DNA damage-mediated apoptosis induced by selenium compounds

Tumor Targeted Cuprous-Based Nanocomposite as Responsive Cascade Nanocatalyst for Efficient Tumor Synergistic Therapy

The single-functionality of traditional chemodynamic therapy (CDT) reagents usually limits the therapeutic efficacy of cancer treatment. Synergistic nanocomposites that involve cascade reaction provide a promising strategy to achieve satisfactory anticancer effects. Herein, a cuprous-based nanocomposite (CCS@GOx@HA) is fabricated, which owns the tumor targeting ability and can undergo tumor microenvironment responsive cascade reaction to enhance the tumor therapeutic efficiency significantly. Surface modification of nanocomposite with hyaluronic acid enables the targeted delivery of the nanocomposite to cancer cells. Acid-triggered decomposition of nanocomposite in cancer cell results in the release of Cu+ , Se2- and GOx. The Cu+ improves the Fenton-like reaction with endogenous H2 O2 to generate highly toxic • OH for CDT. While GOx can not only catalyze the in situ generation of endogenous H2 O2 , but also accelerate the consumption of intratumoral glucose to reduce nutrient supply in tumor site. In addition, Se2- further improves the therapeutic effects of CDT by upregulating the reactive oxygen species (ROS) in tumor cells. Meanwhile, the surface modification endows the nanocomposite the good water dispersibility and biocompatibility. Moreover, in vitro and in vivo experiments demonstrate satisfactory anti-cancer therapeutic performance by the synergistic cascade function of CCS@GOx@HA than CDT alone.

High-Dose Selenium Induces Ferroptotic Cell Death in Ovarian Cancer

Selenium is a promising multi-target chemotherapeutic agent with controversial clinical results. Hence, reassessing the anticancer effects of Se is necessary to clearly understand the potential of high-dose selenium in cancer treatment. Here, we observed that high-dose sodium selenite (SS) significantly decreased the proliferation and increased the death of ovarian cancer cells, mediated by an increased generation of reactive oxygen species. Notably, high-dose SS decreased the levels of glutathione peroxidase (GPx), a selenoprotein with antioxidant properties, without altering other selenoproteins. Furthermore, high-dose SS triggered lipid peroxidation and ferroptosis, a type of iron-dependent cell death, due to dysregulated GPx4 pathways. We demonstrated that intravenous high-dose SS significantly reduced the tumor growth and weight in SKOV3-bearing mice. Consistent with our in vitro results, mice with SKOV3 cells treated with high-dose SS showed decreased GPx4 expression in tumors. Therefore, we highlight the significance of high-dose SS as a potential chemotherapeutic agent for ovarian cancer. High-dose SS-mediated ferroptotic therapy integrating glutathione depletion and ROS generation is a promising strategy for cancer therapy.

Activation of multiple proteolysis systems contributes to acute cadmium cytotoxicity

Cadmium exhibits both toxic and carcinogenic effects, and its cytotoxicity is linked to various cellular pathways, such as oxidative stress, ubiquitin-proteasome, and p53-mediated response pathways. The molecular mechanism(s) underlying cadmium cytotoxicity appears to be complex, but remains largely unclear. Here, we examined the effects of cadmium on the protein catabolism using two surrogate markers, DNA topoisomerases I and II alpha and its contribution to cytotoxicity. We have found that cadmium exposure induced time- and concentration-dependent decreases in the protein level of surrogate markers and therefore suggest that cadmium may be involved in proteolysis system activation. A pharmacological study further revealed the novel role(s) of these proteolytic activities and reactive oxygen species (ROS) in the cadmium-induced acute toxicity: (i) Proteasome inhibition only partially relieved the cadmium-induced proteolysis of topoisomerases; (ii) Moreover, we report for the first time that the activation of metalloproteases, serine proteases, and cysteine proteases contributes to the acute cadmium cytotoxicity; (iii) Consistent with the notion that both ROS generation and proteolysis system activation contribute to the cadmium-induced proteolysis and cytotoxicity, the scavenger N-acetylcysteine and aforementioned protease inhibition not only reduced the cadmium-induced topoisomerase degradation but also alleviated the cadmium-induced cell killing. Taken together, acute cadmium exposure may activate multiple proteolytic systems and ROS formation, subsequently leading to intracellular damage and cytotoxicity. Thus, our results provide a novel insight into potential action mechanism(s) by which cadmium exerts its cytotoxic effect and suggest potential strategies to prevent cadmium-associated acute toxicity.

Selenium deficiency induced apoptosis via mitochondrial pathway caused by Oxidative Stress in porcine gastric tissues

Selenium (Se) is an essential nutrient for the body, which can ensure GSH-Px activity and has antioxidant effect. Se deficiency may lead to apoptosis in various tissues and organs in animals. Pigs as major livestock in the farming industry, Se deficiency can cause various types of diseases such as white muscle disease, and mulberry heart disease.The aim of this experiment was to investigate the effect and mechanism of Se deficiency on apoptosis in porcine gastric tissue. Forty weaned piglets were randomly divided into Se deficiency group and control group, and fed with low Se diet and normal diet for six weeks respectively. The histochemical characteristics, antioxidant indexes, apoptotic genes and apoptotic protein expression of gastric cells in Se-deficient piglets were detected. The results of antioxidant index, TUNEL, RT-PCR and Western blot showed that Se deficiency decreased the activities of CAT, SOD and GSH-Px, increased the apoptotic rate of porcine gastric tissue, increased the expression of Bax and Caspase-3, and decreased the expression of Bcl-2. The results demonstrated that Se deficiency could induce apoptosis in porcine gastric tissue cells through oxidative stress-induced mitochondrial pathway. The stomach was a key target of Se deficiency and may play a key role in the response to Se deficiency. Our study may provide new ideas for the prevention and treatment of swine gastric diseases caused by Se deficiency and is beneficial to the development of pig farming industry.

