Angiotensin II stimulates system y+ and cationic amino acid transporter gene expression in cultured vascular smooth muscle cells

Effect of l-Arginine Supplementation on Blood Pressure in Adults: A Systematic Review and Dose-Response Meta-analysis of Randomized Clinical Trials

A decade-old meta-analytic work indicated that l-arginine supplementation might have a blood pressure (BP)-lowering effect in different populations. However, several relevant investigations have emerged in the last 10 y, and an up-to-date systematic review and meta-analysis on this topic is currently lacking. Therefore, we aimed to examine the impact of l-arginine supplementation on BP by conducting a systematic review and dose-response meta-analysis of randomized placebo-controlled clinical trials (RCTs). We searched online databases using relevant keywords up to April 2021 to identify RCTs using oral l-arginine on systolic BP (SBP) and diastolic BP (DBP) in adults. Inclusion criteria were adult participants and an intervention duration ≥4 d. Exclusion criteria were the use of l-arginine infusion and acute interventions. A random-effects model was used to estimate the weighted mean difference (WMD) and 95% CI. Twenty-two RCTs with 30 effect sizes were included in this meta-analysis. The pooled analysis demonstrated significant decreases in SBP (WMD = -6.40 mmHg; 95% CI: -8.74, -4.05; P < 0.001) and DBP (WMD = -2.64 mmHg; 95% CI: -3.94, -1.40; P < 0.001) after l-arginine supplementation. Subgroup analysis showed significant reductions in SBP and DBP regardless of baseline BP category (normotensive, hypertensive), study duration (≤24 d, >24 d), sex (female, male), health status (healthy, unhealthy), and BMI (normal, overweight, obese). No significant changes were observed with dosages >9 g/d, trial duration >24 d, or in obese individuals. l-Arginine supplementation also appears to decrease DBP more effectively in females than in males. Moreover, meta-regression analysis for DBP demonstrated a significant relation between the dose of l-arginine intake and changes in DBP (P = 0.020). In the nonlinear dose-response analysis, the effective dosage of l-arginine supplementation was detected to be ≥4 g/d for SBP (P = 0.034), independent of trial duration. Overall, l-arginine supplementation may be effective for decreasing BP. This study was registered at PROSPERO as CRD42021242772.

The L-Arginine Transporter Solute Carrier Family 7 Member 2 Mediates the Immunopathogenesis of Attaching and Effacing Bacteria

Solute carrier family 7 member 2 (SLC7A2) is an inducible transporter of the semi-essential amino acid L-arginine (L-Arg), which has been implicated in immune responses to pathogens. We assessed the role of SLC7A2 in murine infection with Citrobacter rodentium, an attaching and effacing enteric pathogen that causes colitis. Induction of SLC7A2 was upregulated in colitis tissues, and localized predominantly to colonic epithelial cells. Compared to wild-type mice, Slc7a2^–/–mice infected with C. rodentium had improved survival and decreased weight loss, colon weight, and histologic injury; this was associated with decreased colonic macrophages, dendritic cells, granulocytes, and Th1 and Th17 cells. In infected Slc7a2^–/–mice, there were decreased levels of the proinflammatory cytokines G-CSF, TNF-α, IL-1α, IL-1β, and the chemokines CXCL1, CCL2, CCL3, CCL4, CXCL2, and CCL5. In bone marrow chimeras, the recipient genotype drove the colitis phenotype, indicative of the importance of epithelial, rather than myeloid SLC7A2. Mice lacking Slc7a2 exhibited reduced adherence of C. rodentium to the colonic epithelium and decreased expression of Talin-1, a focal adhesion protein involved in the attachment of the bacterium. The importance of SLC7A2 and Talin-1 in the intimate attachment of C. rodentium and induction of inflammatory response was confirmed in vitro, using conditionally-immortalized young adult mouse colon (YAMC) cells with shRNA knockdown of Slc7a2 or Tln1. Inhibition of L-Arg uptake with the competitive inhibitor, L-lysine (L-Lys), also prevented attachment of C. rodentium and chemokine expression. L-Lys and siRNA knockdown confirmed the role of L-Arg and SLC7A2 in human Caco-2 cells co-cultured with enteropathogenic Escherichia coli. Overexpression of SLC7A2 in human embryonic kidney cells increased bacterial adherence and chemokine expression. Taken together, our data indicate that C. rodentium enhances its own pathogenicity by inducing the expression of SLC7A2 to favor its attachment to the epithelium and thus create its ecological niche.

Intestinal infections by attaching and effacing (A/E) bacteria widely impact human health, with major social and economic repercussions. Mucosal immunity plays a critical role in determining the outcome of these infections. The amino acid L-arginine regulates inflammatory responses to bacterial pathogens. We studied the role of the L-arginine transporter solute carrier family 7 member 2 (SLC7A2) during infection with the A/E pathogen Citrobacter rodentium. SLC7A2 is induced in colonic epithelial cells during the infection and facilitates the intimate attachment of the bacteria, thus initiating the inflammatory response of the infected mucosa. These data were confirmed in vitro using C. rodentium-infected mouse cells and human colonic epithelial cells infected with enteropathogenic Escherichia coli. Our work describes a mechanism by which A/E bacteria manipulate host response to favor their colonization, thereby positioning SLC7A2 as an unrecognized therapeutic target to limit infection with enterobacteria.

Parallel regulation of arginine transport and nitric oxide synthesis by angiotensin II in vascular smooth muscle cells role of protein kinase C

Experiments were performed to characterize arginine transport in vascular smooth muscle cells (SMCs) and the effect of angiotensin II (Ang II) on this process. In addition, the role of arginine transport in the cytokineinduced nitric oxide (NO) production was assessed. Arginine transport takes place through Na(+)-independent (≈60%) and Na(+)-dependent pathways (≈40%). The Na(+)-independent arginine uptake appears to be mediated by system y(+) because of its sensitivity to cationic amino acids such as lysine, ornithine and homoarginine. The transport system was relatively insensitive to acidification of the extracellular medium. By contrast, the Na(+)-dependent pathway is consistent with system B(0,+) since it was inhibited by both cationic and neutral amino acids (i.e., glutamine, phenylalanine, and asparagine), and did not accept Li(+) as a Na(+) replacement. Treatment of SMCs with 100nM Ang II significantly inhibited the Na(+)-dependent arginine transport without affecting systems y(+), A, and L. This effect occurred in a dose-dependent manner (IC50 of 8.9 ± 0.9nM) and is mediated by the AT-1 receptor subtype because it was blocked by DUP 753, a non-peptide antagonist of this receptor. The inhibition of system B(0,+) by Ang II is mediated by protein kinase C (PKC) because it was mimicked by phorbol esters (phorbol 12-myristate 13-acetate) and was inhibited by staurosporine. Ang II also inhibited the IL-1β induced nitrite accumulation by SMCs. This action was also inhibited by staurosporine and reproduced with phorbol esters, suggesting a coupling between arginine uptake and NO synthesis through a PKC-dependent mechanism. However, arginine supplementation in the medium (10mM) failed to prevent the inhibitory action of Ang II on NO synthesis. These findings suggest that although Ang II inhibits concomitantly arginine transport and NO synthesis in SMCs, the reduction of NO synthesis is not associated with alterations in the cellular transport of arginine.

