Selective putrescine export is regulated by insulin and ornithine in Reuber H35 hepatoma cells

c-MYC-Driven Polyamine Metabolism in Ovarian Cancer: From Pathogenesis to Early Detection and Therapy

c-MYC and its paralogues MYCN and MYCL are among the most frequently amplified and/or overexpressed oncoproteins in ovarian cancer. c-MYC plays a key role in promoting ovarian cancer initiation and progression. The polyamine pathway is a bona fide target of c-MYC signaling, and polyamine metabolism is strongly intertwined with ovarian malignancy. Targeting of the polyamine pathway via small molecule inhibitors has garnered considerable attention as a therapeutic strategy for ovarian cancer. Herein, we discuss the involvement of c-MYC signaling and that of its paralogues in promoting ovarian cancer tumorigenesis. We highlight the potential of targeting c-MYC-driven polyamine metabolism for the treatment of ovarian cancers and the utility of polyamine signatures in biofluids for early detection applications.

Current status of the polyamine research field

This chapter provides an overview of the polyamine field and introduces the 32 other chapters that make up this volume. These chapters provide a wide range of methods, advice, and background relevant to studies of the function of polyamines, the regulation of their content, their role in disease, and the therapeutic potential of drugs targeting polyamine content and function. The methodology provided in this new volume will enable laboratories already working in this area to expand their experimental techniques and facilitate the entry of additional workers into this rapidly expanding field.

A profiling platform for the characterization of transglutaminase 2 (TG2) inhibitors

Huntington's disease (HD) is associated with increased expression levels and activity of tissue transglutaminase (TG2), an enzyme primarily known for its cross-linking of proteins. To validate TG2 as a therapeutic target for HD in transgenic models and for eventual clinical development, a selective and brain-permeable inhibitor is required. Here, a comprehensive profiling platform of biochemical and cellular assays is presented which has been established to evaluate the potency, cellular efficacy, subtype selectivity and the mechanism-of-action of known and novel TG2 inhibitors. Several classes of inhibitors have been characterized including: the commonly used pseudo-substrate inhibitors, cystamine and putrescine (which are generally nonspecific for TG2 and therefore not practical for drug development), the various peptidic inhibitors that target the active site cysteine residue (which display excellent selectivity but in general have poor cellular activity), and the allosteric reversible small-molecule hydrazides (which show poor selectivity and a lack of cellular activity and could not be improved despite considerable medicinal chemistry efforts). In addition, a set of inhibitors identified from a collection of pharmacologically active compounds was found to be unselective for TG2. Moreover, inhibition at the guanosine triphosphate binding site has been examined, but apart from guanine nucleotides, no such inhibitors have been identified. In addition, the promising pharmacological profile of a TG2 inhibitor is presented which is currently in lead optimization to be developed as a tool compound.

Identification and characterization of a diamine exporter in colon epithelial cells

SLC3A2, a member of the solute carrier family, was identified by proteomics methods as a component of a transporter capable of exporting the diamine putrescine in the Chinese hamster ovary (CHO) cells selected for resistance to growth inhibition by high exogenous concentrations of putrescine. Putrescine transport was increased in inverted plasma membrane vesicles prepared from cells resistant to growth inhibition by putrescine compared with transport in inverted vesicles prepared from non-selected cells. Knockdown of SLC3A2 in human cells, using short hairpin RNA, caused an increase in putrescine uptake and a decrease in arginine uptake activity. SLC3A2 knockdown cells accumulated higher polyamine levels and grew faster than control cells. The growth of SLC3A2 knockdown cells was inhibited by high concentrations of putrescine. Knockdown of SLC3A2 reduced export of polyamines from cells. Expression of SLC3A2 was suppressed in human HCT116 colon cancer cells, which have an activated K-RAS, compared with their isogenic clone, Hkh2 cells, which lack an activated K-RAS allele. Spermidine/spermine N(1)-acetyltransferase (SAT1) was co-immunoprecipitated by an anti-SLC3A2 antibody as was SLC3A2 with an anti-SAT1 antibody. SLC3A2 and SAT1 colocalized on the plasma membrane. These data provide the first molecular characterization of a polyamine exporter in animal cells and indicate that the diamine putrescine is exported by an arginine transporter containing SLC3A2, whose expression is negatively regulated by K-RAS. The interaction between SLC3A2 and SAT1 suggests that these proteins may facilitate excretion of acetylated polyamines.