A comprehensive review on the neuropathophysiology of selenium

As an essential micronutrient, selenium (Se) exerts its biological function as a catalytic entity in a variety of enzymes. From a toxicological perspective, however, Se can become extremely toxic at concentrations slightly above its nutritional levels. Over the last few decades, there has been a growing level of concern worldwide regarding the adverse effects of both inorganic and organic Se compounds on a broad spectrum of neurological functions. A wealth of evidence has shown that exposure to excess Se may compromise the normal functioning of various key proteins, neurotransmitter systems (the glutamatergic, dopaminergic, serotonergic, and cholinergic systems), and signaling molecules involved in the control and regulation of cognitive, behavioral, and neuroendocrine functions. Elevated Se exposure has also been suspected to be a risk factor for the development of several neurodegenerative and neuropsychiatric diseases. Nonetheless, despite the various deleterious effects of excess Se on the central nervous system (CNS), Se neurotoxicity and negative behavioral outcomes are still disregarded at the expense of its beneficial health effects. This review focuses on the current state of knowledge regarding the neurobehavioral effects of Se and discusses its potential mode of action on different aspects of the central and peripheral nervous systems. This review also provides a brief history of Se discovery and uses, its physicochemical properties, biological roles in the CNS, environmental occurrence, and toxicity. We also review potential links between exposure to different forms of Se compounds and aberrant neurobehavioral functions in humans and animals, and identify key knowledge gaps and hypotheses for future research.

Effects of waterborne exposure to environmentally relevant concentrations of selenite on reproductive function of female zebrafish: A life cycle assessment

Recently, bioaccumulation of dietary organic selenium (Se) in the ovaries and inhibition of reproduction in female aquatic animals have been reported. However, there is limited data on the subtle reproductive impacts of waterborne exposure to inorganic Se in fish. Here, zebrafish embryos (2 h post-fertilization) were exposed to solutions with environmentally relevant levels of Na₂SeO₃ with concentrations of 0 (control), 7.98 ± 0.31, 25.14 ± 0.15, and 79.60 ± 0.81 μg Se/L for 120 d until they reached sexual maturity. Female zebrafish were selected for reproductive toxicity assessment. In the early embryonic stage, whole-mount in situ hybridization of zebrafish embryos showed that waterborne Na₂SeO₃ exposure did not affect the observed location of vasa expression in primordial germ cells at 24, 48, and 72 h post-fertilization. Life-cycle exposure to 25.14 ± 0.15 and 79.60 ± 0.81 μg Se/L Na₂SeO₃ did not change the testosterone and 17β-estradiol contents in female zebrafish at the endpoint of exposure, but significantly reduced the proportion of early vitellogenic oocytes and mature oocytes. Follicle maturity retardation was accompanied by changes in transcriptional levels of the genes related to the hypothalamus-pituitary-gonad-liver (HPGL) axis. Transcriptional levels of cyp19a and lhr in the ovary were down-regulated, while the transcriptional level of fshr in the ovaries was up-regulated. In the 21-day cumulative spawning experiment, Na₂SeO₃ (25.14 ± 0.15 and 79.60 ± 0.81 μg Se/L) caused fewer eggs to be produced. Additionally, the malformation of zebrafish offspring significantly increased in the group exposed to 79.60 ± 0.81 μg Se/L. In conclusion, for the first time, this study shows that life-cycle exposure to environmentally relevant concentrations of waterborne Na₂SeO₃ significantly delays ovarian maturation and reduces the fertility of the female zebrafish.

Selenium Compounds as Novel Potential Anticancer Agents

The high number of new cancer incidences and the associated mortality continue to be alarming, leading to the search for new therapies that would be more effective and less burdensome for patients. As there is evidence that Se compounds can have chemopreventive activity, studies have begun to establish whether these compounds can also affect already existing cancers. This review aims to discuss the different classes of Se-containing compounds, both organic and inorganic, natural and synthetic, and the mechanisms and molecular targets of their anticancer activity. The chemical classes discussed in this paper include inorganic (selenite, selenate) and organic compounds, such as diselenides, selenides, selenoesters, methylseleninic acid, 1,2-benzisoselenazole-3[2H]-one and selenophene-based derivatives, as well as selenoamino acids and Selol.

Apoptosis oxidative damage-mediated and antiproliferative effect of selenylated imidazo[1,2-a]pyridines on hepatocellular carcinoma HepG2 cells and in vivo

Imidazo[1,2-a]pyridines (IP) and organoselenium compounds have been widely exploited in medicinal chemistry due to their pharmacological activities. Hepatocellular carcinoma (HCC) has few treatment options, and unfortunately, the prognosis is poor. Thus, the development of novel therapeutic drugs is urgent. The present study aimed at evaluating the antitumor mechanism of selenylated IP against HepG2 cells and in vivo. The selenylated IP named IP-Se-06 (3-((2-methoxyphenyl)selanyl)-7-methyl-2-phenylimidazol[1,2-a]pyridine) showed high cytotoxicity against HepG2 cells (half-maximal inhibitory concentration [IC₅₀ ] = 0.03 µM) and selectivity for this tumor cell line. At nontoxic concentration, IP-Se-06 decreased the protein levels of Bcl-xL and increased the levels of p53, leading to inhibition of cell proliferation and apoptosis. This compound decreased the level of extracellular signal-regulated kinase 1/2 protein and changed the levels of proteins involved in the drive of the cell cycle, tumor growth, and survival (cyclin B1, cyclin-dependent kinase 2). In addition, IP-Se-06 decreased the number of cells in the S phase. In addition, IP-Se-06 led to increased generation of reactive oxygen species, changed antioxidant defenses, and caused DNA fragmentation. Finally, IP-Se-06 significantly inhibited the growth of Ehrlich ascites tumors in mice, increased survival time, and inhibited angiogenesis. Therefore, IP-Se-06 may be an important compound regarding the development of a therapeutic drug for HCC treatment.

