Paradoxical effects of arsenic in the lungs

Arsenic can increase the risk of lung cancer, especially at high concentrations. The relationship between arsenic and lung disease was first found because of the widespread use of arsenic in industries [27]. Subsequently, more epidemiological evidence relates exposure to arsenic to lung cancer [28‐30]. Minerals and tobacco, even water and foodstuffs, contain appreciable amounts of arsenic [31], making it necessary to study the relationship between arsenic and lung cancer from an epidemiological perspective. At high concentrations, the higher the exposure concentration, the greater the risk of lung cancer [25, 32, 33]. After the WHO set the maximum arsenic concentration in water at 10 μg/L, various regions have adjusted their allowable concentration of arsenic in water. The risk of lung cancer in high arsenic concentration decreased after intervention [34‐37]; therefore, we believe that high arsenic exposure is related to the occurrence of lung cancer. As for medium- and low-concentration arsenic exposure, the results seem to differ. Do medium and low concentrations of arsenic affect the incidence of lung cancer? We sought and analyzed studies on the relationship between low to medium concentrations of arsenic and lung cancer (Table 1).

As shown previously, the results vary: five studies show that arsenic exposure at low to medium concentrations is related to the occurrence of lung cancer, while the other four consider it unrelated. Through the analysis of the results, the lifetime lung cancer risks were assessed in different age groups, with arsenic exposure at concentrations from 7.61 to 9.25 μg/L. The result showed that the lifetime risk of lung cancer was 3.54 × 10⁻⁵ and the lung cancer burden was 1.20 × 10⁻⁵ per person-year (ppy) [38]. Under the average arsenic concentration of 19.3 μg/L, the mortality risk of lung cancer was found to be 2.61 in men and 2.09 in women [6]. Though there are differences between the sexes, the risk ratios differ significantly compared with the non-exposed group, that is, low to medium-concentration arsenic exposure is related to the occurrence and mortality increase of lung cancer. In addition, there are several studies devoted to exploring the dose-dependent nature of the relationship between arsenic exposure and lung cancer. With the increase of arsenic exposure concentration, the incidence and mortality of lung cancer are also increasing. A case-control study found that lung cancer odds ratios (ORs) and 95% confidence intervals (CIs) for the groups exposed to 10–29 μg/L, 30–49 μg/L, and 50–199 μg/L arsenic were 1.60 (0.50–5.30), 3.90 (1.20–12.30), and 5.20 (2.30–11.70) respectively [7]. Similarly, studies in the USA and northern Chile have also found that the incidence of lung cancer increases with increasing arsenic exposure levels [37, 39]. Besides, we found that the occurrence of lung cancer is different across genders. Some studies have shown that the risk of lung cancer and the burden of disease in men are higher than in women [6, 43], indicating that there may be gender differences in the carcinogenic risk of arsenic exposure. In addition to the five epidemiological studies that found the exposure to low to medium concentrations of arsenic is related to lung cancer, we analyzed the other four studies that are unrelated. A study in north-eastern Taiwan found that the relative risk (RR) of lung cancer was 1.10 (95% CI: 0.74–1.63) and 0.99 (95% CI: 0.59–1.68) after arsenic exposure concentration of 10–49.9 μg/L and 50–99.9 μg/L, respectively, suggesting that