A stable, inducible, dose-responsive ODC overexpression system in human cell lines

ODC is a labile protein subject to rapid turnover, and a conditional expression system providing long-term overexpression may be helpful in further understanding the biochemical properties of this enzyme and elucidating aspects of the polyamine biosynthetic pathway that have otherwise been difficult to study. HEK293 and LNCaP cell lines were engineered to stably and inducibly overexpress ODC using a Tet-on inducible construct. Clones from both cell lines were characterized by evaluating ODC mRNA expression, ODC activity, intracellular and extracellular polyamine levels, SSAT activity and growth kinetics. The ODC-inducible cell lines were time- and dose-responsive providing a mechanism to increase ODC and putrescine accumulation to a desired level in a flexible and controllable manner. The findings demonstrate that LNCaP ODC overexpressing cells maintained over a 100-fold increase in ODC activity and over a 10-fold increase in intracellular putrescine after 6 h. ODC induction at the highest levels was accompanied by a slight decline in intracellular spermidine and spermine levels and this observation was supported by the finding that SSAT activity was induced over 40-fold under these conditions. Growth rate remained unaffected following at least 12 h of ODC overexpression. Similar results were observed in the HEK293 ODC overexpressing cells.

Effects of L-Ornithine on Metabolic Processes of the Urea Cycle in Human Keratinocytes

The complex metabolic function of L-ornithine has led to a great interest in studying its pharmacotherapeutic potential. L-Ornithine is known to be crucial for the metabolism of keratinocytes, especially in the synthesis of urea, polyamines and precursors of collagen synthesis. In this study, we investigated in vitro the cytotoxicity of L-ornithine, and its influence on urea synthesis and arginase expression in primary human keratinocytes. L-Ornithine (> or =1 mM) induced a decrease in the de novo urea synthesis of keratinocytes and an increase (> or =10 mM) in the expression of the urea-generating enzyme arginase. Up to 20 mM, L-ornithine showed no cytotoxic potential, whereas higher concentrations induced apoptosis in keratinocytes in a concentration- and time-dependent manner.

A streamlined method for the isolation and quantitation of nanomole levels of exported polyamines in cell culture media

A number of years ago, our laboratory published a method for the isolation of small amounts of polyamines from cell culture media using the ion-exchange resin Bio-Rex 70. We have used this technique extensively to study the export of putrescine and cadaverine from cultured mammalian cells. Unfortunately, this method was highly inefficient in isolating the polyamines spermidine and spermine and was incapable of recovering the acetylated polyamine N(1)-acetylspermidine. In response to these shortcomings, we modified our previous protocol to quantitatively isolate the polyamines N(1)-acetylspermidine, putrescine, cadaverine, N(1)-acetylspermine, spermidine, and spermine. The new method, which is much faster to perform and more efficient than the one previously described, employs the use of disposable minicolumns and a single resin washing step using a weak solution of sodium carbonate at pH 9.3. This new protocol also eliminates the column elution step in favor of directly derivatizing the polyamines with dansyl chloride on the ion-exchange resin. High-performance liquid chromatography analysis of the dansylated polyamines isolated by this procedure showed that 75% of N(1)-acetylspermidine and nearly 100% of the other polyamines present in nanomolar levels were recovered from small amounts of cell culture medium. This new protocol is a valuable new tool for the study of the intracellular/extracellular dynamics of polyamine pools in cultured cells. [A detailed laboratory protocol for this procedure (containing all of the information in this paper but in a condensed form) can be requested by e-mailing the authors.]

Characterization of a diamine exporter in Chinese hamster ovary cells and identification of specific polyamine substrates

Export of the diamine putrescine was studied using inside-out plasma membrane vesicles prepared from Chinese hamster cells. Putrescine uptake into vesicles was a saturable and an ATP- and antizyme-independent process. Excess amounts of a series of diamines or monoacetyl spermidine, but not monoacetyl putrescine, spermidine, or spermine, inhibited putrescine transport. Putrescine uptake into vesicles prepared at pH 7.4 was suppressed at pH 5, compared with pH 7.4; was stimulated approximately 2.5-fold at pH 7.4 in vesicles prepared at pH 6.25, compared with vesicles prepared at pH 7.4; and was not inhibited by valinomycin in the presence of potassium ions. Reserpine and verapamil blocked [3H]putrescine uptake into inverted vesicles. Verapamil treatment caused an increase in intracellular contents of putrescine, cadaverine, and N8-acetylspermidine, in unstressed proliferating cells, or of N1-acetylspermidine, in cells subjected to heat shock to induce acetylation of spermidine at N1. These data indicate that putrescine export in Chinese hamster cells is mediated by a non-electrogenic antiporter capable of using protons as the counter ion. Physiological substrates for this exporter include putrescine, cadaverine, and monoacetyl spermidine and have the general structure NH3+-(CH2)n-NH2 + R at acidic or neutral pH.

