Regulation of all members of the antizyme family by antizyme inhibitor

Preventing Cancer by Inhibiting Ornithine Decarboxylase: A Comparative Perspective on Synthetic vs. Natural Drugs

This perspective delves into the investigation of synthetic and naturally occurring inhibitors, their patterns of inhibition, and the effectiveness of newly utilized natural compounds as inhibitors targeting the Ornithine decarboxylase enzyme. This enzyme is known to target the MYC oncogene, thereby establishing a connection between polyamine metabolism and oncogenesis in both normal and cancerous cells. ODC activation and heightened polyamine activity are associated with tumor development in numerous cancers and fluctuations in ODC protein levels exert a profound influence on cellular activity for inhibition or suppressing tumor cells. This perspective outlines efforts to develop novel drugs, evaluate natural compounds, and identify promising inhibitors to address gaps in cancer prevention, highlighting the potential of newly designed synthetic moieties and natural flavonoids as alternatives. It also discusses natural compounds with potential as enhanced inhibitors.

EPLIN-β is a novel substrate of ornithine decarboxylase antizyme 1 and mediates cellular migration

Polyamines promote cellular proliferation. Their levels are controlled by ornithine decarboxylase antizyme 1 (Az1, encoded by OAZ1), through the proteasome-mediated, ubiquitin-independent degradation of ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine biosynthesis. Az1-mediated degradation of other substrates such as cyclin D1 (CCND1), DNp73 (TP73) or Mps1 regulates cell growth and centrosome amplification, and the currently known six Az1 substrates are all linked with tumorigenesis. To understand whether Az1-mediated protein degradation might play a role in regulating other cellular processes associated with tumorigenesis, we employed quantitative proteomics to identify novel Az1 substrates. Here, we describe the identification of LIM domain and actin-binding protein 1 (LIMA1), also known as epithelial protein lost in neoplasm (EPLIN), as a new Az1 target. Interestingly, between the two EPLIN isoforms (α and β), only EPLIN-β is a substrate of Az1. The interaction between EPLIN-β and Az1 appears to be indirect, and EPLIN-β is degraded by Az1 in a ubiquitination-independent manner. Az1 absence leads to elevated EPLIN-β levels, causing enhanced cellular migration. Consistently, higher LIMA1 levels correlate with poorer overall survival of colorectal cancer patients. Overall, this study identifies EPLIN-β as a novel Az1 substrate regulating cellular migration.

A novel mechanism for A-to-I RNA-edited AZIN1 in promoting tumor angiogenesis in colorectal cancer

Adenosine (A) to inosine (I) RNA editing catalyzed by adenosine deaminases acting on RNA (ADAR) enzymes is a post-transcriptional modification that emerged as a key player in tumorigenesis and cancer progression. Antizyme inhibitor 1 (AZIN1) is one of the most frequent A-to-I RNA alterations in many human cancers. RNA-edited AZIN1 is known to confer a gain-of-function phenotype associated with aggressive tumors. However, the functional impact of RNA-edited AZIN1 in cancer angiogenesis remains unexplored. We showed here that RNA-edited AZIN1 promoted tumor angiogenesis through the upregulation of IL-8 via in vitro and in vivo experiments. And we subsequently demonstrated that delaying c-Myc degradation by OAZ2-mediated ubiquitin-independent proteasome pathway contributed to increase mRNA level and the secretion of angiogenic factor IL-8. Our study suggests an important contribution of RNA-edited AZIN1 to the tumor vascular microenvironment and highlights its translational potential. Thus, we revealed a potential approach to explore small-molecule antagonists such as reparixin attenuating IL-8 signaling for treatment of human cancer patients detected with hyper-editing.

Genome-wide RNAi screen identifies novel players in human 60S subunit biogenesis including key enzymes of polyamine metabolism

Ribosome assembly is an essential process that is linked to human congenital diseases and tumorigenesis. While great progress has been made in deciphering mechanisms governing ribosome biogenesis in eukaryotes, an inventory of factors that support ribosome synthesis in human cells is still missing, in particular regarding the maturation of the large 60S subunit. Here, we performed a genome-wide RNAi screen using an imaging-based, single cell assay to unravel the cellular machinery promoting 60S subunit assembly in human cells. Our screen identified a group of 310 high confidence factors. These highlight the conservation of the process across eukaryotes and reveal the intricate connectivity of 60S subunit maturation with other key cellular processes, including splicing, translation, protein degradation, chromatin organization and transcription. Intriguingly, we also identified a cluster of hits comprising metabolic enzymes of the polyamine synthesis pathway. We demonstrate that polyamines, which have long been used as buffer additives to support ribosome assembly in vitro, are required for 60S maturation in living cells. Perturbation of polyamine metabolism results in early defects in 60S but not 40S subunit maturation. Collectively, our data reveal a novel function for polyamines in living cells and provide a rich source for future studies on ribosome synthesis.

