Identification of carrier systems in plasma membranes of mammalian cells involved in transport of L-arginine

Extracellular arginine rapidly dilates in vivo intestinal arteries and arterioles through a nitric oxide mechanism

OBJECTIVE

Arginine used for nitric oxide formation can be from intracellular stores or transported into cells. The study evaluated the rapidity, and primary site of NO and vascular resistance responses to arginine at near physiological concentrations (100-400 microM).

METHODS

Arginine was applied to a single arteriole through a micropipette to determine the fastest possible responses. For vascular blood flow and [NO] responses, arginine was added to the bathing media.

RESULTS

Dilation of single arterioles to arginine began in 10-15 seconds and application over the entire vasculature increased [NO] in approximately 60-90 seconds, and flow increased within 120-300 seconds. Resting periarteriolar [NO] for arterioles was 493.6 +/- 30.5 nM and increased to 696.1 +/- 68.2 and 820.1 +/- 110.5 nM at 200 and 400 microM L-arginine. The blood flow increased 50% at 400-1200 microM L-arginine. The reduced arterial resistance during topical arginine was significantly greater than microvascular resistance at 100 and 200 microM arginine. All responses were blocked by L-NAME.

CONCLUSIONS

This study demonstrated arterial resistance responses are as or more responsive to arginine induced NO formation as arterioles at near physiological concentrations of arginine. The vascular NO and resistance responses occurred rapidly at L-arginine concentrations at and below 400 microM, which predict arginine transport processes were involved.

Increased L-arginine transport via system b0,+ in human proximal tubular cells exposed to albumin

Albumin has complex effects on PTECs (proximal tubular epithelial cells) and is able to stimulate growth or injury depending on its bound moieties. Albumin itself is a mitogen, inducing proliferation through a number of pathways. In PTEC exposed to purified albumin, polyamines are required for entry into the cell cycle and are critical for proliferation. Polyamines are synthesized from L-ornithine (itself derived by the action of arginase on L-arginine), and the transport and availability of L-arginine may thus be important for subsequent polyamine-dependent proliferation. In the present study we investigated radiolabelled cationic amino-acid transport in cultured PTEC exposed to 20 mg/ml ultrapure recombinant human albumin, describing the specific kinetic characteristics of transport and the expression of transporters. L-[3H]Arginine transport capacity in human PTEC is increased after exposure for 24 h to human albumin, mediated by the broad-scope high-affinity system b0,+ and, to a lesser extent, system y+L (but not system y+) transport. Increased transport is associated with increased b0,+-associated transporter expression. Inhibition of phosphoinositide 3-kinase, a key regulator of albumin endocytosis and signalling, inhibited proliferation, but had no effect on the observed increase in transport. PTEC proliferated in response to albumin. L-Lysine, a competitive inhibitor of L-arginine transport, had no effect on albumin-induced proliferation; however, arginine deprivation effectively reversed the albumin-induced proliferation observed. In conclusion, in PTEC exposed to albumin, increased L-arginine transport is mediated by increased transcription and activity of the apical b0,+ transport system. This may make L-arginine available as a substrate for the downstream synthesis of polyamines, but is not critical for cell proliferation.

Albumin stimulates cell growth, L-arginine transport, and metabolism to polyamines in human proximal tubular cells

BACKGROUND

Pure albumin stimulates proximal tubular epithelial cell (PTEC) proliferation, and may have a role in homeostasis in health, as well as in disrupted PTEC turnover in proteinuric nephropathies. We investigated a role for arginine and its metabolites, the polyamines, in this process, given the ability of polyamines to trigger proliferation in other mammalian cells.

METHODS

[(3)H]-L-arginine uptake was examined after incubation with 20 mg/mL recombinant human serum albumin (rHSA) in HK-2 PTEC monolayers. Nitric oxide synthase (NOS) and arginase activity was measured; NOS, arginase, and ornithine decarboxylase (ODC) expression was identified by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Polyamine synthesis and intracellular amino acid concentrations were compared using high-performance liquid chromatography, and cell growth measured by [(3)H]-thymidine incorporation.

RESULTS

In HK-2 PTEC exposed to 20 mg/mL rHSA for 24 hours, cell proliferation as determined by [(3)H]-thymidine incorporation was increased. In parallel, L-arginine transport capacity was increased in a dose- and time-dependent manner. This effect was specific to rHSA, and was not seen with transferrin or immunoglobulin G. The intracellular concentration of L-arginine remained unchanged, although L-ornithine was increased with rHSA incubation. rHSA up-regulated type II arginase mRNA, and increased arginase activity, although no difference in nitric oxide synthase expression or activity was seen. ODC mRNA was increased, as were intracellular polyamine concentrations. alpha-Difluoromethylornithine (DFMO), an ODC inhibitor, reduced intracellular polyamine concentrations and rHSA-induced cell proliferation to control levels.

