Role of polyamines at the G1/S boundary and G2/M phase of the cell cycle

Abstract The role of polyamines at the G 1 /S boundary and in the G 2 /M phase of the cell cycle was studied using synchronized HeLa cells treated with thymidine or with thymidine and aphidicolin. Synchronized cells were cultured in the absence or presence of α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, plus ethylglyoxal bis(guanylhydrazone) (EGBG), an inhibitor of S -adenosylmethionine decarboxylase. When polyamine content was reduced by treatment with DFMO and EGBG, the transition from G 1 to S phase was delayed. In parallel, the level of p27 Kip1 was greatly increased, so its mechanism was studied in detail. Synthesis of p27 Kip1 was stimulated at the level of translation by a decrease in polyamine levels, because of the existence of long 5′-untranslated region (5′-UTR) in p27 Kip1 mRNA. Similarly, the transition from the G 2 /M to the G 1 phase was delayed by a reduction in polyamine levels. In parallel, the number of multinucleate cells increased by 3-fold. This was parallel with the inhibition of cytokinesis due to an unusual distribution of actin and α-tubulin at the M phase. Since an association of polyamines with chromosomes was not observed by immunofluorescence microscopy at the M phase, polyamines may have only a minor role in structural changes of chromosomes at the M phase. In general, the involvement of polyamines at the G 2 /M phase was smaller than that at the G 1 /S boundary.