Miyuki Yokoro

Involvement of advanced glycation end product-induced asymmetric dimethylarginine generation in endothelial dysfunction

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase inhibitor, plays a role in endothelial dysfunction, an initial step of atherosclerosis. Advanced glycation end products (AGEs) also contribute to accelerated atherosclerosis. However, a pathophysiological crosstalk between ADMA and AGEs remains unclear. In this study, we investigated the relationship between ADMA and AGE level in patients with end-stage renal disease (ESRD) due to diabetic nephropathy. We also examined whether and how AGEs increased ADMA generation by cultured endothelial cells (ECs). Plasma ADMA levels were positively associated with serum AGE level and were inversely correlated with endothelial function determined by flow-mediated vasodilatation. AGEs dose dependently increased reactive oxygen species (ROS) generation in ECs, which was blocked by antisense DNA raised against receptor for AGEs (RAGE). Furthermore, AGEs decreased messenger RNA (mRNA) level of dimethylarginine dimethylaminohydrolase (DDAH)-II, an enzyme for ADMA degradation, reduced its total enzymatic activity and resultantly increased ADMA, all of which were completely blocked by an antioxidant, N-acetylcysteine. These results suggest that the AGE-RAGE-mediated ROS generation could be involved in endothelial dysfunction in diabetic ESRD patients partly by increasing the ADMA generation via suppression of DDAH activity in ECs.

Oral L-carnitine supplementation increases trimethylamine-N-oxide but reduces markers of vascular injury in hemodialysis patients.

Objectives: Food or supplement-derived L-carnitine is changed to trimethylamine (TMA) by interstinal microbiota, which is further metabolized to trimethylamine-N-oxide (TMAO), being involved in the promotion of atherosclerosis in animal models. Meanwhile, carnitine deficiency has played a role in accelerated atherosclerosis in hemodialysis (HD) patients. However, effects of oral L-carnitine supplementation on circulating levels of TMAO and markers of vascular injury and oxidative stress in patients on HD remain unclear. In this study, we addressed the issue. Methods: Thirty-one HD patients with carnitine deficiency were treated with oral L-carnitine (900 mg/d) for 6 months. At baseline and after treatment, clinical variables including circulating levels of carnitine fractions, TMA, TMAO, advanced glycation end products (AGE), soluble forms of intracellular adhesion molecule-1 (sICAM-1), vascular cell adhesion molecule-1 (sVCAM-1), and malondialdehyde (MDA) were measured. Results: Oral L-carnitine supplementation significantly increased total, free, acyl carnitine, and plasma TMA and TMAO levels, whereas it decreased markers of vascular injury and oxidative stress such as sICAM-1, sVCAM-1, and MDA levels. TMA and TMAO levels at baseline were correlated with each other, and free carnitine was independently associated with TMAO levels. Furthermore, change in AGE values from baseline ([INCREMENT]AGE) was positively correlated with [INCREMENT]sICAM-1 (P = 0.043) and was a sole independent determinant of [INCREMENT]sICAM-1 (R2 = 0.133, P = 0.043). Conclusions: This study demonstrated that although oral L-carnitine supplementation was associated with increased TMAO levels, it might be beneficial on vascular injury in patients on HD. Vasculoprotective properties of L-carnitine supplementation in HD patients might be ascribed partly to its inhibitory actions on AGE.

Mitochonic Acid 5 Binds Mitochondria and Ameliorates Renal Tubular and Cardiac Myocyte Damage

Mitochondrial dysfunction causes increased oxidative stress and depletion of ATP, which are involved in the etiology of a variety of renal diseases, such as CKD, AKI, and steroid-resistant nephrotic syndrome. Antioxidant therapies are being investigated, but clinical outcomes have yet to be determined. Recently, we reported that a newly synthesized indole derivative, mitochonic acid 5 (MA-5), increases cellular ATP level and survival of fibroblasts from patients with mitochondrial disease. MA-5 modulates mitochondrial ATP synthesis independently of oxidative phosphorylation and the electron transport chain. Here, we further investigated the mechanism of action for MA-5. Administration of MA-5 to an ischemia-reperfusion injury model and a cisplatin-induced nephropathy model improved renal function. In in vitro bioenergetic studies, MA-5 facilitated ATP production and reduced the level of mitochondrial reactive oxygen species (ROS) without affecting activity of mitochondrial complexes I-IV. Additional assays revealed that MA-5 targets the mitochondrial protein mitofilin at the crista junction of the inner membrane. In Hep3B cells, overexpression of mitofilin increased the basal ATP level, and treatment with MA-5 amplified this effect. In a unique mitochondrial disease model (Mitomice with mitochondrial DNA deletion that mimics typical human mitochondrial disease phenotype), MA-5 improved the reduced cardiac and renal mitochondrial respiration and seemed to prolong survival, although statistical analysis of survival times could not be conducted. These results suggest that MA-5 functions in a manner differing from that of antioxidant therapy and could be a novel therapeutic drug for the treatment of cardiac and renal diseases associated with mitochondrial dysfunction.

