Role of Selenium in Cancer, HIV Infection and Human Health

硒被认为是一种预防癌症和心脏病的化学预防药物，作为一种抗病毒药物，它可能在延缓艾滋病毒阳性患者的衰老过程和延缓艾滋病的进展方面发挥作用。这种元素的有益效果至少部分归因于它作为氨基酸--硒半胱氨酸(SEC)存在于硒蛋白中。我们的计划集中在SEC被结合到蛋白质中的方式，特定的硒蛋白在人类健康中的作用，以及硒在HIV感染中的作用。由于Sec tRNA是硒蛋白生物合成的关键分子，我们建立了编码2~20个野生型或2~40个异戊烯基腺苷(I6A)突变型Sec tRNA转基因基因的转基因小鼠，以检测其表达对硒代谢的影响。野生型Sec tRNA的过度表达不影响硒蛋白的合成，表明Sec tRNA在蛋白质翻译中不受限制。然而，I6A突变体SEC tRNA的表达选择性地抑制内源SEC tRNA的成熟和特定硒蛋白的翻译。DNA微阵列分析了硒蛋白缺乏对这些i6A突变SEC tRNA小鼠的全球影响，结果表明，参与氧化还原反应的许多其他酶都大大减少。这些数据表明，硒蛋白的表达与参与氧化还原反应的其他酶之间存在细胞内联系。此外，我们参与(与MJB，见合作者)确定了负责与硒半胱氨酸tRNA结合的特定延伸因子，以将SEC插入蛋白质中。在我们对特异性硒蛋白在人类健康中作用的研究中，我们发现一个15 kDa的硒蛋白(9月15)位于染色体1p31上人类癌症中常见的突变或缺失的位置。该基因全长51kb，由5个外显子和4个内含子组成。它在人类群体中以两个不同单核苷酸多态位点不同的等位基因存在。其中一个多态性似乎需要更高水平的硒来表达Sep15。Sep15基因在前列腺、肝脏、睾丸、肾脏和脑中的表达水平最高，而在相应的恶性组织或细胞中的表达水平显著降低。这些观察结果中的每一个都表明9月15日在癌症的发展中起到了作用。有趣的是，Sep15存在于哺乳动物细胞的内质网中，与UDP-葡萄糖：糖蛋白葡萄糖转移酶形成复合体，这表明硒在蛋白质折叠的质量控制中发挥了作用。在一项关于硒对人类健康影响的相关研究中，我们研究了艾滋病毒感染和随后的艾滋病发展导致患者硒和谷胱甘肽过氧化物酶(一种硒蛋白)缺乏的途径。接种了猴免疫缺陷病毒的恒河猴在疾病的发展过程中表现出典型的硒和谷胱甘肽过氧化物酶缺乏，这在艾滋病毒阳性患者中发现。这一观察为我们研究与艾滋病相关的硒缺乏提供了一个模型。我们之前观察到，HIV感染的细胞不像未感染的对照细胞那样有效地标记硒蛋白群体。我们已经完全排除了HIV Tat蛋白在这一过程中的作用。然而，我们观察到，在慢性感染的HIV细胞中诱导HIV表达会导致通常在缺硒组织中看到的SEC tRNA亚型的分布，而相应的未诱导细胞中的SEC tRNA分布与在富硒组织中观察到的相似。因此，SEC tRNA群体似乎在这种疾病的硒缺乏中起到了直接作用。为了更好地了解tRNA在HIV和其他哺乳动物逆转录病毒核糖体移码中的作用，我们检测了苯丙氨酸tRNA在其抗密码子3侧缺乏高度修饰的wybutoxosine碱基刺激移码事件的能力。这种低修饰的tRNA极大地增强了移码，提示了一种通过在细胞内保持完全修饰的tRNA来抑制某些逆转录病毒表达的替代途径。

Selenium is recognized as a chemopreventive agent against cancer and heart disease, as an antiviral agent and it likely plays a role in delaying the aging process and in delaying the progression of AIDS in HIV positive patients. The beneficial effects of this element are due, at least in part, to its presence in selenoproteins as the amino acid, selenocysteine (Sec). Our program focuses on the means by which Sec is incorporated into protein, the role of specific selenoproteins in human health and the role of selenium in HIV infection. As Sec tRNA is the key molecule in selenoprotein biosynthesis, we have produced transgenic mice encoding from 2 to 20 wild type or 2 to 40 isopentenyladenosine (i6A) mutant Sec tRNA transgenes to examine the effect of their expression on selenium metabolism. Over-expression of wild type Sec tRNA does not affect selenoprotein synthesis demonstrating that Sec tRNA is not limiting in protein translation. Expression of i6A mutant Sec tRNA, however, selectively inhibits endogenous Sec tRNA maturation and translation of specific selenoproteins. DNA microarray analysis of the global effects of selenoprotein deficiency in these i6A mutant Sec tRNA mice show that many other enzymes involved in redox reactions are substantially reduced. These data suggest an intracellular connection between selenoprotein expression and other enzymes involved in redox reactions. In addition, we participated (with MJB, see collaborators) in identifying the specific elongation factor that is responsible for binding with selenocysteyl-tRNA for insertion of Sec into protein. In our studies on examining the role of specific selenoproteins in human health, we found that a 15 kDa selenoprotein (Sep15) is localized on chromosome 1p31 at a site commonly mutated or deleted in human cancers. The gene spans 51 kb consisting of 5 exons and 4 introns. It exists as two alleles in the human population that differ by two single nucleotide polymorphic sites. One of the polymorphisms appears to require higher levels of selenium for Sep15 expression. Sep15 gene expression occurs in highest levels in prostate, liver, testes, kidney and brain and the level of expression was reduced dramatically in the corresponding malignant tissues or cells. Each of these observations suggests that Sep15 has a role in cancer development. Interestingly, Sep15 exists in the endoplasmic reticulum of mammalian cells in a complex with UDP-glucose:glycoprotein glucosyltransferase suggesting a role of selenium in quality control of protein folding. In a related study on the effects of selenium on human health, we have examined the means by which HIV infection and the subsequent development of AIDS results in selenium and glutathione peroxidase (a selenoprotein) deficiency in afflicted patients. Rhesus monkeys that are inoculated with simian immunodeficiency virus become characteristically selenium and glutathione peroxidase deficient in the progression of the resulting disease as is found in HIV positive patients. This observation provides us with a model for studying selenium deficiency associated with AIDS. We previously observed that HIV infected cells do not label the selenoprotein population as efficiently as uninfected control cells. We have completely ruled out a role of the HIV TAT protein in this process. However, we observed that induction of HIV expression in chronically infected HIV cells results in the distribution of the Sec tRNA isoforms that is normally seen in selenium deficient tissues, while the Sec tRNA distribution in the corresponding uninduced cells is like that observed in selenium sufficient tissues. Thus, the Sec tRNA population appears to have a direct role in selenium deficiency in this disease. To better understand the role of tRNA in ribosomal frameshifting in HIV and other mammalian retroviruses, we examined the ability of phenylalanine tRNA lacking the highly modified wybutoxosine base on the 3 side of its anticodon to stimulate the frameshift event. This hypomodified tRNA dramatically enhances frameshifting suggesting an alternative avenue for inhibiting expression of some retroviruses by maintaining fully modified tRNAs intracellularly.