Stable siRNA-mediated silencing of antizyme inhibitor: regulation of ornithine decarboxylase activity

Role of Antizyme Inhibitor Proteins in Cancers and Beyond

Polyamines are multivalent organic cations essential for many cellular functions, including cell growth, differentiation, and proliferation. However, elevated polyamine levels are associated with a slew of pathological conditions, including multiple cancers. Intracellular polyamine levels are primarily controlled by the autoregulatory circuit comprising two different protein types, Antizymes (OAZ) and Antizyme Inhibitors (AZIN), which regulate the activity of the polyamine biosynthetic enzyme ornithine decarboxylase (ODC). While OAZ functions to decrease the intracellular polyamine levels by inhibiting ODC activity and exerting a negative control of polyamine uptake, AZIN operates to increase intracellular polyamine levels by binding and sequestering OAZ to relieve ODC inhibition and to increase polyamine uptake. Interestingly, OAZ and AZIN exhibit autoregulatory functions on polyamine independent pathways as well. A growing body of evidence demonstrates the dysregulation of AZIN expression in multiple cancers. Additionally, RNA editing of the Azin1 transcript results in a "gain-of-function" phenotype, which is shown to drive aggressive tumor types. This review will discuss the recent advances in AZIN's role in cancers via aberrant polyamine upregulation and its polyamine-independent protein regulation. This report will also highlight AZIN interaction with proteins outside the polyamine biosynthetic pathway and its potential implication to cancer pathogenesis. Finally, this review will reveal the protein interaction network of AZIN isoforms by analyzing three different interactome databases.

The antizyme family for regulating polyamines

The polyamines spermidine, spermine, and their precursor putrescine are organic polycations involved in various cellular processes and are absolutely essential for cellular proliferation. Because of their crucial function in the cell, their intracellular concentration must be maintained at optimal levels. To a large extent, this regulation is achieved through the activity of an autoregulatory loop that involves two proteins, antizyme (Az) and antizyme inhibitor (AzI), that regulate the first enzyme in polyamine biosynthesis, ornithine decarboxylase (ODC), and polyamine uptake activity in response to intracellular polyamine levels. In this Minireview, I will discuss what has been learned about the mechanism of Az expression and its physical interaction with both ODC and AzI in the regulation of polyamines.

Antizyme Inhibitors in Polyamine Metabolism and Beyond: Physiopathological Implications

The intracellular levels of polyamines, cationic molecules involved in a myriad of cellular functions ranging from cellular growth, differentiation and apoptosis, is precisely regulated by antizymes and antizyme inhibitors via the modulation of the polyamine biosynthetic and transport systems. Antizymes, which are mainly activated upon high polyamine levels, inhibit ornithine decarboxylase (ODC), the key enzyme of the polyamine biosynthetic route, and exert a negative control of polyamine intake. Antizyme inhibitors (AZINs), which are proteins highly homologous to ODC, selectively interact with antizymes, preventing their action on ODC and the polyamine transport system. In this review, we will update the recent advances on the structural, cellular and physiological functions of AZINs, with particular emphasis on the action of these proteins in the regulation of polyamine metabolism. In addition, we will describe emerging evidence that suggests that AZINs may also have polyamine-independent effects on cells. Finally, we will discuss how the dysregulation of AZIN activity has been implicated in certain human pathologies such as cancer, fibrosis or neurodegenerative diseases.

RNA editing of AZIN1 induces the malignant progression of non-small-cell lung cancers

RNA editing is a widespread post-transcriptional mechanism that confers specific and reproducible nucleotide changes in selected RNA transcripts and plays a critical role in many human cancers. However, little is known about how RNA editing operates in non-small-cell lung cancers. Here, we measured the sequence and expression level of genes of antizyme inhibitor 1 and adenosine deaminase acting on RNA family in 30 non-small-cell lung cancer patient samples and 13 cell lines and revealed RNA editing S367G in antizyme inhibitor 1 is a high-frequent molecular events. We determined overexpression of antizyme inhibitor 1 with RNA editing, implying the oncogenic function of this alteration. We also detected the association of adenosine deaminase acting on RNA overexpression with RNA editing occurred in antizyme inhibitor 1. Furthermore, the RNA editing could cause a cytoplasmic-to-nuclear translocation of antizyme inhibitor 1 protein and conferred the malignant phenotype of non-small-cell lung cancer cells. The in vivo experiment confirmed that this RNA editing confers higher capacity of tumor migration as well. In conclusion, antizyme inhibitor 1 RNA editing and its involvement in tumorigenesis of non-small-cell lung cancer pave a new way for potential clinical management of non-small-cell lung cancer.

