Selenium and cancer prevention: observations and complexity

Relationship between Serum Levels of Selenium and Thyroid Cancer: A Systematic Review and Meta-Analysis

Thyroid cancer is one of the most malignant tumors and a serious threat to human health. Selenium (Se) is an essential trace element that is critical for thyroid function. Since the relationship between Se and thyroid cancer remains unclear, a meta-analysis was performed to clarify the relationship. A total of five databases (PubMed, Web of Science, Scopus, Embase and Cochrane library) were searched for case-control studies and cohort studies on serum levels of Se and thyroid cancer published up to 13 July 2022. Seven articles consisting of 10 case-control studies and comprised of 2,205 subjects met the inclusion criteria for meta-analysis. From the 10 selected studies, pooled analysis indicated that thyroid cancer patients had lower serum levels of Se than healthy controls [standardized mean difference = -1.25, 95% confidence interval = (-2.07, -0.44), P = 0.003]. Our meta-analysis supports a significant relationship between serum levels of Se and thyroid cancer.

Trace elements and the thyroid

Trace elements, such as iodine and selenium (Se), are vital to human health and play an essential role in metabolism. They are also important to thyroid metabolism and function, and correlate with thyroid autoimmunity and tumors. Other minerals such as iron (Ir), lithium (Li), copper (Co), zinc (Zn), manganese (Mn), magnesium (Mg), cadmium (Cd), and molybdenum (Mo), may related to thyroid function and disease. Normal thyroid function depends on a variety of trace elements for thyroid hormone synthesis and metabolism. These trace elements interact with each other and are in a dynamic balance. However, this balance may be disturbed by the excess or deficiency of one or more elements, leading to abnormal thyroid function and the promotion of autoimmune thyroid diseases and thyroid tumors.The relationship between trace elements and thyroid disorders is still unclear, and further research is needed to clarify this issue and improve our understanding of how trace elements mediate thyroid function and metabolism. This paper systematically reviewed recently published literature on the relationship between various trace elements and thyroid function to provide a preliminary theoretical basis for future research.

Wpływ suplementacji diety selenem na przebieg autoimmunologicznego zapalenia tarczycy – przegląd badań klinicznych przeprowadzonych w populacji europejskiej

Wiele obserwacji wskazuje na zależność między zmniejszonymi zasobami selenowymi organizmu a różnymi schorzeniami tarczycy, w tym także autoimmunologicznym zapaleniem tarczycy (AZT). W większości prac poświęconych temu zagadnieniu zmiana stężenia przeciwciał anty-TPO była główną miarą oceny skuteczności suplementacji selenem (Se) diety pacjentów z AZT. Stężenie przeciwciał anty-TPO ma wpływ na intensywność nacieków limfocytarnych w gruczole tarczowym i stopień jego uszkodzenia, dlatego zmniejszenie ich stężenia w wyniku zwiększonego spożycia Se może się przyczynić do skuteczniejszego leczenia AZT. W części prac omówionych w artykule ewaluacja immunoregulacyjnego działania Se w przebiegu AZT została rozpatrzona w odniesieniu do szerszego zakresu parametrów biochemicznych i immunologicznych (głównie dotyczy to zmian w wytwarzaniu cytokin i chemokin). Wielu autorów potwierdziło wpływ Se na zmniejszanie stężenia przeciwciał anty-TPO, jednak wyniki wszystkich prac nie są jednoznaczne. Przyczyną rozbieżności może być odmienny stopień wysycenia tarczycy Se i jodem w różnych grupach pacjentów, różny stopień zaawansowania choroby, różne dawki Se podawanego pacjentom, stosowanie lub niestosowanie jednoczesnej terapii L-tyroksyną oraz polimorfizmy pojedynczych nukleotydów (SNP) występujące w genach kodujących poszczególne selenobiałka.

Na podstawie dostępnej literatury można wnioskować, iż zagadnienie dotyczące roli Se w AZT jest nadal słabo poznane. Istnieje potrzeba kontynuowania badań oceniających wpływ suplementacji diety Se na przebieg tej choroby, które powinny obejmować poza anty-TPO inne parametry kliniczne. Dopiero wyniki tak szeroko ujętych badań mogą być pomocne w formułowaniu nowych wytycznych dotyczących wspomagania leczenia przez odpowiednią modyfikację diety z uwzględnieniem suplementacji Se.