there is no significant correlation between low to medium concentration arsenic exposure and lung cancer [41]. In this study, a total of 178 newly diagnosed lung cancer cases were identified. After adjusting for variables including age, gender, years of schooling, cigarette smoking status, and habitual alcohol consumption, different histological types show different RR. In squamous cell carcinomas, the RR was 0.53 and 1.32, whereas in other types, the RR in the 50–99.9 μg/L group was lower than that in the 10–49.9 μg/L group, so those lung cancers caused by arsenic may be related to specific tumor histological types. Besides, the residents involved in this study were followed for 11 years, and although lacking in statistical precision, the risk of lung cancer increased with the exposure time, which means that the exposure period is also a vital factor requiring evaluation. While in another study, the standardized mortality ratio (SMR) was 0.9 in some parts of the USA, where the concentration of arsenic in drinking water was between 3–59 μg/L. Studying this article, we found that the RR was 1.00, 1.0, and 0.98/0.97 (males and females) with a median arsenic exposure concentration of 3.0 μg/L, 3.1–9.9 μg/L, and 10–59.9 μg/L, respectively, that is, the arsenic in underground water is unrelated to mortality as a result of lung cancer [12]. We did also find that in this study, the authors only adjusted for age and ethnicity. Cigarette smoking and occupational exposure also played a significant role in the occurrence of lung cancer. Considering that RR is around 1.0, we believe that further adjustments to factors related to lung cancer such as cigarette smoking may obtain more accurate results. Similarly, in the other two studies, they both think that arsenic in low to medium concentrations is unrelated to lung cancer, but we found that the lack of data of early exposure in a mobile population, the insufficient number of samples, and short exposure time may have a significant influence on the results. In overview, the first five research projects mentioned above show that it is clear that lung cancer risk increased with the increasing concentration of arsenic exposure, especially in Chile. In central Italy, the mean concentration of arsenic exposure is 19.3 ug/L, but the hazard ratio (HR) is over 2. Compared with other results under a similar concentration, the risk of lung cancer is higher. Exploring the difference further, we noticed that the average exposure time is 39.5 years in central Italy, far exceeding the exposure time in the other four studies. Therefore, we believe that not only the dose, but also the duration, affects the occurrence of lung cancer caused by arsenic. Upon inclusion of more people, the longer the observation, the closer to reality the modeling: if accurate concentration detection is difficult, then longer data collection is even more difficult. In the last four studies that considered arsenic exposure to be unrelated to the risk of lung cancer, the exposure time was about 10 years, which is far from enough compared to other studies lasting nearly 40 years. Therefore, the analysis of arsenic exposure and the risk of lung cancer may need a longer follow-up. In summary, we have more reason to believe that low to medium concentrations of arsenic are related to the occurrence of lung cancer. Arsenic exposure time and other bias factors also affected the risk of lung cancer. Further research is necessary to provide a more accurate understanding of the relationship between low- to medium-level arsenic exposure and lung cancer.

