Feedback repression of polyamine uptake into mammalian cells requires active protein synthesis

P5B-ATPases in the mammalian polyamine transport system and their role in disease

Polyamines (PAs) are physiologically relevant molecules that are ubiquitous in all organisms. The vitality of PAs to the healthy functioning of a cell is due to their polycationic nature causing them to interact with a vast plethora of cellular players and partake in numerous cellular pathways. Naturally, the homeostasis of such essential molecules is tightly regulated in a strictly controlled interplay between intracellular synthesis and degradation, uptake from and secretion to the extracellular compartment, as well as intracellular trafficking. Not surprisingly, dysregulated PA homeostasis and signaling are implicated in multiple disorders, ranging from cancer to neurodegeneration; leading many to propose rectifying the PA balance as a potential therapeutic strategy. Despite being well characterized in bacteria, fungi and plants, the molecular identity and properties of the PA transporters in animals are poorly understood. This review brings together the current knowledge of the cellular function of the mammalian PA transport system (PTS). We will focus on the role of P5B-ATPases ATP13A2-5 which are PA transporters in the endosomal system that have emerged as key players in cellular PA uptake and organelle homeostasis. We will discuss recent breakthroughs on their biochemical and structural properties as well as their implications for disease and therapy.

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.

Unusual aspects of the polyamine transport system affect the design of strategies for use of polyamine analogues in chemotherapy

One strategy for inhibiting tumour cell growth is the use of polyamine mimetics to depress endogenous polyamine levels and, ideally, obstruct critical polyamine-requiring reactions. Such polyamine analogues make very unusual drugs, in that extremely high intracellular concentrations are required for growth inhibition or cytotoxicity. Cells exposed to even sub-micromolar concentrations of such analogues can achieve effective intracellular levels because these compounds are incorporated by the very aggressive polyamine uptake system. Once incorporated to these levels, many of these analogues induce the synthesis of a regulatory protein, antizyme, which inhibits both polyamine synthesis and the transporter they used to enter the cell. Thus this feedback system allows steady-state maintenance of effective cellular doses of such analogues. Accordingly, effective cellular levels of polyamine analogues are generally inversely related to their capacity to induce antizyme. Antizyme activity is down-regulated by interaction with several binding partners, most notably antizyme inhibitor, and at least a few tumour tissues exhibit deficiencies in antizyme expression. Our studies explore the role of antizyme induction by several polyamine analogues in their physiological response and the possibility that cell-to-cell differences in antizyme expression may contribute to variable sensitivities to these agents.

Antizyme and antizyme inhibitor activities influence cellular responses to polyamine analogs

Close structural analogs of spermidine and spermine, polyamine mimetics, are potential chemotheraputic agents as they depress cellular polyamines required for tumor growth. Specific mimetic analogs stimulate synthesis of the regulatory protein antizyme (AZ), which not only inactivates the initial enzyme in polyamine biosynthesis but also inhibits cellular uptake of polyamines. The role of AZ induction in influencing cellular uptake of representative analogs was investigated using three analogs produced by Cellgate Inc., CGC-11047, CGC-11102, and CGC-11144, which exhibit markedly distinct AZ-inducing potential. An inverse correlation was noted between the AZ-inducing activity of a compound and the steady-state levels accumulated in cells. As some tumor cells over express AZI as a means of enhancing the polyamines required for aggressive growth, analog sensitivity was examined in transgenic CHO cells expressing exogenous antizyme inhibitor protein (AZI). Although AZI over expression increased cell sensitivity to analogs, the degree of this affect varied with the analog used.

Antizyme induction mediates feedback limitation of the incorporation of specific polyamine analogues in tissue culture

Spermidine, spermine and putrescine are essential for mammalian cell growth, and there has been a pervasive effort to synthesize analogues of these polyamines that will disrupt their function and serve as tools to inhibit cell proliferation. Recently, we demonstrated that a number of such polyamine analogues are also capable of inducing the regulatory protein AZ (antizyme). In the present study the incorporation of a few sample analogues [mimics of bis(ethyl)spermine] was shown to be significantly limited by a decrease in the V(max) for the polyamine transport system in response to analogue-induced AZ. This creates an unusual circumstance in which compounds that are being designed for therapeutic use actually inhibit their own incorporation into targeted cells. To explore the impact of this feedback system, cultures of rat hepatoma HTC cells were pre-treated to exhibit either low or high polyamine uptake activity and then exposed to polyamine analogues. As predicted, regardless of initial uptake activity, all cultures eventually achieved the same steady-state levels of the cellular analogue and AZ. Importantly, analogue-induced AZ levels remained elevated with respect to controls even after the native polyamines were reduced by more than 70%. To model the insufficient AZ expression found in certain tumours, GS-CHO (GS Chinese-hamster ovary) cells were transfected to express high levels of exogenic AZI (AZ inhibitor). As anticipated, this clone incorporated significantly higher levels of the polyamine analogues examined. This study reveals a potential limitation in the use of polyamine-based compounds as therapeutics, and strategies are presented to either circumvent or exploit this elegant transport feedback system.

