Selenite activates caspase-independent necrotic cell death in Jurkat T cells and J774.2 macrophages by affecting mitochondrial oxidant generation

Implications of reactive oxygen species in lung cancer and exploiting it for therapeutic interventions

Lung cancer is the second (11.4%) most commonly diagnosed cancer and the first (18%) to cause cancer-related deaths worldwide. The incidence of lung cancer varies significantly among men, women, and high and low-middle-income countries. Air pollution, inhalable agents, and tobacco smoking are a few of the critical factors that determine lung cancer incidence and mortality worldwide. Reactive oxygen species are known factors of lung carcinogenesis resulting from the xenobiotics and their mechanistic paths are under critical investigation. Reactive oxygen species exhibit dual roles in cells, as a tumorigenic and anti-proliferative factor, depending on spatiotemporal context. During the precancerous state, ROS promotes cancer origination through oxidative stress and base-pair substitution mutations in pro-oncogenes and tumor suppressor genes. At later stages of tumor progression, they help the cancer cells in invasion, and metastases by activating the NF-kB and MAPK pathways. However, at advanced stages, when ROS exceeds the threshold, it promotes cell cycle arrest and induces apoptosis in cancer cells. ROS activates extrinsic apoptosis through death receptors and intrinsic apoptosis through mitochondrial pathways. Moreover, ROS upregulates the expression of beclin-1 which is a critical component to initiate autophagy, another form of programmed cell death. ROS is additionally involved in an intermediatory step in necroptosis, which catalyzes and accelerates this form of cell death. Various therapeutic interventions have been attempted to exploit this cytotoxic potential of ROS to treat different cancers. Growing body of evidence suggests that ROS is also associated with chemoresistance and cancer cell immunity. Considering the multiple roles of ROS, this review highlights the exploitation of ROS for various therapeutic interventions. However, there are still gaps in the literature on the dual roles of ROS and the involvement of ROS in cancer cell immunity and therapy resistance.

Selenocompounds and Sepsis-Redox Bypass Hypothesis: Part B-Selenocompounds in the Management of Early Sepsis

Significance: Endothelial barrier damage, which is in part caused by excess production of reactive oxygen, halogen and nitrogen species (ROHNS), especially peroxynitrite (ONOO^-), is a major event in early sepsis and, with leukocyte hyperactivation, part of the generalized dysregulated immune response to infection, which may even become a complex maladaptive state. Selenoenzymes have major antioxidant functions. Their synthesis is related to the need to limit deleterious oxidant redox cycling by small selenocompounds, which may be of therapeutic cytotoxic interest. Plasma selenoprotein-P is crucial for selenium transport from the liver to the tissues and for antioxidant endothelial protection, especially against ONOO^-. Above micromolar concentrations, sodium selenite (Na₂SeO₃) becomes cytotoxic, with a lower cytotoxicity threshold in activated cells, which has led to cancer research. Recent Advances: Plasma selenium (<2% of total body selenium) is mainly contained in selenoprotein-P, and concentrations decrease rapidly in the early phase of sepsis, because of increased selenoprotein-P binding and downregulation of hepatic synthesis and excretion. At low concentrations, Na₂SeO₃ acts as a selenium donor, favoring selenoprotein-P synthesis in physiology, but probably not in the acute phase of sepsis. Critical Issues: The cytotoxic effects of Na₂SeO₃ against hyperactivated leukocytes, especially the most immature forms that liberate ROHNS, may be beneficial, but they may also be harmful for activated endothelial cells. Endothelial protection against ROHNS by selenoprotein-P may reduce Na₂SeO₃ toxicity, which is increased in sepsis. Future Direction: The combination of selenoprotein-P for endothelial protection and the cytotoxic effects of Na₂SeO₃ against hyperactivated leukocytes may be a promising intervention for early sepsis. Antioxid. Redox Signal. 37, 998-1029.

