Efficacy of cancer prevention by high-selenium garlic is primarily dependent on the action of selenium

Natural Sources of Selenium as Functional Food Products for Chemoprevention

Cancer is one of the leading causes of death worldwide, the incidence of which is increasing annually. Interest has recently grown in the anti-cancer effect of functional foods rich in selenium (Se). Although clinical studies are inconclusive and anti-cancer mechanisms of Se are not fully understood, daily doses of 100–200 µg of Se may inhibit genetic damage and the development of cancer in humans. The anti-cancer effects of this trace element are associated with high doses of Se supplements. The beneficial anti-cancer properties of Se and the difficulty in meeting the daily requirements for this micronutrient in some populations make it worth considering the use of functional foods enriched in Se. This review evaluated studies on the anti-cancer activity of the most used functional products rich in Se on the European market.

Selenium Uptake, Transport, Metabolism, Reutilization, and Biofortification in Rice

Selenium (Se) is an essential trace element for humans and other animals. The human body mainly acquires Se from plant foods, especially cereal grains. Rice is the staple food for more than half of the world's population. Increasing the Se concentration of rice grains can increase the average human dietary Se intake. This review summarizes recent advances in the molecular mechanisms of Se uptake, transport, subcellular distribution, retranslocation, volatilization, and Se-containing protein degradation in plants, especially rice. The strategies for improving Se concentration in rice grains by increasing Se accumulation, reducing Se volatilization, and optimizing Se form were proposed, which provide new insight into Se biofortification in rice by improving the utilization efficiency of Se.

Nutrition by Design: Boosting Selenium Content and Fresh Matter Yields of Salad Greens With Preharvest Light Intensity and Selenium Applications

Selenium (Se) is an essential mineral in multiple human metabolic pathways with immune modulatory effects on viral diseases including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and HIV. Plant-based foods contain Se metabolites with unique functionalities for the human metabolism. In order to assess the value of common salad greens as Se source, we conducted a survey of lettuce commercially grown in 15 locations across the USA and Canada and found a tendency for Se to accumulate higher (up to 10 times) in lettuce grown along the Colorado river basin region, where the highest amount of annual solar radiation of the country is recorded. In the same area, we evaluated the effect of sunlight reduction on the Se content of two species of arugula [Eruca sativa (E. sativa) cv. “Astro” and Diplotaxis tenuifolia (D. tenuifolia) cv. “Sylvetta”]. A 90% light reduction during the 7 days before harvest resulted in over one-third Se decline in D. tenuifolia. The effect of light intensity on yield and Se uptake of arugula microgreens was also examined under indoor controlled conditions. This included high intensity (HI) (160 μ mol⁻² s⁻¹ for 12 h/12 h light/dark); low intensity (LI) (70 μ mol m⁻² s⁻¹ for 12 h/12 h light/dark); and HI-UVA (12 h light of 160 μ mol m⁻² s⁻¹, 2 h UVA of 40 μ mol m⁻² s⁻¹, and 10 h dark) treatments in a factorial design with 0, 1, 5, and 10 ppm Se in the growing medium. HI and HI-UVA produced D. tenuifolia plants with 25–100% higher Se content than LI, particularly with the two higher Se doses. The addition of Se produced a marked increase in fresh matter (>35% in E. sativa and >45% in D. tenuifolia). This study (i) identifies evidence to suggest the revision of food composition databases to account for large Se variability, (ii) demonstrates the potential of introducing preharvest Se to optimize microgreen yields, and (iii) provides the controlled environment industry with key information to deliver salad greens with targeted Se contents.

A comparative study on the accumulation, translocation and transformation of selenite, selenate, and SeNPs in a hydroponic-plant system

Plants can play important roles in overcoming selenium (Se) deficiency and Se toxicity in various regions of the world. Selenite (SeIV), selenate (SeVI), as well as Se nanoparticles (SeNPs) naturally formed through reduction of SeIV, are the three main Se species in the environment. The bioaccumulation and transformation of these Se species in plants still need more understanding. The aims of this study are to investigate the phytotoxicity, accumulation, and transformation of SeIV, SeVI and SeNPs in garlic, a relatively Se accumulative plant. The spatial distribution of Se in the roots were imaged using synchrotron radiation micro-focused X-ray fluorescence (SR-μXRF). The chemical forms of Se in different plant tissues were analyzed using synchrotron radiation X-ray absorption spectroscopy (SR-XAS). The results demonstrate that 1) SeNPs which has the lowest phytotoxicity is stable in water, but prone to be converted to organic Se species, such as C-Se-C (MeSeCys) upon uptake by root. 2) SeIV is prone to concentrate in the root and incorporated into C-Se-C (MeSeCys) and C-Se-R (SeCys) bonding forms; 3) SeVI with the lowest transformation probability to organic Se species has the highest phytotoxicity to plant, and is much easier to translocate from root to leaf than SeNPs and SeIV. The present work provides insights into potential impact of SeNPs, selenite and selenate on aquatic-plant ecosystems, and is beneficial for systematically understanding the Se accumulation and transformation in food chain.

The role of selenium in nutrition – A review

The role of selenium has been changed over the last decade. The element that was previously considered to be toxic turned out to be present in the human body in amounts of 10–15 mg, and almost every cell of our body contains it. Selenium contributes to growth, supports healthy muscle activity, reproductive organs, reduces the toxicity of certain elements such as mercury, supports the immune system, and even delays the spread of certain viruses (influenza, Ebola, HIV). Selenium-deficient areas of Europe could be a risk for their populations. The recommended daily intake (RDA) of selenium is 55 µg/day, while WHO and FAO have set up the daily tolerable dose at 400 µg/day. We must count with the harmful effects of selenium overdose, but it is almost impossible to introduce this amount into our body solely with food. Our selenium sources can be refilled with food supplements or selenium-enriched functional foods. In the review article, we report about the role of selenium in the environment, selenium-enriched plants, selenium-enriched yeast, the role of selenium in animal feed and in the human body, the opportunities of selenium restoration, selenium-enriched animal products, and the selenium content of milk.