Putrescine transport in hypoxic rat main PASMCs is required for p38 MAP kinase activation

Hypoxic pulmonary vascular remodeling in rats is associated with increased polyamine transport in pulmonary artery smooth muscle cells (PASMCs). We therefore defined constitutive and hypoxia-induced polyamine transport properties of rat cultured PASMCs and determined the impact of polyamine transport blockade on hypoxia-induced accumulation of p38 MAP kinase. PASMCs exhibited polyamine transport pathways that were characterized by Michaelis-Menten kinetics. RNA synthesis inhibition attenuated while inhibition of protein synthesis increased polyamine uptake, thus suggesting regulation by ornithine decarboxylase-antizyme. The presence of two transporters with overlapping selectivities, one for putrescine and another for all three polyamines, was inferred by cross-competition studies and by findings that only putrescine uptake was sodium dependent and that hypoxia caused a selective, time-dependent induction of putrescine transport. The pathophysiological significance of augmented putrescine import was suggested by the observation that polyamine transport inhibition suppressed hypoxia-induced p38 MAP kinase phosphorylation. These results indicate that rat PASMCs express two polyamine transporters and that a specific increase in the putrescine uptake pathway is necessary for hypoxia-induced activation of p38 MAP kinase.

Effect of spermine conjugation on the cytotoxicity and cellular transport of acridine

Polyamines are believed to be potent vectors for the selective delivery of chemotherapeutic agents into cancer cells. In this paper, we report the effect of spermine conjugation on the cytotoxic and transport properties of acridine. Six derivatives, composed of a spermine chain attached at its N(1) position to an acridine via an aliphatic chain, were synthesized. The aliphatic linker, comprised of 3-5 methylene units, was connected to the position-9 of the heterocycle through either an amide (amidoacridines 8-10) or an amine (aminoacridines 11-13) linkage. Independently of their architecture, all ligands showed a high affinity for DNA binding but a limited DNA sequence selectivity. In a whole cell assay with L1210 and Chinese hamster ovary (CHO) cells, the aminoacridines (IC(50) values around 2 microM) were more potent than the amidoacridines (IC(50) values between 20 and 40 microM). This was related to a less efficient transport for the latter. As determined from competitive uptake studies with [(14)C]spermidine, all conjugates had a high affinity for the polyamine transport system (PTS). However, on the basis of competitive studies with an excess of spermidine and on the differential effect on cell growth and accumulation in CHO and in the mutant PTS deficient CHO-MG cells, the accumulation of the conjugates through the PTS was found to be poor but still more efficient for the aminoacridines. alpha-Difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, which induces an up-regulation of the activity of the PTS, enhanced accumulation of all acridine conjugates through the PTS and had a synergistic effect on the potency of the acridine conjugates to inhibit cell growth. Despite their high affinity for the PTS, the low amount of derivatives transiting through the PTS is likely to be related to their ability to repress rapidly and efficiently the activity of the PTS and, consequently, to inhibit their own uptake via this system.

Spermine oxidation leads to necrosis with plasma membrane phosphatidylserine redistribution in mouse leukemia cells

Oxidation by copper/quinone-containing serum amine oxidases (SAO) is a well-known cause of polyamine cytotoxicity. Spermine oxidation exerts potent immunosuppressive effects in animal cells, but the cell death mechanism involved remains unclear. We compared biochemical and morphological parameters of SAO-mediated cell death in L1210 mouse leukemia cells with normal or amplified ornithine decarboxylase gene expression with those observed during apoptosis induced by deregulated polyamine uptake or by okadaic acid. None of the characteristic features of apoptotic cell death (e.g., chromatin condensation, nuclear fragmentation, internucleosomal DNA cleavage, poly(ADP-ribose) polymerase cleavage) were observed during spermine oxidation-mediated cell death, which was clearly necrotic by morphological criteria. Inhibition of a wide spectrum of caspases did not prevent SAO-dependent cell death, whereas N-acetylcysteine completely abolished the cytotoxic effects of spermine oxidation. Catalase only delayed spermine oxidation-induced cell death without affecting its modality or preventing depletion of intracellular glutathione, suggesting that both H(2)O(2) and aminoaldehydes generated by SAO-mediated spermine oxidation contribute to SAO-induced necrosis. Interestingly, redistribution of phosphatidylserine to the outer leaflet of the plasma membrane, usually a diagnostic feature of apoptosis, preceded necrotic cytolysis triggered by spermine oxidation. Thus, L1210 cell death caused by SAO-mediated spermine oxidation has all the attributes of primary necrosis, but is also accompanied by loss of phospholipid asymmetry, indicating that the latter phenomenon may not be unique to apoptosis. Phosphatidylserine exposure, a potent engulfment signal for phagocytes, might contribute to the immunosuppressive effects of plasma polyamines through a controlled and rapid necrotic process involving SAO.

Interactions between agmatine and polyamine uptake pathways in rat pulmonary artery endothelial cells

Agmatine, a product of arginine metabolism in vascular endothelial cells, is structurally similar to the natural polyamines, putrescine, spermidine and spermine. To test the hypothesis that agmatine and polyamines interacted at the level of the polyamine transporter, we determined if polyamines competed with agmatine for import and whether interventions modulating polyamine import exerted coordinate effects on agmatine uptake. Multiple lines of evidence were obtained to suggest that agmatine enters pulmonary artery endothelial cells (PAECs) via the polyamine transporter, though its intracellular disposition after uptake appears different from the natural polyamines.

