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Effects of Environmental, Genetic, and Epigenetic Factors on Platelet Glycoproteins and the Development of Diabetic Retinopathy
C H A P T E R
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Effects of Environmental, Genetic, and Epigenetic Factors on Platelet Glycoproteins and the Development of Diabetic Retinopathy Daniel Petrovič Institute of Histology and Embryology, Medical Faculty Ljubljana, University of Ljubljana, Ljubljana, Slovenia
INTRODUCTION Changes in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes, obesity, and related vascular complications.1,2 Both type 1 and type 2 diabetes are associated with hyperglycemia, oxidant stress, and inflammation and a significantly increased risk of macrovascular and microvascular complications (Table 54.1).1,2 Diabetic retinopathy (DR) is associated with both environmental and genetic factors. Several m etabolic abnormalities are implicated in its pathogenesis; however, the exact mechanism remains to be d etermined. While several studies have been devoted to the evaluation of genetic factors related to diabetes and its complications (including DR), much less is known about environmental factors, nutrition, and epigenetic mechanisms related to DR.3–5 So far, several genes and their polymorphisms have been implicated in the pathogenesis (Table 54.2).5–7
PATHOGENESIS OF DIABETIC RETINOPATHY Epidemiologic and experimental studies indicate that besides the duration of diabetes and glycemic control, several other factors are involved in the development of DR (Table 54.3).8,9 DR is characterized by increased vascular permeability, hemostatic abnormalities, increased tissue ischemia, and neoangiogenesis.2 The pathogenetic mechanisms
Handbook of Nutrition, Diet, and the Eye http://dx.doi.org/10.1016/B978-0-12-401717-7.00054-X
of DR are very complex. Although many hyperglycemia-induced metabolic abnormalities are implicated in its pathogenesis, the exact mechanism of the development of retinopathy remains elusive. Alteration in retinal blood flow, metabolic changes, hemostatic abnormalities, increased oxidative stress, increased polyol pathway flux, activation of protein kinase C isoforms, increased h exosamine pathway flux, and increased advanced g lycation end-product formation, nonenzymatic glycosylation of collagen, and other tissue proteins are observed during long-term hyperglycemia.2,9 It is also important to know that many of the systemic abnormalities in both type 1 and type 2 diabetic patients are prothrombotic: hyperreactive platelets, decreased vascular prostacyclin p roduction, endothelial dysfunction resulting in increased circulating levels of von Willebrand factor and leukocyte adhesion molecules, hypercoagulability, and decreased fibrinolysis. Increased levels of intercellular adhesion molecule-1 and plasminogen activator inhibitor-1 messenger RNA (mRNA), as well as decreased levels of tissue plasminogen activator, are specifically found in retinal vessels of diabetic compared with nondiabetic individuals.10,11
TABLE 54.1 Chronic Complications of Diabetes Microvascular
Macrovascular
Retinopathy
Coronary heart disease
Nephropathy
Cerebrovascular disease
Neuropathy
Peripheral arterial obstructive disease
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© 2014 Elsevier Inc. All rights reserved.
536
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TABLE 54.2 Pathways and Genes Implicated in the Pathogenesis of Diabetic Retinopathy Pathway/System
Gene
Polyol pathway
Aldose reductase
Renin–angiotensin system
Renin Angiotensinogen Angiotensin-1 converting enzyme Aldosterone
TABLE 54.3 Risk Factors for Diabetic Retinopathy Modifiable
Nonmodifiable
Cigarette smoking
Genetic factors
Blood pressure
Duration of diabetes
Plasma cholesterol
Age of onset of diabetes
Fasting glycemia
Positive family history
HbA1c Proteinuria
Advanced glycation endproducts
Receptor for advanced glycation end-products
Growth factors
Vascular endothelial growth factor
Waist-to-hip ratio
Basic fibroblast growth factor
Vitamin B12 deficiency
Insulin-like growth factor Peroxisome proliferatoractivated receptor
Peroxisome proliferator-activated receptor Coactivator for peroxisome proliferator-activated receptor
Inflammatory genes
Interleukins Tumor necrosis factor
Thrombotic system
Fibrinogen
Platelet function
Integrin
Oxidative system
Manganese superoxide dismutase Catalase Myeloperoxidase Glutathione S-transferase NADPH oxidase Endothelial nitric oxide synthase Inducible nitric oxide synthase
Adhesion molecules
Intercellular adhesion molecule Vascular cell adhesion protein Platelet endothelial cell adhesion molecule
Extracellular matrix homeostasis
Matrix homeostasis genes Matrix metalloproteinase
Hormones/vitamins
Growth hormone Vitamin D
Undefined
Glucose transporter-1 Growth hormone
NADPH: reduced nicotinamide adenine dinucleotide.
Body mass index
Insulin treatment
HbA1c: glycosylated hemoglobin.
PLATELETS AND DIABETIC RETINOPATHY Platelets are thought to be involved in the p athogenesis of DR.12 Platelets from diabetic patients may interact with exposed subendothelial fibrillar collagens, major components of the subendothelial matrix. Moreover, the amount of nonenzymatically glycosylated collagen, which is prone to interact with platelets, was demonstrated to be higher in diabetic patients than in nondiabetic controls.10 Platelets activated by contact with collagens can trigger thrombus formation and small vessel occlusion. Platelets from diabetic patients are hyperreactive to aggregating agents, such as c ollagen, thrombin, and adenosine diphosphate (ADP).12 The platelet membrane glycoprotein Ia/IIa, α2β1 integrin, serves as a platelet receptor for collagen.13 Some genetic variations of the glycoprotein involved in platelet adhesion, aggregation, and activation in favor of thrombogenesis may be considered as risk factors for thrombotic events.13
C825T POLYMORPHISM OF THE G-PROTEIN-COUPLED RECEPTOR GENE Platelet aggregation that varies among individuals and genetic factors may alter platelet activation through G-protein-coupled receptors.14 Recently, Dusse and co-workers demonstrated that the C825T polymorphism of the gene GNB3 encoding the G-protein β3 subunit influences platelet aggregation. In human whole blood, the GNB3 825CC genotype has been reported to be associated with enhanced platelet aggregation.14 The study evaluating the effect of this polymorphism related to DR development has not been reported yet.
