Consequences of Spermine Synthase or Spermidine/Spermine N- Acetyltransferase Deficiency in Polyamine Metabolism

A vast number of studies have linked polyamines to numerous cellular functions but less is known about the possible physiological functions of the individual polyamines. The specific gene targeting technique has provided a new potential way to resolve this mystery. We generated two separate embryonic stem (ES) cell lines where either spermine synthase gene (SPMSy) or spermidine/spermine N-acetyltransferase gene (SSAT) was disrupted. The SSAT-deficient ES cells enabled the further creation of SSAT knockout mice. We wanted to study the actual role of spermine in mammalian cells as this polyamine is lacking from bacterial cells and also to gain more information of SSAT, which is considered to be the key enzyme in polyamine catabolism. Gene-disrupted models showed only slightly changed polyamine homeostasis at the single polyamine level and the total polyamine pools remained practically unchanged. Spermine-deficient cells compensated for the lack of spermine by significantly increasing the amount of spermidine. SPMSy knockout cells had normal growth rates compared with control cells but showed enhanced sensitivity to antiproliferative drugs. However, spermine may be essential during mouse embryogenesis as the generation of SPMSydeficient mice failed. SSAT knockout cells proliferated also normally and were predictably more resistant than wild-type cells to the cytotoxicity evoked by the polyamine analogue N, N-diethylnorspermine (DENSPM). Polyamine interconversion studies showed that SSAT activity was absolutely necessary for spermidine catabolism, as indicated by the increased spermidine levels in SSATdeficient cells. Interestingly, SSAT seemed to have only marginal importance in spermine catabolism because SSAT knockout cells were able to backconvert spermine to spermidine even more efficiently than control cells. Since SSAT is regarded to be crucial in maintaining normal polyamine pools and therefore preventing the accumulation of toxic levels of higher polyamines, it was somewhat surprising that a lack of a functional SSAT gene would have no effect on the viability, fertility or phenotype of the mice. The indispensability of SSAT in spermidine back-conversion was also detected in vivo as the molar ratio of spermidine to spermine was elevated in most of SSAT knockout tissues. DENSPM showed divergent results in vitro and in vivo, hence SSAT-deficient mice were unexpectedly more sensitive to the drug than their wild-type littermates. DENSPM is considered to exert cytotoxic effects through SSAT induction and thus depletion of higher polyamine pools, but the toxicity of this drug appears to be mediated by other mechanisms in the SSAT-deficient mice. Furthermore, the results with the SSAT knockout mice supported the role of polyamine catabolism in the regulation of glucose and energy metabolism, as these mice were insulin resistant at old age. National Library of Medicine Classification: QU 61, QU 325, QU 450, QY 60.R6 Medical Subject Headings: polyamines/metabolism; spermine synthase; acetyltransferases; stem cells; gene targeting; animals, genetically modified; mice, knockout; energy metabolism; glucose/metabolism; insulin resistance "Ihmeellistä", sanoi orava hämmästyneenä. "Tuota pahvilaatikkoa ei ollut täällä aikaisemmin. Tässä on varmasti jotakin vinossa. Tai sitten tämä on kokonaan väärä luola. Tai minä olen ehkä väärä orava, mutta sitä en oikein uskoisi." Tove Jansson, Taikatalvi