Intracellular putrescine and spermidine deprivation induces increased uptake of the natural polyamines and methylglyoxal bis(guanylhydrazone)

Polyamines regulate cell fate by altering the activity of histone-modifying enzymes

Polyamines are polycationic alkyl-amines abundant in proliferating stem and cancer cells. How these metabolites influence numerous cellular functions remains unclear. Here we show that polyamine levels decrease during differentiation and that inhibiting polyamine synthesis leads to a differentiated-like cell state. Polyamines concentrate in the nucleus and are further enriched in the nucleoli of cells in culture and in vivo . Loss of polyamines drives changes in chromatin accessibility that correlate with altered histone post-translational modifications. Polyamines interact electrostatically with DNA on the nucleosome core, stabilizing histone tails in conformations accessible to modifying enzymes. These data reveal a mechanism by which an abundant metabolite influences chromatin structure and function in a non-sequence specific manner, facilitating chromatin remodeling during reprogramming and limiting it during fate commitment.

Dual targeting of polyamine synthesis and uptake in diffuse intrinsic pontine gliomas

Diffuse intrinsic pontine glioma (DIPG) is an incurable malignant childhood brain tumor, with no active systemic therapies and a 5-year survival of less than 1%. Polyamines are small organic polycations that are essential for DNA replication, translation and cell proliferation. Ornithine decarboxylase 1 (ODC1), the rate-limiting enzyme in polyamine synthesis, is irreversibly inhibited by difluoromethylornithine (DFMO). Herein we show that polyamine synthesis is upregulated in DIPG, leading to sensitivity to DFMO. DIPG cells compensate for ODC1 inhibition by upregulation of the polyamine transporter SLC3A2. Treatment with the polyamine transporter inhibitor AMXT 1501 reduces uptake of polyamines in DIPG cells, and co-administration of AMXT 1501 and DFMO leads to potent in vitro activity, and significant extension of survival in three aggressive DIPG orthotopic animal models. Collectively, these results demonstrate the potential of dual targeting of polyamine synthesis and uptake as a therapeutic strategy for incurable DIPG.

Upregulation of Polyamine Transport in Human Colorectal Cancer Cells

Polyamines are essential growth factors that have a positive role in cancer cell growth. Their metabolic pathway and the diverse enzymes involved have been studied in depth in multiple organisms and cells. Polyamine transport also contributes to the intracellular polyamine content but this is less well-studied in mammalian cells. As the polyamine transporters could provide a means of selective drug delivery to cancer cells, a greater understanding of polyamine transport and its regulation is needed. In this study, transport of polyamines and polyamine content was measured and the effect of modulating each was determined in human colorectal cancer cells. The results provide evidence that upregulation of polyamine transport depends on polyamine depletion and on the rate of cell growth. Polyamine transport occurred in all colorectal cancer cell lines tested but to varying extents. The cell lines with the lowest basal uptake showed the greatest increase in response to polyamine depletion. Kinetic parameters for putrescine and spermidine suggest the existence of two separate transporters. Transport was shown to be a saturable but non-polarised process that can be regulated both positively and negatively. Using the polyamine transporter to deliver anticancer drugs more selectively is now a reality, and the ability to manipulate the polyamine transport process increases the possibility of using these transporters therapeutically.

Serendipitous Discovery of Leucine and Methionine Depletion Agents during the Search for Polyamine Transport Inhibitors

Targeting polyamine metabolism is a proven anticancer strategy. Cancers often escape the polyamine biosynthesis inhibitors by increased polyamine import. Therefore, there is much interest in identifying polyamine transport inhibitors (PTIs) to be used in combination therapies. In a search for new PTIs, we serendipitously discovered a LAT-1 efflux agonist, which induces intracellular depletion of methionine, leucine, spermidine, and spermine, but not putrescine. Because S-adenosylmethioninamine is made from methionine, a loss of intracellular methionine leads to an inability to biosynthesize spermidine, and spermine. Importantly, we found that this methionine-depletion approach to polyamine depletion could not be rescued by exogenous polyamines, thereby obviating the need for a PTI. Using ³H-leucine (the gold standard for LAT-1 transport studies) and JPH-203 (a specific LAT-1 inhibitor), we showed that the efflux agonist did not inhibit the uptake of extracellular leucine but instead facilitated the efflux of intracellular leucine pools.

Alpha-Difluoromethylornithine, an Irreversible Inhibitor of Polyamine Biosynthesis, as a Therapeutic Strategy against Hyperproliferative and Infectious Diseases

The fluorinated ornithine analog α-difluoromethylornithine (DFMO, eflornithine, ornidyl) is an irreversible suicide inhibitor of ornithine decarboxylase (ODC), the first and rate-limiting enzyme of polyamine biosynthesis. The ubiquitous and essential polyamines have many functions, but are primarily important for rapidly proliferating cells. Thus, ODC is potentially a drug target for any disease state where rapid growth is a key process leading to pathology. The compound was originally discovered as an anticancer drug, but its effectiveness was disappointing. However, DFMO was successfully developed to treat African sleeping sickness and is currently one of few clinically used drugs to combat this neglected tropical disease. The other Food and Drug Administration (FDA) approved application for DFMO is as an active ingredient in the hair removal cream Vaniqa. In recent years, renewed interest in DFMO for hyperproliferative diseases has led to increased research and promising preclinical and clinical trials. This review explores the use of DFMO for the treatment of African sleeping sickness and hirsutism, as well as its potential as a chemopreventive and chemotherapeutic agent against colorectal cancer and neuroblastoma.

