Eukaryotic initiation factor 4E (eIF-4E) is rate-limiting for translational initiation and therefore is a key mechanism for regulating the rate of protein synthesis. This project is focused on the role of eIF- 4E in regulating the rate of protein synthesis in the adult cardiac muscle cell, or cardiocyte. Important findings of this project were that 1) eIF- 4E activity was increased in response to an acute increase in load is measured by the extent of eIF-4E phosphorylation and by incorporation of eIF-4E into the translationally activity eIF-4F complex, 2) eIF-4E phosphorylation was dependent upon active tension development using both an in vitro cardiocyte model of electrically stimulated contraction and an in vivo model of acute pressure overload, and 3) eIF-4E phosphorylation in the cardiocyte correlated with an increased rate of translational initiation and an accelerated rate of protein synthesis. The hypothesis underlying this continuation proposal is that eIF-4E activity is causally involved in controlling translational efficiency, and is a primary endpoint for load-induced acceleration of the rate of protein synthesis in the adult cardiocyte. The hypothesis will be tested by employing recombinant adenoviruses as a method for gene transfer of either wild-type or mutated forms of eIF-4E into adult cardiocytes. Using this strategy, the eIF-4E phenotype will be manipulated in terms of activity and function and the effects on translational efficiency determined. The ability of increased load to accelerate the rate of protein synthesis will be tested in cardiocytes by expressing a non-phosphorylatable eIF-4E mutant using electrically stimulated contractile activity as a model of active tension development.
The specific aims are as follows 1) Determine how alterations in eIF-4E activity and function affect translational efficiency in the adult cardiocyte. 2) Determine if contraction-induced load accelerates cardiocyte protein synthesis after specific alterations in eIF-4E activity or function. 3) Define linkage between activation of specific protein kinase C isoforms and eIF-4E phosphorylation in adult cardiocytes, since eIF-4E is a putative substrate for protein kinase C. Because increased eIF-4E activity was also linked to an accelerated rate of protein synthesis in a canine model of pressure overload in vivo, eIF-4E is a potentially important endpoint for determining the mechanisms that regulate the rate of protein synthesis during load-induced hypertrophic growth in the adult myocardium.
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