Selenite is a potent cytotoxic agent for human primary AML cells

The Role of the Thioredoxin Detoxification System in Cancer Progression and Resistance

The intracellular redox homeostasis is a dynamic balancing system between the levels of free radical species and antioxidant enzymes and small molecules at the core of cellular defense mechanisms. The thioredoxin (Trx) system is an important detoxification system regulating the redox milieu. This system is one of the key regulators of cells’ proliferative potential as well, through the reduction of key proteins. Increased oxidative stress characterizes highly proliferative, metabolically hyperactive cancer cells, which are forced to mobilize antioxidant enzymes to balance the increase in free radical concentration and prevent irreversible damage and cell death. Components of the Trx system are involved in high-rate proliferation and activation of pro-survival mechanisms in cancer cells, particularly those facing increased oxidative stress. This review addresses the importance of the targetable redox-regulating Trx system in tumor progression, as well as in detoxification and protection of cancer cells from oxidative stress and drug-induced cytotoxicity. It also discusses the cancer cells’ counteracting mechanisms to the Trx system inhibition and presents several inhibitors of the Trx system as prospective candidates for cytostatics’ adjuvants. This manuscript further emphasizes the importance of developing novel multitarget therapies encompassing the Trx system inhibition to overcome cancer treatment limitations.

Selenium stimulates the antitumour immunity: Insights to future research

Selenium is an essential trace element for regulating immune functions through redox-regulating activity of selenoproteins (e.g. glutathione peroxidase), protecting immune cells from oxidative stress. However, in cancer, selenium has biological bimodal action depending on the concentration. At nutritional low doses, selenium, depending on its form, may act as an antioxidant, protecting against oxidative stress, supporting cell survival and growth, thus, plays a chemo-preventive role; while, at supra-nutritional higher pharmacological doses, selenium acts as pro-oxidant inducing redox signalling and cell death. To date, many studies have been conducted on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level, indicating that likely selenium functions as an immunostimulator, i.e. reversing the immunosuppression in tumour microenvironment towards antitumour immunity by activating immune cells (e.g. M₁ macrophages and CD8⁺ T-lymphocytes) and releasing pro-inflammatory cytokines such as interferon-gamma; whereas, fewer studies have explored the effects of supra-nutritional or pharmacological doses of selenium in cancer immunity. This review, thus, systematically analyses the current knowledge about how selenium stimulates the immune system against cancer and lay the groundwork for future research. Such knowledge can be promising to design combinatorial therapies with Selenium-based compounds and other modalities like immunotherapy to lower the adverse effects and increase the efficacy of treatments.

The Antitumor Activity of Sodium Selenite Alone and in Combination with Gemcitabine in Pancreatic Cancer: An In Vitro and In Vivo Study

Sodium selenite acts by depleting enzymes that protect against cellular oxidative stress. To determine its effect alone or in combination with gemcitabine (GMZ) in pancreatic cancer, we used PANC-1 and Pan02 cell lines and C57BL mice bearing a Pan02-generated tumor. Our results demonstrated a significant inhibition of pancreatic cancer cell viability with the use of sodium selenite alone and a synergistic effect when associated with GMZ. The molecular mechanisms of the antitumor effect of sodium selenite alone involved apoptosis-inducing factor (AIF) and the expression of phospho-p38 in the combined therapy. In addition, sodium selenite alone and in association with GMZ significantly decreased the migration capacity and colony-forming ability, reduced tumor activity in multicellular tumor spheroids (MTS) and decreased sphere formation of cancer stem cells. In vivo studies demonstrated that combined therapy not only inhibited tumor growth (65%) compared to the untreated group but also relative to sodium selenite or GMZ used as monotherapy (up to 40%), increasing mice survival. These results were supported by the analysis of C57BL/6 albino mice bearing a Pan02-generated tumor, using the IVIS system. In conclusion, our results showed that sodium selenite is a potential agent for the improvement in the treatment of pancreatic cancer and should be considered for future human clinical trials.

Selenium-Containing Nanoparticles Combine the NK Cells Mediated Immunotherapy with Radiotherapy and Chemotherapy

Considering the limited clinical benefits of individual approaches against malignancy, natural killer (NK) cell-mediated immunotherapy is increasingly utilized in combination with radiotherapy and target therapeutics. However, the interplay of targeted agents, immunotherapy, and radiotherapy is complex. An improved understanding of the effect of chemotherapy or radiotherapy on specific molecular pathways in immune cells would help to optimize the synergistic antitumor efficiency. In this study, the selenium-containing nanoparticles (NPs) could deliver the chemotherapeutic drug doxorubicin (DOX) to tumor sites by systemic administration. Radiation stimuli facilitate DOX release and enhance chemotherapy efficiency. Moreover, radiation could oxidize diselenide-containing NPs to seleninic acid, which have both synergistic antitumor effect and immunomodulatory activity through enhancing NK cells function. These results indicate that the selenium-containing NPs would be a potential approach to achieve simultaneous treatments of immunotherapy, chemotherapy, and radiotherapy by a simple but effective method.

