Arsenite-induced endoplasmic reticulum-dependent apoptosis through disturbance of calcium homeostasis in HBE cell line

Endoplasmic reticulum stress responses and epigenetic alterations in arsenic carcinogenesis

Arsenic is a well-known human carcinogen whose environmental exposure via drinking water, food, and air impacts millions of people across the globe. Various mechanisms of arsenic carcinogenesis have been identified, ranging from damage caused by excessive production of free radicals and epigenetic alterations to the generation of cancer stem cells. A growing body of evidence supports the critical involvement of the endoplasmic stress-activated unfolded protein response (UPR) in promoting as well as suppressing cancer development/progression. Various in vitro and in vivo models have also demonstrated that arsenic induces the UPR via activation of the PERK, IRE1α, and ATF6 proteins. In this review, we discuss the mechanisms of arsenic-induced endoplasmic reticulum stress and the role of each UPR pathway in the various cancer types with a focus on the epigenetic regulation and function of the ATF6 protein. The importance of UPR in arsenic carcinogenesis and cancer stem cells is a relatively new area of research that requires additional investigations via various omics-based and computational tools. These approaches will provide interesting insights into the mechanisms of arsenic-induced cancers for prospective target identification and development of novel anti-cancer therapies.

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

This review analyzes the causes and consequences of apoptosis resulting from oxidative stress that occurs in mitochondria and cells exposed to the toxic effects of different-valence heavy metals (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+). The problems of the relationship between the integration of these toxic metals into molecular mechanisms with the subsequent development of pathophysiological processes and the appearance of diseases caused by the accumulation of these metals in the body are also addressed in this review. Such apoptosis is characterized by a reduction in cell viability, the activation of caspase-3 and caspase-9, the expression of pro-apoptotic genes (Bax and Bcl-2), and the activation of protein kinases (ERK, JNK, p53, and p38) by mitogens. Moreover, the oxidative stress manifests as the mitochondrial permeability transition pore (MPTP) opening, mitochondrial swelling, an increase in the production of reactive oxygen species (ROS) and H2O2, lipid peroxidation, cytochrome c release, a decline in the inner mitochondrial membrane potential (ΔΨmito), a decrease in ATP synthesis, and reduced glutathione and oxygen consumption as well as cytoplasm and matrix calcium overload due to Ca2+ release from the endoplasmic reticulum (ER). The apoptosis and respiratory dysfunction induced by these metals are discussed regarding their interaction with cellular and mitochondrial thiol groups and Fe2+ metabolism disturbance. Similarities and differences in the toxic effects of Tl+ from those of other heavy metals under review are discussed. Similarities may be due to the increase in the cytoplasmic calcium concentration induced by Tl+ and these metals. One difference discussed is the failure to decrease Tl+ toxicity through metallothionein-dependent mechanisms. Another difference could be the decrease in reduced glutathione in the matrix due to the reversible oxidation of Tl+ to Tl3+ near the centers of ROS generation in the respiratory chain. The latter may explain why thallium toxicity to humans turned out to be higher than the toxicity of mercury, lead, cadmium, copper, and zinc.

Angiotensin II-induced calcium overload affects mitochondrial functions in cardiac hypertrophy by targeting the USP2/MFN2 axis

Ubiquitination, a common type of post-translational modification, is known to affect various diseases, including cardiac hypertrophy. Ubiquitin-specific peptidase 2 (USP2) plays a crucial role in regulating cell functions, but its role in cardiac functions remains elusive. The present study aims to investigate the mechanism of USP2 in cardiac hypertrophy. Animal and cell models of cardiac hypertrophy were established using Angiotensin II (Ang II) induction. Our experiments revealed that Ang II induced USP2 downregulation in the in vitro and in vivo models. USP2 overexpression suppressed the degree of cardiac hypertrophy (decreased ANP, BNP, and β-MHC mRNA levels, cell surface area, and ratio of protein/DNA), calcium overload (decreased Ca²⁺ concentration and t-CaMKⅡ and p-CaMKⅡ, and increased SERCA2), and mitochondrial dysfunction (decreased MDA and ROS and increased MFN1, ATP, MMP, and complex Ⅰ and II) both in vitro and in vivo. Mechanically, USP2 interacted with MFN2 and improved the protein level of MFN2 through deubiquitination. Rescue experiments confirmed that MFN2 downregulation neutralized the protective role of USP2 overexpression in cardiac hypertrophy. Overall, our findings suggested that USP2 overexpression mediated deubiquitination to upregulate MFN2, thus alleviating calcium overload-induced mitochondrial dysfunction and cardiac hypertrophy.

