• Characterization of the stress inducible proteinhnRNP A18 and its role in post-transcriptional regulation of thioredoxin

      Yang, Ruiqing; Weber, David J., Ph.D. (2006)
      Thioredoxin (TRX) is a key protein of the cellular redox metabolism, which expression is increased in several tumors especially gastric tumors. Even though UV and hypoxia specifically induce TRX, the mechanisms that lead to increased TRX levels are still ill defined. We have previously shown that the UV-inducible RNA-binding protein A18 (hnRNP A18) plays a protective role in the genotoxic stress response. It has been shown before that A18 increases mRNA stability and enhances translation of stress-responsive transcripts such as the replication protein A (RPA2). Here, we have further investigated the role of A18 hnRNP in oxidative stress response through a model containing A18 hnRNP and its mRNA target thioredoxin (TRX). Here, we show that the hnRNP A18 RNA Binding Domain (RBD) and the arginine, glycine (RGG) rich domain can bind TRX 3'UTR independently but both domains are required for maximal binding. Immunoprecipitation of hnRNP A18-mRNAs complexes and co-localization of hnRNP A18 and TRX transcripts on ribosomal fractions confirm the interaction of hnRNP A18 with TRX transcripts in cells. Moreover, down regulation of hnRNP A18 correlates with a significant reduction of TRX protein levels. In addition, hnRNP A18 increases TRX translation and interacts with the eukaryotic Initiation Factor 4G (eIF4G), a component of the general translational machinery. Furthermore, hnRNP A18 phosphorylation by the hypoxia inducible GSK3beta increases hnRNP A18 RNA-binding activity in vitro and in RKO cells in response to UV radiation. These data support a regulatory role for hnRNP A18 in TRX post-transcriptional expression possibly through a kissing loop model bridging TRX 3' and 5' UTRs through eIF4G.