CONCLUSION

The arginine-ornithine-polyamine pathway appears enhanced in PTEC incubated with rHSA and is involved in cellular proliferation; this may offer novel approaches to understanding progressive proteinuric nephropathies.

Transport of extracellular l-arginine via cationic amino acid transporter is required during in vivo endothelial nitric oxide production

In cultured endothelial cells, 70-95% of extracellular l-arginine uptake has been attributed to the cationic amino acid transporter-1 protein (CAT-1). We tested the hypothesis that extracellular l-arginine entry into endothelial cells via CAT-1 plays a crucial role in endothelial nitric oxide (NO) production during in vivo conditions. Using l-lysine, the preferred amino acid transported by CAT-1, we competitively inhibited extracellular l-arginine transport into endothelial cells during conditions of NaCl hyperosmolarity, low oxygen, and flow increase. Our prior studies indicate that each of these perturbations causes NO-dependent vasodilation. The perivascular NO concentration ([NO]) and blood flow were determined in the in vivo rat intestinal microvasculature. Suppression of extracellular l-arginine transport significantly and strongly inhibited increases in vascular [NO] and intestinal blood flow during NaCl hyperosmolarity, lowered oxygen tension, and increased flow. These results suggest that l-arginine from the extracellular space is accumulated by CAT-1. When CAT-1-mediated transport of extracellular l-arginine into endothelial cells was suppressed, the endothelial cell NO response to a wide range of physiological stimuli was strongly depressed.

Role of L-citrulline transport in nitric oxide synthesis in rat aortic smooth muscle cells activated with LPS and interferon-gamma

(1) l-citrulline, a coproduct of nitric oxide synthase (NOS)-catalysed metabolism of l-arginine to nitric oxide (NO), is an important intermediate of the urea cycle and a precursor for l-arginine biosynthesis in vascular cells. (2) In the present study, we have examined the characteristics of l-citrulline transport, regulation by lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) and the ability of l-citrulline to sustain NO synthesis in rat cultured aortic smooth muscle cells. (3) l-citrulline transport was saturable with an apparent Km=1.6+/-0.2 mm and Vmax=5.9+/-0.6 pmol microg-1 protein min-1. Transport was pH-insensitive, partially Na+-dependent and markedly inhibited by substrates selective for amino-acid transport systems L and N but not by l-arginine or substrates for systems A, ASC, xc- or XAG. Moreover, transport was not altered in cells treated with LPS (100 microg ml-1) and IFN-gamma (50 U ml-1) for 0-24 h. (4) Unlike l-arginine, l-citrulline could not sustain maximal NO production in cells expressing iNOS. (5) Our findings provide the first evidence in vascular smooth muscle cells that l-citrulline transport is mediated via a low-affinity carrier with characteristics resembling systems L and N. Moreover, in l-arginine-deprived rat aortic smooth muscle cells, l-citrulline cannot sustain maximal NO release via iNOS.

The importance of cationic amino acid transporter expression in human skin

Inducible nitric oxide synthase and arginase activities are acknowledged as important players in human skin epidermal function. For proper enzyme function the substrate availability of L-arginine for both enzymes and thus its transport across the cell membrane via the y+-system (also named cationic amino acid transporters) is critical. Here, we examine the expression of cationic amino acid transporters and their functional role in modulating inducible nitric oxide synthase and arginase activities in human skin and primary keratinocytes, fibroblasts and endothelial cells as well as their impact on keratinocyte proliferation. Skin biopsies were found to express constitutively both cationic amino acid transporter-1 and cationic amino acid transporter-2 mRNA, an expression pattern known to occur in hepatocytes and muscle cells only. To determine the cellular components expressing cationic amino acid transporter, we analyzed the expression patterns in the different human skin cell types in vitro, i.e., in fibroblasts, dermal endothelial cells, and keratinocytes as well as in the HaCaT cell line. An ubiquitous cationic amino acid transporter-1 mRNA expression was found in all cells, whereas constitutive cationic amino acid transporter-2 mRNA expression occurs in resident keratinocytes and dermal endothelial cells only. De novo induction of cationic amino acid transporter-2 and inducible nitric oxide synthase by proinflammatory cytokines was seen in fibroblasts and HaCaT. Competitive inhibition of the cationic amino acid transporter-mediated L-arginine transport by culturing primary human keratinocytes in the presence of increased L-lysine concentration led to decreased inducible nitric oxide synthase and arginase activities with a concomitant significant decrease in keratinocyte proliferation. In summary, our results demonstrate that human keratinocytes constitutively express cationic amino acid transporters 1 and 2 and that cationic amino acid transporter mediated L-arginine influx, is essential for both inducible nitric oxide synthase and arginase enzyme activities, which in turn modulate proliferation and differentiation of human epidermal skin cells.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Microglial function in human APOE3 and APOE4 transgenic mice: altered arginine transport

