**3. Asbestos**

## **3.1 Physiological and molecular impact of exposure**

Asbestos is a term used to define a group of mineral fibers incorporated in a wide variety of products, including talcum powder, brake pads, and construction materials. While more than 50 countries have banned the use of asbestos-containing materials, more than 2 million metric tonnes are still produced every year, which still poses a great public health risk for asbestos-related diseases [50, 51]. There are two main classes of asbestos: chrysotile (spiral-shaped, the most common form) and amphibole (needle-shaped). Other elements such as iron (which can constitute up to 30% of the weight of asbestos fibers) embedded in the surface of fibers can potentiate asbestos-related pathogenic effects [52, 53]. Importantly, all identified forms of asbestos have been classified as carcinogens to humans (Group 1) by the IARC [54].

Exposure to asbestos fibers has been strongly linked to the development of malignant mesothelioma, but it is also a known contributor to the development of lung cancer [55–57]. Between 5 and 7% of all lung cancer cases worldwide have records of high levels of asbestos, mostly derived from occupational exposure (e.g., mining) [10]. Exposure is usually determined by the presence of pleural plaques (areas of fibrosis associated with past exposure to asbestos), or by detection of asbestos fibers in bronchoalveolar lavage (BAL) and lung tissue [58]. The primary source of asbestos exposure comes from inhaled fibers [54]. However, the mechanism of disruption that occurs as a result of asbestos exposure is determined by the efficiency of fiber clearance from airway cells. Longer fibers are cleared at a slower rate than short fibers, and are associated with higher carcinogenic potential [59]. Similarly, thin fibers (width <0.25 μm) are more carcinogenic than thicker ones [60], likely because they can penetrate deeper in airways. Accumulation of asbestos fibers in the lung leads to fibrosis, inflammation, and carcinogenesis, although specific effects depend on the cumulative dose and the type of fiber inhaled [61, 62].

Asbestos-related carcinogenesis is thought to primarily result from the ability of the fibers to induce oxidative stress (**Figure 2**), although the specific mechanisms are not yet fully understood [63]. Asbestos induces the recruitment of alveolar macrophages, followed by an inflammatory reaction [64–66]. Failed phagocytosis of these fibers by macrophages results in the generation of ROS, together with the release of cytokines, chemokines, proteases, and growth factors further amplifying deleterious effects of asbestos [10, 56, 67]. Additionally, the iron contained in asbestos fibers deposits in the lungs and cycles between the reduced and oxidized forms, potentially inducing further oxidative DNA damage in nearby cells via the Fenton reaction which converts H2O2 into more reactive ROS [10, 56, 68, 69].

In lungs, oxidative stress following asbestos exposure can activate several signaling pathways including mitogen-activated protein kinases (MAPK), NF-κB, and activator protein 1 (AP1). All of these pathways have been linked to increases in early response genes (e.g., *JUN* and *FOS*) that govern cell proliferation, apoptosis, and inflammatory signaling [55, 56].

## **3.2 Carcinogenic mechanisms**

The most frequent asbestos-induced alterations in cancer-related genes have been reported in tumor suppressor genes (TSGs). Activation of p53 and p21 are frequently described, both in animal models and lung cancer patients with asbestosis (reviewed in [63]). This likely represents the initial DNA-damage response following exposure to asbestos-induced oxidative stress. In lung cancer patients, the frequency of *TP53* gene mutations is similar between asbestos-exposed and

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**Figure 2.**

exposure to asbestos fibers [78, 79].

*Molecular mechanisms of asbestos-induced carcinogenesis.*

matrix remodeling [81].

*Oncogenetics of Lung Cancer Induced by Environmental Carcinogens*

unexposed NSCLC cases; however, a higher frequency of G:C to T:A transversions in the sequence of *TP53* is observed in asbestos-exposed cases [70, 71]. Contrarily, other tumor suppressor genes such as *CDKN2A* are inactivated in asbestos-exposed lung cancer cases, mostly via segmental copy-number losses [72]. In murine models, chrysotile fibers are able to induce the activity of the c-Jun and c-Fos oncogenes and inactivate p53 and p16 tumor suppressors, both at the mRNA and protein levels [73]. Additionally, other well-known lung cancer genes and pathways have been shown to display aberrant functions in response to asbestos exposure. Different mechanisms of asbestos-mediated activation of the EGFR pathway have been described. Asbestos-induced chronic inflammation has been associated with activation of the EGFR-related and extracellular signal-regulated kinase (ERK) signaling pathway that promote lung epithelial cell and fibroblast proliferation [55, 56, 74]. Also, asbestos fibers can induce over-expression of EGFR mRNA and induce protein dimerization, phosphorylation, and subsequent pathway activation by directly interacting with the surface portion of the receptor [63, 75, 76]. On the other hand, DNA mutations affecting EGFR do not seem to be main mechanisms of asbestosinduced EGFR activation. Asbestos-exposed patients displayed a significantly lower rate of *EGFR* mutations compared to non-exposed patients [77]. Moreover, it is unclear if there is a causal relationship between the mutations found in *EGFR* and

