BACKGROUND.
Hyperthermia can enhance the clinical response of chemotherapeutic agents in prostate cancer, but optimal sequencing of this combination therapy needs to be developed. Given the role of heat shock proteins (HSPs) in the development of resistance (thermotolerance) to subsequent hyperthermic stresses as well as to certain chemotherapeutics, the study of HSP regulation is important in the establishment of effective schedules in multimodal treatment strategies. METHODS. In this study we evaluated the effects of the chemotherapeutic agents cisplatin, 5-fluorouracil, and adriamycin in combination with hyperthermia. (43ยฐC, 1 h) on clonogenic survival and inducible HSP70 regulation in Dunning rat adenocarcinoma of the prostate. HSP70 was analyzed by Western blot and by measuring โค-galactosidase produced by cells stably transfected with a gene construct containing the E. coli โค-galactosidase gene driven by the Drosophila HSP70 promoter. RESULTS. Colony formation assays revealed a sensitizing effect of hyperthermia when simultaneously combined with each chemotherapeutic agent, resulting in a potentiated cytotoxicity compared to subsequenced treatments. Thermotolerant cells showed a significantly better survival when treated with adriamycin alone, but also when each chemotherapeutic agent was combined with hyperthermia. This enhanced survival was correlated with inducible HSP70 accumulation. The chemotherapeutics modified the HSP70 promoter activation induced by hyperthermia, suggesting changes in the development of cellular thermotolerance. CONCLUSIONS. Our data reveal synergistic cytotoxic effects of the synchronous application of chemotherapeutic agents and hyperthermia on this model of prostate cancer. Furthermore, they demonstrate that the induction of HSPs in thermotolerant cells, as measured by HSP70 induction, results in a modulation the chemotherapeutic-mediated cytotoxicity. Therefore, HSP70 is a useful marker of cellular resistance in multimodal approaches combining hyperthermia and chemotherapeutic agents in the treatment of locally advanced prostate carcinoma.