Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/25/2003
Publication Date: 9/1/2003
Citation: Hintze, K.J., Wald, K., Jeffery, E.H., Finley, J.W., Zeng, H. 2003. Thioredoxin reductase in human hepatoma cells is transcriptionally regulated by sulforaphane and other electrophiles via an antioxidant response element. Journal of Nutrition. 133:2721-2727. Interpretive Summary: Thioredoxin reductase is an antioxidant enzyme that increases in amount and activity with increases in dietary selenium. We have investigated whether this enzyme may be 'turned on' by minor compounds found in plant-based diets, and whether this is part of an overall response to reduce cancer risk. We have demonstrated that thioredoxin reductase production in cells can be stimulated by the minor plant compound sulforaphane. We have proven that this increase occurs through a mechanism different from the way dietary selenium increases the activity of thioredoxin reductase. Dietary selenium has been demonstrated to be anticarcinogenic in animal and human studies. Elevated activity of thioredoxin reductase resulting from increased dietary selenium may partially explain the anticarcinogenic effect of selenium. Therefore, diets high in both selenium and sulforaphane may be especially beneficial
Technical Abstract: Previously our lab has demonstrated in-vitro and in-vivo, induction of TR activity by sulforaphane (SF) purified from broccoli. The objective of the present study was to determine if this induction is mediated through the promoter region of the gene. Luciferase reporter constructs were built using the TR promoter sequence. TR constructs transfected into HepG2 cells showed a dose dependent increase in transcription when exposed to electrophillic compounds such as SF (0.5 to 2 µM; P < 0.0001). The addition of tert-butylhydroquinone and ß-Napthoflavone to the medium also increased transcription in a dose dependent manner (P < 0.0001). Similar patterns were observed for cells transfected with quinone reductase promoter constructs, which are inducible by electrophillic compounds via an antioxidant response element-dependent pathway. In another experiment, TR construct transfected cells were exposed to SF and/or Se to test whether SF modulates TR transcription with or without Se. The design was a 2X2 factorial:.2.0µM Se + 2.0µM SF, 2.0µM Se, 2.0µM SF, or control media. Combined Se/SF and SF alone both showed a two-fold increase in transcription compared to controls (P < 0.0001), Se had no effect on transcription. However, both Se and SF independently increased TR activity compared to controls (P < 0.0001) and when combined increased TR activity synergistically (P = 0.036). These data suggest TR is transcriptionally responsive to electrophillic compounds through a mechanism novel from the known Se dependent induction of selenoproteins.