Role of Selenium in Cancer and Health

硒在预防各种形式的癌症(如结肠癌、前列腺癌、肺癌和肝癌)、心脏病以及其他心血管和肌肉疾病方面发挥作用。它也是一种抗病毒药物，在延缓衰老过程、延缓艾滋病毒阳性患者的艾滋病进展、免疫功能、哺乳动物发育和雄性生殖方面发挥作用。尽管人们对硒的潜在代谢影响知之甚少，但至少在一定程度上，这是由于硒蛋白中这种元素作为氨基酸--硒半胱氨酸(SEC)的存在。我们的计划集中在SEC被结合到蛋白质中的方法以及特定的硒蛋白在人类健康和发育中的作用。在过去的一年里，我们发现了SEC生物合成中一种重要酶的基因，并对其产物磷酸丝氨酰-tRNASerSec激酶(PSTK)进行了鉴定。此外，我们正在使用RNAi敲除几种硒蛋白和其他参与SEC生物合成的蛋白(例如SLA、PSTK、SPS1和2)，以进一步阐明它们在硒代谢中的作用，并确定这种元素是如何结合到蛋白质中的。由于SEC tRNA是唯一已知的控制一整类蛋白质--硒蛋白表达的tRNA，我们干扰了SEC tRNA的合成，以更好地了解硒和硒蛋白在健康中的作用。在过去的几年里，我们已经培育出了转基因、基因敲除、条件基因敲除和基因敲除的细胞和小鼠，为阐明硒蛋白在健康和发育中的作用提供了模型。我们拯救了携带野生型或突变型SEC tRNA转基因小鼠的标准SEC tRNA基因敲除小鼠，这些小鼠是胚胎致命的，并分析了哪些硒蛋白被拯救。突变的SEC tRNA不能在34位核糖的2‘-O-羟基位置上形成甲基，这是SEC tRNA成熟的最后一步。用野生型转基因拯救的小鼠正常表达所有硒蛋白，但用突变转基因拯救的小鼠不表达几种与压力相关的硒蛋白，但表达管家硒蛋白的量正常或减少。这种新的蛋白质表达调控发生在翻译水平上，并表现出组织特异性的模式。这些数据表明，2‘-O-羟基核糖基上的SEC tRNA甲基化在调节多种哺乳动物硒蛋白的表达中起作用，并且是合成非必需的应激相关硒蛋白所必需的。利用loxP-Cre技术，我们将条件基因敲除小鼠与不同的Cre小鼠交配，以选择性地移除包括肝脏、前列腺、乳房、内皮细胞以及骨骼肌和心肌在内的各种组织中的SEC tRNA基因(从而改变硒蛋白的表达)。通过这种方式，我们可以评估硒蛋白在发育、组织和器官功能以及健康中的作用。在过去的一年里，我们已经消除了内皮细胞、心脏和骨骼肌中硒蛋白的表达。在内皮细胞中缺乏硒蛋白表达的动物在出生14.5天后死于多种异常，而在心肌和骨骼肌中缺乏硒蛋白表达的动物在出生后10天左右突然死于心脏病。后者的研究清楚地表明，硒蛋白参与内皮细胞和心肌的发育和正常功能。在特定靶向的组织和器官中条件敲除SEC tRNA为研究硒蛋白在发育和健康中的作用提供了一个很好的模型。

Selenium has a role in preventing various forms of cancer (e.g., colon, prostate, lung and liver), heart disease and other cardiovascular and muscle disorders. It also 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. Although the underlying metabolic effects of selenium are poorly understood, they are due, at least in part, to the presence of this element in selenoproteins as the amino acid, selenocysteine (Sec). Our program focuses on the means by which Sec is incorporated into protein and the role of specific selenoproteins in human health and development. In the last year, we discovered the gene for an important enzyme in Sec biosynthesis and characterized its product, phosphoseryl-tRNASerSec kinase (PSTK). In addition, we are using RNAi to knock down several selenoproteins and other proteins involved in Sec biosynthesis (e.g., SLA, PSTK, SPS1 and 2) to further elucidate their roles in selenium metabolism and determine how this element is incorporated into protein. Since Sec tRNA is the only known tRNA that governs the expression of an entire class of proteins, the selenoproteins, we have perturbed Sec tRNA synthesis to better understand the role of selenium and selenoproteins in health. Over the last few years, we have generated transgenic, knockout, conditional knockout and knockdown cells and mice involving the Sec tRNA gene to provide models for explicating the role of selenoproteins in health and development. We rescued standard Sec tRNA gene knockout mice that are embryonic lethal with transgenic mice carrying either a wild type or mutant Sec tRNA transgene and analyzed which selenoproteins are rescued. The mutant Sec tRNA is incapable of forming a methyl group at the 2'-O-hydroxy position on the ribose in position 34 which is the last step in the maturation of Sec tRNA. Mice rescued with the wild type transgene expressed all selenoproteins normally, but mice rescued with the mutant transgene did not express several stress-related selenoproteins, but expressed housekeeping selenoproteins in normal or reduced amounts. This novel regulation of protein expression occurred at the level of translation and manifested a tissue-specific pattern. The data indicate that Sec tRNA methylation at the 2'-O-hydroxylribosyl site plays a role in regulating the expression of various mammalian selenoproteins and is required for synthesis of non-essential, stress-related selenoproteins. Using loxP-Cre technology, we mated the conditional knockout mice with different Cre mice to selectively remove the Sec tRNA gene (and therefore alter selenoprotein expression) in various tissues including liver, prostate, breast, endothelial cells and skeletal and heart muscle. In this way, we can assess the role of selenoproteins in development, in tissue and organ function and in health. This past year we have knocked out selenoprotein expression in endothelial cells and heart and skeletal muscle. Animals lacking selenoprotein expression in endothelial cells died after 14.5 days dpc from multiple abnormalities, while those animals lacking selenoprotein expression in heart and skeletal muscle died abruptly at about 10 days after birth from heart disease. The latter studies clearly show that selenoproteins are involved in the development and normal function of endothelial cells and heart muscle. The conditional knockout of Sec tRNA in specifically targeted tissues and organs provides an excellent model for examining the role of selenoproteins in development and health.