Results of current intraperitoneal carcinogenicity studies with mineral and vitreous fibres

Airborne fiber particles: Types, size and concentration observed in Beijing

Airborne fibers are of public concern because of their potential threat to the environment, however their physical and chemical properties are poorly understood. Fibers are defined as having an aspect ratio >3:1. Fiber particles were collected in the near surface air, surface deposited dust and building materials in Beijing. They were examined using analytical scanning electron microscopy. The particles were initially classified into two categories: organic and inorganic. Organic fibers comprised microplastic and natural organic fiber particles. Inorganic fibers were mainly man-made mineral fibers (MMMFs), asbestos (represented by chrysotile), calcium sulfate and metal fiber particles. Microplastic and MMMFs fibers were most abundant, accounting for 34.6% and 40.3% in total, respectively, followed by asbestos (7.8%), calcium sulfate (7.2%), metal fibers (5.6%) and natural organic fiber particles (4.5%). The number-concentration of these particles was about 16.7 × 10^-3 fibers/ml at 1.5 m above the ground and about 14.1 × 10^-3 fibers/ml at about 18 m, suggesting the particles were mainly derived from surface and were re-suspended. Approximately 80% of the airborne fiber were smaller than 20 μm in length, which is possibly the critical size for fiber particles to re-suspend into the air. Surface dust and construction sites were speculated to be the major contributors of the fiber particles.

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.

Toxicological and epidemiological studies on effects of airborne fibers: coherence and public [corrected] health implications

Airborne fibers, when sufficiently biopersistent, can cause chronic pleural diseases, as well as excess pulmonary fibrosis and lung cancers. Mesothelioma and pleural plaques are caused by biopersistent fibers thinner than ∼0.1 μm and longer than ∼5 μm. Excess lung cancer and pulmonary fibrosis are caused by biopersistent fibers that are longer than ∼20 μm. While biopersistence varies with fiber type, all amphibole and erionite fibers are sufficiently biopersistent to cause pathogenic effects, while the greater in vivo solubility of chrysotile fibers makes them somewhat less causal for the lung diseases, and much less causal for the pleural diseases. Most synthetic vitreous fibers are more soluble in vivo than chrysotile, and pose little, if any, health pulmonary or pleural health risk, but some specialty SVFs were sufficiently biopersistent to cause pathogenic effects in animal studies. My conclusions are based on the following: 1) epidemiologic studies that specified the origin of the fibers by type, and especially those that identified their fiber length and diameter distributions; 2) laboratory-based toxicologic studies involving fiber size characterization and/or dissolution rates and long-term observation of biological responses; and 3) the largely coherent findings of the epidemiology and the toxicology. The strong dependence of effects on fiber diameter, length, and biopersistence makes reliable routine quantitative exposure and risk assessment impractical in some cases, since it would require transmission electronic microscopic examination, of representative membrane filter samples, for determining statistically sufficient numbers of fibers longer than 5 and 20 μm, and those thinner than 0.1 μm, based on the fiber types.

Mouse models of mesothelioma: strengths, limitations and clinical translation

SUMMARY 

Mouse models of cancer are invaluable for obtaining detailed knowledge about tumor development and for screening therapeutic and preventive approaches. Mesothelioma is an unusual cancer because the same carcinogen, asbestos, causes a similar disease in both humans and animals. Unlike most other cancers, murine mesothelioma can therefore be regarded as a disease homolog, rather than a model as such. However, because asbestos-induced cancer has low penetrance and a long lag time, most translational studies have utilized more efficient models such as tumor transplantation. In consequence, many promising results have not translated into positive findings in patients. Here, we describe the widely used murine mesothelioma models and critically discuss their relative advantages and disadvantages. We emphasize the use of the appropriate model for the specific research question and the need to use multiple models in order to obtain robust and translatable data.

Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers

In 2011, SCOEL classified RCF as a secondary genotoxic carcinogen and supported a practical threshold. Inflammation was considered the predominant manifestation of RCF toxicity. Intrapleural and intraperitoneal implantation induced mesotheliomas and sarcomas in laboratory animals. Chronic nose-only inhalation bioassays indicated that RCF exposure in rats increased the incidence of lung cancer and similar exposures resulted in mesothelioma in hamsters, but these studies may have been compromised by overload. Epidemiological studies in the US and Europe showed an association between exposure and prevalence of respiratory symptoms and pleural plaques, but no interstitial fibrosis, mesotheliomas, or increased numbers of lung tumors were observed. As the latency of asbestos induced mesotheliomas can be up to 50 years, the relationship between RCF exposure and respiratory malignances has not been fully determined. Nonetheless, it is possible to offer useful perspectives. RCF and rock wool have similar airborne fiber dimensions and biopersistence. Therefore, it is likely that these fibers have similar toxicology. Traditional rock wool has been the subject of numerous cohort and case control studies. For rock wool, IARC (2002) concluded that the epidemiological studies did not provide evidence of carcinogenicity. Based on analogies with rock wool (read across), it is reasonable to believe that increases in lung cancer or any mesotheliomas are unlikely to be found in the RCF-exposed cohort. RCF producers have developed a product stewardship program to measure and control fiber concentrations and to further understand the health status of their workers.

Determinants of carbon nanotube toxicity

In the last few years questions have been raised regarding the potential toxicity of carbon nanotubes (CNTs) to humans and environment. It is believed that the physico-chemical characteristics of these materials are key determinants of CNT interaction with living organisms, and hence determine their toxicity. As for other nanomaterials, the most important of these characteristics are the length, diameter, surface area, tendency to agglomerate, bio-durability, presence and nature of catalyst residues as well as chemical functionalization of the CNT. This review highlights the recent advancements in the understanding of the CNT properties which are essential in determining CNT toxicity. Hence the focus is on CNT dimensions, surface properties, bio-durability and corona formation as these fields have evolved greatly in recent years. A deeper understanding of these events and their underlying mechanisms could provide a molecular explanation of the biological and physiological responses following CNT administration and therefore help in the development of safe by design materials.

Man-made mineral fibers and the respiratory tract

Man-made mineral fibers are produced using inorganic materials and are widely used as thermal and acoustic insulation. These basically include continuous fiberglass filaments, glass wool (fiberglass insulation), stone wool, slag wool and refractory ceramic fibers. Likewise, in the last two decades nanoscale fibers have also been developed, among these being carbon nanotubes with their high electrical conductivity, mechanical resistance and thermal stability. Both man-made mineral fibers and carbon nanotubes have properties that make them inhalable and potentially harmful, which have led to studies to assess their pathogenicity. The aim of this review is to analyze the knowledge that currently exists about the ability of these fibers to produce respiratory diseases.

Induction of mesothelioma by a single intrascrotal administration of multi-wall carbon nanotube in intact male Fischer 344 rats

The present study assessed a carcinogenic hazard of multi-wall carbon nanotube (MWCNT) in intact (not genetically modified) rodents. MWCNT (1 mg/kg body weight, 7 animals), crocidolite (2 mg/kg body weight, 10 animals) or vehicle (2% carboxymethyl cellulose, 5 animals) was administered to male Fischer 344 rats (12 weeks old) by a single intrascrotal injection. Rats were autopsied immediately after death, when becoming moribund or at the end of the maximal observation period scheduled to be 52 weeks. After 37-40 weeks, however, 6 MWCNT-treated animals died or became moribund due to intraperitoneally disseminated mesothelioma (6/7, 85.7%) with bloody ascites. Peritoneal mesothelium was generally hypertrophic, and numerous nodular or papillary lesions of mesothelioma and mesothelial hyperplasia were developed. While mesothelioid cells were predominant in relatively early stage tumors, advanced stage mesotheliomas were constituted by 2 portions occupied by mesothelioid cells on the surface and spindle-shaped sarcomatous cells in the depth. In the latter, the histological transition was apparently observed between these 2 portions. Mesotheliomas were invasive to adjacent organs and tissues, and frequently metastasized into the pleura. Only 1 rat survived for 52 weeks in the MWCNT-treated group, and similar findings except mesothelioma were observed. All 10 crocidolite-treated and 5 vehicle-treated rats survived for 52 weeks without any particular changes except deposition of asbestos in the former case. It is thus indicated that MWCNT possesses carcinogenicity causing mesothelioma at a high rate in intact male rats under the present experimental conditions. The present data identifies a carcinogenic hazard of MWCNT and will serve as one of the indispensable evidences to be used for the risk assessment crucial for not only protection and improvement of human health and welfare, but also safe and acceptable development and prevalence of this and similar upcoming materials.

