Tests for effect of asbestos on benzo[a]pyrene carcinogenesis in the respiratory tract

Systemic Carcinogens (N-Nitroso Compounds) and Synergistic or Additive Effects in Respiratory Carcinogenesis

Systemic carcinogens induce respiratory tract tumors in various animals species. Typical examples of systemic carcinogens are the N-nitroso compounds, which require metabolic conversion to exert their carcinogenic effect. In this review particular emphasis is put on the comparative studies performed in the same species with different nitrosamines, providing an experimental model in which is possible to examine and correlate the biochemical changes preceding the development of detectable tumors and the carcinogenic effect. In addition studies describing synergistic or additive effects in respiratory carcinogenesis are reviewed and discussed.

[A comprehensive review of literature to investigate development of global knowledge and consensus on asbestos carcinogenicity: up to the report and recommendations by UICC Working Group in 1964]

This author comprehensively reviewed the literature on asbestos carcinogenicity up to the Report and Recommendations by Union Internationale Contra Cancrum (UICC) Working Group on asbestos and cancer in 1964. The first cases of mesothelioma and lung cancer in necropsied patients with asbestosis were reported in 1933 and 1934, respectively. After that, various studies examining the association between each of the diseases and asbestos exposure had been carried out until the meeting of the UICC Working Group: case report studies, case series studies, prevalence studies, historical cohort studies, and case-control studies. Newly reported studies including experimental studies in that meeting all supported the association. These findings on asbestos and cancer correspond well with Hill's criteria, which were just then advocated for evaluating causality epidemiologically. The Report and Recommendations by the Working Group concluded, "There is evidence of an association between exposure to asbestos and malignant neoplasia." and "The types of tumors ... are ... (1) carcinoma of the lungs, and (2) diffuse mesothelioma of the pleura and peritoneum." This author considers that the causal association between lung cancer or mesothelioma and asbestos was established at the meeting of UICC Working Group in 1964, not by the report on asbestos carcinogenicity in ILO (International Labour Organization) or IARC (International Agency for Research on Cancer) expert meetings in 1972, as the Japanese government announced. The amount of asbestos import in Japan doubled from 130,000 to 280,000 tons annually from 1964 to 1972. The government should have recognized the global knowledge on asbestos carcinogenicity in 1964; the amount of asbestos import could have been reduced greatly.

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.

Ascorbic acid inhibits the squamous metaplasia that results from treatment of tracheal explants with asbestos or benzo[a]pyrene-coated asbestos

Hamster tracheal explants were maintained in culture in the presence or absence of benzo[a]pyrene (BP), crocidolite asbestos, or BP-coated crocidolite. Dose-dependent squamous metaplasia was observed in the treated samples. L-Ascorbic acid and DL-alpha-tocopherol were able to partially protect the tracheal explants from the metaplastic response induced by crocidolite. Furthermore, ascorbic acid reduced the extent of metaplasia observed in hamster tracheal explants that were exposed to BP-crocidolite.

Problems of experimental tumourigenesis by fibrous dusts, especially by asbestos (with regard to stanton's hypothesis)

Asbestos dust has got a very great importance because cancerogenicity is imputed into it in occupational and, perhaps, in common environmental conditions too. The short review deals with general problems in experimenting with fibrous dusts, especially the modes of its application, the transferability of results to human pathology and the cause and pathway of cancerisation by fibres and crystals.

Asbestos catalyzes the formation of the 6-oxobenzo[a]pyrene radical from 6-hydroxybenzo[a]pyrene

Crocidolite asbestos catalyzes the oxidation of 6-hydroxybenzo[a]pyrene, a metabolite of benzo[a]pyrene, to the 6-oxobenzo[a]pyrene radical as determined by electron spin resonance spectroscopy. This may be a mechanism whereby inhaled asbestos enhances the incidence of lung cancer induced by cigarette smoke, which contains benzo[a]pyrene.

Effect of particulates on metabolism and mutagenicity of benzo[a]pyrene

Mechanisms of co-carcinogenicity of particulates, such as iron oxide and asbestos, and benzo[a]pyrene (B[a]P) are not completely understood. Particulates dramatically alter rates of uptake of B[a]P into membranes, a factor which could account for co-carcinogenicity. However, B[a]P must be activated to reactive forms to be carcinogenic and mutagenic so alterations in metabolism of B[a]P by particulates also could result in co-carcinogenesis. To elucidate mechanisms of particulate-B[a]P co-carcinogenesis, we have correlated rates of uptake of B[a]P into microsomes with metabolism of B[a]P and with mutagenicity of B[a]P in the Ames test. In general, aryl hydrocarbon hydroxylase (AHH) activity paralleled rates of uptake of B[a]P, though some inhibition of AHH activity by particulates which was not attributable to availability of B[a]P was evident. This inhibition was studied further by assaying separately mixed function oxidase and epoxide hydrase activities in the presence of particulates. Both chrysotile and iron oxide inhibited O-deethylation of 7-ethoxyresorufin and hydration of B[a]P-4,5-oxide. To determine effects of this inhibition on activation of B[a]P to reactive forms, we studied profiles of metabolites of B[a]P and mutagenicity of B[a]P. The only alteration in profiles of B[a]P metabolites produced by particulates was that due to effects on rates of uptake. Similarly, mutagenicity of B[a]P was positively correlated with rates of uptake into microsomes. We conclude that the predominant effects of chrysotile and iron oxide are in altering rates of uptake of particle-adsorbed B[a]P. Changes in uptake rates then result in alterations of B[a]P metabolism and mutagenicity.