Regulation of CAT: Cationic amino acid transporter gene expression

The majority of mammalian cationic amino acid transport is mediated by the transport system y(+) which facilitates Na(+) independent cationic amino acid (arginine, lysine, & ornithine) transport and Na(+) dependent zwitterionic amino acid (glutamine & homoserine) transport. Other transport systems y(+)L, b(0,+) and B(0,+) also mediate cationic amino acid transport. Their broad substrate specificities and overlapping expression patterns confound biochemical analysis. The isolation of cDNA clones has permitted an analysis of their regulation and opens the opportunity to define the role of each protein in specific cell types. Two genes,Cat1 andCat2 encode transporters with properties similar to the y(+) transport system. Thecat2 gene from the mouse encodes two distinct proteins. mCAT2, and mCAT2A via alternate splicing; each protein has distinctly different transport properties. The regulation of mCAT1, mCAT2 and mCAT2A proteins are reviewed here. The implications of this gene specific regulation on cationic amino acid transport is discussed.

Amino acid transport system L activity in developing mouse ovarian follicles

BACKGROUND

Little is known about metabolic processes in the developing ovarian follicle. Using mouse ovarian follicles, we investigated uptake of L-leucine by follicles at varying stages of maturity in the presence of insulin-like growth factor (IGF)-1. METHODS Mouse ovarian follicles were cultured in vitro for 5 days in increasing concentrations of IGF-1, and follicle diameter and atresia measured as endpoints for growth. Uptake of (3)H-leucine was measured in follicles at different stages of development. In optimal IGF-1-mediated growth conditions, competitive inhibition of (3)H-leucine uptake by 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), a non-metabolizable substrate analogue of L-leucine, was performed to demonstrate specificity of influx, via system L transporters. To test whether uptake rates were dependent on intracellular amino acid availability, follicles from in vitro cultures were pre-treated with L-phenylalanine prior to (3)H-leucine uptake.

RESULTS

Follicle development (P< 0.001) and survival (P< 0.001) increased with IGF-1 treatment. As pre-antral follicles progressed to late antral stage, we observed an increase in L-leucine uptake, which was reduced in pre-ovulatory follicles. BCH decreased L-leucine uptake rates in early antral (P< 0.05), antral (P< 0.001) and pre-ovulatory follicles (P< 0.01). L-leucine influx increased in follicles preloaded with phenylalanine (trans-stimulation). In follicles lacking free intracellular amino acids (zero-trans suppression), uptake rate was reduced (P< 0.05).

CONCLUSIONS

These results demonstrate, for the first time, evidence of specific system L amino acid transport in intact, mouse ovarian follicles and profile L-leucine uptake during folliculogenesis. A better understanding of ovarian follicle metabolic pathways is necessary for improved in vitro maturation as well as determining the impact of altered metabolism on fertility.

Role of L-arginine in nitric oxide production in health and hypertension

1. l-Arginine is the substrate for vascular nitric oxide (NO) formation. Under normal physiological conditions, intracellular l-arginine levels far exceed the K(m) of NO synthase for l-arginine. However, endogenous NO formation is dependent on extracellular l-arginine concentrations, giving rise to the concept of the 'l-arginine paradox'. 2. Nitric oxide production in epithelial and endothelial cells is closely coupled to cellular l-arginine uptake, indicating that l-arginine transport mechanisms play a major role in the regulation of NO-dependent function. 3. Consistent with the data in endothelial and epithelial cells are functional data indicating that exogenous l-arginine can increase renal vascular and tubular NO bioavailability and thereby influence kidney perfusion, function and arterial pressure. The integrated effect of increased cellular l-arginine transport is to lower arterial pressure. Therefore, the use of l-arginine in the treatment of hypertension warrants investigation. 4. Low NO bioavailability is central to the development and maintenance of hypertension and to related endothelial dysfunction and target organ damage. We propose that l-arginine can interrupt the vicious cycle that initiates and maintains low NO in hypertension by increasing the formation of NO.

Tubular and glomerular L-arginine transport (uptake and transporters) and the nitric oxide synthases in ischemic acute renal failure (iARF) in streptozotocin-induced diabetic rats (STZ-DM)

BACKGROUND

L-arginine or its metabolites may be important pathogenetic factors in ischemic acute renal failure (iARF) in rats. It was found that the L-arginine-nitric oxide synthase-nitric oxide system plays an important role in the renal hemodynamic alterations in the early stages of diabetes. The iARF in diabetic rats is much more severe than the normal rats exposed to a same ischemia time. The purpose of the present study was to evaluated L-arginine uptake and its transporters and nitric oxide synthase isoform expression in tubuli and glomeruli of STZ-induced diabetic rats with iARF.

METHODS

iARF was induced by right nephrectomy and left renal artery clamping for 60 min followed by a 60 min reflow period. iARF was induced in STZ diabetes rats two weeks after intraperitoneal streptozotocin (60 mg/kg body weight) and in normal control rats. L-arginine uptake, L-arginine transporters (CAT1 and CAT2) and nitric oxide synthases (iNOS, eNOS, and bNOS) were determined by RT-PCR) in both glomeruli and tubuli preparations.

RESULTS

The STZ diabetic rats compared with the non diabetic normal rats have a higher glomerular L-arginine uptake, higher iNOS mRNA, lower eNOS mRNA, and lower tubular CAT1 mRNA, eNOS mRNA, and bNOS mRNA. The diabetic iARF after one hour of reperfusion had lower glomerular L-arginine uptake, lower CAT1 mRNA, lower eNOS mRNA, lower bNOS, and higher tubular iNOS mRNA compared with iARF in normal rats.

CONCLUSIONS

Our findings suggest a prolonged and more severe post-glomerular vasoconstriction very early after the reflow in the iARF of STZ diabetic rats compared with the iARF in the normal control rats. That may be a plausible explanation to the very significant decline in GFR and tubular necrosis that characterize the iARF in diabetic rats.

Apelin activates L-arginine/nitric oxide synthase/nitric oxide pathway in rat aortas

Apelin was recently found to be an inotropic polypeptide in isolated rat hearts, and intravenous injection of apelin can induce a transient decrease in blood pressure. To illustrate the mechanism of apelin-induced vasodilation, we observed the in vitro effects of apelin on the L-arginine (L-Arg)/nitric oxide (NO) pathway in the incubated, isolated rat aorta. Apelin stimulated vascular NO(2)(-) product and NOS activation in a concentration- and time-dependent manner. Compared with no apelin treatment, incubation with apelin (10(-9), 10(-8), and 10(-7)mol/L) increased NO(2)(-) product by 33%, 46%, and 69% (all p<0.01), respectively, and Ca(2+)-dependent constitutive NOS (cNOS) activity by 200%, 460%, and 550% (all p<0.01), respectively. However, Ca(2+)-independent NOS (iNOS) activity was not significantly altered (p>0.05). Apelin incubation (10(-9), 10(-8), and 10(-7)mol/L) increased L-Arg uptake by 130%, 180%, and 240% (all p<0.01), respectively. The mRNA level of cationic amino acid transporters, CAT-1 and CAT-2B, in rat aortic tissues treated with 10(-7)mol/L apelin was increased by 110% and 128%, respectively (both p<0.01). Incubation with 10(-7)mol/L apelin elevated eNOS mRNA and protein levels, by 53% (p<0.05) and 319% (p<0.01), respectively. Collectively, these results demonstrate that apelin directly activated the vascular L-Arg/NOS/NO pathway, which could be one of the important mechanisms of apelin-regulated vascular function.

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.

