Minalrestat and leukocyte migration in diabetes mellitus

BACKGROUND

We recently demonstrated that aldose reductase inhibition was effective in restoring the reduced migratory capacity of leukocytes in diabetic rats. To investigate the mechanism(s) involved in the restoring effect, we used minalrestat, an aldose reductase inhibitor.

METHODS

In sodium pentobarbital-anesthetized (40 mg/kg, intraperitoneally) alloxan-diabetic or galactosemic male Wistar rats, the internal spermatic fascia was exteriorized, and the number of leukocytes rolling along the venular endothelium and the number of leukocytes sticking to the vascular wall after topical application of zymosan-activated plasma or leukotriene B(4) (1 ng/ml), as well as after the application of a local irritant stimulus (carrageenan, 100 microg), were determined using intravital microscopy. Data from animals that were treated with and those that were not treated with minalrestat (10 mg/kg/d by gavage) were compared.

RESULTS

The reduced number of leukocytes rolling along the venular endothelium (by about 70%) and the number of adhered and migrated leukocytes in postcapillary venules (by 60%) were significantly restored to control values after minalrestat treatment. Total or differential leukocyte counts, venular blood flow velocity or wall shear rate were not altered by minalrestat treatment. The expression of ICAM-1 and P-selectin, cell adhesion molecules involved in the interaction of leukocyte-endothelium, reduced in diabetic rats was restored by minalrestat treatment.

CONCLUSION

We conclude that an enhanced flux through the polyol pathway might be involved in the reduced expression of ICAM-1 and P-selectin contributing to the impaired leukocyte-endothelial interactions in diabetes mellitus and that aldose reductase inhibition restores the defect, restoring the reduced expression.

Neuropathy in diabetic mice overexpressing human aldose reductase and effects of aldose reductase inhibitor

The present study was designed to examine the effect of aldose reductase (AR) overexpression on the development of diabetic neuropathy by using mice transgenic for human AR. At 8 weeks of age, transgenic mice (Tg) and non-transgenic littermates (Lm) were made diabetic with streptozotocin. After 8 weeks of untreated diabetes, plasma glucose levels and the reduction in body weight were similar between the groups of diabetic animals. Despite the comparable levels of hyperglycaemia, levels of sorbitol and fructose were significantly greater in the peripheral nerve of diabetic Tg than in diabetic Lm (both P < 0.01). Ouabain sensitive Na(+),K(+)-ATPase activity was similarly decreased in both diabetic Tg and Lm. Protein kinase C activity in the sciatic nerve membrane fraction was unaffected by diabetes in Lm, but was reduced by nearly 40% in the diabetic Tg. Although both groups of diabetic animals exhibited a significant decrease in tibial nerve motor nerve conduction velocity (MNCV), this decrease was significantly more severe (P < 0.01) in diabetic Tg than in diabetic Lm. Consistent with these findings, nerve fibre atrophy was significantly more severe in diabetic Tg than in diabetic Lm (P < 0.01). These findings implicate increased polyol pathway activity in the pathogenesis of diabetic neuropathy. In support of this hypothesis, treating diabetic Tg with an aldose reductase inhibitor (WAY121-509, 4 mg/kg/day) for 8 weeks significantly prevented the accumulation of sorbitol, the decrease in MNCV and the increased myelinated fibre atrophy in diabetic Tg.

Streptozotocin-induced diabetes causes metabolic changes and alterations in neurotrophin content and retrograde transport in the cervical vagus nerve

Abnormal availability of neurotrophins, such as nerve growth factor (NGF), has been implicated in diabetic somatosensory polyneuropathy. However, the involvement of neurotrophins in diabetic neuropathy of autonomic nerves, particularly the vagus nerve which plays a critical role in visceral afferent and in autonomic motor functions, is unknown. To assess the effects of hyperglycemia on the neurotrophin content and transport in this system, cervical vagus nerves of streptozotocin (STZ)-induced diabetic rats were studied at 8, 16, and 24 weeks after the induction of diabetes. Elevations in vagus nerve hexose (glucose and fructose) and polyol levels (sorbitol), and their normalization with insulin treatment, verified that the STZ treatment resulted in hyperglycemia-induced metabolic abnormalities in the nerve. Neurotrophin (NGF and neurotrophin-3; NT-3) content and axonal transport were assessed in the cervical vagus nerves from nondiabetic control rats, STZ-induced diabetic rats, and diabetic rats treated with insulin. The NGF, but not the NT-3, content of intact vagus nerves from diabetic rats was increased at 8 and 16 weeks (but not at 24 weeks). Using a double-ligation model to assess the transport of endogenous neurotrophins, the retrograde transport of both NGF and NT-3 was found to be significantly reduced in the cervical vagus nerve at later stages of diabetes (16 and 24 weeks). Anterograde transport of NGF or NT-3 was not apparent in the vagus nerve of diabetic or control rats. These data suggest that an increase in vagus nerve NGF is an early, but transient, response to the diabetic hyperglycemia and that a subsequent reduction in neuronal access to NGF and NT-3 secondary to decreased retrograde axonal transport may play a role in diabetes-induced damage to the vagus nerve.

Correction of nerve conduction and endoneurial blood flow deficits by the aldose reductase inhibitor, tolrestat, in diabetic rats

Increased activation of the first half of the polyol pathway, the conversion of glucose to sorbitol by aldose reductase, has been implicated in aldose reductase inhibitor-preventable neurochemical changes that may contribute to the aetiology of diabetic neuropathy. Tolrestat has been used as a standard aldose reductase inhibitor to dissect out polyol pathway-dependent mechanisms in many experimental studies; however, doubt has been cast upon its ability to prevent nerve conduction velocity deficits in diabetic rats. Nerve dysfunction has also been linked to abnormal endoneurial blood flow and oxygenation via increased vasa nervorum polyol pathway flux. The aim of this study was to test whether tolrestat could correct sciatic conduction velocity and perfusion defects in diabetic rats. Sciatic motor conduction velocity, 21% reduced by 1 month of streptozotocin-induced diabetes, was corrected by 23% and 84% with 1 month of tolrestat treatment at doses of 7 and 35 mg/kg/day respectively. Endoneurial blood flow, 44-52% reduced by untreated diabetes, was within the nondiabetic range with high-dose tolrestat treatment and the flow deficit was 39% corrected by the low dose. Sciatic sorbitol and fructose concentrations were approximately 13-fold and approximately 4-fold elevated by untreated diabetes. This was 32-50% attenuated by low-dose tolrestat and sorbitol and fructose content was suppressed below the nondiabetic level by high dose treatment. A 58% nerve myo-inositol deficit was partially (32%) corrected by high-dose tolrestat treatment. We conclude that tolrestat restores defective conduction and blood flow in diabetic rats and is a good pharmacological tool for studies on polyol pathway effects in peripheral nerve.

ATP-sensitive K(+) channel effects on nerve function, Na(+), K(+) ATPase, and glutathione in diabetic rats

Some vasodilators correct nerve conduction velocity and endoneurial blood flow deficits in diabetic rats. It is not known whether vasa nervorum has ATP-sensitive K(+) (K(ATP)) channels that mediate vasodilation, or whether K(ATP) channels could modulate peripheral nerve function. Therefore, we examined the effects of 2 weeks treatment with the K(ATP) channel openers, celikalim and WAY135201 (R-4-[3, 4-dioxo-2-(1, 2, 2-trimethyl-propylamino)-cyclobut-1-1-enylamino]-3-methoxy-+ ++benzonitri le), on sciatic nerve blood flow, conduction velocity, Na(+)-K(+) ATPase activity and glutathione content after 6 weeks of untreated streptozotocin-diabetes in rats. Blood flow and motor conduction velocity, 47.6% and 20.3% reduced by diabetes, respectively, were completely restored by both celikalim and WAY135201 treatments. Diabetes diminished sciatic Na(+)-K(+) ATPase activity by 47.6% and this was 80-90% corrected by the K(ATP) channel openers. Sciatic nerve glutathione content, 30.3% reduced by diabetes, was unaffected by celikalim or WAY135201. Thus, K(ATP) channel openers had marked beneficial effects on nerve perfusion and function in experimental diabetic neuropathy, and may be suitable for further study in clinical trials.

