Pathogenesis of silicosis: current concepts and hypotheses

Investigation of Blood and Urine Malondialdehyde Levels in Mice Exposed to Silica Dust

Background and Objectives:

Occupational exposure to silica dust can lead to biochemical damage. Malondialdehyde (MDA) can be considered as a primary marker for measuring the level of oxidative stress in a living organism. This study was conducted in order to evaluate the level of MDA in blood and urine of mice exposed to silica dust.

Material and Methods:

In this experimental study, 72 mice (BALB/c) were randomly allocated to five exposed groups and 1 control group. Exposure of mice to pure 99% silica dust was done in closed containers. Blood sampling was performed from the heart of mice and urine sampling fulfilled by insertion into a metabolic cage. The RAOet al. method was used to measure MDA.

Results:

The highest level of plasma MDA in group 1 in the 4thmonth was 8.4±0.41 nmol/l and the lowest level of MDA was 1.3±0.2 nmol/l in the third sampling in the control group, also the highest amount of urine MDA in the first and second groups and 4 months after exposure was 1.16±0.51 nmol/l, and the lowest in the control group and in the third sampling was 0.48±0.06 nmol/l. A significant difference was found between the levels of MDA in all exposed groups at different times except for the 5thgroup with the lowest concentration (P< 0.05).

Conclusion:

MDA in blood and urine could be proposed as a good biomarker for the evaluation of biochemical damages caused by silica dust. Measuring MDA is also a simple and inexpensive method that does not require complex equipment and can be used as an early detection test for biochemical damages caused by silica.

A review of the fate of inhaled α-quartz in the lungs of rats

The distribution of dust particles within the lungs and their excretion are highly associated with their pulmonary toxicity. Literature was reviewed to discern pulmonary translocation pathways for inhaled α-quartz compared to those for inhaled TiO₂. Accordingly, it was hypothesized α-quartz particles in the alveoli were phagocytized by alveolar macrophages but silica-containing macrophages remained in the alveoli for longer time in contrast to the rapid elimination from the alveoli seen for TiO₂-containing macrophages. In addition, it was presumed that free silica particles are translocated in the interstitium, possibly through the cytoplasm of Type I epithelial cells, as observed with TiO₂. Free silica particles are presumed to be phagocytized by interstitial macrophages soon after the particles penetrate the interstitium; these dust cells are then translocated to the ciliated airway regions in the lumen through bronchus-associated lymphoid tissue (BALT). The pulmonary retention half-time of dust particles in rats exposed to α-quartz is several times longer than that of rats exposed to TiO₂, as long as the lung dust burden is ≈ 3 mg. The reduced pulmonary particle clearance ability in rats exposed to α-quartz aerosol is presumably attributed to the long-term retention of dust cells both in the alveoli and in the interstitium; this retention may be caused by the reduced chemotactic abilities of α-quartz-containing dust cells. However, the accumulation of α-quartz-containing dust cells in the lungs is not associated with the occurrence of pulmonary inflammation.

Genome-wide analysis of aberrantly expressed microRNAs in bronchoalveolar lavage fluid from patients with silicosis

Background To identify differentially expressed miRNAs profiles in bronchoalveolar lavage fluid (BALF) from patients with silicosis and consider the potential contribution of miRNAs to silicosis.Methods miRNAs expression profiling were performed in the cell fraction of BALF samples obtained from 9 subjects (3 silicosis observation subjects, 3 stage I and stage II silicosis patients, respectively). The differential expression of two selected miRNAs hsa-miR-181c-5p and hsa-miR-29a-3p were confirmed by RT-qPCR. Furthermore, miRNAs Gene Ontology Enrichment categories and target mRNAs were determined based on miRWalk.Results We found 110 dysregulated miRNAs in silicosis samples, most of which showed a down-regulation trend. Microarray results were confirmed by RT-qPCR. With the observation group samples set as standards, stage I samples showed 123 differentially expressed miRNAs, and stage II 46. 23 miRNAs were dysregulated in both stages. Finally, functional enrichment analysis indicated that these miRNAs played an important role in various biological processes, including ECM-receptor interaction and endocytosis.Conclusions This is the first time to acquire the BALF-derived microRNAs expression profiling targeting to human silicosis. These results contribute to unravelling miRNAs involved in the pathogenesis of silicosis, and provide new tools of potential use of as biomarkers for diagnosis and/or therapeutic purposes.

Roles of microRNA-146a and microRNA-181b in regulating the secretion of tumor necrosis factor-α and interleukin-1β in silicon dioxide-induced NR8383 rat macrophages

Despite increasing evidence to suggest that microRNA (miR)-146a and miR-181b are involved in the regulation of immune responses and tumor progression, their roles in silicosis remain to be fully elucidated. Therefore, the present study examined the roles of miR-146a and miR-181b in inflammatory responses, and their effect on the expression of the tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) inflammatory chemokines in silicon dioxide (SiO₂)-induced NR8383 rat macrophages. Alterations in the expression levels of miR-146a and miR-181b in rats with silicosis have been previously investigated using miRNA arrays. In the present study, the expression levels of miR-146a and miR-181b were assessed using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The NR8383 cells were transfected with miRNA-146a and miR-181b mimics or inhibitors, and the cells and culture supernatants were collected following SiO₂ treatment for 12 h. The expression levels of TNF-α and IL-1β were detected using western blotting, RT-qPCR and ELISA. Analysis of variance and Student's two-tailed t-test were used to perform statistical analyses. The expression level of miR-146a was significantly increased, while the expression level of miR-181b was significantly decreased in the fibrotic lungs of the rats with silicosis, compared with the levels in the normal rats. It was observed that, following treatment of the NR8383 cells with SiO₂ for 12 h, the levels of TNF-α were significantly increased following miR-181b knockdown and the levels of IL-1β were significantly increased following miR-146a knockdown, compared with the inhibitor-treated controls (P<0.05). By contrast, miR-181b mimic transfection led to a significant reduction in the levels of TNF-α (P<0.05), and miR-146a mimics were responsible for the decrease in IL-1β (P<0.05). The results of the present study provide evidence supporting the roles of miR-146a and miR-181b in the pathogenesis of silicosis, and suggest that they may be candidate therapeutic target in this disease.

Comparing the role of silica particle size with mineral fiber geometry in the release of superoxide from rat alveolar macrophages

Particulate air pollutants and mineral fibers activate inflammatory cells to release oxidants, which contribute to inflammation and injury in the lower respiratory tract. Our aim was to compare the role of silica particle size with mineral fiber length and width in the ability to induce superoxide release from rat alveolar macrophages. We estimated the ability of four types of silica particle samples, with different mode diameter, and three types of mineral fiber samples, with different geometric mean lengths and widths, to induce lucigenin-dependent chemiluminescence (CL) from the cells per number of dust particles (i.e., silica particles and mineral fibers). A close positive correlation was observed between dust size and the ability to induce CL in silica as well as mineral fiber samples. Moreover, the ability of silica samples to induce CL was weaker than that of long mineral fiber sample. This ability increased at a larger rate in small silica particle and thin mineral fiber samples than in large silica particle and thick mineral fiber samples at the initial stage of administration. These results suggest that the kinetics of the induction superoxide release from macrophages is similar between silica particles and mineral fibers; moreover, this depends on silica particle size and mineral fiber geometry. Finally, large silica particles were more active than small ones.

