Phosphorylation and binding of AUF1 to the 3'-untranslated region of cardiomyocyte SERCA2a mRNA

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

AUF1 ligand circPCNX reduces cell proliferation by competing with p21 mRNA to increase p21 production

Mammalian circRNAs can influence different cellular processes by interacting with proteins and other nucleic acids. Here, we used ribonucleoprotein immunoprecipitation (RIP) analysis to identify systematically the circRNAs associated with the cancer-related protein AUF1. Among the circRNAs interacting with AUF1 in HeLa (human cervical carcinoma) cells, we focused on hsa_circ_0032434 (circPCNX), an abundant target of AUF1. Overexpression of circPCNX specifically interfered with the binding of AUF1 to p21 (CDKN1A) mRNA, thereby promoting p21 mRNA stability and elevating the production of p21, a major inhibitor of cell proliferation. Conversely, silencing circPCNX increased AUF1 binding to p21 mRNA, reducing p21 production and promoting cell division. Importantly, eliminating the AUF1-binding region of circPCNX abrogated the rise in p21 levels and rescued proliferation. Therefore, we propose that the interaction of circPCNX with AUF1 selectively prevents AUF1 binding to p21 mRNA, leading to enhanced p21 mRNA stability and p21 protein production, thereby suppressing cell growth.

mRNA Post-Transcriptional Regulation by AU-Rich Element-Binding Proteins in Liver Inflammation and Cancer

AU-rich element-binding proteins (AUBPs) represent important post-transcriptional regulators of gene expression. AUBPs can bind to the AU-rich elements present in the 3'-UTR of more than 8% of all mRNAs and are thereby able to control the stability and/or translation of numerous target mRNAs. The regulation of the stability and the translation of mRNA transcripts by AUBPs are highly complex processes that occur through multiple mechanisms depending on the cell type and the cellular context. While AUBPs have been shown to be involved in inflammatory processes and the development of various cancers, their important role and function in the development of chronic metabolic and inflammatory fatty liver diseases (FLDs), as well as in the progression of these disorders toward cancers such as hepatocellular carcinoma (HCC), has recently started to emerge. Alterations of either the expression or activity of AUBPs are indeed significantly associated with FLDs and HCC, and accumulating evidence indicates that several AUBPs are deeply involved in a significant number of cellular processes governing hepatic metabolic disorders, inflammation, fibrosis, and carcinogenesis. Herein, we discuss our current knowledge of the roles and functions of AUBPs in liver diseases and cancer. The relevance of AUBPs as potential biomarkers for different stages of FLD and HCC, or as therapeutic targets for these diseases, are also highlighted.

The proarrhythmic features of pathological cardiac hypertrophy in neonatal rat ventricular cardiomyocyte cultures

