Overexpression of antizyme-inhibitor in NIH3T3 fibroblasts provides growth advantage through neutralization of antizyme functions.

Antizyme inhibitor (AzI) is a homolog of ornithine decarboxylase (ODC), a key enzyme of polyamine synthesis. Antizyme inhibitor retains no enzymatic activity, but exhibits high affinity to antizyme (Az), a negative regulator of polyamine homeostasis. As polyamines are involved in maintaining cellular proliferation, and since AzI may negate Az functions, we have investigated the role of AzI in regulating cell growth. We show here that overexpression of AzI in NIH3T3 cells increased growth rate, enabled growth in low serum, and permitted anchorage-independent growth in soft agar, while reduction of AzI levels by AzI siRNA reduced cellular proliferation. Moreover, AzI overproducing cells gave rise to tumors when injected into nude mice. AzI overexpression resulted in elevation of ODC activity and of polyamine uptake. These effects of AzI are a result of its ability to neutralize Az, as overexpression of an AzI mutant with reduced Az binding failed to alter cellular polyamine metabolism and growth properties. We also demonstrate upregulation of AzI in Ras transformed cells, suggesting its relevance to some naturally occurring transformations. Finally, increased uptake activity rendered AzI overproducing and Ras-transformed cells more sensitive to toxic polyamine analogs. Our results therefore imply that AzI has a central and meaningful role in modulation of polyamine homeostasis, and in regulating cellular proliferation and transformation properties.

Ubiquitin dependent and independent protein degradation in the regulation of cellular polyamines.

Protein degradation mediated by the ubiquitin/proteasome system is the major route for the degradation of cellular proteins. In this pathway the ubiquitination of the target proteins is manifested via the concerted action of several enzymes. The ubiquinated proteins are then recognized and degraded by the 26S proteasome. There are few reports of proteins degraded by the 26S protesome without ubiquitination, with ornithine decarboxylase being the most notable representative of this group. Interestingly, while the degradation of ODC is independent of ubiquitination, the degradation of other enzymes of the polyamine biosynthesis pathway is ubiquitin dependent. The present review describes the degradation of enzymes and regulators of the polyamine biosynthesis pathway.

Screening for modulators of spermine tolerance identifies Sky1, the SR protein kinase of Saccharomyces cerevisiae, as a regulator of polyamine transport and ion homeostasis.

Although most cells are capable of transporting polyamines, the mechanism that regulates polyamine transport in eukaryotes is still largely unknown. Using a genetic screen for clones capable of restoring spermine sensitivity to spermine-tolerant mutants of Saccharomyces cerevisiae, we have demonstrated that Sky1p, a recently identified SR protein kinase, is a key regulator of polyamine transport. Yeast cells deleted for SKY1 developed tolerance to toxic levels of spermine, while overexpression of Sky1p in wild-type cells increased their sensitivity to spermine. Expression of the wild-type Sky1p but not of a catalytically inactive mutant restored sensitivity to spermine. SKY1 disruption results in dramatically reduced uptake of spermine, spermidine, and putrescine. In addition to spermine tolerance, sky1Delta cells exhibit increased tolerance to lithium and sodium ions but somewhat increased sensitivity to osmotic shock. The observed halotolerance suggests potential regulatory interaction between the transport of polyamines and inorganic ions, as suggested in the case of the Ptk2p, a recently described regulator of polyamine transport. We demonstrate that these two kinases act in two different signaling pathways. While deletion or overexpression of SKY1 did not significantly affect Pma1p activity, the ability of overexpressed Sky1p, Ptk1p, and Ptk2p to increase sensitivity to LiCl depends on the integrity of PPZ1 but not of ENA1.

Transcriptional activation of mammalian ornithine decarboxylase during stimulated growth.

Cloned ornithine decarboxylase (ODC) (EC 4.1.1.17) cDNA was used to investigate the mechanisms which mediate the mitogenic induction of mammalian ODC. Stimulation of quiescent BALB/c 3T3 mouse fibroblasts with purified fibroblast and platelet-derived growth factors and with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate results in a rapid and dramatic increase in ODC mRNA, similar to the increase caused by serum stimulation. Using nuclear runoff transcriptional analysis, we demonstrate that an increase in ODC transcription accounts for the mitogenic induction of ODC mRNA, and using cycloheximide together with the stimulating mitogen, we found that the mitogenic induction of ODC is dependent on ongoing protein synthesis in the stimulated cells.