Molecular mechanism(s) of insulin action on the expression of the angiotensinogen gene in kidney proximal tubular cells

To investigate the molecular mechanism(s) of insulin action on the expression of the angiotensinogen (ANG) gene in kidney proximal tubular cells, we constructed a fusion gene, pOGH (hANG N-1064/+27), containing the 5'-flanking regulatory sequence of the human ANG gene fused with the human growth hormone (hGH) gene as a reporter and stably integrated the fusion gene into the opossum kidney (OK) cell genomes. The level of expression of pOGH (hANG N-1064/+27) was quantified by the amount of immunoreactive hGH secreted into the medium. The addition of a high level of D(+)-glucose (25 mM) or phorbol 12-myristate 13-acetate (PMA, 10(-7) M) stimulated the expression of the fusion gene in OK cells. The stimulatory effect of glucose (25 mM) was blocked by insulin and tolrestat (an inhibitor of aldose reductase). Tolrestat also inhibited the increase of cellular DAG and PKC activity stimulated by 25 mM glucose. While insulin did not affect the cellular DAG and PKC activity, it did block the stimulatory effect of high glucose (25 mM) and PMA on the expression of the fusion gene. Finally, PD98059 (an inhibitor of mitogen-activated protein kinase kinase (MEK)) enhanced the stimulatory effect of high levels of glucose and blocked the inhibitory effect of insulin on the expression of the fusion gene as well as on the phosphorylation of MEK and mitogen-activated protein kinase (MAPK). In contrast, Wortmannin (an inhibitor of phosphatidylinositol-3-kinase) did not block the inhibitory effect of insulin on the ANG gene expression. These studies demonstrate that the action of insulin, blocking the stimulatory effect of a high level of D(+)-glucose (25 mM) on the ANG gene expression is mediated, at least in part, via the 5'-flanking region of the ANG gene and MAPK signal transduction pathway.

Retinal capillary dilation: early diabetic-like retinopathy in the galactose-fed rat model

The purpose of this study was to determine whether capillary dilation is one of the earliest structural changes in the diabetic-like retinopathy of the galactose-fed rat model and thus may represent a stage where intervention treatment might still be effective. Weanling female Sprague-Dawley rats were randomized into 3 groups and fed Purina laboratory chow plus one of the following for 4 months: 50% starch (CONTROL); 50% D-galactose (Galactose); or 50% D-galactose with ARI-509 (25 mg/kg body wt/day) (Inhibitor). One eye from each of 5 rats per treatment group was processed for retinal vasculature wholemounts using elastase digestion, stained with a standard periodic-acid-Schiff reaction and counterstained with hematoxylin. Average capillary width, overall capillary density and total capillary length were measured, using computerized image analysis, within an arc-shaped area (4.4 mm2) of each vasculature surrounding, but separated from, the optic disc margin by approximately 0.7 mm. Galactose rats exhibited significant (p < 0.001) increases in capillary width (Mean +/- SEM: 7.56 +/- 0.07 microm) and density (42.78 +/- 0.37%) compared with CONTROL rats (6.68 +/- 0.11 microm and 37.18 +/- 0.30%, respectively). These increases were prevented with inhibitor treatment (6.58 +/- 0.16 microm and 35.88 +/- 0.97%, respectively). Capillary length remained unchanged at 4 months (

CONTROL

246.66 +/- 2.46 mm; Galactose: 250.75 +/- 1.26 mm; Inhibitor: 242.25 +/- 8.43 mm). Retinal capillary dilation, expressed as increased width and density, is one of the earliest detectable lesions in galactose-fed rats. In these rats, the lesion occurs as early as retinal capillary basement membrane thickening (RCBMT), one of the earliest reported changes in human diabetic retinopathy. Like RCBMT, capillary dilation can be prevented in rats with aldose reductase inhibitor treatment. Unlike RCBMT, capillary dilation could be clinically detectable and may be useful for the diagnosis of early retinopathy and for determining the timing of therapeutic intervention.

Molecular modeling of the aldose reductase-inhibitor complex based on the X-ray crystal structure and studies with single-site-directed mutants

Aldose reductase (AR) has been implicated in the etiology of the secondary complications of diabetes. This enzyme catalyzes the reduction of glucose to sorbitol using nicotinamide adenine dinucleotide phosphate as an essential cofactor. AR has been localized at the sites of tissue damage, and inhibitors of this enzyme prevent the development of neuropathy, nephropathy, retinopathy, and cataract formation in animal models of diabetes. The crystal structure of AR complexed with zopolrestat, a potent inhibitor of AR, has been described.(1) We have generated a model of the AR-inhibitor complex based on the reported Calpha coordinates of the protein and results of a structure-activity relationship study using four structurally distinct classes of inhibitors, recombinant human AR, and four single-site-directed mutants of this enzyme. The effects of the site-directed mutations on residues within the active site of the enzyme were evaluated by average interaction energy calculations and by calculations of carbon atom surface area changes. These values correlated well with the IC(50) values for zopolrestat with the wild-type and mutant enzymes, validating the model. On the basis of the zopolrestat-binding model, we have proposed binding models for 10 other AR inhibitors. Our models have enabled us to gain a qualitative understanding of the binding domains of the enzyme and how different inhibitors impact the size and shape of the binding site.

Influence of tolrestat on the defective leukocyte-endothelial interaction in experimental diabetes

One of the most devastating secondary complications of diabetes is the blunted inflammatory response that becomes evident even in the very early stages of poorly controlled diabetes mellitus. While the etiology of this diminished response is not clearly understood, it has been linked to a decrease in the respiratory burst of neutrophils, as well as a decrease in microvessel response to inflammatory mediators and defective leukocyte-endothelial interactions. Using video microscopy to visualize vessels of the internal spermatic fascia, we have characterized leukocyte-endothelial interactions in alloxan-induced diabetic and in galactosemic rats by quantitating the number of leukocytes rolling along the venular endothelium and the number of leukocytes sticking to the vascular wall after topical application of zymosan-activated plasma or leukotriene B(4) (1 ng/ml), as well as after the application of a local irritant stimulus (carrageenan, 100 microg). We observed that while 33 days of alloxan-induced diabetes or 7 days of galactosemia had no effect on total or differential leukocyte counts and on the wall shear rate, both treatments significantly (P<0.001) reduced the number of leukocytes rolling along the venular endothelium by about 70% and the number of adhered leukocytes in postcapillary venules by 60%. These effects were not observed in diabetic and galactosemic animals treated with an aldose reductase inhibitor. The results suggest that impaired leukocyte-endothelial cell interactions are a consequence of an enhanced flux through the polyol pathway.

Protein kinase C effects on nerve function, perfusion, Na(+), K(+)-ATPase activity and glutathione content in diabetic rats

AIMS/HYPOTHESIS

Increased protein kinase C activity has been linked to diabetic vascular complications in the retina and kidney, which were attenuated by protein kinase C antagonist treatment. Neuropathy has a vascular component, therefore, the aim was to assess whether treatment with WAY151 003 or chelerythrine, inhibitors of protein kinase C regulatory and catalytic domains respectively, could correct nerve blood flow, conduction velocity, Na(+),K(+)-ATPase, and glutathione deficits in diabetic rats.

METHODS

Diabetes was induced by streptozotocin. Sciatic nerve conduction velocity was measured in vivo and sciatic endoneurial perfusion was monitored by microelectrode polarography and hydrogen clearance. Glutathione content and Na(+),K(+)-ATPase activity were measured in extracts from homogenised sciatic nerves.

RESULTS

After 8 weeks of diabetes, sciatic blood flow was 50 % reduced. Two weeks of WAY151 003 (3 or 100 mg/kg) treatment completely corrected this deficit and chelerythrine dose-dependently improved nerve perfusion. The inhibitors dose-dependently corrected a 20 % diabetic motor conduction deficit, however, at high doses ( > 3.0 mg/kg WAY151003; > 0.1 mg/kg chelerythrine) conduction velocity was reduced towards the diabetic level. Sciatic Na(+),K(+)-ATPase activity, 42 % reduced by diabetes, was partially corrected by low but not high dose WAY151 003. In contrast, only a very high dose of chelerythrine partially restored Na(+),K(+)-ATPase activity. A 30 % diabetic deficit in sciatic glutathione content was unchanged by protein kinase C inhibition. The benefits of WAY151 003 on blood flow and conduction velocity were blocked by nitric oxide synthase inhibitor co-treatment.

CONCLUSION/INTERPRETATION

Protein kinase C contributes to experimental diabetic neuropathy by a neurovascular mechanism rather than through Na(+),K(+)-ATPase defects.

Subnormal retinal oxygenation response precedes diabetic-like retinopathy

PURPOSE

Determining which patients are at risk for the development of diabetic retinopathy is expected to greatly improve existing prevention and treatment options. In this study, using an animal model of diabetic retinopathy, the hypothesis was tested that magnetic resonance imaging (MRI) and a carbogen inhalation challenge provides important diagnostic information regarding the risk of developing diabetic retinopathy.

