mRNA decay factor AUF1 maintains normal aging, telomere maintenance, and suppression of senescence by activation of telomerase transcription

AUF1-RGG peptides up-regulate the VEGF antagonist, soluble VEGF receptor-1 (sFlt-1)

The macrophage is essential to the innate immune response, but also contributes to human disease by aggravating inflammation. Under severe inflammation, macrophages and other immune cells over-produce immune mediators, including vascular endothelial growth factor (VEGF). The VEGF protein stimulates macrophage activation and induces macrophage migration. A natural inhibitor of VEGF, the soluble VEGF receptor (sFlt-1) is also produced by macrophages and sFlt-1 has been used clinically to block VEGF. In macrophages, we have shown that the mRNA regulatory protein AUF1/hnRNP D represses VEGF gene expression by inhibiting translation of AURE-regulated VEGF mRNA. Peptides (AUF1-RGG peptides) that are modeled on the arginine-glycine-glycine (RGG) motif in AUF1 also block VEGF expression. This report shows that the AUF1-RGG peptides reduce two other AURE-regulated genes, TNF and GLUT1. Three alternative splice variants of sFlt-1 contain AURE in their 3'UTR, and in an apparent paradox, AUF1-RGG peptides stimulate expression of these three sFlt-1 Variants. The AUF1-RGG peptides likely act by distinct mechanisms with complimentary effects to repress VEGF gene expression and over-express the endogenous VEGF blocking agent, sFlt-1. The AUF1-RGG peptides are novel reagents that reduce VEGF and other inflammatory mediators, and may be useful tools to suppress severe inflammation.

The hallmarks of aging

Aging is characterized by a progressive loss of physiological integrity, leading to impaired function and increased vulnerability to death. This deterioration is the primary risk factor for major human pathologies, including cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. Aging research has experienced an unprecedented advance over recent years, particularly with the discovery that the rate of aging is controlled, at least to some extent, by genetic pathways and biochemical processes conserved in evolution. This Review enumerates nine tentative hallmarks that represent common denominators of aging in different organisms, with special emphasis on mammalian aging. These hallmarks are: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. A major challenge is to dissect the interconnectedness between the candidate hallmarks and their relative contributions to aging, with the final goal of identifying pharmaceutical targets to improve human health during aging, with minimal side effects.

Telomere length and telomerase activity; a Yin and Yang of cell senescence

Telomeres are repeating DNA sequences at the tip ends of the chromosomes that are diverse in length and in humans can reach a length of 15,000 base pairs. The telomere serves as a bioprotective mechanism of chromosome attrition at each cell division. At a certain length, telomeres become too short to allow replication, a process that may lead to chromosome instability or cell death. Telomere length is regulated by two opposing mechanisms: attrition and elongation. Attrition occurs as each cell divides. In contrast, elongation is partially modulated by the enzyme telomerase, which adds repeating sequences to the ends of the chromosomes. In this way, telomerase could possibly reverse an aging mechanism and rejuvenates cell viability. These are crucial elements in maintaining cell life and are used to assess cellular aging. In this manuscript we will describe an accurate, short, sophisticated and cheap method to assess telomere length in multiple tissues and species. This method takes advantage of two key elements, the tandem repeat of the telomere sequence and the sensitivity of the qRT-PCR to detect differential copy numbers of tested samples. In addition, we will describe a simple assay to assess telomerase activity as a complementary backbone test for telomere length.

NF90 coordinately represses the senescence-associated secretory phenotype

A hallmark trait of cellular senescence is the acquisition of a senescence-associated secretory phenotype (SASP). SASP factors include cytokines and their receptors (IL-6, IL-8, osteoprotegerin, GM-CSF), chemokines and their ligands (MCP-1, HCC4), and oncogenes (Gro1 and Gro2), many of them encoded by mRNAs whose stability and translation are tightly regulated. Using two models of human fibroblast senescence (WI-38 and IDH4 cells), we report the identification of RNA-binding protein NF90 as a post-transcriptional repressor of several SASP factors. In ‘young’, proliferating fibroblasts, NF90 was highly abundant, associated with numerous SASP mRNAs, and inhibited their expression. By contrast, senescent cells expressed low levels of NF90, thus allowing SASP factor expression to increase. NF90 elicited these effects mainly by repressing the translation of target SASP mRNAs, since silencing NF90 did not increase the steady-state levels of SASP mRNAs but elevated key SASP factors including MCP-1, GROa, IL-6, and IL-8. Our findings indicate that NF90 contributes to maintaining low levels of SASP factors in non-senescent cells, while NF90 reduction in senescent cells allows SASP factor expression to rise.

mRNA fate: Life and death of the mRNA in the cytoplasm

The life of an mRNA molecule begins with transcription and ultimately ends in degradation. In the course of its life, however, mRNA is examined, modified in various ways and transported before eventually being translated into proteins. All these processes are performed by proteins and non-coding RNAs whose complex interplay in the cell contributes to determining the proteome changes and the phenotype of cells. On May 23‒26, 2012, over 150 scientists from around the world convened in the sunny shores of Riva del Garda, Italy, for the workshop entitled: "mRNA fate: Life and Death of mRNA in the Cytoplasm." Sessions included mRNA trafficking, mRNA translational control, RNA metabolism and disease, RNA-protein structures and systems biology of RNA. This report highlights some of the prominent and recurring themes at the meeting and emerging arenas of future research.

