Skip Navigation LinksHome > January 2013 - Volume 16 - Issue 1 > L-arginine, tetrahydrobiopterin, nitric oxide and diabetes
Current Opinion in Clinical Nutrition & Metabolic Care:
doi: 10.1097/MCO.0b013e32835ad1ef
PROTEIN, AMINO ACID METABOLISM AND THERAPY: Edited by Olav Rooyackers and John Brosnan

L-arginine, tetrahydrobiopterin, nitric oxide and diabetes

Hoang, Hai H.; Padgham, Samuel V.; Meininger, Cynthia J.

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Author Information

Department of Systems Biology and Translational Medicine, Texas A&M Health Science Center, Temple, Texas, USA

Correspondence to Cynthia J. Meininger, Department of Systems Biology and Translational Medicine, Texas A&M Health Science Center, 702 SW H.K. Dodgen Loop, Medical Research Building Room 138, Temple, TX 76504, USA. Tel: +1 254 742 7037; fax: +1 254 742 7145; e-mail: meininger@medicine.tamhsc.edu

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Abstract

Purpose of review: The endothelial isoform of nitric oxide synthase (eNOS) is constitutively expressed but dynamically regulated by a number of factors. Building our knowledge of this regulation is necessary to understand and modulate the bioavailability of nitric oxide, central to the cardiovascular complications of diabetes and other diseases. This review will focus on the eNOS substrate (L-arginine), its cofactor (tetrahydrobiopterin), and mechanisms related to the uncoupling of eNOS activity.

Recent findings: The global arginine bioavailability ratio has been proposed as a biomarker reflective of L-arginine availability, arginase activity, and citrulline cycling, as all of these processes impact eNOS activity. The failure of oral supplementation of tetrahydrobiopterin to recouple eNOS has emphasized the importance of the tetrahydrobiopterin to dihydrobiopterin ratio. Identification of transporters for biopterin species as well as signals that regulate endogenous arginine production have provided insight for alternative strategies to raise endothelial tetrahydrobiopterin levels while reducing dihydrobiopterin and alter eNOS activity. Finally, new information about redox regulation of eNOS itself may point to ways of controlling oxidative stress in the vasculature.

Summary: Restoring proper eNOS activity is key to ameliorating or preventing cardiovascular complications of diabetes. Continued investigation is needed to uncover new means for maintaining endothelial nitric oxide bioavailability.

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INTRODUCTION

L-arginine is a substrate for the family of nitric oxide (NO) synthase (NOS) enzymes that generate NO, a key chemical involved in normal endothelial function and, hence, cardiovascular health. Reduced NO bioavailability is central to the endothelial dysfunction that underlies vascular complications of diabetes [1]. The cofactor 6R-5,6,7,8-tetrahydrobiopterin (BH4) is crucial for proper functioning of all NOS isoforms, including the endothelial NOS (eNOS). When levels of BH4 are insufficient or the ratio of BH4 to its oxidized 7,8-dihydrobiopterin (BH2) form (BH4/BH2) falls, eNOS is said to be ‘uncoupled’ from the oxidation of L-arginine and superoxide (O2) is produced rather than NO [2]. This O2 can react with and rapidly inactivate any NO that is formed, further reducing NO bioavailability. L-arginine is also the substrate for the arginase enzyme, which converts arginine to ornithine and limits NO bioavailability in endothelial cells through increased arginine consumption [3]. Under physiological conditions, formation of reactive oxygen species (ROS) (such as O2) and their elimination are delicately balanced in the vascular wall. However, in disease states like diabetes, enhanced activity of pro-oxidant enzymes and/or reduced activity of antioxidant enzymes result in oxidative stress, leading to endothelial cell dysfunction.

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L-ARGININE

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There is widespread interest in L-arginine biochemistry because it is involved in multiple metabolic processes. However, much of the recent interest stems from the central role of NO in vascular homeostasis and the realization that L-arginine is the source of the nitrogen atom in the biosynthesis of NO.

