Role of load in regulating eIF-4F complex formation in adult feline cardiocytes

Role of the 5'-untranslated region in regulating translational efficiency of specific mRNAs in adult cardiocytes

It has been hypothesized that translational efficiency is determined by the amount of secondary structure in the 5'-untranslated region (5'-UTR) of mRNA. Here, we examined whether specific 5'-UTRs with excessive secondary structure selectively regulate translational efficiency in adult cardiocytes. Recombinant adenoviruses were generated to express reporter mRNAs consisting of the 5'-UTR derived from c-jun or ornithine decarboxylase (ODC) fused to beta-galactosidase (betaGal) coding sequence. Each adenovirus expressed GFP mRNA as a control for 5'-UTRs with minimal secondary structure. Subsequently, cardiocytes were electrically stimulated to contract at 1 Hz to accelerate protein synthesis as compared to quiescent controls. Translational efficiency was calculated by measuring protein expression as a function of mRNA levels. Translational efficiency of c-jun/betaGal mRNA increased significantly by 3.7-fold in contracting vs. quiescent cardiocytes, but ODC/betaGal mRNA was unchanged. Contraction increased c-jun/betaGal mRNA levels in polyribosomes by 2.3-fold, which indicates that translational efficiency was enhanced by mobilization. A short, unstructured 5'-UTR was sufficient for efficient translation of betaGal mRNA in quiescent and contracting cardiocytes. GFP mRNA produced similar results. These studies demonstrate that the 5'-UTR functions as a determinant of translational efficiency of specific mRNAs, such as c-jun, that regulate growth of the adult cardiocyte.

Regulation of c-jun mRNA expression in adult cardiocytes by MAP kinase interacting kinase-1 (MNK1)

Hypertrophic growth of adult myocardium is associated with increased expression of the early response gene c-jun. The purpose of this study was to determine whether eukaryotic initiation factor (elF) 4E (eIF4E) regulates translational efficiency of c-jun mRNA as measured by flux into polysomes. Adult feline cardiomyocytes in primary culture were treated with 0.2 microM 12-O-tetradecanoylphorbol 13-acetate (TPA), and c-jun mRNA was quantified in total, monosome, and polysome fractions by real-time polymerase chain reaction. After 1 h, TPA increased total c-jun mRNA by 10.5-fold. The corresponding flux into polysomes was significantly lower (5-fold). Adenoviral-mediated overexpression of either eIF4E or a nonphosphorylatable mutant (S209/A) did not affect total c-jun mRNA or its flux between monosomes and polysomes. Similar results were obtained following overexpression of the eIF4E kinase Mnk1. Thus, translational efficiency of c-jun mRNA was not affected by changes in activity or amount of eIF4E. In contrast, a kinase-deficient Mnk1 mutant significantly reduced total c-jun mRNA from 9.8-fold to 6.0-fold while flux between monosomes and polysomes remained constant. The decrease in total c-jun mRNA resulted from increased decay of c-jun mRNA incorporated into the polysomes. We conclude that Mnk1 activity stabilizes c-jun mRNA in polysomes independent of eIF4E phosphorylation.

Meal feeding enhances formation of eIF4F in skeletal muscle: role of increased eIF4E availability and eIF4G phosphorylation

