Mobilization of iron from crocidolite asbestos by certain chelators results in enhanced crocidolite-dependent oxygen consumption

Morphological and chemical mechanisms of elongated mineral particle toxicities

Much of our understanding regarding the mechanisms for induction of disease following inhalation of respirable elongated mineral particles (REMP) is based on studies involving the biological effects of asbestos fibers. The factors governing the disease potential of an exposure include duration and frequency of exposures; tissue-specific dose over time; impacts on dose persistence from in vivo REMP dissolution, comminution, and clearance; individual susceptibility; and the mineral type and surface characteristics. The mechanisms associated with asbestos particle toxicity involve two facets for each particle's contribution: (1) the physical features of the inhaled REMP, which include width, length, aspect ratio, and effective surface area available for cell contact; and (2) the surface chemical composition and reactivity of the individual fiber/elongated particle. Studies in cell-free systems and with cultured cells suggest an important way in which REMP from asbestos damage cellular molecules or influence cellular processes. This may involve an unfortunate combination of the ability of REMP to chemically generate potentially damaging reactive oxygen species, through surface iron, and the interaction of the unique surfaces with cell membranes to trigger membrane receptor activation. Together these events appear to lead to a cascade of cellular events, including the production of damaging reactive nitrogen species, which may contribute to the disease process. Thus, there is a need to be more cognizant of the potential impact that the total surface area of REMP contributes to the generation of events resulting in pathological changes in biological systems. The information presented has applicability to inhaled dusts, in general, and specifically to respirable elongated mineral particles.

Apoptosis induced by crocidolite asbestos in human lung epithelial cells involves inactivation of Akt and MAPK pathways

Exposure of human lung epithelial (A549) cells to asbestos fibers causes apoptosis, which is largely attributed to release of iron and generation of reactive oxygen species (ROS) within the cells. To mimic the highly oxidative environment generated by asbestos exposure in the absence of the actual fibers, we used two chemicals; buthione sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis and ferric ammonium citrate (FAC), a source of iron. Here, we report that exposure of A549 cells to crocidolite asbestos led to a significant time-dependent inactivation of signaling proteins, i.e. Akt and all mitogen-activated protein kinases (MAPKs) (p38, ERK1/2 and SAPK/JNK), and subsequently to apoptosis. Unlike crocidolite treatment, the use of BSO and FAC, independently or combined, did not change the phosphorylation status of proteins, nor did it induce apoptosis. Taken together, our results presented herein point to the possibility that crocidolite-induced apoptosis of human lung epithelial cells is not a mere consequence of generation of oxidants but also requires inactivation of major cell growth and differentiation pathways.

In vitro evaluation of orthopedic composite cytotoxicity: assessing the potential for postsurgical production of hydroxyl radicals

Hydroxyl radical (*OH)-induced inflammation is a primary mode for in vivo cytotoxicity. A legitimate concern is whether particulate wear debris from orthopedic composites can stimulate inflammation via ferrous ion (Fe2+)-mediated production of *OH. The purpose of this research was to utilize electron paramagnetic resonance (EPR) spin trapping in investigating and comparing the potential for postsurgical cytotoxicity induced specifically by *OH in the presence of two composites: Simplex P and the novel, hybrid, CORTOSS. Cytotoxicity is evaluated based on the composites competitively chelating catalytic Fe2+ or readily reducing ferric ions (Fe3+), in facilitating the Fenton reaction (FR). *OH that are produced were then validated by a radical scavenger to confirm a genuine radical signal and mechanism. Spin adduct peak areas decreased in the presence of CORTOSS as opposed to increasing in the presence of Simplex P, evaluated against their respective controls. A plausible theory elucidating this finding is that CORTOSS may sequester the Fe form, by virtue of its monomers. Principally, direct comparison of composites indicated that Simplex P had greater tendency to produce *OH, yielding 25.6 and 48.7% greater spin adduct peak areas when chelated and non-chelated Fe2+ are used, respectively. Moreover, the rate of FR accelerated when chelated Fe2+ was used, leading to the formation of a ternary complex with the composites. This was more prominent in Simplex P, as coordination of chelated Fe2+ occurs on its surface via an electrostatic attraction to allow a seventh coordination site for ligand exchange in the ternary complex, stabilized by Ba2+. Conversely, the silica found in CORTOSS possesses radical quenching abilities that deactivate generated *OH in impeding the efficiency of FR. Neither composite demonstrated a capacity to readily reduce Fe3+ to the relevant Fe2+, as validated by a non-radical pathway. Instead, the artificial spin adduct signal attained when employing chelated Fe3+ was due to the nucleophilic addition of water onto DMPO. Simplex P may also serve as a template for surface catalysis of the nucleophilic addition of water onto DMPO involving chelated Fe3+. CORTOSS is thought not to induce cytotoxicity, whereas the propensity of Simplex P in promoting Fenton chemistry is a serious issue that must be addressed.

