Selenium is an essential micronutrient in the diet of humans and other mammals. Many health benefits have been attributed to selenium that include preventing various forms of cancer (e.g., colon cancer, prostate cancer, lung cancer and liver cancer), heart disease and other cardiovascular and muscle disorders, inhibiting viral expression, delaying the progression of acquired immunodeficiency syndrome (AIDS) in human immunodeficiency virus (HIV)-positive patients, slowing the aging process, and having roles in mammalian development, including male reproduction and immune function. Numerous human clinical trails have been undertaken in recent years to assess the role of this element in cancer prevention, delaying the progression of AIDS, etc., at a cost of billions of dollars, but little is known about the mechanism of how selenium acts at the metabolic level in mammals to exert these many health benefits. We have proposed that the health benefits of selenium are due largely to its presence in selenoproteins as the selenium-containing amino acid, selenocysteine (Sec). Our program therefore focuses on: 1) developing mouse models to assess the role of selenium and selenoproteins in cancer prevention and development, 2) characterizing and elucidating the function of various selenoproteins and their roles in cancer prevention and development, and 3) identifying the means by which Sec is biosynthesized and incorporated into protein. The project discussed herein examines our research on the development of various mouse models for determining the role of selenium in cancer, cancer prevention and development. During the past year, we have focused on completing our studies on characterizing mouse models that: 1) knockout of the Sec tRNA gene (designated Trsp) and consequently the loss of selenoprotein expression in (a) T cells, (b) macrophage, (c) epidermal skin tissue, and (d) liver, and subsequent rescue or partial rescue of selenoprotein expression with a wild-type or mutant Trsp transgene. Our direction of focus in this project is shifting towards generating mouse models that deal with the role of selenoproteins in cancer initiation and progression and on the targeted removal of individual selenoproteins instead of knocking out all selenoproteins through the removal of Trsp. This past year, we completed our study on examining the molecular mechanism of selenoproteins in cell immunity. Our findings with knocking out all selenoproteins in T cells provided novel insights into the immune function of selenium and physiological antioxidants. The removal of selenoproteins in macrophage was found to manifest largely normal inflammatory responses, but selenoprotein loss had abnormal expression of extracellular matrix-related genes and a reduced migration of macrophages in a protein gel matrix. We have recently begun to target the removal of only thioredoxin reductase 1 (TR1) or glutathione peroxidase 4 (GPx4) in both T cells and macrophage. In addition, we completed our study on the targeted removal of Trsp in skin and have focused on the removal of a single selenoprotein, GPx4. Loss of GPx4 virtually mimicked the loss of all selenoproteins with the exception that the mice recover after day 10, when they routinely died with the Trsp knockout mice, and appear to live a virtually normal life. Removal of selenoproteins in skin epidermal cells resulted in a runt phenotype and premature death of the progeny. The GPx4 knockout mice had alopecia along with a flaky and fragile skin and histological studies revealed epidermal hyperplasia along with changes in hair follicle appearance. These observations highlight a hitherto unknown role of selenoproteins and GPx4 in cutaneous development. We have initiated in vivo carcinogenesis studies involving TR1 and selenoprotein 15 (Sep15) knockdown mice to provide mouse models to examine our in vitro findings elucidating the role of these selenoproteins in cancer. Clearly, TR1 is a double sword in having roles in protecting normal cells from cancer and then once the malignancy in initiated, the selenoenzyme has a role in the developing cancer (see Hatfield, Thioredoxin reductase 1: A double-edged sword in cancer prevention and promotion. CCR Frontiers in Science 6: 8-10, 2007). We initially carried out pilot studies involving only a few mice to both determine the role of the genetic background of the mice in terms of their susceptibility to the cancer under study and to assure us that our approaches were correct. In our study on the role of Sep15 in colon cancer, the Sep15 knockout mice were in a C57BL/6 (B6)-SV129 (SV) background. We initially examined aberrant crypt formation in the Sep15 knockout mouse against pure B6 and B6-SV lines treated with azoxymethane. After 3 months, we found the Sep15 knockout mice had far fewer aberrant crypts than control mice (but, as expected, the B6 line had less than the mixed background line). We have initiated an ongoing study involving a larger number of mice wherein the control consists of sibling mice with the identical genetic background. In the two mouse models used to examine the role of TR1 reduction on breast and liver cancer progression, the transgenic mouse lines used are the genetically sensitive FVB and encode a shRNA targeting vector (designed by us) transgene that is inducible by doxycycline. Expression of the shRNA partially knocks down TR1 expression. We initially examined tumor production in breast tissue of control and TR1 inducible mice treated with 7,12-DMBA. After 4 months, both mouse lines had tumors and TR1 expression was highly elevated in tumor tissues. We currently have a larger number of mice either fed or not fed doxycycline for examining whether TR1 reduction has an effect on breast cancer. In the liver cancer study, we have an ongoing pilot experiment involving a few transgenic mice encoding the TR1 shRNA targeting vector that have been treated with DEN. This initial study has been ongoing for about six months and will be completed in about another six months at which time it will be evaluated and a decision will be made on how the project will be pursued.
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