Effects of difluoromethylornithine on proliferation, polyamine content and plating efficiency of cultured human carcinoma cells.

We investigated the effects of an irreversible inhibitor of ornithine decarboxylase, difluoromethylornithine (DFMO), on the proliferation, polyamine content, and plating efficiency of five human adenocarcinoma cell lines in vitro. DFMO inhibited the growth of all five lines when added at concentrations between 0.1 and 5.0 mM. The cell lines varied in their sensitivity to DFMO-induced cytostasis, HuTu-80 being most sensitive and HT-29 being most resistant. These differences appeared to be related to the ability of DFMO to prevent continued production of putrescine in the treated cells. Exogenous putrescine (5-50 microM) reversed the growth inhibition for all five cell lines when added 48 h after DFMO treatment. The lowest concentration of exogenous putrescine (5 microM) only restored intracellular polyamine content of DFMO-treated cells to control levels for the first 24 h after its addition. After that time, spermidine content declined once again to that observed for cells treated with DFMO alone. The higher concentrations of exogenous putrescine restored the content of all three polyamines to control levels for as much as 3 days after its addition, but did not cause a greater increase in cell growth rates than did 5 microM putrescine. These data suggest that human adenocarcinoma cell proliferation is dependent on continued polyamine biosynthesis, but that the basal content of intracellular polyamines is greatly in excess of the minimum level required to support cell growth. In the 1.0-5.0 mM concentration range, DFMO treatment for 48 h caused a slight, but statistically significant, reduction in plating efficiency for three of our cell lines, and had no significant effect on the two others.