Asbestos is a fibrous silicate with particular physical/chemical properties and tensile strength that makes it an ideal material for various construction and covering purposes. For this reason it was widely used for more than 100 years and is currently still used in some countries, despite the alarming reports about its toxicity and possible carcinogenicity1
. Asbestos exposure has clearly been associated with development of pulmonary diseases including bronchogenic carcinoma, mesothelioma, pleural plaque and asbestosis (pulmonary fibrosis due to asbestos exposure). A very recent report of the International Agency for Research on Cancer (IARC) classifies all types of asbestos as carcinogenic to humans with sufficient evidence for different types of cancers2
While asbestos toxicity is undeniable, the exact pathogenic mechanisms and the precise co-factors by which asbestos fibres trigger pulmonary toxicity and neoplastic transformation have not been fully understood. Many groups are continuously reconsidering and researching the mechanisms relevant to asbestos's pathogenicity. These include generation of Fe derived free radicals and reactive nitrogen species, release of cytokines, as well as induced genotoxicity and alteration of immune responses. In addition, ionizing radiation and simian virus 40 (SV40) may play some roles specifically in the induction of mesothelioma3,4,5,6,7
Following decades of fibre toxicology studies, asbestos toxicity is considered a consequence of some characteristic physico-chemical properties of the material8,9,10
. For fibres in general, the pathogenicity is described within a toxicology structure-activity paradigm, the main determinants being fibre length, diameter and bio-persistence8
. For asbestos, specific chemical composition of the materials and surface properties should also to be considered as potentially affecting bio-persistence and thus carcinogenic effects10
The presence of transition metals in the fibres and/or the ability of the fibres to adsorb and accumulate them are the first mechanisms suggested for explaining the toxic and particularly carcinogenic effects of asbestos11,12,13
. The presence of iron in the fibres (which may contain up to 30% of Fe w/w) and more importantly the intrinsic ability to attract it from the surrounding environment seems to be also a key factor for asbestos toxicity and for the formation in the lung of the asbestos bodies that are the hallmarks of asbestos exposure5,14
The formation of an asbestos body is an intriguing phenomenon that results in the deposition of endogenous iron, iron containing proteins (as ferritin), mucopolysaccharides and other material on bio-persistent fibres in the lungs. On one hand, it is believed that the shell that is formed isolates the fibre from the tissue and reduces its damaging effect. On the other hand, the locally altered homeostasis of iron produced by the reaction to asbestos fibres and body formation, together with the presence of a potentially reversible iron reservoir constituted by the iron-containing protein aggregates, is considered as responsible for an increase of iron mediated ROS production. This may trigger asbestos related diseases, with potential DNA damage and apoptosis resistance5,14,15
Not all asbestos fibres found in lung are coated (variable percentages of free fibres are observed by different techniques), however correlation between asbestos related diseases and the relative abundance of coated to uncoated fibres has not been demonstrated yet. Similarly, lung tissue burdens of asbestos fibres have long been used as an index of exposure, but the studies have not established correlations between this index and cancer development16,17
It is interesting that according to the most recent views, the central role of iron in asbestos toxicity and related diseases pathogenesis is consistent with a more general picture of a steadily growing number of diseases characterized by imbalance of the iron metabolism in cells and tissues. Concerning this issue, the recent reviews by D. Kell18,19
analyse the wide but dispersed literature via a systems biology approach, and indicate that iron that is weakly complexed boosts many biochemical processes leading to a large variety of apparently unrelated diseases. Among the most prominent examples are Alzheimer's and Parkinson's diseases, metabolic syndrome, diabetes, atherosclerosis and even cancer. Understanding the mechanisms of iron participation in the aetiology of these pathologies may lead to novel therapeutic targets.
Iron in living cells is a trace element and has a crucial role, acting as a redox component of fundamental enzymes and proteins. However, the same divalent character of iron that plays an important biological role, may cause toxicity by sustaining oxidative stress conditions. Due to the presence of high-affinity iron binding and storage proteins, the actual toxicity of iron is restricted to the so called “labile iron pool”. Chronic increase of such ‘free’ iron forms are associated with pathological conditions and possible complex homeostasis unbalance for this element in cells and tissues, relevant to asbestos toxicity and related diseases18,19,20
It is known that airway epithelial cells and alveolar macrophages in lung have an active iron metabolism. The airway epithelial cells, acting as the first line defence against environmental insults, including asbestos, may be efficient in reducing non-transferrin-bound iron, converting it to less toxic protein-bound iron21,22
. However, the major lung defence is performed by one class of macrophages (alveolar macrophages) that, interacting and responding to various stresses, are capable of iron scavenging too, protecting lung tissue against oxidative damage23,24,25
. These macrophages are also involved in the pathogenesis of asbestosis and cancer5,25,26
. During inflammation, iron efflux from alveolar macrophages may be induced via several mechanisms. The paths involved in the overall iron mobilization and utilization by alveolar macrophages as well as epithelial cells are largely unknown. One of the reasons is the limited performance of specific conventional histological staining procedures used for such investigations.
Little is also known about the cellular structures that are involved in transient storing of iron and the real fate and chemical reactivity of metal ions under normal and pathological conditions.
An important initial step towards unravelling all these issues is the identification and localization of the metal in native physiological environments in tissues and cells. In this respect synchrotron-based X-ray microscopy approaches are becoming very desirable tools providing correlated morphology and chemical information of the specimen. The combination of X-ray imaging with X-ray Fluorescence microscopy (μ-XRF) allows monitoring of the distribution of elements in tissue samples without artefacts27,28
, as demonstrated in our recent study of asbestos bodies29
. In this first study we also revealed the contribution of magnesium to the asbestos body formation, which appears to be related to the presence of iron as well.
The present study further explores the chemistry of tissue reaction to the presence of asbestos by primarily using a medium energy XRF microscopy set-up which allowed us to investigate and resolve at high spatial resolution the presence of iron and other biologically relevant elements in the lungs of asbestos exposed patients. Correlative analyses for light elements were performed on the same samples under a low energy X-ray microscopy set-up. In addition, we report X-ray absorption near edge spectroscopy (XANES) analyses, which for the first time show the speciation of iron in the asbestos bodies.