Different factors may trigger arrhythmias in diseased hearts, including fibrosis, cardiomyocyte hypertrophy, hypoxia, and inflammation. This makes it difficult to establish the relative contribution of each of them to the occurrence of arrhythmias. Accordingly, in this study, we used an in vitro model of pathological cardiac hypertrophy (PCH) to investigate its proarrhythmic features and the underlying mechanisms independent of fibrosis or other PCH-related processes. Neonatal rat ventricular cardiomyocyte (nr-vCMC) monolayers were treated with phorbol 12-myristate 13-acetate (PMA) to create an in vitro model of PCH. The electrophysiological properties of PMA-treated and control monolayers were analyzed by optical mapping at day 9 of culture. PMA treatment led to a significant increase in cell size and total protein content. It also caused a reduction in sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2 level (32%) and an increase in natriuretic peptide A (42%) and α1-skeletal muscle actin (34%) levels, indicating that the hypertrophic response induced by PMA was, indeed, pathological in nature. PMA-treated monolayers showed increases in action potential duration (APD) and APD dispersion, and a decrease in conduction velocity (CV; APD₃₀ of 306 ± 39 vs. 148 ± 18 ms, APD₃₀ dispersion of 85 ± 19 vs. 22 ± 7 and CV of 10 ± 4 vs. 21 ± 2 cm/s in controls). Upon local 1-Hz stimulation, 53.6% of the PMA-treated cultures showed focal tachyarrhythmias based on triggered activity (n = 82), while the control group showed 4.3% tachyarrhythmias (n = 70). PMA-treated nr-vCMC cultures may, thus, represent a well-controllable in vitro model for testing new therapeutic interventions targeting specific aspects of hypertrophy-associated arrhythmias.NEW & NOTEWORTHY Phorbol 12-myristate 13-acetate (PMA) treatment of neonatal rat ventricular cardiomyocytes (nr-vCMCs) led to induction of many significant features of pathological cardiac hypertrophy (PCH), including action potential duration prolongation and dispersion, which provided enough time and depolarizing force for formation of early afterdepolarization (EAD)-induced focal tachyarrhythmias. PMA-treated nr-vCMCs represent a well-controllable in vitro model, which mostly resembles to moderate left ventricular hypertrophy (LVH) rather than severe LVH, in which generation of a reentry is the putative mechanism of its arrhythmias.

AUF1 regulation of coding and noncoding RNA

AUF1 is a family of four RNA-binding proteins (RBPs) generated by alternative pre-messenger RNA (pre-mRNA) splicing, with canonical roles in controlling the stability and/or translation of mRNA targets based on recognition of AU-rich sequences within mRNA 3' untranslated regions. However, recent studies identifying AUF1 target sites across the transcriptome have revealed that these canonical functions are but a subset of its roles in posttranscriptional regulation of gene expression. In this review, we describe recent developments in our understanding of the RNA-binding properties of AUF1 together with their biochemical implications and roles in directing mRNA decay and translation. This is then followed by a survey of newly discovered activities for AUF1 proteins in control of miRNA synthesis and function, including miRNA assembly into microRNA (miRNA)-loaded RNA-induced silencing complexes (miRISCs), miRISC targeting to mRNA substrates, interplay with an expanding network of other cellular RBPs, and reciprocal regulatory relationships between miRNA and AUF1 synthesis. Finally, we discuss recently reported relationships between AUF1 and long noncoding RNAs and regulatory roles on viral RNA substrates. Cumulatively, these findings have significantly expanded our appreciation of the scope and diversity of AUF1 functions in the cell, and are prompting an exciting array of new questions moving forward. WIREs RNA 2017, 8:e1393. doi: 10.1002/wrna.1393 For further resources related to this article, please visit the WIREs website.

Physiological networks and disease functions of RNA-binding protein AUF1

Regulated messenger RNA (mRNA) decay is an essential mechanism that governs proper control of gene expression. In fact, many of the most physiologically potent proteins are encoded by short-lived mRNAs, many of which contain AU-rich elements (AREs) in their 3'-untranslated region (3'-UTR). AREs target mRNAs for post-transcriptional regulation, generally rapid decay, but also stabilization and translation inhibition. AREs control mRNA turnover and translation activities through association with trans-acting RNA-binding proteins that display high affinity for these AU-rich regulatory elements. AU-rich element RNA-binding protein (AUF1), also known as heterogeneous nuclear ribonucleoprotein D (HNRNPD), is an extensively studied AU-rich binding protein (AUBP). AUF1 has been shown to regulate ARE-mRNA turnover, primarily functioning to promote rapid ARE-mRNA degradation. In certain cellular contexts, AUF1 has also been shown to regulate gene expression at the translational and even the transcriptional level. AUF1 comprises a family of four related protein isoforms derived from a common pre-mRNA by differential exon splicing. AUF1 isoforms have been shown to display multiple and distinct functions that include the ability to target ARE-mRNA stability or decay, and transcriptional activation of certain genes that is controlled by their differential subcellular locations, expression levels, and post-translational modifications. AUF1 has been implicated in controlling a variety of physiological functions through its ability to regulate the expression of numerous mRNAs containing 3'-UTR AREs, thereby coordinating functionally related pathways. This review highlights the physiological functions of AUF1-mediated regulation of mRNA and gene expression, and the consequences of deficient AUF1 levels in different physiological settings.

