Loss of HuR is linked to reduced expression of proliferative genes during replicative senescence

Tissue- and age-dependent expression of RNA-binding proteins that influence mRNA turnover and translation

Gene expression patterns vary dramatically in a tissue-specific and age-dependent manner. RNA-binding proteins that regulate mRNA turnover and/or translation (TTR-RBPs) critically affect the subsets of expressed proteins. However, very little is known regarding the tissue- and age-dependent expression of TTR-RBPs in humans. Here, we use human tissue arrays containing a panel of organ biopsies from donors of different ages, to study the distribution and abundance of four TTR-RBPs: HuR, AUF1, TIA-1, and TTP. HuR and AUF1 were expressed with remarkably similar patterns. Both TTR-RBPs were present in high percentages of cells and displayed elevated intensities in many age groups and tissues, most notably in the gastrointestinal and reproductive systems; they were moderately expressed in the urinary and immune systems, and were almost undetectable in muscle and brain. TIA-1 was also abundant in many tissues and age groups; TIA-1 was expressed at high levels in the gastrointestinal, immune, urinary, and reproductive systems, and at low levels in brain and muscle. By contrast, TTP-expressing cells, as well as TTP signal intensities declined with advancing age, particularly in the immune, nervous, and muscular systems; however, TTP levels remained elevated in the gastrointestinal tract. The widespread abundance of HuR, AUF1, and TIA-1 throughout the body and in all age groups was in stark contrast with their declining levels in human diploid fibroblasts (HDFs) undergoing replicative senescence, a cultured-cell model of aging. Conversely, TTP levels increased in senescent HDFs, while TTP levels decreased with advancing age. Our studies provide a framework for the study of human TTR-RBP function in different tissues, throughout the human life span.

Alternative polyadenylation variants of the RNA binding protein, HuR: abundance, role of AU-rich elements and auto-Regulation

The RNA-binding protein, HuR, is involved in the stabilization of AU-rich element-containing mRNAs with products that are involved in cell-cycle progression, cell differentiation and inflammation. We show that there are multiple polyadenylation variants of HuR mRNA that differ in their abundance, using both bioinformatics and experimental approaches. A polyadenylation variant with distal poly(A) signal is a rare transcript that harbors functional AU-rich elements (ARE) in the 3'UTR. A minimal 60-nt region, but not a mutant form, fused to reporter-3'UTR constructs was able to downregulate the reporter activity. The most predominant and alternatively polyadenylated mature transcript does not contain the ARE. HuR itself binds HuR mRNA, and upregulated the activity of reporter from constructs fused with ARE-isoform and the HuR ARE. Wild-type tristetraprolin (TTP), but not the zinc finger mutant TTP, competes for HuR binding and upregulation of HuR mRNA. The study shows that the HuR gene codes for several polyadenylation variants differentially regulated by AU-rich elements, and demonstrates an auto-regulatory role of HuR.

RNase L downmodulation of the RNA-binding protein, HuR, and cellular growth

Ribonuclease L (RNase L) is an intracellular enzyme that is vital in innate immunity, but also is a tumor suppressor candidate. Here, we show that overexpression of RNase L decreases cellular growth and downmodulates the RNA-binding protein, HuR, a regulator of cell-cycle progression and tumorigenesis. The effect is temporal, occurring in specific cell-cycle phases and correlated with the cytoplasmic localization of RNase L. Both cellular growth and HuR were increased in RNASEL-null mouse fibroblast lines when compared to wild-type cells. Moreover, the stability of HuR mRNA was enhanced in RNASEL-null cells. The HuR 3' untranslated region (UTR), which harbors U-rich and adenylate-uridylate-rich elements, was potently responsive to RNase L when compared to control 3' UTR. Our results may offer a new explanation to the tumor suppressor function of RNase L.

Ubiquitin-mediated proteolysis of HuR by heat shock

The RNA-binding protein HuR regulates the stability and translation of numerous mRNAs encoding stress-response and proliferative proteins. Although its post-transcriptional influence has been linked primarily to its cytoplasmic translocation, here we report that moderate heat shock (HS) potently reduces HuR levels, thereby altering the expression of HuR target mRNAs. HS did not change HuR mRNA levels or de novo translation, but instead reduced HuR protein stability. Supporting the involvement of the ubiquitin-proteasome system in this process were results showing that (1) HuR was ubiquitinated in vitro and in intact cells, (2) proteasome inhibition increased HuR abundance after HS, and (3) the HuR kinase checkpoint kinase 2 protected against the loss of HuR by HS. Within a central, HS-labile approximately 110-amino-acid region, K182 was found to be essential for HuR ubiquitination and proteolysis as mutant HuR(K182R) was left virtually unubiquitinated and was refractory to HS-triggered degradation. Our findings reveal that HS transiently lowers HuR by proteolysis linked to K182 ubiquitination and that HuR reduction enhances cell survival following HS.

Evidence for LKB1/AMP-activated protein kinase/ endothelial nitric oxide synthase cascade regulated by hepatocyte growth factor, S-adenosylmethionine, and nitric oxide in hepatocyte proliferation

S-adenosylmethionine (SAMe) is involved in numerous complex hepatic processes such as hepatocyte proliferation, death, inflammatory responses, and antioxidant defense. One of the most relevant actions of SAMe is the inhibition of hepatocyte proliferation during liver regeneration. In hepatocytes, SAMe regulates the levels of cytoplasmic HuR, an RNA-binding protein that increases the half-life of target messenger RNAs such as cyclin D1 and A2 via inhibition of hepatocyte growth factor (HGF)-mediated adenosine monophosphate-activated protein kinase (AMPK) phosphorylation. Because AMPK is activated by the tumor suppressor kinase LKB1, and AMPK activates endothelial nitric oxide (NO) synthase (eNOS), and NO synthesis is of great importance for hepatocyte proliferation, we hypothesized that in hepatocytes HGF may induce the phosphorylation of LKB1, AMPK, and eNOS through a process regulated by SAMe, and that this cascade might be crucial for hepatocyte growth. We demonstrate that the proliferative response of hepatocytes involves eNOS phosphorylation via HGF-mediated LKB1 and AMPK phosphorylation, and that this process is regulated by SAMe and NO. We also show that knockdown of LKB1, AMPK, or eNOS with specific interference RNA (iRNA) inhibits HGF-mediated hepatocyte proliferation. Finally, we found that the LKB1/AMPK/eNOS cascade is activated during liver regeneration after partial hepatectomy and that this process is impaired in mice treated with SAMe before hepatectomy, in knockout mice deficient in hepatic SAMe, and in eNOS knockout mice.

