Diabetes can alter the signal transduction pathways in the lens of rats

Association between Life's Essential 8 and cataract among US adults

Currently, a comprehensive assessment of the relationship between ideal cardiovascular health (CVH) indicators and cataract risk is lacking. Life's Essential 8 (LE8) is the latest concept proposed by the American Heart Association to comprehensively reflect CVH status. LE8 includes four health behaviors (diet, physical activity, smoking, and sleep) and four health factors (blood lipid, blood sugar, blood pressure, and body mass index). This study tried to evaluate the association between LE8 and cataract using data from National Health and Nutrition Examination Survey (NHANES) 2005-2008, a continuous research program which aims to monitor and evaluate the health and nutrition status of the US population. A cross-sectional study of 2720 non-cataract participants and 602 cataract participants. All participants were assigned to the poor, intermediate, and ideal CVH status groups based on LE8 score. Weighted multiple logistic regression was used to investigate the correlation between the LE8 score and cataract, as well as the correlation between each of the eight subitems and cataract, with potential confounding variables being adjusted. Then, restricted cubic spline analysis was used to further explore whether there was a nonlinear relationship between LE8 score and cataract. The proportion of cataract participants was 14.1%, 18.2%, and 20.6% in the ideal, intermediate, and poor CVH groups, respectively (P < 0.05). LE8 score was inversely associated with cataract risk, with each 10-point increase in LE8 score associated with a 14% reduction in cataract risk [odds ratio (OR) = 0.86, 95% confidence interval (CI): 0.79-0.93, P < 0.01]. Among all the LE8 subitems, physical activity, sleep, and blood glucose were significantly associated with cataract risk (all P < 0.05). Better CVH, defined by a higher LE8 score, is associated with a lower cataract risk. Efforts to improve LE8 score (especially when it comes to physical activity, sleep, and blood glucose) may serve as a novel strategy to help reduce the risk of cataract.

Taxifolin Shows Anticataractogenesis and Attenuates Diabetic Retinopathy in STZ-Diabetic Rats via Suppression of Aldose Reductase, Oxidative Stress, and MAPK Signaling Pathway

Background:

Due to the increased prevalence of diabetes-associated complications of the eye like diabetic retinopathy and cataract, the need for a novel therapeutic agent is urgent. Due to the advantages that the polyphenolic compounds enjoy in diabetes and associated complications, we postulated that Taxifolin (TXF), a poly-phenolic flavanol, could show anti-retinopathic and anti-cataract effect in diabetes-induced rats.

Methods:

TXF at a dose of 10, 25, and 50 mg/kg was given by oral route to STZ mediated diabetic rats for a time period of 10 weeks. The opacity of lens was studied after every 7 days of treatment till 10 weeks; evaluation of the severity of cataract and changes in the histology of lens as well as retina was done. Tissue homogenates of lens isolated after the end of the study were evaluated for markers of oxidative stress, levels of aldose reductase, p38MAPK, VEGF, and ERK1/2.

Results:

Outcomes suggested that TXF improved retinopathy and cataract in diabetes-induced rats. The treatment of TXF also improved the status of oxidative stress and inhibited the levels of p38MAPK, VEGF, and ERK1/2. The treatment also improved the lens opacity in diabetic rats. The results suggest that the protective effect of TXF against cataract and retinopathy may be due to the anti-oxidative potential of TXF and its inhibiting effect on VEGF, ERK1/2, p38MAPK, and aldose reductase.

Conclusion:

The study confirms that TXF is a potential candidate showing a protective effect against diabetic induced retinopathy and cataract..

Prevention of Diabetic Complications by Walnut Leaf Extract via Changing Aldose Reductase Activity: An Experiment in Diabetic Rat Tissue

BACKGROUND

Increased activity of aldose reductase (AR) is one of the mechanisms involved in the development of diabetic complications. Inhibiting AR can be a target to prevent diabetes complications. This study is aimed at evaluating the effect of cyclohexane (CH) and ethanol extracts (ET) of walnut leaves on AR activity in the lens and testis of diabetic rats.

