Reactive oxygen species (ROS) and reactive nitrogen species (RNS) generation by silica in inflammation and fibrosis

Principles for characterizing the potential human health effects from exposure to nanomaterials: elements of a screening strategy

The rapid proliferation of many different engineered nanomaterials (defined as materials designed and produced to have structural features with at least one dimension of 100 nanometers or less) presents a dilemma to regulators regarding hazard identification. The International Life Sciences Institute Research Foundation/Risk Science Institute convened an expert working group to develop a screening strategy for the hazard identification of engineered nanomaterials. The working group report presents the elements of a screening strategy rather than a detailed testing protocol. Based on an evaluation of the limited data currently available, the report presents a broad data gathering strategy applicable to this early stage in the development of a risk assessment process for nanomaterials. Oral, dermal, inhalation, and injection routes of exposure are included recognizing that, depending on use patterns, exposure to nanomaterials may occur by any of these routes. The three key elements of the toxicity screening strategy are: Physicochemical Characteristics, In Vitro Assays (cellular and non-cellular), and In Vivo Assays. There is a strong likelihood that biological activity of nanoparticles will depend on physicochemical parameters not routinely considered in toxicity screening studies. Physicochemical properties that may be important in understanding the toxic effects of test materials include particle size and size distribution, agglomeration state, shape, crystal structure, chemical composition, surface area, surface chemistry, surface charge, and porosity. In vitro techniques allow specific biological and mechanistic pathways to be isolated and tested under controlled conditions, in ways that are not feasible in in vivo tests. Tests are suggested for portal-of-entry toxicity for lungs, skin, and the mucosal membranes, and target organ toxicity for endothelium, blood, spleen, liver, nervous system, heart, and kidney. Non-cellular assessment of nanoparticle durability, protein interactions, complement activation, and pro-oxidant activity is also considered. Tier 1 in vivo assays are proposed for pulmonary, oral, skin and injection exposures, and Tier 2 evaluations for pulmonary exposures are also proposed. Tier 1 evaluations include markers of inflammation, oxidant stress, and cell proliferation in portal-of-entry and selected remote organs and tissues. Tier 2 evaluations for pulmonary exposures could include deposition, translocation, and toxicokinetics and biopersistence studies; effects of multiple exposures; potential effects on the reproductive system, placenta, and fetus; alternative animal models; and mechanistic studies.

The action of the mast cell product tryptase on cyclooxygenase-2 (COX2) and subsequent fibroblast proliferation involves activation of the extracellular signal-regulated kinase isoforms 1 and 2 (erk1/2)

The mast cell product tryptase, via protease-activated receptor 2 (PAR2), induces cyclooxygenase-2 (COX2) and 15-deoxy-prostaglandin J2 (15d-PGJ2) synthesis. 15d-PGJ2, through the nuclear peroxisome proliferator activated receptor gamma (PPARgamma), subsequently causes fibroblast proliferation. In this study we attempted to determine initial events of the tryptase/PAR2 signaling pathway leading to COX2 induction and fibroblast proliferation. In human fibroblasts (HFFF2), cDNA array, RT-PCR and Western blotting studies demonstrated that tryptase, but not 15d-PGJ2, up-regulates c-jun, c-fos and COX2 expression, and phosphorylates the extracellular signal-regulated kinase isoforms 1 and 2 (erk1/2). Furthermore, tryptase effects on erk1/2, c-jun, c-fos, COX2 and cell proliferation were prevented by PD98059, an inhibitor of the mitogen-activated protein kinase kinase (MEK). Other kinases [P38, stress-activated protein kinase/c-jun N-terminal kinase (SAPK/JUNK), erk5], intracellular Ca(2+) or cAMP were not affected by tryptase/PAR2. Our study identifies crucial intracellular events leading to induction of COX2 and fibroblast proliferation, i.e. a cornerstone of fibrosis.