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Genetic Factors and Glycoprotein IIb-IIIa
GENETIC FACTORS AND α2β1 INTEGRIN Genetic variations in α2β1 integrin were reported to affect the density of α2β1 receptors on the platelet surface.10 Namely, an association between genetic variations in α2β1 and the density of α2β1 receptors on the platelet surface was reported.10 Kunicki et al.10 demonstrated that the density of α2β1 receptors on the platelet surface affects platelet adhesion to collagen, contributing to an increased risk of thrombosis.10 Matsubara et al.15 demonstrated for the first time in 2000 the association between genetic variations in α2β1 integrin and DR in the Japanese population. This finding was confirmed a few years later in a Caucasian population by Petrovic and co-workers.7 In both studies, the BglII (+/+) genotype of the gene polymorphism of the α2β1 integrin gene was reported to be an independent risk factor for DR in Caucasians with type 2 diabetes (Table 54.4). Moreover, Matsubara et al.15 demonstrated an association between genetic variations in α2β1 integrin and another microvascular complication of diabetes, diabetic nephropathy, whereas Tsai et al.16 failed to find such an association with diabetic nephropathy in a Chinese population with type 2 diabetes.
GENETIC FACTORS AND GLYCOPROTEIN IIB-IIIA Glycoprotein (GP) IIb-IIIa (αIIbβ3-integrin), also known as integrin β3 or antigen CD61, is the central receptor of platelet aggregation. GPIIIa is a surface protein found in various tissues, participating in cell adhesion and cell-surface mediated signaling. It builds glycoprotein IIb/IIIa in the platelet membrane and constitutes a fibrinogen receptor, which also exhibits an ability to bind the von Willebrand factor, fibronectin, and thrombospondin.17 Activated GPIIb-IIIa binds fibrinogen or von Willebrand factor, which forms molecular bridges between aggregating platelets. GPIIb-IIIa is a calcium-dependent heterodimer that is expressed in megakaryocytes, platelets, and mast cells.18
On the surface of resting platelets, GPIIb-IIIa is exhibited in a low-affinity conformation, in which the ligand-binding site is not exposed. Following the activation of platelets by agonists such as ADP, thrombin, or collagen, inside–out 3 signaling occurs, giving rise to the exposure of the ligand binding site of GPIIb-IIIa.19 The binding of fibrinogen to the active form of GPIIb-IIIa is the final step leading to platelet aggregation and the formation of thrombus.19,20 Both GPIIb and GPIIIa are known to bear a number of single amino acid substitutions affecting conformational changes and the ligand binding function with little or no effect on platelet function. A platelet-specific antigen (PlA1/A2) polymorphism has been by far the most investigated GPIIIa gene polymorphism. A substitution of cytosine for thymidine at position 1565 in exon 2 of the GPIIIa gene leads to an amino acid difference at position 33: a leucine (A1 allele) or a proline (A2 allele).17 The mentioned polymorphism can influence both platelet activation and aggregation21 and affect postoccupancy signaling by the platelet fibrinogen receptor IIb/IIIa.22 The presence of one or two PlA2 alleles is associated with an increased binding affinity to fibrinogen as well as with platelet aggregability in response to epinephrine, ADP, and collagen in vitro.23 It has also been suggested that the PlA2 allelic variant causes an increased sensitivity to platelet aggregation by various agonists and an altered sensitivity to aspirin24–26 (Table 54.5). Platelet receptor polymorphism has also been implicated in the pathogenesis of glucose metabolism. It has been suggested that the function of platelet GPIIIa is modulated by a cysteine protease, calpain 10, which is not characteristic of platelets. Calpain 10 was reported to influence glucose metabolism, insulin secretion, and insulin action.27 Studies on the association of the PlA1/ A2 polymorphism of glycoprotein IIIa and type 2 diabetes have reported conflicting results17,28,29 (Table 54.6). Although there have been many studies on this polymorphism, conflicting results on its association with stroke, coronary artery disease, and myocardial infarction have been reported in a number of case-controlled clinical studies30–38 (Table 54.7). A meta-analysis of 12 epidemiologic studies showed that there was an association between the PlA2 variant and an increased risk of
TABLE 54.4 Reported Studies of Polymorphism of α2β1 Integrin and Diabetic Retinopathy (DR) in Subjects with Type 2 Diabetes Population
Cases with DR* (n)
Control Group† (n)
Bgl II (+/+) Genotype: Cases (%)
Bgl II (+/+) Genotype: Controls* (%)
OR (P Value)‡
Reference
Japanese
119
108
23.5
11.2
2.1 (0.04)
Matsubara et al.15
Caucasians
163
95
19.2
7.4
2.4 (< 0.05)
Petrovic et al.6
*Diabetic retinopathy. †Subjects with duration of type 2 diabetes of more than 10 years. ‡Odds ratio and P value in logistic regression analysis.
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54. EFFECTS OF ENVIRONMENTAL, GENETIC, AND EPIGENETIC FACTORS ON PLATELET GLYCOPROTEINS
TABLE 54.5 Reported Polymorphisms of Glycoprotein IIIa (GPIIIa) and Their Effect on Platelet Function Gene
Polymorphic Site
Effect on Platelet Function
Reference
GPIIIa
Arg143Gln
No effect
Bajt and Lotus26
GPIIIa
Asp119
No effect
Bajt and Lotus26
GPIIIa
Ser121
No effect
Bajt and Lotus26
GPIIIa
Ser123
No effect
Bajt and Lotus26
GPIIIa
Ser130
No effect
Bajt and Lotus26
GPIIIa
Asp126
No effect
Bajt and Lotus26
GPIIIa
Asp127
No effect
Bajt and Lotus26
GPIIIa
−400 C/A*
No available information
Kozierdska et al.17
GPIIIa
−425 A/C*
No available information
Kozierdska et al.17
GPIIIa
−468 G/A*
No available information
Kozierdska et al.17
GPIIIa
1565 C/T
Influence on platelet activation and aggregation
Feng et al.21
*Polymorphic site on the promoter of the gene.
coronary heart disease.36 The studies were not directly comparable because they differed greatly in their patient pool and also in the way they were analyzed.36 The epigenetic effect was analyzed in only one study. Oksala and co-workers reported that smokers with stable coronary artery disease and carriers of the PlA2 allele were at an increased risk of subsequent cardiac events in comparison with nonsmokers with the PlA2 allele.39 The PlA1/PlA2 polymorphism of the GPIIIa gene was also implicated in the pathogenesis of DR (Table 54.8). Recently, the A2A2 genotype of the GPIIIa PlA1/A2 polymorphism has been reported to be a protective factor in the development of DR in Slovene Caucasians with type 2 diabetes. Contrary to this study, Pucci and co-workers40 failed to demonstrate an important contribution of the PlA1/PlA2 polymorphism of the GPIIIa gene on either nephropathy or retinopathy development in type 2 diabetes. This study recruited 605 subjects with type 2 diabetes. The PlA1/PlA2 polymorphism of the GPIIIa gene failed to contribute to the development of nephropathy or retinopathy in type 1 and type 2 diabetes. Although there have been many studies on this polymorphism, conflicting results on its association with stroke, myocardial infarction, or microvascular complications of diabetes have been reported in a number
TABLE 54.6 Reported Studies of the PlA1/A2 Polymorphism of Glycoprotein IIIa and Type 2 Diabetes Population
Cases* (n)
Control Group* (n)
A2 Allele: Cases* (%)
A2 Allele: Controls† (%)
OR (P Value)‡
Reference
Caucasians
112
59
34.8
14.6
3.1 (P < 0.01)
Tschoepe et al.28
Caucasians
113
95
20.4
21.5
1.0 (ns)
Kozieradzka et al.17
Caucasians
1051
2247
15.6
15.6
1.0 (ns)
Marz et al.29
*Subjects with type 2 diabetes. †Subjects without diabetes. ‡odds ratio and P value; ns: not statistically significant.