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Arginine bioavailability/arginase activity

Individuals with diabetes face an increased risk for cardiovascular disease, and it is endothelial dysfunction brought about by reduced NO bioavailability that is associated with this increased risk [4]. Treatment of the multiple modifiable risk factors for complications in patients with type 2 diabetes (e.g., hyperglycemia, dyslipidemia, and hypertension) results in reduced macrovascular events [5]. Plasma arginine levels have been reported to be low in diabetic patients, likely due to increased arginase activity in these individuals [6]. Tripolt et al.[7▪] report data from a subgroup of the multifactorial treatment of Cardiovascular Risk in Patients with Diabetes Mellitus Type 2: Identification of Treatment Non-Responders – the CARDIONOR Study – which is an ongoing study aimed at investigating early markers of progressive atherosclerosis in type 2 diabetic patients. They show that intensified risk factor management significantly improved the global arginine bioavailability ratio [GABR, defined as [L-arginine]/([L-ornithine]+[L-citrulline])], which is proposed to be a new cardiovascular surrogate parameter that is reduced in diabetic individuals [8]. The GABR, as well as the arginine to ornithine ratio (indirect measure of arginase activity), is inversely correlated with intima–media thickness [7▪], another surrogate parameter for cardiovascular outcome, and with biochemical markers of endothelial dysfunction [9]. Interestingly, in a group of 2236 patients recruited within the LUdwigshafen RIsk and Cardiovascular Health Study, those with type 2 diabetes had a significantly lower GABR than individuals without diabetes [9]. The GABR may provide a noninvasive assessment of endothelial function, as it not only accounts for the substrate arginine but also for the metabolic products citrulline and ornithine, overcoming the poor prognostic value of L-arginine levels alone [8], but this requires verification.

Sourij et al.[9] found that the GABR was not superior to the arginine to ornithine ratio in predicting cardiovascular outcome, presumably because the prognostic importance of these ratios is driven mainly by the arginase pathway (only ornithine levels were associated with cardiovascular outcome). Endothelial cells contain two isoforms of arginase: arginase I (located in the cytoplasm) and arginase II (located in the mitochondria) [3]. Two recent studies demonstrate the involvement of arginase I in vascular dysfunction of diabetic patients [10▪] and mice [11]. Increased protein expression of arginase I was found in coronary arterioles from diabetic patients, and expression colocalized with eNOS in endothelial cells [10▪]. This close localization of arginase to eNOS supports the concept of competition of enzymes for the L-arginine substrate and the demonstration that increased arginase I activity leads to reduced availability of L-arginine for eNOS in coronary arteries of diabetic rats [6]. Examination of subcellular localization of arginase, eNOS, and their common substrate L-arginine may be more pertinent to the issue of reciprocal regulation of eNOS by arginase than a comparison of the Km for these enzymes [1–20 mmol/l for arginase compared to 1–5 μmol/l for eNOS], which would not provide an appropriate measure of relative rates of reactions of these two enzymes [12]. Different intracellular pools of L-arginine are proposed to exist, with at least one accessible to eNOS and arginase but not exchangeable with extracellular L-arginine. The relative L-arginine consumption rates for arginase and eNOS vary on the basis of the molar ratios of eNOS/arginase, with arginase activity being favored as the eNOS/arginase molar ratio falls, and are likely influenced by local substrate concentrations [12].

Unfortunately, recent studies have failed to elucidate the underlying mechanism(s) leading to selective upregulation of arginase I in vessels of diabetic individuals. Romero et al.[11] suggest that arginase II may contribute to the diabetes-induced arginase I expression/activity in diabetic mice, as arginase II knockout mice exhibited less elevation of arginase activity and arginase I expression than wild-type mice. Beleznai et al.[10▪], on the contrary, postulate that insulin resistance may lead to reduced insulin-induced suppression of expression/activity of enzymes in the urea synthesis pathway, including arginase, but this remains to be tested. Hyperglycemia is known to increase arginase expression in cultured endothelial cells. Chandra et al.[13] demonstrate that peroxynitrite and hydrogen peroxide, ROS upregulated in diabetes, increase the expression/activity of arginase in endothelial cells through protein kinase C-mediated activation of the RhoA/Rho kinase pathway. Interestingly, arginase I expression was increased, whereas that of arginase II was not affected.