AUF1 contributes to Cryptochrome1 mRNA degradation and rhythmic translation

In the present study, we investigated the 3' untranslated region (UTR) of the mouse core clock gene cryptochrome 1 (Cry1) at the post-transcriptional level, particularly its translational regulation. Interestingly, the 3'UTR of Cry1 mRNA decreased its mRNA levels but increased protein amounts. The 3'UTR is widely known to function as a cis-acting element of mRNA degradation. The 3'UTR also provides a binding site for microRNA and mainly suppresses translation of target mRNAs. We found that AU-rich element RNA binding protein 1 (AUF1) directly binds to the Cry1 3'UTR and regulates translation of Cry1 mRNA. AUF1 interacted with eukaryotic translation initiation factor 3 subunit B and also directly associated with ribosomal protein S3 or ribosomal protein S14, resulting in translation of Cry1 mRNA in a 3'UTR-dependent manner. Expression of cytoplasmic AUF1 and binding of AUF1 to the Cry1 3'UTR were parallel to the circadian CRY1 protein profile. Our results suggest that the 3'UTR of Cry1 is important for its rhythmic translation, and AUF1 bound to the 3'UTR facilitates interaction with the 5' end of mRNA by interacting with translation initiation factors and recruiting the 40S ribosomal subunit to initiate translation of Cry1 mRNA.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

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.

mRNA stability as a function of striated muscle oxidative capacity

A change in mRNA stability alters the abundance of mRNA available for translation and is emerging as a critical pathway influencing gene expression. Variations in the stability of functional and regulatory mitochondrial proteins may contribute to the divergent mitochondrial densities observed in striated muscle. Thus we hypothesized that the stability of mRNAs encoding for regulatory nuclear and mitochondrial transcription factors would be inversely proportional to muscle oxidative capacity and would be facilitated by the activity of RNA binding proteins (RBPs). The stability of mitochondrial transcription factor A (Tfam), peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), and nuclear respiratory factor 2α (NRF-2α) mRNA was assessed in striated muscles with distinct oxidative capacities using in vitro decay assays. All three mitochondrial regulators were rapidly degraded in cardiac and slow-twitch red (STR) muscle, resulting in a ∼60–65% lower ( P < 0.05) mRNA half-life ( t1/2) compared with fast-twitch white (FTW) fibers. This accelerated rate of Tfam mRNA decay was matched by a 2.5-fold increase in Tfam transcription in slow- compared with fast-twitch muscle ( P = 0.05). Protein expression of four unique RBPs [AU-rich binding factor 1 (AUF1), human antigen R (HuR), KH-homology splicing regulatory protein (KSRP), and CUG binding protein 1 (CUGBP1)] believed to modulate mRNA stability was elevated in cardiac and STR muscles ( P < 0.05) and was moderately associated with the decay of Tfam, PGC-1α, and NRF-2α mRNA. Variable rates of transcript degradation were apparent when comparing all transcripts within the same muscle type. Thus the distribution of RBPs appears to follow a fiber-type specific pattern and subsequently functions to alter the stability of specific mitochondrial regulators in a transcript- and tissue-specific fashion.

