Diabetes is a huge health burden in the United States and around the world, both in decreased quality of life and in cost. This proposal addresses basic mechanisms controlling the function of pancreatic p-cells. Greater knowledge will facilitate the preservation of p-cell function and the development of safe and effective anti-diabetic therapies. The MAP kinases (MAPKs) ERK1 and ERK2 participate in the signal transduction mechanisms that integrate nutrient and hormonal inputs to the maintenance of insulin gene transcription in p-cells. The focus of this proposal is the elucidation of the functions of ERK1/2 in the normal physiology of pancreatic p-cells and in p-cell dysfunction. ERK1/2 enhance the activity of factors essential for insulin gene transcription;inhibiting ERK1/2 activity impairs insulin gene transcription. ERK1/2 are also implicated in multiple conditions in which p-cell function is impaired, including hyperglycemia and immunosuppression. High concentrations of circulating glucose that can occur in diabetes cause prolonged hyperactivationof ERK1/2 and formation of ERK1/2-sensitive transcription factor complexes associated with decreased insulin gene transcription;thus, ERK1/2 also contribute to the reduced ability of p-cells to produce insulin during prolonged hyperglycemia. We have demonstrated that at least six transcription factors that regulate insulin gene transcription are ERK1/2targets in p-cells including PDX-1, Beta2, MafA, NFAT, and C/EBP-p.
The aims are to define mechanisms by which ERK1/2 enhance transcription, to define mechanisms by which ERK1/2 inhibit transcription, and to determine the impact on p-cells of interactions of ERK1/2 with PEA-15, a protein over-expressed in type II diabetes. MafA and NFAT form an ERK1/2-dependent complex associated with increased insulin gene promoter activity. We will examine how ERK1/2 regulate their activities and explore the role of ERK1/2 in recruitment of coactivators and other proteins to transcription factor complexes. Inhibition of ERK1/2 in p-cells chronically exposed to high glucose results in a marked increase inmRNA encoding the stress-induced C/EBP homologous factor CHOP-10 (GADD153). We will define howERK1/2 inhibit expression of the CHOP-10 mRNA and explore its functions in p-cells. Exposure of p-cells to high glucose for more than 12 h inhibits insulin gene transcription in part due to induction of C/EBP-p. Blocking ERK1/2 disrupts a DNA-bound C/EBP-p complex that inhibits insulin gene transcription. Thus, we will determine mechanisms of C/EBP-p regulation by ERK1/2. PEA-15 is overexpressed in type 2 diabetes, causes hyperglycemia when expressed in mice, and impaired insulin release in MIN6 cells. PEA-15 binds directly to ERK1/2 and prevents their nuclear accumulation in fibroblasts. We will determine if overexpression of PEA-15 interferes with ERK1/2 function in p-cells and if ERK1/2 localization affects the functions of their substrates.
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