Under normal conditions, synthesis of L-arginine from L-citrulline recycling represents 5–15% of L-arginine production, making endogenously produced L-citrulline a more efficient NO donor than exogenously supplied L-arginine that may be metabolized by arginase [14]. An alternative mechanism to explain reduced levels of arginine in diabetes is the demonstration that insulin drives recycling of citrulline to arginine to support NO production via the stimulation of argininosuccinate synthase (ASS1) in endothelial cells and reduced ASS1 may contribute to endothelial dysfunction [15▪]. Indeed, overexpression of ASS1 in cultured endothelial cells increased NO production with no change in eNOS protein expression or phosphorylation, suggesting that ASS1 may actively participate in regulating endothelial NO production by supplying L-arginine to eNOS [16▪]. ASS1 phosphorylation at Ser-328 supports the calcium-dependent stimulation of eNOS in endothelial cells and is mediated by protein kinase Cα [17▪▪]. An integral role for the subsequent argininosuccinate lyase (ASL) step in endogenous arginine production and later NO production was also recently identified [18▪▪]. A proposed multiprotein complex, including ASS1, ASL, and eNOS, functions to regulate NO production from both endogenously synthesized and exogenously supplied L-arginine. It remains to be tested whether a diabetes-driven decrease in ASS1 expression would affect formation of this complex and subsequent NO production in vascular cells.

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L-arginine supplementation as a therapeutic approach

Support for the use of L-arginine supplementation as a therapeutic treatment in diabetes is inconsistent (likely due to variation in concentrations utilized and length of treatment), but some recent studies have demonstrated beneficial effects. For example, 2 months of oral supplementation with L-arginine (3 × 2 g/day) significantly increased NO concentration and total antioxidant status in diabetic patients (matching levels in normal, healthy volunteers), without affecting fasting glucose concentration or hemoglobin A1c levels [19]. This finding is in contrast to earlier studies showing that long-term oral administration of L-arginine enhanced insulin sensitivity and glucose metabolism while improving endothelial function in type 2 diabetic patients [20,21]. Recently, Monti et al.[22▪▪] examined whether long-term L-arginine therapy could prevent or delay the onset of diabetes in patients with impaired glucose tolerance and metabolic syndrome. They found that L-arginine (6.4 g/day) in conjunction with structured lifestyle intervention did not significantly reduce the incidence of diabetes, but did significantly increase regression to normal glucose tolerance, presumably due to improvement in insulin sensitivity shown in these patients [22▪▪] and in several diabetic and obese rat models [23]. The ability of L-arginine to attenuate the many deleterious effects of methylglyoxal, a reactive metabolite of glucose (upregulated three to four fold in diabetic patients) and major precursor for the formation of advanced glycation end products (AGE), may underlie some of the beneficial effects of L-arginine supplementation in diabetic individuals [24]. Both D-arginine and L-arginine could attenuate the increased arginase I and II expression (but not arginase activity), oxidative stress, endothelial dysfunction, and AGE formation induced by methylglyoxal and high glucose, suggesting an eNOS-independent scavenging of methylglyoxal. Indeed, L-arginine can bind and inactivate methylglyoxal, preventing methylglyoxal-induced pancreatic β cell dysfunction as well as reduced adipose tissue glucose uptake [24], potentially explaining the improved insulin sensitivity of patients in the Monti study [22▪▪].

We have shown that oral L-arginine supplementation can increase transcription of guanosine triphosphate (GTP) cyclohydrolase I, the first enzyme in the de-novo synthesis of BH4 [25], increasing BH4 and NO production while also reducing weight loss and plasma glucose in type 1 diabetic rats [26]. BH4, one of the most potent antioxidants in the cell, plays a crucial role not only in increasing the rate of NO synthesis by eNOS but also in reducing the formation of O2 in endothelial cells [2]. Increasing levels of both the eNOS substrate, L-arginine, and its critical cofactor, BH4, are necessary to ensure adequate endothelial NO synthesis.

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TETRAHYDROBIOPTERIN

The function of all NOS isoforms is directly associated with NOS–BH4 stoichiometry. BH4 is bound at the interface between two NOS monomers, where it stabilizes the active dimer and participates in L-arginine oxidation through the N-hydroxyl-L-arginine intermediate and in the subsequent generation of NO. However, BH4 is also highly redox sensitive and, thus, can be readily oxidized, reducing its intracellular availability.