CONCLUSION

We have identified an LKB1/AMPK/eNOS cascade regulated by HGF, SAMe, and NO that functions as a critical determinant of hepatocyte proliferation during liver regeneration after partial hepatectomy.

Oxidative Stress Increases the Number of Stress Granules in Senescent Cells and Triggers a Rapid Decrease in p21waf1/cip1 Translation

Very little is known as to how the accumulation of senescent cells during aging may affect our ability to cope with various stresses. Here we show that the assembly of stress granules (SGs) is part of the early events used by senescent cells to respond to certain stresses. Although SGs can form in response to stress during senescence activation, their number significantly increases once the cells are fully senescent. This increase correlates with a rapid decrease in the expression levels of the cyclin kinase inhibitor p21, an important activator of senescence. Throughout stress, p21 mRNA is stabilized and localizes to SGs, but only during late senescence does this localization interferes with its translation. Additionally, we observed that when the stress is relieved, senescent cells produce lower levels of p21 protein, which correlates with a small delay in SG disassembly. Therefore, our data suggest that SG formation and the reduction in p21 protein levels represent two main events by which senescent cells respond to stress.

miR-519 reduces cell proliferation by lowering RNA-binding protein HuR levels

Gene expression is potently regulated through the action of RNA-binding proteins (RBPs) and microRNAs (miRNAs). Here, we present evidence of a miRNA regulating an RBP. The RBP HuR can stabilize and modulate the translation of numerous target mRNAs involved in cell proliferation, but little is known about the mechanisms that regulate HuR abundance. We identified two putative sites of miR-519 interaction on the HuR mRNA, one in its coding region (CR), one in its 3'-untranslated region (UTR). In several human carcinoma cell lines tested, HeLa (cervical), HCT116 and RKO (colon), and A2780 (ovarian), overexpression of a miR-519 precursor [(Pre)miR-519] reduced HuR abundance, while inhibiting miR-519 by using an antisense RNA [(AS)miR-519] elevated HuR levels. The influence of miR-519 was recapitulated using heterologous reporter constructs that revealed a greater repressive effect on the HuR CR than the HuR 3'-UTR target sequences. miR-519 did not alter HuR mRNA abundance, but reduced HuR biosynthesis, as determined by measuring nascent HuR translation and HuR mRNA association with polysomes. Modulation of miR-519 leading to altered HuR levels in turn affected the levels of proteins encoded by HuR target mRNAs. In keeping with HuR's proliferative influence, (AS)miR-519 significantly increased cell number and [(3)H]-thymidine incorporation, while (Pre)miR-519 reduced these parameters. Importantly, the growth-promoting effects of (AS)miR-519 required the presence of HuR, because downregulation of HuR by RNAi dramatically suppressed its proliferative action. In sum, miR-519 represses HuR translation, in turn reducing HuR-regulated gene expression and cell division.

NF-kappaB activates transcription of the RNA-binding factor HuR, via PI3K-AKT signaling, to promote gastric tumorigenesis

BACKGROUND & AIMS

HuR is a RNA-binding factor whose expression is commonly upregulated in some human tumor types. We explored the molecular mechanism underlying HuR elevation and its role in gastric cancer tumorigenesis.

METHODS

HuR expression and subcellular localization were determined by polymerase chain reaction, immunoblot, and immunohistochemical analyses. Its effect on tumor growth was characterized using flow cytometry, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, and soft agar analyses. Luciferase reporter, chromatin immunoprecipitation, and electrophoretic mobility shift assays were used to measure transcriptional activation by nuclear factor kappaB (NF-kappaB) signaling.

RESULTS

Compared with normal gastric tissues, HuR was expressed at higher levels in gastric tumors, particularly in advanced versus early tumors; this increase was associated with enhanced cytoplasmic translocation of HuR. HuR overexpression increased proliferation of tumor cells, activating the G(1) to S transition of the cell cycle, DNA synthesis, and anchorage-independent growth. Small interfering RNA-mediated knockdown of HuR expression reduced tumor cell proliferation and response to apoptotic stimuli. No genetic or epigenetic alterations of HuR were observed in gastric tumor cell lines or primary tumors; overexpression depended on phosphatidylinositol 3-kinase/AKT signaling and NF-kappaB activity. AKT activation increased p65/RelA binding to a putative NF-kappaB binding site in the HuR promoter, the stability of HuR target transcripts, and the cytoplasmic import of HuR.

CONCLUSIONS

HuR is a direct transcription target of NF-kappaB; its activation in gastric cancer cell lines depends on phosphatidylinositol 3-kinase/AKT signaling. HuR activation by this pathway has proliferative and antiapoptotic effects on gastric cancer cells.

Phosphorylated HuR shuttles in cycles

HuR is a ubiquitous RNA-binding protein (RBP) that associates with many mRNAs encoding proliferative proteins. Although predominantly nuclear, HuR translocation to the cytoplasm is linked to its ability to stabilize target mRNAs and modulate their translation. We recently reported that HuR phosphorylation by Cdk1 at S202 (within the HuR hinge region that is necessary for nucleocytoplasmic shuttle) increases HuR association with 14-3-3 and contributes to its nuclear retention. In the next issue of Cell Cycle we report that residue S242 also regulates HuR's cytoplasmic localization, influences cyclin expression, and modulates cell proliferation. Together with evidence of other post-translational HuR modifications, we propose that HuR phosphorylation ensures the timely mobilization of HuR across the nuclear envelope. In this manner, HuR helps to schedule gene expression programs in a cell cycle-dependent manner.

Hu antigen R (HuR) functions as an alternative pre-mRNA splicing regulator of Fas apoptosis-promoting receptor on exon definition

Exclusion of exon 6 by alternative RNA splicing of the primary transcript of the apoptosis receptor Fas produces a soluble isoform that prevents programmed cell death. I report that antiapoptotic regulator Hu antigen R (HuR, ELAVL1), a member of the embryonic lethal, abnormal vision, Drosophila-like (ELAVL) family, promotes Fas exon 6 skipping by binding to an exonic splicing silencer. HuR inhibits the association of U2 small nuclear ribonucleoprotein (snRNP) auxiliary factor 65 kDa (U2AF65) with the upstream 3' splice site, without decreasing recognition of the downstream 5' splice site by U1 snRNP but by antagonizing the role of TIA-1 (T-cell intracellular antigen 1)/TIAR (TIA-1 related protein) on exon definition. Remarkably, U1 snRNP-mediated recognition of the 5' splice site is partially required for efficient U2AF65 inhibition. Further, the silencing capacity of HuR as splicing regulator resides in the RRM1 and hinge-RRM3 domains. Taken together, these results support a functional link between HuR as repressor of alternative Fas splicing and the molecular mechanisms modulating programmed cell death.

