The presence of erionite in North American geologies and the estimated mesothelioma potency by region.

OBJECTIVE: Erionite is a naturally occurring fibrous mineral found in soils in some geographical regions. Known for its potency for causing mesothelioma in the Cappadocia region of Turkey, the erionite fiber has attracted interest in the United States due to its presence in a band of rock that extends from Mexico to Montana. There are few toxicology studies of erionite, but all show it to have unusually high chronic toxicity. Despite its high potency compared to asbestos fibers, erionite has no occupational or environmental exposure limits. This paper takes what has been learned about the chemical and physical characteristics of the various forms of asbestos (chrysotile, amosite, anthophyllite, and crocidolite) and predicts the potency of North American erionite fibers. MATERIALS AND METHODS: Based on the fiber potency model in Korchevskiy et al. (2019) and the available published information on erionite, the estimated mesothelioma potency factors (the proportion of mesothelioma mortality per unit cumulative exposure (f/cc-year)) for erionites in the western United States were determined. RESULTS AND DISCUSSION: The model predicted potency factors ranged from 0.19 to 11.25 (average ∼3.5), depending on the region. For reference, crocidolite (the most potent commercial form of asbestos) is assigned a potency factor ∼0.5. CONCLUSION: The model predicted mesothelioma potency of Turkish erionite (4.53) falls in this same range of potencies as erionite found in North America. Although it can vary by region, a reasonable ratio of average mesothelioma potency based on this model is 3,000:500:100:1 comparing North American erionite, crocidolite, amosite, and chrysotile (from most potent to least potent).

Low Exposures to Amphibole or Serpentine Asbestos in Germline Bap1-mutant Mice Induce Mesothelioma Characterized by an Immunosuppressive Tumor Microenvironment.

Asbestos and BAP1 germline mutations are risk factors for malignant mesothelioma (MM). While it is well accepted that amphibole asbestos is carcinogenic, the role of serpentine (chrysotile) asbestos in MM has been debated. To address this controversy, we assessed whether minimal exposure to chrysotile could significantly increase the incidence and rate of MM onset in germline Bap1-mutant mice. With either crocidolite or chrysotile, and at each dose tested, MMs occurred at a significantly higher rate and earlier onset time in Bap1-mutant mice than in wild-type littermates. To explore the role of gene-environment interactions in MMs from Bap1-mutant mice, we investigated proinflammatory and protumorigenic factors and the tumor immune microenvironment (TIME). IHC and immunofluorescence staining showed an increased number of macrophages in granulomatous lesions and MMs. The relative number of CD163-positive (CD163+) M2 macrophages in chrysotile-induced MMs was consistently greater than in crocidolite-induced MMs, suggesting that chrysotile induces a more profound immunosuppressive response that creates favorable conditions for evading immune surveillance. MMs from Bap1-mutant mice showed upregulation of CD39/CD73-adenosine and C-C motif chemokine ligand 2 (Ccl2)/C-C motif chemokine receptor 2 (Ccr2) pathways, which together with upregulation of IL6 and IL10, promoted an immunosuppressive TIME, partly by attracting M2 macrophages. Interrogation of published human MM RNA sequencing (RNA-seq) data implicated these same immunosuppressive pathways and connections with CD163+ M2 macrophages. These findings indicate that increased M2 macrophages, along with upregulated CD39/CD73-adenosine and Ccl2/Ccr2 pathways, contribute to an immunosuppressive TIME in chrysotile-induced MMs of Bap1-mutant mice, suggesting that immunotherapeutic strategies targeting protumorigenic immune pathways could be beneficial in human BAP1 mutation carriers who develop MM. SIGNIFICANCE: We show that germline Bap1-mutant mice have enhanced susceptibility to MM upon minimal exposure to chrysotile asbestos, not only amphibole fibers. Chrysotile induced a more profound immune tumor response than crocidolite in Bap1-mutant mice by upregulating CD39/CD73-adenosine and Ccl2/Ccr2 pathways and recruiting more M2 macrophages, which together contributed to an immunosuppressive tumor microenvironment. Interrogation of human MM RNA-seq data revealed interconnected immunosuppressive pathways consistent with our mouse findings.

An updated evaluation of reported no-observed adverse effect levels for chrysotile, amosite, and crocidolite asbestos for lung cancer and mesothelioma.

