Effects of organic and inorganic selenium compounds on rat mammary tumor cells

Selenium-Containing Agents Acting on Cancer-A New Hope?

Selenium-containing agents are more and more considered as an innovative potential treatment option for cancer. Light is shed not only on the considerable advancements made in understanding the complex biology and chemistry related to selenium-containing small molecules but also on Se-nanoparticles. Numerous Se-containing agents have been widely investigated in recent years in cancer therapy in relation to tumour development and dissemination, drug delivery, multidrug resistance (MDR) and immune system-related (anti)cancer effects. Despite numerous efforts, Se-agents apart from selenocysteine and selenomethionine have not yet reached clinical trials for cancer therapy. The purpose of this review is to provide a concise critical overview of the current state of the art in the development of highly potent target-specific Se-containing agents.

Selenium and selenoproteins: it's role in regulation of inflammation

Abstract

Selenium is an essential immunonutrient which holds the human’s metabolic activity with its chemical bonds. The organic forms of selenium naturally present in human body are selenocysteine and selenoproteins. These forms have a unique way of synthesis and translational coding. Selenoproteins act as antioxidant warriors for thyroid regulation, male-fertility enhancement, and anti-inflammatory actions. They also participate indirectly in the mechanism of wound healing as oxidative stress reducers. Glutathione peroxidase (GPX) is the major selenoprotein present in the human body, which assists in the control of excessive production of free radical at the site of inflammation. Other than GPX, other selenoproteins include selenoprotein-S that regulates the inflammatory cytokines and selenoprotein-P that serves as an inducer of homeostasis. Previously, reports were mainly focused on the cellular and molecular mechanism of wound healing with reference to various animal models and cell lines. In this review, the role of selenium and its possible routes in translational decoding of selenocysteine, synthesis of selenoproteins, systemic action of selenoproteins and their indirect assimilation in the process of wound healing are explained in detail. Some of the selenium containing compounds which can acts as cancer preventive and therapeutics are also discussed. These compounds directly or indirectly exhibit antioxidant properties which can sustain the intracellular redox status and these activities protect the healthy cells from reactive oxygen species induced oxidative damage. Although the review covers the importance of selenium/selenoproteins in wound healing process, still some unresolved mystery persists which may be resolved in near future.

Graphic abstract

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Selenium Species: Current Status and Potentials in Cancer Prevention and Therapy

Selenium (Se) acts as an essential trace element in the human body due to its unique biological functions, particularly in the oxidation-reduction system. Although several clinical trials indicated no significant benefit of Se in preventing cancer, researchers reported that some Se species exhibit superior anticancer properties. Therefore, a reassessment of the status of Se and Se compounds is necessary in order to provide clearer insights into the potentiality of Se in cancer prevention and therapy. In this review, we organize relevant forms of Se species based on the three main categories of Se-inorganic, organic, and Se-containing nanoparticles (SeNPs)-and overview their potential functions and applications in oncology. Here, we specifically focus on the SeNPs as they have tremendous potential in oncology and other fields. In general, to make better use of Se compounds in cancer prevention and therapy, extensive further study is still required to understand the underlying mechanisms of the Se compounds.

Selenides and Diselenides: A Review of Their Anticancer and Chemopreventive Activity

Selenium and selenocompounds have attracted the attention and the efforts of scientists worldwide due to their promising potential applications in cancer prevention and/or treatment. Different organic selenocompounds, with diverse functional groups that contain selenium, have been reported to exhibit anticancer and/or chemopreventive activity. Among them, selenocyanates, selenoureas, selenoesters, selenium-containing heterocycles, selenium nanoparticles, selenides and diselenides have been considered in the search for efficiency in prevention and treatment of cancer and other related diseases. In this review, we focus our attention on the potential applications of selenides and diselenides in cancer prevention and treatment that have been reported so far. The around 80 selenides and diselenides selected herein as representative compounds include promising antioxidant, prooxidant, redox-modulating, chemopreventive, anticancer, cytotoxic and radioprotective compounds, among other activities. The aim of this work is to highlight the possibilities that these novel organic selenocompounds can offer in an effort to contribute to inspire medicinal chemists in their search of new promising derivatives.

