Effects of glucose and SNK-860, an aldose reductase inhibitor, on the polyol pathway and chemiluminescence response of human neutrophils in vitro

Common infections in diabetes: pathogenesis, management and relationship to glycaemic control

Specific defects in innate and adaptive immune function have been identified in diabetic patients in a range of in vitro studies. However, the relevance of these findings to the integrated response to infection in vivo remains unclear, especially in patients with good glycaemic control. Vaccine efficacy seems adequate in most diabetic patients, but those with type 1 diabetes and high glycosylated haemoglobin levels are most likely to exhibit hypo-responsiveness. While particular infections are closely associated with diabetes, this is usually in the context of extreme metabolic disturbances such as ketoacidosis. The link between glycaemic control and the risk of common community-acquired infections is less well established but could be clarified if infection data from large community-based observational or intervention studies were available. The relationship between hospital-acquired infections and diabetes is well recognized, particularly among post-operative cardiac and critically ill surgical patients in whom intensive insulin therapy improves clinical outcome independent of glycaemia. Nevertheless, further research is needed to improve our understanding of the role of diabetes and glycaemic control in the pathogenesis and management of community- and hospital-acquired infections.

NADPH-dependent reductases and polyol formation in human leukemia cell lines

Because of the limited availability of human tissues, leukemia cell lines are often utilized as the models for human leukocytes. In this study, we investigated the NADPH-dependent reductases and polyol pathway in commonly utilized human leukemia cell lines. The relative amounts of aldose and aldehyde reductases were estimated by separating two enzymes with chromatofocusing. The flux of glucose through the polyol pathway was examined by 19F-NMR using 3-fluoro-3-deoxy-D-glucose (3FG) as substrate. Sugar alcohol analysis was conducted by gas chromatography. In myelocytic leukemia cells, the major reductase was aldehyde reductase, and levels of aldose reductase were extremely low. Although lymphocytic cells also contained both aldose and aldehyde reductases, the levels of aldose reductase appeared to be higher in lymphocytic cells than myeolcytic cells. In two lymphocytic cells MOLT-4 and SKW6.4, aldose reductase is clearly dominant. When incubated in medium containing D-galactose, all cell lines quickly accumulated galactitol. There was correlation between galactitol levels and aldose reductase levels. The aldose reductase inhibitor FK 366 significantly reduced the formation of galactitol. 19F-NMR of the cells cultured with 3FG as substrate demonstrated the formation of 3-fluoro-3-dexoy-sorbitol in all the cell lines examined in this study. The relative amounts of sorbitol and fructose varied significantly among the cells. The data confirm that the polyol pathway is present in both myelocytic and lymphocytic leukemia cell lines. However, there is a large variation among the cell lines in the levels of enzymes and flux of glucose through the polyol pathway.

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.

Effect of aldose reductase inhibitors on glucose-induced changes in sorbitol and myo-inositol metabolism in human neutrophils

AIM

To investigate the influence of glucose and the efficacy of two different aldose reductase (AR) inhibitors, epalrestat and SNK-860, on the polyol pathway and myo-inositol metabolism in human neutrophils.

METHODS

We incubated neutrophils with various concentrations of glucose and AR inhibitors. The neutrophils from healthy volunteers were incubated in the media containing 5-40 mmol/l glucose with or without an AR inhibitor. The sorbitol and myo-inositol contents, and myo-inositol uptake were measured by high performance liquid chromatography and radio isotope technique with 2-[3H]-myo-inositol.

RESULTS

After 2 h incubation, the sorbitol content increased with rising extracellular glucose concentrations, while the myo-inositol content decreased. Both AR inhibitors reduced the sorbitol content in neutrophils exposed to 40 mmol/l glucose medium. A 70% fall in the myo-inositol content in neutrophils exposed to 40mmol/glucose medium was attenuated approximately 40% by the addition of AR inhibitors. myo-Inositol uptake into neutrophils was inhibited by high glucose. AR inhibitors significantly ameliorated the decrease in myo-inositol uptake, but did not completely normalize it.

CONCLUSIONS

Our present in vitro studies showed that the glucose-induced metabolic alterations in human neutrophils were similar to those in tissues prone to diabetic complications, and that AR inhibitors effectively corrected glucose-induced imbalances of the polyol pathway and myo-inositol uptake in neutrophils. In addition, our study suggests that glucose-induced metabolic alterations may result in the neutrophil dysfunction and that an AR inhibitor may be capable ameliorating it.

Aldose reductase: a window to the treatment of diabetic complications?

Kinetic studies on the aldose reductase protein (AR2) have shown that it does not behave as a classical enzyme in relation to ring aldose sugars. These results have been confirmed by X-ray crystallography studies, which have pinpointed binding sites for pharmacological "aklose reductase inhibitors" (ARIs). As with non-enzymic glycation reactions, there is probably a free-radical element involved derived from monosaccharide autoxidation. In the case of AR2, there is free radical oxidation of NADPH by autoxidising monosaccharides, enhanced in the presence of the NADPH-binding protein. Whatever the behaviour of AR2, many studies have showed that sorbitol production is not an initiating aetiological factor in the development of diabetic complications in humans. Vitamin E (alpha-tocopherol), other antioxidants and high fat diets can delay or prevent cataract in diabetic animals even though sorbitol and fructose levels are not modified; vitamin C acts as an AR1 in humans. Protein post-translational modification by glyc-oxidation or other events is probably the key factor in the aetiology of diabetic complications. There is now no need to invoke AR2 in xylitol biosynthesis. Xylitol can be produced in the lens from glucose, via a pathway involving the enzymes myo-inositol-oxygen oxidoreductase, D-glucuronate reductase. L-gulonate NAD(+)-3-oxidoreductase and L-iditol-NAD(+)-5-oxidoreductase, all of which have recently been found in bovine and rat lens. This chapter investigates the molecular events underlying AR2 and its binding and kinetics. Induction of the protein by osmotic response elements is discussed, with detailed analysis of recent in vitro and in vivo experiments on numerous ARIs. These have a number of actions in the cell which are not specific, and which do not involve them binding to AR2. These include peroxy-radical scavenging and recently discovered effects of metal ion chelation. In controlled experiments, it has been found that incubation of rat lens homogenate with glucose and the copper chelator o-phenanthroline abolishes production of sorbitol. Taken together, these results suggest AR2 is a vestigial NADPH-binding protein, perhaps similar in function to a number of non-mammalian crystallins which have been recruited into the lens. There is mounting evidence for the binding of reactive aldehyde moieties to the protein, and the involvement of AR2 either as a 'housekeeping' protein, or in a free-radial-mediated 'catalytic' role. Interfering with the NADPH binding and flux levels--possibly involving free radicals and metal ions--has a deleterious effect. We have yet to determine whether aldose reductase is the black sheep of the aldehyde reductase family, or whether it is a skeleton in the cupboard, waiting to be clothed in the flesh of new revelations in the interactions between proteins, metal ions and redox metabolites.