Oxidative stress in the pathogenesis of type 1 diabetes: the role of adipocyte xanthine oxidase

Rationale. According to modern understanding, oxidative stress plays an essential role in the underlying mechanisms for the emergence of type 1 and type 2 diabetes and development of complications in these conditions. The sources of reactive oxygen species in diabetes are protein glycosylation, mitochondrial respiratory chain, membrane-bound NADPH oxidase, and other enzymes. An important enzymatic source of the superoxide anion radical and Н2О2 is xanthine oxidoreductase. Under physiological conditions, it exists mainly in the xanthine dehydrogenase form and may be reversibly or irreversibly converted to xanthine oxidase, following oxidative modification with formation of disulfide bonds in the protein molecule. In rodents, xanthine oxidoreductase in adipose tissue is expressed at a higher rate, as opposed to in other tissues.

The aim of the study was to establish the role of xanthine oxidase in development of oxidative stress in rat adipocytes in experimental type 1 diabetes.

Materials and methods. The study was conducted in male Wistar rats with experimental diabetes induced by two different diabetogenic agents – alloxan and streptozotocin. Serum concentration of insulin in the control and experimental rats was determined by the immunoradiometric assay. The levels of glucose and uric acid were measured by the enzymatic methods. The concentration of lipid hydroperoxides, primary products of lipid peroxidation, was detected by the FOX-2 method. The activity of xanthine oxidase in isolated adipocytes of epididymal adipose tissue and the expression of xanthine dehydrogenase mRNA were determined by fluorometry.

Results. The rats from the experimental group developed hyperglycemia-induced oxidative stress. The rise in the lipid hydroperoxide concentration in adipocytes was observed against the backdrop of the increased xanthine oxidase activity. The boost in the enzymatic activity under oxidative stress in adipocytes of the experimental rats was accompanied by the increase in the proportion of the xanthine oxidase activity in the total xanthine dehydrogenase plus oxidase activity. It resulted in the elevated xanthine oxidase/xanthine dehydrogenase activity ratio. Oxidative stress in rat adipocytes with experimental type 1 diabetes caused oxidative post-translational modification of the enzyme and its conversion from the xanthine dehydrogenase form to the xanthine oxidase one. It resulted in the subsequent increase in reactive oxygen species production. The inhibitor of xanthine oxidase reduced the level of lipid hydroperoxides in rat adipocytes. Thus, xanthine oxidase may be a potential target to protect from oxidative stress in rat adipose tissue in type 1 diabetes.

Conclusions. Oxidative stress in adipose tissue of the rats with alloxan- and streptozotocin-induced diabetes is determined, to a certain extent, by the increased expression of xanthine dehydrogenase mRNA as well as by post-translational oxidative modification of the enzyme activity from dehydrogenase to oxidase. Allopurinol, the inhibitor of xanthine oxidase, decreases the alloxan-induced elevated level of lipid hydroperoxides in rat serum and isolated adipocytes. It indicates a crucial role of xanthine oxidase in development of oxidative stress in adipocytes in experimental type 1 diabetes. 

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