p53 controls the switch between autophagy and apoptosis through regulation of PLSCR1 in sodium selenite-treated leukemia cells

Coordinated regulation of autophagy and apoptosis helps to enhance the antitumor effects of sodium selenite. However, the potential molecules that act as switch nodes in the crosstalk between autophagy and apoptosis is still elusive. Phospholipid scramblase 1 (PLSCR1) has been shown to regulate leukocyte differentiation, while its role in autophagy/apoptosis toggle switch remains unexplored. In this study, we showed that sodium selenite switched protective autophagy to apoptosis in p53-wild type NB4 cells without obvious caspase-8/apoptosis-inducing factor (AIF) axis activation, while induced autophagy-dependent caspase-8/AIF axis activation in p53-mutant Jurkat cells. Additionally, p53 was demonstrated as a positive regulator of PLSCR1. p53-dependent up-regulation of PLSCR1 accounted for the differential regulation of autophagy and apoptosis induced by sodium selenite. Furthermore, sodium selenite induced the release of AIF from mitochondria to cytosol with the facilitation of caspase-8 in Jurkat cells, while not in NB4 cells. The released AIF further enhanced autophagy flux through interacting with PLSCR1, which hereby resulting in the disassociation of PLSCR1 from Atg5-Atg12 complex. Our results indicate that PLSCR1 plays a critical role in p53-dependent regulation of autophagy and apoptosis in sodium selenite-treated leukemia cells. Manipulation of p53-PLSCR1 cascade might be beneficial to enhance the anti-tumor effects of sodium selenite.

Redox-Active Selenium in Health and Disease: A Conceptual Review

Although it is generally accepted that selenium (Se) is important for life, it is not well known which forms of organic and/or inorganic Se compound are the most biologically active. In nature Se exists mostly in two forms, namely as selenite with fourvalent and selenate with sixvalent cations, from which all other inorganic and organic species are derived. Despite a small difference in their electronic structure, these two inorganic parent compounds differ significantly in their redox properties. Hence, only selenite can act as an oxidant, particularly in the reaction with free and/or protein- bound sulhydryl (SH) groups. For example, selenite was shown to inhibit the hydroxyl radicalinduced reduction and scrambled reoxidation of disulfides in human fibrinogen thus preventing the formation of highly hydrophobic polymer, termed parafibrin. Such a polymer, when deposited within peripheral and/or cerebral circulation, may cause irreversible damage resulting in the development of cardiovascular, neurological and other degenerative diseases. In addition, parafibrin deposited around tumor cells produces a protease-resistant coat protecting them against immune recognition and elimination. On the other hand, parafibrin generated by Ebola's protein disulfide isomerase can form a hydrophobic 'spike' that facilitates virus attachment and entry to the host cell. In view of these specific properties of selenite this compound is a potential candidate as an inexpensive and readily available food supplement in the prevention and/or treatment of cardiovascular, neoplastic, neurological and infectious diseases.

Selenium nanoparticles are more efficient than sodium selenite in producing reactive oxygen species and hyper-accumulation of selenium nanoparticles in cancer cells generates potent therapeutic effects

We have previously demonstrated that selenium nanoparticles (SeNPs) administered via oral route possess similar capacities of increasing selenoenzyme activities as the extensively examined sodium selenite, selenomethionine and methylselenocysteine, and yet display the lowest toxicity among these selenium compounds in mouse models. However, the low toxicity of SeNPs found in mammalian systems would lead to the interpretation that the punctate distribution of elemental selenium found in cultured cancer cells subjected to selenite treatment that triggers marked cytotoxicity represents a detoxifying mechanism. The present study found that SeNPs could be reduced by the thioredoxin- or glutaredoxin-coupled glutathione system to generate ROS. Importantly, ROS production by SeNPs in these systems was more efficient than by selenite, which has been recognized as the most redox-active selenium compound for ROS production. This is because multiple steps of reduction from selenite to selenide anion are required; whereas only a single step reduction from the elemental selenium atom to selenide anion is needed to trigger redox cycling with oxygen to produce ROS. We thus speculated that accumulation of SeNPs in cancer cells would result in a strong therapeutic effect, rather than serves a detoxification function. Indeed, we showed herein that preformed SeNPs generated a potent therapeutic effect in a mouse model due to rapid, massive and selective accumulation of SeNPs in cancer cells. Overall, for the first time, we demonstrate that SeNPs have a stronger pro-oxidant property than selenite and hyper-accumulation of SeNPs in cancer cells can generate potent therapeutic effects.