Structural motifs involved in ubiquitin-mediated processing of the NF-kappaB precursor p105: roles of the glycine-rich region and a downstream ubiquitination domain.

The ubiquitin proteolytic system plays a major role in a variety of basic cellular processes. In the majority of these processes, the target proteins are completely degraded. In one exceptional case, generation of the p50 subunit of the transcriptional regulator NF-kappaB, the precursor protein p105 is processed in a limited manner: the N-terminal domain yields the p50 subunit, whereas the C-terminal domain is degraded. The identity of the mechanisms involved in this unique process have remained elusive. It has been shown that a Gly-rich region (GRR) at the C-terminal domain of p50 is an important processing signal. Here we show that the GRR does not interfere with conjugation of ubiquitin to p105 but probably does interfere with the processing of the ubiquitin-tagged precursor by the 26S proteasome. Structural analysis reveals that a short sequence containing a few Gly residues and a single essential Ala is sufficient to generate p50. Mechanistically, the presence of the GRR appears to stop further degradation of p50 and to stabilize the molecule. It appears that the localization of the GRR within p105 plays an important role in directing processing: transfer of the GRR within p105 or insertion of the GRR into homologous or heterologous proteins is not sufficient to promote processing in most cases, which is probably due to the requirement for an additional specific ubiquitination and/or recognition domain(s). Indeed, we have shown that amino acid residues 441 to 454 are important for processing. In particular, both Lys 441 and Lys 442 appear to serve as major ubiquitination targets, while residues 446 to 454 are independently important for processing and may serve as the ubiquitin ligase recognition motif.

Poliovirus 2A protease induces apoptotic cell death.

A cell line was generated that expresses the poliovirus 2A protease in an inducible manner. Tightly controlled expression was achieved by utilizing the muristerone A-regulated expression system. Upon induction, cleavage of the eukaryotic translation initiation factor 4GI (eIF4GI) and eIF4GII is observed, with the latter being cleaved in a somewhat slower kinetics. eIF4G cleavage was accompanied by a severe inhibition of protein synthesis activity. Upon induction of the poliovirus 2A protease, the cells displayed fragmented nuclei, chromatin condensation, oligonucleosome-size DNA ladder, and positive TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) staining; hence, their death can be characterized as apoptosis. These results indicate that the expression of the 2A protease in mammalian cells is sufficient to induce apoptosis. We suggest that the poliovirus 2A protease induces apoptosis either by arresting cap-dependent translation of some cellular mRNAs that encode proteins required for cell viability, by preferential cap-independent translation of cellular mRNAs encoding apoptosis inducing proteins, or by cleaving other, yet unidentified cellular target proteins.

c-Myc and Max transregulate the mouse ornithine decarboxylase promoter through interaction with two downstream CACGTG motifs.

Ornithine decarboxylase (ODC), the first enzyme in the biosynthesis of polyamines, is essential for the process of cellular proliferation. ODC is a typical delayed early gene, as its mitogenic activation requires ongoing protein synthesis in the stimulated cells. This study provides evidence that the immediate early c-Myc protein is a potential transactivator of the ODC gene. We demonstrate that overexpression of c-Myc results in efficient activation of the ODC promoter, whereas overexpression of Max exerts a repressive effect. Both effects depend on the presence of two evolutionary conserved CACGTG motifs found in the first intron of the ODC gene. Transactivation of the ODC promoter also requires the dimerization of c-Myc with Max. Interestingly, over-expression of USF, a member of the same family of proteins which efficiently binds these two CACGTG motifs, fails to transregulate the ODC promoter. Our data suggest that c-Myc and Max are potential transcriptional regulators of the ODC promoter.

The 26S proteasome degrades mouse and yeast ornithine decarboxylase in yeast cells.

