Importance of the 3' untranslated region of ornithine decarboxylase mRNA in the translational regulation of the enzyme.

Translational regulation of ornithine decarboxylase (ODC), which catalyses the first step in the biosynthesis of polyamines, appears to be an important mechanism in the strong feedback control as well as in the hypotonic induction of the enzyme. However, the exact mechanisms are not yet understood. The ODC mRNA has long 5' and 3' untranslated regions (UTRs) which may be involved in the translational control of the enzyme. In the present study we have used a series of stable transfectants of Chinese Hamster ovary cells expressing ODC mRNAs with various truncations in the 5' and 3' UTRs to investigate the importance of these regions. It is demonstrated that neither the 5' UTR nor the 3' UTR appears to be involved in the polyamine-mediated feedback control of ODC synthesis. The hypotonic induction of ODC, on the other hand, was shown to be highly dependent on the presence of the 3' UTR, but not on the 5' UTR, of ODC mRNA. Cells expressing ODC mRNAs lacking the 3' UTR showed no, or only a very slight, induction of ODC whether the 5' UTR was present or not, whereas the cell lines expressing ODC mRNAs containing the 3' UTR (with or without the 5' UTR) markedly induced ODC after a hypotonic shock. The present finding of a role for the ODC mRNA 3' UTR in the hypotonic induction of ODC is the first demonstration of a specific effect of the 3' UTR in the regulation of ODC.

Regulation of mammalian ornithine decarboxylase. Studies on the induction of the enzyme by hypotonic stress.

One of the cellular responses to hypotonic stress is a marked induction of a key regulatory enzyme in the polyamine biosynthetic pathway, i.e. ornithine decarboxylase (ODC). This increase in ODC activity appears to be a physiological response since the elevated putrescine production seen after the hypotonic shock renders the cells less sensitive to the decrease in osmolarity. In the present study, we have investigated the mechanisms by which the hypotonicity may induce ODC activity. We provide support for a translational mechanism, closely related to the polyamine-mediated feedback regulation of ODC synthesis. In addition, we have examined whether the long G+C-rich 5' untranslated region of the ODC mRNA, which has been demonstrated to negatively affect the translatability of the message, is of any importance for the induction of ODC by hypotonic stress. Chinese hamster ovary (CHO) cells expressing ODC mRNA, with or without the 5' untranslated region, were isolated after transfecting ODC-deficient CHO cells with the appropriate constructs. Hypotonic treatment of the stable transfectants, however, revealed no major difference in ODC induction between the cells expressing a full-length ODC mRNA and those expressing an ODC mRNA deleted of its 5' untranslated region, demonstrating that this part of the message was not essential for the osmotic effects on ODC expression.