PALM BEACH, FL—Researchers studying the actions of selenium said there is in vitro evidence that selenium may prevent thyroid cancer cell proliferation. “Treatment of thyroid cancer cell lines with selenium compounds caused a significant decrease in cell proliferation,” said Byrne Lee, MD, a resident in the Department of Surgery at Lenox Hill Hospital in New York City, in his poster presentation here at the 75th annual meeting of the American Thyroid Association. Dr. Lee noted, however, that the decrease in cell proliferation does not occur due to apoptosis. He suggested that the mechanism of action that has an impact on the cell lines might be due to cell cycle arrest. He and his colleagues attempted to find out what effects selenium supplementation has on thyroid cancer cell lines. “Accumulating evidence suggests an anticarcinogenic effect of selenium supplementation in human epithelial cancers, including prostate, lung, and colon carcinoma,” Dr. Lee explained. Role of Selenoproteins The exact mechanism of action is not known, he noted, but it is hypothesized that selenoproteins may play a role in the anticarcinogenic effects of selenium. “Through molecular profiling techniques, we have recently identified a decreased expression of selenoprotein P in human papillary and follicular thyroid cancers,” he said. Selenoprotein P aids in the reduction of free radicals and prevents oxidative damage in cells and may play a major role in cancer prevention, he noted. In the study, Dr. Lee and colleagues at New York Presbyterian Hospital, Weill-Cornell Medical School treated three thyroid carcinoma lines with two selenium compounds, methylselenocysteine and selenomethionine. The cell lines, FRO, ARO, NPA, which all harbor p53 mutations, were plated in 96 well plates, serum starved for 24 hours, and then were treated with increasing concentrations of methylselenocysteine and selenomethionine for 24 and 48 hours. Each experiment was repeated five times. Assays measured cell proliferation and apoptotic rate in the selenium-treated cells. Cell proliferation was compared with control cell lines that were not treated with the selenium compounds. “Supplemental selenium significantly reduced thyroid cancer cell growth over 24 to 48 hours in all three cell lines at physiologic concentrations,” Dr. Lee said. At 24 and 48 hours, the FRO cells from anaplastic thyroid cancer, which is particularly aggressive, showed a significant decrease in cell proliferation when treated with methylselenocysteine. NPA cells cultured from follicular thyroid carcinomas exhibited significant reductions in cell proliferation as well when treated with methylselenocysteine and selenomethionine. The ARO thyroid cancer cell line also exhibited reduced proliferation when treated with methylselenocysteine and selenomethionine. The experiments found that selenium treatment was not associated with a significant increase in apoptosis. Instead, treatment of thyroid cancer cell lines with selenium not only caused a significant decrease in cell proliferation, but also strongly indicated that cell cycle arrest may be a possible mechanism of action of selenium supplementation. Earlier Research The study builds on earlier research indicating that selenium compounds can have an effect of cancer, Dr. Lee noted. “Recent reports of microarray data as well as quantitative real-time polymerase chain reactions have noted a decrease in selenoprotein P in human prostate cancer samples.” In addition, other in vitro studies suggest that selenium supplementation induces DNA repair mechanisms in human fibroblasts that have suffered DNA damage after exposure to ultraviolet light. He also cited additional research that noted the appearance of cell cycle arrest at the G1 phase in human epithelial cell lines as well as apoptosis and G2/M cell cycle arrest in human prostate cancer cell lines after selenium supplementation. Selenium supplementation, however, remains controversial, said Simon Yeung, Clinical Coordinator of the Integrative Medicine Service at Memorial Sloan-Kettering Cancer Center. “Although selenium is a good antioxidant, people using selenium should be aware of its toxicity. The suggested dosage of over-the-counter selenium is 100 to 200 mg a day. People who use supplements of 400 mg or more a day could experience the equivalence of heavy metal poisoning.” Interest in selenium supplementation as a cancer prevention treatment does remain high, however, he remarked, pointing to the large ongoing SELECT (Selenium and Vitamin E Cancer Prevention Trial) 10-year study.