Molecular Mechanisms Involved in Insulin- and Leptin-mediated Regulation of Hypothalamic Proglucagon Gene Expression and Action of Glucagon-like Peptides on Hypothalamic Neuropeptides

The hypothalamus is a central regulator of energy homeostasis. Recently, proglucagon-derived peptides have emerged as potential appetite regulators. The proglucagon gene is expressed in the periphery and also in selective hypothalamic neurons. The regulation of hypothalamic proglucagon by two key regulators of energy balance, insulin and leptin, remains unstudied. Central glucagon-like peptide (GLP)-1 receptor (GLP-1R) activation by exendin-4, a long-acting GLP-1R agonist, induces anorexia; however, the specific hypothalamic neuronal populations activated by exendin-4 remain largely unknown. The role of GLP-2 as a central appetite regulator is poorly understood. In this thesis, using murine hypothalamic cell lines and mice as experimental models, mechanisms involved in the direct regulation of proglucagon gene by insulin and leptin were studied, and the actions of exendin-4 and GLP-2 on hypothalamic neuropeptides were determined. It was found that insulin and leptin regulate hypothalamic proglucagon mRNA by activating Akt and signal transducer and activator of transcription 3, respectively. Insulin and leptin did not regulate human proglucagon promoter regions, but affected proglucagon mRNA stability. In mice, intracerebroventricular exendin-4 and GLP-2 induced anorexia, activated proopiomelanocortin- and neuropeptide Y-expressing neurons in the arcuate nucleus and neurotensin- and ghrelin-expressing neurons in major hypothalamic appetite-regulating regions. In the hypothalamic neuronal models, exendin-4 and GLP-2 activated cAMP-response element-binding protein/activating transcription factor-1, and regulated neurotensin and ghrelin mRNA levels via a protein kinase A-dependent mechanism. Overall, the in vivo and in vitro findings suggest that these neuropeptides may serve as potential downstream mediators of exendin-4 and GLP-2 action. This research demonstrates direct regulation of hypothalamic proglucagon by insulin and leptin in vitro, and reports a previously unrecognized link between central GLP-1R and GLP-2R activation and regulation of hypothalamic neuropeptides. A better understanding of the regulation of hypothalamic proglucagon and central GLP-1R and GLP-2R activation is important to further expand our knowledge of feeding circuits.Ph