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Nitric oxide synthase-dependent 6R-5,6,7,8-tetrahydrobiopterin effects on vascular function

Although oxidative stress has been proposed to be responsible for diminished NO bioavailability in diabetics, the vast majority of studies utilizing standard antioxidant therapy in humans have failed to significantly improve NO-mediated dilation of vessels. In contrast, acute studies with high levels of BH4 alone or BH4 combined with L-arginine treatment showed improved vascular function in diabetic patients [27,28] and prompted the initiation of clinical studies to determine whether oral BH4 supplementation could provide a therapeutic approach to improve endothelial function. Unfortunately, several recent clinical trials involving oral supplementation of BH4 for various types of cardiovascular disease failed to show significant improvement in clinical endpoints, including dilation or NOS coupling in ex-vivo vessels [29,30]. However, one of these trials [31▪▪], involving patients with coronary artery disease, provided some mechanistic insight into the limitations of oral BH4 supplementation as a therapeutic approach to treating cardiovascular disease in humans. Oral BH4 was able to increase BH4 levels in plasma and saphenous vein (but not internal mammary artery) relative to placebo-treated control patients, but BH4-treated patients exhibited a concomitant increase in plasma levels of BH2 (which lacks eNOS cofactor activity and competitively inhibits eNOS), and no significant improvement in the BH4/BH2 ratio, conversion of L-arginine to L-citrulline, or superoxide production [31▪▪]. This study suggests that the ability of BH4 to recouple NOS in patients with diabetes and/or cardiovascular disease may be limited by BH4 oxidation, BH2 accumulation, and failure to increase BH4/BH2 ratios. Indeed, raising endogenous BH2 levels in the presence of adequate BH4 levels was sufficient to cause eNOS uncoupling and increased O2 production in rats in vivo[32]. Thus, alternative strategies are likely required to target BH4-dependent endothelial function in vascular disease states, such as diabetes, potentially by increasing endogenous BH4 stores without changing BH2 levels in endothelial cells. This approach has been validated in studies utilizing statins, which increase GTP cyclohydrolase I activity in endothelial cells and increase vascular BH4 bioavailability in atherosclerosis patients [33▪].

The recycling of BH2 to BH4 is under the control of dihydrofolate reductase (DHFR) and its activity is critical for maintaining the BH4/BH2 ratio and, thus, eNOS coupling [2]. This may be particularly important under pathological conditions that favor formation of BH2 in contrast to normal physiological conditions in which the de novo pathway for BH4 synthesis may be sufficient to maintain cellular BH4 levels. Youn et al.[34▪] show that oral administration of folic acid, which can recouple eNOS in angiotensin II-induced hypertensive mice via restoration of endothelial DHFR content and activity, completely attenuates eNOS uncoupling in streptozotocin-induced diabetic mice via restoration of DHFR function. Folic acid supplementation or DHFR overexpression reduced O2 production and restored NO-dependent endothelial function in these diabetic mice, demonstrating the importance of DHFR activity in regulating eNOS uncoupling in vivo. Further support for the importance of the BH2 reductase activity of DHFR in maintaining BH4 levels in vivo was the finding that DHFR activity is required to maintain eNOS coupling under conditions of genetic BH4 deficiency [35]. Thus, net cellular NO bioavailability likely reflects the balance between de novo BH4 synthesis, loss of BH4 by oxidation to BH2, and regeneration of BH4 by DHFR [34▪].

Both BH2 and BH4 are transported into cultured endothelial cells and the majority of uptake occurs via the equilibrative nucleoside transporter 2 (ENT2) in cultured rat aortic endothelial cells [36,37▪]. The ENT2 transporter is predominantly localized on the apical surface, suggesting a mechanism for preferential uptake of BH2 from plasma by endothelial cells [37▪]. Interestingly, accumulation of BH4 in cultured cells continued linearly, whereas that of BH2 reached a plateau within 10 min, suggesting that BH2 accumulation was counterbalanced by conversion to BH4 via DHFR [37▪]. The relationship between plasma BH4 and endothelial cell BH4 is not clear, and this complicates assessment of BH4 in target tissues in vivo. ENT2 may transport BH2 into endothelial cells for BH4 production via the DHFR salvage pathway, but in cases in which DHFR activity is too low, the bidirectional nature of the ENT2 transporter may move more of the BH2 out of the cell and into the plasma [36]. Hence, higher plasma levels of BH2 may be indicative of the greater oxidative stress within the endothelial cell, but this remains to be verified.

