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

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.

Possible involvement of tetrahydrobiopterin in the disturbance of redox homeostasis in sepsis - Induced brain dysfunction

BACKGROUND AND AIM

Tetrahydrobiopterin (BH4) is an essential co-factor that regulates nitric oxide (NO) and reactive oxygen species (ROS) production by nitric oxide synthases (NOS). In this study, we evaluated the effects of sepsis on BH4 level and redox status in the brain by using the rat model of sepsis-induced by cecal ligation and puncture (CLP) and examined whether BH4 and/or acetyl-L-carnitine (ALC) could prevent the neuronal apoptosis and neurological changes induced by sepsis.

MATERIAL AND METHOD

Male albino rats were randomly and blindly divided into 8 groups: sham, sham + BH4, sham + ALC, sham +BH4+ ALC, CLP, CLP + BH4, CLP + ALC, and CLP+BH4+ ALC. We measured neurological indicators, brain levels of BH4, guanosine triphosphate cyclohydrolase (GTPCH), sepiapterin reductase (SR) and dihydropteridine reductase (DHPR) genes expression (Essential enzymes in BH4 biosynthesis and recycling pathways). We investigated also brain redox status and both endothelial and inducible NOS expressions.

RESULTS

Brain of septic rats demonstrated a reduced BH4 bioavailability, downregulation of BH4 synthetic enzymes, increased production of hydrogen peroxide and impaired antioxidant enzymes activities. Treatments with BH4 and/or ALC increased BH4 level, upregulated BH4 synthetic enzymes expressions, and attenuated oxidative-induced neuronal apoptosis.

CONCLUSION

Our results suggest that BH4 and/or ALC might protect the brain against oxidative stress induced neuronal apoptosis by restoring bioavailability of BH4 and upregulating of BH4 synthetic enzymes in the brain during sepsis.

Dual effect of nickel on L-arginine/nitric oxide system in RAW 264.7 macrophages

The immunogenic mechanisms of the potent contact allergen nickel are not completely clear. Nitric oxide (NO) serves as a fundamental signalling and effector molecule in the immune system, but little is known about its possible role in immune reactions elicited by nickel. We investigated the effects of nickel on the L-arginine/inducible NO synthase (iNOS) system in a murine macrophage cell line, RAW 264.7. Both LPS-stimulated and non-stimulated RAW 264.7 cells were incubated in the presence of 0-100 μM nickel sulphate for 24 h. Subsequently, NO production, iNOS protein expression, L-arginine uptake and gene expression of iNOS and cationic amino acid transporter systems (CAT) were measured. We found that 100 μM NiSO4 increased LPS-induced nitrite production as well as the formation of [(3)H]-L-citrulline from [(3)H]-L-arginine in the RAW 264.7 cells. Correspondingly, the expression of iNOS gene and protein was also remarkably enhanced. Nevertheless, nickel had an inhibitory effect on L-arginine transport which disappeared gradually upon LPS-stimulation in parallel with an increase in NO output. LPS was found to significantly amplify CAT-3 as well as CAT-2 mRNA expression, mirroring the increase in L-arginine transport. In the range of 1-10 μM, NiSO4 did not have any additional effect on CAT mRNA expression, but at 100 μM it was able to enhance CAT-1 and CAT-3 mRNA expression upon LPS stimulation. Our data indicate that nickel interferes with macrophages' L-arginine/NOS system on multiple levels. Considering the potent biological effects of NO, these influences may contribute to nickel toxicity.

Dexamethasone suppresses eNOS and CAT-1 and induces oxidative stress in mouse resistance arterioles

Long-term treatment with glucocorticoids is associated with mild to moderate hypertension. We reported previously that downregulation of endothelial NO synthase (eNOS) expression and activity is likely to contribute to this increase in blood pressure. In the present study, we tested the effects of dexamethasone on the vasodilation of microvascular arterioles using implanted dorsal skin-fold chambers in anesthetized C57BL/6J mice. Experiments were performed on control mice or on mice treated with dexamethasone (0.1-3 mg/kg of body wt). Endothelium-dependent vasodilation in response to ACh (0.1-10 microM) was reduced by dexamethasone in a dose-dependent fashion. Comparable inhibition was seen in tissues superfused with 30 microM N(G)-nitro-L-arginine methyl ester. In contrast, endothelium-independent vasodilation in response to S-nitroso-N-acetyl-D,L-penicillamine (10 microM) was not influenced by either dexamethasone or N(G)-nitro-L-arginine methyl ester. Levels of eNOS mRNA in murine hearts and NO(2)(-)/NO(3)(-) in serum were suppressed by dexamethasone (down to 63 and 50% of control values, respectively, at 3 mg/kg of body wt) along with a reduction in eNOS protein to 85.6%. Dexamethasone also concentration dependently reduced the expression of the cationic amino acid transporter-1 in murine hearts and cultured endothelial cells. The suppression by dexamethasone of the ACh-induced vasodilation could be partially reversed by dietary L-arginine (50 mg/kg of body wt) and by dietary vitamin C (10 g/kg of diet). We conclude that suppression by dexamethasone of the endothelium-mediated microvascular vasodilation involves several mechanisms including 1) downregulation of eNOS, 2) downregulation of cationic amino acid transporter-1, and 3) generation of reactive oxygen species. The demonstration that L-arginine and vitamin C can partially offset the effects of dexamethasone on microvascular arterioles suggests the potential clinical usefulness of these agents for the reduction of glucocorticoid-induced hypertension.

