Interaction between the Polyol Pathway and Non-Enzymatic Glycation on Mesangial Cell Gene Expression

Physiological and Pathological Roles of Aldose Reductase

Aldose reductase (AR) is an aldo-keto reductase that catalyzes the first step in the polyol pathway which converts glucose to sorbitol. Under normal glucose homeostasis the pathway represents a minor route of glucose metabolism that operates in parallel with glycolysis. However, during hyperglycemia the flux of glucose via the polyol pathway increases significantly, leading to excessive formation of sorbitol. The polyol pathway-driven accumulation of osmotically active sorbitol has been implicated in the development of secondary diabetic complications such as retinopathy, nephropathy, and neuropathy. Based on the notion that inhibition of AR could prevent these complications a range of AR inhibitors have been developed and tested; however, their clinical efficacy has been found to be marginal at best. Moreover, recent work has shown that AR participates in the detoxification of aldehydes that are derived from lipid peroxidation and their glutathione conjugates. Although in some contexts this antioxidant function of AR helps protect against tissue injury and dysfunction, the metabolic transformation of the glutathione conjugates of lipid peroxidation-derived aldehydes could also lead to the generation of reactive metabolites that can stimulate mitogenic or inflammatory signaling events. Thus, inhibition of AR could have both salutary and injurious outcomes. Nevertheless, accumulating evidence suggests that inhibition of AR could modify the effects of cardiovascular disease, asthma, neuropathy, sepsis, and cancer; therefore, additional work is required to selectively target AR inhibitors to specific disease states. Despite past challenges, we opine that a more gainful consideration of therapeutic modulation of AR activity awaits clearer identification of the specific role(s) of the AR enzyme in health and disease.

Multifunctional agents based on benzoxazolone as promising therapeutic drugs for diabetic nephropathy

Diabetic nephropathy (DN) is resulted from activations of polyol pathway and oxidative stress by abnormal metabolism of glucose, and no specific medication is available. We designed a novel class of benzoxazolone derivatives, and a number of individuals were found to have significant antioxidant activity and inhibition of aldose reductase of the key enzyme in the polyol pathway. The outstanding compound (E)-2-(7-(4-hydroxy-3-methoxystyryl)-2-oxobenzo[d]oxazol-3(2H)-yl)acetic acid was identified to reduce urinary proteins in diabetic mice suggesting an alleviation in the diabetic nephropathy, and this was confirmed by kidney hematoxylin-eosin staining. Further investigations showed blood glucose normalization, declined in the polyol pathway and lipid peroxides, and raised glutathione and superoxide dismutase activity. Thus, we suggest a therapeutic function of the compound for DN which could be attributed to the combination of hypoglycemic, aldose reductase inhibition and antioxidant.

Vitamin K1 prevents diabetic cataract by inhibiting lens aldose reductase 2 (ALR2) activity

This study investigated the potential of vitamin K1 as a novel lens aldose reductase inhibitor in a streptozotocin-induced diabetic cataract model. A single, intraperitoneal injection of streptozotocin (STZ) (35 mg/kg) resulted in hyperglycemia, activation of lens aldose reductase 2 (ALR2) and accumulation of sorbitol in eye lens which could have contributed to diabetic cataract formation. However, when diabetic rats were treated with vitamin K1 (5 mg/kg, sc, twice a week) it resulted in lowering of blood glucose and inhibition of lens aldose reductase activity because of which there was a corresponding decrease in lens sorbitol accumulation. These results suggest that vitamin K1 is a potent inhibitor of lens aldose reductase enzyme and we made an attempt to understand the nature of this inhibition using crude lens homogenate as well as recombinant human aldose reductase enzyme. Our results from protein docking and spectrofluorimetric analyses clearly show that vitamin K1 is a potent inhibitor of ALR2 and this inhibition is primarily mediated by the blockage of DL-glyceraldehyde binding to ALR2. At the same time docking also suggests that vitamin K1 overlaps at the NADPH binding site of ALR2, which probably shows that vitamin K1 could possibly bind both these sites in the enzyme. Another deduction that we can derive from the experiments performed with pure protein is that ALR2 has three levels of affinity, first for NADPH, second for vitamin K1 and third for the substrate DL-glyceraldehyde. This was evident based on the dose-dependency experiments performed with both NADPH and DL-glyceraldehyde. Overall, our study shows the potential of vitamin K1 as an ALR2 inhibitor which primarily blocks enzyme activity by inhibiting substrate interaction of the enzyme. Further structural studies are needed to fully comprehend the exact nature of binding and inhibition of ALR2 by vitamin K1 that could open up possibilities of its therapeutic application.