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.

Polyamines in mammalian pathophysiology

Polyamines (PAs) are essential organic polycations for cell viability along the whole phylogenetic scale. In mammals, they are involved in the most important physiological processes: cell proliferation and viability, nutrition, fertility, as well as nervous and immune systems. Consequently, altered polyamine metabolism is involved in a series of pathologies. Due to their pathophysiological importance, PA metabolism has evolved to be a very robust metabolic module, interconnected with the other essential metabolic modules for gene expression and cell proliferation/differentiation. Two different PA sources exist for animals: PA coming from diet and endogenous synthesis. In the first section of this work, the molecular characteristics of PAs are presented as determinant of their roles in living organisms. In a second section, the metabolic specificities of mammalian PA metabolism are reviewed, as well as some obscure aspects on it. This second section includes information on mammalian cell/tissue-dependent PA-related gene expression and information on crosstalk with the other mammalian metabolic modules. The third section presents a synthesis of the physiological processes described as modulated by PAs in humans and/or experimental animal models, the molecular bases of these regulatory mechanisms known so far, as well as the most important gaps of information, which explain why knowledge around the specific roles of PAs in human physiology is still considered a "mysterious" subject. In spite of its robustness, PA metabolism can be altered under different exogenous and/or endogenous circumstances so leading to the loss of homeostasis and, therefore, to the promotion of a pathology. The available information will be summarized in the fourth section of this review. The different sections of this review also point out the lesser-known aspects of the topic. Finally, future prospects to advance on these still obscure gaps of knowledge on the roles on PAs on human physiopathology are discussed.

Purification and Characterization of Ornithine Decarboxylase from Aspergillus terreus; Kinetics of Inhibition by Various Inhibitors

l-Ornithine decarboxylase (ODC) is the rate-limiting enzyme of de novo polyamine synthesis in humans and fungi. Elevated levels of polyamine by over-induction of ODC activity in response to tumor-promoting factors has been frequently reported. Since ODC from fungi and human have the same molecular properties and regulatory mechanisms, thus, fungal ODC has been used as model enzyme in the preliminary studies. Thus, the aim of this work was to purify ODC from fungi, and assess its kinetics of inhibition towards various compounds. Forty fungal isolates were screened for ODC production, twenty fungal isolates have the higher potency to grow on L-ornithine as sole nitrogen source. Aspergillus terreus was the most potent ODC producer (2.1 µmol/mg/min), followed by Penicillium crustosum and Fusarium fujikuori. These isolates were molecularly identified based on their ITS sequences, which have been deposited in the NCBI database under accession numbers MH156195, MH155304 and MH152411, respectively. ODC was purified and characterized from A. terreus using SDS-PAGE, showing a whole molecule mass of ~110 kDa and a 50 kDa subunit structure revealing its homodimeric identity. The enzyme had a maximum activity at 37 °C, pH 7.4-7.8 and thermal stability for 20 h at 37 °C, and 90 days storage stability at 4 °C. A. terreus ODC had a maximum affinity (K_m) for l-ornithine, l-lysine and l-arginine (0.95, 1.34 and 1.4 mM) and catalytic efficiency (k_cat/K_m) (4.6, 2.83, 2.46 × 10^-5 mM^-1·s^-1). The enzyme activity was strongly inhibited by DFMO (0.02 µg/mL), curcumin (IC₅₀ 0.04 µg/mL), propargylglycine (20.9 µg/mL) and hydroxylamine (32.9 µg/mL). These results emphasize the strong inhibitory effect of curcumin on ODC activity and subsequent polyamine synthesis. Further molecular dynamic studies to elucidate the mechanistics of ODC inhibition by curcumin are ongoing.

Developing a novel FRET assay, targeting the binding between Antizyme-AZIN

Antizyme inhibitor (AZIN) stimulates cell proliferation by binding to and sequestering the cell cycle suppressor antizyme. Despite the important role of the antizyme-AZIN protein-protein interaction (PPI) in cell cycle regulation, there are no assays for directly measuring the binding of AZIN to antizyme that are amenable to high throughput screening. To address this problem, we developed and validated a novel antizyme-AZIN intramolecular FRET sensor using clover and mRuby2 fluorescent proteins. By introducing alanine mutations in the AZIN protein, we used this sensor to probe the PPI for key residues governing the binding interaction. We found that like many PPIs, the energy of the antizyme-AZIN binding interaction is distributed across many amino acid residues; mutation of individual residues did not have a significant effect on disrupting the PPI. We also examined the interaction between Clover-AZIN and antizyme-mRuby2 in cells. Evidence of a direct interaction between Clover-AZIN and antizyme-mRuby2 was observed within cells, validating the use of this FRET sensor for probing intracellular antizyme-AZIN PPI. In conclusion, we have developed and optimized a FRET sensor which can be adapted for high throughput screening of either in vitro or intracellular activity.

