Polyamine depletion arrests cell cycle and induces inhibitors p21(Waf1/Cip1), p27(Kip1), and p53 in IEC-6 cells

The polyamines spermidine and spermine and their precursor putrescine are i ntimately involved in and are required for cell growth and proliferation. T his study examines the mechanism by which polyamines modulate cell growth, cell cycle progression, and signal transduction cascades. IEC-6 cells were grown in the presence or absence of DL-alpha-difluoromethylornithine (DFMO) , a specific inhibitor of ornithine decarboxylase, which is the first rate- limiting enzyme for polyamine synthesis. Depletion of polyamines inhibited growth and arrested cells in the G(1) phase of the cell cycle. Cell cycle a rrest was accompanied by an increase in the level of p53 protein and other cell cycle inhibitors, including p21(Waf1/Cip1) and p27(Kip1). Induction of cell cycle inhibitors and p53 did not induce apoptosis in IEC-6 cells, unl ike many other cell lines. Although polyamine depletion decreased the expre ssion of extracellular signal-regulated kinase (ERK)-2 protein, a sustained increase in ERK-2 isoform activity was observed. The ERK-1 protein level d id not change, but ERK-1 activity was increased in polyamine-depleted cells . In addition, polyamine depletion induced the stress-activated protein kin ase/c-Jun NH2-terminal kinase (JNK) type of mitogen-activated protein kinas e (MAPK). Activation of JNK-1 was the earliest event; within 5 h after DFMO treatment, JNK activity was increased by 150%. The above results indicate that polyamine depletion causes cell cycle arrest and upregulates cell cycl e inhibitors and suggest that MAPK and JNK may be involved in the regulatio n of the activity of these molecules.