1-Acetyl-5-phenyl-1H-pyrrol-3-ylacetate: An aldose reductase inhibitor for the treatment of diabetic nephropathy

Diabetic nephropathy (DN) is the most common and serious complication in diabetes mellitus, but the efficacy of available strategies for preventing this disorder remains poor. The aim of this study was to investigate the possible beneficial effects of 1-acetyl-5-phenyl-1H-pyrrol-3-ylacetate (APPA), an aldose reductase inhibitor, on DN. In the present study, a model of rat glomerular mesangial cells (HBZY-1) damaged by high glucose was used to confirm the protective effects of APPA in vitro. Then, a rat model of streptozotocin-induced diabetes was used to assess the effects of APPA in vivo. APPA increased viability and reduced apoptosis in HBZY-1 cells. In vivo, APPA improved the signs of DN as determined by measurements of blood glucose, urinary microalbumin, serum total antioxidant capacity, serum catalase activity, serum glutathione levels, and serum total superoxide dismutase activity. Hematoxylin and eosin staining of kidney tissue confirmed the protective effect. Moreover, APPA reduced the levels of transforming growth factor-β1, collagen IV, and laminin in HBZY-1cells incubated in high glucose, and in serum in DN rats. In summary, APPA can effectively prevent apoptosis and the symptoms of streptozotocin-induced diabetes by inhibiting the polyol pathway in rats. This suggests that APPA could be a potential drug in treating DN.

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.

s-Allyl cysteine, s-ethyl cysteine, and s-propyl cysteine alleviate β-amyloid, glycative, and oxidative injury in brain of mice treated by D-galactose

The neuroprotective effects of s-allyl cysteine, s-ethyl cysteine, and s-propyl cysteine in D-galactose (DG)-treated mice were examined. DG treatment increased the formation of Aβ(1-40) and Aβ(1-42), enhanced mRNA expression of β-amyloid precursor protein (APP) and β-site APP cleavage enzyme 1 (BACE1), and reduced neprilysin expression in brain (P < 0.05); however, the intake of three test compounds significantly decreased the production of Aβ(1-40) and Aβ(1-42) and suppressed the expression of APP and BACE1 (P < 0.05). DG treatments declined brain protein kinase C (PKC) activity and mRNA expression (P < 0.05). Intake of test compounds significantly retained PKC activity, and the expression of PKC-α and PKC-γ (P < 0.05). DG treatments elevated brain activity and mRNA expression of aldose reductase (AR) and sorbitol dehydrogenase as well as increased brain levels of carboxymethyllysine (CML), pentosidine, sorbitol, and fructose (P < 0.05). Test compounds significantly lowered AR activity, AR expression, and CML and pentosidine levels (P < 0.05). DG treatments also significantly increased the formation of reactive oxygen species (ROS) and protein carbonyl and decreased the activity of glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (P < 0.05); however, the intake of test compounds in DG-treated mice significantly decreased ROS and protein carbonyl levels and restored brain GPX, SOD, and catalase activities (P < 0.05). These findings support that these compounds via their anti-Aβ, antiglycative, and antioxidative effects were potent agents against the progression of neurodegenerative disorders such as Alzheimer's disease.