METHODS

MRI was used to measure noninvasively the change in oxygen tension along the entire inner retina (i.e., from superior ora serrata to inferior ora serrata) during a carbogen (95% O2/5% CO2) inhalation challenge (IOVS 1996;37:2089). Two animal groups were examined by this MRI method at two time points: (1) rats fed either normal rat chow (n = 20) or a 50% galactose diet (n = 20) for 3.5 months (i.e., before the appearance of extensive retinal lesions) or (2) rats fed either normal rat chow (n = 3) for 15 months or a 30% galactose diet (n = 4) for 15 to 18 months (i.e., when lesions are present). Retinal biochemical and morphometric measurements were also obtained.

RESULTS

After 3.5 months of galactosemia, before the appearance of extensive retinal morphologic lesions, a significant (P < 0.05) reduction in the panretinal oxygenation response was observed in the galactosemic group compared with its age-matched control. These galactose-fed animals also displayed a significantly (P < 0.05) larger oxygenation response in the inferior hemiretina than in the superior hemiretina. After 15 to 18 months of galactosemia, during the period when lesions are present, the panretinal oxygenation response remained significantly (P < 0.05) lower in the galactose-fed animals than in their age-matched controls. In contrast to the 3.5-month results, the oxygenation response in galactosemic animals at 15 to 18 months was significantly (P < 0.05) larger in the superior than in the inferior hemiretina. Hemiretinal oxygenation responses were not different in normal controls at either duration.

CONCLUSIONS

MRI measurement of the retinal oxygenation response to a carbogen challenge appears to be a powerful new and noninvasive approach that may be useful for assessing aspects of pathophysiology underlying the development of diabetic retinopathy in galactosemic rats. These results support our working hypothesis and suggest that further research into the diagnostic potential of this MRI approach for predicting the development of diabetic retinopathy is warranted.

Molecular mechanisms of glucose action on angiotensinogen gene expression in rat proximal tubular cells

BACKGROUND

Clinical studies have shown that the angiotensin-converting enzyme (ACE) inhibitors or angiotensin II (Ang II) receptor antagonists decrease proteinuria and slow the progression of nephropathy in diabetes, indicating that Ang II plays an important role in the development of nephropathy. We have previously reported that high levels of glucose stimulate the expression of rat angiotensinogen (ANG) gene in opossum kidney (OK) proximal tubular cells. We hypothesized that the stimulatory effect of D(+)-glucose on the expression of the ANG gene in kidney proximal tubular cells is mediated via de novo synthesis of diacylglycerol (DAG) and the protein kinase C (PKC) signal transduction pathway.

METHODS

Immortalized rat proximal tubular cells (IRPTCs) were cultured in monolayer. The stimulatory effect of glucose on the activation of polyol pathway and PKC signal transduction pathway in IRPTCs was determined. The immunoreactive rat ANG (IR-rANG) in the culture medium and the cellular ANG mRNA were measured with a specific radioimmunoassay and a reverse transcription-polymerase chain reaction assay, respectively.

RESULTS

D(+)-glucose (25 mM) markedly increased the intracellular levels of sorbitol, fructose, DAG, and PKC activity as well as the expression of IR-rANG and ANG mRNA in IRPTCs. These stimulatory effects of D(+)-glucose (25 mM) were blocked by an inhibitor of aldose reductase, Tolrestat. PKC inhibitors also inhibited the stimulatory effect of D(+)-glucose (25 mM) on the expression of the IR-rANG in IRPTCs. The addition of phorbol 12-myristate 13-acetate further enhanced the stimulatory effect of D(+)-glucose (25 mM) on the expression of the IR-rANG in IRPTCs and blocked the inhibitory effect of Tolrestat.

CONCLUSION

These studies suggest that the stimulatory effect of a high level of D(+)-glucose (25 mM) on the expression of the ANG gene in IRPTCs is mediated, at least in part, via the de novo synthesis of DAG, an activator of PKC signal transduction pathway.

Diabetic-like retinopathy: early and late intervention therapies in galactose-fed rats

PURPOSE

To determine whether the diabetic-like thickening of retinal capillary basement membrane (RCBM) that develops in the galactose-fed rat model of diabetic ocular complications could be halted or ameliorated after 4 or 8 months of galactosemia by treatment with ARI-509, a potent new aldose reductase inhibitor (ARI), or by withdrawal of the galactose diet.

METHODS

Weanling female Sprague-Dawley rats were randomized into eight groups and fed laboratory chow plus 50% starch, control group (CON); 50% D-galactose, galactose-fed group (GAL); 50% D-galactose with ARI-509 at 25 mg/kg or 10 mg/kg body wt per day, high-dose prevention group (HDP) and low-dose prevention group (LDP), respectively; 50% D-galactose for 4 or 8 months and then intervention by addition of ARI-509 (25 mg/kg body wt per day), 4-month intervention group (4IN) and 8-month intervention group (8IN), respectively; or 50% D-galactose for 4 or 8 months and then intervention by withdrawing galactose and replacing it with the 50% starch diet, 4-month galactose withdrawal group (4GW) and 8-month galactose withdrawal group (8GW), respectively. After 4, 8, 16, and 24 months of the experimental diets, the levels of carbohydrates in tissues and the extent of RCBM thickening in capillaries of the outer plexiform layer were determined in all groups.

RESULTS

Retinal polyol was reduced by 95% in all ARI-treated groups and by 100% in the 4GW and 8GW groups after withdrawal of the galactose. The mean RCBM thickness increased rapidly in GAL rats, becoming almost two times greater (189 +/- 9.4 nm) than in CON rats (103 +/- 3.4 nm) by 24 months. Treatment with ARI-509 in high and low doses (HDP, LDP) initiated with the introduction of the galactose diet significantly prevented RCBM thickening at all time points (P < 0.05). In contrast, intervention by withdrawing galactose from the diet or by adding the high dose of ARI-509 had no significant effect (P < 0.05) on RCBM thickening until the 24-month time point (4IN, 166 +/- 10.3 nm; 8IN, 161 +/- 8.2 nm; 4GW, 136 +/- 5.1 nm; 8GW, 163 +/- 9.6 nm).

CONCLUSIONS

Both early and late interventions decreased RCBM thickening compared with that in untreated GAL rats. The decreased thickening, however, was not evident until 16 to 20 months after the intervention. Because RCBM thickening is one of the earliest changes in diabetic and galactosemic retinopathy, the findings suggest that RCBM thickening and possibly subsequent retinal lesions are caused by early biochemical alterations induced by the galactose diet that are not readily reversed. The delayed response to therapy is consistent with that observed in the Diabetes Control and Complications Trial. The cumulative evidence indicates that intervention should begin as early after onset of diabetes as possible, and long follow-up periods should be used to evaluate efficacy.

Probing the inhibitor-binding site of aldose reductase with site-directed mutagenesis

Aldose reductase (AR) has been implicated in the etiology of the secondary complications of diabetes, and enzyme inhibitors have been proposed as therapeutic agents. While effectively preventing the development of diabetic complications in animals, results from clinical studies of AR inhibitors have been disappointing, possibly due to poor potency in man. To assist in the design of more potent and specific inhibitors, crystallographic studies have attempted to identify enzyme-inhibitor interactions. Resolution of crystal complexes has suggested that the inhibitors bind to the enzyme active site and are held in place through hydrogen bonding and van der Waals interactions formed within two hydrophobic pockets. To confirm and extend these findings we quantified inhibitor activity with single, site-directed, mutant, human AR enzymes in which the apolar active-site residues tryptophan 20, -79, -111 and phenylalanine 115 were replaced with alanine or tyrosine, decreasing the potential for van der Waals interactions. Consistent with molecular models, the inhibitory activity of Tolrestat, Sorbinil and Zopolrestat decreased 800-2000-fold when tested with the mutant enzyme in which Trp20 was replaced with alanine. Further, alanine substitution for Trp111 decreased Zopolrestat's activity 400-fold, while mutations to Trp79 and Phe115 had little effect on the activity of any of the inhibitors. The alanine mutation at Trp111 had no effect on Tolrestat's activity but decreased the activity of Sorbinil by about 1000-fold. These latter effects were unanticipated based on the number of non-bonded interactions between the inhibitors, Tolrestat and Sorbinil, and Trp20 and Trp111 that have been identified in the crystal structures. In spite of these unexpected findings, our results are consistent with the hypothesis that AR inhibitors occupy the enzyme active site and that hydrophobic interactions between the enzyme and inhibitor contribute to inhibitor binding stability.

