect the surface membrane, its subsequent internalization and cytoplasmic processing. This interaction could be one of the key steps in the damaging effects of silica containing dust.

Iron inhibits the nitric oxide synthesis elicited by asbestos in murine macrophages

Crocidolite fibers stimulated nitric oxide synthase (NOS) activity and expression in glial and alveolar murine macrophages: this effect was inhibited by iron supplementation and enhanced by iron chelation. We suggest that in these cells crocidolite stimulates NOS expression by decreasing the iron bioavailability and activating an iron-sensitive transcription factor.

Fas ligand triggers pulmonary silicosis

We investigated the role of Fas ligand in murine silicosis. Wild-type mice instilled with silica developed severe pulmonary inflammation, with local production of tumor necrosis factor (TNF)-alpha, and interstitial neutrophil and macrophage infiltration in the lungs. Strikingly, Fas ligand-deficient generalized lymphoproliferative disease mutant (gld) mice did not develop silicosis. The gld mice had markedly reduced neutrophil extravasation into bronchoalveolar space, and did not show increased TNF-alpha production, nor pulmonary inflammation. Bone marrow chimeras and local adoptive transfer demonstrated that wild-type, but not Fas ligand-deficient lung macrophages recruit neutrophils and initiate silicosis. Silica induced Fas ligand expression in lung macrophages in vitro and in vivo, and promoted Fas ligand-dependent macrophage apoptosis. Administration of neutralizing anti-Fas ligand antibody in vivo blocked induction of silicosis. Thus, Fas ligand plays a central role in induction of pulmonary silicosis.

Interaction of alveolar macrophages with inhaled mineral particulates

Pulmonary disorders triggered by inhalation of occupational and environmental mineral particulates can be endpoints of a chronic inflammatory process in which alveolar macrophages (AMs), as a first line of defense, play a crucial role. The biological processes involved in particulate-induced activation of AMs include indirect or direct interactions of particulates with the cell membrane, subsequent stimulation of signal transduction pathways, and activation of gene transcription. Depending on the nature of particulate involved, particulate-induced activation of AMs has been shown to result in the release of potent mediators, such as reactive oxygen and nitrogen species, cytokines, eicosanoids, and growth factors. The prolonged and enhanced production of such effector molecules may result in a complex cascade of events that can contribute to the development of pulmonary disorders. This paper will give a short review of the present knowledge of AM interaction with inhaled mineral particulates and of the possible implications these interactions may have in the development of pulmonary disorders.

Asbestos causes apoptosis in alveolar epithelial cells: role of iron-induced free radicals

Asbestos causes asbestosis and malignancies by mechanisms that are not fully understood. Alveolar epithelial cell (AEC) injury by iron-induced reactive oxygen species (ROS) is one important mechanism. To determine whether asbestos causes apoptosis in AECs, we exposed WI-26 (human type I-like cells), A549 (human type II-like cells), and rat alveolar type II cells to amosite asbestos and assessed apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate-biotin nick end labeling (TUNEL) staining, nuclear morphology, annexin V staining, DNA nucleosome formation, and caspase 3 activation. In contrast to control medium and TiO2, amosite asbestos and H2O2 each caused AEC apoptosis. A role for iron-catalyzed ROS was suggested by the finding that asbestos-induced AEC apoptosis and caspase 3 activation were each attenuated by either an iron chelator (phytic acid and deferoxamine) or a.OH scavenger (dimethyl-thiourea, salicylate, and sodium benzoate) but not by iron-loaded phytic acid. To determine whether asbestos causes apoptosis in vivo, rats received a single intratracheal instillation of amosite (5 mg) or normal saline solution, and apoptosis in epithelial cells in the bronchoalveolar duct regions was assessed by TUNEL staining. One week after exposure, amosite asbestos caused a 3-fold increase in the percentage of apoptotic cells in the bronchoalveolar duct regions as compared with control (control, 2.1% +/- 0.35%; asbestos, 7.61% +/- 0.15%; n = 3). However, by 4 weeks the number of apoptotic cells was similar to control. We conclude that asbestos-induced pulmonary toxicity may partly be caused by apoptosis in the lung epithelium that is mediated by iron-catalyzed ROS and caspase 3 activation.

Lung tissue mechanics and extracellular matrix composition in a murine model of silicosis

The dynamic mechanical properties of lung tissue and its contents of collagen and elastic fibers were studied in strips prepared from mice instilled intratracheally with saline (C) or silica [15 (S15) and 30 days (S30) after instillation]. Resistance, elastance, and hysteresivity were studied during oscillations at different frequencies on S15 and S30. Elastance increased from C to silica groups but was similar between S15 and S30. Resistance was augmented from C to S15 and S30 and was greater in S30 than in S15 at higher frequencies. Hysteresivity was higher in S30 than in C and S15. Silica groups presented a greater amount of collagen than did C. Elastic fiber content increased progressively along time. This increment was related to the higher amount of oxytalan fibers at 15 and 30 days, whereas elaunin and fully developed elastic fibers were augmented only at 30 days. Silicosis led not only to pulmonary fibrosis but also to fibroelastosis, thus assigning a major role to the elastic system in the silicotic lung.

