Effect of sorbitol dehydrogenase inhibition on sugar cataract formation in galactose-fed and diabetic rats

Oxidative Stress-Induced TRPV2 Expression Increase Is Involved in Diabetic Cataracts and Apoptosis of Lens Epithelial Cells in a High-Glucose Environment

Cataracts are a serious complication of diabetes. In long-term hyperglycemia, intracellular Ca²⁺ concentration ([Ca²⁺]_i) and reactive oxygen species (ROS) are increased. The apoptosis of lens epithelial cells plays a key role in the development of cataract. We investigated a potential role for transient receptor potential vanilloid 2 (TRPV2) in the development of diabetic cataracts. Immunohistochemical and Western blotting analyses showed that TRPV2 expression levels were significantly increased in the lens epithelial cells of patients with diabetic cataracts as compared with senile cataract, as well as in both a human lens epithelial cell line (HLEpiC) and primary rat lens epithelial cells (RLEpiCs) cultured under high-glucose conditions. The [Ca²⁺]_i increase evoked by a TRPV2 channel agonist was significantly enhanced in both HLEpiCs and RLEpiCs cultured in high-glucose media. This enhancement was blocked by the TRPV2 nonspecific inhibitor ruthenium red and by TRPV2-specific small interfering (si)RNA transfection. Culturing HLEpiCs or RLEpiCs for seven days in high glucose significantly increased apoptosis, which was inhibited by TRPV2-specific siRNA transfection. In addition, ROS inhibitor significantly suppressed the ROS-induced increase of TRPV2-mediated Ca²⁺ signal and apoptosis under high-glucose conditions. These findings suggest a mechanism underlying high-glucose–induced apoptosis of lens epithelial cells, and offer a potential target for developing new therapeutic options for diabetes-related cataracts.

Prevention of α-crystallin glycation and aggregation using l-lysine results in the inhibition of in vitro catalase heat-induced-aggregation and suppression of cataract formation in the diabetic rat

The principle role of α-crystallin is chaperoning activity that protect s other proteins against different stresses. High glucose concentration induces the osmotic stress and results in biomacromolecules glycation, which is subsequently cause their conformational and functional changes. Here, the roles of l-lysine (Lys) on the prevention of α-crystallin glycation in both in vitro and in vivo conditions are investigated. The catalase (CAT) activity was considered as a marker of α-crystallin functionality in both conditions. Streptozotocin-induced diabetic rats were treated with 0.1% of the Lys in drinking water. The purified α-crystallin was also incubated with glucose, in the presence or absence of the Lys and its structure-function was compared. The results showed that the visual cataract score was significantly lower in the diabetic rats treated with Lys. After Lys treatment, CAT, superoxide dismutase, aldose reductase and other biochemical parameters in the lens and serum of the diabetic rats returned to the normal value. Formation of the advanced glycation endproducts (AGEs), protein cross-linking, and the hydrophobicity of α-crystallin were changed due to glycation, but they were reversed by Lys treatment. The glycated α-crystallin lost its chaperone activity against heat denatured-CAT, but in the presence of Lys, it preserved its activity and prevented CAT aggregation. In conclusion, Lys treatment significantly inhibited the progression of diabetic cataract in rats. These effects were due to the Lys antiglycating and antioxidant effects, in addition to its protective effect against α-crystallin chaperoning activity.

Selenite and ebselen supplementation attenuates D-galactose-induced oxidative stress and increases expression of SELR and SEP15 in rat lens

Selenite and ebselen supplementation has been shown to possess anti-cataract potential in some experimental animal models of cataract, however, the underlying mechanisms remain unclear. The present study was designed to evaluate the anti-cataract effects and the underlying mechanisms of selenite and ebselen supplementation on galactose induced cataract in rats, a common animal model of sugar cataract. Transmission electron microscopy images of lens fiber cells (LFC) and lens epithelial cells (LEC) were observed in D-galactose-induced experimental cataractous rats treated with or without selenite and ebselen, also redox homeostasis and expression of proteins such as selenoprotein R (SELR), 15kD selenoprotein (SEP15), superoxide dismutase 1 (SOD1), catalase (CAT), β-crystallin protein, aldose reductase (AR) and glucose-regulated protein 78 (GRP78) were estimated in the lenses. The results showed that D-galactose injection injured rat lens and resulted in cataract formation; however, selenite and ebselen supplementation markedly alleviated ultrastructural injury of LFC and LEC. Moreover, selenite and ebselen supplementation could mitigate the oxidative damage in rat lens and increase the protein expressions of SELR, SEP15, SOD1, CAT and β-crystallin, as well as decrease the protein expressions of AR and GRP78. Taken together, these findings for the first time reveal the anti-cataract potential of selenite and ebselen in galactosemic cataract, and provide important new insights into the anti-cataract mechanisms of selenite and ebselen in sugar cataract.

