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

ZFP36L1 and AUF1 Induction Contribute to the Suppression of Inflammatory Mediators Expression by Globular Adiponectin via Autophagy Induction in Macrophages

Adiponectin, a hormone predominantly originated from adipose tissue, has exhibited potent anti-inflammatory properties. Accumulating evidence suggests that autophagy induction plays a crucial role in anti-inflammatory responses by adiponectin. However, underlying molecular mechanisms are still largely unknown. Association of Bcl-2 with Beclin-1, an autophagy activating protein, prevents autophagy induction. We have previously shown that adiponectin-induced autophagy activation is mediated through inhibition of interaction between Bcl-2 and Beclin-1. In the present study, we examined the molecular mechanisms by which adiponectin modulates association of Bcl-2 and Beclin-1 in macrophages. Herein, we demonstrated that globular adiponectin (gAcrp) induced increase in the expression of AUF1 and ZFP36L1, which act as mRNA destabilizing proteins, both in RAW 264.7 macrophages and primary peritoneal macrophages. In addition, gene silencing of AUF1 and ZFP36L1 caused restoration of decrease in Bcl-2 expression and Bcl-2 mRNA half-life by gAcrp, indicating crucial roles of AUF1 and ZFP36L1 induction in Bcl-2 mRNA destabilization by gAcrp. Moreover, knock-down of AUF1 and ZFP36L1 enhanced interaction of Bcl-2 with Beclin-1, and subsequently prevented gAcrp-induced autophagy activation, suggesting that AUF1 and ZFP36L1 induction mediates gAcrp-induced autophagy activation via Bcl-2 mRNA destabilization. Furthermore, suppressive effects of gAcrp on LPS-stimulated inflammatory mediators expression were prevented by gene silencing of AUF1 and ZFP36L1 in macrophages. Taken together, these results suggest that AUF1 and ZFP36L1 induction critically contributes to autophagy induction by gAcrp and are promising targets for anti-inflammatory responses by gAcrp.

Novel Roles for Peroxynitrite in Angiotensin II and CaMKII Signaling

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) oxidation controls excitability and viability. While hydrogen peroxide (H₂O₂) affects Ca²⁺-activated CaMKII in vitro, Angiotensin II (Ang II)-induced CaMKIIδ signaling in cardiomyocytes is Ca²⁺ independent and requires NADPH oxidase-derived superoxide, but not its dismutation product H₂O₂. To better define the biological regulation of CaMKII activation and signaling by Ang II, we evaluated the potential for peroxynitrite (ONOO⁻) to mediate CaMKII activation and downstream Kv4.3 channel mRNA destabilization by Ang II. In vitro experiments show that ONOO⁻ oxidizes and modestly activates pure CaMKII in the absence of Ca²⁺/CaM. Remarkably, this apokinase stimulation persists after mutating known oxidation targets (M281, M282, C290), suggesting a novel mechanism for increasing baseline Ca²⁺-independent CaMKII activity. The role of ONOO⁻ in cardiac and neuronal responses to Ang II was then tested by scavenging ONOO⁻ and preventing its formation by inhibiting nitric oxide synthase. Both treatments blocked Ang II effects on Kv4.3, tyrosine nitration and CaMKIIδ oxidation and activation. Together, these data show that ONOO⁻ participates in Ang II-CaMKII signaling. The requirement for ONOO⁻ in transducing Ang II signaling identifies ONOO⁻, which has been viewed as a reactive damaging byproduct of superoxide and nitric oxide, as a mediator of GPCR-CaMKII signaling.

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.

Delayed endosome-dependent CamKII and p38 kinase signaling in cardiomyocytes destabilizes Kv4.3 mRNA

The Kv4.3 transient outward current (I(to)) channel, which produces early repolarization in human cardiomyocytes, is downregulated with cardiac pathology. This is evident in cultured neonatal rat cardiomyocytes in which Angiotensin II (Ang II) acts via p38 mitogen-activated protein kinase (p38K) to increase apoptosis and induce Kv4.3 mRNA destabilization to downregulate the channel protein. However, it is not understood how p38K activation, which is activated transiently for minutes, induces downstream effects hours later. Here we show that there is a second phase of p38K activation. Inhibiting this delayed p38K activation eliminated Kv4.3 mRNA destabilization. Furthermore, inhibiting endosome generation left the transient activation of p38K intact, but blocked delayed p38K activation and the Kv4.3 effect. CamKII was also found to be required for delayed p38K activation and Kv4.3 mRNA destabilization. Finally, CamKII methionine oxidation and activation are biphasic, with the delayed phase requiring endosomes. Hence, in addition to participating in channel traffic, cardiomyocyte endosomes control channel mRNA expression by mediating delayed oxidative CamKII-p38K signaling.

Simvastatin modulates remodeling of Kv4.3 expression in rat hypertrophied cardiomyocytes

Objectives: Hypertrophy has been shown to be associated with arrhythmias which can be caused by abnormal remodeling of the Kv4-family of transient potassium channels. Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (statins) have recently been shown to exert pleiotropic protective effects in cardiovascular diseases, including anti-arrhythmias. It is hypothesized that remodeling of Kv4.3 occurs in rat hypertrophied cardiomyocytes and is regulated by simvastatin.

Methods: Male Sprague-Dawley rats and neonatal rat ventricular myocytes (NRVMs) underwent abdominal aortic banding (AAB) for 7 weeks and angiotensin II (AngII) treatment, respectively, to induce cardiac hypertrophy. Kv4.3 expression by NRVMs and myocardium (subepicardial and subendocardial) in the left ventricle was measured. The transient outward potassium current (I_to) of NRVMs was recorded using a whole-cell patch-clamp method.

