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Effects of inhibitors of the heat shock protein 90 (HSP90) chaperone activity and inhibitors of the heat shock protein (HSP) expression on sensitivity of HeLa tumor cells to hyperthermia were studied. It was found that nanomolar concentrations of inhibitors of the HSP90 activity (17AAG or radicicol) slowed down chaperone-dependent reactivation of a thermo-labile reporter (luciferase) in heat-stressed HeLa cells and slightly enhanced their death following incubation for 60 min at 43 degrees C. Herein, the inhibitors of HSP90 activity stimulated de novo induction of additional chaperones (HSP70 and HSP27) that significantly increased the intracellular HSP levels. If the cells were treated with 17AAG or radicicol along with an inhibitor of the HSP induction (e.g. quercetin or triptolid, or NZ28), this fully prevented the increase in intracellular chaperone levels resulting from the inhibition of HSP90 activity and subsequent heating. Importantly, in the case of conjunction of all the three treatments (an inhibitor of the HSP90 activity + an inhibitor of the HSP induction + 43 degrees C for 60 min), the reporter reactivation was retarded yet stronger while the cell death was sharply (2-3-fold) enhanced. Such an enhancement of the cytotoxicity appears to occur owing to the "chaperone deficiency" when prior to heat stress both the functional activity of constitutive HSP90 and the expression of additional (inducible) chaperones are blocked in the cells.