Ornithine decarboxylase is degraded by the 26S proteasome without ubiquitination

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.

Autoregulatory frameshifting in decoding mammalian ornithine decarboxylase antizyme

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.

Cloning of antizyme inhibitor, a highly homologous protein to ornithine decarboxylase

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 ODC-antizyme 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.

Agmatine suppresses proliferation by frameshift induction of antizyme and attenuation of cellular polyamine levels

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 full-length 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.

Conservation of polyamine regulation by translational frameshifting from yeast to mammals

Regulation of ornithine decarboxylase in vertebrates involves a negative feedback mechanism requiring the protein antizyme. Here we show that a similar mechanism exists in the fission yeast Schizosaccharomyces pombe. The expression of mammalian antizyme genes requires a specific +1 translational frameshift. The efficiency of the frameshift event reflects cellular polyamine levels creating the autoregulatory feedback loop. As shown here, the yeast antizyme gene and several newly identified antizyme genes from different nematodes also require a ribosomal frameshift event for their expression. Twelve nucleotides around the frameshift site are identical between S.pombe and the mammalian counterparts. The core element for this frameshifting is likely to have been present in the last common ancestor of yeast, nematodes and mammals.

Analyses of ornithine decarboxylase antizyme mRNA with a cDNA cloned from rat liver

Ornithine decarboxylase antizyme is a unique inhibitory protein induced by polyamines and involved in the regulation of ornithine decarboxylase. A cDNA was isolated from a rat liver cDNA library by the screening with monoclonal antibodies to rat liver antizyme as probes. The expression products of the cDNA in bacterial systems inhibited rat ornithine decarboxylase activity in a manner characteristic of antizyme and rabbit antisera raised against its direct expression product reacted to rat liver antizyme, confirming the authenticity of the cDNA. On RNA blot analysis with the cDNA probe, an antizyme mRNA band of 1.3 kb was detected in rat tissues. Antizyme mRNA did not increase upon administration of putrescine, an inducer of antizyme, and its half-life after actinomycin D treatment was as long as 12 h in rat liver, suggesting that antizyme mRNA is constitutively expressed and antizyme synthesis is regulated at the translational level. Similar-sized mRNAs hybridizable to the cDNA were also found in various mammalian and non-mammalian vertebrate tissues under physiological conditions. In addition, chicken and frog antizymes showed immunocrossreactivity with rat antizyme. The ubiquitous presence and the evolutionally conserved structure of antizyme in vertebrate tissues suggest that it has an important function.

Cloning and characterization of a rat gene encoding ornithine decarboxylase antizyme

We cloned an ornithine decarboxylase antizyme-encoding gene (Oaz) from a rat liver genomic library. The entire gene was located on a 4367-bp EcoRI fragment, which corresponded to one of two fragments hybridizable with the antizyme-encoding cDNA, Z1, on Southern blot analysis. Sequence analysis of the cloned gene showed that it consisted of five exons which were identical with the cDNA. The transcription start points of the Oaz mRNA were located 75 and 76 nucleotides upstream from the first ATG codon, as determined by S1 nuclease protection and primer extension analyses. The 5'-flanking region of the gene contained typical promoter motifs, such as a TATA box and Sp1-binding sites. Introduction of a chimeric gene consisting of the 5'-flanking region and the bacterial cat gene into Chinese hamster ovary cells revealed a promoter activity in the region, which was comparable in strength to that of the simian virus 40 promoter. In addition, we isolated a 12-kb EcoRI fragment, the other sequence hybridizable to the cDNA. Sequence analysis showed that it represented a processed Oaz pseudogene and was not able to encode any active protein.

Antizyme, a protein induced by polyamines, accelerates the degradation of ornithine decarboxylase in Chinese-hamster ovary-cell extracts

Ornithine decarboxylase (ODC), the key regulatory enzyme for polyamine biosynthesis, is known to have a short intracellular half-life, and antizyme, an ODC-binding protein induced by polyamines, has been suggested to be involved in the process of ODC degradation. In the present study we demonstrated that antizyme markedly accelerated ATP-dependent degradation of ODC in vitro in an extract from ODC-overproducing Chinese-hamster ovary cells.

