Selenium in Nutrition and Toxicity

1. Selenium in Nutrition and Toxicity Dr. Karin Hotz Jeannine Baumgartner ETH Laboratory of Human Nutrition Institute of Food, Nutrition and Health

2. Overview … deficiency and overload … dietary reference intakes and sources … species: inorganic and organic compounds … absorption, metabolism & excretion … and its functions in selenoproteins … importance to human health Picture: www.buzzle.com

3. Selenium as an essential trace element:symptoms of deficiency and overload • Selenium deficiency • Impaired muscle function • Keshan disease (Se as causative cofactor) • Kashin-Beck disease (Se as causative cofactor) • Selenium overload • Acute overdosing: • garlic odor on the breath • nausea • stomach ache • liver damage • chronic overdosing: • hair loss • brittle fingernails • neurological damage cardio-myopathy deformingarthritis pictures: www.wikipedia.org

4. Dietary Reference Intakes • Recommended daily intakes (SGE) children 25-60 μg/d adolescents 30-70 μg/d adults 30-70 μg/d (U.S. RDA: 55 μg/d) • Safe upper limit (SGE) adults 400 μg/d (symptoms of chronic overload above 800 μg/d) narrow window between ingested amounts resulting in deficiency and those resulting in toxicity!

5. Dietary Reference Intakes and Sources • Recommended daily intake of 30 μg found in: lobster tuna mussels kidney (calf) codfish oysters liver (cow) egg pasta pork meat eggs whole grain bread SGE

6. Dietary Reference Intakes and Sources Soil concentration, dietary intake and serum levels recommended intake 30-70 μg/d n.d. n.d. n.d. n.d. n.d. n.d. Dumont E. (2006)

7. Dietary Reference Intakes and Sources Soil concentration, dietary intake and serum levels Upper safe limit 400 μg/l Dumont E. (2006)

8. Selenium metabolism in plants Se2- accumulated in certain plants (CH3)-Se-Cys (CH3)-Se-Met phytovolatilization (detoxification) (CH3)2Se Dumont E. (2006)

9. Selenium metabolism in plants Se-accumulating plants normal: < 100 μg/g Se (dry weight) Se-accumulators: > 1000 μg/g Se (dry weight) common mushroom(agaricus bisporus) garlic(allium sativum) broccoli (brassica oleracea) Dumont E. (2006) pictures: www.wikipedia.org

10. Se species: inorganic and organic Se compounds Se2- selenide (Se-II) H selenoproteins (Se-Cys in active site) selenocysteine selenite (SeIV) methyl-selenide S Me-selenocysteine Se-containing proteins (e.g. Met replaced by Se-Met, no specific function for Se) selenate (SeVI) dimethyl-selenide selenomethionine Dumont E. (2006)

11. Se species: inorganic and organic Se compounds • Bioavailability • organic selenium species have a higher bioavailability than inorganic species picture: www.magen-darm-grippe.de Targeting depending on the species, Se is specifically targeted to functional compartments or unspecifically distributed between tissues picture: www.monetized.co.za

12. Selenium metabolism in humans A) low Se intake human body SeO32- SeO32- Seleno-proteins GSH (glutathione) SeO42- GSH H2Se (Se-Cys in active site) -lyase Se-Cys Se-Cys methyl-transferase Se-Met demethylase Se-Met g-lyase CH3SeH Yanghorbani 1999 Ip 1998 Brozmanova 2010 urine Wolffram 1989

13. Selenium metabolism in humans B) high Se intake human body SeO32- SeO32- Seleno-proteins GSH (glutathione) SeO42- GSH H2Se (Se-Cys in active site) -lyase Se-Cys Se-Cys Se-sugars methyl-transferase Se-Met demethylase Se-Met g-lyase CH3SeH Se-Met-containingproteins (Se stores) (Se-Met incorporated instead of Met) (CH3)2Se (CH3)3Se+ Yanghorbani 1999 Ip 1998 Brozmanova 2010 breath urine Wolffram 1989

