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Department of Surgery, University of Maryland School of Medicine and Baltimore Veterans Affairs Medical Center, Baltimore, Maryland 21201
The nuclear phosphoprotein p53 acts as a transcription factor
and is involved in growth inhibition and apoptosis. The present study
was designed to examine the effect of decreasing cellular polyamines on
p53 gene expression and apoptosis in small intestinal epithelial
(IEC-6) cells. Cells were grown in DMEM containing 5% dialyzed fetal
bovine serum in the presence or absence of
-difluoromethylornithine (DFMO), a specific inhibitor of polyamine biosynthesis, for 4, 6, and
12 days. The cellular polyamines putrescine, spermidine, and spermine
in DFMO-treated cells decreased dramatically at 4 days and remained
depleted thereafter. Polyamine depletion by DFMO was accompanied by a
significant increase in expression of the p53 gene. The p53 mRNA levels
increased 4 days after exposure to DFMO, and the maximum increases
occurred at 6 and 12 days after exposure. Increased levels of p53 mRNA
in DFMO-treated cells were paralleled by increases in p53 protein.
Polyamines given together with DFMO completely prevented increased
expression of the p53 gene. Increased expression of the p53 gene in
DFMO-treated cells was associated with a significant increase in
G1 phase growth arrest. In
contrast, no features of programmmed cell death were identified after
polyamine depletion: no internucleosomal DNA fragmentation was
observed, and no morphological features of apoptosis were evident in
cells exposed to DFMO for 4, 6, and 12 days. These results indicate
that 1) decreasing cellular
polyamines increases expression of the p53 gene and
2) activation of p53 gene expression after polyamine depletion does not induce apoptosis in intestinal crypt
cells. These findings suggest that increased expression of the p53 gene
may play an important role in growth inhibition caused by polyamine depletion.
growth inhibition; proliferation; tumor suppressor gene; ornithine decarboxylase; IEC-6 cells
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