Mitochondrial ROS, ER Stress, and Nrf2 Crosstalk in the Regulation of Mitochondrial Apoptosis Induced by Arsenite

Long-term ingestion of arsenicals, a heterogeneous group of toxic compounds, has been associated with a wide spectrum of human pathologies, which include various malignancies. Although their mechanism of toxicity remains largely unknown, it is generally believed that arsenicals mainly produce their effects via direct binding to protein thiols and ROS formation in different subcellular compartments. The generality of these mechanisms most probably accounts for the different effects mediated by different forms of the metalloid in a variety of cells and tissues. In order to learn more about the molecular mechanisms of cyto- and genotoxicity, there is a need to focus on specific arsenic compounds under tightly controlled conditions. This review focuses on the mechanisms regulating the mitochondrial formation of ROS after exposure to low concentrations of a specific arsenic compound, NaAsO₂, and their crosstalk with the nuclear factor (erythroid-2 related) factor 2 antioxidant signaling and the endoplasmic reticulum stress response.

Chronic arsenic increases cell migration in BEAS-2B cells by increasing cell speed, cell persistence, and cell protrusion length

There is a strong association between arsenic exposure and lung cancer development, however, the mechanism by which arsenic exposure leads to carcinogenesis is not clear. In our previous study, we observed that when BEAS-2B cells are chronically exposed to arsenic, there is an increase in secreted TGFα, as well as an increase in EGFR expression and activity. Further, these changes were broadly accompanied with an increase in cell migration. The overarching goal of this study was to acquire finer resolution of the arsenic-dependent changes in cell migration, as well as to understand the role of increased EGFR expression and activity levels in the underlying mechanisms of cell migration. To do this, we used a combination of biochemical and single cell assays, and observed chronic arsenic treatment enhancing cell migration by increasing cell speed, cell persistence and cell protrusion length. All three parameters were further increased by the addition of TGFα, indicating EGFR activity is sufficient to enhance those aspects of cell migration. In contrast, EGFR activity was necessary for the increase in cell speed, as it was reversed with an EGFR inhibitor, AG1478, but was not necessary to enhance persistence and protrusion length. From these data, we were able to isolate both EGFR-dependent and -independent features of cell migration that were enhanced by chronic arsenic exposure.

Arsenite impinges on endoplasmic reticulum-mitochondria crosstalk to elicit mitochondrial ROS formation and downstream toxicity

Arsenite is an important carcinogen and toxic compound, causing various deleterious effects through multiple mechanisms. In this review, we focused on mitochondrial ROS (mitoROS) and discussed on the mechanisms mediating their formation. The metalloid promotes direct effects in mitochondria, resulting in superoxide formation only under conditions of increased mitochondrial Ca²⁺ concentration ([Ca²⁺]_m). In this perspective, the time of exposure and concentration requirements for arsenite were largely conditioned by other effects of the metalloid in specific sites of the endoplasmic reticulum (ER). Arsenite induced a slow and limited mobilization of Ca²⁺ from IP₃R via a saturable mechanism, failing to increase the [Ca²⁺]_m. This effect was however associated with the triggering of an intraluminal crosstalk between the IP₃R and the ryanodine receptor (RyR), causing a large and concentration dependent release of Ca²⁺ from RyR and a parallel increase in [Ca²⁺]_m. Thus, the Ca²⁺-dependent mitoO₂^-. formation appears to be conditioned by the spatial/functional organization of the ER/mitochondria network and RyR expression. We also speculate on the possibility that the ER stress response might regulate the above effects on the intraluminal crosstalk between the IP₃R and the RyR via oxidation of critical thiols mediated by the H₂O₂ locally released by oxidoreductin 1α.

Pulmonary Exposure to Copper Oxide Nanoparticles Leads to Neurotoxicity via Oxidative Damage and Mitochondrial Dysfunction

Copper oxide nanoparticles (CuONPs) are widely used in pharmaceutical, food, and textile industries. They have been shown to cause lung, liver, and kidney damage. However, whether an intratracheal instillation of CuONPs would affect the brain and its underlying mechanisms remain poorly studied. In this study, healthy C57BL/6J male mice were equally subdivided into control group, low-dose (30 μg/animal), medium-dose (50 μg/animal), and high-dose (100 μg/animal) CuONPs-treated groups. Mice were subjected to acute exposure of CuONPs via intratracheal instillation. Brain histopathology, inflammatory factors, oxidative stress markers, and mitochondrial function-related protein expression were determined. Our results demonstrated that CuONPs caused a dose-dependent brain damage in mice. Histopathological changes in the brain, elevation of inflammatory factors (Tnf, Il-6), and significant alterations in oxidative stress markers were also observed after treatment with CuONPs. Intriguingly, we did not observe infiltration of macrophage cell. Moreover, Tim23, TFAM, and MFN2 protein expression levels showed the decreasing trend after treatment with CuONPs. Taken together, these results indicate that pulmonary exposure to CuONPs induces pathological damage, inflammation, oxidative stress, and mitochondrial dysfunction in the cerebral cortex, suggesting that neurotoxicity caused by pulmonary exposure of CuONPs needs more attention from the public and relevant departments.

