Diabetic Eye Disease: Biochemistry, Ocular Hemodynamics and New
Strategies For Prevention
A. Paul Chous, M.A., O.D.
Chous EyeCare Associates
Tacoma, WA
practice emphasizing diabetes care and education
Type 1 diabetic since 1968
author of Diabetic Eye Disease: Lessons From A Diabetic Eye Doctor – How To Avoid
Blindness and Get Great Eye Care (Fairwood Press, 2003)
I. Introduction
Epidemiology of Diabetes Mellitus – A Looming Public Health Crisis
Microvascular Complications
 leading cause of ESRD – 100k/yr
leading cause of non-traumatic amputation – 80k/yr
leading cause of new blindness in those <74yo - 12-24k/yr
third leading cause of blindness overall
Macrovascular Complications
5th-7th leading cause of death (100k to 400k/yr) – due to
macrovascular disease (CHD, CVA)
Hyperglycemia and insulin resistance are associated with
Dyslipidemia (atherogenic glycation of LDL & hypercoagulability)
II. Pathophysiology
Macrovascular Insult (CAD, CVD, PVD))
insulin resistance associated with dyslipidemia
elevated triglycerides and LDL cholesterol
LDL particles are more atherogenic (smaller and denser)
hyperglycemia itself promotes an unfavorable lipid profile
 hyperinsulinemia increases BP and promotes platelet adhesion
 glycation of proteins and cells results in atherogenic vascular lesions
Microvascular Insult (retinopathy, nephropathy, neuropathy)
 Four Biochemical Pathways Identified
 polyol pathway increased sorbitol flux with osmotic
pressure (as in diabetic cataract) and  oxidative stress
 non-enzymatic glycation of proteins advanced glycation
endproducts (AGEs), altered protein function and  oxidative stress
 hyperglycemiaincreased intracellular fructose-6-phosphate via
the hexosamine flux pathway alteration in gene expression,
primarily through inflammatory cytokeines like PAI-1 and TNF
beta-1 tissue sclerosis (as in diabetic nephropathy) and  PKC-B
 increased endothelial glucoseincreased diacyl glycerol (DAG)
activates Protein Kinase C- beta (PKC-B) which alters
endothelial tight junctions (as in diabetic macular edema) and
promotes vascular endothelial growth factor (VEGF), a necessary
component for retinal neovascularization (as in PDR)


Each of these pathways is dependant upon underlying mitochondrial
over-production of reactive oxygen species (i.e. superoxide) which
has been shown to occur when mitochondria are exposed to excess
glucose:
euglycemia mitochondria ATP

hyperglycemia mitochondria ATP +Superoxide
Superoxide inhibitsthe enzyme glyceraldehyde-3- phosphate
dehydrogenase (GAPDH), leading to a “log jam” in normal glucose
metabolism, increasing levels of glucose, glucose-6 phosphate,
fructose-6 phosphate and glyceraldehyde-3 phosphate
Inhibition of Superoxide (O2- ) prevents multiple mechanisms of
hyperglycemic damage. Inhibition can be achieved by:
 tight blood sugar control
increased intracellular Superoxide dismutase (SOD)
this gives a biochemical rationale for recommending
antioxidant supplementation to all patients with diabetes
pharmacologic therapies to block individual pathways are being developed
and tested (PKC inhibitors, VEGF inhibitors, aldose reductase inhibitors)
Anti-Sense GAPDH Blocks
Four Pathways of Hyperglycemic Damage



Glucose
NADPH
NADP+
NAD+
Sorbitol

NADPH
Fructose
Polyol Pathway
Glucose -6-P

Gln
Glu
Fructose-6-P
GFAT
Glucosamine -6-P UDP-Glc-NAc + IPA-1
+ TNF-B-1
Hexosamine Pathway




NADH
Glyceraldehyde-3-P DHAP
NAD
NAD
a-Glycerol-PDAGPKC
Protein Kinase C Pathway
NADH  GAPDH O2- (Superoxide)
Methylglyoxal  AGEs
Advanced Glycation Endproduct (AGE) Pathway
1,3 Diphosphoglycerate
(useable, harmless glucose metabolite)
Adapted Source: Michael A. Brownlee, M.D.
“Hyperglycemia, oxidative damage, and protein kinase c”
III.Benfotiamine
synthetic, lipid soluble thiamine (Vitamin B1)
used for 12 years in Europe (Germany and Spain) for treatment of
painful peripheral neuropathies, including diabetic neuropathy
Thiamine is a necessary cofactor for the action of Transketolase
Transketolase catalyzes the intracellular breakdown of the dangerous
glucose metabolites fructose-6- phosphate and glyceraldehyde-3phosphate via the “pentose phosphate shunt” (see diagram below)
diabetics are thiamine deficient due to poor absorption and oxidative
depletion, resulting in reduced transketolase activity in the
hyperglycemic environment where that activity is needed most
thiamine deficiency is particularly notable in vascular endothelium
thiamine is water soluble (requiring active transport across the cell
membrane), and oral supplementation elevates transketolase activity
by 20-30% at most
”allithiamins” (lipid soluble thiamine derivatives) are passively
absorbed and achieve much higher intracellular concentrations
Benfotiamine & The Four Pathways of Hyperglycemic Damage