Results: Expression of the Kv4.3 transcript and protein was significantly reduced in myocardium (subepicardial and subendocardial) in the left ventricle and in NRVMs. Simvastatin partially prevented the reduction of Kv4.3 expression in NRVMs and subepicardial myocardium but not in the subendocardial myocardium. Hypertrophied NRVMs exhibited a significant reduction in the I_to current and this effect was partially reversed by simvastatin.

Conclusions: Simvastatin alleviated the reduction of Kv4.3 expression, I_to currents in hypertrophied NRVMs and alleviated the reduced Kv4.3 expression in subepicardial myocardium from the hypertrophied left ventricle. It can be speculated that among the pleiotropic effects of simvastatin, the anti-arrhythmia effect is partly mediated by its effect on Kv4.3.

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.

The role of AUF1 in regulated mRNA decay

Messenger ribonucleic acid (mRNA) turnover is a major control point in gene expression. In mammals, many mRNAs encoding inflammatory cytokines, oncoproteins, and G-protein-coupled receptors are destabilized by the presence of AU-rich elements (AREs) in their 3'-untranslated regions. Association of ARE-binding proteins (AUBPs) with these mRNAs promotes rapid mRNA degradation. ARE/poly(U)-binding/degradation factor 1 (AUF1), one of the best-characterized AUBPs, binds to many ARE-mRNAs and assembles other factors necessary to recruit the mRNA degradation machinery. These factors include translation initiation factor eIF4G, chaperones hsp27 and hsp70, heat-shock cognate protein hsc70, lactate dehydrogenase, poly(A)-binding protein, and other unidentified proteins. Numerous signaling pathways alter the composition of this AUF1 complex of proteins to effect changes in ARE-mRNA degradation rates. This review briefly describes the roles of mRNA decay in gene expression in general and ARE-mediated decay (AMD) in particular, with a focus on AUF1 and the different modes of regulation that govern AUF1 involvement in AMD.

Estradiol up-regulates expression of the A + U-rich binding factor 1 (AUF1) gene in the sheep uterus

The A+U-rich binding factor 1 (AUF1 or HNRPD) gene produces predominant RNA binding proteins. The AUF1 transcript is alternatively spliced to produce four protein isoforms that stabilize or destabilize hundreds of mRNAs. Previously, we discovered that estradiol (E2) treatment of ovariectomized sheep increased concentrations of AUF1p45 protein which stabilized estrogen receptor alpha (ER) mRNA in the uterus. This study examined E2 regulation of AUF1 mRNAs in the sheep uterus. Northern analysis determined that E2 treatment increased concentrations of total AUF1 mRNAs twofold in endometrial and myometrial tissue compartments. In situ hybridization indicated that the increase was most intense in the glandular epithelium of endometrium. In a well characterized in vitro RNA stability assay, AUF1 3'UTR sequences were much more stable in uterine extracts from E2-treated ewes compared to extracts from control ewes. AUF1 mRNAs with alternative splicing of exons 2 and 7 (in the coding sequence) and exon 9 (in the 3'UTR) were identified. The only effect of E2 treatment on alternative splicing was that it reduced the percentage of AUF1 mRNAs containing exon 9-derived sequences. These data indicate that E2 up-regulates AUF1 and ER genes coordinately by a post-transcriptional mechanism.

Downregulated Kv4.3 expression in the RVLM as a potential mechanism for sympathoexcitation in rats with chronic heart failure

Elevated central angiotensin II (ANG II) plays a critical role in the sympathoexcitation of chronic heart failure (CHF) by stimulating upregulated ANG II type 1 receptors (AT(1)R) in the rostral ventrolateral medulla (RVLM). However, the link between enhanced ANG II signaling and alterations in the electrophysiological characteristics of neurons in the RVLM remains unclear. In the present experiments, we screened for potentially altered genes in the medulla of rats with CHF that are directly related to neuronal membrane conductance using the Rat Genome 230 2.0 Array GeneChip. We found that CHF rats exhibited a 2.1-fold reduction in Kv4.3 gene expression, one of the main voltage-gated K(+) channels, in the medulla. Real-time RT-PCR and Western blot analysis confirmed the downregulation of Kv4.3 in the RVLM of CHF rats. In intact animals, we found that microinjection of the voltage-gated potassium channel blocker, 4-aminopyridine, into the RVLM evoked a sympathoexcitation and hypertension in both normal and CHF rats. CHF rats exhibited smaller responses to 4-aminopyridine than did normal rats. Finally, we used a neuronal cell line (CATH.a neurons) to explore the effect of ANG II on Kv4.3 expression and function. We found that ANG II treatment significantly downregulated mRNA and protein expression of Kv4.3 and decreased the A-type K(+) current. Employing this cell line, we also found that the ANG II-induced inhibition of Kv4.3 mRNA expression was attenuated by the superoxide scavenger Tempol and the p38 MAPK inhibitor SB-203580. The effects of ANG II were abolished by the AT(1)R antagonist losartan. We conclude that the sympathoexcitation observed in the CHF state may be due, in part, to an ANG II-induced downregulation of Kv4.3 expression and subsequent decrease in K(+) current, thereby increasing the excitability of neurons in the RVLM. The ANG II-induced inhibition of Kv4.3 mRNA expression was mediated by ANG II-AT(1)R-ROS-p38 MAPK signaling.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.