WR1065 conjugated to thiol-PEG polymers as novel anticancer prodrugs: broad spectrum efficacy, synergism, and drug resistance reversal

The lack of anticancer agents that overcome innate/acquired drug resistance is the single biggest barrier to achieving a durable complete response to cancer therapy. To address this issue, a new drug family was developed for intracellular delivery of the bioactive aminothiol WR1065 by conjugating it to discrete thiol-PEG polymers: 4-star-PEG-S-S-WR1065 (4SP65) delivers four WR1065s/molecule and m-PEG6-S-S-WR1065 (1LP65) delivers one. Infrequently, WR1065 has exhibited anticancer effects when delivered via the FDA-approved cytoprotectant amifostine, which provides one WR1065/molecule extracellularly. The relative anticancer effectiveness of 4SP65, 1LP65, and amifostine was evaluated in a panel of 15 human cancer cell lines derived from seven tissues. Additional experiments assessed the capacity of 4SP65 co-treatments to potentiate the anticancer effectiveness and overcome drug resistance to cisplatin, a chemotherapeutic, or gefitinib, a tyrosine kinase inhibitor (TKI) targeting oncogenic EGFR mutations. The CyQUANT®-NF proliferation assay was used to assess cell viability after 48-h drug treatments, with the National Cancer Institute COMPARE methodology employed to characterize dose-response metrics. In normal human epithelial cells, 4SP65 or 1LP65 enhanced or inhibited cell growth but was not cytotoxic. In cancer cell lines, 4SP65 and 1LP65 induced dose-dependent cytostasis and cytolysis achieving 99% cell death at drug concentrations of 11.2 ± 1.2 µM and 126 ± 15.8 µM, respectively. Amifostine had limited cytostatic effects in 11/14 cancer cell lines and no cytolytic effects. Binary pairs of 4SP65 plus cisplatin or gefitinib increased the efficacy of each partner drug and surmounted resistance to cytolysis by cisplatin and gefitinib in relevant cancer cell lines. 4SP65 and 1LP65 were significantly more effective against TP53-mutant than TP53-wild-type cell lines, consistent with WR1065-mediated reactivation of mutant p53. Thus, 4SP65 and 1LP65 represent a unique prodrug family for innovative applications as broad-spectrum anticancer agents that target p53 and synergize with a chemotherapeutic and an EGFR-TKI to prevent or overcome drug resistance.

Polyamine-Based Nanostructures Share Polyamine Transport Mechanisms with Native Polyamines and Their Analogues: Significance for Polyamine-Targeted Therapy

Polyamines are small polycationic alkylamines involved in many fundamental cellular processes, including cell proliferation, survival, and protection from oxidative stress. Polyamine homeostasis is tightly regulated through coordinated biosynthesis, catabolism, and transport. Due to their continual proliferation, cancer cells maintain elevated intracellular polyamine pools. Both polyamine metabolism and transport are commonly dysregulated in cancer, and as such, polyamine analogues are a promising strategy for exploiting the increased polyamine requirement of cancer cells. One potential polyamine analogue resistance mechanism is the downregulation of the poorly defined polyamine transport system. Recent advances in nanomedicine have produced nanostructures with polyamine analogue-based backbones (nanopolyamines). Similar nanostructures with non-polyamine backbones have been shown to be transported by endocytosis. As these polyamine-based nanoparticles could be a method for polyamine analogue delivery that bypasses polyamine transport, we designed the current studies to determine the efficacy of polyamine-based nanoparticles in cells lacking intact polyamine transport. Utilizing polyamine transport-deficient derivatives of lung adenocarcinoma lines, we demonstrated that cells unable to transport natural polyamines were also resistant to nanopolyamine-induced cytotoxicity. This resistance was a result of transport-deficient cells being incapable of importing and accumulating nanopolyamines. Pharmacological modulation of polyamine transport confirmed these results in polyamine transport competent cells. These studies provide additional insight into the polyamine transport pathway and suggest that receptor-mediated endocytosis is a likely mechanism of transport for higher-order polyamines, polyamine analogues and the nanopolyamines.