Rational design and optimization of selenophenes with basic side chains as novel potent selective estrogen receptor modulators (SERMs) for breast cancer therapy

To increase the diversity of estrogen receptor (ER) ligands having novel structures and activities, series of selenophene derivatives with a basic side chain (BSC) were synthesized and their biological activity as subtype-selective antagonists for the ER was explored. Compared with the selenophenes without a BSC, most compounds showed an increase in binding affinity, and several compounds displayed enhanced antagonist potency and antiproliferative activity. Especially, compound 16c exhibited excellent transcriptional activity for ERα (IC50 = 13 nM) which made this compound the most potent antagonist for ERα of the whole series and is 66-fold better than the best selenophene compound without a BSC. Moreover, several compounds showed values of IC50 better than that of 4-hydroxytamoxifen in breast cancer MCF-7 cells. The modeling study indicated that the basic side chain might contribute to their increased antagonist potency and antiproliferative activity. These new ligands have the potential to be further developed as novel agents to improve therapeutics that target the estrogen receptor.

Disturbed homeostasis of zinc and other essential elements in the prostate gland dependent on the character of pathological lesions

Pathophysiological changes in the prostate take the form of benign prostate hyperplasia (BPH) and prostate adenocarcinoma (PCa). In prostate, zinc is particularly important to its normal functioning, especially in terms of the consequences of hormone disturbance. The aim of this study was to assess the levels of Zn, Cu, Ca, Mg, and Se in the prostate dependent on the character of patological changes. Zinc, copper, magnesium and calcium were determined by AAS and selenium with spectrofluorometric method. Zn levels in BPH patients were over twofold higher than in controls. On the other hand, in the patients with PCa, the levels of Zn were found almost three times lower than in BPH patients and by almost 50% lower than in controls. In this study, significant changes in the levels of other essential elements were observed. The results apparently confirm the disturbed homeostasis of zinc and other essential elements in the etiology of BPH and PCa.

Titanium surfaces with adherent selenium nanoclusters as a novel anticancer orthopedic material

Current orthopedic implants have several problems that include poor osseointegration for extended periods of time, stress shielding and wear debris-associated bone cell death. In addition, numerous patients receive orthopedic implants as a result of bone cancer resection, yet current orthopedic materials were not designed to prevent either the occurrence or reoccurrence of cancer. The objective of this in vitro study was to create a new biomaterial which can both restore bone and prevent cancer growth at the implant-tissue interface. Elemental selenium was chosen as the biologically active agent in this study because of its known chemopreventive and chemotherapeutic properties. It was found that when selenite salts were reduced by glutathione in the presence of an immersed titanium substrate, elemental selenium nucleated and grew into adherent, hemispherical nanoclusters that formed a nanostructured composite surface. Three types of surfaces with different selenium surface densities on titanium were fabricated and confirmed by SEM images, AFM, and XPS profiles. Compared to conventional untreated titanium, a high-density selenium-doped surface inhibited cancerous bone cell proliferation while promoting healthy bone cell functions (including adhesion, proliferation, alkaline phosphatase activity and calcium deposition). These findings showed for the first time the potential of selenium nanoclusters as a chemopreventive titanium orthopedic material coating that can also promote healthy bone cell functions.

Differential effects of nanoselenium doping on healthy and cancerous osteoblasts in coculture on titanium

In the present study, selenium (Se) nanoclusters were grown through heterogeneous nucleation on titanium (Ti) surfaces, a common orthopedic implant material. Normal healthy osteoblasts (bone-forming cells) and cancerous osteoblasts (osteosarcoma) were cultured on the Se-doped surfaces having three different coating densities. For the first time, it is shown that substrates with Se nanoclusters promote normal osteoblast proliferation and inhibit cancerous osteoblast growth in both separate (mono-culture) and coculture experiment. This study suggests that Se surface nanoclusters can be properly engineered to inhibit bone cancer growth while simultaneously promoting the growth of normal bone tissue.

Diphenylmethyl selenocyanate inhibits DMBA–croton oil induced two-stage mouse skin carcinogenesis by inducing apoptosis and inhibiting cutaneous cell proliferation

Numerous epidemiological and experimental studies have showed the inverse relationship between dietary selenium intake and different types of cancer. Continuous efforts are going on to develop suitable organoselenium compounds, which can be used as cancer chemopreventive agents for human. In the present study, a synthetic organoselenium compound diphenylmethyl selenocyanate was evaluated for its ability to arrest cell proliferation and to induce apoptosis against 7,12-dimethylbenz[a]anthracene-croton oil induced two-stage mouse skin carcinogenesis model. Reduction in the incidence and number of papilloma, the preneoplastic lesion, was considered to be the mean of assessment. Significant decrease in the level of cell proliferation (p<0.01) and significant enhancement in the level of apoptosis (p<0.01) were found. Caspase-3, which contribute a part in the process of cellular apoptosis to prevent further cellular differentiation was also elevated significantly (P<0.01) during the treatment with the Se compound. These observations seem to be correlated with the significant reduction in the corresponding number of skin papilloma formation after 12 weeks of experiment. Thus the compound, diphenylmethyl selenocyanate may be considered for further research to establish it as an effective cancer chemopreventive agent.