Basic pathogenetic mechanisms in silicosis: current understanding

PURPOSE OF REVIEW

Silicosis continues to be a common cause of chronic lung diseases, despite evidence that these diseases can be prevented by environmental dust control. Silicosis has been studied extensively by basic and clinical scientists, yet little is known about the crucial cellular and molecular mechanisms that initiate and propagate the process of inflammation and scarring.

RECENT FINDINGS

Recent in vivo, in vitro, and human studies have focused on several main areas of investigation into the causes and processes of the development of silicosis. These areas of investigation include the variability of pathogenic potential of different varieties of silica; the role of activated alveolar macrophages products in the development and progression of silicosis; and the direct role played by the silica particle surface in triggering adverse biologic reactions, such as generating ROS and RNS. The generation of oxidants by silica particles and by silica-activated cells results in cell and lung damage; increased expression of inflammatory cytokines, including TNF-alpha, IL 1 beta, and TGF-beta; activation of cell signaling pathways, including the MAP kinase pathways; and phosphorylation and activation of specific transcription factors (e.g., NFkB). The ROS, RNS, and NO generated by the silica particles also induce apoptosis in macrophages and other cells.

SUMMARY

Further research on the molecular mechanisms involved in the inflammatory processes important for progression to fibrotic diseases is needed for the development of effective treatment of silicosis. Potential therapeutic strategies include inhibition of cytokines such as IL-1, TNF alpha, the use of anti-oxidants, and the inhibition of apoptosis.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

Silica Particles Enhance Peripheral Thrombosis

RATIONALE

Inflammation and thrombosis are related via interactions between leukocytes, platelets, the vasculature, and the coagulation system. However, the mechanisms behind these interactions remain poorly understood.

OBJECTIVES

We have investigated the effects of the well known pulmonary inflammation induced by silica for the development of peripheral thrombogenicity in a hamster model of thrombosis. In addition, the consequences of pulmonary macrophage and circulating monocyte and neutrophil depletion on the thrombogenicity were investigated.

METHODS

Silica particles (2-200 mug/hamster) were intratracheally instilled, and experimental thrombosis in photochemically induced femoral vein lesions was assessed 24 hours later, in association with cellular infiltration in the lung.

MEASUREMENTS AND MAIN RESULTS

Intratracheally instilled silica particles (20 and 200 mug/hamster) triggered pulmonary inflammation, together with stimulation of peripheral platelet-rich thrombus formation. Both the selective depletion of lung macrophages by intratracheal administration of clodronate liposomes and the combined depletion of circulating monocytes and neutrophils by intraperitoneal injection of cyclophosphamide significantly reduced silica-induced influx of macrophages and neutrophils in bronchoalveolar lavage, and reduced peripheral thrombogenicity. Silica-induced lung inflammation was accompanied by increased neutrophil elastase levels in bronchoalveolar lavage and in plasma. Specific neutrophil elastase inhibition in the lung did not affect lung inflammation but reduced peripheral thrombogenicity.

CONCLUSION

These findings uncover pulmonary macrophage-neutrophil cross-talk releasing neutrophil elastase into the blood circulation. Elastase, triggering activation of circulating platelets, may then predispose platelets to initiate thrombotic events on mildly damaged vasculature.

The radiation-induced fibroatrophic process: therapeutic perspective via the antioxidant pathway

