Abstract
A necessary mediator of cardiac myocyte enlargement is protein synthesis, which is controlled at the levels of both translation initiation and elongation. Eukaryotic elongation factor-2 (eEF2) mediates the translocation step of peptide-chain elongation and is inhibited through phosphorylation by eEF2 kinase. In addition, p70S6 kinase can regulate protein synthesis by phosphorylating eEF2 kinase or via phosphorylation of ribosomal protein S6. We have recently shown that eEF2 kinase is also controlled by phosphorylation by AMP-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis. Moreover, the mammalian target of rapamycin has also been shown to be inhibited, indirectly, by AMPK, thus leading to the inhibition of p70S6 kinase. Although AMPK activation has been shown to modulate protein synthesis, it is unknown whether AMPK could also be a regulator of cardiac hypertrophic growth. Therefore, we investigated the role of AMPK activation in regulating protein synthesis during both phenylephrine- and Akt-induced cardiac hypertrophy. Metformin and 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside were used to activate AMPK in neonatal rat cardiac myocytes. Activation of AMPK significantly decreased protein synthesis induced by phenylephrine treatment or by expression of constitutively active Akt. Activation of AMPK also resulted in decreased p70S6 kinase phosphorylation and increased phosphorylation of eEF2, suggesting that inhibition of protein synthesis involves the eEF2 kinase/eEF2 axis and/or the p70S6 kinase pathway. Together, our data suggest that the inhibition of protein synthesis by pharmacological activation of AMPK may be a key regulatory mechanism by which hypertrophic growth can be controlled.
Highlights
A necessary mediator of cardiac myocyte enlargement is protein synthesis, which is controlled at the levels of both translation initiation and elongation
Effect of Metformin and aminoimidazole-4-carboxamide 1-D-ribofuranoside (AICAR) on Protein Synthesis in Phenylephrine-treated Neonatal Rat Cardiac Myocytes—Neonatal rat cardiac myocytes treated with 10 M phenylephrine displayed a significant 1.7-fold increase in protein synthesis over vehicle-treated cardiac myocytes (Fig. 1A)
Effect of Metformin and AICAR on AMPK Activation in Phenylephrine-treated Neonatal Rat Cardiac Myocytes—To confirm that metformin and AICAR treatment resulted in AMPK activation, lysates from neonatal rat cardiac myocytes treated with 10 M phenylephrine in the presence or absence of metformin or AICAR were subjected to immunoblot analysis using an antibody against the ␣ subunit of AMPK (␣-AMPK) phosphorylated at Thr-172
Summary
A necessary mediator of cardiac myocyte enlargement is protein synthesis, which is controlled at the levels of both translation initiation and elongation. We have recently shown that eEF2 kinase is controlled by phosphorylation by AMP-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis. An alteration in cardiac energy substrate utilization involving a decrease in fatty acid oxidation and an increase in glucose utilization [3, 4] can become maladaptive [5] This switch in substrate utilization is consistent with re-induction of several fetal gene products that regulate metabolism in the hypertrophied heart (see Ref. 5 for review). Horman et al [17] have shown that AMPK activation leads to increased eEF2 phosphorylation via eEF2 kinase activation, resulting in the inhibition of protein synthe-. Because AMPK may inhibit protein synthesis via a number of different pathways, it is possible that AMPK is a key regulator of cardiac hypertrophy
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