Selective modulation of the therapeutic efficacy of anticancer drugs by selenium containing compounds against human tumor xenografts

PURPOSE

Studies were carried out in athymic nude mice bearing human squamous cell carcinoma of the head and neck (FaDu and A253) and colon carcinoma (HCT-8 and HT-29) xenografts to evaluate the potential role of selenium-containing compounds as selective modulators of the toxicity and antitumor activity of selected anticancer drugs with particular emphasis on irinotecan, a topoisomerase I poison.

EXPERIMENTAL DESIGN

Antitumor activity and toxicity were evaluated using nontoxic doses (0.2 mg/mouse/day) and schedule (14-28 days) of the selenium-containing compounds, 5-methylselenocysteine and seleno-L-methionine, administered orally to nude mice daily for 7 days before i.v. administration of anticancer drugs, with continued selenium treatment for 7-21 days, depending on anticancer drugs under evaluation. Several doses of anticancer drugs were used, including the maximum tolerated dose (MTD) and toxic doses. Although many chemotherapeutic agents were evaluated for toxicity protection by selenium, data on antitumor activity were primarily obtained using the MTD, 2 x MTD, and 3 x MTD of weekly x4 schedule of irinotecan.

RESULTS

Selenium was highly protective against toxicity induced by a variety of chemotherapeutic agents. Furthermore, selenium increased significantly the cure rate of xenografts bearing human tumors that are sensitive (HCT-8 and FaDu) and resistant (HT-29 and A253) to irinotecan. The high cure rate (100%) was achieved in nude mice bearing HCT-8 and FaDu xenografts treated with the MTD of irinotecan (100 mg/kg/week x 4) when combined with selenium. Administration of higher doses of irinotecan (200 and 300 mg/kg/week x 4) was required to achieve high cure rate for HT-29 and A253 xenografts. Administration of these higher doses was possible due to selective protection of normal tissues by selenium. Thus, the use of selenium as selective modulator of the therapeutic efficacy of anticancer drugs is new and novel.

CONCLUSIONS

We demonstrated that selenium is a highly effective modulator of the therapeutic efficacy and selectivity of anticancer drugs in nude mice bearing human tumor xenografts of colon carcinoma and squamous cell carcinoma of the head and neck. The observed in vivo synergic interaction is highly dependent on the schedule of selenium.

Mechanisms of mammary cancer chemoprevention by organoselenium compounds

Searching for optimal diets and for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to finding ways to control and prevent cancer. To date, the use of the micronutrient selenium in human clinical trials is limited but the outcome of these investigations indicates that selenium is one of the most promising agents. Data presented in this mini-review indicate that the dose and the form (structure) in which selenium is used are the most critical determinants of success in future clinical trials. The focus of this mini-review is on the mechanisms of mammary cancer chemoprevention by organoselenium compounds. Among the naturally occurring organoselenium compounds, Se-Methylselenocysteine is more efficacious than the most extensively studied forms, such as selenomethionine. However, we showed that synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal side effects by structural modifications; it is evident that synthetic agents are superior to the inorganic selenite, naturally occurring selenium compounds and their sulfur-containing analogs. We have demonstrated that 1,4-phenylenebis (methylene) selenocyanate (p-XSC) and its putative metabolite glutathione conjugate (p-XSeSG) are highly promising agents in the chemoprevention of mammary carcinogenesis in the 7,12-dimethylbenz[a]anthracene (DMBA)-rat mammary tumor model system. Both compounds inhibit the initiation phase of carcinogenesis by inhibiting DMBA-DNA adduct formation in the target organ in vivo. cDNA microarray analysis indicates that both selenium compounds alter genes in a manner that leads to inhibition of cell proliferation and induction of apoptosis; modulation of apoptosis and cell proliferation can account for chemoprevention during the post-initiation phase of mammary carcinogenesis. Using a rat mammary cancer cell line, we compared p-XSC and p-XSeSG as inhibitors of cell proliferation; depending on the selenium dose and time point selected, p-XSC was comparable to or better than p-XSeSG. Collectively, the results described here, suggest that the molecular targets modulated by organoselenium compounds are highly useful indicators of success in clinical cancer chemoprevention trials.