High-dose N-acetylcysteine decreases silica-induced lung fibrosis in the rat

Objective

To study the potential of high-dose N-acetylcysteine (NAC) to attenuate silica-induced pulmonary fibrosis in the rat.

Methods

Rats exposed to intratracheal instillation of silica particles were treated with 500 mg/kg NAC orally every day for 7 days, before and up to 28 days after silica administration ( n = 32), or received no treatment following silica exposure ( n = 32); a third group received intratracheal saline ( n = 32). Fibrosis score, and hydroxyproline (HYP) and malondialdehyde (MDA) content, were assessed in lung tissue. Bronchoalveolar lavage fluid (BALF) and serum levels of tumour necrosis factor (TNF)-α, interleukin (IL)-8 and high-sensitivity C-reactive protein (hsCRP) were assessed by enzyme-linked immunosorbent assay.

Results

Histopathology revealed inflammation and fibrosis in lung tissue from rats exposed to silica, but not in saline controls. The fibrosis score was significantly lower in animals treated with NAC compared with silica-exposed untreated rats. HYP and MDA content were significantly lower at all timepoints, following NAC treatment versus no treatment, in silica-exposed rats. NAC attenuated silica-induced increases in TNF-α, IL-8 and hsCRP in BALF and serum.

Conclusions

Oral treatment with high-dose NAC during early silica exposure can ameliorate the activity of proinflammatory cytokines, thus attenuating subsequent lung fibrosis. These results suggest that NAC has potential as a treatment for silica-induced lung fibrosis.

Regulation of TXB(2) and PGE(2) production by TGF-beta(1) in in vitro silica dust-exposed rat alveolar macrophage

We investigated the effect of transforming factor factor-beta(1) (TGF-beta(1)) on thromboxane B(2) (TXB(2)) and prostaglandin E(2) (PGE(2)) production in in vitro silica dust-exposed rat alveolar macrophages (AM). In the presence of 5 mug of anti-TGF-beta(1) antibodies, TXB(2) production decreased, but PGE(2) production increased. Addition of 2 ng of TGF-beta(1) to the culture medium potentiated TXB(2) production, but PGE(2) production apparently did not change. At 50 ng of TGF-beta(1), TXB(2) production decreased, and PGE(2) production varied. Our data suggest that in rat AM: (1) both endogenous and exogenous TGF-beta(1) regulate TXB(2) production; and (2) in the absence of endogenous TGF-beta(1) the liberation of PGE(2) increases; however, exogenous TGF-beta(1) does not have a regulatory effect on PGE(2).

Anti-inflammatory effects of Stephania tetrandra S. Moore on interleukin-6 production and experimental inflammatory disease models

Deregulation of interleukin-6 (IL-6) expression caused the synthesis and release of many inflammatory mediators. It is involved in chronic inflammation, autoimmune diseases, and malignancy. Stephania tetrandra S. Moore is a Chinese medicinal herb which has been used traditionary as a remedy for neuralgia and arthritis in China. To investigate the anti-inflammatory effects of S. tetrandra S. Moore in vitro and in vivo, its effects on the production of IL-6 and inflammatory mediators were analysed. When human monocytes/macrophages stimulated with silica were treated with 0.1–10 μg/ml S. tetranda S. Moore, the production of IL-6 was inhibited up to 50%. At these concentrations, it had no cytotoxicity effect on these cells. It also suppressed the production of IL-6 by alveolar macrophages stimulated with silica. In addition, it inhibited the release of superoxide anion and hydrogen peroxide from human monocytes/macrophages. To assess the anti-fibrosis effects of S. tetrandra S. Moore, its effects on in vivo experimental inflammatory models were evaluated. In the experimental silicosis model, IL-6 activities in the sera and in the culture supernatants of pulmonary fibroblasts were also inhibited by it. In vitro and in vivo treatment of S. tetrandra S. Moore reduced collagen production by rat lung fibroblasts and lung tissue. Also, S. tetrandra S. Moore reduced the levels of serum GOT and GPT in the rat cirrhosis model induced by CCL₄, and it was effective in reducing hepatic fibrosis and nodular formation. Taken together, these data indicate that it has a potent anti-inflammatory and antifibrosis effect by reducing IL-6 production.

Fas/FasL pathway-mediated alveolar macrophage apoptosis involved in human silicosis

In vitro and in vivo studies have demonstrated that lung cell apoptosis is associated with lung fibrosis; however the relationship between apoptosis of alveolar macrophages (AMs) and human silicosis has not been addressed. In the present study, AM apoptosis was determined in whole-lung lavage fluid from 48 male silicosis patients, 13 male observers, and 13 male healthy volunteers. The relationships between apoptosis index (AI) and silica exposure history, soluble Fas (sFas)/membrane-bound Fas (mFas), and caspase-3/caspase-8 were analyzed. AI, mFas, and caspase-3 were significantly higher in lung lavage fluids from silicosis patients than those of observers or healthy volunteers, but the level of sFas demonstrated a decreasing trend. AI was related to silica exposure, upregulation of mFas, and activation of caspase-3 and -8, as well as influenced by smoking status after adjusting for confounding factors. These results indicate that AM apoptosis could be used as a potential biomarker for human silicosis, and the Fas/FasL pathway may regulate this process. The present data from human lung lavage samples may help to understand the mechanism of silicosis and in turn lead to strategies for preventing or treating this disease.

An overview of caspase: Apoptotic protein for silicosis

Silicosis is a chronic lung disease characterized by granulomatous and fibrotic lesions, which occurs due to accumulation of respirable silica mineral particles. Apoptosis is an important phenomenon of cell death in silicosis. The relationship between silica dust and its exposure is well established. But, the complex chain of cellular responses, which leads to caspase activation in silicosis, has not been fully discovered. Caspase activation plays a central role in the execution of apoptosis. Silica-induced apoptosis of the alveolar macrophages could potentially favor a proinflammatory state, occurring in the lungs of silicotic patients, resulting in the activation of caspase prior to induction of the intrinsic and extrinsic apoptosis pathways. Recent studies indicated that apoptosis may involve in pulmonary disorders. This review summarizes the current knowledge about the underling mechanism of biochemical pathways in caspase activation that have been ignored so far in silicosis. In addition, caspase could be a key apoptotic protein that can be used as an effective biomarker for the study of occupational diseases. It may provide an important link in understanding the molecular mechanisms of silica-induced lung pathogenesis.

The acute and long-term effects of Middle East sand particles on the rat airway following a single intratracheal instillation

Military personnel deployed in the Middle East have emphasized concerns regarding high levels of dust generated from blowing desert sand and the movement of troops and equipment. Airborne particulate matter levels (PM(10); PM < 10 μm) in the region may exceed 1500 μg/m(3), significantly higher than the military exposure guideline (MEG) of 50 μg/m(3). Increases in PM(10) have been linked to a rise in incidences of asthma, obstructive pulmonary disease, lung cancer, and cardiovascular diseases. Male Sprague-Dawley rats received a single intratracheal (IT) instillation of 1, 5, or 10 mg of Middle East PM(10) collected at a military occupied site in Kuwait, silica (positive control), or titanium dioxide (TiO(2); negative control) suspended in 400 μl sterile saline, or saline alone (vehicle control). Twenty-four hours, 3 d, 7 d and 6 mo postexposure (n = 15/group), organs including lung were evaluated for histopathological changes and for particle contaminants. Bronchoalveolar fluid (BALF) was also analyzed for cellular and biochemical parameters, including cytokines and chemokines. Instillation of silica resulted in early, pronounced, sustained inflammation indicated by significant increases in levels of total protein and neutrophils, and activities of lactate dehydrogenase activity and β-glucuronidase activity. Lower magnitude and transient changes using the same markers were observed in animals exposed to TiO(2) and Middle East PM(10). The results suggest that for acute exposures, this Middle East PM(10) is a nuisance-type dust with relatively low toxicity. However, since average deployment of military personnel to the Middle East is 180 d with potential for multiple follow-on tours, chronic exposure studies are needed to fully understand the pulmonary effects associated with Middle East PM exposure.

