A major component of our research in the Diabetes Section is understanding what controls beta cell mass of the pancreas. With age and type 2 diabetes, there is a failure of expansion of beta cell mass. This is possibly due to increased apoptosis of existing beta cells as well as decreased beta cell neogenesis. We have been investigating the mechanisms of action of GLP-1, a gut hormone, as they relate to insulin release. We found that not only is GLP-1 a potent insulinotropic agent, it upregulates insulin biosynthesis, increases translocation of pdx-1, a transcription factor necessary for maintenance of the beta cell phenotype, to the nucleus and it increases glucokinase protein levels (the essential glucose sensor in beta cells). We also found that it increases beta cell mass by increasing beta cell proliferation in islets of Langerhans. Other investigators demonstrated antiapototic effects of GLP-1 in beta cell lines and whole islets. A GLP-1 receptor agonist, exendin-4, has recently become available for treating type 2 diabetes.? A component of our basic work is investigating how GLP-1 increases beta cell turnover. Of relevance to aging, we have found that the ability of GLP-1 receptor agonists to increase beta cell turnover is reduced in islets from old rodents:however, continuous, unremitting treatment of the animals with GLP-1 receptor agonists can overcome this. While attempting to uncover how GLP-1 receptor activation leads to increased beta cell proliferation, we found that all of the prototypic components of the Notch pathway, which is a transmembrane receptor pathway, are present in mature islets. Activation of the pathway requires gamma-secretase (a protein complex) to cleave the C-terminus of the Notch receptor (NICD), which then translocates to the nucleus. NICD controls pancreatic endocrine differentiation and determines cell fate (endocrine vs. exocrine vs. duct cell type) in the embryonal pancreas: we have now found all of the major components of the Notch pathway to be present in mature beta cells and insulinoma cell lines. The gamma-secretase complex exists in beta cells and, upon treating beta cells with GLP-1, gamma-secretase activity increases 6-fold in rodent islets. This was confirmed by measuring gamma-secretase activity in response to GLP-1 and showing that inhibition of gamma-secretase in vitro and in vivo by gamma-secretase inhibitors prevents GLP-1R mediated proliferation of beta cells. The mechanism by which GLP-1 activates gamma-secretase is though activation of PKA. The GLP-1 receptor is a G-protein coupled receptor and, when it is engaged, cAMP is generated, which in turn activates PKA. Inhibition of PKA prevents GLP-1-mediated gamma-secretase activation and prevents GLP-1-mediated proliferation of beta cells. This provides a direct link between activation of the Notch signaling pathway, GLP-1R activation and beta cell proliferation. Downstream of Notch lies Hes, a basic helix-loop-helix transcriptional repressor. Activation of Notch leads to increased transcription of Hes. In beta cells it represses ngn3, a transcription factor that leads to differentiation of precursor endocrine cells to beta cells and by repressing ngn3 Hes allows further expansion of precursor cells, thereby favoring the generation of a larger pool of undifferentiated cells in the embryo. We found that islets from old rodents actually have increased amounts of Hes protein (not decreased), suggesting a downstream block on cellular expansion via Hes. Investigating the Notch pathway in adult islets is at its infancy and further work is needed to continue to undestand how this pathway operates in the mature beta cell.
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