| Diabetes mellitus is a disease characterized by hyperglycemia and insulin deficiency. Keeping circulating glucose at physiological concentrations after a meal involves insulin production from beta cells in the pancreas and insulin-induced glucose uptake in peripheral tissues. Type 1 diabetes is caused by autoimmune destruction of beta cells, and type 2 diabetes mellitus results from a combination of genetic and environmental factors that leads to failure of functional beta cell mass to compensate for insulin resistance. This dissertation aimed to contribute to better understanding of beta cell function, particularly how beta cell mitochondria and reactive oxygen species form a system of intracellular signaling.;The islet beta cell is thought to be susceptible to damaging reactive oxygen species (ROS) due to low expression of endogenous antioxidant proteins such as superoxide dismutases (SOD1 and SOD2) that convert superoxide into hydrogen peroxide and glutathione peroxidase (GPX1) and catalase (CAT) which dispose of hydrogen peroxide to water. We report antioxidant protein expression at the mRNA, protein, and activity level in islets. Surprisingly, we found little to no hydrogen peroxide disposal activity, but high levels of SOD2. This system of antioxidants produces a four-fold increase in hydrogen peroxide production by islets within two minutes of glucose stimulation that is maintained over 90 minutes. Modulation of hydrogen peroxide concentration by overexpressing CAT or SOD2 in beta cells altered mammalian target of rapamycin (mTOR) activation, specifically in the context of mTORC1. Reduction in hydrogen peroxide decreased mTORC1 activation and downstream protein synthesis. In addition, hydrogen peroxide enhanced glucose-stimulated insulin secretion. We propose acute glucose-produced hydrogen peroxide to be a novel second messenger in multiple beta cell signaling pathways.;However, chronic hyperglycemia causes mitochondrial fission and dysfunction in beta cells. Mitochondrial metabolic response decreases after chronic high glucose exposure, measured by NAD(P)H production and mitochondrial membrane potential. Furthermore, chronic high glucose alters cytosolic calcium response, decreases store-operated calcium entry by 75%, and significantly reduces mitochondrial calcium uptake. These studies provide novel evidence for inapt mitochondrial signaling during chronic hyperglycemia, and the potential of chronic glucose-produced hydrogen peroxide contributing to beta cell dysfunction in the pathogenesis of diabetes mellitus. |