IL-1beta differently involved in IL-8 and FGF-2 release in crystalline silica-treated lung cell co-cultures

Background

Inhalation of crystalline silica particles is in humans associated with inflammation and development of fibrosis. The aim of the present study was to investigate the effect of crystalline silica on the release of the fibrosis- and angiogenesis-related mediator FGF-2 and the pro-inflammatory mediator IL-8, and how IL-1β and TNF-α were involved in this release from various mono- and co-cultures of monocytes, pneumocytes and endothelial cells.

Results

Silica exposure induced an increase of IL-8 release from monocytes and from pneumocytes alone, and the FGF-2 level in the medium increased upon silica exposure of pneumocytes. Both the responses were enhanced in non-contact co-cultures with endothelial cells. The FGF-2 release seemed to increase with the silica-induced decrease in the number of pneumocytes. The release of IL-8 and FGF-2 was partially suppressed in cultures with pneumocytes in contact with monocytes compared to non-contact cultures. Treatment with anti-TNF-α and the IL-1 receptor antagonist revealed that release of IL-1β, and not TNF-α, from monocytes dominated the regulation of IL-8 release in co-cultures. For release of FGF-2, IL-1ra was without effect. However, exogenous IL-1β reduced the FGF-2 levels, strongly elevated the FGF-2-binding protein PTX3, and prevented the reduction in the number of pneumocytes induced by silica.

Conclusion

IL-1β seems to be differently involved in the silica-induced release of IL-8 and FGF-2 in different lung cell cultures. Whereas the silica-induced IL-8 release is regulated via an IL-1-receptor-mediated mechanism, IL-1β is suggested only indirectly to affect the silica-induced FGF-2 release by counteracting pneumocyte loss. Furthermore, the enhanced IL-8 and FGF-2 responses in co-cultures involving endothelial cells show the importance of the interaction between different cell types and may suggest that both these mediators are important in angiogenic or fibrogenic processes.

Understanding the chemical properties of macerals and minerals in coal and its potential application for occupational lung disease prevention

Recent increases in oil price further strengthen the argument that coal and coal products will play an increasingly important role in fulfilling the energy needs of our society. Coal is an aggregate of heterogeneous substances composed of organic (macerals) and inorganic (minerals) materials. The objective of this review was to assess whether some chemical parameters in coal play a role in producing environmental health problems. Basic properties of coal--such as chemical forms of the organic materials, structure, compositions of minerals--vary from one coal mine region to another as well as from coals of different ranks. Most importantly, changes in chemical properties of coals due to exposure to air and humidity after mining--a dynamic process--significantly affect toxicity attributed to coal and environmental fate. Although coal is an extremely complex and heterogeneous material, the fundamental properties of coal responsible for environmental and adverse health problems are probably related to the same inducing components of coal. For instance, oxidation of pyrite (FeS2) in the coal forms iron sulfate and sulfuric acid, which produces occupational lung diseases (e.g., pneumoconiosis) and other environmental problems (e.g., acid mine drainage and acid rain). Calcite (CaCO3) contained in certain coals alters the end products of pyrite oxidation, which may make these coals less toxic to human inhalation and less hazardous to environmental pollution. Finally, knowledge gained on understanding of the chemical properties of coals is illustrated to apply for prediction of toxicity due to coal possibly before large-scale mining and prevention of occupational lung disease during mining.

Interleukin-12 is not essential for silicosis in mice

BACKGROUND

Silicosis features foci of inflammation where macrophages and lymphocytes precede and accompany fibroblast proliferation, alveolar epithelial hyperplasia, and increased deposition of connective tissue matrix material. In the mouse following silica inhalation there is recruitment of natural killer-, B-, and CD4+ and CD8+ lymphocytes to the alveolar spaces, enlargement of bronchial-associated lymphoid tissues (BALT), and aggregation of lymphocytes surrounding small airways and blood vessels. A substantial fraction of the recruited lung lymphocytes produce interferon-gamma (IFN-gamma), and IFN-gamma gene-deleted mice develop less silicosis than wild-type mice. Interleukin-12 (IL-12) is an important pathway for driving the adaptive immune response towards a TH1-like phenotype. We hypothesized that IL-12 might stimulate lymphocyte activation and the up-regulation of IFN-gamma, and consequently be an essential mediator for silicosis.

RESULTS

C57Bl/6 wild-type (WT) and IL-12 deficient (IL-12 KO) mice were exposed to sham-air or crystobalite silica (61 mg/m3) by inhalation for 5 hours/day for 12 days and then studied from 1 to 112 days after exposure. Mice exposed to sham-air had normal lung histology at all time points. WT mice exposed to titanium dioxide (72 mg/m3) showed pulmonary macrophage recruitment but no increase in lung collagen. Both WT and IL-12 KO mice exposed to silica showed similar progressive lung pathology, increased wet lung weight and increased total lung collagen (hydroxyproline). IL-12 p35 mRNA was not increased in either strain after silica exposure; IL-12 p40 mRNA was up-regulated after silica in WT mice and constitutively absent in the IL-12 KO mice. IL-18 mRNA was not increased after silica exposure. The expression of IL-15 (an important driver for innate immunity, Natural Killer cell activation, and IFN-gamma production) was abundant in air-exposed mice and was increased slightly in the lungs of mice with silicosis.

CONCLUSION

The axis of IL-12 driving IFN-gamma production is not essential for the full manifestations of silicosis in mice exposed to a crystobalite silica aerosol.

Roles of plasma proteins in the formation of silicotic nodules in rats

The contribution of plasma protein(s) to the stabilization of fibroids formed in rat lungs exposed to acute silica dust inhalation was examined. Antibodies against component proteins of the nodules remaining insoluble in 2% SDS, 10M urea and 40 mM sulfhydryl reagents under prolonged boiling conditions were raised in rabbits and used to capture plasma proteins, which were identified by 2D-gel electrophoresis and MALDI-TOF analysis. The silica particles were encapsulated with extracellular protein composites whose amino acid compositions showed high levels of alanine, i.e., above those of glycine and proline, a building block of collagen. Antibody-captured plasma proteins showed the dominant presences of fibrinogen, albumin, and prealbumin (transthyretin), and other minor proteins, which included alpha-1-protease inhibitor, contraspin-like protease inhibitor, cathepsin B, etc. The presence of the N(epsilon)-(gamma-glutamyl) lysine isopeptide bond in the nodules was evidenced by direct chemical methods and by immunoreactivity for anti-isopeptide bonds. Immunostaining of affected lung tissue and of the fibroid regions showed elevated levels of transglutaminase (TGase) E and plasma factor XIII (F-XIII), but showed no reactivity towards other TGases. These findings suggest that the silica encapsulated nodules are a mixture of extracellular proteins that include collagen type I, fibrin and transthyretin, which is stabilized by TGase catalyzed crosslinking between plasma and extracellular proteins during fibrosis to eventually form insoluble nodules.