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Nitric oxide synthase-independent effects of 6R-5,6,7,8-tetrahydrobiopterin

BH4 also exhibits vasoprotective effects that are independent of NOS. BH4 is a growth factor for endothelial cells [38] and endothelial progenitor cells [39], and thus may play a role in wound healing in diabetes. BH4 is also a powerful antioxidant with scavenging capabilities that help preserve NO bioavailability [40]. Recently, BH4 was shown to protect soluble guanylate cyclase against oxidative inactivation that prevents NO binding and renders this enzyme insensitive to activation by NO [41].

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NITRIC OXIDE

It is well accepted that uncoupled eNOS switches production of NO to production of O2, but the precise molecular mechanisms that underlie this switch are not entirely clear. Recently, it was demonstrated that the S-glutathionylation of critical thiols reversibly uncouples eNOS, and the resultant O2 generation at the reductase domain of S-glutathionylated eNOS is not suppressed by N-nitro-L-arginine methyl ester or calcium removal. This S-glutathionylation uncoupling mechanism would, thus, be different from that induced by lack of substrate L-arginine, increases in competitive methylarginines, or alteration in NOS-bound BH4 [42,43]. Binding of methylarginines to eNOS, in addition to inhibiting NO formation, stimulates O2 production and uncouples eNOS [44]. ROS can lead to accumulation of methylarginines, further uncoupling eNOS and generating positive feedback for more ROS [43]. The O2 produced by uncoupled eNOS can generate thiyl radical formation at Cys-908 in eNOS, which in turn can react with reduced glutathione to bring about eNOS S-glutathionylation, a unique mechanism for redox regulation of eNOS [45]. The oxidant stress associated with diabetes may trigger S-glutathionylation of eNOS, inducing endothelial dysfunction, but, at the same time, this post-translational modification of eNOS may actually prevent irreversible oxidation of cellular components by limiting formation of peroxynitrite from NO and O2[43]. Interestingly, sequestration of eNOS by caveolin-1 has been described as another means of controlling eNOS-derived O2 production and the effect of caveolin-1 on the Akt/eNOS axis is differentially regulated by the cellular biopterin status [46].

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CONCLUSION

NO bioavailability is a major determinant of vascular homeostasis and is reduced in diabetes. New information is emerging to elucidate how eNOS activity and, hence, NO production is dynamically regulated in endothelial cells normally and during diabetes (Fig. 1) [47]. Basic science is driving efforts to identify targets for intervening in the uncoupling of eNOS as well as successfully augmenting BH4 levels, and, at the same time, suppressing BH2 increases in order to achieve the optimal therapeutic benefit of BH4 supplementation. Clinical trials need to continue to determine the optimal concentrations and combinations of L-arginine, BH4, and/or antioxidants to combat the oxidative stress driving down NO production and to address the impact of arginase on cardiovascular outcomes in humans.

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Acknowledgements

This work was supported in part by grants from the National Institutes of Health (R21 HL093689) and American Heart Association (11GRNT7930004) to C.J.M.

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Conflicts of interest

There are no conflicts of interest.

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REFERENCES AND RECOMMENDED READING

Papers of particular interest, published within the annual period of review, have been highlighted as:

▪ of special interest

▪▪ of outstanding interest

Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 116).

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REFERENCES

1. Alp NJ, Channon KM. Regulation of endothelial nitric oxide synthase by tetrahydrobiopterin in vascular disease. Arterioscl Thromb Vasc Biol 2004; 24:413–420.

2. Crabtree MJ, Channon KM. Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease. Nitric Oxide 2011; 25:81–88.

3. Morris SM. Recent advances in arginine metabolism: roles and regulation of the arginases. Br J Pharmacol 2009; 157:922–930.

4. Halcox JP, Schenke WH, Zalos G, et al. Prognostic value of coronary vascular endothelial dysfunction. Circulation 2002; 106:653–658.

5. Gaede P, Vedel P, Larsen N, et al. Multifactorial intervention and cardiovascular disease in patients with type 2 diabetes. N Engl J Med 2003; 348:383–393.

