Molecular mechanism of cytotoxic synergism induced by arsenic trioxide and EGCG

C56 Arsenic trioxide (ATO) has shown substantial efficacy in treating patients with acute myelocytic leukemia as well as solid tumors. However, its side effects to human health have significantly limited its applicable dose range in clinical application. Alternatives such as combination with other drugs might provide a useful strategy for the sustainable use of ATO in cancer therapy. Previously, we have described the synergistic effect of ATO with green tea extract, EGCG, to human cancer cell lines. In this report we are going to present the results on the mechanism elucidation of this synergistic action. In our investigation, ATO and EGCG, when applied jointly, induced oxidative stress and apoptosis significantly, whereas did not so when treated each by itself. The deprivation of ferrous iron decreased while that of ferric iron increased the synergistic effect, indicating the involvement of Fenton reaction, probably through the reacting with EGCG-induced peroxide species. Inhibition of heme oxygenase-1 (HO-1) also decreased the synergistic effect significantly. This revealed that ferrous iron probably was provided by the HO-1-catalyzed heme degradation induced by ATO. Confirmation also came from the Western analysis showing that EGCG inhibited the ferritin, which has been known to play a vital role in ferrous iron degradation. Based on those observations, we tentatively proposed hypotheses to explain the synergistic effect of ATO and EGCG as follows:
 (1) ATO provided ferrous iron through the mediation of heme degradation;
 (2) The degradation of transient ferrous iron was retarded due to the inhibition of ferritin by EGCG;
 (3) Ferrous irons thereby react with EGCG-produced hydrogen peroxide to process Fenton reaction and induced the cytotoxicity.