Absence of carcinogenic response to multiwall carbon nanotubes in a 2-year bioassay in the peritoneal cavity of the rat

Toxicological investigations of carbon nanotubes have shown that they can induce pulmonary toxicity, and similarities with asbestos fibers have been suggested. We previously reported that multiwall carbon nanotubes (MWCNT) induced lung inflammation, granulomas and fibrotic reactions. The same MWCNT also caused mutations in epithelial cells in vitro and in vivo. These inflammatory and genotoxic activities were related to the presence of defects in the structure of the nanotubes. In view of the strong links between inflammation, mutations and cancer, these observations prompted us to explore the carcinogenic potential of these MWCNT in the peritoneal cavity of rats. The incidence of mesothelioma and other tumors was recorded in three groups of 50 male Wistar rats injected intraperitoneally with a single dose of MWCNT with defects (2 or 20 mg/animal) and MWCNT without defects (20 mg/animal). Two additional groups of 26 rats were used as positive (2 mg UICC crocidolite/animal) and vehicle controls. After 24 months, although crocidolite induced a clear carcinogenic response (34.6% animals with mesothelioma vs. 3.8% in vehicle controls), MWCNT with or without structural defects did not induce mesothelioma in this bioassay (4, 0, or 6%, respectively). The incidence of tumors other than mesothelioma was not significantly increased across the groups. The initial hypothesis of a contrasting carcinogenic activity between MWCNT with and without defects could not be verified in this bioassay. We discuss the possible reasons for this absence of carcinogenic response, including the length of the MWCNT tested (< 1 mum on average), the absence of a sustained inflammatory reaction to MWCNT, and the capacity of these MWCNT to quench free radicals.

The Carcinogenicity of WHO Fibers of Silicon Carbide: SiC Whiskers Compared to Cleavage Fragments of Granular SiC

A steep dose-response relationship for tumors was reported after application of 0.05 to 25 mg of whiskers of SiC in 1991 in a chronic intraperitoneal injection study in rats. In contrast, doses of 250 and 1000 mg of granular SiC did not cause tumors. The whiskers contained 107,000,000 fibers longer than 5 mum, per milligram while the granular sample was assumed to be free of such fibers. However, it has since been realized that even granular SiC may contain fibrous fragments that fulfill the definition of WHO fibers. It was the objective of our study to reanalyze the quantity of WHO fibers that were administered with the granular SiC. In addition, we examined whether the potency per WHO fiber was different for the SiC fragments and the SiC whiskers. Samples of the original granular and fibrous SiC were suspended in water and filtered. One half of each filter was analyzed by scanning electron microscopy (SEM, magnification x 2500), and transmission electron microscopy (TEM, x 10,000) was also performed for the SiC whiskers. The concentration of WHO fibers was 58,000 fibers/mg for the granular sample compared to 48,000,000 (SEM) and 42,000,000 (TEM) fibers/mg for the whiskers. The aspect ratio of the WHO fibers exceeded 10/1 for only 3.3% of the fragments but in each analysis for 96% of the whiskers. In addition, 0% of the fragments compared to 44% and 30% of the whiskers were more than 10 mum long. In total, 15 and 58 x 106 WHO fibers were injected with the granular SiC even though only 0.8% and 0% tumors were recorded. However, 20.1% and 43.3% tumors would have been expected if the carcinogenic potency were the same for the fragments and for the whiskers. We therefore conclude that carcinogenic potency is a function of the shapes of the WHO fibers and is much lower for SiC fragments than for whiskers. Hence carcinogenicity mainly is restricted to a subgroup of WHO fibers longer than about 10 and thinner than about 1 mum. This conclusion is in line with the optimal exposure index, which recently has been proposed by the U.S. Environmental Protection Agency (EPA) for asbestos.

