Diabetes is a disease of world-wide epidemic proportion derived from a failure of beta cells to produce relatively sufficient insulin to maintain euglycemia. Recent studies indicate that nutrient fluctuations and insulin resistance drive beta cells to synthesize insulin beyond their capacity for protein folding and secretion and activates the unfolded protein response (UPR). The UPR is an adaptive signaling pathway to promote cell survival upon accumulation of unfolded protein in the endoplasmic reticulum (ER). A subpathway of the UPR is signaled through activation of the ER kinase PERK and phosphorylation of eukaryotic translation initiation factor 2 on the alpha subunit to transiently attenuate protein synthesis, thereby reducing the biosynthetic burden on the beta cell. Recently, we demonstrated that beta cells selectively require phosphorylation of eIF2a and translation attenuation to preserve cell function. However, unresolved ER dysfunction and chronic activation of UPR subpathways increases expression of the proapoptotic transcription factor CAAT-enhancer binding protein homologous protein (CHOP). Chop deletion in insulin-resistant mice profoundly increases beta cell mass and improves beta cell function to maintain glucose-stimulated insulin secretion and prevent progression of diabetes. The results suggest that inhibition of CHOP may have therapeutic value in treatment of human diabetes to increase the function and/or mass of beta cells. To realize this potential, it is necessary to elucidate how CHOP causes beta cell failure. In pursuit of this goal, we have demonstrated that islets from Chop-/- mice are protected from oxidative stress that occurs upon accumulation of unfolded proteins in the ER. In addition, our preliminary unpublished findings show that CHOP does not significantly bind promoter elements of genes that encode apoptotic functions, but rather binds to promoters of genes that encode functions in protein synthesis. These novel findings inspire the hypothesis that CHOP causes beta cell failure by elevating protein synthesis to cause oxidative stress, and suggest an unprecedented link by which protein misfolding in the ER causes oxidative stress. The findings provide an unprecedented link by which protein synthesis and protein folding in the ER causes oxidative stress. In order to test this hypothesis we propose five specific aims to answer five fundamental questions.
Aim 1, How does CHOP expression induce oxidative stress and beta cell failure? We use both genetic and pharmacological approaches to test the hypothesis that eIF2a phosphorylation prevents oxidative stress through control of protein synthesis and that CHOP conversely induces oxidative stress by promoting protein synthesis.
Aim 2, What is the relationship between protein folding and oxidative stress? We will evaluate proinsulin folding and processing and mitochondrial function in these studies.
Aim 3, How do eIF2a phosphorylation and CHOP expression prevent cell death? We will evaluate the role of ATF4 in cell death and screen for genes that regulate ER stress induced cell death.
Aim 4, How do eIF2a phosphorylation and CHOP expression alter gene transcription? mRNA expression profiling and analysis of gene promoters that bind CHOP and ATF4 will identify the transcriptional network regulated by CHOP by eIF2/ATF4/CHOP signaling.
Aim 5, Does the PERK/eIF2a/CHOP signaling pathway alter mRNA translation in a quantitative and/or qualitative manner? The impact of eIF2a phosphorylation and CHOP expression on mRNA translational efficiency and AUG initiation codon selection will be measured using a new technique of ribosome profiling. The results from all these studies will provide fundamental insight and needed information toward an understanding of how eIF2a phosphorylation and translational control maintain beta cell function and how CHOP causes beta cell failure and should encourage the development of small molecules to modulate the UPR to preserve beta cell function and mass in human diabetes patients.
Diabetes is a world-wide epidemic characterized by an abnormality in glucose homeostasis that results from a failure of beta cells to produce insulin. Recent studies indicate that regulation of protein synthesis is essential for beta cells to elevate insulin production and for beta cell survival. The proposed studies on protein synthesis will provide vital information that should encourage the development of novel therapeutics to treat diabetes.
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