Mammalian cell polyamine homeostasis is altered by the radioprotector WR1065

Mammalian cells become more susceptible to radiation-induced death and mutagenesis when restricted in their production of the natural polyamines putrescine, spermidine and spermine. The effects of polyamine deprivation are reversed by N-(2-mercaptoethyl)-1, 3-diaminopropane (WR1065), a simple aminothiol that has been extensively studied for its radioprotectant properties. Because this compound and its oxidized derivative WR33278 bear some resemblance to the polyamines, it was hypothesized that radioprotection by WR1065 or its metabolites is derived, at least in part, from their ability to supplement the natural polyamines. To evaluate the ability of these aminothiol compounds to emulate polyamine function in intact cells, rat liver hepatoma (HTC) cells were treated with radioprotective doses of WR1065; the ability of this compound to affect various aspects of normal polyamine metabolism was monitored. Although cellular WR1065 was maintained at levels exceeding those of the polyamines, this aminothiol did not have any polyamine-like effect on the initial polyamine biosynthetic enzyme, ornithine decarboxylase, or on polyamine degradative reactions. On the contrary, treatment with relatively low levels of WR1065 resulted in an unexpected increase in putrescine and spermidine synthesis. WR1065 treatment enhanced the stability, and consequently the activity, of ornithine decarboxylase. This stabilization seems to result from a WR1065-induced delay in the synthesis of antizyme, a critical regulatory protein required in the feedback modulation of polyamine synthesis and transport. The increase in cellular spermidine induced by WR1065 might explain its antimutagenic properties, but is probably not a factor in protection against cell killing by radiation. This is the first evidence that compounds can be designed to control polyamine levels by targeting the activity of the regulatory protein antizyme.

Selective sites for polyamine binding to rabbit intestinal brush-border membranes

The intestinal polyamine transporters have not yet been identified. Our aim was to characterize specific polyamine binding sites in rabbit intestinal brush-border membranes (IBBM) as a starting step for identification of polyamine transporters. This was investigated at 4 degrees and at low membrane concentration. Saturation isotherms for [3H]putrescine (PUT) binding indicated a single population of sites (puT) with a dissociation equilibrium constant Kd of 3.8 microM and a density of sites Bmax of 58 pmol/mg of protein. [3H]spermidine (SPD) binding also involved only one class of sites (spD), albeit with a lower affinity (Kd = 106 microM) and higher abundance (Bmax = 1240 pmol/mg of protein) than puT. On the contrary, [14C]spermine (SPM) bound two classes of sites (spM1 and spM2) differing in their affinity (Kd = 2.5 and 31.4 microM) and abundance (Bmax = 467 and 1617 pmol/mg of protein, respectively). Membrane association of SPM at 4 degrees was much faster than that of SPD and PUT, both of which proceeded at a similar rate. In contrast to PUT and SPD dissociation, SPM dissociation at 23 degrees did not follow a first-order reaction. Specifically bound [3H]PUT, unlike [3H]SPD and [14C]SPM, dissociated at 23 degrees independently of the addition of nonradioactive polyamine. Methylglyoxal-bis-(guanylhydrazone) was an extremely potent inhibitor of PUT binding (Ki = 3.2 +/- 1.5 nM), but as with PUT and cadaverine (CAD), it did not alter [3H]SPD and [14C]SPM binding substantially. The intestinal brush-border membrane may contain at least three sites specific for polyamine binding and exhibiting different ligand selectivity. Site puT might be associated with the transport system already described for intestinal uptake of PUT.

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

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

The spermidine transport system is regulated by ligand inactivation, endocytosis, and by the Npr1p Ser/Thr protein kinase in Saccharomyces cerevisiae

We have characterized the regulation of spermidine transport in yeast and identified some of the genes involved in its control. Disruption of the SPE2 gene encoding S-adenosylmethionine decarboxylase, which catalyzes an essential step in polyamine biosynthesis, upregulated the initial velocity of spermidine uptake in wild-type cells as well as in the polyamine transport-deficient pcp1 mutants. Exogenous spermidine rapidly inactivated spermidine transport with a half-life of approximately 10-15 min via a process that did not require de novo protein synthesis but was accelerated by cycloheximide addition. Conversely, reactivation of spermidine influx upon polyamine deprivation required active protein synthesis. The stability of polyamine carrier activity was increased 2-fold in polyamine-depleted spe2 deletion mutants, indicating that endogenous polyamines also contribute to the down-regulation of spermidine transport. Ligand-mediated repression of spermidine transport was delayed in end3 and end4 mutants that are deficient in the initial steps of the endocytic pathway, and spermidine uptake activity was increased 4- to 5-fold in end3 mutants relative to parental cells, although the stability of the transport system was similar in both strains. Disruption of the NPR1 gene, which encodes a putative Ser/Thr protein kinase essential for the reactivation of several nitrogen permeases, resulted in a 3-fold decrease in spermidine transport in NH4(+)-rich media but did not prevent its down-regulation by spermidine. The defect in spermidine transport was more pronounced in NH4(+)- than proline-grown npr1 cells, suggesting that NPR1 protects against nitrogen catabolite repression of polyamine uptake activity. These results suggest that (a) the polyamine carrier is an unstable protein subject to down-regulation by spermidine via a process involving ligand inactivation followed by endocytosis and that (b) NPR1 expression fully prevents nitrogen catabolite repression of polyamine transport, unlike the role predicted for that gene by the inactivation/reactivation model proposed for other nitrogen permeases.