Synthetic vitreous fibers: a review toxicology, epidemiology and regulations

This review addresses the characteristics which differentiate synthetic vitreous fibers (SVFs, e.g., fiber glass, stonewool, slagwool, refractory ceramic fibers, etc.), how these influence the potential biopersistence and toxicity, the most recent epidemiological results and the integration of these findings into the health and safety regulations in Europe and the United States. Also presented is the historical basis for the European classification directive. The use and equivalence of the chronic inhalation toxicology and chronic intraperitoneal injection studies in laboratory rodents for evaluation of fiber toxicology is assessed as well as the impact of dose selection and design on the validity of the study. While synthetic vitreous fibers can span a wide range of chemistries, recognition and understanding of the importance of biopersistence (ability to persist in the lung) in fiber toxicity has led to the development of more and more biosoluble fibers (that break down rapidly in the lung). Still, the epidemiological data available which are largely based upon the use of fibers in past decades, indicate that the SVF do not present a human health risk at current exposure levels. The animal toxicology and biopersistence data provide a coherent basis for understanding and evaluating the parameters which affect SVF toxicity. The current regulations are based upon an extensive knowledge base of chronic studies in laboratory rodents which confirm the relationship between chronic adverse effects and the biopersistence of the longer fibers that can not be fully phagocytised and efficiently cleared from the lung. The amorphous structure of synthetic vitreous fibers facilitates designing fibers in use today with low biopersistence. Both the epidemiological data and the animal studies database provide strong assurance that there is little if any health risk associated with the use of SVFs of low biopersistence. IARC (2001) reclassified these fibers from Category 2b to Category 3 (with RCF and special purpose fibers remaining in 2b) an event which has not been common in the history of these monographs.

Special-purpose fiber type 475--toxicological assessment

Type 475 special-purpose glass fiber is rather unique among the family of synthetic mineral fibers. It is used not for insulation but for "high-end" filtration products designed for high and ultra-high purity filtration of air and liquids. The designation for these types of filters varies with country and includes HEPA, ULPA, EU 10-13, EN1822, and S3. In its evaluation, type 475 has been grouped together with E-glass another special-purpose fibre often with little distinction made in terms of its chemistry and corresponding toxicological response. The detailed review of the available toxicology data on type 475 glass fibers clearly shows that following inhalation of this fiber even at relatively high doses, which likely exceed that at which lung overload in the rat is known to occur, type 475 glass fibers are not fibrogenic and do not cause tumors. These data clearly show an important differentiation in potency between type 475 glass fibers and E-glass and support treating these two types of fibers independently and not equating them though the term "special-purpose fibers." Analysis of the intraperitoneal studies taking into account fibre dimensions shows that at 109 fibers injected, there was a 0.3 tumor incidence. While these studies indicate according to the European Commission (EC) classification criteria that 475 should not be fully exonerated as a carcinogen, the results of the inhalation study fully support classification in category 3. The IP results are more difficult to interpret, however, the IP study itself provides no toxicological basis for determining what range of dose-response should correspond to EU category 3 or 2. Following the EC classification criteria, the toxicological data clearly indicate that 475 fibers are appropriately classified in EC category 3.

Classification of man-made vitreous fibers: Comments on the revaluation by an IARC working group

In 2001, an IARC working group revaluated the carcinogenic risks of man-made vitreous fibers (MMVF). Compared with the IARC evaluation in 1987, the overall evaluations of insulation glass wool, rock (stone) wool, and slag wool were changed from Group 2B to Group 3. These changes ensued from an alteration in the evidence for cancer in humans and in experimental animals: Instead of "sufficient," the evidence for cancer in experimental animals is now looked upon as "limited" if there is a carcinogenic response after intraperitoneal injection but not after recently conducted inhalation experiments. For these studies, it is argued that they did properly address the technological limitations of earlier inhalation experiments. For Maxim and McConnell [Maxim L.D., McConnell E.E., 2001. Interspecies comparisons of the toxicity of asbestos and synthetic vitreous fibers: a weight-of-the-evidence approach. Regul. Toxicol. Pharmacol. 33, 319-342], well-conducted inhalation studies are very sensitive and rats may be more sensitive than humans in detecting the carcinogenic potential of MMVF. However, their arguments are highly questionable. The explanations of the IARC working group for preferring the newer inhalation studies are not sufficiently supported by the published data. Having in mind the higher sensitivity of humans compared to rats after inhalation of asbestos, more emphasis should have been given to the carcinogenic response after intraperitoneal injection.