The Paradoxical Leishmanicidal Effects of Superoxide Dismutase (SOD)-Mimetic Tempol in Leishmania braziliensis Infection in vitro

Leishmaniasis is an infectious disease caused by protozoans of the genus Leishmania. The macrophage is the resident cell in which the parasite replicates and it is important to identify new compounds that can aid in parasite elimination since the drugs used to treat leishmaniasis are toxic and present side effects. We have previously shown that treatment of Leishmania braziliensis-infected macrophages with DETC (Diethyldithiocarbamate) induces parasite killing, in vivo. Thus, the objective of this study was to further evaluate the effect of oxidants and antioxidants in L. braziliensis-infected macrophages, following treatment with either oxidizing Hydrogen Peroxide, Menadione, DETC, or antioxidant [NAC (N-Acetyl-Cyteine), Apocynin, and Tempol] compounds. We determined the percentage of infected macrophages and number of amastigotes. Promastigote survival was also evaluated. Both DETC (SOD-inhibitor) and Tempol (SOD-mimetic) decreased the percentage of infected cells and parasite load. Hydrogen peroxide did not interfere with parasite burden, while superoxide-generator Menadione had a reducing effect. On the other hand, NAC (GSH-replenisher) and Apocynin (NADPH-oxidase inhibitor) increased parasite burden. Tempol surfaces as an interesting candidate for the chemotherapy of CL with an IC₅₀ of 0.66 ± 0.08 mM and selectivity index of 151. While it remains obscure how a SOD-mimetic may induce leishmanicidal effects, we suggest the possibility of developing Tempol-based topical applications for the treatment of cutaneous leishmaniasis caused by L. braziliensis.

Responses of an American eel brain endothelial-like cell line to selenium deprivation and to selenite, selenate, and selenomethionine additions in different exposure media

The effect of selenium deprivation and addition on the American eel brain endothelial cell line (eelB) was studied in three exposure media: complete growth medium (L15/FBS), serum-free medium (L15), and minimal medium (L15/ex). L15/ex contains only galactose and pyruvate and allowed the deprivation of selenium on cells to be studied. In L15/ex, without any obvious source of selenium, eelB cells survived for at least 7 d, formed capillary-like structures (CLS) on Matrigel, and migrated to heal wounds. Three selenium compounds were added to cultures: selenite, selenate, and selenomethionine (SeMet). Adding selenite or selenate to eelB cell cultures for 24 h caused dose-dependent declines in cell viability, regardless of the exposure media. Although varying with exposure media and viability end point, selenite was approximately 70-fold more cytotoxic than selenate. By contrast, 24 h exposures to either DL- or L-SeMet in the three media caused little or no cytotoxicity. However for 7 d exposures in L15/ex, DL- and L-SeMet were very cytotoxic, even at the lowest tested concentration of 31 μM. By contrast in L15 and L15/FBS, cytotoxicity was only observed with 500 and 1000 μM L-SeMet. In L15/FBS, eelB continued to migrate and form CLS in the presence of SeMet but at 500 μM, cell migration appeared stimulated. As judged from a colony-forming assay over 14 d in L15/FBS, 500 and 1000 μM DL- and L-SeMet inhibited cell proliferation. Overall, the responses of eel cells to selenium depended on the selenium form, concentration, and exposure media, with responses to SeMet being most dependent on exposure media.

Reactive oxygen species and cancer paradox: To promote or to suppress?