Plasma malondialdehyde and erythrocyte glutathione levels in workers with cement dust-exposure [corrected]

BACKGROUND

Chronic exposure to cement dust has been reported to lead to several health problems. Acute and chronic exposure to quartz are associated with the provocation of an inflammatory response and triggers an extensive host defense mechanism. These inflammatory reactions result in the secretions of cytokines, eicosanoids, lytic enzymes, chemotactic factors and reactive oxygen species (ROS). This study was designed to investigate the plasma oxidant and antioxidant status in cement plant workers.

METHODS

Forty-eight non-smoker volunteer male cement plant workers and 28 non-smoker volunteer office male workers (control) aged between 27 and 56 were recruited. The concentrations of plasma malondialdehyde (p-MDA), and the erythrocyte glutathione (GSH) were measured in both groups. Pulmonary function tests, and exposed free silica fractions were also measured in different working places.

RESULTS

Plasma MDA levels were found to be increased, while erythrocyte GSH levels to be decreased in cement workers (p < 0.001). Although no statistically significance was observed, pulmonary function tests were found to be decreased in cement workers. A negative correlation was observed between MDA levels and FEV1 and FEV1% levels. Although no statistically significant difference was found, MDA levels were found to be increased and GSH levels decreased in the working areas where silisium dioxide concentrations were found to be higher.

CONCLUSIONS

In conclusion, the results presented in this study show that direct measurement of plasma MDA and erythrocyte GSH could be accepted as an indicator of oxidative injury in workers exposed to cement dust.

Inflammatory response and cathepsins in silica-exposed Hermansky-Pudlak syndrome model pale ear mice

Hermansky-Pudlak syndrome (HPS) is a hereditary disorder involving the sorting processes of intracellular organelles such as lysosomes of reticuloendothelial cells. Pale ear (ep) mouse is known to have the HPS1 gene mutation, which is seen in patients with HPS and pulmonary fibrosis. As pulmonary fibrosis is not spontaneously observed in ep mice, we hypothesized that external stimuli are necessary for the genetic predisposition of its development. We used silica as the external stimulus to induce the alveolar macrophage-mediated inflammatory response and evaluated the pathological changes of the lung and biochemical analysis of collagenolytic lysosomal enzymes cathepsins L and B in ep mice. Treatment with silica induced the following: persistent accumulation of activated macrophages; delayed clearance of silica from alveolar spaces; and increased collagen fibers in alveolar tissues, which were shown with trichrome staining in ep mice. The comparison of bronchoalveolar lavage cells between the naïve ep and control mice revealed: decreased enzymatic activities but increased antigenic levels of cathepsins L and B, resulting in significantly lower ratios of activity to antigen; increased ceroid deposits and cathepsin L antigens in lysosomes; and no abnormal forms of cathepsins were detected. After silica instillation, activities of cathepsin L in the ep mice increased but ratios of activity to antigen were still significantly low. These phenomena induced by silica suggest that external stimuli bring forth fibrogenesis in the animal models or humans that have HPS1 gene mutation.

Estudo de imunoglobulinas, complementos e auto-anticorpos em 58 trabalhadores expostos à sílica

INTRODUÇÃO: A silicose é a doença pulmonar ocupacional de maior prevalência em nosso meio. O agente patogênico da silicose é a poeira de sílica-livre ou dióxido de silício (SiO2) na forma cristalina. O processo inflamatório envolvendo o sistema imunológico na silicose ainda não está bem esclarecido. OBJETIVO: Estudar o perfil de IgG, IgM, IgA, C3, C4 e alguns auto-anticorpos no soro de trabalhadores expostos à sílica, com ou sem silicose, através de avaliação laboratorial imunológica, abrangendo tanto a imunidade inespecífica quanto a específica. MÉTODO: Foi examinada uma amostra de 58 pacientes ambulatoriais, do sexo masculino, constituída por trabalhadores expostos à sílica. Foram realizadas avaliações imunológica, radiológica e funcional pulmonar. Foram dosadas as imunoglobulinas IgG, IgA e IgM, os componentes C3 e C4 do sistema complemento e determinados auto-anticorpos. RESULTADOS: Vinte trabalhadores apresentaram radiograma normal e 38 apresentaram-no alterado, compatível com silicose. As médias dos valores de IgG no grupo com silicose foi maior que no grupo com radiograma normal (p < 0,05). Para a IgA e IgM, assim como para o C3 e C4, não houve diferença estatisticamente significativa nas médias (p > 0.05). O percentual de positividade dos auto-anticorpos foi maior no grupo silicótico em relação ao grupo com radiograma normal. CONCLUSÃO: O aumento de IgG nos doentes com silicose constitui um achado importante pois pode indicar a continuidade da reação granulomatosa, mesmo com o trabalhador afastado da exposição. Entretanto, ainda são necessários estudos que possibilitem a compreensão do processo imunogênico na silicose.

Phosphorylation of tumor necrosis factor receptor 1 (p55) protects macrophages from silica-induced apoptosis

Macrophages play a fundamental role in silicosis in part by removing silica particles and producing inflammatory mediators in response to silica. Tumor necrosis factor alpha (TNFalpha) is a prominent mediator in silicosis. Silica induction of apoptosis in macrophages might be mediated by TNFalpha. However, TNFalpha also activates signal transduction pathways (NF-kappaB and AP-1) that rescue cells from apoptosis. Therefore, we studied the TNFalpha-mediated mechanisms that confer macrophage protection against the pro-apoptotic effects of silica. We will show that exposure to silica induced TNFalpha production by RAW 264.7 cells, but not by IC-21. Silica-induced activation of NF-kappaB and AP-1 was only observed in RAW 264.7 macrophages. ERK activation in response to silica exposure was only observed in RAW 264.7 macrophages, whereas activation of p38 phosphorylation was predominantly observed in IC-21 macrophages. No changes in JNK activity were observed in either cell line in response to silica exposure. Silica induced apoptosis in both macrophage cell lines, but the induction of apoptosis was significantly larger in IC-21 cells. Protection against apoptosis in RAW 264.7 cells in response to silica was mediated by enhanced NF-kappaB activation and ERK-mediated phosphorylation of the p55 TNFalpha receptor. Inhibition of these two protective mechanisms by specific pharmacological inhibitors or transfection of dominant negative mutants that inhibit IkappaBalpha or ERK phosphorylation significantly increased silica-induced apoptosis in RAW 264.7 macrophages. These data suggest that NF-kappaB activation and ERK-mediated phosphorylation of the p55 TNF receptor are important cell survival mechanisms in the macrophage response to silica exposure.