Intermedin1-53 activates l-arginine/nitric oxide synthase/nitric oxide pathway in rat aortas

Intermedin (IMD), a novel member of the calcitonin/calcitonin gene-related peptide (CGRP) family, has similar or more potent vasodilatory and hypotensive actions than adrenomedullin (ADM) and CGRP. The present study was designed to observe the effects of synthetic rat IMD1-53 on L-arginine (L-Arg) cellular transport, nitric oxide synthase (NOS) activity, and nitric oxide (NO) production in the isolated rat aortic ring to illustrate its direct effect on the L-Arg/NOS/NO pathway in vasculature. IMD1-53 significantly increased NO production and cNOS activity in rat aortas and was more potent than equivalent ADM. But the peptides of both IMD and ADM had no effect on inducible NOS expression and activity. Otherwise, IMD1-53 induced a concentration-dependent increase in [3H]L-Arg transport and its effect was more potent than that of an equivalent concentration of ADM. Semiquantitative RT-PCR revealed that IMD1-53 significantly increased cationic amino acid transport (CAT)-1 and CAT-2B mRNA expression, and its effect was similar to that of ADM. All these results suggest that IMD1-53 might regulate vessel function homeostasis via upregulating the L-Arg/NOS/NO pathway.

Asymmetrical dimethylarginine plasma clearance persists after acute total nephrectomy in rats

Elevated plasma concentrations of symmetrical dimethylarginine (SDMA) and asymmetrical dimethylarginine (ADMA) are repeatedly associated with kidney failure. Both ADMA and SDMA can be excreted in urine. We tested whether renal excretion is necessary for acute, short-term maintenance of plasma ADMA and SDMA. Sprague-Dawley rats underwent sham operation, bilateral nephrectomy (NPX), ureteral ligation, or ureteral section under isoflurane anesthesia. Tail-snip blood samples (250 microl) were taken before and at 6- or 12-h intervals for 72 h after operation. Plasma clearance was assessed in intact and NPX rats. High-performance liquid chromatography determined SDMA and ADMA concentrations. Sodium, potassium, creatinine, blood urea nitrogen (BUN), and body weight were also measured. Forty-eight hours after NPX, SDMA increased 25 times (0.23 +/- 0.03 to 5.68 +/- 0.30 microM), whereas ADMA decreased (1.17 +/- 0.08 to 0.73 +/- 0.08 microM) by 38%. Creatinine and BUN increased, paralleling SDMA. Sham-operated animals showed no significant changes. Increased SDMA confirms continuous systemic production of SDMA and its obligatory renal excretion, much like creatinine. In contrast, decreased plasma ADMA suggests that acute total NPX either reduced systemic ADMA formation and/or systemic hydrolysis of ADMA increased 48-h post-NPX. However, plasma clearance of ADMA appeared unchanged 48 h after NPX. We conclude that renal excretory function is needed for SDMA elimination but not needed for acute, short-term ADMA elimination in that systemic hydrolysis is fully capable of clearing plasma ADMA.

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.

Recent advances in the regulation of nitric oxide in the kidney

Nitric oxide (NO) plays important roles in the regulation of renal function and the long-term control of blood pressure. New roles of NO have been proposed recently in diabetes, nephrotoxicity, and pregnancy. NO derived from all 3 NOS isoforms contributes to the overall regulation of kidney function, and recent advances in our understanding of their regulation have been made lately. In this regard, substrate and cofactor availability play important roles in regulating nitric oxide synthase (NOS) activity not only by limiting enzyme activity but also by influencing the coupling of NOS with its cofactors, tetrahydrobiopterin and NADPH. Protein-protein interactions are now recognized to be important negative and positive regulators of NOS. Phosphorylation is another component of the mechanism whereby NOS is activated or deactivated. Increased NOS expression can also influence enzyme activity; however, the degree of expression does not always correlate with enzyme activity because increased NO levels can result in inhibition of NOS. Finally, other potential regulators of NOS such as endogenous L-arginine analogs may also be important. In this article, we summarize recent advances in the regulation of activity and expression of the NOS isoforms within the kidney.

Angiotensin II early regulated genes in H295R human adrenocortical cells

Evidence for the dysregulation of aldosterone synthesis in cardiovascular pathophysiology has renewed interest in the control of its production. Cellular mechanisms by which angiotensin II (ANG II) stimulates aldosterone synthesis in the adrenal zona glomerulosa are incompletely understood. To elucidate the mechanism of intracellular signaling by ANG II stimulation in the adrenal, we have studied immediate-early regulated genes in human adrenal H295R cells using cDNA microarrays. H295R cells were stimulated with ANG II for 3 h. Gene expression was analyzed by microarray technology and validated by real-time RT-PCR. Eleven genes were found to be upregulated by ANG II. These encode the proteins for ferredoxin, Nor1, Nurr1, c6orf37, CAT-1, A20, MBLL, M-Ras, RhoB, GADD45α, and a novel protein designated FLJ45273 . Maximum expression levels for all genes occurred 3–6 h after ANG II stimulation. This increase was dose dependent and preceded maximal aldosterone production. Other aldosterone secretagogues, K+and endothelin-1 (ET-1), also induced the expression of these genes with variable efficiency depending on the gene and with lower potency than ANG II. ACTH had negligible effect on gene expression except for the CAT-1 and Nurr1 genes. These ANG II-stimulated genes are involved in several cellular functions and are good candidate effectors and regulators of ANG II-mediated effects in adrenal zona glomerulosa.

Differential regulation of glomerular arginine transporters (CAT-1 and CAT-2) in lipopolysaccharide-treated rats

The decrease in glomerular filtration rate (GFR) that is characteristic of sepsis has been shown to result from inhibition of glomerular endothelial nitric oxide synthase (eNOS) by nitric oxide (NO) generated from the inducible isoform of NOS (iNOS). Although l-arginine is the sole precursor for NO biosynthesis, its intracellular availability in glomeruli from septic animals has never been investigated. Arginine uptake was measured in freshly harvested glomeruli from the following experimental groups: 1) untreated rats; 2) rats pretreated with LPS (4 mg/kg body wt, 4 h before experiments); 3) rats treated with LPS as above with either l-N(6)-(1-iminoethyl)lysine hydrochloride (l-NIL), a selective iNOS antagonist, or 7-nitroindazole, a selective neuronal NOS antagonist; and 4) rats treated with l-NIL only. Both glomeular and mesangial arginine transport characteristics were found compatible with a y(+) system. Arginine uptake was augmented in glomeruli from LPS-treated rats. Treatment with l-NIL completely abolished this effect whereas l-NIL alone had no effect. Similar results were obtained when primary cultures of rat mesangial cells were preincubated with LPS (10 microg/ml for 24 h) with or without l-NIL. Using RT-PCR, we found that in vivo administration of LPS resulted in a significant increase in glomerular cationic amino acid transporter-2 (CAT-2) mRNA expression whereas CAT-1 mRNA was undetected. Northern blotting further confirmed a significant increase in glomerular CAT-2 by LPS. In mesangial cells, the expression of both CAT-1 and CAT-2 mRNA was augmented after incubation with LPS. In conclusion, in vivo administration of LPS augments glomerular arginine transport through upregulation of steady-state CAT-2 mRNA while downregulating CAT-1 mRNA. These results may correspond to the changes in glomerular iNOS and eNOS activity in sepsis.

