Clozapine suppresses NADPH oxidase activation, counteracts cytosolic H2O2, and triggers early onset mitochondrial dysfunction during adipogenesis of human liposarcoma SW872 cells

Long-term treatment of schizophrenia with clozapine (CLZ), an atypical antipsychotic drug, is associated with an increased incidence of metabolic disorders mediated by poorly understood mechanisms. We herein report that CLZ, while slowing down the morphological changes and lipid accumulation occurring during SW872 cell adipogenesis, also causes an early (day 3) inhibition of the expression/nuclear translocation of CAAT/enhancer-binding protein β and peroxisome proliferator-activated receptor γ. Under the same conditions, CLZ blunts NADPH oxidase-derived reactive oxygen species (ROS) by a dual mechanism involving enzyme inhibition and ROS scavenging. These effects were accompanied by hampered activation of the nuclear factor (erythroid-derived2)-like 2 (Nrf2)-dependent antioxidant responses compared to controls, and by an aggravated formation of mitochondrial superoxide. CLZ failed to exert ROS scavenging activities in the mitochondrial compartment but appeared to actively scavenge cytosolic H2O2 derived from mitochondrial superoxide. The early formation of mitochondrial ROS promoted by CLZ was also associated with signs of mitochondrial dysfunction. Some of the above findings were recapitulated using mouse embryonic fibroblasts. We conclude that the NADPH oxidase inhibitory and cytosolic ROS scavenging activities of CLZ slow down SW872 cell adipogenesis and suppress their Nrf2 activation, an event apparently connected with increased mitochondrial ROS formation, which is associated with insulin resistance and metabolic syndrome. Thus, the cellular events characterised herein may help to shed light on the more detailed molecular mechanisms explaining some of the adverse metabolic effects of CLZ.

Mechanisms Associated with Cognitive and Behavioral Impairment Induced by Arsenic Exposure

Arsenic (As) is a metalloid naturally present in the environment, in food, water, soil, and air; however, its chronic exposure, even with low doses, represents a public health concern. For a long time, As was used as a pigment, pesticide, wood preservative, and for medical applications; its industrial use has recently decreased or has been discontinued due to its toxicity. Due to its versatile applications and distribution, there is a wide spectrum of human As exposure sources, mainly contaminated drinking water. The fact that As is present in drinking water implies chronic human exposure to this metalloid; it has become a worldwide health problem, since over 200 million people live where As levels exceed safe ranges. Many health problems have been associated with As chronic exposure including cancer, cardiovascular diseases, gastrointestinal disturbances, and brain dysfunctions. Because As can cross the blood-brain barrier (BBB), the brain represents a target organ where this metalloid can exert its long-term toxic effects. Many mechanisms of As neurotoxicity have been described: oxidative stress, inflammation, DNA damage, and mitochondrial dysfunction; all of them can converge, thus leading to impaired cellular functions, cell death, and in consequence, long-term detrimental effects. Here, we provide a current overview of As toxicity and integrated the global mechanisms involved in cognitive and behavioral impairment induced by As exposure show experimental strategies against its neurotoxicity.

ERO1α primes the ryanodine receptor to respond to arsenite with concentration dependent Ca2+ release sequentially triggering two different mechanisms of ROS formation

A 6 h exposure of U937 cells to 2.5 μM arsenite stimulates low Ca2+ release from the inositol 1, 4, 5-triphosphate receptor (IP3R), causing a cascade of causally connected events, i.e., endoplasmic reticulum oxidoreductin-1α (ERO1α) expression, activation of the ryanodine receptor (RyR), mitochondrial Ca2+ accumulation, mitochondrial superoxide formation and further ERO1α expression. At greater arsenite concentrations, the release of the cation from the IP3R and the ensuing ERO1α expression remained unchanged but were nevertheless critical to sequentially promote concentration-dependent increases in Ca2+ release from the RyR, NADPH oxidase activation and a third mechanism of ERO1α expression which, in analogy to the one driven by mitochondrial superoxide, was also mediated by reactive oxygen species (ROS) and devoid of effects on Ca2+ homeostasis. Thus, concentration-independent stimulation of Ca2+ release from the IP3R is of pivotal importance for the effects of arsenite on Ca2+ homeostasis. It stimulates the expression of a fraction of ERO1α that primes the RyR to respond to the metalloid with concentration-dependent Ca2+-release, triggering the formation of superoxide in the mitochondrial respiratory chain and via NADPH oxidase activation. The resulting dose-dependent ROS formation was associated with a progressive increase in ERO1α expression, which however failed to affect Ca2+ homeostasis, thereby suggesting that ROS, unlike IP3R-dependent Ca2+ release, promote ERO1α expression in sites distal from the RyR.