METHODS

Fifty-six male rats classified into seven groups as control and treatment groups and treated for 30 days. The treatment groups were treated with different concentrations of ET and CH. The diabetic control (DC) group was exposed to streptozotocin. AR activity was measured in the lens and testis. The expression of AR in the testis was evaluated by the immunohistochemistry method.

RESULTS

Both extracts significantly reduced the AR activity (ng/mg of tissue protein) in the testis (0.034 ± 0.004, 0.038 ± 0.010, and 0.040 ± 0.007 in the treatment groups vs. 0.075 ± 0.007 in the DC group) and lens (1.66 ± 0.09, 2.70 ± 0.47, and 1.77 ± 0.20 in the treatment groups vs. 6.29 ± 0.48 in the DC group) of the treatment group compared to those of the DC group (P < 0.05). AR expression in the testes of the treatment groups was decreased compared with that of the DC group (P < 0.0001).

CONCLUSION

Walnut leaf extracts can reduce the activity and localization of AR in the testes and its activity in the lens of diabetic rats.

Anticataractogenesis and Antiretinopathy Effects of the Novel Protective Agent Containing the Combined Extract of Mango and Vietnamese Coriander in STZ-Diabetic Rats

The novel protectant against diabetic cataract and diabetic retinopathy is currently required due to the increased prevalence and therapeutic limitation. Based on the advantage of polyphenol on diabetic eye complications, we hypothesized that the combined extract of mango seed Vietnamese coriander (MPO), a polyphenol-rich substance, should possess anticataractogenesis and antiretinopathy in streptozotocin- (STZ-) diabetic rats. MPO at doses of 2, 10, and 50 mg/kg·BW were orally given to STZ-diabetic rats for 10 weeks. Lens opacity was evaluated every week throughout a study period whereas the evaluation of cataract severity and histological changes of both rat lens epithelium and retina together with the biochemical assays of oxidative stress status, aldose reductase, p38MAPK, ERK1/2, and VEGF were performed at the end of experiment. Our data showed that MPO improved cataract and retinopathy in STZ-diabetic rats. The improved oxidative stress status and the decreased p38MAPK, ERK1/2, and VEGF were also observed. Therefore, anticataractogenesis and antiretinopathy of MPO might occur partly via the decreased oxidative stress status and the suppression of aldose reductase, p38MAPK, ERK1/2, and VEGF. This study points out that MPO is the potential candidate protectant against diabetic cataract and diabetic retinopathy. However, the exploration for possible active ingredient (S) still requires further researches.

Aldose Reductase Mediates Transforming Growth Factor β2 (TGF-β2)-Induced Migration and Epithelial-To-Mesenchymal Transition of Lens-Derived Epithelial Cells

PURPOSE

Cataract surgery involves removal of lens tissue, but is associated with a high complication rate due to regrowth of residual lens epithelial cells to produce posterior capsule opacification (PCO) and diminished visual acuity. As inhibitors of aldose reductase (AR) have been shown to suppress markers of PCO, our studies were designed to identify a role for AR in the pathogenesis of PCO.

METHODS

Sorbinil-mediated AR inhibition was determined by measuring sorbitol accumulation. Cell migration was measured using both transwell and scratch assays. Proteins in the SMAD signaling pathway were measured by Western blotting. The interactions of AR and SMADs were demonstrated by co-immunoprecipitation (Co-IP) and proximity ligation assay (PLA). Epithelial-to-mesenchymal transition (EMT) expression was measured by Western blot and quantitative PCR (q-PCR). Matrix metalloproteinase (MMP)-2 and MMP-9 activities were measured in conditioned medium by zymography.

RESULTS

We observed that either Sorbinil-mediated AR inhibition or siRNA-mediated AR gene knockdown prevented migration of lens epithelial cells following exposure to TGF-β2. AR inhibition or AR knockdown reduced SMAD and MMP activation triggered by TGF-β2. In addition, we demonstrated AR inhibition or AR knockdown decreased TGF-β2-induced expression of EMT markers. Co-IP studies and PLA were used to demonstrate that AR and SMAD2 interact either directly or in close concert with additional factor(s) in a nonenzymatic manner.