Antizyme inhibitor 1: a potential carcinogenic molecule

Polyamines are multivalent and organic cations essential for cellular growth, proliferation, differentiation, and apoptosis. Increased levels of polyamines are closely associated with numerous forms of cancer. An autoregulatory circuit composed of ornithine decarboxylase (ODC), antizyme (AZ) and antizyme inhibitor (AZI) govern the intracellular level of polyamines. Antizyme binds with ODC to inhibit ODC activity and to promote the ubiquitin‐independent degradation of ODC. Antizyme inhibitor binds to AZ with a higher affinity than ODC. Consequently, ODC is released from the ODC–AZ complex to rescue its activity. Antizyme inhibitor increases the ODC activity to accelerate the formation of intracellular polyamines, triggering gastric and breast carcinogenesis as well as hepatocellular carcinoma and esophageal squamous cell carcinoma development. Antizyme inhibitor 1 (AZIN1), a primary member of the AZI family, has aroused more attention because of its contribution to cancer. Even though its conformation is changed by adenosine‐to‐inosine (A→I) RNA editing, it plays an important role in tumorigenesis through regulating intracellular polyamines. Encouragingly, AZIN1 has been revealed to have an additional function outside the polyamine pathway so as to bypass the deficiency of targeting the polyamine biosynthetic pathway, promising to become a critical target for cancer therapy. Here, we review the latest research advances into AZIN1 and its potential contribution to carcinogenesis.

A role for antizyme inhibitor in cell proliferation

The polyamines are important for a variety of cellular functions, including cell growth. Their intracellular concentrations are controlled by a complex network of regulatory mechanisms, in which antizyme (Az) has a key role. Az reduces the cellular polyamine content by down-regulating both the enzyme catalysing polyamine biosynthesis, ornithine decarboxylase (ODC), and the uptake of polyamines. The activity of Az is repressed by the binding of a protein, named Az inhibitor (AzI), which is an enzymatically inactive homologue of ODC. Two forms of AzI have been described: AzI1, which is ubiquitous, and AzI2 which is expressed in brain and testis. In the present study, we have investigated the role of AzI1 in polyamine homeostasis and cell proliferation in breast cancer cells. The results obtained showed that the cellular content of AzI increased transiently after induction of cell proliferation by diluting cells in fresh medium. Inhibition of polyamine biosynthesis induced an even larger increase in the cellular AzI content, which remained significantly elevated during the 7-day experimental period. However, this increase was not a consequence of changes in cell cycle progression, as demonstrated by flow cytometry. Instead, the increase appeared to correlate with the cellular depletion of polyamines. Moreover, induced overexpression of AzI resulted in an increased cell proliferation with a concomitant increase in ODC activity and putrescine content. During mitosis, AzI1 was localised in a pattern that resembled that of the two centrosomes, confirming earlier observations. Taken together, the results indicate that AzI fulfils an essential regulatory function in polyamine homeostasis and cell proliferation.

Adenosine-to-inosine RNA editing mediated by ADARs in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC), the major histologic form of esophageal cancer, is a heterogeneous tumor displaying a complex variety of genetic and epigenetic changes. Aberrant RNA editing of adenosine-to-inosine (A-to-I), as it is catalyzed by adenosine deaminases acting on RNA (ADAR), represents a common posttranscriptional modification in certain human diseases. In this study, we investigated the status and role of ADARs and altered A-to-I RNA editing in ESCC tumorigenesis. Among the three ADAR enzymes expressed in human cells, only ADAR1 was overexpressed in primary ESCC tumors. ADAR1 overexpression was due to gene amplification. Patients with ESCC with tumoral overexpression of ADAR1 displayed a poor prognosis. In vitro and in vivo functional assays established that ADAR1 functions as an oncogene during ESCC progression. Differential expression of ADAR1 resulted in altered gene-specific editing activities, as reflected by hyperediting of FLNB and AZIN1 messages in primary ESCC. Notably, the edited form of AZIN1 conferred a gain-of-function phenotype associated with aggressive tumor behavior. Our findings reveal that altered gene-specific A-to-I editing events mediated by ADAR1 drive the development of ESCC, with potential implications in diagnosis, prognosis, and treatment of this disease.

Multiple forms of mouse antizyme inhibitor 1 mRNA differentially regulated by polyamines

Antizyme inhibitor 1 (Azin1), a positive regulator of cellular polyamines, is induced by various proliferative stimuli and repressed by polyamines. It has been reported that the translational repression of Azin1 by polyamines involves an upstream open reading frame on the mRNA, but little has been known about polyamine effect on its transcription or splicing. We found multiple forms of Azin1 transcripts formed by alternative splicing and initiation of transcription from putative alternative start sites. One of the novel splice variants, Azin1-X, has a premature termination codon on 5′ extension of exon 7, encodes a C-terminal truncated form of protein (Azin1ΔC), and is subject to nonsense-mediated mRNA decay. 2-Difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis, increased both transcription from the canonical transcription start site and the ratio of the full-length mRNA to Azin1-X mRNA, whereas polyamines show the opposite effect. Thus, polyamines regulate two novel steps of Azin1 expression, namely the transcription and a particular splicing pattern, both of which may affect the level of mRNA encoding the full-length active Azin1 protein.

