IJCRR

IJCRR - 9(11), June, 2017

Pages: 48-52

Date of Publication: 12-Jun-2017

Significance of Aldose Reductase in Diabetic cataract

Author: Geeta Bhatia, Subodhini Abhang, A. N. Sontakke

Category: Healthcare

Abstract:Background: Cataract is a major cause of blindness. Diabetes mellitus is a major risk factor for the development of cataract. Diabetic patients have 25 times higher risk of cataract than non-diabetic population. The metabolic changes accompanying hyperglycemia is increased activity of the polyol pathway and aldose reductase (AR) is a key enzyme of polyol pathway. Aldose reductase is responsible for generation of more oxidative stress by decreasing GSH in diabetic patients which is thought to be a major factor to initiate the process of cataractogenesis. The present study was designed to determine significance of aldose reductase diabetic cataract and its correlation with reduced glutathione (GSH) and Malondialdehyde (MDA) in diabetic cataract patients.
Methods: In this study we measured MDA as oxidative stress marker andGlycated Hb (HbA1c) glycemic index marker levels of GSH and AR in erythrocytes of Type2 diabetic cataract patients (n = 30) and non diabetic senile cataract patients (n=30) compared with age matched normal controls(n=30).
Results: We found increased levels of AR, HbA1c and MDA, and decreased levels of GSH in diabetic cataract patients compared to non diabetic senile cataract patients and normal controls.
Conclusions: From the result it is concluded that AR play a major role in generation of more oxidative stress in diabetic patients which may be the cause of early cataractogenesis in diabetic patients as compared to non diabetic senile cataract patients.

Keywords: Cataract, Aldose Reductase (AR), Malondialdehyde (MDA), Reduced Glutathione (GSH), Glycated Hb (HbA1c).

Full Text:

INTRODUCTION:

Population growth, ageing, urbanization, sedentary lifestyles and an increasing prevalence of obesity are increasing the number of people with diabetes mellitus. The global prevalence of diabetes was estimated to be 2.8% in 2000 and is expected to reach 4.4% by 2030. Cataract is the leading cause of blindness in the world, responsible for 48% of blindness worldwide. A putative cause for age-related  cataract is oxidative  stress ¹ . Chronic hyperglycemia is a major determinant in the development of secondary complications of diabetes, such as diabetic cataract. Evidence indicate that both the duration of diabetes and the quality of glycemic control are the most important risk factors for cataract formation ². According to WHO survey, India  will be the world’s diabetic  capital in near future.  Globally, cataract remains the leading cause of blindness, affecting approximately 18 million people. Cataract occurs at an earlier age and is 2–5 times more frequent in patients with diabetes, thus the visual loss has a significant impact on the working population ³

Diabetes mellitus is recognized as a leading located in the eye (cornea, retina, lens) is a key enzyme of polyol pathway. Under normal glycemic conditions, only a small fraction of glucose is metabolized through the polyol pathway, as the majority is phosphorylated by hexokinase, and the resulting product, glucose-6-phosphate, is utilized a substrate for glycolysis or pentose phosphate metabolism⁴.  However, in response to the chronic hyperglycemia found in diabetics, glucose flux through the polyol pathway is significantly increased. Up to 33% of total glucose utilization in some tissues  e.g. eye  can be through the polyol pathway⁵.

In  hyperglycemia excessive amount of   glucose  is   diverted  to   the  polyol pathway,  where  AR  reduces   glucose   into   sorbitol   at   the   expense  of NADPH. Sorbitol is as  an   osmolyte leads to osmotic swelling ,changes in the membrane permeability ,leakage of glutathione, myoinositol ,the generation of free radicals and hydrogen peroxide which primarily causing the diabetic complications such as cataract, retinopathy and neuropathy^6.  Since  NADPH   is  essential     for  generation   of      GSH   (intracellular antioxidant)   from   GSSG, the depletion   of   NADPH   by   the   AR   pathway   may impair intracellular   antioxidant   defence.   Sorbitol   is   then    converted     to fructose   by  SDH   with   the   production   of   NADH,   potentially    leading   to increased ROS  via   NADH  oxidase  Activity  of aldose reductase  is dependent on NADPH. GSH   is required for regeneration of NADPH. So indirectly the activity of AR is in turn depends on GSH. So in present study was designed  to  determine  the levels of  aldose reductase and GSH  and their role in contribution of oxidative stress to diabetic cataract.

MATERIALS AND METHODS:

The study comprises of total 90 subjects  were divided into three groups  aged between

 50 -80 years. The subjects were selected from ophthalmic OPD of   B. J. Medical College and Sassoon Hospitals Pune

GROUP  I - (n = 30)    Senile cataract patients.

