Low doses of arsenic, via perturbing p53, promotes tumorigenesis

Chronic arsenic exposure suppresses proteasomal and autophagic protein degradation

Ubiquitin Proteasomal System (UPS) and autophagy dysregulation initiate cancer. These pathways are regulated by zinc finger proteins. Trivalent inorganic arsenic (iAs) displaces zinc from zinc finger proteins disrupting functions of important cellular proteins. The effect of chronic environmental iAs exposure (100 nM) on UPS has not been studied. We tested the hypothesis that environmental iAs exposure suppresses UPS, activating autophagy as a compensatory mechanism. We exposed skin (HaCaT and Ker-CT; independent quadruplicates) and lung (BEAS-2B; independent triplicates) cell cultures to 0 or 100 nM iAs for 7 or 8 weeks. We quantified ER stress (XBP1 splicing employing Reverse Transcriptase -Polymerase Chain Reaction), proteasomal degradation (immunoblots), and initiation and completion of autophagy (immunoblots). We demonstrate that chronic iAs exposure suppresses UPS, initiates autophagy, but suppresses autophagic protein degradation in skin and lung cell lines. Our data suggest that chronic iAs exposure inhibits autophagy which subsequently suppresses UPS.

SIRT1/P53 pathway is involved in the Arsenic induced aerobic glycolysis in hepatocytes L-02 cells

Arsenic is a known human carcinogen. Low doses of arsenic can induce cell proliferation, but the mechanism remains elusive. Aerobic glycolysis, also known as the Warburg effect, is one of the characteristics of tumour cells and rapidly proliferating cells. P53 is a tumour suppressor gene that has been shown to be a negative regulator of aerobic glycolysis. SIRT1 is a deacetylase that inhibits the function of P53. In this study, we found that P53 was involved in low dose of arsenic-induced aerobic glycolysis through regulating HK2 expression in L-02 cells. Moreover, SIRT1 not only inhibited P53 expression but also decreased the acetylation level of P53-K382 in arsenic-treated L-02 cells. Meanwhile, SIRT1 influenced the expression of HK2 and LDHA, which then promoted arsenic-induced glycolysis in L-02 cells. Therefore, our study demonstrated that the SIRT1/P53 pathway is involved in arsenic-induced glycolysis, thereby promoting cell proliferation, which provides theoretical basis for enriching the mechanism of arsenic carcinogenesis.

Temporal Modulation of Differential Alternative Splicing in HaCaT Human Keratinocyte Cell Line Chronically Exposed to Arsenic for up to 28 Wk

BACKGROUND

Chronic arsenic exposure via drinking water is associated with an increased risk of developing cancer and noncancer chronic diseases. Pre-mRNAs are often subject to alternative splicing, generating mRNA isoforms encoding functionally distinct protein isoforms. The resulting imbalance in isoform species can result in pathogenic changes in critical signaling pathways. Alternative splicing as a mechanism of arsenic-induced toxicity and carcinogenicity is understudied.

OBJECTIVE

This study aimed to accurately profile differential alternative splicing events in human keratinocytes induced by chronic arsenic exposure that might play a role in carcinogenesis.

METHODS

Independent quadruplicate cultures of immortalized human keratinocytes (HaCaT) were maintained continuously for 28 wk with 0 or 100 nM sodium arsenite. RNA-sequencing (RNA-Seq) was performed with poly(A) RNA isolated from cells harvested at 7, 19, and 28 wk with subsequent replicate multivariate analysis of transcript splicing (rMATS) analysis to detect and quantify differential alternative splicing events. Reverse transcriptase-polymerase chain reaction (RT-PCR) for selected alternative splicing events was performed to validate RNA-Seq predictions. Functional enrichment was performed by gene ontology (GO) analysis of the differential alternative splicing event data set at each time point.