Rapid induction of apoptosis by deregulated uptake of polyamine analogues

Treatment of Chinese hamster ovary cells with alpha-difluoromethylornithine for 3 days, followed by exposure to cycloheximide, led to an unregulated, rapid and massive accumulation of polyamine analogues. This accumulation led to cell death by apoptosis within a few hours. Clear evidence of DNA fragmentation was seen in response to both N-terminally ethylated polyamines and to polyamines containing methyl groups on the terminal carbon atoms. Programmed cell death was induced within 2-4 h of exposure to 1 microM or higher concentrations of N1,N11-bis(ethyl)norspermine. The presence of cycloheximide increased the uptake of the polyamine analogues and therefore led to cell death at lower analogue concentrations, but it was not essential for the induction of apoptosis, since similar effects were seen when the protein synthesis inhibitor was omitted and the concentration of N1, N11-bis(ethyl)norspermine was increased to 5 microM or more. The induction of apoptosis was blocked both by the addition of the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, or by the addition of the polyamine oxidase inhibitor N1-methyl-N2-(2,3-butadienyl)butane-1,4-diamine (MDL 72,527). These experiments provide evidence to support the concepts that: (1) polyamines or their oxidation products may be initiators of programmed cell death; (2) regulation of polyamine biosynthesis and uptake prevents the accumulation of toxic levels of polyamines; and (3) the anti-neoplastic effects of bis(ethyl) polyamine analogues may be due to the induction of apoptosis in sensitive tumour cells.

The STK2 gene, which encodes a putative Ser/Thr protein kinase, is required for high-affinity spermidine transport in Saccharomyces cerevisiae

Eukaryotic polyamine transport systems have not yet been characterized at the molecular level. We have used transposon mutagenesis to identify genes controlling polyamine transport in Saccharomyces cerevisiae. A haploid yeast strain was transformed with a genomic minitransposon- and lacZ-tagged library, and positive clones were selected for growth resistance to methylglyoxal bis(guanylhydrazone) (MGBG), a toxic polyamine analog. A 747-bp DNA fragment adjacent to the lacZ fusion gene rescued from one MGBG-resistant clone mapped to chromosome X within the coding region of a putative Ser/Thr protein kinase gene of previously unknown function (YJR059w, or STK2). A 304-amino-acid stretch comprising 11 of the 12 catalytic subdomains of Stk2p is approximately 83% homologous to the putative Pot1p/Kkt8p (Stk1p) protein kinase, a recently described activator of low-affinity spermine uptake in yeast. Saturable spermidine transport in stk2::lacZ mutants had an approximately fivefold-lower affinity and twofold-lower Vmax than in the parental strain. Transformation of stk2::lacZ cells with the STK2 gene cloned into a single-copy expression vector restored spermidine transport to wild-type levels. Single mutants lacking the catalytic kinase subdomains of STK1 exhibited normal parameters for the initial rate of spermidine transport but showed a time-dependent decrease in total polyamine accumulation and a low-level resistance to toxic polyamine analogs. Spermidine transport was repressed by prior incubation with exogenous spermidine. Exogenous polyamine deprivation also derepressed residual spermidine transport in stk2::lacZ mutants, but simultaneous disruption of STK1 and STK2 virtually abolished high-affinity spermidine transport under both repressed and derepressed conditions. On the other hand, putrescine uptake was also deficient in stk2::lacZ mutants but was not repressed by exogenous spermidine. Interestingly, stk2::lacZ mutants showed increased growth resistance to Li+ and Na+, suggesting a regulatory relationship between polyamine and monovalent inorganic cation transport. These results indicate that the putative STK2 Ser/Thr kinase gene is an essential determinant of high-affinity polyamine transport in yeast whereas its close homolog STK1 mostly affects a lower-affinity, low-capacity polyamine transport activity.

Extracellular calcium stimulates Na(+)-dependent putrescine uptake in B16 melanoma cells

The regulation of putrescine transport in difluoromethylornithine-treated B16 melanoma cells by extracellular Ca2+ has been investigated. It was found that physiological concentrations of Ca2+ were essential for optimum uptake of putrescine and spermidine, Mg2+, albeit at higher concentrations, also could potentiate polyamine transport. The maximum rate of putrescine uptake increased from 1698 +/- 67 pmol/min/min/mg DNA in the absence of Ca2+ to 3100 +/0 98 pmol/min/mg DNA in the presence of 0.5 mM Ca2+. There was no change in Km. While Ca2+ enhanced transport of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca2+ fluxes suggesting that the two cations do not share a common transport system. The effects of Ca2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca2+ did not potentiate putrescine uptake in the presence of A23187 and secondly, because the effects of Ca2+ were completely inhibited by the lanthanide Tb3+, which binds to calcium-dependent proteins and does not readily cross biological membranes, Ca2+ did not affect putrescine transport in the absence of extracellular Na+. Moreover, the rate of putrescine uptake in the absence of Ca2+ was similar to that in the absence of extracellular Na+. The results from this study indicate that polyamine transport is stimulated by extracellular Ca2+ and suggest that Ca2+ is required for activity of the Na(+)-dependent transporter only. This transporter appears to possess a regulatory binding site for divalent cations.