This analysis updates two previous analyses that evaluated the exposure-response relationships for lung cancer and mesothelioma in chrysotile-exposed cohorts. We reviewed recently published studies, as well as updated information from previous studies. Based on the 16 studies considered for chrysotile (<10% amphibole), we identified the "no-observed adverse effect level" (NOAEL) for lung cancer and/or mesothelioma; it should be noted that smoking or previous or concurrent occupational exposure to amphiboles (if it existed) was not controlled for. NOAEL values ranged from 2.3-<11.5 f/cc-years to 1600-3200 f/cc-years for lung cancer and from 100-<400 f/cc-years to 800-1599 f/cc-years for mesothelioma. The range of best-estimate NOAELs was estimated to be 97-175 f/cc-years for lung cancer and 250-379 f/cc-years for mesothelioma. None of the six cohorts of cement or friction product manufacturing workers exhibited an increased risk at any exposure level, while all but one of the six studies of textile workers reported an increased risk at one or more exposure levels. This is likely because friction and cement workers were exposed to much shorter chrysotile fibers. Only eight cases of peritoneal mesothelioma were reported in all studies on predominantly chrysotile-exposed cohorts combined. This analysis also proposed best-estimate amosite and crocidolite NOAELs for mesothelioma derived by the application of relative potency estimates to the best-estimate chrysotile NOAELs for mesothelioma and validated by epidemiology studies with exposure-response information. The best-estimate amosite and crocidolite NOAELs for mesothelioma were 2-5 f/cc-years and 0.6-1 f/cc-years, respectively. The rate of peritoneal mesothelioma in amosite- and crocidolite-exposed cohorts was between approximately 70- to 100-fold and several-hundred-fold higher than in chrysotile-exposed cohorts, respectively. These findings will help characterize potential worker and consumer health risks associated with historical and current chrysotile, amosite, and crocidolite exposures.

Effects of SETD2 on telomere length and malignant transformation property of Met-5A after one-month crocidolite exposure.

Crocidolite is a carcinogen contributing to the pathogenesis of malignant mesothelioma. This study aimed to characterize the possible telomere-related events mediating the malignant transformation of mesothelial cells with and without SETD2 under crocidolite exposure. The crocidolite concentration resulting in 90% viable SETD2 knockout Met-5A (Met-5ASETD2-KO) and Met-5A were estimated to be 0.71 µg/cm2 and 1.8 µg/cm2, respectively, during 72 h of exposure, which was further employed in chronical crocidolite exposure during a 72 h exposure interval per time up to 1 month. Chronical crocidolite-exposed Met-5ASETD2-KO (chronical Cro-Met-5ASETD2-KO) had higher colony formation and increased telomerase reverse transcriptase (TERT) protein levels than chronical crocidolite-exposed Met-5A (chronical Cro-Met-5A) and Met-5ASETD2-KO. Chronical Cro-Met-5ASETD2-KO had longer telomere length (TL) than chronical Cro-Met-5A, although there were no changes in TL for either chronical Cro-Met-5A or chronical Cro-Met-5ASETD2-KO compared with their corresponding cells without crocidolite exposure. BIBR 1532, an inhibitor targeting TERT, partially reduced colony formation and TL for chronical Cro-Met-5ASETD2-KO, while BIBR 1532 reduced TL but had no effect on colony formation for chronical Cro-Met-5A. Therefore, SETD2 deficient mesothelial cells are susceptible to malignant transformation during chronical crocidolite exposure, and TERT-dependent TL modification likely partially drives SETD2 loss-mediated early onset of mesothelial malignant transformation.

Characteristics of transcription profile, adhesion and migration of SETD2-loss Met-5A mesothelial cells exposed with crocidolite.