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.

Role of tetrahydrobiopterin in adrenocorticotropic hormone-induced hypertension in the rat

Adrenocorticotropic hormone (ACTH)-induced hypertension in the rat is characterized by nitric oxide deficiency. Tetrahydrobiopterin (BH4) is an essential cofactor for the enzyme nitric oxide synthase and glucocorticoids have been reported to reduce cytokine-induced BH4 production. Accordingly we hypothesized that ACTH-induced hypertension would be reversed by BH4 supplementation. Male Sprague-Dawley rats (n = 33) were treated with BH4 in vehicle (10 mg/kg/day i.p.) or vehicle alone (5 mg/kg/day i.p. of ascorbic acid in 4 mM HCl) for 10 days. ACTH (0.2 mg/kg s.c.) or saline daily injection was started 2 days after BH4 or vehicle treatment and continued for 8 days. Systolic blood pressure (SBP) was measured on alternate days using the tail cuff method. Treatment with HCl, ascorbic acid or BH4 alone had no effect on SBP. In saline treated rats, neither BH4 nor its vehicle modified SBP. In ACTH treated rats, SBP was increased in both BH4 (from 128 +/- 6 to 142 +/- 4 mmHg, T0 to T10, P < 0.0005, one way ANOVA) and vehicle groups (from 127 +/- 3 to 158 +/- 7 mmHg, T0 to T10, P < 0.001, one way ANOVA). There was no significant difference in SBP between BH4 + ACTH treated and vehicle + ACTH treated rats. Thus, daily injection of BH4 (10 mg/kg i.p.) failed to prevent the development of ACTH-induced hypertension in rat.

Folic acid says NO to vascular diseases

OBJECTIVES

The possible link between folic acid or folate and tetrahydrobiopterin (H(4)B), vitamin C, polyunsaturated fatty acids (PUFAs), and nitric oxide (NO), which may explain the beneficial actions of these nutrients in various vascular conditions, was investigated.

METHODS

The literature pertaining to the actions of folic acid/folate, H(4)B, vitamin C, PUFAs, and NO was reviewed.

RESULTS

Impaired endothelial NO (eNO) activity is an early marker for cardiovascular disease. Most risk factors for atherosclerosis are associated with impaired endothelium-dependent vasodilatation due to reduced NO production. Folate not only reduces plasma homocysteine levels but also enhances eNO synthesis and shows anti-inflammatory actions. It stimulates endogenous H(4)B regeneration, a cofactor necessary for eNO synthesis, inhibits intracellular superoxide generation, and thus enhances the half-life of NO. H(4)B in turn enhances NO generation and augments arginine transport into the cells. Folic acid increases the concentration of omega-3 PUFAs, which also enhance eNO synthesis. Vitamin C augments eNO synthesis by increasing intracellular H(4)B and stabilization of H(4)B. Insulin stimulates H(4)B synthesis and PUFA metabolism, suppresses the production of proinflammatory cytokine tumor necrosis factor-alpha and superoxide anion, and enhances NO generation. The ability of folate to augment eNO generation is independent of its capacity to lower plasma homocysteine levels.

CONCLUSIONS

The common mechanism by which folic acid, H(4)B, vitamin C, omega-3 fatty acids, and L-arginine bring about their beneficial actions in various vascular diseases is by enhancing eNO production. Hence, it remains to be determined whether a judicious combination of folic acid, vitamins B12, B6, and C, H(4)B, L-arginine, and omega-3 fatty acids in appropriate amounts may form a novel approach in the prevention and management of various conditions such as hyperlipidemias, coronary heart disease, atherosclerosis, peripheral vascular disease, and some neurodegenerative conditions.

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.

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.

Expression, regulation and function of carrier proteins for cationic amino acids

Different carrier proteins exhibiting distinct transport properties participate in cationic amino acid transport. There are sodium-independent systems, such as b+, y+, y+L and b0,+, and a sodium-dependent system B0,+, most of which have now been identified at the molecular level. In most non-epithelial cells, members of the cationic amino acid transporter (CAT) family mediating system y+ activity seem to be the major entry pathway for cationic amino acids. CAT proteins underlie complex regulation at the transcriptional, post-transcriptional and activity levels. Recent evidence indicates that individual CAT isoforms are necessary for providing the substrate for nitric oxide synthesis, for example CAT-1 for Ca2+-independent nitric oxide production in endothelial cells and CAT-2B for sustained nitric oxide production in macrophages.