Estimation of polyamine binding to macromolecules and ATP in bovine lymphocytes and rat liver

Polyamines enhance synthesis of the RNA polymerase sigma 38 subunit by suppression of an amber termination codon in the open reading frame

The mechanisms by which polyamines stimulate synthesis of the RNA polymerase sigma(38) subunit in Escherichia coli were studied. Polyamine stimulation was observed only in strains in which the 33rd codon of RpoS mRNA is a UAG termination codon instead of a CAG codon for glutamine in wild-type E. coli. Readthrough of the termination codon by Gln-tRNA(supE) was stimulated by polyamines. This stimulation was found to be caused by an increase in both the level of suppressor tRNA(supE) and the binding affinity of Gln-tRNA(supE) for ribosomes. The stimulatory effect was observed with a UAG termination codon but not with UGA and UAA codons. Readthrough of the UAG termination codon at the 270th amino acid position of RpoS mRNA was also stimulated by polyamines, indicating that polyamines stimulate readthrough of a UAG codon regardless of its location within the RpoS mRNA. When cell viability of an E. coli strain having a termination codon in the 33rd position of RpoS mRNA was compared using cells cultured with or without putrescine, it was higher in cells cultured with putrescine than in cells cultured without putrescine. The level of sigma(38) subunit in the cells cultured with putrescine was higher than that in cells cultured without putrescine on days 2, 4, and 8, but the level of sigma(70) subunit was almost the same in cells cultured with or without putrescine. These results confirm that elevated expression of the rpoS gene is important for cell viability at late stationary phase.

Regulation of Ca2+ channel and phosphatase activities by polyamines in intestinal and vascular smooth muscle--implications for cellular growth and contractility

Polyamines added extracellularly to intestinal and vascular smooth muscle cells cause relaxation through inhibition of Ca2+ channel activity. Intracellularly applied polyamines also affect Ca2+ channel properties. Polyamines do not readily pass over the plasma membrane because of their positive charges but in permeabilized smooth muscle preparations they have free access to the cytoplasm. In this system they increase sensitivity of the contractile machinery to Ca2+ through inhibition of myosin phosphatase activity. The magnitude of Ca2+ channel and phosphatase inhibition depends on the number of positive charges on the polyamine molecule. Polyamines have an obligatory, but yet undefined, role in regulation of cell growth and proliferation. Several groups of protein kinases, such as tyrosine and mitogen activated protein (MAP)-kinases transmit the growth signal from the plasma membrane to the cell nucleus where mitosis and protein synthesis are initiated. The data reviewed here show that polyamines may affect such signal transmission via inhibition of phosphatase activity.

Polyamine-mediated post-transcriptional regulation of COX-2

Polyamines are involved in various cellular processes including embryonic development, cell growth and cell cycle regulation, apoptosis and carcinogenesis. Growing evidence suggests the importance of RNA processing and stability mediated by oncogenes and tumor suppressor genes in the development of cancer. Polyamines, which are found to be increased in neoplastic cells and tissues compared to normal, bind to RNA to influence its structure and function. Polyamines mediate RNA processing through the spermidine-regulated protein, eIF-5A. To further investigate how polyamines influence RNA expression, we characterized the polyamine-dependent RNA expression of a cancer-related gene, cyclooxygenase-2 (COX-2), which contains eIF-5A consensus binding elements. Depletion of polyamines by DL-alpha-difluoromethylornithine (DFMO) treatment caused an induction of COX-2 mRNA steady-state levels. Polyamines appear to regulate expression of COX-2 by a post-transcriptional mechanism.

Inward rectifier K(+) current under physiological cytoplasmic conditions in guinea-pig cardiac ventricular cells

The outward current that flows through the strong inward rectifier K(+) (K(IR)) channel generates I(K1), one of the major repolarizing currents of the cardiac action potential. The amplitude and the time dependence of the outward current that flows through K(IR) channels is determined by its blockage by cytoplasmic cations such as polyamines and Mg(2+). Using the conventional whole-cell recording technique, we recently showed that the outward I(K1) can show a time dependence during repolarization due to competition of cytoplasmic particles for blocking K(IR) channels. We used the amphotericin B perforated patch-clamp technique to measure the physiological amplitude and time dependence of I(K1) during the membrane repolarization of guinea-pig cardiac ventricular myocytes. In 5.4 mM K(+) Tyrode solution, the density of the current consisting mostly of the sustained component of the outward I(K1) was about 3.1 A F(-1) at around -60 mV. The outward I(K1) showed an instantaneous increase followed by a time-dependent decay (outward I(K1) transient) on repolarization to -60 to -20 mV subsequent to a 200 ms depolarizing pulse at +37 mV (a double-pulse protocol). The amplitudes of the transients were large when a hyperpolarizing pre-pulse was applied before the double-pulse protocol, whereas they were small when a depolarizing pre-pulse was applied. The peak amplitudes of the transients elicited using a hyperpolarizing pre-pulse were 0.36, 0.63 and 1.01 A F(-1), and the decay time constants were 44, 14 and 6 ms, at -24, -35 and -45 mV, respectively. In the current-clamp experiments, a phase-plane analysis revealed that application of pre-pulses changed the current density at the repolarization phase to the extents expected from the changes of the I(K1) transient. Our study provides the first evidence that an outward I(K1) transient flows during cardiac action potentials.

Spermidine is essential for normal proliferation of trypanosomatid protozoa

Trypanosomatid parasites containing a metabolically unstable ornithine decarboxylase (ODC) are naturally resistant to high levels of alpha-difluoromethylornithine (DFMO) because this ODC inhibitor, though causing a drastic reduction of intracellular putrescine, elicits only a moderate decrease of the spermidine endogenous pool. In this study we have used a combination of DFMO with cyclohexylamine (CHA; bis-cyclohexylammonium sulfate), an inhibitor of spermidine synthase, to reach a more complete depletion of spermidine. Under these conditions we have observed the arrest of proliferation not only in trypanosomatids with stable ODC but also in parasites with an enzyme of high turnover rate. In all cases the reinitiation of proliferation occurred only after the addition of exogenous spermidine, and neither putrescine nor spermine were able to induce the same effect.

