Mechanical activity in heart regulates translation of alpha-myosin heavy chain mRNA but not its localization

Myofibril growth during cardiac hypertrophy is regulated through dual phosphorylation and acetylation of the actin capping protein CapZ

The mechanotransduction signaling pathways initiated in heart muscle by increased mechanical loading are known to lead to long-term transcriptional changes and hypertrophy, but the rapid events for adaptation at the sarcomeric level are not fully understood. The goal of this study was to test the hypothesis that actin filament assembly during cardiomyocyte growth is regulated by post-translational modifications (PTMs) of CapZβ1. In rapidly hypertrophying neonatal rat ventricular myocytes (NRVMs) stimulated by phenylephrine (PE), two-dimensional gel electrophoresis (2DGE) of CapZβ1 revealed a shift toward more negative charge. Consistent with this, mass spectrometry identified CapZβ1 phosphorylation on serine-204 and acetylation on lysine-199, two residues which are near the actin binding surface of CapZβ1. Ectopic expression of dominant negative PKCɛ (dnPKCɛ) in NRVMs blunted the PE-induced increase in CapZ dynamics, as evidenced by the kinetic constant (Kfrap) of fluorescence recovery after photobleaching (FRAP), and concomitantly reduced phosphorylation and acetylation of CapZβ1. Furthermore, inhibition of class I histone deacetylases (HDACs) increased lysine-199 acetylation on CapZβ1, which increased Kfrap of CapZ and stimulated actin dynamics. Finally, we show that PE treatment of NRVMs results in decreased binding of HDAC3 to myofibrils, suggesting a signal-dependent mechanism for the regulation of sarcomere-associated CapZβ1 acetylation. Taken together, this dual regulation through phosphorylation and acetylation of CapZβ1 provides a novel model for the regulation of myofibril growth during cardiac hypertrophy.

Upstream open reading frame in 5'-untranslated region reduces titin mRNA translational efficiency

Titin is the largest known protein and a critical determinant of myofibril elasticity and sarcomere structure in striated muscle. Accumulating evidence that mRNA transcripts are post-transcriptionally regulated by specific motifs located in the flanking untranslated regions (UTRs) led us to consider the role of titin 5'-UTR in regulating its translational efficiency. Titin 5'-UTR is highly homologous between human, mouse, and rat, and sequence analysis revealed the presence of a stem-loop and two upstream AUG codons (uAUGs) converging on a shared in frame stop codon. We generated a mouse titin 5'-UTR luciferase reporter construct and targeted the stem-loop and each uAUG for mutation. The wild-type and mutated constructs were transfected into the cardiac HL-1 cell line and primary neonatal rat ventricular myocytes (NRVM). SV40 driven 5'-UTR luciferase activity was significantly suppressed by wild-type titin 5'-UTR (∼ 70% in HL-1 cells and ∼ 60% in NRVM). Mutating both uAUGs was found to alleviate titin 5'-UTR suppression, while eliminating the stem-loop had no effect. Treatment with various growth stimuli: pacing, PMA or neuregulin had no effect on titin 5'-UTR luciferase activity. Doxorubicin stress stimuli reduced titin 5'-UTR suppression, while H2O2 had no effect. A reported single nucleotide polymorphism (SNP) rs13422986 at position -4 of the uAUG2 was introduced and found to further repress titin 5'-UTR luciferase activity. We conclude that the uAUG motifs in titin 5'-UTR serve as translational repressors in the control of titin gene expression, and that mutations/SNPs of the uAUGs or doxorubicin stress could alter titin translational efficiency.

Motility, survival, and proliferation

Airway smooth muscle has classically been of interest for its contractile response linked to bronchoconstriction. However, terminally differentiated smooth muscle cells are phenotypically plastic and have multifunctional capacity for proliferation, cellular hypertrophy, migration, and the synthesis of extracellular matrix and inflammatory mediators. These latter properties of airway smooth muscle are important in airway remodeling which is a structural alteration that compounds the impact of contractile responses on limiting airway conductance. In this overview, we describe the important signaling components and the functional evidence supporting a view of smooth muscle cells at the core of fibroproliferative remodeling of hollow organs. Signal transduction components and events are summarized that control the basic cellular processes of proliferation, cell survival, apoptosis, and cellular migration. We delineate known intracellular control mechanisms and suggest future areas of interest to pursue to more fully understand factors that regulate normal myocyte function and airway remodeling in obstructive lung diseases.