A Novel Polyamine-Targeted Therapy for BRAF Mutant Melanoma Tumors

Mutant serine/threonine protein kinase B-Raf (BRAF) protein is expressed in over half of all melanoma tumors. Although BRAF inhibitors (BRAFi) elicit rapid anti-tumor responses in the majority of patients with mutant BRAF melanoma, the tumors inevitably relapse after a short time. We hypothesized that polyamines are essential for tumor survival in mutant BRAF melanomas. These tumors rely on both polyamine biosynthesis and an upregulated polyamine transport system (PTS) to maintain their high intracellular polyamine levels. We evaluated the effect of a novel arylpolyamine (AP) compound that is cytotoxic upon cellular entry via the increased PTS activity of melanoma cells with different BRAF mutational status. Mutant BRAF melanoma cells demonstrated greater PTS activity and increased sensitivity to AP compared to wild type BRAF (BRAF^WT) melanoma cells. Treatment with an inhibitor of polyamine biosynthesis, α-difluoromethylornithine (DFMO), further upregulated PTS activity in mutant BRAF cells and increased their sensitivity to AP. Furthermore, viability assays of 3D spheroid cultures of mutant BRAF melanoma cells demonstrated greater resistance to the BRAFi, PLX4720, compared to 2D monolayer cultures. However, co-treatment with AP restored the sensitivity of melanoma spheroids to PLX4720. These data indicate that mutant BRAF melanoma cells are more dependent on the PTS compared to BRAF^WT melanoma cells, resulting in greater sensitivity to the PTS-targeted cytotoxic AP compound.

Polyamine-DNA Interactions: Possible Site of New Cancer Chemotherapeutic Intervention

The effects of polyamines on the structure of nucleic acids in cell-free systems and in cell culture systems are reviewed. Evidence suggests that polyamine depletion inhibits cell growth and may cause conformational changes in DNA. These effects may be exploited to cause changes in the action of drugs and may be used to advantage in combination treatment protocols. A discussion of theoretical models of the interactions, physicochemical evidence for conformational changes, and the effects of anticancer drugs in cells depleted of polyamines is presented.

Bidirectional fluxes of spermine across the mitochondrial membrane

The polyamine spermine is transported into the mitochondrial matrix by an electrophoretic mechanism having as driving force the negative electrical membrane potential (ΔΨ). The presence of phosphate increases spermine uptake by reducing ΔpH and enhancing ΔΨ. The transport system is a specific uniporter constituted by a protein channel exhibiting two asymmetric energy barriers with the spermine binding site located in the energy well between the two barriers. Although spermine transport is electrophoretic in origin, its accumulation does not follow the Nernst equation for the presence of an efflux pathway. Spermine efflux may be induced by different agents, such as FCCP, antimycin A and mersalyl, able to completely or partially reduce the ΔΨ value and, consequently, suppress or weaken the force necessary to maintain spermine in the matrix. However this efflux may also take place in normal conditions when the electrophoretic accumulation of the polycationic polyamine induces a sufficient drop in ΔΨ able to trigger the efflux pathway. The release of the polyamine is most probably electroneutral in origin and can take place in exchange with protons or in symport with phosphate anion. The activity of both the uptake and efflux pathways induces a continuous cycling of spermine across the mitochondrial membrane, the rate of which may be prominent in imposing the concentrations of spermine in the inner and outer compartment. Thus, this event has a significant role on mitochondrial permeability transition modulation and consequently on the triggering of intrinsic apoptosis.

A novel assay platform for the detection of translation modulators of spermidine/spermine acetyltransferase

Spermidine/spermine-N1-acetyltransferase (SSAT) is a mitochondrial-localized enzyme that is highly inducible and tightly controlled and is the rate-limiting enzyme in polyamine catabolism. It is known that SSAT is induced when polyamine level increases. Although multiple mechanisms have been implicated, translational control is thought to be paramount. Previous studies with transgenic and knockout mice suggested that for certain human conditions, the modulation of SSAT levels could offer therapeutic benefits. Besides polyamines and their analogs, certain stimuli can increase SSAT levels, suggesting that the development of reporters for high throughput screening can lead to the identification of novel pharmacophores that can modulate SSAT translation. Here we report the development and validation of a luciferase-based biosensor system for the identification of compounds that are able to either promote or prevent the translation of SSAT. The system uses HEK293T cells transfected with a construct composed of SSAT mRNA modified to lack upstream open reading frame (uORF) function, is mutated to reduce translational repression and is linked with luciferase. As a proof of principle of the utility of the SSAT translation sensor, we screened the Prestwick drug library (1,200 FDA Approved compounds). The library contained 15 compounds that activated SSAT translation by at least 40% more than the basal expression, but none exceeded the positive control N1, N11-diethylnorspermine. On the other hand, 38 compounds were found to strongly inhibit SSAT translation. We conclude that this biosensor can lead to the identification of novel pharmacophores that are able to modulate the translation of SSAT.

Involvement of polyamines in iron(III) transport in human intestinal Caco-2 cell lines

Natural polyamines such as putrescine (Put), spermidine (Spd), and spermine (Spm), which are present in the human diet in large amounts, associated with their active transporter, are assumed to play a role in non-heme iron uptake and iron bioavailability from nutrients. Enterocytes and hepatocytes play pivotal roles in the regulation of body iron homeostasis. In this study, we report the effects of natural polyamines on iron transport in the Caco-2 cell line. In enterocyte-like Caco-2 cells, polyamines did not significantly modulate the transepithelial iron flux across the cell monolayer cultured on permeable membranes. In contrast, Spd, Spm, and to a lesser extent, Put were shown to activate Caco-2 cell iron uptake and to induce an increase in the ferritin level. This iron co-transport in enterocytes, which involved an interaction between iron and polyamine then cell uptake of the polyamine-iron complexes by the polyamine transport system, was more pronounced in proliferating than in differentiated Caco-2 cells. Moreover, it was observed at physiological concentrations of both polyamines and iron. It could thus play a role in the rapid renewal of enterocytes. These data suggest the involvement of polyamines as components of the pool of transferrin-independent iron-chelating vectors. Further investigations are needed to demonstrate their biological relevance in physiological situations.

Polyamine uptake by the intraerythrocytic malaria parasite, Plasmodium falciparum

Polyamines and the enzymes involved in their biosynthesis are present at high levels in rapidly proliferating cells, including cancer cells and protozoan parasites. Inhibition of polyamine biosynthesis in asexual blood-stage malaria parasites causes cytostatic arrest of parasite development under in vitro conditions, but does not cure infections in vivo. This may be due to replenishment of the parasite's intracellular polyamine pool via salvage of exogenous polyamines from the host. However, the mechanism(s) of polyamine uptake by the intraerythrocytic parasite are not well understood. In this study, the uptake of the polyamines, putrescine and spermidine, into Plasmodium falciparum parasites functionally isolated from their host erythrocyte was investigated using radioisotope flux techniques. Both putrescine and spermidine were taken up into isolated parasites via a temperature-dependent process that showed cross-competition between different polyamines. There was also some inhibition of polyamine uptake by basic amino acids. Inhibition of polyamine biosynthesis led to an increase in the total amount of putrescine and spermidine taken up from the extracellular medium. The uptake of putrescine and spermidine by isolated parasites was independent of extracellular Na(+) but increased with increasing external pH. Uptake also showed a marked dependence on the parasite's membrane potential, decreasing with membrane depolarization and increasing with membrane hyperpolarization. The data are consistent with polyamines being taken up into the parasite via an electrogenic uptake process, energised by the parasite's inwardly negative membrane potential.