AUF1 and HuR: possible implications of mRNA stability in thyroid function and disorders

Abstract

RNA-binding proteins may regulate every aspect of RNA metabolism, including pre-mRNA splicing, mRNA trafficking, stability and translation of many genes. The dynamic association of these proteins with RNA defines the lifetime, cellular localization, processing and the rate at which a specific mRNA is translated. One of the pathways involved in regulating of mRNA stability is mediated by adenylate uridylate-rich element (ARE) binding proteins. These proteins are involved in processes of apoptosis, tumorigenesis and development. Out of many ARE-binding proteins, two of them AUF1 and HuR were studied most extensively and reported to regulate the mRNA stability in vivo. Our previously published data demonstrate that both proteins are involved in thyroid carcinogenesis. Several other reports postulate that mRNA binding proteins may participate in thyroid hormone actions. However, until now, exacts mechanisms and the possible role of post-transcriptional regulation and especially the role of AUF1 and HuR in those processes remain not fully understood. In this study we shortly review the possible function of both proteins in relation to development and various physiological and pathophysiological processes, including thyroid function and disorders.

MKP-1 regulates cytokine mRNA stability through selectively modulation subcellular translocation of AUF1

MAPK phosphatase-1 (MKP-1)/dual specificity protein phosphatase-1 (DUSP-1) is a negative regulator of the host inflammatory response to infection. However, the mechanisms underlying the regulation of cytokine expression by MKP-1, especially at the post-transcriptional level, have not been fully delineated. In the current study, MKP-1 specifically dephosphorylated activated MAPK responses and attenuated LPS-induced IL-6, IL-10, and TNF-α expression. In addition, MKP-1 was important in destabilizing cytokine mRNAs. In LPS-stimulated rat macrophages with overexpressed MKP-1, half-lives of IL-6, IL-10 and TNF-α mRNAs were significantly reduced compared to controls. Conversely, half-lives of IL-6, IL-10, and TNF-α mRNAs were significantly increased in bone marrow macrophages derived from MKP-1 knock out (KO) mice compared with macrophages derived from MKP-1 wild type (WT) mice. Furthermore, MKP-1 promoted translocation of RNA-binding protein (RNA-BP) ARE/poly-(U) binding degradation factor 1 (AUF1) from the nucleus to the cytoplasm in response to LPS stimulation as evidenced by Western blot and immunofluorescent staining. Knockdown AUF1 mRNA expression by AUF1 siRNA in MKP-1 WT bone marrow macrophages significantly delayed degradation of IL-6, IL-10 and TNF- α mRNAs compared with controls. Finally, AUF1 was immunoprecipitated with the RNA complex in cellular lysates derived from bone marrow macrophages of MKP-1 KO vs. WT mice, which had increased AUF1-bound target mRNAs, including IL-6, IL-10, and TNF-α in WT macrophages compared with MKP-1 KO macrophages. Thus, this work provides new mechanistic insight of MKP-1 signaling and regulation of cytokine mRNA stability through RNA binding proteins in response to inflammatory stimuli.

Phorbol ester and endothelin-1 alter functional expression of Na+/Ca2+ exchange, K+, and Ca2+ currents in cultured neonatal rat myocytes

Endothelin-1 (ET-1) and activation of protein kinase C (PKC) have been implicated in alterations of myocyte function in cardiac hypertrophy and heart failure. Changes in cellular Ca2+ handling and electrophysiological properties also occur in these states and may contribute to mechanical dysfunction and arrhythmias. While ET-1 or PKC stimulation induces cellular hypertrophy in cultured neonatal rat ventricular myocytes (NRVMs), a system widely used in studies of hypertrophic signaling, there is little data about electrophysiological changes. Here we studied the effects of ET-1 (100 nM) or the PKC activator phorbol 12-myristate 13-acetate (PMA, 1 μM) on ionic currents in NRVMs. The acute effects of PMA or ET-1 (≤30 min) were small or insignificant. However, PMA or ET-1 exposure for 48-72 h increased cell capacitance by 100 or 25%, respectively, indicating cellular hypertrophy. ET-1 also slightly increased Ca2+ current density (T and L type). Na+/Ca2+ exchange current was increased by chronic pretreatment with either PMA or ET-1. In contrast, transient outward and delayed rectifier K+ currents were strongly downregulated by PMA or ET-1 pretreatment. Inward rectifier K+ current tended toward a decrease at larger negative potential, but time-independent outward K+ current was unaltered by either treatment. The enhanced inward and reduced outward currents also result in action potential prolongation after PMA or ET-1 pretreatment. We conclude that chronic PMA or ET-1 exposure in cultured NRVMs causes altered functional expression of cardiac ion currents, which mimic electrophysiological changes seen in whole animal and human hypertrophy and heart failure.