The APOE4 genotype is a known risk factor for Alzheimer's disease (AD) and is associated with poorer outcomes after neuropathological insults. To understand APOE's function, we have examined microglia, the CNS specific macrophage, in transgenic mice expressing the human APOE3 and APOE4 gene allele. Our data demonstrate that arginine uptake is enhanced in APOE4 microglia compared to APOE3 microglia. The increased arginine uptake in APOE4 Tg microglia is associated with an increased expression of mRNA for cationic amino acid transporter-1 (Cat1), a constuitively expressed member of the arginine selective transport system (the y+ transport system) found in most cells. The macrophage-associated transporter, cationic amino acid transporter 2B (Cat2B) did not demonstrate a change in mRNA expression. This change in microglial arginine transport suggests a potential impact of the APOE4 gene allele on those biochemical pathways such as NO production or cell proliferation to which arginine contributes.

System y+ localizes to different membrane subdomains in the basolateral plasma membrane of epithelial cells

We report here that the system y+ cationic amino acid transporter ATRC1 localized to clusters within the basolateral membrane of polarized Madin-Darby canine kidney and human embryonic kidney (HEK) cells, suggesting that the transporters are restricted to discrete membrane microdomains in epithelial cells. Based on solubility in nonionic detergents, two populations of ATRC1 molecules existed: approximately half of the total ATRC1 in HEK cells associated with the actin membrane cytoskeleton, whereas another one-fourth resided in detergent-resistant membranes (DRM). In agreement with these findings, cytochalasin D reduced the amount of ATRC1 associated with the actin membrane cytoskeleton. Although some ATRC1 clusters in HEK cells colocalized with caveolin, the majority of ATRC1 did not colocalize with this marker protein for a type of DRM called caveolae. This distribution of ATRC1 is somewhat different from that reported for pulmonary artery endothelial cells in which transporters cluster predominantly in caveolae, suggesting that differences in the proportion of ATRC1 in specific membrane microdomains correlate with differences in the physiological role of the transporter in polarized kidney epithelial vs. vascular endothelial cells.

Dinitrobenzene-mediated production of peroxynitrite by neuronal nitric oxide synthase

Neuronal nitric oxide synthase (nNOS) is a modular enzyme that consists of a flavin-containing reductase domain and a heme-containing oxygenase domain, fused by a calmodulin (CaM)-binding sequence. Within the central nervous system, nNOS is localized in the cerebellum. CaM binding to nNOS activates both intradomain as well as interdomain electron transfer, and thus activity. The nNOS reductase shares many characteristics with NADPH-cytochrome P450 reductase (CPR), such as catalyzing the reduction of exogenous electron acceptors such as quinones and nitroarenes. The nitroarene 1,3-dinitrobenzene (1,3-DNB) is a cerebellar neurotoxicant in rats. 1,3-DNB is metabolized by CPR in liver, and it was proposed that metabolism of 1,3-DNB to reactive intermediates is involved in mediating the cerebellar neurotoxicity. We have found that, in a manner similar to CPR, nNOS can interact with 1,3-DNB and generate superoxide anion radical (O2*-). Electron transfer through the nNOS reductase is not limiting for nitric oxide (NO.) and L-citrulline production, even in the presence of certain exogenous electron acceptors such as 1,3-DNB. Therefore, NO., L-citrulline, and O2*- are simultaneously produced by nNOS in the presence of 1,3-DNB and other nitroarenes. The simultaneous production of NO. and O2*- leads to peroxynitrite (ONOO-) formation via the combination of these two radicals at a near-diffusion-controlled reaction rate. We present convincing data supporting the hypothesis that in the presence of 1,3-DNB, nNOS is converted from a purely NO. and L-citrulline synthase to a ONOO- and L-citrulline synthase, and propose that the resulting nitosative stress plays a role in the cerebellar neurotoxicity of 1,3-DNB. This paper introduces a new and novel enzymatic mechanism with direct toxicological implications whereby nNOS is converted into a ONOO- synthase by certain nitroarenes.

