Seiya Okuda

Asymmetrical Dimethylarginine, an Endogenous Nitric Oxide Synthase Inhibitor, in Experimental Hypertension

NG,NG-dimethyl-L-arginine (ADMA) is an endogenously synthesized nitric oxide (NO) synthase inhibitor which has potent pressor/vasoconstrictor effects. Dimethylargininase metabolizes ADMA to L-citrulline and plays a key role in determining the in vivo levels of ADMA. To investigate the role of ADMA in the pathogenesis of hypertension, we measured 24-hour urinary excretion of ADMA (UADMA) and nitrate/nitrite (NOx) in Dahl salt-sensitive hypertensive rats and spontaneously hypertensive rats (SHR). In Dahl salt-resistant rats, high-salt diet (8% NaCl) did not increase blood pressure and increased urinary NOx (P < .01) without changes in UADMA compared with low-salt diet (0.3% NaCl). In contrast, in Dahl salt-sensitive rats, high-salt diet increased blood pressure (P < .01), did not change urinary NOx excretion, and increased UADMA (P < .01). There was a significant (r = .65, P < .01) correlation between UADMA and the level of blood pressure in Dahl salt-sensitive rats. Plasma levels of NOx and ADMA and renal dimethylargininase content were comparable among them. These results may suggest that in Dahl salt-resistant rats, blood pressure is kept constant during high-salt intake, possibly due to the compensatory increased production of NO, and that in Dahl salt-sensitive rats, high-salt intake increases the production of ADMA, attenuates the compensatory increases in NO, and increases blood pressure. These results also suggest that the systemic production of ADMA is not dependent on renal dimethylargininase. SHR had significantly greater urinary NOx excretion (P < .05) and smaller UADMA than Wistar-Kyoto rats (P < .05), and UADMA was inversely correlated with their mean arterial pressure (r =.64, P < .05). In conclusion. ADMA, independently of the renal dimethylargininase content, may play a role in the pathogenesis in Dahl salt-sensitive hypertensive rats but not in SHR.

Tetrahydrobiopterin restores endothelial function in long-term smokers

OBJECTIVES We sought to test whether tetrahydrobiopterin (BH4) supplementation improves nitric oxide (NO) bioactivity in smokers. BACKGROUND In smokers, endothelium-derived NO bioactivity is impaired. BH4 is an essential cofactor of NO synthase, and its deficiency decreases NO bioactivity. METHODS Sapropterin hydrochloride, an active analogue of BH4 (2 mg/kg body weight), was administered orally to healthy male smokers and age-matched nonsmokers. Before and 3 and 24 h after sapropterin, we measured plasma levels of BH4 and examined flow-mediated vasodilation (FMV) of the brachial artery by high resolution ultrasonography, a noninvasive test of endothelial function. RESULTS Basal plasma levels of BH4 were not different between smokers and nonsmokers. Sapropterin administration increased plasma levels of BH4 by threefold at 3 h, which returned to the baseline at 24 h. Before sapropterin, FMV was significantly smaller in smokers (p = 0.0002). Sapropterin significantly augmented endothelium-dependent vasodilation in smokers, but did not affect it in nonsmokers (p = 0.001 by analysis of variance [ANOVA]). Coadministration of N(G)-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor (20 micromol), into the brachial artery completely abolished the vasodilatory effects of sapropterin (p = 0.002 by ANOVA). Endothelium-independent vasodilation by glyceryl trinitrate was not different between smokers and nonsmokers and was not altered by BH4. CONCLUSIONS We demonstrated that BH4 supplementation improved bioactivity of endothelium-derived NO in smokers. These observations strongly suggest that decreased NO-dependent vasodilation in smokers could be related to reduced bioactivity of BH4.

Molecular Mechanism for Elevation of Asymmetric Dimethylarginine and Its Role for Hypertension in Chronic Kidney Disease

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase. ADMA is generated by protein methyltransferase (PRMT) and is metabolized mainly by dimethylarginine dimethylaminohydrolase (DDAH). ADMA levels are reported to increase in patients with chronic kidney disease (CKD), thereby playing a role in the pathogenesis of accelerated atherosclerosis in this population. However, the precise mechanism underlying ADMA accumulation in these patients is not fully understood. This study investigated the molecular mechanism for the elevation of ADMA levels in CKD, using a rat remnant kidney model that represents progressive CKD. After male Sprague-Dawley rats underwent baseline measurement of BP and renal function, 5/6 subtotal nephrectomy (5/6Nx) and 4/6 nephrectomy were performed. Plasma and urinary levels of ADMA and symmetric dimethylarginine, an inert isomer of ADMA, were measured by HPLC. Expression levels of PRMT genes and DDAH proteins were analyzed by semiquantitative reverse transcription-PCR and Western blotting, respectively. Plasma ADMA levels were elevated in the Nx groups in proportion to the degree of nephrectomy despite marked increases in renal clearance of ADMA. In contrast, renal clearance of symmetric dimethylarginine was decreased and its plasma levels were increased in the Nx groups. Furthermore, both liver and kidney gene expression of PRMT was increased, whereas DDAH protein expression was decreased in the 5/6Nx group. Plasma ADMA levels were correlated with systolic BP levels. Moreover, adenovirus-mediated DDAH gene transfer into the 5/6Nx rats prevented the elevation of BP levels, which was associated with the reduction of plasma and urinary ADMA levels. The results presented here suggest that decreased DDAH levels as well as increased PRMT gene expression could cause the elevation of plasma ADMA levels, thereby eliciting hypertension in CKD. Substitution of DDAH protein or enhancement of its activity may become a novel therapeutic strategy for the treatment of hypertension-related vascular injury in CKD.