Comparative Toxicity of Standard Nickel and Ultrafine Nickel in Lung after Intratracheal Instillation

A comparison was made of the bronchoalveolar lavage fluid (BALF) response to ultrafine nickel (Uf-Ni) and standard-sized nickel (Std-Ni). Rats were intratracheally instilled with 0, 0.1, 0.5, 1 and 5 mg Uf-Ni and Std-Ni, respectively. At 3 d after instillation, the body weight and wet lung weight were determined. At the same time, BALF was analyzed for lactate dehydrogenase (LDH), total protein (TP), tumor necrosis factor-alpha (TNF-alpha), and total cell and differential cell counts. The results showed that indicators of lung injury and inflammation in BALF were markedly raised with increased Uf-Ni and Std-Ni for each from 0 to 1 mg, and there were no differences in the indices between instillation of Uf-Ni at 1 mg and 5 mg. The results also showed that the effects of Uf-Ni on the indices were significantly higher than those of Std-Ni. Additional groups of rats were intratracheally instilled with 1 mg of Uf-Ni or Std-Ni, and wet lung weight and BALF profiles were analyzed at 1, 3, 7, 15 and 30 d later. The effect of Uf-Ni and Std-Ni on indices that can be presumed to reflect epithelial injury and permeability (LDH or TP), and release of proinflammatory cytokine (TNF-alpha) were increased throughout the 30 d post-exposure and the effects of Uf-Ni on these indices were significantly higher than those of Std-Ni from 1 to 30 d after instillation. Moreover, the number of neutrophils and LDH activity in BALF of rats after exposure to Uf-Ni were significantly greater than those of Std-Ni-exposed rats up to 30 d after instillation. Our findings suggest that Uf-Ni has a much more toxic effect on the lung than St-Ni, but the mechanism remains to be elucidated.

Alterations in immune parameters in foundry and pottery workers

To assess the immune competence of workers occupationally exposed to mainly silica, peripheral blood lymphocytes, serum immunoglobulins (IgG, IgA and IgM), C3 and C4 complement protein concentrations of foundry and pottery workers were evaluated and compared to healthy controls with no history of silica and other chemical exposure. The absolute number and percentage of functionally different subsets of peripheral blood mononuclear lymphocytes, i.e. T, T-suppressor and natural killer cells were unchanged. However, T-helper lymphocytes in pottery (P<0.05) and B cells in foundry (P<0.01) workers were significantly lower when compared to their controls. In addition, silica-exposed foundry workers had a significant reduction in the IgG, IgA and IgM levels. No significant differences were observed in the serum complement C3 and C4 levels of the workers. These results suggest that human chronic exposure to mainly silica and other chemicals originating from foundry and pottery settings may be detrimental to the immune system.

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.

Interleukin-9 reduces lung fibrosis and type 2 immune polarization induced by silica particles in a murine model

We examined the effect of interleukin (IL)-9, a cytokine active on B and T lymphocytes and associated with bronchial asthma, on the development of lung fibrosis induced by crystalline silica particles. Therefore, we compared the response to silica (1 and 5 mg/animal, intratracheally) in transgenic mice that constitutively express high levels of IL-9 (Tg5) and their wild-type counterparts (FVB). At 2 and 4 mo after treatment with silica, histologic examination and measurement of lung hydroxyproline content showed that the severity of fibrosis was significantly less important in Tg5 mice than in their wild-type counterparts. Intraperitoneal injection of IL-9 in C57BL/6 mice also reduced the amplitude of silica-induced lung fibrosis. The reduction of lung fibrosis by IL-9 was associated with a significant expansion of the B-lymphocyte population, both in bronchoalveolar lavage (BAL) and in the pulmonary parenchyma. In wild-type animals, silica-induced fibrosis correlated with markers of a T helper 2-like response such as upregulation of IL-4 levels in lung tissue and an increased immunoglobulin (Ig) G1/IgG2a ratio in BAL. Immunohistochemical studies demonstrated that the upregulation of IL-4 associated with the development of fibrosis was mainly localized in inflammatory alveolar macrophages. In transgenic mice, the level of IL-4 in lung homogenates was not significantly affected by silica treatment, and a reduced IgG1/IgG2a ratio was observed upon treatment with silica. The levels of interferon-gamma were significantly decreased after silica treatment in both strains. Together, these observations point to an antifibrotic effect of IL-9 in pulmonary fibrosis associated with a limitation of the type 2 polarization which accompanies lung fibrosis.

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.

Rat alveolar and interstitial macrophages in the fibrosing stage following quartz exposure

Exposure to quartz induces pulmonary inflammation and development of fibrosis. In order to study the fibrosing process, we investigated morphology, function and phenotype of alveolar (AMs) and interstitial (IMs) macrophages at an early stage of fibrosis in rats. Rats were exposed by intratracheal instillations of 10 mg quartz (n=8) or saline (n=8) and studied 3 months later. AMs were obtained by bronchoalveolar lavage and IMs by mechanical fragmentation, followed by enzymatic digestion of lung tissue. Histology revealed subacute silicosis, with early focal fibrosis and alveolar lipoproteinosis. AM quartz exposure increased phagocytic activity and expression of major histocompatibility complex (MHC) Ia antigens, the latter being associated with cellular antigen presenting capacity. IM had an even more pronounced expression of MHC than AM after quartz exposure. Both macrophage fractions had a higher expression of OX-42 (complement receptor 3, CR3) than controls, but the increase in the IM fraction might be explained by the remaining AM in the IM fraction. Exposed AM adhered less to extracellular matrix components (vitronectin and fibronectin) than controls. In contrast, the adhesion of IM to vitronectin increased after exposure. Besides increased adhesion, the effects on IM were scarce. Our results therefore do not support the hypothesis that IM has a key role in the process of inflammation, including fibrosis.

Surfactant protein A prevents silica-mediated toxicity to rat alveolar macrophages

Silicosis is a serious occupational lung disease associated with irreversible pulmonary fibrosis. The interaction between inhaled crystalline silica and the alveolar macrophage (AM) is thought to be a key event in the development of silicosis and fibrosis. Silica can cause direct injury to AMs and can induce AMs to release various inflammatory mediators. Acute silicosis is also characterized by a marked elevation in surfactant apoprotein A (SP-A); however, the role of SP-A in silicosis is unknown. We investigated whether SP-A directly affects the response of AMs to silica. In this study, the degree of silica toxicity to cultured rat AMs as assessed by a (51)Cr cytotoxicity assay was shown to be dependent on the time of exposure and the concentration and size of the silica particles. Silica directly injured rat AMs as evidenced by a cytotoxic index of 32.9 +/- 2.5, whereas the addition of rat SP-A (5 microg/ml) significantly reduced the cytotoxic index to 16.6 +/- 1.2 (P < 0. 001). This effect was reversed when SP-A was incubated with either polyclonal rabbit anti-rat SP-A antibody or D-mannose. These data indicate that SP-A mitigates the effect of silica on AM viability, and this effect may involve the carbohydrate recognition domain of SP-A. The elevation of SP-A in acute silicosis may serve as a normal host response to prevent lung cell injury after exposure to silica.

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.

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.