TABLE 54.7 Reported Studies of Polymorphism of α2β1 Integrin and Myocardial Infarction Population
Cases with MI (n)
Control Group (n)
Bgl II (+/+) Genotype: Cases (%)
Bgl II (+/+) Genotype: Controls (%)
OR (P Value)*
Reference
Caucasians
71
68
39
19
2.7 (0.04)
Weiss et al.30
Men, USA†
375
14,212
13.5
14.8
0.9 (0.4)
Ridker et al.31
Caucasians
242
209
1.2
2.4
0.9 (0.6)
Samani et al.32
Caucasians
156
216
§
§
1.66 (0.007)
Carter et al.33
Caucasians
1066
512
§
§
1.1 (0.9)
Gardemann et al.34
Caucasians
225
170
33.8
26.9
1.47 (0.06)
Anderson et al.35
Caucasians
529
1191
3.6
2.9
1.4 (0.01)
Grove et al.37
325
11.6
14.1
1.0 (0.9)
Starčević and Petrovič38
Caucasians‡ 229
*Odds ratio and P value in logistic regression analysis. †Men participating in the prospective Physicians’ Health Study. ‡Subjects with type 2 diabetes; §data not available.
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Environmental Factors, Nutrition, and Diabetic Retinopathy
of case-controlled clinical studies. Genetic association studies are prone to beta-statistical error and populationspecific genotype effects, all of which make the results difficult to reproduce. One could speculate that discrepancies between different studies may occur owing to other factors and mechanisms, such as differences in environmental factors (e.g., nutrition) and epigenetic mechanisms. Because of this, further studies enrolling larger numbers of patients from different populations with consideration of various environmental and epigenetic factors are needed to confirm the results of genetic studies published so far on the importance of platelet function in DR.
EPIGENETIC FACTORS AND DIABETIC RETINOPATHY Microthrombosis due to platelet dysfunction is implicated in the pathogenesis of DR via complex interactions of environmental and genetic factors, none of which can be fully and solely responsible for the higher risk of developing DR and for DR progression. Good glycemic control, if started in the initial stage of diabetes, prevents the development of retinopathy but, if reinstituted after a period of poor control, fails to halt its development, suggesting a metabolic memory phenomenon.4 Patients in the conventional treatment regimen during the Diabetes Complications and Control Trial had a higher incidence of c omplications several years after switching to intensive therapy than patients in the intensive control group.41,42 Studies in rats have demonstrated that the retina continues to experience oxidative stress, manganese superoxide dismutase (MnSOD) remains compromised, and nuclear factorκB is activated for at least 6 months after reinstitution of good glycemic control that has followed 6 months of poor control. This phenomenon has recently been reported to be due to the global acetylation of retinal histone H3.4 The epigenetic regulation has been demonstrated in the MnSOD gene,4 whereas it has not been studied in any integrin gene yet. A similar mechanism (i.e., global acetylation of retinal histone H3) has been proposed in the pathogenesis of the progression of DR. Epigenetic changes occur without alterations in the DNA sequence and can affect gene transcription
in response to environmental changes and nutrition. Switching between the active and inactive state of chromatin is the central mechanism of gene regulation, and this is defined as epigenetic factor. Several pathways may be involved in epigenetic regulation, such as DNA methylation, histone acetylation, and noncoding RNAs or microRNAs.3,4 Modulation of epigenetic changes by pharmaceutical means may provide a potential strategy to retard the progression of DR. Besides intense medical management, these strategies include dietary measures and the introduction of epigenetic drugs, such as inhibitors of DNA methylation and histone demethylases.
ENVIRONMENTAL FACTORS, NUTRITION, AND DIABETIC RETINOPATHY The impact of nutrition on the manifestation and progression of DR and other retinal disease has become an important and controversial topic in recent years. Awareness of this topic in the general population has increased partly as a result of commercial advertisements for supplements and diets. Although well-designed clinical trials have demonstrated the importance of some environmental factors (e.g., smoking, arterial hypertension), the significance of other factors (e.g., nutritional factors, vitamins) remains to be confirmed in well-designed clinical trials.43,44 So far, predominantly cross-sectional and interventional studies have been performed to address this question. Several environmental factors have been reported to be associated with DR and the progression of DR.41,43–46 Cigarette smoking was reported to be significantly associated with the incidence of DR and the rate of progression of DR.43,44 The consumption of fatty acids and dietary fiber was reported to be significantly associated with the rate of progression of DR.41 In particular, studies on patients with DR have implicated an impact of higher cholesterol levels on the progression of the disease.44 Fibrates, like statins, may act directly to decrease the progression of diabetic complications through their lipid-lowering effects
TABLE 54.8 Reported Studies of the PlA1/PlA2 Polymorphism of the GPIIIa Gene and Diabetic Retinopathy in Subjects with Type 2 Diabetes Population
Cases with DR (n)
Control Group* (n)
A2A2 Genotype: Cases (%)
A2A2 Genotype: Controls* (%)
OR (P Value)†
Reference
Caucasians, Italy
339
266
1.0
4.0
0.9 (0.6)
Pucci et al.40
Caucasians, Slovenia
222
120
12.6
22.2
0.61 (0.037)
Nikolajević et al.7
*Subjects with duration of type 2 diabetes of more than 10 years. †Odds ratio and P value in logistic regression analysis.