Posttranscriptional gene regulation by RNA-binding proteins during oxidative stress: implications for cellular senescence

To respond adequately to oxidative stress, mammalian cells elicit rapid and tightly controlled changes in gene expression patterns. Besides alterations in the subsets of transcribed genes, two posttranscriptional processes prominently influence the oxidant-triggered gene expression programs: mRNA turnover and translation. Here, we review recent progress in our knowledge of the turnover and translation regulatory (TTR) mRNA-binding proteins (RBPs) that influence gene expression in response to oxidative damage. Specifically, we identify oxidant damage-regulated mRNAs that are targets of TTR-RBPs, we review the oxidant-triggered signaling pathways that govern TTR-RBP function, and we examine emerging evidence that TTR-RBP activity is altered with senescence and aging. Given the potent influence of TTR-RBPs upon oxidant-regulated gene expression profiles, we propose that the senescence-associated changes in TTR-RBPs directly contribute to the impaired responses to oxidant damage that characterize cellular senescence and advancing age.

Development of a human mitochondrial oligonucleotide microarray (h-MitoArray) and gene expression analysis of fibroblast cell lines from 13 patients with isolated F1Fo ATP synthase deficiency

Background

To strengthen research and differential diagnostics of mitochondrial disorders, we constructed and validated an oligonucleotide microarray (h-MitoArray) allowing expression analysis of 1632 human genes involved in mitochondrial biology, cell cycle regulation, signal transduction and apoptosis. Using h-MitoArray we analyzed gene expression profiles in 9 control and 13 fibroblast cell lines from patients with F₁F_o ATP synthase deficiency consisting of 2 patients with mt9205ΔTA microdeletion and a genetically heterogeneous group of 11 patients with not yet characterized nuclear defects. Analysing gene expression profiles, we attempted to classify patients into expected defect specific subgroups, and subsequently reveal group specific compensatory changes, identify potential phenotype causing pathways and define candidate disease causing genes.

Results

Molecular studies, in combination with unsupervised clustering methods, defined three subgroups of patient cell lines – M group with mtDNA mutation and N1 and N2 groups with nuclear defect. Comparison of expression profiles and functional annotation, gene enrichment and pathway analyses of differentially expressed genes revealed in the M group a transcription profile suggestive of synchronized suppression of mitochondrial biogenesis and G1/S arrest. The N1 group showed elevated expression of complex I and reduced expression of complexes III, V, and V-type ATP synthase subunit genes, reduced expression of genes involved in phosphorylation dependent signaling along MAPK, Jak-STAT, JNK, and p38 MAP kinase pathways, signs of activated apoptosis and oxidative stress resembling phenotype of premature senescent fibroblasts. No specific functionally meaningful changes, except of signs of activated apoptosis, were detected in the N2 group. Evaluation of individual gene expression profiles confirmed already known ATP6/ATP8 defect in patients from the M group and indicated several candidate disease causing genes for nuclear defects.

Conclusion

Our analysis showed that deficiency in the ATP synthase protein complex amount is generally accompanied by only minor changes in expression of ATP synthase related genes. It also suggested that the site (mtDNA vs nuclear DNA) and the severity (ATP synthase content) of the underlying defect have diverse effects on cellular gene expression phenotypes, which warrants further investigation of cell cycle regulatory and signal transduction pathways in other OXPHOS disorders and related pharmacological models.

Sirtuins: critical regulators at the crossroads between cancer and aging

Sirtuins (SIRTs 1-7), or class III histone deacetylases (HDACs), are protein deacetylases/ADP ribosyltransferases that target a wide range of cellular proteins in the nucleus, cytoplasm, and mitochondria for post-translational modification by acetylation (SIRT1, -2, -3 and -5) or ADP ribosylation (SIRT4 and -6). The orthologs of sirtuins in lower organisms play a critical role in regulating lifespan. As cancer is a disease of aging, we discuss the growing implications of the sirtuins in protecting against cancer development. Sirtuins regulate the cellular responses to stress and ensure that damaged DNA is not propagated and that mutations do not accumulate. SIRT1 also promotes replicative senescence under conditions of chronic stress. By participating in the stress response to genomic insults, sirtuins are thought to protect against cancer, but they are also emerging as direct participants in the growth of some cancers. Here, we review the growing implications of sirtuins both in cancer prevention and as specific and novel cancer therapeutic targets.

mRNA stability control: a clandestine force in normal and malignant hematopoiesis

This review addresses the scope of influence of mRNA decay on cellular functions and its potential role in normal and malignant hematopoiesis. Evidence is emerging that leukemic oncogenes and hematopoietic cytokines interact with mRNA decay pathways. These pathways can co-regulate functionally related genes through specific motifs in the 3'-untranslated region of targeted transcripts. The steps that link external stimuli to transcript turnover are not fully understood, but include subcellular relocalization or post-transcriptional modification of specific transcript-stabilizing or -destabilizing proteins. Improper functioning of these regulators of mRNA turnover can impede normal cellular differentiation or promote cancers. By delineating how subsets of transcripts decay in synchrony during normal hematopoiesis, it may be possible to determine whether this post-transcriptional regulatory pathway is hijacked in leukemogenesis.

Phosphorylation of HuR by Chk2 regulates SIRT1 expression

The RNA binding protein HuR regulates the stability of many target mRNAs. Here, we report that HuR associated with the 3' untranslated region of the mRNA encoding the longevity and stress-response protein SIRT1, stabilized the SIRT1 mRNA, and increased SIRT1 expression levels. Unexpectedly, oxidative stress triggered the dissociation of the [HuR-SIRT1 mRNA] complex, in turn promoting SIRT1 mRNA decay, reducing SIRT1 abundance, and lowering cell survival. The cell cycle checkpoint kinase Chk2 was activated by H(2)O(2), interacted with HuR, and was predicted to phosphorylate HuR at residues S88, S100, and T118. Mutation of these residues revealed a complex pattern of HuR binding, with S100 appearing to be important for [HuR-SIRT1 mRNA] dissociation after H(2)O(2). Our findings demonstrate that HuR regulates SIRT1 expression, underscore functional links between the two stress-response proteins, and implicate Chk2 in these processes.