14. Physiological Function of Selenoproteins (Examples) R-O-H selenoprotein P glutathione peroxidases (GPx) antioxidant, probably involved in Se-homeostasis antioxidants R-O-O-H selenoprotein W T4 iodothyroninedeiodinases needed for muscle function T3 rT3 DNA-bound spermatic selenoprotein may protect developing sperm 3,3’T2 mitochondrial capsule selenoprotein oxidized ascorbate NADPH NADP+ protects developing sperm from ox. damage, polymerizes into structural protein required for stability/motility of mature sperm thioredoxinreductases reducedascorbate reducedthioredoxin cell growth and survival oxidizedthioredoxin selenophosphatesynthetase required for selenoprotein synthesis Rayman 2000

15. The importance of selenium to human health Immune function Viral infection Thyroid function Cardiovascular disease Cancer and chemoprevention

16. Role of selenium in immune function Se Se Immune system Innate “nonadaptive” immunity Acquired ”adaptive” immunity humoral cellular humoral cellular Cytokines - Interferon - Interleukins (eg. IL-3, IL-2, etc) Lymphocytes - T-cell - B-cell Antibodies - Immunoglobulin (eg. IgG, IgA, etc) Macrophages Granulocytes - Neutrophils - Basophils - Eosinophils Natural-Killer (NK) cells Figure: Modified from http://www.ndt-educational.org/muellerslide2006txt.asp • Stimulation of vaccine-induced immunity • More antibody-producing B-cells • Macrophage and neutrophil activity • Involvement of Se in immune system: • Increased activity of NK cells • Proliferation of T-cells • Increased interferon production • Increased interleukin receptors Tinggi, 2007

17. ….proposed mechanisms • Selenoproteins influence three broad areas of cell function an therefore affect immune function through: • Antioxidant activities • Thyroid hormone metabolism • Regulation of the activity of redox-active proteins • Example: Neutrophil function • Neutrophils produce peroxi-derived radicals to kill invading microbes, but also the neutrophils themselves need to be protected from the radicals by the system. • Radical production and protection depends on GPx activity in neutrophils, which is impaired in Se-deficiency. Arthur, 2003

18. Role of selenium in viral infection • In Se-deficient mice harmless viruses can become virulent. ( e.g. coxsackie virus causing cardiomyopathy in Keshan disease) • Subjects supplemented with Se showed less mutations in poliovirus compared to placebo group. • HIV patients are nearly 20 times more likely to die from HIV-related causes than those with adequate levels. And low Se levels in HIV-infected children are related to faster disease progression. Infection with virus Se + Se - GPx Adequate GPx deficient ↑ Oxidative stress ↑↑↑ Oxidative stress Normal immune response Altered immune response Viral mutation Mild to no tissue damage Moderate to severe tissue damage Figure: Beck, 2001 Rayman, 2000

19. Role of selenium in thyroid function • Deiodinasesare Se-containing enzymes playing important roles in thyroid hormone metabolism. • Low plasma T3:T4 ratios found in people with low Se intake. • A combined deficiency of iodine and selenium is associated with severe endemic myxedematouscretinism (in Democratic Republic of Congo). • Se supplementation in elderly subjects decreased plasma thyroxine (T4) concentrations , increased deiodinase activity and improved conversion of the active hormone T3. • Se Impaired thyroid hormone metabolism has adverse effects on immune function. Figure:  Myxedematous endemic cretinism in the Democratic Republic of Congo Tinggi, 2008; Rayman, 2000

20. Role of selenium in cardiovascular disease (CVD) Increased production of ROS can cause oxidative stress and cause damage to cellular lipids, proteins and DNA, leading to CVD. • Selenoproteins may help to combat oxidative modification of lipids and to reduce platelet aggregation. • Epidemiological studies produced mixed findings (examples): • 2 to 3-fold increase in CV morbidity and mortality in subjects with serum Se conc. below 45 µg/L compared to subjects above at baseline. • Increased risk of ischaemic heart disease in Danish men with serum Se below 79 μg/L. • Inverse association between toenail Se and risk of myocaridal infarction only in areas with the lowest Se status. Thus, effect maybe only apparent in populations of low Se status. • Ebselen, a synthetic mimic of glutathione reductaseshowed cardio- protective effects in mice. Tinggi, 2008, Rayman 2000

