Rakesh Chibber

Pathophysiology of diabetic retinopathy.

Diabetes is now regarded as an epidemic, with the population of patients expected to rise to 380 million by 2025. Tragically, this will lead to approximately 4 million people around the world losing their sight from diabetic retinopathy, the leading cause of blindness in patients aged 20 to 74 years. The risk of development and progression of diabetic retinopathy is closely associated with the type and duration of diabetes, blood glucose, blood pressure, and possibly lipids. Although landmark cross-sectional studies have confirmed the strong relationship between chronic hyperglycaemia and the development and progression of diabetic retinopathy, the underlying mechanism of how hyperglycaemia causes retinal microvascular damage remains unclear. Continued research worldwide has focussed on understanding the pathogenic mechanisms with the ultimate goal to prevent DR. The aim of this paper is to introduce the multiple interconnecting biochemical pathways that have been proposed and tested as key contributors in the development of DR, namely, increased polyol pathway, activation of protein kinase C (PKC), increased expression of growth factors such as vascular endothelial growth factor (VEGF) and insulin-like growth factor-1 (IGF-1), haemodynamic changes, accelerated formation of advanced glycation endproducts (AGEs), oxidative stress, activation of the renin-angiotensin-aldosterone system (RAAS), and subclinical inflammation and capillary occlusion. New pharmacological therapies based on some of these underlying pathogenic mechanisms are also discussed.

Impairment of vascular endothelial nitric oxide synthase activity by advanced glycation end products

Endothelial damage is believed to play a key role in the development of both micro‐ and macrovascular disease in diabetes, and advanced glycation end products (AGEs) may contribute importantly to this. To determine whether glucose‐derived AGEs can cause endothelial dysfunction, we examined the effects of albumin AGE‐modified by glucose (AGE‐Glu) both in vivo, after injection into rabbit femoral artery, and in vitro on rabbit aortic rings and cultured human umbilical vein endothelial cells (HUVEC). Exposure of blood vessels to AGE‐Glu, in vivo and in vitro, inhibited endothelium‐dependent vasorelaxation, whereas unmodified albumin did not. In isolated rabbit aorta, this effect was reversible after AGE‐Glu washout, and the response to the endothelium‐independent vasodilator sodium nitroprusside was unaffected by AGE‐Glu. In HUVEC, AGE‐Glu inhibited endothelial nitric oxide synthase activity, and this was associated with a decrease in serine phosphorylation of this enzyme. Longer term (72 h) incubation decreased HUVEC viability. Use of specific antibodies demonstrated that these effects were mediated by Nε‐(carboxymethyl)lysine (CML), an important AGE found in vivo, and by the AGE‐R1 receptor. Furthermore, these effects all occurred at CML concentrations similar to those found in the plasma of diabetic patients. These results suggest an important role of AGE in the pathogenesis of diabetic vasculopathy.

Glycated Proteins Stimulate Reactive Oxygen Species Production in Cardiac Myocytes Involvement of Nox2 (gp91phox)-Containing NADPH Oxidase

Background— Nonenzymatic glycation that results in the production of early-glycation Amadori-modified proteins and advanced-glycation end products may be important in the pathogenesis of diabetic complications. However, the effects of early-glycated proteins, such as glycated serum albumin (Gly-BSA), are poorly defined. In this study, we investigated the effects of Gly-BSA on reactive oxygen species (ROS) production by cardiomyocytes. Methods and Results— Cultured neonatal rat cardiomyocytes were incubated with Gly-BSA or vehicle (bovine serum albumin [BSA]) for up to 48 hours. Gly-BSA dose-dependently increased in situ ROS production (whole-cell dichlorodihydrofluorescein fluorescence), with an optimum effect at 400 &mgr;g/mL after 24-hour incubation (152±10% versus BSA 100%; P<0.01). Treatment with the NADPH oxidase inhibitor apocynin, a Nox2 (gp91phox) antisense oligonucleotide (Nox2 AS), or the peptide gp91ds-tat significantly reduced Gly-BSA–induced ROS production at 24 hours (68.5±2.2%, 61.4±8.3%, and 53.2±5.4% reduction, respectively). NADPH-dependent activity in cell homogenates was also significantly increased by Gly-BSA at 24 hours (161±8% versus BSA) and was inhibited by diphenyleneiodonium, apocynin, NOX2AS, and the protein kinase C inhibitor bisindolylmaleimide I but not by a nitric oxide synthase inhibitor or mitochondrial inhibitors. Furthermore, bisindolylmaleimide I prevented Gly-BSA–stimulated Rac1 translocation, an essential step for NADPH oxidase activation. Gly-BSA–induced increases in ROS were associated with apocynin-inhibitable nuclear translocation of nuclear factor-&kgr;B and an increase in atrial natriuretic factor mRNA expression. Conclusions— Gly-BSA stimulates cardiomyocyte ROS production through a protein kinase C–dependent activation of a Nox2-containing NADPH oxidase, which results in nuclear factor-&kgr;B activation and upregulation of atrial natriuretic factor mRNA. These findings suggest that early-glycated Amadori products may play a role in the development of diabetic heart disease.