Loss of intracellular putrescine pool-size regulation induces apoptosis

Synthesis and uptake are two important regulated mechanisms by which eukaryotic cells maintain polyamine levels. The role that loss of synthesis and/or uptake regulation plays in mediating putrescine toxicity was investigated by comparing toxicity in an ornithine decarboxylase (ODC)-deficient Chinese hamster ovary cell line (C55.7) with a functional putrescine transport system and an ODC-overproducing rat hepatoma cell line (DH23b), which are transport regulation deficient. When C55.7 cells were transfected with either mouse ODC (M) or trypanosome ODC (Tb), intracellular putrescine content increased slightly in C55.7(Tb-ODC), compared to C55.7(M-ODC), due to the lack of response of Tb-ODC to polyamine regulation. The increase in putrescine content resulting from loss of ODC regulation had no impact on cell growth and viability. When the feedback repression of polyamine uptake was blocked with cycloheximide, C55.7 cells transfected with either ODC construct accumulated very high levels of putrescine from the medium, and underwent apoptosis in a putrescine dose-dependent manner. A similar correlation of deregulated putrescine uptake and increased apoptotic cells was observed in DH23b cells. These data demonstrate that loss of feedback regulation on the polyamine transport system, but not ODC activity, is sufficient to induce apoptosis. Thus, downregulation of the transport system is necessary to prevent accumulation of cytotoxic putrescine levels in rodent cells.

2,2'-Dithiobis(N-ethyl-spermine-5-carboxamide) is a high affinity, membrane-impermeant antagonist of the mammalian polyamine transport system

We have synthesized 2,2'-dithiobis(N-ethyl-spermine-5-carboxamide) (DESC), its thiol monomer (MESC), and the mixed MESC-cysteamine disulfide (DEASC) as potential inhibitors of polyamine transport in mammalian cells. DESC was the most potent antagonist of spermine transport in ZR-75-1 human breast cancer cells, with Ki values of 5. 0 +/- 0.7, 80 +/- 31, and 16 +/- 3 microM for DESC, MESC, and DEASC, respectively. DESC also strongly blocked putrescine and spermidine uptake in ZR-75-1 cells (Ki = 1.6 +/- 0.5 and 2.7 +/- 1.1 microM, respectively). While DESC and MESC were purely competitive inhibitors of putrescine transport, DEASC was a mixed competitive/noncompetitive antagonist. Remarkably, DESC was virtually impermeant in ZR-75-1 cells despite its low Ki toward polyamine transport. The marked difference in affinity between DESC and MESC was essentially due to the tail-to-tail juxtaposition of two spermine-like structures, suggesting that dimeric ligands of the polyamine transporter might simultaneously interact with more than one binding site. While DESC strongly decreased the initial rate of [3H]spermidine transport, even a 40-fold molar excess of antagonist could not completely abolish intracellular spermidine accumulation. Moreover, as little as 0.3 microM spermidine fully restored growth in ZR-75-1 cells treated with an inhibitor of polyamine biosynthesis in the presence of 50 microM DESC, thus emphasizing the importance of uptake of trace amounts of exogenous polyamines. Thus, reducing the exogenous supply of polyamines with a potent competitive inhibitor may be kinetically inadequate to block replenishment of the polyamine pool in polyamine-depleted tumor cells that display high transport capacity. These results demonstrate that polyamine analogues cross-linked into a dimeric structure such as DESC interact with high affinity with the mammalian polyamine carrier without being used as substrates. These novel properties provide a framework for the design of specific irreversible inhibitors of the polyamine transporter, which should present advantages over competitive antagonists for an efficient blockade of polyamine transport in tumor cells.

JAR human placental choriocarcinoma cells actively synthesize, take up and release polyamines

Uptake of polyamines has been investigated extensively in many cells, but not in placenta, where the polyamine-polyamine oxidase system is supposed to have an immunoregulatory function in pregnancy. Due to the importance of the transfer in this tissue, we have started this study. JAR human placental choriocarcinoma cells in monolayer at confluency were used as a model for measuring the key enzymes of polyamine synthesis and interconversion, rate of uptake and efflux, and the polyamine content. Polyamines were taken up by JAR cells and released by an independent mechanism. Ornithine decarboxylase and spermidine acetyltransferase activities and the rate of transport in and out of the cell were much higher than in other cells, such as L1210 cells. However the systems used for uptake and release appear in many respects to be similar to those observed in L1210 cells, but different from others. The uptake appears to be regulated by an inhibitory protein. Moreover, protein kinase C appears to be involved in the process. The efflux also is regulated as in L1210 cells, through control of H+ and Ca2+ concentration. In conclusion, this study shows that, in JAR cells, ornithine decarboxylase and spermidine acetyltransferase activities were much higher than in other cells, and so was the rate of transport in and out of the cells. As a result, a much higher polyamine content was observed.