Methylseleninic Acid Sensitizes Ovarian Cancer Cells to T-Cell Mediated Killing by Decreasing PDL1 and VEGF Levels

Redox active selenium (Se) compounds at sub toxic doses act as pro-oxidants with cytotoxic effects on tumor cells and are promising future chemotherapeutic agents. However, little is known about how Se compounds affect immune cells in the tumor microenvironment. We demonstrate that the inorganic Se compound selenite and the organic methylseleninic acid (MSA) do not, despite their pro-oxidant function, influence the viability of immune cells, at doses that gives cytotoxic effects in ovarian cancer cell lines. Treatment of the ovarian cancer cell line A2780 with selenite and MSA increases NK cell mediated lysis, and enhances the cytolytic activity of T cells. Increased T cell function was observed after incubation of T cells in preconditioned media from tumor cells treated with MSA, an effect that was coupled to decreased levels of PDL1, HIF-1α, and VEGF. In conclusion, redox active selenium compounds do not kill or inactivate immune cells at doses required for anti-cancer treatment, and we demonstrate that MSA enhances T cell-mediated tumor cell killing via PDL1 and VEGF inhibition.

Selenium-Related Transcriptional Regulation of Gene Expression

The selenium content of the body is known to control the expression levels of numerous genes, both so-called selenoproteins and non-selenoproteins. Selenium is a trace element essential to human health, and its deficiency is related to, for instance, cardiovascular and myodegenerative diseases, infertility and osteochondropathy called Kashin–Beck disease. It is incorporated as selenocysteine to the selenoproteins, which protect against reactive oxygen and nitrogen species. They also participate in the activation of the thyroid hormone, and play a role in immune system functioning. The synthesis and incorporation of selenocysteine occurs via a special mechanism, which differs from the one used for standard amino acids. The codon for selenocysteine is a regular in-frame stop codon, which can be passed by a specific complex machinery participating in translation elongation and termination. This includes a presence of selenocysteine insertion sequence (SECIS) in the 3′-untranslated part of the selenoprotein mRNAs. Nonsense-mediated decay is involved in the regulation of the selenoprotein mRNA levels, but other mechanisms are also possible. Recent transcriptional analyses of messenger RNAs, microRNAs and long non-coding RNAs combined with proteomic data of samples from Keshan and Kashin–Beck disease patients have identified new possible cellular pathways related to transcriptional regulation by selenium.

Selenite and methylseleninic acid epigenetically affects distinct gene sets in myeloid leukemia: A genome wide epigenetic analysis

Selenium compounds have emerged as promising chemotherapeutic agents with proposed epigenetic effects, however the mechanisms and downstream effects are yet to be studied. Here we assessed the effects of the inorganic selenium compound selenite and the organic form methylseleninic acid (MSA) in a leukemic cell line K562, on active (histone H3 lysine 9 acetylation, H3K9ac and histone H3 lysine 4 tri-methylation, H3K4me3) and repressive (histone H3 lysine 9 tri-methylation, H3K9me3) histone marks by Chromatin immunoprecipitation followed by DNA sequencing (ChIP-Seq). Both selenite and MSA had major effects on histone marks but the effects of MSA were more pronounced. Gene ontology analysis revealed that selenite affected genes involved in response to oxygen and hypoxia, whereas MSA affected distinct gene sets associated with cell adhesion and glucocorticoid receptors, also apparent by global gene expression analysis using RNA sequencing. The correlation to adhesion was functionally confirmed by a significantly weakened ability of MSA treated cells to attach to fibronectin and linked to decreased expression of integrin beta 1. A striking loss of cellular adhesion was also confirmed in primary patient AML cells. Recent strategies to enhance the cytotoxicity of chemotherapeutic drugs by disrupting the interaction between leukemic and stromal cells in the bone marrow are of increasing interest; and organic selenium compounds like MSA might be promising candidates. In conclusion, these results provide new insight on the mechanism of action of selenium compounds, and will be of value for the understanding, usage, and development of new selenium compounds as anticancer agents.

