An emerging role for agmatine

Structure of the E. coli agmatinase, SPEB

Agmatine amidinohydrolase, or agmatinase, catalyzes the conversion of agmatine to putrescine and urea. This enzyme is found broadly across kingdoms of life and plays a critical role in polyamine biosynthesis and the regulation of agmatine concentrations. Here we describe the high-resolution X-ray crystal structure of the E. coli agmatinase, SPEB. The data showed a relatively high degree of pseudomerohedral twinning, was ultimately indexed in the P31 space group and led to a final model with eighteen chains, corresponding to three full hexamers in the asymmetric unit. There was a solvent content of 38.5% and refined R/Rfree values of 0.166/0.216. The protein has the conserved fold characteristic of the agmatine ureohydrolase family and displayed a high degree of structural similarity among individual protomers. Two distinct peaks of electron density were observed in the active site of most of the eighteen chains of SPEB. As the activity of this protein is known to be dependent upon manganese and the fold is similar to other dinuclear metallohydrolases, these peaks were modeled as manganese ions. The orientation of the conserved active site residues, in particular those amino acids that participate in binding the metal ions and a pair of acidic residues (D153 and E274 in SPEB) that play a role in catalysis, are similar to other agmatinase and arginase enzymes and is consistent with a hydrolytic mechanism that proceeds via a metal-activated hydroxide ion.

Comprehensive metabolomic study of the response of HK-2 cells to hyperglycemic hypoxic diabetic-like milieu

Diabetic nephropathy (DN) is the leading cause of chronic kidney disease. Although hyperglycaemia has been determined as the most important risk factor, hypoxia also plays a relevant role in the development of this disease. In this work, a comprehensive metabolomic study of the response of HK-2 cells, a human cell line derived from normal proximal tubular epithelial cells, to hyperglycemic, hypoxic diabetic-like milieu has been performed. Cells simultaneously exposed to high glucose (25 mM) and hypoxia (1% O2) were compared to cells in control conditions (5.5 mM glucose/18.6% O2) at 48 h. The combination of advanced metabolomic platforms (GC-TOF MS, HILIC- and CSH-QExactive MS/MS), freely available metabolite annotation tools, novel databases and libraries, and stringent cut-off filters allowed the annotation of 733 metabolites intracellularly and 290 compounds in the extracellular medium. Advanced bioinformatics and statistical tools demonstrated that several pathways were significantly altered, including carbohydrate and pentose phosphate pathways, as well as arginine and proline metabolism. Other affected metabolites were found in purine and lipid metabolism, the protection against the osmotic stress and the prevention of the activation of the β-oxidation pathway. Overall, the effects of the combined exposure of HK-cells to high glucose and hypoxia are reasonably compatible with previous in vivo works.

Agmatine protects Müller cells from high-concentration glucose-induced cell damage via N-methyl-D-aspartic acid receptor inhibition

Neural injury is associated with the development of diabetic retinopathy. Müller cells provide structural and metabolic support for retinal neurons. High glucose concentrations are known to induce Müller cell activity. Agmatine is an endogenous polyamine, which is enzymatically formed in the mammalian brain and has exhibited neuroprotective effects in a number of experimental models. The aims of the present study were to investigate whether agmatine protects Müller cells from glucose-induced damage and to explore the mechanisms underlying this process. Lactate dehydrogenase activity and tumor necrosis factor-α mRNA expression were significantly reduced in Müller cells exposed to a high glucose concentration, following agmatine treatment, compared with cells not treated with agmatine. In addition, agmatine treatment inhibited glucose-induced Müller cell apoptosis, which was associated with the regulation of Bax and Bcl-2 expression. Agmatine treatment suppressed glucose-induced phosphorylation of mitogen-activated protein kinase (MAPK) protein in Müller cells. The present study demonstrated that the protective effects of agmatine on Müller cells were inhibited by N-methyl-D-aspartic acid (NMDA). The results of the present study suggested that agmatine treatment protects Müller cells from high-concentration glucose-induced cell damage. The underlying mechanisms may relate to the anti-inflammatory and antiapoptotic effects of agmatine, as well as to the inhibition of the MAPK pathway, via NMDA receptor suppression. Agmatine may be of use in the development of novel therapeutic approaches for patients with diabetic retinopathy.