Relationship of fiber surface iron and active oxygen species to expression of procollagen, PDGF-A, and TGF-beta(1) in tracheal explants exposed to amosite asbestos

To investigate the role of iron and active oxygen species (AOS) in asbestos-induced fibrosis, we loaded increasing amounts of Fe(II)/Fe(III) onto the surface of amosite asbestos fibers and then applied the fibers to rat tracheal explants. Explants were harvested after 7 d in air organ culture. Asbestos by itself doubled procollagen gene expression, and a further increase was seen with increasing iron loading; actual collagen content measured as hydroxyproline was increased in a similar pattern. Iron loading also increased gene expression of platelet-derived growth factor (PDGF)-A and transforming growth factor (TGF)-beta(1). Neither asbestos alone nor iron-loaded asbestos affected gene expression of PDGF-B, tumor necrosis factor-alpha, or TGF-alpha. The AOS scavenger tetramethylthiourea or treatment of fibers with the iron chelator deferoxamine prevented asbestos-induced increases in procollagen, PDGF-A, and TGF-beta gene expression, whereas glutathione had no effect. The proteasome inhibitor MG-132 abolished asbestos-induced increases in procollagen gene expression but did not affect increases in PDGF-A or TGF-beta(1) expression, whereas the extracellular signal-regulated protein kinase (ERK) inhibitor PD98059 had exactly the opposite effect. We conclude that surface iron as well as the iron-catalyzed generation of AOS play a role in asbestos-induced matrix (procollagen) production and that this process is driven in part through oxidant-induced nuclear factor kappa B activation. Surface iron and AOS also play a role in PDGF-A and TGF-beta gene expression, but through an ERK-dependent mechanism.

Different accumulation of activated extracellular signal-regulated kinases (ERK 1/2) and role in cell-cycle alterations by epidermal growth factor, hydrogen peroxide, or asbestos in pulmonary epithelial cells

The extracellular signal-regulated kinase (ERK) pathway is induced by cytokines and oxidative stress. In this study we examined the patterns of localization of phosphorylated ERK proteins in relationship to subsequent phenotypic responses by the mitogenic agent epidermal growth factor (EGF) (5 ng/ ml); hydrogen peroxide (H(2)O(2)) (100 to 300 microM), an inducer of apoptosis; and crocidolite asbestos (5 microg/cm(2) dish) in a nontransformed murine alveolar type II epithelial cell line (C10). Laser scanning cytometry and flow cytometry were used to determine: (1) whether expression of phosphorylated ERKs was cell cycle-related; and (2) whether cell-cycle alterations by agents could be modified after addition of the mitogen-activated protein kinase/ERK kinase (MEK) 1 inhibitor PD98059. In contrast to other stimuli which induced transient increases in phosphorylated ERKs, asbestos caused fiber-associated localization of phosphorylated ERKs that were elevated from 1 to 24 h (P < or = 0.05), and striking apoptosis followed by increased numbers of cells in the S phase at 72 h. In both control and asbestos-exposed cells, the percentage of phosphorylated ERK-positive cells was greatest in cells in the G(2)/M and S phases of the cell cycle. All stimuli caused increased proportions of cells in G(2)/M at 24 h that were inhibited by PD98059 (30 microM). Increases in G(2)/M cells by H(2)O(2) and asbestos also were decreased at 48 h by the MEK1 inhibitor. In addition, PD98059 abrogated elevations in S-phase cells by EGF and H(2)O(2) at 24 h and by asbestos at 72 h. Our results suggest that ERKs mediate cell-cycle alterations during the development of epithelial cell apoptosis or proliferation by diverse ERK stimuli.

Tumors are unique organs defined by abnormal signaling and context

Many cancer investigations have focussed on the eradication of the cancer cell itself and in doing so, overlook the inherent complexity and heterogeneity of solid tumors. Here, we argue that, in many cases, it is the altered communication within the tumor, rather than mutations per se, that is the defining characteristic of cancer. As a result, tumorigenesis can be indirectly initiated by environmental or inherited factors that affect the stromal cells. We propose that anticancer research might be more effective if aimed at eradicating the cause of abnormality rather than just treating the end result.

Differential responses of rat alveolar and peritoneal macrophages to man-made vitreous fibers in vitro

Different approaches, including inhalation and intraperitoneal injection assays, have been used to assess the potential health effects of man-made vitreous fibers (MMVF). The purpose of this study was to compare the phagocytic activity and the formation of reactive oxygen species by rat alveolar macrophages (AM) and peritoneal macrophages (PM) upon exposure to MMVF10 glass wool and MMVF21 rock wool fibers. Macrophage (Mphi) phagocytosis of mineral fibers was assessed by optical videomicroscopy and computer-aided image analysis. Mphi were classified as cells not associated with fibers, cells with attached fibers, cells with incompletely phagocytized fibers (an appearance known as "frustrated phagocytosis"), and cells with completely phagocytized fibers. The production of superoxide anions by AM and PM upon incubation with MMVF10 and MMVF21 fibers was determined by the superoxide dismutase-inhibitable reduction of ferricytochrome C. PM were found to have a lower phagocytic activity than AM. A significantly higher percentage of AM than of PM underwent frustrated phagocytosis of MMVF10 and MMVF21 fibers. In line with these findings, AM generated higher levels of oxygen radicals than PM upon exposure to MMVF21 fibers. In contrast, MMVF10 fibers failed to induce the generation of reactive oxygen species by both AM and PM. Our in vitro results show that the phagocytic activity, in particular the frustrated phagocytosis of mineral fibers, was significantly lower in PM than in AM. The data support the idea that the durability and biopersistence of mineral fibers are higher in the peritoneal cavity than in the lung.