Except for lung cancer, arsenic exposure may also play an important role in other non-malignant diseases of the lungs. Large-scale population studies have found that arsenic is closely related to lung function [44]: in the low to medium dose range, its harmful effects are obvious, the higher the arsenic concentration, the lower the forced expiratory volume in 1 s (FEV1) and the forced vital capacity (FVC) [44]. Chronic arsenic exposure caused obstructive lung damage, and the severity of that damage increased with increasing arsenic exposure [6, 45]. In Bengal, compared with the normal skin group, the skin lesions caused by high levels of arsenic in the occurrence of chronic cough and chronic bronchitis were more numerous and more severe [46]. In early childhood, exposure to arsenic can increase the mortality of bronchiectasis [47]. The studies suggested that arsenic exposure may play an important role in non-malignant lung disease, though the reports remain limited: it is necessary to study the relationship between, and dose dependence of, arsenic and non-malignant lung diseases.

Epidemiological studies differ as to whether low to medium concentrations of arsenic are pathogenic, so many people focus on animals and cells. Merrick et al. found that, the incidences of lung cancer in the 50 ppb and 500 ppb groups of lifetime arsenic exposure in CD1 mice were 51% and 54%, respectively, which were significantly higher than that in the control group (22%) [48]. Wang et al. proved that arsenic can induce human lung epithelial cell malignant transformation [49]. Subsequently, under the exposure to arsenic at concentrations of between 0.5 and 2.5 μM and for times ranging from 13 weeks to 26 weeks, many other researchers also found that lung epithelial cells transformed successfully. Studies have shown that the regulation of cell proliferation, apoptosis, angiogenesis, and metastasis play important roles in malignant transformation [49‐58]. In addition, the inhibition of deoxyribonucleic acid (DNA) damage repair, DNA methylation, and oxidative stress are also involved in carcinogenesis [58‐63]. As mentioned above, tumorigenesis is a complex process: in animal models, arsenic exposure was found to disrupt immune function, and epithelial barrier function [64, 65]. Genetic analysis after intrauterine exposure of mice found that the level of genes related to lung immunity and mucociliary function changed significantly [66]. These changes may be factors initiating tumorigenesis, but there is still a long way from these changes to the occurrence of lung cancer, and the gaps between the two parts are the key points of carcinogenesis in arsenic exposure. Significant changes were found in cell models. Low concentrations of arsenite can induce cell proliferation, which can promote the cell cycle from G1 to S phase, and upregulate the expression of cyclin D1 through activation of the c-Jun N-terminal kinase (JNK1/c-Jun) pathway in human embryonic lung fibroblast (HELF) cell lines [67]. Similarly, in BEAS-2B, low concentrations of arsenite are involved in the malignant transformation of cells by upregulating cyclin D, which was mediated by the p52-Bcl3 complex [68]. MicroRNA (miRNA) also was found to regulate cell proliferation. Inhibition of miR-222 and miR-301a can decrease the proliferation rates of arsenic-transformed (As-T) cells, in which phosphatase and tensin homolog (PTEN) and interleukin 6 and signal transducer and activator of transcription 3 (IL-6/STAT3) signaling are involved, respectively [50, 53]. In As-T cells, reactive oxygen species (ROS) levels are low and have apoptotic resistance. Increasing ROS by inhibiting catalase can restore the apoptosis ability of arsenic-transformed BEAS-2B [69]. Further research showed that high levels of nuclear factor, erythroid 2 like 2 (Nrf2), upregulated the expression of antioxidant proteins catalase and superoxide dismutase, and anti-apoptotic proteins Bcl-2 and Bcl-xl, which reduced ROS production and enhanced the resistance to apoptosis in arsenic-transformed BEAS-2B cells [53, 70]. In addition, under arsenic exposure, IL-6 can regulate Mcl-1 by STAT3 and mediate the binding of Mcl-1 and Beclin 1 to inhibit apoptosis [71]. In angiogenesis, ROS upregulated by arsenic can upregulate the expression of hypoxia-inducible factor 1 (HIF-1) and vascular endothelial growth factor (VEGF) by activating AKT and mitogen-activated protein kinase (ERK1/2) signaling pathways [72]. Under arsenic exposure, HIF-1α accumulated in a dose- and concentration-dependent manner depending on the degree of protein stability, and affected the unanchored growth of transformed cells by mediating glycolysis [73]. Meanwhile, HIF-2α participated in arsenic-induced human bronchial epithelial (HBE) cell transformation by regulating IL-6 and IL-8 [74], and by regulating Twist1 and Bmi1 in epithelial–mesenchymal transition (EMT). Among them, Bmi1 was thought to be related to the maintenance of stem cells mediated by arsenite [75]; However, some studies found arsenic accumulation induced by inhibiting ubiquitination of HIF-2, which participates in the malignant transformation of arsenic-induced cells by inhibiting P53 protein [76]. Those changes are all related to the metastasis of the tumor. Arsenic inhibits DNA repair by suppressing the expression of related genes and inhibiting the base excision repair (BER) and nucleotide excision repair (NER), which is commonly seen in the combined effect of arsenic and other carcinogens, such as benzo[a]pyrene diol epoxide (BPDE), radon and solar ultraviolet radiation [77‐81]. DNA methylation that can control gene expression is involved in the occurrence of lung cancer. In A/J mice, arsenic exposure decreased the expression of Ras association domain family member 1 (RASSF1A) by hyper-methylating its promoter region [82]. Similarly, DNA methylation changes were observed in mice exposed to AS for 90 days by whole-genome DNA methylation and gene expression analysis [83]. Except for DNA methylation, arsenic can induce oxidative stress; accordingly, the related oxidant and enzyme, including glutathione (GSH) and gamma-glutamylcysteine synthetase (gamma-GCS), were changed [84]. Although there are few studies of those mechanisms, there is no doubt that these mechanisms have broadened our thinking and further research is necessary to enable a deeper understanding of the pathogenic effects of arsenic.

