Increased erythrocyte aldose reductase activity in type 1 diabetic patients

Genetic components in diabetic retinopathy

Diabetic retinopathy (DR) is a serious complication of diabetes, which is fast reaching epidemic proportions worldwide. While tight glycemic control remains the standard of care for preventing the progression of DR, better insights into DR etiology require understanding its genetic basis, which in turn may assist in the design of novel treatments. During the last decade, genomic medicine is increasingly being applied to common multifactorial diseases such as diabetes and age-related macular degeneration. The contribution of genetics to the initiation and progression of DR has been recognized for some time, but the involvement of specific genes and genetic variants remains elusive. Several investigations are currently underway for identifying DR susceptibility loci through linkage studies, candidate gene approaches, and genome-wide association studies. Advent of next generation sequencing and high throughput genomic technologies, development of novel bioinformatics tools and collaborations among research teams should facilitate such investigations. Here, we review the current state of genetic studies in DR and discuss reported findings in the context of biochemical, cell biological and therapeutic advances. We propose the development of a consortium in India for genetic studies with large cohorts of patients and controls from limited geographical areas to stratify the impact of the environment. Uniform guidelines should be established for clinical phenotyping and data collection. These studies would permit identification of genetic loci for DR susceptibility in the Indian population and should be valuable for better diagnosis and prognosis, and for clinical management of this blinding disease.

Aldose reductase (-106) C/T gene polymorphism and possibility of macrovascular complications in Egyptian type 2 diabetic patients

INTRODUCTION

Over the past three decades, the number of people with diabetes mellitus (DM) has more than doubled globally, making it one of the most important public health challenges to all nations. Aldose reductase (AR) is a rate-limiting enzyme in the polyol pathway, which has been implicated in the pathogenesis of diabetic microvascular complications; however, the association of the AR gene with diabetic macrovascular complications has rarely been investigated.

AIM

The study aimed to identify the possible association between C(-106) T polymorphism of the AR gene and diabetic macroangiopathy in a cohort of Egyptian patients with type 2 DM.

SETTINGS AND DESIGN

This study was conducted on 100 Egyptian subjects, the control group (n = 20) and the patient group (n = 80) with type 2 diabetes which were further subdivided into two subgroups with (n = 48) and without macroangiopathic complications (n = 32) as evidenced by carotid intima-media thickness, electrocardiography (ECG) ischemic changes, cerebrovascular insufficiency, and peripheral vascular insufficiency.

SUBJECTS AND METHODS

All studied subjects were subjected to detailed history taking, clinical examination, ECG, carotid ultrasonography, routine laboratory investigations, and molecular studies including the detection of AR C(-106) T gene polymorphisms using the polymerase chain reaction (PCR)/restriction fragment length polymorphism technique.

RESULTS

The genotype distribution and allele frequency of AR C(-106) T showed no statistical significance also the genotypes were not associated with any of the different studied parameters.

CONCLUSIONS

The results suggest that the C(-106) T polymorphism in the AR gene is not involved in the pathogenesis of macroangiopathy in type 2 diabetes.

Aldose reductase activity and glucose-related opacities in incubated lenses from dogs and cats

OBJECTIVE

To determine responses of canine and feline lenses to incubation in a medium with a high glucose concentration.

SAMPLE POPULATION

Lenses from 35 dogs and 26 cats.

PROCEDURE

Glucose concentrations were measured in paired lenses from 25 dogs and 17 cats after incubation for 14 days in high-glucose (30 mmol of glucose/L) or control (6 mmol of glucose/L) medium. Aldose reductase activity was measured spectrophotometrically in the incubated lenses and in freshly frozen lenses from 10 dogs and 9 cats. Two lenses of each group were studied histologically.