Stabilization of Snail by HuR in the process of hydrogen peroxide induced cell migration

Snail functions as a key regulator in the induction of a phenotypic change called epithelial to mesenchymal transition (EMT). Aberrant expression of Snail prevails in the onset and development of tumor. Here, we have observed increased expression of Snail under the treatment of hydrogen peroxide (H(2)O(2)). Investigation into the underlying mechanisms revealed that stabilization of Snail mRNA contributes partially to this process. H(2)O(2)-induced the luciferase activity of the reporter construct contains the 3'UTR of Snail. Deletion of the AU-rich elements in the UTR eliminated the response of the reporter to H(2)O(2), suggesting the potential role of HuR in the process. Lowering of endogenous HuR levels through knockdown of HuR by siRNA greatly reduced the inducability and half-life of Snail mRNA, which consequently inhibited the downregulation of E-cadherin by H(2)O(2). Our findings indicate that HuR plays a major role in regulating H(2)O(2)-induced Snail expression by enhancing Snail mRNA stability, which in turn enhances cell migrating ability through repressing expression of E-cadherin.

Post-transcriptional regulation of RNase-L expression is mediated by the 3'-untranslated region of its mRNA

RNase-L mediates critical cellular functions including antiviral, pro-apoptotic, and tumor suppressive activities; accordingly, its expression must be tightly regulated. Little is known about the control of RNASEL expression; therefore, we examined the potential regulatory role of a conserved 3'-untranslated region (3'-UTR) in its mRNA. The 3'-UTR mediated a potent decrease in the stability of RNase-L mRNA, and of a chimeric beta-globin-3'-UTR reporter mRNA. AU-rich elements (AREs) are cis-acting regulatory regions that modulate mRNA stability. Eight AREs were identified in the RNase-L 3'-UTR, and deletion analysis identified positive and negative regulatory regions associated with distinct AREs. In particular, AREs 7 and 8 served a strong positive regulatory function. HuR is an ARE-binding protein that stabilizes ARE-containing mRNAs, and a predicted HuR binding site was identified in the region comprising AREs 7 and 8. Co-transfection of HuR and RNase-L enhanced RNase-L expression and mRNA stability in a manner that was dependent on this 3'-UTR region. Immunoprecipitation demonstrated that RNase-L mRNA associates with a HuR containing complex in intact cells. Activation of endogenous HuR by cell stress, or during myoblast differentiation, increased RNase-L expression, suggesting that RNase-L mRNA is a physiologic target for HuR. HuR-dependent regulation of RNase-L enhanced its antiviral activity demonstrating the functional significance of this regulation. These findings identify a novel mechanism of RNase-L regulation mediated by its 3'-UTR.

14-3-3 protein binds to the low molecular weight neurofilament (NFL) mRNA 3′ UTR

We have previously reported that altered stability of low molecular weight neurofilament (NFL) mRNA in lumbar spinal cord homogenates in amyotrophic lateral sclerosis (ALS) is associated with altered expression of trans-acting 3' UTR mRNA binding proteins. We have identified two hexanucleotide motifs as the main cis elements and, using LC/MS/MS of peptide digests of NFL 3' UTR interacting proteins from human spinal cord, observed that 14-3-3 proteins interact with these motifs. 14-3-3 beta, zeta, tau, gamma, and eta isoforms were found to be expressed in human spinal cord. Each isoform was expressed in vitro and shown to interact with NFL 3' UTR mRNA. Mutation of one or both motifs resulted in decreased 14-3-3 interaction, changes in predicted mRNA structure or alteration in stability of the mRNA. These data show a novel interaction for 14-3-3 with NFL mRNA, and suggests that 14-3-3 may play a role in regulating NFL mRNA stability.

Age- and cell cycle-dependent changes in EPC-1/PEDF promoter activity in human diploid fibroblast-like (HDF) cells

The changes in gene expression during senescence are very interesting. Early population doubling cDNA-1 (EPC-1, also known as pigment epithelial derived factor, PEDF) is one of the genes whose expression decreases dramatically during cellular aging. We examined whether or not EPC-1/PEDF promoter activity was affected by the cellular ageing using human diploid lung fibroblast cells in culture. Here we showed the promoter/enhancer region of EPC-1/PEDF existed at more than 1760 bp upstream from the transcriptional initiation site of the gene, and was regulated by both aging and cell cycle. These findings suggest that the expression of the EPC-1/PEDF gene is, at least in part, regulated transcriptionally in the cells. The analysis of the promoter region of the EPC-1/PEDF gene in this paper suggests the age- and cell cycle-dependent expression of specific transcriptional factor(s).

Translational control of cytochrome c by RNA-binding proteins TIA-1 and HuR

Stresses affecting the endoplasmic reticulum (ER) globally modulate gene expression patterns by altering posttranscriptional processes such as translation. Here, we use tunicamycin (Tn) to investigate ER stress-triggered changes in the translation of cytochrome c, a pivotal regulator of apoptosis. We identified two RNA-binding proteins that associate with its approximately 900-bp-long, adenine- and uridine-rich 3' untranslated region (UTR): HuR, which displayed affinity for several regions of the cytochrome c 3'UTR, and T-cell-restricted intracellular antigen 1 (TIA-1), which preferentially bound the segment proximal to the coding region. HuR did not appear to influence the cytochrome c mRNA levels but instead promoted cytochrome c translation, as HuR silencing greatly diminished the levels of nascent cytochrome c protein. By contrast, TIA-1 functioned as a translational repressor of cytochrome c, with interventions to silence TIA-1 dramatically increasing cytochrome c translation. Following treatment with Tn, HuR binding to cytochrome c mRNA decreased, and both the presence of cytochrome c mRNA within actively translating polysomes and the rate of cytochrome c translation declined. Taken together, our data suggest that the translation rate of cytochrome c is determined by the opposing influences of HuR and TIA-1 upon the cytochrome c mRNA. Under unstressed conditions, cytochrome c mRNA is actively translated, but in response to ER stress agents, both HuR and TIA-1 contribute to lowering its biosynthesis rate. We propose that HuR and TIA-1 function coordinately to maintain precise levels of cytochrome c production under unstimulated conditions and to modify cytochrome c translation when damaged cells are faced with molecular decisions to follow a prosurvival or a prodeath path.

Enhanced proliferation of cultured human vascular smooth muscle cells linked to increased function of RNA-binding protein HuR

In dividing cells, the RNA-binding protein HuR associates with and stabilizes labile mRNAs encoding proliferative proteins, events that are linked to the increased cytoplasmic presence of HuR. Here, assessment of HuR levels in various vascular pathologies (intimal hyperplasia, atherosclerosis and neointimal proliferation, sclerosis of arterialized saphenous venous graft, and fibromuscular dysplasia) revealed a distinct increase in HuR expression and cytoplasmic abundance within the intima and neointima layers. On the basis of these observations, we postulated a role for HuR in promoting the proliferation of vascular smooth muscle cells. To test this hypothesis directly, we investigated the expression, subcellular localization, and proliferative influence of HuR in human vascular smooth muscle cells (hVSMCs). Treatment of hVSMCs with platelet-derived growth factor increased HuR levels in the cytoplasm, thereby influencing the expression of metabolic, proliferative, and structural genes. Importantly, knockdown of HuR expression by using RNA interference caused a reduction of hVSMC proliferation, both basally and following platelet-derived growth factor treatment. We propose that HuR contributes to regulating hVSMC growth and homeostasis in pathologies associated with vascular smooth muscle proliferation.

