Gallegos A, Berggren M, Gasdaska J R, Powis G
Arizona Cancer Center, University of Arizona, Tucson 85724-5024, USA.
Cancer Res. 1997 Nov 1;57(21):4965-70.
Selenium is an essential trace element, the deficiency of which is associated with an increased incidence of some human cancers. Dietary supplementation with selenium has been reported to produce a decrease in the incidence of some cancers in humans. Thioredoxin reductase (TR) is a newly discovered homodimeric selenocysteine (SeCys)-containing protein that catalyzes the NADPH-dependent reduction of the redox protein thioredoxin (Trx). Trx is overexpressed by a number of human tumors, and experimental studies have shown that Trx contributes to the growth and to the transformed phenotype of some human cancer cells. Thus, TR, by reducing Trx, could play a role in regulating the growth of normal and cancer cells. We have investigated mechanisms by which selenium, in the form of sodium selenite, added to serum-free growth medium regulates TR activity in cancer cell lines. Selenium caused a dose-dependent increase in cellular TR activity. The increase in TR activity produced by 1 microM Se compared to medium with no added selenium was: for MCF-7 breast cancer cells, 37-fold; for HT-29 colon cancer cells, 19-fold; and for A549 lung cancer cells, 8-fold. In contrast, Jurkat and HL-60 leukemia cells showed no increase in TR activity. The half-life of the time course of induction of TR in HT-29 cells after adding selenium was 10 h. The increase in TR activity was accompanied by an increase in TR protein levels up to 3-fold and an increase in the specific activity of the enzyme of 5-32-fold, depending on the cell line. Studies using 75Se showed that the amount of selenium incorporated into TR increased with increasing selenium concentration up to a ratio of 1 selenium per TR monomer. There was an increase in TR mRNA levels of 2-5-fold at 1 microM selenium and an increase in the stability of TR mRNA with a half-life for degradation of 21 h compared to 10 h in the absence of selenium. Trx mRNA and protein levels and Trx mRNA stability were not affected by selenium. The results of the study show that the increase in TR activity caused by selenium is specific and due to several effects, including an increase in the stability of TR mRNA leading to increased TR mRNA levels, an increase in TR protein, but predominantly to an increase in the specific activity of TR associated with increased incorporation of selenium into the enzyme.
硒是一种必需的微量元素，其缺乏与某些人类癌症发病率的增加有关。据报道，饮食中补充硒可使人类某些癌症的发病率降低。硫氧还蛋白还原酶（TR）是一种新发现的含硒代半胱氨酸（SeCys）的同型二聚体蛋白，它催化依赖NADPH的氧化还原蛋白硫氧还蛋白（Trx）的还原。许多人类肿瘤中Trx过表达，实验研究表明Trx有助于某些人类癌细胞的生长和转化表型。因此，TR通过还原Trx，可能在调节正常细胞和癌细胞的生长中发挥作用。我们研究了以亚硒酸钠形式添加到无血清生长培养基中的硒调节癌细胞系中TR活性的机制。硒导致细胞TR活性呈剂量依赖性增加。与未添加硒的培养基相比，1 microM硒产生的TR活性增加倍数为：MCF-7乳腺癌细胞为37倍；HT-29结肠癌细胞为19倍；A549肺癌细胞为8倍。相比之下，Jurkat和HL-60白血病细胞的TR活性没有增加。在HT-29细胞中添加硒后，TR诱导过程的半衰期为10小时。TR活性的增加伴随着TR蛋白水平增加高达3倍，以及酶的比活性增加5至32倍，这取决于细胞系。使用75Se的研究表明，掺入TR的硒量随着硒浓度的增加而增加，直至每个TR单体含1个硒的比例。在1 microM硒时，TR mRNA水平增加2至5倍，并且TR mRNA的稳定性增加，降解半衰期为21小时，而在无硒情况下为10小时。Trx mRNA和蛋白水平以及Trx mRNA稳定性不受硒的影响。研究结果表明，硒引起的TR活性增加是特异性的，并且是由多种效应引起的，包括TR mRNA稳定性增加导致TR mRNA水平升高、TR蛋白增加，但主要是与硒掺入酶增加相关的TR比活性增加。
