Roles of endothelin-1 and selected proinflammatory cytokines in the pathogenesis of proliferative diabetic retinopathy: Analysis of vitreous samples

Abstract

Purpose

To investigate the roles of endothelin-1(ET-1), TNF-α, IL-6 in the pathogenesis of proliferative diabetic retinopathy (PDR) in type 2 diabetes.

Methods

Vitreous and blood serum samples were collected during vitrectomy from 19 patients with PDR and 15 patients who underwent vitrectomy for other reasons. The concentrations of ET-1, TNF-α, IL-6, vWF, sE-selectin were determined by ELISA.

Results

Intraocular and serous concentrations of ET-1, TNF-α, IL-6, vWF, sE-selectin were higher in patients with PDR than in the control group. The vitreous ET-1/plasma ET-1 ratios the group of diabetic patients and in the control group were similar. Also TNF-α, IL-6 vitreous/plasma ratio were not statistically different between the analysed groups. Correlation between intraocular ET-1 and TNF-α concentrations in patients with PDR and between the increases in both factors in the vitreous and HbA₁c concentration were shown. In the vitreous the increase in vWF depended on elevated levels of vWF in the serum. E-selectin concentration correlated with diastolic blood pressure.

Conclusion

These data provide evidence of the activation of the local synthesis of ET-1, TNF-α, IL-6 in PDR. The relationship between the increase in vitreous ET-1, TNF-α concentrations and HbA₁c concentration is a important confirmation of the necessity to optimise diabetes treatment.

Introduction

Various mechanism have been proposed to explain how hyperglycaemia and components of metabolic syndrome can affect endothelial function in diabetes [1], [2], [3], [4]. An increase in intracellular glucose will lead to an increase in the flux of glucose to sorbitol via the polyol pathway, an increase in glucosamine-6-phosphativie through the hexosamine pathway, and the activation of PKC (protein kinase C) via de novo synthesis of DAG (diacyloglicerol). In addition, glucose and glucose-derived dicarbonyl compounds react non-enzymatically with the basic amino acids lysine and arginine in proteins to form AGEs. These different pathways are interrelated and potentiate each other [5], [6]. These disturbances are undoubtedly involved in the pathogenesis of diabetic retinopathy. The DAG/PKC pathway determines blood flow dysregulation by decreasing endothelial NOS activity and/or increasing the synthesis of ET-1, one of the strongest vasoconstrictive factors [7]. In diabetic animals, an oral PKC-β inhibitor prevented diabetes-induced abnormalities in the mRNA expressions of TGF-β₁, type IV collagen, and fibronectin [8].

It is necessary to emphasize the fact that ET-1 acts on different vascular areas with different strength [9]. Therefore, various vessels’ response to the activity of this factor may result from different concentrations of receptors on the vessel wall [10]. Observations indicate that the participation of endothelin in coagulation disorders is also essential for the development of PDR (proliferative diabetic retinopathy). Authors [11] point out the fact that thrombosis in the rat microcirculation and a DIC-like process in the rabbit circulation develop under the influence of ET-1. An important element in the development of this disturbance is the documented MAPK (mitogen-activated protein kinase)-dependent ET-1 production [12].

Non-enzymatic glycation of proteins can interfere with endothelial functions in several ways [13], [14]. It is a prominent activation pathway of proinflammatory cytokine synthesis and of growth factors such as IL-1, IL-6, TNF-α, VEGF, and TGF-β [15], [16], [17]. Inflammatory cytokines increase vascular permeability, alter vasoregulatory responses, increase leukocyte adhesion to endothelium, and facilitate thrombus formation by inducing pro-coagulant activity and inhibiting anti-coagulant pathways [18]. Transcription factor NF-kB, the main pathway for the cytokine regulation of gene expression in endothelial cells, is activated not only by TNF-α, IL-1, and IL-6, but also by hyperglycaemia, AGEs, oxidized lipids, and insulin [5].

The aim of this analysis of patients with clinical proven PDR was to establish the role of the inflammatory-proliferative process of the endothelium in the pathogenesis of this diabetic complication on the basis of ET-1, TNF-α, and IL-6 concentration measurement in the vitreous body obtained intraoperatively. The obtained data were analysed in the context of endothelium damage (vWF, sE-selectin) and the clinical course of type 2 diabetes.