Leukocytes in diabetic retinopathy.

Diabetic retinopathy is one of the most common diabetic complications, and is a major cause of new blindness in the working-age population of developed countries. Progression of vascular abnormalities, including the selective loss of pericytes, formation of acellular capillaries, thickening of the basement membrane, and increased vascular permeability characterizes early nonproliferative diabetic retinopathy (NPDR). Capillary occlusion, as shown on fluorescein angiograms, is also one of the earliest clinically recognizable lesion of NPDR. In response to capillary non-perfusion, there is dilation of neighbouring capillaries, leading to early blood-retinal barrier breakdown, capillary non-perfusion, and endothelial cell injury and death. The resulting ischemia leads to increased production of growth factors, and the development of proliferative diabetic retinopathy (PDR), which is characterized by growth of new vessels and potential severe and irreversible visual loss. The exact pathogenic mechanism by which capillary non-perfusion occurs is still unclear but growing evidence now suggests that increased leukocyte-endothelial cell adhesion and entrapment (retinal leukostasis) in retinal capillaries is an early event associated with areas of vascular non-perfusion and the development of diabetic retinopathy. The leukocytes in diabetic patients are less deformable more activated, and demonstrate increased adhesion to the vascular endothelium. This review summarizes the current literature on the role of leukocytes in the pathogenesis of capillary occlusion, and discusses the potential of leukostasis as a new promising target in the treatment of diabetic retinopathy.

Is inflammation a common retinal-renal-nerve pathogenic link in diabetes?

The global diabetes burden is predicted to rise to 380 million by 2025 and would present itself as a major health challenge. However, both Type 1 and Type 2 diabetes increase the risk of developing micro-vascular complications and macro-vascular complications which in turn will have a devastating impact on quality of life of the patients and challenge health services Worldwide. The micro-vascular complications that affect small blood vessels are the leading cause of blindness (diabetic retinopathy) in the people of the working-age, end-stage renal disease (diabetic nephropathy) the most common cause of kidney failure today, and foot amputation (diabetic neuropathy) in patients with Type 1 and Type 2 diabetes. It is accepted that hyperglycemia is a major causative factor for the development of these complications, there is also growing evidence for the role of inflammation. Here we discuss low-grade inflammation as a common retinal-renal-nerve pathogenic link in patients with Type 1 and Type 2 diabetes. This review summarizes evidence showing a link between circulating and locally produced inflammatory biomarkers, such as cell adhesion molecules (vascular adhesion cell molecule-1, VCAM-1; intracellular adhesion molecule-1, ICAM-1), pro-inflammatory cytokines (interleukin-6, IL-6; tumour necrosis factor-alpha, TNF-α; C-reactive protein, CRP) with the development and progression of diabetic micro-vascular complications.