Electron paramagnetic resonance analyses of surface radical chemistries of γ-sterilized orthopedic materials: Implications pointing to cytotoxicity via wear debris-induced inflammation

In this research, electron paramagnetic resonance (EPR) spin-trapping was utilized to determine if surface radical chemistries occur for gamma (gamma)-sterilized orthopedic materials-ultra-high molecular weight polyethylene (UHMWPE) and the novel, hybrid, diurethane dimethacrylate (DUDMA)-based RHAKOSS. The materials' ability to competitively chelate catalytic ferrous ions (Fe(2+)) or readily reduce ferric ions (Fe(3+)), and hydrogen peroxide (H(2)O(2)) directly, in facilitating the Fenton reaction (FR), is indicative of cytotoxicity. Validations with a radical scavenger aids to confirm a radical mechanism. In conjunction, materials were thermally annealed and characterized by attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) spectroscopy in order to explore accelerated oxidative degradation induced by residual radicals evolving from gamma-sterilization. Particularly, there was a significant decrease in spin-adduct peak areas obtained from the reduction of H(2)O(2) in the presence of RHAKOSS or UHMWPE, evaluated against their respective controls. Additionally, chelated Fe(2+) accelerated the rate of FR. This phenomenon suggests that the materials are not better chelators than the Fe-activating chelator, edta. Neither material had the propensity to readily reduce Fe(3+) to the relevant Fe(2+), as certified by a nonradical mechanism. Alternatively, the false spin-adduct signal acquired when chelated Fe(3+) is employed arises via the nucleophilic addition of water onto the DMPO spin trap. Residual radicals in UHMWPE did not recombine/terminate following thermal annealing in an inert atmosphere. The radicals in RHAKOSS, however, did recombine under mild heating in an oxidizing or inert atmosphere. Both materials displayed quenching of ( )OH; however, for UHMWPE, this mechanism was jointly accountable for its accelerated degradation, evidenced by ATR-FTIR. Quenching of ( )OH by the silica found in RHAKOSS manifested in a competing effect that counterbalanced the observed FR. Implanted RHAKOSS is not likely to promote cytotoxicity and should not degrade, but the damaging effect of gamma sterilization on UHMWPE is a serious dilemma confronting its long-term durability and biocompatibility.

The γ-Glutamylcysteine Synthetase and Glutathione Regulate Asbestos-induced Expression of Activator Protein-1 Family Members and Activity

Asbestos fibers cause persistent increases in activator protein-1 (AP-1) family member proto-oncogenes in lung epithelial and mesothelial cells that are linked to proliferation and cell transformation. Using lung epithelial cells, the progenitor cells of lung cancers, we report that crocidolite asbestos initially depletes intracellular glutathione followed by up-regulation of both catalytic and modifier subunits of gamma-glutamylcysteine synthetase. In vivo asbestos inhalation experiments confirm increased protein levels of gamma-glutamylcysteine synthetase in mouse lungs. We also show that asbestos-induced mRNA levels of fos/jun proto-oncogenes, fra-1 transactivation, and AP-1 to DNA binding activity are glutathione-dependent. Epidermal growth factor receptor activity by asbestos is blocked by N-acetyl-l-cysteine, suggesting that it is an initial redox-activated event leading to downstream AP-1 proto-oncogene up-regulation. The overexpression of subunits of gamma-glutamylcysteine synthetase in combination completely blocked asbestos-induced up-regulation of AP-1 proto-oncogene expression. However, when overexpressed individually, the modifier subunit had more dramatic effects than the catalytic subunit. Our work shows that the glutathione-controlled redox status of the epithelial cell plays a pivotal role in asbestos-induced epidermal growth factor receptor and proto-oncogene activation as well as AP-1 activity.

