Control of ornithine decarboxylase in Chinese hamster ovary cells by polyamines. Translational inhibition of synthesis and acceleration of degradation of the enzyme by putrescine, spermidine, and spermine

Abstract

We have recently isolated, without using any inhibitors, a mutant of Chinese hamster ovary cell line which greatly overproduces ornithine decarboxylase in serum-free culture. Addition of polyamines (putrescine, spermidine, or spermine, 10 microM) or ornithine (1 mM), the precursor of polyamines, to the culture medium of these cells caused a rapid and extensive decay of ornithine decarboxylase activity. At the same time the activity of S-adenosylmethionine decarboxylase showed a less pronounced decrease. Notably, the polyamine concentrations used were optimal for growth of the cells and caused no perturbation of general protein synthesis. Spermidine and spermine appeared to be the principal regulatory amines for both enzymes, but also putrescine, if accumulated at high levels in the cells, was capable of suppressing ornithine decarboxylase activity. The amount of ornithine decarboxylase protein (as measured by radioimmunoassay) declined somewhat more slowly than the enzyme activity, but no more than 10% of the loss of activity could be ascribed to post-translational modifications or inhibitor interaction. Some evidence for inactivation through ornithine decarboxylase-antizyme complex formation was obtained. Gel electrophoretic determinations of the [35S]methionine-labeled ornithine decarboxylase revealed a rapid reduction in the synthesis and acceleration in the degradation of the enzyme after polyamine additions. No decrease in the amounts of the two ornithine decarboxylase-mRNA species, hybridizable to a specific cDNA, was detected, suggesting that polyamines depressed ornithine decarboxylase synthesis by selectively inhibiting translation of the message.