Other genes, such as *MAP4K3, CEBPZ, QPCT, FANCG, IGFBPL1, CCL19, MELK, FANCM*, and *CDKL1* have shown aberrant gene expression in human epithelial bronchial cell lines (Beas-2B), following asbestos exposure [80]. Asbestos inhalation also causes up-regulation of mRNA levels of matrix metalloproteinase family members in rat lungs, suggesting induction of extracellular

*DOI: http://dx.doi.org/10.5772/intechopen.81064*

*Oncogenetics of Lung Cancer Induced by Environmental Carcinogens DOI: http://dx.doi.org/10.5772/intechopen.81064*

*Oncogenes and Carcinogenesis*

**3.1 Physiological and molecular impact of exposure**

Asbestos is a term used to define a group of mineral fibers incorporated in a wide variety of products, including talcum powder, brake pads, and construction materials. While more than 50 countries have banned the use of asbestos-containing materials, more than 2 million metric tonnes are still produced every year, which still poses a great public health risk for asbestos-related diseases [50, 51]. There are two main classes of asbestos: chrysotile (spiral-shaped, the most common form) and amphibole (needle-shaped). Other elements such as iron (which can constitute up to 30% of the weight of asbestos fibers) embedded in the surface of fibers can potentiate asbestos-related pathogenic effects [52, 53]. Importantly, all identified forms of asbestos have been classified as carcinogens to humans (Group 1) by the IARC [54]. Exposure to asbestos fibers has been strongly linked to the development of malignant mesothelioma, but it is also a known contributor to the development of lung cancer [55–57]. Between 5 and 7% of all lung cancer cases worldwide have records of high levels of asbestos, mostly derived from occupational exposure (e.g., mining) [10]. Exposure is usually determined by the presence of pleural plaques (areas of fibrosis associated with past exposure to asbestos), or by detection of asbestos fibers in bronchoalveolar lavage (BAL) and lung tissue [58]. The primary source of asbestos exposure comes from inhaled fibers [54]. However, the mechanism of disruption that occurs as a result of asbestos exposure is determined by the efficiency of fiber clearance from airway cells. Longer fibers are cleared at a slower rate than short fibers, and are associated with higher carcinogenic potential [59]. Similarly, thin fibers (width <0.25 μm) are more carcinogenic than thicker ones [60], likely because they can penetrate deeper in airways. Accumulation of asbestos fibers in the lung leads to fibrosis, inflammation, and carcinogenesis, although specific effects depend on the cumulative dose and the type of fiber inhaled [61, 62]. Asbestos-related carcinogenesis is thought to primarily result from the ability of the fibers to induce oxidative stress (**Figure 2**), although the specific mechanisms are not yet fully understood [63]. Asbestos induces the recruitment of alveolar macrophages, followed by an inflammatory reaction [64–66]. Failed phagocytosis of these fibers by macrophages results in the generation of ROS, together with the release of cytokines, chemokines, proteases, and growth factors further amplifying deleterious effects of asbestos [10, 56, 67]. Additionally, the iron contained in asbestos fibers deposits in the lungs and cycles between the reduced and oxidized forms, potentially inducing further oxidative DNA damage in nearby cells via the Fenton reaction which converts H2O2 into more reactive ROS [10, 56, 68, 69]. In lungs, oxidative stress following asbestos exposure can activate several signaling pathways including mitogen-activated protein kinases (MAPK), NF-κB, and activator protein 1 (AP1). All of these pathways have been linked to increases in early response genes (e.g., *JUN* and *FOS*) that govern cell proliferation, apoptosis,

The most frequent asbestos-induced alterations in cancer-related genes have been reported in tumor suppressor genes (TSGs). Activation of p53 and p21 are frequently described, both in animal models and lung cancer patients with asbestosis (reviewed in [63]). This likely represents the initial DNA-damage response following exposure to asbestos-induced oxidative stress. In lung cancer patients, the frequency of *TP53* gene mutations is similar between asbestos-exposed and

**3. Asbestos**

**76**

and inflammatory signaling [55, 56].

