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

A Low Arginine/Ornithine Ratio is Associated with Long-Term Cardiovascular Mortality

AIMS

The long-term prognostic value of the bioavailability of L-arginine, an important source of nitric oxide for the maintenance of vascular endothelial function, has not been investigated fully. We therefore investigated the relationship between amino acid profile and long-term prognosis in patients with a history of standby coronary angiography.

METHODS

We measured the serum concentrations of L-arginine, L-citrulline, and L-ornithine by high-speed liquid chromatography. We examined the relationship between the L-arginine/L-ornithine ratio and the incidence of all-cause death, cardiovascular death, and major adverse cardiovascular events (MACEs) in 262 patients (202 men and 60 women, age 65±13 years) who underwent coronary angiography over a period of ≤ 10 years.

RESULTS

During the observation period of 5.5±3.2 years, 31 (12%) patients died, including 20 (8%) of cardiovascular death, while 32 (12%) had MACEs. Cox regression analysis revealed that L-arginine/L-ornithine ratio was associated with an increased risk for all-cause death (unadjusted hazard ratio, 95% confidence interval) (0.940, 0.888-0.995) and cardiovascular death (0.895, 0.821-0.965) (p＜0.05 for all). In a model adjusted for age, sex, hypertension, hyperlipidemia, diabetes, current smoking, renal function, and log10-transformed brain natriuretic peptide level, cardiovascular death (0.911, 0.839-0.990, p=0.028) retained an association with a low L-arginine/ L-ornithine ratio. When the patients were grouped according to an L-arginine/L-ornithine ratio of 1.16, the lower L-arginine/L-ornithine ratio group had significantly higher incidence of all-cause death, cardiovascular death, and MACEs.

CONCLUSION

A low L-arginine/L-ornithine ratio may be associated with increased 10-year cardiac mortality.

Profound Decrease in Glomerular Arginine Transport by CAT (Cationic Amino Acid Transporter)-1 Contributes to the FLT-1 (FMS-Like Tyrosine Kinase 1) Induced Preeclampsia in the Pregnant Mice

Endothelial dysfunction because of nitric oxide inactivation has been suggested to play a role in the pathogenesis of preeclampsia. During pregnancy, L-arginine transport by CAT-1 (cationic amino acid transporter 1), the only transporter for eNOS (endothelial nitric oxide synthase) is inhibited. We hypothesize that maternal arginine deficiency contributes to the development of preeclampsia. Adenovirus-mediated overexpression of sFlt-1 (soluble fms-like tyrosine kinase 1) in virgin and pregnant mice resulted in glomerular endotheliosis, hypertension, and albuminuria. L-arginine prevented the increase in blood pressure and albuminuria in Flt-1 pregnant but not in Flt-1 virgin mice. Flt-1 augmented arginine transport in pregnant but not in virgin dames. Ex vivo inhibition of CAT-2 leaving exclusively CAT-1 activity, decreased arginine transport velocities in Flt-1 animals more prominently in pregnant dames. Phosphorylated CAT-1/CAT-1 increased in pregnant, sFlt-1-pregnant, and sFlt-1 virgin mice. CAT-2 increased in Flt-1-pregnant and Flt-1-virgin dames. L-arginine augmented arginine transport in pregnant and Flt-pregnant mice and prevented the increase in pCAT-1 and CAT-2 expression. Glomerular cGMP (cyclic guanosine monophosphate) generation as a measure of eNOS activity was decreased in all Flt-1 treated animals. L-arginine abolished the decrease in cGMP levels only in Flt-1-pregnant mice. In conclusion, glomerular endothelial NO generation is compromised in Flt-1-pregnant mice because of CAT-1 inhibition induced by a combined effect of pregnancy and preeclampsia which involves: phosphorylation of CAT-1 and induction of CAT-2. These processes contribute to the clinical syndrome of preeclampsia in mice and are prevented by L-arginine.

Vascular Endothelial Growth Factor Augments Arginine Transport and Nitric Oxide Generation via a KDR Receptor Signaling Pathway

BACKGROUND/AIMS

Vascular endothelial growth factor (VEGF) is an endothelium-specific peptide that stimulates angiogenesis via two receptor tyrosine kinases, Flt-1 and KDR. Endothelial nitric oxide synthase (eNOS) plays a major role in VEGF signaling. Delivery of arginine to membrane bound eNOS by the cationic amino acid transporter-1 (CAT-1) has been shown to modulate eNOS activity. The current studies were designed to test the hypothesis that VEGF enhances eNOS activity via modulation of arginine transport by CAT-1.

METHODS

Using radio-labeled arginine, {[3H] L-arginine} uptake was determined in human umbilical vein endothelial cells (HUVEC) following incubation with VEGF with and without silencing the VEGF receptors Flt-1 or KDR. Subsequently, western blotting for CAT-1, PKCα, ERK 1/2, JNK, and their phosphorylated forms were performed. NO generation was measured by the Griess reaction.

RESULTS

VEGF (50 and 100 ng/ml) significantly augmented endothelial arginine transport in a time dependent manner, an effect which was prevented by Sunitinib (2 µM), a multi targeted receptor tyrosine kinase inhibitor. The increase in arginine transport velocities by VEGF was not affected by silencing Flt-1 while silencing KDR abrogated VEGF effect. Furthermore, incubating cells with 50 and 100 ng of VEGF for 30 minutes significantly augmented CAT-1 abundance. The expression of PKC-α, JNK, and ERK1/2 and their phosphorylated forms were unchanged following incubation of HUVEC with VEGF. The concentration of NO2/NO3 following incubation with VEGF was significantly higher than from untreated cells. This increase was significantly attenuated by silencing KDR.

