Role of Arginase in Selective Impairment of Endothelium-Dependent Nitric Oxide Synthase-Mediated Dilation of Retinal Arterioles during Early Diabetes

Oxidative stress and epigenetics in ocular vascular aging: an updated review

Vascular aging is an inevitable process with advancing age, which plays a crucial role in the pathogenesis of cardiovascular and microvascular diseases. Diabetic retinopathy (DR) and age-related macular degeneration (AMD), characterized by microvascular dysfunction, are the common causes of irreversible blindness worldwide, however there is still a lack of effective therapeutic strategies for rescuing the visual function. In order to develop novel treatments, it is essential to illuminate the pathological mechanisms underlying the vascular aging during DR and AMD progression. In this review, we have summarized the recent discoveries of the effects of oxidative stress and epigenetics on microvascular degeneration, which could provide potential therapeutic targets for DR and AMD.

H2O2 Mediates VEGF- and Flow-Induced Dilations of Coronary Arterioles in Early Type 1 Diabetes: Role of Vascular Arginase and PI3K-Linked eNOS Uncoupling

In diabetes, the enzyme arginase is upregulated, which may compete with endothelial nitric oxide (NO) synthase (eNOS) for their common substrate L-arginine and compromise NO-mediated vasodilation. However, this eNOS uncoupling can lead to superoxide production and possibly vasodilator hydrogen peroxide (H₂O₂) formation to compensate for NO deficiency. This hypothesis was tested in coronary arterioles isolated from pigs with 2-week diabetes after streptozocin injection. The NO-mediated vasodilation induced by flow and VEGF was abolished by NOS inhibitor L-NAME and phosphoinositide 3-kinase (PI3K) inhibitor wortmannin but was not affected by arginase inhibitor N^ω-hydroxy-nor-L-arginine (nor-NOHA) or H₂O₂ scavenger catalase in control pigs. With diabetes, this vasodilation was partially blunted, and the remaining vasodilation was abolished by catalase and wortmannin. Administration of L-arginine or nor-NOHA restored flow-induced vasodilation in an L-NAME sensitive manner. Diabetes did not alter vascular superoxide dismutase 1, catalase, and glutathione peroxidase mRNA levels. This study demonstrates that endothelium-dependent NO-mediated coronary arteriolar dilation is partially compromised in early type 1 diabetes by reducing eNOS substrate L-arginine via arginase activation. It appears that upregulated arginase contributes to endothelial NO deficiency in early diabetes, but production of H₂O₂ during PI3K-linked eNOS uncoupling likely compensates for and masks this disturbance.

Arginase: Biological and Therapeutic Implications in Diabetes Mellitus and Its Complications

Arginase is a ubiquitous enzyme in the urea cycle (UC) that hydrolyzes L-arginine to urea and L-ornithine. Two mammalian arginase isoforms, arginase1 (ARG1) and arginase2 (ARG2), play a vital role in the regulation of β-cell functions, insulin resistance (IR), and vascular complications via modulating L-arginine metabolism, nitric oxide (NO) production, and inflammatory responses as well as oxidative stress. Basic and clinical studies reveal that abnormal alterations of arginase expression and activity are strongly associated with the onset and development of diabetes mellitus (DM) and its complications. As a result, targeting arginase may be a novel and promising approach for DM treatment. An increasing number of arginase inhibitors, including chemical and natural inhibitors, have been developed and shown to protect against the development of DM and its complications. In this review, we discuss the fundamental features of arginase. Next, the regulatory roles and underlying mechanisms of arginase in the pathogenesis and progression of DM and its complications are explored. Furthermore, we review the development and discuss the challenges of arginase inhibitors in treating DM and its related pathologies.

Serum Iba-1, GLUT5, and TSPO in Patients With Diabetic Retinopathy: New Biomarkers for Early Retinal Neurovascular Alterations? A Pilot Study

Purpose

This study explored the possibility of highlighting early retinal neurovascular alterations of diabetic retinopathy (DR) by monitoring in DR patients the serum levels of microglial biomarkers ionized calcium-binding adapter molecule 1 (Iba-1), glucose transporter 5 (GLUT5), and translocator protein (TSPO), along with serum changes of the endothelial dysfunction marker arginase-1.

Methods

Serum markers were determined by enzyme-linked immunosorbent assay in 50 patients: 12 non-diabetic subjects, 14 diabetic patients without DR, 13 patients with non-proliferative DR (NPDR), and 11 patients with proliferative DR (PDR). The results were correlated with hyperreflective retinal spots (HRS), observed with optical coherence tomography (OCT).

Results

Although HRS were absent in diabetic patients without DR, NPDR patients showed an average of 4 ± 1 HRS, whereas the highest presence was detected in PDR patients, with 8 ± 1 HRS (P < 0.01 vs. NPDR). HRS were positively correlated (P < 0.01) with serum levels of arginase-1 (r = 0.91), Iba-1 (r = 0.96), GLUT5 (r = 0.94), and TSPO (r = 0.88). Moreover, serum proinflammatory cytokines and chemokines showed a positive correlation (P < 0.01) with HRS number and the serum markers analyzed.

