**1. Introduction**

It is well known that asbestos fibers cause lung fibrosis as well as certain malignant diseases such as malignant mesothelioma, lung cancer, and other diseases (the International Agency for Research on Cancer (IARC) indicated that asbestos exposure results in a significant increased risk for ovarian and laryngeal cancers) [1–4]. A consideration of the carcinogenic mechanisms of asbestos suggests that various factors may be related. One factor involves DNA damage caused by reactive oxygen species (ROS) produced by asbestos fibers, especially iron-containing fibers such as crocidolite (CR) and amosite [5–7]. In addition to this aspect, ROS are also produced by alveolar macrophages which handle asbestos fibers as a foreign substance. However, they are not able to completely process the fibers because of the rigid and long morphological features of the fibers [8, 9]. Thus, these cells fail as a "frustrated macrophage" and begin to produce ROS [8, 9]. Another mechanism is the direct damage to DNA in cells located near the fibers since the cells possess a tendency to incorporate these foreign fibers into their interiors, but the fibers reach and damage cellular DNA directly because of the physiological features of the fibers [10, 11]. Furthermore, inhaled asbestos fibers may be found in the lung, related lymph nodes, and other pulmonary areas for a long time since they are not removed easily from the human body. Various carcinogenic substances existing in inspired air such as tobacco smoke and air pollutants are adsorbed onto the surface of the asbestos fibers. These additional substances also cause DNA damage to cells surrounding fibers [12, 13].

Asbestos fibers may, therefore, affect human immune cells. To investigate the effects of asbestos fibers on human immune cells, especially T cells, a human T-cell leukemia virus 1 (HTLV-1) immortalized human polyclonal T cell line, MT-2 [30], was selected for use in the establishment of a cell line model of asbestos exposure to immune cells. To choose an MT-2 cell line, various human T or B cell-derived tumorous or virus immortalized cell lines were transiently exposed to asbestos fibers, namely, chrysotile (CH) [31]. We selected chrysotile because of its wide use around the world, and the most exposed populations are thought to have resulted mainly through inhaled chrysotile fibers, although other iron-containing fibers such as crocidolite and amosite are known to possess a much higher potential for carcinogenic activity. Among the various cell lines, MT-2 was the most sensitive (growth inhibition was the strongest). The MT-2 cell line was then used to investigate the mechanisms of cell death in MT-2 cells exposed to asbestos fibers using transient and relatively high doses (doses causing cell death in at least half of the cells) [31, 32]. Thereafter, changes of cell death in MT-2 cells by a continuous and relatively low dose (doses causing cell death in less than half of cells) were investigated to explore cellular and molecular alterations in T cells by long-term exposure to asbestos. Exposure to asbestos in a human population is thought to involve a continuous, recurrent, and low-dose exposure, even for immune cells, because the existence of asbestos fibers in the lung and related lymph nodes can cause repeated encounters between immune

Cytotoxicity Caused by Asbestos Fibers and Acquisition of Resistance by Continuous Exposure…

http://dx.doi.org/10.5772/intechopen.72064

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**3. Transient and high-dose exposure to asbestos fibers in MT-2 cells:** 

pleural mesothelial cells were previously reported [14–17].

The left side of **Figure 1** shows findings concerning the transient and high-dose exposure to asbestos fibers in MT-2 cells. The cells proceed to apoptosis just as alveolar epithelial cells and

Asbestos exposure caused production of ROS. **Figure 1** shows the production of superoxide

molecules in the mitogen-activated protein kinase (MAPK) pathway such as JNK and p38 were then phosphorylated. Cytochrome c in mitochondria was then released into the cytoplasm. As a result, the proapoptotic molecule BAX was upregulated in the cells. These findings indicated that the mitochondrial apoptotic pathway was activated by asbestos exposure. Caspase 9 and 3 were then truncated into active forms to cause apoptosis of cells [31, 32].

In addition, cellular phenomena such as growth inhibition, appearance of apoptosis analyzed by annexin V staining (as an early event), activation of caspase 3, positivity of Tunel staining (a late event), and ROS production were compared between MT-2 cells exposed to fibers of chrysotile and crocidolite (CH and CR, respectively in **Figure 1**). Since CR contains a massive level of iron compared with CH, ROS production was higher in MT-2 cells exposed to CR. However, other events (the degree of growth inhibition, appearance of apoptosis assayed by different methods) were stronger in MT-2 cells exposed to CH compared to CR, although these were just comparisons between these two fibers and apoptosis was certainly caused by

) as positive for hydroethidine analyzed by flow cytometry. Proapoptotic signaling

cells and fibers [32].

**Cytotoxicity**

anion (O<sup>2</sup>

−

asbestos exposure on MT-2 cells [31–33].

Cytotoxicity caused by asbestos fibers, particularly DNA damage caused by fibers, has been investigated in alveolar epithelial cells and pleural mesothelial cells since these cells are the targets of asbestos-induced cancers [14–17]. DNA damage was found when asbestos fibers were exposed to these cells using transient and relatively high doses, which cause apoptosis of cells. Subsequently, the accumulation of relatively small DNA damage that does not cause a quick cell death and/or escape from the apoptotic pathway by continuous or recurrent and relatively low-dose exposure which may exist in the bodies of asbestos-exposed populations is thought to represent the mechanism by which cancers occur in these populations.