A novel liquid-liquid extraction and stable isotope dilution NCI-GC-MS method for quantitation of agmatine in postmortem brain cortex

The group of biologically important amines includes putrescine, spermidine and spermine, as well as agmatine, which is a guanidino-amine. There is considerable evidence supporting a role of these amines in the etiology and pathology of mental disorders. We have previously developed a quantitative GC-MS method for simultaneous measurement of three major polyamines to support our studies linking polyamines to mental disorders. However, a unique GC-MS method is required for agmatine. To efficiently extract agmatine from postmortem brain tissues, we developed an isopropanol based liquid-liquid extraction protocol using potassium carbonate as a salting-out agent which showed a much greater recovery than n-butanol used in earlier methods. The GC-MS analysis employed hexafluoroacetylacetone as derivatization reagent and was carried out using negative chemical ionization with total ion and selected ion monitoring. (15)N(4)-agmatine was synthesized from (15)N(4)-L-arginine and used as internal standard in a conventional stable isotope dilution assay. This method accurately measures the level of agmatine from very small quantities (10-20 mg) of postmortem brain tissue, with a quantitation limit down to 1 ng/g of wet tissue. The limit of detection is 0.01 ng/g of wet tissue.

Agmatine transport in brain mitochondria: a different mechanism from that in liver mitochondria

The diamine agmatine (AGM), exhibiting two positive charges at physiological pH, is transported into rat brain mitochondria (RBM) by an electrophoretic mechanism, requiring high membrane potential values and exhibiting a marked non-ohmic force-flux relationship. The mechanism of this transport apparently resembles that observed in rat liver mitochondria (RLM), but there are several characteristics that strongly suggest the presence of a different transporter of agmatine in RBM. In this type of mitochondria, the extent of initial binding and total accumulation is higher and lower, respectively, than that in liver; saturation kinetics and the flux-voltage relationship also exhibit different trends, whereas idazoxan and putrescine, ineffective in RLM, act as inhibitors. The characteristics of agmatine uptake in RBM lead to the conclusion that its transporter is a channel with two asymmetric energy barriers, showing some characteristics similar to those of the imidazoline receptor I(2) and the sharing with the polyamine transporter.

Arginine metabolism: boundaries of our knowledge

Arginine has multiple metabolic fates and thus is one of the most versatile amino acids. Not only is it metabolically interconvertible with the amino acids proline and glutamate, but it also serves as a precursor for synthesis of protein, nitric oxide, creatine, polyamines, agmatine, and urea. These processes do not all occur within each cell but are differentially expressed according to cell type, age and developmental stage, diet, and state of health or disease. Arginine metabolism also is modulated by activities of various transporters that move arginine and its metabolites across the plasma and mitochondrial membranes. Moreover, several key enzymes in arginine metabolism are expressed as multiple isozymes whose expression can change rapidly and dramatically in response to a variety of different stimuli in health and disease. As illustrated by the questions raised in this article, we currently have an imperfect and incomplete picture of arginine metabolism for any mammalian species. It has become clear that a more complete understanding of arginine metabolism will require integration of information obtained from multiple approaches, including genomics, proteomics, and metabolomics.

The antiproliferative effects of agmatine correlate with the rate of cellular proliferation

Polyamines are small cationic molecules required for cellular proliferation. Agmatine is a biogenic amine unique in its capacity to arrest proliferation in cell lines by depleting intracellular polyamine levels. We previously demonstrated that agmatine enters mammalian cells via the polyamine transport system. As polyamine transport is positively correlated with the rate of cellular proliferation, the current study examines the antiproliferative effects of agmatine on cells with varying proliferative kinetics. Herein, we evaluate agmatine transport, intracellular accumulation, and its effects on antizyme expression and cellular proliferation in nontransformed cell lines and their transformed variants. H-ras- and Src-transformed murine NIH/3T3 cells (Ras/3T3 and Src/3T3, respectively) that were exposed to exogenous agmatine exhibit increased uptake and intracellular accumulation relative to the parental NIH/3T3 cell line. Similar increases were obtained for human primary foreskin fibroblasts relative to a human fibrosarcoma cell line, HT1080. Agmatine increases expression of antizyme, a protein that inhibits polyamine biosynthesis and transport. Ras/3T3 and Src/3T3 cells demonstrated augmented increases in antizyme protein expression relative to NIH/3T3 in response to agmatine. All transformed cell lines were significantly more sensitive to the antiproliferative effects of agmatine than nontransformed lines. These effects were attenuated in the presence of exogenous polyamines or inhibitors of polyamine transport. In conclusion, the antiproliferative effects of agmatine preferentially target transformed cell lines due to the increased agmatine uptake exhibited by cells with short cycling times.