Increased Transglutaminase 2 and GLUT-1 Expression in Breast Tumors not Susceptible to Chemoprevention with Antioxidants

Goals

Expression of GLUT-1 and transglutaminase 2 is increased in aggressive breast cancer, whereas claudin-1, which is expressed in normal tissues, is absent in such tumors. This experimental study was undertaken to establish the aggressiveness and prognosis of DMBA-induced mammary tumors in female Wistar rats based on the assessment of these markers.

Materials and methods

The rats were divided into two groups, a control group (n = 70) and a chemoprevention group (n = 70). Breast tumors were induced in both groups by administration of 7,12-dimethylbenz[a]anthracene (DMBA). The chemoprevention group also received alpha-tocopherol and a solution of micronutrients containing ascorbic acid and selenium. Neoplastic lesions of both groups were randomly selected for immunohistochemical assessment of the expression of GLUT-1, transglutaminase 2 and claudin-1.

Results

A higher proportion of mammary tumors expressed GLUT-1 and transglutaminase 2 in the chemoprevention group. Claudin-1 expression was absent in all tumors of both groups.

Conclusions

These results are suggestive of increased aggressiveness of tumors not susceptible to chemoprevention by the agents used in this study.

Selenium and Sulfur to Produce Allium Functional Crops

Selenium is an element that must be considered in the nutrition of certain crops since its use allows the obtaining of biofortified crops with a positive impact on human health. The objective of this review is to present the information on the use of Se and S in the cultivation of plants of the genus Allium. The main proposal is to use Allium as specialist plants for biofortification with Se and S, considering the natural ability to accumulate both elements in different phytochemicals, which promotes the functional value of Allium. In spite of this, in the agricultural production of these species, the addition of sulfur is not realized to obtain functional foods and plants more resistant; it is only sought to cover the necessary requirements for growth. On the other hand, selenium does not appear in the agronomic management plans of most of the producers. Including S and Se fertilization as part of agronomic management can substantially improve Allium crop production. Allium species may be suitable to carry out biofortification with Se; this practice can be combined with the intensive use of S to obtain crops with higher production and sensory, nutritional, and functional quality.

Cancer chemoprevention research with selenium in the post-SELECT era: Promises and challenges

The negative efficacy outcomes of double-blinded, randomized, placebo-controlled Phase III human clinical trials with selenomethionine (SeMet) and SeMet-rich selenized-yeast (Se-yeast) for prostate cancer prevention and Se-yeast for prevention of nonsmall cell lung cancer (NSCLC) in North America lead to rejection of SeMet/Se-yeast for cancer prevention in Se-adequate populations. We identify 2 major lessons from the outcomes of these trials: 1) the antioxidant hypothesis was tested in wrong subjects or patient populations, and 2) the selection of Se agents was not supported by cell culture and preclinical animal efficacy data as is common in drug development. We propose that next-generation forms of Se (next-gen Se), such as methylselenol precursors, offer biologically appropriate approaches for cancer chemoprevention but these are faced with formidable challenges. Solid mechanism-based preclinical efficacy assessments and comprehensive safety studies with next-gen Se will be essential to revitalize the idea of cancer chemoprevention with Se in the post-SELECT era. We advocate smaller mechanism-driven Phase I/II trials with these next-gen Se to guide and justify future decisions for definitive Phase III chemoprevention efficacy trials.

Plants and microbes assisted selenium nanoparticles: characterization and application

Selenium is an essential trace element and is an essential component of many enzymes without which they become inactive. The Se nanoparticles of varying shape and size may be synthesized from Se salts especially selenite and selenates in presence of reducing agents such as proteins, phenols, alcohols and amines. These biomolecules can be used to reduce Se salts in vitro but the byproducts released in the environment may be hazardous to flora and fauna. In this review, therefore, we analysed in depth, the biogenic synthesis of Se nanoparticles, their characterization and transformation into t- Se, m-Se, Se-nanoballs, Se-nanowires and Se-hollow spheres in an innocuous way preventing the environment from pollution. Their shape, size, FTIR, UV-vis, Raman spectra, SEM, TEM images and XRD pattern have been analysed. The weak forces involved in aggregation and transformation of one nano structure into the other have been carefully resolved.

Plants and microbes assisted selenium nanoparticles: characterization and application

Selenium is an essential trace element and is an essential component of many enzymes without which they become inactive. The Se nanoparticles of varying shape and size may be synthesized from Se salts especially selenite and selenates in presence of reducing agents such as proteins, phenols, alcohols and amines. These biomolecules can be used to reduce Se salts in vitro but the byproducts released in the environment may be hazardous to flora and fauna. In this review, therefore, we analysed in depth, the biogenic synthesis of Se nanoparticles, their characterization and transformation into t- Se, m-Se, Se-nanoballs, Se-nanowires and Se-hollow spheres in an innocuous way preventing the environment from pollution. Their shape, size, FTIR, UV-vis, Raman spectra, SEM, TEM images and XRD pattern have been analysed. The weak forces involved in aggregation and transformation of one nano structure into the other have been carefully resolved.

Fertilizing soil with selenium fertilizers: impact on concentration, speciation, and bioaccessibility of selenium in leek (Allium ampeloprasum)

Leek was fertilized with sodium selenite (Na(2)SeO(3)) and sodium selenate (Na(2)SeO(4)) in a green house to assess the impact of selenium (Se) fertilization on Se uptake by the crop and its speciation in the crop. The bioaccessibility of Se in the Se-enriched leek was assessed using an in vitro extraction protocol mimicking the human gastrointestinal tract (stomach, small intestine, and colon). The lowest Se uptake was observed when Na(2)SeO(3) was used as a fertilizer, which results in a higher risk for Se accumulation in the soil on a longer term. When soil was amended with Na(2)SeO(4), 55 ± 5% of total Se in the leek occurred in an inorganic form, while only 21 ± 8% was inorganic when Na(2)SeO(3) was applied. Se-methylselenocysteine and selenomethione were the major organic species in both treatments. However, concentrations of Se-methylselenocysteine and γ-glutamyl-Se-methyl-selenocysteine, which were previously reported to induce positive health effects, were lower as compared to other Allium species. The majority of the Se in the leek was found to be bioaccessible in the stomach (around 60%) and small intestine (around 80%). However, a significant fraction also has good chances to reach the colon, where it seems to be taken up by the microbial community and may also induce positive health effects.

Effect of selenium treatment on mineral nutrition, bulb size, and antioxidant properties of garlic (Allium sativum L.)