Polyamine enhancement of the synthesis of adenylate cyclase at the translational level and the consequential stimulation of the synthesis of the RNA polymerase sigma 28 subunit

The effects of polyamines on the synthesis of various final sigma subunits of RNA polymerase were studied using Western blot analysis. Synthesis of final sigma(28) was stimulated 4.0-fold and that of final sigma(38) was stimulated 2.3-fold by polyamines, whereas synthesis of other final sigma subunits was not influenced by polyamines. Stimulation of final sigma(28) synthesis was due to an increase in the level of cAMP, which occurred through polyamine stimulation of the synthesis of adenylate cyclase at the level of translation. Polyamines were found to increase the translation of adenylate cyclase mRNA by facilitating the UUG codon-dependent initiation. Analysis of RNA secondary structure suggests that exposure of the Shine-Dalgarno sequence of mRNA is a prerequisite for polyamine stimulation of the UUG codon-dependent initiation.

Spermine mediates inward rectification in potassium channels of turtle retinal Müller cells

Retinal Müller cells are highly permeable to potassium as a consequence of their intrinsic membrane properties. Therefore these cells are able to play an important role in maintaining potassium homeostasis in the vertebrate retina during light-induced neuronal activity. Polyamines and other factors present in Müller cells have the potential to modulate the rectifying properties of potassium channels and alter the Müller cells capacity to siphon potassium from the extracellular space. In this study, the properties of potassium currents in turtle Müller cells were investigated using whole cell voltage-clamp recordings from isolated cells. Overall, the currents were inwardly rectifying. Depolarization elicited an outward current characterized by a fast transient that slowly recovered to a steady level along a double exponential time course. On hyperpolarization the evoked inward current was characterized by an instantaneous onset (or step) followed by a slowly developing sustained inward current. The kinetics of the time-dependent components (block of the transient outward current and slowly developing inward current) were dependent on holding potential and changes in the intracellular levels of magnesium ions and polyamines. In contrast, the instantaneous inward and the sustained outward currents were ohmic in character and remained relatively unaltered with changes in holding potential and concentration of applied spermine (0.5--2 mM). Our data suggest that cellular regulation in vivo of polyamine levels can differentially alter specific aspects of potassium siphoning by Müller cells in the turtle retina by modulating potassium channel function.

Regulation of estrogenic and nuclear factor kappa B functions by polyamines and their role in polyamine analog-induced apoptosis of breast cancer cells

The natural polyamines -putrescine, spermidine, and spermine- are essential for cell growth and differentiation. Polyamines are involved in several gene regulatory functions, although their mechanism(s) of action has not been elucidated. We investigated the role of polyamines in the function of NF-kappa B and estrogen receptor-alpha (ER alpha), two transcription factors implicated in breast cancer cell proliferation and cell survival, using MCF-7 breast cancer cells. We found that spermine facilitated the binding of ER alpha and NF-kappa B to estrogen response element (ERE)- and NF-kappa B response element (NRE), respectively, and enhanced ER alpha-mediated transcriptional activation in transient transfection experiments. We also found that the association of the co-regulatory protein CBP/p300 with ER alpha and NF-kappa B was increased by spermine treatment of MCF-7 cells. Spermine also increased the nuclear translocation of NF-kappa B compared to the control. In contrast, treatment of MCF-7 cells with polyamine analogs, BE-3-4-3 and BE-3-3-3, resulted in transcriptional inhibition of both ERE- and NRE-driven reporter plasmids. In addition, polyamine analogs inhibited the association of ER alpha and NF-kappa B with CBP/p300 and were unable to facilitate nuclear translocation of NF-kappa B. APO-BRDU assay demonstrated that polyamine analogs induced apoptosis, with a loss of the anti-apoptotic protein Bcl-2. These data show a gene regulatory function of polyamines involving transcriptional activation of ER alpha and NF-kappa B, potentially leading to the up-regulation of genes involved in breast cancer cell proliferation. Our results with BE-3-4-3 and BE-3-3-3 suggest that down-regulation of ER alpha- and NF-kappa B-regulated genes is a possible mechanism for the action of polyamine analogs in inducing apoptosis of breast cancer cells.

Control of rectification and permeation by two distinct sites after the second transmembrane region in Kir2.1 K+ channel

1. The rectification property of the inward rectifier K+ channel is chiefly due to the block of outward current by cytoplasmic Mg2+ and polyamines. In the cloned inward rectifier K+ channel Kir2.1 (IRK1), Asp172 in the second transmembrane region (M2) and Glu224 in the putative cytoplasmic region after M2 are reported to be critical for the sensitivity to these blockers. However, the difference in the inward rectification properties between Kir2.1 and a very weak inward rectifier sWIRK could not be explained by differences at these two sites. 2. Following sequence comparison of Kir2.1 and sWIRK, we focused this study on Glu299 located in the centre of the putative cytoplasmic region after M2. Single-point mutants of Kir2.1 (Glu224Gly and Glu299Ser) and a double-point mutant (Glu224Gly-Glu299Ser) were made and expressed in Xenopus oocytes or in HEK293T cells. 3. Their electrophysiological properties were compared with those of wild-type (WT) Kir2.1 and the following observations were made. (a) Glu299Ser showed a weaker inward rectification, a slower activation upon hyperpolarization, a slower decay of the outward current upon depolarization, a lower sensitivity to block by cytoplasmic spermine and a smaller single-channel conductance than WT. (b) The features of Glu224Gly were similar to those of Glu299Ser. (c) In the double mutant (Glu224Gly-Glu299Ser), the differences from WT described above were more prominent. 4. These results demonstrate that Glu299 as well as Glu224 control rectification and permeation, and suggest the possibility that the two sites contribute to the inner vestibule of the channel pore. The slowing down of the on- and off-blocking processes by mutation of these sites implies that Glu224 and Glu299 function to facilitate the entry (and exit) of spermine to (and from) the blocking site.

Regulation of cellular polyamines by antizyme

Proteins that are degraded by the proteasome are first modified by a set of enzymes that attach multiple copies of ubiquitin to substrate lysines, but a tiny minority, including the polyamine-synthesizing enzyme ornithine decarboxylase, is handled differently. This enzyme is targeted for destruction by another protein--antizyme. Why does ornithine decarboxylase have its own dedicated destruction mechanism, how does it work, and is it the only protein to be targeted to the proteasome in this way?