Antiapoptotic function of RNA-binding protein HuR effected through prothymosin alpha

We report the antiapoptotic effect of RNA-binding protein HuR, a critical regulator of the post-transcriptional fate of target transcripts. Among the most prominent mRNAs complexing with HuR is that encoding prothymosin alpha (ProTalpha), an inhibitor of the apoptosome. In HeLa cells, treatment with the apoptotic stimulus ultraviolet light (UVC) triggered the mobilization of ProTalpha mRNA to the cytoplasm and onto heavier polysomes, where its association with HuR increased dramatically. Analysis of a chimeric ProTalpha mRNA directly implicated HuR in regulating ProTalpha production: ProTalpha translation and cytoplasmic concentration increased in HuR-overexpressing cells and declined in cells in which HuR levels were lowered by RNA interference. Importantly, the antiapoptotic influence engendered by HuR was vitally dependent on ProTalpha expression, since use of oligomers that blocked ProTalpha translation abrogated the protective effect of HuR. Together, our data support a regulatory scheme whereby HuR binds the ProTalpha mRNA, elevates its cytoplasmic abundance and translation, and thereby elicits an antiapoptotic program.

mRNA openers and closers: modulating AU-rich element-controlled mRNA stability by a molecular switch in mRNA secondary structure

Approximately 3 000 genes are regulated in a time-, tissue-, and stimulus-dependent manner by degradation or stabilization of their mRNAs. The process is mediated by interaction of AU-rich elements (AREs) in the mRNA's 3'-untranslated regions with trans-acting factors. AU-rich element-controlled genes of fundamentally different functional relevance depend for their activation on one positive regulator, HuR. Here we present a methodology to exploit this central regulatory process for specific manipulation of AU-rich element-controlled gene expression at the mRNA level. With a combination of single-molecule spectroscopy, computational biology, and molecular and cellular biochemistry, we show that mRNA recognition by HuR is dependent on the presentation of the sequence motif NNUUNNUUU in single-stranded conformation. The presentation of the HuR binding site in the mRNA secondary structure appears to act analogously to a regulatory on/off switch that specifically controls HuR access to mRNAs in cis. Based on this finding we present a methodology for manipulating ARE mRNA levels by actuating this conformational switch specifically in a target mRNA. Computationally designed oligonucleotides (openers) enhance the NNUUNNUUU accessibility by rearranging the mRNA conformation. Thereby they increase in vitro and endogenous HuR-mRNA complex formation which leads to specific mRNA stabilization (as demonstrated for TNFalpha and IL-2, respectively). Induced HuR binding both inside and outside the AU-rich element promotes functional IL-2 mRNA stabilization. This opener-induced mRNA stabilization mimics the endogenous IL-2 response to CD28 stimulation in human primary T-cells. We therefore propose that controlled modulation of the AU-rich element conformation by mRNA openers or closers allows message stabilization or destabilization in cis to be specifically triggered. The described methodology might provide a means for studying distinct pathways in a complex cellular network at the node of mRNA stability control. It allows ARE gene expression to be potentially silenced or boosted. This will be of particular value for drug-target validation, allowing the diseased phenotype to ameliorate or deteriorate. Finally, the mRNA openers provide a rational starting point for target-specific mRNA stability assays to screen for low-molecular-weight compounds acting as inhibitors or activators of an mRNA structure rearrangement.

Increased stability of the p16 mRNA with replicative senescence

Expression of p16(INK4a) is elevated during ageing and replicative senescence. Here, we report the presence of an instability determinant within the 3'-untranslated region (UTR) of the p16 messenger RNA in WI-38 human diploid fibroblasts. The p16 3'UTR was found to be a specific target of AUF1, an RNA-binding protein implicated in promoting mRNA decay. Both AUF1 levels and AUF1-p16 mRNA associations were strikingly more abundant in early-passage than late-passage fibroblast cultures. Moreover, short interfering RNA-based reductions in AUF1 levels increased the stability of p16 3'UTR-containing transcripts, elevated the expression of p16 and accentuated the senescence phenotype. Together, our findings show that p16 mRNA turnover decreases during replicative senescence and that the instability-conferring region is located within the 3'UTR of p16, as well as identifying AUF1 as a critical mediator of these regulatory events.

Cell cycle and developmental regulations of replication factors in mouse embryonic stem cells

Embryonic stem (ES) cells can grow rapidly and permanently while maintaining their differentiation capacity. To gain insight into how the cell cycle progression of undifferentiated murine ES cells is regulated, we have examined the expression patterns of various replication and cell cycle regulators. Most factors including cyclins, Cdc6, and geminin are rather constitutively expressed during the cell cycle of ES cells. Furthermore, the transcript levels of almost all the cell cycle regulators we investigated except for p21 and p27 are higher in undifferentiated ES cells than in murine embryonic fibroblasts (MEFs), and the increased stability of mRNA in ES cells may be partially responsible for this at least with some of the factors. More strikingly, the transcriptional levels of these factors are strongly correlated with the acetylated state of histone H3 at their promoter regions. However, the methylation state of histone or CpG methylation of the promoter region is not generally correlated significantly with the expression pattern of these factors in both cell types. On the protein level, Cdc6, ASK, cyclin A2, and cyclin B1 are extremely abundant in ES cells compared with MEFs. Furthermore, they are rapidly down-regulated upon induction of differentiation of ES cells. The significance of these findings is discussed in relation to the unusual proliferative properties of ES cells in an undifferentiated state.

Mutant Copper-Zinc Superoxide Dismutase Binds to and Destabilizes Human Low Molecular Weight Neurofilament mRNA

The mechanism by which mutated copper-zinc superoxide dismutase (SOD1) causes familial amyotrophic lateral sclerosis is believed to involve an adverse gain of function, independent of the physiological antioxidant enzymatic properties of SOD1. In this study, we have observed that mutant SOD1 (G41S, G85A, and G93A) but not the wild type significantly reduced the stability of the low molecular weight neurofilament mRNA in a dosage-dependent manner. We have also demonstrated that mutant SOD1 but not the wild type bound directly to the neurofilament mRNA 3'-untranslated region and that the binding was necessary to induce mRNA destabilization. These observations provide an explanation for a novel gain of function in which mutant SOD1 expression in motor neurons alters an intermediate filament protein expression.