NADPH
Glucose
NADP+


NAD+
Sorbitol
NADPH
Fructose
Polyol Pathway
Glucose -6-P

Gln
Fructose-6-P
Glu
GFAT
Glucosamine -6-P UDP-Glc-NAc + IPA-1
+ TNF-B-1
Hexosamine Pathway
transketolase fructose-6-P
glyceraldehyde-3-P

Pentose Phosphate Shunt
 Thiamine




NADH
Glyceraldehyde-3-P DHAP
NAD
NAD
a-Glycerol-PDAGPKC
Protein Kinase C Pathway
NADH  GAPDH O2- (Superoxide)
Methylglyoxal  AGEs
Advanced Glycation Endproduct (AGE) Pathway
1,3 Diphosphoglycerate


3 recent studies (Hammes, H. et al, 2003; Ascher, E. et al, 2001;
Pomero, F. et al, 2001) have demonstrated that high levels of
intercellular thiamine block vascular damage caused by hyperglycemia
The recent study published in Nature:Medicine (by an international
team of diabetes researchers including, in the US, Dr. Michael
Brownlee of the Albert Einstein College of Medicine) showed that:
1. benfotiamine increased transketolase activity by 300-400%
2. benfotiamine reduced activity in the hexosamine pathway
3. benfotiamine reduced production of DAG and PKC
4. benfotiamine reduced production of AGEs
These 4 results were demonstrated in cultured endothelial cells
5. benfotiamine totally prevented both clinically and
microscopically detectable evidence of diabetic retinopathy in
a well established animal model of the disease (all control
animals developed DRT)
Questions: safety, efficacy, optimal dosage in humans???
How exactly does benfotiamine prevent diabetic retinopathy?
IV. The Protein KInase C Pathway- A Closer Look
PKC Beta damages retinal vascular endothelium, leading to vessel leakage, capillary
closure and recruitment of vascular endothelial growth factor (VEGF) and other
vasoproliferative substances which, in turn, further activate PKC-B in a vicious cycle
leading to PDR and CSME
Protein Kinase C Beta in Diabetic Retinopathy
adapted source: Aaron Vinik, M.D., Ph.D. From “Impact of PKC inhibition on the
pathogenesis of diabetic microvascular complications”
Hyperglycemia