Reactive oxygen species (ROS), a group of highly reactive ions and molecules, are increasingly being appreciated as powerful signaling molecules involved in the regulation of a variety of biological processes. Indeed, their role is continuously being delineated in a variety of pathophysiological conditions. For instance, cancer cells are shown to have increased ROS levels in comparison to their normal counterparts. This is partly due to an enhanced metabolism and mitochondrial dysfunction in cancer cells. The escalated ROS generation in cancer cells contributes to the biochemical and molecular changes necessary for the tumor initiation, promotion and progression, as well as, tumor resistance to chemotherapy. Therefore, increased ROS in cancer cells may provide a unique opportunity to eliminate cancer cells via elevating ROS to highly toxic levels intracellularly, thereby, activating various ROS-induced cell death pathways, or inhibiting cancer cell resistance to chemotherapy. Such results can be achieved by using agents that either increase ROS generation, or inhibit antioxidant defense, or even a combination of both. In fact, a large variety of anticancer drugs, and some of those currently under clinical trials, effectively kill cancer cells and overcome drug resistance via enhancing ROS generation and/or impeding the antioxidant defense mechanism. This review focuses on our current understanding of the tumor promoting (tumorigenesis, angiogenesis, invasion and metastasis, and chemoresistance) and the tumor suppressive (apoptosis, autophagy, and necroptosis) functions of ROS, and highlights the potential mechanism(s) involved. It also sheds light on a very novel and an actively growing field of ROS-dependent cell death mechanism referred to as ferroptosis.

Novel CHOP activator LGH00168 induces necroptosis in A549 human lung cancer cells via ROS-mediated ER stress and NF-κB inhibition

AIM

C/EBP homologous protein (CHOP) is a transcription factor that is activated at multiple levels during ER stress and plays an important role in ER stress-induced apoptosis. In this study we identified a novel CHOP activator, and further investigated its potential to be a therapeutic agent for human lung cancer.

METHODS

HEK293-CHOP-luc reporter cells were used in high-throughput screening (HTS) to identify CHOP activators. The cytotoxicity against cancer cells in vitro was measured with MTT assay. The anticancer effects were further examined in A549 human non-small cell lung cancer xenograft mice. The mechanisms underlying CHOP activation were analyzed using luciferase assays, and the anticancer mechanisms were elucidated in A549 cells.

RESULTS

From chemical libraries of 50 000 compounds, LGH00168 was identified as a CHOP activator, which showed cytotoxic activities against a panel of 9 cancer cell lines with an average IC₅₀ value of 3.26 μmol/L. Moreover, administration of LGH00168 significantly suppressed tumor growth in A549 xenograft bearing mice. LGH00168 activated CHOP promoter via AARE1 and AP1 elements, increased DR5 expression, decreased Bcl-2 expression, and inhibited the NF-κB pathway. Treatment of A549 cells with LGH00168 (10 μmol/L) did not induce apoptosis, but lead to RIP1-dependent necroptosis, accompanied by cell swelling, plasma membrane rupture, lysosomal membrane permeabilization, MMP collapse and caspase 8 inhibition. Furthermore, LGH00168 (10 and 20 μmol/L) dose-dependently induced mito-ROS production in A549 cells, which was reversed by the ROS scavenger N-acetyl-L-cysteine (NAC, 10 mmol/L). Moreover, NAC significantly diminished LGH00168-induced CHOP activation, NF-κB inhibition and necroptosis in A549 cells.

CONCLUSION

LGH00168 is a CHOP activator that inhibits A549 cell growth in vitro and lung tumor growth in vivo.

Aspects of a Distinct Cytotoxicity of Selenium Salts and Organic Selenides in Living Cells with Possible Implications for Drug Design

Selenium is traditionally considered as an antioxidant element and selenium compounds are often discussed in the context of chemoprevention and therapy. Recent studies, however, have revealed a rather more colorful and diverse biological action of selenium-based compounds, including the modulation of the intracellular redox homeostasis and an often selective interference with regulatory cellular pathways. Our basic activity and mode of action studies with simple selenium and tellurium salts in different strains of Staphylococcus aureus (MRSA) and Saccharomyces cerevisiae indicate that such compounds are sometimes not particularly toxic on their own, yet enhance the antibacterial potential of known antibiotics, possibly via the bioreductive formation of insoluble elemental deposits. Whilst the selenium and tellurium compounds tested do not necessarily act via the generation of Reactive Oxygen Species (ROS), they seem to interfere with various cellular pathways, including a possible inhibition of the proteasome and hindrance of DNA repair. Here, organic selenides are considerably more active compared to simple salts. The interference of selenium (and tellurium) compounds with multiple targets could provide new avenues for the development of effective antibiotic and anticancer agents which may go well beyond the traditional notion of selenium as a simple antioxidant.