Mitochondrial fission proteins regulate programmed cell death in yeast

The possibility that single-cell organisms undergo programmed cell death has been questioned in part because they lack several key components of the mammalian cell death machinery. However, yeast encode a homolog of human Drp1, a mitochondrial fission protein that was shown previously to promote mammalian cell death and the excessive mitochondrial fragmentation characteristic of apoptotic mammalian cells. In support of a primordial origin of programmed cell death involving mitochondria, we found that the Saccharomyces cerevisiae homolog of human Drp1, Dnm1, promotes mitochondrial fragmentation/degradation and cell death following treatment with several death stimuli. Two Dnm1-interacting factors also regulate yeast cell death. The WD40 repeat protein Mdv1/Net2 promotes cell death, consistent with its role in mitochondrial fission. In contrast to its fission function in healthy cells, Fis1 unexpectedly inhibits Dnm1-mediated mitochondrial fission and cysteine protease-dependent cell death in yeast. Furthermore, the ability of yeast Fis1 to inhibit mitochondrial fission and cell death can be functionally replaced by human Bcl-2 and Bcl-xL. Together, these findings indicate that yeast and mammalian cells have a conserved programmed death pathway regulated by a common molecular component, Drp1/Dnm1, that is inhibited by a Bcl-2-like function.

Concurrent versus individual binding of HuR and AUF1 to common labile target mRNAs

RNA-binding proteins HuR and AUF1 bind to many common AU-rich target mRNAs and exert opposing influence on target mRNA stability, but the functional interactions between HuR and AUF1 have not been systematically studied. Here, using common target RNAs encoding p21 and cyclin D1, we provide evidence that HuR and AUF1 can bind target transcripts on both distinct, nonoverlapping sites, and on common sites in a competitive fashion. In the nucleus, both proteins were found together within stable ribonucleoprotein complexes; in the cytoplasm, HuR and AUF1 were found to bind to target mRNAs individually, HuR colocalizing with the translational apparatus and AUF1 with the exosome. Our results indicate that the composition and fate (stability, translation) of HuR- and/or AUF1-containing ribonucleoprotein complexes depend on the target mRNA of interest, RNA-binding protein abundance, stress condition, and subcellular compartment.

Proteasome inhibition increases HuR level, restores heat-inducible HSP72 expression and thermotolerance in WI-38 senescent human fibroblasts

At the end of their replicative potential in vitro, late passage WI-38 human diploid fibroblasts (HDF) have a low basal expression of heat shock protein 72 (HSP72) and an attenuated ability to induce it in response to heat shock. The transient exposure to the specific and reversible proteasome inhibitor MG132 during a mild heat shock induced late passage HDF to synthesize and accumulate high levels of HSP72. This HSP72 expression was long-lasting and appeared to result from both increased cytoplasmic levels and enhanced translation of HSP72 mRNA. The level of HuR, a stabilizing mRNA-binding protein, increased following the MG132 treatment. This result is consistent with the proposed role of HuR in assisting mRNA export to the cytoplasm and in antagonizing its degradation. Furthermore, the previous exposure of late passage HDF to a mild heat shock in the presence of MG132 protected these cells against the otherwise lethal effect of a subsequent severe heat shock. This acquisition of thermotolerance appeared to be correlated with the level of HSP72.

Identification of a target RNA motif for RNA-binding protein HuR

HuR, a protein that binds to specific mRNA subsets, is increasingly recognized as a pivotal posttranscriptional regulator of gene expression. Here, HuR was immunoprecipitated under conditions that preserved HuR-RNA interactions, and HuR-bound target mRNAs were identified by cDNA array hybridization. Analysis of primary sequences and secondary structures shared among HuR targets led to the identification of a 17- to 20-base-long RNA motif rich in uracils. This HuR motif was found in almost all mRNAs previously reported to be HuR targets, was located preferentially within 3' untranslated regions of all unigene transcripts examined, and was conserved in >50% of human and mouse homologous genes. Importantly, the HuR motif allowed the successful prediction and subsequent validation of novel HuR targets from gene databases. This study describes an HuR target RNA motif and presents a general strategy for identifying target motifs for RNA-binding proteins.

Competition of CUGBP1 and calreticulin for the regulation of p21 translation determines cell fate

Induction of p21 in senescent human fibroblasts plays a key role in the inactivation of cyclin-dependent kinases and the resulting irreversible growth arrest in the early stages of cell senescence. We found that RNA-binding proteins are critical regulators of p21 during senescence. Two RNA-binding proteins, CUGBP1 and calreticulin (CRT), interact with the same nucleotide sequences within the 5' region of p21 mRNA, but have opposite effects on the translation of p21 mRNA. CUGBP1 increases translation of p21 mRNA, whereas CRT blocks translation of p21 via stabilization of a stem-loop structure within the 5' region of the p21 mRNA. CUGBP1 and CRT compete for binding to p21 mRNA and thereby the regulation of p21 translation. In senescent fibroblasts, CUGBP1 displaces CRT from the p21 mRNA and releases CRT-dependent repression of p21 translation leading to growth arrest and development of a senescent phenotype. These data present evidence that competition between RNA-binding proteins for the regulation of p21 translation determines cell fate.

Tissue distribution of AU-rich mRNA-binding proteins involved in regulation of mRNA decay

Short lived cytokine and proto-oncogene mRNAs are destabilized by an A+U-rich element (ARE) in the 3'-untranslated region. Several regulatory proteins bind to AREs in cytokine and proto-oncogene mRNAs, participate in inhibiting or promoting their rapid degradation of ARE mRNAs, and influence cytokine expression and cellular transformation in experimental models. The tissue distribution and cellular localization of the different AU-rich binding proteins (AUBPs), however, have not been uniformly characterized in the mouse, a model for ARE mRNA decay. We therefore carried out immunoblot and immunohistochemical analyses of the different AUBPs using the same mouse tissues. We show that HuR protein, a major AUBP that stabilizes the ARE mRNAs, is most strongly expressed in the thymus, spleen (predominantly in lymphocytic cells), intestine, and testes. AUF1 protein, a negative regulator of ARE mRNA stability, displayed strong expression in thymus and spleen cells within lymphocytic cells, moderate expression in the epithelial linings of lungs, gonadal tissues, and nuclei of most neurons in the brain, and little expression in the other tissues. Tristetraprolin, a negative regulator of ARE mRNA stability, displayed a largely non-overlapping tissue distribution with AUF1 and was predominantly expressed in the liver and testis. KH-type splicing regulatory protein, a presumptive negative regulator of ARE mRNA stability, was distributed widely in murine organs. These results indicate that HuR and AUF1, which functionally oppose each other, have generally similar distributions, suggesting that the balance between HuR and AUF1 is likely important in control of short lived mRNA degradation, lymphocyte development, and/or cytokine production, and possibly in certain aspects of neurological function.