Feeding promotes protein accretion in skeletal muscle through a stimulation of the mRNA translation initiation phase of protein synthesis either secondarily to nutrient-induced rises in insulin or owing to direct effects of nutrients themselves. The present set of experiments establishes the effects of meal feeding on potential signal transduction pathways that may be important in accelerating mRNA translation initiation. Gastrocnemius muscle from male Sprague-Dawley rats trained to consume a meal consisting of rat chow was sampled before, during, and after the meal. Meal feeding enhanced the assembly of the active eIF4G.eIF4E complex, which returned to basal levels within 3 h of removal of food. The increased assembly of the active eIF4G.eIF4E complex was associated with a marked 10-fold rise in phosphorylation of eIF4G(Ser(1108)) and a decreased assembly of inactive 4E-BP1.eIF4E complex. The reduced assembly of 4E-BP1.eIF4E complex was associated with a 75-fold increase in phosphorylation of 4E-BP1 in the gamma-form during feeding. Phosphorylation of S6K1 on Ser(789) was increased by meal feeding, although the extent of phosphorylation was greater at 0.5 h after feeding than after 1 h. Phosphorylation of mammalian target of rapamycin (mTOR) on Ser(2448) or Ser(2481), an upstream kinase responsible for phosphorylating both S6K1 and 4E-BP1, was increased at all times during meal feeding, although the extent of phosphorylation was greater at 0.5 h after feeding than after 1 h. Phosphorylation of PKB, an upstream kinase responsible for phosphorylating mTOR, was elevated only after 0.5 h of meal feeding for Thr(308), whereas phosphorylation Ser(473) was significantly elevated at only 0.5 and 1 h after initiation of feeding. We conclude from these studies that meal feeding stimulates two signal pathways in skeletal muscle that lead to elevated eIF4G.eIF4E complex assembly through increased phosphorylation of eIF4G and decreased association of 4E-BP1 with eIF4E.

TNFalpha mediates sepsis-induced impairment of basal and leucine-stimulated signaling via S6K1 and eIF4E in cardiac muscle

Decreased translation initiation adversely impacts protein synthesis and contributes to the myocardial dysfunction produced by sepsis. Therefore, the purpose of the present study was to identify sepsis-induced changes in signal transduction pathways known to regulate translation initiation in cardiac muscle and to determine whether the stimulatory effects of leucine can reverse the observed defects. To address this aim, sepsis was produced by cecal ligation and puncture (CLP) in anesthetized rats and the animals studied in the fasted condition 24 h later. Separate groups of septic and time-matched control rats also received an oral gavage of leucine. To identify potential mechanisms responsible for regulating cap-dependent mRNA translation in cardiac muscle, several eukaryotic initiation factors (eIFs) were examined. Under basal conditions, hearts from septic rats demonstrated a redistribution of the rate-limiting factor eIF4E due to increased binding of the translational repressor 4E-BP1 with eIF4E. However, this change was independent of an alteration in the phosphorylation state of 4E-BP1. The phosphorylation of mTOR, S6K1, the ribosomal protein (rp) S6, and eIF4G was not altered in hearts from septic rats under basal conditions. In control rats, leucine failed to alter eIF4E distribution but increased the phosphorylation of S6K1 and S6. In contrast, in hearts from septic rats leucine acutely reversed the alterations in eIF4E distribution. However, the ability of leucine to increase S6K1 and rpS6 phosphorylation in septic hearts was blunted. Sepsis increased the content of tumor necrosis factor (TNF)-alpha in heart and pre-treatment of rats with a TNF antagonist prevented the above-mentioned sepsis-induced changes. These data indicate that oral administration of leucine acutely reverses sepsis-induced alterations eIF4E distribution observed under basal conditions but the anabolic actions of this amino acid on S6K1 and rpS6 phosphorylation remain blunted, providing evidence for a leucine resistance. Finally, TNFalpha, either directly or indirectly, appears to mediate the sepsis-induced defects in myocardial translation initiation.

IGF-I activates the eIF4F system in cardiac muscle in vivo

IGF-I acutely stimulates protein synthesis in cardiac muscle through acceleration of mRNA translation. In the present study, we examined the regulatory signaling pathways and translation protein factors that potentially contribute to the myocardial responsiveness of protein synthesis to IGF-I in vivo. IGF-I was injected IV into rats and 20 min later the hearts were excised and homogenized for assay of regulatory proteins. IGF-I increased assembly of the translationally active eukaryotic initiation factor (eIF)4G.eIF4E complex. The increased assembly of eIF4G.eIF4E was associated with an enhanced eIF4G phosphorylation and increased availability of eIF4E. Increased availability of eIF4E occurred as a consequence of diminished abundance of the inactive 4E-BP1.eIF4E complex following IGF-I. The assembly of the 4E-BP1.eIF4E complex appeared to be decreased through an IGF-I-induced phosphorylation of 4E-BP1. IGF-I also caused an increase in the phosphorylation of S6K1. Activation of the potential upstream regulators of 4E-BP1 and S6K1 phosphorylation via PKB and mTOR was also observed. In contrast, there was no effect of IGF-I on phosphorylation of elongation factor (eFE)2. The results suggest the major impact of IGF-I in cardiac muscle occurred via stimulation of translation initiation rather than elongation. Furthermore, the results are consistent with a role for assembly of active eIF4G.eIF4E complex and activation of S6K1 in mediating the stimulation of mRNA translation initiation by IGF-I through a PKB/mTOR signaling pathway.