Eukaryotic cells possess two high-molecular-mass proteases, the 700 kDa, 20S proteasome, as well as the even larger 1,400 kDa, 26S proteasome. It has been demonstrated that ornithine decarboxylase is degraded, in vitro, by the 26S proteasome that contains the 20S protease as its catalytic core, but not by the free 20S proteasome. Recently, by demonstrating severe inhibition of mouse and yeast ODC degradation in a mutant yeast cell line, defective in the chymotripsin-like activity of the yeast 20S proteasome, we implicated the 20S proteasome in the degradation of ODC, in vivo, in yeast cells. Here we show that the degradation of ODC is also severely inhibited in the mutant yeast cell lines, cim3-1 and cim5-1, containing a specific lesion in subunits that are unique to the yeast 26S proteasome. We therefore, conclude, that as illustrated in vitro, also in intact cells, it is the 26S proteasome, not the free 20S proteasome, that degrades ODC. We also demonstrate, that while deficiency in the proteasome chymotrypsine-like activity (in the yeast pre1-1 mutant) inhibits the degradation of both yeast and mouse ODCs, deficiency in the peptidyl-glutamyl-peptide-hydrolyzing (PGPH) activity inhibits only yeast ODC degradation. Similarly, we have noted that whereas the putative ATPase activity of both the CIM3 and CIM5 subunits is essential for the degradation of mouse ODC, only that of the CIM3 subunit is required for the degradation of yeast ODC. These results suggest differential utilization of individual proteasomal subunits in the recognition and degradation of individual short-lived proteins.

The 20S proteasome mediates the degradation of mouse and yeast ornithine decarboxylase in yeast cells.

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines, is one of the most rapidly degraded proteins in mammalian cells. Recently it has been demonstrated that mammalian ODC is degraded in vitro by the 26S protease that contains the 20S proteasome as its catalytic core, in a reaction that does not require ubiquitin. Here, we show that yeast and mouse ODC are both rapidly degraded in yeast cells and that their degradation severely inhibited in a mutant yeast cell line defective in the chymotryptic activity of proteinase yscE, the yeast 20S proteasome. These results provide compelling genetic support to previous biochemical studies suggesting the involvement of the 20S proteasome in the degradation of ornithine decarboxylase.

Mechanisms of ubiquitin-mediated, limited processing of the NF-kappaB1 precursor protein p105.

In most cases, target proteins of the ubiquitin system are completely degraded. In several exceptions, such as the first step in the activation of the transcriptional regulator NF-kappaB, the substrate, the precursor protein p105, is processed in a limited manner to yield the active subunit p50. p50 is derived from the N-terminal domain of p105, whereas the C-terminal domain is degraded. The mechanisms involved in this unique process have remained elusive. We have shown that a Gly-rich region (GRR) at the C-terminal domain of p50 is one important processing signal and that it interferes with processing of the ubiquitinated precursor by the 26S proteasome. Also, amino acid residues 441-454 are important for processing under non-stimulated conditions. Lys 441 and 442 serve as ubiquitination targets, whereas residues 446-454 may serve as a ligase recognition motif. Following IkappaB kinase (IKK)-mediated phosphorylation, the C-terminal domain of p105, residues 918-934, recruits the SCF(beta-TrCP) ubiquitin ligase, and ubiquitination by this complex leads to accelerated processing. The two sites appear to be recognized under different physiological conditions by two different ligases, targeting two distinct recognition motifs. We have shown that ubiquitin conjugation and processing of a series of precursors of p105 that lack the C-terminal IKK phosphorylation/TrCP binding domain, is progressively inhibited with increasing number of ankyrin repeats. Inhibition is due to docking of active NF-kappaB subunits to the ankyrin repeat domain in the C-terminal half of p105 (IkappaBgamma). Inhibition is alleviated by phosphorylation of the C-terminal domain that leads to ubiquitin-mediated degradation of the ankyrin repeat domain and release of the anchored subunits. We propose a model that may explain the requirement for two sites: a) a basal site that may be involved in co-translational processing prior to the synthesis of the ankyrin repeat domain; and b) a signal-induced site that is involved in processing/degradation of the complete molecule following cell activation, with rapid release of stored, transcriptionally active subunits.

Isolation and characterization of the Drosophila ornithine decarboxylase locus: evidence for the presence of two transcribed ODC genes in the Drosophila genome.