Inhibition of agmatine transport in liver mitochondria by new charge-deficient agmatine analogues

The charge of the agmatine analogues AO-Agm [N-(3-aminooxypropyl)guanidine], GAPA [N-(3-aminopropoxy)guanidine] and NGPG [N-(3-guanidinopropoxy)guanidine] is deficient as compared with that of agmatine and they are thus able to inhibit agmatine transport in liver mitochondria. The presence of the guanidine group is essential for an optimal effect, since AO-Agm and NGPG display competitive inhibition, whereas that of GAPA is non-competitive. NGPG is the most effective inhibitor (Ki=0.86 mM). The sequence in the inhibitory efficacy is not directly dependent on the degree of protonation of the molecules; in fact NGPG has almost the same charge as GAPA. When the importance of the guanidine group for agmatine uptake is taken into account, this observation suggests that the agmatine transporter is a single-binding, centre-gated pore rather than a channel.

Ifosfamide-induced nephrotoxicity: mechanism and prevention

The efficacy of ifosfamide (IFO), an antineoplastic drug, is severely limited by a high incidence of nephrotoxicity of unknown etiology. We hypothesized that inhibition of complex I (C-I) by chloroacetaldehyde (CAA), a metabolite of IFO, is the chief cause of nephrotoxicity, and that agmatine (AGM), which we found to augment mitochondrial oxidative phosphorylation and beta-oxidation, would prevent nephrotoxicity. Our model system was isolated mitochondria obtained from the kidney cortex of rats treated with IFO or IFO + AGM. Oxidative phosphorylation was determined with electron donors specific to complexes I, II, III, or IV (C-I, C-II, C-III, or C-IV, respectively). A parallel study was done with (13)C-labeled pyruvate to assess metabolic dysfunction. Ifosfamide treatment significantly inhibited oxidative phosphorylation with only C-I substrates. Inhibition of C-I was associated with a significant elevation of [NADH], depletion of [NAD], and decreased flux through pyruvate dehydrogenase and the TCA cycle. However, administration of AGM with IFO increased [cyclic AMP (cAMP)] and prevented IFO-induced inhibition of C-I. In vitro studies with various metabolites of IFO showed that only CAA inhibited C-I, even with supplementation with 2-mercaptoethane sulfonic acid. Following IFO treatment daily for 5 days with 50 mg/kg, the level of CAA in the renal cortex was approximately 15 micromol/L. Taken together, these observations support the hypothesis that CAA is accumulated in renal cortex and is responsible for nephrotoxicity. AGM may be protective by increasing tissue [cAMP], which phosphorylates NADH:oxidoreductase. The current findings may have an important implication for the prevention of IFO-induced nephrotoxicity and/or mitochondrial diseases secondary to defective C-I.

Agmatine is transported into liver mitochondria by a specific electrophoretic mechanism

Agmatine, a divalent diamine with two positive charges at physiological pH, is transported into the matrix of liver mitochondria by an energy-dependent mechanism the driving force of which is DeltaPsi (electrical membrane potential). Although this process showed strict electrophoretic behaviour, qualitatively similar to that of polyamines, agmatine is most probably transported by a specific uniporter. Shared transport with polyamines by means of their transporter is excluded, as divalent putrescine and cadaverine are ineffective in inhibiting agmatine uptake. Indeed, the use of the electroneutral transporter of basic amino acids can also be discarded as ornithine, arginine and lysine are completely ineffective at inducing the inhibition of agmatine uptake. The involvement of the monoamine transporter or the existence of a leak pathway are also unlikely. Flux-voltage analysis and the determination of activation enthalpy, which is dependent upon the valence of agmatine, are consistent with the hypothesis that the mitochondrial agmatine transporter is a channel or a single-binding centre-gated pore. The transport of agmatine was non-competitively inhibited by propargylamines, in particular clorgilyne, that are known to be inhibitors of MAO (monoamine oxidase). However, agmatine is normally transported in mitoplasts, thus excluding the involvement of MAO in this process. The I2 imidazoline receptor, which binds agmatine to the mitochondrial membrane, can also be excluded as a possible transporter since its inhibitor, idazoxan, was ineffective at inducing the inhibition of agmatine uptake. Scatchard analysis of membrane binding revealed two types of binding site, S1 and S2, both with mono-co-ordination, and exhibiting high-capacity and low-affinity binding for agmatine compared with polyamines. Agmatine transport in liver mitochondria may be of physiological importance as an indirect regulatory system of cytochrome c oxidase activity and as an inducer mechanism of mitochondrial-mediated apoptosis.