Foliar selenium (Se) treatment of garlic at concentrations of 10, 50, and 100 μg of Se/mL was carried out in open field conditions in 2008 and 2009 in Estonia. Bulb weight and yield structure, content of total Se, S, N, P, K, Ca, and Mg, ascorbic acid content (AAC), pungency, total phenolics, and total antioxidant capacity (TAC) were determined. The highest level of Se decreased total S, K, and Ca in both years; no negative impact on bulb weight was observed. In 2009 Se10 treatment had significantly more bulbs with the largest diameter compared to the other treatments. In 2008, the AAC was decreased by Se50 and the content of total phenolics by all Se treatments; however, TAC was increased. Foliar Se fertilization of garlic at rates of 10-50 μg of Se/mL can be recommended to increase the number of large bulbs and increase bulb antioxidant capacity.

Selenium, but not lycopene or vitamin E, decreases growth of transplantable dunning R3327-H rat prostate tumors

Background

Lycopene, selenium, and vitamin E are three micronutrients commonly consumed and supplemented by men diagnosed with prostate cancer. However, it is not clear whether consumption of these compounds, alone or in combination, results in improved outcomes.

Methodology/Principal Findings

We evaluated the effects of dietary lycopene (250 mg/kg diet), selenium (methylselenocysteine, 1 mg/kg diet), and vitamin E (γ-tocopherol, 200 mg/kg diet) alone and in combination on the growth of androgen-dependent Dunning R3327-H rat prostate adenocarcinomas in male, Copenhagen rats. AIN-93G diets containing these micronutrients were prefed for 4 to 6 weeks prior to tumor implantation by subcutaneous injection. Tumors were allowed to grow for ∼18 weeks. Across diet groups, methylselenocysteine consumption decreased final tumor area (P = 0.003), tumor weight (P = 0.003), and the tumor weight/body weight ratio (P = 0.003), but lycopene and γ-tocopherol consumption intake did not alter any of these measures. There were no significant interactions among nutrient combinations on tumor growth. Methylselenocysteine consumption also led to small, but significant decreases in body weight (P = 0.007), food intake (P = 0.012), and body weight gain/food intake ratio (P = 0.022). However, neither body weight nor gain/food intake ratio was correlated with tumor weight. Methylselenocysteine, lycopene, and γ-tocopherol consumed alone and in combination did not alter serum testosterone or dihydrotestosterone concentrations; tumor proliferation or apoptosis rates. In addition, the diets also did not alter tumor or prostate androgen receptor, probasin, selenoprotein 15, selenoprotein P, or selenium binding protein 2 mRNA expression. However, using castration and finasteride-treated tissues from a previous study, we found that androgen ablation altered expression of these selenium-associated proteins.

Conclusions

Of the three micronutrients tested, only methylselenocysteine consumption reduced growth of transplantable Dunning R3327-H prostate tumors, albeit through an unresolved mechanism.

Chemopreventive doses of methylselenocysteine alter circadian rhythm in rat mammary tissue

It is known that organic forms of selenium inhibit chemically induced rat mammary carcinogenesis, although the molecular basis remains to be elucidated. To identify signaling pathways involved in carcinogenesis that are also modulated by methylselenocysteine, we compared the global gene expression profiles in mammary tissues from pubescent female rats maintained on a selenium-supplemented (3 ppm) diet with those on a standardized diet after N-nitroso-N-methylurea. Whereas the selenium-enriched diet altered the steady-state levels of genes involved in various cellular functions, the most dramatic effect was the coordinated changes in the expression of multiple genes that regulate circadian rhythm. Normal mammary tissue of rats fed a standardized diet showed little circadian oscillation relative to liver tissue. By contrast, mammary tissue of rats maintained on the selenium-enriched diet showed a progressive, time-dependent increase in the expression of circadian gene Per2 and circadian-regulated transcription factor DBP. Our results further showed that the expression of Per2 and DBP mRNAs was significantly decreased in mammary tumors arising in rats on the selenium-enriched diet, but not in tumors of rats on the control diet, suggesting that selenium-induced elevation in the expression of circadian genes was incompatible with mammary carcinogenesis. Given the previously reported role of Per2 as a tumor suppressor, these observations suggest that Per2 is an important target of methylselenocysteine during chemoprevention in N-nitroso-N-methylurea-induced rat mammary carcinogenesis, and for the first time provide a link between chemoprevention and circadian rhythm.

Effect of garlic on lipid peroxidation and antioxidation enzymes in DMBA-induced skin carcinoma

OBJECTIVE

Naturally occurring phytochemicals display an active cancer preventive strategy to inhibit, delay, or reverse human carcinogenesis. Studies have indicated that certain daily-consumed dietary phytochemicals have cancer protective effects mediated by carcinogens. Lipid peroxide plays a detrimental role in all cancers including skin carcinogenesis. Garlic, a phytochemical, has acquired a special position in the folklore of many cultures as a formidable prophylactic and therapeutic medicinal agent. In this report, we pursue the chemopreventive effect of aqueous garlic on skin carcinogenesis.

METHODS

"Swiss albino mice" were divided into five groups depending on the combination of skin cancer-inducing 7,12-dimethylbenz[a]anthracene and garlic treatments. Histology of the affected skin and biochemical assays for lipid peroxide, catalase, superoxide dismutase, glutathione-S-transferase, and glutathione peroxidase were performed to demonstrate the effect of garlic in mice. Immunoblotting was performed with cyclo-oxygenase-2, p53, and caspase-3 to demonstrate expressions of the respective proteins in skin lysates.

RESULTS

Garlic extracts inhibited the oxidative modification of lipids, thus protecting cells from injury by the oxidized molecules. The best chemopreventive action of garlic was observed in mice in which garlic treatment was performed before and after the induction of skin carcinogenesis. Garlic ingestion delayed formation of skin papillomas in animals and simultaneously decreased the size and number of papillomas, which was also reflected in the skin histology of the mice treated.

CONCLUSION

The protective effects against skin cancer elicited by garlic in mice are believed to be due at least in part to the induction cellular defense systems.