Expansion of interferon-gamma-producing lung lymphocytes in mouse silicosis

Silicosis is characterized by mononuclear cell inflammation with macrophage activation, accumulation of lymphocytes, and fibrosis. Interferon-gamma (IFN-gamma) is a lymphocyte cytokine with broad effects, particularly macrophage activation. Mice exposed to an aerosol of cristobalite silica (70 mg/m3, 12 d, 5 h/d) developed diffuse pulmonary pathologic changes with macrophage, lymphocyte, and neutrophil recruitment, and increased lung collagen. IFN-gamma messenger RNA (mRNA) was more abundant by semiquantitative reverse transcription-polymerase chain reaction in the lungs of silica-exposed mice than in control animals. IFN-gamma mRNA transcripts were detected by in situ hybridization with digoxigenin-labeled complementary DNA probes in normal mouse lung tissue within bronchial-associated lymphoid tissues (BALT). In silica- exposed mice, mononuclear cells with IFN-gamma mRNA were more numerous in the silicotic lesions and enlarged BALT structures. Lung-cell suspensions were prepared by enzyme digestion, stained with fluorescent-labeled antibodies against intracellular cytokines, and enumerated by flow cytometry. The percentage of cells producing IFN-gamma was increased in silicotic mice (19% versus 11%). Interleukin (IL)-4 mRNA transcripts were less abundant in the lung tissue from silica-exposed mice than in control mice. Cells staining for IL-4 mRNA were found rarely in either the air-sham or the silica-exposed mouse lungs, and almost all appeared to be within BALT structures. Approximately 3% of cells stained for IL-4 in the digested lungs from both groups. Similar cytokine patterns were observed in mediastinal lymph node/thymus and spleen tissues. The augmented IFN-gamma response, with IL-4 unchanged or decreased, in the lung lesions and lymphoid tissue of mice with silicosis suggests a Th-1-like lymphocyte-mediated immune-inflammatory response.

Regulation of extracellular matrix synthesis by TNF-alpha and TGF-beta1 in type II cells exposed to coal dust

Type II pulmonary epithelial cells respond to anthracite coal dust PSOC 867 with increased synthesis of extracellular matrix (ECM) components. Alveolar macrophages modulate this response by pathways that may involve soluble mediators, including tumor necrosis factor-alpha (TNF-alpha) or transforming growth factor-beta1 (TGF-beta1). The effects of TNF-alpha (10 ng/ml) and/or TGF-beta1 (2 ng/ml) were thus investigated in dust-exposed primary type II cell cultures. In control day 1 or day 3 cultures, TNF-alpha and/or TGF-beta1 had little or no effect on the synthesis of type II cellular proteins, independent of whether the cells were exposed to dust. With PSOC 867 exposure, where ECM protein synthesis is elevated, TNF-alpha and TGF-beta1 further increased both the absolute and relative rates of ECM synthesis on day 3 but had little effect on day 1. Each mediator increased expression of fibronectin mRNA, as well as of ECM fibronectin content, in a manner qualitatively similar to their effects on synthesis. Thus TNF-alpha and TGF-beta1 modulate both ECM synthesis and fibronectin content in coal dust-exposed type II cell cultures.

Oxidative stress in silicosis: evidence for the enhanced clearance of free radicals from whole lungs

We hypothesized that reactive oxygen species (ROS) may be involved in the pathogenesis of silicosis. To investigate ROS' dependent pathophysiological processes during silicosis we studied the kinetic clearance of instilled stable nitroxide radicals (TEMPO). Antioxidant enzymes' superoxide dismutase (SOD) and glutathione peroxidase (GPx), and lipid peroxidation were also studied in whole lungs of rats exposed to crystalline silica (quartz) and sham exposed controls. Low frequency L-band electron spin resonance spectroscopy was used to measure the clearance of TEMPO in whole-rat lungs directly. The clearance of TEMPO followed first order kinetics showing significant differences in the rate for clearance between the diseased and sham exposed control lungs. Comparison of TEMPO clearance rates in the sham exposed controls and silicotic rats showed an oxidative stress in the rats exposed to quartz. Studies on the antioxidant enzymes SOD and GPx in the lungs of silicotic and sham exposed animals supported the oxidative stress and accelerated clearance of TEMPO by up regulated levels of enzymes in quartz exposed animals. Increased lipid peroxidation potential in the silicotics also supported a role for enhanced generation of ROS in the pathogenesis of silica-induced lung injury. These in vivo experiments directly demonstrate, for the first time, that silicotic lungs are in a state of oxidative stress and that increased generation of ROS is associated with enhanced levels of oxidative enzymes and lipid peroxidation. This technique offers great promise for the elucidation of ROS induced lung injury and development of therapeutic strategies for the prevention of damage.

Non-fibrous inorganic particles in bronchoalveolar lavage fluid of pottery workers

AIM

To study the actual exposure of pottery workers to silica particles, as their risk of silicosis is potentially high because of the presence of inhalable crystalline silica particles in the workplace.

METHODS

Nine pottery workers underwent bronchoalveolar lavage. The recovered fluid was analysed for cytological and mineralogical content by analytical transmission electron microscopy. The data were compared with those obtained from a control group composed of seven patients with sarcoidosis and six patients with haemoptysis.

RESULTS

Cytological results showed a similar profile in exposed workers and controls, whereas in patients with sarcoidosis a lymphocytic alveolitis was found. Microanalysis of the particulate identified the presence of silicates, CRSs, and metals. Pottery workers had higher numbers of total particles and CRSs, and had a higher silicate/metal ratio. In five workers, the presence of zirconium silicate was also detected. Patients with sarcoidosis had the lowest number of particles, and an inverted silicate/metal ratio.

CONCLUSION

Microanalysis by transmission electron microscope can provide useful information to assess occupational exposure to dusts.

T lymphocytes and silica-induced pulmonary inflammation and fibrosis in mice

BACKGROUND

Silica-induced pulmonary inflammation and fibrosis in animals provides a good model for chronic pulmonary inflammation and fibrosis. Although lymphocytes are implicated in the pathogenesis of pulmonary fibrosis, experimental models using silica-treated athymic nude mice have not been successful in showing the fibrogenic mechanism regulated by T cells. The aim of this study was to re-evaluate the role of T lymphocytes in the development of silicosis by comparing the response to silica administration of nude athymic mutants with that of euthymic animals.

METHODS

Suspensions of silica particles were transnasally administered to nude athymic mice (Balb/c nu/nu) as well as to their euthymic littermates (Balb/c nu/+). The degree of pulmonary inflammation and fibrosis was assessed on days 14, 28, and 56 based upon histological observation, analysis of collagen deposition in the lungs, and analysis of the cellular constituent, protein, and phospholipid content in the bronchoalveolar lavage fluid.

RESULTS

Histologically, athymic mice developed less severe interstitial pneumonitis than euthymic mice. In euthymic mice the lung hydroxyproline content increased with time after silica administration from 6.48 (0.38) micrograms hydroxyproline/mg dry lung weight on day 0 to 8.87 (0.41) micrograms/mg on day 56. A gradual increase in lung hydroxyproline content was also observed in athymic mice but the increase was significantly smaller than in euthymic mice (6.63 (0.43) micrograms/mg on day 0, 7.90 (0.19) micrograms/mg on day 56). Administration of silica resulted in an increase in the number of macrophages and neutrophils and in the total protein and phospholipid content of the bronchoalveolar lavage (BAL) fluid in both mouse strains. No significant difference was detected between athymic and euthymic mice in the numbers of macrophages, but the increase in neutrophils in the BAL fluid of athymic mice was significantly smaller than in euthymic mice on days 14 and 56. The total protein and phospholipid content of the BAL fluid from athymic mice was lower than that from euthymic mice.

CONCLUSIONS

T lymphocytes appear to be involved in the pathogenesis of silica-induced pneumonitis. Since pulmonary fibrosis develops even in nude athymic mice, T cells do not seem to play a primary part in fibrogenic response but they regulate, at least to some extent, the response of inflammatory cells and fibrogenesis of the lung.

