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

Translation efficiency affects the sequence-independent +1 ribosomal frameshifting by polyamines

Antizyme (AZ) interacts with ornithine decarboxylase, which catalyzes the first step of polyamine biosynthesis and recruits it to the proteasome for degradation. Synthesizing the functional AZ protein requires transition of the reading frame at the termination codon. This programmed +1 ribosomal frameshifting is induced by polyamines, but the molecular mechanism is still unknown. In this study, we explored the mechanism of polyamine-dependent +1 frameshifting using a human cell-free translation system. Unexpectedly, spermidine induced +1 frameshifting in the mutants replacing the termination codon at the shift site with a sense codon. Truncation experiments showed that +1 frameshifting occurred promiscuously in various positions of the AZ sequence. The probability of this sequence-independent +1 frameshifting increased in proportion to the length of the open reading frame. Furthermore, the +1 frameshifting was induced in some sequences other than the AZ gene in a polyamine-dependent manner. These findings suggest that polyamines have the potential to shift the reading frame in the +1 direction in any sequence. Finally, we showed that the probability of the sequence-independent +1 frameshifting by polyamines is likely inversely correlated with translation efficiency. Based on these results, we propose a model of the molecular mechanism for AZ +1 frameshifting.

Roles of polyamines in translation

Polyamines are organic polycations that bind to a variety of cellular molecules, including nucleic acids. Within cells, polyamines contribute to both the efficiency and fidelity of protein synthesis. In addition to directly acting on the translation apparatus to stimulate protein synthesis, the polyamine spermidine serves as a precursor for the essential post-translational modification of the eukaryotic translation factor 5A (eIF5A), which is required for synthesis of proteins containing problematic amino acid sequence motifs, including polyproline tracts, and for termination of translation. The impact of polyamines on translation is highlighted by autoregulation of the translation of mRNAs encoding key metabolic and regulatory proteins in the polyamine biosynthesis pathway, including S-adenosylmethionine decarboxylase (AdoMetDC), antizyme (OAZ), and antizyme inhibitor 1 (AZIN1). Here, we highlight the roles of polyamines in general translation and also in the translational regulation of polyamine biosynthesis.

Evaluation of Polyamine Transport Inhibitors in a Drosophila Epithelial Model Suggests the Existence of Multiple Transport Systems

Increased polyamine biosynthesis activity and an active polyamine transport system are characteristics of many cancer cell lines and polyamine depletion has been shown to be a viable anticancer strategy. Polyamine levels can be depleted by difluoromethylornithine (DFMO), an inhibitor of the key polyamine biosynthesis enzyme ornithine decarboxylase (ODC). However, malignant cells frequently circumvent DFMO therapy by up-regulating polyamine import. Therefore, there is a need to develop compounds that inhibit polyamine transport. Collectively, DFMO and a polyamine transport inhibitor (PTI) provide the basis for a combination therapy leading to effective intracellular polyamine depletion. We have previously shown that the pattern of uptake of a series of polyamine analogues in a Drosophila model epithelium shares many characteristics with mammalian cells, indicating a high degree of similarity between the mammalian and Drosophila polyamine transport systems. In this report, we focused on the utility of the Drosophila epithelial model to identify and characterize polyamine transport inhibitors. We show that a previously identified inhibitor of transport in mammalian cells has a similar activity profile in Drosophila. The Drosophila model was also used to evaluate two additional transport inhibitors. We further demonstrate that a cocktail of polyamine transport inhibitors is more effective than individual inhibitors, suggesting the existence of multiple transport systems in Drosophila. Our findings reinforce the similarity between the Drosophila and mammalian transport systems and the value of the Drosophila model to provide inexpensive early screening of molecules targeting the transport system.

Ribosomal frameshifting and transcriptional slippage: From genetic steganography and cryptography to adventitious use

Genetic decoding is not ‘frozen’ as was earlier thought, but dynamic. One facet of this is frameshifting that often results in synthesis of a C-terminal region encoded by a new frame. Ribosomal frameshifting is utilized for the synthesis of additional products, for regulatory purposes and for translational ‘correction’ of problem or ‘savior’ indels. Utilization for synthesis of additional products occurs prominently in the decoding of mobile chromosomal element and viral genomes. One class of regulatory frameshifting of stable chromosomal genes governs cellular polyamine levels from yeasts to humans. In many cases of productively utilized frameshifting, the proportion of ribosomes that frameshift at a shift-prone site is enhanced by specific nascent peptide or mRNA context features. Such mRNA signals, which can be 5′ or 3′ of the shift site or both, can act by pairing with ribosomal RNA or as stem loops or pseudoknots even with one component being 4 kb 3′ from the shift site. Transcriptional realignment at slippage-prone sequences also generates productively utilized products encoded trans-frame with respect to the genomic sequence. This too can be enhanced by nucleic acid structure. Together with dynamic codon redefinition, frameshifting is one of the forms of recoding that enriches gene expression.

Molecular characterization, tissue expression and nucleotide variation of the porcine AZ1 gene

Antizyme 1 (AZ1) is a member of the antizyme family that is involved in many biological processes. As a natural inhibitor, AZ1 controls the normal level of polyamines, which is indispensable to cellular function. Our prior research showed that the expression of the AZ1 gene in Longissimus doris of Landrace pigs was higher (P<0.05) than in Lantang pigs. The AZ1 gene might be involved in the development of muscle and potentially serves as an important target for muscle improvement in pigs. In this study, the molecular characterization, tissue expression, and sequence variation of porcine AZ1 gene were analyzed. A 4082 bp sequence including the 5'-flanking region and gene sequence was obtained through RACE and sequencing. The genomic sequence of AZ1 gene consists of six exons and five introns. The mRNA of AZ1 gene contains three elements: 5'-untranslated regions (UTR, 79 bp), CDS (684 or 207 bp), and 3'-UTR (161 or 423 bp for the two transcripts). Three termination signals of AATAAA were found in the longer 3'-UTR. The mRNA sequence of the AZ1 gene contained two ORFs with a frameshifting site at the 69th codon. The amino acid sequence from the porcine AZ1 gene was similar to other vertebrates and exhibited the highest similarity to cattle. The partial 5'-flanking region was 852 bp with typical cis-regulatory elements such as TATA boxes and CAAT boxes. By DNA pooling and sequencing, nine and 12 single nucleotide polymorphisms (SNPs) were detected in the 5'-flanking region and introns, respectively. Except the SNP at -713, the other eight SNPs were found in putative cis-acting elements and might alter the binding of trans-acting factors. Expression patterns showed that in Lantang pig, the AZ1 gene is differentially expressed in various tissues and displayed higher expression in lung and skeletal muscle.

