Recent advances in diabetic retinopathy

Cardiovascular risk factors and collateral artery formation

Arterial lumen narrowing and vascular occlusion is the actual cause of morbidity and mortality in atherosclerotic disease. Collateral artery formation (arteriogenesis) refers to an active remodelling of non-functional vascular anastomoses to functional collateral arteries, capable to bypass the site of obstruction and preserve the tissue that is jeopardized by ischaemia. Hemodynamic forces such as shear stress and wall stress play a pivotal role in collateral artery formation, accompanied by the expression of various cytokines and invasion of circulating leucocytes. Arteriogenesis hence represents an important compensatory mechanism for atherosclerotic vessel occlusion. As arteriogenesis mostly occurs when lumen narrowing by atherosclerotic plaques takes place, presence of cardiovascular risk factors (e.g. hypertension, hypercholesterolaemia and diabetes) is highly likely. Risk factors for atherosclerotic disease affect collateral artery growth directly and indirectly by altering hemodynamic forces or influencing cellular function and proliferation. Adequate collateralization varies significantly among atherosclerotic patients, some profit from the presence of extensive collateral networks, whereas others do not. Cardiovascular risk factors could increase the risk of adverse cardiovascular events in certain patients because of the reduced protection through an alternative vascular network. Likewise, drugs primarily thought to control cardiovascular risk factors might contribute or counteract collateral artery growth. This review summarizes current knowledge on the influence of cardiovascular risk factors and the effects of cardiovascular medication on the development of collateral vessels in experimental and clinical studies.

Prevalence of retinopathy and proteinuria in type 1 diabetics in Iceland

The prevalence of retinopathy and proteinuria was assessed in 212 and 230 type 1 diabetics in Iceland respectively. They represent 78% and 84% of all such patients identified in the country. Retinopathy was present in 33.5%, background lesions only in 27.4% and more severe retinopathy in 6.1%. After 5-9 years of diabetes (mean 6.9) the prevalence of retinopathy was 18.8%, including 2.1% proliferative, and rose to 76.7% after 20 years or more of diabetes (mean 26.7), including 16.2% in the proliferative stage. Blindness was found in 2.4% of the patients. Diabetics diagnosed at the age of 0-19 years had a lower prevalence of retinopathy during their first 20 years of diabetes than those diagnosed later in life (p less than 0.05). Proteinuria was present in 14%, intermittent in 10% and continuous in 4%. After 10 years or more of diabetes (mean 19.3) the prevalence of intermittent and continuous proteinuria was 13.0% and 8.3%, respectively.

Influence of diabetes mellitus on coronary collateral flow: an answer to an old controversy

OBJECTIVES

To determine the influence of diabetes mellitus on coronary collateral flow by accurate means of collateral flow measurement in a large population with variable degrees of coronary artery disease.

METHODS

200 patients (mean (SD) age 64 (9) years; 100 diabetic and 100 non-diabetic) were enrolled in the study. Coronary collateral flow was assessed in 174 stenotic and in 26 angiographically normal vessels with a pressure guidewire (n = 131), Doppler guidewire (n = 36), or both (n = 33) to calculate pressure or flow velocity derived collateral flow index (CFI). Diabetic patients were perfectly matched with a non-diabetic control group for clinical, haemodynamic, and angiographic parameters.

RESULTS

CFI did not differ between the diabetic and the non-diabetic patients (0.21 (0.12) v 0.19 (0.13), not significant). Likewise, CFI did not differ when only angiographically normal vessels (0.20 (0.09) v 0.15 (0.08), not significant) or chronic total coronary occlusions (0.30 (0.14) v 0.30 (0.17), not significant) were compared. Fewer patients in the diabetic group tended to have angina pectoris during the one minute vessel occlusion (60 diabetic v 69 non-diabetic patients, p = 0.15).

CONCLUSION

Quantitatively measured coronary CFI did not differ between diabetic and non-diabetic patients with stable coronary artery disease.