WITHDRAWN: AUF1-RGG peptides up-regulate the VEGF antagonist, soluble VEGF receptor-1 (sFlt-1)

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Splicing factor SF2/ASF rescues IL-2 production in T cells from systemic lupus erythematosus patients by activating IL-2 transcription

T cells from patients with systemic lupus erythematosus (SLE) produce insufficient amounts of the vital cytokine IL-2. We previously showed that SLE T cells express decreased levels of the T-cell receptor-CD3ζ chain and forced expression of CD3ζ into SLE T cells restores IL-2 production. We recently showed that the serine arginine protein splicing factor 2/alternative splicing factor (SF2/ASF) enhances the expression of CD3ζ chain by limiting the production of an unstable splice variant. Here we demonstrate that SF2/ASF levels are decreased in patients with SLE and more so in those with active disease. More importantly, we reveal a function of SF2/ASF, independent of T-cell receptor/CD3 signaling, whereby it is recruited to the IL-2 promoter, increases transcriptional activity, and enhances IL-2 production in SLE T cells. Our results demonstrate that SF2/ASF regulates IL-2 production and that decreased SF2/ASF expression in SLE T cells contributes to deficient IL-2 production.

Post-transcriptional control of gene expression by AUF1: mechanisms, physiological targets, and regulation

AUF1 is a family of four proteins generated by alternative pre-mRNA splicing that form high affinity complexes with AU-rich, mRNA-destabilizing sequences located within the 3' untranslated regions of many labile mRNAs. While AUF1 binding is most frequently associated with accelerated mRNA decay, emerging examples have demonstrated roles as a mRNA stabilizer or even translational regulator for specific transcripts. In this review, we summarize recent advances in our understanding of mRNA recognition by AUF1 and the biochemical and functional consequences of these interactions. In addition, unique properties of individual AUF1 isoforms and the roles of these proteins in modulating expression of genes associated with inflammatory, neoplastic, and cardiac diseases are discussed. Finally, we describe mechanisms that regulate AUF1 expression in cells, and current knowledge of regulatory switches that modulate the cellular levels and/or activities of AUF1 isoforms through distinct protein post-translational modifications. This article is part of a Special Issue entitled: RNA Decay mechanisms.

AUF1/hnRNP D is a novel protein partner of the EBER1 noncoding RNA of Epstein-Barr virus

Epstein-Barr virus (EBV)-infected cells express two noncoding RNAs called EBV-encoded RNA (EBER) 1 and EBER2. Despite their high abundance in the nucleus (about 10(6) copies), the molecular function of these noncoding RNAs has remained elusive. Here, we report that the insertion into EBER1 of an RNA aptamer that binds the bacteriophage MS2 coat protein allows the isolation of EBER1 and associated protein partners. By combining MS2-mediated selection with stable isotope labeling of amino acids in cell culture (SILAC) and analysis by mass spectrometry, we identified AUF1 (AU-rich element binding factor 1)/hnRNP D (heterogeneous nuclear ribonucleoprotein D) as an interacting protein of EBER1. AUF1 exists as four isoforms generated by alternative splicing and is best known for its role in destabilizing mRNAs upon binding to AU-rich elements (AREs) in their 3' untranslated region (UTR). Using UV crosslinking, we demonstrate that predominantly the p40 isoform of AUF1 interacts with EBER1 in vivo. Electrophoretic mobility shift assays show that EBER1 can compete for the binding of the AUF1 p40 isoform to ARE-containing RNA. Given the high abundance of EBER1 in EBV-positive cells, EBER1 may disturb the normal homeostasis between AUF1 and ARE-containing mRNAs or compete with other AUF1-interacting targets in cells latently infected by EBV.

AUF1/HnRNP D RNA binding protein functions in telomere maintenance

In this issue of Molecular Cell, Pont et al. (2012) show that AUF1/hnRNP D promotes TERT transcription, which is required for telomere maintenance in mice.

Concurrent binding and modifications of AUF1 and HuR mediate the pH-responsive stabilization of phosphoenolpyruvate carboxykinase mRNA in kidney cells

Onset of metabolic acidosis leads to a pronounced increase in renal expression of phosphoenolpyruvate carboxykinase (PEPCK). This response, which is mediated in part by stabilization of PEPCK mRNA, is effectively modeled by treating LLC-PK(1)-F(+)-9C cells with an acidic medium. siRNA knockdown of HuR prevented the pH-responsive increase in PEPCK mRNA half-life suggesting that HuR is necessary for this response. A recruitment assay, using a reporter mRNA in which the pH response elements of the PEPCK 3'-UTR were replaced with six MS2 stem-loop sequences, was developed to test this hypothesis. The individual recruitment of a chimeric protein containing the MS2 coat protein and either HuR or p40AUF1 failed to produce a pH-responsive stabilization. However, the concurrent expression of both chimeric proteins was sufficient to produce a pH-responsive increase in the half-life of the reporter mRNA. siRNA knockdown of AUF1 produced slight increases in basal levels of PEPCK mRNA and protein, but partially inhibited the pH-responsive increases. Complete inhibition of the latter response was achieved by knockdown of both RNA-binding proteins. The results suggest that binding of HuR and AUF1 has opposite effects on basal expression, but may interact to mediate the pH-responsive increase in PEPCK mRNA. Two-dimensional gel electrophoresis indicated that treatment with acidic medium caused a decrease in phosphorylation of HuR, but may increase phosphorylation of the multiple AUF1 isoforms. Thus, the pH-responsive stabilization of PEPCK mRNA requires the concurrent binding of HuR and AUF1 and may be mediated by changes in their extent of covalent modification.