The role of thioredoxin reductase 1 in melanoma metabolism and metastasis.

Although significant progress has been made in targeted and immunologic therapeutics for melanoma, many tumors fail to respond, and most eventually progress when treated with the most efficacious targeted combination therapies thus far identified. Therefore, alternative approaches that exploit distinct melanoma phenotypes are necessary to develop new approaches for therapeutic intervention. Tissue microarrays containing human nevi and melanomas were used to evaluate levels of the antioxidant protein thioredoxin reductase 1 (TR1), which was found to increase as a function of disease progression. Melanoma cell lines revealed metabolic differences that correlated with TR1 levels. We used this new insight to design a model treatment strategy that creates a synthetic lethal interaction wherein targeting TR1 sensitizes melanoma to inhibition of glycolytic metabolism, resulting in a decrease in metastases in vivo. This approach holds the promise of a new clinical therapeutic strategy, distinct from oncoprotein inhibition.

Methods to detect protein glutathionylation.

Glutathionylation is a posttranslational modification that results in the formation of a mixed disulfide between glutathione and the thiol group of a protein cysteine residue. Glutathionylation of proteins occurs via both nonenzymatic mechanisms involving thiol/disulfide exchange and enzyme-mediated reactions. Protein glutathionylation is observed in response to oxidative or nitrosative stress and is redox-dependent, being readily reversible under reducing conditions. Such findings suggest that glutathionylation plays an important role in mediating redox-sensitive signaling. Indeed, glutathionylation can affect protein function by altering activity, protein-protein interactions, and ligand binding. Glutathionylation may also serve to prevent cysteine residues from undergoing irreversible oxidative modification. Thus, determining the ability of a given protein to become glutathionylated can provide insight into its redox regulation and putative role in dictating cellular response to oxidative and nitrosative stress. Methods to measure protein glutathionylation using immunoblotting and mass spectrometry are described.

Major differences among chemopreventive organoselenocompounds in the sustained elevation of cytoprotective genes.

Cytoprotective enzyme elevation through the nuclear erythroid 2 p45-related factor 2 (Nrf2)-Kelch-like ECH-associated protein 1/antioxidant response element pathway has been promulgated for cancer prevention. This study compares the redox insult and sustained cytoprotective enzyme elevation by organoselenocompounds and sulforaphane (SF) in lung cells. SF elicited a rise in reactive oxygen species (ROS) and drop in glutathione (GSH) at 2 h; nuclear accumulation of Nrf2 at 4 h; and a GSH rebound and elevation in NAD(P)H quinone oxidoreductase (NQO1), thioredoxin reductase (TR1), and glutamate-cysteine ligase (GCL) at 24 h. Selenocystine (SECY) elicited a similar 24 h response, despite lesser earlier time-point changes. 2-Cyclohexylselenazolidine-4-carboxylic acid effects were similar to SECY's but with a larger Nrf2 change and the largest 24 h increase in GSH, GCL, TR1, and NQO1 of any compound investigated. Selenomethionine elicited a similar acute rise in ROS, but lesser depletion of GSH, no 4 h increase in nuclear Nrf2, only minor 24 h elevations in TR1 and NQO1, and a GCL elevation insufficient to elevate GSH.

Thioredoxin reductase 1 knockdown enhances selenazolidine cytotoxicity in human lung cancer cells via mitochondrial dysfunction.

Thioredoxin reductase (TR1) is a selenoprotein that is involved in cellular redox status control and deoxyribonucleotide biosynthesis. Many cancers, including lung, overexpress TR1, making it a potential cancer therapy target. Previous work has shown that TR1 knockdown enhances the sensitivity of cancer cells to anticancer treatments, as well as certain selenocompounds. However, it is unknown if TR1 knockdown produces similar effect on the sensitivity of human lung cancer cells. To further elucidate the role of TR1 in the mechanism of selenocompounds in lung cancer, a lentiviral microRNA delivery system to knockdown TR1 expression in A549 human lung adenocarcinoma cells was utilized. Cell viability was assessed after 48 hr treatment with the selenocysteine prodrug selenazolidines 2-butylselenazolidine-4(R)-carboxylic acid (BSCA) and 2-cyclohexylselenazolidine-4-(R)-carboxylic acid (ChSCA), selenocystine (SECY), methylseleninic acid (MSA), 1,4-phenylenebis(methylene)selenocyanate (p-XSC), and selenomethionine (SEM). TR1 knockdown increased the cytotoxicity of BSCA, ChSCA, and SECY but did not sensitize cells to MSA, SEM, or p-XSC. GSH and TR1 depletion together decreased cell viability, while no change was observed with GSH depletion alone. Reactive oxygen species generation was induced only in TR1 knockdown cells treated with the selenazolidines or SECY. These three compounds also decreased total intracellular glutathione levels and oxidized thioredoxin, but in a TR1 independent manner. TR1 knockdown increased selenazolidine and SECY-induced mitochondrial membrane depolarization, as well as DNA strand breaks and AIF translocation from the mitochondria. These results indicate the ability of TR1 to modulate the cytotoxic effects of BSCA, ChSCA and SECY in human lung cancer cells through mitochondrial dysfunction.

Modulation of redox status in human lung cell lines by organoselenocompounds: selenazolidines, selenomethionine, and methylseleninic acid.

Cancer prevention strategies utilizing selenium-containing compounds have demonstrated reduced cancer mortality and efficacy for some cancer types but considerable differences in cellular effects exist among the selenocompounds employed. The variability of the effects on cell viability, redox modulation, and disruption of subcellular compartments by the conventional selenium-containing amino acid, selenomethionine, the oxidized selenosugar metabolite, methylseleninic acid, and selenazolidines was investigated in A549 and BEAS-2B human lung cell lines. Selenomethionine had little effect whereas methylseleninic acid increased cellular thiols and stress in the endoplasmic reticulum. The cyclohexylselenazolidine increased mild oxidative stress in the adenocarcinoma cell line, A549, but the effects were attenuated in the normal, but virally transformed cell line, BEAS-2B. These data demonstrate that all selenocompounds are not equal and that the form of the organic selenocompound is a major determinant in the expected cellular response.