Monday, May 6, 2024
5/6/2024
Workload-induced pancreatic islet ß-cell dysfunction, loss-of identity, and cell death, commonly known as ß-cell failure, is the hallmark of type 2 diabetes (T2D). This disease usually starts with obesity-induced insulin resistance, when peripheral tissues need higher levels of circulating insulin for glucose storage and usage. Islet ß-cells compensate by expanding ß-cell mass and increasing insulin output per cell, which requires upregulated insulin biosynthesis and oxidative glucose metabolism. These produce unfolded proinsulin in the ER and reactive oxygen species (ROS) in mitochondria, which at high levels can decimate ß cells. Thus, ß cells constantly activate stress response by stimulating the activity of several early-stage SRGs, including Atf6, IRE1¿, PERK, Hsf1, and Nurf2, to lead to: 1) attenuated overall protein translation; 2) enhanced translation of some SRG mRNAs that have special features such as upstream open reading frame (uORF) 5’ to the main ORF; 3) upregulated expression of some late-stage SRGs. The overall effect of these responses is to remove unfolded proteins/ROS for proteomic homeostasis and sustainable ß-cell function. However, over-activating some late-stage SRGs such as Atf4 and Hsps induces ß-cell failure by turning on some proapoptotic genes or by exceedingly lowering overall protein translation. Thus, it is imperative for ß-cells to limit the levels of failure- causing SRGs for sustainable high-level of insulin output. An emerging model from our recent published findings is that a transcriptional complex containing Myt TFs and Sin3 can selectively repressing these failure- causing SRGs. Myt TFs are a family of three myelin transcription factors (Myt1, 2, and 3) highly expressed in islet cells. Sin3, including Sin3a and Sin3b, is a coregulator that represses transcription by recruiting histone deacetylases (HDACs) to modify histones. We showed that Myt TFs and Sin3 can form a transcription complex in ß cells. Inactivating these genes in mouse and human ß cells causes cell dysfunction and/or death while overactivating late-stage ß-cell-failure-causing SRGs but not early stage SRGs. Intriguingly, Myt TFs, particularly Myt3, is induced by obesity-related stressors in mouse and human ß cells, likely mediated by an uORF in 5’ flanking region of Myt3 mRNA. Importantly, MYT3 down-regulation accompanies human ß-cell failure in T2D development. Our overarching hypothesis is that the stress-responsive Myt TFs, particularly Myt3, promote ¿-cell function/survival by repressing late-stage SRGs via Sin3-mediated histone de-acetylation under both normal physiology and metabolic stress. Aim 1 will establish how MYT TFs repress SRG expression in a human ß cell line and how manipulating MYT-TF levels will affect primary human ß-cell function and survival. Aim 2 will define how metabolic stressors up-regulate Myt3 production and how this upregulation enable ß-cell compensation under metabolic stress. We expect to uncover a tunable mechanism that can be explored for preventing/delaying ß-cell failure and T2D.
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