The overall objective of this proposal is to elucidate the molecular mechanisms by which elevated levels of glucose and fatty acids adversely affect the pancreatic ?-cell, a phenomenon referred to as glucolipodysfunction. The underlying hypothesis, based on our previous findings and preliminary data, proposes that early stages of glucolipodysfunction involve two complementary mechanisms: 1- Inhibition of expression of the enzyme Per Arnt Sim kinase (PASK) which results in diminished expression and activity of the transcription factors pancreatic-duodenal homeobox-1 (Pdx-1) and mammalian homologue of avian MafA/l-Maf (MafA), leading to decreased insulin expression via alterations of the histone code and a closed chromatin conformation;and 2- Nutrient-induced ?-cell proliferation driven by elevated circulating levels of fibroblast growth factor 21 (FGF21) activating FoxM1 signaling and leading to a dysfunctional ?-cell mass.
In specific Aim 1 we will determine how inhibition of PASK expression impairs insulin gene expression in glucolipodysfunction. Our working hypothesis is that PASK phosphorylates, and thereby inactivates, glycogen synthase kinase (GSK) 3? which alleviates proteasomal degradation of Pdx-1 and MafA. Using rodent genetic models we propose to further delineate the functional relationship between PASK and GSK3 ? and its consequences on Pdx-1 and MafA expression and function under conditions of glucolipodysfunction.
In specific Aim 2 we will characterize the modifications of the histone code and DNA methylation profile at the Pdx-1, MafA, and insulin promoters associated with glucolipodysfunction. Our working hypothesis is that Pdx-1 deficiency in glucolipodysfunction results in defective recruitment of the histone methyltransferase Set7/9 and alterations of the histone methylation profile at the insulin, Pdx-1, and MafA promoters. Using ex vivo and in vivo models we propose to identify the epigenetic modifications responsible for the initiation of glucolipodysfunction.
In specific Aim 3 we will ascertain how insulin resistance induces ? -cell proliferation in glucolipodysfunction. Our working hypothesis is that insulin resistance in response to nutrient excess in 6-mo-old rats is associated with a rise in circulating factors, FGF21 being a likely candidate, which stimulate FoxM1- mediated ?-cell proliferation. Using in vivo models we propose to identify the mechanisms whereby insulin resistance promotes ?-cell growth under conditions of nutrient excess. We expect that the studies described in this application will reveal the molecular signature of glucolipodysfunction in the pancreatic ?-cell. We anticipate that these findings will serve as a basis to design novel therapeutic approaches to prevent the deterioration of ?-cell function in T2D.
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