Three-dimensional model of angiogenesis: coculture of human retinal cells with bovine aortic endothelial cells in the NASA bioreactor

Ocular angiogenesis is the leading cause of blindness and is associated with diabetic retinopathy and age-related macular degeneration. We describe, in this report, our preliminary studies using a horizontally rotating bioreactor (HRB), developed by the National Aeronautics and Space Administration (NASA), to explore growth and differentiation-associated events in the early phase of ocular angiogenesis. Human retinal (HRet) cells and bovine endothelial cells (ECs) were cocultured on laminin-coated Cytodex-3 microcarrier beads in an HRB for 1-36 days. Endothelial cells grown alone in the HRB remained cuboidal and were well differentiated. However, when HRet cells were cocultured with ECs, cordlike structures formed as early as 18-36 h and were positive for von Willebrand factor. In addition to the formation of cords and capillary-like structures, ECs showed the beginning of sprouts. The HRB seems not only to promote accelerated capillary formation, but also to enhance differentiation of retinal precursor cells. This leads to the formation of rosette-like structures (which may be aggregates of photoreceptors that were positive for rhodopsin). Upregulation of vascular endothelial growth factor and basic fibroblast growth factor was seen in retinal cells grown in the HRB as compared with monolayers and could be one of the factors responsible for accelerated capillary formation. Hence, the HRB promotes three-dimensional assembly and differentiation, possibly through promoting cell-to-cell interaction and/or secretion of growth and differentiation factors.

A Bayesian analysis of the 4-year follow-up data of the Wisconsin epidemiologic study of diabetic retinopathy

The Wisconsin epidemiologic study of diabetic retinopathy (WESDR) is a population-based epidemiologic study carried out in Southern Wisconsin during the 1980s of the last century. The resulting data were analysed by different statisticians and ophthalmologists during the last two decades. Most of the analyses were carried out on the baseline data, although there were two follow-up studies on the same population. In this present paper, we provide a Bayesian analysis of the first follow-up data which were taken 4 years after the baseline study. Our Bayesian analysis provides estimates of the associated covariate effects. Choice of the best model in terms of the covariate inclusion is also performed. The baseline data were used to set the prior for the parameters. Extensive numerical computations illustrate our present methodology.

Abnormal angiogenesis in diabetes mellitus

The adverse long-term effects of diabetes mellitus have been well described and involve many organ systems. While diabetes management has largely focused on control of hyperglycemia, the presence of abnormalities of angiogenesis may cause or contribute to many of the clinical manifestations of diabetes. When compared with non-diabetic subjects, diabetics demonstrate vascular abnormalities of the retina, kidneys, and fetus. Diabetics have impaired wound healing, increased risk of rejection of transplanted organs, and impaired formation of coronary collaterals. In each of these conditions, and possibly in diabetic neuropathy as well, abnormalities of angiogenesis can be implicated in the pathogenesis. A perplexing feature of the aberrant angiogenesis is that excessive and insufficient angiogenesis can occur in different organs in the same individual. In this review, the clinical features, molecular mechanisms, and potential therapeutic options of abnormal angiogenesis in diabetes will be reviewed.

Oligodeoxynucleotides inhibit retinal neovascularization in a murine model of proliferative retinopathy

Diseases characterized by retinal neovascularization are among the principal causes of visual loss worldwide. The hypoxia-stimulated expression of vascular endothelial growth factor (VEGF) has been implicated in the proliferation of new blood vessels. We have investigated the use of antisense phosphorothioate oligodeoxynucleotides against murine VEGF to inhibit retinal neovascularization and VEGF synthesis in a murine model of proliferative retinopathy. Intravitreal injections of two different antisense phosphorothioate oligodeoxynucleotides prior to the onset of proliferative retinopathy reduced new blood vessel growth a mean of 25 and 31% compared with controls. This inhibition was dependent on the concentration of antisense phosphorothioate oligodeoxynucleotides and resulted in a 40-66% reduction in the level of VEGF protein, as determined by Western blot analysis. Control (sense, nonspecific) phosphorothioate oligodeoxynucleotides did not cause a significant reduction in retinal neovascularization or VEGF protein levels. These data further establish a fundamental role for VEGF expression in ischemia-induced proliferative retinopathies and a potential therapeutic use for antisense phosphorothioate oligodeoxynucleotides.

A pilot study of chronic recombinant interferon-alfa 2a for diabetic proliferative retinopathy: metabolic effects and opthalmologic effects

