Over 300 million people worldwide are suffering from diabetes mellitus type 2, a complex disease consisting of decreased insulin sensitivity, increased gluconeogenesis, impaired insulin secretion and apoptosis of the beta-cells. Furthermore, chronic inflammation and oxidative stress caused by free radicals play a major role in its pathophysiology. A defence mechanism against oxidative stress is redox signalling. Major actors in re-establishingredox homeostasis are thioredoxin family proteins, including glutaredoxins, peroxiredoxins and thioredoxins. They modify their substrates by oxidation and reduction reactions. We studied the consequences of an obese phenotype on the Islets of Langerhans in comparison to the lean one. As an animal model we chose db mice. Homozygotes of this strain of C57BL/6 mice suffer from leptin resistance due to a mutation in the transmembrane leptin receptor Ob-Rb, developing an obese, diabetic phenotype. We delineated homozygotes to their heterozygote counterparts and explored the distinctions in vital parameters, islets and redoxin expression. Blood glucose and body weight were measured weekly from 6 to 18 weeks, confirming obesity and diabetic metabolism in homozygotes. Islet isolation and pancreatectomy were carried out at 6, 12 and 18 weeks. Homozygotes held significantly more islets, larger mean islet area and higher proliferation rates as well as increased apoptosis and failing secretion of insulin. Heterozygotes in contrast had higher insulin expression together with stable blood glucose and body weight. Trx family proteins distinguished both groups of animals further, displaying varying expression patterns in immunohistology. We analysed gene and protein expression of Grx1 and 5 more detailed and found significantly higher expression in heterozygote animals. Grx1 levels fluctuated together with the metabolic situation in homozyotes while Grx5 decreased with age. Formation of reactive oxygen species was measured in isolated islets and revealed elevated ROS in db/db specimens. To investigate the role of leptin, we cultivated non-gluco-responsive MIN6 cells with this adipose-derived hormone using different concentrations. Protein levels for Grx1 and 5 as well as insulin measured by ELISA turned out to be not influenced by leptin without glucose as a cofactor. We demonstrated that Trx family protein expression differs significantly between the obese, diabetic and the lean phenotype and that they are connected to the progress of obesity and hyperglycaemia. In conclusion, we propose that the redox state of the beta-cell and Grx1 and 5 are closely connected to impaired insulin secretion and destruction of the islets of Langerhans.
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