Apparent post-transcriptional modification of ornithine decarboxylase accounts for its induction in IEC-6 cells in culture.

Putrescine, the product of the ornithine decarboxylase (ODC)-catalyzed reaction, stimulates macromolecular synthesis in a duodenal crypt cell line, IEC-6 cells, grown in culture. In addition, supplementation of medium with putrescine alone reverses the inhibition of proliferation produced by inhibition of ODC with difluoromethylornithine (DFMO). A series of experiments was initiated in the IEC-6 cell line to study the regulation of induction of ODC, as this enzyme is rate-limiting in putrescine synthesis. Five percent fetal bovine serum (FBS) and 10 nM IGF-II stimulated a 21-fold and 6-fold induction of ODC activity, respectively. Kinetic analysis indicated that the effect was on the Vmax of the reaction and not on the Km, suggesting an increase in total ODC protein. This was verified by measuring [3H]DFMO binding; serum-stimulated induction of activity was accompanied by a corresponding 20-fold increase in the specific binding of DFMO to ODC. In contrast, Northern analysis demonstrated only a two-fold change in ODC mRNA level during induction. Measurement of enzyme stability showed that the half-life of the ODC protein was increased three-fold above basal level in the induced state. Inhibition of induction was produced by pretreatment with either the calmodulin antagonist, W-7, or the product of the ODC-catalyzed reaction, putrescine. Further analysis illustrated that the inhibition produced by these agents was partly the result of destabilization of the enzyme and not a decrease in message level. These results demonstrate that the induction of ODC by trophic agents is the result of post-transcriptional events rather than at the level of RNA synthesis.