Agmatine stimulates hepatic fatty acid oxidation: a possible mechanism for up-regulation of ureagenesis

We demonstrated previously in a liver perfusion system that agmatine increases oxygen consumption as well as the synthesis of N-acetylglutamate and urea by an undefined mechanism. In this study our aim was to identify the mechanism(s) by which agmatine up-regulates ureagenesis. We hypothesized that increased oxygen consumption and N-acetylglutamate and urea synthesis are coupled to agmatine-induced stimulation of mitochondrial fatty acid oxidation. We used 13C-labeled fatty acid as a tracer in either a liver perfusion system or isolated mitochondria to monitor fatty acid oxidation and the incorporation of 13C-labeled acetyl-CoA into ketone bodies, tricarboxylic acid cycle intermediates, amino acids, and N-acetylglutamate. With [U-13C16] palmitate in the perfusate, agmatine significantly increased the output of 13C-labeled beta-hydroxybutyrate, acetoacetate, and CO2, indicating stimulated fatty acid oxidation. The stimulation of [U-13C16]palmitate oxidation was accompanied by greater production of urea and a higher 13C enrichment in glutamate, N-acetylglutamate, and aspartate. These observations suggest that agmatine leads to increased incorporation and flux of 13C-labeled acetyl-CoA in the tricarboxylic acid cycle and to increased utilization of 13C-labeled acetyl-CoA for synthesis of N-acetylglutamate. Experiments with isolated mitochondria and 13C-labeled octanoic acid also demonstrated that agmatine increased synthesis of 13C-labeled beta-hydroxybutyrate, acetoacetate, and N-acetylglutamate. The current data document that agmatine stimulates mitochondrial beta-oxidation and suggest a coupling between the stimulation of hepatic beta-oxidation and up-regulation of ureagenesis. This action of agmatine may be mediated via a second messenger such as cAMP, and the effects on ureagenesis and fatty acid oxidation may occur simultaneously and/or independently.

Arginine: beyond protein

Arginine, a semiessential or conditionally essential amino acid in humans, is one of the most metabolically versatile amino acids and serves as a precursor for the synthesis of urea, nitric oxide, polyamines, proline, glutamate, creatine, and agmatine. Arginine is metabolized through a complex and highly regulated set of pathways that remain incompletely understood at both the whole-body and the cellular levels. Adding to the metabolic complexity is the fact that limited arginine availability can selectively affect the expression of specific genes, most of which are themselves involved in some aspect of arginine metabolism. This overview highlights selected aspects of arginine metabolism, including areas in which our knowledge remains fragmentary and incomplete.

Extraneuronal monoamine transporter and organic cation transporters 1 and 2: a review of transport efficiency

The extraneuronal monoamine transporter (EMT) corresponds to the classical steroid-sensitive monoamine transport mechanism that was first described as "uptake2" in rat heart with noradrenaline as substrate. The organic cation transporters OCT1 and OCT2 are related to EMT. The three carriers share basic structural and functional characteristics. Hence, EMT, OCT1 and OCT2 constitute a group referred to as non-neuronal monoamine transporters or organic cation transporters. After a brief general introduction, this review focuses on the critical analysis of substrate specificity. We calculate from the available literature and compare consensus transport efficiency (clearance) data for human and rat EMT, OCT1 and OCT2, expressed in transfected cell lines. From the plethora of inhibitors that have been tested, the casual observer likely gets the impression that these carriers indiscriminately transport very many compounds. However, our knowledge about actual substrates is rather limited. 1-Methyl-4-phenylpyridinium (MPP+) is an excellent substrate for all three carriers, with clearances typically in the range of 20-50 microl min(-1) mg protein(-1). The second-best general substrate is tyramine with a transport efficiency (TE) range relative to MPP+ of 20%-70%. The TEs of OCT1 and OCT2 for dopamine, noradrenaline, adrenaline and 5-HT in general are rather low, in the range relative to MPP+ of 5%-15%. This suggests that OCT1 and OCT2 are not primarily dedicated to transport these monoamine transmitters; only EMT may play a significant role in catecholamine inactivation. For many substrates, such as tetraethylammonium, histamine, agmatine, guanidine, cimetidine, creatinine, choline and acetylcholine, the transport efficiencies are markedly different among the carriers.

