Development of cross-resistance between heat and cisplatin or hydroxyurea treatments in FaDu squamous carcinoma cells

Background. Induction of hyperthermia by radiofrequency ablation is gaining popularity in treating a variety of solid tumors. This study examined an impact of sublethal heat treatment interacted with chemotherapeutic drugs on the survival of head and neck squamous carcinoma cells using in vitro model. Materials and methods. FaDu cells were subjected to heat treatment at 42°C or 45°C for 15 min either before or after exposure to cisplatin or hydroxyurea. The survival of cells was determined by 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide assay. The RNA and protein levels of various heat shock proteins were examined by reverse transcription polymerase chain reaction and Western blot analysis, respectively. Cell cycle progression was analyzed by flow cytometry with propidium iodide staining. Results. FaDu cells preheated to 45°C exhibited an increased resistance to hydroxyurea but not to cisplatin. The heat treatment resulted in induction of HSP70 expression at transcript and protein levels, but there was no change in expression of HSP90β and HSP27. After heat treatment, cells accumulated in S-phase at 3 h and proceeded to G₂/M phase at 24 h. When cells pre-exposed to drugs for 24 h, the cisplatin-treated cells exhibited a higher thermotolerance than the hydroxyurea-treated cells at heat treatment of 45°C. Cisplatin and hydroxyurea caused cells to accumulate in S-phase and increased the protein expression of HSP27 but not HSP90β and HSP70. Conclusion. FaDu cells surviving the heat treatment expressed HSP70 and disrupted cell cycle progression, which resulted in developing a resistance to subsequent hydroxyurea treatment. However, the heat treatment did not have an effect on the sensitivity to cisplatin. In the reversed procedure, pre-exposure to hydroxyurea and cisplatin resulted in developing a thermotolerance.