CONCLUSIONS

VEGF increases arginine transport via modulation of CAT-1 in endothelial cells. This effect is exclusively dependent on KDR rather than Flt-1.

Expression Profile of Cationic Amino Acid Transporters in Rats with Endotoxin-Induced Uveitis

Purpose. The transcellular arginine transportation via cationic amino acid transporter (CAT) is the rate-limiting step in nitric oxide (NO) synthesis, which is crucial in intraocular inflammation. In this study, CAT isoforms and inducible nitric oxide synthase (iNOS) expression was investigated in endotoxin-induced uveitis (EIU). Methods. EIU was induced in Lewis rats by lipopolysaccharide (LPS) injection. In the treatment group, the rats were injected intraperitoneally with the proteasome inhibitor bortezomib before EIU induction. After 24 hours, leukocyte quantification, NO measurement of the aqueous humor, and histopathological examination were evaluated. The expression of CAT isoforms and iNOS was determined by reverse transcription-polymerase chain reaction, western blotting, and immunofluorescence staining. Nuclear factor-kappa B (NF-κB) binding activity was evaluated by electrophoretic mobility shift assay. The mouse macrophage cell line RAW 264.7 was used to validate the in vivo findings. Results. LPS significantly stimulated iNOS, CAT-2A, and CAT-2B mRNA and protein expression but did not affect CAT-1 in EIU rats and RAW 264.7 cells. Bortezomib attenuated inflammation and inhibited iNOS, CAT-2A, and CAT-2B expression through NF-κB inhibition. Conclusions. CAT-2 and iNOS, but not CAT-1, are specifically involved in EIU. NF-κB is essential in the induction of CAT-2 and iNOS in EIU.

Estradiol augments while progesterone inhibits arginine transport in human endothelial cells through modulation of cationic amino acid transporter-1

Decreased generation of nitric oxide (NO) by endothelial NO synthase (eNOS) characterizes endothelial dysfunction (ECD). Delivery of arginine to eNOS by cationic amino acid transporter-1 (CAT-1) was shown to modulate eNOS activity. We found in female rats, but not in males, that CAT-1 activity is preserved with age and in chronic renal failure, two experimental models of ECD. In contrast, during pregnancy CAT-1 is inhibited. We hypothesize that female sex hormones regulate arginine transport. Arginine uptake in human umbilical vein endothelial cells (HUVEC) was determined following incubation with either 17β-estradiol (E2) or progesterone. Exposure to E2 (50 and 100 nM) for 30 min resulted in a significant increase in arginine transport and reduction in phosphorylated CAT-1 (the inactive form) protein content. This was coupled with a decrease in phosphorylated MAPK/extracellular signal-regulated kinase (ERK) 1/2. Progesterone (1 and 100 pM for 30 min) attenuated arginine uptake and increased phosphorylated CAT-1, phosphorylated protein kinase Cα (PKCα), and phosphorylated ERK1/2 protein content. GO-6976 (PKCα inhibitor) prevented the progesterone-induced decrease in arginine transport. Coincubation with both progesterone and estrogen for 30 min resulted in attenuated arginine transport. While estradiol increases arginine transport and CAT-1 activity through modulation of constitutive signaling transduction pathways involving ERK, progesterone inhibits arginine transport and CAT-1 via both PKCα and ERK1/2 phosphorylation, an effect that predominates over estradiol.

Arginine and citrulline and the immune response in sepsis

Arginine, a semi-essential amino acid is an important initiator of the immune response. Arginine serves as a precursor in several metabolic pathways in different organs. In the immune response, arginine metabolism and availability is determined by the nitric oxide synthases and the arginase enzymes, which convert arginine into nitric oxide (NO) and ornithine, respectively. Limitations in arginine availability during inflammatory conditions regulate macrophages and T-lymfocyte activation. Furthermore, over the past years more evidence has been gathered which showed that arginine and citrulline deficiencies may underlie the detrimental outcome of inflammatory conditions, such as sepsis and endotoxemia. Not only does the immune response contribute to the arginine deficiency, also the impaired arginine de novo synthesis in the kidney has a key role in the eventual observed arginine deficiency. The complex interplay between the immune response and the arginine-NO metabolism is further underscored by recent data of our group. In this review we give an overview of physiological arginine and citrulline metabolism and we address the experimental and clinical studies in which the arginine-citrulline NO pathway plays an essential role in the immune response, as initiator and therapeutic target.

Frog urinary bladder epithelial cells express TLR4 and respond to bacterial LPS by increase of iNOS expression andl-arginine uptake

As in mammals, epithelium of the amphibian urinary bladder forms a barrier to pathogen entry and is a first line of defense against penetrating microorganisms. We investigated the effect of Escherichia coli LPS on generation of nitric oxide (NO), a critically important mediator during infectious processes, by primary cultured frog ( Rana temporaria) urinary bladder epithelial cells (FUBEC). It was found that FUBEC constitutively express Toll-like receptor 4 (TLR4), a receptor of LPS, and respond to LPS (10 μg/ml) by stimulation of inducible nitric oxide synthase (iNOS) mRNA/protein expression and NOS activity measured by nitrite produced in the culture medium and by citrulline assay. We characterized uptake of l-arginine, a precursor in NO synthesis, by FUBEC and showed that it is mediated mainly by the y+ cationic amino acid transport system. LPS stimulated l-arginine uptake, and this effect was blocked by the iNOS inhibitor 1400W. Arginase II was found to be expressed in FUBEC. Inhibition of arginase activity by (S)-(boronoethyl)-l-cysteine increased generation of NO, suggesting contribution of arginase to NO production via competing with NOS for the substrate. LPS altered neither total arginase activity nor arginase II expression. Among epithelial cells, phagocytic macrophage-like cells were observed, but they did not contribute to LPS-induced NO production. These data demonstrate that amphibian urinary bladder epithelial cells recognize LPS and respond to it by increased generation of NO via stimulation of iNOS expression and l-arginine uptake, which appears to be essential for the regulation of the innate immune response and the inflammation in bladder epithelium.