Silica and its antagonistic effects on transforming growth factor-beta in lung fibroblast extracellular matrix production

BACKGROUND

Silicosis, a pneumoconiosis marked by interstitial pulmonary fibrosis, is caused by inhalation of free crystalline silica particles. When silica particles are injected into the lower lung, they are translocated across the epithelium into the interstitial space, where macrophage-derived growth factors affect lung fibroblast proliferation and collagen deposition. We hypothesized that silica may act directly on pulmonary fibroblasts modifying extracellular matrix (ECM) synthesis and that the effects of silica may be mediated by transforming growth factor-beta (TGFbeta) overproduction.

METHODS

To test this hypothesis, we studied a human lung fibroblast cell line (WI-1003) exposed to silica in vitro. We investigated cell morphology by electron microscopic procedure, cell growth, collagen production, and glycosaminoglycans (GAG) composition by radiolabeled precursors. Cytokine and growth factor synthesis were evaluated by specific enzyme-linked immunoadsorbent assay kits and Northern blotting analysis.

RESULTS

Pulmonary fibroblasts internalized silica particles without detectable cell damage. Silica directly stimulated collagen synthesis and decreased the amount of 3H-glucosamine-labeled GAG. Silica-treated fibroblasts secreted less TGFbeta than untreated controls, antagonized the stimulatory effect of TGFbeta on ECM synthesis, and reversed TGFbeta-induced inhibition of cell proliferation. Northern blotting analysis showed increased interleukin-1alpha (IL-1alpha) mRNA after silica treatment. IL-1alpha had no influence on collagen synthesis but increased the number of WI-1003 fibroblasts.

CONCLUSIONS

These results support our hypothesis that lung fibroblasts are direct silica targets. However, contradicting our hypothesis, silica antagonized TGFbeta activities through a TGFbeta downregulation and an IL-1alpha upregulation. The complex pattern of TGFbeta and IL-1alpha regulation in pulmonary fibroblasts is imbalanced by silica exposure and might play a key role in silica-mediated pulmonary fibrosis.

Effects of crocidolite asbestos on human bronchoepithelial-dependent fibroblast stimulation in coculture: the role of IL-6 and GM-CSF

Cocultures of human pulmonary epithelial cells (BEAS-2B) and lung fibroblasts (WISTAR-38), representing two cell types of central regulatory potential in (chronic) lung disease, were used as an in vitro model to study the role of interleukin 6 (IL-6) and of granulocyte macrophage-colony stimulating factor (GM-CSF) in early fibrogenesis. For this purpose, epithelial cells were pre-exposed to UICC crocidolite asbestos fibers or titanium dioxide (TiO2) particles for 96 h and subsequently cocultured with fibroblasts for additional 72 h. Gene expression of IL-6 or GM-CSF in both cell types as well as of alpha1 procollagens types I and III in fibroblasts was determined by RT-PCR. Synthesis of IL-6, GM-CSF or collagen I was quantified using IL-6 bioassay or ELISA tests, respectively. Both mediators were directly induced in bronchoepithelial cells by crocidolite but not by TiO2. Likewise, steady-state mRNA levels of procollagens as well as collagen synthesis were upregulated in cocultured fibroblasts. As a result of coculture, cytokine concentrations were synergistically enhanced and further increased by crocidolite in a dose-dependent manner. Suppression of cytokine induction by corresponding neutralizing antibodies consistently abrogated collagen enhancement. Direct stimulation of fibroblast monocultures with recombinant human IL-6 or GM-CSF significantly increased collagen synthesis and transcription in a dose-dependent manner. Thus, our results demonstrate that crocidolite selectively stimulated production of IL-6 and GM-CSF in bronchoepithelial cells. In epithelial-fibroblast interactions, these mediators appear to play a key role in regulating fibroblast activity, indicating a close correlation between these cytokines and the fibrogenic potential of particulates.

Silicose: correlação da tomografia computadorizada de alta resolução com a anatomopatologia

Neste trabalho foram analisados os padrões observados nas tomografias computadorizadas de alta resolução do tórax de 20 pacientes com silicose, e correlacionados com os achados anatomopatológicos de cinco pacientes. O principal aspecto observado foram os nódulos, com distribuição randômica, na maior parte dos casos com confluência, fazendo, por vezes, massas conglomeradas. Eles predominaram nos terços superiores dos pulmões, nas regiões posteriores. Outro achado importante foi o enfisema.