Selenium Anticancer Properties and Impact on Cellular Redox Status

(1) Background: In this review, we provide information published in recent years on the chemical forms, main biological functions and especially on antioxidant and prooxidant activities of selenium. The main focus is put on the impact of selenoproteins on maintaining cellular redox balance and anticancerogenic function. Moreover, we summarize data on chemotherapeutic application of redox active selenium compounds. (2) Methods: In the first section, main aspects of metabolism and redox activity of selenium compounds is reviewed. The second outlines multiple biological functions, asserted when selenium is incorporated into the structure of selenoproteins. The final section focuses on anticancer activity of selenium and chemotherapeutic application of redox active selenium compounds as well. (3) Results: optimal dietary level of selenium ensures its proper antioxidant and anticancer activity. We pay special attention to antioxidant activities of selenium compounds, especially selenoproteins, and their importance in antioxidant defence. It is worth noting, that data on selenium anticancer properties is still contraversive. Moreover, selenium compounds as chemotherapeutic agents usually are used at supranutritional doses. (4) Conclusions: Selenium play a vital role for many organism systems due to its incorporation into selenoproteins structure. Selenium possesses antioxidant activity at optimal doses, while at supranutritional doses, it displays prooxidant activity. Redox active selenium compounds can be used for cancer treatment; recently special attention is put to selenium containing nanoparticles.

An overview on role of some trace elements in human reproductive health, sperm function and fertilization process

Human semen contains several trace elements such as calcium (Ca), copper (Cu), manganese (Mn), magnesium (Mg), zinc (Zn) and selenium (Se) which are necessary for reproductive health, normal spermatogenesis, sperm maturation, motility and capacitation, as well as normal sperm function. In this review, the potential role of these trace elements in male reproductive health, normal function of spermatozoa and fertility potency were considered. We selected and reviewed articles that considered crucial roles of trace elements in human sperm function and fertility. Ca is essential for sperm motility and its hyperactivation, sperm capacitation and acrosome reaction, as well as sperm chemotaxis. Sodium (Na) and potassium (K) are involved in sperm motility and capacitation. Mg is necessary for normal ejaculation, spermatogenesis and sperm motility. Zn is one of the most significant nutrients in human semen. Seminal deficiency of Zn can be associated with delayed testicular development, impaired spermatogenesis, deficiency of sex hormones, oxidative stress and inflammation, and apoptosis. Se is another significant element which has antioxidative properties and is essential for spermatogenesis and the maintenance of male fertility. Mn is a potent stimulator for sperm motility; however, increased level of seminal plasma Se can be toxic for sperm. Like Se, Cu has antioxidative properties and has a positive effect on sperm parameters. Decreased level of these trace elements can negatively affect human reproductive health, semen quality, sperm normal function and as the result, fertility potency in men. Measurement of these trace elements in men with idiopathic infertility is necessary.

Effect of the Interaction Between Selenium and Zinc on DNA Repair in Association With Cancer Prevention

Cancer is the most common cause of death worldwide. Annually, more than ten million new cancer cases are diagnosed, and more than six million deaths occur due to cancer. Nonetheless, over 80% of human cancer may be preventable through proper nutrition. Numerous nutritional compounds are effective in preventing cancer. Selenium and zinc are essential micronutrients that have important roles in reducing oxidative stress and protecting DNA from the attack of reactive oxygen species. Selenium is an essential trace element that possesses several functions in many cellular processes for cancer prevention. Meanwhile, zinc may have protective effects on tumor initiation and progression, and it is an essential cofactor of several mammalian proteins. Results show that both selenium and zinc provide an effective progression of DNA repair system; thus, cancer development that originated from DNA damage is decreased. Results mostly focus on the separate effects of these two elements on different cell types, tissues, and organs, and their combined effects are largely unknown. This review aimed to emphasize the joint role of selenium and zinc specifically on DNA repair for cancer prevention.

Esophageal cancer genetics in South Africa

Esophageal cancer (EC) is an extremely aggressive cancer with one of the highest mortality rates. The cancer is generally only diagnosed at the later stages and has a poor 5-year survival rate due to the limited treatment options. China and South Africa are two countries with a very high prevalence rate of EC. EC rates in South Africa have been on the increase, and esophageal squamous cell carcinoma is the predominant subtype and a primary cause of cancer-related deaths in the black and male mixed ancestry populations in South Africa. The incidence of EC is highest in the Eastern Cape Province, especially in the rural areas such as the Transkei, where the consumption of foods contaminated with Fusarium verticillioides is thought to play a major contributing role to the incidence of EC. China is responsible for almost half of all new cases of EC globally. In China, the prevalence of EC varies greatly. However, the two main areas of high prevalence are the southern Taihang Mountain area (Linxian, Henan Province) and the north Jiangsu area. In both countries, environmental toxins play a major role in increasing the chance that an individual will develop EC. These associative factors include tobacco use, alcohol consumption, nutritional deficiencies and exposure to environmental toxins. However, genetic polymorphisms also play a role in predisposing individuals to EC. These include single-nucleotide polymorphisms that can be found in both protein-coding genes and in non-coding sequences such as miRNAs. The aim of this review is to summarize the contribution of genetic polymorphisms to EC in South Africa and to compare and contrast this to the genetic polymorphisms observed in EC in the most comprehensively studied population group, the Chinese.

PFG acted as an inducer of premature senescence in TIG-1 normal diploid fibroblast and an inhibitor of mitosis in the HeLa cells

Our previous work has reported an anti-proliferative compound from moutan cortex, paeoniflorigenone which can induce cancer-selective apoptosis. However, its anti-proliferative mechanism is still unknown. According to morphology changes (hypertrophy and flattening), we hypothesized that PFG can induce senescence or inhibit cell mitosis. Here we show that PFG can induce cellular senescence, evidenced by the expression of senescence-associated β-galactosidase, G0/G1 cell cycle arrest and permanent loss of proliferative ability, in normal TIG-1 diploid fibroblast but not cancerous HeLa cells. In cancerous HeLa cells, PFG inhibited proliferation by inducing S and G2/M cell cycle arrest and mitosis inhibition. DNA damage response was activated by PFG, interestingly the reactive oxygen species level was suppressed instead of escalated. To sum up, we report 3 new roles of PFG as, 1. inducer of premature senescence in normal TIG-1 cells, 2. inhibitor of mitosis in cancerous HeLa cells, 3. ROS scavenger. Abbreviations: PFG: Paeoniflorigenone; ROS: reactive oxygen species; ATM: ataxia telangiectasia mutated; t-BHP: tert-butyl hydroperoxide; SA-β-gal: senescence-associatedβ-galactosidase; DNA-PK_cs: DNA-dependent protein kinase; γ-H2AX: H2AX phosphoryla-tion at Ser-139.