Zolfino landrace (Phaseolus vulgaris L.) from Pratomagno: general and specific features of a functional food

Background

The Zolfino bean is a variety of Phaseolus vulgaris, which is cultivated in a limited area of Tuscany, Italy, and is widely appreciated for its flavor and culinary uses.

Objectives

A yellow Zolfino landrace cultivated in the Leccio-Reggello area was characterized and compared with three other varieties of Phaseolus vulgaris (i.e. the Borlotto, Cannellino, and Corona beans) in terms of its general features and potential as an antioxidant/anti-inflammatory agent.

Design

The length, width, thickness, equatorial section surface, weight, volume, and seed coat section were measured in all the beans. The seed surface area was also estimated by an original empirical method. The ability of the different beans to interfere with the enzymes of the polyol pathway (that is, aldose reductase (AR) and sorbitol dehydrogenase) was tested using the supernatant after soaking the beans at room temperature and after thermal treatment, which simulated the bean-cooking process in a controlled fashion.

Results

Concerning the general features, Zolfino was comparable with other beans, except Corona, in terms of surface–volume ratio, which possesses the lowest tegument thickness. Moreover, Zolfino appears the most effective in inhibiting AR activity. The inhibitory ability is unaffected by thermal treatment and appears to be associated with compound(s) present in the coat of the bean.

Conclusions

The ability of Zolfino to inhibit AR, thus reducing the flux of glucose through the polyol pathway, highlights the features of Zolfino as a functional food, potentially useful in treating the dysfunctions linked to the hyperactivity of AR, such as diabetic complications or inflammatory responses.

In vitro and in vivo inhibitory activities of four Indian medicinal plant extracts and their major components on rat aldose reductase and generation of advanced glycation endproducts

The polyol enzyme aldose reductase (AR) and advanced glycation endproducts (AGEs) play an important role in diabetic complications such as cataracts. The purpose of this study was to investigate four standardized plant extracts used for the treatment of diabetes and related diseases, and their principal components for AR inhibitory activity and to find out their influence in diabetic complications. Thus, Boswellia serrata Triana & Planch. (Burseraceae), Lagerstroemia speciosa (L.) Pers. (Lythraceae), Ocimum gratissimum (L.) (Lamiaceae) and Syzygium cumin (L.) Skeels. (Myrthaceae) and their respective major constituents, boswellic acid, corosolic acid, ursolic acid and ellagic acid, were studied for their inhibitory activity against rat lens AR, rat kidney AR, human recombinant AR and generation of AGEs. In addition, in vivo inhibition of lens galactitol accumulation by the major constituents of the plants in galactose-fed rat has been studied. The results revealed that all the tested extracts and their active ingredients possess significant AR inhibitory actions in both in vitro and in vivo assays with urosolic acid showing the most potent effect. Furthermore, the study indicates the potential of the studied plants and their major constituents as possible protective agents against long-term diabetic complications.

GP-1447, an inhibitor of aldose reductase, prevents the progression of diabetic cataract in rats

We examined the effects of GP-1447 (3-[(4,5,7-trifluorobenzothiazol-2-yl)methyl]-5-methylphenyl acetic acid) on existing cataracts and sorbitol content in the lens in rats with streptozotocin-induced diabetes. GP-1447 is an inhibitor of aldose reductase, which is the first enzyme in the polyol pathway. Cataracts in the central region of the lens were observed in 7 of 14 eyes (50%) by the fifth week after induction of diabetes, and development of mature cataracts was observed in most lenses by the ninth week. In diabetic rats that received GP-1447 treatment beginning in the fifth week after induction of diabetes, progression of cataracts was observed for 1 week after initiation of treatment. Thereafter, the severity of cataracts did not change substantially. Sorbitol levels in the lens peaked during the first week of diabetes, and this increase was maintained during the 9-week observation period. Elevated sorbitol levels in the lenses of diabetic rats gradually declined after GP-1447 treatment was started on the fifth week after induction of diabetes. Cataracts and sorbitol elevation were not observed in the lenses of controls or diabetic rats treated with GP-1447 immediately after induction of diabetes. These results suggest that the polyol pathway plays an important role in both the appearance and progression of cataracts in diabetic rats. Inhibition of aldose reductase could significantly prevent progression of existing cataracts.

