Tongjian Zhao, Nils Welsh
Abstract Type 1 and type 2 diabetes are characterized by beta cell dysfunction and insulin deficiency, and the appropriate turnover of MYC expression appears to control the beta-cell fate. Therefore, it is important to better understand the mechanism that controls MYC expression. Hence, the current study aimed to elucidate the interactions of the long non-coding RNA molecule PHAROH at the interface of MYC mRNA and the RNA-binding protein TIA1. For this purpose, we studied MYC mRNA levels in glucagon-producing alpha-TC1-6 cells, insulin-producing MIN6 cells, and in human pancreatic islets. We observed that in insulin-producing cells, but not in alpha cells, MYC mRNA levels were increased in response to stress (proinflammatory cytokines or palmitate + high glucose). Levels of PHAROH were also increased in insulin-producing cells, but not in glucagon-producing cells. To evaluate whether the increased PHAROH levels participated in the induction of MYC mRNA, we treated MIN6 cells with synthetic antisense LNA GapmeR oligonucleotides (ASO) designed to target PHAROH. We observed that the PHAROH ASO 1 increased both PHAROH and MYC mRNA expression. MYC protein levels were also increased by PHAROH ASO 1, both in the absence and presence of pro-inflammatory cytokines. Using an RNA immunoprecipitation strategy, we observed that PHAROH ASO 1 decreased MYC mRNA binding to TIA1, and instead increased PHAROH binding to TIA1. It is concluded that beta cells increase their MYC expression during stress and that this occurs via increased PHAROH-mediated sequestration of TIA1 and subsequent release of MYC mRNA from TIA1-induced inhibition.