Activation of p38 and changes in mitochondria accompany autophagy to premature senescence-like phenotype switch upon chronic exposure to selenite in colon fibroblasts

Effects of chronic exposure to supranutritional sodium selenite (Se) were investigated in colonic fibroblasts. Initially, Se did not produce any gross changes in exposed cells; however, basal levels of autophagy were transiently increased and p38 activity was stimulated. From the 3rd week onwards, Se decreased cell proliferation, with corrensponding changes in cell cycle distribution. Also, in exposed cells oxidative stress and DNA damage slowly but gradually increased along with decreasing mitochondrial function and upon continued elevated activity of p38 kinase. Towards the end of the experiment, premature senescence features became more prominent in treated cells. Pharmacological inhibition as well as gene knockdown of these processes confirmed the involvement of p38 in balancing autophagy and premature senescence in cells exposed to Se and suggests that this element may in a given time frame compromise selected cell populations in digestive system.

Selenium compounds as therapeutic agents in cancer

BACKGROUND

With cancer cells encompassing consistently higher production of reactive oxygen species (ROS) and with an induced antioxidant defense to counteract the increased basal ROS production, tumors have a limited reserve capacity resulting in an increased vulnerability of some cancer cells to ROS. Based on this, oxidative stress has been recognized as a tumor-specific target for the rational design of new anticancer agents. Among redox modulating compounds, selenium compounds have gained substantial attention due to their promising chemotherapeutic potential.

SCOPE OF REVIEW

This review aims in summarizing and providing the recent developments of our understanding of the molecular mechanisms that underlie the potential anticancer effects of selenium compounds.

MAJOR CONCLUSIONS

It is well established that selenium at higher doses readily can turn into a prooxidant and thereby exert its potential anticancer properties. However, the biological activity of selenium compounds and the mechanism behind these effects are highly dependent on its speciation and the specific metabolic pathways of cells and tissues. Conversely, the chemical properties and the main molecular mechanisms of the most relevant inorganic and organic selenium compounds as well as selenium-based nanoparticles must be taken into account and are discussed herein.

GENERAL SIGNIFICANCE

Elucidating and deepening our mechanistic knowledge of selenium compounds will help in designing and optimizing compounds with more specific antitumor properties for possible future application of selenium compounds in the treatment of cancer. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Mechanism(s) of Toxic Action of Zn and Selenite: A Study on AS-30D Hepatoma Cells and Isolated Mitochondria

Mitochondria of AS-30D rat ascites hepatoma cells are found to be the main target for Zn²⁺ and sodium selenite (Na₂SeO₃). High [mu]M concentrations of Zn²⁺ or selenite were strongly cytotoxic, killing the AS-30D cells by both apoptotic and necrotic ways. Both Zn²⁺ and selenite produced strong changes in intracellular generation of reactive oxygen species (ROS) and the mitochondrial dysfunction via the mitochondrial electron transport chain (mtETC) disturbance, the membrane potential dissipation, and the mitochondrial permeability transition pore opening. The significant distinctions in toxic action of Zn²⁺ and selenite on AS-30D cells were found. Selenite induced a much higher intracellular ROS level (the early event) compared to Zn²⁺ but a lower membrane potential loss and a lower decrease of the uncoupled respiration rate of the cells, whereas the mtETC disturbance was the early and critical event in the mechanism of Zn²⁺ cytotoxicity. Sequences of events manifested in the mitochondrial dysfunction produced by the metal/metalloid under test are compared with those obtained earlier for Cd²⁺, Hg²⁺, and Cu²⁺ on the same model system.