Protective effects of taurine against alloxan-induced diabetic cataracts and refraction changes in New Zealand White rabbits

The present study examined the protective effects of taurine on alloxan-induced diabetic cataracts and lens damage in male New Zealand White rabbits. The animals were randomly divided into three treatment groups: (1) normal control (vehicle administration); (2) diabetes (100 mg/kg alloxan administration); and (3) diabetes + taurine (1% [w/v] taurine dissolved in drinking water and alloxan administration). The results showed that alloxan-induced diabetes caused significant (p < 0.05) hyperglycemia, hyperopic refraction shifts, cataract formation and lens damage compared with the normal control group. In contrast, the administration of taurine for 24 weeks significantly ameliorated the alloxan-induced elevated levels of blood glucose, level of hyperopic refraction error shifts in the eyes and progression of diabetic cataract formation in the lens in rabbits. Moreover, histopathology showed that the taurine supplement reduced the incidence of lens lesions induced by hyperglycemia. Overall, the studies demonstrate that taurine exhibits potent protective effects against alloxan-induced diabetic cataracts and refraction changes in rabbits.

Osmotic stress, not aldose reductase activity, directly induces growth factors and MAPK signaling changes during sugar cataract formation

In sugar cataract formation in rats, aldose reductase (AR) activity is not only linked to lenticular sorbitol (diabetic) or galactitol (galactosemic) formation but also to signal transduction changes, cytotoxic signals and activation of apoptosis. Using both in vitro and in vivo techniques, the interrelationship between AR activity, polyol (sorbitol and galactitol) formation, osmotic stress, growth factor induction, and cell signaling changes have been investigated. For in vitro studies, lenses from Sprague Dawley rats were cultured for up to 48 h in TC-199-bicarbonate media containing either 30 mM fructose (control), or 30 mM glucose or galactose with/without the aldose reductase inhibitors AL1576 or tolrestat, the sorbitol dehydrogenase inhibitor (SDI) CP-470,711, or 15 mM mannitol (osmotic-compensated media). For in vivo studies, lenses were obtained from streptozotocin-induced diabetic Sprague Dawley rats fed diet with/without the ARIs AL1576 or tolrestat for 10 weeks. As expected, lenses cultured in high glucose/galactose media or from untreated diabetic rats all showed a decrease in the GSH pool that was lessened by ARI treatment. Lenses either from diabetic rats or from glucose/galactose culture conditions showed increased expression of basic-FGF, TGF-β, and increased signaling through P-Akt, P-ERK1/2 and P-SAPK/JNK which were also normalized by ARIs to the expression levels observed in non-diabetic controls. Culturing rat lenses in osmotically compensated media containing 30 mM glucose or galactose did not lead to increased growth factor expression or altered signaling. These studies indicate that it is the biophysical response of the lens to osmotic stress that results in an increased intralenticular production of basic-FGF and TGF-β and the altered cytotoxic signaling that is observed during sugar cataract formation.

Inhibitory effects of chlorogenic acid on aldose reductase activity in vitro and cataractogenesis in galactose-fed rats

Chlorogenic acid (5-O-caffeoylquinic acid, CA), a phenolic compound found ubiquitously in plants, has antidiabetic effect in diabetic animal models. In this study, we investigated the inhibitory effect of CA on diabetic cataractogenesis. We evaluated the aldose reductase (AR) activity during cataract development in 50% galactose-fed rats, an animal model of sugar cataract. Galactose-fed rats were treated orally with CA (10 and 50 mg/kg body weight) once a day for 2 weeks. In vehicle-treated galactose-fed rats, lens opacity was increased, and lens fiber swelling and membrane rupture were observed. In addition, AR protein was highly expressed in lens epithelial cells and lens cortical fibers of galactose-fed rats. However, CA inhibited the rat AR activity in vitro, and the administration of CA prevented the development of sugar cataract through the inhibition of AR activity. These observations suggest that CA is useful for the treatment of sugar cataract.

Orally active multi-functional antioxidants delay cataract formation in streptozotocin (type 1) diabetic and gamma-irradiated rats

BACKGROUND

Age-related cataract is a worldwide health care problem whose progression has been linked to oxidative stress and the accumulation of redox-active metals. Since there is no specific animal model for human age-related cataract, multiple animal models must be used to evaluate potential therapies that may delay and/or prevent cataract formation.