Effect of the aldose reductase inhibitor tolrestat on nerve conduction velocity, Na/K ATPase activity, and polyols in red blood cells, sciatic nerve, kidney cortex, and kidney medulla of diabetic rats

Long-term prospective studies comparing the effects of conventional and intensive insulin therapy have linked diabetic hyperglycemia to the development of diabetic retinopathy, nephropathy, and neuropathy. The mechanisms through which glucose metabolism leads to the development of these secondary complications, however, are incompletely understood. In animal models of diabetic neuropathy, the loss of nerve function in myelinated nerve fibers has been related to a series of biochemical changes. Nerve glucose, which is in equilibrium with plasma glucose levels, rapidly increases during diabetic hyperglycemia because glucose entry is independent of insulin. This excess glucose is metabolized in large part by the polyol pathway. Increased flux through this pathway is accompanied by the depletion of myo-inositol, a loss of Na/K ATPase activity and the accumulation of sodium. Supportive evidence linking these biochemical changes to the loss of nerve function has come from studies in which aldose reductase inhibitors block polyol pathway activity, prevent the depletion of myo-inositol and the accumulation of sodium and preserve Na/K ATPase activity, as well as nerve function. The kidney and red blood cells (RBCs) are two additional sites of diabetic lesions that have been reported to develop biochemical changes similar to those in the nerve. We observed that polyol levels in the kidney cortex, medulla, and RBCs increased two- to ninefold in rats following 10 weeks of untreated diabetes. Polyol accumulation was accompanied by a 30% decrease in myo-inositol levels in the kidney cortex, but no change in RBCs or the kidney medulla. Na/K ATPase activity was decreased by 59% in RBCs but was unaffected in the kidney cortex or medulla. Aldose reductase inhibitor treatment that preserved myo-inositol levels, Na/K ATPase, and conduction velocity in the sciatic nerve also preserved Na/K ATPase activity in RBCs. Our results suggest that the pathophysiologic mechanisms underlying diabetic neuropathy are different from those of diabetic nephropathy. Our results also suggest that RBCs maybe a surrogate tissue for the assessment of diabetes-induced changes in nerve Na/K ATPase activity.

An aldose reductase inhibitor and aminoguanidine prevent vascular endothelial growth factor expression in rats with long-term galactosemia

OBJECTIVE

To study the effects of an aldose reductase inhibitor (ARI-509, Wyeth-Ayerst, Princeton, NJ) and aminoguanidine (AMG), agents that have been reported to prevent or delay diabetic retinopathy, on retinal vascular abnormalities and the immunocytochemical expression in the retina of vascular endothelial growth factor (VEGF) in rats maintained for up to 2 years on a 50% galactose diet.

METHODS

Albino rats were placed on a control diet, a diet containing 50% galactose, or the 50% galactose diet containing either ARI-509 or AMG. Treatment with ARI-509 or AMG was initiated at the beginning of the experiment or after 12 months of galactose feeding. After 22 to 24 months, the rats were killed and the retinal vasculature from half of one eye was isolated by trypsin-elastase digestion for semiquantitative evaluation of retinal vascular lesions. The other half of the retina was prepared for immunocytochemistry and stained for the presence of VEGF, factor VIII, vimentin, and glial fibrillary acidic protein. Red blood cells, sciatic nerves, and a portion of the retina from the second eye were assayed for glucose, galactose, fructose, sorbitol, galactitol, and myo-inositol. Red blood cells were also assayed for galactosylated hemoglobin.

RESULTS

Galactose-fed animals developed a vascular retinopathy characterized by severe cellular loss in the retinal capillaries and intensification of periodic acid-Schiff staining of the vascular basement membranes. Some animals also displayed dilation and hypercellularity of vessels in the posterior retina. These changes were substantially reduced in animals receiving ARI-509 from the beginning of the galactose diet, but were unaffected in all of the other treatment groups. None of the rats receiving ARI-509 or AMG treatment, whether initiated from the onset or after 12 months of galactosemia, demonstrated VEGF immunoreactivity. With the exception of the animals receiving ARI-509 from the beginning of the experiment, all of the galactose-fed animals developed dense cataracts within 6 weeks of the beginning of the galactose diet. Galactitol levels in animals receiving ARI-509 were 86% to 93% lower in red blood cells, retina, and sciatic nerve than those in the other galactose-fed groups.

CONCLUSIONS

Although ARI-509 and AMG have different abilities to delay or prevent the diabetic-like retinopathy in galactosemic rats, even when substantial retinal microvascular acellularity occurs, both drugs prevent the immunocytochemical expression of VEGF. These results suggest that factors other than hypoxia may be responsible for VEGF expression in the retina, and that aldose reductase inhibitors and AMG have potential roles in preventing such expression and, thus, perhaps preventing retinal neovascularization.

Comparison of the effects of inhibitors of aldose reductase and sorbitol dehydrogenase on neurovascular function, nerve conduction and tissue polyol pathway metabolites in streptozotocin-diabetic rats

Aldose reductase inhibitors (ARIs) attenuate diabetic complications in several tissues, including lens, retina, kidney, blood vessels, striated muscle and peripheral nerve. However, it is unclear whether their action in diabetes mellitus depends directly on inhibiting the conversion of glucose to sorbitol by aldose reductase or indirectly by reducing the sorbitol available for subsequent metabolism to fructose by sorbitol dehydrogenase. To identify the polyol pathway step most relevant to complications, particularly neuropathy, we compared the biochemical effects of a sorbitol dehydrogenase inhibitor, WAY-135706, (250 mg.kg-1.day-1) and an ARI, WAY-121509, (10 mg.kg-1.day-1) on a variety of tissues, and their effects on nerve perfusion and conduction velocity. After 6 weeks of untreated streptozotocin diabetes, rats were treated for 2 weeks. Sorbitol was elevated 2.1-32.6-fold by diabetes in lens, retina, kidney, aorta, diaphragm, erythrocytes and sciatic nerve; this was further increased (1.6-8.2-fold) by WAY-135706 whereas WAY-121509 caused a marked reduction. Fructose 1.6-8.0-fold elevated by diabetes in tissues other than diaphragm, was reduced by WAY-135706 and WAY-121509, except in the kidney. Motor and sensory nerve conduction velocities were decreased by 20.2 and 13.9%, respectively with diabetes. These deficits were corrected by WAY-121509, but WAY-135706 was completely ineffective. A 48.6% diabetes-induced deficit in sciatic nutritive endoneurial blood flow was corrected by WAY-121509, but was unaltered by WAY-135706. Thus, despite profound sorbitol dehydrogenase inhibition, WAY-135706 had no beneficial effect on nerve function. The data demonstrate that aldose reductase activity, the first step in the polyol pathway, makes a markedly greater contribution to the aetiology of diabetic neurovascular and neurological dysfunction than does the second step involving sorbitol dehydrogenase.

The efficacy of tolrestat in the treatment of diabetic peripheral neuropathy. A meta-analysis of individual patient data

OBJECTIVE

The aim of this meta-analysis was to review the existent evidence on the effectiveness of tolrestat in the treatment of diabetic peripheral neuropathy.

RESEARCH DESIGN AND METHODS

Individual patient data on 738 subjects from the three randomized clinical trials published on this topic were analyzed using changes in motor nerve conduction velocities (NCVs) as endpoints. Nerves investigated included median, ulnar, tibial, and peroneal.

RESULTS

The pooled analysis of NCV taken as a continuous measurement showed a significant treatment effect, the magnitude of this benefit being approximately equal to 1 m/s for all the nerves investigated. When looking at the proportion of patients experiencing a loss of NCV of at least 1 or 2 m/s in at least two out of the four nerves investigated, it emerged that treatment reduced by > 40% the risk of such outcomes after adjusting for patients' characteristics. The odds ratios relative to the placebo group were 1.82 (1.30-2.52) and 1.70 (1.15-2.48) for a decrease of 1 and 2 m/s, that is, placebo-treated patients have an 82 and 70% increased risk for a loss of nerve function of 1 and 2 m/s, respectively. No statistically significant difference in treatment effect emerged after stratification according to baseline motor NCV and glycated hemoglobin levels.

CONCLUSIONS

After a treatment duration ranging between 24-52 weeks, patients treated with tolrestat had a reduced risk for developing nerve function loss compared with placebo-treated patients. Future long-term trials are needed to evaluate the impact of the treatment on more clinically meaningful endpoints such as the development of foot complications.