Tolerance to putrescine toxicity in Chinese hamster ovary cells is associated with altered uptake and export

When Chinese hamster ovary (CHO) cells were cultured with low concentrations of putrescine (< 5 mM) their cell cycle time increased significantly and a fraction of the cells died. A cell line tolerant to the cytotoxic and growth inhibitory effects of millimolar concentrations of putrescine was developed by growing CHO cells over many months in increasing concentrations of the polyamine. A putrescine-tolerant cell line was obtained which was capable of growing in concentrations up to 25 mM putrescine and displayed growth and cell division rates similar to the original untreated/parental CHO cells. The tolerant cells grown in putrescine displayed relatively high intracellular putrescine yet the cell-associated putrescine concentration was estimated to be 10-fold less than the culture medium level. This high concentration of cellular putrescine diminished within 60 min when the cells were changed to non-putrescine-containing media. The putrescine-tolerant phenotype was further characterized in regards to the mechanisms involved in putrescine uptake, efflux, and biosynthesis. The parental and tolerant cell lines had similar or identical levels of cellular spermidine and spermine and no differences in the acetylated polyamine pools or diamine oxidase activity. The activity of ornithine decarboxylase was also similar in the two cell lines in both the presence and the absence of ornithine. The tolerant cells, however, had a decreased uptake rate for putrescine. The tolerant cell line also showed a greatly enhanced ability to export putrescine, especially when treated with ornithine, suggesting that an upregulated polyamine export system may be present in the tolerant cells which could be responsible for the increased cell survival in high putrescine concentrations. The data are discussed in regard to the potential for identifying the transport protein(s) responsible for the maintenance of nontoxic intracellular concentrations of putrescine in a tolerant cell line grown in putrescine.

Putrescine export in Xenopus laevis oocytes occurs against a concentration gradient: evidence for a non-diffusional export process

Putrescine export was found to occur by a non-diffusional highly regulated process using Xenopus oocytes as a model system of polyamine transport. Untreated oocytes were observed to possess high endogenous intracellular putrescine and spermidine levels with no detectable polyamine interconversion or biosynthesis over the assay intervals. The putrescine uptake process demonstrated a rapid saturation within a 5 min interval. Spermidine demonstrated a relatively larger uptake capacity with only a minimal ability to export. A kinetic analysis of the concentration-dependence of the putrescine and spermidine uptake processes indicated that the putrescine uptake process may possess two concurrent uptake components while spermidine uptake may possess a two-component process with an allosteric regulation. Elevated intracellular putrescine levels were observed to decrease against a 10-fold higher extracellular concentration gradient in a rapid and specific manner. No noticeable changes in the intracellular levels of other polyamines were observed over the same time interval. The uptake and export rates of putrescine transport also showed a concurrent, rapid and cyclical regulation. These findings support a non-diffusional putrescine export process which is highly regulated.

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.

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.

A kinetic characterization of putrescine and spermidine uptake and export in human erythrocytes

Using human erythrocytes as a model system for the study of mammalian polyamine transport, detailed kinetic parameters regarding the uptake and export of putrescine and spermidine were determined. The putrescine uptake data indicated a multi-component uptake system comprised of a low-capacity saturable component and a non-saturable component. The saturable putrescine uptake component demonstrated a calculated Km of 21.0 microM and a V(max) of only 6.52 x 10(-13) M/s. The non-saturable linear putrescine uptake rate was defined by a significant pH dependence, a lack of uptake inhibition by related polyamines, and a permeability pi of 3.19 x 10(-8) s-1. These findings suggested that non-saturable putrescine uptake involved a process of simple diffusion. Spermidine uptake exhibited Michaelis-Menten kinetics with a Km and Vmax of 12.5 microM and 1.36 x 10(-12) M/s, respectively. Spermidine uptake did not demonstrate pH dependence and was not significantly inhibited by any of the tested polyamines. The Arrhenius plot of spermidine uptake was determined to be biphasic with calculated activation energies of spermidine uptake of 135.2 kJ/mol for 19-21 degrees C and 59.3 kJ/mol for 21-35 degrees C. These data suggest the possibility of multiple spermidine uptake processes which are not mediated by simple diffusion across the cell membrane. The putrescine export process demonstrated both saturable and non-saturable components. The calculated Km, V(max) and pi for putrescine export were 33.8 microM, 1.19 x 10(-11) M/s and 2.81 x 10(-7) s-1, respectively. The spermidine export process was non-saturable up to intracellular spermidine concentrations of 4 microM. At similar intracellular and extracellular concentrations of putrescine and spermidine, however, export processes displayed rates which were an order of magnitude greater than their respective uptake rates. This finding supports the possible presence of mediated putrescine and spermidine export processes different than simple diffusion.

