Endocytosis machinery is required for β1-adrenergic receptor-induced hypertrophy in neonatal rat cardiac myocytes

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Abstract
Cardiac hypertrophy by activation of the β-adrenergic receptor (βAR) is mediated more efficiently by the β1-AR than by the β2-AR. We investigated the signalling mechanism by which the β1-AR mediates cardiac hypertrophy. Experiments were performed in cultured neonatal rat cardiomyocytes. Hypertrophy was determined by the protein/DNA content and atrial natriuretic factor transcription. Phosphorylation of Akt and Src was assessed by immunoblotting. Isoproterenol (ISO, 10 µM), a non-selective β-AR agonist, caused selective downregulation of the β1-AR (control β1 vs. β2: 35 vs. 65%, Bmax 78 ± 4 fmol/mg; 4 h, 10 vs. 90%, 61 ± 5 fmol/mg). Concanavalin A (Con A, 0.5 µg/mL), an inhibitor of endocytosis, prevented downregulation of β1-ARs by ISO treatment (4 h, 35 vs. 65%, 73 ± 8 fmol/mg), suggesting that β1-ARs selectively undergo endocytosis. Interference with β1-AR endocytosis by Con A, carboxyl terminal peptide of β-AR kinase-1, dominant negative (DN) β-arrestin-1, or DN dynamin inhibited β-adrenergic hypertrophy, suggesting that the endocytosis machinery plays a key role in mediating β-adrenergic hypertrophy. Activation of Akt by the β1-AR was blocked by inhibition of the endocytosis machinery, suggesting that endocytosis mediates activation of Akt. Akt plays a critical role in β-adrenergic hypertrophy, since DN Akt blocked ISO-induced hypertrophy. β-Adrenergic activation of Akt is mediated by Src, which associates with the endocytosis machinery and is necessary and sufficient to mediate β-adrenergic hypertrophy. Activation of the endocytosis machinery is required for activation of Akt, which, in turn, critically mediates β1-AR-induced cardiac hypertrophy.