Novel Enteroendocrine Cell Receptors Regulating Incretin Secretion and Glucose Homeostasis

Abstract The proglucagon‐derived peptides (PGDP) are expressed in islet alpha and gut enteroendocrine L cells. Although glucagon, glucagon‐like peptide‐1 (GLP‐1), and glucagon like peptide‐2 (GLP‐2) are derived from the same proglucagon gene, energy ingestion and nutrient assimilation represses proglucagon biosynthesis in the α‐cell, but stimulates the synthesis and secretion of GLP‐1 and GLP‐2 from the gut L cell. In the work presented in this thesis, I have identified novel G protein‐coupled receptors that stimulate GLP‐1 secretion and improve glucose homeostasis. G protein‐coupled receptor 119 (GPR119) is expressed in enteroendocrine cells and islets and is activated by nutrients (fatty acid derivatives) and small specific synthetic agonists. Activation of GPR119 enhances glucosestimulated insulin secretion from islet β‐cells and promotes incretin release from enteroendocrine cells in a cyclic AMP (cAMP)‐dependent manner. To determine the importance of gut hormones for the glucoregulatory actions of GPR119, I examined GPR119 activation in normal mice, isolated islets, and in mice with inactivation of gut hormone receptors. GPR119 activation directly stimulates insulin secretion from islets in vitro, yet requires intact incretin receptor signaling and enteral glucose exposure for optimal improvement of glucose tolerance in vivo. In contrast, activation of GPR119 inhibits gastric emptying independent of incretin receptors through GPR119‐dependent pathways. Another important feature of β‐cell GPCRs coupled to cAMP generation is their ability to protect the β‐cell from external injury. I have shown that mice lacking GPR119 (GPR119‐/‐) are more susceptible to streptozotocin (STZ)‐induced apoptosis while pharmacological activation of GPR119 failed to protect the β‐cell from STZ‐induced injury. Furthermore, GPR119‐/‐ mice iv display impaired incretin secretion and beta cell function when chronically fed a high fat (HF) diet. Conversely, abrogation of GPR119 signaling does not affect the beta‐cell adaptation (increased islet number and size) to the metabolic demand of high‐fat feeding. Mechanisms to increase β‐cell mass and function may be useful tools for the treatment of type 2 diabetes. GLP‐1 stimulates insulin biosynthesis, β‐cell proliferation and exerts antiapoptotic actions on β‐cells. To delineate novel mechanisms, important for the regulation of proglucagon gene expression and GLP‐1 secretion in the enteroendocrine L‐cell, I carried out a microarray‐based gene expression profiling and transcriptional networks analysis using RNA from murine gut GLUTag cells. To identify mechanisms unique to enteroendocrine L‐cells, I used the islet αTC1 cell line for comparative purposes. I identified a novel progesterone mediated signaling pathway involving activation of membrane GPCRs for the control of GLP‐1 secretion. In summary, these studies establish that GPR119 engages multiple complementary pathways for control of glucose homeostasis and suggest that endogenous GPR119 signaling plays a critical role in β‐cell adaptation to cytotoxic injury and nutrient excess. The studies provide evidence for a novel role for progesterone, regulating GLP‐1 secretion and controllingglucose homeostasis.Ph