Aldose reductase in the etiology of diabetic complications: 2. Nephropathy

The Role of Aldose Reductase in Polyol Pathway: An Emerging Pharmacological Target in Diabetic Complications and Associated Morbidities

The expression of aldose reductase leads to a variety of biological and pathological effects. It is a multifunctional enzyme which has a tendency to reduce aldehydes to the corresponding sugar.alcohol. In diabetic conditions, the aldose reductase enzyme converts glucose into sorbitol using nicotinamide adenine dinucleotide phosphate as a cofactor. It is a key enzyme in polyol pathway which is a surrogate course of glucose metabolism. The polyol pathway has a significant impact on the aetiology of complications in individuals with end-stage diabetes. The exorbitant level of sorbitol leads to the accumulation of intracellular reactive oxygen species in diabetic heart, neurons, kidneys, eyes and other vasculatures, leading to many complications and pathogenesis. Recently, the pathophysiological role of aldose reductase has been explored with multifarious perspectives. Research on aldose reductase suggest that besides implying in diabetic complications, the enzyme also turns down the lipid-derived aldehydes as well as their glutathione conjugates. Although aldose reductase has certain lucrative role in detoxification of toxic lipid aldehydes, its overexpression leads to intracellular accumulation of sorbitol which is involved in secondary diabetic complications, such as neuropathy, cataractogenesis, nephropathy, retinopathy and cardiovascular pathogenesis. Osmotic upset and oxidative stress are produced by aldose reductase via the polyol pathway. The inhibition of aldose reductase alters the activation of transcription factors like NF-ƙB. Moreover, in many preclinical studies, aldose reductase inhibitors have been observed to reduce inflammation-related impediments, such as asthma, sepsis and colon cancer, in diabetic subjects. Targeting aldose reductase can bestow a novel cognizance for this primordial enzyme as an ingenious strategy to prevent diabetic complications and associated morbidities. In this review article, the significance of aldose reductase is briefly discussed along with their prospective applications in other afflictions.

Osmolarity and glucose differentially regulate aldose reductase activity in cultured mouse podocytes

Podocyte injury is associated with progression of many renal diseases, including diabetic nephropathy. In this study we examined whether aldose reductase (AR), the enzyme implicated in diabetic complications in different tissues, is modulated by high glucose and osmolarity in podocyte cells. AR mRNA, protein expression, and activity were measured in mouse podocytes cultured in both normal and high glucose and osmolarity for 6 hours to 5 days. Hyperosmolarity acutely stimulated AR expression and activity, with subsequent increase of AR expression but decrease of activity. High glucose also elevated AR protein level; however, this was not accompanied by respective enzyme activation. Furthermore, high glucose appeared to counteract the osmolarity-dependent activation of AR. In conclusion, in podocytes AR is modulated by high glucose and increased osmolarity in a different manner. Posttranslational events may affect AR activity independent of enzyme protein amount. Activation of AR in podocytes may be implicated in diabetic podocytopathy.

Oxidative stress and diabetic kidney disease

The number of people with diabetic kidney disease continues to increase worldwide despite current treatments. Of the pathophysiologic mechanisms that have been identified in the development and progression of diabetic nephropathy, oxidative stress (more accurately described as increased levels of reactive oxygen species; ROS) is of major importance. The increase in ROS is due to both increased production and to decreased and/or inadequate antioxidant function. To date, human clinical trials with antioxidants have not been shown to be effective. This is likely due, at least in part, to the lack of specificity of current agents. Recent research has determined both major sources of high glucose-induced cellular ROS production as well as high glucose-induced changes in antioxidant function. Treatments targeted at one or more of the specific diabetes-induced alterations in the regulation of ROS levels will likely lead to effective treatments that prevent the development and progression of diabetic kidney disease.

myo-Inositol oxygenase: a radical new pathway for O(2) and C-H activation at a nonheme diiron cluster