Asymmetric dimethylarginine accumulates in the kidney during ischemia/reperfusion injury

Ischemia/reperfusion injury is the leading cause of acute tubular necrosis. Nitric oxide has a protective role against ischemia/reperfusion injury; however, the role of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, in ischemia/reperfusion injury remains unclear. ADMA is produced by protein arginine methyltransferase (PRMT) and is mainly degraded by dimethylarginine dimethylaminohydrolase (DDAH). Here we examined the kinetics of ADMA and PRMT and DDAH expression in the kidneys of ischemia/reperfusion-injured mice. After the injury, DDAH-1 levels were decreased and renal and plasma ADMA values were increased in association with renal dysfunction. Renal ADMA was correlated with 8-hydroxy-2′-deoxyguanosine, a marker of oxidative stress. An antioxidant, N-acetylcysteine, or a proteasomal inhibitor, MG-132, restored these alterations. Infusion of subpressor dose of ADMA exacerbated renal dysfunction, capillary loss, and tubular necrosis in the kidneys of ischemia/reperfusion-injured wild mice, while damage was attenuated in DDAH transgenic mice. Thus, ischemia/reperfusion injury–induced oxidative stress may reduce DDAH expression and cause ADMA accumulation, which may contribute to capillary loss and tubular necrosis in the kidney.

Role of Asymmetric Dimethylarginine in Cardiorenal Syndrome

Cardiorenal syndrome (CRS) is a condition in which there is a complex interrelationship between cardiovascular disease (CVD) and chronic kidney disease (CKD). Impairment of one organ could accelerate pathological processes in the other, which in turn accelerates the progression of failure of both. Although clinical studies hint at a specific bidirectional interaction between cardiovascular system and kidney, insight into the pathogenesis of CRS remains unknown. One possible factor that could explain this link is endothelial dysfunction (ED). ED is not only involved in initiation and progression of atherosclerosis, but also contributes to progression of renal injury. Asymmetric dimethylarginine (ADMA) is an endogenous NO synthase inhibitor found in the plasma and cells. Plasma ADMA levels are increased in CKD patients and known to be an independent biomarker and predictor for future cardiovascular events as well as the progression of CKD. These findings indicate that accumulated ADMA-mediated ED may play important roles in CRS in CKD patients. In this review, we discuss the roles of ADMA in the development of ED, especially focusing on its roles in CRS.

Protective role of JAK/STAT signaling against renal fibrosis in mice with unilateral ureteral obstruction.

Inflammation is involved in renal fibrosis, a final common pathway for kidney diseases. To clarify how JAK/STAT/SOCS system was involved in renal fibrosis, UUO was induced in BALB/c or SOCS3(+/-) mice in the presence or absence of JAK inhibitor-incorporated nanoparticle (pyridine6-PGLA). UUO increased pSTAT3 and subsequently elevated SOCS3 levels in the obstructed kidneys. pSTAT3 levels were further increased in SOCS3(+/-) mice. UUO-induced renal fibrosis was markedly suppressed in SOCS3(+/-) mice, while it was aggravated by pre-treatment with pyridine6-PGLA. Although there were no differences in renal mRNA levels of TGF-β and collagens between wild and SOCS3(+/-) mice, MMP-2 activity was enhanced in SOCS3(+/-) UUO mice. Activated MMP-2 was completely suppressed by pyridine6-PGLA-pre-treatment. TNF-α one of JAK/STAT activators, increased pSTAT3 levels and subsequently induced MMP-2 activation in proximal tubular cells. These results suggest that JAK/STAT3 signaling may play a role in repair process of renal fibrosis in UUO partly via MMP-2 activation.

