Polyamine depletion delays apoptosis of rat intestinal epithelial cells

The polyamines spermidine, spermine, and their precursor putrescine are ess ential for cell growth and the regulation of the cell cycle. Recent studies suggest that excessive accumulation of polyamines favors either malignant transformation or apoptosis, depending on the cell type and the stimulus. T his study examines the involvement of polyamines in the induction of apopto sis by the DNA topoisomerase I inhibitor, camptothecin. In IEC-6 cells, cam ptothecin induced apoptosis within 6 h, accompanied by detachment of cells. Detached cells showed DNA laddering and caspase 3 induction, characteristi c features of apoptosis. Depletion of putrescine, spermidine, and spermine by DL-alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithi ne decarboxylase (ODC) that is the first rate-limiting enzyme for polyamine biosynthesis, decreased the apoptotic index. Delayed apoptosis was accompa nied by a decrease in caspase 3 activity in polyamine-depleted cells. Addit ion of putrescine restored the induction of apoptosis as indicated by an in crease in the number of detached cells and caspase 3 activity. Polyamine de pletion did not change the level of caspase 3 protein. Inhibition of S aden osylmethionine decarboxylase by a specific inhibitor [diethylglyoxal bis-(g uanylhydrazone); DEGBG] led to depletion of spermidine and spermine with a significant accumulation of putrescine and induction of ODC. The DEGBG-trea ted cells showed an increase in apoptosis, suggesting the importance of put rescine in the apoptotic process. Addition of putrescine to DFMO-treated ce ll extracts did not increase caspase 3 activity. The above results indicate that polyamine depletion delays the onset of apoptosis in IEC-6 cells and confers protection against DNA damaging agents, suggesting that polyamines might be involved in the caspase activating signal cascade.