Asbestos is a fibrous silicate mineral exhibiting biopersistence and carcinogenic properties and contributes to mesothelioma. Despite the concept of gene-environmental interaction in pathogenesis of mesothelioma, the possible pathophysiological changes of mesothelial cells simultaneously with SET domain containing 2 (SETD2) loss and asbestos exposure remains obscure. Herein, CRISPR/Cas9-mediated SETD2 knockout Met-5A mesothelial cells (Met-5ASETD2-KO ) were established and exposed with crocidolite, an amphibole asbestos. Cell viability of Met-5ASETD2-KO appeared to dramatically decrease with ≥2.5 µg/cm2 crocidolite exposure as compared with Met-5A, although no cytotoxicity and apoptosis changes of Met-5ASETD2-KO and Met-5A was evident with 1.25 µg/cm2 crocidolite exposure for 48 h. RNA sequencing uncovered top 50 differentially expressed genes (DEGs) between 1.25 µg/cm2 crocidolite exposed Met-5ASETD2-KO (Cro-Met-5ASETD2-KO ) and 1.25 µg/cm2 crocidolite exposed Met-5A (Cro-Met-5A), and ITGA4, THBS2, MYL7, RAC2, CADM1, and CLDN11 appeared to be the primary DEGs involved with adhesion in gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Cro-Met-5ASETD2-KO had strong migration but mild adhesion behavior as compared with Cro-Met-5A. Additionally, crocidolite tended to increase migration of Met-5ASETD2-KO but inhibited migration of Met-5A when compared with their corresponding cells without crocidolite exposure, although no further adhesion property changes was evident for both cells in response to crocidolite. Therefore, crocidolite may affect adhesion-related gene expression and modify adhesion and migration behavior for SETD2-depleted Met-5A, which could provide preliminary insight regarding the potential role of SETD2 in the cell behavior of asbestos-related malignant mesothelial cell.

Manufactured doubt and the EPA 2020 chrysotile asbestos risk assessment.

While all forms of asbestos have been determined to be carcinogenic to humans by the International Agency for Research on Cancer (IARC) as well as other authoritative bodies, the relative carcinogenic potency of chrysotile continues to be argued, largely in the context of toxic tort litigation. Relatively few epidemiologic studies have investigated only a single form of asbestos; however, one study that included an asbestos textile plant located in Marshville, North Carolina that processed chrysotile asbestos was used by the United States Environmental Protection Agency (EPA) in 2020 to help inform the agency's chrysotile asbestos risk assessment. During the EPA proceedings toxic tort defense consultants submitted comments to the EPA docket and made public presentations asserting that the Marshville plant had processed amphibole asbestos types and should not be used for the chrysotile risk assessment. A detailed evaluation of defense consultant assertions and supporting information and a full assessment of the available information concerning asbestos types used at the Marshville plant was undertaken. The preponderance of evidence continues to support the conclusion that neither amosite nor crocidolite were likely to have been processed in the Marshville textile plant. Defense consultants' assertions about chrysotile use are not supported by the preponderance of evidence and constitute an example of manipulation of information to cast uncertainty and doubt rather than to seek truth and contribute to the body of scientific evidence.

Quantitative assessment of mesothelioma and lung cancer risk based on Phase Contrast Microscopy (PCM) estimates of fibre exposure: an update of 2000 asbestos cohort data.

An earlier meta-analysis of mortality studies of asbestos-exposed worker populations, quantified excess mesothelioma and lung cancer risks in relation to cumulative exposure to the three main commercial asbestos types. The aim of this paper was to update these analyses incorporating new data based on increased follow-up of studies previously included, as well as studies of worker populations exposed predominantly to single fibre types published since the original analysis. Mesothelioma as a percentage of expected mortality due to all causes of death, percentage excess lung cancer and mean cumulative exposure were abstracted from available mortality studies of workers exposed predominantly to single asbestos types. Average excess mesothelioma and lung cancer per unit of cumulative exposure were summarised for groupings of studies by fibre type; models for pleural and peritoneal mesothelioma risk and lung cancer risk in terms of cumulative exposure for the different fibre types were fitted using Poisson regression. The average mesothelioma risks (per cent of total expected mortality) per unit cumulative exposure (f/cc.yr), RM, were 0.51 for crocidolite, 0.12 for amosite, and 0.03 for the Libby mixed amphiboles cohort. Significant heterogeneity was present for cohorts classed as chrysotile, with RM values of 0.01 for chrysotile textiles cohorts and 0.0011 for other chrysotile-exposed cohorts. Average percentage excess lung cancer risks per unit cumulative exposure, RL, were 4.3 for crocidolite and amosite combined, 0.82 for Libby. Very significant heterogeneity was present for chrysotile-exposed cohorts with RL values spanning two orders of magnitude from 0.053 for the Balangero mine to 4.8 for the South Carolina textiles cohort. Best fitting models suggest a non-linear exposure-response in which the peritoneal mesothelioma risk is proportional to approximately the square of cumulative exposure. Pleural mesothelioma and lung cancer risk were proportion to powers of cumulative exposure slightly less than one and slightly higher than one respectively.