Knockdown of antizyme inhibitor decreases prostate tumor growth in vivo

The endogenous protein antizyme inhibitor (AZI) is a potential oncogene which promotes cell growth by both inhibiting antizyme (AZ) activity and releasing ornithine decarboxylase (ODC) from AZ-mediated degradation. High levels of ODC and polyamines are associated with numerous types of neoplastic transformation, and the genomic region including AZI is frequently amplified in tumors of the ovary and prostate. To determine whether AZI functionally promotes prostate tumor growth, we made PC3M-LN4 (human) and AT6.1 (rat) cancer cell lines stably expressing shRNA to knockdown antizyme inhibitor 1 (AZI). AZI knockdown was confirmed by western blot, quantitative real-time PCR, and immunofluorescence. To examine the ability of these cells to form tumors in vivo, 1 × 10(6) cells were injected subcutaneously into nude mice either with (PC3M-LN4) or without (AT6.1) Matrigel. Tumor growth was measured two times per week by caliper. We found that cells in which AZI levels had been knocked down by shRNA formed significantly smaller tumors in vivo in both human and rat prostate cancer cell lines. These results suggest that not only does AZI promote tumor growth, but also that AZI may be a valid therapeutic target for cancer treatment.

Evidence of a role for antizyme and antizyme inhibitor as regulators of human cancer

Antizyme and its endogenous antizyme inhibitor have recently emerged as prominent regulators of cell growth, transformation, centrosome duplication, and tumorigenesis. Antizyme was originally isolated as a negative modulator of the enzyme ornithine decarboxylase (ODC), an essential component of the polyamine biosynthetic pathway. Antizyme binds ODC and facilitates proteasomal ODC degradation. Antizyme also facilitates degradation of a set of cell cycle regulatory proteins, including cyclin D1, Smad1, and Aurora A kinase, as well as Mps1, a protein that regulates centrosome duplication. Antizyme has been reported to function as a tumor suppressor and to negatively regulate tumor cell proliferation and transformation. Antizyme inhibitor binds to antizyme and suppresses its known functions, leading to increased polyamine synthesis, increased cell proliferation, and increased transformation and tumorigenesis. Gene array studies show antizyme inhibitor to be amplified in cancers of the ovary, breast, and prostate. In this review, we summarize the current literature on the role of antizyme and antizyme inhibitor in cancer, discuss how the ratio of antizyme to antizyme inhibitor can influence tumor growth, and suggest strategies to target this axis for tumor prevention and treatment.

Differential expression of ornithine decarboxylase antizyme inhibitors and antizymes in rodent tissues and human cell lines

Ornithine decarboxylase antizyme inhibitors, AZIN1 and AZIN2, are regulators and homologous proteins of ornithine decarboxylase (ODC), the rate limiting enzyme in the biosynthesis of polyamines. In this study, we have examined by means of real-time RT-PCR the relative abundance of mRNA of the three ODC paralogs in different rodent tissues, as well as in several cell lines derived from human tumors. With the exception of mouse and rat testes, ODC mRNA was the most expressed gene in all tissues examined (values higher than 60%). AZIN2 was more expressed than AZIN1 in testis, epididymis, brain, adrenal gland and lung, whereas the opposite was found in liver, kidney, heart, intestine and pancreas, as well as in all the cell lines examined. mRNA abundance of the three antizymes (AZ1, AZ2 and AZ3) that interact with ODC and antizyme inhibitors was also analyzed. AZ1 and AZ2 mRNA were ubiquitously expressed, AZ1 mRNA being more abundant than that of AZ2, although the ratio was dependent on the mouse tissue. In carcinoma-derived cells AZ1 was more expressed than AZ2, whereas in neuroblastoma-derived cells AZ2 mRNA was much more abundant than that of AZ1. AZ3 was expressed exclusively in rodent testes, where it was the most abundant of the three antizymes (~80%). This study is the first comparative-quantitative analysis on the expression of antizymes and antizyme inhibitors in different types of mammalian cells.