RESULTS

Canine and feline lenses in high-glucose medium developed glucose-specific opacities of variable localization and extent. Canine lenses developed equatorial vacuoles, but severity of the lesions was not associated with the age of the dog. Lenses from young cats (< or = 4 years old) developed extensive posterior cortical opacities, whereas those from older cats (> 4 years old) did not. Glucose concentrations were similar in all lenses incubated in high-glucose medium; however aldose reductase activity was significantly lower in lenses from older cats, compared with lenses from young cats and from dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

High aldose reductase activity and glucose-related opacities suggest a central role for this enzyme in the pathogenesis of diabetic cataracts in dogs and cats. Because onset of diabetes mellitus usually occurs in cats > 7 years of age, low activity of aldose reductase in lenses of older cats may explain why diabetic cataracts are rare in this species despite hyperglycemia.

Aldose reductase inhibitors: therapeutic implications for diabetic complications

The 'late complications' of diabetes mellitus, i.e., nephropathy, neuropathy and retinopathy are firmly rooted in inadequate control of blood glucose: hyperglycaemia. Hyperglycaemia causes elevated cytosolic glucose and/or rates of glucose metabolism, i.e., 'hyperglysolia,' within cells of vulnerable tissues. Although the molecular basis for the pathogenic effects of hyperglysolia remains to be proven, substantial evidence points to a key role for increased glucose metabolism through a cytosolic enzyme, aldose reductase (AR). Recent human genetic and biochemical data link polymorphisms of the AR gene (technically called the AR2 gene) and elevated tissue levels of AR with strongly altered risks for diabetic complications. Despite several genetic reports failing to confirm such an association, there are now ten concordant reports from five continents that certain polymorphisms of the AR gene are associated with an ~ 3- to 20-fold higher risk for diabetic complications. Moreover, in US and European diabetic study populations the principle allele of the AR gene associated with elevated disease risk, the Z-2 allele, correlates with an ~ 2- to 3-fold increase in AR expression. These results, together with recent clinical, experimental and pharmacological data, provide powerful new support for the rationale for research and development of aldose reductase inhibitors (ARIs) targeted at slowing the progression of diabetic complications. Although past clinical trials of ARIs have been disappointing, this has stemmed from overly optimistic expectations, inadequate trial designs and lack of pharmacological robustness and/or acceptable systemic toleration of the agents tested. However, a more realistic and encouraging perspective for therapeutic expectations for ARIs has arisen from recent data revealing that the seemingly modest short-term effects of intensified glycaemic control and of pancreatic transplantation are followed by substantial long-term benefits on diabetic complications. In addition, robust inhibition of AR in human nerve has recently yielded dose-dependent efficacy on nerve structure and function. Thus, the quest for well-tolerated, potent ARIs continues to be a worthy and more urgent objective than ever before.

Aldose reductase: a window to the treatment of diabetic complications?

Kinetic studies on the aldose reductase protein (AR2) have shown that it does not behave as a classical enzyme in relation to ring aldose sugars. These results have been confirmed by X-ray crystallography studies, which have pinpointed binding sites for pharmacological "aklose reductase inhibitors" (ARIs). As with non-enzymic glycation reactions, there is probably a free-radical element involved derived from monosaccharide autoxidation. In the case of AR2, there is free radical oxidation of NADPH by autoxidising monosaccharides, enhanced in the presence of the NADPH-binding protein. Whatever the behaviour of AR2, many studies have showed that sorbitol production is not an initiating aetiological factor in the development of diabetic complications in humans. Vitamin E (alpha-tocopherol), other antioxidants and high fat diets can delay or prevent cataract in diabetic animals even though sorbitol and fructose levels are not modified; vitamin C acts as an AR1 in humans. Protein post-translational modification by glyc-oxidation or other events is probably the key factor in the aetiology of diabetic complications. There is now no need to invoke AR2 in xylitol biosynthesis. Xylitol can be produced in the lens from glucose, via a pathway involving the enzymes myo-inositol-oxygen oxidoreductase, D-glucuronate reductase. L-gulonate NAD(+)-3-oxidoreductase and L-iditol-NAD(+)-5-oxidoreductase, all of which have recently been found in bovine and rat lens. This chapter investigates the molecular events underlying AR2 and its binding and kinetics. Induction of the protein by osmotic response elements is discussed, with detailed analysis of recent in vitro and in vivo experiments on numerous ARIs. These have a number of actions in the cell which are not specific, and which do not involve them binding to AR2. These include peroxy-radical scavenging and recently discovered effects of metal ion chelation. In controlled experiments, it has been found that incubation of rat lens homogenate with glucose and the copper chelator o-phenanthroline abolishes production of sorbitol. Taken together, these results suggest AR2 is a vestigial NADPH-binding protein, perhaps similar in function to a number of non-mammalian crystallins which have been recruited into the lens. There is mounting evidence for the binding of reactive aldehyde moieties to the protein, and the involvement of AR2 either as a 'housekeeping' protein, or in a free-radial-mediated 'catalytic' role. Interfering with the NADPH binding and flux levels--possibly involving free radicals and metal ions--has a deleterious effect. We have yet to determine whether aldose reductase is the black sheep of the aldehyde reductase family, or whether it is a skeleton in the cupboard, waiting to be clothed in the flesh of new revelations in the interactions between proteins, metal ions and redox metabolites.

