Inhibition of polyamine synthesis induces p53 gene expression but not apoptosis

The nuclear phosphoprotein p53 acts as a transcription factor and is involv ed in growth inhibition and apoptosis; The present study was designed to ex amine the effect of decreasing cellular polyamines an p53 gene expression a nd apoptosis in small intestinal epithelial (IEC-S) cells. Cells were grown in DMEM containing 5% dialyzed fetal bovine serum in the presence or absen ce of a-difluoromethylornithine (DFMO), a specific inhibitor of polyamine b iosynthesis, for 4, 6, and 12 days. The cellular polyamines putrescine, spe rmidine, and spermine in DFMO-treated cells decreased dramatically at 4 day s and remained depleted thereafter. Polyamine depletion by DFMO tvas accomp anied 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 p 53 gene. Increased expression of dhe p53 gene in DFMO-treated cells was ass ociated with a significant increase in G(1) phase growth arrest. In contras t, no features of programmmed cell death were identified after polyamine de pletion: no internucleosomal DNA fragmentation observed, and no morphologic al features of apoptosis were evident in cells exposed to DFMO for 4, 6, an d 12 days. These results indicate that 1) decreasing cellular polyamines in creases expression of the p53 gene and 2) activation of p53 gene expression after polyamine depletion does not induce apoptosis in intestinal crypt ce lls. These findings suggest that increased expression of the p53 gene may p lay an important role in growth inhibition caused by polyamine depletion.