New insights of polyamine metabolism in testicular physiology: A role of ornithine decarboxylase antizyme inhibitor 2 (AZIN2) in the modulation of testosterone levels and sperm motility

The specific role of polyamines in the testis physiology is not fully understood. Antizymes (OAZs) and antizyme inhibitors (AZINs) are modulators of ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis and polyamine uptake. Although the three known OAZs are expressed in the testis, only OAZ3 is testis specific and has been proven to have an essential role in male fertility. Regarding the two existing AZINs, AZIN2 is the most abundantly expressed member in this gonad. Whereas previous studies suggested that AZIN2 might participate in mouse spermatogenesis, immunohistological analysis of human testicular sections revealed that AZIN2 is also detected in the steroidogenic Leydig cells but not in the germinal epithelium. In the present study, we found a close ontogenic similarity in the mRNA levels of OAZs and AZINs between mice and rats, but an opposite expression pattern of ODC activity. Further analysis of AZIN2 and OAZ3 in the testis of mice with different alterations in spermatogenesis and fertility, induced either genetically or pharmacologically, corroborated that both AZIN2 and OAZ3 are mainly expressed in the haploid germinal cells. Finally, by using transgenic mice with a truncated Azin2 gene fused to the bacterial lacZ gene, we studied the expression of Azin2 in testes, epididymides and spermatozoa. AZIN2 was detected in spermatids and spermatozoa, as well as in Leydig cells, and in epithelial epidydimal cells. Azin2 knock-out male mice were fertile; however, they showed marked decreases in testicular putrescine and plasma and testicular testosterone levels, and a dramatic reduction in the sperm motility. These results suggest an important role for AZIN2 in testicular cells by modulating polyamine concentrations, testosterone synthesis and sperm function. Overall, our data corroborate the relevance of polyamine regulation in testis functions, where both AZIN2 and OAZ3 play fundamental roles.

A Modified Tripeptide Motif of RS1 (RSC1A1) Down-Regulates Exocytotic Pathways of Human Na+-d-glucose Cotransporters SGLT1, SGLT2, and Glucose Sensor SGLT3 in the Presence of Glucose

A domain of protein RS1 (RSC1A1) called RS1-Reg down-regulates the plasma membrane abundance of Na⁺-d-glucose cotransporter SGLT1 by blocking the exocytotic pathway at the trans-Golgi. This effect is blunted by intracellular glucose but prevails when serine in a QSP (Gln-Ser-Pro) motif is replaced by glutamate [RS1-Reg(S20E)]. RS1-Reg binds to ornithine decarboxylase (ODC) and inhibits ODC in a glucose-dependent manner. Because the ODC inhibitor difluoromethylornithine (DFMO) acts like RS1-Reg(S20E), and DFMO and RS1-Reg(S20E) are not cumulative, we raised the hypothesis that RS1-Reg(S20E) down-regulates the exocytotic pathway of SGLT1 at the trans-Golgi by inhibiting ODC. We investigated whether QEP down-regulates human SGLT1 (hSGLT1) like hRS1-Reg(S20E) and whether human Na⁺-d-glucose cotransporter hSGLT2 and the human glucose sensor hSGLT3 are also addressed. We expressed hSGLT1, hSGLT1 linked to yellow fluorescent protein (hSGLT1-YFP), hSGLT2-YFP and hSGLT3-YFP in oocytes of Xenopus laevis, injected hRS1-Reg(S20E), QEP, DFMO, and/or α-methyl-d-glucopyranoside (AMG), and measured AMG uptake, glucose-induced currents, and plasma membrane-associated fluorescence after 1 hour. We also performed in vitro AMG uptake measurements into small intestinal mucosa of mice and human. The data indicate that QEP down-regulates the exocytotic pathway of SGLT1 similar to hRS1-Reg(S20E). Our results suggests that both peptides also down-regulate hSGLT2 and hSGLT3 via the same pathway. Thirty minutes after application of 5 mM QEP in the presence of 5 mM d-glucose, hSGLT1-mediated AMG uptake into small intestinal mucosa was decreased by 40% to 50%. Thus oral application of QEP in a formulation that optimizes uptake into enterocytes but prevents entry into the blood is proposed as novel antidiabetic therapy.