CONCLUSIONS

This study demonstrates that AR participates in the response of lens epithelial cells to TGF-β2. Our studies raise the possibility that AR inhibition may be effective in preventing development of PCO by disrupting the TGF-β2/SMAD pathway.

Aldose reductase expression as a risk factor for cataract

Aldose reductase (AR) is thought to play a role in the pathogenesis of diabetic eye diseases, including cataract and retinopathy. However, not all diabetics develop ocular complications. Paradoxically, some diabetics with poor metabolic control appear to be protected against retinopathy, while others with a history of excellent metabolic control develop severe complications. These observations indicate that one or more risk factors may influence the likelihood that an individual with diabetes will develop cataracts and/or retinopathy. We hypothesize that an elevated level of AR gene expression could confer higher risk for development of diabetic eye disease. To investigate this hypothesis, we examined the onset and severity of diabetes-induced cataract in transgenic mice, designated AR-TG, that were either heterozygous or homozygous for the human AR (AKR1B1) transgene construct. AR-TG mice homozygous for the transgene demonstrated a conditional cataract phenotype, whereby they developed lens vacuoles and cataract-associated structural changes only after induction of experimental diabetes; no such changes were observed in AR-TG heterozygotes or nontransgenic mice with or without experimental diabetes induction. We observed that nondiabetic AR-TG mice did not show lens structural changes even though they had lenticular sorbitol levels almost as high as the diabetic AR-TG lenses that showed early signs of cataract. Over-expression of AR led to increases in the ratio of activated to total levels of extracellular signal-regulated kinase (ERK1/2) and c-Jun N-terminal (JNK1/2), which are known to be involved in cell growth and apoptosis, respectively. After diabetes induction, AR-TG but not WT controls had decreased levels of phosphorylated as well as total ERK1/2 and JNK1/2 compared to their nondiabetic counterparts. These results indicate that high AR expression in the context of hyperglycemia and insulin deficiency may constitute a risk factor that could predispose the lens to disturbances in signaling through the ERK and JNK pathways and thereby alter the balance of cell growth and apoptosis that is critical to lens transparency and homeostasis.

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.

Aldose reductase, oxidative stress, and diabetic mellitus

Diabetes mellitus (DM) is a complex metabolic disorder arising from lack of insulin production or insulin resistance (Diagnosis and classification of diabetes mellitus, 2007). DM is a leading cause of morbidity and mortality in the developed world, particularly from vascular complications such as atherothrombosis in the coronary vessels. Aldose reductase (AR; ALR2; EC 1.1.1.21), a key enzyme in the polyol pathway, catalyzes nicotinamide adenosine dinucleotide phosphate-dependent reduction of glucose to sorbitol, leading to excessive accumulation of intracellular reactive oxygen species (ROS) in various tissues of DM including the heart, vasculature, neurons, eyes, and kidneys. As an example, hyperglycemia through such polyol pathway induced oxidative stress, may have dual heart actions, on coronary blood vessel (atherothrombosis) and myocardium (heart failure) leading to severe morbidity and mortality (reviewed in Heather and Clarke, 2011). In cells cultured under high glucose conditions, many studies have demonstrated similar AR-dependent increases in ROS production, confirming AR as an important factor for the pathogenesis of many diabetic complications. Moreover, recent studies have shown that AR inhibitors may be able to prevent or delay the onset of cardiovascular complications such as ischemia/reperfusion injury, atherosclerosis, and atherothrombosis. In this review, we will focus on describing pivotal roles of AR in the pathogenesis of cardiovascular diseases as well as other diabetic complications, and the potential use of AR inhibitors as an emerging therapeutic strategy in preventing DM complications.