Degradation of ornithine decarboxylase by the 26S proteasome

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. Turnover of ODC is extremely rapid and highly regulated, and is accelerated when polyamine levels increase. Polyamine-stimulated ODC degradation is mediated by association with antizyme (AZ), an ODC inhibitory protein induced by polyamines. ODC, in association with AZ, is degraded by the 26S proteasome in an ATP-dependent, but ubiquitin-independent, manner. The 26S proteasome irreversibly inactivates ODC prior to its degradation. The inactivation, possibly due to unfolding, is coupled to sequestration of ODC within the 26S proteasome. This process requires AZ and ATP, but not proteolytic activity of the 26S proteasome. The carboxyl-terminal region of ODC presumably exposed by interaction with AZ plays a critical role for being trapped by the 26S proteasome. Thus, the degradation pathway of ODC proceeds as a sequence of multiple distinct processes, including recognition, sequestration, unfolding, translocation, and ultimate degradation mediated by the 26S proteasome.

Methylation of the ribosyl moiety at position 34 of selenocysteine tRNA[Ser]Sec is governed by both primary and tertiary structure

The selenocysteine (Sec) tRNA[Ser]Sec population in higher vertebrates consists of two major isoacceptors that differ from each other by a single nucleoside modification in the wobble position of the anticodon (position 34). One isoacceptor contains 5-methylcarboxymethyluridine (mcmU) in this position, whereas the other contains 5-methylcarboxymethyluridine-2'-O-methylribose (mcmUm). The other modifications in these tRNAs are N6-isopentenyladenosine (i6A), pseudouridine (psi), and 1-methyladenosine (m1A) at positions 37, 55, and 58, respectively. As methylation of the ribose at position 34 is influenced by the intracellular selenium status and the presence of this methyl group dramatically alters tertiary structure, we investigated the effect of the modifications at other positions as well as tertiary structure on its formation. Mutations were introduced within a synthetic gene encoded in an expression vector, transcripts generated and microinjected into Xenopus oocytes, and the resulting tRNA products analyzed for the presence of modified bases. The results suggest that efficient methylation of mcmU to yield mcmUm requires the prior formation of each modified base and an intact tertiary structure, whereas formation of modified bases at other positions, including mcmU, is not as stringently connected to precise primary and tertiary structure. These results, along with the observations that methylation of mcmU is enhanced in the presence of selenium and that this methyl group affects tertiary structure, further suggest that the mcmUm isoacceptor must have a role in selenoprotein synthesis different from that of the mcmU isoacceptor.

Forced expression of antizyme abolishes ornithine decarboxylase activity, suppresses cellular levels of polyamines and inhibits cell growth

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. It is a short-lived protein and negatively regulated by its products, polyamines. Its degradation is accelerated by the binding of antizyme, an ODC-inhibitory protein induced by polyamines. To evaluate the physiological importance of antizyme we examined the effect of forced expression of antizyme on cellular ODC and polyamine levels and cell growth. Antizyme almost completely abolished the induction of ODC by growth stimuli. This may have been caused by antizyme-induced rapid degradation of newly synthesized ODC, since the half-life of ODC complexes with antizyme was less than 5 min. Forced expression of antizyme caused reductions of cellular putrescine and spermidine levels, and inhibited cell growth, which was partially restored by the addition of putrescine. These observations suggested a critically important role of antizyme in polyamine metabolism.

ATP-Dependent inactivation and sequestration of ornithine decarboxylase by the 26S proteasome are prerequisites for degradation

The 26S proteasome is a eukaryotic ATP-dependent protease, but the molecular basis of its energy requirement is largely unknown. Ornithine decarboxylase (ODC) is the only known enzyme to be degraded by the 26S proteasome without ubiquitinylation. We report here that the 26S proteasome is responsible for the irreversible inactivation coupled to sequestration of ODC, a process requiring ATP and antizyme (AZ) but not proteolytic activity. Neither the 20S proteasome (catalytic core) nor PA700 (the regulatory complex) by itself contributed to this ODC inactivation. Analysis with a C-terminal mutant ODC revealed that the 26S proteasome recognizes the C-terminal degradation signal of ODC exposed by attachment of AZ, and subsequent ATP-dependent sequestration of ODC in the 26S proteasome causes irreversible inactivation, possibly unfolding, of ODC and dissociation of AZ. These processes may be linked to the translocation of ODC into the 20S proteasomal inner cavity, centralized within the 26S proteasome, for degradation.

Properties and fluctuations in vivo of rat liver antizyme inhibitor

Antizyme inhibitor was highly purified from rat liver by using affinity chromatography. It has some structural resemblance to ornithine decarboxylase (ODC), as judged from Mr, immunoreactivity and reversible binding with antizyme. However, unlike hepatic amounts of ODC and ODC-antizyme complex, that of antizyme inhibitor did not show much fluctuation upon putrescine treatment, whereas it decreased as rapidly as ODC decay in the presence of cycloheximide. These results suggested that antizyme inhibitor is an independent regulatory protein rather than a derivative of ODC. Changes in hepatic amounts of antizyme inhibitor, antizyme and ODC upon feeding suggested that antizyme inhibitor may play a role in ODC regulation by trapping antizyme and thereby suppressing ODC degradation. A monoclonal antibody to rat liver antizyme inhibitor was obtained. This antibody was shown to be utilizable for a simple assay of antizyme-inhibitor activity in tissue extracts.