An Array-Based Ligand Discovery Platform for Proteins With Short Half-Lives

Many promising therapeutic protein targets were previously considered "undruggable" due to a deficit in structural information to guide drug design and/or a lack of an obvious binding pocket. Fortunately, array-based methods for evaluating protein binding against large chemical libraries, such as small-molecule microarray screening, have provided one of several emerging inroads to ligand discovery for these elusive targets. Despite the advance in the area of ligand discovery for poorly structured and intrinsically disordered proteins provided by array-based technologies involving cell lysates, the extension of this technology for screening proteins with short half-lives in physiologically relevant conformations has been technically challenging. In this chapter we present a protocol for leveraging in vitro translation strategies to enable array-based screening of short-lived proteins against large small-molecule libraries for ligand discovery.

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.

RefSeq curation and annotation of antizyme and antizyme inhibitor genes in vertebrates

Polyamines are ubiquitous cations that are involved in regulating fundamental cellular processes such as cell growth and proliferation; hence, their intracellular concentration is tightly regulated. Antizyme and antizyme inhibitor have a central role in maintaining cellular polyamine levels. Antizyme is unique in that it is expressed via a novel programmed ribosomal frameshifting mechanism. Conventional computational tools are unable to predict a programmed frameshift, resulting in misannotation of antizyme transcripts and proteins on transcript and genomic sequences. Correct annotation of a programmed frameshifting event requires manual evaluation. Our goal was to provide an accurately curated and annotated Reference Sequence (RefSeq) data set of antizyme transcript and protein records across a broad taxonomic scope that would serve as standards for accurate representation of these gene products. As antizyme and antizyme inhibitor proteins are functionally connected, we also curated antizyme inhibitor genes to more fully represent the elegant biology of polyamine regulation. Manual review of genes for three members of the antizyme family and two members of the antizyme inhibitor family in 91 vertebrate organisms resulted in a total of 461 curated RefSeq records.

Filarial parasites possess an antizyme but lack a functional ornithine decarboxylase

In eukaryotes, the key player in polyamine metabolism is the ornithine decarboxylase (ODC) that catalyses the first and rate limiting step in cellular polyamine synthesis. The half life of ODC is strictly regulated by the antizyme (AZ), which promotes its degradation. Older reports on the polyamine situation in filarial parasites indicate a lack of ornithine decarboxylation activity and an increased uptake of polyamines. Our in silico analysis of the Brugia malayi genome revealed only an ODC-like protein that lacks essential residues. Consequently, the recombinant protein had no enzymatic ODC activity. Furthermore, only ODC-like genes were found in the available draft genomes of other filarial parasites. In this ODC-free scenario, we set out to investigate the AZ of O. volvulus (OvAZ). The expression of the recombinant protein allowed us to analyse the localization of OvAZ in different O. volvulus stages as well as to identify it as target for the human humoral immune response. Strong immunostaining was observed in the outer zone of the uterine epithelium as well as in the uterus lumen around the periphery of the developing parasite, indicating a potential role of the OvAZ in the control of polyamine levels during embryonic development. By employing a novel in vivo method using Caenorhabditis elegans, we postulate that the OvAZ enters the secretory pathway. Even though the ODCs are absent in filarial parasites, OvAZ has the ability to bind to various ODCs, thereby demonstrating the functionality of the conserved AZ-binding domains. Finally, pull-down assays show an interaction between B. malayi AZ and the B. malayi ODC-like protein, indicating that the B. malayi ODC-like protein might function as an AZI. Taken together, our results suggest that filarial species do not possess the ODC while retaining the ODC-regulatory proteins AZ and AZI. It is tempting to speculate that both proteins are retained for the regulation of polyamine transport systems.

Minimal antizyme peptide fully functioning in the binding and inhibition of ornithine decarboxylase and antizyme inhibitor

Antizyme (AZ) is a protein with 228 amino acid residues that regulates ornithine decarboxylase (ODC) by binding to ODC and dissociating its homodimer, thus inhibiting its enzyme activity. Antizyme inhibitor (AZI) is homologous to ODC, but has a higher affinity than ODC for AZ. In this study, we quantified the biomolecular interactions between AZ and ODC as well as AZ and AZI to identify functional AZ peptides that could bind to ODC and AZI and inhibit their function as efficiently as the full-length AZ protein. For these AZ peptides, the inhibitory ability of AZ_95-228 was similar to that of AZ_WT. Furthermore, AZ_95-176 displayed an inhibition (IC₅₀: 0.20 µM) similar to that of AZ-95-228 (IC₅₀: 0.16 µM), even though a large segment spanning residues 177–228 was deleted. However, further deletion of AZ_95-176 from either the N-terminus or the C-terminus decreased its ability to inhibit ODC. The AZ_100-176 and AZ_95-169 peptides displayed a noteworthy decrease in ability to inhibit ODC, with IC₅₀ values of 0.43 and 0.37 µM, respectively. The AZ_95-228, AZ_100-228 and AZ_95-176 peptides had IC₅₀ values comparable to that of AZ_WT and formed AZ-ODC complexes with K_d,AZ-ODC values of 1.5, 5.3 and 5.6 µM, respectively. Importantly, our data also indicate that AZI can rescue AZ peptide-inhibited ODC enzyme activity and that it can bind to AZ peptides with a higher affinity than ODC. Together, these data suggest that these truncated AZ proteins retain their AZI-binding ability. Thus, we suggest that AZ_95-176 is the minimal AZ peptide that is fully functioning in the binding of ODC and AZI and inhibition of their function.

