RAPAMYCIN INHIBITS ALPHA(1)-ADRENERGIC RECEPTOR-STIMULATED CARDIAC MYOCYTE HYPERTROPHY BUT NOT ACTIVATION OF HYPERTROPHY-ASSOCIATED GENES -EVIDENCE FOR INVOLVEMENT OF P70 S6 KINASE

The 70-kD S6 kinase (p70(S6K)) has been implicated in the regulation o f protein synthesis in many cell types and in the angiotensin II-stimu lated hypertrophy of cardiac myocytes. Our purpose was to determine wh ether p70(S6K) plays a role in cardiomyocyte hypertrophy induced by th e alpha(1)-adrenergic receptor (alpha(1)-AR) agonist phenylephrine (PE ). PE stimulated the activity of p70(S6K) >3-fold, and this increase w as blocked by rapamycin, an immunosuppressant macrolide that selective ly inhibits p70(S6K). When administered for 3 days, PE stimulated a 30 % increase in total protein content, a 2-fold increase in the incorpor ation of [C-14]phenylalanine (C-14-Phe) into protein, and a 50% increa se in two-dimensional myocyte area. Rapamycin pretreatment (greater th an or equal to 500 pg/mL) significantly inhibited each of these PE-sti mulated changes. Two days of PE treatment resulted in a 1.6-fold incre ase in total RNA yield per dish, a 2-fold increase in incorporation of [C-14]uridine into myocyte RNA, and increases in relative mRNA levels of the hypertrophy-associated atrial natriuretic factor (ANF, 2.1-fol d) and skeletal alpha-actin (SK, 2.2-fold) genes. Although rapamycin a bolished the PE-stimulated increases in total RNA and incorporation of [C-14]uridine, it had no effect on the induction of the ANF and SK ge nes. LY294002, a specific inhibitor of phosphatidylinositol 3-kinase ( PI3-K) activity, inhibited PE-stimulatcd increases in p70(S6K) activit y and the incorporation of labeled precursors into myocyte protein and RNA. These results demonstrate that p70(S6K) is activated by the hype rtrophic agent PE and that a PI3-K or PI3-K-like activity is required for p70(S6K) activation and myocyte hypertrophy. Tile data suggest tha t p70(S6K) activation may be required for PE-stimulated hypertrophy of cardiac myocytes. Our results demonstrate that intracellular signalin g pathways responsible for transcriptional and translational responses diverge early after alpha(1)-AR stimulation in cardiac myocytes.