The radiation-induced fibroatrophic process (RIF) constitutes a late, local and unavoidable sequela to high-dose radiotherapy, traditionally considered irreversible. Today, this process is partly reversible, thanks to recent progress in understanding the physiopathology of the lesions it causes and the results of recent clinical trials using antioxidant therapy. This review includes a synthetic description of the static and dynamic features of the RIF process, as reflected by its clinical, instrumental and histopathological characteristics, and by its cellular and molecular regulation. Schematically, three successive clinical and histopathological phases can be distinguished: a pre-fibrotic aspecific inflammatory phase, a constitutive fibrotic cellular phase, and a matrix densification and remodelling phase, possibly ending in terminal tissular necrosis. The respective roles of the chief actors in the RIF process are defined, as well as their development with time. A fibroblastic stromal hypothesis is suggested revolving around a 'gravitational effect' exerted by the couple ROS (reactive oxygen species)--fibroblasts, and partly mediated by TGF-beta1. A variety of strategies have been tested for the management of RIF. In the light of the mechanisms described, a curative procedure has been proposed via the antioxidant pathway. In particular, it was showed that superoxide dismutase and combined pentoxifylline-tocopherol treatment enables the process of established radiation-induced fibroatrophy to be greatly reduced or even reversed, both in clinical practice and animal experiments. The efficacy of combined pentoxifylline-tocopherol treatment in superficial RIF was confirmed in a randomised clinical trial, and then in successful phase II trials especially in uterine fibroatrophy and osteoradionecrosis. It is of critical importance to evaluate these new management approaches in larger clinical trials and to improve the recording of results for better outcome analysis. Mechanistic studies are always necessary to improve understanding of the RIF process and the antifibrotic drug action.

Chrysotile asbestos is progressively converted into a non-fibrous amorphous material by the chelating action of lichen metabolites

A natural deactivation of chrysotile asbestos occurs on serpentinite rocks where lichens selectively grow on the fibres and secrete metabolites, including oxalic acid, which, in the long term, turn the fibres into a non-toxic amorphous material.

Dietary rutin, but not its aglycone quercetin, ameliorates dextran sulfate sodium-induced experimental colitis in mice: attenuation of pro-inflammatory gene expression

Oxidative stress has been shown to play a pivotal role in the onset of inflammatory bowel disease (IBD) and carcinogenesis. We evaluated the effects of two dietary anti-oxidants, rutin and its aglycone quercetin, on dextran sulfate sodium (DSS)-induced experimental colitis in mice. Female ICR mice were fed a diet containing 0.1% rutin or 0.1% quercetin for 2 weeks, and given 5% DSS in drinking water during the second week to induce colitis. We also examined the dose-dependency of rutin and quercetin (0.01% and 0.001% each) as well as their therapeutic efficacy, which was evaluated following DSS administration, on DSS-induced colitis. The protein level of interleukin (IL)-1 beta in both colonic mucosa and peritoneal macrophages was quantified by enzyme-linked immunosorbent assay. Further, mRNA expression levels of IL-1 beta, tumor necrosis factor-alpha, IL-6, granulocyte macrophage-colony stimulating factor, inducible nitric oxide synthase, and cyclooxygenase (COX)-1 and COX-2 in colonic mucosa were determined by reverse transcription-polymerase chain reaction. A diet containing 0.1% rutin, but not quercetin, attenuated DSS-induced body weight loss and shortening of the colorectum (P<0.01 and <0.05, respectively), and dramatically improved colitis histological scores. Further, DSS-induced increases in colonic mucosal IL-1 beta levels were blunted significantly in rutin-, but not quercetin-, fed mice (P<0.01), while dietary rutin attenuated the expressions of IL-1 beta and IL-6 mRNA in colonic mucosa (each, P<0.01). As for dose dependency, 0.01%, but not 0.001%, dietary rutin significantly reduced mucosal IL-1 beta levels (P<0.01). Notably, a 0.1% rutin diet given 3 days after DSS treatment significantly suppressed both colorectal shortening and IL-1 beta production (P<0.05 and <0.01, respectively). Dietary rutin ameliorates DSS-induced colitis, presumably by suppressing the induction of pro-inflammatory cytokines. Our results suggest that rutin may be useful for the prevention and treatment of IBD and colorectal carcinogenesis via attenuation of pro-inflammatory cytokine production.