RESULTS

At least 600 differential alternative splicing events were detected at each time point tested, comprising all the five main types of alternative splicing and occurring in both open reading frames (ORFs) and untranslated regions (UTRs). Based on functional relevance ELK4, SHC1, and XRRA1 were selected for validation of predicted alternative splicing events at 7 wk by RT-PCR. Densitometric analysis of RT-PCR data corroborated the rMATS predicted alternative splicing for all three events. Protein expression validation of the selected alternative splicing events was challenging given that very few isoform-specific antibodies are available. GO analysis demonstrated that the enriched terms in differential alternatively spliced mRNAs changed dynamically with the time of exposure. Notably, RNA metabolism and splicing regulation pathways were enriched at the 7-wk time point, when the greatest number of differentially alternatively spliced mRNAs are detected. Our preliminary proteomic analysis demonstrated that the expression of the canonical isoforms of the splice regulators DDX42, RMB25, and SRRM2 were induced upon chronic arsenic exposure, corroborating the splicing predictions.

DISCUSSION

These results using cultures of HaCaT cells suggest that arsenic exposure disrupted an alternative splice factor network and induced time-dependent genome-wide differential alternative splicing that likely contributed to the changing proteomic landscape in arsenic-induced carcinogenesis. However, significant challenges remain in corroborating alternative splicing data at the proteomic level. https://doi.org/10.1289/EHP9676.

Connections between endoplasmic reticulum stress-associated unfolded protein response, mitochondria, and autophagy in arsenic-induced carcinogenesis

Arsenic exposure in contaminated drinking water is a global health issue, as more than 200 million people are affected globally. Arsenic has been known to cause skin, liver, lung, bladder and prostate cancers. Accordingly, it has been categorized as a group I human carcinogen by the International Agency for Research on Cancer (IARC). Various natural and anthropogenic activities lead to the release of arsenic in the environment, contaminating air, water and food sources. Traditionally, genetic mutations have been the center of cancer research. However, emerging studies have now focused on the importance of epigenetics, metabolism and endoplasmic reticulum (ER) stress in cancer. Arsenic is highly capable of inducing stress in the cells via the generation of free radicals causing oxidative stress, epigenetic and genetic alterations, mitochondrial dysfunction, activation of intracellular signaling pathways, and impairment of autophagy and DNA repair systems. The cancer cells are able to utilize the unfolded protein response (UPR) to overcome these internal stresses in various stages of arsenic-induced carcinogenesis, from cancer growth to immune responses. The UPR is an evolutionarily conserved stress response that has both survival and apoptotic outcomes. PERK, IRE1α and ATF6α are the three ER stress sensors that are activated to maintain cellular proteostasis, which can also promote apoptosis on prolonged ER stress. The dual nature of UPR in different cancer types and stages is a challenge for researchers. We must investigate the role and the connections among ER stress-associated UPR, mitochondrial dysfunction and autophagy in arsenic malignancies to identify key targets for cancer prevention and therapeutics.

Linc-ROR promotes arsenite-transformed keratinocyte proliferation by inhibiting P53 activity

Linc-ROR is an oncogenic long non-coding RNA over-expressed in many kinds of cancer that promotes cancer cell proliferation. Arsenite is a determined carcinogen that increases the risk of skin cancer, but the carcinogenic mechanism of arsenite remains unclear. To explore whether and how linc-ROR plays a role in arsenite-induced carcinogenesis of skin cancer, we established arsenite-transformed keratinocyte HaCaT cells by exposing them to 1 μM arsenite for 50 passages. Then we examined the linc-ROR expression during the transformation and explored the effect of linc-ROR on the cell proliferation of arsenite-transformed HaCaT cells. We found that the linc-ROR level in HaCaT cells was gradually increased during arsenite-induced malignant transformation, and the activity of P53 was decreased, but the P53 expression was not significantly altered, indicating that linc-ROR may play a role in arsenite-induced HaCaT cell transformation that is associated with P53 activity but not P53 expression. We further demonstrated that linc-ROR down-regulation by siRNA significantly inhibited the cellular proliferation and restored P53 activity in arsenite-transformed HaCaT cells, suggesting that linc-ROR promotes proliferation of arsenite-transformed HaCaT cells by inhibiting P53 activity. Moreover, linc-ROR siRNA also down-regulated the PI3K/AKT pathway in arsenite-transformed HaCaT cells, and treatment with AKT inhibitor wortmannin restored P53 activity, implying that linc-ROR inhibits P53 activity by activating the PI3K/AKT pathway. Taken together, the present study shows that linc-ROR promotes arsenite-transformed keratinocyte proliferation by inhibiting P53 activity through activating PI3K/AKT, providing a novel carcinogenic mechanism of arsenite-induced skin cancer.