microRNA-33-3p involved in selenium deficiency-induced apoptosis via targeting ADAM10 in the chicken kidney

Selenium (Se) deficiency induces typical clinical and pathological changes and causes various pathological responses at the molecular level in several different chicken organs; the kidney is one of the target organs of Se deficiency. To explore the mechanisms that underlie the effects of microRNA-33-3p (miR-33-3p) on Se deficiency-induced kidney apoptosis, 60 chickens were randomly divided into two groups (30 chickens per group). We found that Se deficiency increased the expression of miR-33-3p in the chicken kidney. A disintegrin and metalloprotease domain 10 (ADAM10) was verified to be a target of miR-33-3p in the chicken kidney. The overexpression of miR-33-3p decreased the expression levels of β-catenin, cyclinD1, T-cell factor (TCF), c-myc, survivin, and Bcl-2; it increased the expression levels of E-cadherin, Bak, Bax, and caspase-3; and it increased the number of chicken kidney cells in the G0/G1 phase. In addition, Se deficiency caused the ultrastructure of the kidney to develop apoptotic characteristics. The results of flow cytometry analysis and AO/EB staining showed that the number of apoptotic chicken kidney cells increased in the miR-33-3p mimic group. All these results suggest that Se deficiency-induced cell cycle arrest and apoptosis in vivo and in vitro in the chicken kidney via the regulation of miR-33-3p, which targets ADAM10.

Organic selenium compounds as potential chemotherapeutic agents for improved cancer treatment

Selenium(Se)-containing compounds have attracted a growing interest as anticancer agents over recent decades, with mounting reports demonstrating their high efficacy and selectivity against cancer cells. Typically, Se compounds exert their cytotoxic effects by acting as pro-oxidants that alter cellular redox homeostasis. However, the precise intracellular targets, signalling pathways affected and mechanisms of cell death engaged following treatment vary with the chemical properties of the selenocompound and its metabolites, as well as the cancer model that is used. Naturally occurring organic Se compounds, besides encompassing a significant antitumor activity with an apparent ability to prevent metastasis, also seem to have fewer side effects and less systemic effects as reported for many inorganic Se compounds. On this basis, many novel organoselenium compounds have also been synthesized and examined as potential chemotherapeutic agents. This review aims to summarize the most well studied natural and synthetic organoselenium compounds and provide the most recent developments in our understanding of the molecular mechanisms that underlie their potential anticancer effects.

Boosting ATM activity alleviates aging and extends lifespan in a mouse model of progeria

DNA damage accumulates with age (Lombard et al., 2005). However, whether and how robust DNA repair machinery promotes longevity is elusive. Here, we demonstrate that ATM-centered DNA damage response (DDR) progressively declines with senescence and age, while low dose of chloroquine (CQ) activates ATM, promotes DNA damage clearance, rescues age-related metabolic shift, and prolongs replicative lifespan. Molecularly, ATM phosphorylates SIRT6 deacetylase and thus prevents MDM2-mediated ubiquitination and proteasomal degradation. Extra copies of Sirt6 extend lifespan in Atm-/- mice, with restored metabolic homeostasis. Moreover, the treatment with CQ remarkably extends lifespan of Caenorhabditis elegans, but not the ATM-1 mutants. In a progeria mouse model with low DNA repair capacity, long-term administration of CQ ameliorates premature aging features and extends lifespan. Thus, our data highlights a pro-longevity role of ATM, for the first time establishing direct causal links between robust DNA repair machinery and longevity, and providing therapeutic strategy for progeria and age-related metabolic diseases.

As cells live and divide, their genetic material gets damaged. The DNA damage response is a network of proteins that monitor, recognize and fix the damage, which is also called DNA lesions. For example, an enzyme called ATM senses when DNA is broken and then begins a process that will get it repaired, while another enzyme known as SIRT6 participates in the actual mending process.

When organisms get older, the repair machinery becomes less efficient, and the number of DNA lesions and errors increases. The accumulation of DNA damage may cause the ‘symptoms’ of old age – from cancer, to wrinkles and the slowing down of the body’s chemical processes. In fact, individuals with defective ATMs (who thus struggle to repair their DNA) age abnormally fast; conversely, SIRT6 promotes longevity. If declining repair mechanisms cause aging, would boosting the DNA damage response slow down this process?

Chloroquine is a drug used to combat malaria, but it can also enhance the activity of ATM without damaging DNA. Qian, Liu et al. show that chloroquine helps cells remove broken DNA and keep dividing for longer. In animals, the drug increases the lifespan of worms and prolongs the lives of mice who have mutations that make them age quicker.

Qian, Liu et al. also demonstrate that ATM works by chemically altering the pro-longevity enzyme SIRT6. These changes make SIRT6 more stable, and keep it safe from cellular processes that destroy it. In addition, mice that are genetically engineered to lack ATM can survive for longer if they also produce extra SIRT6. These experiments show that enhancing the DNA damage response can slow down aging, thus linking the DNA repair machinery to longevity.

Progeria is a group of rare genetic conditions with inefficient DNA repair; people with progeria age fast and die young. The results by Qian, Liu et al., if confirmed in humans, could provide a new way of treating these diseases.

Relationships between seminal plasma metals/metalloids and semen quality, sperm apoptosis and DNA integrity

This study aimed to investigate the relationships between environmental exposure to metals/metalloids and semen quality, sperm apoptosis and DNA integrity using the metal/metalloids levels in seminal plasma as biomarkers. We determined 18 metals/metalloids in seminal plasma using an inductively coupled plasma-mass spectrometry among 746 men recruited from a reproductive medicine center. Associations of these metals/metalloids with semen quality (n = 746), sperm apoptosis (n = 331) and DNA integrity (n = 404) were evaluated using multivariate linear and logistic regression models. After accounting for multiple comparisons and confounders, seminal plasma arsenic (As) quartiles were negatively associated with progressive and total sperm motility using multivariable linear regression analysis, which were in accordance with the trends for increased odds ratios (ORs) for below-reference semen quality parameters in the logistic models. We also found inverse correlations between cadmium (Cd) quartiles and progressive and total sperm motility, whereas positive correlations between zinc (Zn) quartiles and sperm concentration, between copper (Cu) and As quartiles and the percentage of tail DNA, between As and selenium (Se) quartiles and tail extent and tail distributed moment, and between tin (Sn) categories and the percentage of necrotic spermatozoa (all P_trend<0.05). These relationships remained after the simultaneous consideration of various elements. Our results indicate that environmental exposure to As, Cd, Cu, Se and Sn may impair male reproductive health, whereas Zn may be beneficial to sperm concentration.