Amino acids regulate expression of antizyme-1 to modulate ornithine decarboxylase activity

In a glucose-salt solution (Earle's balanced salt solution), asparagine (Asn) stimulates ornithine decarboxylase (ODC) activity in a dose-dependent manner, and the addition of epidermal growth factor (EGF) potentiates the effect of Asn. However, EGF alone fails to activate ODC. Thus, the mechanism by which Asn activates ODC is important for understanding the regulation of ODC activity. Asn reduced antizyme-1 (AZ1) mRNA and protein. Among the amino acids tested, Asn and glutamine (Gln) effectively inhibited AZ1 expression, suggesting a differential role for amino acids in the regulation of ODC activity. Asn decreased the putrescine-induced AZ1 translation. The absence of amino acids increased the binding of eukaryotic initiation factor 4E-binding protein (4EBP1) to 5'-mRNA cap and thereby inhibited global protein synthesis. Asn failed to prevent the binding of 4EBP1 to mRNA, and the bound 4EBP1 was unphosphorylated, suggesting the involvement of the mammalian target of rapamycin (mTOR) in the regulation of AZ1 synthesis. Rapamycin treatment (4 h) failed to alter the expression of AZ1. However, extending the treatment (24 h) allowed expression in the presence of amino acids, indicating that AZ1 is expressed when TORC1 signaling is decreased. This suggests the involvement of cap-independent translation. However, transient inhibition of mTORC2 by PP242 completely abolished the phosphorylation of 4EBP1 and decreased basal as well as putrescine-induced AZ1 expression. Asn decreased the phosphorylation of mTOR-Ser(2448) and AKT-Ser(473), suggesting the inhibition of mTORC2. In the absence of amino acids, mTORC1 is inhibited, whereas mTORC2 is activated, leading to the inhibition of global protein synthesis and increased AZ1 synthesis via a cap-independent mechanism.

Knockdown of ornithine decarboxylase antizyme 1 causes loss of uptake regulation leading to increased N1, N11-bis(ethyl)norspermine (BENSpm) accumulation and toxicity in NCI H157 lung cancer cells

Ornithine decarboxylase antizyme 1 (AZ1) is a major regulatory protein responsible for the regulation and degradation of ornithine decarboxylase (ODC). To better understand the role of AZ1 in polyamine metabolism and in modulating the response to anticancer polyamine analogues, a small interfering RNA strategy was used to create a series of stable clones in human H157 non-small cell lung cancer cells that expressed less than 5-10% of basal AZ1 levels. Antizyme 1 knockdown clones accumulated greater amounts of the polyamine analogue N (1),N (11)-bis(ethyl)norspermine (BENSpm) and were more sensitive to analogue treatment. The possibility of a loss of polyamine uptake regulation in the knockdown clones was confirmed by polyamine uptake analysis. These results are consistent with the hypothesis that AZ1 knockdown leads to dysregulation of polyamine uptake, resulting in increased analogue accumulation and toxicity. Importantly, there appears to be little difference between AZ1 knockdown cells and cells with normal levels of AZ1 with respect to ODC regulation, suggesting that another regulatory protein, potentially AZ2, compensates for the loss of AZ1. The results of these studies are important for the understanding of both the regulation of polyamine homeostasis and in understanding the factors that regulate tumor cell sensitivity to the anti-tumor polyamine analogues.

Plant ornithine decarboxylase is not post-transcriptionally feedback regulated by polyamines but can interact with a cytosolic ribosomal protein S15 polypeptide

The formation of putrescine by ornithine decarboxylase (ODC) is a key regulatory step in polyamine biosynthesis in metazoa and fungi. Excess polyamines post-transcriptionally induce the synthesis of a unique non-competitive protein inhibitor of ODC, termed antizyme. Binding of antizyme to an ODC monomer subunit results in enzymatic inhibition, rapid ubiquitin-independent degradation of ODC by the 26S proteasome and recycling of antizyme. Plants possess an additional route for synthesizing putrescine via arginine decarboxylase (ADC). No homologue of ODC antizyme has been detected in plant genomes but several biochemical studies have reported plant ODC antizyme proteins of 9 and 16 kDa. Here we show that plant cells grown in liquid culture do not exhibit any substantial post-transcriptional, polyamine-responsive feedback regulation of ODC or ADC. However, using the yeast two hybrid system, a plant ODC-binding polypeptide was detected: the C-terminal 84-87 amino acids of cytosolic ribosomal protein (rp) S15. The Arabidopsis rpS15 polypeptide interacted specifically with plant ODC but not with human or Saccharomyces cerevisiae ODCs. Co-expression of either the full length or C-terminal rpS15 polypeptides with a plant ODC in yeast did not reduce ODC enzymatic activity. Only the full length mRNA encoding rpS15 was detected in Arabidopsis cells, suggesting that the C-terminal rpS15 polypeptide is encoded by a low abundance mRNA or the polypeptide is not physiologically relevant in plants. These results confirm the primacy of S-adenosylmethionine decarboxylase as the key regulatory enzyme in plant polyamine biosynthesis.

Protocols for studying antizyme expression and function

Antizyme (AZ) is a key molecule in feedback regulation of cellular polyamines. It is induced by polyamines through stimulation of ribosomal frameshifting during its translation. In mammals, AZ is diverged into three paralogs, AZ1-3. Tissue and subcellular distribution are different among the paralogs, as determined by immunochemical methods or expression of fluorescent-tagged proteins. Only AZ2 is known to be phosphorylated. AZ regulates cellular polyamine levels through multiple mechanisms. It binds to ornithine decarboxylase (ODC) to form an inactive complex and to trigger degradation of ODC by 26S proteasomes. The AZ activity to promote ODC degradation can be measured both in vitro and in cells. AZ also inhibits cellular uptake of polyamines. This chapter comprises seven subchapters describing methods for studying expression and function of AZ.

Antizyme 3 inhibits polyamine uptake and ornithine decarboxylase (ODC) activity, but does not stimulate ODC degradation

Azs (antizymes) are small polyamine-induced proteins that function as feedback regulators of cellular polyamine homoeostasis. They bind to transient ODC (ornithine decarboxylase) monomeric subunits, resulting in inhibition of ODC activity and targeting ODC to ubiquitin-independent proteasomal degradation. Az3 is a mammalian Az isoform expressed exclusively in testicular germ cells and therefore considered as a potential regulator of polyamines during spermatogenesis. We show here that, unlike Az1 and Az2, which efficiently inhibit ODC activity and stimulate its proteasomal degradation, Az3 poorly inhibits ODC activity and fails to promote ODC degradation. Furthermore, Az3 actually stabilizes ODC, probably by protecting it from the effect of Az1. Its inhibitory effect is revealed only when it is present in excess compared with ODC. All three Azs efficiently inhibit the ubiquitin-dependent degradation of AzI (Az inhibitor) 1 and 2. Az3, similar to Az1 and Az2, efficiently inhibits polyamine uptake. The potential significance of the differential behaviour of Az3 is discussed.