BRCA1 haploinsufficiency impairs iron metabolism to promote chrysotile-induced mesothelioma via ferroptosis resistance.

Malignant mesothelioma (MM) is still a social burden associated with asbestos exposure. Local iron accumulation thereby represents the major pathogenesis, followed by oxidative DNA strand breaks and genomic alterations in the mesothelium. BRCA1 is a critical component of homologous recombination repair directed to DNA double-stranded breaks, whereas BRCA1 germline mutation is an established risk for breast/ovarian cancer, its role in MM development remains to be elucidated. Murine Brca1 mutant models so far have not reproduced human phenotypes. However, a rat Brca1 mutant model (Mut; L63X/+ ) recently reproduced them at least partially. Here we describe the differential induction of MM in Brca1 mutant rats by intraperitoneal injection of chrysotile or crocidolite. Only Mut males injected with chrysotile revealed a promotional effect on mesothelial carcinogenesis in comparison with wild-type and/or females, with all the MMs Brca1 haploinsufficient. Array-based comparative genomic hybridization of MMs disclosed a greater extent of chromosomal deletions in Brca1 mutants, including Cdkn2a/2b accompanied by Tfr2 amplification, in comparison with wild-type tumors. Mutant MMs indicated iron metabolism dysregulation, such as an increase in catalytic Fe(II) and Ki67-index as well as a decrease in Fe(III) and ferritin expression. Simultaneously, mutant MMs revealed ferroptosis resistance by upregulation of Slc7A11 and Gpx4. At an early carcinogenic stage of 4 weeks, induced Brca1 expression in mesothelial cells was significantly suppressed in chrysotile/Mut in comparison with crocidolite/Mut, whereas significant preference to iron with a decrease in Fe(III) has been already established. In conclusion, chrysotile exposure can be a higher risk for MM in BRCA1 mutant males, considering the rat results.

The Association between the Histological Subtypes of Mesothelioma and Asbestos Exposure Characteristics.

Asbestos mining operations have left South Africa with a legacy of asbestos contamination and asbestos-related diseases continue to be a problem. The large-scale mining of three types of asbestos presents a unique opportunity to study malignant mesothelioma of the pleura (mesothelioma) in South Africa. This study aimed to describe the demographics of deceased individuals diagnosed with mesothelioma and explore any associations between the histological morphology of mesothelioma and asbestos characteristics. We reviewed the records of all deceased miners and ex-miners from the Pathology Automation System (PATHAUT) database of the National Institute of Occupational Health (NIOH) that were histologically diagnosed with mesothelioma in the period from January 2006-December 2016 (11 years). The study population does not include all cases of mesothelioma in South Africa but rather those that reached the compensation system. Crocidolite asbestos fibres were identified in the majority of mesothelioma cases (n = 140; 53.4%). The epithelioid subtype was most commonly present in both occupational and environmental cases. Cases with the sarcomatous subtype were older at death and fewer female cases were diagnosed with this subtype. No relationship between mesothelioma subtype and asbestos type or asbestos burden or fibre size was established.

Acute cytotoxicity of mineral fibres observed by time-lapse video microscopy.

Inhalation of mineral fibres is associated with the onset of an inflammatory activity in the lungs and the pleura responsible for the development of fatal malignancies. It is known that cell damage is a necessary step for triggering the inflammatory response. However, the mechanisms by which mineral fibres exert cytotoxic activity are not fully understood. In this work, the kinetics of the early cytotoxicity mechanisms of three mineral fibres (i.e., chrysotile, crocidolite and fibrous erionite) classified as carcinogenic by the International Agency for Research on Cancer, was determined for the first time in a comparative manner using time-lapse video microscopy coupled with in vitro assays. All tests were performed using the THP-1 cell line, differentiated into M0 macrophages (M0-THP-1) and exposed for short times (8 h) to 25 µg/mL aliquots of chrysotile, crocidolite and fibrous erionite. The toxic action of fibrous erionite on M0-THP-1 cells is manifested since the early steps (2 h) of the experiment while the cytotoxicity of crocidolite and chrysotile gradually increases during the time span of the experiment. Chrysotile and crocidolite prompt cell death mainly via apoptosis, while erionite exposure is also probably associated to a necrotic-like effect. The potential mechanisms underlying these different toxicity behaviours are discussed in the light of the different morphological, and chemical-physical properties of the three fibres.