Critical factors governing the difference in antizyme-binding affinities between human ornithine decarboxylase and antizyme inhibitor

Both ornithine decarboxylase (ODC) and its regulatory protein, antizyme inhibitor (AZI), can bind with antizyme (AZ), but the latter has a higher AZ-binding affinity. The results of this study clearly identify the critical amino acid residues governing the difference in AZ-binding affinities between human ODC and AZI. Inhibition experiments using a series of ODC mutants suggested that residues 125 and 140 may be the key residues responsible for the differential AZ-binding affinities. The ODC_N125K/M140K double mutant demonstrated a significant inhibition by AZ, and the IC₅₀ value of this mutant was 0.08 µM, three-fold smaller than that of ODC_WT. Furthermore, the activity of the AZ-inhibited ODC_N125K/M140K enzyme was hardly rescued by AZI. The dissociation constant (K_d) of the [ODC_N125K/M140K]-AZ heterodimer was approximately 0.02 µM, which is smaller than that of WT_ODC by approximately 10-fold and is very close to the K_d value of AZI_WT, suggesting that ODC_N125K/M140K has an AZ-binding affinity higher than that of ODC_WT and similar to that of AZI. The efficiency of the AZI_K125N/K140M double mutant in the rescue of AZ-inhibited ODC enzyme activity was less than that of AZI_WT. The K_d value of [AZI_K125N/K140M]-AZ was 0.18 µM, nine-fold larger than that of AZI_WT and close to the K_d value of ODC_WT, suggesting that AZI_K125N/K140M has an AZ-binding affinity lower than that of AZI_WT and similar to that of ODC. These data support the hypothesis that the differences in residues 125 and 140 in ODC and AZI are responsible for the differential AZ-binding affinities.

Antizyme restrains centrosome amplification by regulating the accumulation of Mps1 at centrosomes

The failure to degrade Mps1 at centrosomes causes centrosome overproduction, but the factors that target Mps1 for degradation are unknown. This study shows that antizyme, a mediator of ubiquitin-independent degradation, binds to Mps1 and modulates centrosomal Mps1 via the proteasome, revealing a role for Mps1 in procentriole assembly.

Extra centrosomes are found in many tumors, and their appearance is an early event that can generate aberrant mitotic spindles and aneuploidy. Because the failure to appropriately degrade the Mps1 protein kinase correlates with centrosome overproduction in tumor-derived cells, defects in the factors that promote Mps1 degradation may contribute to extra centrosomes in tumors. However, while we have recently characterized an Mps1 degradation signal, the factors that regulate Mps1 centrosomal Mps1 are unknown. Antizyme (OAZ), a mediator of ubiquitin-independent degradation and a suspected tumor suppressor, was recently shown to localize to centrosomes and modulate centrosome overproduction, but the known OAZ substrates were not responsible for its effect on centrosomes. We have found that OAZ exerts its effect on centrosomes via Mps1. OAZ promotes the removal of Mps1 from centrosomes, and centrosome overproduction caused by reducing OAZ activity requires Mps1. OAZ binds to Mps1 via the Mps1 degradation signal and modulates the function of Mps1 in centrosome overproduction. Moreover, OAZ regulates the canonical centrosome duplication cycle, and reveals a function for Mps1 in procentriole assembly. Together, our data suggest that OAZ restrains the assembly of centrioles by controlling the levels of centrosomal Mps1 through the Cdk2-regulated Mps1 degradation signal.

Developmental alterations in expression and subcellular localization of antizyme and antizyme inhibitor and their functional importance in the murine mammary gland

Ornithine decarboxylase (ODC), antizyme (AZ), and antizyme inhibitor (AIn) play a key role in regulation of intracellular polyamine levels by forming a regulatory circuit through their interactions. To gain insight into their functional importance in cell growth and differentiation, we systematically examined the changes of their expression, cellular polyamine contents, expression of genes related to polyamine metabolism, and beta-casein gene expression during murine mammary gland development. The activity of ODC and AZ1 as well as putrescine level were low in the virgin and involuting stages, but they increased markedly during late pregnancy and early lactation when mammary cells proliferate extensively and begin to augment their differentiated function. The level of spermidine and expression of genes encoding spermidine synthase and AIn increased in a closely parallel manner with that of casein gene expression during pregnancy and lactation. On the other hand, the level of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA and AZ2 mRNA decreased during those periods. Immunohistochemical analysis showed the translocation of ODC and AIn between the nucleus and cytoplasm and the continuous presence of AZ in the nucleus during gland development. Reduction of AIn by RNA interference inhibited expression of beta-casein gene stimulated by lactogenic hormones in HC11 cells. In contrast, reduction of AZ by AZsiRNA resulted in the small increase of beta-casein gene expression. These results suggested that AIn plays an important role in the mammary gland development by changing its expression, subcellular localization, and functional interplay with AZ.

