Effects of Conditional Overexpression of Spermidine/Spermine N-Acetyltransferase on Polyamine Pool Dynamics, Cell Growth, and Sensitivity to Polyamine Analogs*

Acetylation of polyamines by spermidine/spermine N-acetyltransferase (SSAT) has been implicated in their degradation and/or export out of the cell. The relationship of SSAT to polyamine pool dynamics and cell growth is not yet clearly understood. MCF-7 human breast carcinoma cells were transfected with tetracycline-regulated (Tet-off) SSAT human cDNA or murine gene. Doxycycline removal for >2 days caused a ;20-fold increase in SSAT RNA and a ;10-fold increase in enzyme activity. After 4 days, intracellular putrescine and spermidine pools were markedly lowered, and cell growth was inhibited. Growth inhibition could not be prevented with exogenous polyamines due to a previously unrecognized ability of SSAT to rapidly acetylate influxing polyamines and thereby prevent restoration of the endogenous pools. Instead, cells accumulated high levels of N-acetylspermidine, N-acetylspermine, and N,Ndiacetylspermine, a metabolite not previously reported in mammalian cells. Doxycycline deprivation before treatment with N,N-diethylnorspermine markedly increased analog induction of SSAT mRNA and activity and enhanced growth sensitivity to the analog by ;100-fold. Overall, the findings demonstrate that conditional overexpression of SSAT lowers polyamine pools, inhibits cell growth, and markedly enhances growth sensitivity to certain analogs. The enzyme also plays a remarkably efficient role in maintaining polyamine pool homeostasis during challenges with exogenous polyamines.