Cisplatin-resistance modulates the effect of protein synthesis inhibitors on spermidine/spermine N-1-acetyltransferase expression

Cisplatin (DDP)-resistance confers a deficient expression of spermidine/spermine N-1-acetyltransferase (SSAT) gene in response to the spermine analog N-1,N-12-bis(ethyl)spermine (BESpm) in the DDP-resistant human ovarian carcinoma cell line (C13*), compared with their parental DDP-sensitive 2008 cells. This SSAT gene deficiency is correlated with a reduced growth sensitivity to spermine analogs. This study was performed to determine whether SSAT gene expression of resistant cells was kept suppressed by labile repressor proteins developed during resistance selection. We show here that inhibitory concentrations of cycloheximide (CHX) and anisomycin (ANISO) differentially affect BESpm-induced SSAT activity in 2008 and in C13* cells in a concentration-dependent manner and allow resistant cells to reach activation levels comparable to those of the sensitive cells. Northern blot analysis revealed that both CHX and ANISO in combination with BESpm caused a synergistic BESpm-mediated accumulation of SSAT mRNA in C13* cells, with respect to each drug alone, while in 2008 cells only a slight increase was observed. The more pronounced effect of inhibitors on the SSAT activity induced by BESpm in the resistant cells was also the result of a more prolonged stabilization of SSAT mRNA and enzyme protein. By contrast, sub-inhibitory concentrations of CHX and ANISO did not significantly stimulate BESpm-induced SSAT transcription and activity. These results suggest that labile repressor proteins, related to DDP-resistance phenotype, play a regulatory role in SSAT gene expression, and further indicate that by overcoming this inhibitory control it is possible to recover BESpm response.