**3.2 Carcinogenic mechanisms**

**Figure 2.** *Molecular mechanisms of asbestos-induced carcinogenesis.*

unexposed NSCLC cases; however, a higher frequency of G:C to T:A transversions in the sequence of *TP53* is observed in asbestos-exposed cases [70, 71]. Contrarily, other tumor suppressor genes such as *CDKN2A* are inactivated in asbestos-exposed lung cancer cases, mostly via segmental copy-number losses [72]. In murine models, chrysotile fibers are able to induce the activity of the c-Jun and c-Fos oncogenes and inactivate p53 and p16 tumor suppressors, both at the mRNA and protein levels [73].

Additionally, other well-known lung cancer genes and pathways have been shown to display aberrant functions in response to asbestos exposure. Different mechanisms of asbestos-mediated activation of the EGFR pathway have been described. Asbestos-induced chronic inflammation has been associated with activation of the EGFR-related and extracellular signal-regulated kinase (ERK) signaling pathway that promote lung epithelial cell and fibroblast proliferation [55, 56, 74]. Also, asbestos fibers can induce over-expression of EGFR mRNA and induce protein dimerization, phosphorylation, and subsequent pathway activation by directly interacting with the surface portion of the receptor [63, 75, 76]. On the other hand, DNA mutations affecting EGFR do not seem to be main mechanisms of asbestosinduced EGFR activation. Asbestos-exposed patients displayed a significantly lower rate of *EGFR* mutations compared to non-exposed patients [77]. Moreover, it is unclear if there is a causal relationship between the mutations found in *EGFR* and exposure to asbestos fibers [78, 79].

Other genes, such as *MAP4K3, CEBPZ, QPCT, FANCG, IGFBPL1, CCL19, MELK, FANCM*, and *CDKL1* have shown aberrant gene expression in human epithelial bronchial cell lines (Beas-2B), following asbestos exposure [80]. Asbestos inhalation also causes up-regulation of mRNA levels of matrix metalloproteinase family members in rat lungs, suggesting induction of extracellular matrix remodeling [81].

At the epigenetic level, alterations affecting tumor suppressor genes have been observed in lung cancer cases associated with asbestos exposure, including those in the promoter regions of *RASSF1A* and *CDKN2A* (p16) [82]. Additionally, a genome-wide DNA methylation study identified differentially methylated CpGs in regions nearby the transcription start site of genes such as *NPTN, NRG2, GLT25D2* and *TRPC3* to be significantly associated with asbestos exposure [83].

The effect of asbestos on micro RNA (miRNA) expression has been also investigated. miRNAs are short (~22 nucleotide) RNA transcripts that negatively regulate gene expression through direct interaction with mRNAs. Interestingly, the overexpression of miR-148b has been described in multiple independent studies. This miRNA was part of an asbestos-related signature in lung tumors, also composed of seven other overexpressed (miR-374a, miR-24-1\*, let-7d, Let-7e, miR-199b-5p, miR-331-3p, and miR-96) and five miRNAs with decreased expression in tumors (miR-939, miR-671-5p, miR-605, miR-1224-5p, and miR-20) [84]. Additionally, miR-148b was found to be overexpressed in asbestos-related lung cancer compared to tumors in non-exposed individuals, and three of its targets (*GADD45A, LTBP1* and *FOSB*) were down-regulated in asbestos-exposed patients [84].

Despite the known genetic and epigenetic abnormalities resulting from asbestos exposure, a relatively small proportion of exposed individuals develop thoracic malignancies (mesothelioma or lung cancer). It has been hypothesized that specific genetic variants may confer increased risk of developing asbestos-related diseases [85]. Thus, recent studies have investigated the association between genomic variants and risk of lung cancer following asbestos exposure. In a genome-wide association study (GWAS) performed in the Texas lung cancer GWAS dataset, the authors did not find statistical evidence for gene-asbestos interaction in the etiology of lung cancer [86]. However, the Fas signaling pathway (regulation of tissue homeostasis in the immune system by inducing apoptosis) was identified as the most significant pathway associated with asbestos exposure in the etiology of lung cancer. Another study identified three single nucleotide polymorphisms (SNPs) in the *MIRLET7BHG* (*MIRLET7B* host gene located at 22q13.31) significantly associated with increased lung cancer risk among individuals exposed to asbestos [36].

The identification of risk variants linked with asbestos-related lung cancer is a challenging task. Sample sizes for asbestos-related lung cancer cohorts are particularly limited by the number of cases that can be unequivocally attributed to asbestos exposure despite other well-known factors (e.g., smoking). Thus, focusing on the genes and chromosomal regions found by these preliminary studies might be useful for more targeted strategies aiming to validate these results.