High-salt diet increases sensitivity to NO and eNOS expression but not NO production in THALs

L-Arginine inhibits thick ascending limb (THAL) NaCl absorption by activating endothelial NO synthase (eNOS) and increasing NO production. Inhibition of renal NO production combined with a high-salt diet produces hypertension, and the THAL has been implicated in salt-sensitive hypertension. We hypothesized that a high-salt diet enhances the inhibitory action of L-arginine on NaCl absorption by THALs because of increased eNOS expression and NO production. To test this, we used isolated THALs from rats on a normal-salt (NS) or high-salt diet (HS) for 7 to 10 days. L-Arginine (1 mmol/L) decreased chloride absorption by 56+/-10% in THALs from rats on a HS diet, but only 29+/-3% in THALs from rats on a NS diet. eNOS expression in isolated THALs from rats on a HS diet was increased by 3.9-fold compared with NS (P<0.03). However, L-arginine increased NO levels to the same extent in THALs from both groups, as measured with DAF-2 DA or a NO-sensitive electrode. To determine whether a HS diet increases the sensitivity of the THAL to NO, we tested the effects of the NO donor spermine NONOate on chloride absorption. In THALs from rats on a HS diet, 1 and 5 micromol/L spermine NONOate reduced chloride absorption by 35+/-5% and 58+/-6%, respectively. In contrast, these same concentrations of spermine NONOate reduced chloride absorption by 4+/-4% (P<0.03 versus HS diet) and 43+/-9% in THALs from rats on a NS diet. We conclude that a HS diet enhances the effect of NO in the THAL. L-Arginine-stimulated NO production was not enhanced by a HS diet, despite increased eNOS protein.

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.

L-Arginine transport is augmented through up-regulation of tubular CAT-2 mRNA in ischemic acute renal failure in rats

BACKGROUND

Ischemic acute renal failure (iARF) is associated with increased nitric oxide (NO) production during the reperfusion period, as endothelial nitric oxide synthase (eNOS) is maximally activated, and renal tubular inducible NOS (iNOS) is stimulated. Increased NO production leads to augmented tubular injury, probably through the formation of peroxynitrite. l-Arginine (l-Arg), the only precursor for NO, is transported into cells by cationic amino acid transporters, CAT-1 and CAT-2. We hypothesized that the increased NO production observed in iARF may result from increased l-Arg uptake, which would be reflected in the augmented expression of l-Arg transporter(s).

METHODS

Ischemic acute renal failure was induced in rats by right nephrectomy + left renal artery clamping for 60 minutes. l-Arg uptake was examined in freshly harvested glomeruli and tubuli from control, sham operated, and animals subjected to 15, 30, and 60 minutes, and 24 hours of reperfusion, following 60 minutes of ischemia. Using RT-PCR, renal tissues were examined further for the expression of iNOS, CAT-1, CAT-2, arginase I and arginase II.

RESULTS

Tubular expression of iNOS mRNA was initiated by ischemia, continued to increase after 60 minutes of reperfusion, and decreased after 24 hours. l-Arg transport into glomeruli was similar in all experimental groups. l-Arg uptake into tubuli was markedly augmented following the 60-minute reperfusion, while it moderately increased after 24 hours of reperfusion. This was accompanied by a parallel, preferential increase in tubular CAT-2 mRNA expression at 60 minutes of reperfusion. CAT-1 mRNA expression was unchanged, as detected by RT-PCR. In addition, the expression of arginase II and arginase I mRNA was attenuated by 30 minutes and one hour of reperfusion, and returned to baseline values after 24 hours of reperfusion.

CONCLUSIONS

Ischemic ARF is associated with augmented tubular CAT-2 mRNA expression, which leads to enhanced l-Arg transport and increased NO production. This may contribute to the renal injury exhibited in iARF.

Two-way arginine transport in human endothelial cells: TNF-alpha stimulation is restricted to system y(+)

Human umbilical vein endothelial cells transport arginine through two Na(+)-independent systems. System y(+)L is insensitive to N-ethylmaleimide (NEM), inhibited by L-leucine in the presence of Na(+), and referable to the expression of SLC7A6/y(+)LAT2, SLC7A7/y(+)LAT1, and SLC3A2/4F2hc. System y(+) is referable to the expression of SLC7A1/CAT1 and SLC7A2/CAT2B. Tumor necrosis factor-alpha (TNF-alpha) and bacterial lipopolysaccharide induce a transient stimulation of arginine influx and efflux through system y(+). Increased expression of SLC7A2/CAT2B is detectable from 3 h of treatment, while SLC7A1 expression is inhibited at later times of incubation. System y(+)L activity and expression remain unaltered. Nitric oxide synthase type 2 mRNA is not detected in the absence or presence of TNF-alpha, while the latter condition lowers nitric oxide synthase type 3 expression at the mRNA and the protein level. Nitrite accumulation is comparable in cytokine-treated and control cells up to 48 h of treatment. It is concluded that modulation of endothelial arginine transport by TNF-alpha or lipopolysaccharide occurs exclusively through changes in CAT2B and CAT1 expression and is dissociated from stimulation of nitric oxide production.

Tetrahydrobiopterin augments arginine transport in rat cardiac myocytes through modulation of CAT-2 mRNA

Tetrahydrobiopterin (BH4) has been shown to be required for dimerization and acquisition of nitric oxide (NO) generating capacity by nitric oxide synthase (NOS). In the present study we have investigated the hypothesis that BH4 may affect NOS activity through a novel mechanism-namely, modulating arginine transport in rat cardiac myocytes. Cardiac myocytes have been previously shown to express cationic amino acid transport proteins (y+ system) CAT-1 and CAT-2. Increasing extracellular BH4 concentrations up to 0.5 mmol/L augments arginine transport in 1 mmol/L arginine media (no BH4, 558 +/- 42 fmol arginine/microg protein/min; 0.1 mmol/L BH4, 580 +/- 11 fmol arginine/microg protein/min; 0.5 mmol/L BH4, 944 +/- 71* fmol arginine/microg protein/min; 1.0 mmol/L BH4, 983+/-84* fmol arginine/microg protein/min, n = 4; *: P <.05 vs no BH4). Treating the cells with lipopolysaccharide (LPS) (10 microg/mL) significantly augmented arginine transport only in the presence of BH4 (no BH4, 600 +/- 33 fmol arginine/microg protein/min; 0.1 mmol/L BH4, 691 +/- 29*dagger fmol arginine/microg protein/min; 0.5 mmol/L BH4, 1123 +/- 32*dagger fmol arginine/microg protein/min; 1.0 mmol/L BH4, 1296 +/- 42*dagger fmol arginine/microg protein/min, n = 4; *: P <.01 vs no BH4, dagger: P <.05 vs no LPS). The administration of biopterin, sodium nitroprusside (NO donor), 2,4-diamino-6-hydroxy-pyrimidine (inhibitor of BH4 synthesis), and sepiapterin (the precursor of de novo synthesis of BH4) to unstimulated cells had no effect on arginine uptake values. Using reverse trancriptase-polymerase chain reaction, we next studied the steady state levels for CAT-1 and CAT-2 mRNA. Incubation with BH4 significantly increased CAT-2 mRNA expression in a concentration-dependent manner in 0.1, 0.5, and 1 mmol/L BH4, respectively. Northern blotting analysis further confirmed this observation. We also found that in the presence of BH4 in these concentrations, CAT-1 mRNA expression was abolished. We suggest that BH4 augments intracellular arginine availability by modulating CAT-2 mRNA expression and suggest that its presence is required for the LPS effect on trans-membrane arginine traffic.

Transforming growth factor-beta(1) stimulates L-arginine transport and metabolism in vascular smooth muscle cells: role in polyamine and collagen synthesis

BACKGROUND

Transforming growth factor-beta(1) (TGF-beta(1)) contributes to arterial remodeling by stimulating vascular smooth muscle cell (VSMC) growth and collagen synthesis at sites of vascular injury. Because L-arginine is metabolized to growth-stimulatory polyamines and to the essential collagen precursor L-proline, we examined whether TGF-beta(1) regulates the transcellular transport and metabolism of L-arginine by VSMCs.