The objective of this study was to evaluate the metabolic effects and opthalmologic effects of alpha-interferon therapy in diabetes mellitus patients with proliferative diabetic retinopathy (PDR). Three volunteer patients [insulin-dependent diabetes mellitus (IDDM), insulin requiring non-insulin-dependent diabetes mellitus (NIDDM), and maturity onset diabetes of the young (MODY)] threatened with blindness due to progressive PDR were treated with alpha interferon for 4 months and were evaluated at intervals of 1-2 weeks to monitor the drug effects on carbohydrate tolerance and possible beneficial therapeutic effects on the preexisting PDR. Metabolic studies included basal and postsustacal glucose, c-peptide and glucagon, fasting serum cortisol, free fatty acids, growth hormone, insulin-like growth factor-1, and urinary microalbumin excretion. Ophthalmologic studies included visual acuity, slit lamp examination, gonioscopy, fluorescein angiography, and standard colored fundus photographs. In all subjects, hyperglycemia worsened with duration of increasing dosage of interferon therapy, requiring progressively higher daily insulin requirements of 17%-68% above pretreatment values. Lowered levels of stimulated C-peptide were observed in the NIDDM and MODY subjects. The counterregulatory hormones (cortisol, growth hormone, and glucagon) were elevated during the 4 months of interferon therapy. In all subjects, visual acuity appeared to stabilize. No new retinal hemorrhages occurred during the 4 months of interferon administration, although all subjects experienced hemorrhage within 6 weeks of termination of the drug. Although only three subjects were investigated, the 1-2 week frequency of metabolic and opthalmologic studies permit some conclusions. The metabolic effects of alpha interferon in our diabetic subjects were consistent worsening of carbohydrate tolerance associated with impaired beta-cell secretion and increased insulin resistance. The extensive opthalmologic investigation suggested protection from retinal hemorrhage while receiving interferon, but further studies are indicated to validate these proposed and antiangiogenic properties.

Vascular endothelial growth factor in ocular fluid of patients with diabetic retinopathy and other retinal disorders

BACKGROUND

Retinal ischemia induces intraocular neovascularization, which often leads to glaucoma, vitreous hemorrhage, and retinal detachment, presumably by stimulating the release of angiogenic molecules. Vascular endothelial growth factor (VEGF) is an endothelial-cell-specific angiogenic factor whose production is increased by hypoxia.

METHODS

We measured the concentration of VEGF in 210 specimens of ocular fluid obtained from 164 patients undergoing intraocular surgery, using both radioimmuno-assays and radioreceptor assays. Vitreous proliferative potential was measured with in vitro assays of the growth of retinal endothelial cells and with VEGF-neutralizing antibody.

RESULTS

VEGF was detected in 69 of 136 ocular-fluid samples from patients with diabetic retinopathy, 29 of 38 samples from patients with neovascularization of the iris, and 3 of 4 samples from patients with ischemic occlusion of the central retinal vein, as compared with 2 of 31 samples from patients with no neovascular disorders (P < 0.001, P < 0.001, and P = 0.006, respectively). The mean (+/- SD) VEGF concentration in 70 samples of ocular fluid from patients with active proliferative diabetic retinopathy (3.6 +/- 6.3 ng per milliliter) was higher than that in 25 samples from patients with nonproliferative diabetic retinopathy (0.1 +/- 0.1 ng per milliliter, P = 0.008), 41 samples from patients with quiescent proliferative diabetic retinopathy (0.2 +/- 0.6 ng per milliliter, P < 0.001), or 31 samples from nondiabetic patients (0.1 +/- 0.2 ng per milliliter, P = 0.003). Concentrations of VEGF in vitreous fluid (8.8 +/- 9.9 ng per milliliter) were higher than those in aqueous fluid (5.6 +/- 8.6 ng per milliliter, P = 0.033) in all 10 pairs of samples obtained simultaneously from the same patient; VEGF concentrations in vitreous fluid declined after successful laser photocoagulation. VEGF stimulated the growth of retinal endothelial cells in vitro, as did vitreous fluid containing measurable VEGF. Stimulation was inhibited by VEGF-neutralizing antibodies.

CONCLUSIONS

Our data suggest that VEGF plays a major part in mediating active intraocular neovascularization in patients with ischemic retinal diseases, such as diabetic retinopathy and retinal-vein occlusion.