Arginine de novo and nitric oxide production in disease states

Arginine is derived from dietary protein intake, body protein breakdown, or endogenous de novo arginine production. The latter may be linked to the availability of citrulline, which is the immediate precursor of arginine and limiting factor for de novo arginine production. Arginine metabolism is highly compartmentalized due to the expression of the enzymes involved in arginine metabolism in various organs. A small fraction of arginine enters the NO synthase (NOS) pathway. Tetrahydrobiopterin (BH4) is an essential and rate-limiting cofactor for the production of NO. Depletion of BH4 in oxidative-stressed endothelial cells can result in so-called NOS3 "uncoupling," resulting in production of superoxide instead of NO. Moreover, distribution of arginine between intracellular transporters and arginine-converting enzymes, as well as between the arginine-converting and arginine-synthesizing enzymes, determines the metabolic fate of arginine. Alternatively, NO can be derived from conversion of nitrite. Reduced arginine availability stemming from reduced de novo production and elevated arginase activity have been reported in various conditions of acute and chronic stress, which are often characterized by increased NOS2 and reduced NOS3 activity. Cardiovascular and pulmonary disorders such as atherosclerosis, diabetes, hypercholesterolemia, ischemic heart disease, and hypertension are characterized by NOS3 uncoupling. Therapeutic applications to influence (de novo) arginine and NO metabolism aim at increasing substrate availability or at influencing the metabolic fate of specific pathways related to NO bioavailability and prevention of NOS3 uncoupling. These include supplementation of arginine or citrulline, provision of NO donors including inhaled NO and nitrite (sources), NOS3 modulating agents, or the targeting of endogenous NOS inhibitors like asymmetric dimethylarginine.

Attenuated glomerular arginine transport prevents hyperfiltration and induces HIF-1α in the pregnant uremic rat

Pregnancy worsens renal function in females with chronic renal failure (CRF) through an unknown mechanism. Reduced nitric oxide (NO) generation induces renal injury. Arginine transport by cationic amino acid transporter-1 (CAT-1), which governs endothelial NO generation, is reduced in both renal failure and pregnancy. We hypothesize that attenuated maternal glomerular arginine transport promotes renal damage in CRF pregnant rats. In uremic rats, pregnancy induced a significant decrease in glomerular arginine transport and cGMP generation (a measure of NO production) compared with CRF or pregnancy alone and these effects were prevented by l-arginine. While CAT-1 abundance was unchanged in all experimental groups, protein kinase C (PKC)-α, phosphorylated PKC-α (CAT-1 inhibitor), and phosphorylated CAT-1 were significantly augmented in CRF, pregnant, and pregnant CRF animals; phenomena that were prevented by coadministrating l-arginine. α-Tocopherol (PKC inhibitor) significantly increased arginine transport in both pregnant and CRF pregnant rats, effects that were attenuated by ex vivo incubation of glomeruli with PMA (a PKC stimulant). Renal histology revealed no differences between all experimental groups. Inulin and p-aminohippurate clearances failed to augment and renal cortical expression of hypoxia inducible factor-1α (HIF-1α) significantly increased in CRF pregnant rat, findings that were prevented by arginine. These studies suggest that in CRF rats, pregnancy induces a profound decrease in glomerular arginine transport, through posttranslational regulation of CAT-1 by PKC-α, resulting in attenuated NO generation. These events provoke renal damage manifested by upregulation of renal HIF-1α and loss of the ability to increase glomerular filtration rate during gestation.

Nitric oxide suppression of cellular proliferation depends on cationic amino acid transporter activity in cytokine-stimulated pulmonary endothelial cells

Inducible nitric oxide (NO) synthase (iNOS) is a stress response protein upregulated in inflammatory conditions, and NO may suppress cellular proliferation. We hypothesized that preventing L-arginine (L-arg) uptake in endothelial cells would prevent lipopolysaccharide/tumor necrosis factor-α (LPS/TNF)-induced, NO-mediated suppression of cellular proliferation. Bovine pulmonary arterial endothelial cells (bPAEC) were treated with LPS/TNF or vehicle (control), and either 10 mM L-leucine [L-leu; a competitive inhibitor of L-arg uptake by the cationic amino acid transporter (CAT)] or its vehicle. In parallel experiments, iNOS or arginase II were overexpressed in bPAEC using an adenoviral vector (AdiNOS or AdArgII, respectively). LPS/TNF treatment increased the expression of iNOS, arginase II, CAT-1, and CAT-2 mRNA in bPAEC, resulting in greater NO and urea production than in control bPAEC, which was prevented by L-leu. LPS/TNF treatment resulted in fewer viable cells than in controls, and LPS/TNF-stimulated bPAEC treated with L-leu had more viable cells than LPS/TNF treatment alone. LPS/TNF treatment resulted in cleaved caspase-3 and cleaved poly(ADP-ribose) polymerase expression, which was attenuated by L-leu. AdiNOS reduced viable cell number, and treatment of AdiNOS transfected bPAEC with L-leu preserved cell number. AdArgII increased viable cell number, and treatment of AdArgII transfected bPAEC with L-leu prevented the increase in cell number. These data demonstrate that iNOS expression in pulmonary endothelial cells leads to decreased cellular proliferation, which can be attenuated by preventing cellular L-arg uptake. We speculate that CAT activity may represent a novel therapeutic target in inflammatory lung diseases characterized by NO overproduction.