Increased white cell aldose reductase mRNA levels in diabetic patients

This paper examines the question of whether diabetes in humans is associated with changes in aldose reductase and sorbitol dehydrogenase gene expression. The polyol pathway, which comprises the enzymes aldose reductase and sorbitol dehydrogenase, is recognised to play a central role in the pathogenesis of the diabetic complications. Whilst it is known that experimental diabetes in the rat is associated with increased aldose reductase gene expression, possibly as an osmoregulatory response to hyperglycaemia, little is known about aldose reductase and sorbitol dehydrogenase gene expression in diabetes in humans. White cell aldose reductase mRNA levels were increased in patients with insulin-dependent (by 135%, P < 0.05) and non-insulin-dependent (by 132%, P < 0.05) diabetes compared to levels in healthy volunteers. Levels of glycosylated haemoglobin were also increased (P < 0.001) in diabetes but there was no correlation between white cell aldose reductase mRNA and glycosylated haemoglobin levels. In contrast to aldose reductase, levels of white cell sorbitol dehydrogenase mRNA were not affected by diabetes. These results establish for the first time that diabetic patients show increases in white cell aldose reductase mRNA levels, possibly consistent with increased aldose reductase gene expression. This finding may have implications for the use of aldose reductase inhibitors in the treatment of the diabetic complications.

Structure-function relationships within peripheral nerves in diabetic neuropathy: the hydration hypothesis

To define the quantitative relationship between peripheral nerve structure and function imposed by endoneurial oedema in the diabetic state, we determined values for sural nerve hydration structure as measured by magnetic resonance spectroscopy, and for neurological function with scores for nerve conduction properties (NCV-score), neuropathic symptoms (NS-score), and examination signs (NE-score). The coefficient of sural nerve hydration was elevated to 30 +/- 6% (p < 0.05) in 79 symptomatic neuropathic diabetic subjects with an average of 15 years of diabetes mellitus, compared to a value of 25 +/- 3% in 72 non-diabetic control subjects. In contrast, in 75 asymptomatic diabetic subjects with an average of 6 additional years of diabetes, the mean hydration coefficient was only 28 +/- 5% (p < 0.05). A nerve hyperhydration state was identified with a prevalence of 25% within the asymptomatic group characterized by nerve hydration greater than the 95th percentile, early changes in nerve electrophysiology and neurological examination, but with no symptomatology of neuropathy. Stratification of the symptomatic neuropathic group by worsening nerve electrophysiology, demonstrates a coincident deterioration in neurological examination (RR = 5.39 at maximum NCV-score), and neuropathy symptomatology (RR = 4.80 at maximum NE-score). The present data are consistent with the hypothesis that endoneurial oedema initiates deterioration sequentially in nerve electrophysiology, followed by abnormal findings on neurological examination, preceding the patient's final perception of symptomatic stocking glove peripheral diabetic neuropathy.