Antizyme, a mediator of ubiquitin-independent proteasomal degradation

Ornithine decarboxylase (ODC) is among the small set of proteasome substrates that is not ubiquitinated. It is instead degraded in conjunction with the protein antizyme (AZ). ODC and AZ are participants in a regulatory circuit that restricts pools of polyamines, the downstream products of ODC enzymatic activity. Functional studies using directed mutagenesis have identified regions of ODC and AZ required for the process of ODC degradation. Within ODC, there is a region that is required for AZ binding which lies on the surface of an alpha-beta barrel forming one domain of the ODC monomer. A carboxy-terminal ODC domain is needed for both AZ-dependent and AZ-independent degradation. Within AZ, the carboxy-terminal half molecule is sufficient for binding to ODC, but an additional domain found within the AZ amino terminus must be present for stimulation of ODC degradation by the proteasome. Recently, the AZs have been found to consist of an ancient gene family. Within vertebrate species, multiple isoforms are found, with distinct functions that remain to be sorted out. Although AZ homologs have been found in some yeast species, homology searches have failed to identify an AZ homolog in Saccharomyces cerevisiae. Nevertheless, the close parallel between polyamine-induced ODC degradation in S. cerevisiae and in animal cells suggests that this organism will also be found to harbor an AZ-like protein.

Inactivation and recovery of sodium currents in cerebellar Purkinje neurons: evidence for two mechanisms

We examined the kinetics of voltage-dependent sodium currents in cerebellar Purkinje neurons using whole-cell recording from dissociated neurons. Unlike sodium currents in other cells, recovery from inactivation in Purkinje neurons is accompanied by a sizeable ionic current. Additionally, the extent and speed of recovery depend markedly on the voltage and duration of the prepulse that produces inactivation. Recovery is faster after brief, large depolarizations (e.g., 5 ms at +30 mV) than after long, smaller depolarizations (e.g., 100 ms at -30 mV). On repolarization to -40 mV following brief, large depolarizations, a resurgent sodium current rises and decays in parallel with partial, nonmonotonic recovery from inactivation. These phenomena can be explained by a model that incorporates two mechanisms of inactivation: a conventional mechanism, from which channels recover without conducting current, and a second mechanism, favored by brief, large depolarizations, from which channels recover by passing transiently through the open state. The second mechanism is consistent with voltage-dependent block of channels by a particle that can enter and exit only when channels are open. The sodium current flowing during recovery from this blocked state may depolarize cells immediately after an action potential, promoting the high-frequency firing typical of Purkinje neurons.

Polyamines as gating molecules of inward-rectifier K+ channels

Inward-rectifier potassium (Kir) channels comprise a superfamily of potassium (K+) channels with unique structural and functional properties. Expressed in virtually all types of cells they are responsible for setting the resting membrane potential, controlling the excitation threshold and secreting K+ ions. All Kir channels present an inwardly rectifying current-voltage relation, meaning that at any given driving force the inward flow of K+ ions exceeds the outward flow for the opposite driving force. This inward-rectification is due to a voltage-dependent block of the channel pore by intracellular polyamines and magnesium. The present molecular-biophysical understanding of inward-rectification and its physiological consequences is the topic of this review. In addition to polyamines, Kir channels are gated by intracellular protons, G-proteins, ATP and phospholipids depending on the respective Kir subfamily as detailed in the following review articles.

Polyamines: mysterious modulators of cellular functions

In recent years the functions of polyamines (putrescine, spermidine, and spermine) have been studied at the molecular level. Polyamines can modulate the functions of RNA, DNA, nucleotide triphosphates, proteins, and other acidic substances. A major part of the cellular functions of polyamines can be explained through a structural change of RNA which occurs at physiological concentrations of Mg(2+) and K(+) because most polyamines exist in a polyamine-RNA complex within cells. Polyamines were found to modulate protein synthesis at several different levels including stimulation of special kinds of protein synthesis, stimulation of the assembly of 30 S ribosomal subunits and stimulation of Ile-tRNA formation. Effects of polyamines on ion channels have also been reported and are gradually being clarified at the molecular level.

A cell cycle-dependent internal ribosome entry site

The eukaryotic mRNA 5' cap structure facilitates translation. However, cap-dependent translation is impaired at mitosis, suggesting a cap-independent mechanism for mRNAs translated during mitosis. Translation of ornithine decarboxylase (ODC), the rate-limiting enzyme in the biosynthesis of polyamines, peaks twice during the cell cycle, at the G1/S transition and at G2/M. Here, we describe a cap-independent internal ribosome entry site (IRES) in the ODC mRNA that functions exclusively at G2/M. This ensures elevated levels of polyamines, which are implicated in mitotic spindle formation and chromatin condensation. c-myc mRNA also contains an IRES that functions during mitosis. Thus, IRES-dependent translation is likely to be a general mechanism to synthesize short-lived proteins even at mitosis, when cap-dependent translation is interdicted.

Dietary induction of ornithine decarboxylase in male mouse kidney

In male mouse kidney, ornithine decarboxylase (ODC) is induced after feeding, and the induction depends on dietary protein content. 24 h after feeding with 50% casein-containing meal, ODC activity and amount of immunoreactive ODC protein increased more than 10-fold, ODC mRNA level increased 2-fold, and the ODC half-life extended 7-fold. The renal ODC induction after feeding is, therefore, due mainly to stabilization of ODC protein. Urinary excretion of putrescine increased in response to the ODC induction, but the renal polyamine contents scarcely changed. Consistently, the level of antizyme, a polyamine-inducible protein, determined as the ODC-antizyme complex level, scarcely changed after feeding, and the antizyme/ODC ratio in the kidney largely decreased, resulting in the stabilization of ODC protein. The present results suggest that the strong excretion system of the kidney for newly synthesized polyamines enables renal ODC escape from antizyme-mediated feedback regulation.

RNA switches the higher-order structure of DNA

By the direct observation of single duplex DNA molecules by fluorescence microscopy, we found that RNA molecules have the potential to change discretely the higher-order structure of individual DNA molecules between the compact and elongated states. We performed an experiment with a linear giant DNA (T4 DNA, 166 kbp) and a circular DNA (cosmid vector, 42 kbp), and examined the effect of single-strand RNA on their conformations under a physiological concentration of spermidine. Individual DNA chains compacted by spermidine were elongated in an abrupt manner with an increase in the RNA concentration. This finding is discussed in view of the effect of the interplay between the dynamics of chromosomal DNA and the production of RNA in the cytoplasmic environment.