Biosynthesis of agmatine in isolated mitochondria and perfused rat liver: studies with 15N-labelled arginine

An important but unresolved question is whether mammalian mitochondria metabolize arginine to agmatine by the ADC (arginine decarboxylase) reaction. 15N-labelled arginine was used as a precursor to address this question and to determine the flux through the ADC reaction in isolated mitochondria obtained from rat liver. In addition, liver perfusion system was used to examine a possible action of insulin, glucagon or cAMP on a flux through the ADC reaction. In mitochondria and liver perfusion, 15N-labelled agmatine was generated from external 15N-labelled arginine. The production of 15N-labelled agmatine was time- and dose-dependent. The time-course of [U-15N4]agmatine formation from 2 mM [U-15N4]arginine was best fitted to a one-phase exponential curve with a production rate of approx. 29 pmol x min(-1) x (mg of protein)(-1). Experiments with an increasing concentration (0- 40 mM) of [guanidino-15N2]arginine showed a Michaelis constant Km for arginine of 46 mM and a Vmax of 3.7 nmol x min(-1) x (mg of protein)(-1) for flux through the ADC reaction. Experiments with broken mitochondria showed little changes in Vmax or Km values, suggesting that mitochondrial arginine uptake had little effect on the observed Vmax or Km values. Experiments with liver perfusion demonstrated that over 95% of the effluent agmatine was derived from perfusate [guanidino-15N2]arginine regardless of the experimental condition. However, the output of 15N-labelled agmatine (nmol x min(-1) x g(-1)) increased by approx. 2-fold (P<0.05) in perfusions with cAMP. The findings of the present study provide compelling evidence that mitochondrial ADC is present in the rat liver, and suggest that cAMP may stimulate flux through this pathway.

Inhibition of inducible nitric oxide synthase alters Thy-1 glomeruonephritis in rats

BACKGROUND/AIMS

Inducible nitric oxide (NO) synthase (iNOS) generated NO increases in the early phase of Thy-1 glomerulonephritis concurrently with mesangiolysis and reduction in glomerular filtration rate (GFR). Activation of ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine biosynthesis, is upregulated to allow mesangial cell proliferation which constitutes the repair phase in this model. Antiproliferative high-output NO generation inhibits proproliferative ODC activity, thereby temporally separating the early 'bactericidal' phase from the later 'growth' repair phase.

METHODS

Renal function, ODC protein expression, arginine, ornithine, and polyamines by high-performance liquid chromatography, and histological changes were assessed in rats after induction of Thy-1 nephritis with and without NOS inhibition.

RESULTS

Thy-1 significantly reduced the GFR relative to untreated controls. Treatment with a nonspecific NOS inhibitor, but not a selective iNOS inhibitor, further decreased the GFR at day 1. This implys a protective role for constitutive NOS in the early phase of this inflammatory model. Selective iNOS inhibition abrogated increased plasma NO(2)/NO(3) levels in Thy-1 glomerulonephritis, but did not significantly reduce mesangiolysis. However, inhibition of iNOS did result in significantly more nuclei/glomerulus during the proliferative phase, increasing the hypercellularity component of this disease model. This correlates with increased levels of polyamines, ornithine, and arginine beyond those observed with Thy-1 administration alone.

CONCLUSIONS

These studies provide evidence that NO generation from different NOS isoforms can be protective in the temporal course of Thy-1 glomerulonephritis. The finding that iNOS attenuates hypercellularity in the repair phase of this inflammatory model adds cautionary insight in the therapeutic use of selective iNOS inhibition in vivo.

Metabolic fate of extrahepatic arginine in liver after burn injury

Increased nitrogen loss in the form of urea is a hallmark of the metabolic aberrations that occur after burn injury. As the immediate precursor for urea production is arginine, we have conducted an investigation on the metabolic fate of arginine in the liver to shed light on the metabolic characteristics of this increased nitrogen loss. Livers from 25% total surface burn (n = 8) and sham burn rats (n = 8) were perfused in a recycling fashion with a medium containing amino acids and stable isotope labeled l-[(15) N(2)-guanidino, 5,5-(2)H(2)]arginine for 120 minutes. The rates of glucose and urea production and oxygen consumption were measured. The rate of unidirectional arginine transport and the intrahepatic metabolic fate of arginine in relation to urea cycle activity were quantified by tracing the disappearance rate of the arginine tracer from and the appearance rate of [(15)N(2)]urea in the perfusion medium. Perfused livers from burned rats showed higher rates of total urea production (mean +/- SE, 4.471 +/- 0.274 v 3.235 +/- 0.261 mumol. g dry liver(-1). min(-1); P <.01). This was accompanied by increased hepatic arginine transport (1.269 +/- 0.263 v 0.365 +/- 0.021 mumol. g dry liver(-1). min(-1)) and an increased portion of urea production from the transported extrahepatic arginine (12.9% +/- 2.9% v 3.5% +/- 0.4%, P <.05). The disposal of arginine via nonurea pathways was also increased (0.702 +/- 0.185 v 0.257 +/- 0.025 mumol/g dry weight(-1)/min(-1); P <.05). We propose that increased inward transport and utilization of extrahepatic arginine by the liver contributes to the accelerated urea production after burn injury and accounts, in part, for its conditional essentiality in the nutritional support of burn patients.