Protective effects of an aqueous extract from pepino (Solanum muricatum Ait.) in diabetic mice

BACKGROUND

This study analysed the content of ascorbic acid, phenolic acids and flavonoids in aqueous and ethanol extracts of pepino (Solanum muricatum Ait.), and examined the protective effects of pepino aqueous extract (PAE) in a mouse model of diabetes. PAE at 1, 2 and 4% was supplied for 5 weeks.

RESULTS

Aqueous and ethanol extracts had similar levels of total phenolic acids, but PAE had a higher content of ascorbic acid and total flavonoids than the ethanol extract. PAE treatments at 2% and 4% significantly lowered plasma glucose level (P < 0.05); however, only the 4% PAE significantly elevated plasma insulin level at week 5 (P < 0.05). PAE treatments significantly decreased the levels of malonyldialdehyde and reactive oxygen species in kidney (P < 0.05); however, only the 2% and 4% treatments significantly reduced oxidised glutathione formation, increased glutathione level, and retained renal glutathione peroxidase and catalase activities (P < 0.05). PAE treatments at 2% and 4% significantly lowered renal interleukin (IL)-6 and tumour necrosis factor-α levels (P < 0.05); however, only the 4% treatments significantly diminished renal IL-1β and levels of monocyte chemoattractant protein-1 (P < 0.05). PAE treatments at 4% significantly decreased aldose reductase activity and sorbitol production in kidney (P < 0.05).

CONCLUSION

These findings support the suggestion that pepino aqueous extract could attenuate the progression of diabetes via its antioxidative, anti-inflammatory and antiglycative effects.

Aldose reductase pathway contributes to vulnerability of aging myocardium to ischemic injury

Aging men and women display both increased incidence of cardiovascular disease and complications of myocardial infarction and heart failure. We hypothesized that altered glucose metabolism, in particular, flux of glucose via the polyol pathway (PP) may be responsible, in part, for the enhanced vulnerability of aging myocardium to ischemic injury, even in the absence of superimposed disease processes linked to PP flux, such as diabetes. To test our hypothesis, we determined the expression and products of PP enzymes aldose reductase (AR) and sorbitol dehydrogenase (SDH) in hearts from Fischer 344 aged (26 months) and young (4 months) rats subjected to global ischemia followed by reperfusion in the presence or absence of blockers of PP and the measures of ischemic injury and functional recovery were determined. Expression and activities of AR and SDH were significantly higher in aged vs. young hearts, and induction of ischemia further increased AR and SDH activity in the aged hearts. Myocardial ischemic injury was significantly greater in aged vs. young hearts, and blockade of AR reduced ischemic injury and improved cardiac functional recovery on reperfusion in aged hearts. These data indicate that innate increases in activity of the PP enzymes augment myocardial vulnerability to I/R injury in aging, and that blockers of PP protect the vulnerable aging hearts.

Genetic deficiency of aldose reductase counteracts the development of diabetic nephropathy in C57BL/6 mice

AIMS/HYPOTHESIS

The aim of the study was to investigate the effects of genetic deficiency of aldose reductase in mice on the development of key endpoints of diabetic nephropathy.

METHODS

A line of Ar (also known as Akr1b3)-knockout (KO) mice, a line of Ar-bitransgenic mice and control C57BL/6 mice were used in the study. The KO and bitransgenic mice were deficient for Ar in the renal glomeruli and all other tissues, with the exception of, in the bitransgenic mice, a human AR cDNA knockin-transgene that directed collecting-tubule epithelial-cell-specific AR expression. Diabetes was induced in 8-week-old male mice with streptozotocin. Mice were further maintained for 17 weeks then killed. A number of serum and urinary variables were determined for these 25-week-old mice. Periodic acid-Schiff staining, western blots, immunohistochemistry and protein kinase C (PKC) activity assays were performed for histological analyses, and to determine the levels of collagen IV and TGF-β1 and PKC activities in renal cortical tissues.