Inhibition of cyclophoshamide-induced mutagenicity by microsized powder of selenium-enriched green tea in mice

Previously, the antioxidant activity of Se-enriched green tea extracts has been studied in vitro. In the present study, an in vivo micronuclei test was employed to assess the antimutagenic effect of microsized Se-enriched green tea powder (MSTP) in mice bone marrow. Pretreatments of MSTP, micrometer-sized regular tea powder (MRTP), selenite, and MRTP + selenite were given by gavage for 29 consecutive days prior to cyclophoshamide (CP) treatment. Certain key antioxidant enzymes were also investigated to elucidate the mechanism of antimutagenic effect. Results indicated that MSTP and MRTP or selenite alone did not significantly induce micronuclei at either concentration, confirming its nonmutagenicity. In the CP-treated groups, significant suppressions in the micronuclei were recorded following pretreatment with MSTP, MRTP, and selenite administration. The antimutagenic effect of MSTP was evidently observed by significant reduction in the frequencies of micronuclei in bone marrow cells when compared to a positive control group. The administration of MSTP, selenite, and MRTP + selenite also increased the levels of selenium concentration, glutathione peroxidase (GPx), and superoxide dismutase (SOD) enzymes in both blood and liver. However, no pronounced differences in activities of GPx and SOD were found among MSTP, selenite, and MRTP + selenite. The present findings demonstrate that the antimutagenic potential of MSTP could not be solely related to the enhancment of antioxidant enzymes of GPx and SOD.

Induction of Phase II Enzyme Activity by Various Selenium Compounds

Twenty-seven selenium compounds and sixteen structurally related organosulfur compounds were tested for quinone reductase (QR) and glutathione-S-transferase (GST) inducing activity in murine hepatoma (Hepa 1c1c7) cells. Sixteen selenium compounds were able to double QR activity, and seven of them also doubled GST activity. The nine most potent compounds, dimethyl diselenide, 2,5-diphenyl- selenophene, dibenzyl diselenide, methylseleninic acid, diphenyl diselenide, benzeneseleninic acid, benzene selenol, triphenylselenonium chloride, and ebselen (2-phenyl- 1,2-benzisoselenazol-3(2H)-one), doubled QR-specific activity at levels lower than 7 microM. The concentration-dependence of QR induction and cell growth inhibition were linearly correlated (P < 0.001, r2 = 0.96) among the group of organoselenium compounds with putative selenol-generating potential, implying that both responses of Hepa 1c1c7 cells were based on these selenol metabolites.

Cancer chemoprevention by garlic and garlic-containing sulfur and selenium compounds

As early as 1550 B.C., Egyptians realized the benefits of garlic as a remedy for a variety of diseases. Many epidemiological studies support the protective role of garlic and related allium foods against the development of certain human cancers. Natural garlic and garlic cultivated with selenium fertilization have been shown in laboratory animals to have protective roles in cancer prevention. Certain organoselenium compounds and their sulfur analogs have been identified in plants. Organoselenium compounds synthesized in our laboratory were compared with their sulfur analogs for chemopreventive efficacy. Diallyl selenide was at least 300-fold more effective than diallyl sulfide in protecting against 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary adenocarcinomas in rats. In addition, benzyl selenocyanate inhibited the development of DMBA-induced mammary adenocarcinomas and azoxymethane-induced colon cancer in rats and benzo[a]pyrene-induced forestomach tumors in mice. The sulfur analog, benzyl thiocyanate, had no effect under the same experimental conditions. Furthermore, we showed that 1,4-phenylenebis(methylene)selenocyanate, but not its sulfur analog, significantly inhibited DMBA-DNA adduct formation and suppressed DMBA-induced mammary carcinogenesis. Collectively, these results indicate that structurally distinctive organoselenium compounds are superior to their corresponding sulfur analogs in cancer chemoprevention. Additionally, synthetic aromatic selenocyanates are more effective cancer chemopreventive agents than the naturally occurring selenoamino acids. Because plants are capable of utilizing selenium in a manner similar to that in sulfur assimilation pathways, future studies should aim at determining whether, under appropriate conditions, these potent cancer chemopreventive synthetic selenium compounds can be synthesized by garlic and related allium foods.

Liquid chromatography–mass spectrometry (LC–MS): a powerful combination for selenium speciation in garlic (Allium sativum)

Liquid chromatography (LC) hyphenated with both elemental and molecular mass spectrometry has been used for Se speciation in Se-enriched garlic. Different species were separated by ion-pair liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS) after hot-water extraction. They were identified by on-line reversed-phase liquid chromatography-electrospray ionization tandem mass spectrometry (RPLC-ESI-MS-MS). Se-methionine and Se-methylselenocysteine were determined by monitoring their product ions. Another compound, gamma-glutamyl-Se-methylselenocysteine, shown to be the most abundant form of Se in the garlic, was determined without any additional sample pre-treatment after extraction and without the need for a synthesized standard. Product ions for this dipeptide were detected by LC-ESI-MS-MS for three isotopes of Se-78 Se, 80Se: and 82Se. The method was extended to the species extracted during in-vitro gastrointestinal digestion. Because both Se-methylselenocysteine and gamma-glutamyl-Se-methylselenocysteine have anticarcinogenic properties, their extractability and stability during human digestion are very important. Garlic was also treated with saliva, to enable detection and analysis of species extracted during mastication. Detailed information on the extractability of selenium species by both simulated gastric and intestinal fluid are given, and variation of the distribution of Se among the different species with time is discussed. Although the main species in garlic is the dipeptide gamma-glutamyl-Se-methylselenocysteine, Se-methylselenocysteine is the main compound present in the extracts after treatment with gastrointestinal fluids. Two more, so far unknown compounds were observed in the chromatogram. The extracted species and their transformations were analysed by combining LC-ICP-MS and LC-ESI-MS-MS. In both the simulated gastric and intestinal digests, Se-methionine, Se-methylselenocysteine, and gamma-glutamyl-Se-methylselenocysteine could be determined by LC-ESI-MS-MS by measuring their typical product ions.

