Role of Asymmetrical Dimethylarginine in Diabetic Microvascular Complications :

In Diabetes, the chronic hyperglycemia and associated complications affecting peripheral nerves are one of the most commonly occurring microvascular complications with an overall prevalence of 50–60%. Among the vascular complications of diabetes, diabetic neuropathy is the most painful and disabling, fatal complication affecting the quality of life in patients. Several theories of etiologies surfaced down the lane, amongst which the oxidative stress mediated damage in neurons and surrounding glial cell has gained attention as one of the vital mechanisms in the pathogenesis of neuropathy. Mitochondria induced ROS and other oxidants are responsible for altering the balance between oxidants and innate antioxidant defence of the body. Oxidative-nitrosative stress not only activates the major pathways namely, polyol pathway flux, advanced glycation end products formation, activation of protein kinase C, and overactivity of the hexosamine pathway, but also initiates and amplifies neuroinflammation. The cross talk between oxidative stress and inflammation is due to the activation of NF- κ B and AP-1 and inhibition of Nrf2, peroxynitrite mediate endothelial dysfunction, altered NO levels, and macrophage migration. These all culminate in the production of proinflammatory cytokines which are responsible for nerve tissue damage and debilitating neuropathies. This review focuses on the relationship between oxidative stress and neuroinflammation in the development and progression of diabetic neuropathy.

Diabetic distal symmetric sensorimotor polyneuropathy (DSPN) represents a major health problem, associated with excruciating neuropathic pain, increased morbidity and impaired quality of life. The understanding of its epidemiology is difficult due to methodological issues. Inconsistency in the selection of diagnostic procedures renders comparison between studies problematic. Further problems arise from selection bias due to the inclusion of hospital-based populations. DSPN affects approximately 30% of hospital-based populations, 20% of community-based samples, and 10% of the diabetic population identified by screening. Chronic painful DSPN is present in 13-26% of diabetic patients. Between 25% and 62% of patients with idiopathic peripheral neuropathy have prediabetes. Among pre-diabetic subjects, 11-25% exhibit peripheral neuropathy and 13-26% neuropathic pain. Evidence from population-based studies indicates that there is a gradient in the prevalence of neuropathy. Indeed, the highest frequency is found in patients with manifest diabetes mellitus, followed by individuals with impaired glucose tolerance, then impaired fasting glucose and, finally, those with normoglycemia. The most important etiologic factors are poor glycemic control, age, diabetes duration, visceral obesity, height, hypertension, age, smoking, hypoinsulinemia, and dyslipidemia. Clinic-based data suggest that DSPN is associated with increased mortality in diabetes, but confirmatory prospective population-based studies are required.

Advanced glycation end products (AGEs) and their receptor (RAGE) play a role in diabetic nephropathy. Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, contributes to diabetic nephropathy. We have found that glucagon-like peptide-1 (GLP-1) inhibits the AGE-induced inflammatory reactions in endothelial cells. However, effects of GLP-1 on the AGE-RAGE-ADMA axis are unknown. This study examined the effects of GLP-1 on reactive oxygen species (ROS) generation, gene expression of protein arginine methyltransfetase-1 (PRMT-1), an enzyme that mainly generates ADMA, and ADMA levels in human proximal tubular cells. Streptozotocin-induced diabetic rats received continuous i.p. infusion of 0.3 μg of vehicle or 1.5 μg of the GLP-1 analog exendin-4 per kilogram of body weight for 2 weeks. We further investigated whether and how exendin-4 treatment reduced ADMA levels and renal damage in streptozotocin-induced diabetic rats. GLP-1 inhibited the AGE-induced RAGE and PRMT-1 gene expression, ROS, and ADMA generation in tubular cells, which were blocked by small-interfering RNAs raised against GLP-1 receptor. Exendin-4 treatment decreased gene expression of Rage, Prmt-1, Icam-1, and Mcp-1 and ADMA level; reduced urinary excretions of 8-hydroxy-2'-deoxyguanosine and albumin; and improved histopathologic changes of the kidney in diabetic rats. Our present study suggests that GLP-1 receptor agonist may inhibit the AGE-RAGE-mediated ADMA generation by suppressing PRMT-1 expression via inhibition of ROS generation, thereby protecting against the development and progression of diabetic nephropathy. Copyright © 2013 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.