Rapamycin Inhibits α 1 -Adrenergic Receptor–Stimulated Cardiac Myocyte Hypertrophy but Not Activation of Hypertrophy-Associated Genes

Abstract

Abstract The 70-kD S6 kinase (p70 S6K ) has been implicated in the regulation of protein synthesis in many cell types and in the angiotensin II–stimulated hypertrophy of cardiac myocytes. Our purpose was to determine whether p70 S6K plays a role in cardiomyocyte hypertrophy induced by the α 1 -adrenergic receptor (α 1 -AR) agonist phenylephrine (PE). PE stimulated the activity of p70 S6K >3-fold, and this increase was blocked by rapamycin, an immunosuppressant macrolide that selectively inhibits p70 S6K . When administered for 3 days, PE stimulated a 30% increase in total protein content, a 2-fold increase in the incorporation of [ 14 C]phenylalanine ( 14 C-Phe) into protein, and a 50% increase in two-dimensional myocyte area. Rapamycin pretreatment (≥500 pg/mL) significantly inhibited each of these PE-stimulated changes. Two days of PE treatment resulted in a 1.6-fold increase in total RNA yield per dish, a 2-fold increase in incorporation of [ 14 C]uridine into myocyte RNA, and increases in relative mRNA levels of the hypertrophy-associated atrial natriuretic factor (ANF, 2.1-fold) and skeletal α-actin (SK, 2.2-fold) genes. Although rapamycin abolished the PE-stimulated increases in total RNA and incorporation of [ 14 C]uridine, it had no effect on the induction of the ANF and SK genes. LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K) activity, inhibited PE-stimulated increases in p70 S6K activity and the incorporation of labeled precursors into myocyte protein and RNA. These results demonstrate that p70 S6K is activated by the hypertrophic agent PE and that a PI3-K or PI3-K–like activity is required for p70 S6K activation and myocyte hypertrophy. The data suggest that p70 S6K activation may be required for PE-stimulated hypertrophy of cardiac myocytes. Our results demonstrate that intracellular signaling pathways responsible for transcriptional and translational responses diverge early after α 1 -AR stimulation in cardiac myocytes.