Functional regions of ornithine decarboxylase antizyme

Degradation of ornithine decarboxylase, a key enzyme in polyamine biosynthesis, is accelerated by the binding of antizyme, an ornithine decarboxylase inhibitory protein induced by polyamines. In the present study, we examined the effects of a series of deletion mutants of rat antizyme. The results indicated that two regions of antizyme, one internal (amino acids 122-144) and the other near the C-terminus (amino acids 211-218) are necessary for its binding to ornithine decarboxylase and inhibition of its activity, and an additional internal region (amino acids 88-118, especially 113-118) is necessary for its destabilization.

Cell culture analysis of the regulatory frameshift event required for the expression of mammalian antizymes

BACKGROUND

Antizyme is a critical regulator of cellular polyamine levels due to its effect on polyamine transport and its ability to target ornithine decarboxylase for degradation. Antizyme expression is autoregulatory, through dependence on an unusual +1 translational frameshift mechanism that responds to polyamine levels.

RESULTS

HEK293 cells were depleted of polyamines by treatment with an ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO), and grown in the presence or absence of exogenous polyamines prior to the analysis of ribosomal frameshifting levels. Results obtained using an optimized dual luciferase assay system reveal a 10-fold dynamic range of frameshifting, which correlates positively with polyamine addition. Polyamine addition to cells, which have not been pre-treated with DFMO, also resulted in an increase in antizyme frameshifting but to a lesser degree (1.3 to 1.5-fold). In addition, the constructs with the 3' deletion were more responsive to stimulation by polyamine addition than those with the 5' deletion.

CONCLUSIONS

The observed regulation of antizyme frameshifting demonstrates the efficiency of a polyamine homeostatic mechanism, and illustrates the utility of a quantifiable cell-based assay for the analysis of polyamines or their analogues on translational frameshifting.

Antizyme to ornithine decarboxylase is present in the liver of starved rats

Antizyme to ornithine decarboxylase (ODC) and ODC-antizyme complex were both present in liver cytosols of starved rats. The antizyme was identified by its molecular weight, kinetic properties, formation of a complex with ODC, and reversal of its inhibition by antizyme inhibitor. The average amount of antizyme in liver cytosols of starved rats was 0.1 unit/mg of protein, roughly corresponding to basal hepatic ODC activity in rats fed ad libitum. The presence of ODC-antizyme complex was detected by using antizyme inhibitor. These results indicate that antizyme participates in the regulation of ODC activity in vivo under physiological conditions.

Programmed frameshifting in the synthesis of mammalian antizyme is +1 in mammals, predominantly +1 in fission yeast, but -2 in budding yeast

The coding sequence for mammalian ornithine decarboxylase antizyme is in two different partially overlapping reading frames with no independent ribosome entry to the second ORF. Immediately before the stop codon of the first ORF, a proportion of ribosomes undergo a quadruplet translocation event to shift to the +1 reading frame of the second and main ORF. The proportion that frameshifts is dependent on the polyamine level and, because the product antizyme is a negative regulator of intracellular polyamine levels, the frameshifting acts to complete an autoregulatory circuit by sensing polyamine levels. An mRNA element just 5' of the shift site and a 3' pseudoknot are important for efficient frameshifting. Previous work has shown that a cassette with the mammalian shift site and associated signals directs efficient shifting in the budding yeast Saccharomyces cerevisiae at the same codon to the correct frame, but that the shift is -2 instead of +1. The product contains an extra amino acid corresponding to the shift site. The present work shows efficient frameshifting also occurs in the fission yeast, Schizosaccharomyces pombe. This frameshifting is 80% +1 and 20% -2. The response of S. pombe translation apparatus to the mammalian antizyme recoding signals is more similar to that of the mammalian system than to that of S. cerevisiae. S. pombe provides a good model system for genetic studies on the mechanism of at least this type of programmed mammalian frameshifting.

