SYNERGISTIC INDUCTION OF ORNITHINE DECARBOXYLASE BY ASPARAGINE AND GUT PEPTIDES IN INTESTINAL CRYPT CELLS

The objective of this study was to determine whether the amino acid as paragine stimulated the activity of ornithine decarboxylase (ODC) syne rgistically with epidermal growth factor (EGF) or gastrin in IEC-B cel ls, a line of normal rat small intestinal crypt cells. Cells were grow n in DMEM containing 5% dialyzed fetal bovine serum, and serum was dep rived for 24 h before experiments. Exposure to EGF or gastrin alone in creased ODC activity 4.5- to 6-fold. Asparagine alone increased the en zyme activity 10- to 13-fold in IEC-6 cells. Simultaneous addition of asparagine and EGF or gastrin, however, increased ODC activity more th an 40-fold. In contrast, there was no synergistic induction of ODC act ivity when gastrin and EGF mere added together. Increased ODC activity in cells treated with asparagine and EGF or gastrin was associated wi th an increase in ODC mRNA and protein levels. The rate of transcripti on of the ODC gene was significantly increased by exposure to EGF or g astrin. Asparagine alone had little or no effect on the rate of transc ription of the ODC gene. When given together with EGF or gastrin, aspa ragine also had no additional effect on the transcription rate of the ODC gene. The half-life of mRNA for ODC in unstimulated IEC-6 cells wa s similar to 30 min and increased to more than 2 h in cells exposed to asparagine, although neither gastrin nor EGF prolonged the stability of ODC mRNA. The half-life of mRNA for ODC after combined addition of asparagine and EGF or gastrin was extended to similar to 2 h, similar to asparagine alone. Combined addition of asparagine and EGF or gastri n also significantly increased DNA synthesis compared with cells expos ed to each of the three agents alone. In conclusion, 2) simultaneous a ddition of asparagine and EGF or gastrin increases ODC activity in a s ynergistic manner and 2) asparagine increases ODC mRNA levels through completely distinct mechanisms from EGF or gastrin. EGF or gastrin spe cifically stimulates transcription of the ODC gene, whereas asparagine affects a posttranscriptional process.