Deep Learning Accurately Predicts Estrogen Receptor Status in Breast Cancer Metabolomics Data

Metabolomics holds the promise as a new technology to diagnose highly heterogeneous diseases. Conventionally, metabolomics data analysis for diagnosis is done using various statistical and machine learning based classification methods. However, it remains unknown if deep neural network, a class of increasingly popular machine learning methods, is suitable to classify metabolomics data. Here we use a cohort of 271 breast cancer tissues, 204 positive estrogen receptor (ER+), and 67 negative estrogen receptor (ER-) to test the accuracies of feed-forward networks, a deep learning (DL) framework, as well as six widely used machine learning models, namely random forest (RF), support vector machines (SVM), recursive partitioning and regression trees (RPART), linear discriminant analysis (LDA), prediction analysis for microarrays (PAM), and generalized boosted models (GBM). DL framework has the highest area under the curve (AUC) of 0.93 in classifying ER+/ER- patients, compared to the other six machine learning algorithms. Furthermore, the biological interpretation of the first hidden layer reveals eight commonly enriched significant metabolomics pathways (adjusted P-value <0.05) that cannot be discovered by other machine learning methods. Among them, protein digestion and absorption and ATP-binding cassette (ABC) transporters pathways are also confirmed in integrated analysis between metabolomics and gene expression data in these samples. In summary, deep learning method shows advantages for metabolomics based breast cancer ER status classification, with both the highest prediction accuracy (AUC = 0.93) and better revelation of disease biology. We encourage the adoption of feed-forward networks based deep learning method in the metabolomics research community for classification.

Integrative Analysis with Monte Carlo Cross-Validation Reveals miRNAs Regulating Pathways Cross-Talk in Aggressive Breast Cancer

In this work an integrated approach was used to identify functional miRNAs regulating gene pathway cross-talk in breast cancer (BC). We first integrated gene expression profiles and biological pathway information to explore the underlying associations between genes differently expressed among normal and BC samples and pathways enriched from these genes. For each pair of pathways, a score was derived from the distribution of gene expression levels by quantifying their pathway cross-talk. Random forest classification allowed the identification of pairs of pathways with high cross-talk. We assessed miRNAs regulating the identified gene pathways by a mutual information analysis. A Fisher test was applied to demonstrate their significance in the regulated pathways. Our results suggest interesting networks of pathways that could be key regulatory of target genes in BC, including stem cell pluripotency, coagulation, and hypoxia pathways and miRNAs that control these networks could be potential biomarkers for diagnostic, prognostic, and therapeutic development in BC. This work shows that standard methods of predicting normal and tumor classes such as differentially expressed miRNAs or transcription factors could lose intrinsic features; instead our approach revealed the responsible molecules of the disease.

Polyamines and cancer: implications for chemotherapy and chemoprevention

Polyamines are small organic cations that are essential for normal cell growth and development in eukaryotes. Under normal physiological conditions, intracellular polyamine concentrations are tightly regulated through a dynamic network of biosynthetic and catabolic enzymes, and a poorly characterised transport system. This precise regulation ensures that the intracellular concentration of polyamines is maintained within strictly controlled limits. It has frequently been observed that the metabolism of, and the requirement for, polyamines in tumours is frequently dysregulated. Elevated levels of polyamines have been associated with breast, colon, lung, prostate and skin cancers, and altered levels of rate-limiting enzymes in both biosynthesis and catabolism have been observed. Based on these observations and the absolute requirement for polyamines in tumour growth, the polyamine pathway is a rational target for chemoprevention and chemotherapeutics. Here we describe the recent advances made in the polyamine field and focus on the roles of polyamines and polyamine metabolism in neoplasia through a discussion of the current animal models for the polyamine pathway, chemotherapeutic strategies that target the polyamine pathway, chemotherapeutic clinical trials for polyamine pathway-specific drugs and ongoing clinical trials targeting polyamine biosynthesis.

Recent advances in the molecular biology of metazoan polyamine transport

Very limited molecular knowledge exists about the identity and protein components of the ubiquitous polyamine transporters found in animal cells. However, a number of reports have been published over the last 5 years on potential candidates for metazoan polyamine permeases. We review the available evidence on these putative polyamine permeases, as well as establish a useful "identikit picture" of the general polyamine transport system, based on its properties as found in a wide spectrum of mammalian cells. Any molecular candidate encoding a putative "general" polyamine permease should fit that provided portrait. The current models proposed for the mechanism of polyamine internalization in mammalian cells are also briefly reviewed.