Changes in DNA 8-hydroxyguanine levels, 8-hydroxyguanine repair activity, and hOGG1 and hMTH1 mRNA expression in human lung alveolar epithelial cells induced by crocidolite asbestos

We examined 8-hydroxyguanine (8-OH-Gua) formation and 8-OH-Gua repair enzyme activity in pulmonary type-II-like epithelial cells to determine whether oxidative stress induced by asbestos plays a role in its carcinogenic mechanism. A549 cells were incubated with crocidolite asbestos at concentrations of 0, 10, 50 and 100 microg/ml over 27 h. We then evaluated 8-OH-Gua formation, 8-OH-Gua repair enzyme activity and gene expression of 8-oxoguanine-DNA glycosylase 1 (hOGG1) and human MUtT homologue (hMTH1). This was done using a high-performance liquid chromatography system equipped with an electrochemical detector, endonuclease nicking assay and reverse transcription polymerase chain reaction, respectively. Crocidolite induced the formation of 8-OH-Gua in DNA at concentrations of 50 and 100 microg/ml. 8-OH-Gua levels increased at 9 h and had declined to near baseline at 27 h, whereas 8-OH-Gua repair enzyme activity peaked at 18 h post-crocidolite exposure. hOGG1 and hMTH1 mRNA levels were also increased by crocidolite exposure. These data suggest that crocidolite asbestos is associated with epithelial cell injury in the process of carcinogenesis through oxidative stress.

Tumor necrosis factor receptor deficiency alters matrix metalloproteinase 13/tissue inhibitor of metalloproteinase 1 expression in murine silicosis

Murine exposure to silica is associated with enhanced tumor necrosis factor alpha (TNF-alpha) expression and matrix deposition. The regulation of TNF is mediated through TNF receptor (TNFR) activation of transcription factors. In the present work we have studied the importance of the individual TNFR in silica-induced lung inflammation and matrix deposition in mice. We studied RNA expression of TNF, alpha1(I) collagen, interstitial collagenase (MMP-13), and its inhibitor (TIMP-1) in the lungs of silica-treated mice. Furthermore, we correlated MMP-13/TIMP-1 RNA abundance with activation of the transcription factors AP-1 and NF-kappaB in the lungs of C57BL/6 mice, and of mice deficient in one of the two types of TNFR (p55(-/-) or p75(-/-)), exposed to silica (0.2 g/kg) or saline by intratracheal instillation. Animals were killed 28 d after exposure and lung hydroxyproline (HP), TNF, alpha1(I) collagen, MMP-13, and TIMP-1 RNA abundance was measured. AP-1 and NF-kappaB activation was studied by gel-shift assays. Compared with C57BL/6 mice, p55(-/-) and p75(-/-) mice significantly (*p < 0.05) decreased lung HP accumulation in response to silica. All murine strains enhanced TNF and alpha1(I) collagen mRNA in response to silica. Enhanced (p < 0.05) MMP-13 RNA expression was also observed in all murine strains in response to silica. Enhanced (p < 0.05) TIMP-1 RNA expression was observed in C57BL/6 mice, but not in p55(-/-) or p75(-/-) mice, in response to silica. NF-kappaB activation was observed in all murine strains, whereas AP-1 activation was observed only in C57BL/6 mice after silica treatment. These data suggest that TNFR deletion modifies MMP-13/ TIMP-1 expression in favor of matrix degradation.

Reactive oxygen species and caspase activation mediate silica-induced apoptosis in alveolar macrophages

Alveolar macrophages (AMs) are the principal target cells of silica and occupy a key position in the pathogenesis of silica-related diseases. Silica has been found to induce apoptosis in AMs, whereas its underlying mechanisms involving the initiation and execution of apoptosis are largely unknown. The main objective of the present study was to examine the form of cell death caused by silica and the mechanisms involved. Silica-induced apoptosis in AMs was evaluated by terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling assay and cell cycle/DNA content analysis. The elevated level of reactive oxygen species (ROS), caspase-9 and caspase-3 activation, and poly(ADP-ribose) polymerase (PARP) cleavage in silica-treated AMs were also determined. The results showed that there was a temporal pattern of apoptotic events in silica-treated AMs, starting with ROS formation and followed by caspase-9 and caspase-3 activation, PARP cleavage, and DNA fragmentation. Silica-induced apoptosis was significantly attenuated by a caspase-3 inhibitor, N-acetyl-Asp-Glu-Val-Asp aldehyde, and ebselen, a potent antioxidant. These findings suggest that apoptosis is an important form of cell death caused by silica exposure in which the elevated ROS level that results from silica exposure may act as an initiator, leading to caspase activation and PARP cleavage to execute the apoptotic process.

Determination of the platelet activating factor in silicotic patients and its effect on fibroblasts

Platelet-activation factor (PAF), one of the potent proinflammatory mediators, is produced from a large range of cells, including polymorphonuclear neutrophils, monocytes, and natural killer cells. To study the role of PAF in the pathogenesis of silicosis, we determined the PAF in silicotic patients and in healthy persons. The results showed that the concentration of PAF in the plasma of silicotic patients was significantly higher than that of healthy persons. Ourin vitro experimental results showed that the total numbers of fibroblasts were markedly raised with added PAF from 0 to 1 μ g/ml. Adding 1 μ g/ml PAF significantly increased the total numbers of fibroblasts after culture for 48, 72, 96 hrs. Therefore, we suggest that PAF be possibly involved in the pathogenesis of silicosis. However, the mechanism remains to be further elucidated.