METHODS/PRINCIPAL FINDINGS

Proof of concept studies were conducted to evaluate 4-(5-hydroxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 4) and 4-(5-hydroxy-4,6-dimethoxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 8), multi-functional antioxidants that can independently chelate redox metals and quench free radicals, on their ability to delay the progression of diabetic "sugar" cataracts and gamma radiation-induced cataracts. Prior to 15 Gy of whole head irradiation, select groups of Long Evans rats received either diet containing compound 4 or 8, or a single i.p. injection of panthethine, a radioprotective agent. Compared to untreated, irradiated rats, treatment with pantethine, 4 and 8 delayed initial lens changes by 4, 47, and 38 days, respectively, and the average formation of posterior subcapsular opacities by 23, 53 and 58 days, respectively. In the second study, select groups of diabetic Sprague Dawley rats were administered chow containing compounds 4, 8 or the aldose reductase inhibitor AL1576. As anticipated, treatment with AL1576 prevented cataract by inhibiting sorbitol formation in the lens. However, compared to untreated rats, compounds 4 and 8 delayed vacuole formation by 20 days and 12 days, respectively, and cortical cataract formation by 8 and 3 days, respectively, without reducing lenticular sorbitol. Using in vitro lens culture in 30 mM xylose to model diabetic "sugar" cataract formation, western blots confirmed that multi-functional antioxidants reduced endoplasmic reticulum stress.

CONCLUSIONS/SIGNIFICANCE

Multi-functional antioxidants delayed cataract formation in two diverse rat models. These studies provide a proof of concept that a general cataract treatment focused on reducing oxidative stress instead of a specific mechanism of cataractogenesis can be developed.

Evaluation of the aldose reductase inhibitor fidarestat on ischemia-reperfusion injury in rat retina

This study evaluated the effects of retinal ischemia-reperfusion (IR) injury and pre-treatment with the potent and specific aldose reductase inhibitor fidarestat on apoptosis, aldose reductase and sorbitol dehydrogenase expression, sorbitol pathway intermediate concentrations, and oxidative-nitrosative stress. Female Wistar rats were pre-treated with either vehicle (N-methyl-D-glucamine) or fidarestat, 32 mg kg(-1) d(-1) for both, in the right jugular vein, for 3 consecutive days. A group of vehicle- and fidarestat-treated rats were subjected to 45-min retinal ischemia followed by 24-h reperfusion. Ischemia was induced 30 min after the last vehicle or fidarestat administration. Retinal IR resulted in a remarkable increase in retinal cell death. The number of TUNEL-positive nuclei increased 48-fold in the IR group compared with non-ischemic controls (p<0.01), and this increase was partially prevented by fidarestat. AR expression (Western blot analysis) increased by 19% in the IR group (p<0.05), and this increase was prevented by fidarestat. Sorbitol dehydrogenase and nitrated protein expressions were similar among all experimental groups. Retinal sorbitol concentrations tended to increase in the IR group but the difference with non-ischemic controls did not achieve statistical significance (p=0.08). Retinal fructose concentrations were 2.2-fold greater in the IR group than in the non-ischemic controls (p<0.05). Fidarestat pre-treatment of rats subjected to IR reduced retinal sorbitol concentration to the levels in non-ischemic controls. Retinal fructose concentrations were reduced by 41% in fidarestat-pre-treated IR group vs. untreated ischemic controls (p=0.0517), but remained 30% higher than in the non-ischemic control group. In conclusion, IR injury to rat retina is associated with a dramatic increase in cell death, elevated AR expression and sorbitol pathway intermediate accumulation. These changes were prevented or alleviated by the AR inhibitor fidarestat. The results identify AR as an important therapeutic target for diseases involving IR injury, and provide the rationale for development of fidarestat and other AR inhibitors.