Regulation of the efflux of putrescine and cadaverine from rapidly growing cultured RAW 264 cells by extracellular putrescine

Cultures of the macrophage-like RAW 264 cells were adapted to divide normally in a synthetic serum-supplemented culture medium lacking any polyamines and diamine oxidase activity. These rapidly dividing cells actively effluxed large amounts of putrescine and cadaverine, compared with the intracellular levels, into the culture medium. The efflux of putrescine was stimulated by the amino acid ornithine, whereas efflux of cadaverine was inhibited. Relatively low levels of spermidine and N1-acetyl-spermidine, compared with those of exported putrescine, were observed to accumulate in the culture medium. A careful analysis of the changes in the intracellular concentration of putrescine relative to the steady-state net rate of putrescine export, as the doubling time of the cultures increased from 16 h to 22 h, indicated that an inverse relationship existed between these two parameters. As the intracellular putrescine concentrations increased, the net rate of putrescine export decreased markedly. Determination of the rate of putrescine uptake indicated that putrescine uptake also decreased significantly as the cultures neared confluency, and at no time during the growth of the culture did the rate of putrescine uptake approximate to the high rate of putrescine efflux. The decrease in the putrescine export rate seen as the cells grew toward confluency was determined to be primarily due to the inhibitory effect of the effluxed putrescine in the medium (Ki = 2 microM), and not to contact inhibition. The data suggested that the efflux of putrescine and cadaverine is not mediated to a significant degree by a process involving simple diffusion.

Characterization of putrescine and cadaverine export in mammalian cells. A pharmacological approach

We characterized the mechanism(s) involved in the efflux of putrescine/cadaverine from cultured mammalian cells using various pharmacological agents. Verapamil and quinine inhibited putrescine and cadaverine export in monocytic-leukemic RAW 264 and H35 hepatoma cells in a concentration-dependent manner with an IC50 in the micromolar range. Verapamil, which inhibits L-type calcium channels, inhibited putrescine export, regardless of whether calcium was present in the extracellular medium or not. Furthermore, the export of putrescine in the absence of verapamil did not appear to depend upon extracellular calcium. Neither intracellular calcium, external sodium, changes in intracellular pH nor phosphorylation affected the levels of putrescine export independently from changes in intracellular putrescine levels. The data suggest that verapamil and quinine inhibit putrescine/cadaverine efflux from the cell by binding directly to an integral membrane protein.

Ornithine decarboxylase activity and polyamine concentrations in fetal rat brain: response to chronic hypoxic-hypoxia and/or carbon monoxide-hypoxia

Ornithine decarboxylase activity (ODC; E.C. 4.1.1.17), is significantly elevated in fetal and newborn rat brain in response to acute hypoxia. Because relatively little is known about ODC activities and polyamine metabolism in hypoxia and also because ODC and the polyamines are essential for normal growth and development, we examined the effect of chronic maternal hypoxic-hypoxia (16-10.5% O2), carbon monoxide-hypoxia (100-200 ppm CO) and their combination, on fetal weight, fetal brain ODC activity and polyamine concentrations. Time-dated pregnant Sprague-Dawley rats were chronically exposed to hypoxia from gestational day (E-15), to gestational day 21 (E-21), in individual chambers. Pair-fed controls were given an amount equivalent to that eaten by a hypoxic dam over the previous 24 h. We measured fetal weight, as well as brain ODC activity and polyamine concentrations on both E-19 and E-21. Pair-feeding had no effect on fetal weight, ODC activity or polyamine concentrations. On both E-19 and E-21, however, fetal weights were significantly reduced with higher levels of hypoxic-hypoxia (e.g., 10.5% O2). At 100 or 200 ppm, carbon monoxide alone appeared not to affect fetal weight; however, combined with even mild hypoxia (16% O2), fetal weights were reduced almost 20%, suggesting that together, CO- and hypoxic-hypoxia exert a synergistic effect of fetal weight decrements. (1) There was no consistent pattern of ODC activity changes which correlated to the fetal weight losses or levels of hypoxia. These results suggest that ODC activity may not be a good marker for chronic, as opposed to acute hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)