Polyamine-dependent facilitation of postsynaptic AMPA receptors counteracts paired-pulse depression

At many glutamatergic synapses in the brain, calcium-permeable alpha - amino - 3 - hydro - 5 - methyl - 4 - isoxazolepropionate receptor (AMPAR) channels mediate fast excitatory transmission. These channels are blocked by endogenous intracellular polyamines, which are found in virtually every type of cell. In excised patches, use-dependent relief of polyamine block enhances glutamate-evoked currents through recombinant and native calcium-permeable, polyamine-sensitive AMPAR channels. The contribution of polyamine unblock to synaptic currents during high-frequency stimulation may be to facilitate currents and maintain current amplitudes in the face of a slow recovery from desensitization or presynaptic depression. Here we show, on pairs and triples of synaptically connected neurons in slices, that this mechanism contributes to short-term plasticity in local circuits formed by presynaptic pyramidal neurons and postsynaptic multipolar interneurons in layer 2/3 of rat neocortex. Activity-dependent relief from polyamine block of postsynaptic calcium-permeable AMPARs in the interneurons either reduces the rate of paired-pulse depression in a frequency-dependent manner or, at a given stimulation frequency, induces facilitation of a synaptic response that would otherwise depress. This mechanism for the enhancement of synaptic gain appears to be entirely postsynaptic.

Polyamine stimulation of the synthesis of oligopeptide-binding protein (OppA). Involvement of a structural change of the Shine-Dalgarno sequence and the initiation codon aug in oppa mRNA

We previously suggested that the degree of polyamine stimulation of oligopeptide-binding protein (OppA) synthesis is dependent on the secondary structure and position of the Shine-Dalgarno (SD) sequence of OppA mRNA. To study the structural change of OppA mRNA induced by polyamines and polyamine stimulation of initiation complex formation, four different 130-mer OppA mRNAs containing the initiation region were synthesized in vitro. The structural change of these mRNAs induced by polyamines was examined by measuring their sensitivity to RNase T(1), specific for single-stranded RNA, and RNase V(1), which recognizes double-stranded or stacked RNA. In parallel, the effect of spermidine on mRNA-dependent fMet-tRNA binding to ribosomes was examined. Our results indicate that the secondary structure of the SD sequence and initiation codon AUG is important for the efficiency of initiation complex formation and that spermidine relaxes the structure of the SD sequence and the initiation codon AUG. The existence of a GC-rich double-stranded region close to the SD sequence is important for spermidine stimulation of fMet-tRNA binding to ribosomes. Spermidine apparently binds to this GC-rich stem and causes a structural change of the SD sequence and the initiation codon, facilitating an interaction with 30 S ribosomal subunits.

Spermine triggers the activation of caspase-3 in a cell-free model of apoptosis

Polyamines are ubiquitous organic cations required for cell proliferation. However, some evidence suggested that their excessive accumulation can induce apoptosis. We show here that, in a post-nuclear extract from U937 cells, the addition of spermine triggers the death program, represented by cytochrome c exit from mitochondria, the dATP-dependent processing of pro-caspase-3 and the onset of caspase activity. Spermine is more effective than spermidine, whereas putrescine has no effect. Polyamine acetylation abolishes their pro-apoptotic power. These data demonstrate a direct mechanism responsible for polyamine toxicity and also suggest that an excessive elevation of free polyamines could be involved in the transduction of a death signal.

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).

Formation of a complex containing ATP, Mg2+, and spermine. Structural evidence and biological significance

The conformation of ATP in the presence of Mg2+ and/or spermine was studied by 31P and 1H NMR, to clarify how polyamines interact with ATP. Spermine predominantly interacted with the beta- and gamma-phosphates of ATP in the presence of Mg2+. A conformational change of the beta- and gamma-phosphate of ATP with spermine could not be observed in the absence of Mg2+ by 31P NMR. It was found by 1H NMR that the conformation of adenosine moiety of ATP was not influenced significantly by spermine. The binding of Mg2+ to ATP was slightly inhibited by spermine and vice versa. The results indicate that the binding sites of Mg2+ and spermine on ATP only partially overlap. The PotA protein, an ATP-dependent enzyme, was used as a model system to study the biological role of the ATP-Mg2+-spermine complex. The ATPase activity of PotA was greatly enhanced by spermine. Double reciprocal plots at several concentrations of spermine as an activator indicate that spermine interacts with ATP, but not with PotA. The activity of protein kinase A was also stimulated about 2-fold by spermine. The results suggest that a ternary complex of ATP-Mg2+-spermine may play an important role in some ATP-dependent reactions in vivo and in the physiological effects of endogenous polyamines.

Polyamine modulation of mitochondrial calcium transport. II. Inhibition of mitochondrial permeability transition by aliphatic polyamines but not by aminoglucosides

In this study, the effects of polyamines and analogous compounds on mitochondrial permeability transition were characterized to distinguish between these effects and those on mitochondrial Ca2+ uptake, which are described in an accompanying report (Rustenbeck et al., Biochem Pharmacol 8: 977-985, 1998). When a transitional Ca2+ release from Ca2+-loaded mitochondria was induced by an acute increase in Ca2+ concentration in a cytosol-adapted incubation medium (Ca2+ pulse), this process was inhibited, but not abolished by spermine in the concentration range of 0.4 to 20 mM. The aminoglucoside, gentamicin, and the basic polypeptide, poly-L-lysine, which like spermine are able to enhance mitochondrial Ca2+ accumulation (preceding paper), had no or only a minimal inhibitory effect, while the aliphatic polyamine, bis(hexamethylene)triamine, which is unable to enhance mitochondrial Ca2+ accumulation, achieved a complete inhibition at 4 mM. The conclusion that the Ca2+ efflux was due to opening of the permeability transition pore was supported by measurements of mitochondrial membrane potential, ATP production, and oxygen consumption. Mg2+, a known inhibitor of mitochondrial membrane permeability transition, did not mimic the effects of spermine on mitochondrial Ca2+ accumulation, while ADP, the main endogenous inhibitor, showed both effects. However, a combination of spermine and ADP was significantly more effective than ADP alone in restoring low Ca2+ concentrations after a Ca2+ pulse. Two different groups of spermine binding sites were found at intact liver mitochondria, characterized by dissociation constants of 0.5 or 4.7 mM and maximal binding capacities of 4.6 or 19.7 nmol/mg of protein, respectively. In contrast to aminoglucosides, the aliphatic polyamine bis(hexamethylene)triamine did not displace spermine from mitochondrial binding sites. The total intracellular concentration of spermine in hepatocytes was measured to be ca. 450 microM and the free cytoplasmic concentration was estimated to be in the range of 10-100 microM. In conclusion, the enhancement of mitochondrial Ca2+ uptake by spermine is not an epiphenomenon of the inhibition of permeability transition. The physiological role of spermine appears to be that of an enhancer of mitochondrial Ca2+ accumulation rather than an inhibitor of permeability transition.