GROUP II- (n =30)    Diabetic  cataract patients and

GROUP II- (n =30)    Normal healthy controls

Inclusion criteria – Senile cataract subjects had normal fasting blood glucose level with no history of diabetes. Diabetic cataract subjects having diabetes for last 12-15 years and were using oral hypoglycaemic agents

Exclusion criteria: - Patients with ocular surgery, trauma, infection inflammation of eye, known cases of cardiovascular disorders, rheumatoid arthritis and carcinomas where free radical damages has been commonly incriminated were excluded from the study.

The study was approved by the Institutional Ethics Committee of B.J. Medical college Pune. A written informed consent was taken from the subjects.10 ml of venous blood was collected during preoperative period in plain vacutainer, EDTA and ACD (Acid Citrate Dextrose)  bulb  under aseptic precaution. Serum and hemolysate was used for investigations.

Blood samples of  all three groups were analyzed  for following  parameters .

Aldose reductase activity (AR) –by Hayman and Kinoshita 1965 ⁷

Glycated Hb (HbA1c) - by Ion exchange resin method ⁸

Reduced glutathione (GSH) -   by  Beutler  et al.1963 ⁹

Malondialdehyde (MDA)  - by Buege and Aust. 1978 method ¹⁰

Statistical analysis: The data were expressed as mean ± standard deviation. Mean values were compared by one-way ANNOVA . Differences between comparison groups were considered to be significant where p<0.05. Post hoc donnet test for inter group comparison and correlation was used.

RESULTS:

Table no 1: The mean  age of diabetic cataract patients ( Group II) was significantly lowered as compared to controls (Group III),and as compared to senile cataract patients (Group I) (Difference in age in between Senile and control were not statistically significant.

Table no 2: It is observed that  the levels of HbA1C were  significantly increased  in diabetic cataract patients(Group II)  as compared to controls (Group III) and the levels of HbA1C in  diabetic cataract  were  significantly  higher as compared to senile cataract patients

Difference   of mean of HbA1C between senile and control was  not statistically significant.

It   is observed that activity of AR  in diabetic cataract  patients  were significantly increased as compared to controls .Also activity of AR was significantly higher in diabetic cataract patients  as compared to senile cataract patients .

 There  was no  statistically significant difference was seen in  activity of AR of senile cataract patients as compared to controls

MDA   represents   lipid peroxidation and act as  oxidative stress marker . It is observed that   levels of MDA in diabetic cataract patients (Group II) were significantly increased as compared  to controls (Group III)

 MDA levels in senile cataract patients (Group I) were significantly increased as compared to controls (Group III) )

  In diabetic cataract patients (Group II) the levels  of  MDA were significantly higher as compared to senile cataract patients (Group I)

It  is  observed that levels of   reduced glutathione were significantly decreased in senile cataract (Group I) and   diabetic cataract patients (Group II) as compared to controls statistically significant difference in reduced glutathione levels between senile and diabetic cataract were seen .

Table no 3 Showed  Pearson’s coefficient  Correlation  (r)  between AR and other parameters in    study group I (senile cataract patients).It is observed that There was no correlation between ,AR and HbA1c AR and MDA and AR and GSH in Senile cataract patients.

Table no 4 :Showed Pearson’s coefficient  Correlation  (r)  between AR and other parameters in    study group II (Diabetic cataract patients) It is observed that There is no correlation between  AR and mean age and between  AR and duration of diabetes  of diabetic cataract patients . ii) There is highly significant positive correlation between AR and HbA1c AR and MDA in diabetic cataract patients iii) There was a significant  negative  correlation between AR and GSH.

Discussion:

In this study, we attempted to evaluate the role of Aldose Reductase in cataractogenesis of  diabetic  patients. In present  study  it  is found increased levels of HbA1C, AR, MDA and decreased levels of  GSH  in  diabetic cataract  patients as compared to senile cataract patients  and as compared to controls.The mechanism of cataractogenesis in diabetes is associated with oxidative stress and osmotic stress.  Eye is an organ that is continuously  being exposed to oxidative stress throughout the life. Aging is a major risk factor for cataract and the prevalence is slightly higher in women considering all types of cataract. Increasing age is directly related with all types of cataracts¹¹.

The mean age of diabetic cataract patients were significantly lower as compared to senile cataract patients (Table no 1).From the results  it is observed that  the process of cataractogenesis occur  at early  an age in diabetic patients as compared to non diabetic senile cataract patients. Our results are supported by the findings of studies of Deepa K et al ¹² and  Anjuman Gul et al ¹³.

Complications in diabetes depending on duration of diabetes and glycemic control HbA1c ( glycated Hb)  is a good indicator of glycemic control . Measurement HbA1c  reflects glycemic control for last 4-6 weeks

In present study the levels of HbA1c were significantly increased in diabetic cataract patients as compared to senile cataract patients and as compared to controls (Table no 2). Increased levels of HbA1c in diabetic cataract patients indicated the poor glycemic control which might be responsible for early cataract formation in diabetic patients as compared to non diabetic senile cataract patients .From this It is clear that an increased glycated haemoglobin level is associated with increased risk of cataract in patients with diabetes^14,15. Our results are strongly supported by studies of various investigators M Lind et al ¹⁶, Anjuman Gul et al¹³, Bhavna et al ¹⁷.

