| Increased dietary selenium (Se) may be protective against cancer. Broccoli intake is also associated with cancer protection, and broccoli enriched with Se is more protective against colon cancer than unenriched broccoli. This series of studies has examined the regulation of the antioxidant selenoprotein thioredoxin reductase (TR) in an attempt to determine whether it is associated with cancer protection. Dietary Se from broccoli or Se salts increased TR activity and mRNA, but not protein, compared to Se-deficient diets. Inhibition of kidney TR activity by aurothioglucose injection was partially ameliorated by increasing dietary Se. Rats fed diets containing broccoli or the glucosinolate sulforaphane (SF) had increased TR activity and protein, but decreased glutathione peroxidase activity and protein. Se plus SF synergistically increased TR activity more than either treatment alone in cultured cells. To investigate the molecular mechanisms of SF-mediated TR upregulation, TR promoter constructs were built and found to be responsive to SF. Mutation of a putative antioxidant response element (ARE) in the promoter ablated this activity, and the presence of an ARE was confirmed by gel shift assay. Further experimentation determined that SF is the most important component of broccoli for ARE induction and that physiological doses of ascorbic acid also induce TR. siRNA technology was used to inhibit TR expression: knockdown (80%) did not affect cell growth or cycle, but increased total glutathione. Total protein carbonyl formation was increased relative to controls in knockdown cells. Taken together, these studies demonstrate that TR is important in preventing protein oxidation; its expression is regulated both like a classical selenoprotein and a phase II antioxidant enzyme. |