Induction of 8-hydroxydeoxyguanosine by man made vitreous fibres and crocidolite asbestos administered intraperitoneally in rats

Inhaled fibres with certain physico-chemical properties are known to induce mesothelioma in humans. The induction of reactive oxygen (ROS) or nitrogen species (RNS) have been suggested as molecular mechanism of fibre induced carcinogenesis. In earlier studies we were able to demonstrate that crocidolite asbestos in vivo induces mutations in transgenic rats with a specific molecular spectrum that indicates the involvement of 8-hydroxydeoxyguanosine (8-OHdG) as pre-mutagenic adduct. 8-OHdG may be induced by primary (direct) and/or secondary (cellular mediated) mechanisms. Therefore, the induction of 8-OHdG as well as the inflammatory response of animals treated with fibre samples significantly differing in their physico-chemical characteristics was investigated. As appropriate system to study mesothelioma carcinogenesis, intraperitoneal injection in rats was used with samples of UICC crocidolite, crocidolite with reduced iron content, and a vitreous fibre (MMVF 11). Equal numbers of carcinogenic fibres from each sample revealed significant comparable increases in 8-OHdG induction. Parameters of inflammation (percentage of macrophages and TNF-alpha secretion) correlated significantly with the induction of 8-OHdG, 10 weeks after treatment.

Man-made mineral (vitreous) fibres: evaluations of cancer hazards by the IARC Monographs Programme

Man-made vitreous (glass-like) fibres are non-crystalline, fibrous inorganic substances (silicates) made primarily from rock, slag, glass or other processed minerals. These materials, also called man-made mineral fibres, include glass fibres (used in glass wool and continuous glass filament), rock or stone wool, slag wool and refractory ceramic fibres. They are widely used for thermal and acoustical insulation and to a lesser extent for other purposes. These products are potentially hazardous to human health because they release airborne respirable fibres during their production, use and removal. Man-made mineral fibres and man-made vitreous fibres have been the subject of reviews by IARC Monographs Working Groups in 1987 and 2001, respectively, which resulted in evaluations of the carcinogenic hazard to humans from exposure to these materials. These reviews and evaluations have been published as Volumes 43 and 81 of the IARC Monographs series [IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 43, Man-made Mineral Fibres and Radon (1988); IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 81, Man-made Vitreous Fibres (2002)]. The re-evaluation in 2001 was undertaken because there have been substantial improvements in the quality of the epidemiological information available on the carcinogenicity to humans of glass fibres, continuous glass filament and rock/slag wool. The new evaluations have addressed the limitations of earlier cohort studies, particularly concerning the lack of adjustment with respect to concomitant risk factors such as smoking and other sources of occupational exposure. In addition, the evaluation of the evidence for carcinogenicity of glass fibres to experimental animals has been refined, by making a distinction between insulation glass wool and special-purpose glass fibres. The results of the evaluations in 1987 and 2001 are thus different in several aspects. In this paper, the reviews and evaluations of the carcinogenic hazards of exposure to man-made mineral fibres (MMMF, Monograph volume 43, [1]) and man-made vitreous fibres (MMVF, Monograph volume 81, [2]) are summarised, and the differences explained. In particular, the considerations of the respective IARC Monographs Working Groups (1987, 2001) in reaching their conclusions are discussed in some detail.

beta1-integrin mediates asbestos-induced phosphorylation of AKT and ERK1/2 in a rat pleural mesothelial cell line

Integrin-mediated signalling has been implicated in asbestos-induced carcinogenesis. In studies here, we examined signal transduction events associated with integrin-directed cell reactions triggered by crocidolite asbestos in the pleural mesothelial cell line 4/4 RM-4. Crocidolite fibres induced a significant time- and dose-dependent activation of the extracellular-signal-regulated kinases ERK1 and ERK2. ERK activation was specifically inhibited by integrin-blocking agents, that are integrin-binding peptides containing the sequence arginine-glycine-aspartic acid (RGD), and monoclonal antibodies against the integrin beta1-chain. Integrin-dependent activation of ERK1/2 in response to asbestos appeared to be independent of focal adhesion kinase pp125FAK (FAK) since FAK autophosphorylation remained unaffected in crocidolite-exposed mesothelial cells. Instead, we observed striking similarities in the kinetics of asbestos-induced ERK1/2 responses and phosphorylation of protein kinase B (AKT) at serine 473, a possible target residue for integrin-linked kinase. As with ERK activation, asbestos-induced AKT stimulation was significantly blocked by both the RGD-peptide and the beta1-integrin antibodies. These studies are the first to establish that in mesothelial cells ERK1/2 and AKT are simultaneously phosphorylated upon asbestos exposure in a beta1-integrin-dependent manner.

Application of a method to evaluate the quality of work histories and document the exposure assessment process

BACKGROUND

Review of work history records by industrial hygienists is an important component of many occupational epidemiologic studies. A number of factors may influence the hygienist, such as the quality of the data and his or her previous experience. As part of a case-control study of mesothelioma, a system was developed to capture data on several factors that can be considered in a review of work history information.