Biological Chemistry of Hydrogen Selenide

There are no two main-group elements that exhibit more similar physical and chemical properties than sulfur and selenium. Nonetheless, Nature has deemed both essential for life and has found a way to exploit the subtle unique properties of selenium to include it in biochemistry despite its congener sulfur being 10,000 times more abundant. Selenium is more easily oxidized and it is kinetically more labile, so all selenium compounds could be considered to be "Reactive Selenium Compounds" relative to their sulfur analogues. What is furthermore remarkable is that one of the most reactive forms of selenium, hydrogen selenide (HSe- at physiologic pH), is proposed to be the starting point for the biosynthesis of selenium-containing molecules. This review contrasts the chemical properties of sulfur and selenium and critically assesses the role of hydrogen selenide in biological chemistry.

Yeast as a model system to study metabolic impact of selenium compounds

Inorganic Se forms such as selenate or selenite (the two more abundant forms in nature) can be toxic in Saccharomyces cerevisiae cells, which constitute an adequate model to study such toxicity at the molecular level and the functions participating in protection against Se compounds. Those Se forms enter the yeast cell through other oxyanion transporters. Once inside the cell, inorganic Se forms may be converted into selenide through a reductive pathway that in physiological conditions involves reduced glutathione with its consequent oxidation into diglutathione and alteration of the cellular redox buffering capacity. Selenide can subsequently be converted by molecular oxygen into elemental Se, with production of superoxide anions and other reactive oxygen species. Overall, these events result in DNA damage and dose-dependent reversible or irreversible protein oxidation, although additional oxidation of other cellular macromolecules cannot be discarded. Stress-adaptation pathways are essential for efficient Se detoxification, while activation of DNA damage checkpoint and repair pathways protects against Se-mediated genotoxicity. We propose that yeast may be used to improve our knowledge on the impact of Se on metal homeostasis, the identification of Se-targets at the DNA and protein levels, and to gain more insights into the mechanism of Se-mediated apoptosis.

Roles of oxidative stress and endoplasmic reticulum stress in selenium deficiency-induced apoptosis in chicken liver

Oxidative stress and endoplasmic reticulum (ER) stress are involved in different types of stress-induced injuries. The aim of the present study was to evaluate the effect of Se deficiency on oxidative stress, ER stress and apoptosis in chicken livers. Chickens (1 day old, n = 180) were randomly divided into two groups: the L group [fed with a Se-deficient (Se 0.033 mg/kg) diet] and the control group [fed with a normal (Se 0.2 mg/kg) diet]. Factor-associated oxidative stress, catalase (CAT) activity, H2O2 production and the inhibition of hydroxyl radicals (·OH) in the chicken liver were determined on days 15, 25, 35, 45, 55 and 65, respectively. In addition, ER stress-related genes (GRP78, GRP94, ATF4, ATF6 and IRE) and apoptosis-related genes (caspase3 and Bcl-2) were examined by fluorescence quantitative PCR or western blot analysis. Apoptosis levels were also measured using ultrastructural observations and the TdT-mediated dUTP nick end labeling assay. The results showed that CAT activity and ·OH inhibition were decreased and that H2O2 production was increased in the low-Se group, which demonstrated that oxidative stress occurred in the chicken liver. The ER stress-related genes (GRP78, GRP94, ATF4, ATF6 and IRE) and the apoptosis-related gene caspase3 were increased (p < 0.05), while Bcl-2 was decreased (p < 0.05) by Se deficiency. In addition, apoptosis and ER lesions were observed by ultrastructural observations of the chicken liver in the low-Se group. The level of apoptosis and the number of apoptotic cells increased with time. These results indicated that the oxidative-ER stress pathway participates in Se deficiency-induced apoptosis in the chicken liver.

Effects of Selenium Yeast on Oxidative Stress, Growth Inhibition, and Apoptosis in Human Breast Cancer Cells

Recent evidence suggests that selenium (Se) yeast may exhibit potential anti-cancer properties; whereas the precise mechanisms remain unknown. The present study was aimed at evaluating the effects of Se yeast on oxidative stress, growth inhibition, and apoptosis in human breast cancer cells. Treatments of ER-positive MCF-7 and triple-negative MDA-MB-231 cells with Se yeast (100, 750, and 1500 ng Se/mL), methylseleninic acid (MSA, 1500 ng Se/mL), or methylselenocysteine (MSC, 1500 ng Se/mL) at a time course experiment (at 24, 48, 72, and 96 h) were analyzed. Se yeast inhibited the growth of these cancer cells in a dose- and time-dependent manner. Compared with the same level of MSA, cancer cells exposure to Se yeast exhibited a lower growth-inhibitory response. The latter has also lower superoxide production and reduced antioxidant enzyme activities. Furthermore, MSA (1500 ng Se/mL)-exposed non-tumorigenic human mammary epithelial cells (HMEC) have a significant growth inhibitory effect, but not Se yeast and MSC. Compared with MSA, Se yeast resulted in a greater increase in the early apoptosis in MCF-7 cells as well as a lower proportion of early and late apoptosis in MDA-MB-231 cells. In addition, nuclear morphological changes and loss of mitochondrial membrane potential were observed. In conclusion, a dose of 100 to 1500 ng Se/mL of Se yeast can increase oxidative stress, and stimulate growth inhibitory effects and apoptosis induction in breast cancer cell lines, but does not affect non-tumorigenic cells.