Anti-inflammatory and Antioxidant Effects of Selenium on Orbital Fibroblasts of Patients With Graves Ophthalmopathy

PURPOSE

In the present study, the authors investigated the effects of selenium on inflammation, hyaluronan production, and oxidative stress in primary cultured orbital fibroblasts of patients with Graves ophthalmopathy (GO).

METHODS

Orbital adipose/connective tissue specimens were obtained during the course of orbital surgery for patients with GO (n = 7) and other noninflammatory problems (n = 5). After incubation with various concentrations of sodium selenite for 48 hours, supernatants from primary cultures were collected. Hyaluronan and cytokine levels were measured using commercially available enzyme-linked immunosorbent assay kits. To determine the effect of selenium on reactive oxygen species (ROS) production stimulated by H2O2 (100 μM) for 30 minutes, the cells were pretreated with various concentrations of sodium selenite for 60 minutes.

RESULTS

Interleukin (IL)-6 and tumor necrosis factor-alpha levels were significantly higher in orbital fibroblasts of patients with GO than in orbital fibroblasts of control patients. Hyaluronan production was suppressed by selenium in cultured orbital fibroblasts of patients with GO. Inflammatory cytokines such as IL-1α, IL-8, and tumor necrosis factor-alpha were suppressed by selenium in cultured orbital fibroblasts of patients with GO. IL-1β and IL-6 were not suppressed by selenium in cultured orbital fibroblasts of patients with GO. Selenium pretreatment reduced intracellular ROS generation stimulated by H2O2 in cultured orbital fibroblasts of patients with GO.

CONCLUSIONS

In conclusion, hyaluronan production, inflammatory cytokines, and intracellular ROS generation were suppressed by selenium in cultured orbital fibroblasts of patients with GO. Several inflammatory cytokines may be suppressed by selenium in cultured orbital fibroblasts of patients with GO. This study provide the basis for use of selenium in the treatment of GO.

Selenium and thyroid cancer: a systematic review

The aim of the study was to investigate the association between blood and tissue levels of selenium and thyroid cancer through a systematic review. We searched for observational studies written in English, Spanish, and Portuguese indexed in PubMed, LILACS, and Scielo without date restriction, that evaluated the association between selenium levels in whole-blood, serum, or plasma and/or thyroid tissue and thyroid cancer, both in individuals with cancer of thyroid as in healthy individuals. Then data were extracted and analyzed. Of the 570 articles identified, five cross-sectional studies were included in the review. In one study, lower concentrations of selenium were found in whole-blood (0.543 μg/ml) and in the thyroid (0.88 μg/g) of thyroid cancer patients compared to controls. Another study showed a decrease in serum selenium concentrations in patients with follicular carcinoma and papillary types (0.077 ± 0.021 μg/ml and 0.080 ± 0.020 μg/ml, respectively). On the other hand, other studies showed no difference in plasma selenium content or glutathione peroxidase activity among patients and healthy volunteers. The available evidence on this issue is inconclusive. Additional studies are needed to elucidate the association between serum and/or tissue levels of selenium and the development of thyroid cancer.

Design of alveolate Se-inserted TiO2 and its effect on osteosarcoma cells and osteoblasts

The experiments in vitro/vivo evidence that an alveolate TiO₂@Se nano-grid with controllable diameter shows good anti-tumor properties and biocompatibility.

Selenium and the thyroid

PURPOSE OF REVIEW

This article provides an update on the role of the essential trace element selenium and its interaction with the other trace elements iodine and iron that together contribute to adequate thyroid hormone status. Synthesis, secretion, metabolism and action of thyroid hormone in target tissues depend on a balanced nutritional availability or supplementation of these elements. Selenium status is altered in benign and malignant thyroid diseases and various selenium compounds have been used to prevent or treat widespread diseases such as goiter, autoimmune thyroid disease or thyroid cancer.

RECENT FINDINGS

Several studies, most with still too low numbers of cases, indicate that selenium administration in both autoimmune thyroiditis (Hashimoto thyroiditis) and mild Graves' disease improves clinical scores and well-being of patients and reduces thyroperoxidase antibody titers. However, published results are still conflicting depending on basal selenium status, dose, time and form of selenium used for intervention. Evidence for sex-specific selenium action, lack of beneficial effects in pregnancy and contribution of genetic polymorphisms (selenoprotein S) has been presented.