Increased toxicity of a trinuclear Pt-compound in a human squamous carcinoma cell line by polyamine depletion

BACKGROUND

Mononuclear platinum anticancer agents hold a pivotal place in the treatment of many forms of cancers, however, there is a potential to improve response to evade resistance development and toxic side effects. BBR3464 is a promising trinuclear platinum anticancer agent, which is a polyamine mimic. The aim was to investigate the influence of polyamine pool reduction on the cytotoxic effects of the trinuclear platinum complex BBR3464 and cisplatin. Polyamine pool reduction was achieved by treating cells with either the polyamine biosynthesis inhibitor α-difluoromethylornithine (DFMO) or the polyamine analogue N1,N11-diethylnorspermine (DENSPM).

METHODS

A human squamous cell carcinoma cell line, LU-HNSCC-4, established from a primary head and neck tumour was used to evaluate cellular effects of each drug alone or combinations thereof. High-performance liquid-chromatography was used to quantify intracellular polyamine contents. Inductively coupled mass spectroscopy was used to quantify intracellular platinum uptake. Cells were exposed to DFMO or DENSPM during 48 h at concentrations ranging from 0 to 5 mM or 0 to 10 μM, respectively. Thereafter, non-treated and treated cells were exposed to cisplatin or BBR3464 during 1 h at concentrations ranging from 0 to 100 μM. A 96-well assay was used to determine cytotoxicity after five days after treatment.

RESULTS

The cytotoxic effect of BBR3464 on LU-HNSCC-4 cells was increased after cells were pre-treated with DENSPM or DFMO, and the interaction was found to be synergistic. In contrast, the interaction between cisplatin and DFMO or DENSPM was near-additive to antagonistic. The intracellular levels of the polyamines putrescine and spermidine were decreased after treatment with DFMO, and treatment with DENSPM resulted in an increase in putrescine level and concomitant decrease in spermidine and spermine levels. The uptake of BBR3464 was significantly increased after pre-treatment of the cells with DFMO, and varied dependent on the concentration of DENSPM. The uptake of cisplatin was unchanged.

CONCLUSIONS

Taken together, these results demonstrate that combinations of polyamine synthesis inhibitors with BBR3464 appear to be a promising approach to enhance the anticancer activity against HSCC.

Polyamine-regulated translation of spermidine/spermine-N1-acetyltransferase

Rapid synthesis of the polyamine catabolic enzyme spermidine/spermine-N(1)-acetyltransferase (SSAT) in response to increased polyamines is an important polyamine homeostatic mechanism. Indirect evidence has suggested that there is an important control mechanism involving the release of a translational repressor protein that allows the immediate initiation of SSAT protein synthesis without RNA transcription, maturation, or translocation. To identify a repressor protein, we used a mass spectroscopy-based RNA-protein interaction system and found six proteins that bind to the coding region of SSAT mRNA. Individual small interfering RNA (siRNA) experiments showed that nucleolin knockdown enhances SSAT translation. Nucleolin exists in several isoforms, and we report that the isoform that binds to SSAT mRNA undergoes autocatalysis in the presence of polyamines, a result suggesting that there is a negative feedback system that helps control the cellular content of polyamines. Preliminary molecular interaction data show that a nucleolin isoform binds to a 5' stem-loop of the coding region of SSAT mRNA. The glycine/arginine-rich C terminus of nucleolin is required for binding, and the four RNA recognition motif domains are included in the isoform that blocks SSAT translation. Understanding SSAT translational control mechanisms has the potential for the development of therapeutic strategies against cancer and obesity.

Regulation of polyamine metabolism by translational control

Polyamines are low molecular weight, positively charged compounds that are ubiquitous in all living cells. They play a crucial role in many biochemical processes including regulation of transcription and translation, modulation of enzyme activities, regulation of ion channels and apoptosis. A strict balance between synthesis, catabolism and excretion tightly controls the cellular concentration of polyamines. The concentrations of rate-limiting enzymes in the polyamine synthesis and degradation pathways are regulated at different levels, including transcription, translation and degradation. Polyamines can modulate the translation of most of the enzymes required for their synthesis and catabolism through feedback mechanisms that are unique for each enzyme. Translational control is associated with cis-acting and trans-acting factors that can be influenced by the concentration of polyamines through mechanisms that are not completely understood. In this review, we present an overview of the translational control mechanisms of the proteins in the polyamine pathway, including ornithine decarboxylase (ODC), ODC antizyme, S-adenosylmethionine decarboxylase and spermidine/spermine N(1) acetyltransferase, highlighting the areas where more research is needed. A better understanding of the translational control of these enzymes would offer the possibility of a novel pharmacological intervention against cancer and other diseases.

Levels of rectal mucosal polyamines and prostaglandin E2 predict ability of DFMO and sulindac to prevent colorectal adenoma

BACKGROUND & AIMS

Combination of polyamine and prostaglandin E2 (PGE2)-synthesis inhibitors reduced the risk of colorectal adenoma (CRA) by 70% in patients who received polypectomies. We studied effects of the combination of difluoromethylornithine (DFMO) and sulindac on biomarkers and investigated factors that modify their efficacy.

METHODS

We analyzed rectal mucosal levels of polyamines (spermidine, spermine, and putrescine) and PGE2, treatment regimens, and risk of CRA in 267 participants of a phase IIb/III chemoprevention trial of DFMO/sulindac.