Effect of exposure to silica on human alveolar macrophages in supporting growth activity in type II epithelial cells

BACKGROUND

The proliferative response of type II cells is an important event following silica-induced lung injury. Alveolar macrophages, when activated by fibrogenic agents, secrete various biological mediators which affect cell growth.

METHODS

Human alveolar macrophages from normal volunteers were incubated in serum-free medium or in the presence of increasing concentrations of silica. Alveolar macrophage conditioned media were diluted and added to type II cell cultures for proliferation studies. Purified type II pneumocytes were isolated from fetal rat lungs for bioassays. Growth factor activities were partially characterised by size exclusion chromatography. Each fractionated mitogenic peak was preincubated with monoclonal antibody against platelet derived growth factor (PDGF) or antisera against insulin-like growth factor 1 (IGF-1) or fibroblast derived growth factor (FGF) in order to study the nature of each activity.

RESULTS

Conditioned media from alveolar macrophages exposed to silica induced an increase in type II cell DNA synthesis and cell number over that seen when type II cells were incubated with unstimulated alveolar macrophage supernatants. Size exclusion of alveolar macrophage supernatants exposed to silica showed four peaks of type II cell stimulating activity with apparent molecular weights of 38, 22, 16, and 8 kDa. Anti-PDGF antibody significantly reduced the activity of the first and second peaks, antiserum against IGF-1 partially reduced the activity of the first and fourth peaks, and antiserum against FGF reduced only the third peak of activity.

CONCLUSIONS

Human alveolar macrophages exposed to silica in vitro release mitogens for type II pneumocytes including PDGF-like, IGF-1-like, and FGF-like molecules. These agents are likely to be involved in the epithelial repair and type II cell hyperplasia observed in silicosis.

Altered regulation of surfactant phospholipid and protein A during acute pulmonary inflammation

Biochemical changes in the pulmonary surfactant system caused by exposure to toxicants are often accompanied by an influx of inflammatory cells into the lungs. We have investigated the possibility that the inflammatory and surfactant biochemical effects might be connected. Co-treatment with dexamethasone, a synthetic anti-inflammatory glucocorticoid, mitigated the increases in free cells and total intracellular surfactant phospholipid normally seen in animals given silica alone, suggesting a relationship between the free cell population of the alveoli and the surfactant system during alveolitis. Furthermore, we have investigated whether induction of the surfactant system is a universal response to alveolar inflammation. Inflammation was induced in the lungs by intratracheal injections of titanium dioxide, silica, bleomycin or lipopolysaccharide (LPS) suspended in isotonic saline. Inflammatory cell and surfactant responses were measured at 3 days and 14 days following injection. There was a distinct alveolar inflammatory cell profile following administration of each agent, at each time point, indicating a dynamic inflammatory cell population during the course of the study. Furthermore, surfactant phospholipid and protein A (SP-A) pools exhibited unique responses to the inflammatory agents. Only silica-treated lungs maintained elevated levels of surfactant phospholipids and SP-A throughout the course of the experiment. We conclude that both the surfactant components and the inflammatory cell population of the alveoli undergo dynamic changes following treatment with these inflammatory agents and that activation of the surfactant system is not a universal response to alveolar inflammation, since surfactant components were not always elevated during times of increased alveolar cellularity. The unique inflammatory cell infiltrate elicited by silica is of particular interest in that surfactant components were elevated throughout the course of the experiment in this group. Indeed, we have shown that the size of the intracellular pool of surfactant is directly proportional to the number of polymorphonuclear leukocytes but not alveolar macrophages or lymphocytes in the alveoli following silica treatment. Finally, our data suggest that the phospholipid and SP-A components of surfactant respond differentially to the pulmonary toxicants in this study.

Differential regulation of the murine ribosomal protein L26 gene in macrophage activation

In mouse RAW 264.7 macrophages, the gene for ribosomal protein L26 is positively regulated by silica. In order to study L26 gene expression a near full-length cDNA for mouse L26 was isolated and characterized. Sequence analysis revealed that mouse L26 is a 145 amino acid protein highly homologous to other vertebrate L26 proteins. The treatment of RAW 264.7 cells with the inflammatory mediators LPS and IFN gamma induced the expression of L26 mRNA, but the patterns of expression obtained differed markedly from silica. On the contrary, TNF alpha acted as a down-regulator of L26 gene. Our results suggest that the synthesis of ribosomal components in response to macrophage activators is inducer-specific. Mouse genomic DNA analysis revealed the presence of multiple (10-12) sequences related to the L26 gene.

N-acetyl-beta-D-glucosaminidase activity within BAL from macaques exposed to generic coal dusts

N-acetyl-beta(beta)-D-glucosaminidase is a lysosomal enzyme secreted by alveolar macrophages in response to phagocytosis of particulate material. Alveolar macrophages participate in the degradation and fibrosis of pulmonary tissue that results in pneumoconiosis. Known quantities of four characterized respirable dusts were bronchoscopically placed into the right caudal lung lobe of macaque monkeys. Bronchoalveolar lavage (BAL) samples were collected from dust-exposed right lung and unexposed left lung of the same individuals at 2-week intervals for 12 weeks after dust instillation. The samples were tested for N-acetyl-beta-D-glucosaminidase activity to determine if the enzyme levels could serve as an indicator of pulmonary injury induced by generic coal dusts when compared to known fibrogenic and nuisance dusts. Installation of generic quartz, anthracite, or TiO2 dusts produced significant elevations of enzyme activity and increased numbers of macrophages in the dust-exposed lobes. Elevations in enzymatic activity and macrophage numbers were greatest in response to generic quartz dust. These results suggest that quantitative levels of N-acetyl-beta-D-glucosaminidase activity may be a useful indicator of acute and chronic lung injury following exposure to fibrogenic and nonfibrogenic dusts.

Silica increases tumor necrosis factor (TNF) production, in part, by upregulating the TNF promoter

Silica causes release of tumor necrosis factor (TNF) from mononuclear phagocytes. One hypothesis is that silica increases TNF production, in part, by upregulating the TNF gene. To evaluate this hypothesis, THP-1 cells (a myelomonocytic cell line) were exposed to various amounts of silica and then the TNF gene transcription was evaluated. In this study silica caused a dose-dependent increase in TNF mRNA and the peak response occurred at 3 h following stimulation. A transient transfection assay also showed that silica upregulated expression of a TNF CAT construct in THP-1 cells. Furthermore, a nuclear run-on assay demonstrated that silica particles induce increased TNF gene transcription in exposed cells. THP-1 cells cultured for various periods of time in the presence of silica released TNF into the cell supernatants. These studies show that silica can upregulate the TNF gene, which results in the release of TNF protein from the cells.

