The islet ?-cell is the key regulatory element of the glucose homeostasis system. Changes in insulin sensitivity and/or ?-cell mass elicit precise adaptations from the remaining ?-cells so normoglycemia is maintained. How is that accomplished? What signaling pathways and ?-cell molecular processes are involved? This application continues our studies of the ?-cell adaptive mechanisms to a reduction in ?-cell mass such as occurs in evolving type 1 diabetes, likely also type 2 diabetes, using the experimental model of 60% pancreatectomy (Px) in normally insulin sensitive rodents. These rodents are normoglycemic following the reduction in ?-cell mass because of a multifacted adaptive response in islet ?-cells that results in maintenance of a normal level of secreted insulin. We plan to test a welldefined mechanistic schema for the adaptive responses that is based on findings from the prior funding period. Specifically, we propose an essential role for PPAR? through its transcriptional regulation of key genes that impact ?-cell function and survival (Pdx-1), the incretin system (GIP receptor), and mitochondrial fuel metabolism (pyruvate carboxylase). Also, we propose upstream regulation of PPAR? by the forkhead transcription factor FOXO1. This implies a broader role for this regulatory system than simply post-Px to include states of ?-cell adaptation that act through IRS-2/Akt such as insulin resistance. We will test these hypotheses in mice with genetically altered expression of key elements in the proposed regulatory pathways ? pancreas specific PPAR? knockout and heterozygous FOXO1 knockout ? that undergo 60% Px or fat feeding. Parallel in vitro studies to identify the molecular details of the observed adaptive responses also will be performed.
Aim #1 will determine the role of PPAR? in the enhanced ?-cell function and survival following a reduction in ?-cell mass. We will test the hypothesis of a necessary role for PPAR? in the post-Px ?cell adaptation by performing a 60% Px in pancreas-specific PPAR? knockout mice, predicting hyperglycemia along with failed upregulation of the identified genes.
Aim #2 will determine the molecular basis and functional significance of FOXO1 regulation of PPAR? expression in ?-cells using in vitro siRNA studies for PPAR? and FOXA2, and by determining the expression of Pdx-1 and PPAR?-regulated genes following Akt activators (GLP-1 and insulin) in FOXO1 heterozygous knockout mice and normal mice.
Aim #3 will determine the role of the FOXO1/PPAR?/Pdx-1 pathway in the ?-cell adaptation to dietinduced obesity using fat feeding of FOXO1 heterozygous knockout mice and pancreas-specific PPAR? knockout mice.