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but may also go beyond that via pleiotropic effects 44 (Table 54.9). Data on some chronic disorders (e.g., asthma, chronic obstructive pulmonary disease, diabetes) indicate that an increase in vegetable and fruit consumption may contribute to the prevention of these diseases, whereas there is insufficient evidence for DR regarding an association with the consumption of fruits and vegetables.47 Vitamin B12 deficiency has been recently demonstrated to be an independent risk factor for the development of DR,48 although more data are needed on the role of vitamins. Nutritional supplementation is receiving increasing interest with regard to DR via different mechanisms (e.g., antioxidants, platelet function). Vitamin C was reported to affect retinal blood flow in animal models and human supplementation trials.49 Vitamin C reduces platelet aggregation50 and p rotects against oxidative stress caused by nonenzymatic glycation and metabolic stress in people with diabetes.51 Vitamin E was reported to reduce oxidative stress at levels of 200 mg/day or more.51 Epidemiologic evidence does not support a relationship between antioxidant intake and reduced risk for DR; however, intervention trials indicate that further research is needed to clarify the role of several nutritional factors. Pycnogenol was reported to protect vitamin E from oxidation and prevent platelet activity in humans without increasing bleeding time.52 So far, only a few studies on agents affecting platelet function have been reported.50 The platelet proteomics approach revealed novel insights into the regulation of cellular biomarkers of atherogenic and thrombotic pathways by a dietary 80:20 cis9,trans11-conjugated linoleic acid blend.46 Recently, the effect of delphinidin-3-glucoside (Dp-3-g), one of the predominant bioactive compounds of anthocyanins in many plant foods, on platelet function has been demonstrated.53 This study found that Dp3-g significantly inhibited human and murine platelet aggregation. Platelet activation markers were examined via flow cytometry, and Dp-3-g significantly inhibited the TABLE 54.9 Agents and Drugs Used to Prevent or Slow the Progression of Diabetic Retinopathy Agent/Drug
Mechanism of Action
Statins
Lipid-lowering effect plus pleiotropic effect
Vitamin B12
Antioxidants, platelet function
Vitamin C
Reduces platelet aggregation and protects against oxidative stress
Vitamin E
Reduces oxidative stress
Delphinidin-3-glucoside
Inhibits platelet activation
expression of P-selectin, CD63, and CD40L, which reflect platelet α- and δ-granule release, and cytosol protein secretion, respectively. They further demonstrated that Dp-3-g downregulated the expression of active integrin αIIbβ3 on platelets and attenuated fibrinogen binding to platelets following agonist treatment without interfering with the direct interaction between fibrinogen and integrin αIIbβ3. Thus, Dp-3-g significantly inhibits platelet activation, and this may have a favorable effect on the progression of DR.53 Optimizing the medical management of DR should address the control of glycemia, blood pressure, and lipids, and based on recent trials, specific therapies using fenofibrate with a statin and candesartan should be considered. Results from experimental studies indicate that further prospective clinical studies are warranted on the prevention and inhibition of the progression of DR.
CONCLUSIONS Changes in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes and related macrovascular and microvascular complications. Several factors, such as hyperglycemia, oxidative stress, inflammation, and platelet dysfunction, are implicated in the pathogenesis of DR. So far, several studies have demonstrated the importance of several environmental and genetic factors, whereas much less is known about gene–environment interactions and epigenetic changes. Alarming estimates indicate that the rate of diabetes and associated complications (including DR) are rapidly increasing; therefore, additional strategies to arrest these trends are needed. Besides intense medical management, these strategies include dietary measures and the introduction of epigenetic drugs, such as inhibitors of DNA methylation and histone demethylases. Finally, based on current knowledge, optimizing the medical management of DR should address the control of glycemia, blood pressure, and lipids, and specific therapies using fenofibrate with a statin and candesartan should be considered. To conclude, the impact of nutritional factors is still insufficiently understood for patients with DR, and well-designed prospective randomized clinical trials are needed to address the role of nutritional factors.
TAKE-HOME MESSAGES • C hanges in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes and related macrovascular and microvascular complications.
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References
• S everal biochemical mechanisms and genetic factors have been implicated in the pathogenesis of diabetic retinopathy (DR). • Owing to the influences of gene–environmental interactions, epigenetic mechanisms regulate at least some of the pathologic mechanisms in the development of DR. • Epigenetic changes occur without alterations in the DNA sequence and can affect gene transcription in response to environmental changes and nutrition. • Optimal medical management of DR should address the control of glycemia, blood pressure, and lipids, and specific therapies using fenofibrate with a statin and candesartan should be considered. • The impact of nutritional factors is still insufficiently understood for patients with DR. • Well-designed prospective randomized clinical trials are needed to address the role of nutritional factors.
Acknowledgment The authors thank Ms Visam Bajt, BA, for revising the English.
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32. Samani NJ, Lodwick D. Glycoprotein IIIa polymorphism and risk of myocardial infarction. Cardiovasc Res 1997;33:693–7. 33. Carter AM, Ossei-Gerning N, Wilson IJ, Grant PJ. Association of the platelet Pl(A) polymorphism of glycoprotein IIb/IIIa and the fibrinogen Bbeta 448 polymorphism with myocardial infarction and extent of coronary artery disease. Circulation 1997;96:1424–31. 34. Gardemann A, Humme J, Stricker J, Nguyen QD, Katz N, Philipp M, et al. Association of the platelet glycoprotein IIIa PlA1/A2 gene polymorphism to coronary artery disease but not to nonfatal myocardial infarction in low risk patients. Thromb Haemost 1998;80:214–7. 35. Anderson JL, King GJ, Bair TL, Elmer SP, Muhlestein JB, Habashi J, Carlquist JF. Associations between a polymorphism in the gene encoding glycoprotein IIIa and myocardial infarction or coronary artery disease. J Am Coll Cardiol 1999;33:727–33. 36. Burr D, Doss H, Cooke GE, Goldschmidt-Clermont PJ. A meta-analysis of studies on the association of the platelet PlA polymorphism of glycoprotein IIIa and risk of coronary heart disease. Statist Med 2003;22:1741–60. 37. Grove EL, Ørntoft TF, Lassen JF, Jensen HK, Kristensen SD. The platelet polymorphism PlA2 is a genetic risk factor for myocardial infarction. J Intern Med 2004;255:637–44. 38. Nikolajević-Starčević J, Petrovič D. The a1/a2 polymorphism of the glycoprotein IIIa gene and myocardial infarction in Caucasians with type 2 diabetes. Mol Biol Rep 2013;40:2077–81. 39. Oksala NK, Heikkinen M, Mikkelsson J, Pohjasvaara T, Kaste M, Erkinjuntti T, Karhunen PJ. Smoking and the platelet fibrinogen receptor glycoprotein IIb/IIIA PlA1/A2 polymorphism interact in the risk of lacunar stroke and midterm survival. Stroke 2007;38:50–5. 40. Pucci L, Lucchesi D, Fotino C, Grupillo M, Miccoli R, Penno G, Del Prato S. Integrin Beta 3 PlA1/PlA2 polymorphism does not contribute to complications in both type 1 and type 2 diabetes. G Ital Nefrol 2003;20:461–9. 41. Cundiff DK, Nigg CR. Diet and diabetic retinopathy: insights from the Diabetes Control and Complications Trial (DCCT). MedGenMed 2005;7:3. 42. Writing Team for the Diabetes Control and Complications Trial/ Epidemiology of Diabetes Interventions and Complications Research Group. Sustained effect of intensive treatment of type 1 diabetes mellitus on development and progression of diabetic nephropathy: the Epidemiology of Diabetes Interventions and Complications (EDIC) study. JAMA 2003;290:2159–67.