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.

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.

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.

Regulation of the nitrogen transfer pathway in the arbuscular mycorrhizal symbiosis: gene characterization and the coordination of expression with nitrogen flux

The arbuscular mycorrhiza (AM) brings together the roots of over 80% of land plant species and fungi of the phylum Glomeromycota and greatly benefits plants through improved uptake of mineral nutrients. AM fungi can take up both nitrate and ammonium from the soil and transfer nitrogen (N) to host roots in nutritionally substantial quantities. The current model of N handling in the AM symbiosis includes the synthesis of arginine in the extraradical mycelium and the transfer of arginine to the intraradical mycelium, where it is broken down to release N for transfer to the host plant. To understand the mechanisms and regulation of N transfer from the fungus to the plant, 11 fungal genes putatively involved in the pathway were identified from Glomus intraradices, and for six of them the full-length coding sequence was functionally characterized by yeast complementation. Two glutamine synthetase isoforms were found to have different substrate affinities and expression patterns, suggesting different roles in N assimilation. The spatial and temporal expression of plant and fungal N metabolism genes were followed after nitrate was added to the extraradical mycelium under N-limited growth conditions using hairy root cultures. In parallel experiments with (15)N, the levels and labeling of free amino acids were measured to follow transport and metabolism. The gene expression pattern and profiling of metabolites involved in the N pathway support the idea that the rapid uptake, translocation, and transfer of N by the fungus successively trigger metabolic gene expression responses in the extraradical mycelium, intraradical mycelium, and host plant.

Ornithine decarboxylase antizyme inhibitor 2 regulates intracellular vesicle trafficking

Antizyme inhibitor 1 (AZIN1) and 2 (AZIN2) are proteins that activate ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis. Both AZINs release ODC from its inactive complex with antizyme (AZ), leading to formation of the catalytically active ODC. The ubiquitously expressed AZIN1 is involved in cell proliferation and transformation whereas the role of the recently found AZIN2 in cellular functions is unknown. Here we report the intracellular localization of AZIN2 and present novel evidence indicating that it acts as a regulator of vesicle trafficking. We used immunostaining to demonstrate that both endogenous and FLAG-tagged AZIN2 localize to post-Golgi vesicles of the secretory pathway. Immuno-electron microscopy revealed that the vesicles associate mainly with the trans-Golgi network (TGN). RNAi-mediated knockdown of AZIN2 or depletion of cellular polyamines caused selective fragmentation of the TGN and retarded the exocytotic release of vesicular stomatitis virus glycoprotein. Exogenous addition of polyamines normalized the morphological changes and reversed the inhibition of protein secretion. Our findings demonstrate that AZIN2 regulates the transport of secretory vesicles by locally activating ODC and polyamine biosynthesis.

Subcellular localization and phosphorylation of antizyme 2

Antizymes (AZs) are polyamine-induced proteins that negatively regulate cellular polyamine synthesis and uptake. Three antizyme isoforms are conserved among mammals. AZ1 and AZ2 have a broad tissue distribution, while AZ3 is testis specific. Both AZ1 and AZ2 inhibit ornithine decarboxylase (ODC) activity by binding to ODC monomer and target it to the 26S proteasome at least in vivo. Both also inhibit extra-cellular polyamine uptake. Despite their being indistinguishable by these criteria, we show here using enhanced green fluorescent protein (EGFP)-AZ2 fusion protein that in mammalian cells, the subcellular location of AZ2 is mainly in the nucleus, and is different from that of AZ1. The C-terminal part of AZ2 is necessary for the nuclear distribution. Within a few hours, a shift in the distribution of EGFP-AZ2 fusion protein from cytoplasm to the nucleus or from nucleus to cytoplasm is observable in NIH3T3 cells. In addition, we found that in cells a majority of AZ2, but not AZ1, is phosphorylated at Ser-186, likely by protein kinase CK2. There may be a specific function of AZ2 in the nucleus.