The role of endoplasmic reticulum stress in lead (Pb)-induced mitophagy of HEK293 cells

It is well-documented that lead (Pb) toxicity can affect almost all systems in living organisms. It can induce selective autophagy of mitochondria (mitophagy) by triggering reactive oxygen species production. Emerging evidence has suggested that Pb-induced autophagy can also be activated by the endoplasmic reticulum (ER) stress pathway. However, the interplay between ER stress and mitophagy remains to be elucidated. In this study, human embryonic kidney HEK293 cells were employed to investigate the role of ER stress in Pb-induced mitophagy. The results showed that the cell viability was decreased and cell damage was induced after exposure to Pb (0, 0.5, 1, 2, and 4 mM) for 24 h in a dose-dependent manner. Moreover, the expression of LC3-Ⅱ was significantly increased, and the expression of HSP60 was dramatically decreased after exposure to 1 mM and 2 mM Pb, indicating the induction of mitophagy following Pb exposure. Meanwhile, the expressions of activating transcription factor 6, inositol-requiring protein-1α, CCAAT/enhancer binding protein homologous protein, and glucose-regulated protein 78 were dramatically increased after Pb treatment, signifying the initiation of ER stress. Notably, the mitophagic effect was significantly compromised when ER stress was inhibited by 0.5 mM 4-phenylbutyrate, which was evidenced by lesser decreases in HSP60 expression and level of LC3-Ⅱ, suggesting Pb-induced mitophagy may be activated by the ER stress. Taken together, these findings provide a better understanding of Pb toxicity and suggest that Pb-induced ER stress may play a regulatory role in the upstream of mitophagy.

Long non-coding RNA ROR confers arsenic trioxide resistance to HepG2 cells by inhibiting p53 expression

Arsenic trioxide is an effective drug in the treatment of hematologic malignancies, but it has no obvious therapeutic effect on liver cancer. Long non-coding RNA ROR is a newly found long-noncoding RNA that has been reported to get involved in the regulation of chemo-resistance in multiple cancers. However, whether and how long non-coding RNA ROR gets involved in the resistance to arsenic trioxide in liver cancer has not been explored. In this study, We found that cellular apoptosis was increased by arsenic trioxide in liver cancer HepG2 cells; P53 expression was also increased by arsenic trioxide at both mRNA level and protein level, indicating that P53-dependent apoptosis is the main mechanism for arsenic trioxide to induce cytotoxicity in liver cancer HepG2 cells. Meanwhile, we found an obvious increase in the level of long non-coding RNA ROR in arsenic trioxide-treated HepG2 cells. By measuring the level of reactive oxygen species, glutathione, superoxide dismutase, and malondialdehyde, the product of lipid peroxidation, we further demonstrated that oxidative stress was a potential factor for both the activation of P53 expression and the increase in long non-coding RNA ROR expression. Through the knock-down of long non-coding RNA ROR by siRNA, we revealed that the activated long non-coding RNA ROR ameliorated arsenic trioxide-induced apoptosis by inhibiting P53 expression. Together, our study reported that long non-coding RNA ROR conferred arsenic trioxide resistance to liver cancer cells through inhibiting P53 expression, and long non-coding RNA ROR might be a novel sensitizing target for liver cancer treatment.

The Rho kinase inhibitor fasudil attenuates Aβ1-42-induced apoptosis via the ASK1/JNK signal pathway in primary cultures of hippocampal neurons