43. Matthews DR, Stratton IM, Aldington SJ, Holman RR, Kohner EM. UK Prospective Diabetes Study Group. Risks of progression of retinopathy and vision loss related to tight blood pressure control in type 2 diabetes mellitus: UKPDS 69. Arch Ophthalmol 2004;122:1631–40. 44. Stratton IM, Kohner EM, Aldington SJ, Turner RC, Holman RR, Manley SE, Matthews DR. UKPDS 50: risk factors for incidence and progression of retinopathy in type II diabetes over 6 years from diagnosis. Diabetologia 2001;44:156–63. 45. Lee CT, Gayton EL, Beulens JW, Flanagan DW, Adler AI. Micronutrients and diabetic retinopathy: a systematic review. Ophthalmology 2010;117:71–8. 46. Bachmair EM, Bots ML, Mennen LI, Kelder T, Evelo CT, Horgan GW, et al. Effect of supplementation with an 80:20 cis9,trans11 conjugated linoleic acid blend on the human platelet proteome. Mol Nutr Food Res 2012;56:1148–59. 47. Boeing H, Bechthold A, Bub A, Ellinger S, Haller D, Kroke A, et al. Critical review: vegetables and fruit in the prevention of chronic diseases. Eur J Nutr 2012;51:637–63. 48. Satyanarayana A, Balakrishna N, Pitla S, Reddy PY, Mudili S, Lopamudra P, et al. Status of B-vitamins and homocysteine in diabetic retinopathy: association with vitamin-B12 deficiency and hyperhomocysteinemia. PLoS ONE 2011;6:e26747. 49. Cunningham JJ, Mearkle PL, Brown RG, Vitamin C: an aldose reductase inhibitor that normalizes erythrocyte sorbitol in insulindependent diabetes mellitus. J Am Coll Nutr 1994;13:344–50. 50. Wilkinson IB, Megson IL, MacCallum H, Sogo N, Cockcroft JR, Webb DJ. Oral vitamin C reduces arterial stiffness and platelet aggregation in humans. J Cardiovasc Pharmacol 1999;34:690–3. 51. Baynes JW, Thorpe SR. Role of oxidative stress in diabetic complications: a new perspective on an old paradigm. Diabetes 1999;48:1–9. 52. Pütter M, Grotemeyer KH, Würthwein G, Araghi-Niknam M, Watson RR, Hosseini S, Rohdewald P. Inhibition of smoking-induced platelet aggregation by aspirin and pycnogenol. Thromb Res 1999;95:155–61. 53. Yang Y, Shi Z, Reheman A, Jin JW, Li C, Wang Y, et al. Plant food delphinidin-3-glucoside significantly inhibits platelet activation and thrombosis: novel protective roles against cardiovascular diseases. PLoS ONE 2012;7:e37323.
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Effects of Environmental, Genetic, and Epigenetic Factors on Platelet Glycoproteins and the Development of Diabetic Retinopathy Daniel Petrovič Institute of Histology and Embryology, Medical Faculty Ljubljana, University of Ljubljana, Ljubljana, Slovenia
INTRODUCTION Changes in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes, obesity, and related vascular complications.1,2 Both type 1 and type 2 diabetes are associated with hyperglycemia, oxidant stress, and inflammation and a significantly increased risk of macrovascular and microvascular complications (Table 54.1).1,2 Diabetic retinopathy (DR) is associated with both environmental and genetic factors. Several m etabolic abnormalities are implicated in its pathogenesis; however, the exact mechanism remains to be d etermined. While several studies have been devoted to the evaluation of genetic factors related to diabetes and its complications (including DR), much less is known about environmental factors, nutrition, and epigenetic mechanisms related to DR.3–5 So far, several genes and their polymorphisms have been implicated in the pathogenesis (Table 54.2).5–7
PATHOGENESIS OF DIABETIC RETINOPATHY Epidemiologic and experimental studies indicate that besides the duration of diabetes and glycemic control, several other factors are involved in the development of DR (Table 54.3).8,9 DR is characterized by increased vascular permeability, hemostatic abnormalities, increased tissue ischemia, and neoangiogenesis.2 The pathogenetic mechanisms
Handbook of Nutrition, Diet, and the Eye http://dx.doi.org/10.1016/B978-0-12-401717-7.00054-X
of DR are very complex. Although many hyperglycemia-induced metabolic abnormalities are implicated in its pathogenesis, the exact mechanism of the development of retinopathy remains elusive. Alteration in retinal blood flow, metabolic changes, hemostatic abnormalities, increased oxidative stress, increased polyol pathway flux, activation of protein kinase C isoforms, increased h exosamine pathway flux, and increased advanced g lycation end-product formation, nonenzymatic glycosylation of collagen, and other tissue proteins are observed during long-term hyperglycemia.2,9 It is also important to know that many of the systemic abnormalities in both type 1 and type 2 diabetic patients are prothrombotic: hyperreactive platelets, decreased vascular prostacyclin p roduction, endothelial dysfunction resulting in increased circulating levels of von Willebrand factor and leukocyte adhesion molecules, hypercoagulability, and decreased fibrinolysis. Increased levels of intercellular adhesion molecule-1 and plasminogen activator inhibitor-1 messenger RNA (mRNA), as well as decreased levels of tissue plasminogen activator, are specifically found in retinal vessels of diabetic compared with nondiabetic individuals.10,11
TABLE 54.1 Chronic Complications of Diabetes Microvascular
Macrovascular
Retinopathy
Coronary heart disease
Nephropathy
Cerebrovascular disease
Neuropathy
Peripheral arterial obstructive disease
535
© 2014 Elsevier Inc. All rights reserved.
536
54. EFFECTS OF ENVIRONMENTAL, GENETIC, AND EPIGENETIC FACTORS ON PLATELET GLYCOPROTEINS
TABLE 54.2 Pathways and Genes Implicated in the Pathogenesis of Diabetic Retinopathy Pathway/System
Gene
Polyol pathway
Aldose reductase
Renin–angiotensin system
Renin Angiotensinogen Angiotensin-1 converting enzyme Aldosterone
TABLE 54.3 Risk Factors for Diabetic Retinopathy Modifiable
Nonmodifiable
Cigarette smoking
Genetic factors
Blood pressure
Duration of diabetes
Plasma cholesterol
Age of onset of diabetes
Fasting glycemia
Positive family history
HbA1c Proteinuria
Advanced glycation endproducts
Receptor for advanced glycation end-products
Growth factors
Vascular endothelial growth factor
Waist-to-hip ratio
Basic fibroblast growth factor
Vitamin B12 deficiency
Insulin-like growth factor Peroxisome proliferatoractivated receptor
Peroxisome proliferator-activated receptor Coactivator for peroxisome proliferator-activated receptor
Inflammatory genes
Interleukins Tumor necrosis factor
Thrombotic system
Fibrinogen
Platelet function
Integrin
Oxidative system
Manganese superoxide dismutase Catalase Myeloperoxidase Glutathione S-transferase NADPH oxidase Endothelial nitric oxide synthase Inducible nitric oxide synthase
Adhesion molecules
Intercellular adhesion molecule Vascular cell adhesion protein Platelet endothelial cell adhesion molecule
Extracellular matrix homeostasis
Matrix homeostasis genes Matrix metalloproteinase
Hormones/vitamins
Growth hormone Vitamin D
Undefined
Glucose transporter-1 Growth hormone
NADPH: reduced nicotinamide adenine dinucleotide.