Arsenic also plays an important role in non-malignant lung diseases. It can impair ATP-mediated Ca²⁺ signaling mechanisms and wound repair through reducing P2Y and P2X receptor function, destroying the innate immunity of airway epithelial cells at concentrations of 10 ppb or 25 ppb [85]. The destruction of innate immune defenses can increase the systemic transport of inhaled pathogens and small molecules, resulting in the increased possibility of viral and bacterial infection in mice after early exposure to arsenic [86, 87]. Besides, for the offspring of C57BL/6 mice after intrauterine exposure to 100 μg/L arsenic, lung tissue genetic analysis shows significant changes in lung development, immunity, and mucociliary function [66], in which lung inflammation and autophagy are involved in the damage of the epithelial barrier and may increase the risk of infection [86, 87]. The relationship between arsenic and lung non-malignant diseases is close. Arsenic may increase the occurrence of diseases by destroying the innate immune system and affecting lung development. The inflammation and autophagy may mediate the occurrence of cancer, but the mechanism of lung disease caused by arsenic exposure is uncertain, especially in non-tumorous lung diseases. Further exploration of the mechanism is significant. Due to the limited research into non-tumorous lung diseases under exposure to arsenic, we sought articles on other diseases related to arsenic exposure, which may provide clues to the relationship between lung disease and arsenic. Arsenic can induce atherosclerosis by upregulating monocyte chemoattractant protein-1 (MCP-1), and tumor necrosis factor α, IL-6 [88, 89]. Arsenic-induced hypertension can be explained by increasing calcium sensitization and vascular endothelial dysfunction [90]. Chronic arsenic exposure can cause malignant transformation of liver epithelium cells, and preneoplastic lesions, including fibrosis, and cirrhosis can also occur, impairing the repair of DNA damage, hyperproliferation, and DNA methylation, all of which may cause the aforementioned diseases [91‐94]. Renal diseases [95] and neurological disorders [96] can also be induced by chronic arsenic exposure. In summary, arsenic is a pollutant that can affect many organs, and the various organs of the body are interconnected, work together, and have similarities, which may be useful for us in our analysis of the relationship between lung diseases and arsenic.

To further understand how arsenic is pathogenic, the metabolism of arsenic has to be mentioned. Arsenic exists as inorganic arsenic (iAS) in nature and often accumulates in the human body in various forms after being metabolized. We used to find that the methylation of arsenic is a detoxification process, but recent research indicates that this may be a toxic process [97]. The recognized pathway for arsenic metabolism is that iAs (III) is methylated by arsenic-3-methyltransferase (AS3MT), using s-adenosylmethionine (SAM) as a methyl donor to form monomethylarsonic acid (MMAsV); MMAV is reduced to monomethyl arsenous form (MMAsIII), which is then methylated by AS3MT to form dimethylarsinic acid (DMAsV). DMAsV can be excreted in urine [98] (Fig. 1).