6. Romero MJ, Platt DH, Tawfik HE, et al. Diabetes-induced coronary vascular dysfunction involves increased arginase activity. Circ Res 2008; 102:95–102.

7▪. Tripolt NJ, Meinitzer A, Eder M, et al. Multifactorial risk factor intervention in patients with type 2 diabetes improves arginine bioavailability ratios. Diabet Med 2012; 29:e365–e368. doi:10.1111/j.1464-5491.2012.03743.x

First interventional study data to show that multifactorial risk factor management can improve arginine bioavailability ratios in patients with type 2 diabetes.

8. Tang WH, Wang Z, Cho L, et al. Diminished global arginine bioavailability and increased arginine catabolism as metabolic profile of increased cardiovascular risk. J Am Coll Cardiol 2009; 2061–2067.

9. Sourij H, Meinitzer A, Pilz S, et al. Arginine bioavailability ratios are associated with cardiovascular mortality in patients referred to coronary angiography. Atherosclerosis 2011; 218:220–225.

10▪. Beleznai T, Feher A, Spielvogel D, et al. Arginase I contributes to diminished coronary arteriolar dilation in patients with diabetes. Am J Physiol Heart Circ Physiol 2011; 300:H777–H783.

This study is the first to provide evidence supporting the contribution of arginase I to diminished NO-mediated coronary dilation in human diabetic patients.

11. Romero MJ, Iddings JA, Platt DH, et al. Diabetes-induced vascular dysfunction involves arginase I. Am J Physiol Heart Circ Physiol 2012; 302:H159–H166.

12. Santhanam L, Christianson DW, Nyhan D, Berkowitz DE. Arginase and vascular aging. J Appl Physiol 2008; 105:1632–1642.

13. Chandra S, Romero MJ, Shatanawi A, et al. Oxidative species increase arginase activity in endothelial cells through the RhoA/Rho kinase pathway. Br J Pharmacol 2012; 165:506–519.

14. El-Hattab AW, Emrick LT, Craigen WJ, Scaglia F. Citrulline and arginine utility in treating nitric oxide deficiency in mitochondrial disorders. Mol Genet Metab 2012; doi: 10.1016/j.ymgme.2012.06.018 [Epub ahead of print].

15▪. Haines RJ, Corbin KD, Pendleton LC, et al. Insulin transcriptionally regulates argininosuccinate synthase to maintain vascular endothelial function. Biochem Biophy Res Commun 2012; 421:9–14.

This examination of insulin regulation of argininosuccinate synthase suggests that one action of insulin may be to poise the endothelium to more effectively respond to physiological cues by promoting both argininosuccinate synthase and eNOS expression.

16▪. Mun GI, Kim I-S, Lee B-H, Boo YC. Endothelial argininosuccinate synthase 1 regulates nitric oxide production and monocyte adhesion under static and laminar shear stress conditions. J Biol Chem 2011; 286:2536–2542.

Results of this study indicate that argininosuccinate synthase actively participates in the regulation of endothelial NO production in response to laminar shear stress by providing the L-arginine substrate for eNOS.

17▪▪. Haines RJ, Corbin KD, Pendleton LC, Eichler DC. Protein kinase Cα phosphorylates a novel argininosuccinate synthase site at serine 328 during calcium-dependent stimulation of endothelial nitric-oxide synthase in vascular endothelial cells. J Biol Chem 2012; 187:26168–26176.

The results of this study represent the first demonstration of a biologically relevant phosphorylation site for the enzyme argininosuccinate synthase. These results help to verify the coordinated regulation of eNOS and argininosuccinate synthase relative to vascular endothelial NO production.

18▪▪. Erez A, Nagamani SCS, Shchelochkov OA, et al. Requirement of argininosuccinate for systemic nitric oxide production. Nature Med 2011; 17:1619–1626.

These mechanistic studies show that ASL has a structural function in addition to its catalytic activity that allows it to contribute to the formation of a multiprotein complex required for NO production. This previously unappreciated role of ASL may represent a new target for manipulating NO production in disease states.