Pharmacological characteristics of the specific transporter for the endogenous cell growth inhibitor agmatine in six tumor cell lines

BACKGROUND AND AIMS

This study examined agmatine transport into six human intestinal tumor cell lines and compared the pharmacological properties of this transporter with those of the agmatine carrier previously characterized in human glioblastoma cells.

METHODS

Carrier-mediated uptake was determined as specific accumulation of [(14)C]agmatine in the cells. The changes in intracellular agmatine concentration in the tumor cells after 24 h incubation with 1 mM agmatine was analyzed by high-performance liquid chromatography.

RESULTS

Specific [(14)C]agmatine accumulation was found in the six human intestinal tumor cell lines Caco2, Cx1, Colo320, HT29, Colo205E, and SW480. Specific [(14)C]agmatine accumulation was inhibited by phentolamine, putrescine, spermine, clonidine, and decynium-22 but not by corticosterone, O-methylisoprenaline, or l-carnitine. Incubation with exogenous agmatine for 24 h increased intracellular agmatine content in all cell lines by a multiple of the basal endogenous content. Transfection of HEK293 cells with cDNA encoding either hOCT1, hOCT2, or hOCT3 did not enhance [(14)C]agmatine accumulation compared to nontransfected cells.

CONCLUSION

All intestinal tumor cell lines investigated express a functional specific agmatine transporter which exhibit pharmacological characteristics similar to those of the agmatine transporter in glioblastoma cells. This agmatine carrier is not identical with any so far known organic cation transport system.

Influence of dietary arginine on sexual dimorphism of arginine metabolism in mice

We have studied the influence of dietary arginine on tissue arginine content, and arginine metabolism in CD1 mice. Dietary arginine restriction produced by feeding mice with a low arginine diet (0.06%) produced a marked decrease in arginine concentrations in the plasma, skeletal muscle and kidney of female mice (72%, 67% and 54%, respectively) while in male mice the decreases were smaller (58% in blood and 18% in the skeletal muscle). This diet abolished not only the sexual dimorphism in arginine content observed in mice fed with the diet containing 1% arginine, but also reduced renal activities of arginase and nitric oxide synthase in the female mice and ornithine decarboxylase and the decarboxylation of arginine in the male mice. Urinary putrescine excretion was dramatically reduced by arginine restriction in the male mice whereas orotic acid excretion increased about 30 fold in both sexes; urea and creatinine excretion did not change. Taken together our results indicate that dietary arginine plays a relevant role in the maintenance of the sexual dimorphism in arginine content and arginine metabolism in CD1 mice, and that this may have physiological significance because of the important effects that arginine-derived products exert on a variety of cellular processes.

Agmatine is efficiently transported by non-neuronal monoamine transporters extraneuronal monoamine transporter (EMT) and organic cation transporter 2 (OCT2)

Agmatine has received considerable attention recently. Available evidence suggests that agmatine functions as a neurotransmitter and inhibits, via induction of antizyme, cellular proliferation. Because of its positive charge, agmatine will not appreciably cross cellular membranes by simple diffusion. Indeed, all physiological models require a channel or transporter protein in the plasma membrane to effect inactivation or nonexocytotic release of agmatine. However, a transport mechanism for agmatine has not been identified on a molecular level so far. In the present study, the non-neuronal monoamine transporters, organic cation transporter (OCT) 1, OCT2, and extraneuronal monoamine transporter (EMT) (gene symbols SLC22A1-A3), both from human and rat, were examined, stably expressed in 293 cells, for [(3)H]agmatine transport. Our results indicate that OCT2 and EMT, but not OCT1, efficiently translocate agmatine. The structural homolog putrescine was not accepted as substrate. Uptake of agmatine via EMT and OCT2 was saturable, with K(m) values of 1 to 2 mM. The affinity of OCT1 was 10-fold lower. Carrier-mediated efflux of agmatine was documented in a trans-stimulation experiment. Finally, uptake of agmatine increased dramatically with increasing pH. Thus, only the singly charged species of agmatine is accepted as substrate. In conclusion, both EMT and OCT2 must be considered for the control of agmatine levels in rat and human.

