Spermidine/spermine N1-acetyltransferase transient over-expression restores sensitivity of resistant human ovarian cancer cells to N1,N12-bis(ethyl)spermine and to cisplatin

The limited induction of spermidine/spermine N1-acetyltransferase (SSAT) activity has been implicated as an important determinant of the reduced response, to the spermine analogue N1,N12-bis(ethyl)spermine (BESpm) by the cisplatin (cDDP)-resistant human ovarian carcinoma cell line (C13*). We checked whether under conditions of SSAT over-expression, enzyme induction and cell sensitivity to both BESpm and cDDP were restored to levels comparable with those of more responsive cDDP-sensitive 2008 cells. We transiently transfected the SSAT repressed C13* cells with two expression vectors driving human SSAT over-expression by diverse promoters, and then we analysed their responses in the absence and in the presence of BESpm. SSAT activity was promptly but briefly expressed by transfection with both pOP/SSAT and pCMV-SSAT plasmids. However, only in the presence of BESpm, did SSAT activity reach the highest levels of induction for longer times, with different time-courses for the two vectors, which paralleled the effect on cell growth. Under these conditions, growth sensitivity to BESpm of the less-responsive C13* cells was 25% reverted to cell growth inhibition displayed by 2008 cells. More interestingly, the sensitivity to cDDP cytotoxicity also increased in parallel to SSAT over-expression. BESpm induction of pCMV-SSAT-transfected cells caused a further 20-30% reduction of cell survival induced by cDDP, almost recovering the sensitivity of 2008 cells. The enhanced effectiveness of cDDP was also confirmed by the comet assay, showing an increase in the number and length of tails of damaged DNA. These findings confirm that SSAT over-expression inhibits cell growth and enhances growth sensitivity to BESpm in C13* cells, showing for the first time that restoring high inducibility of SSAT activity subverts the reduced sensitivity to cDDP of SSAT-deficient cells, making them almost indistinguishable from the responsive parental 2008 cells.