Expression of solute carrier 7A4 (SLC7A4) in the plasma membrane is not sufficient to mediate amino acid transport activity

Member 4 of human solute carrier family 7 (SLC7A4) exhibits significant sequence homology with the SLC7 subfamily of human cationic amino acid transporters (hCATs) [Sperandeo, Borsani, Incerti, Zollo, Rossi, Zuffardi, Castaldo, Taglialatela, Andria and Sebastio (1998) Genomics 49, 230-236]. It is therefore often referred to as hCAT-4 even though no convincing transport activity has been shown for this protein. We expressed SLC7A4 in Xenopus laevis oocytes, but could not detect any transport activity for cationic, neutral or anionic amino acids or for the polyamine putrescine. In addition, human glioblastoma cells stably overexpressing a fusion protein between SLC7A4 and the enhanced green fluorescent protein (EGFP) did not exhibit an increased transport activity for l-arginine. The lack of transport activity was not due to a lack of SLC7A4 protein expression in the plasma membrane, as in both cell types SLC7A4-EGFP exhibited a similar subcellular localization and level of protein expression as functional hCAT-EGFP proteins. The expression of SLC7A4 can be induced in NT2 teratocarcinoma cells by treatment with retinoic acid. However, also for this endogenously expressed SLC7A4, we could not detect any transport activity for l-arginine. Our data demonstrate that the expression of SLC7A4 in the plasma membrane is not sufficient to induce an amino acid transport activity in X. laevis oocytes or human cells. Therefore, SLC7A4 is either not an amino acid transporter or it needs additional (protein) factor(s) to be functional.

Coordinate regulation of L-arginine uptake and nitric oxide synthase activity in cultured endothelial cells

Despite intracellular L-arginine concentrations that should saturate endothelial nitric oxide synthase (eNOS), nitric oxide production depends on extracellular L-arginine. We addressed this 'arginine paradox' in bovine aortic endothelial cells by simultaneously comparing the substrate dependence of L-arginine uptake and intracellular eNOS activity, the latter measured as L-[3H]arginine conversion to L-[3H]citrulline. Whereas the Km of eNOS for L-arginine was 2 microM in cell extracts, the L-arginine concentration of half-maximal eNOS stimulation was increased to 29 microM in intact cells. This increase likely reflects limitation by L-arginine uptake, which had a Km of 108 microM. The effects of inhibitors of endothelial nitric oxide synthesis also suggested that extracellular L-arginine availability limits intracellular eNOS activity. Treatment of intact cells with the calcium ionophore A23187 reduced the L-arginine concentration of half-maximal eNOS activity, which is consistent with a measured increase in L-arginine uptake. Increases in eNOS activity induced by several agents were closely correlated with enhanced L-arginine uptake into cells (r = 0.89). The 'arginine paradox' may be explained in part by regulated L-arginine uptake into a compartment, probably represented by caveolae, that contains eNOS and that is distinct from the bulk cytosolic L-arginine.

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.

The transport activity of the human cationic amino acid transporter hCAT-1 is downregulated by activation of protein kinase C

1. The human cationic amino acid transporter hCAT-1 contains several consensus sequences for phosphorylation by protein kinase C (PKC). This study investigates the effect of PKC activation on hCAT-1-mediated transport. 2. When expressed in Xenopus laevis oocytes, hCAT-1-mediated L-arginine transport was reduced to 44+/-3% after a 30 min treatment of the oocytes with 100 nM phorbol-12-myristate-13-acetate (PMA). 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD, 100 nM) had no effect. 3. In EA.hy926 endothelial cells, maximal inhibition of hCAT-1-mediated L-arginine transport (to 3 -- 11% of control) was observed after treatment of the cells with 100 nM PMA for 4 h. A 20 -- 30 h exposure of the cells to 100 nM PMA led to the recovery of the L-arginine uptake rate that was now resistant to a second application of PMA. Phorbol-12,13-dibutyrate had similar effects as PMA, whereas 4 alpha-PDD had no effect. One microM bisindolylmaleimide I reduced the PMA effect significantly. 4. Interestingly, a 4 h treatment with 100 nM PMA increased the expression of hCAT-1 mRNA 3 -- 5 fold. hCAT-1 protein levels were unchanged for up to 4 h after PMA treatment and then increased slightly between 8 -- 28 h. 5. It is concluded that PMA downregulates the intrinsic activity of hCAT-1 by a pathway involving protein kinase C.

The preferred source of arginine for high-output nitric oxide synthesis in blood vessels

L-arginine is the substrate for nitric oxide (NO) production by each of the 3 NO synthase (NOS) isoforms encoded by the mammalian genome. Despite the pivotal roles of NO in mammalian physiology and pathophysiology, the source of arginine for NO synthesis is not clearly defined. In this context, it is notable that cell types that do not have a complete urea cycle often possess the urea cycle enzymes argininosuccinate synthase and argininosuccinate lyase; together, these enzymes confer the ability to regenerate arginine from the NOS product, L-citrulline. Herein, the authors summarize evidence to support the view that argininosuccinate synthase and argininosuccinate lyase function in an arginine-citrulline cycle, providing a ready source of arginine for high-output NO synthesis. The arginine-citrulline cycle is induced in vascular cells by the same cytokines that trigger iNOS expression and provides the preferred source of substrate for NO production. Evidence suggests that argininosuccinate synthase activity is rate-limiting to high-output NO synthesis and, hence, represents a novel target for the treatment of pathophysiological conditions arising from NO overproduction.