Selenite promotes all-trans retinoic acid-induced maturation of acute promyelocytic leukemia cells

Selective targeting of the PML/RARα oncoprotein demonstrates a successful molecular targeted therapy in acute promyelocytic leukemia (APL) with a typical t(15:17) chromosomal translocation. The zinc-thiolate coordination is critical for structural stability of zinc finger proteins, including the PML moiety of PML/RARα. Based on the known interaction of redox-active selenium compounds with thiolate ligands of zinc, we herein have investigated the abrogatory effects of selenite alone or in combination with all-trans retinoic acid on PML/RARα and the possible effects on differentiation in these cells. At pharmacological concentrations, selenite inhibited the proliferation and survival of APL originated NB4 cells. In combination with ATRA, it potentiated the differentiation of NB4 cells without any differentiating effects of its own as a single agent. Concordant with our hypothesis, PML/RARα oncoprotein expression was completely abrogated by selenite. Increased expression of RARα, PU.1 and FOXO3A transcription factors in the combined treatment suggested the plausible basis for increased differentiation in these cells. We show that selenite at clinically achievable dose targets PML/RARα oncoprotein for degradation and potentiates differentiation of promyelocytic leukemic cells in combination with ATRA. The present investigation reveals the hitherto unknown potential of selenite in targeted abrogation of PML/RARα in APL cells with prospective therapeutic value.

Pathophysiological significance of protein hydrophobic interactions: An emerging hypothesis

Fibrinogen is a unique protein that is converted into an insoluble fibrin in a single enzymatic event, which is a characteristic feature of fibrinogen due to its susceptibility to fibrinolytic degradation and dissolution. Although thrombosis is a result of activated blood coagulation, no explanation is being offered for the persistent presence of fibrin deposits in the affected organs. A classic example is stroke, in which the thrombolytic therapy is effective only during the first 3-4 h after the onset of thrombosis. This phenomenon can now be explained in terms of the modification of fibrinogen structure induced by hydroxyl radicals generated during the period of ischemia caused, in turn, by the blocking of the blood flow within the obstructed vessels. Fibrinogen modification involves intra-to intermolecular disulfide rearrangement induced by the reductive power of hydroxyl radicals that result in the exposition of buried hydrophobic epitopes. Such epitopes react readily with each other forming linkages stronger than the peptide covalent bonds, thus rendering them resistant to the proteolytic degradation. Also, limited reduction of human serum albumin (HSA) generates hydrophobic polymers that form huge insoluble complexes with fibrinogen. Consequently, such insoluble copolymers can be deposited within the circulation of various organs leading to their dysfunction. In conclusion, the study of protein hydrophobic interactions induced by a variety of nutritional and/or environmental factors can provide a rational explanation for a number of pathologic conditions including cardiovascular, neurologic, and other degenerative diseases including cancer.

The Regulation of Pathways of Inflammation and Resolution in Immune Cells and Cancer Stem Cells by Selenium

Cancer is a complex disease where cancer stem cells (CSCs) maintain unlimited replicative potential, but evade chemotherapy drugs through cellular quiescence. CSCs are able to give rise to bulk tumor cells that have the capability to override antiproliferative signals and evade apoptosis. Numerous pathways are dysregulated in tumor cells, where increased levels of prooxidant reactive oxygen and nitrogen species can lead to localized inflammation to exacerbate all three stages of tumorigenesis: initiation, progression, and metastasis. Modulation of cellular metabolism in tumor cells as well as immune cells in the tumor microenvironment (TME) can impact inflammatory networks. Altering these pathways can potentially serve as a portal for therapy. It is well known that selenium, through selenoproteins, modulates inflammatory pathways in addition to regulating redox homeostasis in cells. Therefore, selenium has the potential to impact the interaction between tumor cells, CSCs, and immune cells. In the sections later, we review the current status of knowledge regarding this interaction, with reference to leukemia stem cells, and the importance of selenium-dependent regulation of inflammation as a potential mechanism to affect the TME and tumor cell survival.

Gastrin-releasing peptide contributes to the regulation of adult hippocampal neurogenesis and neuronal development

In the postnatal hippocampus, newly generated neurons contribute to learning and memory. Disruptions in neurogenesis and neuronal development have been linked to cognitive impairment and are implicated in a broad variety of neurological and psychiatric disorders. To identify putative factors involved in this process, we examined hippocampal gene expression alterations in mice possessing a heterozygous knockout of the calcium/calmodulin-dependent protein kinase II alpha heterozygous knockout gene (CaMK2α-hKO), an established model of cognitive impairment that also displays altered neurogenesis and neuronal development. Using this approach, we identified gastrin-releasing peptide (GRP) as the most dysregulated gene. In wild-type mice, GRP labels NeuN-positive neurons, the lone exception being GRP-positive, NeuN-negative cells in the subgranular zone, suggesting GRP expression may be relevant to neurogenesis and/or neuronal development. Using a model of in vitro hippocampal neurogenesis, we determined that GRP signaling is essential for the continued survival and development of newborn neurons, both of which are blocked by transient knockdown of GRP's cognate receptor (GRPR). Furthermore, GRP appears to negatively regulate neurogenesis-associated proliferation in neural stem cells both in vitro and in vivo. Intracerebroventricular infusion of GRP resulted in a decrease in immature neuronal markers, increased cAMP response element-binding protein (CREB) phosphorylation, and decreased neurogenesis. Despite increased levels of GRP mRNA, CaMK2α-hKO mutant mice expressed reduced levels of GRP peptide. This lack of GRP may contribute to the elevated neurogenesis and impaired neuronal development, which are reversed following exogenous GRP infusion. Based on these findings, we hypothesize that GRP modulates neurogenesis and neuronal development and may contribute to hippocampus-associated cognitive impairment.