Selenium and cancer chemoprevention: hypotheses integrating the actions of selenoproteins and selenium metabolites in epithelial and non-epithelial target cells

The trace element nutrient selenium (Se) discharges its well-known nutritional antioxidant activity through the Se-dependent glutathione peroxidases. It also regulates nuclear factor activities by redox mechanisms through the selenoprotein thioredoxin reductases. Converging data from epidemiological, ecological, and clinical studies have shown that Se can decrease the risk for some types of human cancers, especially those of the prostate, lung, and colon. Mechanistic studies have indicated that the methylselenol metabolite pool has many desirable attributes of chemoprevention, targeting both cancer cells and vascular endothelial cells, whereas the hydrogen selenide pool in excess of selenoprotein synthesis can lead to DNA single strand breaks, which may be mediated by some reactive oxygen species. We propose a new paradigm based on a consideration of the post-initiation biology of avascular early lesion expansion microenvironment, physiochemistry of Se delivery, and the obligatory need for angiogenesis to sustain lesion progression. Our model integrates the roles of selenoproteins and specific Se metabolites to account for cancer risk reduction or enhancement. For future studies, speciation (profiling) methods for Se metabolites and for Se forms in foods and supplements are much needed for hypothesis testing and for the development of mechanism-based Se status markers for cancer prevention. Randomized cancer prevention trials are necessary to test the efficacy of methyl selenium compounds.

Proposed criteria for assessing the efficacy of cancer reduction by plant foods enriched in carotenoids, glucosinolates, polyphenols and selenocompounds

BACKGROUND AND AIMS

The cancer-protective properties of vegetable consumption are most likely mediated through 'bioactive compounds' that induce a variety of physiologic functions including acting as direct or indirect antioxidants, regulating enzymes and controlling apoptosis and the cell cycle. The 'functional food' industry has produced and marketed foods enriched with bioactive compounds, but there are no universally accepted criteria for judging efficacy of the compounds or enriched foods.

SCOPE

Carotenoids, glucosinolates, polyphenols and selenocompounds are families of bioactive compounds common to vegetables. Although numerous studies have investigated the agricultural and human health implications of enriching foods with one or more of these compounds, inadequate chemical identification of compounds, lack of relevant endpoints and inconsistencies in mechanistic hypotheses and experimental methodologies leave many critical gaps in our understanding of the benefits of such compounds. This review proposes a decision-making process for determining whether there is reasonable evidence of efficacy for the both the compound and the enriched food. These criteria have been used to judge the evidence of efficacy for cancer prevention by carotenoids, polyphenols, glucosinolates and selenocompounds.

CONCLUSIONS

The evidence of efficacy is weak for carotenoids and polyphenols; the evidence is stronger for glucosinolates and lycopene, but production of enriched foods still is premature. Additionally there is unacceptable variability in the amount and chemical form of these compounds in plants. The evidence of efficacy for selenocompounds is strong, but the clinical study that is potentially the most convincing is still in progress; also the variability in amount and chemical form of Se in plants is a problem. These gaps in understanding bioactive compounds and their health benefits should not serve to reduce research interest but should, instead, encourage plant and nutritional scientists to work together to develop strategies for improvement of health through food.

Antimutagenicity of selenium-enriched rice on mice exposure to cyclophosphamide and mitomycin C

The in vivo antimutagenicity of Se-enriched rice was evaluated by bone marrow micronucleus and testicle chromosomal aberrations assay in mice exposed to cyclophosphamide (CP) and mitomycin C (MMC). Regular rice did not alter the occurrence of chemical-induced mutation. However, the addition of Se-enriched rice or selenite significantly inhibited the incidence of CP-induced micronuclei and MMC-induced chromosomal aberration in mice and the effect was dose-dependent. Providing selenite or Se-enriched rice also significantly increased the activity of glutathione peroxidase in liver and the selenium concentration in blood compared to regular rice. No significant differences were found in mice body weight gain. These results revealed the antimutagenic potential of Se-enriched rice against chemical-induced mutation.

Inhibition of DNA alkylation damage with inorganic salts

Human exposure to alkylating agents metabolized from tobacco- and food-borne carcinogens occurs regularly. Dietary inorganic compounds such as selenium and vanadium have been shown previously to provide chemoprotective benefits in rat and human trials. Here, we present biochemical data on the ability of inorganic compounds to protect DNA from alkylation damage. An enzyme cleavage assay is used to observe alkylated DNA. Simple salts (e.g., NaCl or NiCl(2)) did not prevent DNA alkylation, whereas anionic oxo species (e.g., Na(2)SeO(4) or Na(3)VO(4)) did inhibit alkylation. We propose that these oxo species behave as nucleophilic targets for the electrophilic alkylating agents, thereby preventing DNA damage.

Organosulfur compounds from alliaceae in the prevention of human pathologies

A strong association between elevated plasma low-density-lipoprotein (LDL) and the development of cardiovascular diseases (CVD) has been established. Oxidation of LDL (Ox-LDL) promotes vascular dysfunction, enhances the production and release of inflammatory mediators such as reactive oxygen species and contribute to the initiation and progression of atherosclerosis. In addition, Ox-LDL enhances the production and release of tumor necrosis factor (TNF-alpha), interleukin (IL)-6, arachidonic acid metabolites and nitric oxide (NO) that are responsible for various human pathologies including cancer. Organosulfur compounds (OSC) from alliaceae modulate the glutathione (GSH) redox cycle and inhibits NFkappa-B activation in human T cells. Furthermore, OSC bioactivities include antioxidant, antibacterial, anticarcinogenic, antiatherogenic, immunostimulatory, and liver protection potential.

Reduction of cancer risk by consumption of selenium-enriched plants: enrichment of broccoli with selenium increases the anticarcinogenic properties of broccoli

Plant-based diets and phytochemicals present in plants are associated with decreased risk of cancer. Brassica species, and broccoli in particular, are associated with reduced risk of several important cancers. Selenium (Se) is an essential nutrient that is covalently bound in a number of different chemical forms found in plants. Broccoli accumulates Se many-fold beyond the concentration of Se in the soil, and the chemical form of Se in broccoli is similar to the chemical form in high-Se garlic, a food with unique chemoprotective properties. Se from broccoli grown to accumulate more than 500 micro g Se/g did not accumulate in rat tissues or increase glutathione peroxidase enzyme activity to the same extent as Se salts or seleno-amino acids. Se from high-Se broccoli decreased the incidence of aberrant crypts in rats with chemically induced colon cancer by more than 50%, compared with controls. Se from high-Se broccoli also decreased the incidence of mammary tumors in rats treated with 7,12-dimethylbenz(a)anthracene (DMBA) and tumor number and volume in APC(min) mice. These results suggest that development of methods to increase the natural accumulation of Se in broccoli may greatly enhance its health-promoting properties.