Both calcium and ROS as common signals mediate Na(2)SeO(3)-induced apoptosis in SW480 human colonic carcinoma cells

Recent studies have shown that reactive oxygen species (ROS) play a crucial role in Se-induced cell apoptosis. A number of studies have demonstrated that perturbed cellular calcium homeostasis has been implicated in apoptosis. The main objective of this study was to evaluate the role of Ca(2+) in Na(2)SeO(3)-induced apoptosis and the relationship between Ca(2+) and ROS in human colonic carcinoma cells SW480. When SW480 cells were exposed to 25-100 microM Na(2)SeO(3), both cell apoptosis and growth inhibition were observed by flow cytometric analysis and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Na(2)SeO(3) was able to induce increase of [Ca(2+)](i) and ROS production and disrupt mitochondrial membrane potential (Delta Psi m) in SW480 cells monitored by using a confocal laser scanning microscope. Ca(2+) channel inhibitor CoCl(2) and an intracellular Ca(2+) chelator o-phtalaldehyde, 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA) completely inhibited [Ca(2+)](i) increase, but catalase had no effect on Na(2)SeO(3)-induced increase of [Ca(2+)](i). BAPTA-AM, CoCl(2), and mitochondrial Ca(2+) uptake inhibitor ruthenium red blocked Delta Psi m dissipation. The increase of ROS was also suppressed by CoCl(2), BAPTA, ruthenium red, N-acetylcysteine and catalase, respectively. The mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) completely inhibited Na(2)SeO(3)-induced ROS increase. This showed that ROS increase is due to mitochondrial Ca(2+) overload. The Na(2)SeO(3)-induced apoptosis of SW480 cells was also inhibited by CoCl(2), BAPTA, ruthenium red, N-acetylcysteine, and catalase, respectively. The results mentioned above imply that both calcium and Ca(2+)-dependent ROS as a signal molecule mediate apoptosis induced by Na(2)SeO(3) in SW480 cells.

Comparative action of 1,4-phenylenebis(methylene)selenocyanate and its metabolites against 7,12-dimethylbenz[a]anthracene-DNA adduct formation in the rat and cell proliferation in rat mammary tumor cells

1,4-phenylenebis(methylene)selenocyanate (p-XSC) inhibits 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary carcinogenesis and DMBA-DNA binding in the rat mammary gland. Tetraselenocyclophane (TSC) was identified in rat feces as a metabolite of p-XSC. This led us to postulate the metabolic pathway: p-XSC-->glutathione conjugate (p-XSeSG)-->aromatic selenol (p-XSeH)-->TSC. Whether p-XSC or one of its metabolites is responsible for cancer prevention is the focus of this study. We utilized the DMBA-DNA binding assay with p-XSC as a positive control to evaluate the chemopreventive potential of p-XSC metabolites at dietary selenium levels of 10 ppm. Rats were fed AIN-76A diet supplemented with various selenium compounds for 1 week prior to the oral administration of a single dose of [3H]DMBA (5 mg per rat, specific activity 51.3 mCi/mmol). The rats were sacrificed 24 h later and DNA was isolated from the mammary fat pads. Relative levels of total binding were: [pmol/mg DNA, mean +/- S.D., n=6]; DMBA [7.2 +/- 1.6]; DMBA+p-XSC [3.5 +/- 2.7]; DMBA+p-XSeSG [2.2 +/- 1.1]; DMBA+TSC [5.6 +/- 2.9]. All selenium compounds, except TSC, significantly inhibited DMBA-DNA adduct formation; however, the difference between p-XSC and p-XSeSG was not statistically significant. The inhibition of total binding was attributed to a reduction in the formation of the three major adducts derived from bay-region diol epoxides of DMBA. On the basis of their chromatographic characteristics, these were identified as anti-diol-epoxide:deoxyguanosine, syn-diol-epoxide:deoxyadenosine, and anti-diol-epoxide:deoxyadenosine. Our results suggest that p-XSeSG, but not TSC, is the likely inhibitor of mammary cancer. Selenium levels measured by atomic absorption spectroscopy in the target organ (mammary fat pads) and in plasma following the dietary administration of selenium compounds were in the order of p-XSeSG congruent with p-XSC>TSC. These results appear to be consistent with their order of inhibitory effects on total DMBA-DNA binding. Further in vitro studies of the effect of selenium compounds on cell proliferation suggest that, depending on the dose and time point selected, p-XSC is comparable to or better than p-XSeSG; but both are more effective than TSC. Collectively, our in vivo and in vitro results indicate that p-XSC and its conjugate are better candidates than TSC for future studies on mammary cancer chemoprevention.