Body mass index
Insulin treatment
HbA1c: glycosylated hemoglobin.
PLATELETS AND DIABETIC RETINOPATHY Platelets are thought to be involved in the p athogenesis of DR.12 Platelets from diabetic patients may interact with exposed subendothelial fibrillar collagens, major components of the subendothelial matrix. Moreover, the amount of nonenzymatically glycosylated collagen, which is prone to interact with platelets, was demonstrated to be higher in diabetic patients than in nondiabetic controls.10 Platelets activated by contact with collagens can trigger thrombus formation and small vessel occlusion. Platelets from diabetic patients are hyperreactive to aggregating agents, such as c ollagen, thrombin, and adenosine diphosphate (ADP).12 The platelet membrane glycoprotein Ia/IIa, α2β1 integrin, serves as a platelet receptor for collagen.13 Some genetic variations of the glycoprotein involved in platelet adhesion, aggregation, and activation in favor of thrombogenesis may be considered as risk factors for thrombotic events.13
C825T POLYMORPHISM OF THE G-PROTEIN-COUPLED RECEPTOR GENE Platelet aggregation that varies among individuals and genetic factors may alter platelet activation through G-protein-coupled receptors.14 Recently, Dusse and co-workers demonstrated that the C825T polymorphism of the gene GNB3 encoding the G-protein β3 subunit influences platelet aggregation. In human whole blood, the GNB3 825CC genotype has been reported to be associated with enhanced platelet aggregation.14 The study evaluating the effect of this polymorphism related to DR development has not been reported yet.
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Genetic Factors and Glycoprotein IIb-IIIa
GENETIC FACTORS AND α2β1 INTEGRIN Genetic variations in α2β1 integrin were reported to affect the density of α2β1 receptors on the platelet surface.10 Namely, an association between genetic variations in α2β1 and the density of α2β1 receptors on the platelet surface was reported.10 Kunicki et al.10 demonstrated that the density of α2β1 receptors on the platelet surface affects platelet adhesion to collagen, contributing to an increased risk of thrombosis.10 Matsubara et al.15 demonstrated for the first time in 2000 the association between genetic variations in α2β1 integrin and DR in the Japanese population. This finding was confirmed a few years later in a Caucasian population by Petrovic and co-workers.7 In both studies, the BglII (+/+) genotype of the gene polymorphism of the α2β1 integrin gene was reported to be an independent risk factor for DR in Caucasians with type 2 diabetes (Table 54.4). Moreover, Matsubara et al.15 demonstrated an association between genetic variations in α2β1 integrin and another microvascular complication of diabetes, diabetic nephropathy, whereas Tsai et al.16 failed to find such an association with diabetic nephropathy in a Chinese population with type 2 diabetes.
GENETIC FACTORS AND GLYCOPROTEIN IIB-IIIA Glycoprotein (GP) IIb-IIIa (αIIbβ3-integrin), also known as integrin β3 or antigen CD61, is the central receptor of platelet aggregation. GPIIIa is a surface protein found in various tissues, participating in cell adhesion and cell-surface mediated signaling. It builds glycoprotein IIb/IIIa in the platelet membrane and constitutes a fibrinogen receptor, which also exhibits an ability to bind the von Willebrand factor, fibronectin, and thrombospondin.17 Activated GPIIb-IIIa binds fibrinogen or von Willebrand factor, which forms molecular bridges between aggregating platelets. GPIIb-IIIa is a calcium-dependent heterodimer that is expressed in megakaryocytes, platelets, and mast cells.18
On the surface of resting platelets, GPIIb-IIIa is exhibited in a low-affinity conformation, in which the ligand-binding site is not exposed. Following the activation of platelets by agonists such as ADP, thrombin, or collagen, inside–out 3 signaling occurs, giving rise to the exposure of the ligand binding site of GPIIb-IIIa.19 The binding of fibrinogen to the active form of GPIIb-IIIa is the final step leading to platelet aggregation and the formation of thrombus.19,20 Both GPIIb and GPIIIa are known to bear a number of single amino acid substitutions affecting conformational changes and the ligand binding function with little or no effect on platelet function. A platelet-specific antigen (PlA1/A2) polymorphism has been by far the most investigated GPIIIa gene polymorphism. A substitution of cytosine for thymidine at position 1565 in exon 2 of the GPIIIa gene leads to an amino acid difference at position 33: a leucine (A1 allele) or a proline (A2 allele).17 The mentioned polymorphism can influence both platelet activation and aggregation21 and affect postoccupancy signaling by the platelet fibrinogen receptor IIb/IIIa.22 The presence of one or two PlA2 alleles is associated with an increased binding affinity to fibrinogen as well as with platelet aggregability in response to epinephrine, ADP, and collagen in vitro.23 It has also been suggested that the PlA2 allelic variant causes an increased sensitivity to platelet aggregation by various agonists and an altered sensitivity to aspirin24–26 (Table 54.5). Platelet receptor polymorphism has also been implicated in the pathogenesis of glucose metabolism. It has been suggested that the function of platelet GPIIIa is modulated by a cysteine protease, calpain 10, which is not characteristic of platelets. Calpain 10 was reported to influence glucose metabolism, insulin secretion, and insulin action.27 Studies on the association of the PlA1/ A2 polymorphism of glycoprotein IIIa and type 2 diabetes have reported conflicting results17,28,29 (Table 54.6). Although there have been many studies on this polymorphism, conflicting results on its association with stroke, coronary artery disease, and myocardial infarction have been reported in a number of case-controlled clinical studies30–38 (Table 54.7). A meta-analysis of 12 epidemiologic studies showed that there was an association between the PlA2 variant and an increased risk of
TABLE 54.4 Reported Studies of Polymorphism of α2β1 Integrin and Diabetic Retinopathy (DR) in Subjects with Type 2 Diabetes Population
Cases with DR* (n)
Control Group† (n)
Bgl II (+/+) Genotype: Cases (%)
Bgl II (+/+) Genotype: Controls* (%)
OR (P Value)‡
Reference
Japanese
119
108
23.5
11.2
2.1 (0.04)
Matsubara et al.15
Caucasians
163
95
19.2
7.4
2.4 (< 0.05)
Petrovic et al.6
*Diabetic retinopathy. †Subjects with duration of type 2 diabetes of more than 10 years. ‡Odds ratio and P value in logistic regression analysis.
