Regulation of ornithine decarboxylase in a transformed cell line that overexpresses translation initiation factor...

pMV7-4E cells (4E-P2), derived from NIH-3T3 cells, overexpress eIF-4E and exhibit characteristics of transformation, possibly due to translational relief of mRNAs encoding proteins that regulate cell growth. Ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is induced in 4E-P2 cells, and this induction appears to be related to the transformed phenotype of these cells. ODC mRNA contains extensive secondary structure in its 5' untranslated region (5'UTR) and may be regulated by eIF-4E, which melts mRNA secondary structure. To better understand this regulation, cDNA constructs containing the wild-type 5'UTR of ODC or deletion mutants inserted ahead of the luciferase gene were transfected into 4E-P2 and 3T3 cells. Expression of luciferase was higher in 4E-P2 cells in all cases, suggesting that the secondary structure of the ODC 5'UTR inhibits expression in 3T3 cells, and this inhibition is overcome by the high eIF-4E levels in 4E-P2 cells. When a small open reading frame present in the 5'UTR of ODC was destroyed by a point mutation, this luciferase construct was expressed about 6-fold over that containing the wild-type 5'UTR in both cell lines, although both of these 5'UTRs contain the same predicted secondary structure. Thus, factors in addition to eIF-4E may be involved in the regulation of ODC. To examine the differences in ODC regulation by polyamines in normal and transformed cells, the effect of N1,N12-bis(ethyl)spermine (BE-3-4-3) on the synthesis and degradation of ODC was examined. ODC activity in 4E-P2 cells was 10 times less sensitive to reduction by BE-3-4-3 compared to 3T3 cells, suggesting that high ODC levels in eIF-4E-overexpressing cells are the result of decreased regulation by polyamines as well as relief of translational regulation by eIF-4E.