Signaling pathways controlling the production of inflammatory mediators in response to crystalline silica exposure: role of reactive oxygen/nitrogen species

Occupational exposure to crystalline silica has been linked to pulmonary fibrosis and lung cancer. Surface properties of crystalline silica are critical to the production of oxidant species, chemokines, inflammatory cytokines, and proliferative factors involved in the initiation and progression of silica-induced damage, inflammation, alveolar type II cell hyperplasia, fibroblast activation, and disease. The transcription factors nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1) have been shown to play key roles in gene promotion for inflammatory mediators, oncogenes, and growth factors. This review summarizes evidence that in vitro and in vivo exposure to crystalline silica results in activation of NF-kappaB and AP-1. Signaling pathways for activation of these transcription factors are described. In addition, the role of silica-induced reactive oxygen species and nitric oxide in the activation of these signaling events is presented. Last, the generalizability of mechanisms regulating silica-induced pulmonary responses to pulmonary reactions to other occupational particles is discussed.

Signal transduction pathways relevant for neoplastic effects of fibrous and non-fibrous particles

Apart from their genotoxic effects, both fibrous and non-fibrous particles are known to induce signalling pathways involved in the development of malignant lung diseases. Different direct effects of particles as well as indirect cellular effects are believed to induce changes in apoptosis or proliferation in target cells. Signalling events, e.g. the induction of mitogen-activated protein kinase (MAPK) cascades resulting in the activation of the transcription factor AP-1, as well as the induction of the transcription factor NFkappaB which mainly mediates the expression of pro-inflammatory genes are discussed. There is some insight into the molecular mechanisms eliciting these pathways. Therefore, this review aims to give an overview on signalling pathways as well as initial events including effects of reactive oxygen and nitrogen species, membrane receptors and particle uptake.

Inhaled particles and lung cancer. Part A: Mechanisms

Both occupational and environmental exposure to particles is associated with an increased risk of lung cancer. Particles are thought to impact on genotoxicity as well as on cell proliferation via their ability to generate oxidants such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). For mechanistic purposes, one should discriminate between a) the oxidant-generating properties of particles themselves (i.e., acellular), which are mostly determined by the physicochemical characteristics of the particle surface, and b) the ability of particles to stimulate cellular oxidant generation. Cellular ROS/RNS can be generated by various mechanisms, including particle-related mitochondrial activation or NAD(P)H-oxidase enzymes. In addition, since particles can induce an inflammatory response, a further subdivision needs to be made between primary (i.e., particle-driven) and secondary (i.e., inflammation-driven) formation of oxidants. Particles may also affect genotoxicity by their ability to carry surface-adsorbed carcinogenic components into the lung. Each of these pathways can impact on genotoxicity and proliferation, as well as on feedback mechanisms involving DNA repair or apoptosis. Although abundant evidence suggests that ROS/RNS mediate particle-induced genotoxicity and mutagenesis, little information is available towards the subsequent steps leading to neoplastic changes. Additionally, since most of the proposed molecular mechanisms underlying particle-related carcinogenesis have been derived from in vitro studies, there is a need for future studies that evaluate the implication of these mechanisms for in vivo lung cancer development. In this respect, transgenic and gene knockout animal models may provide a useful tool. Such studies should also include further assessment of the relative contributions of primary (inflammation-independent) and secondary (inflammation-driven) pathways.

Silica-induced apoptosis in mouse alveolar macrophages is initiated by lysosomal enzyme activity

Past studies in our laboratory have shown that silica (-quartz) particle exposure of a mouse alveolar macrophage cell line (MH-S) elicits mitochondrial depolarization and caspase 3 and 9 activation, contributing to apoptosis. However, cellular pathways leading to these outcomes have not been extensively investigated. Initial studies revealed that silica exposure elicits lysosomal permeability after 1 h, as evidenced by leakage of FITC-conjugated dextran and acridine orange. We next evaluated a role for the lysosomal acidic compartment in apoptosis. Cells pretreated with the lysosomotropic weak base ammonium chloride, to increase lysosomal pH, showed decreased caspase activation and apoptotic DNA fragmentation. MH-S cells pretreated with pepstatin A, an inhibitor of lysosomal cathepsin D, showed decreased caspase 9 and 3 activation as well as a decreased percentage of cells that became apoptotic. DNA fragmentation and caspase 9 and 3 activation were also decreased in cells pretreated with despiramine, an inhibitor of lysosomal acidic sphingomyelinase. Silica pretreated with aluminum lactate (to blunt surface active sites) reduced caspase activation and apoptosis. Although aluminum lactate-treated silica still induced lysosomal permeability (by FITC-dextran leakage), one measure of lysosome integrity and function suggested a reduction in the extent and/or nature of lysosomal injury (by acridine orange retention). A role for reactive oxygen species (ROS) was investigated to explore another pathway for silica-induced apoptosis in addition to lysosomal enzymes; however, no role for ROS was apparent. Thus, following silica exposure, lysosomal injury precedes apoptosis, and the apoptotic signaling pathway includes cathepsin D and acidic sphingomyelinase.