METHODS AND RESULTS

TGF-beta(1) increased L-arginine uptake, and this was associated with a selective increase in cationic amino acid transporter-1 (CAT-1) mRNA. In addition, TGF-beta(1) stimulated L-arginine metabolism by inducing arginase I mRNA and arginase activity. TGF-beta(1) also stimulated L-ornithine catabolism by elevating ornithine decarboxylase (ODC) and ornithine aminotransferase (OAT) activity. TGF-beta(1) markedly increased the capacity of VSMCs to generate the polyamine putrescine and L-proline from extracellular L-arginine. The TGF-beta(1)-mediated increase in putrescine and L-proline production was reversed by methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by hydroxy-L-arginine, an arginase inhibitor. Furthermore, the formation of putrescine was inhibited by the ODC inhibitor alpha-difluoromethylornithine, and L-proline generation was blocked by the OAT inhibitor L-canaline. L-Canaline also inhibited TGF-beta(1)-stimulated type I collagen synthesis.

CONCLUSIONS

These results demonstrate that TGF-beta(1) stimulates polyamine and L-proline synthesis by inducing the genes that regulate the transport and metabolism of L-arginine. In addition, they show that TGF-beta(1)-stimulated collagen production is dependent on L-proline formation. The ability of TGF-beta(1) to upregulate L-arginine transport and direct its metabolism to polyamines and L-proline may contribute to arterial remodeling at sites of vascular damage.

Increased L-arginine uptake and inducible nitric oxide synthase activity in aortas of rats with heart failure

L-Arginine crosses the cell membrane primarily through the system y(+) transporter. The aim of this study was to investigate the role of L-arginine transport in nitric oxide (NO) production in aortas of rats with heart failure induced by myocardial infarction. Tumor necrosis factor-alpha levels in aortas of rats with heart failure were six times higher than in sham rats (P < 0.01). L-Arginine uptake was increased in aortas of rats with heart failure compared with sham rats (P < 0.01). Cationic amino acid transporter-2B and inducible (i) nitric oxide synthase (NOS) expression were increased in aortas of rats with heart failure compared with sham rats (P < 0.05). Aortic strips from rats with heart failure treated with L-arginine but not D-arginine increased NO production (P < 0.05). The effect of L-arginine on NO production was blocked by L-lysine, a basic amino acid that shares the same system y(+) transporter with L-arginine, and by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Treatment with L-lysine and L-NAME in vivo decreased plasma nitrate and nitrite levels in rats with heart failure (P < 0.05). Our data demonstrate that NO production is dependent on iNOS activity and L-arginine uptake and suggest that L-arginine transport plays an important role in enhanced NO production in heart failure.

SSeCKS gene expression in vascular smooth muscle cells: regulation by angiotensin II and a potential role in the regulation of PAI-1 gene expression

Rat aortic smooth muscle cells (RASM) express the src suppressed C-kinase substrate (SSeCKS), which is thought to be an integral regulatory component of cytoskeletal dynamics and G-protein coupled-receptor signaling modules. The specific sub-classes of growth factor receptors that regulate the genomic changes in SSeCKS expression in smooth muscle cells have not been characterized. In this study we identify SSeCKS as an angiotensin type 1 (AT(1)) receptor-dependent target gene in RASM cells treated with angiotensin II (Ang II). SSeCKS mRNA levels increase up to three-fold relative to the control within 3.5 h of Ang II treatment and are followed by a slight decrease of mRNA relative to the control levels after 24 h of stimulation. SSeCKS gene expression and plasminogen activator inhibitor-1 (PAI-1) gene expression correlate in RASM cells treated with Ang II. By co-transfecting plasmids bearing recombinant-SSeCKS and a PAI-1-promoter/luciferase reporter into Cos-1 cells, we show that alternative forms of recombinant-SSeCKS protein differentially influence PAI-1 promoter activity. These data indicate a biochemical linkage between SSeCKS activity and one or more of the cytoplasmic signaling pathways that are involved in the control of PAI-1 promoter activity. Finally, we show that the alternative forms of recombinant-SSeCKS protein differentially influence cell-spreading when ectopically expressed in ras -transformed rat kidney (KNRK) fibroblasts. Taken together, our data suggest that SSeCKS interacts with intracellular signaling pathways that control cytoskeletal remodeling and extracellular matrix remodeling following Ang II stimulation of the RASM cell.

Renal expression of constitutive NOS and DDAH: separate effects of salt intake and angiotensin

BACKGROUND

Nitric oxide (NO) is generated from NO synthase (NOS) isoforms. These enzymes can be inhibited by asymmetric dimethylarginine, which is inactivated by N(G)-N(G)-dimethylarginine dimethylaminohydrolase (DDAH). The neuroneal (nNOS) type I and endothelial (eNOS) type III constitutive NOS isoforms are expressed predominantly in the macula densa and microvascular endothelium of the renal cortex, respectively. DDAH is expressed at sites of NOS expression. Since NO may coordinate the renal responses to angiotensin II (Ang II) and changes in salt intake, we tested the hypothesis that salt intake regulates the expression of nNOS, eNOS and DDAH by Ang II acting on type 1 (AT(1)) receptors.

METHODS

Groups (N = 6) of rats were adapted to low-salt (LS) or high-salt (HS) intakes for 10 days. Other groups of LS and HS rats received the AT(1) receptor antagonist losartan for six days (to test the effects of salt independent of AT(1) receptors). A further group of HS rats received an infusion of Ang II for six days (to test the effect of Ang II independent of salt intake).

RESULTS

Compared with HS rats, there was a significant (P < 0.05) increase in LS rats of nNOS protein in kidney and immunohistochemical expression in the macula densa, and of eNOS protein expression and immunohistochemical expression in the microvascular endothelium, and of DDAH protein expression. Losartan prevented these effects of salt on the expression of eNOS or DDAH, both of which were also increased by Ang II infusions in HS rats. In contrast, losartan did not prevent the effects of salt on nNOS expression, which was unresponsive to Ang II infusion. The generation of NO(2)(-) released by slices of renal cortex, in the presence of saturating concentrations of L-arginine, was increased by LS, compared to HS, independent of losartan and by Ang II during HS.

CONCLUSION

The expressions of eNOS in cortical microvascular endothelium and DDAH in kidney are enhanced by Ang II acting on AT(1) receptors. The expression of nNOS in the macula densa is enhanced by salt restriction independent of Ang II or AT(1) receptors.

Physiological cyclic stretch directs L-arginine transport and metabolism to collagen synthesis in vascular smooth muscle

Application of cyclic stretch (10% at 1 hertz) to vascular smooth muscle cells (SMC) increased L-arginine uptake and this was associated with a specific increase in cationic amino acid transporter-2 (CAT-2) mRNA. In addition, cyclic stretch stimulated L-arginine metabolism by inducing arginase I mRNA and arginase activity. In contrast, cyclic stretch inhibited the catabolism of L-arginine to nitric oxide (NO) by blocking inducible NO synthase expression. Exposure of SMC to cyclic stretch markedly increased the capacity of SMC to generate L-proline from L-arginine while inhibiting the formation of polyamines. The stretch-mediated increase in L-proline production was reversed by methyl-L-arginine, a competitive inhibitor of L-arginine transport, by hydroxy-L-arginine, an arginase inhibitor, or by the ornithine aminotransferase inhibitor L-canaline. Finally, cyclic stretch stimulated collagen synthesis and the accumulation of type I collagen, which was inhibited by L-canaline. These results demonstrate that cyclic stretch coordinately stimulates L-proline synthesis by regulating the genes that modulate the transport and metabolism of L-arginine. In addition, they show that stretch-stimulated collagen production is dependent on L-proline formation. The ability of hemodynamic forces to up-regulate L-arginine transport and direct its metabolism to L-proline may play an important role in stabilizing vascular lesions by promoting SMC collagen synthesis.

