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.