A study of polyamine efflux from human cancer cells in culture
The aim of this study was to investigate the relationship between cellular growth status and polyamine efflux, to examine the functional link between acetylation of polyamines and their release from the cell, and to analyse the membrane transport mechanism involved in the polyamine efflux process. The study was based on a human colonic cancer cell line, HT115. Efflux of polyamines was monitored by radiolabelling cellular polyamines and then tracing their appearance in the culture medium. Although the predominant cellular polyamine was spermine, the polyamine almost exclusively effluxed from HT115 cells under normal conditions was N1-acetylspermidine. Actively growing cells released a small but constant amount of N1-acetylspermidine. When cell growth was retarded by serum-deprivation or inhibited by treatment with 5-FU, polyamine efflux increased. The increase in efflux corresponded to the degree of growth inhibition, with maximum release occurring from cells treated with toxic doses (10 M) of 5-FU. Efflux was predominantly of N1-acetylspermidine. Growth inhibition was reversed by re-addition of serum to the culture medium or removal of 5-FU. In response to the re-initiation of cell growth, polyamine efflux decreased. Treatment with 10 M 5-FU, in addition to causing maximum inhibition of cell growth, induced SAT, the enzyme responsible for the formation of N1-acetylspermidine. Upregulated polyamine backconversion coupled with increased efflux of the resulting N1-acetylspermidine was accompanied by a decrease in the total cellular polyamine content. Serum deprivation or doses of 5-FU that were less growth inhibitory did not induce SAT and did not decrease cellular polyamine levels.