Multiple levels of regulation of selenoprotein biosynthesis revealed from the analysis of human glioma cell lines

To gain a better understanding of the biological consequences of the exposure of tumor cells to selenium, we evaluated the selenium-dependent responses of two selenoproteins (glutathione peroxidase and the recently characterized 15-kDa selenoprotein) in three human glioma cell lines. Protein levels, mRNA levels, and the relative distribution of the two selenocysteine tRNA isoacceptors (designated mcm⁵U and mcm⁵Um) were determined for standard as well as selenium-supplemented conditions. The human malignant glioma cell lines D54, U251, and U87 were maintained in normal or selenium-supplemented (30 nM sodium selenite) conditions. Northern blot analysis demonstrated only minor increases in steady-state GSHPx-1 mRNA in response to selenium addition. Baseline glutathione peroxidase activity was 10.7 ± 0.7, 7.6 ± 0.7, and 4.3 ± 0.7 nmol NADPH oxidized/min/mg protein for D54, U251, and U87, respectively, as determined by the standard coupled spectrophotometric assay. Glutathione peroxidase activity increased in a cell line-specific manner to 19.7 ± 1.4, 15.6 ± 2.1, and 6.7 ± 0.5 nmol NADPH oxidized/min/mg protein, respectively, as did a proportional increase in cellular resistance to H₂O₂, in response to added selenium. The 15-kDa selenoprotein mRNA levels likewise remained constant despite selenium supplementation. The selenium-dependent change in distribution between the two selenocysteine tRNA isoacceptors also occurred in a cell line-specific manner. The percentage of the methylated isoacceptor, mcm⁵Um, changed from 35.5 to 47.2 for D54, from 38.1 to 47.3 for U251, and from 49.0 to 47.6 for U87. These data represent the first time that selenium-dependent changes in selenoprotein mRNA and protein levels, as well as selenocysteine tRNA distribution, were examined in human glioma cell lines. Copyright (C) 2000 Elsevier Science Inc.