METHODS

The overall quality of the work history record was described by noting the completeness and the consistency of the information; for any potential exposures, the reviewer experience on which the decision was based and the relative quality of the information were categorized. Because of the potential for mesothelioma cases and their next-of-kin to have undergone rigorous questioning about previous asbestos exposure an evaluation of the knowledge of the respondent was included. The frequency and intensity of exposure were also evaluated.

RESULTS

Evaluation of 3,444 work records is described. The importance of data completeness in the overall evaluation of quality is shown; follow-up questions regarding specific work tasks provide information not elicited in the standard interview process. The use of the literature was an important resource to the reviewer. Asbestos was reported by the respondent as an exposure on 149 work records; of these, 111 (74%) were judged to represent an unusual level of knowledge for a next-of-kin respondent.

CONCLUSIONS

The approach presented allows capture of information about data quality and experience of the reviewer in an epidemiologic analysis. The ratings of frequency and intensity of exposure allow exploration of differences in exposure-response analyses using various exposure metrics.

Hemizygosity of Nf2 is associated with increased susceptibility to asbestos-induced peritoneal tumours

Biallelic NF2 gene inactivation is frequently found in human malignant mesothelioma. In order to assess whether NF2 hemizygosity may enhance susceptibility to asbestos fibres, we investigated the Nf2 status in mesothelioma developed in mice presenting a heterozygous mutation of the Nf2 gene (Nf2(KO3/+)), after intraperitoneal inoculation of crocidolite fibres. Asbestos-exposed Nf2(KO3/+) mice developed tumoural ascites and mesothelioma at a higher frequency than their wild-type (WT) counterparts (P&<0.05). Six out of seven mesothelioma cell lines established from neoplastic ascitic fluids of Nf2(KO3/+) mice exhibited loss of the WT Nf2 allele and no neurofibromatosis type 2 protein expression was found in these cells. The results show the importance of the NF2 gene in mesothelial oncogenesis, the potential association of asbestos exposure and tumour suppressor gene inactivation, and suggest that NF2 gene mutation may be a susceptibility factor to asbestos.

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

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

Production of reactive oxygen species by phagocytic cells after exposure to glass wool and stone wool fibres - effect of fibre preincubation in aqueous solution

The potential of four man-made vitreous fibres (MMVFs) (glass wool Code A, stone wool Code G, HT-N and MMVF 21) and of two natural mineral fibres (crocidolite, erionite) to induce production of reactive oxygen species (ROS) by differentiated HL-60 cells (HL-60-M cells) was investigated by determination of luminol-enhanced chemiluminescence (CL). Quartz served as positive control. The same system was used to uncover possible influences of fibre preincubation in aqueous solutions on the ROS-generating potential. Following preincubation in unbuffered saline over about 4 weeks, Code A and G fibres showed decreased ROS-generating potential as compared to freshly suspended fibres. On the other hand, MMVF 21 and HT-N fibres as well as crocidolite and erionite showed no decreased CL after incubation in aqueous solutions. The observed decrease of the ROS-generating potential of Code A and G fibres after preincubation may be an expression of fibre surface alterations (leaching, initiation of dissolution) that influences the response of exposed phagocytic cells. After incubation of both fibres in buffered solutions at different pH values (5.0, 7.4) a reduced ROS-generating potential was still discernible as compared to freshly suspended fibres.

P53 mutations in tumours induced by intraperitoneal injection of crocidolite asbestos and benzo[a]pyrene in rats

Mutation analysis of the tumour suppressor gene p53 in tumours induced in the peritoneal cavity of rats revealed differences in the mutational pattern with regard to the carcinogenic substances applied. In tumours induced by benzo[a]pyrene a considerable amount of p53 mutations resulting in an altered protein structure could be detected. For the development of these tumours an escape from the p53 mediated cell cycle control can be assumed. However, in tumours of the same tumour type induced by crocidolite asbestos no mutations could be observed. Since there were even no spontaneous p53 mutations detectable in this tumour group, it is obvious that in these tumours the escape from cell cycle control does not take place via inactivation of p53. Therefore, it is concluded that the molecular mechanisms of carcinogenesis and tumour development in this tumour type depend on the type of carcinogen applied.