Correlation of in vitro genotoxicity and oncogenicity induced by radiation and asbestos fibres

The in vitro cytotoxicity and oncogenic potential of both native and acid leached asbestos fibres were studied using the C3H 10T1/2 cell model. Both native and leached fibres induced a dose-dependent toxicity. At high fibre concentrations, acid leached fibres were significantly less toxic than their untreated counterparts. While asbestos fibres alone do not induce oncogenic transformation at the concentration examined, it was found that both leached and native fibres substantially enhanced the oncogenicity of gamma-irradiation in a more than additive fashion. Although no significant chromosomal aberrations or sister chromatid exchanges (SCE) were found in asbestos treated cultures, a significantly higher number of SCEs was observed in cells treated with both asbestos and radiation compared to cells receiving radiation alone. The results suggest that the enhancement in radiation induced oncogenicity by asbestos fibres may be attributed to the mere physical presence of the fibres rather than any chemical contaminants the fibres may contain. Furthermore, the carcinogenicity of asbestos may be unrelated to genotoxicity.

Lung response to particulates with emphasis on asbestos and other fibrous dusts

Many theories have been proposed to explain asbestosis and asbestos-related pulmonary disease. However, none of the theories give a completely plausible explanation for the pathogenesis. Recently, attention has been drawn to a theory that the fibrogenicity or carcinogenicity of fibrous dust particles is related to fiber diameter and length rather than to chemical properties. This theory may help partially elucidate the disease process but is still far from solving the enigma of pulmonary fibrosis or carcinogenesis. The theory cannot explain the absence of these pathological effects among fiberglass workers or experimental animals exposed by inhalation (even though mesotheliomas are induced by intrapleural implantation and fiber dimension-related fibrogenicity is demonstrated by intratracheal injection). Little information regarding the pulmonary response to manmade fibrous particles is available in animals following inhalation exposure. Attempts should be made to confirm the absence of adverse effects using animal inhalation experiments even though to this point there is no conclusive evidence that either lung cancer or pulmonary diseases can be produced among employees in manmade fiber industries. A new research trend seems concentrated on testing the durability of asbestos or manmade fibers. This is based on the concept that biological effects of fibrous particles are the result of relative durability and that particles which can be fragmented or shortened may be less pathogenic. In the last two decades, considerable understanding about pulmonary fibrosis and carcinogenesis of asbestos has been achieved by clinical and animal experiments. In vitro tests including cytotoxicity, hemolysis, immunology, and enzyme biochemistry have provided important information on the interrelationships among these various biological effects of asbestos.

The influence of asbestos dust on the oncogenic transformation of C3H10T 1/2 cells

The cell transforming ability of asbestos dust was investigated using C3H10T 1/2 murine fibroblasts. In a series of experiments both crocidolite and amosite caused no increase in the number of transformed foci over that seen in cultures from untreated cells. The dusts, were, however, capable of augmenting the oncogenic effect of benzo[a]pyrene (BP). This putative synergistic effect was evident when fibres and chemicals were added to cultures as simple mixtures and when BP was adsorbed to the surface of the fibres.

Possible effects on occupational lung cancer from smoking related changes in the mucus content of the lung

A number of recent epidemiological studies of lung cancer among occupations that expose workers to dusts or toxic aerosols have reported a larger prevalence of lung cancer among non-smoking than among smoking workers. One recent study using Beagle dogs has also reported significantly fewer respiratory tumors among animals exposed to radon, radon daughters, uranium dust and tobacco smoke than among animals exposed similarly except for tobacco smoke. The evidence is summarized here that the increased mucus in the lungs of smokers may help in the elimination of dust and toxic substances by facilitating migration of particles and aerosols and possibly also adding protection by thickening of the mucus layer. Recent developments in use of in vivo magnetometric measurements of dust content of workers' lungs in fact indicate that lungs of chronically exposed miners and millers contain less dust if they smoke than if they do not. The hypothesis that increasing mucus in the lung of exposed workers may protect them against lung disease now needs experimental verification in appropriate animal studies. If protective qualities of lung mucus are verified, means need to be explored that will increase mucus flow in non-smoking (and smoking) workers exposed to respiratory hazards.

Measurement of blood flow and vascular permeability changes in response to 12-O-tetradecanoyl-phorbol-13-acetate and to asbestos fibers

The hyperemia (increased blood flow) and vascular permeability changes induced in rabbit skin by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate and by asbestos fibers have been quantitated. The resultant hyperemia, measured with radiolabeled microspheres, was inhibited in both cases by steroidal and nonsteroidal anti-inflammatory drugs. Since the inflammatory responses resulting from exposure to these agents might be important for the subsequent development of neoplasms, it is proposed that such a specific and quantitative technique be used to analyze the components of these inflammatory responses in more detail.

Histopathological studies on experimentally induced pulmonary, pleural and peritoneal neoplasms in mice by intraperitoneal injection of chrysotile asbestos and N-methyl-N-nitrosourethane

The cocarcinogenic effects of asbestos are presented. In lung carcinomas induced in mice, the number of carcinomas and the time of detection of the first carcinoma per tumor-bearing animals were greater and faster in the group with chrysotile plus MNU than either chrysotile or MNU alone. This suggested that chrysotile asbestos had a promoting or cocarcinogenic effect on some carcinogens in the respiratory tract. In the group treated with chryotile alone, a tumor was found in the right pleural cavity at 15 months. This tumor microscopically was similar to the biphasic form of the human diffuse mesothelioma. Microvilli, basement membrane, and junctional apparatus were seen by electron microscope, but other cytoplasmic organelles of the tumor cells were relatively scanty. Two peritoneal tumors developed in gastric and intestinal serosa at 11 and 12 months. Light and electron microscopic studies suggested that the tumors were probably myosarcomas or fibrosarcoms.