Impairment of afferent arteriolar myogenic responsiveness in the galactose-fed rat is prevented by tolrestat

By permitting the separation of increased aldose reductase activity from hyperglycaemia and insulin deficiency, galactose-fed rats have constituted a useful model for investigating diabetic complications. Such rats manifest an impaired afferent arteriolar responsiveness to pressure similar to that of rats 4 to 6 weeks after induction of diabetes with streptozotocin. In the present study, we investigated whether treatment of galactose-fed rats with the aldose reductase inhibitor tolrestat prevent this autoregulatory defect and whether the blunted afferent arteriolar responsiveness to pressure is associated with impaired responsiveness to angiotensin II. Pressure-induced vasoconstriction of afferent arterioles was assessed in kidneys made hydronephrotic to allow direct visualization of renal microvessels by computer-assisted image processing. Vessel diameters were quantitated following stepwise increments of renal perfusion pressure (RAP; from 80 to 180 mm Hg) in kidneys of control rats and rats fed a diet for 2 weeks with 50% galactose with or without tolrestat. Subsequent to the pressure studies, angiotensin II (0.3 nmol/l) was added to the perfusate, and vessel diameters were reassessed. Control rats exhibited progressive afferent arteriolar vasoconstriction when RAP was increased from 80 to 180 mm Hg (-17.2 +/- 1.0%; p < 0.001). In contrast, myogenic responses to increases in pressure were absent in the arterioles of the galactose-fed rats (-4.1 +/- 1.9%; N.S.). Treatment with tolrestat completely prevented this impairment in afferent arteriolar responsiveness (-16.5 +/- 1.8%; p < 0.001). The angiotensin II-induced vasoconstriction did not differ between control rats and galactose-fed rats. We conclude that increased aldose reductase activity contributes to impaired renal auto-regulation in galactose-fed rats, a model of diabetic nephropathy, but is not involved in the loss of afferent arteriolar responsiveness to angiotensin II.

Diabetic-like retinopathy ameliorated with the aldose reductase inhibitor WAY-121,509

PURPOSE

To evaluate the efficacy of WAY-121,509, a potent new aldose reductase inhibitor (ARI), in preventing the retinopathy that develops in the galactose-fed rat model of diabetic ocular complications.

METHODS

Sprague-Dawley rats were randomized into treatment and duration groups and fed diets with either 50% starch of 50% galactose with or without WAY-121,509 (25 mg/kg body weight per day). Progression of cataracts was monitored by slit-lamp biomicroscopy. After duration of 4, 8, 16, and 24 months, levels of plasma glucose and glycated hemoglobin, as well as erythrocyte and retinal galactose and galactitol, were measured in rats in each group. Retinal vasculatures of the 24-month rats were isolated by elastase digestion and analyzed by computer-assisted morphometry.

RESULTS

Mature, diabetic-like cataracts developed within 5 weeks in all the galactose-fed, untreated rats, but only nonprogressive anterior cortical opacities were present in lenses of 85% of the ARI-treated galactosemic animals after 3 months. Plasma glucose remained the same in all groups. Erythrocyte and retinal galactose and glycated (galactosylated) hemoglobin were elevated with galactosemia and were unaffected by ARI treatment. Erythrocyte and retinal galactitol levels were decreased by 91% and 95%, respectively, with inhibitor treatment. At 24 months, capillary length, width, density, the number of microaneurysms, and the percent of capillary length involved in intraretinal microvascular abnormalities, expressed as hypercellular channels with diameters > 20 microns, were significantly increased by galactosemia and were attenuated in the galactose-fed, ARI-treated group.

CONCLUSIONS

A dose of WAY-121,509 sufficient to reduce retinal polyol levels by 95% ameliorated the development of galactose-induced cataracts and diabetic-like retinopathy but was insufficient to prevent early lens opacifications or all the diabetic-like retinal microangiopathies.

Interactions between essential fatty acid, prostanoid, polyol pathway and nitric oxide mechanisms in the neurovascular deficit of diabetic rats

Impaired omega-6 essential fatty acid metabolism and exaggerated polyol pathway flux contribute to the neurovascular abnormalities in streptozotocin-diabetic rats. The potential interactions between these mechanisms were examined by comparing the effects of threshold doses of aldose reductase inhibitors and evening primrose oil, alone and in combination, on neurovascular deficits. In addition, high-dose aldose reductase inhibitor and evening primrose oil treatment effects were challenged by co-treatment with the cyclo-oxygenase inhibitor, flurbiprofen, or the nitric oxide synthase inhibitor, NG-nitro-L-arginine. Eight weeks of diabetes caused an 18.9% reduction in sciatic motor conduction velocity (p < 0.001). This was only modestly ameliorated by a 0.1% dietary supplement of evening primrose oil or the aldose reductase inhibitors ZD5522 (0.25 mg.kg-1.day-1 and WAY121 509 (0.2 mg.kg-1.day-1 for the final 2 weeks. However, joint treatment with primrose oil and ZD5522 or WAY121 509 caused marked 71.5 and 82.4% corrections, respectively, of the conduction deficit. Sciatic nutritive blood flow was 43.1% reduced by diabetes (p < 0.001) and this was corrected by 67.8% with joint ZD5522 and primrose oil treatment (p < 0.001). High-dose WAY121 509 (10 mg. kg-1.day-1 and primrose oil (10% dietary supplement) prevented sciatic conduction velocity and nutritive blood flow deficits in 1-month diabetic rats (p < 0.001). However, these effects were abolished by flurbiprofen (5 mg.kg(-1).day-1 and NG-nitro-L-arginine (10 mg.kg-1.day-1) co-treatment (p < 0.001). Thus, the data provide evidence for synergistic interactions between polyol pathway/nitric oxide and essential fatty acid/cyclo-oxygenase systems in the control of neurovascular function in diabetic rats, from which a potential therapeutic advantage could be derived.

Reversal of defective peripheral nerve conduction velocity, nutritive endoneurial blood flow, and oxygenation by a novel aldose reductase inhibitor, WAY-121,509, in streptozotocin-induced diabetic rats

The main aim was to investigate whether 1 month of aldose reductase inhibitor treatment could correct a deficit in sciatic nerve nutritive blood flow following 1 month of untreated streptozotocin-induced diabetes in rats. Treatment was with two doses of WAY-121,509, both of which completely blocked neuronal sorbitol accumulation. The high dose fully corrected a motor conduction velocity deficit, whereas the low dose caused 51.3% amelioration. Nutritive endoneurial blood flow, monitored by hydrogen clearance, was 43.4% reduced after 1 month of diabetes. This was completely corrected by the high dose of WAY-121,509. In addition, vascular conductance was supranormal and there was a decrease in arteriovenous shunt flow. Low dose treatment caused a 55.6% improvement of the nutritive endoneurial blood flow deficit, paralleling the conduction velocity effect. WAY-121,509 did not alter nerve perfusion in nondiabetic rats. Data from multiple sciatic nerve penetrations by oxygen sensitive microelectrodes revealed a 42.0% deficit in mean endoneurial oxygen tension with diabetes, whereas tensions were in the nondiabetic range for high dose WAY-121,509 treatment. Thus, the data highlight neurovascular actions of aldose reductase inhibition, and suggest that neuronal polyol pathway metabolite levels are a poor predictor of functional efficacy.

Aminoguanidine does not inhibit aldose reductase activity in galactose-fed rats

Aminoguanidine, nucleophilic hydrazine derivative, has been shown to inhibit diamine oxidase, the formation of advanced glycation endproducts, nitric oxide synthase, and catalase. Prompted by the reports that aminoguanidine also inhibits aldose reductase (AR), we have investigated the effect of aminoguanidine, 1,3-diaminoguanidine, and methylguanidine on AR activity in vitro, and in vivo. In vitro, we have measured the inhibition of AR isolated from bovine lenses; in vivo, we have examined the effect on the galactitol levels in the red blood cells, sciatic nerve, retina, and lens of rats administered the test compounds for 11 days in the drinking water and, for the last 4 days, given access to a 20% galactose diet. Two known, structurally distinct AR inhibitors, tolrestat and compound WAY-121,509, were used as reference. In vitro, at concentrations up to 1.0 mmol/L, none of the tested guanidine derivatives had any effect on AR. As a corollary, in vivo, at doses ranging from 201 to 349 mg/kg/day, none of the guanidine derivatives affected tissular galactitol levels. We conclude that, in short-term galactose-fed rats, at the doses tested, aminoguanidine, 1,3-diaminoguanidine, and methylguanidine do not inhibit AR.