PKC B activation

Retinal Vascular Injury

Capillary nonperfusion
PKC B activation
PKC B activation
vascular leakage
VEGF/VPF production
PKC B activation
Neovascularization
Macular edema
Proliferative retinopathy
What other forms of diabetic eye disease might benfotiamine inhibit?
V. The AGE Pathway- A Closer Look
Advanced Glycation Endproducts form in all mammals with age as a function of the
non-enzymatic glycation of proteins. This process is analogous to “carmelization.”
Hyperglycemia accelerates the production of AGEs and causes the premature aging of
persons with DM
Advanced Glycation Endproducts: glucose metabolites bind to
the amino groups of proteins (including RBCs, vascular
endothelium and collagen) resulting in heterogeneous
inter- and intramolecular crosslinking and altered function of
those proteins
AGE levels are found in higher concentrations with age and
hyperglycemia (also implicated in Alzheimer’s, pulmonary
fibrosis, male ED and atherosclerosis)
Method of food preparation directly affects AGE levels in foods
and body tissues
  AGEs with high temperature, low humidity
preparation (baking and broiling) versus lower
temperature, high humidity preparation (boiling)
AGE receptors (e.g.” RAGE”) have been identified that may
promote or inhibit harmful biological effects
AGEs and Diabetic Eye Disease
Glaucoma
High levels of AGE have been found in the optic nerve head of
diabetic patients and those with POAG
structural changes in the cribriform plates and glial
tissues that support/protect optic nerve axons (Amano
et al., 2001; Albon et al., 1995)
Retinopathy
 AGEs are found in retinal vascular endothelium of diabetics
and are believed to induce pericyte loss (Stitt et al., 1997;
Yamagishi et al., 1999) and  levels of PKC
Keratopathy
 AGEs are found in Bowman’s Membrane of diabetics and
have been implicated in hemidesmosomal weakness
and RCE syndrome (Kaji et al., 2000)
Cataract
Markedly  AGEs in diabetic lenses  covalent crosslinkings of
lens crystallins (premature presbyopia), generation of
hydroxyl free radicals and copper-mediated oxidation of
ascorbate (cataract). Similar AGE formation seen in smokers
(Saxena, et al., 2000)
Vitreopathy
 Vitreous liquefaction and PVD occur prematurely in diabetes,
where  AGEs cause abnormal crosslinking of collagen fibrils
and dissociation from hyaluronin (Stitt, et al., 1998)
VI. Ocular Hemodynamics in Diabetes
Diabetic eyes have defective vascular autoregulation
Hyperglycemia impairs endothelial pericyte contractility
Increased volume of retinal blood flow per unit of time precedes
ophthalmoscopically detectable retinopathy
Increased flow results in mechanical injury to retinal capillaries, shunting of
flow to larger caliber vessels, capillary leakage and non-perfusion
the
Retinal perfusion pressure (RPP) denotes the force of blood flow into the eye via
ophthalmic artery and predicts retinal vascular injury:
RPP is measured directly via ophthaldynamometry (ODM) but may be
closely
approximated (assuming no asymmetric carotid stenosis) by
measurement of
blood pressure and intraocular pressure
RPP = 2/3 (MAP) – IOP
where MAP = mean arterial pressure
MAP = (systolic BP – diastolic BP)/3 + diastolic BP
Higher IOP effectively lowers RPP, but less so than does lowering blood
pressure (examples):
Case 1
BP = 120/80
MAP = 93.3
and (a) IOP = 20 or (b) IOP = 10
(a) RPP = 42.2 (b) RPP = 52.2
Case 2
BP = 150/100 and (a) IOP = 20 or (b) IOP = 10
MAP = 116.6
(a) RPP = 57.8 or (b) RPP = 67.8
Conditions that increase retinal blood flow increase the risk and severity of DRT
Hyperglycemia
Systemic Hypertension
Pregnancy
Ocular Hypotension
Highest Risk of sight-threatening retinopathy (4-6 times RR)* when:
RPP > 50.1 mm (Type 1 DM)
MAP > 97.1 mm (Type 2 DM)
*Source: Sailesh, S. Kallis, C et al. Clinical and Hemodynamic risk factors for the presence of
sight threatening retinopathy at presentation to a diabetic retinopathy clinic. International
Diabetes Federation, Paris, 2003
Concordant with findings of the UKPDS but more predictive of STR
 Diabetic eyes cannot tolerate elevated blood pressure
Beware unnecessarily treating ocular HTN in diabetics with poor glycemic
control, uncontrolled systemic HTN and/or longstanding disease
Always measure blood pressure in patients with DM
VII. A New Model For Predicting and Preventing Diabetic Eye Disease
Increased Risk of Diabetic Eye Disease occurs with:
degree of hyperglycemia (enhanced superoxide mediated
microvasculopathy)
pregnancy (increased RPP and MAP)
elevated blood pressure (increased RPP and MAP)
disease duration (longer exposure to injurious glucose metabolites)
puberty (impact of growth hormone)
presence of other diabetes complications (convergent pathophysiologies)
lack of patient understanding, motivation, support

For All Eye Care Providers
Understand diabetes
Ask for the HBA1c; ask to see their log books; perform in-office glucometry
Measure their BP and calculate MAP & RPP
Look for each type of diabetic eye disease
Listen to, talk with, and counsel your patients with diabetes
Communicate your findings and recommendations to your
diabetic patients’ other health care providers
References on Benfotiamine
Ascher, E. et al. Thiamine reduces hyperglycemia-induced dysfunction in cultured endothelial cells.
Surgery. 130: 851-8 (2001)
Bitsch, R. et al. Bioavailability assessment of the lipophilic benfotiamine as compared to a water soluble
thiamin derivative. Ann. Nutr. Metab. 35: 292-6 (1991)
Brownlee, M. Biochemistry and molecular cell biology of diabetic complications. Nature. 414:
813-20 (2001)
Hammes, H. et al. Benfotiamine blocks three major pathways of hyperglycemic damage and
prevents diabetic retinopathy. Nature: Medicine. 9: 294-9 (2003)
Pomero, F.et al. Benfotiamine is similar to thiamine in correcting endothelial cell defects
induced by high glucose. Acta Diabetol. 38: 135-8 (2001)
References on AGEs in Ocular Disease
Albon, J. et al. Changes in the collagenous matrix in the aging human lamina cribrosa. BJO. 79:
368-75 (1995)
Amano, S. et al. Advanced glycation end products in human optic nerve head. BJO. 85: 52-5
(2001)
Kaji, Y. et al. Advanced glycation endproducts in diabetic corneas. Invest Ophthalmol Vis Sci.
41: 362-8 (2000)
Saxena, P. et al. Transition metal-catalyzed oxidation of ascorbate in human cataract extracts:
possible role of advanced glycation end products. Invest Ophthalmol Vis Sci. 41: 1473-81
(2000)
Stitt, AW et al. Advanced glycation endproducts (AGEs) colocalise with AGE receptors in the
retinal vasculature of diabetic and AGE infused rats. Am J Pathol. 150: 523-32 (1997)
Stitt, AW et al. Advanced glycation endproducts in vitreous: structural and functional
implications for diabetic vitreopathy. Invest Ophthalmol Vis Sci. 39: 2517-23 (1998)
Yamagishi, S. et al. Advanced glycation endproducts accelerate calcification in microvascular
pericytes. Biochem Biophys Res Comm. 258: 353-7 (1999)
Dr. Paul Chous’s Web Site: www.diabeticeyes.com
Dr. Chous’s email: [email protected]