77Se and 125Te NMR spectroscopy on a selectivity study of organochalcogenanes with l-amino acids

Organochalcogenanes exhibited a remarkably high selectivity for l-cysteine which was monitored by ⁷⁷Se and ¹²⁵Te NMR spectroscopy.

Cellular mechanisms of the cytotoxicity of the anticancer drug elesclomol and its complex with Cu(II)

The potent anticancer drug elesclomol, which forms an extremely strong complex with copper, is currently undergoing clinical trials. However, its mechanism of action is not well understood. Treatment of human erythroleukemic K562 cells with either elesclomol or Cu(II)-elesclomol caused an immediate halt in cell growth which was followed by a loss of cell viability after several hours. Treatment of K562 cells also resulted in induction of apoptosis as measured by annexin V binding. Elesclomol or Cu(II)-elesclomol treatment caused a G1 cell cycle block in synchronized Chinese hamster ovary cells. Elesclomol and Cu(II)-elesclomol induced DNA double strand breaks in K562 cells, suggesting that they may also have exerted their cytotoxicity by damaging DNA. Cu(II)-elesclomol also weakly inhibited DNA topoisomerase I (5.99.1.2) but was not active against DNA topoisomerase IIα (5.99.1.3). Elesclomol or Cu(II)-elesclomol treatment had little effect on the mitochondrial membrane potential of viable K562 cells. NCI COMPARE analysis showed that Cu(II)-elesclomol exerted its cytotoxicity by mechanisms similar to other cytotoxic copper chelating compounds. Experiments with cross-resistant cell lines overexpressing several ATP-binding cassette (ABC) type efflux transporters showed that neither elesclomol nor Cu(II)-elesclomol were cross-resistant to cells overexpressing either ABCB1 (Pgp) or ABCG2 (BCRP), but that cells overexpressing ABCC1 (MRP1) were slightly cross-resistant. In conclusion, these results showed that elesclomol caused a rapid halt in cell growth, induced apoptosis, and may also have inhibited cell growth, in part, through its ability to damage DNA.

Selenium action in neuro-oncology

The trace element selenium and selenocysteine-carrying selenoproteins play a pivotal role in the brain. Beside the essential function during development and maintenance of brain action, selenium has also been associated with several neurological and neuro-oncological conditions. Reliable supply of selenium is important since selenium compounds can affect tumor microenvironment and neoangiogenesis in malignant gliomas (WHO grade III and IV [glioblastoma, GBM]) via induction of apoptosis and alteration of matrix metalloproteinases expression. Here, we summarize recent findings focusing on the anti-toxicity and cancer-preventive properties of selenium and their implication in current multimodal therapies including temozolomide (Temodal), cyclophosphamide (Endoxan), and cisplatin (DDP, Platiblastin, and Platinol). We shed light on unintended side effects in chemotherapy and the developments of novel combinatorial chemotherapeutics with selenium compounds. We found that selenium and selenium compounds have dual action profiles with direct anti-cancer and chemotherapy-intensifier effects as well as neuroprotective and cytoprotective agents. Current selenium trials and selenium supplementation with focus on neuro-oncology will be discussed with regard to low-adequate-to-high/toxic selenium status.

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.

Arsenic and selenium toxicity and their interactive effects in humans

Arsenic (As) and selenium (Se) are unusual metalloids as they both induce and cure cancer. They both cause carcinogenesis, pathology, cytotoxicity, and genotoxicity in humans, with reactive oxygen species playing an important role. While As induces adverse effects by decreasing DNA methylation and affecting protein 53 expression, Se induces adverse effects by modifying thioredoxin reductase. However, they can react with glutathione and S-adenosylmethionine by forming an As-Se complex, which can be secreted extracellularly. We hypothesize that there are two types of interactions between As and Se. At low concentration, Se can decrease As toxicity via excretion of As-Se compound [(GS3)2AsSe](-), but at high concentration, excessive Se can enhance As toxicity by reacting with S-adenosylmethionine and glutathione, and modifying the structure and activity of arsenite methyltransferase. This review is to summarize their toxicity mechanisms and the interaction between As and Se toxicity, and to provide suggestions for future investigations.

Phosphorylated (pT371)TRF1 is recruited to sites of DNA damage to facilitate homologous recombination and checkpoint activation

TRF1, a duplex telomeric DNA-binding protein, plays an important role in telomere metabolism. We have previously reported that a fraction of endogenous TRF1 can stably exist free of telomere chromatin when it is phosphorylated at T371 by Cdk1; however, the role of this telomere-free (pT371)TRF1 has yet to be fully characterized. Here we show that phosphorylated (pT371)TRF1 is recruited to sites of DNA damage, forming damage-induced foci in response to ionizing radiation (IR), etoposide and camptothecin. We find that IR-induced (pT371)TRF1 foci formation is dependent on the ATM- and Mre11/Rad50/Nbs1-mediated DNA damage response. While loss of functional BRCA1 impairs the formation of IR-induced (pT371)TRF1 foci, depletion of either 53BP1 or Rif1 stimulates IR-induced (pT371)TRF1 foci formation. In addition, we show that TRF1 depletion or the lack of its phosphorylation at T371 impairs DNA end resection and repair of nontelomeric DNA double-strand breaks by homologous recombination. The lack of TRF1 phosphorylation at T371 also hampers the activation of the G2/M checkpoint and sensitizes cells to PARP inhibition, IR and camptothecin. Collectively, these results reveal a novel but important function of phosphorylated (pT371)TRF1 in facilitating DNA double-strand break repair and the maintenance of genome integrity.

MPK-09, a small molecule inspired from bioactive styryllactone restores the wild-type function of mutant p53

In the search for more efficacious and less toxic cancer drugs, the tumor suppressor p53 protein has long been a desirable therapeutic target. In the recent past, few independent studies have demonstrated that the antitumor activity of wild-type p53 can be restored in cancer cells harboring mutant form of p53 using small molecule activators. In this study, we describe a novel small molecule MPK-09, which is selective and highly potent against allele specific p53 mutations mainly, R175H, R249S, R273H, R273C, and E285K . Except E285K, all other mutations tested are among the six "hot spot" p53 mutations reported in majority of human cancer. Furthermore, our study conclusively demonstrates that the apoptotic activity of the small molecule MPK-09 against cancer cells harboring R273C and E285K mutations is due to restoration of the wild-type conformation to the corresponding mutant form of p53.