Facilitation of currents through rat Ca2+-permeable AMPA receptor channels by activity-dependent relief from polyamine block

1. In outside-out patches excised from human embryonic kidney (HEK) 293 cells expressing Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptor (AMPAR) channels, currents activated by 1 ms glutamate pulses at negative membrane potentials facilitated during and following a repetitive (2 to 100 Hz) agonist application. The degree of facilitation depended on subunit type, membrane potential and stimulation frequency being antagonized by a slow recovery from desensitization. 2. Activity-dependent current facilitation occurred in Ca2+-permeable but not in Ca2+-impermeable AMPAR channels. Current facilitation, however, does not depend on Ca2+ flux. Rather it reflects a relief from the block of Ca2+-permeable AMPARs by intracellular polyamines since facilitation occurred only in the presence of polyamines and since facilitated currents had a nearly linear current-voltage relation (I-V). 3. Relief from polyamine block was use dependent and occurred mainly in open channels. The relief mechanism was determined primarily by membrane potential rather than by current flow. 4. In closed channels the degree of polyamine block was independent of membrane potential. The voltage dependence of the rate of relief from the block in open channels rather than the voltage dependence of the block underlies the inwardly rectifying shape of the I-V at negative potentials. 5. Currents through native Ca2+-permeable AMPAR channels in outside-out or nucleated patches from either hippocampal basket cells or a subtype of neocortical layer II nonpyramidal cells also showed facilitation. 6. It is concluded that a use-dependent relief from polyamine block during consecutive AMPAR channel openings underlies current facilitation. This polyamine-AMPAR interaction may represent a new activity-dependent postsynaptic mechanism for control of synaptic signalling.

Neutralization of aspartate residues in the murine inwardly rectifying K+ channel IRK1 affects the substate behaviour in Mg2+ block

1. To investigate the molecular basis of the sublevels induced in the outward current during block by intracellular Mg2+, single-channel currents through inwardly rectifying K+ (IRK1) channels were studied. 2. cDNA encoding a functional murine IRK1 channel was transfected into COS-1 cells (a Green Monkey kidney cell line) using the liposome method, and voltage clamp experiments were done after 48-72 h. 3. Intracellular Mg2+ at micromolar concentrations induced sublevels in the outward current at one-third and two-thirds of the unitary amplitude seen in wild-type channels. Replacing Asp 172 with Asn (D172N) and Gln (D172Q) abolished these sublevels, i.e. the channel showed only the fully open and fully blocked states. 4. Both mutations reduced the Mg2+ sensitivity of the channel at 2 microM Mg2+. However, the Mg2+ sensitivity did not differ significantly at higher concentrations (10 microM) and voltages (+70 mV). 5. Channels expressed from D172E showed the sublevels, indicating that a negative charge is indispensable to the substate behaviour. 6. Channels from tandem tetramers of IRK1 with one and two D172N mutant subunits mainly showed sublevels with two-thirds amplitude, while those from tetramers with three D172N mutant subunits showed no sublevels. 7. These findings suggest that differences in Mg2+ binding patterns lead to different conductive states in a single-barrelled channel.

Polyamines and cerebral ischemia

It has been well established that alterations in polyamine metabolism are associated with animal models of global ischemia. Recently, this has been extended to include models of focal ischemia and traumatic brain injury. There is much evidence to support the idea that polyamines may play a multifaceted detrimental role following ischemia reperfusion. Due to the deficit of knowledge about their physiology in the CNS, the link between ischemia-induced alterations in polyamine metabolism and neuronal injury remains to be substantiated. With the recent revelation that polyamines are major intracellular modulators of inward rectifier potassium channels and certain types of NMDA and AMPA receptors, the long wait for the physiologic relevance of these ubiquitous compounds may be in sight. Therefore, it is now conceivable that the alterations in polyamines could have major effects on ion homeostasis in the CNS, especially potassium, and thus account for the observed injury after cerebral ischemia.

Two mechanisms for inward rectification of current flow through the purinoceptor P2X2 class of ATP-gated channels

1. The ATP receptor subunit P2X2 was expressed in Xenopus oocytes and human embryonic kidney (HEK) 293 cells. ATP-activated currents were studied with two-electrode voltage clamp recordings from oocytes, whole-cell recordings from HEK 293 cells, and outside-out patch clamp recordings from both cell types. The steady-state current-voltage (I-V) relation showed profound inward rectification in all recording configurations. 2. Recordings from outside-out patches demonstrated that inward rectification does not require intracellular Mg2+ or polyamines, and that inward rectification was present when the same solution was used on both sides of the patch. 3. Voltage jump experiments were performed to evaluate the voltage dependence of channel gating. After fast voltage jumps, instantaneous current jumps were followed by substantial relaxations to the steady state. The time course of the current relaxations could be fitted by single exponential functions. The instantaneous I-V relation was less inwardly rectifying than the steady-state I-V relation; however, it was not linear. 4. Single channel recordings indicated that the single channel conductance became smaller when the membrane potential became more positive. This decrease could quantitatively account for inward rectification of the instantaneous I-V relation. 5. We conclude that inward rectification of P2X2 is due to two mechanisms: voltage-dependent gating and voltage dependence of the single channel conductance.

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

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

Complex inhibition of OmpF and OmpC bacterial porins by polyamines

The effects of four polyamines (putrescine, cadaverine, spermidine, and spermine) on the activity of bacterial porins OmpC and OmpF were investigated by electrophysiology. Membrane vesicles made from the outer membrane of Escherichia coli strains expressing only OmpC or OmpF were reconstituted into liposomes probed by patch clamp. The channel activity was recorded in control solutions and in the presence of increasing concentrations of a specific polyamine. In all cases, concentration- and voltage-dependent inhibitory effects were observed. They include both the suppression of channel openings and the enhancement of channel closures as well as the promotion of blocked or inactivated states. OmpF and OmpC, although highly homologous, have distinct sensitivities to modulation, especially by spermine. This compound inhibits OmpF in the nanomolar range, which is in agreement with its potency on eukaryotic channels. Putrescine was the least effective (upper millimolar range) and also had inhibitory effects qualitatively distinct from those exerted by the other polyamines. The compounds appear to bind to at least two distinct binding sites, one of which resides within the pore. The potencies to this site are lower when the polyamines are applied from the extracellular side than from the periplasmic side, suggesting an asymmetric binding site.