Expression of antizyme inhibitor 2 in mast cells and role of polyamines as selective regulators of serotonin secretion

Background

Upon IgE-mediated activation, mast cells (MC) exocytose their cytoplasmic secretory granules and release a variety of bioactive substances that trigger inflammatory responses. Polyamines mediate numerous cellular and physiological functions. We report here that MCs express antizyme inhibitor 2 (AZIN2), an activator of polyamine biosynthesis, previously reported to be exclusively expressed in the brain and testis. We have investigated the intracellular localization of AZIN2 both in resting and activated MCs. In addition, we have examined the functional role of polyamines, downstream effectors of AZIN2, as potential regulators of MC activity.

Methodology/Principal Findings

Immunostainings show that AZIN2 is expressed in primary and neoplastic human and rodent MCs. We demonstrate that AZIN2 localizes in the Vamp-8 positive, serotonin-containing subset of MC granules, but not in tryptase-containing granules, as revealed by double immunofluorescence stainings. Furthermore, activation of MCs induces rapid upregulation of AZIN2 expression and its redistribution, suggesting a role for AZIN2 in secretory granule exocytosis. We also demonstrate that release of serotonin from activated MCs is polyamine-dependent whereas release of histamine and β-hexosaminidase is not, indicating a granule subtype-specific function for polyamines.

Conclusions/Significance

The study reports for the first time the expression of AZIN2 outside the brain and testis, and demonstrates the intracellular localization of endogenous AZIN2 in MCs. The granule subtype-specific expression and its induction after MC activation suggest a role for AZIN2 as a local, in situ regulator of polyamine biosynthesis in association with serotonin-containing granules of MCs. Furthermore, our data indicates a novel function for polyamines as selective regulators of serotonin release from MCs.

Antizyme and antizyme inhibitor, a regulatory tango

The polyamines are small basic molecules essential for cellular proliferation and viability. An autoregulatory circuit that responds to the intracellular level of polyamines regulates their production. In the center of this circuit is a family of small proteins termed antizymes. Antizymes are themselves regulated at the translational level by the level of polyamines. Antizymes bind ornithine decarboxylase (ODC) subunits and target them to ubiquitin-independent degradation by the 26S proteasome. In addition, antizymes inhibit polyamine transport across the plasma membrane via an as yet unresolved mechanism. Antizymes may also interact with and target degradation of other growth-regulating proteins. An inactive ODC-related protein termed antizyme inhibitor regulates polyamine metabolism by negating antizyme functions. The ability of antizymes to degrade ODC, inhibit polyamine uptake and consequently suppress cellular proliferation suggests that they act as tumor suppressors, while the ability of antizyme inhibitors to negate antizyme function indicates their growth-promoting and oncogenic potential.

Critical factors determining dimerization of human antizyme inhibitor

Ornithine decarboxylase (ODC) is the first enzyme involved in polyamine biosynthesis, and it catalyzes the decarboxylation of ornithine to putrescine. ODC is a dimeric enzyme, whereas antizyme inhibitor (AZI), a positive regulator of ODC that is homologous to ODC, exists predominantly as a monomer and lacks decarboxylase activity. The goal of this paper was to identify the essential amino acid residues that determine the dimerization of AZI. The nonconserved amino acid residues in the putative dimer interface of AZI (Ser-277, Ser-331, Glu-332, and Asp-389) were substituted with the corresponding residues in the putative dimer interface of ODC (Arg-277, Tyr-331, Asp-332, and Tyr-389, respectively). Analytical ultracentrifugation analysis was used to determine the size distribution of these AZI mutants. The size-distribution analysis data suggest that residue 331 may play a major role in the dimerization of AZI. Mutating Ser-331 to Tyr in AZI (AZI-S331Y) caused a shift from a monomer configuration to a dimer. Furthermore, in comparison with the single mutant AZI-S331Y, the AZI-S331Y/D389Y double mutant displayed a further reduction in the monomer-dimer K(d), suggesting that residue 389 is also crucial for AZI dimerization. Analysis of the triple mutant AZI-S331Y/D389Y/S277R showed that it formed a stable dimer (K(d) value = 1.3 microm). Finally, a quadruple mutant, S331Y/D389Y/S277R/E332D, behaved as a dimer with a K(d) value of approximately 0.1 microm, which is very close to that of the human ODC enzyme. The quadruple mutant, although forming a dimer, could still be disrupted by antizyme (AZ), further forming a heterodimer, and it could rescue the AZ-inhibited ODC activity, suggesting that the AZ-binding ability of the AZI dimer was retained.