RESULTS

Diabetes-induced extracellular matrix accumulation and collagen IV overproduction were completely prevented in diabetic Ar-KO and bitransgenic mice. Ar deficiency also completely or partially prevented diabetes-induced activation of renal cortical PKC, TGF-β1 and glomerular hypertrophy. Loss of Ar results in a 43% reduction in urine albumin excretion in the diabetic Ar-KO mice and a 48% reduction in the diabetic bitransgenic mice (p < 0.01).

CONCLUSIONS/INTERPRETATION

Genetic deficiency of Ar significantly ameliorated development of key endpoints linked with early diabetic nephropathy in vivo. Robust and specific inhibition of aldose reductase might be an effective strategy for the prevention and treatment of diabetic nephropathy.

Aldose reductase and cardiovascular diseases, creating human-like diabetic complications in an experimental model

Hyperglycemia and reduced insulin actions affect many biological processes. One theory is that aberrant metabolism of glucose via several pathways including the polyol pathway causes cellular toxicity. Aldose reductase (AR) is a multifunctional enzyme that reduces aldehydes. Under diabetic conditions AR converts glucose into sorbitol, which is then converted to fructose. This article reviews the biology and pathobiology of AR actions. AR expression varies considerably among species. In humans and rats, the higher level of AR expression is associated with toxicity. Flux via AR is increased by ischemia and its inhibition during ischemia reperfusion reduces injury. However, similar pharmacological effects are not observed in mice unless they express a human AR transgene. This is because mice have much lower levels of AR expression, probably insufficient to generate toxic byproducts. Human AR expression in LDL receptor knockout mice exacerbates vascular disease, but only under diabetic conditions. In contrast, a recent report suggests that genetic ablation of AR increased atherosclerosis and increased hydroxynonenal in arteries. It was hypothesized that AR knockout prevented reduction of toxic aldehydes. Like many in vivo effects found in genetically manipulated animals, interpretation requires the reproduction of human-like physiology. For AR, this will require tissue specific expression of AR in sites and at levels that approximate those in humans.

Polyol pathway and RAGE: a central metabolic and signaling axis in diabetic complications

There are multiple metabolic and molecular consequences of hyperglycemia. This review will focus on the roles of the polyol pathway and the receptor for advanced glycation end products (RAGE) in the pathogenesis of diabetic complications. The lead enzyme of the polyol pathway, aldose reductase, transduces maladaptive effects of hyperglycemia by multiple mechanisms, at least in part via the generation of the products of nonenzymatic glycation of proteins, the advanced glycation end products (AGEs). Furthermore, seminal shifts in metabolic flux in the intracellular space stimulated by aldose reductase action activate signal transduction pathways, which alter gene expression and change cellular phenotype. Among the ligands of the multi-ligand receptor RAGE are the AGEs. AGE-RAGE stimulation mediates vascular and target cell dysfunction. The intersection and interdependence of the polyol pathway-RAGE connection suggest that targeting this axis may provide benefit in reducing the complications of diabetes.

Poly(Adenosine 5'-diphosphate-ribose) polymerase inhibition counteracts multiple manifestations of experimental type 1 diabetic nephropathy