Studies on the inhibition of human cytochromes P450 by selenocysteine Se-conjugates

1. To investigate whether cytochrome P450 (P450) inhibition can contribute to the chemopreventive activity of selenocysteine Se-conjugates (SeCys conjugates), 21 SeCys conjugates were screened for their inhibitory potency towards seven of the most important human P450s. 2. The majority of the SeCys conjugates produced near complete inhibition of CYP1A1 at a concentration of 250 microm. The most potent inhibitor, Se-benzyl-L-selenocysteine, displayed an IC50 of 12.8 +/- 1.2 microm. CYP2C9, -2C19 and -2D6 were moderately (50-60%) inhibited by the SeCys conjugates. CYP1A2, -2E1 and -3A4 were least inhibited. 3. Studies on the susceptibility of CYP1A1 to SeCys conjugates implicated a thiol-reactive intermediate, as evidenced by reduced inhibition levels in the presence of glutathione and N-acetyl cysteine. Uncoupling of the P450-catalytic cycle was of no importance as ROS scavengers did not influence inhibition levels. 4. P450 inhibition by two physiologically relevant metabolite classes of SeCys conjugates was also studied. N-acetylation of SeCys conjugates consistently increased the inhibitory potency towards CYP1A2, -2C19, -2E1 and -3A4. Beta-lyase catalysed bioactivation of alkyl-substituted SeCys conjugates or Se-benzyl-L-selenocysteine produced little or no additional inhibition of P450 activity. For Se-phenyl-L-selenocysteine, however, significant increases in P450 inhibition were obtained by beta-lyase pre-incubation. 5. It is concluded that the potent and relatively selective CYP1A1 inhibition exerted by SeCys conjugates may contribute to their chemopreventive activity.

Selenium from selenium-rich Spirulina is less bioavailable than selenium from sodium selenite and selenomethionine in selenium-deficient rats

The bioavailabilty of selenium (Se) from selenium-rich Spirulina (SeSp) was assessed in Se-deficient rats by measuring tissue Se accumulation and glutathione peroxidase (GSH-Px) activity. For 42 d, rats were subjected to dietary Se depletion by consumption of a Torula yeast (TY)-based diet with no Se; controls were fed the same diet supplemented with 75 microg Se/kg diet as sodium selenite. Se-deficient rats were then repleted with Se (75 microg/kg) by the addition of sodium selenite, selenomethionine (SeMet) or SeSp to the TY basal diet. Selenium speciation in SeSp emphasized the quasi-absence of selenite (2% of total Se); organic Se comprised SeMet (approximately 18%), with the majority present in the form of two selenoproteins (20-30 kDa and 80 kDa). Gross absorption of Se from SeSp was significantly lower than from free SeMet and sodium selenite. SeMet was less effective than sodium selenite in restoring Se concentration in the liver but not in kidney. SeSp was always much less effective. Similarly, Se from SeSp was less effective than the other forms of Se in restoring GSH-Px activity, except in plasma and red blood cells where no differences were noted among the three sources. This was confirmed by measuring the bioavailability of Se by slope-ratio analysis using selenite as the reference form of Se. Although Se from SeSp did not replenish Se concentration and GSH-Px activity in most tissues to the same degree as the other forms of Se, we conclude that it is biologically useful and differently metabolized due to its chemical form.

Garlic and garlic-derived compounds inhibit human squalene monooxygenase

Although extracts of garlic inhibit cholesterol biosynthesis in cultured hepatocytes, the inhibitory components of garlic and the site or sites of inhibition in the cholesterol biosynthetic pathway have not been established. To elucidate potential mechanisms of inhibition, we examined the effect of fresh garlic extract and 16 water- or lipid-soluble compounds derived from garlic on purified recombinant human squalene monooxygenase. Squalene monooxygenase catalyzes the second and likely rate-limiting step in the downstream pathway for cholesterol biosynthesis. A 50% inhibitory concentration (IC(50)) of squalene epoxidation was achieved with 1 g/L of fresh garlic extract; of the 16 garlic compounds tested, only selenocystine (IC(50) = 65 micromol/L), S-allylcysteine (IC(50) = 110 micromol/L), alliin (IC(50) = 120 micromol/L), diallyl trisulfide (IC(50) = 195 micromol/L), and diallyl disulfide (IC(50) = 400 micromol/L) substantially inhibited the enzyme. Kinetic analysis showed that the inhibition by garlic and by these compounds was slow and irreversible, suggestive of covalent binding to the enzyme; the ability of thiol-containing compounds such as glutathione and 2,3-dimercaptopropanol to prevent and reverse the inhibition indicated that the garlic compounds were reacting with sulfhydryl groups on the protein. Dithiols were better reversal agents than monothiols, further suggesting that these inhibitors bind to the proposed vicinal sulfhydryls present on this enzyme. These results indicate that squalene monooxygenase may be one of the target enzymes through which garlic inhibits cholesterol biosynthesis.

Organosulfur compounds from Allium and the chemoprevention of cancer

Allium vegetables and their associated organosulfur constituents are extensively studied for their chemopreventive potential against cancer. This article overviews their anticarcinogenic and antigenotoxic properties. Epidemiological studies (mostly case-control studies) provide strong evidence that Allium vegetable consumption reduces the incidence of gastric and colon cancer while the association between Allium vegetable consumption and other cancers is less convincing. Furthermore, many experimental studies have demonstrated that organosulfur compounds and Allium extracts have inhibitory effects on carcinogenesis in animals. These inhibitory effects are supported by many diverse mechanisms, including inhibition of carcinogen formation, modulation of carcinogen metabolism, inhibition of mutagenesis and genotoxicity, inhibition of cell proliferation and increase of apoptosis, inhibition of angiogenesis, and immune system enhancement. Before such constituents or extracts can be used in chemopreventive trials, it is important to verify their lack of toxicity and to investigate further their precise mechanisms of action throughout the whole process of carcinogenesis.