Insect ornithine decarboxylase (ODC) complements SPE1 knock-out of yeast Saccharomyces cerevisiae

Ornithine decarboxylase (ODC) is a rate-limiting enzyme in the biosynthesis of polyamines, which are essential for cell growth, differentiation, and proliferation. This report presents the characterization of an ODC-encoding cDNA (SlitODC) isolated from a moth species, the tobacco cutworm, Spodoptera litura (Lepidoptera); its expression in a polyamine-deficient strain of yeast, S. cerevisiae; and the recovery in polyamine levels and proliferation rate with the introduction of the insect enzyme. SlitODC encodes 448 amino acid residues, 4 amino acids longer than B. Mori ODC that has 71% identity, and has a longer C-terminus, consistent with B. mori ODC, than the reported dipteran enzymes. The null mutant yeast strain in the ODC gene, SPE1, showed remarkably depleted polyamine levels; in putrescine, spermidine, and spermine, the levels were > 7, > 1, and > 4%, respectively, of the levels in the wild-type strain. This consequently caused a significant arrest in cell proliferation of > 4% of the wild-type strain in polyaminefree media. The transformed strain, with the substituted SlitODC for the deleted endogenous ODC, grew and proliferated rapidly at even a higher rate than the wild-type strain. Furthermore, its polyamine content was significantly higher than even that in the wild-type strain as well as the spe1-null mutant, particularly with a very continuously enhanced putrescine level, reflecting no inhibition mechanism operating in the putrescine synthesis step by any corresponding insect ODC antizymes to SlitODC in this yeast system.

Antizyme inhibitor 2: molecular, cellular and physiological aspects

Polyamines are small organic polycations essential for cell proliferation and survival. Antizymes (AZs) are small proteins regulated by polyamines that inhibit polyamine biosynthesis and uptake in mammalian cells. In addition, antizyme functions are also regulated by antizyme inhibitors, homologue proteins of ornithine decarboxylase lacking enzymatic activity. There are two antizyme inhibitors (AZIN), known as AZIN1 and AZIN2, that bind to AZs and negate their effects on polyamine metabolism. Here, we review different molecular and cellular properties of the novel AZIN2 with particular emphasis on the role that this protein may have in brain and testis physiology. Whereas AZIN1 is ubiquitously found in mammalian tissues, AZIN2 expression appears to be restricted to brain and testis. In transfected cells, AZIN2 is mainly located in the endoplasmic reticulum-Golgi intermediate compartment and in the cis-Golgi network. AZIN2 is a labile protein that is degraded by the proteasome by a ubiquitin-dependent mechanism. Regarding its physiological role, spatial and temporal analyses of AZIN2 expression in the mouse testis suggest that this protein may have a role in spermiogenesis.

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.

Role of ornithine decarboxylase antizyme inhibitor in vivo

Ornithine decarboxylase (ODC) antizyme inhibitor (AZI) has been shown to regulate ODC activity in cell cultures. However, its biological functions in an organism remain unknown. An embryonic stem (ES) cell clone was established, in which the Azin1 gene was disrupted by the gene trap technique. To identify the function of Azin1 gene in vivo, a mutant mouse line was generated using these trapped ES cells. Homozygous mutant mice died at P0 with abnormal liver morphology. Further analysis indicated that the deletion of Azin1 in homozygous mice resulted in the degradation of ODC, and reduced the biosynthesis of putrescine and spermidine. Our results thus show that AZI plays an important role in regulating the levels of ODC, putrescine and spermidine in mice, and is essential for the survival of mice.

Expression of antizyme inhibitor 2 in male haploid germinal cells suggests a role in spermiogenesis

Recently, we have found that the antizyme inhibitor 2, a novel member of the antizyme binding proteins related to polyamine metabolism, was expressed mainly in the adult testes, although its function in testicular physiology is completely unknown. Therefore, in the present work, the spatial and temporal expression of antizyme inhibitor 2, and other genes related to polyamine metabolism were studied in the mouse testis, in an attempt to understand the role of antizyme inhibitor 2 in testicular functions. For that purpose, the temporal expression of different genes, during the first wave of spermatogenesis in postnatal mice, was studied by real-time RT-PCR, and the spatial distribution of transcripts and protein in the adult testis was examined by both RNA in situ hybridization and immunocytochemistry. The results indicated that antizyme inhibitor 2 was specifically expressed in the haploid germinal cells, similarly to antizyme 3, the testis specific antizyme. Conversely, ornithine decarboxylase mRNA was mainly found in the outer part of the seminiferous tubules where spermatogonia and spermatocytes are located. Functional transfection assays and co-immunoprecipitation experiments corroborated that antizyme inhibitor 2 counteracts the negative action of antizyme 3 on polyamine biosynthesis and uptake. All these results indicate that the expression of antizyme inhibitor 2 is postnatally regulated and strongly suggest that antizyme inhibitor 2 may have a role in spermiogenesis.

