Role of Selenium in Development and Health

硒是人类和其他哺乳动物饮食中必需的微量营养素。许多健康益处归因于硒，包括预防各种形式的癌症（例如，结肠癌、前列腺癌、肺癌和肝癌）、心脏病和其它心血管和肌肉疾病，抑制病毒表达，延缓人免疫缺陷病毒（HIV）阳性患者中获得性免疫缺陷综合征（AIDS）的进展，减缓衰老过程，以及在哺乳动物发育（包括雄性生殖和免疫功能）中发挥作用。近年来，已经进行了许多人类临床试验，以评估这种元素在预防癌症、延缓艾滋病进展等方面的作用，但人们对硒如何在哺乳动物的代谢水平上发挥作用以发挥这些健康益处的机制知之甚少。 我们已经提出，硒的健康益处主要是由于其存在于硒蛋白中作为含硒氨基酸，硒代半胱氨酸（Sec）。因此，我们的计划重点是：1）开发小鼠模型，以评估硒和硒蛋白在癌症预防和发展中的作用，2）表征和阐明各种硒蛋白的功能及其在癌症预防和发展中的作用，3）确定Sec生物合成和整合到蛋白质中的方法。本文讨论的项目检查了我们对各种小鼠模型的开发研究，以确定硒在癌症，癌症预防和发展中的作用。在过去的几年中，我们一直专注于完成我们的研究，对表征小鼠模型：1）敲除Sec tRNA基因（指定Trsp），并因此导致硒蛋白表达的损失（a）T细胞，（B）巨噬细胞，（c）表皮皮肤组织，和（d）肝脏，和随后的救援或部分救援硒蛋白表达的野生型或突变型Trsp转基因。在过去的一年里，我们在这个项目中的重点方向几乎完全转向产生小鼠模型，这些模型处理硒蛋白在癌症进展中的作用，并有针对性地去除单个硒蛋白，而不是通过去除Trsp敲除所有硒蛋白。 在较早的财政年度，我们完成了研究硒蛋白在细胞免疫中的分子机制的研究，在过去的一年里，我们强调提供足够的小鼠，以完成与Melinda Beck博士的合作研究，以确定缺乏，充足，和补充硒摄入水平对T细胞介导的免疫应答的影响，关键的、持家的硒蛋白（但不是非必需的、应激相关的硒蛋白）。我们继续进行了一项针对各种组织中仅去除硫氧还蛋白还原酶1（TR 1）和/或谷胱甘肽过氧化物酶4（GPx 4）的研究，以研究这两种必需硒蛋白在几种器官和组织中的作用。表皮组织（本文称为皮肤）中GPx 4的损失实际上模拟了所有硒蛋白的损失，除了小鼠在第10天后恢复，此时它们在Trsp敲除小鼠中常规死亡，并且似乎过着实际上正常的生活。GPx 4基因敲除小鼠出现沿着脱发，皮肤粗糙且脆弱，组织学研究显示表皮增生沿着伴有毛囊外观的变化。这些观察结果表明GPx 4在皮肤发育中的先前未知作用，进一步证明了这种硒蛋白在细胞发育中具有许多重要功能。靶向去除皮肤中的TR 1似乎没有任何表型改变。 在我们的体内结肠癌发生研究中，我们能够证明Sep 15基因敲除小鼠，与野生型或杂合子同窝对照相比，似乎对氧化偶氮甲烷诱导的肿瘤前病变（异常隐窝病灶）的形成有保护作用。此外，使用微阵列以及结肠粘膜的实时rtPCR分析表明，鸟苷酸结合蛋白1在Sep 15基因敲除小鼠中高度上调。这种干扰素-γ-调节蛋白和结肠癌中15 kDa硒蛋白之间可能的联系将在未来的实验中进行研究。此外，膳食硒在我们使用Sep 15基因敲除小鼠的结肠癌模型中的可能影响将进一步阐明。此外，我们的另一个小鼠研究模型，涉及使用肝脏TR 1缺陷小鼠暴露于肝脏致癌物质二乙基亚硝胺（DEN），在过去的一年中取得了成果。我们发现，肝脏特异性TR 1基因敲除小鼠比对照小鼠更容易发生肿瘤（肝腺瘤，肝癌和肝胆管细胞腺瘤）。18只基因敲除小鼠中有16只发生肿瘤，而19只对照小鼠中有2只发生肿瘤。此外，我们发现TR 1缺陷肝脏中Nrf 2调节基因的上调，包括几种谷胱甘肽转移酶和硒蛋白谷胱甘肽过氧化物酶2，其在肿瘤中甚至进一步上调。

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. In the past several years, we have focused on completing our studies on characterizing mouse models: 1) that knockout the Sec tRNA gene (designated Trsp) and consequently result in 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 has shifted in the past year almost totally towards generating mouse models that deal with the role of selenoproteins in the progression of cancer and the targeted removal of individual selenoproteins instead of knocking out all selenoproteins through the removal of Trsp. In an earlier fiscal year, we completed our study on examining the molecular mechanism of selenoproteins in cell immunity and in the past year, we emphasized providing sufficient mice to complete a collaborative study with Dr. Melinda Beck in determining the effects of deficient, adequate, and supplemental levels of selenium intake on the immune response mediated by T cells in mice that do not express selenoproteins in T cells or express only the critical, housekeeping selenoproteins (but not the non-essential, stress-related selenoproteins). We have continued a study on targeting the removal of only thioredoxin reductase 1 (TR1) and/or glutathione peroxidase 4 (GPx4) in various tissues to examine the role of these two essential selenoproteins in several organs and tissues. Loss of GPx4 in epidermal tissue (designated skin herein) virtually mimicked the loss of all selenoproteins with the exception that the mice recover after day 10, when they routinely died in Trsp knockout mice, and appear to live a virtually normal life. 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 demonstrate a prior unknown role of GPx4 in cutaneous development providing further evidence that this selenoprotein has many essential functions in cell development. The targeted removal of TR1 in skin did not appear to have any alteration in phenotype. In our in vivo colon carcinogenesis study, we were able to demonstrate that Sep15 knockout mice, compared to wild type or heterozygote littermate controls, appeared to be protected against azoxymethane-induced formation of pre-neoplastic lesions (aberrant crypt foci). Furthermore, using microarrays as well as real-time rtPCR analysis of the colonic mucosa showed that guanylate-binding protein 1 was highly up-regulated in Sep15 knockout mice. The possible connection between this interferon-gamma-regulated protein and the 15kDa selenoprotein in colon cancer will be investigated in future experiments. In addition, the possible influence of dietary selenium in our colon cancer model using Sep15 knockout mice will be further elucidated. Furthermore, our other mouse study model, which involved using liver TR1 deficient mice exposed to the liver carcinogen, diethylnitrosamine, (DEN), came to fruition in the past year. We found that liver-specific TR1 knockout mice developed tumors (hepatic adenomas, hepatic carcinomas and hepatocholangiocellular adenomas) much more readily than control mice. 16 of 18 knockout mice developed tumors compared to 2 of 19 control mice. In addition, we found an upregulation of Nrf2-modulated genes in the TR1 deficient livers, including several of the glutathione transferases and the selenoprotein, glutathione peroxidase 2, which was even further upregulated in tumors.