Chronic low dose arsenic exposure preferentially perturbs mitotic phase of the cell cycle

Environmental pollution is a big challenge for human survival. Arsenic compounds are well-known biohazard, the exposure of which is closely linked to onsets of various human diseases, particularly cancers. Upon chronically exposing to arsenic compounds, genomic integrity is often disrupted, leading to tumor development. However, the underlying mechanisms by which chronic, low dose arsenic exposure targets genetic stability to initiate carcinogenesis still remain not fully understood. In this study, human lung epithelial BEAS-2B cells and keratinocytes were treated with 0.5 μM of sodium arsenite for one month (designated as BEAS-2B-SA cells or keratinocytes-SA), and its effect on cell cycle responses was analyzed. After being arrested in mitotic phase of the cell cycle by nocodazole treatment, BEAS-2B-SA cells or keratinocytes-SA were delayed to enter next cytokinesis. The lagging exit of the cells from mitosis was accompanied by a sustained Plk1 phosphorylation, which led to a persistent activation of the mitotic regulators BubR1 and Cdc27. As the result, cyclin B1 (clnB1) degradation was attenuated. BEAS-2B-SA cells or keratinocytes-SA also expressed a constitutively active Akt. The cytogenetic analysis showed an increased numbers of aneuploidy in these cells. The suppression of Akt reversed the aberrant expressions of the mitotic regulators, delay of mitotic exit as well as chromosomal aberrations. Our findings suggest that a long-term exposure to low dose sodium arsenite aberrantly retains the catenation of mitosis, which facilitates establishing genetic instability and predisposes the cells to tumorigenesis.

Sulphur dioxide and arsenic affect male reproduction via interfering with spermatogenesis in mice

As two potential environmental hazards, sulphur dioxide (SO₂) and arsenic have adverse effects on male reproduction, but the mechanism of which and their combined toxicity are not clear. In this study, we investigate male reproductive toxicity with a focus on spermatogenesis by treating mice with 5 mg/m³ SO₂ and/or 5 mg/L arsenic. Our results showed that arsenic exposure caused significant decreases in water and food consumption and body weight in mice, whereas these changes were not observed in the SO₂-only group. Both SO₂ and arsenic reduced sperm counts, increased the percentage of sperm malformation, and induced abnormal testicular pathological changes. Elevated H₂O₂ and MDA contents, declined T-SOD activity, decreased spermatogenic cell counts, enhanced caspase-3 activity, and increased TUNEL-positive cells were also observed in mice exposed to SO₂ and/or arsenic. Moreover, SO₂ and arsenic co-exposure changed the mRNA levels of Bax and Bcl-2, decreased serum testosterone levels, and downregulated the expression of steroidogenic-related genes (LHR, StAR, and ABP) in mice. These findings provide a new theoretical basis for understanding how SO₂ and arsenic interfere with spermatogenesis leading to infertility. These results also suggest that SO₂ and arsenic co-exposure likely result in an additive effect on male reproductive toxicity in mice.