Na+/H+-exchanger-1 inhibition counteracts diabetic cataract formation and retinal oxidative-nitrative stress and apoptosis

The Na⁺-H⁺-exchanger-1 (NHE-1) controls intracellular pH and glycolytic enzyme activities, and its expression and activity are increased by diabetes and high glucose. NHE-1-dependent upregulation of the upper part of glycolysis, under conditions of inhibition (lens) or insufficient activation (retina) of glyceraldehyde 3-phosphate dehydrogenase, underlies diversion of the excessive glycolytic flux towards several pathways contributing to oxidative stress, a causative factor in diabetic cataractogenesis and retinopathy. This study evaluated the role for NHE-1 in diabetic cataract formation and retinal oxidative stress and apoptosis. Control and streptozotocin-diabetic rats were maintained with or without treatment with the NHE-1 inhibitor cariporide (Sanofi-Aventis, 10 mgkg⁻¹d⁻¹) for 3.5 months. In in vitro studies, bovine retinal pericytes and endothelial cells were cultured in 5 or 30 mM glucose, with or without 10 μM cariporide, for 7 days. A several-fold increase of the by-product of glycolysis, α-glycerophosphate, indicative of activation of the upper part of glycolysis, was present in both rat lens and retina at an early (1-month) stage of streptozotocin-diabetes. Cariporide did not affect diabetic hyperglycemia and counteracted lens oxidative-nitrative stress and p38 MAPK activation, without affecting glucose or sorbitol pathway intermediate accumulation. Cataract formation (indirect ophthalmoscopy and slit-lamp examination) was delayed, but not prevented. The number of TUNEL-positive cells per flat-mounted retina was increased 4.4-fold in diabetic rats (101±17 vs. 23±8 in controls, P<0.01), and this increase was attenuated by cariporide (45±12, P<0.01). Nitrotyrosine and poly(ADP-ribose) fluorescence and percentage of TUNEL-positive cells were increased in pericytes and endothelial cells cultured in 30 mM glucose, and these changes were at least partially prevented by cariporide. In conclusion, NHE-1 contributes to diabetic cataract formation, and retinal oxidative-nitrative stress and apoptosis. The findings identify a new therapeutic target for diabetic ocular complications.

Toxicological characterization of N-methyl-N-nitrosourea-induced cataract in rats by LC/MS-based metabonomic analysis

Cataract is one of the most serious drug-induced side effects that can terminate the development of drug candidates, and pharmaceutical companies consider it important to evaluate cataract-inducing potential in the early phases. Metabonomics is expected to be a powerful approach for the safety evaluation of drug candidates. In this study, we conducted a toxicological characterization of N-methyl-N-nitrosourea (MNU)-induced cataract in rats by LC/MS-based metabonomic analysis. MNU was intraperitoneally administered once to 15-day old rats at 70 mg kg(-1) . After that, animals were kept for 3 weeks waiting for cataract formation. Lens samples for metabonomic analysis were collected on 7, 14 and 21 days after MNU administration. Comprehensive analyses of lens metabolites were conducted using an LC/MS system, and multivariate data for each sample were compared by principal component analysis (PCA) to find any changes in lens metabolites. Lens opacity was confirmed by ophthalmoscopy 14 days after dosing, and even by gross observation 21 days after dosing. PCA of the lens samples for the control and MNU-treated groups revealed that the metabolite profiles of lens differed from each other, and several lens metabolites, such as lots of α-amino acids and gluthathione, decreased after MNU treatment. In conclusion, metabonomic analysis enabled us to identify new marker candidates for cataract and provided a better understanding of the mechanism related to MNU-induced cataract. It was considered that metabonomics is a useful approach for the characterization of drug-induced toxicity.

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.

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.

Protein Kinase C-γ Activation in the Early Streptozotocin Diabetic Rat Lens

PURPOSE

The purpose of this study is to demonstrate the early activation of the protein kinase C-gamma (PKC-gamma) pathway in the streptozotocin (STZ)-induced diabetic rat lens.

