AU-rich element (ARE) motifs are cis-acting elements present in the 3′UTR of mRNA transcripts that encode many inflammation-and cancer-associated genes. The TIS11 family of RNA-binding proteins, composed of TTP, BRF-1, and BRF-2 play a critical role in regulating the expression of ARE-containing mRNAs. Through their ability to bind and target ARE-containing mRNAs for rapid degradation, this class of RNA-binding proteins serves a fundamental role in limiting the expression of a number of critical genes, thereby exerting anti-inflammatory and anti-cancer effects. Regulation of TIS11 family members occurs on a number of levels through cellular signaling events to control their transcription, mRNA turnover, phosphorylation status, cellular localization, association with other proteins, and proteosomal degradation, all of which impact TIS11 members' ability to promote ARE-mediated mRNA decay along with decay-independent functions. This review summarizes our current understanding of post-transcriptional regulation of ARE-containing gene expression by TIS11 family members and discuss their role in maintaining normal physiological processes and the pathological consequences in their absence. KeywordsTristetraprolin; Butyrate Response Factor; AU-rich element; mRNA decay; post-transcriptional regulation Messenger RNA turnover is a tightly regulated process that is critical in controlling mammalian gene expression. The importance of this level of regulation is evident in a variety of diseases where loss of post-transcriptional gene regulation directly contributes to the overexpression of many genes encoding growth factors, inflammatory cytokines, and proto-oncogenes [1,2]. A characteristic feature present within the 3′ untranslated region (3′UTR) of these mRNAs is the adenylate-and uridylate (AU)-rich element (ARE). The significance of this conserved cis-acting RNA element is apparent in its frequency since it is currently estimated that approximately 8% of the human transcriptome contains AREs [3]. A primary function of the ARE is to target specific mRNAs for rapid decay through interaction with trans-acting RNA-binding proteins. Initially discovered in 1989 [4], the tristetraprolin (TTP) protein and its related family members butyrate response factors 1 and 2 (BRF-1 and * Correspondence: Dan A. Dixon, Department of Biological Sciences and Cancer Research Center, University of South Carolina, 712 Main St., Jones Physical Sciences Center, Room 614, Columbia, SC 29208, ddixon@biol.sc.edu, Tel: 803-777-4686; Fax: 803-777-1173 . Cross-ReferencesOverview -RNA decay as a major mediator of gene expression and QC Overview -RNA decay in bacteria and eukaryotes Cis acting elements that regulate mRNA decay Networking between mRNA decay and other cellular processes RNA-binding domains-Zn-F Mechanisms of deadenylation-dependent decay Stress granules and P bodies NIH Public Access Author ManuscriptWiley Interdiscip Rev RNA. Author manuscript; available in PMC 2011 January 28. NIH-PA Author ManuscriptNIH-PA Author Manuscript ...

Messenger RNA decay is a critical mechanism to control the expression of many inflammation- and cancer-associated genes. These transcripts are targeted for rapid degradation through AU-rich element (ARE) motifs present in the mRNA 3’ untranslated region (3’UTR). Tristetraprolin (TTP) is an RNA-binding protein that plays a significant role in regulating the expression of ARE-containing mRNAs. Through its ability to bind AREs and target the bound mRNA for rapid degradation, TTP can limit the expression of a number of critical genes frequently overexpressed in inflammation and cancer. Regulation of TTP occurs on multiple levels through cellular signaling events to control transcription, mRNA turnover, phosphorylation status, cellular localization, association with other proteins, and proteosomal degradation, all of which impact TTP’s ability to promote ARE-mediated mRNA decay along with decay-independent functions of TTP. This review summarizes the current understanding of post-transcriptional regulation of ARE-containing gene expression by TTP and discusses its role in maintaining homeostasis and the pathological consequences of losing TTP expression.

Previously, it has been shown that pancreatic ductal adenocarcinoma (PDA) tumors exhibit high levels of hypoxia, characterized by low oxygen pressure (pO2) and decreased O2 intracellular perfusion. Chronic hypoxia is strongly associated with resistance to cytotoxic chemotherapy and chemoradiation in an understudied phenomenon known as hypoxia-induced chemoresistance. The hypoxia-inducible, pro-oncogenic, serine-threonine kinase PIM1 (Proviral Integration site for Moloney murine leukemia virus 1) has emerged as a key regulator of hypoxia-induced chemoresistance in PDA and other cancers. Although its role in therapeutic resistance has been described previously, the molecular mechanism behind PIM1 overexpression in PDA is unknown. Here, we demonstrate that cis-acting AU-rich elements (ARE) present within a 38-base pair region of the PIM1 mRNA 3'-untranslated region mediate a regulatory interaction with the mRNA stability factor HuR (Hu antigen R) in the context of tumor hypoxia. Predominantly expressed in the nucleus in PDA cells, HuR translocates to the cytoplasm in response to hypoxic stress and stabilizes the PIM1 mRNA transcript, resulting in PIM1 protein overexpression. A reverse-phase protein array revealed that HuR-mediated regulation of PIM1 protects cells from hypoxic stress through phosphorylation and inactivation of the apoptotic effector BAD and activation of MEK1/2. Importantly, pharmacological inhibition of HuR by MS-444 inhibits HuR homodimerization and its cytoplasmic translocation, abrogates hypoxia-induced PIM1 overexpression and markedly enhances PDA cell sensitivity to oxaliplatin and 5-fluorouracil under physiologic low oxygen conditions. Taken together, these results support the notion that HuR has prosurvival properties in PDA cells by enabling them with growth advantages in stressful tumor microenvironment niches. Accordingly, these studies provide evidence that therapeutic disruption of HuR's regulation of PIM1 may be a key strategy in breaking an elusive chemotherapeutic resistance mechanism acquired by PDA cells that reside in hypoxic PDA microenvironments.

Cancer aggressiveness may result from the selective pressure of a harsh nutrient-deprived microenvironment. Here we illustrate how such conditions promote chemotherapy resistance in pancreatic ductal adenocarcinoma (PDAC). Glucose or glutamine withdrawal resulted in a 5- to 10-fold protective effect with chemotherapy treatment. PDAC xenografts were less sensitive to gemcitabine in hypoglycemic mice compared with hyperglycemic mice. Consistent with this observation, patients receiving adjuvant gemcitabine (n = 107) with elevated serum glucose levels (HgbA1C > 6.5%) exhibited improved survival. We identified enhanced antioxidant defense as a driver of chemoresistance in this setting. ROS levels were doubled in vitro by either nutrient withdrawal or gemcitabine treatment, but depriving PDAC cells of nutrients before gemcitabine treatment attenuated this effect. Mechanistic investigations based on RNAi or CRISPR approaches implicated the RNA binding protein HuR in preserving survival under nutrient withdrawal, with or without gemcitabine. Notably, RNA deep sequencing and functional analyses in HuR-deficient PDAC cell lines identified isocitrate dehydrogenase 1 (IDH1) as the sole antioxidant enzyme under HuR regulation. HuR-deficient PDAC cells lacked the ability to engraft successfully in immunocompromised mice, but IDH1 overexpression in these cells was sufficient to fully restore chemoresistance under low nutrient conditions. Overall, our findings highlight the HuR–IDH1 regulatory axis as a critical, actionable therapeutic target in pancreatic cancer.