Pneumocystis mediates overexpression of antizyme inhibitor resulting in increased polyamine levels and apoptosis in alveolar macrophages

Pneumocystis pneumonia (PcP) is the most common opportunistic disease in immunocompromised patients. Alveolar macrophages are responsible for the clearance of Pneumocystis organisms; however, they undergo a high rate of apoptosis during PcP due to increased intracellular polyamine levels. In this study, the sources of polyamines and mechanisms of polyamine increase and polyamine-induced apoptosis were investigated. The level of ornithine decarboxylase (ODC) was elevated in alveolar macrophages, and the number of alveolar macrophages that took up exogenous polyamines was increased 20-fold during PcP. Monocytes, B lymphocytes, and CD8+ T lymphocytes that were recruited into the lung during PcP expressed high levels of ornithine decarboxylase, suggesting that these cells are sources of polyamines. Both protein and mRNA levels of antizyme inhibitor (AZI) were increased in alveolar macrophages during PcP. This AZI overexpression correlated with increased polyamine uptake by alveolar macrophages, because AZI expression knockdown decreased the polyamine uptake ability of these cells. AZI expression knockdown also decreased the apoptosis rate of alveolar macrophages. Pneumocystis organisms and zymosan A were found to induce AZI overexpression in alveolar macrophages, suggesting that beta-glucan, which is the major component of the Pneumocystis cell wall, induces AZI overexpression. The levels of mRNA, protein, and activity of polyamine oxidase were increased in alveolar macrophages during PcP, indicating that the H(2)O(2) generated during polyamine catabolism caused alveolar macrophages to undergo apoptosis. Taken together, results of this study indicate that Pneumocystis organisms induce AZI overexpression in alveolar macrophages, leading to increased polyamine synthesis and uptake and apoptosis rate of these cells.

The antiproliferative effects of agmatine correlate with the rate of cellular proliferation

Polyamines are small cationic molecules required for cellular proliferation. Agmatine is a biogenic amine unique in its capacity to arrest proliferation in cell lines by depleting intracellular polyamine levels. We previously demonstrated that agmatine enters mammalian cells via the polyamine transport system. As polyamine transport is positively correlated with the rate of cellular proliferation, the current study examines the antiproliferative effects of agmatine on cells with varying proliferative kinetics. Herein, we evaluate agmatine transport, intracellular accumulation, and its effects on antizyme expression and cellular proliferation in nontransformed cell lines and their transformed variants. H-ras- and Src-transformed murine NIH/3T3 cells (Ras/3T3 and Src/3T3, respectively) that were exposed to exogenous agmatine exhibit increased uptake and intracellular accumulation relative to the parental NIH/3T3 cell line. Similar increases were obtained for human primary foreskin fibroblasts relative to a human fibrosarcoma cell line, HT1080. Agmatine increases expression of antizyme, a protein that inhibits polyamine biosynthesis and transport. Ras/3T3 and Src/3T3 cells demonstrated augmented increases in antizyme protein expression relative to NIH/3T3 in response to agmatine. All transformed cell lines were significantly more sensitive to the antiproliferative effects of agmatine than nontransformed lines. These effects were attenuated in the presence of exogenous polyamines or inhibitors of polyamine transport. In conclusion, the antiproliferative effects of agmatine preferentially target transformed cell lines due to the increased agmatine uptake exhibited by cells with short cycling times.

Ribosomal frameshifting in decoding antizyme mRNAs from yeast and protists to humans: close to 300 cases reveal remarkable diversity despite underlying conservation

The protein antizyme is a negative regulator of intracellular polyamine levels. Ribosomes synthesizing antizyme start in one ORF and at the codon 5′ adjacent to its stop codon, shift +1 to a second and partially overlapping ORF which encodes most of the protein. The ribosomal frameshifting is a sensor and effector of an autoregulatory circuit which is conserved in animals, fungi and protists. Stimulatory signals encoded 5′ and 3′ of the shift site act to program the frameshifting. Despite overall conservation, many individual branches have evolved specific features surrounding the frameshift site. Among these are RNA pseudoknots, RNA stem-loops, conserved primary RNA sequences, nascent peptide sequences and branch-specific ‘shifty’ codons.

Novel antizyme gene in Danio rerio expressed in brain and retina

The synthesis of the protein antizyme requires a +1 ribosomal frameshift event. The frameshifting serves as a regulatory sensor. Antizyme homologs have been identified in diverse organisms ranging from yeast to human and characterized in a disparate subset. Most vertebrates have multiple antizyme paralogs. Here we present identification in the zebrafish Danio rerio of a heretofore unknown member of the antizyme gene family. This novel antizyme does not correspond to any of the known orthologous groups in vertebrates and unlike most other antizymes is preferentially expressed in the retinal ganglion cell layer of the eye. In addition to the retina, it is also expressed in the brain and somites.

Mitochondrial localization of antizyme is determined by context-dependent alternative utilization of two AUG initiation codons

Ornithine decarboxylase-antizyme (Az), a polyamine-induced protein that targets ornithine decarboxylase (ODC) to rapid degradation, is synthesized as two isoforms. Studies performed in vitro indicated that the 29 and 24.5 kDa isoforms originate from translation initiation at two alternative initiation codons. Using transient transfections we demonstrate here that also in cells the two isoforms are synthesized from two AUG codons with the second being utilized more efficiently. The more efficient utilization of the second AUG is due to its location within a better sequence context for translation initiation. By using immunostaining we demonstrate that only the less expressed long form of Az is localized in the mitochondria. Moreover, this long isoform of Az and not the more efficiently expressed short isoform is imported into mitochondria in an in vitro uptake assay. Our data therefore demonstrate that a single Az transcript gives rise to two Az proteins with different N-terminal sequence and that the longer Az form containing a potential N-terminal mitochondrial localization signal is transported to mitochondria.