SUMMARY

Adequate nutritional supply of selenium that saturates expression of circulating selenoprotein P, together with optimal iodine and iron intake, is required for a healthy and functional thyroid during development, adolescence, adulthood and aging.

Selenium supplementation in thyroid associated ophthalmopathy: an update

The therapeutic effect of selenium (Se) has already been proven in thyroid disease and thyroid associated ophthalmopathy (TAO). In spite of clear scientific proof of its benefits in TAO, there appears to be no clear agreement among the clinicians regarding its optimum dose, duration of the treatment, efficacy and safety to date. In this review, the author summarises the findings of 135 English language articles published on this subject over the past four decades from 1973 to 2013. The regulation and metabolism of thyroid hormones require a steady supply of Se and recent studies have revealed several possible mechanisms by which Se improves the severity of thyroid disease and TAO. These mechanisms include 1) inhibitory effect of HLA-DR molecule expression on thyrocytes; 2) profound reductions of thyroid stimulating hormone (TSH) receptor antibodies (TSHR-Ab) and TPO antibodies (TPO-Ab); 3) prevention of dysregulation of cell-mediated immunity and B cell function; 4) neutralising reactive oxygen species (ROS) and inhibition of redox control processes required for the activation, differentiation and action of lymphocytes, macrophages, neutrophils, natural killer cells involved in both acute and chronic orbital inflammation in TAO; 5) inhibition of expression of pro-inflammatory cytokines and 6) inhibition of prostaglandin and leukotriene synthesis. An increased oxidative stress has been observed in both acute and chronic phases of thyroid disease with raised tissue concentrations of ROS. The benefits of Se supplementation in individuals with TAO appear to be proportionate to the degree of systemic activity of the thyroid disease. The maximal benefit of Se supplementation is therefore seen in the subjects who are hyperthyroid. Restoration of euthyroidism is one of the main goals in the management of TAO and when anti-thyroid drugs are combined with Se, the patients with Graves' disease (GD) and autoimmune thyroiditis (AIT) achieved euthyroidism faster than those treated with anti-thyroid drugs alone. Se status of normal adult humans can vary widely and Se supplementation may confer benefit only if serum Se levels are insufficient. The author recommends that serum Se levels of patients with TAO to be assessed prior to and during Se supplementation at regular intervals to avoid potential iatrogenic chronic Se overdose.

Potential health risks of heavy metals in cultivated topsoil and grain, including correlations with human primary liver, lung and gastric cancer, in Anhui province, Eastern China

Environmental exposure to heavy metals is a well-known risk factor for cancers. To evaluate potential health risks of heavy metals (Cr, Cd, Pb, As and Hg) and Se in cultivated topsoil and grains, we investigated the concentrations of Hg, As and Se using atomic fluorescence spectrometry and Cr, Cd and Pb using inductive coupled plasma emission spectrometry (ICP-MS). We also analyzed human cancer tissues for heavy metals. Potential health risks for local residents were evaluated by calculating the hazard index (HI) and the total carcinogenic risk (TCR) for soil heavy metals and the target hazard quotient (THQ) and the carcinogenic risk (CR) for grain heavy metals. A bioconcentration factor (BCF) was applied to quantify the bioaccumulation of heavy metals. Our results demonstrated that the mean concentrations of heavy metals in soil were all within the safety limits set by FAO/WHO and Chinese regulations; however, the mean concentrations of Cr and Hg in grain exceeded the safety limits. HI and TCR for soil heavy metals were all within acceptable levels, but the THQ for four grain heavy metals exceeded the target value of 1 (Cr, 2.64; Pb, 1.41; As, 1.24; Hg, 1.07; Cd, 0.39). The grain CR for Cr, Pb and As exceeded the accepted risk level of 10(-6). BCF values indicated that the bioaccumulation capacity decreased in the following sequence: Hg>Se>Cd>Cr>Pb>As. We also observed statistically significant correlations of topsoil Pb concentration with human gastric cancer and grain Hg with human liver cancer. Therefore, long-term low dose exposure of heavy metals may play a key role in tumorigenesis, and it may not be necessary to accumulate a high concentration of heavy metals in the human body for those metals to induce tumorigenesis.

Selenium and the thyroid

PURPOSE OF REVIEW

To provide information on the role of the essential trace element selenium, which enables appropriate thyroid hormone synthesis, secretion, and metabolism, and to discuss supplementation with various selenium compounds, which prevent thyroid diseases such as goiter and exert beneficial effects in thyroid autoimmune diseases.