Inducible nitric oxide synthase inhibitor SD-3651 reduces proteinuria in MRL/lpr mice deficient in the NOS2 gene

Several studies have demonstrated the effectiveness of arginine analog nitric oxide synthase (NOS) inhibitor therapy in preventing and treating murine lupus nephritis. However, MRL/MpJ-FAS (MRL/lpr) mice lacking a functional NOS2 (inducible NOS [iNOS]) gene (NOS2) develop proliferative glomerulonephritis in a fashion similar to their wild-type (wt) littermates. This finding suggests that the effect of arginine analog NOS inhibitors is through a non-iNOS-mediated mechanism. This study was designed to address this hypothesis.NOS2 mice were given either vehicle or a NOS inhibitor (SD-3651) to determine if pharmacological NOS inhibition prevented glomerulonephritis, using wt mice as positive controls. Urine was collected fortnightly to measure albumin. At the time of full disease expression in wt mice, all mice were killed, and renal tissue was examined for light, immunofluorescence, and electron microscopic evidence of disease. Serum was analyzed for anti-double-stranded DNA antibody production.NOS2 mice had higher serum anti-double-stranded DNA antibody antibody levels than those of wt mice. SD-3651 therapy reduced proteinuria, glomerular immunoglobulin G deposition, and electron microscopic evidence of podocytopathy and endothelial cell swelling without affecting proliferative lesions by light microscopy.These studies confirm that genetic iNOS deficiency alone is insufficient to prevent proliferative glomerulonephritis and suggest that iNOS activity may inhibit autoantibody production. These results also suggest that SD-3651 therapy acts via a non-iNOS-mediated mechanism to prevent endothelial cell and podocyte pathology. Studies that elucidate this mechanism could provide a useful drug target for the treatment of nephritis.

Exogenous arginine in sepsis

Sepsis is a severe condition in critically ill patients and is considered an arginine deficiency state. The rationale for arginine deficiency in sepsis is mainly based on the reduced arginine levels in sepsis that are associated with the specific changes in arginine metabolism related to endothelial dysfunction, severe catabolism, and worse outcome. Exogenous arginine supplementation in sepsis shows controversial results with only limited data in humans and variable results in animal models of sepsis. Since in these studies the severity of sepsis varies but also the route, timing, and dose of arginine, it is difficult to draw a definitive conclusion for sepsis in general without considering the influence of these factors. Enhanced nitric oxide production in sepsis is related to suggested detrimental effects on hemodynamic instability and enhanced oxidative stress. Potential mechanisms for beneficial effects of exogenous arginine in sepsis include enhanced (protein) metabolism, improved microcirculation and organ function, effects on immune function and antibacterial effects, improved gut function, and an antioxidant role of arginine. We recently performed a study indicating that arginine can be given to septic patients without major effects on hemodynamics, suggesting that more studies can be conducted on the effects of arginine supplementation in septic patients.

HO-1 overexpression attenuates endotoxin effects on CAT-2 isozymes expression

BACKGROUND

l-arginine transport mediated by type-2 cationic amino acid transporter (CAT-2) isozymes is one crucial mechanism that regulates nitric oxide (NO) production via inducible nitric oxide synthase (iNOS). We sought to investigate the effects of heme oxygenase-1 (HO-1) overexpression on CAT-2 isozymes, e.g., CAT-2, CAT-2A, and CAT-2B.

MATERIALS AND METHODS

Adult male Sprague Dawley rats were allocated to receive lipopolysaccharide (LPS), normal saline, hemin (a HO-1 inducer), tin protoporphyrin (SnPP, a HO-1 inhibitor), LPS plus hemin, or LPS plus hemin plus SnPP. After maintaining for 6 h, rats were sacrificed and the expression and activity of individual enzyme was evaluated.

RESULTS

LPS increased HO activity, HO-1 concentration, NO production, l-arginine transport, and concentrations of iNOS, CAT-2, and CAT-2B in rat lungs and kidney. LPS also increased HO activity, HO-1 concentration, NO production, l-arginine transport, and iNOS concentration but decreased CAT-2 and CAT-2B concentrations in rat liver. LPS increased CAT-2A concentration in rat liver but did not affect CAT-2A concentration in rat lungs and kidney. Hemin further increased HO activity and induced HO-1 overexpression in the lungs, kidney, and liver from LPS-treated rats. In addition, the effects of LPS on NO production, l-arginine transport, and concentrations of iNOS and CAT-2 isozymes were significantly attenuated by hemin. SnPP, on the other hand, reversed the effects of hemin.

CONCLUSIONS

HO-1 overexpression significantly attenuates endotoxin-induced increases in NO production and l-arginine transport. Induction of HO-1 overexpression also significantly attenuates the effects of endotoxin on the expression of iNOS and CAT-2 isozymes in septic rats.

Biomarkers of arginine and lysine excess

Arginine supplementation is used in several disease states. In arginine-deficient states, supplementation is a logical choice of therapy. However, the definition of an arginine-deficient state is complex. For example, plasma arginine levels could be within normal range but intracellular arginine levels could be reduced because of membrane transport problems. Lysine competes with arginine for transport into the cell. In these situations, arginine supplementation of higher than required levels is proposed. Arginine has several important functions in metabolism as it is a precursor of metabolically active components such as nitric oxide (NO), ornithine, creatine, and polyamines. Supplementing arginine in excess could potentially overstimulate metabolism via enhanced production of NO. NO is a reactive component that, via production of radicals, will inactivate proteins. NO is also a powerful vasodilator, which could lead to severe hemodynamic instability. A good marker for excess supplementation of arginine or lysine could be an increased or reduced production rate of NO. However, NO production is difficult to measure because NO is a very labile component and is rapidly oxidized in blood. Stable isotope-labeled arginine and citrulline are used to trace the arginine-NO route. During supplementation of arginine in septic pigs or patients in septic shock, NO production, measured with stable isotope technology, is enhanced.