Phosphorylation of tristetraprolin by MK2 impairs AU-rich element mRNA decay by preventing deadenylase recruitment

mRNA turnover is a critical step in the control of gene expression. In mammalian cells, a subset of mRNAs regulated at the level of mRNA turnover contain destabilizing AU-rich elements (AREs) in their 3' untranslated regions. These transcripts are bound by a suite of ARE-binding proteins (AUBPs) that receive information from cell signaling events to modulate rates of ARE mRNA decay. Here we show that a key destabilizing AUBP, tristetraprolin (TTP), is repressed by the p38 mitogen-activated protein kinase (MAPK)-activated kinase MK2 due to the inability of phospho-TTP to recruit deadenylases to target mRNAs. TTP is tightly associated with cytoplasmic deadenylases and promotes rapid deadenylation of target mRNAs both in vitro and in cells. TTP can direct the deadenylation of substrate mRNAs when tethered to a heterologous mRNA, yet its ability to do so is inhibited upon phosphorylation by MK2. Phospho-TTP is not impaired in mRNA binding but does fail to recruit the major cytoplasmic deadenylases. These observations suggest that phosphorylation of TTP by MK2 primarily affects mRNA decay downstream of RNA binding by preventing recruitment of the deadenylation machinery. Thus, TTP may remain poised to rapidly reactivate deadenylation of bound transcripts to downregulate gene expression once the p38 MAPK pathway is deactivated.

Alternatively expressed domains of AU-rich element RNA-binding protein 1 (AUF1) regulate RNA-binding affinity, RNA-induced protein oligomerization, and the local conformation of bound RNA ligands

AU-rich element RNA-binding protein 1 (AUF1) binding to AU-rich elements (AREs) in the 3'-untranslated regions of mRNAs encoding many cytokines and other regulatory proteins modulates mRNA stability, thereby influencing protein expression. AUF1-mRNA association is a dynamic paradigm directed by various cellular signals, but many features of its function remain poorly described. There are four isoforms of AUF1 that result from alternative splicing of exons 2 and 7 from a common pre-mRNA. Preliminary evidence suggests that the different isoforms have varied functional characteristics, but no detailed quantitative analysis of the properties of each isoform has been reported despite their differential expression and regulation. Using purified recombinant forms of each AUF1 protein variant, we used chemical cross-linking and gel filtration chromatography to show that each exists as a dimer in solution. We then defined the association mechanisms of each AUF1 isoform for ARE-containing RNA substrates and quantified relevant binding affinities using electrophoretic mobility shift and fluorescence anisotropy assays. Although all AUF1 isoforms generated oligomeric complexes on ARE substrates by sequential dimer association, sequences encoded by exon 2 inhibited RNA-binding affinity. By contrast, the exon 7-encoded domain enhanced RNA-dependent protein oligomerization, even permitting cooperative RNA-binding activity in some contexts. Finally, fluorescence resonance energy transfer-based assays showed that the different AUF1 isoforms remodel bound RNA substrates into divergent structures as a function of protein:RNA stoichiometry. Together, these data describe isoform-specific characteristics among AUF1 ribonucleoprotein complexes, which likely constitute a mechanistic basis for differential functions and regulation among members of this protein family.