METHODS

Twelve-week-old male and female Sprague-Dawley rats were injected with 80 mg/kg (body weight) of STZ (N-[methylnitrosocarbamoyl]-D-glucosamine) intraperitoneally. Very high glucose (VHG) diabetes was defined as a nonfasting blood glucose level of at least 450 mg/dl, confirmed by daily monitoring with Accu-Check Advantage test strips, and occurred about 2 weeks after STZ administration. All assayed lenses were from VHG or age-matched control rats, harvested within 24 hr of VHG detection. PKC-gamma activation was measured by enzyme activity assay and by Western blotting to show autophosphorylation on Thr514. Cellular insulin-like growth factor-1 (IGF-1), PKC-gamma phosphorylation of Cx43 on Ser368, and activation of phospholipase C-gamma 1 (PLC-gamma 1), extracellular signal-regulated kinase (ERK1/2), and caspase-3 were determined by Western blotting. Endogenous diacylglycerol (DAG) levels were measured with a DAG assay kit. Lens gap junction activity was determined by the microinjection/Lucifer yellow dye transfer assay. Electron microscopy was applied to affirm fiber cell damage in the VHG diabetic lenses.

RESULTS

In the lenses of VHG diabetic rats, PKC-gamma enzyme was activated. PKC-gamma could be further activated by 400 nM phorbol-12-myristate-13-acetate (PMA), but the PKC-gamma protein levels remained constant. No elevation of IGF-1 level was observed. Western blots showed that activation of PKC-gamma may be due to activation of PLC-gamma 1, which synthesized endogenous DAG, a native PKC activator. The level of PKC-gamma -catalyzed phosphorylation of Cx43 on Ser368 and resulting inhibition of lens gap junction dye transfer activity was increased in the VHG diabetic lenses. At this early time period, the diabetic lens showed no activation of either caspase-3 or ERK1/2. Only a single fiber cell layer deep within the cortex (approximately 90 cell layers from capsule surface) showed vacuoles and damaged cell connections.

CONCLUSIONS

Early activation of PLC-gamma 1 and elevated DAG were observed within VHG diabetic lenses. These were correlated with activation of PKC-gamma, phosphorylation of Cx43 on Ser368, and inhibition of dye transfer. Abnormal signaling from PKC-gamma to Cx43 in the epithelial cells/early fiber cells, observed within VHG diabetic lenses, may be responsible for fiber cell damage deeper in the lens cortex.

Diabetes and the peripheral nerve

Diabetes-induced damage to peripheral nerve culminates in development of peripheral diabetic neuropathy (PDN), one of the most devastating complications of diabetes mellitus and a leading cause of foot amputation. The pathogenesis of PDN occurs as a consequence of complex interactions among multiple hyperglycemia-initiated mechanisms, impaired insulin signaling, inflammation, hypertension, and disturbances of fatty acid and lipid metabolism. This review describes experimental new findings in animal and cell culture models as well as clinical data suggesting the importance of 1) previously established hyperglycemia-initiated mechanisms such as increased aldose reductase activity, non-enzymatic glycation/glycooxidation, activation of protein kinase C, 2) oxidative-nitrosative stress and poly(ADP-ribose) polymerase activation; 3) mitogen-activated protein kinase and cyclooxygenase-2 activation, impaired Ca(++) homeostasis and signaling, and several other mechanisms, in PDN.

Long-term treatment with ranirestat (AS-3201), a potent aldose reductase inhibitor, suppresses diabetic neuropathy and cataract formation in rats

We investigated the chronic functional and histopathological changes in the sciatic nerve and lens of streptozotocin (STZ)-diabetic rats and evaluated the preventive effects of ranirestat (AS-3201), a potent aldose reductase inhibitor, on these changes. Sorbitol levels in the sciatic nerve and lens, motor nerve conduction velocity (MNCV), and development of cataracts were measured in STZ-diabetic rats given a ranirestat-admixed diet (0.0005%) for 35 weeks. Ranirestat reduced sorbitol accumulation in the sciatic nerve and improved the decrease in MNCV of STZ-diabetic rats. Morphological and morphometric examination of changes in sural nerve revealed that treatment with ranirestat prevented both the deformity of myelinated fibers and the decrease in their axonal and myelin areas (atrophy). Ranirestat also averted the changes in the size frequency histogram of myelinated fibers. Finally, STZ-diabetic rats developed early lens opacities 8 weeks after STZ injection and had cataract by the end of the experimental period. However, in the ranirestat-treated diabetic rats, no lens opacity was observed in any rat throughout the entire experimental period. This study suggests that the polyol pathway plays an important role in the progress of diabetic neuropathy and cataract formation in STZ-diabetic rats. Ranirestat should be a promising agent for the treatment of complications associated with diabetes, especially neuropathy.