Inhibition of activity/expression, or genetic deletion, of ERO1α blunts arsenite geno- and cyto-toxicity

Our recent studies suggest that arsenite stimulates the crosstalk between the inositol 1, 4, 5-triphosphate receptor (IP₃R) and the ryanodine receptor (RyR) via a mechanism dependent on endoplasmic reticulum (ER) oxidoreductin1α (ERO1α) up-regulation. Under these conditions, the fraction of Ca²⁺ released by the RyR via an ERO1α-dependent mechanism was promptly cleared by the mitochondria and critically mediated O₂^-. formation, responsible for the triggering of time-dependent events associated with strand scission of genomic DNA and delayed mitochondrial apoptosis. We herein report that, in differentiated C2C12 cells, this sequence of events can be intercepted by genetic deletion of ERO1α as well as by EN460, an inhibitor of ERO1α activity. Similar results were obtained for the early effects mediated by arsenite in proliferating U937 cells, in which however the long-term studies were hampered by the intrinsic toxicity of the inhibitor. It was then interesting to observe that ISRIB, an inhibitor of p-eIF2 alpha, was in both cell types devoid of intrinsic toxicity and able to suppress ERO1α expression and the resulting downstream effects leading to arsenite geno- and cyto-toxicity. We therefore conclude that pharmacological inhibition of ERO1α activity, or expression, effectively counteracts the deleterious effects induced by the metalloid via a mechanism associated with prevention of mitochondrial O₂^-. formation.

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.

Crosstalk between ERO1α and ryanodine receptor in arsenite-dependent mitochondrial ROS formation

Arsenite, a well-established human carcinogen and toxic compound, promotes the formation of mitochondrial superoxide (mitoO₂^-) via a Ca²⁺-dependent mechanism, in which an initial stimulation of the inositol 1, 4, 5-trisphosphate receptor (IP₃R) is followed by the activation of the ryanodine receptor (RyR), critical for providing Ca²⁺ to the mitochondria. We now report that, under the same conditions, arsenite triggers endoplasmic reticulum (ER) stress and a threefold increase in ER oxidoreductin 1α (ERO1 α) levels in proliferating U937 cells. EN460, an inhibitor of ERO1 α, recapitulated all the effects associated with RyR inhibition or downregulation, including prevention of RyR-induced Ca²⁺ accumulation in mitochondria and the resulting O₂^-. formation. Quantitatively similar results were obtained in inhibitor studies performed in terminally differentiated wild type C2C12 cells. Moreover, ERO1 α knockout C2C12 myotubes responded to arsenite as their wild type counterpart supplemented with EN460. As a final note, arsenite enhanced the expression of ERO1 α via a mechanism mediated by Ca²⁺ release from both the IP₃R and RyR. We therefore conclude that arsenite activates a positive feedback amplification cycle between Ca²⁺ levels and ERO1 α in the ER, by which IP₃R-dependent Ca²⁺ induces ERO1 α and ERO1 α promotes Ca²⁺ release via RyR, thereby amplifying the initial Ca²⁺ load and causing the mitochondrial accumulation of the cation, critical for mitoO₂^- formation.

Overexpression of CERKL Protects Retinal Pigment Epithelium Mitochondria from Oxidative Stress Effects

The precise function of CERKL, a Retinitis Pigmentosa (RP) causative gene, is not yet fully understood. There is evidence that CERKL is involved in the regulation of autophagy, stress granules, and mitochondrial metabolism, and it is considered a gene that is resilient against oxidative stress in the retina. Mutations in most RP genes affect photoreceptors, but retinal pigment epithelium (RPE) cells may be also altered. Here, we aimed to analyze the effect of CERKL overexpression and depletion in vivo and in vitro, focusing on the state of the mitochondrial network under oxidative stress conditions. Our work indicates that the depletion of CERKL increases the vulnerability of RPE mitochondria, which show a shorter size and altered shape, particularly upon sodium arsenite treatment. CERKL-depleted cells have dysfunctional mitochondrial respiration particularly upon oxidative stress conditions. The overexpression of two human CERKL isoforms (558 aa and 419 aa), which display different protein domains, shows that a pool of CERKL localizes at mitochondria in RPE cells and that CERKL protects the mitochondrial network—both in size and shape—against oxidative stress. Our results support CERKL being a resilient gene that regulates the mitochondrial network in RPE as in retinal neurons and suggest that RPE cell alteration contributes to particular phenotypic traits in patients carrying CERKL mutations.