Increased capillary permeability to albumin in diabetic rat myocardium

To clarify the mechanism for the well-known increase in microvascular permeability that occurs with diabetes mellitus, we investigated capillary permeability to albumin in diabetic rat myocardium by electron microscopy using albumin-gold (Alb-Au) complexes as a tracer. Diabetes was induced by an intravenous injection of streptozotocin. After 24-32 weeks, hearts from diabetic rats and age-matched control rats were perfused with Krebs-Henseleit bicarbonate buffer containing Alb-Au for 5 or 20 minutes and then fixed and processed for electron microscopy. The binding and transport of Alb-Au by capillary endothelium was quantitatively evaluated. In control rats, Alb-Au particles were found preferentially bound to the luminal plasmalemmal vesicles. In diabetic rats, the labeling of luminal vesicles was more extensive and more pronounced after 5 minutes of perfusion when compared with control vesicles. The plasma membrane proper was also heavily labeled in diabetic rats. After 20 minutes, Alb-Au particles were transported across the capillary endothelium via plasmalemmal vesicles, but they did not penetrate the intercellular junctions in either control or diabetic rats. The vesicular transport of Alb-Au across the capillary endothelium was significantly increased in the diabetic myocardium when compared with control myocardium (percentage of abluminal labeled vesicles, 25.9 +/- 5.5% versus 1.3 +/- 0.5%; p < 0.01). The study on food-restricted rats with body weights close to those of diabetic rats suggested that caloric deficiency alone did not have much effect on capillary permeability. The data indicate that capillary permeability to albumin is markedly increased in diabetic myocardium because of enhanced vesicular transport. This may play an important role in the pathogenesis of diabetic cardiomyopathy.

Activation of protein kinase C by elevation of glucose concentration: proposal for a mechanism in the development of diabetic vascular complications

Hyperglycemia is believed to be the major cause of diabetic vascular complications involving both microvessels and arteries as in the retina, renal glomeruli, and aorta. It is unclear by which mechanism hyperglycemia is altering the metabolism and functions of vascular cells, although changes in nonenzymatic protein glycosylation and increases in cellular sorbitol levels have been postulated to be involved. Previously, we have reported that the elevation of extracellular glucose levels with cultured bovine retinal capillary endothelial cells causes an increase in protein kinase C (PKC) activity of the membranous pool with a parallel decrease in the cytosol without alteration of its total activity. Now we demonstrate that the mechanism for the activation of PKC is due to an enhanced de novo synthesis of diacylglycerol as indicated by a 2-fold increase of [14C]diacylglycerol labeling from [14C]glucose. The elevated diacylglycerol de novo synthesis is secondarily due to increased formation of precursors derived from glucose metabolism; this formation is enhanced by hyperglycemia as substantiated by elevated [3H]glucose conversion into water. This effect of hyperglycemia on PKC is also observed in cultured aortic smooth muscle and endothelial cells and the retina and kidney of diabetic rats, but not in the brain. Since PKC in vascular cells has been shown to modulate hormone receptor turnover, neovascularization in vitro, and cell growth, we propose that this mechanism of enhancing the membranous PKC activities by hyperglycemia plays an important role in the development of diabetic vascular complications.

Role of platelets in diabetic microangiopathy--an additional factor

A study of platelet aggregation in 324 diabetes mellitus (DM) patients and 24 patients with microvascular complications of DM (retinopathy) is reported. Platelet aggregation tests showed that DM patients can be divided into two groups: "normal" and "prone." The mean reading and standard deviation (SD) of the normal group was found to be 22.6% (SD 6.99%). The mean reading and standard deviation of the prone group was found to be 60.5% (SD 12.7%) respectively. All patients with microangiopathy had a significant increase, all of them being in the prone range, with a mean of 69.5% (SD 7.2%). Platelet hyperfunction appears to be a high-risk factor for the development of diabetic microangiopathies. A hypothesis postulating platelet hyperfunction as an additional factor along with altered glycoprotein metabolism and explaining the chain of events in the genesis of diabetic microangiopathies is presented.

Differential responsiveness to insulin of endothelial and support cells from micro- and macrovessels

The pathologies of diabetic micro- and macroangiopathy are different, suggesting that diabetes affects these two types of vascular tissue in a dissimilar manner. We have compared insulin receptors and the effects of insulin on cultured endothelium from calf retinal capillaries and aorta, and the vascular supporting cells, retinal pericytes, and aortic smooth muscle cells. 125I-insulin binds to high affinity insulin receptors on all four cell types. Receptor concentrations were similar except for aortic smooth muscle cells, which have 10-fold fewer receptors than the other cell types. Insulin at a concentration of 10 ng/ml stimulated [14C]glucose incorporation into glycogen in retinal endothelial cells and pericytes and aortic smooth muscle cells, but had no effect on aortic endothelium. Insulin over a concentration range of 10 ng/ml-10 microgram/ml, stimulated [3H]thymidine incorporation into the DNA of retinal pericytes, and endothelial cells and aortic smooth muscle cells but had no effect on aortic endothelial cells. These data suggested that a differential response to insulin may exist between endothelium of micro- and macrovasculature, and suggest that retinal capillary endothelium and retinal pericytes are both very insulin-sensitive tissues.