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 (2Hz, 5ms 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 24h and produced a corresponding increase in eIF4F content. However, it did not accelerate total protein synthesis rates in quiescent 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 in quiescent and contracting cardiocytes. Overexpression of a mutant that blocked eIF4E phosphorylation (eIF4E/S209A) increased the quantity of eIF4F without any significant effect on total protein synthesis rates in quiescent or contracting cardiocytes. Overexpression of the eIF4E kinase 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 cap binding affinity of eIF4F is a rate-limiting determinant for total protein synthesis rates; and (3) increases in the quantity of eIF4F alone or in eIF4E phosphorylation are not sufficient to accelerate total protein synthesis rates.
Abbreviations used: eIF, eukaryotic initiation factor; 4E-BP, eIF4E-binding protein; 7m-Gppp, 7-methylguanosine cap; Mnk-1, MAP kinase signal-integrating kinase; HA, influenza haemagglutinin epitope.