Antiproliferative effect of 1,4-phenylenebis(methylene)selenocyanate (p-XSC) on colonic epithelium of patients with adenomatous polyps in vitro

We have consistently shown that the organoselenium compound 1,4-phenylenebis(methylene)selenocyanate (p-XSC) is a superior cancer chemopreventive agent and less toxic than selenite or certain naturally-occurring selenoamino acids. To elucidate the effects of p-XSC on human colonic mucosa, biopsies from endoscopically normal sigmoid colon of 30 patients with adenomatous polyps were incubated with p-XSC at concentrations of 1, 2 and 5 micromol/l dissolved in dimethylsulphoxide (DMSO). Biopsies incubated with DMSO or pure culture medium served as a control. Proliferating cells were labelled by bromodeoxyuridine immunohistochemistry and the labelling index (LI) was computed. Upper crypt labelling index (LI of crypt compartments 4+5) and Phih value, which are both discriminators of the expansion of the proliferative zone, were significantly lower after incubation with 1 and 5 micromol/l p-XSC, respectively (LI 4+5: 0.8 and 1.0; Phih value: 2.1 and 2.4), as compared with DMSO (LI 4+5: 3.6 and 4.5; Phih value: 7.0 and 8.3) or culture medium (LI 4+5: 3.3 and 4.5; Phih value: 7.2 and 8.1) (P<0.005 and P<0.05 by Friedman's block test). A trend towards lower levels of LI 4+5 (P=0.059) and Phih value (P=0.075) were seen after 2 micromol/l p-XSC incubation compared with DMSO. Since hyperproliferation of colonic crypt cells with expansion of the proliferative zone is regarded as a biomarker of increased cancer risk, the antiproliferative effects of p-XSC especially on upper crypt LI and Phih value may indicate a possible protective effect of this organoselenium compound in the prevention of human colon cancer development.

Multiorgan sensitivity to anticarcinogenesis by the organoselenium 1,4-phenylenebis(methylene)selenocyanate

The data in this report clearly indicate that the form (structure) in which selenium is used is the most critical determinant of success in future clinical trials. Synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal toxic side effects by structural modifications. We demonstrated that 1,4-phenylenebis(methylene)selenocyanate is a powerful chemopreventive agent against the development of experimental colon, mammary, lung, and oral carcinogenesis. On the basis of metabolism studies of organoselenium compounds and those reported in the literature, our working hypothesis is that aromatic selenol intermediates are important entities in cancer chemoprevention. In addition, we suggest that 1,4-phenylenebis(methylene)selenocyanate not only serves as a chemopreventive agent, but it may be valuable in preventing metastatic diseases in future studies in the clinic.

Molecular mechanisms of cancer prevention by selenium compounds

Selenium compounds that are chemopreventive in animal models inhibit cell growth and induce apoptosis in vitro, and this could explain how they reduce the outgrowth of tumor cells in vivo. Our recent work has shown that primary cultures of oral carcinoma biopsies are significantly more sensitive than normal oral mucosa cultures to induction of apoptosis by a natural selenium metabolite [selenodiglutathione (SDG)], and this is associated with induction of Fas ligand, a well-known mediator of apoptosis in other contexts, and activation of so-called stress kinase signaling pathways, particularly the Jun NH2-terminal kinase (JNK). Heme oxygenase, another marker of stress responses, is also induced by selenite and SDG. The selective activation of the Fas pathway in carcinomas could be responsible directly for their destruction by apoptosis or target them for attack by immunologic responses. In contrast, although the potent pharmacological selenium chemopreventive agent 1,4-phenylenebis(methylene)selenocyanate (p-XSC) also induces Fas ligand, heme oxygenase, and stress kinase pathways, apoptosis/Fas induction is not so strongly JNK-dependent and p-XSC does not show tumor selectivity. These differences in mechanism between SDG and p-XSC may be due to the manner in which they induce redox changes in the cells, since although the effects of SDG and p-XSC are prevented by antioxidants such as glutathione or N-acetylcysteine, hydroxyl radical scavengers such as mannitol or pyrrolidine dithiocarbamate only protect against the effects of p-XSC.