Effect of chronic contractile activity on mRNA stability in skeletal muscle

Repeated bouts of exercise promote the biogenesis of mitochondria by multiple steps in the gene expression patterning. The role of mRNA stability in controlling the expression of mitochondrial proteins is relatively unexplored. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 or 6 h/day) rat muscle for 7 days. Chronic contractile activity (CCA) increased the protein expression of PGC-1alpha, c-myc, and mitochondrial transcription factor A (Tfam) by 1.6-, 1.7- and 2.0-fold, respectively. To determine mRNA stability, we incubated total RNA with cytosolic extracts using an in vitro cell-free system. We found that the intrinsic mRNA half-lives (t(1/2)) were variable within control muscle. Peroxisome proliferator-activated receptor-gamma, coactivator-1alpha (PGC-1alpha) and Tfam mRNAs decayed more rapidly (t(1/2) = 22.7 and 31.4 min) than c-myc mRNA (t(1/2) = 99.7 min). Furthermore, CCA resulted in a differential response in degradation kinetics. After CCA, PGC-1alpha and Tfam mRNA half-lives decreased by 48% and 44%, respectively, whereas c-myc mRNA half-life was unchanged. CCA induced an elevation of both the cytosolic RNA-stabilizing human antigen R (HuR) and destabilizing AUF1 (total) by 2.4- and 1.8-fold, respectively. Increases in the p37(AUF1), p40(AUF1), and p45(AUF1) isoforms were most evident. Thus these data indicate that CCA results in accelerated turnover rates of mRNAs encoding important mitochondrial biogenesis regulators in skeletal muscle. This adaptation is likely beneficial in permitting more rapid phenotypic plasticity in response to subsequent contractile activity.

Factors controlling the activity of the SERCA2a pump in the normal and failing heart

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

AUF1 is upregulated by angiotensin II to destabilize cardiac Kv4.3 channel mRNA

Expression of cardiac myocyte Kv4 channels (Kv4.3 for human, Kv4.2 and Kv4.3 for rodents) is downregulated with hypertrophy in vivo leading to a decrease in the transient outward current (Ito). This effect is recapitulated in vitro with rat neonatal cardiac myocytes treated with angiotensin II (Ang II), which acts via AT(1) receptors, NADPH oxidase and p38 MAP kinase to destabilize the 3' untranslated region (3'UTR) of the Kv4.3 channel messenger RNA (mRNA). Here deletion analysis and mutagenesis identify an AU-rich element (ARE) in the Kv4.3 3'UTR that is required for Ang II-induced destabilization. Overexpression of AUF1 (ARE/poly-(U)-binding/degradation factor 1), an RNA destabilizing protein, mimics and occludes the Ang II effect, while RNA interference targeted against AUF1 blocks the Ang II effect on the Kv4.3 3'UTR. Ang II upregulates AUF1 by activating AT(1) receptors, NADPH oxidase and p38 MAP kinase. Finally, pull-down assays establish that Ang II increases AUF1 binding to the ARE required for destabilization, while binding of the mRNA stabilizing protein HuR is unaffected. Hence, Ang II acts via AT(1) receptors, NADPH oxidase and p38 MAP kinase to upregulate AUF1, which in turn binds to an ARE in the Kv4.3 3'UTR to destabilize the channel mRNA.

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.