Selenium and the selenoprotein thioredoxin reductase in the prevention, treatment and diagnostics of cancer

Selenium is an essential element that is specifically incorporated as selenocystein into selenoproteins. It is a potent modulator of eukaryotic cell growth with strictly concentration-dependant effects. Lower concentrations are necessary for cell survival and growth, whereas higher concentrations inhibit growth and induce cell death. It is well established that selenium has cancer preventive effects, and several studies also have shown that it has strong anticancer effects with a selective cytotoxicity on malignant drug-resistant cells while only exerting marginal effects on normal and benign cells. This cancer-specific cytotoxicity is likely explained by high affinity selenium uptake dependent on proteins connected to multidrug resistance. One of the most studied selenoproteins in cancer is thioredoxin reductase (TrxR) that has important functions in neoplastic growth and is an important component of the resistant phenotype. Several reports have shown that TrxR is induced in tumor cells and pre-neoplastic cells, and several commonly used drugs interact with the protein. In this review, we summarize the current knowledge of selenium as a potent preventive and tumor selective anticancer drug, and we also discuss the potential of using the expression and modulation of the selenoprotein TrxR in the diagnostics and treatment of cancer.

Outer membrane VDAC1 controls permeability transition of the inner mitochondrial membrane in cellulo during stress-induced apoptosis

Voltage-dependent anion channel (VDAC)1 is the main channel of the mitochondrial outer membrane (MOM) and it has been proposed to be part of the permeability transition pore (PTP), a putative multiprotein complex candidate agent of the mitochondrial permeability transition (MPT). Working at the single live cell level, we found that overexpression of VDAC1 triggers MPT at the mitochondrial inner membrane (MIM). Conversely, silencing VDAC1 expression results in the inhibition of MPT caused by selenite-induced oxidative stress. This MOM-MIM crosstalk was modulated by Cyclosporin A and mitochondrial Cyclophilin D, but not by Bcl-2 and Bcl-X(L), indicative of PTP operation. VDAC1-dependent MPT engages a positive feedback loop involving reactive oxygen species and p38-MAPK, and secondarily triggers a canonical apoptotic response including Bax activation, cytochrome c release and caspase 3 activation. Our data thus support a model of the PTP complex involving VDAC1 at the MOM, and indicate that VDAC1-dependent MPT is an upstream mechanism playing a causal role in oxidative stress-induced apoptosis.

Treatment of human cancer cells with selenite or tellurite in combination with auranofin enhances cell death due to redox shift

Selenium is an essential trace element incorporated as selenocysteine in 25 human selenoproteins. Among them are thioredoxin reductases (TrxR) and glutathione peroxidases, all central proteins in the regulation of the cellular thiol redox state. In this paper the effects of selenite and tellurite treatment in human cancer cells are reported and compared. Our results show that both selenite and tellurite, at relatively low concentrations, are able to increase the expression of mitochondrial and cytosolic TrxR in cisplatin-sensitive (2008) and -resistant (C13*) phenotypes. We further investigated the cellular effects induced by selenite or tellurite in combination with the specific TrxR inhibitor auranofin. Selenite pretreatment induced a dramatic increase in auranofin cytotoxicity in both resistant and sensitive cells. Investigation of TrxR activity and expression levels as well as the cellular redox state demonstrated the involvement of TrxR inhibition and redox changes in selenite and auranofin combined action.

Phenotype-dependent apoptosis signalling in mesothelioma cells after selenite exposure

Background

Selenite is a promising anticancer agent which has been shown to induce apoptosis in malignant mesothelioma cells in a phenotype-dependent manner, where cells of the chemoresistant sarcomatoid phenotype are more sensitive.

Methods

In this paper, we investigate the apoptosis signalling mechanisms in sarcomatoid and epithelioid mesothelioma cells after selenite treatment. Apoptosis was measured with the Annexin-PI assay. The mitochondrial membrane potential, the expression of Bax, Bcl-XL, and the activation of caspase-3 were assayed with flow cytometry and a cytokeratin 18 cleavage assay. Signalling through JNK, p38, p53, and cathepsins B, D, and E was investigated with chemical inhibitors. Furthermore, the expression, nuclear translocation and DNA-binding activity of p53 was investigated using ICC, EMSA and the monitoring of p21 expression as a downstream event. Levels of thioredoxin (Trx) were measured by ELISA.

