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

Agmatine transport into spinal nerve terminals is modulated by polyamine analogs

Agmatine (decarboxylated arginine) is an endogenous amine found in the CNS that antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits hyperalgesia evoked by inflammation, nerve injury and intrathecally administered NMDA. These actions suggest an antiglutamatergic neuromodulatory role for agmatine in the spinal cord. Such a function would require a mechanism of regulated clearance of agmatine such as neuronal or glial uptake. Consistent with this concept, radiolabeled agmatine has been shown to accumulate in synaptosomes, but the mechanism of this transport has not been fully characterized. The present study describes an agmatine uptake system in spinal synaptosomes that appears driven by a polyamine transporter. [(3)H]Agmatine uptake was Ca(2+), energy and temperature dependent. [(3)H]Agmatine transport was not moderated by L-arginine, L-glutamate, glycine, GABA, norepinephrine or serotonin. In contrast, [(3)H]agmatine uptake was concentration dependently inhibited by unlabeled putrescine and by unlabeled spermidine (at significantly higher concentrations). Similarly, [(3)H]putrescine uptake was inhibited in a concentration-dependent manner by unlabeled agmatine and spermidine. The polyamine analogs paraquat and methylglyoxal bis (guanylhydrazone) inhibited, whereas the polyamine transport enhancer difluoromethylornithine increased, [(3)H]agmatine transport. Taken together, these results suggest that agmatine transport into spinal synaptosomes may be governed by a polyamine transport mechanism.

Extracellular polyamines regulate fluid secretion in rat colonic crypts via the extracellular calcium-sensing receptor

BACKGROUND AND AIMS

Polyamines are essential for the normal postnatal development, maintenance, and function of gastrointestinal epithelia. The extracellular Ca(2+) (Ca(2+)(o)/nutrient)-sensing receptor is expressed on both luminal and basolateral membranes of colonocytes, and, in other cell systems, this receptor has been shown to respond to polyamines. Thus, the Ca(2+)-sensing receptor could provide a mechanism for modulation of colonocyte function by dietary and systemic extracellular polyamines. In the present study, we investigated the interaction of polyamines, particularly spermine, and extracellular Ca(2+) on second messenger generation by, and on function of, rat distal colonic crypts.

METHODS

Calcium-sensing receptor activation was assessed in colonic epithelial cells and intact crypts freshly isolated from distal colon by monitoring intracellular IP(3) and Ca(2+) accumulation using radioimmunoassay and Fluo-3 fluorometry, respectively. Interactions of extracellular Ca(2+) and spermine on regulation of both basal and forskolin-stimulated fluid transport were measured in crypts microperfused in vitro.

RESULTS

Polyamine (spermine > spermidine > putrescine)-mediated enhancement of intracellular D-myo-inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) accumulation required extracellular Ca(2+), and the EC(50) for extracellular Ca(2+)-mediated activation of the calcium-sensing receptor was reduced by polyamines. Extracellular spermine modulated both basal and forskolin-stimulated fluid secretion in perfused colonic crypts, and the EC(50) for spermine-induced reduction in forskolin-stimulated fluid secretion was inversely dependent on extracellular Ca(2+) (Ca(2+)(o)).

CONCLUSIONS

The interactions of extracellular Ca(2+) and polyamines on second messenger accumulation and fluid secretion support a role for the luminal and basolateral calcium-sensing receptors in mediating some of the effects of polyamines on distal colonic epithelial cells.

Mode of Ca2+ action on ciliary beat frequency in single ovine airway epithelial cells

1. We analysed the kinetics of coupling between cytoplasmic calcium ([Ca2+]i) and ciliary beat frequency (CBF) using simultaneous single cilium recording and single cell [Ca2+]i measurements from cultured ovine tracheal epithelial cells. 2. CBF and [Ca2+]i (indicated by fura-2) were measured at rest and in response to activation of the G-protein coupled M3 muscarinic receptor by 10 microM acetylcholine (ACh). 3. Fourier transform analysis of 3 s data segments of light intensity from phase-contrast microscopy showed no significant delay between changes in [Ca2+]i and CBF during a 2 min exposure to ACh and subsequent washout. 4. CBF time resolution was improved by computing instantaneous beat frequency. This revealed that CBF lagged the rapid increase in [Ca2+]i in response to ACh with a delay of less than 1 beat cycle (143 ms at 7 Hz). When CBF was estimated by an improved Fourier method, this delay was observed to be 70 +/- 30 ms (mean +/- s.e.m.; n = 20 cilia). During the slower return to baseline, a lag of 8 +/- 3.2 s was observed, indicative of hysteresis. 5. While calmodulin inhibitors (calmidazolium and W-7; each n = 5) decreased baseline CBF by an average of 1.1 +/- 0.1 Hz, they did not alter the kinetic relationship between [Ca2+]i and CBF. Similarly, phosphatase inhibitors (okadaic acid and cyclosporin A; each n = 5), changed neither baseline CBF nor the kinetic coupling between [Ca2+]i and CBF. 6. These data suggest that the timing of Ca2+ action on CBF in ovine airway epithelial cells, is unlikely to be determined by phosphorylation reactions involving calmodulin or kinase/phosphatase reactions. 7. A simple model for Ca2+ stimulation of CBF is presented. Fits of the model to the data suggest four or more Ca2+ ions bind cooperatively to speed up CBF.

Polyamines. An overview

The polyamines spermine, spermidine, and putrescine are small organic molecules one or more of which are present in all living organisms. Many natural products contain polyamine residues. Polyamines are synthesized by a highly regulated pathway from arginine or ornithine and also can be transported in and out of cells. Polyamines are degraded to a variety of compounds the functions of which are largely unknown. Polyamines influence the transcriptional and translational stages of protein synthesis, stabilize membranes, and, in mammalian systems, modulate neurophysiological functions and may act as intracellular messengers. However, at the molecular level the mode of action of the polyamines is largely unknown.