RESULTS

In the group that received DFMO/sulindac, spermidine-to-spermine ratio (Spd:Spm) in rectal mucosa decreased between baseline and 12- and 36-month follow-up examinations (0.30, 0.23, and 0.24, respectively; P < .001 for both comparisons to baseline). Putrescine levels decreased between baseline and 12 months (0.46 vs 0.15 nmol/mg protein; P < .001) but rebounded between 12 and 36 months (0.15 vs 0.36 nmol/mg protein; P = .001). PGE2 levels did not change, although aspirin use was significantly associated with lower baseline levels of PGE2. No significant associations were observed between changes in biomarker levels and efficacy. However, drug efficacy was greatest in subjects with low Spd:Spm and high PGE2 at baseline; none of these subjects, versus 39% of those given placebo, developed CRA (P < .001). Efficacy was lowest in subjects with high Spd:Spm and low PGE2 at baseline; 28% developed CRA, compared with 36% of patients given placebo (P = .563).

CONCLUSIONS

A combination of DFMO and sulindac significantly suppressed production of rectal mucosal polyamines but not PGE2. No relationship was found between changes in biomarker levels and response. However, baseline biomarker levels modified the effect of DFMO/sulindac for CRA prevention.

Lipophilic lysine-spermine conjugates are potent polyamine transport inhibitors for use in combination with a polyamine biosynthesis inhibitor

Cancer cells can overcome the ability of polyamine biosynthesis inhibitors to completely deplete their internal polyamines by the importation of polyamines from external sources. This paper discusses the development of a group of lipophilic polyamine analogues that potently inhibit the cellular polyamine uptake system and greatly increase the effectiveness of polyamine depletion when used in combination with DFMO, a well-studied polyamine biosynthesis inhibitor. The attachment of a length-optimized C(16) lipophilic substituent to the epsilon-nitrogen atom of an earlier lead compound, D-Lys-Spm (5), has produced an analogue, D-Lys(C(16)acyl)-Spm (11) with several orders of magnitude more potent cell growth inhibition on a variety of cultured cancer cell types including breast (MDA-MB-231), prostate (PC-3), melanoma (A375), and ovarian (SK-OV-3), among others. These results are discussed in the context of a possible membrane-catalyzed interaction with the extracellular polyamine transport apparatus. The resulting novel two-drug combination therapy targeting cellular polyamine metabolism has shown exceptional efficacy against cutaneous squamous cell carcinomas (SCC) in a transgenic ornithine decarboxylase (ODC) mouse model of skin cancer. A majority (88%) of large, aggressive SCCs exhibited complete or nearly complete remission to this combination therapy, whereas responses to each agent alone were poor. The availability of a potent polyamine transport inhibitor allows, for the first time, for a real test of the hypothesis that starving cells of polyamines will lead to objective clinical response.

New insights into the design of inhibitors of human S-adenosylmethionine decarboxylase: studies of adenine C8 substitution in structural analogues of S-adenosylmethionine

S-adenosylmethionine decarboxylase (AdoMetDC) is a critical enzyme in the polyamine biosynthetic pathway and depends on a pyruvoyl group for the decarboxylation process. The crystal structures of the enzyme with various inhibitors at the active site have shown that the adenine base of the ligands adopts an unusual syn conformation when bound to the enzyme. To determine whether compounds that favor the syn conformation in solution would be more potent AdoMetDC inhibitors, several series of AdoMet substrate analogues with a variety of substituents at the 8-position of adenine were synthesized and analyzed for their ability to inhibit hAdoMetDC. The biochemical analysis indicated that an 8-methyl substituent resulted in more potent inhibitors, yet most other 8-substitutions provided no benefit over the parent compound. To understand these results, we used computational modeling and X-ray crystallography to study C(8)-substituted adenine analogues bound in the active site.

HIV-Tat protein transduction domain specifically attenuates growth of polyamine deprived tumor cells

Polyamines are essential for tumor cell growth, and the polyamine pathway represents an attractive target for cancer treatment. Several polyamine transport proteins have been cloned and characterized in bacteria and yeast cells; however, the mechanism of polyamine entry into mammalian cells remains poorly defined, although a role for proteoglycans has been suggested. Here, we show that the HIV-Tat transduction peptide, which is known to enter cells via a proteoglycan-dependent pathway, efficiently inhibits polyamine uptake. Polyamine uptake-deficient mutant cells with intact proteoglycan biosynthesis (CHO MGBG) displayed unperturbed HIV-Tat uptake activity compared with wild-type cells, supporting the notion that HIV-Tat peptide interferes with polyamine uptake via competition for proteoglycan binding sites rather than a putative downstream transporter. HIV-Tat specifically inhibited growth of human carcinoma cells made dependent on extracellular polyamines by treatment with the polyamine biosynthesis inhibitor alpha-difluoromethylornithine; accordingly, the Tat peptide prevented intracellular accumulation of exogenous polyamines. Moreover, combined treatment with alpha-difluoromethylornithine and HIV-Tat efficiently blocked tumor growth in an experimental mouse model. We conclude that HIV-Tat transduction domain and polyamines enter cells through a common pathway, which can be used to target polyamine-dependent tumor growth in the treatment of cancer.

Targeting polyamine metabolism: a viable therapeutic/preventative solution for cancer?

The polyamine pathway has been identified as a target for the design of new antiproliferative drugs, due to the strong positive relationship between intracellular polyamine content and cell, particularly cancer cell growth. A number of single enzyme inhibitors have been synthesised against the two key biosynthetic enzymes, ornithine decarboxylase and S-adenosylmethionine decarboxylase, but their success in the clinic has been limited due to incomplete polyamine depletion and induction of compensatory mechanisms that counteract the effects of enzyme inhibition. Overall, clinical trials of these agents as chemotherapeutic drugs have proved disappointing, with either little clinical efficacy or unacceptable toxicity. The polyamine analogues provide an alternative strategy that shows promise, particularly against diseases other than cancer. Combination of the polyamine inhibitors with classic cytotoxic agents may be an alternative strategy that is showing some promise, at least in vitro. An avenue that is, however, presently more promising is the use of polyamine inhibitors or analogues as chemopreventative agents against a range of human cancers. It seems likely that the future use of these drugs will be in disease prevention rather than treatment. With regard to the newer agents with restricted conformation that are now undergoing clinical trials, it is too early to say whether they will be chemotherapeutic and/or chemopreventative. This article focuses on the clinical use and responses to inhibitors of polyamine metabolism.