The polymerase chain reaction (PCR) was used to isolate two Drosophila ornithine decarboxylase (ODC) genes. Two mixtures of degenerate oligonucleotides corresponding to peptides that are fully conserved among ODCs from widely diverged species were used as opposing primers in the PCR with cDNA or genomic DNA as templates. Sequence analysis of the resulting DNA products confirmed their identity as ODC fragments. The genomic PCR product was then used as a probe for screening a Drosophila genomic library, resulting in the isolation of genomic clones representing two distinct ODC genes (dODC1 and dODC2). Sequence analysis of both genes demonstrated that although varying at their coding and noncoding regions, their overall structure is extremely similar containing 6 exons and 5 short introns. Southern blot and sequence analyses revealed that the two ODC genes are arranged in a tandem head-to-tail configuration. Both ODC genes were assigned by in situ hybridization analysis to position 44A on the right arm of the second chromosome. The isolation of cDNA clones corresponding to these two ODC genes demonstrated that both are transcribed in the adult fly. We hope that the isolation of genomic and cDNA clones of Drosophila ODC will permit the investigation of the expression of ODC during Drosophila development and the role of polyamines in this process.

Depressive symptoms are associated with food insufficiency and nutritional deficiencies in poor community-dwelling elderly people.

UNLABELLED: Depression is associated with nutritional deterioration in older persons and is highly prevalent among people of low socioeconomic status (LSES). OBJECTIVES: To determine the prevalence of depressive symptoms and food insufficiency, and to examine the relationship between dietary intake, food insufficiency and depression, in LSES community dwelling elderly. DESIGN: Cross-sectional study. SETTING: Lod, a town in the central Israel. PARTICIPANTS: Community-dwelling welfare recipients aged 60 to 92. MEASUREMENTS: Depression was assessed by 15-item Geriatric Depression Scale (GDS-short version), using a score ≥ 10 as the cut off point for clinically important depressive symptoms. Dietary intake was evaluated using a 24-hour dietary recall. Food insufficiency was defined by participants reporting that they did not have enough food to eat " sometimes " or " often " . RESULTS: This study reports on 112 persons aged 60 years and above (27.1% men). The prevalence of depression in this population was 47%; 25% of the study sample was classified as " food insufficient " . Macronutrients intake was similar for depressed and non-depressed persons, except for polyunsaturated fats which was lower among the depressed group (7.9 ± 4.9 vs.11.0 ± 7.5 g/day in the non-depressed, p=0.03). Vitamins and minerals intake was lower than recommended for both groups; vitamin E intake was associated with depression. In regression models controlling for confounding variables, an increase of 1 mg in vitamin E intake and 1 gram in polyunsaturated fatty acids (PUFA) intake was associated with lower risk for depression (OR=0.73, p=0.008 and OR=0.86, p=0.007 respectively) Participants who reported food insufficiency were 10 times more likely to be depressed compared with those who reported sufficient food. CONCLUSIONS: Given the evaluated adverse association between depressive symptoms and food insufficiency, more efforts are needed to guarantee adequate food intake, particularly foods rich in vitamin E and PUFA, in poor elderly people. Further studies are needed to clarify the temporal relationship between the emotional and nutritional domains in this vulnerable population.

Antizyme inhibitor: a defective ornithine decarboxylase or a physiological regulator of polyamine biosynthesis and cellular proliferation.

ODC (ornithine decarboxylase) is a central regulator of cellular polyamine synthesis. ODC is a highly regulated enzyme stimulated by a variety of growth-promoting stimuli. ODC overexpression leads to cellular transformation. Cellular ODC levels are determined at transcriptional and translational levels and by regulation of its degradation. Here we review the mechanism of ODC degradation with particular emphasis on AzI (antizyme inhibitor), an ODC homologous protein that appears as a central regulator of ODC stability, cellular polyamine homoeostasis and cellular proliferation.

Mechanism of polyamine tolerance in yeast: novel regulators and insights.