Effect of an aldose reductase inhibitor on alveolar bone loss associated with periodontitis in diabetic rats

Periodontitis is a lesser known but frequent complication of diabetes mellitus and is the major cause of tooth loss in patients with diabetes. Dental therapy for this complication is primarily focused on the control of oral infections. No current therapy directly addresses the potential effects of diabetes itself on this complication. In studies conducted in young normal control and streptozotocin diabetic rats (100 g) treated with and without the aldose reductase inhibitor (ARI) imirestat, experimental periodontitis was induced in one side of the mouth by 3 injections of lipopolysaccharide (LPS) from Escherichia coli 055:B5 9 into the palatal gingiva between the first and second maxillary molars at 48-hour intervals. The other control side was injected with phosphate buffered saline (PBS). Fourteen days after the final injection, all rats were euthanized and the heads were defleshed. The maxillary area was separated from the remaining skull. The cleaned maxillary alveoli were stained in 5% aqueous toluidine blue to identify the cemento-enamel junction (CEJ) on the molars. Alveolar bone loss was measured according to standard methods by determining both the distance between the CEJ and the alveolar bone on the 2 molars between which the injections were made, and by measuring the ratio of root area/enamel area in the same region. These measurements showed that LPS injections resulted in significant bone loss compared with PBS injections in both control and diabetic rats, and that this bone loss was not present in the ARI-treated diabetic rats (P < 0.05). These results suggest that the sorbitol pathway plays a critical role in the pathophysiological mechanism(s) of diabetic periodontitis and that AR may be a direct pharmacological target for the treatment for this disease.

Aldose reductase deficiency improves Wallerian degeneration and nerve regeneration in diabetic thy1-YFP mice

This study examined the role of aldose reductase (AR) in diabetes-associated impaired nerve regeneration using thy1-YFP (YFP) mice. Sciatic nerves of nondiabetic and streptozotocin-induced diabetic AR(+/+)YFP and AR(-/-)YFP mice were transected after 4 weeks of diabetes. Wallerian degeneration and nerve regeneration were evaluated at 1 and 2 weeks postaxotomy by fluorescence microscopy. Motor nerve conduction velocity recovery and regenerating nerve morphometric parameters were determined at 10 and 20 weeks, respectively. There was no difference in the extent of Wallerian degeneration, size of regenerating stump, motor nerve conduction velocity recovery, or caliber of regenerating fibers between nondiabetic AR(+/+)YFP and AR(-/-)YFP mice. In diabetic AR(+/+)YFP mice, Wallerian degeneration was delayed, associated with slower macrophage invasion and abnormal vascularization. Those mice had smaller regenerating stumps, slower motor nerve conduction velocity, and smaller regenerating fibers compared with nondiabetic mice. These features of impaired nerve regeneration were largely attenuated in diabetic AR(-/-)YFP mice. Retarded macrophage invasion and vascularization associated with Wallerian degeneration were normalized in diabetic AR(-/-)YFP mice. These results indicate that AR plays an important role in diabetes-associated impaired nerve regeneration, in part by affecting vascularization and macrophage invasion during Wallerian degeneration. The thy1-YFP mice are valuable tools for further investigation of the mechanism of diabetes-associated nerve regeneration.

Diabetic cataracts: mechanisms and management

Diabetes mellitus is associated with a 5-fold higher prevalence of cataracts, which remains a major cause of blindness in the world. Typical diabetic cataracts contain cortical and/or posterior subcapsular opacities. Adult onset diabetic cataracts also often contain nuclear opacities. Mechanisms of diabetic cataractogenesis have been studied in less detail than those of other diabetic complications. Both animal and human studies support important contribution of increased aldose reductase activity. Surgical extraction is the only cure of diabetic cataract today. An improved understanding of pathogenetic mechanisms, together with finding effective therapeutic agents, remain highest priority for diabetic cataract-related research and pharmaceutical development.

ZAC1 is up-regulated by hypertonicity and decreases sorbitol dehydrogenase expression, allowing accumulation of sorbitol in kidney cells

Affymetrix GeneChip technology was employed to detect differentially expressed genes in inner medullary collecting duct (IMCD3) cells grown under isotonic and hypertonic conditions. A marked up-regulation was found for the zinc-finger protein ZAC1 under hypertonic stress (219-fold, p < 0.001). Changes in expression for ZAC1 were verified by quantitative PCR for message and Western blotting for protein. In mouse and human kidney tissues, ZAC1 expression was substantial in the papilla and was absent in the cortex. Furthermore, ZAC1 expression significantly increased in the papilla of mice following 36 h of fluid restriction and decreased in polyuric mice consuming sucrose in water. Because ZAC1 has been described to be a potential negative regulator of sorbitol dehydrogenase (SDH) in hippocampal cells, we examined whether this relationship also occurs in kidney cells under hypertonic stress. We found that stable IMCD3 clones silenced for ZAC1 to varying levels demonstrated an inverse effect on SDH expression. ZAC1 binds to a consensus repression site within the promoter of SDH, pointing to a mechanism whereby ZAC1 acts by repressing SDH transcriptional activity during hypertonic conditions. Taken together, these data strongly suggest that ZAC1 is up-regulated under hypertonic stress and negatively regulates expression of SDH, allowing for accumulation of sorbitol as a compatible organic osmolyte.

