Antizyme inhibitor is rapidly induced in growth-stimulated mouse fibroblasts and releases ornithine decarboxylase from antizyme suppression

Ornithine decarboxylase (ODC) catalyses the first step in the synthesis of the polyamines putrescine, spermidine and spermine. The polyamines are esse ntial for cell growth, but at elevated levels they may be tumorigenic, toxi c, or may induce apoptosis. Therefore, ODC activity is highly regulated. It is induced when cells are stimulated to grow, and it is subjected to feedb ack inhibition by the polyamines. By causing ribosomal frameshifting, polya mines induce the synthesis of antizyme, a 23-kDa protein, which binds to OD C, inhibits its activity and promotes its degradation by the 26 S proteasom e. Antizyme, in turn, is inhibited by antizyme inhibitor (AZI). We describe the cloning of a mouse AZI cDNA, encoding a protein with high homology to mouse ODC. Using purified recombinant proteins, we show that AZI (which has no ODC activity) can release enzymically active ODC from antizyme suppress ion in vitro. We also show that ODC reactivation takes place in mouse fibro blasts upon transient transfection with an AZI-expressing plasmid construct . Finally we demonstrate that the AZI mRNA content of mouse fibroblasts inc reases significantly within an hour of growth stimulation, i.e. much earlie r than ODC transcripts. Our results indicate that induction of AZI synthesi s may represent a means of rescuing ODC molecules that have been inactivate d and tagged for degradation by antizyme, when culture conditions improve a nd polyamine production is needed far cell growth and proliferation.