Search for a functional glucocorticoid receptor in the mammalian lens

Prolonged glucocorticoid treatment of medical conditions such as rheumatoid arthritis or asthma can lead to the formation of a posterior subcapsular cataract as a negative side effect. Currently, the only treatment for this cataract is surgery because very little is known about the mechanism of glucocorticoid action in the mammalian lens. Understanding of a lens glucocorticoid response is essential for the treatment and prevention of a steroid induced cataract. It has been suggested that glucocorticoids exert their effects on the lens indirectly, non-specifically, or through non-classical mechanisms. While these modes of action may contribute to the formation of glucocorticoid induced posterior subcapsular cataract, the finding of a classical, specific, functional lens glucocorticoid receptor suggests that glucocorticoids target lens epithelial cells directly, specifically, and similar to what has been observed in other cells types. This review explores the discovery of the glucocorticoid receptor in humans lens epithelial cells and the lens specific glucocorticoid response. The distinct changes in lens epithelial cell signaling pathways (MAPK and PI3K-AKT) suggest that glucocorticoids modulate several cellular functions and may explain why a lens glucocorticoid response has been difficult to elucidate.

The Etiology of Steroid Cataract

Steroid-induced posterior subcapsular cataracts (PSCs) exhibit three main distinctive characteristics: (i) association only with steroids possessing glucocorticoid activity, (ii) involvement of aberrant migrating lens epithelial cells, and (iii) a central posterior location. The first characteristic suggests a key role for glucocorticoid receptor activation and subsequent changes to the transcription of specific genes. Glucocorticoid receptor activation is associated in many cell types with proliferation, suppressed differentiation, a reduced susceptibility to apoptosis, altered transmembrane transport, and enhancement of reactive oxygen species activity. Glucocorticoids may be capable of inducing changes to the transcription of genes in lens epithelial cells that are related to many of these cellular processes. This review examines the various mechanisms that have been proposed to account for the development of PSC in the context of recent DNA array studies. Additionally, given that the glucocorticoid receptor can also engender wide-ranging indirect activities, glucocorticoids could also indirectly affect the lens through the responses of other cells within the ocular compartment and/or through effects on cells at more remote locations. These indirect mechanisms, which, for example, could be mediated through alterations to the intraocular levels of growth factors that normally orchestrate lens development and maintain lens homeostasis, are also discussed. Although the mechanism of steroid cataract induction remains unknown, glucocorticoid-induced gene transcription events in lens epithelial cells, and also other intraocular or systemic cells, likely interact to generate steroid cataracts. Finally, although evidence for glucocorticoid-protein adduct formation in the lens is inconclusive, the generation of such adducts cannot yet be discounted as a contributing factor and must necessarily be retained in discussions of the etiology of steroid cataract.

Methylglyoxal downregulates Raf-1 protein through a ubiquitination-mediated mechanism

