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

The majority of pancreatic ductal adenocarcinomas (PDAC) 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 PDAC cells to PARP inhibitors (PARPi). PDAC cells treated with PARPi stimulated translocation of HuR from the nucleus to the cytoplasm, specifically promoting stabilization of a new target, poly (ADP-ribose) glycohydrolase (PARG) mRNA, by binding a unique sequence embedded in its 3' untranslated region. 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 PARP-1 protein binds and posttranslationally modifies HuR in PARPi-treated PDAC cells. In a mouse xenograft model of human PDAC, PARPi monotherapy combined with targeted silencing of HuR significantly reduced tumor growth compared with PARPi therapy alone. Our results highlight the HuR-PARG axis as an opportunity to enhance PARPi-based therapies. Cancer Res; 77(18); 5011-25. ©2017 AACR.

Abstract 1995: HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells

Introduction: Despite our deep understanding of genetic drivers of the disease, pancreatic ductal adenocarcinoma (PDA) continues to be associated with dismal survival rates. Targeting the DNA repair machinery has emerged as a promising therapeutic strategy to treat pancreatic cancer patients carrying DNA damage repair (DDR) mutations. Such mutations promote tumorigenesis, but also paradoxically render tumor cells particularly susceptible to platinum-based agents and PARP inhibitors (PARPi). However, despite promising preclinical and clinical results, early data demonstrate that eventually most tumors, regardless of DDR status, become resistant to PARPi-therapies.

The mRNA-binding protein HuR, predominantly expressed in the nucleus, translocates to the cytoplasm upon tumor-associated stress where it post-transcriptionally regulates select mRNA cargo, resulting in resistance to DNA damaging agents in a harsh tumor microenvironment. Here, we sought to evaluate the role of HuR in regulating PARPi efficacy.

Results: In response to cellular stress induced by IC50 dosages of a panel of PARPis such as Veliparib, Olaparib, Rucaparib, Talazoparib and Niraparib, nuclear localized HuR undergoes cytoplasmic translocation. Silencing of HuR via siRNA, CRISPR and a DOX-inducible system resulted in significant decrease in long and short- term PDA cell survival, irrespective of DDR status. To complement and validate in vitro findings, we employed a heterotropic mouse xenograft model using Mia.sh290 cells wherein DOX induction significantly reduced HuR expression. Olaparib mediated PARP inhibition (50mg/kg, 5 days a week) combined with DOX-induced HuR silencing resulted in significant reduction in tumor volumes, compared to Olaparib alone or DOX alone.

Mechanistically, we demonstrate that the pro-survival protein HuR facilitates PDA cells to recover from PARPi insult by, in part, regulating poly ADP ribose glycohydrolase (PARG), the major enzyme responsible for hydrolyzing poly-ADP ribose (PAR) polymers, on chromatin and associated proteins. HuR binds to two 41- 43bp long sites in the 3’ untranslated region (3’UTR) of PARG, increasing its mRNA stability and protein expression. Increased PARG activity, further validated via exogenous overexpression, promotes DNA repair efficiency and increases PDA cell survival. Functional analysis indicate that such inhibition of HuR and/or PARG significantly enhances PARPi sensitivity in PDA cells, via increased accumulation of DNA damage γH2AX foci, preventing efficient removal of PAR polymers, and enhancing detrimental trapping of PARP1 on chromatin.

Conclusions: Taken together, our results indicate that HuR- mediated upregulation of PARG acts as a universal pro-survival mechanism and HuR inhibition could significantly potentiate PARPi therapy in PDA, irrespective of DNA repair status.

Citation Format: Saswati N. Chand, Mahsa Zarei, Akshay R. Kamath, Matthew J. Schiewer, Carmella Romeo, Joseph A. Cozzitorto, Nicole Meisner- Kober, Eric Londin, Isidore Rigoutsos, Karen Knudsen, John M. Pascal, Charles J. Yeo, Jordan M. Winter, Jonathan R. Brody. HuR dependent inhibition of PARG enhances PARP inhibitor therapy for DNA repair proficient and deficient pancreatic cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1995.

Abstract 5476: A novel PARP inhibitor resistance mechanism mediated by the RNA-binding protein HuR

Introduction: Pancreatic ductal adenocarcinoma (PDA) is the 4th leading cause of cancer-related deaths in the United States, and the 3rd most common cancer associated with BRCA mutations. Frontline therapies have significant toxicities and only minimally extend overall survival, highlighting the need to optimize targeted therapies.

Poly-ADP ribose polymerase (PARP) inhibitors (PARPi), a ‘poster child’ for personalized medicine, depend on the concept of synthetic lethality where the combined perturbation of DNA repair genes, via genetic mutations within the tumor cells, and pharmacological PARP inhibition effectively targets BRCA-deficient tumors. Although PARPi have delivered promising preclinical and clinical results, initially- responsive patients ultimately develop resistance.

A unique mechanism elucidated by our lab demonstrates that the mRNA-binding protein HuR mediates resistance to DNA damaging agents through post-transcriptional regulation of select mRNA cargo. Predominantly expressed in the nucleus, HuR translocates to the cytoplasm upon tumor-associated stress. Cytoplasmic HuR binds and stabilizes unique pro-survival transcripts, resulting in resistance to a harsh tumor microenvironment. Here, we sought to evaluate the role of HuR in regulating PARPi response.

Methods and Results: Through immunofluorescence and western blot of fractionated lysates, we demonstrate that the PARP inhibitors Veliparib, Olaparib, and Rucaparib induced cytoplasmic HuR localization. Conversely, pre-treatment with MS-444 (Novartis), an established small molecule inhibitor of HuR, abrogated its nuclear export induced by PARPi treatment. Consistent with these findings, the growth-inhibitory effects of PARPi treatment were significantly potentiated upon HuR silencing whereas ectopic HuR overexpression promoted resistance, as observed in short term cell survival and long-term anchorage-independent growth assays. Additionally, silencing of HuR enhanced PARPi-induced cytotoxicity, assessed by increased accumulation of DNA damage (γH2Ax) foci and Poly ADP-ribose (PAR) polymers. Ribonucleotide protein immunoprecipitation (RNP-IP) assays demonstrated that HuR binds and upregulates Poly-ADP Ribose Glycohydrolase (PARG) mRNA, the major enzyme responsible for catabolism of PAR. Taken together, when PDA cells are exposed to PARPi, HuR mediates upregulation of PARG, thereby decreasing PARylation and facilitating DNA repair. Conversely, HuR inhibition results in detrimental accumulation of PAR and enhanced DNA damage, which ultimately leads to increased PARPi-conferred cytotoxicity.

Discussion: These results demonstrate that HuR imposes a significant barrier to PARPi therapy by orchestrating a strong chemoresistance mechanism. Thus, we provide evidence that HuR (and/or its target) inhibition via an HuR inhibitor (MS-444) can optimize PARPi-based therapies for better patient outcomes.

Citation Format: Saswati N. Chand, Akshay R. Kamath, Nicole Meisner-Kober, Charles J. Yeo, Jordan M. Winter, Jonathan R. Brody. A novel PARP inhibitor resistance mechanism mediated by the RNA-binding protein HuR. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5476. doi:10.1158/1538-7445.AM2015-5476