Ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is the most rapidly turned over mammalian enzyme. We have shown that its degradation is accelerated by ODC antizyme, an inhibitory protein induced by polyamines. This is a new type of enzyme regulation and may be a model for selective protein degradation. Here we report the identification of the protease responsible for ODC degradation. Using a cell-free degradation system, we demonstrate that immunodepletion of proteasomes from cell extracts causes almost complete loss of ATP- and antizyme-dependent degradation of ODC. In addition, purified 26S proteasome complex, but not the 20S proteasome, catalyses ODC degradation in the absence of ubiquitin. These results strongly suggest that the 26S proteasome, widely viewed as specific for ubiquitin-conjugated proteins, is the main enzyme responsible for ODC degradation. The 26S proteasome may therefore have a second role in ubiquitin-independent proteolysis.

Rat antizyme gene expression requires programmed, ribosomal frameshifting. A novel autoregulatory mechanism enables modulation of frameshifting according to the cellular concentration of polyamines. Antizyme binds to, and destabilizes, ornithine decarboxylase, a key enzyme in polyamine synthesis. Rapid degradation ensues, thus completing a regulatory circuit. In vitro experiments with a fusion construct using reticulocyte lysates demonstrate polyamine-dependent expression with a frameshift efficiency of 19% at the optimal concentration of spermidine. The frameshift is +1 and occurs at the codon just preceding the terminator of the initiating frame. Both the termination codon of the initiating frame and a pseudoknot downstream in the mRNA have a stimulatory effect. The shift site sequence, UCC-UGA-U, is not similar to other known frameshift sites. The mechanism does not seem to involve re-pairing of peptidyl-tRNA in the new frame but rather reading or occlusion of a fourth base.

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Polyamines are required for entry and progression of the cell cycle. As such, augmentation of polyamine levels is essential for cellular transformation. Polyamines are autoregulated through induction of antizyme, which represses both the rate-limiting polyamine biosynthetic enzyme ornithine decarboxylase and cellular polyamine transport. In the present study we demonstrate that agmatine, a metabolite of arginine via arginine decarboxylase (an arginine pathway distinct from that of the classical polyamines), also serves the dual regulatory functions of suppressing polyamine biosynthesis and cellular polyamine uptake through induction of antizyme. The capacity of agmatine to induce antizyme is demonstrated by: (a) an agmatine-dependent translational frameshift of antizyme mRNA to produce a fulllength protein and (b) suppression of agmatine-dependent inhibitory activity by either anti-antizyme IgG or antizyme inhibitor. Furthermore, agmatine administration depletes intracellular polyamine levels to suppress cellular proliferation in a transformed cell line. This suppression is reversible with polyamine supplementation. We propose a novel regulatory pathway in which agmatine acts as an antiproliferative molecule and potential tumor suppressor by restricting the cellular polyamine supply required to support growth.Polyamines (putrescine, spermidine, and spermine) are required for DNA replication, proliferation, and cell homeostasis (1-3). Ornithine decarboxylase (ODC) 1 is the first rate-limiting enzyme of polyamine biosynthesis and one of the most highly regulated eukaryotic enzymes. Cellular polyamine transporters are stimulated by many of the same factors that induce ODC activity, and similarly, enhanced cellular polyamine uptake occurs both in normal but rapidly proliferating cells (4) and in tumor cell lines (5-8). Cells in vivo can acquire polyamines released into the circulation by other cells, dietary sources, and gut flora. Polyamines have been demonstrated to play an important role in the transformation process. Conversely, polyamine depletion results in growth arrest (9, 10).Intracellular polyamine concentrations are autoregulated by the induction of the protein antizyme (11). Antizyme is the only known endogenous protein that binds to ODC, inhibiting activity and accelerating its degradation (12). In addition to inhibiting polyamine biosynthesis, antizyme has recently been shown to concurrently suppress polyamine transporter(s) (13,14). Pharmacological inhibition of ODC activity, however, has been shown to result in compensatory cellular polyamine uptake (6). Beneficial therapeutic intervention must therefore address both polyamine transport as well as biosynthesis (for review see Ref. 15).The metabolism of arginine to agmatine by ADC has only recently been demonstrated in mammals (16). As agmatine and polyamines are structurally analogous polycationic molecules derived from distinct arginine-dependent pathways (6), we speculated that the ADC metabolite agmatine may play a role in regulating intracellul...

The degradation of ornithine decarboxylase (ODC) catalyzed by the 26 S proteasome is accelerated by antizyme, an ODC inhibitory protein induced by polyamines. Previously, we have found another possible regulatory protein of ODC degradation, antizyme inhibitor. Antizyme inhibitor binds to the antizyme with a higher affinity than that of ODC, releasing ODC from ODCantizyme complex. We report here the cDNA sequence of rat heart antizyme inhibitor. The deduced sequence of the protein is highly similar to, but distinct from, sequences of ODCs from various species. Antizyme inhibitor contains amino acid residues required for formation of active sites of ODC, but it completely lacks ODC activity. Antizyme inhibitor has no homology with peptide sequence in the mammalian ODC carboxyl terminus, which is needed for rapid turnover of ODC. It inhibits antizyme-dependent ODC degradation, but, unlike ODC, its degradation is not accelerated by antizyme.Ornithine decarboxylase (ODC) 1 is a key enzyme in polyamine biosynthesis pathway (1, 2). The turnover of ODC is very rapid and highly regulated (3, 4). The degradation of ODC catalyzed by the 26 S proteasome is accelerated by ODC antizyme (5, 6), an inhibitory protein induced by polyamines (7). Strict regulation of ODC appears to be important for cell growth, because overproduction of ODC is associated with neoplastic transformation (8, 9), whereas overproduction of antizyme inhibits cell growth (10, 11). We previously found in rat liver and heart another possible regulator of ODC degradation, antizyme inhibitor (12). Antizyme inhibitor binds to the antizyme with a higher affinity than that of ODC and releases ODC from the ODC-antizyme complex (12)(13)(14). The physiological fluctuation of antizyme inhibitor in vivo suggested that it is another regulatory protein that stabilizes ODC by trapping antizyme (14). However, the possibility that antizyme inhibitor is a post-translationally modified product of ODC could not be ruled out. In this report, we describe the cloning and expression of antizyme inhibitor and show that the sequence of antizyme inhibitor is closely related to, but distinct from, that of ODC. EXPERIMENTAL PROCEDUREScDNA Cloning-Oligo(dT)-primed cDNA was synthesized from poly(A) ϩ RNA from the hearts of isoproterenol-treated Wistar rats (10 mg/kg, 2 h) and inserted into ZAPII vector (Stratagene) through EcoRI adaptors to construct a library. One positive clone was selected from 10 5 recombinants by screening with a monoclonal antibody to rat heart antizyme inhibitor (14) as a probe. This monoclonal antibody does not react with rat ODC (14). The selected clone, A1, which carried a cDNA insert of about 1.9 kb in length was purified and sequenced. Two more positive clones were selected by plaque hybridization with a probe of the partial length cDNA A1. These clones, A2 and A3, carried cDNA inserts of about 2.2 and 4 kb, respectively, and were sequenced. All the DNA sequences were determined from both strands.Northern Blot Analysis of Antizyme Inhibitor mRNA-Poly(A) ϩ ...