Polyamine analogues – an update

The polyamines are growth factors in both normal and cancer cells. As the intracellular polyamine content correlates positively with the growth potential of that cell, the idea that depletion of polyamine content will result in inhibition of cell growth and, particularly tumour cell growth, has been developed over the last 15 years. The polyamine pathway is therefore a target for development of rationally designed, antiproliferative agents. Following the lessons from the single enzyme inhibitors (alpha-difluoromethylornithine DFMO), three generations of polyamine analogues have been synthesised and tested in vitro and in vivo. The analogues are multi-site inhibitors affecting multiple reactions in the pathway and thus prevent the up-regulation of compensatory reactions that have been the downfall of DFMO in anticancer chemotherapy. Although the initial concept was that the analogues may provide novel anticancer drugs, it now seems likely that the analogues will have wider applications in diseases involving hyperplasia.

De novo ceramide biosynthesis is associated with resveratrol-induced inhibition of ornithine decarboxylase activity

Previous studies could demonstrate, that the naturally occuring polyphenol resveratrol inhibits cell growth of colon carcinoma cells at least in part by inhibition of protooncogene ornithine decarboxylase (ODC). The objective of this study was to provide several lines of evidence suggesting that the induction of ceramide synthesis is involved in this regulatory mechanisms. Cell growth was determined by BrdU incorporation and crystal violet staining. Ceramide concentrations were detected by HPLC-coupled mass-spectrometry. Protein levels were examined by Western blot analysis. ODC activity was assayed radiometrically measuring [(14)CO(2)]-liberation. A dominant-negative PPARgamma mutant was transfected in Caco-2 cells to suppress PPARgamma-mediated functions. Antiproliferative effects of resveratrol closely correlate with a dose-dependent increase of endogenous ceramides (p<0.001). Compared to controls the cell-permeable ceramide analogues C2- and C6-ceramide significantly inhibit ODC-activity (p<0.001) in colorectal cancer cells. C6-ceramide further diminished protein levels of protooncogenes c-myc (p<0.05) and ODC (p<0.01), which is strictly related to the ability of ceramides to inhibit cell growth in a time- and dose-dependent manner. These results were further confirmed using inhibitors of sphingolipid metabolism, where only co-incubation with a serine palmitoyltransferase (SPT) inhibitor could significantly counteract resveratrol-mediated actions. These data suggest that the induction of ceramide de novo biosynthesis but not hydrolysis of sphingomyelin is involved in resveratrol-mediated inhibition of ODC. In contrast to the regulation of catabolic spermidine/spermine acetyltransferase by resveratrol, inhibitory effects on ODC occur PPARgamma-independently, indicating independent pathways of resveratrol-action. Due to our findings resveratrol could show great chemopreventive and therapeutic potential in the treatment of colorectal cancers.

Polyamine modulation of iron uptake in CHO cells

Polyamines are ubiquitous molecules, which, like iron, are essential for cell growth. All eukaryotic cells are equipped with a specific polyamine transport system (PTS). Polyamines have primary and secondary amino groups which chelate bivalent metal cations such as Fe and Cu. In the present study, we investigated the potential contribution of naturally occurring polyamines and their active transport system to iron uptake. In presence of subtoxic Fe(III) (10microM), treatment of CHO cells with spermine, and to a lesser extent with spermidine (10-100microM), resulted in a marked cytotoxic effect. This cytotoxicity was prevented by the addition of an iron-chelator, deferioxamine, and was not observed in CHO-MG cells, a mutant cell line devoid of polyamine transport activity. Experiments using 14C-polyamines and 55Fe(III) revealed that these toxic effects were related to polyamine-modulation of iron uptake, and were dependent on the presence of the active PTS. These results demonstrated active uptake of polyamine-iron complexes via the PTS. The number of amino groups affected the efficacy of the studied natural polyamines to transport iron via the PTS. Spermine, a tetramine, was more efficient than the triamine spermidine. Co-transport of iron by the diamine putrescine was not observed. These results demonstrate that the cell polyamine transport system is a potential cell entry pathway for iron. The studied polyamines, spermine and spermidine, may be components of the pool of transferrin-independent iron-chelating vectors, which have recently attracted the attention of many investigators.

A perspective of polyamine metabolism

Polyamines are essential for the growth and function of normal cells. They interact with various macromolecules, both electrostatically and covalently and, as a consequence, have a variety of cellular effects. The complexity of polyamine metabolism and the multitude of compensatory mechanisms that are invoked to maintain polyamine homoeostasis argue that these amines are critical to cell survival. The regulation of polyamine content within cells occurs at several levels, including transcription and translation. In addition, novel features such as the +1 frameshift required for antizyme production and the rapid turnover of several of the enzymes involved in the pathway make the regulation of polyamine metabolism a fascinating subject. The link between polyamine content and human disease is unequivocal, and significant success has been obtained in the treatment of a number of parasitic infections. Targeting the polyamine pathway as a means of treating cancer has met with limited success, although the development of drugs such as DFMO (alpha-difluoromethylornithine), a rationally designed anticancer agent, has revolutionized our understanding of polyamine function in cell growth and provided 'proof of concept' that influencing polyamine metabolism and content within tumour cells will prevent tumour growth. The more recent development of the polyamine analogues has been pivotal in advancing our understanding of the necessity to deplete all three polyamines to induce apoptosis in tumour cells. The current thinking is that the polyamine inhibitors/analogues may also be useful agents in the chemoprevention of cancer and, in this area, we may yet see a revival of DFMO. The future will be in adopting a functional genomics approach to identifying polyamine-regulated genes linked to either carcinogenesis or apoptosis.