In the female Kunming mouse model, the measurement of the distribution of arsenic in different tissues after a single injection of arsenite found that iAS is the most abundant in the liver and kidney, while the concentration of DMA in the lung and bladder is maximal [99]; therefore, the distribution of iAS and its methylated metabolites may be tissue specific. Similarly, the main metabolite in the lung was found to be DMA [100]. The adult female B6C3F1 mouse model also implied that after a single exposure to arsenate, the percentage of DMA in the lung was the highest, and increased with repeated exposure [101], so DMA may play an essential role in the lungs. Studies have shown that DMA can induce DNA damage, cytotoxicity, chromosomal abnormalities, apoptosis, and gene mutations by inducing oxidative stress [102‐104], and its reductive metabolites have been proven to be genotoxic and tumorigenic [105]. In the study of other non-malignant diseases of the lungs, it was found that under the influence of low-dose arsenic exposure, neither iAS nor DMA could change the cytokine secretion induced by Pseudomonas aeruginosa. In contrast, MMA increased the secretion of IL-8, IL-6, and chemokine ligand 2 (CXCL2) induced by Pseudomonas aeruginosa. These results indicate that MMA may negatively affect the innate immune response of human bronchial epithelial cells to Pseudomonas aeruginosa [106]; therefore, we speculate that the two main metabolites of sodium arsenite, MMA and DMA, may play an important role in the development of various diseases in the lungs. To further understand how arsenic works, and what role it plays in the occurrence and development of lung diseases, we need to understand it from a more comprehensive perspective.

In lung cancer, ATO can play an anti-cancer effect by inhibiting cell proliferation [113], inducing apoptosis [114] and anti-angiogenesis [115], and inhibiting tumor metastasis [116], as shown in Fig. 2.

The anticancer mechanisms of ATO involve inhibiting cell proliferation, inducing apoptosis and anti-angiogenesis, and inhibiting tumor metastasis. ATO can inhibit proliferation by downregulating cyclin-dependent protein kinase (CDC2) and cyclinB1, causing cell cycle arrest. ATO treatment can affect mitochondrial function, regulate related molecules, promote the formation of apoptotic bodies, and cause cell apoptosis. ATO can affect CXCRT and RNDI by inhibiting NFAT and calcineurin, thereby inhibiting tumor metastasis. ATO can induce anti-angiogenesis by affecting key molecules of angiogenesis including MMPs and VEGF.

In addition to the important role of ATO in lung tumors, ATO also plays an important role in other non-malignant diseases of the lungs. In the pulmonary fibrosis (PF) rat model, ATO inhibits rat PF by upregulating miR-98 and inhibiting its downstream Stat3. Cell experiments further showed that As₂O₃ can prevent lung interstitial thickening and inhibit type I collagen and hydroxyproline, thereby preventing the development of PF [109]. In the female BALB/c mouse asthma model, ATO can reduce the severity of asthma attacks. Exploring the mechanism of action thereof may be related to the apoptosis of CD+T cells involved in the ER stress-C/EBP homologous protein pathway [110]. In the model of c immunization, ATO reduces airway responsiveness, airway inflammation, and mucus hyperplasia. Further studies have shown that ATO can cause mitochondrial dysfunction, Ca²⁺-overload, and promote caspase-12 activation, that is, ATO may have important significance in the treatment of asthma [111]. It is also found in the OVA-immunized mouse model that As₂O₃ reduces the occurrence of airway hyperresponsiveness (AHR) and cell infiltration into the airway by downregulating the expression of eosinophils. In vitro experiments have further shown that ATO can significantly inhibit the secretion of eosinophil chemokine when it induces a certain apoptosis in primary lung cells [112].

Sodium arsenite and ATO play different roles in lung diseases, but what caused the difference remains unknown. Analysis of cell proliferation, cell cycle distribution, oxidative stress, genetic damage, and apoptotic index of the A549 cell line exposed to sodium arsenite and ATO show that As₂O₃ was more cytotoxic than NaAsO₂. As₂O₃ is more effective than NaAsO₂ in arresting cells in the G2/M phase. As₂O₃ is more capable of inducing DNA damage and chromosome breakage than NaAsO₂ and is more genotoxic. Compared with As₂O₃, NaAsO₂ significantly increased the ROS-level in cells [130]. This result is significant to our understanding of the different roles of arsenic in lung diseases, but further research is necessary to determine how arsenic actually affects lung diseases.