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Department of Physiology, College of Medicine, University of Tennessee, Memphis, Memphis, Tennessee 38163
The polyamines spermidine, spermine, and their precursor
putrescine are essential 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. This study examines the involvement of
polyamines in the induction of apoptosis by the DNA topoisomerase I
inhibitor, camptothecin. In IEC-6 cells, camptothecin induced apoptosis
within 6 h, accompanied by detachment of cells. Detached cells showed
DNA laddering and caspase 3 induction, characteristic features of
apoptosis. Depletion of putrescine, spermidine, and spermine by
DL-
-difluoromethylornithine (DFMO), a specific inhibitor
of ornithine decarboxylase (ODC) that is the first rate-limiting enzyme
for polyamine biosynthesis, decreased the apoptotic index. Delayed
apoptosis was accompanied by a decrease in caspase 3 activity in
polyamine-depleted cells. Addition of putrescine restored the induction
of apoptosis as indicated by an increase in the number of detached
cells and caspase 3 activity. Polyamine depletion did not change the
level of caspase 3 protein. Inhibition of S-adenosylmethionine
decarboxylase by a specific inhibitor [diethylglyoxal
bis-(guanylhydrazone); DEGBG] led to depletion of spermidine and
spermine with a significant accumulation of putrescine and induction of
ODC. The DEGBG-treated cells showed an increase in apoptosis,
suggesting the importance of putrescine in the apoptotic process.
Addition of putrescine to DFMO-treated cell 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.
caspases; DL-
-difluoromethylornithine; putrescine; spermidine; spermine
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