Comparison of the phagocytic response of rat and hamster alveolar macrophages to man-made vitreous fibers in vitro

Rats and hamsters are well known for their disparate response to inhaled mineral fibers/particles. Alveolar macrophages (AM) play an important role in the pulmonary clearance and retention of mineral fibers/particles mainly through the process of phagocytosis. The aim of this study was to investigate whether there exist differences in the phagocytic response and release of reactive oxygen species (ROS) between rat and hamster AM upon exposure to man-made vitreous fibers (MMVF) in vitro. AM were obtained by bronchoalveolar lavage and macrophage-enriched cultures were exposed to MMVF10 and MMVF21 fibers for 20 h. The phagocytic response of macrophages was determined by computer-assisted video-microscopy and the superoxide anion production was evaluated by cytochrome c reduction. A significantly higher percentage of rat AM underwent frustrated phagocytosis of both types of MMVF compared to hamster AM. This was associated with a higher ROS release by rat AM compared to hamster AM. These data may help to explain the cellular mechanisms underlying the disparate pulmonary response of rat and hamster to inhaled particulate matter.

Bleomycin-induced pulmonary injury in mice deficient in SPARC

SPARC (secreted protein, acidic and rich in cysteine) is a component of the matrix that appears to regulate tissue remodeling. There is evidence that it accumulates in the lung in the setting of pulmonary injury and fibrosis, but direct evidence of its involvement is only now emerging. We therefore investigated the development of pulmonary fibrosis induced by bleomycin administered either intratracheally or intraperitoneally in mice deficient in SPARC. Bleomycin (0.15 U/mouse) given intratracheally induced significantly more pulmonary fibrosis in mice deficient in SPARC compared with that in wild-type control mice, with the mutant mice demonstrating greater neutrophil accumulation in the lung. However, in wild-type and SPARC-deficient mice given intraperitoneal bleomycin (0.8 U/injection x 5 injections over 14 days), the pattern and severity of pulmonary fibrosis, as well as the levels of leukocyte recruitment, were similar in both strains of mice. These findings suggest that the involvement of SPARC in pulmonary injury is likely to be complex, dependent on several factors including the type, duration, and intensity of the insult. Furthermore, increased neutrophil accumulation in the peritoneal cavity was also observed in SPARC-null mice after acute chemical peritonitis. Together, these data suggest a possible role for SPARC in the recruitment of neutrophils to sites of acute inflammation.

TNF-alpha increases tracheal epithelial asbestos and fiberglass binding via a NF-kappaB-dependent mechanism

Tumor necrosis factor (TNF)-alpha is released from alveolar macrophages after phagocytosis of mineral fibers. To determine whether TNF-alpha affects the binding of fibers to epithelial cells, we exposed rat tracheal explants to TNF-alpha or to culture medium alone, followed by a suspension of amosite asbestos or fiberglass (MMVF10). Loosely adherent fibers were removed from the surface with a standardized washing technique, and the number of bound fibers was determined by scanning electron microscopy. Increasing doses of TNF-alpha produced increases in fiber binding. This effect was abolished by an anti-TNF-alpha antibody, the proteasome inhibitor MG-132, and the nuclear factor (NF)-kappaB inhibitor pyrrolidine dithiocarbamate. Gel shift and Western blot analyses confirmed that TNF-alpha activated NF-kappaB and depleted IkappaB in this system and that these effects were prevented by MG-132 and pyrrolidine dithiocarbamate. These observations indicate that TNF-alpha increases epithelial fiber binding by a NF-kappaB-dependent mechanism. They also suggest that mineral particles may cause pathological lesions via an autocrine-like process in which the response evoked by particles, for example, macrophage TNF-alpha production, acts to enhance subsequent interactions of particles with tissue.

Inhaled particulate matter causes expression of nuclear factor (NF)-kappaB-related genes and oxidant-dependent NF-kappaB activation in vitro

High levels of ambient air pollution are associated with exacerbation of asthma and respiratory morbidity, yet little is known concerning the mechanisms of inflammation and toxicity by components of inhaled particulate matter (PM). Brief inhalation of PM(2.5) (particles of an aerodynamic diameter of < 2.5 microns) (300 microg/m(3) air for 6 h followed by a period of 24 h in clean air) by either C3H/HeJ or C57/BL6 mice caused significant (P </= 0.05) increases in steady-state messenger RNA (mRNA) levels of a number of nuclear factor (NF)-kappaB-associated and/ or -regulated genes, including tumor necrosis factor-alpha and -beta, interleukin-6, interferon-gamma, and transforming growth factor-beta. Lung mRNA levels of lymphotoxin-beta and macrophage migration inhibitory factor were unchanged. In murine C10 alveolar cells and an NF-kappaB-luciferase reporter cell line, exposure to PM(2.5) at noncytotoxic concentrations resulted in increases in transcriptional activation of NF-kappaB-dependent gene expression which were inhibited in the presence of catalase. Early and persistent increases in intracellular oxidants, as measured by flow cytometry and cell imaging using the oxidant probe 2'-7'-dichlorofluoroscin diacetate, were observed in epithelial cells exposed to PM(2.5) and ultrafine carbon black particles. Studies here are the first to show NF-kappaB-related inflammatory and cytokine gene expression after inhalation of PM(2.5) and oxidant-dependent induction of NF-kappaB activity by PM(2.5) in pulmonary epithelial cells.