LEUCINE ACUTELY REVERSES BURN-INDUCED ALTERATIONS IN TRANSLATION INITIATION IN HEART

Myocardial dysfunction is a common manifestation of thermal injury, the etiology of which appears to be multifactorial. We have previously demonstrated that burn injury impairs cardiac protein synthesis at the level of translation initiation. The purpose of the present study was to determine whether oral administration of leucine, which is known to stimulate translation initiation in skeletal muscle, can ameliorate burn-induced changes in signal transduction pathways known to regulate protein synthesis in cardiac muscle. To address this aim, thermal injury was produced by a 40% total body surface area full-thickness scald burn in anesthetized rats, and the animals were studied in the fasted condition 24 h later; appropriate time-matched nonburned control rats were also included. Separate groups of control and burn rats also received an oral gavage of leucine. To identify potential mechanisms responsible for regulating mRNA translation in cardiac muscle, several eukaryotic initiation factors (eIFs) were examined using immunoprecipitation and immunoblotting techniques. Hearts from burned rats demonstrated a redistribution of eIF4E as evidenced by the increased binding of the translational repressor 4E-BP1 with eIF4E, a decreased amount of eIF4E bound with eIF4G, and a decreased amount of the hyperphosphorylated gamma-isoform of 4E-BP1. Furthermore, constitutive phosphorylation of mTOR, the ribosomal protein S6, and eIF4G was also decreased in hearts from burned rats. In control rats, leucine failed to alter eIF4E distribution but did increase the phosphorylation of S6K1 and S6. However, in hearts from burn rats, leucine acutely reversed the alterations in eIF4E distribution as well as the changes in S6, eIF4G, and mTOR phosphorylation. These data suggest that oral administration of leucine can acutely reverse multiple defects in cardiac translation initiation produced by thermal injury.

Regulation of protein synthesis by eIF4E phosphorylation in adult cardiocytes: the consequence of secondary structure in the 5'-untranslated region of mRNA

In adult cardiocytes, eIF4E (eukaryotic initiation factor 4E) activity and protein synthesis are increased concomitantly in response to stimuli that induce hypertrophic growth. We tested the hypothesis that increases in eIF4E activity selectively improve the translational efficiency of mRNAs that have an excessive amount of secondary structure in the 5'-UTR (5'-untranslated region). The activity of eIF4E was modified in primary cultures of adult cardiocytes using adenoviral gene transfer to increase either the amount of eIF4E or the extent of endogenous eIF4E phosphorylation. Subsequently, the effects of eIF4E on translational efficiency were assayed following adenoviral-mediated expression of luciferase reporter mRNAs that were either 'stronger' (less structure in the 5'-UTR) or 'weaker' (more structure in the 5'-UTR) with respect to translational efficiency. The insertion of G+C-rich repeats into the 5'-UTR doubled the predicted amount of secondary structure and was sufficient to reduce translational efficiency of the reporter mRNA by 48+/-13%. Translational efficiency of the weaker reporter mRNA was not significantly improved by overexpression of wild-type eIF4E when compared with the stronger reporter mRNA. In contrast, overexpression of the eIF4E kinase Mnk1 [MAP (mitogen-activated protein) kinase signal-integrating kinase 1] was sufficient to increase the translational efficiency of either reporter mRNA, independent of the amount of secondary structure in their respective 5'-UTRs. The increases in translational efficiency produced by Mnk1 occurred in association with corresponding decreases in mRNA levels. These findings indicate that the positive effect of eIF4E phosphorylation on translational efficiency in adult cardiocytes is coupled with the stability of mRNA.

