Bradykinin-stimulated protein synthesis by myocytes is dependent on the MAP kinase pathway and p70(S6K)

Bradykinin (BK) has a direct hypeptrophic effect on rat ventricular cardiom yocytes (VCM) as defined by an increase in protein synthesis and an increas e in atrial natriuretic peptide mRNA and secretion. In the current study, w e have examined the dependence of BK-induced protein synthesis an activatio n of 90-kDa ribosomal 86 kinase (p90(rsk)) and 70-kDa S6 kinase (p70(S6K)). Both of these kinases possess the ability to phosphorylate the ribosomal p rotein S6, which plays an important role in initiating mRNA translation. St imulation of adult VCM with 10 mu M BK increased p90(rsk) activity by 2.5 /- 0.3-fold and increased p70(S6K) activity by 2.0 +/- 0.3-fold. p90(rsk) i s a terminal kinase in the mitogen-activated protein (MAP) kinase pathway. Inhibition of MAP kinase kinase activation by Raf in the MAP kinase pathway with PD-098059 (25 mu M) blocked BK-stimulated activation of p90(rsk) by 7 0% and unexpectedly blocked p70(S6K) by 72%. Rapamycin inhibited BK-stimula ted p70(S6K) activity by 93% but had no effect on p90(rsk) activation by BK . Inhibition of the MAP kinase pathway and p70S6K With PD-098059 was parall eled by changes in protein synthesis. BK (10 mu M) increased [H-3]phenylala nine incorporation by 27 +/- 3 and 39 +/- 6% in cultured adult and neonatal VCM, respectively. Treatment with PD-098059 or rapamycin abolished the inc rease in protein synthesis stimulated by BK. These results suggest that 1) BK activates p70(S6K) and p90(rsk); 2) although both p70(S6K) and p90(rsk) have the potential to phosphorylate the ribosomal S6 protein, p70(S6K) and not p90(rsk) is the predominant kinase involved in increasing protein synth esis by BK; and 3) p70(S6K) activation is dependent on stimulation of the M AP kinase pathway at a point distal to Raf.