Results

In both cell lines, 10 μM selenite caused apoptosis and a marked loss of mitochondrial membrane potential. Bax was up-regulated only in the sarcomatoid cell line, while the epithelioid cell line down-regulated Bcl-XL and showed greater caspase-3 activation. Nuclear translocation of p53 was seen in both cell lines, but very little p21 expression was induced. Chemical inhibition of p53 did not protect the cells from apoptosis. p53 lost its DNA binding ability after selenite treatment and was enriched in an inactive form. Levels of thioredoxin decreased after selenite treatment. Chemical inhibition of MAP kinases and cathepsins showed that p38 and cathepsin B had some mediatory effect while JNK had an anti-apoptotic role.

Conclusion

We delineate pathways of apoptosis signalling in response to selenite, showing differences between epithelioid and sarcomatoid mesothelioma cells. These differences may partly explain why sarcomatoid cells are more sensitive to selenite.

Nutritional lipid-induced oxidative stress leads to mitochondrial dysfunction followed by necrotic death in FaO hepatocytes

OBJECTIVE

Mitochondrial dysfunction and hepatocyte cell death have been reported in fatty liver and non-alcoholic steatohepatitis. Our aim in this study was to evaluate whether direct exposure of hepatocytes to extracellular fat could facilitate such deleterious effects.

METHODS

FaO hepatic cells treated with fat was used as an in vitro model for steatosis. FaO hepatocytes were exposed to 0.1% triacylglycerols using commercially available lipid emulsion (LE) for various periods and studied for production of reactive oxygen species (ROS), mitochondrial function, and cell death parameters. To study the type of cell death, high-mobility group box chromosomal protein 1cellular levels, DNA fragmentation, and caspase activity were evaluated.

RESULTS

Cells incubated with LE for 6 h exhibited a marked increase in the production of intracellular ROS. Using treatments with peroxisome proliferator-activated receptor activators, mitochondrial electron-transfer chain inhibitor, and different sources of LE that did or did not contain medium-chain triacylglycerols, the mitochondria were found to be the source of ROS. LE treatment resulted in phosphorylation of adenosine monophosphate-activated protein kinase, accompanied by a decrease in adenosine triphosphate levels. Changes in intracellular ROS and energy levels were followed by cell death. FaO hepatocytes showed a significant reduction in high-mobility group box chromosomal protein-1 and little DNA fragmentation. Incubation with LE for 24 h did not change caspase-3 activity, indicating that hepatocyte death was necrotic. The antioxidant N-acetylcysteine was able to attenuate the changes in intracellular energy levels and ROS levels and to prevent cell death after exposure to LE.

CONCLUSION

These results suggest that exposure of FaO cells to LE leads to an increase in mitochondrial ROS production and a decrease in cellular energy levels followed by necrotic cell death.

Cancer chemoprevention: a radical perspective

Cancer chemopreventive agents block the transformation of normal cells and/or suppress the promotion of premalignant cells to malignant cells. Certain agents may achieve these objectives by modulating xenobiotic biotransformation, protecting cellular elements from oxidative damage, or promoting a more differentiated phenotype in target cells. Conversely, various cancer chemopreventive agents can encourage apoptosis in premalignant and malignant cells in vivo and/or in vitro, which is conceivably another anticancer mechanism. Furthermore, it is evident that many of these apoptogenic agents function as prooxidants in vitro. The constitutive intracellular redox environment dictates a cell's response to an agent that alters this environment. Thus, it is highly probable that normal cells, through adaption, could acquire resistance to transformation via exposure to a chemopreventive agent that promotes oxidative stress or disrupts the normal redox tone of these cells. In contrast, transformed cells, which typically endure an oxidizing intracellular environment, would ultimately succumb to apoptosis due to an uncontrollable production of reactive oxygen species caused by the same agent. Here, we provide evidence to support the hypothesis that reactive oxygen species and cellular redox tone are exploitable targets in cancer chemoprevention via the stimulation of cytoprotection in normal cells and/or the induction of apoptosis in transformed cells.