L-arginine uptake and L-phosphoarginine synthesis in Trypanosoma cruzi

A very specific L-arginine transporter showing high affinity has been characterized in Trypanosoma cruzi epimastigotes. Uptake was found to be dependent on L-arginine concentration and it was saturable. Values for maximum velocity and Km ranged between 48.1-57.5 pmol.min-1 per 3 x 10(7) cells and between 4.2-5.5 microM, respectively. The calculated activation energy and Q10 were 31.1 KJ.mol-1, and 1.7, respectively. Uptake velocity significantly increased when cells were preincubated in the absence of L-arginine. Cells retained the labeled amino acid independently of the presence or absence of exogenous L-arginine. The specificity of L-arginine uptake was demonstrated by competition assays in the presence of 80-fold molar excess of natural amino acids and several L-arginine derivatives. The highest levels of inhibition were caused by L-homoarginine, D-arginine, L-canavanine, L-ornithine, and L-citrulline. L-arginine uptake by T. cruzi epimastigotes was not affected by the presence of potassium or sodium ions in the incubation mixture or by pH changes in the range between 5.5-8.5. The major product of L-arginine uptake was characterized as phosphoarginine. Moreover, arginine kinase activity was detected in soluble extracts from T. cruzi epimastigotes.

Phosphatidylinositol 3-kinase is required for growth factor-induced amino acid uptake by vascular smooth muscle cells

Although accumulating evidence suggests that phosphatidylinositol 3-kinase (PI3K) is a common signaling molecule for growth factor-induced amino acid uptake by the cell, the role of PI3K in the uptake of different amino acids was not tested under the same conditions. In this study, we asked whether PI3K mediates platelet-derived growth factor (PDGF) -stimulated uptake of different amino acids that are taken up through 3 major amino acid transporters expressed in rat vascular smooth muscle cells and other cell types and whether PI3K mediates amino acid uptake stimulated with different growth factors and vasoactive substances. PDGF increased the uptake of [(3)H]leucine, [(3)H]proline, and [(3)H]arginine in a dose- and time-dependent fashion. Two different PI3K inhibitors, wortmannin (100 nmol/L) and LY294002 (10 micromol/L), completely inhibited the amino acid uptake stimulated by PDGF. Chinese hamster ovary cells expressing both PDGF receptor-beta and a dominant-negative PI3K did not increase their leucine uptake when stimulated with PDGF, whereas the same cells expressing only PDGF receptor-beta did. Transforming growth factor-beta, as well as insulin-like growth factor-I and angiotensin II, increased leucine uptake by vascular smooth muscle cells. Wortmannin and LY294002 inhibited this increase. We also found that transforming growth factor-beta stimulated PI3K activity and the phosphorylation of Akt, a downstream signaling molecule of PI3K. A similar effect of PI3K inhibitors on amino acid uptake was observed in Swiss 3T3 cells. We conclude that PI3K mediates the uptake of different amino acids by vascular smooth muscle cells and other cell types stimulated with a variety of growth factors, including transforming growth factor-beta. Our findings suggest that PI3K may play an important role in vascular pathophysiology by regulating amino acid uptake.

Neuron-specific expression of cationic amino acid transporter 3 in the adult rat brain

CAT3 (cationic amino acid transporter 3) is a member of the murine CAT family which bears a system y(+) transport activity. On the Northern blot of adult rat tissues, the expression of CAT3 is restricted to the brain. In the present study, cellular localization of CAT3 mRNA and protein in the adult rat brain sections was examined by in situ hybridization with cRNA and immunostaining with a CAT3-specific antiserum, respectively. CAT3 mRNA was present both in the cerebral and cerebellar gray matter but most prominently in the nuclei located in the ventromedial part of the brain. These included preoptic nucleus, hypothalamic nucleus, reticular nucleus of thalamus, substantia nigra, central gray around the third ventricle and amygdala. CAT3 protein was also detected both in the cerebral and cerebellar gray matter and strong immunostaining was obtained in the olfactory cortex, hippocampus and cerebellar granular and Purkinje cell layers. Observations at higher magnifications revealed that both mRNA and protein were expressed by neurons but neither by glial nor endothelial cells. These results confirm the neuron-specificity of CAT3 in the adult rat brain and indicate that CAT3 is responsible for the neuronal system y(+) activity. The discrepancy between the distribution of mRNA and its translation product suggests a regional difference in the translation rate of the CAT3 transcript.

Cationic amino acid transporter gene expression in cultured vascular smooth muscle cells and in rats

Immunostimulants trigger vascular smooth muscle cells (VSMC) to express the inducible isoform of NO synthase (iNOS) and increased arginine transport activity. Although arginine transport in VSMC is considered to be mediated via the y+ system, we show here that rat VSMC in culture express the cat-1 gene transcript as well as an alternatively spliced transcript of the cat-2 gene. An RT-PCR cloning sequence strategy was used to identify a 141-base nucleotide sequence encoding the low-affinity domain of alternatively spliced CAT-2A and a 138-base nucleotide sequence encoding the high-affinity domain of CAT-2B in VSMC activated with lipopolysaccharide (LPS) in combination with interferon-gamma (IFN). With this sequence as a probe, Northern analyses showed that CAT-1 mRNA and CAT-2B mRNA are constitutively present in VSMC, and the expression of both mRNAs was rapidly stimulated by treatment with LPS-IFN, peaked within 4 h, and decayed to basal levels within 6 h after LPS-IFN. CAT-2A mRNA was not detectable in unstimulated or stimulated VSMC. Arginine transporter activity significantly increased 4-10 h after LPS-IFN. iNOS activity was reduced to almost zero in the absence of extracellular arginine uptake via system y+. Induction of arginine transport seems to be a prerequisite to the enhanced synthesis of NO in VSMC. Moreover, this work demonstrates tissue expression of CAT mRNAs with use of a model of LPS injection in rats. RT-PCR shows that the expression of CAT-1 and CAT-2B mRNA in the lung, heart, and kidney is increased by LPS administration to rats, whereas CAT-2A mRNA is abundantly expressed in the liver independent of LPS treatment. These findings suggest that together CAT-1 and CAT-2B play an important role in providing substrate for high-output NO synthesis in vitro as well as in vivo and implicate a coordinated regulation of intracellular iNOS enzyme activity with membrane arginine transport.

