The effect of aldose reductase inhibition with ponalrestat on the width of the capillary basement membrane in diabetes mellitus

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.

Pericapillary basement membrane thickening in human skeletal muscles

The basement membrane (BM) surrounding capillaries in skeletal muscles varies physiologically in thickness according to age, physical fitness, and anatomical site in humans. Furthermore, the pericapillary BM thickness (CBMT) increases pathophysiologically during several common disease states, including peripheral arterial disease and diabetes mellitus. This review on CBM thickening in human skeletal muscles is two pronged. First, it addresses the advantages/disadvantages of grid- and tablet-based measuring and morphometric techniques that are implemented to assess the CBMT on transmission electron micrographs. Second, it deals with the biology of CBM thickening in skeletal muscles, particularly its possible causes, molecular mechanisms, and functional impact. CBM thickening is triggered by several physical factors, including diabetes-associated glycation, hydrostatic pressure, and inflammation. Increased biosynthesis of type IV collagen expression or repetitive cycles in pericyte or endothelial cell degeneration/proliferation appear to be most critical for CBM accumulation. A thickened CBM obviously poses a greater barrier for diffusion, lowers the microvascular elasticity, and impedes transcytosis of inflammatory cells. Our own morphometric data reveal the CBM enlargement to be not accompanied by the pericyte coverage. Owing to an overlap or redundancy in the capillary supply, CBM thickening in skeletal muscles might not be such a devastating occurrence as in organs with endarterial circulation (e.g., kidney and retina). CBM growth in skeletal muscles can be reversed by training or administration of antidiabetic drugs. In conclusion, CBM thickening in skeletal muscles is a microvascular remodeling process by which metabolic, hemodynamic, and inflammatory forces are integrated together and which could play a hitherto underestimated role in etiology/progression of human diseases.

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.

The efficacy of aldose reductase inhibitors in the management of diabetic complications. Comparison with intensive insulin treatment and pancreatic transplantation

Recently, aldose reductase inhibitors (ARIs) have been registered in several countries for the improvement of glycaemic control. However, their efficacy is still controversial. ARIs inhibit the enhanced flux of glucose through the polyol pathway. As such, they can never be more effective than normoglycaemia, and so their potential benefits and limitations should be considered relative to the effects of prolonged euglycaemia. The clinical effects of ARIs can be put into perspective by assessing the effects of improved glycaemic control attained in randomised trials of intensive insulin treatment [such as the Diabetes Control and Complications Trial (DCCT)] and after pancreatic transplantation. Although direct comparison of these 3 interventions is hampered by differences in patient populations, duration and methods of follow-up and in the potency of ARIs, the effects of these 3 metabolic interventions and their course in time appear remarkably similar. For neuropathy, all 3 interventions induce an increase in average motor nerve conduction velocity of approximately 1 m/sec during the first months of treatment. At the same time, improvement of painful symptoms may occur. These changes probably largely represent a metabolic amelioration of the condition of the nerves. Around the second year of treatment with all 3 forms of metabolic improvement, an acceleration of nerve conduction of a similar magnitude occurs, with signs of structural nerve regeneration and some sensory recuperation. Experience with ARIs in nephropathy is still limited, but similar improvements in glomerular filtration rate and, less consistently, in urinary albumin excretion were found during short term normoglycaemia produced by all 3 forms of treatment. Comparison of a small number of studies, however, shows differences between intensive insulin regimens, pancreatic transplantation and ARIs in effects on retinopathy. Retinopathy often temporarily deteriorates in the early phases of improved glycaemic control, but this is not noted with ARIs. New microaneurysm formation was slightly reduced in a single long term study with the ARI sorbinil, but the preventive effects on the overall levels of retinopathy seemed less strong than in normoglycaemia trials of similar duration. However, the pharmacodynamic effects on inhibiting the polyol pathway differ among ARIs, and the half-life of the inhibiting effect of sorbinil may have been too short for a complete reduction of polyol pathway activity. The trials of prolonged intensive insulin therapy and pancreatic transplantation have demonstrated that very strict metabolic control must be maintained continuously for many years before a significant reduction of complications can be demonstrated.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of myo-inositol treatment on basement membrane thickening in the BB/W-rat retina

A polyol-pathway related perturbation of myo-inositol metabolism has been invoked in the pathogenesis of diabetic complications, including retinal microvasculopathy. Previous studies have demonstrated a beneficiary effect of aldose reductase inhibition on basement membrane thickening of retinal microvessels in diabetic animals. In the present study we demonstrate a significant but partial effect on basement membrane thickening following myo-inositol supplementation. Qualitative structural changes, such as nodular swellings, fibrillar changes and basement membrane projections were not effected by myo-inositol supplementation, suggesting that although abnormal myo-inositol tissue levels may play a role in basement membrane thickening, other factors may be of primary pathogenetic importance.