Polyamine transport in mammalian cells. An update

The uptake and release of the natural polyamines putrescine, spermidine and spermine by mammalian cells are integral parts of the systems that regulate the intracellular concentrations of these biogenic amines according to needs. Although a general feature of all tissues, polyamine uptake into intestinal mucosa cells is perhaps the most obvious polyamine transport pathway of physiological and pathophysiological importance. Mutant cell lines lacking the ability to take up polyamines from the environment are capable of releasing polyamines. This indicates that uptake and release are functions of two different transport systems. The isolation of a transporter gene from a mammalian cell line is still lacking. Overaccumulation of polyamines is controlled by release and by a feedback regulation system that involves de novo synthesis of antizyme, a well known protein that also regulates the activity of ornithine decarboxylase. Recent work has demonstrated that Ca(2+)-signalling pathways are also involved. Although there is consensus about the importance of polyamine uptake inhibitors in the treatment of neoplastic disorders, a practically useful uptake inhibitor is still missing. However, the attempts to target tumours, and to increase the selectivity of cytotoxic agents by combining them with the polyamine structure, are promising. New, less toxic and more selective anticancer drugs can be expected from this approach.

Regulation of polyamine synthesis and transport by fibroblast growth factor in aortic smooth muscle cells

Basic-FGF (FGF2) is implicated as a regulator of smooth muscle cell proliferation that develops after arterial injury. Polyamines are essential for cell growth and differentiation and may mediate some of the FGF2-elicited responses. To examine this possibility, the effect of FGF2 on polyamine synthesis and uptake was tested on rat arterial smooth muscle cells. Exposure of cells to FGF2 for 24 and 48 h resulted in increased intracellular polyamine content. Ornithine decarboxylase (ODC) activity increased in FGF2-treated cells after 6 h of treatment, whereas no increases were detected in ODC mRNA steady-state levels. Basic-FGF increased maximal polyamine transport rate without changes in Km. Treatment with actinomycin D decreased polyamine transport. The effect of cyclohexamide on polyamine uptake was dose dependent. These studies indicate that treatment of vascular smooth muscle cells with FGF2 results in increases in intracellular polyamine content, polyamine synthetic activity, and polyamine transport.

Use of aminopropyltransferase inhibitors and of non-metabolizable analogs to study polyamine regulation and function

The polyamines spermidine and spermine are essential for the growth of mammalian cells. This review describes the properties of the two aminopropyltransferases that are responsible for their biosynthesis, the synthesis and use of specific aminopropyltransferase inhibitors, and the use of analogs of the polyamines to investigate polyamine transport and function. Highly specific and potent multisubstrate adduct inhibitors of these enzymes have been synthesized while less potent inhibitors have been obtained by the synthesis of amines that bind at the active site. Studies with these inhibitors indicate that polyamines are needed for a normal rate of growth and that, although some of the functions of polyamines may be interchangeable, other functions may have a specific requirement for spermidine or spermine. Two groups of growth-promoting polyamine analogs can be distinguished: the many that are effective in short-term experiments compared to the few that can act over a prolonged period. The more stringent structural requirements for long-term growth are probably due to a need for spermidine, or a closely related analog, as a precursor of hypusine in the protein eIF-5A. Metabolically resistant polyamine analogs can be used as model substrates for studies of the polyamine transport system, which plays a critical role in maintaining normal cellular polyamine levels. The feedback regulation by high levels of polyamines that downregulates transport is essential to prevent the accumulation of polyamines at toxic levels. Such accumulation may be associated with apoptosis and, therefore, polyamine analogs are useful tools for investigating the mechanism(s) of polyamine-mediated toxicity.

Rapid and regulated degradation of ornithine decarboxylase

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Stable amplification of the S-adenosylmethionine decarboxylase gene in Chinese hamster ovary cells

