What is selenium?
Selenium is an essential trace element that is contained in various proteins and has both nutritional and potentially toxic properties. Selenium mainly exists as selenocysteine (Sec) and selenomethionine amino acids. These selenium containing residues are transported around the body in special selenoproteins.
In humans, selenium is important as several disorders are associated with its deficiency. Disorders such as Keshan disease, Kashin-Beak disease, and myxedematous endemic cretinism. Humans also use selenium in the synthesis of glutathione peroxidases (GPx). Research has also shown that selenium is an essential trace element in all mammals; which was exemplified by disrupting several selenoprotein genes in mice embryos – those encoding for thioredoxin reductase (TR)1, TR3, and GPx4 – which proved to be lethal.
Both prokaryotes and eukaryotes have been analyzed for their containment and usage of selenoproteins over several studies. These studies concluded that both domains have a sporadic expression of selenium with certain fungi, vascular plants, and insects utilizing no selenium at all. This sporadic expression was further demonstrated by research that showed mammals contain an average of 24 selenoproteins but simpler eukaryotes and prokaryotes contain far fewer selenoproteins, with Plasmodium containing only 4 and Escherichia coli containing only 3.
It is clear that there is some evolutionary contribution to the increased amount of selenoproteins in complex mammals. These observations have outlined selenium genetics as a useful model system for evolution which aims to address the evolutionary changes in the use of selenium by different organisms.
Many studies have used selenoprotein P (SelP) as a marker for selenium utilization in humans. SelP could be a more accurate marker than the previously used GPx3. It has been demonstrated that evolutionary changes in SelP exhibit numerous Sec residues of the protein being replace by cysteine (Cys), effectively decreasing the amount of selenium in the protein. However, fish show a higher Sec content in their SelP than mammals and also exhibit no replacement of Sec by Cys nor any loss of selenoproteins. This clearly shows that the evolution of humans from our fish-like ancestors carried a decrease in the utilization of Sec in selenoproteins. This reduced reliance on selenium raises suspicion on the widely accepted importance of selenium within our diets.
Can selenium prevent cancer?
Selenium is mainly gained through the consumption of food. As previously discussed, small amounts are essential, however, higher amounts can pose as a toxicity risk. This gives selenium a narrow window of safe exposure.
Many studies performed in the 1960s showed that individuals with higher selenium levels in their diets had a lower risk of developing cancer and that selenium can even inhibit the growth of cancer cells. This revelation led to a drastically increase of selenium supplements being used for cancer prevention. Following this time period, studies have become more accurate, therefore further enhancing the understanding of the relationship between selenium and cancer risk.
Foods containing selenium. Image Credit: Bitt24 / Shutterstock
More recent studies have aimed to assess if selenium could prevent prostate cancer. The conclusion of these studied showed that selenium has no or little effect at reducing the risk of cancer. However, as these were observational studies, they may have misclassified the selenium exposure of the participants which limits their validity. Clearly, more research is needed to determine if selenium can reduce the risk of cancer in certain individuals.
Selenium for medical discovery and drug delivery
Selenium research has discovered pleiotropic effects on therapy and drug delivery. Selenium has been proven to aid in the following processes:
- Endocrine regulation
- Enhancement of the immune system and host defense
- Combating tumors
- Effecting hair and skeletal systems
- Improving cognitive ability
- Reducing mental and psychiatric stress
- Improving fertility
Selenium nanoparticles (SeNPs) are a Nano form of selenium that allows selenium to be applied to nanomedicine and nanotechnology techniques. This is advantageous over free selenium as it allows for more diverse and accurate applications of selenium research. SeNPs possess many anticancer mechanisms including inhibiting DNA mutation, inducing cancer cells apoptosis, and limiting tumor invasion & metastasis.
SeNPs can reduce hyperglycemia and hyperlipidemia in diabetic patients by mimicking an insulin response. The surface Properties of SeNPs allow them to be used as customizable drug delivery carriers.