EFFECTS OF POSTTRANSLATIONAL MODIFICATION OF TRANSCRIPTION FACTOR GLI-SIMILAR 3 BY SUMOYLATION ON INSULIN TRANSCRIPTION IN PANCREATIC β CELLS

The ability to control blood glucose levels is a fundamental component of vertebrates. In these organisms, blood glucose homeostasis is achieved through a fine-tuned mechanism that largely involves the secretion of hormones from the endocrine pancreas into the bloodstream. These hormones include glucagon, which is secreted by the α cells of the pancreas and initiates the release of glucose into the bloodstream through gluconeogenesis in the liver, and insulin, which is secreted from the β cells and signals the uptake of excess blood glucose by the peripheral tissue. Gli-similar 3 (Glis3) is a transcription factor that has previously been shown to play a critical role in both the development of β cells and transcriptional activation of the insulin gene. The mechanism by which the activity of Glis3 is modulated is still largely enigmatic. The posttranslational modification of proteins by covalent attachment of the small ubiquitin-like modifier protein (SUMO) has become an interest of many labs in recent years. This is largely because of the multiple identified substrates for SUMOylation as well as the wide variety of affects that modification by SUMOylation can have on these substrates. A mammalian two-hybrid assay identified enzymes that play an important role in the process of SUMOylation as potential Glis3 interactors. Subsequently, potential SUMOylation sites were identified within a phylogenetically conserved N-terminal region of Glis3. We found that Glis3 was SUMOylated by PIAS-family proteins (PIAS1 and PIASy) at two conserved lysine residues within the N-terminal region of Glis3. Additionally, it was found that posttranslational modification of Glis3 by SUMOylation regulated the transactivational activity of Glis3 in a tissue-dependent manner; down-regulating target genes in a β cell environment and up-regulating genes in a non-β cell environment. These observed effects were due to mechanisms both dependent and independent of Glis3 SUMOylation as effects were observed after the mutation of the modified conserved lysine residues. The transactivation activity of MafA, another transcription factor that plays a major role in activating target genes in β cells has previously been shown to be modulated by SUMOylation. It has also been demonstrated previously that MafA and Glis3 work synergistically to activate target genes. We found that PIASy promoted MafA SUMOylation which disrupts synergism between MafA and Glis3 at the insulin promoter. Finally, we found that chronic exposure of β cells to hyperglycemic conditions promoted Glis3 and MafA SUMOylation which was coincident with a sharp decrease in insulin transcription. These data suggest a potential mechanism that could underlie β cell dysfunction preceding type 2 diabetes and could potentially explain one mechanism by which insulin secretion is negatively affected in individuals that are incapable of regulating their blood glucose levels. Collectively, the results of this project provided novel information regarding transcriptional control of the insulin gene that is modulated by a combination of blood glucose levels and posttranslational modification