Mechanism of spermidine uptake in cultured mammalian cells and its inhibition by some polyamine analogues

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

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

The metabolic coupling of arginine metabolism to nitric oxide generation by astrocytes

Arginine, the only known precursor of nitric oxide, enters the brain parenchyma from the blood through the endothelial cells or from the cerebral spinal fluid through the ependymal cells. Astrocytes, whose processes abut the endothelium and ependymum, take up arginine through cationic amino acid transporters and release arginine through this transport system to the synapses that astrocytes shield. Some of these synapses are excitatory, and liberate glutamate into the synaptic cleft. Glutamate induces arginine release from astrocytes, making it available to the neuron. Neurons can take up arginine to be used in nitric oxide-mediated processes, such as neurotransmission. Thus, neural and nonneural cells act in concert to affect neuron physiology in an elegantly integrated system. This review focuses on the components of the interaction between astrocytes and neurons in nitric oxide biology.

Identification and Characterization of a Polyamine Permease from the Protozoan Parasite Leishmania major

The proteins that mediate polyamine translocation into eukaryotic cells have not been identified at the molecular level. To define the polyamine transport pathways in eukaryotic cells we have cloned a gene, LmPOT1, that encodes a polyamine transporter from the protozoan pathogen, Leishmania major. Sequence analysis of LmPOT1 predicted an unusual 803-residue polytopic protein with 9-12 transmembrane domains. Expression of LmPOT1 cRNA in Xenopus laevis oocytes revealed LmPOT1 to be a high affinity transporter for both putrescine and spermidine, whereas expression of LmPOT1 in Trypanosoma brucei stimulated putrescine uptake that was sensitive to inhibition by pentamidine and proton ionophores. Immunoblot analysis established that LmPOT1 was expressed predominantly in the insect vector form of L. major, and immunofluorescence demonstrated that LmPOT1 was localized predominantly to the parasite plasma membrane. To our knowledge this is the first molecular identification and characterization of a cell surface polyamine transporter in eukaryotic cells.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

QSAR analysis of polyamine transport inhibitors in L1210 cells

PURPOSE

In this paper, the authors attempt to construct a mathematical model to correlate the biological activities of 63 polyamine transport inhibitors in L1210 cells with their physicochemical parameters.

METHOD

The inhibitory constants (Ki) were obtained from the published work of Bergeron et al. Non-weighted least square method was used in deriving the regression equations with a BMDP program. An AM1 subroutine of the HyperChem program was used to optimize the geometry and calculate the molecular dipole moments and the distance between two terminal amino groups. A CQSAR program was used to calculate Clog P (oct./w.).

RESULTS

A good correlation (r2 = 0.81) was obtained by using a five-parameter equation including the distance between two terminal amino groups (d), the number of cationic charge (Charge), molecular weight (MW), dipole moment (mu), and hydrogen bond forming ability (Hb).

CONCLUSION

This model accounts for 81% of the variance in the data and can be used to estimate transport-inhibitory activity of many other polyamine analogues. It gives some quantitative information about the relationship between the polyamine analogues' function as transport inhibitors and their molecular structures.

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

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

Polyamine transport in mammalian cells. An update

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

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.

Protective effect of spermine on DNA exposed to oxidative stress

Pathological conditions that cause oxidative stress can affect DNA integrity. The aim of this research was to study the protective effect of spermine against DNA damage induced by an oxygen-radical generating system. Deoxyguanosine and DNA were separately dissolved in phosphate buffer and incubated for 1 h at 40 degrees C in the presence of 50 mM H2O2/10 mM ascorbic acid. Single nucleosides and their products of oxidation were then obtained by enzymatic digestion of DNA. The compounds were separated by micellar electrokinetic capillary chromatography (MECC) with SDS-modified mobile phase and detected at 254 nm. Two major products of DNA oxidation have been identified as derivatives of deoxyguanosine with electrophoretic properties different from 8-hydroxy-2'-deoxyguanosine. When the oxidation of DNA was carried out in the presence of 0.1 mM spermine, the formation of the two by-products of deoxyguanosine was markedly reduced. On the contrary, spermine did not prevent the oxidation of deoxyguanosine alone, suggesting that the polyamine should be bound to the DNA strands to exert its antioxidative effect.