The effects of selenium and the GPx-1 selenoprotein on the phosphorylation of H2AX

BACKGROUND

Significant data supports the health benefits of selenium although supplementation trials have yielded mixed results. GPx-1, whose levels are responsive to selenium availability, is implicated in cancer etiology by human genetic data. Selenium's ability to alter the phosphorylation of the H2AX, a histone protein that functions in the reduction of DNA damage by recruiting repair proteins to the damage site, following exposure to ionizing radiation and bleomycin was investigated.

METHODS

Human cell lines that were either exposed to selenium or were transfected with a GPx-1 expression construct were exposed to ionizing radiation or bleomycin. Phosphorylation of histone H2AX was quantified by flow cytometry and survival by the MTT assay. Phosphorylation of the Chk1 and Chk2 checkpoint proteins was quantified by western blotting.

RESULTS

In colon-derived cells, selenium increases GPx-1 and attenuated H2AX phosphorylation following genotoxic exposures while the viability of these cells was unaffected. MCF-7 cells and transfectants that express high GPx-1 levels were exposed to ionizing radiation and bleomycin, and H2AX phosphorylation and cell viability were assessed. GPx-1 increased H2AX phosphorylation and viability following the induction of DNA damage while enhancing the levels of activated Chk1 and Chk2.

CONCLUSIONS

Exposure of mammalian cells to selenium can alter the DNA damage response and do so by mechanisms that are dependent and independent of its effect on GPx-1.

GENERAL SIGNIFICANCE

Selenium and GPx-1 may stimulate the repair of genotoxic DNA damage and this may account for some of the benefits attributed to selenium intake and elevated GPx-1 activity.

Topoisomerase II-mediated DNA cleavage and mutagenesis activated by nitric oxide underlie the inflammation-associated tumorigenesis

AIMS

Both cancer-suppressing and cancer-promoting properties of reactive nitrogen and oxygen species (RNOS) have been suggested to play a role in tumor pathology, particularly those activities associated with chronic inflammation. Here, we address the impact of nitric oxide (NO) on the induction of DNA damage and genome instability with a specific focus on the involvement of topoisomerase II (TOP2). We also investigate the contribution of NO to the formation of skin melanoma in mice.

RESULTS

Similar to the TOP2-targeting drug, etoposide (VP-16), the NO-donor, S-nitrosoglutathione (GSNO), induces skin melanomas formation in 7,12-dimethyl- benz[a]anthracene (DMBA)-initiated mice. To explore the mechanism(s) underlying this NO-induced tumorigenesis, we use a co-culture model system to demonstrate that inflamed macrophages with inducible NO synthase (iNOS) expression cause γ-H2AX activation, p53 phosphorylation, and chromosome DNA breaks in the target cells. Inhibitor experiments revealed that NO and TOP2 isozymes are responsible for the above described cellular phenotypes. Notably, NO, unlike VP-16, preferentially induces the formation of TOP2β cleavable complexes (TOP2βcc) in cells. Moreover, GSNO induced TOP2-dependent DNA sequence rearrangements and cytotoxicity. Furthermore, the incidences of GSNO- and VP-16-induced skin melanomas were also observed to be lower in the skin-specific top2β-knockout mice. Our results suggest that TOP2 isozymes contribute to NO-induced mutagenesis and subsequent cancer development during chronic inflammation.

INNOVATION AND CONCLUSIONS

We provide the first experimental evidence for the functional role of TOP2 in NO-caused DNA damage, mutagenesis, and carcinogenesis. Notably, these studies contribute to our molecular understanding of the cancer-promoting actions of RNOS during chronic inflammation.

Effect of oxygen free radicals and nitric oxide on apoptosis of immune organ induced by selenium deficiency in chickens

Selenium is an essential element with antioxidant roles in immune regulation, but there is little understanding of how Se acts in apoptosis in the immune organs of birds. The aim of study was to evaluate the influence of Se deficiency on oxygen free radicals, NO and apoptosis in immune organ of chickens. 160 1-day-old chickens were randomly assigned to two groups of 80 each and were fed on a low-Se diet (0.032 mg/kg Se) or a control diet (0.282 mg/kg Se), respectively. OFR production in blood was determined on days 30, 45, 60 and 75, respectively. The iNOS-NO system activity in immune organ (thymus, spleen, bursa of fabricius) was identified by NO content and NOS activity assay on days 30, 45, 60 and 75, respectively. Apoptosis was measured by DNA ladder analysis, ultrastructural observations, TdT-mediated dUTP nick end labeling TUNEL assay and flow cytometric analysis of apoptotic DNA. The transcription of factor-associated suicide, caspase-3 mRNA was tested by fluorescence quantitative PCR. The results showed that OFR production, NO and inducible NO synthases (iNOS) activity in the low-Se group were significantly increased (p < 0.05) than in the control group. In addition, apoptosis was observed in chicken immune organ in the low-Se group. The degree and the number of apoptotic cells rose in a time-dependent manner. The expression of Fas and caspase-3 mRNA increased (p < 0.05) than in the control group. It indicated that the oxidative stress and NO played a causative role in the apoptosis of immune tissues induced by selenium deficiency.

Rational incorporation of selenium into temozolomide elicits superior antitumor activity associated with both apoptotic and autophagic cell death

Background

The DNA alkylating agent temozolomide (TMZ) is widely used in the treatment of human malignancies such as glioma and melanoma. On the basis of previous structure-activity studies, we recently synthesized a new TMZ selenium analog by rationally introducing an N-ethylselenocyanate extension to the amide functionality in TMZ structure.

