Modifications of eukaryotic initiation factor 4F (elF4F) in adult cardiocytes by adenoviral gene transfer: differential effects on elF4F activity andtotal protein synthesis rates
An. Saghir et al., Modifications of eukaryotic initiation factor 4F (elF4F) in adult cardiocytes by adenoviral gene transfer: differential effects on elF4F activity andtotal protein synthesis rates, BIOCHEM J, 356, 2001, pp. 557-566
In adult feline cardiocytes, increases in eukaryotic initiation factor 4F (
eIF4F) activity are correlated with accelerated rates of total protein synt
hesis 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 stimul
ated to contract (2 Hz, 5 ms pulses). Non-stimulated cardiocytes were used
as controls. Adenovirus-mediated overexpression of wildtype eIF4E increased
the total eIF4E pool by 120-140% above endogenous levels after 24h and pro
duced a corresponding increase in eIF4F content. However, it did not accele
rate 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 decreas
ed binding affinity for the mRNA cap. Overexpression of eIF4E/W56F increase
d the quantity of eIF4F but the rate of total protein synthesis was decreas
ed in quiescent and contracting cardiocytes. Overexpression of a mutant tha
t blocked eIF4E phosphorylation (eIF4E/S209A) increased the quantity of eIF
4F without any significant effect on total protein synthesis rates in quies
cent or contracting cardiocytes. Overexpression of the eIF4E kinase Mnk-1 i
ncreased eIF4E phosphorylation without a corresponding increase in eIF4F co
mplex formation or in the rate of total protein synthesis. We conclude the
following: (1) eIF4F assembly is increased by raising eIF4E levels via aden
oviral gene transfer; (2) the cap binding affinity of eIF4F is a rate-limit
ing 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.