Molecular mechanisms of diabetic nephropathy and its therapeutic intervention.

Diabetic nephropathy is a leading cause of end-stage renal failure, which could account for disabilities and high mortality rates in patients with diabetes. Diabetic nephropathy seems to occur as a result of an interaction between metabolic and hemodynamic factors, which activate common pathways that lead to renal damage. Recent large landmark clinical studies have shown that intensive glucose control reduces the risk of the development and progression of diabetic nephropathy, and the blockade renin-angiotensin system (RAS) is also an important target for both metabolic and hemodynamic derangements in diabetic nephropathy. However, diabetic nephropathy remains the leading cause of end-stage renal failure in developed countries. Therefore, to develop novel therapeutic strategies that specifically target diabetic nephropathy may be helpful for most patients with diabetes. High glucose, via various mechanisms such as increased production of oxidative stress and advanced glycation end products (AGEs), and activation of the RAS and protein kinase C (PKC), elicits vascular inflammation and alters gene expression of growth factors and cytokines, thereby it might be involved in the development and progression of diabetic nephropathy. This article summarizes the molecular mechanisms of diabetic nephropathy and the potential therapeutic interventions that may prevent this devastating disorder even in the presence of hyperglycemia, control of which is often difficult with current therapeutic options.

Agents that block advanced glycation end product (AGE)-RAGE (receptor for AGEs)-oxidative stress system: a novel therapeutic strategy for diabetic vascular complications

Background: Diabetic vascular complications are leading causes of acquired blindness, end-stage renal failure, a variety of neuropathies, and accelerated atherosclerosis, which together could account for disabilities and high mortality rates in patients with diabetes. Since there is accumulating evidence that the advanced glycation end product (AGE)–RAGE (receptor for AGEs)–oxidative stress axis is involved in diabetic vascular complications, inhibition of the AGE–RAGE system may be a promising target for therapeutic intervention in these devastating disorders. Objective: In this review, we discuss several types of agent that may be able to inhibit the AGE–RAGE–oxidative stress system, and their therapeutic implications in vascular complications in diabetes. Methods: We have analyzed currently available scientific literature in the field of AGE–RAGE to create a comprehensive review on novel therapeutic agents for vascular complications in diabetes. Results/conclusion: Inhibition of AGE formation, blockade of the AGE–RAGE interaction, and suppression of RAGE expression or its downstream pathways may be novel therapeutic strategies for the treatment of vascular complications in diabetes.

Role of AGEs in Diabetic Nephropathy

Diabetic nephropathy is the most common cause of end-stage renal disease in the world, and accounts for a significant increase in morbidity and mortality in patients with diabetes. Therapeutic options such as strict blood pressure and/or glycemic control are effective for preventing the development and progression of diabetic nephropathy, but the number of diabetic patients on hemodialysis is still increasing. Therefore, a novel therapeutic strategy that could halt the progression of diabetic nephropathy should be developed. Advanced glycation end products (AGEs) are heterogeneous cross-linked sugar-derived proteins which could accumulate in glomerular basement membrane, mesangial cells, endothelial cells, and podocytes in patients with diabetes and/or end-stage renal failure. AGEs are thought to be involved in the pathogenesis of diabetic nephropathy via multifactorial mechanisms such as oxidative stress generation and overproduction of various growth factors and cytokines. Further, recently, the cross-talk between AGEs and the renin-angiotensin system (RAS) has been proposed to participate in diabetic nephropathy. In addition, activation of the RAS elicits ROS generation and subsequently stimulates growth factor and cytokine production by kidney cells as well. These observations suggest that combination therapy with inhibitors of the RAS and blockers of the AGEs formation and/or their downstream pathway may be a novel therapeutic option for preventing diabetic nephropathy. In this paper, we review the role of AGEs and their receptor system in the pathogenesis of diabetic nephropathy. We further discuss here the cross-talk between AGEs and the RAS in the development and progression of diabetic nephropathy.