19. Jablecka A, Bogdanski P, Balcer N, et al. The effect of oral L-arginine supplementation on fasting glucose, HbA1c, nitric oxide and total antioxidant status in diabetic patients with atherosclerotic peripheral arterial disease of lower extremities. Eur Rev Med Pharmacol Sci 2012; 16:342–350.

20. Piatti PM, Monti LD, Valsecchi G, et al. Long-term oral L-arginine administration improves peripheral and hepatic insulin sensitivity in type 2 diabetic patients. Diabetes Care 2001; 24:875–880.

21. Lucotti P, Setola E, Monti LD, et al. Beneficial effects of a long-term oral L-arginine treatment added to a hypocaloric diet and exercise training in obese, insulin-resistant type 2 diabetic patients. Am J Physiol Endocrinol Metab 2006; 291:E906–E912.

22▪▪. Monti LD, Setola E, Lucotti PCG, et al. Effect of a long-term oral L-arginine supplementation on glucose metabolism: a randomized, double-blind, placebo-controlled trial. Diabetes Obes Metab 2012; 14:893–900. doi: 10.1111/j.1463-1326.2012.01615.x.

This interventional study demonstrated that dietary L-arginine supplementation in conjunction with lifestyle intervention could significantly increase the conversion of impaired glucose tolerance and metabolic syndrome to normal glucose tolerance. This study provides a potential population-directed approach to prevention of type 2 diabetes.

23. Wu G, Bazer FW, Davis TA, et al. Arginine metabolism and nutrition in growth, health and disease. Amino Acids 2009; 37:153–168.

24. Dhar I, Dhar A, Wu L, Desai K. Arginine attenuates methylglyoxal- and high glucose-induced endothelial dysfunction and oxidative stress by an endothelial nitric oxide synthase-independent mechanism. J Pharmacol Exp Therapeut 2012; 342:196–204.

25. Wu G, Kelly KA, Hatakeyama K, et al. Regulation of endothelial tetrahydrobiopterin synthesis by L-arginine. In: Thony B and Blau N, editors. Pterins, Folates, and Neurotransmitters in Molecular Medicine. SPS Verlagsgesellschaft mbh, Heilbronn, Germany; 2004. pp. 54–59.

26. Kohli R, Meininger CJ, Haynes TE, et al. Dietary L-arginine supplementation enhances endothelial nitric oxide synthesis in streptozotocin-induced diabetic rats. J Nutr 2004; 134:600–608.

27. Heitzer T, Krohn K, Albers S, Meinertz T. Tetrahydrobiopterin improves endothelium-dependent vasodilation by increasing nitric oxide activity in patients with type II diabetes mellitus. Diabetologia 2000; 43:1435–1438.

28. Settergren M, Bohm F, Malmstrom RE, et al. L-arginine and tetrahydrobiopterin protects against ischemia/reperfusion-induced endothelial dysfunction in patients with type 2 diabetes mellitus and coronary artery disease. Atherosclerosis 2009; 204:73–78.

29. Moens AL, Kietadisorn R, Lin JY, Kass D. Targeting endothelial and myocardial dysfunction with tetrahydrobiopterin. J Mol Cell Cardiol 2011; 51:559–563.

30. Alkaitis MS, Crabtree MJ. Recoupling the cardiac nitric oxide synthases: tetrahydrobiopterin synthesis and recycling. Curr Heart Fail Rep 2012; 9:200–210.

31▪▪. Cunnington C, Van Assche T, Shirodarla C, et al. Systemic and vascular oxidation limits the efficacy of oral tetrahydrobiopterin treatment in patients with coronary artery disease. Circulation 2012; 125:1356–1366.

First in-vivo study to demonstrate the correlation between high BH2 and compromised endothelial function. This detailed mechanistic evaluation of the pharmacokinetics and pharmacodynamics of oral BH4 treatment suggests that alternative strategies are required to target BH4-dependent endothelial function in established vascular disease states.

32. Noguchi K, Hamadate N, Matsuzaki T, et al. Increasing dihydrobiopterin causes dysfunction of endothelial nitric oxide synthase in rats in vivo. Am J Physiol Heart Circ Physiol 2011; 301:H721–H729.

33▪. Antoniades C, Bakogiannis C, Leeson P, et al. Rapid, direct effects of statin treatment on arterial redox state and nitric oxide bioavailability in human atherosclerosis via tetrahydrobiopterin-mediated endothelial nitric oxide synthase coupling. Circulation 2011; 124:335–345.