Relationship between the state of the surface of four commercial quartz flours and their biological activity in vitro and in vivo

Four commercial quartz dusts (flours), two inflammogenic in vivo and activating macrophages in vitro (Qz 2/1-c and Qz 3/1-c) and two mostly inert (Qz 5/1-c and Qz 11/1-c), have been compared regarding their surface properties, in order to detect chemical differences which may account for their different biological behaviour. The following features have been examined: 1) extent of the amorphous fraction (heat associated alpha<-->beta transition of quartz) and its solubility in HF; 2) potential to cleave a carbon-hydrogen bond with consequent generation of carbon centred radicals (spin trapping technique, EPR); 3) evolution of surface functionalities upon heating (FTIR spectroscopy); 4) mechanisms of adsorption of water on dusts outgassed at 150 degrees and at 800 degrees C (adsorption calorimetry). HCl treated samples have also been examined. The two "less toxic" quartzes are more resistant to HF attack, coordinate irreversibly H2O molecules and exhibit strong adsorption sites, which are absent in the other two and in a very pure quartz dust. Conversely all samples show the same potential to release free radicals. The different behaviour of the two sets of dust is consistent with a different level of impurities, namely aluminium ex kaolin, carbon and alkaline ions. The less inflammogenic quartzes appear to be covered by aluminium ions (and possibly iron) which strongly holds molecular water or carbonates, thus reducing the silanol patches to a large extent and changing the surface properties of the particles. We hypothesize that cellular response, and particularly macrophage activation and death, is mediated by strong interactions between silanol patches and some cell membrane components, but inhibited when the surface of the particle is modified by the presence of aluminium ions, surface carbonates and other metal contaminants. This hypothesis suggests that grinding procedures with little appropriate additives, e.g. kaolin, alumina, can reduce the biological activity of quartz dusts.

The Role of the Flavodiiron Proteins in Microbial Nitric Oxide Detoxification

The flavodiiron proteins (first named as A-type flavoproteins) constitute a large superfamily of enzymes, widespread among anaerobic and facultative anaerobic prokaryotes, from both the Archaea and Bacteria domains. Noticeably, genes encoding for homologous enzymes are also present in the genomes of some pathogenic and anaerobic amitochondriate protozoa. The fingerprint of this enzyme family is the conservation of a two-domain structural core, built by a metallo-beta-lactamase-like domain, at the N-terminal region, harbouring a non-heme diiron site, and a flavodoxin-like domain, containing one FMN moiety. These enzymes have a significant nitric oxide reductase activity, and there is increasing evidence that they are involved in microbial resistance to nitric oxide. In this review, we will discuss available data for this novel family of enzymes, including their physicochemical properties, structural and phylogenetic analyses, enzymatic properties and the molecular genetic approaches so far used to tackle their function.

