Altered spermidine/spermine N-1-acetyltransferase activity as a mechanism of cellular resistance to bis(ethyl)polyamine analogues

To develop a model system to investigate mechanisms of antiproliferative ac tion of bis(ethyl)polyamine analogues, intermittent analogue treatments fol lowed by recovery periods in drug-free medium were used to select an N-1,N- 12-bis(ethyl)spermine-resistant derivative of the Chinese hamster ovary cel l line C55.7. The resulting C55,7Res line was at least 10-fold resistant to N-1,N-11-bis(ethyl)spermine and N-1,N-11-bis(ethyl)norspermine. The stabil ity of the resistance in the absence of selection pressure was greater than or equal to 9 months, indicating that a heritable genotypic change was res ponsible for the resistance phenotype. Polyamine transport alterations and multi-drug resistance were eliminated as causes of the resistance. Spermidi ne/spermine N-1-acetyltransferase (SSAT) activity and regulation were alter ed in C55,7Res cells as basal activity was decreased, and no activity induc tion resulted from exposure to analogue concentrations, which caused 300-fo ld enzyme induction in parental cells. SSAT mRNA levels in the absence and presence of analogue were unchanged, but no SSAT protein was detected in C5 5,7Res cells. A point mutation, which results in the change leucine156 (a f ully conserved residue) to phenylalanine, was identified in the C55,7Res SS AT cDNA. Expression of wtSSAT activity in C55.7Res cells restored sensitivi ty to bis(ethyl)polyamines. These results provided definitive evidence that SSAT activity is a critical target of the cytotoxic action of these analog ues.