Identification of a new antizyme mRNA +1 frameshifting stimulatory pseudoknot in a subset of diverse invertebrates and its apparent absence in intermediate species

The expression of eukaryotic antizyme genes requires +1 translational frameshifting. The frameshift in decoding most vertebrate antizyme mRNAs is stimulated by an RNA pseudoknot 3' of the frameshift site. Although the frameshifting event itself is conserved in a wide variety of organisms from yeast to mammals, until recently no corresponding 3' RNA pseudoknot was known in invertebrate antizyme mRNAs. A pseudoknot, different in structure and origin from its vertebrate counterparts, is now shown to be encoded by the antizyme genes of distantly related invertebrates. Identification of the 3' frameshifting stimulator in intermediate species or other invertebrates remains unresolved.

Decreased ornithine decarboxylase activity in the kidneys of Dahl salt-sensitive rats

To assess the roles of polyamines (putrescine, spermidine, and spermine) and ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine synthesis, in the development of salt-sensitive hypertension, we evaluated activity and expression of ODC, urinary polyamine excretion, and antizyme (endogenous ODC inhibitor protein) expression in Dahl salt-sensitive (SS) and salt-resistant (SR) rats after they were fed on a low (0.3%) or high (4%) salt diet for 4 weeks. We also examined the effects of spermidine and difluoromethylornithine (DFMO: a specific inhibitor of ODC) on the systolic blood pressure and ODC protein expression in SS rats fed a high salt diet. Renal ODC activity and urinary polyamine excretion in SS rats were lower than those in SR rats after 4 weeks treatment with a low or high salt diet. The renal ODC protein expression of SS rats was paradoxically increased as compared to the SR group. A high salt diet did not alter ODC activity but increased ODC protein only in SS rats. ODC mRNA and antizyme protein expressions were not significantly different among the four groups. Spermidine supplementation attenuated and DFMO exaggerated hypertension in SS rats fed a high salt diet. Spermidine down-regulated and DFMO up-regulated renal ODC protein in SS rats on a high salt diet. ODC activity was decreased but protein was paradoxically increased in kidneys of SS rats. ODC protein was suggested to increase in compensation for the inhibition of its activity. Impaired ODC activity and polyamine production in the kidney may exaggerate salt-sensitive hypertension in SS rats.

Antizyme induction by polyamine analogues as a factor of cell growth inhibition

The polyamines spermidine and spermine and their diamine precursor putrescine are essential for mammalian cell growth and viability, and strategies are sought for reducing polyamine levels in order to inhibit cancer growth. Several structural analogues of the polyamines have been found to decrease natural polyamine levels and inhibit cell growth, probably by stimulating normal feedback mechanisms. In the present study, a large selection of spermine analogues has been tested for their effectiveness in inducing the production of antizyme, a key protein in feedback inhibition of putrescine synthesis and cellular polyamine uptake. Bisethylnorspermine, bisethylhomospermine, 1,19-bis-(ethylamino)-5,10,15-triazanonadecane, longer oligoamine constructs and many conformationally constrained analogues of these compounds were found to stimulate antizyme synthesis to different levels in rat liver HTC cells, with some producing far more antizyme than the natural polyamine spermine. Uptake of the tested compounds was found to be dependent on, and limited by, the polyamine transport system, for which all these have approximately equal affinity. These analogues differed in their ability to inhibit HTC cell growth during 3 days of exposure, and this ability correlated with their antizyme-inducing potential. This is the first direct evidence that antizyme is induced by several polyamine analogues. Selection of analogues with this potential may be an effective strategy for maximizing polyamine deprivation and growth inhibition.

Recoding: translational bifurcations in gene expression

During the expression of a certain genes standard decoding is over-ridden in a site or mRNA specific manner. This recoding occurs in response to special signals in mRNA and probably occurs in all organisms. This review deals with the function and distribution of recoding with a focus on the ribosomal frameshifting used for gene expression in bacteria.

Ornithine decarboxylase-antizyme is rapidly degraded through a mechanism that requires functional ubiquitin-dependent proteolytic activity

Antizyme is a polyamine-induced cellular protein that binds to ornithine decarboxylase (ODC), and targets it to rapid ubiquitin-independent degradation by the 26S proteasome. However, the metabolic fate of antizyme is not clear. We have tested the stability of antizyme in mammalian cells. In contrast with previous studies demonstrating stability in vitro in a reticulocyte lysate-based degradation system, in cells antizyme is rapidly degraded and this degradation is inhibited by specific proteasome inhibitors. While the degradation of ODC is stimulated by the presence of cotransfected antizyme, degradation of antizyme seems to be independent of ODC, suggesting that antizyme degradation does not occur while presenting ODC to the 26S proteasome. Interestingly, both species of antizyme, which represent initiation at two in-frame initiation codons, are rapidly degraded. The degradation of both antizyme proteins is inhibited in ts20 cells containing a thermosensitive ubiquitin-activating enzyme, E1. Therefore we conclude that in contrast with ubiquitin-independent degradation of ODC, degradation of antizyme requires a functional ubiquitin system.

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.

Regulation of cellular polyamines by antizyme

Proteins that are degraded by the proteasome are first modified by a set of enzymes that attach multiple copies of ubiquitin to substrate lysines, but a tiny minority, including the polyamine-synthesizing enzyme ornithine decarboxylase, is handled differently. This enzyme is targeted for destruction by another protein--antizyme. Why does ornithine decarboxylase have its own dedicated destruction mechanism, how does it work, and is it the only protein to be targeted to the proteasome in this way?

Hepatocyte growth factor induces pro-apoptotic genes in HepG2 hepatoma but not in B16-F1 melanoma cells

Hepatocyte growth factor (HGF) exerts a cytostatic effect on HepG2 and B16-F1 cell lines. To evaluate the possible involvement of the apoptotic process in this effect, we performed studies at cellular and molecular levels. HGF induced apoptosis only in HepG2 hepatoma cells at day 3 in about 20% of the cells undergoing growth inhibition, while hallmarks of apoptosis did not occur in B16-F1 melanoma cells. During the first 24 h after HGF treatment, enhanced expression of the pro-apoptotic genes bax and c-Myc was observed at level of mRNA and protein. Concomitant induction of antizyme (AZ) might lower ornithine decarboxylase (ODC) protein level though a huge increase in ODC mRNA level took place. This was suggested as a signal for apoptosis decisional phase. The levels of the proteins examined except that of AZ fell down thereafter when HepG2 cells underwent apoptosis. In B16-F1 cells, only ODC and AZ protein levels were elevated probably in relation to the initial elevated growth rate and the absence of apoptosis involvement in the following cytostatic effect of HGF in melanoma cells. Consistent with this hypothesis, bax mRNA and protein levels were unchanged or even lower relative to control values.