Comparison of a left-handed Z-DNA molecular structure determined by X-rays with that simulated by a molecular dynamics

The 1.0 A resolution X-ray crystal structures of the left-handed Z-DNA(Z-I and Z-II conformations) were compared with that of the simulated molecular dynamics(MD) structure both in vacuo and in solution. Whilst the X-ray structure showed a tendency for the d(CG)3 molecule to take on a Z-II conformation in high salt solution or in strongly ionized conditions, the MD simulation with Na ion for 30 ps revealed that the left-handed d(CG)3 structure with the Z-I conformation was transformed into the Z-II conformation in the torsion angles of the C3, G4 and C5 phosphate groups, and furthermore, when K+ ion was used as the counterion instead of Na+ ion, the torsion angles in almost the entire d(CG)3 molecule were preserved. On the other hand, the MD calculation resulted in some very important changes on the sugar puckerings; the simulation with Na+ ion indicated that all the sugar puckerings of cytosine residues were changed from C2'-endo to C3'-endo, while those for guanosine residues tended to keep unchanged (C3'-endo) except for a terminal residue (G6).

Molecular mechanism of polyamine stimulation of the synthesis of oligopeptide-binding protein

Polyamine stimulation of the synthesis of oligopeptide-binding protein (OppA) was shown to occur mainly at the level of translation by measuring OppA synthesis and its mRNA level. Several artificial oppA genes were constructed by site-directed mutagenesis. These synthesize different kinds of OppA mRNAs: mRNAs differing in the size of 5'-untranslated region; mRNAs having the Shine-Dalgarno (SD) sequence in a different position; mRNAs having different secondary structure in the region of the SD sequence; and fusion mRNAs consisting of the 5'-untranslated region of OppA mRNA and the open reading frame of beta-galactosidase. By measuring the synthesis of OppA or beta-galactosidase from these mRNAs, we found that the 171-nucleotide 5'-untranslated region and 145 nucleotides of the ORF of OppA mRNA are involved in the polyamine stimulation of OppA synthesis. When the secondary structure of the above region of OppA mRNA was analyzed by optimal computer folding, it was shown that the degree of polyamine stimulation of OppA protein synthesis was dependent on the structure of the SD sequence in addition to its position. Loose base pairing of the SD sequence with other regions of the mRNA caused strong polyamine stimulation, while intense base pairing of the SD sequence with other regions of the mRNA resulted in insignificant or weak polyamine stimulation.

Time-dependent outward currents through the inward rectifier potassium channel IRK1. The role of weak blocking molecules

Outward currents through the inward rectifier K+ channel contribute to repolarization of the cardiac action potential. The properties of the IRK1 channel expressed in murine fibroblast (L) cells closely resemble those of the native cardiac inward rectifier. In this study, we added Mg2+ (0.44-1.1 mM) or putrescine (approximately 0.4 mM) to the intracellular milieu where endogenous polyamines remained, and then examined outward IRK1 currents using the whole-cell patch-clamp method at 5.4 mM external K+. Without internal Mg2+, small outward currents flowed only at potentials between -80 (the reversal potential) and approximately -40 mV during voltage steps applied from -110 mV. The strong inward rectification was mainly caused by the closed state of the activation gating, which was recently reinterpreted as the endogenous-spermine blocked state. With internal Mg2+, small outward currents flowed over a wider range of potentials during the voltage steps. The outward currents at potentials between -40 and 0 mV were concurrent with the contribution of Mg2+ to blocking channels at these potentials, judging from instantaneous inward currents in the following hyperpolarization. Furthermore, when the membrane was repolarized to -50 mV after short depolarizing steps (> 0 mV), a transient increase appeared in outward currents at -50 mV. Since the peak amplitude depended on the fraction of Mg(2+)-blocked channels in the preceding depolarization, the transient increase was attributed to the relief of Mg2+ block, followed by a re-block of channels by spermine. Shift in the holding potential (-110 to -80 mV), or prolongation of depolarization, increased the number of spermine-blocked channels and decreased that of Mg(2+)-blocked channels in depolarization, which in turn decreased outward currents in the subsequent repolarization. Putrescine caused the same effects as Mg2+. When both spermine (1 microM, an estimated free spermine level during whole-cell recordings) and putrescine (300 microM) were applied to the inside-out patch membrane, the findings in whole-cell IRK1 were reproduced. Our study indicates that blockage of IRK1 by molecules with distinct affinities, spermine and Mg2+ (putrescine), elicits a transient increase in the outward IRK1, which may contribute to repolarization of the cardiac action potential.

Modulation and block of ion channels: a new biology of polyamines

The endogenous polyamines, spermine, spermidine, and putrescine have effects on several types of cation channels. Intracellular polyamines, in particular spermine, contribute to intrinsic gating and rectification of strong inward rectifier K+ channels. Intracellular spermine is also responsible for inward rectification of some types of Ca(2+)-permeable AMPA and kainate receptors. Spermine has a number of effects on the activity of the NMDA subtype of glutamate receptor, involving two or more extracellular polyamine binding sites on the NMDA receptor. In K+ channels and glutamate receptors, some of the amino acids in the receptor/channel structure that influence to polyamines have been identified, leading to a partial understanding of the effects of polyamines at a molecular level. Block of K+ channels by intracellular polyamines is likely to be an important receptors by intracellular spermine and modulation by extracellular spermine may affect excitability and the influx of Ca2+ in neurons and glial cells of the nervous system.

Impairment of DNA replication within one cell cycle after seeding of cells in the presence of a polyamine-biosynthesis inhibitor

Chinese hamster ovary (CHO) cells in the plateau phase were seeded in the absence or presence of 5 mM 2-difluoromethylornithine (F2MeOrn), an enzyme-activated irreversible inhibitor of ornithine decarboxylase. The thymidine analogue bromodeoxyuridine (BrdUrd, 5 microM) was added to the culture medium 30 min before sampling of the cells, which occurred 1-17 h after seeding. Using flow cytometry, coupled with an indirect immunofluorescence technique, which utilized monoclonal BrdUrd and secondary fluorescein-isothiocyanate-conjugated antibodies, and the DNA stain propidium iodide, cellular BrdUrd and DNA contents were quantified. To determine if there was a perturbation in the progression of cells through the S phase, the distribution of BrdUrd-labelled cells in the S phase was evaluated in two ways: (a) by calculating the mean DNA content of BrdUrd-labelled cells in relation to the mean DNA contents of G1 and G2 cells (relative movementzero) and (b) by studying DNA histograms of BrdUrd-labelled cells. By using both evaluation methods, we show that DNA replication was impaired during the first cell cycle that was initiated after seeding CHO cells in the presence of F2MeOrn. The cells appeared to enter the S phase normally but were then delayed in their progression through this phase. The impairment of F2MeOrn treatment on DNA replication was apparent at 9 h after seeding, a time point at which the putrescine pool was depleted, the spermidine pool was approximately halved, and the spermine pool was unaffected, when compared to corresponding pools of control cells. When cells were seeded in the presence of F2MeOrn and putrescine, the effect on DNA replication was prevented. The rates of incorporation of [3H]uridine and [3H]leucine into RNA and protein, respectively, were the same in control and in F2MeOrn-treated cells for at least up to 11 h after seeding.