Fifty years of radical ideas

My role in the free radical theory of oxygen toxicity is discussed. Rebeca Gerschman and I published several papers on this subject. This sparked my interest in geochemistry and I developed the idea that oxygen was the best qualified biological potential energy source for the following reasons: great abundance, easily accessible, possession of a high thermodynamic potential, and its slow reaction rate. Ionization radiation can be viewed as a catalyst for reactive oxygen species since a killing dose imparts an infinitesimal small amount of energy. Next, Carol A. Colton and I showed that in the mammalian brain that stimulated microglia produce the superoxide radical anion and its implications in Alzheimer's disease is discussed. More recently, I have become interested in the role of sulfhydryl groups in transcription factors.

Levels of 8-hydroxy-2'-deoxyguanosine in DNA of white blood cells from workers highly exposed to asbestos in Germany

Asbestos fibers have genotoxic effects and are a potential carcinogenic hazard to occupationally exposed workers. The ability of inhaled asbestos fibers to induce the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the DNA of white blood cells (WBC) of workers highly exposed at the workplace has been studied. The 8-OHdG adduct level of asbestos-exposed workers was significantly increased (p<0.001) compared to that in the control group in all three years of the study. Asbestos-exposed individuals showed a mean value of 2.61+/-0.91 8-OHdG/10(5) dG (median 2.49, n=496) in 1994-1995, 2.96+/-1.10 8-OHdG/10(5) dG (median 2.76, n=437) in 1995-1996 and 2.55+/-0.56 8-OHdG/10(5) dG (median 2.53, n=447) in 1996-1997. For the control subjects, a mean of 1.52+/-0.39 (median 1.51, n=214) was determined. The results indicate that human DNA samples from exposed individuals contain between 1.7 times and twice the level of oxidative damage relative to that found in control samples in all 3 years of the study. The studies presented here show that asbestos exposure can result in oxidative DNA damage. Our data confirm that oxidative DNA damage occurs in the WBC of workers highly exposed to asbestos fibers, thus supporting the hypothesis that asbestos fibers damage cells through an oxidative mechanism. These in vivo findings underline the importance of oxidative damage in asbestos-induced carcinogenesis and highlight the need for exploring the molecular basis of asbestos-induced diseases, and for more effective diagnosis, prevention and therapy of mesothelioma, lung cancer and pulmonary fibrosis. In addition, preventive and therapeutic approaches using antioxidants may be relevant.

Association between 8-hydroxy-2'-deoxyguanosine levels in DNA of workers highly exposed to asbestos and their clinical data, occupational and non-occupational confounding factors, and cancer

In the preceding paper [B. Marczynski, P. Rozynek, T. Kraus, St. Schlösser, H.J. Raithel, X. Baur, Levels of 8-hydroxy-2'-deoxyguanosine in DNA of white blood cells from workers highly exposed to asbestos in Germany, Mutat. Res. (2000) submitted] we described significant increases (p<0.001) in the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG) adducts in the DNA of white blood cells (WBC) of workers highly exposed to asbestos fibers at the workplace relative to those found in the control group in all three study years (period between 1994 and 1997). The results show that the oxidative DNA damage in exposed individuals is between 1.7 times and twice that found in control samples for all 3 years of the study (p<0.001). The aim of this study was to examine the association between the 8-OHdG levels in WBC DNA of workers highly exposed to asbestos fibers at the workplace and clinical data, occupational and non-occupational confounding factors, and cancer. There is no obvious correlation between the steady-state levels of 8-OHdG in the circulating WBC DNA of asbestos workers and possible confounding factors, such as the presence of benign asbestos-associated diseases, the duration of asbestos exposure, the latency period, the fixed cumulative fibrous dust dose ("fiber years"), age, smoking status, acute febrile infections, medicines, aspirin, calcium (Ca(2+)), magnesium (Mg(2+)), and the hormone and vitamin intake. This indicates that previous inhalation of asbestos fibers is the major factor responsible for the difference observed in oxidative DNA damage between asbestos workers and controls. For patients suffering from respiratory cancer, cancer of the gastrointestinal tract, mouth/pharynx/larynx, and urogenital tract the mean DNA-adduct level was significantly higher (p<0.01) than that found in controls, but not significantly higher (p>0.05) than that for asbestos-exposed patients without tumours. The formation of 8-OHdG adduct levels in WBC DNA of patients with hematopoietic cancer, chondrosarcomas and multiform glioblastomas was not significantly higher than that found in the control group (p>0.05). Our results support the hypothesis that oxidative DNA damage in man caused by asbestos fibers plays a role in the formation of malignant tumours.

