Role of Selenium in Cancer and Health

硒是人类和其他哺乳动物饮食中必需的微量营养素，已知在预防各种癌症（如结肠癌、前列腺癌、肺癌和肝癌）、心脏病和其他心血管和肌肉疾病方面发挥作用。此外，它还作为抗病毒药物，在延缓衰老过程、延缓HIV阳性患者的艾滋病进展、免疫功能、哺乳动物发育和男性生殖中发挥作用。硒发挥这些健康益处的方式尚不清楚，但我们提出，这主要是由于硒蛋白中硒元素作为氨基酸硒半胱氨酸（Sec）的存在。因此，我们的项目将重点放在硒结合到蛋白质中的方式以及特定硒蛋白在人类健康和发育中的作用。由于Sec tRNA是唯一已知的控制整个蛋白质（硒蛋白）表达的tRNA，我们已经开发了几种转基因、敲除、条件敲除和敲除的细胞和小鼠系，这些细胞和小鼠系干扰Sec tRNA的合成，从而干扰硒蛋白的表达，作为更好地理解硒和硒蛋白代谢及其在健康中的作用的模型。在过去的一年里，我们的项目专注于使用RNAi技术敲除几种硒蛋白和其他参与硒生物合成的蛋白质（例如SLA， SECp43， PSTK， SPS1和2），以阐明它们在硒代谢中的作用，并确定该元素如何被纳入蛋白质中。本课课组进一步鉴定了SECp43和SLA在Sec生物合成及其掺入蛋白中的作用。此外，我们开发了一种新的方法来研究硒蛋白的功能，包括敲低/敲入，并使用这种技术来研究许多以前不容易接近的硒蛋白功能的新参数。利用loxP-Cre技术，我们靶向去除小鼠肝脏中硒蛋白的表达，然后分别用野生型或两种不同的突变型Sec tRNA转基因完全或部分替代硒蛋白群体。突变的Sec tRNA转基因涉及tRNA的突变，该突变阻止了在反密码子的摆动位置37位添加甲基，2'- o -甲基核糖（指定为Um34）。Um34的缺失导致应激相关硒蛋白的表达缺失，而管家硒蛋白几乎完全表达。我们之前通过使用野生型和突变型转基因对标准敲除小鼠的硒蛋白群体进行修复，表明Um34参与了与应激相关的硒蛋白的合成。然而，我们现在针对特定器官或组织的能力将使我们能够专注于硒和硒蛋白在特定细胞类型中的作用。应该进一步指出的是，这种新的蛋白质表达调控发生在翻译水平，并表现出组织特异性模式。我们还扩大了在各种组织和器官中硒蛋白表达的靶向敲除，并明确显示了硒蛋白在发育和疾病预防中的作用。在过去的一年中，我们在这一领域的主要焦点之一是硒蛋白在免疫功能中的作用。我们以去除t细胞中硒蛋白的表达为目标，观察到CD8群体减少了约50%，mIL-13在敲除小鼠的雄性和雌性中分别富集了55倍和80倍以上。在靶向小鼠中，IgG2a增加了约两倍。这些研究表明，t细胞中的硒蛋白与炎症过敏免疫反应有关，在t细胞的发育/分化/成熟和功能中起重要作用，是决定Th1 vs Th2反应不可或缺的因素。

Selenium is an essential micronutrient in the diet of humans and other mammals and is known to have roles in preventing various forms of cancer (e.g., colon, prostate, lung and liver), heart disease and other cardiovascular and muscle disorders. In addition, it serves as an antiviral agent and plays a role in delaying the aging process, in delaying the progression of AIDS in HIV positive patients, in immune function, in mammalian development and in male reproduction. The means by which selenium exerts these many health benefits are poorly understood, but we have proposed that they are due largely to the presence of this element in selenoproteins as the amino acid, selenocysteine (Sec). Our program therefore focuses on the means by which Sec is incorporated into protein and the role of specific selenoproteins in human health and development. Since Sec tRNA is the only known tRNA that governs the expression of an entire class of proteins, the selenoproteins, we have developed several transgenic, knockout, conditional knockout and knockdown cell and mouse lines that perturb Sec tRNA synthesis which in turn perturbs selenoprotein expression as models to better understand selenium and selenoprotein metabolism and their roles in health. This past year, our program has focused on using RNAi technology to knockdown several selenoproteins and other proteins involved in Sec biosynthesis (e.g., SLA, SECp43, PSTK, SPS1 and 2) to elucidate their roles in selenium metabolism and to determine how this element is incorporated into protein. Our group further characterized the roles of SECp43 and SLA in the biosyntheisis of Sec and its incorporation into protein. In addition, we developed a novel assay for studying selenoprotein function that involved knockdown/knock-in and are using this technique to study many new parameters of selenoprotein function that could not readily be approached previously. Using loxP-Cre technology, we targeted the removal of selenoprotein expression in mouse liver and then replaced the selenoprotein population either fully or partially with a wild type or two different mutant Sec tRNA transgenes, respectively. The mutant Sec tRNA transgenes involve mutations in the tRNA that prevent the addition of a methyl group, 2'-O-methylribose (designated Um34), at position 37 which is the wobble position of the anticodon. The absence of Um34 results in the loss of expression of stress-related selenoproteins, while the housekeeping selenoproteins are virtually completely expressed. We had shown earlier through rescue of the selenoprotein population in a standard knockout mouse using wild type and mutant transgenes that Um34 was involved in synthesis of stress-related selenoproteins. However, our ability now to target a specific organ or tissue will permit us to focus on the role of selenium and selenoproteins in a given cell type. It should be further noted that this novel regulation of protein expression occurred at the level of translation and manifested a tissue-specific pattern. We are also expanding our targeted knockout of selenoprotein expression in various tissues and organs and have shown unequivocally a role of selenoproteins in development and disease prevention. One of our major focuses in this area over the last year has been on the role of selenoproteins in immune function. We targeted the removal of selenoprotein expression in T-cells and observed about a 50% reduction in the CD8 population and that mIL-13 was enriched more than 55 fold in males and 80 fold in females of knockout mice. IgG2a was increased about two-fold in targeted mice. These studies show thus far that selenoproteins in T-cells are associated with inflammatory allergic immune responses, have an essential role in the development/differentiation/maturation and function of T-cells and are indespensible factors in determining Th1 vs Th2 responses.