Abnormal accumulation of methylglyoxal, a physiological glucose metabolite, is considered a potential link between hyperglycemia and diabetes complications. Evidence has shown that methylglyoxal modifies cellular proteins by glycation and oxidation, resulting in dysfunction or loss of cellular proteins. Raf-1 protein-serine/threonine kinase serves as a central switch board in the transmission of many growth and developmental signals. It was reported that Raf-1 levels appear to decrease in some diabetic subjects. But the potential mechanisms have not yet been clarified. Here, we tested the hypothesis that methylglyoxal-mediated proteolysis might contribute to the downregulation of Raf-1 levels. We observed that a rapid and detectable decrease in Raf-1 protein levels was induced by methylglyoxal, which was accelerated by treating with a Raf-1 activator, phorbol-12-myristate-13-acetate, and by expressing active forms of Raf-1 and Ras. Moreover, immunoprecipitation and immunoblotting assays showed that co-treatment of cells with methylglyoxal and phorbol-12-myristate-13-acetate caused dramatic ubiquitination in both total intracellular proteins and Raf-1. Blocking phosphorylation with the protein kinase C inhibitor bisindolylmaleimide, or inhibiting intracellular oxidation by addition of the antioxidant N-acetyl-l-cysteine could reverse the ubiquitination and downregulation of Raf-1 induced by methylglyoxal and phorbol-12-myristate-13-acetate. These results suggest that methylglyoxal-mediated intracellular oxidation and ubiquitin/proteasome-dependent proteolysis are involved in the downregulation of Raf-1, which may be closely related to the development complications in diabetes.

Water extract of Aralia elata prevents cataractogenesis in vitro and in vivo

The water extract of Aralia elata (Aralia extract) has been used in Korean traditional medicine to treat diabetes mellitus. Here, we investigated the aldose reductase inhibitory activity, antioxidant activity and anticataract capacity of Aralia extract using various experimental systems. To determine its aldose reductase inhibitory activity and its antioxidant effect, we used rat lens homogenates. Rat lens cultures and a rat model were used to observe anticataract activity. The resulting IC50 value of Aralia extract in vitro as an aldose reductase inhibitor was 11.3 microg/ml and as an antioxidant was 25.1 microg/ml. Rat lenses in media containing 20 mM xylose developed a distinctly opaque ring in 9h, and treatment with Aralia extract at a concentration of 1mg/ml lowered lens opacity by 36.4 and 31.3% after 24 and 48 h, respectively. In vivo experiments were performed with streptozotocin (STZ) induced diabetic rats. The diabetic control animals developed cataracts at 11 weeks after STZ injection, while oral Aralia extract administered at 300 and 600 mg/kg body weight for 11 weeks reduced cataract formation by 15 and 12%, respectively. The present study shows that Aralia extract inhibits aldose reductase and acts in vitro as an antioxidant, and suggests that these activities have a preventive effect on cataractogenesis in xylose containing lens organ cultures and in in vivo in STZ induced diabetic rats.

Aldose reductase inhibition counteracts oxidative-nitrosative stress and poly(ADP-ribose) polymerase activation in tissue sites for diabetes complications

This study evaluated the effects of aldose reductase inhibition on diabetes-induced oxidative-nitrosative stress and poly(ADP-ribose) polymerase (PARP) activation. In animal experiments, control and streptozotocin-induced diabetic rats were treated with or without the aldose reductase inhibitor (ARI) fidarestat (16 mg . kg(-1) . day(-1)) for 6 weeks starting from induction of diabetes. Sorbitol pathway intermediate, but not glucose, accumulation in sciatic nerve and retina was completely prevented in diabetic rats treated with fidarestat. Sciatic motor nerve conduction velocity, hindlimb digital sensory nerve conduction velocity, and sciatic nerve concentrations of two major nonenzymatic antioxidants, glutathione and ascorbate, were reduced in diabetic versus control rats, and these changes were prevented in diabetic rats treated with fidarestat. Fidarestat prevented the diabetes-induced increase in nitrotyrosine (a marker of peroxynitrite-induced injury) and poly(ADP-ribose) immunoreactivities in sciatic nerve and retina. Fidarestat counteracted increased superoxide formation in aorta and epineurial vessels and in in vitro studies using hyperglycemia-exposed endothelial cells, and the DCF test/flow cytometry confirmed the endothelial origin of this phenomenon. Fidarestat did not cause direct inhibition of PARP activity in a cell-free system containing PARP and NAD(+) but did counteract high-glucose-induced PARP activation in Schwann cells. In conclusion, aldose reductase inhibition counteracts diabetes-induced nitrosative stress and PARP activation in sciatic nerve and retina. These findings reveal the new beneficial properties of fidarestat, thus further justifying the ongoing clinical trials of this specific, potent, and low-toxic ARI.