In vitro genotoxicity assessment of commercial quartz flours in comparison to standard DQ12 quartz

Crystalline silica has been classified as a human carcinogen, but there is still considerable debate on its variable fibrogenic and carcinogenic potential. We investigated genotoxicity of a panel of four quartz flours in comparison to DQ12 standard quartz with similar size and surface area, using single cell gel electrophoresis (SCGE) or comet assay. A549 human lung epithelial cells were incubated for 4 hours with different concentrations of quartz ranging from 1.6 to 200 micrograms/cm2 and cytotoxicity was assessed using leakage of lactate dehydrogenase (LDH), trypan blue exclusion and conversion of a metabolic substrate (MTT). DNA strand breakages were seen with all quartzes at an in vitro concentration of 200 micrograms/cm2. At this concentration all tests and quartz samples showed significant cytotoxicity. The most toxic quartz flour (Qz 2/1-C) but not DQ12, showed an increase in strand breaks at 40 micrograms/cm2 in cell culture. At this concentration no cytotoxicity was seen with LDH and MTT, but a significant increase in cells with trypan blue uptake was noted. No differences in tail moment percentage were observed at equal concentrations of different quartz flours. Also no correlation between DNA damage and OH-radical generation or surface radicals as measured by electron spin resonance was observed. We conclude that quartzes do not cause strand breaks without concomitant cell toxicity and a sufficient in vitro concentration of > 40 micrograms/cm2 can only be reached in vivo with instillation of massive doses (> 100 mg). Therefore, in vitro genotoxicity found here is unlikely to explain the genotoxicity observed in in vivo studies with the same and other quartzes.

Surface reactivity of volcanic ash from the eruption of Soufrière Hills volcano, Montserrat, West Indies with implications for health hazards

The fine-grained character of volcanic ash generated in the long-lived eruption of the Soufrière Hills volcano, Montserrat, West Indies, raises the issue of its possible health hazards. Surface- and free-radical production has been closely linked to bioreactivity of dusts within the lung. In this study, electron paramagnetic resonance (EPR) techniques have been used, for the first time, on volcanic ash to measure the production of radicals from the surface of particles. Results show that concentrations of hydroxyl radicals (HO*) in respirable ash are two to three times higher than a toxic quartz standard. The dome-collapse ash contains cristobalite, a crystalline silica polymorph that may cause adverse health effects. EPR experiments indicate, however, that cristobalite in the ash does not contribute to HO* generation. Our results show that the main cause of reactivity is removable divalent iron (Fe2+), which is present in abundance on the surfaces of the particles and is very reactive in the lung. Our analyses show that fresh ash generates more HO* than weathered ash (which has undergone progressive oxidation and leaching of iron from exposed surfaces), an effect replicated experimentally by incubating fresh ash in dilute acid. HO* production experiments also indicate that iron-rich silicate minerals are responsible for surface reactivity in the Soufrière Hills ash.

Reaction of cysteine and glutathione (GSH) at the freshly fractured quartz surface: a possible role in silica-related diseases?

The reactivity of quartz dusts towards glutathione (GSH) and cysteine (Cys) has been investigated. Cys and GSH react, without being adsorbed (UV-Vis spectroscopy), with commercial quartz dusts in an exposed surface-dependent way, but not with amorphous silica. GSH and Cys have been contacted with freshly ground quartz (agate jar QZg-a and steel jar QZg-s) and quartz heated in air at 500 degrees C (QZs-500) and with a dust generated from a purified quartz (99.9999%) to detect the nature of the reacting surface sites. With both GSH and Cys, the highest reactivity was found on the particles ground in a steel jar, while pure quartz was fully inactive. Detection of the radical GS* (spin trapping) suggests a radical mechanism of oxidation to disulphide onto surface-bound iron traces, more abundant on QZg-s and absent on the pure quartz. Oxidation of thiol groups occurs at surface sites different from those involved in the homolytic rupture of a C-H bond. Both reactions are more pronounced on freshly ground samples, but the C-H rupture takes place at silicon-based surface radicals and Fe2+ centers, while oxidation of GSH and Cys requires Fe3+ centers. As all commercial quartz dusts contain surface iron as an impurity, depletion of extracellular or intracellular GSH may contribute to the oxidative damage caused by particle-derived and cell-derived reactive oxygen species.