Rapamycin limits formation of active eukaryotic initiation factor 4F complex following meal feeding in rat hearts

Feeding promotes protein synthesis in cardiac muscle through a stimulation of the messenger RNA translation initiation phase of protein synthesis by enhancing assembly of active eukaryotic initiation factor (eIF)4F complex. The experiments reported herein examined the potential role for a rapamycin-sensitive signaling pathway in increasing formation of active eIF4G-eIF4E complex during meal feeding. Hearts from male Sprague-Dawley rats fed a meal consisting of rat nonpurified diet were sampled prior to and 3 h following the meal in the presence or absence of treatment with rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) complex 1. Rapamycin prevented the meal feeding-induced stimulation of myocardial protein synthesis. Inhibition of mTOR with rapamycin decreased the association of rapamycin-associated TOR protein with mTOR and prevented the feeding-induced assembly of eIF4G-eIF4E complex. In contrast, the abundance of eIF4E binding protein-1 (4E-BP1)-eIF4E complex was unaffected by either meal feeding or rapamycin. Pretreatment with rapamycin completely prevented the feeding-induced phosphorylation of eIF4G(Ser(1108)), whereas the inhibitor only partially attenuated meal feeding-induced 70-kDa ribosomal protein S6 kinase1(Thr(389)) phosphorylation and extent of 4E-BP1 in the gamma-form. Meal feeding-induced phosphorylation of protein kinase B on either Ser(473) or Thr(308) was unaffected by rapamycin. These findings suggest the extent of phosphorylation of eIF4G following meal feeding occurs by a rapamycin-sensitive mechanism in cardiac muscle. Furthermore, the rapamycin-sensitive reductions in phosphorylation of eIF4G may also lead to decreased formation of active eIF4G-eIF4E complex.