Control of expression of the gene for the arginine transporter Cat-1 in rat liver cells by glucocorticoids and insulin

Hepatic arginine and lysine uptake is partly regulated by changes in the transport activity of a group of cell surface proteins exhibiting properties of the transport system y+. The Cat-1 gene encodes a sodium-independent high-affinity cationic amino acid transporter of the y+ system which is nearly undetectable in the quiescent liver. In this paper we investigate the regulation of expression of Cat-1 in the quiescent rat liver by glucocorticoids and insulin, two hormones which play a critical role in amino acid dependent pathways of hepatic metabolism. Injection of insulin and glucocorticoids resulted in a rapid (15-30 min, 4-5 fold) increase in transcription which returned to basal levels within 4 hours. In contrast to the rapid single peak of transcriptional induction of the Cat-1 gene, the accumulation of the Cat-1 mRNAs occurred transiently with two peaks, the first at 30 minutes and the second at 2-4 hours following hormone treatment. These data indicate that expression of the Cat-1 gene in the quiescent liver can be transiently induced by both transcriptional and post-transcriptional mechanisms. In FTO2B rat hepatoma cells, expression of the gene is constitutive and accumulation of Cat-1 mRNAs in response to dexamethasone and insulin was dependent on transcription and protein synthesis. Furthermore, the accumulation of the basal level of the Cat-1 mRNAs was reduced by 70%, upon treatment of cells with inhibitors of protein synthesis for 6 h, when the transcription rate of the gene did not decrease significantly. We conclude the following: (i) under normal physiologic conditions, expression of the Cat-1 gene in the quiescent liver is negligible, probably to prevent unnecessary transport and metabolism of arginine by the hepatic arginase in the hepatocytes. (ii) in the cases when hepatic cationic amino acid transport is needed, such as following feeding, cellular growth and illness, glucocorticoids and insulin induce expression of the Cat-1 gene in liver cells through induction of transcription and stabilization of the mRNA. (iii) constitutive Cat-1 mRNA accumulation in rat hepatoma cells depends on protein synthesis through a labile regulated factor. Overall, constitutive expression of Cat-1 is associated with hepatic cellular growth and transformation.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Overexpression of protein kinase C alpha enhances lipopolysaccharide-induced nitric oxide formation in vascular smooth muscle cells

Our previous studies showed that lipopolysaccharide (LPS)-induced nitric oxide (NO) synthesis in cardiovascular tissues is attenuated by protein kinase C (PKC) inhibitors. In the current study, we identify a specific PKC isotype involved in the LPS signal transduction pathway that leads to NO formation in rat vascular smooth muscle cells (VSMC). VSMC were transfected with a mammalian expression vector containing a full length PKCalpha cDNA insert, and a stable transfectant overexpressing PKCalpha was obtained as evidenced by increased expression of PKCalpha mRNA and protein. In response to 100 ng/ml LPS stimulation, the PKCalpha transfectants showed a 1.8-fold increase in PKC activity at 30 min and a twofold increase in NO production over 24 hr compared with cells transfected with control plasmids. The LPS-stimulated increase in NO synthesis in PKCalpha transfectants was blocked by the specific PKCalpha inhibitor Gö 6976. After 6 hr LPS treatment, PKCalpha-transfected and control cells showed equivalent increases in mRNA and protein for the inducible NO synthase. NO synthase activity of the cell extracts assayed in the presence of excess substrate and cofactors was not significantly different between PKCalpha-transfected and control cells after LPS stimulation. However, mRNA levels for GTP cyclohydrolase I, a key enzyme in (6R)-tetrahydro-L-biopterin synthesis, and cationic amino acid transporter-2, involved in L-arginine transport, was enhanced in cells overexpressing PKCalpha compared with control cells. These results suggest that PKCalpha plays an important role in LPS-induced NO formation and that a significant portion of this effect may be by means of enhanced substrate availability to the inducible nitric oxide synthase enzyme.

Thrombin stimulates vascular smooth muscle cell polyamine synthesis by inducing cationic amino acid transporter and ornithine decarboxylase gene expression

Thrombin, a serine protease, is a potent mitogen for vascular smooth muscle cells (SMCs), but its mechanism of action is not known. Since L-ornithine is metabolized to growth-stimulatory polyamines, we examined whether thrombin regulates the transcellular transport and metabolism of L-ornithine by vascular SMCs. Treatment of SMCs with thrombin initially (0 to 2 hours) decreased L-ornithine uptake, whereas longer exposures (6 to 24 hours) progressively increased transport. Kinetic studies indicated that thrombin-induced inhibition was associated with a decrease in affinity for L-ornithine, whereas stimulation was mediated by an increase in transport capacity. Thrombin induced the expression of both cationic amino acid transporter (CAT)-1 and CAT-2 mRNA. Furthermore, thrombin stimulated L-ornithine metabolism by inducing ornithine decarboxylase (ODC) mRNA expression and activity. The stimulatory effect of thrombin on both L-ornithine transport and ODC activity was reversed by hirudin, a thrombin inhibitor, and was mimicked by a 14-amino acid thrombin receptor-activating peptide. Thrombin also markedly increased the capacity of SMCs to generate putrescine, a polyamine, from extracellular L-ornithine. The thrombin-mediated increase in putrescine production was reversed by N(G)-methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by alpha-difluoromethylornithine (DFMO), an ODC inhibitor. DFMO also inhibited thrombin-induced SMC proliferation. These results demonstrate that thrombin stimulates polyamine synthesis by inducing CAT and ODC gene expression and that thrombin-stimulated SMC proliferation is dependent on polyamine formation. The ability of thrombin to upregulate L-ornithine transport and direct its metabolism to growth-stimulatory polyamines may contribute to postangioplasty restenosis and atherosclerotic lesion formation.

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

Lysophosphatidylcholine regulates cationic amino acid transport and metabolism in vascular smooth muscle cells. Role in polyamine biosynthesis

Lysophosphatidylcholine (lyso-PC) is a major component of atherogenic lipids that stimulate vascular smooth muscle cell (SMC) proliferation. Because cationic amino acids are metabolized to growth-stimulatory polyamines, we examined whether lyso-PC regulates the transcellular transport and metabolism of cationic amino acids by vascular SMC. Treatment of SMC with lyso-PC initially (0-2 h) decreased cationic amino acid uptake, whereas longer exposures (6-24 h) progressively increased transport. Kinetic studies indicated that lyso-PC-induced inhibition was associated with a decrease in affinity for cationic amino acids, but the stimulation was mediated by an increase in transport capacity. Lyso-PC strongly induced the expression of cationic amino acid transporter-2 mRNA while modestly elevating the level of cationic amino acid transporter-1 mRNA. In addition, lyso-PC stimulated intracellular cationic amino acid metabolism by inducing ornithine decarboxylase activity and mRNA expression and also by inducing arginase activity in vascular SMC. In contrast, lyso-PC inhibited the catabolism of L-arginine to nitric oxide by blocking inducible nitric oxide synthase expression. Lyso-PC increased markedly the capacity of SMC to generate putrescine, a polyamine, from extracellular L-ornithine and L-arginine. The lyso-PC-mediated increase in the production of putrescine was reversed by NG-methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by alpha-difluoromethylornithine, an ornithine decarboxylase inhibitor. The formation of putrescine from L-arginine was also prevented by arginase inhibitor NG-hydroxy-L-arginine. These results demonstrate that lyso-PC stimulates polyamine synthesis in vascular SMC by inducing the expression of the genes that regulate both the transport and metabolism of cationic amino acids. The actions of lyso-PC in stimulating cationic amino acid uptake and directing their metabolism to growth-stimulatory polyamines while simultaneously inhibiting the synthesis of antiproliferative NO, may contribute to lyso-PC-induced SMC proliferation and atherosclerotic lesion formation.

A new member of the cationic amino acid transporter family is preferentially expressed in adult mouse brain

We have isolated and characterized a novel member (CAT3) of the cationic amino acid transporter (CAT) family. In oocyte injection assays, CAT3 cRNA exhibited a saturable, sodium ion-independent transport activity with high affinity for L-arginine and L-lysine (Km = 40-60 and 115-165 microM, respectively). Transport of L-arginine was effectively competed only by cationic amino acids in L-form: arginine, lysine, ornithine, and 2,4-diamino-n-butyric acid but not by 2,3-diaminopropionic acid. The presence of L-arginine in the incubation medium stimulated the efflux rate of L-arginine, indicating that CAT3 is subject to trans-stimulation. All these results are consistent with the idea that CAT3, along with CAT1 and CAT2, constitutes the transport activity originally assigned to system y+. Like CAT2, but unlike CAT1, the expression of CAT3 is regulated in a highly tissue-specific manner; when various adult tissues were examined, significant levels of CAT3 transcript were detectable only in brain. In situ hybridization on brain sections revealed that CAT3 transcripts were localized predominantly along the midbrain-thalamus-hypothalamus axis, whereas neither CAT1 nor CAT2 transcripts demonstrated a similar localization. In contrast to its highly localized expression during the primitive streak stage and in the adult stage, CAT3 expression was detected more widely in 13.5 day post-coitum mouse embryos.