This study was aimed at evaluating the role for poly(ADP-ribose) polymerase (PARP) in early nephropathy associated with type 1 diabetes. Control and streptozotocin-diabetic rats were maintained with or without treatment with one of two structurally unrelated PARP inhibitors, 1,5-isoquinolinediol (ISO) and 10-(4-methyl-piperazin-1-ylmethyl)-2H-7-oxa-1,2-diaza-benzo[de] anthracen-3-one (GPI-15427), at 3 mg/kg(-1) x d(-1) ip and 30 mg/kg(-1) x d(-1), respectively, for 10 wk after the first 2 wk without treatment. PARP activity in the renal cortex was assessed by immunohistochemistry and Western blot analysis of poly(ADP-ribosyl)ated proteins. Variables of diabetic nephropathy in urine and renal cortex were evaluated by ELISA, Western blot analysis, immunohistochemistry, and colorimetry. Urinary albumin excretion was increased about 4-fold in diabetic rats, and this increase was prevented by ISO and GPI-15427. PARP inhibition counteracted diabetes-associated increase in poly(ADP-ribose) immunoreactivities in renal glomeruli and tubuli and poly(ADP-ribosyl)ated protein level. Renal concentrations of TGF-beta(1), vascular endothelial growth factor, endothelin-1, TNF-alpha, monocyte chemoattractant protein-1, lipid peroxidation products, and nitrotyrosine were increased in diabetic rats, and all these changes as well as an increase in urinary TNF-alpha excretion were completely or partially prevented by ISO and GPI-15427. PARP inhibition counteracted diabetes-induced up-regulation of endothelin (B) receptor, podocyte loss, accumulation of collagen-alpha1 (IY), periodic acid-Schiff-positive substances, fibronectin, and advanced glycation end-products in the renal cortex. In conclusion, PARP activation is implicated in multiple changes characteristic for early nephropathy associated with type 1 diabetes. These findings provide rationale for development and further studies of PARP inhibitors and PARP inhibitor-containing combination therapies.

Poly(ADP-ribose) polymerase (PARP) inhibition counteracts multiple manifestations of kidney disease in long-term streptozotocin-diabetic rat model

Evidence for the important role for poly(ADP-ribose) polymerase (PARP) in the pathogenesis of diabetic nephropathy is emerging. We previously reported that PARP inhibitors counteract early Type 1 diabetic nephropathy. This study evaluated the role for PARP in kidney disease in long-term Type 1 diabetes. Control and streptozotocin-diabetic rats were maintained with or without treatment with the PARP inhibitor 10-(4-methyl-piperazin-1-ylmethyl)-2H-7-oxa-1,2-diaza-benzo[de] anthracen-3-one (GPI-15,427, Eisai Inc.), 30mgkg(-1)d(-1), for 26 weeks after first 2 weeks without treatment. PARP activity in the renal cortex was assessed by Western blot analysis of poly(ADP-ribosyl)ated proteins. Urinary albumin, isoprostane, and 8-hydroxy-2'-deoxyguanosine excretion, and renal concentrations of transforming growth factor-beta(1), vascular endothelial growth factor, soluble intercellular adhesion molecule-1, fibronectin, and nitrotyrosine were evaluated by ELISA, and urinary creatinine and renal lipid peroxidation products by colorimetric assays. PARP inhibition counteracted diabetes-associated increase in renal cortex poly(ADP-ribosyl)ated protein level. Urinary albumin, isoprostane, and 8-hydroxy-2'-deoxyguanosine excretions and urinary albumin/creatinine ratio were increased in diabetic rats, and all these changes were at least partially prevented by GPI-15,427 treatment. PARP inhibition counteracted diabetes-induced renal transforming growth factor-beta(1), vascular endothelial growth factor, and fibronectin, but not soluble intercellular adhesion molecule-1 and nitrotyrosine, accumulations. Lipid peroxidation product concentrations were indistinguishable among control and diabetic rats maintained with or without GPI-15,427 treatment. In conclusion, PARP activation plays an important role in kidney disease in long-term diabetes. These findings provide rationale for development and further studies of PARP inhibitors and PARP inhibitor-containing combination therapies, for prevention and treatment of diabetic nephropathy.

