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Sensitization of Chinese Hamster Ovary Cells to Heat Shock by -Difluoromethylornithine¹

Department of Radiation Oncology, University of Arizona Health Sciences Center, Tucson, Arizona 85724

When exposed to -difluoromethylornithine (DFMO), an inhibitor of polyamine biosynthesis, Chinese hamster ovary cells become increasingly sensitive to the cytotoxic effects of elevated temperatures (D. J. M. Fuller and E. W. Gerner, Cancer Res., 42: 5046–5049, 1982). This sensitization becomes marked at times greater than 24 h after drug removal, and by 48 h, polyamine-depleted cells that have been exposed to 43°C for 90 min have clonogenic survival values more than two orders of magnitude lower than control populations. Dose response studies demonstrate that, when measured 36 h after removal of the drug, hyperthermic cytotoxicity is maximally potentiated by exposure to DFMO for times as short as 2 to 4 h. A drug concentration of 1 mM for 8 h also elicits maximal response. An additional 8-h drug treatment 24 h after the first fails to further reduce survival in response to heat shock, suggesting the effects of the first exposure are persistent. Intracellular putrescine pools are depleted by the drug within 8 h, and spermidine levels continue to decline for up to 50 h. Consistent with these observations, ornithine decarboxylase (EC 4.1.1.17) activity is found to be reduced for up to 48 h after drug removal. The concomitant depression of spermidine is reflected in the elevation of S-adenosylmethionine decarboxylase (EC 4.1.1.50), which is substrate limited. Putrescine and spermidine show no sign of reaccumulation until approximately 4 days after DFMO exposure. Exposure to exogenous putrescine reversed the sensitization to heat shock induced by DFMO. This effect is quite specific for putrescine (1,4-diaminobutane) and is not replicated by other diamine homologues ranging from 1,3-diaminopropane to 1,8-diaminooctane. Polyamine-depleted cells express thermotolerance with kinetics similar to control cells although overall survival levels are lower. These results suggest that the mechanism of induction and expression of thermotolerance is independent of the role of acid-soluble polyamine pools in cellular responses to heat shock.

¹ This work was supported by USPHS Grants CA-30052 and CA-17343.

² To whom requests for reprints should be addressed.

Received 12/26/85. Revised 9/ 3/86. Accepted 10/21/86.