A Chinese hamster ovary cell subline (CHO/664) > 1000-fold resistant to the S-adenosylmethionine decarboxylase (AdoMetDC) inhibitor, CGP-48664 (4-(aminoiminomethyl)-2,3-dihydro-1H-inden-1-one diaminomethylenehydrazone), has been developed and characterized. The cells were also cross-resistant to the highly specific nucleoside analog inhibitor of AdoMetDC, MDL-73811. These unique cells stably overexpress AdoMetDC due to a 10-16-fold amplification of the AdoMetDC gene, which resulted in a similar increase in AdoMetDC transcript levels. In the presence of 100 microM CGP-48664, the CHO/664 cells displayed AdoMetDC activities similar to the parental line. Following removal of the inhibitor, AdoMetDC activity increased steadily over 20 days to 10-12 times that found in parental CHO cells. Decarboxylated (dc) AdoMet pools accumulated rapidly from < 5 pmol/10(6) cells to approximately 1000-1500 pmol/10(6) cells at 3 days due to diffusion away of intracellular inhibitor and to the depletion of putrescine and spermidine as aminopropyl acceptors in dcAdoMet-mediated synthase reactions. Polyamine pools shifted as putrescine, and spermidine pools were processed forward to spermine. During the period from 3 days to 20 days, dcAdoMet pools fell steadily and eventually stabilized at 100-200 pmol/10(6) cells. Providing excess putrescine at this time as an aminopropyl acceptor rapidly lowered dcAdoMet pools and led to a near normalization of polyamine pools, indicating that both dcAdoMet and putrescine are essential in maintaining steady-state polyamine pool profiles. As with cell line variants that overproduce ornithine decarboxylase, polyamine transport was found to be increased in CHO/664 cells due to an apparent inability of the system to down-regulate polyamine transport in response to polyamine excess. Given the unique metabolic disturbances seen in these cells, we anticipate that in addition to providing a useful system for evaluating the specificity of newly developed AdoMetDC inhibitors, they will undoubtedly prove valuable for investigating the various regulatory interrelationships involved in polyamine homeostasis and possibly other aspects of purine metabolism.

Characterization of putrescine transport across the intestinal epithelium: study using isolated brush border and basolateral membrane vesicles of the enterocyte

Putrescine transport was investigated in isolated brush border and basolateral membrane vesicles prepared from the rabbit enterocyte. Brush border vesicles were oriented right-side-out and basolateral vesicles inside-out, forming a model representing uptake and extrusion across the intestinal epithelium. Putrescine transport across both membranes was initially rapid, and 66% of the equilibrium uptake was achieved within the first minute. According to osmoplots and measurements at 4 degrees C, 20% of total incorporation presented binding to the membrane. In order to estimate actual uptake into the vesicles, Km was calculated from the differences in putrescine incorporation at 37 degrees C and 4 degrees C, and was 12.7 mumol L-1 for brush border uptake and 38.2 mumol L-1 for basolateral extrusion. Putrescine uptake into brush border and basolateral membrane vesicles was not enhanced in the presence of an Na+ gradient. When Na+ was substituted with an uncharged solute, mannitol, putrescine incorporation was increased, indicating that putrescine uptake is not Na(+)-dependent and that cations might interfere with the carrier. Paraquat and methylglyoxalbis(guanylhydrazone), known to share the polyamine transport system, inhibited putrescine incorporation in both membrane vesicle preparations. Basolateral carrier showed significantly higher sensitivity to cations. We conclude that putrescine uptake across the apical membrane and extrusion across the basolateral membrane of the enterocyte are mediated by two different and independent carriers which differ in their electrical properties.

Inorganic cation dependence of putrescine and spermidine transport in human breast cancer cells

The mechanism of polyamine uptake in mammalian cells is still poorly understood. The role of inorganic cations in polyamine transport was investigated in ZR-75-1 human breast cancer cells. Although strongly temperature dependent, neither putrescine nor spermidine uptake was mediated by a Na+ cotransport mechanism. In fact, Na+ and cholinium competitively inhibited putrescine uptake relative to that measured in a sucrose-based medium. On the other hand, ouabain, H+, Na+, and Ca2+ ionophores, as well as dissipation of the K+ diffusion potential, strongly inhibited polyamine uptake in keeping with a major role of membrane potential in that process. Polyamine transport was inversely dependent on ambient osmolality at near physiological values. Putrescine transport was inhibited by 70% by decreasing extracellular pH from 7.2 to 6.2, whereas spermidine uptake had a more acidic optimum. Deletion of extracellular Ca2+ inhibited putrescine uptake more strongly than chelation of intracellular Ca2+. In fact, bound divalent cations were absolutely required for polyamine transport, as shown after brief chelation of the cell monolayers with EDTA. Either Mn2+, Ca2+, or Mg2+ sustained putrescine uptake activity with high potency (Km = 50-300 microM). Mn2+ was a much stronger activator of spermidine than putrescine uptake, suggesting a specific role for this metal in polyamine transport. Other transition metals (Co2+, Ni2+, Cu2+, and Zn2+) were mixed activators/antagonists of carrier activity, while Sr2+ and Ba2+ were very weak agonists, while not interfering with Ca2+/Mg(2+)-dependent transport. Thus, polyamine uptake in human breast tumor cells is negatively affected by ionic strength and osmolality, and is driven, at least in part, by the membrane potential, but not by the Na+ electrochemical gradient. Moreover, the polyamine carrier, or a tightly coupled accessory component, appears to have a high-affinity binding site for divalent cations, which is essential for the uptake mechanism.