Agmatine modulates the in vivo biosynthesis and interconversion of polyamines and cell proliferation

Agmatine has recently gained wide interest as a bioactive arginine metabolite with a multitude of physiological functions. This study evaluates the in vivo role of agmatine in the modulation of metabolism and intracellular level of polyamines. Here, we report that agmatine, administered to mice, differentially affects the renal and liver activity of the two key enzymes regulating polyamine biosynthesis and interconversion/degradation. Thus, agmatine exerts a negative regulation of ODC activity and protein content, and positive regulation of SSAT activity, having no effect on ODC and SSAT transcript level. Agmatine modulation of ODC and SSAT activities is noticeably augmented by the inhibitor of its catabolism, aminoguanidine. Antizyme and eIF4E protein content appears to be affected by agmatine only insignificantly and apparently do not contribute to agmatine-induced down-regulation of ODC content. The homeostasis of spermidine and spermine is preserved after agmatine injection, while the putrescine level decreases. Furthermore, when tested in a mouse kidney injury model, agmatine, partially but significantly, reduces [3H] thymidine incorporation into DNA. This is consistent with suppressed renal tubule epithelial cell proliferation. The findings provide in vivo evidence of a substantial role of agmatine as a modulator of polyamine biosynthesis and degradation and suggest its suppressive effect on cell proliferation.

Agmatine inhibits arginine vasopressin-stimulated urea transport in the rat inner medullary collecting duct

BACKGROUND

Agmatine, a putative endogenous ligand for imidazoline receptors, induces numerous biological effects. The agonist clonidine binds to alpha-2 (alpha2) adrenoceptors and imidazoline receptors, and inhibits arginine vasopressin (AVP)-stimulated urea permeability (Pu) in the rat inner medullary collecting duct (IMCD). Dexmedetomidine, a selective alpha2 agonist, does not inhibit AVP-stimulated Pu. This study was conducted to determine if agmatine affects Pu in the rat IMCD and to investigate the possibility of an imidazoline-mediated mechanism.

METHODS

The isolated-perfused tubule technique was used to measure Pu in IMCDs from Wistar rats. AVP at 220 pmol/L or 8-chlorophenylthio cyclic adenosine monophosphate (8CPT cAMP) was used to stimulate Pu. Agmatine and other agents were added to the bath.

RESULTS

Agmatine at 1 micromol/L inhibited AVP-stimulated Pu by 50%. Agmatine-induced inhibition could not be separated completely from inhibition produced by the non-imidazoline, catecholamine epinephrine. Of three antagonists selective for alpha2 adrenoceptors (rauwolscine, yohimbine, and RX821002), only rauwolscine reversed inhibition, whereas each of the three imidazoline-selective antagonists tested (atipamezole, idazoxan, and BU239) produced a significant reversal. Agmatine did not affect basal Pu or inhibit 8CPTcAMP-stimulated Pu.

CONCLUSION

Our results indicate that agmatine inhibits AVP stimulated Pu by a cAMP-dependent mechanism. Imidazoline receptors are probably not involved. The possibility exists of an unknown agmatine-selective receptor modulating urea transport in the rat IMCD.

Regulation of urea synthesis by agmatine in the perfused liver: studies with 15N

Administration of arginine or a high-protein diet increases the hepatic content of N-acetylglutamate (NAG) and the synthesis of urea. However, the underlying mechanism is unknown. We have explored the hypothesis that agmatine, a metabolite of arginine, may stimulate NAG synthesis and, thereby, urea synthesis. We tested this hypothesis in a liver perfusion system to determine 1) the metabolism of l-[guanidino-15N2]arginine to either agmatine, nitric oxide (NO), and/or urea; 2) hepatic uptake of perfusate agmatine and its action on hepatic N metabolism; and 3) the role of arginine, agmatine, or NO in regulating NAG synthesis and ureagenesis in livers perfused with 15N-labeled glutamine and unlabeled ammonia or 15NH4Cl and unlabeled glutamine. Our principal findings are 1) [guanidino-15N2]agmatine is formed in the liver from perfusate l-[guanidino-15N2]arginine ( approximately 90% of hepatic agmatine is derived from perfusate arginine); 2) perfusions with agmatine significantly stimulated the synthesis of 15N-labeled NAG and [15N]urea from 15N-labeled ammonia or glutamine; and 3) the increased levels of hepatic agmatine are strongly correlated with increased levels and synthesis of 15N-labeled NAG and [15N]urea. These data suggest a possible therapeutic strategy encompassing the use of agmatine for the treatment of disturbed ureagenesis, whether secondary to inborn errors of metabolism or to liver disease.