Polyamine regulation of ornithine decarboxylase and its antizyme in intestinal epithelial cells

Ornithine decarboxylase (ODC) is feedback regulated by polyamines. ODC antizyme mediates this process by forming a complex with ODC and enhancing its degradation. It has been reported that polyamines induce ODC antizyme and inhibit ODC activity. Since exogenous polyamines can be converted to each other after they are taken up into cells, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone) (DEGBG), to block the synthesis of spermidine and spermine from putrescine and investigated the specific roles of individual polyamines in the regulation of ODC in intestinal epithelial crypt (IEC-6) cells. We found that putrescine, spermidine, and spermine inhibited ODC activity stimulated by serum to 85, 46, and 0% of control, respectively, in the presence of DEGBG. ODC activity increased in DEGBG-treated cells, despite high intracellular putrescine levels. Although exogenous spermidine and spermine reduced ODC activity of DEGBG-treated cells close to control levels, spermine was more effective than spermidine. Exogenous putrescine was much less effective in inducing antizyme than spermidine or spermine. High putrescine levels in DEGBG-treated cells did not induce ODC antizyme when intracellular spermidine and spermine levels were low. The decay of ODC activity and reduction of ODC protein levels were not accompanied by induction of antizyme in the presence of DEGBG. Our results indicate that spermine is the most, and putrescine the least, effective polyamine in regulating ODC activity, and upregulation of antizyme is not required for the degradation of ODC protein.

Antizyme expression: a subversion of triplet decoding, which is remarkably conserved by evolution, is a sensor for an autoregulatory circuit

The efficiency of programmed ribosomal frameshifting in decoding antizyme mRNA is the sensor for an autoregulatory circuit that controls cellular polyamine levels in organisms ranging from the yeast Schizosaccharomyces pombe to Drosophila to mammals. Comparison of the frameshift sites and flanking stimulatory signals in many organisms now permits a reconstruction of the likely evolutionary path of the remarkably conserved mRNA sequences involved in the frameshifting.

BCR-ABL prevents c-jun-mediated and proteasome-dependent FUS (TLS) proteolysis through a protein kinase CbetaII-dependent pathway

The DNA binding activity of FUS (also known as TLS), a nuclear pro-oncogene involved in multiple translocations, is regulated by BCR-ABL in a protein kinase CbetaII (PKCbetaII)-dependent manner. We show here that in normal myeloid progenitor cells FUS, although not visibly ubiquitinated, undergoes proteasome-dependent degradation, whereas in BCR-ABL-expressing cells, degradation is suppressed by PKCbetaII phosphorylation. Replacement of serine 256 with the phosphomimetic aspartic acid prevents proteasome-dependent proteolysis of FUS, while the serine-256-to-alanine FUS mutant is unstable and susceptible to degradation. Ectopic expression of the phosphomimetic S256D FUS mutant in granulocyte colony-stimulating factor-treated 32Dcl3 cells induces massive apoptosis and inhibits the differentiation of the cells escaping cell death, while the degradation-prone S256A mutant has no effect on either survival or differentiation. FUS proteolysis is induced by c-Jun, is suppressed by BCR-ABL or Jun kinase 1, and does not depend on c-Jun transactivation potential, ubiquitination, or its interaction with Jun kinase 1. In addition, c-Jun-induced FUS proteasome-dependent degradation is enhanced by heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and depends on the formation of a FUS-Jun-hnRNP A1-containing complex and on lack of PKCbetaII phosphorylation at serine 256 but not on FUS ubiquitination. Thus, novel mechanisms appear to be involved in the degradation of FUS in normal myeloid cells; moreover, the ability of the BCR-ABL oncoprotein to suppress FUS degradation by the induction of posttranslational modifications might contribute to the phenotype of BCR-ABL-expressing hematopoietic cells.

Discovery of a spermatogenesis stage-specific ornithine decarboxylase antizyme: antizyme 3

Previous studies with mice overproducing ornithine decarboxylase have demonstrated the importance of polyamine homeostasis for normal mammalian spermatogenesis. The present study introduces a likely key player in the maintenance of proper polyamine homeostasis during spermatogenesis. Antizyme 3 is a paralog of mammalian ornithine decarboxylase antizymes. Like its previously described counterparts, antizymes 1 and 2, it inhibits ornithine decarboxylase, which catalyzes the synthesis of putrescine. Earlier work has shown that the coding sequences for antizymes 1 and 2 are in two different, partially overlapping reading frames. Ribosomes translate the first reading frame, and just before the stop codon for that frame, they shift to the second reading frame to synthesize a trans-frame product. The efficiency of this frameshifting depends on polyamine concentration, creating an autoregulatory circuit. Antizyme 3 cDNA has the same arrangement of reading frames and a potential shift site with definite, although limited, homology to its evolutionarily distant antizyme 1 and 2 counterparts. In contrast to antizymes 1 and 2, which are widely expressed throughout the body, antizyme 3 transcription is restricted to testis germ cells. Expression starts early in spermiogenesis and finishes in the late spermatid phase. The potential significance of antizyme 3 expression during spermatogenesis is discussed in this paper.

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.

Identification and characterization of testis specific ornithine decarboxylase antizyme (OAZ-t) gene: expression in haploid germ cells and polyamine-induced frameshifting

BACKGROUND

Polyamines are known to play important roles in the proliferation and differentiation of many types of cells. However, in the testis, where polyamines such as spermidine and spermine exist in high concentrations, their roles still remains to be elucidated.

RESULTS

We have cloned a testis-specific gene encoding an ornithine decarboxylase antizyme known to control intracellular concentrations of polyamines in a feedback manner. The mRNA encoding the protein named ornithine decarboxylase antizyme in testis (OAZ-t) was specifically expressed in haploid germ cells. In contrast, the mRNA level of somatic ornithine decarboxylase antizyme 1 (OAZ1) decreased markedly at the late stages of haploid germ cell differentiation. OAZ-t mRNA was first observed in 23-day-old mice, whereas the OAZ-t protein was detected much later, at 35 days after birth. Further experiments on OAZ-t revealed that polyamines were capable of inducing a frameshifting at the frameshift sequence of OAZ-t mRNA, resulting in the translation of OAZ-t, as was the case with the somatic OAZ1. Transfection of OAZ-t cDNA inactivated the ornithine decarboxylase activity in the HEK293 cells.

CONCLUSIONS

Results indicate that the expression of OAZ-t is controlled at both transcriptional and translational levels, and that OAZ-t likely plays a key role in spermatogenesis by regulating the intracellular concentration of polyamines in haploid germ cells.