Polyamines are small charged molecules essential for various cellular functions, but at high levels they are cytotoxic. Two yeast kinases, SKY1 and PTK2, have been demonstrated to regulate polyamine tolerance. Here we report the identification and characterization of additional genes involved in regulating polyamine tolerance: YGL007W, FES1 and AGP2. Deletion of YGL007W, an open reading frame located within the promoter of the membrane proton pump PMA1, decreased Pma1p expression. Deletion of FES1 or AGP2 resulted in reduced polyamine uptake. While high-affinity spermine uptake was practically absent in agp2Delta cells, fes1Delta cells displayed only reduced affinity towards spermine. Despite the reduced uptake, the resistant strains accumulated significant levels of polyamines and displayed increased ornithine decarboxylase activity, suggesting reduced polyamine sensing. Interestingly, fes1Delta cells were highly sensitive to salt ions, suggesting different underlying mechanisms. These results indicate that mechanisms leading to polyamine tolerance are complex, and involve components other than uptake.

Rearrangement between ornithine decarboxylase and the switch region of the gamma 1 immunoglobulin gene in alpha-difluoromethylornithine resistant mouse myeloma cells.

We demonstrate here that the functional ornithine decarboxylase (ODC) gene of alpha-difluoromethylornithine (alpha-DFMO, a suicide inhibitor of ODC) resistant mouse myeloma 653-1 cells has been rearranged with the immunoglobulin heavy chain locus in a c-myc like manner. Structural analysis of a molecular clone representing this gene revealed that it is joined in a head to head configuration to the switch region of the gamma 1 immunoglobulin gene. Comparison of this rearranged ODC gene to a germline ODC gene isolated from mouse liver DNA revealed identity in the region downstream to the breakpoint which was mapped to position -1371 +/- 1 relative to the transcription initiation site (position +1). In the switch region of the gamma 1 immunoglobulin gene the breakpoint falls within a 49 bp repeat, in a sequence frequently involved in class switching. This finding further supports the notion that in B cells the immunoglobulin gene clusters are prone to random rearrangements which under selection for a tumorigenic phenotype involve oncogenes. However, as demonstrated here, employment of specific pharmacological selection can reveal rearrangements with non-oncogenic genes.

Isolation and characterization of the mouse ornithine decarboxylase gene.

Mouse ornithine decarboxylase (ODC) genomic clones were isolated from a bacteriophage lambda genomic library representing mouse myeloma 653-1 cells which over-produce ODC due to amplification of an active ODC gene. Sequence analysis of the amplified ODC gene revealed that ODC mRNA is encoded by 12 exons, 10 of which (exons 3 to 12) code for the ODC protein. Exon 12 also corresponds to the 3' noncoding region of the two species of ODC mRNA which are formed by alternative utilization of two polyadenylation signals separated from each other by 422 nucleotides. The transcription initiation site was mapped by S1 nuclease protection and by primer extension analysis. The 5' flanking region is extremely rich in G + C and contains typical promoter motifs such as the TATA box and SP1 transcription factor binding sites. Joining the 5' flanking region to the Escherichia coli chloramphenicol acetyltransferase structural gene and its introduction into mouse cells resulted in the expression of a high level of chloramphenicol acetyltransferase activity. Comparing the sequence of the ODC gene to our previously published sequence of ODC cDNA revealed a disagreement between the sequences located 5' to the AvaI site and demonstrated that this region of our previously reported cDNA represents a cloning artifact. The portion of the correct 5' noncoding region encoded by exon 1 is extremely rich in G + C and includes potential secondary structures which may be involved in translational regulation of ODC mRNA.

Isolation of cloned cDNA encoding mammalian ornithine decarboxylase.