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.

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.

Human ornithine decarboxylase paralogue (ODCp) is an antizyme inhibitor but not an arginine decarboxylase

ODC (ornithine decarboxylase), the rate-limiting enzyme in polyamine biosynthesis, is regulated by specific inhibitors, AZs (antizymes), which in turn are inhibited by AZI (AZ inhibitor). We originally identified and cloned the cDNA for a novel human ODC-like protein called ODCp (ODC paralogue). Since ODCp was devoid of ODC catalytic activity, we proposed that ODCp is a novel form of AZI. ODCp has subsequently been suggested to function either as mammalian ADC (arginine decarboxylase) or as AZI in mice. Here, we report that human ODCp is a novel AZI (AZIN2). By using yeast two-hybrid screening and in vitro binding assay, we show that ODCp binds AZ1-3. Measurements of the ODC activity and ODC degradation assay reveal that ODCp inhibits AZ1 function as efficiently as AZI both in vitro and in vivo. We further demonstrate that the degradation of ODCp is ubiquitin-dependent and AZ1-independent similar to the degradation of AZI. We also show that human ODCp has no intrinsic ADC activity.

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, a natural ornithine decarboxylase inhibitor, induces apoptosis of haematopoietic cells through mitochondrial membrane depolarization and caspases' cascade

Antizymes delicately regulate ornithine decarboxylase (ODC) enzyme activity and polyamine transportation. One member of the family, antizyme-1, plays vital roles in molecular and cellular functions, including developmental regulation, cell cycle, proliferation, cell death, differentiation and tumorigenesis. However, the question of how does it participate in the cell apoptotic mechanism is still unsolved. To elucidate the contribution of human antizyme-1 in haematopoietic cell death, we examine whether inducible overexpression of antizyme enhances apoptotic cell death. Antizyme reduced the viability in a dose- and time-dependent manner of human leukemia HL-60 cells, acute T leukemia Jurkat cells and mouse macrophage RAW 264.7 cells. The apoptosis-inducing activities were determined by nuclear condensation, DNA fragmentation, sub-G(1) appearance, loss of mitochondrial membrane potential (Deltapsi( m )), release of mitochondrial cytochrome c into cytoplasm and proteolytic activation of caspase 9 and 3. Following conditional antizyme overexpression, all protein levels of cyclin-dependent kinases (Cdks) and cyclins are not significantly reduced, except cyclin D, before their entrance into apoptotic cell death. However, introduced cyclin D1 into Jurkat T tetracycline (Tet)-On cell system still couldn't rescue cells from apoptosis. Antizyme doesn't influence the expression of tumor suppressor p53 and its downstream p21, but it interferes in the expressions of Bcl-2 family. Inducible antizyme largely enters mitochondria resulting in cytochrome c release from mitochondria to cytosol following Bcl-xL decrease and Bax increase. According to these data, we suggest that antizyme induces apoptosis mainly through mitochondria-mediated and cell cycle-independent pathway. Furthermore, antizyme induces apoptosis not only by Bax accumulation reducing the function of the Bcl-2 family, destroying the Deltapsi( m ), and releasing cytochrome c to cytoplasm but also by the activation of apoptosomal caspase cascade.

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.

Mouse ornithine decarboxylase-like gene encodes an antizyme inhibitor devoid of ornithine and arginine decarboxylating activity

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines, is a labile protein that is regulated by interacting with antizymes (AZs), a family of polyamine-induced proteins. Recently, a novel human gene highly homologous to ODC, termed ODC-like or ODC-paralogue (ODCp), was cloned, but the studies aimed to determine its function rendered contradictory results. We have cloned the mouse orthologue of human ODCp and studied its expression and possible function. mRNA of mouse Odcp was found in the brain and testes, showing a conserved expression pattern with regard to the human gene. Transfection of mouse Odcp in HEK 293T cells elicited an increase in ODC activity, but no signs of arginine decarboxylase activity were evident. On the other hand, whereas the ODCp protein was mainly localized in the mitochondrial/membrane fraction, ODC activity was found in the cytosolic fraction and was markedly decreased by small interfering RNA against human ODC. Co-transfection experiments with combinations of Odc, Az1, Az2, Az3, antizyme inhibitor (Azi), and Odcp genes showed that ODCp mimics the action of AZI, rescuing ODC from the effects of AZs and prevented ODC degradation by the proteasome. A direct interaction between ODCp and AZs was detected by immunoprecipitation experiments. We conclude that mouse ODCp has no intrinsic decarboxylase activity, but it acts as a novel antizyme inhibitory protein (AZI2).