Protein kinase C as a molecular target for cancer prevention by selenocompounds

Selenium is a very effective cancer-preventive agent, suppressing tumor promotion and early stages of tumor progression. However, the mechanisms by which selenium exerts these cancer-preventive actions are not known. Protein kinase C (PKC) is a receptor for certain tumor promoters and also plays a crucial role in events related to tumor progression. Therefore, it is not only a potential target for the cancer-preventive activity of selenium, but also it has the structural basis for interaction with selenium. Redox-active selenocompounds can inactivate PKC, particularly the Ca(2+)-dependent isozymes, by reacting with the critical cysteine-rich regions present within the catalytic domain while, in some cases, also reacting with the cysteine residues present within the zinc-fingers of the regulatory domain. The selenoprotein thioredoxin reductase (TR), acting through thioredoxin, reverses the inactivation of PKC induced by selenometabolites. Furthermore, TR, through a direct interaction involving its selenosulfur center with the zinc-thiolates of PKC, can reverse the redox modification of this kinase induced by selenometabolites. Thus the selenometabolite-induced toxicity is reversed by a selenoprotein, and therefore an interrelationship exists between these two mechanisms of selenium actions. Moreover, this also explains how a resistance to selenium develops in advanced tumor cells probably due to an overexpression of functional TR. Selenium-induced inactivation of PKC may, at least in part, be responsible for the selenium-induced inhibition of tumor promotion, cell growth, invasion, and metastasis, as well as for the induction of apoptosis.

Se-methylselenocysteine induces apoptosis through caspase activation and Bax cleavage mediated by calpain in SKOV-3 ovarian cancer cells

Se-methylselenocysteine (Se-MSC) is a potent chemopreventive agent in many test systems and has been shown to inhibit tumor promotion and induce apoptosis, but its mechanism of action is still not well understood. The present study was designed to assess the mechanism of Se-MSC on the induction of apoptosis in SKOV-3 ovarian cancer cells. Se-MSC displayed strong inhibitory effects on cell proliferation and viability of SKOV-3 cells in dose and time dependent manners and induced apoptosis. Investigation of the mechanism of Se-MSC-induced apoptosis revealed that treatment with Se-MSC produced morphological features of apoptosis and DNA fragmentation. This was associated with caspase-3 activation and cleavage of poly(ADP-ribose) polymerase and phospholipase C-gamma1 proteins. However, SKOV-3 cells treated with Se-MSC did not demonstrate cytochrome c accumulation in the cytosol during apoptosis induction. Pretreatment of cells with the caspase inhibitors (z-VAD-fmk and DEVD-CHO) prevented Se-MSC-induced apoptosis. These results suggested that Se-MSC induces apoptosis through cytochrome c-independent caspase-3 activation in SKOV-3 cells. In late stage of apoptosis, p18kDa fragment of Bax was generated with the down-regulation of the expressions of survivin, X-linked inhibitor of apoptosis protein, and human inhibitor of apoptosis protein 1 following Se-MSC treatment, suggesting that the modulation of Bax and IAP (inhibitors of apoptosis) family proteins play some role in Se-MSC-mediated apoptosis. Pre-treatments of z-VAD-fmk and the calpain inhibitor, calpeptin inhibited Bax cleavage. These results suggested that Bax cleavage is mediated by calpain, and calpain activation may be a caspase-dependent one. Taken together, the chemopreventive effects of Se-MSC may be related in part to the caspase-3 activation, the down-regulation of IAP family proteins, and Bax cleavage mediated by caspase-dependent calpain activation.

Osteosarcoma cell apoptosis induced by selenium

An essential nutrient selenium has been reported to be a potential cancer preventive and inhibitory agent, although no exact mechanism has yet been proposed. Since little is known about the anti-proliferative effect of selenium on osteosarcoma, this issue was addressed in the present study in vitro using three osteosarcoma cell lines, and in vivo using an osteosarcoma transplantable to nude mice. Selenium inhibited the tumor growth in vitro and morphological changes indicative of apoptosis were demonstrated. Osteosarcomas in nude mice were inhibited in growth by selenium with no cytotoxic change in normal tissues. The findings suggested that selenium may offer a novel therapeutic modality for osteosarcoma.

