Optimization of Selenium Chemoprevention in Prostate Cancer Cells

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Xiuqi Li and Merrill Christensen, Nutrition, Dietetics, and Food Science

Introduction

Prostate cancer is the most frequently diagnosed cancer and the second-leading cause of cancer death in men in the United States (1). However, most men will die with prostate cancer rather then from it (2). Because prostate cancer exhibits a long latency period, high incidence rate, and significant morbidity but low mortality, its prevention remains an active field of research.

Selenium (Se), an essential micronutrient, is one of the most promising agents for prostate cancer prevention. Interestingly, unlike many other nutrients, dietary Se exists in multiple chemical forms each with varying degrees of anti-cancer effects and achieves chemoprevention by different mechanisms (3-4). In this study we aim to systematically combine different forms of Se to optimize its prostate-cancer-prevention efficacy.

Appropriate statistical modeling is crucial in experiments involving combinations of multiple substances. Response surface methodology (RSM) is often adopted as an optimization method (5). In this study, we utilized RSM to describe the growth inhibition effects of mixtures containing three different chemical forms of Se – methylseleninic acid (MSA), sodium selenite, and Se nanoparticles (Nano-Se) – in PC-3 cells.

Methodology

While MSA and sodium selenite were purchased from Sigma-Aldrich, Nano-Se was synthesized in our lab using a modified procedure of Zhang et al (6). Subsequently, Nano-Se was characterized by transmission electron microscopy (TEM) to determine its size and morphology.

PC-3 cells cultured in F-12K medium with 10% fetal bovine serum supplementation were used in all experiments. Cells were seeded in 48-well plates and allowed to attach before they were treated with different Se species at various concentrations for 48 hours. Cell viability was then measured using Alamar Blue assay (AbD Serotec). Statistical analysis was performed under the guidance of Dr. John Lawson, professor of statistics at Brigham Young University.

Results

Our freshly synthesized Nano-Se particles were well dispersed in water, forming a stable and clear red solution. TEM images suggested a core-shell morphology for the particles. Statistical analysis revealed that the diameters of the synthesized Nano-Se particles averaged 28 nm, with the majority between 20 and 35 nm.

MSA, Nano-Se, and sodium selenite exhibited varying degrees of growth inhibitory effects in PC-3 cells. Utilizing the IC-50 calculations established by Sebaugh (7) we showed with our empirical data that, when used individually, MSA was more effective in inhibiting PC-3 cell growth than sodium selenite, and sodium selenite was slightly more effective than Nano-Se.2211

The combination of MSA and sodium selenite served as a proof of principle that combinations of different Se species could achieve greater chemopreventive effect than single compounds (Figure 1). When all three Se compounds were combined, we discovered that different combinations of MSA, sodium selenite, and Nano-Se varied in their antiproliferative effects (Figure 2). Based on the RSM utilized, the optimum combination of the three Se compounds was approximately 5 μmol/L MSA, 5 μmol/L sodium selenite, and 0 μmol/L Nano-Se. To further confirm our results and analysis, we selected a few data point to test empirically (Figure 3). Within reasonable experimental error, these data confirmed the effectiveness of RSM in modeling the antiproliferative effects of combinations of Se compounds.33

Discussion

Chemoprevention relies on the use of nontoxic natural or synthetic agents to reverse, arrest, or prevent the formation of clinically significant diseases. Selenium was among the first nutrients that scientists acknowledged as a chemopreventive agent.

In the past decade, Nano-Se has emerged as a promising anticancer agent. To our knowledge, we are the first to test the effect of Nano-Se in PC-3 cells. The reason that Nano-Se did not exhibit the expected superior anticancer activity in our study when compared to MSA may be the ARnull nature of PC-3 cells.

By systematically combining multiple forms of Se compounds, we showed that combinations exhibit much potent anti-cancer effects. Interestingly, the optimum combination of the three tested Se compounds in inhibiting PC-3 cell growth occurred at the 50:50 mixture of MSA and sodium selenite. This along with other evidence lead us to speculate that the mechanisms by which Nano-Se inhibits cell growth may overlap with those of sodium selenite, resulting in Nano-Se’s slight contribution in a combination of the three tested Se compounds.

Conclusion

In this study, we demonstrated the anti-proliferative effect of a combination of three Se compounds. In this process, we showed the effectiveness of RSM to assist researchers and oncologists in systematically combining different agents together in search for an optimal combination. The use of RSM can be extended beyond the three chemical forms of the same element and into any combinations of nutrients, phytochemicals, and chemotherapeutics.

References

  1. American Cancer Society. Cancer Facts & Figures 2014. Atlanta: American Cancer Society; 2014.
  2. Bostwick DG, Burke HB, Djakiew D, et al. Human prostate cancer risk factors. Cancer. 2004;101:2371-490.
  3. Wu M, Kang MM, Schoene NW, Cheng WH. Selenium compounds activate early barriers of tumorigenesis. J Biol Chem. 2010;285:12055-62.
  4. Zhang J, Wang L, Li G, et al. Mouse prostate proteomes are differentially altered by supranutritional intake of four selenium compounds. Nutr Cancer. 2011;63:778-89.
  5. Bas D, Boyaci IH. Modeling and optimization I: Usability of response surface methodology. Journal of Food Engineering. 2007;78:836-45.
  6. Zhang JS, Gao XY, Zhang LD, Bao YP. Biological effects of a nano red elemental selenium. Biofactors. 2001;15:27-38.
  7. Sebaugh JL. Guidelines for accurate EC50/IC50 estimation. Pharm Stat. 2011;10:128-34.