Alzheimer's disease (AD), a chronic, progressive, neurodegenerative disorder, is the most common type of dementia. Beta amyloid (Aβ) peptide aggregation and phosphorylated tau protein accumulation are considered as one of the causes for AD. Our previous studies have demonstrated the neuroprotective effect of the Rho kinase inhibitor fasudil, but the mechanism remains elucidated. In the present study, we examined the effects of fasudil on Aβ_1-42 aggregation and apoptosis and identified the intracellular signaling pathways involved in these actions in primary cultures of mouse hippocampal neurons. The results showed that fasudil increased neurite outgrowth (52.84%), decreased Aβ burden (46.65%), Tau phosphorylation (96.84%), and ROCK-II expression. In addition, fasudil reversed Aβ_1-42-induced decreased expression of Bcl-2 and increases in caspase-3, cleaved-PARP, phospho-JNK(Thr183/Tyr185), and phospho-ASK1(Ser966). Further, fasudil decreased mitochondrial membrane potential and intracellular calcium overload in the neurons treated with Aβ_1-42. These results suggest that inhibition of Rho kinase by fasudil reverses Aβ_1-42-induced neuronal apoptosis via the ASK1/JNK signal pathway, calcium ions, and mitochondrial membrane potential. Fasudil could be a drug of choice for treatment of Alzheimer's disease.

Exposure to carbon black nanoparticles during pregnancy persistently damages the cerebrovascular function in female mice

Maternal exposure to carbon black nanoparticles (CBNPs) during pregnancy have been well documented to induce harmful outcomes of offspring on brain function. However, it remains largely unknown whether females exposed to CBNPs during sensitive period of pregnancy can cause the neurotoxic effects on their own body after parturition. In this study, our results showed that pregnancy CBNPs exposure induced the persistent pathological changes in the cerebral cortex tissues and impaired cerebrovascular function of mice manifested by significant alterations of endothelin-1, endothelial nitric oxide synthase, vascular endothelial growth factor-A and ATP-binding cassette transporter G1. Intriguingly, we observed that these deleterious effects on brain and cerebrovascular functions in mice could persist for 49 days after delivery of pups. By using in vitro human umbilical vein endothelial cells, we further verified the potential vascular dysfunction after CBNPs exposure. In summary, our results provide the first evidence that pregnancy CBNPs exposure-induced brain pathological changes and cerebrovascular dysfunction can persist for a relative long time. These finding suggest exposure to CBNPs during sensitive stages of pregnancy may not only show the harmful effects on offspring neurodevelopment, but also result in the irreversible brain damage on mother body.

Low-level arsenic causes proteotoxic stress and not oxidative stress

Prolonged exposure to arsenic has been shown to increase the risk of developing a number of diseases, including cancer and type II diabetes. Arsenic is present throughout the environment in its inorganic forms, and the level of exposure varies greatly by geographical location. The current recommended maximum level of arsenic exposure by the EPA is 10μg/L, but levels>50-1000μg/L have been detected in some parts of Asia, the Middle East, and the Southwestern United States. One of the most important steps in developing treatment options for arsenic-linked pathologies is to understand the cellular pathways affected by low levels of arsenic. Here, we show that acute exposure to non-lethal, low-level arsenite, an environmentally relevant arsenical, inhibits the autophagy pathway. Furthermore, arsenite-induced autophagy inhibition initiates a transient, but moderate ER stress response. Significantly, low-level arsenite exposure does not exhibit an increase in oxidative stress. These findings indicate that compromised autophagy, and not enhanced oxidative stress occurs early during arsenite exposure, and that restoring the autophagy pathway and proper proteostasis could be a viable option for treating arsenic-linked diseases. As such, our study challenges the existing paradigm that oxidative stress is the main underlying cause of pathologies associated with environmental arsenic exposure.

MicroRNA-155 regulates arsenite-induced malignant transformation by targeting Nrf2-mediated oxidative damage in human bronchial epithelial cells

Arsenite is a well-documented human lung carcinogen but the detailed mechanisms of carcinogenesis remain unclear. In this study, human bronchial epithelial (16-HBE) cells were continuously exposed to 2.5μM arsenite for about 13 weeks to induce the phenotypes of malignant transformation. Our results showed that Nrf2 expression was gradually decreased whereas no significant change was observed on NF-κB activation with increased time of arsenite exposure. To test the roles of Nrf2-meidtaed oxidative damage in the arsenite-induced malignant transformation, we compared the levels of cGMP, PKG and oxidative damage-related indicators between arsenic-transformed cells and control cells. Our data demonstrated there were no significantly differences on the contents of cGMP, PKG, MDA and the production of ROS, but the levels of GSH and NO, the activities of SOD, tNOS and iNOS were significantly enhanced in the arsenic-transformed cells. Importantly, Nrf2 inactivation could be modulated by miR-155, and inhibition of miR-155 remarkably attenuated the malignant phenotypes and promoted apoptotic cell death in the arsenic-transformed cells. Together, our findings provide the novel mechanism that miR-155 may regulate arsenite-induced cell malignant transformation by targeting Nrf2-mediated oxidative damage, indicating that inhibition of miR-155 may be a potential strategy against lung carcinogenesis of arsenite.