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54. EFFECTS OF ENVIRONMENTAL, GENETIC, AND EPIGENETIC FACTORS ON PLATELET GLYCOPROTEINS
TABLE 54.5 Reported Polymorphisms of Glycoprotein IIIa (GPIIIa) and Their Effect on Platelet Function Gene
Polymorphic Site
Effect on Platelet Function
Reference
GPIIIa
Arg143Gln
No effect
Bajt and Lotus26
GPIIIa
Asp119
No effect
Bajt and Lotus26
GPIIIa
Ser121
No effect
Bajt and Lotus26
GPIIIa
Ser123
No effect
Bajt and Lotus26
GPIIIa
Ser130
No effect
Bajt and Lotus26
GPIIIa
Asp126
No effect
Bajt and Lotus26
GPIIIa
Asp127
No effect
Bajt and Lotus26
GPIIIa
−400 C/A*
No available information
Kozierdska et al.17
GPIIIa
−425 A/C*
No available information
Kozierdska et al.17
GPIIIa
−468 G/A*
No available information
Kozierdska et al.17
GPIIIa
1565 C/T
Influence on platelet activation and aggregation
Feng et al.21
*Polymorphic site on the promoter of the gene.
coronary heart disease.36 The studies were not directly comparable because they differed greatly in their patient pool and also in the way they were analyzed.36 The epigenetic effect was analyzed in only one study. Oksala and co-workers reported that smokers with stable coronary artery disease and carriers of the PlA2 allele were at an increased risk of subsequent cardiac events in comparison with nonsmokers with the PlA2 allele.39 The PlA1/PlA2 polymorphism of the GPIIIa gene was also implicated in the pathogenesis of DR (Table 54.8). Recently, the A2A2 genotype of the GPIIIa PlA1/A2 polymorphism has been reported to be a protective factor in the development of DR in Slovene Caucasians with type 2 diabetes. Contrary to this study, Pucci and co-workers40 failed to demonstrate an important contribution of the PlA1/PlA2 polymorphism of the GPIIIa gene on either nephropathy or retinopathy development in type 2 diabetes. This study recruited 605 subjects with type 2 diabetes. The PlA1/PlA2 polymorphism of the GPIIIa gene failed to contribute to the development of nephropathy or retinopathy in type 1 and type 2 diabetes. Although there have been many studies on this polymorphism, conflicting results on its association with stroke, myocardial infarction, or microvascular complications of diabetes have been reported in a number
TABLE 54.6 Reported Studies of the PlA1/A2 Polymorphism of Glycoprotein IIIa and Type 2 Diabetes Population
Cases* (n)
Control Group* (n)
A2 Allele: Cases* (%)
A2 Allele: Controls† (%)
OR (P Value)‡
Reference
Caucasians
112
59
34.8
14.6
3.1 (P < 0.01)
Tschoepe et al.28
Caucasians
113
95
20.4
21.5
1.0 (ns)
Kozieradzka et al.17
Caucasians
1051
2247
15.6
15.6
1.0 (ns)
Marz et al.29
*Subjects with type 2 diabetes. †Subjects without diabetes. ‡odds ratio and P value; ns: not statistically significant.
TABLE 54.7 Reported Studies of Polymorphism of α2β1 Integrin and Myocardial Infarction Population
Cases with MI (n)
Control Group (n)
Bgl II (+/+) Genotype: Cases (%)
Bgl II (+/+) Genotype: Controls (%)
OR (P Value)*
Reference
Caucasians
71
68
39
19
2.7 (0.04)
Weiss et al.30
Men, USA†
375
14,212
13.5
14.8
0.9 (0.4)
Ridker et al.31
Caucasians
242
209
1.2
2.4
0.9 (0.6)
Samani et al.32
Caucasians
156
216
§
§
1.66 (0.007)
Carter et al.33
Caucasians
1066
512
§
§
1.1 (0.9)
Gardemann et al.34
Caucasians
225
170
33.8
26.9
1.47 (0.06)
Anderson et al.35
Caucasians
529
1191
3.6
2.9
1.4 (0.01)
Grove et al.37
325
11.6
14.1
1.0 (0.9)
Starčević and Petrovič38
Caucasians‡ 229
*Odds ratio and P value in logistic regression analysis. †Men participating in the prospective Physicians’ Health Study. ‡Subjects with type 2 diabetes; §data not available.
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539
Environmental Factors, Nutrition, and Diabetic Retinopathy
of case-controlled clinical studies. Genetic association studies are prone to beta-statistical error and populationspecific genotype effects, all of which make the results difficult to reproduce. One could speculate that discrepancies between different studies may occur owing to other factors and mechanisms, such as differences in environmental factors (e.g., nutrition) and epigenetic mechanisms. Because of this, further studies enrolling larger numbers of patients from different populations with consideration of various environmental and epigenetic factors are needed to confirm the results of genetic studies published so far on the importance of platelet function in DR.
EPIGENETIC FACTORS AND DIABETIC RETINOPATHY Microthrombosis due to platelet dysfunction is implicated in the pathogenesis of DR via complex interactions of environmental and genetic factors, none of which can be fully and solely responsible for the higher risk of developing DR and for DR progression. Good glycemic control, if started in the initial stage of diabetes, prevents the development of retinopathy but, if reinstituted after a period of poor control, fails to halt its development, suggesting a metabolic memory phenomenon.4 Patients in the conventional treatment regimen during the Diabetes Complications and Control Trial had a higher incidence of c omplications several years after switching to intensive therapy than patients in the intensive control group.41,42 Studies in rats have demonstrated that the retina continues to experience oxidative stress, manganese superoxide dismutase (MnSOD) remains compromised, and nuclear factorκB is activated for at least 6 months after reinstitution of good glycemic control that has followed 6 months of poor control. This phenomenon has recently been reported to be due to the global acetylation of retinal histone H3.4 The epigenetic regulation has been demonstrated in the MnSOD gene,4 whereas it has not been studied in any integrin gene yet. A similar mechanism (i.e., global acetylation of retinal histone H3) has been proposed in the pathogenesis of the progression of DR. Epigenetic changes occur without alterations in the DNA sequence and can affect gene transcription
in response to environmental changes and nutrition. Switching between the active and inactive state of chromatin is the central mechanism of gene regulation, and this is defined as epigenetic factor. Several pathways may be involved in epigenetic regulation, such as DNA methylation, histone acetylation, and noncoding RNAs or microRNAs.3,4 Modulation of epigenetic changes by pharmaceutical means may provide a potential strategy to retard the progression of DR. Besides intense medical management, these strategies include dietary measures and the introduction of epigenetic drugs, such as inhibitors of DNA methylation and histone demethylases.
