Sensitization of human Ewing's tumor cells to chemotherapy and heat treatment by the bioflavonoid quercetin

Background: The bioflavonoid quercetin, a polyphenolic compound widely distributed in the plant kingdom, has been demonstrated to exert cytostatic activity against a variety of tumor cells in vitro and in vivo. It may be useful in cancer therapy as a thermosensitizer by increasing the cell killing effect of hyperthermia and chemotherapy because of its ability to suppress heat-shock protein expression. Materials and Methods: We investigated the effect of quercetin combined with two cytotoxic agents, cDDP (cis-diamminedichloroplatinum II) and VP-16 (etoposide), under various heat-shock conditions in two Ewing's tumor cell lines SK-ES-1 and RD-ES, using XTT-assay and Western blot analysis. Results: Induction of thermotolerance by a sublethal heat-shock (42degreesC, 1 hour) led to a transient resistance against subsequent heat treatment alone or combined thermochemotherapy with the crosslinking agent cDDP or the topoisomerase II inhibitor VP-16. Quercetin (greater than or equal to 50 muM) applied for 24 hours inhibited cell proliferation, increased the cytotoxic activity of cDDP or VP-16 alone or combined with simultaneous hyperthermia and suppressed the development of thermotolerance. Hyperthermia (43degreesC, 45degreesC for 1 hour) induced high expression of the inducible form of HSP70, whereas HSP27, which is constitutively expressed at normothermic conditions, is only slightly induced by 43degreesC and nearly completely suppressed at 45degreesC. Induction of thermotolerance is accompanied by an elevated expression of both HSP70 and HSP27. Quercetin (? 50 muM), alone as well as in combination with thermochemotherapy, inhibited the expression of both HSP70 and HSP27. Conclusion: These data suggest that the bioflavonoid quercetin potentially may be useful in clinical trials for optimizing the efficacy of hyperthermia in combination with chemotherapy.