The human carnitine transporter SLC22A16 mediates high affinity uptake of the anticancer polyamine analogue bleomycin-A5

Bleomycin is used in combination with other antineoplastic agents to effectively treat lymphomas, testicular carcinomas, and squamous cell carcinomas of the cervix, head, and neck. However, resistance to bleomycin remains a persistent limitation in exploiting the full therapeutic benefit of the drug with other types of cancers. Previously, we documented that the Saccharomyces cerevisiae L-carnitine transporter Agp2 is responsible for the high affinity uptake of polyamines and of the polyamine analogue bleomycin-A5. Herein, we document that the human L-carnitine transporter hCT2 encoded by the SLC22A16 gene is involved in bleomycin-A5 uptake, as well as polyamines. We show that NT2/D1 human testicular cancer cells, which highly express hCT2, are extremely sensitive to bleomycin-A5, whereas HCT116 human colon carcinoma cells devoid of detectable hCT2 expression or MCF-7 human breast cancer cells that only weakly express the permease showed striking resistance to the drug. NT2/D1 cells accumulated fluorescein-labeled bleomycin-A5 to substantially higher levels than HCT116 cells. Moreover, L-carnitine protected NT2/D1 cells from the lethal effects of bleomycin-A5 by preventing its influx, and siRNA targeted to hCT2 induced resistance to bleomycin-A5-dependent genotoxicity. Furthermore, hCT2 overexpression induced by transient transfection of a functional hCT2-GFP fusion protein sensitized HCT116 cells to bleomycin-A5. Collectively, our data strongly suggest that hCT2 can mediate bleomycin-A5 and polyamine uptake, and that the rate of bleomycin-A5 accumulation may account for the differential response to the drug in patients.

Molecular correlates of the action of bis(ethyl)polyamines in breast cancer cell growth inhibition and apoptosis

Polyamines are known to be involved in cell growth regulation in breast cancer. To evaluate the efficacy of bis(ethyl)polyamine analogs for breast cancer therapy and to understand their mechanism of action we measured the effects of a series of polyamine analogs on cell growth, activities of enzymes involved in polyamine metabolism, intracellular polyamine levels, and the uptake of putrescine and spermidine using MCF-7 breast cancer cells. The IC50 values for cell growth inhibition of three of the compounds, N1,N12-bis(ethyl)spermine, N1,N11-bis(ethyl)norspermine, and N1,N14-bis(ethyl)homospermine, were in the range of 1-2 microM. Another group of three compounds showed antiproliferative activity at about 5 microM level. These compounds are also capable of suppressing colony formation in soft agar assay and inducing apoptosis of MCF-7 cells. The highly effective growth inhibitory agents altered the activity of polyamine biosynthetic and catabolic enzymes and down-regulated the transport of natural polyamines, although each compound produced a unique pattern of alterations in these parameters. HPLC analysis showed that cellular uptake of bis(ethyl)polyamines was highest for bis(ethyl)spermine. We also analyzed polyamine analog conformations and their binding to DNA minor or major grooves by molecular modelling and molecular dynamics simulations. Results of these analyses indicate that tetramine analogs fit well in the minor groove of DNA whereas, larger compounds extend out of the minor groove. Although major groove binding was also possible for the short tetramine analogs, this interaction led to a predominantly bent conformation. Our studies show growth inhibitory activities of several potentially important analogs on breast cancer cells and indicate that multiple sites are involved in the mechanism of action of these analogs. While the activity of an analog may depend on the sum of these different effects, molecular modelling studies indicate a correlation between antiproliferative activity and stable interactions of the analogs with major or minor grooves of DNA.

Polyamine-depletion induces p27Kip1 and enhances dexamethasone-induced G1 arrest and apoptosis in human T lymphoblastic leukemia cells

Glucocorticoid-induced apoptosis is preceded by G1 arrest and supposed to be up-regulated by polyamine-depletion, which also induces G1, arrest. In CEM leukemia cells, dexamethasone showed an antileukemic effect by inducing G1 arrest and apoptosis. DFMO, which depleted cellular polyamines by inhibiting ornithine decarboxylase, induced G1 arrest but without apoptosis, though it enhanced dexamethasone-induced G1 arrest and apoptosis. The G1 arrest was associated with hypophosphorylation of pRb. Dexamethasone inhibited the increase of mutated p53 expression but had little effect on p2Wafl/Cip1 expression. The p27Kip1, level was increased by dexamethasone or and DFMO in line with the kinetics of G1 arrest. Therefore, the up-regulation of dexamethasone-induced apoptosis by polyamine-depletion may be associated with additive down-regulation of G1 progression via the p27Kip1-pRb pathtway.