Two zebrafish (Danio rerio) antizymes with different expression and activities

Cellular polyamines are regulated by a unique feedback mechanism involving ornithine decarboxylase (ODC) antizyme. The synthesis of mammalian antizyme requires a programmed translational frameshift event induced by polyamines. Antizyme represses ODC, a key enzyme for polyamine synthesis, through accelerating enzyme degradation by the 26 S proteasome. Antizyme also inhibits the cellular uptake of polyamines. In the present study we isolated two distinct zebrafish (Danio rerio) antizyme cDNA clones (AZS and AZL) from an embryonic library. Their sequences revealed that both clones required translational frameshifting for expression. Taking account of +1 frameshifting, AZS and AZL products were 214 and 218 residues long respectively and shared 51.8% amino acid identity. In rabbit reticulocyte lysates, both mRNA species were translated through spermidine-induced frameshifting. The presence of the two antizyme mRNA species in embryos, adult fish and a cultured cell line was confirmed by Northern blot analysis. The ratio of AZS mRNA to AZL mRNA in the adult fish was 1.8-fold higher than in the embryos. Whole-mount hybridization in situ demonstrated that both mRNA species are expressed in every tissue in embryo, but predominantly in the central nervous system and the eyes. Bacterial expression products of both cDNA species inhibited ODC activity, but only the AZS product accelerated ODC degradation in vitro. These results show that both zebrafish antizymes are induced by polyamines but their mRNA species are expressed differently during development. The difference in activities on ODC degradation suggests their functional divergence.

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.

The cellular localization of the L-ornithine decarboxylase/polyamine system in normal and diseased central nervous systems

Natural polyamines, spermidine and spermine, and their precursor putrescine, are of considerable importance for the developing and mature nervous system. They exhibit a number of neurophysiological and metabolic effects in the nervous system, including control of nucleic acid and protein synthesis, modulation of ionic channels and calcium-dependent transmitter release. The polyamine system is also known to be involved in various brain pathologic events (seizures, stroke, Alzheimer's disease and others). While cerebral polyamine concentrations and the activities of polyamine-metabolizing enzymes have been studied in great detail, much less is known about the cells that are responsible for cerebral polyamine synthesis and interconversion. With the present review the attempt is made to show how exact knowledge about the regional distribution and cellular localization of polyamines and the polyamine-synthesizing enzymatic machinery (and especially of L-ornithine decarboxylase) may help to better understand the functional interplay between polyamines and other endogenous agents (transmitters, receptors, growth factors neuroactive drugs etc.). Polyamines have been localized both in neurones and glial cells. However, the main cellular locus of the ODC is the neuron--both in the immature and adult central nervous system. Each period of normal brain development and ageing seems to have its own, characteristic temporo-spatial pattern of neuronal ODC expression. During strong functional activation (kindling, epileptic seizures, neural transplantation) astrocytes and other non-neuronal cells do also express ODC and other polyamine-metabolizing enzymes. Astroglial expression of ODC is accompanied by an increase in glial fibrillary acidic protein in these cells. This shift in the cellular mechanisms of polyamine metabolism is currently far from being understood. In human brain diseases (Alzheimer's disease, schizophrenia) certain neurones show an increased expression of ODC, the first and rate-limiting enzyme of polyamine metabolism. Since polyamines are structurally related to psychoactive drugs (neuroleptics, antidepressants) the polyamine system might be of importance as a putative target for drug intervention in psychiatry.

Inhibition of proteasome function prevents thymocyte apoptosis: involvement of ornithine decarboxylase

We have previously shown that polyamine levels rapidly decrease in thymocytes undergoing apoptosis, and that ornithine decarboxylase increases early but too transiently to maintain elevated polyamine levels. These data led us to suppose that a precocious ornithine decarboxylase degradation might be responsible for the imbalance of polyamine metabolism. Ornithine decarboxylase is known to be degraded by the cytosolic 26S proteasome that plays an essential role in thymocyte apoptosis. In this paper we demonstrate that the inhibition of proteasome function preserves ornithine decarboxylase activity and prevents thymocytes from undergoing apoptosis after dexamethasone treatment. Since intracellular polyamine levels are also preserved, ornithine decarboxylase seems to be functionally active in maintaining polyamine homeostasis after proteasome inhibition in thymocytes. Our proposed role for the proteasome in quiescent cells upon an apoptotic stimulus is to degrade proteins like ornithine decarboxylase that are involved in the control of the cell cycle and cell survival.

A second mammalian antizyme: conservation of programmed ribosomal frameshifting

A second mammalian ornithine decarboxylase antizyme was discovered. The deduced protein sequence of the human antizyme2 is 54% identical and 67% similar to human antizyme1 but 99.5% identical to mouse antizyme2. Polyamine-regulated programmed ribosomal frameshifting is used in decoding antizyme2 mRNA as it is for antizyme1 mRNA. The mRNA signals for the programmed frameshifting are similar in the mRNAs for the two antizymes. However, in the stimulatory pseudoknot 3' of the shift site, while the sequences of the stems are highly conserved, the sequences of the loops are divergent. Functional distinctions between antizymes seem likely, but no distinction in the tissue distribution of human antizyme1 and 2 mRNAs was distinguished, though antizyme2 mRNA is 16-fold less abundant than its antizyme1 counterpart. In addition to the previously characterized human antizyme1 mRNA, a second antizyme1 mRNA with an additional 160 nucleotides at its 3' end was identified, and it has a tissue distribution different from that of the shorter antizyme1 mRNA.

The Drosophila gene for antizyme requires ribosomal frameshifting for expression and contains an intronic gene for snRNP Sm D3 on the opposite strand

Previously, a Drosophila melanogaster sequence with high homology to the sequence for mammalian antizyme (ornithine decarboxylase antizyme) was reported. The present study shows that homology of this coding sequence to its mammalian antizyme counterpart also extends to a 5' open reading frame (ORF) which encodes the amino-terminal part of antizyme and overlaps the +1 frame (ORF2) that encodes the carboxy-terminal three-quarters of the protein. Ribosomes shift frame from the 5' ORF to ORF2 with an efficiency regulated by polyamines. At least in mammals, this is part of an autoregulatory circuit. The shift site and 23 of 25 of the flanking nucleotides which are likely important for efficient frameshifting are identical to their mammalian homologs. In the reverse orientation, within one of the introns of the Drosophila antizyme gene, the gene for snRNP Sm D3 is located. Previously, it was shown that two closely linked P-element transposon insertions caused the gutfeeling phenotype of embryonic lethality and aberrant neuronal and muscle cell differentiation. The present work shows that defects in either snRNP Sm D3 or antizyme, or both, are likely causes of the phenotype.