Selenium from high selenium broccoli protects rats from colon cancer

Colon cancer is the third most common newly diagnosed cancer in the United States and the third most common cause of cancer-related deaths. Previous supplementation studies have demonstrated the efficacy of selenium (Se) for prevention of colon cancer in humans. The metabolism of Se depends on its chemical form, and studies have shown that the chemical form of Se in broccoli does not accumulate in the body as fast as other forms of Se and may be especially beneficial for prevention of cancer. In the first experiment of the present study, Fisher F-344 rats (n = 45) were allotted randomly to torula yeast-based diets supplemented with the following: 1) no Se; 2) 0.1 microg Se/g diet as selenate; 3) 1.0 microg Se/g diet as selenate; 4) 0.1 microg Se/g diet as selenized broccoli (Se concentration of approximately 500 microg/g); or 5) 1.0 microg Se/g diet as selenized broccoli. In Experiment 2, rats (n = 80) were allotted randomly to the same basal diet supplemented with the following: 1) no added Se; 2) 2.0 microg Se/g diet as selenite; 3) 2. 0 microg Se/g diet as selenite + low Se broccoli; and 4) 2.0 microg Se/g diet as selenized broccoli. Rats were fed the diets for 2 wk and injected with a chemical carcinogen (3,2 dimethyl 4-amino biphenyl or dimethyl-hydrazine in Experiment 1 or dimethyl hydrazine in Experiment 2; 2 rats/treatment were used as vehicle controls). Supranutritional amounts of Se supplied as high Se broccoli significantly decreased (P: < 0.05) the incidence of aberrant crypts (AC) and aberrant crypt foci (ACF; preneoplastic lesions indicative of colon cancer) compared with other dietary treatments. Diets were controlled for the presence or absence of broccoli and for the total amount of Se. The reduction in AC and ACF was a function of Se in high Se broccoli and not a result of broccoli alone or Se alone. Adequate dietary Se supplied as high Se broccoli did not accumulate in tissues or increase glutathione peroxidase activity as well as other forms and amounts of Se. Thus, Se from high Se broccoli may be metabolized in a manner that diverts much of the Se into a pool that provides protection against colon cancer.

Chemoprevention of DMBA-Induced Mammary Tumors in Rats by a Combined Regimen of Alpha-Tocopherol, Selenium, and Ascorbic Acid

This experimental study was designed to evaluate the efficacy of associated naturally occuring antioxidants in the prevention of chemically induced breast cancer using DMBA in virgin female Wistar rats. Rats were randomly allocated to three groups: control group (CG; n = 20), induction group (IG; n = 100), and prevention group (PG; n = 70). A single dose (65 mg/kg) of DMBA was administered in the IG and PG animals at 50 days of age. PG animals also received a single dose of alpha-tocopherol (200 mg/rat) 1 hour after DMBA administration and an association of selenium (p-XSC, 40 ppm/day/rat) and ascorbic acid (540 mg/day/rat) in drinking water, daily, from carcinogenic induction until necropsy. Macroscopic study and pathology revealed a significantly lower development of neoplasms in the PG animals (p < 0.05); the number of rats with mammary tumors, breast cancer incidence, and the number of malignant breast tumors per rat as well as per tumor-bearing rat were significantly decreased in the PG animals. Other types of primary neoplasms existing in the IG animals totally disappeared in the PG animals. Immunostaining to hormone steroid receptors (ER and PR) and cathepsin D was similar in both groups. Overexpression of p53 and metallothioneine was significantly increased in the PG animals (p < 0.05) and immunostaining to bromodeoxiuridin and Ki-67 was also stronger in the remaining tumors in the PG animals. These data thus add to the accumulating evidence that those micronutrients in combination seem to be effective in reducing the incidence of malignant tumors. Nevertheless, remaining tumors seem to present more aggressive behavior and characteristics of drug resistance.

Selenium-induced inhibition of angiogenesis in mammary cancer at chemopreventive levels of intake

The trace element nutrient selenium (Se) has been shown to possess cancer-preventive activity in both animal models and humans, but the mechanisms by which this occurs remain to be elucidated. Because angiogenesis is obligatory for the genesis and growth of solid cancers, we investigated, in the study presented here, the hypothesis that Se may exert its cancer-preventive activity, at least in part, by inhibiting cancer-associated angiogenesis. The effects of chemopreventive levels of Se on the intra-tumoral microvessel density and the expression of vascular endothelial growth factor in 1-methyl-1-nitrosourea-induced rat mammary carcinomas and on the proliferation and survival and matrix metalloproteinase activity of human umbilical vein endothelial cells in vitro were examined. Increased Se intake as Se-enriched garlic, sodium selenite, or Se-methylselenocysteine led to a significant reduction of intra-tumoral microvessel density in mammary carcinomas, irrespective of the manner by which Se was provided: continuous exposure (7-wk feeding) with a chemoprevention protocol or acute bolus exposure (3 d) after carcinomas had established. Compared with the untreated controls, significantly lower levels of vascular endothelial growth factor expression were observed in a sizeable proportion of the Se-treated carcinomas. In contrast to the mammary carcinomas, the microvessel density of the uninvolved mammary glands was not altered by Se treatment. In cell culture, direct exposure of human umbilical vein endothelial cells to Se induced cell death predominantly through apoptosis, decreased the gelatinolytic activities of matrix metalloproteinase-2, or both. These results indicate a potential for Se metabolites to inhibit key attributes (proliferation, survival, and matrix degradation) of endothelial cells critical for angiogenic sprouting. Therefore, inhibition of angiogenesis associated with cancer may be a novel mechanism for the anticancer activity of Se in vivo, and multiple mechanisms are probably involved in mediating the anti-angiogenic activity.