Asymmetric Dimethylarginine Contributes to the Impaired Response to Erythropoietin in CKD-Anemia

Erythropoietin-resistant anemia is associated with adverse cardiovascular events in patients with ESRD, but the underlying mechanism remains unclear. Here, we evaluated the role of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA). In 54 patients with advanced CKD, erythrocyte but not plasma ADMA levels independently associated with low hemoglobin values, although levels of both types of ADMA were elevated compared with those in healthy volunteers. Furthermore, erythrocyte ADMA level associated with the erythropoietin resistance index in patients receiving a weekly injected dose of erythropoiesis-stimulating agents standardized for hemoglobin levels and body weight, whereas it correlated with the erythropoietin demand index (plasma erythropoietin units divided by the hemoglobin value) in patients not receiving erythropoiesis-stimulating agents. Compared with sham-operated controls, wild-type mice with 5/6 subtotal nephrectomy (Nx), a remnant kidney model with advanced CKD, had decreased hemoglobin, hematocrit, and mean corpuscular volume values but increased erythrocyte and plasma ADMA and plasma erythropoietin levels. In comparison, dimethylarginine dimethlaminohydrolase-1 transgenic (DDAH-1 Tg) mice, which efficiently metabolized ADMA, had significant improvements in all of the values except those for erythropoietin after 5/6 Nx. Additionally, wild-type Nx mice, but not DDAH-1 Tg Nx mice, had reduced splenic gene expression of erythropoietin receptor and erythroferrone, which regulates iron metabolism in response to erythropoietin. This study suggests that erythrocyte ADMA accumulation contributes to impaired response to erythropoietin in predialysis patients and advanced CKD mice via suppression of erythropoietin receptor expression.

RAGE-aptamer attenuates deoxycorticosterone acetate/salt-induced renal injury in mice

The mineralocorticoid receptor (MR) and its downstream signaling play an important role in hypertensive renal injury. The interaction of advanced glycation end products (AGE) with their receptor (RAGE) is involved in the progression of renal disease. However, the pathological crosstalk between AGE–RAGE axis and MR system in kidney derangement remains unclear. We screened DNA-aptamer directed against RAGE (RAGE-apt) in vitro and examined its effects on renal injury in uninephrectomized deoxycorticosterone acetate (DOCA)/salt-induced hypertensive mice. RAGE, GTP-bound Rac-1 (Rac1), and MR were co-localized in the podocytes of DOCA mice. The deletion of RAGE gene significantly inhibited mesangial matrix expansion and tubulointerstitial fibrosis in DOCA mice, which was associated with the reduction of glomerular oxidative stress, MR, Rac1, and urinary albumin excretion (UAE) levels. RAGE-apt attenuated the increase in carboxymethyllysine (CML), RAGE, nitrotyrosine, Rac1, and MR levels in the kidneys and reduced UAE in DOCA mice. Aldosterone (Aldo) increased nitrotyrosine, CML, and RAGE gene expression in murine podocytes, whereas CML stimulated MR and Rac1 levels, which were blocked by RAGE-apt. The present study indicates the crosstalk between the AGE–RAGE axis and Aldo–MR system, suggesting that RAGE-apt may be a novel therapeutic tool for the treatment of MR-associated renal diseases.

L-carnitine Supplementation Improves Self-rating Depression Scale Scores in Uremic Male Patients Undergoing Hemodialysis

Background: Depression is highly prevalent in uremic patients undergoing hemodialysis (HD). We previously found that low free-carnitine levels are associated with depression severity in male patients undergoing HD. However, whether L-carnitine supplementation improves the depression state in male patients undergoing HD remains unclear. Methods: Sixteen male patients undergoing HD were orally administered 900 mg L-carnitine daily or intravenously administered 1000 mg L-carnitine immediately after undergoing HD for 3 months. The depression state and various types of carnitine levels were evaluated using the self-rating depression scale (SDS) and tandem mass spectrometry, respectively, at baseline and 3 months after treatment. Results: L-carnitine supplementation significantly increased serum levels of free and other acylcarnitine types, associated with improved SDS scores in male patients undergoing HD. Univariate analysis revealed that low baseline butyryl- and isovaleryl-/2-methylbutyryl-carnitine levels were significantly correlated with SDS scores after treatment. Multiple regression analysis revealed that butyryl-carnitine levels were a sole independent predictor of SDS scores after treatment (r2 = 0.533). Conclusion: L-carnitine supplementation for 3 months improved the depression state in uremic male patients undergoing HD. Thus, low butyryl-carnitine levels may predict the clinical response to L-carnitine supplementation in male patients undergoing HD and who have mild depression.