Final results from a 90-day quantitative inhalation toxicology study evaluating the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos: Histopathological examination, confocal microscopy and collagen quantification of the lung and pleural cavity.

The final results from this multi-dose, 90-day inhalation toxicology study in the rat with life-time post-exposure observation have shown a significant fundamental difference in pathological response and tumorgenicity between brake dust generated from brake pads manufactured with chrysotile or from chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos. The groups exposed to brake dust showed no significant pathological or tumorigenic response in the respiratory track compared to the air control group at exposure concentrations and deposited doses well above those at which humans have been exposed. Slight alveolar/interstitial macrophage accumulation of particles was noted. Wagner grades were 1-2 (1 = control group), similar to the TiO2 particle control group. Chrysotile was not biopersistent, exhibiting in the lung a deterioration of its matrix which results in breakage into particles and short fibers which can be cleared by alveolar macrophages and which can continue to dissolve. Particle-laden macrophage accumulation was observed, leading to a very-slight interstitial inflammatory response (Wagner grade 1-3). There was no peribronchiolar inflammation, occasional very-slight interstitial fibrosis (Wagner grade 4), and no exposure-related tumorigenic response. The pathological response of crocidolite and amosite compared to the brake dust and chrysotile was clearly differentiated by the histopathology and the confocal analysis. Crocidolite and amosite induced persistent inflammation, microgranulomas, persistent fibrosis (Wagner grades 4), and a dose-related lung tumor response. Confocal microscopy quantified extensive inflammatory response and collagen development in the lung, visceral and parietal pleura as well as pleural adhesions. These results provide a clear foundation for differentiating the innocuous effects of brake dust exposure from the adverse effects following amphibole asbestos exposure.

In vitro toxicity of fibrous glaucophane.

The health hazard represented by the exposure to asbestos may also concern other minerals with asbestos-like crystal habit. One of these potentially hazardous minerals is fibrous glaucophane. Fibrous glaucophane is a major component of blueschist rocks of California (USA) currently mined for construction purposes. Dust generated by the excavation activities might potentially expose workers and the general public. The aim of this study was to determine whether fibrous glaucophane induces in vitro toxicity effects on lung cells by assessing the biological responses of cultured human pleural mesothelial cells (Met-5A) and THP-1 derived macrophages exposed for 24 h and 48 h to glaucophane fibres. Crocidolite asbestos was tested for comparison. The experimental configuration of the in vitro tests included a cell culture without fibres (i.e., control), cell cultures treated with 50 µg/mL (i.e., 15.6 µg/cm2) of crocidolite fibres and 25-50-100 µg/mL (i.e., 7.8-15.6-31.2 µg/cm2) of glaucophane fibres. Results showed that fibrous glaucophane may induce a decrease in cell viability and an increase in extra-cellular lactate dehydrogenase release in the tested cell cultures in a concentration dependent mode. Moreover, it was found that fibrous glaucophane has a potency to cause oxidative stress. The biological reactivity of fibrous glaucophane confirms that it is a toxic agent and, although it apparently induces lower toxic effects compared to crocidolite, exposure to this fibre may be responsible for the development of lung diseases in exposed unprotected workers and population.

Functional effects of differentially expressed microRNAs in A549 cells exposed to MWCNT-7 or crocidolite.