Effect of short-term exogenous pulmonary surfactant treatment on acute lung damage associated with the intratracheal instillation of silica

The objective of our study was to investigate whether coating the surface of silica with Survanta, a commercially available, bovine pulmonary surfactant, would reduce the in vitro cytotoxicity to alveolar macrophages (AMs), as well as attenuate lung damage in vivo following intratracheal instillation of silica. In the in vitro studies, alveolar macrophages from male Fischer 344 rats were incubated for 1 and 24 h with native or Survanta-treated silica (0.5 mg/ml). At both time points, the native, uncoated silica caused a dramatic loss of AM viability. The AMs were protected, however, when the silica was treated with the Survanta surfactant. This protective effect was significantly greater after 1 h when compared with 24 h. In the in vivo studies, a high dose of silica (10 mg/100 g body weight) was suspended in Survanta and intratracheally instilled into the lungs of male Fischer 344 rats. A number of biochemical and cellular parameters were measured within the bronchoalveolar lavage fluid (BALF) 1 and 14 d after the instillation exposures to assess lung damage. One day after the instillations, the suspension of silica in Survanta resulted in significant reductions in the silica-induced increases in total protein, beta-glucuronidase activity, and influx of neutrophils (PMNs) into the airspaces of the lungs. Fourteen days after the instillation exposures, this protective effect was lost. When Survanta was instilled into the lungs 15 min after the intratracheal instillation of silica, a significant reduction also was demonstrated in the silica-induced elevations in BALF total protein, beta-glucuronidase activity, and influx of PMNs 1 d after the instillation exposures. In an attempt to protect silica-exposed lungs over a longer period of time, Survanta was instilled into the lung 15 min after the silica instillation, and then every other day over a 7-d treatment period. Twenty-four hours after the last Survanta instillation, slight but significant decreases in the silica-induced elevations in BALF total protein and beta-glucuronidase activity were observed. The Survanta treatment, however, had no effect in preventing the infiltration of PMNs into the airspaces of the lungs. The results of this study indicate that artificially coating the silica with surfactant phospholipid offers short-term protection against its toxicity under both in vitro and in vivo conditions.

Bacillus Calmette-Guérin (BCG) and immunoglobulins synergistically enhance mineral dust-induced production of reactive oxygen metabolites by human monocytes

The modulating effect of BCG and polyclonal immunoglobulin on mineral dust-induced production of reactive oxygen metabolites (ROM) by human monocytes was studied using luminol-dependent chemiluminescence. BCG and immunoglobulin synergistically amplified the ROM production induced by chrysotile asbestos and quartz particles, and BCG caused a sharper dose response for poly-immunoglobulin added to the mineral dusts. Immunoglobulins did not affect zymosan yeast-induced ROM production, which was enhanced strongly by BCG. As there is evidence that phagocyte-derived ROM are of importance in mineral dust-induced lung injury, we suggest that the observed synergism between host response inflammatory mediators (poly-immunoglobulin) and exogenic irritants (BCG) may contribute to the outcome of exposure of mineral dusts, and thus in part explain the individual variations in susceptibility to mineral dust-induced diseases.

Interleukin-6, interferon-gamma, and phospholipid levels in the alveolar lining fluid of human lungs. Profiles in coal worker's pneumoconiosis and idiopathic pulmonary fibrosis

Cytokines are widely involved in physiologic as well as immunoinflammatory and fibrosing processes of the lung. The aim of this work was to study, by bronchoalveolar lavage, two groups of human interstitial lung diseases (ILD) with fibrosing propensity (ie, idiopathic pulmonary fibrosis [IPF], n = 10; and coal worker's pneumoconiosis [CWP], n = 15). Patients were compared with nonsmoker control subjects (n = 20). Cellularity, proteins, and phospholipids were determined in the alveolar fluids. In addition, two cytokines (interleukin-6 [IL-6] and interferon-gamma [IFN-gamma]), which are presumed to possess respective antifibrotic and profibrotic activities, were measured in the respiratory tract. Compared with control subjects, IPF and simple CWP showed alveolar hypercellularity (p < 0.05) and relative lymphocytosis (p < 0.05). Both exhibited increased alveolar permeability (ie, increased albumin/urea ratio, p < 0.05), with enhanced IL-6 and decreased IFN-gamma in the alveolar spaces (p < 0.05). On the other hand, IPF displayed an associated polymorphonuclear alveolitis, enhanced alveolar epithelial lining fluid (AELF) volume and low surfactant phospholipid levels (p < 0.05 vs control), whereas simple CWP shared an exclusive lymphocytosis, normal AELF volume, and a surfactant lipid overflow (p < 0.05 vs control). Relationships among all of these parameters were found only between alveolar cellularity, neutrophils and IL-6 levels in the AELF of IPF (respectively, r = 0.85, p = 0.0009, and r = 0.89, p = 0.0006). In summary, common alterations of cellular and cytokine turnover were observed in IPF and simple CWP and may reflect activity of the antifibrotic fight in these diseased lungs. Surfactant phospholipid levels are likely to represent a specific disturbance among IPF and CWP, but no clear relationship with respect to the other parameters could be established for explaining the difference in time course outcome.

Cobalt bioavailability from hard metal particles. Further evidence that cobalt alone is not responsible for the toxicity of hard metal particles

Hard metal is an alloy of tungsten carbide (WC) in a matrix of cobalt metal (Co). The inhalation of hard metal dust can cause an alveolitis which may progress to interstitial fibrosis. This study was undertaken to compare, both in vivo and in vitro, the bioavailability of cobalt metal when mixed or not with WC and to assess whether this factor had any influence on the cellular toxicity of hard metal particles. In vivo, non-toxic doses of cobalt metal were administered intratracheally in the rat, alone (Co, 0.03 mg/100 g) or mixed with tungsten carbide (WC-Co, 0.5 mg/100 g containing 6.3% of cobalt metal particles). Sequential measurements of cobalt in the lung and in urine demonstrated that the retention time of the metal in the lung was longer in Co- than in WC-Co-treated animals. In vitro, the cellular cobalt uptake was higher when the metal was presented to the macrophages as WC-Co. However, there was no relationship between the cellular uptake of cobalt and the occurrence of toxicity, since the intracellular concentration of cobalt associated with the occurrence of a cytotoxic effect of WC-Co particles was insufficient to exert the same effect when resulting from exposure to Co alone. This clearly indicates that increased bioavailability of cobalt is not the mechanism by which hard metal particles exhibit their cellular toxicity. These observations confirm and extend our previous findings supporting the view that cobalt is not the only component responsible for the toxicity of hard metal particles which should be considered as a specific toxic entity.

Enhanced clearance of silica from mouse lung after instillation of a leukocyte chemotactic factor

It has been suggested that increased recruitment of phagocytes and subsequent clearance of particles may follow instillation of a leukocyte chemoattractant to lungs containing silica. The present study quantitated serially the silica content in alveolar spaces, in lung tissue and in hilar lymph nodes of mice that received 2 mg silica only, compared to a group that also received 100 micrograms intratracheal chemotactic factor N-formyl-L-methionyl-leucyl-phenylalanine (FMLP) at 2 and 3 weeks after silica. These mice showed a supplemental increase in alveolar macrophages and neutrophils, and an increase in silica was measured in lavaged cells and fluid soon after FMLP injection. At all times to 16 weeks, the silica content of lung tissue was significantly lower in mice that also received FMLP, and in this group, pulmonary fibrosis was much reduced, as shown morphologically and biochemically. In addition, there was reduced translocation of silica to lymph nodes in FMLP-treated mice. The results indicate that induction of a controlled inflammatory response in the alveoli at a time when particles are present in the pulmonary interstitium can accelerate clearance by increasing phagocyte traffic to the alveoli. The subsequent reduction in particle content of the lung is associated with a lower level of pulmonary fibrosis.