Control of protein expression through mRNA stability in calcium signalling

Specific sequences (cis-acting elements) in the 3'-untranslated region (UTR) of RNA, together with stabilizing and destabilizing proteins (trans-acting factors), determine the mRNA stability, and consequently, the level of expression of several proteins. Such interactions were discovered initially for short-lived mRNAs encoding cytokines and early genes like c-jun and c-myc. However, they may also determine the fate of more stable mRNAs in a tissue and disease-dependent manner. The interactions between the cis-acting elements and the trans-acting factors may also be modulated by Ca(2+) either directly or via a control of the phosphorylation status of the trans-acting factors. We focus initially on the basic concepts in mRNA stability with the trans-acting factors AUF1 (destabilizing) and HuR (stabilizing). Sarco/endoplasmic reticulum Ca(2+) pumps, SERCA2a (cardiac and slow twitch muscles) and SERCA2b (most cells including smooth muscle cells), are pivotal in Ca(2+) mobilization during signal transduction. SERCA2a and SERCA2b proteins are encoded by relatively stable mRNAs that contain cis-acting stability determinants in their 3'-regions. We present several pathways where 3'-UTR mediated mRNA decay is key to Ca(2+) signalling: SERCA2a and beta-adrenergic receptors in heart failure, renin-angiotensin system, and parathyroid hormones. Other examples discussed include cytokines vascular endothelial growth factor, endothelin and endothelial nitric oxide synthase. Roles of Ca(2+) and Ca(2+)-binding proteins in mRNA stability are also discussed. We anticipate that these novel modes of control of protein expression will form an emerging area of research that may explore the central role of Ca(2+) in cell function during development and in disease.

Regulation of the sarcoplasmic reticulum Ca2+-ATPase expression in the hypertrophic and failing heartThis paper is part of a series in the Journal's “Made in Canada” section. The paper has undergone peer review

The sarcoplasmic reticulum (SR) plays a central role in the contraction and relaxation coupling in the myocardium. The SR Ca2+-ATPase (SERCA2) transports Ca2+ inside the SR lumen during relaxation of the cardiac myocyte. It is well known that diminished contractility of the hypertrophic cardiac myocyte is the main factor of ventricular dysfunction in the failing heart. A key feature of the failing heart is a decreased content and activity of SERCA2, which is the cause of some of the physiological defects observed in the hypertrophic cardiomyocyte performance that are important during transition of compensated hypertrophy to heart failure. In this review different possible mechanisms responsible for decreased transcriptional regulation of the SERCA2 gene are examined, which appear to be the primary cause for decreased SERCA2 expression in heart failure. The experimental evidence suggests that several signalling pathways are involved in the downregulation of SERCA2 expression in the hypertrophic and failing cardiomyocyte. Therapeutic upregulation of SERCA2 expression using replication deficient adenoviral expression vectors, pharmacological interventions using thyroid hormone analogues, β-adrenergic receptor antagonists, and novel metabolically active compounds are currently under investigation for the treatment of uncompensated cardiac hypertrophy and heart failure.

JNK activation decreases PP2A regulatory subunit B56alpha expression and mRNA stability and increases AUF1 expression in cardiomyocytes

A central feature of heart disease is a molecular remodeling of signaling pathways in cardiac myocytes. This study focused on novel molecular elements of MAPK-mediated alterations in the pattern of gene expression of the protein phosphatase 2A (PP2A). In an established model of sustained JNK activation, a 70% decrease in expression of the targeting subunit of PP2A, B56alpha, was observed in either neonatal or adult cardiomyocytes. This loss in protein abundance was accompanied by a decrease of 69% in B56alpha mRNA steady-state levels. Given that the 3'-untranslated region of this transcript contains adenylate-uridylate-rich elements known to regulate mRNA degradation, experiments explored the notion that instability of B56alpha mRNA accounts for the response. mRNA time-course analyses with real-time PCR methods showed that B56alpha transcript was transformed from a stable (no significant decay over 1 h) to a labile form that rapidly degraded within minutes. These results were supported by complementary experiments that revealed that the RNA-binding protein AUF1, known to destabilize target mRNA, was increased fourfold in JNK-activated cells. A variety of other stress-related stimuli, such as p38 MAPK activation and phorbol ester, upregulated AUF1 expression in cultured cardiac cells as well. In addition, gel mobility shift assays demonstrated that p37AUF1 binds with nanomolar affinity to segments of the B56alpha 3'-untranslated region. Thus these studies provide new evidence that signaling-induced mRNA instability is an important mechanism that underlies the changes in the pattern of gene expression evoked by stress-activated pathways in cardiac cells.