Inhibitory effects of Zingiber officinale Roscoe derived components on aldose reductase activity in vitro and in vivo

Ginger (Zingiber officinale Roscoe) continues to be used as an important cooking spice and herbal medicine around the world. Scientific research has gradually verified the antidiabetic effects of ginger. Especially gingerols, which are the major components of ginger, are known to improve diabetes including the effect of enhancement against insulin-sensitivity. Aldose reductase inhibitors have considerable potential for the treatment of diabetes, without increased risk of hypoglycemia. The assay for aldose reductase inhibitors in ginger led to the isolation of five active compounds including 2-(4-hydroxy-3-methoxyphenyl)ethanol (2) and 2-(4-hydroxy-3-methoxyphenyl)ethanoic acid (3). Compounds 2 and 3 were good inhibitors of recombinant human aldose reductase, with IC50 values of 19.2 +/- 1.9 and 18.5 +/- 1.1 microM, respectively. Furthermore, these compounds significantly suppressed not only sorbitol accumulation in human erythrocytes but also lens galactitol accumulation in 30% of galactose-fed cataract rat model. A structure-activity relationship study revealed that the applicable side alkyl chain length and the presence of a C3 OCH3 group in the aromatic ring are essential features for enzyme recognition and binding. These results suggested that it would contribute to the protection against or improvement of diabetic complications for a dietary supplement of ginger or its extract containing aldose reductase inhibitors.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

Naphtho[1,2-d]isothiazole Acetic Acid Derivatives as a Novel Class of Selective Aldose Reductase Inhibitors

Acetic acid derivatives of naphtho[1,2-d]isothiazole (NiT) were synthesized and tested as novel aldose reductase (ALR2) inhibitors. The parent compound 11 exhibited a fair inhibitory activity (IC(50) = 10 muM), which was enhanced by 2 orders of magnitude by introducing a second carboxylic group at position 4 (13 and 14: IC(50) = 0.55 and 0.14 muM, respectively). Substitution of the acetic acid function with an apolar group gave inactive (29) or poorly active (25, 26, 30) compounds, thus demonstrating that the 2-acetic group is involved in the enzyme pharmacophoric recognition while the 4-carboxylic moiety has only an accessory role. The potent compounds 11, 13, 14, 26 all proved to be selective for ALR2, since none of them inhibited aldehyde reductase, sorbitol dehydrogenase, or glutathione reductase. The isopropyl ester 31, a prodrug of 14, was found to be effective in preventing cataract development in severely galactosemic rats, when administered as an eyedrop solution. The theoretical binding mode of 13 and 14, obtained by docking simulations into the ALR2 crystal structure, was fully consistent with the structure-activity relationships in the NiT series.

Anticataract Activity of Analogs of a Sorbitol Dehydrogenase Inhibitor

The initiation of sugar cataract formation by the aldose reductase catalyzed accumulation of sorbitol in diabetic rats, and its prevention by the administration of aldose reductase inhibitors at the onset or early stages of diabetes, has been well established. In contrast, the inhibition of sorbitol dehydrogenase by 4-[4-(N,N-dimethylsulfamoyl)piperazino]-2-hydroxymethylpyrimidine (SDI-1) has been observed to increase the onset in severity of sugar cataract formation in diabetic rats. Two analogs of SDI-1 have been synthesized, where the 4-(2-hydroxymethyl)pyrimidine ring has been replaced with either a 4-(2,6-dimethoxy)-pyrimidine ring or a 2-pyrimidine ring. Neither compound, 2-[4-(N,N-dimethylsulfamoyl)piperazino]-pyrimidine (SRA-1) or 4-[4-(N,N-dimethylsulfamoyl) piperazino]-2,6-dimethoxypyrimidine (SRA-2), demonstrated significant sorbitol dehydrogenase or aldose reductase inhibition. Oral administration of these compounds to streptozotocin diabetic rats, however, delayed cataract formation without reducing the levels of hyperglycemia or lens polyol.

