Nuclear translocation of antizyme and expression of ornithine decarboxylase and antizyme are developmentally regulated

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Abstract
The polyamines are important regulators of cell growth and differentiation. Cells acquire polyamines by energy-dependent transport and by synthesis where the highly regulated ornithine decarboxylase (ODC) catalyzes the first and rate-controlling step. Inactivation of ODC is mainly exerted by antizyme (AZ), a 20--25 kDa polyamine-induced protein that binds to ODC, inactivates it, and targets it for degradation by the 26S proteasome without ubiquitination. In the present study, we have performed a systematic analysis of the expression of ODC and AZ, at the mRNA and protein levels, during mouse development. The expression patterns for ODC and AZ were found to be developmentally regulated, suggesting important functions for the polyamines in early embryogenesis, axonogenesis, epithelial-mesenchymal interaction, and in apoptosis. In addition, AZ protein was found to translocate to the nucleus in a developmentally regulated manner. The nuclear localization is consistent with the fact that the amino acid sequence of AZ exhibits features that characterize nuclear proteins. Interestingly, we found that cultivation of mandibular components of the first branchial arch in the presence of a selective proteasome inhibitor caused ODC accumulation in the nucleus of a subset of cells, suggesting that the observed nuclear translocation of AZ is linked to proteasome-mediated ODC degradation in the nucleus. The presence of AZ in the nucleus may suggest that nuclear ODC activity is under tight control, and that polyamine production can be rapidly interrupted when those developmental events, which depend on access to nuclear polyamines, have been completed.