The effect of polyamine depletion on the survival response of human lung carcinoma cells (A-549) to acute heating at 45°C and its effect on the induction and decay of thermotolerance were investigated in exponential and plateau-phase cells. A 48-h exposure to 1 mM α-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, was used to deplete intracellular levels of putrescine and spermidine. Inhibition of polyamine synthesis had no effect on the survival of exponential cells to heating at 45°C, but slightly enhanced the killing of slowly proliferating plateau-phase cells. While DFMO treatment did not inhibit the development of thermotolerance, it caused a reduction in the thermotolerance ratio of exponential cells from 2.6 to 1.80, and from 1.66 to 1.59 in plateau-phase cells. DFMO caused thermotolerance to decay more rapidly in polyamine-depleted cells as well. Flow cytometry demonstrated that DFMO did not alter the cell cycle distribution of plateau-phase cells (i.e., >73% in G1/ G0), but caused a block and time-dependent accumulation of exponential cells in G1/ G0. The cytostatic properties of DFMO in exponential cells which favor its use with phase-specific agents, and its ability to alter the magnitude and decay of thermotolerance in human carcinoma cells suggest a potential role for this nontoxic agent in clinically oriented hyperthermia studies.

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