Redox regulation of genome stability by effects on gene expression, epigenetic pathways and DNA damage/repair

Reactive oxygen and nitrogen species (e.g. H2O2, nitric oxide) confer redox regulation of essential cellular signaling pathways such as cell differentiation, proliferation, migration and apoptosis. In addition, classical regulation of gene expression or activity, including gene transcription to RNA followed by translation to the protein level, by transcription factors (e.g. NF-κB, HIF-1α) and mRNA binding proteins (e.g. GAPDH, HuR) is subject to redox regulation. This review will give an update of recent discoveries in this field, and specifically highlight the impact of reactive oxygen and nitrogen species on DNA repair systems that contribute to genomic stability. Emphasis will be placed on the emerging role of redox mechanisms regulating epigenetic pathways (e.g. miRNA, DNA methylation and histone modifications). By providing clinical correlations we discuss how oxidative stress can impact on gene regulation/activity and vise versa, how epigenetic processes, other gene regulatory mechanisms and DNA repair can influence the cellular redox state and contribute or prevent development or progression of disease.

Selenium and Human Health: Witnessing a Copernican Revolution?

In humans, selenium was hypothesized to lower the risk of several chronic diseases, mainly due to the antioxidant activity of selenium-containing proteins. Recent epidemiologic and laboratory studies, however, are changing our perception of the biological effects of this nutritionally essential trace element. We reviewed the most recent epidemiologic and biochemical literature on selenium, synthesizing the findings from these studies into a unifying view. Randomized trials have shown that selenium did not protect against cancer and other chronic diseases, but even increased the risk of specific neoplasms such as advanced prostate cancer and skin cancer, in addition to type 2 diabetes. Biochemical studies indicate that selenium may exert a broad pattern of toxic effects at unexpectedly low concentrations. Furthermore, its upregulation of antioxidant proteins (selenium-dependent and selenium-independent) may be a manifestation of self-induced oxidative stress. In conclusion, toxic effects of selenium species occur at lower concentrations than previously believed. Those effects may include a large range of proteomic changes and adverse health effects in humans. Since the effects of environmental exposure to this element on human health still remain partially unknown, but are potentially serious, the toxicity of selenium exposure should be further investigated and considered as a public health priority.

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.

Comparison of selected gene expression profiles in sensitive and resistant cancer cells treated with doxorubicin and Selol

AIM OF THE STUDY

Cellular resistance is strongly correlated with the risk of failure in doxorubicin (DOX) treatment, and the knowledge of the mechanisms of resistance and its possible modulation is still very limited.

MATERIAL AND METHODS

In this study, we assessed the effect of 5% Selol and DOX on the expression of genes that affect cell proliferation in the resistant KB-V1 and sensitive HeLa cell lines, using RT2 ProfilerTM PCR Array matrix "Human Cancer Drug Resistance and Metabolism" (SABiosciences).

RESULTS

We showed that HeLa and KB-V1 cell lines, characterised by varying susceptibility to DOX, have different genetic profiles as regards the studied genes. KB-V1 cells show overexpression of MYC and BCL2 genes, which encode proteins with anti apoptotic properties. Selol, when used in KB-V1 cells, reduced the expression of MYC and BCL2 genes, suggested as a new therapeutic target in the treatment of cancers resistant to cytostatic drugs.

CONCLUSIONS

The results suggest that Selol could be used as a modulator that enhances the cytotoxic effects of doxorubicin, particularly in cells resistant to this drug.