The change of antizyme inhibitor expression and its possible role during mammalian cell cycle

Antizyme inhibitor (AIn), a homolog of ODC, binds to antizyme and inactivates it. We report here that AIn increased at the G1 phase of the cell cycle, preceding the peak of ODC activity in HTC cells in culture. During interphase AIn was present mainly in the cytoplasm and turned over rapidly with the half-life of 10 to 20 min, while antizyme was localized in the nucleus. The level of AIn increased again at the G2/M phase along with ODC, and the rate of turn-over of AIn in mitotic cells decreased with the half-life of approximately 40 min. AIn was colocalized with antizyme at centrosomes during the period from prophase through late anaphase and at the midzone/midbody during telophase. Thereafter, AIn and antizyme were separated and present at different regions on the midbody at late telophase. AIn disappeared at late cytokinesis, whereas antizyme remained at the cytokinesis remnant. Reduction of AIn by RNA interference caused the increase in the number of binucleated cells in HTC cells in culture. These findings suggested that AIn contributed to a rapid increase in ODC at the G1 phase and also played a role in facilitating cells to complete mitosis during the cell cycle.

Antizyme inhibitor 2 (AZIN2/ODCp) stimulates polyamine uptake in mammalian cells

One of the processes that regulate intracellular levels of polyamines in mammalian cells is polyamine uptake. We have measured polyamine uptake in COS7 cells for putrescine, spermidine, and spermine, obtaining K(m) values of 4.5, 1.0, and 0.8 mum, respectively. Treatment of nonconfluent cells with cycloheximide stimulated polyamine uptake and prevented the inhibitory effect found in cells preloaded with polyamines, suggesting the existence of a feedback repression mechanism mediated by antizymes. Transient transfected cells with mutated antizyme forms of AZ1, AZ2, and AZ3, which do not require frameshifting, showed a total blockade of polyamine uptake. Transfection of COS7 cells with mouse or human AZIN2, a novel member of the antizyme inhibitor family, recently characterized by our group, markedly stimulated polyamine uptake and counteracted the action of any of the three antizymes in co-transfected cells. The stimulatory effect of AZIN2 on polyamine uptake was abrogated when the putative antizyme binding sequence, formed by residues 117-140 in AZIN2, was deleted. Real time reverse transcription-PCR analysis of antizyme inhibitor transcripts revealed that in brain and testes AZIN2 is more expressed than AZIN1, especially in the testes where the relative expression was about 25-fold higher. Collectively, our results clearly indicate that AZIN2 affects polyamine homeostasis not only by increasing ornithine decarboxylase activity but also by stimulating polyamine uptake, through negating the inhibitory effect of the antizymes. This finding may have physiological relevance, mostly in testes where AZ3 and AZIN2 are mainly expressed.

Prediction of relapse in paediatric pre-B acute lymphoblastic leukaemia using a three-gene risk index

Despite high cure rates 25% of children with acute lymphoblastic leukaemia (ALL) relapse and have dismal outcome. Crucially, many are currently stratified as standard risk (SR) and additional markers to improve patient stratification are required. Here we have used diagnostic bone marrow specimens from 101 children with pre-B ALL to examine the use of gene expression profiles (GEP) as predictors of long-term clinical outcome. Patients were divided into two cohorts for model development and validation based on availability of specimen material. Initially, GEP from 55 patients with sufficient material were analysed using HG-U133A microarrays, identifying an 18-gene classifier (GC) that was more predictive of outcome than conventional prognostic parameters. After feature selection and validation of expression levels by quantitative reverse transcription polymerase chain reaction (qRT-PCR), a three-gene qRT-PCR risk index [glutamine synthetase (GLUL), ornithine decarboxylase antizyme inhibitor (AZIN), immunoglobulin J chain (IGJ)] was developed that predicted outcome with an accuracy of 89% in the array cohort and 87% in the independent validation cohort. The data demonstrate the feasibility of using GEP to improve risk stratification in childhood ALL. This is particularly important for the identification of patients destined to relapse despite their current stratification as SR, as more intensive front-line treatment options for these individuals are already available.

ODCp, a brain- and testis-specific ornithine decarboxylase paralogue, functions as an antizyme inhibitor, although less efficiently than AzI1

ODC (ornithine decarboxylase), the first enzyme in the polyamine biosynthesis pathway in mammalian cells, is a labile protein. ODC degradation is stimulated by Az (antizyme), a polyamine-induced protein, which in turn is regulated by an ODC-related protein termed AzI (Az inhibitor). Recently, another ODCp (ODC paralogue) was suggested to function as AzI, on the basis of its ability to increase ODC activity and inhibit Az-stimulated ODC degradation in vitro. We show in the present study that ODCp is indeed capable of negating Az functions, as reflected by its ability to increase ODC activity and polyamine uptake and by its ability to provide growth advantage in stably transfected cells. However, ODCp is less potent than AzI1 in stimulating ODC activity, polyamine uptake and growth rate. The superiority of AzI1 to ODCp in inhibiting the Az-stimulated ODC degradation is also demonstrated using an in vitro degradation assay. We show that the basis for the inferiority of ODCp as an AzI is its lower affinity towards Az (Az1 and Az3). Further, we show here that ODCp, like AzI, is degraded in a ubiquitin-dependent manner, in a reaction that does not require either interaction with Az or the integrity of its C-terminus. Interaction with Az actually stabilizes ODCp by interfering with its ubiquitination. This results in sequestration of Az into a stable complex with ODCp, which is the central feature contributing to the ability of ODCp to function as AzI.