Temporal correlation of morphological and biochemical changes with the recruitment of different mechanisms of reactive oxygen species formation during human SW872 cell adipogenic differentiation

Human SW872 preadipocyte conversion to mature adipocytes is associated with time-dependent changes in differentiation markers' expression and with morphological changes accompanied by the accumulation of lipid droplets (LDs) as well as by increased mitochondriogenesis and mitochondrial membrane potential. Under identical conditions, the formation of reactive oxygen species (ROS) revealed with a general probe was significant at days 3 and 10 of differentiation and bearly detectable at day 6. NADPH oxidase (NOX)-2 activity determined with an immunocytochemical approach followed a very similar pattern. There was no evidence of mitochondrial ROS (mROS), as detected with a selective fluorescence probe, at days 3 and 6, possibly due to the triggering of the Nrf-2 antioxidant response. mROS were instead clearly detected at day 10, concomitantly with the accumulation of very large LDs, oxidation of both cardiolipin and thioredoxin 2, and decreased mitochondrial glutathione. In conclusion, the morphological and biochemical changes of differentiating SW872 cells are accompanied by the discontinuous formation of ROS derived from NOX-2, increasingly implicated in adipogenesis and adipose tissue dysfunction. In addition, mROS formation was significant only in the late phase of differentiation and was associated with mitochondrial dysfunction.

Functional organization of the endoplasmic reticulum dictates the susceptibility of target cells to arsenite-induced mitochondrial superoxide formation, mitochondrial dysfunction and apoptosis

Arsenite induces many critical effects associated with the formation of reactive oxygen species (ROS) through different mechanisms. We focused on Ca²⁺-dependent mitochondrial superoxide (mitoO₂^-.) formation and addressed questions on the effects of low concentrations of arsenite on the mobilization of the cation from the endoplasmic reticulum and the resulting mitochondrial accumulation. Using various differentiated and undifferentiated cell types uniquely expressing the inositol-1, 4, 5-triphosphate receptor (IP₃R), or both the IP₃R and the ryanodine receptor (RyR), we determined that expression of this second Ca²⁺ channel is an absolute requirement for mitoO₂^-. formation and for the ensuing mitochondrial dysfunction and downstream apoptosis. In arsenite-treated cells, RyR was recruited after IP₃R stimulation and agonist studies provided an indirect indication for a close apposition between RyR and mitochondria. It was also interesting to observe that arsenite fails to promote mitochondrial Ca²⁺ accumulation, mitoO₂^-. formation and mitochondrial toxicity in RyR-devoid cells, in which the IP₃R is in close contact with the mitochondria. We therefore conclude that low dose arsenite-induced mitoO₂^- formation, and the resulting mitochondrial dysfunction and toxicity, are prerequisite of cell types expressing the RyR in close apposition with mitochondria.

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.

SVCT2-Dependent plasma and mitochondrial membrane transport of ascorbic acid in differentiating myoblasts

The Na⁺-dependent Vitamin C transporter 2 (SVCT2) is expressed in the plasma and mitochondrial membranes of various cell types. This notion was also established in proliferating C2C12 myoblasts (Mb), in which the transporter was characterised by a high and low affinity in the plasma and mitochondrial membranes, respectively. In addition, the mitochondrial expression of SVCT2 appeared particularly elevated and, consistently, a brief pre-exposure to low concentrations of Ascorbic Acid (AA) abolished mitochondrial superoxide formation selectively induced by the cocktail arsenite/ATP. Early myotubes (Mt) derived from these cells after 4 days of differentiation presented evidence of slightly increased SVCT2 expression, and were characterised by kinetic parameters for plasma membrane transport of AA in line with those detected in Mb. Confocal microscopy studies indicated that the mitochondrial expression of SVCT2 is well preserved in Mt with one or two nuclei, but progressively reduced in Mt with three or more nuclei. Cellular and mitochondrial expression of SVCT2 was found reduced in day 7 Mt. While the uptake studies were compromised by the poor purity of the mitochondrial preparations obtained from day 4 Mt, we nevertheless obtained evidence of poor transport of the vitamin using the same functional studies successfully employed with Mb. Indeed, even greater concentrations of/longer pre-exposure to AA failed to induce scavenging of mitochondrial superoxide in Mt. These results are therefore indicative of a severely reduced mitochondrial uptake of the vitamin in early Mt, attributable to decreased expression as well as impaired activity of mitochondrial SVCT2.