NF-κB activation and susceptibility to apoptosis after polyamine depletion in intestinal epithelial cells

The maintenance of intestinal mucosal integrity depends on a balance between cell renewal and cell death, including apoptosis. The natural polyamines, putrescine, spermidine, and spermine, are essential for mucosal growth, and decreasing polyamine levels cause G₁ phase growth arrest in intestinal epithelial (IEC-6) cells. The present study was done to determine changes in susceptibility of IEC-6 cells to apoptosis after depletion of cellular polyamines and to further elucidate the role of nuclear factor-κB (NF-κB) in this process. Although depletion of polyamines by α-difluoromethylornithine (DFMO) did not directly induce apoptosis, the susceptibility of polyamine-deficient cells to staurosporine (STS)-induced apoptosis increased significantly as measured by changes in morphological features and internucleosomal DNA fragmentation. In contrast, polyamine depletion by DFMO promoted resistance to apoptotic cell death induced by the combination of tumor necrosis factor-α (TNF-α) and cycloheximide. Depletion of cellular polyamines also increased the basal level of NF-κB proteins, induced NF-κB nuclear translocation, and activated the sequence-specific DNA binding activity. Inhibition of NF-κB binding activity by sulfasalazine or MG-132 not only prevented the increased susceptibility to STS-induced apoptosis but also blocked the resistance to cell death induced by TNF-α in combination with cycloheximide in polyamine-deficient cells. These results indicate that 1) polyamine depletion sensitizes intestinal epithelial cells to STS-induced apoptosis but promotes the resistance to TNF-α-induced cell death, 2) polyamine depletion induces NF-κB activation, and 3) disruption of NF-κB function is associated with altered susceptibility to apoptosis induced by STS or TNF-α. These findings suggest that increased NF-κB activity after polyamine depletion has a proapoptotic or antiapoptotic effect on intestinal epithelial cells determined by the nature of the death stimulus.