Crystallographic and biochemical studies revealing the structural basis for antizyme inhibitor function

Antizyme inhibitor (AzI) regulates cellular polyamine homeostasis by binding to the polyamine-induced protein, Antizyme (Az), with greater affinity than ornithine decarboxylase (ODC). AzI is highly homologous to ODC but is not enzymatically active. In order to understand these specific characteristics of AzI and its differences from ODC, we determined the 3D structure of mouse AzI to 2.05 A resolution. Both AzI and ODC crystallize as a dimer. However, fewer interactions at the dimer interface, a smaller buried surface area, and lack of symmetry of the interactions between residues from the two monomers in the AzI structure suggest that this dimeric structure is nonphysiological. In addition, the absence of residues and interactions required for pyridoxal 5'-phosphate (PLP) binding suggests that AzI does not bind PLP. Biochemical studies confirmed the lack of PLP binding and revealed that AzI exists as a monomer in solution while ODC is dimeric. Our findings that AzI exists as a monomer and is unable to bind PLP provide two independent explanations for its lack of enzymatic activity and suggest the basis for its enhanced affinity toward Az.

Polyamines and neoplastic growth

Studies over many years have suggested that increased polyamine synthesis may be necessary for neoplastic growth. This review summarizes recent work on the regulation of putrescine production both de novo and via the degradation of higher polyamines and provides a summary of studies using transgenic mice in which the levels of proteins that regulate these processes (L-ornithine decarboxylase, antizyme and spermidine/spermine-N(1)-acetyltransferase) are altered.

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.

Antizyme, a mediator of ubiquitin-independent proteasomal degradation and its inhibitor localize to centrosomes and modulate centriole amplification

The potential tumor suppressor antizyme and its endogenous inhibitor (antizyme inhibitor, AZI) have been implicated in the ubiquitin-independent proteasomal degradation of proteins involved in cell proliferation as well as in the regulation of polyamine levels. We show here that both antizyme and AZI concentrate at centrosomes and that antizyme preferentially associates with the maternal centriole. Interestingly, alterations in the levels of these proteins have opposing effects on centrosomes. Depletion of antizyme in various cell lines and primary cells leads to centrosome overduplication, whereas overexpression of antizyme reduces numerical centrosome abnormalities. Conversely, silencing of the antizyme inhibitor, AZI, results in a decrease of numerical centrosome abnormalities, whereas overexpression of AZI leads to centrosome overduplication. We further show that the numerical centrosome abnormalities are due to daughter centriole amplification. In summary, our results demonstrate that alterations in the antizyme/AZI balance cause numerical centrosomal defects and suggest a role for ubiquitin-independent proteasomal degradation in centrosome duplication.

Polyamines: metabolism to systems biology and beyond

Polyamines and the metabolic and physiopathological processes in which they are involved represent an active field of research that has been continuously growing since the seventies. In the last years, the trends in the focused areas of interest within this field since the 1970s have been confirmed. The impact of "-omics" in polyamine research remains too low in comparison with its deep impact on other biological research areas. These high-throughput approaches, along with systems biology and, in general, more systemic and holistic approaches should contribute to a renewal of this research area in the near future.

Ornithine decarboxylase activity in tumor cell lines correlates with sensitivity to cell death induced by histone deacetylase inhibitors

Inhibitors of histone deacetylases (HDAC) show significant promise as targeted anticancer agents against a variety of hematologic and solid tumors. HDAC inhibitors arrest the growth of primary cells, but they induce apoptosis or differentiation of tumor cells. Although the precise mechanism is unknown, differences in cell cycle checkpoints and chromatin structure may be responsible. Cellular polyamines regulate both cell cycle progression and chromatin structure. In tumors, polyamines are abundantly produced because of increased activity of the rate-limiting enzyme in polyamine synthesis, ornithine decarboxylase (ODC). To determine if polyamines contribute to the cellular response to HDAC inhibitors, we inhibited ODC activity with alpha-difluoromethylornithine. Polyamine depletion increased resistance to apoptosis induced by HDAC inhibitors. In addition, we found that ODC activity levels correlated with sensitivity to HDAC inhibitors in a panel of tumor cell lines. We conclude that polyamines participate in the cellular response to HDAC inhibitors and that ODC activity correlates with sensitivity to HDAC inhibitor-induced apoptosis. Thus, elevated polyamine levels might be a biomarker for tumor sensitivity to HDAC inhibitor-induced apoptosis. These findings warrant clinical evaluation of tumor samples to determine if high ODC activity levels predict sensitivity to HDAC inhibitors.