Molecular requirements for targeting the polyamine transport system. Synthesis and biological evaluation of polyamine-anthracene conjugates

A series of nine N(1)-(9-anthracenylmethyl)tetraamines (e.g., Ant-4,4,4-tetraamine) were synthesized and evaluated for cytotoxicity in L1210, alpha-difluoromethylornithine (DFMO)-treated L1210, Chinese hamster ovary (CHO), and CHO-MG cell lines. Surprisingly, the 3,3,4- and 3,4,3-tetraamine motifs had the same or decreased cytotoxicity in DFMO-treated L1210 cells, whereas the rest of the tetraamine systems were usually more cytotoxic and gave lower IC(50) values in this treated cell line. The most sensitive derivatives to DFMO treatment were the Ant-4,4,3- and Ant-4,4,4-tetraamine analogues, which were 7 and 5 times more cytotoxic in DFMO-treated L1210 cells, respectively. K(i) values for each of the anthracenylmethyl(Ant)-polyamine conjugates were determined in L1210 cells and revealed that these systems are high-affinity ligands for the polyamine transporter (PAT). Mixed results were observed in the CHO and CHO-MG assays. The 4,4,4- and 5,4,4-tetraamine motifs were 3 times more toxic to CHO cells with active polyamine transporters. For example, the Ant-4,4,4-tetraamine conjugate displayed IC(50) values of 11 microM in CHO cells and 33 microM in CHO-MG cells, a PAT-deficient cell line. This suggested that these derivatives used the PAT in part to access cells. However, most of the other tetraamine derivatives had similar potencies in both the CHO and CHO-MG cell lines. In terms of vector design, higher affinity for the PAT (lower K(i) values) did not translate into higher potency for the tetraamine conjugate. In contrast, the related triamine systems, which had micromolar K(i) values in L1210 cells, were more efficacious and selective. In one case, the 4,4-triamine motif imparted 150-fold higher potency in CHO cells than the CHO-MG mutant. A deconvolution microscopy study in A375 melanoma cells revealed a rapid internalization of the Ant-4,4-triamine as fluorescent vesicles, whereas the Ant-4,4,4-tetraamine remained mostly at the cell surface. These findings help define the key characteristics required for selective delivery of polyamine-drug conjugates into cell types with active polyamine transporters.

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.

(Z)-1,4-diamino-2-butene as a vector of boron, fluorine, or iodine for cancer therapy and imaging: synthesis and biological evaluation

Polyamine vectors are attractive for tumor targeting. We envisaged (Z)-1,4-diamino-2-butene (Z-DAB), an unsaturated analogue of putrescine as vector of (10)B, (18)F and (131)I for boron neutron capture therapy (BNCT), and tumor imaging by positron emission tomography or scintigraphy respectively. In the present work, the synthesis and characterization of new derivatives of Z-DAB were reported. Z-DAB was actively transported in cells via the polyamine transport system and converted into the spermidine analogue.(E)-2-iodo-1,4-diamino-2-butene (E-I-DAB) was not taken up by the polyamine transport system and may not be suitable for tumor imaging. In contrast, (Z)-2-[4-(5,5-dimethyl-dioxaborinan-2-yl)phenyl]methyl-1,4-diamino-2-butene (Z-4-Bbz-DAB) was a substrate of the transport system and allowed significant boron accumulation in 3LL cells. Its potential in BNCT will be evaluated.

Amino acid/spermine conjugates: polyamine amides as potent spermidine uptake inhibitors

In this paper we describe the synthesis and characterization of a series of simple spermine/amino acid conjugates, some of which potently inhibit the uptake of spermidine into MDA-MB-231 breast cancer cells. The presence of an amide in the functionalized polyamine appeared to add to the affinity for the polyamine transporter. The extensive biological characterization of an especially potent analogue from this series, the Lys-Spm conjugate (31), showed this molecule will be an extremely useful tool for use in polyamine research. It was shown that the use of 31 in combination with DFMO led to a cytostatic growth inhibition of a variety of cancer cells, even when used in the presence of an extracellular source of transportable spermidine. It was furthermore shown that this combination effectively reduced the cellular levels of putrescine and spermidine while not affecting the levels of spermine. These facts together with the nontoxic nature of 31 make it a novel lead for further anticancer development.

Nonsteroidal anti-inflammatory drugs stimulate spermidine/spermine acetyltransferase and deplete polyamine content in colon cancer cells

BACKGROUND

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit colonic tumourigenesis and have an established usefulness in cancer prevention. Because polyamines are essential for neoplastic cell growth, the aim of this study was to evaluate the effect of NSAIDs (indomethacin, a nonselective COX-1 and COX-2 inhibitor) on polyamine metabolism in colon cancer cells.

METHODS

Both cell counting and thymidine incorporation into cellular DNA were used to assess colon cancer cell growth. Activities of polyamine-metabolising enzymes, polyamine content (HPLC) and ODC and c-myc protein expression (Western blot) were measured in colon cancer cells treated with indomethacin during logarithmic phase of proliferation.

RESULTS

Indomethacin impaired growth of human colon cancer cells (Caco-2 and HCT-116). As a result, ornithine decarboxylase activity and c-myc protein expression were decreased. Treatment with indomethacin induced intracellular oxidant formation in colon cancer cells significantly increased the spermidine/spermine-acetyltrasferase activity (SSAT) and enhanced polyamine acetylation and efflux from colon cancer cells. Impairment of cell growth by indomethacin could not be reversed by exogenous polyamines.

CONCLUSION

Taken together, our results suggest that NSAIDs affect polyamine metabolism in colon cancer cells by inducing SSAT activity, and that polyamine depletion in NSAID-treated colon cancer cells is mainly due to enhanced polyamine acetylation and irreversible depletion of intracellular polyamine pools.

Polyamines and colon cancer

Colorectal cancer is a major health problem in the western world and is associated with significant morbidity and mortality. Diet makes a significant contribution to the disease, with high fat, low fibre diets correlating positively with a high incidence of colorectal cancer. Intracellular polyamine concentrations and ornithine decarboxylase activity are both increased in colorectal cancer tissue and in premalignant polyps. Measurement of the polyamine content of serum and urine of individuals has been proposed as a diagnostic marker of malignancy but a number of false positives make this idea untenable. There may, however, still be a role for the measurement of urinary polyamine content as a means of monitoring the efficacy of therapy. Inhibition of polyamine metabolism by polyamine analogues or by non-steroidal anti-inflammatory drugs may be useful in the chemotherapy and/or chemoprevention of colorectal cancer. Preliminary results suggest that a low polyamine diet might be helpful as part of a health care plan for cancer patients.

Binding of spermidine and putrescine to energized liver mitochondria

The binding of spermidine and putrescine to mitochondrial membranes was studied by applying a thermodynamic model of ligand-receptor interactions developed both for equilibrium and far-from-equilibrium binding processes (V. Di Noto, L. Dalla Via, A. Toninello, and M. Vidali Macromol. Theory Simul. 5, 165-181, 1996). Results demonstrate the presence of two monocoordinated binding sites (S1 and S2) for spermidine and one monocoordinated binding site (S2) for putrescine, all exhibiting high capacity and low affinity. It is proposed that differences in the polyamines' flexibility and hydrophilicity perhaps contributes to the observed variations in their interactions with the two sites. A comparison of the binding parameters of these polyamines with those of spermine reveals differences in the specific function of the S1 and S2 sites, identified in studies of spermine binding (L. Dalla Via, V. Di Noto, D. Siliprandi, and A. Toninello Biochim. Biophys. Acta 1284, 247-252, 1996).