ENVIRONMENTAL FACTORS, NUTRITION, AND DIABETIC RETINOPATHY The impact of nutrition on the manifestation and progression of DR and other retinal disease has become an important and controversial topic in recent years. Awareness of this topic in the general population has increased partly as a result of commercial advertisements for supplements and diets. Although well-designed clinical trials have demonstrated the importance of some environmental factors (e.g., smoking, arterial hypertension), the significance of other factors (e.g., nutritional factors, vitamins) remains to be confirmed in well-designed clinical trials.43,44 So far, predominantly cross-sectional and interventional studies have been performed to address this question. Several environmental factors have been reported to be associated with DR and the progression of DR.41,43–46 Cigarette smoking was reported to be significantly associated with the incidence of DR and the rate of progression of DR.43,44 The consumption of fatty acids and dietary fiber was reported to be significantly associated with the rate of progression of DR.41 In particular, studies on patients with DR have implicated an impact of higher cholesterol levels on the progression of the disease.44 Fibrates, like statins, may act directly to decrease the progression of diabetic complications through their lipid-lowering effects
TABLE 54.8 Reported Studies of the PlA1/PlA2 Polymorphism of the GPIIIa Gene and Diabetic Retinopathy in Subjects with Type 2 Diabetes Population
Cases with DR (n)
Control Group* (n)
A2A2 Genotype: Cases (%)
A2A2 Genotype: Controls* (%)
OR (P Value)†
Reference
Caucasians, Italy
339
266
1.0
4.0
0.9 (0.6)
Pucci et al.40
Caucasians, Slovenia
222
120
12.6
22.2
0.61 (0.037)
Nikolajević et al.7
*Subjects with duration of type 2 diabetes of more than 10 years. †Odds ratio and P value in logistic regression analysis.
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54. EFFECTS OF ENVIRONMENTAL, GENETIC, AND EPIGENETIC FACTORS ON PLATELET GLYCOPROTEINS
but may also go beyond that via pleiotropic effects 44 (Table 54.9). Data on some chronic disorders (e.g., asthma, chronic obstructive pulmonary disease, diabetes) indicate that an increase in vegetable and fruit consumption may contribute to the prevention of these diseases, whereas there is insufficient evidence for DR regarding an association with the consumption of fruits and vegetables.47 Vitamin B12 deficiency has been recently demonstrated to be an independent risk factor for the development of DR,48 although more data are needed on the role of vitamins. Nutritional supplementation is receiving increasing interest with regard to DR via different mechanisms (e.g., antioxidants, platelet function). Vitamin C was reported to affect retinal blood flow in animal models and human supplementation trials.49 Vitamin C reduces platelet aggregation50 and p rotects against oxidative stress caused by nonenzymatic glycation and metabolic stress in people with diabetes.51 Vitamin E was reported to reduce oxidative stress at levels of 200 mg/day or more.51 Epidemiologic evidence does not support a relationship between antioxidant intake and reduced risk for DR; however, intervention trials indicate that further research is needed to clarify the role of several nutritional factors. Pycnogenol was reported to protect vitamin E from oxidation and prevent platelet activity in humans without increasing bleeding time.52 So far, only a few studies on agents affecting platelet function have been reported.50 The platelet proteomics approach revealed novel insights into the regulation of cellular biomarkers of atherogenic and thrombotic pathways by a dietary 80:20 cis9,trans11-conjugated linoleic acid blend.46 Recently, the effect of delphinidin-3-glucoside (Dp-3-g), one of the predominant bioactive compounds of anthocyanins in many plant foods, on platelet function has been demonstrated.53 This study found that Dp3-g significantly inhibited human and murine platelet aggregation. Platelet activation markers were examined via flow cytometry, and Dp-3-g significantly inhibited the TABLE 54.9 Agents and Drugs Used to Prevent or Slow the Progression of Diabetic Retinopathy Agent/Drug
Mechanism of Action
Statins
Lipid-lowering effect plus pleiotropic effect
Vitamin B12
Antioxidants, platelet function
Vitamin C
Reduces platelet aggregation and protects against oxidative stress
Vitamin E
Reduces oxidative stress
Delphinidin-3-glucoside
Inhibits platelet activation
expression of P-selectin, CD63, and CD40L, which reflect platelet α- and δ-granule release, and cytosol protein secretion, respectively. They further demonstrated that Dp-3-g downregulated the expression of active integrin αIIbβ3 on platelets and attenuated fibrinogen binding to platelets following agonist treatment without interfering with the direct interaction between fibrinogen and integrin αIIbβ3. Thus, Dp-3-g significantly inhibits platelet activation, and this may have a favorable effect on the progression of DR.53 Optimizing the medical management of DR should address the control of glycemia, blood pressure, and lipids, and based on recent trials, specific therapies using fenofibrate with a statin and candesartan should be considered. Results from experimental studies indicate that further prospective clinical studies are warranted on the prevention and inhibition of the progression of DR.
CONCLUSIONS Changes in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes and related macrovascular and microvascular complications. Several factors, such as hyperglycemia, oxidative stress, inflammation, and platelet dysfunction, are implicated in the pathogenesis of DR. So far, several studies have demonstrated the importance of several environmental and genetic factors, whereas much less is known about gene–environment interactions and epigenetic changes. Alarming estimates indicate that the rate of diabetes and associated complications (including DR) are rapidly increasing; therefore, additional strategies to arrest these trends are needed. Besides intense medical management, these strategies include dietary measures and the introduction of epigenetic drugs, such as inhibitors of DNA methylation and histone demethylases. Finally, based on current knowledge, optimizing the medical management of DR should address the control of glycemia, blood pressure, and lipids, and specific therapies using fenofibrate with a statin and candesartan should be considered. To conclude, the impact of nutritional factors is still insufficiently understood for patients with DR, and well-designed prospective randomized clinical trials are needed to address the role of nutritional factors.
TAKE-HOME MESSAGES • C hanges in dietary habits and lifestyles associated with rapid economic growth have dramatically increased the incidence of diabetes and related macrovascular and microvascular complications.
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References
• S everal biochemical mechanisms and genetic factors have been implicated in the pathogenesis of diabetic retinopathy (DR). • Owing to the influences of gene–environmental interactions, epigenetic mechanisms regulate at least some of the pathologic mechanisms in the development of DR. • Epigenetic changes occur without alterations in the DNA sequence and can affect gene transcription in response to environmental changes and nutrition. • Optimal medical management of DR should address the control of glycemia, blood pressure, and lipids, and specific therapies using fenofibrate with a statin and candesartan should be considered. • The impact of nutritional factors is still insufficiently understood for patients with DR. • Well-designed prospective randomized clinical trials are needed to address the role of nutritional factors.
Acknowledgment The authors thank Ms Visam Bajt, BA, for revising the English.
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