Polyamine sulfonamides with NMDA antagonist properties are potent calmodulin antagonists and cytotoxic agents

N1-Dansylspermine and related sulfonamides of the natural polyamines are very potent blockers of NMDA-type glutamate receptors. They exhibit pharmacological properties which were not predicted from the constituents of the conjugates. Cytotoxicity and calmodulin antagonism of N1-dansylspermine were especially impressive. Calmodulin antagonism implies that N1-dansylspermine prevents induction of ornithine decarboxylase and inhibits its own active uptake via the polyamine transport system. Structure-activity considerations demonstrated that an aromatic character of the substituent is not required; amide bond formation with an aliphatic sulfonic acid is sufficient to transform spermine into a highly toxic calmodulin antagonist. Cytotoxicity and calmodulin antagonism are properties which are intrinsic to spermine, but they are observed only at very high concentrations. Amide bond formation at N1 with a lipophilic residue appears to 'amplify' these normally latent properties. The use of polyamine conjugates structurally related to the amides described in this work for targeting tumours may be marred by their calmodulin antagonism.

Spermine inhibits proinflammatory cytokine synthesis in human mononuclear cells: a counterregulatory mechanism that restrains the immune response

The local production of proinflammatory cytokines mediates the host response to inflammation, infection, and injury, whereas an overexpression of these mediators can injure or kill the host. Recently, we identified a class of multivalent guanylhydrazone compounds that are effective inhibitors of proinflammatory cytokine synthesis in monocytes/macrophages. The structure of one such cationic molecule suggested a molecular mimicry with spermine, a ubiquitous endogenous biogenic amine that increases significantly at sites of inflammation and infection. Here, we addressed the hypothesis that spermine might counterregulate the innate immune response by downregulating the synthesis of potentially injurious cytokines. When spermine was added to cultures of human peripheral blood mononuclear cells stimulated with lipopolysaccharide (LPS), it effectively inhibited the synthesis of the proinflammatory cytokines tumor necrosis factor (TNF), interleukin-1 (IL-1), IL-6, MIP-1alpha, and MIP-1beta. The inhibition of cytokine synthesis was specific and reversible, with significant inhibition of TNF synthesis occurring even when spermine was added after LPS. The mechanism of spermine-mediated cytokine suppression was posttranscriptional and independent of polyamine oxidase activity. Local administration of spermine in vivo protected mice against the development of acute footpad inflammation induced by carrageenan. These results identify a distinct molecular counterregulatory role for spermine in downregulating the monocyte proinflammatory cytokine response.

Extracellular calcium stimulates Na(+)-dependent putrescine uptake in B16 melanoma cells

The regulation of putrescine transport in difluoromethylornithine-treated B16 melanoma cells by extracellular Ca2+ has been investigated. It was found that physiological concentrations of Ca2+ were essential for optimum uptake of putrescine and spermidine, Mg2+, albeit at higher concentrations, also could potentiate polyamine transport. The maximum rate of putrescine uptake increased from 1698 +/- 67 pmol/min/min/mg DNA in the absence of Ca2+ to 3100 +/0 98 pmol/min/mg DNA in the presence of 0.5 mM Ca2+. There was no change in Km. While Ca2+ enhanced transport of both putrescine and spermidine it did not affect the uptake of deoxyglucose, thymidine or leucine. Putrescine did not alter Ca2+ fluxes suggesting that the two cations do not share a common transport system. The effects of Ca2+ on putrescine uptake appeared to be mediated extracellularly firstly because Ca2+ did not potentiate putrescine uptake in the presence of A23187 and secondly, because the effects of Ca2+ were completely inhibited by the lanthanide Tb3+, which binds to calcium-dependent proteins and does not readily cross biological membranes, Ca2+ did not affect putrescine transport in the absence of extracellular Na+. Moreover, the rate of putrescine uptake in the absence of Ca2+ was similar to that in the absence of extracellular Na+. The results from this study indicate that polyamine transport is stimulated by extracellular Ca2+ and suggest that Ca2+ is required for activity of the Na(+)-dependent transporter only. This transporter appears to possess a regulatory binding site for divalent cations.

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.

Putrescine modulation of depolarization-induced [3H]serotonin release from fish brain synaptosomes

In this study, we investigated the effects of putrescine, a precursor of polyamine metabolism, on depolarization (potassium chloride, KCl)-induced [3H]serotonin (5-hydroxytryptamine (5-HT)) release from brain synaptosomes of rainbow trout (Oncorhynchus mykiss). An irreversible inhibitor of polyamine metabolism, difluoromethyl-ornithine (DFMO), inhibited the KCl-induced [3H]serotonin efflux, and addition of putrescine (PUT) reversed the inhibitory effects of the former in synaptosomal preparations. In Fura-2/AM loaded synaptosomes, KCl stimulated Ca2+ influx. Hence, prior addition of EGTA abolished the KCl-induced Ca2+ responses, indicating that KCl-induced depolarization recruited Ca2+ from extracellular medium. The DFMO curtailed the KCl-evoked Ca2+ influx, and addition of PUT reversed the inhibitory effects of DFMO. Furthermore, fish brain was found to contain polyamines in high amounts (e.g. putrescine from 600 +/- 20 to 850 +/- 30 nmol/g protein). These results suggest that polyamine putrescine, present in fish brain, might be involved in [3H]serotonin efflux via Ca2+ influx mechanisms.

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.