The role of polyamine depletion and accumulation of decarboxylated S-adenosylmethionine in the inhibition of growth of SV-3T3 cells treated with α-difluoromethylornithine

The effects of α-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, on cell growth rate, polyamine content and the content of decarboxylated S-adenosylmethionine in SV-3T3 transformed mouse fibroblasts were studied. DL-α-Difluoromethylornithine at 1 mM or higher concentrations decreased the growth rate by over 90% after 2 or more days of exposure, but the cells remained viable, although quiescent for at least 9 days. Addition of 10 μM-spermidine or -spermine or 50 μM-putrescine at any time throughout this period completely reversed the inhibition of growth. Treatment with α-difluoromethylornithine decreased putrescine and spermidine contents by more than 98% and that of spermine by 60%, but cells exposed to exogenous polyamines did not require complete replenishment of the polyamine pools to resume growth. In fact, a virtually normal growth rate was obtained in cells lacking putrescine, having 2% of normal spermidine content and 156% of normal spermine. These results suggest that the well-known increase in putrescine and spermidine in cells stimulated for growth is not essential for this to occur and that mammalian cells can utilize spermine as their only polyamine. A substantial reversal of the growth-inhibitory effect of α-difluoromethylornithine was produced by a number of polyamines not normally found in mammalian cells, including the spermidine analogues aminopropylcadaverine and sym-homospermidine, which were partially converted into their respective spermine analogues by addition of an aminopropyl group within the cell. The spermine analogue sym-norspermine was also effective, but the maximal growth rate produced by these unphysiological polyamines was only 60-70% of that produced by the normal polyamines. These results indicate that spermidine and spermine have the optimal length for activation of the cellular processes critically dependent on polyamines and should help in identifying these processes. Exposure to α-difluoromethylornithine leads to an enormous rise in the concentration of decarboxylated S-adenosylmethionine, which reached a peak at 530-fold after 3 days of exposure and steadily declined to 140-fold after 11 days. This increase was abolished by addition of exogenous polyamines, which rapidly decreased the activity of S-adenosylmethionine decarboxylase. The increase in decarboxylated S-adenosylmethionine is unlikely to be solely responsible for the decrease in growth rate, since decarboxylated S-adenosylmethionine content was decreased to the same extent by spermine, sym-norspermidine and sym-homospermidine, which produce 97%, 16% and 60% of the control growth rate, respectively. However, the change in the content of this nucleoside may contribute to the effects of α-difluoromethylornithine in these cells.