Manganese superoxide dismutase (MnSOD/SOD2) is a mitochondria-resident enzyme that governs the types of reactive oxygen species egressing from the organelle to affect cellular signaling. Here, we demonstrate that MnSOD upregulation in cancer cells establishes a steady flow of H2O2 originating from mitochondria that sustains AMP-activated kinase (AMPK) activation and the metabolic shift to glycolysis. Restricting MnSOD expression or inhibiting AMPK suppress the metabolic switch and dampens the viability of transformed cells indicating that the MnSOD/AMPK axis is critical in support cancer cell bioenergetics. Recapitulating in vitro findings, clinical and epidemiologic analyses of MnSOD expression and AMPK activation indicated that the MnSOD/AMPK pathway is most active in advanced stage and aggressive breast cancer subtypes. Taken together, our results indicate that MnSOD serves as a biomarker of cancer progression and acts as critical regulator of tumor cell metabolism.

Selenocysteine (Sec) tRNA (tRNA([Ser]Sec)) serves as both the site of Sec biosynthesis and the adapter molecule for donation of this amino acid to protein. The consequences on selenoprotein biosynthesis of overexpressing either the wild type or a mutant tRNA([Ser]Sec) lacking the modified base, isopentenyladenosine, in its anticodon loop were examined by introducing multiple copies of the corresponding tRNA([Ser]Sec) genes into the mouse genome. Overexpression of wild-type tRNA([Ser]Sec) did not affect selenoprotein synthesis. In contrast, the levels of numerous selenoproteins decreased in mice expressing isopentenyladenosine-deficient (i(6)A(-)) tRNA([Ser]Sec) in a protein- and tissue-specific manner. Cytosolic glutathione peroxidase and mitochondrial thioredoxin reductase 3 were the most and least affected selenoproteins, while selenoprotein expression was most and least affected in the liver and testes, respectively. The defect in selenoprotein expression occurred at translation, since selenoprotein mRNA levels were largely unaffected. Analysis of the tRNA([Ser]Sec) population showed that expression of i(6)A(-) tRNA([Ser]Sec) altered the distribution of the two major isoforms, whereby the maturation of tRNA([Ser]Sec) by methylation of the nucleoside in the wobble position was repressed. The data suggest that the levels of i(6)A(-) tRNA([Ser]Sec) and wild-type tRNA([Ser]Sec) are regulated independently and that the amount of wild-type tRNA([Ser]Sec) is determined, at least in part, by a feedback mechanism governed by the level of the tRNA([Ser]Sec) population. This study marks the first example of transgenic mice engineered to contain functional tRNA transgenes and suggests that i(6)A(-) tRNA([Ser]Sec) transgenic mice will be useful in assessing the biological roles of selenoproteins.

Selenium has been implicated in cancer prevention, but the mechanism and possible involvement of selenoproteins in this process are not understood. To elucidate whether the 15-kDa selenoprotein may play a role in cancer etiology, the complete sequence of the human 15-kDa protein gene was determined, and various characteristics associated with expression of the protein were examined in normal and malignant cells and tissues. The 51-kilobase pair gene for the 15-kDa selenoprotein consisted of five exons and four introns and was localized on chromosome 1p31, a genetic locus commonly mutated or deleted in human cancers. Two stemloop structures resembling selenocysteine insertion sequence elements were identified in the 3-untranslated region of the gene, and only one of these was functional. Two alleles in the human 15-kDa protein gene were identified that differed by two single nucleotide polymorphic sites that occurred within the selenocysteine insertion sequence-like structures. These 3-untranslated region polymorphisms resulted in changes in selenocysteine incorporation into protein and responded differently to selenium supplementation. Human and mouse 15-kDa selenoprotein genes manifested the highest level of expression in prostate, liver, kidney, testis, and brain, and the level of the selenoprotein was reduced substantially in a malignant prostate cell line and in hepatocarcinoma. The expression pattern of the 15-kDa protein in normal and malignant tissues, the occurrence of polymorphisms associated with protein expression, the role of selenium in differential regulation of polymorphisms, and the chromosomal location of the gene may be relevant to a role of this protein in cancer.The essential trace element selenium occurs in several proteins found in bacteria (1), archaea (2), and eukaryotes (1, 3). All of the known selenoproteins described to date incorporate selenium into protein co-translationally as the amino acid selenocysteine (Sec) 1 in response to the UGA codon (1, 3, 4), with the exception of several bacterial molybdenum-containing enzymes such as Clostridium barkeri nicotinic acid hydroxylase that contains an active center dissociable selenium species (5-7). Among the selenoproteins thus far identified in mammals are four glutathione peroxidases, three thioredoxin reductases, three thyroid hormone deiodinases, selenophosphate synthetase, selenoprotein P, selenoprotein W, selenoprotein T, selenoprotein R (also called selenoprotein X), selenoprotein N, and a 15-kDa selenoprotein (1, 3, 8 -10).The 15-kDa selenoprotein was initially identified as a strongly labeled protein that was detected when human T cells were grown in the presence of [75 Se]selenite (11). The open reading frame within the human 15-kDa protein cDNA coded for 162 residues and contained an in-frame TGA codon that would result in the incorporation of Sec at codon position 93. A putative stem-loop structure, designated the Sec insertion sequence (SECIS) element, was predicted in the 3Ј-untranslated region (3Ј-UTR). SECIS elements are ...

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Two isoacceptors of selenocysteine tRNA[Ser]Sec are present in higher vertebrates which are responsible for donating selenocysteine to protein. One such selenocysteine containing protein, glutathione peroxidase, requires selenium for its translation and transcription. Since tRNA[Ser]Sec is a critical component of the glutathione peroxidase translational machinery, the levels and distributions of its isoacceptors were examined from both human and rat cells grown in chemically defined media with and without selenium. Not only did the level of the selenocysteine tRNA[Ser]Sec population increase approximately 20% in cells grown in the presence of selenium, but the distributions of the two isoacceptors also changed relative to each other.