L-Arginine supplementation in sepsis: beneficial or harmful?

PURPOSE OF REVIEW

Recent results are reviewed on the effects of L-arginine supplements in excess of standard nutritional practices during severe sepsis and septic shock.

RECENT FINDINGS

Septic shock has been alternatively viewed as an L-arginine-deficient state or as a syndrome caused by excess nitric oxide, a vasoactive product of L-arginine metabolism. L-Arginine has many physiologic and pharmacologic effects that indicate its potential to affect survival in septic patients. Animal studies have documented immunologic effects of L-arginine and of commercial 'immune-enhancing' diets. However, survival studies in small animals have not consistently favored L-arginine. L-Arginine monotherapy in a canine model of septic shock found significant harm at infusion rates of less than twice that administered in standard formulations of total parenteral nutrition. Meanwhile, clinical studies have suffered from lack of statistical power, patient heterogeneity, randomization failures, and use of complex nutritional formulas. Meta-analyses have noted heterogeneity between the effects of immune-enhancing diets in surgical versus medical patients and mixed critically ill populations that include subjects with sepsis, indicating that these results may not be reliably pooled.

SUMMARY

To date, published evidence has not established the safety and efficacy of L-arginine at doses above standard dietary practices in severe sepsis or septic shock.

Arginine uptake is attenuated, through post-translational regulation of cationic amino acid transporter-1, in hyperlipidemic rats

Endothelial cell dysfunction (ECD) is a common feature of hypercholesterolemia. Defective nitric oxide (NO) generation due to decreased endothelial nitric oxide synthase (eNOS) activity is a crucial parameter characterizing ECD. L-arginine is the sole precursor for NO biosynthesis. Among several transporters that mediate L-arginine uptake, cationic amino acid transporter-1 (CAT-1) acts as a specific arginine transporter for eNOS. Our hypothesis implies that CAT-1 is a major determinant of eNOS activity in hypercholesterolemia. We studied aortic arginine uptake, CAT-1 and CAT-2 mRNA expression, and CAT-1, and PKC alpha protein in: (a) control, untreated animals (CTL), (b) rats fed with 4% cholesterol+1% cholate and 2% corn oil for 6 weeks (CHOL) and (c) rats with hypercholesterolemia treated orally with either atorvastatin (CHOL+ATORVA, 20mg/kg BW/day) or arginine 1% (CHOL+ARG) in the drinking water (modalities which have been shown to enhance CAT-1 activity and improve endothelial function). Serum cholesterol levels significantly increased in cholesterol fed animals, an increase which was blocked by atorvastatin (CTL: 66.8+/-15, CHOL: 133.9+/-22, CHOL+ARG: 128.2+/-20, CHOL+ATORVA: 77+/-15 mg/dl). Arginine transport was significantly decreased in CHOL. Treatment with neither arginine nor atorvastatin had an effect. Using RT-PCR, we found no change in aortic CAT-1 and CAT-2 mRNA expression in CHOL as well as following arginine or atorvastatin administration. The abundance of CAT-1 protein was significantly augmented in cholesterol fed rats and was not affected by arginine or atorvastatin. PKC alpha protein content, which was previously shown to regulate CAT-1 activity, increased significantly in CHOL and was neither affected by atorvastatin nor arginine. In conclusion, aortic arginine uptake is attenuated in hypercholesterolemia, through post-translational modulation of CAT-1 protein, possibly via upregulation of PKC alpha.

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.

Lidocaine inhibition of inducible nitric oxide synthase and cationic amino acid transporter-2 transcription in activated murine macrophages may involve voltage-sensitive Na+ channel

Lidocaine has been reported to inhibit nitric oxide (NO) production in activated murine macrophages, but the role of inducible NO synthase (iNOS) in lidocaine-induced inhibition of NO has not been explored. In addition, type-2 cationic amino acid transporter (CAT-2) and guanosine triphosphate cyclohydrolase I (GTPCH) also regulate iNOS activity. The effects of lidocaine on CAT-2 and GTPCH are unknown. To explore further these effects, confluent immortalized murine macrophages (RAW264.7 cells) were incubated with lipopolysaccharide (LPS) or in combination with lidocaine (5, 50, or 500 microM) for 18 h before harvesting. We also used tetrodotoxin (TTX) and veratridine to elucidate the possible role of voltage-sensitive Na+ channel. Our data demonstrated that LPS significantly upregulated transcription of iNOS and CAT-2 but not GTPCH in stimulated macrophages. In a dose-dependent manner, lidocaine significantly attenuated the LPS-induced upregulation of iNOS and CAT-2. Conversely, lidocaine significantly increased GTPCH transcription in LPS-stimulated macrophages. The effects of TTX on iNOS, CAT-2, and GTPCH expression were comparable to those of lidocaine. In addition, veratridine significantly attenuated the effects of lidocaine and TTX. We therefore concluded that lidocaine significantly inhibits iNOS and CAT-2 and, in turn, enhances GTPCH transcription in LPS-stimulated macrophages via a mechanism that possibly involves the voltage-sensitive Na+ channel.

Platonin attenuates LPS-induced CAT-2 and CAT-2B induction in stimulated murine macrophages

BACKGROUND

Platonin, a cyanine photosensitizing dye, is a potent immunomodulator that suppresses acute inflammation. Platonin not only inhibits interleukin (IL)-1beta, IL-6, and tumor necrosis factor (TNF)-alpha production but also improves circulatory failure in septic rats. In addition, platonin reduces plasma nitric oxide (NO) formation during sepsis. However, the effects of platonin on inducible NO synthase (iNOS) and cationic amino-acid transporter (including CAT-2, CAT-2 A, and CAT-2B) expressions during sepsis remain uninvestigated.