Residues affecting the catalysis and inhibition of rat lens aldose reductase

Aldose reductase (AR), the first enzyme of the polyol pathway, has been implicated in diabetic complications. Results of recent clinical studies have shown that compounds that inhibit aldose reductase (ARIs) and block the flux of glucose through the polyol pathway have provided benefit to diabetic neuropathic patients. Since many ARIs show broad substrate specificity, emphasis on the structure-function properties of the AR enzyme will help in the refinement and design of future inhibitors. To this end, catalysis and inhibition of rat lens aldose reductase was examined following site-directed mutagenesis. Replacement of tyrosine 48 with phenylalanine (Y48F) resulted in an enzyme form with less than 0.25% activity with DL-glyceraldehyde and no detectable activity with p-nitrobenzaldehyde or xylose, although circular dichroism spectra and NADPH binding affinity were similar to wild-type AR. Mutation of histidine 110 to glutamine (H110Q) also resulted in a less active protein with an approximate 3-fold decrease in kcat for the reduction of DL-glyceraldehyde; slight or no activity was measured with other substrates and an increase of 195-fold over wild type was observed in the Km for glyceraldehyde. H110Q was less sensitive to inhibition by aldose reductase inhibitors. The most dramatic change was seen with imeristat, which showed an 1800-fold increase in IC50. Mutation of cysteine 298 to serine (C298S) affected enzyme function by increasing kcat 2- to 4-fold and increasing Km 15- to 48-fold, with DL-glyceraldehyde, p-nitrobenzaldehyde or xylose as substrates. As a result kcat/Km, catalytic efficiency, dropped to approx. 10% of control. Inhibition of C298S was not noticeably different from wild type. Substitution of histidine 187 or 200 with glutamine (H187Q, H200Q) had little effect on AR catalysis or inhibition. Based on structural and mutagenesis studies of human AR and the conservation of amino acids between human and rat, these data would indicate that Y48, H110, and C298 are important residues in the active site of rat AR and that Y48 is most likely the proton donor during substrate reduction by rat lens aldose reductase. In addition, these studies indicate that mutagenesis of H110 also affects aldose reductase inhibition.

Na,K-ATPase response to osmotic stress in primary dog lens epithelial cells

PURPOSE

Na,K-ATPase activity increases in lens cells exposed to hypertonic stress. To test whether the increase in activity involves stimulation of Na,K-ATPase expression, dog lens epithelial cells were subjected to hypertonic stress, and the time course of Na,K-ATPase protein and mRNA response was measured.

METHODS

Primary cultures of dog lens epithelial cells were maintained in isotonic or hypertonic media over the course of several days. Rubidium-86 uptake measurements, immunoreactive protein, and northern blot analysis were performed.

RESULTS

Dog lens epithelial cells exposed to hypertonic stress from culture medium supplemented with 150 mM NaCl or 250 mM cellobiose showed a twofold increase in Na,K-ATPase activity. The increase in activity was blocked by cycloheximide and was reversible when the cells were returned to isotonic medium. This activity was unaffected by the aldose reductase inhibitor, tolrestat. Na,K-ATPase protein and mRNA levels increased in cells exposed to medium containing 150 mM NaCl. Northern blot analysis showed that the alpha-1 and beta-1 mRNA levels increased as early as 6 hours and maximally increased 1.5-fold to twofold by 12 to 24 hours.

CONCLUSIONS

Elevation of Na,K-ATPase activity in dog lens epithelial cells exposed to hypertonic stress was associated with increased expression of Na,K-ATPase subunit mRNAs and was dependent on protein synthesis. These results suggest that upregulation of the enzyme activity is the result of an induction of Na,K-ATPase.

Impairment of afferent arteriolar myogenic responsiveness in the galactose-fed rat

Previous studies from our laboratory have demonstrated impaired afferent arteriolar responsiveness to pressure in rats 4-6 weeks after the induction of diabetes mellitus. Although the responsible mechanisms mediating this renal autoregulatory defect have not been fully defined, increased polyol metabolism has been implicated as a possible factor involved in the pathogenesis of diabetic complications. We therefore investigated the possible role of this metabolic disturbance in renal autoregulation using the galactose-fed rat, a model characterized by increased polyol pathway activity independent of hyperglycemia or insulin deficiency. Hydronephrosis was induced to permit direct visualization of renal microvessels. Pressure-induced vasoconstriction of afferent arterioles was assessed by quantitating vessel diameter following stepwise increments of renal perfusion pressure (RAP; from 80 to 180 mm Hg) in the hydronephrotic kidneys from control rats and rats fed a 50% galactose diet for 2 or 4 weeks. Vessel diameters were measured from video images by computer-assisted image processing. Control rats exhibited progressive afferent arteriolar vasoconstriction when RAP was increased from 80 to 180 mm Hg (-17.3% +/- 1.0%; P < 0.001). In contrast, myogenic responses to increases in pressure were absent in the afferent arterioles of rats fed a 50% galactose diet for either 2 (-4.1% +/- 1.9%; not significant) or 4 weeks (-2.9 +/- 3.4%; not significant). Our demonstration that the impairment of afferent arteriolar responsiveness to increasing RAP in the normoglycemic galactose-fed rat was identical to that observed in the STZ-diabetic rat suggests that increased polyol accumulation may contribute to the impairment of renal autoregulation in the diabetic rat.

Galactosemia produces ARI-preventable nodal changes similar to those of diabetic neuropathy

The present study was designed to examine the development of structural changes, characteristic of diabetic neuropathy, in chronic galactosemia and their responsiveness to inhibition of the polyol-pathway. Sprague-Dawley rats weighing 70-90 g were given a 50% galactose diet continued for 4 or 8 months. Half of these animals were simultaneously given the aldose reductase inhibitor (ARI) WAY 121-509. ARI-treatment normalized galactitol and myoinositol levels in the sciatic nerve. At 4 months, sciatic nerve conduction velocity (NCV) in galactosemic rats was reduced by 30% which was prevented in ARI-treated rats. At 8 months galactosemia reduced NCV to 58% of control values, while ARI-treatment for 8 months improved NCV to 71% of control values. ARI-treatment prevented in galactosemic rats nodal structural changes characteristic of diabetic neuropathy, whereas axonal atrophy was not affected by ARI-treatment, which may in part account for the only partial prevention of the NCV slowing at 8 months. Nerve fiber regeneration was increased 4-fold in ARI-treated rats compared with untreated galactosemic rats. These data suggest that chronic galactosemia produces a neuropathy structurally similar to diabetic neuropathy. The lack of an ARI-treatment effect on axonal atrophy suggests that this defect is not polyol related in galactosemia.

Novel spirosuccinimide aldose reductase inhibitors derived from isoquinoline-1,3-diones: 2-[(4-bromo-2-fluorophenyl)methyl]-6- fluorospiro[isoquinoline-4(1H),3'-pyrrolidine]-1,2',3,5'(2H)-tetrone and congeners. 1

The high concentrations of plasma glucose formed during diabetic hyperglycemia rapidly translate into high levels of glucose in tissues where glucose uptake is independent of insulin. In these tissues that include the lens, retina, nerve, and kidney, this excess glucose enters the sorbitol (polyol) pathway. The first enzyme in this pathway, aldose reductase, reduces glucose to sorbitol. The diabetes-induced increased flux of glucose through the polyol pathway is believed to play an important role in the development of certain chronic complications of diabetes mellitus. Compounds that inhibit aldose reductase activity and block the flux of glucose through the polyol pathway prevent the development of neuropathy and nephropathy in diabetic animals and interrupt the progression of neuropathy in diabetic patients. Here we describe the preparation and characterization of novel aldose reductase inhibitors. These spiro[isoquinoline-4(1H),3'-pyrrolidine]-1,2',3,5'-(2H)-tetrones, based on the isoquinoline-1,3-dione framework, were evaluated in vitro for their ability to inhibit glyceraldehyde reduction, using a partially purified bovine lens aldose reductase preparation, and in vivo for their ability to inhibit galactitol accumulation in the lens and sciatic nerve of galactose-fed rats. Substitution at the N-2 position of the isoquinoline-1,3-dione framework with diverse structural substituents (i.e., aralkyl, benzothiazolylmethyl, methyl) produced several excellent series of ARIs. Optimization of these new series of spirosuccinimides through structure-activity relationship (SAR) studies, including analogy from other drug series (ponalrestat, zopolrestat), led to the design of the clinical candidate 2-[(4-bromo-2-fluorophenyl)methyl]-6-fluorospiro[isoquinoline-4(1H ),3'- pyrrolidine]-1,2',3,5'(2H)-tetrone (41). Compound 41 exhibited exceptional oral potency in two animal models of diabetic complications, the 14-day galactose-fed and streptozocin-induced diabetic rats, with ED50 values for the sciatic nerve of 0.1 and 0.09 mg/kg/day, respectively. Both enantiomeric forms of 41 exhibited similar inhibitory activity in both in vitro and in vivo assays possibly due to their rapid interconversion. In an ex vivo experiment, the pharmacodynamic effect of 41 in the plasma of rats and dogs, after a single dose, appeared to be comparable to that of tolrestat.