The enzyme myo-inositol oxygenase (MIOX) catalyzes conversion of myo-inositol (cyclohexan-1,2,3,5/4,6-hexa-ol or MI) to d-glucuronate (DG), initiating the only known pathway in humans for catabolism of the carbon skeleton of cell-signaling inositol (poly)phosphates and phosphoinositides. Recent kinetic, spectroscopic and crystallographic studies have shown that the enzyme activates its substrates, MI and O(2), at a carboxylate-bridged nonheme diiron(ii/iii) cluster, making it the first of many known nonheme diiron oxygenases to employ the mixed-valent form of its cofactor. Evidence suggests that: (1) the Fe(iii) site coordinates MI via its C1 and C6 hydroxyl groups; (2) the Fe(ii) site reversibly coordinates O(2) to produce a superoxo-diiron(iii/iii) intermediate; and (3) the pendant oxygen atom of the superoxide ligand abstracts hydrogen from C1 to initiate the unique C-C-bond-cleaving, four-electron oxidation reaction. This review recounts the studies leading to the recognition of the novel cofactor requirement and catalytic mechanism of MIOX and forecasts how remaining gaps in our understanding might be filled by additional experiments.

Simultaneous determination of cell aging and ATP release from erythrocytes and its implications in type 2 diabetes

Glucose-6-phosphate dehydrogenase (G6PD) is a determinant in the antioxidant status of the red blood cell (RBC) and is also used as an indicator of cell age. However, it is unknown if the relationship among antioxidant status, cell age, and RBC-derived adenosine triphosphate (ATP) occurs immediately or over a period of time. Therefore, the development of a simultaneous determination of G6PD activity (via the determination of nicotinamide adenine dinucleotide phosphate (NADPH)) in RBCs and the determination of deformation-induced RBC-derived ATP is described. The NADPH and ATP were determined while undergoing a chemically induced aging process via inhibition of G6PD with dehydroepiandroesterone (DHEA). Upon incubation with DHEA for 30 min, NADPH levels measured in a flow stream decreased to 7.96+/-1.10 microM from an original value of 13.20+/-1.80 microM in a 0.02% solution of RBCs. In order to demonstrate a direct relationship between G6PD activity and deformation-induced ATP release from RBCs, a simultaneous microflow determination of G6PD activity and ATP release was performed. Upon inhibition with DHEA, NADPH levels decreased to 8.62+/-0.29 microM from its original value of 12.73+/-0.50 microM while ATP release decreased from 0.21+/-0.07 microM to 0.06+/-0.02 microM. These values were validated by an examination of NADPH levels in, and ATP release from, RBC fractions containing younger and older cells (separated by cell density centrifugation). This determination provides evidence that antioxidant status in the RBC and its ability to release ATP, a known stimulus of nitric oxide production, are closely related.

Intervention with the aldose reductase inhibitor, tolrestat, in renal and retinal lesions of streptozotocin-diabetic rats

The progressive increase in urinary albumin excretion, which precedes the development of diabetic nephropathy, can be prevented in diabetic rats if the aldose reductase inhibitor, tolrestat, is administered at the initiation and throughout the duration of hyperglycaemia. We therefore determined the ability of tolrestat to intervene in the further progression of already established urinary albumin excretion of streptozotocin-diabetic female Wistar rats. Two months after streptozotocin injection, diabetic rats were grouped as low-urinary albumin excretion (0.2-1.0 mg albumin/day) or high-urinary albumin excretion (1.9-5.9 mg albumin/day), at which time tolrestat intervention (25 mg/kg per day) was begun for half of the diabetic rats in each urinary albumin excretion group. After six months of treatment tolrestat caused a significant reduction in the urinary albumin excretion rate of the low-urinary albumin excretion group only. The diabetes-induced rise of total urinary protein in both groups was significantly reduced by tolrestat. Furthermore, the diabetes-induced increase (49%) in the thickness of the basement membranes of retinal capillaries from the outer plexiform layer was significantly diminished by tolrestat administration. In conclusion, intervention therapy with the aldose reductase inhibitor, tolrestat, can reduce the progression of urinary albumin excretion and retinal basement membrane thickening in long-term diabetic rats.