METHODS

Five groups of confluent murine macrophages (RAW264.7 cells) were randomly allocated to receive a 1-h pretreatment of one of five doses of platonin (0.1 microM, 1 microM, 10 microM, 100 microM, or 1000 microM) followed by lipopolysaccharide (LPS; 100 ng ml(-1)). For negative, positive, and platonin control, three other groups of cell cultures were randomly allocated to receive phosphate-buffered saline, LPS, or platonin (1000 microM). The cultures were harvested after exposing them to LPS for 18 h or a comparable duration in those groups without LPS. NO production, L-arginine transport, and expression of the relevant enzymes were then evaluated.

RESULTS

Platonin significantly attenuated LPS-induced up-regulation of iNOS expression and NO production in stimulated murine macrophages in a dose-dependent manner. Platonin also significantly inhibited up-regulation of CAT-2 and CAT-2B expression as well as L-arginine transport in LPS-stimulated murine macrophages in a dose-dependent manner. In contrast, CAT-2 A expression in murine macrophages was not affected by LPS and/or platonin.

CONCLUSIONS

Platonin attenuates NO production and L-arginine transport in LPS-stimulated murine macrophages possibly through inhibiting iNOS, CAT-2, and CAT-2B expression.

Arginine uptake is attenuated through modulation of cationic amino-acid transporter-1, in uremic rats

Endothelial cell dysfunction (ECD) is a common feature of chronic renal failure (CRF). Defective nitric oxide (NO) generation due to decreased endothelial NO synthase (eNOS) activity is a crucial parameter characterizing ECD. L-arginine is the sole precursor for NO biosynthesis. Among several transporters that mediate L-arginine uptake, cationic amino-acid transporter-1 (CAT-1) acts as the specific arginine transporter for eNOS. Our hypothesis implies that CAT-1 is a major determinant of eNOS activity in CRF. We studied glomerular and aortic arginine uptake, CAT-1, and CAT-2 messenger ribonucleic acid (mRNA) expression, and CAT-1 protein in: (a) rats 6 weeks following 5/6 nephrectomy (CRF), (b) sham-operated animals, and (c) rats with CRF treated orally with either atorvastatin or arginine in drinking water (modalities which have been shown to enhance eNOS activity and improve endothelial function). Both glomerular and aortic arginine transport were significantly decreased in CRF. Treatment with either arginine or atorvastatin abolished the decrease in arginine uptake in CRF rats. Using reverse transcriptase-polymerase chain reaction and Northern blotting, we found a significant increase in glomerular and aortic CAT-1 mRNA expression in CRF. Western blotting revealed that CAT-1 protein was decreased in CRF, but remained intact following arginine and atorvastatin administration. Renal and systemic arginine uptake is attenuated in CRF, through modulation of CAT-1 protein. These findings provide a possible novel mechanism to eNOS inactivation and endothelial dysfunction in uremia.

Catecholamines' enhancement of inducible nitric oxide synthase-induced nitric oxide biosynthesis involves CAT-1 and CAT-2A

Catecholamines enhance inducible nitric oxide synthase (iNOS) expression that results in nitric oxide (NO) overproduction in lipopolysaccharide (LPS)-stimulated macrophages. L-arginine transport mediated by cationic amino acid transporters (including CAT-1, CAT-2, CAT-2A, and CAT-2B) is crucial in regulating iNOS activity. We sought to assess the effects of catecholamines on L-arginine transport and CAT isozyme expression in stimulated macrophages. Confluent RAW264.7 cells were cultured with LPS with or without catecholamines (epinephrine or norepinephrine, 5 x 10(-6) M) for 18 h. NO production, L-arginine transport, and enzyme expression were determined. Our data revealed that LPS co-induced iNOS, CAT-2, and CAT-2B expression, whereas CAT-1 and CAT-2A expression remained unaffected. Significant increases in NO production and L-arginine transport (approximately eight-fold and three-fold increases, respectively) were found in activated macrophages. Catecholamines significantly enhanced NO production and L-arginine transport (approximately 30% and 20% increases, respectively) in activated macrophages. Catecholamines also enhanced the expression of iNOS, CAT-1, and CAT-2A but not CAT-2 or CAT-2B in LPS-stimulated macrophages. Furthermore, the enhancement effects of catecholamines were inhibited by either dexamethasone or propranolol. We provide the first evidence to indicate that L-arginine transport in activated macrophages could be enhanced by catecholamines. Furthermore, this catecholamine-enhanced L-arginine transport might involve CAT-1 and CAT-2A but not CAT-2 or CAT-2B.

NF-kappaB inhibitors stabilize the mRNA of high-affinity type-2 cationic amino acid transporter in LPS-stimulated rat liver

BACKGROUND

Induction of inducible nitric oxide synthase (iNOS) results in nitric oxide (NO) overproduction during endotoxemia. Cellular uptake of L-arginine, modulated by the isozymes of type-2 cationic amino acid transporters (CAT), including CAT-2, CAT-2A and CAT-2B, has been reported to be a crucial factor in the regulation of iNOS activity. We sought to elucidate the expression of CAT-2 isozymes and the role of nuclear factor-kappaB (NF-kappaB) in this expression in lipopolysaccharide (LPS)-treated rat liver.

METHODS

Adult male Sprague-Dawley rats were randomly given intravenous (i.v.) injections of normal saline (N/S), LPS, LPS preceded by an NF-kappaB inhibitor (PDTC, dexamethasone or salicylate) or an NF-kappaB inhibitor alone. After injection, rats were sacrificed at different times and enzyme expression and liver injury were examined. Hepatic and systemic NO production were also measured.