Single amino-acid replacement is responsible for the stabilization of ornithine decarboxylase in HMOA cells

The half-life of ornithine decarboxylase (ODC) in HMOA cells, a variant cell line derived from hepatoma tissue culture (HTC) cells, is markedly increased compared with that in the parental cell line. In the present study, we examined which of the three relevant factors is responsible for the ODC stabilization in HMOA cells, namely ODC itself, a regulatory protein antizyme and an ODC-degrading activity. SDS/PAGE analysis of radiolabeled ODC revealed that ODC from HMOA cells migrated somewhat faster than that from HTC cells, suggesting that HMOA ODC was structurally altered. Direct sequencing of reverse-transcription/polymerase-chain-reaction (RT-PCR) products of ODC mRNA from HMOA cells revealed a T to G replacement, causing a Cys441-->Trp replacement near the C-terminus. No alteration was found in the whole coding region of antizyme mRNA. An authentic mutant ODC cDNA with the same replacement was transfected and expressed in C55.7 ODC-deficient Chinese hamster ovary cells. Upon cycloheximide treatment, the mutant ODC activity did not decrease appreciably for at least 3 h, whereas wild-type ODC activity decreased with a half-life of 1 h. In-vitro-synthesized mutant ODC with the Cys441-->Trp (or Ala) replacement was also stable in a reticulocyte-lysate ODC-degradation system. Metabolically labeled and purified mouse ODC was degraded in HMOA cell extracts in the presence of ATP and antizyme as rapidly as in HTC cell extracts, indicating that HMOA cells have a normal ODC degrading activity. These results indicated that the single amino acid replacement, Cys441-->Trp, is responsible for the stabilization of ODC in HMOA cells and that Cys441 is important for rapid ODC turnover.

Involvement of the proteasome and antizyme in ornithine decarboxylase degradation by a reticulocyte lysate

Ornithine decarboxylase (ODC) degradation in a freshly prepared reticulocyte lysate was examined. Immunodepletion of proteasomes from the reticulocyte lysate resulted in almost complete loss of ODC degradation. In contrast with the previously reported degradation in extracts of hepatoma tissue-culture (HTC) and Chinese-hamster ovary (CHO) cells or that by the purified 26 S proteasome, efficient degradation of ODC was observed in the lysate without exogenous antizyme, an ODC protein inhibitor induced by polyamines, owing to the presence of a significant amount of antizyme in the lysate. The degradation of ODC in the lysate was strongly suppressed on inactivation of antizyme in the lysate with antizyme inhibitor, a protein which binds to the antizyme and releases ODC from the ODC-antizyme complex. Thus the main pathway for ODC degradation in a reticulocyte lysate was essentially the same as that characterized previously in extracts of HTC and CHO cells, namely an ATP- and antizyme-dependent 26 S proteasome-catalysed pathway that is presumed to be responsible for ODC degradation in whole cells.

Upstream stimulators for recoding

Recent progress in elucidation of 5' stimulatory elements for translational recoding is reviewed. A 5' Shine-Dalgarno sequence increases both +1 and -1 frameshift efficiency in several genes; examples cited include the E. coli prfB gene encoding release factor 2 and the dnaX gene encoding the gamma and tau subunits of DNA polymerase III holoenzyme. The spacing between the Shine-Dalgarno sequence and the shift site is critical in both the +1 and -1 frameshift cassettes; however, the optimal spacing is quite different in the two cases. A frameshift in a mammalian chromosomal gene, ornithine decarboxylase antizyme, has recently been reported; 5' sequences have been shown to be vital for this frameshift event. Escherichia coli bacteriophage T4 gene 60 encodes a subunit of its type II DNA topoisomerase. The mature gene 60 mRNA contains an internal 50 nucleotide region that appears to be bypassed during translation. A 16 amino acid domain of the nascent peptide is necessary for this bypass to occur.

Antizyme regulates the degradation of ornithine decarboxylase in fission yeast Schizosaccharomyces pombe. Study in the spe2 knockout strains

The mechanism of the regulatory degradation of ornithine decarboxylase (ODC) by polyamines was studied in fission yeast, Schizosaccharomyces pombe. To regulate cellular spermidine experimentally, we cloned and disrupted S-adenosylmethionine decarboxylase gene (spe2) in S. pombe. The null mutant of spe2 was devoid of spermidine and spermine, accumulated putrescine, and contained a high level of ODC. Addition of spermidine to the culture medium resulted in rapid decrease in the ODC activity caused by the acceleration of ODC degradation, which was dependent on de novo protein synthesis. A fraction of ODC forming an inactive complex concomitantly increased. The accelerated ODC degradation was prevented either by knockout of antizyme gene or by selective inhibitors of proteasome. Thus, unlike budding yeast, mammalian type antizyme-mediated ODC degradation by proteasome is operating in S. pombe.