The protective role of selenium on genetic damage and on cancer

Collectively, results from epidemiologic studies, laboratory bioassays, and human clinical intervention trials clearly support a protective role of selenium against cancer development. Several hypotheses have been proposed to explain these observations. Increased genomic instability, either inherent or induced by exogenous agents (mutagens or carcinogens), has been considered as a primary event leading to neoplastic transformation. This report deals specifically with the evidence for a role of selenium in the inhibition of carcinogen-induced covalent DNA adduct formation and retardation of oxidative damage to DNA, lipids and proteins, and for modulating cellular and molecular events that are critical in cell growth inhibition and in the multi-step carcinogenesis process. At present, the bulk of our knowledge on the role of selenium on genetic stability is based primarily on animal data and from studies conducted in in vitro systems. Studies performed in vitro showed that the dose and form of selenium compounds are critical factors with regard to cellular responses. Inorganic (at doses up to 10microM) and organic selenium compounds (at doses equal to or greater than 10microM) elicit distinctly different cellular responses. The recommended daily allowance (RDA) is 50-70 microgramSe per day for healthy adults; with 40 microgramSe as minimum requirement. Less than 11 microgramSe will definitely put people at risk of deficiency that would be expected to cause genetic damage. Daily doses of 100-200 microgramSe inhibited genetic damage and cancer development in humans. About 400 microgramSe per day is considered an upper limit. Clearly, doses above the RDA are needed to inhibit genetic damage and cancer. However, it has been hypothesized that the intake of excessive doses of selenium may cause oxidative damage, leading to genomic instability. The use of a cocktail consisting of selenium, and other vitamins and minerals appears to be a promising approach to inhibit genetic damage and the development of cancer. It is the author's recommendation that development of mechanism-based hypotheses that can be tested in pilot studies in different populations prior to a large-scale clinical trial in humans, is of paramount importance in order to better understand the role of selenium on genetic stability and cancer.

Selenium and signal transduction: roads to cell death and anti-tumour activity

Accumulated evidence from prospective studies, intervention trials and studies on animal models of cancer have suggested a strong inverse correlation between selenium intake and cancer incidence. Several putative mechanisms have been suggested to mediate the chemopreventive activities of selenium: of these, the inhibition of cellular proliferation and the induction of apoptosis are particularly attractive. The mitogen activated protein kinase (MAPK) pathways are known to be important regulators of cell death and our recent work has focused on the involvement of these pathways in selenium-induced apoptosis in primary cultures of oral cancers and corresponding normal mucosa derived from biopsy material. Using this system, the oral carcinoma cells were found to have enhanced sensitivity to apoptosis when treated with certain selenium compounds compared to normal oral mucosa. Induction of Fas ligand was associated with selenium-induced apoptosis. Signal transduction studies suggests that selenium induces several changes in the MAPK signalling pathways but functional intervention/inhibitor studies indicate that activation of the JNK pathway seems to be most important.

Se-methylselenocysteine activates caspase-3 in mouse mammary epithelial tumor cells in vitro

Se-methylselenocysteine (MSC) inhibits mouse mammary epithelial tumor cell (TM6) growth. When synchronized TM6 cells were exposed to 50 microM MSC, either for 30 minutes or continuous, the 116 kDa poly(ADP-ribose)polymerase (PARP) was cleaved to an 85 kDa fragment indicative of cells undergoing apoptosis. The earliest cleaved PARP appears at 24 hr time point followed by elevated levels of 85 kDa fragment at 34 hr and 48 hr time points when the cells were exposed to continuous treatment with MSC. Results also showed that MSC increased caspase-3 activity at 24 hr time point. In addition, continuous treatment with MSC induced DNA fragmentation at 34 hr and 48 hr time points with caspase-3 gene expression moderately increased at 16 hr and 24 hr time points. Caspase-6 and -8 were also involved in the MSC-induced apoptosis but to a lesser extent. These results suggest that MSC mediates cleavage of PARP and apoptosis by activating one or more caspases in synchronized TM6 cells and the events are dependent on the duration of treatment.

Molecular targets for selenium in cancer prevention

Mounting evidence reveals that selenium is a dietary constituent with anticarcinogenic and antitumorigenic properties. Various forms of selenium appear to be effective in bringing about these effects, although preclinical studies suggest that differences may arise as the quantity provided is reduced. The literature also documents the greater sensitivity of neoplastic cells to selenium than their nonneoplastic counterparts. Unfortunately, the minimal amount needed to bring about a positive effect in humans remains elusive. If there is a positive response to exaggerated intakes, it will likely be dependent on many factors, including the consumption of other dietary constituents, as well as variation in a host of genetic pathways involved with cancer. Although the biological basis of the reduction in cancer risk ascribed to selenium remains to be established, its consistency in retarding various experimentally induced tumors and suppressing the growth of various types of neoplasms in vitro and in vivo suggests that several mechanisms are involved. Depressed carcinogen bioactivation, reduced cell proliferation, and increased apoptosis raise the possibility that selenium works at a number of specific molecular targets involved with the cancer process. This review will focus on molecular targets involved with cell proliferation and apoptosis as possible mechanisms by which selenium might alter the cancer process.