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

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

Effects of the S-adenosylmethionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, on cell growth and polyamine metabolism and transport in Chinese hamster ovary cell cultures

The regulation of polyamine transport and the roles of polyamine transport and synthesis in cell growth were investigated using cultured Chinese hamster ovary (CHO) cells and CHOMG cells which are mutants lacking polyamine-transport activity. Metabolically stable methylated polyamine analogues were used to measure polyamine accumulation, and the irreversible S-adenosyl-L-methionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (AbeAdo), was used to inhibit synthesis. Exposure to AbeAdo lead to a dose-dependent decrease in growth for both cell lines, although CHOMG cells were more sensitive. Intracellular putrescine levels were greatly increased in AbeAdo-treated CHO cells and to a lesser extent in CHOMG cells, whereas intracellular spermidine and spermine levels were substantially reduced in both. Treatment with AbeAdo increased putrescine content in the culture medium to a much greater extent in CHOMG cultures indicating that a portion of the excess putrescine synthesized in response to AbeAdo treatment is excreted, but that CHO cells salvage this putrescine whereas it is lost to CHOMG cells which cannot take up polyamines. AbeAdo treatment increased polyamine transport into CHO cells despite high intracellular putrescine, suggesting that spermidine and/or spermine, and not putrescine, are the major factors regulating transport activity. The accumulation of either 1-methylspermidine or 1,12-dimethylspermine was significantly increased by AbeAdo treatment. Accumulation was increased even further when protein synthesis was blocked by cycloheximide, indicating that a short-lived protein is involved in the regulation of polyamine uptake. In the presence of cycloheximide and AbeAdo or alpha-difluoromethylornithine, methylated polyamine derivatives accumulated to very high levels leading to cell death. These results show that the polyamine-transport system plays an important role in retaining intracellular polyamines and that down-regulation of the transport system in response to increased intracellular polyamine content is necessary to prevent accumulation of toxic levels of polyamines.

Antizyme protects against abnormal accumulation and toxicity of polyamines in ornithine decarboxylase-overproducing cells

Exposure of ornithine decarboxylase (ODC; L-ornithine carboxy-lyase, EC 4.1.1.17)-overproducing mouse FM3A cells to micromolar levels of spermine or spermidine caused abnormal accumulation and toxicity of polyamines. This was apparently due to the inefficiency of negative feedback control of polyamine transport by polyamines in ODC-overproducing cells. Since antizyme is the only protein thus far recognized that can interact with ODC, depletion of free antizyme was regarded as the reason for the abnormal accumulation of polyamines. Accordingly, ODC-overproducing cells were transfected with pMAMneoZ1 possessing rat antizyme cDNA under the control of a glucocorticoid-inducible promoter. In the transfected cells, the addition of dexamethasone caused an increase in the amount of antizyme with an apparent molecular mass of 27 kDa, a decrease in the amount of ODC, a decrease in the polyamine transport activity, and the recovery of growth inhibition or cell death. The results indicate that antizyme can regulate not only the amount of ODC but also the activity of polyamine transport.

Feedback repression of polyamine transport is mediated by antizyme in mammalian tissue-culture cells

Antizyme, a spermidine-induced protein that binds and stimulates ornithine decarboxylase degradation, is now shown also to mediate the rapid feedback inhibition of polyamine uptake into mammalian cells. Using a cell line (HZ7) transfected with truncated antizyme cDNA, and mutant ornithine decarboxylase cell lines, we demonstrate that this newly discovered action of antizyme is distinct from its role in modulating polyamine biosynthesis.

Regulation of polyamine transport by polyamines and polyamine analogs

Regulation of polyamine transport in murine L1210 leukemia cells was characterized in order to better understand its relationship to specific intracellular polyamines and their analogs and to quantitate the sensitivity by which it is controlled. Up-regulation of polyamine uptake was evaluated following a 48-hr treatment with a combination of biosynthetic enzyme inhibitors to deplete intracellular polyamine pools. The latter declined gradually over 48 hr and was accompanied by a steady increase in spermidine (SPD) and spermine (SPM) transport as indicated by rises in Vmax to levels approximately 4.5 times higher than control values. Restoration of individual polyamine pools during a 6-hr period following inhibitor treatment revealed that SPD and SPM uptake could not be selectively affected by specific pool changes. The effectiveness of individual polyamines in reversing inhibitor-induced stimulation of uptake was as follows: putrescine < SPD < SPM = the SPM analog, N1, N12-bis(ethyl)spermine (BESPM). In contrast to stimulation of transport, down-regulation by exogenous polyamines or analogs occurred rapidly and in response to subtle increases in intracellular pools. Following a 1-hr exposure to 10 microM BESPM, Vmax values for SPD and SPM fell by 70%, whereas the analog pool increased to only 400-500 pmol/10(6) cells--about 15-20% of the total polyamine pool (approximately 2.8 nmol/10(6) cells). SPM produced nearly identical regulatory effects on transport kinetics. Both BESPM and SPM were even more effective at down-regulating transport that had been previously stimulated four to fivefold by polyamine depletion achieved with enzyme inhibitors. A dose response with BESPM at 48 hr revealed a biphasic effect on uptake whereby concentrations of analog < 3 microM produced an increase in SPD and SPM Vmax values, whereas concentrations 3 microM and higher produced a marked suppression of these values. Cells treated with 3 microM BESPM for 2 hr and placed in analog-free medium recovered transport capability in only 3 hr. Thus, whereas stimulation of polyamine transport is a relatively insensitive and slowly responsive process that tends to parallel polyamine depletion, down-regulation of polyamine transport by exogenous polyamines and analogs and its reversal are rapidly responsive events that correlate with relatively small (i.e., 15-20%) changes in intracellular polyamine pools.