N-substituted spirosuccinimide, spiropyridazine, spiroazetidine, and acetic acid aldose reductase inhibitors derived from isoquinoline-1,3-diones. 2

The isoquinoline-1,3-dione framework featured in our clinical candidate (1) and its congener was used as the template in the design of several new series of aldose reductase inhibitors (ARIs). These series included N'-substituted spirosuccinimide, spiropyridazine, spiroazetidine, and acetic acid analogues. Compounds within these series were evaluated in vitro for their ability to inhibit glyceraldehyde reduction by bovine lens aldose reductase and in vivo by their ability to inhibit galactitol accumulation in the lens and sciatic nerve of galactose-fed rats. The N'-amino- and N'-alkyl-substituted spiro[isoquinoline-4(1H),3'-pyrrolidine]-1,2',3,5'(2H)- tetrones 6 exhibited high oral potency, even though they were devoid of any intrinsic activity for the aldose reductase enzyme. Similar results were observed for the closely related spiropyridazines 8. Both of these groups are also considered to be prodrugs since they exhibited good oral potency, even though they were devoid of any intrinsic activity for the aldose reductase enzyme. In contrast, the isoquinoline-1,3-dione acetic acids 9 exhibited very high intrinsic activity for the aldose reductase enzyme, although minimal or no in vivo activity. The absence of in vivo activity for some of these compounds may be due to poor tissue penetration. In support of this suggestion, the more lipophilic acetyl alkyl carbamate derivatives of these isoquinoline-1,3-dione acetic acids, exhibited enhanced oral potency. The spiroazetidines 7 exhibited good activity for the aldoe reductase enzyme in both the in vitro and in vivo assays. The findings of this study demonstrate the utility of the isoquinoline-1,3-dione framework, as a versatile template for the design of divese series of potent ARIs.

Hydrolase compartmentalization limits rate of digestion in Acanthamoeba

The kinetics of lysosomal enzyme acquisition by newly formed phagosomes was studied by following the rate of digestion of radiolabeled yeast fed to Acanthamoeba. The distribution of hydrolases among phagosomes was assessed by electron microscopic acid phosphatase cytochemistry and by measurement of three glycosidases in isolated early and late phagosomes. The results show that compartmentalization of hydrolases limit the digestion of large phagocytic loads. The hydrolases appear to be sequestered into the early phagosomes and not to be distributed either by small vesicle transport or phagosome-phagosome fusion to those formed later. We infer from these results that newly internalized surface membrane in phagosomes is not rapidly randomized with internal pools, but is recycled to the surface as a function of the digestive process.

Effect of hyperglycemia and the aldose reductase inhibitor tolrestat on sural nerve biochemistry and morphometry in advanced diabetic peripheral polyneuropathy. The Tolrestat Study Group

Tolrestat is a well tolerated nonhydantoin aldose reductase inhibitor that has been reported to improve nerve conduction in diabetic animals and humans. Its effects on nerve biochemistry and structure have not been studied in patients with diabetic neuropathy. Patients with advanced diabetic neuropathy treated with long-term open-label tolrestat were randomly assigned to continuation on drug treatment or to placebo-controlled drug withdrawal for 12 months. At the end of this period, sural nerve biopsies were obtained for measurement of glucose, sorbitol, and fructose content, and for detailed morphometric analysis. Tolrestat ameliorated the glucose-mediated increase in sorbitol and fructose in sural nerve tissue. No statistically significant differences in nerve morphometry emerged between the two groups; however, both treatment groups exhibited increased nerve-fiber regeneration and normalization of axo-glial dysfunction and segmental demyelination following long-term tolrestat treatment. These findings are similar to those previously reported in a placebo-controlled sequential nerve biopsy study with the aldose reductase inhibitor sorbinil. Thus tolrestat is a biochemically effective aldose reductase inhibitor in human diabetic nerve with potential therapeutic efficacy for diabetic neuropathy.

Novel spirosuccinimides with incorporated isoindolone and benzisothiazole 1,1-dioxide moieties as aldose reductase inhibitors and antihyperglycemic agents

Compounds from two novel series of spirosuccinimides were prepared. Analogs of series 2 possessed a spiro-fused isoindolone moiety while those of series 3 contained a spiro-fused benzisothiazole S,S-dioxide group. These compounds were evaluated as aldose reductase inhibitors (ARI) in vitro by their ability to inhibit glyceraldehyde reduction using a partially purified bovine lens aldose reductase preparation and in vivo as inhibitors of galactitol accumulation in the lens, sciatic nerve, and diaphragm of galactose-fed rats. Many members from the isoindolone series 2, particularly those containing an isoindolone N-methyl moiety, showed good in vitro and in vivo potency. The most potent member, the 6-chloro analog 32, was resolved, and aldose reductase activity was found to reside almost exclusively in the (+)-enantiomer. Compound 32 was approximately equipotent in the sciatic nerve of the galactose-fed rat to other cyclic imide ARI's of similar in vitro activity, namely sorbinil and ADN-138 and also to tolrestat, an acetic acid-based ARI (ED50's 4-8 mg/kg). Compounds from both series, 2 and 3, were also found to lower plasma glucose levels of genetically obese db/db and ob/ob mice with potency similar to that of ciglitazone. However, members from these series failed to lower insulin levels of the ob/ob mouse at the doses tested.

Hypertonic stress induces alpha B-crystallin expression

Alpha B-crystallin, a major lens protein, was induced in primary cultures of dog lens epithelial cells and glomerular endothelial cells when they were grown under conditions of hypertonic stress. With Western blot analysis using a specific alpha B-crystallin antibody, we observed a significant increase in the concentration of alpha B-crystallin protein in cells grown for 4-6 days in media supplemented with 150 mM NaCl or 250 mM cellobiose. These supplements increased the osmolarity of the medium from 300 to 550-600 mosmol kg-1. Alpha B-crystallin mRNA was also increased reaching a maximum four-fold increase in lens and 16-fold increase in kidney cells within 1-2 days. These studies demonstrate a type of regulation of alpha B-crystallin expression in cells from lenticular and non-lenticular tissues.

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

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

Induction of aldose reductase expression in rat kidney mesangial cells and Chinese hamster ovary cells under hypertonic conditions

Rat kidney cortex mesangial cells (MES) and Chinese hamster ovary cells (CHO) responded to hypertonicity (600 mosmol/kg) in culture by accumulating sorbitol. The accumulation of sorbitol was due to increased aldose reductase (AR) activity, apparently brought about by increased levels of AR mRNA and protein. The levels of AR mRNA increased approximately 60-fold in MES cells and 30-fold in CHO cells by 24 h in culture media (300 mosmol/kg supplemented with 150 mM NaCl, 600 mosmol/kg total). AR activity also markedly increased (14- to 16-fold above control), but MES took 4 days and CHO 6 days to reach this maximum. Other osmolytes, raffinose and sorbitol (at concentrations of 250 to 300 mM) elicited the same response as that of 150 mM NaCl. These data show that AR expression is induced in MES and CHO cells under hypertonic conditions. Of special interest is the induction of large amounts of AR in rat kidney cortex mesangial cells, a target tissue of diabetes and a site where excessive accumulation of sorbitol is suspected to be a critical factor in diabetic nephropathy.

Diabetic-like retinopathy in rats prevented with an aldose reductase inhibitor

The earliest histopathologic signs of diabetic retinopathy include selective loss of intramural pericytes and thickening of capillary basement membranes. Previous evidence from animal models indicated that aldose reductase inhibitors could prevent these capillary wall lesions, but only recently have aldose reductase inhibitors been tested for prevention of the subsequent retinal complications of diabetes, such as microaneurysms. In the present study, Sprague-Dawley rats were fed diets containing 50% galactose with or without an aldose reductase inhibitor (tolrestat). After 28 months of galactose feeding, the retinal capillaries in whole mounts exhibited a marked increase in periodic acid-Schiff (PAS) staining, extensive pericyte loss, endothelial cell proliferation, acellularity, diffuse dilation, occluded lumens, microaneurysms, and complex microvascular abnormalities including gross dilation and formation of multiple shunt networks. The PAS hyperchromaticity of basement membrane material and pericyte loss occurred throughout the retinal vasculature, while while the microaneurysms and complex lesions were limited to the capillaries of the central and paracentral retina. The changes were associated with both the arterial and venous portions of the capillary plexus. Treatment with orally administered tolrestat prevented essentially all of the vessel abnormalities. Thus, long-term galactose feeding of rats induced microvascular lesions simulating those occurring in background diabetic retinopathy in humans, and these lesions were prevented by treatment with an aldose reductase inhibitor.