Decrease in cell viability due to the accumulation of spermidine in spermidine acetyltransferase-deficient mutant of Escherichia coli

Physiological functions of spermidine acetyltransferase in Escherichia coli have been studied using the spermidine acetyltransferase (speG) gene-deficient mutant CAG2242 and the cloned speG gene. The growth of E. coli CAG2242 in the defined M9 medium was normal in the presence and absence of 0.5mM spermidine. However, cell viability of E. coli CAG2242 at 48 h after the onset of growth decreased greatly by the addition of 0.5 mM spermidine. The amount of spermidine accumulated in the cells was approximately 3-fold that in the cells grown in the absence of spermidine. Transformation of the cloned speG gene to E. coli CAG2242 recovered the cell viability. Decreased in cell viability of E. coli CAG2242 was observed even when 0.5mM spermidine was added at 24 h after the onset of growth. The results indicate that accumulated spermidine functions at the late stationary phase of growth. The accumulation of spermidine caused a decrease in protein synthesis but not in DNA and RNA synthesis at 28 h after the onset of growth. The synthesis of several kinds of proteins was particularly inhibited. They included ribosome modulation factor and OmpC protein. Since the ribosome modulation factor is essential for cell viability at the stationary phase of growth (Yamagishi, M., Matsushima, H., Wada, A., Sakagami, M., Fujita, N., and Ishihama, A. (1993) EMBO J. 12, 625-630), the decrease in the protein was thought to be one of the reasons for the decrease in cell viability. The decrease in the ribosome modulation factor mainly occurred at the translational level.

Inward rectification of both AMPA and kainate subtype glutamate receptors generated by polyamine-mediated ion channel block

CA2+-permeable glutamate receptors assembled from subunits containing a GLN residue at the RNA editing site in membrane domain 2 show strong inward rectification. In HEK 293 cells transfected with the kainate receptor subunit GluR6(Q), inward rectification is lost in outside-out patches, suggesting a role for diffusible, cytoplasmic factors. Inclusion of different polyamines in the internal solution restored inward rectification, whereas Mg2+ (1 mM) was inactive. Spermidine (Kd[0 mV] = 5.5 microM) was of higher affinity than spermidine (Kd[0 mV] = 25.4 microM) or putrescine (Kd[0 mV] = 1.2 mM). AMPA receptors assembled from GluRA(flip) showed even higher affinity for spermine (Kd[0 mV] = 1.5 microM). Analysis of the voltage dependence of whole-cell responses predicted intracellular free spermine and spermidine concentrations of 51 and 153 muM, respectively.

Spermine gates inward-rectifying muscarinic but not ATP-sensitive K+ channels in rabbit atrial myocytes. Intracellular substance-mediated mechanism of inward rectification

The effect of spermine, a low molecular mass aliphatic amine with positive charges, on the strongly inwardly rectifying muscarinic K+ (KACh) channel was examined in rabbit atrial myocytes. In inside-out patch membranes, the single channel current-voltage relationship of KACh channels activated by guanosine 5'-3-O-(thio)triphosphate became linear in the absence of intracellular Mg2+. The open probability (Po) of the channels did not show significant voltage dependence under these conditions. Spermine specifically reduced Po of outwardly flowing KACh channel currents without affecting the unitary current amplitude at depolarized potentials, but had no effect on inward KACh currents under hyperpolarization. This voltage dependence of Po of KACh channels in the presence of spermine resembled that normally observed in the whole cell or open cell-attached configurations. Spermine (300 nM to 3 microM) also restored the relaxation of KACh currents which had been lost in the inside-out configuration. The effect of spermine was concentration-dependent with IC50 of approximately 10 nM at +40 mV. The order of potency of polyamines in reducing Po at +40 mV was spermine > or = spermidine > putrescine > ornithine; arginine had no significant effect. Intracellular Mg2+ antagonized the effect of spermine. Neither the single channel conductance nor Po of the ATP-sensitive K+ channel, a weak inward rectifier, was affected by spermine. Because submillimolar concentrations of spermine and spermidine are available in the cytosol of most cells, these substances may be the unidentified intracellular gating factors for strong inward rectifiers such as KACh and IK1 channels.

Effect of polyphosphates on the activity of amine oxidases

The interaction between polyphosphates and polyamines was investigated by 31P-NMR spectroscopy and by amine oxidase activity measurements. An apparent competition between negatively charged polyphosphates (ATP, ADP, AMP, tripolyphosphate and pyrophosphate) and positively charged polyamine, for the active site of bovine serum and soybean seedling amine oxidases, was observed by activity measurements. This behavior was explained by formation of polyamine-polyphosphate complexes and the stability constants of these complexes were calculated by 31P NMR. However, at a given concentration of polyphosphate, the amine oxidase activity was found higher than that expected on the basis of the free amine concentration calculated according to the NMR stability constant. This fact, and the different extent of inhibition of the spermidine oxidase activity of soybean seedling and of bovine serum amine oxidases observed in the presence of a given polyphosphate, suggest that amine oxidases may be active also on the polyamine-polyphosphate complexes. This hypothesis was supported by the strong dependence of the kcat/Km of bovine serum amine oxidase on ionic strength, indicating an electrostatic interaction between the charged amine and the active site, while no effect of ionic strength on kcat/Km was observed in the presence of ATP. A kinetic model of this behavior was found to fit the experimental data.

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

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

Strong voltage-dependent inward rectification of inward rectifier K+ channels is caused by intracellular spermine

Inward rectifier K+ channels mediate the K+ conductance at resting potential in many types of cell. Since these K+ channels do not pass outward currents (inward rectification) when the cell membrane is depolarized beyond a trigger threshold, they play an important role in controlling excitability. Both a highly voltage-dependent block by intracellular Mg2+ and an endogenous gating process are presently assumed to underly inward rectification. It is shown that strong voltage dependence of rectification found under physiological conditions is predominantly due to the effect of intracellular spermine. Physiological concentrations of free spermine mediate strong rectification of IRK1 inward rectifier K+ channels even in the absence of free Mg2+ and in IRK1 mutant channels that have no endogenous rectification.