Caracterização imunofenotípica das subpopulações de linfócitos do lavado broncoalveolar de pacientes com silicose

A lavagem broncoalveolar é um procedimento simples e seguro, na avaliação das pneumopatias relacionadas à exposição a poeiras minerais. O objetivo do estudo foi caracterizar as subpopulações celulares no lavado broncoalveolar (LBA) de pacientes silicóticos. A lavagem broncoalveolar foi realizada em 26 trabalhadores com diferentes formas de silicose: forma simples (n = 12), complicada (n = 13) e um paciente com a forma aguda da doença. Como grupo controle, foram incluídos sete indivíduos sadios. Os pacientes com silicose apresentaram intensa pleocitose com predomínio de macrófagos alveolares e tendência à linfocitose. As subpopulações de linfócitos presentes no lavado broncoalveolar (LBA) dos indivíduos sadios apresentaram fenótipo de células maduras. A grande maioria era constituída por células CD2+TCR<FONT FACE=Symbol>ab</FONT> (87,3%) e somente 2,9% das células T apresentaram marcação CD2+TCR<FONT FACE=Symbol>gd</FONT>. A relação CD4/CD8 foi de 1,8, com poucas (16%) células T imaturas duplo-negativas CD4-CD8-. Em contraste, pacientes com silicose apresentaram redução acentuada das subpopulações dos linfócitos maduros CD2+CD4+, CD2+CD8+ e aumento marcante (47%) de células imaturas (DN) duplo-negativas (CD4-CD8-). Não foi observado aumento das células NK (CD56+). A análise do conteúdo protéico e a determinação da relação Ig/albumina permitiram caracterizar produção local de imunoglobulinas no microambiente pulmonar. Como não foi observado aumento percentual de plasmócitos e linfócitos B (CD19+) no LBA desses pacientes, é possível concluir que as células produtoras de imunoglobulinas estão possivelmente localizadas no interstício pulmonar. Estes resultados sugerem que, durante a evolução da silicose, ocorre o desenvolvimento de linfopoese extratímica e surgimento de órgão linfóide terciário, no microambiente pulmonar desses pacientes.

Malignant pleural disease

The vast majority of pleural neoplasms invade the pleura secondarily and can be seen in patients with bronchogenic carcinoma, breast cancer, lymphoma, and ovarian or gastric carcinoma. Primary pleural neoplasms are less common, although they have developed notoriety since the up-surge of malignant mesothelioma and the knowledge of its connection to asbestos exposure. Other malignant primary tumors include localized fibrous tumor and pleural liposarcoma. In most patients with diffuse malignant pleural disease the chest radiograph shows pleural effusion with or without pleural thickening. Computed tomography (CT) usually provides precise localization and extent of the disease and may be of value in assessing chest wall and mediastinal involvement. In specific situations, magnetic resonance (MR) may be useful as a problem-solving tool when CT findings of chest wall or diaphragmatic invasion are equivocal or in patients with contraindication to intravenous administration of ionic contrast material.

Pathology and pathophysiology of pneumoconiosis

Cellular and molecular mechanisms, as well as associated gene expressions, in silicosis and asbestosis are widely investigated, and compound mechanisms involved in initiating inflammation and progression to fibrosis are comprehensively studied, though not yet totally understood. Recent advances in this field, especially concerning pathophysiology of these pneumoconioses, are reviewed in this article. Silicosis and asbestosis are two major types of pneumoconiosis. Although the clinico-pathologic features presented are apparently different, silicosis and asbestosis are both interstitial lung diseases caused by chronic exposure to airborne inorganic dusts, and the pathology of these two diseases is essentially a fibrosis.

Involvement of oxidative stress in crystalline silica-induced cytotoxicity and genotoxicity in rat alveolar macrophages

Alveolar macrophages (AMs) occupy a key position in silica-induced pulmonary fibrosis, although the mechanisms are yet to be elucidated. In the present study we examined the involvement of oxidative stress and reactive oxygen species formation in silica-induced cytotoxicity and genotoxicity in cultured rat AMs. A lucigenin-dependent chemiluminescence test was used to determine superoxide anion (O(-)(2)), and a 2',7'-dichlorofluorescin diacetate fluorescence test was employed to measure the hydrogen peroxide (H(2)O(2)) level. The cytotoxic and genotoxic effects caused by silica in AMs were examined by lactate dehydrogenase (LDH) leakage and single-cell gel electrophoresis (comet assay), respectively. The results showed that silica enhanced O(-)(2) and H(2)O(2) formation in AMs. There were clear dose- and time-dependent relationships in silica-induced cytotoxicity and genotoxicity. Furthermore, superoxide dismutase and catalase were able to reduce silica-induced LDH leakage and DNA damage, with concurrent significant inhibition on silica-induced oxidative stress in AMs. These findings provide convincing evidence that oxidative stress mediates the silica-induced cytotoxicity and genotoxicity. The understanding of such a mechanism may provide a scientific basis for the possible application of antioxidants in preventing the hazardous effects of silica.