Transport and metabolism of polyamines in wild and multidrug resistant human leukemia (K 562) cells

Multidrug resistance (MDR) can be defined as the resistance of cancer cells not just to chemotherapeutic agents to which they have been exposed but also to other apparently unrelated compounds. This MDR phenotype is commonly associated with the high expression of levels of 170 kDa P-glycoprotein, encoded by MDR genes. In the present study, the uptake kinetics of polyamines and their biosynthesis were studied in wild and multidrug resistant (MDR) K 562 cells in culture. The rate (Vmax) of polyamine uptake was significantly lower in MDR cells than that in wild type cells, whereas the Km for the uptake was not significantly different in these cells, suggesting that polyamine transporter is not modified in MDR cells, though their different physiological state influences the uptake process. In a 32 h chase, the transported radioactive polyamines were gradually interconverted. [14C]putrescine was converted into [14C]spermidine following between 15 min and 32 h of culture, and into [14C]-spermine after 16 h of culture, in both the cell types; however, the levels of interconverted radioactive polyamines were always lower in MDR cells as compared with wild type cells. Similarly, internalized [14C]spermidine was converted into [14C]spermine, but not into [14C]putrescine in both the cells types. [14C]spermidine is metabolized into [14C]spermine after 4 h of culture in wild type cells, whereas in MDR cells the interconversion of [14C]spermidine into [14C]spermine is seen only after 16 h of culture. Blocking of the transmembrane drug efflux pump, expressed in the MDR cells, by preincubation in the presence of verapamil, did not influence the uptake of either of the two polyamines (putrescine and spermidine) by MDR cells. On the contrary, this kind of preincubation of wild type cells in the presence of verapamil significantly increased the uptake of these two polyamines. The levels of intracellular polyamine contents in MDR cells were always lower than those in the parental cell line. These results demonstrate that MDR cells are defective in both the uptake of polyamines and their biosynthesis as compared with wild type cells.

Endothelial polyamine uptake: selective stimulation by L-arginine deprivation or polyamine depletion

Uptake of putrescine and spermidine by cultured porcine aortic endothelial cells was time dependent and linear for 60 min. Transport, against a 5- to 10-fold concentration gradient, demonstrated both saturable and non-saturable components. Apparent concentration giving one-half maximal transport (Kt) values for putrescine and spermidine were 9 and 0.6 microM, respectively. Transport was reduced at 0 degrees C, suggesting that the process is energy requiring; inhibition by N-ethylmaleimide or p-chloromercuribenzoate suggested a requirement for sulfydryl groups. Transport of putrescine, but not spermidine, was partially activated by Na+. Spermidine and spermine did not inhibit putrescine uptake, and putrescine and spermine did not inhibit spermidine uptake, suggesting the presence of a separate transporter for each polyamine. Pretreatment with DL-2-difluoromethy-lornithine increased the uptake of putrescine but not spermidine. The endothelial cell putrescine transporter is thus sensitive to polyamine depletion, suggesting that transport from the extracellular space may be an important source of polyamines. L-Ornithine or L-arginine were not inhibitory, indicating that polyamine and cationic amino acid transport is mediated by independent systems. The sensitivity of putrescine transport to L-arginine but not to L-ornithine deprivation suggests that intracellular levels of arginine rather than ornithine regulate polyamine metabolism and transport in these cells. Thus factors that affect arginine utilization may also influence polyamine metabolism.

Regulation of spermidine transport in L1210 cells

1. 1 mM 2-amino isobutyric acid (AIB), glutamine or asparagine when preincubated for 3 hr with L1210 cells promoted a marked increase in the rate of spermidine uptake. 2. Cycloheximide also increased the transport rate and completely prevented the increase due to AIB. 3. Trifluoperazine and iso-H7 inhibited the uptake of spermidine, much less the uptake of AIB. 4. Adenosine promoted an increase in the uptake of AIB, a decrease in that of spermidine. 5. Hypotonic stress also increased the rate of spermidine transport. This modification was only partially prevented by cycloheximide. 6. Okadaic acid had no effect on this increase, whereas it prevented the increase of ODC activity.