Multi-walled carbon nanotubes (MWCNT) are engineered nanomaterials widely used in industrial and biomedical applications. Yet, MWCNT inhalation may induce pulmonary adverse effects, and the MWCNT-7 (Mitsui-7) has been classified as possibly carcinogenic to humans. However, its molecular mechanisms of action are poorly understood and there are no biomarkers of exposure for occupational monitoring. Several pulmonary diseases, including lung cancer, have been associated with alterations in microRNA expression that are used as biomarkers of disease progression, and differentially-expressed microRNAs (DE miRNAs) can also allow understanding the molecular effects induced by a toxicant. In this study, we identify DE miRNAs in A549 alveolar epithelial cells following 24 h exposure to MWCNT-7 or crocidolite, as well as their enriched cellular functional pathways. These indicate that both materials change cell survival, differentiation and proliferative properties under the influence of AMPK, FoxO, TGF-ß and Hippo pathways, and their metabolic activity and cell-to-cell communication. In addition, MWCNT-7 affects the actin cytoskeleton, ubiquitin mediated proteolysis, and ECM-receptor interactions; crocidolite the PI3K-Akt and mTOR pathways, endocytosis, and central carbon metabolism. Since deregulation of these pathways may be related to carcinogenesis, an interaction network of DE miRNAs and corresponding target cancer-related genes was constructed, highlighting the carcinogenic potential of Mitsui-7.

Cytotoxicity and genotoxicity of MWCNT-7 and crocidolite: assessment in alveolar epithelial cells versus their coculture with monocyte-derived macrophages.

In the past years, several in vitro studies have addressed the pulmonary toxicity of multi-walled carbon nanotubes (MWCNT) and compared it with that caused by asbestos fibers, but their conclusions have been somewhat inconsistent and difficult to extrapolate to in vivo. Since cell coculture models were proposed to better represent the in vivo conditions than conventional monocultures, this work intended to compare the cytotoxicity and genotoxicity of MWCNT-7 (Mitsui-7) and crocidolite using A549 cells grown in a conventional monoculture or in coculture with THP-1 macrophages. Although a decrease in A549 viability was noted following exposure to a concentration range of MWCNT-7 and crocidolite, no viability change occurred in similarly exposed cocultures. Early events indicating epithelial to mesenchymal transition (EMT) were observed which could explain apoptosis resistance. The comet assay results were similar between the two models, being positive and negative for crocidolite and MWCNT-7, respectively. An increase in the micronucleus frequency was detected in the cocultured A549-treated cells with both materials, but not in the monoculture. On the other hand, exposure of A549 monocultures to MWCNT-7 induced a highly significant increase in nucleoplasmic bridges in which those were found embedded. Our overall results demonstrate that (i) both materials are cytotoxic and genotoxic, (ii) the presence of THP-1 macrophages upholds the viability of A549 cells and increases the aneugenic/clastogenic effects of both materials probably through EMT, and (iii) MWCNT-7 induces the formation of nucleoplasmic bridges in A549 cells.

Frequent homozygous deletion of Cdkn2a/2b in tremolite-induced malignant mesothelioma in rats.

The onset of malignant mesothelioma (MM) is linked to exposure to asbestos fibers. Asbestos fibers are classified as serpentine (chrysotile) or amphibole, which includes the crocidolite, amosite, anthophyllite, tremolite, and actinolite types. Although few studies have been undertaken, anthophyllite has been shown to be associated with mesothelioma, and tremolite, a contaminant in talc and chrysotile, is a risk factor for carcinogenicity. Here, after characterizing the length and width of these fibers by scanning electron microscopy, we explored the cytotoxicity induced by tremolite and anthophyllite in cells from an immortalized human mesothelial cell line (MeT5A), murine macrophages (RAW264.7), and in a rat model. Tremolite and short anthophyllite fibers were phagocytosed and localized to vacuoles, whereas the long anthophyllite fibers were caught on the pseudopod of the MeT5A and Raw 264.7 cells, according to transmission electron microscopy. The results from a 2-day time-lapse study revealed that tremolite was engulfed and damaged the MeT5A and RAW264.7 cells, but anthophyllite was not cytotoxic to these cells. Intraperitoneal injection of tremolite in rats induced diffuse serosal thickening, whereas anthophyllite formed focal fibrosis and granulomas on peritoneal serosal surfaces. Furthermore, the loss of Cdkn2a/2b, which are the most frequently lost foci in human MM, were observed in 8 cases of rat MM (homozygous deletion [5/8] and loss of heterozygosity [3/8]) by array-based comparative genomic hybridization techniques. These results indicate that tremolite initiates mesothelial injury and persistently frustrates phagocytes, causing subsequent peritoneal fibrosis and MM. The possible mechanisms of carcinogenicity based on fiber diameter/length are discussed.