Regulation of eotaxin gene expression by TNF-alpha and IL-4 through mRNA stabilization: involvement of the RNA-binding protein HuR

During inflammatory responses, a major posttranscriptional regulation of early response and inflammatory gene expression occurs through modulation of mRNA turnover. We report that two potent inducers of the CC chemokine eotaxin, TNF-alpha and IL-4, regulate its production in airway epithelial cells by increasing eotaxin mRNA stability. In experiments using the transcriptional inhibitor actinomycin D, eotaxin mRNA half-life was significantly prolonged by cell stimulation with TNF-alpha or IL-4, with the combination of the two cytokines being the most effective in extending the mRNA half-life. Involvement of the eotaxin 3' untranslated region in the mRNA-stabilizing effect was tested by transient transfection of a construct expressing a chimeric transcript carrying a serum-inducible beta-globin reporter linked to the eotaxin 3' untranslated region. The half-life of the chimeric mRNA was markedly increased in cells stimulated with TNF-alpha and IL-4. Evidence that the mRNA-stabilizing protein HuR participated in the cytokine effect was obtained: first, HuR presence in the cytoplasm, believed to be required for HuR-mediated mRNA stabilization, increased in both transformed (BEAS-2B cell line) and primary bronchial epithelial cells following treatment with TNF-alpha and IL-4. Second, endogenous eotaxin mRNA was found to bind to HuR in vivo, as detected by immunoprecipitation of HuR-containing messenger ribonucleoprotein complexes followed by real-time RT-PCR analysis; such association increased after cell treatment with TNF-alpha and IL-4. Third, overexpression of HuR in BEAS-2B cells significantly increased the expression of eotaxin mRNA and protein. Our findings implicate mRNA stabilization in the cytokine-mediated increase in eotaxin expression and strongly suggest a role for HuR in this effect.

A role for the exosome in the in vivo degradation of unstable mRNAs

In mammals, the mRNAs encoding many proteins involved in inflammation bear destabilizing AU-rich elements (AREs) in the 3'-untranslated region. The exosome, a complex of 3' --> 5' exonucleases, is rate limiting in the destruction of such mRNAs in a mammalian in vitro system, but a role in vivo has not been demonstrated. The phenomenon of ARE-mediated degradation also occurs in the protist parasite Trypanosoma brucei. Messenger RNAs with 3'-untranslated region U-rich elements, which strongly resemble AREs, are extremely unstable in the trypanosome form that parasitizes mammals. The first step in degradation of these mRNAs in vivo is rapid destruction of the 3'-untranslated region; subsequently the mRNA is destroyed by exonucleases acting in both 5' --> 3' and 3' --> 5' directions. We here investigated the roles of three subunits of the trypanosome exosome complex, RRP45, RRP4, and CSL4, in this process, depleting the individual subunits in vivo by inducible RNA interference. RRP45 depletion, which probably disrupts exosome integrity, caused a delay in the onset of degradation of the very unstable RNAs, but did not affect degradation of more stable species. Depletion of RRP4 or CSL4 does not affect the stability of the residual exosome and did not change mRNA degradation kinetics. We conclude that the exosome is required for the initiation of rapid degradation of unstable mRNAs in trypanosomes.

Role of the RNA-binding protein HuR in colon carcinogenesis

Immunohistochemical analysis of paired tumor and normal tissue specimens revealed that the expression and cytoplasmic abundance of the RNA-binding protein HuR increased with malignancy, particularly in colon carcinomas. Interventions to modulate HuR expression in human RKO colon cancer cells altered gene expression profiles and identified beta-catenin mRNA as a novel HuR target. Subcutaneous injection of HuR-overexpressing RKO cells into nude mice produced significantly larger tumors than those arising from control populations; conversely, RKO cells expressing reduced HuR through small interference RNA- or antisense HuR-based approaches developed significantly more slowly. We propose that HuR-regulated target mRNA expression contributes to colon cancer growth. Our results suggest a pivotal function for HuR in colon carcinogenesis.

Increased AMP:ATP ratio and AMP-activated protein kinase activity during cellular senescence linked to reduced HuR function

Cytoplasmic export of the RNA-binding protein HuR, a process that critically regulates its function, was recently shown to be inhibited by the AMP-activated protein kinase (AMPK). In the present investigation, treatment of human fibroblasts with AMPK activators such as 5-amino-imidazole-4-carboxamide riboside, antimycin A, and sodium azide inhibited cell growth and lowered the expression of proliferative genes. As anticipated, AMPK activation also decreased both the cytoplasmic HuR levels and the association of HuR with target radiolabeled transcripts encoding such proliferative genes. HuR function was previously shown to be implicated in the maintenance of a "young cell" phenotype in models of replicative cellular senescence. We therefore postulated that AMPK activation in human fibroblasts might contribute to the implementation of the senescence phenotype through mechanisms that included a reduction in HuR cytoplasmic presence. Indeed, AMP:ATP ratios were 2-3-fold higher in senescent fibroblasts compared with young fibroblasts. Accordingly, in vitro senescence was accompanied by a marked elevation in AMPK activity. Evidence that increased AMPK activity directly contributed to the implementation of the senescent phenotype was obtained through two experimental approaches. First, use of AMPK activators triggered senescence characteristics in fibroblasts, such as the acquisition of senescence-associated beta-galactosidase (beta-gal) activity and increased p16INK4a expression. Second, infection of cells with an adenoviral vector that expresses active AMPK increased senescence-associated beta-gal activity, whereas infection with an adenovirus that expresses dominant-negative AMPK decreased senescence-associated beta-gal activity. Together, our results indicate that AMPK activation can cause premature fibroblast senescence through mechanisms that likely involve reduced HuR function.