Effects of methylselenocysteine on PKC activity, cdk2 phosphorylation and gadd gene expression in synchronized mouse mammary epithelial tumor cells

Methylselenocysteine (MSC), an organic selenium compound is an effective chemopreventive agent against mammary cell growth both in vivo and in vitro but its mechanism of action is still not understood. We have previously demonstrated that MSC is able to inhibit growth in a synchronized TM6 mouse mammary epithelial tumor cell line at 16 h time point followed by apoptosis at 48 h. The decrease in cdk2 kinase activity was coincident with prolonged arrest of cells in S-phase. The present set of experiments showed that cdk2 phosphorylation was reduced by 72% in the MSC-treated cells at 16 h time point. Expression for gadd34, 45 and 153 was elevated 2.5 to 7 fold following MSC treatment only after 16 h time point. In order to investigate a possible upstream target for MSC, we analyzed protein kinase C (PKC) in this model. Total PKC activity was reduced in TM6 cells by MSC (50 microM) within 30 min of treatment, both in cytosolic (55.4 and 77.6%) and membrane (35.2 and 34.1%) fractions for calcium-dependent and independent PKCs, respectively. PMA significantly elevated the PKC activity in membrane fraction (P < 0.01) and MSC inhibited this activation by more than 57%. The effect of MSC was selenium specific as selenomethionine and sulfurmethyl-L-cysteine (SMC) did not alter PKC activity either in cytosolic or membrane fraction. Immunoblot analysis showed that PKC-alpha was translocated to the membrane by PMA and MSC did not alter this translocation. PKC-delta was faintly detectable in membrane fractions of control and MSC-treated cells. MSC treatment slightly reduced levels of PKC-e (in cytosolic and membrane fractions) and PKC-zeta (cytosolic fractions). The data presented herein suggest that PKC is a potential upstream target for MSC that may trigger one or all of the downstream effects; i.e. the decrease of cdk2 kinase activity, decreased DNA synthesis, elevation of gadd gene expression and finally apoptosis.

Dietary selenium reduces the formation of aberrant crypts in rats administered 3,2'-dimethyl-4-aminobiphenyl

Human epidemiologic studies suggest that low selenium status is associated with increased cancer risk and that selenium supplementation is associated with reduction in the incidence of several cancers, including colorectal cancer. Aromatic and heterocyclic amine carcinogens are thought to be important in the etiology of human colorectal cancer, but no information is available on the effects of selenium on aromatic amine-induced colon cancer. In order to investigate this effect, aberrant crypt foci (ACF), the putative preneoplastic lesions of colon cancer in humans and rodents, were used as a biomarker to test the hypothesis that selenium supplementation can reduce aromatic amine-induced colon carcinogenesis. Male weanling F344 inbred rats were fed a basal torula yeast selenium-deficient diet supplemented with 0, 0.1, or 2. 0 mg selenium/kg diet as selenite, selenate, or selenomethionine (SeMet). Animals were fed the diets for 4 weeks and then administered 1 sc injection/week for 2 weeks of 3, 2'-dimethyl-4-aminobiphenyl (DMABP; 100 mg/kg) or vehicle (peanut oil). At 12 weeks, the rats were euthanized and the colon and rectum were removed, opened longitudinally, and fixed in 70% ethanol. Glutathione peroxidase activities in erythrocytes and liver cytosol and selenium concentrations in the colon/rectum and kidney increased significantly (p < 0.05) and in a dose-dependent manner with each of the three selenium diets. No ACF were identified in vehicle-treated rats. In DMABP-treated rats, ACF frequencies decreased significantly (p < 0.05) in groups supplemented with 0.1 or 2.0 mg selenium/kg diet as selenite and selenate but not SeMet. There were no significant differences in ACF and aberrant crypts between rats fed 0.1 vs 2.0 mg selenium/kg diet. These results suggest that dietary selenium, depending on chemical form, can reduce aromatic amine-induced colon carcinogenesis.

Selenium-enriched Agaricus bisporus mushrooms suppress 7,12-dimethlybenz[a]anthracene bioactivation in mammary tissue

The present studies compared dietary Se (1.0 microg/g) when provided as either fortified Agaricus bisporus mushrooms, or sodium selenite on the in vivo metabolism of 7,12-dimethylbenz(a)anthracene (DMBA). Dietary addition of Se unenriched A. bisporus mushrooms at 2% did not alter the occurrence of DMBA induced DNA adducts or the activity of glutathione S-transferase (GST). However, the addition of Se as enriched mushrooms, or as selenite, significantly increased both liver and mammary GST activity. Providing sodium selenite, or enriched mushrooms also significantly reduced total and anti-3,4-dihydrodiol-1,2-epoxide-deoxyguanosine adducts compared to feeding the basal diet (P < 0.05). These investigations provide evidence that Se enriched mushrooms can be used as an effective method to retard chemically induced tumors.

Lessons from basic research in selenium and cancer prevention

The article reviews the progress in basic research of selenium and cancer prevention during the past decade. Special emphasis is placed on the following four major areas of discussion: 1) chemical forms of selenium and anticarcinogenic activity; 2) selenium-enriched food; 3) in vitro effects of selenite vs. monomethylated selenium; and 4) aromatic selenium compounds. It is clear that basic research has contributed new knowledge to our understanding of selenium biochemistry, anticancer efficacy and regulation of cell growth. Some of this information could be ready for incorporation into the design of a second-generation selenium trial in humans.

Modulation of phase I and phase II xenobiotic-metabolizing enzymes by selenium-enriched garlic in rats

Previous research showed that treatment with selenium-enriched garlic (Se-garlic) was able to inhibit the initiation phase of mammary carcinogenesis in the dimethyl-benz[a]anthracene (DMBA) model in rats. The present study was designed to investigate the following parameters: 1) DMBA-DNA adduct formation in liver and mammary gland, 2) urinary excretion of DMBA metabolites, 3) phase I and phase II xenobiotic-metabolizing enzymes, and 4) tissue selenium levels as a function of Se-garlic supplementation. Prior feeding with an Se-garlic-containing diet (at 3 ppm Se) for two weeks resulted in a consistent reduction of all DMBA adducts in liver and mammary gland. This was accompanied by a 40% increase in urinary excretion of DMBA metabolites over a two-day period. Several liver P-450 enzymes were examined in rats fed a diet supplemented with 1, 2, or 3 ppm Se. Compared with controls receiving 0.1 ppm Se, no significant alteration in activity was detected with respect to P-450 1A1 (responsible for DMBA activation), 1A2, 2B1, 2E1, and 3A4. In contrast, glutathione S-transferase and uridine 5'-diphosphate-glucuronyltransferase activities were elevated to a maximum of 2- to 2.5-fold in liver and kidney. As expected, there was a dose-dependent elevation of selenium concentrations in liver, kidney, mammary gland, and plasma as a function of the level of Se-garlic supplementation. Our data seem to suggest that an increased detoxification of carcinogen via the phase II conjugating enzymes might represent a mechanism of tumor suppression by Se-garlic.