Polyol and vacuole formation in cultured canine lens epithelial cells

Polyol accumulation and myo-inositol depletion were accompanied by extensive vacuole formation in cultured canine lens epithelial cells that were incubated for up to 96 hr in growth medium supplemented with 30 mM D-galactose or 30 mM D-glucose. These changes did not occur in cells incubated in a hypergalactosemic or hyperglycemic medium which also contained an aldose reductase inhibitor (20 microM sorbinil). In addition, these changes were not observed in lens cells incubated in growth medium supplemented with either 30 mM mannitol, which is known to enter cells only slowly, or in 30 mM L-galactose, which is not a substrate for aldose reductase. The vacuoles were visible at the ultrastructural level after 6 hr of incubation in 30 mM D-galactose and increased in both number and size with time. These vacuoles had a unique fine structure. They did not result from swelling of mitochondria or other cell organelles. As demonstrated cytochemically, they did not represent either lysosomes or Golgi saccules. The proliferation pattern of cells incubated with 30 mM D-galactose was clearly different from that of control cells, but approached normal when an aldose reductase inhibitor was added to the incubation medium. Together these findings suggest that vacuole formation and altered cell proliferation were caused by polyol accumulation and/or myo-inositol loss, both of which result from aldose reductase activity.

Aldose reductase and polyol in cultured pericytes of human retinal capillaries

Aldose reductase has been found in several tissues involved in the various complications of long-term diabetes. Since the loss of intramural pericytes is typical of early microangiopathy of the diabetic retina, the presence of aldose reductase and its activity were investigated in pericytes cultured from capillaries of human retinas. Verification that contaminating cells had been removed was made on the basis of the pericytes' distinctive appearance in culture, inability to internalize acetylated-low-density lipoprotein, and immunoreactivity for muscle actin. Using pure cultures of human pericytes, the presence of aldose reductase was demonstrated immunohistochemically with antibodies directed against human placental aldose reductase, and aldose reductase activity was shown biochemically by monitoring the accumulating of xylitol in cells incubated with xylose. Xylitol accumulation was inhibited by the inclusion of an aldose reductase inhibitor in the xylose-containing medium.

Purification of plasma membrane from Acanthamoeba castellanii

A simple method for isolation of plasma membrane from Acanthamoeba using self-generating gradients of Percoll is described. To obtain a membrane marker, intact amoebae were radioiodinated and the distribution of the radiolabel was followed through the plasma membrane isolation procedure. The purity of isolated plasma membrane was assessed by enrichment of radiolabel, by electron microscopy, and by enzymatic assays for contaminating membranes. As judged from enrichment of radiolabel, a 37-fold purification of plasma membrane was obtained. We estimate that 80% of the total protein was from plasma membrane and 10% from membrane-associated actin.

Characterization of mRNA and genes for aldose reductase in rat

Aldose reductase (AR; E.C. 1. 1. 1. 21) has been implicated in a variety of diabetic complications. To investigate the expression of this enzyme in target tissues susceptible to such complications, mRNA encoding AR was characterized by Northern blot hybridization in various tissues and cultured cell preparations. The size of mRNA for AR (approximately 1500 bases) was in good agreement with the size determined by sequence analysis. A cDNA probe for AR from rat lens hybridized to the same size species of RNA isolated from cultured dog lens epithelial cells, cultured human retinal capillary pericytes (mural cells), and Y 79 human retinoblastoma cells. In rat tissues, a substantial amount of mRNA was expressed not only in lens, but also in retina, sciatic nerve and kidney medulla. AR mRNA seemed to be less abundant in rat skeletal muscle and brain, and was scarcely present in liver. Furthermore, Southern blot analysis of rat genomic DNA indicated that there are multiple sequences related to that for AR, probably indicating the existence of a multi-gene family.

Relationship between proteins encoded by three human gamma-crystallin genes and distinct polypeptides in the eye lens

Although individual gamma-crystallins from the human eye lens have not been successfully purified and sequenced, most of the genes coding for these lens-specific structural proteins have been cloned and characterized. To investigate the relationship between these genes and the gamma-crystallins of the human lens, we made use of mouse cell lines which contain stably integrated copies of the coding sequences for three of the human gamma-crystallin genes coupled to the human metallothionein IIA promoter. The proteins produced by these hybrid genes in cell culture were detected immunologically and compared by physical characteristics with the gamma-crystallins from the human lens. The protein encoded by the G3 gene showed properties identical to those of the 21,000-molecular-weight gamma-crystallin from 11-month-old lens. The protein isolated from the cells expressing the G4 gene was similar to a 19,000-molecular-weight lens gamma-crystallin, while gene G5 encodes a highly basic gamma-crystallin which may be synthesized in only limited amounts in the human lens. These correlations provide a basis for future investigations on the relationship between putative mutations in human gamma-crystallin genes and altered proteins in hereditary lens cataracts.

Vacuolar pH is one factor that regulates hydrolase secretion

Lysosomal hydrolases are continually secreted by Acanthamoeba as a consequence of membrane cycling between the vacuolar compartment and the cell surface. In pinocytosing amoebae acid hydrolases can be separated into two groups on the basis of their secretion kinetics. We have previously shown that in Acanthamoeba acid hydrolases are almost exclusively restricted to a single compartment, digestive vacuoles, and that pH-dependent differential binding of hydrolases to vacuolar membrane can account for the different rates of hydrolase secretion from this compartment. In this report we show that the hydrolase secretion pattern changes and that all of the hydrolases are released with the same kinetics after phagocytosis of yeast or in growth media supplemented with ammonium acetate or chloroquine, but not after phagocytosis of polystyrene beads. The changes in the pattern of hydrolase secretion correlate with changes in vacuolar pH. The vacuolar pH of pinocytosing amoebae and amoebae saturated with beads is about 4.8. This value is increased to 6.8 by accumulation of weak bases and to about 6.1 when digestive vacuoles are saturated with yeast. These results indicate that vacuolar pH modulates hydrolase transport and secretion.

Hydrolase secretion is a consequence of membrane recycling

Acanthamoeba releases lysosomal hydrolases continuously into the culture medium. This release is specific for lysosomal hydrolases, but not other cellular proteins, and is energy dependent. The secreted hydrolases can be separated into two groups on the basis of their secretion kinetics: one is secreted at approximately 15% of the cellular activity per hour and the other at approximately 5%. Intracellularly the lysosomal hydrolases are restricted almost exclusively to secondary lysosomes where the hydrolases demonstrate a differential pH-dependent binding to membrane. Hydrolase secretion is not the result of secondary lysosomes' fusing with the plasma membrane since soluble and particulate lysosomal contents are not released at the same rate. Together the data suggest that the secreted hydrolases are trapped in shuttle vesicles that cycle membrane from secondary lysosomes to the cell surface. The inner membrane and content of these vesicles undergo a marked pH shift when, following fragmentation from lysosomes, these vesicles fuse with plasma membrane. This rapid pH shift and the differential pH-dependent membrane binding of hydrolases appear to account for the heterogeneous hydrolase secretion kinetics.

Phagosome-lysosome fusion inhibited by algal symbionts of Hydra viridis

Certain species of Chlorella live within the digestive cells of the fresh water cnidarian Hydra viridis. When introduced into the hydra gut, these symbiotic algae are phagocytized by digestive cells but avoid host digestion and persist at relatively constant numbers within host cells. In contrast, heat-killed symbionts are rapidly degraded after phagocytosis. Live symbionts appear to persist because host lysosomes fail to fuse with phagosomes containing live symbionts. Neither acid phosphatase nor ferritin was delivered via lysosomes into phagosomes containing live symbionts, whereas these lysosomal markers were found in 50% of the vacuoles containing heat-killed symbionts 1 h after phagocytosis. Treatment of symbiotic algae before phagocytosis with polycationic polypeptides abolishes algal persistence and perturbs the ability of these algae to control the release of photosynthate in vitro. Similarly, inhibition of photosynthesis and hence of the release of photosynthetic products as a result of prolonged darkness and 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) treatment also abolishes persistence. Symbiotic algae are not only protected from host digestive attack but are also selectively transported within host cells, moving from the apical site of phagocytosis to a basal position of permanent residence. This process too is disrupted by polycationic polypeptides, DCMU and darkness. Both algal persistence and transport may, therefore, be a function of the release of products from living, photosynthesizing symbionts. Vinblastine treatment of host animals blocked the movement of algae within host cells but did not perturb algal persistence: algal persistence and the transport of algae may be initiated by the same signal, but they are not interdependent processes.