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.

Polyamine transport system mediates agmatine transport in mammalian cells

Agmatine is a biogenic amine with the capacity to regulate a number of nonreceptor-mediated functions in mammalian cells, including intracellular polyamine content and nitric oxide generation. We observed avid incorporation of agmatine into several mammalian cell lines and herein characterize agmatine transport in mammalian cells. In transformed NIH/3T3 cells, agmatine uptake is energy dependent with a saturable component indicative of carrier-mediated transport. Transport displays an apparent Michaelis-Menten constant of 2.5 microM and a maximal velocity of 280 pmol x min(-1) x mg(-1) protein and requires a membrane potential across the plasma membrane for uptake. Competition with polyamines, but not cationic molecules that utilize the y+ system transporter, suppresses agmatine uptake. Altering polyamine transporter activity results in parallel changes in polyamine and agmatine uptake. Furthermore, agmatine uptake is abrogated in a polyamine transport-deficient human carcinoma cell line. These lines of evidence demonstrate that agmatine utilizes, and is dependent on, the polyamine transporter for cellular uptake. The fact that this transport system is associated with proliferation could be of consequence to the antiproliferative effects of agmatine.

Agmatine uptake by cultured hamster kidney cells

Agmatine, the product of arginine decarboxylation, has been recently found in a wide variety of animal tissues. In spite of the emergent interest on agmatine in animals, the mechanism of agmatine uptake in mammalian cells has been scarcely studied. An analysis of radiolabeled agmatine uptake was carried out by using a classical, kinetic approach with BHK-21 hamster kidney cells in culture. A high affinity, temperature- and energy-dependent agmatine transport system in BHK-21 kidney cells is here kinetically characterized which seems to be a "general" transporter shared by di- and triamines and different to a highly specific carrier for the tetraamine spermine.

Agmatine inhibits cell proliferation and improves renal function in anti-thy-1 glomerulonephritis

Changes in the expression of alternate arginine metabolic pathways have been implicated in the pathogenesis of experimental glomerulonephritis. Agmatine, decarboxylated arginine, has been shown in vitro to suppress both inducible nitric oxide synthase and the rate-limiting enzyme of polyamine biosynthesis, ornithine decarboxylase (ODC). This study was undertaken to determine whether agmatine administration could reduce tissue injury by decreasing nitric oxide, and reduce cell proliferation, by diminishing ODC activity, in experimental mesangial proliferative glomerulonephritis (Thy-1 nephritis). Agmatine treatment (50 mg/kg per d intraperitoneally) in Thy-1 nephritis rats prevented a reduction in GFR at day 1. Agmatine treatment decreased nitric oxide production in Thy-1 nephritis rats by 23% and 41% at days 1 and 4, respectively. Agmatine treatment also reduced ODC activity and glomerular (3)H-thymidine incorporation on days 1, 4, and 7. Histologic evaluation revealed a decline in mesangial cell proliferation and extracellular matrix accumulation associated with agmatine treatment administered before or 24 h after Thy-1 antibody, and this was confirmed by a reduction in the number of cells expressing proliferating cell nuclear antigen on days 4 and 7. These studies provide the first in vivo evidence that agmatine administration can reduce cellular proliferation in Thy-1 nephritis and attenuate the initial reduction in renal function associated with this model.

Is agmatine a novel neurotransmitter in brain?

Recent evidence suggests that agmatine, which is an intermediate in polyamine biosynthesis, might be an important neurotransmitter in mammals. Agmatine is synthesized in the brain, stored in synaptic vesicles in regionally selective neurons, accumulated by uptake, released by depolarization, and inactivated by agmatinase. Agmatine binds to alpha2-adrenoceptors and imidazoline binding sites, and blocks NMDA receptor channels and other ligand-gated cationic channels. Furthermore, agmatine inhibits nitric oxide synthase, and induces the release of some peptide hormones. As a result of its ability to inhibit both hyperalgesia and tolerance to, and withdrawal from, morphine, and its neuroprotective activity, agmatine has potential as a treatment of chronic pain, addictive states and brain injury.