Effects of selenium status and supplementary seleno-chemical sources on mouse T-cell mitogenesis

Although selenium is thought to be essential for various immune responses, the excess supplementation may have an adverse effect on certain immunological functions. The present study was designed to determine the effective chemical forms of selenium and their optimal levels on T-cell mitogenesis with splenic cells from mice given a selenium-deficient diet for 8 weeks to avoid effects of cellular selenium sources. Although selenium in tissues, except for spleen and thymus, was almost depleted by feeding selenium-deficient diet, the lymphoid organs still contained low levels of selenium. Both activities of cellular glutathione peroxidase (cGPx) and thioredoxin reductase (TR) in liver and splenic cells showed a tendency to decrease by selenium deficiency. However, splenic cells were tolerant against decrease of the selenoenzyme activities, and TR was also more tolerant than cGPx. T-cell proliferation of the selenium-insufficient splenic cells induced by concanavalin A was increased by addition of Na2SeO3, Na2SeO4, Na2Se, seleno-DL-cystine, seleno-L-methionine and selenocystamine. Their promoting action was observed at levels lower than 0.1 micromol/L and was completely suppressed at the highest concentration (1 micromol/L), except for selenocystamine. Na2SeO3 was one of the efficient selenocompounds for the mitogenesis, which was concomitant with the significant induction of cGPx and TR. However, recovery of cGPx activity in the selenium-insufficient cells by supplementary Na2SeO3 was only partial,while TR activity was readily recovered from selenium deficiency. These results therefore indicate that only low levels of selenium is essential for T-cell mitogenesis even in selenium-insufficient splenic cells, and TR, which is readily recovered by Na2SeO3, may be the critical enzyme.

Selenium supplementation increases liver MnSOD expression: molecular mechanism for hepato-protection

Selenium is recognized as essential in animal and human nutrition. Several hypotheses have been advanced for its biological activity. The aim of this study was to investigate the in vivo effect of selenium on rat liver manganese superoxide dismutase (MnSOD), a key antioxidant enzyme, under naïve and inflammatory conditions. Rats received sodium selenite supplementation and LPS injection. Whole-liver samples, isolated hepatocytes, Kupffer cells and blood samples were subjected to protein, RNA and biochemical analysis. Liver enrichment with selenium increased whole-liver MnSOD levels due to an increase in MnSOD transcription in hepatocytes. This was due to an increase in the ratio of specificity protein 1 to activating enhancer binding protein 2 DNA-binding activity. The inflammatory stimulus further elevated MnSOD levels in the whole-liver that was abrogated in sodium selenite supplementation due to reduced transcription of MnSOD in Kupffer cells. Moreover, selenium enrichment decreased Kupffer cells IL-6 transcription in LPS-injected animals. Anti-inflammatory activity of selenium was demonstrated by normalized blood levels of ALT and IL-6 in LPS-injected animals. In conclusion, selenium up-regulates hepatocytes MnSOD expression, probably improving their anti-oxidant defense, while decreasing MnSOD and IL-6 transcription in Kupffer cells in the presence of inflammatory stimuli, attenuating their inflammatory response. This selective mechanism may explain the anti-inflammatory and hepato-protective effect of selenium.

Selenium activates p53 and p38 pathways and induces caspase-independent cell death in cervical cancer cells