During the growth of mammalian cells the level of ornithine decarboxylase ( OrnDCase ; L-ornithine carboxy-lyase, EC 4.1.1.17), the first enzyme in polyamine biosynthesis, undergoes rapid changes. As an initial step in the study of possible genetic mechanisms involved in these changes, we have isolated cDNA clones encoding OrnDCase . To obtain RNA enriched for OrnDCase messenger, mouse myeloma cells that overproduce OrnDCase were selected in the presence of the OrnDCase inhibitor, difluoromethylornithine. A pBR322 cDNA library was prepared from poly(A)+ RNA isolated from difluoromethylornithine-resistant cells, and the library was probed with [32P]cDNA representing mRNA sequences from resistant or parental (sensitive) cells. All clones hybridizing preferentially to the resistant cell probe shared nucleotide sequences. A representative clone containing 1.1 kilobases of cDNA was shown to encode OrnDCase sequences by in vitro translation of hybrid-selected mRNA followed by precipitation of the translation products with anti- OrnDCase antiserum. Using this cDNA clone as a probe, we found that mouse DNA yielded several restriction fragments that react with the OrnDCase cDNA. In the difluoromethylornithine-resistant myeloma cells, one of these DNA segments is amplified and the level of OrnDCase mRNA is greatly increased compared with that in parental plasmacytoma cells. The level of OrnDCase mRNA is also increased in cultured 3T3 cells stimulated with serum and in mouse kidneys after administration of androgen, indicating that OrnDCase gene transcription and/or mRNA stability are regulated during cell growth.

Polyamines regulate the expression of ornithine decarboxylase antizyme in vitro by inducing ribosomal frame-shifting.

We provide here an example of a mammalian cellular gene expressed by frame-shifting. Conventional reading of the sequence of ornithine decarboxylase-antizyme mRNA (a protein that modulates the rate of ornithine decarboxylase degradation) results in premature termination at an in-frame termination codon (stop-1), located shortly after the initiation codon. By translating, in vitro in reticulocyte lysate, antizyme mRNA with a full coding capacity and various mutants derived from it, we demonstrate that antizyme expression requires that ribosomes shift from the first open reading frame (termed ORF0) to a second +1 open reading frame (ORF1). Our studies show that this frame-shifting, which occurs at maximal efficiency of approximately 20%, is stimulated by polyamines and requires the functional integrity of the stop codon (stop-1) of ORF0. By introducing in-frame deletions, we have shown that an 87-nt segment surrounding stop-1 enhances frame-shifting efficiency, whereas the 6 nt located just upstream to stop-1 are absolutely essential for this process. Because this segment does not contain sequences that were previously characterized as shifty segments, our results suggest that another mechanism of frame-shifting is involved in mediating antizyme expression.

Nucleotide sequence of murine ornithine decarboxylase mRNA.

Ornithine decarboxylase (OrnDCase; L-ornithine carboxy-lyase, EC 4.1.1.17) is the first and rate-limiting enzyme in the biosynthesis of polyamines in mammalian cells. During cell growth the enzyme is regulated by rapid changes in the level of its mRNA and protein. To explore the molecular basis of these changes, we cloned a full-length cDNA copy of the major 2.4-kilobase OrnDCase mRNA from mouse cells and determined its sequence. The cDNA contains 2465 nucleotides derived from OrnDCase mRNA, consisting of a 737-nucleotide-long 5' noncoding segment, a coding segment of 1383 nucleotides terminated by a TAG triplet, and a 342-nucleotide 3' noncoding segment. The encoded protein of 461-amino acid residues has a molecular weight of 51,105 and has a potential site for phosphorylation by casein kinase II. In the unusually long 5' leader sequence, there are four ATG triplets, each of which is followed by an in-phase termination signal; the presence of these upstream ATGs could explain the low in vitro translational activity of OrnDCase mRNA reported earlier. A restriction digest of mouse genomic DNA was probed with a defined OrnDCase coding sequence, revealing a multimembered family of OrnDCase-related genes.

Translational regulation of mammalian ornithine decarboxylase by polyamines.

Ornithine decarboxylase, which catalyses the formation of putrescine, is the first and rate-limiting enzyme in the biosynthesis of polyamines in mammalian cells. The enzyme is highly regulated, as indicated by rapid changes in its mRNA and protein during cell growth. Here we report that ornithine decarboxylase is regulated at the translational level by polyamines in difluoromethylornithine-resistant mouse myeloma cells that overproduce the enzyme due to amplification of an ornithine decarboxylase gene. When such cells are exposed to putrescine or other polyamines, there is a rapid and specific decrease in the rate of synthesis of ornithine decarboxylase, assayed by pulse-labeling. Neither the cellular content of ornithine decarboxylase mRNA nor the half-life of ornithine decarboxylase protein is affected. Our results indicate that polyamines negatively regulate the translation of ornithine decarboxylase mRNA, thereby controlling their own synthesis.