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.

Organic cation transporters in intestine, kidney, liver, and brain

This review focuses on sodium-independent transport systems for organic cations in small intestine, liver, kidney, and brain. The roles of P-glycoproteins (MDR) and anion transporters (OATP) in organic cation transport are reported, and two members of the new transporter family OCT are described. The OCT transporters belong to a superfamily that includes multidrug-resistance proteins, facilitative diffusion systems, and proton antiporters. They mediate electrogenic transport of small organic cations with different molecular structures, independently of sodium and proton gradients. The current knowledge of the distribution and functional properties of cloned cation transport systems and of cation transport measured in intact plasma membranes is used to postulate identical or homologous transporters in intestine, liver, kidney, and brain.

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.

Spermine binding to liver mitochondria

Non-equilibrium binding of spermine to mitochondrial membranes is studied in rat liver mitochondria by applying a new thermodynamic treatment of ligand-receptor interactions (Di Noto, V., Dalla Via, L., Toninello, A. and Vidali, M. (1996) Macromol. Theory Simul. 5, 165-181). The presence on mitochondrial membranes of two spermine binding sites, both with monocoordination, is demonstrated. The calculated binding energy is characteristic for weak interactions. The treatment allows also to evaluate the variations of the molar fraction ratio of spermine bound to sites 1 and 2 as function of total bound spermine. The possible role of the two sites is discussed.

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.

Isolation of a polyamine transport deficient cell line from the human non-small cell lung carcinoma line NCI H157

In an effort to study the mechanism underlying the observed phenotype-specific response of human lung cancer cell lines to a polyamine analogue, N1,N12-bis(ethyl)spermine(BESpm), we have isolated a BESpm resistant cell line from the BESpm-sensitive large cell lung carcinoma line NCI H157. The mutant line exhibits identical growth rates in the presence or absence of the analogue. However, the overall growth of mutant cells reaches stationary phase earlier than that of the parental cells. In contrast to the parental cells, where a superinduction of spermidine/spermine N1-acetyltransferase (SSAT) is associated with BESpm toxicity, treatment of this resistant line with BESpm did not induce SSAT mRNA or enzyme activity. BESpm treatment was not effective in depleting the intracellular polyamine pools and very low intracellular BESpm levels were detected. This BESpm resistance is not mediated by multidrug resistance (MDR) protein, since these cells maintain their sensitivity to the antineoplastic agent adriamycin. Treatment of these cells with methylglyoxal bis(guanylhydrazone) (MGBG), an AdoMetDC inhibitor which enters cell using polyamine transport system, shows no inhibition of cell growth. Our data suggest that these mutant cells are deficient in polyamine transport. Consistent with this hypothesis, exogenous polyamines did not prevent difluoromethylornithine (DFMO) induced growth inhibition in the mutant cells.

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.

Polyamines in brain tumor therapy

In the search for ways to augment current brain tumor therapies many have sought to exploit the fact that adult brain tissue is virtually lacking in cell division. This endorses a special appeal to therapeutic approaches which target the dependence on cell division for brain tumor growth. Polyamines play an essential role in the proliferation of mammalian cells and depletion results in inhibition of growth. As a result, there are investigations into the feasibility of controlling tumor growth by targeting the enzymes in polyamine metabolism with specific enzyme inhibitors. DFMO, an inhibitor of putrescine synthesis, is a cytostatic agent which in combination with tritiated radioemitters or cytotoxic agents such as, MGBG or BCNU is an effective antitumor agent, but the effectiveness of DFMO in vivo is reduced by tumor cell uptake of polyamines released into the circulation by normal cells and from gut flora or dietary sources. However, DFMO therapy combined with elimination of exogenous polyamines inhibits tumor growth but also results in body weight loss, reduced protein synthesis and evidence of toxicity. Furthermore, tumor growth recurs upon termination of treatment. In contrast, competitive polyamine analogs function in the homeostatic regulation of polyamine synthesis but fail to fulfill the requirements for growth and they continue to inhibit tumor growth for several weeks after cessation of treatment. Analogs are now in clinical trials. However, their action may be highly specific and differ from one cell type to another. We suggest that the effectiveness of polyamine based therapy would be enhanced by two approaches: local delivery by intracerebral microdialysis and tumor cell killing by internal radioemitters such as tritiated putrescine or tritiated thymidine which are taken up in increased amounts by polyamine depleted tumor cells. The growth inhibition by polyamine depletion prevents the dilution of the radioactive putrescine and thymidine. The overload of radioactivity kills the growth inhibited cells so that growth cannot recur when treatment terminates.

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.

MGBG: teaching an old drug new tricks

Methylglyoxalbisguanylhydrazone or MGBG is an agent with a unique mechanism of action (polyamine biosynthesis inhibition). MGBG was discarded in the 1960s because of severe mucositis and other toxicities. New clinical trials in the late 1970s and early 1980s utilized weekly administration and indicated MGBG had significant activity in patients with chemotherapy-refractory Hodgkin's and non-Hodgkin's lymphoma. In addition, some activity was noted in patients with head and neck, prostate, esophageal, and endometrial cancer. The toxicities on the weekly schedule were minimal and no myelosuppression was noted. Based on MGBG's spectrum of antitumor activity and its activity in severely debilitated patients, we hypothesize that MGBG may have greater antitumor activity in patients who are malnourished (possibly based on polyamine depletion). MGBG is a good candidate for treatment of AIDS-associated NHL because it has proven activity in patients with NHL which is not associated with AIDS, crosses the blood brain barrier, is non-myelosuppressive, and appears to work in patients with inanition (no polyamines available to reverse MGBG's antitumor effects). Clinical trials are ongoing to determine the activity of MGBG in AIDS-associated NHL and other diseases. Based on encouraging initial results, it appears MGBG may become part of our therapeutic armamentarium.