Osmotic stress induces variation in cellular levels of ornithine decarboxylase-antizyme

The polyamines, and especially putrescine, play an integral role in the physiological response of cells to varying extracellular osmotic conditions. Ornithine decarboxylase (ODC) synthesis and stability, as well as the activity of the polyamine transporter, had all been reported to be very sensitive to media osmolarity in different cells and tissues, yet the mechanism of this complex, co-ordinated response was not known. In this study we have determined that all these aspects of osmotic-shock response may be mediated by the common regulatory protein, ODC-antizyme. HTC cells were induced for antizyme and then exposed to media of reduced osmotic strength. Both antizyme activity and protein decreased rapidly, under these conditions, to new steady-state levels that depended upon the degree of reduction in media tonicity. This antizyme reduction was found to be due to a rapid increase in antizyme degradation, with a half-life decrease from 75 min down to 45 min occurring immediately upon exchanging media. In complementary experiments, increased media tonicity induced elevated antizyme levels and stability. The sensitivity of antizyme turnover to osmotic conditions was also observed in DH23b cells, which contain elevated levels of more stable antizyme. Interestingly, the two main antizyme proteins, AZ-1 and AZ-2 (presumably products from the first and second translational start sites), differed in their responses to these changing osmotic conditions. Just as feedback regulation of antizyme synthesis provides an effective mechanism for maintaining stable polyamine levels, these studies suggest that alteration in the rate of antizyme degradation may be the mechanism whereby cells adjust steady-state polyamine levels in response to stimulation or stress.

Characterization of the human antizyme gene

Antizyme is a polyamine-inducible protein involved in feedback regulation of cellular polyamine levels. Recently, we isolated genomic clones for the human antizyme gene and determined its chromosomal location (Matsufuji et al., Genomics 38 (1996) 112-114). In the present study, we report complete nucleotide sequence and organization of the human antizyme gene. The organizations of human and rat genes are very similar, but their introns show divergency in terms of the length and nucleotide sequence. Luciferase reporter assay revealed that the 5'-flanking region of the human gene had a strong transcriptional activity in NIH-3T3 with and without addition of spermidine. The promoter was also effective in transfected COS7 and HeLa cells. A 223-bp region at the proximity of the transcriptional start points carries several regulatory sequence motifs including a TATA box, CAAT boxes and GC boxes, and was shown to be important for the strong transcriptional activity.

Polyamines regulate both transcription and translation of the gene encoding ornithine decarboxylase antizyme in mouse

The degradation of ornithine decarboxylase (ODC) is mediated by antizyme, a protein regulated by the end-products of ODC activity, the polyamines. High levels of polyamines induce a +1 ribosomal frameshift in the translation of the rat antizyme message leading to the expression of a full-length protein. We have studied whether the regulation of antizyme expression occurs only at the level of translation or whether polyamine levels also affect the transcription of the antizyme gene. Thus, we have cloned and sequenced the mouse homologues of the rat ODC-antizyme gene and cDNA. Northern blot analysis shows that although high concentrations of polyamines do not affect the steady-state levels of antizyme message in L1210 leukemia cells, polyamine depletion using 2-(difluoromethyl)ornithine [Orn(F2Me)] leads to a marked decrease in mRNA levels. Results of transient transfections of luciferase-reporter-gene constructs driven by antizyme promoter fragments in untreated and Orn(F2Me)-treated Balb/C 3T3 cells indicate that the transcription of the antizyme gene is altered upon polyamine depletion. The amount of antizyme protein on Western blots was also altered by polyamine depletion and addition, and the polysomal distribution of antizyme message suggests a general translational increase of the message when polyamine concentrations are high. These results indicate a role for polyamines in the transcriptional and translational regulation of ornithine decarboxylase antizyme.

Identification of regulatory region of antizyme necessary for the negative regulation of polyamine transport

Antizyme is a negative regulator of ornithine decarboxylase (ODC) and of polyamine transport. Regions of antizyme necessary for the negative regulation of polyamine transport were determined by transfecting ODC-overproducing EXOD-1 cells with mutant antizyme genes containing different size deletions in the NH2- and COOH-terminal of antizyme (AZ69-227). When peptide 119-144 or peptide 211-216, which are responsible for the binding of ODC, were deleted from antizyme, the mutant antizyme could not reverse the inhibition of growth of EXOD-1 cells produced by spermine. In parallel with the decrease in antizyme effect on cell growth, spermine transport activity and the accumulation of spermine in EXOD-1 cells were not significantly altered by the mutant antizyme, whereas wild-type antizyme decreased spermine transport and accumulation. When the peptide 69-118, which is responsible for the degradation of ODC, was deleted from antizyme, the mutant antizyme showed a smaller effect compared with the normal antizyme in terms of the inhibition of spermine transport and the recovery from the spermine inhibition of cell growth. The results indicate that regions 119-144 and 211-216 in antizyme are necessary for the negative regulation of polyamine transport and that these regions overlap with ODC binding sites.

Characterization of a diamine exporter in Chinese hamster ovary cells and identification of specific polyamine substrates

Export of the diamine putrescine was studied using inside-out plasma membrane vesicles prepared from Chinese hamster cells. Putrescine uptake into vesicles was a saturable and an ATP- and antizyme-independent process. Excess amounts of a series of diamines or monoacetyl spermidine, but not monoacetyl putrescine, spermidine, or spermine, inhibited putrescine transport. Putrescine uptake into vesicles prepared at pH 7.4 was suppressed at pH 5, compared with pH 7.4; was stimulated approximately 2.5-fold at pH 7.4 in vesicles prepared at pH 6.25, compared with vesicles prepared at pH 7.4; and was not inhibited by valinomycin in the presence of potassium ions. Reserpine and verapamil blocked [3H]putrescine uptake into inverted vesicles. Verapamil treatment caused an increase in intracellular contents of putrescine, cadaverine, and N8-acetylspermidine, in unstressed proliferating cells, or of N1-acetylspermidine, in cells subjected to heat shock to induce acetylation of spermidine at N1. These data indicate that putrescine export in Chinese hamster cells is mediated by a non-electrogenic antiporter capable of using protons as the counter ion. Physiological substrates for this exporter include putrescine, cadaverine, and monoacetyl spermidine and have the general structure NH3+-(CH2)n-NH2 + R at acidic or neutral pH.