Induction of Gamma-Glutamyltransferase Activity and Consequent Pro-oxidant Reactions in Human Macrophages Exposed to Crocidolite Asbestos.

Asbestos is the main causative agent of malignant pleural mesothelioma. The variety known as crocidolite (blue asbestos) owns the highest pathogenic potential, due to the dimensions of its fibers as well as to its content of iron. The latter can in fact react with macrophage-derived hydrogen peroxide in the so called Fenton reaction, giving rise to highly reactive and mutagenic hydroxyl radical. On the other hand, hydroxyl radical can as well originate after thiol-dependent reduction of iron, a process capable of starting its redox cycling. Previous studies showed that glutathione (GSH) is one such thiol, and that cellular gamma-glutamyltransferase (GGT) can efficiently potentiate GSH-dependent iron redox cycling and consequent oxidative stress. As GGT is expressed in macrophages and is released upon their activation, the present study was aimed at verifying the hypothesis that GSH/GGT-dependent redox reactions may participate in the oxidative stress following the activation of macrophages induced by crocidolite asbestos. Experiments in acellular systems confirmed that GGT-mediated metabolism of GSH can potentiate crocidolite-dependent production of superoxide anion, through the production of highly reactive dipeptide thiol cysteinyl-glycine. Cultured THP-1 macrophagic cells, as well as isolated monocytes obtained from healthy donors and differentiated to macrophages in vitro, were investigated as to their expression of GGT and the effects of exposure to crocidolite. The results show that crocidolite asbestos at subtoxic concentrations (50-250 ng/1000 cells) can upregulate GGT expression, which raises the possibility that macrophage-initiated, GSH/GGT-dependent pro-oxidant reactions may participate in the pathogenesis of tissue damage and inflammation consequent to crocidolite intoxication.

The Wittenoom legacy.

The Wittenoom crocidolite (blue asbestos) mine and mill ceased operating in 1966. The impact of this industry on asbestos-related disease in Western Australia has been immense. Use of the employment records of the Australian Blue Asbestos Company and records of the Wittenoom township residents has permitted two cohorts of people with virtually exclusive exposure to crocidolite to be assembled and studied. Follow-up of these two cohorts has been conducted through data linkage with available hospital, mortality and cancer records. The evolution of asbestos-related disease has been recorded and, with the establishment of exposure measurements, quantitative exposure-response relationships have been estimated. There has been an ongoing epidemic of mortality from lung cancer and malignant mesothelioma and, less so, from asbestosis. Wittenoom crocidolite was used extensively in asbestos-cement products in Western Australia. As a result, the state has recorded a higher malignant-mesothelioma mortality rate than in any other Australian state and in any defined general population in the world. Thus, the legacy of Wittenoom has extended beyond the mine and the town, and is still evident more than 50 years after the closure of the mine.

Evaluation of the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos in a 90-day quantitative inhalation toxicology study - Interim results Part 2: Histopathological examination, Confocal microscopy and collagen quantification of the lung and pleural cavity.

The interim results from this 90-day multi-dose, inhalation toxicology study with life-time post-exposure observation has shown an important fundamental difference in persistence and pathological response in the lung between brake dust derived from brake-pads manufactured with chrysotile, TiO2 or chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos. In the brake dust exposure groups no significant pathological response was observed at any time. Slight macrophage accumulation of particles was noted. Wagner-scores, were from 1 to 2 (1 = air-control group) and were similar to the TiO2 group. Chrysotile being biodegradable, shows a weakening of its matrix and breaking into short fibers & particles that can be cleared by alveolar macrophages and continued dissolution. In the chrysotile exposure groups, particle laden macrophage accumulation was noted leading to a slight interstitial inflammatory response (Wagner-score 1-3). There was no peribronchiolar inflammation and occasional very slight interstitial fibrosis. The histopathology and the confocal analyses clearly differentiate the pathological response from amphibole asbestos, crocidolite and amosite, compared to that from the brake dust and chrysotile. Both crocidolite and amosite induced persistent inflammation, microgranulomas, and fibrosis (Wagner-scores 4), which persisted through the post exposure period. The confocal microscopy of the lung and snap-frozen chestwalls quantified the extensive inflammatory response and collagen development in the lung and on the visceral and parietal surfaces. The interim results reported here, provide a clear basis for differentiating the effects from brake dust exposure from those following amphibole asbestos exposure. The subsequent results through life-time post-exposure will follow.