Pharmacological prevention of diabetic cataract

Cataract--opacification of the lens--is closely related to diabetes as one of its major late complications. This review deals with three molecular mechanisms that may be involved in the development of diabetic cataract: nonenzymatic glycation of eye lens proteins, oxidative stress, and activated polyol pathway in glucose disposition. Implications resulting from these mechanisms for possible pharmacological interventions to prevent diabetic cataract are discussed. The article reviews research on potential anticataract agents, including glycation inhibitors, antioxidants, and aldose reductase inhibitors. Information on possible benefits of putative anticataract agents comes from a variety of approaches, ranging from laboratory experiments, both in vitro and in vivo, to epidemiological studies in patients.

Polyol pathway and diabetic peripheral neuropathy

This chapter critically examines the concept of the polyol pathway and how it relates to the pathogenesis of diabetic peripheral neuropathy. The two enzymes of the polyol pathway, aldose reductase and sorbitol dehydrogenase, are reviewed. The structure, biochemistry, physiological role, tissue distribution, and localization in peripheral nerve of each enzyme are summarized, along with current informaiton about the location and structure of their genes, their alleles, and the possible links of each enzyme and its alleles to diabetic neuropathy. Inhibitors of pathway enzyme and results obtained to date with pathway inhibitors in experimental models and human neuropathy trials are updated and discussed. Experimental and clinical data are analyzed in the context of a newly developed metabolic odel of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reuctase. Overall, the data will be interpreted as supporting the hypothesis that metabolic flux through the polyol pathway, rather than nerve concentration of sorbitol, is the predominant polyol pathway-linked pathogeneic factor in diabetic preipheral nerve. Finally, key questions and future directions for bsic and clinical research in this area are considered. It is concluded that robust inhibition of metabolic flux through the polyol pathway in peripheral nerve will likely result in substantial clinical benefit in treating and preventing the currently intractable condition of diabetic peripheral neuropathy. To accomplish this, it is imperative to develop and test a new generation of "super-potent" polyol pathway inhibitors.

A biodegradable injectable implant sustains systemic and ocular delivery of an aldose reductase inhibitor and ameliorates biochemical changes in a galactose-fed rat model for diabetic complications

PURPOSE

To fabricate and characterize in vitro and in vivo performance of a sustained release biodegradable implant for N-4-(benzoylaminophenylsulfonyl glycine) (BAPSG), a novel aldose reductase inhibitor.

METHODS

The ability of BAPSG to inhibit aldose reductase activity and glucose-induced vascular endothelial growth factor (VEGF) expression was assessed in a retinal pigment epithelial cell line (ARPE-19). A poly (DL-lactic-co-glycolic acid) implant containing 50% w/w BAPSG was fabricated and characterized for drug loading, in vitro drug release, and the thermal behavior of the drug and the polymer. Implants were injected subcutaneously into a galactose-fed diabetic rat model and cataract scores, plasma and tissue drug levels, galactitol levels in the lens and the retina, glutathione levels in the plasma, lens, cornea and retina and VEGF expression in the retina were determined on or until 18 days.

RESULTS

BAPSG inhibited aldose reductase activity and reduced VEGF expression in ARPE-19 cells. Implants (1 x 4 mm), with a loading efficiency of 106 +/- 7% for BAPSG, were fabricated. Upon implant fabrication, while the glass transition temperature of the polymer decreased, the melting point of the drug was not affected. In vivo drug release correlated well with in vitro release, with approximately 44% drug release occurring in vivo by the end of 18 days. The implant reduced galactitol accumulation, glutathione depletion, cataract scores, and VEGF expression in galactose-fed rats.

CONCLUSIONS

An injectable biodegradable implant of BAPSG sustained drug release in vitro and in vivo, and reduced galactitol accumulation, glutathione depletion, cataract scores, and VEGF expression in galactose-fed rats.

Can drugs or micronutrients prevent cataract?

Cataract is the major cause of blindness and of visual impairment worldwide, so its prevention is of the greatest importance. At present no drug therapy is licensed for use in the UK or the US, so the only treatment for cataract is by surgery, which is expensive and has adverse effects. This article reviews research on prevention of cataract by a variety of agents, including micronutrients as well as drugs. Benefits have been claimed for many compounds or mixtures and this review concentrates on those most extensively studied. Information on possible benefits of putative anticataract agents comes from a variety of approaches, from laboratory experiments, both in vitro and in vivo, to epidemiological studies in patients. Sorbitol-lowering drugs were the first to be examined systematically and progressed to clinical trials which were disappointing, and now the entire rationale for their use in prevention of cataract is questionable. Micronutrients showed little promise in animals but came to clinical trial in patients with cataract without the publication of any major benefit. Pantethine showed more promise in animal studies but the only clinical trial was abandoned early. A variety of laboratory and epidemiological evidence supports the benefits of aspirin-like drugs but there has been no trial specifically in patients with cataract. Add-on studies to trials of aspirin for other indications have not been encouraging. Research into other compounds is interesting but less advanced.