Dimethylarginine Dimethylaminohydrolase Prevents Progression of Renal Dysfunction by Inhibiting Loss of Peritubular Capillaries and Tubulointerstitial Fibrosis in a Rat Model of Chronic Kidney Disease

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, is mainly degraded by dimethylarginine dimethylaminohydrolase (DDAH). It was recently reported that reduced DDAH expression could contribute to ADMA accumulation and subsequent elevation of BP in an experimental model of chronic kidney disease (CKD). ADMA is a strong predictor of the progression of CKD as well. However, a role for the ADMA-DDAH in the pathogenesis of CKD remains to be elucidated. This study investigated the effects of DDAH-elicited ADMA lowering on renal function and pathology in a rat remnant kidney model. Four weeks after five-sixths subtotal nephrectomy (Nx), the rats were given tail-vein injections of recombinant adenovirus vector encoding DDAH-I (Adv-DDAH) or control vector expressing bacterial beta-galactosidase (Adv-LZ) or orally administered 20 mg/kg per d hydralazine (Hyz), which served as a BP control model. In comparison with Adv-LZ or Hyz administration, Adv-DDAH decreased plasma levels of ADMA and inhibited the deterioration of renal dysfunction. Plasma levels of ADMA were associated with decreased number of peritubular capillaries, increased tubulointerstitial fibrosis, and proteinuria levels in Nx rats. These changes were progressed in Adv-LZ-or Hyz-treated Nx rats, which were ameliorated by DDAH overexpression. In addition, semiquantitative reverse transcriptase-PCR and immunohistochemistry for TGF-beta revealed that Adv-DDAH inhibited upregulation of TGF-beta expression in Nx rats. These data suggest that ADMA may be involved in peritubular capillary loss and tubulointerstitial fibrosis, thereby contributing to the progression of CKD. Substitution of DDAH protein or enhancement of its activity may become a novel therapeutic strategy for the treatment of CKD.

Food-Derived Advanced Glycation end Products (AGEs): A Novel Therapeutic Target for Various Disorders

Non-enzymatic modification of proteins by reducing sugars, a process that is also known as Maillard reaction, leads to the formation of advanced glycation end products (AGEs) in vivo. It is now well established that formation and accumulation of AGEs progress during normal aging, and at an extremely accelerated rate under diabetes, thus being implicated in various types of AGE-related disorders such as diabetic vascular complications, neurodegenerative diseases and cancers. Further, there is accumulating evidence that AGEs and their receptor RAGE interaction elicits oxidative stress generation and subsequently alters gene expression in various types of cells. In addition, digested food-derived AGEs are found to play an important role in the pathogenesis of the AGE-related disorders as well. Indeed, restriction of diet-derived AGEs not only blocks the progression of atherosclerosis and renal injury, but also improves insulin resistance in animal models. AGE-poor diets reduce serum levels of inflammatory biomarkers in patients with diabetes or chronic renal failure. These observations suggest that the restriction of food-derived AGEs or the inhibition of absorption of dietary AGEs may be a novel target for therapeutic intervention in the AGE-related disorders. In this paper, we review the pathological role of food-derived AGEs in various types of disorders and discuss the potential utility of oral adsorbent that inhibits the absorption of AGEs in these devastating diseases.

Role of AGEs-RAGE System in Cardiovascular Disease

Advanced glycation end products (AGEs) are a heterogenous group of molecules formed during a non-enzymatic reaction between proteins and sugar residues. Recently, AGEs and their receptor (receptor for AGEs; RAGE) play a central role in the pathogenesis of cardiovascular disease (CVD), which accounts for disability and high mortality rate in patients with diabetes. AGEs initiate diabetic micro- and macrovascular complications through the structural modification and functional alteration of the extracellular matrix proteins as well as intracellular signaling molecules. Engagement of RAGEs with AGEs elicits intracellular reactive oxygen species (ROS) generation and subsequently activates mitogen-activated protein kinase (MAPK) and nuclear factor kappa-B (NF-κB) signaling, followed by production of several inflammatory and/or profibrotic factors such as vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), plasminogen activator inhibitor-1 (PAI-1) and monocyte chemoattractant protein-1 (MCP-1), thereby being involved in the progression of atherosclerosis. Administration of soluble form of RAGE (sRAGE) could work as a decoy receptor for AGEs and might inhibit the binding of AGEs to RAGE, preventing the development and progression of atherosclerosis in animal models. Furthermore, AGEs/high mobility group box-1 (HMGB-1)-RAGE interaction is involved in heart failure, abdominal aortic aneurysm (AAA) and vascular calcification as well. Thus, blockade of the AGEs/HMGB-1-RAGE system may be a promising therapeutic target for preventing diabetes- and/or age-related CVD. We review here the pathological role of the AGEs/HMGB-1-RAGE system in various types of CVD.

Asymmetric dimethylarginine (ADMA) is a novel emerging risk factor for cardiovascular disease and the development of renal injury in chronic kidney disease

Endothelial dysfunction due to the reduced bioavailability of nitric oxide (NO) is involved in the course of atherosclerotic cardiovascular disease as well as chronic kidney disease (CKD). NO is synthesized from L-arginine via the action of NO synthase, which is blocked by endogenous L-arginine analogues such as asymmetric dimethylarginine (ADMA). ADMA is a naturally occurring amino acid found in plasma and various types of tissues. The plasma level of ADMA is reported to be associated with cardiovascular risk factors such as hypertension, diabetes, hyperlipidemia, and CKD, and is a strong predictor for cardiovascular disease and the progression of CKD. In this review, we discuss the biology of ADMA, the molecular mechanisms of the elevation of ADMA levels in CKD, and the pathological role of ADMA in patients with CKD.