Synergistic Cytotoxic Effects of Arsenic Trioxide plus Dithiothreitol on Mice Oral Cancer Cells

The anti-tumor properties of arsenic trioxide have attracted extensive attention after successfully inducing apoptosis of acute promyelocytic leukemia cells. However, the therapeutic spectrum should not only be restricted to acute promyelocytic leukemia, but should also extend into other types of tumor cells. In this study, we aimed at investigating its potential application to clinical therapeutics in oral cancer. In this preclinical animal test, primarily cultured cells from the tumor sites and normal sites of a two-drug (200 mu g/ml 4-nitroquinoline 1-oxide (4NQO) plus 500 mu g/ml arecoline)-induced oral cancer C57BL/6J Narl mice model were examined for their viabilities after treatments of arsenic trioxide with/without other drugs. In this model, the mice were treated with 4NQO plus arecoline (NA) in their drinking water for eight weeks (8-w), and the drugs were withdrawn for another 10 or 20 weeks (18-w and 28-w, respectively). The results showed that 2 mu M of arsenic trioxide 24-h treatment suppressed the viabilities of cells primarily cultured from the tumor sites of 8-w, 18-w and 28-w NA-treated mice to 72.9%, 71.5% and 65.6%. However, it also suppressed the viabilities of cells from the sham-treated mice of 8-w, 18-w and 28-w to 76.8%, 73.4% and 75.7%, respectively. Therefore, 0.5 mu M of arsenic trioxide treatment for 24 h, which suppressed the viabilities of cells primarily cultured from the tumor sites of 28-w NA-treated and sham-treated mice to 15.6% and 9.1%, was examined for its synergistic effects on the two primarily cultured cell lines with other drugs. The results showed that 10-20 mu M dithiothreitol enhanced the cytotoxic effects of arsenic trioxide to 43.3 similar to 62.1%, better than those of 4 J/m(2) UVC, 20 mu M H2O2 or 100 mu M buthionine sulfoximine (21.3%, 13.2%, and 14.2%, respectively). At the same time, 10-20 mu M dithiothreitol plus 0.5 mu M arsenic trioxide treatments caused only 12.3% and 15.2% of cell death in the control group. The cytotoxicity of dithiothreitol and arsenic trioxide combination on primarily cultured cells from this oral cancer model should be confirmed in human oral cancer cell lines before its application in clinical therapy, and the detailed mechanism is worth further investigation.