Antiradical and chelating effects in flavonoid protection against silica-induced cell injury

Quercetin, dihydroquercetin, and rutin are capable of scavenging superoxide anion (rate constants of the reaction with superoxide at pH 10 were 1.7 x 10(5), 1.5 x 10(5), and 0.5 x 10(5) M-1 s-1, respectively). At the same time rutin and quercetin but not dihydroquercetin are iron ion chelators. These substances were used to elucidate the role of radical scavenging and iron chelating in flavonoid protection against asbestos-induced oxidative cellular injury. Exposure of rat peritoneal macrophages to chrysotile asbestos fibers resulted in "frustrated" phagocytosis, cell injury, and a LDH release. Quercetin, dihydroquercetin, and rutin were effective in protecting the phagocytic cells against injury caused by asbestos. Moreover, these flavonoids exhibited cellular protection in the same order of effectiveness as that observed for the quenching of superoxide: quercetin > dihydroquercetin > rutin. Exposure of human red blood cells to asbestos fibers also caused progressive cell injury and lysis. Quercetin and rutin protected the red cells (quercetin > rutin), whereas dihydroquercetin was ineffective in preventing asbestos-induced hemolysis. The protective ability of quercetin and rutin may be related to their iron-chelating activity. Due to this these flavonoids can be located on asbestos surface in sites of initiation of free radical reactions and their antiradical moieties can scavenge reactive oxygen species immediately after the appearance. Thus, both antiradical and chelating effects appear to be involved in the flavonoid protection against silica-induced cell injury.

Role of extracellular signal-regulated protein kinases in apoptosis by asbestos and H2O2

Stimulation of cell signaling cascades by oxidants may be important in the pathogenesis of pulmonary and pleural diseases. Here, we demonstrate in rat pleural mesothelial cells that apoptotic concentrations of crocidolite asbestos and H2O2 induce phosphorylation and activation of extracellular signal-regulated protein kinases (ERK). Activation of c-jun-NH2-terminal protein kinases (JNK)/stress-activated protein kinases was also observed in response to H2O2. In contrast, asbestos caused more protracted activation of ERK without JNK activation. Both H2O2- and asbestos-induced activation of ERK was abolished by catalase. Moreover, chelation of surface iron from crocidolite fibers or addition of N-acetyl-L-cysteine prevented ERK activation and apoptosis by crocidolite, indicating an oxidative mechanism of cell signaling. The MEK1 inhibitor PD-98059 abrogated asbestos-induced apoptosis, confirming a causal relationship between ERK activation and apoptosis. These results suggest that distinct cell-signaling cascades may be important in phenotypic responses elicited by oxidant stresses.

Iron mobilization from crocidolite as enhancer of collagen content in rat lung fibroblasts

Asbestos exposure causes pulmonary fibrosis by mechanisms that remain uncertain. There is increasing evidence that iron from asbestos is responsible for many of its effects. In this paper, we investigated the effect of iron mobilized from crocidolite asbestos on collagen content in rat lung fibroblast cultures under serum-free conditions. Crocidolite (2, 4, 6 microg/cm2 well) increased collagen content in a dose-dependent manner (+42 +/- 8, +92 +/- 10, and +129 +/- 13% vs controls). This effect was specific for collagen, since it did not alter total protein content and was inhibited by the iron chelator deferoxamine (DFO). Preincubation of crocidolite with citrate (1 mM) for 48 hr resulted in iron mobilization (51 microM) and increased collagen production (>3-fold) in treated cells. These effects occurred without the intervention of serum factors. The absence of cell damage, proliferation or lipid peroxidation leads to the supposition that iron from crocidolite per se may act as a profibrogenic agent. Although the in vivo participation of other cells and factors cannot be excluded, we conclude that iron released from crocidolite plays a role in collagen increase occurring during asbestosis.

Induction of ferritin synthesis in human lung epithelial cells treated with crocidolite asbestos

