The roles of protein O-GlcNAc in pancreatic beta-cell apoptosis and hormone secretion

The pancreatic β cells respond in the long-term to hyperglycemia both with an increased capacity for insulin production and, in susceptible individuals, with apoptosis. Apoptosis offsets the increasing β cell capacity resulting in a net loss of β cell function and the development of type 2 diabetes. Although this glucotoxicity has been recognized for a long time, its molecular mechanism is largely unknown. O-glycosylation is a cytoplasmic form of glycosylation that involves the O-linkage of the monosaccharide, N-acetylglucosamine (O-GlcNAc), to cytoskeletal and nuclear protein. Protein O-GlcNAc modification is reversible with addition catalyzed by O-GlcNAc transferase (OGT), and removal catalyzed by N-acetyl-β-D-glucosaminidase (O-GlcNAcase). OGT is highly enriched in pancreatic β cells. The β cell-specific toxin, streptozotocin (STZ), specifically inhibits the activity of O-GlcNAcase. In this dissertation, we have studied the effect of the modification of intracellular proteins with O-GlcNAc on β cell apoptosis. We found that β cells display a massive and specific shift in O-GlcNAc content in respond to changes in blood glucose (Glc) in intact animals, and STZ specifically blocks O-GlcNAc removal from proteins and causes O-GlcNAc accumulation in β cells. This β cell-specific up-regulated O-GlcNAc appears to potentiate the cytotoxicity of STZ and result in β cell apoptosis. Using transgenic mice, we proved that this change of protein O-GlcNAc modification is essential for the STZ-induced β-cell apoptosis. These results provide a causal link between β cell apoptosis and Glc metabolism through glucosamine to O-GlcNAc, implicating this pathway of Glc metabolism in β cell Glc toxicity. In addition, the first phase insulin secretion function of type 2 diabetes is largely impaired. In this dissertation, we also provide genetic and physiological evidence to correlate this β cell secretion defect with impaired O-GlcNAcase and up-regulated β cell O-GlcNAc modification both in vitro and in vivo. In concert with our finding that pituitary somatotrophin cells also contain highly expressed O-GlcNAc transferase, we showed that impaired O-GlcNAcase also significantly inhibits stimulated growth hormone secretion in intact animal models. These data further support the possible physiological role of O-GlcNAc in general hormone secretion control and its pathological role in β cell dysfunction of type 2 diabetes.