OVERPRODUCTION OF ORNITHINE DECARBOXYLASE CAUSED BY RELIEF OF TRANSLATIONAL REPRESSION IS ASSOCIATED WITH NEOPLASTIC TRANSFORMATION

The mRNAs for two key enzymes in polyamine biosynthesis, ornithine dec arboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), bo th have long 5' untranslated regions (5'UTRs) that could be important in the regulation of enzyme levels by affecting the translation of the se mRNAs. In order to test this hypothesis, ODC and AdoMetDC activitie s were measured in 3T3 cells and in 3T3 cells overexpressing eIF-4E (P 2 cells). eIF-4E has been reported to be a limiting factor in the tran slation of mRNAs with extensive secondary structures in the 5'UTR. Ado MetDC activity was not greatly different in the two cell lines, but OD C activity was much greater in the P2 cells. These results were confir med by transfecting these cells with plasmids containing a luciferase complementary DNA fused to follow the 5'UTR from ODC or AdoMetDC. The ODC 5'UTR construct produced a higher luciferase activity in the P2 ce lls. The high level of expression of ODC may be a critical factor in t he transformed phenotype of the P2 cells since the ability of these ce lls to grow in soft agar was blocked by levels of the ODC inhibitor, a lpha-difluoromethylornithine, that reduced the ODC activity to values comparable to those of the parent 3T3 cells. These results provide mor e evidence for a critical role of ODC activity in neoplastic transform ation and for the importance of its translational regulation in cell g rowth and transformation.