Cancer-Induced Alterations of NK-Mediated Target Recognition: Current and Investigational Pharmacological Strategies Aiming at Restoring NK-Mediated Anti-Tumor Activity

Despite evidence of cancer immune-surveillance, which plays a key role in tumor rejection, cancer cells can escape immune recognition through different mechanisms. Thus, evasion to Natural killer (NK) cell-mediated anti-tumor activity is commonly described and is mediated by various mechanisms, mainly cancer cell-induced down-regulation of NK-activating receptors (NCRs, NKG2D, DNAM-1, and CD16) as well as up-regulation of inhibitory receptors (killer-cell immunoglobulin-like receptors, KIRs, NKG2A). Alterations of NK cells lead to an impaired recognition of tumor cells as well as a decreased ability to interact with immune cells. Alternatively, cancer cells downregulate expression of ligands for NK cell-activating receptors and up-regulate expression of the ligands for inhibitory receptors. A better knowledge of the extent and the mechanisms of these defects will allow developing pharmacological strategies to restore NK cell ability to recognize and lyse tumor cells. Combining conventional chemotherapy and immune modulation is a promising approach likely to improve clinical outcome in diverse neoplastic malignancies. Here, we overview experimental approaches as well as strategies already available in the clinics that restore NK cell functionality. Yet successful cancer therapies based on the manipulation of NK cell already have shown efficacy in the context of hematologic malignancies. Additionally, the ability of cytotoxic agents to increase susceptibility of tumors to NK cell lysis has been studied and may require improvement to maximize this effect. More recently, new strategies were developed to specifically restore NK cell phenotype or to stimulate NK cell functions. Overall, pharmacological immune modulation trends to be integrated in therapeutic strategies and should improve anti-tumor effects of conventional cancer therapy.

Selenium induces a multi-targeted cell death process in addition to ROS formation

Selenium compounds inhibit neoplastic growth. Redox active selenium compounds are evolving as promising chemotherapeutic agents through tumour selectivity and multi-target response, which are of great benefit in preventing development of drug resistance. Generation of reactive oxygen species is implicated in selenium-mediated cytotoxic effects on cancer cells. Recent findings indicate that activation of diverse intracellular signalling leading to cell death depends on the chemical form of selenium applied and/or cell line investigated. In the present study, we aimed at deciphering different modes of cell death in a single cell line (HeLa) upon treatment with three redox active selenium compounds (selenite, selenodiglutathione and seleno-DL-cystine). Both selenite and selenodiglutathione exhibited equipotent toxicity (IC₅₀ 5 μM) in these cells with striking differences in toxicity mechanisms. Morphological and molecular alterations provided evidence of necroptosis-like cell death in selenite treatment, whereas selenodiglutathione induced apoptosis-like cell death. We demonstrate that selenodiglutathione efficiently glutathionylated free protein thiols, which might explain the early differences in cytotoxic effects induced by selenite and selenodiglutathione. In contrast, seleno-DL-cystine treatment at an IC₅₀ concentration of 100 μM induced morphologically two distinct different types of cell death, one with apoptosis-like phenotype, while the other was reminiscent of paraptosis-like cell death, characterized by induction of unfolded protein response, ER-stress and occurrence of large cytoplasmic vacuoles. Collectively, the current results underline the diverse cytotoxic effects and variable potential of redox active selenium compounds on the survival of HeLa cells and thereby substantiate the potential of chemical species-specific usage of selenium in the treatment of cancers.

A bioassay-driven discovery of an unexpected selenophene and its cytotoxicity

During the reaction of methyl 3β-acetoxy-glycyrrhetinate (1) with SeO2 significant amounts of a cytotoxic hitherto unprecedented triterpenoic selenophene 3 are formed. This compound stops cell proliferation and acts by apoptosis.

Redox control of leukemia: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species (ROS) play both positive and negative roles in the proliferation and survival of a cell. This dual nature has been exploited by leukemia cells to promote growth, survival, and genomic instability-some of the hallmarks of the cancer phenotype. In addition to altered ROS levels, many antioxidants are dysregulated in leukemia cells. Together, the production of ROS and the expression and activity of antioxidant enzymes make up the primary redox control of leukemia cells. By manipulating this system, leukemia cells gain proliferative and survival advantages, even in the face of therapeutic insults. Standard treatment options have improved leukemia patient survival rates in recent years, although relapse and the development of resistance are persistent challenges. Therapies targeting the redox environment show promise for these cases. This review highlights the molecular mechanisms that control the redox milieu of leukemia cells. In particular, ROS production by the mitochondrial electron transport chain, NADPH oxidase, xanthine oxidoreductase, and cytochrome P450 will be addressed. Expression and activation of antioxidant enzymes such as superoxide dismutase, catalase, heme oxygenase, glutathione, thioredoxin, and peroxiredoxin are perturbed in leukemia cells, and the functional consequences of these molecular alterations will be described. Lastly, we delve into how these pathways can be potentially exploited therapeutically to improve treatment regimens and promote better outcomes for leukemia patients.