Principal Findings

This TMZ-Se analog showed a superior cytotoxicity to TMZ in human glioma and melanoma cells and a more potent tumor-inhibiting activity than TMZ in mouse glioma and melanoma xenograft model. TMZ-Se was also effective against a TMZ-resistant glioma cell line. To explore the mechanism underlying the superior antitumor activity of TMZ-Se, we compared the effects of TMZ and TMZ-Se on apoptosis and autophagy. Apoptosis was significantly increased in tumor cells treated with TMZ-Se in comparison to those treated with TMZ. TMZ-Se also triggered greater autophagic response, as compared with TMZ, and suppressing autophagy partly rescued cell death induced by TMZ-Se, indicating that TMZ-Se-triggered autophagy contributed to cell death. Although mRNA level of the key autophagy gene, Beclin 1, was increased, Beclin 1 protein was down-regulated in the cells treated with TMZ-Se. The decrease in Beclin 1 following TMZ-Se treatment were rescued by the calpain inhibitors and the calpain-mediated degradation of Beclin1 had no effect on autophagy but promoted apoptosis in cells treated with TMZ-Se.

Conclusions

Our study indicates that incorporation of Se into TMZ can render greater potency to this chemotherapeutic drug.

Excess dietary sodium selenite alters apoptotic population and oxidative stress markers of spleens in broilers

Three hundred 1-day-old avian broilers were fed on a basic diet (0.2 mg/kg selenium) or the same diet amended to contain 1, 5, 10, and 15 mg/kg selenium supplied as sodium selenite (n = 60/group). In comparison with those of 0.2 mg/kg selenium group, the percentages of annexin V-positive splenocytes were increased in 5, 10, and 15 mg/kg selenium groups. TUNEL assay revealed that apoptotic cells with brown-stained nuclei distributed within the red pulp and white pulp of the spleens with increased frequency of occurrence in 10 and 15 mg/kg selenium groups in comparison with that of 0.2 mg/kg Se group. Sodium selenite-induced oxidative stress in spleens of chickens was evidenced by decrease in glutathione peroxidase, superoxide dismutase, and catalase activities and increase in malondialdehyde contents. The results indicate that excess dietary selenium in the range of 5-15 mg/kg of feed causes oxidative stress, which may be mainly responsible for the increased apoptosis of splenocytes in chickens.

Role for p53 in selenium-induced senescence

The tumor suppressor p53 and the ataxia-telangiectasia mutated (ATM) kinase play important roles in the senescence response to oncogene activation and DNA damage. It was previously shown that selenium-containing compounds can activate an ATM-dependent senescence response in MRC-5 normal fibroblasts. Here, the shRNA knockdown approach and other DNA damage assays are employed to test the hypothesis that p53 plays a role in selenium-induced senescence. In MRC-5 cells treated with methylseleninic acid (MSeA, 0-10 μM), depletion of p53 hampers senescence-associated expression of β-galactosidase, disrupts the otherwise S and G2/M cell cycle arrest, desensitizes such cells to MSeA treatment, and increases genome instability. Pretreatment with KU55933, an ATM kinase inhibitor, or NU7026, an inhibitor of DNA-dependent protein kinase, desensitizes MSeA cytotoxicity in scrambled but not p53 shRNA MRC-5 cells. These results suggest that p53 is critical for senescence induction in the response of MRC-5 noncancerous cells to selenium compounds.

Dietary phytochemicals, HDAC inhibition, and DNA damage/repair defects in cancer cells

Genomic instability is a common feature of cancer etiology. This provides an avenue for therapeutic intervention, since cancer cells are more susceptible than normal cells to DNA damaging agents. However, there is growing evidence that the epigenetic mechanisms that impact DNA methylation and histone status also contribute to genomic instability. The DNA damage response, for example, is modulated by the acetylation status of histone and non-histone proteins, and by the opposing activities of histone acetyltransferase and histone deacetylase (HDAC) enzymes. Many HDACs overexpressed in cancer cells have been implicated in protecting such cells from genotoxic insults. Thus, HDAC inhibitors, in addition to unsilencing tumor suppressor genes, also can silence DNA repair pathways, inactivate non-histone proteins that are required for DNA stability, and induce reactive oxygen species and DNA double-strand breaks. This review summarizes how dietary phytochemicals that affect the epigenome also can trigger DNA damage and repair mechanisms. Where such data is available, examples are cited from studies in vitro and in vivo of polyphenols, organosulfur/organoselenium compounds, indoles, sesquiterpene lactones, and miscellaneous agents such as anacardic acid. Finally, by virtue of their genetic and epigenetic mechanisms, cancer chemopreventive agents are being redefined as chemo- or radio-sensitizers. A sustained DNA damage response coupled with insufficient repair may be a pivotal mechanism for apoptosis induction in cancer cells exposed to dietary phytochemicals. Future research, including appropriate clinical investigation, should clarify these emerging concepts in the context of both genetic and epigenetic mechanisms dysregulated in cancer, and the pros and cons of specific dietary intervention strategies.

Dietary isothiocyanate-induced apoptosis via thiol modification of DNA topoisomerase IIα

Studies in animal models have indicated that dietary isothiocyanates (ITCs) exhibit cancer preventive activities through carcinogen detoxification-dependent and -independent mechanisms. The carcinogen detoxification-independent mechanism of cancer prevention by ITCs has been attributed at least in part to their ability to induce apoptosis of transformed (initiated) cells (e.g. through suppression of IκB kinase and nuclear factor κB as well as other proposed mechanisms). In the current studies we show that ITC-induced apoptosis of oncogene-transformed cells involves thiol modification of DNA topoisomerase II (Top2) based on the following observations. 1) siRNA-mediated knockdown of Top2α in both SV40-transformed MEFs and Ras-transformed human mammary epithelial MCF-10A cells resulted in reduced ITC sensitivity. 2) ITCs, like some anticancer drugs and cancer-preventive dietary components, were shown to induce reversible Top2α cleavage complexes in vitro. 3) ITC-induced Top2α cleavage complexes were abolished by co-incubation with excess glutathione. In addition, proteomic analysis revealed that several cysteine residues on human Top2α were covalently modified by benzyl-ITC, suggesting that ITC-induced Top2α cleavage complexes may involve cysteine modification. Interestingly, consistent with the thiol modification mechanism for Top2α cleavage complex induction, the thiol-reactive selenocysteine, but not the non-thiol-reactive selenomethionine, was shown to induce Top2α cleavage complexes. In the aggregate, our results suggest that thiol modification of Top2α may contribute to apoptosis induction in transformed cells by ITCs.