Cloning and characterization of a brain-specific cationic amino acid transporter

rCAT3 (rat cationic amino acid transporter 3), a cDNA that encodes a novel member of the murine CAT family was isolated. The protein encoded by rCAT3 contained 619 amino acids, 53-58% of which were identical with those of the murine CAT family proteins previously described (mouse CAT1, CAT2a, CAT2b, and rat CAT1). Transient expression of rCAT3 and L-[14C]arginine incorporation experiments in COS7 cells verified a high affinity system y+ transporter activity of rCAT3. First, rCAT3-mediated L-[14C]arginine incorporation was time-dependent and saturable with half-saturation constant (Km) values of 103 +/- 12 microM (mean +/- S.E., n = 3). Second, the incorporation was specific for cationic amino acids as evidenced from the inhibition by L-arginine, L-lysine, and L-ornithine. Third, neither sodium nor chloride ions in the extracellular medium were required for the activity. Fourth, the incorporation was inhibited by high potassium-induced membrane depolarization. On Northern blot using RNAs from various rat tissues, the expression of rCAT3 mRNA was restricted to the brain. These results indicated a role of rCAT3 in the system y+ transporter activity in the nervous tissue.

Molecular sites of regulation of expression of the rat cationic amino acid transporter gene

Cat-1 is a protein with a dual function, a high affinity, low capacity cationic amino acid transporter of the y+ system and the receptor for the ecotropic retrovirus. We have suggested that Cat-1 is required in the regenerating liver for the transport of cationic amino acids and polyamines in the late G1 phase, a process that is essential for liver cells to enter mitosis. In our earlier studies we had shown that the cat-1 gene is silent in the quiescent liver but is induced in response to hormones, insulin, and glucocorticoids, and partial hepatectomy. Here we demonstrate that cat-1 is a classic delayed early growth response gene in the regenerating liver, since induction of its expression is sensitive to cycloheximide, indicating that protein synthesis is required. The peak of accumulation of the cat-1 mRNA (9-fold) by 3 h was not associated with increased transcriptional activity of the cat-1 gene in the regenerating liver, indicating post-transcriptional regulation of expression of this gene. Induction of the cat-1 gene results in the accumulation of two mRNA species (7.9 and 3.4 kilobase pairs (kb)). Both mRNAs hybridize with the previously described rat cat-1/2.9-kb cDNA clone. However, the 3' end of a longer rat cat-1 cDNA (rat cat-1/6.5-kb) hybridizes only to the 7.9-kb mRNA transcript. Sequence analysis of this clone indicated that the two mRNA species result from the use of alternative polyadenylation signals. The 6. 5-kb clone contains a number of AT-rich mRNA destabilizing sequences which is reflected in the half-life of the cat-1 mRNAs (90 min for 7. 9-kb mRNA and 250 min for 3.4-kb mRNA). Treatment of rats with cycloheximide superinduces the level of the 7.9-kb cat-1 mRNA in the kidney, spleen, and brain, but not in the liver, suggesting that cell type-specific labile factors are involved in its regulation. We conclude that the need for protein synthesis for induction of the cat-1 mRNA, the short lived nature of the mRNAs, and the multiple sites for regulation of gene expression indicate a tight control of expression of the cat-1 gene within the regenerating liver and suggest that y+ cationic amino acid transport in liver cells is regulated at the molecular level.

Platelet-derived growth factor regulates vascular smooth muscle cell proliferation by inducing cationic amino acid transporter gene expression

Since recent studies demonstrated that platelet-derived growth factor (PDGF) induces vascular smooth muscle cell (SMC) proliferation by stimulating polyamine synthesis, we examined whether the transcellular transport of L-ornithine, the cationic amino acid precursor of polyamines, could regulate the mitogenic response of PDGF. Treatment of SMC with PDGF stimulated DNA and putrescine synthesis, and this was enhanced further by increasing the extracellular concentration of L-ornithine. The potentiating effect of L-ornithine was reversed by the competitive inhibitor of cationic amino acid transport, methyl-L-arginine, or by preventing putrescine formation with alpha-difluoromethylornithine. Cationic amino acid uptake by SMC was Na+-independent and was mediated by both a high and low affinity carrier system. Treatment of SMC with PDGF initially (0-2 h) decreased basic amino acid transport, while longer exposures (6-24 h) progressively increased uptake. Kinetic studies indicated that PDGF-induced inhibition was associated with a decrease in affinity for cationic amino acids, while the stimulation was mediated by an increase in transport capacity. Endogenous PDGF released by collagen-activated platelets likewise up-regulated cationic amino acid transport in SMC. Reverse transcriptase-polymerase chain reaction detected the presence of mRNA encoding two distinct cationic amino acid transporter (CAT) proteins, CAT-1 and CAT-2B. Treatment of SMC with PDGF strongly induced the expression CAT-2B mRNA and modestly elevated the level of CAT-1 mRNA. These results demonstrate that PDGF-induced polyamine synthesis and SMC mitogenesis are dependent on the transcellular transport of L-ornithine. The capacity of PDGF to up-regulate the transport of L-ornithine by inducing the expression of the genes for CAT-1 and CAT-2B may modulate its mitogenic effect by providing SMC with the necessary intracellular precursor for polyamine biosynthesis.

Interleukin-1 beta and tumor necrosis factor-alpha stimulate the cat-2 gene of the L-arginine transporter in cultured vascular smooth muscle cells

The production of nitric oxide (NO) from L-arginine by nitric oxide synthase (NOS) in cytokine-stimulated vascular smooth muscle cells (VSMC) is thought to play an important role in the pathophysiology of several vascular disease states including septic shock. This study examines the relationship between cytokine-stimulated NO production and L-arginine transport in cultured VSMC. Cultured VSMC from rat aorta were stimulated with interleukin-1 beta, tumor necrosis factor-alpha, and/or angiotensin II (Ang II); and the accumulation of nitrite, a stable product of NO metabolism, in the culture media and the rates of net L-arginine uptake were measured. Interleukin-1 beta and tumor necrosis factor-alpha, alone or in combination, stimulated both the uptake of L-arginine and the accumulation of nitrite in the culture media in a dose-dependent manner. Inhibition of NOS activity by substituted analogues of L-arginine had no effect on cytokine-stimulated L-arginine transport. Ang II in the presence of cytokines up-regulated L-arginine transport while inhibiting nitrite accumulation. Two forms of the L-arginine transporter, cat-1b and cat-2, are expressed in VSMC. Northern analysis revealed that the cytokine-stimulated increase in L-arginine transport coincided with increased levels of cat-2 mRNA. In contrast, cat-1b does not appear to be regulated by cytokines at the mRNA level, although significant increases in response to Ang II were observed. These results show that, while cytokines can stimulate both NOS activity and L-arginine uptake, NO production is not required to signal the increase in L-arginine transport. Furthermore, Ang II and cytokine stimulation of L-arginine uptake involves the differential regulation of the cationic amino acid transporter (cat) genes.