A structural determinant of differential sensitivity of cloned inward rectifier K+ channels to intracellular spermine

Large subtype-specific differences in the sensitivity of cloned inward-rectifier K+ channels of the IRK1, BIR10 and ROMK1 subtype to being blocked by intracellular spermine (SPM) are described. It is shown, by site-directed mutagenesis, that the four orders of magnitude larger SPM sensitivity of BIR10 channels compared to ROMK1 channels may be explained by a difference in a single amino acid in the putative transmembrane segment TMII. This residue, a negatively charged glutamate in BIR10, is homologous to the residue in IRK1 and ROMK1 which has previously been shown to change gating properties and Mg2+ sensitivity. Differential block by physiological SPM concentrations is suggested as a major functional difference between subtypes of inward-rectifier K+ channels.

Involvement of polyamines in epidermal growth factor (EGF), transforming growth factor (TGF)-alpha and -beta 1 action on culture of L6 and fetal bovine myoblasts

1. alpha-Difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase significantly abolished stimulation of protein synthesis evoked by EGF, TGF-alpha or -beta 1 in L6 and fetal bovine myoblasts. 2. The participation of polyamines in early events evoked by growth factors was shown by a significant stimulation of ornithine decarboxylase and S-adenosylmethionine decarboxylase activity as well as increased concentration of spermidine and spermine in L6 cells exposed to TGF-alpha and EGF. 3. TGF-beta 1 at a high concentration (1 ng/ml) increased protein synthesis in L6 myoblasts but inhibited it in fetal bovine myoblasts. Metabolic effects of TGF-beta 1 in L6 cells was associated with an enhancement of decarboxylase activities, however there were no significant changes in cellular polyamine concentrations. Presented data suggest that polyamines are involved in the signal transduction pathway of EGF, TGF-alpha, and -beta 1 in L6 and fetal bovine myoblasts.

Correlation between the inhibition of cell growth by bis(ethyl)polyamine analogues and the decrease in the function of mitochondria

The antiproliferating effect of nine kinds of bis(ethyl)polyamine analogues [three kinds each of bis(ethyl)triamine, bis(ethyl)tetraamine and bis(ethyl)pentaamine] was compared using FM3A cells. The inhibitory effect was in the order BE4444 > BE3443 > BE4334 > or = BE444 > BE343 > BE333 > BE44 > BE34 > BE33. Our results indicate that not only polyamine deficiency but also the accumulation of polyamine analogues is involved in the inhibition of cell growth. Accumulation of bis(ethyl)polyamine analogues caused the inhibition of protein synthesis and the decrease in the ATP content. The protein synthetic system in mitochondria was more strongly inhibited by bis(ethyl)polyamine analogues than that in the cytoplasm. Under conditions such that cytoplasmic protein synthesis was inhibited by 50% by bis(ethyl)polyamine analogues, mitochondrial protein synthesis was almost completely inhibited. Mitochondrial Ile-tRNA formation was inhibited by bis(ethyl)polyamine analogues at the concentrations that cytoplasmic Ile-tRNA formation was stimulated. This may be one of the reasons for the selective inhibition of mitochondrial protein synthesis. This inhibition was followed by the decrease in ATP content, swelling of mitochondria and depletion of mitochondrial DNA. These results suggest that the early event of metabolic change caused by bis(ethyl)polyamine analogues in cells is the inhibition of protein synthesis, especially of mitochondrial protein synthesis.

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.

Correlation between the inhibition of cell growth by accumulated polyamines and the decrease of magnesium and ATP

The mechanism of the antiproliferation effect of spermidine and spermine was studied using a cell culture system of mouse FM3A cells. The addition of either 10 mM spermidine or 2 mM spermine to the growth medium containing 0.9 mM Mg2+ greatly inhibited cell growth (more than 90%). A decrease in the Mg2+ concentration to 50 microM in the growth medium, but without the polyamine addition, did not influence cell growth. However, the concentrations of spermidine and spermine necessary for the inhibition of cell growth when cells were cultured in the presence of 50 microM Mg2+ were much smaller (2 mM spermidine and 0.15 mM spermine). Nevertheless, the amount of polyamines accumulating in cells which could cause the inhibition of cell growth was almost the same, regardless of the large difference in the added polyamine concentrations. At the early stage of polyamine accumulation, the inhibition of cell growth correlated with the decrease of Mg2+ content, but not with a decrease of the ATP content. The decrease in Mg2+ content correlated well with the inhibition of macromolecular synthesis, especially protein synthesis. Thus, the inhibition of cell growth at the early stage of polyamine accumulation was thought to be due to the inactivation of ribosomes through the replacement of Mg2+ on magnesium-binding sites by polyamines. The decrease in Mg2+ content was mainly caused by the inhibition of Mg2+ transport by polyamines. At the later stage of polyamine accumulation, a decrease in ATP content was also observed. This was followed by swelling of the mitochondria, which may be a symptom of the subsequent cell death.

A new enzyme-linked immunosorbent assay (ELISA) for studying immunocytochemical procedures using an antiserum produced against spermidine as a model

Antiserum was produced in rabbits against the polyamine spermidine (Spd) conjugated to bovine serum albumin (BSA). The reactivity of the serum to Spd and a variety of structurally related compounds was quantified by a new immunocytochemical model system incorporating an enzyme-linked immunosorbent assay (ELISA) binding test. This is based on the principle of coupling these compounds to the wells of microtiter plate activated with poly-L-lysine and glutaraldehyde and incubating the wells by the indirect immunoperoxidase method. The antiserum showed a 25% cross reaction with spermine (Spm), putrescine (Put), and cadaverine (Cad), and a 1% cross reaction with 1,3-diaminopropane (Dap), but no cross reaction with monoacetyl polyamines and amino acids. The antibody binding was inhibited most effectively by absorption of the antiserum with N1-acetylspermidine and Spd in the ELISA inhibition test. Also, immunoblot analysis of the antiserum with nitrocellulose paper gave completely identical results to the ELISA binding tests. Spd-like immunoreactivities in human melanoma BD and neuroblastoma IMR 32 cell lines are presented as examples of the staining pattern obtained with the antiserum. Absorption of the serum with N1-acetylspermidine and Spd was demonstrated to abolish the immunostaining reaction. The immunohistochemical model is simple: amines and amino acids are bound in the same way as in aldehyde-fixed tissues and, in comparison to immunoblot analysis, the immunoreactivity can be more easily and accurately quantified by assay with the antibody. The model should prove useful in assessing the specificity of other antisera.