Mutagenic activity of 4-nitroquinoline-N-oxide in upper aerodigestive tissue in lacZ mice (MutaMouse) and the effects of 1, 4-phenylenebis(methylene)selenocyanate

4-Nitroquinoline-N-oxide (4-NQO) was administered to lacZ mice at a concentration of 20 microg/ml in drinking water for 2 weeks, and the mutagenic fractions in a number of organs were assayed. The mutant fractions in tongue, esophagus and other pooled oral tissues were, respectively, 117+/-26, 73+/-15, and 48+/-15 mutants/10(5) plaque-forming units (pfu) (ca. 15-40xbackground). 4-NQO was not mutagenic in lung, liver or colon at conditions used here. We had previously demonstrated that the synthetic organoselenium compound, 1,4-phenylenebis(methylene)selenocyanate (p-XSC), an established chemopreventive agent, greatly reduced carcinogenicity in 4-NQO in rat tongue, and we observed here that administration of p-XSC (10 ppm se) in the diet for 6 weeks (2 weeks before, during, and 2 weeks after 4-NQO) resulted in a 33% decrease in mutagenesis in oral tissue, a 17% decrease in esophagus, and a slight increase in tongue. Only the decrease in oral tissue reached statistical significance (p<0.04). The results reported here demonstrate that 4-NQO was extremely mutagenic in lacZ mouse tongue, with lower, but highly significant activities in esophagus and other pooled oral tissues. The high activity of 4-NQO in lacZ mouse tongue is consistent with the organ specificity of 4-NQO in the rat. Inhibition of 4-NQO-induced mutagenesis by p-XSC was observed mainly in pooled oral tissues, other than tongue. Possible reasons for the difference between inhibition of mutagenesis and carcinogenesis in tongue are discussed, as well as advantages and disadvantages of in vivo mutagenesis assays as surrogates for carcinogenicity assays in chemoprevention studies.

Chemopreventive mechanisms of selenium

The element selenium (Se) was recognized only 40 years ago as being essential in the nutrition of animals and humans. It is recognized as being an essential component of a number of enzymes in which it is present as the amino acid selenocysteine (SeCys). Selenium compounds have also been found to inhibit tumorigenesis in a variety of animal models and recent studies indicate that supplemental Se in human diets may reduce cancer risk. Anti-tumorigenic activities have been associated with Se intakes that are more than sufficient to correct nutritionally deficient status; that is, Se appears to be anti-tumorigenic at intakes that are substantially greater than those associated with maximal expression of the known SeCys-containing enzymes. Therefore, while some cancer protection may involve one or more Se-enzymes, it is probable that anti-tumorigenic functions of Se are discharged by certain Se-metabolites produced in significant amounts at relatively high Se intakes. Thus, Se supplementation of individuals with relatively low or frankly deficient natural intakes of the element can be expected to support enhanced anti-oxidant protection due to increased expression of the Se-dependent glutathione peroxidases and thioredoxin reductase. Higher levels of Se-supplementation can be expected to affect other functions related to tumorigenesis: carcinogen metabolism, immune function, cell cycle regulation and apoptosis. Thus, according to this 2-stage model of the roles of Se in cancer prevention, even individuals with nutritionally adequate Se intakes may benefit from Se-supplementation.

Environmental factors as regulators and effectors of multistep carcinogenesis

This review highlights current knowledge of environmental factors in carcinogenesis and their cellular targets. The hypothesis that environmental factors influence carcinogenesis is widely supported by both epidemiological and experimental studies. The fact that only a small fraction of cancers can be attributed to germline mutations in cancer-related genes further buttresses the importance of environmental factors in carcinogenesis. Furthermore, penetrance of germline mutations may be modified by either environmental or other genetic factors. Examples of environmental factors that have been associated with increased cancer risk in the human population include chemical and physical mutagens (e.g. cigarette smoke, heterocyclic amines, asbestos and UV irradiation), infection by certain viral or bacterial pathogens, and dietary non-genotoxic constituents (e.g. macro- and micronutrients). Among molecular targets of environmental influences on carcinogenesis are somatic mutation (genetic change) and aberrant DNA methylation (epigenetic change) at the genomic level and post-translational modifications at the protein level. At both levels, changes elicited affect either the stability or the activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Together, via multiple genetic and epigenetic lesions, environmental factors modulate important changes in the pathway of cellular carcinogenesis.

Trace elements and apoptosis

It is known that apoptosis is considered to be responsible for selective deletion of cells during embryogenesis, the homeostasis of cell populations in continuously renewing tissues (i.e., serving as a counterbalance to mitosis), and tissue involution in response to chemical or physical stimuli. There are many publications on these questions. On the other hand, the intracellular processes that contribute to apoptosis are incompletely understood. Therefore, the role of apoptosis in the intracellular accumulation and outflow of minerals is of considerable importance in light of both their essential functions and toxic effects.