21. Role of selenium in chemoprevention Possible mechanisms of cancer prevention by selenium Mechanisms of Se From homepage of German Cancer Research Center

22. …history of selenium in chemoprevention • In 1969 Shamberger found that his cancer patients had selenium blood levels only 60 to 80 % as high as non-cancer patients. • In 1977, Schrauzer reported that selenium intake in 27 countries was inversely correlated with breast cancer death rates. • Several human epidemiological studies found a statistically significant inverse relationship between Se level and risk of cancer overall, particularly in men. • Also early animal studies have shown that selenium added to the diet reduced cancer incidence. Tinggi, 2008, Rayman 2000

23. Se in chemoprevention • In the first RCT, supplementation with selenized yeast (200 µg/day), predominantly in the form of selenomethionine (Se-Met) for 4 years, led to a reduction of nearly 50% in overall cancer morbidity (Clark, 1996). • Average selenium intake of the study subjects 90 µg/day, well above levels needed for optimal selenoprotein activity. • This suggests additional chemopreventive mechanisms. • Inorganic forms, such as selenite/selenate, were more effective than commonly used organic form Se-Met in fighting cancer. • Because cells can not distinguish Se-Met from essential amino acid methionine, it is incorporated into general body proteins. • However, inorganic Se is more toxic compared to organic Se forms. Brozmanova, 2010, Rayman 2000, Ip, 1998

24. Methylselenocystein (Met-Se-Cys): Less toxic alternative • Is formed naturally in various plants grown on high selenium soil. • Met-Se-Cys rich foods have shown good anticancer activity, without • excess tissue accumulation or toxicity. • Met-Se-Cys is converted into methylselenol (CH3SeH), which has been shown to be an active anticancer form of Se. • Mechanism of action: Apoptosis and inhibition of angiogenesis. toxic Monomethylated Se compounds, SeMC and methylselenic acid (MSA), were shown to be more effective than other Se compounds in chemoprevention. And it is less toxic! methyl-transferase β-lyase SeMC H2Se CH3SeH MSA GSH demethylase (CH3)2Se MSA breath urine Brozmanova, 2010, Rayman 2000, Ip, 1998

25. Selenium in human health – A double-edged sword • Several organic and inorganic Se compounds have been investigated as Se supplements. Their safety and efficacy differ markedly because of their differential metabolic processing by the body. • The inorganic Se compound, sodium selenite, due to its prooxidant character, represents a promising alternative for cancer therapy. • However, this Se compound is highly toxic compared to organic Se forms. • To achieve chemopreventive effects, Se had to be given at Se levels 5-10 fold higher than what would be required for normal nutritional need. • Due to a broad interest in the positive effects of Se on human health and cancer therapy, studies investigating the negative effects such as toxicity and DNA damage are highly required.

26. Literature • Rayman M.P. (2000); The importance of selenium to human health. Lancet 356: 233-41 • Dumont E., Vanhaecke F., Cornelis R. (2006); Selenium speciation from food source to metabolites: a critical review. Anal. Bioanal. Chem. 385: 1304-1323 • www.sge-ssn.ch (homepage of SGE, “Schweizerische Gesellschaft für Ernährung”) • Brozmanova J., Manikova D., Vlckova V., Chovanec M. (2010); Selenium: a double-edged sword for defense and offence in cancer. Arch. Toxicol. 84: 919-938 • Ip, C. (1998); Lessons from Basic Research in Selenium and Cancer Prevention. J. Nutr. 128(11): 1845-1854 • Janghorbani M., Xia Y., et. al. (1999); Metabolism of Selenite in Men with Widely Varying Selenium Status. J. Am. Coll. Nutr. 18(5), 462-469 • Tinngi U (2008); Selenium: its role as antioxidant in human health. Environ Health Prev Med. 13:102-108 • Arthur J.R, McKenzie RC, Beckett J (2003); Selenium in the Immune System. J. Nutr 133: 1457S-1459S • Beck MA (2001); Antioxidants and Viral Infections: Host Immune Response and Viral Pathogenicity. J. Am College of Nutrition. 20(5), 384S-388S • http://www.dkfz.de/en/tox/c010-2_projects/list_assays.html (homepage of German Cancer research Center)