RESULTS

CAT-2, CAT-2A and CAT-2B were constitutively expressed in un-stimulated rat liver. LPS stimulation not only significantly increased iNOS mRNA and NO concentrations but also decreased the mRNA concentrations of CAT-2 and CAT-2B, but not CAT-2A, in a time-dependent manner. LPS-induced hepatic and systemic NO overproduction was associated with hepatocellular injury. Pre-treatment with NF-kappaB inhibitors significantly attenuated LPS-induced iNOS induction as well as CAT-2/CAT-2B mRNA destabilization, which was associated with significant inhibition of NO biosynthesis and less liver injury.

CONCLUSION

NF-kappaB inhibitors stabilize CAT-2 and CAT-2B mRNA in LPS-stimulated rat liver. The hepatic CAT-2/CAT-2B pathway may be a constitutive part of cytoprotective mechanisms against sepsis.

Sepsis: an arginine deficiency state?

OBJECTIVE

Sepsis is a major health problem considering its significant morbidity and mortality rate. The amino acid L-arginine has recently received substantial attention in relation to human sepsis. However, knowledge of arginine metabolism during sepsis is limited. Therefore, we reviewed the current knowledge about arginine metabolism in sepsis.

DATA SOURCE

This review summarizes the literature on arginine metabolism both in general and in relation to sepsis. Moreover, arginine-related therapies are reviewed and discussed, which includes therapies of both nitric oxide (NO) and arginine administration and therapies directed toward inhibition of NO.

DATA

In sepsis, protein breakdown is increased, which is a key process to maintain arginine delivery, because both endogenous de novo production from citrulline and food intake are reduced. Arginine catabolism, on the other hand, is markedly increased by enhanced use of arginine in the arginase and NO pathways. As a result, lowered plasma arginine levels are usually found. Clinical symptoms of sepsis that are related to changes in arginine metabolism are mainly related to hemodynamic alterations and diminished microcirculation. NO administration and arginine supplementation as a monotherapy demonstrated beneficial effects, whereas nonselective NO synthase inhibition seemed not to be beneficial, and selective NO synthase 2 inhibition was not beneficial overall.

CONCLUSIONS

Because sepsis has all the characteristics of an arginine-deficiency state, we hypothesise that arginine supplementation is a logical option in the treatment of sepsis. This is supported by substantial experimental and clinical data on NO donors and NO inhibitors. However, further evidence is required to prove our hypothesis.

Pulmonary transcription of CAT-2 and CAT-2B but not CAT-1 and CAT-2A were upregulated in hemorrhagic shock rats

Hemorrhagic shock stimulates nitric oxide (NO) biosynthesis through upregulation of inducible NO synthase (iNOS) expression. Trans-membrane l-arginine transportation mediated by the isozymes of cationic amino acid transporters (e.g. CAT-1, CAT-2, CAT-2A, and CAT-2B) is one crucial regulatory mechanism that regulates iNOS activity. We sought to assess the effects of hemorrhage and resuscitation on the expression of these regulatory enzymes in hemorrhage-stimulated rat lungs. Twenty-four rats were randomized to a sham-instrumented group, a sustained shock group, a shock with blood resuscitation group, or a shock with normal saline resuscitation group. Hemorrhagic shock was induced by withdrawing blood to maintain MAP between 40 and 45mmHg for 60min. Resuscitation by infusing blood/saline mixtures (blood resuscitation group) or saline alone (saline resuscitation group) was then performed. At the end of the experiment (300min after hemorrhage began), rats were sacrificed and enzymes expression as well as pulmonary NO biosynthesis and lung injuries were assayed. Our data revealed that hemorrhage-induced pulmonary iNOS, CAT-2, and CAT-2B transcription which was associated with pulmonary NO overproduction and subsequent lung injury. Resuscitation significantly attenuated the hemorrhage-induced enzyme upregulation, pulmonary NO overproduction, and lung injury. Blood/saline mixtures were superior to saline as a resuscitation solution in treating hemorrhage-induced pulmonary NO overproduction and lung injury. Hemorrhage and/or resuscitation, however, did not affect the expression of pulmonary CAT-1 and CAT-2A. It is, therefore, concluded that the expression of pulmonary iNOS, CAT-2, and CAT-2B is inducible and that of CAT-1 and CAT-2A is constitutive in hemorrhagic shock rat lungs.

Augmented arginine uptake, through modulation of cationic amino acid transporter-1, increases GFR in diabetic rats

BACKGROUND

It is suggested that either arginine or its metabolites, nitric oxide and polyamines play a role in the renal hemodynamic alterations observed in the early stages of diabetes. Yet, the regulation of arginine transport in diabetic kidneys has never been studied.

METHODS

Arginine uptake was determined in glomeruli harvested from control rats; diabetic rats (2 weeks following an intraperitoneal injection of streptozotocin, 60 mg/kg body weight); rats, 4 days following left nephrectomy (a nondiabetic model of hyperfiltration); diabetes + lysine (0.5% in the drinking water to attenuate arginine uptake); and control + lysine.

RESULTS

Glomerular arginine transport was significantly increased in diabetic rats, but remained unchanged following uninephrectomy. Lysine abolished the increase in arginine uptake in diabetic rats but had no effect in controls. The increase in creatinine clearance observed in diabetes was completely abolished by lysine. Using reverse transcription-polymerase chain reaction (RT-PCR), Northern blotting, and immunohistochemistry, we found a significant increase in glomerular cationic amino acid transporter-1 (CAT-1) expression in diabetic animals, which was unaffected by lysine. When human endothelial cells were incubated with arginine end products no effect on arginine transport was observed. However, only in the presence of 0.5 mM/L sodium nitroprusside (SNP) an augmented steady-state CAT-1 mRNA was demonstrated by RT-PCR.