Conclusions

Serum markers of microglial activation positively correlate with retinal HRS in NPDR and PDR patients.

Translational Relevance

These data corroborate the possibility of highlighting early retinal neurovascular changes due to diabetes by monitoring circulating microglial markers.

Retinal blood flow dysregulation precedes neural retinal dysfunction in type 2 diabetic mice

We investigated and compared the susceptibility of retinal blood flow regulation and neural function in mice developing type 2 diabetes. The longitudinal changes in retinal neuronal function and blood flow responses to a 10-min systemic hyperoxia and a 3-min flicker stimulation were evaluated every 2 weeks in diabetic db/db mice and nondiabetic controls (db/m) from age 8 to 20 weeks. The retinal blood flow and neural activity were assessed using laser speckle flowgraphy and electroretinography (ERG), respectively. The db/db mice had significantly higher blood glucose levels and body weight. The resting retinal blood flow was steady and comparable between two groups throughout the study. Hyperoxia elicited a consistent decrease, and flicker light an increase, in retinal blood flow in db/m mice independent of age. However, these flow responses were significantly diminished in db/db mice at 8 weeks old and then the mice became unresponsive to stimulations at 12 weeks. Subsequently, the ERG implicit time for oscillatory potential was significantly increased at 14 weeks of age while the a-wave and b-wave amplitudes and implicit times remained unchanged. The deficiencies of flow regulation and neurovascular coupling in the retina appear to precede neural dysfunction in the mouse with type 2 diabetes.

Activation of Coronary Arteriolar PKCβ2 Impairs Endothelial NO-Mediated Vasodilation: Role of JNK/Rho Kinase Signaling and Xanthine Oxidase Activation

Protein kinase C (PKC) activation can evoke vasoconstriction and contribute to coronary disease. However, it is unclear whether PKC activation, without activating the contractile machinery, can lead to coronary arteriolar dysfunction. The vasoconstriction induced by the PKC activator phorbol 12,13-dibutyrate (PDBu) was examined in isolated porcine coronary arterioles. The PDBu-evoked vasoconstriction was sensitive to a broad-spectrum PKC inhibitor but not affected by inhibiting PKCβ2 or Rho kinase. After exposure of the vessels to a sub-vasomotor concentration of PDBu (1 nmol/L, 60 min), the endothelium-dependent nitric oxide (NO)-mediated dilations in response to serotonin and adenosine were compromised but the dilation induced by the NO donor sodium nitroprusside was unaltered. PDBu elevated superoxide production, which was blocked by the superoxide scavenger Tempol. The impaired NO-mediated vasodilations were reversed by Tempol or inhibition of PKCβ2, xanthine oxidase, c-Jun N-terminal kinase (JNK) and Rho kinase but were not affected by a hydrogen peroxide scavenger or inhibitors of NAD(P)H oxidase and p38 kinase. The PKCβ2 protein was detected in the arteriolar wall and co-localized with endothelial NO synthase. In conclusion, activation of PKCβ2 appears to compromise NO-mediated vasodilation via Rho kinase-mediated JNK signaling and superoxide production from xanthine oxidase, independent of the activation of the smooth muscle contractile machinery.

A Cross-Talk between the Erythrocyte L-Arginine/ADMA/Nitric Oxide Metabolic Pathway and the Endothelial Function in Subjects with Type 2 Diabetes Mellitus

(1) Background: Type-2-diabetes-mellitus (DM) is one the most important cardiovascular-risk-factors. Among many molecules regulating vascular tone, nitric oxide appears to be the most pivotal. Although micro- and macrovascular-abnormalities are extensively studied, the alterations in the nitric-oxide-metabolic-pathway require further investigations. Additionally, the role of erythrocytes in the vascular tone regulation has not been extensively explored. The aim of this study was to evaluate the endothelial-function and the nitric-oxide-metabolic-pathway in erythrocytes and plasma of diabetic individuals. (2) Methods: A total of 80 subjects were enrolled in this cross-sectional study, including 35 patients with DM and 45 healthy individuals. The endothelial-function was evaluated in response to different stimuli. (3) Results: In the DM group, decreased Arginine and citrulline concentrations in the plasma compartment with reduced Arginine/ADMA and ADMA/DMA-ratios were observed. Preserved nitric-oxide-metabolism in erythrocytes with reduced citrulline level and significantly higher NO-bioavailability were noted. Significant endothelial dysfunction in DM individuals was proved in response to the heat-stimulus. (4) Conclusions: DM patients at an early stage of disease show significant differences in the nitric-oxide-metabolic-pathway, which are more pronounced in the plasma compartment. Erythrocytes constitute a buffer with a higher nitric-oxide-bioavailability, less affected by the DM-related deviations. Patients at an early-stage of DM reveal endothelial-dysfunction, which could be diagnosed earlier using the laser-Doppler-flowmetry.