Pulmonary artery smooth muscle hypertrophy: roles of glycogen synthase kinase-3beta and p70 ribosomal S6 kinase

Increased medial arterial thickness is a structural change in pulmonary arterial hypertension (PAH). The role of smooth muscle hypertrophy in this process has not been well studied. Bone morphogenetic proteins (BMPs), transforming growth factor (TGF)-beta1, serotonin (or 5-hydroxytryptamine; 5-HT), and endothelin (ET)-1 have been implicated in PAH pathogenesis. We examined the effect of these mediators on human pulmonary artery smooth muscle cell size, contractile protein expression, and contractile function, as well on the roles of glycogen synthase kinase (GSK)-3beta and p70 ribosomal S6 kinase (p70S6K), two proteins involved in translational control, in this process. Unlike epidermal growth factor, BMP-4, TGF-beta1, 5-HT, and ET-1 each increased smooth muscle cell size, contractile protein expression, fractional cell shortening, and GSK-3beta phosphorylation. GSK-3beta inhibition by lithium or SB-216763 increased cell size, protein synthesis, and contractile protein expression. Expression of a non-phosphorylatable GSK-3beta mutant blocked BMP-4-, TGF-beta1-, 5-HT-, and ET-1-induced cell size enlargement, suggesting that GSK-3beta phosphorylation is required and sufficient for cellular hypertrophy. However, BMP-4, TGF-beta1, 5-HT, and ET-1 stimulation was accompanied by an increase in serum response factor transcriptional activation but not eIF2 phosphorylation, suggesting that GSK-3beta-mediated hypertrophy occurs via transcriptional, not translational, control. Finally, BMP-4, TGF-beta1, 5-HT, and ET-1 treatment induced phosphorylation of p70S6K and ribosomal protein S6, and siRNAs against p70S6K and S6 blocked the hypertrophic response. We conclude that mediators implicated in the pathogenesis of PAH induce pulmonary arterial smooth muscle hypertrophy. Identification of the signaling pathways regulating vascular smooth muscle hypertrophy may define new therapeutic targets for PAH.

Airway smooth muscle in asthma

Airway smooth muscle plays a multifaceted role in the pathogenesis of asthma. We review the current understanding of the contribution of airway myocytes to airway inflammation, airway wall remodeling, and airflow obstruction in this prevalent disease syndrome. Together, these roles make airway smooth muscle an attractive target for asthma therapy.

Factors controlling cardiac myosin-isoform shift during hypertrophy and heart failure

Myosin is a molecular motor, which interacts with actin to convert the energy from ATP hydrolysis into mechanical work. In cardiac myocytes, two myosin isoforms are expressed and their relative distribution changes in different developmental and pathophysiologic conditions of the heart. It has been realized for a long time that a shift in myosin isoforms plays a major role in regulating myocardial contractile activity. With the recent evidence implicating that alteration in myosin isoform ratio may be eventually beneficial for the treatment of a stressed heart, a new interest has developed to find out ways of controlling the myosin isoform shift. This article reviews the published data describing the role of myosin isoforms in the heart and highlighting the importance of various factors shown to influence myosin isofrom shift during physiology and disease states of the heart.

Transforming growth factor-beta induces airway smooth muscle hypertrophy

Although smooth muscle hypertrophy is present in asthmatic airways, little is known about the biochemical pathways regulating airway smooth muscle protein synthesis, cell size, or accumulation of contractile apparatus proteins. We sought to develop a model of airway smooth muscle hypertrophy in primary cells using a physiologically relevant stimulus. We hypothesized that transforming growth factor (TGF)-beta induces hypertrophy in primary bronchial smooth muscle cells. Primary human bronchial smooth muscle cells isolated from unacceptable lung donor tissue were studied. Cells were seeded on uncoated plastic dishes at 50% confluence and TGF-beta was added. Experiments were performed in the absence of serum. TGF-beta increased cell size and total protein synthesis, expression of alpha-smooth muscle actin and smooth muscle myosin heavy chain, formation of actomyosin filaments, and cell shortening to acetylcholine. Further, TGF-beta increased airway smooth muscle alpha-actin synthesis in the presence of the transcriptional inhibitor actinomycin D, evidence that translational control is a physiologically important element of the observed hypertrophy. TGF-beta induced the phosphorylation of eukaryotic translation initiation factor-4E-binding protein, a signaling event specifically involved in translational control. Finally, two inhibitors of 4E-binding protein phosphorylation, the phosphoinositol 3-kinase inhibitor LY294002 and a phosphorylation site mutant of 4E-binding protein-1 that dominantly inhibits eukaryotic initiation factor-4E, each blocked TGF-beta-induced alpha-actin expression and cell enlargement. We conclude that TGF-beta induces hypertrophy of primary bronchial smooth muscle cells. Further, phosphorylation of 4E-binding protein is required for the observed hypertrophy.