Thermal injury impairs cardiac protein synthesis and is associated with alterations in translation initiation

The purpose of the present study was to determine whether burn injury decreases myocardial protein synthesis and potential contributing mechanisms for this impairment. To address this aim, thermal injury was produced by a 40% total body surface area full-thickness scald burn in anesthetized rats, and the animals were studied 24 h late. Burn decreased the in vivo-determined rate of myocardial protein synthesis and translation efficiency by 25% but did not alter the protein synthetic rate in skeletal muscle. To identify potential mechanisms responsible for regulating mRNA translation in cardiac muscle, we examined several eukaryotic initiation factors (eIFs) and elongation factors (eEFs). Burn failed to alter eIF2B activity or the total amount or phosphorylation status of either eIF2 alpha or eIF2B epsilon in heart. In contrast, hearts from burned rats demonstrated 1) an increased binding of the translational repressor 4E-BP1 with eIF4E, 2) a decreased amount of eIF4E associated with eIF4G, and 3) a decreased amount of the hyperphosphorylated gamma-form of 4E-BP1. These changes in eIF4E availability were not seen in gastrocnemius muscle where burn injury did not decrease protein synthesis. Furthermore, constitutive phosphorylation of mTOR, S6K1, the ribosomal protein S6, and eIF4G were also decreased in hearts from burned rats. Burn did not appear to adversely affect elongation because there was no significant difference in the myocardial content of eEF1 alpha or eEF2 or the phosphorylation state of eEF2. The above-mentioned burn-induced changes in mRNA translation were associated with an impairment of in vitro myocardial performance. Finally, 24 h postburn, the cardiac mRNA content of IL-1 beta, IL-6, and high-mobility group protein B1 (but not TNF-alpha) was increased. In summary, these data suggest that thermal injury specifically decreases cardiac protein synthesis in part by decreasing mRNA translation efficiency resulting from an impairment in translation initiation associated with alterations in eIF4E availability and S6K1 activity.

Modifications of eukaryotic initiation factor 4F (eIF4F) in adult cardiocytes by adenoviral gene transfer: differential effects on eIF4F activity and total protein synthesis rates

In adult feline cardiocytes, increases in eukaryotic initiation factor 4F (eIF4F) activity are correlated with accelerated rates of total protein synthesis produced in response to increased load. Adenoviral gene transfer was employed to increase either eIF4F complex formation or the phosphorylation of eIF4E on Ser-209. To simulate load,cardiocytes were electrically stimulated to contract (2 Hz,5 ms pulses). Non-stimulated cardiocytes were used as controls.Adenovirus-mediated overexpression of wild-type eIF4E increased the total eIF4E pool by 120-140% above endogenous levels after 24 h and produced a corresponding increase in eIF4F content.However, it did not accelerate total protein synthesis rates inquiescent cardiocytes; neither did it potentiate the increase produced by contraction. To modify the affinity of eIF4F, cardiocytes were infected with a mutant (eIF4E/W56F) with a decreased binding affinity for the mRNA cap. Overexpression of eIF4E/W56F increased the quantity of eIF4F but the rate of total protein synthesis was decreased inquiescent and contracting cardiocytes. Overexpression of a mutant that blocked eIF4E phosphorylation (eIF4E/S209A) increased the quantity ofeIF4F without any significant effect on total protein synthesis rates in quiescent or contracting cardiocytes. Overexpression of the eIF4Ekinase Mnk-1 increased eIF4E phosphorylation without a corresponding increase in eIF4F complex formation or in the rate of total protein synthesis. We conclude the following: (1) eIF4F assembly is increased by raising eIF4E levels via adenoviral gene transfer; (2) the capbinding affinity of eIF4F is a rate-limiting determinant for total protein synthesis rates; and (3) increases in the quantity of eIF4Falone or in eIF4E phosphorylation are not sufficient to accelerate total protein synthesis rates.