Poor glycemic control leads to hyperglycemia and hyperglycemia  induces polyol pathway. Aldose reductase is  a key enzyme of this pathway.

We found high levels of AR in diabetic cataract patients as compared to senile cataract patients and as compared to controls (Table no 2). Hyperglycemia causes more and more glucose to enter in the polyol pathway which stimulates Aldose reductase (AR) that might facilitate the process of cataractogenesis.

A significant positive correlation between AR and HbA1c in diabetic cataract group (Table no 4) indicates that a significant increase in Aldose reductase activity in diabetic cataract patients due to hyperglycemia. Aldose Reductase (AR ) catalyzes the reduction of glucose to sorbitol through polyol pathway. The sorbitol pathway is stimulated in diabetes in those tissues that do not require insulin for cellular glucose uptake, such as the retina, kidney, peripheral nerves and blood vessels¹⁸. So there is free entry to glucose in lens and retina. AR reduces glucose to sorbitol. Which accumulates in lens and exerts osmotic stress leads to cataract formation. Our results strongly support the hypothesis that hyperglycemia induces/stimulates polyol pathway at the same time the diabetes reduces age of cataractogenesis.

In our study we found significant decreased levels of  GSH in diabetic cataract group as compared to controls and as compared to senile cataract patients (Table no 2).

In addition to this a significant negative correlation was found in between AR and GSH (Table 4). Hyperglycemia is characterised by increase oxidative stress. Thus hyperglycemia is also responsible for increased flux of glucose in polyol pathway increasing the AR activity. In this Sorbitol is formed by AR  at the expense  of NADPH. Increased activity of AR decreases the concentration of GSH as there is less availability of NADPH   for generation of GSH. This is explained by figure 1. Thus these results shows that depletion in GSH level might be responsible for increased oxidative stress. Our results are supported by the work of various investegators who reported the increase in oxidative stress  in diabetic cataract by Oshi et al¹⁹  and G. Bhanuprakash Reddy  et al ²⁰.

Oxidative stress is characterized by an increase in the concentration of free radicals  which can cause damage at different levels of cellular organization. Oxidative stress causes ocular membrane lipid peroxidation as well as lens protein oxidation MDA is a indicator of lipid peroxidation²¹.

In our study we have observed high levels of MDA in diabetic and senile cataract patients  as compared to controls. An increase in MDA levels in diabetic cataract patients is more as compared to senile cataract patients may be due to poor glycemic control because  a strong significant positive correlation was found in between MDA and HbA1c in diabetic cataract patients ( r = 0.88,p< 0.0001).These findings are supported by the work of Garg et al²² and Donma et al ²³.  They showed that increased levels of lipid peroxidation product (MDA) in diabetes was due to increased production of reactive oxygen species caused by hyperglycemic status, hyperinsulemia and hyperlipidemia, which are commonly associated with  diabetes ^24-26. This indicates that in diabetes due to hyperglycemia   there is increase in lipid peroxidation which might be responsible for additional oxidative stress in diabetics .Based on the results of the AR in current study ,it is reasonable to hypothesize  that AR activity above the threshold level in diabetics, might be predisposed to develop cataract. Secondly polyol pathway is responsible for generation of osmotic stress and oxidative stress in diabetics

Conclusions: From the result it is concluded that the extent of oxidative stress in diabetic cataract patients are more as compared to non diabetic senile cataract patients and AR is responsible  for   that generation of  more oxidative stress. More oxidative stress may be the cause of early cataractogenesis in diabetic patients as compared to non diabetic senile cataract patients.

Further studies are required for therapeutic use of ARI (Aldose reductase inhibitors) to diabetic patients which may be beneficial in delaying the cataract formation in diabetics.

Acknowledgement:

Authors acknowledge the immense help received from the scholars whose articles are cited and included in references of this manuscript. The authors are also grateful to authors / editors / publishers of all those articles, journals and books from where the literature for this article has been reviewed and discussed.

DECLARATIONS:

Sources of Funding: No funding sources

Conflict of Interest: None declared

Ethical approval: The study was approved by the institutional ethics committee of B.J. Medical college  Pune.

FIGURE 1| :  Role of aldose reductase (AR) in hyperglycemia-induced oxidative  stress. Excessive amount of  glucose is shunted to the polyol pathway,  where AR reduces glucose into sorbitol at the expense of NADPH. Since NADPH   is essential for generation of GSH (intracellular antioxidant) from GSSG, the depletion of NADPH by the AR pathway may impair intracellular antioxidant defense .Sorbitol is then converted to fructose by SDH with the production of NADH ,potentially leading to increased ROS via NADH oxidase

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