Involvement of antizyme-like regulatory protein in polyamine-caused repression of ornithine decarboxylase in insect-derived Trichoplusia ni 5 cells

Addition of spermidine to culture medium of insect cells, Trichoplusia ni 5, at a low cellular density suppressed ornithine decarboxylase (ODC; EC 4.1.1.17) activity and induced ODC inhibitory activity. The inhibitory factor was non-dialyzable, temperature-sensitive, and could reversibly form an inactive complex with ODC. It showed a time-independent and non-stoichiometric pattern of inhibition. Upon addition of spermidine to cultured cells with preinduced ODC, the enzyme decayed more rapidly than after addition of cycloheximide. These data strongly suggested that ODC of Tn5 cells is under negative feedback control by polyamines, in which an antizyme-like regulatory protein plays an essential role.

Structure, organization and expression of the mouse ornithine decarboxylase antizyme gene

Ornithine decarboxylase antizyme is a protein that participates in the regulation of cellular polyamine levels. In this study we have isolated and sequenced the mouse gene encoding antizyme protein. Transfection of various cell lines with a 5.5 kb genomic fragment containing the antizyme locus resulted in the production of a 29 kDa antizyme protein, confirming that this locus contained a functional gene. Comparison of the mouse gene with the corresponding rat gene [Miyazaki, Matsufuji and Hayashi, (1992) Gene 113, 191-197] revealed an identical exon/intron organization and high level of nucleotide sequence conservation that was 89% for the entire transcription unit. Protein-coding regions of the two genes exhibited 97% nucleotide sequence identity and there were only four amino acid differences between the 227-residue antizyme protein sequences of the mouse and rat. The promoter of the antizyme gene was functional in mouse (N2A and NIH/3T3) and hamster (CHO) cell lines. The presence of 0.1 mM spermidine in culture medium increased the amount of immunoreactive antizyme protein in cells transfected with the antizyme gene or antizyme cDNA, possibly owing to facilitated frameshifting in the translation of antizyme mRNA. Recombinant antizyme protein was also produced in Escherichia coli and used to raise specific polyclonal antibodies in rabbits and to devise immunological methods for the measurement of antizyme concentration.

Feedback repression of ornithine decarboxylase synthesis mediated by antizyme

The induction of antizyme by spermidine and the resulting enhancement of ornithine decarboxylase (ODC) degradation have been well studied; however, little is known about the mechanism whereby elevated spermidine levels decrease synthesis of the polyamine biosynthetic enzyme. To evaluate the relative contribution of inhibited synthesis, as distinct from enhanced degradation of ODC, spermidine levels were manipulated in a variant cell line that overproduces a stable form of ODC. Spermidine did not selectively inhibit ODC synthesis in these variant cells, supporting the concept that spermidine diminishes ODC synthesis in normal cells owing to enhanced degradation of the protein in the presence of elevated antizyme levels. This model was further investigated in vitro by use of rabbit reticulocyte lysate, which catalyses simultaneous ODC mRNA translation and antizyme-stimulated degradation of ODC protein. Antizyme strongly repressed the incorporation of labelled amino acids into normal rat ODC. Unexpectedly it also diminished the apparent translation of ODC mRNA species coding for enzyme forms that are not destabilized by the post-translational addition of antizyme. The effect of antizyme on ODC translation was not observed in wheatgerm extract, in which there is no antizyme-induced degradation. Further, deletion of a short segment of antizyme necessary for the destabilization of ODC (amino acid residues 113-118) resulted in a form that bound ODC but did not diminish its apparent translation. These results suggest that the co-translational addition of antizyme to ODC results in a complex that is different from, and innately less stable than, that formed when antizyme is added post-translationally.

The N terminus of antizyme promotes degradation of heterologous proteins

Regulated degradation of ornithine decarboxylase (ODC) is mediated by its association with the inducible protein antizyme. The N terminus of antizyme (NAZ), although unneeded for the interaction with ODC, must be present to induce degradation. We report here that covalently grafting NAZ to ODC confers lability that normally results from the non-covalent association of native antizyme and ODC. To determine whether NAZ could act similarly as a modular functional domain when grafted to other proteins, we fused it to a region of cyclin B (amino acids 13-90) capable of undergoing degradation or to cyclin B (amino acids 13-59), which is not subject to degradation. The association with NAZ made both NAZ-cyclin B13-90 and NAZ-cyclin B13-59 unstable. Furthermore, NAZ and cyclin B 13-59 were together able to induce in vitro degradation of Trypanosoma brucei ODC, a stable protein. The ODC-antizyme complex bound to the 26 S protease but not the 20 S proteasome, consistent with the observation that ODC degradation is mediated by the 26 S protease. The association was shown to be independent of NAZ, suggesting that NAZ does not act as a recognition signal.

Control of ornithine decarboxylase activity by polyamines and absence of antizyme in Tetrahymena

1. In cells of Tetrahymena pyriformis and thermophila, ODC activity was significantly suppressed but ODC decay was not stimulated by putrescine. 2. Free antizyme and ODC-antizyme complex were both not detected in extracts of cells of T. pyriformis treated with putrescine. 3. It was concluded that in Tetrahymena, unlike vertebrate cells, ODC is not subject to polyamine-induced destabilization mediated by antizyme.

The expression of ornithine decarboxylase antizyme mRNA and protein in rat motoneurons

The distribution of ornithine decarboxylase antizyme messenger ribonucleic acid (AZ mRNA) and AZ-like immunoreactivity (LI) was studied in the brainstem and spinal cord motoneurons and in the extraocular and triceps surae muscles of rat. In situ hybridization showed AZ mRNA in the gray matter of the spinal cord at different levels of spinal cord with highest AZ mRNA levels in the ventral horn of the spinal cord. No apparent changes in AZ mRNA contents were seen after unilateral transection of the sciatic nerve in lumbar motoneurons. AZ-immunoreactive (IR) motoneurons were observed in the nucleus of the VI cranial nerve and in the ventral horn of the spinal cord. These motoneurons also showed ornithine decarboxylase (ODC)-LI. Subcellularly, AZ-LI was observed both in the nuclei and cytoplasm of labeled motoneurons. Heavily stained AZ-IR nerve fibers and myoneural junctions were observed among muscle fibers in different muscles. In addition, the nuclei of muscle fibers showed AZ-LI.

Nucleotide sequence of ornithine decarboxylase antizyme cDNA from Xenopus laevis

An ornithine decarboxylase antizyme cDNA was obtained from Xenopus laevis liver and its sequence was determined. The cDNA consists of two major open reading frames as found in mammalian antizymes, which require +1 ribosomal frameshifting for its translation. Sequences important for frameshifting, namely the frameshift site and downstream stimulatory pseudoknot determined in the rat mRNA, are conserved.