RECENT FINDINGS

Selenium administration in both autoimmune thyroiditis (M. Hashimoto) and mild Graves' disease improves clinical scores and well-being of patients and reduces autoimmune antibody titres in several prospective, placebo-controlled supplementation studies.

SUMMARY

Adequate nutritional supply of selenium, together with the two other essential trace elements iodine and iron, is required for a healthy thyroid during development and adolescence, as well as in the adult and aging populations.

A pilot study of serum selenium, vitamin D, and thyrotropin concentrations in patients with thyroid cancer

BACKGROUND

Low serum selenium concentrations have been associated with a diagnosis of differentiated thyroid cancer in small studies in selenium deficient areas. We conducted a pilot study to explore associations between selenium concentrations and the diagnosis of thyroid cancer in an area of selenium sufficiency in the United States. As low 25-hydroxyvitamin D concentrations have been associated with several malignancies, we also examined 25-hydroxyvitamin D levels.

METHODS

This study was designed as a pilot study of prediagnostic selenium and 25-hydroxyvitamin D concentrations. We identified 65 euthyroid patients at an academic medical center who were scheduled for thyroidectomy for thyroid cancer, suspicion of thyroid cancer, or nodular disease. Blood samples were obtained two to four weeks prior to thyroidectomy. Samples were analyzed for thyrotropin (TSH), free thyroxine, total triiodothyronine, selenium, and 25 hydroxyvitamin D levels. Concentrations of these analytes were correlated with whether the patient was diagnosed with benign or malignant disease following their thyroidectomy. In patients with thyroid cancer, the concentrations of selenium and 25-hydroxyvitamin D were correlated with various prognostic features.

RESULTS

Although selenium concentrations were not significantly lower in patients with thyroid cancer, serum selenium concentrations were inversely correlated with disease stage (p = 0.011). There were no associations between vitamin D concentration and a diagnosis of thyroid cancer. Within the thyroid cancer patients, vitamin D concentrations were not associated with disease stage or any other prognostic features. In contrast, TSH concentrations were significantly higher in patients with thyroid cancer, and were positively correlated with the number of involved lymph nodes (p = 0.011) and disease stage (p = 0.022).

CONCLUSION

These data confirm the association between serum TSH and advanced thyroid cancer. In addition, they also suggest a potential association between selenium concentrations and higher thyroid cancer stage. No such association was seen for 25-hydroxyvitamin D concentrations. Larger prospective studies will be required to confirm this association. If confirmed, future studies would need to determine if the association is causative in nature. If causation exists, it seems likely that selenium concentrations would influence thyroid cancer development via an independent mechanism from that of TSH.

Neutralization by metal ions of the toxicity of sodium selenide

Inert metal-selenide colloids are found in animals. They are believed to afford cross-protection against the toxicities of both metals and selenocompounds. Here, the toxicities of metal salt and sodium selenide mixtures were systematically studied using the death rate of Saccharomyces cerevisiae cells as an indicator. In parallel, the abilities of these mixtures to produce colloids were assessed. Studied metal cations could be classified in three groups: (i) metal ions that protect cells against selenium toxicity and form insoluble colloids with selenide (Ag⁺, Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺ and Zn²⁺), (ii) metal ions which protect cells by producing insoluble metal-selenide complexes and by catalyzing hydrogen selenide oxidation in the presence of dioxygen (Co²⁺ and Ni²⁺) and, finally, (iii) metal ions which do not afford protection and do not interact (Ca²⁺, Mg²⁺, Mn²⁺) or weakly interact (Fe²⁺) with selenide under the assayed conditions. When occurring, the insoluble complexes formed from divalent metal ions and selenide contained equimolar amounts of metal and selenium atoms. With the monovalent silver ion, the complex contained two silver atoms per selenium atom. Next, because selenides are compounds prone to oxidation, the stabilities of the above colloids were evaluated under oxidizing conditions. 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), the reduction of which can be optically followed, was used to promote selenide oxidation. Complexes with cadmium, copper, lead, mercury or silver resisted dissolution by DTNB treatment over several hours. With nickel and cobalt, partial oxidation by DTNB occurred. On the other hand, when starting from ZnSe or FeSe complexes, full decompositions were obtained within a few tens of minutes. The above properties possibly explain why ZnSe and FeSe nanoparticles were not detected in animals exposed to selenocompounds.