Modelling studies of the active site of human sorbitol dehydrogenase: an approach to structure-based inhibitor design of the enzyme

The program GRID was used to design novel potential inhibitors of human sorbitol dehydrogenase based on a model of the holoenzyme in complex with the inhibitor WAY135 706. Replacement of the methyl hydroxyl group of the inhibitor with methyl phosphate and methyl carboxylate functional groups increased the net binding energy of the complex by 2.0- and 1.7-fold, respectively. This study may be useful in the development of potent and more specific inhibitors of the enzyme.

Modelling studies on the binding of substrate and inhibitor to the active site of human sorbitol dehydrogenase

This study reports a molecular modelling investigation of human sorbitol dehydrogenase complexed with the substrate sorbitol and the inhibitor WAY135 706 based on the structures of human beta3 alcohol dehydrogenase, human sigma alcohol dehydrogenase and horse liver alcohol dehydrogenase. The tertiary structure of human beta3 alcohol dehydrogenase was used as a template for the construction of the model. The rms positional deviation between the main-chain atoms of the initial and final models of sorbitol dehydrogenase is 1.37 A. Similar residue interactions exist between sorbitol dehydrogenase and both sorbitol and inhibitor. Binding of sorbitol in the substrate-binding site results in interactions with Lys-294, Tyr-50, His-69, Glu-150, and NAD+ while WAY135 706 interacts with Ser-46, Lys-294 and Phe-59. The enzyme-inhibitor interactions revealed by this study will be useful in the design of more specific inhibitors.

Interaction between osmotic and oxidative stress in diabetic precataractous lens: studies with a sorbitol dehydrogenase inhibitor

Both sorbitol accumulation-linked osmotic stress and "pseudohypoxia" [increase in NADH/NAD+, similar to that in hypoxic tissues, and attributed to increased sorbitol dehydrogenase (1-iditol:NAD+ 5-oxidoreductase; EC 1.1.1.14; SDH) activity] have been invoked among the mechanisms underlying oxidative injury in target tissues for diabetic complications. We used the specific SDH inhibitor SDI-157 [2-methyl-4(4-N,N-dimethylaminosulfonyl-1-piperazino)pyrimid ine] to evaluate the role of osmotic stress versus "pseudohypoxia" in oxidative stress occurring in diabetic precataractous lens. Control and diabetic rats were treated with or without SDI-157 (100 mg/kg/day for 3 weeks). Lens malondialdehyde (MDA) plus 4-hydroxyalkenals (4-HA), MDA, GSH, and ascorbate levels, as well as the GSSG/GSH ratios, were similar in SDI-treated and untreated control rats, thus indicating that SDI-157 was not a prooxidant. Intralenticular osmotic stress, manifested by sorbitol levels, was more severe in SDI-treated diabetic rats (38.2+/-6.8 vs 21.2+/-3.5 micromol/g in untreated diabetic and 0.758+/-0.222 micromol/g in control rats, P<0.01 for both), while the decrease in the free cytosolic NAD+/NADH ratio was partially prevented (120+/-16 vs 88+/-11 in untreated diabetic rats and 143+/-13 in controls, P<0.01 for both). GSH and ascorbate levels were decreased, while MDA plus 4-HA and MDA levels were increased in diabetic rats versus controls; both antioxidant depletion and lipid aldehyde accumulation were exacerbated by SDI treatment. Superoxide dismutase (superoxide:superoxide oxidoreductase; EC 1.15.1.1), GSSG reductase (NAD[P]H:oxidized-glutathione oxidoreductase; EC 1.6.4.2), GSH transferase (glutathione S-transferase; EC 2.5.1.18), GSH peroxidase (glutathione:hydrogen-peroxide oxidoreductase; EC 1.11.1.9), and cytoplasmic NADH oxidase activities were increased in diabetic rats versus controls, and all the enzymes but GSH peroxidase were up-regulated further by SDI. In conclusion, sorbitol accumulation and osmotic stress generated oxidative stress in diabetic lens, whereas the contribution of "pseudohypoxia" was minor. SDIs provide a valuable tool for exploring mechanisms of oxidative injury in sites of diabetic complications.