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Identification of polymorphisms and balancing selection in the male infertility candidate gene, ornithine decarboxylase antizyme 3

Background

The antizyme family is a group of small proteins that play a role in cell growth and division by regulating the biosynthesis of polyamines (putrescine, spermidine, spermine). Antizymes regulate polyamine levels primarily through binding ornithine decarboxylase (ODC), an enzyme key to polyamine production, and targeting ODC for destruction by the 26S proteosome. Ornithine decarboxylase antizyme 3 (OAZ3) is a testis-specific antizyme paralog and the only antizyme expressed in the mid to late stages of spermatogenesis.

Methods

To see if mutations in the OAZ3 gene are responsible for some cases of male infertility, we sequenced and evaluated the genomic DNA of 192 infertile men, 48 men of known paternity, and 34 African aborigines from the Mbuti tribe in the Democratic Republic of the Congo. The coding sequence of OAZ3 was further screened for polymorphisms by SSCP analysis in the infertile group and an additional 250 general population controls. Identified polymorphisms in the OAZ3 gene were further subjected to a haplotype analysis using PHASE 2.02 and Arlequin 2.0 software programs.

Results

A total of 23 polymorphisms were identified in the promoter, exons or intronic regions of OAZ3. The majority of these fell within a region of less than two kilobases. Two of the polymorphisms, -239 A/G in the promoter and 4280 C/T, a missense polymorphism in exon 5, may show evidence of association with male infertility. Haplotype analysis identified 15 different haplotypes, which can be separated into two divergent clusters.

Conclusion

Mutations in the OAZ3 gene are not a common cause of male infertility. However, the presence of the two divergent haplotypes at high frequencies in all three of our subsamples (infertile, control, African) suggests that they have been maintained in the genome by balancing selection, which was supported by a test of Tajima's D statistic. Evidence for natural selection in this region implies that these haplotypes may be associated with a trait other than infertility. This trait may be related to another function of OAZ3 or a region in tight linkage disequilibrium to the gene.

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.

Solution structure of a conserved domain of antizyme: a protein regulator of polyamines

Antizyme and its isoforms are members of an unusual yet broadly conserved family of proteins, with roles in regulating polyamine levels within cells. Antizyme has the ability to bind and inhibit the enzyme ornithine decarboxylase (ODC), targeting it for degradation at the proteasome; antizyme is also known to affect the transport of polyamines and interact with the antizyme inhibitor protein (AZI), as well as the cell-cycle protein cyclin D1. In the present work, NMR methods were used to determine the solution structure of a stable, folded domain of mammalian antizyme isoform-1 (AZ-1), consisting of amino acid residues 87-227. The protein was found to contain eight beta strands and two alpha helices, with the strands forming a mixed parallel and antiparallel beta sheet. At the level of primary sequence, antizyme is not similar to any protein of known structure, and results show that antizyme exhibits a novel arrangement of its strands and helices. Interestingly, however, the fold of antizyme is similar to that found in a family of acetyl transferases, as well as translation initiation factor IF3, despite a lack of functional relatedness between these proteins. Structural results, combined with amino acid sequence comparisons, were used to identify conserved features among the various homologues of antizyme and their isoforms. Conserved surface residues, including a cluster of acidic amino acids, were found to be located on a single face of antizyme, suggesting this surface is a possible site of interaction with target proteins such as ODC. This structural model provides an essential framework for an improved future understanding of how the different parts of antizyme play their roles in polyamine regulation.

The antizyme family: Polyamines and beyond

The family of antizymes functions as regulators of polyamine homeostasis. They are a class of small, inhibitory proteins, whose expression is regulated by a unique ribosomal frameshift mechanism. They have been shown to inhibit cell proliferation and possess anti-tumor activity. Antizymes bind ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis. They inhibit its enzymatic activity and promote the ubiquitin-independent degradation of ODC by the 26S proteasome. In addition, they also negatively regulate polyamine transport. Antizyme-mediated, ubiquitin-independent degradation of ODC is conserved from yeast to man. But recent data suggest that this degradation pathway might not be restricted to ODC alone and could involve newly discovered antizyme binding partners. Interestingly, antizyme proteins have been strictly preserved over a vast evolutionary timeframe. Antizymes consequently represent an important class of proteins that regulate cell growth and metabolism by a diverse set of mechanisms that include protein degradation, inhibition of enzyme activity, small molecule transport and other, potentially not yet discovered properties.