Molecular basis for the antiproliferative effect of agmatine in tumor cells of colonic, hepatic, and neuronal origin

The aim of the present study was to challenge potential mechanisms of action underlying the inhibition of tumor cell proliferation by agmatine. Agmatine inhibited proliferation of the human hepatoma cells HepG2, the human adenocarcinoma cells HT29, the rat hepatoma cells McRH7777, and the rat pheochromocytoma cells PC-12. Inhibition of proliferation of HepG2 cells was associated with an abolition of expression of ornithine decarboxylase (ODC) protein and a doubling of mRNA content encoding ODC. In HepG2 cells, silencing of ODC-antizyme-1, but not of antizyme inhibitor, by RNA interference resulted in an increase of agmatine's antiproliferative effect. Thus, the distinct decrease in intracellular polyamine content by agmatine was due to a reduced translation of the synthesizing protein ODC but was not essentially mediated by induction of ODC-antizyme or blockade of antizyme inhibitor. In interaction experiments 1 mM L-arginine, 1 mM D-arginine, 1 mM citrulline, 100 microM N(omega)-nitro-L-arginine methyl ester, 1 and 10 microM sodium nitroprusside, and 1 microM N1-guanyl-1,7-diaminoheptane failed to alter agmatine's antiproliferative effect. Hence, the antiproliferative effect of agmatine in HT29 and HepG2 cells is due to an interaction with neither the NO synthases, the hypusination of eIF5A, nor an agmatine-induced reduction in availability of intracellular L-arginine. L-Arginine and citrulline, but not d-arginine, inhibited tumor cell proliferation by themselves. Their inhibitory effect was abolished after silencing of arginine decarboxylase (ADC) expression by RNA interference indicating the conversion to agmatine by ADC. Finally, in the four cell lines under study, agmatine-induced inhibition of cell proliferation was paralleled by an increase in intracellular caspase-3 activity, indicating a promotion of apoptosis.

Regulation of cell proliferation by the antizyme inhibitor: evidence for an antizyme-independent mechanism

The antizyme inhibitor was discovered as a protein that binds to the regulatory protein antizyme and inhibits the ability of antizyme to interact with the enzyme ornithine decarboxylase (ODC). Blocking antizyme activity subsequently leads to increased intracellular levels of ODC and increased ODC enzymatic activity. We now report that antizyme inhibitor is a positive modulator of cell growth. Overexpression of antizyme inhibitor in NIH-3T3 mouse fibroblasts or in AT2.1 Dunning rat prostate carcinoma cells resulted in an increased rate of cell proliferation and an increase in saturation density of the cultured cells. This was accompanied by an increase in intracellular levels of the polyamine putrescine. In AT2.1 cells, antizyme inhibitor overexpression also increased the ability of the cells to form foci when grown under anchorage-independent conditions. In order to determine the role of antizyme on antizyme inhibitor activity we created an antizyme inhibitor mutant, AZI(Delta117-140), which lacks the putative antizyme-binding domain. We show that this mutant fails to bind to antizyme, but remains capable of inducing increased rates of cell proliferation, suggesting that antizyme inhibitor has antizyme-independent functions. Silencing antizyme inhibitor expression leads to diminished levels of cyclin D1 and to reduced cell proliferation. Antizyme inhibitor is capable of preventing cyclin D1 degradation, and this effect is at least partially independent of antizyme. We show that wild-type antizyme inhibitor and the AZI(DeltaY) mutant are capable of direct interaction with cyclin D1 suggesting a potential mechanism for the antizyme-independent effects. Together, our data suggest a novel function for antizyme inhibitor in cellular growth control.

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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.

Regulation of ornithine decarboxylase

Ornithine decarboxylase (ODC) initiates the polyamine biosynthetic pathway. The amount of ODC is altered in response to many growth factors, oncogenes, and tumor promoters and to changes in polyamine levels. Susceptibility to tumor development is increased in transgenic mice expressing high levels of ODC and is decreased in mice with reduced ODC due to loss of one ODC allele or elevated expression of antizyme, a protein that stimulates ODC degradation. This review describes key factors that contribute to the regulation of ODC levels, which can occur at the levels of transcription, translation, and protein turnover.