Evaluating β-catenin related pathways in insulin secretion: Role of Ser552 β-catenin phosphorylation in pancreatic β cells

Abstract

One important component of the glucose homeostatic process is the ability of key tissues to detect changes in glucose levels and respond appropriately. Insulin is the major hormone in the body for lowering blood glucose and is produced only by β cells. Both the transcriptional function and the adhesion function of β-catenin binding have been implicated as essential for β cells in releasing insulin. β-catenin has also been reported to associate with some PDZ domain containing proteins, which act as scaffold proteins during membrane motility of synapses. Our research group has previously found that an increase in glucose levels dramatically increases the Ser552 phosphorylation of β-catenin in β cell lines. This thesis has focused on understanding the role this event may play in regulating glucose homeostasis. The main approach was to use mutagenesis at Ser552 of β-catenin and investigate the interactions of these mutants by co-immunoprecipitations in β cells. Coimmunoprecipitations were also used to detect the changes in β-catenin associations under glucose or incretin stimulation. Mutation of mass-spectrometry was applied to look for interactions with β-catenin. Furthermore, Ser552 of β-catenin in a knock-in mouse model was generated to investigate its physiological role. Surprisingly, β-catenin did not interact with its transcriptional co-factor TCF7L2 in INS- 1 β cells and long term high glucose raised most proteins of interest in this thesis, but not TCF7L2. In β cells, β-catenin bound both N-cadherin and E-cadherin, and Ser552 phosphorylation of β-catenin did not significantly affect these interactions. In addition, Shank3, a PDZ domain containing protein, associated with β-catenin at the overexpressed level in β cells and this interaction was affected by glucose and Ser552 phosphorylation of β- catenin. Interestingly, the first round of animal experiments indicated that the heterozygous S552A β-catenin knock-in mice display a protective role against high fat diet induced obesity. All things considered, Ser552 is an important site in β-catenin for its physiological functions. More specifically, Ser552 phosphorylation of β-catenin may function in β cells’ response to glucose through its interaction with N-cadherin and Shank3. Accordingly, further investigation on β-catenin interactions during glucose stimulated insulin secretion (GSIS) in β cells is still needed.