Role of HuR in skeletal myogenesis through coordinate regulation of muscle differentiation genes

In this report, we investigate the role of the RNA-binding protein HuR during skeletal myogenesis. At the onset of myogenesis in differentiating C2C12 myocytes and in vivo in regenerating mouse muscle, HuR cytoplasmic abundance increased dramatically, returning to a predominantly nuclear presence upon completion of myogenesis. mRNAs encoding key regulators of myogenesis-specific transcription (myogenin and MyoD) and cell cycle withdrawal (p21), bearing AU-rich regions, were found to be targets of HuR in a differentiation-dependent manner. Accordingly, mRNA half-lives were highest during differentiation, declining when differentiation was completed. Importantly, HuR-overexpressing C2C12 cells displayed increased target mRNA expression and half-life and underwent precocious differentiation. Our findings underscore a critical function for HuR during skeletal myogenesis linked to HuR's coordinate regulation of muscle differentiation genes.

found in neurons, a third member of the Drosophila elav gene family, encodes a neuronal protein and interacts with elav

elav, a gene necessary for neuronal differentiation and maintenance in Drosophila, encodes the prototype of a family of conserved proteins involved in post-transcriptional regulation. We identified found in neurons (fne), a gene encoding a new ELAV paralogue. We showed that FNE binds RNA in vitro. fne transcripts are present throughout development and contain long untranslated regions. Transcripts and proteins are restricted to neurons of the CNS and PNS during embryogenesis. These features are reminiscent of elav. However, fne expression is delayed compared to elav's, and FNE protein appears cytoplasmic, while ELAV is nuclear. GAL4-directed overexpression of fne in neurons leads to a reduction of stable transcripts produced from both the fne and elav endogenous loci, suggesting that fne autoregulates and also regulates elav.

ELAV/Hu proteins inhibit p27 translation via an IRES element in the p27 5'UTR

p27Kip1 restrains cell proliferation by binding to and inhibiting cyclin-dependent kinases. To investigate the mechanisms of p27 translational regulation, we isolated a complete p27 cDNA and identified an internal ribosomal entry site (IRES) located in its 5'UTR. The IRES allows for efficient p27 translation under conditions where cap-dependent translation is reduced. Searching for possible regulators of IRES activity we have identified the neuronal ELAV protein HuD as a specific binding factor of the p27 5'UTR. Increased expression of HuD or the ubiquitously expressed HuR protein specifically inhibits p27 translation and p27 IRES activity. Consistent with an inhibitory role of Hu proteins in p27 translation, siRNA mediated knockdown of HuR induced endogenous p27 protein levels as well as IRES-mediated reporter translation and leads to cell cycle arrest in G1.

Regulation of pathways of mRNA destabilization and stabilization

The level of an mRNA in the cytoplasm represents a balance between the rate at which the mRNA precursor is synthesized in the nucleus and the rates of nuclear RNA processing and export and cytoplasmic mRNA degradation. Although most studies of gene expression have focused on gene transcription and in the area of eukaryotic mRNA degradation, but to provide a short general discussion of the importance of mRNA degradation and its regulation and a brief overview of recent findings and present knowledge. The overview is followed by a more in-depth discussion of one of the several pathways for mRNA degradation. We concentrate on the pathway for regulated mRNA degradation mediated by mRNA-binding proteins and endonucleases that cleave within the body of mRNAs. As a potential example of this type of control, we focus on the regulated degradation of the egg yolk precursor protein vitellogenin on the mRNA-binding protein vigilin and the mRNA endonuclease polysomal ribonuclease 1 (PMR-1).

AMP-activated kinase regulates cytoplasmic HuR

While transport of RNA-binding protein HuR from nucleus to cytoplasm is emerging as a key regulatory step for HuR function, the mechanisms underlying this process remain poorly understood. Here, we report that the AMP-activated kinase (AMPK), an enzyme involved in responding to metabolic stresses, potently regulates the levels of cytoplasmic HuR. Inhibition of AMPK, accomplished either through cell treatment or by adenovirus infection to express dominant-negative AMPK, was found to increase the level of HuR in the cytoplasm and to enhance the binding of HuR to p21, cyclin B1, and cyclin A mRNA transcripts and elevate their expression and half-lives. Conversely, AMPK activation, achieved by means including infection to express constitutively active AMPK, resulted in reduced cytoplasmic HuR; decreased levels and half-lives of mRNAs encoding p21, cyclin A, and cyclin B1; and diminished HuR association with the corresponding transcripts. We therefore propose a novel function for AMPK as a regulator of cytoplasmic HuR levels, which in turn influences the mRNA-stabilizing function of HuR and the expression of HuR target transcripts.

Correlation between intrinsic mRNA stability and the affinity of AUF1 (hnRNP D) and HuR for A+U-rich mRNAs

Presence of A+U-rich elements (AREs) within 3'-untranslated regions (3'UTRs) of numerous mRNAs has been associated with rapid mRNA turnover; however, the interaction of specific factors with AREs is also associated with mRNA stabilization. Recently, two ARE binding proteins with putative mRNA destabilizing (AUF1) and stabilizing (HuR) properties have been described. However, no direct comparison ofAUF1 and HuR binding properties has been made. Therefore, we examined the relative affinities of p37AUF1 and HuR for a diverse set ofARE-containing mRNAs encoding beta-adrenergic receptors, a proto-oncogene, and a cytokine. We find that high-affinity AUF1 binding appears to require elements beyond primary nucleotide sequence. In contrast, binding of HuR appears considerably less constrained. As a functional correlate, we determined the ability of these specific mRNA sequences to affect the stability of chimeric beta-globin mRNA constructs. Although the relative affinity ofAUF1 and HuR are generally predictive of mRNA stability, we find that certain mRNA sequences do not conform to these generalizations.

Thermodynamics and kinetics of Hsp70 association with A + U-rich mRNA-destabilizing sequences

Rapid mRNA degradation directed by A + U-rich elements (AREs) is mediated by the interaction of specific RNA-binding proteins to these sequences. The protein chaperone Hsp70 has been identified in a cellular complex containing the ARE-binding protein AUF1 and has also been detected in direct contact with A + U-rich RNA substrates, indicating that Hsp70 may be involved in the regulation of ARE-directed mRNA turnover. By using gel mobility shift and fluorescence anisotropy assays, we have determined that Hsp70 directly and specifically associates with U-rich RNA substrates in solution. With the ARE from tumor necrosis factor alpha (TNFalpha) mRNA, Hsp70 forms a dynamic complex consistent with a 1:1 association of protein:RNA but demonstrates cooperative binding behavior on polyuridylate substrates. Unlike AUF1, the RNA binding activity of Hsp70 is not regulated by ion-dependent folding of the TNFalpha ARE, suggesting that AUF1 and Hsp70 recognize distinct binding determinants on this RNA substrate. Binding of Hsp70 to the TNFalpha ARE is driven entirely by enthalpy at physiological temperatures, indicating that burial of hydrophobic surfaces is likely the principal mechanism stabilizing the Hsp70.RNA complex. Potential roles for the interaction of Hsp70 with ARE-containing mRNAs in the regulation of mRNA turnover and/or translational efficiency are discussed.