Creatine kinase is an alpha myosin heavy chain 3'UTR mRNA binding protein

Altered cardiac workload regulates the translation and localization of the alpha myosin heavy chain (alphaMyHC) messenger RNA through the 3' untranslated region (UTR) by protein-RNA interactions. We used the alphaMyHC 3'UTR from neonatal rat heart tissue in a gel shift analysis to find RNA binding proteins. One was identified by microsequencing as creatine kinase, brain form B (CKBB). The affinity of its binding interaction was evaluated using sense and antisense alphaMyHC 3'UTR and 3'UTR probes from myosin isoforms of 2B and 2X skeletal muscle. Removal of calcium by the chelating agent EGTA had a potentiating effect on the formation of the CKBB/alphaMyHC 3'UTR complex in vitro . Varying the concentration of ATP (0.1-1 mM) also enhanced this interaction, suggesting that autophosphorylation of CKBB is taking place. Our novel finding that CKBB, an energy transduction enzyme, binds to the RNA of the 3'UTR of the faster ATP consuming alphaMyHC suggests a possible regulatory linkage between the metabolic state of the cell and myosin isoform expression.

IL-1 beta transcript stability in monocytes is linked to cytoskeletal reorganization and the availability of mRNA degradation factors

Monocyte extravasation initiates reorganization of the cytoskeleton (CSK) and adhesion-dependent cytokine gene transcription. The actin CSK is thought to be crucial for compartmentalization and translation of mRNA, many of which contain AU-rich (ARE) instability motifs in the 3' untranslated region. We investigated regulation of adhesion-induced IL-1 beta expression by the monocyte CSK. In serum-free adherent monocytes, the induced IL-1 beta mRNA was stable and did not coextract with actin filaments. In contrast, in cells adherent in autologous serum, IL-1 beta transcripts were unstable, coextracted with actin filaments and were associated with only transient activation of the mitogen-activated protein kinases (MAPK). Under both conditions of adherence, the ARE-binding protein AUF1/hnRNP D was readily extracted in the cytosolic fraction. Electro-injection with AUF1/hnRNP D modified the actin CSK and, surprisingly, stabilized IL-1 beta transcripts. These data suggest that the control of mRNA degradation is linked with changes in the CSK. Mitogen-activated protein kinase activation or alterations in the availability of mRNA degradation factors may mediate these effects.

Translation is regulated via the 3' untranslated region of alpha-myosin heavy chain mRNA by calcium but not by its localization

Posttranscriptional regulation plays an important role in alpha-myosin heavy chain (alpha-MyHC) protein synthesis in cardiac muscle cells. In the present study, we test the effects of calcium and mRNA mislocalization on alpha-MyHC translation in order to determine the mechanism(s) contributing to translational block via the 3' untranslated region (3'UTR). Neonatal rat cardiac myocytes were treated for 6 h with L-isoproterenol (10 microM) to enhance beating, with 10 microM verapamil to block beating and mislocalize mRNA, or with 3 microM colchicine to enhance beating but mislocalize mRNA by depolymerization of the microtubules. In order to determine whether translation is regulated by the 3'UTR, either a control (SV40 3'UTR) or the experimental (alpha-MyHC 3'UTR) was placed after a luciferase reporter gene and transfected into the myocytes. The amount of luciferase protein only decreased significantly in verapamil arrested cells transfected with the alpha-MyHC 3'UTR construct (P < 0.01). To control for the possibility that pharmacological treatments might affect transcription or message stability, we analyzed neomycin and luciferase mRNA levels transcribed from the same transfected plasmid. No significant changes were found with an RNase protection assay. These results suggest that calcium but not mRNA localization regulates protein synthesis and further, this is mediated by the 3' untranslated region of alpha-MyHC.