First demonstration in humans that atorvastatin rapidly reduces vascular superoxide generation and improves endothelial function via increased BH4 bioavailability.

34▪. Youn JY, Gao L, Cai H. The p47phox- and NADPH oxidase organizer 1 (NOXO1)-dependent activation of NADPH oxidase 1 (NOX1) mediates endothelial nitric oxide synthase (eNOS) uncoupling and endothelial dysfunction in a streptozotocin-induced murine model of diabetes. Diabetologia 2012; 55:2069–2079.

Novel approaches to inhibit NADPH oxidase and/or improve DHFR function that may have therapeutic potential for diabetic endothelial dysfunction.

35. Crabtree MJ, Hale AB, Channon KM. Dihydrofolate reductase protects endothelial nitric oxide synthesis from uncoupling in tetrahydrobiopterin deficiency. Free Rad Biol Med 2011; 50:1639–1646.

36. Ohashi A, Sugawara Y, Mamada K, et al. Membrane transport of sepiapterin and dihydrobiopterin by equilibrative nucleoside transporters: a plausible gateway for the salvage of tetrahydrobiopterin biosynthesis. Mol Genet Metab 2011; 102:18–28.

37▪. Ohashi A, Mamada K, Tsuboi I, et al. Asymmetric uptake of sepiapterin and 7,8-dihydrobiopterin as a gateway of the salvage pathway of tetrahydrobiopterin biosynthesis from the luminal surface of rat endothelial cells. Mol Genet Metab 2011; 104:404–406.

The asymmetric localization of the ENT2 transporter on the luminal surface of endothelial cells provides a mechanism for the uptake of BH4 precursors from plasma.

38. Marinos RS, Zhang W, Wu G, et al. Tetrahydrobiopterin levels regulate endothelial cell proliferation. Am J Physiol Heart Circ Physiol 2001; 281:H482–H489.

39. He T, Smith LA, Lu T, et al. Activation of peroxisome proliferator-activated receptor-d enhances regenerative capacity of human endothelial progenitor cells by stimulating biosynthesis of tetrahydrobiopterin. Hypertension 2011; 58:287–294.

40. Bowers MC, Hargrove LA, Kelly KA, et al. Tetrahydrobiopterin attenuates superoxide-induced reduction in nitric oxide. Front Biosci 2011; 3:1263–1272.

41. Schmidt K, Neubauer A, Kolesnik B, et al. Tetrahydrobiopterin protects soluble guanylate cyclase against oxidative inactivation. Molec Pharmacol 2012; 82:420–427.

42. Chen CA, Wang TY, Varadharaj S, et al. S-glutathionylation uncouples eNOS and regulates its cellular and vascular function. Nature 2010; 468:1115–1118.

43. Zweier JL, Chen C-A, Druhan LF. S-glutathionylation reshapes our understanding of endothelial nitric oxide synthase uncoupling and nitric oxide/reactive oxygen species-mediated signaling. Anitoxid Redox Signal 2011; 14:1769–1775.

44. Druhan LJ, Forbes SP, Pope AJ, et al. Regulation of eNOS-derived superoxide by endogenous methylarginines. Biochemistry 2008; 47:7256–7263.

45. Chen C-A, Lin C-H, Druhan LJ, et al. Superoxide induces endothelial nitric oxide synthase protein thiyl radical formation, a novel mechanism regulating eNOS function and coupling. J Biol Chem 2011; 286:29098–29107.

46. Karuppiah K, Druhan LJ, Chen C-A, et al. Suppression of eNOS-derived superoxide by caveolin-1: a biopterin-dependent mechanism. Am J Physiol Heart Circ Physiol 2011; 301:H903–H911.

47. Wu G, Haynes TE, Li H, Meininger CJ. Glutamine metabolism in endothelial cells: ornithine synthesis from glutamine via pyrroline-5-carboxylate synthase. Comp Biochem Physiol 2000; 126:115–123.

Keywords:

arginase; dihydrofolate reductase; endothelial dysfunction; endothelial nitric oxide synthase uncoupling; tetrahydrobiopterin

© 2013 Lippincott Williams & Wilkins, Inc.

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