Evaluation of the exposure, dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos in a 90-day quantitative inhalation toxicology study - Interim results Part 1: Experimental design, aerosol exposure, lung burdens and BAL.

This 90-day repeated-dose inhalation toxicology study of brake-dust (BD) (brakes manufactured with chrysotile) in rats provides a comprehensive understanding of the biokinetics and potential toxicology in the lung and pleura. Exposure was 6 h/d, 5d/wk., 13wks followed by lifetime observation (~20 % survival). Control groups included a particle control (TiO2), chrysotile, commercial crocidolite and amosite asbestos. Aerosol fiber distributions of the chrysotile, crocidolite and amosite were similar (fibers L > 20 µm/cm3: chrysotile-Low/High 29/72; crocidolite 24; amosite 47 fibers/cm3; WHO-fibers/cm3: chrysotile-Low/High 119/233; crocidolite 181; amosite 281 fibers/cm3). The number of particles/cm3 in the BD was similar to that in the chrysotile, crocidolite & amosite exposures (BD 470-715; chrysotile 495-614; crocidolite 415; amosite 417 particles/cm3). In the BD groups, few fibers L > 20 µm were observed in the lungs at the end of exposure and no fibers L > 20 µm at 90d post exposure. In the chrysotile groups, means of 204,000 and 290,000 fibers(L > 20 µm)/lung were measured at 89d. By 180d, means of 1 and 3.9 fibers were counted on the filter corresponding to 14,000 and 55,000 fibers(L > 20 µm)/lung. In the crocidolite and amosite groups mean lung concentrations were 9,055,000 and 11,645,000 fibers(L > 20 µm)/lung at 89d. At 180d the means remained similar with 8,026,000 and 11,591,000 fibers(L > 20 µm)/lung representing 10-13% of the total lung fibers. BAL determined the total number of macrophages, lymphocytes, neutrophils, eosinophils, epithelial-cells and IL-1 beta, TNF-alpha and TGF-beta. At the moderate aerosol concentrations used in this study, neutrophil counts increased ~5 fold in the amphibole asbestos exposure groups. All other groups and parameters showed no important differences at these exposure concentrations. The exposure and lung burden results provide a sound basis for assessing the potential toxicity of the brake dust in comparison to the TiO2 particle control and the chrysotile, crocidolite and amosite asbestos control groups. The BAL results provide an initial indication of the differential response. Part 2 presents the presentation and discussion of the histopathological and confocal microscopy findings in this study through 90 days post exposure.

Empirical model of mesothelioma potency factors for different mineral fibers based on their chemical composition and dimensionality.

Context: The potency of various mineral fiber types to produce mesothelioma was previously evaluated for numerous cohorts, but the differences in potencies for distinct fiber types have yet to be explained. Objective: To develop an empirical model that would reconstruct mesothelioma potency factors for various types of fiber based on their chemical composition and dimensionality. Methods: Typical chemical composition and dimensionality metrics (aspect ratios) were obtained and combined with mesothelioma potency factors estimated by Hodgson and Darnton method for Quebec chrysotile, South Africa amosite, South Africa and Australian crocidolite, Russian anthophyllite, Libby amphiboles, and Turkey erionite. The forward stepwise log-log regression method was utilized to determine the best combination of input parameters. Results: Mesothelioma potency factors (RM) for selected cohorts were effectively reconstructed utilizing the median aspect ratio of fibers and equivalent fractions of SiO2, total Fe oxides or total equivalent Fe3+ as Fe2O3, and MgO. Modeled potency factors increase as the aspect ratio, SiO2, and total Fe oxide (or Fe2O3) content grow, and as the MgO content diminishes. Correlation coefficients up to 0.999, p < 0.01, were achieved. The models also yield reasonable estimates of mesothelioma potencies for other fiber types, including Bolivian crocidolite, Russian chrysotile, fluoro-edenite, and others. Conclusion: In spite of the empirical approach, the proposed models provide a starting point for targeted studies of mesothelioma mechanisms by elucidating significant contributing physicochemical factors. The models have an exploratory and preliminary character but can potentially be useful to introduce quantitative structure-activity relationship approaches for the toxicology of fibrous minerals.