Regulation and control of glucose overutilization in erythrocytes by vanadate

The insulin mimetic effect of vanadate in in vitro incubation of erythrocytes with high glucose concentrations showed an increase in sorbitol accumulation and glucose utilization using U-14C-glucose. Aldose reductase inhibitors and vanadate addition reversed the sorbitol accumulation, whereas insulin could not reverse it. Increased glucose utilization was also normalized with vanadium compounds. Increased activity of aldose reductase and sorbitol levels in diabetic animals were also normalized with vanadate treatment.

Association of erythrocyte aldose reductase activity with diabetic complications in type 1 diabetes mellitus

Erythrocyte aldose reductase was isolated and its activity measured in 72 Type 1 (insulin dependent) diabetic patients and 21 age and sex matched non-diabetic subjects. The diabetic patients were categorized into two groups in terms of presence (n = 29) or absence (n = 43) of severe diabetic complications. Age, sex, duration of diabetes and HbA1c levels were matched between the diabetic groups. Erythrocyte aldose reductase (mean +/- SEM) was increased in patients with Type 1 diabetes compared to the non-diabetic subjects (7.22 +/- 0.24 vs 5.66 +/- 0.19 Ul-erythrocytes-1, < 0.0001). There was a four-fold variation in its activity among the diabetic patients (3.38-12.23 Ul-erythrocytes-1). The enzyme activity was significantly higher in patients with complications than those without (8.17 +/- 0.39 vs 6.58 +/- 0.26 Ul-erythrocytes-1, p < 0.002). When the patients were stratified by duration of the disease, the enzyme activity was highest in patients who had developed complications with a duration of less than 20 years and lowest in those without complications for 20 years or longer (8.54 +/- 0.48 vs 6.46 +/- p +/- 0.33 Ul-erythrocytes-1, p < 0.002). Patients who had an aldose reductase activity greater than the mean +/- 2SD of that seen in non-diabetic controls were four times more likely to have diabetic complications than those whose enzyme activity fell within 2SD of non-diabetic individuals (p < 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in cardiac muscle fructose content in streptozotocin-induced diabetic rats

To evaluate the activation of the sorbitol pathway in cardiac muscle in diabetic rats, we measured sorbitol, fructose, and myo-inositol content in cardiac tissue obtained from control and streptozotocin-diabetic rats, with or without an 8-week insulin treatment, using gas chromatography-mass spectrometry (GC-MS). Cardiac fructose and sorbitol content in 10-week diabetic rats increased by 60-fold and 3.9-fold of those of control rats, respectively (P less than .001). In contrast, cardiac myo-inositol content in 10-week diabetic rats decreased to 56% (P less than .025) of the control value. The abnormalities in cardiac fructose, sorbitol, and myo-inositol content were completely normalized by the 8-week insulin treatment, which was initiated 2 weeks after the induction of diabetes. There was no difference in cardiac aldose reductase activity between control and diabetic rats. However, cardiac sorbitol dehydrogenase activity in diabetic rats was 151% (P less than .005) higher than that of control rats, although hepatic sorbitol dehydrogenase activity was not different between the two groups. These results indicate that the sorbitol pathway is significantly activated in cardiac tissue obtained from streptozotocin-induced diabetic rats, which results in the marked cardiac accumulation of fructose.

