selenium, arsenic, thioredoxin reductase, selenoproteins
Arsenic exposure, through various routes, is associated with the development of cancer of the skin, lung, liver, kidney, and bladder. Treatment of cells in culture with trivalent arsenicals has been shown to increase reactive oxygen species (ROS). In particular, monomethylarsonous acid (MMAIII), a trivalent metabolite of arsenite, is highly cytotoxic and possibly carcinogenic. Three trivalent arsenicals; arsenite, arsenic trioxide (ATO), and MMAIII, are also known inhibitors of the selenoprotein thioredoxin reductase (TrxR). Selenium, an essential micronutrient in mammals, is needed in the form of selenocysteine for activity of this enzyme and other selenoproteins. TrxR is part of a key component of the cell's ability to defend against ROS. It has been speculated that TrxR is also involved directly in selenium metabolism, but this has yet to be demonstrated in vivo. The promoter region of the gene encoding the cytosolic TrxR (TrxR1) also contains an antioxidant responsive element (ARE). The ARE is activated by the transcription factor, Nrf2, which is governed by the Nrf2/Keap1 response, and can be triggered by certain oxidants. ATO and arsenite both inhibited incorporation of selenium into selenoproteins. Auranofin, a gold chemotherapeutic inhibitor of TrxR1, also inhibited selenoprotein synthesis. These results seem to support the hypothesis that TrxR1 is needed for selenoprotein synthesis. However, siRNA mediated reduction of TrxR1 did not block incorporation of selenium into selenoproteins. It is likely that ATO and auranofin are forming As-Se and Au-Se complexes, respectively. We also found that exposure of primary lung fibroblasts (WI-38) to MMAIII led to increased synthesis of TrxR1. This increase was dependent on the activation of transcription of the TrxR1 gene, specifically mediated through the ARE element. These results indicate exposure to MMAIII induces the Nrf2 response. The results obtained in these studies aid in both our understanding of the carcinogenic potential of arsenic as well as give new insight into the mechanism of action of emerging cancer drugs.
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Master of Science (M.S.)
Burnett College of Biomedical Sciences
Molecular Biology and Microbiology
Molecular and Microbiology
Length of Campus-only Access
Masters Thesis (Open Access)
Talbot, Sarah Ryann, "The Effects Of Trivalent Arsenicals And Thioredoxin Reductase Inhibitors On Selenium Metabolism In Lung Cell Culture Models" (2007). Electronic Theses and Dissertations. 3376.