A free-radical hypothesis for the instability and evolution of genotype and phenotype in vitro

It has been known for several decades that cultured murine cells undergo a defined series of changes, i.e., an in vitro evolution, which includes crisis, spontaneous transformation ('immortalization'), aneuploidy, and spontaneous neoplastic transformation. These changes have been shown to be caused by the in vitro environment rather than an inherent instability of the murine phenotype or genotype. Serum amine oxidases were recently identified as a predominant cause of crisis. These enzymes generate hydrogen peroxide from polyamine substrates that enter the extracellular milieu. This finding implicates free-radical toxicity as the underlying cause of in vitro evolution. We propose an oxyradical hypothesis to explain each of the stages of in vitro evolution and discuss its significance for cytotechnology and long-term cultivation of mammalian cell types.

Abnormal accumulation and toxicity of polyamines in a difluoromethylornithine-resistant HTC cell variant

Mammalian cells possess an inducible, active polyamine transport system that is stringently regulated by feedback controls. This study provides evidence that DH23b cells, which were initially selected from the rat hepatoma HTC line for overproduction of ornithine decarboxylase, demonstrate an abnormality in the regulation of polyamine transport. Exposure of these cells to micromolar levels of spermidine or spermine resulted in inhibition of protein synthesis and eventual cell lysis. These effects were not due to by-products of polyamine oxidation by serum oxidases as neither inhibition of protein synthesis nor cell lysis was mitigated by aminoguanidine, reduced glutathione, dithiothreitol, or catalase. Although the polyamine transport system in the DH23b cells has the same Km and Vmax as that in the parental HTC line, the variant cells accumulated abnormally high levels of both spermidine (8-times normal) and spermine (4-times normal). In the HTC line, however, transport of both polyamines as well as putrescine was feedback inhibited within approx. 3 h, while in the variant cells uptake was not diminished by 12 h and terminated only with cell lysis. The DH23b cells appear to lack the normal mechanism responsible for feedback control of active polyamine incorporation. This defect provided the opportunity to manipulate intracellular levels of spermidine from 30 to approx. 800% of normal, allowing the demonstration that cellular protein synthesis is as sensitive to spermidine levels as previous in-vitro studies had suggested.

Phorbol esters augment polyamine transport by influencing Na(+)-K+ pump in murine leukemia cells

In this report, we elucidate the role of Na(+)-K+ pump in the regulation of polyamine spermidine (Spd) transport in murine leukemia (L 1210) cells in culture. Ouabain, known to bind extracellularly to the alpha-subunit of the Na(+)-K+ pump, inhibits the pump activity. The L 1210 cells were found to possess ouabain binding sites at 7.5 fmol/10(6) cells. Ouabain significantly inhibited the Spd uptake in a dose-dependent manner. The maximum inhibition of Spd uptake by ouabain was observed beyond 200 microM. Spd transport was inversely correlated with the [3H]ouabain binding to L 1210 cells: an increase in the saturation of ouabain binding to L 1210 cells resulted in a decrease of the Spd uptake process. Treatment of L 1210 cells with protein kinase C activator phorbol esters increased the Spd transport and, also, ouabain-sensitive 86Rb+ uptake, a measure of the activity of the Na(+)-K+ pump. H-7, a protein kinase C inhibitor, significantly inhibited the ouabain-sensitive 86Rb+ uptake by L 1210 cells. Phorbol esters stimulated the level, but not the rate, of 22Na+ influx. Addition of H-7 to L 1210 cells inhibited the 22Na+ influx process. A concomitant phorbol ester-induced increase in 22Na+ influx, [14C]Spd uptake, together with the functioning of Na(+)-K+ pump, indicates the role of the "Na+ cycle" in the regulation of the polyamine transport process.