Inhibition of sphingosine kinase 1 enhances cytotoxicity, ceramide levels and ROS formation in liver cancer cells treated with selenite

High doses of selenite have been shown to induce cell death in acute myeloid leukemia and lung cancer cells. In this study, we combined selenite treatment with modulators of sphingolipid metabolism in the hepatocellular carcinoma cell line Huh7. Treatment with 20 μM of selenite reduced the viability of Huh7 cells by half and increased the levels of long chain C14-, C16-, C18- and C18:1- ceramides by two-fold. Inhibition of neutral sphingomyelinase with 3-O-methylsphingosine significantly reduced the cytotoxic effect of selenite. In line with this result, selenite caused a 2.5-fold increase in the activity of neutral sphingomyelinase. The sphingosine kinase 1 (SK1) inhibitor 2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole (SK1-II) sensitized the cells to the cytotoxic effects of selenite. Preincubation with 10 μM of SK1-II prior to treatment with 10 μM of selenite caused induction of apoptosis and gave rise to a 2.5-fold increase in C14-, C16-, C18- and C18:1- ceramides. Instead, 50 μM of SK1-II combined with 10 μM of selenite caused accumulation of cells in G1/S phases, but less apoptosis and accumulation of ceramides. The formation of reactive oxygen species (ROS) after treatment with 10 μM of selenite was maximally enhanced by 1 μM of SK1-II. Moreover, combined treatment with SK1-II and 10 μM of selenite synergistically reduced the number of viable Huh7 cells, while the non-tumorigenic hepatocyte cell line MIHA remained unaffected by the same treatment. These results raise the possibility that a combination of selenite and SK1 inhibitors could be used to treat liver cancer cells, that are regarded as drug resistant, at doses that are non-toxic to normal liver cells.

The value of thioredoxin family proteins and proliferation markers in dysplastic and malignant gallbladders in patients with primary sclerosing cholangitis

BACKGROUND AND AIMS

Patients with primary sclerosing cholangitis (PSC) have an increased risk for biliary and gallbladder malignancy and markers of early malignancy in PSC are lacking. The aims were to evaluate biomarkers to look for premalignancy/malignancy.

METHODS

All available gallbladder specimens (n = 53) in patients with PSC at Karolinska University Hospital between 1985 and 2006 were reviewed. Immunohistochemical staining for p53, Ki-67, Cyclin D1 and the thioredoxin family redox proteins; Thioredoxin reductase 1 (TrxR1), isoform-TrxR1-v.2.3.5, Thioredoxin (Trx1) and Glutaredoxin1 (Grx1) was performed on tissues from patients with carcinoma (n = 6), dysplasia (n = 7) and non-cancerous gallbladder epithelium (n = 6).

RESULTS

Dysplasia and carcinoma were found in 16/53 (30%) cases. Inflammation and fibrosis of the gallbladder wall were more common in tissue with gallbladder dysplasia/carcinoma than in benign tissue 12/25 (48%) versus 4/28 (12%) (p < 0.01) and in 13/21 (62%) versus 3/32 (9%) (p < 0.0001), respectively. Immunoreactivity for p53, Ki67, Cyclin D1 was detected in significantly more cases of dysplasia/carcinoma of the gallbladder than in non-cancerous epithelium. 2/19 (11%) of the samples were positive in non-cancerous epithelium versus 7/17 (41%) in dysplasia/carcinoma (p < 0.05) for TrxR1-v.2.3.5. Grx1 was down regulated; more specifically 15/19 (79%) positive cases in non-cancerous epithelium versus 7/17 (41%) in dysplasia/carcinoma.

CONCLUSION

PSC patients have a frequency of gallbladder dysplasia/carcinoma of 30% in operative specimens. The overexpression of TrxR1-v2,3,5 and down regulation of Grx1 in dysplastic gallbladder epithelium suggest that these proteins should be further evaluated as possible future immunohistochemical markers in the early diagnosis of biliary malignancy in PSC.

Selenite induces posttranscriptional blockade of HLA-E expression and sensitizes tumor cells to CD94/NKG2A-positive NK cells

CD94/NKG2A is an inhibitory receptor that controls the activity of a large proportion of human NK cells following interactions with the nonclassical HLA class Ib molecule HLA-E expressed on target cells. In this study, we show that selenite (SeO(3)(2-)), an inorganic selenium compound, induces an almost complete loss of cell surface expression of HLA-E on tumor cells of various origins. Selenite abrogated the HLA-E expression at a posttranscriptional level, since selenite exposure led to a dose-dependent decrease in cellular HLA-E protein expression whereas the mRNA levels remained intact. The loss of HLA-E expression following selenite treatment was associated with decreased levels of intracellular free thiols in the tumor cells, suggesting that the reduced HLA-E protein synthesis was caused by oxidative stress. Indeed, HLA-E expression and the level of free thiols remained intact following treatment with selenomethionine, a selenium compound that does not generate oxidative stress. Loss of HLA-E expression, but not of total HLA class I expression, on tumor cells resulted in increased susceptibility to CD94/NK group 2A-positive NK cells. Our results suggest that selenite may be used to potentiate the anti-tumor cytotoxicity in settings of NK cell-based immunotherapies.