A risk assessment for exposure to glass wool

Synthetic vitreous fibers (SVFs) have been widely used as insulation material in places where asbestos was used many years ago and therefore the hazards have been compared. Since the three principal types of asbestos fibers types have caused lung cancer at high exposures, there is a widely held belief that all fibers are carcinogenic if inhaled in large enough doses. Hence, on a morphological basis, SVFs have been studied for their carcinogenic potential. However, there is considerable evidence that differences exist among fibers in their potency to produce a carcinogenic response. In this attempt to carry out a numerical risk assessment for the installers of blown glass wool (fiber) insulation, we start with a characterization of the material; then we review the exposures both in manufacturing and installation. Neither the epidemiological studies of human exposure nor the animal studies have shown a marked hazardous effect from glass wool and we can therefore be sure that any effect that might exist is small. But in this case, as in many other situations where there is a potential hazard, society desires further reassurance and therefore we have made a mechanistic calculation. There are good estimates of the risk associated with exposure to chrysotile asbestos at high exposures and doses. We have therefore taken these numbers and discussed how much less risky an exposure to glass wool fibers might be. We conclude that for a given fiber count, glass wool is five to ten times less risky (and of course the risk might be zero). The risk for a nonsmoking installer of glass wool fiber insulation who wears a respirator is about 6 in a million (and might be zero) per year. This means that out of a million installers there might be six lung cancers from this cause every year or out of 10,000 installers there might be one in 16 years. The low risk of 6 in a million per year of a worker blowing glass wool is consistent with the fact that no one has found any of cancer attributable to the manufacture or installation of glass wool fibers in spite of diligent searches. This is compared with several other occupational risks. Nonetheless common prudence suggests that any installer of blown glass wool fiber insulation wear a respirator.

Asbestos-induced phosphorylation of epidermal growth factor receptor is linked to c-fos and apoptosis

We examined the mechanisms of interaction of crocidolite asbestos fibers with the epidermal growth factor (EGF) receptor (EGFR) and the role of the EGFR-extracellular signal-regulated kinase (ERK) signaling pathway in early-response protooncogene (c-fos/c-jun) expression and apoptosis induced by asbestos in rat pleural mesothelial (RPM) cells. Asbestos fibers, but not the nonfibrous analog riebeckite, abolished binding of EGF to the EGFR. This was not due to a direct interaction of fibers with ligand, inasmuch as binding studies using fibers and EGF in the absence of membranes showed that EGF did not adsorb to the surface of asbestos fibers. Exposure of RPM cells to asbestos caused a greater than twofold increase in steady-state message and protein levels of EGFR (P < 0.05). The tyrphostin AG-1478, which inhibits the tyrosine kinase activity of the EGFR, but not the tyrphostin A-10, which does not affect EGFR activity, significantly ameliorated asbestos-induced increases in mRNA levels of c-fos but not of c-jun. Pretreatment of RPM cells with AG-1478 significantly reduced apoptosis in cells exposed to asbestos. Our findings suggest that asbestos-induced binding to EGFR initiates signaling pathways responsible for increased expression of the protooncogene c-fos and the development of apoptosis. The ability to block asbestos-induced elevations in c-fos mRNA levels and apoptosis by small-molecule inhibitors of EGFR phosphorylation may have therapeutic implications in asbestos-related diseases.

Critical role of GSH in silica-induced oxidative stress, cytotoxicity, and genotoxicity in alveolar macrophages

The main objective of this study was to evaluate the critical role of glutathione (GSH) in silica-induced oxidative stress, cytotoxicity, and genotoxicity in rat alveolar macrophages (AMs). Silica-induced superoxide radical and hydrogen peroxide formation were determined with lucigenin-dependent chemiluminescence and 2', 7'-dichlorofluorescin diacetate fluorescence test, respectively. The cytotoxicity of silica was estimated by lactate dehydrogenase leakage, and a comet assay was used for examining silica-induced DNA damage in AMs. The intracellular GSH content was modulated by N-acetylcysteine, a GSH precursor, and buthionine sulfoximine, a specific GSH synthesis inhibitor. It was found that silica led to a dose- and time-dependent decrease in GSH content in AMs. N-acetylcysteine increased intracellular GSH level and protected against silica-induced reactive oxygen species formation, lactate dehydrogenase leakage, and DNA strand breaks in AMs. In contrast, buthionine sulfoximine pretreatment depleted cellular GSH and enhanced the susceptibility of AMs to the cytotoxic and genotoxic effects of silica. It thus appears that GSH plays a critical role in protecting against silica-induced cell injury, most probably through its antioxidant activity.

Cellular and molecular mechanisms of asbestos-induced fibrosis

Pleural and pulmonary fibrosis (asbestosis) are ramifications of occupational exposures to asbestos fibers, a diverse family of ubiquitous, naturally-occurring minerals. The pathogenesis of asbestos-associated fibrosis involves the participation of a number of cell types and is characterized by an early and persistent inflammatory response that involves the generation of oxidants, growth factors, chemokines, and cytokines. These mediators may also contribute directly to cell injury, proliferation, and fibrogenesis. After interaction with cells, asbestos fibers trigger a number of signaling cascades involving mitogen-activated protein kinases (MAPK) and nuclear factor kappa-B (NF-kappaB). Activation of transcription factors such as NF-kappaB and activator protein-1 (AP-1) may be linked to increases in early response genes (e.g., c-jun and c-fos) which govern proliferation, apoptosis, and inflammatory changes in the cells of the lung. The goal of this article is to review the cellular and molecular mechanisms of asbestos-induced fibrosis that may be critical to the development of effective treatment regimens.