**2.3 Prominent cancer genes affected by arsenic**

*Oncogenes and Carcinogenesis*

**2.2 Carcinogenic mechanisms**

be mediated by the proteasome [31].

radicals (OH∙), superoxide anion radicals (O2∙−), and hydrogen peroxide (H2O2), which can further damage cells through lipid oxidation, protein oxidation, and reduction of the mitochondrial membrane potential [26]. The subsequent liberation of cytochrome c can activate apoptotic pathways through caspases, leading to an abnormal rate of cell death. However in addition to faulty apoptotic signaling, exposure to arsenic can also lead to further aberrations in DNA-repair pathways. Here, arsenic affects the expression of genes involved in both nucleotide- (NER) and base-excision repair (BER) mechanisms, allowing the cell to continue through the cell cycle despite extensive damage and genomic instability [27–30]. Thus, arsenic exposure can induce an array of molecular damage across the genome and

While it is exposure to the methylated metabolic byproducts that yields the largest toxic effects resulting from exposure to environmental arsenic, it is noteworthy that even at very low doses, arsenic may be able to act as a co-mutagen to other known carcinogens, such as ultraviolet light, X-rays, methyl methane sulfonate, and tobacco smoke [15]. ROS are perhaps more immediately damaging to cells, as they can lead to alterations in a variety of lung cancer-specific pathways. As stated previously, arsenic exposure can interfere with DNA damage repair pathways, which exacerbates the effects of ROS generation. In the NER pathway, arsenic can alter the expression of key damage-repair genes, such as XPC, in a process that may

Collectively, aberrations in cellular DNA-damage repair pathways may not only highlight mechanisms of arsenic toxicity, but also its co-mutagenic effects. One of the most common pathways affected in lung cancer is the constitutive activation of the epidermal growth factor receptor (EGFR), especially in women and individuals who have never smoked [32]. Both amplification and mutation can lead to EGFR activation, which subsequently stimulates cell proliferation. AsIII can activate protooncogene c-Src (c-Src) through vicinal sulfhydryl groups, which then promotes phosphorylation events in intracellular EGFR tyrosine residues (Tyr845) [32]. As tyrosine phosphorylation is a key event in EGFR activation, AsIII thus promotes EGFR constitutive signaling. Alternatively, arsenic exposure may also indirectly affect downstream members of the EGFR pathway, through arsenic-induced oxidative stress and ROS, a common mechanism of environmentally-induced lung carcinogenesis. In a mechanism similar to that of EGFR activation, arsenic has been shown to induce the phosphorylation of several potential substrates of protein kinase B (Akt), a regulator of epithelial-to-mesenchymal transition (EMT) and metastasis, inducing cell migration [33]. Specifically, arsenic may affect c-Jun N-terminal kinase (JNK) activation and subsequent activation of signal transducer and activator of transcription 3 (STAT3), resulting in Akt growth and migration signaling [34]. Similarly, arsenic may increase the enzymatic activity of phosphoinositide 3-kinase (PI3K) and Akt phosphorylation, a key pathway in lung cancer tumorigenesis and progression [35]. The mechanism of PI3K/AKT activation has proven elusive, yet evidence suggests that ROS may play a mediating role, as well as alterations in histone modifications and activation of other related pathways, such as EGFR, mammalian target of rapamycin (mTOR), or polo-like kinase 1 (PLK1) signaling [35, 36]. Phenotypically, activation of the PI3K/Akt signaling axis by arsenic can result in a variety of changes, including cellular growth and angiogenesis [37]. There are many other lung cancer-specific pathways that may be altered upon exposure to arsenic and its toxic byproducts, including the nuclear

epigenome, culminating in malignant transformation.

**74**

As evidenced by its genome-wide effects on cellular physiology and molecular pathways, gene expression alterations cause by arsenic exposure can potentiate negative health outcomes. In fact, there are a growing number of genes that have been observed to have abnormal expression resulting from arsenic exposure, in cell lines, mouse, and human samples. Many of these genes have accepted roles in cancer, both as tumor-suppressors and oncogenes. Most notably, the tumor suppressor gene *TP53* has been shown to be epigenetically inactivated in arsenic-exposed cell lines [39]. Similarly, other cell line studies have suggested that low concentrations of arsenic may upregulate the known lung oncogene *Myc* (also related to the cell cycle) through aberrant expression of miRNAs targeting upstream regulators of its transcription [40].

As previously discussed, the frequent disruption of DNA damage repair and stress response pathways is a common feature of arsenic-induced lung tumors. Notably, arsenic has been associated with stimulation of the DNA damage response through the upregulation of critical genes, such as the gene encoding DNA excision repair protein ERCC1 (*ERCC1*) [41], confirming that DNA damage is prevalent in arsenic-exposed individuals. Alternatively, arsenic may induce repression and decreased activity of main DNA repair enzymes, including poly [ADP-ribose] polymerase 1 (PARP1) inhibition (through ROS) [42], proteasomal degradation of xeroderma pigmentosum, complementation group C (XPC) [31], and widespread hypermethylation of NER genes [43]. Additional lung cancer-related genes affected by arsenic include: *EGFR* [44], *cyclin-dependent kinase inhibitor 1A* (*CDKN1A*) [45], and *B-cell lymphoma 2 (BCL2)* [46]. Despite the mounting evidence of the toxic effects of arsenic, the concentration and identity of key damage-related arsenic compounds varies widely between studies. While different arsenic-based compounds affect similar pathways, specific physiological responses may vary greatly depending on compound type and dose response, necessitating closer examination of these factors in future studies.

However, it is important to note that variations in these genes may exist within individuals prior to arsenic exposure, and that certain genetic polymorphisms may make some individuals more susceptible to the genotoxic effects of arsenic. For instance, a single nucleotide polymorphism (rs238406; C > A) in *ERCC2* (part of the DNA-damage response) leads to the inclusion of an alanine residue in the place of a cysteine in the complete protein, increasing an individual's odds ratio for skin cancer to 2.04 [47]. Additionally, polymorphisms in many of the genes involved in the metabolism and biotransformation of arsenic may result in the production of different metabolic byproducts, conferring differential susceptibility and cancer risk [48]. This is exemplified by the rs1191439 polymorphism of As3MT, which is correlated with elevated MMA levels in urine [49]. Thus, the landscape of arsenicinduced carcinogenesis is quite complex, with multiple types and outcomes of the molecular aberrations that can result from chronic exposure. A more comprehensive understanding of the mechanisms at play may result in the identification of the underlying causes of lung cancer in never-smokers, and may help to direct the development of novel treatment strategies for these affected individuals.