Selenium: a double-edged sword for defense and offence in cancer

Selenium (Se) is an essential dietary component for animals including humans and is regarded as a protective agent against cancer. Although the mode of anticancer action of Se is not fully understood yet, several mechanisms, such as antioxidant protection by selenoenzymes, specific inhibition of tumor cell growth by Se metabolites, modulation of cell cycle and apoptosis, and effect on DNA repair have all been proposed. Despite the unsupported results of the last SELECT trial, the cancer-preventing activity of Se was demonstrated in majority of the epidemiological studies. Moreover, recent studies suggest that Se has a potential to be used not only in cancer prevention but also in cancer treatment where in combination with other anticancer drugs or radiation, it can increase efficacy of cancer therapy. In combating cancer cells, Se acts as pro-oxidant rather than antioxidant, inducing apoptosis through the generation of oxidative stress. Thus, the inorganic Se compound, sodium selenite (SeL), due to its prooxidant character, represents a promising alternative for cancer therapy. However, this Se compound is highly toxic compared to organic Se forms. Thus, the unregulated intake of dietary or pharmacological Se supplements mainly in the form of SeL has a potential to expose the body tissues to the toxic levels of Se with subsequent negative consequences on DNA integrity. Hence, due to a broad interest to exploit the positive effects of Se on human health and cancer therapy, studies investigating the negative effects such as toxicity and DNA damage induction resulting from high Se intake are also highly required. Here, we review a role of Se in cancer prevention and cancer therapy, as well as mechanisms underlying Se-induced toxicity and DNA injury. Since Saccharomyces cerevisiae has proven a powerful tool for addressing some important questions regarding Se biology, a part of this review is devoted to this model system.

Redox control systems in the nucleus: mechanisms and functions

Proteins with oxidizable thiols are essential to many functions of cell nuclei, including transcription, chromatin stability, nuclear protein import and export, and DNA replication and repair. Control of the nuclear thiol-disulfide redox states involves both the elimination of oxidants to prevent oxidation and the reduction of oxidized thiols to restore function. These processes depend on the common thiol reductants, glutathione (GSH) and thioredoxin-1 (Trx1). Recent evidence shows that these systems are controlled independent of the cytoplasmic counterparts. In addition, the GSH and Trx1 couples are not in redox equilibrium, indicating that these reductants have nonredundant functions in their support of proteins involved in transcriptional regulation, nuclear protein trafficking, and DNA repair. Specific isoforms of glutathione peroxidases, glutathione S-transferases, and peroxiredoxins are enriched in nuclei, further supporting the interpretation that functions of the thiol-dependent systems in nuclei are at least quantitatively distinct, and probably also qualitatively distinct, from similar processes in the cytoplasm. Elucidation of the distinct nuclear functions and regulation of the thiol redox pathways in nuclei can be expected to improve understanding of nuclear processes and also to provide the basis for novel approaches to treat aging and disease processes associated with oxidative stress in the nuclei.

Selenium compounds are substrates for glutaredoxins: a novel pathway for selenium metabolism and a potential mechanism for selenium-mediated cytotoxicity

The Grx (glutaredoxin) proteins are oxidoreductases with a central function in maintaining the redox balance within the cell. In the present study, we have explored the reactions between selenium compounds and the glutaredoxin system. Selenite, GS-Se-SG (selenodiglutathione) and selenocystine were all shown to be substrates of human Grx1, implying a novel role for the glutaredoxins in selenium metabolism. During the past few years, selenium has further evolved as a potential therapeutic agent in cancer treatment, and a leading mechanism of cytotoxicity is the generation of ROS (reactive oxygen species). Both selenite and GS-Se-SG were reduced by Grx1 and Grx2 in a non-stoichiometric manner due to redox cycling with oxygen, which in turn generated ROS. The role of Grx in selenium toxicity was therefore explored. Cells were treated with the selenium compounds in combination with transient overexpression of, or small interfering RNA against, Grx1. The results demonstrated an increased viability of the cells during silencing of Grx1, indicating that Grx1 is contributing to selenium toxicity. This is in contrast with TrxR (thioredoxin reductase), which previously was shown to protect cells from selenium cytotoxicity, verifying a diverse role between Grx and TrxR in selenium-mediated cytotoxicity. Furthermore, selenium treatment led to a marked increase in protein glutathionylation and cysteinylation that potentially can influence the activity and function of several proteins within the cell.

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.

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.