Characterization of a COS cell line deficient in polyamine transport

In the present study, we describe the isolation and characterization of a COS cell line deficient in polyamine uptake that may provide an important tool for the molecular cloning of polyamine transporter(s). The cells were selected by isolation for resistance against the cytotoxic agent, methylglyoxal bis(guanylhydrazone) (MGBG), which is entering the cells using the same transport system as the polyamines. The isolated cell line was capable of growing in the presence of 100 microM MGBG, which totally inhibited the growth of the wild-type cells. The transport of putrescine and spermidine was markedly decreased in the COS-MGBGr cells. The decrease in putrescine transport was mainly a result of a 14-fold decrease in Vmax, whereas the reduced spermidine uptake was due to a 3-4-fold decrease in Vmax as well as 12-fold increase in Km, indicating the existence of at least two separate transport systems. No major difference in polyamine content was seen between the parental and the COS-MGBGr cells when grown without MGBG. In the presence of MGBG, both cell lines exhibited an increase in putrescine content. Treatment with MGBG also resulted in a decrease in spermidine and spermine contents in the wild-type cells. In the COS-MGBGr cells, on the other hand, there were no statistically significant effects on the spermidine and spermine contents by MGBG treatment. In the wild-type cells, depletion of polyamines, e.g., by treatment with the ornithine decarboxylase inhibitor 2-difluoromethylornithine (DFMO), stimulated the uptake of polyamines (3-7-fold), whereas in the COS-MGBGr cells the effect of DFMO treatment on polyamine transport was only minor. In contrast to the growth-medium of the wild-type cells, large amounts of polyamines accumulated in the medium of the COS-MGBGr cells, presumably indicating that COS cells normally excrete polyamines and then salvage them using the polyamine transport system.

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.

Polyamine transport regulation by calcium and calmodulin: role of Ca(2+)-ATPase

The study was conducted on human leukemia (K 562) cells to characterize the mechanisms implicated in the regulation of the polyamine spermidine (Spd) transport process. The antagonists of calmodulin, trifluoperazine (TFP), W-7 (N-[6-aminohexyl]-5-chloro-1-naphthelenesulfonamide), or mellitin inhibited significantly polyamine Spd uptake in these cells. The translocation of calmodulin towards plasma membrane and a concomitant decrease in its contents in cytosol were directly correlated with the time course increases similar to that of Spd uptake, indicating that calmodulin is recruited towards plasma membrane during the Spd transport process. Diminution of free intracellular calcium, (Ca2+)i, by preincubating the cells in BAPTA (bis[2-amino-5-methylphenoxyl]-ethane-N,N,N',N',-tetraacetate) buffer inhibited Spd transport significantly. Addition of lanthanum (LAN), a molecule known to inhibit Ca2+ efflux via Ca(2+)-ATPase, curtailed Spd uptake by these cells. LAN inhibited Vmax, but not the Km, of Spd uptake, indicating that the former does not directly interact with the polyamine transporter; rather it regulates the transport process, probably via its action on Ca(2+)-ATPase. Calmodulin-stimulated uptake of 45Ca2+ by inside-out vesicles of K 562 cells, a measure of Ca(2+)-ATPase activity. Furthermore, addition of LAN inhibited both basal and calmodulin-stimulated activity of Ca(2+)-ATPase. Thapsigargin (THAP), a molecule known to elevate (Ca2+)i due to its action on the endoplasmic reticulum, increased Spd transport whereas addition of LAN inhibited THAP-stimulated Spd transport activity. THAP increased free (Ca2+)i in these cells, and a pre-addition of LAN to these cells curtailed the THAP-stimulated increases of (Ca2+)i concentrations. Addition of Spd brought about elevations in (Ca2+)i contents. Caffeine also increased (Ca2+)i in these cells; however, it failed to stimulate significantly the Spd uptake process, indicating that (Ca2+)i which is involved in the regulation of polyamine transport pathways does not belong to the calcium-induced calcium-release (CICR) pool. Replacement of Ca2+ from the incubation medium (i.e., 0% Ca2+) resulted in higher uptake activity as compared to that in 100% Ca2+ medium, demonstrating that in 100% Ca2+ medium the calcium efflux process is quickly compensated by calcium refilling/influx from the extracellular medium, while in 0% Ca2+ medium there is perpetual efflux of (Ca2+)i which contributes to higher Spd uptake process. The results of this study suggest that an increase in free (Ca2+)i and its release from the cells via Ca(2+)-ATPase, and concomitant activation of calmodulin, which controls Ca(2+)-pump activity, are involved in the regulation of the Spd uptake process in human leukemia cells.

TGF-beta 1 inhibits polyamine biosynthesis in K 562 leukemic cells

The present study proved that TGF-beta 1 significantly inhibited the growth of K 562 cells. The drop in cell numbers after 24 h incubation with increasing concentrations of TGF-beta 1 (0.01, 0.1, 1.0, 10.0 ng/ml) was accompanied by significant suppression of the activity of two key enzymes of polyamine biosynthesis: ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC). In contrast to ODC and SAMDC activity, TGF-beta 1 did not significantly affect the absolute concentration of spermidine and spermine in K 562 cells. We suppose that the lack of an evident drop in concentration of spermidine and spermine in spite of a significant decrease in ODC and SAMDC activity in K 562 cells exposed to TGF-beta 1 resulted from the uptake of polyamines from the extracellular space.

Induction of spermidine/spermine N1-acetyltransferase in human breast carcinoma cells. A possible role for calcium

Methylglyoxal bis(guanylhydrazone) (MGBG) a structural analogue of spermidine produced a dose-dependent induction of cytosolic spermidine/spermine N1-acetyltransferase (N1-SAT) in the human breast carcinoma cell line, T47-D. Intracellular accumulation of MGBG was found to be saturable and the drug produced characteristic effects on intracellular polyamines, decreasing spermidine and spermine content, with concomitant increases in putrescine levels. The MGBG-induced increase in N1-SAT activity was potentiated by both tetronasin, a calcium ionophore, and felodipine, a calcium channel blocking agent. Only tetronasin was an active inducer of the enzyme when used alone. Both drugs influenced intracellular MGBG content but in opposite directions: tetronasin increased MGBG content while felodipine decreased it. Therefore, the potentiation of N1-SAT induction is not simply the result of increased intracellular accumulation of MGBG but is more likely to be related to the concentration of intracellular free calcium in these cells.

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.