Synergy between sulforaphane and selenium in protection against oxidative damage in colonic CCD841 cells

Dietary isothiocyanates (ITCs) are potent inducers of the NF-E2-related factor-2 (Nrf2) pathway. Sulforaphane (SFN), a representative ITC, has previously been shown to upregulate antioxidant enzymes such as selenium (Se) dependent thioredoxin reductase-1 (TrxR-1) in many tumour cell lines. In the present study, we hypothesized that a combination of SFN and Se would have a synergistic effect on the upregulation of TrxR-1 and the protection against oxidative damage in the normal colonic cell line CCD841. Treatment of cells with SFN and Se significantly induced TrxR-1 expression. Pre-treatment of cells with SFN protects against H2O2-induced cell death; this protection was enhanced by co-treatment with Se. The siRNA knockdown of either TrxR-1 or Nrf2 reduced the protection afforded by SFN and Se co-treatment; TrxR-1 and Nrf2 knockdown reduced cell viabilities to 66.5 and 51.1% respectively, down from 82.4% in transfection negative controls. This suggests that both TrxR-1 and Nrf2 are important in SFN-mediated protection against free radical-induced cell death. Moreover, flow cytometric analysis showed that TrxR-1 and Nrf2 were involved in SFN-mediated protection against H2O2-induced apoptosis. In summary, SFN activates the Nrf2 signaling pathway and protects against H2O2-mediated oxidative damage in normal colonic cells. Combined SFN and Se treatment resulted in a synergistic upregulation of TrxR-1 that in part contributed to the enhanced protection against free radical-mediated cell death provided by the co-treatment.