Determination of nucleotide sequence of cDNA coding rat glutathione peroxidase and diminished expression of the mRNA in selenium deficient rat liver

Modulation of glutathione peroxidase expression by selenium: effect on human MCF-7 breast cancer cell transfectants expressing a cellular glutathione peroxidase cDNA and doxorubicin-resistant MCF-7 cells

We have studied the effect of selenium on the expression of a cellular glutathione peroxidase, GSHPx-1, in transfected MCF-7 cells and in doxorubicin-resistant (Adrr) MCF-7 cells. A GSHPx-1 cDNA with a Rous Sarcoma virus promoter was transfected into a human mammary carcinoma cell line, MCF-7, which has very low endogenous cytosolic glutathione (GSH) peroxidase activity and no detectable message. The transfectant with the highest GSH peroxidase activity among the isolates, MCF-7H6, was characterized. Adrr MCF-7 cells, a subline of MCF-7 cells, also has elevated GSH peroxidase activity. GSH peroxidase expressed by MCF-7H6 and Adrr MCF-7 cells is similar to the endogenous GSHPx-1 based on molecular weight, immunoreactivity, and metabolic labeling with 75Se. MCF-7H6 and Adrr MCF-7 cells grown in Se-deficient media had 2.6 +/- 2.4 (mean +/- S.D.) and 4.2 +/- 3.6 units/mg protein of GSH peroxidase specific activity, respectively. Se supplementation increased GSH peroxidase activity in a concentration- and time-dependent fashion. Enzymatic activity reached a level of 164 +/- 62 in MCF-7H6 cells and 114 +/- 27 in Adrr MCF-7 cells within 5 days of growth in media supplemented with 30 nM Se. Northern analysis revealed that Se-deficient MCF-7H6 cells expressed 2.1 +/- 0.4-fold less GSHPx-1 mRNA than their Se-sufficient counterparts. Similarly, Se-deficient Adrr MCF-7 cells expressed 3.3 +/- 1.8-fold less GSHPx-1 mRNA than their Se-supplemented counterparts after the quantity of mRNA was normalized with beta-actin. These studies suggest that modulation of GSH peroxidase activity by Se in both MCF-7H6 transfectants expressing pRSV-GSHPx-1 and Adrr MCF-7 cells expressing endogenous GSHPx-1 occurs largely at the translational level, and to a lesser degree at the level of mRNA, possibly by stabilizing GSHPx-1 mRNA since the transfected cDNA in MCF-7H6 cells has only 5 nucleotides 5' to the AUG initiation codon.

Role of the glutathione-glutathione peroxidase cycle in the cytotoxicity of the anticancer quinones

Recent studies have suggested that the selenoenzyme glutathione peroxidase, in the presence of reducing equivalents from the tripeptide glutathione, is responsible for detoxifying hydrogen peroxide and lipid hydroperoxides generated as a consequence of the cyclic reduction and oxidation of quinone-containing anticancer agents including doxorubicin, daunorubicin, mitomycin C, diaziquone, and menadione. Alterations in the intracellular levels of glutathione peroxidase or glutathione can significantly affect the activity of these drugs against human tumor cells and the expression of their normal tissue toxicity, especially with respect to the heart. Furthermore, augmentation of the glutathione peroxidase pathway appears to render certain human tumor cells relatively resistant to the anticancer quinones; therefore, the glutathione peroxidase system may, at least in part, modulate certain forms of acquired drug resistance in man. Thus, the glutathione peroxidase cycle appears to play a central role in maintaining intracellular peroxide homeostasis during quinone-induced oxidative stress.

The regulation of glutathione peroxidase gene expression relevant to species difference and the effects of dietary selenium manipulation

Glutathione peroxidase (GSH-Px) contains selenium (Se) as selenocysteine in the active site of the enzyme. GSH-Px activities in the cytosol of all guinea-pig tissues examined were extremely low compared with those in mice and rats, while Se concentrations in tissues were almost the same among three animal species. In addition, no GSH-Px mRNA was detectable in any tissues of guinea-pigs, although the guinea-pig had the same copy number (probably a single copy) of the GSH-Px gene in its genomic DNA as that of the mouse and rat, suggesting that the species difference of GSH-Px activity observed in rodents might be due to incapability of gene transcription. On the other hand, feeding of mice with Se-deficient diet for 6 weeks resulted in a remarkable decrease in GSH-Px mRNA as well as GSH-Px activity both in the liver and kidneys. The detailed time-course experiment revealed that the drop in GSH-Px activity preceded the decrease in the mRNA level in Se-depleted mice and the mRNA level recovered rapidly in contrast to the slow rate of increase in the enzyme activity in Se-repleted mice. These results suggested that the alteration in GSH-Px activity in mice subjected to dietary Se manipulation is attributable not only to transcriptional but also to post-transcriptional regulation.