Physiologic and genetic functions of Foxo1 in the pancreatic beta-cell

Type II diabetes mellitus (T2DM) is a complex metabolic disorder characterized by insulin resistance and a progressive pancreatic beta-cell deficiency or dysfunction in regulating glucose homeostasis. Although key to understanding and potentially alleviating T2DM, mechanisms underlying regenerative capacity, nutrient and growth-factor signaling, and the cell-cycle of pancreatic islet beta-cells remain mostly uncharacterized. Foxo1 is known in neuronal tissues to be a pro-apoptotic and anti-proliferative protein inhibited by the PI-3K/Akt pathway. In other tissues, such as the liver, muscle, and adipose, Foxol plays a unique role in regulating metabolism and cellular differentiation.;The regulation of Foxo1 activity by nutrients and the consequences of its inhibition on insulin secretion, proliferation and response to stress were previously uncharacterized.;A significant portion of this thesis addressed how nutrents, specifically glucose and free fatty acids, mediate Foxo1 in beta-cells. The first experiments determined the glucose inhibition of Foxo1 activity in beta-cells to be dependent on autocrine/paracrine insulin signaling. After establishing that Foxo1 is a dynamic transcription factor in beta-cells, we observed that Foxo1 activity is increased in beta-cells treated with fatty acids, and that inhibiting Foxo1 by expressing a dominant-negative allele decreased ER-stress and fatty acid induced apoptosis. Additionally, we found the expression of another pro-apoptotic transcription factor, Chop, to be regulated at the promoter level by Foxo1.;The effects of inhibiting Foxo1 were tested in vivo by creating transgenic mice expressing the dominant-negative Foxo1 allele in beta-cells. The resulting phenotypes were hyperinsulinemia, which was exacerbated with high-fat feeding, and a resistance to apoptosis induced by palmitate and thapsigargin. Interestingly, there were no appreciable effects of the dominant-negative Foxo1 on beta-cell proliferation in vivo . Microarray analysis comparing gene expression changes with lipotoxic palmitate treatment and dominant-negative Foxo1 expression revealed many genes whose induction with palmitate was suppressed by the dominant-negative Foxo1. Further promoter analysis revealed genes which may be targets of Foxo1.;In conclusion, the data suggest a pivotal role for Foxo1 in potentially regulating insulin secretion and beta-cell survival, both critical aspects of islet physiology and of potential therapeutic value to alleviate the morbidity and mortality of type II diabetes.