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

Abstract

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.

β-Arrestin-mediated Signaling Regulates Protein Synthesis

“…This may reflect a conditional specificity, where Mnk1 and eIF4E increase translation only in response to certain upstream stimuli, as we have reported here for AngII. Numerous others have reported the necessity for Mnk1 or eIF4E in translation, particularly in cardiac cells (11,31,32), supporting the observations made in this report.…”

Mechanisms of translational deregulation in human tumors and therapeutic intervention strategies

“…For example, the effect of eIF4E on stimulating cyclin D1 translation is thought to be through an element in its 3 0 UTR regulating nuclear export (Culjkovic et al, 2005). In addition, overexpression of eIF4E in cardiomyocytes did not stimulate the translation of chimeric reporter mRNAs with increased secondary structure (Tuxworth et al, 2004). Finally, in an empirical analysis of mRNAs translationally stimulated following constitutive overexpression of eIF4E in NIH3T3 cells, there was no obvious enrichment for either long structured 5 0 UTRs, or for many of the mRNAs mentioned above (with the exception of ODC; Larsson et al, 2006).…”

Myc regulates VEGF production in B cells by stimulating initiation of VEGF mRNA translation

“…These findings show that the increased VEGF mRNA translation observed in Myc-overexpressing P493-6 cells is not mediated by increased eIF4E expression. eIF4E activity can be modulated via phosphorylation of serine residue 209 by the Mnk1 kinase, which has been associated with cell growth (Waskiewicz et al, 1999;Scheper and Proud, 2002;Tuxworth et al, 2004). An antibody specific for phosphorylated eIF4E was used to show that eIF4E is phosphorylated in Myc-on and Myc-off P493-6 cells (Figure 8a).…”