Vascular endothelial growth factor in diabetes induced early retinal abnormalities

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Abstract

Increased vascular permeability and blood flow alterations are characteristic features of diabetic retinal microangiopathy. The present study investigated vascular endothelial growth factor (VEGF) and its interactions with endothelin (ET) 1 and 3, endothelial, and inducible nitric oxide synthase (eNOS, iNOS) in mediating diabetes induced retinal vascular dysfunction. Male Sprague Dawley rats with streptozotocin (STZ) induced diabetes, with or without VEGF receptor signal inhibitor SU5416 treatment (high or low dose) were investigated after 4 weeks of follow-up. Colour Doppler ultrasound of the ophthalmic/central retinal artery, retinal tissue analysis with competitive RT-PCR and microvascular permeability were studied. Diabetes caused increased microvascular permeability along with increased VEGF mRNA expression. Increased vascular permeability was prevented by SU5416 treatment. Diabetic animals showed higher resistivity index (RI), indicative of vasoconstriction with increased ET-1 and ET-3 mRNA expression, whereas eNOS and iNOS mRNA expressions were un-affected. SU5416 treatment corrected increased RI via increased iNOS in spite of increased ET-1, ET-3 and VEGF mRNA expression. Cell culture (HUVEC) studies indicate that in part, an SU5416 induced iNOS upregulation may be mediated though a MAP kinase signalling pathway. The present data suggest VEGF is important in mediating both vasoconstriction and permeability in the retina in early diabetes.

Introduction

The major target of diabetic retinopathy is the retinal microvasculature. The integrity of capillary endothelial cells is crucial to maintain homeostasis of the surrounding retinal tissue [1]. Endothelial cells produce and are responsive to the autocrine and paracrine activities of several vasoactive molecules like vascular endothelial growth factor (VEGF), endothelin (ET) and nitric oxide (NO) [2].

Diabetes increases the expression of VEGF secondary to protein kinase C (PKC) activation [3]. The VEGF protein family is comprised of several members including VEGF A, B, C, D, E and placental growth factor [4]. VEGF mediates its activities through interactions with receptor tyrosine kinase proteins; VEGFR1 (Flt-1), VEGFR2 (KDR), or VEGFR3 (Flt-4). A recently discovered co-receptor, neuropilin has been demonstrated to associate with VEGFR2 [5], [6]. VEGF is an important factor promoting angiogenesis during proliferative diabetic retinopathy [7]. However, since its discovery, VEGF has demonstrated the ability to increase vascular tissue permeability in non-diabetic conditions [8].

Reduced microvascular blood flow and increased vascular permeability are two early characteristic abnormalities of diabetic microangiopathy [9]. There are reports outlining a possible pathogenetic role of VEGF activation in early diabetes, where VEGF may play a role in the breakdown of the blood retinal barrier [10], [11]. Recently, it has been demonstrated that VEGF neutralising antibody treatment is capable of preventing the diabetes induced increased permeability [12]. VEGF also reacts extensively with other vasoactive factors. We have reported how an up-regulation of ET-1 and ET-3 mRNA levels in response to short term diabetes, contributed to reduced blood flow in the retina [13]. We have further demonstrated in human umbilical vein endothelial cells both ET-1 and VEGF may be responsible for the production of glucose induced, increased endothelial permeability [14]. Furthermore, a co-stimulatory relationship between glucose induced ET and VEGF may exist [15]. In addition, VEGF effector pathways involve increased nitric oxide synthase (NOS) mRNA expression and NO production [16]. NO has been demonstrated to cause a down-regulation of VEGF [17], [18]. Interestingly, an up-regulation of ET can lead to a down-regulation of NO [19], [20]. Hence, an intricate relationship may exist among these factors.

In order to delineate the pathogenetic mechanisms in early diabetic microangiopathy, the present study investigated the role of VEGF and its interactions with other vasoactive factors in the pathogenesis of increased microvascular permeability in the retina of the streptozotocin (STZ) diabetic rat. We investigated VEGF alterations in mediating retinal blood flow changes in short term diabetes. Finally, we have examined the role of the MAP kinase signalling pathway in inducible nitric oxide synthase (iNOS) upregulation.

Section snippets

Animals

All animals were cared for under the conditions and rules designated by the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research with approval by the University of Western Ontario Animal Care and Ethics Committee. Male Sprague Dawley rats of approximately 200 g received a single intravenous injection of streptozotocin (65 mg/kg in citrate buffer, pH 6.5). Control animals received an equivalent injection of citrate buffer. After confirmation of diabetes (blood glucose > 20 mmol/l

Clinical monitoring

All diabetic animals had higher blood glucose levels, higher glycated haemoglobin levels and reduced body weight gain compared to age matched, non-diabetic control animals. Treatment with SU5416 had no effect in modifying blood glucose and body weight changes [Table 2]. Twenty four hours post injection plasma levels of SU5416 high dose group was 29.86±2.75 ng ml−1, which is within the therapeutic range. SU5416 levels in the low dose group was determined to be 9.48±2.75 ng ml−1.

Permeability alterations in the retina

In non-diabetic

Discussion

In the present study we demonstrated the important role of VEGF by itself and its interaction with other vasoactive substances in the pathogenesis of diabetes induced early functional changes in the retina. Increased vascular permeability and microvascular blood flow abnormalities are characteristic features of diabetic retinopathy [1], [2]. VEGF is a potent factor causing increased vascular permeability. Increases in vascular permeability have been demonstrated in STZ diabetic rats after 1

Acknowledgements

This work was supported in part by grants from the Canadian Diabetes Association in memory of Glenn W. Liebrock as well as the Lawson Health Research Institute Internal Research Fund. The authors wish to thank K. Mukherjee for the histological preparations and immunostaining.

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