Posttranscriptional Regulation of PARG mRNA by HuR Facilitates DNA Repair and Resistance to PARP Inhibitors

Abstract

The majority of pancreatic ductal adenocarcinomas (PDA) rely on the mRNA stability factor HuR (ELAV-L1) to drive cancer growth and progression. Here we show that CRISPR-Cas9-mediated silencing of the HuR locus increases the relative sensitivity of PDA cells to PARP inhibitors (PARPi). PDA cells treated with PARPi stimulated translocation of HuR from the nucleus to the cytoplasm, specifically promoting stabilization of a new target, polyADP-ribose glycohydrolase (PARG) mRNA, by binding a unique sequence embedded in its 3′ untranslated region (UTR). HuR-dependent upregulation of PARG expression facilitated DNA repair via hydrolysis of polyADP-ribose on related repair proteins. Accordingly, strategies to inhibit HuR directly promoted DNA damage accumulation, inefficient PAR removal, and persistent PARP-1 residency on chromatin (PARP-1 trapping). Immunoprecipitation assays demonstrated that the PARP1 protein binds and post-translationally modifies HuR in PARPi-treated PDA cells. In a mouse xenograft model of human PDA, PARPi monotherapy combined with targeted silencing of HuR significantly reduced tumor growth compared to PARPi therapy alone. Our results highlight the HuR-PARG axis as an opportunity to enhance PARPi-based therapies.

Interleukin‐4‒induced posttranscriptional gene regulation of CCL26 by the RNA‐binding protein HuR in primary human nasal polyp‒derived epithelial cells

“…In the process, HuR is thought to bind to target mRNA and then form an HuR‐mRNA complex in the nucleus, which is then exported to the cytoplasm to protect the bound mRNA from degradation. There is mounting experimental evidence suggesting that there is an increase in cytoplasmic HuR after stimulation, which is considered a crucial step in the increased stability of many HuR‐bound mRNAs . In accordance with this model, our immunofluorescent staining findings indicate that IL‐4 can also regulate HuR for nuclear shuttle.…”

“…The posttranscriptional events associated with adenylate‐ and uridylate‐rich elements (AREs)‒containing gene are recognized as important control points in gene regulation . The mechanisms of mRNA decay regulation are closely associated with specific target regions, including AREs in the 3' untranslated regions (3'UTRs) of mRNA .…”

PARP‐1 regulates DNA repair factor availability

“…Furthermore, poly(ADP‐ribose) glycohydrolase (PARG), which hydrolyzes PAR moieties, harbors patient‐derived alterations of unknown relevance, which may impact PARP‐1 activation status by differentially regulating PAR levels. It has recently been reported that PARG impacts the response to PARPi in models of pancreatic cancer (Chand et al , ). Irrespective of the mechanism(s) by which PARP‐1 is hyperactivated in advanced PCa, studies described herein yield novel insights into the downstream functions of elevated PARP‐1 activity.…”

Connections between 3′ end processing and DNA damage response: Ten years later

“…Transcriptome wide analysis of samples from cells exposed to IR radiation revealed a decrease in HuR binding to its target mRNAs due to activated CHK2‐mediated phosphorylation (Masuda et al, ). Upon genotoxic stress, HuR can be PARylated by poly(ADP‐ribose) polymerase 1 (PARP1) facilitating its cytoplasmic translocation and regulation of its binding to target mRNAs (Chand et al, ; Gagné et al, ; Y. Ke et al, ). dePARylation of HuR facilitates its release from target mRNAs and its shuttling back into the nucleus.…”