Dual effects of arsenic trioxide on tumor cells and the potential underlying mechanisms

The human ether-a-go-go related gene (hERG) encodes the rapid delayed rectifier K⁺ channel. hERG not only serves an important role in heart muscle and cardiomyocyte excitability by regulating action potential repolarization, but also represents a selective advantage for cancer cell proliferation. Arsenic trioxide is a traditional Chinese medicine, which has been previously identified as an efficient tumor suppressor, particularly in the treatment of acute pro-myelocytic leukemia. However, studies have also reported that long-term exposure to arsenicals may lead to the formation of malignant tumors. In the present study, the effect of low-dose arsenic trioxide on the proliferation and apoptosis of tumor cells was investigated, as were the potential underlying mechanisms of this effect. The data demonstrated that low-dose arsenic trioxide (0.1 µM) enhanced the viability and apoptosis of tumor cells expressing hERG channels following long-term incubation. However, in tumor cells lacking hERG channels, low-dose arsenic trioxide had no effect. Therefore, we hypothesized that this hormesis effect of low-dose arsenic trioxide on tumor cells may be associated with the hERG channel. Furthermore, low dose arsenic trioxide promoted the hERG-channel current by changing the kinetics of channel gating and prolonging the open-channel stage. Simultaneously, high-dose As₂O₃ (1 or 10 µM) significantly reduced the expression of hERG in tumor cells compared with the control group, which resulted in reduced proliferation rate and promotion of apoptotic rate. The results of the present study demonstrate that the dual effects of arsenic trioxide on hERG channels vary according to concentration, resulting in the dual effects on tumor cells. This provides a theoretical basis for the potential clinical application of arsenic trioxide, suggesting that hERG channels are an important target in preventing and treating tumorigenesis during arsenicosis.

In vivo assessment of polydatin, a natural polyphenol compound, on arsenic-induced free radical overproduction, gene expression, and genotoxicity

Arsenic (As) is a well-known contaminant of global groundwater. Its exposure causes several hazardous effects on animals and human via oxidative stress. The present study examined the effect of polydatin (PD) on free radical overproduction in rats exposed to As. Thirty-five male rats randomly allocated into five equal groups. To the control group, physiological saline was given orally and to the second group only 100 mg/L As was given by drinking water for 60 days. The other groups were treated with As (100 mg/L) and PD orally at 50, 100, and 200 mg/kg/day, respectively. Treatment with As enhanced malondialdehyde level but decreased glutathione level in blood, liver, kidney, brain, lung, and heart of rats. Also, As decreased superoxide dismutase and catalase activities of erythrocyte, liver, kidney, brain, lung, and heart in rats. Furthermore, As treatment gave rise to increased DNA damage and gene expressions of interleukin 1 beta (IL-1β), nuclear factor kappa beta (NFκB), p53, and tumor necrosis factor-α (TNF-α) in the lung, brain, kidney, and liver. However, treatment of PD ameliorated As-exposed lipid peroxidation, antioxidant enzymes activities, DNA damage, gene expressions, and histopathological changes in tissues. In conclusion, PD has a dose-dependent protective effect on lipid peroxidation and antioxidant defense mechanism in rats against As exposure.

Pancreatic islet-autonomous effect of arsenic on insulin secretion through endoplasmic reticulum stress-autophagy pathway

Inorganic arsenic is a worldwide environmental pollutant. Arsenic's relationship with the incidence of diabetes arouses concerns on its etiological mechanism. In this study, the glucose-stimulated insulin secretion (GSIS) from isolated pancreatic islets of As₂O₃-treated mice was significantly lower than that of control mice. It indicated that the effect of As₂O₃-inhibited GSIS was pancreatic islet-autonomous. The level of phospho-PERK (p-PERK), a biomarker of endoplasmic reticulum (ER) stress, in pancreas of As₂O₃-treated mice was increased significantly. After treatment with NaAsO₂, the p-PERK level in INS-1 rat pancreatic β- cells was increased correspondingly. After treatment with PERK inhibitor, the GSIS from isolated pancreatic islets of As₂O₃-treated mice was recovered. Arsenic induced autophagy in pancreatic islets, as evidenced by elevated LC3-II level and depressed P62 level in vivo and in vitro. In NaAsO₂-treated INS-1 cells, the initiation of ER stress preceded the stimulation of autophagy, which was a key factor controlling pancreatic β cell function. Furthermore, knockdown of PERK attenuated NaAsO₂-induced autophagy in INS-1 cells. These data indicated that arsenic impaired β cell function through ER stress-autophagy pathway. The present study will provide new mechanistic insights into arsenic-related diabetes.