The mechanisms of sodium selenite-induced cell death in cervical carcinoma cells were studied during 24 h of exposure in the HeLa Hep-2 cell line. Selenite at the employed concentrations of 5 and 50 micromol/L produced time- and dose-dependent suppression of DNA synthesis and induced DNA damage which resulted in phosphorylation of histone H2A.X. These effects were influenced by pretreatment of cells with the SOD/catalase mimetic MnTMPyP or glutathione-depleting buthionine sulfoximine, suggesting the significant role of selenite-generated oxidative stress. Following the DNA damage, selenite activated p53-dependent pathway as evidenced by the appearance of phosphorylated p53 and accumulation of p21 in the treated cells. Concomitantly, selenite activated p38 pathway but its effect on JNK was very weak. p53- and p38-dependent signaling led to the accumulation of Bax protein, which was preventable by specific inhibitors of p38 (SB 203580) and p53 (Pifithrin-alpha). Mitochondria in selenite-treated cells changed their dynamics (shape and localization) and released AIF and Smac/Diablo, which initiated caspase-independent apoptosis as confirmed by the caspase-3 activity assay and the low effect of caspase inhibitors z-DEVD-fmk and z-VAD-fmk on cell death. We conclude that selenite induces caspase-independent apoptosis in cervical carcinoma cells mostly by oxidative stress-mediated activation of p53 and p38 pathways, but other selenite-mediated effects, in particular mitochondria-specific ones, are also involved.

Contribution of thioredoxin reductase to T-cell mitogenesis and NF-kappaB DNA-binding promoted by selenite

Although the essential role of selenium for cellular immune responses is obvious, delineation of the functions is lacking because selenium can either promote or inhibit cell growth, cytokine production, and activation of transcription factor nuclear factor-kappaB (NF-kappaB). Studies with human thioredoxin-1 (Trx-1)-transgenic (Tg) mice were conducted to evaluate the relationship between stimulation of T-cell mitogenic response by sodium selenite and the intracellular Trx-1 levels, and the activities of selenoenzymes and NF-kappaB-DNA binding. Concanavalin A-induced mitogenesis of wild-type mouse splenic cells was stimulated by exposure to low levels of selenite (0.02-0.1 microM), with augmentation of NF-kappaB-DNA binding activity. Treatment with NF-kappaB nuclear translocation inhibitor SN50 or thioredoxin reductase (TR) inhibitor aurothioglucose depressed this stimulatory action. The mitogenic response of Trx-1-Tg mouse splenic cells was enhanced by exposure to relatively high levels of selenite (> or = 0.05 microM), compared with the wild-type mouse. Selenite also augmented TR activity but not cellular glutathione peroxidase activity in the Trx-1-overexpressed cells. These results suggest that the stimulation of T-cell mitogenic response by the physiological levels of selenite is predominantly caused by increased TR activity, which may lead to reduction of Trx-1 dependent on the intracellular expression level and promotion of DNA binding of NF-kappaB.

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.

Selenium attenuates expression of MnSOD and uncoupling protein 2 in J774.2 macrophages: molecular mechanism for its cell-death and antiinflammatory activity

Selenium can activate cell death. However, the mechanism of action is not yet fully defined. We hypothesized that selenium may impede mitochondrial superoxide dismutation to H2O2 and O2, leading to cell death in macrophages and that this effect may be relevant to antiinflammatory treatment by selenium. In this study, the mechanism of action of selenium was investigated in nonactivated and activated (immune-stimulated) J774.2 macrophages. Sodium selenite treatment decreased dichlorodihydrofluorescein-reacting intracellular reactive oxygen species (ROS) (mainly peroxides and hydroxyl radicals), with no correlation to glutathione peroxidase activity. However, selenite decreased the transcription and expression of manganese superoxide dismutase (MnSOD) and uncoupling protein 2 (UCP2). This cellular effect was due to inhibition of specificity protein-1 (Sp1) binding to its DNA binding site. Following immune stimulation of macrophages using lipopolysaccharides plus interferon-gamma, MnSOD was up-regulated. Activated macrophages showed higher mitochondrial membrane potential, intracellular ROS levels, and cellular resistance to cell death. Selenite treatment attenuated all of these parameters. Selenite prevented nuclear factor-kappaB (NF-kappaB) activation as a mechanism of its inhibitory activity on MnSOD expression in the immune-stimulated cells. In addition, overexpression of human MnSOD protected against death induced by selenite treatment. It is therefore concluded that selenium at high nanomolar to low micromolar concentrations shifts the balance between inflammatory response and cell death toward the latter, through a direct effect on the transcription factors Sp1 and NF-kappaB, and down-regulation of MnSOD and UCP2.