INTRODUCTION TO DIABETES MELLITUS

The chronic metabolic disorder diabetes mellitus is a fast-growing global problem with huge social, health, and economic consequences. It is estimated that in 2010 there were globally 285 million people (approximately 6.4% of the adult population) suffering from this disease. This number is estimated to increase to 430 million in the absence of better control or cure. An ageing population and obesity are two main reasons for the increase. Furthermore it has been shown that almost 50% of the putative diabetics are not diagnosed until 10 years after onset of the disease, hence the real prevalence of global diabetes must be astronomically high. This chapter introduces the types of diabetes and diabetic complications such as impairment of immune system, periodontal disease, retinopathy, nephropathy, somatic and autonomic neuropathy, cardiovascular diseases and diabetic foot. Also included are the current management and treatments, and emerging therapies.

The Effect of Aminoguanidine and Tolrestat on Glucose Toxicity in Bovine Retinal Capillary Pericytes

Cultured bovine retinal capillary pericytes (BRP) were used to investigate the effect of an aldose reductase inhibitor, tolrestat, and an inhibitor of advanced glycation end products (AGE) formation, aminoguanidine, on glucose toxicity. Glucose at high concentration reduced the replicative activity of pericytes in a dose-dependent manner. Tolrestat completely inhibited the production of sorbitol in cells exposed to a high concentration of glucose but failed to protect the cells from glucose toxicity. These results suggest that sorbitol accumulation in cells is probably not the major mechanism for glucose toxicity. In contrast, the addition of aminoguanidine at 10 mM concentration to the culture media protected pericytes from glucose toxicity. The degree of protected pericytes from glucose toxicity. The degree of protection was dose-dependent and evident at aminoguanidine concentration as low as 1 mM. The drug was only slightly toxic to BRP but induced morphological changes in pericytes with the loss of cellular processes and decreased cell spreading. This may suggest some action of aminoguanidine on the pericyte cytoskeleton. High concentration of glucose significantly increased the level of early glycation but not fluorescent AGE formation on BRP proteins. This was inhibited by the addition of aminoguanidine suggesting that glycation of cellular/membrane proteins and other mechanisms may play an important role in the toxic action of high glucose levels in cultured pericytes.

Clinical validation of a link between TNF-α and the glycosylation enzyme core 2 GlcNAc-T and the relationship of this link to diabetic retinopathy

Aims/hypothesisIncreasing evidence suggests that chronic, subclinical inflammation plays an important role in the pathogenesis of diabetic retinopathy. We recently reported that a glycosylating enzyme, core 2 β-1,6-N-acetylglucosaminyltransferase (core 2 GlcNAc-T), is implicated in increased leucocyte–endothelial cell adhesion in diabetic retinopathy via an upregulation mechanism controlled by TNF-α.Subjects, materials and methodsWe examined the functional link between circulating TNF-α and the activity and phosphorylation of core 2 GlcNAc-T in polymorphonuclear leucocytes of patients with type 1 and type 2 diabetes.ResultsPlasma levels of TNF-α, although similar in patients with type 1 and type 2 diabetes, were significantly higher than in age-matched healthy controls, and correlated well with the severity of retinopathy. Core 2 GlcNAc-T activity followed the same trend and was associated with phosphorylation of the enzyme. Finally, the observation that TNF-α levels are also linked to glycaemic values suggests that in patients, as well as in vitro, the glycosylation-mediated cell adhesion process that plays a role in diabetic retinopathy may involve glucose- and TNF-α-induced protein kinase β2 activation, and subsequently raise activity of core 2 GlcNAc-T through increased enzyme phosphorylation.Conclusions/interpretationOur results reveal a novel rationale towards a specific treatment of diabetic retinopathy, based on the inhibition of core 2 GlcNAc-T activity and/or the blockage of cognate glycans.

Diabetic Retinopathy and Atherosclerosis: is there a Link?

Diabetic retinopathy (DR) is the leading cause of blindness amongst the working-age population, and diabetes accelerated cardiovascular disease (CVD) the commonest cause of death in diabetic patients. Although, there is evidence suggesting a close association between DR and CVD, particularly in patients with Type 2 diabetes, the pathophysiology underlying the link is unclear. Here we review common risk factors and pathogenic mechanisms linking DR and CVD, and aim to highlight the need for a more holistic view of the management of diabetes and its complications. The understanding of the link between the two complications could eventually lead to refined management strategies and improved patient outcomes in the expanding diabetes epidemic.