Anti-inflammatory effects of topical ocular MAXIDEX® administration to rabbits following vitrectomy or lensectomy

We conducted a series of surgical studies (n=155) to find out the possible utility of the Dutch Belted rabbits as an ocular test model. Following either vitrectomy (n=59) or lensectomy (n=96) studies using either BSS or BSS Plus, we characterized the corresponding magnitude and duration of inflammatory response of selected endpoints over a one-week period. Preoperative Dutch Belted rabbits served as controls for baseline determination (n=27). Inflammatory endpoints included clinical inflammation, blood aqueous barrier (BAB) changes measured by particle-scatter and fluorophotometry, corneal edema, and prostaglandin-E2 (PGE2). Topical ocular 0.1% dexamethasone (MAXIDEX) served as a positive treatment group. We compared the inflammatory features (treated and untreated) to determine significance. Using either irrigating solution, the endpoints (n=101) were shown increased in both models. While the clinical scores were similar following both types of surgery (ns; p=0.51), the lensectomy study caused a more marked effect on corneal edema (p=0.0004) and PGE2 production (p=0.002) compared with the vitrectomy study. After the lensectomy procedure, BSS Plus (n=52) compared with BSS (n=24) showed a significant improvement (p=0.004) of clinical score during the recovery phase. Further improvement was gained over BSS Plus (n=52) using MAXIDEX treatment. Topical MAXIDEX (lensectomy, n=20/group; vitrectomy, n=12/group) reduced clinical score (p<0.001), decreased BAB breakdown to fluorescein (p<0.01), lessened particle flare (p<0.05), inhibited aqueous PGE2 levels (p<0.001), and reduced corneal edema (p=0.01) in the lensectomy group. The use of the rabbit model offers a convenient test to identify therapeutic agents that could lessen ocular complications after these common ocular surgeries.

Alterations in lens protein tyrosine phosphorylation and phosphatidylinositol 3-kinase signaling during selenite cataract formation

PURPOSE

Protein tyrosine phosphorylation is an important event in the cell signal transduction process. Phosphatidylinositol-3 kinase (PI-3K) is an intracellular signal mediator and plays a key role in many cellular functions. In this study we have examined the changes in lens protein tyrosine phosphorylation and its impact on phosphatidylinositol 3-kinase (PI-3K) signaling during selenite cataract development.

METHODS

Cataract was induced in 10 days old rat pups by a single sub-cutaneous injection of sodium selenite (30 microM/Kg body weight) and lenses were collected at different stages of cataract development. Immunoprecipitation and Western immunoblotting were employed to determine protein tyrosine phosphorylation, PI-3K activity and protein in lens cell extracts. Tyrosine kinase activity in lens membrane preparations was assayed in the presence of a synthetic substrate peptide and [32P]ATP.

RESULTS

Protein tyrosine phosphorylation in the lens was disrupted before the onset of cataract. A decrease in tyrosine phosphorylation of lens proteins was observed within 2-3 days of selenite injection (pre-cataract stage). The effect was much more prominent with the progression of cataract. The decrease in protein tyrosine phosphorylation correlated with the decrease in tyrosine kinase activity associated with the lens membrane fraction. Stimulation of normal rat lenses in organ culture with insulin and IGF-1 caused an increase in the phosphorylation of proteins, whose tyrosine phosphorylation status appeared to be diminished during cataract development. Insulin and IGF-1 also stimulated rat lens PI-3K activity. While there was no change in total PI-3K activity during the onset of cataract, the activity of PI-3K associated with tyrosine phosphorylated proteins decreased markedly in pre-cataract lenses. Further, the ability of IGF-1 to stimulate PI-3K activity was significantly reduced in lens epithelial cells treated with selenium.

CONCLUSIONS

These studies show that signaling events involving the protein tyrosine phosphorylation process and activation of PI-3K are altered during selenite cataract formation and implicate defects in signal transduction mechanisms as contributing factors in the development of cataract.