Crocidolite asbestos is a known human carcinogen containing 27% iron by weight. It has previously been shown that iron was mobilized intracellularly from crocidolite after treatment of human lung epithelial cells (A549) and that the toxicity of the fibers was directly related to how much mobilized iron was in the < 10,000 MW (low-molecular-weight, LMW) fraction [C. C. Chao, L.G. Lund, K. R. Zinn, and A. E. Aust (1994) Arch. Biochem. Biophys. 314, 384-391]. The data here show that iron mobilization from crocidolite began immediately after treatment of the A549 cells and increased linearly with time. However, the synthesis of ferritin, an iron storage protein, did not begin until after 4 h of treatment, reaching a sustained maximum after 12 h. Mobilized iron was preferentially incorporated into the nonferritin-protein fraction up to 7 h after treatment, when the amount of iron mobilized was low and before significant accumulation of newly synthesized ferritin had occurred. This suggested that these cultured cells needed additional iron for synthesis of iron-requiring proteins and that iron mobilized from crocidolite could be utilized directly for this purpose. Subsequent to this, additional mobilized iron was incorporated into newly synthesized ferritin. Even though iron from crocidolite was incorporated into newly synthesized ferritin or into other proteins, the amount of iron from crocidolite in the LMW fraction remained constant during the 24 h. Thus, it appeared that synthesis of ferritin may not have fully protected the cells from the toxic effects of iron mobilized from crocidolite.

Iron associated with asbestos bodies is responsible for the formation of single strand breaks in phi X174 RFI DNA

The ability of amosite cored asbestos bodies isolated from human lungs to catalyse damage to phi X174 RFI DNA in vitro was measured and compared with that of uncoated amosite fibres with a similar distribution of length. Asbestos bodies (5000 bodies) suspended for 30 minutes in 50 mM NaCl containing 0.5 micrograms phi X174 RFI DNA, pH 7.5, did not catalyse detectable amounts of DNA single strand breaks. Addition of the reducing agent ascorbate (1 mM), however, resulted in single strand breaks in 10% of the DNA. Asbestos bodies in the presence of a low molecular weight chelator (1 mM) and ascorbate catalysed the formation of single strand breaks in 21% of the DNA with citrate or 77% with ethylenediamine tetra-acetic acid (EDTA), suggesting that mobilisation of iron may increase damage to DNA. Preincubation for 24 hours with desferrioxamine B, which binds iron (Fe (III)) and renders it redox inactive, completely inhibited the reactivity of asbestos bodies with DNA, strongly suggesting that iron was responsible. Amosite fibres (5000 fibres/reaction), with a similar length distribution to that of the asbestos bodies, did not catalyse detectable amounts of single strand breaks in DNA under identical reaction conditions. The results of the present study strongly suggest that iron deposits on the amosite core asbestos bodies were responsible for the formation of DNA single strand breaks in vitro. Mobilisation of iron by chelators seemed to enhance the reactivity of asbestos bodies with DNA. It has been postulated that the in vivo deposition of the coat material on to fibres may be an attempt by the lung defenses to isolate the fibre from the lung surface and thus offer a protective mechanism from physical irritation. These results suggest, however, that the iron that is deposited on asbestos fibres in vivo may be reactive, potentially increasing the damage to biomolecules, such as DNA, above that of the uncoated fibres.

Oxidative DNA damage by crystalline silica

Using a simple DNA strand breakage assay, we detected the production of oxidant species, probably hydroxyl free radicals, in buffered suspensions of crystalline silica at pH 7.4. DNA damage was affected by the presence of oxygen and was accelerated by superoxide dismutase and by hydrogen peroxide. Deferoxamine blocked damage by hydrogen peroxide and silica but accelerated DNA damage by silica alone and by superoxide dismutase and silica. DNA damage was blocked by catalase and by the scavenging agents dimethyl sulfoxide and sodium benzoate. Chemical etching of crystalline silica to remove impurities by treatment of the surface with hydrofluoric acid resulted in markedly diminished DNA damaging ability. Even preparations of crystalline silica previously characterized as highly pure contained trace iron impurities in amounts significant enough to produce oxygen free radicals in aqueous suspension. Both superoxide and Fenton reaction oxidants were produced. We conclude that silica is able to mediate DNA strand breakage in vitro and that this DNA damage may be an important factor in silica toxicity.

Iron released from an erythrocyte lysate by oxidative stress is diffusible and in redox active form

The incubation of a ghost-free erythrocyte lysate with the oxidizing agent phenylhydrazine resulted in both methemoglobin formation and release of iron in a desferrioxamine (DFO)-chelatable form. The released iron was diffusible, as shown by a dialysis carried out simultaneously with the incubation. When the dialysate was added to erythrocyte ghosts or to microsomes from liver or brain, lipid peroxidation developed in the membranes, indicating that the diffusible iron was in a redox active form. The addition of ATP to the lysate markedly increased both iron diffusion and lipid peroxidation in the membranes subsequently added to the dialysate. The possible implication of these data in some well known pathologies is discussed.

The role of free radicals in asbestos-induced diseases

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