Structural and Thermodynamic Study on Aldose Reductase: Nitro-substituted Inhibitors with Strong Enthalpic Binding Contribution

To prevent diabetic complications derived from enhanced glucose flux via the polyol pathway the development of aldose reductase inhibitors (ARIs) has been established as a promising therapeutic concept. In order to identify novel lead compounds, a virtual screening (VS) was performed successfully suggesting carboxylate-type inhibitors of sub-micromolar to micromolar affinity. Here, we combine a structural characterization of the binding modes observed by X-ray crystallography with isothermal titration calorimetry (ITC) measurements providing insights into the driving forces of inhibitor binding, particularly of the first leads from VS. Characteristic features of this novel inhibitor type include a carboxylate head group connected via an alkyl spacer to a heteroaromatic moiety, which is linked to a further nitro-substituted aromatic portion. The crystal structures of two enzyme-inhibitor complexes have been determined at resolutions of 1.43 A and 1.55 A. Surprisingly, the carboxylic group of the most potent VS lead occupies the catalytic pocket differently compared to the interaction geometry observed in almost all other crystal structures with structurally related ligands and obtained under similar conditions, as an interstitial water molecule is picked up upon ligand binding. The nitro-aromatic moiety of both leads occupies the specificity pocket of the enzyme, however, adopting a different geometry compared to the docking prediction: unexpectedly, the nitro group binds to the bottom of the specificity pocket and provokes remarkable induced-fit adaptations. A peptide group located at the active site orients in such a way that H-bond formation to one nitro group oxygen atom is enabled, whereas a neighbouring tyrosine side-chain performs a slight rotation off from the binding cavity to accommodate the nitro group. Identically constituted ligands, lacking this nitro group, exhibit an affinity drop of one order of magnitude. In addition, thermodynamic data suggest a strongly favourable contribution to binding enthalpy in case the inhibitor is equipped with a nitro group at the corresponding position. To further investigate this phenomenon, we determined crystal structures and thermodynamic data of two similarly constituted IDD-type inhibitors addressing the specificity pocket with either a nitro or halogen-substituted aromatic moiety. As these data suggest, the nitro group provokes the enthalpic contribution, in addition to the H-bond mentioned above, by accepting two "non-classical" H-bonds donated by the aromatic tyrosine side-chain. In summary, this study provides the platform for further structure-guided design hypotheses of novel drug candidates with higher affinity and selectivity.

Immunohistochemical localization of advanced glycation end-products (AGEs) and their receptor (RAGE) in polycystic and normal ovaries

The aim of the present study was to investigate the localization/immunohistochemical distribution of AGEs and RAGE, as well as their putative signalling mediator NF-kappaB in ovaries of women with polycystic ovary syndrome (PCOS) compared to normal. Archival ovarian-tissue samples from biopsies of six women with PCOS and from six healthy of similar age women, were examined immunohistochemically with monoclonal anti-AGEs, anti-RAGE and anti-NF-kappaB(p50/p65) specific antibodies. In healthy women, AGE immunoreactivity was observed in follicular cell layers (granulosa and theca) and luteinized cells, but not in endothelial cells. PCOS specimens displayed AGE immunoexpression in theca interna and granulosa cells as well as in endothelial cells, but staining of granulosa cells was stronger than in that of normal ovaries. RAGE was highly expressed in normal and PCOS tissues. Normal tissue exhibited no staining differences between granulosa cell layer and theca interna. However, in PCOS ovaries, granulosa cells displayed stronger RAGE expression compared to theca interna cells in comparison to controls. NF-kappaB(p50/p65) was expressed in the cytoplasm of theca interna and granulosa cells of both normal and PCOS ovaries; whereas the NF-kappaB p65 subunit was only observed in granulosa cells nuclei in PCOS tissue. In conclusion, these findings demonstrate for the first time that RAGE and AGE-modified proteins with activated NF-kappaB are expressed in human ovarian tissue. Furthermore, a differential qualitative distribution of AGE, RAGE and NF-kappaB p65 subunit was observed in women with PCOS compared to healthy controls, where a stronger localization of both AGE and RAGE was observed in the granulosa cell layer of PCOS ovaries.