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.

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.

Characterization of simian malarial parasite (Plasmodium knowlesi)-induced putrescine transport in rhesus monkey erythrocytes. A novel putrescine conjugate arrests in vitro growth of simian malarial parasite (Plasmodium knowlesi) and cures multidrug resistant murine malaria (Plasmodium yoelii) infection in vivo

A stage-dependent increase in the level of putrescine, spermidine, and spermine during intraerythrocytic growth of Plasmodium knowlesi in rhesus monkey erythrocytes was observed. Further, intraerythrocytic P. knowlesi-induced putrescine influx was found in trophozoite stage-infected erythrocytes and process was time- and temperature-dependent and showed saturable kinetics. Characteristics of induced putrescine influx appears in infected erythrocytes to be close to the normal erythrocytes in terms of affinity of putrescine to the putrescine transporter (Km 34.6 +/- 3.8 microM as normal erythrocytes and Km 37.2 +/- 5.2 microM in infected erythrocytes). However, the difference involves the significant increase in the putrescine influx rate after infection (Vmax = 4.21 nmol/min/10(10) normal erythrocytes, compared with 11.6 nmol/min/10(10) infected erythrocytes). Energy dependence, involvement of -SH group, and noninterference by amino acid, spermidine, and spermine in the putrescine influx process clearly demonstrate the presence of a distinct transporter for putrescine in infected erythrocytes. A putrescine conjugate N1,N4-bis(7-chloroquinoline-4-yl)butane-1, 4-diamine (BCBD) was synthesized, which inhibits the putrescine influx in the P. knowlesi infected erythrocytes (Ki of 43.2 microM) as well as in vitro growth of P. knowlesi (IC50 value, 7.64 +/- 0.97 ng/ml BCBD, 10.8 +/- 0.45 ng/ml chloroquine). Addition of exogenous polyamines failed to reverse the inhibitory effect of BCBD in vitro. Administration of BCBD (24 mg/kg body weight, intraperitoneal, twice a day for 4 days) cured the Swiss mice infected with multidrug-resistant infection of Plasmodium yoelii. Therefore, inhibition of putrescine transport in malaria-infected erythrocytes offers a lead in the search of a new class of chemotherapeutic molecules against malaria.

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.

Growth-independent induction of spermidine transport by IL-4 and IL-13 in ZR-75-1 human breast cancer cells

Polyamine transport is strongly induced by insulin and estradiol (E2) in ZR-75-1 human breast cancer cells. Because signal transduction mechanisms of insulin and interleukin-4 (IL-4) partly overlap, we have compared the ability of these agents as well as that of interleukin-13 (IL-13), a cytokine that often mimics IL-4, to modulate spermidine transport in these cells. In the presence of E2, insulin increased DNA content and the rate of [3H]spermidine uptake by 2.1- and 3.7-fold, respectively, after an 8-day incubation, whereas the sole addition of IL-4 caused a quantitatively similar induction of [3H]spermidine uptake while leaving cell growth unaffected. No comparable induction of spermidine transport was observed with interleukins-1 alpha and -6, and the effect of IL-4 was not additive to that elicited by insulin plus E2. IL-4 and IL-13 stimulated [3H]spermidine uptake to a comparable extent, with half-maximal effects observed at 80 and 400 pg/ml, respectively. Interferon-gamma inhibited IL-4- and IL-13-dependent spermidine uptake to a much greater extent than basal or insulin-induced transport of the polyamine. IL-4 and IL-13 increased the Vmax and K(m) of [3H]spermidine uptake by about 4- and 2.5-fold, respectively. Na(+)-dependent amino acid uptake was increased by insulin but not by IL-4 or IL-13, indicating that the cytokines do not induce a general increase in membrane transport activity. IL-4 and IL-13 did not interfere with feedback inhibition of polyamine uptake, and only modestly decreased polyamine content after prolonged incubation, suggesting that these cytokines stimulate spermidine uptake by increasing total transport capacity rather than by repressing and endogenous inhibitor.

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.

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.