CONCLUSION

In a rat model of early diabetes, glomerular arginine uptake is elevated through modulation of CAT-1 expression, thus, contributing to the pathogenesis of hyperfiltration. Increased nitric oxide formation may play a role in this process.

NF-kappaB involvement in the induction of high affinity CAT-2 in lipopolysaccharide-stimulated rat lungs

BACKGROUND

Endotoxemia stimulates nitric oxide (NO) biosynthesis through induction of inducible NO synthase (iNOS). Cellular uptake of L-arginine, the sole substrate for iNOS, is an important mechanism regulating NO biosynthesis by iNOS. The isozymes of type-2 cationic amino acid transporters, including CAT-2, CAT-2A, and CAT-2B, constitute the most important pathways responsible for trans-membrane L-arginine transportation. Therefore, regulation of CAT-2 isozymes expression may constitute one of the downstream regulatory pathways that control iNOS activity. We investigated the time course of enzyme induction and the role of nuclear factor-kappaB (NF-kappaB) in CAT-2 isozymes expression in lipopolysaccharide-(LPS) treated rat lungs.

METHODS

Adult male Sprague-Dawley rats were randomly given intravenous injections of normal saline (N/S), LPS, LPS plus NF-kappaB inhibitor pre-treatment (PDTC, dexamethasone, or salicylate), or an NF-kappaB inhibitor alone. The rats were sacrificed at different times after injection and enzyme expression and lung injury were examined. Pulmonary and systemic NO production were also measured.

RESULTS

LPS co-induced iNOS, CAT-2, and CAT-2B but not CAT-2A expression in the lungs. Furthermore, NF-kappaB actively participated in LPS-induction of iNOS, CAT-2, and CAT-2B. LPS induced pulmonary and systemic NO overproduction and resulted in lung injuries. Attenuation of LPS-induced iNOS, CAT-2, and CAT-2B induction significantly inhibited NO biosynthesis and lessened lung injury.

CONCLUSION

NF-kappaB actively participates in the induction of CAT-2 and CAT-2B in intact animals. Our data further support the idea that CAT-2 and CAT-2B are crucial in regulating iNOS activity.

Renal transcription of high-affinity type-2 cationic amino acid transporter is up-regulated in LPS-stimulated rodents

OBJECTIVE

Sepsis stimulates renal nitric oxide (NO) biosynthesis through up-regulation of inducible NO synthase (iNOS) expression. Type-2 cationic amino acid transporter (CAT-2) mediation of trans-membrane L-arginine (L-Arg) transportation has been identified as one of the crucial regulatory mechanisms involved in the formation of NO by iNOS. We had previously shown that CAT-2B, a high-affinity alternative-spliced transcript of the CAT-2, is involved in induced NO biosynthesis by iNOS (Nitric Oxide, 2002). In this present study, we sought to assess the effects of sepsis on the expression of CAT-2B in lipopolysaccharide (LPS)-stimulated rat kidney.

METHODS

Forty rats were randomized to either a normal saline (N/S)-treated group or a LPS-treated group. Renal NO production was determined using chemiluminescence. Semi-quantitative RT-PCR was used to determine the mRNA concentrations of iNOS and L-Arg transporters (CAT-1, CAT-2 and CAT-2B) in kidney.

RESULTS

Lipopolysaccharide-coinduced iNOS, CAT-2 and CAT-2B mRNA expression in kidney and caused renal NO overproduction. A significant linear regression relationship was defined between renal NO concentrations and iNOS, CAT-2 and CAT-2B, respectively. On the contrary, CAT-1 expression was not affected by LPS-stimulation.

CONCLUSIONS

We provide the first evidence to illustrate that sepsis/septic shock induces the transcription of high-affinity CAT-2B in renal tissues. Transcription of iNOS, CAT-2 and CAT-2B correlates well with renal NO biosynthesis. Regulation of L-Arg uptake by modulating the expression regulation of induced CAT-2 and CAT-2B might be a potential target for therapies against renal pathologic conditions related to NO overproduction.

Normalization of nitric oxide flux improves physiological parameters of porcine kidneys maintained on pulsatile perfusion

Early endothelial damage and resultant reduction in the beneficial production of nitric oxide (NO) derived from the endothelial NO synthase (eNOS) are phenomena associated with the functional degradation of transplanted kidneys. In contrast, the inflammation characteristic of kidney preservation leads to the later, detrimental expression of the inducible NO synthase (iNOS). We reasoned that provision of low-level NO (to compensate for lack of eNOS) using the chemical NO donor S-nitrosoglutathione (GSNO), along with an iNOS inhibitor (N-omega iminoethyl-L-lysine; L-NIL), might "normalize" NO levels and therefore be beneficial in maintenance of flow. Non-heartbeating donor porcine kidneys were subjected to 30-45 min warm ischemic time and stored from 3 to 30 h, simulating the time required for national sharing. The kidneys were then machine preserved with Belzer MPS (BMP) at a set systolic pressure of 40 mmHg. Eight kidneys were perfused for 5h with BMP only (Group 1 control), 8 kidneys with BMP+GSNO only (Group 2), and 8 kidneys with BMP+GSNO+L-NIL (Group 3). Lower vascular resistance (VR) is a predictor of improved end-organ function. Both Group 2 and 3 kidneys demonstrated statistically significant reduction in VR as compared to Group 1 kidneys, with Group 3 kidneys demonstrating a greater drop in VR than Group 2. Reduced oxygen saturation suggests a higher metabolic rate. Only Group 3 had lower oxygen saturation as compared to Group 1. Increased Ca2+ concentration in the perfusate is a predictor of worse end-organ function. Group 2, but not Group 3, had a higher perfusate Ca2+ concentration than Group 1. The combination of suppression of harmful amounts of NO, while supplying a constant low-level amount of NO, may improve pulsatile kidney preservation.