Correlation between erythrocyte aldose reductase activity and the width of skeletal-muscle capillary basement membrane in insulin-dependent diabetes mellitus

Thickening of capillary basement membrane has been demonstrated in diabetic subjects, and it is considered to be the characteristic pathological lesion of diabetic microvascular disease. There are studies reporting the effects of inhibitors of aldose reductase, the first enzyme of the polyol pathway, on the thickening of the capillary basement membrane. These observations indicate a significant role of the polyol pathway in the development of microvascular disease. However, it is unknown whether or not there is any correlation between the thickness of the capillary basement membrane and the activity of aldose reductase in diabetic patients. To clarify this issue, we measured the width of skeletal-muscle basement membrane and erythrocyte aldose reductase activity in 27 insulin-dependent diabetic and 8 nondiabetic individuals. The results showed that both the aldose reductase activity and the width of capillary basement membrane were increased in diabetic patients as compared to nondiabetic individuals (6.89 +/- 0.38 versus 5.15 +/- 0.60 mL/mU erythrocytes, p < 0.05 and 2257 +/- 166 versus 1136 +/- 69 A, p < 0.0001, respectively) (mean +/- SE), but marked variability was observed in both the enzyme activity and the basement membrane thickness among the diabetic patients. There was a significant correlation between the capillary basement membrane thickness and the activity of erythrocyte aldose reductase (r = 0.51, p < 0.01) in diabetic patients. Our data suggest that the polyol pathway plays an important role in thickening of capillary basement membrane in diabetic individuals, and the variability in aldose reductase activity seen among diabetic patients may result in the varying susceptibility to the development of diabetic microvascular disease.

No correlation between glycemic control and an increase in erythrocyte aldose reductase activity in type I and type II diabetic patients

Aldose reductase, the first enzyme of the polyol pathway, has been related to the pathogenesis of diabetic complications. The regulation of the enzyme in diabetes patients, however, has not yet been clarified. We recently reported that the activity of aldose reductase was increased in erythrocytes of insulin-dependent diabetes mellitus patients but short-term hyperglycemia did not affect the enzyme activity. It is still unclear, however, whether or not the increase in the enzyme activity is caused by long-term hyperglycemia and thus would be seen equally in both type I (insulin-dependent diabetes mellitus) and type 2 (non-insulin-dependent diabetes mellitus) individuals. To further clarify these issues we measured erythrocyte aldose reductase activity in 46 type I patients and 30 type II patients who had variable glucose control and in 16 nondiabetic subjects. We compared the enzyme activity with plasma glucose levels and hemoglobin A1c levels. The results show that erythrocyte aldose reductase activity is increased in both type I and type II patients as compared with nondiabetic subjects (7.1 +/- 0.3 U/L and 6.8 +/- 0.4 U/L erythrocytes versus 5.6 +/- 0.2 U/L erythrocytes, p less than 0.001 and p less than 0.01, respectively), but there were no significant differences between the two groups of diabetic patients. The enzyme activity varied by approximately four times among the diabetic individuals but there was no correlation between the enzyme activity and plasma glucose or hemoglobin A1c levels. We conclude that the increased activity of erythrocyte aldose reductase seen in diabetes is not related to hyperglycemia.

Neutrophil aldose reductase activity as a potential marker for neuropathy and cataract in diabetes

Aldose reductase activity can be measured in the neutrophil and it has been proposed that this may be a marker for risk of complications in diabetes. We have studied aldose reductase activity in neutrophil, nerve, and lens in diabetic patients undergoing sural nerve biopsy or cataract extraction. A correlation was demonstrated between lens and neutrophil aldose reductase activity (r = 0.53, p = 0.01) but no correlations were demonstrated between nerve aldose reductase activities and nerve morphometry, nerve function or neutrophil aldose reductase activity. No significant difference was found between neutrophil aldose reductase activities in groups of patients with severe neuropathy, or cataract, or no complications (24 (interquartile range 16-32) vs 24 (16-40) vs 24 (16-40) nmol NADPH min-1 10(8)-cells-1). In a group of 56 Type 1 diabetic patients screened within 6 years of diagnosis, multiple regression analysis failed to show any relationship between neutrophil aldose reductase activity and abnormalities of neurophysiological function. These results suggest that neutrophil aldose reductase activity cannot be used as a marker for the development of cataract or neuropathy in diabetes.